Natural Aromatic Polymer Research Articles

Sustainable lignin-based high-efficiency flame-retardant epoxy resins with excellent mechanical properties

As an abundant natural aromatic polymer, how to realize the high-value utilization of lignin (LA) is still a difficult problem. Herein, a sustainable flame retardant (LA@APP) was synthesized through hydrogen bonding interactions between LA and ammonium polyphosphate (APP), then flame retarded alone epoxy resin (EP/LA@APP). With loading of 6 wt% LA@APP, the LOI of EP/LA@APP6 rose to 27.9 % and achieved UL-94 V0 rating. Meanwhile, EP/LA@APP6 presented a 63.5 % reduction in peak of heat release rate (pHRR) and an 51.3 % decrease in peak of smoke production rate (pSPR). LA@APP could promote the EP composites to form the denser and more continuous expanded charring residuals than APP during the combustion process, effectively shielding the underlying epoxy substrate and thus enhancing fire safety. Additionally, the shell layer of LA@APP, abundant in hydroxyl groups and methoxy groups, effectively improved the compatibility and interfacial interaction with EP. Therefore, the tensile, flexural and impact strength of EP/LA@APP6 reached 50.8 MPa, 86.1 MPa and 7.81 kJ/m2, higher than those of pure EP at 48.3 MPa, 81.5 MPa and 7.45 kJ/m2. This flame retardant with simultaneously enhanced fire safety and mechanical properties of epoxy resin was expected to realize the high-level utilization of lignin.

‘Rigid–flexible’ strategy for high-strength, near-room-temperature self-healing, photo-thermally functionalised lignin-reinforced polyurethane elastomers

Lignin is the most abundant and only renewable aromatic polymer in nature. Herein, a flexible matrix and the rigid lignin were rationally integrated to prepare high-strength, near-room-temperature self-healing, processable lignin-reinforced polyurethane elastomers (LZPUs). Reversible hydrogen and oxime–amino ester bonds were introduced into the matrix to provide excellent dynamic properties and abundant ligands for lignin–matrix coordination bonds. Abundant metal coordination bonds were constructed between the matrix and lignin via the introduction of Zn2+, which not only effectively enhances the dispersibility and compatibility, but also provides an excellent energy dissipation mechanism for the LZPUs. One of the prepared elastomers, LZPUs, exhibited a high strength of 40.5 MPa, which is twice that of the blank sample and 1.6 times that of the sample without Zn2+. It maintained kinetic stability at mild temperature, but it exhibited a self-healing efficiency of 91.3 % in strength and 99.8 % in elongation at break after decoupling with trace ethanol (≈ 50 μL) at 35 °C. It exhibited a self-healing efficiency of 93.6 % in strength under 1 sun irradiation (0.1 W cm−2) for 4 h. We believe this elastomer offering high mechanical properties with multi-functionality can be applied in flexible drives and photo-thermal power generation.

Lignin Structural Characterization and Its Antioxidant Potential: A Comparative Evaluation by EPR, UV-Vis Spectroscopy, and DPPH Assays.

The natural aromatic polymer lignin and its lignin-like oligomeric fragments have attracted attention for their antioxidant capacity and free radical scavenging activities. In this study, a 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay was employed to assess the antioxidant capacity of fractionated and partially depolymerized organosolv lignin by electron paramagnetic resonance (EPR) and UV-Vis spectroscopy. The results show significant antioxidant activity for both the lignin and oligomeric fragments, with the EPR measurements demonstrating their efficiency in quenching the free radicals. The EPR data were analyzed to derive the kinetic rate constants. The radical scavenging activity (RSA) of lignins was then determined by UV-Vis spectroscopy and the results were compared with the EPR method. This two-method approach improves the reliability and understanding of the antioxidant potential of lignin and its derivatives and provides valuable insights for their potential applications in various industries, including pharmaceuticals, food preservation, and cosmetics.

