Method for determining the shade stability of lacquers

Optics: measuring and testing – By shade or color

Reexamination Certificate

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C356S405000

Reexamination Certificate

active

06628393

ABSTRACT:

This application claims priority under 35 U.S.C. Sec. 120 upon International PCT Application PCT/EP99/04496 and German Patent Application DE 198 33 859.7, filed Jul. 28, 1998.
The present invention relates to a method of determining the shade stability of coating materials and of optimizing a paint formulation in respect of the shade stability.
When paints and other coating compositions are used for industrial coating it is important to observe a shade, once predetermined, or a shade desired by the customer, with the maximum possible accuracy and uniformity, since even the slightest deviations in color are evident to the human eye. It is necessary in particular to ensure that color deviations arising through different film thicknesses of the coating do not reach an intolerable magnitude.
The assessment of a paint is based substantially on the physiological fact that the human eye contains three kinds of receptors having different spectral sensitivities, which roughly speaking can be assigned to the three primary colors red, green and blue (cf. Glasurit-Handbuch Lacke und Farben, Vincentz Verlag Hannover 1984, 11
th
edition, page 220ff.). In order to be able to determine objective data, statistical averaging has been used to specify standard distribution coefficients x(&lgr;), y(&lgr;) and z(&lgr;) for the three sensitivities of the receptors (CIE system, DIN 5033). By integrating an arbitrary color spectrum f(&lgr;) with these standard distribution coefficients it is possible to obtain what are known as the tristimulus values X=∫d&lgr;x(&lgr;)f(&lgr;), Y=∫d&lgr;y(&lgr;)f(&lgr;), Z=∫d&lgr;z(&lgr;)f(&lgr;), which represent objective values for characterizing the color spectrum f and may be illustrated as the extent of stimulation of the three types of receptor in the eye.
In order better to harmonize the above-described chromaticity coordinate system with human sensation in respect of the distance between two colors X
1
,Y
1
,Z
1
and X
2
,Y
2
,Z
2
, moreover, the variables L*,a*,b* were defined in the CIELAB system (DIN 6174) as follows (X
n
,Y
n
,Z
n
are the coordinates of the standardized light used for the illumination):
L
*=116(
Y/Y
n
)

−16
a
*=500[(
X/X
n
)

−(
Y/Y
n

]
b
*=200[(
Y/Y
n
)

−(
Z/Z
n
)

]
The total color difference &Dgr;E* between the two colors is then given, in the L*-a*-b* system, by the Euclidean difference
&Dgr;
E
*=[(&Dgr;
L
*)
2
+(&Dgr;
a
*)
2
+(&Dgr;
b
*)
2
]
½
When the shade behavior of a coating material is to be assessed in practice, then, in accordance with the present state of the art (for example, in a method according to DE 196 40 376.6), sample coatings are measured visually or by colorimetry in order to determine, for example, the values L*,a*,b* for different film thicknesses of the coating and different observation angles. This produces a large volume of data, difficult to comprehend, from which it is necessary to estimate whether the coating material investigated has a sufficient constancy of shade in its area of application. In particular, fluctuations in the film thickness of the coating on, for example, an automobile body, which occur automatically in practice, must not lead to an obvious change in the shade. The above-described estimation of the coating material on the basis of the large volume of data from the sample measurements is of course extremely difficult and requires much effort and experience in evaluation. In any case, the result is highly subjective and dependent on the abilities of the assessor.
It is therefore an object of the present invention to avoid these disadvantages and to develop a method of determining the shade stability of the coating materials which provides a useful, comprehensible and objective measure which also correlates well with the findings from a painting line. The intention is also to specify a method of optimizing paint formulations with regard to the shade stability.
This object is achieved by means of a method of determining the shade stability of coating materials, comprising a first step of
a) measuring, for different film thicknesses FT, the total color difference &Dgr;X=&Dgr;X(FT) between the color at the respective film thickness FT and the color at a predetermined film thickness FT
0
.
Accordingly, the color coordinates of the coating are first of all determined at a reference film thickness FT
0
. This reference film thickness FT
0
is preferably chosen so as to lie at the edge of the film thickness fluctuations which occur in practice. If, for example, fluctuations between 10 and 20 &mgr;m are to be expected, FT
0
=20 &mgr;m (or even FT
0
=25 &mgr;m) would therefore be an appropriate choice. The color coordinates are then determined likewise for other film thicknesses FT; the number of film thicknesses measured must be determined individually, balancing the effort of the measurement against the desired accuracy of the result. The range of the film thicknesses measured will substantially overlap with the range of the film thickness fluctuations which occur in the course of production line painting (above example: 10-20 &mgr;m). Using the color coordinates for the film thicknesses FT, it is then possible to define a total color difference &Dgr;X(FT) in relation to the color coordinates of the reference film thickness FT
0
. If the color coordinates are expressed, for example, in the L*-a*-b* system, &Dgr;X may be chosen in agreement with the known definition
&Dgr;
X=&Dgr;E
*:=[(&Dgr;
L
*)
2
+(&Dgr;
a
*)
2
+(&Dgr;
b
*)
2
]
½
.
However, it should be pointed out expressly that this is only one of several possible choices. With L*-a*-b* values it would also be possible, for example, to use the variables
&Dgr;
X &Dgr;C*:=[a*
2
+b*
2
]
1/2
−[a
0
*
2
+b
0
*
2
]
½
or
&Dgr;
X &Dgr;H
*:=[(&Dgr;
E
*)
2
−(&Dgr;
L
*)
2
−(&Dgr;
C
*)
2
]
1/2
.
The results obtained from component step a) of the method of the invention is therefore the film thickness dependency of the total color difference &Dgr;X(FT) (i.e., the difference between the color at the film thickness FT and the color at the reference film thickness FT
0
).
Then, in the second step of the method of the invention,
b) the slopes
&sgr;(
FT
)=
d&Dgr;X/dFT
 are determined as parameters for the shade stability at the film thickness FT.
Accordingly, the slope &sgr;(FT) at the film thickness FT is the derivation of the total color difference &Dgr;X in accordance with the film thickness FT. This slope indicates how strongly the color at the film thickness FT depends on the change in film thickness.
Surprisingly it has been found that the parameter &sgr;(FT) thus determined is particularly informative in respect of the shade stability of the coating material. A particular advantage in this context is that it bundles three color coordinates (e.g., L*,a*,b*) into a single value. The assessment of shade stability is therefore no longer left to the experience or the feeling of the evaluator but instead can be made on the basis of an objectively and reproducibly determinable parameter.
The method of the invention may preferably be implemented by measuring the color coordinates L*,a*,b* directly using an appropriate apparatus and calculating from these measurements the total color difference &Dgr;X.
The measurement of the total color difference &Dgr;X and/or of the coordinates L*,a*,b* may be performed under various observation angles between 0° and 180°, preferably at 15°, 25°, 45°, 75°, and 110°. This procedure is necessary when the perceived color of a coating is dependent on the direction of observation and/or of illumination. The latter is the case with many coatings, especially the coatings known as effect coatings.
In the cases of angle dependency it has been found that from the &Dgr;X
&agr;
it is possible to form a (preferably geometric)

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