After completing the reactive dyeing, whether it
is of any class viz., cold brand, hot brand, vinylsulphone, bi-functional or
High Exhaust, the practice is to neutralize the alkalinity with an acid and
soaping off the remaining unfixed dye from the fibre.
Even after thorough soaping, some of the deep
shades tend to wash off continuously and may affect the final quality of the
fabric. In order to arrest the further washing off of the dyestuff from the
dyed material, it is a practice to treat the dyed material with a cationic dye
fixing agent.
These cationic dye fixing agents work as
bonding materials between fibre and the dye molecule and forms strong covalent
bonds with the dye and fibres and thus fixes the partially fixed and
superficially adhered dyes and improves the wash and crocking fastness of the
reactive dyed materials.
Cationic Dye fixing is essential under the following
conditions:
-
if the dyed material is to be left standing
for a prolonged period in the wet state
-
if migration problems are likely to occur
after drying
-
if acid hydrolysis is likely
-
if thermo hydrolysis (cracking) is likely at
drying temperatures above 160 to 170C
-
to fix any residues of hydrolyzed dye still
present
Almost all dye fixing agents are a product
formaldehyde and some long chain polymer. The residual quantity of
formaldehyde present in the dye fixing agent is an objectionable chemical and
the EU and Oeko-Tex-100 certification are banning the presence of free
formaldehyde on the finished goods.
Not only these free formaldehyde are
objectionable from certification point of view, they are indeed affect the
light fastness quality of the dyed materials.
Testing the Quality of a Dye Fixing agent:
- Solid Content: Take approximately 3
gram of the sample and weigh it accurately in a glass dish. Place it in the
oven for 4 hours at 110C. After cooling in the decicator, weigh the dish
accurately. Percentage Solid content = Dry weight/Actual weight
x 100.
- Free Formaldehyde content: Take about
2.0 grams of the sample in a glass dish and weigh it accurately. Then
dissolve it in 25 ml of distilled water in a 250 ml beaker. Pour 25 ml of
neutralized sodium Sulphite solution and 10ml of 1.0N HCl. Immediately
titrate with 0.5N NaOH using Thymothaline as the indicator. End point
will be indicated by the appearance of light blue color. Also take the blank reading. Free formaldehyde % = Difference x 0.5 x 3/ weight
- Actual fixing test: Do fixing test of
dyed fabric using the recommended concentration. Test the treated fabric for
a) tone change, b) wash fastness and c) crocking.
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