Reactivity enhancement of organosolv lignin by phenolation for improved bio-based thermosets

Straw Tar Epoxy Resin for Carbon Fiber-Reinforced Plastic: A Review

The massive consumption of fossil fuels has led to the serious accumulation of carbon dioxide gas in the atmosphere and global warming. Bioconversion technologies that utilize biomass resources to produce chemical products are becoming widely accepted and highly recognized. The world is heavily dependent on petroleum-based products, which may raise serious concerns about future environmental security. Most commercially available epoxy resins (EPs) are synthesized by the condensation of bisphenol A (BPA), which not only affects the human endocrine system and metabolism, but is also costly to produce and environmentally polluting. In some cases, straw tar-based epoxy resins have been recognized as potential alternatives to bisphenol A-based epoxy resins, and are receiving increasing attention due to their important role in overcoming the above problems. Using straw tar and lignin as the main raw materials, phenol derivatives were extracted from the middle tar instead of bisphenol A. Bio-based epoxy resins were prepared by replacing epichlorohydrin with epoxylated lignin to press carbon fiber sheets, which is a kind of bio-based fine chemical product. This paper reviews the research progress of bio-based materials such as lignin modification, straw pyrolysis, lignin epoxidation, phenol derivative extraction, and synthesis of epoxy resin. It improves the performance of carbon fiber-reinforced plastic (CFRP) while taking into account the ecological and environmental protection, so that the epoxy resin is developed in the direction of non-toxic, harmless and high-performance characteristics, and it also provides a new idea for the development of bio-based carbon fibers.

Lignin phenolation by graft copolymerization to boost its reactivity

Lignin is the most abundant phenolic biopolymer and a renewable resource of aromatics. It can be used as a phenol substitute in the synthesis of phenolic resins. However, lignin is not as reactive as phenol, so phenolation is generally carried out to improve lignin reactivity. In this work, we suggest a solution to circumvent the limitations of traditional phenolation (e.g., high temperature, strong acids/bases, limited reactivity, and phenol toxicity). We first attempt new lignin phenolation by graft copolymerization in which polymeric phenol, instead of toxic phenol, is introduced to lignin. Organosolv lignin from hardwood was modified with 2-bromoisobutyryl bromide to act as a lignin macroinitiator (L-Br). A protected phenolic monomer, 4-acetoxystyrene, was graft copolymerized onto L-Br using CuBr2/tris[2-(dimethylamino)ethyl]amine as a catalyst/ligand, after which the resultant lignin copolymer was deacetylated to produce lignin grafted with poly(4-hydroxystyrene). This poly-phenolation process was conducted at room temperature without the strong acids/bases and toxic phenol required in conventional phenolation. The poly-phenolated lignin was analyzed using 1H-, 13C-, and 31P NMR spectroscopy and gel permeation chromatography. This novel phenolation process enhanced the reactive sites of lignin more than tenfold. It also reduced the dark color of technical lignins significantly, thereby overcoming a serious obstacle to their applicability.

Green Phenolic Resins from Oil Palm Empty Fruit Bunch (EFB) Phenolated Lignin and Bio-Oil as Phenol Substitutes for Bonding Plywood

Lignin is a natural biopolymer with a complex three-dimensional network and it is rich in phenol, making it a good candidate for the production of bio-based polyphenol material. This study attempts to characterize the properties of green phenol-formaldehyde (PF) resins produced through phenol substitution by the phenolated lignin (PL) and bio-oil (BO), extracted from oil palm empty fruit bunch black liquor. Mixtures of PF with varied substitution rates of PL and BO were prepared by heating a mixture of phenol-phenol substitute with 30 wt.% NaOH and 80% formaldehyde solution at 94 °C for 15 min. After that, the temperature was reduced to 80 °C before the remaining 20% formaldehyde solution was added. The reaction was carried out by heating the mixture to 94 °C once more, holding it for 25 min, and then rapidly lowering the temperature to 60 °C, to produce the PL-PF or BO-PF resins. The modified resins were then tested for pH, viscosity, solid content, FTIR, and TGA. Results revealed that the substitution of 5% PL into PF resins is enough to improve its physical properties. The PL-PF resin production process was also deemed environmentally beneficial, as it met 7 of the 8 Green Chemistry Principle evaluation criteria.

