order to achieve a V-0 rating.
2.6 Aliphatic Phosphates and Phosphonates
Polyurethane (PU) foams encompass a wide range of foamed materials with very different properties starting from low density open cell flexible and rigid foams all the way to high density isocyanurate closed cell foams. From the point of view of response to flame, PU foams are considered to be thermally thick materials. This means that the heat applied to the foam doesn’t dissipate deeply but stays in the surface layer. The surface reaches a high temperature quickly and therefore PU foams are easy to ignite. The ignition of the rigid foams is an interesting phenomenon because the flame flashes over the surface and then quickly retreats. If the heat flux to the surface is not high enough the flame can extinguish. The foam may reignite again if the heating is continued. Since the rigid foam is more densely cross-linked compared to the flexible foam, it doesn’t melt away but undergoes charring. In such a scenario the best strategy to flame retard foam is to leverage both the gas phase and the condensed phase modes of action. This is achieved by combining phosphate ester flame retardants and reactive bromine-based flame retardants. Polyisocyanurate foams (PIR) are made with a significant 2.0-3.5 times excess of isocyanate over polyol. An excess of isocyanate forms an isocyanurate cross-linked network rich in nitrogen which is thermally more stable than the urethane groups. PIR foam is intrinsically more flame retardant than rigid spray PU foams and typically does not require help with brominated flame retardants.
Tris(isopropyl-2-chloro)phosphate (TCPP) is the largest commercially produced phosphorus flame retardant, but it is out of the scope of this chapter because it contains chlorine. Dimethyl methylphosphonate (DMMP, Formula 2.5(a)) for many years was used in rigid PU foams [216], but it was effectively removed from the market in the USA and Europe because it was categorized as a suspected mutagen. It is still used in China for passing stringent fire test requirements for high rise buildings. Diethyl ethylphosphonate (DEEP, Formula 2.5(b)) and dimethyl propylphosphonate (DMPP, Formula 2.5(c)) [217] were sold as replacements of DMMP but didn’t gain a large market share because of a higher cost. Triethyl phosphate (TEP, Formula 2.5(d)), now produced only in Asia, is used in rigid PU foam as a co-additive with TCPP or brominated FRs as a viscosity cutter. TEP also helps with decreasing smoke, however, in fact it doesn’t reduce smoke, but just doesn’t increase it as much as halogen-containing FRs tend to do. For example, 9 parts TEP provides a B-2 rating in DIN 4102 in high density rigid PU foam and shows lower smoke [218] compared to TCPP. Interestingly, TEP allows production of translucent rigid PU foam [219].
One study [220] compared DMMP, DEEP, DMPP and TEP with TCPP and tris(chloroethyl phosphate) (TCEP, removed from the market a decade ago). It was surprisingly found that the halogen-free phosphates and phosphonates show a higher LOI, 25-26.5 compared to chloroalkyl phosphates. It seems that the high volatility of halogen-free FRs compensated for a lack of chlorine. TEP, DEEP and DMPP showed good compatibility with blowing agents n-pentane and water, which resulted in an overall better shelf life of the mixed composition. On the negative side, halogen-free FRs showed lower compression strength and elastic modulus, probably due to stronger plasticization of the PU polymer. Another study [221] found similar FR efficiency of TEP (phosphate) and TCPP (phosphonate) confirming that the volatility of the FR plays an important role, but not the oxidative state of the phosphorus atom. Interesting research involving reactive FRs for rigid PU foams was reported from Korea [222]. A large amount of TEP or trimethyl phosphate or TCPP was added to waste PU foam and the mixture was heated to 190°C for 6 hours. At this temperature PU decomposes and the polyol fragments transesterify phosphate ester thus producing phosphorylated polyol. Rigid foam produced with the addition of this recycled polyol showed a decrease in peak heat release rate as measured by cone calorimeter.
For years low molecular weight phosphates TCEP and DMMP were used in highly filled ATH unsaturated polyester (UPE) systems or in glass-fiber composites with the main purpose of viscosity reduction [223, 224]. For example, 55-60 wt. % ATH and 1-2 wt. % DMMP allows passing the UL 723 test with class I for ventilation stacks [225]. Researchers at the Industrial Technology Research Institute (Taiwan) showed transesterification of simple phosphorus compounds such as DMMP to form phosphorylated unsaturated polyester resins [226]. Similar work was performed in China [227], where it was found that addition of about 15 wt. % DMMP to the reactive mixture in the synthesis results in UPE composites with a V-0 rating. Because the use of TCEP and DMMP was significantly restricted in North American and European markets the use of TCPP or TEP or DMPP was promoted for viscosity reduction in UPE. For example, it was suggested to use 5-10 wt. % DMPP as a viscosity reducer and synergist with APP and ATH [228]. Surprisingly only 10 wt. % ATH, 4 wt. % EDAP and 1 wt. % DMPP provide a V-0 rating in a glass-filled UPE composite [229].
Flexible PU foams have mostly open cell structures. Because of this, flexible foams are very combustible with an LOI in the range of 16-18 [230], and they show fast flame spread and a high heat release rate [231, 232]. The flammability of PU foams strongly depends on the foam density and the openness of the cells (air flow). Light foam with open cells burns very fast. Flexible PU foam is the main and most combustible component of upholstered furniture, mattresses [233] and car seats. Fires involving PU foams are the deadliest. “No ignition – no fire” is the best strategy to mitigate the fire hazard of flexible PU foams. Paradoxically, although PU foams are easy to ignite it is also easy to extinguish the fire when the flame is still small. This relates to the same inherent property of the PU foam being a thermally thick material. Because the heat cannot penetrate to the depth of the foam the heated layer where the foam decomposes and produces combustible gases is shallow. Such small flames can be extinguished by small changes in the fuel supply or by decreasing the heat by means of incomplete combustion. Flame retardants added to the flexible PU foams are specifically designed to extinguish small accidental fires [234]. However, if small flame doesn’t extinguish the foam begins to liquefy and collapses in the liquid pool [235] which creates dangerous conditions for fire spread.
The most common flame retardants used in flexible PU foams are chlorinated phosphate esters. However, in recent years oligomeric or reactive flame retardants which don’t contribute to VOC and do not migrate from the foam started taking market share. An oligomeric ethyl ethylene glycol phosphate (Formula 2.6) has been on the market for two decades. Because of the high 19% phosphorus content, it is quite efficient and as little as 4-8 php is effective in passing FMVSS 302 in a 1.5-1.8 lb/cu.ft. foam [236]. This oligomeric FR has been especially of interest in Europe and Japan, particularly with respect to the low-fogging low-volatiles-emission requirements of the automotive industry. It has been recommended for use in combination with alkylphenyl phosphates, which improve the flame retardant performance and also decrease the additive viscosity [237]. A number of recent patents [238, 239] indicate that a similar oligomeric phosphate but with a diethylene glycol bridging group (Formula 2.7) is in significant commercial development in Europe.
In manufacture of flexible polyurethane