Moisture Plasticization

Viscoelastic Change

The intake of water increases the free volume within the material, allowing the polymer to deform more easily under load. During moisture plasticization, the storage modulus of an adhesive may drop substantially, changing its performance in a structural joint. This change is particularly critical in microelectronics, where the softening of an encapsulant can alter the stress distribution on a delicate chip.

Metrology tools like dynamic mechanical analysis track these changes by measuring the properties of samples at different humidity levels.

Diffusion Rate

Water diffusion speed depends on the temperature and the ambient humidity. Higher temperatures accelerate the diffusion process, making moisture plasticization occur more rapidly in tropical or industrial environments. Sensors are often used to monitor the weight gain of a part to determine the saturation level.

Designers must account for this phenomenon when setting the operating limits for a device. If the glass transition temperature drops below the operating temperature, the material may transition from a hard plastic to a soft rubber, leading to a mechanical failure.

Saturation Boundary

Equilibrium is reached when the rate of water molecules entering the polymer equals the rate of those leaving. This state represents the maximum impact the moisture will have on the material properties under those specific conditions.