## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Additive Modifications of Styrenic Polymers
#### 4.1.5 Functionalisation of Inorganic FRs with Organophosphorus Compounds (P\({}^{+5}\))  
The additive modification of syrenic polymers with organophosphorus compounds can be achieved either by using them individually, or by the functionalisation, and/or in combination with other fire-retarding materials. For example, [PERSON] et al. (2007) used triethylphosphate (TEP, Figure 12a) and diethylphosphonate (DEEP, Figure 12b) through the additive incorporation into the PS systems [38]. The phosphorus content in these compounds was not higher than 3.5 wt%. The fire-retarding mechanism was reported to occur in the gaseous phase, regardless of whether the compound was phosphate (TEP) or phosphonate (DEEP) [38]. Furthermore, several organophosphorus compounds were used as additives in combinations with inorganics or nanomaterials. For instance, a nucleophilic substitution reaction of diphenylphosphinic chloride (DPP-Cl, Figure 12c) with an aminated multi-walled carbon nanotube (A-MWCNT) had led to the formation of a P-N-containing intermediate (DPPA-MWCNT) that was consequently mixed with PS via the melt blending at 190 \({}^{\circ}\)C [80]. The modification of PS with 1 wt% DPPA-MWCNT increased its fire retardance that was evident from the reduction in the toxic emissions such as smoke, CO, and CO\({}_{2}\). Meanwhile, the PHRR values dropped from 907 kW/m\({}^{2}\) for the  
Figure 12: Chemical structures of organophosphorus additive FRs [38, 80, 84]. for the PS/DPPA-MWCNT [80]. The enhanced fire retardance level was attributed to the barrier effect of carbon nanotubes (CNTs), which might provide sufficient time for the functionalised P-containing FR to trap the polymeric radicals, thereby inhibiting the further thermal degradation of PS and also leading to better char formation. Even though the CNT was functionalised with P and N groups, the synergistic effect of those two atoms was not clearly explained by the researchers [80]. In a similar way, the P-functionalised P-MWCNTs were prepared by treating the oxidised MWCNTs with phosphoric acid followed by the incorporation into PS via the solution blending [81]. The incorporation of 2 wt% of P-MWCNT into the polymer resulted in the 27% decrease of the total heat release (THR), while doubling the amount of char residue formed [81]. Graphene oxide (GO) is commonly used as the additive FR for styrene polymers [82; 50; 83]. The functionalisation of GO with organophosphorus compounds can boost the thermal properties of the polymer composites. For instance, [PERSON] et al. (2015) synthesised the organophosphorus oligomer, poly(methylphosphonylbis(hydroxymethyl)hypophosphate) (PMPPD, Figure 12d), for the preparation of the functionalised graphene oxide (FGO) [84]. The incorporation of 5 wt% of FGO into PS resulted in the reduction of the heat release rate by 38%, while the value of Tmax increased by 20 \({}^{\circ}\)C [84]. Along with better fire protection, the mechanical properties of the polymer such as its tensile and impact strengths were also improved due to the synergism between the PMPPD and FGO sheets. However, both the tensile and the impact strength of the PS/FGO composites were reduced when the loading of FGO grew from 0.1 wt% to 5 wt% [84]. Similarly, the functional modification of GO was carried out with the aid of DOPO to obtain the fire-retardant PS composite microspheres [85]. The gaseous phase free radical quenching of the functionalised DOPO and the condensed phase catalytic carbonisation effect of the GO sheets contributed to the overall enhancement of the fire safety of the polymer [85]. The Mannich reaction between the DOPO derivative and \(\gamma\)-aminopropyltriethoxysilane (KH550) resulted in the formation of the FR formulation, KDOPO, containing P, N, and Silicon (Si) atoms (Figure 13) [86]. The silanisation reaction between KDOPO and GO produced the functionalised GO, labelled as DGO, which was consequently applied onto the surface of the EPS through the simple surface-coating method.