Post-consumer Poly(ethylene terephthalate) - properties, problems during reprocessing, and modification by reactive extrusion

• Post Consumer Polyethyleneterephthalat - Eigenschaften, Probleme während des Wiederaufbereiten und Modifizierung durch Reaktivextrusion

Berg, Dennis; Möller, Martin (Thesis advisor); Pich, Andrij (Thesis advisor)

Aachen (2018, 2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

In this thesis, new methods to improve the properties of discolored post-consumer poly(ethylene terephthalate) (PET) during recycling are presented. Especially for the fiber production, the use of 100 % post-consumer PET is challenging because of its discoloration and numerous further problems (e.g. low inherent viscosity). In the fiber industry, white polyesters are indispensable for various products, which leads to the application of mainly virgin PET. Here, new approaches were used, to improve the color of post-consumer PET and to increase its molecular weight, afterwards. Furthermore, innovative masterbatches containing chain extenders based on a polyester matrix were synthesized. Finally, application experiments were performed in reactive extrusion tests in small lab scale extruders and, furthermore, on a pilot plant spinning device. Firstly, the cause of the gray discoloration of post-consumer PET during reprocessing was investigated. The analysis of post-consumer PET materials with the help of various analytical methods revealed that, basically, the antimony (Sb) content which originates from catalyst residues used in PET synthesis has high influence on the gray discoloration obtained during reprocessing of PET due to reduction of Sb3+ to elementary, black-colored Sb during heating at high temperatures during melting of PET. Thereupon, it was focused on the color improvement of discolored post-consumer PET by bleaching. The bleaching of gray post-consumer PET was performed during reactive extrusion by addition of microscopic zinc peroxide (ZnO2) particles to oxidize the metallic antimony in PET. The bleaching of discolored PET materials was successfully achieved by the addition of small amounts of ZnO2 (0.1 - 0.2 wt%) to the melt due to its oxygen release. To compensate an oxidative degradation of PET due to the bleaching with ZnO2, chain extenders were applied during reactive extrusion. Therefore, the impact of chain extended PET with 1,3-phenylene-bis-oxazoline (1,3-PBO) and N,N’-carbonylbiscaprolactam (CBC) on its properties was studied intensively. An improvement of the properties of PET can be obtained by the addition of small amounts of these chain extenders without any significant negative effect on the properties of the polymer such as crystallinity or rheology. Finally, novel masterbatches containing 1,3-PBO or CBC in a non-reactive matrix were synthesized for pilot plant experiments. As non-reactive matrix a cyclic poly(butylene terephthalate) oligomer which has no reactive end groups and which can be polymerized in the presence of catalysts was selected. The spinnability of post-consumer PET with the addition of the developed masterbatches was proven by spinning trials on a pilot plant. The addition of these masterbatches led to an increase of the mechanical properties of polyester fibers. Finally, these spinning tests were also performed with post-consumer PET with addition of a combination of ZnO2 and the chain extender masterbatches. It was successfully shown that fibers can be spun and drawn out from post-consumer PET at very high velocities up to 5000 m/min. In conclusion, discolored post-consumer PET can be bleached and repaired during recycling, which can be an important contribution to protect the environment, and to save resources and energy.