Understanding Discharge

Seldom has a process held so much promise or been so maligned. Although no other process can produce such bright colors with so soft a hand, discharge has not been widely accepted. There are valid reasons for choosing not to use discharge, including the fact that only 100% natural fabric colored with selected dyes discharge well and the fact that dryers designed to cure plastisol are not ideal.

The processes’ failure to gain wide acceptance has little to do with valid drawbacks. The problem has been that false rumors and ignorance take root and grow in the anti-water desert of a plastisol-oriented industry. This article is an attempt to present the facts about safety, substrate requirements and process control requirements so that printers can make intelligent decisions about discharge’s potential and can control the process in order to obtain the best possible results.

Environmental Concerns

The following is a quote from a Federal EPA letter concerning Zinc Formaldehyde Sulfoxylate which is the discharge agent recommended for use in most systems: “A review of the current office of Water Activities reveals that there are no current regulations specifically controlling zinc formaldehyde sulfoxylate (ZFS).

Also, according to the Office of Pesticides and Toxic Substances Hot Line, which has a computer based Federal Register (FR) data base, there have not been any FR notice/publications over the past ten years discussing, proposing, or promulgating rules for ZFS.

Further, we are not aware of any situations where state or local officials have established controls for ZFS or have banned the release of ZFS into water.” As the letter we quoted from is dated June 11, 1991, the Federal EPA and the Toxic Substances Hot Line were contacted again on December 21, 1998 and both stated that the status of ZFS has not changed.

If anyone wants to check for themselves the telephone numbers are: Federal EPA - 202-260-7120 Toxic Substance Hot Line - 202-554-1404

These agencies are there to protect and serve you and seem to enjoy helping people. Giving the CAS number along with the name of any chemical you want to check can speed things up. The CAS number of Zinc Formaldehye Sulfoxylate is 24887-06-7.

The fact that there are no specific laws against putting ZFS into water should not be taken as an invitation to dump it down a drain. Chemicals should be disposed of as specified by local laws and MSDS.

Health and Safety Concerns

Discharging is a chemical reaction that takes place at temperatures close to the boiling point of water. The non-deodorized inks may smell like two-day old sushi at room temperature, but there are no toxic fumes. If the reaction takes place in a well vented oven any fumes emitted should be exhausted.

Only when the reaction takes place in a transfer machine, press (see STEP #6) or under a flash unit are there fumes to worry about and monitor. The problem is that almost everyone monitors for the wrong thing.

The formaldehyde in ZFS is not set free prior to, during or after the reaction. If formaldehyde was released during the reaction, it would combine with urea which is as far as I know in all competitive discharge bases to form urea formaldehyde which is a safe stable compound that is used in paper towels, facial tissues and old fashioned butcher paper.

The possible products of decomposition include carbon monoxide, carbon dioxide, sulfur dioxide and zinc oxides. PEL and TLV for sulfur dioxide is 2 ppm 8-hour (5 ppm STEL). Sulfur oxides can cause irritation of the respiratory tract. PEL and TLV for zinc oxide is 5 mg/m3 /8-hour. Zinc oxide fumes could cause “metal fume fever.” PEL is the abbreviation for Permissible Exposure Limit. TLV is the abbreviation for Threshold Limit Value which is the maximum level recommended for a lifetime of exposure for 8 hours/day. STEL is the abbreviation for Short Term Limit Exposure which, of course, means maximum level recommended for even brief exposure.

As fumes created during pressing or flashing are being generated in a confined area, it should be easy to collect and vent them. It is quite possible that a good air exchange rate in the print area will be enough but air blowing directly on the screens should be avoided as this will cause inks to dry out.

Solvent free discharge bases made with the safest possible ingredients are available but prolonged contact could cause skin irritation in sensitive individuals, especially after the ZFS is added. Dried prints contain chemicals that, although considered nontoxic, are highly water soluble. It is because they are highly soluble (even in perspiration) that the chemicals remaining in the garment are more likely to irritate the skin of sensitive people and thus should be washed out of the garment prior to wearing.

In the writer’s opinion discharge printed garments should be tagged “wash prior to wearing.” I do not know of an actual case where a man looking in the mirror saw a rash on his chest spelling out the same words as were printed on the T-shirt he just took off, but it is possible. I do know of two instances where a person working with discharge inks suffered an asthma attack. The two instances involved the same person but discharge inks from two different companies.

 

What Is Discharge?

Discharge is a chemical reaction that destroys the ability of selected dyes to reflect a color. The reaction takes place at temperatures above 82°C (180° F) while water is present.

What Fabrics Are Dischargeable?

Only selected dyes used on natural fibers are dischargeable. Currently there is only one major garment manufacturer I am aware of that officially offers “dischargeable” garments. Garments from other manufacturers may discharge but it’s a hit or miss affair as of mid-1993. To assist other companies in producing dischargeable garments, we are pleased to provide the following list of some recommended direct dyes.

