Great opportunities for new biobased coatings

In search of a more environmentally-friendly version, researchers took a close look at polyamide powder coatings and waterborne polyurethane coatings. Both coatings consist of polymers with amide bonds (powder coatings) or urethane bonds (waterborne coatings) as a basis. ‘For both coatings, we collaborated with our project partners to find a biobased version that could also be produced with greener chemistry,’ explains Bart Noordover, university lecturer in Polymer Materials Chemistry at Eindhoven University of Technology.

Powder coating

Transparency of the material was an important criterion for the new biobased powder coating as it enables more effective colouring by the producer. The powder coating becomes hazy when the polyamide molecules start forming crystals; a phenomenon the researchers were looking to avoid. ‘Crystals develop once the polymer chains are able to form a regular structure,’ Noordover explains. ‘We were able to prevent this regular arrangement of polymers by using asymmetrical building blocks of varying lengths developed by Food & Biobased Research. This produces an irregular chain structure that prevents crystal formation, keeping the material transparent.’

Powder shelf life

In the development of the new powder coating for project partners AkzoNobel and Nuplex, another important consideration was the shelf life of the powder at temperatures up to around fifty degrees Celsius. If the polyamide powder becomes hotter than fifty degrees, the fine powder particles can start to stick to one another. A bag of powder would then change into a rock hard, unusable mass. This risk is particularly prevalent with transparent polyamides with no crystal structure. These materials are often soft at relatively low temperatures: the polymer chains are flexible and mobile and more likely to adhere to one another. The researchers were able to prevent this from happening by incorporating isoidide diamine (a sugar-based molecule) into the polyamide chain. This made the chains stiff and less mobile. The powder then retains its powdery texture even at higher temperatures. In collaboration with Croda, Food & Biobased Research succeeded in optimising the synthesis of the isoidide diamine. ‘Our successful collaboration with Croda enabled us to quickly scale up the process developed in Wageningen, so that sufficient resin-grade diamine was available for our partners,’ says Daan van Es, senior researcher at Food & Biobased Research.

Toxic substances

The environmental benefits of a biobased version of waterborne polyurethane coatings were even greater than with powder coatings. When producing these coatings, producers use toxic substances, such as isocyanates and phosgene. The researchers came up with an alternative production process which no longer required these substances. ‘We used glycerol as a basis. This is a cheap by-product of the biodiesel industry and is mainly produced from vegetable oils,’ explains Noordover. ‘We used it to make so-called cyclic carbonates. These building blocks react together with amines to produce a polyurethane polymer without the need for toxic substances.’

Critical step

Recent tests conducted at the Eindhoven University of Technology and at participating companies have shown that the new biobased coating has good properties. ‘We already have the basis,’ explains Noordover. ‘The next step is to produce the polymer on a larger scale so that the industry can conduct further tests for specific applications.’ Noordover is convinced this will eventually happen, not only because of rising oil prices, but also because of increasing demand for cleaner, sustainable materials. In addition, EU and Dutch policies are focused on replacing toxic chemicals (‘substances of very high concern’) in the production process. Recent research conducted by Wageningen UR Food & Biobased Research at the request of the Netherlands National Institute for Public Health and the Environment (RIVM) underlined the availability in many cases of good biobased alternatives. Noordover explains, ‘It would be fantastic if we could make a contribution with our research to a cleaner production process.’