NATIONAL BUREAU OF STANDARDS REPORT

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8574

THERMAL CONDUCTIVITY OF

FIVE EPOXY RESIN SPECIMENS

by

T. W. Watson and H. E, Robinson

Report to

Sandia Corporation

Albuquerque, New Mexico

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assuring

maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. Its responsibilities include development and maintenance of the national stand- ards of measurement, and the provisions of means for making measurements consistent with those standards; determination of physical constants and properties of materials; development of methods

for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; assistance

to industry, business, and consumers in the development and acceptance of commercial standards and

simplified trade practice recommendations; administration of programs in cooperation with United

States business groups and standards organizations for the development of international standards of

practice; and maintenance of a clearinghouse for the collection and dissemination of scientific, technical. and engineering information. The scope of the Bureau’s activities is suggested in the following

listing of its four Institutes and their organizational units. Institute for Basic Standards. Electricity. Metrology. Heat. Radiation Physics. Mechanics. Applied Mathematics. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radio Standards Laboratory: Radio Standards Physics; Radio Standards Engineering.** Office of Standard Ref- erence Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.** Office of Standard Reference Materials. Central Radio Propagation Laboratory.** Ionosphere Research and Propagation. Troposphere

and Space Telecommunications. Radio Systems. Upper Atmosphere and Space Physics.

Institute for Applied Technology. Textiles and Apparel Technology Center. Building Research.

Industrial Equipment. Information Technology. Performance Test Development. Instrumentation.

Transport Systems. Office of Technical Services. Office of Weights and Measures. Office of Engineering Standards. Office of Industrial Services.

* NBS Group, Joint Institute for Laboratory Astrophysics at the University of Colorado. ** Located at Boulder, Colorado.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

421.06-30-4210642

NATIONAL BUREAU OF STA

for use within the Government. 0 and review. For this reason, the whole or in part, is not authoriz Bureau of Standards, Washington,

the Report has been specifically p

NBS REPORT

15 October 1964 8574

THERMAL CONDUCTIVITY OF

FIVE EPOXY RESIN SPECIMENS

by

T. W. Watson and H. E. Robinson

Heat Transfer Section

Building Research Division

Institute for Applied Technology

Report to

Sandia Corporation

Albuquerque, New Mexico

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

accounting documents intended

ibjected to additional evaluation

isting of this Report, either in Office of the Director, National

the Government agency for which

ies tor its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Thermal Conductivity of

Five Epoxy Resin Specimens

by

Thomas W. Watson and Henry E. Robinson 1.

INTRODUCTION

This report presents results of thermal conductivity measurements in

the temperature range 20 to 135 °F of five samples of epoxy resin base

submitted by the Sandia Corporation, Sandia Base, Albuquerque, New Mexico.

The samples are intended for use as thermal conductivity reference standards by the Sandia Corporation.- The measurements at NBS were authorized

by Sandia Contract Number 16-2276, dated 18 June 1964.

2.

SPECIMENS

Five different epoxy resin samples were submitted for measurement,

each in the form of a pair of specimens each approximately 9 inches by

9 inches by 1 inch in size.

i4- When received # August, the pairs of specimens were designated by the

numbers 1 through 5 inscribed on the edges of each respective piece and were further identified by letter of 4 September as follows:

1. 89.5% (weight) Shell 828 epoxy resin

10.7% Diethanol amine (DEA)

Cured at 160 °F for 15 hours 40 minutes on 6 July 1964

2. 69.6% 828 resin

22.0% mica filler

8.4% DEA

Cured at 160 °F for 16 hours 30 minutes on 7 July 1964

3 . 44.670 828 resin

50.0% mica filler

5.4% DEA

Cured at 1.60 °F for 18 hours 15 minutes on 8 July 1964

4. 71.4% 828 resin

20.0% mica filler

8.6% DEA

Cured at 160 °F for 66 (sic) hours on 10 July 1964

- 2 -

5. 62.5% 828 resin

30 . 0 % mica filler

7.5% DEA

Cured at 160 °F for 17 hours on 13 July 1964.

‘Hie specimens also were described as having been cast by Sandia as 9-inch by 9-inch by 1-1/8-inch pieces. The 1-1/8-inch thickness was then

ground to be approximately one inch in size as submitted. It is noted

here that the detailed descriptions of the specimens given on Page 1 of

Contract No. 16-2276 differ from those given above.

3. METHOD AND PROCEDURE OF TEST

The thermal conductivity measurements were made in an 8 -inch guarded

hot-plate apparatus (conforming to ASTM C177) having a 4-inch square

central metering section. The tests were conducted in a cabinet in air

maintained at within about two degrees Fahrenheit of the specimen mean

temperature; the exposed edges of the specimens protruded 1 / 2- inch

beyond the hot and cold plates of the apparatus and were covered by

about 1 inch of fine glass-fiber insulation.

