# Advancements in Flame-Retardant Systems for Rigid Polyurethane Foam
## 4 Flame-Retardant Coatings  
Flame-retardant coatings are a popular method to enhance the fire resistance of PUF. These coatings can be applied to the surface of PUF to create a protective layer that slows down the spread of fire and reduces the amount of heat and smoke released during combustion [102; 103]. The coatings are typically made up of flame-retardant additives, such as aerogels, alumina trihydrate, metal hydroxides, and other inorganic materials that have a low flammability and good thermal stability. These additives work by releasing water vapor when exposed to heat, which helps cool down the surface of the foam and prevent it from igniting. In addition to enhancing the fire resistance of RPUF, flame-retardant coatings can also improve other properties, such as mechanical strength, chemical resistance, and UV stability. One example of a flame-retardant coating is the intumescent coating, which can form a char layer when exposed to fire, leading to reduced heat transfer and flame spread. [PERSON] et al. [104] conducted a study in which they developed an intumescent coating by combining a silicone resin (poly-DDPM) and expandable graphite (EG). The resulting poly-DDPM/EG coating tightly adhered to the surface of RPUF and exhibited excellent flame retardancy. Additionally, the compressive strength of the coated RPUF increased significantly, up to 10%. As shown in Figure 8, [PERSON] et al. [105] utilized UV-curable intumescent coatings to treat RPUF. By employing a spray-coating method, they applied a conformal IFR/MXene coating to the foam, aiming to enhance its fire safety. In addition to intumescent coatings, other types of flame-retardant coatings have also been investigated for their effectiveness in enhancing the flame retardancy of RPUF. For example, a study by [PERSON] et al. [45] reported on the development of a flame-retardant coating using alginate/clay aerogel. The coating was shown to reduce the flame spread rate, the HRR, and the THR of the RPUF during combustion as well as inhibit smoke production. The study showed that the facile and inexpensive posttreatment is a promising approach to improve the thermal stability and flame retardancy of RPUF. As depicted in Figure 9, motivated by the interfacial mechanical interlocking and hydrogen-bonding mechanisms observed in tree frogs and snails, [PERSON] et al. [2] successfully produced bioinspired flame-retardant poly(VS-co-HEA) coatings through a free-radical copolymerization process (refer to Figure 9). The thoughtfully engineered microphase-separated micro/nanostructure granted these coatings robust interfacial adhesion to the matrix [106]. The resulting RPUF exhibited notable fire-retardant properties, including a UL-94 V-0 rating and an LOI of 35.5%. Additionally, the treated foam showcased significant reductions in the peak heat release rate (PHRR) by 87.0% and total smoke release (TSR) by 71.0% compared to the untreated foam. Figure 8: (**a**) Schematic illustration of the fabrication procedure of IFR/MXene-coated RPUF and (**b**) HRR, (**c**) THR, (**d**) SPR, and (**e**) TSP for RPUF and coated RPUF [105] (Copyright 2019). Reproduced with permission from Elsevier Science Ltd.  
Figure 9: Preparation of RPUF with poly(VS-co-HEA) coatings [2] (Copyright 2021). Reproduced with permission from American Chemical Society. (**a**) Synthesis of bioinspired flame retardantpoly(VS-co-HEA) coatings and (**b**) a typical phase-separated micro/nanostructure of poly(VS50-co-HEA50). (**c**) Schematic illustration for the preparation process of flame-retardant rigid PU foam (FRPU). (**d**) The chart illustrating the adhesion or shear strength tests for PU foam. (**e**) Digital image of poly(VS-co-HEA) coatings against PU foam after shear tests, during which bulk PU foam broke before interfaces. (**f**) Shear strength of poly(VS-co-HEA) coatings against different substrates in comparison to some previous and commercial adhesives. (**g**) The homemade setup for determining the flammability of PU foam.