Biobased Flame Retardant Materials: Exploring the Balance Between Flame Retardancy Efficiency and Sustainability

With the global focus on environmental protection and sustainable development increasing, industries have set stricter requirements for materials, which not only need excellent performance but also sustainable sources and environmental friendliness. In the field of flame retardant materials, traditional flame retardants such as halogen-based ones have significant flame retardant effects, but they cause serious harm to the environment and human health during production, use, and disposal, such as generating toxic gases and having bioaccumulation problems. Therefore, the development of green and sustainable biobased flame retardant materials has become a current research hotspot and trend.

Biobased flame retardant materials are mainly derived from renewable biomass resources such as lignin, chitosan, cellulose, and starch. These materials not only have good biocompatibility and biodegradability to reduce environmental pressure but also show great potential in flame retardant performance. However, how to ensure high-efficiency flame retardancy while giving full play to their sustainability advantages and achieving a balance between the two is a key issue to be solved in the research and application of biobased flame retardant materials. This article will focus on two typical biobased flame retardant materials, lignin and chitosan, to discuss the research progress, challenges, and future directions in balancing flame retardancy efficiency and sustainability.

Lignin-Based Flame Retardant Materials

Structure and Properties of Lignin

Lignin is an abundant natural polymer in nature, mainly existing in plant cell walls and being the second most abundant biomass resource after cellulose. It is composed of three phenylpropane structural units (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol) connected by ether bonds and carbon-carbon bonds, forming a complex three-dimensional network structure. This unique structure endows lignin with special properties such as good thermal stability, antioxidant activity, and flame retardancy. Due to the large number of benzene rings and phenolic hydroxyl groups in lignin molecules, the benzene rings can absorb heat and carbonize when heated to form a dense char layer, effectively blocking heat and oxygen transfer for flame retardancy; phenolic hydroxyl groups can participate in chemical reactions to further improve its flame retardant performance.

Application Forms of Lignin in Flame Retardant Materials

1. Direct Addition Adding lignin directly to polymer matrices to prepare flame retardant composites is a simple and common method. Researchers have added lignin to polyethylene (PE) and found that the thermal stability and flame retardant performance of the composites gradually improve with increasing lignin content. When the lignin content reaches a certain level, the limiting oxygen index (LOI) of the composites significantly increases, and the vertical combustion test meets certain flame retardant standards. However, this method has problems: due to the poor compatibility between lignin and polymer matrices, agglomeration easily occurs in composites, leading to a decline in mechanical properties.
2. Application After Modification To improve the compatibility between lignin and polymer matrices and enhance its application effect in flame retardant materials, lignin is often modified. Common modification methods include chemical and physical modifications. Chemical modification introduces new functional groups to lignin molecules through chemical reactions, such as hydroxymethylation, phenolation, and esterification. For example, after hydroxymethylation modification, the introduced hydroxymethyl groups increase the reactivity and polarity of lignin, enabling better bonding with polymer matrices. Studies show that hydroxymethylated lignin added to polylactic acid (PLA) not only improves the flame retardant performance of the composites but also enhances their mechanical properties. Physical modification mainly changes the physical morphology and size of lignin through blending, nanosizing, etc., to improve its dispersibility in matrices. Adding lignin nanoparticles to epoxy resin results in composites with excellent flame retardant and mechanical properties.
3. Preparation of Lignin-Based Flame Retardants Using the structural characteristics of lignin to prepare new lignin-based flame retardants through a series of chemical reactions is an important application direction of lignin in the flame retardant field. Taking lignin as a raw material and reacting it with phosphorus-containing compounds, a new phosphorus-lignin flame retardant was synthesized. This flame retardant shows good flame retardant effects in polypropylene (PP), significantly reducing the heat release rate and smoke release amount of PP. This lignin-based flame retardant not only has high-efficiency flame retardancy but also has good environmental friendliness due to its biomass origin.

