Uniformly Dispersed Poly(lactic acid)-Grafted Lignin Nanoparticles Enhance Antioxidant Activity and UV-Barrier Properties of Poly(lactic acid) Packaging Films

Lignin reinforced eco-friendly and functional nanoarchitectonics materials with tailored interfacial barrier performance

Exploring innovative and sustainable routes for the production of biodegradable biomass-based materials is critical to promote a circular carbon economy and carbon neutrality goals. Fossil-based non-biodegradable plastic waste poses a nonnegligible threat to humans and the ecological environment, and biomass-based functional materials are becoming increasingly viable alternatives. Lignin, a naturally occurring macromolecular polymer, is green and renewable resource rich in aromatic rings, with biodegradability, biocompatibility, and excellent processability for eco-friendly composites. Moreover, versatile and high tunable lignins can be valorized into functional materials, which are crucial building blocks in the fabrication of biomass-derived composites. Lignin's unique chemical structure, solvent resistance, anti-aging, and anti-ultraviolet functional properties make it highly potential for the fabrication of sustainable biobased barrier materials. This review systematically summarizes the progress in the fabrication and application of lignin-based functional composites, with a particular focus on barrier materials. First, the structural diversity of lignins from different sources and fractionation methods, and the structural modification strategies of lignins are briefly introduced. Then, the multiple barrier performances of lignin-based composites are listed, and the fabrication methods of different composites based on the polymer matrix systems are elaborated. In terms of diverse applications, this review highlights the multifaceted barrier properties of lignin-based composites in oxygen barrier, water vapor barrier, ultraviolet barrier, flame retardant and antibacterial applications. These functional barrier materials are widely used in food/pharmaceutical packaging, agricultural protection, construction, etc., providing an excellent option for sustainable materials with high barrier performance requirements. Finally, the main challenges faced by lignin-based barrier materials and the future directions are proposed.

Enhancement of polycaprolactone nanofiber film performance by hydrogen bonding interactions with chitosan for food packaging

Hybrid environmentally friendly nanocomposite films were synthesized via electrospinning using polycaprolactone (PCL) and chitosan (CH). The resulting nanofiber films displayed a homogeneous fibrous microstructure with average diameters between 250-270 nm. Molecular simulation experiments revealed a progressive increase in hydrogen bonding over time. The impact of different CH concentrations on surface roughness was investigated, with results showing that PCL/CH (2 %) reduced surface roughness by 240 % compared to pure PCL film. Furthermore, the addition of CH imparted stable hydrophobic properties to the nanofiber film, with a water contact angle remaining steady at 107° after 20 s. L929 cell experiments confirmed that the nanofiber film exhibits good biocompatibility. Practical application studies using blueberries demonstrated that the PCL/CH (2 %) film effectively preserved freshness at room temperature for up to 5 days. These findings indicate that PCL/CH (2 %) films hold significant potential for use in fruit packaging applications.

Transforming lignin into polymer film with improved physiochemical properties by modification with itaconic acid and grafting with polycaprolactone

Owing to the renewability and abundance of lignin, over the last two decades, a greater amount of interest has been shifted towards the development of lignin-based copolymers for renewable and sustainable film preparation. Therefore, due to worldwide demand and attention for this kind of work, a two-stage modification method has been proposed in this work. Where firstly, chemical modification of alkali lignin (AL) was carried out with itaconic acid (IA) to enhance compatibility and reactivity. This itaconic acid-modified lignin (IAL) was further grafted with a monomeric unit of ɛ-caprolactone (CL) to synthesize lignin-grafted polycaprolactone copolymer (IAL-PCL). The chemical modification and grafting were confirmed through FT-IR, NMR, and XPS by analyzing the characteristic peaks of IA and CL, respectively. Changes in thermodynamic properties due to chemical modification and grafting were evaluated from TGA and DSC analysis. An increase in water contact angle (65° to 87°) indicates an increase in grafting when the copolymer was synthesized with a higher CL amount. Excellent flexibility and improved mechanical properties, i.e., 2.2 MPa tensile strength and >38 MPa elastic modulus confirm the improvement in compatibility between lignin and PCL chain of IAL-PCL copolymer film, which was mainly induced due to the incorporation of IA onto lignin.

Exploring nanolignin as a sustainable biomacromolecule in polymer composites: Synthesis, characterization, and applications: A review

Nanolignin is a biomacromolecule nanomaterial obtained from lignin, a natural plant polymer. It has attracted considerable attention as a potential nanofiller for polymer composites owing to its remarkable properties, such as biodegradability, antioxidant activity, and high specific surface area. However, the development of biological macromolecule nano lignin-based polymer composites faces several challenges, mainly due to the complex structure and poor chemical compatibility of lignin with most polymers. A comprehensive summary of the functionalization strategies and synthesis techniques for nanolignin is provided in this article. The morphological, structural, and thermal characterization of nanolignin is also reviewed. Furthermore, this review summarizes the recent progress of nanolignin and its composites prepared using various polymers, such as polyvinyl alcohol, poly (methyl methacrylate), polylactic acid, phenol formaldehyde, and natural rubber. This study concludes by outlining the possible uses of nanolignin-reinforced polymer composites and the potential opportunities and disadvantages of using nanolignin as a reinforcement in polymer composites for several value-added applications.

Lignin Particle Size Affects the Properties of PLA Composites Prepared by In Situ Ring-Opening Polymerization

The present work focuses on the synthesis and characterization of biobased lignin-poly(lactic) acid (PLA) composites. Organosolv lignin, extracted from beechwood, was used as a filler at 0.5, 1.0, and 2.5 wt% loadings, with ultrasonication reducing the lignin particle size to ~700 nm. The PLA-lignin composites were prepared via in situ ring-opening polymerization (ROP) of L-lactide in the presence of lignin. This method ensured uniform lignin dispersion in the PLA matrix due to grafting of PLA chains onto lignin particles, preventing aggregation. Strong polymer-filler interactions were confirmed through spectroscopic analysis (FTIR and XPS) and their effects on static and dynamic glass transitions (DSC). These interactions enhanced mechanical properties, including a two-fold increase in tensile strength and elongation at 1 wt% lignin. Crystallization was suppressed due to shorter PLA chains, and a 15% drop in dynamical fragility was observed via Broadband Dielectric Spectroscopy (BDS). Antioxidant activity improved significantly, with PLA-2.5% ultrasonicated organosolv lignin reducing DPPH• content to 7% after 8 h, while UV-blocking capability increased with lignin content.

