**4. Guaiacol based polybenzoxazine for flame resistant applications**

Kan Zhang research group have developed [78] the fully bio-based tri-furan functional bis-benzoxazine resin (**Figure 16**) and was synthesized using furfural, guaiacol, furfurylamine and paraformaldehyde via a two-step reaction approach. In the first step, bio-based bis-phenol containing furan ring was synthesized using furfural and guaiacol via base mediated condensation reaction. Then the bio-based sustainable tri-furan functional bis-benzoxazine (FBP-fa) was synthesized using bio-based bis-phenol, furfurylamine and paraformaldehyde through greener approach. The flammability behaviour of poly(GU-fa) and poly(FBP-fa) was assessed using LOI. As a result, poly(GU-fa) and poly(FBP-fa) show LOI values of 39.9 and 42.3 respectively. Both PBz's possess better LOI values in the selfextinguishing region (LOI > 28). Also MCC analysis was utilized further to assess the flammability performance of poly(GU-fa) and poly(FBP-fa). From the MCC characterization of poly(GU-fa) and poly (FBP-fa) display HRC values of 79.6 and 30.4 J/gK, respectively (**Table 5**).


#### **Table 5.** *Formulations and flame retardant properties of bio-based polybenzoxazines.*

In addition, the same group have further reported [75] the high thermal and flame resistant PBz-silica hybrid materials were synthesized using eugenol and furfurylamine via sol–gel-techniques (**Figure 14**). An inorganic component of TEOS was introduced into eugenol- benzoxazine (BZ–E–F) with the help of MPTMS as a coupling agent viz. thiol-ene click approach. The thermal studies indicate that the char yield increases to 67.54 from 41.32 (neat PBz) and LOI increased to 44.52 for PBZ–E–F silica hybrid from 34.03 for neat PBz (**Table 2**). Bimlesh Lochab research group have reported [76] the synthesis of sustainable biobased benzoxazine monomers (**Figure 15**) using isomeric phenols, eugenol (E) and iso-eugenol (IE), and furfurylamine (fa) to form E-fa and IE-fa monomer, respectively. The monomers differ in the position of the double bond in the p-substituted propylene unit forming non-conjugated Vs. conjugated alkylene chain with the benzene ring containing benzoxazine in E-fa and IE-fa respectively. In comparison to other bio-based PBz's, both PE-fa and PIE-fa exhibited the higher thermal stability and high flame resistant properties. The char residue values of PE-fa and PIE-fa are 52 and 60 respectively and the LOI values observed are 38.3 and 41.5 respectively (**Table 2**).

*Schematic representation for the preparation of renewable PBZ–E–F– silica hybrids [75].*

*Flame Retardant and Thermally Insulating Polymers*

*Synthesis of renewable bio-based isomeric benzoxazine E-fa and IE-fa.*

**Figure 14.**

**Figure 15.**

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**Figure 17.** *Synthesis of bisguaiacol-F (BGF) and BGF based benzoxazine monomers.*

Further, poly(GU-fa) infers the THR value of 6.5 KJ/g, whereas poly(FBP-fa) displays a fairly lower THR value of 5.8 KJ/g. The data of flame related properties of both PBz's are presented in **Table 5**. Unexpectedly, the HRC value of developed poly(FBP-fa) gives significantly lower value than that of earlier reported polymers. Furthermore, poly(FBP-fa) displays very lower flammability characteristics than that of many other reported PBz's. It is well-known that the HRC value less than 300 J/gK can be observed as self-extinguishing while, whereas the HRC values less than 100 J/gK are designated as non-ignitable materials.

(**Figure 18**) have been produced using resveratrol, different amines (aniline, 4 chloroaniline, 3-aminophenylacetylene, and furfurylamine) and paraformaldehyde. The thermal and flammability performances of the developed PBz's are studied using TGA and MCC respectively. Among the benzoxazine studied, resveratrolbased, tri-functional benzoxazines containing acetylene possesses the highest thermal stability with a value of Tg over the 350°C, 10% weight loss at 465°C, and char yield of 74%. Further, the developed polybenzoxazine infers the extremely low heat

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

MCC analysis was performed to analyse the quantitative features of flame resistance of PBz's. Usually, the extreme value of specific HRR can be used to assess the HRC, which is considered as unique best single interpreters for the flame resistance of materials. Here, poly(RES-a), poly(RES-ch), poly(RES-ac) (**Figure 19**), and poly (RES-fa) were analysed at the heating rate of 1°C/s over the temperature range 100 750°C. As illustrated in MCC results (**Table 5**) of poly(RES-a), poly(RESch), poly(RES-ac) and poly(RES-fa) indicate HRC values of 79.7, 73.4, 30.7, and 54 J/gK, respectively. Moreover poly(RES-a), poly(RES-ch), and poly(RES-fa) exhibit THR values of 15.9, 14.2, and 9.3 KJ/g, while poly(RES-ac) shows a lower THR value of 6.0 KJ/g. The data obtained from thermal and flame resistant

properties for PBz's are presented in **Table 5**. The value of HRC for poly(RES-ac) is significantly lesser than those of other polymers. Moreover, the HRC value of poly (RES-ac) is even lower than that of the thermosets from ortho-amide and orthoimide benzoxazines, which exhibited one of the lowest HRC values of all polymers. Obviously, the PBz formed from the ring opening polymerization along with the cyclo trimerization of acetylene results in a considerable HRC reduction. Thus, the newly developed resveratrol-based tri-benzoxazine monomer containing acetylene group has greatest prospective applications as anti-flammable and fire resistant

**6. Apigenin based polybenzoxazine for flame retardant applications**

Kan Zhang research group reported [79] the synthesis of a new fully bio-based bis-benzoxazine (API-fa) (**Figure 20**) using apigenin as phenolic precursor and

release capacity of 30.7 J/gK and total heat release value of 6.0 kJ/g.

*Schematic representation of proposed thermal behaviors of RES-ac.*

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

matrix for advanced composites.

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**Figure 19.**

M. Sarojadevi research group have also synthesized [51] the BGF and BGF-based benzoxazines (**Figure 17**) viz., bis[(3-(furan-2-yl) methyl)-3,4-dihydro-2H benzoxazin-6-yl] methane and bis[(3-octadecyl)-3,4-dihydro-2H benzoxazin-6-yl] methane as an substitute to BPA. The LOI values (**Table 2**) of the BGF-FPbz and BGF-SPbz was found to be 38.5 and 32.8 respectively indicating the high flame retardancy. As expected, the developed PBz's with these LOI values are greater than 26 endorsing outstanding flame resistant property.
