A comprehensive review of lignin-reinforced lignocellulosic composites: Enhancing fire resistance and reducing formaldehyde emission

The rising environmental concerns and the growing demand for renewable materials have surged across various industries. In this context, lignin, being a plentiful natural aromatic compound that possesses advantageous functional groups suitable for utilization in biocomposite systems, has gained notable attention as a promising and sustainable alternative to fossil-derived materials. It can be obtained from lignocellulosic biomass through extraction via various techniques, which may cause variability in its thermal, mechanical, and physical properties. Due to its excellent biocompatibility, eco-friendliness, and low toxicity, lignin has been extensively researched for the development of high-value materials including lignin-based biocomposites. Its aromatic properties also allow it to successfully substitute phenol in the production of phenolic resin adhesives, resulting in decreased formaldehyde emission. This review investigated and evaluated the role of lignin as a green filler in lignin-based lignocellulosic composites, aimed at enhancing their fire retardancy and decreasing formaldehyde emission. In addition, relevant composite properties, such as thermal properties, were investigated in this study. Markedly, technical challenges, including compatibility with other matrix polymers that are influenced by limited reactivity, remain. Some impurities in lignin and various sources of lignin also affect the performance of composites. While lignin utilization can address certain environmental issues, its large-scale use is limited by both process costs and market factors. Therefore, the exact mechanism by which lignin enhances flame retardancy, reduces formaldehyde emissions, and improves the long-term durability of lignocellulosic composites under various environmental conditions remains unclear and requires thorough investigation. Life cycle analysis and techno-economic analysis of lignin-based composites may contribute to understanding the overall influence of systems not only at the laboratory scale but also at a larger industrial scale.

Decolorization of Lignin for High-Resolution 3D Printing of High Lignin-Content Composites

Lignin, one of the most abundant biomaterials and a large-scale industrial waste product, is a promising filler for polymers as it reduces the amount of fossil resources and is readily available. 3D printing is well-known for producing detailed polymer structures in small sizes at low waste production. Especially light-assisted 3D printing is a powerful technique for production of high-resolution structures. However, lignin acts as a very efficient absorber for UV and visible light limiting the printability of lignin composites, reducing its potential as a high-volume filler. In this work, the decolorization of lignin is presented for high-resolution 3D printing of biocomposites with lignin content up to 40 wt.%. Organosolv lignin (OSL) is decolorized by an optimized low-energy process of acetylation and subsequent UV irradiation reducing the UV absorbance by 71%. By integration of decolorized lignin into bio-based tetrahydrofurfuryl acrylate (THFA), a lignin content of 40 wt.% and a resolution of 250 µm is achieved. Due to the reinforcing properties of lignin, the stiffness and strength of the material is increased by factors of 15 and 2.3, respectively. This work paves the way for the re-use of a large amount of lignin waste for 3D printing of tough materials at high resolution.

Green Flame-Retardant Blend Used to Improve the Antiflame Properties of Polypropylene

The flammability properties of polymers and polymeric composites play an important role in ensuring the safety of humans and the environment; moreover, flame-retardant materials ensure a greater number of applications. In the present study, we report the obtaining of polypropylene (PP) composites contain a mixture of two green flame retardants, lignin and clinoptilolite, by melt extrusion. These additives are abundantly found in nature. Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), mechanical properties, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), cone calorimetry, UL-94, and carbonized residues analysis were carried out. TGA analysis shows that PPGFR-10 and PPGFR-20 compounds presented better thermal stability with respect to PP without flame retardants. The conical calorimetric evaluation of the composites showed that PPGFR-10 and PPGFR-20 presented decreases in peak heat release rates (HRRs) of 9.75% and 11.88%, respectively. The flammability of the composites was evaluated with the UL-94 standard, and only the PPGFR-20 composite presented the V-0 and 5VB classification, which indicates good flame-retardant properties. Additives in the polymer matrix showed good dispersion with few agglomerates. The PPGFR-20 composite showed an FRI value of 1.15, higher percentage of carbonized residues, and UL-94 V-0 and 5VB rating, suggesting some kind of synergy between lignin and clinoptilolite, but only at high flame-retardant concentrations.