Anti-SARS-CoV-2 activity of microwave solvolysis lignin from woody biomass

The global pandemic caused by the novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has had profoundly detrimental effects on our society. To combat this highly pathogenic virus, we turned our attention to an abundant renewable natural aromatic polymer found in wood. Through a chemical modification of Eucalyptus and Japanese cedar wood via acidic microwave solvolysis in equivolume mixture of 2 % (w/w) aqueous H2SO4, ethylene glycol, and toluene at 190 °C. Subsequently, we separated the resulting solvolysis products through extractions with toluene, ethyl acetate, and ethanol. Among these products, the ethyl acetate extract from Eucalyptus wood (eEAE) demonstrated the highest inhibition effects against the novel SARS-CoV-2. We further divided eEAE into four fractions, and a hexane extract from the ethanol-soluble portion, termed eEAE3, exhibited the most substantial inhibitory rate at 93.0 % when tested at a concentration of 0.5 mg/mL. Analyzing eEAE3 using pyrolysis gas chromatography–mass spectrometry revealed that its primary components are derived from lignin. Additionally, 1H–13C edited-heteronuclear single quantum coherence nuclear magnetic resonance analysis showed that the solvolysis process cleaved major lignin interunit linkages. Considering the abundance and renewability of lignin, the lignin-derived anti-SARS-CoV-2 agent presents a promising potential for application in suppressing infections within our everyday environment.

Classical curing of epoxy modified lignin with polyamine for new Epoxy-Polyimine composite

Lignin, which is the most abundant aromatic polymer in nature was used as a green substitute for the toxic bisphenol A. Here we synthesized the epoxy agent from lignin by reacting lignin with epichlorohydrin. Further this product was reacted with the lignin-amine to give the new epoxy-polyimine composites. This study evaluated the curing of bio-based epoxy resins derived from lignin with aromatic and an aliphatic amine. The thermal stability of the cured lignin-based epoxy samples was evaluated. Epoxidation is an interesting way to develop new uses and applications of lignin. Lignin-based epoxy resins were obtained by the epoxidation reaction using the lignin recovered directly from pulping liquor. For the epoxidation reaction it was optimized the influence of the lignin:NaOH (w/w) ratio, temperature and time of the reaction on the properties of the prepared epoxidized lignin. The structures of lignin-based epoxy resins were evaluated by epoxy index test and FTIR spectroscopy. Optimal conditions were obtained for lignin-based epoxy resin produced at lignin/NaOH = 1/3 at 80 °C for 3 h. Thermogravimetry analysis (TGA) revealed that epoxidation enhances the thermal stability of lignin and may allow a wider temperature range for various applications.

3D printed lignin/polymer composite with enhanced mechanical and anti-thermal-aging performance

Lignin is the most abundant natural aromatic polymer globally but is still underutilized as a renewable material, even with its versatile properties. One approach to using lignin is to incorporate it in polymer composites, but this application is often limited by the poor mechanical performance of the resultant composite due to the poor interfacial adhesion. Following the structure–property relationship, stronger interactions were designed to be enhanced by alternating the functional groups of lignin. This study applied a demethylation method to hardwood lignin (Ori-Lig) to introduce more phenolic hydroxyl groups. Research studies with rheology behavior and molecular simulations demonstrated that an increased phenolic hydroxyl content could improve the adhesion between the modified lignin (OH-Lig) and the polymer matrix at the interface. The tensile performance of the samples from the fused depositional modeling (FDM) 3D printing technique was also improved due to the improved interfacial adhesion. Specifically, by adding 10 wt% of the modified lignin, the tensile strength of the 3D printed samples could reach 46.1 MPa (40.2 MPa of Polyamide 12, PA12), and Young’s Modulus could be improved to 1.73 GPa (1.48 GPa of PA12). OH-Lig also improved anti-aging performance so that the tensile strength of the OH-Lig composite after thermo-aging (100 h at 140 °C) remained ∼ 48 MPa. The enhanced mechanical performance with anti-aging properties indicated that the OH-Lig composite may replace PA12 to reduce the use of petrol-based materials. This work showed the method of purposefully designing and modifying lignin structures to fulfill the interaction requirements between lignin and polymer matrix. The as-prepared lignin could be used to prepare functional composites to achieve improved mechanical performance and targeted functions at the same time.

