| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | August 17, 2004 |
| PATENT TITLE |
Method to make circular-knit elastic fabric comprising spandex and hard yarns |
| PATENT ABSTRACT | Circular-knit, elastic, single-knit jersey fabric, of spun and/or continuous filament hard yarns with bare spandex plated in every course, has a cover factor in the range of 1.3 to 1.9, a basis weight from 140 to 240 g/m.sup.2, an elongation of 60% or more and low shrinkage. The circular knit, single-knit jersey fabric is produced by maintaining the draft of the spandex at or below 2.times. (100% elongation) and maintaining the finishing and drying temperature(s) below the spandex heat set temperature. The knit fabric meets the end-use specifications without heat setting |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | October 20, 2003 |
| PATENT REFERENCES CITED |
DuPont: "Single Jersey Fabrics with LYCRA" Technical Bulletin L410, Jan. 24, 2001, pp. 1-14. DuPont: "Wet Processing of Fabrics Containing LYCRA Elastane" Tchnical Bulletin L704, Oct. 15, 1999, pp. 1-21. E.I. du Pont de Nemours and Company: "Dyeing and Finishing Circular Knit Fabrics with LYCRA" Technical Bulletin L713 Replaces Bulletin L712, 1999, pp. 1-29. DuPont Nov. 3, 2000 letter to Customers--North American Lycra.RTM. Seamless Technical Team |
| PATENT CLAIMS |
We claim: 1. In a method for making a circular knit, single jersey fabric in which bare spandex yarn from 17 to 33 dtex is plated with one or more spun or continuous filament hard yarns, or blends thereof, with yarn count from 35 to 85, and in which the spandex and hard yarn(s) are plated in every knit course to produce the circular knit, single jersey fabric with a cover factor of from 1.3 to 1.9, wherein the improvement comprises: controlling the draft on the spandex feed so that the spandex yarn is drafted no more than 2.times. its original length when knit to form the circular knit, single jersey fabric; and finishing and drying the knit fabric while maintaining the fabric at a temperature below such temperature required to heat set the spandex. 2. The method of claim 1, wherein the knit fabric is maintained at temperatures below 160.degree. C. during finishing and drying. 3. The method of claim 1, wherein the knit fabric has a length in the warp direction and is dried or compacted while subject to an overfeed in its length. 4. The method of claim 1, wherein the knit fabric has a spandex content of from 3.5% to 14% by weight based on the total fabric weight per square meter. 5. The method of claim 4, wherein the knit fabric has a spandex content of from 5% to 10% by weight based on the total fabric weight per square meter. 6. The method of claim 1, wherein the cover factor of the knit fabric is 1.4. 7. The method of claim 1, wherein finishing comprises one or more steps selected from the group consisting of: cleaning, bleaching, dyeing, drying, and compacting and any combination of such steps. 8. The method of claim 1, wherein the hard yarn is selected from the group consisting of spun cotton and cotton blended with synthetic fiber or yarn. 9. A method for making a circular knit, elastic, single knit jersey fabric, consisting essentially of: plating a bare spandex yarn with a hard yarn, wherein the spandex is from 17 to 33 dtex and can be heat set within a heat setting efficiency of at least 85% at a heat setting temperature, and wherein the hard yarn has a total yarn count (Nm) of 35 to 85; circular knitting the plated bare spandex yarn and hard yarn in every knit course to form a single knit jersey fabric having a cover factor of from 1.3 to 1.9; controlling the feed of the spandex so that the spandex in the fabric is drafted no more than 2.times. its original length; and maintaining the fabric below the heat setting temperature of the spandex during further processing. 10. The method of claim 9, wherein the spandex is present in the fabric in an amount from 3.5% to 14% by weight based on the total fabric weight per square meter. 11. The method of claim 10, wherein the spandex is present in the fabric in an amount from 5% to 10% by weight based on the total fabric weight per square meter. 12. The method of claim 9, wherein the fabric has a cover factor of 1.4. 13. The method of claim 9, further comprising drying or compacting the fabric in an overfeed condition. 14. The method of claim 9, wherein the fabric is maintained at temperatures below 160.degree. C. during further processing. 15. The method of claim 9, wherein further processing comprises one or more steps selected from the group consisting of: cleaning, bleaching, dyeing, drying, and compacting, and any combination of such steps. 