Determination of the moisture uptake mechanism in closed cell polymeric structural foam during hygrothermal exposure

Polymer composite sandwich construction is increasingly used in the fabrication of marine craft where high specific strength and stiffness are required. While the moisture uptake behaviour of thermoset resins and their fibre-reinforced composites has been the focus of much research, the mechanisms of moisture absorption in foam core materials have received little attention. This paper presents a programme of work carried out to determine the moisture uptake mechanisms in closed cell PVC foam under fresh-water immersion. A comprehensive understanding of the ageing mechanism for the foam has been proposed. A multi-stage process was observed, which exhibited an initial mass increase that was linear with respect to the square root of time, followed by an apparent saturation plateau, subsequently followed by further increases and plateaus. While diffusion into the polymer cell wall network was found to be the primary mechanism, passage of moisture into cell cavities was also observed. The multi-stage process was attributed to diffusion, influenced by internal stress relaxation and the geometry of the local cell structure. A Fickian one-dimensional diffusion model was found to fit the initial part of the experimental results adequately.