Effect of reinforcement and solvent content on moisture absorption in epoxy composite materials

Composites: Part A 31 (2000) 741–748 www.elsevier.com/locate/compositesa

Effect of reinforcement and solvent content on moisture absorption in epoxy composite materials F.U. Buehler, J.C. Seferis* Polymeric Composites Laboratory, University of Washington, Box 351750-1750, Seattle, WA 98195-1750, USA Received 5 July 1999; received in revised form 20 November 1999; accepted 5 December 1999

Abstract The effect of fibrous reinforcement and solvent content on moisture uptake in composite laminate was investigated. Two materials using identical epoxy resin systems but different reinforcements—glass vs. carbon fibers—and of different solvent content—low vs. normal— were examined. Samples were characterized in terms of water absorption and desorption. Mechanical and thermal properties including flexural modulus, flexural strength, and glass transition temperature were measured. Results clearly show the contribution of the fiber reinforcement as well as solvent content on the water absorption rate and mechanical property changes. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Water absorption; B. Mechanical properties

1. Introduction Developments in the field of advanced composite materials over the past decades have significantly altered their future role in structural applications. Composites offer structural designers a material of higher strength, stiffness, and lower distortion than had previously been available from engineering materials. Epoxy resin composites are one example of such a material. These composite systems are finding widespread use in the transportation, aerospace, and even sporting good industries. Several areas need to be investigated before the full potential of epoxy resin composites can be realized in practical applications. Resin and reinforcement properties, structure, and process techniques are but a few of the important relationships which must be investigated to understand existing and potentially new materials. Many investigations have focused on the influence of either chemical or environmental effects on the various composite system properties [1,2]. Environmental effects can cause property alterations due to development of microstresses in the composites [3]. Chemical effects manifest themselves through structural changes (bond scission). Properties such as resin/catalyst stoichiometry, matrix impurities, fiber surface impurities, and other process-related phenomena cause chemical altera* Corresponding author. Tel.: ⫹ 1-206-543-9371; fax: ⫹ 1-206-5438386.

tions in the composite which affect the internal macroscopic structure of the system [4]. Among thermoset epoxy prepregs, several types of systems are available based on their resin flow and/or fiber reinforcement. In the aerospace industry, structures of high strength-to-weight ratio are highly desirable. For this reason, airplane structural components are often made out of honeycomb sandwich structures, which consist of thin high-strength skins bonded on the top and bottom of a honeycomb core [5]. The development of honeycomb structures has identified many desirable prepreg characteristics [5]. These include skin-to-core self-adhesive characteristics and controlled flow resin characteristics. Controlled flow resin prepreg consists of elevated viscosity resins that provide very little flow during cure. As a result, the controlled flow viscosity resin remains in the prepreg skins and, therefore, a consistent resin content and thickness are produced in the cured part. Ideally, controlled flow resin is also self-adhesive so that no film adhesive is required to bond the honeycomb core to the prepreg skins [5]. Since the resin maintains a controlled flow viscosity throughout cure, small fillets are formed between the prepreg and core. Therefore, skin-to-core bond results with no significant reduction in the resin content of the skins. On top of these desired characteristics, other requirements such as fireretardant or damage-tolerant properties may have to be met. Fire-retardant properties are usually met by brominating the resin, while the inclusion of elastomers or

1359-835X/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S1359-835 X( 00)00 036-1

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Fig. 1. Relative stability of epoxy resins according to structure. Reproduced from Ref. [6].

thermoplastics can usually help achieve the desired damagetolerance [5]. 1.1. Background The selection of a particular epoxy resin for composite fabrication can play a significant role in the moisture absorption characteristics of the resultant composite. In general, one would expect epoxy resins which form low cross-link density systems to absorb greater amounts of water than highly cross-linked systems. This has been shown experimentally [6–8]. One study by Fleming and Rose compares the relative amount of absorbed water for a series of different epoxy resins, as seen in Fig. 1 [6]. It is of interest to note that the term “hydrolytic stability”, suggested by this figure, loosely implies the prolonged maintenance of structural properties in moist environments. Plasticization and alteration of the glass transition tempera-

