2. BLEACHING
Bleaching is chemical treatment employed for the removal of
natural colouring matter from the substrate. The source of
natural colour is organic compounds with conjugated double
bonds , by doing chemical bleaching the discoloration takes
place by the breaking the chromosphere , most likely
destroying the one or more double bonds with in this
conjugated system. The material appears whiter after the
bleaching.
In simple terms it is the process of removing paleness from
natural fibres like cotton, wool, linen etc. are off-white in
colour due to colour bodies present in the fibre. The degree
of off-whiteness varies from batch-to-batch. Bleaching
therefore can be defined as the destruction of these colour
bodies.
3. Purpose Of Bleaching
Removal of coloured impurities.
Removal of the seed coats.
Minimum tendering of fibre.
Technically reliable & simple mode of
operation.
Low chemical & energy consumption.
Increasing the degree of whiteness
5. Oxidative Bleaching Agents
Reductive Bleaching Agents
Enzymatic Bleaching Agents
Type of Bleaching Agents
A bleaching agent is a substance that can whiten or
decolorize other substances. Bleaching agents essentially
destroy chromospheres (thereby removing the colour),
via the oxidation or reduction of these absorbing groups.
Thus, bleaches can be classified as either oxidizing
agents or reducing agents .
Bleaching Agent
6. Oxidizing bleaching agents:
Generally oxidative bleachings are carried out hypochloretes, sodium chlorite
or peroxy compounds. Natural fibres like cotton, ramie, jute, wool, bamboo
are all generally bleached with oxidative methods. The double bond breaks in
the oxidative bleaching, which makes the substrate colourless.
Example:
Na-hypochlorite (NaOCl), Ca-Hypochlorite [Ca(OCl)],
Hydrogen peroxide (H2O2), Ozone (O3)
Reducing bleaching agents:
Reductive method of bleaching is done with sodium hydrosulphite sodium
sulphide and sulphur dioxide are powerful reducing agents. Fibres like
polyamide, Polyacrylics and Polyacetates can be bleached using reductive
bleaching technology. Double bond is converted into single bond in case of
reductive bleaching.
Example:
Carbon monoxide (CO), Hydrogen sulphide (H2S), Zinc dust (Zn)
Sulphur di-oxide (SO2), Sodium bi-Sulphate (NaHSO4)
7.
8. SEMI BLEACHING
Bleaching of textiles with Hydrogen Peroxide is known as semi bleaching.
Advantages of Peroxide Bleaching:
Among the oxidizing bleaching agents, only hydrogen peroxide provides a high
bleaching effect at reasonable costs, especially if modern short-term bleaching
processes are used with only a few minutes bleaching time.
Peroxide bleaching keeps the fibre quality intact.
Cotton can be bleached with peroxide in a single stage. Other processes require
two or three bleaching stages,(desize with scour, scour with bleach and desize
with scour and bleach).
No separate pre treatment is necessary because hot, alkaline bleaching has not
only a bleaching but also a cleaning effect, it therefore combines the advantages
of an alkaline extraction with the bleaching treatment.
Animal fibres can only be bleached with peroxide to a high and stable degree of
whiteness.
Corrosion of stainless steel equipment does not occur during peroxide bleaching.
9. Optical Brightening Agents (OBAs)
Optical brighteners, optical brightening agents (OBAs), fluorescent
brightening agents (FBAs) or fluorescent whitening agents (FWAs)
are chemical compounds that absorb light in the ultraviolet and
violet region. These additives are often used to enhance the
appearance of colour of fabric and paper, causing a "whitening"
effect.
OBAs are used to make white coloured or light coloured fabrics
brighter. Mostly bleached white fabrics are treated with these
brighteners.
Optical brighteners have the property of absorbing ultra violet
(U.V) light and re emit energy in the form of weaker energy that is
visible light ( blue light) so that the yellow colour of material
appears white.
10. Optical brighteners aren’t effective unless they remain in the
fabric, and persist after washing. They only last so long, until the
point when they actually burn out and no longer do anything. They
are also subject to fading when exposed long term to UV.
Optical brighteners are synthesized from various chemicals. The
group of chemicals which are called “optical brighteners” consists
of approximately 400 different types.
TEXTILE BRIGHTENERS
UJALA(4 drops only)
Most commonly used
optical brighteners
domestically to keep your
“WHITES” white and
bright for long time.
11. WORKING OF OPTICAL BRIGHTENERS
The sun rays have Ultra Violet (U.V)
rays and due to treatment of
optical brightening agents (OBA’S)
on white fabrics they reflect U.V
rays and the fabric appears to be
white and bright.
Ultra violet rays converts to white
light, when gets into prism.
