## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Additive Modifications of Styrenic Polymers
#### 4.1.1 Ammonium Polyphosphate (P\({}^{+5}\))
A novel formulation containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and calixarene was synthesised by [PERSON] et al. (2018) as per the schemes shown in Figure 6[62]. Subsequently, it was combined with APP and PS by the melt blending at 170 \({}^{\circ}\)C [62]. The total fire retardant loading was fixed at 30 wt% in all the cases. The char formation under the nitrogen atmosphere was increased dramatically from 0.79 wt% for the neat PS to 20.00 wt% for the composite with 15 wt% FR and 15 wt% APP. The fire-retardant effect was also clearly observed from the increased limiting oxygen index (LOI) values: 17 vol% for PS and 23 vol% for the modified PS composites [62]. The char layers formed in the condensed phase seemed not only to prevent the release of combustible gases but also to protect the interior surface of the polymer from the detrimental impact of heat. However, it is important to note here that none of these composites passed the vertical UL-94 burning test, even though during the UL-94 horizontal test all the composites exhibited a lower flame spread and no melt dripping compared to the unmodified counterpart. The results of all the tests carried out may indicate the improved thermo-oxidative resistance and fire retardance of the modified PS, which was due to the catalysing effects of APP on the char formation during combustion along with the synergistic effects between the FR and APP [62]. The APP was combined with clay to develop fire-retarded PS/Nylon-6 blends [63]. The dispersion and the compatibility of FRs with the polymer matrix are the important factors that affect the properties and fire-retarding efficiency of the final products. Ammonium polyphosphate is a hydrophilic inorganic material; hence, the dispersion of any hydrophobic FR with APP would affect the preparation, uniformity, and properties of the obtained composites. To overcome this difficulty, [PERSON] et al. (2019) proposed the surface modification of APP that would improve its dispersion and compatibility with hydrophobic carboxylic-functionalised multi-walled carbon nanotubes (COOH-MWCNTs) [64]. The surface of APP was treated with a silane-coupling agent, 3-aminopropyl triethoxysilane (KH 550), to obtain the modified APP (k-APP). The HIPS nanocomposites were prepared by the melt blending with k-APP and COOH-MWCNTs. The scanning electron microscopy (SEM) micrographs revealed an enhanced dispersion of k-APP in the HIPS matrix that also seemed to prevent the agglomerations of the suspended FR particles compared with the unmodified APP. The uniform distribution of FRs within the polymer matrix was directly reflected in better mechanical properties of the polymers. Indeed, the HIPS composites with 15 wt% k-APP were characterised by the 5.0-7.0% increase in flexural modulus and tensile strength, when compared with those containing the unmodified APP. As opposed to the neat HIPS, the synergistic effect of k-APP and COOH-MWCNTs improved the thermal stability and fire retardance of the composites by introducing 12 wt% k-APP/3 wt% COOH-MWCNTs. This was evident from the increase in the values of T\({}_{i}\) by 13 \({}^{\circ}\)C  
Figure 6: Synthesis of a novel FR based on calixarene and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) [62]. and T\({}_{\rm max}\) by 16 \({}^{\circ}\)C, from the 681 kW/m\({}^{2}\) drop in the PHRR, and from the 0.30 kg/kg reduction in CO yield. Similarly, nano-sized oxides such as alumina and silica, treated with octylsilane in combination with APP, were used to impart the fire resistance to polymethylmethacrylate (PMMA) and PS [65]. This comparative study revealed that hydrophobic nano-oxides with APP have a more positive impact on the thermal stability and fire retardance to PS than the hydrophilic oxides mixed with APP [65].