ture are phenomena often linked to moisture absorption and property alterations. Consequently, considerations such as cure cycle, catalyst type, catalyst amount, and epoxy preparation technique are among the items which influence the absorption of water and alteration of networks. It has also been suggested that water may bond or cluster within the polymer. Such phenomena, as well as the moisture diffusion process itself, are affected by thermal agitation. In some instances, impurities such as halohydrin and organic salts have been linked with increment in moisture absorption [9]. These compounds may persist in the epoxy as a consequence of various manufacturing operations. Since they are quite hygroscopic, even trace amounts of these impurities can lead to large variations in observable moisture absorption [9]. Other factors leading to increase in water absorption are the modifiers used to increase or tailor the matrix properties. Materials such as elastomers or toughening materials can have a profound influence on the water

Table 1 Alkali metal concentration on various commercial carbon fibers. Concentration is reported as parts per million of metal based on weight of fiber. Reproduced from Ref. [6] Manufacturer

Fiber type

Lot

Sodium (ppm)

Potassium (ppm)

Calcium (ppm)

Union Carbide Union Carbide Union Carbide Hercules Hercules Hercules Courtaulds Carbon Morganite Morganite Morganite Gelanese Stockpole Carbon Great Lake Carbon Great Lake Carbon Great Lake Carbon Great Lake Carbon

Thornel 75 Thornel 300 Thornel 400 HM-S HT-S A-S HT-S Modmor I Modmor II Modmor III GY-70 Panex 30-A 3T 4T 5T 6T

4062121-1 TY-3215233 TY-282924 5-2/61-1 5/1-25HZ 3-1/180-3 1CT56C/15R G245 188-1 177-1 2-037 76PB-30 – – – –

0.8 7.8 0.3 15.4 13.0 21.5 310.0 195.0 135.0 230.0 3.0 3400.0 4.4 3.3 3.3 4.5

1.0 290.0 1.0 – 1.3 1.1 8.3 5.5 5.8 10.0 1.0 1.0 – – – –

2.0 9.5 2.4 14.0 – 75.0 33.0 35.5 43.0 74.0 2.0 2.0 2.5 19.0 20.0 14.0

F.U. Buehler, J.C. Seferis / Composites: Part A 31 (2000) 741–748 Table 2 Material abbreviations used in this paper

Glass fiber Carbon fiber

Solvent

No solvent

GF S CF S

GF NS CF NS

absorption. This could be due to polarity and/or plasticization, and is a function backbone structure, phase separation, etc. Although epoxy resins affect the hygrothermal response of composite systems, they alone do not contribute to the end result. Several authors note that various fiber reinforcement trace element impurities can significantly affect the composite material properties and moisture uptake [10,11]. A list of alkali metal trace element concentrations for 16 fiber types representing seven different fiber manufacturers is given in Table 1. Although only Na ⫹, K ⫹, and Ca 2⫹ trace concentrations are given for those fibers listed, it was noted that other (proprietary) fiber contaminants did affect the moisture absorption process. In addition to the observed effects on moisture absorption, composite surface blistering is often reported in conjunction with trace fiber contamination [6,10,11]. Blistering does not usually become evident until after long exposure to a moist environment. Moreover, differences in coefficient of thermal expansion (CTE) between fibers and matrix can favor blistering as well as crack formation and propagation. It can also create low resistance diffusion path at the fiber–matrix interface, leading to an increase in water absorption rate [9]. The purpose of this work was to investigate the influence of matrix solvent content and reinforcing materials on moisture uptake. Two prepreg materials were provided from a single supplier. These materials had an identical resin system, but different fiber reinforcements: glass and carbon fabrics. Both prepreg materials were characterized before being subject to cure. The resulting laminates were evaluated through various mechanical tests before and after exposition to water until saturation. Tests were also performed on samples that underwent a desiccation step after water absorption. Additionally, to determine the influence of the solvent present in the resin systems, both prepregs were exposed prior to lay-up to moderate heat in order to evaporate the solvents. Similar tests were then performed on the ‘solvent-free’ laminates.

