Surface Modification of Lignin with Methyltrimethoxysilane (MTMS) for Enhanced Hydrophobicity

Abstract

Lignin is an abundant aromatic biopolymer source produced from the processes of pulping and biorefining as a major byproduct. Reutilizing this renewable resource in cementitious materials offer a sustainable approach to enhancing their durability. However, the abundant hydroxyl groups on lignin promotes moisture adsorption and strong intermolecular hydrogen bonding, leading to particle agglomeration and limiting its effectiveness in improving the water resistance of cementitious materials. Therefore, surface modification to produce hydrophobic lignin particles is a promising strategy for reducing moisture affinity and enhancing the durability of lignin-modified cementitious materials. This study investigated the use of methyltrimethoxysilane (MTMS) to modify lignin surfaces and evaluated the effects of lignin concentration and MTMS-to-lignin mass ratio on the resulting hydrophobicity. Lignin suspensions (3–7 and 10–70 mg/mL) were prepared in aqueous ethanol and mixed with hydrolyzed MTMS. Initial experiments were performed at a constant MTMS-to-lignin mass ratio of 0.3:1 using lignin concentration of 3–7 mg/mL. After stirring, drying, and being grinded, the samples were examined by optical microscopy and tested using water droplet tests to determine hydrophobicity. Hydrophobicity increased with lignin concentration between 3 and 7 mg/mL, with the 7 mg/mL sample showing the largest contact angle. For the expanded concentration series, the 10 and 30 mg/mL samples showed low hydrophobicity, while the 50 and 70 mg/mL samples showed high hydrophobicity. The mass retentions of the 10, 30, 50, and 70 mg/mL samples were calculated as 70.0%, 74.0%, 72.4%, and 78.9%, respectively. Since lignin concentrations of 50 and 70 mg/mL showed better hydrophobicity results compared to all other lignin concentration samples, they were advanced for evaluation at MTMS:lignin ratios of 0.1:1 and 0.6:1, where the sample with lignin concentration 50 mg/mL at 0.6:1 MTMS:lignin ratio showed to be the best performing sample. These findings demonstrate that lignin particles can be modified to become hydrophobic using MTMS, therefore supporting their potential application as water resistant admixtures in cementitious materials.