Challenges in Balancing Flame Retardancy Efficiency and Sustainability

4. Bottlenecks in Improving Flame Retardancy Efficiency Although lignin itself has certain flame retardancy, its flame retardant efficiency still needs to be improved compared with traditional high-performance flame retardants. In application fields with extremely high flame retardant requirements, such as aerospace and electronics, lignin or lignin-based materials alone are difficult to meet strict flame retardant standards. How to further improve the flame retardant efficiency of lignin-based flame retardant materials to enable their wide application in these high-end fields is an important challenge currently faced.
5. Impacts on Other Material Properties In the process of improving the flame retardant performance of lignin-based materials, other properties of the materials are often negatively affected. As mentioned earlier, direct addition of lignin leads to a decline in the mechanical properties of composites. In addition, the dark color of lignin may affect the appearance and optical properties of materials. In application scenarios with high requirements for material appearance and mechanical properties, such as packaging and architectural decoration, these issues limit the application of lignin-based flame retardant materials.
6. Large-Scale Production and Cost Control At present, the extraction and processing technologies for lignin are not yet mature, leading to high production costs and unstable quality of lignin. This has hindered the large-scale production and wide application of lignin-based flame retardant materials to a certain extent. To achieve large-scale industrial application of lignin-based flame retardant materials, it is necessary to further optimize lignin extraction and processing technologies, reduce production costs, and ensure product quality stability.

Chitosan-Based Flame Retardant Materials

Structure and Properties of Chitosan

Chitosan is a natural cationic polysaccharide obtained by deacetylation of chitin, widely present in the shells of crustaceans such as shrimp and crab, as well as in fungal cell walls. Its molecular structure contains a large number of amino and hydroxyl groups, which endow chitosan with many unique properties, such as good biocompatibility, biodegradability, antibacterial activity, film-forming ability, and adsorption capacity. In terms of flame retardancy, the nitrogen elements in chitosan molecules can form nitrogen-containing free radicals during combustion, which can capture active free radicals in the combustion reaction to inhibit its progress and play a flame retardant role. In addition, chitosan can undergo dehydration carbonization when heated to form a dense char layer, blocking heat and oxygen transfer and further improving the material’s flame retardant performance.

Application Forms of Chitosan in Flame Retardant Materials

7. As a Flame Retardant Additive Directly using chitosan as a flame retardant additive in polymer matrices is one of its common application methods. Researchers added chitosan to polyvinyl alcohol (PVA) to prepare PVA/chitosan composites with good flame retardant performance. As the chitosan content increases, the LOI value of the composites gradually increases, and the heat release rate significantly decreases. Chitosan shows better flame retardant effects when used in combination with other flame retardants. Adding a composite of chitosan and montmorillonite to polypropylene significantly enhances the flame retardant performance of the composites due to the synergistic flame retardant effect between chitosan and montmorillonite.
8. Preparation of Chitosan-Based Flame Retardant Coatings Using the film-forming property of chitosan to prepare chitosan-based flame retardant coatings and apply them to material surfaces is another important application of chitosan in the flame retardant field. Through layer-by-layer self-assembly technology, chitosan is alternately deposited with other flame retardants (such as phytic acid and ammonium polyphosphate) on fabric surfaces to form multi-layer flame retardant coatings. These coatings not only endow fabrics with good flame retardant performance but also have good washability. In addition, chitosan-based flame retardant coatings can be applied to the fire protection of wood, paper, and other materials to effectively improve their flame retardant performance.
9. Synthesis of Chitosan-Based Flame Retardant Derivatives Chemical modification of chitosan to synthesize chitosan-based flame retardant derivatives with higher flame retardant performance has become a research hotspot in recent years. By introducing flame retardant elements such as phosphorus and nitrogen into chitosan molecules, a series of new chitosan-based flame retardant derivatives have been prepared. For example, a phosphorus-containing chitosan flame retardant was synthesized by reacting chitosan with ammonium dihydrogen phosphate. This flame retardant shows excellent flame retardant performance in polylactic acid, increasing the LOI value of polylactic acid to a high level while reducing its heat release rate and smoke release amount.