2. Experimental In this study, two similar prepreg materials were utilized. Both prepreg materials were self-adhesive, controlled-flow prepreg systems with fire-retardant properties. These materials are commonly used in the manufacturing of the airplane secondary structure. Both prepreg materials had the same resin system but differed in their fiber reinforcement: the first prepreg (BMS 8-79) was a glass fiber fabric of

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style 7781, while the second prepreg (BMS 8-168) was a 3k70 carbon fiber plain weave fabric. As both prepreg materials were impregnated by a solution-dip process, the resin matrix contained a certain level of solvents, which have been previously identified as methyl ethyl ketone, xylene, and acetone [12]. In order to investigate the effect of these solvents on environmental attack, part of the materials were subjected to mild heat in a convective oven in order to drive out most of the solvents, leading to quasi-solvent-free prepregs, referred to in this study as NS (no-solvent). Similarly, solvent-containing prepregs were referred to as S, carbon fiber materials as CF, and glass fiber materials as GF. For convenience, Table 2 groups the abbreviations used throughout this paper. Prepreg resin content was determined by weighing a 51 × 51 mm2 prepreg coupon, followed by extracting the resin from the prepreg using acetone and weighing the dried fibers. Five samples were tested and averaged for each prepreg system. Solvent-free prepregs were prepared by placing each prepreg for 30 minutes in a recirculating oven set at 60⬚C. The amount of solvent evaporated was measured gravimetrically. All solvent-free prepregs underwent the same experimental tests as the other prepregs. Quantitative tack measurements were carried out on all four samples according to the method described and developed by Seferis and co-workers [13–16]. All tack measurements were performed on a screw-type Instron 4505 Tensile/Compressive testing machine. To perform the tack test, five plies of 51 × 51 mm2 prepreg were bonded between two metal tabs with double back-tape, and subjected to a compression-to-tension cycle. In a standard fabric prepreg cycle, the prepreg was compressed to 134 N at a displacement of 2.54 mm/min, held at 134 N for 2 s, and then pulled apart in tension at the same displacement rate. Stress/strain data were recorded during the test. Analysis of the data was then performed and five test samples were averaged together for each tack value. From an analysis of the stress/strain graph collected during the test, both compressive energy and toughness factor were obtained to quantitatively define the tack of the materials. Laminates were made by laying-up 18 plies of each prepreg. Laminates were cured for 90 min at 121⬚C using a 2.78⬚C/min ramp. The total consolidation pressure was 620 kPa. Laminates were cut into the required shape for dynamic mechanical analysis (DMA), double cantilever beam (DCB), end-notch flexure (ENF), and porosity tests. A third of these laminates were tested immediately, while the remaining two-thirds were exposed to moisture. The moisture exposure consisted of prolonged immersion in a 71⬚C water bath for nearly 1200 h. Weight change was recorded for each sample periodically. Once the weight increase ceased, half of the moisture-exposed samples were tested for DMA, DCB, ENF, and porosity. The other half underwent desorption for approximately 450 h in a convection oven set at 50⬚C before the tests were performed.

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Fig. 2. Compressive energy of glass and carbon fiber epoxy prepregs of various solvent content. Error bars shown are the standard deviation from five samples.

DMA experiments were performed on the cured laminate with a TA Instrument 983 DMA interfaced to a Thermal Analyst 2100 controller. The experiments were performed in a nitrogen atmosphere with a heating rate of 10⬚C/min, a frequency of 1 Hz, and an oscillation amplitude of 0.10 mm. The glass transition temperature (Tg) was measured from the peak in the loss modulus. Mode I interlaminar fracture toughness was measured using the double cantilever beam (DCB) method [17,18]. The laminates were 18 plies thick, 330 mm long, and had a 51 mm FEP crack starter in the mid-plane of the sample. After cure, the laminates were cut into 12.7 mm wide samples for testing. Each specimen was pre-cracked in the mechanical testing apparatus to provide a sharp crack tip before testing was performed. The fracture specimen was pulled apart in tension at a rate of 25.4 mm/min until a displacement of 63.5 mm was reached and the crack extension marked. A minimum of five samples, each providing one GIc value, were tested and averaged for a reported GIc value. Mode II interlaminar fracture toughness was measured using the end-notch flexure (ENF) test [17,18]. The same laminate preparation was used as for the DCB specimens. A three-point bending apparatus with stationary posts set 101.6 mm apart was used to create shear fracture of the specimen along the mid-plane. The crack tip was set 25.4 mm from the stationary post, and the loading point was set 51 mm from the post. The specimens were precracked in the mechanical testing apparatus to provide a

