Mechanically recyclable melt-spun fibers from lignin esters and iron oxide nanoparticles: towards circular lignin materials

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.

Kraft Lignin: A Valuable, Sustainable Resource, Opportunities and Challenges

Kraft lignin, a by-product from the production of pulp, is currently incinerated in the recovery boiler during the chemical recovery cycle, generating valuable bioenergy and recycling inorganic chemicals to the pulping process operation. Removing lignin from the black liquor or its gasification lowers the recovery boiler load enabling increased pulp production. During the past ten years, lignin separation technologies have emerged and the interest of the research community to valorize this underutilized resource has been invigorated. The aim of this Review is to give (1) a dedicated overview of the kraft process with a focus on the lignin, (2) an overview of applications that are being developed, and (3) a techno-economic and life cycle asseeements of value chains from black liquor to different products. Overall, it is anticipated that this effort will inspire further work for developing and using kraft lignin as a commodity raw material for new applications undeniably promoting pivotal global sustainability concerns.

Effects of Blending Tobacco Lignin with HDPE on Thermal and Mechanical Properties

Depletion of fossil fuels and the detrimental environmental impacts of synthetic plastics have prompted a global interest in bio-based polymers. Lignin is an abundant, unused, and low-value byproduct of pulping and biochemical operations that has the potential to decrease the need for plastics derived from petroleum. Melt blending is one of the easiest strategies for expanding the commercial applications of lignin. Concerns remain, however, regarding the negative effects of lignin on the final composite material’s performance, and the increase in manufacturing costs. This study investigates the effects of blending lignin extracted from tobacco using a novel one-step processing technique on injection molding parameters, and the mechanical, physical, and thermal properties of high-density polyethylene (HDPE). By extruding HDPE pellets and lignin powder, varying blend concentrations (0, 5, 10, 15, and 30% wt.) were produced. Scanning electron microscopy (SEM) and optical microscopy were used to investigate the compatibility of the blend morphology. Results indicated that interfacial interactions were achieved as particles of tobacco lignin were well dispersed and uniformly distributed throughout HDPE. Intermolecular interactions between HDPE and lignin were also discovered through Fourier-transform infrared (FTIR) spectral analyses. The tensile test results showed that increase in lignin content up to 15% wt. had little effect on tensile strength, but at 30% wt., a 19% reduction was observed. With the addition of 5, 10, 15, and 30% wt. of lignin, the tensile modulus increased by 4%, 29%, 25%, and 8%, respectively. TGA results demonstrated that at 15% and 30% wt., tobacco lignin acted as a thermal stabilizer. The processability study revealed that tobacco lignin could be processed easily using injection molding without requiring significant changes to the process parameters. Overall, tobacco lignin showed great promise as a biodegradable HDPE filler.

Room Temperature Fabrication of Macroporous Lignin Membranes for the Scalable Production of Black Silicon

Rising global demand for biodegradable materials and green sources of energy has brought attention to lignin. Herein, we report a method for manufacturing standalone lignin membranes without additives for the first time to date. We demonstrate a scalable method for macroporous (∼100 to 200 nm pores) lignin membrane production using four different organosolv lignin materials under a humid environment (>50% relative humidity) at ambient temperatures (∼20 °C). A range of different thicknesses is reported with densely porous films observed to form if the membrane thickness is below 100 nm. The fabricated membranes were readily used as a template for Ni²⁺ incorporation to produce a nickel oxide membrane after UV/ozone treatment. The resultant mask was etched via an inductively coupled plasma reactive ion etch process, forming a silicon membrane and as a result yielding black silicon (BSi) with a pore depth of >1 μm after 3 min with reflectance <3% in the visible light region. We anticipate that our lignin membrane methodology can be readily applied to various processes ranging from catalysis to sensing and adapted to large-scale manufacturing.

