Screening programme to select a resin for Gravity Probe-B composites

Screening programme to select a resin for Gravity Probe-B composites* E.T. W i l l Lockheed Palo Alto Research Laboratory, 3251 Hanover Street, Palo Alto, CA 94304, USA The Gravity Probe-B (GP-B) programme undertook a screening programme to select a possible replacement resin for the E-787 resin currently used in composite neck tubes and support struts. The goal was to find a resin with good cryogenic and structural properties, low helium permeation and an easily repeatable fabrication process. Cycom 92, SCI REZ 081 and RS-3 were selected for comparison with E-787. Identical composite tubes made from each resin and -),-alumina fibre (85% AI203, 1 5% SiO 2) were evaluated for cryogenic and structural performance and for processability. Cryogenic performance was evaluated by measuring low-temperature permeation and leaks to determine cryogenic strain behaviour. Structural performance was evaluated by comparing the resindominated shear strength of the composites. Processability was evaluated from fabrication comments and GP-B's own experience. SCI REZ 081 was selected as the best overall resin with superior strength and cryogenic performance and consistent processability.

Keywords: composites; permeation; modulus

Gravity Probe-B (GP-B) is intended to test part of Einstein's theory of relatively through orbiting gyroscopes housed in a superfluid helium Dewar. A composite neck tube and support struts will isolate the Dewar thermally from the satellite structure. Problems of inconsistent processing and microcracking have occurred in the present composite, which uses E-787 resin. The GP-B programme undertook a screening programme to select a resin as a possible replacement for E-787. The goal was to find a resin with good cryogenic properties, low helium permeation and an easily repeatable process. The neck tubes must provide a permeation barrier, resist buckling under 1 atm t external pressure, and act as a thermal barrier over a temperature differential of 300 K. Sixteen resin candidates were evaluated to select three candidates for comparison with E-787. The tests of the three candidates and E-787 focussed on three key selection criteria critical to GP-B's cryogenic operating environment: (1) cryogenic performance, as indicated by microcracking and helium permeation, (2) strength performance, as measured by the shear modulus of the material, and (3) processability of the resin using 3'alumina fibre.

Selection of resin candidates A literature review was used to select 16 promising resins. The emphasis was on readily available resins *Paper presented at the 1991 Space Cryogenics Workshop, 1 8 - 2 0 June 1991, Cleveland, OH, USA t l atm = 105 Pa

with a history of cryogenic use. Amine-cured epoxies were preferred because of their chemical compatibility with the currently used film adhesive, American Cyanamide's FM 300; however, one anhydride, one polycyanate and three thermoplastic resins were also included. The criteria used to select three candidates for testing focussed on the specific needs of Gravity Probe-B, with special attention to the operating temperature range of 1.8-360 K. First, resins with a cryogenic history or those developed specifically for cryogenic use were sought. Next, processability was considered, focussing on process techniques and process-related variables of cure temperature, glass transition temperature (Tg) and percentage of volatiles. Finally, availability was considered.

Cryogenic history The evaluation of cryogenic history was based solely on previous experience and published test data. Attention was focussed on information about a resin's resistance to microcracking as an indication of its low-temperature strain characteristics. Existing information on resins is important, since the GP-B programme does not have the resources to characterize a potentially good resin with unknown properties. Very few test results and little operating experience at the low terngerntures at which GP-B operates were found.

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Selection programme for gravity probe-B composites: E. 7-. Will

Processability

RS-3 (manufactured by YLA)

For cryogenic use, the cure temperature should be below 450 K (350°F), in order to minimize the inherent strain within the composite at low operating temperatures. At the same time, the Tg must be above 372 K (210°F) to maintain the structural integrity of the cured composite when the assembled system undergoes a 361 K (190°F) secondary cure. Experience with E-787 indicates that the percentage of volatiles in the resin must be kept to a minimum to prevent delaminations caused by trapped volatiles. Other processability considerations were previous experience with ,y-aluminia fibre and the resin's adaptability to either filament winding or tape lay-up process techniques.

