The present invention relates to a method for preventing curling of stretch fabric around areas which have been intentionally been distressed. The method comprises applying heat and tension for a sufficient time to achieve a heat setting efficiency of at least 50%. Fabrics made using the method of the invention exhibit less curling around the distressed areas.

Background and Summary of the Invention

A popular current fabric trend, particularly for denim fabric, is to subject the fabric to mechanical processes to intentionally destroy a part of the fabric in order to produce a worn-out or broken-in effect. Typically, warp yarns are removed over a limited area leaving the weft yarns intact to give a threadbare look.

Another current popular fabric trend is the incorporation of stretch fibers such as lastol or spandex into the fabric, to allow a more comfortable fit.

While it would be desirable to combine these trends and produce stretch fabric having distressed areas, it has been observed that in fabrics containing stretch fiber, the destruction of a portion of the fabric typically results in curling of the fabric around the destroyed area. This curling is believed to be due to unbalanced shrink forces imparted by the stretch fiber after the yarns in the opposing direction have been removed. In the originally woven stretch fabric, the retractive force of the stretch fiber is balanced by the resistance provided by the opposing yarns. As the opposing yarns are removed, the resistance becomes less, leading the retractive force of the fibers to become greater than the resistance provided by the fabric, causing the fabric to curl around the distressed areas. This curling is not aesthetically pleasing and has caused most clothing producers to restrict the use of such treatments to fabrics containing only rigid fibers.

It is therefore desirable to have a method for distressing fabric containing stretch fiber in a manner which minimizes curling around the distressed area. Ideally the method would be easy to implement without the need for expensive equipment.

It has been discovered that the curling effect can be minimized by the application of heat and tension for a sufficient amount of time to achieve a heat setting efficiency of the stretch fibers at least 50%. Such a treatment can be applied by first placing the fabric under tension such that the elastic fibers are extended past their static length. Then, heat is applied to the fabric around the distressed area. This heat can be applied using common ironing techniques.

Accordingly, one aspect of the present invention is a method for producing fabric having distressed areas comprising the steps of selecting a woven fabric having stretch fibers and rigid fibers; forming at least one distressed area in the fabric by cutting or removing a portion of the fibers; applying tension to the elastic fibers around the distressed areas; and then applying heat to the elastic fibers around the distressed areas, where the heat and tension are applied in a sufficient amount and for a sufficient amount of time to achieve a heat setting efficiency of the elastic fibers of at least 50%.

It has been observed that the application of heat while the fiber is under strain, actually changes the morphology of the crystalline structure of the elastic fibers. Thus, in another aspect of the invention, a method for producing stretch fabric having distressed areas is provided comprising the steps of selecting a woven fabric having stretch fibers and rigid fibers; forming at least one distressed area in the fabric by cutting or removing a portion of the fibers; applying tension to the elastic fibers around the distressed areas; and then applying heat to the elastic fibers around the distressed areas, where the heat and tension are applied in a sufficient amount and for a sufficient amount of time to alter the crystal morphology of the elastic fiber in the area near the distressed area, as demonstrated by the use of x-ray diffraction.

In another aspect of the invention a fabric having stretch fibers is provided which has one or more distressed areas characterized in that the fabric remains flat or non- wrinkled ) around the distressed areas without the application of tension. That is, the fabric can be characterized by having no creased appearance, no obvious crimps in the exposed yarns; and no heavy folds in areas next to the destroyed areas.

Brief Description of the Drawings

Figure 1 is the x-ray diffraction pattern for control fiber which was not exposed to either heat or tension.

Figure 2 is the x-ray diffraction pattern of fiber which has been constrained at 300% and exposed to a temperature of 60°C in air for 15 minutes.

Figure 3 is a graph of heat setting efficiency versus applied temperature for spandex and crosslinked ethylene alpha olefin block copolymer. Detailed Description of the Invention

The following terms shall have the indicated meaning when used in the present patent application:

"Fiber" means a material in which the length to diameter ratio is greater than about 10. Fiber is typically classified according to its diameter. Filament fiber is generally defined as having an individual fiber diameter greater than about 15 denier (17 dtex), usually greater than about 30 denier (33 dtex). Fine denier fiber generally refers to a fiber having a diameter less than about 15 denier. Microdenier fiber is generally defined as a multifilament fiber having less than about 0.9 denier (1 dtex) per filament.

