Desiccant Dryer Operation and Basic Moisture Management in Hygroscopic Polymers
Maintaining desiccant dryers at -40°C dew point prevents hydrolytic degradation, preserving melt viscosity and mechanical performance in hygroscopic polymers.
Hygroscopic Polymers
Meaning
Thermoplastics that possess a high affinity for moisture and actively absorb water molecules from their surrounding environment are categorized as moisture-sensitive materials. This group, representing various hygroscopic polymers, includes resins such as polyamide, polycarbonate, and polymethyl methacrylate. The absorbed water occupies the intermolecular spaces within the polymer matrix, acting as a plasticizer and affecting the physical properties of the material.
These polymers must undergo thorough drying prior to melt processing to prevent hydrolytic degradation and related molding defects.
Absorption Mechanism
The polar chemical groups present along the polymer backbone form hydrogen bonds with water molecules from the atmosphere. This chemical attraction drives the diffusion of water into the bulk of the polymer pellets until a state of equilibrium is reached. The rate and amount of moisture absorption depend on the ambient relative humidity, temperature, and the specific chemical structure of the resin.
For instance, polyamides can absorb several percent of their weight in water, whereas non-polar polymers like polyethylene absorb almost none.
Processing Defect
Melt processing of wet hygroscopic resins results in the thermal cleavage of the polymer chains by the trapped water molecules. This process, known as hydrolysis, significantly reduces the molecular weight of the polymer, leading to a loss in mechanical properties like impact strength. It also causes cosmetic defects on the molded part, such as silver streaks and splay marks.
To avoid these issues, processors must dry the resin to very low moisture levels, typically below zero point zero two percent.
Drying Requirement
Effective drying of these resins requires the use of desiccant dryers that can generate process air with a dew point below minus forty degrees Celsius. The dry air must be heated to a specific temperature that is high enough to drive off the moisture but low enough to prevent thermal degradation of the pellets. The required drying time varies depending on the initial moisture content and the specific polymer type, often taking several hours.
Proper drying is a critical step for ensuring the quality and consistency of molded plastic parts.