UV-resistant transparent lignin-based polyurethane elastomer with repeatable processing performance

Recent Advances in the Preparation and Application of Bio-Based Polyurethanes

Amid environmental pollution and resource depletion, developing and utilizing biomass resources as alternatives to petroleum is a prominent research focus. Driven by environmental protection and sustainable development, the shift from petroleum-based to bio-based polyurethane is a prevailing trend in polyurethane material development. Biomass sources such as vegetable oil, polysaccharides, and lignin offer extensive application prospects in bio-based polyurethane production. Functional modifications of these polyurethanes can further expand their application range. This article explores the preparation of various bio-based polyurethanes, their applications across different fields, and their anticipated future development and uses.

Photothermal-responsive lignin-based polyurethane with mechanically robust, fast self-healing, solid-state plasticity and shape-memory performance

In light of the depletion of petrochemical resources and increase in environmental pollution, there has been a significant focus on utilizing natural biomass, specifically lignin, to develop sustainable and functional materials. This research presents the development of a lignin-based polyurethane (DLPU) with photothermal-responsiveness by incorporating lignin and oxime-carbamate bonds into polyurethane network. The abundant hydrogen bonds between lignin and the polyurethane matrix, along with its cross-linked structure, contribute to DLPU's excellent mechanical strength (30.2 MPa) and toughness (118.7 MJ·m-3). Moreover, the excellent photothermal conversion ability of DLPU (54.4 %) activates dynamic reversible behavior of oxime-carbamate bonds and hydrogen bonds, thereby endowing DLPU with exceptional self-healing performance. After 15 min of near-infrared irradiation, DLPU achieves self-healing efficiencies of 96.0 % for tensile strength and 96.3 % for elongation at break. Additionally, DLPU exhibits photocontrolled solid-state plasticity as well as an excellent phototriggered shape-memory effect (70 s), with shape fixity and recovery ratios reaching 98.8 % and 95.3 %, respectively. By exploiting the spatial controllability and photothermal-responsiveness of DLPU, we demonstrate multi-dimensional responsive materials with self-healing and shape-shifting properties. This work not only promotes the development of multi-functional polyurethanes but also provides a pathway for the high-value utilization of lignin.

Sustainable Strategies for Synthesizing Lignin-Incorporated Bio-Based Waterborne Polyurethane with Tunable Characteristics

In this study, we introduce a novel approach for synthesizing lignin-incorporated castor-oil-based cationic waterborne polyurethane (CWPU-LX), diverging significantly from conventional waterborne polyurethane dispersion synthesis methods. Our innovative method efficiently reduces the required solvent quantity for CWPU-LX synthesis to approximately 50% of that employed in traditional WBPU experimental procedures. By incorporating lignin into the polyurethane matrix using this efficient and reduced-solvent method, CWPU-LX demonstrates enhanced properties, rendering it a promising material for diverse applications. Dynamic interactions between lignin and polyurethane molecules contribute to improved mechanical properties, enhanced thermal stability, and increased solvent resistance. Dynamic interactions between lignin and polyurethane molecules contribute to improved tensile strength, up to 250% compared to CWPU samples. Furthermore, the inclusion of lignin enhanced thermal stability, showcasing a 4.6% increase in thermal decomposition temperature compared to conventional samples and increased solvent resistance to ethanol. Moreover, CWPU-LX exhibits desirable characteristics such as protection against ultraviolet light and antibacterial properties. These unique properties can be attributed to the presence of the polyphenolic group and the three-dimensional structure of lignin, further highlighting the versatility and potential of this material in various application domains. The integration of lignin, a renewable and abundant resource, into CWPU-LX exemplifies the commitment to environmentally conscious practices and underscores the significance of greener materials in achieving a more sustainable future.

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.

