NBSIR 76-1149

A Second Appraisal of

Methods for Estimating

Self-Reaction Hazards

Eugene S. Domalski

Physical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

March 1977

Interim Report for Period January - December 1975

Prepared for Office of Hazardous Materials Operations

Department of Transportation

Washington, D.C. 20024

NBSIR 76-1 149

A SECOND APPRAISAL OF

METHODS FOR ESTIMATING

SELF-REACTION HAZARDS

Eugene S Domalski

Physical CheiViistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D C 20234

March 1977

Interim Report for Period January - December 1975

Prepared for Office of Hazardous Materials Operations

Department of Transportation

Washington, D C. 20024

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Acting Director

Foreword

The work reported here represents a second attempt to

appraise methods employed for the estimation of self-reaction

hazards for the Office of Hazardous Materials Operations of the

U. S. Department of Transportation during calendar year 1975. During

the course of this effort, expertise from various sectors of the

National Bureau of Standards was enlisted. A clearer and more

quantitative definition and identification of thermal instability

was sought with an outlook toward establishing a reactivity scale

for materials with a tendency to self -decompose

.

An assessment of the accidental polymerization of bulk chemicals

was provided by Mr. Daniel W. Brown and Dr. Roland E. Florin of

the NBS Polymer Stability and Reactivity Section. Assistance was

obtained through the NBS Center for Fire Research from Dr. Andrej

Macek. Dr. Bert R. Staples of the NBS Chemical Process Data Evaluation

Section helped in the collection and formating of the physical test

methods, and in the compilation of the corresponding bibliography.

Dr. Kenneth L. Churney of the NBS Thermochemical Standards and

Measurements Section studied relationships between the physical

test methods and the fundamental concepts of thermal explosion

theory. Technical contributions were obtained in the area of

chemical kinetics from Drs . Wing Tsang, Michael J. Kurylo and Alan

H. Laufer of the NBS Physical Chemistry Division,

Table of Content

Foreword it

List of Tables : v

List of Figures vii

I. Introduction 1

II. A More Detailed Statement of the Problem 4

III. Conclusions and Recommendations 8

IV. Collection and Evaluation of Test Methods and Test Data n

A. Sensitivity Tests : 12

1. Thermal Tests 13

2. Mechanical Tests 43

a. Detonability Tests 43

b. Compression Tests 49

c. Confinement Tests 54

d. Friction Tests 57

B. Power Tests 5 7

C. Table of Tests 65

V. Predictive Schemes— 6 7

A. Expandability of the Predictive Schemes to Specific

Reaction Mechanisms 6 7

B. Evaluation of Safe Materials by the CHETAH Program 69

C. CHETAH Calculation of Enthalpies of Polymerization 90

VI. Accidental Polymerization of Bulk Chemicals 98

A. Introduction 98

i B. Polymerization Classifications 98

C. Step Reaction Polymerization Hazards 99

iii
D. Chain Reaction Polymerization Hazards-^ 100

E. Olefin Polymerizability 101

F. Oxidation 109

G. Thermal Initiation 111

H. Inhibition 111

I. Recommendations for Olefinic Monomers 113

J. Polymerizability of Olefin Oxides 116

K. Dimer or Trimer Formation 116

L. ' General Reference Sources 117

VII. Thermal Analysis and Hazard Evaluation 121

A. Thermal Explosion Theory 121

B. Experimental Studies on Hazardous Materials using

DTA, DSC, or Related Measurement Techniques 129

VIII. References 134

—I

IV
List of Tables

Table 1 Decomposition reactions computed by the CHETAH Program

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

Table 10

Table 11

Table 12

Table 13

Table 14

Table 15

Table 16

A. CHO Compound sB. CHN CompoundsC. CHNO CompoundsMaximum Enthalpy of Decomposition

(CHO Compounds)

Maximiim Enthalpy of Decomposition

(CHN Compounds)

Maximum Enthalpy of Decomposition

(CHNO Compounds)

AH (combustion) - AH (decomposition)

(CHO Compounds)

AH (combustion) - AH (decomposi tion)

(CHN Compounds)

Z;ii(combustion)A- AH (decomposi tion)

(CHNO Compounds)

Oxygen Balance (CHO Compounds)-

Oxygen Balance (CHN Compounds)-

Oxygen Balance (CHNO Compounds)-

Modified Enthalpy of Decomposition

(CHO Compounds)

Modified Enthalpy of Decomposition

(CHN Compounds)

Modified Enthalpy of Decomposition

(CHNO Compounds)

Summary of CHETAH Hazard Ratings

(CHO Compounds)

Summary of CHETAH Hazard Ratings

(CHN Compounds)

Summary of CHETAH Hazard Ratings

(CHNO Compounds)

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87
Table 17 Polymerization Process and Enthalpies at 298.15 K

91

Table 18 Enthalpies of Polymerization 93

Table 19 Maximum Enthalpy of Decomposition (Olefins) 94

Table 20 AH (combustion) - Z^H(decomposi tion) (Olefins) 95

Table 21 Oxygen Balance (Olefins) 96

Table 22 Modified Maximum Enthalpy of Decomposition (Olefins)" 97

Table 23 Olefin Polymerizability at 60°C in Radical Reactions- 118

Table 24 Instances of Dimer or Trimer Formation 119

Table 25 Dimer Formation Rates 120

Table 26 Enthalpies of Dimer and Trimer Formation — 120

vi
List of Figures

Figure L Heat Production and Heat Loss Rates for a Sel f -Reacting Material — '-25

Figure 2 Heating Curves (Temperature -Time Curves) for

a Self -Reacting Material— -'-27

vi i

I . Introduction

The need to find a clear definition and a more quantitative

identification of the property known as thermal instability (selfreactivity) of chemical substances was the goal of the efforts put

forward in the Physical Chemistry Division of the National Bureau of

Standards (NBS) during the calendar year 1975. This need is

acknowledged by groups within various sectors involved in the

production, handling, and transport of hazardous materials. The

Office of Hazardous Materials Operations of the U. S. Department of

Transportation (DOT) has supported this investigation as part of its

own program to review the hazard classification system, to re-examine

regulations for hazardous cargoes, and to implement an emergency

response system.

r 1 + A previous evaluation program [ 1 | for DOT performed by NBS in

1974 examined predictive schemes and physical test data to determine

if correlations could be made between the test data and various

thermochemical and kinetic parameters which would allow tlie establishment of

a ranking of substances on the basis of their chemical reactivity.

Included in this previous program was an attempt to derive a better

idea of how thermal instability could be defined. However, because

the correlations obtained were not definitive and because only

qualitative relationships were apparent, an improved definition or

identification of thermal instability did not emerge.

_ Numbers appearing in brackets refer to references at the end of this

report

.

1
Our efforts for DOT during calendar year 1975, to appraise

thermal instability concepts and to seek a better definition and

identification of self -reactivi ty as a function of increasing temperature, pursued the following areas: (1) an evaluation of the

literature which reported explosive sensitivity test methods and

the corresponding test data, (2) an examination of hazards caused

by monomer polymerizabi li ty , and (3) the re-examination of the

predictive schemes.

Although a large array of test methods and test data were found

as a result of performing the literature survey, we were not able to

establish a reliable reactivity ranking which could be applied to

chemical substances as a whole. Some of the test methods and test

data are very empirical in nature, however, others could be linked

to fundamental concepts involving thermal explosion theory. The

broad spectrum of data required to cover the range of reactivity

from nitroglycerine to sand were either unavailable or were present in

a form which could not be merged easily with other related information.

We found that it was difficult to reduce the data obtained from

different test methods to a common level for comparison because of

the inherent dissimilarities in the kinds of methods employed and

parameters measured. ' .

2
Development of the predictive schemes to accommodate various

reaction mechanisms remains a difficult task because of the overall

empirical, and in part arbitrary, character of their formulation.

Reaction mechanisms are established best by performing reaction-rate

experiments on the systems under question. Without experimental

kinetic data, prediction of reaction mechanism is subject to

considerable risk.

From the evaluation of the test methods and the re-examination

of the predictive schemes, it appears that a calorimetric approach

has promise of providing part of the necessary information and support

for concepts from which a reactivity scheme could be developed. One

of the serious di f f iciencies of many of the existing test methods

and the predictive schemes is their overall empirical nature. A

firmer foundation for a definition of thermal instability and for

a method to rank chemical reactivity could be obtained from the theory

of thermal explosion as put forward many years ago by Semenov [2, 3]

and by Frank-Kamenetskii [4, 5], and studied more recently by

Merzhanov and his co-workers [6].

The calorimetric approach offers the advantage of permitting

simultaneous determination of thermochemical, kinetic, and heat

transfer parameters from experimental measurements performed on a

specific thermal decomposition process. The need to calculate

these parameters from the same reacting system, thus providing a common

base for comparison, appears to be an important feature. The fact

that the calorimetric approach should provide the parameters embodied

in the theory of thermal explosion suggests that a ranking for chemical

reactivity could be developed from such investigations.

3
II , A More Detailed Statement of the Problem

The U. S. Department of Transportation is responsible for

the effective administration and coordination of the transportation

programs of the Federal Government. The safe transportation of

hazardous materials is included in these programs. On January 3,

1975 the Hazardous Materials Transportation Safety Act (Public Law

93-633) was passed [7]. This legislation repealed and superseded

the Hazardous Materials Transportation Control Act of 1970 (Public

Law 91-458). Under both Acts the Secretary of Transportation

must prepare and submit to the President for transmittal to the

Congress on or before May 1 of each year a comprehensive report on

the transportation of hazardous materials during the preceding

calendar year. Included in this report is a thorough statistical

compilation of any accidents and casualties involving the

transportation of hazardous materials, and a list and summary of

applicable Federal regulations, criteria, orders, and exemptions

in effect

.

An area within the existing code of Federal regulations which

needs clearer definition and identification is the characterization

of thermally unstable materials. The regulations which cover the

hazardous materials occasionally classify certain chemicals which are

thermally unstable in categories not indicating this property because

the other hazard classification is more outstanding. For example,

picric acid (with not less than 10 percent water) is listed under

4
"Flammable Solids"; anhydrous hydrazine is classed as a "Flammable

Liquid"; and chloropicrin is identified as a "Class B Poison".

These chemicals are all potentially capable of thermal decomposition.

As a result of situations such as those cited above in which only

a partially classification of the hazard potential of a chemical is provided,

the DOT Office of Hazardous Materials Operations has sought to improve the

systematization of its regulations of hazardous materials as part of

a long-range research and development effort. New regulations and

amendments to existing regulations are published on a yearly basis

and should be helpful to shippers and carriers in expediting the

transport of hazardous cargoes.

Expertise in the areas of thermochemistry and chemical kinetics

which are in the NBS Physical Chemistry Division has been employed in

seeking a clearer definition and identification of thermal instability.

Both data analysts and experimentalists in this Division have been

helpful in providing their appraisals of various aspects of thermal

instability, chemical reactivity, se If -decomposi tion, and the

examination of other factors which render a chemical as hazardous.

In conjunction with these appraisals, the files of the Chemical

Thermodynamics Data Center and the Chemical Kinetics Information Center

were made available so that the information at hand was as current

as pos sible

.

5
The problem of defining and identifying thermal instability

in a quantitative manner is linked to the collection of an array

of data, from either the literature and/or the laboratory, from

which a mathematical correlation can be derived. Correlatable

parameters are obtained from test methods for explosive sensitivity

and enthalpies of decomposition, activation energies, and

frequency factors which have been determined from thermochemical

and kinetic studies. Ideally, the correlation should be able to rank

all substances between nitroglycerine and sand in a quantitative

manner according to their chemical reactivity. The correlation

should have a good theoretical foundation and not be strictly

empirical. Our previous attempts to correlate enthalpies of

decomposition with impact test data could only provide us with a

qualitative relationship because the enthalpies of decomposition used

were for other processes than those taking place at impact as imposed by

the falling weight. The impact test is useful and has its place

in hazard evaluation, but is for the most part empirical in origin,

fundamentally difficult to interpret, and limited in the degree to

which it can be applied in certain correlations.

We are hopeful that within the theory of thermal explosion and

the measurement of parameters contained in the self-heating expressions

of that theory, the proper combination of factors necessary to provide

a ranking for a chemical reactivity scale will be found. After a

sufficient number of experiments have been conducted and parameters

calculated, we feel a quantitative relationship can be obtained which

6
will be useful in defining and identifying thermal instability.

The problem of scaling to large samples must also be examined as well

as changes in the ambient conditions to which the chemical is

subjected during handling and shipment.

7
Ill . Conclusions and Reconunendations

Our general conclusions and recommendations are as follows:

1 . Test Methods and Test Data

Thermal tests which provide information about the critical

conditions for thermal explosion are valuable because they give

quantitative data identifying the limit of thermal stability for

a specified material. Examples of such thermal tests are: the

JANAF Thermal Stability Test, the ASTM E-476-73 Confinement Test,

the Thermal Surge Test, and the Explosion Temperature Test. The

adiabatic and isothermal storage tests are useful in estimating the

induction time prior to explosion for a material.

Detonability tests apply only to sensitive explosives, and

hence, give negative results for materials of lesser sensitivity to

initiation. The impact test is probable the most versatile and most widely

used of the compression tests. It usually can give reasonable reactivity

ranking for explosives. Inversions in rank can occur when impact

data from different apparatuses are examined.

The air-blast test is an important test which provides data

on the energy release of explosive materials. It is applicable to

both detonative and sub-detonative explosives.

8
2 . Predictive Schemes

Attempts were made to develop separate predictive schemes

which were oriented toward handling particular reaction mechanisms.

We found that it was not possible to devise or formulate separate

schemes. Realistic enthalpies of decomposition could not be

predicted independent of experimental measurement.

A hazard evaluation study of some selected materials, which

are commonly considered safe, showed that the CHETAH program is

likely to err in labelling safe materials as being hazardous.

The CHETAH program does not appear to provide a clear-cut

ranking of hazards related to polymerization.

3 . Accidental Polymerization of Bulk Chemicals

Free radical polymerization involving olefins is the kind

of polymerization process most likely to occur inadvertently during

transport. Peroxides are the probable source of initiation. Information

regarding the identification of the type of inhibitors and their

required concentrations which would prevent the accummulat ion of

serious amounts of peroxides is considered important.

9
4 . Thermal Explosion Theory and Hazard Evaluation

From an examination of reports of experimental investigations

of hazardous materials and the elements of thermal explosion theory,

a calormetric experiment appeared to offer a more likely and reasonable

recourse to obtain data from which an unambiguous reactivity scheme

could be developed. We feel a test procedure for ranking materials

consistent with such a reactivity scheme should be derivable having

a firm relationship to thermal explosion theory. Properties of

materials which are embodied in this theory (such as the enthalpy

of decomposition, activation energy, pre-exponential factor, heat

transfer coefficient, the temperature below which no violent decomposition

will take place, and the induction time prior to explosion) are

derivable from scanning calorimetric measurements. The calorimetric

approach has the advantage of permitting the simultaneous derivation the

above-mentioned properties from the experiments performed on a

specific decomposition process.

10
IV. Collection and Evaluation of Test Methods and Test Data

* We acknowledge the assistance of Dr. Andre j Macek of the NBS

Center for Fire Research in the preparation of this section of the

report.

The collection and evaluation of test methods and test data have

been approached from the standpoint of both the sensitivity to

decomposition or explosion, and the damage potential or total energy

release which can be expected from a decomposition or explosion. The

sensitivity of a material can be related to the minimum amount of

thermal excitation necessary to initiate the process. In more

fundamental terms, this concept can be associated with the rates and

mechanisms of the initial reaction process. In the case of bond -breaking

processes, the bond dissociation energy and the activation energy

are equivalent, and are useful estimates of material sensitivity

for decomposition. On the other hand, the total energy release (or

power) of a decomposition reaction is related fundamentally to the

thermodynamic aspects of the reaction, and is a function of the

reaction's initial and final states.

