The focus of this contribution is to highlight the challenges of chemical recycling of End-of-Life glass fiber composite (GFRP) waste from wind turbine blades utilizing solvolysis/HTL (hydrothermal liquefaction) methods based on subcritical water as solvent. A multitude of investigations have been published during the years regarding solvolysis of newly produced composite laminates and known thermoset composition (epoxy, polyester, and vinyl ester). However, a real wind turbine blade is more complex and constitutes of thermosets, thermoplastics, and other materials such as balsa wood. It is a very challenging task to separate these materials from each other within the wind turbine blade structure, so the premise for recycling is a mixed waste stream where little is known about the chemical composition. In the present study, the solvolysis process for GFRPs based on sub/supercritical water at 250-370 °C and 100-170 bar process conditions with catalyst (acid and base) and additives (alcohols and glycols) was studied and optimized. The samples used are representative for End-of-Life wind turbine blades. The aim is therefore to investigate if it is possible to develop a general process that can accept all material constituents in a real wind turbine blade, resulting in recycled glass fibers and a hydrocarbon fraction that can be used as a refinery feedstock.

Processing of polymer fibre composites has a remarkable influence on their mechanical performance. These mechanical properties are even more influenced when using recycled reinforcement. Therefore, we place particular attention on the evaluation of micromechanical models to estimate the mechanical properties and compare them against the experimental results of the manufactured composites from recycled carbon fibre material. For the manufacturing process, an epoxy matrix and carbon fibre production cut-offs as reinforcing material are incorporated using a vacuum infusion process. In addition, continuous textile reinforcement in combination with the epoxy matrix is used as reference material to evaluate the degradation of mechanical performance of the recycled composite. The experimental results show higher degradation of the composite strength compared to the stiffness properties. Observations from the modelling also show the same trend as the deviation between the theoretical and experimental results is lower for stiffness comparisons than the strength calculations. Yet still, good mechanical performance for specific applications can be expected from these materials.

A new concept, Re-Fib, was developed within an EU project, REFORM, to recycle carbon and glass fibres from polymeric composite structures, aiming to reduce energy consumption and degradation of fibre properties during recycling. The optimized thermolysis treatment, 24 h at 380 °C, was verified able to recover clean fibres from most tested composite structures containing different thermoset resins (epoxy, vinyl ester, and polyester) and various core materials such as polyvinyl chloride (PVC), polyurethane (PU), and wood. Single-fibre test was performed in dynamic mechanical analysis (DMA). The reduction of strength was found around 26% for carbon fibres and 34-40% for glass fibres. Thermally recycled glass fibres were melt-compounded with recycled polypropylene (rPP); the resultant composites showed promising mechanical properties.

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