Solvent Properties of Isomeric Paraffins Comparative solvent properties, as measured by dilution ratios and viscosities of polymer solutions, have been determined on two pairs of isomeric paraffins. Viscosities were taken with the Gardner-Holdt viscometer, which precludes loss of solvent. Two series of polymers-low-molecularweight polybutenes and alkyd resins-were used at concentrations of 800 grams per liter, with viscosities ranging from 0.1 to 150 poises. Graphically presented data show that, although the solvents possess substantially the same dilution ratios, the highly branched paraffins (neohexane and triptane) produce polymer solutions whose relative viscosities average more than twice those of corresponding solutions in straight-chain solvents. Corrections applied to absolute viscosities for differences in boiling point or critical temperature accentuate the spread.
E
FFORTS directed toward the development of highoctane gasoline have resulted in the production of a number of stocks which contain large proportions of branched paraffins; e. g., the paraffinic hydrocarbon content of certain catalytically cracked naphthas is preponderantly branched (5). Since engine-knocking tendencies of normal and branched paraffins lie poles apart, it might be expected to find marked differences in their properties as solvents. Should it develop, however, that a branched octane with the same volatility as wheptane gives rise to twice or half the viscosity in comparable solutions of, say, a protective coating vehicle, many practical applications would a t once become apparent. From the theoretical standpoint such interesting relations might conceivably provide an additional tool for research-e. g., in the study of polymers. A preliminary survey has therefore been undertaken. Solvent properties may be divided into two general classes --dilution ratio values and viscosity effects. Dilution ratios are usually interpreted as estimates of affinity or relative structural similarity between solute and solvent; viscosity effects may be taken as indicative of both the “goodness” of a solvent (e. g., its ability to cut a resin or thin a varnish) or its “poorness” (e. g., where the “poorer” solvent produces the lower viscosity in a high-polymer solution). Two dilution ratio tests were selected for this study: the familiar kauri-butanol value ( I , 2) and the tolerance to pure n-decanol of a solution of polybutene in the hydrocarbon under test. Table I shows that the most highly branched Ceand C7 paraffins do not differ widely from the corresponding normal paraffins in solvency or affinity, for either a butanol solution of the natural resin or for a low-molecularweight polybutene in the presence of a mild precipitant.
E. H. MCARDLE AND A. E. ROBERTSON Standard oil Development Company, Elizabeth, N. J.
To test the relative effects of the straight-chain and the branched isomers on the viscosity of comparable solutions, two sets of low-molecular-weight’ polymers were chosen. The linear series is represented by four polybutenes, from 3000 t o 12,000 molecular weight, while the three-dimensional series includes three alkyd resins containing from 30 to 50 per cent of polybasic acid glyceryl ester. All concentrations are 800 grams per liter, corresponding to a solids content of more than 50 per cent by weight, a concentration which might serve for practical application. TABLE I. DILUTION RATIO VALUES Neohexane %-Heptane Triptane’ 24.4 25.4 29.1 24.3 29.4 (1, 8) Solvent power by wt. ( 1 ) 16.8 15.8 17.3 20.1 n-Decanol valuea 11.0 9.7 11.9 11.5 0 M1. of n-deoanol tolerated (to cloud point) by a solution of 2.0 grams of 5500 molecular weight polybutene in 10 ml. of solvent, at 25’ C. n-Hexane Kauri-butanol solvent power 25.5
