Thermoplastic Polymer Moisture Absorption Mechanics during Storage
Polymer moisture absorption follows Fickian kinetics, requiring hermetic packaging, Karl Fischer verification, and desiccant drying to prevent hydrolytic cleavage.
Secondary Bound Water
Meaning
Weakly held moisture within a polymer matrix resides in the spaces between polymer chains without forming direct primary chemical bonds. In secondary bound water, these molecules are held by weak dispersion forces. This moisture is easier to remove during the drying process than primary bound water.
Chemical Association
Physical separation of the moisture types is based on the strength of their interaction with the polymer chains. While primary bound water binds directly to the polar amide or ester groups, the secondary molecules occupy the free volume of the polymer. This layer of moisture can migrate through the polymer matrix with a lower activation energy, which allows it to evaporate more quickly when heated in a desiccant drying hopper.
Temperature Threshold
Removing this weakly bound moisture occurs at lower temperatures than those required for primary bound water. However, the drying system must still provide sufficient thermal energy to drive the water molecules to the surface of the polymer pellet. Inadequate drying temperatures will leave this moisture behind, which can cause surface defects and hydrolysis during subsequent processing.
Processing Hazard
Molders who run fast cycles with under-dried materials risk releasing this residual moisture during the melting phase. The water vaporizes under the high temperatures and pressures of the injection barrel, which leads to splay or internal voids in the molded parts. Ensuring complete removal of this moisture fraction is essential for maintaining the cosmetic and structural quality of the product.