- May 01, 2001, Richard M. Podhajny, Ph.D., Contributing Editor
The rapid growth of ink jet technology in various printing sectors forecasts their use in package printing. However, use of ink jet printing in packaging requires product resistance and press speed. Unlike colorants used in other ink jet applications, colorants used in packaging inks require that end-use and product safety concerns be considered in the selection of colorants.
Although ink jet printing was developed to support the high-speed IBM computers of the 1960s, it soon found applications in packaging. By the early 1970s, continuous ink jet (CIJ) printing technology was being used for printing information on beer and beverage cans. This technology utilizes a method of charging ink droplets and deflecting them in flight to create characters. Today, CIJ is used in many applications, including labels, textiles, addressing, forms, and industrial marking.
CIJ used in packaging usually utilizes solvent-based dyes; the solvents are fast drying, such as alcohols or MEK. Viscosity requirements are very low, and nozzle diameters tend to be just a few microns.
Piezo-electric and thermal bubble drop-on-demand (DOD) jet inks dominate desktop office printing, wide format, and postal addressing. Piezo DOD inks also are used in textile and some packaging applications.
Thermal DOD jet printers use a resistive element to create the “bubble” by generating heat, and they require thermally resistive colorants. Viscosity requirements are more liberal, and the drying requirements are somewhat lower than those in CIJ.
Although ink jet printing has been very successful in many applications, its acceptance in packaging has been slow. For lightfastness and resistance to abrasion and chemicals, ink jet formulators have turned to conventional pigments. Pigments, unlike dyes, are colorants that are insoluble in water and most solvents. Dyes typically are salts of sulfonic or carboxylic acids, making them soluble. Since they are soluble, they are ideal for jet printing applications, but they do not provide the resistance characteristics that are desired for packaging applications.
Pigments have substantially superior lightfast properties and chemical resistance compared to dyes. However, their insolubility and particle size instability make them difficult to use in ink jet printing.
Commercial use of pigments in jet inks goes back at least a decade. Use of pigments in jet inks developed in Japan, but it has moved to many applications in the US. While the use of carbon black and titanium dioxide is commonplace, use of other pigments has been limited, although efforts to develop the pigment technology are accelerating.
Whereas dyes are soluble, pigments are insoluble and must be dispersed. This creates two key problems: One is the need to reduce the pigment particle to a size that is submicron, and the other is the need to stabilize the dispersion and limit re-ogglomoration. Pigment particles need to be less than 1 micron for DOD applications and less than 0.5 microns for CIJ.
One approach to making pigments more stable is to modify their chemistry. Recently, a process was described that modifies the carbon black surface (US Patent #5,803,959) with polar functional groups that provide the pigment with indefinite stability in water and particle size in the range of 0.15 microns. The downside of this approach is that the pigments lose some of their water-resistance properties.
Many packaging products require that the ink ingredients be FDA compliant, necessitating careful selection of ink components.
Until recently, jet ink manufacturers were usually companies such as DuPont, Epson, Cannon, etc. Today, packaging ink suppliers, such as Flint Ink Co., have devoted resources to develop and supply jet inks to their packaging printer customers.
Use of jet ink technology in packaging is growing slowly. The degree and rate of penetration will depend on its ability to meet product resistance requirements.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314, e-mail: firstname.lastname@example.org.