Minimizing food oxidation using aromatic polymer: From lignin into nano-lignin

Food loss and waste caused by oxidation result in environmental and economic losses and health threats. Lignin is an abundant aromatic polymer with varied antioxidant capacity, which can reduce food oxidation caused by radical species exposure. The lignin antioxidant strength can be influenced by source, type, structure, processing, degradation products, chemical modifications, and particle size. Lignin in micro- or nano-particles has high reactivity and is associated with increased surface area to improve antioxidant capacity. Lignin can be used as a food additive to suppress lipid and protein oxidation, although its effect on fruit/vegetable oxidation needs to be discussed. The lignin antioxidant properties are promising to be applied in food industries, such as food additives, animal feed supplements, and antioxidant packaging designs. However, there are challenges and limitations to consider, such as the potential for toxicity reactions in some individuals and the need for further research to understand its effects on different food products fully. As a feed nutrition, lignin can improve meat quality. Meanwhile, loading lignin in the packaging matrix can extend the food shelf life through antioxidant and antimicrobial activities, and UV-block. Lignin also improves packaging properties (conventional and 3D-printing fabrication) to maintain food quality, e.g., changes in mechanical properties, hydrophobicity, water vapor permeability, and other influences. This article reviews lignin's role as a natural antioxidant in the food industry. Future directions and discussions relate to prooxidative mechanisms, toxicity, fruit and vegetable preservation mechanisms, inhibition of protein oxidation, activity to food enzymes (fruit ripening enzyme activators and inhibitors of cellulase and β-glucosidase enzyme), dispersity in packaging matrices, and material diversification for 3D printing.

A comprehensive review on polylactic acid/lignin composites - Structure, synthesis, performance, compatibilization, and applications

Today, the world is facing a great problem of plastic pollution due to its non-degradable nature. Alternatively, polylactic acid (PLA), a bio-based and biodegradable polymer, is emerging as a promising substitute for conventional, non-biodegradable plastics. However, its high cost, limited properties, and single functionality hinder its wide application. Lignin, a natural and sustainable biomass derived from plant cell walls, has become a promising filler for PLA. The integration of lignin into PLA composites holds the potential to realize the trifecta of low cost, high performance, and multifunctional properties while maintaining the principles of biodegradation and sustainability. However, the poor compatibility between PLA and lignin severely affects their overall performance, which creates a major challenge for the development of PLA/lignin composites. In recent years, a significant of advancements have been achieved in addressing this challenge. In this review, we provide a systematic insight into PLA/lignin composites, focusing on numerous compatibilization strategies including physical addition and chemical modification, and the progress on the structural characteristics, synthesis methods, performance improvements brought by lignin, and multiple applications. Finally, the existing problems and developmental direction of PLA/lignin composites are discussed. We believe that this review can be useful for future research prospects and industrial applications.

Photoactivated room temperature phosphorescence from lignin

Sustainable photoactivated room temperature phosphorescent materials exhibit great potential but are difficult to obtain. Here, we develop photoactivated room temperature phosphorescent materials by covalently attaching lignin to polylactic acid, where lignin and polylactic acid are the chromophore and matrix, respectively. Initially the phosphorescence of the lignin is quenched by residual O2. However, the phosphorescence is switched on when the residual oxygen is consumed by the triplet excitons of lignin under continuous UV light irradiation. As such, the lifetime increases from 3.0 ms to 221.1 ms after 20 s of UV activation. Interestingly, the phosphorescence is quenched again after being kept under an atmosphere of air for 2 h in the absence of UV irradiation due to the diffusion of oxygen into the materials. Using these properties, as-developed material is successfully used as a smart anti-counterfeiting logo for a medicine bottle and for information recording.

Improved Performance of Polylactic Acid Biocomposites at High Lignin Loadings through Glycidyl Methacrylate Grafting of Melt-Flowable Organosolv Lignin

Glycidyl methacrylate (GMA) was grafted onto a melt-flowable organosolv lignin, called bioleum (BL), using a melt mixing process. Then, up to 40 wt % of BL-g-GMA was blended with polylactic acid (PLA) in the presence of dicumyl peroxide as a free radical initiator utilizing a melt extrusion method. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, and tensile testing were performed to characterize the biocomposites' performance. The FTIR results revealed a successful grafting of GMA onto BL. Overall, BL and PLA compatibility increased significantly with the grafting and resulted in decreased domain size of BL-g-GMA and thus enhanced all the tensile properties (strength, modulus, and elongation at break) at BL loadings as high as 40 wt %. For instance, in the biocomposites containing 30 wt % BL, the GMA grafting increased the tensile strength by 23%. The presence of BL and BL-g-GMA hindered PLA's crystallization even when it was cooled at a rate of 1 °C/min. However, the composites with BL-g-GMA showed a crystallinity value comparable to that of PLA during isothermal crystallization, but with a slower crystallization rate. This work reveals a facile and scalable method that can be adopted to enhance the performance of lignin-based biocomposites.

Epoxidized Soybean Oil Toughened Poly(lactic acid)/Lignin-g-Poly(lauryl methacrylate) Bio-Composite Films with Potential Food Packaging Application

The application of lignin as a filler for poly (lactic acid) (PLA) is limited by their poor interfacial adhesion. To address this challenge, lignin-graft-poly(lauryl methacrylate) (LG-g-PLMA) was first blended with poly (lactic acid), and then epoxidized soybean oil (ESO) was also added to prepare PLA/LG-g-PLMA/ESO composite, which was subsequently hot pressed to prepare the composite films. The effect of ESO as a plasticizer on the thermal, mechanical, and rheological properties, as well as the fracture surface morphology of the PLA/LG-g-PLMA composite films, were investigated. It was found that the compatibility and toughness of the composites were improved by the addition of ESO. The elongation at break of the composites with an ESO content of 5 phr was increased from 5.6% to 104.6%, and the tensile toughness was increased from 4.1 MJ/m3 to 44.7 MJ/m3, as compared with the PLA/LG-g-PLMA composite without ESO addition. The toughening effect of ESO on composites is generally attributed to the plasticization effect of ESO, and the interaction between the epoxy groups of ESO and the terminal carboxyl groups of PLA. Furthermore, PLA/LG-g-PLMA/ESO composite films exhibited excellent UV barrier properties and an overall migration value below the permitted limit (10 mg/dm2), indicating that the thus-prepared biocomposite films might potentially be applied to environmentally friendly food packaging.