Lignin-based materials for electrochemical energy storage devices

Lignin is the most abundant aromatic polymer in nature, which is rich in a large number of benzene ring structures and active functional groups. The molecular structure of lignin has unique designability and controllability, and is a class of functional materials with great application prospects in energy storage and conversion. Here, this review firstly focuses on the concept, classification, and physicochemical property of lignin. Then, the application research of lignin in the field of electrochemical storage materials and devices are summarized, such as lignin-carbon materials and lignin-carbon composites in supercapacitors and secondary batteries. Finally, this review points out the bottlenecks that need to be solved urgently and the prospects for future research priorities.

Depolymerization of lignin using laccase from Bacillus sp. PCH94 for production of valuable chemicals: A sustainable approach for lignin valorization

Lignin is the most abundant aromatic polymer in nature, and its depolymerization offers excellent opportunities to develop renewable aromatic chemicals. In the present study, Bacillus sp. PCH94 was investigated for laccase production and lignin depolymerization. Maximum production of laccase enzyme was achieved within 6.0 h at 50 °C on a natural lignocellulosic substrate. Furthermore, Bacillus sp. PCH94 was used to bioconvert lignin dimeric and polymeric substrates, validated using FT-IR, NMR (1H, 13C), and LCMS. Genome mining of Bacillus sp. PCH94 revealed laccase gene (lacBl) as multicopper oxidase (spore coat CotA). Further, lacBl from Bacillus sp. PCH94 was cloned, expressed, and kinetically characterized. LacBl enzyme showed activity for substrates ABTS (40.64 IU/mg), guaiacol (5.43 IU/mg), and DMP (11.93 IU/mg). The LacBl was active in higher temperatures (10 to 100 °C) and showed a half-life of 36 and 27 h at 50 and 60 °C, respectively. The purified LacBl was able to depolymerize kraft lignin into valuable products (ferulic acid and acetovanillone), which have applications in the pharmaceutical and food industries. Overall, the current study demonstrated the role of bacterial laccase in the depolymerization of lignin and opened a promising prospect for the green production of valuable compounds from recalcitrant lignin.

High value valorization of lignin as environmental benign antimicrobial.

Lignin is a natural aromatic polymer of p-hydroxyphenylpropanoids with various biological activities. Noticeably, plants have made use of lignin as biocides to defend themselves from pathogen microbial invasions. Thus, the use of isolated lignin as environmentally benign antimicrobial is believed to be a promising high value approach for lignin valorization. On the other hand, as green and sustainable product of plant photosynthesis, lignin should be beneficial to reduce the carbon footprint of antimicrobial industry. There have been many reports that make use of lignin to prepare antimicrobials for different applications. However, lignin is highly heterogeneous polymers different in their monomers, linkages, molecular weight, and functional groups. The structure and property relationship, and the mechanism of action of lignin as antimicrobial remains ambiguous. To show light on these issues, we reviewed the publications on lignin chemistry, antimicrobial activity of lignin models and isolated lignin and associated mechanism of actions, approaches in synthesis of lignin with improved antimicrobial activity, and the applications of lignin as antimicrobial in different fields. Hopefully, this review will help and inspire researchers in the preparation of lignin antimicrobial for their applications.

