# Advancements in Flame-Retardant Systems for Rigid Polyurethane Foam
## 2 Reactive-Type Flame Retardants
### Incorporation of Phosphorus-Containing Groups  
Phosphorous-containing flame-retardant polyols are a type of reactive flame retardants, which can be used as a substitute for conventional polyols in the preparation of RPUF to enhance its flame-retardant properties [41; 49]. The effectiveness of phosphorus-based flame retardants in minimizing the flammability of polyurethane foam has been well established, leading to its widespread adoption in industries ranging from construction and transportation to electronics [32; 50]. Polyols containing phosphorus-based flame retardants possess multiple hydroxyl groups that can actively engage in the curing reaction of polyurethane foam. The incorporation of phosphorus-containing flame retardants in polyols enables the formation of chars and reduces the release of flammable gases during combustion [51]. One of the studies in this area focused on the application of biobased flame-retardant polyols. In their study, [PERSON] et al. [52] explored the fire safety of a polyol sourced from limonene, which was chemically modified with phenyl phosphonic acid. According to their findings, adding 1.5 wt.% of phosphorus through chemical modification could decrease the self-extinguishing time from 81 s to 11.2 s. [PERSON] et al. [53] employed the approach illustrated in Figure 2a to produce castor oil phosphate flame-retardant polyol (COFPL), a flame-retardant polyol derived by incorporating a phosphate group into the polyol. The process involves the preparation of glycerolized castor oil (GCO) and the epoxidation of GCO, which is ultimately converted to COFPL through a reaction with diethyl phosphate. [PERSON] et al. [54] developed two biobased flame-retardant polyols (CODEOA and CPPA) from a modified vegetable oil. By incorporating epoxidized polyols (BIO\({}_{2}\)) and derived carbon materials into the RPUF matrix, the heat release rate (HRR), total heat release (THR), and total smoke production (TSP) can be significantly reduced. Currently, green materials such as plant oil, tung oil, and lignin have been chosen as the primary synthetic resources. However, the flame-retardant performance of biobased polyols is hindered by their long-chain structure, resulting in the low con  
Figure 2: Structure of different flame retardants: (**a**) COFPL and (**b**) MADP. elements (phosphorus, nitrogen, silicon, etc.). [PERSON] et al. [55] synthesized tung-oil-based polyols through the ring-opening reaction of epoxidized tung oil and silane-coupling agents. Although the limiting oxygen index of the biobased RPUF prepared from these polyols increased from 19.0 vol.% to 22.6 vol.%, the improvement in the flame retardancy was not significant. This can be attributed to the low phosphorus content in long-chain biobased polyols. In another approach, flame-retardant polyols with short chains are utilized for the synthesis and application of RPUF due to their elevated phosphorus content. [PERSON] et al. [56] developed a hard-segment flame retardant (HSFR) to enhance the fire resistance of RPUF. By incorporating 13.8 wt.% of THPO, the resulting system was able to achieve a UL-94 V-0 rating, with an LOI ranging from 17.0 to 25.5%. During combustion, the HSFR can produce PO\(\bullet\) and PO\({}_{2}\bullet\) radicals in the vapor phase, which then react with flammable free radicals and impede segment decomposition. In the study conducted by [PERSON] et al. [40], a flame-retardant polyol with a substantial phosphorus content was synthesized through a dehydrochlorination reaction. [PERSON] et al. [57] synthesized a flame-retardant polyol with a high phosphorus content by performing a dehydrochlorination reaction. The addition of expandable graphite (EG) to the RPUF/BHPP system significantly enhanced the flame-retardant characteristics of the RPUF composites, achieving a high LOI value of 30.0%. To achieve highly flame-retardant RPUF, [PERSON] et al. [58] successfully synthesized a short-chain flame-retardant polyol.