Improved antioxidant activity of pretreated lignin nanoparticles: Evaluation and self-assembly

Lignin nanoparticles (LNPs) have gained significant attention for their potential as natural antioxidants. This study investigated the effect of various pretreatment methods on the lignin structure and subsequent antioxidant activity of LNPs. Among four pretreated LNPs, hydrothermal LNPs exhibited the highest antioxidant activity, surpassing unpretreated, acid-pretreated and kraft LNPs, with an impressive efficacy of 91.6%. The relationship between LNPs' structure and antioxidant activity was revealed by 2D heteronuclear singular quantum correlation (1H13C HSQC) and 31P nuclear magnetic resonance (NMR). 1H13C HSQC suggested the cleavage of β-O-4 ether bonds, as well as a decrease in ferulic acid and p-coumaric acid, which directly influenced the antioxidant activity of LNPs. 31P NMR demonstrated a positive correlation between the total hydroxyl group content and the antioxidant activity. Besides, an isothermal kinetic model for scavenging free radicals was established based on Langmuir kinetic model instead of Freundlich model. Moreover, multilayer LNPs, based on layer-by-layer self-assembly, were prepared and exhibited remarkable antioxidant activity of 95.8%. More importantly, when blended with pure cosmetic cream, the multilayer LNPs maintained antioxidant activity of 86.7%. These finding may promote the practical applications of biomolecules, e.g. lignin additives in cosmetics and pharmaceuticals.

Application of surface-modified functional packaging in food storage: A comprehensive review

Innovations in food packaging systems could meet the evolving needs of the market; emerging concepts of non-migrating technologies reduce the negative migration of preservatives from packaging materials, extend shelf life, and improve food quality and safety. Non-migratory packaging activates the surface of inert materials through pretreatment to generate different active groups. The preservative is covalently grafted with the resin of the pretreated packaging substrate through the graft polymerization of the monomer and the coupling reaction of the polymer chain. The covalent link not only provides the required surface properties of the material for a long time but also retains the inherent properties of the polymer. This technique is applied to the processing for durable, stable, and easily controllable packaging widely. This article reviews the principles of various techniques for packaging materials, surface graft modification, and performance characterization of materials after grafting modification. Potential applications in the food industry and future research trends are also discussed.

Influence of Low-Molecular-Weight Esters on Melt Spinning and Structure of Poly(lactic acid) Fibers

Poly(lactic acid) has great potential in sectors where degradability is an important advantage due to its polymer nature. The medical, pharmaceutical, and packaging industries have shown interest in using PLA. To overcome the limitations of stiffness and brittleness in the polymer, researchers have conducted numerous modifications to develop fibers with improved properties. One such modification involves using plasticizing modifiers that can provide additional and desired properties. The scientific reports indicate that low-molecular-weight esters (LME) (triethyl citrate and bis (2-ethylhexyl) adipate) affect the plasticization of PLA. However, the research is limited to flat structures, such as films, casts, and extruded shapes. A study was conducted to investigate the impact of esters on the process of forming, the properties, and the morphology of fibers formed through the melt-spinning method. It was found that the modified PLA required different spinning and drawing conditions compared to the unmodified polymer. DSC, FTIR, WAXD, and GPC/SEC analyses were performed for the modified fibers. Mechanical tests and morphology evaluations using SEM microscopy were also conducted. The applied plasticizers lowered the temperature of the spinning process by 40 °C, and allowed us to obtain a higher degree of crystallinity and a better tenacity at a lower draw ratio. GPC/SEC analysis confirmed that the polymer-plasticizer interaction is physical because the booth plasticizer peaks were separated in the chromatographic columns. The use of LME in fibers significantly reduces the temperature of the spinning process, which reduces production costs. Additives significantly change the production process and the structure of the fiber depending on their rate, which may affect the properties, e.g., the rate of degradation. We can master the degree of crystallinity through the variable amount of LME. The degree of crystallization of the polymers has a significant influence on polymer application.

Hydrogels based on lignin extracted from cashew apple bagasse and its application in antimicrobial wound dressings

Hydrogels are versatile materials with a three-dimensional network structure that can retain water and release bioactive compounds. They have found applications in various fields, including agriculture, biomaterial synthesis, and pharmaceuticals. Incorporating natural antimicrobial compounds into hydrogels is a promising approach to developing non-toxic biomedical materials, particularly for wound healing dressings. It was evaluated the extraction and use of cashew apple bagasse lignin (CAB-Lig) due to its healing, anti-inflammatory, and antimicrobial properties for producing a hydrogel-based bandage. The extraction process involved acid and alkali treatments followed by precipitation. The antimicrobial potential of CAB-Lig was evaluated at different concentrations for formulating hydrogels. Hydrogels containing 0.1 % and 3 % lignin showed high swelling and liquid retention abilities. The 3 % lignin hydrogel exhibited effectiveness against Escherichia coli and Staphylococcus aureus. Incorporating CAB-Lig into the hydrogel structure improved its mechanical properties, making it more suitable for application as a bandage. Moreover, the extracted lignin showed low toxicity, indicating its safe use. A bandage was formulated by combining the CAB-Lig-based hydrogel with polyester, which possessed antimicrobial properties and demonstrated biocompatibility (L929 and HaCat cells). The results confirmed the potential of CAB-Lig for synthesizing hydrogels and dressings with antimicrobial properties, offering a sustainable solution for utilizing lignocellulosic biomass.

