Meaning
Chemical reaction that increases the molecular weight of a polymer by joining monomers or prepolymers under heat while releasing a small by-product molecule defines this process. Performing thermal polycondensation in the solid state is a common method to upgrade recycled polyethylene terephthalate and polyamide resins. This reaction requires both elevated temperatures and a driving force to remove the volatile by-products.
Chemical Mechanism
Monomer molecules with reactive end groups combine to form linkages. As the thermal polycondensation progresses, by-products such as water are generated. Efficient removal of these molecules is necessary to drive the reaction forward.
Process Variables
Temperature and vacuum level are the primary controls used to regulate the reaction rate. Running the process just below the polymer melting point prevents the polymer granules from sticking together while maintaining a high reaction rate. High vacuum or inert gas purging is used to sweep away the volatile by-products, ensuring that the molecular weight continues to increase to the desired level.
Resin Enhancement
Continuous viscosity increase during this reaction improves the melt strength of the polymer. Virgin and recycled resins undergo this process to make them suitable for extrusion. By-product removal must be complete, because any residual moisture will cause hydrolytic degradation during subsequent moulding steps.
