Comparative study on the degradation of HDPE, LLDPE and LDPE during multiple extrusions

Stricter regulations of plastics waste and the enforcement of higher recycling quotas has led to an increasing interest in recycling. One of the most prominent recycling strategies is mechanical recycling, which is based on sorting, pre-treatment and remelting of plastics waste. As repeated reprocessing, may be associated with a reduced quality of the obtained recyclates, approaches for the characterization of these materials by analytical techniques need to be developed. These should help to identify recyclates that will not fulfil the requirements but also to provide data allowing to take countermeasures (such as addition of a certain percentage of virgin material and/or polymer stabilizers). Three different polyethylene (PE) types (high-density (HDPE), linear low-density (LLDPE) and low-density (LDPE)) were subjected to long-time extrusion experiments in order to gain a better understanding of degradation occurring during multiple recycling steps (as expected in a circular economy). Unstabilized material as well as PE with two different stabilizers, Irganox 1010 (IX1010) and Irgafos 168 (IF168) were investigated, employing high temperature gel permeation chromatography with infrared spectroscopic detection (HT-GPC-IR), thermo desorption gas chromatography-mass spectrometry (TD-GC-MS) and high performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (HPLC-qTOF-MS) (the latter primarily used for analyzing stabilizer degradation). The direct comparison of the PE types revealed significant differences regarding the degradative behaviour. One main conclusion is the possible use of short n-alkanes (below 30 carbon atoms) as a quality markers in the recycling process, as unique trends in the breakdown of polymer chains were found for each PE type. Both HDPE and LLDPE showed an increase in odd numbered n-alkanes, but only HDPE showed a constant decrease in the even numbered ones. For LDPE odd and the even numbered n-alkanes were increasing during extrusion. Focusing on the effect of the added stabilizers, IF168 alone had little to no impact, whereas the presence of IX 1010 showed a beneficial influence on polymer degradation – a trend that was even intensified when both stabilizers were combined. Also, in this case differences between the three PE types could be observed. In HDPE and LDPE stabilizers were consumed during extrusion to a higher extent than in LLDPE.