and Hsu-Yeh Huang
All nonwoven fabrics are based on a fibrous web. The
characteristics of the web determine the physical properties of the final
product. These characteristics depend largely on the web geometry, which is
determined by the mode of web formation. Web geometry includes the
predominant fiber direction, whether oriented or random, fiber shape
(straight, hooked or curled), the extent of inter-fiber engagement or
entanglement, crimp and z-direction compaction. Web characteristics are also
influenced by the fiber diameter, fiber length, web weight, chemical and
mechanical properties of the polymer.
The choice of methods for forming webs is determined by
fiber length. Initially, the methods for the forming of webs from
staple-length fibers were based on the textile carding process, whereas web
formation from short fibers was based on papermaking technologies. Though
these technologies are still in use, newer methods have been developed. For
example, webs are formed from long, virtually endless filament directly from
bulk polymers; both web and fibers are produced simultaneously.
DRY-LAID NONWOVENS FROM STAPLE FIBERS
These fibers are long enough to be handled by
conventional spinning equipment. The fibers are 1.2 to 20cm or longer, but
FOUR PHASES OF THE DRY-LAID MANUFACTURING SYSTEM
- FIBER SELECTION
Some of the factors to be considered in the selection
of fibers for dry-laid nonwovens are:
- Abrasion resistance
- Bursting strength
- Softness and tear resistance in the fabric
- FIBER PREPARATION
Staple fibers are shipped to the manufacturer in the
form of bales and fiber preparation consists of mechanical and pneumatic
processes of handling from the bale to the point where the fiber is
introduced into the web-forming machine. The following processes are
included in a typical fiber preparation line:
The bales are unstrapped and placed side-by-side in
line with the milling head of a bale opener. The fibers are picked up from
the top of the bales by two opening rolls in conjunction with a partial
air vacuum. The opening head traverses back and forth across the bale
laydown, starting and stopping on demand from the blending hopper. This
ensures maximum efficiency and blending. The objective of an opening line
is to reduce the size of fiber tufts from the bale to the chute feed,
which supplies the web forming machine.
The blending feeders gently open the tufts of fibers by
the interaction of an inclined needle lattice apron and an evener roller
equipped with needles. Blending of the tufts from different bales also
takes place in the opening and mixing achieved by the inclined apron and
the evener roller. The opened tufts are deposited into a weigh pan
controlled by load cells which dump the fibers onto a feed conveyor.
The blending conveyor feeds fiber into an opening roll,
which has a three-lag pin beater (Kirschner beater type) where coarse
opening of the fiber tufts takes place.
The fiber opened by the opening roll is transported by
air to the feed box of the fine opener. The fine opener consists of two
opening rolls, one evener roll and a cylinder roll all of which are wound
with metallic clothing. The opener reduces the tuft size by using the
principle of carding points between rolls A and B and between rolls B and
C. The reduced tufts are transferred to the cylinder roll D which delivers
the opened fiber into an air stream to the web-former.
The feed system to the web-forming machine is selected
based on the type of fiber and the type of web-former. Chute feeding is
normally used to feed fibers up to 60 millimeters in length. For longer
fibers, a hopper feed with a shaker-type chute is used.
Web formation and layering
The dry-web process for making a nonwoven consists of
basically three methods:
- Mechanical web formation (carding or garnetting)
The main objectives of carding are to separate small
tufts into individual fibers, to begin the process of parallelization
and to deliver the fibers in the form of a web. The principle of carding
is the mechanical action in which the fibers are held by one surface
while the other surface combs the fibers causing individual fiber
separation. At its center is a large rotating metallic cylinder covered
with card clothing. The card clothing is comprised of needles, wires, or
fine metallic teeth embedded in a heavy cloth or in a metallic
foundation. The cylinder is partly surrounded by an endless belt of a
large number of narrow, cast iron flats positioned along the top of the
cylinder. The top of the cylinder may be covered by alternating rollers
and stripper rolls in a roller-top card, also.
The fibers are fed by a chute or hopper and condensed
into the form of a lap or batting. This is initially opened into small
tufts by a licker-in, which feeds the fibers to the cylinder. The
needles of the two opposing surfaces of the cylinder and flats or the
rollers are inclined in opposite directions and move at different
speeds. The main cylinder moves faster than the flats and, due to the
opposing needles and difference in speeds, the fiber clumps are pulled
and teased apart. In the roller-top card the separation occurs between
the worker roller and the cylinder. The stripping roller strips the
larger tufts and deposits them back on the cylinder. The fibers are
aligned in the machine direction and forms a coherent web below the
surface of the needles of the main cylinder.
The web is doffed from the surface of cylinder by a
doffer roller and deposited on a moving belt. The orientation ratio of
the web at the doffer of a conventional card is approximately 5:1.
