Meaning
The migration and evacuation of absorbed water molecules from a solid polymer matrix during desiccant heating constitutes a mass transfer process. Hygroscopic resins such as polyamide and polycarbonate absorb ambient moisture into their molecular structures during storage. Hydrolytic desorption occurs inside hopper dryers when heated dry air passes over wet plastic pellets to strip bound moisture.
Unremoved water molecules break polymer backbone chains under high heat during injection processing, severe thermal degradation that drops mechanical impact strength.
Drying Profile
Moisture diffusion speed depends on temperature gradients and air dew point inside the hopper. Dry air flowing across resin surfaces establishes a vapor pressure differential that draws trapped water outward. Effective hydrolytic desorption requires maintaining air dew points below minus forty degrees Celsius throughout the drying cycle.
Insufficient residence time leaves internal core moisture intact despite surface dryness.
Viscosity Impact
Excess moisture during melt processing causes hydrolytic chain scission in condensation polymers. Melt flow index increases rapidly when water cleaves polymer chains, lowering melt viscosity inside the injection barrel.
Regrind Quality
Reprocessed sprues and runners absorb moisture faster than virgin resin due to surface micro-cracks. Inadequate hydrolytic desorption before processing regrind blends yields brittle moulded parts with visible splay surface flaws.
