Nonwoven Fabrics
Nonwoven fabrics are textiles made from long fibers, bonded together by mechanical, chemical, thermal or solvent treatment. The term describes any textile product that is neither woven nor knitted such as a felt which is mechanical interlocked in a random web. These materials typically lack strength unless they are strongly densified, fiber-bonded and/or reinforced by a backing. In principle, all types of fibers can be processed into nonwoven (bonded) fabrics. The choice of fibrous material depends on the application, the performance requirements, the cost target and the processing requirements. To achieve these targets, the right choice of fiber material, fiber length-to-width ratio, and processing method are crucial. Shorter staple fibers generally lead to stronger felt but also degrade faster than nonwovens made from longer staples. Man-made fibers such as nylon and polyester are often a better choice when higher water, stain, wear, and chemical resistance are required whereas natural fibers are typically chosen when the nonwoven material must be biodegradable and/or biocompatible such as disposable and medical textile products. Natural fibers are also cheaper than synthetic fibers because they are often produced from low-grade (cotton) waste. These products have good bulk and bonding properties and high absorption power but are typically less durable than nonwovens made from synthetic fibers because they can be attacked by bacteria and fungi (mildew), and break and wear down over time, whereas most synthetic fibers are non-biodegradable. In some cases, naturally fibers are blended with man-made fibers which are more uniform and stronger. The most common natural fibers are wool, and cotton as well as regenerated cellulose (Rayon) and acetate. The latter two are manufactured from cellulose by chemical treatment. Synthetic fibers such as polyester, acrylics, nylon and olefins are used on a smaller scale but have continuously gained market share over the years and do so.
Nonwovens are mass-produced on a very large scale and are used in numerous common items.1 Important applications of nonwovens include disposable articles such as baby diapers, feminine hygiene, adult incontinence products, filters, wet wipes, wound dressing, crop covers, weed control fabrics, root bags, as well as more durable articles such as blankets, drapes, backing for tufted carpets, greenhouse shadings, drainage and erosion control covers, house wraps, shoe felts, insulators, filters, fiber glass, wall coverings, and tennis ball covers.2
A nonwoven fabric is typically made directly from fibers or filaments without the elaborate yarn preparation (weaving beam)3 required for warp knitting or weaving. In a first step, fibers are oriented in one direction or are deposited in a random manner. This web or sheet is then bonded together by chemical, thermal or mechanical treatment. The most common mechanical bonding method is needle punching. The fibers are entangled with each other by punching triangle needles through the web. The quality of this type of nonwoven fabric - that is the degree of entanglement and web density (bond strength) - depends on the intensity and number of needles and on subsequent treatment steps. Another important mechanical interlocking method is stitch bonding in which the fibers in the web are bonded together by stitches sewn or knitted through the web to produce a fabric. The finished product has the look of corduroy. In some cases, both methods are combined to produce a nonwoven of higher quality. The fibers of a web can also be interlocked (entangled) with each other with high velocity jets of water. This bonding methods is known as hydroentangling or spunlace bonding (not to be confused with spunlaid).
A nonwoven can also be produced by wet or dry (chemical) bonding. In the case of wet bonding, the web is formed by filtering an aqueous suspension of fibers onto a screen conveyor belt or onto a perforated drum similar to a paper making process. This method is very common for natural fibers. A bonding agent (adhesive resin) is added to the suspension which binds the fibers firmly together. Another popular chemical bonding method is saturation adhesive bonding. The web or screen passes through an impregnation bath of adhesive resin and is then laid onto a perforated drum. To remove excess adhesive, the wet web is squeezed through the nip of a pair of rolls and is cured afterwards in an oven to bind the fibers together. Alternatively, a nonwoven web can be bonded by gravure bonding. The gravure system consists of a solid roll that is engraved with numerous minute indentations. The roller is partially immersed into a resin bath. As the roll turns, excess resin is removed by a doctor blade, which leaves only the adhesive binder in the roller’s indentations. An unbonded web is then squeezed through the nip consisting of gravure roll and rubber roll and resin penetrates the web by capillary action. The web is dried and cured afterwards.
Another very popular method of bonding fibers is thermal bonding (spunbond or spunlaid). The method employs spinnerets to produce continuous strands of thermoplastic fibers which are stretched to increase their strength, and then randomly spread on a moving conveyor belt made of a fine screen. In some cases, the filaments are still sufficiently hot to adhere to each other or the fibers are bonded by a calender process.
1The nonwoven market had a volume of about 270 billion square meters and was vlaues US$ 42 billion in 2017.
Source: "The Futre of Global Nonwovens to 2022", Smithers Pira Market Report 2017
2In recent years, nonwoven fabrics have also become an alternative to polyurethane foam products.
3Beaming is the winding of the full width of warp yarns on the weaving beam in a single winding operation.