Thermal and mechanical properties of coconut shell lignin-based polyurethanes synthesized by solvent-free polymerization

In the present work, polyurethanes were synthesized through a three-component system formed by coconut shell ethanosolv lignin (CSEL), polyethylene glycol 400 and toluene diisocyanate (TDI), through a solvent-free polymerization route. The effects of CSEL content and [NCO]/[OH] ratio used in the formulations in a range from 0 to 50% w/w and 1.0–1.5, respectively, on properties such as porosity and crosslinking degree of PUs were investigated. For this purpose, four formulations with increasing CSEL content were chosen to evaluate the effect of its insertion on the structural, morphological, thermal and mechanical properties of these PUs. The CSEL-based PUs presented higher thermal and thermo-oxidative stability in relation to the PU without CSEL. Moreover, an increase in glass transition temperature (T_g) was observed as the CSEL content in the PU formulations increased, suggesting that the addition of lignin caused an increase in the crosslinking degrees of the obtained materials. However, high lignin contents produced PUs with poor mechanical properties, while intermediate lignin contents produced materials with interesting mechanical properties.