Solvent-free synthesis of high-performance polyurethane elastomer based on low-molecular-weight alkali lignin

Tough and UV-resistant biodegradable polyurethane elastomers based on extracted lignin and treated wood flour

Polyurethanes are widely utilized across various fields. In the pursuit of sustainable development, current research prioritizes the development of bio-based, environmentally friendly polyurethanes. Wood flour, a type of biomass waste, faces significant challenges in achieving high-value utilization. In this study, wood flour was pretreated with sodium hydroxide solution to generate extracted lignin (Elig) and alkali-treated wood flour (AWF), which were subsequently employed to fabricate biodegradable polyurethane elastomers. Elig was further modified with polycaprolactone (PCL) to enhance its reactivity and flexibility, serving as a biological macromolecule crosslinker in the synthesis of lignin-based polyurethane [PU(Plig)]. Tough polyurethane composite elastomers were then obtained by incorporating AWF into PU(Plig). The resulting PU composite elastomer, PU(Plig@AWF), containing strong interfacial interaction between Elig and AWF, exhibited high toughness (188.15 MJ/m3), excellent UV resistance (with stress and strain retention rates of 77.92 % and 77.07 %, respectively, after 96 h of ultraviolet aging), and outstanding biodegradability (with a mass loss of 19.06 % observed after 50 days of soil degradation testing). Additionally, leveraging the unique characteristics of lignin and wood flour (WF), the elastomer demonstrated remarkable light/thermal-electric conversion capabilities, effectively powering a fan. This study advances the development of biomass-based, multifunctional polyurethane elastomers with enhanced cost-efficiency.

A robust, stable, and scalable multifunctional composite foam utilizing full components of lignin and cellulose from lychee pruning waste

Foam materials hold great promise in construction and packaging applications. However, the non-biodegradability and poor thermal stability of petroleum-based foams present serious environmental and safety concerns. It is crucial to develop sustainable, eco-friendly foam fabrication methods that balance environmental responsibility with high performance. In this study, a novel high-strength, heat-resistant, and water-stable composite foam (FPLs) made from Lignin-based waterborne polyurethanes (LWPUs) and Cellulose fibers, derived from full-component utilization of lychee pruning waste, is introduced. A eco-friendly and simple method utilizing LWPUs crosslinking to fabricate composite foams has been developed, bypassing the need for special drying and ensuring scalability. The FPLs exhibits a high compressive modulus of 455.8 kPa and a yield strength of 191.2 kPa due to the interaction between the LWPUs adhesive and the cellulose fibers. In addition, it demonstrates natural water resistance (maximum contact angle of 122°), exceptional photothermal conversion performance (reaching a peak temperature of 199.7 °C under infrared laser irradiation), superior thermal stability (no deformation up to 250 °C), and insulation performance (thermal conductivity of 0.038 W/mK), while maintaining excellent degradability and recyclability. These materials hold promise as sustainable alternatives to conventional plastic-based foams, providing a viable solution to mitigate the pervasive issue of "white pollution."

Lignin nanoparticles prepared via lactic acid-based deep eutectic solvents as an emulsifier for waterborne polyurethane

Lignin nanoparticles (LNPs) have been extensively studied as emulsifiers, where the particle size and hydrophilic lipophilic balance value (HLB) are pivotal to the emulsion's stability. This investigation leveraged a range of lactic-based deep eutectic solvents (DES) for the nanoscale modification of lignin. Subsequently, the size characteristics of LNPs were characterized by measuring particle size and size distribution. Combined with the detachment energy theory and the maximum capillary pressure theory, the mechanism of how the size of Pickering emulsifier particles affects emulsion stability was thoroughly analyzed, thereby screening the preparation process for LNPs with suitable particle sizes. Meanwhile, the composition and structure of the LNPs were analyzed using NMR spectroscopy. The HLB was calculated based on NMR information and further refined the preparation process to achieve LNPs with appropriate hydrophilicity/lipophilicity. The research results indicate that LNPs prepared using the LA/AL DES at 100 °C for 2 h are most suitable for the preparation of waterborne polyurethane, with an average particle size of 204 nm and an HLB value of 9.14. When the waterborne polyurethane derived from this LNP was applied to plywood, the resulting adhesive demonstrates wet shear strengths of 1.03 MPa and 0.74 MPa under warm and boiling water conditions, respectively.

