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Bureau of Standarc:

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JAN 2 3 197<J

NIST

PUBLICATIONS

NBS SPECIAL PUBLICATION 338

n

U.S. DEPARTMENT

OF

COMMERCE

National Bureau

of Standards

Status of

Thermal Analysis

UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Status of Thermal Analysis

Proceedings of a Symposium

on the Current Status of Thermal Analysis

Held at Gaithersburg, Maryland, April 21-22, 1970

Oscar Menis, Editor

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

9- "Zt.

Sponsored by the American Society for Testing and Materials with the cooperation of the National Bureau of Standards

L , U ' S I National Bureau of Standards, Special Publication 338 / r « » Nat. Bur. Stand. (U.S.), Spec. Publ. 338. 189 pages (Oct. 1970) CODEN: XNBSA

Issued October 1970

For sale by the Superintendent of Documents, U.S. Government Printing Office. Vi'asliingtun. D.C. 20402

(Order by SD Catalog No. C 13. 10:338), Price $1
NATIONAL BUREAU OF STANDARDS

FEB 1 6 1971

FOREWORD

This series of papers was presented at a Symposium on the

Current Status of Thermal Analysis as part of a joint NBS-ASTM

effort to focus attention on the recent trends and needs in the

field of thermal analysis. The selected papers cover discussions

of the importance of experimental parameters for obtaining information from differential thermal analysis and the evaluation of errors of classical calorimetric measurements. They also include

a review of the progress on the development of Standards for temperature calibration based on dynamic measurements. Other papers

sampled a representative cross section of thermal analysis applications. They encompass areas of high temperature measurements,

biochemical, polymer, and explosive materials. In addition, they

provide an evaluation of errors in differential scanning calorimetry and modifications of thermogravimetric apparatus for kinetic

studies. Many facets of this rapidly expanding technique for re- searchers and students were revealed. It is hoped that this publication may be useful as an overall review for updating textbook

information.

In order to describe adequately experimental procedures, it is

occasionally necessary to identify commercial products and equipment by the manufacturer's name or label. In no instance does such

identification imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the particular product

or equipment is necessarily the best available for that purpose.

The assistance of Mrs. Rosemary Maddock and Mr. J. I. Shultz

in the preparation of this publication is gratefully acknowledged.

The assistance of Dr. Frank E. Karash of Massachusetts University

in organizing the scientific program is greatly appreciated.

Oscar Menis
CONTENTS

STANDARDIZATION OF DIFFERENTIAL THERMAL

ANALYSIS TEST METHODS

Paul D. Gam 1 MACROCALORIMETRY - HOW ACCURATE?

K. L. Churney J G. T. Armstrong ^ and E. D. West ... 23 STATUS OF THERMAL ANALYSIS TEMPERATURE

SCALE STANDARDS

0. Menis and J. T. Sterling 6l

HIGH TEMPERATURE DIFFERENTIAL THERMAL

ANALYSIS

Robert D. Freeman 87

THERMAL STUDIES ON LIPID-WATER SYSTEMS BY DIFFERENTIAL

SCANNING CALORIMETRY WITH REFERENCE TO

ATHEROSCLEROSIS

G. Jerry Davis and Roger S. Porter 99 AN ANALYTICAL EVALUATION OF DIFFERENTIAL

SCANNING CALORIMETRY (DSC)

Joseph H. Flynn 119

APPARATUS FOR RATE STUDIES OF VAPOR

PRODUCING REACTIONS

R. J. MaCarter 137

DIFFERENTIAL THERMAL ANALYSIS OF PRIMARY

EXPLOSIVES - A MODIFIED TECHNIQUE

R. J. Grayhush, F. G. May, and A. C. Forsyth .... 151

THERMOGRAVIMETRY OF VULCANIZATES

John J. Maurer l65

ill
2 DTA Test Methods

The isolation of the specimen reaches an extreme in adiabatic

calorimetry wherein the interchange3 by intent at least, is reduced

to zero. That approach cannot very well be used in differential

thermal analysis , but we can and do isolate to varying degrees for

different purposes.

Then, too, maximizing the signal is not necessarily the best

solution — that is, the desired information may be in the shape

of the peak rather than in the magnitude. Our electronics coworkers

have developed amplifiers far better than we can use effectively,

so we can turn up the gain so much that it becomes virtually a pulse of great magnitude, structureless except for length, truly a single byte of information. Or, if we choose, we can heat the

specimen so fast that the thermal event takes place in a very

short time; since some quantity of energy must be taken in or given

out, the rate of heat transfer must be high and hence the differential temperature is high.

Either way, the result is a peak with little character. So

the question may be asked — Is there any unique feature which

may be hidden?

Let us look at silver sulfate. There is no particular evidence in Figure la that anything is awry. Here the temperature is

also being plotted so we can get a really good look at the behavior. We see only one unique feature in the heating curve, and

that is the lack of a really clear thermal arrest in the temperature plot. These curves were obtained using a block (Fig. 2)

containing a few hundred milligrams. Slowing the heating somewhat

without increasing the gain correspondingly, it becomes clear that

the peak (Fig. lb) is very obviously skewed. Some poorly-defined

rate effect is operating. We may not know what it is yet, but at least we know it's there.

Naturally if we were measuring on the outside of the sample

we would see less structure, but if we choose to use very small

samples, we have no choice but to measure on the outside. A
Garn 3

Figure 2. Sample block with axlally located thermocouples.
4 DTA Test Methods

thermocouple bead would be a substantial part of the sample-thermocouple assembly. Using small samples is, of course, no very difficult task. Even with a relatively low gain, heavily stabilized

amplifier, and with a very simple holding device, (Fig. 3) use of

sub-milligram quantities of materials was not at all difficult [IJ.

The sample cavity was just a dimple in the gold sheet, to which was welded a thermocouple wire. Relative to the sample, the gold sheet

provides a very satisfactory heat sink so that very rapid reactions

can dissipate or absorb heat quickly. So a fraction of a milligram

of potassium sulfate (Fig. 4) is plenty to obtain a good peak.

The new transistor amplifiers could possibly extend the peak a thousand fold and still maintain good stability, but we would learn

no more

^SAMPLE

Pt-io%Rh Pt Pt-io%Rh

Figure 3. Micro free-diffusion sample holder (Garn [1]). The

gold sheet acts as the joining wire of the differential thermocouple. The sample is placed directly

over the junction with the fine wire.

Diffusion of gases into or out of a small sample is very rapid

so dehydrations (Fig. 5) may give small peaks but oxidation reactions proceed very rapidly for the same reason.

We can control the atmosphere in several ways including the

self-generated atmosphere technique, (Fig. 6) so that by using a different sample holder and even a lesser amount of material the

dehydration is seen clearly (Pig. 7). The second reaction is now

the loss of carbon dioxide because oxygen cannot diffuse into the

reaction chamber.
Garn 5

Figure 4. Thermal curve of potassium

sulfate (0.1 mg) (Garn [1]).

The local cooling permits

ready detection of phase

transformations

500 625 TEMPERATURE (°C)

COBALT OXALATE HYDRATE

0.7 MG

Figure 5- Thermal curve of 0.7 mg cobalt

oxalate hydrate (Garn [1]).

The ease of entry of oxygen

permits rapid enough oxidation

to cause a 15 °C exothermic

deflection

100 200 300 400 TEMPERATURE IN DEGREES CENTIGRADE
6 DTA Test Methods

SAMPLE

0.1 -4.0 MG

SHIELDED THERMOCOUPLE

CERAMIC TUBE

Figure Inhibited-dlffusion sample

holder for small samples

(Garn [1]) .

- Even with

shielded thermocouples

very small effects can be

detected

100 200 300 400 500 TEMPERATURE (°C)

Figure 7. Thermal curve of 0=3 mg cobalt oxalate hydrate

(Garn [1]). The inhibited diffusion eliminates

exterior atmosphere effects for small samples as well as for large. Compare with Figure 5.
Gam 7 The first aonolusion we can Teach is that oommercia'l or custom-built instruments today generally have sensitivities

far beyond the needs. And the second is that the principal

attention must be centered on the experimental method.

Next consider a thin processed sample, for example a film.

The DTA curve for the material would presumably represent one or more degradations. Irreversible or not, the degradation will show

different curves if the film is against or separated from the

heater (Fig. 8). For the separated film, not only are both sides

available for escape of products but the temperature must be more

nearly homogeneous. Immediate contact with a heater would require

that the temperature differential between the heater and the far

side of the film be In the film Itself so that at reasonable heating rates the gradient within the film will be substantial; while

if the film is separated, the principal gradient will exist between

heater and film and a lesser gradient within the film Itself. This

may effect not only the clarity of observation, but even change the course of the degradation.

HEATER

B

HEATER

Figure 8. Heater, and A, suspended or, B, adherent film

specimens. The suspended film will be heated

more slowly and uniformly, for the same heater

temperature, than the adherent film.
8 DTA Test Methods

The principal example here comes from pyrolysls experiments

(Fig. 9) by Martin and Ramstad [2]. They used a flash heater to degrade a cellulose sample directly in the inlet to a gas chromatograph. They chose to bring the heater virtually instantaneously

above 600 °C by flash discharge in one experiment, but heated more

gently to 250-350 °C by arc illumination in a second experiment.

The more gentle heating is obviously a closer approach to equilibrium, and the "flash" heating not only produces large gradients

but brings the material to very high temperatures so quickly that

the easiest degradation reactions, presumably rate-dependent,

find themselves competing with other reactions which at near- homogeneity of temperature might never occur because the reactant had been used up in the earlier degradation.

"slow" pyrolysis . "flash" pyrolysis

TIME Figure 9. Chromatographic data on cellulose pyrolysis products

from slow and from flash pyrolysis (Martin and Ramstad

[2],). Different products and proportions of products

are formed at the two temperatures.
Garn 9 So we may fuvthev Qonclude that in some oases the nature of

the v&aotion will he determined or influenced hy the conditions

of the experiment and^ hence, if the study is intended to relate

to some -process outside the experimental apparatus , the experimental apparatus should simulate the real process as closely as po ssihle

,

The study of differences will certainly be a major use of DTA;

not simply differences between standard and reference but also

differences between similar -- but not identical -- specim.ens.

These differences may be inherent to the specimen or imparted to the specimen by treatment. Mineral species, for example, may show

deviate behavior by reason of impurities or strains. A very marked

example is that of brannerite [3] 5 in which the radioactivity has

apparently caused extensive disruption of the system (Pig. 10).

The disruption can be measured, perhaps, by comparison with an annealled sample and, from the present activity and estimated

effect of the disruption, some measure of the mineralogical age

obtained

More delicate measurements would be needed, however, for the

kind of work Keith and Tuttle [4] reported on a single crystal of

quartz. They found that from some crystals they could separate

portions which showed distinctly different a^:^3 transition temperatures (Fig. 11). In other cases the separation was not feasible although the existence of two forms was readily apparent

from the DTA curve. Quartz transitions have been reported over a fifty degree range, but there is no way to assess the reality of

that range because of differences in instrumental parameters.

These can play a large part; l.C.T.A.'s Committee on Standardization received reports varying by 22 degrees for the same quartz

sample

Other differences arise from processing. In polymer technology

this would be a prime source of variation. The work of Schwenker

and Beck [5] showed the effect of a very necessary process upon a very common material subjected to that process (Fig. 12). The un-
DTA Test Methods

100 200 300 400 500 600 700 800 900 1000

Degrees centigrade gure 10. Thermal curves of brannerite (Adler and Pulg [3]).

The variable appearance of thermograms of the

mineral is due partly to its variable composition

but partly also to its history.
Garn 11

Figure 11. Character of the heat effect obtained on two zoned

single crystals. Crystal F3-42'-43 was separated

into two fractions as illustrated and the inversion

of each fraction is shown (solid lines: thermal

effect on heating, dashed lines; thermal effect on cooling) (Keith and Tuttle [4]). The very low

heating rate permits clear distinction of materials

transforming at temperatures only a degree apart.

drawn material shows the expected glass transition and crystallization exotherm, while the drawn material, being highly ordered,

shows neither.

Other treatment effects can also be disclosed by DTA . For

example, Balata D rubber can show a double endotherm when heated

through the 30 to 80 degree range (Fig. 13). When cooled at moderate rates to aa. -100 ° <Z , it apparently retains its rubbery qualities to some extent. If the specimen is somewhat restrained,

mechanically, reheating will bring about mechanical relaxation at

a temperature which is presumably dependent upon the amount of
12 DTA Test Methods

Figure 12. Differential thermal analysis curves of Dacron

(Schwenker and Beck [5])- The undrawn fiber shows

a second order transition at 77 °C and crystalli-

, , zation at I36 °C.

strain. In this case the relaxation occurred oa. 0 °C. If this

same specimen is quenched, the material recry stallized oa . -50 °C

and shows no mechanical movement at 0 degree.

The curves for brannerlte, quartz and Balata rubber were run under substantially different circumstances for very good reason. The brannerlte could be handled by virtually any DTA equipment,

large or small samples, because no very great resolution was needed.

The quartz experiment needed, above all, high resolution of temperature; sensitivity was not a problem even with this low-energy transition and an optical amplifier. The rubber required a relatively

low heat capacity system so the quenching could be done properly.

The new uses for DTA will almost certainly call not just for

good resolution of curves, but more for good resolution of struc-
Garn 13

AT

-30 0 30 60

TEMPERATURE IN °C

Figure 13. Thermal curve of Balata D.

ture of peaks. Even though a few people are trying to do kinetics

calculations from DTA peaks, and others are trying to describe all

peaks by a single and simple equation by adjusting the constants,

the fact Is that considerable variety exists In the shapes of peaks.

Differences exist because of differing mechanisms as well as differing thermal properties. So the equipment we shall use a few years

from now shall have to be able to reproduce faithfully some measure

of the events taking place In the sample rather than the existence

or non-existence of an event. For example It will need to detect

not only the quick departure from the base line but also the misshapen leading side of the peak for potassium nitrate [6]. Laying

a straight edge alongside It leads quickly to the suspicion that

the transition Is not a simple, one-step process and the cooling
Ik DTA Test Methods

COOLING

/ \ / » / \

TIME TIME Figure l4 . Thermal curve of potassium nitrate.

curve shows clearly that it is not (Pig. l4). Potassium perchlorate shows a quick departure, too, (Pig. 15) and some indication of

a rate limiting process on cooling, but no indication of a real

two-step process.

The Qonolusions we may derive from these data are that (I)

there is much more to be seen that we are aooustomed to seeing;

(2) good instrument design will permit us to see more structure;

(3) the disclosure of this "fine structure" will eventually lead

to interpretation and description; and (4) description will lead

in short order to some utility o

One might postulate a composition-of-matter in which one material was to be concentrated at the grain boundaries of the base

material. The first material might be coprecipit ated in the base

material and concentrated by cycling through a phase transforma-
Garn 15

HEATING

I AT

Figure 15. Thermal curve of potassium perchlorate.

tion and the Increasing homogeneity of the base material followed

by observation of the supercooling. If this seems a bit far-out

let us remember that tremendous portions of our technology depend

on such things as controlled discontinuities at levels which were

not even detectable thirty years ago.

What about the shapes of the future? They will be like the

shapes of the past, mostly. Considerable ingenuity has been applied

to the problem of getting useful data, and much of this has been

directed to the sample holder design. New shapes will be brought

forth, but seldom; our problem will be selection from amongst the

many existing possibilities. Again, this choice will depend upon

the intended use.

I shall advance the thesis that the major new knowledge will

come by use of a moderate-size sample with the temperature-measuring

thermocouple in the center or along the axis. The reason is simple;
16 DTA Test Methods

176

174 172

170

CJ °. 168 166

164

162

160

158 6 10 14 18 22 26 30

Heating rate, C° per minute Figure l6. Peak temperature of (diluted) salicylic acid melting

as measured in the block {a,h,c), the reference (d^e),

and the sample (f). Curves ajd, and f refer to the

same sample weight. (Barrall and Rogers [7]).

Variations in thermal lag at the several locations

will be dependent on heating rate; the actual melting

point will not

more of the extraneous parameters are eliminated and the heat effects

seen by the thermocouple are necessarily influenced by the sample.

An example of the elimination of a variable is the Barrall and Rogers

[7] work (Fig. 16) in which they showed that the salicylic acid peak

shifted in temperature both with sample weight and heating rate if

the temperature was measured at other points, but invariant with

heating rate if the measurement was made on the sample axis. Other

shapes will be used for measurement, and I feel sure that most measurements will involve other shapes

The important aonalusion here is that shapes other than cylinders may present some convenience in measurement ^ hut there is no superiority for obtaining meaningful data o The major deduction^ then^

is that we should not allow ours elves to acquire a fondness for a particular type of sample holder^ hut pick and choose according to the

prohlem.
Garn I7

350°C 332°C

Figure 17. DTA-TGA data for PbN6 aged 2 weeks at room temperature in an H2O + CO2 atmpsphere (PalVerneker and Maycock [8]). With the 25°/min

heating rate, 2.0 mg of sample, the detonation

characteristics are very obvious.

So let us look at a further variety of problems and see why

differences are appropriate. PalVerneker and Maycock [8] had a good reason to use small samples of lead nitride; they wanted to use their equipment again (Fig. 17). Here we would not become concerned

about very accurate measurement of the sample temperature; It changes too rapidly to follow anyway. So measuring the temperature

of the sample container suffices. Besides, we can reasonably Infer

that the important measurement is the temperature of the surface of

the first particle to decompose. Hence the surface on which the

particles are resting is the appropriate measurement.
18 DTA Test Methods

TEMPERATURE Figure l8 . Van't Hoff plots for two reactions of the same kind. Even though the two reactions may occur together at some particular pressure a change

in pressure will separate them on the temperature scale. The separation, '^{"'^^j will vary

regularly with pressure.

We must not neglect the utility of gases and pressures of

gases o I have already described the advantages to be gained by

atmosphere control [9] 5 but let us look at it as a means of resolving peaks. The van't Hoff equation (Fig. 18) describes the variation of equilibrium vapor pressure with temperature — or vioe

versa. If the van't Hoff equation is plotted for each of two

materials, we can see that even if they decompose at the same temperature at some pressure, by changing the pressure the reactions

can be separated. A good example here is the calcite-magnesite

system (Fig. I9) reported by Stone [10]. The reactions are resolved at 16.5 mm and higher, but if the pressure were decreased significantly they would coincide.

Note that the gas supplied is the one concerned in the reaction. If other gases are used, special care must be taken or it

may have an adverse effect on the reaction. Helium is sometimes

used when evolved gases are to be measured, but it has particularly
Garn 19

0 200 400 600 800 1000 TEMPERATURE IN DEGREES CENTIGRADE Figure 19 . Differential thermal analysis of magnesite

in carbon dioxide 16. mm of He; 750 mm of Hg (Stone [10]). The calcite

peak is shifted much more than the magnesite.

100 200 300 400 500 600 700 800

Ca Cz O4

ihgO (air)

CaC204

IH2O (under He

100 200 300 400 500 600 700 800

Temperature

,

Figure 20. Differential thermal analysis curves of calcium

oxalate monohydrate (Wendlandt [11]). The peak

(200-300 °C) for the dehydration was used to calculate the activation energy by the method of

Borchardt (Garn [9]).

bad effects. Wendlandt 's [11] work shows the effect on a simple

dehydration, a very substantial peak shift downward in temperature

(Pigo 20). The reason is simple. Gases diffuse in helium with

tremendous velocity, so that the partial pressure at the surface

is low because the water moves away quickly. In air, the diffusion is slower, so a significant partial pressure can accrue.
20 DTA Test Methods

Also 3 there is a considerable difference between air and

oxygen. They both support oxidation but one carries with the ac- tive ingredient a quadruple portion of inert gas. So if diffusion

of oxygen is required in the reaction the supply is quickly depleted and further transfer of oxygen must take place through a layer of nitrogen. May's [12] work on SBR rubber illustrates

this very well (Fig. 21). The need to carry along the nitrogen

as the gas moves Inward slows oxidation reactions sometimes to the point of being imperceptible.

We cannot avoid the question of atmosphere control, not simply as a blanket to avoid reaction but often as a means of supplying one of the reactants. Oxidative stability studies can be

carried out under circumstances which simulate the normal atmos0XY6EN

AIR

NITROGEN

100° 200° 100° 200°

(a) (b) Figure 21. DTA curves for SBR rubber. (a) Without antioxidant.

(b) With antioxidant. (May, Bsharah and Merrifield

[12].
Garn 21 phere, or the inside of a reaction vessel, or the upper atmosphere,

or a deep-sea-diving apparatus without difficulty. And as we introduce new materials we must carry out the appropriate studieSo But

these will become more and more concerned with the structure of the

peak rather than the existence or magnitude of the heat effect.

To summarize and repeat, standardization of the DTA experiment as a general proQ edure is unwise. In DTA we are carrying out

a physical or chemioal process whose heat effect is the resultant

of many factors, We must design each test so that the parameter

we want to evaluate has the maximum effect on the signal we obtain.

This will usually require only a judicious selection from among the

known types of furnace as s emhli es , using the same control and recording ass emhly for all.

REFERENCES

[1] Garn, P. D., Thermal Analysis of Small Samples. Proc

Intern, Symp. Microchem. Techniques, State College,

Pennsylvania, I96I Cheronis, N. D., Editor, pp. 1105-

1109, Wiley, New York, I962.

[2] Martin, S. G., and Ramstad, R. W. , Compact Two-Stage Gas

Chromat ograph for Flash Pyrolysis Studies, Anal, Chem.

31, 982-985 (I96I).

[3] Adler, H. H. and Puig, J. A., Thermal Behavior of Brannerite, Am. Mineralogist 4^, IO86-IO96 (I96I).

[4] Keith, M. L. and Tuttle, 0. P., Significance of Variation

in the High-Low Inversion of Quartz, Am. J. Sci., Bowen

Vol., Pt. 1, 203-252 (1952).

[5] Schwenker, R. F., Jr. and Beck, L. R., Jr., The Differential Thermal Analysis of Textile and Other High Polymeric Materials, Textile Res. J. 30_, 624-626 (I96O).

[6] Garn, P. D., On the Dissimilarities in Solid^ Solid2

Phase Transformations, Anal. Chem. 4l, 447-456 (I969).

[7] Barrall, E. M. and Rogers, L. R., Differential Thermal

Analysis of Organic Samples. Effects of Geometry and

Operating Variables, Anal. Chem. 3l, 1101-1105 (1962).
J 22 DTA Test Methods

[8J PalVerneker, V. R. and Maycock, J. N., Simultaneous Differential Thermal Analy sis-Thermogravimetric Analysis Technique to Characterize the Explosivity of Lead Azide,

Anal. Chem. 40, 1325-29 (1968).

[9] Garn, P. D., "Thermoanalytical Methods of Investigation,"

Academic Press, New York, I965.

[10] Stone, R. L., Thermal Analysis of Magnesite at Carbon

Dioxide Pressures up to Six Atmospheres, J. Am. Ceram.

Soc. 37> 46-^17 (195^) . [11] Wedlandt, W. W. , Reaction Kinetics by Differential

Thermal Analysis, J. Chem. Educ . 3_8, 571-573 (I96I) . [12] May, W. R., Bsharah, L. and Merrifield, D. B., Evaluation

of Antioxidant Activity in Rubber by Differential

Thermal Analysis, I & EC Product Research and Development

7, 57-61, March I9680

V
MACROCALORIMETRY - HOV/ ACCURATE?

K. L. Churney, G. T. Armstrong, E. D. West

Physical Chemistry Division

Institute for Materials Res earoh

National Bureau of Standards

Washington, D. C. 20234

The measurement theory of isoperlbol and adlabatic

shield calorimeters operated by the method of intermittent

heating is reviewed and recent developments indicated.

Implications of the theory in terms of calorimeter design

and tests for systematic errors are discussed. Some idea

of the magnitude of calorimetric errors (as compared to errors arising from, uncertainties in the change of state

of the system under study) is outlined.

Key words: adiabatic shield; calorimetric errors; calorimetry; isoperlbol; measurement theory.

1. INTRODUCTION

The rapid growth in the use of differential thermal methods

has been accompanied by increasing demands in the use of these

methods to measure such thermodynamic properties as heat capacities, heats of fusion, heats of transition, and heats of reaction.

For example, the recent development of differential scanning

calorimeters [1,2] of the temperature-controlled type [3] may be

viewed as an effort to improve the utility of differential thermal

methods in making thermodynamic mieasurements without sacrificing

the principal advantages of these techniques (small sample size

and speed) or their sensitivity.

The measurement theory of differential thermal measurements

amounts to an analysis of the operation of differential calorimeters (see ref. [3-5]) in the dynamic mode [6]. As dynamic

processes, the methods are at a disadvantage whenever attainment

of thermodynamic equilibrium by the sample is slow relative to

23
24 Macrocalorimetry - How Accurate?

the scanning rate [6d]. Also, low heat transfer coefficients

inside the calorimeter {i.e., those among sample, sensor, and

calorimeter) and any large heat loss to the calorimeter environment can lead to a large uncertainty in the determination of the

temperature of the sample in spite of small sample size and low

scanning rates. Neither these ideas nor the idea of operation

of calorimeters in the dynamic mode are new to macrocalorimetry

(where by contrast the mass of sample or quantity of heat are relatively large). When heat losses are minimized, as in adiabatic shield calorimeters [7], and internal temperature gradients

are reduced [8], heat capacities can be obtained whose accuracy

and precision [8-10] are limited only by the inherent uncertainty

(see ref. [11], p. 338) of the dynamic mode of operation.

A reasonable goal in calorimeter design is to make the calorimetric errors less than those associated with specifying the

change in state under study. While this can be achieved for the

most part in conventional calorimetry [12,13] this is apparently

not yet generally the case in differential thermal methods.

Thus, it seems entirely relevant to review some of the concepts

used in the design and operation of certain common types of macrocalorimeters, ouch a review, more or less unencumbered by details,

can afford insight about sources of error in differential thermal

methods because, in concept, the measurement theory is identical

v/ith that of conventional macrocalorimetry.

Numerous reviews [6,12,14,15] of calorimietry and treatises

concerning its application to thermochemistry [l6,17] and thermodynamics [18] can be consulted for details of m.acrocalorimet er design and use.

We shall consider only two types of calorimeters: first,

the "isoperibol" [19] calorimeter, in which the temperature of

the calorimeter environment is independent of time, and second,

the adiabatic shield calorimeter, where the calorimeter is surrounded by a shield whose temperature is adjusted so it matches

that of the calorimeter at all times
Churney, Armstrong, and West 25 In addition, we shall restrict our comments with respect to the operation of the calorimeter to the "Intermittent [11,20] or Interrupted [21] heating method" of operation of the calorimeter.

In this method, the temperature of the calorimeter Is first measured when It Is In a quasl-equlllbrlum state, called a rating

period, when there Is no power Input. It Is then heated at constant power for a known time Interval or allowed to be heated as

a result of a chemical process. After a suitable time Interval

the temperature Is again measured In another quasl-equlllbrlum

state or rating period. The extension of the arguments to the

method of "continuous heating" [20] Is straight-forward.

For the purpose of Illustration and simplification the various concepts will be discussed by associating them with some

slm.ple models of calorimeters. We shall classify these as one- body or two-body models of a calorimeter depending upon whether

the calorimeter Is considered (approximately) as consisting of one or two regions of uniform temperature.

2. ONE-BODY MODEL OF AN ISOPERIBOL CALORIMETER

A. Assumptions and Summary of Design Criteria

Most of the design criteria for calorimetry can be thought

of as originating from this simplest model of a calorimeter.

This model is sketched in Figure 1 and can be described in terms

of the following three assumptions.

(1) The calorimeter, a single body having a heat capacity

C-, , has a uniform but time varying temperature, T-j^, and is sur- rounded by a jacket or environment held at a uniform t em.perature,

T^ . wiiich is independent of time,

u

- (2) The rate of absorption of heat from the environment is given by ^^^(1^-1.^) where h-^^ is the coefficient for heat transfer

between the calorimeter and environment.

(3) C-j^ and h-^^ are assumed to be independent of temperature

and time.
26. Macrocalorimetry - How Accurate?

Figure 1. Sketch of one-body model of an isoperibol

calorimeter

Accordingly 3 while this model of a calorimeter is not attainable in a real calorimeter, a conscious effort is made in the design of practical calorimeters to cause them to conform to the

description given above. To meet assumption (1), the jacket is

usually designed so its temperature is constant and uniform [22].

For the calorimeter itself, if it has large dimensions, we use a closed can containing a liquid and stir so as to promote uniformity of temperature of the calorimeter [22,23]. For calorimeters

of smaller dimensions, construction materials of high thermal

conductivity and diffusivity are used [23]. Within the calorimeter, heat transfer coefficients across solid boundaries are increased by small separations, large contact area, and use of

liquids, greases, or of gases such as helium between solid boundaries [23]. Use of vanes plus helium gas in low temperature calorimetry to increase thermal contact between the sample and con-
Churney, Armstrong, and West 27

tainer is a typical example [21,24], In addition to improving

heat transfer inside the calorimeter, we reduce internal temperature gradients by reducing the heat exchange with the environment

(i.e. J minimizing h-j^^ and keeping T^-T^ as small as is practicable).

