Challenges and possibilities in coating of foam formed structure

Foam formed paper was compared to water formed paper made with same process parameters. The variables in base papers were wet pressing line load, water/foam density and the amount of cellulose microfibrils. Test sheets were coated with a metered size press, with a pre-coating formulation containing CaCO3. Coating layer was analysed with microscopy and burn out tests. The results will be discussed in details. The main results are that the penetration of the coating colour clearly depends mostly on the used line load in wet pressing of the paper machine. With the highest line load, the penetration of the liquid phase is smaller and the coating colour stays more on the surface, while the base papers produced with the lowest line load in wet pressing had more bulky structure and more surface voids. With foam forming, the coating layer coverage is slightly poorer than with water forming. The addition of cellulose microfibrils to the base paper structure can improve the uniformity of the coating layer. In general, the surface of the base paper for coated products should be engineered to be dense