Flame-retarding fibre blends

Providing flame retardancy for fibre blends has proved to be a difficult task. Fibre blends, especially blends of natural fibres with synthetic fibres, usually exhibit a flammability that is worse than that of either component alone. Natural fibres develop a great deal of char during pyrolysis, whereas synthetic fibres often melt and drip when heated. This combination of thermal properties in a fabric made from a fibre blend results in a situation where the melted synthetic material is held in the contact with the heat source by the charred natural fibre. The natural fibre char acts as a candle wick for the molten synthetic material, allowing it to bum readily. This can be demonstrated by the LOl values of cotton (18-19), polyester (20-21) and a 50/50 blend of both (LOl 18), indicating ahigher flammability of the blend as described later (Section 8.11). But a rare case of the opposite behaviour is also known (modacrylic fibres with LOl 33 and cotton in blends from 40-60 % can raise the LOl to 35). 

Even an antagonistic behaviour is reported for wooFpolyester blends. Both Zirpro finished wool and Trevira CS, which is inherently flame retardant modified 

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Since cellulosic fibres are sensitive to acids they can be easily damaged by the acid catalysts used in easy-care, silicone, fluorocarbon and flame-retardant finishes as well as by drops of concentrated acid or faulty dyeing of cellulose/wool blends. 

Ternary systems are becoming more widely reported with, in addition to epoxy and clay, other materials being present such as rubber, thermoplastic or fibres. Synergies need to be sought. likewise, the addition of additives such as flame retardants, either physically blended, or covalently-incorporated with the epoxy or amine need to be examined in nanocomposites, since this is one of the most important, ongoing requirements of transport industries such as aerospace. 

It is probably true to state that FR viscose staple fibres are rarely used alone, except perhaps in nonwoven barrier fabrics, but they are used as blends with more expensive, high performance fibres (see below and section 8.5) where they act as effective flame retardant diluent components. 

Traditionally, Zirpro -finished wool meets the above requirements and decabro-modiphenyl ether/antimony oxide-acrylic resin-fmished cotton fabrics (originally marketed as Caliban, White Chemical) have also been found to be suitable for workers in the aluminium industry. However, as discussed in Chapter 8, this latter finish is currently being withdrawn on environmental grounds and this whole area has recently been reviewed by Makinen, who lists more recent fabrics based on a variety of blends with flame retardant wool, viscose, and inherently flame retardant aramid fibres, for example. However, these factors are all different for molten iron or steel, copper, tin, lead, zinc, or aluminium and so protective aprons and overalls have to be tailored to fit the threat. Examples listed by Makinen for molten aluminium resistance include ... 

However, most items are actually composites of at least two fabrics (see below) and it is the composite that is subjected to the test. Usually, if a single fabric layer or a multilayer comprising the same fibre type is to be tested and the former passes the test, then so will the latter, and no additional flame retardant is required to enable the standard to be achieved. The use of flame retardants in any case is to be avoided, since not only do they add to cost but also they can increase the levels of toxic gases emitted during burning, although currently levels of these are not required to be assessed. If blends of different fibres or combinations of fabrics comprising different fibres are combined, then it is probable that additional flame retardants may be required to pass the test even if component fabrics alone each pass the test. 

Another way to protect the iimer, support component of upholstery is the use of barrier materials in the form of non-flammable interlayers placed between a covering fabric and the filling material. Woven and non-woven fabrics as well as felt made from natural fibres (e.g., cotton, flax and hemp) and their blends with flame retarded synthetic fibres, can be used. 

DuPont-Toray, Ichimura Sangyo and Takayasu in Japan have incorporated aramid fibres in polymers instead of conventional flame retardants, using a special blending method that gives a uniform dispersion. Advantages include better tracking resistance and lower specific gravity than many other flame retarded compositions. 

Visfii and other similar viscose fibres are inherently flame-retardant silicic acid-containing viscose rayon fibres used as a blend component. VisiP is made by wet spinning of alkaline cellulose xanthate (viscose) containing a sodium silicate (equivalent to about 30-33% Si02) with some aluminosilicate component. During fire combustion, flame retards by both endothermic water release and char formation. 

Halogen-containing fibres, such as modacrylic fibres (e.g., Saran fibres ), are also used as flame-retardant components in blends of fire protection equipment.i ° i i Modacrylic fibres are typically copolymers of vinyl chloride or vinylidene dichloride and acrylonitrile. However, while, modacrylic fibres are non-flammable and do not melt or drip, they shrink rapidly when exposed to fire, and thus are rarely used in firefighters clothing. 

The GE LNP Engineering Plastics subsidiary offers THERMOCOMP HT Solder UE-1006 and THERMOCOMP HT Solder ZE-1006 compounds which comprise a matrix of resin blends containing 30% glass. The former compound is based on PPA resin whereas the latter uses a matrix of modified-polyphenylene ether. Both these grades are claimed to offer high distortion temperatures of over 260 °C, excellent dimensional stability and excellent flame retardancy. LNP also manufactures Starflam Eco-Fr compounds which are based on glass-fibre reinforced PPA coupled with ECO friendly flame retardant technology. 

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