Meaning
Chemical compounds belonging to the group of metal oxides or glycolates act as transition-state stabilizers during the high-temperature synthesis of polyester resins. An antimony catalyst accelerates the final transesterification step to build molecular weight quickly. This reaction occurs under high vacuum to remove ethylene glycol.
The catalytic effect is active during polycondensation but remains present in the solidified resin.
Catalyst Activity
High reaction temperatures in polycondensation reactors drive the conversion efficiency of the metal compound. Although other metals can be utilized, the antimony catalyst offers the most favorable balance between reaction rate and polymer stability. It remains dormant during solid-state polymerization but becomes active again if the melt temperature exceeds the safe processing window.
Degradation Control
Thermal limits during extrusion and blow moulding determine the final color profile of the polymer matrix. Excessive dwell times at high temperatures in the presence of an antimony catalyst trigger secondary reactions that cause grey or yellow discoloration. This catalytic degradation can be suppressed by adding phosphoric acid or phosphite stabilizers during polymer synthesis.
Moulders trace these yellowing defects to degraded batch chemistry rather than mechanical shear. Proper stabilizer addition prevents this catalyst from initiating thermal degradation during subsequent moulding cycles.
Polymer Specification
Analytical testing on incoming resin batches verifies the heavy metal concentration before melt processing begins. While alternative metal systems are available for specialty applications, the antimony catalyst remains the industry standard for bottling-grade polyester due to its reliability during processing.
