Abstract
Lignin-based polyols were synthesized through microwave-assisted liquefaction under different microwave heating times (5–30 min). The liquefaction reactions were carried out using polyethylene glycol (PEG-400)/glycerol as liquefying solvents and 97 wt% sulfur acid as a catalyst at 140 °C. The polyols obtained were analyzed for their yield, composition and structural characteristics using gel permeation chromatography (GPC), Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectra. FT-IR and NMR spectra showed that the liquefying solvents reacted with the phenol hydroxyl groups of the lignin in the liquefied product. With increasing microwave heating time, the viscosity of polyols was slightly increased and their corresponding molecular weight (MW) was gradually reduced. The optimal condition at the microwave heating time (5 min) ensured a high liquefaction yield (97.47%) and polyol with a suitable hydroxyl number (8.628 mmol/g). Polyurethane (PU) foams were prepared by polyols and methylene diphenylene diisocyanate (MDI) using the one-shot method. With the isocyanate/hydroxyl group ([NCO]/[OH]) ratio increasing from 0.6 to 1.0, their mechanical properties were gradually increased. This study provided some insight into the microwave-assisted liquefied lignin polyols for the production of rigid PU foam.
Highlights
Lignin, as a nontoxic, low-cost and renewable resource, has been considered as a substitute for some petrochemical products to mitigate the effects of the petroleum resource crisis and environmental pollution caused by non-biodegradable polymers
The liquefaction yield increased with increasing microwave heating time from 5 to 20 min; the liquefaction yield was slightly reduced when the microwave heating time was 30 min
In contrast with the recently published literature, Jin et al [16] reported that the optimal liquefaction yield under conventional heating condition was 98.4%; the hydroxyl values of the liquefied products were relatively low for preparing the rigid PU foam, and the liquefaction time lasted as long as 60 min
Summary
As a nontoxic, low-cost and renewable resource, has been considered as a substitute for some petrochemical products to mitigate the effects of the petroleum resource crisis and environmental pollution caused by non-biodegradable polymers. One of the techniques to obtain the polyols from renewable resources is liquefaction Various lignocellulosic materials, such as wood [10], paper [11], straw [12], bamboo [13], sugar cane [14] and corn stover [15], have been liquefied in different solvents to generate polyols and to subsequently produce the PU foam. This study profoundly looks into the liquefaction behavior of lignin into polyols under microwave heating for the preparation of rigid PU foam. Based on previous studies [16,25], the optimum conditions were selected in microwave heating liquefaction using the mixed liquefying reactants of PEG-400 and glycerol (80/20), lignin/liquefying reactants at a ratio of 0.2, reaction temperature at 140 °C and catalyzed with 1.5% sulfuric acid (based on the weight of PEG-400 and glycerol). The optimal polyols obtained were used to evaluate the performances of the PU foam at different [NCO]/[OH] ratios
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