Cardanol and Eugenol Sourced Sustainable Non-halogen Flame Retardants for Enhanced Stability of Renewable Polybenzoxazines

Abstract

Olefin bonds participate in co-reaction with the benzoxazine functionality of the monomer and are one of the strategies used to affect the crosslink density of a polybenzoxazine network. In general, the double bond incorporation in starting material is usually catalyzed by expensive, rare earth metals affecting the sustainability of the reaction. The natural abundance of feedstocks with inherent double bonds may be a powerful platform for the development of novel greener structures, with potential applications in polymers. Here, we report the design, synthesis, and characterization of a biobased non-halogen flame retardant, consisting of naturally occurring phenols, eugenol (E), and cardanol (C). The presence of a covalently linked phosphazene (P) core allowed the synthesis of hexa-functional flame retardant molecules, abbreviated as EP and CP. The chemical structures of the synthesized EP and CP were confirmed by Fourier transform infrared (FTIR), nuclear magnetic resonance (1 H, 13 C, 31 P NMR), and single crystal XRD (only in the case of EP). Their polymerization with cardanol sourced trioxazine benzoxazine monomer, C-trisapm, was followed by FTIR, NMR, and DSC studies. The thermal stability and flame retardant properties of the hybrid phosphazenebenzoxazine copolymers was determined by thermogravimetry analysis (TGA), limiting oxygen index (LOI), vertical burning, and smoke density analyses. SEM images of the char residues of the polymers with or without the addition of reactive phosphazene molecules confirmed the intumescent flame retarding mechanism. Current work highlights the utility of sustainable origin non-halogen flame retardant (FR) molecules and their utility in polybenzoxazine chemistry.

Development of Halogen Free Sustainable Polybenzoxazine Matrices and Composites for Flame Retardant Applications

“…The larger surface area exhibited by such inter-connected system of voids alleviate altercation of heat and oxygen (air), which in turn contributes to an enhanced resistance against flame. Further, to improve the flame resistant behaviour of polymers, the same research group have reported [72] the introduction of hexacardanolphosphazene (CP) /hexaeugenolphosphazene (EP) as a flame retardant additives in to cardanol based tris-benzoxazine monomer (C-trisapm). The flame retardant properties of resulting matrices were studied using LOI, UL-94, and smoke density analysis.…”

“…The flame retardant properties of resulting matrices were studied using LOI, UL-94, and smoke density analysis. Hexaeugenolcyclotriphosphazene (EP) was synthesized (Figure 4) as per the reported procedure [72] as follows; in a 250 mL RB flask comprising a mixture of acetonitrile and acetone (1:3 ratio) under N 2 atmosphere, K 2 CO 3 (143 mmol) was added. Eugenol (1.15 mmol) was dissolved in acetone and added to the above mixture, followed by the addition of N 3 P 3 Cl 6 (14.3 mmol).…”

“…Hexacardanolcyclotriphosphazene (CP) was synthesized (Figure 4) as per the reported procedure [72] using cardanol (115 mmol) was added with acetonitrile, followed by K 2 CO 3 (143 mmol). A solution of N 3 P 3 Cl 6 (14 mmol) dissolved in acetonitrile was added to the reaction solution.…”

“…An introduction of 1.1 wt% of phosphorous (P) in C-trisapm indicated slow burning features and all other blends exhibited selfextinguishing features. The smoke density results, C-trisapm, which burns with a smoke emission of 37.80%, while those of CP and EP with P weight percentage contents of 4.75 and 8.34 wt% exhibit the value of smoke density of 34.01 and [72]. (Copyright 2020.…”

“…Figure 5.Plausible polymerization reactions: (A) self-crosslinking via double bonds, (B) oxazine ring-opening polymerization, (C) Co-polymerization: Co-reaction of double bonds and oxazine ring[72]. (Copyright 2020.…”