**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 polybenzoxazines.

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 furan [52, 65–70].

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. The CPN monomer showed good compatibility with cardanol and tris-paminophenylmethane 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 with enhancement in flame resistant properties.

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

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

In addition, the flame-retardant behaviour of polymeric materials are considered as

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 nano-

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,

one of the most important criteria to utilize them in the form of sealants, encapsulants, coatings, and matrices for different industrial applications [3–8]. 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

*Flame Retardant and Thermally Insulating Polymers*

retardant behavior is warranted from sustainable bio-resources.

reinforcements/fillers into polymers.

**38**

**Figure 1.** *Divergent approach for the synthesis of cardanol phosphazene benzoxazine monomer (CPN).*

slowburning; 28 to 100% self-extinguishing; and > 100 is considered as inherently non-flammable [71]. From the **Table 1**, an increment in the value of LOI is reliant on the weight percentage content of phosphorous signifying the role played by CPN core in flammability. The vertical burning test (UL-94) was adopted to check the flammability features of poly(CPNx) and the values obtained are presented in **Table 1**. The poly(CPN0) was burnt immediately with a lower combustion time along with fire drippings. The flame resistance properties was increased with an increase in weight content of phosphorus which substantiates with LOI results.

*Thermal and flame resistant (LOI, UL-94 and smoke density) properties of bio-based polybenzoxazine*

**Samples Phosphorus**

*a*

*b*

**41**

**Table 1.**

*Air atmosphere.*

*Renewable phenol (%).*

*matrices and composites.*

**(%)**

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

CPN0 0 70.40 14.0 (0<sup>a</sup>

CPN10 0.30 70.00 24 .0(7<sup>a</sup>

CPN80 2.70 66.80 32.0(26<sup>a</sup>

CPN 3.40 65.70 39.0(28<sup>a</sup>

**Cardanol (%)**

**Char(%) residue**

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

C-trisapm (T) 0 70.40<sup>b</sup> 8.30 20.82 54.60 [72] CP1T3 1.19 23.28<sup>b</sup> 28.50 28.90 51.60 [72] CP3T1 3.56 69.83<sup>b</sup> 42.70 34.58 47.50 [72] CP 4.79 93.10<sup>b</sup> 29.30 29.22 34.01 [72] EP1T3 2.09 21.97<sup>b</sup> 28.70 28.98 35.39 [72] EP3T1 6.26 65.92b 28.50 28.9 39.83 [72] EP 8.34 87.89<sup>b</sup> 32.40 30.46 18.80 [72]

) 23 (18<sup>a</sup>

) 27 (20<sup>a</sup>

) 30 (28<sup>a</sup>

) 33 (29a

**LOI UL-94 Smoke**

**density**

) V-2 77.70 [71]

) V-1 70.70 [71]

) V-0 34.30 [71]

) V-0 33.90 [71]

**Reference**

A smoke density test was carried out to ascertain the relative quantities of smoke produced during burning of poly(CPNx). The neat poly(CPN0) indicated nearly very low residue char which crumbled in the mesh, whereas phosphazene

containing poly(CPN10), poly(CPN80), and poly(CPN) exhibited high char residue and perceived smoke density rating of 77.7, 70.8, 34.3 and 33.9 respectively. The comparative variance in smoke density was observed as 6.9 and 43.4 with 10 wt% and 80 wt.% respectively with introduction of CPN proposing a substantial role frolicked by CPN core in the decrease of smoke density in case of poly(CPN0). Further to confirm the flame retardant mechanism, SEM was performed to check the morphology of the residual char samples after smoke density analysis and are depicted in **Figure 3**. From SEM images, it was noticed that the several undulations and ripples in the cases of poly(CPN0), and poly(CPN10) due to the presence of a higher amount of flammable matrix (**Figure 3a** and **b**). The formation of bigger cracks (20–30 μm) on exterior surface was observed in the residue of poly(CPN0). The cracks were significantly decreased with the introduction of CPN, as observed from the exterior surfaces of poly(CPN10), poly(CPN80), and poly(CPN). Both poly (CPN80), and poly(CPN) formed a extremely compact and thick charred layers with combined enlarged residual char. Analysis of interior surface morphology exhibited the development of bigger porous structures with smoother surface while a several honeycombed micro-structures with fizze detached by precise tinny layers was perceived with higher in weight content of phosphorous in the residue. The larger surface area exhibited by such inter-connected system of voids alleviate

