Meaning
Spontaneous migration of water molecules from an area of high concentration to an area of low concentration within a solid material. In the context of industrial polymers and adhesives, moisture diffusion typically follows Fickian laws where the rate of ingress depends on the temperature and the properties of the barrier. This movement continues until the internal humidity of the material reaches equilibrium with the surrounding environment.
It affects the weight and volume of the finished product. The process is driven by the random thermal motion of molecules rather than external pressure.
Saturation Profile
Time required for water to penetrate the center of a thick part can vary from days to years. Because moisture diffusion is a slow process, the outer layers of a component may be fully saturated while the core remains dry, creating internal stresses. This gradient can cause warping or surface cracking in sensitive materials.
Polymer Interaction
Chemical bonds within a matrix can be weakened by the presence of diffusing water molecules. As moisture diffusion progresses, it can trigger plasticization, which lowers the strength and stiffness of the plastic. This change is often reversible upon drying, but permanent damage occurs if the water causes hydrolysis or leaching of additives.
Barrier Performance
Coatings and sealants are often applied to slow down the rate of water entry. High moisture diffusion in a protective layer indicates that the underlying substrate will be exposed to corrosion or decay sooner than expected. Selecting materials with low permeability is essential for products used in underwater or high humidity applications.
