Lignin-based Epoxy Research Articles

Preparation and properties of novel bio-based epoxy resin thermosets from lignin oligomers and cardanol

A series of lignin-based epoxy resins (LEPs) were prepared by the reaction of epichlorohydrin with lignin oligomers derived from partial reductive depolymerization of lignin. To overcome the high viscosity and brittleness defects in practical applications, the LEPs were blended with renewable epoxied cardanol glycidyl ether (ECGE) and then cured with methyltetrahydrophthalic anhydride (MeTHPA) to form high-performance epoxy thermosets. The effects of degree of lignin depolymerization, chemical composition of lignin oligomers and dosage of ECGE on thermal and mechanical properties of the cured products were investigated. The LEP/MeTHPA thermosets exhibited good thermal and mechanical properties. Especially, by separating monomer-rich fractions from lignin oligomers, the thermal and mechanical properties of the cured product were improved obviously. Notably, the incorporation of ECGE also possessed a positive effect on reinforcing and toughening the cured products. With 20 wt% ECGE loadings, the tensile, flexural and impact strength of the cured product reached the maximum value of 77 MPa, 115 MPa and 14 kJ/m2, respectively, which were equivalent to the commercial bisphenol A epoxy resins thermosets. These findings indicated that the novel bio-based epoxy resins from lignin oligomers and cardanol could be utilized as renewable alternatives for BPA epoxy resins.

Demethylation, phenolation, and depolymerization of lignin for the synthesis of lignin-based epoxy resin via a one-pot strategy

A one-pot method to synthesize demethylated-phenolated lignin (DPL) and demethylated-phenolated lignin-based epoxy resin (DPLINP) using phenol as a phenolic reagent and polymerization inhibitor was proposed in this study. This method avoids the occurrence of polycondensation caused by demethylation and simplifies the synthesis of lignin-based epoxy resins. Phenolated lignin (PL)-based epoxy resin (PLINP) and demethylated lignin (DL)-based epoxy resin (DLINP) were prepared to investigate the effects of different modification methods on the resulting lignin and epoxy resin. The results of 1H nuclear magnetic resonance (NMR), Fourier transform infrared, and gel permeation chromatography analyses indicated that the introduction of phenol could significantly reduce the occurrence of polycondensation while also decreasing the molecular weight and increasing the content of phenolic hydroxyl in DPL compared with DL and PL. 2D heteronuclear single quantum coherence NMR analysis was used to further investigate the structural changes of lignin during the modification process. The results obtained via dynamic mechanical analysis, flexural test, and impact test demonstrated that demethylated-phenolated lignin-based epoxy network (DPLIEN) possessed an improved cross-linking density, glass transition temperature, and mechanical performance compared with all other epoxy resins. Moreover, the polycondensation of DL caused by demethylation deteriorated the mechanical performance of demethylated lignin-based epoxy network (DLIEN). The thermal stability of DPLIEN was decreased compared with phenolated lignin-based epoxy network (PLIEN), as the removal of methoxy groups may have reduced the protective char layer formation in DPLIEN. This study provides a novel method for the modification of lignin and synthesis of lignin-based epoxy resins.

Flame-retardant effect of a functional DOPO-based compound on lignin-based epoxy resins

A simple and environmentally benign synthetic route to lignin-based epoxy resins is highly desirable. Alkali lignin and glyoxal are promising renewable and sustainable alternatives to phenol and formaldehyde, respectively. Their use is demonstrated to produce lignin-phenol-glyoxal novolac epoxy resins (LPG-NERs) through a simple one-pot synthesis. Flame retardancy of LPG-NERs was improved by functionalization with a N, S modified 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) material (termed DBS) obtained via nucleophilic reaction between DOPO and a N, S containing intermediate. FTIR and 1H NMR analyses confirmed the successful fabrication of LPG-NERs and SBD. The introduction of lignin and SBD reduced crosslinking of the epoxy network, weakening mechanical properties, but conferred excellent flame retardancy (including vapor and condensed phases) as determined by LOI (34.5%), UL-94 (V-0 rating), and cone calorimetry tests (lower heat release and smoke production, and higher char formation). The SBD structure imparted a desirable vapor mechanism (blowing out phenomenon caused by the fire quenching effect of PO•/PO2• radicals and dilution effect from non-condensable gases such as NH3, N2, SO2), while lignin (a natural biochar precursor) in synergy with SBD imparted superior charring performance.

