Date of Award

Summer 8-21-2020

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Thomas Schwartz

Second Committee Member

Adriaan Van Heiningen

Third Committee Member

Douglas Bousfield

Abstract

In recent years the demand for packaging paper grades has increased while the demand for printing and writing grades has continue to decline. Packaging grades are increasing in importance and are sold based on a combination of thickness and area rather than basis weight. This issue has driven the industry to produce paper board sheets of low apparent density to save fiber costs. The use of extracted xylan to produce paper with higher strength at a given density has been suggested in the literature; however the direct relationship between hemicellulose addition and apparent sheet density has received little attention in the literature. Our work includes a literature review of the pulp fiber characteristics that affect apparent sheet density. Bleached pulp was treated with hemicellulose (beechwood xylan) in a 0.5M sodium chloride solution at 120°C for two hours. We observe a small decrease in apparent sheet density and a decrease in tensile strength index as the xylan loading increases. Analysis of the handsheets shows a lower hemicellulose uptake than the literature suggested. This discrepancy prompted an investigation into the mechanism by which hemicellulose, particularly xylan, attaches to cellulose surfaces. If the mechanism is understood then the adsorption process can be tuned to maximize xylan uptake on pulp fibers, thus maximizing the effect on apparent sheet density. The literature almost exclusively concludes that hydrogen bonding is the primary mechanism for xylan attachment onto cellulose surfaces, which we verify using a thermodynamics approach. We attempt to calculate an enthalpy of adsorption using the Van ’t Hoff relationship and equilibrium constants from adsorption isotherms. Isotherms were collected in a temperature range of 35-95°C in 0.5M sodium chloride solution using Whatman 42 filter paper as a pure cellulose source. Isotherms were also collected using SigmaCell pure cellulose instead of Whatman 42 filter. Additional experiments were conducted with SigmaCell cellulose using deionized water and 1.0M sodium hydroxide for solvents. The observed isotherms do not appear to be temperature-dependent, suggesting a very weak attraction between hemicellulose and cellulose. However, a decrease in uptake was observed in cases where sodium chloride was omitted from the system and increases in uptake when aqueous sodium hydroxide solution was used as a solvent. The ionic strength provided by the sodium chloride mitigates the negative charge repulsion between the xylan and cellulose. It is likely that the alkaline environment cleaves glucuronic acid substituents from the xylan backbone. This decreases the solubility of the xylan polymer causing it to aggregate, precipitate from solution, and finally adsorb onto cellulose surfaces. As suggested in the literature, hydrogen bonding and Van der Waal’s forces are likely the dominant mechanisms for xylan adsorption onto cellulose.

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