Fig. 3. Toughness factor of glass and carbon fiber epoxy prepregs of various solvent content. Error bars shown are the standard deviation from five samples.

sharp crack tip before testing was performed. A displacement rate of 2.54 mm/min was used to load the specimen in flexure until the load decreased upon crack propagation. The crack front was then located with an optical microscope attachment and moved back to 25.4 mm from the stationary post. This was repeated until the sample tested yielded five GIIc values. These five values were then combined with those of a second specimen to yield a GIIc averaged over 10 values. Flexural properties were determined in a three-point bend test according to ASTM D790 [19]. For each material, five specimens with a length of 75 mm and width of 12.7 mm were tested. The support span was 64 mm and the crosshead speed was 1.7 mm/min. Deflection was measured as cross-head travel. The cured laminates were cut and polished, and optical photomicrographs were taken at 50 × magnification. Void percentage was determined by image analysis by the use of the freeware NIH Image written by Wayne Rasband at the US National Institutes of Health (http://rsb.info.nih.gov/nihimage).

3. Results and discussion 3.1. Properties of uncured prepregs Upon solvent removal by convective heating, the handling characteristics of the prepregs were found to have altered. These characteristics were investigated in terms of tack by measuring the compressive energy and toughness factor as defined by Seferis et al. [13,14,16,20]. As shown in Fig. 2, the carbon prepreg (CF) was found to exhibit a drastic decrease in compressive energy upon solvent removal, while no effect was noticed on the glass fabric prepreg. However, removing solvents greatly increased the toughness factor of the glass fiber prepreg, while the carbon fiber prepreg experienced a decrease in toughness factor, as shown in Fig. 3. In the case of carbon fiber prepreg, moderate heat exposure favored the migration of surface resin further into the fiber bundles or in weave openings under the combined effect of lower viscosity and capillary pressure. Ultimately, this resulted in a higher degree of impregnation, which is known to produce a higher compressive energy and a lower amount of surface resin, giving a slight decrease in toughness factor [16]. In the case of the glass fiber prepreg, the fabric weave was much tighter and did not allow for much resin migration, hence the stationary value of its compressive energy after solvent removal. The surface resin, by losing its solvent, improved its adhesive characteristics, which increased the amount of energy required to pull the specimen apart, resulting in a higher toughness factor [16]. During lay-up of the glass fiber prepreg without solvent (GF NS), the high toughness factor greatly impaired prepreg drape and ply repositioning, which could in certain cases lead to increased porosity.

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Table 3 Properties unconditioned laminates made from epoxy prepreg of various fiber type and solvent content

GF S GF NS CF S CF NS

GIc (J/m 2)

GIIc (J/m 2)

Flexural modulus (Pa)

Flexural strength (Pa)

Tg (⬚C)

2152 ^ 137 861 ^ 15 860 ^ 101 712 ^ 71

4668 ^ 181 2454 ^ 131 3076 ^ 135 2821 ^ 249

20170 ^ 87 18960 ^ 158 41760 ^ 515 42600 ^ 567

646 ^ 20 605 ^ 16 711 ^ 25 729 ^ 27

117.1 134.2 114.0 130.0

3.2. Properties of the unconditioned (reference) cured laminates Laminates were laid-up and tested for mode I and mode II interlaminar fracture toughness, flexural properties, and glass transition temperature. Table 3 summarizes these results. Removing the solvents from the prepreg resin system prior to lay-up and autoclaving were found to slightly decrease the mode I and II fracture toughness of the laminates, while the flexural properties remained essentially unaffected. By removing the solvents from the prepregs, the resulting laminate became more brittle, causing the GIc and GIIc values to decrease as less energy could be absorbed by the material before crack propagation. Under some instances, studies have shown that GIIc can increase with high modulus material at the interface [21], but this was not observed here. Conversely, the flexural properties, which are mostly fiber dominated, did not suffer significantly from the matrix increase in brittleness [22]. Measurements of the glass transition temperature confirmed the plasticization effect of the solvents: Tg was about 16⬚C higher for the no-solvent (NS) samples than for the solventcontaining (S) samples. 3.3. Water absorption and desorption The results of the water absorption and desiccation experiment are shown in Fig. 4. All laminate materials exhibited similar absorption behavior and the total amount of water absorbed after 1200 h in 71⬚C water was 4–5% of the total laminate weight, which corresponds to about 11– 13% of the resin weight. Usually, neat epoxy resins typically absorb 3–10 wt% of water [5]. However, the resin

