ð Thickeners
used in textile printing are high molecular weight compounds giving viscose
pastes in water.
ð These
impart stickiness and plasticity to the printing paste which preserve the
sharpness of the printed design and prevent blurring effect by capillary
action.
ð Four
significantly different approaches may be used to produce thickeners, using:
(1) a low
concentration of a polymer of high relative molecular mass (r.m.m.)
(2) a
high concentration of a material of lower r.m.m. or of highly branched chain
structure
(3) an
emulsion of two immiscible liquids, similar to the emulsions used as cosmetic
creams, or a foam of air in a liquid
(4) a
dispersion of a finely divided solid, such as bentonite.
ð In
the selection of thickening agents, it is necessary to take into account
requirements other than viscosity, which can usefully be classified in five
categories:
(1) Print
paste stability
(2) Good
adhesion of the dried thickener film
(3) Minimum
effect on colour yield
(4) Ease
of removal
(5) Acceptable
cost.
ð Print
paste stability:- The thickener must be stable and
compatible with the dyes and auxiliaries to be used. If a cationic dye is added
to a thickener with anionic charges, the interaction is likely to change the
viscosity and to produce insoluble complexes. The pH of the print paste must be
considered, as some polymers are only usable within a limited pH range and form
gels when acids or strong alkalis are added. The micro-organisms responsible
are present in the air, and thickeners provide nutrients and ideal conditions
for their growth and reproduction. They produce enzymes that break down the
polymer, with a consequent and often rapid fall in viscosity.
ð Properties
of the dried thickener film:- Drying usually follows printing,
and the fabric may be creased and flexed over rollers and tension rails before
fixation of the print occurs. The thickening agent is deposited on the fabric
surface as a dry film that sticks fibres together and contains colorant. Good
adhesion to the fibre is required in order to avoid loss of colorant during mechanical
handling. Otherwise particles of coloured film may break off, leaving white spots
in coloured areas and possibly giving coloured spots in unprinted areas. The
deposition of polymer films on a fabric inevitably causes some stiffening, and washing
is normally required after fixation of the dye to remove thickening agent and any
loose dye. Fortunately, many synthetic thickeners do not form such hard films
as do the natural polymers.
ð Effect
on colour yield:- Printers have found that the fixation of
dye is usually best achieved by steaming. Steam condenses on to the film of
thickening agent, which swells and contains a miniature dye bath on the fibre
surface. Some dye dissolves, and the next step is the diffusion of dye through
the swollen film to the fibre surface. Any affinity between the molecules of
dye and thickening agent will reduce the speed of this diffusion process as well
as the extent of dye transfer to the fibre. If both molecules have ionic
charges of the same sign, the speed and efficiency of the process will be
higher because of the mutual-repulsion effect. In addition to any effect on dye
fixation, the thickener will significantly affect the penetration of print
paste into the yarn and fabric structure, and this may have a dominant effect
on colour yield.
ð Ease
of preparation and removal:- The time taken to prepare a thickener,
and the precautions required to ensure that the paste has satisfactory and
consistent properties, are variables that have greater significance today than
in the past. The extent to which a thickening agent is removed in a high-speed
washing process, especially after a high-temperature steaming operation, is
also a vital consideration in the selection of thickening agents. It is also
difficult to redisperse starches and remove them from the printed fabric. The
removal of thickening agents can also be facilitated by introducing a second component
in the thickener.
ð Cost:- Traditionally, when labour costs were low, it was common to look only at the cost of the thickening agent itself and the cheapest materials were widely used. It must already be obvious, however, that it is essential to consider many other aspects to decide which material will give the required quality at the lowest overall cost. The concentration to be used, cost of preparation, stability, print penetration, colour yield and ease of removal can in total be more important than the basic price of the polymer. The biological oxygen demand of the effluent, due to thickener removed in the washing-off operation, can also be a vital parameter.
ð The
inter relationship between Low and High solid Thickening properties
|
Characteristic
Property |
Type
of Thickening |
|
|
Low
solids |
High
solids |
|
|
Flow |
Short |
Long |
|
Effect of increasing shear on
viscosity |
Viscosity falls off rapidly |
Little or no falling off of viscosity |
|
Yield of dye |
Generally high |
Usually of a lower order than low solids
thickenings |
|
Levelness of print |
More difficult to achieve than with
high solids thickenings |
Generally good |
|
Viscosity |
High |
Medium-low |
|
Handling of dried or baked print |
Flexible film giving moderate-good
handle |
Film tends to be hard, brittle and
many crack off fabric |
|
Nature of polysaccharide chains |
Very long and straight with little or
no side batching |
Shorter than low solids types. Heavily
branched, coiled in solution |
|
Price of made-up thickening |
Relatively low |
More expensive than low solids type |
|
Chemical resistance |
There is no clear demarcation between
the two types |
|
