O o d h e a d p u b L i s h I n g L i m I t e d

6.3
Bonding systems
6.3.1
Needle punch
Figure 6.8
 shows the needle-punching process which is very commonly used
to manufacture medium to heavyweight fabrics. The needles are used at a
Water–fibre slurry
Fabric
Pump
Suction
Perforated belt
6.7 
Wetlaid nonwoven process schematic.
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density of several thousand per square metre and reciprocate through the
fabrics. The barbs on the reciprocating needles catch the fibres and entangle
them to form a felted fabric. A stripper plate above and below the fabric has
a hole for every needle and strips the fabric from the needle as it passes down
or up through the plate.
Often several stages of needling from each side of the fabrics are required
to give the fabric sufficient strength. Because of the reciprocating motion,
the speed is limited by the ‘advance per stroke’, the fabric cannot be pulled
forward very far while the needles are in the fabric without damaging either
the fabric or the needles. Needle board frequencies have now reached 2000
rpm but are commonly lower. Needle selection is crucial to efficient felting.
The shaft diameter, the barb position, the number of barbs and the needle
shape are key parameters. Needle penetration depth can be adjusted on the
machine.
Over-needling can lead to excessive fibre breakage and so there is an
optimum level of needling with respect to fabric strength. For wool nonwoven
textiles, an attractive needling process is the velour needle punch. This machine
needles the fabric into a brush underneath rather than into a steel stripper-
plate with a hole for each needle to strip the fibres from it. The velour needle
punch pushes the fibres through the fabric to the opposite surface in a controlled
way and produces a velour or velvet-like finish to the fabric. The fabric can
also be patterned in this way using special arrangements of needles and
brushes.
A recent development is the Hyperpunch system from Dilo 
(Fig. 6.9)
where the needles follow an elliptical path rather than straight up and down
so that they partially follow the fabric as it advances through the needle-
6.8 
Schematic of the needle-punch process.
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loom, increasing the distance advanced per stroke of the needle board. This
allows higher fabric delivery speeds and hence higher productivity.
6.3.2
Stitch bonding
Because nonwoven fabrics need to derive their strength from the intimate
entanglement of the fibres and this leads to greater stiffness compared with
wovens and knits a compromise is reached in pure-fibre nonwovens between
handle and performance. However, nonwovens can be reinforced so that the
fibres can be more loosely entangled but the strength is provided by the
reinforcement. One means to do this is called stitch-bonding and one example
machine is the Maliwatt™ from Karl Meyer. In stitch bonding sewing threads
are inserted by sewing needles aligned across a cross-lapped web. The threads
provide mostly machine direction strength while the natural fibre orientation
of the cross-lapped web provides the CMD strength. The sewing threads,
constituting only about 5% of the fabric weight, are buried into the fabric in
finishing where a pile may be raised or the fabric lightly wet-felted so that
they are not visible in the final garment. Such wool and wool-blend fabrics
6.9 
The Dilo hyperpunch system.
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can be used in outerwear and have a lower fabric weight limit of around
250 gsm. A fleece-like fabric can also be produced in this way.
An alternative to stitch bonding to give extra strength is the use of a
‘scrim’. These are light woven fabrics that are incorporated into the nonwoven,
usually by insertion between two webs before bonding. Needle punching the
webs through the woven fabric can produce a strong fabric with a lower
degree of entanglement and so give a softer fabric. However, the fibre security
can then be low and pilling and fibre shedding can become a problem. The
cost of the woven fabric often has to be low and so synthetic fibres are used
and for disposables welded nets are favoured. However, for highly specified
technical fabrics, such as some wool-containing paper-making felts, the scrim
is a carefully designed key component rather than a cheap reinforcement.
6.3.3
Hydroentanglement
Also known as Spunlace or Jetlace, hydroentanglement uses rows of fine
high-pressure water jets to entangle the fibres of the web into nonwoven
fabrics as illustrated in Fig. 6.10. The water from the jets is removed by
suction slots behind each injector. Because there are no reciprocating parts
the production speed is not limited as needle-punching is by the ‘advance per
stroke’. The speed can be very high and is limited only by the energy that can
be injected by the water jets into the entanglement process. Speeds of hundreds
6.10
 Hydroentanglement or Spunlace.
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of metres per minute are used on lightweight synthetic fabrics for disposables
but lower speeds are often used for heavier or more durable fabrics. Power
consumption is relatively high but the energy cost per kg remains low because
of the high production rates possible. Only two companies supply large-
scale Spunlace lines, Rieter Perfojet and Fleissner.
The fine, high-pressure water jets are applied against the fabric backed by
either a mesh belt or a drum. The drums have mesh shells or, in the case of
Rieter-Perfojet, may have random perforated shells designed to improve
entanglement and reduce striping by the jets. Spunlace process speeds may
exceed 300 m/min but are usually much lower for wool, which is harder to
entangle than some finer synthetic fibres.
Common Spunlace products are:
• wipes, towels, tissues;
• filters;
• protective apparel;
• surgical gowns and covers;
• synthetic leather;
• sanitary products;
• home furnishings;
• interlinings (some wool).
