## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Additive Modifications of Styrenic Polymers
#### 4.1.1 Ammonium Polyphosphate (P\({}^{+5}\))  
Ammonium polyphosphate (APP) has gained great attention in developing FRs because of the high P- and N- contents. It has been extensively used for different polymers such as epoxy resins [53] and polyamides [54]. Additionally, APP is one of the main components of the intumescent fire-retarding systems, where it functions as an acid catalyst [55]. However, the individual use of APP as a FR for PS was not found to be effective, which is possibly due to the change in the morphology of char [56; 57]. Hence, several attempts have been made to extend the utilisation of APP in PS, specifically by combining it with other agents. For example, [PERSON] et al. (2016) utilised graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\)) as an additive, along with APP, for enhancing the fire-retarding properties of PS [47]. The g-C\({}_{3}\)N\({}_{4}\) has good chemical and thermal stability. The primary (-NH\({}_{2}\)) and secondary (-NH) amino-groups present in graphitic carbon nitride serve as the reactive sites either for hydrogen bonding or for reacting with other functional groups. In this study, g-C\({}_{3}\)N\({}_{4}\) was treated with APP to prepare a FR formulation denoted as carbon nitride ammonium polyphosphate (CNAPP) by the solvent mixing method, followed by mixing with PS at a temperature of 180 \({}^{\circ}\)C [47]. Thermogravimetric analysis (TGA) proved that the thermal stability of the PS composite was enhanced by the addition of 20 wt% CNAPP. The maximum decomposition temperature (T\({}_{\text{max}}\)) of the PS/CNAPP composite was increased by 40 \({}^{\circ}\)C; meanwhile, the peak heat release rate (PHRR), measured through cone calorimetry (CC), was dropped by 65% compared to the neat PS [47]. The real-time infrared (RTIR) spectra revealed the presence of bands characteristic for P-O-C and P-N-C linkages, which were formed during the thermal oxidative degradation of the prepared composites. These cross-linked structures facilitated the formation of a char residue at higher temperatures (750 \({}^{\circ}\)C). Besides, g-C\({}_{3}\)N\({}_{4}\), when present in the PS composites, accelerated the char formation, which resembled a vermiform structure that exhibited a \"fortuous effect\". These char layers protected the PS polymer from further combustion and decomposition [47]. Recently, the syntheses of layered double hydroxides (LDHs) were carried out to obtain FRs suitable for the application in PS [58; 59; 60]. The LDH generally consists of positively charged metal hydroxide layers with interlayers of anions to maintain the overall charge neutrality [61]. [PERSON] et al. (2008) prepared the fire-retarded PS by combining APP with a magnesium (Mg) and aluminium (Al)-based layered inorganic/organic hybrid, Mg-Al-undecenoate LDH (MAU-LDH) [56]. The purpose of using MAU-LDH as an additive FR along with APP is two-fold. Firstly, the combination of APP and LDH improves the dispersion of these additives within the polymer matrix, whilst the synergistic chemical interactions between APP and LDH allow reducing the amounts of FRs required to obtain a sufficient level of fire retardance. Secondly, the MAU-LDH works in the same way as the metal hydroxides, Mg(OH)\({}_{2}\) and Al(OH)\({}_{3}\), by slowing down the depolymerisation processes of PS, while at the same time forming water through dehydroxylation reactions [56]. This study demonstrated that the onset of thermal degradation (T\({}_{\text{i}}\)) for PS was shifted from 399 to 379 \({}^{\circ}\)C by the incorporation of 10 wt% APP alone, whereas the values of T\({}_{\text{i}}\) were increased by nearly 10 \({}^{\circ}\)C as a response to the addition of 5 wt% MAU-LDH with 5 wt% APP. This result indicated the synergistic interactions of APP and MAU-LDH enhancing the thermal stabilisation of PS. However, the authors did not observe any significant increase in the char formation for PS-MAU/APP (5/5 wt%) composites [56].