Rigid polyurethane foams containing lignin oxyalkylated with ethylene carbonate and polyethylene glycol
A “green” oxyalkylation protocol employing ethylene carbonate (EC), polyethylene glycol 400 (PEG400), and water was developed to convert an industry byproduct, kraft lignin (KL), into biopolyols for the production of rigid polyurethane (RPU) foams. EC served as an oxyalkylation reagent for the derivatization of KL’s hydroxyl (OH) groups. PEG400 and water served as a co-solvent and a reactive additive, respectively, to tailor two properties of the KL-derived biopolyols, those being viscosity and OH number. For the oxyalkylated-KL (OKL), as characterized by 31P-nuclear magnetic resonance (31P-NMR), more than 80% of OH content was aliphatic. The 14-fold increase in reactivity towards isocyanates for the OKL, over that for the unmodified KL, was attributed to the derivatization of lignin phenolic OHs to aliphatic OHs, the latter being more accessible and reactive during the RPU foam production. The preferred oxyalkylation conditions were 170 °C, 30 min, PEG400/EC ratios of 50/ 50 to 70/30 (wt/wt), and KL mass loadings of 20–50 wt%. Biopolyols synthesized under these conditions exhibited viscosities from 0.5 to 53 Pa·s, OH numbers from 459 to 659 mg KOH/g, and alkalinity numbers below 4 mg KOH/g. The biopolyols were directly used for RPU foam production. RPU foams produced under optimal conditions showed densities from 51 to 60 kg/m3 and compressive strengths from 139 to 204 kPa.