The limiting oxygen index (LOI) of the polymers was calculated from TGA using van Krevelen's equation and the values obtained are presented in **Table 1**. If a polymer possesses the value of LOI is less than 20.9%, it burn simply in air; 21–28%


*Development of Halogen Free Sustainable Polybenzoxazine Matrices and Composites for Flame… DOI: http://dx.doi.org/10.5772/intechopen.98470*

#### **Table 1.**

*Thermal and flame resistant (LOI, UL-94 and smoke density) properties of bio-based polybenzoxazine matrices and composites.*

slowburning; 28 to 100% self-extinguishing; and > 100 is considered as inherently non-flammable [71]. From the **Table 1**, an increment in the value of LOI is reliant on the weight percentage content of phosphorous signifying the role played by CPN core in flammability. The vertical burning test (UL-94) was adopted to check the flammability features of poly(CPNx) and the values obtained are presented in **Table 1**. The poly(CPN0) was burnt immediately with a lower combustion time along with fire drippings. The flame resistance properties was increased with an increase in weight content of phosphorus which substantiates with LOI results.

A smoke density test was carried out to ascertain the relative quantities of smoke produced during burning of poly(CPNx). The neat poly(CPN0) indicated nearly very low residue char which crumbled in the mesh, whereas phosphazene containing poly(CPN10), poly(CPN80), and poly(CPN) exhibited high char residue and perceived smoke density rating of 77.7, 70.8, 34.3 and 33.9 respectively. The comparative variance in smoke density was observed as 6.9 and 43.4 with 10 wt% and 80 wt.% respectively with introduction of CPN proposing a substantial role frolicked by CPN core in the decrease of smoke density in case of poly(CPN0).

Further to confirm the flame retardant mechanism, SEM was performed to check the morphology of the residual char samples after smoke density analysis and are depicted in **Figure 3**. From SEM images, it was noticed that the several undulations and ripples in the cases of poly(CPN0), and poly(CPN10) due to the presence of a higher amount of flammable matrix (**Figure 3a** and **b**). The formation of bigger cracks (20–30 μm) on exterior surface was observed in the residue of poly(CPN0). The cracks were significantly decreased with the introduction of CPN, as observed from the exterior surfaces of poly(CPN10), poly(CPN80), and poly(CPN). Both poly (CPN80), and poly(CPN) formed a extremely compact and thick charred layers with combined enlarged residual char. Analysis of interior surface morphology exhibited the development of bigger porous structures with smoother surface while a several honeycombed micro-structures with fizze detached by precise tinny layers was perceived with higher in weight content of phosphorous in the residue. The larger surface area exhibited by such inter-connected system of voids alleviate

The limiting oxygen index (LOI) of the polymers was calculated from TGA using van Krevelen's equation and the values obtained are presented in **Table 1**. If a polymer possesses the value of LOI is less than 20.9%, it burn simply in air; 21–28%

*Ring opening polymerisation of CPN monomer to form polyphosphazene polybenzoxazine.*

*Divergent approach for the synthesis of cardanol phosphazene benzoxazine monomer (CPN).*

*Flame Retardant and Thermally Insulating Polymers*

**Figure 1.**

**Figure 2.**

**40**

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 N2 atmosphere, K2CO3 (143 mmol) was added. Eugenol (1.15 mmol) was dissolved in acetone and added to the above mixture, followed by the addition of N3P3Cl6 (14.3 mmol). The reaction mixture was heated to 80°C and stirred for 18 h, and then allowed to cool to room temperature and evaporate the solvent. The residue was dissolved in ethyl acetate and the organic phase was washed with DM water, followed by addition of 5% NaOH and water until to obtain neutral pH. The organic layer was dried over sodium sulfate and the solvent was evaporated. Finally, the compound was purified by column chromatography using 10% ethyl acetate in hexane yield a white solid of EP.

