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Recent developments in preparatory processing
This article, on recent developments in preparatory processing,
is the first one in a series highlighting the technological advancement in the
processing of woven cotton and blended fabrics through innovations and improvements
in processing machinery.
Dr G P NAIR
Preparatory processing removes impurities, color bodies and size imparted in
woven fabrics either naturally or during the manufacturing operations of spinning
and weaving. As a result, the fabric acquires good absorbency and appearance
and becomes suitable for mercerising, dyeing, printing and finishing.
Steps in preparatory processing
The first steps are mechanical and dry treatments, viz., shearing and singeing
followed by chemical and wet treatments, viz., desizing, scouring and bleaching.
The following are the features of the state of the art machines representing
recent developments in each of these preparatory processing steps:
Shearing
Shearing machine of Kusters Calico of Germany-India can give 100% shearing efficiency
with good output. Its suction mechanism is for getting optimum and all through
results. It has seam protection device and a system for adjusting the distance
between cloth and blade to suit different fabric sorts. The vertical shearing
heads give the best results even with fabrics of loose selvedges. There is a
pre-brushing unit, dust collection and cleaning unit to keep the machine as
well as fabric surface clean.
Singeing
Singeing machine of Osthoff-senge of Germany has ‘Double-Jet’ burners
capable of using all types of gases and an ideal mixture distribution system
for singeing to perfection. Flame is set as per fabric width for energy saving.
Singeing positions include - on to free-guided fabric, on to cooled roller or
tangential singeing for different fabric sorts for the best results. Machine
conforms to European Safety Regulations and has exhaust purification as well
as hot air use.
Desizing, scouring and bleaching
Preparatory wet processing operations on cotton and blended fabrics are carried
out these days mostly in open width form by continuous, semi-continuous or batch-wise
methods.
Continuous method
‘Mega Bleach or Flash Steam Bleach and FlexNip high-add-on application’
is the latest continuous combined 2-stage process developed by Kusters of Germany
in collaboration with Ciba Specialty Chemicals of Switzerland. The first stage
consists of pad-steam enzymatic cracking to thoroughly desize and also to remove
the troublesome alkaline earth and heavy metal ions. Fabric is impregnated with
Ciba’s enzyme Tinozym ADC and cracking agent Invatex MD followed by a short
time steaming and hot wash off. This process combines acid cracking and enzymatic
desizing. In its second stage, the fabric is scoured and bleached at the same
time with the well established ‘Mega Bleach’ using Ciba’s Tinoclarite
CBB Processor and Invatex MD cracking agent. These agents are given a high add-on
application with Flex Nip followed by about 20-minute steaming and hot wash
off. The well-established Mega Bleach combines the conventional steps of alkaline
scouring and peroxide bleaching. The desized, scoured and bleached fabric is
finally neutralised and subjected to an optimised washing process using Ciba’s
Invatex AC. This process is claimed as the most efficient and economical one
giving high quality goods.
Semi-continuous method
Kusters of Germany has also developed a semi-continuous 2-stage desizing, scouring
and bleaching process with Rudolf Chemie of Germany. In its first stage, the
singed fabric is impregnated with the enzyme Rucolase CML 200 at 60-95øC
and padded to 100% liquor pick-up, dwelled at ambient temperature for 6-12 hr
and finally washed off at boil. The second stage of combined scouring-bleaching
of fabric, consists of fabric impregnation with a liquor containing 50% H2O2
(42 ml/kg), 50% NaOH (30 ml/kg), Ruco-Stab OKE, the stabiliser (6 ml/kg) and
Rucogen WBL, scouring and wetting agent (2 ml/kg), padding as in the first stage
followed by steaming at 100øC for 90 sec, final washing at boil, rinsing
hot and cold and neutralising.
Batch-wise method
Open-width preparatory processing involving desizing, scouring and bleaching
can also be carried out on fabric in batch form on Jumbo jiggers. jt-10 Spectra
of Harish, India is an open-width Super Jumbo jigger for desizing, scouring
and bleaching of cotton and blended fabrics in jumbo-size batches up to 1500
mm. Its trough is divided into three compartments with two intermediate squeeze
nips. On the full width of fabric, chemicals and dyes are squeezed to facilitate
rapid processing. Steam injector boxes are positioned in close proximity of
fabric for flash scouring and bleaching. A chemical dosing system maintains
chemical concentration in the compartment. The machine design helps to obtain
temperatures up to 98øC. Its rapid hot wash with water injectors washes
off soil, dirt, waxes and chemicals from fabric by water spray and thermo-mechanical
energy from steam. These impurities are flushed out by an efficient filtration
system. It has also a Counter Meter System with auto-reverse and a crawling
arrangement during stoppage to counter any unbalancing of the wet batch.
With its electronic program, the various process cycles can be pre-programmed
in terms of processing conditions such as temperature, duration, speed and chemical
additions.
Preparatory processed fabric
As a result of preparatory processing, the residual size levels in fabric come
down to 0.5-1.0% in cotton and 0.2-0.5% in blended fabrics. The fabric also
becomes adequately absorbent with drop test readings of 3 seconds or less. Fully
bleached fabrics show brightness levels of 80-85% with yellowness levels of
only 0.05-0.06%. Cuprammonium fluidity, a yardstick to check fabric deterioration
or tendering during the process, will also be as per the norm of 8 or less.
These quality parameters ensure that the preparatory processing is properly
done to make the fabric fit for further processes of dyeing, printing and finishing.
The next article in this series will focus on mercerising.
(The author is technical advisor (textiles), Mumbai)
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