**1. Introduction**

Most of the polymeric materials including synthetic and natural based, are extremely flammable in nature. The extensive usage of polymer based materials led to a substantial increase in fire risk in our day to day-life. For example, about 2 million fire accidents are reported in Europe every year, which cause more than about 4000 casualties and 70000 injuries along with economic loss of about 120 billion € [1, 2]. Thus, there are unwanted and excessive societal and economic burdens on the entire civilization, hence it is most essential to produce polymeric products with efficient flame retardant properties to avoid fire related losses.

In addition, the flame-retardant behaviour of polymeric materials are considered as one of the most important criteria to utilize them in the form of sealants, encapsulants, coatings, and matrices for different industrial applications [3–8].

the preparation, properties of some of the important and valuable halogen free bio-based flame retardant polybenzoxazines matrices and composites are reviewed

*Development of Halogen Free Sustainable Polybenzoxazine Matrices and Composites for Flame…*

**2. Cardanol based polybenzoxazines for flame resistant applications**

Cardanol is a naturally occurring combination of bio-phenolic materials isolated from cashew nut-shell agro- waste with exported worth of 1.39 USD million in the year 2020–2021 (Apr-Nov) by India [58]. The cardanol chemical structure is very exciting due to its reactive functional phenolic-OH group and an alkyl spacer with unsaturation in m-position will be applicable for several chemical reaction and functional group modifications [59–62]. Cardanol bargains one of the better probable material for green synthesis of benzoxazines due to their massive availability and very low cost of production. Also, cardanol competently alternative to petrophenol (for example bisphenol-A (BPA) which is an endocrine disruptor) to prepare BPA-free polybenzoxazines. There are numerous auspicious ingenuities for replacing the petro-based phenolic resources with cardanol to form cardanol based

The flame retardant properties of cardanol based polybenzoxazines are usually lower than those of petroleum based thermosets. However, solutions can be provided by either physical blending with flame retardants or by chemical modification. Several research groups attempted to solve these issues by designing multi amine precursors [63, 64] or by introducing, silica/silicon, phosphorus, boron, and

To improve the flame resistant properties, Bimlesh Lochab research group introduced the [71] halogen free eco-friendly hexa-functional cardanol (bio-based phenolic) benzoxazine with a phosphazene core (cyclophosphazene ring based benzoxazine monomer designated as CPN) as reactive flame retardant precursor.

aminophenylmethane based benzoxazine monomer (CPN0). Higher loading of CPN in the monomer improved both the smoke density rating, vertical burning rating and also led to higher limiting oxygen index (LOI). Finally, the introduction of CPN network shows good compatibility with the polybenzoxazine phenolic thermosets

CPN monomer was synthesized as per the reported procedure (**Figure 1**). The phosphazene core with hexa-amine (2) was synthesised using phosphonitrilic

corresponding Schiff base which is then reduced to compound (4). Compound (4) undergoes intra-molecular cyclization reaction to form CPN (5). The renewable cardanol and phosphorous content in the CPN monomer are 65.7% and 3.4%

The cardanol (CPN0) monomer was synthesised as per the procedure reported [71]. The monomer combinations are named as CPNx, here x is percentage weight of CPN incorporated in the CPN0 monomer and blends developed are named as CPN10, and CPN80. The polymerization of CPN, CPN80, CPN10, andCPN0 was accomplished in a hot air oven at the temperature of 50°C, 100°C, 120°C, 150°C, 180°C, 200°C, 220°C, 240°C for each 1 h and followed by further heating at 240 <sup>o</sup>

for 0 h, 1 h, 1.5 h and 2 h. The prepared poly (CPNX) (**Figure 2**) was utilized for

C

The CPN monomer showed good compatibility with cardanol and tris-p-

chloride trimer (N3P3Cl6), by base facilitated reaction with excess of 4 acetomidophenol to yield compound (1) and, which upon hydrolysis, gave the corresponding hexa-amine compound (2). Later, the synthesized hexa-amine

(2) was reacted with 4-pentadecylsalicylaldehyde (3) to form the

with enhancement in flame resistant properties.

and summarized.

*DOI: http://dx.doi.org/10.5772/intechopen.98470*

polybenzoxazines.

furan [52, 65–70].

respectively.

**39**

further characterizations.

In the recent years, due to the environmental, sustainable and economic aspect, growing attempts have been progressed for the synthesis of polymeric resin from natural renewable/sustainable feedstock instead of using fossil fuel/petroleum based raw materials [9–12]. The production of polymeric materials at present are mostly based on the petroleum feedstock and are used for wide range of industrial applications [13–16]. Bio-based matrices/composites already found numerous applications in the diverse fields of our everyday lives, such as in the automotive industry or in building and construction, aerospace, and so forth [17–23]. In this context, in order to improve their safe utility and also widen their area of application, it is essential to increase their fire/flame resistant behavior to make them suitable for indented applications. Hence, the development of polymeric materials with enhanced flame retardant behavior is warranted from sustainable bio-resources.

Usually, halogen based flame resistant additive materials has been extensively used into polymer matrix to enhance its flame resistant properties. The release the corrosive smoke and toxic gases by these additives during pyrolysis, consequently, give rise to serious ecological problems. Consequently, the development of polymeric materials free from halogens are considered to be a versatile approach. The method of enhancing the flame resistant properties of polymers without use of halogen based flame additives can be deliberated by the following three methodologies to obtain the flame retardant of polymers: (i) using intrinsically flame retardant polymeric materials, (ii) to suitable structural modification of existing industrial polymers, (iii) to introduce intrinsic flame resistant nanoreinforcements/fillers into polymers.

As a result, new research fields have been emerged in the material science in particular in the field of polymers and directed toward a completely or partial replacement of the petroleum-based materials. In this regard, polybenzoxazines possess very useful perspective due to their extraordinary molecular design flexibility [24–32], which easily facilitates and allows bio-based precursors for the production of varied molecular structured benzoxazines [33–57], consequently reducing/replacing the considerable utilization of the petroleum-based raw materials. The nitrogen atom in the skeleton of benzoxazines significantly enhances their flame retardancy, making them suitable for the fabrication of flame retardant components. Fire, Smoke, and Toxicity (FST) reduction behaviour of benzoxazine based materials have attracted industrial and academic attention as federal regulations become strict and new technologies emerge. It was reported that the many efforts have been made to syntheses of benzoxazines exploiting natural renewable resources using cardanol, urushiol, coumarine, eugenol, guaiacol, furfuryl amine and stearyl amine. In addition, silica/phosphorous/carbon reinforced benzoxazine hybrid materials expected to possess radiation resistant behaviour coupled with inherent flame retardant properties. Probable flame resistant mechanism of the polybenzoxazine resin is owing to the release of gaseous species on the surface. The gases including CO2, NO2, and H2O might able to diminish the O2 concentration around the burning area and diminish the heat of the surface. Another probable mechanism of PBz polymer, with flame resistant layer through the formation of char on the surface. The inert flame resistant layer might protect the benzoxazine from the external heat from the flame, thus performing as the O2 protection layer. The high strength, excellent flame retardancy, radiation resistance, high thermal stability, low moisture absorption, low temperature cure, low shrinkage and low-k dielectric behaviour have made benzoxazine resins become an attractive for electronics and aerospace applications. Hence, in the present chapter,

the preparation, properties of some of the important and valuable halogen free bio-based flame retardant polybenzoxazines matrices and composites are reviewed and summarized.
