GB1589705A - Method for paint colour control - Google Patents

Description

(54) METHOD FOR PAINT COLOUR CONTROL (71) We, PFIZER, INC., a Corporation organized under the laws of the State of Delaware United States of America, of 235 East 42nd Street, New York, State of New York, United States of America, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with the color control of paint. More specifically, it is concerned with color matching and batch corrections in mimicking standard paint preparations.

Computer color matching and batch correction of paints has been used commercially for more than a decade. Traditionally, the color characteristics of the colorants used to make a paint are determined by measuring the reflectance values of dry paint films containing the colorants and calculating their absorption and scattering parameters by means of a computer program. Likewise, dry paint films are used as standards for color matching, and batches of in-process paints are prepared and measured as dry paint films to determine if the color of the batch matches the standard within established acceptance limits. If a batch is not an acceptable match for the standard, computer programs are used to predict corrective additions of colorants to the batch.

Thus, all color measurements for the purpose of establishing the color characteristics of the colorants, batch and standard are normally made on dry paint film. It is a primary objective of the present invention to provide a procedure which will eliminate this time-consuming approach to paint color control.

Previous attempts to improve on this traditional method of color matching include the measurement of wet films, as reported in the Journal of Coatings Technology, Vol. 48, No.

619, August 1976, page 58. This approach is not particularly satisfactory, since a wet paint film is a continuously changing system with both the medium and the pigment particles undergoing rapid changes that affect the color appearance of the system. Measurement of wet paint in bulk, which presents a more homogenous and stable system, has been suggested by I.G.H. Ishak for monitoring the color of in-process paint batches, Journal of the Oil and Colour Chemists' Association, Vol. 54, No. 2, February 1971 page 129. This method, however, contemplates the establishment of a correlation between the wet bulk and dry film samples for several batches of each shade of paint manufactured, a laborious and costly undertaking.

It has now been found that color matching and batch correction of paints can be easily and reliably accomplished when the color properties of the colorants, standard and batch are all determined wet in bulk.

Accordingly, the present invention entails a method for mimicking the color of a standard paint which comprises the steps of (a) determining by wet measurement the reflectance values at several wavelengths throughout the visible spectrum of the standard paint in bulk, (b) determining the optical absorption and light scattering properties of the individual colorant components of the standard paint by wet measurement of the reflectance values at several wavelengths throughout the visible spectrum of paint vehicle dispersions of the individual colorant components in bulk, (c) combining the colorant components in proportion based on their optical properties to prepare a wet paint approximately the color of the standard paint, (d) determining by wet measurement the reflectance values at several wavelengths throughout the visible spectrum of the prepared paint in bulk, and (e) introducing further amounts of at least one of the colorant components into the prepared paint in proportions based on thc optical properties of the components and on the observed discrepan cies in the reflectance values of the standard and the prepared paint to adjust the color of the prepared paint to within an acceptable tolerance of the color of the standard paint.

The invention further entails a method for adjusting the color properties of a prepared paint containing at least two colorant components, to mimic the color properties of a standard paint comprising the steps of: (a) determining the optical absorption and light scattering properties of individual color ant components of the standard paint by wet measurement of the reflectance values at several wavelengths throughout the visible spectrum of paint vehicle dispersions of the individual colorant components in bulk, (b) determining, by wet measurement, the reflectance values at several wavelengths throughout the visible spectrum of the standard paint and the prepared paint in bulk, and (c) introducing an amount of at least one colorant component into the prepared paint, wherein each amount introduced is calculated using the reflectance measurements from (a) and (b) above.

In either method, the last two steps of the method may be repeated at least once to further adjust the color properties of the prepared paint to within predetermined tolerances of the color properties of the standard paint.

The methods of the present invention may be applied to the color control of all paint systems including the oll-based paints such as alkyds as well as water-based latex paint. By measuring all samples wet in bulk. these methods offer distinct advantages over the prior art approaches to color control in that they reuuire no time delav for preparing or drying paint films, no correlation between wet and dry measurement is required, and they lend themselves to automation since all steps from measuring the colors to corrective addition of colorants can be done mechanically under computer control.

