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INDUSTRIAL AND ENGINEERING CHEMISTRY
Acknowledgment
VOL. 32, NO. 2
tory for his encouragement, advice, and permission to release the results.
Grateful acknowledgment is due some of the leading lacquer chemists for to suggestions’ The author is the assistance Of Dunlap and the variOUB members of the staff of this laboratory who rendered and assistance in the work* The author wishes also to thank J. K. Stewart, director of this labora-
Literature Cited (1) Dorsch, J.B., andstewart, J. K., IND. ENQ.CHEM.,30,325 (1938). (2) Stewart, J. K., Dorsch, J. B., and Hopper, C. B., Ibid., 29, 899 (1937). PRESENTED before the Division of Paint and Varnish Chemistry a t the 97th lleeting of the American Chemical Sooiety, Baltimore, &Id.
Colloidal Structure of Rubber in Solution Colloidal Aspects of Vulcanization S. D. GEHMAN AND J. E. FIELD The Goodyear Tire & Rubber Company, Akron, Ohio Colloidal changes in solutions of purified rubber due to a vulcanizing system consisting of piperidinium pentamethylene dithiocarbamate, sulfur, and zinc propionate have been followed by means of measurements of the depolarization and intensity of the transversely scattered light and by viscosity measurements. A study wab made of the time required for vulcanization of benzene solutions of rubber with the above vulcanizing agents. The temperature coefficient of vulcanization was approximately the same as that for cures of solid sfocks. The light scattering results are interpreted as showing that, upon the addition of the vulcanizing
HE work here reported is a continuation of studies T(8,9 on) the colloidal structure of rubber in solution, by means of viscosity measurements and measurements of the intensity and depolarization of the transversely scattered light. The previous papers dealt with reversible colloidal changes. The work has now been extended to cover the irreversible changes which occur when a solution of rubber is gelled or vulcanized by a system of curing agentsnamely, piperidinium pentamethylene dithiocarbamate (P. P. D.), zinc propionate, and sulfur. The advantage of studying the colloidal aspects of vulcanization in solution rather than in solid cures lies in the fact that the colloidal changes are more accessible for direct observation in solution. In a gelled solution, the colloidal structure of the solid rubber may be thought of as existing in an expanded condition, so that in a sense the structure under examination is magnified. Chemical explanations of vulcanization restricted to the rate of sulfur addition and the type of sulfur bonding (3, 11, 19) cannot be expected by themselves to account for the physical properties of the vulcanizates because these properties depend upon the colloidal structure formed during vulcanization. Van Rossem (14) discussed the status of theories of vulcanization and emphasized that a chemical theory should regard vulcanization as a type of polymerization in which a three-dimensional network of primary valence link-
agents to the solutions, the colloidal units become larger. The decrease in viscosity is explained as being due to a diminished “interlocking” of the units. In the vulcanization of the solutions, an equilibrium of colloidal processes occurs which results in a constant viscosity for the larger part of the time required for vulcanization, although the light scattering measurements show a continuous change. The magnitude of the light scattering changes indicates that the molecular clusters in the gelled solution are probably not very different from those in the original solution. The viscosity measurements show that the forces between the clusters have been radically strengthened.
ages is built up between large molecules. Such a structure is essentially colloidal in nature, but van Rossem restricted the use of the word to theories of vulcanization in which soft vulcanized rubber is regarded as a colloidal dispersion of hard rubber. I n a broader sense than this greatly restricted historical usage, the vulcanization of rubber should be regarded as a colloidal process initiated by chemical reactions. Furthermore, it appears that the amount of chemical action, by which is meant change in primary valence linkages, required to produce the colloidal changes and affect the physical properties is small (4, 17). Presumably a single cross linkage can tie together more or less effectively entire colloidal units or clusters of molecules instead of just single large molecules, as van Rossem considered the structure to be. The importance of paying more attention to the colloidal aspects of the situation thus becomes apparent. The colloidal nature of the process of vulcanization has not been entirely ignored in fundamental work on vulcanization. Williams (18) made a detailed study of colloidal phenomena in gels of lightly vulcanized rubber. Many instances of vulcanization in solution are reported in the literature. Vulcanization with solutions of sulfur chloride was studied by Garvey (7) and earlier workers. Garvey’s suggestion that a change occurs in the cis-trans relations of the units in the long-chain molecules upon vulcanization with sulfur chloride
ml
1 (I
INDUSTRIAL AND ENGINEERING CHEMISTRY
FEBRUARY, 1940
GELLING O F 2 % S O L RUBBER IN ETHYL ETHER (2% P.P.D. > 4 f S ; S f Z N P R O P I O N A T E )
A
CONTROL PREVIOUSLY IRRADIATED
283
LIGHT SCATTERING MEASUREMENTS SOLUTIONS OF FIG. I
ON THE-
FIG. 2
.? 5-
7-
I
Y
I
FIG. I
OL 0
I
20
I
I
40 TIME
60
I 80
I 100
(HRS. A T 2 d C . )
is not consistent with x-ray diffraction results. Boiry ( I ) investigated the gelling of rubber solutions by means of sulfur and the application of heat. The action of ultra-accelerators in rubber solutions has been reported on by Thiollet (16) and by Bourbon (2), whose work includes results with a variety of solvents.
