Coatings World - March 2016

Christine Louis, Mike Peck and Jim Reader, Air Products

The introduction of Point-of-Sale Tinting systems in the 1950s changed forever the way that decorative paints are sold. Prior to World War II and during the immediate post-war period, paints were supplied in a limited range of colors and retailers had to stock a large range of ready-made colors to meet customer demand. The arrival of tinting systems led to dramatic changes; all retailers needed were base paints and a tinting machine to supply an immense range of colors, while inventory and production for both the supplier and paint store was reduced. Most countries and paint producers quickly adapted to this change and soon this became the standard way of supplying decorative paints.1

Paint tinting systems are designed to deliver color to achieve the exact shade desired by the end customer. The tinting systems include colorants, dispensing and mixing equipment, software and a color formula database matched to the color marketing tools.2 The selected colors are prepared by accurately dosing the colorants into a base paint, followed by mixing to produce the finished color. In order to achieve maximum operational efficiency, paint producers will use universal colorants to tint a wide range of different base paint chemistries, carriers and formulations.

Most universal colorants have tradi-
tionally been based on glycols to ensure
compatibility with different carriers, and
contain the appropriate pigments, fill-
ers, dispersants, thickeners and other
additives. 3, 4 However, recent changes
in VOC regulations, such as the South
Coast Air Quality Management District
(SCAQMD) requiring colorants for archi-
tectural coatings to have a VOC of 50g/L
or less, have led to many colorants being
reformulated with much lower levels of
glycol and co-solvent. While these may
be identical in terms of color, these low-
VOC colorants may give different perfor-
mance in both dispensing machines and
the base paints.

An additional challenge is that many paint producers do not manufacture their own colorants, buying them from another supplier or different division of the company. Therefore, changes in colorant will often require formulation changes in the base paint. The higher level of water present in these low-VOC colorants can affect properties such as gloss, dry time and other application properties, as well as consistency in achieving the intended final color. A recent customer survey conducted by Air Products in Europe found that issues related to colorant compatibility and color development are some of the top concerns of architectural paint formulators and certainly some of the most common and difficult problems to solve.

Color and Color
Development

The term “color development” describes the degree of color quality and color uniformity of a tinted paint. Good color development implies that the color appears uniform and of the expected strength.

Color development in a tinted base paint is
dependent on both the stability of the pig-
ments and fillers dispersed in both the base
paint and colorant. When the colorant and
paint are mixed, the additives present in
both can re-equilibrate among all the
pigments and fillers present. Because many
base paints are under-stabilized, this often
means that some of the additives needed
to stabilize the colorant pigment are re-
distributed away from the color pigment
to pigments and fillers in the base paint.
This leads to the loss of pigment stabili-
zation and flocculation of the colorant or
paint pigments leading to an immediate or
gradual change in color (Figure 1).

Pigment dispersion formulation, in both base paints and colorants, can be enhanced by using surface active agents to wet, disperse and stabilize solid particles or pigments in the medium, as well as to provide application performance. Typically, formulations contain at least two, and often three or more, surface active components that are combined to provide optimal properties. These materials include classical polymeric and oligomeric dispersants as well as lower-molecular-weight surfactants. These surfactants can also help to improve the compatibility of the dispersion for letdown into other systems and minimize shock and color acceptance when tinted. It is thought that these surfactants work by migrating to the surfaces created when the dispersing additives re-distribute upon mixing, thus preventing the destabilization of the pigments (Figure 2).

Nonionic Surfactants as
Compatibilizers

Nonionic surfactants are widely used in coatings as wetting agents, emulsifiers and co-dispersants. 4 The structure of a simple nonionic surfactant is shown in Figure 3, where a hydrophilic head group is bonded to a hydrophobic (lipophilic)