A late discovery for the candidate list was the polycyanate resin RS-3. This resin cures at 450 K (350°F) with a Tg of 483 K (410°F) and no measurable volatiles. It has excellent resistance to microcracking and is currently a candidate for liquid hydrogen tanks on the National Aerospace Plane.

Evaluation criteria Cryogenic performance

Characteristics of the selected test candidates

Permeation and leak testing are good indicators of both low-temperature permeation and low-temperature strain characteristics. Permeation is measured at room temperature before and after the tube is thermally cycled in liquid nitrogen, and then again at temperatures close to liquid nitrogen temperature (100 K). Thermal cycling strains the bond between the resin and fibre because of the difference in their thermal expansion rates. This strain can break the bond and cause a series of microcracks, if microcracking is extensive, it will provide a leak path through the composite. Cryogenic testing magnifies the leak path by opening up the microcracks as the two materials contract.

The three test candidates selected and the baseline E-787 are described below.

Structural performance

Availability Availability of the resin in the future is also important, since GP-B requires composite parts up to 1997. Lack of availability and cryogenic data eliminated all new or experimental resins, even those specifically developed for cryogenic use.

E-787 (manufactured by BP Chemicals) E-787 is an epoxy resin system made from Shell 828 and 1031 resins with an anhydride curing agent of nadic methyl anhydride-benzyl dimethyl amine (NMA-BDMA). It has a relatively high volatiles content of 4%. The cure temperature was lowered to 425 K (305°F) from the original 450 K (350°F) to reduce strain, while the cure time was increased to maintain a Tg above 372 K (210°F). Extensive cryogenic material properties data are available for E-787, based on its use in cryogenic dewars at Lockheed Palo Alto Research Laboratories.

Cycom 92 (manufactured by American Cyanamide) Cycom 92 is a diglycidyl ether abisphenol-A (DGEBA) type resin with an amine curing agent. It cures at 450 K (350°F) with a Tg of 383 K (230°F) and a volatiles content of 0.41%. Its cryogenic use has been for space satellites in European space programmes.

SCI REZ 081 (manufactured by Structural Composites Industries) SCI REZ 081 is an anhydride-cured epoxy that cures at 416 K (290°F) with a Tg of 388 K (240°F) and a volatiles content of 0 . 1 3 - 2 4 % . Extensive cryogenic material properties are available for SCI REZ 081, based on its use in dewar support straps with alumina fibre. Test data are also available from NIST 1. In spite of FM 300's poor compatibility with anhydride curing agents, SCI REZ 081 was included because FM 300 would be limited to providing adhesion of the titanium foil barrier and could not be used within the lay-up as was the case with E-787.

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During the data reduction of the strain results, the most significant value for resin comparison was found to be the shear modulus. The conclusion of the analysis is that 'the shear modulus showed much smaller variation over a wide range of assumed fibre direction moduli. This indicates the shear modulus is only weakly dependent on the fibre, and is primarily a function of the matrix material. Since the purpose of testing is to provide a discriminator which could be used in matrix material selection, it seems reasonable to use the shear modulus for stiffness considerations when selecting the matrix material (other factors notwithstanding)' 2. The ultimate strength was not used as a selection criteria for two reasons: (1) the design application for these tubes is buckling-critical, and ultimate tensile load at failure is not a good indicator of a tube's buckling performance, and (2) tensile failures require a much larger statistical base to provide meaningful information for comparison.

Processability Processability evaluations are subjective and were based on comments from the fabricators of the test samples and, on GP-B's own experience with E-787. All sample tubes were to be made under conditions that were as identical as possible. The same construction lay-up, fibre, mandrel and fabrication techniques were used by Programmed Composites of Brea, CA, USA, for all resins except SCI REZ 081. SCI REZ 081 is proprietary and was fabricated by SCI at their facility. It was also important that the resin could be used in a four-ply lay-up and still meet all the basic process requirements.