Productivity of older roller cards is about 30-50
kg/hour at the width of 1.5~2m. Nowadays, the roller cards of
performance up to 1000kg/hour in width 2~3.5m are delivered. Flat
carding machines are usually 1m wide and process about 5~50kg/hour.
Spinning preparation and carding of staple fibers
have been and still are the subjects of studies and publications
concerning various aspects. There is even research on the carding of
Garnetts are similar to roller-top cards. R.L. Street
has described the garnett as "a group of rolls placed in an order that
allows a given wire configuration, along with certain speed
relationships, to level, transport, comb and interlock fibers to a
degree that a web is formed." Garnetts are mostly used to process
waddings and for making pads for automobile and bedding industries. It
delivers a more random web than a card. Most webs from garnetts are
layered by crosslapping to build up the desired finished nonwoven
Aerodynamic web formation (air-lay)
The orientation created by carding is effectively
improved by capturing fibers on a screen from an air-stream. This is
done on a Rando-Webber component. Starting with a lap or plied card webs
fed by a feed roller, the fibers are separated by a licker-in or spiked
roller and introduced into an air-stream.
The total randomization excludes any preferred
orientation when the fibers are collected on the condenser screen. The
web is delivered to a conveyor for transporting to the bonding area.
Feeding of the Rando-Webber by the cards increases the uniformity of the
web. The length of fibers used in air-laying varies from 2 to 6 cm. The
shorter lengths allow higher production speeds. Longer fibers require
higher air volume, i.e., a lower fiber density to avoid tangling.
Problems associated with air-laying are speed, web uniformity and weight
limitations. Due to uniformity problems, it has not been practical to
make isotropic webs lighter than 30g/m2. Air-laying is slower
than carding and, hence, more expensive.
The aerodynamic web forming process has some typical
advantages and disadvantages:
Among the advantages are:
Isotropic structure of the web
Voluminous webs can be produced
- Wide variety of processable fibers such as natural, synthetic,
glass, steel, carbon, etc.
The main disadvantages are as follows:
Low level of opening fiber material by licker-in
Variable structures of web in width of layer due to irregular air
flow close to walls of duct
Possible entanglement of fibers in air stream
Centrifugal dynamic web formation (random card)
The centrifugal dynamic random card forms a web by
throwing off fibers from the cylinder onto a doffer with fiber inertia,
which is subject to centrifugal force, in proportion to the square of
the rotary speed. Orientation in the web is three-dimensional and is
random or isotropic. The random card produces a 12 to 50 g/ m2
web with fine fibers of 1.5 den and a web up to 100 g/m2 with
coarse fibers. The production of the random card is generally about 30
to 50% higher than conventional cards. The machine direction versus the
cross-direction strength is better than those produced in the
conventional card, but not as good as that of the air-laid webs. The
number of machines required in the nonwovens line for the production of
multi-layered webs can be reduced by the use of the random card. The
broad scope of adaptability of the random card for producing a wide
range of nonwovens has led to innovations in this method.
Web formations can be made into the desired web
structure by the layering of the webs from either the card or garnett.
Layering can be accomplished in several ways to reach the desired weight
and web structure.
Cross layering(most common)
- Bonding and stabilization of webs
The type of bonding and finishing must also be
considered when determining the chemical changes in the fiber properties
that may develop, which might affect the end product. Equally important
is the performance of the fiber in fiber preparation (opening and
blending) and web formation.
Needle punching is a process of bonding nonwoven web
structures by mechanically interlocking the fibers through the web.
Barbed needles, mounted on a board, punch fibers into the web and then
are withdrawn leaving the fibers entangled. The needles are spaced in a
non-aligned arrangement and 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. Kevlar
is used for strength in the fabric for protective vests. Lycra
is used for stretch in the fabric. Home furnishings are a big market for
these fabrics. Other uses are vacuum bags, geotextiles, 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
or 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 coverstock industry for baby diapers. Polypropylene has been the
most suitable fiber with a low melting point of approximately 165
C. It is also soft to touch. The fiber web is passed between heated
calender 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
Bonding a web by means of a chemical 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.
Hydroentanglement 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 belt.
Entanglement occurs when the water strikes the web and the fibers are
deflected. The vigorous agitation within the web causes the fibers to
.Turbak, Albin.F., Nonwovens: Theory, Process,
Performance And Testing.
 Street, R.L., "Mechanical Web Formations," 1981
Fiber Fill Conference Proceedings, INDA, Charlotte, NC, p.1.
 Oldrich Jirsak and Larry C. wadsworth; "Nonwoven
textiles", Carolina academic Express, 1999
 F. Leifeld; "Carding Micro-fibers", Textile
Technology, Melliand English, 2/1993 E43
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