Present address: Energy Research and Development Administration (ERDA),

Division of Fossil Energy Research, Washington, D. C. 20545.

11
A . Sensitivity Tests

There is a wide variety of standardized tests designed to

evaluate the sensitivity of self-reacting materials to external

stimuli. The tests can be divided conveniently into two classes,

thermal and mechanical. Thermal tests measure the inherent stability

of materials and thus allow an explanation of the sensitivity to

some degree in fundamental terms. Mechanical tests, on the other

hand, are very difficult to analyze in fundamental terms, because

the stimulus usually is an intricate mixture of several fundamental

processes (e.g., compression, friction, viscous flow) none of which

is easy to reduce to quantitative values, and all of which are

difficult to scale. It is, therefore, clear that, while both types

of tests are important, the approach to the problem of evaluation of

the two types of tests must be different.

The question of thermal sensitivity can be approached in a

general way, so there even appears to be a possibility of specifying

a single testing procedure, applicable to all materials. The approach

to mechanical tests ought to be much more empirical. The only reasonable

procedure at this point is to seek tests which simulate some aspects

of the anticipated practical exposure to stimuli. This means that

there are two reasons why one should not expect to be able to specify a

simple mechanical test for evaluation of sensitivity. First, there is

a variety of possible hazardous stimuli. Second, simulation by

laboratory tests is never quite satisfactory, so cumulative ratings

by several tests are desirable for increased confidence.

12
1 . Thermal Tests

We are concerned only with materials which self-react exothermica

when exposed to sufficiently high temperatures. UTten this happens,

the temperature T in the interior of the sample generally increases

above the temperature of the surroundings, T^, and heat is transferred

from the sample to the surroundings. Steady states are possible at

sufficiently low temperatures. However, if the temperature exceeds

a critical value, T^^, the heat generation rate becomes so large

that heat loss cannot keep up with it, and catastrophic self-heating

ensues, usually ending in an explosion. These processes have been

described quantitatively by Semenov, Frank-Kamenetskii , and others

[3, 5, 8 and a relatively simple relationship can be derived for the

critical temperature from their treatments which is provided below:

where R is the gas constant, a is the radius of a sphere or cylinder

or the half -thickness of a slab, - is the density, Q is the heat of

reaction during the self-heating process, A is the pre -exponenti al

factor and E the activation energy from the Arrhenius expression,

' is the thermal conductivity, and 5 is the shape factor (0.88 for

infinite slabs, 2.00 for infinite cylinders, and 3.32 for spheres).

A maximum value for • can be calculated for each of the three

geometries which corresponds to the steady state situation.

E

T

Rln a^^

cr

(T )SfR

cr

13
An important assumption which is embodied in the latter equation

is that the Arrhenius form of expressing rate processes is applicable

to the self-heating problem. The assumption is not trivial and can

lead to difficulties when experiments are designed poorly. However,

if we presume that experiments are designed properly, pre-exponential

factors and activation energies can be derived which will lead to a

realistic description of the problem.

14
Test Methods and Test Data

General format for the exposition of the test methods and test data

is as follows:

1. Purpose of Test :

A brief identification of the physical property which is being

measured is provided, or the parameter which the measurement is

directly related to is mentioned.

2 . Operating Principle :

The definitive experimental condition is given under which

the sample is subject in the test method, or its relationship to

the measurable property.

3 . Test Description:

A brief description of the test method is provided.

4 . Test Evaluation :

This section discusses the advantages, disadvantages,

limi tations, and outstanding or unique features of the method.

5. Data:

The hazardous compounds studied are listed along with the

parameters which were calculated. Individual values are not always

supplied. References are always provided, however, in some cases,

because of the large amount of data or the complexity of its

exposition, only references are supplied.

15
a. JANAF Thermal Stability Test Niimber Six for Liquid Propellants [9]

1. Purpose of Test ;

To determine the maximum temperatures to which thermally unstable

liquids can be subjected for short periods of time without danger of

explosive decomposition,

2 . Operating Principle :

Under confinement in a microbomb a liquid sample is either heated

rapidly and held at a pre-determined temperature for an

arbitrary time interval, or heated at a constant rate until evidence of

rapid decomposition appears. Spot immersion is also possible, where

the microbomb containing the sample is immersed into the bath at

some elevated temperature.

3 . Test Description: • "

A microbomb which is drilled and tapped for a thermocouple and

3 burst disc fitting, has an internal volume of 1.3 cm . A liquid

sample of 0.5 ml volume is used and burst diaphrams ranging from 300

to 8400 psi failure pressure can be used. The microbomb is immersed

in a bath containing a bismuth-lead alloy, which melts in the range

150 to 250*F. Maintenance of the bath around 200*? and of the heating rate

at 20*F per minute, allows detection of rates of decomposition of 2*'-5*'F per

minute. An air-vibrator is used to agitate the bath and the sample

in order to establish the desired heat transfer between bath and

sample. The sample temperature and the temperature difference between

the bath and sample are recorded as a functions of time. The temperature

at which se If -decomposi ti on begins and the rate of decomposition

can be derived.

16
4. Test Evaluation:

This test utilizes small samples of material in good thermal

contact with thermos tat ted surroundings. The temperature of the

sample can be increased with time at such a slow rate that quasisteady states are maintained and a relationship identified by the

equation on page 10 is applicable.

Rates of decomposition can be estimated from plots of the sample

temperature vs. time, and from plots of the temperature difference

between the sample and bath vs. time. The slope of the temperature

differential curve represents the rate of heat transfer between the

sample and the bath. Factors which need to be taken into account are

the rate at which the bath is being heated, heating from the selfreaction of the sample, and temperature gradients in the microbomb.

From a plot of the self-heating rate of the sample vs. the reciprocal

of the temperature, a linear slope proportional to the activation

energy should result. The precision of activation energies derived

in this manner is about =15 percent.

5 . Data :

The activation energy of nitromethane was calculated to be 51

kcal mol""^ [9]

,

17
b. ASTM Standard Method of Test E-476-73 [10]

Thermal Instability of Confined Condensed Phase Systems (Confinement Test)

1 . Purpose of Test :

To determine the temperature at which a chemical mixture will

commence a reaction, liberating appreciable heat or pressure, when

subject to a programmed temperature rise. This method applies to

solids or liquids in a closed system in air or some other atmosphere

present initially under normal laboratory conditions.

2. Operating Principle :

The sample to be tested is confined in closed vessel equipped

with a burst diaphram, pressure transducer, and thermocouple. The

apparatus is equilibrated in a bath at room temperature and subsequently

heated at a constant rate. The temperature difference between the

bath and sample, the pressure in the closed vessel, and the bath temperature are recorded continuously during the course of the test.

3. Test Description :

This apparatus is a modification of that described under the

JANAF Thermal Stability Test. The sample (300 mg.) is placed in the

3

test cell or vessel (volume ~ 1 cm ) and is in intimate contact with

a thermocouple. The apparatus also has a burst diaphram-vent tube

system to release gases formed during decomposition if the pressure

reaches too high a value, and a pressure transducer to provide

measurement of the total pressure inside the vessel as heat is supplied

from a bath at a constant rate. The nominal heating rate of the bath

is 8 to 10 *C per minute. Silicone oil is used in the range 0 to 370 *C.

18
and a low-melting alloy (i.e., Wood's metal) in the range 100° to

500°C. Recorders are used to monitor, first, the difference between

the sample temperature, T, and bath temperature, T^, as a function

of bath temperature, and, second, pressure, P, as a function of bath

temperature. No agitation to minimize thermal lag is used.

4 . Test Evaluation :

The threshold temperature is the lowest temperature at the lefthand base of the positive r peak r which appears rr in the plot r of T-T vs T . O O

The threshold temperature is an indication of the onset of thermal

instability in the sample. A potential hazard exists, therefore,

when the temperature of the sample exceeds this value. The instantaneous

rate of pressure rise with bath temperature (dP/dT) can be obtained

from the plot of pressure vs. bath temperature. The maximum pressure

generated and the rate of pressure rise are useful hazard parameters

related to rough approximations of reaction time, and damage potential.

Examination of the rate of temperature rise of the sample,

dT/dt, and rate of temperature rise of the bath, dT^/dt, not only

allows an evaluation of the Arrhenius constants, but also provides

for arbitrary scaling of the process. A simpler, and probably

preferable procedure, may be to record only T^ corresponding to a

runaway condition (e.g., a specificed value dT/d t>>dT^ /d t, or rupture

of a pressure disk; there is some arbitrariness in the definition

of the runaway criterion, but this feature may not be serious).

19
and then repeat the experiment with a different sample diameter, d.

The Frank-Kamenetskii condition then gives the value of E from

- (d./d )2 = (T /T )^ exp[(E/R) (1/T .-1/T -) ]

.

i / oi oz oi oZ

This procedure obviates the necessity of evaluating A and \^ and

allows immediate scaling to any size.

5. Data :

None provided [10],

20
c . Self-Heating Adiabatic Test [11]

1 . Purpose of Test :

To follow the thermal decomposition of poorly conducting

materials (i.e. wood) under conditions such that heat exchange

between the sample and its surroundings is kept to a minimum (i.e.,

adiabatic conditions).

2 . Operating Principle :

A fuimace is designed with such controls so as to keep the

furnace temperature as close to the temperature at the center of

the sample as possible. After a sample had been brought into

thermal equilibrium at a given temperature, the furnace temperature

control system is changed automatically to allow the sample to selfheat up to a temperature of 500" C with minimal heat exchange to the

surroundings inside the furnace.

3 . Test Description :

The sample is suspended in the furnace by a wire fastened to a

strain-gauge dynamometer. Thermocouples are mounted at the center

and near the surface of the specimen as well as in the air within

the furnace. The operating range of the furnace is from 30° to 500°C.

When the temperature of the interior of the sample reaches the air

temperature within the furnace and slightly over-shoots this

temperature because of se If -heating , heat is supplied to the furnace

by means of a control system so as to maintain as small a temperature

diffe rence as possible between the sample interior and the gases

inside the furnace. A recorder provides a continuous record of the furnace

temperature, two or more temperatures within the sample, and the weight

of the sample. ^
4 . Test Evaluation :

Since there is no heat-transfer balance, no steady-state is

possible in the self-heating adiabatic test, and the 6 -parameter

criterion does not apply directly. This test can be applied in two

ways. First, one can measure the time to thermal runaway as a function

of T^ . The runaway times so measured will correspond to the

predicted behavior of actual materials in the limit of perfect

insulation or of very large size. Thus, while the procedure to obtain

this information is straightforward in principle, the test will

probably be too severe for all but a very few practical situations.

The second way is to use the test for measurement of fundamental

parameters A and E--in the manner described by Gross and Robertson

[12]. The information so obtained can be used for calculation of

F rank -Kamenet ski i parameters (6, critical size as function of T^)

.

5. Data :

Values have been reported [12, 13] using a modification of the

above apparatus, for the activation energy, heat-generation coefficient

(product of the Arrhenius pre-exponential factor and heat of

decomposition), thermal conductivity, heat capacity, and density

over specific temperature ranges for: wood fiberboard, cotton linters,

sugar pine, cork, crepe rubber, GRS rubber, foam rubber, raw linseed

oil, rapeseed oil, sperm oil, olive oil, castor oil, neatsfoot oil,

ammonium perchlorate, and nitrocellulose plastic.

22
d. Vacuum Thermal Stability Test [14]

1 . Purpose of Test :

To determine the explosive character or tendency toward

decomposition of materials at elevated temperatures (200° to 350°C)

in vacuum in a storage environment.

2 . Operating Principle :

This test measures the amount of gas evolved by a sample at a

fixed temperature.

3 . Test Description :

The test chamber consists of a cylindrical aluminum block

(radius 4", height 12") and contains a sample holder with twelve

sample holes some of which are used to accommodate thermocouples,

nichrome heating elements, and thermometers. Mullite tubes are used

to protect thermometers and other sensing elements. The vacuum

stability test chambers are controlled from 35° to 350°C to about

±0.2°C. Continous AC power is attained by adjustment of two variacs

for constant temperature. Temperature control is obtained by shorting

out a resistor in series with each of two heating elements. Samples (about

0.2 gram each) are sealed in evacuated tubes. Each tube which contains

a sealed sample is also connected to a mercury manometer. The enclosed

samples and manometer units are placed in the test chamber at 260*C after

room temperature and barometer readings are taken. Manometer and barometer

readings are taken for a two-hour period. The volume of gas evolved from

the thermal decomposition is calculated as ml. of gas per gram of sample

per hour at 260°C. The test may be conducted at temperatures as high as

350*0.

23
4. Test Evaluation :

Evolution of surface moisture or occluded volatile impurities

can give higher pressure readings than appropriate for testing the

sample especially in the first 20 minutes of readings. Outgassing

and drying of samples prior to testing should be given particular

attention. Sublimation of the sample to cooler parts of the system

can also occur.

5. Data :

Vacuum stability data for 24 explosives are given in reference

[19]. Some samples are:

Amatol 50/50: 1 cm"^/40 hrs at 120**

C

PETN: 0.5 cm^/40 hrs at 100*

C

Tetryl: 1 cm"^/40 hrs at 120*C

TNT: 0.23 cm^/40 hrs at 120*

C

24
e . Thermal Surge Test [15]

1 . Purpose of Test :

To determine explosion temperatures (temperatures for which

there is a delay time of 250 M sec before explosion).

2 . Operating Principle :

The discharge of a capacitor across a thin-walled tube provides

the thermal stimulus to initiate explosive decomposition. The timetemperature profile of the decomposition is obtained from oscillographic

records. Although the tubes are thin-walled (0.089 mm), they have

considerable strength and provide a state of heavy confinement for

the explosive or unstable material.

3 . Test Description :

A test sample is loaded into hypodermic needle tubing which is

heated, essentially instantaneously, by a capacitor discharge. The

temperature and time of the explosive event are recorded from a continuous

measurement of the electrical resistance of the tubing by means of an

oscilloscope. The test is particularly suited to liquid material but solids

can also be accommodated by melting prior to their insertion into hypodermic

needle tubing. Materials are subject to temperatures in the range of

260° to 1100°C and delay times of 50 m sec to 50 ^ sec. The delay time,

T is given by A exp (B/RT) where A and B are constants (somewhat related

to the Arrhenius pre-exponential factor and activation energy), R is the

gas constant, and T is the absolute temperature.
4 . Test Evaluation :

The thermal surge test supplies data on explosion temperatures

which represent conditions of minimal heat transfer. This

test measures the true induction time of an explosive rather than

the time required to heat up the sample. Wenograd [15] was able

to show a correspondence between the temperature of the system

250 |J sec prior to explosion and impact test data. The activation

energy parameter obtained in thermal surge test measurements

under dynamic conditions are considerably lower than those determined

in other measurements under isothermal conditions. This test is

probably one of the best available approximations to a point source

heat initiation of an unstable material in a multi component system.

5. Data : w^; -

Results are provided in reference [15] for the critical temperature

for explosion in 250 (a sec, "apparent" activation energy, and

"apparent" logarithm of the Arrhenius frequency factor for: nitroglycerine, pentaerythritoltetranitrate (PETN), bis (2, 2, 2-trinitroethyl)

nitramine (BTNEN), 2 , 2, 2 - trini troethy 1-4,4,4 - trini trobuty rate (TNETB),

2, 2-dinitropropy 1-4,4,4-trinitrobutyrate (DNPTB), tetryl, trini trobenzene,

and trinitrotoluene. More data are available in reference [16] which

include some of the above explosives and also: ethyl nitrate

chloropicrin, trini troani so le, 2,4-dinitrophenol, nitrobenzene,

aery loni tri le, crotonaldehyde , ni trome thane, m-dini trobenzene, 1,1-

dimethy Ihydrazine , 95% hydrazine, and 85% hydrazine hydrate.

