Urushi, an oriental lacquer, is sensitive to light and easily deteriorates upon light exposure, leading to the fading and loss of gloss of its coatings. In this study, we attempted to improve the lightfastness of black urushi by utilizing the UV-absorbing properties of kraft lignin. Oleic acid-esterified kraft lignin was prepared and added to the black urushi to improve the compatibility. No aggregation of oleic acid-esterified lignin occurred in the black urushi, and the physical properties of the films were maintained to a large extent. A possible chemical interaction between lignin and black urushi was suggested upon thermal analysis. It was also found that the addition of lignin improved the thermal dimensional stability of black urushi. Finally, the high gloss factor of the black urushi film was maintained by adding oleic acid-esterified lignin after 168 h of light irradiation. Oleic acid-esterified lignin therefore appears as a promising functional additive to improve photostability of urushi coatings.

The more widespread applicability of photopolymerization-based three-dimensional (3D) printing is limited by the availability of light-curable resins, most of which are based on fossil-derived compounds. We developed a biobased lignin-derivable resin by utilizing methacrylated derivatives of vanillin, vanillyl alcohol, and eugenol as aromatic monomers. Lignin nanoparticles (LNPs) were incorporated as functional fillers that enhance print resolution and material properties. The crosslinking degree, and thereby the tensile properties, was modulated through the use of mono- or dimethacrylated vanillin derivatives in the resin formulation. The LNPs acted as UV absorbers, conferring better control of the photopolymerization process by preventing light penetration across unintended layers, leading to enhanced print resolution. The LNPs showed excellent dispersion stability due to their size and morphology. The inclusion of up to 2 wt% of LNPs improved the ductility of the 3D printed nanocomposites through toughening mechanisms enabled by the rigid nanoparticles. Finally, exploiting the differences in crosslinking degree of the resin formulations, a multi-material model featuring both soft and rigid domains was fabricated. This study demonstrates a simple but effective strategy for the design of biobased photocurable resins with tailorable mechanical properties that are suitable for high-resolution and multi-material 3D printing. 

The inferior thermoplastic properties have limited production of melt-spun fibers from lignin. Here we report on the controlled esterification of softwood kraft lignin (SKL) to enable scalable, solvent-free melt spinning of microfibers using a cotton candy machine. We found that it is crucial to control the esterification process as melt-spun fibers could be produced from lignin oleate and lignin stearate precursors with degrees of esterification (DE) ranging from 20-50%, but not outside this range. To fabricate a functional hybrid material, we incorporated magnetite nanoparticles (MNPs) into the lignin oleate fibers by melt blending and subsequent melt spinning. Thermogravimetric analysis and X-ray diffraction studies revealed that increasing the weight fraction of MNPs led to improved thermal stability of the fibers. Finally, we demonstrated adsorption of organic dyes, magnetic recovery, and recycling via melt spinning of the regular and magnetic fibers with 95% and 83% retention of the respective adsorption capacities over three adsorption cycles. The mechanical recyclability of the microfibers represents a new paradigm in lignin-based circular materials.

Lignin nanoparticles (LNPs) are considered as intriguing green, renewable alternatives to fossil-based nanomaterials. However, the predisposition of LNPs to dissolve under alkaline conditions makes covalent surface functionalisation in the dispersion state difficult and limits applications demanding morphological stability under challenging pH conditions. Mechanistic studies suggest that during the formation of LNPs by nanoprecipitation the higher molecular weight fractions of lignin likely start precipitating first, while the low molecular weight fractions tend to deposit later and thus locate on the outer shell. Capitalising this aggregation pattern, the present work presents a strategy to prepare surface-functionalised LNPs that can find applications as adhesives and alkaline stable LNPs. The entire process is based on a single-step solvent fractionation of lignin using either ethanol or ethyl acetate, subsequent functionalisation of selected fractions with epichlorohydrin, and recombination according to the original mass proportions in line with the so-called zero waste principle. Aqueous colloidal dispersions of lignins were synthesised by nanoprecipitation of epoxidised low molecular weight (M_W) fractions combined with the corresponding unmodified high M_W ones, and vice versa. Upon thermal treatment, LNPs containing the epoxidised insoluble fraction underwent intraparticle crosslinking, proving dimensional stability at pH 12. Conversely, LNPs including epoxidised solvent-soluble fractions resulted in interparticle crosslinking upon heating, which confirmed the surface localisation of such low M_W fractions. The latter system was exploited to develop green LNP-based adhesives for aminated glass with lap shear strength outperforming prior adhesive systems based on lignin particles.