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

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

Hexacardanolcyclotriphosphazene (CP) was synthesized (**Figure 4**) as per the reported procedure [72] using cardanol (115 mmol) was added with acetonitrile, followed by K2CO3 (143 mmol). A solution of N3P3Cl6 (14 mmol) dissolved in acetonitrile was added to the reaction solution. The mixture was heated to 85°C and stirred for 36 h followed by work up as per the above procedure (EP). Finally, the crude compound was purified by column to get CP as a brown transparent liquid. Polymer blend were prepared with three varying weight percentage of EP/CP, C-trisapm of 1:3, 1:1, and 3:1, by simple mixing of CP/EP and C-trisapm using tetrahydrofuran (THF) (**Figure 5**). After vaporizing the THF under the vacuum, the attained resultant mixture was thermally treated at 50, 100, 120,160, 180, 200, 230, and 250°C for each 1 h. The blends are abbreviated as CPxTy or EPxTy where x and y are the weight percentage ratios in the blends, and T is represented for

The LOI value of poly(C-trisapm) was changed from 20 to higher values (34.58 for CP3T1) in the case of blends. 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

*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.*

*Reproduced with permission from Frontiers in Chemistry).*

C-trisapm.

**Figure 5.**

**43**

#### **Figure 3.**

*SEM images of exterior (a, b, c, d) and interior (a', b', c', d') surfaces of residual char (a, a') poly(CPN0), (b, b') poly(CPN10), (c, c') poly(CPN80), and (d, d') poly(CPN) samples [71].(Copyright 2018. Reproduced with permission from American Chemical Society).*

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.

**Figure 4.** *Synthesis of EP and CP.*

### *Development of Halogen Free Sustainable Polybenzoxazine Matrices and Composites for Flame… DOI: http://dx.doi.org/10.5772/intechopen.98470*

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 N2 atmosphere, K2CO3 (143 mmol) was added. Eugenol (1.15 mmol) was dissolved in acetone and added to the above mixture, followed by the addition of N3P3Cl6 (14.3 mmol). The reaction mixture was heated to 80°C and stirred for 18 h, and then allowed to cool to room temperature and evaporate the solvent. The residue was dissolved in ethyl acetate and the organic phase was washed with DM water, followed by addition of 5% NaOH and water until to obtain neutral pH. The organic layer was dried over sodium sulfate and the solvent was evaporated. Finally, the compound was purified by column chromatography using 10% ethyl acetate in hexane yield a white solid of EP.

Hexacardanolcyclotriphosphazene (CP) was synthesized (**Figure 4**) as per the reported procedure [72] using cardanol (115 mmol) was added with acetonitrile, followed by K2CO3 (143 mmol). A solution of N3P3Cl6 (14 mmol) dissolved in acetonitrile was added to the reaction solution. The mixture was heated to 85°C and stirred for 36 h followed by work up as per the above procedure (EP). Finally, the crude compound was purified by column to get CP as a brown transparent liquid.

Polymer blend were prepared with three varying weight percentage of EP/CP, C-trisapm of 1:3, 1:1, and 3:1, by simple mixing of CP/EP and C-trisapm using tetrahydrofuran (THF) (**Figure 5**). After vaporizing the THF under the vacuum, the attained resultant mixture was thermally treated at 50, 100, 120,160, 180, 200, 230, and 250°C for each 1 h. The blends are abbreviated as CPxTy or EPxTy where x and y are the weight percentage ratios in the blends, and T is represented for C-trisapm.

The LOI value of poly(C-trisapm) was changed from 20 to higher values (34.58 for CP3T1) in the case of blends. 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

#### **Figure 5.**

altercation of heat and oxygen (air), which in turn contributes to an enhanced

*SEM images of exterior (a, b, c, d) and interior (a', b', c', d') surfaces of residual char (a, a') poly(CPN0), (b, b') poly(CPN10), (c, c') poly(CPN80), and (d, d') poly(CPN) samples [71].(Copyright 2018. Reproduced*

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.

resistance against flame.

*with permission from American Chemical Society).*

*Flame Retardant and Thermally Insulating Polymers*

**Figure 3.**

**Figure 4.**

**42**

*Synthesis of EP and CP.*

*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. Reproduced with permission from Frontiers in Chemistry).*

18.80 wt% respectively. Reduction in values of smoke density are due to the higher amount of phosphazene moiety present in the polymers. The UL-94 test is a vertical burning test that decides the vertical burning properties of a polymers (**Table 1**). Neat polyhexacardanolcyclotriphosphazene (poly(CP)) and its reactive blends did not catch fire instantly, dissimilar to poly(C-trisapm). The flame resistant characteristics of cross-linked CP/C-trisapm blends was found to increase with increase in P weight percent content, which supports the results of values of LOI.

