The effect of moisture on composite materials occurs when the composite materials are exposed to humid air, water or any liquid. Depending on the
or loses moisture as manifested by weight gain or weight loss [Springer, 1981]. Moisture absorption and desorption in a composite material eventually leads to the degradation in its properties. Moisture affects the composite materials at the matrix or the fiber level, and even at the interface of fiber/matrix level.
Effect of moisture in resins
Moisture content in resins depends on various factors such as type of resins, fillers, additives, nanomers, type of liquid to which the resin is exposed to, temperature, etc. First, moisture penetrates through the resin and later moisture is transferred through the cracks [Springer, 1981]. Neat resins show higher moisture absorption than the composites show, because in the resins the matrix swells when exposed to moisture. The swelling of the matrix causes stress within the material, which eventually tends to decrease the strength and stiffness. However, many resin systems tend to recover their properties upon drying. Further, the percentage of moisture absorption varies from resin to resin based on their chemical structures. Chin [1999] observed the moisture uptake for vinyl ester and isopolyester resin exposed to distilled water, salt water and concrete pore solution at 22oC and 60oC. For vinyl ester (at both ambient temperature and 60oC), salt-water uptake was higher than pure aqueous or alkali solution uptake. Similar results were observed in the case of isopolyester at ambient temperature, but at 60oC, mass loss occurred after a certain period in alkaline solution and salt water. This is attributed to the fact that in a polyester resin, ester groups are distributed along the main chain, making them more available to hydrolysis reactions at higher temperatures. The structure and morphology of a resin affect the moisture uptake. In general, a high concentration of polar functional groups can promote increased sorption of polar penetrants [Apicella et al., 1982]. Among epoxy, vinlyester, and isopolyester resins, the sorption was greater for epoxy compared to the other two resins because more hydroxyl groups are present in epoxy matrix. Most of the resins or composites follow the Fickian process, and a non-Fickian behavior occurs when the resins or composites undergo damages such as cracks. Diffusion in all three liquids (water, salt, alkaline) and in three resins (epoxy, vinyl ester and isopolyester) followed the Fickian process [Chin, 1996].
Effect of moisture in glass fibers
Studies conducted by Ehrestein and Spaude [1984] showed that both glass fiber and glass fiber-resin bond are susceptible to degradation through moisture content. Glass fibers can lose up to 10% of their bending strength when exposed to moisture. While resins recover their lost properties while drying, glass fibers do not recover their properties but tend to corrode, eventually leading to loss of effective cross-section. Results from a companion study of Karbhari et al. [1998] show that water accelerates the rate of crack growth in glass with the degradation being more severe and following different mechanisms with higher temperature exposures. Acceleration is thought to be the result of two factors: 1) reduction in surface energy of glass fibers after exposure to moisture that reduces the energy required for interfacial crack formation, or debonding and 2) reduction in energy required to break the Si-O bonds. The strength of E-Glass is time dependent in the presence of moisture and is susceptible to stress corrosion. Stress corrosion in turn is dependent on the type of attacking fluid, wherein more concentration of corrosive fluid leads to greater detriment to glass fibers.
Effect of moisture in composites
Moisture content in composites depends on the type of composites and environmental conditions that the composite is exposed to over a certain period and range of temperature. When composites with polymer matrix are placed in a wet environment, the matrix begins to absorb moisture. The moisture absorption of most fibers used in practice is negligible; however aramid fibers alone absorb significant amount of moisture when exposed to high humidity [MIL-HDBK-17, 1997]. The effect of temperature and moisture on Kevlar/epoxy laminates was studied by Allred [1984], who showed that in the saturated state, the room temperature flexural strength of the laminates decreased by 40% over the dry state. Similarly, at an elevated temperature, the strength drop was found to be in the range of 60 – 70%.
When composites are exposed to humid air, the moisture content depends on the relative humidity in the air. When composites are exposed to acidic or alkaline
moisture content in the Graphite/epoxy composite was found to be lower in salt solution (1.25%) than in distilled water [Springer, 1981]. A similar trend was noted in the E- glass/vinyl ester composite with maximum moisture content of 0.18 ~ 0.29% in the salt solution compared to the moisture content of 0.22 ~ 0.33% in the tap water [Vijay, 1998]. The maximum moisture content was nearly twice in the alkaline solution compared to tap or salt water. This hypothesizes that the degradation of the material has started at the interface of the fiber/matrix level. In the case of E-glass /polyester composites, the interface tends to become more hydrophilic when exposed to moisture and the following is noted: 1) The fibers weaken due to crack growth that is accelerated by water in the resin; 2) The resin swelling produces radial stresses at interface that is reinforced by water pressure, and leads to fiber debonding and consequent weakening of composite; 3) plasticization of resin by water results in increase in viscoelasticity. When a composite absorbs moisture, the swelling coefficient of fiber is lower than the matrix. Free swelling of layers does not take place and, consequently, internal stresses are developed. These internal stresses can be calculated as in MIL-HDBK-17 [1997].