Effects of Functionalized Kraft Lignin Incorporation on Polypropylene Surface Energy and Practical Adhesion

Polypropylene (PP) is a multifunctional and widely applied polymer. Nevertheless, its low energy surface and poor adhesion are well-known and might impair some prospective applications. Aiming to overcome these limitations, PP composites can be applied as a tool to enhance PP surface energy and then increase its practical adhesion. In this work, Kraft lignin (KL) was chemically modified and blended with PP. In short, KL was hydroxypropylated and further reacted with acetic anhydride (A-oxi-KL) or maleic anhydride (M-oxi-KL). Lignin modifications were confirmed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). PP-composites with different lignin contents, as well as pristine PP, were characterized in terms of their thermal behavior, morphology, surface energy, and practical adhesion by DSC, scanning electron microscopy (SEM), contact angle measurement, and peeling tests, respectively. Lignin incorporation did not affect the PP degree of crystallization. The lignin modifications led to a better compatibility with the PP matrix and surface energies up to 86% higher than neat PP. Increases of up to 66% in the peel strength were verified. Composites with M-oxi-KL showed the best adhesion performance, confirming the lignin functionalization is an efficient approach to improve the practical adhesion of PP films.

Investigating the Effects of Tobacco Lignin on Polypropylene

The utilization of eco-friendly materials, such as lignin, for higher value product applications became increasingly important as environmental concerns due to global warming increased. Melt blending is one of the easy ways to increase the usage of lignin in commercial applications. However, the degradation of the final product performance and increase in the production time and costs are of major concern. In the current work, the effects of blending lignin, extracted from tobacco plants, with polypropylene (PP) on the injection molding parameters, physical, thermal and mechanical properties are investigated. Blends of lignin (5, 15 and 30% by wt.) with PP were prepared using a Filabot single screw extruder. Results show that tensile strength decreases by 3.2%, 9.9% and 5.4% at 5 wt. %, 15 wt. %, and 30 wt. % of lignin addition, respectively. The tensile stiffness was almost unaffected by the addition of up to 15% lignin, but a 23% increase was observed at 30 wt. % loading. When compared to lignin processed via expensive processes, such as acetylation, tobacco lignin showed superior performance. The DSC results show unaffected crystallization and melting temperatures but a decrease in enthalpies and percentage of crystallinity. The SEM and optical micrographs of the coupon cross-sections show that the extrusion process has achieved a uniform distribution of lignin particles in the PP. Thermogravimetric analysis results show that tobacco lignin accelerates the onset decomposition temperature but does not influence the decomposition peak temperature. The increase in lignin content did not have a significant influence on the injection molding parameters, implying no additional processing costs for adding lignin to the PP. Overall, the performance of the tobacco lignin is comparable, if not better, than that of processed lignin reported in the literature.

Mechanically Reinforced, Flexible, Hydrophobic and UV Impermeable Starch-Cellulose Nanofibers (CNF)-Lignin Composites with Good Barrier and Thermal Properties

Bio-based composite films have been widely studied as potential substitutes for conventional plastics in food packaging. The aim of this study was to develop multifunctional composite films by introducing cellulose nanofibers (CNF) and lignin into starch-based films. Instead of costly and complicated chemical modification or covalent coupling, this study optimized the performance of the composite films by simply tuning the formulation. We found that starch films were mechanically reinforced by CNF, with lignin dispersing as nanoparticles embedded in the matrix. The newly built-up hydrogen bonding between these three components improves the integration of the films, while the introduction of CNF and lignin improved the thermal stability of the starch-based films. Lignin, as a functional additive, improved hydrophobicity and blocked UV transmission. The inherent barrier property of CNF and the dense starch matrix provided the composite films with good gas barrier properties. The prepared flexible films were optically transparent, and exhibited UV blocking ability, good oxygen-barrier properties, high hydrophobicity, appreciable mechanical strength and good thermal stability. These characteristics indicate potential utilization as a green alternative to synthetic plastics especially for food packaging applications.

Lignin-based polymers

On the basis of the world’s continuing consumption of raw materials, there was an urgent need to seek sustainable resources. Lignin, the second naturally abundant biomass, accounts for 15–35% of the cell walls of terrestrial plants and is considered waste for low-cost applications such as thermal and electricity generation. The impressive characteristics of lignin, such as its high abundance, low density, biodegradability, antioxidation, antibacterial capability, and its CO2 neutrality and enhancement, render it an ideal candidate for developing new polymer/composite materials. In past decades, considerable works have been conducted to effectively utilize waste lignin as a component in polymer matrices for the production of high-performance lignin-based polymers. This chapter is intended to provide an overview of the recent advances and challenges involving lignin-based polymers utilizing lignin macromonomer and its derived monolignols. These lignin-based polymers include phenol resins, polyurethane resins, polyester resins, epoxy resins, etc. The structural characteristics and functions of lignin-based polymers are discussed in each section. In addition, we also try to divide various lignin reinforced polymer composites into different polymer matrices, which can be separated into thermoplastics, rubber, and thermosets composites. This chapter is expected to increase the interest of researchers worldwide in lignin-based polymers and develop new ideas in this field.

