Non-halogenated Flame Retardant Handbook. Группа авторов

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СКАЧАТЬ 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.

(2.33)

      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 T_g > 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].

(2.34)

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 T_g > 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.

(2.35)

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].

(2.36)

2.9 Phosphine Oxides

Phosphine oxides have three P-C bonds which are hydrolytically stable, and they seem to be ideal flame-retardant candidates for critical applications where there is exposure to moisture. On the other hand, phosphine oxides, especially aromatic ones are difficult to produce, and they tend to be more expensive than other organophosphates. This somehow limits the broad the use of phosphine oxides as flame retardants. One of the oldest applications of phosphine oxides is in textile finishing where the leading commercial products are tetrakis(hydroxymethyl) phosphonium chloride (THPC) or sulfate (THPS) [420]. THPC and THPS are water-soluble, but non-hydrolysable phosphonium salts that ensure exceptional durability. In the finishing process THPC or THPS is reacted with urea first and the product obtained is used to impregnate textile which is then dried and cross-linked with gaseous ammonia. At this stage some methylol groups react with cotton OH groups to permanently fix this finish on the textile [421]. Finally, the textile is treated with aqueous hydrogen peroxide which oxidizes phosphine into a more thermally stable phosphine oxide. The idealized structure [422] which doesn’t have hydrolyzable bonds is shown in Formula 2.37. As a result, this finish is durable for 100 industrial launderings with alkaline detergent, more durable than any other flame-retardant cotton finish [423]. The need for using gaseous ammonia is the major disadvantage of this process and it requires special equipment.

(2.37)

Recently, another commercial phosphine oxide type flame retardant p-xylenebis(diphenyl phosphine oxide) (Formula 2.38) was introduced to the market. Although DOPO based flame retardants are most common in printed wiring boards the hydrolytic and thermal stability of DOPO sometimes is not sufficient for multiple pressing and reflow operations. Since phosphine oxide is not soluble in the common solvents used by PWB laminators it is applied as a filler in high frequency formulations [424]. p-Xylenebis(diphenyl phosphine oxide) provides a mostly gas phase flame retardant mode of action. In order to boost its efficiency it can be combined with resorcinol bis(di-2,6-xylyl phosphate) (Formula 2.26) which allows a decrease in the total FR loading [425].

(2.38)

2.10 СКАЧАТЬ