Enhancing Mechanical Performance of High-Lignin-Filled Polypropylene via Reactive Extrusion

Polypropylene (PP) is one of the most extensively used commodity plastics. In terms of eco-friendliness, it is worth considering preparing high-lignin-filled PP. This study explores the incorporation of high lignin content, derived from acetic acid lignin (AAL) and Kraft lignin (KL), into PP through twin-screw extrusion and injection molding. The challenge lies in maintaining mechanical performance. A compatibilizer-specifically, maleic anhydride-grafted polypropylene (MAPP)-is employed to enhance lignin-PP compatibility by chemically bonding with lignin and physically associating with the PP phase. Results indicate that KL maintains better dispersity than AAL. Compatibilizers with a high maleic anhydride (MA) level (≥0.8 wt.%) and moderate melt flow index (MFI) in the range of 60-100 g 10 min⁻¹ prove favorable in constructing a reinforced PP/KL network. Optimizing with 40 wt.% lignin content and 10 parts per hundred (pph) of compatibilizer yields blends with mechanical performance comparable to neat PP, exhibiting a notable increase in modulus and heat deflection temperature (HDT). Furthermore, utilizing PP/lignin blends can lead to a 20% reduction in expenses and approximately 40% reduction in PP-induced greenhouse gas (GHG) emissions. This approach not only reduces PP costs but also adds value to lignin utilization in a sustainable and cost-effective manner.

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.

Biocatalytic selective acylation of technical lignins: a new route for the design of new biobased additives for industrial formulations

In this article, we describe a proof of concept of the potential use of a biocatalytic process for the functionalization of technical soda lignins from wheat straw through the selective acylation of primary hydroxy groups of lignin oligomers by acetate or hexanoate, thus preserving their free, unreacted phenols. The selectivity and efficiency of the method, although they depend on the structural complexity of the starting material, have been proven on model compounds. Applied to technical lignins, the acylation yield is only moderate, due to structural and chemical features induced by the industrial mode of preparation of the lignins rather than to the lack of efficiency of the method. However, most of the physicochemical properties of the lignins, including their antioxidant potential, are preserved, advocating the potential use of these modified lignins for industrial applications.

In Situ Dispersion of Lignin in Polypropylene via Supercritical CO2 Extrusion Foaming: Effects of Lignin on Cell Nucleation and Foam Compression Properties

Supercritical CO₂ (scCO₂) extrusion foamed high-melt-strength (HMS) polypropylene (PP) often suffers from low cell density, large cell sizes, and poor cell structure uniformity due to the poor nucleation rates of CO₂ in the PP. To remedy this, various inorganic fillers have been used as heterogeneous nucleation agents. Although their efficient nucleation effects have been demonstrated, the preparation of these fillers causes some adverse effects on the environment/human health or involves relatively expensive processes or non-eco-friendly chemicals. In this work, biomass-based lignin is studied as a sustainable, lightweight, and cost-effective nucleating agent. It is found that scCO₂ could assist in situ dispersion of lignin in the PP in the foaming process, leading to significantly increased cell density, smaller cells, and improved cell uniformity. The Expansion Ratio is also simultaneously improved due to reduced diffusive gas loss. The PP/lignin foams with low lignin loadings exhibit higher compression moduli and plateau strengths than the PP foams with the same densities owing to the improved cell uniformity and probably also the reinforcing effect of the small lignin particles in cell walls. Moreover, the energy absorption capability of the PP/lignin foam with 1 wt% lignin could match the PP foam with similar compression plateau strengths; even the density of the former is 28% lower than the latter. Therefore, this work provides a promising approach to a cleaner and more sustainable production of HMS PP foams.

A review of the extraction methods and advanced applications of lignin-silica hybrids derived from natural sources

The global trend of increasing energy demand along the large volume of wastewater generated annually from the paper pulping and cellulose production industries are considered as serious dilemma that may need to be solved within these current decades. Within this discipline, lignin, silica or lignin-silica hybrids attained from biomass material have been considered as prospective candidates for the synthesis of advanced materials. In this study, the roles and linking mechanism between lignin and silica in plants were studied and evaluated. The effects of the extraction method on the quality of the obtained material were summarized to show that depending on the biomass feedstocks, different retrieval processes should be considered. The combination of alkaline treatment and acidic pH adjustment is proposed as an effective method to recover lignin-silica with high applicability for various types of raw materials. From considerations of the advanced applications of lignin and silica materials in environmental remediation, electronic devices and rubber fillers future valorizations hold potential in conductive materials and electrochemistry. Along with further studies, this research could not only contribute to the development of zero-waste manufacturing processes but also propose a solution for the fully exploiting of by-products from agricultural production.