Sustainable Esterification of a Soda Lignin with Phloretic Acid

In this work, a sustainable chemical process was developed through the Fischer esterification of Protobind^® lignin, a wheat straw soda lignin, and phloretic acid, a naturally occurring phenolic acid. It aimed at increasing the reactivity of lignin by enhancing the number of unsubstituted phenolic groups via a green and solvent-free chemical pathway. The structural features of the technical and esterified lignins were characterized by complementary spectroscopic techniques, including ¹H, ¹³C, ³¹P, and two-dimensional analysis. A substantial increase in p-hydroxyphenyl units was measured (+64%, corresponding to an increase of +1.3 mmol g⁻¹). A full factorial design of the experiment was employed to quantify the impact of critical variables on the conversion yield. The subsequent statistical analysis suggested that the initial molar ratio between the two precursors was the factor predominating the yield of the reaction. Hansen solubility parameters of both the technical and esterified lignins were determined by solubility assays in multiple solvents, evidencing their high solubility in common organic solvents. The esterified lignin demonstrated a better thermal stability as the onset of thermal degradation shifted from 157 to 220 °C, concomitantly to the shift of the glass transition from 92 to 112 °C. In conclusion, the esterified lignin showed potential for being used as sustainable building blocks for composite and thermoset applications.

Hydrophobization and Photo-Stabilization of Radiata Pinewood: The Effect of the Esterification on Thermal and Mechanical Properties

Wood protection through chemical modification has received increasing interest over the last decades due to the environmental issues related to conventional biocides or protecting products. Consequently, a wide range of new treatments are developed in laboratories, which are later scaled up in the industrial environment. The main goal of modifying wood for indoor–outdoor application is to change its hydrophilic character, which in turn improves the intrinsic properties of the material and its durability against external factors. Wood can be esterified through its hydroxyl groups to obtain a hydrophobic and photo-stable material. Chemical modifications of Pinus radiata D. Don wood using hexanoyl chloride (P6), dodecanoyl chloride (P12), and stearoyl chloride (P18) were carried out at different concentrations. Esterification was confirmed by Fourier Transform Infrared Spectroscopy (FTIR) technique combined with a discriminatory analysis. Weight percent gain was associated with the number of carbons of the aliphatic chain of the fatty acid (P6 > P12 > P18). Moreover, an increase of wood density as a consequence of modification treatments was observed. A substantial improvement of the hydrophobicity of wood was observed by dynamic contact angle measurements. In addition, the effect of ultraviolet (UV) radiation on color changes was reduced with the treatments. Furthermore, the P6 treatment presented acceptable values of modulus of elasticity (MOE) and modulus of rupture (MOR), being suitable for similar mechanical uses as non-treated pinewood. However, only treatments P12 and P18 enhanced thermal resistance of the pinewood in an oxidative atmosphere.

Willow Lignin Recovered from Hot-Water Extraction for the Production of Hydrogels and Thermoplastic Blends

A blend of commercial willow cultivars (Salix spp.) was subjected to hydrothermal pretreatment: hot-water extraction in a pilot plant 65-ft³ digester. Lignin recovered from the autohydrolyzate (willow lignin, WL) was used to produce hydrogels and thermoplastic blends. Lignin-kaolin-acrylamide (WL 10.7 % w/w) and lignin-poly(ethylene)glycol diglycidyl ether (WL 66.7 % w/w) hydrogels exhibited favorable adsorption and absorption properties, respectively. Dye adsorption kinetics, swelling capacities, fragrance emanation, and compression moduli were measured for the hydrogels, along with SEM characterization. Additionally, WL was esterified by acetylation (C₂ , WAc) and acylation with lauroyl chloride (C₁₂ , WFAE and WFAE_H ). The crude and esterified lignin samples were blended (1-12 % w/w) with polylactic acid (PLA) via lab-scale melt extrusion to produce thermoplastic blends. The physicochemical properties of the blends were evaluated. They exhibited an increase in degradation temperature and UV absorbance with potential to hinder photodegradation of PLA and may be leveraged in packaging applications.

