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Ink Jet PigmentsThe Land of the Small

If you've ever had your ink jet-printed copy get wet, you saw all the inks became soluble and you had a mess on your hands. On the contrary, if you spilled water on this magazine, the printing inks would be quite water resistant. Although office printers are not the best examples of water-resistant jet inks, they do illustrate the difference in water resistance of dye-based jet inks compared to conventional pigmented inks.

Desire to improve water resistance and light-fastness of conventional dye-based jet inks has created a demand for more-resistant dyes as well as for the use of conventional colorants. Considerable effort has been put into developing these products with higher resistance, but this has been a challenging effort.

Dye-based jet inks utilize a variety of soluble dyes, but most commonly these are acid, reactive, and direct dyes. Acid dyes provide brilliant color but have limited light resistance. Direct dyes have similar brilliant color and better light resistance. Reactive dyes originally designed for textile printing can provide improved resistance to light and chemicals.

Some lightfast dyes can be produced from pigments. For instance, copper phthalocyanine pigment, which is lightfast and water resistant, can be modified chemically to make it soluble to produce complex dyes based on its chemistry. As such, its resistance properties fall between dyes and pigments.

Typically, two-year shelf life stability is expected for an ink jet formulation. Unlike dyes that are dissolved in the jet ink fluid, pigments must be dispersed. The dispersed pigment particles must be kept apart to prevent agglomeration and achieve the desired shelf life.

Similarly, carbon black pigments have been modified chemically to provide surface functionality to make stable water pigment dispersions. The particle size of these pigments is well below 1 micron. These dispersions have good lightfastness but lower water resistance than conventional black pigments.

Most pigments can be milled and dispersed well below 1 micron. However, high-density pigments can settle in low-viscosity jet ink formulations. Typically, jet inks contain about 3%-5% pigment. At submicron size, pigments at this level provide adequate color strength and hiding power.

The problem of particle size becomes evident in the jet ink filtration using a 1-micron absolute filter. If the average particle size is not below 0.5 microns, the filtration time becomes commercially impractical as the filter readily gets clogged and the filtration time extended.

Although shelf life often is determined by exposing the ink to freeze, thaw, and heat cycles, the rate of filtration is an excellent way to assess the ink particle size stability. For example, 300 ml of jet ink should pass through a filter in just a few minutes or less, and the filter essentially shows no pigment.

This same filtration time test also is quite sensitive to pigment agglomeration. The change in the flow rate due to agglomeration can be compared to the change in particle size distribution. As the ink agglomerates, the flow rate decreases through the filter, and the particle size distribution shifts noticeably.

In water-based jet inks (most drop-on-demand type inks), the pigment dispersions are stabilized by charge-charge repulsion. This can be increased through dispersants or resins that can increase the stability of the ink formulation.

Whereas conventional inks use pigments in the size range of microns, jet inks typically use finely dispersed pigments in the size of 0.1-0.3 microns. The chemistry of each pigment is quite different, so particle size and stability can vary.

Pigmented jet inks provide water resistance and lightfastness. Today, these inks are being used in more demanding packaging applications, but as the technology evolves, their use will become commonplace and displace many of the dye-based jet inks.

Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; rpodhajny@colorcon.com


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