Modified starches for concentrated water-based binder systems with application in paints and varnishes

Our work aimed at the optimization of the structure of native starch for the preparation of concentrated water-based dispersions with film forming properties for the application in paints and varnishes. Potato and smooth pea starches were degraded via ManOx and hypochlorite conversion (9.4-0.2BL106 g/mol) and used as starting materials for esterification and etherification reactions with different chain lengths (C2-C6) and varied degrees of substitution (DS) / molar substitutions (MS) (0.05-1.1) for an increase in their hydrophobicity. The applied modification conditions led to cold water soluble or dispersible products with 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'') than the storage modulus (G'). Continuous films were observed with starch esters in the DS range of 0.1-0.8 and in the case of the starch ethers with MS values of 0.7-0.9. Wet and dry layer thicknesses of 25-50 mm and 7-12 mm were obtained (doctor blade: 100 mm). In the cross cut test on glass plates the esters had clearly better results with values of 0 and 1 (ISO class), in contrast to the tested ether with values of 4. The compatibility tests with 3 commercial binders showed that two Acronal binders (A 754 and S 559) (42%) showed compatibility with the modified starches (58%). Selected starch esters showed also good adhesion (class 0 and 1) on aluminium plates while the tested ether could be classified in the worst class (5). In the short-time corrosion stability tests which were evaluated by electrochemical impedance spectroscopy, the barrier properties corresponded to the hydrophobicity of the starch ethers and esters. The starch butyrate with a DS of 0.65 had the best performance.