Lignin Polyurethane Research Articles

Green co-solvent-assisted one-pot synthesis of high-performance flexible lignin polyurethane foam

Although lignin is a promising candidate as a source of polyol for producing eco-friendly polyurethane foam (PUF), its direct application has limitations, primarily stemming from the brittleness of the resulting products and poor processibility at high lignin loadings. To address these technical challenges, we introduced an effective co-solvent for the lignin-based polyurethane foams (LPUFs) with up to 80 wt% of lignin substitution for petroleum polyols. As a green co-solvent that involves carbon dioxide fixation during its production, propylene carbonate (PC), was blended with lignin and improved the distribution of lignin by decreasing the viscosity, providing a room-temperature reaction. The LPUFs demonstrated a comparable thermal insulation performance (R-values greater than 5 in−1) with commercially available PUF with a uniform size of closed-cell structure induced by the reduced viscosity of lignin-based polyol. LPUFs also showed enhanced fire retardancy and moisture resistance compared to the control PUFs, benefiting from lignin’s intrinsic fire-resistant and hydrophobic properties. To mitigate the brittleness issue at high lignin loadings, flexible polyethylene glycols (PEGs) with a high molecular weight (i.e., PEG-1000) were dissolved in PC at room temperature to further improve the mechanical strength of LPUFs. The compressive strength of LPUFs with 80 wt% lignin substitution content was remarkably improved to 226 kPa by the employment of co-solvent and PEG-1000 compared to the control PUF (<140 kPa). Moreover, the flexibility of the LPUFs was successfully controlled by simply adjusting the PEG-400/PEG-1000 ratio. The results of this study provide valuable insights for accelerating and broadening the utilization of lignin in sustainable materials and manufacturing areas.

Mild condition lignin modification enabled high-performance anticorrosive polyurethane coating

The diverse active hydroxyl groups of lignin pose challenges in the preparation of lignin-based polyurethane coatings with exceptional long-term anticorrosive properties. Here, the dense and defect-free lignin-based polyurethane coating with a thickness of 25 ± 5 μm was successfully synthesized using a mild hydroxypropyl lignin modification approach, exhibiting outstanding barrier properties (|Z| > 109 Ω cm2) and long-term anti-corrosion performance exceeding 120 d. Under ambient conditions (i.e., 25 °C and atmospheric pressure), propylene oxide was directly blended with the alkali solution of lignin to effectively convert phenolic hydroxyl groups into more reactive aliphatic hydroxyl groups, while also minimizing the significant increase in molecular weight caused by lignin condensation. As a result, the high crosslinking density of lignin polyurethane coatings effectively prevented the penetration of corrosive media and enhanced the long-term corrosion resistance of the coatings. Overall, the results demonstrate that a mild hydroxypropyl modification process is an effective and facile strategy to prepare highly reactive lignin-based polyols, which is crucial for the development of high-performance bio-based polyurethane anticorrosive coatings.

Successive organic solvent fractionation and homogenization of technical lignin for polyurethane foam with high mechanical performance

This work demonstrates an organic solvent fractionation method for lignin homogenization, which can effectively reduce the lignin heterogeneity and use each lignin fraction to prepare polyurethane foams (PUFs) with excellent mechanical properties. Such fractions were fully characterized by GPC, NMR (31P, 2D-HSQC), FTIR, and TG to obtain a detailed description of the structures and properties. The properties of PUFs from each lignin fraction showed higher compatibility than that from unfractionated industrial lignin, as studied by morphology and DSC analysis. The improvement of compatibility between the fractionated lignin fractions and polyethylene glycol can effectively enhance the mechanical properties of the prepared PUFs. The hysteresis loss (43.10%–51.85%) and resilience (95.81%–98.81%) of the fractionated lignin polyurethane foams (LPUFs) were better than that from the unfractionated LPUFs (hysteresis loss 41.64%, resilience 94.67%) at the lignin content of 5%. Subsequently, the strong relationships between lignin structures and PUF properties were demonstrated in detail. The suggested approach provides greater possibilities to prepare LPUFs with tunable properties based on real industrial lignin fractions, rather than modified lignin.

