Selecting Pigments for Optimal Performance
Meeting customers’ color demands has evolved from simply providing colors tinted in the factory, to providing any desired color tinted on demand at the point of sale (POS). This is accomplished with the use of a universal tinting system and color formulae taken either from preset formulae or a formula generated from an in-store computer color match. Commonly POS tinting systems use a limited number of tinting pastes, generally from 10–16, but most often 12. This limitation places increasing importance on pigment selection in formulating colorants used at the tinting stations. It is crucial that the optimal combination of pigments are selected to provide not only the largest color gamut possible, but also to provide the technical characteristics required to meet a variety of performance requirements.
Pigment SelectionThe choice of what pigments to use in a tinting paste would be a simple process if only earth tone colors were made, which could be made with inorganic pigments. But the consumer’s preference for bright shades, such as Alpine Pink, Golden Dome, or Sassy Yellow, requires the use of organic pigments. Organic pigments can provide the range of shades necessary to meet formulators’ color needs. There is also an equally wide variation in performance properties of individual pigments. Performance of a POS tinting system depends on both availability of pigments and which pigment is selected when several different pigment chemistries can provide similar hues.
One important characteristic of paint is its resistance to loss of color — “fading.” Any loss of color in paint is also perceived as a loss of performance of the paint film. As paint performance warranties are extended for longer periods, it becomes more important to select the most durable pigment for POS systems colorants.
Color loss can have several causes, including attacks from aggressive substrates such as stucco, strong cleaning solutions such as bleach, or environmental factors such as light and weather.
Determining the resistance of a pigment to various chemical agents is clearly definable, and the performance of a pigment can be identified by carefully designed laboratory tests. In contrast, evaluating the resistance to the effect of environmental factors (weathering) is more problematical. Factors such as solar radiation, temperature, moisture, coating PVC and composition all affect the performance of a coating and its weatherability.
The only method to accurately determine the performance of a colorant is to tint the coating, apply to the surface in question and expose it at the site of its intended use for the warranty period. This is usually an impractical test in terms of time and cost, and may not be useful in predicting performance in other coatings or at other locations.
The general practice is to conduct either limited time exposure at commonly accepted exposure sites or to use various accelerated exposure apparatuses to simulate the UV component of solar radiation. The advantage of accelerated exposure devices is a constant radiation source independent of local weather conditions and the day/night cycle. While the test conditions are well defined, correlation to actual exposure is not linear. The result is that the final coating performance must be deduced from test data and general rules of thumb as to which test period is equal to real-time exposure. The common question is: How many hours equal one year of exposure?
Most test methods can clearly identify those pigments that fall in the poor performance-rating region. The time period used for accelerated evaluations typically ends at 1,500–2,000 hours, and this will differentiate among those pigments considered to be good performers. The crucial element is to identify those pigments that will provide superior performance. The selection process must be done by predication rather than direct measurement.
Decision CriteriaSeveral choices are available about which decision criteria to use in selecting test pigments or formulations (see Table 1). Adopting methods used in failure analysis, several criteria were found that can be applied when making a decision among candidates.
The first decision criterion is more appropriately applied to mechanical devices that have a definite failure mode — or, more simply, it breaks. The second criterion is useful when the degree of acceptable change is clearly defined, and the test method has good correlation to real-time performance. This criterion does have merit when evaluating lightfastness and can be used to select between different pigments. The parameter change model while useful has no predictive power. The third criterion, rate of change, does provide insight into how rapidly a particular composition will change. It can be used in predicting performance beyond the test period and in selecting closely performing pigments.
Analysis of FadeFading is actually a process in which ultraviolet energy changes the chemical composition of a pigment to a compound with poor or no color-producing properties. The effect is not the result of a loss of pigment concentration, but non-linear chemical processes in which the chromophores that provide a pigment its color chromophores are degraded, and its original color is altered. Figure 1 compares the change in chroma resulting from changes in concentration and from the fading process. This plot compares the non-linear change of the fading process to the linear effect resulting from concentration changes.
When evaluating changes in color, several tools are available to quantify the observed change, including visual rating scales and color difference measurements. Rating scales are more aptly applied to exposure programs where surface defects such as cracking make instrumental analysis difficult. The integrating ability of the human mind can compensate for surface imperfections and provide meaningful, if subjective measures of change.
Color difference measurements are useful in that they provide a means to quantify change in a manner that can be analyzed mathematically. When CIElab* measurements are applied to fading studies, there is the temptation to use (delta)E as the evaluation parameter. While (delta)E has, as a single measure of change, a certain attraction for use in defining color change, its components can lead to a false assessment of change particularly when darkening occurs. Measuring the change in chroma (DC) more accurately describes the changes seen visually in fading/weathering studies and is a more useful measurement.
Usually when an exposure panel is evaluated, the first change noticed is a loss of brightness (chroma), which may be accompanied in some instances by a change in hue. Normally, these changes can be quantified by measuring the degree of change in chroma and hue angle. In those instances when a pigment darkens on exposure, the change in hue angle (DH) provides an estimate of this phenomenon by defining the change in hue that results from darkening.
Figure 2 shows plots of the change in chroma of several yellow colorants. The absolute value (ABS) of DC was plotted rather than the measured value to make visual interpretation easier and to facilitate curve fitting. Using these plots, the pigments of obviously poorer fastness YP11, YP3, YP12 and YP10 can be eliminated. Two pigments give similar performance: YP2 and YP6. How to decide between these two is the question. One selection tool that can be used is the rate of change of the fitted curves.
Rate of Change ModelPlotting the change over time and then fitting the data with a trend line will provide a means to select between similarly performing pairs. Using the curve fitting parameters one can estimate the rate of change between pigments and thereby estimate future performance.
The curves of yellows YP2 and YP6 are apparently converging, and in order to select one over the other, one must project where values are anticipated to extend in the future. In this instance, the curves are best fitted with a logarithmic curve. Evaluating the exponents of the fitted curves as the rate of change, it can be predicted how rapidly each of the pigments is changing, and select the one changing at the lowest rate. In this instance, the sample YP6 is not only the lowest in absolute terms, it is also changing at the lower rate (see Table 2).
The rate-of-change model is a useful selection tool for selecting pigments. When it is coupled with an understanding of an acceptable magnitude change of DC, better decisions can be made in providing the maximum color gamut and fastness in a color system.
Future of Color SystemsDelivering the desired color has changed from factory-tinted paint to POS-tinted paint. It is increasingly linked to a specific assortment of colorants, which, in many instances, is specified by the retailer rather than the coating manufacturer. The increasing use of POS-tinting systems worldwide places the responsibility for color permanence in the hands of the color system designers rather than the coatings manufacturer. The formulation of the tinting pastes is a crucial element in performance and customer satisfaction.
The next stage in POS systems will be to provide collections of colors to select from and a personalized POS display customized to each manufacturer’s or retailer’s preference. Additionally, to meet both the need of the general trade and the requirement of a special higher performance applications system, colorants will be available in both standard and enhanced performance grades to meet the quality and cost profile of the coating manufacturers and retailers.