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312
The solutions containing the higher concentrations of stearic acid and benzoyl peroxide also formed separate phases upon cooling, but in contrast to the cetyl alcohol the separation of these compounds caused no increase in power factor or conductivity. It was found by microscopic examination that in the case of the stearic acid and benzoyl peroxide the phase which separated was in the form of minute crystals which remained in suspension in the oil. Although the number of cases examined is too limited to permit defhite conclusions to be drawn, the data indicate that from a practical point of view the power factor and the conductivity of an insulating oil, as exemplified by the liquid paraffi, are only slightly changed by the presence in homogeneous solution of moderately large quantities of certain types of dry oxidation products. The work is being continued uvon both oils and oil-and-DarJer insulations. In addition to s&ble oxidation products,&insolubleoxidation products and other typesof impurities which may be either found in or formed in cable insulation are being investigated.
VOL. 28, NO. 3
Literature Cited (1)Boeseken, J., and Gaster, A,, Rec. trav. chim., 49, 102 (1930). (2) Dietrich, W., Helv. Chim. Acta, 8, 149 (1925). (3) Gelissen, H., and Hermans, P. H., Ber., 59B,662 (1926). (4) Hickman, K. C.D., private communication describing improvement in apparatus given in J. Franklin Inst., 213,119 (1932). (5) Jackson, W., J . Inst. Elec. Engrs. (London), 75, No. 451, 93 (1934). and Muller, Hans, Phys. Rev.,32, 979 (1928); (6) Kitchin, D. W., Kitchin, D. W., Trans. Am. Inst. Elec. Engrs., 48,495 (1929). Race, H.H., S.Phys. Chem., 36, 1928 (1932). Schering, H., Z.Instrumentenlc., 1920, 124. (9) Shanklin, G. B., and Mackay, G. M. J., Trans. Am. Inst. Elec. Engrs., 48,338 (1929). (10) Shrader, J. E., J. Franklin Inst., 199,513 (1925). (I1) Whitehead, J. B., Trans. Electrochm. SOC.,65,35 (1934). (12) Wyatt, K.S.,Spring, E. W., Fellows, C . H., Trans. Am. Inst. Elec. Engrs., 52, 1035 (1933). RECEIVED September 21, 1936. This work forms part of an inveetigation on the deterioration of high-voltage underground cable being conducted b y The Detroit Edison Company under the direction of C. F. Hirshfeld, chief of research.
NITROCELLULOSE LACQUERS Rate of Evaporation of Liquids F. A. BENT AND S. N. WIK Shell Development Company, Emeryville, Calif.
@ I
N FORMULATING thinners and lacquers, data are needed on the relative rates of evaporation of the various solvents and diluents in order to determine whether, in the practical applications of these solvents and diluents, their initial solubility relations for nitrocellulose and resins may be disturbed during the drying. If a proper balance of solubility is not maintained throughout the drying of the lacquer, the solid ingredients may be precipitated with a consequent loss of the desirable qualities of the resulting h. I n spite of the importance of a knowledge of the rate of evaporation of solvents and thinners, no standard method of determining this property of liquids has been adopted by the lacquer industry. Id an important article Hofmann (3) correlates the results of various methods of determining rates of evaporation with the results he obtained using a modification of a method described by Polcich and Fritz (4). This modified method cannot, however, be used in the rapid determination af rates of evaporation of several liquids a t the same time; hence a single determination cannot give comparative data on the rate of a number of liquids. A method similar to that of the writers for measuring the relative rates of evaporation of liquids is mentioned in an article (I) which appeared while the present report was being prepared. Almost certainly the results obtainable with the two methods will check closely. Data on single liquids or mixtures of liquids are taken with the apparatus described in the following pages. The absolute rates of evaporation of solvents and diluents usually change
A rapid method of determining rates of evaporation of liquids which has given results serving as a reliable guide in the formulation of lacquer thinners has been developed. A comparison of this method with others prevailing in the lacquer industry is made. The rates of evaporation were determined for various classes of organic solvents and diluents used in the lacquer solvents and diluents used in the lacquer industry. The rates of evaporation of the liquid components of a lacquer influence the structure and appearance of the final film. somewhat when these materials are added to lacquer mixtures, but the relative rates, which are measured with this apparatus, are preserved during the evaporation unless the materials form constant-evaporating mixtures.