Thermal performance and thermal decomposition kinetics of a novel lignin-based epoxy resin containing phosphorus and nitrogen elements

Thermal performance and thermal decomposition kinetics of a novel lignin-based epoxy resin containing phosphorus and nitrogen elements

Lignin doped epoxy acrylate sandwich electromagnetic shielding material synergized with Fe3O4 and CNT

A kind of novel electromagnetic shielding sandwich coating material with excellent electromagnetic interference shielding and coating properties was explored in this work. The interlayer was obtained through the in situ reaction of lignin, epoxy and acrylate doped with modified Fe3O4 nanoparticles (Fe3O4) and multi-wall carbon nanotubes (CNT) (FCLBEA). The FCLBEA layer was coated by two layers of lignin-based epoxy acrylate (LBEA) on both sides, and the total thickness was only around 1 mm. The SEM image showed that the Fe3O4 and CNT are uniformly dispersed in the LBEA matrix. The electromagnetic interference shielding efficiency (EMI SE) of LBEA@FCLBEA@LBEA sandwich material (LFL) was tested in the frequency range of 8.2–18 GHz, and the EMI SE value was higher than 10 dB in the entire measurement range. When the content of Fe3O4 and CNT was both 5% and the lignin content was 15%, the maximum EMI SE reached 14.8 dB. The LFL showed excellent hardness (4H) and adhesion (standard 1). The lignin enhanced the mechanical properties of LFL due to its benzene ring skeleton structure. In addition, the benzene ring of lignin interacted with CNT through π-π bond, which enhanced the dispersion of carbon nanotubes and Fe3O4 in the matrix, and promoted a complete electronic network formed in the system. The most important, the benzene ring in lignin has certain electromagnetic shielding effect, and can also form a synergistic effect with Fe3O4 and CNT to enhance the electromagnetic shielding performance. It was illustrated that the LBEA@FCLBEA@LBEA sandwich material with excellent overall properties can be promising in wide applications, such as EMI coatings, dark room and military fields.

Sustainable alternative for bisphenol A epoxy resin high-performance and recyclable lignin-based epoxy vitrimers

Epoxy vitrimers based on dynamic covalent bond chemistry have been developed in recent years as they can solve the problem of recycling and reuse for traditional thermosetting materials. Lignin as an aromatic raw material has recently used to partially replace commercial diglycidyl ether bisphenol A (DGEBA) in the epoxy vitrimers, however, it is difficult to achieve high lignin replacement rate, excellent mechanical property and reprocessability in a facile method at the same time. In this work, high mechanical strength and lignin replacement rate of lignin-based vitrimers were successfully synthesized by gradually replacing DGEBA with glycidyl ether of lignin. The mechanical properties of the lignin-based epoxy vitrimers can be tuned by different ratios of glycidyl ether of lignin and DGEBA, the Young’s modulus and tensile strength of those vitrimers can reach up to 2.00 GPa and 46.8 MPa, respectively. It's worth noting that the tensile strength and elongation at break were 39.5 MPa and 5.81 % when the lignin replacement rate was 100 %. All the lignin-based epoxy vitrimers exhibit excellent self-healing, reprocessing and shape memory property. The crack widths of the vitrimers were repaired over 80 % in just 5 min at 160 ℃, the tensile strength can reach up to 97 % healing efficiency after being hot reprocessed. The great properties of lignin-based epoxy vitrimers indicate that they can be used as renewable alternatives for bisphenol A epoxy vitrimers.

