Jadel Baptista, Technical Director, LANSCO COLORS -Technical Center- pigments.com
Introduction
Pigments are designed and produced to be insoluble particles
used to impart color in a variety of materials. They can be
classified as organic, inorganic, effect, and functional. The diverse range of pigment chemistries, the end use requirements,
and the broad range of colors available create a challenge for
chemists in selecting the optimal pigment or additive for a
given application. Proper pigment and additive selection is
critical in determining the performance of the finished coating
as well as the production efficiency. Understanding the various
pigment types, properties and chemical families will help the
formulator in determining the most effective pigment chemistry and manufacturing processing, thus avoiding potential
problems with the final coating and application. This paper focuses on pigment families and their application in waterborne,
solventborne and universal dispersions. The theory behind
processing, selection of wetting and dispersing agents, and the
differences between co-grinding and single pigment dispersion
are discussed.
Pigments vs. Dyes – Is Solubility
Important?
Pigments are inorganic or organic colorants that are virtually
insoluble in the application medium while dyes are colorants
that are soluble in the application medium. Understanding
this difference is the most important concept to help with
the correct colorant choice. Dyes are organic molecules with
brighter undertone, higher transparency, and they generally
demonstrate inferior fastness to UV exposure versus pigments.
Organic pigments vary regarding their chemical structure and
surface treatment; some can behave like a dye, losing their
crystal structure when exposed to solubilizing conditions.
This structural change will negatively impact the pigments
fastness properties. A good example of this is Pigment Yellow
74, widely used in architectural paints. This pigment will have
better fastness properties when used in the mentioned applica-
tion, than in most industrial paint applications. The difference
in performance is due to the solvents used in these systems.
Solvent-based architectural paints, are based on acrylic and/or
medium/long oil alkyd resins, and the preferred solvent used in
this application is mineral spirits. In waterborne applications,
acrylic emulsions or water-soluble acrylics are used. Pigment
Yellow 74 is slightly soluble in mineral spirits but is not sol-
uble in water (Figure 1), therefore it will maintain its crystal
structure. However, in industrial coatings applications, where
the most commonly used solvents are aromatics, esters and
ketones, Yellow 74 will have higher solubility which will have
a negative influence on its fastness properties such as heat sta-
bility, weathering, overspray migration, and opacity. Choosing
the correct pigment for the desired application should be the
first step during the coatings formulation process in order to
formulate a stable system. Formulators are advised to consult
their pigment suppliers in order to obtain the appropriate pig-
ment solubility information.
In addition to solubility, resistance to acids and alkalis, heat
stability and weatherfastness/lightfastness, should be considered
in order to formulate a stable system that meets end use requirements. Figure 2 shows some important points to be considered
during the initial formulating stage. The formulator should always keep in mind that the same pigment chemistry may not
perform equally in all coatings systems.
Organics vs. Inorganics – Are they
different?
Organic pigments are synthetic materials based on carbon usually derived from petrochemicals. They generally are not stable
at elevated temperatures and have partial solubility in strong
solvents, but do not dissolve in water. Inorganic pigments are
metal salts and oxides, some natural and some synthetic, that
generally are stable at elevated temperatures and do not dissolve
Starting from Scratch.
Four Steps to Effective
Pigment Dispersions
