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.
2.8 Aromatic Phosphinates
In general, the flame retardancy of phosphorus-containing polyester and polyamide fibers is mostly achieved by enhanced melt flow and melt drip, presumably catalyzed by phosphoric acid species produced in the process of oxidative degradation during combustion. Although it was a significant effort to try to introduce phosphate or phosphonate types of flame-retardant monomers into polyesters and polyamides, none of them led to a commercial produc [382]. The problem is that undesirable transesterification and hydrolysis reactions occur during the copolymerization. However, these side reactions do not seem to be a problem with phosphinates which have two non-reactive and not hydrolysable P-C bonds. For many years cyclic 2-methyl-2,5-dioxa-1,2-phospholane was copolymerized with ethylene glycol and dimethyl terephthalate to produce flame retardant PET fibers. About a decade ago this product was discontinued because one of the raw materials in its production was strictly regulated. This cyclic phosphinate was replaced with an adduct of benzenephosphinic acid and acrylic acid also known as CEPPA (Formula 2.30). CEPPA can be co-polymerized in the PET chain at 0.3-0.9 wt.% which leads to a significant increase in the LOI of PET fibers [383]. Interestingly, CEPPA also helps to improve the color stability of PET fibers [384]. Reportedly it can also be copolymerized in polyamide 6.6 fibers [385] to produce flame retardant carpets.
Some time ago it was discovered that the product of the reaction of o-phenyl phenol and phosphorus trichloride [386] followed by hydrolysis [387] resulted in a unique cyclic 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide structure, also known as DOPO (Formula 2.31). Its P-H bond is more reactive than a similar bond in many other phosphinates or phosphonates and therefore DOPO can be reacted with alkenes, epoxies and aldehydes to form different flame retardants.
For example, the adduct of DOPO and dimethyl itaconate [388] (Formula 2.32 (a)) is a reactive flame retardant commercially used as a co-monomer in polyester fibers [389]. Like CEPPA, this product is efficient in PET fibers at a low concentration of 0.3-0.65 wt. % phosphorus and maintains good fiber properties [390]. It was found that placing the phosphorus ester linkage in the side chain, instead of the main chain, afforded superior hydrolysis resistance 391 and thermal stability [392, 393]. The adduct of DOPO and itaconic acid (Formula 2.32(b)) can be further copolymerized with diols and maleic anhydride to form an unsaturated ester prepolymer [394, 395] which can be cured with styrene to produce a thermoset resin.
The printed wiring boards (PWB) which are produced with epoxy resins must pass the UL-94 test with a rating of V-1 or V-0. Phosphorus-based FRs can be added to epoxy as an additive or can be incorporated in the epoxy network by phosphorylation of the epoxy resin or in the form of phosphorus-based cross-linking agents [396]. Reactive FRs are more preferred in epoxy because they show fewer negative effects on the physical properties, mostly glass-transition temperature and hydrolytic stability. Although DOPO is monofunctional, it was adopted by the industry