Print mottle of wood-containing paper, the effect of fines and formation

grantor: University of TorontoThe spatial unevenness of print density, print mottle, is due to the small-scale variation in the ink transfer and in the ink penetration depth. Print mottle is affected by both paper properties and printing conditions, and is a principle factor affecting the printability of paper. The objectives of this study were to develop a sensitive method to describe print mottle, to conduct a fundamental study of the effects of paper properties on print mottle of wood-containing paper, and to develop a model to predict the print mottle tendency of paper. The principle pulp used in the study was thermomechanical pulp and the printing method used was an IGT proof printer to simulate offset printing. A texture analysis method, based on the spatial gray level dependence method (SGLD), was developed and used to evaluate the structural characteristics of both the mass density map and the print density map of handsheets, dynamic former sheets and machine made paper. Texture features of homogeneity, contrast and correlation, combined with the coefficient of variation (CV) gave a complete description of the uniformity. Structural variation and print density variation due to changes in orientation, wire marks and uneven mixing were identified by the texture analysis method. Four types of sheets: handsheets, dynamic former sheets, pilot machine sheets and commercial newsprint were used to determine the effects of paper structure on print density. The fines content level of the handsheets and dynamic former sheets were controlled to 0%, 20%, 30% and 40%. Formation was varied by changing the settling times, i.e. the time before drainage. While the fibre orientations in the dynamic former sheets were changed by varying the jet/wire ratios. Print mottle was affected by the fines content level, as well as the fines distribution, and by the formation level. The handsheet results indicated the importance of not only the absolute amount of fines, but also the uniform distribution of fines on print mottle. The study on oriented samples showed that sheets with 30% fines gave the best printing results, and the structure of these sheets was close to that of commercial newsprint. The principle print mottle factor in wood-containing paper was shown to be the uniformity of ink penetration, a conclusion supported by a pointwise study of the dependence of the local print density, mass density and ink amount for newsprint sample. A model was developed based on a two-layered Kubelka-Munk model to relate print density variations to other variations. The model is able to predict the experimental observations such as a 30% fines content in sheets made with a dynamic former is an optimal fines level for offset printing.Ph.D