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

Douglas J. Gardner

Third Committee Member

William Gramlich

Abstract

Cellulose Nanofibrils (CNFs) are promising materials for reinforcement of polymer matrices attributable to their impressive physical and mechanical properties, as well as their biodegradability. However, the utilization of these materials in composites is made challenging by the water content of CNF slurries, the tendency of CNFs to agglomerate as they dry, and incompatibility between hydrophilic CNFs and hydrophobic polymer matrices. The most commercially viable drying methods to produce small-scale dry CNFs, such as spray drying, are very energy intensive, can only dry the materials down to micron-scale agglomerates, and do not preserve fibrillar aspect ratios. “Contact dewatering,” or the removal of bound water from CNF suspensions using wood flour (WF) and mechanical pressing, may provide a solution to the challenge of excess water and preservation of nanoscale CNF dimensions after drying. This project explored the use of contact dewatering with maple wood flour to remove water from CNF suspensions and incorporate dry CNF into poly(lactic) acid (PLA) composites as part of hybrid WF-CNF furnishes. This concept was further explored by using cryocrushed PLA powder instead of wood flour to dewater and process CNF into PLA-CNF composites. The use of WF showed preservation of nanoscale CNFs on the surface of wood particles after drying and high energy efficiency in water removal from CNF suspensions, with mixed results in terms of mechanical properties when incorporated into wood-plastic composites. These results included a slight increase in tensile modulus compared to PLA with WF at the lowest wt.%CNF/LCNF hybrid furnish, a drop in tensile strength with higher wt.%CNF/LCNF loading levels, and a reduction in flexural strength at lower wt.%CNF/LCNF loading levels. When successfully scaled-up to produce 50lbs of a PLA/WF/CNF composite for additive manufacturing, the process showed an 84% decrease in specific drying energy per gram of dried CNF compared to empirical values for spray-dried CNF (SDCNF). The use of small polymer particles as opposed to wood in the contact dewatering process for drying and direct compounding of CNF into composites has not been extensively studied, so the viability of contact dewatering with poly(lactic) acid (PLA) powder as a dewatering method for compounded PLA/CNF composites in terms of energy efficiency, preservation of nanoscale CNF morphology, and mechanical properties was also evaluated. Initial results that utilized high or equivalent amounts of CNF to PLA in the dewatering process showed some preservation of nanoscale morphology of dry CNFs, but with overall structures that favored spherical agglomerates with low aspect ratios that reduced the mechanical properties of the final composites compared to PLA and PLA/SDCNF. Using lower amounts of CNF in the dewatering process reduced the formation of larger-scale agglomerates, which was optimized to produce micron-to-nano-scale fibrillar CNF structures observed under polarized light microscopy (PLM). Attempting to model CNF agglomeration around PLA particles by assuming the formation of spherical agglomerates based on CNF weight percent and PLA particle size was not accurate to observed results for agglomeration based on the weight percent of CNF. Using PLA for contact dewatering showed more efficient water removal from the CNF suspension than previous wood flour-CNF furnishes, and specimens laser cut from compression molded films of dried materials showed an increase in tensile strength of up to 31% compared to pure PLA. Shear-mixed specimens showed an increase in tensile strength and modulus of 1.7% and 4.2% compared to PLA at 1 and 2wt.%CNF loading levels, respectively, and had equivalent or better properties than PLA-Spray Dried CNF at the same loading levels. None of these results were statistically significant save for significantly higher tensile modulus in dewatered CNF composites compared to SDCNF composites at 1 and 2wt.% total CNF loading levels post-shear-mixing. Theoretical energy for drying dewatered CNF to produce micron-to-nanoscale dry fibrils after one press was 67-205x lower than empirical values for drying SDCNF depending on initial CNF loading level.

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