NATL INST. OF STAND & TECH

AlllOb T7A8bl

NBSIR 81-1654

LOW TEMPERATURE MECHANICAL PROPERTIES OF

A POLYURETHANE FOAM

J.M. Arvidson

L.L. Sparks

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

Prepared for:

Gas Research Institute

8600 West Bryn Mawr Avenue

Chicago, Illinois 60631

100

,1156

81-1654

1981

L c. 2

November 1981

auwati BVREAt

09 rr*KDARUS

LIBRARY

FEB 1 0 1982

NBSIR 81-1654

LOW TEMPERATURE MECHANICAL PROPERTIES OF

A POLYURETHANE FOAM

J.M. Arvidsont

L.L. Sparks f-jt Fracture and Deformation Division

National Measurement Laboratory

tt Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

November 1981

Prepared for:

Gas Research Institute

8600 West Bryn Mawr Avenue

Chicago, Illinois 60631

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
> „ ..¥
Low Temperature Mechanical Properties of a Polyurethane Foamt

J. M. Arvidson and L. L. Sparks

Polyurethane foam, having a density of 32 kg/m3

, was tested at 295, 111, 76, and 45 K

in helium gas. The material properties reported are Young's modulus, proportional limit,

yield strength (at 0.2% offset), tensile, shear, and compressive strengths, and elongation

(elastic and plastic). To perform these tests, a unique apparatus was developed. This

apparatus permits tension, compression, and shear testing of materials at any temperature

ranging from 295 to 4 K. Strain is measured with a concentric, overlapping-cylinder

capacitance extensometer that is highly sensitive and linear in output.

Key words: Compressive strength; elongation; foam; insulation; low temperature; mechanical properties; proportional limit; shear strength; tensile strength; yield strength;

Young's modulus.

1 . INTRODUCTION

Safe, economical storage and transportation of liquefied gases depend to a large

extent on the materials used in capital equipment, i.e., storage facilities, transfer

terminals, liquefiers, pipelines, and ships. Thermal insulations constitute a critical

link in the materials for use in LNG and LPG oriented energy efforts because of the

cryogenic nature of the liquids. Accurate data, predictive capability, and standardized

methods and materials will improve the selection and development of insulating materials

for these applications. The mechanical properties reported are part of a broader effort

to understand and be able to predict the thermal and mechanical behavior of expanded

plastics (foams) in cryogenic environments.

The tensile, compressive, and shear properties of polyurethane foam, having a density

of 32 kg/m3

, were determined using a test fixture developed specifically for this program.

This fixture provides the capability of determining the above material properties at any

temperature from ambient (295 K) to near 4 K, in a cold helium-gas atmosphere. Tests can

also be done in liquids, such as helium (4 K) , nitrogen (76 K) , and dry ice and alcohol

(195 K) . The fixture can also be used for tests on materials at the above temperatures

while under static pressures ranging from atmospheric to approximately 0.3 MPa (30 psig)

.

A strain extensometer was developed for use with soft viscoelastic materials. The

technique [1] utilizes concentric overlapping cylinders and the change in electrical

capacitance to detect specimen strain. This type of extensometer does not attach directly

to the specimen so that the effect of instrumentation on the experimental results is

minimized. The extensometer works well in cryogenic environments, is accurate, and is

linear for large strains [2,3]. The capacitance extensometer system had a linearity range

in excess of 2.5 cm (1.0 in) when used with the current foam specimen geometry (5.1 cm diameter by 10.2 cm long). Similar extensometers can be designed for specific sensitivities and extents of linearity [4]. As long as the capacitance extensometer is situated in

a stable fluid (i.e., single phase—no boiling) the result is a low-noise, output signal.

These sensors function in gases as well as in liquids. The original calibration of the

system, for example, could have been performed at room temperature in air. To conduct a test in any other media (e.g. , liquid nitrogen) the original calibration need only be

corrected for the change in dielectric constant [5].

2. MATERIAL CHARACTERIZATION

The material tested in this study is a nominal 32 kg/m3 polyurethane foam (PU)

designated as GM30. This amorphous, organic polymer is a thermosetting foam. Our supply

of this material was obtained from the NBS Office of Standard Reference Materials (OSRM)

,

Washington, D.C. The OSTil distributed this and other expanded plastics for the Products

Research Committee [6]. These materials were commercially produced and designated as General Materials.

tThis work was supported by the Gas Research Institute, 8600 West Bryn Mawr Avenue,

Chicago, Illinois 60631.