Because of the low thermal resistance, and rigidity, of the speci- mens the tests were made using a composite-specimen technique* to minimize thermal contact problems, and which avoided need for thermocouples

applied to the specimen faces. A resilient pad (1/8-inch commercial

gum rubber, 30 to 45 Durometer floating stock. Federal Specification

MIL-R- 88QA) was interposed between each face of the specimen and the

adjacent metal hot or cold plate of the apparatus at a pressure of approximately 50 lb/ft2

, so that there was on each side of the hot plate

a composite specimen consisting of pad, specimen, pad. The total temperature drop through the composite specimen was measured by means of

the permanent thermocouples installed in grooves in the metal faces of

the cold and hot plates. The test yielded as its result the average

thermal resistance per unit area of the two composite specimens over the

central 4-inch square area. By subtracting from the average compositespecimen resistance the average thermal resistance of the two rubber

pads at the same mean temperature, as determined in separate tests

using the same permanent plate thermocouples, the average net thermal

resistance of the principal specimens was calculated. The average

thickness (apparent thickness) of the principal specimens during test was determined by subtracting from the measured value of the distance

between the cold plates during the composite-specimen test the measured distance between the cold plates determined in the separate tests

at the same mean temperature, in which the rubber pads alone constituted

the test specimen.

*See AS1M C177-63, paragraph 4(h)

- 3

-

In the separate tests of the two pairs of 8-inch by 8-inch by 1/8-

inch gum rubber pads the thermal resistance increased linearly with

temperature from a value of 0.240 deg F/ (Btu/hr ft2

) at 20 °F to 0.252

at 130 °F, with none of the six values obtained departing by as much as 0.2 percent from the linear relationship. Similarly, the average total

thickness of the two rubber pads measured in the apparatus increased

linearly from 0.247 inch at 20 °F to 0.255 at I30 °F, with no departure

exceeding 0.2 percent.

4. RESULTS AND DISCUSSION

The results of the thermal conductivity measurements are summarized

in Table 1. A plot of thermal conductivity versus density at about

130 °F is shown in Figure 1; the conductivity of these specimens varied

smoothly with density.

As shown in Table 1, two tests at about the same mean temperature were made on Specimen No. 1. At the conclusion of the first test

(HC90164) there was no visible warping of the specimens in the apparatus,

nor was there a significant thickness change during the test. However,

after leaving the specimens stacked together on a table overnight it was discovered the following day that the specimens had become concavo- convex in shape. The concave face of each specimen was the face that

had been toward the hot-plate in the conductivity test. Because of the

dishing, which amounted to about 0.5 millimeter at the center, the

specimens were compressed between flat metal plates under a load of

124 pounds for seven days. After this period, there was no perceptible

change in the degree of concavity. Nevertheless, a repeat test (HC92564) was made on the specimen by the composite technique, with the specimens

reversed so that the previous ly-colder side (now convex) faced the hot

plate, in the hope that the test temperature gradient might reduce the

dishing. Actually, the degree of dishing increased slightly during the

test. As Table 1 shows, the final apparent thickness of the specimen

in the second test was 0.017 inch greater than in the first test, indicating a degree of dishing too extreme to be accommodated by the rubber

pads. Consequently, the conductivity result obtained in this test

(1.40) is not valid (is low), and is recorded only as a matter of interest.

Because of the warping of Specimen No. 1 after the first test, no further valid conductivity measurements could be made with it at lower

temperatures. Fortunately, perceptible warping did not occur for any of

the other specimens, and it was feasible to make all of the desired

measurements on them, as recorded in Table 1. The slight changes in

thickness with temperature recorded in Table 1 appear to indicate an effect of thermal expansion, but the precision of the specimen thickness

determination is not enough to afford a clear indication, although it is

believed adequate to indicate that warping was not appreciable for the

last four specimens.

- 4 -

No information is available to us as to the effect of aging or elevated temperatures on the thermal conductivity of these materials. It

is believed that any change in the conductivity of the material would be

relatively unimportant as compared to the effects of warping of the

specimen which might result from aging, insofar as the use of the speci- mens as references is concerned. Because of the rigidity of the specimens, and their high conductances, plate surface thermal contact resistances arising either from imperfect flatness of contacting surfaces

or warping of the specimen can introduce serious errors in their use as references. The composite-specimen technique used in these measurements

was adopted to minimize such errors, and it is believed was successful

in this direction in the tests of Specimens Nos. 2 to 5, and in the first

test of Specimen No. 1. Equal avoidance of thermal contact errors is

of course necessary in using these specimens as thermal conductivity

references.

Summary

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Discussion

THERMAL

CONDUCTIVITY

OF

EPOXY

RESIN

SPECIMENS

VERSUS

DENSITY

AT

ABOUT

130

Lu

DENSITY,

lb/ft 5

FIG.