INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1927
vents, such as butyl acetate, will stand the most dilution with non-solvent. These curves also illustrate the fallacy of comparing dilution ratios obtained for different solvents where a uniform initial concentration of nitrocellulose-say, 10 per cent-is employed, and the final concentration is disregarded. I n such cases the natural difference of dilution ratios is exaggerated. For example, with a solvent having a high dilution ratio the final concentration of the nitrocellulose, after all the noli-solvent has been added, will be less than the final concentration of nitrocellulose obtained in testing a solvent having a lower dilution ratio, to which less non-solvent may be added. The curves clearly indicate that a t low final concentrations of nitrocellulose the dilution ratios of the solvents may be expected to be larger than a t the higher concentrations, which are more comparable with actual lacquers. In the preparation of a satisfactory lacquer the use of diluent ratios must be tempered by the evaporation balance of the liquid mixture. Diluent ratios represent merely the
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limit of tolerance of solvent for non-solvent in the complete lacquer, and are no indication of tolerance during the drying period. If solvent and non-solvent present are adjusted in evaporation rates so that neither is completely evaporated before the lacquer film sets during drying, the diluent ratio mill not be exceeded and a satisfactory film should result. This is not always the case. For example, the data presented show that a mixture comprising 1 part of ethyl acetate and 3 parts of xylene will retain 8 per cent of nitrocellulose in homogeneous colloidal solution. But if such a mixture is employed as a lacquer a badly blushed film of poor adherence and low tensile strength will result, since all of the solvent will evaporate before the film dries and the remaining xylene will coagulate (blush) the nitrocellulose. Acknowledgment
C. W. Simms, of this laboratory, performed many of the laboratory tests on which this paper is based and the authors' thanks are due him.
Lacquer Surfacers' By F. M. Beegle and C. M . Simmons T H E CLEVELAND VARNISHCOMPANY, CLEVELAND, OHIO
K DET'ELOPISG a suitable lacquer surfacer for use in automobile production, it was thought desirable to grind the pigments on a burr stone mill, such as all paint factories possess, thereby obviating the necessity of purchasing special equipment. Various combinations of chemical plasticizers, oils, and gum were therefore tried in order to secure a liquid that would wet sufficient pigment to give satisfactory filling and feed through the mill. A mixture of castor oil, linseed oil, and dibutyl tartrate gave a very good grinding mixture when Titanox, Keystone filler, talc, and iron oxide formed the bulk of the pigment.
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Gums 111 accordance with good varnish reasoning it was believed that the less gum used the tougher and more waterproof would be the product. Consequently, a batch of surfacer was made with the ingredients mentioned above, without any gum. This surfacer was very elastic and held out the finishing lacquer in a very satisfactory manner, but on spraying an automobile body with it and sending to the rubbing deck after sufficient drying, it rubbed very tough, and in spots where the water lay for a time it blistered from the primer. Considerable work was then done to find the cause of the blistering, retaining the idea that no gum should be used. Grinds were made with the same pigment combinations and oils, but substituting Lindol, dibutyl phthalate, and diamyl phthalate for the dibutyl tartrate. A11 combinations were better than the one using dibutyl tartrate, because this plasticizer seemed to allow the water to pass quickly through the film. From this point considerable stress was placed on a water test and all batches were sprayed on, force-dried 1 hour a t 66" C. (150" F.), sanded to a smooth surface, and stood in a pail of tap water overnight. The formula for any surfacer showing a tendency to blister was discarded. The varnish theory had to be discarded because it was found necessary to include gum in the formulas in order to get adhesion and a material to stand a satisfactory water 1
Received April 23, 1927.
test. The tendency t o blister was found to decrease as the gum increased up to an optimum point. Fortunately, the gum content giving optimum waterproofness was about the same as the gum content giving satisfactory rubbing properties. The addition of the gum solution t o the oils and chemical plasticizer gave a very satisfactory grinding liquid. After the proper ratio of oil, chemical plasticizer, gum, total amount of pigments, and cotton was established to give proper adhesion, waterproofness, sufficient filling and sanding, work was started to determine the gum or combination of gums to be used to secure the best results. Shellac, dammar, ester gum, and kauri gum solutions were used. On all tests that could be completed in a week or 10 days it was decided that the gums gave the best results in the order named above. However, very good results could he obtained by using a combination of dammar and ester gum. It was found that a very valuable test could be made on lacquer surfacers by applying two coats on top of a good, long oil-baked primer, force-drying 1 hour a t 66" C. (150" F.),rubbing to a surface, then coating with two double coats of lacquer. After the film had dried about an hour, the panels were hung in an electric oven and baked overnight at 121" C. (250" F.). This treatment caused many lacquer surfacers to crack open and pull the finishing lacquer with them, giving the appearance of sun-baked mud. Optimum Cotton Solution
Surfacers were made using the forrnula with the best oil, chemical plasticizer, pigments, and gum conibinations with l/r-second cotton solution in one case, and with 7-second cotton solutions in other cases, after reducing the latter to the viscosityof the '/Tsecond solution by use of various catalysts. The same number of grams of cotton per liter of solvents was used in all cases. The surfacer made from '/*-second cotton was the best of the series, irrespective of the catalyst used with the 7-second cotton, even though the catalyst were only heat and pressure. A weather test on the roof bore out the conclusions of the quick tests in this case.
