Producing Bark-based Polyols through Liquefaction: Effect of Liquefaction Temperature

Abstract

Bark-based polyols were synthesized through a solvent liquefaction in a polyethylene glycol (PEG)/glycerol cosolvent. Liquefaction reactions were carried out at temperatures of 90, 130, and 160 °C. The bark-based polyols were analyzed for their yield, composition, and structural characteristics using the standard titration method for hydroxyl value, combined with gel permeation chromatography (GPC), Fourier transform infrared (FTIR), and liquid state phosphorus (31P), carbon (13C), and proton (1H) NMR analyses. As the liquefaction temperature increased, viscosity of the polyols became higher with a corresponding broadening of the molecular weight (MW) distributions that also shifted toward higher MW. The liquefaction of biomass induced a high degree of modification to the bark components. These polyols had similar hydroxyl values but differed greatly in molecular structures. The polyol obtained through liquefaction at 90 °C had more secondary alcohols and contained sugars. Meanwhile, sugars were degraded into levulinate and formic esters in the polyols obtained at 130 and 160 °C. None of the polyols had condensed tannins, neither in their polymeric or monomeric state. Instead, aromatic ethers were seen in the carbon NMR spectra and various carboxyl functionalities were observed from the FTIR analysis. These results demonstrated the influence of the liquefaction temperature on the liquefaction behaviors of the bark biopolymers and provided an insight into the physical and structural properties of these bark-based polyols.