Tensile, fracture and thermal properties of polyarylates at room and cryogenic temperatures

Tensile, fracture and thermal properties of polyarylates at room and cryogenic temperatures K. Humer, E.K. Tschegg* and H . W . Weber Atominstitut der Osterreichischen Universit~iten, A - 1 0 2 0 Vienna, Austria *lnstitut f~ir Angewandte und Technische Physik, TU Vienna, A - 1 0 4 0 Vienna, Austria

Received 22 September 1992 In the present paper, measurements of the modulus of elasticity, the ultimate tensile strength (UTS) and the ultimate tensile strain of polyarylates (ISARYL 1 5X and ISARYL 25X) carried out at room temperature, 77 and 4.2 K are reported on. In addition, the fracture behaviour in mode I on cylindrical circumferentially sharp notched samples as well as the thermal expansion have been investigated. The results show an increase in both the modulus of elasticity and the UTS by about a factor of t w o on decreasing the temperature to 77 K. Further cooling to 4.2 K leads to a decrease of = 10% in these properties. On the other hand, the ultimate tensile strain of ISARYL 1 5X decreases by = 65% at 4.2 K, whereas a non-uniform temperature dependence is found for ISARYL 25X. The fracture toughness of both materials increases by about a factor of three upon decreasing the temperature to 77 K, but decreases by a factor of t w o at lower temperatures. These results are discussed in conjunction with additional observations of the fracture behaviour made with optical and scanning electron microscopes.

Keywords: mechanical properties; thermal properties; polyarylates

Due to their excellent mechanical, electrical and thermal properties over a wide range of temperatures, as well as because of their low density, the application of plastics is of special interest in several areas of technology. Emerging applications in mechanical and chemical engineering as well as in the transportation industry require high durability but low wear and/or good chemical resistance of components at room and elevated temperatures. Good electrical insulation properties, non-magnetic behaviour and high resistance against nuclear radiation have led to new applications of plastics in the aviation and space industry, as well as in low temperature technology, as insulating and support materials for superconducting magnets, e.g. for particle accelerators or nuclear fusion devices and power plants. Some test results from previous work pertaining to the low temperature properties of plastics should be mentioned at this point. Tensile tests on various epoxies were carried out at room temperature and at 77 K by Michael et a l . I . At room temperature these materials are relatively brittle and show plastic behaviour just before fracture. At 77 K no plasticity was found, and the modulus of elasticity increased by a factor of three while the ultimate tensile strain decreased by = 5 0 % . Measurements of the stress-strain behaviour, as well as of the fracture behaviour in mode I, carried out on a polyimide at room temperature, 77 and 4.2 K, were 0011-2275/93/070686-06 © 1993 Butterworth-Heinemann Ltd 686

Cryogenics 1993 Vol 33, No 7

recently reported 2. The modulus of elasticity and the ultimate tensile strength (UTS) increased by = 4 0 and = 6 0 % , respectively, upon cooling to 77 K, but no further change was observed at 4.2 K. The ultimate tensile strain decreased continuously (by =25%) down to 4.2 K, while the fracture toughness K1c (1.6 MPa m 1/2 at room temperature) increased in a similar way (by = 10 %). The stress-strain behaviour was strictly linear at 77 and 4.2 K, whereas additional plastic deformation was observed shortly before fracture at room temperature. In addition, no sample size dependence was detected for either the UTS or the Klc values 3. Compact tension specimens made of epoxy and polyethylene have been investigated by Hartwig et al.4 at 77 K. The K~c values reported for these two materials were 1.9 and 9.0 MPa m ~/2, respectively. Polycarbonate, polyethylene and several epoxies were investigated with respect to the influence of deformation rate on the UTS at low temperature 4'5. Further work 6 refers to gamma irradiation induced changes of both the flexural and the compressive strength of various polyimides at room temperature and at 77 K. In this work, the mechanical strength was found to increase by a factor of two at 77 K and no significant change occurred following gamma irradiation. Nishijima et al. 7 tested several epoxies prior to and following neutron and gamma irradiation in the three-point bending test.

