Synthesis of novel high-performance all-biobased wood flame retardant materials using phytic acid and β-cyclodextrin

Wood, as an indispensable component of our daily lives, has been widely applied due to its excellent acoustic properties, easy processability, and renewable nature [1], [2], [3]. However, the inherent flammability of wood often poses a significant challenge, causing potential property damage and personal injuries, thereby imposing certain limitations on its application [4], [5]. Therefore, flame-retardant treatment of wood is crucial to enhance its fire safety performance. However, the utilization of halogenated flame retardants has been prohibited owing to their poor environmental compatibility and the release of toxic gases during combustion [6], [7], [8]. Inorganic flame retardants can adversely affect the mechanical properties of wood, hindering their widespread application as a construction material [9], [10]. Given the escalating environmental concerns, the selection of eco-friendly, biodegradable, and renewable flame-retardant materials has become increasingly urgent [11]. Biobased flame retardants hold great promise for future applications. However, single biobased flame retardants often exhibit suboptimal performance in terms of smoke suppression and mechanical properties [12], [13], [14]. Therefore, developing synergistic composite flame retardants is necessary to comprehensively improve the overall performance of wood [15], [16], [17]. Phytic acid, lignin, cyclodextrin, proteins, and chitosan are increasingly recognized as environmentally friendly and effective flame retardants. Guo et al. developed a fully bio-based intumescent flame retardant (IFR) by utilizing lignin, casein, and phytic acid through a straightforward and eco-friendly method, yielding polyacrylonitrile (PAN) with remarkable flame retardancy and effective char formation. This FR-PAN exhibited rapid self-extinguishing properties upon removal from the flame, with a limiting oxygen index (LOI) value increased to 32.5 % [18]. Tan et al. synthesized an innovative flame retardant based on phytic acid, urea, kaolin, and sodium carbonate, providing an environmentally friendly, lightweight, and efficient flame-retardant solution. By conducting ionic complex assembly of phytic acid and urea in water and employing polydopamine as a binder, they created scratch-resistant flame-retardant expanded polystyrene (EPS) insulation boards (EPS-PAUK) that achieved a V-0 rating in the UL-94 test. These boards exhibited excellent smoke suppression and compressive strength while maintaining the lightweight properties of EPS insulation [19]. Liu et al. assembled ammonium polyphosphate (APP), chitosan (CS), and carboxylated silicone oil in an aqueous phase to synthesize a bio-based core-shell flame retardant, denoted as APP@CS@Si. With a loading of merely 2 wt%, the LOI value of PLA composites was increased to 28.2 %, effectively suppressing smoke release. [20]

Phytic acid (PA), a naturally occurring compound found in plant seeds, cereals, and other plant-based materials, serves as a form of storage of phosphorus in plants [21], [22]. With a phosphorus content that can be as high as 28 %, PA is an ideal biobased flame retardant derived from plant sources and can function as the acid source in flame retardant formulations [23], [24]. The synthesis of a flame retardant from PA and urea enables the covalent bonding between the flame retardant and the cellulosic fiber in wood, enhancing the flame retardancy and durability of the wood [25], [26]. The nitrogen in urea can work synergistically with the phosphorus in PA to form P-N bonds, which facilitate the formation of a protective char layer during the combustion of wood [27].

β-Cyclodextrin (β-CD) is a cyclic oligosaccharide structure stemming from plant-based resources. It offers advantages such as widespread availability, low cost, and non-toxicity [28], [29]. The high hydroxyl content of β-CD endows it with excellent chemical reactivity and a positive role in promoting char formation, making it a valuable biobased carbon source [30], [31], however, the poor water solubility and chemical stability of β-CD have limited its scope of application. Chemical modifications are often required to enhance the performance of β-CD, such as the introduction of hydroxymethyl or methyl substituents [32], [33]. Previous research showed that the replacement of hydroxyl groups with other functional groups can improve the efficiency of utilization of β-CD. In the present work, succinic acid (SA) was selected to modify β-CD [34], [35] Importantly, the entire modification process was conducted without the use of organic solvents, providing an eco-friendly, convenient approach.

A highly efficient all-biobased flame retardant was developed through a green, eco-friendly approach, involving the synthesis of ammonium phytate (PAAn) and SA-modified β-cyclodextrin (SA-β-CD). The synergistic effect between PAAn and SA-β-CD endowed the resulting flame-retardant treated wood with a comprehensive flame retardancy mechanism, incorporating gas, acid, and carbon sources. The treated wood exhibited excellent flame-retardant, smoke-suppressing, and thermal insulation performance, while also demonstrating notable improvements in mechanical properties. This mitigated the adverse effects of the strong acidity of PA on the physico-mechanical properties of wood [36], [37], [38]. The flame-retardant treated wood was comprehensively characterized to investigate the influence of the synergistic flame retardant on the combustion behavior and thermal stability of the wood. In this study, we developed an effective strategy to maximize the benefits of β-cyclodextrin while addressing its limitations, focusing on critical issues in its application for flame-retardant system construction. This method notably increased the solubility of β-cyclodextrin in water and enhanced its thermal stability. We combined it with ammonium phytate, synthesized from phytic acid and urea, to evaluate its effectiveness as a flame-retardant modifier. The bio-based wood flame-retardant composite materials demonstrated not only excellent flame retardancy and thermal stability but also effectively reduced the negative impact of phytic acid’s strong acidity on the mechanical properties of wood. This led to enhanced dimensional stability, reduced water absorption, and improved mechanical strength of the materials. The flame-retardant treated wood was comprehensively characterized, possible flame retardancy mechanisms were proposed, providing new insights and theoretical guidance for the design of green, efficient, sustainable, and renewable all-biobased flame retardants, as well as the development of novel eco-friendly wood-based materials with superior fire safety performance. The flame-retardant treated wood can find applications in indoor furniture, outdoor construction materials, among others.