Meaning
Polymerization reactions carried out below the melting point of the polymer allow the molecular weight of the resin to be increased without thermal degradation. Initiating solid state transesterification involves heating polyester chips in an inert gas stream, promoting ester exchange reactions and removing volatile byproducts. This process governs the production of high-viscosity resins for bottle or fiber applications where melt polymerization would cause excessive degradation or discoloration.
It is limited by the crystallinity of the starting polymer, as the reaction occurs solely in the amorphous regions.
Resin Synthesis
Polyethylene terephthalate requires a high molecular weight to achieve sufficient melt strength for blow molding. Heating the crystallized resin chips under vacuum drive the reaction forward by continuously extracting ethylene glycol. This technique produces high-grade packaging resins.
Viscosity Enhancement
Extrusion of heavy-gauge sheet requires high melt viscosity to prevent sheet sag. Increasing the molecular weight through solid-state reactions improves the melt strength of the recycled resin. This enhancement allows the use of regrind material in demanding applications.
Degradation Prevention
High temperatures in melt polymerization cause side reactions that generate acetaldehyde and yellow color. Solid-state processing occurs at lower temperatures, preserving polymer purity and optical clarity. This preservation is critical for food-grade beverage containers.
