Flame retarded and inherently flame retardant synthetic fibres

Notwithstanding their associated thermoplasticity and often fusible behaviour (see Table 15.1), these fibres may, however, be used as minor components in blends with char-forming fibres, particularly cellulosic and wool and these have been discussed above. Rarely when present as a minor component are these synthetic fibres individually treated (Horrocks, 2003) and they rely on the flame retarded majority component to reduce any flammable tendency. Their presence is often included to enhance tear strength and abrasion resistance as well as conferring some easy-care character but their generally non-char-forming behaviour remains a problem, unless heavily back-coated (see section 15.3.6).

However, some synthetic fibres modified during production by either incorporation of a flame retardant additive in the polymer melt or solution prior to extrusion or by copolymeric modification can yield inherent flame retardancy, although still thermoplasticity remains a challenge. Perhaps the oldest available fibres in this group (see Table 15.3) are the modacrylics which have been commercially available for forty years or so although at present few manufacturers, such as the Kaneka Corporation of Japan and Montefibre in Italy (see Table 15.3) continue to produce them. These fibres have some char-forming capacity because of the comonomeric chloro-species present. It is this char-forming capacity that makes their presence in the previously mentioned Protex M blends with cotton so effective along with the ability of the chlorine radicals released by the Kanecaron Protex component to flame retard adjacent cotton fibres. Not surprisingly, therefore, the chlorofibres such as PVC and PVDC themselves (see Table 15.3) are highly char-forming and also belong to this group. However, because both these fibre groups have poor thermal physical properties and release hydrogen chloride gas during burning, they will not find application in closed environments such as exist in aircraft and other transport systems.

On the other hand, one group which continues to be successful is FR polyester (see Table 15.3) typified by the well-established Trevira CS (Horrocks, 1996), which contains the phosphinic acid comonomer. Toyobo's latest version of its Heim FR polyester is also based on a phosphorus-containing, phosphinate diester and this has claimed superior hydrolytic stability (Weil and Levchik, 2004). Other flame retardant systems, believed to be based on phosphorus-containing additives, such as Fidion FR (Montefibre) are also commercially available. However, these FR polyester variants do not promote char but function mainly by reducing the flaming propensity of molten drips normally associated with unmodified polyester. Thus they are rarely used in protective applications and only if blended with compatible flame and/or heat resistant, char-forming fibres as the major element.

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