Printing – part 3

19 01 2012

Yes, we’re still talking about the printing process!  As I warned you, it’s complicated.

For the past two weeks we’ve concentrated on the first two steps of the basic 5 steps in printing a fabric, which  are:

1. Preparation of the print paste.

2. Printing the fabric.

3. Drying the printed fabric.

4. Fixation of the printed dye or pigment.

5. Afterwashing.

So let’s look at the rest of the steps – drying, fixation and afterwashing.

Actually, the printing process begins even before passing  the fabric thru the printing presses, because the fabric must be conditioned.  The cloth must always to be brushed, to free it from loose nap, flocks and dust that it picks up while stored. Frequently, too, it has to be sheared by being passed over rapidly revolving knives arranged spirally round an axle, which rapidly and effectually cuts off all filaments and knots, leaving the cloth perfectly smooth and clean and in a condition fit to receive impressions of the most delicate engraving. Some figured fabrics, especially those woven in checks, stripes and crossovers, require very careful stretching and straightening on a special machine, known as a stenter, before they can be printed with certain formal styles of pattern which are intended in one way or another to correspond with the cloth pattern. Finally, all descriptions of cloth are wound round hollow wooden or iron centers into rolls of convenient size for mounting on the printing machines.

Immediately after printing, the fabric must be dried  in order to retain a sharp printed mark and to facilitate handling between printing and subsequent processing operations.

Two types of dryers are used for printed fabric, steam coil or natural gas fired dryers, through which the fabric is conveyed on belts, racks, etc., and steam cans, with which the fabric makes direct contact. Most screen printed fabrics and practically all printed knit fabrics and terry towels are dried with the first type of dryer, not to stress the fabric. Roller printed fabrics and apparel fabrics requiring soft handling are dried on steam cans, which have lower installation and operating costs and which dry the fabric more quickly than other dryers.

After printing and drying, the fabric is often cooled by blowing air over it or by passing it over a cooling cylinder to improve its storage properties prior to steaming, which is the process which fixes the color into the fabric.  Steaming may be likened to a dyeing operation.  Before steaming, the bulk of the dyestuff is held in a dried film of thickening agent.  During the steaming operation, the printed areas absorb moisture and form a very concentrated dyebath, from which dyeing of the fiber takes place.  The thickening agent prevents the dyestuff from spreading outside the area originally printed, because the printed areas act as a concentrated dyebath that exists more in the form of a gel than a solution and restricts any tendency to bleed.  Excessive moisture can cause bleeding, and insufficient moisture can prevent proper dyestuff fixation.  Steaming is generally done with atmospheric steam, although certain tyepes of dyestuffs, such as disperse dyes, can be fixed by using superheated steam or even dry heat.  In a few instances, acetic or formic acid is added to the steam to provide the acid atmosphere necessary to fix certain classes of dyes.  Temperatures in the steamer must be carefully controlled to prevent damage from overheating due to the heat swelling of the fabric, condensation of certain chemicals, or the decomposition of reducing agents.

Flash aging is a special fixation technique used for vat dyes. The dyes are printed in the insoluble oxidized state by using a thickener which is very insoluble in alkali. The dried print is run through a bath containing alkali and reducing agent, and then directly into a steamer, where reduction and color transfer take place.

After steaming, the printed fabric must not be stored for too long prior to washing because reducing agent residues may continue to decompose, leading to heat build up in the stacked material and defective dyeing or even browning of the fibers. If a delay of several hours is anticipated before the wet aftertreatment the fabric should be cooled with air (called “skying”) to oxidize at least some of the excess reducing agent.

Finally, printed goods must be washed thoroughly to remove thickening agent, chemicals, and unfixed dyestuff.  Washing of the printed material begins with a thorough rinsing in cold water.  After this, reoxidation is carried out with hydrogen peroxide in the presence of a small amount of acetic acid at 122 – 140 degrees F. A soap treatment with sodium carbonate at the boiling point should be begun only after this process is complete. This washing must be carefully done to prevent staining of the uncolored portions of the fabric.  Drying of the washed goods is the final operation of printing. 

And there you have it – a beautifully printed fabric that you can proudly display. Bet you know the subject of the next post – the environmental consequences of all this. Stay tuned.


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