INDUSTRIAL AND ENGINEERING CHEMISTRY
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acknowledged; several samples of highly purified olefins and paraffins from H. C. Buc of the Standard Oil Development Co.; a number of aromatics containing unsaturated side chains (up to Ca side chain) from President Conant of Harvard University; several aliphatic diolefins from Frank Cortese; and a number of valuable terpenes, sesquiterpenes, and related compounds from Schimmel & Co., through the courtesy of F. E. Watermeyer, president of Fritzsche Bros., their ilmerican representatives.
Literature Cited (1) Aubert and AubrBe, Compt. rend., 182, 577 (1926). (2) Davis, IND. ENQ.CHEM.,Anal. Ed., 1, 61 (1929).
VOL. 7, NO. 4
“Identifioation of Pure Organic Compounds,” Vol. I, New York, John Wiley & Sons, 1908.
(3) Mulliken,
(4) Ibid., p. 199.
(5) Mulliken, J. Am. Chem. SOC.,32, 1049 (1910). (6) Mulliken and Wakeman, IND.ENG.CHEM., Anal. Ed., 7, $9 (1935). (7) Mulliken and Wakeman, Rec. traw. chim., 54, 366 (1935). (8) Norris and Rigby, J. Am. Chem. SOC.,54, 2088 (1932). (9) Wakeman, Ph.D. thesis, Massachusetts Institute of Technology, 1930. RECEIVEDFebruary 8, 1935. Abstract of a portion of a dissertation submitted t o the faculty of the Massachusetts Institute of Technology by R. L. Wakeman in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1930. Contribution No. 121 from the Research Laboratory of Organic Chemistry, Massachusetts Institute of Technology.
The Evap-0-Rotor A Device for Comparing the Evaporation Rates of Lacquer Solvents J. HERBERT LOWELL E. I. du Pont de Nernours-tk Co., Inc., Parlin, N. J.
A
MOKG the tests employed in the lacquer industry
to control and judge the performance of the product, the determination of rates of evaporation of the volatile solvents holds an important place. This test, if carried out on a comparative basis, furnishes a measure of the relative drying times of lacquers, and as a consequence yields valuable preliminary information pertaining to the probable flowing-out characteristics and blush-resistance of the drying film. It is generally accepted that, other factors being constant, the blush-resistance varies inversely with the rate of evaporation of the solvents. Flow, although not a function of the volatility alone, is governed by it to a considerable extent. The maintenance of the proper balance between active solvents and diluents during the drying process is extremely important from the standpoint of film characteristics, and this is largely determined by the relative rates of evaporation of the active and inactive components of the solvent system. The information derived from simple evaporation rate comparisons of solvents and solvent mixtures, although unquestionably of value in predicting the behavior of the finished lacquer in many important respects, is open to question from several angles. I n the first place, as the test is usually conducted in the absence of nitrocellulose, plasticizer, or other film-forming ingredients, the effect of these nonvolatile constituents of the finished lacquer is not taken into account. Nitrocellulose preferentially holds certain solvents by virtue of solvation or association. This property tends to distort the normal vapor pressure relationships as determined for the components of the solvent system alone. Furthermore, mixtures of certain solvents may behave abnormally because of the formation of azeotropic combinations; so that the evaporation rate of the solvent mixtures is not always predictable from the behavior of the individual constituents. Also, some of the methods in general use in the industry for determining evaporation rates do not yield reliable and reproducible results. I n measuring any physical constant, whether for purely scientific or for industrial application, it is highly desirable, whenever possible, to express results in absolute units. Evaporation, however, is affected by such a variety of external variables that it is virtually impossible, where simplicity and speed are of prime importance, to establish sufficiently controlled conditions to allow absolute expression
of results. Hofmann (3) expresses the evaporation rates of organic liquids in terms of the slopes of the evaporation rate curves, selecting n-butyl acetate as a standard of comparison and assigning to it an arbitrary value of 100. It is obvious that only pure liquids, the slopes of which are constant, can be satisfactorily treated in this way. I n any case, the results, however expressed, are not reliable unless a method of determination is employed which will yield reproducible data. The ideal method for measuring evaporation rates should provide for rigid control over many variables such as temperature, pressure, humidity, velocity of the air, and the amount of liquid surface exposed. A method described by Polcich and Fritz (4) takes these factors into account perhaps as well as any yet devised. Air of definite temperature and humidity is passed a t a given rate over the sample of liquid contained in a flask maintained a t a constant temperature. The volume of air required to evaporate the sample is used as a measure of the rate of evaporation. Careful standardization of the manner in which the air is led over the liquid is important, however, and the method offers the disadvantage that only one liquid can be studied a t a time. Realizing the