Cellulose Ester Solutions EVAPORATION IN BINARY SOLVENT MIXTURES CHARLES R. FORDYCE
AND DAVID R; SIMONSEN Eastmun Koduk Company, Rochester, N . Y.
Solutions of cellulose derivatives in mixtures of organic solvents are employed for a variety of technical applications. A n understanding of the mechanism of evaporation of euch solvent mixtures is important to proper control of properties of surface coatings. Measurements during evaporation of several binary solvent mixtures have been made and correlated with vapor pressures. Solvent mixtures in absence of cellulose derivatives have been found to evaporate primarily according to liquid-vapor equilibrium characteristics during exposure to ordinary atmospheric conditions. In the presence of cellulose esters
I
evaporation in many cases deviates from this behavior, showing selective retention of certain types of solvents.
T
HE use of solutions of cellulose derivatives for lacquers, for cloth and paper coatings, and for casting of sheets often
involves the use of mixtures of two or more volatile solvents. These compositions are used sometimes in viscous and at other times in comparatively thin solutions and with solvents of high or low boiling points depending upon the desired rate of drying. I n addition t o cellulose derivatives or resins, such solutions may contain plasticizers, pigments, and coloring materials as required. Whatever the composition, the solutions are applied generally by suitable means as comparatively thin coatings, according to particular application, after which the solvents are allowed to evaporate. The manner in which such compositions evaporate upon drying is important to proper understanding of the coating operation and the quality of product obtained. It has been the object of this study to examine the evaporation behavior of several binary solvent mixtures in order to determine the effect of one solvent upon another and to determine whether or not the presence of cellulose derivatives in solution alters this behavior. Published data on evaporation of solvent mixtures suggest that it may follow a complicated behavior. Lewis and Squires (2, 3) have shown that evaporation under turbulent conditions proceeds according to vapor pressure characteristics, while under a quiet atmosphere diffusion of solvent vapors at the liquidvapor interface causes a modified behavior. Bennett and Wright ( I , 6) have indicated, however, that evaporation in an open atmosphere proceeds according to vapor pressure. It is, there-
I
. q ,I , , 2 500
40
20
,
, 60
,
, 80
,
I 100
100-
A C E T O N E - E T H Y L E N E CHLORIDE
80
20
40
60
80
100
0 2
a-1
PER C E N T ACETONE
701 0
'
20
t
40
60
80
-
8 P. AT ATM. PRESSURE
-
I
IO0
0
PER CENT ISOPROPYL ALCOHOL
PER CENT ACETONE IN VAPOR
Figure 2. Liquid-Vapor Composition Diagram of Acetone-Ethylene Chloride Mixtures
Figure 1. Boiling Point-Composition Diagram of Binary Solvent Mixtures a t Atmospheric Pressure
104
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
January 1949
I
TABLEI. LTQUID-VAPOR COMPOSITION DATA CONSTANT PRESSURE Methyl Alcohol-Ethylene Chloride, Atmospheric Pressure % Acetone In In Temp., Liquid vapor c. 0.0 0.1 10.0 0.6 22.4 2.2 7.1 30.3 14.3 34.7 38.1 30.2 42.2 45.3 48.9 59.5 58.6 73.5 74.8 87.5 99.1 100.0 Acetone-Chloroform, Atmospheric Pressure liquid In
vapor In
0.0 4.5 9.9 14.6 19.2 28.2 41.2 55.3 75.7 100.0
0.4 3.3 7.8 12.7 18.6 31.5 49.5 67.3 85.0 99.8
T:y' 60.2 61.4
...
63.0 63.2 62.9 61.5 59.6 57.4 55.3
Acetone-Methyl Alcohol, 100 Mm. yo Acetone In In Temp., liquid vapor c. n n 17.6 li.0 22.5 15.8 19.0 36.0 12.6 34.0 51 .O 49.0 65.0 10.8' 9.2 70.5 80.5 85.5 88.5 8.0 100.0 100.0 7.4
A C E T O N E - E T H Y L E N E CHLORJOE
Propylene Chloride-Isopropyl Alcohol, Atmospheric Pressure
% Isopropyl Alcohol In liquid 0.0 1.1 2.8 6.9 17.1 27.2 38.0 48.5 62.3 75.3 88.0 100.0
In vapor 0.1 5.8 13.6 25.2 35.3 41.0 45.7 49.8 56.3 65.0 76.8 96.8
Temp.,
c.
