Moisture conditioning

Polymeric composites absorb moisture and this may have a deleterious effect. Hence, evaluation of material should be carried out after relevant moisture exposure representing the worst case. The degree of moisture in a composite laminate will depend on the thickness, material, relative humidity and temperature. Hence the conditioning procedure should account for the diffusion process and terminate with the moisture content nearly uniform through the thickness.

Moisture diffusivity and moisture equilibrium content are obtained using gravimetric tests (such as ASTM D 5229) that exposes an initially dry specimen to a humid environment and documents moisture mass gain versus the square-root of time. The diffusion process may be accelerated by increasing the exposure temperature. During early exposure, the increase in mass will be linear with root time and this linear relationship gives the diffusion rate or moisture diffusivity. In time, the mass uptake will reach equilibrium or saturation and the specimen will gain no more significant weight. The weight percent mass gain at this point is the equilibrium moisture content. If the specimens cannot be tested shortly after conditioning, they should be placed in sealed bags to prevent moisture escape.