26
f . Explosion Temperature Test [17]

1 . Purpose of Test :

To determine the temperature at which a material explodes,

ignites, or decomposes after a five second immersion in a Wood's

metal bath.

2 . Operating Principle :

This test gives an estimate of how close the explosion temperature

is to ambient condition for a material, and, hence, provides a

measurable indication of thermal instability.

3 . Test Description :

The material to be tested (~ 25 mg.) is placed in a copper test

tube (high thermal conductivity) and immersed in a Wood's metal bath.

This test is made at a series of bath temperatures, and the time lag prior

to explosion at each temperature is recorded. The bath temperature is

lowered until a temperature is reached at which explosion ignition,

or apparent decomposition does not occur. The bath temperature working

range from about 125° to 400°C. The sample is removed from the bath

after 5 minutes if no explosion had occurred at 360°C.

27
4 . Test Evaluation :

The explosion time is very nearly independent of sample size

provided the sample size is in the range 10 to 40 mg. In this

regard, data for all 25 mg samples should be comparable to data

for all 3 mg samples. Particle size is also important in providing

consistent results for a group of materials. Rapid equilibration

of the sample upon contact with the high temperature bath will

depend upon the heat capacity and thermal conductivity of the

material, and could be a major uncertainty in this test. Explosion

temperature data as a function of time serve as useful indicators

to assist in maintaining safe thermal condition during handling

and transport . • ;

.

5. Data :

Explosion temperature, explosion times, and activation ene;rgies

are provided for [17]: trini tropheny Imethy Ini tramine (tetryl),

N,N-dini tro-N,N ' -di (I' -ni troxyethy 1) oxamide, black powder, lead

azide, ammonium nitrate, lead styphnate, picric acid,

tetrame thy lolcyclopentanone te t rani t rate, e thy It rime thy lolme thane

trinitrate, pentaery thritol tetrani trate, nitroglycerine, erythritol

tetranitrate, tetrame thy lolcyc lohexano pentani trate, nitrocellulose,

mercury fulminate, diazodini trophenol, and ethy lened ini tramine

.

Considerable explosive temperature-explosive time data are

available in reference [18],

28
g. Adlabatic Storage Test [20]

1. Purpose of Test ;

To determine the time interval (induction period) during which

the gradual increase of temperature due to self -heating will result

in explosion of the material under test,

2. Operating Principle ;

A temperature -time profile is obtained for a substance from a

preselected starting temperature until rapid exothermic decomposition

results. An indication is provided of what constitutes an unfavorable

storage environment under adiabatic conditions.

3 . Test Description ;

A Dewar vessel which can accommodate a sample volume of about

one liter is placed inside a furnace chamber. A stainless steel lid,

which has a protective insulating material on its inner side, covers

the Dewar vessel. The furnace chamber is surrounded by an insulating

mantle. A coiled heater is immersed into the sample and raises its

temperature to a preselected starting value. The temperature at the

center of the sample and at the wall of the furnace chamber are monitored

using thermocouples. The temperature difference between the two

thermocouples controls the voltage input to the furnace chamber so

that the temperature difference is practically reduced to zero for the

duration of the test, thus, rendering the Dewar vessel with its sample

under adiabatic conditions, i.e., virtually no heat exchange between

the sample and its surroundings. Heat effects as small as 10 mW per

kg. can be detected. This is equivalent to measuring temperature

increments of as little as 0.5°C per day and still maintaining adiabatic

conditions

.

29
4 . Test Evaluation :

Testing of a sample can take days or weeks if the preselected

starting temperature is too low. This situation is difficult to

avoid especially if the substance being examined has not been studied

before

.

5. Data :

The induction period for the adiabatic storage and apparent

activation energy of t-buty Iperbenzoate is given [20].

30
h. Isothermal Storage Test [20]

1 . Purpose of Test ;

To determine the heat generation rate as a function of time and

an estimate of the induction period at a given temperature for a

material.

2 . Operating Principle :

A sample begins to self-decompose and to generate heat inside

a large aluminum block held at a constant temperature. The heat flow

from the sample to the block takes place via a Peltier element which

produces an electric signal. The rate of heat generation as a

function of time is observed.

3. Test Description :

A large aluminum block is surrounded by an insulating mantle

and maintained isothermal via an electrical heater and the rmo regulator

system. Two sample containers which can hold up to 50 grams are

located at the center of the block. An inert material is placed in

one of the containers and used as a reference temperature junction.

A Peltier element is located below each of the sample containers and heat

flow from the sample to the aluminum block takes place via the Peltier

element, producing an electric signal. The apparatus is 1 sensitive

enough to detect 1 mW per kg.

31
4 . Test Evaluation :

Test can be made with a large variation in sample size (up to

50 gramsi The critical radius of the sample can be calculated from

the results. This test also allows one to examine the degree of self

decomposition as a function of time at a constant temperature.

Testing can take days, weeks, or months depending upon the number

and range desired for various parameters.

5. Data ;

' "' ' ' ' '

A plot of the heat generated as a function of time is given for

t-butyl perbenzoate at 58.2* and 67 A" C, Mean activation energies

are calculated by constructing iso-conversion lines [20].

32
i . Exothermic Decomposition Meter Test [20]

1 . Purpose of Test :

To determine the self-heating of a sample at small to moderate

heat generation rates as a function of temperature or time.

2 . Operating Principle :

A cylindrical aluminiom block contains a cavity which has a

Peltier element attached at the bottom and a sample is placed on the

Peltier element. Heat flow from the block to sample is measured by

means of the Peltier element which provides an electrical signal to

a recording device.

3 . Test Description :

A sample vessel constructed of stainless steel (volume, ~ 2 cm )

is positioned over a Peltier element, and both are housed inside the

cavity of a cylindrical aluminum block. This central block is

surrounded by mantles containing electrical heating elements in addition

to an insulating layer. The electrical input to the block and mantles

is maintained in such a manner as to keep the temperature difference

between the block and mantles as small as possible while the block

is heated linearly at about 10*0 per hour. The heat flow from the

aluminum block to the sample is measured by the Peltier element. As

soon as the sample begins self-reaction the heat flux to the sample

starts to decrease. From a plot of the heat generation of the sample

vs. the reciprocal of the absolute temperature, the activation

energy can be calculated.

33
4 . Test Evaluation :

Changes in the heat capacity of the aluminum block over the

temperature range 20° to 200°C will cause the temperature increase

over this range to be slightly non-linear. The Peltier element is

temperature dependent, and calibration using a pure copper sample

having known thermal properties is recommended.

5 . Data :

The rate of heat generation as a function of the

temperature is shown for t-butylperbenzoate [20].

34
j . Homogeneous Explosion Test [20]

1 . Purpose of Test :

To determine the pressure-time profile of the thermal explosion

of solid or liquid materials.

2 . Operating Principle :

A sample is heated under adiabatic conditions in a closed vessel

until explosion occurs. The maximum rate of pressure rise and the

maximum overpressure are measured as a function of time at different

heat input rates.

3. Test Description :

'

About 100 ml of a sample is introduced into the lower part of a

stainless steel vessel. The lower section is sealed off from a larger

upper section above by a membrane (breaking pressure ~ 1 bar). The

larger upper section serves as a free space for the expansion of

reactant or product vapors. During the main part of the induction

period, pressure equalization is accomplished by a capillary tube

connecting the upper and lower sections of the vessel. The two -compartment

vessel is placed inside a larger vessel of 20 liter capacity which

seals the former from the external surroundings. A heating mantle

around latter vessel allow heating of the inner vessel to take place as

near to adiabatic conditions as possible. Around the sample vessel there

is also an auxiliairy heater which heats the sample at a constant (but

adiabatic) rate until explosion occurs. When explosion takes place,

the membrane is ruptured and expansion into the larger volume takes

place. A piezo-electric pressure transducer records the pressure

prior to, during, and after explosion.

35
4 . Test Evaluation :

Differentiation of materials which give large rates of pressure

rise and overpressures can be singled out from those which give low

values. Subsequent precautions for shipping, transport, and storage

can be taken.

5. Data :

Maximum rates of pressure rise and maximum overpressures are

given for various heat input rates for t-butyl perbenzoate. Similar

data for the self-heating process are also provided [20].

36
k Self-Heating Test for Organic Peroxides [21]

1 . Purpose of Test ;

To determine the minimum ambient temperatures for the selfheating to explosion of thermally unstable compounds in charges of

specified shape but varying size.

2. Operating Principle :

The thermal decomposition of organic peroxides is observed from

studying temperature-time plots to obtain the critical temperatures

for explosion, heat transfer coefficient data, and apparent activation

energies

.

A circulating fan located within the working space of the furnace

provides temperature control to within ±0,5°C.

3 . Test Description :

A cylindrical tube furnace constructed of steel housed an aluminum

open-topped cylindrical container which could hold 40 to 60 grams

of organic peroxide. The furnace was heated electrically over the

range 50** to SSO^C and could be maintained at a fixed temperature to

within ±0.3**C. The progress of self-heating in the peroxide sample relative

to the furnace was observed by using a differential thennocouple at the

center of the sample. A second thermocouple attached to the side of the

container monitored the surface temperature. Temperature- time plots

were recorded for different cylindrical diameters for the samples and

critical temperatures were calculated.

37
Explosion studies were carried out with sample amounts as large

as 800 grams using a somewhat modified apparatus, and similar parameters

examined.

4 . Test Evaluation :

The chief disadvantage of the method is the long period over which

readings must be recorded and the long time required for the furnace

to stabilize following a large change in operating temperature.

5 . Data :

, -

Critical temperatures leading to explosion, heat transfer coefficients,

maximum temperature increase due to self-heating (AT max) in charges

which did not explode are tabulated for various charge diameters for

dry benzoyl peroxide and benzoyl peroxide paste (containing 35%

dimethyl phthalate) [21],

References [22-25] contain information on reviews of the kinetics of

thermal decomposition of organic peroxides, hazard classification of

peroxides, and other test methods.

38
1 Differential Thermal Analysis (DTA) Test

1 . Purpose of Test :

To determine exothermic and endothermic reactions in a material

as heat is applied at a particular input rate.

2 . Operating Principle :

The material under test and a stable reference material are

heated simultaneously at the same rate. Exothermic and endothermic

traces are measured using a recorder providing a temperature-time

plot of the reaction process.

3 . Test Description'

The material to be tested (5 to 25 mg) and a reference material

(such as alumina or glass beads) are placed into identical compartments

in an aluminum block. Heat is supplied to both compartments at the

same constant rate of input. Temperatures are measured using thermocouples in conjunction with automatic recording devices so that a plot

of temperature vs. time is obtained. A shift in the base line results

from a change in the heat capacity or mass of the material under test.

Particular care must be given to the type of temperature sensor used

and to the choice of its location in the compartment inside the

aluminum block. The geometry of the sample and thermal characteristics

(such as thermal conductivity) of the sample will affect the shape

of the DTA curve.

39
4 . Test Evaluation :

From the exo therms and endo therms of the DTA curve decomposition

temperatures corresponding to various rates of temperature rise can

be obtained. Kinetic parameters can be calculated as a result of

properly varying the heating rates and assuming a constant degree of

conversion of reactant when a specific thermal event (such as the

peak temperature of a given exotherm) takes place. When the

temperature sensors are placed in the path of the heat flow the DTA

apparatus can measure the enthalpies of processes such as heats of

decomposition or transition.

5. Data :

See references [26-32],

As a specific example, approximate heats of explosion where

determined using DTA for the following compounds: trinitrotoluene (TNT),

cyclotrimethy lene trinitramine (RDX), pentaerythri to 1 tetranitrate

(PETN), Tetryl, nitroglycerine, N-5 Propellant, JPN Propellant, ABL-2056

Propellant, tetranitromethane and some laboratory prepared composite

propellants.

40
m Shock Tube Test [33]

1 . Purpose of Test :

To determine the expected linear relationship between the

ignition delay time required to detonate fine dusts or sprays of

explosives and the reflected shock temperature.

2 . Operating Principle :

Liquid or finely divided solid samples are instantaneously

subjected to very hot gaseous environments of known composition.

The sample is dispersed by the hot flow behind the incident shock

and the particles thus formed are ignited with a measurable time

delay by the twice-shocked gases behind the reflected shockwave.

3. Test Description :

Finely-ground solid samples (less than 20 \i diam) are supported

on a fuse wire coil 6 mm from the end plate of the low pressure channel.

Liquids are similarly supported but placed about 50 mm from the end

plate to ensure spray formation behind the incident shock wave. The

diaphram separating the high pressure section from the low pressure

channel of the shock tube is ruptured and the incident shock provided

by the driver gas disperses and thereafter ignites the sample. The

reflected shock wave temperature is calculated from the velocity of

the decelerating incident shock at the end plate. The explosion time

is calculated from the oscillograph trigger of the incident shockwave

and the photocell trace. Apparent high temperature activation energies

are calculated from:

ln(tg/T^^) = A + E/R T^

41
where is the explosion time, is the reflected shock temperature,

A is a constant, E is the activation energy, and R is the gas constant.

4. Test Evaluation :

Only a few tests are required to characterize a given sample which

needs to be available in only milligram quantities. Environment

de-sensi tization may be studied using this technique by studying

test responses with a variety of channel gases. The shock tube data

were shown to correlate well with impact test data.

5. Data :

High temperature activation energies were calculated for:

pentaery thritol tetranitrate (PETN), nitroglycerine (NG), lead azide,

hydrazine nitrate, lead styphnate, tetracene, and ni trosoguanidine

\33]. . .

42
2 . Mechanical Tests

A great many sensitivity tests using mechanical stimuli have

been devised, mostly by the military, hence generally intended for

the rating of sensitive energetic materials (explosives and propellants)

.

Since we are interested mostly in commercial materials of lower

sensitivity (although often shipped in larger amounts), the main

problem is to select a few suitable tests from the large number of

existing ones.

a. Detonability (Shock-Initiation) Tests

First a word about tests which do not appear particularly relevant.

All tests which depend on development of steady-state (Chapman-Jouguet)

detonation for a positive result ("Go" in a "Go - No Go" test) are in

this class.

Two such tests , the Card-Gap (or "Booster") test [35, 36] and the

Critical Detonation Diameter test [40, 41] may illustrate why these

are less important in the context of our study. In both tests, the

detonation of a standard donor charge transmits a strong Shockwave

to the test sample, initiating, or failing to initiate, detonation

in the latter. In the Card-Gap test, the sample diameter is specified

and constant (usually either 1" or 2.5"), and the shock strength

from the donor is decreased until detonation fails to develop

in the acceptor. In the Critical Detonation Diameter test, the shock

strength from the donor is held at the maximum (unattenuated) value,

and the sample diameter is decreased until there is no detonation.

While the results of both tests are unusually clear-cut and easily

interpretable, they apply only to sensitive explosives. For example,

43
three energetic and potentially hazardous materials, ammonium

perchlorate, ammonium nitrate and dinitrotoluene, are all rated

negative by both tests even under the maximum stimulus (detonation

pressure of Tetryl) and largest diameter (2.5") specified by the

tests. The tests do not tell anything about detonability in larger

geometries, and, what is worse, nothing about the possibility of

destructive sub-detonation explosions. Both tests can be scaled

up, in principle, but this is not a promising approach for two

reasons. First, scaling to practical cargo diameters would be

immensely expensive, and could be done only on veiry large proving

grounds; and, second, the specified stimulus is so strong (shock

pressure of the order of 10^ atmospheres) that it can be generated

only by detonation of another closely adjacent charge (which could

hardly ever arise in the context of problems under this study),

or possibly by a high velocity projectile.

