In a new study, the photooxidative stability of condensed tannin additives in acrylic-based surface coatings under exterior exposure was investigated.
Condensed tannins, also known as proanthocyanidins, have been added as functional additives to acrylic-based clear coatings to promote polymer coating longevity and also stabilise timber colour on accelerated and exterior weathering. When added at < 0.5% w/w content, native tannins and tannins modified with maleate groups extended coating performance > 20% compared to commercial hindered amine light stabilisers (HALS) and phenolic stabilisers used at comparable coating loadings. Other tannin chemical modifications such as methylcarboxylate groups were also associated with greater coating longevity beyond that achieved with the synthetic UV and antioxidant additives.
Photostabilising timber colouring on exterior exposure
The study has also revealed that tannin addition to acrylic coatings can act to photostabilise timber colouring on exterior exposure. While tannin use can contribute colour to coatings, any colour was rapidly bleached on UV exposure with a dependency on tannin content, degree of modification, and coating formulation pH. Moreover, through manipulating tannin addition levels, this tannin decolourisation could offset typical wood darkening associated with UV exposure to provide low or no wood colour change over longer exposure periods. With the photooxidative stability efficacy of tannins established on exterior exposure, condensed tannins in native and modified form offer potential as sustainable functional additives for the coatings sector.
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