US3454460A - Bicomponent polyester textile fiber - Google Patents

Description

United States Patent 3,454,460 BICOMPONENT POLYESTER TEXTILE FIBER David Emerson Bosley, Grifton, N.C., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Sept. 12, 1966, Ser. No. 578,443 Int. Cl. D01d 5/28, 5/22 US. Cl. 161-173 8 Claims This invention relates to synthetic textile fibers possessing permanent crimp and more particularily to polyester yarns which possess a latent ability to develop and retain crimp under tension.

Highly crimped synthetic thermoplastic fibers are desirable for textile use especially because fabrics produced therefrom have good cover, i.e., the ability to hide a background, pleasing tactile aesthetics, and exhibit a certain degree of recoverable stretch. Articles of clothing produced from such fabrics are comfortable because they yield to the actions of the wearer. Highly crimped polyester fibers are particularly desirable because they possess the added features of low moisture adsorption, durability, high tenacity and excellent aesthetic properties.

It is known that very high levels of crimp are obtained in synthetic biomponent fibers, i.e., fibers having at least two components of different polymeric species in intimate adherence along their length and in eccentric relationship (frequently side-by-side). The components exhibit different amounts of shrinkage or extension upon treatment with certain media such as hot fluids, and such treatment causes the filaments to assume a helical configuration. These fibers have the advantage of being inherently crimpable, that is, the crimp-developing forces are in the fiber from its inception and no external means, such as a stutter-box, is necessary to produce these forces. Sisson describes some of these fibers in US. Patent No. 2,439,815, dated Apr. 20, 1948, and in US. Patent No. 2,443,711, dated June 22, 1948.

To make a spun yarn with recoverable stretch from composite filaments, two requirements must be met: first, the fibers contemplated for such yarns must be amenable to carding, i.e., they must not be highly crimped prior to, or during carding, or else they will tangle and obstruct fiber passageways in the machinery; and second, the fibers must develop a high level of crimp while they are in the spun yarn, i.e., must be capable of crimping while under the influence of the relatively strong external forces imposed by virtue of their crowded position in the yarn.

The polyester composite filaments of the art do not possess both of the above requirements to a satisfactory extent and thus have not been entirely satisfactory in making spun yarns having recoverable stretch.

This invention provides an inherently-crimpable, composite polyester fiber which has latent crimpability. Another provision is a textile fiber which inherently develops a considerable amount of helical crimp. A further provision is a crimpable composite polyester fiber which develops crimp while under substantial external longitudinal force. A still further provision is a polyester fiber which possesses a high value of crimp elongation. Other provisions will appear hereinafter.

The stated advantages of the present invention are provided in a polyester textile fiber of two distinct components, extending along the fiber and intimately adhered along their lengths in eccentric relationship to the fiber cross section, having, as one of the components, a saturated glycol-terephthalate homopolyester of fiberforming molecular weight characterized by a relative viscosity of 15 to 25 and a T shrinkage value of 45 to 75 C.; and having, as the second of the components, a copolyester of (A) said saturated glycol-terephthalate and (B) either a different saturated glycol-terephthalate or a saturated glycol-isophthalate, the mole ratio of (A) to (B) being from 8:2 to 9:1, the copolyester being of fiber-forming molecular weight characterized by a relative viscosity at least 5 units higher than the homopolyester component and having a T shrinkage value within plus or minus 8 C. of the value for the homopolyester component. Preferably, the weight ratio of homopolyester component to copolyester component is from 40:60 to 60:40.

To facilitate the understanding of this invention an explanation of the terms used in the specification follows:

The term bicomponent fiber as used herein is a single fiber comprising two separate and distinct polymeric components in intimate adherence and in either side-byside or eccentric sheath-core relationship, each component extending longitudinally through the fiber.

Crimp refers to the non-straight configuration of a fiber, which in the case of the fibers of this invention, is a helical configuration. Crimp recovery refers to the ability of a fiber to regain its original crimped configuration after being pulled straight. Crimp recovery is indicated by the Crimp Elongation property measurement shown below.