"Filament fiber" or "monofilament fiber" means a single, continuous strand of material of indefinite (i.e., not predetermined) length, as opposed to a "staple fiber" which is a discontinuous strand of material of definite length (i.e., a strand which has been cut or otherwise divided into segments of a predetermined length).

The term "yarn" includes both a monofilament fiber as well as a number of fibers twisted or otherwise joined together to form a continuous strand.

An "elastic fiber" or "stretch fiber" is one that will recover at least about 50 percent, more preferably at least about 60% even more preferably 70% of its stretched length after the first pull and after the fourth pulls of four consecutive pulls to 100% strain (that is, double the length). One suitable way to do this test is based on the one found in the

International Bureau for Standardization of Manmade Fibers, BISFA 1998, chapter 7, option A. Under such a test, the fiber is placed between grips set 4 inches apart, and the grips are then pulled apart at a rate of about 20 inches per minute to a distance of eight inches and then allowed to immediately recover. It is preferred that the elastic textile articles of the present invention have a high percent elastic recovery (that is, a low percent permanent set) after application of a biasing force.

"Elastic materials" are also referred to in the art as "elastomers" and "elastomeric". For purposes of this invention, an "elastic article" is one that comprises elastic fiber. Thus an "elastic fabric" is simply a fabric which contains some amount of elastic fiber.

"Nonelastic", "rigid" or "hard" fiber means a fiber that is not elastic as defined above.

It should be understood that despite the normal definition of these terms, for purposes of this application "nonelastic", "rigid" or "hard" fibers are not necessarily completely incapable of being extended and may have the ability to be stretched to some extent under a biasing force, and further these fibers may exhibit some recovery when the biasing force is released after such stretching, just not to the extent which would make them "elastic" as defined above.

"Core spun yarn" means a yarn which has been made by twisting fibers around a core which is another filament or a previously spun yarn, thus at least partially concealing the core.

The term "Elongation" means the amount the fabric or fiber lengthens after applying a load over a given length of time expressed as a percentage of the initial fabric or fiber dimension. Elongation of fabric is determined using the following procedure. Three fabric samples, each of 10 cm length and 5 cm width, are subject to two load (to 36N) and unload (to 0% elongation) cycles lengthwise, one sample at a time, in an Instron Universal testing Machine with the strain rate set at 400 mm/min. The elongation is measured as the average extension of the three samples at 36N load in the second cycle. The test is performed with samples cut in cross (or width) and machine (or length) direction and each direction attains its own value of elongation (Em = elongation machine direction; Ec = elongation cross direction). The overall fabric Elongation (E _f ) is then calculated according to the formula: E _f = -^E _m ² + E _C ² .

The term "Growth" when referring to fabrics of the present invention refers to dimensional changes of the fabric under prolonged strain conditions. Growth is evaluated in this patent as follows: First, sample specimens are cut from the fabric: one on machine direction and the other one on cross direction. The short dimension of the specimen is always cut 10 cm in length whereas the long dimension varies depending on the level of strain at which the growth will be measured. Typically, three strain levels are evaluated: 15%, 25% and 35 %. . Second, the samples are converted into loops by sewing the extremes of the long dimension in such a manner as to ensure that the ends do not separate during testing. Next, two sets of marks are made with a ruler and a pen marker on the surface of the sample specimen; one in the front or top of the loop layer and another one in the back or bottom of the loop. Then, both ends of the loops are fixed to a frame with two protruding ends long enough to ensure that the entire loop fits over the protruding end. The protruding ends are at a fixed distance apart from each other. Given the distance between these protruding ends, the size of the loop can be set so as to achieve the desired strain (typically 15%, 25% and 35%) when the loop is stretched to reach both protruding ends. The stretched specimens can be placed in air ("dry growth") or in water (tap water is used for the present invention but it could be, for example, a chlorine solution - "wet growth"). The specimens are kept under this strain and environmental condition (dry or wet) for 24 hours at room temperature. After 24 hours, the specimens are taken out of the environment selected (dry or wet) and removed from the frame and the distance between marks is measured after 1 minute (sometimes referred to as "instantaneous growth") and again after 24 hours (unless otherwise stated, the distance after 1 minute is the measurement referred to in the present application). The growth at a given time and a given direction (machine or cross) is calculated as: ((distance after exposure - initial distance) / initial distance) * 100 in machine and cross direction. The overall Fabric Growth (G _f ) is calculated as -^Gm ² + Gc ² where G _m is growth in machine direction and G _c is the growth in cross direction.