Cyclohexane-Ethylene Chloride, Atmospheric Pressure yo Cyclohexane In In Temp., c. liquid vapor 0.0 0.0 82.4 77.5 10.3 21.7 21.9 75.4 33.8 74.5 41.6 34.3 47.8 74.1 48.8 56.5 61.9 74.4 66.7 76.1 75.2 80.7 89.3 77.0 l0OIO 79.9 100 I O
yo Acetone
105
Acetone-Ethylene Chloride, Atmospheric Pressure YoAcetone In In Tempt, liquid vapor c. 0 0 82.4 17.7 34.2 73.3 36.3 59.7 67.0 52.5 75.2 62.8 75.0 90.0 58.7 100.0 100.0 55 3
CONSTANT TEMPERATURB Propylene Chloride-Isopropyl Alcohol, 30° C. yo Isopropyl Alcohol In liquid I n vapor 93.0 83.4 81.2 64.3 66.6 49.2 49.7 36.3 38.2 32.8 33.1 31 .O 20.3 25.4 19.1 25.5 6.3 13.5
PER CENT
Figure 3.
EVAPORATED
Evaporation Characteristics 4- Calculated
0 Observed
ethylene chloride. Boiling point-oomposition curves of these mixtures determined a t atmospheric pressure are plotted in Figure 1. I n the boiling point-composition curves, the upper curve represents the composition of the vapor in equilibrium with the liquid a t the boiling point. For the system having no maximum or minimum, it can be seen that the vapor is always richer in the lower boiling material, and so, on the evaporation of any mixture of this type, the solution should become continuously richer in the higher boiling component. For the system having a maximum boiling point, illustrated by acetone-chloroform, it can likewise be shown that any mixture on evaporation should approach the composition of the constant boiling mixture. By similar reasoning for the minimum boiling point systems, all solutions except those having a composition of the azeotropic or minimum boiling point mixtures on evaporation should become
Toluene-Ethylene Chloride, 25O C.
% Toluene In liquid 11.2 23.1 33.2 54.5 74.4 83.6 92.4
I n vapor 5.3 10.0 16.1 31 . o 51.3 65.8 82.5
P R O P Y L E N E CHLORIDE
Pressure, mm. 50 47 40 36 26 23 22
-
Acetone-n-Butyl Alcohol, 25' C.
__
% Acetone
I n liquid 93.6 87.9 74.6 59.4 38.1 12.5 0.0
I n vapor 98.8 98.6 97.9 96.8 95.1 88.8 6.3
Pressure, mm. 190 182 164 149 116 73 < 50
fore, desirable to confirm whether or not, under conditions similar to lacquer coatings, evaporation may be explained by vapor pressure properties. Classification according to vapor pressure relationships includes three general types of binary solvent mixtures, in which the compositions show: a maximum vapor pressure, a minimum vapor pressure, and no maximum or minimum vapor pressure. Examples of each of these types are, respectively, propylene chloride-isopropyl alcohol, acetone-chloroform, and acetone-
B P AT ATM. PRESSURE
a
20
40
60
80
I
PER CENT ISOPROPYL ALCOHOL IN VAPOR
Figure 4. Liquid-Vapor Composition Diagram of Propylene Chloride-Isopropyl Alcohol Mixtures at Boiling Point
0 Observed
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
106
1
2 5-
PROPYLENE: CHLORIDE -ISOPROPYL NEAR 0 . P
Vol. 41, No. 1
ALCOHOL
40
W
i)
a W
20
20
40
60
80
PER CENT EVAPORATED
Figure 5 , Evaporation Characteristics of Propylene Chloride-Isopropyl Alcohol Mixtures
Figure 6. Liquid-Vapor Composition Diagram o f Propylene Chloride-Isopropyl Alcohol 3Xixtures
9 Calculated
0 Observed
8 Obaerred
I
P R O P Y L E N E C H L O R I D E - ISOPROPYL ALCOHOL
PER CENT
Figure 7.
EVAPORATED
Evaporation Characteristics of Propjlerie Chloride-Isopropyl Alcoh~lhlixtrares C Calculated 0 Observed
richer in either one or the other component depending upon Ihe original composition of the mixture. The pure component which was in greater proportion than that required for the azeotropic mixture should remain. If these solvent mixturcs in different proportions are allowed to evaporate from an open surface, such as in a shallow dish, the course of the evaporation may be followed by analysis of the rcsidual solvent. Also, assuming that the increments of vapor removed in evaporation are in equilibrium with the residual liquid, calculations of changes in composition of the mixtures on evaporation may be made from liquid-vapor composition data for the temperature
0
20
40
60
BO
1
PER CEYT ACETONE IN VAPOR
Figure 8.
