GB1578810A - Flame-retardant copolyester - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO FLAME-RETARDANT COPOLYESTER (71) We, AVTEX FIBERS INC., a Corporation organised and existing under the laws of the State of New York, United State of America, of 580 East Swedesford Road, 9 Executive Mall, P.O. Box 880, Valley Forge, Pennsylvania 19482, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to filament-forming random copolyester resins, such resins in shaped form, especially in the form of textile fibre, blends of such textile fibre, and a method of making such a resin.

It is known that polyester resins have a high carbon content and are quite flammable.

They can be rendered flame-retardant by incorporating inorganic and organic materials therein, especially those containing bromine. chlorine, phosphorous, antimony, zinc and alumina. The main drawback of the prior systems is the adverse effects they have on fibres produced therefrom, since appreciable quantities of flame-retardant agents are required to promote the required level for flame-retardance. As a result, these materials can render the fibre brittle or otherwise affect the usually outstanding physical properties of the polyester.

Another disadvantage to the additive approach is the fact that such additives can often be easily leached out or removed during normal laundering and dry cleaning, causing an adverse effect on the flame-retardancy of the fabric.

An alternative method to improve the flame-retardancy of a polyester resin is the incorporation of a comonomer which contains one or more of the elements that are known to impart flame-retardancy. The major disadvantage of this approach is that the molar concentration of the comonomer, which is required to achieve the desired level of flame-retardance, is generally so high that the resultant resin manifests physical properties not usually associated with polyester. For example, a large proportion of comonomer could lower the melting point and. as a result. limit the utility of the polyester. Likewise, certain comonomers lower the crystallinity of the polymer, producing amorphous polymers which are incapable of producing commercially suitable textile fibres. If a flame-retardant copolyester could be produced with good physical and thermal properties, it would receive widespread acceptance for the manufacture of shaped articles.

The need for a polyester fibre which has good physical properties and high flame-retardancy is most critical for yarn and fabric blends of polyester fibres and cellulosic fibres. Polyester fibres are thermoplastic and, when exposed to a flame, burn and melt away from the flame, thus extinguishing themselves. If polyester fibres are blended with flammable cellulosic fibres and exposed to a flame, the polyester is more likely to continue burning even when melting since the burning cellulosic fibre continuously ignites it. If polyester fibres are blended with flame-retardant cellulose fibres and the blend ignited, the flame-retardant cellulosic fibres burn only in the area of flame contact. However, the flame-retardant cellulosic fibre acts as a scaffold or support and prevents the polyester fibre from dripping or shrinking away from the flame, and thus the polyester continues to burn.

According to the first aspect of the present invention there is provided a filament-forming random copolyester resin of at least 75 mol % of ethylene-2.6-naphthalene dicarboxylate units and from 5 mol /o to 25 mol % of alkylene-2,5-dibromoterephthalate units, the alkylene radical of which contains from 2 to 10 carbon atoms per radical.

According to the second aspect of the present invention there is provided a shaped article, especially textile fibre, of the resin of the first aspect.

According to the third aspect of the present invention there is provided a textile fibre blend comprising textile fibre of the second aspect blended with flame-retardant cellulosic textile fibre in an amount of from 10 to 90% by weight of copolyester fibre and from 90 to 10% by weight of cellulosic fibre. Preferably, the cellulosic textile fibre is regenerated cellulose fibre containing a flame retardant amount of a water-insoluble, liquid polymer of di-n-propyl phosphonitrilate.

According to the fourth aspect of the present invention there is provided a method of making a filament-forming random copolyester resin, which method comprises effectively copolymerising at least 75 mol % of ethylene-2,6-naphthalene dicarboxylate units and from 5 mol % to 25 mol % of alkylene-2,5-dibromoterephthalate units, the alkylene radical of which contains from 2 to 10 carbon atoms per radical.