Melt Stable Functionalized Organosolv and Kraft Lignin Thermoplastic

A shift towards an economically viable biomass biorefinery concept requires the use of all biomass fractions (cellulose, hemicellulose, and lignin) for the production of high added-value products. As lignin is often underutilized, the establishment of lignin valorization routes is highly important. In-house produced organosolv as well as commercial Kraft lignin were used in this study. The aim of the current work was to make a comparative study of thermoplastic biomaterials from two different types of lignins. Native lignins were alkylate with two different alkyl iodides to produce ether-functionalized lignins. Successful etherification was verified by FT-IR spectroscopy, changes in the molecular weight of lignin, as well as 13C and 1H Nuclear Magnetic Resonance (NMR). The thermal stability of etherified lignin samples was considerably improved with the T2% of organosolv to increase from 143 °C to up to 213 °C and of Kraft lignin from 133 °C to up to 168 °C, and glass transition temperature was observed. The present study shows that etherification of both organosolv and Kraft lignin with alkyl halides can produce lignin thermoplastic biomaterials with low glass transition temperature. The length of the alkyl chain affects thermal stability as well as other thermal properties.

Modification of Kraft Lignin with Dodecyl Glycidyl Ether

Kraft lignin (KL) is extensively produced in industry but is mainly burned as fuel. To broaden its use, KL was grafted with dodecyl glycidyl ether to alter its thermal properties. The reaction of KL with dodecyl glycidyl ether (DGE) was analyzed using nuclear magnetic resonance (NMR), Fourier infrared spectroscopy (FT-IR) and elemental analysis. Alternatively, KL was methylated to mask its phenolic hydroxy groups to investigate how phenolic hydroxy groups impact the grafting of the alkyl chain of DGE onto lignin (methylated Kraft lignin, MKL). The methylation facilitated the molecular weight enhancement and thermal stability reduction of Kraft lignin via grafting with DGE. The influence of grafting alkyl chains on the structural and thermal properties of KL and MKL was studied using thermogravimetric analysis and differential scanning calorimetry analysis. Our data suggest that, due to their high molecular weights and lower glass transition temperatures, the produced lignin derivatives may be promising feedstocks for composite production.

Acidic Ionic Liquid as Both Solvent and Catalyst for Fast Chemical Esterification of Industrial Lignins: Performances and Regioselectivity

Lignin can be considered an essential under-exploited polymer from lignocellulosic biomass representing a key for a profitable biorefinery. One method of lignin valorization could be the improvement of physico-chemical properties by esterification to enhance miscibility in apolar polyolefin matrices, thereby helping the production of bio-based composites. The present work describes for the first time a succeeded chemical esterification of industrial lignins with maleic anhydride in an acidic ionic liquid: 1-butyl-3-methyl imidazolium hydrogen sulfate without additional catalyst. This efficient strategy was applied to four industrial lignins: two softwood Kraft lignins (Indulin AT, Wayagamack), one hardwood Kraft lignin (Windsor), and one softwood organosolv lignin (Lignol), distinct in origin, extraction process and thus chemical structure. The chemical, structural, and thermal properties of modified lignins were characterized by ³¹P nuclear magnetic resonance, infrared spectroscopy and thermal analyses, then compared to those of unmodified lignins. After 4 h of reaction, between 30 to 52% of the constitutive hydroxyls were esterified depending on the type of lignin sample. The regioselectivity of the reaction was demonstrated to be preferentially orientated toward aliphatic hydroxyls for three out of four lignins (66.6, 65.5, and 83.6% for Indulin AT, Windsor and Lignol, respectively, vs. 51.7% for Wayagamack). The origin and the extraction process of the polymer would thus influence the efficiency and the regioselectivity of this reaction. Finally, we demonstrated that the covalent grafting of maleyl chain on lignins did not significantly affect thermal stability and increased significantly the solubility in polar and protic solvent probably due to additional exposed carboxylic groups resulted from mono-acylation independently of H/G/S ratio. Blending with polyolefins could then be considered in regard of compatibility with the obtained physico-chemical properties.

