Meaning
Thermodynamic limits governing the concentration of volatile species in a polymer melt determine the driving force for devolatilization. The equilibrium partial pressure of a volatile byproduct represents the pressure at which the byproduct ceases to migrate out of the polymer. This value depends strongly on melt temperature and the concentration of the volatile substance.
Removal Kinetics
Gas-phase stripping or vacuum application reduces the ambient gas pressure to accelerate byproduct extraction. When the system pressure is lower than the equilibrium partial pressure, volatile byproducts evaporate from the reaction mixture. This pressure difference is the primary force for polycondensation or solid-state polymerization.
A larger difference results in faster chain growth.
Processing Variable
Controlling melt temperature and vacuum level allows operators to optimize polymerization throughput. Because the equilibrium partial pressure increases with temperature, higher processing temperatures can accelerate the removal of byproducts. However, this temperature must be kept below the thermal degradation limit of the polymer to prevent discolored or brittle resin.
Moisture Control
Drying operations before extrusion prevent hydrolytic degradation of sensitive polymer chains. In the drying of polyesters, the equilibrium partial pressure of water determines the minimum moisture level achievable before processing. If the dryer cannot reduce the water vapor pressure below this thermodynamic threshold, the resin will undergo molecular weight loss in the extruder.
This hydrolytic degradation reduces the impact strength of molded products. Inadequate control of this variable leads to degraded product performance in the final application.