Method and Apparatus The time required for the complete evaporation of a definite amount of each liquid exposed to a draught of air is the important measurement in the empirical method developed for comparing the rates of evaporation of liquids: An electric fan, mounted on a turntable which revolves at a speed of 4 revolutions per minute, extends through a hole in the center of a platform. The blades of the fan, inclined at an angle of 70" to the horizontal, send an intermittent current of air over and into a set of aluminum pans which contain the evaporating liquids. The aluminum pans, 2 inches ( 5 cm.) in diameter, 0.5 inch (1.27 cm.) in depth, and containing a close-fitting filter paper, are fixed in position on the platform in a circle around the fan. The filter paper insures uniform wetting and hence a uniform evaporating surface during the final stages of the evaporation, when the various liquids may otherwise expose greatly different evaporation areas. Figure 1 is a view of the platform from above showing the pans arranged about the fan: Figure 2 shows, from beneath the platform, the mechanism driving the turntable.
INDUSTRIAL AND ENGINEERING CHEMISTRY
MARCH, 1936
In making the test, duplicate portions of 5 cc. of each of the liquids under test and of the liquid taken as a standard are
measured with a pipet into the aluminum pans which are placed in position on the platform while the fan and turntable are in motion. The time from the initial exposure to the current of air to the time for complete evaporation of the liquids is noted. The time of evaporation of the standard liquid (usually whutyl acetate) is divided by the time of evaporation of the liquid under examination; in this way the value 1.00 is assigned to the rate of evaporation of the standard liquid. Thus, values greater than 1.00 indicate meater rates of evanoration than that of the standard liquid: those under 1.00, &aller rates. When it is desired to obtain an evaporation curve of a liquid, the Dam are removed at reeular intervals of time durine the evafiration, tightly cove red^ and weighed on an analFtica1 balance. The curve is constructed by plotting loss of weight against time on rectangular coiirdinates.
Precision of Method The method and apparatus described have been used during the last three years under different conditions of laboratory temperature and probably of humidity, although the latter was not recorded. During that time, consistent and reproducible results have been obtained by different operators with an average deviation of *0.11 unit; the maximum deviation is not greater than 5.6 per cent of the mean of the rates. The effect of ordinary variation in laboratory temperatures on the relative rates of evaporation of n-butyl acetate and of toluene is as follows: Rnte of Evapn.'
No. of Detna. Aversped
Room
Temp. 0
?0
1.96 1.94
22
1.95
19
21
1.95
2 2
6
8
8 Ratio oi n-butyl seetnte t o toluene. 23
1
1.92
c.
24 25 26 27
Rate of Ev*pn.* 1.95 1.95 1.95 1.93
No. oi Dctns. Averaged
9 5
1
2
It is apparent that the average variation of temperature throughout the year in this laboratory has little effect on the relative rates of evaporation of toluene and n-butyl acetate.
Evaporation Process When liquids are evaporated using this method, eqiiilibrium between the liquid and vapor phases is never attained, because the molecules are swept away from the surface of the liquid by the draught of air. In this way, evaporation of the liquid is greatly accelerated. The necessary,heatrequired for vaporization is supplied from the surroundmg a u and the platform, with the result that evaporation always goes on at temperatures below that of the room. The thermal and molecular properties which control evaporation during this process are not dicussed, for they have not been investigated separately; but such inherent properties, although affecting an absolute messurement of the rate of evaporation, necessarily all affecta relative measurement of the rate of evaporation of a liquid. However, i t may he of interest to consider the effectof probable association of molecules as indicated by the latent heat of vaporination (measured a t the hoiling point) by coinparing this property with the relative rates of evaporation and the boiling p o i n t s of s e v e r a l aliphatic alcohols and ethers. T h e r e l a t i v e r a t e s of evaporation of alcohols and ethers arranged in the order
313
of their boiling points are shown in Table I, referred to the rate of n-hutyl acetate as unity.