16. A circular knit, elastic, single knit jersey fabric made by the method of claim 1. 17. The circular knit, elastic, single knit jersey fabric of claim 16, wherein the hard yarn is cotton or a cotton blend, and the fabric has a basis weight of from 140 to 240 g/m.sup.2. 18. The circular knit, elastic, single knit jersey fabric of claim 16, wherein the fabric has an elongation of at least 60% in its length (warp) direction. 19. The circular knit, elastic, single knit jersey fabric of claim 16, wherein the fabric has a shrinkage of 7% or less after washing. 20. A circular knit, elastic, single knit jersey fabric made by the method of claim 9. 21. The circular knit, elastic, single knit jersey fabric of claim 20, wherein the hard yarn is cotton or a cotton blend, and the fabric has a basis weight of from 140 to 240 g/m.sup.2. 22. The circular knit, elastic, single knit jersey fabric of claim 20, wherein the fabric has an elongation of at least 60% in its length (warp) direction. 23. The circular knit, elastic, single knit jersey fabric of claim 20, wherein the fabric has a shrinkage of 7% or less after washing. 24. A garment made from the circular knit, elastic, single knit jersey fabric of claim 16. 25. A garment made from the circular knit, elastic, single knit jersey fabric of claim 17. 26. A garment made from the circular knit, elastic, single knit jersey fabric of claim 20. 27. A garment made from the circular knit, elastic, single knit jersey fabric of claim 21. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention relates to circular knitting yarns into fabrics, and specifically to elastic single-knit jersey fabrics comprising both spun and/or continuous filament hard yarns, and bare spandex yarns. BACKGROUND OF THE INVENTION Single-knit jersey fabrics are broadly used to make underwear and top-weight garments, such as T-shirts. Compared to woven structures, the knit fabric can more easily deform, or stretch, by compressing or elongating the individual knit stitches (comprised of interconnected loops) that form the knit fabric. This ability to stretch by stitch rearrangement adds to the wearing comfort of garments made from knit fabrics. Even when knit fabrics are constructed of 100% hard yarns, such as cotton, polyester, nylon, acrylics or wool, for example, there is some recovery of the knit stitches to original dimensions after imposed forces are removed. However, this recovery by knit stitch rearrangement generally is not complete because hard yarns, which are not elastomeric, do not provide a recovery force to rearrange the knit stitches. As a consequence, single-knit fabrics may experience permanent deformations or `bagging` in certain garment areas, such as at the elbows of shirt sleeves, where more stretching occurs. To improve the recovery performance of circular, single-knit fabrics, it is now common to co-knit a small amount of spandex fiber with the companion hard yarn. As used herein, "spandex" means a manufactured fiber in which the fiber-forming substance is a long-chain synthetic polymer comprised of at least 85% of a segmented polyurethane. The polyurethane is prepared from a polyether glycol, a mixture of diisocyanates, and a chain extender and then melt-spun, dry-spun or wet-spun to form the spandex fiber. For jersey knit constructions in circular knit machines, the process of co-knitting spandex is called "plating." With plating, the hard yarn and the bare spandex yarn are knitted parallel, side-by-side relation, with the spandex yarn always kept on one side of the hard yarn, and hence on one side of the knitted fabric. FIG. 1 is a schematic illustration of plated knit stitches 10 wherein the knitted yarn comprises spandex 12 and a multi-filament hard yarn 14. When spandex is plated with hard yarn to form a knit fabric, additional processing costs are incurred beyond the added cost of the spandex fiber. For example, fabric stretching and heat setting usually are required in the finishing steps when making elastic knit jersey fabrics. By "circular knitting" is meant a form of weft knitting in which the knitting needles are organized into a circular knitting bed. Generally, a cylinder rotates and interacts with a cam to move the needles reciprocally for knitting action. The yarns to be knitted are fed from packages to a carrier plate that directs the yarn strands to the needles. The circular knit fabric emerges from the knitting needles in a tubular form through the center of the cylinder. The steps for making elastic circular-knit fabrics according to one known process 40 are outlined in FIG. 4. Although process