1
Our bulk supply of GM30 was in the form of a 0.1 x 0.2 x 1.8 m slab. The physical

tests were conducted on specimens taken from the area indicated in figure 1. The orientation of the elongated cell axis for the material used in the physical properties tests was determined optically from statistical evaluation or microphotographs. The ratios of cell

height to cell width for the principal orthogonal planes of the physical test specimens

were: x/y = 1.33 ± 0.24, z/x = 1.02 ± 0.18, z/y = 1.40 ± 0.26. The uncertainties given

represent estimates of one standard deviation. The cell orientation relative to the

orthogonal axes of the bulk slab are shown in figure 2 .

The chemical formula of the polyurethane resin is CiQ. 47 Hu. 9 gO2 . 38 N. The cellular

gas content, although of secondary importance to the properties being reported here, was

found by mass spectrography to be (in mole percent): CCI 3 F, 71.2%; N2 , 6.1%; 0 2 , 21.8%;

A, C0 o , CO and F, 1%. The cellular gas pressure was approximately 66 kPa (0.65 atm). The

apparent density of the resin-gas composite was found to be 32.4 ± 0.5 kg/m3 at ambient

temperature and humidity. Conditioning at 23° C and 50% relative humidity caused an estimated 2.7% volume increase and a resulting density of 31.7 kg/m3

.

3. SPECIMENS

The tensile specimens were rods 9.9 cm long and 2.9 cm in diameter. For the determination of ultimate strength, a reduced section specimen was used. The gage length was approximately 5.1 cm and 1.9 cm in diameter. All tensile specimens were epoxied to threaded polycarbonate grips. The reduced section geometry forced fracture to occur

' within the gage length, thereby eliminating premature failure at the grip ends due to biaxial stresses. All other mechanical properties, such as Young's modulus, proportional

limit, and yield stress, were derived using the rod geometry.

The compression specimens were 2.54 cm long and 2.9 cm in diameter.

Shear specimens were approximately 1.9 cm x 2.54 cm x 0.4 cm. These specimens were

epoxied to flat plates and each plate was attached to a pull-rod. An aluminum cylinder

was slipped over the specimen and plates to help maintain alignment during testing and to minimize induced torque.

5. RESULTS

The results are presented in tables 1 through 3 and in figures 1 through 7. The error bars on the figures show the data spread from repeated tests. Scatter is typically

higher for compression and shear tests than for tensile tests, since the former tests are more sensitive to misalignment. Two orientations, longitudinal and transverse, are designated in the data. These orientations are relative to the axes of the bulk supply,

not the true cell orientation.

As shown in the figures, Young's modulus, proportional limit, yield strength, ultimate strength, and shear strength increase as temperature is lowered from 295 to 76 K. The longitudinal specimens are, in general, stronger and stiffer than those tested in the

transverse direction (figs. 1, 2, and 4). An exception to this is seen in the compressive

yield strength (fig. 5) where the longitudinal and transverse curves cross between 76 and

111 K. Tensile strain decreases with temperature (fig. 3); the material is completely

brittle for temperatures below 111 K. Figure 6 indicates a significant deformation

capability in compression at low temperatures. The apparent plastic strain is probably

caused by a collapse of the cell structure. The polyurethane resin is actually brittle at cryogenic temperatures and cannot exhibit true plastic deformation.

6 . DISCUSSION

In many respects the polyurethane foam behavior is typical of polymers in general.

Expected behavior for a temperature reduction from 295 K to 76 K includes large mechanical

property changes such as complete loss of ductility and doubling of Young's modulus in

tension. Longitudinal shear strength and longitudinal Young's modulus in compression

exhibit a maximum between 7 6 and 2 95 K. The cause of this behavior is not known at the

present time.

2
Sectioning after testing revealed that cell orientation varied somewhat from the

orientation of the mold axes. It is generally thought that foam specimens from the center

of a large billet have uniform cells oriented with respect to the vertical. But this is

not true in the case of foams formed on a continuous or bun-line production facility. The

cell orientation reflects the movement of the foaming resin. Cells in the x-z plane of

the material reported here have nearly a 45° inclination to the x-axis (fig. 2), whereas

in the y-z plane there is no inclination from the z-axis. Future tests will include

measurement of physical properties perpendicular and parallel to the cell axes, rather

than the mold axes as reported in this study.

