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:
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 fabrics with different properties.
, 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.
Spunmelt is a generic term describing the manufacturing of nonwoven webs directly from thermoplastic polymers. It encompasses 2 processes and the combination of both.
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 (also 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.
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.
The principle of wetlaying is similar to paper manufacturing. The difference lies in the amount of synthetic fibres present in a wetlaid nonwoven. 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.
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.
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 such as electrostatic spinning.
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.