The cause of the grey discoloration of PET prepared by the use of antimony‐catalysts

Heating three antimony catalysts with several glycols and other model compounds in open and sealed vessels with various degrees of oxygen availability was followed by analyses of the volatile products in the head‐space and of the nonvolatile products. Bis(hydroxyethylene terephthalate)(Bis‐HET) and terephthalic acid (TA) plus ethylene glycol (EG) were separately polymerized to poly(ethylene terephthalate)(PET) and TA plus triethylene glycol (TEG) was polymerized to poly(triethyleneglycol terephthalate) (PTEGT). Analyses of the polymers and of the model compounds revealed that: (1) Free glycols or the glycolate moieties in polyesters degrade upon heating in the presence of oxygen to produce equal amounts of CO and CO2. (2) In the presence of Sb(III) species, the CO oxidizes to CO2 at elevated temperatures with the concomitant reduction of Sb(III) to very fine black particles of the elemental Sb(O). This is the cause for the grey discoloration of PET made with Sb‐catalysts. The redox reaction starts at about 200°C and is fully dominant at about 235°C and above. (3) At temperatures lower than ˜200°C, the Sb(III) usually converts to the white Sb2O3 and the CO does not oxidize the CO2.