# Advancements in Flame-Retardant Systems for Rigid Polyurethane Foam
## 3 Additive-Type Flame Retardants
### Addition of Nanoclay and Other Nanoparticles  
Adding nanoclay and other nanoparticles to both flexible and rigid polyurethane foam is a promising strategy for decreasing the production of smoke particles and toxic gases during combustion [88; 89; 90]. Clay nanosheets are frequently used as nanoparticle fillers in polymer nanocomposites because of their low cost, widespread availability, and flexibility. Incorporating nanoclay (a two-dimensional nanoparticle) into rigid polyurethane foam can form a physical barrier that obstructs gas diffusion and heat transfer, resulting in reduced smoke production and increased thermal stability [34; 35; 91]. Furthermore, the platelet structure of nanoclay is attributed to the significant enhancement in the mechanical properties of RPUF, such as its strength, stiffness, and toughness [92]. [PERSON] et al. [93] conducted a study on the flame retardancy and thermal stability of clay nanosheets, where halloysite nanotubes (HNTs) were synthesized and added to biobased RPUF. The study  
Figure 6: Preparation and expansion processes of expandable graphite [85] (Copyright 2019). Reproduced with permission from Elsevier Science Ltd.  
showed that increasing the HNT load from 1 wt.% to 5 wt.% led to a corresponding increase in the thermal stability of the RPUF. Moreover, RPUF blended with 5 wt.% HNTs exhibited the highest residual char of 18.1% at 600 \({}^{\circ}\)C compared to pristine RPUF with only 7.6%, indicating that the addition of HNTs significantly improved the char-forming ability of RPUF.  
Similarly, nanoparticles, such as cuprous oxide (Cu2O), titanium dioxide (TiO2), nickel oxide (NiO), and silica (SiO2), have also been investigated for their potential in reducing smoke production and toxic gas emissions during rigid polyurethane foam's combustion [94; 95; 96]. These nanoparticles act as flame retardants by catalyzing the formation of integral and compact chars, reducing the production of volatile compounds, and increasing the thermal stability of RPUF. A comparative study was conducted by [PERSON] et al. [97] to investigate highly efficient catalysts for reducing toxic gas generation in RPUF nanocomposites at various temperatures. The primary objective of the study was to determine the most effective catalyst for minimizing the release of harmful gases during the combustion of RPUF. The findings indicated that both NiO and NiMoO4 were efficient catalysts in reducing toxic gas emissions. Furthermore, a comprehensive quantitative analysis of the gaseous degradation products, such as HCN, NOx, and CO, was performed for various polyurethane composite materials, including PU/Cu2O, PU/NiO, PU/MoO3, PU/CuMoO4, and PU/NiMoO4. This analysis was conducted using a tubular furnace method at both 650 \({}^{\circ}\)C and 850 \({}^{\circ}\)C.  
[PERSON] et al. [98] utilized a straightforward wet chemical method to synthesize cuprous oxide (Cu2O) crystals of varying sizes and investigated their impact on the combustion performance of RPUF. According to the study, the addition of 2 wt.% Cu2O to the RPUF matrix resulted in a notable decrease in the peak rate of carbon monoxide (CO) production and total smoke production. Furthermore, as illustrated in Figure 7, the reduction of Cu2+-Cu+-Cu+ by degraded gases and the oxidation of Cu0-Cu+-Cu2+ by oxygen were involved in the conversion of CO to CO2 and the complete combustion of RPUF.  
Figure 7: Schematic illustration for the mechanism of catalytic oxidation and catalytic carbonization of pristine RPUF (**a**), RPUF/Cu2O-11\(\upmu\)m (**b**), RPUF/Cu2O-100 nm (**c**) and RPUF/Cu2O-100 nm (**d**) [98] (Copyright 2021). Reproduced with permission from Elsevier Science Ltd.