Properties of polyarylates : K. Humer et al.

Investigations concerning the influence of electron irradiation on the tensile properties of various thermoplastic polyimides at room and elevated temperatures should also be mentioned 89. Finally, the thermal expansion of polymers was measured from 4.2 K up to room temperature by Schwarz'°. Whereas a small coefficient of thermal expansion was found for polyimides, polyethersulphones and polymethylmethacrylates, larger values have been obtained for polyethylene and polytetrafluoroethylene. In order to extend our knowledge of the low temperature properties of various plastics which have become available over the past few years, the present contribution presents results on the mechanical, fracture and thermal properties of recently developed sintered polyarylates (ISARYL 15X and ISARYL 25X) at room temperature and at cryogenic temperatures.

Experimental details Materials

ISARYL 15 and ISARYL 25 are developed and produced by ISONOVA GmbH, Wiener Neudorf, Austria, and belong to the family of high performance plastics. They are polyarylates of high molecular weight. The polymer powders are sintered and commercially available in the form of semi-complete or finished components and products. The chemical structure of both materials is shown in Figure 1. Because of their chemical composition and structure (ISARYL consists of carbon, oxygen and hydrogen only), they show low moisture uptake, high dimensional stability, inherent flame resistance and favourable burning properties (low heat release value and low toxicity of the combustion gases), as well as excellent electrical and good mechanical properties. For example, the ultimate tensile strength of ISARYL 15X at 150°C decreases to only =50% of its room temperature value. On the other hand, the ultimate tensile strength of ISARYL 25X shows no decrease up to = 150°C, but decreases to only

=70% of its room temperature value at 250°C. ISARYL 15X and 25X have been employed to fabricate components such as insulators, plugs, sealings and valves. Test procedures All mechanical tests were made with a 200 kN top

loading tensile testing machine ~ operating in the temperature range 4 . 2 - 3 0 0 K. During the measurements, the cross-head speed was set at 0.5 mm rain -~ and both the force and the sample elongation were recorded on an XY recorder. The elongation was measured through the cross-head displacement with LVDT. In view of the high stiffness of the testing machine, as well as the small loads required in these experiments, this displacement is identical to the elongation assessed using LVDT directly on the sample, as has been confirmed experimentally before. After the experiments, fractographic investigations were made using optical and scanning electron microscopes. The tensile tests were carried out on standard tensile samples according to DIN 53455 (thickness 4 mm, width 10 mm) at room temperature, 77 and 4.2 K. A schematic view of the test geometry and the sample dimensions is shown in Figure 2. The modulus of elasticity, the UTS and the ultimate tensile strain were calculated from the initial slope of the load-elongation curve, and the load and the sample elongation at breaking point, respectively. In order to determine average data, three measurements were made on each material at room temperature, and five measurements at 77 and 4.2 K, respectively. Concerning the fracture toughness tests, previous work on several metallic materials has shown, firstly, that the fracture toughness, K~c, can be determined from measurements on precracked tensile samples with a circumferential notch and, secondly, that excellent agreement is achieved for the determination of the stress intensity factor, K,, using the standardized tests with compact tension (CT) samples and the new test. As a consequence, precracked samples with a circumferential notch have been used previously 2'3 to evaluate the fracture toughness, K~c, of a polyimide and good agreement has been found with the K~. wflues from recent

I S A R Y L ® 15 X

c\,

f ISARYL ® 25 X

1 150

I -

o

o

o

"

'

o

o-co

~

,o

o

,

0

~

~. 25 _J

l

i

j__

,_

60 °

Figure 1

Chemical structure of ISARYL 15X and ISARYL 25X

Figure2 Geometry and dimensions of tensile (top) and fracture toughness (bottom) test samples