Specimens tested in liquid nitrogen appeared to have slightly lower shear strength

values than those tested at the same temperature in cold helium gas. This may be an indication of environmental sensitivity. Nitrogen at low temperatures is deleterious to a number of other polymeric materials. Thermoplastics have exhibited crazing or reduced

fracture strengths, or both when tensile tested in cold liquid or gaseous nitrogen, as compared with vacuum or helium environments [8]. Environmental effects must ultimately be

taken into account in design applications.

7. BIBLIOGRAPHY

[1] R. P. Reed, J. M. Arvidson, and R. L. Durcholz, Tensile properties of polyurethane

and polystyrene foams from 76 to 300 K, in: "Advances in Cryogenic Engineering,"

Vol. 18, K. D. Timmerhaus, ed. , Plenum Press, New York (1973), pp. 184-193.

[2] J. M. Roberts, R. B. Herring, and D. E. Hartman, The use of capacitance gauge sensors

to make precision mechanical property measurements, in: "Materials Technology,"

American Society for Mechanical Engineers, New York (1968), pp. 87-96.

[3] "High-Temperature Capacitive Strain Measurement System," NASA Tech. Brief B75-10069,

NASA (1975).

[4] P. C. F. Woldendale, Capacitive displacement transducers with high accuracy and

resolution, J. Sci. In strum. ( J . Phys . E

.

) 1:817 (1968).

[5] G. R. White, Measurement of thermal expansion at low temperatures, Cryogenics , 2:151

(1961).

[6] "Materials Bank Compendium of Fire Property Data," Products Research Committee, J. W. Lyons, chairman. National Bureau of Standards, Washington, D.C. (1980).

[7] W. G. Jurevic, "Structural Plastics Applications Handbook, Supplement 1 Test

Methods," Technical Report AFML-TR-67-332 (1969).

[81 A. Hiltner and E. Baer, Mechanical properties of polymers at cryogenic temperatures,

Polymer 15:805 (1974)

.

3
3 Table 1. Summary of tensile test results for a 32 kg/m

polyurethane foam (average values)

Material Specimen

Property Orientation* 295 K 111 K 76 K 45 K

Young ' s L 13.61 (1,980) 24.41 (3,540) 26 .37 (3,820) 26.95 (3,910)

Modulus

MPa (psi)

T 7.26 (1,050) 12.81 (1,860) 14 .27 (2,070)

Proportional L 0.166 (23.91) - - - Limit

MPa (psi) T 0.077 (11.17)

Yield Strength L 0.265 (38.46) - -

(0 . 2% Offset)

MPa (psi)

T 0.130 (18.93)

Ultimate Ten- L 0.458 (66.43) 0.491 (71.23) 0 . 490 (71.08)

sile Strength,

Mpa (psi)

T 0.365 (52.85) 0.368 (53.43) 0 . 386 (55.93)

Elongation % Elastic L 3.1 2.0 2.0 _ Plastic 2.5 0 0 - Total 5.6 2.0 2.0 -

Elastic T 5.0 2.9 2.7 - Plastic 10.6 0 0 - Total 15.6 2.9 2.7 -

*L : longitudinal; T: transverse

4
3 Table 2. Summary of compression test results for a 32 kg/m

polyurethane foam (average values).

Material Specimen

Property Orientation 295 K 111 K 76 K

Yo un g ' s L 8.48 (1,230) 14.88 (2,160) 13.26 (1,920)

Modulus

MPa (psi)

T 6.03 ( 880) 6.69 ( 970) 6.40 ( 930)

Proportional L 0.067 ( 9.70) 0.079 (11.47) 0.152 (22.03)

Limit

,

MPa (psi)

T 0.073 (10.63) 0.150 (21.75) 0.118 (17.14)

Yield Strength L 0.114 (16.57) 0.146 (21.15) 0.204 (29.60)

(0.2% Offset),

MPa (psi) T 0.109 (15.80) 0.179 (26.04) 0.172 (24.98)

Maximum Compres- L 0.287 (41.62) 0.311 (45.12) 0.338 (49.02)

sive Strength,

Mpa (psi)

T 0.218 (31.60) 0.247 (35.91) 0.227 (32.93)

Elongation % Elastic L 3.3 1.9 2.5

Plastic 4.4 12.4 9.4

Total 7.7 14.3 11.9

Elastic T 3.0 3.4 3.3

Plastic 5.4 10.5 15.2

Total 8.4 13.9 18.5

Table 3. Summary of shear test results for a polyurethane foam (average values)

.

32 kg/m3

Shear Strength

Test Temperature, K Specimen Orientation

MPa psi

295 L 0.217 31.47

T 0.217 31.43

111 L 0.266 38.62

T 0.219 31.74

76 L 0.242 35.14

T 0.225 32.65

76* L 0.241 34.95

T 0.216 31.39

*Test conducted in

5
6

Figure

1.