44
a-1 Card-Gap Test (Booster Test) [34. 35, 36. 38. 39]

1. Purpose of Test ;

To establish the ease with which a potentially hazardous material

will detonate as a result of detonating a Tetryl charge in its

proximity.

2. Operating Principle ;

The test provides an estimate of the distance a Shockwave must

travel from a booster charge in a confined steel tube through a

series of Plexiglas disks to detonate a potentially explosive material,

3 . Test Description ;

The working apparatus consists of a confined steel tube.

The donor charge consists of two Tetryl pellets (25g. each) and is

initiated by a No. 8 electric blasting cap. The shock attenuator

has the same diameter as the Tetryl pellets, is made of Plexiglas or

cellulose acetate, and separates the booster charge from the material

under test. From 25 to 50 test firings are required to determine a

reasonable threshold gap value. High-velocity detonations are

characterized by means of detonation velocity probes, witness plate

damage, and the size of fragments from the steel tube.

45
4. Test Evaluation ;

The card gap test has some application to situations in which

a hazardous material in storage is near other hazardous materials more

prone to detonation. For such cases, this test serves to compare

the relative ease of initiation to detonation and also provides an estimate

of the force needed to bring about detonation,

5. Data ;

References [18, 34, 35, 36, 38, 39]. This test is applicable to

gels, slurries, cast solids, granular mixes, powders, and liquids.

46
a-2 Critical Diameter and Detonation Velocity Test [40. 41]

1, Purpose of Test ;

To determine the critical internal diameter in a given type of

metal or plastic tubing below which propagation of stable high velocity

detonation will not take place. Detonation rates can also be measured.

The test described here is oriented toward liquid samples although

solids can be tested provided some modifications in apparatus are

implemented

.

2 , Operating Principle ;

A high velocity detonation is initiated from a blasting cap

and booster. The detonation is propagated through the material under

test which is contained in a tube constructed of a certain material

(such as steel, aluminum, plastic, etc.).

3, Test Description ;

The apparatus is constructed of stainless steel and consists

of three sections; the first section (length 2", dlam, 1") contains

a high-energy booster (Pentolite) and a No. 8 blasting cap, the

second or donor section (length 5", dlam. 1") contains the liquid to

be tested; the third or "test" section (length 30", dlam. variable,

0.125" to 1") also is filled with the liquid under test. The end of

the third section is closed by plugging, clamping, or crimping. If,

after detonation of the booster, the "test" section of the apparatus

Is completely fragmented over its entire length, the critical diameter

of the tubing has been exceeded. Fragmentation over a short distance

Indicates that the tubing is less than the critical diameter.

47
A ten-MHz counter, or an oscilloscope with a 5 microsecond

per cm. sweep frequency is used to measure the time of propagation

between timing stations which have been set up. Detonation

velocities are calculated from the measured time lapses over the

distance between stations.

4. Test Evaluation ;

' ! '-^ -

Critical diameter values are reported as the largest diameter

at which an explosive reaction will not be propagated for an explosive

material under certain specified conditions. Application of the

data to real situations must be done with care to be sure that

conditions are comparable with those present when the test data were

determined. .

- n ' ^

5. Data ;

References [40, 41],

.

' . ^ . " r" '-..ail:

48
b . Compression Tests

Two types of standard tests and one non-standard compression

test will be mentioned. The most common among the compression tests

are impact (drop-weight) tests, which are used extensively both by

military and civilian agencies. An extensive correlation of the

results from the Bureau of Mines [42] and the Picatinny Arsenal [43]

impact testers has already been made in the first phase of this study [1],

It should be mentioned that the correlation between the results from

the two testers is better for liquids than for solids. This is

perhaps not surprising, because processes such as crushing of the

granules and friction depend crucially on geometrical details and

size of the sample. It should also be mentioned that even for liquids,

the agreement between laboratories is far from perfect. JANAF Liquid

Propellant Test No. 4 is a standardized impact test for liquids [44], and is

identical with ASTM Test D 2540-70, Standard Method of Test for DropWeight Sensitivity of Liquid Monopropellants [45].

The JANAF Liquid Propellant Test No. 5 [46] is a piston-compression

test which may realistically simulate some aspects of accidents (e.g. tankcar collisions). The procedure is to accelerate a liquid sample to

a specified velocity and then stagnate it against a target plate.

The correspondence of the test stimulus to anticipated realistic

conditions remains to be determined. The equipment for this test is

elaborate and probably expensive.

49
b-1 Impact Test

1 . Purpose of Test ;

To determine the minimum drop height of a falling weight

which strikes an explosive material and produces either a mild or

violent decomposition reaction. Both falling weight and explosive

material have a fixed and constant mass.

2. Operating Principle ;

Impact energy is supplied to an explosive by a weight of constant

mass which is dropped from varying heights to establish the minimum

height to provide detonation, decomposition, or charring. The impact

provides rapid compression and crushing of the sample (which may

involve a frictional component of crystals rubbing against crystals)

and detonation ensues,

3. Test Description ;

^ :

The two most prevalent impact tests are those by Picatinny

Arsenal (PA) [43] and the Bureau of Mines (BM) [42].'

In the PA apparatus a sample is placed in the recess of a small

steel die cup, and capped with a thin brass cover, A cylindrical

steel plug is placed in the center of the cover, which contains a

slotted -vent and the impact of the 2 kilogram weight is transferred

to the steel plug.

In the BM apparatus a 20 mg, weight is always employed while

the PA sample size may be varied for each experiment. The explosive

sample is held between two flat parallel plates made of hardened

steel and impact is transmitted to the sample by means of the upper

plate. Sample decomposition is detectable by audible, visual or other

sensory means.

50
In an apparatus used by the Bureau of Explosives (part of the

Association of American Railroads) and cited in Title 49 CFR (DOT

Hazardous Materials Regulations) a falling weight is guided by a pair of

rigid uprights into a hammer -anvil assembly containing a 10 mg, sample of

explosive. Reproducibility can become a problem here because of a nonideal collisions between the drop weight and the impact hammer since

only a fraction of the drop-weight energy is transmitted to the sample,

4. Test Evaluation ;

Greater confinement of the sample will limit the translational

component of the impulse to a smaller area as is the case with the

PA apparatus. Factors which play an influential role in the test are:

materials of construction, sample thickness, sample density, hammer

geometry, mass of drop weight, impact area, surface finish, the

surrounding atmosphere, temperature, and pressure. Modifications can

also be made to accommodate cast and liquid samples.

Impact tests suffer from the drawback that the fundamental

processes leading to energy release are complicated and poorly

understood. Failure of good agreement between various impact tests

shows that these tests contain uncontrolled parameters. On the other

hand, (1) partial correlations do exist, (2) the history of the test

indicates rough agreement with field experience, (3) the stimulus

is of reasonable severity, (4) the tests are widely known and

relatively easy to use. These facts make them useful for a partial

definition of hazards.

5. Data :

Impact test data can be found in the following references: [8,

18, 19, 36, 42, 43, 44, 47, 48, 49, 52, 54, 55, 56].

51
b-2 Adlabatic Compression Sensitivity Test [46]

1. Purpose of Test ;

To determine the sensitivity of hazardous liquids to the initiation

of decomposition in the presence of rapidly compressed gas bubbles.

2. Operating Principle :

A gas bubble in contact with a liquid is compressed rapidly by

a gas driven piston. The minimum kinetic energy input per unit volume

of gas bubble which is needed to initiate decomposition of the liquid

is measured.

3 . Test Description ;

The apparatus consists of two cylinders with mechanicallycoupled pistons axially alligned. Pressure multiplication is achieved

by having a small piston in the test chamber cylinder and a large

piston area in the driving gas cylinder. When the apparatus is fired,

gas from a high pressure tank is admitted to the cylinder thereby

driving the end of the small piston against the liquid. The area ratio

of the pistons is 20:1; maximum working pressure of the apparatus is

20,000 psi.

Instantaneous pressure in the test chamber is measured by a

strain guage on the burst diaphram which is used to seal the chamber.

The piston movement is recorded by the signal from a linear motion

potentiometer on the large end of the driving piston.

52
4. Test Evaluation:

The sensitivity of liquids to ignition in the presence of a

rapidly compressed air bubbles can be used as a sensitivity rating

scale. The sensitivity parameter is based on the minimum compression

energy per unit volume of gas bubble necessary to bring about gas

phase ignition followed by propagation to the liquid. The sensitivity

parameter appears to be constant over the range of bubble volumes

(0.2 ml to 0.8 ml) tested.

5 . Data ;

For the materials tested, the sensitivity ratings in the presence

of air bubbles are the following [46]:

Liquid

60% ethyl nitrate, 40% propyl nitrate

n-propyl nitrate

nitrome thane

methylacetylene

hydrogen peroxide

hydrazine

unsyrametrical dimethyl hydrazine

ethylene oxide

Small variation in test temperature did not appear to be an important

effect relative to other random experimental errors.

Sensitivity,

kg-cm/ml

4.0 + 0.8

6,7 ± 1.2

10.4 ± 1.7

86 ± 12

>144

>144

>144

>144

53
c. Confinement Tests

Among non-standard tests, the Heavy-Confinement Cap Test merits

a mention. In this test a blasting cap is set off inside a confined

liquid, but the liquid need not, in fact usually does not, detonate.

Instead, the occurrence and the extent of energy release is judged.

Thus the test combined a sensitivity measurement with power measurement. The main feature of the test is that the stimulus is one of

high pressure and short duration.

Heavy Confinement Cap Test [57]

1 . Purpose of Test ;

To determine the sensitivity of a chemical (liquid or solid)

as a result of detonating a blast cap immersed in it under conditions

of confinement. Decomposition takes place by a combination of shock,

heat, and pressure. A quantitative measured of the energy release is

obtained.

2 . Operating Principle ;

The sensitivity of a chemical to decomposition under confinement

leads to an estimate of the energy release obtained by the decomposi tiai.

The height to which a weight is impelled is related to the combination

of the sensitivity - energy release parameter observed in this test.

54
3 . Test Description ;

A cylinder of cold-rolled steel (5 in. diam. ; 6 in. high)

contains a central borehole (1 in. diam.; 4 in. deep). The top-face

of the cylinder is grooved radially by a cut (0.125 in. wide;

0.0625 deep). The sample (~ 4.0 ml, or 4.0 grams) is placed in a

Pyrex test tube along with an Atlas no. 8 electric dynamite cap.

A cylindrical test weight made of steel and weighting 5 lbs. is

placed symmetrically over the borehole containing the test sample.

Upon firing the dynamite cap, the test weight is elevated by the

explosion and the maximum height is observed visually. Heights above

20 feet are calculated using a stop watch to measure the time of

flight of the test weight from explosion to impact on the ground.

If tests are conducted between 10° to 30° C, temperature has little

effect upon test results.

4 . Test Evaluation ;

The energy release measured by this test using unreactive materials

such as water, cyclohexane, and benzene show some degree of correlation

with the enthalpy of vaporization of the compound tested. Heights

attained are usually 2 to 5 feet. Reactive materials indicate an energy

release which does not correlate with the enthalpy of vaporization, enthalpy

of combustion, enthalpy of decomposition, or enthalpy of formation. It

appears that decomposition kinetics probably limit the energy release

measured.

55
This test is particularly applicable to compounds yielding a

zero card-gap test value, and is useful in initial screening

operations of various hazardous or semi -hazardous formulations.

Results are believed indicative of adiabatic compression hazard.

5. Data :

Test results for a few common hazardous materials are [57]:

Compound Quantity, ml. Height, Feet

Nitromethane 4.0 48

907o Hydrogen Peroxide 4.0 39

Hydrazine 4.0 39

n-Propyl nitrate 4.0 29

Ethylene Oxide 4.0 10

Methyl Acetylene 4.0 8

56
d. Friction Tests [54. 58. 59. 60. 611

The existing (military) friction-sensitivity tests are not quite

pertinent to our problem, because they give negative results for

all but the most sensitive explosives. For example, even such high

explosives as TNT and Composition B are unaffected by the Picatinny

Arsenal friction test [59]. This is unfortunate, because friction is

a common stimulus in realistic accidents, and should not be discounted

a priori. However, a realistically severe friction test from which

definitive rankings or conclusions can be obtained, appears not yet

developed.

B, Power Tests

The sensitivity tests discussed so far deal with the ease of

initiation, hence fundamentally with chemical kinetics. Another

aspect of the hazards problem is the magnitude of energy release.

The distinction between "sensitivity" tests and "power" tests has

been stated clearly in our previous report [1], While the sensitivity

tests present a more crucial aspect of the problem, because the

objective is to avoid the release of any energy accidentally, the power problem

should not be neglected for two reasons. First, a large energy

obviously represents a relatively high hazard. Second, the problems

of power and sensitivity are coupled: materials having high

exothermicity (relative to heat capacity) will self-heat to high

temperatures and thereby represent a high sensitivity hazard.

57
There are great many explosives tests designed to evaluate the

energy release (sometimes also the rate of energy release) under

detonative conditions. These tests almost invariably give a measure

of energy release by the effect on the surroundings: size of crater

or dent in a plate generated by detonation, displacement of a target,

or generation of a blast wave in air. Among these, the measurement

of the blast wave appears especially useful.

Blast-wave measurement has the following advantages: (a) it

is simple and relatively inexpensive; it is done by the placement of

a piezo-electric gauge at a suitable distance with a suitable readout; (b)

the method of interpretation of records is highly developed, and

extensive tables and monographs are available relating the energy

release to blast parameters (usually in terms of "TNT equivalents");

(c) while the method has been developed primarily for detonative

energy release, it is also suitable for subdetonative explosions;

(d) the records give some (usually qualitative) information about the

rate of energy release; (e) if the material is deficient in oxygen

(which many materials are, the notable exceptions being oxidizers or

highly oxidized explosives), the measurement will include the

involvement of air in the experiment (or accident).

Since power tests deal directly with the effect of explosions

and only indirectly with the manner of initiation, the initiation

stimulus must be specified separately. It could be almost any

technique discussed earlier in this report. For example, blast gauges

could be used in conjunction with an impact test, a friction test,

the Heavy-Confinement Cap Test, or even thermal tests, to evaluate

the extent and the rate of energy release.

58

'
a. Plate Dent Test [8. 19]

1 . Purpose of Test ;

To determine the brlsance of an explosive, or its shattering

power. This is an intensive property and not depend upon the quantity

of explosive,

2 . Operating Principle ;

The depth of a dent produced in a steel plate is compared to the

dent produced by a charge of TNT.

3 . Test Description ;

An explosive cartridge of a certain diameter is placed on end

in contact with a steel plate of given dimensions. The explosive is

detonated from the opposite end of the cartridge. The depth of the

dent in the plate is measured. The test is repeated using TNT, and

the dent produced compared to that made by the test charge.

4 . Test Evaluation ;

The depth of the dent produced by the detonation is not

dependent upon the length (quantity) of the cartridge used in the

test and is probably related closely to the detonation pressure.

5. Test Data ;

Reference [18].

59
b. Ballistic Mortar Test [37. 42. 62]

1. Purpose of Test ;

To determine the maximum available energy which can be released

in an explosion.

2 . Operating Principle :

A standard weight of explosive is fired in a suspended mortar

with a tightly fitting steel projectile and the angle of recoil of

the mortar is measured.

