epoxy prepregs by electrical stimulation

Strength modification of carbon/ epoxy prepregs by electrical stimulation A. B. STRONG and P. HAUWILLER (Brigham Young University, Utah, USA) Increases in the specific strength of composites have been a key to their use in an ever widening variety of application areas. These strength increases have been accomplished by improving the fibre manufacturing methods, controlling the matrix materials, and improving the bond between fibres and matrices. The realization of higher strengths is especially important in applications where the environment surrounding the composite could cause a decrease in the strength. This research explored how electrification of fibres effects the tensile strength of a composite. The fibres were stimulated during and after cure at various current densities and for various durations. Two effects were seen: at high currents and moderate duration following cure (4 amps for 10 min or 6 amps for 2 min), the tensile strength decreased; and in the low current range when done during the cure (0.16 amps), a tensile strength increase was seen. That increase was small (6%) but statistically significant.

Key words: carbon~epoxy prepreg; electrical stimulation; strength modification

Composite materials have been used in electrical applications for some time. Graphite fibres generally have a resistivity of 4 × 10 -5 which is about 20 times that of copper. ~Some of the uses for carbon fibre reinforced composites are electron shielding, radar absorbers, heating elements, even space antennas. 2 These uses require the composite part to assume a structural role while being used in an electrical environment. The electrical current involved and the subsequent magnetic field can have an effect on the composite structure. Some work in this area has been performed in the Soviet Union 3' 4 and in the United States. 5 These works consider the effect of exposing composites to externally generated magnetic fields to enhance the epoxy properties, and thus the composite properties, and is limited to during-cure stimulation at a low current level (0.14 amps). The present work examined subjecting the material to electrical fields and the effects of performing the stimulation both during and after the cure. Several current levels (0.16-2.00 A during cure and 2.00-6.00 A after cure) were tested.

EXPERIMENTAL Hercules AS-4/3501-5A three inch (76 mm) wide unidirectional tape (unsized, surface oxidized fibres) cut into six inch (152 mm) long strips was used. Single ply samples were used to ensure a good electrical contact. (Attempts were made to use multi-ply samples, but a repeatable and complete electrical contact, all fibres being in the circuit, could not be achieved even if the ends were immersed in mercury.) A resistance measurement of each composite was used to ensure that the pressure contact was good and uniform for each sample. Prior to electrical connections being made, the ends of the strips were cleaned with MEKto remove the partially cured epoxy resin, thereby allowing a good electrical contact to be made with the fibres (Fig. 1). Each end of the sample was pressed between plates to establish intimate contact between the fibres and between the fibres and plate electrode. The resistance of each sample was 0.4-0.5 f2. (The lower the resistance the better the electrical contact since each fibre acts as a parallel resistor in the circuit.) The range

0010-4361/89/050437-04 $3.00(~)1989 Butterworth & Co (Publishers) Ltd COMPOSITES. VOLUME 20. NUMBER 5. SEPTEMBER 1989

437

Electrodeplates

177°C

24°C

,~

112°C r ~

45 min

60 min 45 min

r~66o0 60 min

45 min

Fig.2 Curecycle

Fig.1 Schematicof experimentalapparatus of stimulation was selected as that which would adequately span the range of currents expected in normal electrical applications. For electrical stimulation during cure, the DC currents used were: 0.167, 1.333 and 2.000 A for 3.5 h (voltage range 0.083-1.00 V). For electrical stimulation after cure, the DC currents were: 2, 4 and 6 A for 2 min and 10 min durations, depending on the sample (voltage range 1.00-3.00 V). Electrical stimulation of the fibres created noticeable heat at the 4 A (8 W) level. However, this and higher current levels were used only on post cured samples which are not affected by heat within their use temperatures (< 149°C). The temperatures reached were, for 4 A (8 W), 49°C and for 6 A (18 W), 61°C. The temperatures were determined using a thermocouple mounted under the sample during stimulation. The composites were cured under vacuum only using the cure cycle depicted in Fig. 2 (excluding external pressure). The curing was performed in an oven with a thermocouple placed on the centre of the tool between the samples. All samples were prepared, stimulated and cured within 12 h of out time. The single ply unidirectional samples were then cut into half inch (12 mm) wide strips (along the 0° orientation) and tabbed with 12 mm × 25 mm aluminium tabs which