Spunlace fabric weights have an upper limit, if the fabric is to be entangled
throughout its thickness, of about 400 gsm. The main advantage of the Spunlace
process for wool is that lighter-weight fabrics can be produced compared
with needle-punch nonwovens. Also a higher degree of entanglement can be
achieved with less fibre damage than needle-punching. Reinforcing scrims
can also be used in spunlace fabrics to add strength. While there is currently
very little commercial production of spunlace wool fabrics, research and
development is ongoing and is expected to provide commercial outcomes in
the near future.
Hydroentanglement jets have very high energy density and can fibrillate
fibres with weak tranverse strength compared with their longitudinal strength.
This makes the process ideal for making micro-fibre fabrics from fibres
deliberately designed to fibrillate. These ‘splitable’ microfibres are
manufactured in bicomponent fibre extrusion systems where two different
polymers are co-extruded into one filament with a cross-section such as that
shown in 
Fig. 6.11.
When these fibres are formed into a web via carding the adhesion between
the two polymers is sufficient to avoid fribillation during all processes up to
hydroentanglement. During hydroentanglement the fibres break up into the
microfibres and produce super-soft flexible fabrics with interesting technical
as well as aesthetic characteristics. If blended with wool the microfibres can
provide improved softness and drape compared with pure wool or normal
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wool-blend nonwovens. Because of the problems of poor stretch recovery,
reinforcing scrims can be used to prevent bagging of fabrics during use or
wear. Such fabrics are still stiffer than woven fabrics but can be made with
equivalent quality to milled wool outer-wear fabrics at lower cost. It should
be noted, however, that much of the cost in wool fabrics is in the fibre itself
and in dyeing and finishing processes. Thus while the full top-making and
spinning processes are avoided, the savings may not be as high as expected
at first glance and there is usually a quality penalty.
Many technical attributes of wool favour its use in nonwovens. These
include its inherent fire resistance, odour absorption, electrostatic effects,
moisture absorbency and natural image. It also does not melt, is durable and
when it wears it produces a soft non-abrasive powder and so is still used in
felts for protecting bearings in various machines.
The electrostatic properties can be used in electret filters where it is
combined with a resin or fibre with opposite electrical affinity as defined in
the triboelectric series. The charged fibres attract dust particles and so enhance
the filtration efficiency but with low pressure drop and high dust-holding
capacity, which are the key quality parameters for filters. A lower pressure
drop means less energy is used to filter the air.
6.3.4
Chemical bonding
Latex bonding is the common chemical bonding technique used where a
binder is applied to the fibre batt. The term latex is used loosely in the
textiles industry for historical reasons to refer to any curable polymer binder
6.11
 Citrus-style microfibres.
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rather than true latex binders. The resin application can be achieved in several
ways, the most popular being kiss-roll (Fig. 6.12), saturation, foam and
gravure roller application. Saturation application involves the fabric being
run through a bath of the binder, the fabric wets out and excess binder is
removed by passing the fabric through nip or squeeze rollers. Gravure
application is by gravure rollers having a fine textured surface that picks up
and transfers a layer of the resin emulsion onto the surface of the fabric and
then passes the fabric through nip or squeeze rollers. Other methods include
spraying of the resin as well as foam applicators. All methods require drying
and curing of the binder and this is an added cost because of the high energy
required to remove the water component of the binder.
6.3.5
Thermal bonding
For wool nonwovens thermal bonding is used with blends of wool and
thermoplastic bonding fibres. These bonding fibres are usually sheath–core
bicomponent fibres with cross-sections as shown in 
Fig. 6.13.
 The low melt-
temperature component melts and wets out the other fibres during heating
with hot air, usually in a through-air bonding oven as shown in 
Fig. 6.14.
This process usually produces an open lofty structure often used for acoustic
or thermal insulation. The fraction of bonding fibre used affects the stiffness,
strength and resilience of the nonwoven.
In polyester bonding fibres the low melt-temperature component is usually
a copolymer of the high melt-temperature polyester component. A range of
melt temperatures is available from 110 to 160
°C. Polyamide (nylon) bonding
6.12
 Kiss roller application (courtesy of Andritz Kusters).
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fibres are available with PA6 as the low melt-temperature component and
PA66 as the high melt-temperature component, the difference is only around
20
°C (about 225 versus 245 °C). Nylon is often used with wool when the
fabric is to be dyed because nylon readily takes up the same dyes as wool.
Polyester is cheaper and easier to thermally bond than nylon and can also be
dyed, but dyeing is slightly more difficult and requires higher temperatures.
Often thermal bonding is used in insulation layers such as bedding products
that do not require dyeing. The wool provides good moisture management
properties and some odour management effects.
Thermal bonding is also possible using heated calendar rollers whereby
the fabric is passed through a high-pressure nip point and heat is transferred
via conduction. A variation on calendar bonding is point bonding where
a patterned or embossing roller is used (
Fig. 6.15).
 When the raised
section of the roller comes in contact with the fabric and the base roller it
creates a bond only on this site, simultaneously generating a pattern in the
fabric.
6.13
 Sheath–core and side-by-side thermal bonding fibre cross-
sections.
Air ducts
Fabric
Permeable belts
6.14
 Through-air bonding oven.
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