Date of Award

4-2024

Degree Name

Doctor of Philosophy

Department

Chemical and Paper Engineering

First Advisor

Kecheng Li, Ph.D.,

Second Advisor

Abdus Salam, Ph.D.

Third Advisor

Alexandra Pekarovicova, Ph.D.

Fourth Advisor

Yirong Mo, Ph.D.

Keywords

Enzymes, fiber bonding, fiber morphology, paper recycling, stickies removal, sticky contaminants

Abstract

Producing paper with recycled paper consumes about 30-70% less energy and emits less GHG than using virgin wood. However, with current recycling practices, more foreign matters are commingled with the recycled paper including hot-melt adhesives, food residues, inks, resins/sizing agents, and coating adhesives, which result in stickies issues in the paper remanufacturing process. On the other hand, recycled fibers inherently have lower quality due to mechanical treatment, heating, chemical processing, and aging in various types of application/use of paper products, limiting the production of high-quality paper grades from recycled fibers. The goal of this dissertation research is to address both the knowledge and technology gaps in contamination removal and fiber quality restoration in paper recycling.

Various recycled fibers including containerboard, newsprint, and residential wastepaper were analyzed using solvent extraction, FTIR, SEM, GC-MS and colloidal titration. The results indicate that the quantity and size and number of stickies vary among different paper grades. The wastepaper from curb side contains nearly 3 times more stickies than most of the wastepaper grades. Sticky contaminants, predominantly composed of polyvinyl acetate, styrene butadiene rubber, paraffin wax, resin, and polyamines, were found not only to create sticky issues on the paper machine and generate anionic trash (2-4.5 mmol/L), but also negatively impact paper strength.

Enzymatic technology was developed for stickies removal with three enzyme formulations. Critical factors influencing the enzymatic treatment process, including temperature, pH, residence time, and enzyme dosage, were systematically investigated. Enzyme treatment under optimal conditions, i.e., 65°C, 30-60 minutes, and pH 8.0, resulted in the removal of up to 93.4% of stickies. Microscopic analysis using SEM and AFM revealed that stickies deposits, appeared as tar-like materials, adhere to fibers and cover microfibrils on the fiber surface, and with the enzyme process developed, the stickies were effectively eliminated.

In fiber quality study, changes in both cell wall structure and chemical components were investigated with using SEM, FTIR, XRD, and with reducing sugars, WRV, and FQA analysis. The results indicate that enzymes preferentially attack the amorphous regions of cellulose and hydrolyzed cellulose into glucose. The fiber cell wall structure and inner layers were loosened, making the fibers more flexible and collapsible. As a result, the swelling capacity of recycled fiber was improved by up to 20%, and the physical properties of remanufactured papers were increased by 18-66% depending on the paper grades.

This comprehensive research identified and investigated the two most challenging technical issues facing the paper recycling industry. The enzyme-based technologies developed for both stickies removal and fiber quality restoration in this research will help the industry in using more recycled fibers in paper remanufacturing and produce more high-quality paper from recycled fibers, thus, promoting environmentally friendly industry practice for a sustainable future. The research work also reveals the fundamental mechanisms behind enzyme modification of wood fiber ultrastructure and the chemistry behind enzyme-assisted dissolution of stickies substance from fiber suspension system, providing an avenue for further development of enzyme-based green technologies for other industry applications.

Access Setting

Dissertation-Open Access

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