Lignin derivatives-based hydrogels for biomedical applications

Recently, numerous studies have been conducted on renewable polymers derived from different natural sources, exploring their suitability for diverse biomedical applications. Lignin as one of the main components of lignocellulosic has garnered significant attention as a promising alternative to petroleum-based polymers. This interest is primarily due to its cost-effectiveness, biocompatibility, eco-friendly nature, as well as its antioxidant and antimicrobial properties. These characteristics could be more beneficial when incorporating lignin into the formulation of value-added products. Although lignin has a chemical structure that is suitable for various applications, these characteristics require modifications to guarantee that the resultant materials display the desired biological, chemical, and physical properties when applied in the creation of biodegradable hydrogels, particularly for biomedical purposes. This study delineates the recent modification approaches that have been employed in the creation of lignin-based hydrogels. These strategies encompass both chemical and physical interactions with other polymers. Additionally, this review encompasses an examination of the current applications of lignin hydrogels, spanning their use as scaffolds for tissue engineering, carriers for pharmaceuticals, materials for wound dressings and biosensors, and elements in flexible and wearable electronics. Finally, we delve into the challenges and constraints associated with these materials, discuss the necessary steps required to attain the appropriate properties for the development of innovative lignin-based hydrogels, and derive conclusions based on the presented findings.

Exploring the potential of lignin nanoparticles in enhancing the mechanical, thermal, and bioactive properties of poly (butylene adipate-co-terephthalate)

The preparation and characterization of lignin nanoparticles (LNPs) were described. LNPs were produced via the precipitation technique. Nanocomposites of LNPs with poly (butylene adipate-co-terephthalate) (PBAT) were prepared by melt mixing with various concentrations up to 6 wt% of LNPs. The assessment of the effects of LNP addition on the mechanical, thermal, morphological, cytotoxicity, antioxidant, antibacterial, and antiviral properties of nanocomposites was carefully performed. The addition of LNPs to PBAT enhances the thermal stability of the nanocomposites. The antioxidant effect of LNPs on PBAT increased with increasing filler content. LNPs showed higher efficiency as antioxidant agents than lignin particles (LP). The tensile modulus increased by 20 % for the nanocomposites with 6 % LNPs in comparison with neat PBAT. The crystallization peak temperature of PBAT was 80 °C, which increased to 104.6 °C with the addition of 6 wt% of LNPs, suggesting their strong nucleation activity. Antibacterial tests demonstrated the bacteriostatic activities of LNP, LP, and nanocomposites. Both LP and LNP showed considerable antiviral activity against herpes simplex virus type 1 and human coronavirus 229e. The antiviral activity of LNP was concentration-dependent. The findings suggest that LNP is a promising bio-additive for PBAT and can enhance its properties for various applications, including food packaging.

Kraft (Nano)Lignin as Reactive Additive in Epoxy Polymer Bio-Composites

The demand for high-performance bio-based materials towards achieving more sustainable manufacturing and circular economy models is growing significantly. Kraft lignin (KL) is an abundant and highly functional aromatic/phenolic biopolymer, being the main side product of the pulp and paper industry, as well as of the more recent 2nd generation biorefineries. In this study, KL was incorporated into a glassy epoxy system based on the diglycidyl ether of bisphenol A (DGEBA) and an amine curing agent (Jeffamine D-230), being utilized as partial replacement of the curing agent and the DGEBA prepolymer or as a reactive additive. A D-230 replacement by pristine (unmodified) KL of up to 14 wt.% was achieved while KL-epoxy composites with up to 30 wt.% KL exhibited similar thermo-mechanical properties and substantially enhanced antioxidant properties compared to the neat epoxy polymer. Additionally, the effect of the KL particle size was investigated. Ball-milled kraft lignin (BMKL, 10 μm) and nano-lignin (NLH, 220 nm) were, respectively, obtained after ball milling and ultrasonication and were studied as additives in the same epoxy system. Significantly improved dispersion and thermo-mechanical properties were obtained, mainly with nano-lignin, which exhibited fully transparent lignin-epoxy composites with higher tensile strength, storage modulus and glass transition temperature, even at 30 wt.% loadings. Lastly, KL lignin was glycidylized (GKL) and utilized as a bio-based epoxy prepolymer, achieving up to 38 wt.% replacement of fossil-based DGEBA. The GKL composites exhibited improved thermo-mechanical properties and transparency. All lignins were extensively characterized using NMR, TGA, GPC, and DLS techniques to correlate and justify the epoxy polymer characterization results.

Modified Biomass-Reinforced Polylactic Acid Composites

Polylactic acid (PLA), as a renewable and biodegradable green polymer material, is hailed as one of the most promising biopolymers capable of replacing petroleum-derived polymers for industrial applications. Nevertheless, its limited toughness, thermal stability, and barrier properties have restricted its extensive application. To address these drawbacks in PLA, research efforts have primarily focused on enhancing its properties through copolymerization, blending, and plasticization. Notably, the blending of modified biomass with PLA is expected not only to effectively improve its deficiencies but also to maintain its biodegradability, creating a fully green composite with substantial developmental prospects. This review provides a comprehensive overview of modified biomass-reinforced PLA, with an emphasis on the improvements in PLA's mechanical properties, thermal stability, and barrier properties achieved through modified cellulose, lignin, and starch. At the end of the article, a brief exploration of plasma modification of biomass is presented and provides a promising outlook for the application of reinforced PLA composite materials in the future. This review provides valuable insights regarding the path towards enhancing PLA.

One-Pot Reversible Complexation-Mediated Polymerization (RCMP) from Benzylic Alcohols for Facile Access to Polymer-Grafted Lignin

One-pot synthesis of methacrylic and acrylic polymers from benzylic alcohols (R-OH) used as initiating moieties was developed. R-OH was converted to alkyl iodide (R-I), and the generated R-I was used as an initiator without purification or isolation in the subsequent reversible complexation mediated polymerization (RCMP), leading to one-pot RCMP from R-OH. As a useful application, this technique was exploited for one-pot polymer-grafting from lignin that is the second most abundant renewable carbon-source on earth and bears benzylic alcohols. The direct initiation from lignin eliminates tedious initiator attachment and purification, offering a facile access to polymer-grafted lignin. The obtained polymer-grafted lignin was utilized to form an efficient UV-absorbing film with high transparency in visible region. One-pot RCMP may serve as a practical method to obtain value-added functional lignin-polymer composites.

