The Smell of Progress in the Circular Plastics Economy: Odor Characterization and Reduction in Polymer Recyclates and HDPE-Lignin Blends

With respect to the general resource scarcity but the at the same time still rising need for plastic material, the sustainability of future applications and developments in the plastics sector becomes increasingly important. Encouraging the bioeconomic thinking, possible solution approaches which tackle the ‘plastic problem’ of the modern society include the promotion of plastic recycling or the replacement with natural substances and materials. Both approaches address the conservation of resources via reducing the need for fossil feedstocks. However, they feature associated obstacles, such as off-odors which are especially pronounced in recyclates and natural substance-containing polymers. The detailed elucidation of the causative odor-active substances is addressed in thisthesis, and aims to build the knowledge base for subsequent investigations on odor reduction techniques. By combining and correlating human sensory evaluations with multi-dimensional gas chromatographic and mass spectrometric measurements, the odors of post-consumer recyclates and HDPE-lignin blends were characterized. Likewise, efficiencies of diverse odor reduction techniques were assessed. In detail, odorous contaminants detected in post-consumer recycled polypropylene (PP) were significantly reduced by applying an industrial scale hot water washing step. This was reflected by the overall odor reduction of the samples by 45%. In contrast, color sorting of the initial waste fraction did neither affect the overall odor of the recyclates nor foster trends within single odorants. Following an even more environmentally friendly approach, probiotic bacteria were added to post consumer plastic packaging waste to manage the smell effects. This resulted in a reduction of odorous microbial metabolites in the mixed plastic waste fraction and demonstrated the potential effect of probiotics to inhibit other off-odor producing microorganisms. However, odor levels of both treated samples were not comparable to the smell neutrality of virgin polymers. Therefore, another bioeconomy-oriented approach comprised the partial replacement of virgin plastics with the natural polymer lignin. Smell properties and causative odorants of HDPE-lignin composites were comprehensively elucidated on a molecular level. Amongst others, complexly substituted phenolic odorants and, especially in the case of kraft lignins, diverse sulfur compounds were reported for the first time as odor contributors of lignin and HDPE-lignin blends. Incorporating the new findings, the latter were target for a subsequent smell reduction by the in-extrusion application of different processing additives. Especially activated carbon proved to be powerful for kraft HDPE-lignin blends, reducing odors by 48%. Evidently, the applied techniques of both approaches did not completely solve the odor problem of recyclates or lignin-containing polymers. However, fundamental knowledge on causative odorants as well as odor reduction principles were gained, possibly transferable to the smell optimization of other material systems. In the end, only the combination of advanced techniques and innovative solution approaches can pave the way towards odor-optimized alternatives to virgin plastics.