Application of three types of aminated lignins for efficient removal of Cd(II) and Pb(II) ions in aqueous solution

Lignin is a renewable natural aromatic polymer that is generated as a co-product during lignocellulosic biorefinery processes, and it has been applied widely as a functional biomaterial. In this study, the adsorption behavior of Cd(II) and Pb(II) metal ions was investigated via ion chelation using aminated lignins (ALs) with primary, secondary, and tertiary amine groups. ALs exhibited optimal Cd(II) and Pb(II) adsorption capacities in solution under neutral conditions due to their chelating, electrostatic, and cationic–π interactions with metal ions. The AL with the primary amine group showed the highest adsorption capacities for both Cd(II) and Pb(II), reaching 83.2 and 159.7 mg·g-1, respectively, followed by the ALs with secondary and tertiary amine groups. The adsorption kinetics and isotherm analysis demonstrated that all adsorption behaviors followed the Langmuir isotherm and pseudo-second-order kinetics. Thermodynamic studies revealed that the adsorption processes of Cd(II) and Pb(II) using the ALs were spontaneous and endothermic. These results demonstrate that ALs are promising adsorbents for the removal of Cd(II) and Pb(II) metal ions.

Lignin as a Renewable Building Block for Sustainable Polyurethanes.

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

Mechanically Robust, Self-Repairable, Shape Memory and Recyclable Ionomeric Elastomer Composites with Renewable Lignin via Interfacial Metal-Ligand Interactions.

Lignin, the most abundant aromatic polymer in nature, is one of the most promising renewable feedstocks for value-added polymer products. However, it is challenging to prepare high-performance and multifunctional polymer materials with renewable lignin because of its poor compatibility with the elastomer matrix. In fact, lignin often requires solvent fractionation, chemical modification, or prohibitively expensive additives. This work develops a cost-effective strategy to prepare ionomeric elastomer composites based on a commercial carboxyl elastomer and a high content of lignin without purification or chemical modification. The compatibility between the elastomer and lignin is improved by the incorporation of zinc oxide which creates metal-ligand coordination at the interfaces between the carboxyl groups of the elastomer and the oxygen-bearing groups of the lignin. This results in fine dispersion of the lignin in the elastomer matrix, even when its content reaches 50 wt %. The lignin/elastomer composites show excellent mechanical properties, which are attributed to the reinforcing effect of the lignin domains and the presence of abundant sacrificial coordination bonds. Moreover, ionic bonds and ionic aggregates created by the neutralization of the zinc ions with the carboxyl groups of the elastomer behave as physical crosslinks which endow the composites with excellent recyclability; namely, their mechanical properties are retained or even improved after multiple reprocessing cycles. They also show good self-repairability and shape memory. Hence, this work may open up new avenues to utilize lignin as a renewable alternative to petroleum derivatives for designing and fabricating high-performance and multifunctional elastomer materials.

The performance and applications of lignin based hydrogels: a review

Lignin is the only natural aromatic polymer and the second most abundant lignocellulosic resource in nature after cellulose, and its derived high-value products can be used in a variety of fields. An efficient, highly value-added and high-quality production of lignin is critical for lignocellulose biorefinery, but the complex and variable structure of lignin macromolecules, poor activity of reactions, and redundant functional groups make it difficult to prepare polymeric materials with stable properties. With the increasing research on lignin modification, the application of lignin composite hydrogels has also received great attention. In this paper, the preparation of lignin composite hydrogels based on the basic structural composition and reaction properties of lignin was briefly outlined. The current applications of lignin composite hydrogels, including biosensors, controlled release materials, environmentally responsive materials, adsorbent materials, electrode materials, and other materials, were summarized. Moreover, future perspectives of lignin-based composite hydrogels were prospected.

Green synthesis and chemometric characterization of hydrophobic xanthan matrices: Interactions with phenolic compounds

Polysaccharides such as xanthan, locust bean gum or chitosan are easily crosslinked and purified using citric acid in an ecofriendly process. In order to achieve an improved sorption capability towards hydrophobic solutes, β-cyclodextrin, a cyclic oligosaccharide, and lignin, a natural aromatic polymer, are incorporated in the same process. Once crosslinked, the influence of these on the sorption capacities towards model solutes has been assessed by comparing the sorption isotherms of matrices with or without the hydrophobic modifications. The sorption capacities of these materials for different phenolic compounds have also been tested to ascertain their efficiencies as a function of their affinities to β-cyclodextrin cavities and/or their partition coefficients. In addition, these functionalized carbohydrate matrices were successfully characterized by principal component analysis, which is a useful tool to select the most appropriate polymers to interact with a specific molecule.

