A brief chemo-timeline: In 1960, a quality T-shirt print meant that it was spelled correctly; less than five years later it needed to pass the wash test as well. Forward 50 years and we’ve become far more discriminating. The consumer has combined comfort with color and contrast and has very rigid ideas as to their combination. The image has to look and feel just right.
There was a point in time when the solutions to comfort and texture (particularly here in the USA) would be built entirely out of plastisol inks, but no more. Today the solution is likely found in a pail of water-based, water-based discharge, HSAs (high-solids acrylic) and/or plastisol, the latter of which appears to be on the endangered species list. The same government which taxes cigarette smoking and gasoline for internal combustion engines purports they have turned a blind-eye to the unprecedented hazards of phthalate plasticizers and PVC resin long enough and, God willing, justice will prevail!
Trouble is, we don’t really know just how much justice the Feds have in mind to administer, but even the largest of the U.S. plastisol makers are finally and aggressively promoting water-based alternatives—a fact enough to cause anyone to peek out of their plastisol-complacent hole.
We can continue to turn a blind eye to the reality or cowboy-up and learn, then master printing techniques with water-based ink systems. In the words of philosopher Soren Kierkegaard, “There are two ways to be fooled. One is to believe what isn’t true; the other is to refuse to believe what is true.” The truth has been upon us for some time.
What’s in the pail?
Water-based coatings are low molecular weight acrylic resin systems emulsified with solvents and water into a screen-printable fluid. Emulsifiers are added to “connect” the otherwise incompatible liquids and surfactants are added to keep the emulsion stable on the shelf. Anti-fungicides are added to keep the water component from forming mold and humectants are added to keep the ink stable in the screen.
Some inks have colorants that are pre-mixed into a toner that is mixed into the ink. Depending on how you buy and/or blend the product, the raw material costs of water-based inks can be very low.
If you are a discharge practitioner, you begin with a water-based compound where the discharging reaction takes place in water. The most common discharging chemical is ZFS (zinc formaldehyde sulfoxolate) that can be added to a compatible water-based system as recommended by your ink supplier. This agent will alter the pH and, over time will destabilize the emulsion, so it should be added immediately prior to use and any left-over product should be properly disposed of. You may be advised to add some additional humectant in an attempt to keep the ink from immediately plugging and filling in the screen. If you want the discharging paste to color the garment you will either use RFU colors or add toners to your discharging base.
HSA ink systems are relatively new to the market. They are a gallant attempt by ink manufacturers to offer textile printers a product more akin to plastisol. These are so varied by brand and type a separate article is warranted, but suffice it to say they are a reasonable compromise of the two disparate systems (plastisol and water-based). As you investigate these, they bear a high price tag and have a far greater conversion cost than our old friend plastisol.
Assuming you will be purchasing either RFU (ready-for-use) colors or a premixed base into which you add the appropriate toners, the best way to blend a water-based compound is to use a homogenizer, even when the product you are purchasing is ready for use. However, you will want to be very careful what you attempt to add to the mix as not to challenge the emulsion stability. Water-based inks are very sensitive to the water and oil balance. If you start throwing in too much of either, you will destabilize the existing emulsion and it can separate… perhaps in the pail, perhaps on the press where it’s warmer and the ink is under high shear.
Further, the base and RFU colors will have a relatively high pH in order to keep fungus (such as mold) from growing in the stagnant water in the ink. If your additives are acidic they will change the pH of the ink, so a pre-adding call to the manufacture is in order, although a higher pH will restrict substrate wetting and ink transfer.
Tools of the trade-up
Perhaps the most critical selection in printing water-based inks is the screen mesh. It should have a thin thread, the limiting factor for highlight dots, and a large opening, which limits the shadow dots, resulting in a low mesh count. Go for the thinnest and lowest for line colors. Keep in mind that this ink is made to penetrate the knitted construction of the T-shirt and the mesh must keep this thin ink from dripping through.
The stencil must be resistant to water-based chemistry (water and the other chemicals) and must be both adequately dry before exposure and completely exposed. Note: You can’t expose the water out of a screen. And, a screen that isn’t adequately exposed will not withstand the abrasion that will come from regularly wiping both sides of the screen with a wet rag to keep the screen opened.
The press needs to be equipped with a front stop setting to leave screens flooded with ink during dwell. Flash curing (when needed) is the Achilles heel of the water-based system for now. The cycle is vicious: You use heat to flash, helping plug the mesh with ink and warming up the platens, which, in turn, increases the vapor pressure of the ink, causing it to plug. With the current equipment available this step is far more art than science—be patient.
The squeegee should have an edge compatible with coarser mesh counts in order that a tight seal is made between blade and mesh. The key is to apply the highest fluid pressure possible in order to thin the ink adequately so it will purge any ink residue from the mesh and penetrate well into the garment. To do so, select a blade that will allow you to print at top press speeds. The speed limit is predicated on emulsion stability and phase separation—run as fast as you can without causing the ink to separate. The flood-bar should be used in “pre-print” mode in order to seal the mesh for on-press screen stability and to pre-load ink into the cells as required.
The dryer is very often another weak part in the process. The ideal situation is to allow the heavy, wet ink to sit on the garment long enough to wick on synthetic fibers and to absorb onto natural fibers. As the temperature increases, the “solvents” evaporate one by one and all must eventually be evaporated. The softened resin then affixes the pigment to the surface of the fabric. You will need to spend time to empirically optimize your drying conditions.
Water-based systems are not going to be as clear-cut and user-friendly as plastisol inks. Whether you start out with a discharge underbase and overprint plastisol, start with a simple water-based colored image on white, or whether you start a conversion of your shop, the operative word is “start” and the best time has already passed. You won’t learn the nuances of the system overnight. There is an axiom; “what got you to this point won’t be enough to get you to where you need to go” and this may become the epitaph of plastisol inks for T-shirts.