NATIONAL BUREAU OF STANDARDS REPORT

6111

THERMAL AND SELF-IGNITION PROPERTIES

OF THREE SOLID PROPELLANTS

by

J. J. Loftus

and

D. Gross

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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national bureau or standards report

THERMAL AND SELF-IGNITION PROPERTIES

OF THREE SOLID PROPELLANTS

by

J. J. Loftus

and

D. Gross

for

Department of the Navy

Naval Ordnance Laboratory

V^hite Oak

Ref. No. 60921/5089/5b

NBS PROJECT

1000-30-4820 August 11, 1958

i«jB8 REPORT

6111

to reproduce additional <NBS>

U. S. DEPARTMENT OF COMMEI^ei

NATIONAL BUREAU OF STANDARDS
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Thermal and Self -Ignition Properties

of Three Solid Propellants

J. J. Loftus and D. Gross

ABSTRACT

Measurements have been made of the specific heat,

thermal conductivity and kinetic properties of

three solid propellants. Based upon these measurements, estimates are provided of the self-ignition

hazard associated with the use and bulk storage

of such propellants.

INTRODUCTION

An understanding of the mechanism of the thermal

decomposition of explosives, propellants, and fuels requires

knowledge of the kinetics of the reactions involved. One

application of this knowledge is in the analysis of the selfignition hazard associated with the use or storage of these

materials. With the present trend toward the design and use

of large rocket propellant grains, basic information on the

size limitations for storage of such propellants is important.

At the request of the Naval Ordnance Laboratory, letter

dated 12/23/57) s/G. K. Hartman, determination of the thermal

and kinetic properties of several solid propellants whs

initiated. Based upon an analysis of the self-heating

reaction and the magnitude of these properties, an estimation

may be made of the critical size and temperature conditions

for self-ignition.

MATERIALS

The three solid propellant materials were furnished by

the Naval Ordnance Laboratory in the shapes required for test

and were Identified as A, B and C in the chronological order

of their receipt.
A

'• 'rl'.
2

EXPERIMENTAL WORK

A. Thermal Conductivity Measurements

For these measurements, a heat-flow meter type of thermal

conductivity apparatus^ was used. In operation, two slabs

each 10 by 10 by 1-in. , comprising a sample pair of the

specimen under test are placed one on each side of a heatflow meter and then the faces of a hot plate and of a cold

plate are brought together to make intimate contact with the

specimens. At steady state, the heat flowing through the

meter from the hot plate maintained at 32.8°C (91°F) to

the cold plate maintained at 13. 3° C (56°F) produces an emf

proportional to the temperature gradient through the meter.

The coefficient of thermal conductivity for each specimen

was determined from the heat-flow meter output, the thickness

of the specimen and the temperature drop in the specimen as

measured by thermocouples. The heat-flow meter was calibrated

by tests made on two materials of known conductivities as

previously determined using a guarded hot plate (ASTM C 177-^5)*

Two tests, one "forward" and one "reverse", were performed on

propellants A and B. Since only one 10 by 10 by 1-in.

specimen of propellant C was available for test, this propellant

was tested in only one direction.

The results are summarized in Table 1.

Table 1. Thermal Conductivity Measurements

Material Density

g/cm-^

Mean

Temperature Thermal Conductivity

cal/sec-cm-deg C

Propellant A 1.621 23.1 0.0005253

0.0005315 0 . 000527- 1^ Propellant B 1.766 23.1 0.001322

0.001329 0.001326=^'0. %

Propellant C 1.63^ 31.0 0.001002-1^
<C*'

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- 3 -

B. Specific Ii,eat Measurements

For these measurements, each propellant sample measured

1 by 2 by 2-1/2-in. To eliminate the possibility of interaction between the propellants and the water bath of the

calorimeter, propellants A and B were sealed in individual

plastic envelopes while propellant C was spray-coated with

a plastic film for test. The specific heats of the samples

were measured by substituting them, separately, for 100 g

of water in a calorimeter normally containing 600 g, and

measuring the energy equivalent of the calorimeter and contents

electrically.. From the value obtained for the calorimeter

with 600 g of water and the values obtained when the

propellants were tested, the specific heats of the propellants

including their plastic envelopes were obtained. The

specific heat of the propellant was determined using a

value of 0.69 cal/g-deg C for , the specific heat of the

plastic envelope. The specific heat of water was taken as

0.99921 cal/g at 22.5°C. Measurements were made over a 3

to 4 deg C range between 20 and 25°C. This method of

measurement groups all the errors of measurement into the

value obtained for the specific heat of the propellant.

