## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Additive Modifications of Styrenic Polymers
#### 4.1.3 Phosphorus-Containing Intumescent Formulations (P\({}^{+5}\))  
The intumescent fire retardants (IFRs) have been used for a long time for fire protecting a variety of materials and surfaces. Typically, an IFR system consists of an acid source (APP, phosphoric acid, etc. ), a char-forming agent such as pentaerythnitol (PER), and a blowing agent (e.g., melamine (MEL), or polyamides) [72]. When the IFR is exposed to fire, the charring and acid agents trigger the formation of a charred layer followed by its growth and expansion with the aid of a blowing agent. The expanded carbonised layer generated on the surface of a polymeric substrate acts as an insulating barrier, not only preventing the heat transfer between the heat source and the surface, but also limiting the fuel transfer and the diffusion of oxygen into the material [72; 73]. [PERSON] and [PERSON] (2010) used APP and tripentaerythnitol (TPE) as an acid source and carbonisation agents, respectively, for developing an IFR system [74]. The PS/IFR composites were prepared by melt blending at 180 \({}^{\circ}\)C with 20 wt% of the IFR in the final material. A partial substitution of the IFR with the small amounts (2 wt%) of different nanofillers produced the PS/IFR composites containing montmorillonite (MMT) clay, MWCNTs, iron oxide (Fe\({}_{2}\)O\({}_{3}\)), nickel (Ni) catalyst, and zirconium (Zr) phosphate. Thermal studies demonstrated that the composites with MMT clay and Ni catalyst exhibited the highest efficiency: the values of the PHRR were reduced by almost 70% compared to the unmodified PS [74]. The CC results proved that the flammability of the PS/IFR composites was dependent on the nature of the nanofiller and was reduced in the following order: MMT clay \(>\) Ni catalyst \(>\) Fe\({}_{2}\)O\({}_{3}\)\(>\) MWCNTs \(>\) Zr phosphates. This pattern was in agreement with the results obtained from TGA, particularly in terms of the char yields generated by each composite [74]. The higher fire retardance of the PS composite with MMT clay and Ni catalyst can be explained by the formation of an aluminophosphate structure and the catalytic effect of Ni in the cross-linking and carbonisation processes. Similarly, APP was used as an acid source along with a novel carbonisation agent (CA) triazin-2-aminoethanol diethylenetriamine in the IFR added to the polymers in the amount of 30 wt% [75]. The flammability studies demonstrated that the composite with a 3:1 weight ratio of APP to CA exhibited better efficiency than the materials with 1:1 and 2:1 weight ratios of APP:CA. The CC results for the PS/APP/CA (APP:CA = 3:1) composite revealed that the time to ignition (TTI) was decreased from 52 s to 31 s measured for the untreated PS [75]. It is a common trend that the TTI values of fire-retarded polymers are lower than those of the unmodified versions, which is associated with the decomposition of the added FRs during the initial stages of combustion. The synergistic effect between APP and CA resulted in the improved fire retardance of the composite styrene materials, which was gauged from the increase in the T\({}_{\text{max}}\) by 16 \({}^{\circ}\)C; better char production (up to 17 wt%); the decrease by 938 kW/m\({}^{2}\) in the PHRR, and the lower total heat release (40 MJ/m\({}^{2}\)) compared to the neat PS [75]. In addition, the smoke production rate (SPR) was reduced from 0.254 m\({}^{2}\) s\({}^{-1}\) for PS to 0.008 m\({}^{2}\)-s\({}^{-1}\) for the PS/APP/CA composite, whilst the CO emission decreased by 88% [75]. These changes can be explained by the decomposition of APP and a formation of polyphosphoric acid, which reacts with the molecules of triazin-2-aminoethanol diethylenetriamine, eventually promoting the dehydration processes between OH groups present in the CA (Figure 10). The obtained P-O-C cross-links are transformed further, at relatively high temperatures, through the aromatisation, producing P-rich charring residues, thus having a positive impact on the fire retardance as well as on the thermal stability of the composites [75].