Ultrastrong, Hydrostable, and Degradable Straws Derived from Microplastic-Free Thermoset Films for Sustainable Development

Single-use plastics such as straws have caused intricate environmental challenges since they are not readily assimilated into nature at the end of life. Paper straws, on the contrary, become soggy and collapse in drinks resulting in an obnoxious user experience. Here, all-natural, biocompatible, degradable straws and thermoset films are engineered by integrating economical natural resources-lignin and citric acid-into edible starch and poly(vinyl alcohol), making them the casting slurry. The slurries were cast on a glass substrate, partially dried, and rolled on a Teflon rod to fabricate the straws. The straws are perfectly adhered at the edges by the strong hydrogen bonds from the crosslinker-citric acid-during drying, thus eliminating the need for adhesives and binders. Further, curing the straws and films in a vacuum oven at 180 °C results in enhanced hydrostability and endows the films with excellent tensile strength, toughness, and ultraviolet radiation shielding. The functionality of the straws and films surpassed paper and plastic straws, making them quintessential candidates for all-natural sustainable development.

A modified ionization difference UV-vis method for fast quantitation of guaiacyl-type phenolic hydroxyl groups in lignin

An ionization difference UV-Vis method (Δε-spectrum method) is the most potentially simple method for fast quantitation of phenolic hydroxyl groups (ph-OH) in lignin. However, the underestimated results were calculated from the conventional Δε-spectrum method using one- or two-point wavelengths measurement. In this study, a modified Δε-spectrum method using multi-point wavelengths measurement was developed and the negative absorbance was also considered. Four main typical lignin models, e.g. vanilla alcohol, 5-5 biphenyl, stilbenoid and vanillin, were applied as the guaiacyl-type (G-type) phenolic models for the determination of ph-OH by the modified Δε-spectrum method. The 2-methoxyethanol/water/acetic acid = 8/2/0.2 (V/V/V) was used as the acidic solvent system and the 2-methoxyethanol/0.2 M NaOH solution = 1/9 (V/V) was used as the alkaline solvent system. The ph-OH contents in the spruce milled wood lignin (SMWL) and the spruce Kraft lignin (SKL) were respectively quantified by the modified Δε-spectrum method as 1.078 and 4.348 mmol/g, which were comparable to the counterparts determined by ³¹P Nuclear Magnetic Resonance Spectroscopy (³¹P NMR). The results revealed that the modified Δε-spectrum method can provide more accurate and reliable results compared to the conventional method.

Effects of Alcell Lignin Methylolation and Lignin Adding Stage on Lignin-Based Phenolic Adhesives

To investigate the effects of lignin methylolation and lignin adding stage on the resulted lignin-based phenolic adhesives, Alcell lignin activated with NaOH (AL) or methylolation (ML) was integrated into the phenolic adhesives system by replacing phenol at various adhesive synthesis stages or directly co-polymerizing with phenolic adhesives. Lignin integration into phenolic adhesives greatly increased the viscosity of the resultant adhesives, regardless of lignin methylolation or adding stage. ML introduction at the second stage of adhesive synthesis led to much bigger viscosity than ML or AL introduction into phenolic adhesives at any other stages. Lignin methylolation and lignin adding stage did not affect the thermal stability of lignin based phenolic adhesives, even though lignin-based adhesives were less thermally stable than NPF. Typical three-stage degradation characteristics were also observed on all the lignin-based phenolic adhesives. Three-ply plywoods can be successfully laminated with lignin based adhesives, and it was interesting that after 3 h of cooking in boiling water, the plywoods specimens bonded with lignin-based phenolic adhesives displayed higher bonding strength than the corresponding dry strength obtained after direct conditioning at 20 °C and 65% RH. Compared with NPF, lignin introduction significantly reduced the bonding strength of lignin based phenolic adhesives when applied for plywood lamination. However, no significant variation of bonding strength was detected among the lignin based phenolic adhesives, regardless of lignin methylolation or adding stages.

A Comparison among Lignin Modification Methods on the Properties of Lignin-Phenol-Formaldehyde Resin as Wood Adhesive

The research aim of this work is to determine the influence of lignin modification methods on lignin–phenol–formaldehyde (LPF) adhesive properties. Thus, glyoxal (G), phenol (P), ionic liquid (IL), and maleic anhydride (MA) were used to modify lignin. The modified lignins were used for phenol substitution (50 wt%) in phenol–formaldehyde adhesives. The prepared resins were then used for the preparation of wood particleboard. These LPF resins were characterized physicochemically, namely by using standard methods to determine gel time, solids content, density, and viscosity, thus the physicochemical properties of the LPF resins synthesized. The panels dimensional stability, formaldehyde emission, bending modulus, bending strength, and internal bond (IB) strength were also measured. MA-modified lignin showed by differential scanning calorimetry (DSC) the lowest temperature of curing than the resins with non-modified lignin and modified with IL, phenolared lignin, and glyoxal. LPF resins with lignin treated with maleic anhydride presented a shorter gel time, higher viscosity, and solids content than the resins with other lignin modifications. Equally, the particleboard panels prepared with LPF resins with maleic anhydride or with ionic liquid had the lowest formaldehyde emission and the highest mechanical strength among all the synthesized resins. The dimensional stability of all panels bonded with modified lignin LPF resins presented no difference of any significance.

