Thermoset composites, such as those used in wind turbine blades, are difficult to reuse. That is why the research group led by Dr Albert ten Busschen, associate professor at Windesheim University of Applied Sciences, is developing new composite formulations. By using thermoplastic acrylic resin, the researchers aim to create a material with properties comparable to conventional thermoset composites, but with better recycling options.
Wind turbine blades need to be replaced after approximately 25 years. At present, the composite material can be shredded and mixed with resin to make new products. However, the resulting material does not have the same properties as the original composite. The fibres are shorter and therefore lack the strength needed to make new blades. Shredded blade material is currently used mainly as a filler in construction materials or in composites for less demanding applications, such as automotive components, bank protection, noise barriers and planters.
Finding the right processing conditions
One approach for future wind turbine blades is to use thermoplastic acrylic resin. Unlike in thermoset composites, its polymer chains are not permanently crosslinked. This creates opportunities to separate and reprocess the resin and fibres using heat, suitable solvents or chemical recycling.
But this approach does not solve the waste problem posed by existing blades, Ten Busschen points out. In the TKI project Acrylic resin for pultrusion EoL composite, he is therefore investigating whether recyclable acrylic resin can be combined with shredded composite waste.
Acrylic resin cures differently from conventional thermoset resins. Among other things, the researchers need to control how the temperature develops during processing. “We have found that the starting temperature is an important factor, which we can influence by preheating the moulds,” Ten Busschen explains.
The process can also be controlled by selecting different peroxides to initiate the reaction. “Some peroxides that react very strongly with polyester work very slowly, or not at all, with acrylic resin. We have found that certain peroxides can start the process at very low temperatures, while others can complete the reaction.”
Focus shifts to pultrusion
During the first phase of the one-year project, the researchers largely mapped out the chemistry behind this staged curing process. Their focus is now shifting to pultrusion: pulling a bundle of fibres, composite flakes and resin through a heated die.
The team is investigating the use of a preliminary die to shape the material before the resin is preheated and injected. The material then passes through the final heated die.
Another line of research involves digital modelling of the process, which could make it easier to identify the right polymerisation conditions. Initial attempts have proved difficult because the chemical reactions are complex. Ten Busschen hopes to explore this further with colleagues.
Industry collaboration is essential
The project is funded in part by ChemistryNL. Alongside Windesheim researchers and students, industry partners include AOC, Parthian Technology, Pontis Engineering, PBS Machinery and Nouryon. “For the second part of the research, we are working extensively at AOC because they have the pultrusion equipment we need for the trials.”
PBS Machinery is helping to develop the dies and the production process. The project aims to present a standardised test specimen of the new composite at a meeting in late November or early December. The specimen will be 5 cm long, with a cross-sectional dimension of approximately 2.5 cm.
Further scale-up will be a task for industry, as it requires large, costly equipment. According to Ten Busschen, the composites industry is interested in circular innovation, partly in response to market demand and stricter environmental legislation. Companies also want more clarity about which technologies are most likely to succeed. Experience with these acrylic resins remains limited in the Netherlands.
“Companies consider not only economic costs, but also government policy, environmental costs and purchasing requirements,” he says. The business case will need to be convincing in both practical and economic terms before companies adopt the technology at scale.
