What is Twist?
In yarn construction, twist is one of the main factors that affects how a yarn behaves in fabric. All staple yarns and some filament yarns have twist applied to them to hold the fibers together. Twist, the spiral arrangement of the fibers around the yarn’s axis, is produced by rotating one end of a fiber strand while holding the other end stationary. It binds the fibers together and contributes strength to the spun yarn. The amount of twist in a given yarn can be measured and expressed as turns per meter (tpm), turns per centimeter (tpcm), or turns per inch (tpi). The degree of twist given to a yarn affects a number of aspects of its appearance, behavior, and durability.
Effects of Twist
The amount of twist affects a number of yarn properties, especially for staple yarns. Bulkiness is one such property. As a general rule, increasing twist decreases apparent yarn size. Think again of the crude formation of a yarn by twisting a loose strand of cotton fibers. The more one twists, the smaller in diameter the yarn will become.
Strength increases in staple yarns as twist increases up to a certain point. Beyond this point, the strength of the yarn begins to decrease, and yarns with exceptionally high, tight twist may become brittle and weak. Elasticity is increased if yarns are twisted tightly. Tightly twisted yarns are known as crepe yarns, which are generally very fine. The twist of crepe yarns is so high that they curl up unless they are held under tension, as they would be on a loom during weaving.
A simple and easy test to identify crepe yarns is to unravel a yarn from the fabric and run it between the fingernails, thus removing the tension produced by sizing material. If the yarn curls up into corkscrewlike curls, it has been creped. This tendency of crepe yarn to twist makes fabrics constructed from these yarns less dimensionally stable than other fabrics, as they are more elastic and have an uneven fabric surface.
More tightly twisted yarns shed soil more easily. Because their surface is smoother, there are fewer loose fiber ends to attract and hold soil. In yarns made from absorbent fibers, absorbency is lower in more tightly twisted yarns. Abrasion resistance is increased by tighter twist. This is a logical result because in a more tightly twisted yarn, many fibers are held in such a way that they appear on the surface, then sink into the center of the yarn, then come back to the surface again. As a result, more fibers are subject to a relatively even distribution of abrasion. Loose surface fibers in low-twist yarns also snag and pull up, creating points of wear. The appearance of a fabric can be determined to a large extent by the twist of the yarn. For example, if filament yarns of higher luster are given only low twist, they will reflect greater quantities of light in mirrorlike fashion and, therefore, appear brighter than the same yarns when they are more tightly twisted. Crepe fabrics achieve a nubby, somewhat crinkled effect by using creped yarns that have a less even surface texture. Unevenly twisted slub yarns can also produce a rough surface. The fabric designer takes advantage of these effects to create a wide variety of surface textures and designs.
Direction of Twist
In twisting fibers together to form yarns, the fibers can be twisted either to the right or to the left. In textile industry terminology, this twist is called S-twist or Z-twist. Z-twisted yarns are twisted so the direction of the twist follows the center bar of the letter Z. Z twist is also known as right twist. In S-twist yarn, the twist direction follows the center bar of the letter S. S twist is also known as left twist.
A simple way to see this is to take a small bunch of absorbent cotton and make a vertical line on the fibers with a pen. Then, suspending the fibers from the thumb and forefinger of the left hand to those of the right, rotate the fibers to the right with the upper left-hand fingers. The line you have made will take a diagonal direction similar to that of the bar in a Z. If the upper fingers are twisted to the left, the line will follow the bar of the S.
Most yarns are made with a Z twist. However, in certain fabric constructions, special effects can be achieved by combining yarns in which the fibers have been twisted in either the same or opposite directions. Z-twist is also more common for weaving yarns. Hand spinners of yarn typically spin singles clockwise with a Z-twist and ply counterclockwise with an S-twist.
Amount of Twist
The amount of twist varies with fiber length, yarn size, and intended use. Increasing the amount of twist to the point of perfect fiber-to-fiber cohesion increases yarn strength. However, excessive twist places the fibers at right angles to the yarn axis, causing a shearing action among fibers and a weaker yarn. Increasing the amount of twist also impacts yarn hairiness, comfort, cost, and linting. Yarns with lower twist tend to be hairier, pill and lint more, have better comfort when in contact with the skin, and cost less. Combed yarns with long fibers do not require as much twist as carded yarns with short fibers. Long, parallel fibers have more points of contact per unit length, producing a stronger yarn for the same amount of twist. Fine yarns require more twist than coarse yarns. Knitting yarns have less twist than filling yarns used in weaving. In better-quality yarns, twist is evenly distributed throughout the yarn and the tpi is toward the high end of the range for that yarn type. The number of twists determines, in part, yarn and product performance and yarn cost.
Monofilament and Low-Twist Yarns
Monofilament yarns have no twist. With only a single fiber forming the yarn, there is no need to twist the yarn. Low twist is used to maintain integrity within smooth multifilament yarns. Two to three turns per inch keeps filament yarns from separating into individual fibers.
Napping Twist
Napping twist, 8 to 15 tpi, produces lofty spun yarns. They are used in filling yarns of fabrics that are to be napped. Napping teases out the ends of the staple fibers and creates the soft, fuzzy surface.
Average Twist
Average twist, 15 to 30 tpi, is used most commonly for yarns made of staple fibers, but seldom for filament yarns. Yarns with average twist are the most durable spun yarns. Combed, worsted, and line (flax) yarns are made of long-staple fibers with a parallel arrangement. Carded, woolen, and tow (flax) yarns are made of short-staple fibers with a less parallel arrangement.
Hard Twist or Voile Twist
Hard twist or voile twist yarns, 30 to 40 tpi, have a harsher hand and more turns per inch. Yarn hardness results when twist brings the fibers closer together and compacts the yarn. This effect is more pronounced when a twist-on-twist ply yarn is used. Twist-on-twist means that the direction of twist in the single is the same as that of the plying twist, thus increasing the total amount of yarn twist.
Crepe Twist
Crepe twist or crepe yarns have the highest number of turns per inch, 40 to 80 tpi, inserted in the yarn. These spun or filament yarns are also known as unbalanced yarns, since they twist and kink when removed from the fabric. These yarns are so lively that they must be twist-set, a yarn-finishing process, before they can be woven or knitted. To identify crepe yarns, test a yarn removed from the fabric by first pulling on the yarn and then letting one end go. A crepe twist yarn should resemble the shape of a corkscrew. Oftentimes, the crepe twist yarn will knot, twist, or curl when removed from a fabric. Most crepe fabrics have crepe yarns in the crosswise direction, although some are in the lengthwise direction and some have crepe yarns in both directions. Crepe fabrics may or may not include crepe yarns.
Increasing the amount of crepe yarn twist and alternating the direction of twist increases the amount of crinkle in a crepe fabric. For example, a fabric made of crepe twist yarns in a band of six S-filling yarns followed by a band of six Z-filling yarns produces a more prominent crinkle than bands of two S-filling yarns followed by bands of two Z-filling yarns.
Conclusion
Twist is a small change in yarn construction, but it affects strength, bulk, elasticity, soil shedding, absorbency, abrasion resistance, and appearance. The direction of twist, whether S or Z, also matters in yarn behavior and fabric effects. From no twist in monofilament yarns to crepe twist in lively yarns, each range serves a different purpose. As yarn development continues, twist will remain a basic tool for controlling both performance and surface character.