Test plan For testing and final evaluation, composite tubes (6in long x 1.5in inner diameter) were fabricated from each

Selection programme for gravity probe-B composites: E.T. Will

Figure 1

Composite test tube, with and w i t h o u t end caps

resin and Sumitomo's 3,-alumina fibre (85% A1203, 15% SiO2) (see Figure 1). A +40 ° wind angle, symmetrical about the longitudinal axis, was selected to duplicate the design of GP-B's neck tube. The two-ply tubes used a (+40, -40) lay-up and the four-ply tubes used a (+40, - 4 0 , - 4 0 , +40)s balanced lay-up, each with the same 0.508 mm (0.020in) wall thickness. Helium permeation and leak rates, strain vs pressure and strain vs tensile load were measured for each resin system. All two- and four-ply tubes were tested in the same manner. Each tube was first tested for permeation and leaks. After permeation testing, the strain of the composite was measured with 1 atm of internal and external pressure in the tube, and again by loading the tube in tension to failure.

Figure 2 Schematic diagram of permeation test. 1, Composite tube; 2, aluminium end caps; 3, copper cold plates; 4, cooling line; 5, cold shield; 6, thermocouples; 7, dewar

Permeation tests

Test apparatus Aluminium caps were bonded to each end of the tube with Epibond 1210/9615-10 epoxy to seal the ends and provide a fitting for connection to the leak detector. Veeco 120SSA and Leybold Ultratest F leak detectors were used for all tests. The tubes were tested in a simple dewar with connections for a leak detector, GHe and LHe supplies and purge pump. The tube was supported within the dewar by bolting it to copper plates brazed to LN2 cooling lines. Thermocouples monitored the temperature of the composite, end plate and cold plate. Figure 2 illustrates schematically the test set-up and plumbing and Figure 3 shows a tube bolted to the LN 2 cooling coils. All data were recorded on a Schlumberger 3531D data acquisition system. GHe was introduced inside the tube to minmize helium saturation of the dewar and provide a more uniform temperature distribution throughout the composite during cold testing by utilizing the thermal conduction of the gas. A cold shield was strapped to the outside of the tubes to reduce thermal radiation to the tubes.

Figure 3 View of composite tube and cooling coils (shown with cold shield removed)

Test procedure With background permeation levels established, helium was introduced into the tube with a 1 atm pressure differential across the tube wall. Permeation was measured until a steady-state value had been maintained for 2 h. The tube was removed and thermally cycled ten times by

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Selection programme for gravity probe-B composites: E.T. Will submersion in LN2. A second permeation measurement was made, again at room temperature. The final permeation measurement was made after circulating LN2 through the cooling coils to stabilize the composite temperature at 100 K.

Test procedure All modulus tests were performed at room temperature. Strain vs pressure was measured by slowly increasing the pressure from - 1 4 . 7 to +14.7 psid ( - 1 0 1 to + 101 kPa) while monitoring the strain in all six channels. Strain vs tension was measured by slowly pulling to failure while monitoring strain, load and deflection.

Modulus tests Test results Test apparatus

Each tube was fitted with two three-channel rosette strain gauges mounted on opposite sides at the centre of the tube. A positive pressure source was attached to the tube for the strain vs positive 1 atm measurements. The same set-up was used with a vacuum source for the strain vs negative 1 atm measurements. The tube was then mounted in standard tensile test fixture and pulled directly by the end caps to obtain strain and deflection vs tensile load.

Table 1

Cryogenic test results In general, the permeation curves of each resin system have a steep rise time from a baseline value before leveling out to a steady-state permeation value. This rise time varied from 1 to 5 h, depending on the resin. The steady-state permeation values shown in Table 1 are those used for evaluation. With two exceptions, the permeation at room temperature rose slightly after thermal cycling, with an increase of less than an order of