Facile synthesis of lignin-based epoxy resins with excellent thermal-mechanical performance

Up to now, various approaches have been used to fabricate lignin-based epoxy thermosets by utilizing lignin or lignin-derivatives, but there is still lack of a simple, effective and environmental-friendly pathway for producing lignin-based epoxy resins from industrial lignin. In this work, a novel strategy - one-pot to synthesize phenolated lignin incorporated novolac epoxy networks (PLIENs) was proposed. As expected, PLIENs obtained from the novel route exhibited preferable mechanical and thermal properties compared with the epoxy resins which obtained from common route. Moreover, increasing the loading of lignin did not significantly deteriorate the thermal-mechanical performance of cured epoxy resins. However, the Tg of PLIENs was slightly lowered compared with conventional petroleum-based epoxy resins (DGEBA). Nonetheless, the flexural strength and storage modulus of PLIENs were higher than that of DGEBA. Especially, the char yield of PLIENs at 800 °C was up to 28.9%, much higher than that of DGEBA (only 6.9%), which indicated that lignin has a certain promoting effect on the flame retardancy of epoxy resins. This research provides a new insight for producing commercially viable lignin-based epoxy thermosets.

Preparation of carboxylated lignin-based epoxy resin with excellent mechanical properties

Lignin, a renewable biopolymer recovered from the chemical processing of woody biomass, has potential to enhance the mechanical properties of epoxy resins with the added benefit of reduced contents of non-renewable petroleum-based polymers. Lignin-based epoxy resins were prepared by employing a carboxylated lignin as a co-hardener using a “one-pot” method. The effect of lignin content and preparation method on mechanical properties, apparent morphology, and thermodynamic properties of the resulting epoxy resins were then studied. The results showed that elongation at break increased from 2.2 to 5.7%, and flexural strength increased from 23.3 to 101 MPa, with the addition of lignin up to 20 wt%. The highest increase in tensile strength was obtained for the lignin-based resin having a lignin content of 10 wt%. The incorporation of carboxylated lignin at an appropriate amount demonstrated simultaneous reinforcing and toughening effects on the epoxy resin, which can be attributed to the rigid and highly-branched structure of lignin.

Study on extraction of lignin and synthesis of lignin-based epoxy resins using ionic liquid

Lignin molecules contain alcohol hydroxyl and phenolic hydroxyl, so lignin can be used instead of toxic bisphenol A (BPA) to synthesize epoxy resin, which can not only reduce the amount of formaldehyde emission but also achieve the purpose of waste utilization and environmental protection. In this study, the lignin of Eucalyptus urophylla was extracted with ionic liquid as the extraction agent, and subsequently phenolated by methylphenol as phenolic agent with concentrated sulfuric acid as catalyst to prepare phenolated lignin (PLG). Then, using NaOH as catalyst, the PLG was epoxidized with epoxy chloropropane (ECH) to synthesize the lignin-based epoxy resin (LGEP). The extraction of lignin and the synthesis of LGEP were investigated by the single-factor and orthogonal test, and the Eucalyptus urophylla raw materials, lignin, PLG, and LGEP were characterized by UV, FTIR, XRD, SEM, and TGA. The results demonstrated that when the ionic liquid was [ChCl][Gly], the temperature was 90 °C, the time was 4 h, and the liquid-solid ratio was 20:1; the extraction rate of lignin reached 93.73%, and the purity of the regenerated lignin was 96.3%. The mass ratio of PLG to ECH was 1:2.5, the mass fraction of NaOH was 20% of the mass of PLG, the reaction time was 3 h, and the reaction temperature was 95 °C. Under this condition, the epoxy value of LGEP was 0.364.

