Date of Award

Summer 8-19-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Forest Resources

Advisor

Mehdi Tajvidi

Second Committee Member

Jinwu Wang

Third Committee Member

Douglas W. Bousfield

Additional Committee Members

William M. Gramlich

Abstract

Self-standing cellulose nanofibril (CNF) films are regarded as one of the promising alternatives to current petroleum-based packaging materials. The mechanical and barrier properties of CNF films are not yet up to the mark for certain applications, especially at high relative humidity. Those properties of CNF films can be tuned by the drying methods of films, degree of fibrillation, cross-linking, and controlled shrinkage. A comprehensive understanding of these processes and their influence on the structure and properties of CNF films have been presented in this thesis.

First, we prepared CNF films from CNF suspensions with two different degrees of fibrillation- standard CNF (90% fine) and high-fine CNF (97% fine) by casting and filtration. These were dried in four different ways: air, oven, heat gun, and hot press drying. The CNF films made by hot press drying showed the highest tensile strength (98.82 MPa) and lowest water vapor permeability (13.91 g.mm/m2.day.kPa). A facile thermal compression on the dried films further improved the strength by 13.1%, reduced the water vapor permeability by 22% and oxygen permeability by 43%.

With the hot-press drying and thermal compression technique, we created self-standing films of lignin-containing cellulose nanofibrils (LCNFs) derived from recycled old corrugated cardboard (OCC) pulp that cost considerably less than bleached softwood Kraft (BSK) pulp yet only use half as much energy for refining to obtain the same fines content. The low zeta potential (-3.83 mV) of OCC-derived LCNFs (OCC-LCNFs) resulted in aggregation of the fibrils in aqueous suspension, leading to considerable unpredictability in oxygen permeability values (coefficient of variation 36%). The addition of 3 wt.% (based on the dry weight of LCNF) carboxymethyl cellulose (CMC) lowered the coefficient of variation with an average oxygen permeability of 1478 (cc.μm/m2.atm.day) at 80% relative humidity. We demonstrated that ionic crosslinking with Al3+ or covalent crosslinking with polyamide epichlorohydrin could decrease the oxygen permeability by 30% at 23 °C and 80% relative humidity, while also significantly enhancing the tensile strength and modulus.

Finally, the shrinkage in CNF films upon drying has been studied. The shrinkage was classified into the radial and vertical directions. Two types of CNF films were prepared: one in a restrained condition that did not allow shrinkage in the radial direction but enabled it in the vertical direction and another one with 11% radial shrinkage but limited vertical shrinkage. The radial shrinkage led to a more porous and less dense structure than the vertical shrinkage, which brought about poorer oxygen and moisture barrier performance than its counterpart. Interestingly, radial shrinkage resulted in 140% and 90% higher strain at break and toughness in films with a significant sacrifice in strength and modulus. The structural changes caused by the radial and vertical shrinkage were revealed by scanning electron and optical microscope images.

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