What are pigment dyes

Properties of dyes and pigments

by Ansgar Wessendorf,

Colorant is the collective name for all coloring substances, which are divided into two groups: soluble (dyes) and insoluble colorants (pigments). Both have advantages and disadvantages in terms of lightfastness, durability and migration potential.

by Bernd Miller

 

content

Pigments

Dyes

Fanals and other semi-soluble dyes

Advantages and disadvantages of pigments and dyes

Conclusion

 

Pigments are superior to dyes in the manufacture of printing inks. Almost all printing inks are therefore based on pigments, while the soluble dyes are used, for example, to color textiles, leather or food.

The Color Index (C.I. for short) has been an official reference work for all common colorants since 1925. It contains information about the chemical structure and common names of colorants as well as their synthesis, reaction behavior, solubility and patent literature. The Color Index is published by the British Society of Dyers and Colourists and the American Association of Chemists and Colorists.

 

Pigments

In contrast to the dyes, which are soluble, the pigments are insoluble particles. A distinction is made between organic and inorganic pigments. A pigment-based printing ink is therefore a suspension of solids in a liquid.

When the paint with these pigment particles dries or hardens, the pigment particles are enclosed by the binder and held in place. Because the pigment particles are relatively large, they cannot move in the dried paint film, which usually prevents migration.

Inorganic pigments are used when the structure of the pigment does not contain any hydrocarbon compounds. Inorganic pigments are mostly mineral products, such as titanium dioxide, by far the most widespread white pigment. The inorganic pigments also include all iridescent pigments made up of mica or silicon dioxide.

Organic pigments are different chemical compounds whose basic structure is based on hydrocarbon compounds. Examples are Pigment Yellow 13 and Pigment Blue 15: 3, both of which are widely used for yellow and cyan in four-color printing.

When manufacturing pigment colors, the fineness of the pigments is an important point. In order for a pigment in a printing ink to be able to bring out its full color strength, it must on the one hand be completely enclosed and wetted by the binder system (e.g. UV acrylate binder) and on the other hand have the required fineness (= grain size of the particles). A particle size of 0.5 to 1 μm is usually aimed for - in any case, the particles must not be larger than 10 μm. If the pigments are not fine enough (> 10 μm), printing problems can quickly occur.

Good wetting of the pigment is achieved by “making a paste” of the dry pigment in the binder and grinding it either using a ball mill or a 3-roller mill. To speed up the process, a dispersant can be added to the pigment paste, which ensures faster and better wetting of the pigment.

 

Dyes

Dyes are chemical compounds which have the property of coloring other materials and which are present in solution in the application medium. It is not a dispersion like the pigments, but a so-called real solution of the dye in the liquid application medium, which consists of a solvent, an aqueous solution or a binder.

The dye molecules are mostly of an organic nature and have a simple chemical structure. They move freely as monomolecular particles in the solvent or binder. When the paint dries with these dye particles, the particles are enclosed by the binder and held in place, but because they consist of individual small molecules and the binder molecules are also small, the dye particles can remain to a certain extent despite the dried paint film Move measure - that is, migrate. The synthetically produced group of low-molecular azo dyes, known for their strong colors, is an example of the dyes category.

 

Fanals and other semi-soluble dyes

The term Fanal in connection with pigments is actually a protected name for a pigment product range from BASF in Ludwigshafen. The name Fanal pigments is often used for all semi-soluble pigments, even if these products do not come from BASF. Fanal pigments are pigments that completely or partially dissolve in a color and show the properties of dyes. A widely used luminescent agent - an optical brightener - is mentioned here as an example. This substance largely dissolves in the paint system and tends to migrate in the film due to the very small particle size.

 

Advantages and disadvantages of pigments and dyes

Resistance: Pigments are generally more resistant than dyes. Resistance is tested with various test media, such as acids, bases, fats, oils, solvent mixtures or hydrogen peroxide, provided that the binder system itself (UV binder, aqueous solution, solvent) can withstand the test medium. The reason for the better resistance of pigments is that their chemical structure is usually more resistant than that of dyes. However, the higher resistance simply depends on the fact that the pigments are particles that are embedded in the binder system of the paint and cannot move freely. The statements about the pigment resistance apply as a rough rule of thumb, which is not always correct in certain individual cases. It is therefore advisable to check the specific application beforehand.

 

bleed out

In the case of dyes and fanals, due to the mobility of the dye particles in the film, there is a risk that individual particles migrate into the environment. Extended periods of time and high temperatures encourage this migration. Typical cases of bleeding are visible discoloration on the back of the print carrier (set-off), but also bleeding into overprint varnishes or into the print carrier itself with direct contact with the back in the stack or in the roll.

Warning: if a dye bleeds, it will also migrate. This means that in the case of dyes that bleed out, you must also comply with the foodstuffs regulations, such as those that apply to photoinitiators and binders.

 

Migration potential

Unfortunately, there are printing ink manufacturers who still pay too little attention to the migration potential - especially when the printing inks are used for food packaging. No dyes or fanals should be used in the production of such printing inks, unless compliance with the migration limit values ​​is carefully monitored. Even in the manufacture of pigment colors, the necessary caution is not yet used everywhere. The relative size of the particles in pigment paints usually prevents migration, but not in every case. Especially with pigment-based inks for printing on food packaging, it is one of the manufacturer's obligations to ensure that the migration limits can be met with them.

In addition to the binders and additives (e.g. photo initiators), certain residual compounds from the manufacturing process of the pigments used can also migrate: e.g. aromatic compounds from cheap carbon blacks or cheap violet-23 pigments and aromatic amines from azo pigments, such as numerous yellow or orange colors from Asian Delivery.

 

Lightfastness

Because inorganic pigments do not contain any thermally labile hydrocarbon compounds, their lightfastness is usually very good. In the case of organic pigments, the light stability strongly depends on the pigment in question; the lightfastness (according to the wool scale) ranges from poor (level 1 to 3) to very good (level 6 to 8).
In the case of dyes and fanals, the lightfastness is generally poor and hardly ever reaches a 2 on the wool scale. Fanale Reflex Blue, for example, has lightfastness 2; the real pigment-based Reflex Blue achieves level 7 on the wool scale.

 

Color strength and purity

The color strength is usually better with dyes and fanals than with pigments. The reason for this is that all color molecules are dissolved and participate in the "coloring". In the case of pigments, so-called shadow effects result, which leads to a decrease in color strength. Dyes and fanals are usually also purer and brighter than pigments. A classic example is the reflex blue: With a pigment color mixture of Pigment Violet 23 and Pigment Blue 15: 3 you will never achieve the color strength and purity of a fantastic reflex blue.

 

Conclusion

Which color system is chosen for the specific application must be clarified on a case-by-case basis. Basically, it is advisable to avoid the use of dyes and beacons, because the negative properties predominate when printing inks.

 

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