-________
~
_-
TABLE I. COMPARIGON O F VOLATILITY OF ETHERS AND ALcoaoL8 Latent
Substnnee
Rate of
E>-apn."
B. P.st 760 M m . 0
Methyl terbhutyl ether Methyl alcohol Ethyl lert-butyl ether Ethyl alcohol Isopropy1 alcohol Methyl lert-amyl ether aec-Rutyl aloohol Ethyl w - a m y l ethei sa-Butyl tcrl-butyl ether pec-Amyl alcohol
7.40
3.70 6.40 2.05 2.08 4.37 0.96
3.43 2.25
0.47
%-Butylaleohol 0.44 n-Butyl Dt-butyl ether 1.26 rB"tv1 acetate = 1.00, ~
Mol. Weight
c.
55.2 64.7 72.8
78.3
82.4
86.3 99.5 101 114 116-119 117 125
Heat .of vapor,.*-
tion
Cd./O. 88 32 102 46
60
102 74 116 130 88 74
130
76.8 74.3 202.2 161. I 72.8
262.2
134.4
73.1
...
1ii:z
...
~-
Table I shows clearly that the boiling point of a liquid is not indicative of it.s rate of evaporation, except for homologs; for instance, isopropyl alcohol has a lower boiling point than methyl tat-amyl ether, yet isopropyl alcohol has a rate of evaporation less than half that of the ether. Hygroscopic liquids absorb water from the atmosphere during their evaporation, and probably the evaporation of the liquid is retarded as a result; hut as was previously pointed out: the method described is relative and affords a measurement which does not require correction for practical usage.
Rates of Evaporation of Single Liquids The rates of evaporation of members of sevcral homologous series of liquids were determined. In the test.s, the liquids and a sample of a referenceliquid were evaporated to complete dryness at the same t i e and under the same conditions. In all cases the materials examined were of a conimcrcial grade of purity. No attempt was made to control the temperature of the room, the humidity of the atmosphere, or the temperature in the evaporating liquid.
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TABLE 11. RATEOF EVAPORATION OF VARIOUS LIQUIDS Substance
Rate of Evaporationa
Boiling Point O
c.
mol. Weight
Acetic Esters of Lower Aliphatic rllcohols 74 7.15 57 Methyl acetate 88 77 4.95 Ethyl acetate 102 3.13 92 Isopropyl acetate 116 112 1.78 see-Butyl acetate 116 1.00 126.5 n-Butyl acetate 130 131-133.5 0.83 sec-Amyl acetate 130 126-155 0.56 Pentecetate 144 146-156 0.34 see-Hexyl acetate Ketones 7.70 56 58 Acetone Methyl ethyl ketone 4.59 79.6 72 Methyl propyl ketone 2.02 102 86 Meth 1 butyl ketone 0.60 127 90 Cy oloKexanone 0.24 155 98 Lower Alcohols 32 3.70 64.7 46.1 78.3 2.65 78.1 1.86 60.1 82.4 2.08 74.1 0.44 117 74.1 107 1.48 74.1 99.5 0.96 74.1 1.92 82.3 88.1 0.47 116-1 19 88.1 0.85 101.8 Ethers 88 55 7.40 Methyl tert-butyl 102 86.3 4.57 Methyl terl-amyl 102 6.40 72.8 Ethyl tert-butyl 116 101 3.43 Ethyl tert-amyl 130 123 1.26 n-Butyl tert-butyl 130 114 2.25 see-Butyl lert-butyl Aromatic Hydrocarbons 4.76 80.2 78.1 Benzene 1.96 110.7 92.1 Toluene 106.1 0.55 131.5 Xylene 0.89 135.8 106.1 Ethylbenzene a Ratio of time of evaporation of n-butyl acetate to that of the liquid.
...
