Throughout this specification, the term"blended fibre"will be used to include intimate blends (where staple fibre of two or more different fibres are spun together to form a single yarn). Staple fibres are formed by chopping different extruded fibre into short predetermined lengths.

Such staple fibres have the required physical properties to enable the spinning of intimate blends containing two or more fibres.

The term"blended fabric"will be used to include fabrics made entirely from a single blended fibre, fabrics made from two or more blended fibres and union fabrics (where warp and weft are each made from a different fibre, at least one of which is a blended fibre).

Blended fibres are used in the production of fabrics, in woven and knitted constructions, in order to impart to the finished fabric the optimised properties of the blend components, for example strength, wear, handle, drape, dyeing capacity, resistance to chemical attack and flame- retardancy.

Flame-retardant fabrics are known and can include fibres which have been rendered flame-retardant and/or fibres which are inherently flame- retardant.

Where a fibre is to be rendered flame-retardant, it is convenient for this to be done after the fibre has been converted into fabric. Cellulosic fibres are frequently treated in this way.

A known process for the flame-retardant treatment of fabrics including cellulosic (e. g. cotton) fibres consists of impregnation of the fabric with an aqueous solution of a poly (hydroxyorgano) phosphonium compound, for example, a tetrakis (hydroxyorgano) phosphonium (THP+) salt.

Alternatively, the poly (hydroxyorgano) phosphonium compound may comprise a condensate with a nitrogen containing compound such as urea.

Following impregnation, the fabric is then dried and cured with ammonia to produce a cured, water-insoluble polymer which is mechanically fixed within the fibres of the fabric. After curing, the polymer is oxidised to convert trivalent phosphorus to pentavalent phosphorus and the fabric is washed and dried.

Fabrics treated according to the aforesaid process and garments made from such treated fabrics are sold under the Registered Trade Mark PROBANT of Rhodia Consumer Specialties Limited.

A particularly preferred treatment is disclosed in the applicant's granted patent GB 2 294 479, which provides a composition adapted to confer both flame-retardant and fabric-softening properties on a textile material, the composition comprising an aqueous solution of the product obtained by the reaction of: (a) a tetrakis (hydroxyorgano) phosphonium (THP+) salt; (b) an organic nitrogen compound; and (c) an aliphatic, hydroxyl-reactive compound containing at least one alkyl group having 12 or more carbon atoms.

An alternative known process for rendering cotton fibres flame-retardant comprises treating the fibres (either before or after incorporation into a fabric) with N-methylol dimethyl phosphonopropionamide (DMPP).

Compositions including DMPP are available under the Registered Trade Mark PYROVATEX of Ciba Speciality Chemicals.

It is also known to produce flame-retardant fabrics using the inherently flame-retardant polyamide imide fibres, available under the Registered Trade Mark KERMEL of Rhodia Performance Fibres, Polyamide Division. As well as inherent flame-retardant properties, these fibres have high tensile strength and tear strength.

It has also been proposed to produce a blend of cotton and poly m-phenylene isothphalamide (aramid) fibres. The cotton fibre in these blends is a cotton fibre which has been rendered flame-retardant by means of a composition such as PYROVATEX°. The applicants have found that these blends of aramid fibre and flame- retardant cotton do not possess all of the properties required in industrial worker end-use applications. This remains the case even if a cotton treated with a flame-retardant based on N-methylol dimethyl phosphonopropionamide is blended with fibres that possess superior strength properties (such as polyamide imide fibres). Blends of polyamide imide and cotton fibres treated with N-methylol dimethyl phosphonopropionamide have been found to have low physical strength and poor resistance to ultra-violet radiation. The poor performance of such blends in terms of resistance to ultra-violet radiation is attributable to both the inferior performance of cotton treated with N-methylol dimethyl phosphonopropionamide and of the polyamide imide fibre.

Flame-retardant finishes based on N-methylol dimethyl phosphonopropionamide have also been found to be susceptible to acid hydrolysis over storage, wear and wash in-use cycles. We have found that

acid hydrolysis of N-methylol dimethyl phosphonopropionamide treated cotton can exhibit a degrading effect on the flame-retardant performance of blends of such treated cotton fibres with polyamide imide fibres.

