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Web Formation
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 form molten polymer
granules.
Four basic methods are used to form a web. The
nonwoven are usually referred to by one of these
technologies.
Drylaid
Airlaid
Spunlaid/Meltblown
Wetlaid
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Drylaid
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. The fibres are then combed into a web by
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
machine direction, 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 cross direction.
Relative speeds and web composition can be varied to
produce a wide range of properties.[Picture
of Drylaid production process]
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Airlaid
The second
method of drylaying is airlaying. 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 can 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 fibre blends that can be used. [Picture of Airlaid production process]
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Spunlaid/Meltblown
In this
process polymer granules are melted and molten polymer
is extruded through spinnerets. The continuous filaments
are cooled and deposited on to a conveyer 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.
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.[Picture
of Spunlaid production
process] |
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Wetlaid
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 binder 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. [Picture of Wetlaid production
process] | |
Technologies 
