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2.7 Aromatic Phosphates and Phosphonates
ОглавлениеTriphenyl phosphate (TPP, Formula 2.20(a)), a white low melting (48°C) solid, and tricresyl phosphate (TCP, Formula 2.20(b)), a liquid, were introduced into commercial use early in the twentieth century, initially for cellulose nitrate and later for cellulose acetate. TPP is usually produced in the form of flakes or shipped in heated vessels as a liquid. TPP has also been used as a flame-retardant additive for engineering thermoplastics such as polyphenylene ether–high impact polystyrene (PPE/HIPS) [283] and polycarbonate-ABS (PC/ABS)[284] blends. The largest use of TCP has been as a plasticizer for PVC [285]. In recent years some adverse aquatic toxicity data and later discovery of endocrine disruption effects of TPP led to a significant decrease in its use. Manufacturers of aromatic phosphates are now in the process of eliminating it from other products where it is present as a blend component or as an impurity. Originally tricresyl phosphate was made from petroleum-derived or coal-tar-derived cresols. Discovery of the toxicity of ortho-cresyl phosphates led manufacturers to switch to synthetic cresols having very little o-cresol, but this significantly increased the cost of TCP and almost eliminated it from the market. Another product from the same family, cresyl diphenyl phosphate (CDP, Formula 2.20(c)), took the market share of TCP, but mostly in Europe.
Typical applications of TCP and CDP are in PVC tarpaulins, mine conveyer belts, air ducts, cable insulation, and vinyl films. These phosphates are usually used in blends with phthalates. The proportion of the more expensive phosphate is usually chosen such as to permit the product to reliably pass the flammability specifications. Other uses of CDP are in various rubbers, polyisocyanurate foams [286], semi-rigid PU foams in combination with expandable graphite [287], in phenolic and epoxy laminates [288] and as a plasticizer in epoxy-based coatings [289].
In the late 60s, the use of more economical synthetic isopropyl- and tert-butylphenols as alternatives to cresols was developed [290, 291]. Commercial triaryl phosphates are based on partially isopropylated or tert-butylated phenols. Made from the product of isopropylation of phenol by propylene, isopropylphenyl phenyl phosphate (Formula 2.21(a)) is a mixture of mainly ortho- and para-isomers and contains a distribution of different levels of alkylation [292, 293]. The plasticizer performance of isopropylphenyl phenyl phosphate is close to that of TCP. Mixed tert-butylphenyl phenyl phosphate (Formula 2.21(b)), is a slightly less efficient plasticizer for PVC by itself but it is quite effective in blends with phthalate plasticizers. Both commercial isopropylated and tert-butylated phosphates contain a significant amount of TPP. tert-Butylphenyl phenyl phosphate is the least volatile and the most oxidatively stable in the family of alkylphenyl phosphates [294].
Apart from PVC both of these mixed phosphates found some use in flexible foam formulations [295] sometimes in combination with bromine-containing additives [296]. Even though these phosphates are not very efficient they have good hydrolytic stability and low volatility which is important for automotive foams. tert-Butylphenyl phenyl phosphate was also shown to be a viable flame retardant in high index PIR foams [297]. It has also been used as a flame retardant in PPE/HIPS pallets to pass the large scale UL 2335 test [298], in PPE/elastomer blends [299] and in PC/ABS where it shows good stress cracking resistance [300]. tert-Butylphenyl phenyl phosphate is often added to PPE-based blends as a processing aid even if flame retardancy is not required. Another large application of alkylphenyl phosphates is a plasticizer for thick intumescent coating for offshore oil rigs [301].
