Thickener:
Thickener is a thick mass which imparts stickiness and
plasticity to the print paste so that it may be applied on the fabric surface
without bleeding or spreading and be capable of maintain the design out lines.
Function of thickener:
i.
To give the required viscosity to the printing
paste.
ii.
To hold the ingredients of the print paste on
the fabric.
iii.
To prevent premature reaction between the
chemicals contained in the print paste.
Four significant approaches to produce thickeners using:
1. A
low concentration of a polymer of high molecular weight.
2. A
high concentration of material of lower molecular weight or highly branched
chain structure.
3. An
emulsion of two immiscible liquids.
4. A
dispersion of a finely divided solid, such as bentonite.
Quality of print paste:
Ø
Print paste stability.
Ø
Properties of the dried thickener film.
Ø
Effect on color yield, such as diffusion,
fixation.
Ø
Preparation and removal of the thickener.
Ø
Cost.
Ø
Biological oxygen demand.
Ø
Styles of printing.
Ø
Methods of printing
Ø
Type of material to be printed
Ø
quality of material to be printed
Ø
Compatibility with dyes and chemicals.
Classification of thickeners:
1. Natural
thickeners:
i.
Cereals starch: i.e. Marine starch, wheat starch
etc.
ii.
Plant exudates: i.e. Gum tragacanth, Karaya gum
etc.
iii.
Root and seeds: i.e. Locust beam gum.
iv.
Sea weeds: i.e. Na-alginate.
2. Synthetic
thickeners:
i.
Acrylic: i.e. Polyacrylic acid, polyacrylic
amides etc.
ii.
Vinyl: i.e. poly vinyl alcohol.
3. Modified
natural thickeners:
i.
Starch derivatives: i.e. British gum, C.N.C etc.
ii.
Cellulose derivatives: i.e. C.M.C , HEC etc.
iii.
Gum derivatives: i.e. Meypro gum,indalca etc
Principles of selecting thickening agents:
I.
The print paste auxiliaries and dyes should be
compatible with the thickener and the thickener should be stable. If a cationic
dye is added to a thickener with anionic chares the viscosity will be charged
and it will produce insoluble complexes. Similarly anionic thickeners can’t be
used when metal salts such as Al2(SO4) are to be added.
II.
The pH of the print paste must be
considered because some thickeners like Na-alginate are only usable within a
imitated pH range and form gels when acids or strong alkalis are
added.
III.
Print paste is required to be stable for at
least one week. But most natural thickeners are biodegradable. To avoid this
hazard phenol or formaldehyde are recommended as preservatives.
IV.
Good adhesion of the thickeners to fibre is
required in order to avoid the less of colorant during mechanical handling.
Otherwise particles of colored films may break off, leaving white spots in
colored areas, so the thickener film should be flexible and should have good
adhesion properties.
V.
The choice of thickener to use with a particular
class of dyestuffs depends on the method of application and on the fabric to be
printed. Thus in screen printing chocking of the silk screen fabric can occur
if an improper thickener is used. Again in block printing, number of suitable
thickener is more limited.
VI.
thickeners may be classed as having either high
or low solid content. Gum tragacanth is a high solid content thickener suitable
for screen printing on fine filament rayon fabric.
VII.
The extent to which a thickening agent isremoved
in a high speed washing process is a vital consideration in the selection of
thickener.
VIII.
The cost and availability of thickener is
considered in selection a thickener.