Challenges in Balancing Flame Retardancy Efficiency and Sustainability

10. Coordination Between Flame Retardant Performance and Biocompatibility Although chitosan itself has good biocompatibility, modifying it to improve flame retardant performance may introduce groups or substances that affect biocompatibility. Some chemical modification methods may cause changes in the molecular structure of chitosan, thereby affecting its degradability and compatibility in biological systems. In the development of chitosan-based flame retardant materials, how to maintain good biocompatibility while ensuring high-efficiency flame retardant performance is one of the key issues to be solved.
11. Source and Quality Stability of Chitosan Chitosan is mainly derived from the shells of crustaceans, whose sources are affected by factors such as season, region, and raw material quality, leading to differences in chitosan quality. Different batches of chitosan may vary in key indicators such as degree of deacetylation and molecular weight, which can significantly affect the performance of chitosan-based flame retardant materials. To ensure the performance stability of chitosan-based flame retardant materials, it is necessary to establish a stable supply channel for chitosan raw materials and improve the quality control system.
12. Expansion of Application Scope and Technical Challenges At present, chitosan-based flame retardant materials have been applied in some fields, but their application scope is still relatively narrow compared with traditional flame retardant materials. In some large-scale engineering materials and high-performance material fields, chitosan-based flame retardant materials still face many technical challenges, such as compatibility with matrix materials and processing performance. To further expand the application scope of chitosan-based flame retardant materials, it is necessary to strengthen relevant technical research and solve these technical bottlenecks.

Strategies and Prospects for Balancing Flame Retardancy Efficiency and Sustainability

Multidisciplinary Research Approaches

The research on biobased flame retardant materials involves multiple disciplines such as materials science, chemistry, biology, and engineering. To achieve a balance between flame retardancy efficiency and sustainability, multidisciplinary research approaches are needed. Combining knowledge from materials science and chemistry to develop new modification methods and synthesis technologies to improve the performance of biobased flame retardant materials; using biological principles to deeply study the biodegradation mechanism and biocompatibility of biobased materials to provide theoretical support for material design and application; and optimizing material preparation processes and processing methods through engineering means to achieve large-scale production and application.

Construction of Synergistic Flame Retardant Systems

Single biobased flame retardant materials often struggle to meet both high-efficiency flame retardancy and sustainability requirements. Therefore, constructing synergistic flame retardant systems is an effective strategy to improve the performance of biobased flame retardant materials. Different biobased flame retardants can be compounded to enhance flame retardancy efficiency through their synergistic effects. For example, compounding lignin and chitosan and adding them to polymer matrices can significantly improve the flame retardant performance of composites due to the complementary flame retardant mechanisms of the two. In addition, biobased flame retardants can be used synergistically with traditional flame retardants to reduce the dosage of traditional flame retardants and improve material sustainability while ensuring flame retardant performance.

Integrating the Concept of Sustainable Development Throughout the Material Lifecycle

From a sustainable development perspective, the research and application of biobased flame retardant materials need to integrate the concept of sustainable development throughout the entire material lifecycle, including raw material acquisition, production processing, use, and waste disposal. In the raw material acquisition stage, renewable, abundant, and environmentally friendly biomass resources should be prioritized; in the production processing stage, green and environmentally friendly processes and technologies should be adopted to reduce energy consumption and pollutant emissions; during use, materials should have good performance and stability to extend their service life; and in waste disposal, materials should be biodegradable or recyclable to achieve resource circularity.

Biobased flame retardant materials, as green and sustainable flame retardant materials, have broad application prospects in the future flame retardant field. Although there are still many challenges in balancing flame retardancy efficiency and sustainability, these issues are expected to be gradually resolved with the deepening of research and technological progress. Through multidisciplinary research approaches, the construction of synergistic flame retardant systems, and the implementation of sustainable development concepts, biobased flame retardant materials are expected to fully leverage their sustainability advantages while achieving high-efficiency flame retardancy, making important contributions to promoting sustainable social development.

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