Estrogenic activity of lignin-derivable alternatives to bisphenol A assessed via molecular docking simulations

Lignin-derivable bisphenols are potential alternatives to bisphenol A (BPA), a suspected endocrine disruptor; however, a greater understanding of structure-activity relationships (SARs) associated with such lignin-derivable building blocks is necessary to move replacement efforts forward. This study focuses on the prediction of bisphenol estrogenic activity (EA) to inform the design of potentially safer BPA alternatives. To achieve this goal, the binding affinities to estrogen receptor alpha (ERα) of lignin-derivable bisphenols were calculated via molecular docking simulations and correlated to median effective concentration (EC50) values using an empirical correlation curve created from known EC50 values and binding affinities of commercial (bis)phenols. Based on the correlation curve, lignin-derivable bisphenols with binding affinities weaker than ∼-6.0 kcal mol-1 were expected to exhibit no EA, and further analysis suggested that having two methoxy groups on an aromatic ring of the bio-derivable bisphenol was largely responsible for the reduction in binding to ERα. Such dimethoxy aromatics are readily sourced from the depolymerization of hardwood biomass. Additionally, bulkier substituents on the bridging carbon of lignin-bisphenols, like diethyl or dimethoxy, were shown to weaken binding to ERα. And, as the bio-derivable aromatics maintain major structural similarities to BPA, the resultant polymeric materials should possess comparable/equivalent thermal (e.g., glass transition temperatures, thermal decomposition temperatures) and mechanical (e.g., tensile strength, modulus) properties to those of polymers derived from BPA. Hence, the SARs established in this work can facilitate the development of sustainable polymers that maintain the performance of existing BPA-based materials while simultaneously reducing estrogenic potential.

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.

Recent Research Progress on Lignin-Derived Resins for Natural Fiber Composite Applications

By increasing the environmental concerns and depletion of petroleum resources, bio-based resins have gained interest. Recently, lignin, vanillin (4-hydroxy-3-methoxybenzaldehyde), and divanillin (6,6'-dihydroxy-5,5'-dimethoxybiphenyl-3,3'-dicarbaldehyde)-based resins have attracted attention due to the low cost, environmental benefits, good thermal stability, excellent mechanical properties, and suitability for high-performance natural fiber composite applications. This review highlights the recent use of lignin, vanillin, and divanillin-based resins with natural fiber composites and their synthesized processes. Finally, discussions are made on the curing kinetics, mechanical properties, flame retardancy, and bio-based resins' adhesion property.

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.

Nano-Structured Lignin as Green Antioxidant and UV Shielding Ingredient for Sunscreen Applications

Green, biocompatible, and biodegradable antioxidants represent a milestone in cosmetic and cosmeceutical applications. Lignin is the most abundant polyphenol in nature, recovered as a low-cost waste from the pulp and paper industry and biorefinery. This polymer is characterized by beneficial physical and chemical properties which are improved at the nanoscale level due to the emergence of antioxidant and UV shielding activities. Here we review the use of lignin nanoparticles in cosmetic and cosmeceutical applications, focusing on sunscreen and antiaging formulations. Advances in the technology for the preparation of lignin nanoparticles are described highlighting structure activity relationships.

Novel lignin-containing high-performance adhesive for extreme environment

The gradual depletion of petroleum is a main challenge restricting the development for the fine chemicals, such as epoxy resin adhesive. In this study, a novel lignin-containing high-performance epoxy resin adhesive is synthesized using lignin as precursor material. Lignin is a unique biomacromolecule with three dimensional network structure, large molecular weight, and aromatic structure. The lignin is simply hydrolyzed and modified by epichlorohydrin to obtain lignin-based epoxy prepolymer. The hydrolysis process effectively reduces the molecular weight and improves the chemical reactivity of lignin, thus increasing the number of modified functional groups and the dispersibility of lignin concurrently. With the introduction of the lignin-based epoxy prepolymers, the shear strength of the adhesive increases obviously and reaches 10.42 MPa, which displays 228% of the shear strength of commercial epoxy resin adhesives. Furthermore, the lignin-containing epoxy resin adhesive still displays excellent mechanical properties in extreme environments, including extreme temperature and high humidity environment.