Enhancement of mechanical properties in reactive polyurethane film via in-situ assembly of embedded cellulose nanocrystals

Comparing to the solvent-based and waterborne polyurethanes (PU), the solvent-free reactive PU (RPU) is prepared via in-situ polymerization and film-formation of isocyanate-capped prepolymers and macromolecular polyols in solvent-free system. Thus, the carbon emissions and environmental pollutions are significantly reduced. However, the rapid polymerization also challenges the well control of structure and properties, especially the ordered microstructures. In this work, an efficient approach is applied to enhance the mechanical property of RPU via chemically bonding cellulose nanocrystals (CNCs) with isocyanates in a solvent-free system. The well distributed spot-like structure in the RPU film is realized and significantly improve the mechanical property due to the synergistic effect of hydrogen bonds and urethane bonds. Only adding 1 wt% of CNCs, the tensile strength and elongation at break are profoundly increased to 25.4 ± 1.5 MPa and 748 ± 50.0%, respectively. Both values are 490% and 16% better than that without CNCs. Additionally, the thermal stability is also improved. The initial decomposition temperature is 29 °C higher than that without CNCs. This approach provides a simple method for developing bio-based RPU with well-ordered structures, showing great potential in applications such as environmentally friendly materials and high-performance polymers.

A One-Stone-Two-Birds Strategy for Photothermal Shape Memory Polyurethane Utilizing Lignin as Monomer and Internal Photothermal Agent

Photothermal-triggering shape memory polyurethane allows for precise and controllable shape transformation under remote light stimulation, making it highly desirable for applications in intelligent devices. This study develops a sustainable and high-performance lignin-based polyurethane (LPU) using a one-stone-two-birds strategy, wherein lignin serves as both a synthetic monomer and an internal photothermal agent. The incorporation of lignin significantly improved the mechanical properties of LPU, achieving a tensile strength of 42.1 MPa and an impressive elongation at break of 1558%. Additionally, the LPU exhibited exceptional photothermal heating capabilities due to the inherent intramolecular π-π conjugations and intermolecular π-π stacking effects of lignin, which facilitated the precise and contactless remote photoheating. Furthermore, the rigid structure of lignin and robust hydrogen bonding interactions provided LPU with excellent multi-cycle shape memory performance, with shape fixation and shape recovery rates exceeding 93% after five cycles. Under near-infrared irradiation, LPU demonstrated precise non-contact heating and remote photothermal shape-control capabilities. This research not only offers a sustainable and high-value application for lignin but also advances the development of environmentally friendly intelligent shape memory polyurethane materials.

Improving fire retardancy and mechanical properties of polyurethane elastomer by acid hydrotropic lignin

Improving flame retardancy and mechanical strength of lignin-containing polyurethane is a great challenge. In this study, lignin with favorable reactivity and dispersity was extracted from poplar using acid hydrotrope p-TsOH in EtOH. The extracted acid hydrotrope lignin (AHL) was subsequently functionalized with nitrogen and phosphorus (FHL) and reacted with isocyanate to fabricate a fire-retardant polyurethane (FHL-PU). The resulting FHL-PU exhibited a five-fold increase in fracture toughness and remarkable reprocessability, attributable to the dual cross-linked network formed by dynamic hydrogen bonds and carbamate bonds between AHL and PU. Furthermore, the FHL can effectively prevent the release of heat and smoke through mechanisms like forming a char layer at elevated temperatures, generating non-combustible gases and sequestering free radicals. As a result, the FHL led to a reduction in the peak heat release rate and total heat release of PU from 946.8 kW/m2 and 86.9 MJ/m2 to 383.5 kW/m2 and 28.4 MJ/m2, respectively. This acid hydrotrope lignin modified polyurethane, holds tremendous potential for a wide range of practical applications.

