Solid State Polymerization Volatile Stripping Mass Transfer Limits in rPET
Solid state polymerization stripping rate is dictated by internal Fickian diffusion limits of condensation by-products and volatiles within rPET pellet cores.
Desiccant Nitrogen Drying
Meaning
Industrial conditioning processes use a dry inert gas to extract moisture from hygroscopic polymer resins before extrusion or injection molding. Implementing desiccant nitrogen drying is necessary for moisture-sensitive engineering plastics such as polycarbonate and polyethylene terephthalate. This process prevents oxidation and hydrolytic degradation by replacing ambient air with heated, ultra-dry nitrogen gas during the drying cycle.
Equipment Operation
The operational cycle of desiccant nitrogen drying utilizes a closed-loop system where nitrogen passes through a desiccant bed to achieve a dew point below minus forty degrees Celsius. The dry gas is then heated and circulated through the polymer hopper, where it absorbs moisture from the resin pellets. After exiting the hopper, the gas is cooled and passed through a regeneration bed to strip the absorbed water before re-entering the cycle.
This continuous recirculation ensures that the polymer maintains an extremely low moisture content without being exposed to atmospheric oxygen.
Material Moisture
Polymers with high sensitivity to oxygen at elevated temperatures benefit from desiccant nitrogen drying by avoiding yellowing and thermal degradation. Traditional hot air dryers can cause oxidation, which generates gel particles and reduces the optical clarity of the molded parts. By using an inert nitrogen atmosphere, high-performance resins are protected from thermal oxidation, ensuring that the regrind can be reused without sacrificing mechanical strength.
Operational Efficiency
While the initial capital expenditure for nitrogen drying is higher than that of standard compressed air systems, the reduction in scrap rates offset this cost. Consistent drying maintains stable melt viscosity, which ensures repeatable cycle times and precise part dimensions.