Effect of competitive interactions on the structure and properties of blends prepared from an industrial lignosulfonate polymer

To explore the possibility of applying lignin in practice, an industrial lignosulfonate (0-50 vol%) was blended with four ionomers. The concentrations of carboxyl and carboxylate groups were systematically varied in the ethylene-acrylic acid copolymers to study the competition of hydrogen and ionic bonds forming between the components. The mechanical properties of the blends were determined by tensile testing. The structure was investigated by scanning electron microscopy, while deformation and failure processes were studied by acoustic emission measurements and microscopy. Interfacial interactions were quantitatively characterized by analyzing local deformation processes and by evaluating the composition dependence of the tensile strength using appropriate models. Molecular dynamics simulations indicated that carboxylate groups preferably form clusters in the ionomer phase, consequently, the increasing degree of neutralization results in ionomers with more and more self-interactions of components deteriorating ionomer-lignin interactions. The novel combination of experiments, modeling, and simulation was done for the first time on such materials, and it pointed out that the role of hydrogen bonds is more critical in determining blend properties. Blends can be prepared for practical applications with a good combination of stiffness (0.8 GPa), tensile strength (22 MPa), and elongation-at-break (25 %) at 30 vol% lignosulfonate content and 33 % neutralization.

Enhancement of elongation at break and UV-protective properties of poly(lactic acid) film with cationic ring opening polymerized (CROP)-lignin

In this study, the intrinsic brittleness of poly(lactic acid) (PLA) was overcome by chemical modification using ethyl acetate-extracted lignin (EL) via cationic ring-opening polymerization (CROP). The CROP was conducted to promote homopolymerization under starvation of the initiator (oxyrane). This method resulted in the formation of lignin-based polyether (LPE). LPE exhibited enhanced interfacial compatibility with nonpolar and hydrophobic PLA owing to the fewer hydrophilic hydroxyl groups and a long polyether chain. In addition, because of the UV-protecting and radical-scavenging abilities of lignin, LPE/PLA exhibited multifunctional properties, resulting in improved chemical properties compared with the neat PLA film. Notably, one of the LPE/PLA films (EL_MCF) exhibited excellent elongation at break of 297.7 % and toughness of 39.92 MJ/m3. Furthermore, the EL_MCF film showed superior UV-protective properties of 99.52 % in UVA and 88.95 % in UVB ranges, both significantly higher than those of the PLA film, without sacrificing significant transparency in 515 nm. In addition, the radical scavenging activity improved after adding LPE to the PLA film. These results suggest that LPEs can be used as plasticizing additives in LPE/PLA composite films, offering improved physicochemical properties.

The mechanism of self-assembly of lignin in deep eutectic solvent based on sulfamic acid and urea through molecular dynamics simulation

Due to the diversity of industrial lignin sources and the complexity of its structure, its application as a high-value material is limited. Lignin nanoparticles (LNPs) have emerged as a hotspot for research due to their advantages of high specific surface area and high dispersion and the solvent transfer method is commonly used for the preparation of LNPs. In this paper, LNPs were prepared by solvent transfer method using DES based on sulfamic acid and urea (S/U DES) as solvent and water as anti-solvent. To explore the internal mechanism of the self-assembly of nanoparticles, a theoretical model of the solvent system and model lignin compound was constructed with the assistance of quantum chemistry and molecular dynamics theories. Through classical molecular dynamics (MD) simulations, the interaction energy, radius of gyration (ROG), solvent accessible surface area (SASS), radial and spatial distribution function (RDFs/SDFs), hydrogen bonding, and the morphology changes were analyzed to reveal the internal mechanism of self-assembly of model lignin compounds in S/U DES. This study is useful in revealing the mechanism of interaction between lignin and DES, as well as providing a benchmark for the green and efficient preparation of lignin nanoparticles.

Recent advances in lignin antioxidant: Antioxidant mechanism, evaluation methods, influence factors and various applications

Lignin, a by-product of processing lignocellulosic materials, has a polyphenolic structure and can be used as an antioxidant directly or synergistically with synthetic types of antioxidants, leading to different applications. Its antioxidant mechanism is mainly related to the production of ROS, but the details need to be further investigated. The antioxidant property of lignin is mainly related to the content of phenolic hydroxyl group, but methoxy, purity will also have an effect on it. In addition, different methods to detect the antioxidant properties of lignin have different advantages and disadvantages. In this paper, the antioxidant mechanism of lignin, the methods to determine the antioxidant activity and the progress of its application in various fields are reviewed. In addition, the current research on the antioxidant properties of lignin and the hot directions are provided, and an outlook on the research into the antioxidant properties of lignin is provided to broaden its potential application areas.

Bioactive and Hemocompatible PLA/Lignin Bio-Composites: Assessment of In Vitro Antioxidant Activity for Biomedical Applications

In this study, acetylated soda lignin (ASL) and non-acetylated soda lignin (SL) were extruded with PLA in different concentrations to fabricate antioxidant polylactic acid (PLA)/lignin composites for potential biomedical applications. After lignin acetylation, good compatibility was observed between PLA and lignin in scanning electron microscopy images. All the PLA/ASL composites displayed higher mechanical properties than PLA/SL composites. PLA/ASL5 displayed the highest mechanical characteristics with elongation at break of 10% and tensile strength of 57 MPa, while PLA/SL15 and PLA/SL20 demonstrated superior UV-blocking potential with UV transmittance less than 10%. Addition of ASL in PLA lead to an increase in the hydrophobic character, with all the PLA/ASL displaying a higher water contact angle. The antioxidant test using 2,2-diphenyl-1-picrylhydrazyl assay showed that PLA/SL composites rendered superior radical scavenging activity (RSA), with PLA/SL20 composites displaying an RSA of 80%. Furthermore, in vitro antioxidant activity and cytocompatibility were analyzed using human colon cancer cells (HCT-15) and gastric epithelial cells (NCC-24). In vitro antioxidant activity, evaluated by H2O2 exposure was confirmed by a live/dead assay. PLA/SL composites protected both types of cells from oxidative stress. In addition, all PLA/SL and PLA/ASL composites promoted cell proliferation compared to PLA. PLA/SL5 and PLA/SL10 displayed the highest cell proliferation of all composites. Lastly, all PLA/SL and PLA/ASL composites had a hemoglobin release less than 2%. The antioxidant properties, cytocompatibility, and hemocompatibility of lignin/PLA demonstrated in our study indicate that these lignin/PLA composites possess the desirable attributes for potential biomedical applications.