Formaldehyde-free self-polymerization of lignin-derived monomers for synthesis of renewable phenolic resin

Most phenolic resins are synthesized with non-renewable petroleum-based phenol and formaldehyde, which have adverse effects on the environment and human health. To achieve green and sustainable production of phenolic resins, it is important to replace non-renewable toxic phenol and formaldehyde. Herein, a new strategy was proposed to completely replace phenol and formaldehyde, using lignin-derived monomers to synthesize renewable phenolic resins. Lithium aluminum hydride was utilized to reduce lignin-derived monomers, including vanillin, methyl vanillate, and syringaldehyde, to generate the corresponding vanillyl and syringic alcohol. With oxalic acid as the catalyst, vanillyl and syringic alcohol could be polymerized to phenolic resins without using formaldehyde. The structure of the phenolic resins based on lignin-derived monomers was analyzed by Fourier transform infrared spectroscopy and ¹³C and ³¹P nuclear magnetic resonance spectroscopy. Differential scanning calorimetry and thermogravimetric analysis were performed to characterize the thermal properties of the phenolic resins. The phenolic resins based on lignin-derived monomers exhibited excellent adhesion strength (6.14 MPa), glass transition temperature (T_g) (107-115 °C), and thermal stability, and its performance was similar to that of the commercial Novolak phenolic resin. This study presents a promising green and sustainable approach to synthesize renewable phenolic resins based on lignin-derived monomers without using formaldehyde.

Naphthalene Structures Derived from Lignins During Phenolation

Phenolation is a commonly used method to improve the reactivity of lignin for various applications. In this study, resinol lignin models (syringaresinol and pinoresinol) and eucalyptus alkali lignin were treated under acid-catalyzed phenolation conditions to investigate the products derived from resinol (β-β) structures of lignins. The phenolation products were characterized by means of GC-MS and NMR spectroscopy following separation using flash chromatography and thin-layer chromatography. A series of new naphthalene products were identified from phenolation of syringaresinol, and the corresponding guaiacyl analogs were also identified by GC-MS. The C1-Cα bond of these resinol compounds was cleaved to release syringol or guaiacol during phenolation. In addition, diphenylmethane products formed from phenol or phenol and syringol/guaiacol were found in the phenolation products. Comparatively, more naphthalene products were obtained by phenolation from syringaresinol than those obtained from pinoresinol. HSQC NMR characterization of the phenolated alkali lignin revealed that naphthalene structures formed in the phenolated lignin.

Bio-Based Alternatives to Phenol and Formaldehyde for the Production of Resins

Phenol-formaldehyde (PF) resin continues to dominate the resin industry more than 100 years after its first synthesis. Its versatile properties such as thermal stability, chemical resistance, fire resistance, and dimensional stability make it a suitable material for a wide range of applications. PF resins have been used in the wood industry as adhesives, in paints and coatings, and in the aerospace, construction, and building industries as composites and foams. Currently, petroleum is the key source of raw materials used in manufacturing PF resin. However, increasing environmental pollution and fossil fuel depletion have driven industries to seek sustainable alternatives to petroleum based raw materials. Over the past decade, researchers have replaced phenol and formaldehyde with sustainable materials such as lignin, tannin, cardanol, hydroxymethylfurfural, and glyoxal to produce bio-based PF resin. Several synthesis modifications are currently under investigation towards improving the properties of bio-based phenolic resin. This review discusses recent developments in the synthesis of PF resins, particularly those created from sustainable raw material substitutes, and modifications applied to the synthetic route in order to improve the mechanical properties.