variations exist for different fabric knit constructions and fabric end uses, the steps shown in FIG. 4 are representative for making jersey knit elastic fabrics with spun hard yarns, such as cotton. The fabric is first circular knit 42 at conditions of high spandex draft and feed tensions. For example, for single-knit jersey fabrics made with bare spandex plated in every knit course, the prior-art feed tension range is 2 to 4 cN for 22 dtex spandex; 3 to 5 cN for 33 dtex; and 4 to 6 cN for 44 dtex (DuPont Technical Bulletin L410). The fabric is knit in the form of a tube, which is collected under the knitting machine either on a rotating mandrel as a flattened tube, or in a box after it is loosely folded back and forth. In open-width finishing, the knitted tube is then slit open 44 and laid flat. The open fabric is subsequently relaxed 46, either by subjecting it to steam, or by wetting it by dipping and squeezing (padding). The relaxed fabric is then applied to a tenter frame and heated (for heat setting 46) in an oven. The tenter frame holds the fabric on the edges by pins, and stretches it in both the length and width directions in order to return the fabric to desired dimensions and basis weight. This heat setting is accomplished before subsequent wet processing steps and, consequently, heat setting is often referred to as "pre-setting" in the trade. At the oven exit, the flat fabric is released from the stretcher and then tacked 48 (sewed) back into a tubular shape. The fabric then is processed in tubular form through wet processes 50 of cleaning (scouring) and optional bleaching/dyeing, e.g., by soft-flow jet equipment, and then dewatered 52, e.g., by squeeze rolls or in a centrifuge. The fabric is then "de-tacked" 54 by removing the sewing thread and re-opening the fabric into a flat sheet. The flat, still wet, fabric is then dried 56 in a tenter-frame oven under conditions of fabric overfeed (opposite of stretching) so that the fabric is under no tension in the length (machine) direction while being dried at temperatures below heat-setting temperatures. The fabric is slightly tensioned in the width direction in order to flatten any potential wrinkling. An optional fabric finish, such as a softener, may be applied just prior to the drying operation 56. In some cases a fabric finish is applied after the fabric is first dried by a belt or tenter-frame oven, so that the finish is taken up uniformly by fibers that are equally dry. This extra step involves re-wetting the dried fabric with a finish, and then drying the fabric again in a tenter-frame oven. Heat setting "sets" spandex in an elongated form. This is also known as redeniering, wherein a spandex of higher denier is drafted, or stretched, to a lower denier, and then heated to a sufficiently high temperature, for a sufficient time, to stabilize the spandex at the lower denier. Heat setting therefore means that the spandex permanently changes at a molecular level so that recovery tension in the stretched spandex is mostly relieved and the spandex becomes stable at a new and lower denier. Heat setting temperatures for spandex are generally in the range of 175 to 200.degree. C. For the prior art process 40 shown in FIG. 4, the heat setting 46 commonly is for about 45 seconds or more at about 190.degree. C. If heat-setting is not used to "set" the spandex, after the fabric is knitted and released from the constraints of the circular knitting machine, the stretched spandex in the fabric will retract to compress the fabric stitches so that the fabric is reduced in dimensions compared to what those dimensions would be if the spandex were not present. Compression of the stitches in the knitted fabric has three major effects that are directly related to elastic knit fabric properties, and thereby usually renders the fabric inappropriate for subsequent cut and sew operations. First, stitch compression reduces fabric dimensions and increases fabric basis weight (g/m.sup.2) beyond desired ranges for single jersey knit fabrics for use in garments. As a result, the traditional finishing process for elastic circular-knit fabric includes a fabric stretching and heating step, at sufficiently high temperatures and sufficiently long residence time, so that the spandex yarn in the knit will "set" at desired stretched dimensions. After heat setting, the spandex yarn will either not retract, or will retract only modestly below its heat-set dimension. Thus, the heat-set spandex yarn will not significantly compress the knit stitches from the heat-set dimensions. Stretching and heat setting parameters are chosen to yield the desired fabric basis