Test results Modulus

Permeation Permeation constant (cm 3 cm s -1 atm -1 cm -2 Room temp,

Resin system

Room Plies temperature

Cycom 92

2

4

RS-3

SCI-REZ 081

E787

9.10 x 10 - l °

-

-

b

with 10 thermal cycles 100 K a 1.60 x 10 -9

< 7 . 1 0 × 10 -13

8.00 X 10-10

Leaked

2 2

6.10 x 10 -9 5.50 x 10 -9

9.20 x 10 -9 7.30 x 10 -9

< 2 . 6 0 x 10 -11 < 1 . 6 0 × 10 -12

4

4.30 x 10 .9

6.10 x 10 .9

< 3 . 9 0 × 10 -12

2 2

1.30 × 10 -9 1.40 x 10 -9

3.30 × 10 -9 1.40 x 10 -9

< 9 . 0 0 × 10 -12 < 2 . 7 0 × 10 -12

4

1.30 x 10 -9

2.70 x 10 .9

< 7 . 3 0 x 10 -13

2

1.90 × 10 -9

1.30 x 10 -9

< 4 . 7 0 x 10 -13

2

4.00 × 10 -9

Leaked

Leaked

Comments No visible microcracks were seen on the 2-ply. Permeation of 2-ply improved at low temp.,

indicating resistance to low-temp, strain Visible microcracks were seen and tube leaked at LN 2 No visible microcracks were seen on 2-ply. Permeation improved at low temp., indicating resistance to low-temp. strain

2-ply tubes had visible microcracks, but they

did not cause leaks at low temp. No microcracking seen on 4-ply tube

Both tubes had visible cracking after thermal cycling Second tube leaked at RT after thermal cycling

Shear (Gpa)

Strength

10 s psi

(N) (Ib) c

7.92 (1.15)

14346 (3224)

7.92 (1.15)

14729 (3310)

7.58 (1.1)

15032 (3378) d 9790 (2200) d

8.95 (1.3)

12535 (2817) d

9.65 (1.4)

12446 (2797)

11.02 (1.6)

16478 (3703) d 17666 (3970)

6.61 (0.96)

17924 (4028)

6.27 (0.91)

17550 (3944)

aThese permeation values are at the lower limit of the test apparatus. Actual coupon permeation may be lower unless otherwise specified bTurbo pump and leak detector were running simultaneously, causing erroneous readings CValues in parentheses dEnd cap pulled off

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Selection programme for gravity probe-B composites: E.T. Will Table 2

Resin selection summary

Resin system

Cryo performance

Processability

Strength

Conclusions

Medium shear modulus

Unacceptable Microcracking and leaks No thermal data available

Cycom 92

Visible microcracks were seen in all coupons The 4-ply coupon leaked during lowtemp. test

RS-3

No microcracks were seen No leaks at any temperature

No problems

Medium shear modulus Epibond epoxy unable to bond with polycyanate resin

Unacceptable Poor bonding with Epibond epoxy No microcracks or leaks Lowest water absorption by factor of 10 No thermal data available

SCI-REZ 081

Visible microcracks were seen in the 2ply coupons only No leaks at any temp.

Proprietary resin

Highest shear modulus

First choice Highest shear modulus Thermal data available Microcracks in the 2ply No leaks i

E787

Visible microcracks were seen in all coupons One coupon leaked during RT and lowtemperature tests

LMSC is only customer of old system Unable to meet fibre vol. fraction requirement. GP-B had the same problem Unable to make 4ply lay-up

Lowest shear modulus

Unacceptable Lowest shear modulus Microcracking and leaks Fails fibre vol. fraction requirement Unable to make 4ply lay-up Thermal data available

magnitude. As expected, the permeation of all resins decreased at cryogenic temperatures, by two to four orders of magnitude. The only exceptions were leaks with the Cycom 92 and E-787 resins. Permeation of the Epibond 1210/9615-10 epoxy used is not a factor in the overall permeation. The epoxy contribution at room temperature to the total permeation is 6.63 × 10 -~' cm 3 s -~ He, well below the sensitivity of the leak detectors.

Cycom 92 Both two- and four-ply tubes showed good permeation performance at room temperature with no significant changes after thermal cycling. The permeation of the two-ply tube improved at 100 K, indicating a resistance to microcracking. A lack of visible microcracks seemed to support this. The performance of the four-ply tube was far worse, however, with a major leak at low temperature and with microcracks easily seen on the exterior of the tube.