Liquid-'Vapor Composition Diagram of Acetone-Chloroform Mixtures
a t which the evaporation is to take place. Data for liquid-vapor equilibrium curves were obtained by use of an Othmer apparatus (4, Figure l), operated at either atmospheric or reduced pressures according to the desired temperature range. Since i t is necessary to know only the relationship between the compositions of the liquid and vapor, data have been plotted in the form of X - Y diagrams. The data for X - Y diagrams are given in Table I. The components in the graphs and tables are in per cent by weight in all cases. If an equation is obtained for the curve of the X - Y diagram, the composition of the residual liquid can be calculated from the integrated form of the Rayleigh equation which is:
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1949 80
ACETONE NEAR
+\+
107
- CHLOROFORM
6.P AT ATM. PRESSURE
I + \ + \ kt +' +-----+\
\+
+--+
++-
0
20
40
80
60
100
PER C E N T EVAPORATED
Figure 9. Evaporation Characteristics
+0 Calrrulated Obmerved PER CENT M E T H Y L ALCOHOL IN VAPOR
Figure 10. Liquid-Vapor Composition Diagram of Ethylene Chloride-Methyl Alcohol Mix tures
E T H Y L E N E CHLORIDE - M E T H Y L
ALCOHOL
N E A R 8. P. AT ATM. PRESSURE
z w
U
40
t-4
.
--+---a+
PER CENT EVAPORATED
Figure 11. Evaporation Characteristics
+0 Calculated Observed
B
LZ
LT
ds Y--2
in which L1is the original amount in weight or moles of a liquid mixture and Lzis the final amount; y is the weight or mole per cent of the most volatile component in the vapor in equilibrium with x weight or mole per cent of the most volatile cornponefit in the liquid; dx is the increment removed by evaporation; 2 1 is the weight or mole per cent of the most volatile component at the beginning of the evaporation; and 5 2 is the weight or mole per cent at the end. The equations for most X-Y curves are of a high order and so are difficult to use. Therefore, practically all of the calculations of changes in composition of binary mixtures on evaporation have
PER CENT ACETONE IN VAPOR
Figure 12. Liquid-Vapor Composition Diagram of Acetone-Methyl Alcohol Mixtures
been made stepwise by the use of finite increments. By keeping the increments small and using the mean composition during the evaporation of an increment, the calculations were made as accurate as the X-Y data warranted. An example of the stepwise method of calculation would be the evaporation of a mixture of 50% acetone and 5ooj, ethylene chloride. The X-Y diagram of this system is shown in Figure 2. The vapor in equilibrium with that mixture has 73.0% acetone. A preliminary calculation shows that approximately 7.3 grams of acetone would be removed if 10 grams of liquid were evaporated from a 100-gram sample of the mixture. By averaging the concentration of acetone at the beginning ancEat the end of the remova1 of the 10 grams it is found that 7.19 grams of acetone are removed. The composition of the residual solvent is then calculated to be 47.5% acetone. Then the calculations can
INDUSTRIAL AND ENGINEERING CHEMISTA Y
Vol. 41, No. I
+‘ ACETONE-METHYL ABOUT 25OC.
ALCOHOL
La
0
+\
6
-
PER CENT EVAPORATED
+\
.
/
0 0
20 20
:
Figure 13. E+aporationCharacteristics
PER CENT
Figure 14.
