## 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 modified foam exhibited better fire-retarding performance: the LOI values were up by 29%, and the V-0 rating (UL-94 test) was achieved by the addition of 20 g of the DGO to the EPS. In this study, it was shown that after the addition of DGO, the values of thermal conductivity were increased by 6.8%, although this is not sufficient to alter the thermal insulation property of the EPS [86]. The flammability studies revealed that each element within the FR composition, i.e., P, N, and Si, has a well-defined role to play in the fire-retarding mechanism. The P-containing fragment of the FR structure functioned in both the gaseous and the condensed phases, and the Si-containing group generated a positive silica layer on the polymer surface to protect the polymer matrix, thereby boosting the condensed phase mechanism. Meanwhile, the N-containing part in the structure of DGO supported the gaseous phase mode of action through the release of inert gases, diluting the combustible products and reducing the heat from the fire [86]. Figure 13: A synthetic route for the preparation of the KDOPO FR formulation [86]. KDOPO: Mannich reaction between the 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivative and \(\gamma\)-aminopropyltriethoxysilane (KH550). The P-containing compounds such as triphenylphosphate (TPP) (Figure 14a) in the amount of 3 wt% and hexaphenoxycyclotriphosphazene (HPCTP) (Figure 14b), at 3 wt%, were used in combination with 25 wt% of EG and 25 wt% of melamine phosphate (MP) to prepare the foams by a supercritical carbon dioxide foaming [87]. As a result of this modification, the thermal stability and fire retardance of PS foams increased in the presence of TPP or HPCTP within the composition of the formulation. As for the thermal conductivity of the foam, its value was reduced from 0.0435 W/m\(\cdot\)K in the neat PS to 0.0372 W/m\(\cdot\)K and 0.0363 W/m\(\cdot\)K in the samples of PS foam with TPP and HPCTP, respectively [87]. The fire-retarding mechanism revealed that the highly viscous phosphate-based products formed during the degradation of MP were able to cover the surface of the EG, which further prevented its oxidation and generated a thick char layer. At the same time, TPP and HPCTP acted as synergists, thus enhancing the fire retardance of the modified styrene material. The thermal decomposition of these compounds generated different P-centred radicals, which then quenched down other free radicals, thus inhibiting the combustion process [87]. [PERSON] et al. (2003) evaluated a series of phosphate compounds to identify the best fire-retardant solution for the PS/nanoclay composite [88]. From the analysis, the phosphorus compounds such as tricosylphosphate (TCP, Figure 14c), trixylylphosphate (TXP, Figure 14d), and resorcinol diphosphate (RDP, Figure 14e) were determined as the suitable fire retardants [88]. The PS nanocomposites with clay (1-10%) and phosphates (5-25%) were prepared by bulk polymerisation to achieve the fire retardance through synergy between these conventional phosphorus FRs and nano-sized additives [88]. In another study, 55% and 75% of diphenyl-4-vinylphenylphosphate (DPVPP) (Figure 14f) were used to modify clay that was then used as the additive FR for PS [89]. Even though the flammability of PS was decreased by this type of modification, high levels of FR loadings negatively affected the mechanical and other useful properties of PS [89]. As mentioned earlier, the effective strategy to improve the fire retardance of styrene polymers adopted by many researchers is to employ the P-containing formulations, which act in both the gaseous phase and in the condensed phase. This improvement is often achieved through the specific interactions of some elements such as phosphorous-nitrogen (P-N), phosphorous-silicon (P-Si), phosphorous-nitrogen-silicon (P-N-Si), and phosphorous-nitrogen-sulphur (P-N-S). The interactions  
Figure 14: Chemical structures of different P-containing additive FRs [87; 88; 89].