Coating of surface-modified papers with poly(vinyl alcohol)

The influence of the physical and chemical characteristics of a paper substrate coated with a thin homogeneous film of poly(vinyl alcohol) (PVA) has been investigated. Some of the paper specimens were temperature-gradient (TG) calendered (at high temperatures) before pilot-scale coating. Two different paper grades differing in structure and sizing formulation were used. For one of the grades, the high-temperature calendering had a slight effect on the chemical composition of the paper surface as evaluated by electron spectroscopy for chemical analysis. For the other grade, no such effect was noted. In the former case, it was suspected that the sizing in the paper substrate degraded and, as a consequence, that volatile hydrocarbons evaporated from the surface. The slight change in surface chemistry had no significant direct effect on the homogeneity of the applied polymer film. The results indicated that the surface roughness of the substrate was the governing factor. The low surface roughness obtained by TG-calendering was beneficial for the homogeneity and barrier properties. In one series of experiments, some of the paper substrates (uncalendered and calendered) were corona-treated before PVA-coating. This treatment greatly increased the wettability of the surface by water. In general, the corona treatment improved the homogeneity and the barrier properties (grease resistance, air permeability and water vapour transmission) of the applied film. The surface smoothness was clearly important also for the corona-treated papers.