Browsing by Author "Richard A. Venditti, Committee Co-Chair"

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    Behavior of Adhesive Materials in Screening Devices for Paper Recycling
    (2004-04-28) Lucas, Bradley Earle; Hasan Jameel, Committee Co-Chair; Martin A. Hubbe, Committee Member; Saad A. Khan, Committee Member; Richard A. Venditti, Committee Co-Chair
    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.
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    Preventing Strength Loss of Unbleached Kraft Fibers
    (2003-10-30) Zhang, Min; Samuel M. Hudson, Committee Member; Richard A. Venditti, Committee Co-Chair; John A. Heitmann, Committee Member; Martin A. Hubbe, Committee Chair
    The purpose of this study was to understand the mechanism of paper strength loss that occurs when paper made from chemical pulps is recycled. It is found that due to drying, unbleached kraft pine fibers lost cellulose viscosity, water retention value, fiber flexibility and accessible surface area. Handsheets made from dried fibers had lower paper strength and lower apparent density compared to corresponding primary handsheets made from never-dried fibers. With the increase in drying temperature of virgin fibers, the above properties of dried fibers and recycled handsheets experienced larger effects. It was hypothesized that adding certain chemicals to virgin fibers before drying could prevent strength loss upon recycling. Results showed that relatively low molecular weight additives (such as sucrose and glucose) appeared to interfere with the mechanism of pore closure during drying and improved the strength of recycled paper. Higher molecular weight chemicals added to never dried virgin fibers (such as starch) also increased the strength of the recycled paper but this was attributed to residual chemical being retained on the fiber surface during recycling. Although the effect of adding certain chemicals to virgin fibers before drying could significantly prevent strength loss in recycled paper, it was determined that improvements of recycled paper strength due to refining were of much larger magnitude. It is found that recycled handsheets had lower paper strength compared with virgin handsheets at all pH values considered during paper formation within the range of pH 3 to pH 8. There was no significant effect of pH on paper strength within this range. The fiber flexibility tests showed that the method is useful to determine the flexibility of individual fibers. In the case of sugar treatment, treated fibers showed higher flexibility compared to untreated fibers after drying, and glucose was found to have larger effect than sucrose. With respect to papermaking conditions, fibers were more flexible under alkaline conditions than fibers under acidic conditions, but fibers became less flexible with increasing salt concentration and hardness.