f Calaulated 0 Observed
40 40
60 60
80 80
I/ 0
ACETONE IN VAPOR
Liquid-Vapor Composition Diagram of Acetone-Butyl Alcohol Mixtures
loo[-
ACETONE
- N--BUTYL
ALCOHOL
PER CENT TOLUENE !N VAPOR
PER CENT EVAPORATED
Figure 15. Evaporation Characteristics -k Calculated
Figure
16. Liquid-Vapor Composition Diagram (~i‘ Toluene-Ethylene Chloride Mixtures
0 Observed
be repeated for another 10 grams At more advanced steps in the evaporation smaller increments should be taken unless the composition of the liquid changes very little. Agreement between the observed changes in composition of the binary mixtures on evaporation and the changes in compositions predicted by liquid-vapor composition data is shown on the accompanying graphs. Figures 4, 5, 6, and 7 illustrate the effect of temperature on the way in which mixtures of isopropyl alcohol and propylene chloride evaporate. For both near the boiling point and at room temperature, the agreement is good between the observed values and those calculated for the respective temperaturm. I n figures on rvaporation characteristics, those labeled “near the boiling point’’ were carried out on a steam bath, adjusted to maintain IL temperature just below the boiling point,
while the evaporations a t “near 25” C.” were made in the laboratory a t room temperature. Since there is a close agreement on all of the systems studied, which included each of the types of binary mixtures, it appears that changes in composition of binary mixtures and probably more complex mixtures can be expected to proceed according to vapor pressure characteristics. Application of this information to studies of lacquer, film, or coating processes requires consideration of whether or not evaporation characteristics are affected by the presence of cellulose esters. Suitable laboratory methods have been set up therefore to study the evaporation of volatile solvents from cellulose ester solutions from the point of the original viscous solutiou through partially cured films to the final stages of residual solvent in essentially cured coatings.
ETHYLENE CHLORIDE -CYCLOHEXANE B
5
ai
80
t
0 0
20
40 PER CENT
60
80
100
EVAPORATED
Figure
Figure 17. Evaporation Characteristics 4- Calculated
0
Oboewed
P. AT ATM. PRESSURE
//
20 40 60 80 PER CENT CYCLOHEXANE IN VAPOR
too
18. Liquid-Vapor Composition Diagram Ethylene ChlorideCyclohexane Mixtures
of
the receiver by a stream of dry air which was allowed to escape through a vent in the stopper.
The experimental technique used in making the measurements of residual solvent during evaporation involved distillation of residual solvent from partially cured films. The compositions which were studied were coated on glass plates, from which samples were removed after desired periods of time and transferred t o distillation equipment for analysis. The distillations were carried out in an apparatus made especially for the purpose (Figure 20).
As the solvent mixtures were binary, they could be analyzed by the measurement of suitable physical constants. An Abbe refractometer was used to make refractive index measurements,
The film (15 to 20 grams) from which residual solvent was to be distilled was wrapped around the glass tube of the still head which extends down into the distilling tube. Dbtilling tubes, all 6 inches long, Qf varying capacity from 30 t o 100 ml. were used. The larger tubes were used for the films having least amount of residual solvent. The distilling tube was heated by a sulfuric acid bath at about 170' C. for approximately 2 hours. The receiver was cooled by a dry ice-methyl Cellosolve mixture. The last vapors of the solvent during distillation were forced over into
80
E T H Y L E N E CHLORIDE -CYCLOHEXANE
0
20
40
60
80
100
PER CENT EVAPORATED
Figure 19. Evaporation Characteristics
C Calaulated
0
Obmed
Figure 20. Distillation Apparatus for Recovery of Residual Solvents in Films
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
110
.-80/
ISOPROPYL ALCOHOL
_I
I.
t
P R O P Y L E N E CHLORIDE-
- --- - - -
Vol. 41, No. 1
- ---
---a
-e---*---
->-: \
80 W
z W
u
4
t Z
60
w
U
e l [L
W
--- - - - - - L
_----------
i
a
40
ACETONE- M E T H Y L ALCOHOL
2 o - - - - - - - - --*--e ..---------b I
0
I
I
I
I
40
20
PER CENT
I
1
60
I
I
40
60
80
IO0
PER CENT EVAPORATED
I
80
I
20
0
1
EVAPORATED
Figure 21. Evaporation of JIixtures of Propylene Chloride and Isopropyl Alcohol from Solutions Containing Cellulose Acetate Butyrate
Figure 22. Evaporation of Mixtures of Acetone and Methyl Alcohol from Solutions Containing Cellulose Acetate @
Observed
0 Observed
A C E T O N E - E T H Y L E N E CHLORIDE
80 W
Z
PW 0
Q
c
z W u a W
0.