Bulk

supply

of

the

bun-line

produced,

32

kg/m

3

polyurethane

foam.

Material

used

in

the

mechanical

property

tests

is

indicated

by

the

shaded

area.
Figure

2.

Cell

orientation

relative

to

the

major

coordinate

axes

of

the

hulk

supply

ot

32

kg/m

3

polyurethane

foam.

The

straight

line

segments

do

not

represent

the

actual

cell

shape

but

are

rather

artifacts

of

the

statistical

evaluation

of

microphotographs

of

the

test

material.
Figure 3. Tensile Young’s modulus versus temperature.

TENSILE

YOUNG'S

MODULUS

(E

T

),

psi
Proportional

Limit

Yield

Strength

Ultimate

Ten

tie

Strength

Op

L

.

MPa

°0.2%’

MP®

^max*

MPa

Figure 4

0.4 -

0.2b

0

MATERIAL:

Polyurethane Foam

(32 kfl/m^)

8PECIMEN ORIENTATION

Longitudinal

o Transverse

80

60

40

m

a

StX

20

«0 b

e

5

-JO

60

40

20

^0

m

a

#

Osl b°

m

a

b

0-

Tensile test results: proportional limit, yield strength, and ultimate

strength versus temperature.

9
so

*5

w

15

“10

5

©

5

4

3

2

1

©

25

20

15

10

6

0

Figure 5. Tensile strain versus temperature.

10
COMPRESSIVE

YOUNG'S

MODULUS.

(E

c

).

MPa

Figure 6. Compressive Young’s modulus versus temperature.

11

COMPRESSIVE

YOUNG'S

MODULUS

(Ec).

pal
Figure

©

C

®

£ ae

co>

« a, oe 4

£ 2

a • O E0.2

g

5

K

GJ

a* T

MATERIAL:

Polyurethan® Foam (32 kg/m^)

”9

5J

80

60

40

-20

-0

60

40

20

0

40

20

0

«

a

c

£

7. Compression test results: proportional limit, yield strength, and maximum

compressive strength versus temperature.

PL*

M

0.2%'

max
30

25

20

15

10

5

0

5

4

3

2

1

0

25

20

15

10

5

0

TOTAL

ELASTIC

MATERIAL: Polyurethane Foam (32 kg/m^)

SPECIMEN ORIENTATION

PLASTIC 1 1 1 1 I 50 100 150 200 250 300

TEMPERATURE. K

Figure 8. Compressive strain versus temperature.

13
0.30

'H10N3U1S BV3HS

w O « © ** CM o

^ CO CO CO CO CO

i t i 1 1 1 r

o d

o

©

CO

©

o

CM

O

o

o

UJ

QC

Z>

<

QC

UJ

a

2

UJ

'H19N3H1S HV3HS

14

Figure

9.

Shear

strength

versus

temperature.
M6S-114A (REV. 2-80

U.S. DEPT. OF COMM. 1 . BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR 2 REPORT NO.

. Performing Organ. Report No. NBSIR 81-1654

3. Publication Date November 1981

4. TITLE AND SUBTITLE

Low Temperature Mechanical Properties of a Polyurethane Foam

5. AUTHOR(S)

J. M. Arvidson and L. L. Sparks

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No,

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Gas Research Institute

8600 West Bryn Mawr Avenue

Chicago, Illinois 60631

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. I f document includes a significant

bibliography or literature survey, mention it here)

3 Polyurethane foam, having a density of 32 kg/m , was tested at 295, 111, 76,

and 45 K in helium gas. The material properties reported are Young's modulus, proportional limit, yield strength (at 0,2% offset), tensile, shear, and compressive

strengths, and elongation (elastic and plastic). To perform these tests, a unique

apparatus was developed. This apparatus permits tension, compression, and shear

testing of materials at any temperature ranging from 295 to 4 K. Strain is measured

with a concentric, overlapping-cylinder capacitance extensometer that is highly

sensitive and linear in output.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Compressive strength; elongation; foam; insulation; low temperature; mechanical

properties; proportional limit; shear strength; tensile strength; yield strength;

Young's modulus.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

Uni imited

1 | For Official Distribution. Do Not Release to NTIS 17

1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. 15. Price

fX] Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $5.00

USCOMM-DC 6043-P80

U.S. GOVERNMENT PRINTING OFFICE: 1981-576-001/1216 Region No. 8