Selection of Thickeners for Printing (Textiles)

 

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