Gaseous free convection is eliminated by evacuation of the space

between the calorimeter and jacket whenever possible or by minimizing the space and temperature difference between the calorimeter

and jacket [23]. This also eliminates gas conduction. Heat transfer by radiation is reduced by using materials of low emissivity

for the outer surface of the calorimeter and inner surface of the

jacket. Conduction by solids is reduced by using a minimum of

connections between the jacket and calorimeter and these are made

of small cross-sectional area and, where possible, of low thermal

conductivity. When evacuation is not practical, gaseous conduction is reduced by using the maximum separation concordant with

eliminating free convection [25].

Attempts are made to conform to assumptions (2) and (3) by

eliminating heat transfer by convection, minimizing omittances,

and keeping T-i-T^^ and the range of T in any one experiment small.

B. Results Predicted by the One-body Model

Applying the principal of conservation of energy one obtains

equation (1), which is a statement that the rate of energy in- crease of the calorimeter equals the rate at which heat is transferred from the jacket to the calorimeter and work is done on the

calorimeter. Expansion work is neglected.*

C. dT-, (t

)

In eq. (1), dT^/dt is the rate of temperature rise of the calorimeter, P° is the time invariant, and P-,(t) the time- varying rate

The overall expansion work done on the calorimeter will tend

to cancel in the substitution method outlined in section 2,C. In any event, this term is small.
28 Macrocalorimetry - How Accurate?

at which work is done on the calorimeter.

When P^(t) = 0, a rating period, we find:

where

dT- = b(T3_^-T^) (2a) dt

h P°

b = -1^ . T = T + — (2b) C' loo Q h

b is called the cooling constant and T-j^^ the convergence

temperature.

When P-|^(t) 7^ 0 one obtains:

T^(t) = T^^ + (T^(0)-T^^)e-^^+e-^^/ e''^'^ dx (3a)

0 ^

where T^(0) is the value of at t = 0. For P^(t) = 0[U(t)-

U(t-t^)], where U(t) is the unit step function, 3 is the magnitude

of the power, and t^ is the heating interval one obtains:

0 < t < t^

t > t, (3b)

e-^^ / e^b-p^(T)dT = e-^^ | [e^^-l]

0 -bt 3 [e^^h-l]

b

A rough sketch of a temperature-time curve of a typical calorimetric experiment in which the idealized calorimeter conforms to

eqs. (3a) and (3b) is shown in Figure 2. Equation (3b) indicates

that at time t = 0 (initiation of heating) the calorimeter temperature starts to rise with a slope of 3 added to the previous
Churney, Armstrong, and West 29

ZDh-<

cr

LJX

LlJ

1oo

Figure 2. Temperature-time curve for a cne-tcdy model

of an Isoperlbol calorimeter for an electrical

calibration experim.enr.

rate of rise, b(T^^-T-j^). Because b is sm.all this continues ivith only a slight diminution until t, , at which tim.e the rate of h

temperature rise is restored abruptly to that appropriate tc eq

(2a). Before t = 0 and any time after t,^ , eq. (2) is obeyed and

the calorimeter is said to be in a rating period. For later

purposes, we take the final rating period after tim.e t^ and call

the time between 0 and t^ the main period.

To express our results in the conventional form: for calculation, we integrate eq . (1) between 0 and t^ to give

1- C. T, /t l(^f) - ^1^0) - b / (Tioo-Ti)dt = f

F^(t)dt = W (^a)
30 Macrocalorimetry - How Accurate?

Equation (^b) is the conventional "working" equation of isoperibol calorimetry —the product of the heat capacity*, C-^ , times a corrected temperature rise, A6-j^, equals VJ, the work done on the

calorimeter by the power source, P. The quantities T-|^(t^),

T-^(O), b, and T^^ of eq. (^a) are determined from rating period

data, and the integral is evaluated from the main period data.**

For the idealized one-body calorimeter A6^ will be independent

of the value of t^ so long as t„ >: t, .

It is to be noted that the assumption that and h^^ are independent of temperature makes eq. (1) a linear differential

equation and allows the principle of superposition to be applied.

This principle is shown explicitly in eq . (3a) where the temperature is a superposition of that due to heat transfer from the

environment and that due to the power source P-|^(t).

C . Limitations of the One-body Model

Real isoperibol calorimeters do not conform exactly to the

assumptions and predicted performance of the one body model.

During the heating interval the temperature gradient inside the

calorimeter [23,2^] and on the calorimeter surface [23] can be

quite large compared to those when P-j^(t) = 0. Also, the rate

of rise of the temperature, dT-^^/dt, does not abruptly increase

by the amount 3 as predicted by eq. (3b) at time t = 0 or drop

to b(T^^-T^) just after t^. Only for the time t^ >> t^ [25,27]

is eq. (2) obeyed. Provided, however, we define the final

rating period to be a period of time when eq , (2) is again

obeyed, we do find that eq . (h) is obeyed if A9-^ is evaluated

from the actual temperature-time curve of the main period. • That is, AS-j^ is independent of t^ and is independent of the

heating rate 3. The experimental finding that C-j^ is independent of 3 suggests that any error in eq. (4) due to the temperature gradient during the main period is proportional to W.

Heat capacity is later altered to energy equivalent.

For details see [25-28].
Churney, Armstrong, and West 31 A second objection to the one-body model arises because we

do not know how to exactly locate the boundary surface separating the environment and the calorimeter. If a calorimeter met

assumption (1) of the one-body model in section 2, A, the boundary could be located by determining the location of an infinite

temperature gradient! For real calorimeters no such gradient

exists. Nothing in our experimental results or one-body theory

suggests an alternative method for locating the boundary experimentally. For further discussion of this point see [11]. Since

the boundary of the calorimeter is indefinite, it follows that

it is impossible to compute the heat capacity of the calorimeter

even assuming C-j^ is the sum of heat capacities. An experimental

calibration must be made as outlined below.

In light of the above considerations, we can then modify

the application of the ideal one-body theory to handle real

calorimeters as follows:

(1) Take t^ sufficiently larger than t^ so that eq . (2) is

obeyed. That is, eq . (2) will be used to define a rating period.

Evaluate A0 from the actual temperature-time curve of the main

period after obtaining b and t-j^^ from the rating period data.

(2) Calibrate the calorimeter experimentally in such a way

that the boundary need not be known. For example, in a heat

capacity experiment, determine the heat capacity with and without

the sample. If is the heat capacity of the sample, is that

of the calorimeter, and heat capacities can be added, we have for

the two experiments:

c^Ae- =

(3) Attempt to duplicate the temperature-time curves for

the various calorimetric determinations (for instance, for the

full calorimeter, eq . (5a), and empty calorimeter, eq. (5b))

so that errors in evaluation of the heat exchange by means of

(5a)

(5b)
32 Macrocalorlmetry - How Accurate?

eq . (4) due to the temperature gradients during the main

period will tend to cancel.

The last two procedures constitute the elements of the

substitution method in calorimetry [29530]. As will be seen

in subsequent sections, the last condition can be replaced.

Since heat capacities are assumed not to change during an experiment, and large non-uniformities of temperature are not

provided for, the one-body model cannot handle in any simple

way the cases of a chemical reaction, drop calorimetry with an isoperibol "receiving" calorimeter, or the measurement of a heat of fusion. In these cases the heat capacity -of the calorimeter plus contents does change in an abrupt way or a large

deviation from uniformity of temperature occurs during an ex- periment so that a more sophisticated model of a calorimeter

is required.

3. THE TWO-EODY MODEL OF AN ISOPERIBOL CALORIMETER

A. Internal Temperature Differences

We now assume the calorimeter consists of two bodies having

constant heat capacities C.^^ and and uniform but time varying

temperatures T.^ and T^ as shown in Figure 3- We assume that

only body one exchanges heat with the environment and the rate

of this exchange is given by ^-j^q ( T^-T.^ ) as before. Applying the

principle of conservation of energy to the calorimeter as a whole, we have:

dT dT

^1 d^ ^ ^2 dt- = ^lo(V^l) ^ n ^ Pl^t) ^ ^2^^) (6)

In eq. (6) PJ, P° are the time invariant and P^(t), ?^{t) are

the time varying rate of doing work on bodies one and two,

respectively.
Churney, Armstrong, and V/est 33

///

/ / / / /

10

T

0

Figure 3. Sketch of a tvjo-body model of an Isoperlbol

calorimeter Involving Internal temperature

differences

Investigating first the temperatures when P-j^(t) = P^Ct) = 0 we assume that heat exchange between body one and body two Is proportional to T^-T.^ . Applying conservation of energy to body

two, we have:

dT,

'2 dT ^21(^1-^2)

po

2 (7a)

In eq. (7a) h^^ (equal to h^^^ the coefficient for heat transfer bodies one and two. It Is taken to be large com.pared to h^^

and Is also taken to be Independent of temperature and tlm.e. As has been shown previously [31], the solution of eqs . (6) and (7a)

when P.(t) = P^(t) = 0 Is as follows:
.34 Macrocalor Imetry - How Accurate?

^1 "

^loo + Ae'^l"^ + Be~^2^ (8a)

^2 = + Aq-^e-^l^ + Bq2e-^2^ (8b)

The quantities T^^, T^^, q^, q2^ b^j and in eqs . (8a) and (8b) are defined as follows:

^100 = + (P° + P°)/h^2 ^^^^

(^12/^2)

(h^./Co)

12/C,)

(8e,f)

•10 + C^q-L

12 10 12 (8g,h)

Since we have selected (i.e.j designed) our calorimeter so h-j^^ is

small and h-^^ large, b-^ is much less than b^* • Thus we obtain

from eqs. (8a) and (8b) after a sufficient length of time so the

second exponential term is negligible:

* h h /h h^A2

For b^ « b^. c^cT" [c^ ^ cT-j

g^^^^

^10^12

C.C. h, ,+h,^ h,„
Churney, Armstrong, and West 35

dT dT^ ^ = bi(Ti^-T^) ; = ^1(^2.-^2) (9a)

Equations (9a) are of the same form as eq . (2a) for the

one-body model. . While eq . (2a) is valid for all times when the

time varying component of the rate of doing work on the calorimeter is zero, eqs . (9a) require in addition that certain transients must die out before a rating period can exist. Thus, the

two-body model provides a justification of the first modification of the one-body theory to allow application to real calorimeters given in section 2,C. It also gives two new results.

First, the cooling constant is the same (i.e.^ b-^) wherever the

temperature is measured in the calorimeter but the convergence

temperatures (i.e.^ T.^^ or T^^) will be different. Second, the

cooling constant is a function of ratios of heat transfer coefficients divided by heat capacities and will vary to a certain

extent if the internal heat transfer coefficients {i.e.j, h^^^

of the calorimeter should change.

In order to investigate the temperature differences present

during rating periods, we also obtain from eqs. (8) that

dT^ dT, ^ dt ^1 dt

po ^ po po (^2-^1) = (Ti-Tq) - (li-l) + 4

" (9=)

We see that provided one temperature is known, the other is automatically fixed during rating periods. Further, the temperature

difference, T^-T.^, varies linearly with the temperature difference between the calorimeter and jacket, T-^-T^, as expected.

Since
36 Macrocalorimetry - How Accurate?

to the first order of small quantities when h^^ << h^^s the model

also confirms the design criterion suggested to reduce internal

temperature differences, i,e,, ^10-^^12 * '^^^ internal temperature differences will not be small, however, unless ^\/^^2

and '^\/^Y2 small. In the intermittent heating method P° and P° are made small (i.e., only the temperature sensor cur- rent and in the case of stirred liquid, the heat of stirring,

contribute to P° , P"). In the continuous heating method, P° and P° are large and it is necessary to make h-j^^ large [12].

In order to obtain working equations analogous to eq . (4), one can first integrate eq . (6) directly, using eq . (9b), to obtain:

t (C^ 4- C2qi)[T^(t^) - T^(0)] f

(T^^-T^)dt + W

0 ^f

where ^ = f [^i^^) + P2(t)]dt. Using eq . (8g), we obtain

0

(^1 + C2^l)k(^f) - ^1^0^ - h f' (^Ico-Ti) c L n

= w (10)

It should be noted that in eq . (6) and in deriving eq . (10) we

did not make any assumption about the manner of heat exchange

between body one and two during the main period. The importance of this comment will be evident in section 3jC.

For comparison, we can derive an equation analogous to

eq . (10) involving only the temperature of the inner body. For

this purpose, we assume that during the main period the heat

exchange between bodies one and two is proportional to T2~'^l and modify eq . (7a) as follows:

dT^

(7b)
Churney , ..rmstr-ong, and West 37 Eliminating T-, from the right hand side of eq. (6) with eq . (7b)}

integrating, and sim.pllfying terms one obtains:

In eq . (11a) we have assumed:

P^(t) = f^g(t); ?^(t) = r^git); f^ +

That is, we assume the rate of doing work on the calorimeter is

a product of a function of time and position. (This is reasonable

for electrical calibration experiments but will not apply in general to chemical reactions.) The left hand side of eq . (11a) is the increase in internal energy of the calorimeter while the quantity in the brackets, [], on the right hand side is the heat absorbed by the calorimeter during the main period. We see that the error incurred in using the temperature to measure the heat ex- change is indeed proportional to W as proposed in section 3}C.

Rearranging eq . (11a), one obtains .

= 1; g(t)dt = W (lib)

(c^ + c q ) r ^f

1

( f, ^ f,(h,^.h^^)/h^^ )r2(^f)-^2^Q)-^ J (T2oo-T2)dtJ (12)

Comparison of eqs . (10) and (12) shows that wherever the temperature is measured, the equation has the form: the product of

what we will now call an energy equivalent and the corrected temperature rise is equal to the work done on the calorimeter. This

is the equivalent of White's [32a] statement "when the calorimeter

is calibrated, it is calibrated, lag effects and all."
38 Macrocalorimetry - How Accurate?

A new result given by the two-body model is that the energy

equivalent is not the sum of heat capacities. For example, in

eq . (10) the energy equivalent is equal to C-j^+C^q-j^.

q-j^ is not

equal to one and is a function of the heat transfer coefficients

h^Q and h^^ well as the heat capacities C.^ and C^- Since in

practice it is impossible to measure these heat transfer coefficients accurately, we conclude that this is a second reason (besides the inherently indefinite location of the calorimeter discussed in section 2jC) for determining the energy equivalent of

the calorimeter experimentally.

A second consequence of the above result Is that heat capacities of materials cannot be determined with an isoperibol calorimeter by only a single combination of an "empty" and "full" calorimeter experiment. For example, suppose we could assume the empty

calorimeter is an ideal one-body calorimeter with heat capacity

and the full calorimeter is an ideal two-body calorimeter containing a sample with heat capacity C^- In place of eqs . (5a) and (5b), we have

(C-^ + C^Q-l) Ae^ = W (13a)

C^Ae^ = (13b)

Additional experiments are then required to determine q.^

As Soes to zero, eq . (9d) says q.^^ approaches one and

hence, the energy equivalent in eq . (13a) approaches C^+C^. This

constitutes the basis for one method of determining q^-, extrapolating energy equivalents to zero values of h-^^ (see [2^,33]).

In practice, the energy equivalents of both the empty and full

calorimeter are extrapolated to zero values of h-j^^ [33] since

neither the full nor the empty calorimeter can be approximated as an ideal one or two-body calorimeter. To illustrate the magnitude of this extrapolation, in one case at room temperature (where

h-, ^ is large) [33] the corrections to the energy equivalents of
Churney , Ann., orong, and. West 39

the empty and full calorimeter were 1 percent and 0.3 percent

respectively.

An alternative use of the fact that q-j^ approaches one as h-j^^ approaches zero is to interpose a closed shield between the calorimeter and the environment so that h-^^ is forced to be extremely

small. If the temperature of the shield is kept equal to that of

the calorimeter, one has an adiabatic shield calorimeter.

Another approach to the problem of non-additivity of heat

capacities is to determine experimentally whether or not, for

example, the energy equivalent Increases linearly with the amount

of sample added to the calorimeter [3^].

Equations (10) and (12) show that the energy equivalent

depends in general on the location of the thermometer and the

geometrical distribution of the power supplied to the calorimeter {i.e. J, f^ and f^ of eq . (12)). The implication of the

latter is particularly important because, if not recognized, it can lead to a systematic error. For example, suppose one wished

to calibrate a calorimeter electrically for the purposes of

measuring the change in energy accompanying a chemical reaction.

Assume that the heat liberated by the reaction can be treated as equivalent to that generated by doing electrical work on the

calorimeter and that the change in the heat capacities of the

various parts of the calorimeter can be neglected. Then eq . (7b)

applies. It would then be necessary to show that f-j^ and f^ are the same in both cases or that variations in f^ and f^ have a negligible effect on the energy equivalent. This can be studied

experimentally by determining the variation of the energy equivalent for various positions of the electrical heater (see

Prosen [35] or Dickinson [36]).

Both eqso (10) and (12) state that the energy equivalent of

the calorimeter is independent of the heating rate, i.e., it det . pends on f

g(t)dt instead of g(t) in eqs . (11a) and (12).

Thus, we can now modify our original formulation of the substi-
40 Macrocalorlmetry - How Accurate?

tution method to the following: calibrate experimentally and

also determine experimentally whether the energy equivalent depends linearly upon the amount of sample added and is independent

of the geometrical distribution of the power source. These

tests must be affirmative if system.atic errors are to be avoided.

B. Surface Temperature Differences

An alternative point of view arises from a literal interpretation of eq , (10). This suggests that if the temperature^ T-j^, of the surface of the calorimeter is uniform and we measure it,

the energy equivalent will be independent of the geometrical distribution of the power supplied to the calorimeter. This may be

thought of as the origin of the idea of measurement of the surface temperature, so to speak, of the calorimeter.

To consider this in more detail, let us acknowledge that

there are temperature differences on the surface of the calorimeter. We can consider, then, the two-body model portrayed in

Figure 4 and assume a term h2Q(TQ-T2'' been added to eqs

(6), (7a), and (7b). h^Q is the coefficient for heat transfer

between body two and the environment. In this case, the form

of eqs. (8a), (8b), (9a), and (9b) are the same (assuming again

^1 ^2^ ^^'^ °"1 given by

1 {h,2+h2o)/C2-b^

Using the method of derivation of eq. (12), one can show that,

for example, the result analagous to eq . (10), where the

temperature of body one, T-j^, is measured now becomes

L

1 2j

(14)
Churney, Armstrong, and West

T

0

Figure 4. Sketch of a two-body model of an Isoperlbol

calorimeter Involving both Internal and

surface temperature differences.

A similar result will be obtained If the temperature of body

two Is measured. That Is, the energy equivalent In either case

Is a function of f.^^ and f^

One can show this result Is a consequence of the problem of

evaluating the heat absorbed by the calorimeter during the main

period by the measurement of the temperature of the calorimeter

in a single location. From a design point of view, the problem

can be reduced if the temperature gradient is minimized. Better,

it can be eliminated even without minimizing the surface gradients if the temperature gradient on the calorimeter surface is
42 Macro calorlmetry - How Accurate?

Independent of where heat is generated inside the calorimeter

[23] c Then the energy equivalent will be Independent of differences between f-j^ and f^ for a temperature measurement made anyV7here on the surface of the calorimeter.

The latter design criterion is given the title of "equivalent

sources" [37]. In light of this design criterion, the experimental test for determining the dependence of the energy equivalent on the variation of f-^ and f^, mentioned in section 3, A,

is called the "test for equivalent sources." The test amounts

to studying the dependence of the energy equivalent on variations

in the temperature gradients on the surface of the calorimeter

(produced by variations in the positions of the sources of energy

in the calorimeter) . The design criterion for "equivalent

sources" constitutes the final revision of the substitution

method in calorlmetry.

In practice, this design criterion is nearly satisfied in

stirred-water calorimeters when the stirring action of the propeller mixes hot water from various sources before the water

reaches the calorimeter surface. (The temperature gradient on the surface of the calorimeter^ in any event, is usually small

[38].) It is also the origin of White's [32b] suggestion that

the dependence of the energy equivalent on the temperature

gradient on the calorimeter surface can be studied by altering

the rate of stirring to vary this gradient. Two designs, one

for a stirred-water calorimeter [38] and the other for aneroid

calorimeters [39}23] fulfill this design criterion in concept

as well as in practice.

It will be noted that the design criterion and test for

equivalent sources is associated with the desire to make temperature measurem.ents at a single location even though the temperature on the calorimeter surface is non-uniform. An alternative

approach sometimes adopted [24] is to abandon this desire and

attempt to measure some average temperature of the calorimeter

surface. In the case of the two-body model, it can be shown
Churney, Armstrong, and West ^3

that, when body two exchanges heat directly with the environment,

the energy equivalent is indeed independent of and provided

one is able to measure the average temperatures, S, given in

eq. (15)

h T + h T ^^01 1 02 2 (15)

^01 ^02

As noted by White [40], the experimental difficulty with such a measurement is proving that the placement of thermocouples , for

example, is such as to weight the temperatures at the various

junctions in proportion to the local heat transfer coefficients

to the environment o Certainly one proof might be obtained by

the test for equivalent sources.

C. Change in State

The measurement of the change in energy associated with a chemical or physical {i , e . y heat of fusion) change in state can

be handled in terms of the two-body model of Figure 3 discussed

in section SjA. Let us assume that the change in state takes

place at the location of body two and replace eq. (6) by eq . (l6a)

dT dE

^1 dt^ ^ dt^ = ^10(^1^-^1) ^ ^1^^^ ^ ^2^^) ' (16a)

In eq. (l6a), dE^/dt replaces C^dT^/dt and is the time rate of

change of the energy of body two. T^^ is given by eq. (8c).

Integrating between the rating period ending at time t = 0 and

the one beginning at time t = t^, we have

C^ [T^(t^)- T^(0)] +[Ep(t^) - E^(0)]= h^Q / (T-L^-T-L)dt + W

0 (16b)
44 Macro calorimetry - How Accurate?

In eq. (l6b), E^Ct^) and E^ ( 0 ) are the energies of the products

at time t = t^ and the reactants at time t = 0, respectively.

W Is the work done on the calorimeter to cause or Initiate the

change In state. To obtain results In a more conventional form,

let us first add and subtract from the left hand side of eq . (I6b),

the quantity E^ ( 0 ) . E^CO) Is the energy of the products at time

t = 0 assuming the change In state had occurred before the Initial

rating period. Rearranging, one obtains

^f

Ci[Ti(t^) - T^(0)] + [Ep(t^) - Ep(0)] - h^Q / (Ti^-T^)dt - W =

0

- [£^(0) - E (0)1 = -AE ^ . ^ ^ (16c)

^ p change m state

We make the further assumption that In the Initial rating period

the actual temperature distribution In the calorimeter at time

t = 0 Is close to that obtained If the products of the change In state were present. This will be nearly so. If at time t = 0, the

calorimeter Is near Its convergence temperature. Then we may

write

Ep(tf) - Ep(0) = C^l^^it^) - T2(0)] (I6d)

In eq . (l6d) Is the heat capacity of the products. Inserting

eq , (l6d) Into (l6c) we obtain the familiar result

(^1 ^ ^iC2)[ti(V) - ^1^0) - ^ f (Tioo-Ti)dt] -W =

0

change In state (17)
Churney, Armstrong, and West ^5

Thus, in this case, if one calibrates the calorimeter electrically when the products are present in the calor j.meter , one obtains the change in energy associated with the change in state

as if it took place isothermally at the temperature of body two

at time t = Oo

If we had added and subtracted E^(t^) from the left hand

side of eq. (l6c), we would assume that in the final rating

period that the actual temperature distribution inside the calorimeter is close to that obtained if the reactants were present in

the calorimeter. This assumption would be nearly fulfilled if the

calorimeter is near its convergence temperature during the final

rating period. Then the energy equivalent is determined electrically with the reactants present in the calorimeter and we obtain

the change in energy associated as if it occurred isothermally at the temperature of body two at time t = t^.

Since we measure the temperature of body one, the error in

assigning the value T-j^(O) to the temperature of the isothermal

change in state in eq. (17) will also be smaller if the calorimeter is near its convergence temperature at time t = 0 (see

eq. (9c)). . Calculation of the change in energy at some other temperature

requires a knowledge of the heat capacity of the products and reactants which must be determined in other experiments. If the change

in energy for fusion, for example, is to be calculated, then the

melting point must also be determined. Since in general the sample

will contain impurities, this will involve a melting point - purity

determination (see refs. [21, 4l]) and premelting corrections to the

heat capacities (see ref. [21]).

It should be noted that, provided one can assume that body two

does not exchange heat directly with the environment, eqs. (16) and

(17) do not depend upon whether or not heat exchange during the

main period {i.e., 0 to t^) between bodies one and two are linearly

proportional to T -T or not. This is particularly important for a
46 Macrocalorlmetry - How Accurate?

chemical reaction where this is most certainly not the case.

An analogous treatment can be given for drop calorimetry with

an "isoperibol" receiving calorimeter (see ref. [31]).

4. ADIABATIC SHIELD CALORIMETERS

The need for adiabatic shield calorimeters can be thought of

as arising for three reasons:

(1) To obtain a calorimeter for heat capacity measurements

whose energy equivalent is more nearly the sum of heat capacities,

(2) To obtain a calorimeter in which the tem.perature distribution is more nearly uniform during rating periods

(3) To obtain a calorimeter whose heat exchange with the

environment is small at all times so that one can study changes

in state that require long times to reach thermodynamic equilibrium, without introducing excessively large corrections for heat

exchange

,

Two-body Model of an Adiabatic Shield Calorimeter

To see how the needs itemized above are more closely met by

adiabatic-shield calorimeters, consider a model of an adiabaticshield calorimeter as shown in Figure 5. We assume the calorimeter, itself, can be idealized as shown in Figure 3. Surrounding

the calorimeter is an adiabatic shield whose temperature, T^, is

assumed to be uniform, but is varied with time so it is nearly

always equal to the temperature, T^, of the outer surface of the

calorimeter. The coefficient for heat transfer between the shield

and calorimeter is h-j^^. Since this shield cannot be exactly perfect we will allow for this by assuming there is some heat exchange

(i.e., the value of h^Q i- 0) between the calorimeter and the en- vironment which is held at some constant uniform temperature, T . Applying conservation of energy to each of the bodies, one and two, of the calorimeter we have:
To solve eqs. (l8) it is necessary to specify how T^-T^

varies with time. The model shown in Figure 5 omits any consideration of the details of the adiabatic shield and the associated

temperature control problem (for details see refs « [11, 20, 21]).
48 Macrocalorimetry - How Accurate?

For our purposes it will be sufficient to appeal to experiment to specify (T^-T^).

During rating periods, when g(t) = 0 , we assume that (T^-T-^)

reaches a constant very small value which we will call a. We will assume a is the same for all rating periods. Under these conditions, the form of eqs . (8) is virtually unchanged except that a term ^h-^^/h-^^ must be added to T-j^^ in eq . (8c) and T^^ in eq . (8d)

If, again, we assume b^ << b^ we obtain:

dT P +P +ah.

dt^ = b^(Ti.-Ti) = ^'c^lc^^^ + ^^1(^0-^1) - (19a)

dT^ P^^+P.^+ah. / P^^

In eqs. (19a) and (19b) we have factored out the terms in T^^ and

T^^ that are inversely proportional to h-^^ using eqo (8g). Since

a can in principle be positive or negative, depending upon how

the controller for the shield operates, the physical meaning of a convergence temperature must be relinquished

„

During the main period T^-T^ = 6(t) may vary with time. We assume that whatever this "offset", 6(t), we measure it experimentally during the main period. Then the equations analogous to those obtained in section SjB are:

(20a)

= W (20b)
Churney, Armstrong, and West ^9

where we have written:

AT^ = T^(t^) - T^(0) (21a)

AT^ = T^{t^) - T^iO) (21b)

' = ^T^^ ^'''^

It will be noted that eqs . (20a) and (20b) state that the

corrected temperature rise in the case of an adiabatic shield

calorimeter (the quantity multiplying C,+C^q-, in eq. (20a) , for

t^

example) consists of two parts. One part, AT-^-b^

J*

(T-^^^-T-^ ) dt

,

is of the same form as that for an isoperibol calorimeter. The

second part, -y J'^6it)dt, is peculiar to an adiabatic shield

calorimeter. ItOcorrects for the heat transfer to the shield

during the main period and is usually quite small or negligible

in comparison to the first part. The parameter y can be calculated from the change in drift rate of the calorimeter during

rating periods caused by a deliberate known offset of the shield

temperature as can be seen from eqs. (19) [21].

In order to convert the corrected temperature rise to the

conventional form (see ref. [21]), we note that it can be shown

^f that the term b-j^ J

( T-j^^-T-j^ )d t is given by the expression [27]:

0

t

/•f dT.(O) dT. (t .) 1 J (^l«.-^l)^^ = (VO) ^ -4-^ (^f-^x) (21d)

^.f where (t^-t^)= J

[T^ ( t

) -T^ ( 0 )] dt/ [T^ (t ^) -T^ ( 0 )] . (21e)
50 Macrocalorlmetry - How Accurate?

In eq. (21d) dT-^(0)/dt and dT^(t^)/dt are the time rate of change

of the temperature, T.^^ , at times t = 0 and t = t^. respectively.

