Approved for public release by the

Director of the National Institute of + Standards and Technology (NIST) fenced

on October 9, 2015. ^ c NATIONAL BUREAU OF STANDARDS REPORT <fr

4L

7794

AN OVEN TEST FOR SELF-HEATING TENDENCY OF MATERIALS

by

Marjorie W. Sandholzer

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress,

March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means

and methods for making measurements consistent with these standards; the determination of

physical constants and properties of materials; the development of methods and instruments

for testing materials, devices, and structures; advisory services to government agencies on

scientific and technical problems; invention and development of devices to serve special needs

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Publications

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A complete listing of the Bureau’s publications can be found in National Bureau of Standards Circular 460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1.25),

and the Supplement to National Bureau of Standards Circular 460, July 1947 to June 1957

($1.50), and Miscellaneous Publication 240, July 1957 to June 1960 (includes Titles of Papers

Published in Outside Journals 1950 to 1959) ($2.25); available from the Superintendent of

Documents, Government Printiiig Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1002 - 12-10120 February 6, 1963

NBS REPORT

7791+

AN OVEN TEST FOR SELF-HEATING TENDENCY OF MATERIALS

by

Marjorie W. Sandfctolzer

IMPORTANT NOTICE NATIONAL BUREAU OF STANDARDS REPORTS are usually preliminary or progress accounting documents intended

for use within the Government. Before material in the reports is formally published it is subjected to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or open-literature listing of this Report, either in whole or in part, is not authorized unless permission is obtained in writing from the Office of the Director, National Bureau of Standards, Washington 25, D.C. Such permission is not needed, however, by the Government agency for which

the Report has been specifically prepared if that agency wishes to reproduce additional copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

AN OVEN TEST FOR SELF-HEATING TENDENCY OF MATERIALS

Ly

Marjorie W. Sandholzer

ABSTRACT

A simple oven test for the approximate

evaluation of self-heating tendency in materials

is described. The method appears suitable for

preliminary screening purposes, and its possible

usefulness for detecting a hazardous degree of

self-heating in materials in general household use

is discussed.

1. Introduction

In assessing the possible self-ignition hazard of a

material, a simple and convenient means of obtaining a general

idea of the self-heating tendency can be very helpful. Ah

approximate determination may frequently be all that the

situation requires, or it can serve to indicate an appropriate

range for more precise measurement with refined equipment.

A simple oven test, designed as a means of roughly surveying

a group of foam rubber materials, appears promising for somewhat broader applicability and usefulness. The method is

described with the thought that it may prove suitable for^-the

detection of potential self-ignition hazards among materials

in general household use.

2. Test Method

The method consists essentially in holding a wellinsulated test specimen of the material in an oven at an

appropriate temperature for a certain period of time, to

determine whether ignition occurs under the selected conditions.

With a judicious choice of temperatures, a considerable range

of conditions may be studied.
2

A test' specimen of about 8-10 in.^~ is used. The specimen

form is not critical but it should be fairly compact, with ctl Tensions as uniform as maybe readily feasible. In any case, the

longest dimension should not exceed 3 inches and the shortest

should not be less than 1 inch. Liquids may be distributed on

cotton gauze, and pulverized or chipped materials may be confined

in a gauze or glass cloth bag.

The specimen is wrapped in an initial covering of glass wool

free of dressing or binder (preferably hard glass, available from

most scientific supply houses) to provide an inert separation of

the specimen from the outer insulation. About 1/3 ounce of glass

wool is usually sufficient for the purpose. An outer wrapping of

about 6 ounces of cotton batting is then applied uniformly to make

a test ball 8-9' inches in diameter.

The oven in which the specimen is heated should have a chamber

large enough to provide 2 or 3 inches of free space on all sides

of the test ball. It should be equipped with forced air circulation, with the heating elements located in the air stream and

well removed from the chamber walls, so that the temperature

throughout the chamber is fairly uniform. The insulated specimen

is placed in the preheated oven and held at a temperature ofl30±2°C

for a period of 7 hours, or until ignition occurs. The production

of smoke and char, usually accompanied by glow, is considered to

constitute ignition, and is indicated by smoke issuing from the

oven. With some materials, such as the rubbers, the odor developed

provides a slightly earlier indication of ignition, but smoke also

appears shortly and is considered a sufficiently accurate criterion

for the purposes of an approximate test. Examination of the specimen upon removal from the oven supplies confirmation, or # the final

determination, of ignition. The photograph shows a specimen just

after ignition.