3. Test Description ;

The apparatus is a compound pendulum consisting of a mortar

supported by a framework. The framework is suspended on knife

edges arranged so that the recoil distance of the mortar is automatically

recorded. The mortar has two chambers; an outer chamber for the

steel projectile; an inner chamber for the explosive charge. The

apparatus is arranged so that the center of percussion coincides with

the axis of the firing chamber. Three tests are run with the sample

and three tests run with a reference (such as TNT) . The quantity

of explosive being tested which gives the same recoil as 10 grams of

TNT is usually used as the basis for expressing the relative power

or strength of the explosive sample.

4. Test Evaluation ;

Ballistic mortar test data correlate poorly with the heat released

during the explosion of the sample. Care must be exercised during

interpretation.

5. Test Data :

Reference [18].

60
c. Air-Blast Test [64, 66. 67l

1 . Purpose of Test :

To determine the energy release from an explosive charge from

pressure measurements of the resulting Shockwave,

2 . Operating Principle ;

The effectiveness of an explosive charge which has been detonated

in air, can be obtained in terms of an equivalent weight of a standard

explosive (such as TNT) to produce the same air-blast overpressure

as the test explosive. Measurements of the pressure parameters are

made at fixed distances from the explosive charges and can be shown

to be proportional to the initial energy of the explosive.

3 . Test Description ;

An explosive charge is detonated in air with piezo-electric

pressure transducers placed at suitable distances from the charge. This

equipment is in turn connected to oscillographic and recording equipment.

As the Shockwave generated by an explosive charge travels away from

the source, the peak pressure decreases and the pressure behind the

shock front falls off in a regular manner. Measurement of the shockwave velocity provides data on the peak pressures and positive impulses.

The methods of interpretation of the data are highly developed and

extensive tables are available relating the energy release to air-blast

overpressures, usually in terms of equivalents of a standard explosive

(such as TNT) .

61
I

4. Test Evaluation ;

Peak pressure and impulse data for equivalent weights of a

variety of explosives as compared to a standard, such as TNT or

TNETB (trinitroethyl trinitrobutyrate) suggest that a direct

relationship exists between the equivalent weight and the heat of

detonation. Careful examination of blast test data indicate that

relatively large post -detonation reactions of the products with the

oxygen in the air can take place. The extent of these post-detonation

reactions can be estimated from firings of the explosive charge in

a nitrogen atmosphere followed by comparison to test data for firings

performed in air.

5. Data ;

References [18, 47, 64, 66].

62
d. Underwater-Blast Test [37, 64, 65]

1. Purpose of Test :

To determine explosive performance and energy release as a

result of detonating an explosive charge under water and measuring the

shock and bubble energies which are produced.

2 . Operating Principle:

Shock and bubble pressure are measured by two piezoelectric

pressure transducers in the water and attached to signal conditioning

equipment and oscilloscopes.

3 . Test Description :

About a kilogram charge with a suitable booster is suspended

in the center of a pond at a depth of 12 feet. Transducers are placed

12 feet deep and at a horizontal distance 12 feet from the charge.

The explosive charge is detonated under water and the shock produced

by the detonation front is transmitted to the surrounding water as

a pressure discontinuity followed by an exponential pressure decay.

The pressure front moves radially outward as a Shockwave in the

water until its velocity decays to sonic values.

63
An expanding gas bubble follows the Shockwave, This bubble is

composed of reaction products under high pressure. Expansion,

contraction, and collapse of the gas bubble follow with bubble

oscillations emitting secondary pressure pulses. The period of

oscillation is related to the internal energy of the gas and the

equilibrium pressure.

4. Test Evaluation :

Underwater explosive effects are more difficult to interpret in

comparison to those effects obtained in air. The energy release by

the detonation of the explosive charge is partitioned into (1) that

of the Shockwave, (2) that dissipated in the water during of the travel

of the Shockwave from the charge to the measuring instruments, and

(3) and that remaining in the oscillating bubble formed by the

detonation products. Comparison of the measured shockwave energy

with the detonation energy shows a rough linear relationship; a

direct proportion does not appear to exist.

5. Data ;

Reference [64].

64
C. Summary of Tests

An assessment of the relative importance of various tests

with respect to establishing and identifying the thermal stability

of a material is given below. As stated earlier, the state-of-theart is such that several tests are desirable for an assessment of

sensitivity.

Important

JANAF Thermal Stability Test No. 6

ASTM Standard Method of Test E-A76-73

Thermal Surge Test

Explosion Temperature Test

Exothermic Decomposition Meter Test

Adiabatic Storage Test

Differential Thermal Analysis Test

Impact Test

Air-Blast Test

Useful

Self -Heating Adiabatic Test

Isothermal Storage Test

Homogeneous Explosion Test

Vacuum Thermal Stability Test

Self -Heating Test for Organic Peroxides

Adiabatic Compression Sensitivity Test

Underwater-Blast Test

65
Marginal

Heavy Confinement Cap Test

(Only when used in conjunction with the Air-Blast Test)

Shock Tube Test .

'

Card -Gap Test

(only to determine whether sample is detonable for

diameters equal to 2.5 inches)

Friction Test

(Requires further development)

Less Important

Critical Diameter and Detonation Velocity Test

Plate Dent Test

Ballistic Mortar Test

66
V, Predictive Schemes

A. Expandability of the Predictive Schemes to Specific Reaction

Mechanisms

The predictive schemes, which had been examined in detail as

part of our research agreement with the DOT Office of Hazardous

Materials Operations in 1974 [1], were studied further to seek

improvement in their ability to estimate the hazard potential of

chemicals. The idea of developing separate schemes which could be

associated with different reaction mechanisms such as molecular

elimination reactions or bond fission processes, was explored. In

this connection, the critical review of kinetic data on thermally

induced, unimolecular, homogeneous, gas-phase reactions of molecules

and free radicals by Benson and O'Neal [68] was studied and was found

to be very helpful. The latter authors provided evaluated kinetic

data for over 160 simple bond-fission processes and 150 molecular

elimination reactions.

Although self -decomposition reactions are not all unimolecular

processes, such processes constitute a desirable focus for this

investigation for the following reasons: (1) more information Is

generally available for unimolecular processes than for other

processes, (2) their overall treatment is simpler, and (3) they

represent many explosive reactions adequately.

67
Certain generalizations regarding the gas-phase kinetics of

unimolecular reactions are possible. The generalizations are based

upon the assumption that the transition state theory is valid and

that the thermal contributions from radical recombination reactions

are essentially zero. In the case of simple bond-fission processes,

the endothermicity of the unimolecular rate-determining process is

equivalent to its activation energy. Exothermic effects in such a

process results from subsequent fast reactions. Also, the pre-exponential

or frequency factor does not appear to vary widely for the types of

compounds which had been reviewed. For most reactions, the

logarithm of the frequency factor was about 16 ± 1.5.

Some generalizations regarding molecular elimination reactions

are also possible. The enthalpy accompanying an elimination reaction

appears to be less positive than the corresponding activation energy.

In the case of organic chlorine and bromine compounds which have been

evaluated by Benson and O'Neal [68], the difference between the

reaction heat and the activation energy was usually about 30 to 40

kcal mol ^. The logarithm of the frequency factor was about 13 ± 1.5.

The temperature range over which researchers had studied molecular

elimination reactions were often 100 to 200 K lower than the range

for bond-fission processes.

After spending considerable time in attempting to develop

separate schemes for particular reaction mechanisms, and consulting

with several experimental kineticists on the feasibility of this effort,

we were not able to formulate a procedure which could compute and

predict realistic heats of decomposition independent of experimental

data. At present, there appears to be no simple substitute for

experimental kinetic data.

68
B. Evaluation of Safe Materials by the CHETAH Program

Our earlier appraisal of the CHETAH and CRUISE programs for

estimating the hazard potential of chemicals showed that both programs

over-emphasize explosive power as opposed to explosive sensitivity.

This over-emphasis is not a serious problem in some cases where the

hazardous chemical or material under investigation gives a reasonably

larger energy release as a result of decomposition. This Is the

case with explosives, such as: nitroglycerine, TNT, PETN, and

others. Thus, the philosophy of erring on the "safe-side" suggests

that it is very likely that these programs will label a hazardous

material as being hazardous, and not safe.

In our earlier appraisal we neglected to say much about the

prediction capabilities of the programs with respect to safe materials.

It is important that a safe compound be labelled as "safe", and not

be labelled as a hazardous material. As a model study, we had taken

the CHETAH program and have tested its response toward identifying

safe materials among the four CHETAH hazard criteria.

A group of 22 compounds have been selected (arbitrarily)

which were regarded generally throughout the chemical industry as

being safe from any tendency to self -decompose . The group

consists of eight CHO compounds, eight CHN compounds, and six CHNO

compounds. Certain vulnerabilities have revealed themselves. The

information is presented in a series of tables (see Tables 1-16).

69
Table 1, Decomposition Reactions Computed by the CHETAH Program

A. CHO Compounds

1. formic acid; 46.0259; - -90. 5^ [^9]

CH^Og -» + O.5CO2 + 0.5C(s)

2. ethylene glycol; 62.0689; ^f* = -93.9 [69]

S"6°2 * ^"2° ^'^^4 l-5C(s)

3. methyl acetate; 74.0798; AHf = -94.9 [68]

SV2 ^^2^ 0.5CH^ + 2.5C(s)

4. glycerol; 92.0954; AHf = -139.3 [69]

SV3 *^"2° ^'^^ 2.5C(s)

5. ribose; 150.1325; ^f" = -232 [72, 112]

S"l0°5 ^2° ^^^^^

6. pentaerythritol; 136.1490; = -185.63 [69]

^5^12^4 ^"2° ^"4 "^^^^^

7. ascorbic acid; 176.1264; AHf* = -257 [72, 112]

C^HgOg * ^"2° ^°2 ^^^^^

8. sucrose; 342.3025; mf = -505 [72, 112]

^12"22°11 "* "2° "** ^^^^^^

Datum following the compound name is the formula weight.

(**) -1 1 1 Values of AHf* are in kcal mol ; 1 kcal mol = 4.1840 kJ mol" ,

Formulas in equations represent substances in the gaseous

state unless otherwise indicated.

70
Table 1. Decomposition Reactions Computed by the CHETAH Program

B. CHN compounds

1. acetonitrile; 41.0529; /^f* = 15.74 [71]

CgH^N 0.75CH^ + 1.25C(s) + O.5N2

2. acrylonitrile; 53.064; AHf = 42.95 [71]

C^H^N 0.75CH^ + 2.25C(s) + O.5N2

3. malononltrile; 66.0628; AHf* = 63.5 [69]

S"2**2 °-5^4 2.5C(s) +

4. pyrimldlne; 80.0899; mf* -47.0 [69]

C.H.K- -» CH, + 3C(s) + N4 4 2 4 2

5. pyridine; 79.1023; ^f* = 34.35 [69]

C^H^N -» 1.25CH, + 3.75C(s) + 0.5N,

5 5 4 2

6. adiponitrile; 108.1441; AHf =40.2 [72, 146]

CgHgN^ 2CH^ + 4C(s) + N2

7. triethylenediamine; 112.1759; AHf*" = 11.4 [69]

S"l2^2 ^^4 ^^(^^ ^2

8. benzonitrile; 103.1246; AHf = 51.54 [69]

C^HjN -» 1.25CH^ + 5.75C(s) + O.5N2

71
Table 1, Decomposition Reactions Computed by the CHETAH Program

C. CHNO Compounds

1. formamide; 45.042; Z^Hf" - -44.5 [70]

CH^NO H-0 + 0.25 CH, + t).75C(s) + 0.5N_

3 2 4 2.

2. urea; 60.0558; ^f* = -58.7 [69]

CH^N^O H^O + 0.5 CH^ + 0.5C(s) '

3. glycine; lb.Oeil\ AHf = -93.72 [69]

S 5^^2 "* ^^2° **" °-25CH^ + 1.75C(s) + O.5N2

4. N, N-dlme thy 1 formamide; 73.0953; AH.f = -45.8 [69]

C^H^NO -» H^O + 1.25CH^ + 1.75C(s) + O.5N2

5. dlmethylglyoxime; 116.1206; ^f* = -19.3 [69]

^4V2°2 ^"2° ^4 ^^^^^ ^2.

6. e-caprolactam; 113.1607; AHf° = -58.8 [69]

C^H^jNO + 2.25ai^ + 3.75(s) + O.5N2

72
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87
The ratings provided by the four CHETAH hazard criteria are

sunmiarized in tables 14, 15, and 16, Although the selection of the 22

compounds was arbitrary, the first impression one probably obtained was

that they are all safe compounds. Also, one might feel that the CHETAH

program should rate all or most of the compounds as being of low

hazard. The end results, however, reveal that none of the 22 compounds

were ranked across-the-board by all the CHETAH hazard criteria as

being of low hazard. If one were to adopt the general criterion for

CHETAH which defines a hazardous material as one having a rating high

or medium hazard by the maximum enthalpy of decomposition (criterion

1), enthalpy difference (criterion 2), or oxygen balance (criterion 3),

then all of the so-called safe compounds above would be rated as

hazardous

.

Closer inspection of functional groups or other structural

parameters of the compounds selected provides some of the answers as

to why certain hazard ratings were obtained.

Compounds of the CHO-type were generally classed as being of

medium hazard because of the large percentage of oxygen which

makes up their formula weights. The oxygen ends up as gaseous water

in the decomposition products making the maximum enthalpy of

decomposition high. Organic cyanides, or nitriles, are particularly

vulnerable to a rating of high hazard for the maximum enthalpy of

decomposition. Nitriles, in general, have a positive enthalpy of

formation which makes for a higher than usual maximum

enthalpy of decomposition.

88
The enthalpy difference and oxygen balance criteria come out with a

"low" hazard rating for nitriles as expected. The compounds containing

the elements CHNO yield hazard ratings showing poor consistency.

Formamide and urea are classed as low, medium, and high among the

four hazard criteria, Dimethy Igloxime should be suspected as being

particular hazardous with ratings of: high, medium, medium, and

medium. This compound, however, does contain a carbon atom doublybonded to a nitrogen atom (> C = N-) which might suggest a hazard.

Dimethy Iglyoxime has been used by chemists for a long time as

an analytical reagent for determining nickel. Experience has not

identified it as a hazardous compound. Chemists as a whole would

identify the 22 compound studied here as safe from self -decomposition,

but none have received a hazard rating of "low" for all four hazard

criteria.

We feel this study points out the general trend of the CHETAH

Program to not necessarily miss identifying a true hazardous compound

as being hazardous, i.e., never giving such a compound a rating of

"low" under all four criteria. However, we find an over-emphasis of

hazard affecting the ratings for safe compounds. Hence, CHETAH will

probably identify a hazard compound as such, but there is a reasonably

high probability that CHETAH will indicate a large number of safe

compounds as being hazardous

.

89
C. CHETAH Calculation of Enthalpies of Polymerization

We have examined ten olefinic monomers commonly produced in

large volume which are routinely polymerized to yield products for

various applications. In table 17, we have listed the reaction for

the process: monomer (gas) = polymer (solid), the respective enthalpies

of formation at 298.15 K and corresponding references, and the

resulting enthalpies of polymerization. In table 18, we have

tabulated the enthalpies of polymerization of the ten olefinic

monomers and ranked them in a manner equivalent to the ranking used

for the CHETAH maximum enthalpy of decomposition. The usual

four CHETAH hazard criteria were used to calculate the hazard ratings

for the monomers in tables 19 through 22.