Chemical modification of lignin derived from spent coffee grounds for methylene blue adsorption

In this work, spent coffee grounds (SCG) were treated using sulfuric acid hydrolysis in order to isolate the sulfuric acid lignin (SAL). The reactivity of SAL was improved through phenolation and acetylation. Spectroscopic analysis showed that the isolated lignin is composed of GHS type and it was characterized by a high amount of (C-C) and β-O-4 bonds. The thermal analysis showed that the phenolated sulfuric acid lignin (Ph-SAL) present higher thermal stability compared to SAL and acetylated sulfuric acid lignin. In addition, the phenolic hydroxyl group content increases from 2.99 to 9.49 mmol/g after phenolation. Moreover, a methylene blue (MB) adsorption test was established in order to find out the sorption capacity of different samples. The study showed that the adsorbed amount of dye increase after the chemical modification of SAL, especially after phenolation. The removal efficiency was enhanced after modification to reach 99.62% for Ph-SAL. The evaluation of the adsorption experimental data with the theoretical models of Langmuir and Freundlich showed that the best fitting was expressed by the Langmuir model for all samples. Finally, this study showed that lignin isolated from SCG can be simply and easily chemical modified and exhibits excellent adsorption ability towards cationic dyes (MB) in aqueous solutions. As a renewable, low-cost, and natural biomass material, lignin from SCG shows a promising practical and economical application of biomass in the field of wastewater purification.

Preparation and characterization of a nanolignin phenol formaldehyde resin by replacing phenol partially with lignin nanoparticles

A new strategy for the preparation of a lignin phenol formaldehyde (LPF) resin has been developed. Nanolignin with high specific surface area and porous structure with an average particle size of about 300 nm was prepared, used as the raw material to substitute phenol partially, and combined with formaldehyde to produce a wood adhesive. The results show that the artificial board prepared with a nanolignin phenol formaldehyde (NLPF) resin with nanolignin substitution degree of 40% wt for phenol could give a dry bond strength of 1.30 ± 0.08 MPa, which is 1.85 times that of the Chinese national grade 1 plywood standard (0.7 MPa) and whose formaldehyde emission of 0.40 mg L-1 meets the standard of GB/T 14732-2006 (E 0, 0.5 mg L-1). TG and DSC analyses show that the replacement of phenol by nanolignin could improve the thermal stability and decrease the curing temperature of the prepared lignin-based resin, with the residual ratio of 40% NLPF being 45% wt at 800 °C and the curing exothermic peak being 145.4 °C, which are much better than that of the 40% LPF resin with the residual ratio being 40% wt and the exothermic peak being 186 °C, respectively. The present study provides a new thought for preparation of LPF resins.

Bio-Crude by Acidic Phenolation and Carbamation for the Preparation of Phenolic Thermosetting Resin and Its Application in Thermoresistant Laminates

Fir sawdust was liquefied in phenol solvent under acidic catalyst at 135, 150 and 165 °C, respectively; after neutralization, bio-crude was obtained where contained oil-like liquid and tiny powder-like residue. The bio-crude was chemically modified with urea at high temperature (e. g. > 130 °C) to form carbamate so as to improve chemical reactivity of bio-crude in phenolic resin synthesis. The carbamate-containing bio-crude was condensed with paraformaldehyde into thermosetting phenolic resin. Finally, this biomass-derived phenolic resin matrixed silica fabric laminates were processed. The uncured and thermally cured bio-based resins were characterized by the techniques of Differential Scanning Calorimetry (DSC), Fourier Transform Infrared spectrum (FT-IR), rheology and Thermogravimetric Analysis (TGA), and the laminates’ structure and mechanical performances were studied using the methods of Scanning Electron Microscopy (SEM), three point bending mechanical test and Dynamic Mechanical Analysis (DMA). The results showed: (1) the chemical reactivity of bio-crude was highly improved by carbamation; (2) biomass-derived thermosetting phenolic resin was thermally curable at 150–250 °C (with two exothermic peaks at 185 °C and 220 °C); (3) the char yield was about 47 %, which was not in apparent relationship with sawdust liquefaction temperatures; (4) flexural strength of silica fabric laminates at room temperature was around 357 MPa (similar with that of conventional phenolic laminate); (5) glass transition temperature of silica fabric laminate was above 270 °C (much higher than Tg of conventional phenolic resin laminate, which is normally at 215 °C). The biomass-derived phenolic resin is expected to be widely used as cost-effective and environment-friendly thermosetting resin in the application of high-performance composites.

Intensified ozonolysis of lignins in a spray reactor: insights into product yields and lignin structure

A safe and efficient spray ozonolysis to valorize grass and hardwood lignins, selectively yielding aromatic aldehydes without their overoxidation or lignin bleaching.

Reactivity improvement by phenolation of wheat straw lignin isolated from a biorefinery process

This work describes an effective phenolation process to improve wheat straw biorefinery lignin reactivity.

Benzoxazines with enhanced thermal stability from phenolated organosolv lignin

Phenolated lignin was modified with propargyl amine and aniline to benzoxazines, which exhibit high thermal stability and were classified as self-extinguishing.