Skein Shrinkage (8.8.) is the measure of fiber shrinkage afer immersion in boiling water notwithstanding the eifect of the crimp so induced. A simple procedure to determine this measurement is used: From the known denier of the yarn the number of turns on a skein reel (of any convenient size) required to achieve a denier of 1500 (167 Tex.) is calculated, using the formula:

Where T designates turns on the skein reel and d is the denier of the yarn. T is rounded 01f to the nearest whole number of turns. It will be obvious that such a skein must be considered as 3000 denier (334 Tex.) when loaded as a loop. The skein is then hung and a 1500-gram weight is applied at the bottom of the loop. The skein is exercised gently 4 times. After 15 seconds the initial length of the skein is measured (L The 300- gram weight is replaced with a 4.5-gram weight and the skein is immersed into boiling water for 15 minutes. The skein is removed from the water and dried at ambient temperature under no load. The length of the skein (L is then measured under the 4.5-gra1n load. The 300-gram load is then reapplied and the skein is exercised as described above and the extended skein length (L is measured. The skein shrinkage is calculated by the formula:

8.8. (percent) It is noted that this property is an important consideration for determining the economic value of the fiber.

Crimp. Development (CD) is the measure of the amount of crimp in a fiber notwithstanding the effect of skein shrinkage, supra. It is measured according to the procedure shown above and calculated by the formula:

(ID. (percent) L It should be noticed that CD. is measured under a measure this property: Using the procedure employed above for measuring Skein Shrinkage, one further step is added: After the measurement of L the 300-gram load is replaced by a 4.5-gram load and a second measurement of recovered length, (L under this load is made. Crimp Elongation is calculated by the formula:

Crimp Index (C.I.) is a measurement of as produced crimp; i.e., before appreciable crimp is induced in the fiber. It is measured by the following procedure: Initial preparation of the yarn to form a skein as shown in the procedure for Skein Shrinkage, supra, is performed. A weight of 0.1 gram per denier is applied to the skein and a measurement of the skein length is made, (L The weight is removed and replaced by a weight of 1.5 milligram/denier and a second length measurement is made, (L Crimp Index is calculated by the formula:

(LsLw) 100 e This measurement illustrates an important variable affecting staple yarn processing. Low Crimp Index indicates a low degree of crimp upon initial yarn production, which facilitates staple processing.

Second-order transition temperature (T as used herein, is the temperature at which the rate of shrinkage of a fiber in water increases substantially as the temperature of the fiber is raised. This measurement is made on undrawn fibers to minimize the elfect of crystallinity. Relatively large length changes occur in undrawn yarn over narrow temperature changes. A small amount of the polymer is extruded through an orifice to form a fiber. The T value of the fiber is referred to as the T shrinkage value of the polymer. T measured in water is usually lower than T measured in air, presumably because of a reduction of intra-molecular forces. Since, as illustrated herein, aqueous media are conveniently used to induce crimp in a fiber, T of the polymers described herein is measured in water unless otherwise specified. The general procedure for determining T is similar to that of previous methods based on other thermodynamic properties, such as the technique disclosed in Pace US. Patent No. 2,556,295 dated June 12, 1951.

It is noted that the T of a polymer measured by one technique may be difierent from the T measured by another technique. Therefore, the limitations as to the T values of polymers used in the present invention pertain to values obtained as described above, using the same analytical technique for each polymer.

Relative Viscosity (R.V.) is the ratio of the viscosity of a 10% by weight solution of polymer in a mixture of 10 parts of phenol and 7 parts of 2,4,6-trichlorophenol to the viscosity of the mixture, per se, measured in the same units at 25 C.

Intrinsic Viscosity (1 is the limit of the fraction as concentration approaches zero, where r is the relative viscosity as defined above, except that the relative viscosity is measured at several concentrations to facilitate extrapolation to zero concentration; and the solvent employed in this measurement is a mixture of three parts of methylene chloride and one part of trifluoroacetic acid (by weight).

Surprisingly, a polyester textile fiber has now been found which not only exhibits a C1. of less than 10%, thus indicating satisfactory staple processing, but also exhibits a CD. of greater than 10%, thus indicating its ability to imaprt to fabrics woven therefrom a very high degree of stretch.

In the practice of the present invention each polymer of the composite filament is prepared separately. The

C'.E. (percent) Cl. (percent) polyester homopolymer is prepared according to wellknown procedures such as disclosed in US. Patent No. 2,465,319, dated Mar. 22, 1949, or US. Patent No. 3,018,272, dated Jan. 23, 1962; the polyester copolymer may be prepared according to the procedures stated in US. Patent No. 2,965,613, date'd Dec. 2, 1960. The polymers are separately metered to and extruded from a spinneret such as one of the type shown in French Patent No. 1,442,768, to form composite filaments as herein described. The fiber is subsequently drawn and wound according to well-known polyester processing procedures. Crimp may be imparted to the fiber at any time subsequent to drawing. Of course, if a staple-spun yarn is contemplated, the fiber is cut to staple and processed into a yarn before appreciable crimp is developed.