impracticability of attempting to control all the variables encountered, lacquer technologists have endeavored to minimize their effect by resorting to a method whereby the evaporation rate of the solvent under investigation is determined simultaneously with that of some standard liquid. This allows a direct comparison which should not alter appreciably with moderate changes in conditions. As usually carried out, this method consists of pipetting equal volumes of the standard and experimental solvents into uniform cups or pans of predetermined weight. The weights of the samples are then quickly determined t o the nearest milligram, and the cups are placed close together on an exactly level platform where the solvents are allowed t o evaporate under the prevailing atmospheric conditions of the laboratory. At intervals the samples are reweighed and the loss in weight and the time are recorded. The percentage evaporated versus the time is finally plotted on coordinate paper and the graph furnishes a convenient record of the desired comparison. To simplify weighing, several devices have been proposed, among which are the Jolly balance and the Hart balance (a). The latter employs an evaporation pan attached indirectly t o a pointer, by means of which the weight evaporated can be read directly from a scale at any time during the course of the determination.
JULY 15, 1935
ANALYTICAL EDITION
This general method, which may be called the "static method," although widely used, is far from accurate. Local temperature differences, inconsistent air currents, and stagnation of the soIvent vapors over some of the cups (when a large number of comparisans are being made a t one time) account, for the major portion of the discrepancies observed. The use of guards for the purpose of minimizing the effect of air currents serves only to accentuate the stagnation effect which may be accompanied by a certain amount of condensation that is apt to occur in any partially enclosed space. Wilson and Worster (6) have attempted to improve upon this method by placing the evaporation cups in a tunnel through which air is forced by means of an electric fan. While this is a step in the right direction, it appears doubtful whether the velocity of the air is actually constant over the entire mea occupied by the samples, especially when half a dozen or more liquids are being studied simultaneously.
Evap-0-Rotor This brings us to a discussion of a device developed by the author for a more accurate determination of evaporation rates. The instrument, which has been christened the Evap-0-Rotor, consists of a circular turntable 40.5 cm. (15 inches) in diameter rotating at a constant rate of 1 revolution per minute. The table is constructed of two parallel aluminum plates spaced about 1.25 cm. (0.5 inch) apart. Around the outer edge of the upper plate are ten equally spaced holes of suitable diameter to receive the evaporation cups and allow them to rest squarely on the level surface below. An adjustable circuIar guard about 15 cm. (6 inches) in height surrounds the rotating parts; this can be raised, lowered, or removed altogether as desired. Power is furnished by a 0.25-horsepower, vapor-proof, electric motor operating the table through a gear-reduction unit. The entire apparatus is mounted on a heavy iron base which is bolted t o a bench i n such a manner that the rotating table is in an exactly level position. A coiivenient type of evaporation cup to use with this instrument ie a flat-bottomed, straight-sided aluminum pan 2.5 cm. (1 inch) deep by 6.25 cm. (2.5 inches) in diameter. For accurate results it is important that the evaporation cups possess a high degree of uniformity. Slight variations in depth, small dents, or corrosion pits in the metal and in general any irregularity in the bottom of the cup will often disturb the rate of evaporation, particularly near the dry-point of the determination. Some iiivestigators (1) prefer to use a cup with a very slightly concave bottom. This is advantageous in that the last traces of solvent tend to collect at the center instead of in the corners as sometimes occurs with the flat-bottomed type. The procedure followed in carrying out the determination is essentially the same as that described under the static method. Two to five cubic centimeters of liquid (depending on the vapor pressure of the material under test) are pipetted into the cups aiod, after weighing to the nearest milligram, these are placed in the depressions on the rotating table. The time is recorded as each successive sample is started, and at intervals the C U ~ Sare reweighed in the same order that they were originally placed on the machine. A maximum of ten samples can be handled conveniently in this way. The principle of the Evap-0-Rotor is simple and does not require a detailed discussion. Obviously the device does not actually control variable conditions in the laboratory, but its value lies in the fact that it distributes the effect of these variables evenly over all the samples under test and thus prevents any marked distortion of the comparative results The rotary principle insures that all the cups are successively subjected to any local differences in air currents and temperature which may occur within the area occupied by them. The slow continuous motion is also effective in preventing stagnation of vapors over the evaporating liquids.
