Improving mechanical properties and functionality of bio-foams

Abstract

In various industries, there is a growing need for circular alternatives to plastics, particularly for protective coatings and foams. Lignin, the second most abundant natural component, remains underutilized due to its complex structure. This thesis focuses on effectively utilizing lignin to create value-added structures, specifically hydrophobic coatings and bio-foams. Coating and foaming suspensions were prepared by mixing methylcellulose (MC) and organosolv lignin powders with water. These suspensions were used to coat glass wafers via spin-coating and to produce foams through a laboratory-scale process. Additionally, the substitution of lignin with citric acid (CA) in foam production was explored. Surface morphology of the coated wafers and foams was examined using optical and scanning electron microscopy, while water contact angle tests were conducted to assess surface wettability. Mechanical properties of lignin-based foams were evaluated through compression tests, and Fourier Transform Infrared (FTIR) spectroscopy was utilized to analyze chemical bands.

The water contact angle findings of the coatings indicate that increasing the lignin content in the suspension, while maintaining a constant MC content, enhances the coatings' hydrophobicity up to an optimal weight fraction (C = 2.5 wt.\%), where the initial contact angle reaches 114°. Beyond this point, the coated surface becomes more hydrophilic. Similar trends were observed in both lignin and CA-based foams, with optimal weight fraction of C = 2 wt.\% for lignin and C = 1.5 wt.\% for CA, respectively. This behavior was attributed to the optimal surface coverage achieved by lignin particles on the coatings and foams at the optimal lignin weight fraction. However, for CA-based foams, this phenomenon was attributed to limitations in the chemical reaction between CA and MC. Additionally, compression tests revealed enhanced mechanical properties of the lignin-based foams. Heat-pressing lignin-based foams indicated potential for enhancing their water resistance, depending on the heating temperature. In summary, this study presents an effective approach for utilizing organosolv lignin to create hydrophobic coatings and bio-foams, while also improving the mechanical properties of the resulting bio-foams. This method holds promise for developing sustainable alternatives to plastics, particularly in industries such as packaging and electronics, where lignin-based bio-foams could replace Styrofoams.