Hydrocarbons Studied It is intended to study the solvent properties of a number of C6to Cs isomers. Pertinent physical properties have been determined on the eight already assembled, as shown in Table 11. Degrees of purity are indicated by their lack of Cottrell spread and by their practically correct refractive indices. It is hoped that two pure octanes, and possibly other pairs, may be added to the list for future study. Meanwhile, measurements have been confined to pairs of hexanes and heptanes. PROPERTIES OF PARAFFINS USED“ TABLE 11. PHYSICAL ~ ~ i l Refractive i ~ ~ Specific Cottrell Point, O C. Index*, n%’ Gravityc, d:’ Spread None n-Pentane 35.9 1.3580 (1.3577) 0.626 None 1.3541 (1.3539 0.620 Isopentsne 27.9 0.659 None 1.3751 n-Hexane 68.4 None 0.653 1.3715 2-Methylpentane 60. 0 None 2 3-Dimethylbutane 5 7 . 8 58.00) 1 ,3749 (1.3750) 0.661 None deohexaned 49.6 1 ,3689 (1.3689) 0.649 None n-Heptane 98.5 98.42) 1 ,3876 (1.3876) 0.683 None Triptanes 80.6 80.88) 1.3894 (1.3895) 0.689 a Values in parenthasea are those recently assembled by Francis (4). b Determined in a Pulfrich refractometer. c Determined in a Westphal balance. d 2,2-Dimethylbutane. e 2,2,3-Trimethylbutane. 3
Solutions were made in one-ounce wide-mouth bottles in a constant-temperature room at 25’ C. Eight grams of polymer were weighed into a bottle to a n accuracy of 0.25 per cent. Solvent was pipetted. The bottle was quickly corked, and the selected oversize cork was slowly forced well into the neck. The bottle was then weighed. After solu1005
1006
VoI. 34, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion was effected (occasionally several days were required), the bottle was again weighed. I n no case did a weighable loss appear. Solution was hastened by tumbling a t 25" C. on a wheel revolving a t 15 r. p. m. It has been shown (8) that no depolymerization occurs under such circumstances with polybutenes of low molecular weight.
30,000
29000 I5,OOO
lop00 8POO 6,000
5000 4,000 3,000
2,000 PO0
Viscosity Measurement After solution, the liquid was poured as rapidly as possible into a Gardner-Holdt sample tube ( 5 ) , and the tube quickly corked. It is believed that this technique involves a lower solvent loss than does any other method of preparing for viscosity measurement. Once tightly corked, the sample tube was used for a series of determinations a t four temperatures. Measurements below 25" C. were taken under water, with tubes inverted and upright, and no leakage was observed in any case. . Since Gardner-Holdt tube stand0 5 ards are translatable to poises only a t 25' C., comparisons with sample tubes at the lower temperatures (15", lo", and 5" C., * 0.2" C.) mere made in the following manner. A tube holder for six tubes was fastened to a hinge, mounted on a ring-stand clamp. After standing 20 minutes in a basket in the thermostat, a set of sample tubes was clipped into the holder, which had been previously submerged. After several minutes the hinged assembly was then inverted manually a t the instant a standard tube (also clipped to a holder) was inverted in the room (at 25" * 0.2" C.). For the higher viscosities it is possible for a single operator to invert both holders and obtain sufficiently accurate comparisons. The estimates given in Table 111, however, were made by the authors together, one inverting each holder. After a little practice it was found possible to check results repeatedly. The submerged tube holder was best placed near one front corner of the glass thermostat, and thus the standard tube holder could be simultaneously inverted some 3 or 4 inches distant. Care was taken to keep the standard tube sets, as well as individual standard tubes when not being inverted, several feet from the thermostat; for although the laboratory air was changed every 3 minutes, slight differences in temperature were noted in the immediate surroundings of the bath. Over-all accuracy and precision of the method are attested by the close coincidence of the relative. viscosity-temperature curves for the two normal paraffin solutions of the 5500 and 9900 molecular weight polybutenes (Figure 1). Fair concordance is also shown in the relative viscosities of resinn-paraffin solutions in Figure 2 . Conversely, the poor showing of fhe relative viscosities of the 3000 molecular weight polybutene in the two normal paraffins demonstrates the difficulty of using the Gardner-Holdt "extSemely light bodied" tubes (A1 to A5) in the present manipulation. Convergence of the two curves approaching 0" C.-i. e., above 50 centipoises, the beginning of the viscosity range of the "varnish series" (tubes A to T)-is doubtless significant.