In applying the instant invention to the mimicking of the color of a standard paint, the reflectance values of the wet standard in bulk are initially determined, preferably through a glass interface, at several wavlengths throughout the visible spectrum. This determination can be readily accomplished, for example, with a spectrophotometer or abridged spec trophotometer simply by placing the wet paint in a glass cell on the instrument and reading the reflectance values of the paint at 20 nanometer intervals between wavelengths of about 400 and 700 nanometers.

The reflectance values of paint vehicle dispersions comprising the individual colorant components of the standard paint are measured in the same way as for the standard paint; preferably the dispersions are intermixes of the color components with white and with black as illustrated in the example which follows. From these reflectance values, the optical absorption and light scattering properties of the invidual colorants are determined. While a number of methods may be used for this determination, the Kubelka-Munk 2-constant (K and S) method is preferred. A discussion of this and other methods as well as a general discussion of color science may be found in D.B. Judd and G. Wyszecki, Color in Business, Science and Industry, third edition, 1975.The actual K and S calculations can be performed with the assistance of standard computer programs such as the IICAL program offered by Applied Color Systems, Inc. (ACS), Princeton, New Jersey.

Using the reflectance values of the standard and the optical absorption and light scattering properties of its colorant components, the proportion of the individual colorants required to produce a paint approximating the color of the standard is determined. Standard computer programs such as the ACS IIMAT can assist in this calculation. The paint is then prepared by combining the colorants in the indicated amounts, and reflectance values of the prepared wet paint in bulk arc determined in the same manner as with the standard.

The color difference values between the standard and prepared paints are calculated from their reflectance values, usually for daylight illumination using CIE (Commission Inter nationale de l'Eclairagc) Illuminant D65 and the standard CIE 1976 (L* a* b*) color difference formula (CIE Publication No. 15, Supplement No. 2, May 1976). From these differences, the corrections to be made in the prepared paint composition can be calculated, assisted by such standard computer programs as ACS IICOR. The indicated additional amounts of colorants arc then added to the prepared paint, and the process of comparison and correction is repeated until the color difference between the standard and prepared paints is within a predetermined limit.

In applying the instant invention to the adjustment of the color properties of a prepared paint to mimic the color properties of a standard paint, the same steps as above, with the omission of those involving the preparation of the prepared paint itself, are followed.

As indicated hereinbefore, the methods of the present invention readily lend themselves to automation. For example, when adjusting the color properties of a prepared paint to mimic the color properties of a standard paint, the colorant components can be introduced into the prepared paint by automatic metering controlled by the reflectance measurements of the prepared paint. In one such system, a sample of the prepared paint is periodically pumped from the paint mixing tank through a flow-through glass cell where its reflectance values are measured. The reflectance values are fed to a computer which calculates the colorant additions required to bring the color of the paint within a specified tolerance of the standard paint. These colorant additions are in turn automatically metered into the mixing tank.

The following example is merely illustrative and it is not to be construed as limiting the invention, the scope of which is defined by the appended claims.

EXAMPLE The following paints and color concentrates, manufactured by the Sherwin Williams Company, Cleveland, Ohio, were used throughout this example: White - Dover White (K7W97) latex paint Black - Lampblack (B43311) universal color concentrate Yellow - Hansa Yellow L (B43Y]O) universal color concentrate Green - Permanent Oxide Green (B43G9) universal color concentrate Red - Dragon Red (B43R1) universal color concentrate Clear - Latex Clear (B45VC2) latex paint All color measurements in this example were made on a Model 400016 Diano/Hardy spectrophotometer manufactured by Diano Corporation, Mansfield, Massachussetts. The wet paint measurements were made by pouring a paint into a glass cell and measuring the color through the glass. These rectangular (55 x 35 x 10 mm) cells, made of fused optical glass, were manufactured by Hellma Cells, Inc., Jamaica, New York.Dry paint films were made by drawing down the liquid paint on cardboard supplied by the Morest Company, Inc., Freeport, New York, using a .006" (.015 cm) doctor blade. The drawdowns were allowed to dry for at least 12 hours at room temperature before their colors were measured.

