k k
Chapter 7
Color Measurement and
k k 118 C O L O R I N G T H E C O S M E T I C W O R L D
The six basic principles of interest to us are as follows:
Selection and Validation of a Test Method: In order to eliminate apparent differences in results between testing laboratories, it is very important that they use a common test methodology. This principle is especially important when trying to compare results between the color additive manufacturer’s test facility and one belonging to the cosmetic producer who is evaluating the color manufacturer’s pigment prior to purchase. As with any type of experimental work, the fewer the variables involved in the process, the better the chances of success. Further, it is important to ensure that the test method is reproducible. This means that the same batch, when tested several times, will always produce the same results.
Establishment of a Standard: In order to objectively evaluate the accept- ability of batches of any given color additive, it is essential that they be compared to the same standard every time. Since the batches will vary slightly from one to another, the practice of adopting the last batch used in production would cause havoc in the evaluation process, akin to shoot- ing at a moving target. The standard established for evaluation of incoming receipts should be selected in consultation with their manufacturer. Color additive manufacturers have master standards against which they evaluate their productions, as well as acceptable tolerances on either side that allow them to approve or reject batches in a consistent manner. These tolerances, also called specifications, can be subjective (if the batches are judged by the human eye) or objective (if they are judged by a color computer).
The necessity of consulting the manufacturer of a color concerning stan- dard selection is illustrated in Figures 7.1 and 7.2. In Figure 7.1, a color manufacturer would approve both batch A and batch B for shipment. The two batches are both within the established tolerances, though they are on opposite sides of the standard. Figure 7.2 illustrates what happens if a cosmetic producer arbitrarily selects batch A as the standard and then receives batch B as a shipment. Even though both of these batches are within commercial tolerance of the color manufacturer’s standard, they are very far apart from each other. The cosmetic producer would perceive the difference as an inability on the part of the color manufacturer to pro- duce additives consistently, when, in reality, the problem is one of standard selection.
Preparation of the Standard: In order to save time and lab effort, it appears on the surface that it would be best to prepare a large dispersion of the standard in the appropriate system and use it to test numerous batches of color additives as they are received. Unfortunately, this practice
k k Color Measurement and Pigment Testing 119
Figure 7.1 Left: batch strength 95% of standard; Center: standard; Right: batch strength 105% of standard.
Source: Courtesy of Sun Chemical Corp.
will lead to problems and erroneous results in testing. The major reason for this is that the color additives will continue to “wet out” over time, as the vehicle will work its way into the spaces between the agglomerates, causing them to separate. The result is a greater surface area, which allows more light to be reflected, resulting in more color strength. The increase in strength of a standard dispersion of color over time will again provide the technician with a moving target at which to aim.
The other reason that the making of a large dispersion of a standard leads to problems is that it introduces new variables into the situation: the dis- persing equipment and the technician. Each piece of dispersing equipment (e.g., a Hoover Muller) will disperse color additives to different degrees, giving different results in terms of color values. If the standard and the sam- ple are made on two different pieces of equipment, one cannot be certain whether any color differences seen are a result of a real difference from the standard or only of differences in the dispersing equipment.
k k 120 C O L O R I N G T H E C O S M E T I C W O R L D
Figure 7.2 Left: batch strength 95% of standard; Right: batch strength 105% of standard.
Source: Courtesy of Sun Chemical Corp.
Likewise, using two different technicians to run the standard and sample might also lead to erroneous results, due to subtle differences in the way they perform the test procedure. Therefore, it is imperative to prepare the standard and the sample fresh each time, using the same equipment and the same technician. This will eliminate the possibility of color differences coming from inconsistencies in these two sources.
Evaluation in the Appropriate System: Color additives will not perform equally in different end use systems. For example, a high dye content D&C Red No. 6 Ba Lake will yield one result when used in nail lacquer and another when used in a blush. Colors of this nature are difficult to dis- perse, so they will appear weak and dull in powder applications because the dispersion process normally employed is not very rigorous. On the other hand, the nitrocellulose chipping process used in making dispersions for
k k Color Measurement and Pigment Testing 121
nail polish is very effective, so the high dye content colorant will appear bright and strong. With this type of difference in mind, it is necessary to test color additives in the same system in which they will be used in pro- duction. If this principle is not observed, a batch that tests “OK” in the laboratory evaluation may perform poorly in the factory; conversely, a per- fectly acceptable one may be rejected if the QC test does not really measure how it will work in production.
Evaluation at the Appropriate Use Level: As color additive perfor- mance is system-dependent, it is also affected by use level. Just as a color additive will exhibit different dispersion and color characteristics at disparate use levels, so an entirely different color profile will show itself when a pigment is used at 8–10% in a particular formulation than when it is used at 3–5%. Therefore, as with testing in a real system, it is also necessary to test color additives at the level at which they will be used in the actual production situation.
Maintenance of Constant Light Conditions: Have you ever observed how the color of your car looks different when it is illuminated by the sun than when it sits under yellowish sodium vapor lights in a parking lot at night? This phenomenon is called metamerism, which simply means that colored substances appear different in hue under different light conditions.
Cosmetic color additives are no exception, so, in order to ensure consis- tency of color test results, it is necessary to always use the same source of illumination when observing test displays. There is no right or wrong, good or bad light source; rather, it is all about consistency. To maximize consistency, it is advisable to procure a light box with several different artifi- cially generated types of light, including simulated daylight. By using such a device, it is possible to eliminate the vagaries of natural sunlight and the changes in spectral output of incandescent and fluorescent lightbulbs as they age.
Observance of these six principles will serve to minimize the possibil- ity of test results varying among laboratories. In addition, following them closely will prevent many of the color problems commonly encountered dur- ing the cosmetic product manufacturing process.