ð The
importance of knowing which fibre or fibres are present in a fabric has already
been stressed. Because the many fibres available differ in their chemical and
physical properties the method used to prepare fabrics made from them for
printing also vary.
ð Depending
on the origin of their component fibres, fabrics may contain impurities. The
nature of these impurities varies with different fibres, and so do the methods
adopted to remove them.
ð Such
impurity-removal operations are known as Scouring treatments and they are often
followed by bleaching.
ð It is
important to distinguish between chemical bleaching done with sodium
hypochlorite or hydrogen peroxide, and optical bleaching.
ð It is
generally advisable to have the material supplied scoured and bleached where
appropriate, ready for printing.
Polyester
ð It is
necessary to remove size from polyester fabric in order to obtain maximum
colour yield. Starch is removed by enzyme desizing. When water soluble PVA is
used as sizing agent non-ionic surfactant is used as desizing agent and when
polyacrylic sizing agent is used non-ionic surfactant and soda ash is used as
desizing agent.
ð A
simple scour to remove lubricants is given. The material is treated as below:
Lissapol D : 1.5
parts
Soda ash : 2.0 parts
Total liquor : 1000.0
parts
Temperature : 70
˚C
Time : 30
minutes
ð The
bleaching carried out by treating the fabric in a bath containing
Sodium chlorite : 2-3
parts
Formic acid : 2 parts for pH =
3
Sodium nitrate : 1 parts
At boil for 1 hour.
ð The
fabric is heat-set by exposing it to a temperature of 220˚ to 230˚C for a few
seconds in a stenter or curing chamber with hot air or radiant heat produced by
infrared burners.
Nylon
ð Nylon
material is heat-set by dry heat at 210˚C to prevent the formation of permanent
wrinkles during processing and to make it dimensional stable.
ð Desizing
and scouring is done by batch and continuous process.
Batch
process Continuous process
Soda ash
1-3 gpl ---
Sodium pyrophosphate --- 3-6 gpl
Anionic surfactant
1-2 gpl 1-2 gpl
Non-ionic surfactant 2-3 gpl 2-3 gpl
Temperature / Time 70 ˚C /20-30 mi 90 ˚C /10-20 min
ð Bleaching
of nylon is necessary and therefore material is treated in a bath containing
Wetting
agent : 2 gpl
Sodium
chlorite (80%) : 2 gpl
Sodium
nitrate : 2 gpl
Time
/ Temperature : 30 min / 85 ˚C
Acetic
acid for maintain pH 3.5 – 4
ð After
bleaching, it is washed well and treated with Sodium bi-sulphite liquor to
remove chlorine and washed.
Acrylic
ð The
fabric is scoured with Non-ionic detergent : 1-2 gpl
Wetting agent :
2 gpl
Formic acid (85%) :
2 ml/l
Temperature / Time : 75
˚C / 1 hour
ð Bleaching
is carried out with
Sodium
chlorite (80%) : 2.5 gpl
Sodium
nitrate : 3
gpl
Noigen
HC : 1
gpl
Conc.
Sulphuric acid : 1.5 gpl (pH 3-3.5)
Temperature
/ Time : 95 ˚C / 30 min
The fabric is washed and then given an
anti-chlor treatment with 2 gpl sodium thiosulphate solution at 60 ˚C for 20
mins, washed and dried.
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