Читать книгу Non-halogenated Flame Retardant Handbook - Группа авторов - Страница 39

Cyclic phenoxyphosphazenes are thermally and hydrolytically stable phosphorus-nitrogen products. Typically, these are mixtures of hexaphenoxy-tricyclophosphazene as a major component (Formula 2.39) with some octaphenoxytetraphosphazene and a smaller fraction of linear oligomers. Because cyclic phenoxyphosphazenes have low polarity and favorable electrical properties they found use in high end epoxy [426], cyanate ester [427], benzoxazine [428] and PPE [429] based printed wiring boards. On the negative side the low melting point of cyclic phenoxyphosphazene of 110°C can cause excessive resin flow in the lamination process and as a result its loading level in the formulation is limited. Therefore, it is often applied in combination with other FRs. Another commercial use of cyclic phenoxyphosphazene is in PC/ABS where it is effective at 12-15 wt. % loading [430]. Reportedly, it shows a higher heat distortion temperature compared to aromatic bisphosphates [431]. It also provides excellent hydrolytic stability and low temperature ductility. However, the relatively high cost of cyclic phenoxyphosphazene compared to traditional bisphosphates limits its use in PC/ABS. Cyclic phenoxyphosphazenes can be further functionalized with cyanate groups [432] for specific applications with cyanate esters. Cyclic phenoxyphosphazenes functionalized with allylic groups [433] are used in acrylic terminated PPE laminates. However, at the time of writing this chapter the commercial status of these specialty phenoxyphosphazenes is unclear.

(2.39)