The Role of Interfacial Adhesion in Polymer Composites Engineered from Lignocellulosic Agricultural Waste

This paper presents a comprehensive study about the application of a lignocellulosic agricultural waste, sunflower husk in different polymer composites. Two types of milled sunflower husk with different geometrical factors were incorporated into polypropylene, low-density and high-density polyethylene, polystyrene (PS), glycol-modified polyethylene terephthalate (PETG) and polylactic acid (PLA). The filler content of the composites varied between 0 and 60 vol%. The components were homogenized in an internal mixer and plates were compression molded for testing. The Lewis-Nielsen model was fitted to the moduli of each composite series, and it was found that the physical contact of the filler particles is a limiting factor of composite modulus. Interfacial interactions were estimated from two independent approaches. Firstly, the extent of reinforcement was determined from the composition dependence of tensile strength. Secondly, the reversible work of adhesion was calculated from the surface energies of the components. As only weak van der Waals interactions develop in the interphase of polyolefins and sunflower husk particles, adhesion is weak in their composites resulting in poor reinforcement. Interfacial adhesion enhanced by specific interactions in the interphase, such as π electron interactions for PS, hydrogen bonds for PLA, and both for PETG based composites.

Polypropylene/lignin blend monoliths used as sorbent in oil spill cleanup

With increasing industrial development, frequent oil spillages in water; therefore, it is imperative and challenging to develop absorbents materials that are eco-efficiency, cost-effective, and pollution prevention. In this study, sorbents obtained from Lignin incorporated with Polypropylene in different levels loading 0, 10, 20 % wt using thermally induced phase separation Technique (TIPS). The Polypropylene/Lignin blend monoliths were fabricated and compared in terms of morphological, thermal, and wetting characterizations. The successfully blending of different lignin concentrations with preserved the chemical structure of the polymer was confirmed by FTIR analysis. Thermogravimetric tests displayed that the existence of Lignin has changed the onset temperature (Tonset) of the blending sorbents, decreasing as the loading of Lignin is increased. The contact angle measurement showed a decrease in the hydrophobicity of sorbents with increasing lignin loading, Polypropylene/Lignin blend monoliths showed better absorption toward oils (soybean - engine) as compared to Polypropylene itself. PP10L showed an improvement in the oil sorption capacity around 2 times compared to the Polypropylene. These excellent features make Polypropylene/Lignin blend monoliths more competitive promising candidates than commercial absorbent.

Is Green Chemistry a feasible tool for the implementation of a circular economy?

The main goal of this research is to evaluate the contributions of Green Chemistry as a potential tool to drive the transition to circularity. For this, we have carried out a bibliographic study, analyzing those documents, process, or experiences that dealt jointly with the Green Chemistry aspects related to circularity such circular economy, industrial ecology, and closed loop. Findings show that few authors have treated that disciplines together in the last 10 years. Based on an analysis of academic literature, common strategies (design, raw materials, life cycle assessment, processes, normative, new business, and collaboration), specific experiences (catalyst, biobased products or methods, recycling, and reusing), and difficulties to overcome (metrics, transdisciplinary research, unawareness, and competitiveness) have been identified. Finally, different kind of measures, as behind such joint metrics, informal open spaces, closer the industry, education, standards and label are proposed to facilitate the development of Green Chemistry, circular economy, industrial ecology, and closed loop with the ultimate goal of improving sustainable development.From the evidences found, we finally conclude that it is possible to use Green Chemistry and its principles as a tool to drive the transition to circularity, being the development of open spaces for exchange information between different actors from academia, governments and regulatory actors, business and industrial sectors, with the aim of promoting disruptive advances in sustainability.

Automotive wastes

A review of the literature from 2018 related to automotive wastes is presented. Topics include solid wastes from autobodies, batteries, tires, and vehicle electronics, in addition to sustainable building materials. Treatment and control of automotive pollution is also discussed, as well as potential toxicological and health risks. PRACTITIONER POINTS: Life cycle analyses and sustainable development automotive components were a key focus of the 2018 literature. Automobiles have been identified as point sources for air and water pollution. Research into energy consumption, emissions control, and catalytic converter behavior was conducted to mitigate the impact of automobile emissions. Potential non-point source control measures were identified. These included biological and chemical degradation of pollutants, as well as adsorption of heavy metals.