Direct Garment Dyes Recommended for Subsequent Discharge Printing

Listed in Table 1 are some recommended direct dyes for discharge printing both from a neutral and an alkaline discharge medium.

Process Control Step #1 Checking Discharge Ability

Everyone understands that when working with garments not manufactured specifically for discharge, it is necessary to check at least a garment from the top of each carton and one from the bottom prior to assuming that the whole carton is dischargeable.

If possible have the supplier certify that all garments in a carton or cartons are from the same dye lot. What everyone does not understand is that there can be variations from lot to lot of “discharge able” garments and perhaps also within a lot. Black garments are normally made from unbleached cotton, and the color of unbleached cotton can vary. For this reason and because the discharge process can do no more than take a garment back to its natural color, “discharge ability” of a garment as defined by how close to white the natural base color is, will vary greatly in black garments.

One should also be aware that other colors are sometimes overdyed with black. In checking the discharge ability of various fabrics, we have found black that discharged to bright red. It is also possible to have colors other than black created by combining dischargeable and non-dischargeable dyes. One of my favorites was a dark green that discharged to bright yellow.

If the printer is to maintain any semblance of color uniformity throughout a run and from run to run, garments should be discharged to the same base color. If variation in dischargeability can be detected prior to printing, the printer can at least try to adjust ink color to compensate. The only way I know to check dischargeability of a fabric is to discharge it under controlled conditions. Controlled conditions means:

1. Using the same fresh unpigmented bases with the same level of discharge agent every time.
2. Printing each test in the same way though the same mesh with the same squeegee on the same base.
3. Discharging at the same temperature, time and airflow.

It’s the last item that is the killer for most printers. Production ovens are not scientific instruments. Oven temperatures are apt to vary by 30 degrees F or more from time to time and by up to 50 degrees F with belt loading.

Too many people test dischargeability or try to formulate colors by running samples through production ovens whose speed, temperature and air are adjusted for whatever is being run at the time. In my opinion the best solution is to discharge the wet print with a transfer machine or press press.

This will approximate what can be achieved in a perfect oven. Small printers with ovens too short to handle discharge are discharging via transfer machine prior to drying. 160°C - 177°C (320°F to 350°F) for 10 seconds should be enough. Note: Temperature under 166°C (330° F) may be required to prevent killing some reds.

Process Control Step #2 Determining Best Discharge Agent Level

Few pigments used to color discharge inks are unaffected by the discharge agent. Many pigments such as violet may be half as intense when used in a discharge base containing the recommended maximum of 8% ZFS as when used in a base containing no discharge agent. Of course, without the agent they would have to be printed on white material for this to be apparent, but the fact is that ZFS will let down some colors. Some pigments can shift in shade if held overnight after ZFS is added.

The higher the level of ZFS in the ink, the more the let down or shade shift. This is why people who do not measure the ZFS accurately get more color variation and why people who like to use more than recommended, tend to get less than optimum results.

If a fabric being printed discharges to near white with only 3 to 4% ZFS, use only 3 to 4%. We recommend that fabrics be tested with 4%, 6% and 8% ZFS in discharge base to be used. If there is little or no difference between the 4% and the 8% levels, we suggest that only 4% be used in the pigmented press ready ink. More is not always better.

Process Control Step #3 Creating Colors

Variations in the shade garments discharge to, variations in ink viscosity due to evaporation of water from ink in the screen, print technique variations, and oven temperature variation make absolute control of color impossible.

The first thing I tell printers is to avoid exact color matches. I also upset quite a few printers who want to order premixed colors by telling them that it is impossible for us to mix a color based on results achieved in our lab, even if they provide samples of the material to be discharged, and guarantee that the same color will be obtained in production. Colors should be mixed based on prints made approximating production conditions.

A lot of trial and error should be expected prior to developing the required rapport between the ink department and production. An understanding of the process and its limitations and willingness to work together is required if success is to be more than an accident.

Printers often add more pigment than the base will handle to achieve richer colors and wind up with prints that will not wash. Standard color pigment levels over 12% usually are not recommended. Pastel shades should be created by letting down standard colors with white pigment. A total pigment loading of 8% to 10% is usually recommended with white replacing color until the desired tint is attained.

Process Control Step #4 Print Technique

Transparent discharge inks must penetrate beyond visible range in order to destroy the garment color so it will not show. It is not necessary for the color of the garment to be destroyed all the way through, but it doesn’t hurt in the case of lightweight T-shirts unless print clarity is destroyed in the process.

Best penetration is achieved with beveled or very slanted squeegees and very fast strokes. Some good manual printers use print strokes so fast that it’s almost comical to watch when they are printing discharge. Experiment until you find what works best for you and then BE CONSISTENT. I like working with automatics because there is less variation. As surface deposit effects color intensity, it is possible to brighten the last color down by combining a heavy penetration stroke with a light second stroke on manuals. Hiding colors such as black, blues, greens and browns should be printed first. Reds should be printed last or second to last and yellows or fluorescents last.