Chemical and thermal analysis of lignin streams from Robinia pseudoacacia L. generated during organosolv and acid hydrolysis pre-treatments and subsequent enzymatic hydrolysis

Lignin streams produced in biorefineries are commonly used to obtain energy. In order to increase the competitiveness of this industry, new lignin valorization routes are necessary, for which a depth characterization of this biological macromolecule is essential. In this context, this study analyzed lignin streams of Robinia pseudoacacia L. generated during organosolv and acid hydrolysis pre-treatments and during the subsequent enzymatic hydrolysis. These lignins included dissolved lignins from pre-treatment liquors and saccharification lignins from pre-treated materials. Chemical composition and structural features were analyzed by analytical standard methods and Fourier Transform Infrared spectroscopy (FTIR), size exclusion chromatography (SEC), ¹³C solid state nuclear magnetic resonance (¹³C NMR) and ¹H-¹³C two-dimensional nuclear magnetic resonance (2D NMR); while thermal characterization included thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). In general, all studied lignins contained a predominance of β-O-4' aryl ether linkages, followed by resinol (β-β') and phenylcoumaran (β-5'), with a predominance of syringyl over guaiacyl and hydroxyphenyl units. Nevertheless, the dissolved lignins revealed a removal of linkages, especially β-O-4', leading to an enrichment of phenolic groups. Moreover, high thermal stability and good thermoplasticity were characteristics of these lignins. Contrary, the saccharification lignins exhibited a more intact structure, but with an important remaining carbohydrates content.

The Palladium-Catalyzed Carbonylative Telomerization Reaction with Phenols, Polyphenols and Kraft Lignin

An efficient carbonylative coupling reaction of two equivalents of 1,3-butadiene, yielding aryl nona-3,8-dienoate esters, is performed with phenols as nucleophile, and promoted by palladium-based catalysts. Optimization study reveals the key role of benzoic acid as a cocatalyst. The suggested catalyst combination enables the conversion of a wide scope of variously substituted phenols into corresponding esters with a high yield. Further tests were performed with diphenols, naturally-occurring phenols and an industrial grade Kraft lignin, thus, indicating the scope of this reaction for transforming industrially relevant polyphenolic structures.

Acylation of Lignin with Different Acylating Agents by Mechanical Activation-Assisted Solid Phase Synthesis: Preparation and Properties

Acylated lignins with substituents consisting of different lengths of carbon chains were prepared by a mechanical activation-assisted solid phase synthesis (MASPS) technology with a customized stirring ball mill as a reactor. The structures and properties were analyzed by UV/Vis, FTIR, NMR, SEM, DSC, and TG. The results showed that the acylated lignins were successfully prepared with either non-cyclic or cyclic anhydrides as the acylating agents. Both aliphatic hydroxyl and phenolic hydroxyl groups of lignin reacted with non-cyclic anhydrides, and different reactivity of acylating agents resulted in different relative contents of phenolic and aliphatic substituents in the products. The reactivity of the cyclic anhydrides was weaker than that of the non-cyclic anhydrides, and the reactivity of the acylating agents decreased with increasing carbon chain length and unsaturated bonds of acyl groups. All of the acylated lignins except maleylated lignin had a lower glass transition temperature (Tg) than the original lignin. The acylated lignins prepared with non-cyclic anhydrides had better thermal stability than original lignin, and the thermal stability increased, but Tg decreased with an increasing chain length of the acyl groups. The acylated lignins prepared with cyclic anhydrides had higher a Tg than those with non-cyclic anhydrides with the same carbon number, and the thermal stability was not obviously improved.

Palladium-Catalyzed Functionalization of Kraft Lignin: Ether Linkages through the Telomerization Reaction

Kraft lignin was efficiently functionalized with octadienyl ether linkages through the palladium-catalyzed telomerization of 1,3-butadiene. Comparison with molecular model substrates assessed the grafting of phenols and alcohols and an optimization study led to up to 69 % conversion of the total number of hydroxyl groups present in lignin. This catalytic study evidenced the partial oxidation of triphenylphosphine into triphenylphosphine oxide and triphenylphosphine sulfide by contaminants present in the industrial grade kraft lignin. The telomerised lignin is a malleable material and a preliminary study of the thermal properties showed a decrease in the glass transition in comparison with the starting material.