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Comparative Tests In order to test the efficacy of the instrument, numerous tests were conducted comparing the static and Evap-ORotor methods. The general procedure employed was to pipet exactly 5 cc. of pure toluene into each of ten standard evaporation cups and to record the total time required for each sample to evaporate. Obviously if the method used were ideal, these times would all be identical. Early tests using the static method showed variations from the mean time of more than 12 per cent in some cases. While it was possible by use of the Evap-0-Rotor to show that about one-fourth of this error could be attributed to slight inconsistencies in the cups used, recent tests conducted with highly standardized cups have confirmed the inaccuracy of the static method. A typical test has been selected to illustrate the advantages of the Evap-0-Rotor over the older method. Twenty evaporation cups of high uniformity were selected and 5 cc. of toluene were pipetted into each. Ten of these were placed on the Evap-0-Rotor and the remainder were grouped close by on an exactly level platform. The samples were allowed to evaporate in the absence of any direct draft except for incidental air currents in the laboratory. Table I shows the results of this direct comparison of the two methods. While the maximum deviation from the mean by the static method is 5.4 per cent, the Evap-0-Rotor reduces this error to a maximum of 1.4 per cent. TABLEI. STATICMETHODvs. EVAP-0-ROTOR METHOD (Test liquid, 5 cc. of toluene) Methoda7 Evap-0-Rotor- b Variation Variation Evaporation from mean I Evaporation from mean CUP time time CUP time time iMin. % Min. % -5.4 1 306 -1.4 -4.2 2 309 -0.4 -1.4 3 3 12 f0.5 -1.4 4 312.7 +0.8 f1.2 5 310.6 0.0 +1.8 6 312.2 +0.6 +0.4 7 311 +0.2 +2.6 8 309.7 -0.2 +4.7 9 309.6 -0.3 +1.3 10 310 -0.1 Mean time 310.3 0 Temperature, 24.5' to 25.5' C. b Temperature, 23.5" to 25.0' C.
-Static
Whether the shield surrounding the turntable has any bearing on the accuracy of the results has not been determined. The principle on which the device is based, however, would indicate that the shield probably is not essential. For determining the evaporation rates of solvents of low vapor pressure, it should be possible to employ an electric fan in conjunction with the Evap-0-Rotor. While the author has not experimented along this line, other users of the instrument report satisfactory results with such a combination.
Acknowledgment I n closing, the writer wishes to acknowledge the credit due J. P. Burke of the du Pont Parlin Laboratory, not only for his assistance in designing the Evap-0-Rotor but also for his cooperation in obtaining the data used in this paper.
Literature Cited (1) (2) (3) (4) (5)
Bridgman, IND. EKQ. cam^., 20, 184 (1928). Hart, Am. Paint Varnish Mfrs. Assoc., Circ. 360 (1930). Hofmann, IND. ENG. CHEM., 24, 135-40 (1932). Polcich and Fritz, Brennstoff C h m . , 5, 371 (1924). Wilson and Worster, IND.ENQ.CHEM.,21, 592 (1929).
R E C ~ I ~ April E D 19, 1934. Presented before the Division of Paint and Varnish Chemistry at the 89th Meeting of the American Chemical Society, New York, N. Y., April 22 to 26, 1935.