Pyrolysis mechanism and kinetics of high-performance modified lignin-based epoxy resins

Lignin used was extracted from the bio-ethanol production residue by enzymatic hydrolysis method. In this study, a novel lignin-based flame retardant agent (Lig-F) was synthesized by modification using 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and piperazine (PA) as grafting reactants, and characterized by thermogravimetric analysis coupled with Fourier transformation infrared spectroscopy (TG-FTIR). Three degradation stages were observed by TG analysis. FTIR results showed that significant variations in functional groups such as the presence of CN at 3060 cm−1, CaromaticP at 1478 cm−1, PO at 1203 cm−1, PN at 866 cm−1, and Caromatic-O-P at 749 cm−1 had been detected after grafting reaction. Epoxy resin (EP) composites with different amounts of Lig-F (2–10 %) were prepared through a curing agent, 4,4′-diaminodiphenylmethane (DDM). The investigation on thermal stability of EP composites demonstrated that more char residue with high thermal stability was obtained after pyrolysis as compared to EP, which was due to the highly efficient charring performance derived from PA-based DOPO compound. Furthermore, the content of char residue increased with the increase of Lig-F addition. The flame retardancy determined by limiting oxygen index (LOI) presented that EP composite containing 10 % Lig-F exhibited the highest LOI value of 34.8 %. Finally, the activation energy of Lig-F, an important component in the polymeric system of lignin and EP, was investigated, and significant effects of PA-based DOPO were mainly reflected in the early and final stages of decomposition as compared to general lignocellulosic biomass materials.

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.

Biobased Epoxy Thermoset Polymers from Depolymerized Native Hardwood Lignin.

Biobased epoxy thermoset polymers were prepared from lignin hydrogenolysis oils produced from native hardwood lignin. Native lignin in Eucalyptus nitens and Eucalyptus saligna wood was reacted in situ under Pd-catalyzed mild hydrogenolysis conditions to give depolymerized lignin oils in yields up to 98 wt %. Reacting these lignin oils with epichlorohydrin produced biobased epoxy resins. Blending these resins with nonrenewable bisphenol A diglycidyl ether (BADGE) in different proportions, and curing with diethylenetriamine, produced a series of epoxy thermoset polymers with varying biobased content. Up to 67% of the BADGE could be replaced with hardwood lignin-derived epoxy resins while achieving superior or equivalent mechanical properties to the BADGE control polymer. Comparing the performance of lignin-based epoxy polymers from eucalyptus and pine wood provided insights into the advantages and disadvantages of using hardwood versus softwood native lignins in the quest for high performance biobased thermoset polymers.

Epoxidation of Kraft Lignin as a Tool for Improving the Mechanical Properties of Epoxy Adhesive.

Owing to its chemical structure, wide availability and renewable nature, lignin is a promising candidate for the partial replacement of fossil-based raw material in the synthesis of epoxy resins. Its poor compatibility has been reported to be one of the main drawbacks in this domain. On the other hand, a well-established modification method for lignin epoxidation has been used for many years for the improvement of lignin compatibility. However, the extent of the effect of lignin epoxidation on the improvement of bio-based epoxy mechanical properties, applied as adhesives, is still an open question in the literature. In this context, a pristine and industrial grade kraft lignin (AKL) was reacted with epichlorohydrin to yield epoxidized lignin (E-AKL) in this work. Afterwards, AKL or E-AKL were separately blended with petroleum-based epoxy resin at 15 and 30 wt% and cured with a commercial amine. The adhesive curing kinetic was evaluated using a novel technique for thermal transition characterization, Temperature Modulated Optical Refractometry (TMOR); the results showed that the incorporation of AKL reduces the crosslinking rate, and that this effect is overcome by lignin modification. Mechanical tests revealed an improvement of impact and practical adhesion strength for samples containing 15 wt% of E-AKL. These results elucidate the effect of lignin epoxidation on the application of lignin-based epoxy adhesives, and might support the further development and application of these bio-based materials.

Lignin-Based Epoxy Resins: Unravelling the Relationship between Structure and Material Properties.