weight and elongation, within relatively tight limits. For a typical cotton-jersey elastic single-knit, the desired elongation is at least 60%, and the basis weight ranges from about 140 to about 240 g/m.sup.2. Second, the more severe the stitch compression, the more the fabric will elongate on a percentage basis, thus far exceeding minimum standards and practical needs. When a plated knit with elastic yarn is compared with a fabric knit without elastic yarn, it is common for the plated elastic knit fabric to be 50% shorter (more compressed) than the fabric without elastic yarn. The plated knit is able to stretch in length 150% or more from this compressed state, and such excessive elongation is generally undesirable in jersey knits for cut and sew applications. This length is in the warp direction of the fabric. Fabrics with high elongation in length (stretch) are more likely to be cut irregularly, and are also more likely to shrink excessively upon washing. Similarly, stitches are compressed by spandex in the width direction, so that fabric width is reduced about 50% as well, far beyond the 15 to 20% as-knit width reduction normally encountered with rigid (non-elastic) fabrics. Third, the compressed stitches in the finished fabric are at an equilibrium condition between spandex recovery forces and resistance to stitch compression by the companion hard yarn. Washing and drying of the fabric can reduce the hard-yarn resistance, probably in part because of agitation of the fabric. Thus, washing and drying may permit the spandex recovery forces to further compress the knit stitches, which can result in unacceptable levels of fabric shrinkage. Heat-setting the knit fabric serves to relax the spandex and reduce the spandex recovery force. The heat setting operation therefore improves the stability of the fabric, and reduces the amount that the fabric will shrink after repeated washings. Heat setting is not used for all varieties of weft knit elastic fabrics. In some cases a heavy knit will be desired, such as in double knits/ribs and flat sweater knits. In these cases, some stitch compression by the spandex is acceptable. In other cases, the bare spandex fiber is covered with natural or synthetic fibers in a core-spinning or spindle-covering operation, so that the recovery of the spandex and resultant stitch compression is restrained by the covering. In still other cases, bare or covered spandex is plated only on every second or third knit course, thereby limiting the total recovery forces that compress the knit stitches. In seamless knitting, a process wherein tubular knits are shaped for direct use while being knitted on special machines, the fabric is not heat set because dense, stretchy fabrics are intended. For circular-knit jersey elastic fabrics made for cutting and sewing, however, wherein bare spandex is plated in every course, heat setting is almost always required. Heat setting has disadvantages. Heat setting is an extra cost to finish knit elastic fabrics that contain spandex, versus fabrics that are not elastic (rigid fabrics). Moreover, high spandex heat-setting temperatures can adversely affect sensitive companion hard yarns, e.g., yellowing of cotton, thereby requiring more aggressive subsequent finishing operations, such as bleaching. Aggressive bleaching can negatively affect fabric tactile properties, such as "hand," and usually requires the manufacturer to include fabric softener to counteract bleaching. Also, heat-sensitive hard yarns, such as those from polyacryonitrile, wool and acetate, cannot be used in high-temperature spandex heat-setting steps, because the high heat-setting temperatures will adversely affect such heat-sensitive yarns. The disadvantages of heat setting have long been recognized, and, as a result, spandex compositions that heat-set at somewhat lower temperatures have been identified (U.S. Pat. Nos. 5,948,875 and 6,472,494 B2). For example, the spandex defined in U.S. Pat. No. 6,472,494 B2 has a heat set efficiency greater than or equal to 85% at approximately 175-190.degree. C. The heat set efficiency value of 85% is considered a minimum value for effective heat setting. It is measured by laboratory tests comparing the length of stretched spandex before and after heat setting to the before-stretched spandex length. While such lower heat setting spandex compositions provide an improvement, heat setting is still required, and the costs associated with it have not been significantly reduced. The traditional practice of making and heat setting circular-knit fabrics has further disadvantages. The knit fabric emerges from a circular knitting machine