TEXTILES.ORG
For advanced technical textiles, knits offer a diverse and versatile fabric system.
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Textiles come in many forms: fiber, yarn, nonwovens, woven fabric, knits, and many versions and combinations of each. All fabric-forming techniques can be used for advanced textiles. Though the definition of “advanced textiles” may vary, we are usually talking of materials utilizing high performance fibers or fabrics chosen for their unique properties, such as extreme strength, high levels of resistance to specific or a broad range of chemicals, fire retardant or flame-proof, tolerant of high temperatures and others. They are generally high value items. Some, such as medical implants, may require small volume but their use is essential. Which fiber or fabric to use?
Beyond apparel
We are familiar with knit fabrics in apparel—polo shirts, dresses, sweaters and lingerie, for example. When end product manufacturers (EPMs) consider developing a new product for specialized or technical applications, including advanced textiles, knits are not usually considered first, but that could be changing. Knits offer many desirable properties that make them attractive for use in specialty applications. Advanced textiles can take full advantage of mechanical and other properties available from knit fabrics.
While there are no definitive numbers, most seem to agree that knits are used in about 15-20 percent of the $127 billion technical textiles market. That is still a powerful number. While there are many medium-to-large companies, most of those in technical textiles (and those doing highly specialized advanced textiles) may be smaller firms better suited to experimentation and development. Most seem to “fly under the radar” and are not well known. Often, they prefer it that way, not drawing attention to their niche. On the other end, medium-to-large companies doing warp knits and other types, such as multi-axial, turn out large quantities for automotive, medical, protective clothing, composites and geosynthetics applications—and many are highly specialized.
Knit types
There are two major types of knits for technical textiles: circular (weft) and flat (warp) knits.
Circular knits. These are common among smaller knitters and have been around for many years. The emphasis is on the cross-fabric (weft) yarns, laid down in a circular or tubular form from a revolving overhead creel, and looped and interlocked by interplay of special needles as the creel spins around, raising and lowering needles. The machines are of varying diameters, can use different yarns, and produce a seamless circular fabric that may be used as is or slit and opened up for a flat fabric.
Flat or warp knits. Using larger and more complex machines with multi-yarn carriers or creels, these knits are similar to warp yarns feeding into a loom and also have interlocking of yarns with special needles producing flat fabrics of one or multiple widths, depending on the machine capability and design. In flat knits, the emphasis is on the lengthwise warp yarns.
Each knit type has many variations and can be modified in construction or use different yarns for specific properties for intended applications. Warp knitting is popular for many advanced textile applications ranging from those for composites to spacer and open mesh fabrics.
A special warp knit multi-axial machine lays down up to six or more layers of yarns, even fabric, at angles of the same or various material combinations. They are mechanically bonded, producing unique fabric composites used for advanced, specialized products.
Mechanical bonding is often done by stitch-bonding. Basically, a nonwoven web or a “sheet” of yarns, or combinations of fabric and yarn, are literally sewn (sewing is a knit process) across the full width and length of the combination. The essential elements are held in place by the stitching around them, and the yarns are not subject to the tensions involved with the over and under process of weaving. As a case in point, BMW uses stitch-bonding to hold the layers or parallel sheets of untwisted carbon yarn together for subsequent use in composites. Not having the carbon yarns twisted increases resin penetration and the lack of intertwining results in less elongation, crucial in many applications.
There are also a significant number of specialized machines capable of knitting fully formed gloves or other unique shapes.
Selecting knits
How does one determine which type of knit to use, or whether to use knits? The simple answer is that it all depends on the properties of the end product and economics. If you want conformability, stretch, certain patterning, you may want to consider circular knits. If you want open mesh fabrics, a more dimensionally stable closed-up fabric, a more robust fabric with the option of multiple yarns and patterns, you may want warp knits. Sometimes the properties of the fibers or yarns dictate which form to use as some fiber, especially some of the exotic yarns, may be somewhat brittle or fragile, making weaving interlacing impractical.
Many knits, like woven fabrics, are already in use and readily available, materials that may utilize aramids or other specialty fibers, and may offer good economics versus developing a special fabric. Many of the technical textile fabrics being used today are woven materials, which are (or were) readily available in a variety of formats. Though the properties may have been somewhat higher than needed or slightly less, they were chosen simply because “they were there” and were faster and less expensive than designing a special construction.
Variations can often be made more easily with knits as opposed to wovens. For high performance materials, off-the-shelf items will likely not be available, the material has to be designed and custom made.
The circular knit specialty
Typical of the smaller circular knit companies, Draper Knitting Co., Canton, Mass., produces high-pile silver knit fabrics (a fleecy materials for paint rollers and filtration applications), conventional circular knits and nonwoven stitch-bonded fabrics for performance, industrial and safety applications. Many yarn forms, including Kevlar, are used. Draper is not a commodity house, says president and marketing and development manager Kristin Draper, a 6th generation head of this 157-year-old company. Most items, in fact, are custom, and customers normally come to them.
“They typically find us at trade shows, through word-of-mouth, and referrals of other customers,” Draper says. The industry, however, has suffered due to foreign competition. “Why, then, is a small, less well-known company still around?” she asks. “We can work with customers, give them great service, design and deliver what they need quickly, on time and at a reasonable price. Many say this, but we’ve been doing it for years.”
