Flame Resistant Substrates

Fabrics comprising blends of cotton with polyester fibers are of considerable commercial importance because of their widespread use in apparel. However, such fabrics are highly flammable. To reduce the flammability of such blends, a flame retardant must be applied to the fabric. A flame retardant is a substance which increases the resistance of a substrate to burning or charring. Flame retardants are known for 100 percent cotton fabrics, but researchers in this field have concluded that the flammability of blend fabrics cannot be predicted from knowledge of the flammability of fabrics made from a single fiber, G. C. Tesoro, Status and Prospects for Flame Resistant Polyester/Cellulose Blend Fabrics, National Bureau of Standards Report COM-73-11265, March, 1973. One important reason for this unpredictability of blend fabrics, e.g., cotton/polyester blends, is the so-called "grid" or "scaffold" effect by which one component of the blend forms a supporting matrix for continued burning of the other component. With regard to this effect, W. Kruse reported:

"In all textile mixtures containing a component capable of forming a structural network (e.g. cotton) and a thermoplastic component (e.g. polyester) account must be taken of the scaffold effect, as it substantially alters the combustion behavior of thermoplastic synthetic fibers.

"In mixed textiles the framework is built in all cases by organic material. Interestingly enough, it is also possible to arrive at a scaffold effect with inorganic material if, for instance, single component fabrics of thermoplastic fibers are given a framework forming finish (e.g. silicate)." W. Kruse, Melliand Textilber, April 1969, pp. 460-469 (Gottlieb Duttweiler Institute Publication No. 45, pp. 137-161, 1969), Combustibility and Flame Resistant Finishing of Mixed Textiles.

In accordance with existing technology, polyester fibers can be flame retarded. In one method, a brominated component, tetrabromobisphenol-A ethoxylate, is incorporated in the polymerization reaction producing the polyester from which the fiber is spun. In another method, a polyester fiber is treated topically with an aqueous dispersion of tris-(2,3-dibromopropyl)phosphate which, at elevated temperature, undergoes a thermally induced diffusion into the polyester fiber, thereby reducing the flammability of the fiber.

Polyester fibers having tetrabromobisphenol-A ethoxylate incorporated therein or treated with tris-(2,3-dibromopropyl)phosphate are themselves flame retardant, but when blended with cotton the fire retardance of the polyester is insufficient to render the blend flame retardant. It has now been found that treatment according to the present method significantly increases the resistance of such blends of cotton/flame retardant polyester fibers to flame and provides a material which is significantly more flame retardant than presently known materials.

In accordance with the present invention, there is provided a flame resistant substrate of cotton and flame retardant polyester fibers, the fibers having phosphorus-containing flame retardant affixed thereto, the phosphorus-containing flame retardant being a compound of the formula ##STR1## where R.sub.1 is methyl, ethyl, propyl, phenyl, ClH.sub.2 or BrCH.sub.2 ; R.sub.2 is oxygen or sulphur; R.sub.3 is hydrogen, methyl, ethyl or propyl, and R.sub.4 is hydrogen, methyl, ethyl or propyl.

Also in accordance with the present invention, there is provided a method for increasing the flame resistance of a substrate composed of cotton and flame retardant polyester fibers including coating the fibers with a phosphorus-containing flame retardant, the phosphorus-containing flame retardant being a compound of the formula ##STR2## where R.sub.1 is methyl, ethyl, propyl, phenyl, ClCH.sub.2 or BrCH.sub.2; R.sub.2 is oxygen or sulphur; R.sub.3 is hydrogen, methyl, ethyl or propyl, and R.sub.4 is hydrogen, methyl, ethyl or propyl, drying the substrate, and curing the substrate. The flame retardant can be applied to the cotton polyester substrate from an aqueous solution by a standard pad, dry, cure and after wash procedure, or in any other conventional manner.

The polyester fibers may be made by any process well known in the art. The polyester fibers may be flame retarded with well known flame retardants which are conventional in the art. Especially preferred is a polyester fiber component which is flame retarded with a bromine-containing flame retardant compound and most preferable is the polyester having tetrabromobisphenol-A ethoxylate incorporated therein. The fabric may contain from 15% to 85% polyester by weight, the remainder being cotton. Preferably, the fabric may contain from about 35% to about 75% polyester by weight. More preferably, the fabric may contain from about 45% to about 65% polyester by weight. A most especially preferred polyester/cotton blend is a 50/50 weight percent blend.

The phosphorus-containing flame retardants used in the present invention are those compounds having the general formula ##STR3## where R.sub.1 is methyl, ethyl, propyl, phenyl, ClCH.sub.2 or BrCH.sub.2 ; R.sub.2 is oxygen or sulphur; R.sub.3 is hydrogen, methyl ethyl or propyl, and R.sub.4 is hydrogen, methyl, ethyl or propyl. Preferred flame retardants are those where R.sub.1 is an ethyl or methyl group, and R.sub.3 and R.sub.4 are hydrogen atoms. The most preferred is a compound in which R.sub.1 is a methyl group, R.sub.2 is oxygen, and R.sub.3 and R.sub.4 are hydrogen atoms, i.e., methyl phosphonic diamide, hereinafter abbreviated as MPDA. MPDA can be prepared according to the method of Ratz, J. Am. Chem. Soc., 77 4170 (1955), which is hereby incorporated by reference as if fully set forth. Certain halomethyl phosphonic acid bisamides have previously been disclosed as flame retardants in Dutch Patent Publication No. 66/5,460. In addition, flame retardants containing the ##STR4## group have been disclosed in U.S. Pat. No. 2,648,597, which is hereby incorporated by reference.