To achieve the improved properties of the products of this invention the T shrinkage value of the copolyester component of the fiber must be within :8 C. of the T of the homopolyester component and the T shrinkage value of the homopolyester must be within the range of 45 75 C. The T of a few important homopolyesters and copolyesters is shown in the following table.

Homopolyester T C.) Poly(ethylene terephthalate) 57 Poly(1,4-cyclohexylene dimethylene terephthalate) 74 Copolyester Poly(ethylene terephthalate/sebacate) (/15) 43 Poly(ethylene terephthalate/isophthalate) (85/ 15 57 Poly(ethylene/ 2,2 dimethyl 1,3 propylene tereph- Within the above limitations, in the preparation of the copolymeric component, the relative proportion of the reactants may be varied to a certain extent. If a mixture of dibasic acids or their corresponding lower alkyl ester derivatives is condensed with a suitable saturated glycol, such as ethylene glycol, to produce a copolymer, the mole ratio of terephthalic acid or ester thereof to the other dibasic acid or ester constituent may be from approximately 8:2 to approximately 9:1 and is preferably approximately 8.5 :15. If this ratio is greater than approximately 9:1, the CD. of the composite filament will be low; if less than approximately 8:2, the melting point of the copolymeric component is depressed to an extent undesirable for textile purposes. Likewise, if a mixture of glycols is used to produce the copolymeric component, the principal glycol is the same as the glycol used in the homopolyester component and the mole ratio of this glycol to other glycol of the mixture may be from approximately 822 to approximately 9:1 and is preferably approximately 8.5: 1.5.

The polyester components are prepared so that the copolymeric component is of a higher RV than the homopolymeric component and both components are of an RV corresponding to fiber-forming molecular weight. Then when the filaments are treated as a single unit in their processing and in orientation-inducing phases of the operations; e.g., melt-spinning, the component of higher RV will become more oriented than the lower RV component. The RV of the homopolymeric component corresponds to fiber-forming molecular weight and may be from approximately 15 to 25 and, preferably, is approximately 20. The RV of the copolymeric component is preferably approximately 35 and, in a general sense, should be at least approximately 5 units higher than the RV of the homopolymeric component.

In connection with the RV adjustment, it is believed that the crimp potential of the new fibers derives from the difference in the extent of molecular orientation of each component of the fiber. The high RV component will become more oriented than a low RV component upon exposure to an identical degree of orientation inducement during spinning, and the greater the inducement, the greater the difference in orientation between the components. The total degree of molecular orientation of each component is the product of the orientation induced by extruding the polymer from the spinneret and withdrawing it at a finite rate during spinning, and the orientation induced by the subsequent yarn-drawing process. Therefore, the polymers comprising the yarns of this invention should be spun and drawn so as to maximize the total orientation of the copolyester component, and corresponding create a maximum of crimp potential in the yarn. Preferably, the yarn is spun at a speed greater than 300 yds./ min. (275 meters/ min.) and drawn to such an extent that the elongation at break of the composite filaments in the yarn is less than about 20% A further advantage of the new fibers exists in that they are amenable to annealing operations. Since annealing decreases the skein shrinkage of the fibers, the economic advantage is evident. Unlike the bicomponent fibers of the prior art, the new fibers may be annealed at reasonable temperatures without negating their capacity for high crimp development.

The weight proportion of the homopolymeric component to the copolymeric component in the new filament is such that the copolymeric component, which is the highshrinkage member of the filament, is present to such an extent as to produce a high crimp development in the filament. Weight ratios of copolymer to homopolymer between 40:60 and 60:40 are preferred.

Many of the spinnerets used in the prior art for spinning composite filaments may be used in accordance with this invention. Some of the patent disclosures that may be referred to for this purpose include Breen, U.S. Patent No. 2,987,797; Radow et al., U.S. Patent No. 3,039,174; Breen, US, Patent No. 3.038,236; Taylor, U.S. Patent No. 3,038,237; Breen, U.S. Patent No. 2,931,091; and Zimmerman, U.S. Patent No. 3,038,235. In general, any type of spinneret used for melt-spinning which provides means for merging two separate polymer streams to form composite filaments as herein described may he used in the practice of this invention.

