Читать книгу Non-halogenated Flame Retardant Handbook - Группа авторов - Страница 38

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)