Recyclable composite resins from green building blocks

Researchers working on the BIOCRES project have developed two different biobased composite resins. The products made from these new resins are just as strong as those made from traditional polyester resins but the building blocks are made from renewable raw materials and are also less toxic. One of the resins is also reusable. These are extraordinary results for a thermoset resin.

Thermoset composites have become an indispensable part of modern society. These two-component plastics  consisting of a resin and a fibre  are lightweight, strong and heat-resistant. This makes them suitable for a wide range of applications and they are often used in the automotive and aircraft industries. In the production of composite resins, individual building blocks are joined together to create a resin consisting of long chains called polymers. The resin is dissolved in styrene immediately after production to form a liquid mass. Composite products are created by pouring the resin formulation into a mould of the required shape, in which fibre mats have been placed to reinforce the end product. During treatment with heat, the styrene reacts with the resin, cross-linking the polymers to create a strong network. This changes the liquid resin into a hard, sturdy plastic.

Greener alternative

Conventional polyester composite resins are, however, not environmentally-friendly. The polymer chains are constructed from crude oil components and are difficult to recycle. Furthermore, toxic styrene is needed to cross-link the individual polymers together into a network. As a result, the resin can contain thirty to forty per cent of this toxic substance. Researchers believed it was time for a greener alternative: a composite resin based on biobased building blocks that would preferably also be fully recyclable.

Similar properties

Wageningen UR Food & Biobased Research developed a resin based on furandicarboxylic acid. In addition, researchers also succeeded in creating a biobased substitute for styrene. ‘The combination of isosorbide diallyl ether and isosorbide dimethyl acrylate produces properties and applications similar to those of the conventional composite material, but roughly two thirds of the combination consists of biobased components based on sugar,’ explains Rolf Blaauw, researcher at Food & Biobased Research (FBR).

Fully recyclable

Francesco Picchioni, a professor of Chemical Product Engineering at the University of Groningen, also believes in the reuse of composite resins. ‘Going green is not enough: environmental gains are limited when only green building blocks are used,’ he explains. At the same time as FBR, Picchioni’s team developed a different resin, with greater emphasis on recycling. The team developed a new resin composed of a biobased version of bisphenol A and terephthalic acid. No longer does the resin need to be dissolved in styrene, but becomes liquid when heated, after which it can be processed. However, this bisphenol A variant could pose a health risk for resin producers who become exposed to it during the production process. In addition to the new polymer, the team in Groningen also developed an alternative to the cross-linker styrene: a bismaleimide. Less toxic than styrene, it does however do the same job of cross-linking the polymer chains together. This new resin is now fully recyclable, representing a breakthrough in the research conducted in Groningen. ‘By simply heating the resin to approximately 140 degrees Celsius, the network structure disappears,’ explains Picchioni. ‘This returns exactly the same individual polymers again, fully in keeping with the cradle to cradle principle.’

Combining inventions

It seems obvious to the researchers in Wageningen and Groningen to combine their inventions to create a non-toxic, recyclable resin. However, this does require a significant technical step. ‘We could combine the non-toxic resin developed in Wageningen with the bismaleimide cross-linker and the chemistry developed in Groningen,’ Blaauw suggests. Picchioni also sees ample opportunities for combining the best of both products. ‘It would certainly be possible to apply the concept of degradable and recyclable network structures to the resin made in Wageningen,’ he says. ‘All we need now is an interested business partner.’