It is essential that each component of the composite filaments of the present invention be of fiber-forming molecular weight; i.e., a molecular weight associated with useful properties of a fiber for textile purposes.

Two important advantages exist in the products of this invention; viz., the new fibers have a low degree of crimp in the as-produced form and retain their low degree of crimp when stretched cold, as in carding, and they have an inherent ability to develop a considerable amount of crimp upon heating, even under the influence of appreciable longitudinal forces. These characteristics are quantitatively described by the above-mentioned Cl, and CD. measurements, respectively. The new fibers preferably have CI. values of about 2% to Relating this value to staple-spun yarn processing, it has been found that the fibers have a low enough degree of initial crimp or as-produced crimp as not to hamper opening and carding operations, yet have enough crimp to allow staple spinning. The fibers have C.D. values of greater than 10%. This property enables them to impart a considerable degree of stretch to fabrics woven therefrom. The comparison, the CD. value of composite filaments of the prior art is approximately 1%. It is entirely unexpected that a polyester fiber of such a high (ID. value could have a low 01. value.

The crimp of the new fibers is developed by exposure of the fibers to a temperature of approximately 100 C. or reasonably above. Crimp is preferably developed in the yarn subsequent to its being Woven to fabrics and is most conveniently developed in one of the fabric-finishing operations, such as scouring, involving exposure of the fiber to high temperatures.

The fibers of this invention may be used to form continuous filament yarn or they may be converted to staple and spun to a yarn of any count conventionally used for textile purposes. These yarns may be woven to any construction used in the textile trade. For example, continuous filament, 73 denier yarn consisting of the new fibers may be woven to a fabric construction as tight as 90 ends per inch (34.5 per cm.) by 70 picks per inch (27.5 per cm.) and will impart to such fabrics stretch levels as high as 35%.

This invention is further illustrated by the following examples of preferred embodiments, although it is not to be restricted thereto unless otherwise specifically indicated.

EXAMPLE I This example illustrates the improved crimp characteristics of a yarn of the present invention as compared with a typical bicomponent yarn, the T of each component of which dilfers significantly. It further illustrates the significance of the stringent C.D. measurement employed herein.

(A) A 34-filament bicomponent (sidc by-side) yarn is prepared by melt-spinning poly(ethylene terephthalate/ isophthalate) 15 of 30.6 RV and poly(ethylene terephthalate) of 23 RV in a 50:50 weight ratio, simultaneously, at approximately 285 C., from a spinneret of the type described in French Patent No. 1,442,768. The yarn is drawn at C. to 4.26 times its original length and is found to be of 73 denier. The T of the homopolymer is 57 C. The T of the copolymer is approximately 57 C. The yarn is annealed at 170 C. at substantially constant length. The yarn has a CD. of 22%, a C1. of approximately 0.7%, and an S.S. of only 8%.

(B) To illustrate the significance of the stringent C.D. measurement as used in the present specification as compared to CD. determinations of the prior art, a similar yarn is produced in the same manner with the exception that the RV of the copolymer is 31.6, the spinning temperature is 280 C., the draw ratio is 4.54 and the annealing temperature is C. The CD. of this yarn, measured in accordance with procedures shown above with the exception that the 1.5 mg./ denier load is replaced ml a 0.6 mg./ denier load, is 30%.

For comparison, a 34-filament yarn is prepared by spinning poly(ethylene terephthalate/sebacate) (85/15) of 56 RV and poly(ethylene terephthalate) of 20 RV in a 50:50 weight ratio, simultaneously at approximately 284 C., from a spinneret equivalent to the one used to pro duce the test fiber of this example. The yarn is drawn at 100 C. to 4.06 times its original length and is found to be of 49 denier. The T of the copolymer is approximately 43 C. The T of the homopolymer is approximately 57 C. The yarn is annealed at C. at substantially constant length. The yarn has a CD. of only 2.6% and a C1. of 0.8%.

EXAMPLE II This example illustrates the fact that the new fibers may be cut to staple and spun to a yarn and further illustrates the high degree of stretch in fabrics woven from the spun yarn.

