The effect of salt solution on a composite is very low compared to the effect of other environmental aging conditions. This is because salt molecules are larger than other liquid molecules; hence the diffusion rate is slower. The slow diffusion rate prevents early damage of fiber/matrix interface and the glass fibers. In fact, several researchers have observed a strength gain in composites when exposed to salt solution. Strength gain is mainly due to post curing which improves the properties of matrices in the composites. Strength gain (less than 10%) and stiffness loss (less than10%) were observed for a glass/vinyl ester composite when conditioned in salt
water at room temperature [Vijay, 1998]. The reduction in strength and stiffness is greater when composites in salt solution are exposed at elevated temperature because the rate of diffusion increases at elevated temperatures. The tensile strength loss of about 19% was observed in a glass/epoxy composite conditioned in salt solution at 60oC. From the SEM micrographs, it was concluded that the failure of composites was due to matrix degradation; i.e., cohesive failure [Kajorncheappunngam, 1999]. The degradation in tensile strength (when exposed to salt solution) can be reduced to some extent by adding chopped strand mats in the composites. The ester linkages in the outer layers of the continuous strand mat bind the free water through hydrogen bonds in the surface layers, hence reducing its effect through the thickness. In general, the degradation in strength and stiffness of composites exposed to salt solution is insignificant compared to other aging liquids.
Effect of aqueous solution\acid solution
Acid affects on the composite in terms of reduction of strength and stiffness at room temperature. When composites are exposed to acids, the acid diffuses through the matrix and subsequently reaches the surface of glass fibers. Once the acid contacts the glass fibers, the ion exchange takes place between glass fibers and acid, eventually leading to surface shrinkage. The surface shrinkage causes internal stress within the fibers and initiates cracks in the fibers. Sometimes, the ions in the matrix cause fracturing of matrix, thereby increasing the rate of diffusion and leading to debonding of fibers. The reduction in tensile strength of a glass/epoxy composite exposed to acid solution over a 5-month period was 73% at room temperature, but it was only 48% at elevated temperature [Kajorncheappunngam, 1999]. At elevated temperatures, the effect of acid on strength reduction is low because the ion exchange reaction reduces at elevated temperature, and hence the damage to glass fibers is low compared to room temperature damage.
Durability of composites in an aqueous solution greatly depends on the type of fiber that reinforces the composite. For example, boron free glass fiber shows improved
resistance. ECRGLAS (boron-free glass) laminates were found to have 30% higher flexural strength than E-glass laminates. The aqueous solutions cause dramatic increase in hydrolysis of Kevlar 49 yarn, especially in the combination of temperature and stress. The degradation in such glass fibers can be protected to some extent by choosing the appropriate resin system, application of gel coats and providing appropriate protective coatings. The efficacy of application of gel coats and protective coating has been shown by the marine industry to prevent blistering, jackstraw, matrix degradation and fiber attack [Altizer et al., 1996].
Effect of alkaline solution
Significant loss in strength and stiffness occurs when the composites or neat resins are exposed to alkaline conditions. This loss is attributed to the fact that the rate of moisture absorption in composites is more when exposed to alkaline solution than other liquids. The rate of strength and stiffness loss was about twice in alkaline environment for E-glass/vinyl ester composite over that for glass composite exposed to salt environment [Vijay, 1998]. The alkaline solution mainly attacks at the interface of fiber/matrix debonding. The hydroxide ions in an alkaline environment attack the primary component of glass (silica) and cause the breaking of Si-O-Si bonds in glass fiber. This results in fiber corrosion and reduction in strength. At an elevated temperature alkali has a greater effect on the strength and stiffness of the composites. This is attributed to better matrix cross-linking reaction and becomes brittle. The brittleness leads to matrix cracking, thus reducing the strength and stiffness of the composite. A reduction of about 70% in tensile strength and ultimate strain to failure was observed in a glass/polyester composite (Vijay and GangaRao, 1999). The effect of alkali can be anywhere in the composites, i.e., fibers, matrix or at the interface of fiber/matrix. Hence, durability of composites in alkaline solution can be improved by selecting proper fiber and/or resin. For example, corrosive resistant glass fibers and alkali resistant resins can be used to make composites. Boron free glass fibers (ECR) show improved performance over traditional E-glass fibers because of their improved corrosion resistance. Composite with Advantex glass fibers (boron free glass) had
about 95% tensile strength retention, while those with E-glass had only about 85% retention in the tensile strength Devalpura [1998].
Glass fibers are more sensitive to alkali environment when compared to aramid or carbon fibers. The penetration of alkali into such fibers is mostly time dependent. For GFRP bars, the penetration of alkali increases with time. The glass fibers deteriorate in the area where alkali has penetrated, which reduces strength of the bars. The strength of GFRP bars decreased with time when immersed in alkali solution, whereas those of AFRP and CFRP bars were not decreased [Katsuki, 1995]. Higher alkali concentration increases the degradation on composites. GFRP bars immersed in 5gm/L of sodium hydroxide (over a 4 month period) had about 20% reduction in strength while those in 20 gm/L of sodium hydroxide had about 30% reduction in strength [Alsayed, 1998].
With respect to performance of resin in the alkaline solution, vinyl ester has better alkaline resistance compared to other resins and exhibits excellent strength and stiffness properties. The increased distance between cross-linkages in vinyl ester implies that it does not completely polymerize. Performance characteristics of vinyl ester changes with cure time. The ultimate tensile strength of vinyl ester samples after 1300 hours of immersion in alkaline solution was about 70 MPa while that of isopolyester was only about 50 MPa. The pH level of an alkaline solution is another important factor, which acts as a catalyst in degrading the glass fibers. Cementitious extract with pH 10 buffer and water had the greatest degradation in composite strength. This is hypothesized due to greater concentration of Ca ions available for formation of calcium hydroxide crystals at the surface of glass fibers. The effect of alkali solution on composite materials can be potentially reduced by fiber sizings (to promote fiber/resin debonding) and by selecting alkali resistant resins and corrosive resistant fibers [Altizer et al., 1996].