An examination of the CHETAH hazard ratings for the olefinic

monomers shows that collectively all of the monomers could be

classified as hazardous if either a ranking of high or medium is

sufficient to denote hazard. However, the monomers each have received

a ranking of low hazard by one of the four CHETAH hazard criteria,

except for vinylidene chloride, A weak relationship can be implied

between the hazard rating imposed in table 18 upon the enthalpies of

poljnnerization and those in table 19 calculated from the maximum

enthalpies of decomposition. Inspection of table 23 in Section VI

on Accidental Polymerization of Bulk Chemicals suggests that

monomers such as vinyl chloride, vinyl acetate, and methyl acrylate

should be suspect of being more polymerizable than the remaining

olefins. No inference of this kind is obtained from the CHETAH hazard

criteria or their application to the enthalpies of polymerization in

table 18. We feel that the CHETAH program does not present a clear-cut

view of hazards related to polymerization.

90
Table 17

Polymerization Processes and Enthalpies at 298.15 K

1. ethylene (f.w. = 28.0536*)

C2H, (g) (CjH.)^^ (solid polymer)

+ 12.50[70] -l5.50 [73]

AH polym. = -26.0 kcal mol"

= -0.927 kcal g"!

2. 1,3-butadiene (f.w. = 54.0914)

C4H6(g) -» (C4H5)n (solid polymer)

+ 26.33[70] + 3.3[73]

AH polym. = -23.03 kcal mol"-"" = -0.426 kcal g"l

3. isoprene (f.w. = 68.1182)

C5H8(g) (CsHg)^ (solid polymer)

+ 18.10[70] -6.24[73]

ZXH polym. = -24.34 kcal mol"-'- = -0.357 kcal g'^

4. styrene (f.w. = 104.1512)

CsHsCg) (C8H8)n (solid polymer)

+ 35.22[70] + 7.99[73] ^

polym. = -n .23 kcal mol'^

= -0.261 kcal g-1

5. vinyl chloride (f.w. = 62.4987)

C2H3Cl(g) -» (C2H3CI) (solid polymer)

+ 8.40[70] -22.5 [73]

AH polym. = -30.9 kcal mol"^

= -0.494 kcal g"l

6. vinylidene chloride (f.w. = 96.9438)

C2H2Cl2(g) -» (C2H2Cl2)n (solid polymer)

+ 0.3[70] -24.0173]

AH polym. = -24.3 kcal mol"^

= -0.251 kcal g"l

* -1

f.w. = formula weight, g mol

91
acrylonitrile (f.w. = 53.0634)

C3H N(g) (C3H3N) (solid polymer)

+42.95[71] + 16.86r77]

AH polym. = -26.09 kcal mol"-*- = -0.492 kcal g"^

vinyl acetate (f.w. = 86.0902)

C4H602(g) (C4H502)n (solid polymer)

-75.5[69] -105.2[78] _^

polym. = -29.7 kcal mol

= -0.345 kcal g"!

methyl acrylate (f.w. = 86.0902)

C4H602(g) (C4H502)n (solid polymer)

-79.6171] -106.6[78,79)

m polym. = -27.0 kcal mol'-*" = -0.314 kcal g"l

methyl methacrylate (f.w. = 100.1170)

C5H802(8) (C5Ho02)n (solid polymer)

-81.6172) -108.2r78,80]

polym. = -26.6 kcal mol" ' = -0.266kcal g"!

92
Table 18 Enthalpies of Polymerization

olefin name

formula

AH polym.

kcal mol"^

AH polym,

kcal g

^

CHETAH

hazard

rating

1. ethylene

2. 1,3 -butadiene

-26.0

-23.03

-0.927

-0.426

high

medium

3. isoprene

-24.34 -0.357 medium

4. styrene

27.23 -0.261 low

5. vinyl chloride

C2H3CI -30.9 -0.494 medium

6. vinylidene chloride

C2H2CI2 -24.3 , -0.251 low

7. acrylonitrile

C3H3N •26.19 0.492 medium

8. vinyl acetate

^6^2 -29.7 -0.345 medium

9. methyl aery late

C4Hg02 -26.8 -0.314 medium

10. methyl methacrylate

C5H8O2 -21.5 0.266 low

The limits for the hazard rating are equivalent to those for the

CHETAH criterion for the maximum enthalpy of decomposition.

93
Table 19 Maximum Enthalpy of Decomposition (Olefins)

olefin name

formula

maximum Z^j^ (decompositio^)

kcal mol kcal g

CHETAH

hazard

rating

1. ethylene

S«4 -30.4 -1.08 high

2. 1,3-butadiene

-53.2 0.98 high

3. isoprene

S"8 -53.9 -0 . 79 high

4. styrene

^8«8 -71.0 -0.68 medium

5. vinyl chloride

C2H3CI

*

-39.4 -0.63 medium

6. vinylidene chloride

C2H2CI2 -44.4 -0.46 medium

7. acrylonitrile

• S»3^ 56.4 -1.06 high

8. vinyl acetate

^6^2 -49.0 -0.57 medium

9. methyl aery late

^6^2 -44.94 -0.52 medium

10. methyl methacrylate

S»8°2 -51.9 -0.52 medium

CI present in decomposition products as: HCl(g).

94
Table 20 Z^HC combustion) - (decomposition) (Olefins)

olefin name

formula

Zffl(comb.) - Z^H(decomp.)

kcal mol

^ kcal g^

CHETAH

hazard

rating

1. ethylene

S«4 -285.8 -10.19 low

2. 1,3 -butadiene

-522.7 -9.66 low

3. Isoprene

S«8 -665.6 -9.77 low

4. styrene

^8^8 -947.8 -9.10 low

5. vinyl chloride

C2H3CI -237.0 -3.79 medium

6. vlnylldene chloride

C2H2CI2 -188.1 -1.94 high

7. acrylonltrlle

C3H3N -355.4 -6,70 low

8. vinyl acetate

-425.1 -4.94 medium

9. methyl aery late

-425.1 -4.94 medium

10. methyl methacrylate

S«8°2 -567.9 5.67 low

Combustion products are In the gaseous state; C02(g), H20(g), HCl(g), N2(g)

95
Table 21 Oxygen Balance (Olefins)

olefin name

formula

oxygen

balance

CHETAH

hazard

rating

1. ethylene

C2H4 • iA.im- -342.2 low

2. 1,3 -butadiene

-325.4 low

3. isoprene

S»8 -328.8 low

4. styrene

S»8 -307.2 low

5. vinyl chloride

C2H3CI- -166.4 low

6. vinylidene chloride

C2H2CI2 -99.0 medium

7. acrylonitri le

C3H3N -226.1 low

8. vinyl acetate W2 .'S,^- -204.4 low

9. methyl aery late

-204.4 low

10. methyl methacrylate

S«8°2 -223.7 low

oxygen balance (C^H^N^O^j) compounds = -[(32) (a + (b/4) - (d/2))]

(100) /(formula weight).

96
Table 22 Modified Maximum Enthalpy of Decomposition (Olefins)

olefin name

formula

= 10(Z^(decomp.))^^/n*

CHETAH

hazard

rating

1. ethylene

S«4 54.5 medium

2. 1,3 -butadiene

51.9 medium

3. isoprene

S«8 32.7 medium

4. styrene

S»8 30.1 medium

5. vinyl chloride

C2H3CI 41.3 medium

6. vinylidene chloride

C2H2CI2 34.2 meditim

7. acrylonitri le

C3H3N 85.2 medium

8. vinyl acetate

23.3 low

9. methyl aerylate

19.4 low

10. methyl methacrylate

S«8°2 18.0 low

AH (decomp.) in kcal g"^

W is the formula weight

n is the number of atoms in the compound

97
VI. ACCIDENTAL POLYMERIZATION OF BULK CHEMICALS

(prepared by

Daniel W. Brown and Roland E. Florin

NBS Polymer Stability and Reactivity Section, Polymers Division)

A . Introduction

The question of whether or not the shipment of large quantities

of bulk chemicals constitutes a hazard because of accidental

polymerization deserves serious consideration. These chemicals are

mainly conventional monomers but also include other substances that

have non-polymeric uses but some polymerization potential. This report

assesses polymerizability and identifies radical polymerization of

unsaturated monomers as the most likely mechanism of accidental

polymerization during transport. Accidental reaction hazards other

than polymerization and dimerization were not considered.

B. Polymerization Classifications

At one time polymers were- classified as having been formed in

condensation or addition polymerizations [81]. In the formation of

condensation polymers, a small molecule (frequently water) was split out

when larger molecules combined. In addition polymerizations nothing

was split out so the elemental composition of polymer and monomer

were the same. It is now fairly copmion to use cyclic monomers to

form some polymers that previously were called condensation polymers.

Such monomers combine via ring opening reactions and so give polymers

having the same elemental composition as the monomer. The classification now used is based on the nature of the intermediate species

involved in polymer formation. If such intermediates are essentially

V 98
small polymers then the polymer is said to have formed in a

"step reaction polymerization," If the intermediate is chemically

much more reactive and has a very short lifetime as an individual

species the polymer is said to have formed in a "chain reaction

polymerization,"

The different types of intermediate give rise to different

polymerization characteristics [82], In step reaction polymerizations

(1) growth occurs by coupling of any two species (monomer or polymer)

(2) the concentration of the monomer decreases markedly before high

polymer is formed (3) the molecular weight increases with the extent

of reaction; truly high polymer is not formed except at almost complet

conversion (4) the rate of polymerization decreases from the start

of the reaction. Chain reaction polymerizations have very different

characteristics. These are: (1) growth occurs by rapid addition of

monomer to a relatively few active centers (2) polymer of very high

molecular weight forms almost from the start of the reaction (3)

ideally there is little change in the molecular weight of the polymer

or of the poljmierization rate over much of the reaction period.

C. Step Reaction Polymerization Hazards

Monomers used to form polymers by step reactions are unlikely

to be self polymerization hazards. The class includes condensation

poljnnerizations, some of which use different molecular species for

reaction. Moreover, these and self condensation reactions are really

equilibrium processes that require removal of the molecule eliminated

before the reaction can go far. Indeed, substantially all step growth

99
reactions are nearly thermoneutral; this, and the decreasing rate

associated with the decrease in the concentration of reactive

groups as reaction proceeds limit heat effects. Materials used in

step reaction polymerizations include dibasic acids and anhydrides,

glycols, uu-hydroxy acids, urea, amino acids, melamine capro lactams,

isocyanates, silanes and siloxanes, phenol, dichloroalkanes,

polysulfides, aldehydes, bis -phenol A, and phosgene. Of these

materials only formaldehyde and some isocyanates could self polymerize

rapidly and then only if somehow initiated in chain reaction

polymerizations

.

D, Chain Reaction Polymerization Hazards

Chain reaction polymerizations often involve high polymerization

heats (15 - 25 kcal/mol). Polymer usually is highly favored

thermodynamically over monomer. Thus there exists a definite

potential hazard. Olefins and oxides are the most common monomers.

Some of the former are ethylene, propylene, styrene, butadiene,

vinyl chloride, vinyl acetate, vinylidene chloride, acrylonitrile,

methyl acrylate, methyl methacrylate, cy-methyl styrene, isoprene,

tetrafluoroethylene, and chlorotrif luoroethylene . Ethylene and

propylene oxides are the most important oxide monomers . The

polymerization mechanism of olefinic and oxide type monomers is

very different so their polymerization hazards will be discussed

separately.

100
E. Olefin Polymerlzability

Olefins are polymerized by mechanisms involving free radicals,

cations, anions, and complex heterogeneous organo metallic compounds.

The terms describe organic species occurring during the growth of the

poljrmer. Successful polymerization by the last three mechanisms involves

very special conditions and catalysts. The activity of these types of

catalysts is destroyed by traces of water or alcohols. Uncommon materials make

up the catalysts - aluminum chloride, butyl lithium, alumintjm alkyls in

combination with transition metal compounds. Let us assume for

the sake of the present discussion that the possibility of such

compounds being introduced during transport in quantities which

would polymerize a monomer is unlikely.

Free radical polymerizations involving olefins are the

polymerizations thought most likely to occur inadvertently during

transport. Thus behavior in free radical polymerizations is germane

to the present problem.

The basic mechanism underlying most free radical polymerizations

contains three reactions:

Step Reaction Rate

Initiation dR/dt = 2k,f[l]

Termination

Propagation

101
Here I, R, M, P, f, and t represent initiator, radicals, monomer,

polymer, an efficiency factor, and time respectively. The k's are

rate constants; subscripts d, p, and t indicate decomposition,

propagation, and termination reactions. Subscripts n and m

indicate that there are specific numbers of monomer units in the

species being considered.

The mechanism shows initiator I decomposing to give radicals;

only a fraction, f, of these are regarded as initiating polymerization.

It is only such radicals that are of present interest and their rate

of formation is 2 k^f [l]. Monomer may add to these radicals with a

rate constant, k^, that is independent of the size of the radical.

Radicals of any size may terminate, also with a rate constant, k^,

independent of the size of the radical. In real systems, f is very

frequently between 0.5 and 1; k^ is much smaller than k^ which is very

much smaller than k^. Consequently a radical is formed, grows for

a short while, and then is terminated. Values of initiator concentration

and of the k are such that the time between initiation of a growing

chain and its termination is between 0.1 and 100 seconds; in this

time, perhaps 100 to 10000 monomer units may be added. Thus,

conversion of a significant fraction of the total monomer to polymer

requires the initiation of many chains and ordinarily is done over

several hours

.

102
If k is so small that the concentration of initiator

a changes little with time then the total concentration of radicals

becomes nearly constant in times not much longer than the lifetime

of a growing chain. Then,

i| - 0 - 2k^f[I] - 2kj.(i;[R^])^

from which

and

Here, ,^ is the fractional rate of conversion of monomer to

' M dt

polymer. Expressed in this way the instantaneous polymerization rate

changes with time only to the extent that the square root of the

initiator concentration changes with time. Integration of equation

(1), treating the concentration of initiator as a constant, gives

In (CM '""'^ ' o

]/[M]) - ^-AI

(2)

Here [M ] is the initial monomer concentration,

o

103
Conceivably then, substances could be ranked on a polymerizability

scale by comparing their polymerization rates in a standard

initiating situation. However, even if various special considerations

were eliminated, some rates would be too fast or too slow for

practical measurement. Another approach is suggested by looking at

1/2 equation (1) and realizing that only k^/k^ is a property of the

monomer. One can, therefore, choose conditions that give practical

1/2

rates and calculate k /k from such rates by using known values

P

1/2 of k^ and f. Extrapolation of k^/k^ to a common temperature then

provides a number which can reasonably be considered as the monomer

polymerizability at that temperature.

1/2 Such values of ^p/^^. ^re in Table 23 along with the

associated activation energy, - E^/2, the heat of polymerization,

Z^p, and a "Remark" indicating what special considerations are

involved. Note that every monomer has such a remark. Most values

1/2 of kp/k^. probably required two or more man years of effort per reaction.

Furthermore, alternate choices of method give apparent values of

1/2 k /k^ that differ by a factor of five and values of E - E /2

p t ^ P t that differ by several kcal/mol. Thus, the ranking is uncertain.

Probably the only conclusions justified are that hydrocarbons

polymerize less rapidly than electronegatively substituted olefins

and that double substitution on the same carbon gives intermediate

reactivity. The difficulty is that real polymerizations occur

by modifications of the above general mechanism and the methods of

compensating for the deviations are imperfect.

104
Calculation of the polymerization rate from the data of

Table 23 is done as follows. Values of k, have been tabulated

a for many initiators [83], 2,

2 ' -Azobisisobutyronitrile has a well

known k, and values of f between 0.5 and 1.0 for several

a monomers [84]; at 60" C k^ is 10 ^ sec ^. If f is taken as unity

and the concentration of this initiator is made 0.01 molar (about

0.1 mol 7=) then (k^[l]f)^^^ is 0.3 x lO""^ (mol/liter, sec)^''^-

1/2 this multiplied by the listed value of k /k^ gives -dM/Mdt as

p t the fraction of monomer present that is polymerized per second.