Fibers prepared as described in Example I(A) are cut to staple of 1.5 inches (3.76 cm.) length and blended with cotton to a weight ratio of 70% polyester/30% cotton. The staple is carded and spun to 50/1 cc. yarn and woven as the filling into a standard 50/1 cc. 65%- poly(ethylene terephthalate) /35 %-cotton warp. The loom construction is 88 warp ends/inch (30.7 per cm.) by 88 picks/inch (30.7 per cm.). The fabric is scoured and bleached at the boil-The fabric exhibits a hand-stretch of approximately 11%.

As a control, a fabric is produced and finished in the same manner with the expection that a standard 50/1 cc. poly(ethylene terephthalate)/cotton (65/35) blended yarn is used for the filling instead of the bicomponent 7 fiber. The control fiber exhibits a hand-stretch of only approximately 3%.

The term hand-stretch is a measure of the percent extension of a fabric upon applying a force of approximately 2 lbs/inch of a fabric width (357 g./cm.).

EXAMPLE III This example illustrates the preparation of another yarn of this invention and compares its properties to a typical bicomponent yarn of the prior art, the T of the components of which difiers by greater than 8 C.

A 34-filament bicornponent (side-by-side) yarn is prepared by spinning poly(ethylene/2,2-dimethyl 1,3 propylene terephthalate) (85/15) of 49 RV and poly(ethylene terephthalate) of 20 RV in a 50:50 weight ratio, simultaneously, at approximately 298 C. from a spinneret of the type described in French Patent No. 1,442,- 768. The yarn is drawn at approximately 96 C. to 2.08 times its original length and is found to be of 91.4 denier. The T of the homopolymer is 57 C. The T of the copolymer is approximately 58 C. The yarn is annealed at 140 C. at substantially constant length. The yarn has a C.D. of 14.6% and a Cl. of approximately 7.8%.

As a comparison, to show that the T range is critical, a 34-filament bicomponent yarn is prepared by spinning poly(ethylene terephthalate/bibenzoate) (85/15) of approximately 348 RV and po1y(ethylene terephthalate) of 20 RV in a 50:50 weight ratio, simultaneously, at approximately 291 C., from the spinneret used for the test yarn of this example. The yarn is drawn at 100 C. to 2.63 times its original length and found to be of 54.2 denier. The T of the homopolymer is 57 C. The T of the copolymer is approximately 67 C. The yarn is annealed at 152 C. at substantially constant length. The yarn has a C.D. of only 1% and a C1. of approximately 0.8%.

EXAMPLE IV This example illustrates the preparation of another yarn of this invention and, further, illustrates the good stretch properties attained in a fabric woven from this yarn is continuous-filament form.

A 34-filament bicomponent (side-by-side) yarn is prepared by melt-spinning poly(1,4-cyclohexylene dimethylene terephthalate) of 0.45 intrinsic viscosity and poly (1,4-cyclohexylene dimethylene terephthalate/isophthalate) (85/15) of 0.611 intrinsic viscosity in a 50:50 Weight ratio, simultaneously at approximately 302 C. from a spinneret of the type described in French Patent No. 1,442,768. The yarn is drawn at 200 ft./min. (61 meters/ min.) to 1.72 times its original length, at 100 C., onto 110 C. rollers. The T g of each component is approximately 74 C. The resulting SO-denier yarn is woven as a filling into a 70-denier poly(ethylene terephthalate) Warp to give a plain-woven fabric of 96 warp ends per inch by 87 picks per inch (38 by 34 per cm.). A portion of this fabric is scoured min. at the boil. The resulting fabric shows a hand-stretch of 9.5% and an extension of 11.5% upon applying a force of approximately 3 lbs. per inch of fabric width (0.54 kg. per cm.).

It is apparent by perusal of the above examples that fibers of the present invention are superior to the fibers of the prior art in those qualities which are essential in many commercial applications of crimped fibers.

The ability of a fiber to develop and retain crimp under load is extremely important in the staple yarn industry. When staple is processed to yarn, a certain amount of restraint is present on each staple fiber by virtue of its position in the yarn. The fibers of the present invention have such a high degree of crimp development under load that the inherent forces developed causing the fiber to crimp exceed the exterior force imposed by virtue of the fibers position in the yarn, and therefore the staple filaments develop crimp notwithstanding these exterior forces.

An additional advantage of the product of the present invention exists in that substantially no crimp exist in its as-drawn form, a factor which allows and facilitates staple processing.