Grafting Polymerization of Long-Chain Hydrophobic Acrylic Monomer onto Lignin and Its Application in Poly(Lactic Acid)-Based Wholly Green UV Barrier Composite Films

The development of low-cost and high-performance bio-based composites derived from forestry waste lignin and polylactic acid has emerged as a topic of central attention. However, the weak compatibility between lignin and polylactic acid often resulted in high brittleness of the composites. Graft copolymerization is not only the most effective way to modify lignin but also can significantly improve the compatibility of lignin and polylactic acid. In this study, bio-based monomer lauryl methacrylate was grafted onto lignin by feasible radical polymerization to prepare lignin graft copolymers with excellent thermal stability and hydrophobicity, which are expected to improve the compatibility with polylactic acid. Wholly bio-based composites were prepared by compounding this graft copolymer with polylactic acid. The results showed that the crystallization ability of the composite was improved, and the highest crystallinity was increased from 6.42% to 17.46%. With addition of LG-g-PLMA lower than 9%, the thermal stability of the composites was slightly improved. At 5% copolymer addition, the elongation at break and tensile toughness of the composites increased by 42% and 36%, respectively. Observation of the frozen fracture surface of the composite by SEM found that wire drawing and ductile deformation appeared when a small amount of LG-g-PLMA was added. The thus prepared composites also showed excellent UV barrier properties. This approach provides a new idea for the high-value application of lignin.

Lignins as Promising Renewable Biopolymers and Bioactive Compounds for High-Performance Materials

The recycling of biomass into high-value-added materials requires important developments in research and technology to create a sustainable circular economy. Lignin, as a component of biomass, is a multipurpose aromatic polymer with a significant potential to be used as a renewable bioresource in many fields in which it acts both as promising biopolymer and bioactive compound. This comprehensive review gives brief insights into the recent research and technological trends on the potential of lignin development and utilization. It is divided into ten main sections, starting with an outlook on its diversity; main properties and possibilities to be used as a raw material for fuels, aromatic chemicals, plastics, or thermoset substitutes; and new developments in the use of lignin as a bioactive compound and in nanoparticles, hydrogels, 3D-printing-based lignin biomaterials, new sustainable biomaterials, and energy production and storage. In each section are presented recent developments in the preparation of lignin-based biomaterials, especially the green approaches to obtaining nanoparticles, hydrogels, and multifunctional materials as blends and bio(nano)composites; most suitable lignin type for each category of the envisaged products; main properties of the obtained lignin-based materials, etc. Different application categories of lignin within various sectors, which could provide completely sustainable energy conversion, such as in agriculture and environment protection, food packaging, biomedicine, and cosmetics, are also described. The medical and therapeutic potential of lignin-derived materials is evidenced in applications such as antimicrobial, antiviral, and antitumor agents; carriers for drug delivery systems with controlled/targeting drug release; tissue engineering and wound healing; and coatings, natural sunscreen, and surfactants. Lignin is mainly used for fuel, and, recently, studies highlighted more sustainable bioenergy production technologies, such as the supercapacitor electrode, photocatalysts, and photovoltaics.

Optimization of the Electrospray Process to Produce Lignin Nanoparticles for PLA-Based Food Packaging

The development of new processing methods is required in order to meet the continuous demand for thinner films with excellent barrier properties for food packaging and other applications. In this study, rice husk organosolv lignin nanoparticles were prepared using the electrospray method, which were applied to produce polylactic acid (PLA)-based films for food packaging. The effect of the following electrospray parameters has been investigated: lignin concentration (LC) ranging from 5-50 mg/mL, flow rate (FR) from 0.5-1 mL/min, applied voltage from 10-30 kV, and tip-to-collector distance (TCD) from 10-25 cm, on the morphology, size, polydispersity index (PDI), and Zeta potential (ZP) of lignin nanoparticles (LNPs). The response surface methodology with a Box-Behnken design was applied to optimize these parameters, while dynamic light scattering (DLS) and scanning electron microscopy (SEM) analyses were used to characterize the controlled LNPs. The results showed that the LNPs shape and sizes represent a balance between the solvent evaporation, LC, applied voltage, TCD and FR. The application of optimal electrospray conditions resulted in the production of LNPs with a spherical shape and a minimal size of 260 ± 10 nm, a PDI of 0.257 ± 0.02, and a ZP of -35.2 ± 4.1 mV. The optimal conditions were achieved at LC = 49.1 mg/mL and FR = 0.5 mL/h under an applied voltage of 25.4 kV and TCD = 22 cm. Then, the optimized LNPs were used to improve the properties of PLA-based films. Three types of PLA-lignin blend films were casted, namely lignin/PLA, LNPs/PLA and PLA-grafted LNPs. PLA-grafted LNPs exhibited a more uniform dispersion in PLA for lignin contents of up to 10% than other composite samples. Increasing the lignin content from 5% to 10% in PLA-grafted LNPs resulted in a significant increase in elongation at break (up to four times higher than neat PLA). The presence of PLA-grafted lignin led to a substantial reduction in optical transmittance in the UV range, dropping from 58.7 ± 3.0% to 1.10 ± 0.01%, while maintaining excellent transparency to visible light compared to blends containing lignin or LNPs. Although the antioxidant capacity of unmodified lignin is well-known, a substantial increase in antioxidant capacity was observed in LNPs and PLA-grafted LNP films, with values exceeding 10 times and 12 times that of neat PLA, respectively. These results confirm the significant potential of using studied films in food packaging applications.