system present in the studied material is brominated to provide fire-retardant properties [12]. This bromination enables the resin to absorb much more water through a free volume increase mechanism and through enhanced polarity interactions. From Fig. 4, it can be seen that the laminates prepared from solvent-free prepreg (NS) had a higher absorption rate when compared to the solventcontaining (S) counterpart. This suggests that solvent removal freed sites for the water to migrate into, or it may have resulted in a higher driving force for absorption. Lower amount of plasticization may also have led to an increased coefficient of diffusion, or solvent evaporation may have created easier paths for water diffusion. However, the different levels of porosity observed in the materials might have contributed to this rather complex water absorption behavior. In terms of reinforcement effect, the glass fiber laminates were found to have a higher absorption rate than their carbon fiber equivalents. A possible explanation for this is that the glass fibers provided a better path for diffusion, perhaps due to the tightness of the glass fiber weave, or the morphology of the fibers themselves. After 1200 h in 71⬚C water, samples were taken out of the water bath and desiccated at 50⬚C in a convection oven. As shown in Fig. 4, the desorption behavior was almost identical for all laminates and the water content was approximately 3% after 450 h. This identical desorption behavior suggests that the water did not rely heavily on the fiber type or weave geometry to migrate outside of the samples. Fig. 5 shows that 1200 h of water absorption created diffusion paths. These paths could have remained open during desorption, allowing for an efficient removal of the water in a mass transfer controlled regime. It is possible that as one nears the zero water content, the contribution of the fibers becomes more important, as one faces a diffusion controlled regime, but this was not studied in this work. 3.4. Properties of cured laminates after environmental exposure

Fig. 4. Water absorption and desorption curves of laminates made from epoxy prepregs of various fiber reinforcement and solvent content.

Since the resin absorbed about 11–13 wt% water, the properties of the laminate were expected to be affected. Mode I fracture toughness results for the water exposed and desiccated specimens are shown in Fig. 6. Typically, mode I interlaminar fracture toughness is expected to decrease due to the interfacial damage caused by water absorption, and desiccation would be expected to restore part of this loss. This trend is counter-balanced by the

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Fig. 6. Mode I fracture toughness of laminates made from epoxy prepregs of various fiber reinforcement and solvent content. Error bars shown are the standard deviation from five values.

Fig. 5. Cross-sectional area of a carbon fiber laminate laid-up from prepreg containing solvent. Shown are (a) the unconditioned laminate and (b) the laminate after 1200 h of water absorption. Dark areas are void/delamination, gray areas are resin, and white areas are carbon fibers.

plasticization effect of water, which tends to increase in GIc during water absorption. Therefore, the increase in GIc observed in the carbon laminates may be partly attributed to plasticization. However, plasticization itself may not be the sole contributor to the increase in mode I for these samples: increased fiber bridging and more importantly crack tip blunting may have played a role. Blunting refers to the increase of the crack tip radius, which reduces the

stress concentration at the crack tip, leading to a higher effective (measured) GIc value [23]. This blunting phenomenon was likely to have happened as the percentage porosity increased two- to five-fold upon water exposure, as shown in Table 4 and Fig. 5. This latter figure, which is a crosssectional area of a carbon laminate, reveals that water absorption caused void growth as well as delamination. Surface blistering was also observed in these samples. In the mode II fracture toughness, specimen performance was found to decline by ⬃66% after 1200 h of water absorption, as shown in Fig. 7. As water was absorbed and created delamination and void growth, the crack surface was reduced and led to a decrease of energy needed to propagate the crack, largely overcoming the plasticization effect of the water which would allow more energy to be absorbed by the material. Upon water desorption, the GIIc values were found to increase slightly, owing to some reversible damage. This was confirmed by the cross-sectional photomicrograph as the void area was found to slightly shrink. As shown in Fig. 8, the flexural modulus of all samples decreased by about 30% upon water exposure. Although flexural modulus is a filament dominated property, past work has showed that moisture and temperature exposure

