Читать книгу Non-halogenated Flame Retardant Handbook - Группа авторов - Страница 37
2.8 Aromatic Phosphinates
ОглавлениеIn 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 for use in PWB laminates [397] and now it is the largest phosphorus FR used in epoxy. The common practice is to react DOPO with a multifunctional novolac type epoxy [398] in order to achieve a phosphorus content of about 3 wt. % (Formula 2.33). This still leaves on average 2-4 epoxy functionalities unconsumed which allows further curing of the phosphorylated epoxy resin.
Because DOPO is monofunctional it cannot be used with most common difunctional bisphenol A epoxy resins. In novolac type epoxies DOPO provides V-0 at a relatively low phosphorus content of 2.0-2.5 wt. %. [399]. The high efficiency of DOPO compared to other phosphorus FRs is partially attributed to its gas phase action [400]. DOPO can be combined with ATH [401] which is normally not the case with many phosphorus FRs showing mostly condensed phase action. When combined with ATH or fine silica, DOPO-based laminates require only 1 wt. % P or less to achieve a V-0 rating. The main disadvantage of DOPO modified epoxy is a challenge to achieve a high glass transition temperature Tg > 150°C even when combined with multifunctional epoxy [402].
By reacting DOPO with quinone, a phenolic difunctional product can be made (DOPO-HQ, Formula 2.34(a)). It can be incorporated in an epoxy resin through a chain-extension process like tetrabromobisphenol A with difunctional epoxies [403]. Although it provides good physical properties and the required level of flame retardancy, it is not finding broad application because it is low in phosphorus and more expensive than DOPO. Because DOPO-HQ has poor solubility in the common solvents for epoxies, it is not used as a co-curing agent. DOPO-HQ can be co-polymerized into the polyester chain [404, 405], but this polyester seems not to have been commercialized. If naphthoquinone is used instead of quinone DOPO-NQ (Formula 2.34(b)) can be made. Because DOPO-NQ shows gas phase efficiency as well as good charring tendency it is efficient even at relatively low levels of addition [406]. DOPO-NQ is compatible with ATH and magnesium hydroxide (MDH) and when incorporated in multifunctional epoxy shows very good thermal and hydrolytic stability [407]. Because DOPO-NQ is a high melting temperature (295°C) solid [408] it can also be used as an additive in high frequency laminates based on polyphenylene ether where it allows maintaining a low dissipation factor [409]. Interestingly, DOPO-HQ and DOPO-NQ can be further functionalized with cyanate groups [410] instead of OH groups in order to be used in high end cyanate ester laminates [411]. DOPO-HQ can also be reacted with acetic anhydride and then transesterified with isophthalic acid to produce polymeric product which is an active ester that effectively cures epoxy [412].
By the reaction of DOPO with butoxymethylated bisphenol A [413] a mixture of phosphorylated bisphenols (DOPO-BPA) can be made with the major component presented in Formula 2.35. Because DOPO-BPA is a difunctional reactive FR and has a high phosphorus content of about 9% compared to phosphorylated epoxy of 3% (Formula 2.33) it allows production of laminates with a Tg > 175°C and with good thermal stability as measured by a delamination test. It also has good solubility in solvents that are compatible with epoxy lamination processes. Another positive attribute of DOPO-BPA is good electrical properties in epoxy [414] and benzoxazine [415] laminates.
In order to achieve lower thermal expansion, improve heat dissipation and decrease the dissipation factor, a significant amount of silica is added to high end laminates. This new technology also opens the door for use of high melting, non-reactive and non-soluble flame retardants which further improve electrical properties. An example of such an FR is ethylene bis-DOPO phosphinate (Formula 2.36) made by reacting dichloroethane with DOPO [416] or reacting ethylene glycol with DOPO in the presence of sodium iodide [417]. This phosphinate provides a V-0 rating in novolac epoxy-based laminates at 20 wt. % loading. However, the main use of this flame retardant seems to be in non-epoxy polyphenylene ether (PPE) based laminates [418] or in hydrocarbon laminates based mostly on butadiene rubber and some PPE [419].