"Fabric Width" is determined by the average of three measurements of distance between the two edges of the fabric in cross direction.

The "Fabric Density" for the fabrics of the present invention is determined by the average of the mass per unit area of samples taken from the left fabric side, the right fabric side and the center of the fabric. The sample dimension is 100 cm ² .

"Dimensional Stability" means the level of fabric shrinkage during a hot wash and tumble drying sequence. It is measured following the standard AATCC 135-1999 type l;V;Ai. in cross and machine directions (i.e. along the warp direction and along the weft direction).

Woven fabric is formed by interlacing yarns so that they cross each other at right angles. The yarns that run lengthwise are known as the warp yarns whereas the yarns which run crosswise are known as the filling or weft yarns. For much of the elastic fabric commonly produced today, elastic yarns are used for a portion of the weft yarns. The degree of stretch provided will depend on several factors including the retractive force of the elastic yarn used, the percentage of weft yarn in which elastic yarn used, as well as the particular fabric construction. With regards to fabric construction, it will be readily understood by those of ordinary skill in the art that the tighter the weave, the less stretch will be observed, all else being equal. This points out how the fabric offers resistance to the stretch.

In a process to create a worn, threadbare look, a mechanical and/or chemical process can be used to destroy some of the nonelastic yarns, leaving at least a portion of the elastic yarns intact. When this happens there is less resistance provided by the fabric construction, which may result in curling. This curling effect can de diminished or eliminated altogether by use of the following method. First tension is applied to the fabric along the lengthwise direction of the elastic fibers which remain intact (whether in the warp direction the weft direction or both). Then, while the fabric remains under tension; heat is applied to the elastic fibers around the distressed areas. The heat and tension should be applied in a sufficient amount and for a sufficient amount of time to achieve a heat setting efficiency of the elastic fibers of at least 50%.

In fabrics which are made elastic by the incorporation of elastic yarns in the weft direction, the tension will be applied across the width of the garment. The tension should be sufficient to maintain consistent width and flatness across the whole garment, including the distressed area. For most fabrics, tension in the amount of from two to five pounds is applied, but tension up to ten pounds may be useful. The tension can be applied using any means capable of extending the elastic yarns lengthwise. This includes using a template or a dummy that is made according to the size of the garment (slightly larger than the garment when no strain is being applied to the garment), which can then be forced inside the garment to straighten the exposed weft elastic fiber. The tension may also be applied by hand, or by use of a tenter frame. If elastic yarn is used as both the weft and warp yarns, then it will be preferred that tension is applied in both the width-wise and length-wise direction.

Then, while the garment is still under tension in the lengthwise direction of the elastic yarns, heat is applied to elastic yarns in the proximity of the distressed area. The heat can be applied by any means known, but is conveniently applies using an iron. The temperature applied to the fabric is preferably between 130°C and 200°C, more preferably between 150°C and 185°C, or 170°C to 180°C. Care should be exercised to avoid temperatures which may scorch cotton or other fibers present in the fabric, as scorching may lead to undesirable color changes. The heated area is then allowed to cool before the tension is released.

The heat should be applied for a sufficient amount of time, given the particular temperature and tension, to achieve a heat setting efficiency ('HSE") of the elastic fiber of at least 50%. Preferably the HSE is greater than 70%, more preferably greater than 90%), still more preferably greater than 95%. The heat setting efficiency of an elastic fiber is calculated as follows: HSE= (Li - L ₀ ) ÷ (L ₂ -L ₀ ) where L ₀ is the fiber length in the fabric after the distressing process but before the application of heat or tension, Li is the relaxed fiber length after the application of heat with tension and L ₂ is the elongated length under tension but prior to the application of heat. The heat-setting efficiency of the embedded elastic fiber may vary depending on the whether the fiber is covered or not, as well as the companion yarn count in weft direction of the fabric or the elastic fiber yarn count. In general, the lower the companion yarn count, the lower the heat-setting efficiency is, and the higher the yarn count of the elastic fiber, the higher the heat-setting efficiency may be. It may be difficult to determine the heat setting efficiency of the fiber embedded in the fabric. For purposes of this invention, while it is preferred that the heat setting efficiency of the fibers is determined in the fabric, it may be assumed that heat and tension which is sufficient to achieve the desired HSE on the bare fiber will produce the same HSE on covered fiber, even when that covered fiber is embedded in fabric, although it may require a longer exposure to the heat. The lack of retractive force on the fiber after a treatment of heat and pressure is an indication that heat- setting efficiency of at least 50% has been obtained.

