Abstract
Most recycling methods remove the essential sizing from reinforcing fibres, and many studies indicate the importance of applying sizing on recycled fibres, a process we will denote here as resizing. Recycled fibres are not continuous, which dissociates their sizing and composite lay-up processes from virgin fibres. In this study, commercial polypropylene and polyurethane-based sizing formulations with an aminosilane coupling agent were used to resize recycled glass and carbon fibres. The impact of sizing concentration and batch process variables on the tensile properties of fibre-reinforced polypropylene and polyamide composites were investigated. Resized fibres were characterized with thermal analysis, infrared spectroscopy and electron microscopy, and the tensile properties of the composites were analysed to confirm the achievable level of performance. For glass fibres, an optimal mass fraction of sizing on the fibres was found, as an excess amount of film former has a plasticising effect. For recycled carbon fibres, the sizing had little effect on the mechanical properties but led to significant improvement of handling and post-processing properties. A comparison between experimental results and theoretical prediction using the Halpin-Tsai model showed up to 81% reinforcing efficiency for glass fibres and up to 74% for carbon fibres.
Highlights
The manufacturing and use of glass and carbon fibre-reinforced composite materials based on thermoset polymers have increased during the last decades in all engineering applications
Matrix residues on the fibre surface after recycling can significantly hinder the functionality of the new sizing, decreasing adhesion between the matrix and the fibres
SEM analysis prior to the sizing (Figure 1a) revealed some residues of matrix and original sizing on the recycled glass fibres (rGF) surfaces, which were assumed to be present in the resized fibres
Summary
The manufacturing and use of glass and carbon fibre-reinforced composite materials based on thermoset polymers have increased during the last decades in all engineering applications. A common approach to deal with EoL fibre-reinforced composites has been landfilling, it is the least preferred option for waste management, as stated in the European Waste Framework Directive [1]. Much work remains to make the recycling of composites viable in industrial scale. Several methods, such as thermal [2,3], steam [4], electrochemical [5], microwave induced [6] and mechanical [7] recycling, have been used to recover reinforcing materials from composite waste. In most of the recycling processes, the physical properties, surface quality and origin of the recycled fibres cannot be fully controlled leading to non-uniform and degraded fibre properties, which downgrades the further potential application possibilities of the recovered fibres [10]
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