- May 01, 2003, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
Printing inks are a complex recipe of ingredients, of which the most obvious are colorants. Packaging product-resistance requirements necessitate the colorants be very resistant to a variety of conditions.
To this end, inorganic and organic pigments are used in package printing inks. Pigments, as raw materials, are not suitable for printing inks until they are reduced to a fine particle size to optimize color and performance characteristics.
Pigments are reduced in particle size as the inks are processed. Commercially, this can involve various processes from mixers, chip dispersion, ball milling, flushing, three-roll milling, sand mills, or vertical as well as horizontal bead mills. Several of these processes can be combined to provide the necessary production output and quality needed for specific inks.
It's a challenge to produce a laboratory ink formula that has the same level of dispersion you will obtain from production equipment. Ink laboratories typically are equipped with some of the following lab dispersing equipment: a lab mixer or blender, ball mills, a small three-roll mill, and horizontal bead mills.
Shot media can be placed in a container with the ink recipe and on a paint shaker for a specified period. These attempts work in some cases, but they won't deliver the high-velocity rotation and dispersion quality.
Lab mixers come in different shapes and sizes. For dispersion of small pigment samples, one key requirement for a lab mixer is the speed of the propeller blade. Although variable speed is desirable, for dispersion a blade speed of more than 7,000 rpm is preferred with a high-speed, shear-dispersing blade.
These types of dispersing mixers are useful for a variety of applications but have limitations with higher-viscosity formulations and certain pigments.
Higher viscosities and harder pigments are handled more effectively using either a three-roll or horizontal mill. Both mills typically are equipped with water-cooling and temperature gauges.
Horizontal bead mills suitable for lab work come in bead chamber sizes from 50 ml to several liters. The beads used in these mills typically are in the range of 1-2 mm. Some of these mills can be recirculating so that samples can be taken out as a function of time and the color and particle size analyzed. You can achieve good correlation to production equipment.
In a typical test, the pigment is introduced into the dispersion formula, and the color strength is determined after a specified time in the dispersion unit. For a more accurate result, make several color strength evaluations and plot them as a function of time. Normally, this leads to an extrapolation that is the maximum color level achievable for that pigment.
At the same time color strength is determined, particle size is compared using an NPIRI gauge and compared over the same period of the dispersion color evaluation. As the color strength goes up, the particle size goes down. Typically, the dispersion formula will have pigments ranging from 3-7 microns and will be dispersed to submicron level. The NPIRI gauge is a metal block that has two wedge-like troughs that are machined accurately from a maximum depth of 25 microns to zero.
Like everything else, you cannot make a valid comparison between two pigments if they are not dispersed to the same level. Much of the effort that goes into producing a commercial ink goes into milling and reducing the pigment particle size to produce the desired color.
Lab dispersion equipment is not standardized in our industry, and various testing units are in use. The best lab equipment choice is the dispersing unit that correlates best to your production equipment.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; firstname.lastname@example.org.