Colorless, Transparent, and High-Performance Polyurethane with Intrinsic Ultraviolet Resistance and Its Anti-UV Mechanism

Polyurethane (PU) is a widely used polymer material that will age under prolonged exposure to ultraviolet (UV) light, shortening the service life. Several methods have been used to prepare the anti-UV PU, including adding nonreactive anti-UV additives, functional fillers, and biological antioxidant molecules. However, the nonreactive anti-UV additives may migrate during long-term use, the functional fillers may damage the mechanical properties and seriously reduce the light transmittance of the sample, and the biological antioxidant molecules will inevitably color the sample. To solve these problems, in this work, a benzotriazole UV absorber (Chiguard R-455) was introduced into the PU molecular chains by in situ polymerization to prepare the nonmigrating intrinsic anti-UV PU sample with high performance and colorless transparency. The anti-UV PU samples exhibit light transmittance of over 88% in the visible range and superior mechanical properties with tensile strength higher than 65 MPa and elongation at break higher than 900%. After 24 h UV irradiation (200 W, 365 nm), the tensile strength and elongation at break of pure PU sample are significantly reduced to only 8.9 and 15.8% of the original one, respectively. On the contrary, the addition of Chiguard R-455 will endow the PU sample with excellent anti-UV performance. After 24 h UV irradiation, the tensile strength (67.2 ± 1.6 MPa) and elongation at break (917.4 ± 30.0%) of PU-0.5% (the content of Chiguard R-455 is only 0.5 wt %) have changed little compared with the sample without irradiation (67.4 ± 3.5 MPa and 919.4 ± 26.5%). Additionally, the anti-UV mechanism of the PU sample is systematically studied. This work provides a feasible method for preparing colorless, transparent, high-performance, nonmigrating intrinsic UV-shielding PU samples, which can be used as a UV light-shielding material in various fields with visible and aesthetic requirements, such as protection fields and wearable products.

Multifunctional composite coatings with hydrophobic, UV-resistant, anti-oxidative, and photothermal performance for healthcare

Personal protective textiles have attracted extensive interest since Corona Virus Disease 2019 has broken out. Moreover, developing eco-friendly, multifunctional waterproof, and breathable surface is of great importance but still faces enormous challenges. Notably, good hydrophobicity and breathability are necessary for protective textiles, especially protective clothing and face masks for healthcare. Herein, the multifunctional composite coatings with good UV-resistant, anti-oxidative, hydrophobic, breathable, and photothermal performance has been rapidly created to meet protective requirements. First, the gallic acid and chitosan polymer was coated onto the cotton fabric surface. Subsequently, the modified silica sol was anchored on the coated cotton fabric surface. The successful fabrication of composite coatings was verified by RGB values obtained from the smartphone and K/S value. The present work is an advance for realizing textile hydrophobicity by utilizing fluorine-free materials, compared with the surface hydrophobicity fabricated with conventional fluorinated materials. The surface free energy has been reduced from 84.2 to 27.6 mJ/m² so that the modified cotton fabric could repel the ethylene glycol, hydrochloric acid, and sodium hydroxide solutions, respectively. Besides, the composite coatings possesses lower adhesion to deionized water. After 70 cycles of the sandpaper abrasion, the fluorine-free hydrophobic coatings still exhibits good hydrophobicity with WCA of 124.6 ± 0.9°, with overcoming the intrinsic drawback of the poor abrasion resistance of hydrophobic surfaces. Briefly, the present work may provide a universal strategy for rapidly creating advanced protective coatings to meet personal healthcare, and a novel method for detecting RGB values of composite coatings by smartphone.

Catalyst-Free Synthesis of Covalent Adaptable Network (CAN) Polyurethanes from Lignin with Editable Shape Memory Properties

