Modified starch as potential water-based binder system for interior coatings

With its worldwide availability and its low market-price native starch provides the potential for the use as a bio-based material. However, in its native form it is highly hydrophilic, not cold-water dispersible and only forms brittle films. So, our work aimed at the chemical modification of degraded starch for the application in the coating sector as binder, with the focus on paints and varnishes. For this purpose, cold-water dispersible hydrophobized starch esters and ethers with high solid contents (> 30% (w/w)) and viscosities between 100 and 1000 mPaBLs should be synthesized. Additionally, they should form continuous films and show adhesion to different substrates. As starting product a degraded potato starch with a molar mass of 1.18BL105 g/mol was used and modified by carboxylic acid anhydrides (C2-C4) and carboxylic acid vinylesters (C6) in the case of the esters. Epoxy alkane derivatives were used for the synthesis of hydroxyalkyl starch ethers (C3-C4, C6). Beside the type of substituent and the chain length, also the degrees of substitution (DS) and molar substitutions (MS) were varied from 0.05 to 1.1. The modification led to cold water-soluble or dispersible products with applicable concentrations between 30 and 45% (w/w) and viscosities in the range of 102-103 mPaBLs (100 s -1). The most samples had a newtonian flow behaviour and showed viscoelastic properties with higher values for the loss modulus (G''). Continuous films were observed with starch esters in the DS range of 0.11-0.81 and in the case of the starch ethers with MS values of 0.77-0.91. Wet and dry layer thicknesses of 25-50 mm and 7-12 mm were obtained (doctor blade: 100 mm). In the cross cut test the esters had clearly better results with values of 0 and 1, in contrast to the tested ether with values of 4. The compatibility tests with 3 commercial binders showed that two Acronal binders (A 754 & S 559) (42%) showed compatibility with the modified starches (58%). According our experiments and especially based on the results of the film formation and the cross cut test the starch esters and not the ethers could be the preferred class for an application in the coating sector. Additionally, the results showed that the starch esters could be an alternative to petroleum-based polymers as binder in paints and varnishes potentially.