Bio Based Grades Priced against the Composting Route They Need
Matching resin melt stability and wall thickness to regional waste facility residence times prevents unexpected landed part cost escalation.
Transesterification
Meaning
Chemical reaction pathways involving the exchange of organic groups between an ester and an alcohol can alter the molecular structure and properties of polyester resins during melt processing. In polymers like polyethylene terephthalate and polycarbonate, transesterification can occur at high temperatures, especially when blending different polyester resins. This reaction is often driven by residual catalysts from the polymerization process and can lead to polymer degradation or the formation of random copolymers.
Reaction Mechanism
When two different polyesters are melted together in an extruder, the ester linkages on the polymer chains can react with the terminal hydroxyl groups of adjacent chains. This exchange of ester groups rearranges the block structures of the polymer blend into a random copolymer, which can alter the physical properties of the material. This reaction pathway is accelerated by heat and shear.
This chemical rearrangement can lead to a reduction in crystallinity and a shift in the glass transition temperature of the blended polymer.
Processing Impact
In polymer blending and compounding, uncontrolled transesterification can lead to a reduction in the melt strength of the resin, making it difficult to extrude or mold. The loss of molecular weight that occurs during the reaction reduces the impact strength and tensile properties of the molded parts. This degradation is a major concern when processing recycled PET that has been contaminated with other polyesters.
Mitigation Method
Adding chemical stabilizers or catalyst quenchers to the polymer melt during compounding can deactivate the residual catalysts and suppress these exchange reactions. These additives block the active catalyst sites, ensuring that the blend maintains its phase structure and molecular weight during subsequent molding cycles. This stabilization is necessary to produce high-performance parts from recycled or blended polyester feedstocks.
Recycled Polyethylene Terephthalate Moisture Testing Methods
Accurate moisture testing below 50 ppm via Karl Fischer oven titration prevents hydrolytic intrinsic viscosity degradation in recycled PET during melt processing.