The Kubelka-Munk absorption (K) and scattering (S) coefficients of the white paint and color concentrates were determined both as wet paints and dry films using intermixes of each color concentrate both with white and with black. The intermixes with white were made with 4 weight percent color concentrate for yellow and green, and 2 percent color concentrate for red and black. The intermixes with black were made with 39.6 percent color concentrate, 0.4 percent black concentrate and 60 percent clear latex paint, the clear latex (actually a translucent white in color) being employed to obtain a paint film which dried completely in less than 12 hours. The reflectance value of each of the intermixes was then measured, both wet (in the glass cell) and dry (as a drawdown), at 20 nanometer intervals between wavelengths of 400 and 700 nanometers.The K and S calculations were then performed using the calibration program (LOCAL) supplied by Applied Color Systems, Inc. (ACS) Princeton, New Jersey. A specular reflection value of 4.2 percent was used for the wet color measurements while 1.7 percent was used for the dry color measurements, these values being determined by measuring several samples both with the specular component excluded and with it included. The calibrations thus resulted in two separate files of color data: WET - K and S data for each colorant in the wet state; DRY - K and S data for each colorant in the dry state.

Using the white paint as tinting base and the color concentrates, standard paints were prepared for color matching experiments according to the formulas given in Table I. The reflectance values of each standard paint were measured, both wet and dry, in the same manner as with the intermixes. These values were stored in a computer file for reference.

TABLE I Standard Paint Formulas Color No. % Red % Yellow % Green % Black % White R50Y50 1.0 1.0 --- --- 98.0 Y50G50 --- 1.0 1.0 --- 98.0 R4YB2 0.8 0.8 --- 0.4 98.0 A. Color Matching Paint formulas to match each standard paint were predicted by computer using the ACS "IIMAT" program, both from the wet color measurements of the standard together with the colorant data stored in the WET file and from the dry color measurements of the standard together with the colorant data stored in the DRY file; these predicted formulas are shown in Table II.

TABLE II Predicted Paint Formulations for Matching Standard Paints Based on Wet Measurement Color No. % Red % Yellow % Green % Black % White R50Y50 1.118 1.244 ----- ----- 97.637 Y50G50 ----- 1.395 1.138 ----- 97.467 R4Y4B2 0.827 1.021 ----- 0.518 97.634 Based on Dry Measurement Color No. % Red % Yellow % Green % Black % White R50Y50 1.075 1.436 ----- ----- 97 97.489 Y50G50 ----- 1.586 1.084 ----- 97.330 R4Y4B2 0.934 1.262 ----- 0.463 97.341 Paints were prepared using the predicted formulas, and their reflectance values were measured both wet and dry. Color difference values between the standard and prepared paints were then calculated for CIE Illuminant D65 (standard daylight light source) by the CIE 1976 (L* a* b *) color difference formula.Table III summarizes the color difference values determined by comparing the dry drawdown of the prepared paint to the dry drawdown of the standard paint for matches predicted both from wet and from dry color measurement.

TABLE III Color Difference Values Between Standard and Prepared Paints Color Difference Values Prediction Color No. Basis Aa* Ab* AL* AE R50Y50 Wet 0.0 1.29 -1.48 1.38 Dry -0.62 2.47 -0.42 2.58 Y50G50 Wet -0.24 1.28 -0.41 1.36 Dry -0.56 3.34 -0.46 3.42 R4Y4B2 Wet -2.3] -1.45 -1.45 3.09 Dry -1.11 1.53 -1.01 2.14 As expected, the prepared paints were off-shade because of the limitations of the Kubelka Munk theory and the calibration procedure. However, the predictions based on wet measurement were as accurate as those based on dry measurement.

The paints prepared by prediction based on wet measurement were also compared in bulk with the standard paints in bulk. Table IV shows that color difference values obtained by comparing a wet sample to a wet standard correlate well with color difference values obtained by comparing a dry sample to a dry standard.

