Part one of a five-part series on nonwoven manufacturing technologies
Jürg Rupp, Executive Editor
Nonwovens are no doubt among the most promising textile products of the past 10 to 15 years. They are not only replacing traditional textiles, but are also creating new markets for new products.
A carded drylaid process featuring binder impregnation
All illustrations courtesy of EDANA
Asia Becomes Very Important
With the sustained and rapid economic and social growth in Asia, the development of the nonwovens industry has been pushed forward rapidly in many countries on the Asia-Pacific Rim. Analysts have predicted there will be unlimited prospects for the market of nonwoven materials used in medical, health, hygiene, filtering, geotextiles and agrotextiles, and other industries over the next decade. In 2010, the production of nonwovens in China is predicted to exceed 1 million metric tons, and China and India will become the greatest potential markets for nonwovens equipment, machinery and nonwoven products. China’s developing nonwovens industry will need higher-quality man-made fibers and functional fibers as well as more advanced technology and nonwovens production equipment.
Needlepunch technology is quite flexible — recycled cotton waste can be made into needlepunched nonwovens.
Three Production Stages
The production of nonwovens can be described as taking place in three stages: web formation; web bonding; and finishing treatments. However, modern technology allows an overlapping of the stages, and in some cases, all three stages can take place at the same time.
The opportunity to combine different raw materials and different techniques accounts for the diversity of the nonwovens industry and its products. This diversity is enhanced by the ability to engineer nonwovens to have specific properties and perform specific tasks.
Short-fiber airlaid process
Nonwovens production begins with the arrangement of fibers in a sheet or web. The fibers can be staple fibers packed in bales, or filaments extruded from molten polymer granules. Four basic methods are used to form a web, and nonwovens are usually referred to by one of these methods: drylaid; spunlaid; wetlaid; or other techniques.
There are two methods of drylaying — carding and airlaying. Carding is more or less the same starting process as in spinning. It starts with the opening of the fiber bales, natural or man-made, which then are blended according to the requested characteristics of the final product. After homogeneous blending, the fibers are processed into a web by a carding machine. Of utmost importance is the configuration of the carding drums, which gives the final nonwoven product its fabric weight and fiber orientation. The fiber orientation is very important in terms of the machine direction/cross direction ratio of the product. An ideal value would be an isotropic value of 1:1 in both the horizontal and vertical direction. In a further process, the web can be parallel-laid, where most of the fibers are laid in the direction of the web travel, or they can be random-laid. Typical parallel-laid carded webs result in nonwovens with good tensile strength, low elongation and low tear strength in the machine direction; and the reverse in the cross direction. Relative speeds and web composition can be varied to produce a wide range of properties.
A chemical (adhesion) web-bonding process featuring saturation/impregnation
Another technology of transforming fiber material into a nonwoven product is the airlaid technology. One of the major advantages of this technology is the fact that very short fiber material can be applied. This is of special interest for recycled fiber material such as cotton waste from spinning and yarn material, for example.
The fibers are fed into an air stream and from there to a moving belt or perforated drum, where they form a randomly oriented web. Compared with carded webs, airlaid webs have a lower density, a greater softness and an absence of laminar structure. Airlaid webs offer great versatility in terms of the fibers and fiber blends that can be used.
A thermal (cohesion) web-bonding process featuring calendering
After the formation, the web is still a very loose product and has very little strength in its unbonded form. The web must therefore be consolidated in some way. This bonding process also is very important for the final appearance of the nonwovens product. The choice of the bonding technology is at least as important to the ultimate functional properties as the type of fiber material in the web. There are three basic types of bonding: chemical; thermal; and mechanical.
Chemical or adhesion bonding mainly refers to the application of a liquid-based bonding agent to the web. Three groups of materials are commonly used as binders — acrylate polymers and copolymers, styrene-butadiene copolymers and vinyl acetate ethylene copolymers. Water-based binder systems are the most widely used, but powdered adhesives, foam and in some cases organic solvent solutions also are used. There are many ways of applying the binder. It can be applied uniformly by impregnating, coating or spraying; or intermittently, as in print bonding. Print bonding is used when specific patterns are required and where it is necessary to have the majority of fibers free of binder for functional reasons. Due to the application of chemical binders, the final product is generally more rigid than nonwovens bonded using other technologies. However, this type of bonding gives the product very high tensile strength and resilience properties.
Thermal or cohesion bonding uses the thermoplastic properties of certain man-made fibers to form bonds under controlled heating. In some cases, the fiber web itself can be used, but more often, a low-melt polyethylene or bicomponent fiber is the bonding agent. There are several thermal bonding systems in use:
• Calendering uses heat and high pressure applied through rollers to weld the fiber web at a given speed.
• Through-air thermal bonding is the ideal method for bulkier products performing the overall bonding of a web containing low-temperature melting fibers. This takes place in a carefully controlled hot-air stream.
• Drum and blanket systems apply pressure and heat to make products of average bulk.
• Sonic bonding takes place when the molecules of the fibers held under a patterned roller are excited by high-frequency energy that produces internal heating and softening of the fibers.
Mechanical or friction bonding is probably the most applied technology. In mechanical bonding, the strengthening of the web is achieved by inter-fiber friction as a result of the physical entanglement of the fibers. The two types of mechanical bonding are needlepunching and hydroentanglement.
Needlepunching can be used for almost all kinds of fibers. Specially designed needles are pushed and pulled through the web to entangle the fibers. Webs of different characteristics can be needled together to produce a gradation of properties difficult to achieve by other means.
Hydroentanglement or spunlacing is applied mainly to carded or wetlaid webs and uses fine, high-pressure jets of water to cause the fibers to interlace. By applying a design of the perforated drum on the spunlacing machine, the arrangement of the water jets can give a wide variety of aesthetically pleasing effects, such as logos and designs. The water-jet pressure used has a direct bearing on the strength of the web, but system design also plays a part.
As in the traditional textile industry, nonwovens must have some finishing treatment too. This production stage is the final opportunity to meet the needs of the customer even more precisely by modifying or adding to existing properties. A variety of different chemical substances may be employed before or after binding, or various mechanical processes may be applied to the nonwoven after binding. Nonwovens may be made conductive, flame-retardant, water-repellent, porous, antistatic, breathable and absorbent — the list of properties can be very long.
Nonwovens may also be coated, laminated, printed, flocked or dyed, and may be combined with other materials to form composite materials.