- November 01, 2001, Richard M. Podhajny, Ph.D., Contributing Editor
I have received various inquiries about pigments and pigment chemistry. I will try to answer a few of these questions as they relate to package printing and converting.
What is the difference between pigments and dyes?
The primary difference between these colorants is their solubility. Pigments used in printing inks are colorants that are insoluble in water and most solvents. For instance, copper phthalocyanine blue is the primary blue colorant used in packaging and is quite insoluble in water and organic solvents. As the name indicates, it is a blue pigment that includes copper. The rest of its structure is a highly complex aromatic structure of benzene rings that include nitrogen atoms.
Unlike pigments, the number of dye colorants is seemingly infinite. Dyes are organic colorants that are soluble, and they are commonly used in the textile industry and in office products, such as ink jet printers. In packaging applications, dyes do not have the product resistance of pigments. Dyes provide brilliant color and are used in products that do not require long-term resistance. Most have inferior solvent bleed and oxidation resistance.
Many dyes are chemically acids or bases and usually are found in the form of salts. For example, sodium salts of sulfonic or carboxylic acid groups are used to solubilize certain dyes. Dyes used in printing paper can be made insoluble chemically by reacting with components in the paper. For instance, presence of calcium ion or tetra-alkyl ammonium ion will make the dye insoluble and “cement” it to the substrate.
The basic difference between dyes and pigments is their solubility. Whereas pigments are insoluble and must be used as dispersions in inks and coatings, dyes are soluble colorants.
Do organic pigments contain chlorinated aromatics?
The copper phthalocyanine chemical structure is also part of the green copper phthalocyanine pigments, which are chlorinated versions of copper phthalocyanine. These organic green pigments have similar resistance properties to copper phthalocyanine blue and are used extensively in package printing.
Some yellow pigments used in printing inks are based on diaryllide yellows. These azo pigments (pigments that have an N=N double bond) are made from intermediates that are based on dichlorobenzidine. These pigments are vulnerable to very high temperatures and/or chemical degradation. Although such chemical reactions are possible, normal printing and converting operations do not create such chemical or thermal conditions.
Can pigments be a source of cyanide?
Certain pigments used in printing inks have chemical structures that utilize cyanide complexes. They are iron pigments based on ferro and ferric cyanides, such as potash blue or soda blue pigments. Under conditions that decompose the pigment, cyanide can be liberated. Some tests conducted to determine free cyanide in water decompose these pigments in the test protocol and find “free cyanide.” Pigments that may test positive for “free cyanide” under some test protocols include CI Pigment Blue 27, CI Pigment Red 169, as well as PMTA green and violet pigments.
Can certain pigments produce aromatic amines?
This question concerns aromatic amines in packaging products. Like chlorinated aromatics, aromatic amines are a concern due to their potential carcinogenic properties as a chemical class. Many pigments used in printing inks contain nitrogen in their chemical structure. Nitrogen-based pigments contain nitrogen within the pigment skeletal structure, but some may contain secondary and tertiary amine functional groups on the skeletal exterior.
The question of generating amines is likely associated with recent concerns over the potential reactivity of organic azo pigments to strong acids that can break down these pigments into aromatic amines. Azo-based pigments include most yellow and red packaging pigments. Although such chemical reactions are possible, normal printing and converting operations do not create such chemical conditions.
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.