Alkyl diphenyl phosphates are products developed to provide improved low temperature flexibility, a fault of triaryl phosphate plasticizers in PVC [302]. There are three commercial phosphates in this family (Formula 2.22), e.g., 2-ethylhexyl diphenyl phosphate, isodecyl diphenyl phosphate and diphenyl phosphate with a mixture of longer (C12-C14) chains. These phosphates generally provide slightly less flame-retardant efficacy but generally produce less smoke compared to triaryl phosphates when the PVC formulation burns [303]. 2-Ethylhexyl and isodecyl diphenyl phosphate find their use in PVC sheet applications, PVC and TPU based artificial leathers and PVC/nitrile rubber tapes for insulative wrap of conduits. C12-C14-alkyl diphenyl phosphate has lower volatility compared to the other two phosphates and therefore it is used in PVC cable jacketing. Other applications of alkyl diphenyl phosphates are in casted polyurethane-polyurea goods [304] and in combination with intumescent flame retardants in thermoplastic elastomers cable jackets [305].
Aromatic phosphates or aromatic phosphate oligomers (mostly diphosphates) are very widely used in PC/ABS and PPE/HIPS blends. Historically triphenyl phosphate (TPP) was the first phosphorus-based flame retardant used in these blends. Although TPP is soluble in these resins and it doesn’t bloom out at room temperature, it deposits on the mold surfaces during molding. Because of the low melting point of TPP (48°C), it leads to bridging at extrusion feeding ports. The next generation of aromatic phosphate FR in PC/ABS and PPE/HIPS was tert-butylphenyl phenyl phosphate, which is still used nowadays in old formulations. However, now oligomeric aromatic phenyl phosphates (mainly diphosphates) are finding broader application than monophosphates because of better thermal stability and lower volatility.
The first product which became commercial was resorcinol bis(diphenyl phosphate) (RDP, Formula 2.23) which is a mixture of oligomers with two to five phosphorus atoms, but with the distribution heavily shifted towards the diphosphate [306]. In commercial PC/ABS blends where ABS content normally does not exceed 25%, RDP gives a V-0 rating at 8-12 wt. % loading [307]. Poly(tetrafluoroethylene) (PTFE) is a necessary ingredient in the formulation, which is usually added at <0.5 wt.% to retard dripping. Since the glass transition temperature of PTFE is below room temperature, it is soft and difficult to handle. To improve PTFE feeding it can be added during the production of ABS so that it is embedded in the polymer [308], or it can be specially treated to become free flowing [309], or pre-processed as a masterbatch [310]. RDP is somewhat less hydrolytically stable compared to other bisphosphates, which limits its application in humid environments and may cause a problem in recycling. This shortcoming of RDP can be alleviated by adding acid scavengers such as epoxies, oxazolines, or ortho esters [311].
Bisphenol A bis(diphenyl phosphate) (BDP) (Formula 2.24) was introduced to the market in the late 90s as an alternative to RDP [312]. Since bisphenol A is less expensive that resorcinol, BDP is more cost efficient despite a lower phosphorus content (8.9 % P for BDP vs. 10.7 % P for RDP). BDP is significantly more viscous than RDP (12500 cP for BDP vs. 600 cP for RDP at 25°C) and therefore it requires a heated storage tank and heated transfer lines, whereas RDP needs only heated transfer lines. Because of the high viscosity, the oligomers content (n>1, Formula 2.24) in the BDP mixture is usually limited to only 10-15% which creates a problem of potential crystallization of BDP during transportation. Despite the many disadvantages over RDP, BDP became the major product used in PC/ABS and the second largest phosphorus-based flame retardant produced. On the positive side BDP has better hydrolytic stability than RDP [313] and can be used in high humidity applications especially if it is further stabilized by adding epoxy [314] as an acid scavenger. PC/ABS with an ABS content less than 25 wt. % usually needs more than 12 wt. % BDP plus a small co-addition of PTFE in order to assure a V-0 rating [315].
BDP and RDP are also used in PC/PBT and PC/PET but further addition of an impact modifier, for example polyethylene copolymer [316] or core-shell copolymer [317] is needed. Recently, new flame-retardant blends of PC/PMMA [318] (copolymer of methyl methacrylate and phenyl methacrylate) which produce very high gloss and have excellent scratch resistance were introduced to the market. New FR blends using as one component a bio-based polymer PC/PLA [319] (polylactic acid) are also being explored for use in electronic equipment. Further addition of talc improves the heat stability of PC/PLA [320]. The content of bisphosphate in these blends depends mostly on PC content, the higher the PC content, the less bisphosphate required to achieve a V-0 rating.