Two duplicate tests on the same sample were performed and

the results are summarized in Table 2.

Table 2. Specific Heat Measurements

Material Sample

Weight

e

Envelope

Weight

R

Temp.

Rise

des C

Mean

Temp.

° C

Specific

Heat

cal/s-deg C

Propellant A 134.1903 1.8603 3.337

3.325

23.2

22.3

O.3906

0.3731 0.38^0.01

Propellant B 145.5787 1.7949 3.841

3.500

22.9

23.0

0.2827

0.3026 0.29-0.01

Propellant C 140.9178 4.3152* 3.163

3.234

23.4

23.5

0.3124

0.2925 0.30^0.01

*sprayed-on plastic film

C. Kinetic Measurements

For these measurements, two pre-cut wafers 2-in. in

diameter by 1-in. thick as furnished were placed together to

form a cylindrical specimen 2-in. in diameter by 2-in. long.

- 4 -

The specimen was assembled and mounted within a furnace

designed for self -heating studies. ^ During the initial

heating period, the thermostatically controlled furnace

air temperature was gradually increased by small temperature

intervals. At a temperature at which an indication of selfheating was obtained according to the signal from a differential

thermopile, the control system was actuated to provide

"adiabatic" or compensating temperature rise control. The

differential thermopile was composed of four (4) 23 ga.

(B & S) chromel-alumel thermocouples in series to indicate

the mean temperature difference between the specimen and the

furnace atmosphere. The electrical signal from the differential

thermopile was applied to an amplifier and servo control

system so as to automatically compensate for any temperature

rise within the specimen. Under these conditions, the

specimen temperature remains uniform throughout its mass and

the heat generated within the body increases its temperature

according to the relation:

c/3dT/dt = A6 ( 1 ) where T =

t = P =

c = A = E = R =

absolute temperature, K

time, sec

specimen density, g/cm-^

specific heat, cal/g-deg C

^ heat generation coefficient, cal/sec-cm-^

activation energy, kcal/mole

gas constant = O.OOI987 kcal/mole- K

Continuous records. Figures 1, 2, and 3*, of the specimen

and furnace air temperatures were obtained on an automatic

recorder from the signals of 24 ga. (B & S) chromel-alumel

thermocouples. Measurements were made of the slope of the

curve, dT/dt, at a number of temperatures over the self-heating

rar+ge. By taking logarithms and plotting In dT/dt versus

1/T, the resultant line has a slope of -E/R, and intercepts

the In dT/dt axis at l:n A//> c. At least two tests for each

propellant were performed and the results from one test for

each propellant is shown in Figure 4 together with those from

similar tests on nitrocellulose plastic, wood fiberboard and

purified cotton linters.

*These records were only included in 5 copies of this report

including those forwarded to the sponsor.

- 5

-

The plotted points approximate straight lines very closely

and show the rate of temperature rise due to self-heating for

each material under negligible heat loss ^ conditions. ^ Within

the range shown^ the displacement of a line toward higher

values of reciprocal temperature indicates ^If-heating at

relatively lower temperatures. Comparative rates of selfheating at any temperature may be read directly from the

graph as illustrated in Table 3*

Table 3. Comparative Rates of Self -Heating

Rate of self -heating , deg C/min

Material Temperature: 135"^C Temperature: 209°C

Cl/T = 0.002490) (1/T = 0.002075)

Propellant A 1.15 c

Propellant B a

.

0.84

Propellant C 0.07 1.58

Nitrocellulose plastic 0.225 c Wood fiberboard 0.025 2.3

Cotton linters b 0.067

a. No appreciable self-heating until 190‘^C

b. No appreciable self-heating until l80°’C

c. Specimen consumed

At 135°C, propellant A exhibited self-heating at a rate

of 1.15 deg C/min, approximately 5 times that of nitrocellulose

plastic and ^0 times that of wood fiberboard, while propellant

B exhibited no appreciable self-heating at temperatures below

190°C. At this same temperature, propellant C self -heated at

a rate of 0.07 deg C/min. At 209°C, propellant A had been

entirely consumed while propellant B exhibited self -heating at

a rate of O.d^ deg C/min, approximately 1/3 that of wood

fiberboard, and propellant C showed self-heating at a rate

of 1,58 deg C/min, approximately 2/3 that of wood fiberboard^

The values of the kinetic constants determined from the

lines in Figure ^ are listed in Table k for the applicable

temperature ranges.
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Table 4. Kinetic Measurements