Discharge inks with some opacity which are now available and pre-mixed whites require a surface deposit for maximum brightness but enough penetration to discharge and anchor. This means that they must be printed last or second to last unless flashed or press pressed, and that reduction of some colors may be required to achieve adequate penetration. The best test for adequate penetration is the fingernail test. You should not be able to scratch the print off easily with your fingernail. Use a little common sense and remember that you can abrade any surface deposit print if you try hard enough.

Process Control Step #5 Eliminating Misprints

Regular transparent discharge inks are very thin after the ZFS is added. We are often asked what should be used to thicken discharge inks. Our answer is that we do not offer a thickener for discharge inks because thickening will reduce penetration which usually reduces color brightness.

If absolutely required, an anti-wicking agent might be used, but it is not recommended. The major problem with the ink viscosity being low is that ink left in the top of a screen in a clamshell type press will run down onto and through the image area and contaminate the non-image area on the back of the screen. This contamination will discharge color out of the non-image area of an amazing number of shirts.

Wiping the superfluous ink off the back of the screen with a dry rag just spreads it around and causes a larger unwanted, although often interesting discharge pattern on the next half dozen shirts. Ink smears on the back of the screens must be washed off with damp sponges and the area dried prior to resumption of printing.

The secret to avoiding run through is to avoid leaving ink in the top of the screen on a clamshell type printer during any work stoppage. Fingerprints are the most common cause of misprints. If the person loading or unloading shirts has ink on their fingers, their fingerprints will be invisible until the garment comes out of the oven at which time they will have been saved for posterity.

When printing discharge, cleanliness may not be the next thing to godliness, but it is the best defense against a huge percentage of misprints. Luckily all that is required to prevent problems is a container of water and a towel in the print area. Discharge inks wash off fingers very easily.

Process Control Step #6 Discharging

As stated earlier, the discharge reaction takes place at temperatures higher than 82°C (180° F) while water is present. Drying stops the process. For this reason, we have separated the reaction from the drying that stops it. Too bad few dryers are capable of doing so. Discharge was initially a wet process in which the reaction was caused by steam.

The newer “dry” discharge inks are loaded with humectants to hold water long enough for the reaction to take place. Ideally, the wet ink film should be brought up to the temperature required for the reaction to take place and held at that temperature long enough for the reaction to be completed prior to driving the water off so the print can be cured and thus made washable.

Most ovens do what they are designed to do which is to heat the print up and drive off the water as quickly as possible which makes it impossible to obtain the brightest possible discharge prints. The best that most printers can do with ovens designed for plastisol or normal water-based inks is to lower the temperature enough to avoid cutting the reaction short or add urea or other suitable humectant to prolong reaction time. Both options make it more likely that the print will not wash well.

IT IS POSSIBLE TO GET A GREAT DISCHARGE REACTION WITHOUT CURING THE PRINT WELL ENOUGH TO WASH. This is because the binders require close to 149°C (300°F) to crosslink while the discharge reaction best takes place at temperatures lower than 121°C (250°F) in most cases. One felt pool table material we tested discharged best at under 82°C (180°F). Small printers can get around the problem by discharging wet prints on their transfer machine prior to drying.

Five to 10 seconds at 166°C to 177°C (330°F to 350°F) is recommended unless a pigment that is affected by temperatures above 166°C (330°F) (primarily reds) is being printed. Remember to put the transfer machine in front of a window with a fan blowing out or to collect and exhaust the fumes.

I am told that the "Press” we have been trying to get equipment companies interested in making for almost two years will soon be a reality. The press press is, in effect, a heat transfer machine platen mounted on a multi-color textile press, hopefully with built-in venting. This unit will solve a lot of problems for everyone except belt printers. Perhaps belt printers can solve the problem by discharging with steam.

Control Step #7 Drying

By separating drying from discharging, we can make this step short and sweet. After the reaction is complete, give a one side print for all the heat required (short of shifting heat sensitive colors and scorching the garment) to crosslink the binder.

This should be less than 1 minute at over 160°C (320°F). If discharge is taking place in the oven, a minimum of 90 seconds in the oven is recommended. Some systems require a minimum of 3 minutes. If both sides of the garment are to be printed, excessive heat should be avoided the first time through because it can decrease discharge ability of the garment and this gives duller colors on the backside.

This problem is usually attributed to “settling the dyes,” but more often it is lessening in absorbency. You can test this for yourself by putting a drop of water on a garment that has been through the dryer and one that has not been through the dryer.

Some ink makers have addressed the absorbency problem and/or created bases with enough opacity to hide lessening discharge ability for the last or possibility last two colors printed.

Note: It is our understanding that at least one ink manufacturer is presently producing a discharge agent which is not zinc formaldehyde sulfoxylate and contains no ingredients that are listed in any regulatory listing as being hazardous.