A one-step and solvent-free strategy for high lignin-containing polyurethane elastomers with excellent mechanical and shape memory performance

Lignin, a renewable and biodegradable polymer, offers a promising alternative to petroleum-based polyols for polyurethane elastomer synthesis. However, its complex structure poses challenges, such as poor dispersibility and reactivity. This study introduces a novel one-step and solvent-free method for synthesizing lignin-containing polyurethane elastomers (SF-LPUes-ONE) with a high lignin substitution rate of at least 30 wt%. By directly incorporating a dispersion of ethanol-extracted lignin and long-chain polyols into the reaction with isocyanates, we successfully prepared SF-LPUes-ONE with remarkable mechanical properties. The tensile strength, elongation at break, and toughness of the resulting sample reached 42.3 MPa, 584.7 %, and 110.0 MJ/m3, respectively. In addition, the phenolic hydroxyl groups in lignin endowed SF-LPUes-ONE with excellent anti-aging resistance, ensuring sustained high performance under demanding conditions. Furthermore, the dynamic hydrogen bonding and chemical cross-linking dual-network endowed SF-LPUes-ONE with exceptional shape memory capabilities, achieving shape fixation and recovery rates exceeding 99 % after 3 cycles. This work demonstrates a green and efficient approach to high-performance lignin-based polyurethane elastomers, showcasing their potential for broad industrial applications.

Sustainable superhydrophobic lignin-based polyurethane foam: an innovative solution for oil pollutant adsorption

Green, efficient treatment of crude oil spills and oil pollutants is a global challenge, with adsorption technology favored for its efficiency and low environmental impact. The development of an environmentally friendly adsorbent with high hydrophobicity, excellent adsorption performance, and degradability is crucial to overcoming the limitations of petroleum-based adsorbents. Here, a lignin-based polyurethane foam (LPUF) with superhydrophobic and photothermal oil-absorbing properties was fabricated by incorporating octadecyltrimethoxysilane into the foam system. The modified foam showed a 151.4° water contact angle, as long-chain alkyl groups reduced surface energy, giving it superhydrophobicity. The foam adsorbent exhibited remarkable adsorption performance for a variety of organic solvents, achieving a maximum adsorption capacity of 20 g g-1 and an oil-water separation efficiency exceeding 97%. Due to its outstanding elastic recovery properties, the foam exhibited only a 1.5% reduction in adsorption capacity after 10 adsorption-desorption cycles, indicating its strong potential for repeated adsorption and recovery. Under 1 kW m-2 sunlight intensity, the surface temperature of the foam adsorbent rose to 79.7 °C within 350 seconds. The excellent photothermal conversion properties of the foam significantly reduced the viscosity of the surface crude oil, thereby increasing the adsorption rate. In addition, the modified foam adsorbent also demonstrated self-cleaning properties and could be completely degraded after 5 hours of treatment in an alkaline solution. The developed LPUF adsorbent exhibited superior hydrophobicity and oil-water separation capabilities, highlighting its potential for efficient oil pollutant removal, while also offering new avenues for the high-value utilization of renewable resources.

High modulus and strength polyurethane film synthesized from lignin-based polyol with various lignin contents and NCO/OH molar ratios

This study explored the synthesis of lignin-polyol (LP) by liquefaction of Kraft lignin in polyethylene glycol and glycerol. LP23 (23 wt% lignin) and LP41 (41 wt% lignin) were obtained with remarkable liquefaction yields of 97.43 % and 93.75 %, respectively. LP-based polyurethane film (LP-PUF) was then fabricated by reacting LP23 and LP41 with hexamethylene diisocyanate. Mechanical properties and hydrophobicity of LP-PUF were optimized by varying NCO/OH molar ratios. LP23-PUF1.9 with NCO/OH of 1.9 showed excellent tensile strength of 34.28 MPa and Young's modulus of 233.27 MPa, while LP41-PUF2.0 with NCO/OH of 2.0 exhibited good tensile strength of 26.03 MPa and Young's modulus of 260.66 MPa. LP23-PUF2.0 displayed high hydrophobicity as indicated by the water contact angle of 96.8° and low surface free energy of 15.68 mN/m. Glass transition temperature (Tg) of LP-PUF varied depending on lignin content and NCO/OH molar ratio. Specifically, the increase of KL content from 23 % to 42 % induced big rise of Tg by 8 °C. The NCO/OH induced change of Tg was relatively weak, as indicated by the small increase of -2.71 °C of LP41-PU1.4 to -1.81 °C of LP41-PUF2.0. The results of this work offer insights for lignin utilization as structural component of high modulus and high strength polyurethane film.