Polylactic Acid/Lignin Composites: A Review

With the gradual depletion of petroleum resources and the increasing global awareness of environmental protection, biodegradable plastics are receiving more and more attention as a green substitute for traditional petroleum-based plastics. Poly (lactic acid) is considered to be the most promising biodegradable material because of its excellent biodegradability, biocompatibility, and good processability. However, the brittleness and high cost limit its application in more fields. Lignin, as the second largest renewable biopolymer in nature after cellulose, is not only rich in reserves and low in cost, but it also has an excellent UV barrier, antioxidant activity, and rigidity. The molecular structure of lignin contains a large number of functional groups, which are easy to endow with new functions by chemical modification. Currently, lignin is mostly treated as waste in industry, and the value-added utilization is insufficient. The combination of lignin and poly (lactic acid) can on the one hand solve the problems of the high cost of PLA and less efficient utilization of lignin; on the other hand, the utilization of lignocellulosic biomass in compounding with biodegradable synthetic polymers is expected to afford high-performance wholly green polymer composites. This mini-review summarizes the latest research achievements of poly (lactic acid)/lignin composites. Emphasis was put on the influence of lignin on the mechanical properties of its composite with poly (lactic acid), as well as the compatibility of the two components. Future research on these green composites is also prospected.

Poly(Lactic Acid) Composites with Lignin and Nanolignin Synthesized by In Situ Reactive Processing

Poly(lactic acid) (PLA) composites with 0.5 wt% lignin or nanolignin were prepared with two different techniques: (a) conventional melt-mixing and (b) in situ Ring Opening Polymerization (ROP) by reactive processing. The ROP process was monitored by measuring the torque. The composites were synthesized rapidly using reactive processing that took under 20 min. When the catalyst amount was doubled, the reaction time was reduced to under 15 min. The dispersion, thermal transitions, mechanical properties, antioxidant activity, and optical properties of the resulting PLA-based composites were evaluated with SEM, DSC, nanoindentation, DPPH assay, and DRS spectroscopy. All reactive processing-prepared composites were characterized by means of SEM, GPC, and NMR to assess their morphology, molecular weight, and free lactide content. The benefits of the size reduction of lignin and the use of in situ ROP by reactive processing were demonstrated, as the reactive processing-produced nanolignin-containing composites had superior crystallization, mechanical, and antioxidant properties. These improvements were attributed to the participation of nanolignin in the ROP of lactide as a macroinitiator, resulting in PLA-grafted nanolignin particles that improved its dispersion.

Modification of poly(L-lactic acid)-based films and evaluation of physical and antibacterial properties by using multivariate data analysis

The aim of this study is to prepare new packaging materials with improved physical and antimicrobial properties that prevent the growth of microorganisms. Poly(L-lactic acid) (PLA) based packaging films were prepared by the solvent-casting method using spruce resin (SR), epoxidized soybean oil, an essential oil mixture (calendula and clove oil), and silver nanoparticles (AgNPs). The AgNPs were synthesized by the polyphenol reduction method, using spruce resin dissolved in methylene chloride. The prepared films were tested for antibacterial activity and physical properties, such as tensile strength (TS), elongation at break (EB), elastic modulus (EM), water vapor permeability (WVP), and UV-C blocking effect. The addition of SR decreased the water vapor permeation (WVP) of the films, whereas the addition of essential oils (EOs) increased this property due to their higher polarity. The morphological, thermal, and structural properties were characterized using SEM, UV-Visible spectroscopy, FTIR, and DSC. The agar disc well method showed that SR, AgNPs, and EOs imparted antibacterial activity to the PLA-based films against Staphylococcus aureus and Escherichia coli. Multivariate data analysis tools, such as principal component and hierarchical cluster analysis, were used to discriminate PLA-based films by simultaneously evaluating their physical and antibacterial properties.

Newly Synthesized Lignin Microparticles as Bioinspired Oral Drug-Delivery Vehicles: Flavonoid-Carrier Potential and In Vitro Radical-Scavenging Activity

The aim of the present study was to synthesize lignin microparticles, to evaluate their physicochemical, spectral, morphological and structural characteristics, to examine their encapsulation and in vitro release potential and behaviour towards the flavonoid morin in simulated physiological medium and to assess the in vitro radical-scavenging potential of the morin-loaded lignin microcarrier systems. The physicochemical, structural and morphological characteristics of alkali lignin, lignin particles (LP) and morin-encapsulated lignin microparticles (LMP) were determined based on particle size distribution, SEM, UV/Vis spectrophotometric, FTIR and potentiometric titration analyses. The encapsulation efficiency of LMP was 98.1%. The FTIR analyses proved that morin was successfully encapsulated in the LP without unexpected chemical reactions between the flavonoid and the heteropolymer. The in vitro release performance of the microcarrier system was successfully mathematically described by Korsmeyer–Peppas and the sigmoidal models outlining the general role of diffusion during the initial stages of the in vitro release process in simulated gastric fluid (SGF), and the predominant contribution of biopolymer relaxation and erosion was determined in simulated intestinal medium (SIF). The higher radical-scavenging potential of LMP, as compared to that of LP, was proven via DPPH and ABTS assays. The synthesis of lignin microcarriers not only provides a facile approach for the utilization of the heteropolymer but also determines its potential for the design of drug-delivery matrices.

Opportunities and Challenges for Lignin Valorization in Food Packaging, Antimicrobial, and Agricultural Applications

The exploration of renewable resources is essential to help transition toward a more sustainable materials economy. The valorization of lignin can be a key component of this transition. Lignin is an aromatic polymer that constitutes approximately one-third of the total lignocellulosic biomass and is isolated in huge quantities as a waste material of biofuel and paper production. About 98% of the 100 million tons of lignin produced each year is simply burned as low-value fuel, so this renewable polymer is widely available at very low cost. Lignin has valuable properties that make it a promising material for numerous applications, but it is far from being fully exploited. The aim of this Perspective is to highlight opportunities and challenges for the use of lignin-based materials in food packaging, antimicrobial, and agricultural applications. In the first part, the ongoing research and the possible future developments for the use of lignin as an additive to improve mechanical, gas and UV barrier, and antioxidant properties of food packaging items will be treated. Second, the application of lignin as an antimicrobial agent will be discussed to elaborate on the activity of lignin against bacteria, fungi, and viruses. Finally, the use of lignin in agriculture will be presented by focusing on the application of lignin as fertilizer.

