The concept of stickies exposure for paper recycling processes

Fragments of converting adhesive films (namely stickies) are the cause of severe runability problems in paper recycling processes. These tacky particles are picked-up by the paper machine clothes, inducing their clogging, which is often at the origin of the break of the running paper web. To investigate stickies related problems, these particles are usually characterized by their concentration in the pulp. Industrial experience has shown that it is often difficult to relate the frequency of paper web breaks to the concentration of stickies in the pulp. In this work, we propose that paper machine runability issues are related to the amount of stickies that is effectively exposed at the surface of the sheet to the machine clothes, so that their concentration is not sufficient to describe the problem. A definition of the stickies exposure concept is proposed, through statistical geometry methods. The effect of stickies morphological parameters on their exposure is studied theoretically. Their orientation and distribution within the sheet thickness is found to be of importance as well. Hopefully, this new concept will help to better understand runability problems on paper machines using recycled raw material. Admittedly, stickies exposure only represents the geometrical aspect of stickies deposition problems. Hence, this approach is only the first step towards a more general description of the stickies related problems, which would take into account other parameters, such as adhesion and paper strength.