'Rigid-flexible' strategy for high-strength, near-room-temperature self-healing, photo-thermally functionalised lignin-reinforced polyurethane elastomers

Lignin is the most abundant and only renewable aromatic polymer in nature. Herein, a flexible matrix and the rigid lignin were rationally integrated to prepare high-strength, near-room-temperature self-healing, processable lignin-reinforced polyurethane elastomers (LZPUs). Reversible hydrogen and oxime-amino ester bonds were introduced into the matrix to provide excellent dynamic properties and abundant ligands for lignin-matrix coordination bonds. Abundant metal coordination bonds were constructed between the matrix and lignin via the introduction of Zn2+, which not only effectively enhances the dispersibility and compatibility, but also provides an excellent energy dissipation mechanism for the LZPUs. One of the prepared elastomers, LZPUs, exhibited a high strength of 40.5 MPa, which is twice that of the blank sample and 1.6 times that of the sample without Zn2+. It maintained kinetic stability at mild temperature, but it exhibited a self-healing efficiency of 91.3 % in strength and 99.8 % in elongation at break after decoupling with trace ethanol (≈ 50 μL) at 35 °C. It exhibited a self-healing efficiency of 93.6 % in strength under 1 sun irradiation (0.1 W cm-2) for 4 h. We believe this elastomer offering high mechanical properties with multi-functionality can be applied in flexible drives and photo-thermal power generation.

Research Status of Lignin-Based Polyurethane and Its Application in Flexible Electronics

Polyurethanes (PU) have drawn great attention due to their excellent mechanical properties and self-healing and recyclable abilities. Lignin is a natural and renewable raw material in nature, composed of a large number of hydroxyl groups, and has a great potential to replace petroleum polyols in PU synthesis. This review summarizes the recent advances in modification methods such as the liquefaction, alkylation, and demethylation of lignin, and a systematic analysis of how to improve the reactivity and monomer substitution of lignin during polyurethane synthesis for the green manufacturing of high-performance polyurethanes was conducted. Polyurethane can be used in the form of films, foams, and elastomers instead of conventional materials as a dielectric or substrate material to improve the reliability and durability of flexible sensors; this review summarizes the green synthesis of polyurethanes and their applications in flexible electronics, which are expected to provide inspiration for the wearable electronics sector.

Recycling of Commercially Available Biobased Thermoset Polyurethane Using Covalent Adaptable Network Mechanisms

Until recently, recycling thermoset polyurethanes (PUs) was limited to degrading methods. The development of covalent adaptable networks (CANs), to which PUs can be assigned, has opened novel possibilities for actual recycling. Most efforts in this area have been directed toward inventing new materials that can benefit from CAN theory; presently, little or nothing has been applied to industrially producible materials. In this study, both an industrially available polyol (Sovermol780®) and isocyanate (Tolonate X FLO 100®) with percentages of bioderived components were employed, resulting in a potentially scalable and industrially producible material. The resultant network could be reworked up to three times, maintaining the crosslinked structure without significantly changing the thermal properties. Improvements in mechanical parameters were observed when comparing the pristine material to the material exposed to three rework processes, with gains of roughly 50% in elongation at break and 20% in tensile strength despite a 25% decrease in Young's modulus and crosslink density. Thus, it was demonstrated that theory may be profitably applied even to materials that are not designed including additional bonds but instead rely just on the dynamic urethane bond that is naturally present in the network.

Self-Healing and Recyclable Polyurethane/Nanocellulose Elastomer Based on the Diels-Alder Reaction

With the background of the fossil fuel energy crisis, the development of self-healing and recyclable polymer materials has become a research hotspot. In this work, a kind of cross-linking agent with pendent furan groups was first prepared and then used to produce the Polyurethane elastomer based on Diels-Alder chemistry (EPU-DA). In addition, in order to further enhance the mechanical properties of the elastomer, cellulose nanofibers (CNFs) were added into the Polyurethane system to prepare a series of composites with various contents of CNF (wt% = 0.1~0.7). Herein, the FTIR and DSC were used to confirm structure and thermal reversible character. The tensile test also indicated that the addition of CNF increased the mechanical properties compared to the pure Polyurethane elastomer. Due to their reversible DA covalent bonds, the elastomer and composites were recycled under high-temperature conditions, which extends Polyurethane elastomers' practical applications. Moreover, damaged coating can also be repaired, endowing this Polyurethane material with good potential for application in the field of metal protection.