Thus, it is an object of the present invention to enable the provision of a polyester resin having improved flame-retardance and being suitable for the formation of fibres which will meet more stringent flame-retardant test requirements while maintaining good fibre physical properties, to enable the provision of a flame-retardant fibre blend of a flame-retardant polyester fibre and a flame-retardant cellulose fibre which fibre blend has good physical properties, and to enable the provision of fabrics having good permanent flame-retardancy and good physical properties which fabrics are prepared from polyester fibres and cellulosic fibres.

In the resin according to the invention, the alkylene radical is derived from an alkylene glycol which can be structurally shown as HO(CH2)nOH where n is an integer of from 2 to 10.

The copolyester can contain up to 10 mol % of other interpolymerised ester units as is well known in this art to vary the properties of the resin as desired without deleteriously affecting the flame-retardant properties of the resin. Other ester units, which may be interpolymerised constituents of the copolyester chain in amounts up to 10 mol % are well known in the art and are usually derived from other diacids and diols. Some examples of these diacids and diols include terephthalic acid, isophthalic acid, bibenzoic acid, sulphoisophthalic acid, sulphoalkoxyisophthalic acid, diphenyl sulphone dicarboxylic acid, malonic acid and glutaric acid: alkylene glycols having from 3 to 12 carbon atoms, gem-dialkyl glycols, bis(hydroxymethyl) cyclohexane and diethylene glycol.

The copolyesters of this invention are preferably prepared by reacting (polycondensing) the 2,5-dibromoterephthalic acid with a low molecular weight prepolymer of ethylene 2,6-naphthalene dicarboxylate and an alkylene glycol having from 2 to 1() carbon atoms, preferably ethylene glycol. The reaction of the dialkyl ester of the brominated acid with the prepolymer is not recommended since its reactivity is severely limited. The dibromoterephthalic acid can also be used as an initial reactant in a direct esterification procedure with 2,6-naphthalene dicarboxylic acid and glycol.

The copolyester resins of this invention generally have an intrinsic viscosity of at least 0.25, and preferably at least 0.4 as determined in a 60 weight 9c phenol and 40 weight % tetrachloroethane solution at 3() C.

The copolyester resin described herein can have various additives incorporated therein to improve the resin properties. For example. heat. oxidation and ultra-violet light stabilisers, antistatic agents, plasticisers, dyes. and pigments can be employed.

Additionally, a metal compound selected from antimony oxides, for example antimony trioxide; antimony salts of a-hydroxycarboxylic or a.li-dicarboxylic acid (see German Offenlegungsschrift No. 2121186). zinc oxide. alumina and mixtures thereof can be mixed into the copolyester resin to provide additional improvement in flame-retardant properties.

The metal compound is preferably present in an amount such that the metal is present in an amount of from 0.5 to 5%. based on the weight of the resin.

While the copolyester resin of this invention can be formed into various shaped articles including filaments. bands. sheets and moulded articles. it is especially useful when formed into textile fibres and yarns. These fibres are used. for example. to prepare flame-retardant clothing, carpets and draperies.

Fibres or filaments are usually formed by melt extrusion of the copolyester resin composition through a multihole spinneret in a conventional manner. The as-spun yarn is then conventionallv oriented to produce textile yarn of the continuous filament or staple fibre type.

A most preferred embodimcnt of this invention comprises a mixture of fibres of the flame-retardant copolyester resin described above and flame-retardant cellulosic fibres, especially those having permanent flame-retardant properties. Mixtures or blends of these flame-retardant copolyestcr fibres and flame-retardant cellulosic fibres can provide textile fabrics having the highlv desirable wear characteristics of polyester textiles with the highly desirable comfort characteristics of cellulosic material.

The flame-retardant cellulosic fibres preferably include cotton, rayon or cellulose acetate fibres which have been combined, impregnated or coated with flame-retardant chemicals which provide substantially permanent flame-retardant properties therefor without degrading the physical properties of the fibre. That is, the cellulosic fibres or fabrics produced therefrom should be capable of withstanding periodic washing or cleaning with conventional dry cleaning solvents without losing much of their flame-retardant properties.

