Improved Lignin Polyurethane Properties with Lewis Acid Treatment

Abstract

Chemical modification strategies to improve the mechanical properties of lignin-based polyurethanes are presented. We hypothesized that treatment of lignin with Lewis acids would increase the concentration of hydroxyl groups available to react with diisocyanate monomers. Under the conditions used, hydrogen bromide-catalyzed modification resulted in a 28% increase in hydroxyl group content. Associated increases in hydrophilicity of solvent-cast thin films were also recorded as evidenced by decreases in water contact angle. Polyurethanes were then prepared by first preparing a prepolymer based on mixtures of toluene-2,4-diisocyanate (TDI) and unmodified or modified lignin, then polymerization was completed through addition of polyethylene glycol (PEG), resulting in mass ratios of TDI:lignin:PEG of 43:17:40 in the compositions investigated here. The mixture of TDI and unmodified lignin resulted in a lignin powder at the bottom of the liquid, suggesting it did not react directly with TDI. However, a homogeneous solution resulted when TDI and the hydrogen bromide-treated lignin were mixed, suggesting demethylation indeed increased reactivity and resulted in better integration of lignin into the urethane network. Significant improvements in mechanical properties of modified lignin polyurethanes were observed, with a 6.5-fold increase in modulus, which were attributed to better integration of the modified lignin into the covalent polymer network due to the higher concentration of hydroxyl groups. This research indicates that chemical modification strategies can lead to significant improvements in the properties of lignin-based polymeric materials using a higher fraction of an inexpensive lignin monomer from renewable resources and a lower fraction an expensive, petroleum-derived isocyanate monomer to achieve the required material properties.

Demethylation of Wheat Straw Alkali Lignin for Application in Phenol Formaldehyde Adhesives

“…This may be due to part cleavage of ether bonds during the demethylation process [18]. The results showed that the WSAL was demethylated successfully and more efficiently than in other reported methods [21,25]. The methods reported in the past, employed demethylation processes with harsher conditions and had a lower yield of phenolic hydroxyl groups.…”

“…Lignin has many methoxyl groups and it has been shown that lignin can also be demethylated using a Lewis acid [23,24]. Furthermore, Chung and Washburn [25] reported demethylation of softwood kraft lignin (SKL) by hydrobromic acid and phase transfer catalyst hexadecyltri-n-butylphosphonium bromide (TBHDPB) in a H 2 O-DMF mixture. The hydroxyl group in SKL increased by 28% and the demethylated SKL was more reactive than the unmodified SKL when used for polyurethane preparation.…”

The Effect of Plant Source on the Properties of Lignin-Based Polyurethanes

“…The majority of isocyanates used such as toluene-2,4-diisocyanate, isophorone diisocyanate, and 4,4-methylene diphenyl diisocyanate are aromatic in structure while aliphatic isocyanates, such as hexamethylene diisocyanate (HMDI), are also used (Teo et al, 1997, Chan-Chan et al, 2010, Prisacariu, 2011, Chung and Washburn, 2012, Zhang et al, 2015. Polyols are usually the more flexible of the two components which allows the formation of flexible PU materials and usually consist of polyesters, polyethers, or glycols such as polyethylene glycol (PEG), 1,4-butanediol, and polypropylene glycol (PPG) (Grassie and Zulfiqar, 1978;Wen et al, 2001;Oprea, 2010;Nozaki et al, 2017).…”

Chemistry of lignin-based materials

“…Hydroxyl group content is best determined after acetylation of hydroxyl groups by acetic anhydride in the presence of pyridine. 33,34 The acetoxy groups produced in this reaction correspond to hydroxyl groups of lignin and appear at chemical shifts of approximately 2 ppm. 34,35 After acetylation, the phenolic hydroxyl group can be determined selectively by relatively fast, selective, deacetylation of phenolic acetyl groups.…”

“…33,34 The acetoxy groups produced in this reaction correspond to hydroxyl groups of lignin and appear at chemical shifts of approximately 2 ppm. 34,35 After acetylation, the phenolic hydroxyl group can be determined selectively by relatively fast, selective, deacetylation of phenolic acetyl groups. 36 Alkylation of hydroxyl group was performed to determine hydroxyl groups in 1 H NMR spectra.…”