THE APPLICATION OF SIZE-EXCLUSION CHROMATOGRAPHY TO STUDY MOLECULAR-WEIGHT CHANGES OF FLAME-RETARDANT FIBER-REINFORCED POLYESTERS

Reinforced plastics based on poly(cyclohexanedimethylene terephthalate) (PCT) are excellent for electrical and electronic applications particularly in the manufacture of electrical connectors. PCT offers a high heat deflection temperature, low cost, and relative ease of processing. For the injection molding process, stability of the melt is an important consideration, especially for materials with high melting points such as PCT. The combination of the polyester resin with flame retardant additives, processing aids, and thermal stabilizers results in a number of competing reactions which can change the molecular weight and molecular-weight distribution (MWD) of the base polymer in the composite. Typical analytical techniques such as melt or dilute solution viscosity do not give adequate means of monitoring these changes so as to allow the polymer chemist to determine the effects of various additives on MWD. Size-exclusion chromatography (SEC), by virtue of providing information on the entire MWD, was found suitable to study molecular-weight changes in the melt due to both branching and chain cleavage, even when both phenomena occur simultaneously. Changes in the MWD over time at processing temperatures can be used to determine kinetic parameters and have been used to optimize PCT additive formulations for best processability and mechanical property retention.