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2.6 Aliphatic Phosphates and Phosphonates
ОглавлениеPolyurethane (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 foams, if the foam reaches an excessively high temperature, “scorch” can occur. Scorch is, at the least, a discoloration of the interior of the slab or bun, and more seriously the loss of mechanical properties because of polymer degradation. Some of the commonly used flame retardants can aggravate scorch. Mechanistic studies showed [240, 241] that scorch is largely the result of the oxidation of aromatic amino groups arising from the hydrolysis of isocyanate groups which became isolated in the PU network. The formation of chromophoric groups is aggravated by the presence of flame retardants with alkylating capabilities such as chloroalkyl or alkyl phosphates because alkylated aminophenyl structures are more easily oxidized to quinoneimines. Ethyl ethylene glycol polyphosphate causes some scorch, especially in low density water blown foam, therefore the foam needs to be stabilized [242].
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 FR for textile finishing don’t need to be di- or multifunctional. For many years the product of the addition of dimethyl phosphite to acrylamide followed by methylolation (Formula 2.15) was marketed for cotton and cotton-based blends [258]. This product is fixed on the cellulose using an amino resin and an acid curing catalyst. A recent academic study [259] shows that use of titanium dioxide as a co-catalyst for cotton textile treatment improves the flame retardant efficiency especially after laundering. It has a mild formaldehyde odor because it contains some components with less well bound formaldehyde [260]. This product is not used in the USA and has limited use in Europe because of potential formaldehyde exposure. There are methods of decreasing formaldehyde release [261] and it is believed that they are used commercially. A recent patent application [262] shows use of this product on lyocell fibers where it is introduced in the spinning solution.
Termosol finishes with phosphorus-based flame retardants have been used for many years in PET textiles [263] and probably in polyamides. The major product used in the thermosol treatment of polyesters is a liquid cyclic phosphonate (Formula 2.16). It is a mixture of diphosphonate and triphosphonate with the ratio mostly shifted towards diphosphonate x=1. Usually, a small concentration of phosphorus 0.3-0.5 wt. %, in PET is needed to pass the textile flammability test NFPA 701 and 0.7 wt.% is needed to pass the vertical FAR 25.853 test for use in aviation airbags [264]. Even being highly soluble in water, after the phosphonate is trapped under the fibers surface it is resistant to laundering and doesn’t leak out. Because this phosphonate has a high phosphorus content it is also attractive for use in rigid or flexible PU foams. However, it has high viscosity and needs to be diluted with an aromatic phosphate [265] or bisphosphate [266]. A version of the spirophosphonate where the ratio is mostly shifted to triphosphonate (Formula 2.16, x is mostly 0) has shown promise as an additive in polyamide fibers via a melt process [267].
About a decade ago pentaerythritol spirobis(methylphosphonate) (Formula 2.17) was introduced for use in polyamides, including melt-spun fibers [268] and for use in combinations with intumescent flame retardants in polyethylene [269]. When combined with a free-radical generator, it is effective for flame-retarding polyethylene foam [270] and thin polyethylene films [271]. Another application of spirobis(methylphosphonate) seems to be in polyurethane based textile backcoatings [272]. In a recent academic publication [273] it was found that spirobis(methylphosphonate) has a higher flame retardant efficiency in molded PET compared to a bisphosphate, bisphosphinate and bis-n,n-naphthtylimide of similar structure. This high efficiency was attributed to the phosphorus gas phase action.
Similarly pentaerythritol spirobis(benzylphosphonate) (Formula 2.18) has been developed in Japan and introduced as a flame retardant for thermoplastic polyesters and styrenics [274]. However, based on later patents the main applications of this product seem to be in polylactic acid [275] and its blends [276] and in bio-based polycarbonate [277]. It doesn’t affect the transparency and the clarity of the polycarbonate. Other potential applications are poly(methyl methacrylate) and its blends [278], backcoating for polyester textiles [279] and polyurethane based artificial leather [280].
1,3,2-dioxaphosphorian-2,2-oxy-bis-(5,5-dimethyl-2-sulphide) (Formula 2.19) is a solid flame retardant additive developed and commercialized in Europe for use in viscose rayon [281]. Despite the anhydride structure, it is remarkably stable, surviving addition to the highly alkaline viscose, the acidic coagulating bath, and also resisting multiple laundering of the rayon fabric. The unusual stability may be attributed to the sulfur atoms, which enhance hydrophobicity, and to the sterically hindering neopentyl groups that retard hydrolysis. The process of producing flame-retardant viscose fibers has been improved in recent years [282] because of increasing use of rayon as a fire barrier in mattresses.
Other commercial aliphatic phosphates e.g., tributyl phosphate, triethoxybutyl phosphate and tri-2-ethylhexyl phosphate can be used as a flame retardants or part of a flame-retardant mixture, but their major use is in other areas. Therefore, they are out of the scope of this chapter.