While not intending to be bound by theory, it is believed that at temperatures above the crystalline melting point of polyolefin based elastic fibers such as lastol, the crystal domains partially or fully melt depending on the dwell time above the melting point. When cooled below the melting point, the crystals reform at the dimensions of the fiber at the time of cooling. For polyurethane based elastic fibers such as spandex, heat exposure partially or fully breaks hydrogen bonds between hard segments of the spandex polymer depending on the temperature, draft and dwell time. When cooled, some dissociation of H-bonds occurs and fiber remains at the length determined by its draft at the time of cooling

For polyolefin based polymers, x-ray diffraction can be used to demonstrate the effect of the heat-setting process on the crystal morphology of the fibers. It is known that exposing ethylene copolymers of low crystallinity to heat can lead the structure transition from pseudo hexagonal to orthorhombic very slowly, for example over several days. It has surprisingly been observed that when the heat is applied while the fiber is under tension, the transition is much faster. This transition towards orthorhombic crystalline structure can be evidenced by x-ray diffraction. Orthorhombic crystalline structures will have their main peak occurring at a higher two-theta (or degree) value. It is therefore preferred that the heat-setting process be conducted for a time sufficient to shift the main x- ray diffraction peak (obtained using the method set out in the examples) at least 1 degree. After the heat-setting process, the elastic fibers will have substantially lower shrink force compared to fibers which have not been subjected to this heat setting process. This lower shrink force in the area of close proximity to the distressed area will deter any curling. In the case of polyolefin based elastic fibers care should be taken during any subsequent heating and relaxation cycles to keep the temperature below the temperature at which a substantial amount of the crystallites may become re-molten as this may result in another heat-setting process, without tension. If this can be avoided then the curling problem should not reoccur.

The dwell time for the heat is set to achieve the desired heat setting efficiency given the temperature being applied to the fabric. In general times of from 10 to 30 seconds are preferred, with dwell times around 15 seconds being preferred. It should be understood that the use of lower temperatures may require longer dwell times. It is also conceived that it may be beneficial in some circumstances that the application of heat may occur in more than once cycle. Thus for example, it may be desirable to apply a temperature of 150°C to 170°C for approximately 15 seconds, then allow the fabric to cool, then apply heat for a second time while the fabric is still under tension.

The present invention is applicable to all elastic fabric. Such fabrics may comprise from about 0.5% to about 60% by weight of elastic yarn, more preferably 1% to 40%), more prefereably 2%-30%. Elastic fibers include certain fibers made from polyolefins such as polyethylene or polypropylene, as well as segmented polyurethane fibers known as spandex or elastane, and polyester bi-component fibers known as elasterell-p. Due to its heat-setting properties, the preferred elastic fiber is a cross linked polyolefin fiber, more preferably a cross linked polyethylene fiber. The elastic fiber may have a random, block, or pseudo block structure. Crosslinked polyethylene fibers include the fibers described in US 6,437,014, (which is hereby incorporated by reference in its entirety) which are generically known as lastol. Such fibers are available from The Dow Chemical Company under the trade name DOW XLA™ fibers. Crosslinked polyethylene fibers also include the segmented ethylene-alpha-olefin block copolymers discussed for example in WO 2005/090427, WO 2005/090425 and WO 2005/090426, each of which are hereby incorporated by reference in their entirety. It is also contemplated that the elastic fiber used in a particular fabric may comprise more than material. The elastic yarn may be a monofilament fiber, or a multifilament fiber or may be a covered yarn such as a core spun yarn where the elastic fiber comprises the core, and a hard yarn such as polyester or cotton is wrapped around the core.

If the elastic yarn is a monofilament fiber or a multifilament fiber then it will have a count ranging from 11 to 1000 dtex, preferably from 40 to 800 dtex and most preferably from 70 to 800 dtex, as determined by standard industry methods known to the person skilled in the art.