poo 800 600
500 400 300 200
150 100
80 60 50 OCENTIGRADE
10 15 20 OCENTIGRADE
FIGURE 1 (Left). RELATIVE VISCOSITY-TEMPERATURE CURVES FOR SOLUTIONSOF POLYBUTENES IN PUREPARAFFINS FIGURE2 (Above). RELATIVE VISCOSITY-TEMPERATURE CURVESFOR SOLUTIONS OF ALKYDTYPERESINS IN PUREPARAFFINS
25
Effect of Branching Figure 3 shows the absolute viscosity of the solution of 5500 molecular weight polybutene in neohexane to be two and a half to three times the absolute viscosity of the corresponding solution in n-hexane; the absolute viscosity of
TABLE111. SOLUTION VISCOSITIES c -
Temp., C. n-Hexane
+-
Beckosol No. 31 solids
25 15 10
U 22 84
Beckosol No. 19 solids
25
E J
10 5
P
T
Beckosol No. 18 solids
25
A3 A
Polybutene3000
Polybutene 5500
Polybutene9900
Polybutene 12,000
15
15
Viscosity, Gardner-Holdt. Iieohexane +Heptane
-
l/4
J U
l/g l/3
++ -+
'g
E'?
A4(?) A4 A4 A3
25
g/C
15 10 5
p+
E
M
I/&
t/3
1/4
A2 '/a A1/A2 A1
A
P
Z
'g
y"
z2 88/24 24
25 15 10
25 Z6 Z6
w + '/3 -
+- '/a
'/4a
l/a
Z5/Z6
T" v
-k
'/a
- l/s
E G
'/a
A4 A3/A4 A3
+ + ... ...
1/3
l/g
...
D
+
"FH; -
M V
$ + '/a -
B F
'/g
H
M A1
A B C
+ '/a
+ '/a
'/3 v- '/E
u '/3
23
$/? Z
++ '/a'/a - '/a
24
'/a
W
Z6
Triptane
w
Z6
A2
l/s
v - '/a
25 15
2/5
A1 B
-
+ '/a
-
F f '/a
1/4
z - 2/s A E G L
25 15 10 5
F
'/a
. ..
X
+
..
,
1/6
+ '/a
+ 23 f
v
Z
'/d
... ...
1/10
-
'/3
;:7 ;$; ZS
.. . ...
...
With '/,-inoh difference in bubble height, the viscosity is estimated a t about 150 t o 155 poises. (i
August, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
K
1007
OCENTIGRADE
a
$
-
FIGURE5. CURVESFOR CALCULATING RELATIVEVISCOSITIES
I 0
w 0 V
m 4
a
2 0
OCENTIGRADE
FIGURE3. ABSOLUTE VISCOSITYOF BRANCHED AND OF STRAIGHT-CHAIN PARAFFIN SOLUTIONS OF POLYBUTENES
5
IO 15 TENTIGRADE
20
25
FIGURE4. ABSOLUTEVISCOSITY BRANCHED AND OF STRAIGHTCHAIN PARAFFINSOLUTIONSOF ALKYD TYPERESINS OF
the triptane solution averages some four times that of the n-heptane solution throughout the temperature range plotted. The viscosity ratios for the 9900 molecular weight polymer solutions are even higher, ranging from 3:l to 5:l for the C6 solvents and from 4 : l to 7:l for the C?isomers. Also noteworthy are the appreciably steeper temperature-viscosity slopes of the branched paraffin solutions of the 5500 and 9900 molecultir weight polybutenes. (Solutions of the 3000 molecular weight polymer in the normal paraffins were too mobile for accurate viscosity measurement, as previously indicated.) Ratios of the absolute viscosities of branched versus straight-chain paraffin solutions of alkyd type resins (Figure 4) show a surprising similarity to those of the polybutene solutions, when it is recalled that the resins are three-dimensional polymers and polybutenes are linear. Beckosol No. 19 in neohexane is two to three times as viscous as the n-hexane solution. The viscosity ratios of the C, solutions, however, are equal to or smaller than those of the Cg, instead of being larger. An interesting anomaly is seen in the higher absolute viscosity of the neohexane ((20,boiling a t 50" C.) cut of Beckosol No. 31 as compared to the triptane (C?, boiling at 81' C.) cut. Beckosol No. 31 is a 50 per cent glyceryl phthalate, soy-bean-oil modified alkyd-i. e., is relatively high in glyceryl phthalate content to be readily soluble in pure paraffins. Structurally neohexane is less branched than triptane. Thus, considering that the long fatty-acid residues in Beckosol No. 31 are connected with rings in unusually great abundance, it can be envisioned that a neohexane molecule could associate its less branched Cr chain with a greater chance of entanglement than can triptane. Factu-