Colloidal lignin nanoparticles are promising buildingblocks forsustainable functional materials. However, their instability in organicsolvents and aqueous alkali limits their applicability. Current stabilizationmethods require nonrenewable and toxic reagents or tedious workupprocedures. Here we show a method to prepare hybrid nanoparticlesusing only natural components. Urushi, a form of black oriental lacquer,and lignin are coaggregated to form hybrid particles, with Urushiacting as a sustainable component that stabilizes the particles viahydration barrier effect and thermally triggered internal cross-linking.The weight fractions of the two components can be adjusted to achievethe desired level of stabilization. Hybrid particles with Urushi content>25 wt % undergo interparticle cross-linking that produces multifunctionalhydrophobic protective coatings that improve the water resistanceof wood. This approach provides a sustainable and efficient methodfor stabilizing lignin nanoparticles and opens up neoteric possibilitiesfor the development of lignin-based advanced functional materials.

The current biorefineries are focused on the comprehensive fractionation of biomass components into separate lignin and carbohydrate fractions for the production of materials, platform chemicals and biofuel. However, it has become obvious that the combination of lignin and carbohydrates can have significant technical, environmental, and economic benefits as opposed to their separate use. Herein, we developed a green, simple, and flexible biorefinery concept for the integrated utilization of all major biomass components for high-value applications with the focus on functional lignin–carbohydrate hybrids (LCHs). The established process consisted of a modified hydrothermal treatment (HTT) of birch wood followed by solvent extraction of the resulting solids and is therefore named AquaSolv Omni (AqSO) biorefinery. The AqSO biorefinery produces three major streams: hydrolysate (hemicellulose-derived products), solvent-extracted lignin–carbohydrate complexes (LCCs) and cellulose-rich fibers. Specific process conditions were found to facilitate the production of LCCs of different types in high yields as a new valuable and industrially realistic process stream. The effect of the process severity and liquid to solid (L/S) ratio on the yields and compositions of the produced fractions as well as on the structure and properties of the extracted LCCs was investigated using state of the art NMR spectroscopy and molar mass distribution analysis among other methods. The high flexibility of the process allows for engineering of the resulting products in a wide range of chemical compositions, structures and physicochemical properties and therefore gives a good opportunity to optimize the products for specific high-value applications. The process can be easily combined with other biorefinery operations (e.g., enzymatic hydrolysis, pulping, bleaching) to be incorporated into existing value chains or create new ones and thus is suitable for different biorefinery scenarios. First examples of high-value applications of AqSO biorefinery LCHs are reported. LCC nanoparticles (LCCNPs) were produced for the first time directly from the solvent extract and their properties were investigated. LCCNPs could efficiently stabilize Pickering emulsions of tetrahydrofurfuryl methacrylate and allowed their free radical polymerization. In addition, AqSO LCHs showed promising results as wood adhesives. Overall, our results provide detailed information on the complex lignocellulosic fractions and bridge the gap from process engineering to sustainable product development.

Sustainable materials are needed to mitigate against the increase in energy consumption resulting from population growth and urbanization. Here, we report fully biobased nanocomposite films and coatings that display efficient photothermal activity and selective absorption of ultraviolet (UV) radiation. The nanocomposites with 20 wt % of lignin nanoparticles (LNPs) embedded in a chitosan matrix displayed an efficient UV blocking of 97% at 400 nm along with solar energy-harvesting properties. The reflectance spectra of the nanocomposite films revealed the importance of well-dispersed nanoparticles in the matrix to achieve efficient UV-blocking properties. Finally, yet importantly, we demonstrate the nanocomposites with 20 wt % LNPs as photothermal glass coatings for passive cooling of indoor temperature by simply tailoring the coating thickness. Under simulated solar irradiation of 100 mW/cm², the 20 μm coating achieved a 58% decrease in the temperature increment in comparison to the system with uncoated glass. These renewable nanocomposite films and coatings are highly promising sustainable solutions to facilitate indoor thermal management and improve human health and well-being.

