## 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\({}^{+5}\))
Approximately 2 wt% of BP and ZnB were added separately to the PS composite along with 28 wt% of PDMPD. The THR values of the PS/PDMPD composites were dropped by 12% and 9% in the presence of BP and ZnB, respectively [97]. Furthermore, the replacement of PDMPD with low amounts of BP or ZnB did not affect the char formation trend in the sytrenic composite. The influence of the FR chemical structure on its mechanism of action was extensively studied by [PERSON] et al. (2008) [98]. According to the authors, PS with added HBCD and S-containing FR additives exhibited an enhanced degradation of the polymeric matrix, whereas PS with P-based structures such TPP or triphenylphosphine oxide (TPPO) had a minimal impact on the polymer degradation behaviour [98]. Interestingly, a combination of S with TPP resulted in much better fire-retarding performance owing to the synergism between S and P within the PS [98]. The fire-retarding performance of the ABS copolymer with different aryl cyclic phosphorous FRs revealed that the phosphonate compounds were more thermally stable and generated higher char yields as opposed to phosphinates [99]. There is a range of publications dedicated to synergistic formulations, for HIPS and ABS, containing red phosphorous combined with other additives: EG [100; 101; 102], Mg(OH)2 [103; 104; 105], and melamine pyrophosphate [106]. In summary, the additive modifications of styrene polymers with different P-containing FRs remain popular approaches to improving their fire performance. The analysis of the latest literature precedents shows that the majority of the P-containing additive FRs are incorporated into the polymer matrix, via the melt or the solution-blending techniques, at relatively high loadings. The evaluation  
Figure 16: Synthesis of PNTMS-PA and PNTMS-POA (a) and the chemical structure of PDMPD (b) [95; 97]. NITS: \(N\)-\(\beta\)-(aminoethyl)-\(\gamma\)-aminopropyltrimethoxysilane, PA: phosphoric acid, H3 PO4, POA: phosphorus acid, H3 PO3, PDMPD: poly(4,4'-diaminodiphenylmethanephenyldichlorophosphate). of mechanical and physical properties proved that these levels of FRs, when added to the polymers, adversely affect the key properties, in particular, the impact and/or tensile strengths, and the density of the foams. In recent years, the multi-component hybrid formulations produced from organophosphorus compounds in combination with either transition metals, or with other inorganic compounds, were used successfully to tackle this problem by lowering the amounts of FRs to 2-10 wt%. Many researchers found that a combination of the gaseous and the condensed phase mechanisms are responsible for the activity of FRs in styrene polymers after the additive modifications. In addition, intumescent fire-retardant actions of some compositions are also explored to passively protect some styrene systems. Although the additive modifications of styrene polymers became the simplest way to impart fire retardance, the dispersion and compatibility, recyclability, and toxicity of modified polymers are still questionable. In addition, developing the new P-containing additives based on inorganic/organic compounds is a cost-consuming task. In this context, the chemical incorporation of the FRs into the polymeric structure, which is discussed in the next section of the review, is believed to overcome these problems and provides some advantages over the additive modification approach.