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Sunday, 22 September 2019
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Raw materials & technologies, Raw materials, Coatings binders

Toxic heavy-metal catalysts banned from paints

Tuesday, 12 February 2013

By enzymatic cross-linking of alkyd resins, Austrian scientists have tried to ban toxic heavy-metal catalysts from paints.

By enzymatic cross-linking of alkyd resins, toxic heavy-metal catalysts were banned from paints

Source: Pavel Losevski/ Fotolia

By enzymatic cross-linking of alkyd resins, toxic heavy-metal catalysts were banned from paints

Source: Pavel Losevski/ Fotolia

By enzymatic cross-linking of alkyd resins, Austrian scientists have tried to ban toxic heavy-metal catalysts from paints. Alkyd resins are polyesters containing unsaturated fatty acids that are used as binding agents in paints and coatings. Chemical drying of these polyesters is based on heavy metal catalyzed cross-linking of the unsaturated fatty acid moieties. Among the heavy-metal catalysts, cobalt complexes are the most effective, yet they have been proven to be carcinogenic. Therefore, strategies to replace the cobalt-based catalyst by environmentally friendlier and less toxic alternatives are under development.

Laccase-mediator system to replace heavy-metal catalyst

In their study, Katrin J. Greimel, Veronika Perz, Klaus Koren et al demonstrate for the first time that a laccase–mediator system can effectively replace the heavy-metal catalyst and cross-link alkyd resins. Interestingly, the biocatalytic reaction does not only work in aqueous media, but also in a solid film, where enzyme diffusion is limited. Within the catalytic cycle, the mediator oxidizes the alkyd resin and is regenerated by the laccase, which is uniformly distributed within the drying film as evidenced by confocal laser scanning microscopy.

During gradual build-up of molecular weight, there is a concomitant decrease of the oxygen content in the film. A new optical sensor to follow oxygen consumption during the cross-linking reaction was developed and validated with state of the art techniques. A remarkable feature is the low sample amount required, which allows faster screening of new catalysts.

The study was published in "Green Chemistry”, issue No. 2, 2013 (DOI: 10.1039/C2GC36666E).

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