Evaluation and Control of Coated Paper Stiffness, 1999 Coating Fundamentals Symposium Proceedings

Koji Okomori * , Toshiharu Enomae and Fumihiko Onabe

Paper Science Laboratory

Stiffness is an important property of paper in printing processes. Operational problems such as jamming during sheet fed offset printing often stems from poor paper stiffness. In the case of coated papers, the Young's modulus of the coating layer affects the stiffness of the whole coated paper because external layers undergo larger strain during bending compared to the internal layers. A method to theoretically calculate the Young's modulus of a one-sided coated sheet is presented in terms of material

mechanics. The accurate interpretation of Clark stiffness enables us to estimate the Young's modulus of the coating layer from Clark stiffness. The accurate calculation of Young's modulus from the Clark stiffness is shown to require an impervious film because basepaper, when wetted with water, swells and roughens, giving an incorrect value for the Young's modulus.

Formulation with starch improved the Young's modulus of starch/SB-latex blend films, suggesting that the Young's

modulus of a coating layer of the same binder composition would tend to increase with starch content. The coating layer containing plastic pigment (PP) of smaller particle diameter had higher Young's modulus compared to large pigments, but the value was close to that of calcium carbonate. However, the lower density of the PP was expected to give some advantage to bending by its own weight. In double coating, increasing the starch level on the top coating layer was found to improve the total paper stiffness.