Лако-красочные материалы — производство

1.3.2.1

General

The instruments used for color measurements are nowadays spectrophotometers de­termining the reflectance of a sample. Three-filter colorimeters, trying to mimic the spectral response of the human eye are now next to obsolete. For applications in the field of uni-pigments photometer illuminating/viewing geometries are standardized as methods A and B, and are designed to suit the individual application (see Section 1.4.2). For standards, see Table 1.1 (“Color Differences, Conditions/ Evaluation…”).

Method A. The geometry d/8 (diffuse illumination, viewing from an angle of 8°) or 8/d (illumination from an angle of 8°, diffuse viewing), including specular reflection,

enables total surface reflection and reflection from the interior of a sample to be measured. An amount representing the surface reflection has to be subtracted from the measured value. Thus, measured color variations can be ascribed to differences or changes of the colorants in the interior of the sample.

Method B. Here, color differences in samples are evaluated in almost the same way as in visual evaluation by exclusion of gloss effects. Suitable geometries are d/8, d/0, 8/d, and 0/d with a gloss trap, and 45/0 and 0/45.

After the color of the sample and reference pigments has been measured, color differences are usually calculated by transformation of the X, Y, and Z values into the CIELAB system to calculate color differences. Color measurement results of black and white pigments can be expressed more simply because they only amount to a determination of the relative color undertone. For this, the environment of the reference pigment is divided into eight sectors, these being filled with color names from “red” to “violet”. The octant in which the CIELAB color position of the sample is located is found by calculation.

When colored or black systems are reduced with white pigments, an undertone is observed, which is a particle-size effect of the white pigment (see Section 1.3.1). These undertones can be conveniently expressed as CIELAB color differences. The effects can, however, also be measured by using the difference Rz-Rx between the values obtained with the blue and red reflectometer values. The undertone measured in this way depends on the lightness, and has a maximum at Y = 41.4. The lightness of a gray paste should therefore have this value to ensure that undertone differences between white pigments are comparable [1.44, 1.45].

Measured color differences are only true (i. e., significant) when they are not fal­sified by measuring errors. A significance test standard (see Table 1.1, “Color differ­ences, significance”) has been developed to check this [1.45]. The numerical value of a color difference must be higher than a critical value, which is statistically calculated using the standard deviation.

Problems concerning the acceptance (tolerance) of color differences (e. g., in pro­duction quality control or in computer color matching) should also be solved by mathematical statistics [1.22].

The gloss of pigmented coatings is not a true pigment property. The pigment can, however, influence the luster quality, mainly via its dispersing properties (see Section 1.5.2). The degree of gloss ofa coating can range from high gloss (specular reflection) to an ideally matt surface (complete scattering). Gloss haze is due to a disturbance of specular reflection: the reflected objects appear as if seen through a veil, this is caused by halation effects. Gloss retention is discussed in Section 1.4.1; a method of gloss measurement is described in Section 1.4.2. Special problems arise in the measurement of black [1.46] and fluorescent pigments [1.47].

1.3.2.2