Understanding Polyester Printing And Dye Migration
Seldomly, a week goes by without at least one or more customers calling in and asking how to print on polyester fabrics and how to avoid dye migration. Although we have touched on the subject briefly on a couple of our blogs, here is Ed Branigan, our Print Applications Manager, with a more in-depth insight on what the cause of dye migration (bleeding) is, and how to best avoid it:
”To combat dye migration on polyester garments you need to understand two things: What causes dye migration in the first place and how plastisol inks cure. It really comes down to information and if you can get your head around these two things it will go a long way towards minimizing the problem and saving yourself a lot of time and money.
Let’s look at dye migration first. If you were to stretch out a single thread from some cotton yarn it would look kind of fibrillated or hairy. When it’s being dyed, the dye can soak down into the thread like little fingers and grip onto it. We call this a mechanical bond. Natural fabrics absorb the dyes. Polyester, on the other hand, is very different. Stretch out a single thread of polyester and it’s really smooth, like fishing line. There’s nothing for the dye to grip onto or soak into so the dyes are given a chemical bond that’s heat-sealed on. The temperatures used to heat seal the dye onto the polyester fibers range from 230º to 260º Fahrenheit. Keep these numbers in mind because these are really important ones to remember.
Most plastisol inks need to hit 320º Fahrenheit for up to 1 minute to fully cure. The correct terminology to use would actually be fusion. There are two main elements involved, resin – which is a powder, and plasticizer, which usually looks like an oily liquid. When these two are mixed together with a pigment, you get your ink. If stored correctly they’ll stay wet for a long, long time. When we’re running a printed plastisol ink through the dryer, the resin swells and absorbs the plasticizer until they become fully fused, or cured. As mentioned already, most plastisols need to hit 320º F for a period of time (1 minute) to become fully fused.
Here’s where the dye migration occurs: When the plastisol is on its journey through the dryer and the heat passes the 260º F threshold, the polyester dyes will often get released from the thread and they will sublimate, which basically means they turn into a gas. The molecular weight of the dye is important here. If it’s a dye that has a low molecular weight, it means that the dye molecules are small. If it’s a dye that has a high molecular weight, this means that they’re bigger. The plasticizer in the ink wants to pull those dye molecules into itself in the same way that plasticizers disperse plastisol dye molecules. The smaller dye molecules can get pulled up through resin particles and mixed into the ink. This is what causes the ink to change color. The heavier dye molecules won’t because they’re too big to pass through the resin particles. This is why sometimes the garments “bleed” (or have dye migration) and sometimes they don’t.
There are several ways to combat the problem. One is by using a low-bleed or bleed resistant ink. International Coatings carries several of varying strengths depending on the type of material being used i.e. 50/50 versus 100% polyester. (The Product Bulletins page on our website lists our various low-bleed inks, from the 711 to the 7041 Paramount White).
There is no such thing as a No – Bleed ink, though. You need to test every single time that you run production. I always recommend waiting at least two days after doing a test print, or over the weekend if possible.
Another way to combat dye migration is to find an ink that cures at a lower temperature. For example, International Coatings has a low cure additive (3804 Low Cure Additive) that can be used with a poly white. This enables you to lower the dryer temperature to anywhere from 275º to 300º F. The lower curing temperature can go a long way to keeping those dye molecules locked in the polyester fibers.
An important thing to remember though is making sure that the ink is also fully fused, otherwise it will crack and fall off after washing. It’s a very thin line to walk: The lower the temp the more likely a garment won’t bleed, but the ink may not fully fuse. Ultimately the point to remember is that as far as polyester dye migration is concerned, heat is your enemy and time is your friend. If you can get the temperature on the dryer lower, just run the garments through for longer. This will help in attaining fusion.
Once you’re sure that the ink is cured, get those garments cooled down as fast as possible. Even post-cure, the residual heat can entice the polyester dye molecules to come out. Don’t lay the printed garments on top of each other until they’re cooled. This only traps the heat longer and exacerbates the problem.
It is possible to make polyester garments that don’t bleed and to make inks that would resist bleeding, but the cost would make them completely prohibitive. In the meantime, we’re stuck with what we’ve got. It’s all about patience and waiting.”
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