Material Temperature Activation Heat Generation

Ranges Energy E Coefficient A

C kcal/mole cal/ sec-cm-^

Propellant A 100 - 170 38.8 6.99 X

CO 11— 0

I1—

Propellant B 190 - 242 49.3 1.56 X

CM 0

0

1

1

Propellant C l65 - 235 28.6 1.17 X

Nitrocellulose

plastic 134 - 169 42.0 2.43 X 1019

Wood fiberboard 80 - 225 25.7 1.97 X 10?

Cotton linters 180 - 260 34.5 5.30 X loll

In the tests with propellants B and C, a (nearly)

constant temperature phase was observed at a temperature of

about 240°C. One possible explanation for this behavior

would be a change in crystal structure occurring at this

temperature.

In the tests with propellant C, melting, softening or

slumping of the propellant was observed at a temperature of

about l40 to l60°C. For the purposes of this analysis, it

was assumed that the self-heating characteristics of the

melted or slumped propellant were the same as those of the

original solid.

Due to the unexpected behavior of propellant C in the

first few tests, a total of six tests were performed on

specimens of this material and details of each of the tests

are described below.

Test No. 1

At an initial temperature setting of ll4°C, no appreciable

self-heating was observed. After 36 minutes at this temperature, the temperature was raised to 156°C. The specimen selfheated to 243°C in 65 minutes and remained at this temperature

for about 2 minutes. After this short constant-temperature
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phase, very rapid combustion set in and the specimen was

consumed. Based upon the data from this test, an activation

energy of about 55 kcal/mole was computed.

Test No. 2

At an initial temperature setting of 1^-1°C, very little

self-heating was observed. After 136 minutes at this

temperature, the temperature was raised to l49^C. The

specimen self-heated to 162°C in 135 minutes at which time

the adiabatic control operation and consequently the temperature measurements became erratic. It is interesting to note

the behavior of this test specimen after the furnace temperature was increased in an effort to destroy the specimen.

At a temperature of about 195°C, this decision was reconsidered

and the furnace heater was turned off. The specimen continued

to heat reaching a temperature of 225°C before it started

to cool. It had cooled down to 215° C and was continuing to

cool when recording was discontinued for the day. It was

found on the next day that the specimen had been consumed.

Test No. ^ After 25 minutes at a temperature setting of 82^C , the

furnace temperature was raised to l42°C. After 15 minutes,

the adiabatic control operation became erratic as in Test

No. 2. The test was discontinued after approximately 90

minutes during which period the maximum recorded specimen

temperature was l47°C. The furnace and specimen were allowed

to cool over night. Inspection of the furnace chamber the

following day showed that the specimen had melted away from

its holder and from the recording and controlling thermocouples and was found resolidified at the bottom of the

furnace chamber.

On the basis of this finding, it was assumed that

softening or melting had occurred in each of the previous

tests. It is speculated that in Test No. 1 a small quantity

of propellant material had remained in contact with the

control thermopile to permit adiabatic control operation

whereas in Test Nos. 2 and 3? complete separation had

occurred. For all succeeding tests, the cylindrical propellant

specimens were placed in a 2-in. diameter by 2-ln. high

stainless steel wire basket consisting of 40 mesh, 0.010-in.

diameter wires. This basket served to support the softened

or melting propellant specimen in position during the selfheating ^-process.
1

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- 8 -

Test No. ^ From an initial temperature of 130°C, the specimen selfheated to 238°C in about 15 hours. After a short pause

at this temperature, very rapid combustion occurred and the

specimen was consumed. The data on the rate of temperature

rise versus reciprocal temperature for this test was best

approximated by two straight lines yielding an activation

energy of 38.1 kcal/mole for the temperature range l80 to

200° C and an activation energy of 19»9 kcal/mole for the

temperature range 200 to 235°C.

Test No. 3

From an initial temperature of 130°C, the specimen selfheated to 238°C in about 12 hours. After a short delay at

this temperature, very rapid combustion occurred and the

specimen was consumed. Analysis of the data yielded an

activation energy of 24.1 kcal/mole for this test.

