I've written before about ink safety for this publication on a number of previous occasions. But ink safety is such an important issue, it seemed to me to be a good time for an update and overview.
Ink and printer manufacturers have addressed some of the issues and claim to have cleaner, less hazardous inks today. Are they safe? How do you know what the hazards are? In this article we will provide an overview of the issues that remain and should be considered when buying and using ink jet technology.
Water-Based Dye and Pigmented Inks (including Latex)
These are inherently the safest of all the inks available today. For all types of inks hazardous ingredients like nickel, a carcinogen, in the yellow pigment have mostly been eliminated. To be sure your dye or pigmented yellow ink does not contain nickel consult the Materials Safety Data Sheet (MSDS) required by law to be supplied by the manufacturer for any ink sold in the US. They are usually available on line from the printer or ink manufacturer's website or upon request.
If nickel is used in the ink the MSDS must list it as an ingredient and warn you that it is classified as a carcinogen. It is a good practice to review the MSDS of any ink you use so that you understand what you should do to protect yourself, your workers and your customers from the hazards present in them. Be particularly cautious of any MSDS from third-party ink suppliers (sources other than the printer manufacturer) which list too many “Proprietary Ingredients,” as the term can be used as a way to cover the presence of dangerous ingredients. This goes for all types of inks, not just water-based inks. Frequently printer and ink salespeople either gloss over or understate any hazard that is present. This can be a problem with solvent and UV-cure printer manufacturers.
The liquid part of the water based dye and pigmented inks are mostly water with some humectants, usually glycol ether, which is quite safe. Don't drink it but otherwise it is a lot like the glycerin you may have around the house. They usually add some small amounts of alcohol and an ingredient to stop the growth of mold or bacteria colonies, which could plug the nozzles.
The polymers associated with the pigment dispersion of Latex inks are generally acrylic polymers and pose no significant hazard. In Latex inks they are polymer particles that coalesce and form a film when water and co-solvents are evaporated. This is a physical process without a chemical reaction. Although the term “cure” is sometime used for Latex inks—it really means it is heated until it forms a film.
Water-Based Textile Inks.
Direct printing textile inks may be pigmented or dye based. The pigmented inks are very similar to the ones formulated for printing on coated film or paper and contain a binder like “Latex” inks so that they will provide durability on the fabric. The dye based inks are a different story. Depending on the fiber they may contain sublimation dyes (a smaller molecule of disperse dye), disperse dye inks for synthetic fibers, reactive dye inks for cotton and other cellulosic fabrics or acid dye inks for nylon and silk where vivid colors are required.
Sublimation dyes must be heated after printing so that they will diffuse into the synthetic fiber, usually polyester. Dyes that fail to completely diffuse into the fiber must be washed from the fabric. There is no chemical reaction so there are no hazardous byproducts from the process. Whether it is transferred from paper or direct printed, the materials are basically the same. Undiffused dye may cause skin irritation, so it is important to wash the fabric if it is to be used in a clothing application. It’s not necessary for signage or trade show graphics.
The acid dyes and reactive dyes react with the fiber and are chemically bonded to the fabric just as in traditional fabric dying. Pretreatments and post treatments are required to achieve effective chemical reactions. Unreacted dye and byproducts of the reactions as well as pretreatment materials should be removed after treatment so that these materials do not remain on the fabric. In traditional analog printing frequently after dying the manufacturer fails to adequately wash out unreacted or undiffused dye in order to avoid adding the dyes to their wastewater. This is why if you purchase clothing, especially dyed polyester, you should wash alone the first time to remove this undiffused dye.
Solvent and Eco-Solvent Inks
Solvent and eco-solvent inks present a very different set of hazards. These are pigmented inks where instead of water they use an organic solvent as the primary ink vehicle (carrier). As far as safety goes the pigments and polymers in the inks are very similar to water-based inks. However the solvents are a completely different story.
First, solvents make up most of the weight of the ink and are organic, which means they can burn, and when mixed with air they can explode (depending upon the ink's flashpoint). These solvents may be classified as VOCs (Volatile Organic Chemicals) which cause ozone pollution when discharged into the environment, and also may be toxic to humans and fish if breathed in significant quantities or discharged as a liquid into the environment.
Some "strong" or "true" solvent inks—often used for billboard printing—contain solvents with low flashpoints. Flashpoints determine the degree of danger from explosion from ink vapors. Solvent inks provide the best adhesion to vinyl but present a significant safety hazard. Some can cause skin irritation as well. Again, consult the MSDS for the particular inks you are considering. Don't simply take the word of a salesperson promoting the printer or ink because they will usually downplay the hazards.
Eco-solvent (or mild solvent) inks were introduced in an effort by ink chemists to avoid as many of these hazards as feasible without compromising performance significantly. For eco-solvent inks, chemists chose to use solvents with higher flashpoints, which makes drying more difficult. They also selected solvents with less of odor and that eliminated aggressive co-solvents, which somewhat compromised adhesion and durability (compared to true solvents).
Some printer manufacturers may suggest that, because of the low odor you do not need to ventilate the printer using eco-solvent inks. Remember, even if you can't smell it, the operator of the printer is still breathing the solvents evaporating from the printing process and ink aerosols (very tiny ink droplets are always present during printing).
Another consideration with all solvent inks is the out-gassing of residual solvent from the print well after it is dry to the touch. Depending upon the solvents, this can pose a problem both in odor and health if the graphic is used indoors. Strong solvent inks pose the biggest problem, but all solvent inks exhibit this effect.
UV-Curable Inkjet Inks
UV-curable inks do chemically react. They will react with UV light exposure—or on your skin if the liquid ink is spilled. You must wear eye protection because you can have severe damage if ink gets into your eyes (not to mention the potential eye damage from powerful cure lamps).
Make no mistake—these inks are very reactive and contain hazardous chemicals and pose a hazard if not handled properly. Low odor UV inks are being promoted now by several printer manufacturers. This is principally either a reduction or elimination of co-solvents in the formula which are added to the ink and only dry over time, or low-odor monomers, oligomers or photo initiators.
After the ink is laid down, the UV cure lamp initiates crosslinking of the ink's oligomers and monomers, which instantly then become a solid three-dimensional matrix and hopefully provide a chemical and/or physical bond with the substrate being printed. If not cured completely, these unlinked reactive ingredients may remain in the print. Depending on the application this may not matter, but if a partially cured print is used in contact with people or used indoors they can be problematic.
Many UV printer and ink manufacturers claim that their inks react completely when used properly and pose no hazard. Smell the print! If you can smell the ink, it is out-gassing chemicals. This could be due to the substrate, it could be from small amounts of organic solvents used in the ink formulation to adjust the ink for the printheads—or it could be unreacted hazardous ingredients. Whichever the case you should understand the hazard and use the prints accordingly. Don’t forget to check the MSDS and take the precautions that they recommend.
All in all we have made a lot of progress in the design of inks and printing systems in ink jet, but always remember to read the MSDS for the inks so that you understand the hazards and take precautions to avoid hurting yourself, your employees, your customers and the environment.