For electrical calibrations, it is found experimentally that t^

is approximately the midtime, t^, of the heating interval {i.e.,

t^/2 of Figure 1). Thus, one can rearrange eq . (20a) using eq. (21d) to

dT^(O) dT^(t^)

dt t - m dt

0 (22)

In real adiabatic calorimeters b is two or three orders of

magnitude smaller than it would be if no shield were present [42],

According to our model, b.^^ approaches zero and q^ one as h^^ goes

to zero—that is when the adiabatic shield is perfect o When

'10

dT. dF

dT,

dt"

P + P ^20 ^10 ^Is^ (23a)

The temperature difference (T^-T.^^) during rating periods analogous

to eq. (9g) becomes

^T T ) - (^1^1 - ^2^2 ) ^2 ^1^ h^2 + (23b)

When h^Q = 0 eqs . (20a, 20b) or (22) become

(Ci + C^) [aT^ - ^

t^-Y /

6(t)dt]

("l ^ ^2) [^^2 - dt^ ^f-Y J 6(t)dtJ

(23c)

(23d)
Churney, Armstrong, and West 51

dT^/dt or dT^/dt in eqs. (23c, 23d) are given in eq. (23a).

Thus in an adiabatic calorimeter with a nearly perfect shield

both the temperature and the rate of change of temperature during the rating periods become more nearly equal. Further the

energy equivalent of the calorimeter becomes more nearly the sum

of heat capacities.

It is important to note that eqs. (20a and 20b) do not mean

the energy equivalent using a temperature measured anywhere in a real adiabatic calorimeter is independent of the power distribution of the sources even though h-^Q/h^2 t)e negligible. The reason for this is that the model considers only the internal

temperature difference in the calorimeter and not the surface

temperature difference produced by different distributions of

the power sources considered in section 3,C. Further, the question of the temperature gradient in the shield and the effect of

where temperatures on the shield and calorimeter are controlled

to be equal have been totally neglected. Thus, for real adiabatic shield calorimeters, the design relation for equivalent

sources must still be retained.

It should also be noted that while the correction for heat

exchange with its surroundings is greatly reduced in an adiabatic

shield calorimeter as compared to an isoperibol calorimeter, it

does not necessarily follow that the uncertainty in the measurements due to the uncertainty in this heat exchange is also smaller.

5. SUMMARY: COMMENTS ON PRECISION AND ACCURACY

It can be seen that although the simple models of calorimeters discussed in the previous sections are non-quantitative

(in the sense that they do not predict the magnitude of calorimetric errors); they do have utility beyond providing a basis

for discussing calorimeter design. They afford valuable insight

into possible sources of calorimetric error and suggest experiments to test for these errors that are predicted by more sophisticated theories [37,^3].
52 Macrocalorimetry - How Accurate?

The heat exchange of the calorimeter with its environment

and the interrelation of this heat exchange with the temperature gradient in the calorimeter (and on its surface) have been

examined in the preceding material in terms of their effects on the energy equivalent of the calorimetero Particular attention

has been focused on possible errors in the determination of the

energy equivalents when the substitution method is used for

determining heat capacities or the change in energy associated

with a physical or chemical change in state

In particular, we have considered an isoperibol calorimeter,

where the heat exchange between the calorimeter and environment is

large, and the adiabatic shield calorimeter, which. is designed to reduce this heat exchange. The latter type of calorimeter evidently

has advantages for making measurements of heat capacities because

its energy equivalent is more nearly the sum of heat capacities.

Also, it has advantages in making m.easurement s of the change in

energy associated with a change in state that takes place slowly

(i.Soj a slow chemical reaction) or when attainment of thermodynamic equilibrium is not rapid,

V/henever the change in state is completed rapidly (i.Soj most

of reaction calorlmetry and an appreciable part of heat capacity

calor imet ry )

, measurements made using an isoperibol calorimeter

can yield results that are at least competitive with the adiabatic

calorimeterc This can be seen, for example, by intercompar ing the

low temperature {ioB,, 15 to ^00 K) measurements of the heat capacity of benzoic acid made by Busey [44] and Cole, et al . [33] using

isoperibol calorimeters with those of Furukawa, et al . [45],

Osborne, et al . [46], and Busey [44] using adiabatic calorimeters.

(In Busey 's work, the same calorimeter was operated adiabat ically .

)

In all cases, measurements were made on a standard sample, as received, which was supplied by the Calorimetry Conference (see

refo [47]). Also, each worker used the same temperature scalOo

Except below 20 K, agreement of the various measurements was within

the overall estimated uncertainty of the measurements of Furukawa,

et al . : ±0.2 percent from 395 K to 60 K increasing to +1 percent

at 14 K.
Churney, Armstrong, and West 53 The overall uncertainties in heats of transition or reaction

arise from those associated with the specification in the change in

state and those associated with calorimetric measurements themselves.

Using modern techniques of calorimetry, it is possible to make the

overall uncertainty associated with the calorimetric measurements

themselves less than that associated with the specification of the

change in state. To determine whether this has been achieved, calorimetrists can obtain some information about the imprecision and estimated uncertainty associated with their calorimetry by intercomparing

the results of their measurements on standard samples o Some idea of

the current minimum values of these uncertainties can be obtained by

looking at a few of the results of careful investigations.

For heat capacity calorimeters and drop calorimeter:;, which are used to measure differences in heat content above room temperature,

these int ercomparisons are made on n-heptane (25 to 523 K) , alumi- num oxide (15 to 1200 K), benzoic acid (15 to 400 K), and ./ater

(0 to 100 °C) [47]. Results of measurements on a standard saiapxe

of aluminum, oxide [39,48] illustrate the magnitude of precision

and the overall uncertainty due to calorimetric error and precision

obtainable in careful work:

Adlabatic Calorimeter (13 to 380 K) [39]

Imprecision in Heat Capacity: Uncertainty in Heat Capacity

1 to 0.55^ at 13 K ±10^ at 10 K to ±0.1^

to 0.03f^ at 90K . at 90 K

0.03/S from 90 K to 380 K ±0.1^ from 90 K to 380 K

Drop Calorimeter (50 °C to Imprecision in H(t °C)-H(0 °C):

0,1% at 50 °C to 0.01^ from 300 °C to 900 °C

900 °C) [39]

Uncertainty in H(t °C)-H(0 °C)

±0.2f. Uncertainty in Heat Capacity: . ±0.2^ at 100 °C to ±0.4^

at 800 °C
5^ Macrocalorimetry - How Accurate?

Imprecision in Unsmoothed Mean

Heat Capacity

0.1% ^ . .

Adiabatic Calorimeter (312 K to 690 K) [39]

Imprecision in Heat Capacity: Uncertainty in Heat Capacity:

0.05^ at 312 K to ±0.1% at 312 K to

0.14^ at 690 K ±0o2% at 69O K

As an example of the type of precision and uncertainty obtainable in careful measurements of the change in enthalpy associated

with a physical change in state, we give the results of Furukawa,

et at. [49] in Table 1 on the heat of fusion of benzene. In these

measurements 5 benzene was deliberately contaminated with small

amounts of n-heptane (which is known not to form solid solutions

with benzene ) . _

In reaction calorimetry, measurements have been made on the

heat of combustion of benzoic acid (also used for calibration purposes) or iso-octane (bomb calorimetry) and currently are being

made on the heat of reaction of THAM( tris-hydroxymethyl amino

methane) with HCl(aq) (solution calorimetry) [50]o A good illusTable 1. Heat of fusion, triple point, and impurities

in benzene [ 49]

Sample

designation

Known

cont aminat ion

Measured^

contamination

Triple

point^-

Heat of

fusion^'

(mol %) (mol %) (°C) (J mol-1)

A ..000 . 0063± . 0010 5 5209±.005 9844. 5±2.

B .036 .042 ±.005 5 5199± .01 9844 .2±3 .0 C .039 .053 ±.005 5 5164± .01 9842. 5±6 .3 'Stated error is overall uncertainty.

Stated error is standard deviation of the mean of each set of determinations.
Churney, Armstrong, and V/est 55

tration of the magnitude of the overall uncertainty due to calorimetric errors (ioSo, excluding sample impurities, etc.) is given

by the intercomparison of measurements of the heats of combustion

of benzoic acid (under standard bomb conditions [27]). Measurements were made on standard sample 39 (attached subscript indicates

batch number) supplied by NBS or specially purified samples (B,C,

VUS) as shown in Table 2, Precisions of a single measurement were

of the order of 0.01 percent. The uncertainties are the estimated

overall uncertainty due to calorimetric errors given by each

investigator.

Table 2. Heat of combustion of benzoic acid . Author Sample ^^^^r Calorimeter

Jessup, Green (193^)

[51-53]

39d, 39e 26,432.,0 + 2. 6 1(D)

Prosen, Rossini (1939-

1941) [54]

39e 26,434..7 + 2. 2 1(D)

Jessup (1942)

['1,531

39e, 39f, B 26,433..8 + 2.,6 1(D)

Coops, et al. (1946-

1948) [55]

39b, 39f, VUS 26,438,.0 + 4 1(C)

Coops, et al. (1954)

[551

39b, VUS 26,435..0 + 4 1(C)

Challoner, Gundry

,

Meetham (1954) [56]

C 26,436,.0 + 4 1(A)

Churney , Armstrong

(1966) [57]

391 26,434,.4 + 3..3 1(D)

Peters, Tappe (I966)

(571

39h 26,433..3 + 3..0 DE Gundry, Harrop, Head,

Lewis (1969) [59]

39i 26,434 .4 + 1,.8 1(C)

Mosselman, Dekker

(1966) [38]

39i 26,432,.7 + 1,.8 1(C)

'Under standard bomb conditions [27].

Calorimeter designations:

1 = Isoperibol calorimeter

1(A) - Aneroid

1(C) - Coops design

1(D) - Dickinson design

DE = Diphenyl ether (phase change) calorimeter
56 Macrocalorlmetry - How Accurate?

6 . ACKNOWLEDGMENTS

The authors wish to express their thanks to E, J. Prosen

for his constructive criticism and discussion of the contents

of this paper.

7. REFERENCES

[1] Eyraud, C, Groton, G., Trambouze, Y., Tran Huu The, Pettre,

M., Compt. Rend. 2jW, 862 (1955).

[2] Watson, E. S., O'Neill, J. J., Brenner, N., Anal. Chem. 36,

1233 (1964).

[3] O'Neill, M. J., Anal. Chem. 3^, 1238 (1964).'

[4] Smothers, W. J., Chiang, Yao, Handbook of Differential

Thermal Analysis, Chemical Publishing Company, Inc.,

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[5] Gray, A. P., A Simple Generalized Theory for the Analysis of

Dynamic Measurements, p. 209 in Analytical Calorimetry,

Porter, R. S., and Johnson, J. F., Editors, Plenum Press,

New York (I968)

[6] Wilhoit, R. C, J. Chem. Education,

[6a] 44, No. 7, A571 (1967)

[6b] ^4, No. 8, A629 (1967)

[6c] No. 9, A685 (1967)

[6d] No. 10, A853 (1967)

[7] Ginnings, D„ C., Introduction, Chapter 1, McCullough, J. P.,

Scott, D. W. , Editors, Experimental Thermodynamics, Vol. I Calorimetry of Non-Reacting Systems, Plenum Press, New

York (1968).

[8] Dauphinee, T. M., MacDonald, K. C, Preston-Thomas, H., Proc

Roy. Soc. (London) A221, 267 (1954).

[9] Martin, D. L., Can. J. Phys. 38, 17 (I960).

[10] Martin, D. L., Can. J. Phys. II66 (I962).

[11] West, E. D., Westrum, E. F., Jr., Adiabatic Calorimetry from

300 to 800 K, Chapter 9, McCullough, J. P., Scott, D. W.

,

Editors, Experimental Thermodynamics, Vol. I, Calorimetry

of Non-Reacting Systems, Plenum Press, New York, (I968).

[12] Skinner, H, A., Theory, Scope, and Accuracy of Calorimetric

Measurements, Chapter 1, Biochemical Microcalorimetry

,

Brown, D. H. , Editor, Academic Press, New York (I969).
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the Past into Future, p. 95, Thermodynamics and Thermochemistry, Symposium, Lund I963, Planary Lectures,

Butterworths , London (1964).

[14] Sturtevant, J., Calorlmetry, Techniques of Organic Chemistry,

Vol. I, 2nd Edition, Weissberger A., Editor, Wlley-Interscience. New York (I949).

[15] Kubaschewski, 0., Evans, E. LI., Alcock, C. B. Experimental

Methods, Chapter 2, Metallurgical Thermochemistry, 4th

Edition, Pergamon Press, Oxford, (I967).

[16] Rossini, F. D., Editor, Experimental Thermochemistry, Measurements of Heats of Reaction, Intersclence, New York, (1956).

[17] Skinner, H. A., Editor, Experimental Thermochemistry, Vol. II,

Intersclence-John Wiley and Sons, New York, (I962),

[18] McCullough, J. P., Scott, D. W., Editors, Experimental

Thermodynamics, Vol. I, Calorimetry of Non-Reacting Systems

Plenum Press, New York, (I968).

[19] Kubaschewski, 0., Hultgren, R., Metallurgical and Alloy

Thermochemistry, Chapter I6, Skinner, H. A., Editor,

Experimental Thermochemistry, Vol. II, Intersclence-John

Wiley and Sons, New York (I962).

[20] Crulckshank, A. J. B., Ackermann, Th . , Giguere, P. A., Heat

Capacity of Liquids and Solutions Near Room Temperature,

Chapter 12, McCullough, J. P., Scott, D. W. , Editors,

Experimental Thermodynamics, Vol. I, Calorimetry of NonReacting Systems, Plenum Press, New York (I968).

[21] Westrum, E. F., Jr., Furukawa, G. T., McCullough, J. P.,

Adiabatic Low-Temperature Calorimetry, Chapter 5, McCullough,

J. P., Scott, D. W., Editors, Experimental Thermodynamics,

Vol. I, Calorimetry of Non-Reacting Systems, Plenum Press,

New York (I968) . [22] Skinner, H. A., Sturtevant, J. M., Sunner, S., The Design and

Operation of Reaction Calorimeters, Chapter 9, Skinner, H. A.,

Editor, Experimental Thermochemistry, Vol. II, IntersclenceJohn Wiley and Sons, New York, (I962)

[23] Ginnings, D. C, West, E. D., Principles of Calorlmetric

Design, Chapter 4, McCullough, J. P., Scott, D. W. , Editors,

Experimental Thermodynamics, Vol. I, Calorimetry of NonReacting Systems, Plenum Press, New York (I968)

[24] Stout, J. W. , Low-Temperature Calorimetry with Isothermal

Shield and Evaluated Heat Leak, Chapter 6, McCullough, J. P.,

Scott, D. W. , Editors, Experimental Thermodynamics, Vol. I,

Calorimetry of Non-Reacting Systems, Plenum Press, New York

(1968).
58 Macrocalorimetry - How Accurate?

[25] Coops, J., van Nes, K., Rec. Trav. Chim. 66_, I31 (19^7).

[26] Wadso, I., Science Tools, 13, 33 (1966).

[27] Coops, J., Jessup, R. S., van Nes, K., Calibration of Calorimeters for Reactions at Constant Volume, Chapter 3, Rossini,

P. D. , Editor, Experimental Theromchemistry , Measurements

of Heats of Reaction, Interscience , New York (I956)

[28] Prosen, E. J., Combustion in a Bomb of Compounds Containing

Carbon, Hydrogen, Oxygen, Nitrogen, Chapter 6, Rossini,

F. D., Editor, Experimental Thermochemistry Measurements

of Heats of Reaction, Interscience, New York (I956).

[29] Rossini, P. D., Introduction; General Principles of Modern

Thermochemistry, Chapter 1, Rossini, P. D., Editor, Experimental Thermochemistry Measurements of Heats of Reaction,

Interscience, New York (1956).

[30] Rossini, P. D., Calibration of Calorimeters for Reactions in

a Plame at Constant Pressure, Chapter 4, Rossini, P. D.,

Editor, Experimental Thermochemistry Measurements of Heats

of Reaction, Interscience, New York (1956).

[31] West, E. D., Churney, K. L., J. Appl. Phys . 39, ^206 (I968).

[32] White, W. P., The Modern Calorimeter, The Chemical Catalog Co.,

New York (1928)

[32a] p. 88 and section 10, Chapter 2 [32b] p. 36

[33] Cole, A. G., Hutchens, J. 0., Robie, R. A., Stout, J. W.,

J. Am. Chem. Soc . 82, 4807 (i960).

[34] Hu, A. T., Sinke, G. C, J. Chem. Thermodynam. 1, 507 (I969).

[35] Prosen, E. J., Rossini, P. D., J. Res. NBS 31, 255 (19^4).

[36] Dickinson, H. C, Bull. BS 11, I89 (1914).

[37] West, E. D., Churney, K. L., J. Appl. Phys. 4l, 2705 (1970).

[38] Mosselman, C, Dekker, H., Rec. Trav. Chim. 88_, 162 (I969).

[39] West, E. D., Ginnings, D. C, J. Res. NBS 60, 309 (1958).

[40] White, W. P., J. Am. Chem. Soc. 4_0, I858 (I918).

[41] Westrum, E. P., Determination of Purity and Phase Behavior by

Adiabatic Calorimetry, p. 231, Analytical Calorimetry,

Porter, R. S., and Johnson, J. P., Editors, Plenum Press,

New York (I968)

[42] Prosen, E. J., private communication.

[43] West, E. D., J. Res. NBS 67A, 331 (I963)
Churney, Armstrong, and West 59

[44] Busey, R. H., J. Am. Chem. Soc . 78, 3263 (1956); ORNL-I828,

Office of Technical Services, Dept. of Commerce,

Washington, D. C. [45] Furukawa, G. T., McCoskey, R. E., King, G. J., J. Res. NBS

47, 256 (I95I).

[46] Osborne, D. W., Westrum, E. F., Jr., Lohr, H. R., J. Am. Chem.

Soc. 71. 2737 (1955).

[47] Ginnings, D. C, Furukawa, G. T., J. Am. Chem. Soc. 75., 522

(1953)

[48] Furukawa, G. T., Douglas, T. B., McCoskey, R. E., Ginnings,

D. C, J. Res. NBS 57, 67 (1956).

[49] Glasgow, A. R., Ross, G. S., Horton, A. T., Enagio, D., Dixon,

H. R., Saylor, C. P., Furukawa, G. T., Reilly, M. L.,

Henning, J. M., Anal. Chim. Acta 17, 5^ (1957).

[50] Armstrong, G. T., Calorimetric Reference Materials - Status of

the Primary Standard, Paper E-2, First International

Conference on Calorimetry and Thermodynamics, Warsaw,

Aug. 31-Sept. 4, 1969. To be published.

[51] Jessup, R. S., J. Res. NBS 29., 247 (19^2).

[52] Jessup, R. S., Green, C. B., J. Res. NBS 13, 469 (193^).

[53] Jessup, R. S., J. Res. NBS 36, 421 (19^6).

[5^] Prosen, E. J., Rossini, F. D., J. Res. NBS 31, ^39 (19^^).

[55] Coops, J., Adrianse, N., van Nes, K., Rec. Trav. Chim. 75., 237 (1956) . [56] Challoner, A. R., Gundry, H. A., Meetham, A. R., Phil. Trans.

Roy. Soc. (London) A247 , 553 (1955).

[57] Churney, K. L., Armstrong, G. T., J. Res. NBS 72A, 453 (1968).

[58] Peters, H., Tappe, E., Monat sberichte der Deutschen Akademie

der Wissenschaften zu Berlin 9(11), 828 (I967).

[59] Gundry, H. A., Harrop, D., Head, A. J., Lewis, G. B., J. Chem.

Thermodynam. 1, 321 (I969).

STATUS OP THERMAL ANALYSIS

TEMPERATURE SCALE STANDARDS

0. Menis and J. T. Sterling

Analytiaal Chemistry Division

Institute fov Materials Research

National Bureau of Standards

Washington^ D. C, 20234

Current status in the development of suitable

temperature scale standards for differential thermal

analysis is described. It involved the cooperative

effort at NBS in the collection and evaluation of

data on 12 materials studied by the Standards

Committee of the International Committee on Thermal

Analysis. In addition the results of the study

group of an ASTM Committee with the cooperation of

NBS are presented. The selected temperature values

depend on the transition temperature of a number of

compounds as well as two low temperature melting

metals. The temperature scale region of 40 to 900

°C can be covered effectively with the ten materials,

which will serve as calibration standards.

Key words: ASTM Committee; differential thermal

analysis; International Committee on Thermal Analysis;

temperature scale region of 4o to 900 °C; temperature

scale standards.

INTRODUCTION

In the field of thermal analysis , we have witnessed a tremendous growth in the last few years [1]. This is evident from the

activities of new societies and organizations as well as large in- creases in the sales and number of manufacturers of differential

thermal analysis (DTA) and differential scanning calorimetry (DSC)

instruments. Last year saw the formation of the North American

Thermal Analysis Society and the Provisional Subcommittee (in E-1)

on Thermal Test Methods of the American Society for Testing and

Materials (ASTM). In I965 the First International Conference on

61
62 DTA Standards

Thermal Analysis (ICTA) formed a Standards Committee to resolve

disagreements on reported thermal data through the development of

appropriate calibration standards for DTA instruments.

Thermal analyses, akin to all instrumental procedures, require

some form of calibration. Only rarely, as in the case of coulometry,

can Independent physical methods be readily employed. In analytical

techniques, composition materials or high purity substances are used

for this purpose, NBS has therefore carefully characterized and

certified materials for use as calibration standards. These standards are characterized by chemical and physical measurements to ensure that instrumental biases are not perpetuated. Ideally, an effort is made to establish a certified value which is independent

of the instrument for which it is intended. In certain measurements,

hov/ever, such as the determination of gases in metals and in differential thermal analyses, it is not possible to separate the measurements and certification of the true value from the method. In such cases the careful consideration of the phy sicochemical principles

and the evaluation of the sources of errors due to various instrumental characteristics play a predominant role. In developing

temperature scale calibrations for differential thermal analyses,

comparison with data from adiabatic calorlmetry is of great value.

The two previous speakers have discussed the principles and techniques from which one can estimate the errors involved in these two

types of measurements. These two methods, then, when critically

applied to the measurement of selected materials, should enable us to arrive at a more accurate value for transition temperatures

and enthalpies.

THE COOPERATIVE STUDY WITH ICTA

In 1966 the ICTA Standards Committee initiated a cooperative

"round robin" test of twelve compounds to evaluate their potentialities as temperature scale standards. The selection of these materials was based on the following criteria: (I) only compounds with

known phase transition temperatures should be selected; (2) these

temperature values should be well established from literature ther-
Menis and Sterling 63

modynamic data as reported in the NBS Circular 500, of 1952; (3)

enthalpy changes at these transitions should be sufficiently large

to be detected by most instruments; (4) on storage, the compounds

should be stable, with known purity and homogeneity; (5) these

compounds should cover a wide range of temperatures from 100 to 1000 °C; and (6) they might also serve as enthalpy standards for

DTA.

These criteria were established after evaluating all the factors essential for achieving the best accuracy in these measurements. It was felt that transition temperatures are less often

subject to environmental conditions and decomposition or side reactions. For these reasons melting point standards were considered

less reliable, (H. McAdie discussed the latter problem fully at the American Chemical Society meeting at Houston in February 1970.)

The selection of compounds based on the data in NBS Circular 500 was agreed upon in order to avoid pitfalls when lesser known values

are considered. However, even in this case, preliminary information [2] indicates that the reported value for the transition temperature of silver sulphate is incorrect. The need for compounds

which would cover the temperature scale of interest stems from the

experience with the dynamic mode of obtaining thermal curves. Here

many factors, such as the heat capacity and thermal conductivity of

the tested material, may affect the shape of the curve at a given

temperature. Thus, a single temperature reference point may not be

sufficient to allow m.eaningful extrapolation to test conditions at significantly different temperatures. This presents the problem

that ultimately one would like standards having physical properties

(such as thermal conductivity and heat capacity) similar to the re- spective materials encountered in the biological, polymer and inorganic fields.

RESULTS OF THE FIRST ICTA TEST

In the "round robin" evaluation program, we have cooperated

with the ICTA Standards Committee in the two tests. After a preliminary review by the Standards Committee, the data from the
64 DTA Standards

first round robin, involving twenty-four laboratories from eight

nations, were evaluated by a subcommittee consisting of H. McAdie,

Chairman, from the Ontario Research Foundation, P. Garn, University

of Akron, 0. Menis, NBS Analytical Chemistry Division and B. Joiner,

NBS Statistical Engineering Section, who acted as a consultant. In the preliminary review, four compounds were rejected because of

difficulties encountered in their measurements. These were (1)

potassium bisulfate, (2) sodium sulphate, (3) potassium carbonate

and (4) sodium carbonate. In the first, the decomposition temperature partially overlapped the transition temperature; in the

second, the thermal curves were not reproducible and depended on

the pretreatment of the sample. In the latter two, the materials

tended to lose CO^ at comparatively low temperatures and the thermal effects were below the limit of resolution of some instruments.

The data from the twenty-four collaborating laboratories are summarized in Figures 1, 2 and 3- These are plots of the data

from thermal analysis curves of eight compounds-- (1) potassium

nitrate, (2) potassium perchlorate, (3) silver sulphate, (4) quartz,

(5) potassium sulphate, (6) potassium chromate, (7) barium carbonate, and (8) strontium carbonate, all of which met the specified

criteria for a standard. In Figure 1 plots of point A, the 1st

departure from the base line is recorded, point B in Figure 2 represents the intersection of the forward-extrapolated baseline

with the backward-extrapolated initial side of the peak, and in

Figure 3, point D is the peak temperature. From these plots of

data it is evident that the intersection point, B, yields values

which have the smallest deviation from the thermodynamic transition

temperatures given in the literature. About two-thirds of the laboratories reported values within ±5 °C of the literature value. A brief review of the data was reported at the Proceedings of the

Second International Conference [31.
Menis and Sterling 65

I 3 ^"^ 7 13 5 7 13 5 7 13 5 7 13 5 7

I I I I I I I I I I I I I I I I

I I I I I I I I I I I I I I I I I M I M I I 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 MATERIALS

Figure 1. Data from 24 ICTA cooperating laboratories for point A heating cycle, (The numbers in brackets represent the

laboratories j the groups of numbers in the abscissa

represent the eight materials; the dashed lines represent the values from the NBS Circular 500.)
66 DTA Standards

Figure 2. Data from 24 ICTA cooperating laboratories for

point B heating cycle.
Menls and Sterling 67

Figure 3. Data from 24 ICTA cooperating laboratories for

point D heating cycle.
68 DTA Standards

PRELIMINARY RESULTS AT NBS FOR THE SECOND ICTA TEST

As a result of the above evaluation, the ICTA Standards Committee

initiated another cooperative study. In this case more rigorous

requirements for describing experimental conditions were included.

Mainly, however, the purpose was to characterize large quantities

(suitable for future distribution as Standards) of the eight previously screened compounds and to add indium and tin to the list as mielting point standards. The results of these tests are not yet

available from these laboratories. However, we present the data

obtained at the Analytical Division at NBS for comparison. In Table 1, results are presented from the first and second tests and are compared with the values in NBS Circular 500. The second series

was measured after considerable modification of equipment which will

be described in the next section. The data from the two tests are similar, but, some of the large deviations for the transitions temperatures above 500 °C have been reduced significantly in the

second test. The melting point standard behaved almost ideally on the heating cycle. In the second set of values, there is a definite

positive bias which may be due to the rate of heat transfer which is

a controlling factor in a dynamic DTA measurement.

Table I. NBS first and second ICTA DTA study

Compound

No . Compoundformula

NBS

Cir. 500

°C

1st NBS

DTA "B" ^C

2nd NBS

DTA "B" °C

1 KNO^ 127.7 129 129.5

2 KCIO^ 299.5 300 300 . 8 3 Ag^SO^ 426 427.5

4 SiO^ 573 570 574.4

5 K^SO^ 583 577 586.0

6 K^CrO^ 665 668 668.5

7 BaCO^ 810 791

8 SrCO^ 925 905 929.3

Sn 231.9 233.0

In 157 157.8
Menls and Sterling 69

The changes in the apparatus and procedure prior to the

measurement of the second set were as follov7s:

MODIFICATION OF INSTRUMENT

The commercial Instrument Initially used in this study v/as lim.ited in its capability to provide precise and accurate values.

This instrument was equipped with thermocouple v/ires whose cold

junction was dependent on the ambient temperature. Thus the temperature measurements would vary several degrees during the day.

The hot junction was also located outside the sample compartment.

The high programming rate, together with a low sensitive recorder,

were limiting factors of the initial apparatus

The following modifications were made to ensure its use as a research tool, especially to determine more accurately the transition temperatures of selected compounds for use as standard

reference materials

1. Sim.ultaneous use of the same thermocouple for both sample

temperature readings and delta-T readings.

2. Insertion of ice bath junctions in all temperature thermocouple circuits for voltage measurements with respect to 0 °C.

3. Simultaneous recording on an x-y recorder and two-pen strip

recorder. The areas of these thermal curves correspond more, in

theory, to heats of transitions than do the areas delta-T vs. temperature plots.

4. Slov/er scan rates were provided for minimizing gradients

between the sample thermocouple and the sample itself, and for approximating adiabatic conditions

5o Addition of a stable bucking voltage source was provided for use with a sensitive setting of the temperature vs, time recorder.

This increased the precision of measurement of the sample thermocouple voltage by a factor of ten, by permitting the scanning of 5 millivolts from the thermocouple in ten one-half millivolt recorder

spans
70 DTA Standards

60 Provision for temperature calibration of the apparatus:

ao Replacement of sample thermocouple with calibrated wire,

b. Provision for rapid comparison of calibrated thermocouple with standard voltage source,

Co Construction of non-metallic sample holders for use with metal freezing point standards

EXPERIMENTAL

Modification of Thermocouple Wiring and Readout Arrangement.

The commercial unit was provided with various alloy thermocouples together v/ith a foam insulated box containing cold junction

connections for the temperature thermocouple. However, regardless of

the intermetallic junctions involved, the voltage of the hot thermocouple was always measured with reference to a cold junction or junctions completing the circuit. The temperature of this thermocouple

was thus read with respect to ambient temperature together with the

temperatures in the apparatus at the junctions of the alloys. Also,

the hot junction supposedly measuring the sample temperature was not

inside the sample cup (this one was used for the differential measurement only) but was actually the furnace control thermocouple

The following changes were made:

a , Changes in Thermocouple Arrangement

(1) All thermocouple wiring was removed and replaced

with platinum or platinum-10 percent rhodium wires leading to an ice

bath (see Fig. 4). From the ice bath, all copper leads went to the

recorders and amplifiers.

(2) The furnace control thermocouple previously used

for both the temperature measurement and furnace control was now

limited to furnace control » This provided the required flexibility

for its optimum placement and avoided interference from the programmer electronics,

(3) The thermocouple in the sample cup was used for

temperature measurement (with respect to the ice bath) rather than

the thermocouple in the reference cupo (See Pig. 4.)
Menis and Sterling 71

J i

feed through furnace insulation

bead weld

furnace control t. c. spotweld to can

all Pt or Rh leads

heat test applied here "TT

all Pt or Rhmleads

screw-type terminal

strip on furnace platform

vacuum feed through L

ice bath

1> all copper

leads

i solder Pt or Rh

to copper

to furnace programmer

—

o

t

Leads

1. sample - Rh (Rh = Platinum/10% Rhodium)

2. sample, ref. - Pt

3. furnace control - Rh

4. furnace control - Pt

5. reference - Rh

G. spare - Pt i

calibrate

calibrated voltage source

X-Y recorder jx (temp) 1 y Y (A temp) ^

^ temperature (V^

[with bucking ^

iv. source! ^

I

D C amplifier

Figure 4. Modified thermocouple wiring, calibration and

recording arrangement.

(4) Screw-type terminal strips were provided for direct

connections of the thermocouple leads to the ice bath leads of the

same metal. Tests with a hot screwdriver showed a slight potential

shift due to the small dissimilarities of the Pt and Pt/Rh thermocouples. These shifts, as expected, were small enough to be negligible to ambient temperatures. Since then, the terminals have been

replaced with directly soldered connectionso

All wiring was external with hookup wire for easy changes. Hum

and chatter were suppressed by shielding with wire braid and copper

screening and shunting the leads with 0,1 mfd capacitors at the

recorder terminals.
72 DTA Standards

b o Recorder Changes

The two-pen recorder was used to obtain a plot of temperature and differential temperature vso tlmOo To make It possible

to expand the temperature scale by a factor of ten, without Introducing great distortion, bucking sources were combined with more sensitive scales. Because of the sensitivity due to the scale factor expansion. It was necessary to thermally Insulate the bucking

voltage divider and associated resistors and dry cells to suppress

drift. This was done with plastic packing material. At high sensitivity, the recorder was susceptible to static electric pickup from

personnel moving nearby o It was thus necessary to surround the

apparatus with grounded copper screening.

Switches were added as shown (Flgo 4) for setting the recorders

to zero degrees centigrade without Interference to the differential

circuit, and for calibrating the temperature recorders with a standardized voltage source.

c . Lower Scanning Rates

The motor controlling the heating or cooling of the

furnace had a reliable speed range of a factor of less than three

(see Fig. 5) . Using the lowest reliable setting and the gears provided with the lowest ratio of potentiometer turns per motor revolutions, the lowest rate available was about five degrees per minute.

Smaller motor gears were made, and the motor was moved to accommodate

them. Also, the electronics In the furnace programmer were modified

to lower the rate of temperature scan. Because this modification

also decreased the temperature range proportionately, the rate de- crease was only a factor of 2. Using the smallest motor gears, the

minimum heating (cooling) rate was lowered to less than a degree per

minute. However, at the lower heating rates, the feedback from the

furnace sensing thermocouple was slow enough so that the furnace

would cycle above and below the proper temperature by several degrees

with periods of 10 minutes or less. To speed the feedback, the fur- nace control thermocouple was welded to the nickel sample can (Fig. 4)

which was the closest stable point to the furnace heater windings.

Even with this modification and the optimum adjustment of the furnace
Menis and Sterling 73

Figure 5. Programmer motor calibration after modification.

controller, the minimum heating rate that was free of the cycling was about one degree per minute. Moving the thermocouple outside the can, thus exposing it directly to the furnace windings has further improved

performance

d . Automatic Voltage Divider for Bucking Voltage Source

A bucking voltage source of the following properties

was designed and built (see Fig. 6):

(1) Automatic - able to reset the recorder (temperature)

when it went off scale in either directiono

(2) Isolated from the ac power line and from ground.

(3) Stable with respect to time and temperature to within at least 0.1 percent.

(h) Able to provide 20 increments of approximately

1/2 millivolt each.

The voltage divider was made from two percent carbon resistors

and trimmer potentiometers to compensate for the expected four percent resistance variation. This portion was shielded from the control circuits, and offered no noticeable hum.
DTA Standards

47K NE-2

Figure 6. Automatic voltage divider for providing bucking

voltage

The control circuits were made from relays because space and

speed were not at a premium and simplicity of operation was desired.

The basic control units were stepping relays that would advance and

reset, and a magnetic read switch (whose Inherent hysteresis prevented chatter) that was timed to advance the stepping relays once every

two seconds. Stepping relays advance or reset continuously until

the recorder pen is on scale. Micro switches were added to both

ends of the recorder scale, and either would activate the recorder

circuit. Additional relays provided power for the steppers to prevent skipping Immediately after resetting, and to electrically isolate the limit switches for reasons of safety. Only 6 volts ac were

on the limit switch binding posts, and either one could be grounded,

if necessary

The bucker can be set manually to any of its 20 positions and

reset from any position. When the limit switch binding posts are
Menis and Sterling 75 connected, the position of the divider will be advanced about once every two seconds (increasing the voltage output) until it reaches

the 4th, 9th, l4th or 20th position. It will then reset to the zero position.

Indicator lamps were provided for monitoring the magnetic read

switch (continually driven by the motor and magnets), the position

of the voltage divider and a lamp fuse.

Because the bucker was designed for use with a potentiometric

recorder or other high-impedance device, it exhibited high impedance

(40,000 ohms) and drew very little battery current. It was therefore not expected to supply current. Its purpose v/as for a voltage

source only <, The bucking voltage source was used either with a 1.5 volt dry

cell or a mercury cell in series with an 8.2 megohm resistor so that

each step would swing a 1/2 millivolt pen recorder almost to full

scale

Because of the scale expansion, the temperature recorder was very sensitive to the drift of the dry cell or m.ercury cell. The

temperature and time stability of dry cells and size D mercury

batteries x^ere checked separately with a potentiometer o The performance varied widely except for new cells. All cells were temperature sensitive. The mercury cells performed best, and remained

stable to better than 0.1 percent if insulated from ambient temperature by several inches of plastic foamo To suppress further drift,

the bucker was enclosed in several inches of insulating material,

because of the temperature sensitivity of the carbon resistors in

the voltage divider. The bucker-recorder combination seldom drifted

over several microvolts per hour. Short term drift was checked and was not apparent o The effect of long-term drift was checked and was

not apparent. The effect of long-term drifts (over 20 minutes) on the results was not critical because the entire recorder system was calibrated with a separate potentiometer and was provided with temperature regulated standard cells, which were used frequently for

calibration.
76 DTA Standards

TEMPERATURE CALIBRATION

1 , Thermocouple Changes

Platinum and platinum-10 percent rhodium (15 mil) wires

were calibrated by the Heat Division at NBS

The thermocouple over which the sample cup rested was removed,

and the calibrated wires were inserted several inches beyond the

ceramic tube. The ends of the wires were welded into a bead with an oxy-gas torch, then placed in the same position as those of the previous thermocouple. The reference thermocouple was withdrawn slightly to match the position of the calibrated thermocouple. These precautions were taken to avoid bending the leads, especially where they

enter the bottom of the ceramic tubes and where large heat gradients

were expected. The leads were directed to the ice bath containing

the copper junction.

2. Calibration Procedure

With the zero-run switch in the run position, the temperature recorders were switched from the sample thermocouple to a calibrated voltage source which in this case was a high precision

potentiometer o The approximate transition temperatures under investigation were known, so the potentiometer could be set to approximately the correct potential. This allowed the fiducial marks to be

on scale, within several equivalent degrees of the thermal curve. Fiducial marks were taken within minutes before and after the thermogram. Several points gave the correct scale factor for interpolating to the potential reading of the thermocouple.

As a further check on the temperature readings, NBS freezing

point standards, lead, aluminum and tin were carefully prepared.

Non-metallic cups were used with these materials to avoid alloying

with the sample c Graphite rod was chosen because of its availability, purity, and good thermal conductivity. This rod was cut to size, drilled, rinsed several times in acetone, boiled in HCl, and

heated to incandescence before use.
Menis and Sterling 77

3 o Sample Cups

Platinum, nickel, stainless steel, aluminum and graphite

cups were tested. Stainless steel was unsatisfactory because of

oxidation and resultant contribution to the thermal curve. Nickel

and aluminum cups were adapted and the thermocouples v;ere inserted

directly into the sample and reference o This did not improve the

thermal curves perhaps because the thermocouple contacted only the

sm.all layer of sample immediately surrounding it. This layer acted

as an insulator for the remainder of the samples, which were powders

or sm.all crystals having relatively poor thermal conductivity. This

approach was abandoned because the thermal curves shov/ed no improvement and since the thermocouple was not protected, the apparatus v/as more difficult to load and clean. Therefore, the closed v;ell v/as used throughout this study.

4o Results

Scanning rates of 1 to 5 degrees per minute were used over the tem.perature range investigated for potassium nitrate and quartz

for use as DTA standards. Temperatures could be determined to a precision of about 0.2 degree (2 microvolts). Data for several

melting point standards were within 2 degrees of their certified

values. Repetitive temperatures of a given sample v/ere consistent

to at least one-half degree for a particular heating rate and at all heating rates.

Typical thermal curves of two proposed Standard Reference

Materials, KNO^ and SiO^, are presented in Figure 7a and 7b.

Those for metal melting point standards are given in Figure 8. The departure point. A, and the transition peak D are marked.

INDUCTIVE PERIOD AND SUPERCOOLING

The transition temperature peak for potassium nitrate on the

first programming was always several degrees higher than on subsequent repetitive cyclings (without ever going through the melting

point). The displacement of the transition temperature may also be

avoided by preheating the potassium nitrate crystals in a drying
78 DTA Standards

HEAT

A = 129.0 B = 129.6 D = 131.4

POTASSIUM NITRATE

172.0 X

— 147.0 X COOL A =124.0 B =123.8 D =123.8

122.0 X

A,'B Figure 7a. Thermal curve, heating and cooling of potassium

nltrateo

A ,B QUARTZ

HEAT

A = 570.2

B =574.4

D = 575 .4

602.0 X

579.0 X

COOL

*C A= 578.4 B = 575.4 D = 574.4

555.0 "C

Figure 7b. Thermal curve, heating of quartz

„
Menis and Sterling 79

ALUMINUM LEAD TIN

Figure 8. Thermal curves, heating and cooling, of metals.

oven at 150 °C. This effect is probably due to the presence of

water within the crystals. Kracek [4] who investigated the effects

of water reports also its effect on the phase transition on the

cooling cycle o He observed that on cooling two peaks occur and

postulated that phase I transforms to phase III and then to phase

II. In an experiment in a sealed quartz tube in the presence of

water, he demonstrated that a rapid direct transition occurs from

phase I to phase II. Data on the cooling cycle for other compounds

are shown in Table 2. As can be noted in these examples, this type

of supercooling or induction period is not uncommon for the com- pound and for most metals tested. In this study, quartz, potassium

sulfate and potassium chromate did not show a large Induction type

temperature spread.
80 DTA Standards

Table 2. Supercooling in selected materials.

Transition temperature - °C

NBS NBS-DTA NBS-DTA

Compound Circular-50G Heating-"B" Cooling-"B" A.S.^-°C

KCIO^ 299.5 , 300.8 290.^ +10.8

KCrO^ 665.0 668.5 671.2 -2.7

AgSO^ 427.5 407.0 +20.5

Sn(Metal) 231.9 233.0 213.0 +20.0

In(Metal) 157.0 157.8 155.2 +2.6

^A . S . = apparent supercooling = difference bt^tween average

transition temperatures on heating and on cooling.

COOPERATIVE STUDIES OP STANDARDS SECTION

OF ASTM E-1 PROVISIONAL SUBCOMMITTEE

In another effort to develop Standard Reference Materials,

studies with two organic compounds and sulfur were initiated which

covered lower transition temperatures. This effort was carried out with the cooperation of members of ASTM Committee E-1, Provisional

Subcommittee on Thermal Analytical Test Methods. The results from

nine cooperating laboratories on sulfur, stearic acid and hexachloroethane are presented in the three Figures 9a-llb, Again it

is evident that point B shows the least variation in interlaboratory

comparisons. The poorest reproducibility between laboratories can be noted with sulfur. It is evident from this study and the experience of others, that it would not be suitable as a standard. The

melting point of stearic acid and the transition temperature of

hexachloroethane offers greater promise. The latter compound, especially because of its two transition temperatures, would be very useful for calibration purposes. The cooperative values agree closely

within 1 degree of the accepted literature values of the triclinic

to cubic transformation at 72 °C and to within 5 °C to the rhombic

to triclinic at 44 °C. Based on these data, it is planned to purify

larger quantities of hexachloroethane for certification as a Standard

Reference Material.
Menis and Sterling 81

126 124

122

120

118

116

114

112 no

108 106

104

1 \ \ \—I—I\ \ 1 \ 1—

A

A -

SULFUR MELTING POINT

J \ L_J J \ \ L I L 01 2 3 3a 456789 10 LABORATORIES

II 0.0

108.0 106.0

104.0 102.0

100.0 98.0

96.0

94.0 92.0

1 \ \ \ \1

\—\—\—

SULFUR SOLID (D-^SOLIDIT

I I I I I

\ L I I I I 01 23 3a 456789 10 LABORATORIES

Figure 9. Cooperative data for ASTM committee members

(one member with two data points)

a) Sulfur Solid

b) Sulfur melting point
82 DTA Standards

In conclusion^ Figure 12 shows schematically a list of proposed

Standard Reference Materials which should meet the requirement for

temperature scale calibration over a temperature range from 4o to 900 °C. The peaks and the areas indicate qualitatively the relative

sensitivity and enthalpy changes of these materials. Two of these,

potassium nitrate and quartz, shown also in Figure 7, have been

issued under a provisional certificate, NBS-SRM 755 and 756.
Figure 11. Cooperative data from ASTM committee members - for a) hexachloroethane, rhombic triclinic

b) hexachloroethane triclinia cubic
84 DTA Standards

800

(925) SrC03

929.3 900 1000 1100 1200

(419.505) (412) Zn Ag2S04

419.7 (4275) 400

(573.) (583) Si02 K2SO4

574.4 586.0

(665) K2Cr04

668.5 500 700

I 800

(43.6) (71.1) C2Clg C2CI5 479 72.4

(127.7) KNO3

129.5

(156) in 1578(232) Sn

233.0

(299.5) KCIO4 300.8

100 200 300 400

Figure 12. Selected DTA temperature reference materials over the temperature range of 4o to 900 °C.

Sample homogeneity, the effect of particle size and recycling

through the transition temperature below the melting points of the

two compounds were studied and are summarized in Figures 13 and l4.

It must be noted that KNO^ , as described previously, in its first

thermal cycle, reveals a large deviation at point D. Subsequent

thermal curves of the same sample indicate consistent reproducibility. The transition peaks in these figures show a maximum

spread of ±0.5 °C which also reflects the variation in particle

size and source of materials.
Menls and Sterling 85

HOMOGENEITY OF SRM POTASSIUM NITRATE

SIEVE MATERIALS MESH

100-200 200-325

FINES AS IS

LOT 2RUNRUN-2 AS IS

RUN-3 AS IS

«-0.5'C-» *-0,5'C-» INITIAL • SUBSEQUENT •

INITIAL o • SUBSEQUENT •

INITIAL • • SUBSEQUENT • •

INITIAL • • SUBSEQUENT • •

INITIAL o SUBSEQUENT • SUBSEQUENT • • SUBSEQUENT • •

INITIAL o SUBSEQUENT « • SUBSEQUENT • SUBSEQUENT • • SUBSEQUENT 9 •

INITIAL 0 SUBSEQUENT • •

INITIAL 0 SUBSEQUENT •

POINT OF DEPARTURE

Figure 13. Homogeneity and r-'i^oet : 1 ve cycling through the

transition point for potassium nitrate.

Future efforts should extend to cover the subamblent temperature region which Is of Interest In blo-cllnical work, and rhe

high temperature range, of Interest In transportation and space tec nology. It Is anticipated also that many more organic or polyr:er

type standards could be usefulo Finally, It can be expected that 1 addition to temperature scale standards the establishment cf known

enthalpy values could also aid science and technology In quantltati

analysis of various materials.
DTA Standards

HOMOGENEITY OF QUARTZ

MATERIALS

SIEVE MESH NBS- LOT 1 >I00 • <-0.5'C-»

100-200

• 200-325 • •

FINES « • NBS- LOT 2 100-200

•• NBS- LOT 3 100-200 •

« NBS- LOT 4 100-200 •

•

0. L BRAZILIAN 100-200 •

•

ICTA •

•

• 0.5 'C-* •e

•

•

•

•

•

» •

••

c •

•

• COOLING TRANSITION PEAK

HEATING TRANSITION PEAK

igure 14. Homogeneity and cooling and heating transition

peak

ACKNOWLEDGMENT

The assistance of D. S. Bright in this study is acknowledged.

REFERENCES

Chemical and Engineering News, p. ^6, August l8, 1969-

H. McAdie, private communication.

Schwenker, R. P., Jr. and Garn, P. D., Editors, Thermal Analysis

Vol. II, Appendix 3, Academic Press, New York (I969).

Kracek, F. C, J. of Phys. Chem. 31, 225 (1930).
HIGH TEMPERATURE DIFFERENTIAL THERMAL ANALYSIS

Robert D. Freeman

Department of Chemistry

Oklahoma State University

Stillwater^ Oklahoma 74074

Three areas are discussed: (I) high temperature

(>1900 K) differential thermal analysis (DTA) ; (2) some problems in conventional high temperature calorImetry, which may be relevant to high temperature dynamic calorimetry; and (3) a brief description of our efforts to achieve dynamic calorimetry to at least

1300 K. Key words: high temperature chemistry; differential

thermal analysis; dynamic calorimetry

INTRODUCTION

High temperature chemists [1] have debated at some length the

appropriate definition of "high temperature." For the purposes of

this paper I shall define "high temperature (HT)" to mean a temperature higher than the range in which conventional, well-known techniques and equipment produce acceptable results.

Conventional DTA (AT vs. T or t) may be more or less routinely

accomplished at temperatures up to ^^^1900 K. The techniques and

equipment are described in books by Garn [2], Wendlandt [3], Smothers

and Chiang [4], and Mackenzie [5], and in literature readily available from the various manufacturers of DTA instruments. Therefore,

I shall consider "high temperature DTA" to indicate temperatures

above 1900 K.

On the other hand, dynamic calorimetry {e.g., "Differential

scanning calorimetry") is not a well established technique above

^900 K; accordingly, for this technique "high temperature" shall

indicate temperatures above 900 K. 8?
88 DTA

This discussion will focus on three areas: (1) high temperature DTA (as defined above); (2) some problems in conventional

high temperature calorimetry, which may be relevant to HT dynamic

calorimetry; and, finally (3) a brief description of our efforts to achieve dynamic calorimetry to at least 1300 K. HIGH TEMPERATURE DIFFERENTIAL THERMAL ANALYSIS

One of the rules of thumb of high temperature chemistry [6] is

At high temperatures, everything reacts with everything else. Like

most rules of thumb, this one is not strictly valid, but it does serve to emphasize the problems associated with choosing suitable

thermocouples, sample holders, furnace elements and insulation,

etc., to extend conventional DTA to temperatures above I8OO-I9OO K. For specific problems, one may be able to use tungsten-rhenium

thermocouples [7] and to operate tungsten, molybdenum, tantalum or graphite furnace elements in an inert atmosphere, but the major

efforts in the development of generally applicable HT DTA apparatus

have been directed toward vacuum furnaces and radiation detectors

as temperature sensors.

One of the first of these systems was described by Rupert [8]

in 1963. A rather straight-forward vacuum system contained a vertical section of lO-cm o.d. Pyrex pipe around which was located a radiofrequency induction coil and inside of which was an "eddy

current concentrator." The crucible-sample holder was supported

in the center of the "concentrator" by a tungsten rod. Radiation

from a small hole in the top of the crucible passed through a quartz window in the vacuum envelope, and impinged on a partially

aluminized mirror which reflected part of the light to an optical

pyrometer, and passed part to a phot omultiplier tube. The output

of the PM tube, as displayed on an oscilloscope (1-2 div/s sweep

rate on horizontal axis), provided a reasonably accurate transform

of the temperature-time behavior of the crucible. This apparatus

produced reliable data from cooling curves, e.g., the temperature

of phase transitions [8], but had no reference cell for differential

temperature sensing.
Freeman 89

A couple of years later Rupert reported a major modification

to, and, improvement of his apparatus [9]. The major changes in

equipment were: (1) provision was made for two sample (or sample

and reference) containers in the crucible which was heated as before; (2) two photodiodes in "identical" light sensing circuits

replaced the PM tube and permitted cooling curves for two samples

to be recorded simultaneously or the differential output between

sample and reference to be recorded; (3) a dual-trace storage

oscilloscope was used to record and display the photodiode outputs

for long periods (hours); the display could then be studied and/or

photographed as desired; (4) electronic circuitry between the

photodiodes and the oscilloscope permitted a variety of thermal

analysis data to be recorded. These various types of data included

(1) thermal analysis (cooling curves, T_ vs . t) , (2) differential

thermal analysis (AT vs. T or t_) , (3) derivative thermal analysis

(dT/dt ys.t), and (4) "thermal derivative thermal analysis,"

(dT/dt vs. T). This last mode for presentation of data appears to

be especially useful in studying the temperature range over which

phase transitions occur. With this Improved apparatus thermal

effects in the range 1300-2800 K could be studied; the sensitivity

is indicated by noting that a 2.5 cal transition in 0.5 g of iron

at 1665 K produced a '^J4-cm deflection on the oscilloscope trace of

the dT/dt signal.

In a parallel effort, Heetderks, Rudy and Eckert [10] developed

an apparatus similar to Rupert's, but with these differences: (1) a low voltage, high amperage resistance furnace with a cylindrical

graphite element provides programmable temperatures from 1000 to 3900 K in vacuum or in inert gas with pressures up to 6 bar; (2) the

problem of matching radiation detectors for the sample and the ref- erence channels was avoided by using only one photodiode which re- ceived radiation from both sample and reference cells as a mechanical chopper alternately passes and intercepts each of the two

light paths. This apparatus is now commercially available [11].
90 DTA

In more recent developments Baldwin, et al . [12] have combined

radiofrequency induction heating, as used by Rupert [9], with a single radiation detector, as used by Heetderks, et al . [10]. The

major innovation is the use of a lead sulfide photoconductor as the radiation sensor. These photoconductors , which were developed

as infra-red radiation sensors, have a usable spectral response

over the range 0.25 to 3-5 mm and therefore responds to changes in

radiation from objects at temperatures as low as 400 K. These various HT DTA apparatuses have been used to study a variety of chemical systems. Some examples are: Mo-C, Zr-C and

U-C [8]; Th~C and U02_^ [12]; Nb-C, U-C, and U-Gd-C [9]; Hf~C

,

Ta-C. ^

HIGH TEMPERATURE CALORIMETRY

There seems to be a growing desire to make DTA more quantitative i.e.y to obtain from peak height or peak area a value which

correlates directly and linearly with the energy or enthalpy change

associated with the transition/reaction. The prime example of this

trend is the differential scanning calorimetry (DSC) [13,1^], but a number of other approaches have also been reported [15-17]. Since

this trend is certain to continue, and since there already is a desire

to extend DSC-type measurements to higher temperatures than the 900 K presently available, we examine briefly some of the problems of con- ventional calorimetry at high temperatures and indicate where detailed

discussions of these problems may be found.

There are two conventional calorimetric techniques for obtaining heat capacity and enthalpy data at temperatures above 500 K. One of these, drop calorimetry, is not particularly suited to automated, dynamic measurements; therefore, the recent publication [l8]

of a comprehensive discussion of the subject is noted, and our attention is directed to the other technique, adiabatic calorimetry.
Freeman 91 As ordinarily used, the term adiahatio aalorimetvy is a misnomer. What is meant is the (precise m.easurement of changes in temperature associated with enthalpy (energy) increments such that the

derived average heat capacity and enthalpy of transition is obtained

with high precision, the measurements being accomplished with the

calorimeter vessel (sample container) surrounded by an adiabatic

shield so that energy exchange between calorimeter and surroundings

is minimized. There are two techniques for producing the enthalpy

(energy) increments: (1) intermittent heating, in which the temperature is measured before and after a heating period, but no power is

supplied to the heater during the temperature measurement; and (2)

continuous heating, in which constant power is supplied to the sample

heater and the time is recorded when the thermometer indicates each

of various selected temperatures. This latter dynamic technique is

obviously closely related to quantitative DTA techniques.

West and Westrum. [19] have recently discussed in detail the

problems associated with adiabatic calorimetry up to 800 K. The

problems arise primarily from the limitation imposed by the effect

of HT on possible materials of construction {e.g, oxidation, melting difficulties in achieving good thermal contact between components)

and from the increased rate of exchange of energy between calorimeter

and adiabatic shield as a result of the T radiation law. West and

Westrum also provide a survey of some 40 intermediate and high temperature adiabatic calorimeters (^300-1800 K) . Only a very few of

these operate above 1200 K; the most notable are two rather massive,

complex devices reported by Backhurst [20] (to I87O K) and by Dench

[21] (to 1670 K)

Other very useful discussions of calorimetry at HT have appeared

in recent years [22-28]. Especially interesting is a discussion [29],

among several leaders in the field, from which the consensus appears

to be that the best approach to HT calorimetry is with heat-flow calorimeters of the Calvet [30] or Wittig [31] type.
92 DTA

Another major effort to overcome the difficulties inherent in

traditional calorimetry at HT has been made by the several developers

of pulse, or high-speed, calorimetry. For example, Kollie [32] has

developed an apparatus in which a dc pulse of several seconds duration is passed through a rod-shaped sample, vjhile a high-speed (400

readings/s) digital voltmeter sequentially scans data channels con- nected to voltage, current, and temperature (thermocouple) probes.

Accuracy within 1 percent and reproducibility within 0.5 percent in

the heat capacity of pure iron over 300-1200 K was obtained.

More recently, Cezairliyan, et al . [33] have described a similar

system in which an individual experiment, with the temperature of

the specimen {e.g., a molybdenum tube) changing from 300 K to near

the melting point, is completed in less than one second. High-speed

digital data acquisition is used, of course, and temperatures are measured by a high-speed photoelectric pyrometer which can make 1200

readings/s. It is interesting that during an experim.ent the pyrometer reads alternately the specimen temperature and the temperature

of a calibrated tungsten filament lamp; in a sense, then, the technique is a type of high-speed DTA. Beckett and Cezairliyan [3^] have

recently reviewed high-speed calorimetric techniques.

DYNAMIC DIFFERENTIAL CALORIMETRY

Our initial interest in "quantitative DTA" arose from a desire

to measure directly the enthalpy of a disproportionation reaction of

the type: k, . —> B. . + C, . for which measurement of the equi-

( s ) ( s ) (g)

librium dissociation pressure [35] presented considerably more than

ordinary difficulties. Our first furnace and sample/reference

assembly was essentially a copy of that of Speros and Woodhouse [15];

the electronics were somewhat more sophisticated in that power to the

sample heater was automatically controlled and the power dissipated

in the heater was monitored by recording the output of an operational

amplifier-Hall-effect multiplier circuit [36]. This apparatus require

a "one-time" calibration, as proposed and discussed by Speros and

Woodhouse [15], of the thermal coupling between sample, heater, and
Freeman 93 thermocouple. Thereafter, it produced reliable data (within 1-2%)

for heats of fusion of metals and for heats of decomposition of various carbonates [36]. It is worth noting that, although the samplecell-heater assembly was considerably larger and more massive than

the DSC units, the ''peak" obtained for a sharp .transition (e.g.,

melting) had the same characteristics as the theoretical shape predicted by Gray [37] for DSC peaks.

From, our experiences with this apparatus and from various con- current developments, we reached the following tentative conclusions:

(1) Furnace programming and control could be improved if the furnace

was small {e.g., 5 cm o.d. and 10 cm, high) and had high thermal con- ductivity and low heat capacity. (2) Energy exchange betv^een fur- nace and sample/reference cells is most easily controlled by minimizing that exchange, i.e., operating adiabatically (or quasi-adiabatically) with the "furnace" becoming, in effect, an adiabatic

shield for the sample/reference cells. (3) Achievement of near-adiabatic conditions in a small furnace (c/. [IJ) at 13OO K is likely to be very difficult] therefore, one should retain the differential m.ode of operation and should make sample and reference structures as nearly symmetrical as possible, so that any nonadiabaticity produces

equal effect on sample and reference.

Very few differential adiabatic dynamic calorimeters (DADC) have

been described; for example, Wilhoit [38], in his survey of "Recent

Developments in Calorimet ry

, " describes five configurations for dynamic calorimeters but does not mention DADC. Clarebrough, et al

[39] used what is essentially a DADC apparatus to measure (up to 800 K) the energy stored in deformed metals. More recently, Bonjour

[40] has described a DADC which operates over the range 75-700 K, but

the sample configuration is a small rectangular strip, 0.5-3-0 mm

thick, held closely to the heating element by clips.

Our ideas about DADC have led to a design in which a sample and

a reference structure similar to those of Speros and Woodhouse [15] are enclosed in a small metal-walled cylindrical furnace with heating

elements evenly distributed over sides and ends. The temperature
94 DTA

Figure 1. DADC control system.

sensing and the power control circuitry are indicated in block diagram in Figure 1; the components are, more or less, conventional.

The circuit which measures the differential power between the sample

and the reference heaters is perhaps not conventional; a schematic

diagram is given in Figure 2. Details about the twin Wheatstone
Freeman 95

Figure 2. Analogue computation of differential power.

bridge with magnetoresistive elements ("Mlstor"), which is the heart

of the power measuring circuity are available elsewhere [41].

The apparatus Is still in the development stage. It has been

established that the circuitry of Figure 2 is quite satisfactory,

and the entire apparatus has been successfully operated in the sense

that it has produced the usual "peaks" for melting samples, but we have not yet obtained data which would establish precision, accuracy,

actual operating limits on temperatures, eta.
96 DTA

REFERENCES

[1] Goldfinger, P., A Definition of High Temperature Chemistry,

"Advamces in High Temperature Chemxistry," Eyring, L., Ed.,

Vol. 1, Academic Press, New York (1967).

[2] Garn, P. D., "Thermoanaly tical Methods of Investigation,"

Academic Press, New York (I965).

[3] Wendlandt, W. W., "Thermal Methods of Analysis," Inters cienceJohn Wiley and Sons, New York (1965).

[4] Smothers, W. J., and Chiang, Y., "Handbook of Differential

Thermal Analysis," Chemical Publishing Co., Inc.,

New York (1965 )

[5] Mackenzie R. C, Ed., "Differential Thermal Anlaysis,"

Vol. 1 and 2, Academic Press, New York (1969.).

[6] Searcy, A. W., in "Proceedings of an International Symposium

on High Temperature Technology," Asilomar, California,

October, 1959, p. 336, McGraw-Hill, New York (1969).

[7] Asamoto, R. R., and Novak, P. E., Rev. Sci. Instr. 33_, 104?

(1967); 39, 1233 (1968).

[8] Rupert, G. N., Rev. Sci. Instr. 3^, 11«3 (I963).

[9] Rupert, G. N., Rev. Sci. Instr. 36, l629 (1965).

[10] Heetderks, H. D., Rudy, E., and Eckert, T., Planseeber.

Pulvermet. 13_, 105 (1965 ). [11] Aerometrics Div., Aerojet-General Corp ., San Ramon, California.

[12] Baldwin, N. L., Danger, S., Kester, F. D., and Hancock, C., Rev. Sci. Instr. 4l, 200 (1970).

[13] O'Neill, M. J., Anal. Chem. 36_, I238 (1964).

[14] Watson, L. S., O'Neill, M. J., Justin, J., and Brenner, N.,

Anal. Chem. 36_, 1233 (1964).

[15] Speros, D. M., and Woodhouse, R. L., J. Phys . Chem. 67,

2164 (1963).

[16] Boersma, S. L., J. Am. Ceram. Soc. 38_, 281 (1955).

[17] Levy, P. F., Amer. Laboratory, p. 46, January 1970.

[18] Douglas, T. B., and King, E. G., "High Temperature Drop

Calorimetry , " Chapt . 8, "Experimental Thermodynamics,"

McCullough, J. P., and Scott, D. W., Eds., Vol. 1,

Plenum Press, New York, (I968).
Freeman 97

[19] West, E. D., and Westrum, E. F., Jr., "Adiabatic Calorimetry

from 300 to 800 K," Chapt . 9, "Experimental Thermodynamics," McCullough, J. P., and Scott, D. W., Eds. Vol. 1,

Plenum Press, New York, (1968).

[20] Backhurst, I., J. Iron Steel Inst. (London) I89 124 (1958).

[21] Bench, W. A., Trans. Faraday Soc . 1279 (I963).

[22] Bockris, J. O'M., White, J. L., and MacKenzie, J. D., Editors,

"Physico-chemical Measurements at High Temperatures."

Butterworths , London (1959).

[23] Kubachewski, 0., Evans, E. LI. and Alcock, C. B.,

"Metallurgical Thermochemistry," 4th Ed., Pergamon

Press, London (I967).

[24] Eyrlng, L., Ed., "Advances in High Temperature Chemistry,"

Vol. 1, Academic Press, New York (I967).

[25] "Thermodynam.ics , " Vols. I and II, Proceedings of a Symposium, Vienna, July, I965. International Atomic

Energy Agency, Vienna (1966).

[26] Rossini, F. C., Ed., "Experimental Thermochemistry,"

Vol. 1, and Skinner, H. A., Ed., Vol. 2, Inters cienceJohn Wiley and Sons, New York (1956 and I962).

[27] McCullough, J. P., and Scott, D. W., Eds., "Experimental

Thermodynamics," Vol. 1, Plenum Press, New York, I968.

[28] "Thermodynamics," Proceedings of a Symposium, PritzensWattens, Austria, 1959. Butterworths, London (196I)

(Reprinted from Pure and Applied Chemistry 2_. Nos . 1-2).

[29] "Thermodynamics," Proceedings of a Symposium, Vienna,

July 1965, p. 89-93, International Atomic Energy Agency,

Vienna (1966).

[30] Calvet, E. and Prat, H., "Recent Progress in Microcalorimetry , " (translated by H. A. Skinner), Macmillan

Co . , New York (1963 ) . [31] Wittig, F. E., "Calorimetric Methods at High Temperatures,"

in Proceedings of a Symposium, Fritzens-Wat tens

,

Austria, 1959. Butterworths, London (I96I) (Reprinted

from Pure and Applied Chemistry 2_ Nos. 1-2, p. I83.)

[32] Kollie, T. G,, Rev. Sci . Instr. 38_, 1^52 (I967).

[33] Cezairliyan, K. A., Morse, M. S., Berman, H. A., and

Beckett, C. W., J. Res. NBS 74A» ^5 (1970).
98 DTA

[34] Beckett, C. W., and Cezairliyan, K. A., "High-speed

Thermodynamic Measurements and Related Techniques,"

Chap. Ik "Experimental Thermodynamics," McCullough, J. P and Scott, D. W., Eds., Vol. 1, Plennum Press, New

York (1967).

[35] Margrave, J. L., Ed. "Characterization of High Temperature Vapors," Chapt . 2-7, John Wiley and Sons, Inc.,

New York (I967)

[36] Gwinup, P. D=, Ph.D. Thesis, Oklahoma State University,

Stillwater (1967).

[37] Gray, A. P., "Analytical Calorimetry , " in Proceedings of

American Chemical Society Symposium, San Francisco,

April, 1968, Porter, R. S. and Johnson, J. F., Eds.,

p. 209, Plenum Press, New York (1968).

[38] Wilhoit, R. C, J. Chem. Educ . 4_4, A571, A629 , A685,

A853 (1967).

[39] Clarebrough, L. M., Hargreaves, M. E., Michell, D.,

West, G. W., Proceedings of the Royal Society, London,

A215, 507 (1952).

[40] Bonjour, E., "Thermal Analysis 1965," Proceedings of First

International Conference on Thermal Analysis, Redferen,

J. D., Ed., Aberdeen, Scotland, p. 56, Macmillan and

Company, Ltd., London, England (I966).

[41] Gitlin, R. M., Analog Dialogue 1, (I967). Published by

Analog Devices, Inc., Cambridge, Massachusetts.
THERMAL STUDIES ON LIPID-WATER SYSTEMS BY DIFFERENTIAL

SCANNING CALORIMETRY WITH REFERENCE TO ATHEROSCLEROSIS

G. Jerry Davis and Roger S. Porter

Folymev Soienoe and Engineering

University of Mas s aohus etts Amhersty Massachusetts 01002

Studies have been performed on liqid-water

systems by differential scanning c.alorlmetry

Three-component systems were investigated. The

components consisted of a phospholipid, cholesterol or one of its esters, and water. The

phospholipids studied included lecithin and

sphingomyelin. The cholesteryl derivatives used

in the ternary systems included cholesterol and

its oleate, linoleate, and stearate esters. The

temperatures and heats of transition for these

systems were investigated. The results reveal

considerable differences in the stability of

complexes formed in these ternary systems with

the unsaturated esters generally leading to the

least stable structures. The morphologies likely

represent compositions and order similar to the

layered structures observed for the same and re- lated phospholipid-cholesteryl derivative molecular organization. The results have implications

concerning the nature of biological membranes and

the deposition of atherosclerotic plaques.

Key words: atherosclerosis; differential scanning

calorimetry; cholesterol and its fatty acid esters;

lecithin; lipid-water systems; phospholipids;

sphingomyelin; thermal studies.

1. INTRODUCTION

Thermal studies on lipid-water systems provide information

which have implications in biological membrane organization in

connection with atherosclerosis. The origin of lipids in atherosclerotic lesions is not known but its possible association with

abnormal plasma lipids has been suggested [1]. Plasma lipids are,

99
100 DTA Llpld^Water Systems

for the most part, a mixture of triglycerides, cholesterol and its

fatty acid esters, and phospholipids, of which the main components

are lecithin and sphingomyelin [1]. These circulate as lipoproteins

which can be separated (by electrophoresis, ultracentrifugation , or chemical fractionation) into two main divisions — the alpha or high

density lipoproteins and the beta or low density lipoproteins . It is the latter group which may be associated with Increased deposits

in clinical atherosclerosis [1].

If the lipids in atherosclerotic plaques are compared with

those in different classes of low density lipoproteins, their com- position is found to be closest to the 0-12 or true-6-llpoprotein [2]. The S^, 0-12 lipoprotein contains 15 to 20 percent

protein [2]. The lipid content of 0-12 lipoprotein has been

analyzed to be in terms of percentages of total lipid, 58.2 percent

cholesteryl ester, 20 percent phospholipid, and 10.2 percent triglyceride [3]- The most abundant fatty acids of the cholesteryl esters

are, in percentage of total fatty acid, oleic (l8:l) at 2^.1 percent

and linoleic (l8:2) at 46.8 percent £3].

Lipoproteins are complexes of lipid and protein that have the

solubility characteristics of protein, i.e. are soluble in water or aqueous salt [4], The lipid links together with the protein, in

most instances, by the association of the non-polar regions of the

lipid molecules with similar regions of the protein molecule, such

as the hydrocarbon tails of the fatty acid moieties of lipids with

the hydrophobic amino acid side chains of the protein [4]. However,

many lipoprotein fractions contain far too much lipid for direct

association of all non-polar regions of lipid residues with non- polar regions of protein [4]. In such cases, the lipoprotein structure must also Involve lipid-lipid interaction. It is hypothesized

here that 0-12 3-lipoprotein is an example of a lipoprotein

which has lipid-lipid interaction.

Lipoproteins can supply the major constituents for mammal membranes, which are composed of lipids and proteins [5]. Carbohydrates

and nucleic acids are minor components at most of membranes [5].
Davis and Porter 101

Electron microscopic observation siaggests that many different kinds

of membranes have similar molecular structures [6], With adequate

resolution, the membranes shov7 a trilaminar structure consisting of

two dense outer laminae separated by a faint interface [6]. Model

studies on phospholipid-water systems of low water content indicate

that the darkly stained laminae are charged and the faint inner layer

is the hydrocarbon region [?]. A trilaminar structure v/hich v/ould be

hydrophilic on the membrane exteriors and hydrophobic internally could

be formed by protein alone, lipoprotein or lipid alone.

Given that the 3-llpoproteins contain excess lipids v/hich interact with each other, these lipids have the capacity to form membranes.

Systems of phospholipid-water [8] and phospholipid-cholesterol-v/ater

[9] have been shown to form into discrete bimolecular layers of lipid

in which the polar regions of each phospholipid layer face outward into a water layer and the hydrocarbon chains "fill" the interior of the

"sandwich." These bimolecular structures would fulfill the trilaminae

requirements of the electron microscopic observations [71-

The experimentally observed phospholipid, dipalmitoy 1-L-alecithin, organizes, in the presence of an equivalent v/eight of

water, into bimolecular layers separated by water, with the hydrophilic groups on the surface separating the lipid and v/ater layers

[9]. This ordering can be studied thermally on a differential

scanning calorimeter (DSC) by the observation of a measurable endothermic transition at a temperature of 4l °C on heating [9]. The

addition of cholesterol to dipalmitoy 1-L-a-lecithin-water systems

has been shown to reduce the heat of transition until at a mole

ratio of 1:1 cholesterol to lecithin, no transition was observed

[9]. The cholesterol appears to become a part of the lecithin

structure, as noted by x-ray examination, up to this 1:1 mole ratio,

but in doing so, causes a reduction in the cohesive forces between

adjacent hydrocarbon chains of the lecithin [9]. These hydrocarbon

chains contract in length [10].

This study is particularly concerned with the effect of cholesteryl esters on phospholipid-water systems. A comparison is made
102 DTA Lipld-Water Systems

with previous systems involving cholesterol. A goal is to see whether this information might help suggest a mechanism by which

lipids accumulate in arteries during development of atherosclerotic

lesions. The idea that the deposits from the plasma accumulate

through membrane formation rather than by direct deposition will be

considered [11]. The phospholipids acquired for these experiments

V7ere dipalmitoy 1-L-a-lecithin , bovine lecithin, and bovine sphingomyelin. Prom a physical-chemical standpoint, sphingomyelin is similar to lecithin, since it has a hydrophobic region of two long-chain

fatty acyl groups and a highly polar region consisting of a phosphoryl

choline zwitterion as does lecithin [12]. The cholestery ester series

of the C-^Q fatty acids, stearic (l8:0), oleic (l8:l), and linoleic

(l8:2) was chosen to be used in conjunction with the phospholipids

because of the presence in 0-12 3-lipoprotein of large quantities

of cholesteryl oleate and linoleate

2. EXPERIMENTAL PROCEDURE

The dipalmitoyl-L-a-lecithin , bovine lecithin, bovine sphingomyelin, cholesterol, and cholesteryl esters of stearic, oleic, and

linoleic acid used in this study are 99 percent pure according to the supplier Applied Science Laboratories, State College, Pennsylvania. The phospholipids and cholesterlc compounds were weighed

directly into the liquid sample pans provided for DSC testing. A six place Mettler balance weighed the samples to 0.001 mg. Spectroscopic grade benzene was added to the pan to dissolve and mix the

phospholipid and cholesterlc compound in situ. The benzene was evaporated under vacuum at 35 °C. The sample was rewelghed as a function

of time to insure that all benzene was removed. Distilled water in

the amount of twice the phospholipid sample weight was added into the

planchet by syringe and the sample lid clamped on. The space between

the crimped sample lid and the pan was sealed with Eastman 910 adhesive. Each sample contained one phospholipid and one cholesterlc

compound. The sealed planchet containing the sample was placed in an oven at a temperature of 70 °C for one hour before testing. This temperature is 25 °C above that necessary to disperse the phospholipid in

the water [8,9]- The temperature choice also Insured that the choles-
Davis and Porter 103

teryl esters would not pass through their major transition between

27 and 57 °C. The phospholipids used in this study have crystalline

melting points well above 70 °C [13].

Transitions within the samples observed on heating (all endothermic) and on cooling (all exothermic) v/ere measured on a PerkinElmer Differential Scanning Calorimeter, DSC-IB. The calibration

of the DSC for temperatures and heats of transition have been described elsewhere [l4]. Equilibrium conditions for the samples were demonstrated by repeating the heating and cooling cycle three times over a period of one hour with observation of effectively the same heats and temperatures of transition in the first and third cycle..

The samples were heated and cooled at a constant rate of 2.5 °C per

minute. The ordinate scale was on the maximum sensitivity of one calorie per second. An accuracy in the heat of transition determination

of ± 5 percent for the dipalmitoyl-L-a-lecithin and ± 10 percent for

the bovine phospholipids is estimated. The precision in temperature

measurements is ± 0.5 °C for the dipalmitoyl-L-a-lecithin and ± 1.5

percent for the bovine phospholipids, natural materials containing a mixture of compounds

3. RESULTS

A. Dipalmitoyl-L-a-lecithin

The temperatures and heats of transition for the dipalmitoylL-a-lecithin alone in water and with various mole fractions of cholesterol or cholesteryl ester in water, both in heating (endotherm) and

cooling (exotherm), are presented in Table 1. The entropy of transi-'

tion per mole of lecithin for the heating and cooling of each sample

has been calculated from its respective transition energy and temperature. The transition energies were relatively constant over several

cycles of heating and cooling the same sample. This process is con- sidered to be entirely reversible, since measured energy on heating

and cooling was, within the accuracy of the experimental technique,

the same. For this reason, the entropy values from the heating and

cooling of each sample were averaged. This average entropy value is
104 DTA Llpld-Water Systems

Table 1. Thermal transitions of dipalmitoyl-L-a -lecithin-water

systems containing cholesteric compounds.

Mole fraction Transi- Measured AH AS ASOTT "•^av cholesteric tion Tempo °C Cal/g. Cal/K- Cal/Kcompound lecithin mole miole

(dry basis) lecithin lecithin

None Heating 43o2 9.36 21o7

Cooling 41.5 9.78 22.8 22.3

Cholesterol^

0 o 092 Heat ing 42.7 8.68 20 0

Cooling 41.4 80IO 18.9 19.5

Heat ing 42.2 6.31 14 »

Cooling 40.8 5o6l 13.1 13.9

0.320 Heat ing 42.4 3.14 7 .

Cooling 41.2 2o56 60O 6.7

0 . 4o4 H e a t i dp* 41.8 1.68 3 Q Cooling 40.9 1.64 3.8 3.9

Cholesterol

Stearate

0.096 Heat ing 43o2 9.15 21 .

Cooling 41.3 9.40 21„9 21.6

0 0 201 H p P t i DP" 42.6 9.36 21 08 Cooling 41.2 8.91 20.8 21.3

0 ^2Q Hp3 t i n p" 42o3 9ol5 21 3 Cooling 40.7 9.21 21.5 21.4

0.484 liC-CX U Xil^ 42.4 8.86 20 7 Cooling 41.0 8.88 20.7 20.7

Choi p^tp"Pnl

Oleate

n n v n U 0 U (J Heating 42.8 9.59

Cooling 41.2 9.00 21.0 21.6

Ocl37 Heating 43.5 8.67 20.2

Cooling 41.3 8.43 19.7 20 .0 0.300 Heating 42.5 9o70 22„6

Cooling 41.0 8cl2 19.0 20.8

0.366 Heating 42.7 80I6 19cO

Cooling 41.0 7.90 18.7 l8o9

0o477 Heating 42.6 8.64 20.1

Cooling 40.9 7.98 18.7 19.4
Davis and Porter 105

Table 1. (continued)

Mole fraction

cholesterlc

compound

(dry basis)

TransitionMeasured AH Temp. °C Cal/g.

lecithin

AS Cal/Kmole

lecithin

AS Cal/Kmole

lecithin

Cholesterol

Linoleate

0.108 Heating 43.1 8o98 20.8

Cooling 41.4 8.36 19.5 20.4

0.244 Heating 43.2 8.10 17.7

Cooling 41.2 6.50 15.2 l6o4

Oo401 Heating 42.5 7.48 17.4

Cooling 40.4 6.23 l4o6 16.0

Oo500 Heating 42.8 7.41 17c 2 Cooling 40.8 6.24 14.6 15.9

^At a mole fraction of 0.500 for cholesterol, no transition was detected on the DSC.

shown in the final column of Table 1. These average transition en- tropies per mole of lecithin are plotted as a function of the mole

fraction of the cholesteric compound present in the sample, on a water-free (dry) basis, in Figure 1. A representative DSC cooling

curve for each cholesteric compound used in the dipalmitoy 1-L-a-lecithin-water system fraction is shown to scale in Figure 2. The cholesteric compound was used in concentrations between 0.32 and 0.40 mole.

A rapid reduction in the transition heat with the addition of

cholesterol to the dipalmitoyl-L-a-lecithin-water system is observed

in Figure 1. This is similar to the results obtained by Ladbrooke,

Williams, and Chapman [9] for the dipalmitoy 1-L-a-lecithin-cholesterol-water system, though the heat of transition values from this

study are about 15 percent lower than those of these investigators.

The temperature of transition changed less than a degree over the

entire range of compositions tested. The transition on heating

occurred at a temperature about 1.5 °C higher than that on cooling.
106 DTA Llpid-Water Systems

0 0.1 0.2 0.3 0.4 0.5 0,6 MOLE FRACTION CHOLESTERIC COMPOUND (DRY BASIS)

Figure 1

(Dipalmltoy 1 )-L-a-lecithin cholesteric compound

transition entropies (in water)
Davis and Porter 107

Ladbrooke, Williams, and Chapman [9] reported about 10 °C temperature decline with increasing cholesterol content. Two possible

reasons for this large temperature effect are a wider curve spread

with increasing cholesterol content than experienced by the present

investigators, and a different method of temperature calculation.

In this study, the peak temperature, corrected for difference between sample temperature and program temperature, the method recom- mended by Perkin-Elmer , the manufacturer [15], is reported, instead

of the temperature the DSC trace leaves the baseline. The peak temperature is the most significant and reproducible value. This observation is supported by Figure 2 which shows that all the transitions

have the same heat temperatures but widely varying ranges. An extensive analysis of DSC curve behavior for endothermic transitions at a heating rate of 2.5 °C/min. is presented in a complete and separate

study by these investigators [l6].

In contrast to cholesterol, cholesteryl stearate has remarkably

little effect either on the entropy or temperature of transition,

even at 0.50 mole fraction or a 1:1 mole ratio of ester to lecithin.

A plot of transition entropy per mole of lecithin vs. mole fraction

of cholesteryl stearate in Figure 1 indicates that perhaps a 5 percent drop in entropy occurred at a 1:1 mole ratio.

Cholesteryl oleate also has a small effect on transition en- tropy of the system, but somewhat more than the lecithin-cholestery

stearate-water systems. The transition entropy decreased about 15 percent at a 1:1 mole ratio. Cholesteryl linoleate has a greater

effect, reducing the transition entropy about 30 percent at a mole

fraction of 0.35 of a 1:2 mole ratio. The presence of either cholesteryl oleate or linoleate has little effect on transition temperature.

B. Bovine Phospholipids

No thermal transition could be detected by the DSC for the

bovine lecithin-water system between the temperatures of 5 and

55 °C. However, the bovine sphingomyelin-water system exhibited

a thermal transition about 75 percent as large in energy as the
108 DTA Llpld-Water Systems

I0 0.1 0.2 0.3 0.4 0.5 MOLE FRACTION CHOLESTERIC COMPOUND {DRY BASIS)

Figure 2. Sphingomyelin ( bovine ) -cholest eric compound transition entropies (in water).

dipalmitoy 1-L-a-lecithin-water system and at the same transition

temperature. This is not a crystalline-liquid transition, since

sphingomyelin melts at a temperature considerably higher than

70 °C [13], but may be similar to the type of organization exhibited by the lecithin-water system [9,10]. The breadth of the

trace extended over a temperature span of 12 °C, about twice that
Davis and Porter 109

of the dipalmitoy l-L~a-lecithin , reflecting the difference between

a natural and a single synthetic compound.

The above result is not unexpected since the bovine lecithin

has two adjustable acyl groups and the bovine sphingomyelin, only

one. Sphingomyelin usually has as one of its two acyl groups, the

hydrocarbon chain provided by sphingosine, though some sphingomyelin

contains dihydrosphingosine rather than sphingosine [5]- With two

variable groups, bovine lecithin is likely to contain many more different molecules which have a broader range of physical properties

than bovine sphingomyelin. These many different molecules in bovine

lecithin could spread the transition energy over such a broad temperature range that the transition could go undetected by the DSC.

However, the possibility exists that the transition temperature for

this material is lower than + 5 °C.

The temperatures and heats of transition for bovine sphingomyelin alone in water and with various mole fractions of cholesterol or cholesteryl esters in water, both in heating (endotherm)

and cooling (exotherm) are presented in Table 2. Given that the

temperature range for all samples tested, either on heating or cooling, were separated by less than 3 °C, the actual temperature

variation for these systems (see Table 2) should be considered as small as that of the dipalmitoy 1-L-a-lecithin systems. The molecular weight of sphingomyelin was estimated at 770 from data supplied by Applied Science Laboratories. Bovine sphingomyelin is

particularly rich in the saturated C^^ amides though about ten

percent of the adjustable acyl group are the saturated C.^^^ and

C-j^g amides.

The average entropies of transitions of the sphinogomyelincholesteric compound-water systems have been calculated in the

same manner as the comparable dipalmitoy 1-L-a-lecithin systems.

These values are shown in the final column of Table 2. These

average transition entropies per mole of sphingomyelin are plotted

as a function of the mole fraction of cholesteric compound present

in the sample (on a dry basis) in Figure 3 (see page 112).
DTA Lipid-Water Systems

Table 2, Thermal transitions of sphingomyelin (bovine) -water

systems containing cholesterlc compounds

Mole fraction Transi- Measured AH AS ASav

cholesterlc tion Temp. °C Cal/

g

Cal/K- Cal/Kcompound RDhi neT)— mole '.lole

( (i "pv h a i ^ ^ myelin c!ID TilnI T-L 1n D" n 1^ myelin myelin

None Heating 43 0 0 7 0 36 17.9

Cooling 4l 7 10 17o4 17 .7 Cholesterol

0.088 Heating 41.4 5.76 14.2

Cooling 39 o7 0.14 15.2 14.7

0.160 Heating 40o5 4.82 11.8

Cooling 39 0 4 b . 1

12.6 12.2

0.250 Heating 40.4 3.42 8.5

Cooling 39.4 4.16 10.3 9-. 4 0.356 Heating 40o3 1.72 4.3

Cooling 3o 0 9 2 0 4 5 60O 5.2

Cholesterol

Stearate

0.080 Heating 41.9 7o54 18.4

Cooling 4 0.4 0 . 5o 16.2 17 0 3 O0I50 Heating 41.9 6.73 16.5

Cooling 39.9 0 0 0 j 16.8 16.7

0.250 Heating 42.9 7.27 17.7

Cooling 41 oO 0 .53 I60O 16.8

0.455 Heating 41.8 6.96 17o0

Cooling 39o6 7.10 17.4 17o2

Cholesterol

Oleate

0.091 Heating 42.7 6 .30 15o6

Cooling 40o5 7.00 17.4 16.5

0.164 Heating 42.0 5.90 14.7

Cooling 40.3 5.80 14.5 14.6

0o272 Heating 42o0 5.66 l4ol

Cooling 40.8 5.41 13c4 13.8

0.390 Heating 41.0 5.19 12.9

Cooling 39.9 6.03 14.9 13.9
Davis and Porter 111

Table 2. Continued

Mole fraction Transi- Measured AH AS ^^av cholesterlc tion Temp. °C Cal/g Cal/K- Cal/Kcompound sphingo- mole mole

(dry basis) myelin sphingo- sphingomyelin myelin

Cholesterol

Linoleate

00091 Heating 42.0 6.46 15o8

Cooling 41.2 5.66 13o9 14 .9 0.166 Heating 43o6 4.97 12a

Cooling 4lo9 4.79 11.7 llo9

0.270 Heating 42.9 4.35 10.6

Cooling 4l„7 5.19 12.7 11.6

Oo4l7 Heating 43.7 3.76 9.2

Cooling 42.7 3.62 8o9 9o0

The results displayed in Figures 1 and 2 indicate that cholesterol reduces the transition entropy of bovine sphingomyelin and

dipalmitoy 1-L-a-lecithin in water sharply; moreover, the transition

entropy change with cholesterol concentration is quite similar in

the two systems. In contrast, cholesteryl stearate has no ability

to change the entropy of transition of either phospholipid. Cholesteryl oleate and linoleate are somewhat more effective in reducing

the entropy of transition of bovine sphingomyelin with increasing

mole fraction than with dipalmitoy 1-L-a-lecithin . The most important observation, though, is that the curves in Figures 1 and 2 for

each cholesterlc compound are so similarly positioned.
112- DTA Lipid-Water Systems

0.1

0.320 MOLE FRACTION CHOLESTEROL

J \ \ L J \ \ \ L

0.1 —

0.401 MOLE FRACTION CHOLESTERYL LINOLEATE

0.366 MOLE FRACTION CHOLESTERYL OLEATE

0.329 MOLE FRACTION CHOLESTERYL STEARATE

0.2 —

0.1

310 315 TEMPERATURE (K) 320

Figure 3. DSC cooling curves of dlpalmlt oyl-L-a-lecit hln-wat er systems containing cholesterlc compounds.

