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.
Intrinsic Viscosity Build
Meaning
Incremental growth of the molecular weight of a polymer through solid-state polymerization increases the melt strength of recycled polyethylene terephthalate. This systematic growth, referred to as intrinsic viscosity build, is essential for transforming recycled flakes into high-grade thermoforming sheet or bottle-to-bottle resin. Without this process, the recycled material would be too brittle and have insufficient melt strength for extrusion.
The progress of the build is monitored by measuring the solution viscosity in a controlled laboratory setting.
Chemical Mechanism
Condensation reactions occurring at elevated temperatures under a vacuum or inert gas flow drive the chain extension of the polymer. As glycol and water are removed from the system, the polymer chains link together, resulting in an intrinsic viscosity build. This reaction must be carefully controlled to prevent thermal degradation of the resin.
Process Parameter
Extended residence times in the solid-state reactor are required to achieve the desired molecular weight distribution. For a successful intrinsic viscosity build, the pellet temperature must be held just below the melting point for several hours while maintaining a dry gas purge. Any moisture in the system will trigger hydrolysis, reversing the reaction.
Quality Threshold
Meeting the precise specification for melt strength ensures that the thermoformer can produce stable parts without sagging. A sufficient intrinsic viscosity build allows the processor to mix a higher percentage of regrind with virgin resin without risking part failure. This capability is essential for meeting circular economy targets.