Meaning
Environmental uptake of atmospheric water vapor into raw polymer pellets affects physical properties and processing stability during melt processing. Moisture absorption causes hydrolytic degradation in hygroscopic resins such as PET and nylon when heated inside an extruder barrel. The metric stops applying to non-hygroscopic polymers such as pure polyolefins that do not absorb water into their molecular structure.
Hygroscopic Expansion
Water molecules diffuse into amorphous regions of resin pellets during storage in humid ambient warehouse environments. Absorbed moisture causes polymer chains to expand slightly, altering pellet bulk density and hopper flow characteristics. Processing damp resin creates steam bubbles inside the extruder barrel, leading to voids and surface blemishes in extruded parts.
Drying Requirement
Desiccant dryers remove moisture from hygroscopic resin down to specified parts-per-million thresholds prior to extrusion. Moisture meters test dried resin samples to ensure moisture content remains below maximum allowable processing limits. Inadequate drying schedules result in severe loss of mechanical properties in molded or extruded products.
Polymer Degradation
Thermal processing of wet hygroscopic resin breaks polymer chains through hydrolysis, reducing molecular weight and melt viscosity. Lower melt viscosity leads to web sagging, parison droop and erratic dimensional control during extrusion. Supplier material data sheets specify maximum moisture limits, but actual plant humidity often accelerates moisture absorption beyond assumed rates.
Testing resin moisture only at receiving fails to detect moisture regain that occurs while pellets reside in unheated machine hoppers. Calling resin dry based on drying time alone without verifying dew point performance risks catastrophic batch failure during production.
