Pretreatment Process for Polyestr/cotton Blended fabric
Singeing
In singeing the short fibres are burnt off from the surface of the fabric by direct or indirect heating systems without damage to the cloth by scorching or burning. The thermal behavior of different kinds of fibres is different and singeing at higher temperature is naturally associated with greater hazards on excessive contact period and may cause thermal degradation of the fibre.
Size normally
contains an adhesive (film former) and a lubricant. For cotton fabrics the film
former is usually starch or a starch derivative. All starches are, by their
very nature, either water-insoluble or only sparingly soluble. For viscose the most
important sizing agent is the cellulose derivative carboxymethylcellulose
(CMC), which has good water solubility. In addition to natural products, such
as starch and starch derivatives, synthetic sizes based on styrenemaleic acid
copolymers, polyvinyl alcohol, polyacrylates or polyacrylamides are used on
polyester/cotton or polyester/viscose, as well as mixtures of starch and
polyvinyl alcohol. The synthetic polymer sizes and carboxymethylcellulose are
also used on continuous-filament warps made from acetate, triacetate, nylon or
polyester. The lubricant in a size formulation is usually tallow, but
spermaceti, paraffin wax and mineral oils are sometimes employed. These
lubricants impart good smoothness and low frictional properties to the yarn and
are therefore beneficial for weaving, but they are insoluble in water and
difficult to remove from the fibre surface, which can lead to severe problems
in desizing. Several chemical manufacturers offer wax like products that are
water-soluble for addition to size formulations to improve suppleness and
smoothness of the yarn; being water-soluble these are relatively easily removed
during desizing.
Singeing
In singeing the short fibres are burnt off from the surface of the fabric by direct or indirect heating systems without damage to the cloth by scorching or burning. The thermal behavior of different kinds of fibres is different and singeing at higher temperature is naturally associated with greater hazards on excessive contact period and may cause thermal degradation of the fibre.
Singeing in PET cotton blends is carried out after
PET dyeing. In singing PET fibre ends up melting and these melt beads have
higher dye affinity than rest of the fabric.
So, if singeing is carried out before dyeing, there will be uneven and
hazy dyeing. Gas singeing or hot plate singeing is used. Amongst
the synthetic fibres polyester has the greatest significance. It melts at
280°-290°C but does not burn till about 500°C. 'Reflector' or 'refractory'
singeing machines produce smears of fused polymer on the surface of the
polyester cloth and therefore unsuitable for polyester material. Thus flame
singeing machine with a powerful flame is needed and also helps in overcoming
the problems of oligomers i.e. the small chain polymers that come to the
surface. High temperature singeing process may sometimes change the glass-transition
temperature (Tg) of synthetic fibres that lead to uneven dyeing.
For blended fibre fabrics singeing conditions are
to be selected depending on the sensitiveness of the kinds of fibres to heat,
blend composition, weight of fabric and fabric geometry.
Desizing
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| Fabric |
Size present on PET – Cotton blend
consists of acrylic monomers or Poly Vinylalcohol (PVA) together with starch, Carboxy
methylcellulose (CMC) and lubricating agents. PVA and acrylic monomers size is
removed with mild alkali treatment using a liquor containing Soda ash (4 g/l)
and detergent (1 g/l) at 60 –70°C.
Synthetic fibres generally do not
contain naturally occurring impurities like natural fibres i.e. cotton and wool. However, spin finishes, processing
and coning oils and antistatic
agents are added to improve the physical and mechanical properties during spinning, weaving and knitting. Other
impurities are dirt and sighting colours
(to distinguish one kind of fibre from the other). The objective of
scouring synthetic fibre fabrics is
to remove the dirt, size, processing lubricants and sighting colours.
At
high concentration of caustic soda, PET gets saponified. PET loose 3 –5 %
weight on treatments with 2 % caustic soda at 100°C
It is essential to optimise scouring
process with respect to minimum loss in strength. Soda ash is safer alkaline
agent for scouring of PET –cotton blends. After boiling with 2% sodium
carbonate for 24 h, PET strength losses are negligible.
Bleaching
Polyester fibre in
blends with cellulosic fibres in the ratios of 65/35 and 50/50 are common
construction. When cellulose portion is rayon, the blends rarely require
bleaching, but when cotton is present bleaching is usually necessary. Bleaching
treatments of such blends are normally required to remove the natural colours
of cotton, sighting colours and if the polyester portion is turned yellow at
the time of heat-setting operation. Chlorine bleaching, peroxide bleaching and
chlorite bleaching are employed widely. If the polyester portion requires
bleaching, then chlorite bleaching is used, as this bleaching agent bleaches
both polyester and cellulose. If the polyester portion does not need bleaching,
then peroxide bleaching is more convenient. Alkaline hydrogen peroxide
bleaching is the most preferred system for polyester/cotton blends.
It
is essential to bleach PET – cotton fabric to improve the whiteness of cotton
components. The usual bleaching agents used for cotton are used for bleaching
of blends. For goods to be dyed to be dyed or printed in deep shades, bleaching
is carried out with sodium hypochlorite or hydrogen peroxide.
For
goods to be sold as white or lightly coloured fabric, bleaching is done with
sodium hypochlorite and or sodium chlorite followed by hydrogen peroxide.
Mercerizing
PET/cotton blend fabric is
mercerised to improve the properties of cotton component without significant
damage to the polyester. Lustre smoothness, dimensional stability tensile
strength etc of cotton are improved by mercerisation. Polyester get saponified
by strong alkali under drastic conditions and fibre become finer in diameter
and smoother in feel which gives silk like handle however due to decrease in
denier of polyester yarn looses tensile strength. So, the mercerisation of
PET/Cotton blend is carried out at low temperature for short time.
Concentration of NaOH around 18% with special wetting agent as wet ability of
fabric is not very high is used at room temperature. An acid souring and
rinsing is done to neutralise the alkali in fabric.
Heat Setting
Short staple polyester fibres are often
blended with cotton or some other cellulosic fibres. Normally heat-setting in
stenter with hot air is carried out at 180°C for 30 secs. Higher temperatures
may discolour the cellulosic portion of the material. For increased stability,
setting may be carried out even at 200°C for 30 secs without serious risk of
damaging the cellulose. The free weft-shrinkage of a typical polyester/ cotton
shirting fabric is about 4% at its normal setting temperature, but this is
restricted to 2-3% (i.e. 1-2% residual shrinkage is allowed) in order to ensure
the removal of creases and to get control over weft straightness.
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