Gray and/or yellow discoloration may occur during repeated heating of poly(ethylene terephthalate) (PET). Both phenomena can cause problems in further application. In this work, the reasons for the discoloration of PET during reprocessing are investigated by physical and chemical analysis such as colorimetry, size exclusion chromatography, viscosimetry (ηinh.), inductively coupled plasma mass spectrometry, X‐ray photoelectron spectroscopy, and MALDI‐ToF‐MS analysis. It is found that the antimony content which originates from catalyst residues used in PET synthesis has high influence on the gray discoloration obtained during reprocessing of PET. Antimony ions are reduced to elementary antimony during heating to temperatures above 230 °C as proven by X‐ray photoelectron spectroscopy. The yellow discoloration is partially generated by polyamide contaminants which are used as barrier layers in PET packaging materials such as soft drink bottles. In conclusion, to prevent discoloration of postconsumer PET during reprocessing different methods such as oxidation of gray metallic antimony or sorting out of polyamide contaminants are needed.
New masterbatches for poly(ethylene terephthalate) (PET) which contain chain extenders such as 1,3‐phenylene‐bis‐oxazoline (1,3‐PBO) and/or N,N’‐carbonylbiscaprolactam (CBC) were developed. Here, cyclic poly(butylene terephthalate) (cPBT) which has no reactive end groups was used as masterbatch matrix. The main advantage of these masterbatches is their polyester‐based matrix, but the incorporated chain extenders are still active after the synthesis. The masterbatches were analyzed by TGA/FT‐IR, SEC, and 1H‐NMR analysis. They were applied to poly(ethylene terephthalate) during extrusion and, afterwards, also during spinning at high velocities on a pilot plant spinning device. The influence of these masterbatches on the quality of PET was analyzed by measurement of the screw force during extrusion, viscosimetry, SEC, and rheometry of the extruded compounds. The fibers spun on the pilot plant were analyzed by viscosimetry and by tensile strength measurements. The results show that the quality of polyester fibers spun from postconsumer PET can be improved by applying the developed masterbatches which contain chain extenders during spinning.
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