Sulfur
Dyeing -The way we do
Chemistry of sulfur dyes according to many books is still
mystery and still could not get exact picture of nature of binding and activity
but, with practical work we know many interesting things about sulfur dyes.
Basic application of sulfur dyes is very simple “FIRST YOU REDUCE AND LATER YOU
OXIDIZE”. Reduction is necessary to make these dyes soluble and to reduce we
have two routes either to go with inorganic based reducing agents like sodium
hydrosulfite or we can also reduce with organic reducer which are more
environment friendly like glucose in presence of strong alkaline medium.
Complete reduction of dyes can be determined through redox potentio meter a
reading of -700 to -750 shows good reduction. It may need PH of above 10 to achieve
that high reduced potential. After complete reduction this bath contain reduced
dyes could be utilized for dyeing cellulose either in exhaust or continuous on
pader. A typical reduction ready to dye recipe could be.
Recipe:
Sequester = 2g/L
Caustic(Solid = 5 to 7g/L
*Reducing agent = 3 to 15g/L
**Sulfur dyes (Liq = 80 to 200g/L
Penetrating agent = 2g/L
*(3 for inorganic based strong reducing agent like sodium
hydrosulfite and 15 for organic based reducer.
** (Depends upon shade depth, preferably use pre-reduced
liquid dyes.
Process:
Pad with 65 to 70% pick up.,
Dry at 120 to 130°C for 2 to 3 Min.
“FIRST YOU REDUCE AND LATER YOU OXIDIZE”. So, let’s oxidize now. Do
you know oxidation of sulfur dyes can also be done by Hydrogen peroxide? Doesn’t
it seem to be interesting as hydrogen peroxide is famous for its bleaching
effect but we also know its oxidizing agent and that’s what we here required? Alternate
to hydrogen peroxide we can also utilize commercially available oxidizing
agents with acetic acid to oxidize.
What
actually oxidation does is to cause insolubility of dyes. This insolubility of
dyes helps in two ways. Firstly, by insoluble dyes got stuck into cellulose and
secondly these dyes have now no affinity toward water and during washing it
does not move away from fabric toward water hence showing average to good
washing fastness.
Problems/Troubleshooting:
Sulfur dyeing is never been easy. It’s been of devils
thing. There are many issues involved in sulfur dyeing and king of them is
oxidation when we do not need of it. Oxidation of dyes in open container
exposed to air. Oxidation of prepared liquors even pre mature oxidation of
padded or exhaust dyed fabric resultantly variation in shades. To counter this
in exhaust mode we utilize Tonello air tight packed machines. On pader we do it
immediately after liquor prepared and never let it to stay and expose to air
for longer.
An other problem is of tailing on padder. These dyes have
high substantivity and it varies from color to color. Let’s study a scenario to
make it easy to understand. Suppose we have a shade in which we are going to
utilize two sulfur based dyes first one is black and second is orange. Black
has more substantibity towards fabric than orange so what does it goanna do.
So, when we started padding fabric as black dye was more substantive it rushed
more quickly toward fabric rather than orange and resultantly after some time
padder contains more orange than black and eventually changing shde tone.
With single component element strength keeps on losing up
as dyeing progresses.
Solution could be to utilize apporx same substantive
dyes.
Use not more than two dyes to make shade.
Pad with low liquor volume.
Go with high speed.
And speed is what counts after all…..
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