Here a new strategy of catalyst-free direct synthesis of covalent adaptable network polyurethanes (LPUs) from lignin with editable shape memory effect is reported. Using unmodified lignin, PEG, and isocyanate under the condition of the isocyanate index less than 1.0 (NCO/OH<1.0), a variety of LPUs are obtained. When NCO/OH=0.8, a stable cross-linked network can be formed (ex. the gel content of LPU50-0.8 was 98±0.3 %). The activation energy (E_a ) value of LPUs is similar to that of polyhydroxyurethanes (PHUs), at around 110 kJ mol^-1 . With an increase of lignin content, the LPUs show a transition from ductile fracture to brittle fracture mode. And the mechanical properties of LPUs are significantly enhanced after extrusion processing, with the maximum modulus reaching 649±26 MPa and the maximum toughness up to 9927±111 kJ m^-3 . The improvement in mechanical properties is due to the homogenization of complex cross-linked network under the powerful external force of the extruder and the lignin that originally was free in the system participated in the exchange reactions. Moreover, LPUs can also be prepared continuously in one step by using an extruder as the reactor. In addition, LPU50-0.8 has an editable shape memory effect. This study develops a novel method for the synthesis of LPU from lignin with NCO/OH<1.0, showcasing new possibilities for value-added utilization of lignin, and expands the bio-based products portfolio from biomass feedstock to help meet future green manufacturing demands.

Lignin as a Renewable Building Block for Sustainable Polyurethanes

Currently, the pulp and paper industry generates around 50-70 million tons of lignin annually, which is mainly burned for energy recovery. Lignin, being a natural aromatic polymer rich in functional hydroxyl groups, has been drawing the interest of academia and industry for its valorization, especially for the development of polymeric materials. Among the different types of polymers that can be derived from lignin, polyurethanes (PUs) are amid the most important ones, especially due to their wide range of applications. This review encompasses available technologies to isolate lignin from pulping processes, the main approaches to convert solid lignin into a liquid polyol to produce bio-based polyurethanes, the challenges involving its characterization, and the current technology assessment. Despite the fact that PUs derived from bio-based polyols, such as lignin, are important in contributing to the circular economy, the use of isocyanate is a major environmental hot spot. Therefore, the main strategies that have been used to replace isocyanates to produce non-isocyanate polyurethanes (NIPUs) derived from lignin are also discussed.

Biocomposite PBAT/lignin blown films with enhanced photo-stability

Lignin can be obtained as a byproduct during cellulose-rich pulp fibers production and it is habitually treated as waste or intended for low-value destinations. However, due to UV absorption and mechanical properties, lignin can contribute to the fabrication of biodegradable blown films with superior performances. In this study, it was established the suitability of lignin for manufacturing biocomposite PBAT blown films with higher stiffness and photo-oxidation resistance. The effect of the filler concentration on the melt rheological behavior in non-isothermal elongational flow was investigated. The results allowed us to choose the correct filler concentration for producing films through a film blowing operation. The PBAT/lignin blown film composites displayed an increase of the elastic modulus if compared to neat PBAT films without affecting their elongation at break. Furthermore, the filler delayed the photo-oxidative degradation of PBAT hence potentially allowing open-air applications.

Phase Change Energy Storage Elastic Fiber: A Simple Route to Personal Thermal Management

A novel thermoplastic polyurethane (TPU) PCFs possessing a high loaded ratio and high elasticity was simply prepared by vacuum absorption following wet spinning, then coated by waterborne polyurethane (WPU). Octadecane (OCC), hexadecanol (HEO), and stearic acid (SA), which have different tendencies to form hydrogen bonds with TPU, were selected as PCMs, and their thermal behavior, thermal storge properties, and elasticity were systematically studied, respectively. The hierarchical pore structure though from the sheath to the core part of TPU filaments weakened the influence of the nonfreezing layer and hydrogen bond on the crystallization behavior of PCMs. The resulting HEO/TPU fiber has the highest enthalpy of 208.1 J/g compared with OCC and SA. Moreover, the HEO/TPU fiber has an elongation at break of 354.8% when the phase change enthalpy is as high as 177.8 J/g and the phase change enthalpy is still 174.5 J/g after fifty cycles. After ten tensile recovery cycles, the elastic recovery rate of HEO/TPU fiber was only 71.3%. When the HEO in the fiber was liquid state, the elastic recovery rate of HEO/TPU fiber promoted to 91.6%. This elastic PCFs have excellent thermal cycle stability, elastic recovery, and temperature sensitivity. It has great application potential in the fields of flexible wearable devices, intelligent fabrics, and temperature sensors.