Surface Analyses of Thin Multiple Layer Barrier Coatings of Poly(vinyl alcohol) for Paperboard

The hypothesis of the present study is that thin multiple layer coatings on paperboard from the aqueous solutions of poly(vinyl alcohol) (PVOH) at high machine speeds is more effective in terms of barrier properties than one or two thick layers. The objectives included attempts to use surface roughness parameters to understand the coating process and mechanisms behind coating defects. The present study is focused on pilot-scaled PVOH coating onto uncoated paperboard at machine speeds of 400 m/min. The multiple coating operation was carried out in six passes with a dry coat weight of about 1 g/m2 in each layer. The concept of thin multiple coatings resulted in coated surfaces without detected pinholes and with Kit rating 12 after only two thin layers. However, the oxygen transmission rates were still fairly high (100 ± 89 cm3/m2 day atm) after six layers, and some coating defects (such as craters and cracks) could be identified. The analyses of surface structure indicated that the surface properties are affected by water uptake during the coating processes. The compression of paperboard beneath the metering element seemed to be required to achieve homogeneous thin layers. However, an analysis of defects revealed flaws and inhomogeneities near objects protruding from the surface, such as surface fibers and craters, caused by blistering. For rough paperboard substrates, the desired barrier properties may require a careful balance between sufficient compression for fiber coverage and gentle compression in order to avoid defects near craters and surface fibers.