Physicochemical properties of lignin nanoparticles from softwood and their potential application in sustainable pre-harvest bagging as transparent UV-shielding films

Technical lignin can be mainly obtained as a waste by-product from pulp industry, and it exhibits unique properties including ultraviolet adsorption, biodegradable, antibacterial, and antioxidant which can be utilized for bioplastic applications. However, common limitations of technical lignin for plastic applications are compatibility mainly due to poor interfacial adhesion, relatively large particle size and impurity. In this study lignin nanoparticles from softwood (S-LNPs) were successfully produced through a continuous-green-scalable antisolvent precipitation and the suitability of S-LNPs for fabrication of bio-composite polybutylene succinate (PBS) films using conventional blown film extrusion was examined. The attained S-LNPs showed lower ash content, higher phenolic content and higher lignin content compared to pristine softwood kraft lignin (S-lignin). Rheological property including shear viscosity and melt-flow index was determined. The obtained PBS/S-LNP composite films showed improved tensile modulus, higher water vapor transmission rate and excellent UV-shielding ability compared to neat PBS and PBS/S-lignin films. Accelerated weathering testing was conducted to replicate outdoor conditions. Degradation indices including carbonyl, vinyl and hydroxyl of the weathered PBS/lignin composites were evaluated for photo-oxidative stability. The S-LNPs as multifunctional bio-additives in biodegradable composite film exhibited superior performances of transparency, UV-absorption and stiffness with high photo-oxidative stability suitable for outdoor applications.

Eco-friendly and facile preparation of chitosan-based biofilms of novel acetoacetylated lignin for antioxidant and UV-shielding properties

The development of eco-friendly, sustainable, biodegradable, and biocompatible green biopolymer composites is becoming increasingly important. In this study, acetoacetylated lignin (ATL) was obtained via an eco-friendly, facile one-step synthesis reaction, and chitosan (CS)-containing ATL films (CSL) were prepared. The chemical structural analysis of ATL confirmed that the acetoacetyl groups were successfully grafted onto kraft lignin (KL). ATL with adequate acetoacetyl groups exhibited enhanced molecular weight and antioxidant and ultraviolet (UV)-shielding properties. In particular, ATL, with a half maximal inhibitory concentration (IC₅₀) of 23.8 μg·mL^-1, exhibited superior antioxidant activity than butylated hydroxytoluene (38.3 μg·mL^-1) and KL (50.0 μg·mL^-1). When ATL was incorporated into the CS solution to prepare biofilms, the antioxidant activity, UV-shielding property, water resistance, and thermal stability of the CSL greatly improved. Notably, the UV-A and UV-B shielding properties of the 2 % CSL were 130 % and 78 % higher than those of the pure CS film, respectively. Therefore, ATL designed with lignin-derived multifunctional properties has potential applications as an antioxidant and UV-shielding bio-additive and shows significant prospects in food packaging and biomedical applications.

Effect of Micro- and Nano-Lignin on the Thermal, Mechanical, and Antioxidant Properties of Biobased PLA-Lignin Composite Films

Bio-based poly(lactic acid) (PLA) composite films were produced using unmodified soda micro- or nano-lignin as a green filler at four different contents, between 0.5 wt% and 5 wt%. The PLA-lignin composite polymers were synthesized by solvent casting to prepare a masterbatch, followed by melt mixing. The composites were then converted into films, to evaluate the effect of lignin content and size on their physicochemical and mechanical properties. Differential scanning calorimetry (DSC), supported by polarized light microscopy (PLM), infrared spectroscopy (FTIR-ATR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) were employed to investigate the PLA crystallization and the interactions with Lignin (L) and Nanolignin (NL). The presence of both fillers (L and NL) had a negligible effect on the glass transition temperature (chain diffusion). However, it resulted in suppression of the corresponding change in heat capacity. This was indicative of a partial immobilization of the PLA chains on the lignin entities, due to interfacial interactions, which was slightly stronger in the case of NL. Lignin was also found to facilitate crystallization, in terms of nucleation; whereas, this was not clear in the crystalline fraction. The addition of L and NL led to systematically larger crystallites compared with neat PLA, which, combined with the higher melting temperature, provided indications of a denser crystal structure in the composites. The mechanical, optical, antioxidant, and surface properties of the composite films were also investigated. The tensile strength and Young's modulus were improved by the addition of L and especially NL. The UV-blocking and antioxidant properties of the composite films were also enhanced, especially at higher filler contents. Importantly, the PLA-NL composite films constantly outperformed their PLA-L counterparts, due to the finer dispersion of NL in the PLA matrix, as verified by the TEM micrographs. These results suggest that bio-based and biodegradable PLA films filled with L, and particularly NL, can be employed as competitive and green alternatives in the food packaging industry.

Synthesis and Characterization of Poly(lactic acid) Composites with Organosolv Lignin

Lignin, being one of the main structural components of lignocellulosic biomass, is considered the most abundant natural source of phenolics and aromatics. Efforts for its valorisation were recently explored as it is mostly treated as waste from heat/energy production via combustion. Among them, polymer-based lignin composites are a promising approach to both valorise lignin and to fine tune the properties of polymers. In this work, organosolv lignin, from beech wood, was used as fillers in a poly (lactic acid) (PLA) matrix. The PLA/lignin composites were prepared using melt mixing of masterbatches with neat PLA in three different lignin contents: 0.5, 1.0 and 2.5 wt%. Lignin was used as-isolated, via the organosolv biomass pretreatment/fractionation process and after 8 h of ball milling. The composites were characterised with Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy, X-ray Diffraction (XRD), and Differential Scanning Calorimetry (DSC). Additionally, their antioxidant activity was assessed with the 2,2-Diphenyil-1-picrylhydrazyl (DPPH) method, the colour was measured with a colorimeter and the mechanical properties were evaluated with tensile testing. Ball milling, at least under the conditions applied in this study, did not induce a further substantial decrease in the already relatively small organosolv lignin primary particles of ~1 μm. All the produced PLA/lignin composites had a uniform dispersion of lignin. Compression-moulded films were successfully prepared, and they were coloured brown, with ball-milled lignin, giving a slightly lighter colour in comparison with the as-received lignin. Hydrogen bonding was detected between the components of the composites, and crystallization of the PLA was suppressed by both lignin, with the suppression being less pronounced by the ball-milled lignin. All composites showed a significantly improved antioxidant activity, and their mechanical properties were maintained for filler content 1 wt%.