Although major compounders of PC based blends are likely to be well equipped with liquid feeding systems, small and medium size compounders prefer to use solid bisphosphates even if they cost more than BDP. Very close to RDP, hydroquinone bis(diphenyl phosphate) (HDP, Formula 2.25) when made relatively pure with low TPP content and low oligomers content is a solid with a melting point of 105-108°C. It can be fed into the extruder without extensive cooling of the feeding zone and therefore some large compounders also use this product where flexibility of changing extrusion lines is desirable. HDP has a phosphorus content of 10.8%, similar to BDP, hydrolytic stability and requires 8-12 wt.% loading in PC/ABS [321] and other PC based blends to achieve a V-0 rating. In talc filled PC only 7 wt. % HDP is needed for a V-0 rating [322].
Resorcinol bis(di-2,6-xylyl phosphate) (RXP, Formula 2.26) [323] has been on the market for over 20 years, but mostly in Asia. Similar to HDP, RXP is mostly pure bisphosphate with very little oligomers present. Because of the specific chemical structure and high purity RXP is a solid with a melting point of 95°C. The steric hindrance provided by the 2,6-xylyl groups makes this product more hydrolytically stable than BDP. RXP has a phosphorus content of 9.0 % and its fire-retardant efficiency is similar to that of BDP; it provides a V-0 rating in PC/ABS at 12–16 wt. % loading [324] and about 7-10% wt. % loading in mineral filled PC/ABS [325].
4,4’-Biphenyl bis(diphenyl phosphate) (Formula 2.27) is a specialty bisphosphate for high temperature molding of glass-filled PC and PC/ABS [326]. It has a melting range of 65-85°C [327] and a phosphorus content of about 9.5 %. It gives a V-0 rating in PC at 3.5wt. % loading and 0.3 wt. % PTFE and at 10 wt. % loading it gives a V-0 rating in PC/ABS and a comparative tracking index (CTI) of 600 V [328]. When used without PTFE it preserves the transparency of polycarbonate [329]. As measured by thermogravimetry, 4,4’-biphenyl bis(diphenyl phosphate) shows a 5 wt. % loss at about 405°C which is significantly higher than other bisphosphates. Because of its low melting point this bisphosphate requires a significant cooling system in order to avoid bridging in the extruder feeding ports. A recently developed variation of the same bisphosphate, but with a significantly higher content of oligomeric fraction (n>1, 30-40%) is a viscous liquid [330].
Academic studies [331, 332] on the mechanism of the flame-retardant action of aromatic phosphates in a PC based blend revealed that BDP shows mostly condensed phase action, RDP shows a mixed condensed phase and gas phase, whereas TPP is mostly gas-phase-active. This was attributed to the temperature of decomposition of PC and phosphates, e.g., TPP evaporates at a relatively low temperature and doesn’t have a chance to react with PC, whereas bisphosphates react with PC [333, 334]. RDP or BDP tend to cause PC to produce more char, decreasing the fuel supply to the flame and decreasing the flame temperature. TPP, which has gas phase activity, becomes more effective in the gas phase with a decrease in the flame temperature. HDP shows significant gas phase efficiency and when mixed with a mostly condensed phase active BDP exhibits a synergistic effect [335]. Another study showed [336] that the hindered structure of RXP slows down the reaction with PC and therefore it shows less condensed phase action compared to RDP. Interestingly talc improves the flame retardant efficiency of bisphosphates because of the better protective properties of the char [337] and on the other hand glass fiber reinforcement deteriorates the flame retardant efficiency because it increases the combustion surface (candlewick effect) [338].