ally it is notable that the relative viscosities of all threeneohexanealkyd resin solutions are higher BECKOSOL NO.31: 9500 than those in triptane, while NO. 19:7000 behavior in the linear polymerpolybutene series is more ortho3000,,5000 7000 9000 !!OOO dox. We hope MOLECULAR WEIGHT to obtain suffici e n t a d d i FIGURE6. CURVES FOR cALCULATING tiona1 da ta MOLECULARWEIGHTS OF ALKYD in future work RESINS to attempt a n explanation. Relative viscosities are plotted in Figures 1 and i. They are obtained by dividing the absolute solution viscosities by the viscosity of the solvent at the several temperatures (Figure 5). Since relative viscosity measures the effect of the solute, it might be expected that the relative viscosities of solutions of a given polybutene or alkyd in n-hexane and n-heptane would be identical as shown (within what is probably experimental error). Relative viscosity should also provide a more reliable measure of the effect of solvent branching. Figure 1 sholvs that the more highly branched triptane produces solutions of the linear polymers with higher relative viscosities than does neohexane; the reverse is true with the three-dimen-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1008
TABLE IV. CORRECTED ABSOLUTEVISCOSITIES 7 -
Paraffin n-Heptane n -H e xa n e n-Heptane Triptane n-Hexane Neohexane
Tpp., C.
-Polybutene--. 9900 mol. wt.
Viscosity, Poises 5500
mol. wt
Corrected for Boiling Point 10.7 25 1.05 1.2 18.5 1.62
25
11.2 25 9
10.7 100
7.8
56
1.05 6.1 0.74 3.34
Reckosol K O . 19
1.5 7.0 1.5 12.8 1.2 13.9
Corrected for Critical Temperature n-Heptane n-Hexane n-Heptane Triptane n-Hexane Neohexane
25 5.5 25 18.7 25 13
10.7 15.2 10.7
63 7.8
44
1.05 1.37 1.05 4.2 0.74 2.8
1.5 5.2 1.5 6.2 1.2 9.3
sional polymers, where the entanglement mentioned previously may be a controlling factor.
Temperature Corrections To compare absolute viscosities in a pair of solvents, each viscosity may be taken a t an absolute temperature which is a constant fraction of the absolute critical temperature or of the absolute boiling point; such temperatures mere calculated and the corresponding viscosities read from Figures 3 and 4. Table IV shows that these temperature corrections merely widen the spread between the normal and the branched paraffins. Molecular Weights Molecular weights (weight averages) for the polybutenes had been calculated from viscosities of dilute solutions in
Vol. 34, No. 8
hydrocarbons other than paraffins. Xevertheless these values are plotted in Figure 6 against the relative solution viscosities of the four polybutenes at a concentration of 800 grams per liter. The best straight line is drawn through the four values obtained for n-hexane (from 3000 to 12,000 molecular weight), and other best straight lines are drawn through the three values for the other solvents; then if viscosity is plotted against molecular weight for a linear polymer and a three-dimensional polymer on the same chart, the alkyd resins seem to have the molecular weights indicated. It is a t least certain that the resins do not have the same weightaverage molecular weights, and also probably that the highest is less than 10,000.