Test No. 6

From an initial temperature of 131°C, the specimen selfheated to 236°C in about 10 hours. Following the characteristic

pause at almost constant temperature, very rapid combustion

occurred and the specimen was consumed. The activation

energy for this test was calculated as 28.6 kcal/mole and

is the value reported in the tables.

CALCULATION OF CRITICAL SIZE

In order to estimate the critical size for ignition of

a mass of self-heating material, the analysis presented by

Enig , Shanks and Southworth8 was used. This relates the

half -thickness of a material of given thermal and kinetic

properties with the temperatures at the center and the surface

under critical steady-state conditions. This analysis has

been applied to wood fiberboard and cotton linters and, fair

agreement with experimental results has been ob;tained.^

The assumption was made that the kinetic properties

measured over higher temperature ranges may be applied to

the 'temperature range of practical interest for ordinary

storage (20 - 100°C}. It was further assumed that the

measured thermal properties may be applied over the whole
i
- 9

-

temperature range. Critical radius determinations for a sphere

have been made for each propellant and for three other common

materials, and these are listed in Table 5 and shown graphically

in Figure 5* It may be noted from the tables in reference

3 that for a given surface temperature, the critical radius

for a cylinder and the critical half thickness for a semiinfinite slab are given very closely by 0.775 Be and 0.51^ Bq

respectively, where Bq is the critical radius for the sphere.

Table 5» Critical Size Calculations

Material Critical Radius of a Sphere

Surface 20‘^C 4d.9°C d2.2^C 100°C

Temp . (6d°F) ( 12Q° F) ( l80°F) (21 2^F ) Propellant A 9800 cm

320 ft

54o cm

18 ft

35 cm

1.2 ft

10 cm

0.33 ft

Propellant B 2.4x10^

7.9x105

6.0x10,^

2.0x10^

1.8x10^

580

3500

120

Propellant C 1.9x10^

633

2351

77

318

10.4

128

4.2

Nitrocellulose

plastic

7.7x10^

2500

3300

110

170

5.5

43

1.4

V/ood fiberboard 4500

150

680 • 22

110

3.7

50

I06

Cotton linters 4.2x10^

1.4x10^

3-3x10^

l.lxio'^

2.9x10^

940

9400

310

SUMMARY

Measurements of the specific heat, thermal conductivity and kinetic properties of 3 propellants have been made.

In the self-heating experiments, propellant A self-heated

• initial temperature of 100*^C to a temperature of 170^^0

temperature, very rapid combustion occurred and the sample was consumed. The activation energy for propellant A over this temperature range was 38.8 kcal/mole. rropellant B self-heated in 2-1/2 hours from an initial
/ J'**'
10

temperature of 190°C to a temperature of 242°C, at which

temperature very rapid combustion occurred and the sample

was consumed. The activation energy of propellant B over

this temperature range was 49.3 kcal/mole. Propellant C

self-heated in 10 hours from an initial temperature of 131^0

to a temperature of 236^0 at which temperature very rapid

combustion occurred and the sample was consumed. The

activation energy of propellant C over this temperature

range was 23. b kcal/mole.

A comparison of critical radius determinations, under

the given assumptions, is presented in Table 5 and Figure 5*

Size limitations on the storage of bulk quantities of

propellant A, propellant C and nitrocellulose plastic, and

careful control of temperature and ventilation conditions

seem justified on the basis of these calculations. Although

wood fiberboard is not normally considered a storage hazard

at temperatures up to 60°C (l40°F), it has been known to selfheat to ignition when stacked hot after drying during the

manufacturing process. Propellant B does not appear likely

to present any storage hazard with respect to self-ignition

at ordinary storage temperatures.

11

REFERENCES

1. Lang, D. L. , "A Quick Thermal Conductivity Test on

Insulating Materials," ASTM Bui. No. 216, pp. 56-60,

Sept. 1956 .

2. Raskin, W. H. and Robertson, A, F. , "An Adiabatic

Apparatus for the Study of Self-Heating of Poorlv

Conducting Materials", Rev. Sci. Inst. 25» PP* 5^1-5^^?

June 195’+*

3 . Enig, J. W. , Shanks, D. and Southworth, R. W. , "The

Numerical Solution of the Heat Conduction Equation

Occurring in the Theory of Thermal Explosions", NAVORD

Report 4377 (1956).

4. Gross, D. and Robertson, A. F. , "Self-Ignition Temperatures

of Materials from Kinetic Reaction Data", to be published.
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U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE national BUREAU OF STANDARDS

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