Table 4 Laminate porosity (%) as determined by image analysis on a cross-sectional area Laminate material

Unconditioned

1200 h absorption

450 h desorption

GF S GF NS CF S CF NS

4.42 6.05 2.35 3.40

9.38 13.75 11.39 6.99

6.86 6.05 7.17 4.16

Fig. 7. Mode II fracture toughness of laminates made from epoxy prepregs of various fiber reinforcement and solvent content. Error bars shown are the standard deviation from 10 values.

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Fig. 8. Flexural modulus for laminates made from epoxy prepregs of various fiber reinforcement and solvent content. Error bars shown are the standard deviation from five samples.

Fig. 10. Glass transition temperature of laminates made from epoxy prepregs of various fiber reinforcement and solvent content. Tg was determined as the peak in loss modulus obtained by DMA.

can lead to inducing a change in failure mode from filament dominated to matrix dominated [22]. Therefore, the observed behavior can be explained by high shear rate induced in the flexural test by matrix plasticization [22]. A more accurate method of measuring the flexural modulus would be to use a four-point bend test and strain gage the lower surface in the center region where the bending moment is constant [22]. Comparatively, flexural strength was even more affected by water absorption than flexural modulus, with a loss of strength of 40–50%, as shown in Fig. 9. This large decrease was possibly due to poor interfacial adhesion and change in failure mode, which has been shown by many researchers to affect flexural and interlaminar strength values [22,24,25]. Dynamic mechanical analysis confirmed that matrix plastization took place during water exposure. As seen in Fig. 10, the glass transition temperature of the samples containing solvent (S) decreased by 38⬚C after 1200 h in 71⬚C water, while the sample containing no solvent (NS) experienced an even sharper decrease in Tg with a drop of 47⬚C. The larger drop of the NS samples is due to their higher Tg to start out with compared to the S samples. Desiccation of the samples proved to have very little effect on the glass

materials (less than 2⬚C recovery), while the carbon materials recovered over 10⬚C. This better recovery in carbon material might explain the better recovery in flexural strength observed previously for these materials. Collectively, this study brought insights into the effect of fiber type on the water absorption in epoxy composites. It showed that solvent-free resins and glass fiber composites absorb water at a faster rate than solvent-containing and carbon fiber epoxy composites. Moreover, brominated resins for fire-retardant purposes were found to absorb as much as 13 wt% of water. These findings are important in designing a honeycomb core part that does not suffer the problem of water ingression [26].

Fig. 9. Flexural strength for laminates made from epoxy prepregs of various fiber reinforcement and solvent content. Error bars shown are the standard deviation from five samples.

4. Conclusions In this work, the effects of fibrous reinforcement and solvent content on water absorption/desorption in epoxy composite laminates were investigated. Two types of reinforcement, glass and carbon, and two levels of resin solvent content were used to conduct the study. Water absorption was found to be facilitated by the glass reinforcement and low solvent content, while the desorption was unaffected by either of these variables. Mode I interlaminar fracture toughness was found to decrease upon water absorption for the glass fiber laminates, while the carbon fiber materials showed an increase in GIc under the same conditions. Lower GIc values were also observed for the samples containing no solvent. Mode II interlaminar fracture toughness as well as flexural modulus and flexural strength were observed to decrease upon water absorption, and to recover slightly after water desorption. Collectively, this work brought a better understanding of the reinforcement and solvent content effects on water diffusion and absorption in epoxy composites. The data showed that water uptake was lower in carbon fiber laminates containing higher level of solvent in their resin system. These findings may help to design improved self-adhesive honeycomb prepregs which would prevent water ingression in sensitive airplane parts.

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Acknowledgements The authors express their appreciation to the US Airforce Office of Scientific Research, AFOSR Grant number F49620-97-1-0163, for support at the Polymeric Composites Laboratory.

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