t I
0
20
40
60
80
PER CENT EVAPORATED
Figure 23. Evaporation of Mixtures of Acetone and Butyl Alcohol from . Solutions Containing Cellulose Acetate @
'
I
I
IC
Observed
and a 1- or 5-ml. pycnometer with graduated capillary arms was used in measurement of specific gravities. All coatings except those involving acetone were made in an unconditioned room which was about 30 O C. Since the acetone solutions were found to absorb water from humid air during the curing of the film, such solutions were coated in room conditioned a t 70" F. and 1070 relative humidity. Also, all coatings were made at a coating-knife setting of 0.040 inch from relatively highly viscous solutions of approximately 18% concentration. The results which were obtained are shown graphically in Figures 21 t o 28. For measurements in Figures 22 and 23, cellulose acetate of 40.5% acetyl content was used as the cellulose
PER C E N T
EVAPORATED
Figure 24. Evaporation of Mixtures of Acetone and Ethylene Chloride from Solutions Containing Cellulose Acetate Butyrate @
Observed
ester and for all others a cellulose acetate butyrate of 31y0acetyl and 17% butyryl content, was used. The esters used had a viscosity of 200 t o 350 centipoiscs a t 25" C. in a 10% acetone solution, Solvent mixtures selected €or such evaporation tests ncccssarily were limited to those which were capable of dissolving the cellulose derivative. It may be seen from the figures that evaporation from cellulow ester solutions deviates more from vapor pressurc calculations than does that of the solvents alone. Acetone in mixtures with ethylene chloride tends to become somewhat mol e concentrated in the residual films than it does in the absence of cellulose ester. In acetone-butyl alcohol mixtures the variation is somewhat inow
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1949
111
80 W
5
40-
N
B E N Z E N E - E T H Y L E N E CHLORIDE
z
W
m e
5V
-
60
9
W
--.--._ _ _ _ _
20’c---------LL
a
- --
--
W
a I
I
I
- - -.---*;,
-
I
I
-3-
I
-
TOLUENE
- ETHYLENE
CHLORIDE
-
2
i
0.
-*--H,c
I
z W
2 I-
40-
Z W
u
PER CENT EVAPORATED
Figure 26. Evaporation of Mixtures of Toluene and Ethylene Chloride from Solutions Containing Cellulose Acetate Butyrate CYCLOHEXANE
-
0 Observed E T H Y L E N E CHLORIDE
80
W
z
2
60-
W
: I
t-
z
c 4
40-
W
/
V
/ ’
a W
e 20
PER CENT EVAPORATED
Figure 27. Evaporation of Mixtures of Cyclohexane and Ethylene Chloride from Solutions Containing Cellulose Acetate Butyrate 0 Obaerved
0
20
40
60
80
I 0
PER CENT EVAPORATED
Figure 28. Evaporation of Mixtures of Neohexane and Ethylene Chloride from Solutions Containing Cellulose Acetate Butyrate
pronounced. A still great,er dcviation occurs with acetonemethyl alcohol mixtures, leading to high acetone concentrations in the final stages. Benzene-ethylene chloride mixtures follow the calculated behavior until the latter stages of evaporation, at which time the benzene concentration increases sharply. Cyclohexane and neohexane (2,2-dimethylbutane) show a much more pronounced effect, becoming very high in concentration in the residual solvent. Toluene, on the other hand, is withheld less in the residual solvent than calculated from vapor pressures. These measurements appear to be sufficient to show that the presence of film-forming materials in many cases results in selective retention of certain solvents. As would be expected, the effect usually becomes most predominant after the film has “set” to a solid state. This suggests that diffusion of the solvents through cellulose esters becomes a controlling factor. No quantitative correlation of this factor has been made, although cyclohexane is known to have a very low diffusion constant for the cellulose esters used. Preliminary measurements of the diffusion constant of acetone, however, would not indicate selective retention..
0 Observed
ACKNOWLEDGMENT
The authors wish to express their appreciation to Harold Vivian and Martin Sal0 for the data used in preparation of t h e graphs on evaporation of propylene chloride and isopropyl alcohol mixtures. LITERATURE CITED
(1) Bennett, G. W., and Wright, W. A., IND. ENG.CHEM.,28, 646
(1936). ( 2 ) Lewis, W. X., and Squires, L., Ibid., 29,109 (1937). (3) Lewis, W. K., Squires, L., and Sanders, C. E., Ibid., 27, 1396 (1935). (4) Othmer, D.F.,X d . , 20,743 (1928). (5) Robinson, E.,Wright, W. A., and Bennett, G. W., J . Phys. Chem., 35,658 (1932). R~CBIVEID May 29, 1947. Presented before the Division of Cellulose Chemistry at the 111th Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantio City, N. J.
