Discharge printing is similar to traditional water-based ink, except that ?ink? is actually formulated to remove the original dye from the garment being printed. Discharge printing involves discharging (or removing) the dye in a textile substrate and is often followed by a traditional screen printing of color on the resulting natural (pre-dyed) color of the fabric. These printing systems only work on garments dyed with dyestuffs that are compatible with discharge systems.
However, the oven time for water-based inks — including discharge — is longer than for plastisol inks, slowing production times. Typical oven conditions are one to two minutes at over 300 F. Discharge requires a heater or dryer that can evaporate all of the water in the time that it takes the textile to pass through the system. Water-based only discharge involves a lot more water to get rid of than the plastisol/water-based combination systems
Discharge inks require an activator or catalyst to function. The most popular system relies on zinc formaldehyde sulfoxylate (ZFS) as its active ingredient. The newer system uses thiourea dioxide chemistry as its active ingredient.
Discharge ink has a limited pot life once the activator is added- typically one workday. Discharge ink can be ?recycled? after once activated as a normal ink on light colored textiles. Waste discharge inks, like all inks in a liquid state, are considered a hazardous waste.
Zinc formaldehyde sulfoxylate is a reactive chemical commonly used in industrial applications for bleaching. It is also known as Rongalite (registered trademark of BASF), sodium hydroxymethylsulfinate. While its heath effects include irritation of skin, eyes and gastrointestinal tract the full toxicological properties of this material have not been fully investigated.
During the heating and curing of discharge inks, formaldehyde and sulfur dioxide gases develop as byproducts of the reduction process. Formaldehyde is a known human carcinogen. Proper ventilation of the ovens and/or workplace is crucial. It would be proactive to have any facilities indoor air quality checked for formaldehyde levels.
Waste discharge ink, like all inks in a liquid state, is considered a hazardous waste and must be handled as such. Even though it cannot function as a discharge ink after its shelf life, it can be ?recycled? by using the ink as a normal ink on light colored textiles.
Garments printed with the ZFS discharge system can have measurable levels of residual formaldehyde that did not off gas from the garment during the curing/heating process. Garments may need to be washed or aired to remove all traces of this chemical. If garments are allowed to sit for a time after printing will outgas much of the residual formaldehyde in a few days, temperature and airflow pending – but consideration of where that formaldehyde is going is of concern if it exposes workers or others, ensure adequate ventilation.
The European Union has a formaldehyde exposure threshold of 75 ppm for apparel for small children and infants. Some EU countries, Japan and other countries law is even more strict – 20ppm for children under 36 months of age. A ZFS discharged garment, which has not been laundered or allowed, to offgas could fail these tests.
Thiourea dioxide activated discharge inks were developed to avoid the formaldehyde exposure and residues associated with ZFS activated discharge inks. They work in a similar fashion with thiourea dioxide as the chemical reducing agent that destroys the garments original dyestuff
Synonyms: aminoiminomethanesulphinic acid, thiourea dioxide, formamidinesulfinic acid, formamidinesulphinic acid, as well as several trade names.
Acute health effects of thiourea dioxide include; eye irritation, chemical conjunctivitis, skin and it may cause gastrointestinal irritation with nausea, vomiting and diarrhea. But like ZFS the toxicological properties have not been fully investigated.
It is important to note that thiourea dioxide is a different chemical than thiourea (without the ?dioxide?). Thiourea is a chemical known to the State of California to cause cancer.
At Econscious we use this ranking of different print systems to help us determine what kinds of inks we should use for a specific print job: