**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

and poly(API-fa)- 2 was presented in **Figure 21**. **Figure 21a**, HRR maximums are perceived at 225°C and 591°C for poly(API-fa)-1 and poly(API-fa)-2, respectively. Also, the values of HRC are observed as 22.5 and 20.2 J/gK respectively. Furthermore, poly(API-fa)-1 indicates the THR value of 11.2 KJ/g, whereas poly(API-fa)-2 shows a moderately lower value of THR (9.4 KJ/g) (**Figure 21b**). These results support the very exceptional flame retardant behaviour of apigenin based

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

**7. Naringenin based benzoxazine for flame retardant applications**

51% for THR and HRC, respectively (**Table 5**).

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

*Synthesis of fully bio-based naringenin-benzoxazine monomer (NAR-fa).*

Kan Zhang et al., synthesized [80] a fully bio-based benzoxazine monomer (NAR-fa) using Naringenin, furfurylamine and paraformaldehyde (**Figure 22**). Thermal properties of the resulting co-polymeric thermosets were maintained or slightly enhanced, while those related to flame retardancy improved to about 38 and

**Figure 23** displays the graphic representation of the HRR as a function of temperature, where the maximum HRR is detected at 455°C. From **Figure 23**, it was calculated a HRC and THR of 31.9 J/gK and 6.6 KJ/g, respectively. The HRC value for poly(NAR-fa) is significantly lower than those reported for PBz's with high

*Microscale combustion calorimetric (MCC) analysis of poly(NAR-fa). (a) heat release rate, and (b) total heat release as a function of the temperature [80]. (Copyright 2020, reproduced permission from Royal Society of*

polybenzoxazines.

**Figure 22.**

**Figure 23.**

*Chemistry).*

**57**

**Figure 20.** *Synthesis of apigenin-based bis-benzoxazine (API-fa).*

furfurylamine as amine precursor both derived from bio-sources. The wellstructured intra-molecular hydrogen bond in API-fa is probable to be a stable latent heat formulation, which prolongs the shelf life of the Bz resin. Besides, the benzopyrone and furan rings with carbon–carbon double bonds in the API-fa are proficient of making further cross-linking networks, which are anticipated to considerably increase the mechanical, thermal and flame resistant properties of the resulting PBz thermosets. Remarkably, the thermal possessions of the API-fa based PBz achieved in this work are considerably higher than those of recently reported PBz's. The resulting polymerized PBz thermoset developed from API-fa indicates the high glass transition temperature of 376°C, a higher char residue of 66%, with an extremely lower heat release capacity of 20.2 J/gK (**Table 5**).

Thus, the LOI values obtained for poly(API-fa)-1 and poly(API-fa)-2 are as high as 42.7 and 43.9, respectively (**Table 5**). From the MCC analysis of poly(API-fa)-1

#### **Figure 21.**

*Microscale combustion calorimetric (MCC) analysis of poly(API-fa)-1 and poly(API-fa)-2. (a) heat release rate, and (b) total heat release as a function of the temperature [79]. (Copyright 2020, reproduced permission from Royal Society of Chemistry).*

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

and poly(API-fa)- 2 was presented in **Figure 21**. **Figure 21a**, HRR maximums are perceived at 225°C and 591°C for poly(API-fa)-1 and poly(API-fa)-2, respectively. Also, the values of HRC are observed as 22.5 and 20.2 J/gK respectively. Furthermore, poly(API-fa)-1 indicates the THR value of 11.2 KJ/g, whereas poly(API-fa)-2 shows a moderately lower value of THR (9.4 KJ/g) (**Figure 21b**). These results support the very exceptional flame retardant behaviour of apigenin based polybenzoxazines.