3- Application of the Method

This oven test was recently used for preliminary work in

a study of the effects of aging and cleaning on the self -heating

tendency of foam rubbers, where it served to roughly assess differences in self -heating and to indicate materials on which more

precise measurements were desirable. The refined measurements

were made by an adiabatic furnace method (1) in which the temperature of the heating specimen is closely followed by the furnace

temperature, and which provides valid information on the rate of

self-heating of the material as a function of temperature.

Although the methods differ decidedly, the furnace determinations

permitted some appraisal of the usefulness of“fhe oven t est.

In Table 1 the ra t e s*" of -'siel-f*- heating of a number^ of rubbers at

130 °C,‘'as determined in the furnace, are listed together with

the times required for the same rubbers to ignite in the oven

(1) Self -Ignition Temperatures of Materials from Kinetic-Reaction

Datja by Gross and Robertson, J. Research of NBS, No. 5**+13-17

(Nov. 19^8). - * -v ,
- 3

-

at- 130°C. Purified cotton linters and raw linseed oil were

also tested to provide some comparison with materials known

to differ widely in heating tendency.

A number of the determinations were based on only one

test and hence took no account of possible non-uniformities

in the material sample. However, in most instances where

replicate tests were made, the individual results appeared

reasonably consistent. Rubber No. 4 was a major exception,

and the individual oven ignition times ranged from 3 hours to

about 8 hours. It seems quite possible that in open storage

the non-uniformity of a rubber sample may appreciably increase

with age, because of the differences in exposure to light and

air between the surfaces of the sample and its interior.

In order to determine the actual temperature behavior of

the oven specimens, several of the oven tests were made with a

thermocouple embedded in the center of the specimen and another

placed on the specimen surface inside the glass wool wrapping.

Automatic recording equipment provided a continuous record of

the thermocouple readings* As would be expected, the temperature at the surface of the specimen was generally somewhat

higher than that at the center, until the oven temperature was

approached or appreciable self-heating developed. The center

temperature appeared the more indicative of characteristic

specimen behavior, and Figure 1 shows the time-temperature curves

obtained for the specimen center with the four materials on

which temperature records were made. The curves for linseed oil

and foam rubber No. 4 represent one determination each, and

those for cotton linters and foam rubber No. 6 represent the

average of duplicate tests. For both of the latter materials,

the two tests recorded gave almost coincident curves, with some

difference in the early stages due, in part at least, to the

variation of a few degrees in starting temperature. The average

values for oven ignition time given in Table 1 include dther

determinations for which time-temperature records were not made.

4. Discussion

The conditions in the oven test and the adiabatic, furnace

differ considerably. The adiabatic furnace does not supply

additional heat to the specimen after self-heating begins, but

maintains an ambient temperature closely following the specimen

temperature so that a minimum of self-generated heat is lost

from the specimen. The rate of rise of the specimen temperature,

therefore, depends entirely on the particular self -heating
reaction involved. The oven, on the other hand, supplies heat

to the specimen steadily up to 130°C, but no higher. The specimen is insulated, however, and the oven heat penetrates the

insulation slowly to raise the specimen temperature very

gradually. If self ->heating begins below 130°C, the specimen

will receive heat from both the oven and its own reaction until

evolution from the latter source exceeds inflow from the oven,

when the self -generated heat will be partially dissipated into

the insulation around the specimen. At specimen temperatures

above 130°C, there can be no inflow from the oven and there will

certainly be some loss of self-generated heat. Hence, the

particular self-heating reaction involved will generally be the

dominant factor in the rate of temperature rise of a strongly

self -heating specimen, but its effects will be more or less

modified by such heat exchange with the oven as may occur. The

time-temperature curves in Figure 1 illustrate the effects of

this heat exchange. In view of the difference in the methods,

only a general relationship between the oven ignition times and

self -heating rates determined in the adiabatic furnace would be

expected.

The cotton linters showed no self -heating below 130°C and,

therefore, the time-temperature curve resulted from heating by

the oven alone. Since the material specimen is small, the

total specimen assembly consists primarily of cotton batting

insulation, with a thin inner layer of glass wool, and these

products are believed to be sufficiently uniform in density and

thermal properties to warrant the assumption that the rate of

heat penetration into the specimen assembly is reasonably similar

in the different tests. The close similarity of the curves

obtained in duplicate tests tends to support this assumption

and to indicate that significant differences in insulation

packing are not likely, at least among specimens prepared by the

same operator. Hence, it would seem permissible to consider the

curve for cotton linters fairly representative of the behavior

of materials in general when heated by the oven alone, and to

assume that pronounced deviations from that curve result from

the effects of self -heating on the rate of rise of the specimen

temperature

.

