Nonwoven businesses, in general, rely on high volume production which is highly engineered and, because of competitive pressures, there is a constant need to innovate to ensure high quality, in-specification products are produced at the minimum cost. The industry is driven by technology developments in machinery, process control and materials and, to have a sustainable future, nonwovens enterprises need to be at the forefront of these developments.
What is a nonwoven fabric?
Nonwoven fabrics are broadly defined as web structures bonded together by entangling fibres mechanically, thermally fusing the fibres or chemically bonding the fibres. Nonwovens are defined more exactly by various bodies one of the most often quoted is the International Nonwovens & Disposables Association (INDA) definition: Nonwovens are a sheet, web, or bat of natural and/or man-made fibres or filaments, excluding paper, that have not been converted into yarns, and that are bonded to each other by any of several means.
The various methods for bonding are:
1. Adding an adhesive.
2. Thermally fusing the fibres to each other or to the other melt able fibres or powders.
3. Fusing fibres by first dissolving, and then re-solidifying their surfaces.
4. Creating physical tangles or tuft among the fibres.
5. Stitching the fibres or filaments in place.
Nonwovens are not made by weaving or knitting and do not require converting the fibres to yarn. Nonwoven fabrics are engineered fabrics that may be single-use disposable or a very durable fabric. They are used in numerous applications, including; baby diapers, adult incontinence products, wet wipes, surgical drapes and covers, liquid cartridge and bag filters, face masks, air-conditioning filters, soil stabilizers and roadway underlayment, erosion control, drainage systems, insulation (fiberglass batting), pillows, cushions, and upholstery padding, carpet backing, automotive headliners and upholstery, house wraps, and disposable clothing (foot coverings, coveralls).
Nonwoven manufacturing process
Nonwoven manufacturing can be described in simple terms as a series of manufacturing steps consisting of forming a fibrous web, entangling or bonding the fibres in the web to impart mechanical integrity to the structure and finishing/converting the fabric to impart some special properties to the fabric that the customer specifies. The manufacturing steps are described below:
The characteristics of the fibrous web are a key determinant of the physical properties of the final product. The choice of methods for forming webs is determined by fibre length. Initially, the methods for the formation of webs from staple-length fibres were based on the textile carding process, whereas web formation from short fibres was based on a wet laid process similar papermaking. These technologies are still in use, but methods based forming a web directly from filaments immediately they exit an extruder (Spun laid) have also been developed.
Fibrous webs have little mechanical strength and a further manufacturing process is necessary to form a fabric with useful properties. There are number of processes which are used to accomplish this as described in next section.
Needle punching is a process of bonding nonwoven web structures mechanically interlocking the fibers through the web. Barbed needles, mounted on a board, punch fibers into the web and then are withdrawn leaving fibers entangled. The needles are spaced in a non-aligned arrangement are designed to release the fiber as the needle board is withdrawn.
Stitch bonding is a method of consolidating fiber webs with knitting elements with or without yarn to interlock the fibers. There are a number of different yarns that can be used. Home furnishings are a market for these fabrics. Other uses are vacuum bags, geo-textiles, filtration and interlinings. In many applications stitch-bonded fabrics are taking the place of woven goods because they are faster to produce and, hence, the cost of production is considerably less.
Thermal bonding is the process of using heat to bond or stabilize a web structure that consists of a thermoplastic fiber. All part of the fibers act as thermal binders, thus eliminating the use of latex or resin binders. Thermal bonding is the leading method used by the cover stock industry for baby diapers. Polypropylene has been the most suitable fiber with a low melting point of approximately 165C. It is also soft to touch. The fiber web is passed between heated calendar rollers, where the web is bonded. In most cases point bonding by the use of embossed rolls is the most desired method, adding softness and flexibility to the fabric. Use of smooth rolls bonds the entire surface of the fabric increasing the strength, but reduces drape and softness.
Chemical bonding is the process of bonding a web by means of a chemical and is one of the most common methods of bonding. The chemical binder is applied to the web and is cured. The most commonly used binder is latex, because it is economical, easy to apply and very effective. Several methods are used to apply the binder and include saturation bonding, spray bonding, print bonding and foam bonding.
Hydro entanglement is a process of using fluid forces to lock the fibers together. This is achieved by fine water jets directed through the web, which is supported by a conveyor beit. Entanglement occurs when the water strikes the web and the fibers are deflected. The vigorous agitation within the web causes the fibers to become entangled.
Finishing and converting
Finishing and converting are the last operations performed on the fabric before it is delivered to the customer. Finishing includes operations such as coating and laminating, calendaring and embossing to impart particular surface properties, corona and plasma treatments to change the wetting properties of the fabric, wet chemical treatments to impart anti-stat" properties, anti-microbial properties, flame retardant properties etc. Aft finishing the fabric, it is usually cut to the width the customer specifies a rewound ready for shipment. This is known as converting.
This article was originally published in the November issue of the New Cloth Market: the Complete Textile Magazine from Textile Technologists