Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) >nced on October 9, 201 5. NATIONAL BUREAU OF STANDARDS REPORT

^012

THERMAL AND SELF IGNITION PROPERTIES OF

AMMONIUM PERCHLORATE AND PETN EXPLOSIVE

by

J. J, Loftus

D. Gross

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NIBS PROJECT NBS REPORT

1002-12-10429 November 9* I960 7012

THERMAL AND SELF IGNITION PROPERTIES OF

AMMONIUM PERCHLORATE AND PETN EXPLOSIVE

by

J. J. Loftus

D. Gross

U. S. DepartmlSI: of the Navy

Naval Ordnance Laboratory

white Oak, Maryland

Ref: 932/60

IMPORTANT. NOTICE

U

i;

I

F.

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<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THERMAL AND SELF IGNITION PROPERTIES OF

AMMONIUM PERCHLORATE AND PETN EXPLOSIVE

by

J. J. Loftus

D. Gross

ABSTRACT

Measurements have been made of the specific heat,

thermal conductivity and kinetic properties of pressed

ammonium perchlorate and PETN explosive. _ Based' upon

these measurements estimates are provided on the self

ignition hazard associated with use and bulk storage

of such materials.

INTRODUCTION

At the request of the Naval Ordnance Laboratory determination of the

thermal and kinetic properties of pressed ammonium perchlorate and PETN

explosive was initiated. This report summarizes the results of such

measurements and supplements results on three solid propellants (l) and

six explosive materials (2) previously reported.

From these measurements and an analysis of the self heating reaction,

an estimation may be made of the self ignition hazard associated with the use and bulk storage of such explosives. These measurements may also be

useful for possible intercorrelation with an interpretation of sensitivity

data.

MATERIALS

The explosives were furnished by the Naval Ordnance Laboratory in the

shapes required for test. Th§r consisted of pure pressed ammonium perchlorate and two PETN samples, one made up of 85$ PETN and 15$ wax and the

other of 75$ PETN and 25$ wax.

- 2-

A. Kinetic Measurements

The apparatus and method used for the self-heating studies. and

analysis of the data obtained have been, described previously J_ 1 , 3.7* Briefly, thermocouples were placed within a cylindrical specimen, 2 in.

in diameter by 2 in. long, which was prepared by placing together 2

precut wafers 2 in. in diameter by 1 in. thick. The specimen was then

placed in a stainless steel beaker and mounted within the furnace designed for the self-heating measurements. At least two tests for each

explosive were performed in order to establish satisfactory reproducibility between duplicate tests. The slope of the time- temperature curve

at several temperatures was plotted against reciprocal absolute temperature and the results are shown in Figure 1. A straight line has been

drawn through one set of points in each case and the activation energy

E determined from the slope. Comparative rates of self-heating at any

temperature may be read directly from the graph. For example, at l60°C

(—0.002309) the pressed explosive PETN exhibited self-heating at a rate

T of 7.1 deg C/min, whereas any self-heating of ammonium perchlorate at

this temperature was below the limit of sensitivity of the apparatus used.

The values of the kinetic constants determined from the lines in

Figure 1 are listed in Table 1 for the applicable temperature ranges.

Test observations and remarks are summarized below:

Self-heating tests on 75$ PETN - 25$ wax and 85$ PETN - 15$ wax

indicated an apparent melting of the wax additive over the temperature

range of 77 - 82°C. The samples exhibited negligible self-heating at

temperatures below 135°C. However, upon reaching 135°C a very rapid

combustion took place and the specimens were destroyed within 15 minutes.

Three tests were run on pressed ammonium perchlorate. From an initial furnace temperature of 190°C the material self-heated in 2 l/i

hours to 240°C, at which temperature an induction period of about 5

minutes' duration occurred. This effect was probably caused by a change

in crystal structure of the material. After this short pause at 240°C,

the ammonium perchlorate underwent a rapid combustion and the specimens

were destroyed.

-3-

B. Specific Heat Measurements

For the specific heat measurements, Varsol was used as the heat transfer fluid because of its low heat capacity of 0.4370 cal/gm deg C, as measured over the experimental temperature range. Specimens for these

measurements consisted of two pieces of explosive, each 1 7/8 by 2 by

1/2 in. , and each piece was sealed for protection against possible attack

by the Varsol by means of cellophane tape. The calorimeter was so con- structed that the two pieces constituting a specimen formed two sides of

a channel through which the Varsol was circulated by a centrifugal pump.