RS-3 As with the Cycom-92, the two- and four-ply RS-3 tubes showed good permeation performance at room tempera-

ture with no significant changes after thermal cycling. The permeation of all tubes improved by two and three orders of magnitude at 100 K with no performance difference between two- or four-ply construction. No microcracking could be seen on either the two- or fourply tubes.

SCI REZ 081 The SCI REZ 081 tubes also showed good room temperature permeation performance with no significant changes after thermal cycling. Both two-ply tubes had visible microcracks, but they did not seem to cause leaks at low temperature. No microcracking could be seen on the four-ply tube.

E-787 Only two-ply tubes were tested with the E-787 resin because of process limitations of the resin supplier (this is discussed further under Processability Results). The first tube had slightly higher room temperature permeation after thermal cycling and extremely low permeation at 100 K, the same as for RS-3 and SCI REZ 081. This tube had visible microcracking, even though it did not leak. The second had visible microcracking that was

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Selection programme for gravity probe-B composites: E. 7-. Will severe enough to cause leakage at room temperature after thermal cycling. This was confirmed with a leak at low temperature.

Shear modulus test results The SCI REZ 081 stood out, with the highest shear modulus of any resin, whereas E-787 had the lowest. The relative difference between SCI REZ and the others is large enough to indicate that it is a significantly stiffer resin. Similarly, the low value for E-787 indicates that it is significantly softer than the others. The results for both Cycom 92 and RS-3 were so close as to be equivalent. The results are given in Table 1. One important result of the tensile test was the discovery that the Epibond 1210/9615-10 epoxy used on GP-B is a filled epoxy and does not adhere well to the polycyanate-based RS-3 resin. This is a major drawback for use in structural support struts. A polycyanate-compatible replacement epoxy or primer would solve this problem.

Processability results As mentioned before, all tubes (except the SCI REZ 081 tubes) were fabricated by the same company, to ensure consistent Ifabrication conditions. All tubes were built by laying preimpregnated panels of fibre and resin over a mandrel at the desired wind angle. In each case the same mandrel was used. A 0.508 mm (0.020in) wall thickness was required for all tubes, regardless of whether a two- or four-ply lay-up was used. E-787 could not meet the four-ply requirement because BP Chemicals does not supply preimpregnated panels thin enough to build a four-ply lay-up. All resins except E-787 were able to meet the requirement of a fibre volume fraction greater than 50 %. This reflected the same experience GP-B has had in the past when using E-787. SCI did not report any problems working with their proprietary resin.

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Conclusion SCI REZ 081 was selected as the best overall resin. Both SCI REZ and RES-3 stood out above the others in cryogenic and processability performance. Neither presented processing problems or was affected by microcracking. Even though SCI REZ tubes were fabricated by a different company, SCI's familiarity with their own resin alleviated any concerns about process problems or inconsistencies. The inability of GPB's Epibond 1210A/9615-10 epoxy to bond to a polycyanate-based resin made RS-3 unacceptable. Before RS-3 could be used, extensive testing would be required to develop a bond primer or to find and qualify a polycyanate-compatible epoxy. Although not considered a specific pass/fail criterion, the availability of existing thermal conductivity data was a major advantage for SCI REZ, and was the final determinant for selection. E-787 and Cycom 92 were unacceptable because each had microcrack-induced leakage. Comments and conclusions about each resin's performance are summarized in Table 2.

Acknowledgements This work was supported through Stanford University, via subcontract PR 4660 and NASA contract 8-36125 with the Marshall Space Flight Center. Valuable technical guidance and input were provided by Richard T. Parmley, Robert Milligan, Tak Aochi and Mark Ferraro. All testing was performed by Margaret Bogan, Dan Welsh and Roger Mihara. Data reduction and modulus analysis were done by Mike Jacoby.

References I Kriz, R.D. and Sparks, L.L. Performance of alumina/epoxy thermal isolation straps Adv Cryo Eng Mater (1988) 34 107-114 2 Jacoby, M.S. GravityProbe-B CompositeNeck TubeMaterial Testing Data Reduction Theory Internal Analysis at Lockheed Palo Alto Research Labs, Palo Alto, CA, USA (1991)