Defence of doctoral thesis: Luísa Rosenstock Völtz – Use of co-rotating extrusion process for the development of resource-efficient biocomposites

Natural fibers have gained popularity for reinforcing polymers due to their renewability, biodegradability, and usually have lower costs than synthetic fibers. The demand to reduce environmental impact has heightened awareness of the importance of adopting resource-efficient alternatives in line with recycling, reusing, and responsible consumption practices. This study integrates resource-efficient use of materials using a co- rotating twin screw extruder (TSE) in the development of biocomposites as an alternative for recycling, reducing landfill disposal, and contributing to the circular economy.

The influence of recycling wood-polymer composites (WPCs) was explored by subjecting the materials to nine extrusion cycles. The properties of the WPCs were evaluated after every other cycle and compared to those of virgin and recycled polypropylene (PP) and WPCs. The results revealed that, although shear forces during extrusion decreased the aspect ratio of the wood fibers and reduced tensile strength after nine recycling cycles, the WPC still maintained higher mechanical properties than virgin PP, highlighting the advantages of using recycled WPC over PP. Recycling WPCs reduces the overconsumption of fossil-based plastics, decreases landfill disposal, and lessens the need for virgin fibers.

The valorization of end-of-life textiles was studied as a potential reinforcement used in biocomposites. The primary challenge was to develop a low-energy consumption method for feeding, fibrillating, and compounding textiles with thermoplastic polymers without requiring chemical or mechanical pretreatments. A continuous extrusion process, recycled-textile long fiber thermoplastic (RT-LFT), was developed to incorporate end-of- life textiles into a PP matrix using a co-rotating TSE. The results demonstrated that RT-LFT is a direct, effective, and energy-efficient method for compounding end-of-life textiles with polymer, enabling the textiles to be separated into individual fibers that are well dispersed within the matrix.

In addition, the performance of fibers from end-of-life textiles was also evaluated using PP, and Bio-flex a biodegradable blend of polybutylene adipate terephthalate (PBAT)/ polylactic acid (PLA). The addition of the fibers did not show improvements in the mechanical properties of the PP matrix but showed great enhancement for the Bio-flex matrix. Adding fibers also improved the flow properties and melt strength of the polymers. Moreover, the disintegration under compositing conditions showed that the addition of fibers delayed the process when compared to neat polymer, but after 75 days cotton and silk biocomposites began to disintegrate.

WPCs are often modified with fossil-based virgin elastomers to enhance their impact and toughness properties. However, the use of these elastomers increases competition for limited resources and raises the costs of WPCs. To address this, the influence of recycled materials as impact modifiers in WPCs was investigated by replacing virgin elastomer with recycled fibers from end-of-life textiles and recycled elastomer. The results indicated that both recycled materials are viable alternatives to virgin elastomer, offering comparable impact and toughness properties while reducing the WPCs’ carbon footprint and costs.

In conclusion, using recycled or end-of-life materials presents an excellent alternative to virgin materials in biocomposites. It contributes to the circular economy, reduces landfill waste, decreases material costs, enhances resource efficiency, promotes recycling, and adds value to end-of-life materials.

Keywords: Co-rotating extrusion; Thermoplastic polymer; Biocomposites; Natural fibers; End-of-life textiles; Recycling; Resource efficiency