Another large application of aromatic bisphosphates and oligomers is in polyphenylene ether (PPE) based blends. Polyphenylene ether cannot be processed alone because of its very high melting temperature, but it is readily compatible with many polymers and can be processed as a blend. Depending on the molecular weight and chain ends PPE can be blended with HIPS, polyamides, styrenic elastomers and even epoxy. Apart from improving the physical properties of the host polymer, PPE is an excellent charring polymer due to its specific thermal decomposition mechanism (Formula 2.28). PPE undergoes Fries isomerization [339] and forms a phenolic type of resin with numerous OH groups which are reactive with phosphorus FRs.
Commercial PPE/HIPS blends, also known as modified PPE, contain from 35 to 65 wt. % PPE. Similarly to PC/ABS, the first FR used in PPE/HIPS was TPP, which was later replaced with RDP and BDP [340]. Typically, between 9 and 15 wt. % of a phosphate ester is needed to achieve V-0 rating; the lower the PPE content in the blend, the higher the phosphate loading required. PTFE is required to prevent dripping. A copolymer of polydimethyl- and polydiphenyl siloxane can prevent dripping and is at the same time synergistic with RDP [341]. Addition of polysiloxane also helps to decrease smoke formation allowing the achievement of HL3 rating in the European mass transit test EN 45545 in PPE/HIPS based glass fiber composites [342]. Apart from flame retardancy, phosphate esters also play an important role in plasticization and resin flow improvement. Therefore, some phosphate esters can be added to PPE/HIPS even if flame retardancy is not needed. Because PPE is not sensitive to hydrolysis, any bisphosphate can be used in high humidity applications, as for example water pipes [343].
Apart from HIPS, PPE is also compatible with styrene based thermoplastic elastomers, such as styrene-ethylene-butylene-styrene (SEBS) block copolymer. A copolymer of SEBS and maleic anhydride is used as a compatibilizer [344]. These blends are mostly used in electric wire jackets and often polyolefins and HIPS are also included in the blends. As the patent literature indicates, aromatic bisphosphates RDP [345] and BDP [346] were originally used as flame retardants in PPE/SEBS blends. Bisphosphates are very compatible and soluble in PPE, but not in TPEs and polyolefins and therefore the total loading of bisphosphates is limited because of potential exudation. To overcome this problem in blends containing less than 50% PPE, solid flame retardants are added along with bisphosphates. The patent literature shows combinations of aromatic phosphates with magnesium hydroxide [347], melamine phosphates [348], ammonium polyphosphate [349] or DEPAL [350].
Mechanistic studies of the flame-retardant action of bisphosphates and TPP in PPE based blends showed that phosphates catalyze the Fries rearrangement [351] (Formula 2.28) and promote charring and improve the morphology of the char by making it intumescent-like [352]. The PPE charring capability is higher than PC, therefore PPE improves the fire retardant performance of RDP in PC/PBT blends [353]. One comprehensive study [354] looked at a large number of substituted aromatic phosphates and bisphosphates and compared them with red phosphorus and aliphatic phosphates. This study concluded that aliphatic phosphates are the least efficient because they decompose at temperatures much lower than the decomposition temperature of PPE. The efficiency of all aromatic phosphates and bisphosphates were similar in the range of experimental error and directly proportional to the phosphorus content. The efficiency of red phosphorus was similar to that of aromatic phosphates at the same phosphorus concentration. However, the strongest factor that controls the flammability of PPE/HIPS blends was the PPE content [355].
Another use of aromatic bisphosphates is in TPU. One of the common commercial halogen-free TPU formulations is based on about 25 wt. % melamine cyanurate and 5 wt. % RDP [356]. This TPU still drips, but the droplets do not ignite cotton and therefore it is rated V-0. Addition of some free isocyanate during processing creates additional cross-links and prevents dripping [357]. Many formulations based on RDP and ATH passing the VW-1 rating in the UL-1581 test for wire and cables applications were developed [358] and some were probably commercialized. Interestingly, the addition of only 2.5 wt. % novolac type epoxy resin provides robustness in passing the test [359] probably by cross-linking and decreasing the resin flow and dripping.