Morphology of both the interior and exterior surface of the char residue attained after smoke density test was determined using SEM analysis and the images are presented in **Figure 6**. With an increasing the weight percentage of CP, the surface exterior morphology reformed to smoother surface from rippled, whereas surface interior indicated a porous honey-combed structures detached by tinny layers of border. The founding of cracks and bubbles are agreed to the stiff outer layer in case of EP comprising co-polymers, which might have rupture to discharge the interchange of heat and oxygen, thus hindering the propagation of fire as a measure of structural safety.

To improve the flame and thermal properties, M. Alagar and his research team have reported [34, 36] hybrid approach using silica (nano-silica through in-situ sol– gel/or bio silica derived from rice-husk ash) as reinforcement in to cardanol based benzoxazine (**Figure 7**). Introduction of inorganic constituent 3-mercaptopropyltrimethoxysilane (MPTMS) in to cardanol/furfurylamine based benzoxazine (BZ-C-F) via thiol-ene reaction followed by in-situ sol–gel techniques using tetraethoxysilane (TEOS). From thermal studies the percentage char yield indicates an increase of about 4.2 times (11.11 to 48.63%) and the values of LOI 21.94 to 36.95 for PBZ-C-F hybrids (**Table 2**) respectively. Functionalized bio-silica (FRHA) (0, 1, 5, 10, 15, and 20%) reinforced tri-substituted cardanol benzoxazine (CBz) (**Figure 8**), the value of LOI was increased with increase in weight percentage content of FRHA. Among the composites developed, 20 wt% FRHA reinforced CBz composites possesses the highest value of LOI of 36%, which is higher than that of values obtained for other composite samples. Also, the UL-94 vertical burning

result (**Table 3**) indicates the neat CBz and CBz/FRHA (1 wt%) exhibit no rating, while 5 and 10 wt% FRHA reinforced CBz composites possess the V-2 rating. 15 and 20 wt% FRHA reinforced CBz composites showed the V-1 rating. The thermal stability and flame resistant behaviour of PBZ-silica hybrid materials are higher than those of neat PBZ as well as traditional PBA-a, due to the hybridization of silica component through chemical interaction. Further, the incorporation of higher amount of silica into the PBZ matrix reduced the volatile decomposition. In addition, the silica components offer the additional heat capacity which restricts the

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

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

In addition, the same research group have also reported [73] the eco-friendly cardanol-based benzoxazines (C-ida and C-pyta) synthesized from hetero-cyclic core amines (**Figure 7**), such as pyridine core triamine (pyta) and tetraarylimidazole core diamines (ida). Further, bio-composites were also prepared using 3-glycidoxypropyltrimethoxysilane (GPTMS)-functionalized bio-silica (1, 3, 5, 7, and 10 wt%) obtained from rice husk. The thermal stability of developed PBz and its composites (**Figure 9**) is analyzed using TGA and the values obtained are presented in **Table 2**. As the biosilica weight percentage content increases, the degradation temperature and the value

materials against thermal degradation.

*Schematic synthesis of cardanol based benzoxazine monomers.*

**Figure 7.**

**45**

#### **Figure 6.**

*Digital images of cured samples of (a) poly(CP), (b) poly(CP3T1), (c) poly(CP1T3) before (a, b, c) and after burning (a*<sup>0</sup> *, b*<sup>0</sup> *, c*<sup>0</sup> *); SEM images of poly(CP), poly(CP3T1), poly(CP1T3) surfaces of residual char: exterior (d, e, f) and interior (as inset) (d*<sup>0</sup> *, e*<sup>0</sup> *, f*<sup>0</sup> *), respectively [72]. (Copyright 2020. Reproduced with permission from Frontiers in Chemistry).*

*Development of Halogen Free Sustainable Polybenzoxazine Matrices and Composites for Flame… DOI: http://dx.doi.org/10.5772/intechopen.98470*