The company has a vertical mill with finishing capabilities, but they often work with others in the supply chain, such as laminators and specialty finishers, to provide the customer a one-source, complete package. While volume is important, she says they can work with a minimum order of as little as 500 yards. Circular knitters often can respond more quickly than flat knit or woven fabric producers. It is advisable to work with knitters who will take the time to develop or supply what is needed, advanced or otherwise.
“You have to innovate, listen, and personalize to stay ahead in this business,” says Charley Norgard, president of Straus Knitting Mills Inc., St. Croix Falls, Wis., a 90-year-old and relatively small circular knitting company with many fingers in the specialty textiles pie. Typical of successful companies providing specialized fabrics, Straus succeeds with attention to the special needs of customers, providing close support and a high level of customer service, outstanding quality and the required technical performance. Norgard says Straus adapted to globalization quite early.
“We no longer compete with other companies, we compete with countries. We are a tough competitor in an equally tough textile industry.” And the company competes well, even numbering China among its largest customers. They succeed with innovation, personalized service, and by constantly reinventing themselves with new fibers, markets and applications.
While many knitters have gone out of business with competition from overseas, many in specialty textiles areas seem to do relatively well. Being in a niche business helps to achieve higher margins and be competitive—at home or overseas.
The flexibility of warp knits
Lutz Heinig, technical manager at Karl Mayer North America, Greensboro, N.C., says the ability to design high-performance specialty fabrics is limited with circular knits. Warp knitting, the specialty of Karl Mayer, is more flexible, he says, making the process easier to customize. Predictably, Heinig feels it is the future of knits for technical textiles. “Look at the economics,” he says. “The output is much faster, wider widths, even multiple widths are possible, and changes and set-ups can be easily done.”
Widely used weft-inserted-warp-knits (WIWK) produce open mesh scrims/fabrics used to reinforce film-to-film laminates. The open mesh allows film-to-film bonding through the interstices rather than to the fabric itself, allowing the high tear strengths for which this type of laminate is noted.
They were formally made as woven scrims, although maintaining uniformity in openings was difficult. Fabrics required special treatments to prevent yarn shifting. WIWK fabrics have very uniform opening size due to a binder yarn knitted around the warp and filling yarns, which are laid on top of each other rather than interlaced themselves. There is no need to finish the fabrics in order to hold the openings straight, helping to make this process cost effective. This economical, high-output system is used by major laminators, many of which have installed their own WIWK machines. The system is extensively used as well in other applications such as safety, construction, geotextiles, mining and the like. High visibility safety vests are an examples of a WIWK fabric application.
A trend Heinig sees, and one often reported by others, is more business coming back from China. While a premium upfront may be needed for most U.S. production, increasing labor costs in China and other considerations are shrinking that advantage. Factoring in often lower quality, large container-full order requirements, and rapidly increasing shipping cost and longer in-transit times make U.S. production more attractive in specialty and certainly advanced textiles, not to mention the requirement for nearby technical support. Heinig says the technology has improved in recent years resulting in increased machine shipments to the U.S.
Growth in spacer fabrics
There are many variations of warp knitting; one receiving a lot of attention is spacer fabrics. In this system, two fabrics—the same or different—are knitted on top of each other and interconnected by a third yarn system with controlled height between the layers. Normally the distance ranges from 1.5-20mm, and with varying openness between the two layers. The connecting yarns can be of many types—monofilaments, filament, even spun—and, depending on construction, create an open area between the layers which allows air flow, cooling, heating and breathability.
Heinig says spacer fabrics may constitute as much as 70 percent of the warp knit fabric market today, and are growing in use, although it is still a niche product in the overall technical textiles area. Large volumes of spacer fabrics are used in composites for huge windmill blades for power generation. (Carbon fiber is often used for strength in the very long blades where extreme force is prevalent due to centrifugal force.)
The outlook for the power market is mixed, says Heinig, and often based on subsidies by the respective governments, but other markets, such as automotives, are growing. The number one criteria for designers in automotive interiors and seating is for “something new, fresh, and unexpected— something they haven’t seen,” says Lee Woodard, global director R&D – Seating, Guilford Performance Textiles, Wilmington, N.C. Knits can supply that, Woodward says; 50 percent of automotive seating in the U.S. uses knits.
Spacer fabrics are used often as a second layer of the seating to offer cushioning and comfort (heating and cooling), especially in luxury cars. Some, like Nissan and Volkswagen, use them as the primary seating surface in mesh or sporty patterns to add cushioning and a “technical” look.
Still, woven fabrics are more robust and durable, especially for seating, and continue to be used extensively. Europe uses about 70 percent woven fabrics for seating, and their use is actually “on the uptick,” says Woodard, but “Knits offers greater versatility and design opportunities and will grow in use.”
Knits are also used in most headliners, an application that has gotten quite complex in recent years with increased use of molded headliners as components in the ceilings of automobiles. With their extensibility and elasticity, knits provide the draw needed in molding. The same property is often of value in advanced textile applications, as well. Nonwovens are used in less complex headliners because they are much less expensive (by about a third), but they don’t have the versatility of knits.
While some of the larger and best known companies promote their technology, many of the smaller ones are quietly working with customers to develop new, specialty fabrics of advanced materials. The range of applications seems limitless, with possible applications in bedding, medical uses, athletics, safety and protection, geotextile, industrial areas and filtration. They are represented in virtually any segment of technical textiles, including military and aerospace applications. Virtually every fiber is utilized. For those looking to come up with new products and applications, whether routine or highly advanced, knits may be the answer.