Various aromatic bisphosphates, more specifically RDP [360] can be incorporated by the exhaust method in PET textiles in the presence of polycaprolactone as a dispersing agent and polyethylene diamine as an auxiliary FR helping to retain RDP in the fiber. An add-on level > 10 wt. % was achieved and the textile passed the stringent DIN 54336 test with immediate extinguishment. A similar result was achieved by dispersing RDP, BDP or RXP in water using a non-ionic surfactant with a small addition of a cationic surfactant and then immersing the PET fibers at 130°C in an autoclave [361]. Emulsified RDP can also be applied as a backcoating to a nylon/cotton fabric blend [362]. In terms of combustion performance films are often close to textiles. About 8 wt. % RDP was used to pass the FMVSS 302 test in polyester films based on ethylene and 1,4-cyclohexanedimethane terephthalate [363] or 3 wt. % in a 45 degree flame spread test in poly(trimethylene terephthalate) film [364].
Independently of the physical form (liquid or solid), aromatic bisphosphates have very limited compatibility with polyolefins. Interestingly it was found [365] that aromatic bisphosphates can be loaded in PP plus EVA at 5 wt. % without visible exudation after heating for 72 hours at 70°C. A solid bisphosphate HDP showed a slightly better performance than liquid RDP. The films with 5% bisphosphate showed an HB rating in the UL-94 test. Interestingly, the maximum loading of triphenyl phosphate achievable in PP and EVA was only 3 wt. %. It is believed that bisphosphates can be used in PP fibers, films and foams to provide some level of flame retardancy. For example, 2.5 wt. % RDP or BDP combined with 1 wt. % aminophenyl disulfide provides a UL-94 HBF rating in PP foam [366]. About 3-8 wt. % of aromatic bisphosphate allows passing the 45 degree angle ISO 11925-2 test in HDPE/EVA flash spun sheets [367].
Some time ago it was discovered that the P-O-C bond in aromatic alkylphosphonates is reactive towards epoxy. Based on this discovery a new curing agent poly(1,3-phenylene methylphosphonate), (PMP, Formula 2.29(a)) for epoxy resins was developed [368]. It is semi-solid at room temperature, but it melts at about 45-55°C. The product is very rich in phosphorus (17.5%) and is thermally stable with a weight loss starting only above 300°C. PMP is qualified as an active ester and it cures epoxy by opening the epoxy group and insertion into the phosphonate ester linkage [369]. Because PMP doesn’t produce secondary aliphatic alcohol groups as typical amine or phenolic curing agents, epoxy resin cured with PMP shows an improved thermal stability and a high glass transition temperature [370]. From 20 to 30 wt. % PMP provides a V-0 flammability rating in epoxy laminates.
Poly(bisphenol A methylphosphonate) (PAMP, Formula 2.29 (b)) was first developed in the ‘80s [371] but commercialized only a quarter of a century later [372]. The homopolymer can be used as an additive in PC or PC/ABS or co-polymerized with PC [373]. Being combined with potassium sulfonates, PAMP or its copolymers give a V-0 rating and good transparency in PC up to a 0.4 mm thickness [374]. Oligomeric and end chain functionalized PAMP are also suitable for special applications such as epoxy resins [375], cyanate resins [376], flexible PU foams [377] and TPU [378]. Despite its many potential applications, the main use of PAMP at the time of writing this chapter seems to be in PET fibers [379] for carpets and in PET films [380].
Interestingly, one of the first phosphonates used in PET fibers was poly (sulfonyldiphenylene phenylphosphonate) (Formula 2.29 (c)) produced in Japan. This oligomer is easily miscible with PET [381] up to 15 wt. % but for fiber applications typically less than 5 wt. % loading is needed. This product was discontinued in Japan in favor of reactive type phosphinates (see next subchapter), but it is reportedly produced now in China.
