## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Reactive Modifications of Styrenic Polymers
#### 4.2.3 Inorganic FRs Modified with Organophosphorus Compounds (P\({}^{+5}\))  
The reactive modification of syrenic polymers with the inorganic P-containing compounds remains rare, which is possibly due to the difficulties associated with the chemical methods of incorporating the FRs into the polymeric chains, i.e., a lack of polymerisable double bonds. Nevertheless, the covalent functionalisation of some inorganics with unsaturated organophosphorus compounds provided an opportunity to achieve this. The use of graphene oxide (GO) as a FR additive was already discussed in this review earlier [50; 82; 83]. Recently, [PERSON] et al. (2018) covalently functionalised GO with a novel organic P-containing compound, 2-((6-oxidodibenzo[c,e][1,2]oxaphosphinin-6-yl)methoxy)acryloxyethyl)chlorophosphate (PACP), the structure of which is shown in Figure 27b [123]. From 1 to 3 wt% of the FGO were incorporated into the PS chain via the bulk polymerisation method, using benzoyl peroxide as the initiator and heating the reaction mixture to 90 \({}^{\circ}\)C. The good dispersion of the FGO within the polymer and the strong interfacial bonds between the FGO and the polymer matrix resulted in the significant improvement of thermal behaviour and fire resistance [123]. The TG char residue of the copolymer with 3 wt% of reactive FR increased from 0.33 to 6.50 wt% under the inert atmosphere. At the same time, the heat release capacity (HRC) of the same copolymer decreased dramatically, from 984 J/(g:K) for the unmodified PS to 599 J/(g:K) for the copolymer [123]. These studies also demonstrated the combined effect of the gaseous and condensed phase activities mainly originating from the FGO moieties by the inhibition of combustion chain reactions and catalysing the char formation, respectively [123]. In another study, the condensation reaction between the carboxyl group on the surface of GO and the amino group of \(N\)-aminoethylpiperazine (AEPZ) resulted in the formation of AEPZ-GO [124]. Then, the AEPZ-GO was reacted with the phosphonate compound, 3,9-bis(chloro)-2,4,8,10-tetraoxa-3,9-diphosphapiro[5.5]undecane-3,9-dioxide, to prepare a hyper-branched FR. In this case, the FGO was prepared by grafting the hyper-branched FR onto the surface of the GO via the Michael addition reaction [124]. The PS-FGO composites were prepared by in situ polymerisation, starting with a very low amounts: 0.1 wt%, followed by 0.5 wt%, 1.0 wt%, and 2.0 wt% of the FGO. The MCC results showed that the PHRR values decreased from 840 W/g for the unmodified polymer to 514 W/g by the incorporation of 2 wt% of the FGO in the polystyrene chains. Meanwhile, the char yield, obtained from TGA under the air atmosphere, was increased from 0.5 wt% in the neat PS to 6.5 wt% in the PS-FGO [124]. In addition, other fire hazards were brought under control, for example by reducing the CO and CO\({}_{2}\) emissions. The enhancement in the fire retardance was attributed, partially, to the uniform dispersion of the FGO layers within the PS matrix. These layers were able to capture oxygen free radicals in the gaseous phase and inhibit the degradation of the polymer chains. The decomposition of the hyper-branched FR also promoted the char formation on the surface of the burning material, thereby reinforcing the barrier effect of the FGO sheets [124]. Another inorganic compound, \(\alpha\)-zirconium phosphate, \(\alpha\)-Zr(HPO\({}_{4}\))\({}_{2}\):H\({}_{2}\)O (labelled as \(\alpha\)-ZrP) was used along with the P-N-containing FR, AEPPA (Figure 27a) to prepare the copolymer by the bulk polymerisation [125]. The comparative MCC studies of different polymeric samples showed that the incorporation of 3 wt% of \(\alpha\)-ZrP and 10 wt% of AEPPA significantly reduced the HRC from 915 J/g:K for the unmodified polymer to 563 J/g:K for the copolymer, while the THR values dropped from 39.0 to 36.6 kJ/g. Similarly, the char residue from the St/AEPPA copolymer was almost doubled after the incorporation of \(\alpha\)-ZrP into the St/AEPPA copolymer.