Here we investigate the relationship between thermomechanical properties and chemical structure of well-characterized lignin-based epoxy resins. For this purpose, technical lignins from eucalyptus and spruce, obtained from the Kraft process, were used. The choice of lignins was based on the expected differences in molecular structure. The lignins were then refined by solvent fractionation, and three fractions with comparable molecular weights were selected to reduce effects of molar mass on the properties of the final thermoset resins. Consequently, any differences in thermomechanical properties are expected to correlate with molecular structure differences between the lignins. Oxirane moieties were selectively introduced to the refined fractions, and the resulting lignin epoxides were subsequently cross-linked with two commercially available polyether diamines (Mn = 2000 and 400) to obtain lignin-based epoxy resins. Molecular-scale characterization of the refined lignins and their derivatives were performed by 31P NMR, 2D-NMR, and DSC methods to obtain the detailed chemical structure of original and derivatized lignins. The thermosets were studied by DSC, DMA, and tensile tests and demonstrated diverse thermomechanical properties attributed to structural components in lignin and selected amine cross-linker. An epoxy resin with a lignin content of 66% showed a Tg of 79 °C from DMA, Young’s modulus of 1.7 GPa, tensile strength of 66 MPa, and strain to failure of 8%. The effect of molecular lignin structure on thermomechanical properties was analyzed, finding significant differences between the rigid guaiacyl units in spruce lignin compared with sinapyl units in eucalyptus lignin. The methodology points toward rational design of molecularly tailored lignin-based thermosets.

Ecoefficiency of Thermal Insulation Sandwich Panels Based On Fly Ash Modified with Colloidal Mesoporous Silica

The current practice of landfilling fly ash generated by waste incineration is nonsustainable, so alternative ways of using this material are needed. Silanization effectively immobilizes the heavy metal contaminants in the incineration fly ash and enables its circular utilization because silanized fly ash (SFA) has market value as a low-cost filler for polymer composites. This study examines the ecoefficiency of a thermal insulation panel that consists of a polyurethane (PU) foam core sandwiched between two epoxy composite skins prepared by reinforcing glass fibers (GF) and SFA in epoxy resin. The ecoefficiency of such panels was evaluated by comparing their life cycle environmental externality costs (LCEE) to their life cycle costs (LCC). The LCEE was calculated by monetizing the panels’ environmental impacts, which were quantified by performing a life cycle assessment (LCA). The results revealed that the ecoefficiency of the composite panels is positive (47%) and superior to that of market incumbent alternatives with PU foam or rockwool cores and steel skins. The two market incumbents have negative ecoefficiencies, primarily due to their high LCEE. The environmental performance of the panel with SFA–GF epoxy composite skins can be further improved by using lignin-based epoxy resin or thermoplastic polypropylene as the polymer matrix of composite skins. Overall, application as a filler in fabricating polymer composite skins of sandwich panels is an upcycling pathway of SFA that combines circular economy prospects with sustainability benefits.

Preparation and toughening of mechanochemically modified lignin-based epoxy

In this work, succinylated lignin (SA-KL) was prepared by reacting a kraft lignin and succinic anhydride (SA) through a solvent-free ball milling process. A bisphenol A/F epoxy resin (DER 353) cured with a commercial alicyclic anhydride and SA-KL (~38 wt% in the composition) exhibited a flexural strength of 48.8 MPa and modulus of 3.62 GPa but a low impact strength of 6.7 kJ/m2 which were comparable to the properties of many anhydride cured epoxies. Toughening of the lignin-based epoxy resin with a carboxyl-terminated butadiene acrylonitrile (CTBN) liquid rubber was investigated. Introduction of the CTBN rubber significantly increased the impact strength and fracture toughness but resulted in reduction of flexural strength. A ductile-brittle transition was observed at 5–12 wt% CTBN rubber. Examination of the fracture surfaces by scanning electron microscopy suggests that rubber particle cavitation and massive shear yielding of the epoxy matrix were the dominant toughening mechanisms. Incorporation of CTBN had little change to the thermal stability of the lignin-based epoxy. This study introduces a green method for synthesis of lignin-derived polyacid and a viable approach for achieving balanced mechanical and thermal properties for lignin-based epoxy materials.