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I N D UXTRIAL A N D ENGINEERING CHEMISTRY Pigments
The next step was to vary the pigment combinations. Into 850 grams of plasticizers and gum solutions the folloming pigments were ground: A- 850 grams Titanox, 850 Keystone filler, and 170 talc B-1190 grams white lead, 1190 Keystone filler, and 170 talc C-1075 grams zinc oxide, 1075 Keystone filler, and 170 talc D-1190 grams Titanox, 1190 white lead, and 170 talc E-1075 grams Titanox, 1075 zinc oxide, and 170 talc F-1190 grams lithopone, 1190 Keystone filler, and 170 talc G-1190 grams silica, 1190 Keystone filler, and 170 talc
These bases were then thinned with l/n-second cotton solution, some additional gum solution, and reducer. Two panels, previously primed with oil primer and baked, were sprayed with two coats of each surfacer. The films were force-dried in an electric oven at 66" C. (150' F.) for 45 minutes. Both panels were sanded to a good surface. One stood in water overnight, while the other was sprayed with two double coats of lacquer and baked 48 hours in an oven a t 121" C. (250" F.). The next morning the panels which were stood in water u-ere cut with a knife, both above and below the water line, to ascertain their relative elasticity and adherence to the primer. All the surfacers were found t o stand a good water and knife test. The combination of Titanox and white lead and the combination of lithopone with Keystone filler all sanded well, while the silica with Keystone filler sanded very hard, and the zinc oxide with Keystone and Titanox with white lead sanded hard. The surfacer containing zinc oxide and Keystone cut the best, both above and below water, but it stood the bending
Vol. 19, No, 9
test poorly. On cutting after baking, the films gave a ribbon in the following order: A, F,B, C, E, D, and G. The bending test after baking gave the following order of results: D, A, F, B, E, C, and G. Panels coated with those surfacers made with the seven pigment combinations were sprayed over half the surface with two coats of blue finishing lacquer and exposed on the roof for about, 6 months. At the end of this time the surfacer made with white lead and Keystone showed the best, both with and without finishing lacquer over it. The combination of lithopone and Keystone was a close second. The combination of Titanox and white lead was the poorest, while the other combinations varied between these two extremes. Conclusions
1-A good lacquer surfacer can be manufactured on a burr stone mill. 2--A combination of oil, chemical plasticizer, and gum solution makes a good grinding medium. 3-A satisfactory lacquer for production work must stand a good water test. 4-Gum is essential for waterproofness, adhesion, and satisfactory rubbing properties. 5-Gums are satisfactory in the following order: shellac, dammar, ester gum, and kauri. However, a combination of dammar and ester gum may be used with satisfactory results. 6-Half-second cotton gives better results than cotton of higher viscosity reduced to '/rsecond viscosity. 7-Of all pigment combinations used, white lead and Keystone filler showed the best results after 6 months' outdoor exposure, both with and without finishing lacquer sprayed over the surfacer.
Uniform Varnish Films for Exposure Tests' By H. A. Gardner and G . G. Sward IKSTITUTE OF P A I N T A N D \ T A R X I S H R E S E A R C H , W A S H I N G T O N ,
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HEN panels of any type are brush-coated with var-
nish, it is considered necessary to apply a t least two and preferably three coats in order to get a continuous film and one of sufficient thickness to give fairly good durability over a period of three months' exposure in the summer. It has been found, however, that various operators working with the same varnish may apply films of marked variation in thickness. Naturally, when exposed the durability of such panels would vary. It was thought that much more uniform results could be obtained if the various operators would use some uniform method of preparing films. The writers, for their own work, have standardized upon one-coat films which are spun upon revolving disks. The apparatus consists of a 12-inch (30-cm.) circular spinning table, set within a drain pan 18 inches (46 cm.) in diameter and 8 inches (20 cm.) high to catch the varnish thrown off by centrifugal force. The power is furnished by a 0.1 horsepower motor. The speed of the apparatus is controlled by varying the size of the pulleys. Walker and Thompson2 have prepared films of paint and varnish by spinning on glass disks. They found that plastic 1 Received April 6, 1927. Presented under the title "Methods of Producing Uniform Films for Exposure Tests" a s a part of the Symposium on Lacquers, Surfacers, and Thinners before the Section of Paint and Varnish Chemistry a t t h e 73rd Meeting of the American Chemical Society, Richmond, Va.,April 11 t o 16, 1927. 2 Proc. A m . SOC.Testing Materials, 22, Pt. 11, 464 (1922).
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materials, such as paint, gave films that were much thicker at the center than at the edge of a 25-em. disk. On the other hand, viscous materials, such as varnish, gave films varying but a few microns over the entire disk. For a study of many properties of the paint they decided that a speed of 300 r. p. m. maintained for 3 minutes produced the best results. No extensive study of varnish films was made. Pulsifer3 found that the thickness of a normal brush coat of a varnish could be expressed as F = (IOV N ) , where F equals the thickness in microns, V equals the absolute viscosity in poises at 25" C., and N equals the percentage of non-volatile matter in the varnish. For example, a varnish possessing a viscosity of 1.4 poises and a non-volatile percentage of 50 should give a film 32 microns thick. None of the above investigators attempted to control the thickness of the film by altering the conditions of preparation to suit the material under consideration.
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Preparation of Films
I n view of the influence of the film thickness upon the results of most tests on varnish films, it was thought desirable to study the spinning method to learn if films of predetermined thickness could be prepared. The method was essentially the same as that of Walker and Thompson,2 but using, in8
Drugs, Ozls, Painls, 39, 354 (1924)
