Polymer-coated paper and board - Role of substrate, polymer characteristics and additives
The surface roughness of paper or board has a significant influence on the homogeneity of an applied polymer film, and this thesis discusses how it may be improved by a surface treatment, e.g. high-temperature calendering or precoating. Hot calendering can in principle affect not only the physical smoothness but also the chemical characteristics of the paper surface. It also reduces the uptake of polymer during the coating operation, and to some extent improves the spreading of the polymer solution or dispersion on the surface, and can improve the homogeneity of the film at low coat weights.
The polymer characteristics govern the barrier properties of the film, but these properties can be improved by additives and by surface treatments like corona discharge. The barrier against grease of a poly(vinyl alcohol)-coated (PVA) substrate with a partially hydrolysed PVA-grade was substantially enhanced when the substrate was hot-calendered. The fully hydrolysed PVA-grade did not perform as well, but the moisture barrier was improved when carboxyl groups were incorporated.
Dispersion coating of paper or board with styrene-butadiene copolymers led to better barrier properties against water vapour if a latex with a relatively high degree of cross-linking and a low degree of carboxylation was used. Too low a glass transition temperature was not beneficial due to the greater segmental mobility and larger free volume available for diffusion. There was a trade-off between the improvement in properties such as smoothness, gloss and moisture barrier, and the blocking tendency. With a smoother coated material, the latter increased, but could be counteracted by a wax addition, which also improved the water vapour barrier. Wax reduced however the surface energy of the coating and the printability, when using water-based inks deteriorated. Corona or plasma treatment of the coated substrate was shown to counteract this negative effect.
Platy mineral pigments, such as talc or kaolin clay improved the water vapour barrier by increasing the degree of structural tortuosity for the diffusing medium. The combination of platy pigments with a high aspect ratio and wax was found to be very effective in reducing the water vapour permeability.