had a 14° taper at one end. The tabs were sandblasted on the bonding side and then adhesively bonded to the samples with REN 556 epoxy. The gauge length for these samples was 100 mm. The samples were tensile tested with a crosshead speed of 12 mm min -~. The width to thickness ratio was nine fold that of a standard ASTM D-3039 sample, which caused some increased variability in the data. The use of the thin laminates (single ply) caused many specimens to fail prematurely due to resin cracking between the fibres during loading. Hercules Aerospace was consulted concerning this phenomenon, and indicated that premature breaking of thin laminates was much more common in tensile loading than with thicker test specimens. Furthermore, the control specimens, even though thin, were found to have the same average tensile strength as the published Hercules data (see Table 1). Therefore it was not thought that the handling of the materials initiated the resin cracking. These prematurly failed samples were excluded from consideration. The remainder of the samples failed in the customary manner. RESULTS The results of the tensile tests are presented in Figs 3 and 4. With post-cure stimulation, when the current is high or the exposure time is long, the composite tensile strength is lowered. The effect of the current is greater than the effect of time over the ranges of current and time considered. At low currents and short duration times for post-cure stimulation, the tensile strength is relatively unchanged.

Table 1. Statistical analysis results

Comparison groups

Df

ttable

t calculated

Results

Ho= 1-2=0 Ha = 1 - 2 = 0 Control/product data 0.16 A during cure/control

6 6

2.477 2.477

2.032 4.019

accept Ho reject Ho

/-/o=1-2=0 Ha = 1 - 2 < 0 Product data/0.16 A during cure 1.33 A during cure/control 2.0 A during cure/control 2 A for 10 m in/control 4 A for 2 rain/control 4 A for 10 min/control 6 A for 2 min/control

8 4 4 4 4 4 7

1.876 2.132 2.132 2.132 2.132 2.132 1.895

-2.700 -0.300 -0.330 -0.740 - 1.940 -4.500 -2.220

reject Ho accept Ho accept Ho accept Ho accept Ho reject Ho reject Ho

438

COMPOSITES. SEPTEMBER 1989

Possible non-electorical causes for strength modifications

300 291.1 (3, 13.3)

280.3 (3, 16.2) 261 5 (6, 13.4) 250 247.5 (3, 3.4)

[Number of specimens, standard deviation]

1

I

I

2

4 Current density

6

Fig. 3 Post-cured tensile test results, varying current for constant t i m e

300

b Control 291.2 (3, 13.3)

2 A, 280.3 • (3, 16.2)

6 A, 261.5 • (6, 13.4)

4 A, 247.5

250 4 A, 250.1 (3, 26.9)

(3, 3.4)

F(Number of specimens, standard deviation)

Many factors can influence the tensile strength of a composite. To confirm that the observed tensile increases were due to the electrical stimulation, potential factors were investigated: possible increases in fibre vol %, possible differences in the percentage of crosslinking; and possible changes resulting from processing of the materials. The fibre weights were investigated using an acid digestion test (ASTM D-3171). The fibre wt % values for all samples were within 1% of each other, and were therefore not an influence on the results. The crosslinking density was investigated using a differential scanning calorimeter (DSC) to measure the glass transition temperature which correlates with the amount of crosslinking. The results for all samples were all within 1.5% of each other, and it was therefore concluded that crosslinking was not a factor. The variability due to possible internal processing peculiarities was checked by comparing the tensile test results obtained for control samples with published results on the same materials. Tests showed that the control samples were statistically similar to Hercules product data properties (Table 1). Therefore processing differences were ruled out as possible causes for increased strength. The differences were attributed to electrical stimulation.

Analysis of post-cure stimulation samples

Fig. 4 Post-cured tensile test results, constant current for v a r y i n g time

Moderate and high currents were applied to samples after they had been cured (2.00-6.00 A). The tensile strength decreased with increasing current, although the effect was minimal up to 2 A. (At 2 A the tensile strength remained statistically similar to the control). However, at 4 A and 6 A the tensile strength was statistically lower than the control (Table 1).

When the samples are electrically stimulated during the cure, medium currents (2 A for 10 min) showed minimal effects on the tensile strength. However, at low currents (0.16 A) the tensile strength was increased. (This increase in strength at low current agreed with work cited previously 3-5 at approximately the same current levels.)

A possible explanation for this reduction in" strength could be that the current disrupts the fibre surface and resin interface (interphase) by providing additional surface energy which may interfere with the bonding. Alternatively, the magnetic field created by the current may be inducing a stress in the already crosslinked molecules. The exact mechanism which causes the strength modification has not as yet been determined, but work is continuing in this area.