Flexing with lignin: lignin-based elastomers synthesised from untreated kraft black liquor

The synthesis and characterisation of a lignin-based elastomer system using lignin-epoxy-resins is presented. Untreated kraft black liquor was used to synthesise glycidyl lignin or black liquor-based epoxy resin (BLER), following a published procedure. A flexible, elastomeric thermoset was produced by cross-linking BLER with succinic anhydride (SA). The produced material was characterised in respect to its chemical, thermal, mechanical and swelling characteristics. In addition, vertical burning tests were performed. The obtained lignin-based elastomeric thermoset had a tensile strength of 1.0 ± 0.20 MPa and elastic moduli of 1.6 ± 1.4 and 0.44 ± 0.35 MPa at 5% and 50% elongation, respectively. A maximum elongation of 151 ± 49% was found.

Strength Enhancement of Polyurethane Film by Solution Annealing

Recently, polyurethane elastomer (TPU) has attracted more and more attention depending on its excellent optical, mechanical, and retreatment properties. The high strength of polyurethane has always been pursued, which can enable its application in more fields. In this work, an aliphatic polyurethane elastomer membrane (HRPU6) was successfully synthesized, and its strength was obviously improved by solvent annealing technology. The tensile strength and adhesion strength can reach 64.56 and 2.58 MPa, but 36.55 and 1.57 MPa only before solvent annealing, respectively. The impact strength of laminated glass based on HRPU has also been significantly improved after solvent annealing, confirmed through drop ball impact testing. It has been confirmed that the increase in strength of HRPU6 is attributed to the enhancement of hydrogen bonding and the improvement of the phase separation degree.

Fractional extraction of lignin from coffee beans with low cytotoxicity, excellent anticancer and antioxidant activities

Lignin, a biopolymer generated from renewable resources, is widely present in terrestrial plants and possesses notable biosafety characteristics. The objective of this work was to assess the edible safety, in vitro antioxidant, and anti-cancer properties of various lignin fractions isolated from commercially available coffee beans often used for coffee preparation. The findings suggest that the phenolic hydroxyl content increased from 3.26 mmol/g (ED70L) to 5.81 mmol/g (ED0L) with decreasing molecular weight, which resulted in more significant antioxidant properties of the low molecular weight lignin fraction. The findings of the study indicate that the viability of RAW 264.7 and HaCaT cells decreased as the quantity of lignin fractions increased. It was observed that concentrations below 200 μg/mL did not exhibit any harmful effects on normal cells. The results of the study demonstrated a significant reduction of cancer cell growth (specifically A375 cells) at a concentration of 800 μg/mL for all lignin fractions, with an observed inhibition rate of 95 %. The results of this study indicate that the lignin extracts derived from coffee beans exhibit significant potential in mitigating diseases resulting from excessive radical production. Furthermore, these extracts show promise as natural antioxidants and anti-cancer agents.

Recent Developments in Synthesis, Properties, Applications and Recycling of Bio-Based Elastomers

In 2021, global plastics production was 390.7 Mt; in 2022, it was 400.3 Mt, showing an increase of 2.4%, and this rising tendency will increase yearly. Of this data, less than 2% correspond to bio-based plastics. Currently, polymers, including elastomers, are non-recyclable and come from non-renewable sources. Additionally, most elastomers are thermosets, making them complex to recycle and reuse. It takes hundreds to thousands of years to decompose or biodegrade, contributing to plastic waste accumulation, nano and microplastic formation, and environmental pollution. Due to this, the synthesis of elastomers from natural and renewable resources has attracted the attention of researchers and industries. In this review paper, new methods and strategies are proposed for the preparation of bio-based elastomers. The main goals are the advances and improvements in the synthesis, properties, and applications of bio-based elastomers from natural and industrial rubbers, polyurethanes, polyesters, and polyethers, and an approach to their circular economy and sustainability. Olefin metathesis is proposed as a novel and sustainable method for the synthesis of bio-based elastomers, which allows for the depolymerization or degradation of rubbers with the use of essential oils, terpenes, fatty acids, and fatty alcohols from natural resources such as chain transfer agents (CTA) or donors of the terminal groups in the main chain, which allow for control of the molecular weights and functional groups, obtaining new compounds, oligomers, and bio-based elastomers with an added value for the application of new polymers and materials. This tendency contributes to the development of bio-based elastomers that can reduce carbon emissions, avoid cross-contamination from fossil fuels, and obtain a greener material with biodegradable and/or compostable behavior.

