Introduction
From windbreakers and backpacks to carpet backing and fishing line, filament yarns show up in products that need different levels of strength, bulk, and appearance. These yarns begin as manufactured fibers and are then grouped, twisted, textured, or finished according to the end use. The way they are made changes luster, comfort, and durability, and it also affects how the fabric behaves in service. The sections below place the main yarn types and texturing methods in a cleaner sequence.
How Filament Yarns Are Made
Filament yarns are made from manufactured fibers, except for the small percentage that is filament silk. Manufactured filament yarns are made by first extruding a polymer solution through a spinneret and solidifying it in fiber form. Then the individual filaments are brought together with or without a slight twist. The grouping of the filaments with the addition of twist creates the filament yarn. The spinning machine then winds the yarn on a bobbin. The yarn is then rewound on spools or cones and is a finished product, unless additional treatments such as crimping, twisting, texturing, or finishing are required. Filament yarns reach their maximum strength at a low twist, 3 to 6 turns per inch, and strength then remains constant or decreases.
Throwing, originally a process for twisting silk filaments, evolved into the twisting of manufactured fibers and then into texturing. Throwing provides the fabricator with the specific yarn needed for a particular product. Some fiber producers texture yarns as a final step in the fiber-spinning process, and occasionally a trade name is associated with a textured filament yarn.
Smooth-Filament Yarns
Filament yarns are more expensive per pound than staple fibers. The number of holes in the spinneret determines the number of filaments in the yarn. Filament fiber cost is almost equal to filament yarn cost, so staple fibers are cheaper at this point, but the cost of producing a spun yarn quickly eats up the lower fiber cost.
Regular, conventional, or smooth-filament yarns are uniform as they come from the spinneret. Their smooth nature gives them greater luster than spun yarns or textured filament yarns, but the luster varies with the amount of delusterant in the fiber and the amount of twist in the yarn. Maximum luster is obtained with bright filaments with little or no twist. Very-high-twist yarns, like crepe yarns, reduce filament luster. With thermoplastic fibers, the twist can be heat-set. Filament yarns generally have either high twist or low twist.
Filament yarns have no protruding ends, so they do not lint; the fibers are very parallel; they resist pilling; and fabrics made from them easily shed soil. Filaments of round cross section pack well into compact yarns that give little bulk, loft, or cover to fabric. Compact yarns are used in wind- and water-resistant fabrics. Compactness is a disadvantage in end uses in which bulk and absorbency are necessary for comfort. Nonround filaments create more open space for air and moisture permeability and produce greater cover.
Filament yarn strength depends on the strength of the individual fibers and the number of filaments in the yarn. The strength of each filament is fully utilized. In order to break the yarn, all the filaments must be broken. The strength of filament fibers is usually greater than that of staple fibers. For example, staple polyester tenacity is 3 to 5.5 g/d, while filament polyester tenacity is 5 to 8 g/d.
Fine-filament yarns are soft, supple, and strong, but not as abrasion-resistant as coarse filaments. For durability, fewer and coarser filaments in the yarn are most advantageous. A practical comparison is the difference in durability between a taffeta used for windbreakers and a taffeta used for backpacks. Both are probably nylon, but the windbreaker taffeta uses finer fibers in a finer yarn, compared with the coarser fibers and larger yarn in the backpack taffeta. The windbreaker taffeta is resistant to wind and water, but flexible and comfortable to wear. The backpack taffeta is stiffer and less pliable, but stronger and more abrasion resistant.
Monofilament Yarns
Monofilament yarns are primarily for technical uses. These yarns consist of a single coarse filament fiber. End uses include sewing thread, fishing line, fruit and vegetable bags, nets, and other woven or knitted fabrics for which low cost and high durability are most important. Metallic fibers are used as monofilament yarns for the sparkle they produce in apparel and interiors and for their ability to minimize static electricity in carpeting, clean-room apparel, and technical applications.
Tape and Network Yarns
Tape yarns are inexpensive yarns produced from extruded polymer films. Extrusion is the standard method of spinning fibers and some films. The split-fiber method is less expensive than the traditional fiber extrusion process and requires a minimal investment in equipment. Although some fiber polymers cannot be processed by the split-fiber method, polypropylene is often processed in this way because it is easy and inexpensive and produces strong yarns. Tape yarns are ribbonlike in appearance but can also take on the more rounded appearance of traditional yarns.
Pellets of polypropylene with appropriate additives are melted and then extruded as a film 0.005 to 0.020 inch thick onto a chilled roll or cooled quickly by quenching in water. The film is slit into tapes approximately 0.1 inch wide and heat-stretched to orient the molecular chains. The stretching can be carried to a point at which the film fibrillates, or splits into fibers, or the film is passed over needles to slit it. Twisting or other mechanical action completes the fibrillation.
Yarns as low as 250 denier have been made from split fibers. Tape yarns are coarse and usually used in carpet backing, rope, cord, fishnets, bagging, and interiors support fabrics for which ribbonlike yarn is needed.
Olefin films are slit into yarns that are used for the same textile products as split-fiber olefin. Slit-film-tape yarns are much more regular and may be thicker than fibrillated film-tape yarns. Tape yarns are slightly more expensive and their production is slower.
