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.
Vacuum Stripping Pressure
Meaning
Operational pressure parameters measure the level of vacuum maintained in a degassing chamber or extruder vent to facilitate the removal of volatile species. The vacuum stripping pressure determines the boiling point and vapor pressure of residual monomers, solvents, and moisture within the polymer melt. This pressure is controlled to draw out unwanted gaseous impurities without suctioning the polymer itself into the vacuum lines.
Thermodynamic Balance
Lowering the pressure increases the driving force for volatilization from the molten polymer. In venting zones, the vacuum stripping pressure must be kept below the vapor pressure of the targeted contaminants to ensure efficient extraction. If the pressure is too high, the decontamination rate drops, which leaves residual volatiles that can cause splay or voids in the moulded parts.
Degasification Rate
The rate of gas extraction is directly proportional to the pressure gradient established between the polymer melt and the vent throat. Under high vacuum, volatile organic compounds migrate rapidly to the surface and escape into the exhaust system.
Physical Integrity
If the vacuum is too deep, the foaming of the polymer melt can lead to entrainment and vent flow-backs. This disruption clogs the equipment and leads to costly downtime for cleaning.