A universal colorant is a concentrated dispersion of colour pigment that is used to tint a base paint. A line of well-designed universal colorants can be used for both water-based paints and solvent-based alkyd paints, most often for architectural applications. Modern universal colorants are solvent-free, water-based materials. The adjustment of colour of the architectural paints can be done either in the production plant (In-Plant tinting) or in the shops where customers buy their paints (Point-of-Sale tinting).
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The challenges of formulating universal colorants
It is already complicated to develop a robust line of water-based colorants that is only used to tint water-based paints. A wide range of different colour pigments must be stabilised by using as few as possible additives. Dispersants are polymeric additives used to provide stability against flocculation. Anchoring, strong adsorption of the dispersant molecules at the surface of the pigment particles, is a key hurdle to take for all types of colorants.
For universal colorants, there are additional problems to overcome.
First, the composition of the water-based colorants must be such that the stabilisation of the pigment particles is assured when the colorants are added to both water-based (WB) and solvent-based (SB) paints.
Most often steric stabilisation is used to provide repulsive forces between the solid particles. The dispersant molecules must contain water-soluble tails of sufficient length to assure steric stabilisation in systems that have an aqueous continuous phase, like the colorants themselves and the water-based base paints to which the colorants are added. Also, tails are needed that dissolve in the nonpolar continuous phase of solvent-based architectural alkyd paints, often based on de-aromated white spirit D40 or D60 as solvent.
Secondly, the water of the colorants must be homogeneously distributed in the solvent-based paint when the colorants are mixed with such a base paint. Upon mixing of paint and colorant, the water must be emulsified and stabilised within the solvent-based system.
Additives for universal colorants
Steric stabilisation in both aqueous and nonpolar media can be arranged by using different sorts of soluble tails, that are both part of the dispersant molecules.
In the colorant itself, the pigment particles repel each other because of steric stabilisation provided by water-soluble tails, possibly combined with electrostatic repulsion. This stabilisation mechanism stays intact when the colorant is added to a water-based paint. The hydrophobic tails are not soluble in the aqueous continuous phase. The hydrophobic tails are coiled when the pigment particles and dispersant are surrounded by water. When the universal colorant is added to nonpolar solvent-based base paint, the hydrophobic tails dissolve and stretch. Because of this, the tails provide steric stabilisation in the nonpolar continuous phase consisting of alkyd resin dissolved in organic solvent.
The water-soluble tails are not soluble in the nonpolar continuous phase of the solvent-based paint. Therefore, the water-soluble tails coil when the particles become surrounded by the nonpolar continuous phase of the solvent-based base paint.
It will come as no surprise that it requires skills to develop dispersants that comply with these criteria. Disperbyk®-2060, for inorganic pigments, and Disperbyk®-2061, for organic pigments, are polymeric dispersants designed to make universal colorants for architectural paints.
Preferably, the water of the universal colorants is emulsified to small droplets when the colorants are added to a solvent-based base paint. Several additives, often referred to as compatibilisers, can be used either in the colorants or in the base paints to accomplish this.1
The final products
Despite all the difficulties that must be overcome, high-quality universal colorants are available. Monicolor™ C is a line of solvent-free universal colorants designed to be compatible with both water-based and solvent-based architectural paints.
The development of fool-proof universal colorants has not yet ended. The quality of adsorption of dispersants at the surface of pigment particles must be improved, especially anchoring on the surface of organic pigments. Secondly, not all universal colorants can cope with the shock in polarity when the water-based colorants are added to solvent-based base paints. Finally, the emulsification of water, when colorant is added to solvent-based paints, is often not good enough.
- Settle Down: Factors that Influence Pigment Settling and Stability
- Mix It Up: High Speed Mixers for Paints, Inks & Coatings
- A Home for Architectural Coatings Resources
- Architectural Coatings: Paint Formulations and their Properties
1. White paper: Past and Future of Universal Colorants in Coatings, Frank Kleinsteinberg (Evonik), October 2017 [PDF]
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