The specific heat of each specimen was measured by substituting it for

63.4 g of the initial 175*4 g of Varsol in the half-pint Dewar fladk

calorimeter, and measuring the energy equivalent of the calorimeter and

contents electrically. Storage batteries supplied electrical energy to

a submerged cartridge heater, measurements being made of the heater cur- rent I, the heater resistance R, and the time of power input t. The

energy input was calculated by the equation

_l£R_L

E = 4.184 calories -

From the value obtained for the calorimeter and 175*4 g of Varsol, and

the values obtained when the specimens were tested, the specific heat of

each specimen, including the cellophane wrapping, was obtained. The

specific heat of the specimen alone was determined using a value of

0.32 cal/gm deg C for the specific heat of the cellophane. Three tests

on each specimen were performed and inspection of the material after test

showed no evidence of any Varsol leakage into the cellophane-packaged

explosives. Results are summarized in Table 2.

C. Thermal Conductivity Measurements

Specimens for these measurements consisted of circular discs of

6-in. diameter and a uniform thickness of approximately 0.5 in.

The thermal conductivity was determined in the conductive disc

apparatus (4) which consists of a stainless steel disc of 6-in. diameter

sandwiched between two specimens, which in turn are sandwiched between

two water-cooled brass plates. The central conductive disc is uniformly

heated at the edge by means of electrical resistance wire set in an edge

-4-

groove. The heat generated at the disc edge tends to flow in the disc

radially toward its center, and also from the disc through the two speci- mens to the two cold plates. These plates are maintained at a uniform

temperature by circulating cold water through copper tubing soldered to

them. Under steady state conditions, the temperature of the conductive

disc decreases toward the center. By measuring the temperature of the

conductive disc at the center and at a suitable radius (temperatures t0

and t respectively) the effective conductance of the specimens can be

calculated if the temperature of the cold plates (tc ) and the conductivity

and thickness of the disc metal are known.

It was decided to use only one explosive specimen for these measurements and therefore a dummy specimen was used in place of a duplicate.

This dummy consisted of a 1 in. thick semi-rigid glass-fiber insulating

board of stable, low thermal conductivity (0.348 mw/cm°C at 50°C). A

thickness of 0.991 in. was maintained during use by three small fiber

pegs thrust perpendicularly through the board. The lower cold plate of

the apparatus, the dummy specimen, and the conductive disc were fastened

together with rubber cement to form a sub-assembly.

During previous use, the apparatus had been calibrated by measurements on six specimens for which thermal conductances in the range 0.3

to 4.4 mw/crrr°C were known from determinations made with the guarded hot

plate apparatus (ASTM C177-45) . A faired curve drawn through the six

points obtained by plotting the known conductance against the measured

ratio

^ (designated ) for each material, provided a calibration

curve

°“ c for the apparatus. Departure of the individual points from

the faired curve did not exceed one percent. Before making measurements

on the PETN and ammonium perchlorate, a check measurement was made on one of the six calibration specimens ( 1.25 cm thick neoprene rubber) and

the value obtained agreed with the previous measurement within two percent.

The specimen conductances were corrected to compensate for the change

in the conductive disc conductivity when its mean temperature departed

from the calibration temperature. Duplicate tests were run on each of

the materials and results are summarized in Table 3.

D. 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

Southworth J_ £[ was used. This related the half- thickness of a material

-5-

of given thermal and kinetic properties with the temperatures at the

center and the surface, under critical steady state conditions. 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 temperature range. These are broad assumptions, particularly for PETN for

which the kinetic measurements were made on the liquid phase. Critical

radius determinations for a sphere have been made for each explosive

and are listed in Table 4 and shown graphically in Figure 2. It may be

noted from the tables in Reference 4 that for a given surface temperature, the critical radius for a cylinder and the critical half-thickness

for a semi-infinite slab are given very closely by 0.775 B c and 0.514

B c , respectively, where B c is the critical radius for the sphere.

SUMMARY

Measurements of the specific heat, thermal conductivity and kinetic

properties of pressed ammonium perchlorate and PETN explosive have been

made and are listed in Tables 1 to 3. In the self-heating experiments

on the PETN specimens an apparent melting of the wax additive occurred

over the temperature range 77 - 82°C. The material exhibited negligible

self-heating at temperatures below 135° C. However, upon reaching 135°C,

a rapid combustion began and the specimens were destroyed within 1

minutes. Activation energy values, as measured by this method, were

32.2 for the 85$PETN - 15$ wax specimens. Comparison of the data

shows close agreement between duplicate tests on each material and

appears to indicate that the different amounts of wax additive present

in the specimens tested had a negligible effect on the self-heating

process of the PETN explosive material.