Cryogenics 1993 Vol 33, No 7

687

Properties of polyarylates: K. Humer et al. data 12 for another polyimide. Furthermore, no significant differences in the fracture toughness have been observed between precracked (fatigue crack) and sharp notched (razor blade) samples. Hence, in the present study circumferentially sharp notched tensile samples were used for the fracture toughness tests. The test geometry as well as the sample dimensions are also shown in Figure 2. The fracture toughness, Klc, was calculated from the maximum tensile load on the basis of standard equations t3. The data refer to average results calculated from three measurements at all test temperatures and on each material. The coefficient of thermal expansion was measured in the temperature range 4 . 2 - 3 0 0 K with a modified version of the measuring device described in reference 14. The samples were cut from cylindrical rods with a diameter of 3.8 mm to a length of 2.3 mm. Three measurements were made on each material in the whole temperature range, and the deviations were within the measuring accuracy (precision) of the device.

25X). A graphical representation of the UTS for both materials as a function of temperature is shown in Figure 3. Note that the UTS of ISARYL 15X is about twice as high as the' UTS of ISARYL 25X. The ultimate tensile strain of ISARYL 15X (13.3% at room temperature) decreases continuously with temperature to 5.9% at 77 K and to 4.9% at 4.2 K, whereas for ISARYL 25X the ultimate tensile strain increases by -- 15% (from 2.6 to 3%) at 77 K and then decreases by --20% (to 2.3%) at 4.2 K. In comparison to ISARYL 15X, the ultimate tensile strain of ISARYL 25X amounts to only --20% at room temperature, but is --50% at 77 and at 4.2 K, respectively. Typical qualitative stress-strain diagrams are shown in Figure 4 for all three test temperatures. Whereas the stress-strain behaviour is strictly linear for ISARYL 25X at all temperatures (Figure 4, right-hand side), additional plastic deformation can be observed for ISARYL 15X in the second part of the stress-strain curves at room temperature (Figure 4, left-hand side).

Results and discussion

160

Test temperatures as well as the results for the modulus of elasticity, the UTS and the ultimate tensile strain of ISARYL 15X and ISARYL 25X are summarized in Tables 1 and 2. For both materials, the results show an increase in the modulus of elasticity by about a factor of two (from 1.2 to 2.7 GPa and from 1.5 to 2.9 GPa, respectively) on decreasing the temperature to 77 K. Further cooling to 4.2 K leads to no further change in the modulus of elasticity for ISARYL 15X, but to a decrease of -- 10% (to 2.6 GPa) for ISARYL 25X. The UTS is 77 MPa for ISARYL 15X and 37 MPa for ISARYL 25X at room temperature, and increases in a similar way to 138 MPa for ISARYL 15X and to 73 MPa for ISARYL 25X at 77 K. Cooling to 4.2 K leads to a slight decrease (-- 10%) of the UTS (to 124 MPa for ISARYL 15X and to 61 MPa for ISARYL

A

140

a. ~-" J~

120

¢"

100

~

80

o'l e~

60

~

40

"5

20

E

0

293

Table 1

Test t e m p e r a t u r e s and average results for modulus of elasticity, UTS and ultimate tensile strain for ISARYL 1 5X

Temperature (K)

Modulus of elasticity (GPa)

Ultimate tensile strength (MPa)

Ultimate tensile strain (%)

293 77 a 4.2 b

1.2 2.7 2.7

77 4- 0.5 138 4- 10.0 124 4- 5.7

13.3 5.9 4.9

a Data refer to results calculated from t w o measurements Data refer to results calculated from three measurements

Figure 3 UTS of ISARYL 15X and ISARYL 2 5 X obtained at room t e m p e r a t u r e (left), 77 K (middle) and 4.2 K (right)

ISARYI 15)( .... u~

Test temperatures and average results for modulus of elasticity, UTS and ultimate tensile sttain for ISARYL 2 5 X (n

Temperature (K)

Ultimate tensile strength (MPa)

Ultimate tensile strain (%)

/ ./-" /../..

