IArDUSTRIAL ALVD ESGINEERING CHEMISTRY
February, 1928
183
LACQUERS AND SOLVENTS Papers presented before the Division of Paint and Varnish Chemistry at the 74th Meeting of the American Chemical Society, Detroit, Mich., September 5 to 10, 1927
Solvent Structure and Solvent Action Bruce K. Brown COMMERCIAL SOLVEKTS CORPORATION, TERREHAUTE,IND.
I
h’ THE manufacture of lacquer it is necessary to produce
a homogeneous liquid mixture containing materials that are not completely compatible. While there are a number of solvent liquids that will dissolve either nitrocellulose or varnish gums, frequently solvents that dissolve either solid ingredient separately will not hold both nitrocellulose and gum in the same solution. In practical usage such esters as ethyl acetate, butyl acetate, and amyl acetate are used t o dissolve nitrocellulose, but in the completed lacquer there will also be found some diluents-i. e., alcohols and hydrocarbons. In such a liquid mixture it may be presumed that each component plays a definite role. Attempts have been made to reason that “like dissolves like” and that, since nitrocellulose is a cellulose ester, the best solvents for it will be esters. However alluring this theory may be, it is not well substantiated in practice, since ketones, keto alcohols, hydroxy esters, and ethers are also notably good nitrocellulose solvents. It may also be noted that the gums most used in lacquers are esters, yet to secure complete compatibility of the ingredients of lacquer the presence of hydrocarbons or alcohols is usually required. There are many instances known in which a mixture of two or more liquids of different molecular structure-that is, having different characteristic solvent groups, such as the carboxyl, carbonyl, and hydroxyl groups-exerts a solvent action on nitrocellulose that is superior to the action of either ingredient separately. It has been reasoned that a single solvent possessing more than one of the solvent groups in its molecule would exert a solvent action superior to that action obtainable with a molecule containing only one solvent group. Keyes’ has discussed this subject briefly and has cited instances of nitrocellulose solvents which possess more than one solvent group. These he characterizes as two-type solvents. No doubt there is a broad field of research open to those who can structurally design a solvent to meet a given need. The solvent properties of a liquid for nitrocellulose may be predicted fairly well when the molecular structure of the liquid is known. For example, most esters and ketones are solvents for nitrocellulose and most alcohols and hydrocarbons are not. With some knowledge of this sort it may be possible for chemists to build up solvent molecules containing exactly the proper solvent groups to do a given job. At present, however, there are distressing discrepancies between theory and fact. For example, it has been reasoned that, since nitrocellulose is readily soluble in esters or in ketones, and since most gums are easily dissolved by alcohols, a solvent combining an alcohol group with an ester or ketone group could be employed to dissolve both nitrocellulose and gum; in which case there would be no necessity of using a special gum solvent in formulating a lacquer. But such Ind Eng Chem., 11, 1120 (1925).
reasoning has not yet been supported by experimental evidence. Diacetone alcohol is a well-known lacquer solvent that contains both a ketone group and an alcohol group. It is, furthermore, an excellent solvent for both nitrocellulose and ester gum if these solutions are made separately. However, when an attempt is made to mix a solution of nitrocellulose and diacetone alcohol with a solution of ester gum in the same solvent, a haze results which persists in the dry film and which can be cured only by adding another high-boiling diluent such as hexalin or “hercosol.” Ethyl lactate is also a nitrocellulose solvent which contains two solvent groups-the ester group and the hydroxyl group. Ethyl lactate dissolves nitrocellulose and also shellac and kauri gum, but ethyl lactate solutions of nitrocellulose and these gums cannot be mixed without producing a cloudy film. The remedy in this case is also to add a high-boiling diluent such as butanol. Exactly the same phenomenon is observed in the case of a one-type solvent such as butyl acetate. Butyl acetate is a good solvent for both nitrocellulose and kauri gum, yet mixtures of these two solutions are not compatible except in the presence of a diluent such as butanol. The ethyl ether of ethylene glycol provides another instance of a two-type nitrocellulose solvent, since it contains an ether group and a hydroxyl group. This solvent dissolves nitrocellulose and also kauri gum, although it does not dissolve ester gum. However, when separate solutions of nitrocellulose and kauri gum in the ethyl ether of ethylene glycol are mixed, incompatibility is observed and turbid lacquer films are produced. The remedy for this is exactly the same as that used in the case of the one-type solvent, butyl acetate. At the same time, while ester gum is not soluble in ethyl ether of ethylene glycol, it may be readily dissolved in a mixture of that material with some highboiling diluent. The instances cited show that however effective a solvent may be in dissolving a variety of single substances, nevertheless, in the practical formulation of lacquers, solvents possessing theoretically desirable solvent groups cannot be relied upon offhand to produce the desired result. It is not to be denied that two-type solvents sometimes possess unique properties. For example, a nitrocellulose solvent which contains a hydroxyl group will ordinarily dissolve some water, and hence solutions of nitrocellulose, and this material will be more tolerant of water than those made up of simple ester solvents. The practical importance of this particular property is doubtful, however, since water has no proper part in present-day lacquer formulation. It is the hydrocarbons that are most intolerant of water, and a lacquer made up with a two-type solvent and a hydrocarbon is no more tolerant of water than one made up with a onetype solvent and a hydrocarbon.