Preferably the phosphorous-containing flame retardant is 100% pure, but various other compounds such as salts may be intermixed with the phosphorus-containing flame retardants when produced commercially. Preferbly, such compounds are water soluble so that they can be easily removed from the substrate by washing. For example, salts such as ammonium chloride, sodium chloride, and the like, may be intermixed with the flame retardant. Such compounds are by-products of one process by which the phosphorus-containing flame retardant can be made, and because of the difficulty in separating them from the flame retardant, it is convenient to have them intermixed therewith. They are permissible and do not detract from the advantages of the present invention. Such other compounds may be intermixed with the flame retardant in amounts of up to about 150% of the flame retardant by weight, more preferably, in amounts of up to about 120% of the flame retardant by weight.

The amount of phosphorus-containing flame retardant may be varied in accordance with the degree of flame retardance desired. The amount of flame retardant is expressed as a percentage which is determined by dividing the weight of the flame retardant solid on the treated fabric by the weight of the untreated fabric and multiplying the quotient by 100. Such a percentage is hereinafter referred to as percent dry add on (and is abbreviated percent DAO). The percent dry add on of the phosphorous-containing flame retardant may vary from about 2% to about 35%, preferably from about 5% to about 20%, and more preferably, from about 7% to about 15%.

The solution with which the cotton polyester substrates are treated contains from about 5% by weight to about 30% by weight of the phosphorus-containing flame retardant, the remainder being water. More preferably, the treating solution contains from about 15% by weight to about 25% by weight of the phosphorus-containing flame retardant, the remainder being water.

The cotton/polyester substrate may be treated or impregnated with a solution of the phosphorus-containing flame retardant, by dipping the substrate into the solution of the phosphorus-containing retardant or by spraying or padding the solution of such flame retardant on the substrate and allowing the substrate to dry. The solution of such flame retardant may also be applied by spraying, dipping, padding, or the like. After the solution of such flame retardant has been applied, the substrate is dried by any conventional means known in the art.

After drying, the substrates are then cured by heating at a temperature sufficiently high enough and for sufficient time to firmly bond the phosphorus-containing flame retardant to the substrate. Curing preferably is carried out at a temperature of from about 330.degree. to about 360.degree. F for a period of from about 1/2 minute to about 5 minutes, or more preferably at a temperature of from about 340.degree. to about 350.degree. F for a period of from about 1/2 minute to about 11/2 minutes. After curing, the substrates may be washed to remove any salt such as ammonium chloride or sodium chloride which may be on the fabrics. After washing the substrates are then redried as in the previous drying.

Several fabrics composed of cotton/polyester blends were treated by dipping the fabric into a solution of the phosphorus-containing flame retardant followed by drying, curing, washing and redrying. The fabrics were then tested according to the procedure of Department of Commerce flammability test FF 3-71. The results are shown in the following table. In the table, the following explanations are used:

1. Fabric

A. 50% cotton/50% woven polyester (by weight) blend fabric, 2.6 oz/yd.sup.2, in which the polyester fiber was made flame retardant prior to making the fabric by incorporating a bromine-containing compound.

B. 35% cotton/65% polyester blend of materials of Fabric A.

2. treating Solution

I. one part crude MPDA (47% MPDA, 53% NH.sub.4 Cl) and two parts water. Parts and percentages by weight.

Ii. two parts crude MPDA (47 % MPDA, 53% NH.sub.4 Cl) and three parts water. Parts and percentages by weight.

3. Drying and Curing Conditions

Where drying is indicated, the fabric was dried at 90.degree. C to constant weight.

Where curing is indicated, the fabric was heated at 340.degree.-350.degree. F for 1 minute.

4. MPDA Efficiency

This term refers to the weight increase of the fabric because of fire retardant treatment divided by the weight of MPDA in the treating solution absorbed by the fabric multiplied by 100.

5. Vertical Flame Test

The test employed was the procedure of Department of Commerce flammability test FF 3-71. W refers to samples tested in the warp direction. F refers to samples tested in the fill direction. BEL is an acronym meaning burned entire length of the 10-inch specimen. Data are given in the table in the following order: (1) direction of test -- W or F meaning warp or fill; (2) char length, in inches; and (3) time of burning, in seconds, e.g., W 4.8 25. The samples were tested as produced and the results indicate initial values.

The following examples, presented in tabular form, should be considered illustrative of the invention and non-limiting. Unless otherwise indicated, all parts and percentages are by weight.

Although acceptance criteria for Flammability Test DOC FF 3-71 require that the average char length for 5 specimens be less than 7 inches and no specimen burns the entire length (BEL), when a BEL result is obtained the burning time is an important indication of relative difficulty of burning compared to other specimens also having BEL results. Thus, when two specimens are reported as "BEL", the one with a longer burning time is considered relatively more resistant to flame. Therefore, specimens of the present invention which burn the entire length can be considered more flame resistant, based on burning time, than the comparative specimens. For example, the specimens of Example 4, having BEL results with burning times of 23 and 21 seconds would be considered more resistant to flame than the comparative specimens, all of which gave BEL results and burned more than twice as fast, i.e., from 9-13 seconds. However, it should be noted that burning time is not considered of relative importance if the specimen did not burn the entire length.

For comparative purposes, samples not in accord with this invention were tested in the Vertical Flame Test for initial results. One sample used a blend of 50% cotton, 50% polyester, by weight, blend without treatment. All specimens burned the entire length within 12-13 seconds. Another sample was the same Fabric A described hereinabove as a 50% cotton/50% polyester in which the polyester has been flame retarded with a bromine-containing compound, but without treatment according to this invention. All specimens burned the entire length in 9-10 seconds.