There is some market interest in reactive flame retardants for rigid and flexible PU foams. The advantage of a reactive FR is its permanence in the foam which is especially important in roofing applications in hot desert and tropical climates where the temperature of the roof can be very high and non-reactive FRs can be lost. Diethyl N,N bis(2-hydroxyethyl) aminomethylphosphonate (Formula 2.8) has been on the market for long time. The main application of this FR is in roofing spray foam and in the insulation foam of large refrigerators. A mechanistic study on this product showed [243] that even though most of the phosphorus splits off and volatilizes from the foam during combustion, it still helps with significant char increase which indicates that this reactive FR provides both condensed phase and gas phase modes of action.
Another long time in the market reactive product is a diol mixture obtained by the reaction of propylene oxide and dibutyl acid pyrophosphate (Formula 2.9) [244]. The product contains 11% phosphorus and it is a mixture of isomers. Its recommended use is in flexible and rigid PU foams and polyurethane based coatings and adhesives.
Recently realization came that a reactive flame retardant doesn’t need to be difunctional, but a monofunctional can still be anchored to the rigid PU network to prevent migration and it can be easily released to the gas phase during the thermal decomposition of the foam due to reverse scission of one urethane bond. A recent patent [245] shows an improved process of manufacturing diethyl hydroxymethyl phosphonate (DEHMP, Formula 2.10) and a number of patents claim advantages of use of this product in rigid PU [246] and low smoke release PIR foams [247].
Similar to rigid foams there is a market desire to have a reactive phosphorus based flame retardant for flexible foams. However, technical development of such a product is more difficult because the cell structure of flexible foams is more sensitive to the variations in the composition compared to rigid foams. For example, diethyl N,N bis(2-hydroxyethyl) aminomethylphosphonate (Formula 2.8) broadly used in rigid PU foams, can be used in flexible foams only as a co-additive at the levels of 1-2 phr because the hydroxyl (OH) number is very high compared to typical flexible foam polyols.
In recent decades significant attempts were made to commercialize halogen-free phosphorus-containing diols for flexible foams. One of these diols is a hydroxyethyl terminated ethyl ethylene glycol phosphate oligomer (Formula 2.11) with about 17% phosphorus content [248, 249]. It is noticeable that this product is like the ethyl ethylene glycol oligomer of Formula 2.6, but it has terminal OH groups. It is primarily recommended for use in molded and high density slabstock flexible foams, where it passes the FMVSS302 test at 7.5 parts. The main advantage of this product is permanency in the flexible foam which allows achievement of low volatile organic compounds (VOC). Another phosphorus ester with about 12% P is a reactive phosphonate [250] made by reacting methyl phosphonic acid with ethylene oxide (Formula 2.12). It is mostly used in automotive flexible PU foams where it reacts in and becomes part of the PU network. It is highly efficient especially in high density foam where passing of the FMVSS302 test is achieved at < 4 parts.
One of the limitations of phosphorus containing diols is their tendency to create closed cell foams, which is not desirable in flexible PU. That is why these diols are used only in high density foams at low concentration. Some time ago it was discovered that monofunctional reactive flame retardants are easier to formulate in flexible PU foams [251]. Because monofunctional flame retardants are anchored on the PU foam chains they do not contribute to VOC and therefore are mostly targeted for automotive foam. An example of such phosphorus containing monohydric alcohol is the product made by reacting cyclic neopentyl acid phosphate with propylene oxide [252] (Formula 2.13) developed in Japan. In spite of a low phosphorus content of 11 % it allows passing FMVSS302 test at 8 parts which is similar to the chloroalkyl phosphates widely used in automotive foam. The market penetration of this cyclic flame retardant was limited because it is a solid, but the PU industry likes to operate with liquid flame retardants.
For many years mixed methyl phosphate methylphosphonate ethylene glycol oligomer was sold as a flame retardant for paper automotive filters. However later it was taken from the market and replaced with a chain end hydroxylterminated version (Formula 2.14) but for textile finishing. It has been shown that this oligomeric product can be curable on cotton or blends using dimethyloldihydroxyethyleneurea (DMDHEU) and trimethylolmelamine [253] or melamine-formaldehyde [254] to obtain a durable finish with low formaldehyde odor. It is also efficient on cotton-nylon [255] and cotton-polyester [256] blends. It can also be used in non-formaldehyde finishes where the bonding to cellulose is achieved by using a polycarboxylic acid such as butanetetracarboxylic acid or citric acid [257]. Now this product is mostly sold in China for military uniforms.
Similarly to PU foams, phosphorus based