I 2

Time (min)

1 10

ANAL YSIS The analysis will be separated into three sections: examination of possible causes for the changes in tensile strength other than the electrical stimulation; the tensile strength modifications due to post-cure stimulation; and the strength modifications due to the during-cure stimulation. The statistics employed for the tensile strength results of the analysis were the 'F' test for variance and the 't' test for population means. All tests were performed at a 95% confidence level (Table 1). All 'F' tests indicated variances which were sufficiently close to perform 't' tests.

COMPOSITES.

SEPTEMBER

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Analysis of during-cure stimulation samples Low to moderate currents were used to stimulate the samples during cure. At low currents (0.167 A), the tensile strength increased 6%, while those specimens stimulated at moderate currents (1.33 and 2.00 A) showed no significant strength modification (Fig. 5). The increase in strength for low current specimens was subjected to a t-test for statistical verification. The increase was statistically significant at the 95 % confidence level (Table 1). This increase may be caused by several possible mechanisms. The current along the fibre surface could 439

and adds additional data in the area of post-cured composites. 307.9 • (5, 42.2)

3OO , 291.2 e(3, 13.3) Y=

=:

286.1 • (3, 19.6)

288.1 • (3 35.1)

p-

It should be noted that at medium currents the tensile strength of the during-cure stimulated samples remained statistically similar to the control (1.33-2.00 A). This would indicate that the mechanism which causes the strength increase at low amperages is altered by increasing amperages. Perhaps the orientation is taken to the point of molecular randomization and the effect is obscured.

(Number of specimens, standard deviation)

I 0.16

Fig. 5

I 1.33 Current density During cure tensile test results

I 2.00

be providing sufficient energy to aid bonding at the fibre/resin interface (interphase). However, at higher currents, this bonding energy effect should be even more prominant; the opposite was observed. Furthermore, the cured samples would also exhibit this effect, but again the reverse was observed. It is therefore concluded that the electrical stimulation does not aid the bonding energy. Alternatively, the magnetic field created by the current may be moving the fibres to a more optimal spacing during the low viscosity phase which exists during cure. This magnetic field may also be orienting the epoxy molecules around the fibre. The rationale for this difference could be the mobility of the interphase molecules which might achieve some bonding orientation in the liquid state but cannot orient in the cured state. This molecular orientation mechanism was suggested as the mechanism for enhanced tensile strengths in previous studies in which magnetic fields were used. 3-5 The work previously done agrees with the results achieved here. DC currents of no more than 0.14 A were used to create a magnetic field, as opposed to 0.16 A here, during the cure cycle. Epoxy molecule orientation also increased with increasing time with some leveling out occurring at 4 h with some epoxies (the time here was 3.5 h). One major difference is that in previous work magnetic fields varied in intensity and the strength increases were slightly larger than here. Also, the magnetic fields were previously created externally while here they were internally created. Despite these variations in procedure, the work carried out here appears to confirm the previous conclusions,

440

SUMMARY Most electrical stimulation of graphite/epoxy composites at medium to high currents will reduce their tensile strength. Low currents on cured parts seem to cause no degradation of the tensile strength. Therefore most electrical applications of composites will not cause a strength decrease. Low applied currents during cure can raise the composite tensile strength. The mechanism may involve orientation of the molecules or modification of the interphase between fibres and resin. Further research will concentrate on optimization of the effect, examination of other systems, and the development of industrial uses for this technique.

REFERENCES 1 Spain, I. L., Ubbelohde, A. R. and Young, D. A. Electronic properties of well oriented graphite' Phil Trans Roy Soc London, Series A 262 (1967) pp. 345-86 2 Sittig, M. 'Carbon and Graphite Fibres Manufacturing and Applications' (Noyes Data Corp., 1980) Chemical Technology Review, No. 162 3 Rodin, Yu. P., Molchanov Yu. M. and Kisis, E. 'Properties of polymeric composite materials formed in the presence of a non-uniform constant magnetic field' Mekhanka Kompozitnykh Materialov 5 (1981) pp. 864-8 4 Rodin, Yu. P., Molchanov Yu. M. and Kharitonova, N. V. 'Anisotropy of the strength properties of plastics and their components treated in a magnetic field', Mekhanka Kompozitnykh Materialov 3 (1984) pp. 503-8 5 Gerzeski, R. H. Masters degree thesis (University of Dayton, OH,

USA, 1987)

AUTHORS The authors are with the Technology Department, Brigham Young University, 435 CTB, Provo, UT 84602, USA. Enquiries should be addressed to A. B. Strong.

COMPOSITES. SEPTEMBER 1989