The Effect of Plant Source on the Properties of Lignin-Based Polyurethanes

This work increases our understanding of the effect of plant source on the mechanical and morphological properties of lignin-based polyurethanes. Lignin is a polymer that is synthesized inside the plant cell wall and can be used as a polyol to synthesize polyurethanes. The specific aromatic structure of the lignin is heavily reliant on the plant source from which it is extracted. These results show that the mechanical properties of lignin-based polyurethanes differ based on lignin’s plant source. The morphology of lignin-based polyurethanes was examined using atomic force microscopy and scanning electron microscopy and the mechanical properties of the lignin-based polyurethane samples were measured using dynamic mechanical analysis and Shore hardness (Type A). The thermal analysis and morphology studies demonstrate that all polyurethanes prepared form a multiphase morphology. In these polyurethanes, better mixing was observed in the wheat straw lignin polyurethane samples leading to higher moduli than in the hardwood lignin and softwood lignin polyurethanes whose morphology was dominated by larger aggregates. Independent of the type of the lignin used, increasing the fraction of lignin increased the rigidity of polyurethane. Among the different types of lignin studied, polyurethane with wheat straw soda lignin exhibited storage moduli ~2-fold higher than those of polyurethanes incorporating other lignins. This study also showed that during synthesis all hydroxyl groups in the lignin are not available to react with isocyanates, which alters the number of cross-links formed within the polyurethane and impacts the mechanical properties of the material.

Mechanical and Thermal Properties of Polyurethane Films from Peroxy-acid Wheat Straw Lignin

In the present work, a type of PU film was made from peroxy-acid-derived wheat straw lignin by reacting varying amounts of lignin ranging from 10 to 50 wt% (with respect to PEG), polyethylene glycol (Mn = 1000), and tolylene 2,4-diisocyanate without catalyst. The effects of lignin content on the tensile property and thermostability of the lignin polyurethane films obtained were investigated. The thermal conductivity of lignin polyurethane films was also studied. Results showed that when the lignin content increased, the glass transition temperature (Tg) and tensile strength of the lignin polyurethane film increased, but the onset decomposition temperature and elongation at break decreased. When the lignin content was 40 wt%, the lignin polyurethane film had the best overall properties. In addition, compared with PU films, lignin PU films showed better heat-insulating properties.

Development of lignin-based polyurethane thermoplastics

Here we report utilizing a tailored feedstock to synthesize mechanically robust thermoplastic polyurethanes at very high lignin contents (65–75 wt%). The molecular weight and glass transition temperature (Tg) of lignin were altered through cross-linking with formaldehyde. The cross-linked lignin was coupled with diisocyanate-based telechelic polybutadiene as a network-forming soft segment. The appearance of two Tg's, around −35 and 154 °C, for the polyurethanes indicates the existence of two-phase morphology, a characteristic of thermoplastic copolymers. A calculated Flory–Huggins interaction parameter of 7.71 also suggests phase immiscibility in the synthesized lignin polyurethanes. An increase in lignin loading increased the modulus, and an increase in crosslink-density increased the modulus in the rubbery plateau region of the thermoplastic. This path for synthesis of novel lignin-based polyurethane thermoplastics provides a design tool for high performance lignin-based biopolymers.

Improved Lignin Polyurethane Properties with Lewis Acid Treatment

Chemical modification strategies to improve the mechanical properties of lignin-based polyurethanes are presented. We hypothesized that treatment of lignin with Lewis acids would increase the concentration of hydroxyl groups available to react with diisocyanate monomers. Under the conditions used, hydrogen bromide-catalyzed modification resulted in a 28% increase in hydroxyl group content. Associated increases in hydrophilicity of solvent-cast thin films were also recorded as evidenced by decreases in water contact angle. Polyurethanes were then prepared by first preparing a prepolymer based on mixtures of toluene-2,4-diisocyanate (TDI) and unmodified or modified lignin, then polymerization was completed through addition of polyethylene glycol (PEG), resulting in mass ratios of TDI:lignin:PEG of 43:17:40 in the compositions investigated here. The mixture of TDI and unmodified lignin resulted in a lignin powder at the bottom of the liquid, suggesting it did not react directly with TDI. However, a homogeneous solution resulted when TDI and the hydrogen bromide-treated lignin were mixed, suggesting demethylation indeed increased reactivity and resulted in better integration of lignin into the urethane network. Significant improvements in mechanical properties of modified lignin polyurethanes were observed, with a 6.5-fold increase in modulus, which were attributed to better integration of the modified lignin into the covalent polymer network due to the higher concentration of hydroxyl groups. This research indicates that chemical modification strategies can lead to significant improvements in the properties of lignin-based polymeric materials using a higher fraction of an inexpensive lignin monomer from renewable resources and a lower fraction an expensive, petroleum-derived isocyanate monomer to achieve the required material properties.

