Evaluation of stabilizer and additive content of post-consumer-polypropylene waste and their influence on new concepts of polypropylene-upcycling

Abstract

Introduction. Thermoplastics account for a large proportion of waste but have the potential to be reintroduced in product cycles. While polyethylene terephthalate as well as polyethylene are reintroduced (more or less) easier polypropylene (PP) is highly prone to thermal oxidation and radical degradation due to its high number of ternary carbon atoms and the abstraction of the hydrogen atom adjacent to the pendant methyl groups. Depending on the certain field of application and processing procedure of PP additives are essential in maintaining physical and mechanical properties during use. Decreasing molar mass, impurities and also heterogeneous composition of stabilizers and additives lead to challenges in reprocessing of post-consumer-PP (pc-PP) waste. [1] For that reason the knowledge about the content of stabilizers (phenolic antioxidants, phosphite-type, hindered amine light stabilizers) and additives remaining as well as their degradation products is essential for chemical or physical modification of post-consumer-PP waste [2], and as a result of this for new and innovative methods in terms of polypropylene upcycling. Methods. In this study a standard series of virgin PP pellets and stabilizer is produced. Irganox 1010, Chimassorb 2020, and Tinuvin 770, commercial products (BASF), and a linear polypropylene (HD601CF, Borealis) are extruded on a laboratory scale. In addition, a zero-stabilized reference PP-sample is tested in order to create a worst case scenario. In addition, a mixture of stabilizer and KBr are pressed to pellets in order to characterize the unperturbed additive. Samples are characterized spectroscopically (FT-IR), thermally (DSC, TGA), thermo-mechanically (rheology) and mechanically by tensile testing. Results. A first overview of the oxidative stability can be given by the isothermal oxidative induction time, however, for a deep insight of thermal stabilization condition more quantitative methods need to be applied. By FTIR the resultant composition of specific functional-groups including their products of degradation is obtained. The ratios of the area under additive bands to polypropylene reference bands are used in order to prevent the influence of sample thickness. A relationship to the concentration of the stabilizers (plus their degradation products) can be shown. By means of a temperature controlled stage further modification in the chemical composition of stabilizers in the PP melt is monitored online with the infrared microscope at elevated temperatures. The actual state of stabilization of the standard series can be quantified by a TG-MS hyphenation which allows the detection and identification of volatiles at a certain time and temperature respectively. As the consumption of stabilizers results in modifications of the molecular structure of a polymer thermo-mechanical tests are performed. Changes in the polypropylene's molar mass are quantified by melt rheology. Their influence on the mechanical properties on a macroscopic scale is ascertained by tensile testing. In a last step the methods applied are evaluated on real post-consumer PP waste - from flakes to restabilized granules.

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