Many flame-retardant treatments for cellulosic fibres are known and several have been found to produce substantially permanent flame-retardancy. It is preferred, in the case of artificially prepared cellulosic fibres such as rayon and cellulose acetate, that the flame-retardant chemical be: incorporated into the cellulosic spinning solution thereby providing cellulosic fibres having the flame-retardant "locked in" the cellulosic matrix.

Examples of the preparation of these types of cellulosic fibres are found in United States Patent Specifications Nos. 2,816,004, 3,266,918, 3,321,330, 3,455,713, 3,645,936 and 3,704,144.

Orie preferred form of this invention involves the use of the flame-retardant regenerated cellulose filaments or fibres described in United States Patent Specification No. 3,455,713.

These fibres have been found to have excellent physical properties and permanent flame-retardancy. In brief, they are regenerated cellulose filaments having dispersed therein a substantially water-insoluble, liquid phosphonitrilate polymer having the general formula:

in which R and R' represent the same or different alkyl or alkenyl radicals containing from one to six carbon atoms and n is an integer of at least three.

These filaments are preferably prepared by incorporating a flame-retarding amount of the phosphonitrilate polymer in filament-forming viscose, and spinning and regenerating filaments.

In another preferred aspect of the present invention, the flame-retardant cellulosic fibres are cellulose acetate fibres prepared by incorporating compounds such as tris-(2,3dibromopropyl) phosphate or similar compounds as disclosed in United States Patent Specification No. 3,321,330 into the acetate spinning dope and wet or dry spinning the fibres.

It is further preferred that fibre blends of the present invention contain from 10 to 90, preferably 20 to 80 weight percent, of polyester fibres and from 90 to 10, preferably 80 to 20 weight percent, of cellulosic fibres.

The blended or combined flame-retardant polyester and cellulosic fibres can be used in various appropriate fibre and fabric constructions including, for example, spun staple yarns, mixed or tangled continuous filament yarns, novelty yarns, knit, woven and non-woven fabrics.

The flame-retardant pqlyester fibres described herein can also be blended with or combined in a fabric with normally flame-retardant fibres including, for example, glass fibres, polyvinyl chloride fibres, asbestos fibres, metal fibres, modacrylic fibres such as those sold under the trademarks "Dynel" and "Verel". and aromatic ring polyamide fibres such as that sold under the trademark "Nomex". Fibre and fabric blends can, of course, comprise more than one of the other known flame-retardant fibres with the flame-retardant polyester fibre of this invention.

It is realized that blends of polyester fibres and cellulosic fibres have been treated, usually in the form of a fabric, with flame-retardant chemicals to provide flame-retardant material.

However, this approach does not usually provide fabrics which will retain their flame-retardant properties after many washings or dry cleaning treatments. Furthermore, such after-treatments tend to stiffen the fabrics to an undesirable extent.

The following Examples further illustrate the present invention.

Example 1 2,5-dibromoterephthalic acid was prepared in accordance with the procedure described in British Patent Specification No. 946.259 except that the brominated terephthalic acid was not esterified but recrystallised from glacial acetic acid.

Filament-forming random copolymer resins containing varied amounts of bromine with or without antimony were prepared by reacting 2,5-dibromoterephthalic acid with bis(2-hydroxyethyl)-2,6-naphthalene dicarboxylate under conventional polycondensation conditions. Varying amounts of antimony trioxide were incorporated with the reactants to obtain the desired antimony content. In a similar manner, various filament-forming random copolymer resins of 2,5-dibromoterephthalic acid and bis(2-hydroxyethyl) terephthalate were prepared.

The copolyester resins were first evaluated for flame-retardancy by grinding the resin sufficiently for the resulting particulate to pass through a 10 mesh screen and pressed into plaques 1/32" x 5 1/2" x 5 1/2". The plaques were prepared as follows: A chrome plated brass plate is placed in a Carver Press; a sheet of 6 1/2" x 6 1/2" "Teflon" coated aluminum foil is placed on the brass plate, followed by a 6" x 6" x 1/32" spacer with inside dimensions of 5 1/2" x 5 1/2". A 6 gram sample of the polymer to be evaluated is spread evenly inside the spacer.

