How to produce Nonwovens
Nonwovens emerged from the textile, paper, plastic and leather industries and a separate, innovative and completely flexible industry has evolved.
As the demand for nonwovens has steadily increased, it has been met by the technology and ingenuity of raw materials and equipment suppliers, and nonwoven producers and converters.
A precise definition of nonwovens is that adopted by the International Standards Organisation – ISO 9092:1988 and by the European Committee for Normalisation (CEN) – EN 29092.
The production of nonwovens can be described as taking place in three stages, although modern technology allows an overlapping of the stages, and in some cases all three stages can take place at the same time.
The three stages are:
- Web Formation
- Web Bonding
- Finishing Treatments
The opportunity to combine different raw materials and different techniques accounts for the diversity of the industry and its products. This diversity is enhanced by the ability to engineer nonwovens to have specific properties and to perform specific tasks.
Nonwoven manufacture starts by the arrangement of fibres in a sheet or web. The fibres can be staple fibres 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:
- Other techniques
There are two methods of drylaying:
Carding is a mechanical process which starts with the opening of bales of fibres which are blended and conveyed to the next stage by air transport. They are then combed into a web by a carding machine, which is a rotating drum or series of drums covered in fine wires or teeth. The precise configuration of cards will depend on the fabric weight and fibre orientation required. The web can be parallel-laid, where most of the fibres are laid in the direction of the web travel, or they can be random-laid. Typical parallel-laid carded webs result in 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.
In airlaying, the fibres, which can be very short, 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 fibres and fibre blends that can be used.
In this process polymer granules are melted and molten polymer is extruded through spinnerets. The continuous filaments are cooled and deposited on to a conveyor to form a uniform web. Some remaining temperature can cause filaments to adhere to one another, but this cannot be regarded as the principal method of bonding. The spunlaid process (sometimes known as spunbonded) has the advantage of giving nonwovens greater strength, but raw material flexibility is more restricted.
Co-extrusion of second components is used in several spunlaid processes, usually to provide extra properties or bonding capabilities.
A dilute slurry of water and fibres is deposited on a moving wire screen and drained to form a web. The web is further dewatered, consolidated, by pressing between rollers, and dried. Impregnation with binders is often included in a later stage of the process.
Wetlaid web-forming allows a wide range of fibre orientations ranging from near random to near parallel. The strength of the random oriented web is rather similar in all directions in the plane of the fabric. A wide range of natural, mineral, synthetic and man-made fibres of varying lengths can be used.
This includes a group of specialised technologies, in which the fibre production, web structure and bonding usually occur at the same time and in the same place.
In meltblown web formation, low viscosity polymers are extruded into a high velocity airstream on leaving the spinneret. This scatters the melt, solidifies it and breaks it up into a fibrous web.
Flash spun webs are made by dissolving a polymer in a suitable solvent and then spraying it into a vessel held at reduced pressure. The solvent evaporates, or flashes off, leaving a cloud of fibres, which are collected and bonded. Other variants of in situ web forming techniques include different methods of fibrillation and the use of complex rotating dies.
Processes are emerging where two or more web forming techniques are used in tandem. The spunlaid/meltblown process is an example, where one or more meltblown webs and spunlaid webs are combined.
Webs, other than spunlaid, have little strength in their unbonded form. The web must therefore be consolidated in some way. This is effected by bonding, a vital step in the production of nonwovens. The choice of method is at least as important to ultimate functional properties as the type of fibre in the web.
There are three basic types of bonding:
Chemical bonding (adhesion bonding)
Chemical 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 are also found.
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 fibres free of binder for functional reasons.
Thermal bonding (cohesion bonding)
This method uses the thermoplastic properties of certain synthetic fibres to form bonds under controlled heating. In some cases the web fibre itself can be used, but more often a low melt fibre or bicomponent fibre is introduced at the web formation stage to perform the binding function later in the process.
There are several thermal bonding systems in use:
Calendering uses heat and high pressure applied through rollers to weld the fibre webs together at speed.
Through-air thermal bonding makes bulkier products by the overall bonding of a web containing low melting fibres. 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 fibres held under a patterned roller are excited by high frequency energy which produces internal heating and softening of the fibres.
Mechanical bonding (friction bonding)
In mechanical bonding the strengthening of the web is achieved by inter-fibre friction as a result of the physical entanglement of the fibres.
There are two types of mechanical bonding:
Needlepunching can be used on most fibre types. Specially designed needles are pushed and pulled through the web to entangle the fibres. Webs of different characteristics can be needled together to produce a gradation of properties difficult to achieve by other means.
Hydroentanglement is mainly applied to carded or wetlaid webs and uses fine, high pressure jets of water to cause the fibres to interlace. Hydroentanglement is sometimes known as spunlacing, as the arrangement of jets can give a wide variety of aesthetically pleasing effects. The water jet pressure used has a direct bearing on the strength of the web, but system design also plays a part.
There is an opportunity to meet the needs of the customer even more precisely by modifying or adding to existing properties. A variety of different chemical substances can be employed before or after binding, or various mechanical processes can be applied to the nonwoven after binding.
Nonwovens can be made conductive, flame retardant, water repellent, porous, antistatic, breathable, absorbent and so on – the list is a very long one. They can also, for example, be coated, printed, flocked or dyed, and can be combined with other materials to form complex laminates.
The nonwoven fabric is now complete and in a roll. Converters can take it a stage nearer its final form by slitting, cutting, folding, sewing or heat sealing.
In this way, the quality, properties and size of the converted nonwoven products can be further tailored to the precise needs of the customer, and the tasks to be performed in an impressively broad range of end-uses.