Heat-Induced changes in oil and grease resistant hydroxypropylated-starch-based barrier coatings

The effects of thermal treatment on the stability of the dispersion barrier coating layer and on the mechanical rigidity of the packaging material were studied via oil resistance measurements, microscopic and topographical analyses, and determinations of liquid spreading and absorption. The barrier coatings consisted of hydroxypropylated starch and talc, with and without latex. The coatings showed mainly excellent oil resistance at 23°C and 60°C, but at 100°C the coatings lost their oil resistance, which was considered to be due to the lower oil viscosity and segregation of coating components due to heat. It was found that two consecutive heat treatments at 100°C significantly increased the contact angle of rapeseed oil on coatings containing talc, making the coating more oleophobic. Without talc, the effect of heat treatment on the contact angle was weak. X-ray Photoelectron Spectroscopy data indicated that heat caused latex movement towards the surface. According to Atomic Force Microscopy adhesion images, poor miscibility of latex and starch resulted in local depletion and the agglomeration of latex particles, and reduced the diffusion. The changes in adhesion forces between the tip and coated surfaces were mostly reversible, whereas topographical changes were partly irreversible as an effect of in-situ thermal treatment. Scanning Electron Microscopy indicated that heat treatment of coatings containing latex may also induce self-healing and thus reduce the number of pinholes.