in the form of a continuous tube. As the tube is formed in knitting, it is either rolled under tension onto a mandrel, or it is collected as a flat tube under the knitting machine by plaiting, or loose folding. In either case, the fabric establishes two permanent creases where the fabric tube has been folded or flattened. Although the fabric is "opened" by slitting the fabric tube along one of the creases, subsequent use and cutting of the fabric usually must avoid the remaining crease. This reduces the fabric yield (or the amount of knit fabric that can be further processed into garments). New methods are sought for making circular-knit, elastic, single-knit jersey fabrics that have bare spandex plated in every knit course, and that avoid the costs and disadvantages associated with heat setting. SUMMARY OF THE INVENTION We have surprisingly found that a circular knit, elastic, single jersey fabric that includes bare spandex plated with spun and/or continuous filament hard yarns can be manufactured with commercially acceptable properties without a need for in-fabric spandex heat setting if: (1) the spandex draft is limited during the knitting process; and (2) certain desired single knit jersey fabric parameters are maintained. "Hard yarns" include spun staple yarns, spun staple and continuous filament yarns and continuous filament yarns. The first aspect of the invention is a method for making a circular knit, single jersey fabric in which bare spandex yarn from 17 to 33 dtex, preferably from 22 to 33 dtex, is plated with a hard yarn of spun and/or continuous filament yarn, or blends thereof, with yarn count (Nm) from 35 to 85, preferably from 44 to 68, most preferably from 47 to 54. Preferably, hard yarn is spun staple yarn of cotton or cotton blended with synthetic fibers or yarn. Other natural and synthetic fibers may be selected for the hard yarn, including nylon, polyester, acrylics and wool, for example. The spandex and the hard yarn are plated in every knit course. The circular knit, single jersey fabric produced by this knitting method has a cover factor of from 1.3 to 1.9. During the knitting, the draft on the spandex feed is controlled so that the spandex yarn is drafted no more than 2.times. its original length when knit to form the circular knit, single jersey fabric. In addition, the knit fabric is finished and dried without heat setting the fabric or the spandex within the fabric. Thus, the fabric is dried at temperatures below the heat setting temperature of the spandex. Finishing may comprise one or more steps, such as cleaning, bleaching, dyeing, drying, and compacting, and any combination of such steps. Preferably, the finishing and drying are carried out at one or more temperatures below 160.degree. C. Drying or compacting is carried out while the knit fabric is in an overfeed condition in the warp direction. The resulting circular knit, elastic, single jersey knit fabric preferably has a spandex content of from 3.5% to 14% by weight based on the total fabric weight per square meter, more preferably from 5% to 10% by weight based on the total fabric weight per square meter. In addition, such fabric preferably has a cover factor of 1.4. The second and third aspects of the invention are the circular knit, elastic, single jersey fabrics made according to the inventive method, and garments constructed from such fabrics. The fabric produced by the inventive method preferably is formed with hard yarns of cotton or cotton blends and has a basis weight of 140 to 240 g/m.sup.2 most preferably of 170 to 220 g/m.sup.2. The fabric preferably also has an elongation of 60% or more, preferably from 60% to 130% in the length (warp) direction, and a shrinkage after washing and drying of about 7% or less, preferably less than 7% in both length and width. Garments may include underwear, t-shirts, and top-weight garments. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates plated knit stitches comprising a hard yarn and spandex; FIG. 2 is a schematic diagram of a portion of a circular knitting machine fed with a spandex feed and a hard yarn feed; FIG. 3 illustrates a series of single jersey knit stitches and highlights one stitch of stitch length "L"; FIG. 3A shows the single stitch of FIG. 3 straightened to illustrate stitch length "L"; FIG. 4 is a flow chart showing prior art process steps for making circular-knit, elastic, single-knit jersey fabrics that have bare spandex plated in every knit course; and FIG. 5 is a flow chart showing the inventive process steps for making circular-knit, elastic, single-knit jersey fabrics that have bare spandex plated in every knit course. While the invention will be described in connection with preferred embodiments below, it is to be understood that the invention is in no way intended to be limited by such description. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the true spirit and scope of the invention as defined by the claims appended hereto. DETAILED DESCRIPTION OF THE INVENTION The subject of this patent is circular knitting, and in particular the manufacture of specific knit elastic fabrics for subsequent `cut and sew` use. Regarding circular knitting, FIG. 2 shows in schematic form one feed position 20 of a circular knitting machine having a series of knitting needles 22 that move reciprocally as indicated by the arrow 24 in response to a cam (not shown) below a rotating cylinder (not shown) that holds the needles. In a circular knitting machine, there are multiple numbers of these feed positions arranged in a circle, so as to feed individual knitting positions as the knitting needles, carried by the moving cylinder, are rotated past the positions. For plating knit operations, a spandex yarn 12 and a hard yarn 14 are delivered to the knitting needles 22 by a carrier plate 26. The carrier plate 26 simultaneously directs both yarns to the knitting position. The spandex yarn 12 and hard yarn 14 are introduced to the knitting needles 22 at the same or at a similar rate to form a single jersey knit stitch 10 like that shown in FIG. 1. The hard yarn 14 is delivered from a wound yarn package 28 to an accumulator 30 that meters the yarn to the carrier plate 26 and knitting needles 22. The hard yarn 14 passes over a feed roll 32 and through a guide hole 34 in the carrier plate 26. Optionally, more than one hard yarn may be delivered to the knitting needles via different guide holes in the carrier plate 26. The spandex 12 is delivered from a surface driven package 36 and past a broken end detector 39 and change of direction roll(s) 37 to a guide slot 38 within the carrier plate 26. The feed tension of the spandex 12 is measured between the detector 39 and drive roll 37, or alternatively between the surface driven package 36 and roll 37 if the broken end detector is not used. The guide hole 34 and guide slot 38 are separated from one another in the carrier plate 26 so as to present the hard yarn 14 and spandex 12 to the knitting needles 22 in side by side, generally parallel relation (plated). The spandex preferably is a commercially available elastane product for circular knitting, such as Lycra.RTM. types T162, T169 and T562. The spandex stretches (drafts) when it is delivered from the supply package to the carrier plate and in turn to the knit stitch due to the difference between the stitch use rate and the feed rate from the spandex supply package. The ratio of the hard yarn supply rate (meters/min) to the spandex supply rate is normally 2.5 to 4 times (2.5.times. to 4.times.) greater, and is known as the machine draft. This corresponds to spandex elongation of 150% to 300%, or more. The feed tension in the spandex yarn is directly related to the draft (elongation) of the spandex yarn. This feed tension is typically maintained at values consistent with high machine drafts for the spandex. We have found that improved results are obtained when the total spandex draft, as measured in the fabric, is kept to about 2.times. or less. This draft value is the total draft of the spandex, which includes any drafting or drawing of the spandex that is included in the supply package of as-spun yarn. The value of residual draft from spinning is termed package relaxation, "PR", and it typically ranges from 0.05 to 0.15 for the spandex used in circular knit, elastic, single jersey fabrics. The total draft of the spandex in the fabric is therefore MD*(1+PR), where "MD" is the knitting machine draft. The knitting machine draft is the ratio of hard yarn feed rate to spandex feed rate, both from their respective supply packages. Because of its stress-strain properties, spandex yarn drafts (draws) more as the tension applied to the spandex increases; conversely, the more that the spandex is drafted, the higher the tension in the yarn. A typical spandex yarn path, in a circular knitting machine, is schematically shown in FIG. 2. The spandex yarn 12 is metered from the supply package 36, over or through a broken end detector 39, over one or more change-of-direction rolls 37, and then to the carrier plate 26, which guides the spandex to the knitting needles 22 and into the stitch. There is a build-up of tension in the spandex yarn as it passes from the supply package and over each device or roller, due to frictional forces