1.5 2.9 2.6

37 4- 0 . 5 73 4- 13.2 61 4- 8.5

688 Cryogenics 1993 Vol 33, No 7

2.6 3.0 2.3

ISARYL 25)( 293K 77K 4.2K

/

/° /

/ / . / ~

strain 293 77 4.2

4.2

test temperature (K)

Table 2

Modulus of elasticity (GPa)

77

P

Figure 4 S t r e s s - s t r a i n diagrams of ISARYL 15X (left) and I S A R Y L 2 5 X (right) at 4.2 K ( . . . . ), 77 K ( - - - ) a n d room temperature ( )

Properties of polyarylates: K. Humer et al. Table 3 Test temperatures and average results for fracture toughness of ISARYL 15X and 25X Fracture toughness, Klc (MPa m 1/2) Temperature (K)

ISARYL 15X

ISARYL 25X

293 77 4.2

2.13 ± 0.09 6.15 ± 0.23 3.46 ± 0.13

1.28 ± 0.13 4.53 + 0.21 2.47 ± 0 . 4 8

Test temperatures, as well as the test results for the fracture toughness, Ktc, are summarized in Table 3. The fracture toughness increases by about a factor of three (from 2.13 to 6.15 MPa m ~/2 and from 1.28 to 4.53 MPa m ~/2, respectively) on decreasing the temperature to 77 K, but decreases by about a factor of two (to 3.46 M P a m 1/2 and to 2.47 M P a m 1/2, respectively) at 4.2 K. A graphical representation of the fracture toughness, K~c, of both materials as a function of temperature is shown in Figure 5. We note that the fracture toughness, Ktc, of ISARYL 15X is = 30% higher than that of ISARYL 25X. A detailed comparison of our test data on ISARYL 15X and ISARYL 25X with other results on polyarylates is difficult, because relevant data are scarce. Hence, the present data should be compared with results on another similar class of materials, the polyimides. Concerning fracture toughness, both ISARYL 15X ( = 2 MPa m 1/2 at room temperature, = 6 MPa m ~/2 at 77 K) and ISARYL 25X ( = 1.3 MPa m ~2 at room temperature, = 4 . 5 MPa m t/2 at 77 K) compare favourably with a polyimide ( = 1 . 6 MPa m ~n at room temperature and = 1.8 MPa m ~/2 at 77 K) investigated previously 2'3. On the other hand, the modulus of elasticity as well as the UTS of both materials are up to = 50% lower than the values reported recently for the same polyimide (especially for the UTS of ISARYL 25X). In addition,

the stress-strain behaviour is similar to that of this polyimide. Additional fractographic investigations of the fracture surfaces made with optical and scanning electron microscopes have led to the following results. In Figure 6 the fracture surfaces of standard tensile samples (DIN 53455, width 10mm, thickness 4ram) are shown following fracture at room temperature (Figures 6a and d), 77 K (Figures 6b and e) and 4.2 K (Figures 6c and ~ . The crack initiation area with its very smooth surface and absence of any obvious structure is situated approximately in the middle of the fracture surface. Further away, where the crack propagates with increasing velocity, the fracture surfaces show increasingly more structure in the form of river patterns. In = 50% of the samples the crack initiation point is situated approximately in the middle of the fracture surface (Figure 6c), and is otherwise at the edge (Figure 6a and b). However, if local inhomogeneities exist in the materials, they always determine the crack initiation points (Figures 6d, e and f). No significant influence of the test temperature on the fracture processes and the fracture surfaces could be observed. Comparison of the fracture surfaces shows that the crack initiation area with its smooth surface is always larger for ISARYL 25X, especially at room temperature, where river patterns are often observed

(Figure 6d). The fracture surfaces of the fracture toughness test samples show similar results as discussed above for the tensile test samples. The crack initiation point is always situated at the notch root. The crack initiation area shows a relatively smooth surface and only a slight degree of formation of river patterns, which begin at the crack initiation point. Further along the crack propagation route, not only river patterns but also ductile

lmm

7 IRARYI

1RY

6

ISARYL 15X

D.. L) 'v"T-u'J

4

(9 ¢.c:

3

ISARYL 25X

,9 ° 2 .--1 (~

1

293

77

4.2

test temperature (K) Figure 5 Fracture toughness, Klc, of ISARYL 15X and ISARYL 25X at room temperature (left), 77 K (middle) and 4.2 K (right)

293K

77K

4.2 K

Figure6 Fracture surfaces of ISARYL 15X Cupper sections) and ISARYL 25X (lower sections) following tensile testing at room temperature (left), 77 K (middle) and 4.2 K (right) (see text for details)

Cryogenics 1993 Vol 33, No 7

689

Properties of polyarylates: K. Humer et al. deformations in the form of dimple patterns could be observed. This may be explained as follows. The specific heat of plastics is very small at low temperature compared to its value at room temperature. Therefore, the energy released during crack propagation heats up the material at the crack tip. Hence, the formation of a small plastic zone around the crack tip will lead to ductile deformations in the form of river and dimple patterns. The fracture surfaces of ISARYL 25X following fracture at room temperature, 77 and 4.2 K are shown in Figure 7. As can be seen in this figure, the formation of a plastic zone, leading to clearly marked dimple patterns, becomes increasingly important at lower test temperatures (77 and 4.2 K), which results in higher values of the fracture toughness at low temperatures (Figure 5). In general, no differences have been found between the fracture surfaces of ISARYL 15X and ISARYL 25X. The formation of river and dimple patterns on fracture surfaces, following fracture at room and cryogenic temperatures, has been observed previously in a similar way on other plastics 2'3'~5'~6. Finally, the results pertaining to the coefficient of thermal expansion, c~, are shown in Figure 8, together with data on aluminium. The values of c~ are about the same for both materials and are similar to those for aluminium in the temperature range from 4.2 to -- 120 K. With increasing temperature, c~ values of both ISARYL 15X and ISARYL 25X exceed the data for aluminium. At room temperature, the coefficient of thermal expansion is --42 x 10 -6 K -~ for ISARYL 15X, --35 x 10 -6 K -~ for ISARYL 25X and --22 x 10 6 K-~ for aluminium. Similar values of c~ ( - - 4 0 x 10 -6 K -t) at room temperature have been measured for polyimides, polymethylmethacrylates and polysulphones, whereas considerably larger values have been obtained for polyethylene and polytetrafluoroethylene ( ~ 110 x 10 - 6 K - 1 ) by Schwarz 1°.

Summary As pointed out at the start of this paper, ISARYL 15X and ISARYL 25X have been shown to possess good mechanical properties at room and elevated

293K

:

77K

temperatures (up to -- 500 K). In the present contribution, measurements of the modulus of elasticity, the UTS and the ultimate tensile strain, as well as the fracture toughness, KIt, of these polyarylates at room temperature, 77 and 4.2 K, have been reported on. The main results of these tests, as well as of fractographic investigations, may be summarized as follows: 1 For both materials the modulus of elasticity shows an increase by about a factor of two on decreasing the temperature to 77 K. Further cooling to 4.2 K does not lead to a further change in this value for ISARYL 15X, but leads to a decrease in the modulus of elasticity of -- 10% for ISARYL 25X. 2 For both materials the UTS increases in a similar way upon decreasing the temperature to 77 K. Cooling to 4.2 K leads to a slight decrease (-- 10%) in this value. 3 The ultimate tensile strain o f I S A R Y L 15X decreases continuously with decreasing temperature, to give a decrease of -- 65% at 4.2 K. For ISARYL 25X, this value increases by -- 15% at 77 K, but decreases by --20% at 4.2 K. 4 For ISARYL 25X the stress-strain behaviour is strictly linear at all temperatures, whereas additional plastic deformation was observed in the second part of the stress-strain curves for ISARYL 15X. 5 The fracture toughness of both materials increases by about a factor of three on decreasing the temperature to 77 K. Further cooling to 4.2 K leads to a decrease of about a factor of two. 6 The fracture surfaces of both materials show riverlike structures following tensile tests and additional formations of dimple patterns following fracture toughness tests at all temperatures. 7 The coefficient of thermal expansion is about the same for both materials and is similar to that of aluminium in the temperature range 4 . 2 - 1 2 0 K. At higher temperatures, the coefficient of thermal expansion is -- 100% higher for ISARYL 15X and -- 50% higher for ISARYL 25X compared to that of aluminium.

