Degradation Pathway
High temperature processing of synthetic polymers can lead to the direct cleavage of covalent bonds within the polymer backbone. This breakdown process, known as thermal scission, occurs when the heat energy exceeds the bond energy of the carbon oxygen and carbon carbon bonds in the polyester chain. The reaction results in shorter polymer chains, which lowers the overall molecular weight of the material.
Structural Damage
Cleavage of the molecular chain generates carboxyl end groups and vinyl ester terminals that further degrade the polymer. These reactive end groups accelerate subsequent breakdown reactions, reducing the polymer’s stability during melt spinning. The damage is irreversible, meaning that once the chains are broken, the polymer cannot regain its original strength without expensive reprocessing.
Process Variable
Moisture content and temperature are the main factors that influence the rate of chain cleavage. Running the extruder at temperatures above the melting point of the resin increases the frequency of thermal scission events. Mills monitor the melt temperature closely to prevent the polymer from degradation during the recycling process.
Mitigation Protocol
Advanced recycling lines use vacuum systems and solid state polymerization to rebuild the damaged polymer chains. This restoration increases the molecular weight and restores the tensile properties required for high quality yarn. Careful control of the extrusion temperature represents the first line of defense against molecular breakdown.
Mill operators also dry the resin pellets to extremely low moisture levels before processing to eliminate hydrolytic reactions that would otherwise worsen the thermal degradation. Dry pellets processed at optimized temperatures ensure that the final fiber retains its structural integrity.