Acknowledgment The authors wish to thank P. J. Flory, of the Esso Laboratories, for helpful advice in planning this work. They are also indebted to M. R. Fenske, of The Pennsylvania State College, for supplying samples of pure n-hexane and neohexane.
Literature Cited L., Morgan, M. D., a n d Troeller, W. J., IKD.Exc. CHEM.,ANAL.ED.,9, 540 (1937). Baldeschwieler, E. L., Troeller, W.J., a n d Morgan, M . D., Ibid., 7, 374 (1935). B a t e s , J. R . , Rose, F. W., Jr., Kurtz, S. S.,J r . , a n d Mills, I. W., IND. ENG.CHEM..34, 147 (1942). Francis, A. W., Ibid., 33, 554 (1941). G a r d n e r , H. A., "Physical a n d Chemical Examination of P a i n t s , Varnishes, Lacquers a n d C,olors", 9th e d . , p. 216 (1939). Thomas, R. M., Zimmer, J. C., T u r n e r , L. B., Rosen, R., a n d Frolich, P. K., IND. ENG.CHEM.,32, 299 (1940).
(1) Baldeschwieler, E.
(2)
(3) (4)
(5) (6)
PRESENTED before the Division of Paint, Tarnish, and Plastics Chemistry at the 103rd Meeting of the AMERICANCHEMICAL SOCIETY, Memphis, Tenn.
METHANE-ISOBUTANE SYSTEM R. H. OLDS, B. H. SAGE, AND W. N. LACEY California Institute of Technology, Pasadena, Calif.
I
N RECENT years there has been an increasing interest in the volumetric and phase behavior of binary hydrocarbon
systems. This has been prompted in part by the desire to obtain basic information to permit the prediction of the behavior of multicomponent hydrocarbon systems of industrial importance. Earlier work upon binary hydrocarbon systems containing methane includes the study of the methane-propane ( 1 4 , methane-ethane (Q), methane-ethylene (6), and methaneen-butane (IO, 1 1 ) systems, as well as studies by Boomer and co-workers upon methane*-pentane (g), meth( I ) , and methane-n-heptane (3) mixtures which an--hejtane contained significant quantities of nitrogen but were nevertheless treated as binary systems. In order to ascertain the behavior of methane with an isoparaffin of low molecular weight, the study of the volumetric and phase behavior of the methane-isobutane system was undertaken at pressures up to 5000 pounds per square inch and at temperatures between 100" and 460" F. The volumetric behavior of methane is already well established by the work of Kvalnes and Caddy (6) and of Michels and Kederbragt (7). The vapor pressure, Joule-Thomson coefficient, latent heat of vaporization of isobutane, and its volumetric behavior in the condensed liquid region have been deter-
mined (12). Information concerning the volumetric behavior of this hydrocarbon a t temperatures as high as 460" F. for pressures up to 5000 pounds per square inch is available ( 8 ) . These data suffice to establish the volumetric and phase behavior of the components of the methane-isobutane system with sufficient accuracy for present purposes.
Methods In principle the method employed in determination of the volumetric behavior of this system iniTolves the addition of known weights of methane and isobutane to a chamber whose effective volume was varied over wide limits by the addition or withdrawal of mercury. The temperature of the equilibrium chamber was maintained a t a uniform value by immersing it in a thermostat bath. The temperature of measurement was related to the international platinum scale by the use of a strain-free platinum resistance thermometer which was compared a t the beginning and end of the investigation with a similar instrument calibrated a t the National Bureau of Standards. The pressure was ascertained by a pressure balance calibrated against the vapor pressure of carbon dioxide a t the ice point. The volume of the chamber occupied by hydrocarbons was determined from measurements of the elera-
