Coatings World - March 2016

Surfactant Influence on Colorant Acceptance
March 2016 www.coatingsworld.com Coatings World | 89

tail; an ethoxylated alcohol is a commonly used nonionic surfactant of this type. The hydrophobe is commonly an aliphatic hydrocarbon derived from oil or natural sources, although oil-derived alkylphenol-containing head groups have been used historically. The behavior of these surfactants is dependent upon many factors, but, in particular, the balance between the hydrophilic head and hydrophobic tail (the hydrophile – lipophile balance or HLB) is known to influence the behavior of these surfactants in many ways. 4, 5

A series of nonionic surfactants were tested to see if they could enhance the color acceptance of different colorants when let down into a water-based acrylic base paint and a solvent-based alkyd paint. The surfactants were post-added to the paints at 1.0% (w/w) dosage and then tested with the most challenging colorants. Based on customer feedback, colorants with the color indexes PV 23, PBk 7 and PB 15: 3 are considered among the most difficult in terms of color acceptance when used to tint either water-based or solvent-based paints.

The influence of the additives on color compatibility and color strength was measured by applying the tinted paints by drawdown onto Leneta charts after 2 and 10 minutes of shaking. A rub-out was performed on each of the drawdowns, and the color difference between the rubbed and un-rubbed parts was measured as well. The color was measured using an X-Rite 939 Spectrodensitometer D65/10 and recorded as a Delta E value. The formulation without post-added surfactants was used as the reference in each case and its color strength was set as 100%.

Poor colorant compatibility may be-
come apparent either through the slow
development of color or loss of color
through flocculation of either the white
pigment in the base paint or the colorant
pigments. Flocculation can be detected
quite easily by the rub-up test, as the
action of the rubbing re-disperses the
pigments leading to a change in color be-
tween the rubbed and unrubbed areas if
flocculation has occurred.

The best result with the PV 23 colorant was obtained with an alkoxylated acetylenic diol-based surfactant (Figure 4). These surfactants are known for their ability to quickly wet many different hydrophobic surfaces, such as organic pigments. It is also a low

Figure 3: Simplified structure of a nonionic surfactant.

Figure 1: Simple mechanism of colorant destabilization: (a) Colorant pigment (orange) and base paint pigment (white) are mixed; (b) Dispersing additives on colorant pigment redistribute to base paint pigment; (c) Colorant pigments flocculate leading to loss of color strength. Symbols represent different polymeric dispersants.

Figure 2: Simple mechanism of colorant destabilization: (a) Colorant pigment (orange) and base paint pigment (white) are mixed; (b) Dispersing additives on colorant pigment re-distribute to base paint pigment;