imparted by each device or roller that touches the spandex. The total draft of the spandex at the stitch is therefore related to the sum of the tensions throughout the spandex path. The spandex feed tension is measured between the broken end detector 39 and the roll 37 shown in FIG. 2. Alternatively, the spandex feed tension is measured between the surface driven package 36 and roll 37 if the broken end detector 39 is not used. The higher this tension is set and controlled, the greater the spandex draft will be in the fabric, and vice versa. The prior art teaches that this feed tension should range from 2-4 cN for 22 dtex spandex, and from 4-6 cN for 44 dtex spandex in commercial circular knitting machines. With these feed tension settings and the additional tensions imposed by subsequent yarn-path friction, the spandex in commercial knitting machines will be drafted significantly more than 2.times.. This invention does not anticipate all the ways that spandex friction can be minimized between the supply package and the knit stitch. The method requires, however, that friction be minimized to keep the spandex feed tensions sufficiently high for reliable spandex feeding when the spandex draft is 2.times. or less. After knitting a circular knit, elastic, single jersey fabric of plated spandex with hard yarn per the method of this invention, the fabric is finished in either of the alternate processes 60 illustrated diagrammatically in FIG. 5. Drying operations can be carried out on circular knit fabric 62 in the form of an open width web (top row of diagram, path 63a), or as a tube (bottom row of diagram, path 63b). For either of these paths, wet finishing process steps 64 (such as scouring, bleaching and/or dyeing) are carried out on the fabric while it is in tubular form. One form of dyeing, called soft-flow jet dyeing, usually imparts tension and some length deformation in the fabric. Care should be taken to minimize any additional tension applied during fabric processing and transport from wet finishing to the dryer, and also enable the fabric to relax and recover from such wet-finishing and transport tensions during drying. Following wet finishing process steps 64, the fabric is dewatered 66, such as by squeezing or centrifuging. In process path 63a, the tubular fabric is then slit open 68 before it is delivered to a finish/dry step 70 for optional finish application (e.g., softener by padding) and subsequent drying in a tenter-frame oven under conditions of fabric length overfeed. In process path 63b, the tubular fabric is not slit open, but is sent as a tube to the finish/dry step 70. Finish, such as softener, can be optionally applied by padding. The tubular fabric is sent through a drying oven, e.g., laid on a belt, and then to a compactor to separately provide fabric overfeed. A compactor commonly uses rolls to transport the fabric, usually in a steam atmosphere. The first roll(s) is driven at a faster speed of rotation than the second roll(s) so that the fabric has an overfeed. Generally, the steam does not "re-wet" the fabric so that no additional drying is required after compacting. The drying step 70 (path 63a) or the compacting step 72 (path 63b), is operated with controlled, high fabric overfeed in the length (machine) direction so that the fabric stitches are free to move and rearrange without tension. A flat, non-wrinkled or non-buckled fabric emerges after drying. These techniques are familiar to those skilled in the art. For open width fabrics, a tenter-frame is used to provide fabric overfeed during drying. For tubular fabrics, forced overfeed is typically provided in a compactor 72, after belt drying. In either open-width or tubular fabric processing, the fabric drying temperature and residence time are set below the values required to heat set the spandex. The structural design of a circular knit fabric can be characterized in part by the "openness" of each knit stitch. This "openness" is related to the percentage of the area that is open versus that which is covered by the yarn in each stitch (see, e.g., FIGS. 1 and 3), and is thus related to fabric basis weight and elongation potential. For rigid, non-elastic weft knit fabrics, the Cover Factor ("Cf") is well known as a relative measure of openness. The Cover Factor is a ratio and is defined as: Cf=(tex).div.L where tex is the grams weight of 1000 meters of the hard yarn, and L is the stitch length in millimeters. FIG. 3 is a schematic of a single knit jersey stitch pattern. One of the stitches in the pattern has been highlighted to show how the stitch length, "L" is defined. For yarns of metric count Nm, the