Next, a 5 1/2" x 5 1/2" square of fibreglass fabric is placed on the resin powder. Another 6 grams of resin is spread on the top surface of the fibreglass, followed by another sheet of "Teflon" coated foil and a second chrome plated brass plate. The press platens (previously heated at 270"C.) are slowly closed to the point where they just begin to touch the top chrome plate. After 3 minutes, the platens are tightly closed and the pressure raised to 10,000-12,000 p.s.i.g. After 1 minute, the pressure was released and the laminate quenched in a cold bath. The resultant plaques were cut into 1/2" x 5 1/2" strips and evaluated in the Standard Method of Test for Flammability of Plastics Using the Oxygen Index Method, ASTM-D-2863-70, commonly called the LOI test. The higher the LOI number, the better the flame-retardant property of the resin. The words "Carver" and "Teflon" are Trade Marks.

The results of the LOI test on various plaques of resins prepared as described above are set forth in the following Table I. The designation PET-DBT in the following table indicates a filament-forming copolyester of ethylene terephthalate units and ethylene-2,5dibromoterephthalate units and the designation PEN-DBT indicates a filament-forming copolyester of ethylene 2,6-naphthalene dicarboxylate units and ethylene-2,5dibromoterephthalate units as described for this invention.

TABLE I Resin Bur.,% Sub.,%* LOI Number (1) PET-DBT 5.22 - 22.4 (2) PET-DBT 9.81 - 26.7 (3) PEN-DBT 4.69 30.5 (4) PET-DBT 5.13 0.94 26.3 (5) PEN-DBT 4.55 0.50 35.2 (6) PET-DBT 9.41 1.00 30.0 (7) PEN-DBT 9.88 0.60 44.0 (8) PET-DBT 13.90 1.06 36.0 * Percent bromine (from brominated comonomer) based on the weight of the resin composition.

Percent antimony (from antimony trioxide) based on the weight of the resin composition.

In the above Table 1. the mol % of ethylene-2,5-dibromoterephthalate in the PEN-DBT resins (3), (5) and (7) were, respectively, 7.38%, 7.15C/c and 16.136/c.

It can be seen from the results in Table I that the copolyester resins of this invention can have much greater flame retardant effectiveness than similar copolyesters of ethylene terephthalate and ethylene-2, 5-dibromoterephthalate.

Example 11 A copolyester of ethylene-2,6-naphthalene dicarboxylate units and ethylene-2,5dibromoterephthalate units was prepared using the following typical proceedure.

A reaction vessel. equipped with a nitrogen inlet. heating means and stirring means, was charged with 498.44 grams (2.0 moles) of dimethyl-2,6-naphthalene dicarboxylate, 260.4 grams (4.2 moles) of ethylene glycol and 0.1147 grams of manganous acetate (0.04 mol % on the moles of the dicarboxylate). The mixture was slowly heated under a nitrogen atmosphere to 1650C. over a period of 45 minutes, at which time the first drop of distillate was observed; after an additional 33.0 minutes, the reaction temperature had reached 193"C., at which time 50% of the theoretical methyl alcohol had been collected. Heating was continued at 193-221"C. for 90 minutes, followed by a final period of 70 minutes at 221-225"C. After cooling, the reaction equipment was dismantled, giving 611.4 grams of white prepolymer. This was used without further purification in the next step.

One hundred thirty-seven and three-tenths grams of the above prepolymer, 12.7 grams of 2,5-dibromoterephthalic acid, 0.039 gram of Sb203 and 0.594 gram of triphenyl phosphite were charged into a reaction vessel: The mixture was heated to 225"C. in 40 minutes under a nitrogen atmosphere. The temperature was raised from 225"C. to 275"C. in 65 minutes while lowering the pressure to 0.6 ntm. of mercury. After 90 minutes, the polycondensation was finished, giving the copolyester resin. The resulting resin was spun into a 10 filament yarn through a spinneret affixed to the reactor bottom and the yarn was uniformly oriented by drawing at a temperature of 149"C. and at a 5.025:1 draw ratio.

