What is a peroxide killer?
After bleaching the cellulosic fiber with
hydrogen peroxide, the fiber is subjected a thorough hot wash cold wash
and neutralization processes. These operations would remove all
superficially available chemicals that were used in bleaching process.
However in practice it is found that the core
alkali i.e. the alkali due to the use of caustic soda or soda ash and
hydrogen peroxide, wetting agents and other auxiliaries would remain in
the core of the fiber processed even after 2 or 3 washes. These
residual chemicals like alkali's and peroxide are detrimental to the
production of uniform flawless fabric production.
The core residual alkali would lead to
immature hydrolisation of the reactive dyes and results in streaky and
uneven dyeing.
The residual peroxide present in the fiber
will oxidize the reactive dyes' chromophore that contain the metal ions
and lead to tonal variation and sometimes uneven dyeing. In some cases the
chrmophore itself is getting destroyed into a colorless product and lead
to pale or white patchy dyeing.
Hence the removal residual alkali and peroxide
are very much essential before starting a good dyeing operation.
So any chemical that kills the residual
peroxide in the fiber is called a peroxide killer. All reducing agents are
in fact peroxide killers. Again we should note that excess presence of
reducing agent in the fiber also lead to destruction of dyestuff molecule.
Hence a careful selection of a peroxide killer is very much essential.
Now in the market there are many enzymatic
peroxide killers available that will remove the residual peroxide and
die-off during subsequent dyeing operations at higher temperatures.
Test Methods for quality and consistency:
Specific gravity:
Accurately weigh 10
ml of the peroxide killer. Specific gravity = Volume/Mass. Note down the
value and do this test whenever your receive a new consignment and
compare against the first supply.
Solid content:
Refer procedure of wetting agent.
pH of 1% solution:
Dissolve 1 gram of
the peroxide killer agent in 100 ml of distilled water and
check the pH.
Ionic Nature:
Check the stability of
this product against the addition of acid and alkali under processing
temperature of 70C.
Enzymatic
Killers: It is difficult to test the quality of enzymatic peroxide
killers at the purview of a dye house, however, by testing the residual
peroxide content using a MERK indicator paper or more rigidly by
extracting the treated fiber in distilled water and testing the extract
for the peroxide content.
Testing the adulteraion in Enzymatic Peroxide Killers:
Generally Enzymatic killers are Catalse enzymes diluted in different concentrations. Catalase is the natural enzyme that specifically search and consume hydrogen peroxide and does not react with cellulose. This specific property of selective reaction is made use of to scavnege the residual peroxide in hydrogen peroxide.
Catalase is an enzyme that requires specific reaction conditions to exhaust the residual peroxide, e.g., pH 5.5 to 6.0 and temperature 50 to 60'C. The reaction is an organic reation that requires at least 5 to 10 minutes of operation.
Nowadays, cheap products are available in the market with the name 'enzymatic peroxide killers'. These products not only kill peroxide at lower dosages but also instantaneously. But they may not be of pure enzyme or even sometimes, not enzyme at all.
Any inorganic reducing agent such as Sodium bisulphite, Sodium thiosulphite, etc., can easily remove residual peroxide. But the residue of these products may inturn affect dyeing process. So to find out whether th product that you are using is an enzyme or not there is a simple test.
Take 250ml conical flask. Add 100 ml distilled water; to this add 3 drops of 0.1 N iodine solution. You can a light yellow color solution.
Add a drop of your peroxide killer. If the yellow color disappears immediately, then your product is not an enzyme; it may be either a mixture of sodium bisulphite and catalase enzyme or a pure inorganic salt solution.
Residual Peroxide on the fabric: Analysis by spot test:
Preparation of Titanyl Chloride
solution:
100 ml titanium (IV) chloride (50%w/v) are
added drop wise, with stirring to 200 ml of conc hydrochloric acid (SG
1.18). The reaction should be carried out in a fume cupboard as the
reaction is exothermic and hydrogen chloride gas is given off. It is
essential that each drop is added slowly and the reaction mixture is
constantly stirred with a magnetic stirrer.
When all the titanyl chloride has been added,
the solution is heated to boil and, after boiling for approximately one
minute, 800 ml of dilute hydrochloric acid (2 parts conc hydrochloric acid
10 1 part water) are added. On cooling the reagent is ready to use.
Procedure for textiles: Spot 2 – 3 drips of
the titanyl chloride solution onto the fabric, and wait for 15-20 seconds
for reaction, if any to occur. If no colour develops, there is no
peroxide. If yellow – orange colour develops then you may compare the
colour with the stripes on the scale L and determine the intensity of
peroxide present. |