Lignin is the most abundant aromatic biopolymer with many promising features but also shortcomings as a filler in polymer blends. The main objective of this work was to improve the processability and compatibility of lignin with poly (lactic acid) (PLA) through etherification of lignin. Commercial kraft lignin (KL) and oxypropylated kraft lignin (OPKL) were blended with PLA at different weight percentages (1, 5, 10, 20, and 40%) followed by injection molding. Low lignin contents between 1 and 10% generally had a favorable impact on mechanical strength and moduli as well as functional properties of the PLA-based composites. Unmodified lignin with free phenolic hydroxyl groups rendered the composites with antioxidant activity, as measured by radical scavenging and lipid peroxidation tests. Incorporating 5–10% of KL or OPKL improved the thermal stability of the composites within the 300–350°C region. DSC analysis showed that the glass transition temperature values were systematically decreased upon addition of KL and OPKL into PLA polymer. However, low lignin contents of 1 and 5% decreased the cold crystallization temperature of PLA. The composites of KL and OPKL with PLA exhibited good stabilities in the migration test, with values of 17 mg kg⁻¹ and 23 mg kg⁻¹ even at higher lignin content 40%, i.e., well below the limit defined in a European standard (60 mg kg⁻¹). These results suggest oxypropylated lignin as a functional filler in PLA for safe and functional food packaging and antioxidant applications.

Spherical lignin nanoparticles are emerging biobased nanomaterials, but instability and dissolution in organic solvents and aqueous alkali restrict their applicability. Here, we report the synthesis of hydroxymethylated lignin nanoparticles and their hydrothermal curing to stabilize the particles by internal cross-linking reactions. These colloidally stable particles contain a high biobased content of 97% with a tunable particle size distribution and structural stability in aqueous media (pH 3 to 12) and organic solvents such as acetone, ethanol, dimethylformamide, and tetrahydrofuran. We demonstrate that the free phenolic hydroxyl groups that are preserved in the cured particles function as efficient reducing sites for silver ions, giving rise to hybrid lignin–silver nanoparticles that can be used for quick and facile sensing of hydrogen peroxide. The stabilized lignin particles can also be directly modified using base-catalyzed reactions such as the ring-opening of cationic epoxides that render the particles with pH-dependent agglomeration and redispersion properties. Combining scalable synthesis, solvent stability, and reusability, this new class of lignin nanoparticles shows potential for its use in circular biobased nanomaterials. 

Weak interfacial binding of lignin within synthetic polymer composites results in unsatisfactory mechanical properties that limit their application prospects. In the present work, polystyrene (PS) and poly(butyl methacrylate) (PBMA) nanocomposites containing lignin nanoparticles (LNPs) are produced by simple melting of polymeric latex dispersions obtained from free radical polymerization of oil-in-water Pickering emulsions stabilized by hybrid LNPs coated with chitosan and glucose oxidase. Owing to the formation of viscous polymer melts, the hybrid LNPs ended up uniformly dispersed within the polymeric matrices, which gave the polymeric nanocomposites markedly improved tensile strength without sacrificing their elasticity in comparison to pure PS and PBMA. Consequently, the composites reinforced with 15 wt% of the hybrid particles showed improvement in toughness by a factor of 3.5 and 15 compared to those of the corresponding pristine PS and PBMA. In addition, the presence of the hybrid particles conferred the nanocomposites with commendable UV-blocking and antioxidant properties which are relevant for protective packaging and coating applications. Overall, our results show a new and green route with excellent material economy (overall mass yield up to 91%) to obtain strong and transparent polymeric nanocomposites reinforced with up to 30 wt% of LNPs, which is expected to attract renewed interest in lignin-polymer composites for a broad range of applications.