This oriented polyester yarn was then combined in a conventional manner with a permanent flame-retardant rayon to provide a 50/50 blended yarn. The rayon was prepared in accordance with United States Patent Specification No. 3,455,713 and contained about 15% by weight of a water-insoluble, liquid polymer of di-n-propyl phosphonitrilate. The resulting yarn blend was knit on a Lawson knitting machine into a sleeve weighing 4.93 oz.

per square yard. The blended fabric was evaluated in a vertical flammability test as defined by the United States Department of Commerce FF 3-71 (37 F.R. 146,424), "Standard for the Flammability of Childrens Sleepwear". The test results are shown in Table II. The word "Lawson" is a Trade Mark.

TABLE 11 Vertical Flammability Test (3 second bone dry) 5 samples, NAF1, 2.11" CL2 NAF "no after flame", meaning material self-extinguishing when flame source was removed 2 CL = "char length" An analysis of the copolyester fibre produced in this Example revealed the presence of 4.58% bromine, an intrinsic viscosity of 0.36, a free carboxyl content of 13 meq./kg., a glass transition temperature of 76"C., and a crystalline melting point of 246"C. The mol % of ethylene-2,5-dibromoterephthalate in the fibre was 7.20%. This Example indicates that the PEN-DBT resins of Example I would also pass the vertical flame test if they had been spun into fibres.

Example 111 (Comparatory) In a manner similar to that described in Example II, 29.2 grams of 2,5dibromoterephthalate acid was added to 150 grams of prepolymer (previously prepared by reacting 2.1 moles of ethylene glycol with 1.0 mole of dimethyl terephthalate in the presence of calcium acetate; 0.92% methoxyl; 7.31% free ethylene glycol; 6 meq./gram, free carboxyl; 1.18% diethylene glycol content). The polycondensation was catalysed by 0.12 grams of antimony trioxide.

The temperature was raised from 200"C. to 250"C. in 2 hours, while lowering the pressure to 0.6 mm. of mercury. After 210 minutes, the polycondensation was finished giving the copolyester resin. The resulting resin was spun into a 10 filament yarn through a spinneret affixed to the reactor bottom. The yarn was uniformly oriented by drawing at a temperature of 121"C. and at a 3.36:1 draw ratio. After drawing, it was combined in a conventional manner with the flame-retardant rayon described in Example II to provide a 50/50 yarn blend. This yarn was knit on a Lawson knitter and weighed 6.95 oz. per square yard. The fabric blend was evaluated in the vertical flame test in the same manner as described in Example II. The test results are shown in the following Table III.

TABLE III Vertical Flammability Test (3 second bone dry) Burn (1) 2.0 sec. AF' - 3.06" CL2 (2) 2.0 sec. AF - 3.19" CL (3) 1.0 sec. AF - 3.6" CL (4) 11.2 sec. AF - 3.56" CL (5) 2.0 sec. AF - 3.31" CL AF = after flame; material continued to burn for the period shown after the flame was removed 2 CL = char length; original sleeve length was 10 inches An analysis of the polyester fibre of this Example before blending revealed that it contained 9.49 weight % bromine, had an intrinsic viscosity of 0.49, a free carboxyl content of 13 meq./kg. and contained 0.4% of diethylene glycol. This data shows that in spite of the increased quantity of bromine in the ethylene terephthalate copolymer, the copolyester containing ethylene 2,6-naphthalene dicarboxylate units is clearly superior. Copolyesters containing ethylene terephthalate units manifest longer periods of burning after the flame is removed causing 50% longer char lengths. The effect of the relatively small amount of bromine plus the synergistic influence of the 2,6-naphthalene dicarboxylate units on the flame-retardancy is quite unexpected.

