Fiber Coverage and Fiber Lifting in LWC, 1998 Coating Conference Proceedings

The visual appearance of light-weight coated (LWC) papers, both before and after printing, often is affected by incomplete coverage of fibers, resulting in fiber-roughness. Through super-calendering this fiber-roughness is reduced significantly in glazed LWC, however, this often leads to fiber-lifting in the heat set offset printing process.

Quality demands are rapidly increasing in today’s print jobs and therefore issues like fiber-roughness and fiber-lifting have been studied, targeted to find the optimum balance in quality, production costs and runnability. Only through optimization of both the base sheet and the coating process, have significant improvements been made.

Enhancing the surface and porosity of the base paper and bonding of individual fibers required:

- adaptation of the Chemi-Thermo-Mechanical Pulping process involved;
- selective application of chemical pulp;and,
- optimization of paper machine operations.

Significant modification of coatings applied addressed:

- interaction with the base-sheet;
- speed of immobilization; and,
- rheology.

By optimizing several unit-operations involved in LWC-production, fiber roughness and fiber lifting improved significantly, with marginal effects on costs and runnability.

This paper does not describe a comprehensive solution to both phenomena. It does report the work being done at SAPPI that is specific for the requirements and needs of the Lanaken mill in Belgium.