Behavior of Adhesive Materials in Screening Devices for Paper Recycling

Abstract

The objective of this research is to understand the behavior of pressure sensitive adhesive (PSA) materials in industrial and laboratory screening devices. The research was initiated by screening pulp containing PSA using an industrial pressure screen with fine slots. The industrial pressure screen removed less than 80% of the feed PSA contaminant. In contrast, an atmospheric laboratory screen of the same slot width, 0.006 inches, removed 99% of the PSA. One of the reasons for the lower removal efficiency was found to be a significant breakage of PSA particles into smaller particles within the industrial pressure screen. The breakage of PSA particles in a shear field was studied using a high shear mixer and the consistency was found to be the most significant variable affecting the breakage of the PSA. It was also of interest to investigate how the PSA material could deform and pass through the fine slot of a pressurized industrial screen. A laboratory screening device with a single slot was developed to study the passage of PSA particles through the slot as a function of pressure drop across the slot. It was observed that at moderate pressure drops, particles with their smallest dimension up to five times the slot width passed through the slot. Increased temperature promoted the passage of the particles through the slot, indicating that the physical properties of the PSA influenced its passage. To understand the relationship between PSA formulation, physical properties, and passage of particles through a fine slot, several PSA formulations of known composition were prepared for analysis. Differences in the PSA formulations were found to affect the yield strain, yield stress, and modulus of the PSA films, which correlated with particle breakage in the pulper. Yield stress and particle area correlated with particle passage through the slot. A mechanical model for passage is in agreement with the experimental results, indicating that yield stress and particle size are significant for PSA particle passage through fine slots. These studies have resulted in important information for screen manufacturers, paper recyclers, and PSA producers to improve the removal of PSA particles from recovered paper.