Adhesive Contaminants (Stickies): Characterization and Their Interaction with Papermaking Components During Paper Recycling

Abstract

The effect of individual paper components on the properties of an acrylate pressure-sensitive adhesive (PSA) after laboratory high-consistency pulping was investigated. The adhesive material was quantified by selectively dyeing the adhesive particles followed by image analysis. It was found that some of the components of copy paper have a significant effect on the size and shape of PSA particles formed during pulping. Further, some copy paper components were also found to have a significant effect on the tackiness and deposition of PSA particles after pulping.

The stability of micro-stickies is of great concern to papermakers as these materials, when they agglomerate, can deposit on equipment and on the paper product. In order to investigate, the charge demand of an acrylic microsphere adhesive emulsion (a model for micro-stickies), softwood market pulp and different starches were titrated by standard polymer solutions. The effects of fibers, different cationic polymers and salts on the stability of the micro-stickies were investigated. The stability of the micro-stickies in water was evaluated using turbidity and a hemacytometer with microscope. The agglomeration of micro-stickies with Poly-DADMAC occurred mainly via a neutralization mechanism. In contrast, the agglomeration of micro-stickies with cationic starch does not correspond with neutralization mechanism. Both Poly-DADMAC and cationic starch could stabilize the micro-stickies if they were charged to the system in excess. The adhesive emulsion particles have a double layer and both salts and cationic polymers have an effect on the stability of micro-stickies. The stability with respect to salts with different valence obeyed the Shultz-Hardy rule. Both the anionic fibers/fines and micro-stickies compete for the cationic polymer in the system.

PSA particles, or stickies, will agglomerate in water with agitation. Under certain conditions, some starches prevent this agglomeration. Both cationic and neutral starches were found to adsorb onto the surface of the negatively charged stickies. This was determined by infra-red spectroscopy, UV/visible spectroscopy, contact angle measurements and charge titration methods. Cationic charge promotes the adsorption of starch onto the surface of stickies and cationic starches adsorbed on the PSA film do not desorb when exposed for 15 minutes to deionized water at room temperature. Fibers compete with stickies for the cationic charge in the system. Over the molecular weight (MW) range studied, MW has no effect on the stabilization of stickies.

The cleanliness efficiencies of macro-stickies and micro-stickies in each operational unit of a recycling mill were qualitatively determined. A deposition tester and a solvent extraction technique were used as complementary methods to analyze macro- and micro- stickies, respectively. The primary coarse screen, secondary fine screen, tertiary fine screen, the cleaners and flotation were found to remove macro-stickies. In contrast, the primary fine screen was observed to have a net macro-stickies cleanliness efficiency of 0%. The primary fine screen caused a significant amount of macro-stickies to change into a more "string-like" shape (like fibers). The deflaker was determined to produce a significant amount of micro-stickies.