tex is 1000.div.Nm, and the Cover Factor is alternatively expressed as follows: Cf=(1000/Nm).div.L. We have found that commercially useful circular knit, elastic, single jersey fabrics plated from bare spandex and a hard yarn can be made without heat setting if the spandex draft is kept about 2.times. or less, and if the knit fabric is designed and manufactured within the following preferred limits: The Cover Factor, which characterizes the openness of the knit structure, is between 1.3 and 1.9, and is preferably 1.4; The hard yarn count, Nm, is from 35 to 85, preferably from 44 to 68, and most preferably from 47 to 54; The spandex has 17 to 33 dtex, preferably 22 to 33 dtex; Preferably, the content of spandex in the fabric, on a % weight basis, is from 3.5% to 14%, and is most preferably from 5% to 10%; The knit fabric so formed has a shrinkage after washing and drying of about 7% or less, preferably less than 7% in both the length and width directions; The knit fabric has an elongation of 60% or more, preferably from 60% to 130%, in the length (warp) direction; and Preferably, the hard yarn is spun staple yarn of cotton or cotton blended with synthetic fibers or yarns. While not wishing to be bound by any one theory, it is believed that the hard yarn in the knit structure resists the spandex force that acts to compress the knit stitch. The effectiveness of this resistance is related to the knit structure, as defined by the Cover Factor. For a given hard yarn count, Nm, the Cover Factor is inversely proportional to the stitch length, L. This length is adjustable on the knitting machine, and is therefore a key variable for control. Because the spandex is not heat set in the process of the invention, the spandex draft should be the same in the circular knit, elastic, single jersey as-knit fabric, the finished fabric, or at fabric-processing steps in-between, within the limits of measurement error. For circular knit, elastic, single jersey fabric, the appropriate gauge of knitting machine is selected according to prior art relationships between hard yarn count and knitting machine gauge. Choice of gauge can be used to optimize circular knit, elastic, single jersey basis weight, for example. The benefits of this invention are evident when the prior art process shown diagrammatically in FIG. 4, is compared with the inventive process shown diagrammatically in FIG. 5. Traditional knitting and finishing require more process steps, more equipment, and more labor-intensive operations than does either alternative method of the invention shown in FIG. 5. Further, by eliminating high-temperature heat set previously required (see FIG. 4), the inventive process reduces heat damage to fibers like cotton, requires less or no bleaching, and thus improves the `hand` of the finished fabric. As a further benefit, heat sensitive hard yarns can be used in the invention process to make circular knit, elastic, single jersey fabrics, thus increasing the possibilities for different and improved products. The use of a softener is optional, but commonly a softener will be applied to the knit fabric to further improve fabric hand, and to increase mobility of the knit stitches during drying. Softeners such as SURESOFT or SANDOPERM SEI are typical. The fabric may be passed through a trough containing a liquid softener composition, and then through the nip between a pair a pressure rollers (padding rollers) to squeeze excess liquid from the fabric. Also surprisingly, circular knit, elastic, single jersey fabrics knitted by the method of the invention and collected by folding (plaiting), do not crease to the same extent as prior art circular knit single jersey fabrics. Fewer or less visible fold creases in the finished fabric can result in an increased yield for cutting and sewing the fabric into garments. Also unexpectedly, the circular knit, elastic, single jersey fabrics of the invention have significantly reduced skew during process in either open-width or tubular finishing processes, compared to prior art fabrics. With excess skew or spirality, fabrics are diagonally deformed and courses are "on the bias", and are unacceptable. Garments made with skewed fabric will twist on the body. The following examples demonstrate the invention and its benefits. The invention is capable of other and different embodiments, and its several details are capable of modifications in various apparent respects, without departing from the scope and spirit of the present invention. Accordingly, the examples are to be regarded as illustrative in nature and not as restrictive. |
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