Reactive flame retardant, also known as structural flame retardant, is a kind of flame retardant that reacts in the process of polymerization or polycondensation and is combined with the main chain or branched chain of the polymer, which plays a flame retardant role. It overcomes the disadvantages of easy migration of additive flame retardant, inability to maintain flame retardant effect for a long time and damage the physical properties of foam. Its stability is good, not easy to disappear, the amount of small, small toxicity, the impact on the performance of the polymer is also small. Flame retardant elements or groups can be introduced by isocyanate or polyol, due to technical and cost reasons, the current less use of the method of introducing flame retardant elements in isocyanate. This kind of flame retardant polyol mainly includes halogen-free phosphorus-containing polyol Exolit OP550, phosphorus-containing reactive flame retardant An-tiblaze PR82, halogenated polyether polyol Ixol M125, organic phosphorus halogen compound reactive flame retardant FRT-4, tetrabromophthalate diol, tris (dipropylene glycol) phosphite, N,N-bis (2-hydroxyethyl) aminomethyl phosphonate diethyl ester, etc. Huang et al. prepared montmorillonite nanocomposite (c-MMT) by ion exchange method. The nitrogen-phosphorus structure containing quaternary ammonium salt was successfully inserted into the interlayer of nano-montmorillonite and added to polyurethane foam. The results of cone calorimeter measurement showed that the peak heat release rate (PHRR) of PU system with c-MMT (mass fraction 20%) was reduced by 25% compared with standard PU. Scanning electron microscopy shows that c-MMT contributes to the formation of carbon in the process of PU combustion and makes its distribution more uniform, which hinders the transfer of heat and air to the internal substrate, thus effectively playing the role of flame retardant. The BASF patent introduces a foaming system composed of halogenated polyols, brominated polyols, flame retardants, catalysts, surfactants, foaming agents and other components, and the flame spread index and smoke density of the prepared PUF products exceed the US flame retardant standard ASTME-84 and FM 4880.

Rotaru et al. synthesized a new polyether polyol with high nitrogen content and good thermal stability by Mannich reaction using diethanolamine, paraformaldehyde, cyanuric acid, propylene oxide and other raw materials, and prepared PUF foam from this Mannich polyol. The results show that due to the presence of isocyanuric ring in the PU foam system, the PUF foamed by this Mannich polyol has better mechanical strength, and has better thermal stability and higher flame retardant efficiency than the traditional aliphatic PU foam system.

Paciorek Sadowska J et al. synthesized a reactive boron-containing flame retardant polyol from N, N'-bis (methylene epoxy -2-hydroxyethyl) urea and boric acid derivatives. Compared with standard PUF, the compressive strength of PUF prepared from this boron-containing flame retardant polyol increased from 241.9 kPa to 398.7 kPa when the mass fraction of boron-containing polyol increased from 1% to 4%. The flame test shows that the PUF with boron-containing polyol can reach the self-extinguishing level, and its retention can reach up to 91.2%, while the retention of standard PUF is only 67.3%. The results show that the boron-containing flame retardant polyol is not only the polyol raw material which reacts with isocyanate, but also the boron and chlorine in the structure of PUF can play a role in flame retardant. Yanchuk prepared a series of vinyl diphosphate salts and applied them to PUF to improve the flame retardant properties of PUF. Experiments show that with the increase of vinyl diphosphate salt content, the flammability of PUF is significantly reduced, and the fire is extinguished. Biomass resources are abundant and renewable in nature, coupled with the introduction of various environmental protection laws and the improvement of people's awareness of energy conservation, biomass PUF has attracted people's attention. Melissa Heinen et al. used epoxidized soybean oil extracted from plants to react with phosphoric acid to prepare phosphorylated polyols, and then reacted with different proportions of phosphorylated polyols with glycerol and ethylene glycol polyester, using pentane as a foaming agent to obtain phosphorylated PUF. SEM, SDT-FTIR and flammability test results show that the flame retardancy of the prepared phosphorylated PUF is as good as that of commercial products, and the LOI of phosphorylated PUF can be even higher.