Fully Bio-based Adhesive from Enzymatic Saccharification Residue

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Fully Bio-based Adhesive from Enzymatic Saccharification Residue

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2018-02

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In this dissertation, the utilization of lignin-rich residue from the solid waste stream of bio-ethanol (fermentable sugar) production was investigated for wood adhesive applications. The novelty of this value-adding conversion lies in the adoption of three key strategies: 1) utilizing the lignin-rich by-product without additional purification or modification steps, 2) reducing the particle size of the residue to compensate for the intrinsic low reactivity of lignin, and 3) using a bio-based crosslinker, citric acid (without catalyst), to induce crosslinking via esterification reactions. The overall objective of this study was to examine the processing factors, bond strength evolution, mechanisms, and performance durability of the bonding. Results show that higher saccharification levels (percent of glucan removal) are favorable for achieving a higher wood bond strength. This validates a co-production opportunity in which the fermentable sugar production itself serves as an “activation” step to prepare its solid residue for adhesive applications. The saccharification residue exhibited stronger bonding to wood if they were ground to a smaller size (increased reactive surfaces), attaining a lap shear strength of up to 6.25 MPa, which is 80% of the wood bond strength of phenol formaldehyde (PF), an industrial wood adhesive from petrochemical sources. The application of citric acid further enhanced the dry and wet bonding properties of the saccharification residue. The bond strength evolution in citric acid curing correlates to the formation of ester linkages as confirmed by FTIR. Curing longer than the optimum five minutes decomposed the esters formed, based on FTIR and thermogravimetric studies. At the same time, lignin condensation occurred, as revealed by solid-state 13C-NMR, possibly compensating for the loss in ester linkages as judged from the unaltered wet bond strength when compared to the optimum 5-min curing. Conductometric titration revealed that two-third of the carboxylic groups of the citric acid (trifunctional) participated in ester formation, supporting the mechanism that the reactions occur via cyclic anhydride intermediates. This mechanism, which suggests that bifunctional carboxylic acids could not participate in crosslinking, was also verified by the use of adipic acid in lieu of citric acid; the former did not markedly improve either the dry or wet bond strength of the saccharification residue. This finding also confirms that citric acid functions as a crosslinker for the residue in wood bonding. Overall, this fully bio-based, formaldehyde-free, and competitive adhesive co-product would add values to bioconversion wastes and increase the economic viability of the cellulosic biofuel production.

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University of Minnesota Ph.D. dissertation. February 2018. Major: Bioproducts/Biosystems Science Engineering and Management. Advisor: William Tze. 1 computer file (PDF); vii, 126 pages.

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Hafez, Islam. (2018). Fully Bio-based Adhesive from Enzymatic Saccharification Residue. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/202905.

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