## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Additive Modifications of Styrenic Polymers
#### 4.1.2 Aluminium Hypophosphate (P\({}^{+1}\))  
Aluminium hypophosphite (AP) belongs to another category of P-containing inorganic fire retardants. AP has been widely used in various polymers because of its excellent fire-retarding performance, cost effectiveness, and environmental friendliness. Initially, AP was incorporated as a single additive to PS composites by melt blending [66]. The V-0 rating in the UL-94 test was achieved in the composites containing high amounts of AP, between 25 and 30 wt%. As a result of this modification, the char residues were increased dramatically, from 0.29 wt% for the neat PS to 16.00 wt% and to 24.00 wt% for the composites with 25 wt% and 30 wt% of AP, respectively. Notably, the char residue formed in the air was approximately 4 wt% higher than that formed under the nitrogen atmosphere for all the PS/AP composites, which was attributed to the formation of oxidised products of AP during thermal degradation of the polymeric materials [66]. The studies of fire-retarding mechanisms in PS/AP composites revealed the combined gaseous phase and condensed phase activity of AP during combustion [66]. The investigation of the decomposition processes of PS/AP composites indicated that during combustion, AP decomposed prior to PS, and the pyrolysis products such as phosphine (PH\({}_{3}\)), hypophosphorus acid (H\({}_{3}\)PO\({}_{2}\)), and red phosphorus (P\({}_{4}\)) were released. Among these products, PH\({}_{3}\) can be further oxidised to generate phosphoric acid (H\({}_{3}\)PO\({}_{4}\)), which can promote charring (Figure 7). The formed P-O cross-linked structure acts as an insulating layer, which can also cover the polymer surface and protect it from fire. Meanwhile, in the gaseous phase, phosphinate ions (PO\({}_{2}\)\({}^{-}\)) formed during combustion seemed to prevent the flame growth and further development of the fire front [66]. The latest reports describe the processes when AP was added along with other compounds, with the purpose of lowering the loadings of the FR and to enhance the fire-retarding efficiency [67; 68; 69; 70; 71]. The study conducted by [PERSON] et al. (2018) described the incorporation of expandable graphite (EG) in combination with AP for preparing fire-retarded syrenic composites, with the AP/EG content being fixed at 15 wt% [67]. The UL-94 vertical burning tests revealed that that the polymers containing  
Figure 7: Fire-retarding mechanisms occurring within PS/AP (aluminium hypophosphite) composites [66]. either 15 wt% of AP or 15 wt% of EG failed them, whereas the PS with 10 wt% of EG and 5 wt% of AP (PS/10 EG/5 AP) passed these tests, achieving the V-0 rating. In addition, this modification resulted in the reduction of the PHRR, measured through the CC, from 730 kW/m\({}^{2}\) for PS to 163 kW/m\({}^{2}\) for the PS/EG/AP composite. The extent of char formation also increased by up to 12 wt% for the modified styrene polymers [67]. The enhanced fire retardance of the composites can be attributed to both the condensed phase and the gaseous phase activities of EG and AP. In the condensed phase, EG expanded to form a 'worm'-like char residue, which turned out to be not compact enough to protect the PS surface from combustion. In comparison, the pyrophosph generated from the decomposition of AP covered the polymer surface, resulting in a more compact char structure, which protected the inner polymer matrix from further decomposition. Meanwhile, in the gaseous phase, AP tends to form several P-containing substances that, in turn, were able to generate phosphorus radicals. These radicals captured highly reactive free radicals formed during the polymer combustion and thus effectively inhibited fire growth [67]. In another work, a coating based on a mixture of AP with thermoplastic phenolic resin (PF) was prepared for the protection of the EPS [68]. The performance of EPS/PF/AP was compared with the one of the EPS containing PF and EG (EPS/PF/EG). It was reported that PF interacted with AP through the hydrogen bonding, as shown in Figure 8[68].