**Figure 7.** *Schematic synthesis of cardanol based benzoxazine monomers.*

result (**Table 3**) indicates the neat CBz and CBz/FRHA (1 wt%) exhibit no rating, while 5 and 10 wt% FRHA reinforced CBz composites possess the V-2 rating. 15 and 20 wt% FRHA reinforced CBz composites showed the V-1 rating. The thermal stability and flame resistant behaviour of PBZ-silica hybrid materials are higher than those of neat PBZ as well as traditional PBA-a, due to the hybridization of silica component through chemical interaction. Further, the incorporation of higher amount of silica into the PBZ matrix reduced the volatile decomposition. In addition, the silica components offer the additional heat capacity which restricts the materials against thermal degradation.

In addition, the same research group have also reported [73] the eco-friendly cardanol-based benzoxazines (C-ida and C-pyta) synthesized from hetero-cyclic core amines (**Figure 7**), such as pyridine core triamine (pyta) and tetraarylimidazole core diamines (ida). Further, bio-composites were also prepared using 3-glycidoxypropyltrimethoxysilane (GPTMS)-functionalized bio-silica (1, 3, 5, 7, and 10 wt%) obtained from rice husk. The thermal stability of developed PBz and its composites (**Figure 9**) is analyzed using TGA and the values obtained are presented in **Table 2**. As the biosilica weight percentage content increases, the degradation temperature and the value

18.80 wt% respectively. Reduction in values of smoke density are due to the higher amount of phosphazene moiety present in the polymers. The UL-94 test is a vertical burning test that decides the vertical burning properties of a polymers (**Table 1**). Neat polyhexacardanolcyclotriphosphazene (poly(CP)) and its reactive blends did not catch fire instantly, dissimilar to poly(C-trisapm). The flame resistant characteristics of cross-linked CP/C-trisapm blends was found to increase with increase in

Morphology of both the interior and exterior surface of the char residue attained after smoke density test was determined using SEM analysis and the images are presented in **Figure 6**. With an increasing the weight percentage of CP, the surface exterior morphology reformed to smoother surface from rippled, whereas surface interior indicated a porous honey-combed structures detached by tinny layers of border. The founding of cracks and bubbles are agreed to the stiff outer layer in case of EP comprising co-polymers, which might have rupture to discharge the interchange of heat and oxygen, thus hindering the propagation of fire as a measure of

To improve the flame and thermal properties, M. Alagar and his research team have reported [34, 36] hybrid approach using silica (nano-silica through in-situ sol– gel/or bio silica derived from rice-husk ash) as reinforcement in to cardanol based benzoxazine (**Figure 7**). Introduction of inorganic constituent 3-mercaptopropyltrimethoxysilane (MPTMS) in to cardanol/furfurylamine based benzoxazine (BZ-

tetraethoxysilane (TEOS). From thermal studies the percentage char yield indicates an increase of about 4.2 times (11.11 to 48.63%) and the values of LOI 21.94 to 36.95 for PBZ-C-F hybrids (**Table 2**) respectively. Functionalized bio-silica (FRHA) (0, 1, 5, 10, 15, and 20%) reinforced tri-substituted cardanol benzoxazine (CBz) (**Figure 8**), the value of LOI was increased with increase in weight percentage content of FRHA. Among the composites developed, 20 wt% FRHA reinforced CBz composites possesses the highest value of LOI of 36%, which is higher than that of values obtained for other composite samples. Also, the UL-94 vertical burning

*Digital images of cured samples of (a) poly(CP), (b) poly(CP3T1), (c) poly(CP1T3) before (a, b, c) and after*

*); SEM images of poly(CP), poly(CP3T1), poly(CP1T3) surfaces of residual char: exterior*

*), respectively [72]. (Copyright 2020. Reproduced with permission*

P weight percent content, which supports the results of values of LOI.

*Flame Retardant and Thermally Insulating Polymers*

C-F) via thiol-ene reaction followed by in-situ sol–gel techniques using

structural safety.

**Figure 6.**

*burning (a*<sup>0</sup>

**44**

*, b*<sup>0</sup> *, c*<sup>0</sup>

*from Frontiers in Chemistry).*

*(d, e, f) and interior (as inset) (d*<sup>0</sup>

*, e*<sup>0</sup> *, f*<sup>0</sup>
