Printing:
Printing is a process for applying color to a substrate.
However, instead of colouring the whole substrate(cloth, carpet or yarn) as in dyeing, printing colour is applied only
to defined area to obtain the desired pattern. By the term “ Textile printing”
we mean the localized application of dyes or pigments and chemicals by any
method which can produce particular effect of colour on the fabric according to
the design. This involves different technique and different machinery with
respect to dyeing, but the physical and chemical processes that take place
between the dye and the fiber are analogous to dyeing.
Step of printing:
Grey textiles/raw materials
Preparation of textile materials
(singeing, desizing, scouring and bleaching)Preparation of printing paste
Printing (with a certain style and
method)
Drying of the printed fabric (in the
drier)Steaming of the printed fabric ( to
transfer dye into fiber, 100-102℃,
15 min in a steamer)
After treatment (soaping of washing)
Steps of printing:
Ø
Color paste preparation: Generally the printing
dyes or pigments are insoluble, during printing textiles, the dyes or pigments
are not in an aqueous liquor instead, it usually finely dispersed in a printing
past, in high concentration.
Ø
Printing: After suitable preparation of printing
paste then it is applied to the substrate using different technique.
Ø
Fixation: After printing the printed textile
material needed to fixation by dried. The prints are fixed mainly with steam or
hot air.
Ø
After treatment: After fixation the specimen is
needed to after treatment.
Printing paste and its ingredients:
Printing paste is viscous solution of printing ingredients
referred below that is used for textile printing. The main three printing
ingredient given below:
i.
Dyestuff or pigments: Reactive, Disperse, Vat
etc.
ii.
Thickeners: Thickener can be defined as a
substance used to increase the viscosity of a print paste or other fluid, which
is controlled the disperse of dyes/pigments and contain the definite design.
iii.
Chemicals and water: During printing many
chemical is necessary such as wetting agents, reducing agents, oxidizing
agents, dispersing agents, solvents, acids, alkalis, hygroscopic agents,
catalyst and carries.
Printing pastes contain a thickening agent & other
auxiliaries. According to their function, the classification given below:
i.
Reducing agents(e.g. formaldehyde sulphoxylates,
tin(‖)chloride, sodium dithionite, thiourea dioxide)
ii.
Oxidizing agents(e.g. sodium chlorate, hydrogen
peroxide, m-nitrobenzene sulphonate)
iii.
Substances with a hydrotropic effect, like urea.
iv.
For discharge printing, Discharging agents
needed such as anthraquinone.
v.
Resist for reactive resist printing such as
sulphonated alkanes.
vi.
Dye solubilisers, which are polar organic
solvents like glycerine, butyl glycol, ethylene glycol etc.
vii.
Defoamers( e.g. esterssilicon compounds etc)
Thickeners:
A thickener is a colorless, viscous paste made with one or
more thickening agents.
Four significantly different approaches may be used to
produced thickeners, using:
Ø
A low concentration of high relative molecular
mass(r.m.m.).
Ø
A high concentration of a material of lower
r.m.m. or highly branched chain structure.
Ø
An emulsion of two immiscible liquid, similar to
the emulsions used as cosmetic creams or a foam of air in a liquid a dispersion
of a finely divided solid.
Selection of thickener:
Ø
Viscosity
Ø
Print paste stability
Ø
Good adhesion of the dried thickener film
Ø
Minimum effect on color yield
Ø
Ease of removal
Ø
Acceptable cost
Important of viscosity:
There are two essential reasons for the importance of the
viscosity and hence the flow, of a print paste.
First: It affects the amounts of paste applied.
Secondly: The spread of paste, on the surface of the textile
material and into its structure.
The upper limit of the viscosity is determined by the
flatness of the fabric surface and the condition of the printing process. The
lower limit also depends on the process conditions, but is mainly determined
sharpness of print.
Examples of natural thickeners:
1. Starch
and its derivatives(rice, potato, wheat etc):
i.
Provides high color yield
ii.
Susceptibility to crush and poor levelness
2. British
gums:
i.
Good stability to alkali
ii.
Used for printing vat dyes
iii.
For resist print, it is used
3. Locust
bean gum:
i.
Required 45℃
or higher for complete dispersion in water
ii.
PH has little effect on the viscosity
over the pH range 3-11
4. Guar
gum:
i.
Can be dispersed in cold water.
5. Alginates:
-
Source in brown phaeophycae seaweeds
-
Very important because of their ready solubility
-
Every after high temperature fixation
treatments.
They are especially important for pastes of reactive dyes
because the extent of interaction is very small.
The stability is good between pH 4 and 10.
Alginate pastes are
compatible with a wide range of materials, including starch and tragacanth.
6. Gum
Arabic:
Gum Arabic has been used as an adhesive more than as a thickening agent.
7. Gum
tragacanth:
It
is less sensitive to additions of electrolyte.
Sequence of printing:
I.
Printing with pigment:
Print
à Dry à Curing(At 150℃
for 30s to 5 min)
II.
Print with dye:
Print
à Dry à Steaming/afeing(At 100-200℃ for 10s to 60 min) à After washing.
Style of printing:
Ø
Direct style
Ø
Discharge
White discharge
Color discharge
Ø
Resist style
Ø
Flock style
Ø
Crimp/Crepon style
Ø
Burn out style
Method of printing:
1) Block
printing
2) Stencil
printing
3) Roller
printing
4) Screen
printing
–Hand screen printing
–Semi automatic flat screen
–Rotary screen
5) Transfer
printing
–Flat bed
–Continuous transfer
–Vacuum transfer
6) Digital
inject printing