-6 -

1

For the monomers of Table 23 rates range from about 10 sec

-4 -1

(isoprene) to 2 x 10 sec (methyl acrylate) or from 0.4 to 707o/h.

The rate of heat release per mole of monomer used is the product

of -dM/Mdt and the heat of polymerization. Rates greater than 5%

per hour probably cannot be kept isothermal because of this heat

release. Therefore, autoacce leration in accord with the Arrhenius

equation would be anticipated in the polymerization of methyl

acrylate

.

1/2 Many olefins polymerize too slowly for k^/k^ to have been

measured. For these the polymerizability may be regarded as zero

and so the monomers often are not hazardous in ordinary radical

reactions. One class of such monomers has abstractable ally lie

atoms. Examples are ck'-methyl styrene, propylene and other a-olefins,

isobutylene, allyl acetate, and allyl chloride. In radical

polymerizations, these monomers undergo transfer reactions that

generate stable radicals:

R + CH- C CH„X y P + CH„C CH. • or CH„C CHX-.

^ ^ 2^

105
Such allylic radicals propagate poorly except at pressures of

several thousand atmospheres [85]. However, it should be kept

in mind that the presence of abstractable allylic atoms is not a

guarantee of non-polymerizability . Both methyl methacrylate

and methacrylonitrile polymerize despite having such atoms.

Probably polar effects account for their activity. Nevertheless,

one should feel that most materials bearing abstractable allylic

atoms will not polymerize by ordinary radical reactions.

Internal olefins, including many 1,2 disubsti tuted ethylenes

constitute another group which can ordinarily be regarded as not

being poljmierization hazards. Many of these bear abstractable atoms

one carbon removed from a double bond. However, steric hindrance

to propagation is probably the decisive cause of their unpolymerizability

.

Some internal olefins, maleic anhydride for example, copolymerize

(combine with another monomer to form a mixed polymer) readily.

Thus, mixtures of monomers might constitute a hazard.

Fluorinated ethylenes are exceptional because of the small size

of the fluorine atom. Such highly fluorinated ethylenes as trifluoroethylene, chlorotrif luoroethylene, and tetrafluoroethy lene homopolymerize

readily. Possibly, this is due to a physical effect described below.

At any rate, the polymerizations are too rapid for determination of

1/2 kp/k^ . Higher perfluoro-cv -olefins, however, are polymerized

with great difficulty, probably because Ep is large [86]. In

passing, it should be noted that organic hydrogen is frequently

avoided in polymerizing f luorocarbons because f luororadicals abstract

hydrogen and the radical produced may not continue the kinetic chain.

106
Acetylenes, despite their high energy content, polymerize

rather poorly under radical conditions. The triple bond resists

radical attack; vinyl acetylene, for example, polymerizes largely

through the vinyl group to form a poljnner with pendant triple

bonds [87]. Thus, it is unlikely that acetylenes themselves are

polymerization hazards.

1/2 It should be stated that the listed values of k /k refer

P t

to homogeneous polymerizations. These may not be realizable in bulk.

Eight monomers in Table 23 may give polymer that is insoluble in

the monomer (note a or b) . The kinetic consequences of this may

be profound. The general result is that the effective termination

rate constant is made smaller because the diffusion of large polymeric

radicals is restricted in the precipitated phase. Monomer molecules,

being small, may still reach these radicals so that the propagation

rate constant is affected to a much smaller degree. Thus, as polymer

forms and separates from monomer, the number of growing radicals in

the system actually increases, leading to an increase in rate. When

the rate reaches a value at which effective temperature control is

lost, further acceleration occurs in accord with the Arrhenius

equation. Extreme temperatures and pressures may result and the

character of the reaction may change. The writers (D. W. Brown and

R. E. Florin) have seen a copolymerization involving 10 g of

tetraf luoroethylene develop 10,000 atm pressure in the course of

perhaps 15 seconds. The product was not polymer, but carbon and

107
tetrafluoromethane. On several occasions 15 kilogram vessels,

which had been tested to 25,000 atm pressure, were split in the

polymerization of 10 g of perf luorocyclobutene , This type of

result is not peculiar to f luorocarbons . Vinyl chloride, for

example, poljmierized in bulk in 50 g quantities in a stirred oil

bath, gave dark poljmier due to the heat-induced evolution of

hydrogen chloride. Nor does the phenomenon of rate acceleration

in bulk polymerization even require insolubility of the polymer.

Solutions of high polymers containing more than a few percent

polymer are exceedingly viscous. With methyl methacrylate, the

increase in viscosity with polymer formation causes a several fold

decrease in k^ and an increase in -dM/Mdt [88], Thus, bulk

polymerizations may give results grossly inconsistent with Table

23 unless little polymer forms.

1/2 Values of ^p/^^. Table 23 were usually obtained for

monomers which give rate accelerations in bulk by working in the

presence of a solvent for the polymer. Industrial polymerizations

of such monomers are frequently accomplished by suspending the

monomer in a large quantity of water and using stirred reactors

equipped for cooling.

It 108
F. Oxidation

Oxidation of olefins may generate peroxides under ambient

conditions [89]. The decomposition of these may initiate

polymerization. If allylic hydrogens are present frequently

allylic peroxides are formed by a chain reaction:

As indicated earlier, the polymerization of monomers having allyl

hydrogens is generally slow so that further reactions of these

peroxides will not be considered.

Olefins which poljrmerize or copolymerize often form

polyperoxides with oxygen by a chain mechanism

The rate constant for the first of these reactions is so large

that the oxygen at low concentrations competes well with the addition

of monomer to the same radical. However, only monomer adds to

the RO^ radical and that relatively slowly. Thus, 1:1 copolymers

of oxygen and monomer tend to form, although so slowly that oxygen

appears to inhibit polymerization. The kinetics and even the nature

of the major product depend somewhat on oxygen pressure. As the

oxygen is exhausted, the polyperoxides act as sources of free radicals

by cleavage of 0 - 0 bonds. If the temperature is high, a rapid

109
polymerization can result. Peroxidic products of tetraf luoroethylene

can induce violent reaction even at room temperature and any contact

of oxygen with this monomer is to be avoided, Polyperoxides are

formed by styrene [90], methyl methacrylate [90], vinyl acetate

[90], methacrylonitrile [91], methyl styrene [92], and isoprene

[93]. Methyl acrylate [91], acrylonitrile [91], and chloroprene

[94] give chiefly carbonyl products at the temperatures studied,

about 60** C. Even if peroxide forms it is not always an effective

initiator; vinyl chloride is an example of this [95].

The complicated effect of oxygen in polymerizations suggests

that special precautions should be taken in doing experimental work.

Most uninhibited monomers will form peroxides on exposure to air.

The time of such exposure should be held to a minimum. Before

polymerization, the monomer should be frozen, pumped on, and thawed

several times in order to remove oxygen. The monomer can then be

used as such, but a better practice would be to evaporate it at

room temperature and condense it directly into a reaction vessel,

thus leaving behind any peroxide. Generally, an appreciable

portion of monomer is left behind in this step, particularly if

peroxide is suspected; some of the latter are explosive when

concentrated.

Industrial organizations use nitrogen to purge equipment and

monomers and add enough additional catalyst to compensate for residual

oxygen and inhibitor. ... . . ;c

110
G. Thermal Initiation

Without the presence of an initiator many monomers will not

polymerize but the presence of oxygen may cause some polymerization

through peroxide formation, even without purposeful addition of

initiator. With methyl methacrylate there may be thermal

polymerization, but there is evidence that exposure to visible

light forms an initiator and observed polymerizations may result

from this [96]. Styrenes are probably the only monomers in which

true thermal initiation is proven. The uncatalyzed poljrmerization

of styrene itself is slow, about 0,1% per hour at 60** C and has an

overall activation energy of 21 kcal/mol [97]. Important monomers

thought not to polymerize thermally include vinyl acetate [98],

methyl aery late [98], vinyl chloride [99], acrylonitrile [100], and

tetraf luoroethylene [101],

H, Inhibition

Free radical polymerizations are relatively easy to inhibit.

The most common inhibitors are phenols and quinones. The mechanism

of action is apparently rather complex and even the stoichiometry

may be uncertain [102]. Kinetically, however, radicals can be

pictured as reacting with inhibitor Z to form non-propagating species:

111
Values of k /k have been tabulated for many inhibltor-roonomer

z p

systems [103]. From these one can calculate the concentration

of inhibitor required to achieve a desired low level of the

polymerization rate in the following manner. If reaction with

inhibitor stops substantially all the growing chains, then when

dR/dt reaches its steady state value

2k k. fill

This may be solved for [Z], the inhibitor concentration.

Relatively low concentrations of inhibitors can be effective.

For styrene and benzoquinone k /k is given as 225 at 60*'C [98],

z p

-2

If [Z] is 10 mol/liter and the initiator is 2,

2 'azobisisobutyronitrile,

-2 -7

also at a concentration of 10 mol/liter then -dM/Mdt will be 10

sec The uninhibited rate, calculated from Eq. 1 by use of Table

I, would be 60 X 10 ^ sec The inhibitor is consumed and if

excess initiator is used there will be inhibition first and then a

retardation period in which neither equation 1 nor 2 applies.

Subsequently, behavior characteristic of the uninhibited reaction

will occur. Thus, commercial styrene, inhibited with p-tert butyl

catechol, will polymerize after standing at room temperature for

a year or so.

112
The same substances that inhibit poljrraerization also inhibit

oxidation [92]. Since true thermal initiation is rare, polymerization

inhibitors may function more by preventing peroxide formation than

by interaction with growing radicals. Indeed, it may be desirable

to add free radical inhibitors to olefins that do not polymerize

in order to prevent their oxidation. The necessity for so doing

is not considered here,

I. Recommendations for Olefinic Monomers

In view of the complexity of free radical polymerization it

is thought that results of test polymerizations should not be used

as a means of rating olefins as to poljrmerizability during transport.

At most such a testing program would tell something about v^at

happens after polymerization has started. Polymerization in bulk

in large containers having a low surface to volume ratio, no

provision for cooling, and no stirring probably would be

substantially adiabatic. Therefore, if polymerization started at

anything but a very small rate it would be expected to continue

at an increasing rate until limited by (1) initiator depletion, (2)

monomer depletion, (3) change in reaction character at high

temperature, or (4) venting of the container. Thus, one feels that

the important thing to consider is the likelihood of polymerization

starting inadvertently. If such polymerization did start peroxides

probably would be the likely source of initiation, since thermal

initiation of pure monomer is quite rare. Important considerations

113
for each monomer then are whether true thermal initiation occurs

and the character of the oxidation--its rate, means of

inhibition, what peroxides, if any form, and \jhethev or not their

decomposition is likely to initiate polymerization. The general

character of the bulk polymerization is worth knowing to judge how

severe an accident might be. However, the ultimate intent would

be to establish the level of peroxide likely to accumulate in each

monomer during transport and specify one or more inhibitors and

concentrations that prevent such accumulation. If it can be

definitely established that an uncatalyzed polymerization does not

occur and that oxidation products do not cause polymerization

shipment of uninhibited monomer could be permitted.

The examination of the literature for information on important

monomers produced and shipped in large quantities in this country

would be a worthwhile activity. Monomers which should be considered

are: ethylene, styrene, 1, 3 -butadiene, vinyl chloride, vinylidene

chloride, vinyl acetate, acrylonitri le, chloroprene, methyl acrylate,

methyl methacry late, isoprene, tetraf luorethylene, vinylidene fluoride,

and chlorotrif luoroethylene . At present, these monomers constitute

the bulk of free radical polymer formers.

114
Information of particular interest would be descriptions of

incidents in which a monomer has polymerized in transit. Data of

interest might be: (1) details regarding the type of inhibitor

employed and its initial concentration, (2) the time interval before

polymerization occurred, and (3) the temperatures prior to and during

the polymerization. Depending upon the nature of the literature

search, an experimental program might even emerge which would be

directed toward the behavior of slightly oxidized monomers and the

detection of the initiation of thermal polymerization.

115
J. Polymerlzablllty of Olefin Oxides

Ethylene oxide and propylene oxide polymerize by anionic and

cationic mechanisms and possibly also be step reactions with alcohols

[104], but not by free radical mechanisms, A difference from olefins

is that catalysts too inactive for olefin polymerization are still

capable of poljmierizing olefin oxides, for example, the cationic

initiators ferric chloride hydrate and activated montmorillonite clay

and such anionic initiators as alkali and alkaline earth hydroxides

can cause appreciable poljmierization in several days at 20 to 100*0

[105], One incident is reported in which calcium hydroxide and water

with ethylene oxide led to exothermic polymerization and explosion

[105], Such initiators could possibly occur as a consequence of gross

contamination of equipment. The enthalpy of polymerization is large;

-AHp = 22,5 kcal/mole for ethylene oxide [106], Aldehydes and ketones

form polyacetals under special conditions, but the reaction is

thermodynamically favored only at low temperature or high pressure,

except for fomaldehyde. No further action in this field is indicated,

K. Dlmer or Trimer Formation

Dimers or trimers, which some persons might class as low polymers,

may form in some instances where high pol3rmer does not form. Their

formation would usually evolve heat, but would be undetected by ordinary

methods except perhaps dilatometry, distillation, or gas chromatographic

analysis. Three cases can be distinguished (a) formation from monomers

usually considered unpolymerizable because of their structure, (b)

116
formation in inhibited monomer, by molecular mechanisms not

susceptible to inhibition, (c) formation as an important part of

the polymerization process. Table 24 lists examples.

In most cases only the fact of formation and the identity of

the dimer or trimer are known. Rates under conceivable storage

conditions are reported only in a few instances (Table 25). In a

few more, the enthalpy or free energy changes have been measured

or estimated (Table 26).

In many cases, notably acetaldehyde and a-methy Istyrene, rapid

dimer or trimer formation requires an acid or alkaline catalyst. The

dangers associated with dimerization seem at present more potential

then real. Where high polymer formation occurs readily, they are

already allowed for, except perhaps when there is exposure to very

high temperatures in the presence of inhibitors. Where high polymer

formation does not occur, the thermal efforts of dimer or trimer

formation tend to be small relative to usual polymerization effects.

L. General Reference Sources

Some general reference sources are:

Flory, P. J., Principles of Polymer Chemistry, Cornell University

Press, Ithaca, New York (1953).

Encylopedia of Polymer Science and Technology . Vol. 1-16, Interscience,

New York, 1964-1972.

Jenkins, A. D., and Ledwith, A., Reactivity. Mechanism, and Structure

in Polymer Chemistry, John Wiley and Sons, New York, 1974.

Brandrup, J., and Immergut, E. H., Polymer Handbook. John Wiley and Sons,

New York, 1966.

Leonard, E. C, Vinyl and Diene Monomers, Interscience, Part 1-3 (1970-1971).

Lundberg, E. 0., Autoxidation and Antioxidants . Vol. 1 and 2, Interscience,

New York (1961)

.

117
Table 23. Olefin Polymerlzablllty at 60'*C in Radical React^on8

Olefin (Z/mol, sec) kcal mol ^ kcal mol Remarks

AbOyxene 7• 4 25

Xf ^^DUbaaicne 7 rin7l 17 b

XBoprene A r 1081 10. 17

9 b/ £ cue 20 7# 17 w

Vinyl chloride 300 2 22 a

Vlnylidene Chloride 30 2 • - 16 a

Acrylonitrile 70 2 18 a

Mcthacrylonitrile 30 8 14 a

Vinyl acetate 300 4 21 d

Methyl acrylate 700 4 19 b

Hethyl methacrylate 90 5 13 c

' , t,

tRemarks: (a) Polymer insoluble in monomer;

(b) Gelation possible;

(c) Thermal initiation possible;

(d) Branched polymer may form.