Depolymerization of lignin into high-value products

Lignin is the most abundant natural aromatic polymer source on the earth; the lignin structure contains three primary monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These units make the lignin structure a suitable raw material to obtain different value-added products, which has attracted the scientific community's attention. However, most lignin produced globally is still used as boiler fuel in carbohydrate processing plants, but lignin has shown innumerable times that due to its low value and high volume it has great potential to produce high value new products. If it is transformed into chemical compounds, its revenue can generate a multibillion-dollar lignin-based bioproducts industry. Numerous approaches have been reported for lignin fragmentation as an alternative way to produce chemicals currently extracted from fossil fuel sources. This review summarizes some of the most common methods to convert lignin into valuable products.

Study of Lignin Extracted from Rubberwood Using Microwave Assisted Technology for Fuel Additive.

Lignin is the most abundant natural aromatic polymer, especially in plant biomass. Lignin-derived phenolic compounds can be processed into high-value liquid fuel. This study aimed to determine the yield of lignin by the microwave-assisted solvent extraction method and to characterize some essential properties of the extracted lignin. Rubberwood sawdust (Hevea brasiliensis) was extracted for lignin with an organic-based solvent, either ethanol or isopropanol, in a microwave oven operating at 2450 MHz. Two levels of power of microwave, 100 W and 200 W, were tested as well as five extraction times (5, 10, 15, 20, 25, and 30 min). The extracted lignin was characterized by Klason lignin, Fourier transform infrared spectroscopy (FT-IR), 2D HSQC NMR, Ultraviolet-visible spectrophotometry (UV-vis), and Bomb calorimeter. The results showed that the yield of extracted lignin increased with the extraction time and power of the microwave. In addition, the extraction yield with ethanol was higher than the yield with isopropanol. The highest yield was 6.26 wt.%, with ethanol, 30 min extraction time, and 200 W microwave power.

Isolation and Characterization of a Novel Laccase for Lignin Degradation, LacZ1.

Lignin is a complex natural organic polymer and is one of the primary components of lignocellulose. The efficient utilization of lignocellulose is limited because it is difficult to degrade lignin. In this study, we screened a lacz1 gene fragment encoding laccase from the macrotranscriptome data of a microbial consortium WSC-6, which can efficiently degrade lignocellulose. The reverse transcription-quantitative PCR (RT-qPCR) results demonstrated that the expression level of the lacz1 gene during the peak period of lignocellulose degradation by WSC-6 increased by 30.63 times compared to the initial degradation period. Phylogenetic tree analysis demonstrated that the complete lacz1 gene is derived from a Bacillus sp. and encoded laccase. The corresponding protein, LacZ1, was expressed and purified by Ni-chelating affinity chromatography. The optimum temperature was 75°C, the optimum pH was 4.5, and the highest enzyme activity reached 16.39 U/mg. We found that Cu2+ was an important cofactor needed for LacZ1 to have enzyme activity. The molecular weight distribution of lignin was determined by gel permeation chromatography (GPC), and changes in the lignin structure were determined by 1H nuclear magnetic resonance (1H NMR) spectra. The degradation products of lignin by LacZ1 were determined by gas chromatography and mass spectrometry (GC-MS), and three lignin degradation pathways (the gentian acid pathway, benzoic acid pathway, and protocatechuic acid pathway) were proposed. This study provides insight into the degradation of lignin and new insights into high-temperature bacterial laccase. IMPORTANCE Lignin is a natural aromatic polymer that is not easily degraded, hindering the efficient use of lignocellulose-rich biomass resources, such as straw. Biodegradation is a method of decomposing lignin that has recently received increasing attention. In this study, we screened a gene encoding laccase from the lignocellulose-degrading microbial consortium WSC-6, purified the corresponding protein LacZ1, characterized the enzymatic properties of laccase LacZ1, and speculated that the degradation pathway of LacZ1 degrades lignin. This study identified a new, high-temperature bacterial laccase that can degrade lignin, providing insight into lignin degradation by this laccase.

