Metallic shades have gained growing popularity as automobile finishing and become widely applied in the mass-production lines. Consequently, metallic enamels containing aluminum powder have obtained an important position among industrial enamels and the control of their shades with the same accuracy as of conventional colors has gradually become very important. Metallic enamels, being essentially polychoromatic, are difficult to control. A theoretical definition has not been established yet as to such colors. Moreover, a metallic enamel tends to give different colors when applied under different conditions. The authors, from a paint manufacturer standpoint, have investigated if such colors could be controlled with the same accuracy as with solid colors, and further if a numerical parameter could be introduced for control using Model CM-20 Color Difference Meter. Metallic enamels are found to show greater variation in colors when the application conditions are controlled at the same level as of solid colors, and hence are difficult to control. Metallic tones among other characteristics show the greatest variation, to which L values are appoximately correlated. The a and b values, which are supposed to correspond to color shades, lessen their correlationship with visual observation while L values disperse considerably. The size of aluminum powder has a characteristic influence on variation of colors. Coarse powder gives smaller panel-to-panel but greater date-to-date variation, while fine powder yields the opposite effect. Among the application conditions, spraying viscosity and delivery rate of enamels are key factors to the reproducibility of metallic colors. With the strict control of such factors, metalic enamels can be controlled to give the similar metallic shades, whose numerical values fall within the like range as solid colors, and correlate reasonably with visual observation. It is essentially impossible to specify metallic colors with L, a and b values, but the numerical control of the predetermined metallic shades with a fair accuracy is believed practically possible under the specified conditions, although the dispersion of those values within a panel is to be further investigated.
Zinc-phosphatized panels made from mild steel were activated in the aotmic pile (at Tokyo Atomic Industrial Research Lab.**) and were used as the anode for electrodeposition coating. The electrodeposition was repeated approx. twenty times using new panels. In this continuous process, the bath solution and the electrodeposited paint film were sampled at various starges of the process, and were analyzed according to the radioactive tracer method. For the analysis every electrodeposited paint film was dissolved in xylene/ butanol solvent, while the bath solution was directly used. They were counted for γ rays by a 256 Channel Pulse Height Analyzer having NaI (T1) detector to detect or Zn, and for β rays by a Geiger-Müller Counter to detect PO4. The paint samples used in this work were ; A) a clear solution of an alkyd/melamine blended varnish where the alkyd resin, having the acid number of about 60, was made from trimeritic anhydride, adipic acid, soybean fatty acid and neopentyl glycol. The blended varnish was solubilized with triethylamine. The apparent pH value of the aqueous solution was 7.3 B) an ammoniacal solution of a varnish made from maleic anhydride and linseed oil of which pH value was 7.4. The acid number of the resin was about 120. The electrodeposition was carried out as follows. For A) paint, the activated panels (the so called “hot panel”) were used continuously for the first ten times, and then the panels which were not activated (the so called “cold panel”) were used continuously for the next ten times. For B) paint, the ten “hot panels” and the ten “cold panels” were used alternately. The results are summerized as follows. 1 Zn2+ ions migrated not only into the electrodeposited paint film, but also into the bath solution. 2 PO3-4 ions also migrated from the anode into the electrodeposited films and into the bath in spite of the fact that they are negatively charged. 3 The concentrations of Zn2+ and PO3-4 ions in the bath increased with increase in the number of panels electrodeposited. In the case of A) paint, they leveled off when several panels were electrodeposited, that is, when the surface of about 0.1m2 was treated out of the one liter of the paint. On the other hanp in the case of B) paint, these concentrations were still increasing even after twenty panels treatment, that is, even after the value of 1 m2/l. was reaches. Itis to be noted that the value of 1 m2/l without any adjustment of the bath is believed to be large enough to know of the running stability of the bath. 4 Zn2+ and PO3-4 ions found in the electrodeposited paint appear to come mainly from the zinc-phosphate film of the panel and partly from the bath solution. Their concentrations increased to some extent in parallel with the ion concentrations in the bath. 5 The concentrations of Zn2+ and PO3-4 ions in the “dipped film” (i. e. the shower rinsible paint adhering to the electrodeposited film when the panel is withdrawn from the bath) increase or decrease along with those in the electrodeposited film. Although the absolute contents of the ions in the “dipped film” are very different from those in the electrodeposited film, it is interesting to note that, when the concentrations are plotted against the number of panels electrodeposited, the curves obtained from the data on the “dipped film” resemble surprisingly to those obtained from the data on the electrodeposited film. These similarities are probably due to the dominant effect of each panel of which phosphate film is not of uniform thickness.