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<\/figure><\/div>\n\n\nFurther chemical analysis using NMR (Nuclear Magnetic Resonance) and GC-MS (Gas Chromatography-Mass Spectrometry) revealed the presence of commonly used additives and their respective oxidation products, as well as terpenes, which are hypothesised to interfere with MAPP functionality. A masterbatch solution was implemented, compensating for the reduced mechanical performance of rPP-based composites, achieving a Young\u2019s modulus of 2.18 GPa and a tensile strength of 30\u2009MPa, compared to 2.24 GPa and 35\u2009MPa for fibre-reinforced virgin materials, respectively, while maintaining a recyclate content of 67%.<\/p>\n\n\n\n
Introduction<\/h3>\n\n\n\n
The demand for renewable, more sustainable alternatives to virgin polymer materials is growing constantly [Citation1<\/a>]. To counteract climate change, alternatives to fossil fuels as carbon sources must be found, namely biomass, carbon dioxide, and recycled materials [Citation2<\/a>]. Various studies have introduced the possibility of combining virgin polypropylene (vPP) with agro-waste streams [Citation3<\/a>,Citation4<\/a>]. The Report \u2018Circular Economy for Plastics\u2019 [Citation5<\/a>] presents a considerable potential towards full circularity, with 17% of plastics from post-consumer waste still being disposed of in landfills in 2020, as well as 11\u201321 million tonnes being discarded into the ocean as of 2021 [Citation3<\/a>,Citation6<\/a>,Citation7<\/a>].<\/p>\n\n\n\n Moreover, when considering the use of rPP, it is crucial to distinguish between post-industrial recyclates (PIR) and post-consumer recyclates (PCR). Furthermore, a distinction must be made between PCR polymers from a single or controlled waste stream and those derived from mixed consumer waste of unknown composition. For controlled waste streams, many exemplary studies have shown successful the reintegration of the waste back into new applications [Citation8<\/a>]. In the present context, however, PCR refers specifically to material derived from mixed-source post-consumer waste. The key difference between PIR and PCR is the controlled sources. Whilst factors such as additives, polymer type, and raw materials are known and considered, this is not the case for PCR. In addition to contamination, PCR derived from mixed-source post-consumer waste may contain a wide range of additives and feedstocks associated with the diverse PP grades produced by different manufacturers. While the composition of PP is known for PIR, the path to obtaining PCR is more complex and involves multiple steps (as illustrated in Figure 1).<\/p>\n\n\n\n Figure 1. Schematic illustration to differentiate between post-industrial and post-consumer recyclates.<\/p>\n\n\n\n <\/a>Display full size<\/p>\n\n\n\n The prospect of economic value creation may therefore stimulate the development of advanced recycling techniques. The combination of rPP (PCR derived from mixed-source post-consumer waste) with agro-waste streams, which has not yet been extensively studied, creates an incentive for further market exploration and marks a step toward filling the knowledge gap. This provides a two-pronged sustainability approach: Firstly, substituting the use of fossil carbon from the geosphere, thereby aligning with the vision of the renewable carbon initiative [Citation9<\/a>]. Secondly, previously supposedly burned crop residue after harvesting can be valorised, decreasing the potential emission of greenhouse gases [Citation10<\/a>]. An agricultural stream with unused potential is flax straw, a by-product of linseed production, which has already been successfully implemented and characterised as a filler in an ethylene-octene copolymer [Citation11<\/a>]. Fibre flax is commonly used for apparel textiles and natural fibre-reinforced composites [Citation12<\/a>]. The combination of textile flax with vPP has been extensively researched [Citation13\u201320<\/a>]. Still, only limited information is available on the use of linseed flax with PP, despite linseed flax showing comparable mechanical characteristics to fibre flax [Citation21<\/a>].<\/p>\n\n\n\n To distribute the load applied onto the composite properly, an adequate fibre-matrix interface needs to be provided. Naturally, due to the hydrophobic nature of PP and the hydrophilicity of natural fibres, their compatibility requires improvement to achieve good mechanical properties [Citation22<\/a>,Citation23<\/a>]. Commonly used adhesion promoters, such as MAPP, have been researched extensively [Citation24<\/a>,Citation25<\/a>]. MAPP is incorporated into the backbone of the PP and provides binding sites for natural fibres through a nucleophilic acryl substitution, resulting in esterification [Citation23<\/a>,Citation25<\/a>,Citation26<\/a>]. These anhydride-binding sites can be utilised for an esterification of the cellulose, spreading out in a brush-like structure from the fibre towards the polymer. In the literature, different concentrations of MAPP in the PP matrix have been successfully applied at the composite level to increase the interfacial shear strength (IFSS) and provide improved mechanical properties [Citation27\u201331<\/a>]. An important contribution by Mattlet et al. [Citation22<\/a>] showed that, in rPP, the molecular mass, representative of the polymer chain length, plays a crucial role in adhesion. However, a comprehensive understanding of the underlying causes for low fibre-matrix adhesion is still missing.<\/p>\n\n\n\n In the interest of using agro-waste streams and recycled polymers to achieve circularity as well as sustainability in composites, it is important to examine the mechanical properties of mixed-source post-consumer recyclate and its interaction with natural fibres. Adequate mechanical properties at the composite level have to be achieved to provide potential for using such a material combination. Thus, adequate interaction between the flax fibre and the rPP is needed.<\/p>\n\n\n\n This study focuses on the mechanical properties of injection-moulded fibre-reinforced composites and on the analysis of the fibre-matrix interface. The following research questions are addressed:<\/p>\n\n\n\n Flax straw (Linum usitatissimum<\/em> L.; linseed variety Agram, harvest 2022) was supplied by Agritec Plant Research Ltd. (\u0160umperk, Czech Republic). The straw was collected directly from the field and cut into smaller pieces approximately 2\u2009cm long. These pieces were directly introduced into the compounding process. For the microbond trials, single fibres\/fibre bundles were manually extracted from the stems by breaking the stems and then manually extracting single fibres or fine fibre bundles. Tensile testing of individual fibre bundles from cut Agram flax yielded a mean cross-sectional area of 0.00304\u2009mm2<\/sup>, a tensile strength of 295.3\u2009MPa, a Young\u2019s modulus of 17,089\u2009MPa, and an elongation at break of 2.05%. The used materials are shown in Figure 2.<\/p>\n\n\n\n Figure 2. Overview of the used material. (a) Shows the cut flax straw used for the compounding process. (b) Shows the broken stems from which single fibres\/fibre bundles could be prepared.<\/p>\n\n\n\n\n
Materials & methods<\/h3>\n\n\n\n
Fibres, polymers and coupling agent<\/h3>\n\n\n\n