198
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 20, No. 2
Lacquer Formulation, with Especial Reference to the Use of Cumar Allen Rogers and Clifford Banta P R A W INSTITUTE,
BROOKLYN, N. Y., AND
HIS paper applies particularly to brushing lacquers, although many of our brushing lacquers are also excellent spraying lacquers, the main difference being in the rate of evaporation or drying. Components A lacquer consists essentially of nitrocellulose as the base material, which has been rendered soluble by suitable solvents, and to which have been added diluents, plasticizers, gums, and oils. Sometimes coloring matter is also added. To be commercially applicable a clear brushing lacquer must possess the following qualities:
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(1) It must be clear. ( 2 ) It must be easily spread with a brush. (3) It must have sufficient body to give the desired flow and protection. (4) It must not lift or draw on repeated applications. ( 5 ) When dry the film should be transparent and show no tendency to blush or give a cloudy appearance. (6) In most cases it should be capable of rubbing to a fine smooth finish.
That grade of nitrocellulose must be selected which is most suitable for the purpose for which it is to be used, It is now possible to obtain any degree of fluidity by mixing the various grades. A number of solvents for the nitrocellulose are now available. Of the low-boiling compounds acetone and ethyl acetate should be mentioned. Of the medium boilers butyl-acetate is probably the most largely employed, although butyl propionate and the monoethyl ether of ethylene glycol are finding favor in the trade. The high-boiling compounds, such as ethyl lactate and diethyl oxalate, are used only in limited amounts. Dibutyl phthalate and tricresyl phosphate are the most commonly employed plasticizers, the chief object of which is to add flexibility to the finished film and to prevent the precipitation of the cotton in the form of a blush or cloud. Diluents are used to reduce the cost of a lacquer and in proper combination do not destroy its valuable properties. Benzene, toluene, and xylene are the principal diluents, toluene seeming to have the preference. In certain combinations it is possible to use petroleum distillates, but this procedure is not general. The amount of diluent must be very carefully adjusted to the amount and evaporation rate of the solvents. Another important constituent of modern lacquers is butanol, whose chief purpose seems t o be as a blending agent between the diluent and solvent. Nitrocellulose lacquers in themselves produce a flat finish to the surface of the film. To secure a gloss and give added body it has been found necessary t o introduce certain gums and oils, The material most largely used for this purpose is ester gum. This product by itself, however, does not give all of the qualities required, and so other gums and resins, such as dammar, elemi, copal, shellac, and synthetic resins, are usually added. Of these, dammar has been most favorably received in the past. As the natural gums are fairly expensive and have other drawbacks, synthetic materials have come more and more into use. Among the synthetic resins one which offers exceptional possibilities is cumar. This is a neutral polymerization product made from certain coal-tar naphthas in very large quantities a t a minimum cost and of a standard degree of purity. For lacquer manufacture cumar has many decided
!hS
BARRETT COMPANY,
PHILADELPHIA,
PA.
advantages. It is cheap. It is hard. It is a clear, lightcolored, transparent solid and is not affected by alkalies or acids. It is soluble in all cotton solvents and in practically all diluents except alcohol. Thus far a satisfactory formula with cumar alone has not been developed, but when mixed with ester gum or natural resins it imparts to the film the necessary degree of hardness and produces a gloss and body superior to that obtained from ester gum or ester gum with natural resins. Being acid-proof, alkali-proof, and waterproof to a high degree, it renders the film more weatherresistant than a combination of other gums and resins. Many experiments with cumar combinations indicate that a ratio of 7 parts of ester gum t o 4 parts of cumar furnishes a gum content which, on repeated weather tests, gives a superior film. Cumar cannot always be used to replace other gums, but by proper regulation of the formula it may be readily substituted and an improved product obtained. To give more flow and body to a lacquer there is a growing tendency to replace part of the gum with an oil. This procedure also makes the lacquer work under the brush more like an oil varnish. Linseed oil, either raw, boiled, or blown, is one of the most important oils used for this purpose. Among the other oils should be mentioned castor and rapeseed, which are introduced as either raw, boiled, or blown, China wood oil, pine oil, and Hercosol. The amount of oil added depends largely upon the other constituents, but generally is comparatively small. Too much oil not only retards the drying, but tends to give a soft film and one which is liable to lift on repeated applications. Formulation Let us now see how the component parts of a lacquer are assembled. We will take a typical formula and follow the steps necessary t o produce a finished product. Typical Lacquer P e r cent b y E'eighl Nitrocelluloie ( ' / I sec., wet with 30 per cent of alcohol) 8 10 Butanol 31 Toluene Ethyl acetate 10
Formula Butyl acetate Dibutyl phthalate Ester g u m Cumar
Per cent
b y weight 25 5
7 4
The cotton is placed in a suitable container for mixing and the butanol and toluene are added. When well wet the ethyl acetate, butyl acetate, and dibutyl phthalate are added and the mass is stirred until all of the cotton is in solution. The mixture of ester gum and cumar is now introduced and the stirring continued until all of the gums have dissolved. This will give a finished lacquer of the quick drying type, which is clear and ready for use. It may be pigmented by adding the necessary amount of pigment ground in oil, or the plasticizer may be used as a grinding medium, or again the clear lacquer may be placed in a ball mill and the dry pigment thus incorporated. I n the case of a lacquer requiring an oil the formula may be modified by simply replacing 3 parts of the toluene by a suitable oil, or the oil may take the place of a part of the gum mixture. The heavy-bodied oils give more viscosity to the product, but in cases where this is not desired a thin or raw oil may be employed. This formula is given to show the general type of lacquer in which cumar can be used to advantage. It can be modified, within reason, by the manufacturer to meet his individual requirements.