Example IV (Comparatory) To a stainless steel polymerisation reactor equipped with stirring and heating means was added 33 pounds of dimethyl terephthalate, 22 pounds of ethylene glycol and 22.1 grams of calcium acetate. After heating for 4 hours at 2()-2250C., the transesterification sequence had been completed. At this point. 4.5 pounds of 2,5-dibromoterephthalic acid and 15.7 grams of antimony trioxide were added to the mixture. Heating and stirring were continued for an additional hour. The pressure was gradually lowered over 75 minutes to 1.0-1.4 mm.

of mercury, while the temperature was simultaneously increased to 250"C. After about 3 hours and 15 minutes, the poly-condensation was terminated. The resultant resin was extruded onto a moving belt and thence diced into small chips.

The resin was yellow in colour; it had an intrinsic viscosity of 0.42; free carboxyl, 16 meq./kg contained 5.760/c bromine and melted at 222"C. The resin was melt spun in the conventional manner to give a 34 filament yarn. The samples could not be drawn using a heated pin (93"C.), thus cold drawing was necessary. Occasional splits occurred during the drawing. The physical properties as shown in Table IV are averages obtained from several trials.

Table IV summarises the physical properties obtained from the polyester yarns prepared in Examples II, III and IV. The yarns were processed in each case to obtain the best physical properties.

TABLE IV Example Bromine Tenacity Elongation (%) (g./d.) (%) PET' Control 0 3.4 39.2 PEN2 Control 0 5.1 34.8 II (PEN-DBT) 4.58 4.0 34.6 III (PET-DBT) 9.49 2.1 55.7 IV (PET-DBT) 5.76 2.4 16.6 PET = poly(ethylene terephthalate) 2 PEN = poly(ethylene-2,6-naphthalene dicarboxylate) The data in Table IV indicate that polyester fibres, obtained from the copolymerisation of 2,5-dibromoterephthalate acid with naphthalene-2,6-dicarboxylic acid and ethylene glycol are unexpectedly superior in physical properties to those obtained from the corresponding copolymer of terephthalic acid and at the same time meet stringent flame-retardant standards. The results shown for the fibres of Example III indicate that yarn blends containing yarns prepared from copolyesters of terephthalic acid and 2,5-dibromoterephthalic acid, although sufficient to impart flame-retardance are physically weaker and as a result will not give the outstanding wear performance typically associated with the non-flame-retardant blend.

WHAT WE CLAIM IS: 1. A filament-forming random copolyester resin of at least 75 mol % of ethylene-2,6naphthalene dicarboxylate units and from 5 mol % to 25 mol % of alkylene-2,5dibromoterephthalate units, the alkylene radical of which contains from 2 to 10 carbon atoms per radical.

2. A copolyester resin according to Claim 1, wherein the alkylene-2,5dibromoterephthalate units are ethylene-2,5-dibromoterephthalate units.

3. A copolyester resin according to Claim 1 or 2, which has dispersed therein a metal compound selected from antimony oxides, antimony salts of a-hydroxycarboxylic or a,(3-dicarboxylic acid, zinc oxide and alumina, such that the metal is present in an amount of from 0.5 to 5% based on the weight of the resin.

4. A copolyester resin according to Claim 1, 2 or 3, in the form of a shaped article.

5. A copolyester resin shaped article according to Claim 4, in the form of textile fibre.

6. Copolyester resin textile fibre according to Claim 5, blended with flame-retardant cellulosic textile fibre in an amount of from 10 to 90% by weight of copolyester fibre and from 90 to 10% by weight of cellulosic fibre.

7. A textile fibre blend according to Claim 6, wherein the cellulosic textile fibre is regenerated cellulose fibre containing a flame-retardant amount of a water-insoluble, liquid polymer of di-n-propyl phosphonitrilate.

8. A textile fibre blend according to Claim 6, wherein the cellulosic textile fibre is cellulose acetate fibre.

9. A textile fibre blend according to Claim 6, wherein the cellulosic textile fibre is cotton fibre.

10. A method of making a filament-forming random copolyester resin, which method comprises effectively copolymerising at least 75 mol % of ethylene-2,6-naphthalene dicarboxylate units and from 5 mol % to 25 mol % of alkylene-2.5-dibromoterephthalate units, the alkylene radical of which contains from 2 to 10 carbon atoms per radical.