TABLE IV Comparison of Standard Paints with Paints Prepared by Wet Measurement Prediction Type of Color Difference Values Color No. Measurements aha * Ab* AL* AE R50Y50 Wet Bulk 0.12 1.05 -0.51 1.17 Dry Film 0.00 1.29 -0.48 1.38 Y50G50 Wet Bulk -0.30 1.05 -0.35 1.14 Dry Film -0.24 1.28 -0.41 1.36 R4Y4B2 Wet Bulk -1.55 -1.14 -1.22 2.28 Dry Film -2.31 -1.45 -1.45 3.09 Corrections for the prepared paint formulas were calculated using the ACS "IICOR" program.The corrections for the wet formulation paints were based on wet measurement of the standards and the initial formulations together with the colorant data stored in the WET file, while those for the dry formulation paints were based on dry film measurement of the standards and the initial formulations together with the colorant data stored in the DRY file.

The corrected paint formulations were then prepared, measured both wet and dry, and compared to the standards. The corrected formulations are shown in Table V, while the color difference values between the dry drawdowns of the standard and corrected paints are summarized in Table VI.

TABLE V Corrected Paint Formulations for Matching Standard Paints Based on Wet Measurement Color No. % Red % Yellow % Green % Black % White R50Y50 1.037 1.043 ----- ----- 97 97.920 Y50G50 ----- 1.211 1.057 ----- 97.732 R4Y4B2 (1) 1.015 1.069 ----- 0.364 97.553 R4Y4B2 (2) 0.892 0.881 ----- 0.367 97.851 Based on Dry Measurement Color No. % Red % Yellow % Green % Black % White R50Y50 1.044 1.080 ----- ----- 97.876 Y50G50 ----- 1.143 1.049 ----- 97.807 R4Y4B2 0.920 0.930 ----- 0.394 97.756 TABLE VI Color Difference Values Between Standard and Corrected Paints Prediction Color Difference Values Color No.Basis Lea * Ab* AL* R50Y50 Wet -0.03 -0.04 -0.12 0.13 Dry -0.13 0.41 -0.17 0.47 Y50G50 Wet 0.12 0.75 -0.12 0.77 Dry 0.20 -0.07 -0.03 0.21 R4Y4B2 Wet (1) 0.47 1.03 0.08 1.14 Wet (2) 0.00 -0.10 0.03 0.10 Dry -0.14 -0.14 -0.16 0.25 Based on experience and visual observation, an acceptable match was defined as one having hE < 1.0 CIE L*a*b* unit. Table VI shows that wet measurement is as capable of predicting acceptable paints as dry measurement. In the case of R4Y4B2, the wet prediction required a second correction to bring the paint within the acceptable limit. In present practice when working with dry drawdowns, it is not uncommon to use two corrections to obtain a close match.The fact that two corrections were required for R4Y4B2 is therefore not considered significant.

B. Batch Correcting New batches of the standard paints were prepared using the same formulas as in Table I but employing different lots of the color concentrates and the white tinting base. As expected, the colors of the new batches did not match the colors of the original standards because of lot-to-lot variations in the colors of the components. Table VII gives the color differences between the original standard paints and the new batches.

TABLE VII Color Difference Values Between New Batches and Original Standard Paints Wet Batch Compared to Wet Standard Color No. Aa* Ab* AL* AE R50Y50 0.51 -1.03 0.08 1.15 Y50G50 0.89 -1.50 0.34 1.77 R4Y4B2 -0.27 -1.54 -0.28 1.58 Dry Batch Compared to Dry Standard Color No. ha* Ab* AL* AE R50Y50 0.56 -1.21 -0.01 1.34 Y50G50 1.11 -1.33 0.24 1.75 R4Y4B2 -0.67 -2.18 -0.21 2.29 Using the ACS "IICOR" program, batch corrections were preducted both from the wet color measurements of the original standard and the new batch together with the colorant data stored in the WET file, and from the dry color measurements of the original standard and the new batch together with the colorant data stored in the DRY file.These batch corrections were made and the reflectance values for the corrected batches were measured both wet and dry. The corrected batch formulations are listed in Table VIII, while the color difference values between the dry drawdowns of the corrected batches and the original standard paints are shown in Table IX.