4:2K

Figure 7 Fracture surfaces of ISARYL25X following fracture toughness testing at room temperature (left), 77 K (middle) and 4.2 K (right)

690

Cryogenics 1993 Vol 33, No 7

Properties of polyarylates: K. Humer et al. 50.00

40.00

o

ISARYL25X

o

lSARYL15X

- -

°oOo° o o o

AI

oO ooCC°

[] o °

30.00 i

oo

oo

o o o

ooo o°° o

o

oo

__

o °

°°

20.00

;

o °°°

c

v

?

°

Do

o

oO o

o

10.00

0.00

-10.00

0

'

50

100

--

'

150 T[KI

'

200

'

250

300

F i g u r e 8 Coefficient of thermal expansion of ISARYL 1 5X (E]), ISARYL 25X ( © ) and aluminium ( ) as a function of temperature

In conclusion, it should be pointed out that the new plastic materials ISARYL 15X and ISARYL 25X show good mechanical properties (especially ISARYL 15X) in the tensile tests at all temperatures down to 4.2 K. The fracture toughness of both materials is similar to that of other materials of this class and increases by about a factor of three at cryogenic temperatures.

Acknowledgements We are indebted to ISONOVA GmbH, Wiener Neudorf, Austria, for providing us with the test samples of ISARYL 15X and 25X. This work is supported in part by the Fends zur F6rderung der wirtschafllichen Forschung, Vienna, by Innovationsund Technologiefonds, and by Bundesministerium ffir Wissenschafi und Forschung, Vienna, under contract No.77.780/2-25/91. Thanks are due to Dr H. Miiller (Institut fiir Experimentalphysik, TU Vienna) for the measurements of the thermal properties, as well as to Ing. W. Hametner for technical support.

References 1

2 3 4 5 6 7 8 9 l0 11 12 13 14 15 16

Michael,

P.C.,

Aized,

D.,

Robinowicz,

E.

and

lwasa,

Y.

Cryogenics (1990) 30 775 Tschegg, E.K., Humer, K. and Weber, H.W. Cryogenics (1991) 31 878 Humer, K., Weber, H.W. and Tschegg, E.K. Adv Cryog Eng (1992) 38A 331 Hartwig, G., Kneifel, B. and P6hlmann, K. Adv Cryog Eng (1986) 32 169 Hartwig, G. and P6hlmann, K. Adv Cry'og Eng (1984) 30 83 Coltman, R.R. Jr and Klabunde, C.E. J Nucl Mat (1981) 103/104 717 Nishijima, S., Ueta, S. and Okada, T. Cryogenics (1981) 21 312 Hirade, T., Hama, Y., Sasuga, T. and Seguchi, T. Polo'met (1991) 32 2499 Sasuga, T. Polymer (1991) 32 1539 Schwarz, G. Cryogenics (1988) 28 248 Tschegg, E.K., Kubasta, E. and Steiner, W. Crvogeni~ (1979) 19 269 Hinkley, J.A. and Mings, S.L. Polymer (1990) 31 75 Murakami, Y. Stress Intensity Factors Handbook Vol 2, Soc Mat Sci Japan (1987) 643 Gr6ssinger, R. and Miiller, H. Rev Sci lnstrum (1981) 52 1528 Zhang, M.J. and Zhi, F.X. Polymer (1988) 29 2152 De Vries, K.L. and Hornberger, L.E. Polymer Degrad Stability (1989) 24 213

Cryogenics 1993 Vol 33, No 7

691