Preparation Of Lignin-based Polyurethane And Its Performance Characterization

Cellulose, lignin and hemicellulose are the major components in the lignocellulosic materials and also the most abundant biopolymers in nature. They are widely available, renewable, biodegradable and low cost which are competitive with petroleum-derived polymers. Cellulose, as a raw material in pulp and paper mills, has been found to have a lot of superior properties when it is incorporated into the composites synthesis. Cellulose nanocrystal (CNC), acid hydrolyzed from cellulose, which can be applied to structural materials and nanocomposites synthesis. Lignin, as a major waste in the pulping and paper process, led to the prosperity of the biobased chemicals and materials instead of being burned directly as a low value fuel. Considering the fact that lignin is a polymer with high concentrations of hydroxyl units (phenolic and aliphatic) and carboxylic groups that own reactive hydrogen, the lignin has the potential to replace commercial polyols in polyurethane (PU) production, one of the most diverse and widely used plastic materials with an ever-growing global market.In this work, the lignin role as a macromonomer in polyurethane synthesis was evaluated in two different approaches:(1) utilisation after chemical modification in producing the lignin based polyols, the lignin converted into the lignin based polyols through oxypropylation, liquefaction and microwave assisted liquefaction were fully discussed; (2) direct utilisation in making the lignin based PU composites, the preparation and properties of the lignin based rigid PU foams reinforced with pulp fiber were investigated, and the lignin based PU films enhanced by the CNC were also studied.Auto-catalyzed ethanol-water lignin converted into liquid polyols through oxypropylation and liquefaction techniques was fully investigated. The oxypropylation reaction was performed by reacting auto-catalyzed ethanol-water lignin (AEL) with propylene oxide under the acidic and alkaline conditions at room temperature, respectively. In contrast, the liquefaction reaction was carried out using the mixed solvents of polyethylene glycol and glycerol at 160℃ with sulfuric acid as a catalyst. The resulting polyols from each method was characterized by Fourier transform-infrared (FT-IR),1H and 31P nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) and thermogravimetric analysis. The results indicated that more secondary hydroxyl groups (2.016 mmol/g) were obtained in the alkaline oxypropylation reaction, whereas more primary hydroxyl groups (4.296 mmol/g) were found in the liquefied product; the alkaline oxypropylated product (Mw 3130 g/mol, Mn 2080 g/mol) and liquefied product (Mw 4990 g/mol, Mn 4630 g/mol) have higher molecular weights than AEL (Mw 2560 g/mol, Mn 1530 g/mol); the polyols have a lower thermal stability than AEL.Lignin-based polyols were synthesized through microwave-assisted liquefaction of alkaline lignin from the corncob under different microwave heating times. The liquefaction reactions were carried out using polyethylene glycol (PEG-400)/glycerol as liquefying solvents and 97wt% sulfur acid as a catalyst at 140 ℃. The polyols obtained were analyzed for their yield, composition, and structural characteristics using GPC, FT-IR and NMR spectra. The results showed that the liquefying solvents reacted with the phenol hydroxyl groups of the lignin in the liquefied products. 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 microwave heating time (5 min) ensured a high liquefaction yield (97.47%) and polyol with suitable hydroxyl number (8.628 mmol/g). PU foams were prepared by the polyols and methylene diphenylene diisocyanate (MDI) using one-shot method. With the isocyanate/hydroxyl group ([NCO]/[OH]) ratio increasing from 0.6 to 1.0, their mechanical properties were gradually increased.The alkaline lignin from the corncob was directly used as co-monomer in combination with a linear polyethylene glycol (PEG). The polyol was replaced with different amounts of lignin (8.33-37.19% w/w) to prepare the lignin based rigid PU foam (LRPF). The LRPF containing 37.19% lignin was further reinforced with different weight ratios (1,2 and 5 wt%) of pulp fiber. The resulting foams were evaluated by their chemical structure, cellular structure, density, compressive strength and thermal property. The results indicated that typical urethane linkages in LRPF were formed; the cell shape is significantly affected by the lignin and pulp fiber content, which both resulted in inhomogenous irregular and large cell shape and further decreased the densities of the LRPF; the compressive strength of the LRPF decreased with increasing lignin, but the additional pulp fiber had no significant effect on their compressive strength; the introduction of the lignin and pulp fiber into the lignin based PU foam would improve the thermal stability of the LRPF.The alkaline lignin from the corncob was used to make the lignin based PU (L-PU) film. The reinforcing effect of CNC in preparation of the L-PU film was investigated. Novel nanocomposite films were synthesized using CNC as a reactive reinforcing filler and L-PU as the matrix through casting and evaporating method. A series of L-PU films were prepared by replacing polyol with lignin from 10 to 50%(molar percentage). L-PU film with maximum addition of lignin content was used as the matrix, which were reinforced with 0.5,1 and 5 wt% dosage of CNC. The structural, mechanical and thermal properties of the resulting films were evaluated by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), tensile test and thermal gravimetric analysis. The results showed that the tensile strength of the L-PU films was significantly improved by the addition of CNC as a reinforcement; the thermal stability of the L-PU film after the addition of CNC was slightly increased.Based on the above results, this study provides irrefutable evidence about the possibility to directly or indirectly incorporate the lignin in making the PU materials. In most cases, the lignin based PU materials exhibit properties similar to those of conventional ones, thus presenting a viable option to rationally valorize an abundant renewable industrial by-product.