I N D U S T R I A L A N D ENGINEERING CHEMISTRY
February, 1928
197
Action of Pigments in Metal Primers E. J. Probeck and H. W. Battle JOKES-DABNEY Co., LOUISVILLE, KY.
BOUT two years ago the predominating lacquer primers on the market were composed of very small amounts of pigment, a large amount of gum, and varying amounts of nitrocellulose and plasticizer. The theory advocated by most experts was that in order to get adhesion to metal it was necessary to have as much varnish gum present as possible.
A
The same surfacer was used as in the previous series, and the lacquer enamels were similar-that is, they were good durable lacquer enamels for exterior work. Table 11-Composition of Special Panels (Figures in per cent) PANEL 6 PANEL 7 PANEL S 18.70 20.00 18.50 Solids Volatile 81.30 80.00 81.50
Experimental Tests
ANALYSIS
SERIES 1-To test out this theory the writers made a series of tests, starting with a primer of this nature. The composition is given in Table I. Black iron panels were thoroughly cleaned and sprayed with a coat of primer. This was allowed to dry 4 hours, and then two coats of lacquer surfacer were applied. The surfacer was stepped down about 11/2 inches for the first coat, and an additional inch for the second coat. This gave a weather test on primer, one and two coats of surfacer, and two coats of finishing lacquer. The lacquer surfacer was made up according to the analysis given in Table I. It was very high in pigment and contained only a very small amount of plasticizer and a comparatively small amount of nitrocellulose, which, of course, made it very easy to sand to a smooth finish. The general trade seems to demand this type of surfacer. It did not have an excessive amount of toughness and was made to sand as near as possible like an oil surfacer. The lacquer surfacer was thoroughly water-sanded to a smooth surface, and then various types of automobile finishing lacquer were sprayed on. Two full wet coats were used. The lacquer was sanded, rubbed, and polished before exposure in order to get conditions as near as possible to actual methods in vogue in the finishing rooms. The panels were exposed on the first day of April, 1926, and were taken in on the first of August, 1927, very thoroughly cleaned, and part of the panel polished. Where oil primers are used, the composition of the base left on the panel after drying usually contains about 2 parts pigment to 1 part varnish base. This shows a very decided difference from the composition of the base of the lacquer where the ratio is about 6 part.s vehicle to 1 part pigment. Table I-Composition of Primers a n d Surfacers (Figures in per cent) SERIES 1 LACQUER SERIES 2 PRIMER SURFACER PRIMER Solids 22.50 66.80 22.70 Volatile 77.50 33.20 77.30 ANALYSIS
OF SOLIDS
Pigment 13.35 Half-second nitrocellulose 31.10 Plasticizer 11.80 Gum 43.75" a Half ester and half dammar. b Dammar.
76.00 8.60 3.10 12,30b
39.00 30 00 11.10 19.90
SERIES 2-In the second series of tests the amount of pigment was increased a t least three times that in the first series; the gum was decreased a t least one-half. This gave a primer that was absolutely solid in covering, somewhat flat, and the first indications are that it did not adhere to the metal as well as the other type. Analysis of the primer for the second series is given in Table I.
Pigment Nitrocellulose Plasticizer Gum
OF SOLIDS
27.10 32.20 14.20 26.50
36.70 25.90 12.80 24.60
32.60 29.00 13.00 25.40
OTHER TEsTs-Three additional panels were run with single coats of different pigment composition-a combination of blues and greens and some of the commonly used white pigments. These broke down very early in the tests, and showed very serious rusting and chalking. All three panels showed excessive chalking and indications of rust in 4 months. I n 6 months all showed rust spots and indications of failure. Analyses of these three special panels are given in Table 11. Conclusion
The results after 16 months' exposure of the panels show conclusively that the primer with a small amount of pigment does not hold up. The primer in Series 1 started to fail in between 3 and 4 months, showing rust spots and then complete disintegration; whereas, in Series 2, even after 18 months, the primer is practically in perfect condition, shows no rust spots, and apparently is an ideal foundation for the lacquer coats. The lacquer enamels in Series 2 are in considerably better condition than those in Series 1. Inasmuch as the same type of lacquer enamels was used, it seems to indicate conclusively that the primer has considerable bearing on the durability of the finishing lacquer enamels. The appearance of the panels indicates that the lacquer enamels in Series 2 were a t least twice as durable as those in Series 1. This would give a maximum durability of the entire lacquer system, as indicated in Series 2, of a t least three years. The results also show that the high pigment concentration of the surfacer was too great. Here we have a ratio of about 4l/4 parts pigment to one part base. Decreasing the pigment and increasing the nitrocellulose, gum, and plasticizer, would give a tougher film and one that would be harder to sand, but such a film would be more durable and would hold up the finishing lacquer better. As for the lacquer surfacer, a maximum of 55 per cent solids should give the most homogeneous film that would still be workable in automobile finishes. In the course of this work no attempt was made to make a complete determination of which pigments were the best for lacquer primers. The test indicated that iron oxide of a high state of purity (at least 90 per cent oxide) is one of the best pigments that can be used for this work. Chrome greens and chrome yellows and some of the commonly used white pigments did not give such good results as iron oxide. The results also indicate that a lacquer primer should contain a t least 39 per cent pigment to give the proper durability to the entire lacquer system.