(Scaled to 1.000 mg Lecithin; Cooling Rate 2.5 °C/miln)
Davis and Porter 113

^. DISCUSSION OP RESULTS

A. Significance to Membrane Formation.

When dipalmitoyl-L-a -lecithin and bovine sphingomyelin organize

structurally in water on cooling to 4l °C, heat must be removed from

the sample causing a reduction in the entropy of the system. The

magnitude of the entropy loss is a measure of the stability of the

system, since the same amount of entropy must be returned to the

sample on heating to disrupt the organization. As cholesterol is

added to the lecithin or sphingomyelin system, the entropy of transition is reduced until at measurements at a 1:1 cholesterol to phospholipid ratio, no energy of transition is recorded by the DSC.

The above information would suggest that in the limit of a 1:1

mole ratio, a cholesterol-lecithin or cholest erol-sphingomyelin

structure is quite unstable. However, unpublished studies by D. M. Small [17.1 indicate that the 1:1 structure is stable up to 100 °C.

Organization of the phospholipid structure did not increase the overall stability of the system measured. But for the cholesterol to participate in the phospholipid structure, as described by other investigators [9jlO], the crystal structure of the cholesterol must be

disrupted. The heat absorbed by the breakdown of the crystal structure would offset the heat given off in the organization of the

water molecules and acyl groups of the phospholipid. (Water molecules

in contact with acyl chains cannot co-hydrogen bond and acyl chains

separated by water cannot join with one another by van der Waal's

forces. At a 1:1 mole ratio of cholesterol to phospholipid, the sum

of these heat changes is zero. This fact could indicate a reversible transition between crystalline cholesterol with unorganized

phospholipid and the cholest erol-phospholipid membrane-like

structure

In contrast to the behavior of cholesterol, cholesteryl stearate has little measurable effect on the entropic change that occurs

as lecithin and sphingomyelin in water are cooled through the transition temperature. The same heat is given off, within five percent,

whether the cholesteryl stearate is present or not.
114 DTA Lipid-Water Systems

The equilibrium temperature of 70 °C applied to all samples

causes the cholesteryl oleate and linoleate to melt. Once melted at 70 °C, the cholesteryl oleate and linoleate will not revert

to crystalline form at temperatures between 27 and 57 °C. The

cholesteryl oleate and linoleate can pass through their mesophase

transitions in the 27 to 57 °C range, but these heats are, at the

maximum mole fraction of sample present, no more than 4 to 6 percent of the heat reported for their ternary transition. The

shape of the cholesteryl oleate ternary transition in Figure 2 at

46 °C does suggest the isotropic liquid to cholesteric transition

expected at this temperature from the previous data of the present

investigators. Since cholesteryl oleate and linoleate cannot be

in crystalline form at 27 to 57 °C, a change in the entropy of the

system can be directly related to the stability of the phospholipid

structure formed.

There is always present the biological possibility of phospholipids (in the blood plasma) organizing to form membranes. The

sphingolipids have been reported to be major lipid constituents

of most mammalian cell membranes [l8J. Sphingomyelin even when

composed of mixtures of molecules containing different acyl groups,

can organize at temperatures close to human biological environment.

It may be significant that the only sphingolipid in erythrocyte

plasma membrane in appreciable percentage is sphingomyelin [5]. In contrast, lecithin containing unsaturated bonds may have liquid

crystalline to crystalline transitions in water many degrees below

the biological environment [19].

S^ 0-12 lipoprotein, in which lipid-lipid interaction is likely,

contains substantial quantities of cholesteryl oleate and linoleate.

A shift in the ratio of these two esters could have a significant

effect on the stability of the structure formed with phospholipids,

in particular sphingomyelin. Should the ratio of cholesteryl linoleate to oleate decline, structures of greater stability may form.

The saturated or monounsaturated C-^^g esters of cholesterol, would

most likely be excluded from the membrane structure owing to the

length of their acyl chain. The diunsaturated fatty esters of choles-
Davis and Porter 115

terolj the linoleate, could participate in the formation of the

membrane- like structure, imparting to the structure a relatively high

thermal instability. This could occur because, as has been reported

[10] J the amount of unsaturation in the fatty acid is important in

determining its molecular shape. For instance, saturated and transsaturated fatty acids are essentially straight, but at cis double

bonds, the chain is bent. The higher the unsaturation, the more

bent a fatty acid chain can be; in arachidonic acid, the molecule

can be curved into a U [20]. The magnitude of the entropy decrease

during the organization of the sphingomyelin or dipalmitoy-l-L-alecithin by esters of cholesterol is likely inversely proportional

to their ability to participate in the membrane structure and can

be expected to be, in order of decreasing effectiveness: linoleate

> oleate > stearate. Or, in other words, cholesteryl esters are not incorporated into the lecithin structure as readily as cholesterol, the solubilities being in the order stearate < oleate < linoleate. , .

5. SIGNIFICANCE TO ATHEROSCLEROSIS

E. B. Smith et at. [3] have been able to distinguish between

perifibrous lipid of the human aorta intima that thickens with

age from abnormal fat-filled cell thickening. Particularly noticeable in comparing the chemical composition of these two types of

thickening, for the under 30, 30-40, and 40-49 age groups, is the

decrease in the percent cholesteryl linoleate and increase in the

percent cholesteryl oleate in the fat-filled membranes or cells

compared to perifibrous lipid [3]. An explanation may lie in the

potential ability of cholesteryl linoleate to inhibit the formation

of stable phospholipid structures from the plasma. The presence of

increased cholesteryl oleate could accelerate the slower natural

structure formation process in the presence of cholesteryl linoleate. The cholesteryl oleate, itself, could be enclosed inside

the forming structure.
116 DTA Llpid-Water Systems

E. Smith et al . [21] in a more recent technical article

describe an amorphous lipid structure in the human aorta which is

extremely high in cholesterol. It is conjectured here that this

amorphous behavior may be due to the presence of the cholesterol

causing the phospholipid to change between ordered and disordered

phospholipid structure. In this same article by E. B. Smith et al

[21], has also provided a table which shows the percent distribution of the main phospholipid fractions in fatty plaques, fibrous

plaques, and amorphous lipid. The percentage of sphingomyelin in

these plaques varies from 30 to 80 percent of the total phospholipid. This percentage is remarkably high, since only from 10 to 25 percent of the lipid phosphorous in plasma is in the form

of sphingomyelin [12]. It is suggested here that the reason for

this difference may lie in the ability of sphingomyelin to form

membrane structure at biological environmental temperatures

The authors wish to express their appreciation to the National

Institutes of Health, HEW 113^2, for support of this work.

[1] Smith, E, B., Lancet 1, 799 (I960).

[2] Smith, E. B., Lancet 2, 530 (1962).

[3] Smith, E. B., Evans, P. H., and Downham, M. D., J. Atheroscler, Res., 7, 171 (1967).

[4] Masoro, E. J., "Physiological Chemistry of Lipids in Mammals,"

p. 179, W. B. Saunders Company, Philadelphia, Pa., 1969-

[5] Masaro, E. J., "Physiological Chemistry of Lipids in Mammals,"

P. 276, W. B. Saunders Company, Philadelphia, Pa., 1969.

[6] Toner P. G., and Carr, K. E., "Cell Structure," p. 5, E. and S. Livingston, Ltd., London, England, I968.

[7] Stein, W. E., "The Movement of Molecules Across Cell Membranes,"

Chapter 1, Academic Press, New York, New York, I968.

6. ACKNOWLEDGEMENT

7. REFERENCES
Davis and Porter 117

[8] Chapman, D., Williams^ R. M. , and Ladbrooke, R. D., Chem.

Phys. Lipids, 1, 445 (196?).

[9] Ladbrooke, B. D., Williams, and Chapman, D., Biochim.

Biophys. Acta, 150_, 333 (1968).

[10] Small, D. M., Bourges, M., and Dervichian, D. C, Nature

211 , 8l6 (1966).

[11] Smith, E. B., Cardiovascular Res., 1, 111 (1967).

[12] Masoro, E. J., "Physiological Chemistry of Lipids in Mammals,"

p. 161, W. B. Saunders Company, Philadelphia, Pa.. 1969.

[13] Dervichian, D. C., Progress in Biophysics and Molecular

Biology, 1_4 , 249 (1964).

[14] Davis, G. J., and Porter, R. S., "Thermal Transitions for

Some Cholesteryl Esters of Saturated Aliphatic Acids,"

to be published in Molecular Crystals and Liquid Crystals

[15] Perkin-Elmer Corporation, Thermal Analysis Newsletter, 5_, Norwalk, Conn. - [16] Davis, G. J., Porter, R. S., "Application of the Differential

Scanning Calorimeter to Purity Measurements," accepted for

publication in Journal of Thermal Analysis.

[17] Personal communication by D. M. Small.

[18] Brady, R. 0., "The Metabolism of Sphingomyelin," in "Membrane

Models and the Formation of Biological Membranes," Bolis, L.,

and Pethica, B. A., Editors, p. Il4, North Holland Publishing

Company, 1968.

[19] Chapman, D., "Physical Studies of Biological Membranes and

Their Constituents," in Membrane Models and the Formation

of Formation of Biological Membranes, Bolis, L., and

Pethica, B. A., Editors, p. 6., North Holland Publishing

Company, I968.

[20] Sinclair, H. M., "Lipid Pharmacology," Chapter 5, Paoletti, R.,

Editor, Academic Press, New York, New York, 1964.

[21] Smith, E. B., Slater, R. S., and Chu, P. K., J. Atheroscler.

Res. , 8 , 399 (1968) .

AN ANALYTICAL EVALUATION OP DIFFERENTIAL

SCANNING CALORIMETRY (DSC)

Joseph H. Flynn

Institute for Materials Research

National Bureau of Standards

Washington^ D. C. 20234

Three instrumental time constants were found in an analysis of the ( Perkin-Elmer DSC-IB) Differential

Scanning Calorimeter: an energy pulse response (1.5 s),

a temperature-averaging network response ('^>15 s) and a temperature programming response (8.0 s). The first con- stant may be determined from the decay of the response to infrared light pulses. It affects the shape of a transition curve but not the heat of transition calculated from

the area. The second constant affects only the base line

and the third, the temperature calibration.

Application of elementary heat flow theory to a wide

range of experimental variations in the interfacial thermal resistance demonstrated that interfacial time constants

are a negligible factor in determining the melting transitions of many substances.

It was concluded that corrections for the various

instrumental and thermal time constants are necessary

if meaningful parameters are to be deduced, especially

at fast heating rates . Key words: analyses of thermal apparatus; differential

scanning calorimetry ; evaluation of heats and temperatures of transition; instrumental time constants; interfacial thermal conductivity; thermal analyses.

I . INTRODUCTION

Differential Scanning Calorimetry (DSC) is a technique from

which a large quantity of experimental data on heat capacities,

heats and temperatures of transitions, reaction kinetics, etc., may

be obtained with an extremely small amount of effort. Therefore,

119
120 An Analytical Evaluation of DSC

there exists a great need to assess critically the quality of the

constantly growing mass of data resulting from its exploitation.

This paper summarizes some of the experimental and theoretical

work in progress on the evaluation of the differential scanning calorimetry technique. The results reported here apply specifically to the Perkin-Elmer Differential Scanning Calorimetry, (DSC-IB)

The instrument is described in literature [1,2] so only a brief

summary is given here. Nearly identical copper sample and reference

holder cups contain heaters and platinum resistance thermometers.

These holders are maintained, even at constant rates of tem.perature

change, close to the same temperature by a temperature averaging network. The difference in power supplied to the two cups, one empty

and one containing a sample, is a direct measure of the change in

enthalpy of the sample as a function of time. This net enthalpy flow

rate and the average temperature of the two cups are read out on a recorder.

The train by which a net thermal flux is transported from the

sample and translated into a recorder readout can be divided into

three sections:

1. Instrument: the instrumental response between the surface

of the sample holder and the recorder readout includes both the thermal lags between the heater, sensor and surface, and the electronic

time constants of the instrument.

2. Interface: this is the region between the surface of the

sample and the surface of the sample holder. It includes heat flow

through any sample container.

3. Sample: this region is defined by the sample geometry.

The character of the energy flux may be influenced and modified

in each of these regions, and, under separate conditions, any one region may be the slow, rate determining step of the sequence.
Flynn 121

2. INSTRUMENTAL TIME CONSTANTS

It is logical to start with the instrument itself. The re- corder response may be a limiting factor during sharp delta-function-like heat pulses which occur during some transitions such as the fusion of a supercooled liquid. An approximate time constant is

often supplied with the recorder. The Leeds and Northrup "Speedo- max W" used in most of these experiments had a zero order response

time of a tenth of a second per inch.

A. Energy Pulse Response

An inspection of the wiring schematic for the energy readout

indicates a time constant for energy response of about one and one half seconds. Since the holder assembly cover has a transparent

window, this constant can be measured quite simply and unambiguously.

Rectangular energy pulses of varied amplitude and duration were produced by shining a beam of infrared light from a heat lamp through

the transparent window onto either the sample or the reference holder

cup. Three typical readouts for pulses of 1.92^ 7.92 and 15-32

seconds are shown in Figure 1. Both the build-up curves and the decay curves were superimposible. Their shape and amplitude were independent of temperature and rate of heating. The build-up and decay curves closely

followed first order kinetics after an initial transitory period,

and from them were obtained time constants of the order of one and

one half seconds.

The energy pulse time constants for two instruments and three

holder assemblies are given in Table 1.

The values vary slightly for each of the three sample holders

and one reference holder. In several of the cases there is a several tenths of a second difference between the calculated time

constants in the endothermic and exothermic directions.
122 An Analytical Evaluation of DSC

/I rr~\

dq

dt

.92 sec 7.92 sec TIME

15.32 sec

Figure 1. Recorder response to Infrared energy pulses.

Chart speed: 0.0667 in/s || (maximum): 0.0

cal/s (dashed line represents rectangular

energy pulse)

Table 1. Time constants for instrumental response.

TIME CONSTANTS (seconds)

Holder Recorder Zero^ Energy^ Temp. Speed

DSC assembly (x inches) knob pulse ave . selector

A 1 0.1 0.63; 1.12 1.15; 1.65 l4.9 8.0

1.^7; 1.55^

A 2 0.1 - 1.58; 1.72 l8.7

B 3 - 0.1 - 1.55; 1.71 9.3

^Average values for "endo and "exothermic" directions.

These two values are for reference holder; all others

are for sample holders.
Flynn 123

If the rectangular energy pulses build up and decay according

to first order kinetics over the entire time interval with time

constants in the endothermic and exothermic directions, '^g^do

T , respectively, the fractional error, e, in the area of the

exo ^ ^ dq/dt vs t curve is

('^endo~'^exo )

where x is the fraction of the steady state value of the energy

flux attained and t is the duration of the pulse. Since the

enthalpy is calculated from the area, "^^j this difference in

time constants could result in large errors when the pulses are

of short duration.

However, in Figure 2, the integrated areas in arbitrary units

are plotted against the time of the light pulse at constant intensity. The least squares straight line is for times of one and one half through twenty seconds, and is given by

AREA = 0.003075 t - 0.000027 + e' (2)

(t < 20 s) -

where z ' represents a random experimental error with a calculated

standard deviation of .3. -4 x 10~^.

The calculated standard deviations of the slope and intercept

in equation (2) are respectively, 1.44 x lo"^ and 1.25 x 10~^. The concurrence of the intercept with the origin is well within its calculated uncertainty, indicating that the time constant difference

must be compensated for during the transitory period before first

order decay becomes operative. Thus it appears that, when the course

of the base-line can be accurately deduced, quite accurate relative

enthalpy values are attainable with this instrument.
124 An Analytical Evaluation of DSC

20

TIME(sec)

Figure 2. Area vs. time of Infrared light pulse.

(Solid line is least squares fit for 0 > t > 20 s . )

B. Temperature Averaging Network Response

The detailed energy balancing between the sample and reference

holders is upset during a change in the rate of heating or cooling

and a deflection of the baseline occurs. This deflection varies

both in its initial amplitude and in its rate of recovery, as may be seen in the next to last column of Table 1, for the different instru-
Flynn 125

j ments and holder assemblies. For one instrument, the effect appeared

to have a somewhat damped harmonic nature.

For instrument 1, holder assembly A, the decay was kinetically

slightly greater than first order and gave an average first order

time constant of 15 seconds. The initial amplitude of the displacement, , obtained from, the extrapolation of the first order rate

equation back to the time of the heating rate change, was found to be

related to the change in heating rate, A3 (K/s) by the equation*

= 1.67 X 10"3 Ae (cal/K) (3)**

A relationship, such as equation (3) may be determined for each

instrument. This slow baseline decay may then be corrected for.

This tem.perature averaging time constant appears to be com- pletely independent of the small exothermic and endotherm.ic energy

pulses which occur x^hen a sample undergoes a transition. Several

examples are shown in Figure 3

The lower curve is a readout for empty holders for vjhich the

temperature was changed from 320 to 38O K by rapid manual operation

of the temperature control knob. During the decay, infrared light

pulses were alternately shone on the sample and reference holders.

The upper left curve in Figure 3 shows the fusion of Wood's

Metal which was suddenly supercooled to 323 K and held isothermally

at that temperature. The upper right curve is again for Wood's

Metal which was heated quickly to 337 K and then scanned at 2.5

K/min through its melting point.

In each of these three cases the independence of the energy

response from the baseline decay is obvious.

* K = degrees Kelvin

1 cal = 4 .1840 .1 .
126- An Analytical Evaluation of DSC

\

\\\

Figure 3. Three rate of enthalpy change vs. time curves illustrating the independence of "energy pulse"

and "temperature averaging" time constants.

recorder readout; — unperturbed

"temperature averaging" decay.

A. Empty holders; T = 320-380 then

isothermal at 38O K, infrared pulses ^ x 1. B. Fusion of Wood's Metal (in reference holder)

supercooled to 323 x 1/4.

C. Melting of Wood's Metal, heated quickly to 337 K and scanned at 6 = 2 . 5 K/min, x 1/4.
Flynn 127

A correction for the temperature-averaging lag should be

applied to the baseline whenever the heating rate has been changed

v;ithin about two minutes before the temperature range of interest

is reached.

The scan speed selector time constant will be discussed later.

An instantaneous change of the "zero knob" includes only part

of the readout circuit, so its response time, indicated in the

fourth column on Table 1., is somewhat smaller than that for an energy pulse.

3. "INTERFACIAL" EFFECTS

For the problem involving the transport of thermal energy from

or to the sample to or from the surface of the copper sample holder,

some sort of assumption for a mechanism of heat flow is needed in

order to compare parameters under diverse conditions.

The simplest case, utilized by Gray [3] and others, is to ass^ume

that the sample and sample holder have much higher thermal conductivities than the interfacial regions so that the temperature gradient

is confined to the latter region and that, due to the small ratio of

sample mass to holder mass, the holder is unperturbed by thermal

effects taking place in the sample.

A number of model cases are assumed. These are illustrated in

Figures 4 and 5. The curves on the left are idealized plots of temperature vs . time at constant heating rate where T^ is the holder

temperature, T^, the sample temperature, T^, the melting temperature

and T^ the fusion temperature; the right-hand curves represent the

resultant enthalpy flow assuming Newton's Law of Cooling.

In the first set of graphs in Figure 4, a temperature gradient

is set up by heating at a constant rate starting at temperature T^^ and stopping at temperature T^. will approach a constant value

until T^ at which point it will decay to the baseline. If, as in
128 An Analytical Evaluation of DSC

Figure 4. Idealized temperature vs. time and ^ vs. time curves.

T-^ = sample temperature

= holder temperature

T^,Tg= isothermal temperatures

= melting temperature

3 = constant

c = heat capacity
Flynn 129

OOOi./f\/(h

rUGION

NO SUPERCOOLING

V

PARTIAL SUPERCOOLING

COMPLETE SUPERCOOLING

V

TIME

Figure 5- Idealized temperature vs. time and ^-r^ vs. time curves. dt

T-j^ = sample temperature

= holder temperature

T^= fusion temperature

3 = constant
130 An Analytical Evaluation of DSC

the second set of graphs, the heat capacity is a function of temperature, the steady value of ^ will not be parallel to the baseline.

If a melting transition takes place (lower set. Figure ^), the

temperature of the sample remains constant and a steady rise in ^

is approached until the melting is complete.

The first set of graphs in Figure 5 illustrate the inverse situation for f4*9-*€ua.. If partial or complete supercooling takes place,

as in the middle and lower sets of Figure 53 there will be a sudden

jump in followed by the usual decay. In each of the above cases, the decay back to the baseline will depend upon an "interfacial con- ductivity" time constant.

All of these idealized curves approximately duplicate curves obtained with the differential scanning calorimeter for the heating

and cooling of pure elements and compounds.

An actual melting curve for 99.99 percent Indium, encapsulated

in an aluminum sample pan with molten Wood's Metal at the aluminumcopper interface, is shown in Figure 6. The three critical parameters with which we concern ourselves

are shown in this figure. They are:

1. The steady slope of a transition curve caused by a linearly

increasing temperature increment and proportional to an interfacial

conductivity term.

2. An onset temperature of the transition, T^, obtained from

the intersection of this slope with the baseline.

3. A decay constant , k^ , back to the baseline at the completion

of the transition. - •

Some simple relationships for these three parameters may be de- • rived from the following four equations
Plynn 131

Figure 6. Melting of 99.99 indium (molten Wood's Metal interface).

Rate of Enthalpy Change vs , Temperature (Time).

dt = (^2 - ^1)

T^ = T^ + 6t (5)

T = T + — dt, C6) 1 o rac J dt ^

= T^ (during transition)

c = constant or c = a + bt (7)
132 An Analytical Evaluation of DSC

Equation 4, "Newton's Law of Cooling," states that the heat flow across the interface is equal to the interfacial thermal conductivity, h, multiplied by the area of the interface, A, and the

temperature difference between the sample and the holder. At constant heating rate, 3, the temperature of the holder, , will

increase, from an initial value, T^, linearly with tim.e eq . (5).

The temperature of the sample, T-^, will depend on the rate of heat

flow into it, where m is the mass of the sample and c, its heat

capacity per gram, eq . (6). At a transition, will remain at the

transition temperature, , for a residence time, t., determined

from

where AH^ is the heat of transition in cal/g. The heat capacity in

equation (7) is assumed to be either constant or linearly increasing

with time at constant heating rate. (During a transition, particularly a phase transition, h. A, and c may change considerably. This

will not be taken into account in this elementary treatment.)

Solving equations (4)-(7) for constant heat capacity, the steady

slope during a melting transition, the onset temperature and the decay constant at the termination of the transition are given by

Slope = 3hA = k^Bcm (9)

T. = T + = T + ^

(10) 1 m hA m k ^

^ m

decay constant = — = k (11) cm m

where ^ = x , the time constant for the interfacial region m
Flynn 133

The effect of a linear heat capacity increase with time considerably complicates the solutions of the equations for the three

parameters. However, in steady regions, it has the effect of introducing a factor of (1 + ^ ^

) where b is the time coefficient in

equation (7)- mb has a value of '^^ 2 x lO"''' cal/s K for 20 mg of

indium heated at 6 = 10 K/min (4). From the steady slope of the

melting transition of indium values of h A range from 3 to l4 x 10

cal/s K, depending upon interfacial conditions. It appears for this

case, the correction term is quite insignificant. This applies only

to the absence of transitions, such as during heat capacity determinations. During a phase transition, the heat capacity may change

quite markedly with time which will introduce much larger correction

terms into equations (9)j (10) and (11).

Values for h A, obtained from the steady slopes of indium melting transition curves also may be used to calculate the time constant

for interfacial thermal resistance at the melting point from equation

(11). Thus, if c = 0.062 cal/g K at 429 K (4) and m = 0.02 g;

T = 1/k = = 0.09 to 0.4 s for Indium for a wide variety of

m m h A Interfacial conditions.

These Interfacial decay time constants are considerably smaller

than the 1.5 s Instrumental decay time constant. Under ordinary con- ditions, viz., a sample encapsulated in a flat-bottomed aluminum cup

in good contact with the copper sample holder—the perturbing effect

of the interfacial resistance upon the return to the baseline following a rapid transition is negligible and the decay constant will be

1.5 seconds. If the surface contact is poor, the decay time constant will increase] however, this requires rather drastic tampering

with the interfacial region.

Equation (10) indicates that the heating rate, 6, approaches

zero, the onset temperature, T^ approaches the true melting temperature, T . Rearranging equation (10), m

^ -

- cm T.-T^ T.-T.i,o

(10')
134 An Analytical Evaluation of DSC

where ^ is the onset temperature extrapolated to 3 = 0 . Thus the

ratio of onset temperature difference to heating rate will give a "temperature lag" time constant.

Figure 7 is a plot of heating rate vs. onset temperature change

for the fusion of indium. Curve B is a composite curve for indium

metal encapsulated in aluminum sample pans . The time constant calculated from its slope is 8.0 seconds. This value is much too large

to be accounted for by the interfacial time constant, whose values,

determined from the slopes of the transition curves, vary over the

range — 0.09 < t < 0.4 seconds. This conclusion is borne out by the

other curves in Figure 7 whose time constants calculated from their

slope are not greatly different from curve B even though there has

A

ATj (°)

Figure 7. Scanning rate vs. fusion onset temperature

increment (Eq. 10').

A. Bare Wood's Metal, x = 7-0 s B. Indium in aluminum (average of 15 determinations)

T = 8.0 s C. Indium (2 shims) x = 9-3 s D. Indium (3 shims) x = 12.0 s
Flynn 135

been a drastic alteration of the Interfaces. Curve A, for the fusion of Wood's Metal resting directly upon the copper holder, yielded

a time constant of 7-0 seconds. Curve C for the fusion of encapsulated indium with two aluminum sample pan covers between the pan and

the sample holder {i.e., three interfaces) had a time constant of

9.3 seconds. Three aluminum sample pan covers (four interfaces) increased the constant to 12.0 seconds. More importantly, a large number of similar curves for melting transition of metals, organics, and

inorganics in aluminum sample pans closely coincided with curve B for

indium. Thus an 0.15 second interfacial time constant has only a slight effect on the temperature calibration, e.g., it would bring

about an onset temperature shift of 0.2 K at 3 = 80 K/min.

It also appears from the coincidence of these curves that the

physical state and thermal resistivity of the samples, themselves,

do not affect greatly the temperature calibration curve.

The source of the 8 seconds time constant which overwhelms interfacial and material effects can be found from an inspection of the

scan-speed-selector circuit. Calculations indicate that there should

be a time constant of about 4 seconds for heating rates of 0.625 and

1.25 K/min, and about 8 seconds at 2.5, 5.0, 10.0, 20.0, 40.0 and

80.0 K/min. This is the third instrumental time constant, given in

the last column of the table. It reflects the average temperature

lag behind the temperature readout. The readout works directly from,

temperature control knob.

^ . EFFECT OF SAMPLE

The effects of the thermal resistance and physical state of the

sample upon the flow of heat to and from it — especially the coupling of heat transport to the process of autogeneration or dissipation of heat and the rigorous application of realistic boundary conditions — defy any simple mathematical approach [5].

It appears, however, from the results discussed in concurrence

with Figure 7 that, so far as the temperature calibration is con-
136 An Analytical Evaluation of DSC

cernedj they can be relatively unimportant. Several experiments designed to test the order of magnitude of these effects, involving

sandwiches and laminates of varied thickness, are being conducted

at present.

\ 5. CONCLUSIONS

The present state of experimental progress permits the reaching

of several tentative general conclusions concerning the reliability

of differential scanning calorimetry

DSC can determine heats of transition within 1 percent accuracy

when the baseline is unambiguous.

The transport of heat through the sample and the interfacial

regions is not a limiting factor in the measurement of transition

temperatures for many materials.

Corrections for the several instrumental time constants must be

incorporated if temperatures of transition or kinetic parameters are

to be determined, particularly at fast scan rates.

• 6 . ACKNOWLEDGMENT

The author thanks Dr. Charles M. Guttman and Dr. Shu-Sing Chang

for their advice and suggestions on many phases of this work.

7 . REFERENCES

[1] Watson, E. S., 0"Neill, M. J., Justin, J., and Brenner, N.,

Anal. Chem. 36_, 1233 (1964).

[2] O'Neill, M. J., Anal. Chem. 36, 1238 (1964).

[3] A. P. Gray, "Analytical Chemistry", Porter, R. S., and Johnson,

J. P., Eds., p. 209, Plenum Press, New York (I968). j [4] K. K. Kelly, "Contributions to the Data on Theoretical Metal- i; lurgy," XIII U. S. Bureau of Mines Bulletin 584, p. 90, 196O. i [5] Lukaszewski, G. M., Laboratory Practice l4, 1277 (1965);

15, 861 (1966). ~
APPARATUS FOR RATE STUDIES

OF VAPOR PRODUCING REACTIONS

R. J. McCarter

Office of Flammable Fahrias

Institute for A-pplied T eohnolo gy National Bureau of Standards

Washington^ D. C. 20234

An apparatus was developed for measuring the rate

at which vapors are evolved during the thermal degradation of materials and thereby deriving the kinetics

of such reactions. Requisite to the operating scheme

of the apparatus is the provision of a high-temperature

zone to convert condensable or tarry vapors into non- condensable form.

The apparatus yields a direct measure of reaction

velocity, rather than the integrated indication obtained

with thermogravimetric analysis. This simplifies the

identification and calculation of kinetic parameters.

Increases in sensitivity and operating range are also

achieved. Flexibility in operation is obtained that

permits the separate recording of reactions that tend

to overlap.