Lignin-Degrading Microorganisms from Organic Soils

The most prevalent aromatic polymer in nature is lignin, produced by higher plants and thought to make up 30-35 percent of the non-fossil organic carbon on the planet. Lignin hydrolyzing enzymes such as lignin peroxidase, laccase, manganese peroxidase, and others produce a variety of aromatic monomers, including ferulic and vanillic acids. However, very little research has been done on the role of microbes in lignin degradation. In the present work, we have isolated 25 ligninolytic bacteria and 25 ligninolytic fungi from organic soils of Koppal, Raichur districts of Karnataka. The bacterial isolates were identified as Pseudomonas putida, Bacillus subtilis, based on biochemical tests, and fungi were identified as Aspergillus niger, Trichoderma viridae, Phanerochaete chrysosporium and Pleurotus ostreatus based on morphological characters. The ligninolytic activity of bacterial isolates was high when compared to fungal isolates. All the isolates produced detectable amounts of lignin peroxidase, manganese peroxidase, and laccase under in vitro conditions. In dye decolorization test, fungal isolates KGST-1, KGST-2, and KKSP could decolorize Ramazol Brilliant Blue R and Congo red.

New deep eutectic solvent assisted extraction of highly pure lignin from maritime pine sawdust (Pinus pinaster Ait.)

Lignocellulosic biomass is a renewable and sustainable feedstock, mainly composed of cellulose, hemicellulose, and lignin. Lignin, as the most abundant natural aromatic polymer occurring on Earth, has great potential to produce value-added products. However, the isolation of highly pure lignin from biomass requires the use of efficient methods during lignocellulose fractionation. Therefore, in this work, novel acidic deep eutectic solvents (DESs) were prepared, characterized and screened for lignin extraction from maritime pine wood (Pinus pinaster Ait.) sawdust. The use of cosolvents and the development of new DES were also evaluated regarding their extraction and selectivity performance. The results show that an 1 h extraction process at 175 °C, using a novel DES composed of lactic acid, tartaric acid and choline chloride, named Lact:Tart:ChCl, in a molar ratio of 4:1:1, allows the recovery of 95 wt% of the total lignin present in pine biomass with a purity of 89 wt%. Such superior extraction of lignin with remarkable purity using a “green” solvent system makes this process highly appealing for future large-scale applications.

Refining the Properties of Softwood Kraft Lignin with Acetone: Effect of Solvent Fractionation on the Thermomechanical Behavior of Electrospun Fibers

The complexity and polydispersity of lignin, the most abundant aromatic polymer in nature, create great challenges for its valorization for value-added materials and products. Reducing the heterogeneity of industrial lignin and identifying the structure–property relationships are a necessity to achieve predictable and high performance materials for polymeric applications. In this study, a simple one-step solvent extraction method using an industrial solvent, acetone, was presented as a tool to improve the homogeneity of softwood kraft lignin (SKL). The effect of acetone fractionation on the molecular weight (GPC), chemical structure (13C and HSQC NMR), and thermal (DSC and TGA) and dynamical mechanical properties (compressive-torsion DMA) of soluble (ASKL) and insoluble (AISKL) lignin fractions was evaluated. Results showed that two fractions had significant differences in Mw, chemical structure, and functional groups, showing distinct thermal flow and viscoelastic behaviors along with large shifts in the glass-to-rubbery transition temperatures of nearly 80 °C. Electrospun submicron fibers composed of 99 wt % lignin were produced from ASKL and AISKL fractions, which had improved spinnability characteristics compared to their parent SKL lignin. Thermomechanical properties of fiber mats were studied by a tensile dynamical mechanical analyzer, and the heat-induced change in the morphology of fibers was evaluated by SEM. Fiber mats produced from ASKL fractions showed increased water resistance and hydrophobicity as revealed by water contact angle measurements.