TABLE VIII Corrected Batch Formulations Based on Wet Color Measurement Color No. % Red % Yellow % Green % Black % White R50Y50 0.988 1.167 ----- ----- 97.846 Y50G50 ----- 1.160 1.304 ----- 97 97.535 R4Y4B2 0.862 1.032 ----- 0.354 97.752 Based on Dry Color Measurement Color No. % Red % Yellow % Green % Black % White R50Y50 0.980 1.151 ----- ----- 97.869 Y50G50 ----- 1.110 1.347 ----- 97 97.543 R4Y4B2 0.906 1.071 ----- 0.348 97.675 TABLE IX Color Difference Values Between Corrected Batches and Original Standard Paints Predicition Color Difference Values Color No. Basis ha* Ab* AL* AE R50Y50 Wet 0.23 -0.60 -0.21 0.68 Dry 0.00 -0.49 0.14 0.51 Y50G50 Wet 0.43 -0.63 -0.57 0.94 Dry 0.34 -1.04 -0.68 1.29 R4Y4B2 Wet 0.08 -0.20 0.51 0.56 Dry 0.06 -0.42 0.09 0.43 Within the limits of sample preparation repeatability, there is no significant difference in accuracy between the wet-predicted and the dry-predicted corrections. Indeed, the dry match for Y50G50 is outside of the defined acceptance limit, and the computer predicted that no further improvement of this match was possible with the colorants available.

Claims (8)

WHAT WE CLAIM IS:

1. A method for adjusting the color properties of a prepared paint containing at least two colorant components, to mimic the color properties of a standard paint comprising the steps of: (a) determining the optical absorption and light scattering properties of individual colorant components of the standard paint by wet measurement of the reflectance values at several wavelengths throughout the visible spectrum of paint vehicle dispersions of the individual colorant components in bulk, (b) determining, by wet measurement, the reflectance values at several wavelengths throughout the visible spectrum of the standard paint and the prepared paint in bulk, and (c) introducing an amount of at least one colorant component into the prepared paint, wherein each amount introduced is calculated using the reflectance measurements from (a) and (b) above.

2. The method of claim 1 wherein said steps (b) and (c) are repeated at least once to further adjust the color properties of said prepared paint to within predetermined tolerances of the color properties of said standard paint.

3. The method of claim 1 wherein said reflectance measurements are made through a glass interface.

4. The method of claim 1 wherein said reflectance measurements are made at intervals of 20 nanometers between wavelengths of about 400 and 700 nanometers.

5. The method of claim 1 wherein said colorant components are introduced into said prepared paint by automatic metering controlled by reflectance measurements of said prepared paint.

6. A method for mimicking the color of a standard paint which comprises the steps of (a) determining by wet measurements the reflectance values at several wavelengths throughout the visible spectrum of said standard paint in bulk, (b) determining the optical absorption and light scattering properties of the individual colorant components of said standard paint by wet measurement of the reflectance values at several wavelengths throughout the visible spectrum of paint vehicle dispersions of said individual colorant components in bulk, (c) combining said colorant components in proportion based on said optical properties to prepare a wet paint approximately the color of said standard paint, (d) determining by wet measurement the reflectance values at several wavelengths throughout the visible spectrum of said prepared paint in bulk, and (e) introducing further amounts of at least one of said colorant components into said prepared paint in proportions based on said optical properties of said components and on the observed discrepancies in said reflectance values of said standard and said prepared paint to adjust the color of said prepared paint to within acceptable tolerances of the color of said standard paint.

7. The method of claim 6 wherein said steps (d) and (e) are repeated at least once to further adjust the color of said prepared paint to within predetermined tolerances of the color of said standard paint.

8. A paint which has had its color properties adjusted according to a method as claimed in any one of claims 1 to 7.