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De Heen (2) found that the quantity of a liquid evaporated in a given time interval is proportional to the product of the vapor pressure and the molecular weight. Hofmann confirmed de Heen’s findings and showed that approximately accurate results can be calculated from the formula: (Vapor pressure X mol. weight)/ll = rate of evaporation
The proportionality factor depends upon the conditions under which the evaporation takes place. This formula, with the shows the approximate relative rate of evaporaconstant tion of a liquid a t 25” C., referred to n-butyl acetate. The rates of several liquids measured by Hofmann are compared in Table N to rates of evaporation calculated by de Heen’s formula and also to rates of ,evaporation measured by the writers. A good agreement is found between the values for the rates of evaporation measured by the modified Polcich-Fritz method and measured by the method developed here. Both methods afford results in fair agreement with the calculated values. Time-evaporation curves of several petroleum aliphatic hydrocarbon diluents obtained from the Shell Petroleum Corporation are shown in Figure 3, compared to time-evaporation curves of toluene and n-butyl acetate. This method of representation has the value of depicting all stages of evaporation of a liquid. Such curves can also be used to express relative rates a t any stage of evaporation by dividing the time required for a certain volume of a reference liquid to evaporate, by the time required to evaporate an equal volume of the liquid under test. Similarly, an instantaneous relative rate of evaporation can be found from the slopes of the curves of the liquid under test and the reference liquid.
Rates of Evaporation -. of Binary
L
I
Mixtures
OF ALIPHATIC HYDROTABLE111. RATESOF EVAPORATIUN CARBON DILVENTS
When a blend of two liquids which do not form a constant-evaporating mixture is evaporated to Manufacturer or Supplier Substance complete dryness, the time of evaporation is alLacquer Diluent A most linear with the initial composition. That Lacquer Diluent B Lacquer Diluent C is, when the times of evaporation to complete TS-11 dryness of a number of mixtures of the two liquids Rotosolve P. & V. Thinner of different composition are plotted against the Benzo-Sol Tolu-Sol initial composition of the mixtures, the resulting Diisobut lene curve is almost a straight line and can be so conUnion Oil‘Solvent N o . 8 “10” Toluene Substitute sidered for practical purposes of formulation. B. & L. Thinner Troluoil Several such mixtures which apparently do not Cook Co.’s. naphtha form constant-evaporating mixtures, are shown Petroleum ether in Figures 4 and 5. The average relative rate of Time of evaporation of standard to that of the liquid. evaporation of a particular composition of any two of the components which are plotted can be TABLEI v . COMPARISON O F FAN METHODWITH MODIFIED obtained by dividing the time of evaporation of the reference METHODUSED BY HOFMANN POLCICH AND FRITZ liquid by the time of evaporation of the mixture. Rate of Evaporation When two liquids form a constant-evaporating mixture, Polcich-Frit. Bent-Wik Calcn. of Rate -Rate of Evaporationa---Toluene = 1.00 n-BuAo = 1.0 2.85 6.38 0.32 0.60 1.20 2.26 0.89 0.44 0.30 0.57 0.22 0.11 4.40 8.65 1.83 3.58 1.68 3.29 1.17 2.36 2.30 1.22 4.25 2.17 2.88 1.44 2.20 1.11 1.55 0.79
0
Substance method method ( X 100) Ethyl acetate 485 495 sec-Butyl acetate 162 178 sec-Amyl acetate 73 83 %-Butylacetate 100 100 95% ethyl alcohol 193 186 sec-Butyl alcohol 110 96 n-Butyl alcohol 45.7 44 Acetone 850 770 Toluene 186 194 65.5 55 Xylene 0 (Vapor pressure X mol. weight)/ll.
of Evapn.a 582 180 85 100 186 87.5 31.7 975 186 62
The rates of evaporation of various liquids are given in Table I1 Aliphatic hydrocarbons derived from petroleum have been used extensively by the lacquer industry as substitutes for toluene. The rates of evaporation of some petroleum diluents commercially available are shown in Table 111.