I claim:

1. A polyester textile fiber comprising two distinct components extending along the fiber and intimately adhered along their lengths in eccentric relationship to the fiber cross-section; one of said components being a saturated glycol-terephthalate homopolyester of fiber-forming molecular weight characterized by having a relative viscosity of 15 to 25 determined as a 10% by Weight solution in mixture of 10 parts phenol and 7 parts 2,4,6-trichlorophenol at 25 C., and by having a T shrinkage value of 45 to C. determined for an undrawn fiber of the homopolyester in water; and the second of said components being a copolyester of (A) said saturated glycol-terephthalate and (B) a member selected from the group consisting of a diiferent saturated glycol-terephthalate and a saturated glycolisophthalate, the mol ratio of (A) to (B) being from 8:2 to 9:1, the copolyester being of fiber-forming molecular weight characterized by a relative viscosity at least 5 units higher than the homopolyester component and having a T shrinkage value within plus or minus 8 C. of the value for the homopolyester component.

2. A polyester textile fiber as defined in claim 1 which has a crimp index value of 2% to 10% and an inherent helical crimp development upon heating to approximately 100 C. characterized by a crimp development value greater than 10%.

3. A polyester textile fiber as defined in claim 1, wherein the weight ratio of homopolyester component to copolyester component is from 40: 60 to 60:40.

4. A polyester textile fiber as defined in claim 1, wherein the copolyester component has a relative viscosity of approximately 35.

5. A polyester textile fiber as defined in claim 1, wherein the homopolyester component is po-ly(ethylene terephthalate) and the copolyester component is poly(ethylene terephthalate/isophthalate) of approximately :15 mole ratio of terephthalate to isophthalate.

6. A polyester textile fiber as defined in claim 1 wherein said homopolyester component is poly(ethylene terephthalate) and said copolyester component is poly (ethylene terephthalate/isophthalate) 7. A polyester textile fiber as defined in claim 1 wherein said homopolyester component is poly(ethylene terephthalate) and said copolyester component is poly(ethylene/2,2-dimethyl- 1 ,3 -propylene terephthalate) 8. A polyester textile fiber as defined in claim 1 wherein said homopolyester component is poly(1,4-cyclohexylenedimethylene terephthalate) and said copolyester component is poly(1,4-cyclohexylenedimethylene terephthalate/isophthalate) References Cited UNITED STATES PATENTS 2,931,091 4/1960 Breen 161173 3,038,235 6/1962 Zimmerman 161-173 3,038,236 6/1962 Breen 161-177 ROBERT F. BURNETT, Primary Examiner.

LINDA M. CARLIN, Assistant Examiner.

U.S. Cl. X.R.

Claims (1)

1. A POLYESTER TEXTILE FIBER COMPRISING TWO DISTINCT COMPONENTS EXTENDING ALONG THE FIBER AND INTIMATELY ADHERED ALONG THEIR LENGTHS IN ECCENTRIC RELATIONSHIP TO THE FIBER CROSS-SECTION; ONE OF SAID COMPONENTS BEING A SATURATED GLYCOL-TEREPHTHALATE HOMOPOLYESTER OF FIBER-FORMING MOLECULAR WEIGHT CHARACTERIZED BY HAVING A RELATIVE VISCOSITY OF 15 TO 25 DETERMINED AS A 10% BY WEIGHT SOLUTION IN MIXTURE OF 10 PARTS PHENOL AND 7 PARTS 2,4,6-TRICHLOROPHENOL AT 25*C., AND BY HAVING A TG SHRINKAGE VALUE OF 45* TO 75*C. DETERMINED FOR AN UNDRAWN FIBER OF THE HOMOPOLYESTER IN WATER; AND THE SECOND OF SAID COMPONENTS BEING A COPOLYESTER OF (A) SAID SATURATED GLYCOL-TEREPHTHALATE AND (B) A MEMBER SELECTED FROM THE GROUP CONSISTING OF A DIFFERENT SATURATED GLYCOL-TEREPHTHALATE AND A SATURATED GLYCOL-ISOPHTHALATE, THE MOL RATIO OF (A) TO (B) BEING FROM 8:2 TO 9:1, THE COPOLYESTER BEING OF FIBER-FORMING MOLECULAR WEIGHT CHARACTERIZED BY A RELATIVE VISCOSITY AT LEAST 5 UNITS HIGHER THAN THE HOMOPOLYESTER COMPONENT AND HAVING A TG SHRINKAGE VALUE WITHIN PLUS OR MINUS 8*C. OF THE VALUE FOR THE HOMOPOLYESTER COMPONENT.