118
Table 24. Instances of Dimer or Trlmer Formation

Remarks

' Tetrafluoroethylehe b

Chlorotrlfluoroethylene b

1,2-Dlchloroethylene «

Trlchloroethylene «

Propylene a

Bexafluoropropene c

Butene-2 a

Isobutene . c Butadiene b

2-Chlorobutadiene b

Isoprene ' b

a-Methylstyrene c

Kethyl methacrylate c

Acetaldehyde —* aldol a

Acetaldehyde —*• paraldehyde a

Acetone —>• diacetone alcohol a

Acetone —^ mesityl oxide + water a

4* A-Diphenylmethane diisocyanate c

Benzonitrile a

(a) High polymer does not form at usual temperature and pressure;

(b) Inhibited monomer;

(c) Polymer forms also.

(No attempt has been made to find references to diracrs as minor products

incidental to formation of high polymer.)

119
Table 25. Dlmer Formation Rates

(wt 2/h) X 10^

Tenp [109] [1101

*C 1.3-Butadlene Isoprene

20 1.5 1.17

40 lA 1.9

60 130 21

80 1200 230

100 11000 2300

Table 26. Enthalpies for Dlmer and Trlmer Formation

^ Data

Reaction -AH kcal mol Source

Propylene, liq 1/2(4 -methy 1-1-pentene) , liq " 10.7 [111]

Propylene, liq polypropylene, isotactic, c 20,3 [111]

Formaldehyde, liq 1/3 trioxane, c 13.6 [111]

Formaldehyde, liq polyoxymethy lene, c 13.0 [111]

Acetaldehyde, liq -* 1/3 paraldehyde, liq 8.77 [112]

120
VII . Thermal Analysis and Hazard Evaluation

A . Thermal Explosion Theory

In 1928, Semenov [2, 3] presented the first quantitative

treatment of the theory of thermal explosion. In this treatment,

the heat generated by a self -reacting material competes with the

heat lost from the exothermic process due to transfer to the

surroundings. As the temperature of the surroundings increases

in a linear fashion, the rate of heat production from self -reaction

increases, exponentially, until a temperature is reached above

which equilibration is not possible and decomposition takes place

rapidly with ignition or explosion.

The theoretical treatment of Semenov provides for the calculation

of the critical conditions in excess of which ignition or explosion

will take place. Assumptions basic to this treatment state that

(1) the self -reacting material maintains a uniform temperature during

decomposition, and (2) the Arrhenius rate law is obeyed. Heat

transfer to the surroundings is not limited to any one mode. The exact

solution of the equations generated in Semenov 's model is not

possible without making some approximations. Necessary data for

certain thermodynamic, kinetic, or heat transfer parameters are also

required before an actual solution can be obtained. In the absence

of data, estimates are usually made.

121
In 1939, Frank-Kamenetskii [4] developed the Semenov model to

accommodate a distribution of temperatures within the self -reacting

material and limited the heat loss process to conduction only. In

many instances, the Frank-Kamenetskii model is a closer approximation

to the actual situation.

Various refinements have been introduced into the mathematical

treatment of the theory of thermal explosion. Articles by Chambre^

[113], Todes [114, 115], Rice, et al. [116, 117], Zinn and Mader

[119] and Enig [120] should be consulted. Thomas [118] compared

the Semenov and Frank-Kamenetskii models and showed that both are

special cases of a more general model. Useful reviews have been

provided by Merzhanov and Dubovitskii [6], and Gray and Lee [121],

In order to facilitate a clearer understanding between the theory

of thermal explosion and hazards caused by thermally unstable

materials, we shall describe some features of a thermal explosion

process. Let us imagine a test tube (constructed of a high thermal

conductivity metal), containing an hazardous material, which has been

immersed in an oil bath initially at some low temperature. To. A

thermocouple imbedded in the material allows a temperature -time

record to be taken. (This situation is analogous to one for the

Explosion Temperature Test described in Section IV). In the first

test, the temperature of the material will be observed to rise to

the bath temperature, then slightly surpass the bath temperature,

and proceed to level off back down to the bath temperature. During

122
this time, the material undergoes decomposition, and if the reaction

products are gaseous, has disappear completely. As the bath

temperature is increased in successive steps, a critical temperature

will be attained, which will be the maximum temperature for maintaining

a steady-state process. At temperatures above the critical temperature,

a steady-state cannot be maintained, and ignition, rapid decomposition,

or explosion results.

The phenomenon of the induction period begins to emerge at this

point. This period is the time interval between immersion of the

material in the bath and explosion.

We shall now examine the rate processes taking place during

heat generation and heat loss. The se If -reacting material produces

heat at the rate dq^^/dt, or q^^, given by:

q^ = dq^^/dt = QVpAexp(-E/RT)

where Q is the heat of decomposition, V and p are the volume and

density of the material, respectively, A is the pre-exponential

factor, E is the activation energy, R is the gas constant, and T

is the temperature of the material.

The material loses heat to the surrounds at a rate according

to Newton's law of cooling:

^2 ^^2^^^^ ^ hSCT-T^) where

h is a heat transfer coefficient, S is the surface area of the

material, and (T-Tq) represents the temperature difference between

the material, T, and its surroundings, T .

123
At the critical temperature, a steady-state is reached when

the rate of heat generation and heat loss are equal,

QVpAexp(-E/RT) = hS(T-T^)

The heat generation rate of the self -reacting material, plotted

as a function of temperature, is shown in Figure 1. The heat loss

rate corresponds to three straight lines marked as the bath temperatures at 200, 208, and 210 *'C. There are three different

relationships which are possible between the exponential curve

representing the heat production rate, dq^/dt or q^ and the straight

lines depicting the heat loss rate, dq2/dt or q^- The net rate at

which heat is generated in the self -reacting material is the sum of

q^^ and q^, and the rate of temperature rise will be expressed by this

sum divided by the heat capacity, C.

If C = mc where m is the mass of material, and c is its

specific heat, and if m = pV, then the rate of temperature rise is

given by:

CdT/dt = QVp exp (-E/RT) + hS(T-T^).

124
125
The first relationship shows the curve cutting the straight

line at two points. Here, the heat generation rate of the material

for a bath temperature of 200°C is greater than the heat loss rate

up to point A, where both are equal. This corresponds to a material

temperature of about 202°C. Beyond point A, the heat loss rate

exceeds that of heat generation and rapid decomposition or explosion

does not take place.

In the second situation, the bath temperature is 208°C, which

is the critical temperature, T^^, for the material. The curve is

tangent to the straight line at point B. The material temperature

reaches about 221**C at point B afterwhich sufficient heat loss to

the surrounds takes place and no explosion results.

The bath temperature is at 210°C in the third situation. The

curve representing heat production via self -reaction of the material

and the line representing heat loss do not meet. This temperature,

210°C, is above the critical temperature, T^^, hence, ignition or

explosion takes place.

A temperature-time plot for the self-reacting material is

shown in Figure 2 for various bath temperatures. Variation in the

induction period can be seen as the bath temperature rises above

T^^. The induction time prior to explosion is very short as the

bath temperature, T^, exceeds the critical temperature, T^^. The

amount of material reacted during the induction period is small.

About 4 percent of the material decomposes before explosion when T^

is 210*C and about 2 percent when T^ is 220°C.

126
DECOMPOSmON RATE, sec

127
A relationship between the heat transfer coefficient, h, and the

surface area, S, of the material, and the induction time seems

apparent; both h and S in turn would affect the critical temperature, T^^.

The mathematical treatment of the theory of thermal explosion

has been presented adequately by Semenov [3], F rank-Kamenet ski

i

[5], Merzhanov and Dubovitskii [6], and Gray and Lee [121], and hence,

will not be delivered in any detail here. An examination of some

of the basic objectives of this theory does demonstrate, however,

that a fundamental basis for identifying and defining thermal

instability are contained in it. These objectives are: (1) to

establish what factors determine whether an explosive reaction will

occur, (2) to find the conditions under which the reaction will

proceed at a steady rate, (3) to determine the temperature distribution

in the self-reacting material under steady state conditions, (4)

to determine the relationship between temperature and time prior

to the attainment of steady state conditions, and (5) to examine

conditions which prevail when no steady state is possible.

.

" *'

.

'^1 PA W

128
B. Experimental Studies on Hazardous Materials using DTA, DSC ,

or Related Measurement Techniques

During the course of examining the literature for physical

test methods and test data, we have encountered a large number of

articles which report measurements on hazardous materials. Some are

studies designed to examine and expand various concepts in thermal

explosion theory while others report only thermochemical or kinetic

data on a particular decomposition reaction. Many investigators use

differential thermal analysis (DTA), thermogravimetric analysis

(TGA) , and differential scanning calorimetry (DSC). Some researchers

have employed the more standard calorimetric or kinetic methods.

These articles supplement those mentioned in Section III of this

report under Thermal Tests.

Rogers [122-124] has shown that the activation energy and preexponential factor can be determined for the decomposition process

of many explosives using DSC methods. Critical temperatures can be

determined on a routine basis using the method described. Agreement

between experimental and calculated values is good for most explosives.

129
An extensive study of the application of the thermal theory

of spontaneous ignition to the explosive decomposition of diethyl

peroxide was reported by Fine, Gray, and MacKinven [125]. Thermal

explosion theory was able to make quantitative predictions of the

maximum temperature rise in the system which just failed to explode

and the shape of the temperature profiles for the critical conditions.

Experiments were in agreement with predictions.

Merzhanov, Abramov, and Abramova [126] reported a method, based

on the heat balance equation, for calculating kinetic constants from

a differential thermogram in which non-isothermal kinetic data are

reduced to isothermal conditions. The method is demonstrated using

the decomposition of tetryl and the polymerization of styrene.

An experimental investigation of the effect of gas removal on

the critical conditions for thermal explosion was performed by Samoilenko,

Abramov, and Merzhanov [127]. Experiments were performed on

compressed pyroxylin samples. Gas removal conditions were varied by

changing the pressure of nitrogen in the system. The authors found

that as the pressure of the gas over the sample was raised, the

critical temperature for thermal explosion fell sharply.

130
Aleksandrov, Bufetov, Pastukhova, and Tukhtaev [128] reviewed

the application of pulsed calorimetry to the determination of

thermochemical and kinetics parameters of various explosives.

References to studies on the decomposition of nitrocellulose,

nitroglycerin powder, ammonium perchlorate, di ni troxydiethy Init rami ne

(DINA) and polymethylmethacrylate were provided.

The thermal stability of tetramethylphosphonium perchlorate,

nitrate, and picrate was determined using DTA and TGA techniques

by Nambiar and Jain [129]. The thermal stability of these compounds

was found to be in the order: perchlorate > nitrate >picrate. The

explosive sensitivity of the compounds was determined from explosive

delay measurements and indicates the same order as the thermal stability.

The remaining list of articles describing the use of DTA, DSC,

TGA, and variation of calorimetric and kinetic measurements to

establish the hazardous character of self-reacting materials is still

large. We have abbreviated our exposition of this section by providing

shorter descriptions of the articles along with the corresponding

references

.

(1) A study of the energy of combustion of ammonium perchlorate

was carried out in a twin-type combustion calorimeter under 11 atm.

of helium [ 130]

.

131
(2) DTA, EGA and TGA methods were used to study the decomposition

of KClO^-lactose mixtures in the temperature range from room temperature to over 500°C [131].

(3) An automatic TGA apparatus designed for non -isothermal kinetic

measurements in the temperature range from 20** to 300*0 is described

[132]. Heating rates of 1 to 100 deg hr ^ are possible for lOg samples.

(4) Thermal decomposition of alkali metal perchlorates were

studied by DTA and TGA methods up to 600*0 [133].

(5) The compatibility of polymers with explosive materials was

determined using DTA methods. [134, 135].

(6) The thermal explosion of lead azide has been studied in vacuum

at temperatures high enough for explosion to occur. Reaction rates

were measured up to the onset of explosion [136].

(7) A calorimetric determination of the thermal decomposition of a

nitroglycine propellant was carried out in vacuum from 70° to 280*0

[137]. .

(8) Heat capacities and phase transitions in HMX were determined

using DSC methods [138].

(9) The critical conditions and induction period for the thermal

explosion of isopropy Ibenzene hydroperoxide in a homogeneous flowthrough reactor system were determined [139].

(10) Several investigations of the thermal decomposition of

di ni t roxydie thy Initramine (DINA) have been conducted under various

conditions [140-145].

132
From an examination of thermal explosion theory and experimental

studies of hazardous materials by various methods, we felt that a

calorimetric approach offered some reasonable promise of providing

the necessary information and support for concepts from which an

unambiguous reactivity scheme could be developed. A test procedure

for ranking materials consistent with such a reactivity scheme

should be derivable which has a strong relationship to the theory

of thermal explosion. Properties of materials which are embodied in

this theory, are obtainable from calorimetric experiments. Moreover,

the calorimetric approach has the advantage of permitting the

simultaneous derivation of such properties from the experimental

measurements performed on a specific decomposition process. The

need to calculate thermochemical parameters (such as the energy of

decomposition) and kinetic parameters (such as the activation energy

of decomposition) from the same reacting system, thus providing the

common base for comparison, cannot be overemphasized. This common

base has been lacking for most materials. The promise of the

calorimetric method to supply the needed ranking for chemical

reactivity is based upon its provision of parameters called for by

the theory of thermal explosion. One can anticipate further that

if self-reacting systems (decomposition and poljrmerization) are

adequately described by this approach, extension to combustion

processes should also be possible.

133
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t

149
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-1149

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A SECOND APPRAISAL OF METHODS FOR ESTIMATING SELF -REACTION

HAZARDS

5. Publication Date March 1977

6. Performing Organization Code

7. AUTHOR(S)

Dr. Eugene S. Domalski

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3160415

11. Contract/Grant No. DOT -AS -4 0028

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of Hazardous Materials Operations

U. S. Department of Transportation

Washington, DC 20590

13. Type of Reporf & Period

Covered Final

1-1-75 to 12-30-75

14. Sponsoring Agency Code

MTH-1

15. SUPPLEMENTARY NOTES Reference to initial study: An Appraisal of Methods for

Estimating Self -Reaction Hazards, June 1974 DOT Report No. TES-20-74-8; (NBSIR 74-551).

16. ABSTRACT (A 200-word or less factual summary of most sigpificant information. If document includes a sigpiticant

bibliography or literature survey, mention it here.)

A literature survey was carried out to collect and evaluate test methods

and test data on hazardous materials which undergo self-decomposition. Some

thermal tests are regarded as valuable because they give quantitative data

indicating the limit of thermal stability for specified materials. With the

assistance of the CHETAH predictive scheme, a hazard evaluation was conducted

on a selected group of compounds normally considered safe. Results showed a tendency to identify these safe materials as hazardous so as not to err in

evaluating a truly hazardous compound. In a study of accidental polymerization

of bulk chemicals, free radical polymerization involving olefins was considered the polymerization process most likely to occur inadvertently

during transport. A calorimetric test procedure appeared to offer the promise

of yielding data from which an unambiguous reactivity scale could be developed.

The test procedure should have a firm relationship to the theory of thermal

explosion.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Accidental polymerization; CHETAH predictive scheme;

hazard evaluation; test data; test methods; thermal explosion theory.

18. AVAILABILITY [x] Unlimited

1 1 For Official Distribution. Do Not Release to NTIS

1 1 Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. CU

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

157

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$6.75

USCOMM-DC 29042-P74