0
10
20
30
40
50
60
70
MINUTES
FIG~URE 3. EVAPORATION RATESOF TOLUENE, R-BUTYLACETATE, BENZO-SOL, AND TOLU-SOL
’
INDUSTRIAL AND ENCSNEERING CHEMISTRY
MARCH, 1936
I
100
(10
60
40
20
0
SCC. AMYL A C S T A Z
PEn CENTBY VOLUME
&urmr;s 4 AND 5. EVAPORATION & m s OF BINAZY MIXTURES(10-Cc. SAMPLES) Flgure 4 F18uie 5
Methyl ethyl ketone and see-amyl acetate scc-Butyl acetate and sec-amyl acetate
the curve showing initial composition of different mixtures plotted against their time of evaporation is not a straight line. Such a CUNC is shown in Figure 6 , representing mixtures of methyl ethyl ketone and a petroleum hydrocarbon diluent. It is not known just which constituent of the diluent wed is responsible for this singularity, but the case illustrated is unique, for methyl ethyl ketone does not form constant evaporating mixtures with the ordinary petroleum hydrocarbon diluents. Constantevaporatmg mixtures are, however, likely to be formed from the components of ordinary lacquers, and unless this is kept in mind, the rate of evaporation of the final mixture will not be that predicted from a consideration of the rates of evaporation of the individual components of the lacquer.
Selection of Evaporation Data There is some difference oE opinion as to the relative value of evaporation data expressed graphically by time-evaporation curves, or expressed as time to total dryness in terms of a standard reference liquid. Some lacquer chemists believe that the first 30 to 40 per cent evaporated is the fraction of the lacquer which introduces water into the film and causes roughness or "orange peel" and that the following 30 to 40 per cent is most effectivein removing any water and roughness of the filmwhich may be present. Another view is that data giving relative evaporation to total dryness are most important, Both methods of expressing evaporation data appear to be of use. Graphical representation either by time-evaporation curves or by average relative evaporation curves is especially applicable to the comparison of mixtures to indicate the evenness of evaporation. The writers do not believe that a timeevaporation curve of the liquid portion of a lacquer can be used to predict whether or not a lacquer will give, on drying, a smooth or a rough film. It seems more probable that the ultimate quality of the film will depend on the vis-
CONSTANT-EVAPORATING MIXTURES(5-Cc SAMPLES)
315
cosity of the nitrocellulose lacquer just prior to drying. A controlling factor of viscosity is the solvent power for nitrocellulose of the liquid in its final evaporation stages; accordingly, the method of expressing data by relative evaporation to total dryness appears to be of more practical value. From such data we can predict the relative time each component will remain in the lacquer during its drying period, and from this knowledge balance the solvent power of the mixture throughout the drying period by a proper combination of diluents (nonsolvents) and solvents for nitrocellulose. This view appears to be supported by the consideration expressed in the folloxving paragraphs.
Effect of Rate of Evaporation of Lacquer Ingredients on Film formation The rates of evaporation of the liquid components of a lacquer determine, in part, the appearance of the film which is obtained by applying the lacquer to a surface. Evaporation of the liquid components begins immediately when the lacquer is applied to an exposed surface. If the lacquer is sprayed, a portion of the liquid components of the lacquer is evaporated during the transit from the spray gun to the surface. After the lacquer has covered the surface, evaporation continues and the lacquer is said to "flow out." A lacquer bas good f l o w w h e n t h e film settles to a perfectly smooth, even surface, with good gloss. If the liquid components leave the drying film too rapidly, the viscosity of the lacquer may i n c r e a s e rapidly, and a pitted uneven surface may result because the excrescences will n o t have had time to level out. In general. fastevaporating lacquers show a coarse structure. To illustrate t h i s point, three different lacquers were made using the same base lacquer blended with thinners of different rates of evaporation. The initial excess tolerance was the same in each case, as were also the proportions of thinner to base lac- ' quer. These lacquers were s p r a y e d i n a similar manner on to
thinner
oontaining isst-evspointine thinner
P. I,Bcquer
INDUSTRIAL AND ENGINEERING CHEMISTRY
316
glass plates, the sprayed portion was next covered, and then the adjoining section of each plate was sprayed with the same base lacquer blended with a slowdrying thinner. The photomicrographs of Figure 7 indicate not only the similarity of spraying technic, by comparing the films on the left of the dividing line in each photograph, but also the differences between the structures of films of fast-drying and slow-drying lacquers, by comparing the films on either side of the line. The ra*te of evaporation of the thinners used in these tests increases in the order of the numbers. I t is clear that in this
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case thinner with the smallest rate of evaporation has yielded the smoothest film.