The activation energy of pressed ammonium perchlorate, as measured

by this method was 29-. 7 k cal/mole over the temperature range of 205°C

to 230°C. It appears, however, that the activation energy may be quite

dependent -upon physical structure as well as temperature. Although an accurate value could not be estimated, the activation energy for the

pressed ammonium perchlorate above the temperature of crystal transformation (240°C) appeared to be somewhat greater than 40 k cal/mole.

ERRATA

Sentence in first paragraph of Summary beginning "Activation energy..."

should read as follows: "Activation energy values, as measured by this

method, were 32.2 K gf* for the 75% PETN - 25% wax specimen and 33.8 k_cal

for the 85% PETN - 15% wax specimens."

-6-

The energy of activation for pure granular ammonium perchlorate of

0.91 gm/cm3 density had previously been determined in the same apparatus,

and mean values for the temperature range 205° to 270°C of E = 41.2 k cal/

mole and A = 1.22 x 10l6 cal/sec cm3 were obtained (6). The present data

on pressed ammonium perchlorate suggested the possibility that, in the

earlier results on the granular perchlorate, some indication of a change

in activation energy at the temperature of crystal transformation might

have escaped notice and been obscured in the mean values reported. Reexamination of the earlier results, however, confirm the belief that the

activation energy of the pure ammonium perchlorate in granular form did

not undergo a significant change at 240°C.

A comparison of critical radius determinations under the given

assumption is presented in Table 4 and Figure 2. Size limitations on

the storage of bulk quantities of PETN and ammonium perchlorate, and

careful control of temperature and ventilation conditions, seem justified

on the basis of these calculations. Consideration must be given to

melting or other phase transformations occurring at relatively low temperatures, for example, the melting of the wax additive in the PETN

explosive specimens at 77° - 82°C.

USCOMM-NBS-DC

REFERENCES

1. Loftus, J. J. and Gross, D. "Thermal and Self Ignition Properties

of Three Solid Propellants," NBS Rept. 6lll.

2. Loftus, J. J. and Gross, D. "Thermal and Self Ignition Properties

of Six Explosives," NBS Rept. 6548.

3. Raskin, W. H. and Robertson, A. F. "An Adiabatic Apparatus for the

Study of Self Heating of Poorly Conducting Materials," Rev. Sci.

Inst. 25, 541-544, June 1954.

4. Robinson, H. E. , Flynn, D. R. and Watson, T. W, "Measurements of

the Thermal Conductivity of Several Explosives by the Conductive

Disc Method," NBS Rept. 6527 .

5. 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 Rept. 4377 (1958). ASTIA Document

No. AD-116 873.

6. Gross, D. and Robertson, A. F. "Self Ignition Temperatures of

Materials from Kinetic Reaction Data," J. Research NBS 6l, 413 - 417 (Nov. 1958).

TABLE 1. KINETIC MEASUREMENTS

Material

Temperature

Range State

Activation

Energy , E

Heat Generation

Coefficient, A

°C k cal /mole cal/sec cm^

75$ PETN - 2 5$ Wax 135 - 175 Liquid 32.2 3.18 x 10*5

8 5$ PETN - 15$ Wax 135 - 175 Liquid 33.8 6.78 x 1015

Ammonium Perchlorate 205 - 230 Solid 29.7 4.83 x 10

11

NH4CIO4

TABLE 2. SPECIFIC HEAT MEASUREMENTS

Energy

Sample Cellophane Temp.

Input

I

2Rt

4.184

Specific Heat

Material Weight Weight Rise individual mean

g deg C Cal cal/g °C cal/g '

75$ PETN - 25$ Wax 90.0339 5.3571 3.524

3.472

3.624

367.92

363.66

375.19

0.3414

0.3419

0.3375 0.340

85$ PETN - 15$ Wax 92.9239 4.8527 3.526

3.507

3.523

366.52

367.71

367.86

0.3277

0.3363

0.3326 0.332

Ammonium Perchlorate

NHqCiq,

124.8187 4.0813 3.400

3.362

3.584

369.65

370.35

392.31

0.2785

0.2883

0.2926 0.286

TABLE

3.

THERMAL

CONDUCTIVITY

MEASUREMENTS

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TABLE

4.

CRITICAL

SIZE

CALCULATIONS

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RATE

OF

TEMPERATURE

RISE

°C/

M

in

FI6.I ADIABATIC SELF - HEATING DATA FOR AMMONIUM

PERCHLORATE AND PETN EXPLOSIVES

CRITICAL

RADIUS

OF

SPHERE

CM

F16.2 COMPUTED CRITICAL RAbU AND CRITICAL SURFACE TEMPERATURES

FOR SPHERICAL PILES OF NH4CIO4 AND PETN EXPLOSIVES

U.S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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