An investigation of the long‐term water uptake behavior and mechanisms of carbon fiber/977‐2 epoxy composites

Abstract

The long-term water absorption behavior of UD and 645 carbon fiber/977-2 epoxy composite and unreinforced 977-2 epoxy resin has been studied at different environments with various temperatures and relative humidities. Fickian behavior was noticed initially followed by a slight and continuing increase in saturation with time, a behavior that continues for at least 4 years. Fickian saturation and diffusion coefficients of all materials were derived using graphical method. Initial Fickian equilibrium moisture contents were found not to be dependent on temperature but the long-term increase in solubility increased with temperature. Absorption rates showed strong dependency on temperature and relative humidity. The activation energy values for all materials in the initial stage found to be much higher (40 kJ/mole) than the long-term stage (10 kJ/mole). E a values of the neat resin were observed to be slightly lower than the composites. The slow continued weight gain in the long-term absorption experiments has been seen to be coupled with an incomplete drying and a higher diffusion rate on drying and subsequent re-conditioning. The possible reasons and mechanisms for the long-term gradual increase in moisture absorption was further investigated. It was found that molecular relaxation is the most likely dominant mechanism. POLYM. ENG. SCI., 00:000-000, 2018. V C 2018 Society of Plastics Engineers Water UptakeThe samples were dried in an oven at 708C and were weighed periodically until the sample weights stabilized. Specimens were then immersed in distilled water at 238C, 408C, 608C, 708C, and 808C. For humidity experiments, other samples were exposed to moisture uptake at different relative humidity 45%, 60%, 85%, 100% at 708C. The percentage of moisture weight gain versus time was monitored by gravimetric methods for all specimens until the specimens approached effective moisture equilibrium. Two to three specimens of each material type were conditioned to get averaged results.

An Assessment of the Effect of Progressive Water Absorption on the Interlaminar Strength of Unidirectional Carbon/Epoxy Composites Using Acoustic Emission

“…However, in small structures (small test specimens), there are well-established standards that allow quantification of moisture in composite samples using gravimetric methods to monitor mass change [ 5 ]. This is then followed by mechanical testing to determine the influence of moisture absorption on the mechanical properties [ 6 , 7 , 8 ] and therefore allows better understanding of the behaviour of the materials at certain moisture absorption levels, leading to safer structural designs. The absorbed water can cause reversible and/or irreversible changes in the physical and chemical properties of the material [ 7 , 9 , 10 , 11 , 12 , 13 , 14 ] because polymers in composite materials chemically react with water since epoxy resins have the ability to absorb substantial amounts of moisture.…”

Development of Novel Thin-Ply Technology for Carbon Fibre Composite Materials

“…Water absorption can occur via diffusion and capillary action [29,44,79]. The process is dependent on many variables, such as: fibre volume fraction [86], fibre orientation [87], material thickness [88], resin type [89], temperature and humidity [86]. Both the amount of moisture absorption (maximum saturation)…”

“…have been shown to be dependent on temperature [86,90]. A possible mechanism for this increased moisture absorption this is that the free volume of the resin increases with temperature, providing more volume for the water to diffuse into.…”

“…In addition, a high temperature environment can cause cracking within the matrix, enabling more water to diffuse inward [86]. Maximum moisture content has also been shown to be strongly dependent on relative humidity, with increasing humidity leading to increased moisture content [86].…”

“…Saturation level is also dependent on fibre volume fraction [86]. This is because the polymer matrix is more susceptible to moisture ingress than the reinforcement [82,86], thus larger volumes of resin allow for more moisture to be absorbed.…”