The fabric of the present invention also comprises one or more nonelastic or hard yarns which may comprise natural and/or synthetic fibers. Natural fibers include cellulosic materials such as cotton, flax, ramie, rayon, viscose and hemp as well as other materials such as wool, silk or mohair, although these materials are not preferred due to the harsh sterilization treatments they will be subjected to. Synthetic materials include materials such as polyester, nylon, polypropylene, and their blends.. Polyester yarn includes materials such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and

poly(trimethylene) terephthalate (PTT). Nylon includes both Nylon 6 and Nylon 6,6.

Polypropylene includes homopolymer polypropylene, random copolymer polypropylene, impact modified polypropylene, olefin block copolymers or statistical multi-block olefin copolymers such as those described in WO 2005/090427 and WO 2005/090426 and WO 2005/090425, functionalized homopolymers or copolymers and propylene based elastomers and plastomers, such as those described in WO03/040442, and US application 60/709688 filed August 19, 2005 (each of which is hereby incorporated by reference in its entirety).

The additional yarn can be a flat or a textured multifilament yarn or staple fiber yarn. "Textured" fibers means that the fiber is subject to a mechanical twist as is known to the skilled artisan. This mechanical twisting imparts a slight amount of elasticity to the fiber.

The nonelastic yarn will have a count ranging from 22 to 1500 dtex, preferably from 50 to 1200 dtex and most preferably from 75 to 1000 dtex, as determined by standard industry methods known to the person skilled in the art. The nonelastic yarn of will comprise about 50% to about 99.5% by weight, preferably about 70 to 98 % by weight of the fabric. This weight percent is based on the total content of nonelastic yarns used. It should be understood that more than one type of nonelastic yarn may be used. The fabrics of the present invention can be made according to any suitable weaving process. The elastic fiber may be used in the warp or the weft (or both) directions, but it is generally preferred that the elastic fiber be used as the weft yarns. Denim fabric is particularly preferred for use in the present invention, with the elastic yarn being used in the weft direction, and a portion of the nonelastic warp yarns being removed to provide the worn look.

Any process to remove some of the non-elastic yarn may be used in order to produce the worn out or threadbare look. These include chemical and mechanical process as known in the art. Chemical methods include processes such as applying sulfuric acid to remove the cotton fibers (in both the warp and weft direction, as well as those covering the elastic fiber) leaving only the elastic fiber. Mechanical processes include using tools such as razor blades, scissors or sandpaper to create a hole and fray the cotton yarn to create the desired design. Bleach or other chemical agents in combination with mechanical means can also be used to create the faded or worn out design.

In addition to the mechanical or chemical process to removing some of the nonelastic yarn, the fabrics can also be subjected to other finishing steps known in the art such as bleaching, dyeing, mercerization, stone washing, softening etc.

EXAMPLES

In the following examples curling is evaluated by visual inspection by two observers with below ratings:

CI - is a wrinkled, creased appearance with obvious crimps in the exposed yarns; heavy folds in areas next to the destroyed areas.

C2 - is a wrinkled, creased appearance with moderate crimps in the exposed yarns; mild folds in areas next to the destroyed areas.

C2.5 - is a fairly smooth appearance with little crimps in the exposed yarns; little folds in areas next to the destroyed areas.

C3 - is a smooth appearance with little to no crimps in the exposed yarns; little to no folds in areas next to the destroyed areas.

A rating for the fabrics is assigned based on the consensus of the two observers. In the event that the first two observers do not agree, a third observer will decide which of the forst two observations was more accurate, and that will be the rating for the fabric. The following fibers were used to make a series of fabrics:

Yarn A is a core spun fiber with 140 denier olefin block copolymer commercially available as DOW Next Generation XL A™ as the core, subjected to a draft of 4.8X then covered with lONe cotton.

Yarn B is a core-spun fiber similar to Yarn A except that the core is 105 denier DOW Next Generation XLA™ whichis subjected to a draft of 4.5x prior to covering.

Yarn C is a core-spun fiber having a core of 70 denier spandex fiber subjected to a draft of 4.8X then covered with cotton.

The above yarns were used to make woven fabrics as indicated in Table I, where the percentage listed is the total amount of elastic fiber by weight present in the fabric.

Table I

After weaving, each fabric is subjected to the following processes: Singeing— Quenching— Desizing by enzyme (two boxes,55°C-60°C)— 85°C Washing 3 boxes— 60°C Washing 2 boxes— 50°C Washing 1 box— Drying at 110°C— Mercerization (pad caustic soda two time)— Rinsing 6 times— Rinsing 9 boxes— Drying— Sanforizing. Running speed is at 50m/min with no tension applied to the weft direction.