Although the apparatus has been operated principally with a combustible gas indicator serving to meter

the evolved product vapors, a number of options are available for the latter function, including flow meters

and various continuous gas analyzers. The applicability

of the method appears promising.

Key words: differential thermal analysis] DTAj kinetics;

pyrolysis; TGA ; thermal analysis; thermal degradation;

thermogravimetric analysis

Part of this work was done while employed by the U. S. Department of Agriculture.

137
138 Thermal Apparatus for Rate Studies

INTRODUCTION

Numerous limitations are encountered in the application of

conventional thermal analytical methods to the investigation of

the pyrolysis and combustion behavior of materials. In the use

of thermogravimetric analysis (TGA), various factors tend to degrade the accuracy of recordings [1], particularly with samples

of low thermal conductivity and as heating rates are increased

above 10 to 20 °C/min toward those encountered in the combustion

of materials. Differential thermal analysis (DTA) involves com- promises in signal quality arising from the conflicting needs

that must be met in design of the equipment [2]. Combining or combined TGA and DTA measurements involve further problems.

Even under favorable circumstances, as when dealing with a single

well-defined reaction, results from these methods are often re- stricted in scope and certainty by unavoidable extraneous experimental effects. When samples undergo more than one reaction proceeding in proximity on the temperature scale, the complications

multiply. A clear interpretation of TGA results can then become

impossibly difficult or inordinately laborious. Such limitations

arise in part from the isothermal or linear-temperature rise operation that is conventional to or required by the methods. The

same appears to apply to other related systems that have been

reported [3-10].

The system of this report functions on a different and less

restricted basis. It is designed to derive the kinetics of pyrolysis reactions from measurements of the rate of vapor evolution,

rather than from weight recordings of solids. The reaction velocity is recorded directly, and does not require computation from

its integrated record as in TGA. The labor, complications, inaccuracies and obfuscations of the latter step are thus avoided.

Additionally and importantly, the restrictions imposed on programming of the reaction temperature are avoided. Thus the reaction

temperature may be freely and usefully manipulated and be directed

through any course desired within the operating limits of the

apparatus.
McCarter 139

The apparatus includes an innovation found necessary to permit

general use of the system. The innovation consists of the insertion

into the flow scheme of a high temperature zone. This is added to avoid the deposition of tarry products on equipment surfaces by

thermally reducing such products to volatile form.

The benefits of simultaneous DTA may be obtained with the

apparatus when it is appropriately operated.

PRINCIPLES OF OPERATION

The operating scheme of the apparatus is indicated in Figure 1. A flow of inert carrier gas is passed over the sample material, cen- tered within the tube furnace. The furnace is designed to afford a slight positive gradient in temperature toward its exit end. A single fine-wire thermocouple imbedded or immersed in the sample

material serves for measurement of the pyrolysis temperature. As volatiles are evolved from, the sample with increasing temperature,

they are carried to the "cracking" coil to be subjected to thermal

decomposition and the reduction of "tars" to non-condensables . The

flow is then conducted to any suitable recording instrument that

will serve to continuously meter the mass flow rate of the pyrolyzed

vapors or a representative portion or a component thereof. Meters

that have been used, other than the combustible gas indicator* diagramed in Figure 1, include:

1. Linear mass flowmeter

2. Thermal conductivity cells, composition and flow

sensitive types**.

3. Thermopile-type CO analyzer

4. Infrared analyzers-H^O , CO, and CO^ types

A portable instrument sometimes termed a "sniffer", used by utilities and industry for the detection of flammable or explosive vapors.

It appears possible that, other gas chromatograph devices, such

as electron capture and hydrogen flame detectors, could be useful

with a smaller-scale apparatus (such is exemplified in part by the

apparatus described in ref [9] and [10]) or with appropriate splitting of a side-stream. Possibilities appear numerous for the use

of meters in the system in parallel or sequential combinations.
140 Thermal Apparatus for Rate Studies

A.C. POWER A.C. POWER

(MANUAL

CONTROL) VARIABLE AUTO- TRANSFORMER

VARIABLE AUTO- TRANSFORMER

THERMOCOUPLE LEADS

ICED I COLD JUNCTION —2-CHANNEL

POTENTIOMETRIC RECORDER

Figure 1. Schematic diagram of apparatus.

The combustible gas indicator, because of its superior response

rate, has received the greater use and will be detailed. With use

of this meter, the flow from the tube furnace. Figure 1, is diluted

into a considerably larger flow of ambient air being drawn through

the meter. This dilution is normally over a hundred-fold in volume.

Therefore, the carrier gas and inert pyrolysis products are seldom

of consequence to the meter response. The combustible gas indicator

[10] employs a hot platinum filament within a bridge circuit. As oxygen and combustibles of the throughput gases contact the platinum

filament, they are catalyzed to react on the platinum surface. The

heat from this surface oxidation reaction increases the temperature

and the resistance of the filament. This causes an imbalance in the

bridge circuit and a consequent proportionate increase in the output

potential of the meter.

In measuring the kinetics of decomposition reactions, the linearity of the meter response to concentrations of evolved vapors is
McCarter 1^11 of prime concern. In this system, as a pyrolyzing compound evolves

combustible vapors, the response of the meter would be proportional

to reaction velocity insofar as:

1. The transit time of the evolved vapors to and through the

filament cell of the combustible gas indicator is appropriately

brief.

2. Absorption and desorption of product vapors in the conduit

tubes is not of significant magnitude.

3. Oxidation of product vapors or secondary reactions of disproport ioning effect do not occur in transit.

4. The decomposition performed by the "cracking" coil is uniform and does not introduce disproportioning effects in the product

vapors in regard to meter response.

5. The response of the meter is linear to the concentration of

combustibles in the filament cell.

6. The recorder registers linearly to its input.

The temperature record would represent the reaction temperature

insofar as thermocouple registry is accurate and temperature gradients within the sample are minor. Tests and a variety of operating

results indicate that these criteria have been met.

It was also found in operating the system that when the furnace

temperature was uniformly increased, departures of the sample temperature from a corresponding increase could be clearly detected and

serve to indicate reaction exotherms and endotherms.

DERIVATIONS

For typical simple reactions, the calculation of kinetic parameters from the recordings is straightforward. For example, the meter

recording for a single first-order reaction with a linear reaction

temperature increase would be of the nature indicated in Figure 2. The total gas-evolving reaction is represented by the area under the

meter curve. At temperature T (K) where the meter output is V (mv),

the curve area to the right of the V ordinate, F (mv sec), represents the proportion of the sample material that has not yet decom-
1H2 Thermal Apparatus for Rate Studies

TIME

Figure 2. Simple reaction curve.

posed, and the specific reaction rate, k (sec ) , is equal to V/F

Any convenient selection of points along the curve can then be used

to assess compliance to an Arrhenius equation by plotting In k vs. 1/T . The goodness of fit to this equation, particularly for reactions conducted over a varied and irregular temperature history, is

taken to validate the derived reaction constants, e.g. Figure 5.

\, ,. , DETAILS OF APPARATUS

The apparatus is diagrammed in Figure 1. The nitrogen

(Matheson prepurified grade), was passed through silica gel and a cotton filter before entry to the flow meter (Fischer and Porter
McCarter 1^3

Minirator or Brooks Type 1355), at a rate generally of about 10 cc/min. Helium was found to be less satisfactory as a carrier

gas in use with the combustible gas indicator because it caused

excessive base-line drift in this meter. Plow through the com- bustible gas indicator (Johnson-Williams Model G or Model SS-P)

was maintained at 2.1 liter/min by bleed valve adjustment before

the air pump (Model 2 Neptune diaphragm-type). Recordings were

taken with a two-channel potentiometric recorder (Moseley Model

7100B or Honeywell Electronik 19^).

Chromel-alumel thermocouple leads of 28 B&S gage wire were

used, with a 2 cm extension of 40 B&S gage wire to the hot junction. The "cracking" coil, maintained at "orange" heat in use, was a 3-5 nim o.d., 3 cm long helix of 24 B&S gage nichrome wire

of about 2 ohms" resistance (cold) with a ceramic inner core. Various tube furnaces have been used. One furnace was 20 cm long,

constructed with a 7.5 nim i.d. copper tube bore which was wound

with about l60 cm of asbestos-covered nichrome wire, with closer

windings toward the tube ends to compensate for end heat losses,

and enclosed with ceramic fiber insulation and a 3-2 cm D. chromeplated brass cover. Another furnace, without insulation and heated by a radiant source within a reflective shell, had less thermal

inertia. This version was less stable in operation but permitted

faster changes in temperature.

Tests of the apparatus' response rates have included:

1. Imposition of a full-scale square-wave input to the com- bustible gas indicator by connecting and disconnecting a premixed

gas sample to the inlet, to which the meter indicated a 70 percent

response in 2 seconds.

2. Measurements indicating a vapor transit time from, sample

to meter of 1 second.

3. A perturbation in power to the radiant-type tube furnace

was observed to effect a change in both the sample temperature and

the meter recordings in less than 3 seconds.

The system's responsiveness and linearity is also indicated by
Thermal Apparatus for Rate Studies

the degree of correlation obtained in kinetic parameters, e.g.

Figure 5. Temperature gradients within sample material are minimized by the convective heat transfer supplied by the inert carrier

gas, and by adjustments in sample size and geometry when required.

PROCEDURES

"

Samples are weighed and loaded into a standard quartz tube for

insertion into the furnace. Figure 1. A small plug of glass wool

inserted ahead of the capillary section of the tube can be used to help in the loading and retention of pulverized or powdered samples.

Circuitry and flows are activated and run until base lines are stabi

lized and air is flushed from the upstream sectiono - The cracking

coil is then activated and, a minute or two later, power to the tube

furnace is switched on and manually advanced. Predetermined voltage

time schedules are used when uniform advances in furnace temperature

are desired. Such a schedule can be useful in initial or exploratory pyrolysis runs. Meter outputs resulting from reactions then

appear in a standard recognizable form, and departures of the sample

temperature from the furnace temperature are apparent, indicating

reaction heat effects. At completion of pyrolysis, the quartz tube

is removed and cooled, and the sample residue weighed.

Sample size requirements vary inversely to the pyrolysis rates

to be used. Samples of 10 to 20 mg weight have yielded adequate

meter outputs at heating rates in the order of 1 °C/sec. Samples

up to 200 mg weight have been used with lower heating rates.

Samples in pulverized, sheet, fabric, and chunk forms have been

used, with the difference in form not of apparent consequence to the results (e.g. a 20 mg block of wood yielded essentially the

same meter recording as 20 mg of sawdust of the same sample.)

Liquid samples and samples of materials that melt before pyrolysis have been pyrolyzed in the apparatus using quartz or metal

boats for sample support o Some materials of this type indicated

excessive wetting or "creep" tendencies. Dispersion of such samples

in a plug of glass wool has served in some cases to provide sufficient wettable surface to contain the material
McCarter 1^ EXPERIMENTAL EXAMPLES

To obtain a direct comparison of the system's performance to

that of thermogravimetry , some vjell-charact er ized polymers were pyrolyzed in the apparatus. The meter recordings for these materials and the respective Arrhenius plots are diagrammed. Figure 3

-4

-6

On,

POLYETHYLENE ^ EST. E=78 KCAL/GMOL

^ \ POLYISOBUTYLENE v EST. E=62 KCAL/GMOL

POLYSTYRENE\^O

EST. E = 72KCAL/GMOL

360

POLYETHYLENE

400 440 480

TEMPERATURE, °C. (HEATING RATE lO'C/MIN.)

Figure 3. Pyrolysis recordings and Arrhenius plots of data

for polymer samples

The derived reaction parameters are compared in Table 1 to those

obtained [12] with thermogravimetry. The values for activation

energies and the temperature datumx points are judged to be within

the limits of experimental error for the respective thermogravimetric results.

A more informative comparison is afforded by the recordings

for the pyrolysis of a cellulosic material. Figure 4. Thermograv-
146 Thermal Apparatus for Rate Studies

Table 1. Comparison of results from thermal methods.

Thermal Method

Material Estimate of

Combustible

gas

indicator^

DuPont 950

Thermograv

Analyzer^ jb

polyisobutylene

(Enjay Chemicals

L80-B)

E, kcal/mol

temp, °C,§20% reacted

°C,@50^ reacted

62

381

394

62 375

400

polystyrene

(Styron 66)

E, kcal/mol

temp, °C,@20% reacted

°C,@50| reacted

72 398

nil

75

390

408

polyethylene

(Marlex 6002)

E, kcal/mol

temp, °C,@20^ reacted

°C,@50^ reacted

78

453

468

71

445

460

^With heating rate of 10 °C/min.

Activation energies, E, estimated by Broido method [13].

averaged from 3 or 4 runs

imetric analysis had not been helpful in defining the pyrolytic

reactions of this material. The thermogram for this material was judged undecipherable, as was its first derivative, recording A of Figure 4. However, the comparable output curve of the combustible gas indicator, recording C of Figure 4, clearly indicated the curve form of Figure 2 for the principal decomposition. The recording was found on calculation to conform closely to first order reac- tion kinetics, with an apparent activation energy of 40 1/2 kcal/mol,

after subtraction of the minor tail area of the curve as representing a secondary decomposition.

The differences between recordings A and C were resolved by

various experiments, including the use of other meters in the apparatus. It was found that the initial pyrolysis reaction of the material (with precautions in sample desiccation) evolved only non- combustible vapors. The combustible gas indicator was not responsive
McCarter 1^7

TEMPERATURE RECORDING, "C

Figure 4. Comparison of system recordings, pyrolysis of treated

cellulose. Materials: Standard grade Whatman Ashless

cellulose powder (<0.01 % ash) with 1.5 Wt % KHCO^, added from aqueous solution. Heating rate: All 60 °C/min.

A. First derivative of thermogravimetric recording,

DuPont 950 Thermogravimetric Analyzer, 11.4 mg sample.

B. Output of water analyzer. Model 300 LIRA Infrared

Analyzer, N2 flow 280 cc/min, tube furnace outflow direct

to analyzer w/o air dilution, analyzer responds to water

and hydrocarbon vapors, 19.5 mg sample. C. Output of

combustible gas indicator, Johnson-Williams Model G,

19.8 mg sample. D. Output of flowmeter. Model LL-100

Hastings Linear Electrical Mass Flowmeter, 13.8 mg

sample

to these vapors. Thus this reaction had been conveniently "filtered"

out of recording C. The reaction was recorded by the infrared HpO
l48 Thermal Apparatus for Rate Studies

analyzer, recording B of Figure 4, and by the linear mass flov/meter,

recording D of Figure 4. The kinetics of the Initial reaction could

then be separated by obvious means: (1) by calculation and subtraction of the principal decomposition from recording D, (2) by

difference, from recordings C and and (3) by recording the reaction separately with the linear mass flowmeter using an appropriate

temperature limit.

The above illustrates capabilities of the apparatus system

provided by options in the metering function that are not available

with TGA.

The kinetics indicated by recording C of Figure 4 was confirmed by a variety of other pyrolysis runs of the material, including

the one represented in Figure 5. In this case, the pyrolysis temperature has been cycled through an irregular course and held below

370 °C to repress secondary decomposition during the principal reaction. The secondary reaction was then recorded as the temperature

was subsequently Increased to 500 °C, an example of separating reactions by appropriate temperature control.

Figure 5- Recordings and Arrhenlus plot, pyrolysis of

treated cellulose.
McCarter 149

DISCUSSION

Other advantages of the system may be noted. There is little

"noise" in the recordings. Dynamic effects from evolved vapors,

convection, ete . are not of consequence to the measurements. A wide range of reaction rates may be accommodated by the apparatus.

Heating rates up to 2 °C/sec have been used without apparent impairment of the recordings. Sensitivity of up to 2 orders of magnitude

greater than that of conventional thermal methods is obtained. The

direct contact of the low-mass temperature sensor with the sample

material is advantageous. For example, during pyrolysis some cellulosic samples have indicated excursions of up to 30 °C from the temperature of the immediate envir onm.ent . This indicates the inaccuracies that can occur in thermal methods where the temperature sensor

is bulky, or is detached from the sample,,

Of particular merit is the simplicity of the system. The

apparatus is slm.ple and expeditious to operate. It does not involve the com.plicat ions and delays of vacuum operation. The re- sults are generally simple to interpret. The "visibility" of the

recordings is such that effects can sometimes be recognized and

explored by adjustments in temperature during the course of pyrolysis runs, in the manner of operating an analog computer. The data

are simple to translate into kinetic terms. The equipment is

relatively inexpensive.

CONCLUSIONS

The described apparatus is an efficient means for measuring

the kinetics of vapor producing reactions and has performed kinetic measurements not attainable by conventional thermal methods.

The system permits a variety of options in effluent sensing and

mode of operation, such that the applicability of the method

appears promising.

ACKNOWLEDGMENT

The assistance, counsel, encouragement, and materials supplied

by Edward M. Barrall II, IBM Corp., are very gratefully acknowledged.
150 Thermal Apparatus for Rate Studies

REFERENCES

[1] Newkirk, A. E., Anal. Chem. 32, 1558 (I96O).

[2] Barrall, E. M. II and Rogers, L. B., Anal. Chem. 3^,

1101 (1962) . ,

, [3] Wilson, Abraham, Rev. Scl. Instr . 38, 1757 (I967).

[4] Ramlah, M. V. and Goring, D. A. I., Cellulose Chemistry

and Technology 1, 277 (1967).

[5] Murphy, E. J., J. Polymer Sci. 58_, 649 (I962).

[6] Bryce, D. J. and Greenwood, C. T., Applied Polymer

Symposia No. 2, l49, Interscience, New York (I966).

[7] Bryce, D. J. and Greenwood, C. T., Die Starke No, 10/15,

359 (1963) . [8] McNeill, I. C, "Application of Thermal Volatility Analysis

with Differential Condensation to Som.e Problems in Poly- mer Chemistry," 2nd International Conference on Thermal

Analysis, Holy Cross College, Worcester, Mass., August I968.

[9] Eggertsen, P. T., Oki, H. M., and Stress, F. H. "A Pyrolysis-Flame Detection Technique for Thermal Analysis",

Second International Symposium on Thermal Analysis,

Holy Cross College, Worcester, Mass., August I968.

[10] Stapp, A. C. and Carle, D. W. "A New Thermal Analysis

Instrument", Pittsburgh Conference -on Analytical Chemistry

and Applied Spectroscopy, Cleveland, Ohio, March I969.

[11] Johnson, K. W. and Bruce, E. L., Instruction Manual,

Johnson and Williams Model G Combustible Gas Indicator.

[12] Barrall II, E. M., Private communication.

[13.] Broido, A., "A Simple, Sensitive Graphical Method of

Treating Thermogravimetr ic Analysis Data," Spring

Meeting of Western States Section, The Combustion

Institute, Univ. of Southern California, Los Angeles,

California, April I968
DIFFERENTIAL THERMAL ANALYSIS OF

PRIMARY EXPLOSIVES - A MODIFIED TECHNIQUE

R. J. Graybush, F. G, May*, A. C. Forsyth

Solid State Branch

Explosive Laboratory J FRLj

Piaatinny Arsenal

Dover^ New Jersey

The usefulness of differential thermal analysis

(DTA) in the study of metastable materials has been hindered by the tendency of samples to explode during the

experiment. A description is given of modifications

which have been made to the remote cell used in conjunction with a Dupont 900 DTA such that the possibility of

explosion is minimized. Using lead azide to evaluate

the technique it is . shown that complete curves for the

exothermic decomposition

PbNg Pb + SN^ are reproducibly obtained. The endotherm in the curve corresponding to the fusion of the product lead serves

as a confirm.ation of the reaction. Illustrations are given to show extensions of the system to other sensitive

compounds, namely lead styphnate , mercury fulminate, and

potassium dinitrobenzfuroxan

Key Words: differential thermal analysis, Dupont 900

DTA remote cell, lead azide, lead styphnate, mercury

fulminate, metastable materials, potassium dinitrobenzfuroxan, primary explosives.

INTRODUCTION

The application of thermal analysis to decomposition studies of

very sensitive materials, such as primary explosives, has been hindered by the possibility of explosions of samples within the apparatus.

* Guest Scientist, Department of Supply, Defence Standards

Laboratories, Melbourne, Australia. c--.
152 DTA of Explosives

Although specimens can be sufficiently small to ensure little risk

of instrument damage this is an undesirable situation since the

amount of significant data which can be extracted from such a curve

is minimal.

Krien [1] investigated the application of combined differential

thermal analysis and thermogravimetry (DTA/TG) to the decomposition

of explosives. Problems associated with this TG apparatus restricted

him to slow heating rates (5 °C/min or less) and as a consequence

the DTA peaks obtained were spread out

In more recent work PaiVerneker and Maycock [2,3] described

the application of simultaneous DTA-TG to lead azide . They were

able to observe thermal decomposition which they attributed to the

reaction

. PbNg -> Pb +

Since the DTA and TG traces are recorded from the same sample it is

correct to assume that analysis obtained for the TG will also be

incorporated in the DTA results. The nitrogen content of lead azide

is 28.8^ therefore it would be expected that if this reaction were

to take place the weight loss recorded by thermogravimetry would be

28.8^. The curves which they obtained show very slow thermal decomposition over a wide temperature range with loses of 21.9 - 27.5^.

Thus the decrease in weight found experimentally indicates conclusively that the products of reaction consist of a mixture of metallic

lead and its oxide. The results can be more adequately explained

from a consideration of reactions of the type

PbNg ^^^^ ^2 PbO + (2)

or formation of higher oxides of lead depending on oxygen concentration. The weight losses predicted for reactions of this type would

be 23.^% for formation of PbO or less for a mixture of PbO plus

higher oxides of lead. The experience of the present investigators

has been that such slow reactions invariably yield oxide mixtures.
Graybush, May and Forsyth 153

or, at best, metal plus oxide mixtures. It is more tenable to propose that the decomposition which was reported was not the formation

of metal plus gaseous nitrogen but decomposition in the presence of

trace amounts of oxygen to yield an ill defined product consisting

of a variable mixture of metal plus oxides with no assurance that

the same reaction path will be followed in any two (2) experiments.

Lead azide is of interest because of its wide use. A study of

the decomposition reaction (eq. 1) is most suitable since it is known

to occur in the detonation process [^] yielding basic lead and nitrogen and therefore can be easily characterized. Thus, it vias considered ideal to use for the evaluation of the modified cell.

EXPERIMENTAL

Remote Cell Modifications

A typical remote cell as currently supplied by E . I. DuPont

(Fig. 1) differs from earlier models in that the heating block has

been raised and is separated from the thermocouple connections by

a large insulating disk of pressed asbestos . Experience with earlier

designs of the cell had shown that it is imperative to protect these

connections from exposure to stray thermal currents. The need to

protect samples from the slightest oxidizing environment necessitated

the cell being evacuated to 10"^ torr. This required removal of the

porous disk and redesign of the support. It was also necessary to seal the electrical connectors, relocate the gas inlets, and modify

the base plate and locking clamps.

The modified cell is shown in Figures 2 and 3, where Figure 2 shows an overall view and Figure 3 gives the dimensions. It was necessary to reconstruct the original cell starting with a new base

having four (4) holes to accomodate vacuum tight cartridge bushings,

supplied by Glass Tite Industries (A-2622), which were in turn

soldered into the base. The pins from the original plug were soldered into the new cartridges to adapt them to the existing thermocouple and heater connections. The new heating block holder and

support consisted of a single unit fabricated from heavy walled glass

and silver coated to minimize thermal losses. The thermocouple con-
15^ DTA of Explosives

Figure 1. Remote cell as supplied by E. I. Dupont.

nectlons were further protected from the development of thermal

gradients by shielding with glass tubing. Evacuation and gas flow

was effected through two (2) pressure-vacuum stopcocks attached to the top of the pyrex dome. The pyrex dome was made vacuum tight by

fastening it to the base with a ring seal and metal ring clamp.
Graybush, May and Forsyth 155

Figure 2. Modified remote cell.

DTA Procedures

Samples investigated were in the form of fine crystalline

powders. Glass beads were used as reference materials and in the

tube containing the heater control thermocouple. Standard heating

blocks of silver or aluminum were used with specimens contained in

either 2 mm or ^ mm capillary tubes. With 2 mm tubes sample weight

was approximately 0.5 mg, in 4 mm tubes a maximum of 2.5 mg was used;
156 DTA of Explosives

THERMOCOUPLES

OV A.C. WIRE PYREX DOME HEATER HOLDER & SUPPORT HEATING BLOCK HEATER

CLAMPING SCREWS

ELECTRICAL FEED THROUGHS

RING CLAMP

CABLE TO INSTRUMENT

2.7 MM DRILL

Figure 3. Schematic diagram of components in the modified remote

DTA cell.
Graybushj May and Forsyth 157

however, the weight could be Increased to 5 mg if the sample was diluted with an equal volume of glass .beads [5]. In all cases samples

-6 were compacted by mild tapping. The cell was evacuated to 10 torr

for several minutes before backfilling and purging with dried helium

gas. A flow rate of 400 cc/min was maintained throughout the experiment. Standard chromel-alumel thermocouples were employed. Heating

rates up to 10 °C/min and maximum differential temperature sensitivity,

0.1 °C/in, could be used.

Fisher Scientific "Thermetic" standards and high purity lead

(99.999^) supplied by Electronic Space Products, Inc. were selected

for instrument calibration. Lead azide of high purity was prepared

by reaction of lead nitrate with hydrozoic acid [6]. Other explosive

samples were of commercial or military production grade stock.

RESULTS AND DISCUSSIONS

The instrument was calibrated through the lead azide temperature

range using melting point standards and was in agreement with the old

cell. The results of pure lead melting at 3l8 °C under the high

helium flow rate is shown in Figure 4a. The lead fusion peak is

indicated as an exotherm at 318 °C in Figure 4b when lead is placed

in the reference tube and analyzed simultaneously with lead azide in

the sample tube. The differences in the peak sizes of the product

lead from that of the added lead is due to differences in amount.

DTA curves for lead azide obtained prior to cell modifications

are shown in Figure 5. Two (2) unacceptable curves, sample detonation (Fig. 5a) and sample oxidation (Fig. 5b) are shown. It is

readily apparent that very little information other than onset temperature can be extracted from those curves in which high order

detonation occurred.

The presence of trace amounts of oxygen causes nonreproducible

distortion of the decomposition exotherm. Depending on the oxygen

concentration this distortion can range from gross deformation and
158 DTA of Explosives

200 250 300 350 j TEMPERATURE '

Figure 4. DTA curves showing the position of the lead m.p.

(a) (1) lead fusion (a) (2) supercooled solidification

of lead (cooling curve). (b) Pb metal in reference

tube + PbNg in sample tube.

extended tailing of the peak to a reduction in peak area in the

presence of extremely minute traces of oxygen. When lead azide

specimens were placed in the modified cell at varying vacuum pressures before backfilling and purging with helium it was found that

the exotherm trace configuration was a function of pressure becoming

constant at 10~^ torr. Although the peak height and shape is affected.
Graybush, May and Forsyth 159

1^0 200 250 300 350 '

' ^ TEMPERATURE °C Figure 5. DTA curves for lead azide. (a) sample detonated.

(b) sample decomposed in the presence of trace amounts

of oxygen.

this treatment did not influence either the onset or peak temperature

except in cases of gross deformation of the trace.

Figure 6 shows decomposition of high purity lead azide to metallic lead as obtained in the modified system. Features of importance

are the decomposition exotherm in the region 287 - 302 °C and the

lead fusion endotherm at 3l8 °C. Subsequent cooling of the sample

gives the exothermic solidification of lead supercooled to 315 °C.

The products of reaction (eq. 1) are confirmed by the lead fusion.
160 DTA of Explosives

lOT/Min

(a)

10°C/Min

(b)

LEAD AZIDE

SUPERCOOLED LEAD SOLIDIFICATION

EAD FUSION

LEAD AZIDE

LEAD FUSION

200 250 300 TEMPERATURE

350

Figure 6. DTA curves for lead azide using modified remote cell.

(a) 2.6 mg lead azide. (b) 3.1 mg lead azide diluted

with glass beads

The extent to which the fusion point is displaced at a constant

heating rate depends on the amount of azide used. Using varying

quantities of lead azide to check out the cell the average deviation

of the lead fusion was ± 1.9 °C at 3l8 °C. This could be significantly reduced by use of a constant mass of sample. Similarly values

of the decomposition onset and apex temperatures could be obtained
Graybush, May and Forsyth 161

with greater accuracy If all experimental parameters were held constant during a set of experiments. It would also be desirable to sieve samples for constant particle size but with sensitive materials

such as lead azide this proves to be extremely time consuming. The

onset and peak temperatures are used for trace description. These

values are given in Table 1 and were obtained from experimental

curves using various instrument settings and sample sizes. This

data gave an average T^^^^^^-^ 287 °C with a deviation of ± 1.0% and

an average T. v 302 °C with a deviation of ± 0.8^.

Table 1. Results of thermal decomposition of lead azide

under various experimental conditions.

T °C, . T °C, s Lead Sample AT Glass Size of {onsez) imax; weight (°C/in) beads capillary

mg tube

287 305 317 2.4 0.5 No 4 289 305 316 2.6 0.5 No 4 288 305 320 1.1 0.2 No 4 293 305 320 1.0 0.2 No 4 294 306 321 0.8 0.2 No 4 290 304 320 0.5 0.2 No . 4 288 304 320 0.4 0.2 No 2 290 304 316 0.4 0.2 No 2 287 302 316 0.5 0.2 No 2 290 303 320 0.3 0.2 No 2 287 302 320 0,3 0,1 No 2 288 300 316 3.0 0.5 Yes 2

It can be seen from the table that considerable flexibility in

choice of instrument settings and sample configuration is possible.

