Montmorillonite for starch-based barrier dispersion coating — Part 1: The influence of citric acid and poly(ethylene glycol) on viscosity and barrier properties

Starch-based coating formulations were draw-down coated on a paper substrate and the effect on the water vapor transmission rate, WVTR, upon montmorillonite, Mt addition was compared to that of the pure starch matrix and for poly(ethylene glycol), PEG, or citric acid, CA, as potential plasticizers. Both PEG and CA were added at a ratio of 3 to 10 to starch. Addition of CA to the starch substantially lowered the WVTR of the coated papers compared to coating with pure starch or PEG plasticized starch. The WVTR of the papers coated with just starch and CA was lower than that obtained for paper coated with the non-plasticized Mt–starch dispersion, and approximately the same as paper coated with Mt–starch dispersions plasticized with PEG or CA. Addition of Mt was effective in reducing the WVTR in the case of starch plasticized by PEG but not with CA. These differences in WVTR reduction were indicated to be due to differences in the viscosity and clay swelling indicating that CA and PEG affected the Mt dispersion differently. CA was also investigated as a dispersing agent by addition in small quantities to the montmorillonite at different pHs. When this Mt dispersion was added to a starch containing PEG, CA gave slightly reduced plastic viscosity and showed a potential to further reduce the WVTR.