Network yarns are made of fibers that are connected in a network arrangement. They have a ribbonlike characteristic similar to tape yarns, but are bulkier and less dense. These yarns are produced by incorporating air into the polymer to create a foam. When the foam is extruded and stretched, tiny air cells rupture, forming an interconnected fibrous web. Although their strength is not as great as that of multifilament, monofilament, or tape yarns, these network yarns have interesting bulk and comfort characteristics. Uses include technical products in which bulk and low density are more important than high strength.
Bulk Yarns and Texturing
What Makes a Bulk Yarn
According to ASTM International, a not-for-profit organization that provides a forum for the development and publication of standards, a bulk yarn is one that has been processed to have greater covering power or apparent volume than that of a conventional yarn of equal linear density and of the same basic material with normal twist. Often these bulk yarns are referred to as bulk-continuous-filament (BCF) yarns or textured filament yarns. BCF yarns include any continuous-filament yarn whose smooth, straight fibers have been displaced from their closely packed, parallel position by the introduction of some form of crimp, curl, loop, or coil.
The characteristics of bulk yarns are quite different from those of smooth-filament yarns. Bulking gives filaments the aesthetic properties of spun yarns by altering the surface characteristics and creating space between the fibers. These yarns have an irregular surface, soft twist, and continuous nonparallel fibers that resist being pulled apart. Fabrics are more absorbent, more permeable to moisture, more breathable, and more comfortable, and they have better bulk, cover, and elasticity. Static buildup is lower. Bulk yarns do not pill or shed.
There are three classes of bulk yarns: bulky yarns, stretch yarns, and textured yarns. They are discussed below after the texturing processes are described.
Texturing Filament Yarns
The texturing processes discussed here are primarily mechanical methods used with thermoplastic fibers. Heat and chemical methods are used to achieve texture with bicomponent fibers.
In the false twist process, the spindle whirls at 600,000 revolutions per minute and generates such an intense sound that it adversely affects health and hearing. In the continuous process, the yarn is twisted, heat-set, and untwisted as it travels through the spindle. The filaments form a relaxed, distorted helical coil. When the yarn is pulled at each end, the yarn stretches as the coils straighten out. This is one of the most important and cheapest methods used to add bulk and stretch to filament yarns.
In draw-texturing, unoriented filaments or partially oriented filaments, often referred to as partially oriented yarns or POY, are fed through the double-heater false-twist spinner, then stretched slightly and heat-set. Draw-texturing is a fast and inexpensive way to make textured bulk yarns.
The stuffer box produces a sawtooth crimp of considerable bulk. Straight-filament yarns are pushed into one end of a heated box and then withdrawn at the other end in crimped form. The volume increase is 200% to 300%, with some elasticity. The stuffer box is a fast, inexpensive, and popular method for carpeting yarns.
Conventional filament yarns are fed over an air jet at a faster rate than they are drawn off. The blast of air forces some of the filaments into very tiny loops; the velocity of the air affects the size of the loops. This is a slow, relatively costly, but versatile process. Volume increases with little or no stretch. Air-jet yarns maintain their size and bulk under tension because the straight areas bear the strain and the loops remain relatively unaffected.
In the knit-deknit process, a small-diameter tube is knit, heat-set, unraveled, and wound on cones. Crimp is varied by changing stitch size and tension. The gauge used to make a fabric must differ from that used in yarn texturing, or pinholes will form when texturing and knit gauges match.
Classes of Bulk Yarns
According to the ASTM, bulky yarns are formed from inherently bulky manufactured fibers that are hollow along part or all of their length or from fibers that cannot be closely packed because of their cross-sectional shape, fiber alignment, stiffness, resilience, or natural crimp. Bulky texturing processes can be used with any kind of filament fiber or spun yarn. The yarns have less stretch than either stretch or textured yarns. Bulky yarns are used in a wide array of products including carpeting, lingerie, sweaters, and shoelaces.
The ASTM defines stretch yarns as thermoplastic filament or spun yarns with a high degree of potential elastic stretch, 300% to 500%, rapid recovery, and a high degree of yarn curl. Stretch yarns have moderate bulk. Stretch yarns of nylon are used extensively in men’s and women’s hosiery, pantyhose, leotards, swimwear, leggings, football pants, and jerseys. Apparel manufacturers like stretch yarns because fewer sizes are needed since one-size items fit wearers of different sizes. Stretch yarns are not the same as yarns made with elastomeric fibers.
The ASTM defines textured or bulked yarns as filament or spun yarns with notably greater apparent volume than a conventional yarn of similar filament count and linear density. These yarns have much lower elastic stretch than stretch yarns, but greater stretch than bulky yarns. They are stable enough to present no unusual problems in subsequent processing or in consumer use. Fabrics made from these yarns maintain their original size and shape during wear and care.
Conclusion
Filament yarns offer a wide range of performance because the spinneret, twist, filament count, and texturing method can all change the final yarn. Smooth-filament yarns give luster and strength, monofilament yarns suit technical uses, and tape and network yarns serve low-cost or low-density applications. Bulk, stretch, and textured yarns extend those choices by adding cover, comfort, and elasticity. As these methods continue to evolve, filament yarns will remain closely tied to specific end uses.
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor to Wikipedia.