Coupling chemistry and biology for the synthesis of advanced bioproducts

Chemical and biological syntheses can both lead to a myriad of compounds. Biology enables us to harness the metabolism of microbial cell factories to produce key target molecules from renewable biomass-derived substrates. Although bio-based feedstocks are sustainably sourced and more benign than the rapidly depleting fossil fuels that chemical processes have historically relied on, limiting pathways solely to biological reactions may not equate to a greener process overall. In fact, bioreactors rely on substantial quantities of water and can be inefficient since organisms typically operate around ambient conditions and are sensitive to perturbations in their environment. Hybridizing biosynthetic pathways with green chemistry can instead be a more potent strategy to reduce our net manufacturing footprint. Emerging chemistries have demonstrated considerable success in performing complex transformations on biological feedstocks without significant solvent use. Many of these transformations would be too slow to perform enzymatically or infeasible altogether. Here, we put forth the concept that by carefully considering the merits and drawbacks of synthetic biology and chemistry as well as one's own use case, there exist many opportunities for coupling the two. Merging these syntheses can unlock a wider suite of functional group transformations, thereby enabling future manufacturing processes to sustainably access a larger space of valuable, platform chemicals.

High-Performance, Light-Stimulation Healable, and Closed-Loop Recyclable Lignin-Based Covalent Adaptable Networks

In this work, high-performance, light-stimulation healable, and closed-loop recyclable covalent adaptable networks are successfully synthesized from natural lignin-based polyurethane (LPU) Zn2+ coordination structures (LPUxZy). Using an optimized LPU (LPU-20 with a tensile strength of 28.4 ± 3.5 MPa) as the matrix for Zn2+ coordination, LPUs with covalent adaptable coordination networks are obtained that have different amounts of Zn. When the feed amount of ZnCl2 is 9 wt%, the strength of LPU-20Z9 reaches 37.3 ± 3.1 MPa with a toughness of 175.4 ± 4.6 MJ m-3 , which is 1.7 times of that of LPU-20. In addition, Zn2+ has a crucial catalytic effect on "dissociation mechanism" in the exchange reaction of LPU. Moreover, the Zn2+ -based coordination bonds significantly enhance the photothermal conversion capability of lignin. The maximum surface temperature of LPU-20Z9 reaches 118 °C under the near-infrared illumination of 0.8 W m-2 . This allows the LPU-20Z9 to self-heal within 10 min. Due to the catalytic effect of Zn2+ , LPU-20Z9 can be degraded and recovered in ethanol completely. Through the investigation of the mechanisms for exchange reaction and the design of the closed-loop recycling method, this work is expected to provide insight into the development of novel LPUs with high-performance, light-stimulated heal ability, and closed-loop recyclability; which can be applied toward the expanded development of intelligent elastomers.

Improving Sustainability through Covalent Adaptable Networks in the Recycling of Polyurethane Plastics

The global plastic waste problem has created an urgent need for the development of more sustainable materials and recycling processes. Polyurethane (PU) plastics, which represent 5.5% of globally produced plastics, are particularly challenging to recycle owing to their crosslinked structure. Covalent adaptable networks (CANs) based on dynamic covalent bonds have emerged as a promising solution for recycling PU waste. CANs enable the production of thermoset polymers that can be recycled using methods that are traditionally reserved for thermoplastic polymers. Reprocessing using hot-pressing techniques, in particular, proved to be more suited for the class of polyurethanes, allowing for the efficient recycling of PU materials. This Review paper explores the potential of CANs for improving the sustainability of PU recycling processes by examining different types of PU-CANs, bond types, and fillers that can be used to optimise the recycling efficiency. The paper concludes that further research is needed to develop more cost-effective and industrial-friendly techniques for recycling PU-CANs, as they can significantly contribute to sustainable development by creating recyclable thermoset polymers.