11. A filament-forming random copolyester resin, substantially as described in foregoing Example I, Resin 3.

12. A filament-forming random copolyester resin, substantially as described in foregoing Example I, Resin 5.

13. A filament-forming random copolyester resin, substantially as described in foregoing Example I, Resin 7.

14. A filament-forming random copolyester resin, substantially as described in foregoing Example II.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. TABLE IV Example Bromine Tenacity Elongation (%) (g./d.) (%) PET' Control 0 3.4 39.2 PEN2 Control 0 5.1 34.8 II (PEN-DBT) 4.58 4.0 34.6 III (PET-DBT) 9.49 2.1 55.7 IV (PET-DBT) 5.76 2.4 16.6 PET = poly(ethylene terephthalate) 2 PEN = poly(ethylene-2,6-naphthalene dicarboxylate) The data in Table IV indicate that polyester fibres, obtained from the copolymerisation of 2,5-dibromoterephthalate acid with naphthalene-2,6-dicarboxylic acid and ethylene glycol are unexpectedly superior in physical properties to those obtained from the corresponding copolymer of terephthalic acid and at the same time meet stringent flame-retardant standards. The results shown for the fibres of Example III indicate that yarn blends containing yarns prepared from copolyesters of terephthalic acid and 2,5-dibromoterephthalic acid, although sufficient to impart flame-retardance are physically weaker and as a result will not give the outstanding wear performance typically associated with the non-flame-retardant blend. WHAT WE CLAIM IS:

1. A filament-forming random copolyester resin of at least 75 mol % of ethylene-2,6naphthalene dicarboxylate units and from 5 mol % to 25 mol % of alkylene-2,5dibromoterephthalate units, the alkylene radical of which contains from 2 to 10 carbon atoms per radical.

2. A copolyester resin according to Claim 1, wherein the alkylene-2,5dibromoterephthalate units are ethylene-2,5-dibromoterephthalate units.

3. A copolyester resin according to Claim 1 or 2, which has dispersed therein a metal compound selected from antimony oxides, antimony salts of a-hydroxycarboxylic or a,(3-dicarboxylic acid, zinc oxide and alumina, such that the metal is present in an amount of from 0.5 to 5% based on the weight of the resin.

4. A copolyester resin according to Claim 1, 2 or 3, in the form of a shaped article.

5. A copolyester resin shaped article according to Claim 4, in the form of textile fibre.

6. Copolyester resin textile fibre according to Claim 5, blended with flame-retardant cellulosic textile fibre in an amount of from 10 to 90% by weight of copolyester fibre and from 90 to 10% by weight of cellulosic fibre.

7. A textile fibre blend according to Claim 6, wherein the cellulosic textile fibre is regenerated cellulose fibre containing a flame-retardant amount of a water-insoluble, liquid polymer of di-n-propyl phosphonitrilate.

8. A textile fibre blend according to Claim 6, wherein the cellulosic textile fibre is cellulose acetate fibre.

9. A textile fibre blend according to Claim 6, wherein the cellulosic textile fibre is cotton fibre.

10. A method of making a filament-forming random copolyester resin, which method comprises effectively copolymerising at least 75 mol % of ethylene-2,6-naphthalene dicarboxylate units and from 5 mol % to 25 mol % of alkylene-2.5-dibromoterephthalate units, the alkylene radical of which contains from 2 to 10 carbon atoms per radical.

11. A filament-forming random copolyester resin, substantially as described in foregoing Example I, Resin 3.

12. A filament-forming random copolyester resin, substantially as described in foregoing Example I, Resin 5.

13. A filament-forming random copolyester resin, substantially as described in foregoing Example I, Resin 7.

14. A filament-forming random copolyester resin, substantially as described in foregoing Example II.

15. Copolyester resin textile fibre, substantially as described in foregoing Example II.