While fabrics including KERMEL fibres have found application in protective clothing for firefighters and in heat-resistant filters, and military clothing, KERMEL is a high cost fibre and as such its use has effectively been restricted to incorporation into textiles as a blend with viscose fibres (which have inherent flame-retardant properties). Such blends exhibit full physical strength, but inferior shrinkage performance, low wet strength and laundering properties. These disadvantages stem from the use of the viscose fibre. In addition to these aforementioned disadvantages it is also not possible to achieve satisfactory colouration of fabrics made from these blends for use in the industrial arena.

The applicants have now found that the use of a blend of PROBANT treated cellulosic fibres and polyamide imide fibres ameliorates many of the above disadvantages.

Accordingly, the present invention provides a flame-retardant fabric made from blended fibres (as hereinbefore defined), said fabric comprising polyamide imide fibres and cellulosic fibres, said cellulosic fibres having been rendered flame-retardant by treatment with a composition comprising the product obtained by reaction of: (a) a tetrakis (hydroxyorgano) phosphonium (THP+) salt; (b) an organic nitrogen compound; and optionally (c) an aliphatic, hydroxyl-reactive compound containing at least one alkyl group having 12 or more carbon atoms.

The THP+ salt is preferably a tetrakis (hydroxyalkyl) phosphonium salt, for example, tetrakis (hydroxymethyl) phosphonium chloride (THPC) or tetrakis (hydroxymethyl) phosphonium sulphate (THPS).

The organic nitrogen compound is preferably an amide, for example, urea or thiourea.

The aliphatic hydroxyl-reactive compound may be any one or more of the following: Primary amines Secondary amines Tertiary amines Diamines Quaternary ammonium salts Ethoxylated amines Ethoxylated diamines Amine oxides Alkyl amino-substituted carboxylic acids Amides Ethoxylated amides Amido-imidazolines Preferably, the aliphatic, hydroxyl-reactive compound contains at least one alkyl group having 14 carbon atoms. For example, the compound may be a primary, secondary or tertiary amine such as n-tetradecylamine.

The cellulosic fibre may be flax, cotton, linen, jute or hessian.

Preferably, the ratio of the cellulosic fibre to the polyamide imide fibre in the blend is in the range to 15: 85 to 85: 15.

More preferably, the ratio of cellulosic fibre to polyamide imide fibre in the blend is 70: 30.

The blended fibres used in accordance with the present invention may be in the form of an intimate or non-intimate blend, a staple blend or a core- spun yarn.

Flame-retardant fabrics according to the present invention may be woven, knitted or non-woven as an intimate blend or as a union fabric. The optimum weight of the fabric will be determined by the intended end-use, but a generally serviceable weight range for such fabrics would be 200 to 500 g/m2 (for example, 250 to 400 g/m2).

The present invention will be illustrated, merely by way of example, as follows: The tensile strength properties of a number of samples consisting of polyamide imide fibres, woven in an intimate blend with flame retardant viscose fibres or with cotton treated with a flame retardant finish, were determined in accordance with BS 2576 N/50mm test strip.

The results are set out in the TABLE (below).

The results obtained confirm the high level of tensile strength of polyamide imide fibres. The results also show the relatively high level of tensile strength that can be obtained from the combination of a polyamide imide fibre and cotton treated with a flame retardant based on tetrakis (hydroxymethyl) phosphonium chloride in an intimate blend (sample (a)).

The high level of tensile strength performance determined in the testing of this blend is especially evident when a comparison is made with the resultant tensile strength properties of intimate blends of polyamide imide

fibres and flame retardant viscose (sample (b) ). This was found to be the case, even when the blend of polyamide imide fibre and viscose contains a significantly higher level of polyamide imide fibre.

The tensile strength of an intimate blend of polyamide imide fibre with cotton treated with a flame retardant finished with N-methylol dimethyl phosphonopropionamide (sample (c) ), was found to be inferior to that of an intimate blend of a polyamide imide fibre with cotton treated with a flame retardant finish based on tetrakis (hydroxymethyl phosphonium chloride) (sample (a)).

The samples (a) to (c) each had a weight of 260 g/m2. Sample Fibres in Blend Ratio Tensile Strength No. (A: B) (N/50 mm strip) (A) (B) Mean Mean (warp) (weft) (a) Cotton: dyed KERMEL 70: 30 1056. 0 853. 9 and treated with PROBANT (b) KERMELO Viscose 50 : 50 1049.8 649.84 (c) Cotton : dyed KERMEL 70 : 30 683. 8 521. 3 and treated with PYROVATEX Control Nil KERMELX N/A 1729.8 1071.0