It was found essential to provide for efficient conduction of heat

away from the sample area since the reaction is highly exothermic

and can result in detonation. Helium was employed as the preferred

carrier gas. When the sample size was increased to the point where

helium gas alone was not sufficient; dilution of the sample with
162 DTA of Explosives

250 300 350

TEMPERATURE °C

Figure 7. DTA curves for iron doped lead azide. (A) PbN^ + Pel^

(B) PbNg + FeBr^. (C) PbNg + FeCl^. (D) PbNg + FeF^;'

glass beads, commonly used for reference material, provided a heat

sink. This use of glass dilution affects peak magnitude but not

position as shown in Figures 7a and 7b.

The new cell and the experimental procedure were evaluated

further using doped lead azide samples and other primary explosives.

From another study [7] it is known that lead azide prepared in the

presence of various dopants has related defect structure. Figure 7 shows curves for a series of lead azide samples doped with ferric

chloride salts. Work is in progress in interpret these results in

terms of observed electron spin resonance phenomena. In general,

initiating compounds were difficult to analyze without the modification of the remote cell. Figure 8 shows some results of the exten-
Figure 8. DTA curves for other primary explosives. (a) potassium

dinltrobenzfuroxan (KDNBF) . (b) lead styphnate.

(c) mercury fulminate.
164 DTA of Explosives

sion of the method to several other metastable compounds. In all

Instances complete curves are obtained and it appears that the

method can be successfully applied to other sensitive materials.

Effort is currently being directed to modifying the accessory

DuPont 950 TG apparatus to enable equivalent thermogravlmetric data

to be obtained.

REFERENCES

1. Krien, G., Explosivst off e 13., 2.05 (1965).

2. PalVerneker, V. R., and Maycock, J. N.^ Anal. Chem. 40, 1325

(1968). —

3. PaiVerneker, V, R., and Maycock, J. N., Proc . of 2nd International

Conference of Thermal Analysis (1968).

4. Garner, W. E., Trans. Farad. Soc . ^ Symposium on Solid Phase

Reactions (1938).

5. Smith, J. R. , and Forsyth, A. C., private communication.

6. Reitzner, B., and Manno, R. P., Nature 198 , 991 (1963).

7. Downs, D. S., Forsyth, A. C, and Fair, H. D., ESR Studies of

Doped Lead Azide, to be published.
THERMOGRAVIMETRY OF VULCANIZATES

John J. Maurer

Enjay Polymer Laboratories

Linden^ New Jersey

Thermogravlmetrlc analysis provides an Important

advance In the analysis of elastomer compounds and

vulcanlzates . This paper reviews the current state

of such Investigations Including a consideration of

key experimental variables, precision and accuracy for

estimating basic composition of different types of

practical formulations. Additional Information can

be obtained by careful study of the oxidation characteristics of the carbon black In the formulation.

The latter Is Influenced by both physical {e.g., surface area) and chemical (type of cure system or polymer) effects. Finally, an Isothermal oxidation procedure Is discussed for demonstrating differences In carbon black(s) and/or curatives In routine comparisons of samples

Key words: carbon black; curatives; elastomer com- pounds and vulcanlzates; oxidation; thermogravlmetrlc analysis.

INTRODUCTION

Elastomer compounds are complex mixtures of polymer(s), carbon

black(s), mineral flller(s), curatives, plast Iclzers , and miscellaneous additives. This complexity Is Increased by the frequent

use of blends of carbon blacks or elastomers . Because of this

situation. It Is a challenging task for the manufacturer to determine the cause of various problems which may arise In the processing and curing operations leading to the finished product,

and also for manufacturer and customer to conduct quality control

analyses on the finished rubber products.

165
166 Thermogravimetry of Vulcanlzates

Methods exist for conducting highly detailed analyses of

rubber formulations, but these are often too expensive or timeconsuming to be justified and/or useful for routine application

[1], Our first-stage TGA objective [2] was to develop rapid

practical procedures for use in certain factory and technical

service related problem areas. Later, it became evident that the

basic concepts could be extended to provide additional information by careful analysis of the oxidation characteristics of the

carbon black in the formulation [3]. The objectives of this

paper are to present a critique of the present methods [4]; demonstrate current strengths; consider refinements to improve precision and accuracy; discuss factors which may influence the results;

suggest new areas of application and new types of information which

may be obtained; and show the potential of the current methods for

Quality Control analysis and "trouble shooting" activities.

Considerations of instrument design, etc., are not included in

this discussion since it will be assumed that one has available a TGA instrument of sound basic operating characteristics. References to such matters are available elsewhere [4]. As a basis for

commenting on several important procedural details, the basic method

adopted in our laboratories for routine inspections of butyl rubber

vulcanlzates is presented below.

Decomposition sequence: heat at 15 °C/min to constant weight

EXPERIMENTAL CONSIDERATIONS

TGA Conditions for Compositional

Analyses of Vulcanlzates

Instrument DuPont Model 950 TGA ' 20 mg (usually in one piece)

Chromel/Alumel

purge with nitrogen for 15 minutes prior to run

Sample

Thermocouple

Atmosphere

Calibration

in nitrogen; admit air, continue at 15 °C/min

to constant weight

Monitor daily via standard vulcanizate to check reference temperatures and composition.
Maurer 167

The conditions listed above were selected because of a desire

to have a relatively rapid analysis, and because a dynamic method

(as opposed to several possible isothermal approaches) would lead

to information about oil and carbon black variations in some systems. An important consideration, regardless of the specific steps

employed, is to insure reliable instrument operation. This was done in the present case by means of daily checks of an extracted

vulcanizate known to contain ^8.8 weight percent polymer and 51-2

weight percent carbon black plus ash. Some representative data are shown in Figure 1, which shows the degree of variation experienced

in the terminal decomposition temperature (T^) for the polymer and

in estimated total polymer content for one such sample. Long-term

data on this type can be translated into a quality control chart

which provides a reference point for judging instrument stability.

Of course, such a procedure also reflects any variations in operator technique and uniformity of the standard, both of which must be

evaluated and controlled. It is useful to point out that the precision of quantities such as shown in Figure 1, especially reference

temperatures, is influenced also by the scale used on the thermo-

'490

480

470

50

g49

48

1—1

\—\—\ \ \ \ \ I \ \ I r

• • •

• • •

•

1 •

1 •1 1 i sO o CO o \ C\J r-l rH \ \

a< (\J C\J rH 1—

1-H

J L J L J L

DATE OF TEST

Figure 1. Analysis of TGA standard sample (extracted)
168 Thermogravlmetry of Vulcanizates

gram. In the present instance, each minor chart division on the

weight scale represented 2 weight percent, and on the temperature

scale represented 5 °C. Expansion of these scales would contribute

to improved precision and accuracy.

ANALYSIS OF "BASIC COMPOSITION" OF VULCANIZATES

The original goal of this work was to provide a rapid estimate

of the "oil," polymer," "carbon black," ash, or mineral filler plus

ash in the formulation. For general unknowns, use of an unextracted

sample would not enable separate determination of the oil content,

thus, the sum of oil and polymer would be obtained in the decomposition in nitrogen. Extraction of the sample prior to TGA can provide

an estimate of the "oil" content but lengthens the time for the

analysis of the vulcanizate.

The kinds of data obtained from this analysis are illustrated in

Figure 2. The upper thermogram depicts the weight losses due to polymer + oil, to carbon black oxidation and the residual weight due

to ash. The lower thermogram (for an extracted sample) shows the

weight losses due to polymer and carbon black, and the large residue

(after achieving constant weight in air) due primarily to mineral

filler but with a small contribution from the ash content as well.

The precision and accuracy of such determinations, as well as a method for estimation of oil content in a "known" system, will be taken

up later in this report.

EXPERIMENTAL VARIABLES INFLUENCING THE ANALYSIS

Other than instrument considerations, the three TGA variables

which could influence the present analysis are heating rate, sample

size and atmosphere control. The first of these is considered in

Table 1, which contains data taken on an extracted commercial vulcanizate. It is seen that the calculated composition in terms of

polymer, carbon black and ash is not significantly influenced by

this range of heating rates. This is an interesting result for two
Maurer 169

Figure 2. Analysis of carbon black and mineral filler.

Table 1. Influence of heating rate on vulcanizate analysis

Vulcanizate composition (Wt %)°" Rate (°C/min) Polymer Carbon black Ash

15 57.0 27.0 3.0

6 55.7 27.8 3.5

0^ 56.0 27.5 3.5

.Commercial sample; 13 percent "oil" removed by extraction.

^Isothermal ( 532 °C)

reasons. First, one may use a rate as high as 15 °C/min if a dynamic test is desirable (reasons for this will be illustrated subsequently). Second, a very rapid estimation of vulcanizate composition is possible in terms of (a) polymer plus oil, and (b) carbon

black plus ash, or mineral filler plus ash. This may be easily

accomplished for the system in Table 1 by isothermal degradation in
170 Thermogravimetry of Vulcanizates

nitrogen at 532 °C. The total time for this isothermal analysis

was less than 5 minutes. It should be noted that heating rate does

influence the decomposition characteristics of polymer and carbon

black, and this effect may be useful in analyses of blends of these

materials. ....

. > , ... ... - . . The influence of particle size is demonstrated in Table 2 for

several vulcanizates, some of which contained polymer blends, of

the same "basic composition." The pulverized samples were obtained

by crushing vulcanizate sections which had been shock cooled in

liquid nitrogen. The data indicate no advantage for using smaller

pieces in this test; therefore, the one piece, solid sample was adopted for convenience. Similar experience was observed for other

vulcanizates. ...

Table 2. Influence of sample size^ on vulcanizate analysis.

Sample Composition (Wt %)^

Q Formulation type Polymer Carbon black + ash

A Solid 49 51

Pulverized \ 51 B Solid 49 51

Pulverized 49 51,

C Solid 49 51

Pulverized 49 . 51

^20 mg in all cases; known:

^^8.8^ polymer; 51.2% black + ash

'Samples differed in elastomer type.

The atmosphere control during the basic TGA procedure consists

of purging the system with nitrogen for 15 minutes prior to initiation of the heating rate. The nitrogen, which is piped to the laboratory from a large, central supply is known to contain a very low

level of oxygen (4-5 ppm) . To date, this has presented no apparent

problem in our TGA studies of basic vulcanizate composition, as in-
Maurer 171

ferred from precision and accuracy data of the type cited elsewhere

in this report. However, rigorous nitrogen purification to remove

trace oxygen would enhance absolute accuracy of the method, and may

assist in other studies which rely on the temperatures at which cer- tain events take place rather than on total weight changes, as such.

FACTORS INFLUENCING ACCURACY OF VULCANIZATE ANALYSIS

As previously noted, the initial objective of this work was to provide a procedure applicable to problems where both time and cost

of the analysis were important factors. It will subsequently be

shown that the standard TGA conditions successfully meet these objectives. For completeness, we first take up the assumptions and

approximations involved in calculating the Basic Vulcanizate Composition. Some of these are also involved in other methods of vulcanizate analysis. These point to the areas where additional work is

required to enable even greater utility of the method.

• Estimation of Oil Content by Preliminary Extraction

Solvent will also remove accelerators, sulfur, resins, waxes, etc.;

also some low ends from the polymer.

•Estimation of Polymer from Weight Loss in Nitrogen

(Extracted Sample)

Polymer is assumed to degrade completely. Organic materials, insoluble in e.g, MEK, and also those of high thermal stability {e.g.,

certain resins) will interfere.

Note: DTA may be useful to detect systems containing major amounts

of these ingredients [2],

• Estimation of Carbon Black Content From Weight Loss in Air

Any residues from polymer, resin, eta., will interfere. Small

weight losses due to certain mineral fillers interfere.

• Estimation of Ash Level from Residue After Weight Loss in Air

Residue contains all non-volatile metallic residues (from ZnO,

accelerators, resins, process aids, etc.).
172 Thermogravimetry of Vulcanlzates

PRECISION AND ACCURACY OF BASIC TGA PROCEDURE

In this section we examine the degree to which the technique

may provide reliable information about basic vulcanizate composition. In Table 3, data are shown for four laboratory-prepared

vulcanlzates J each of which contained a different elastomer type.

Visual inspection of these data indicate the excellent precision of

the analysis. The accuracy is also quite good, recalling that, be- cause of the assumptions involved, exact agreement between known

and observed values will not be obtained. It is interesting to note

for the NR system that incomplete polymer loss may have been involved, with a resulting increase in the apparent carbon black content. Such problems are usually not major, but illustrate the type

of variation in the analysis which must be considered in treating

"unknown" samples. It should also be noted that major differences

in compounds are readily detected; and it is this type of data which

is generally most significant.

Table 3- Precision of vulcanizate analyses for

different elastomers,.

Vulcanizate composition (Wt %)^

Polymer type Polymer Carbon black + ash

Ethylene propylene

t erpolymer

49 . 0 49 .0 50.0

50.0

51.0

51.0

50 .0 50. 0 Butyl rubber 49.0

49 . 0 49 .0 49.0

51 .0 51.0

51.0

51.0

Chlorobutyl rubber 66.0

66.0

66.0

66.0

34 .0 34 .0 34.0

34.0

Natural rubber 64.0

65.0

64.0

64.5

36 .0 35.0

36 .0 35.5

Four separate samples analyzed for each system.
Maurer 173

The vulcanlzates referred to in Table 3 also provided additional

information regarding the characteristics of the polymer decomposition in nitrogen (Table 4). It is noted that there is a v/ide range

of temperatures over which the 50 percent weight loss (T^q) and the

terminal decomposition temperature (T^) occur. This suggests that

for some systems TGA procedures might be devised to determine the

composition of elastomer blends in the vulcanizate. We hope to report on this kind of study at another time. For this type of analysis, it will be necessary to determine whether the effect on poly- mer decomposition J of the fillers, curatives, etc., in the system

varies with elastomer blend composition. Of course, once having

established the pattern for a known compound, T^^ and T^ of the

polymer are two additional quality control statistics v/hich may be

monitored. ...

Table 4. Decomposition characteristics of different

elast omers^-

Polymer type

Polymer decomposition temperature (°C)

"^^0% '^terminal

Ethylene propylene

terpolymer

479

472

472

472

504

496

498'

il98- Butyl rubber 417

417

417

416

•452

449

452

450

Chlorobutyl rubber 407

411

406

404

445

454

449

447

Natural rubber 388

386

388

388

462

462

470

471

'Same vulcanlzates as Table 3.
174 Thermogravimetry of Vulcanlzates

A more Interesting demonstration of the accuracy of the basic

TGA method is to examine the data obtained for different types of

practical elastomer formulations. As shown in Table 5, the method

appears highly useful, even in its simplest form, for studying the

composition of a variety of commercial compounds. However, systems

are known where particular ingredients in the formulation will

interfere with the analysis to varying extents [2]. Examples of

these would be cases where (a) all of the polymers did not degrade

completely in the nitrogen step, or (b) resins, waxes or miscellaneous processing aids are present in substantial amounts.

Further, the latter materials are not readily removed completely

from some formulations during extraction procedures prior to TGA.

It should be noted that even in these cases, the carbon black, ash,

mineral filler plus ash, or "oil plus polymer" values are often

reasonably close to the true values. Of course, the degree to which

such formulations are a problem depends upon the purpose of the analysis. For determinations of the exact composition of unknown formulations, the problem is a serious one, but for routine quality control

considerations it may be insignificant. Those interested in the forTable 5- Accuracy of TGA method for unextracted commercial

vulcanlzates

Formulation

Inner tube known

observed

Oil

12.5

13

Carbon Mineral

Polymer black filler

50 .0 49.5

35.0

34.5

Ash

2.5

3.0

Weather

strip

known

observed

Oil + Polymer

49.4

50 .0 49.5

49 .0 1.1

1.0

Sponge

Engine

mount

known

observed

known

ob served

55.0

56.0

62.0

62.0

14 .

15.0

35.4

35.0

29 .0 29 . 0 1.5

2.6

3.0

Electrical

insulation

known

observed

49 .2 49 .0 8.6

9.0

42.2

42
Maurer 175 mer area may find Differential Thermal Analysis a useful complementary

technique since it can detect the presence of significant amounts of

resins, waxes, ete . ^ in some formulations [2].

EXTENDING THE ANALYSIS TO PROVIDE ADDITIONAL INFORMATION

Oil Content of a System Containing a Known Elastomer

The oil content of a vulcanizate cannot generally be estimated

directly from the thermogram because the oil weight loss region

overlaps that due to the polymer. A method of circumventing this

problem has been found for butyl rubber systems and should also be

applicable to others (including oil-extended polymers) . The

approach involved is to establish a reference temperature (Tt^) at which essentially all of the oil plus only a small amount of polymer have been volatilized from the system. A series of vulcanizates

differing in oil/polymer/carbon black ratio is then extracted and

analyzed in the standard manner. Based upon the resulting thermograms, a correction curve of "Polymer remaining at T„" vs. "Polymer

n Weight Loss at T^^" is constructed (Figure 3).

For an "unknown" system, after completing the polymer decomposition step in nitrogen, one can read the amount of polymer remaining

at T^ from, the thermogram. Reference to the correction curve then

n allows an estimation of the polymer content in the system and thus,

from the knowledge of the total weight loss at T^^ , an estimate of

the oil content as well. Potential methods for improving the analysis of oil content consist of lower (including isothermal) heating

rates, and reduced pressure to facilitate vaporization of the oil.

An example of the possibilities of this method is the first

formulation in Table 5; here, the oil content was estimated by the

Tj^ technique o Our experience indicates that the oil content can usually be estimated within about three weight percent of the known

value for butyl rubber compounds. This is sufficiently accurate for some purposes, and avoids the use of a lengthy extraction step to estimate oil content. In order to use this approach, one must know
176 Thermogravlmetry of Vulcanizates

the decomposition characteristics of the polymer and oil of interest

(assuming isothermal and/or reduced pressure conditions do not enable

the oil to be directly "separated" from the polymer. Here again,

waxes, resins, etc . ^ may interfere if present in large and/or varying

quantities

DETECTION OF CARBON BLACK DIFFERENCES

IN STANDARD FORMULATION

It has been found^that a slight modification of the basic TGA

procedure permits discrimination among several carbon black types

in a standard butyl formulation [3]. This modification consists

of cooling the system to 275 °C (after bringing it to constant

weight in nitrogen) before admitting air, and then continuing the

analysis at 15 °C/min in the usual manner. An example of this

approach is shown in Figure ^ . Both chemical and physical effects

can influence the decomposition characteristics of carbon blacks
Maurer 177

In this procedure. Thus, for a known formulation containing a single accelerator system and differing only in carbon black type,

a relationship between decomposition characteristics and surface

area has been demonstrated (Fig. 5).

Information concerning the degree of chemical effects on carbon

black decomposition was obtained by examining a set of different

types of cure systems for butyl rubber in combination with each of

several different carbon black types. Tables 6 to 9 indicate the

repeatability of the analyses, and Table 10 summarizes the data for

the study. It is evident that the components in the cure system

can influence decomposition of the carbon black. These data suggest

that replicate examination of carbon black decomposition characteristics could be useful to detect unusual carbon black types and/or

acceleration systems in a "known" formulation.
178 Thermogravimetry of Vulcanizates

T

500 525 550 575 600

TEMPERATURE FOR 15% WEIGHT LOSS (°C)

Figure 5. Carbon black decomposition vs. surface area.

Table 6. Influence of cure system on butyl rubber - MPC vulcanizate.

Polymer Carbon black

decomposition (°C) decomposition

Cure

System^

% Polymer% Black% Ash

Temp

for 50 loss

Terminal

temp

Onset

temp . Terminaltemp

66 31 3.0 422 462 490 557

65 31 4.0 421 455 492 542

. 1 ,. . 65. . 31 4.0 419 454 499 554

64 31 5.0 419 453 50 8 560

" 65 31 4.0 422- 457 504 558

65 • 32 3.0 417 452 507 554

2 . 65.. 32 3.0 417 450 518 555

65 32 3.0 416 452 521 564

65 32 3.0 424 464 531 563

- - 66 • 31 3.0 420 450 522 563

.3 , 6.5 , 32 3.0 420 457 527 554

65 32 3.0 419 455 531 562

63 30 .0 7.0 417 464 423 454

6 .4 . 30 .0 6.0 421 462 431 468

63 30.0 7.0 412 466 426 460

^See Table 10 .
Maurer 179

Table 7. Influence of cure system on butyl rubber - HAF vulcanizateLl]

Polymer Carbon black

decomposition (°C) decomposition

Cure

Sys tem^

% Polymer

% Black

% Ash

Temp

for 50 lossTerminal

temp . Onsettemp . Terminal

temp . 65.5 30 .5 4 .0 415 452 526 577

1 65 .0 31 4 .0 419 457 530 584

65.0 31 4 .0 421 457 530 584

66 30 4 .0 413 452 541 583

2 66 30,.5 3 .5 418 455' 541 588

66 30,.5 3 .5 421 457 547 594

66 31 3^.0 421 459 592 607

3 66 31..5 2 .5 422 466 607 616

65.5 31^.5 3^.0 421 466 605 612

63 30 .0 1 .0 412 445 460 471

63 30 7.. 0 412 447 462 479

4 63 30 1 .0 412 447 471 482

63 30 7..0 416 452 479 482

^See Table 10.

Additional points of interest regarding the cure system study

are the following: (1) with the possible exception of system

number four, there does not appear to be a large effect of cure system on polymer decomposition in nitrogen. If this is generally true, it would be possible to ascribe variations in the poly- mer decomposition region of extracted samples primarily to differences in polymer type. (As previously noted, resins could be an interference here); (2) there is a significant change in decomposition characteristics of the different blacks for cure systems 1 and

2 [4]o For systems 3 and 4, three of the blacks have similar onset

temperatures, but larger differences in terminal temperatures are evident
180 Thermogravlmetry of Vulcanlzates

Table 8. Influence of cure system on butyl rubber - PEF vulcanizate [1]

Polymer Carbon black

decomposition (°C) decomposition

Cure

Sys tem^-

% Polymer

% Black

% Ash

Temp

for 50$? loss

Terminal

temp . Onset

temp . Terminal

t em.p . 65 .0 31 4 .0 417 455 549 597

65 .0 31 4 .0 413 447 550 592

1 64 .5 31 .5 4 .0 417 452 550 599

65 .0 32 .0 3..0 417 454 551 595

65 .0 31..0 4,.0 414 449 569 610

65 .0 31..5 3..5 412 448 563 606

2 65,.5 31..5 3..0 416 453 569 611

65 .0 31..0 4,.0 416 452 565 606

65 .5 32,.5 2,.0 421 455 579 591

64 .5 32,.5 3..0 416 452 610 624

3 64 .5 32,.5 3., 0 421 457 597 629

64,.5 32,.5 3.,0 421 457 600 624

63 30,.0 7.,0 416 457 466 ^9 7 62,.5 30,.5 7.,0 414 452 462 497

4 62,.5 30,.5 7.,0 412 450 464 502

63..0 30,.5 6,.5 412 452 464 491

See Table 10.

A new area of study is suggested by comparison of the shapes of

e.g.^ the FEF black decomposition regions for the different accelerator systems. The complex nature of some of these. Figure 6, suggests that TGA monitoring of cure system uniformity is feasible; one certainly detects the oxidative effect of the GMF system. Its uniqueness may enable detection in unknown vulcanlzates. It is also

interesting to note the strong two-step regions in some systems.

These have suggested to us additional studies to determine whether

each region relates to one accelerator component.
Maurer 181

Table 9. Influence of cure system on butyl rubber - SRF vulcanizate [1].

Polymer Carbon black

decomposition (°C) decomposition

C ur e System^

1 Polymer

JO Black

Jo Ash

Temp . for 50^

loss

Terminal

temp . 0ns e t temp . T e rm i na 1 temp . 65 32 3 0 412 452 555 602

1 64.5 31.5 4 0 413 452 546 607

65 32 3 0 412 457 552 607

65 32 3 0 411 452 583 621

2 65 32 3 0 411 452 579 621

65 32 3 0 410 447 582 626

64 32.5 3 5 416 452 608 630

3 64 33 3 0 417 452 593 630

64 33 3 0 416 452 585 625

62 31 7 0 405 438 466 527

63 30 7 0 408 4^7 459 537

4 62 31 7 0 407 438 460 521

63 31 6 .0 407 443 460 532

^See Table 10.

LJ

TEMPERATURE (°C)

Figure 6. PEF oxidation in standard vulcanizate.
182 Thermogravlmetry of Vulcanizates

Table 10. Effects of cure system and black type on decomposition

of butyl rubber vulcanizates.

System'

Average values

Polymer

decomposition (°C)

Temp

Carbon black

decomposition

% % % for 50$? Terminal Onset Terminal

Polymer Black Ash loss temp . temp

temp

65 .0 31.0 4.0 420 456 497 553

65.2 30.8 4.0 418 455 529 582

64.9 31. 4 3.8 416 452 550 596

64.8 31.8 3.3 412 454 551 605

65.0 32,.0 3..3 418 453 513 558

66.0 30,.3 3..7 417 455 543 588

65.1 31..3 3..6 415 451 567 608

65.0 32,.0 3.,0 411 450 581 623

65.3 31..8 3.,0 421 457 528 561

65.8 31..3 2,.8 421 464 601 612

64.8 32,.5 2,.8 420 455 597 617

64.0 33.,0 3..3 416 452 595 628

63.3 30,.0 6,.7 417 464 427 461

63.0 30,.0 7.,0 413 448 468 479

62.8 30,.4 6,.9 414 453 464 497

62.5 31..0 6,.8 407 442 461 529

Cure Systems (phr)

1. Altax (1.0), Tellurac (1.5), Sulfur -1.0, ZnO (5-0), Stearic Acid

(2.0) . 2. Sulfasan R (2.0), Tuads (2.0), ZnO (5-0), Stearic Acid (2.0).

3. SP-1055 (12.0), ZnO (5.0), Stearic Acid (2.0).

4. Altax (4.0), GMF (1.5), Red Lead (5.0), ZnO (5.0), Stearic Acid

(2.0).

EXAMINATION OP CARBON BLACK BLENDS

Isothermal analysis of the carbon black residues obtained after

the nitrogen degradation step of the standard TGA procedure indicated that the degradation characteristics of thermal blacks were significantly different from those of the other blacks tested.

The magnitude of this difference indicated that at least semi-quantitative analyses of carbon black mixtures should be possible where
Maurer 183

10

I-o

LlI

5 10 15 20 25

TIME (MINUTES AT 550°C)

30 35

Figure 7- Isothermal analysis of mixed blacks in TGA residue.

one component is a thermal black. A test of this idea is shown in

Figure 7 which clearly demonstrates the different characteristics

of the two formulations known to contain this type of carbon black

blend. Rough estimates of the composition were made based on the

weight loss at an arbitrary reference temperature [3]- For system.

A, rather good agreement with the known content (25 small/75 large) was obtained. This method obviously over-estimates the sm.all black

content due to failure to correct for the initial weight loss due

to the large black. The other two, "unknown" development compounds

appear to differ significantly in carbon black blend composition.

More quantitative determinations may be possible for known systems

by means of a reference temperature technique similar to that used

in the estimation of oil content of known systems

Recent evidence has indicated that additional insight into variations in carbon black residues may be obtained even for systems

which do not contain thermal blacks. An example of this is shown in

Figures 8 and 9. Figure 8 shows the basic TGA thermogram for ex- tracted samples of commercial product produced in different loca-
184 Thermogravlmetry of Vulcanlzates

0

TEMPERATURE (°C)

Figure 8. Comparison of vulcanlzates by standard TGA conditions.

SAMPLE A

¥r¥r^* SAMPLE B

\ (NITROGEN)

(AIR)

TEMPERATURE (°C)

TIME (MINUTES AT 532°C)

Figure 9. Isothermal analysis of carbon black residue.
Maurer 185

tions. This analysis indicates very similar basic compositions for

the two materials; note especially the similar carbon black decomposition characteristics. However, when the analysis was repeated

with the modification of decomposing the carbon black residues isothermally at 532 "^C (Figure 9), a substantial difference in degradation characteristics is evident . Several suggestions can be made

about the reasons for these differences; (a) different cure systems

were used, (b) one of the samples (B) is a blend of carbon blacks

and the other is not, (c) both are blends but the average particle

size of the two components in one of them is too similar to be detected in this analysis. Our experience in this area to date leads

us to favor interpretation (b). Additional experimental procedures

to magnify further the oxidation rate differences of such blends

are now under consideration in our laboratories.

REFERENCES

[1] ASTM Test D-297-67-T "I968 Book of ASTM Standards, Part 28,

Rubber, Carbon, Carbon Black, Gaskets", p. 114.

[2] Maurer, J. J., Proceedings of the Second International

Conference on Thermal Analysis, Worcester, Mass., I968.

Thermal Analysis, Schwenker, R. F. and Garn, P. D., Eds.,

Vol. 1, p. 373, Academic Press, New York, (I969).

[3] Maurer, J. J., Rubber Chemistry and Technology (Rubber

Reviews), 42, 110, (I969).

[4] Maurer, J. J. Rubber Age, 102, 47 (1970).

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