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.
Desiccant Dryer Operation
Meaning
Industrial processes for removing moisture from hygroscopic polymer pellets using a heated air stream and a moisture-absorbing medium are used before melt processing. This thermal process, known as desiccant dryer operation, utilizes synthetic zeolites or silica gels to absorb water vapor from the circulating air. The dried air is then heated to a specific temperature and blown through the polymer hopper to extract moisture from the resin.
This prevents hydrolytic degradation during subsequent injection moulding or extrusion, preserving the mechanical properties of the finished part.
Process Cycle
Alternating cycles of moisture absorption and thermal regeneration ensure a continuous supply of dry air to the polymer hopper. During the drying phase, the process air passes through one desiccant bed where the water molecules are captured. Concurrently, a second desiccant bed undergoes regeneration, where heated air at high temperatures drives off the trapped moisture.
The system automatically switches flow between the beds when the active desiccant reaches its absorption limit. This dual-bed configuration maintains a constant low dew point throughout the production run.
Material Preservation
Hygroscopic resins such as polycarbonate and nylon require extremely low moisture levels to prevent polymer chain scission during melting. If the residual water content exceeds the specified limit, usually below zero point zero two percent, the polymer chains break down under high heat. This hydrolytic degradation results in reduced impact strength and brittle failure on the molded part surface.
Effective drying ensures that the molecular weight of the resin is maintained during processing.
Energy Efficiency
Optimizing the regeneration temperature and cycle times reduces the electricity consumption of the drying system. Some advanced dryers utilize closed-loop heat recovery to preheat the regenerating air with exhaust heat. This configuration minimizes the energy required to reach the high temperatures needed for desiccant regeneration.
Implementing these energy-saving measures reduces the overall operating cost of the compounding facility.