Water-Based UV-Curable Polyurethane Based on Wheat Straw Lignin Obtained by Ethanol Extraction

The aim of this paper is to discuss the application of ethanol-dissolved lignin in preparing high value products. Lignin was extracted from wheat straw by ethanol and used to prepare water-based UV-curable polyurethane. The preparation of water-based UV-curable polyurethane was completed by a three-step reaction. Besides lignin, toluene- 2, 4- diisocyanate (TDI), dimethylolpropionic acid (DMPA), 2-hydroxyethyl acrylate (HEA) and polyether glycol (PPG) were used. The effect of lignin content on the synthesis process was studied. The prepared lignin polyurethane was neutralized with tertiary amine (TEA) and dispersed in water. The properties of the final product, for example, rheological property, average particle size, UV-curing ability and mechanical property, etc. were characterized. Results indicated that the lignin polyurethanes could cure quickly and the presence of lignin with a proper proportion during the synthesis had a positive effect on the tensile strength of cured film.

Properties of lignin–polyurethane films prepared by casting method

A polyurethane elastomer made from a 1:5:6 mole ratio of poly(ethyleneadipate), ethylene glycol, and 4,4′-diphenylmethanediisocyanate was blended with flax soda lignin to form dimethylformamide-cast films containing between 4.2 and 23.2 wt.% lignin. The spectral, mechanical and thermal properties of this new type of blends were investigated in an attempt to establish their applicative potential. Films containing more than 9.3 wt.% lignin were heterogeneous, without relevance from the standpoint of potential applications, and were not tested for mechanical or thermal stability properties. The thermal degradation range of polyurethane and blends is quite similar, but the presence of lignin accelerates decomposition at lower temperatures. At 4.2 wt.% lignin, blends lost almost one half more weight until 300 °C, and almost twice at 9.3 wt.% lignin than pure polyurethane. The presence of 9.3 wt.% lignin also lowers the main degradation intensity corresponding to the polyurethane soft segment with about 120 °C (from 360 to 235–245 °C). The lignin concentration and polyurethane soft segment properties were shown to be the key degradation factors. The tensile strength increased up to 370%, toughness up to 470% and elongation at break up to 160% at blends compared to the pure polyurethane film. By increasing the strain rate from 4.6 to 120 mm/min the blends behavior generally improves, whereas the polyurethane elastomer is detrimental affected. Lignin contents up to about 10 wt.% were adequate to increase the polyurethane strength in detriment of elasticity. However, the better thermo-mechanical properties were found for the 4.2 wt.% lignin-containing blend.

Reacting Cotton Cellulose with Lignin-Based Polyurethane

Chain-extended lignin was reacted with blocked isocyanate and the product, lignin polyurethane (LPU), was subsequently applied by padding to cotton print cloth at three add-on levels: 1.70, 3.45, and 4.30%. Product syntheses and reactivity were followed by FTIR spectroscopy. LPU as a formaldehyde-free finish improved durable press (DP) and dimensional stability of the unfinished cloth, but the improvement was less than conventional amino resin finishes. Unlike conventional resin finishes, LPU showed strength and abrasion resistance retention. Abrasion resistance improved at the 3.5% add-on level. Grafting LPU onto cotton cloth is a surface reaction, given the molecular size of the product and detection by specular reflectance infrared spectroscopy. Even with the limited accessibility of the reactant groups in the unswollen cloth, a sufficient number of carbamoyl linkages formed to improve properties. Although DP ratings were not high, LPUs tailored by alkyloxyhydroxide chain extensions are likely resins for films, nonwoven consolidants, and composite materials, providing receptivity of cellulose to materials for which it has little affinity.

Mechanical properties of solvolysis lignin‐derived polyurethanes

AbstractIn order to study the tensile properties of solvolysis lignin polyurethanes, polyurethane (PU) films were prepared from the solvolysis lignin (SL)–polyethylene glycol (PEG)–diphenylmethane diisocyanate (MDI) system, and in addition to this further PU films were also prepared from the SL–MDI system. In the SL–PEG‐MDI system, the SL content, the molecular weight of PEG and the NCO:OH ratio were varied in order to control the physical properties. The tensile stress and Young's modulus of the PUs of the SL–PEG–MDI type increased with increasing SL content and NCO:OH ratio. The tensile properties of the PUs from the SL–MDI system showed no NCO:OH ratio dependency; i.e. the mechanical properties of PUs are not influenced by the change in crosslinking. It is possible to control the tensile properties of PUs of the SL–PEG–MDI type by changing the content of PEG and SL at a constant NCO:OH ratio.