The fabric of each example is evaluated for stretch, growth, dimensional stability in both the warp ("MD") and weft ("CD") direction as reported in Table II.

The treated fabrics are then each used to construct a pair of pants.

A mechanical process is then used to form the destroyed areas in the pants: Warp yarns are removed with the weft yarns left as designed. The destroyed areas are maintained to be irregular in shape. For the three samples shown in the table, the holes are made in the similar location of the jeans pants and the the hole size (around 2 inches in diameter) and shape are similar too.

After the destroyed areas are formed, a laundering process is applied according to AATCC Test Method 135-2004 for one cycle at the following conditions: machine wash hot at 60°C, normal cycle, tumble dry at cotton sturdy (66 +/- 5°C).

Curling around the distressed areas is observed for each of the fabrics, with a reported value of CI for each.

The fabric was then stretched along the weft direction so that the fabric laid flat, which required an elongation of from 5% -20% with a 2-5 lbs load. A common household iron at "Dots Three" setting is then used to apply heat (approximately 150°C-

170°C) to the weft yarns surrounding the distressed area for 15 seconds (Example 3 was subjected to a second identical ironing process for a total of 30 seconds). From Figure 3, it can be seen that this temperature is sufficient to impart a heat setting efficiency of about 99% for examples 1, 2, and 4, but only about 60% for Example 3.

The curling rating for Examples 1, 2 and 4 is was improved from CI to C3.

With Example 3, the curling rating remained at CI after ironing twice for 15 seconds at "Dot

Three" iron setting. While the applied conditions were insufficient to prevent the wrinkling in the spandex-based fabric, it is believed that higher temperature and/or longer time which the garment is exposed to the heat would result in increased heat set efficiency of the fiber and hence reduce the amount of wrinkling . However, high temperature and/or longer time might scorch cotton and change the shade from pure white to yellowish white which would be undesired for most denim applications.

Each fabric is then laundered by AATCC Test Method 135-2004 for one cycle at conditions of: machine wash hot at 60°C, normal cycle, tumble dry at cotton sturdy (66 +/- 5°C). Curling performance is evaluated after drying at room temperature and after tumble dried at 70°C. No further ironing step is performed. The curling rating for Examples

1, 2, and 4 remained at C3, while Example 3 remained at CI .

Example 4 19.10% 3.50% -5.00% -4.00%

Table II

Example 5

A second set of experiments was designed to show how the crystal morphology of the fiber may change upon the heat setting treatment. The fibers used in this example are all 70 denier fibers from ethylene-octene copolymer having a density of

0.875g/cc (as determined according to ASTM D-792), and a melt index (I ₂ ) of 3 (as determined according to ASTM D 1238, 190°C, 2.16 kg). The fibers are sampled before and after electron beam crosslinking with 19.2 Mrad dose. Fibers are spun at 500m/min and 280°C using 0.8mm diameter circular dies. Bundles containing approximately 10 well- aligned fibers of ca. 10cm length are manually stretched to the desired elongation, using the tabs attached to the ends of the bundle, and are mounted on cardboard frames. Control samples are mounted on the cardboard frames without any stretch. Samples designated for heat setting treatment are then placed into a temperature-equilibrated convection oven for 15min with the cardboard frames.

The fibers are analyzed using a Wide Angle X-ray Diffraction (WAXD) GADDS system (Bruker-AXS) equipped with a HiStar area detector, a video microscope, as well as a laser pointer for sample alignment. Data are collected using copper (Ka) radiation with a typical sample to detector distance of 6 cm. A 0.3mm beam is used for the measurement. The collected WAXD images are calibrated using Corundum as a standard.

The results are shown in Figure 1 and Figure 2. Figure 1 is the x-ray diffraction pattern for control fiber which was not exposed to either heat or tension. Figure 2 is the x-ray diffraction pattern of fiber which has been constrained at 300% and exposed to a temperature of 60°C in air for 15 minutes. As can be seen from these figures, the crystals of the fiber which have been heat set have a peak which has been shifted more than 1 degree from the fiber which has not been exposed to such treatment. Thus, under the applied heat- setting condition, the crystals with pseudo hexagonal structure changed into orthorhombic. This is a surprising result, given the limited time exposure.