Moisture absorption Fickian diffusion

Whereas the thermodynamic properties of the resin and environment will determine the maximum extent of moisture absorption, diffusion is a kinetic property which is temperature dependent. Normally, the latter is considered to be Fickian in nature. For highly polar polymers, other diffusion models need to be considered, but for most structural composite materials, the Fickian laws are applicable. 

While many resin systems exhibit Fickian diffusion, others show non-Fickian behaviour. The experimental reabsorption diffusion curve shown in Figure 12.2 represents a smooth process which is linear over the hrst 60% before reaching equilibrium. For the resin system used, Fickian diffusion could only be obtained after preconditioning because often the early stage of moisture absorption deviates from Fickian kinetics. This creates difficulties in the calculation of a precise diffusion constant. It is thought that the non-Fickian behaviour results from the reorganisation of the network as the material becomes plasticised [9—11]. 

Figure 3.1 Typical Fickian moisture diffusion curve for an equilibrated resin casting of a bismaleimide modified epoxy resin which had previously been subject to thermally enhanced moisture absorption, followed by desorption prior to reabsorption. The sorption shown does not begin at zero because of a small residual quantity of absorbed moisture from the first conditioning cycle (4).

Figure 3.1 shows an example of a Fickian diffusion curve [4]. In this case, the resin had been previously conditioned and subjected to moisture absorption at 50°C and 96% RH, followed by desorption in vacuum at 50°C, prior to resorption [4]. One-dimensional diffusion can therefore be described by the following characteristics of the M, versus curve ... 

Figure 3.2 gives an example [4] of a moisture absorption curve for an advanced epoxy resin and the theoretical curve obtained from equation [3.4], This illustrates the difhculties of assessing the diffusion coefficient when deviations from Fickian behaviour occur and an equihbrium moisture... 

Another consideration in modeling the uptake of moisture by adhesives is that the uptake behavior can be different in absorption and desorption and can change with number of sorption cycles. This is illustrated in Fig. 31.13b. It can be seen that the rate of moisture absorption and the equilibrium moisture content increase from the first to second absorption cycle, although there is little difference between the second and third absorption cycles. Desorption is faster than absorption and more closely resembles Fickian diffusion. Mubashar et al. (2009b) proposed a method of incorporating the moisture history effects illustrated in Fig. 31.13b in a finite element-based predictive methodology. 

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