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.
Nitrogen Carrier Dew Point
Meaning
Gas purity specifications measure the temperature at which water vapor in a nitrogen stream begins to condense into liquid. In high-temperature drying systems, the nitrogen carrier dew point dictates the limit of moisture removal achievable for moisture-sensitive polymers. This parameter must be maintained at extremely low levels, typically below minus forty degrees Celsius, to prevent hydrolysis during the subsequent melting phase.
Moisture Capacity
The ability of the gas to absorb moisture from the resin surface increases as the dew point of the entering stream drops. A low nitrogen carrier dew point creates a high concentration gradient between the wet polymer pellet and the dry carrier gas. This high gradient is necessary to drive the moisture out of the pellet core within the available residence time in the hopper.
Resin Protection
Using nitrogen instead of air prevents oxidative degradation of the polymer at elevated drying temperatures. If the nitrogen carrier dew point rises, the drying efficiency drops, which introduces both moisture and the risk of polymer degradation into the molding run.
Quality Output
Consistent control of this gas property ensures that the injection-molded parts maintain their molecular weight and impact resistance. Fluctuations in dryness can lead to brittle failure in the finished products.