The curves for the two foam rubbers, No. b and No. 6,

indicated very similar self -heating characteristics until the

specimen temperatures had nearly reached that of the oven,

where heat loss from the specimen undoubtedly increased. The

heat evolved by the reaction of rubber No. 6 at that temperature

was evidently sufficient to continue its acceleration to immediate ignition in spite of the increased loss. The reaction of
rubber No. 4, however, required an additional' 1-1/2' hours'- to

gradually build up to a temperature where it produced' enough

heat to compensate for loss to the oven and' at the same time

cause ignition. The bex-avior of these two rubbers' indicate

that, for a limited range of self -heating susceptibility,

relatively small variations in the rate of heat evolution may

produce widely divergent ignition times. It seems pro’bable

that the large variation in individual ignition times for

rubber No. 4, noted earlier, reflected this situation and may

have resulted from moderate differences in the specimens due

to non-uniform aging or manufacture. The range of self -heating

susceptibility in which this exaggerated diff erentiation occurs

will depend on the oven temperature selected, and may be shifted

to suit varying purposes.

Two tests, one on linseed oil and one on foam rubber* No. 7,

which were made with the oven temperature held at 115°C,

illustrated the effects of a change in oven temperature. The

average ignition times for these two materials at 130 aC, as

shown in Table 1, were quite similar at about 2-1/2 and 3 hours

respectively, but at 115°C the linseed oil ignited at slightly

over 3 hours while rubber No. 7 required 6-1/2 hours to ignite.

Time-temperature records were not obtained for the tests made

at 115°C. However, the curve for linseed oil tested at 130°C

shown in Figure 1 suggests that at ll5°Cthe linseed oil reaction

would have already accelerated sufficiently to ignite the

specimen, and that the ignition time was extended primarily by

the slightly longer time required to reach that temperature.

Presumably, the reaction of rubber No. 7 had not yet accelerated

enough at 115°C to produce ignition, and the ignition time was

decidedly extended by the increased heat loss above 115°C. The

lower oven temperature had shifted the range for sharp differentiation among materials toward those more highly susceptible

to self -heating

.

5. Suggested Applications

Although there are basic differences between the two methods

the oven test results show a broad general agreement with those

of the adiabatic furnace and appear sufficiently definitive to

recommend the test for certain purposes. It can be a useful preliminary survey method, as in the present application to foam

rubbers, where the effects of cleaning noted by the oven test

led to more precise study with the adiabatic furnace. It may

also be suitable for the detection of a hazardous degree of

self -heating among household materials in ordinary consumer use.
- 6 -

For' this purpose a general classification of self-heating- tendency

is sufficient, but it must be applicable to widely diverse

materials and should be reasonably simple to determine. It

should be noted that the oven test provides no information on

the effect of specimen size on the self -heating hazard, and hence

would be of little use in the investigation of warehouse problems

involving large quantity storage.

Considering the possible application of the method to

household materials, the conditions of the test as it has been

described are certainly more severe than would normally occur

in the home, and none of the rubbers tested would be likely to

present any hazard of self -ignition under the usual conditions

of consumer use. Most accidents occur, however, because of

inadvertently abnormal conditions. It is believed that the test

conditions employed may reasonably simulate such probable abnormalities as the development of a "hot spot" by a malfunctioning

heating pad or other electrical appliance, or the careless insulation of a material from proper air circulation, and may

provide a useful criterion for household safety. The potential

hazard of improperly discarded oily rags is well known, and

rubber No. 6 became available for test because of actual ignition

some time after a small area of the auto seat had been overheated

by a malfunctioning electrical appliance. In view of such

instances, the data in Table 1 suggest that it might be appropriate to consider a material which ignites in 3 or 3-1/2 hours

under the test conditions described, as potentially hazardous

for general consumer use. Additional tests at a lower oven

temperature would be informative and perhaps desirable for materials

which ignited within that period. An oven temperature of about

130°C would appear suitable for initial testing, however, because

it would permit considerable confidence in assuming the general

safety of a material which failed to ignite during a 7-hour

exposure period.

6 . Summary

Several characteristics of the oven test which has been

described recommend it as a useful tool in the study of selfheating tendency. It is simple and is applicable to a wide

variety of materials. The range of self -heating tendency

studied may be adjusted by appropriate selection of the oven

temperature

.
- 7 -

The test provides a convenient means of"' making a rough

general survey of the self -heating character" of a material

,

from 'Which the desirability of further investigation may be

judged. In addition, the approximate information obtained

would appear adequate to indicate a potential self-ignition

hazard in materials for general household use.
Table

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300

TIME,

HOURS

FIG.ITIME-TEMPERATURE

CURVES

FOR

OVEN

TEST

II. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. A st in. Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., tW

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering] in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover. WASHINGTON, l). C.

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Dielectrics. High Voltage.

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Office of Weights and Measures. BOULDER, COLO.

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USCOMM—NBS— DC