Effect of DES-NiO System on Modified Lignin and Synthesis of Lignin-Based Epoxy Resin

Effect of DES-NiO System on Modified Lignin and Synthesis of Lignin-Based Epoxy Resin

Preparation and Characterization of Green Epoxy Resin Composites and Its Use in Corrosion Resistance

In this work, appropriate alternative for diglycidyl ether bisphenol A (DGEBA) was found to avoid the destructive effects of bisphenol A. Lignin, an aromatic compound from palm tree leaves, was used as a renewable material to synthesize a bio-based epoxy resin. Lignin extracted using Kraft pulping process. Kraft Lignin was epoxidized with epichlorohydrin in alkaline medium. Nano-titanium dioxide was used as filler with ratio of 10% to prepare the green epoxy composite. The structure of the Kraft lignin and lignin-based epoxy resin was proven via Infrared spectra (FT-IR) were recorded using solid KBr disk by testing Shimadzu (FT-IR-8300) spectrophotometer. The thermal properties of the curing process of lignin-based epoxy resin and composite were investigate using Differential scanning calorimetry (DSC) analysis. Potentiodynamic measurements data revealed that the anti-corrosion performance of the lignin based epoxy resin. The study demonstrates successful of epoxidation of Kraft lignin. In addition, lignin based eopxy resin showed effective inhibitor for carbon steel in 3.5 wt. % NaCl electrolyte solutions

Improved performance of dual-cured organosolv lignin-based epoxy acrylate coatings

Abstract Organosolv lignin (OL) was used to synthesize organosolv-lignin-based epoxy acrylate (OLBEA) coatings. The reaction process was determined by epoxy and acid values. OLBEA coatings were prepared by dual (Ultraviolet (UV) and thermal) curing to achieve a deep and complete process. The structures of lignin-based epoxy (OLBE) and OLBEA were characterized by Fourier transform infrared spectroscopy (FTIR). The properties of the dual- and UV-cured coatings were then compared by measuring gel content and mechanical properties. Results revealed that the dual-cured films had higher cross-linking degree and better mechanical properties than the UV-cured films. The chemical resistance of the dual-cured coatings was also examined. The properties of OLBEA coatings obtained by dual curing were shown to be excellent and promising for wide-ranging applications.

Study on the demethylation of enzymatic hydrolysis lignin and the properties of lignin–epoxy resin blends

To increase hydroxyl content in lignin, demethylation of enzymatic hydrolysis lignin (DML) was conducted using 1-dodecanethiol as nucleophilic reagent and sodium methoxide as catalyst in this work. Under alkaline conditions, the demethylated lignin–epoxy resin (DML-EP) was synthesized by demethylated lignin reacted with epichlorohydrin (ECH). The optimum conditions for the synthesis of DML-EP were: 80 °C, 2.5 h, molar ratio of ECH and NaOH to phenolic hydroxyl of 7.5 and 1.2, respectively. FTIR spectrum showed the epoxy group appeared in the product. Then, the lignin-based epoxy composites were produced by cast molding method. Tensile strength test reflects that DML-EP-reinforced epoxy composites have an obvious improvement in tensile strength and elongation of about 27 and 34% compared to the pure epoxy resin with 5% content. Scanning electron microscopy images showed the fractured surface of DML-EP/epoxy composites was rough. Rheological analysis showed that the cured DML-EP/E-51 epoxy resin composites had better elasticity and bonding effect. TGA experiment showed the thermal stability of DML-EP/epoxy composites was also improved. Non-isothermal method was used to analyze the curing kinetics of E-51 resin and DML-EP/E-51 composites, and the curing apparent activation energy was calculated using the Kissinger and Ozawa methods.