Increasing adsorption by wood flour of quaternary ammonium salts to improve reactivity when preparing thermoplastic lignocellulose

To enhance the bulk butyrylation and flowability of thermoplastic lignocellulose (TPLC) ultimately to be produced by reactive extrusion, understanding the role of quaternary ammonium salts (quats) as a pretreatment of Aspen Wood Flour (AWF) was essential. This study investigated the adsorptive nature of AWF treated with two distinctly different quaternary ammonium surfactants based on their hydrophilicity to improve the extent of butyrylation, filling a gap in knowledge related to how they associate with lignocellulosic species in a varying acidic environment. The pretreatment effectiveness of the two surfactants was evaluated based on their ability to enhance the reactive effectiveness of butyric anhydride with the resulting improvements reported by a visual adsorption test using Congo Red dye, BET surface area analysis, colorimetric titration, contact angle measurement, electrophoretic mobility testing, and elemental analysis. Adsorption efficiency significantly improved from 1.4% for the untreated AWF to peak values of 62.8% (0.70 mmol/g CTAB) and 87.5% (0.35 mmol/g Hyamine). BET analysis revealed increased surface areas, notably peaking at 13.63 m2/g (CTAB) and 14.79 m2/g (Hyamine) at optimal quat concentrations. Butyrylation mirrored these trends, reaching maximum butyryl contents of 7.67 mmol/g (CTAB) and 7.93 mmol/g (Hyamine) compared to 0.25 mmol/g for the untreated AWF. The more hydrophilic surfactant was more likely to increase surface area to the benefit of the butyrylation reaction whereas the hydrophobic surfactant accumulated on the fibrous surface as a coating that closed over pores and reduced surface area, as well as more effectively decreased the negative charge of the acidified lignocellulose.