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Evaporation of Lacquer Solvents
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W. K. LEWIS, LOMBARD SQUIRES, AND C. E. SANDERS Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass.
T
HE development of modern lacquers has
required the use of mixed solvents, but this in turn has introduced the factor of change in solvent composition during drying, caused by unequal evaporation rates of the individual components of the mixture. The attack on this vitally important problem has been based on the assumption that the evaporation of the solvent mixture is essentially a phenomenon of distillation in which, as in the simple distillation of liquid mixtures, the composition of the vapor given off a t any instant is always in equilibrium with the liquid mixture from which it is being evolved. While it has been recognized that constant-evaporating mixtures (i. e., those which evaporate without change in composition during the process) are not identical in composition with the constant-boiling mixture of the same components, the difference has apparently been ascribed solely to change in composition of the constant-boiling mixture with the temperature. Thus, Hoffman (3) not only gives this explanation but shows that he considers the constant-boiling mixture a t the temperature in question as identical with the constant-evaporating mixture by including two of the former mixtures in a table (Table [X) in which the remaining constant-evaporating mixtures were otherwise determined. A study of this problem recently undertaken developed the fact that the relationships are far more complicated. If the vapor evolved from a film is in equilibrium with the liquid in it, it is clear that one way to prevent completely any change of Composition of the solvent mixture in the film is to employ a mixture of constant boiling point, since the equilibrium vapor from such a liquid, however complex the composition, is identical in composition with the liquid itself. To check this point, a lacquer was made up with a binary constant-boiling mixture of methanol and benzene, with the surprising result that, on drying, change in composition occurred as evaporation proceeded. This phenomenon is of special interest since Hoffman (3) classifies this specific mixture as a constant-evaporating mixture because, a t the temperature of the evaporation, it possessed a constant boiling point. Careful investigation of the phenomenon is clearly desirable.
the constant-boiling composition the relative volatility of the mixture is unity. The volatility of a component in a binary mixture is defined as the partial pressure of its vapor in equilibrium with the liquid divided by its mole fraction in the liquid. The relative volatility is the ratio of the volatilities of the components-i. e., for a binary mixture:
where x = mole fraction of one component in the liquid y = its mole fraction in the vapor Figure 1 shows that the constant-boiling mixture becomes poorer in methanol as the temperature decreases; a t 25" C. it contained 51.3 per cent methanol. A 4.5-ml. portion of a lacquer containing 3 per cent of halfsecond nitrocellulose and 3 per cent ester gum in a 59.1 mole per cent methanol mixture with benzene was placed in each of a series of 50-mm. evaporating dishes, covered with a coarse wire mesh rectangle. The set of dishes was placed in a large desiccator, containing 250 ml. of 85 per cent sulfuric acid, to eliminate complicating effects of water vapor. From time t o time a dish was removed from the desiccator, the loss in weight due t o evaporation was determined, and the change in composition of the residual solvent was found by evaporation of the lacquer to dryness under vacuum, by condensation of the evolved solvent in a trap cooled t o -75" C., and by determination of its refractive index. The temperature of evaporation was 22" C., held constant within a degree.
The data so determined are plotted as the dotted lines on Figure 2, where the percentage of the total solvent evaporated and the composition of the residual liquid are shown as functions of the time of evaporation. The same data are replotted in Figure 3 as mole per cent methanol in the liquid
SIKCE evaporation was a t room temperature, it was im@ portant to know the composition of the constant-boiling mixture a t these temperatures. Isothermal vapor-liquid equilibria of methanol-benzene mixtures were determined in an Othmer apparatus a t 58.9", 44.1", and 25" C. (I,,%), The composition of the constant-boiling mixture can be accurately determined by plotting the relative volatility against the mole fraction of methanol in the liquid as in Figure 1. At
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with the liquid a t that point, but the components of the vapor as they diffuse out into the gas move independently. Consequently, their rate of motion will be dictated not only by their concentrations-i. e., the partial pressure differences between the interface and the body of the gas into which they are moving-but also by their specific diffusion rates through the gas film. In general, diffusion rate is higher the lower the molecular weight of the diffusing constituents. Since the partial pressure of all constituents in the main body of the gas is often low enough to be negligible, it is obvious that the constituents of low molecular weight will diffuse rapidly away
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US. the percentage total solvent evaporated. Inspection of the figures brings out the surprising result that, on evaporation the composition of the liquid changes continuously from the initial value, through the constant-boiling mixture to a mixture containing no detectable methanol, in spite of the fact that, if the evaporation were a simple distillation process, the composition should not have fallen below the constantboiling mixture. However, it may well be that the lacquer constituents, particularly the gum, affect the solvents and change the composition of the constant-boiling mixture. To test this point, the solvent mixture was evaporated alone, without the addition of nitrocellulose or gum. The solid lines on Figures 2 and 3 show the results. The effect is similar to that obtained with the lacquer; namely, the composition of the mixture changes during evaporation, even when it has reached the constant boiling point. Apparently the ester gum has some effect, since the gum-free mixtures are always richer in methanol than the lacquer a t corresponding degrees of evaporation. These data prove that it is possible to break up a constant-boiling mixture by evaporation.
ANALYSIS of the mechanism of evaporation makes it 0 possible to show why these results were obtained. In a THESE factors, hitherto unappreciated by lacquer comboiling liquid the vapor evolved pushes back mechanically Q pounders, are of fundamental importance in determining any as or vapor ahead of it and moves bodily away from the change in composition of lacquer solvents during drying. liqui8 surface. There exists no tendency for the vapor, once The assumption hitherto made that a solvent mixture of conevolved, to change its composition. In the evaporation of a liquid mixture into an inert gas, the situation is entirely different. The vapor pressure of the liquid is less than the external pressure of the gas with which it is in contact; hence, the vapor evolved cannot move bodily away from the liquid surface, because it cannot push back the external pressure. The only way the vapor can escape from the liquid surface is by diffusion through a relatively stagnant film of the gas. The vapor a t the liquid-gas interface is in true equilibrium
stant boiling point a t the drying temperature of the film is a constant-evaporating mixture is, in general, false and likely to be grossly misleading.
Literature Cited (1) Castleman, M., Thesis, Mass. Inst. Tech., 1932. (2) Fink, C. E., Ibid., 1933. (3) Hoffman, H. E., IND.ENQ.CHEM.,24, 136 (1932). RECEIVED February 6, 1935.
DIORAMA OF MERRIMAC CHEMICAL COMPANY PLANTAT BOSTON,MASS.