Literature Cited (1) Eastlack, H. E., Paint Oil Chem. Rev., 97,No. 9,22 (1935). (2) Heen, de, J. chim. phys., 11, 205 (1913). (3) Hofmann, IND. ENG.CKEM.,24, 135 (1932). (4) Polcioh and Fritz, Brennstqff-Chem.,5, 371 (1924).
RECEIVED September 12, 1935. Presented before the Division of Paint and Varnish Chemistry at the 90th LMeetingof the American Chemical Society, San Francisco, Calif., August 19 to 23, 1935.
RATES OF SOLUTION OF GASES IN OILS‘
E. A. BERTRAM AND W. N. LACEY California Institute of Technology, Pasadena, Calif.
Rate of Solution of Methane in Oils Filling Spaces between Sand Grains
vv
HEN the pressure within a partially depleted petroleum formation has been increased by injection of natural gas to the gas dome above the oil, there is a tendency for some of the gas to dissolve in the oil, thus lowering its viscosity, surface tension, and density. These possible favorable changes resulting from “repressuring” create interest in the rate a t which the gas may be expected to dissolve under different prevailing conditions. Previous experiments (9,3, 6) have shown that the rate of solution of a gas such as methane or propane in a quiescent body of hydrocarbon oil is controlled primarily by the rate of diffusion of the dissolved gas from the gas-oil interface into the body of the liquid. The rate for such a process is given quantitatively, up to half-saturation, by the equation:
where Q = quantity of gas which has passed through surface C, = final equilibrium concentration of gas in solution A = area of liquid, at right angles t o direction of flow D = diffusion constant 1
=time
Methods of predicting, with sufficient\ accuracy for engineering purposes, the values of D for methane (2) and for propane ( 3 )in various hydrocarbon oils have been proposed. Petroleum is found in the pore spaces or interstices between closely packed sand or rock particles. These particles are often siliceous but in some cases are calcareous. They are in many cases unconsolidated but in others are cemented firmly together to form hard rocklike masses. It is obvious from geometrical considerations that the rate of solution of a gas in a given oil will be 1 For other artidea on this general subject, Bee literature o i t a tiom S.S,B.
different when the oil is held within and completely fills the interstices of such a sand mass than when it is in the form of an oil body free of sand. The effective cross section will’ be less and the path of diffusion will be longer in carrying dissolved gas to the same depth below the gas-oil interface. It has been suggested ( I ) that the case of oil held in a sand may be complicated by surface effects due to the presence of large amounts of sand-oil interface which might assist the solution process. It was the purpose of the present investigation to develop methods of utilizing the constants for gases dissolving in oils determined by previous experiments, for the more practical case of oils held in unconsolidated silica sands. It was also desired to ascertain whether the effect of the presence of sand was merely geometrical in character.
A study of the rate of solution of gaseous methane in hydrocarbon oils entirely filling the interstices of closely packed silica sands has shown that the process is substantially the same as for the case of quiescent oils in the absence of sand. In the case with sands present, the over-all area at right angles to the path of diffusion must be multiplied by the fraction of the total volume of the sand body which is occupied by the oil and by a constant whose value is 0.82. The constancy of this factor has been experimentally verified only with unconsolidated sands and therefore throughout a relatively narrow range of porosities. Electrical conductance experiments upon copper sulfate solutions held in similar sand bodies gave practically the same value of the constant, indicating that the effect of the sand is only geometrical.
