Avoid corrections! Completely!
ISO 787-24 describes color strength as the ability of a pigment to absorb incoming light and thereby have the ability to color or darken, for example, a white color into which it is incorporated.
In practice, for a colored pigment, a batch of paste is mixed with a standard white in a defined ratio, applied and measured. A subsequent batch is processed in the same defined way. The spectra are then compared. So far, so good, if it weren’t for various basic conditions and influencing factors that make life difficult in practice.
On the one hand, there are the application methods:
- Draw down
- Spraying
- Liquid color measurement, static
- Liquid color measurement, dynamic
Problematic with the first two methods are the hiding power and gloss level and, from a process engineering point of view, the drying times. In static liquid coating color measurement, when the material is at rest during the measurement, adhesion or anti-drying effects can influence the measurement results. In all these methods, flocculation effects are common concomitants that also influence the results. In dynamic liquid paint color measurement, these effects do not have a major influence, but since it is a non-contact measurement of material in motion, the measurement distance and shear must be kept constant during the measurement. At ORONTEC, we have developed two measuring devices that enable the non-contact measurement of liquid films in a reproducible and accurate manner.
On the other hand, there are also different calculation methods:
- Comparison of the value in the spectrum at maximum absorption.
- Integral comparison of the area under the spectrum without weighting
- Integral comparison of the area under the spectrum with weighting
Starting from exactly same spectrum, the differences in the results can be very different:
The example shows that not all methods deliver good results for every course of the spectra.
While similar values are displayed for the brightening power and the color strength of the “black”, the integral methods deliver very different values. Especially for the yellow paste, the deviation is very considerable. By means of mixing tests, it was confirmed that the weighted algorithm of ORONTEC delivers exactly the values that resulted in a small acceptable deviation. Completely without taking into account the color strength factors, a readjustment of the example color tone with the second set of mixing components had a deviation of dE* = 2.1. With taking into account the color strength factors of the weighted algorithm, The color difference is dE*=0.2 only.
A method to quantify the quality of the color strength determination will be content of the next blog post.
By definition, the color strength correlates directly with the concentration of a colorant. If the color strength is determined inaccurately or completely incorrectly, the result will be exactly the corrections that constitute its “hidden factory”. The method therefore has a direct influence on the commercial success of your production and your company. Anyone who measures color strength incorrectly or uses incorrectly measured values is giving money away.
In the next article, we will present a simple procedure for finding out how good your color strength method is.
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