Discover the rare interactions between cotton dyes and certain plastisol products.
By Rick Davis
Competition domestically and internationally has forced garment screen printers to be more cost conscious than ever. To keep our budgets in the black, we are often tempted to source less costly supplies and materials. Cheaper doesn't always mean inferior in quality, but as decorators, we must remain aware of the fact that garment components and materials are continually changing, and not always for the better. To ensure that we can maintain a consistent level of quality, we must, on a regular basis, test and monitor the quality of the goods on which we print. This is the only way we can be certain that the printed products we deliver will meet our customers' demands.
In this installment, we will look closely at garment fabrics, particularly those that have undergone some form of dye processing. As we'll see, this is one aspect of our products we have little ability to control, but one that can have a significant impact on our success.
Most of us print on two primary types of fabrics: 100% cotton and 50/50-cotton/polyester blends. Here, we will look at the interactions that can take place between the dyes in cotton garments and the plastisol inks that we print onto them.
Cotton dyes are water based and applied to the cotton by saturating the hydrophilic (water-absorbing) cotton fibers with the water-dispersed dyes. These water-soluble dyes are the ones that cause our white socks to turn pink when we accidentally include a red cotton T-shirt in our washing machines with a load of whites.
One reason that 100%-cotton garments are so popular with textile screen printers is because bleeding of garment dye into the printed ink film is not an issue. Since the dyes used on cotton are water soluble and the plastisol inks we use are petroleum based, the dyes and ink will not intermix, and consequently, bleeding won't normally occur.
But we can't forget that everyone in the garment industry, from printers to soft-goods suppliers, is continually looking for ways to cut costs in order to remain competitive. In relation to the blank garments we buy, these cost-cutting measures can have some surprising and serious ramifications.
To illustrate this point, consider a situation I recently witnessed in which a dyed 100%-cotton garment did bleed into the print. In this situation, a company had printed a large run of 100%-cotton garments dyed in a shade of dark red. The ink was a standard non-bleed-resistant white for printing on cotton. Based on experience, the printer had no reason to be concerned that the garment dye would migrate into the plastisol ink. But over a short period of time after printing, that is precisely what happened.
The cause is believed to have been very cheap water-dispersed dyes that were used to color the garment fabric. For some reason, this dye was able to intermix with the plastisol ink film and alter the appearance of printed images. From a chemical standpoint, the water-based dye and the petroleum-based ink were completely incompatible, and the dye migration should not have occurred. But after some investigation, it was determined that conventional dye migration was not the culprit. Instead, the printers deduced that the garments had an inferior dye that was heat sensitive--so heat sensitive, in fact, that the dye sublimated from the garment into the printed ink film during the drying process.
In some cases, the effect was severe enough that it could be detected in printed areas of the shirts as soon as the garments exited the dryer. And on garments where the effect didn't appear immediately, the dyes often penetrated partially into the print and emerged at the surface later.
While this situation was unique in terms of the materials involved, it does raise some interesting questions about a related and more common printing problem called ghosting and its underlying causes. Ghosting usually occurs when a bleed-resistant ink film comes in contact with a cotton fabric and the agents that give the ink its bleed resistance literally gas the dyes out of any neighboring garment that the first garment comes in contact with. Generally, this happens when the garments are stacked.
With stacked garments, this outgassing and subsequent ghosting primarily occur in two ways. One is when the garments are stacked hot and the bleed-resistant agent (typically an organic peroxide) is released as a gas from the hot ink film and fades the color of the hot garment that is stacked on top.
The second scenario occurs when a bleed-resistant ink is printed onto a cotton garment and slightly undercured. In this case, the bleed-resistant component is not properly fused into the ink film and thus allowed to sublimate out of the ink film and into or onto any other garment it makes contact with. The result is a bleaching effect on the dyes in the second garment that appears as a "ghost" image of the original print. Once the bleed-resistant components have created such ghost effects, the garments are lost for good.
The good news is that there are solutions we can employ to avoid this problem. First, we need to ensure that we actually do need to use a bleed-resistant ink on our 100%-cotton garments. Most well-known and reputable garment manufactures do not take shortcuts on the dyes they use, meaning that their fabric colors are stable, and we don't need to use bleed-resistant inks. But if a bleed-resistant ink is needed, we must make sure that the ink film is properly cured. This way, we minimize the potential for any dye-blocking agents in the ink to gas into adjacent garments after printing.
Another basic solution for avoiding ghosting with bleed-resistant ink is to carefully control the printing process and minimize the thickness of the printed ink film. Thicker ink films require a greater amount of heat and time to properly cure and have the greatest risk of being undercured.
To get thinner, easier-to-cure ink films, we must employ high-tension screens, minimal off-contact, and minimal squeegee pressure. These parameters will deposit less ink and will help ensure that the ink only makes contact with the surface of the fabric. This way, the interface between the ink and the fabric is minimized, and thus, the potential for ghosting is reduced.
In general, higher quality 100%-cotton garments use more reliable dyes, and we should not require bleed-resistant inks to print on them. But some shops have fallen into the habit of using a bleed-resistant ink on all fabrics in the name of simplicity and standardization. This is a philosophy that I can agree with, except for the potential risk of ghosting if the ink film is not properly cured.
One effective solution is to keep a non-bleed-resistant cotton white on hand for jobs on 100%-cotton garments and a bleed-resistant ink for 50/50 garments. If you experience bleeding problems on the cotton, you can always try the bleed-resistant ink for a solution.
A wiser approach, however, is to avoid such garments in the first place. And the trick to identifying potential bad 100%-cotton fabric is to know your garment manufacturer well and to test your fabrics on a regular basis.
A useful way to test dye quality is to print a thin film of non-bleed-resistant white-plastisol onto the 100%-cotton fabric in question, then overheat the sample. Any gassing of the dyes from the fabric onto the thin ink film will be readily apparent and will alert you to the potential for this problem. Then you can decide whether to use a bleed-resistant ink or look for an alternate garment supplier.
Although occurrences are rare in which 100%-cotton dyes result in bleeding, it only takes one bad run for you to lose an important customer and gain a load of wasted goods. Controlling your procedures, printing with a minimal ink-film thickness, testing properly, and using the right products for the job will help you keep these problems in check.
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