Apparatus and Methods The light scatterin measurements were made with apparatus previously described f8,9). To prepare the solutions, the curing ingredients were dissolved in sodium-distilled ethyl ether which was also used as the rubber solvent. The solution of the curing agents was fltered into the tube of the light scattering cell. The intensity of the light scattering from such a filtered solution was found to be negligible in comparison with the intensity from the rubber solutions. The desired amount of rubber, purified by diffusion in petroleum ether, was weighed and introduced into the bulb of the cell. The cell was then evacuated and sealed off. The solvent was distilled onto the rubber, and when the solution was complete, light scattering measurements were made. The curing ingredients, which were left in the tube by the distillation, were then dissolved in the rubber solution by pouring the solution into the tube. Light scattering and viscosity measurements were made at suitable intervals. By this technique any exposure of the rubber solution to air was avoided. The cell was kept in the dark, and when viscosity measurements were being made, it was protected by a wrapping of red Pliofilm. In the experiments where viscositybut not light scattering measurements were made, solutions of milled crepe rubber were used. The rubber was prepared by milling pale crepe rubber so that a one per cent solution in benzene gave a specific viscosity of 6.07. Baker's c. P. benzene was used as the solvent in these ex eriments. &he milled pale crepe rubber was dissolved in benzene to give a 14 per cent solution by volume. Portions were diluted when lower concentrations were required. In carrying out the experiments, glass tubes (15 mm. inside diameter and 30 cm. long) were filled with solutions of the desired concentrations. The curing ingredients were weighed out on an analytical balance and stirred into the solutions with a glass rod. Viscosity measurements were made by timing the fall between marks of a steel ball of suitable diameter, usually inch. The tubes were closed with rubber stoppers protected by aluminum foil but the ball was returned to the top of the tube with a magnet so that it
.
I
I
0
50 TI ME (H RS. A T
100
I
150
25'C.)
was unnecessary to remove the stoppers and insert a ball for each measurement. During the measurements the tubes were immersed in a water thermostat at 30" C. and were protected from the light by a wrapping of red Pliofilm. At other times they were kept in the dark. For cures a t various temperatures, the tubes were immersed in water thermostats or in uniformtemperature ovens with circulating air so that the temperature was uniform and constant to about * 0.25" C.
Light Scattering Measurements The theory which relates the size and shape of colloidal light scattering units to the intensity and depolarization of the transversely scattered beam was outlined in a previous paper (8). Reference should also be made to more recent articles (IO, 12, IS) for the present status of the theory. Lotmar made general criticisms of the type of equipment used by the authors as subject t o error due to secondary scattering. For the systems investigated, the intensity of the light scattering was so low that we do not believe it was a source of appreciable error. Figure 1 shows the viscosity changes for 2 per cent solutions of sol rubber in ethyl ether due to the addition of 2 per cent P. P. D., 4 per cent sulfur, and 3 per cent zinc propionate. For curve B the solution was irradiated with ultraviolet light before the curing ingredients were added. Figure 2 gives light scattering measurements on the same solutions. The curves for the intensity of the scattered light and the depolarization factors, PA and Ap., consistently indicate that the initial drop in viscosity is accompanied by a n increase in the size of the colloidal units. At first thought this may seem contrary to the generally accepted idea of a prevulcaniaation disaggregation. The origin of this concep tion is undoubtedly the intuitive reasoning that the st,rength and elasticity of rubber are associated with the molecular structure of a solid, and that as rubber becomes weaker and more plastic, the continuous structure of the solid is broken. There is no contradiction between this fundamental line of t.hought and a n observation that the colloidal units become larger in the process of disaggregation. As a crude analogy, a block of concrete may be taken as a n example. The
IKDUSTRIAL AND ENGINEERING CHEMISTRY
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MEASUREMENTS ON .l5% S O L RUBBER I N ETHER IRRADIATED WITH ULTRAVIOLET
VOL. 32, NO. 2
3
FIG. 3
2
VULCANIZATION IN SOLUTION
I
4
”
n
n
% SULFUR
(MILLED CREPE IN BENZENE) CURING TEMPERATURE 2 d C .
I
I
0
20
I
I
I
40
60
80
TIME fHR.5.),
I
0
I
40 80 TIME OF EXPOSURE TO U. L! @IN.)
I
I20
aggregates in the block are the pieces of gravel. The block may be disaggregated in such a way that cement will adhere to the gravel and the resulting aggregates will be larger than before. By the prevulcanization disaggregation must be understood a weakening of the continuous structure or “interlocking” of the colloidal units in the rubber and not a decrease in their size. This interpretation of the light scattering measurements and the resultant conception of the disaggregation is in contradiction to the ideas of Staudinger (16) who regarded the viscosity of a rubber solution as due to individual, long, threadlike molecules. For such a structure any decrease in viscosity must of necessity mean a decrease in the size of the colloidal units which are considered to be the individual macromolecules. I n opposition to this, the light scattering measurements show that the effects which have been observed and attributed to a prevulcanization disaggregation are due to a growth in particle size carried out in such a way that the interlocking of the colloidal units is diminished. The degree and amount of interlocking may reasonably be a more important factor in determining viscosity and plasticity than the size of the units. It is easy to visualize a growth in particle size, even for a solution of macromolecules such as postulated by Staudinger, which would lead to a decrease in viscosity. Thus, if the individual molecules align themselves in parallel groups, from simple mechanical considerations the resistance to flow should decrease. Comparison of the curves of Figure 2 with those of Figure 1 shows that colloidal changes are occurring while the viscosity is constant and that the viscosity is by no means a complete specification of the colloidal structure. This constant viscosity must therefore represent an equilibrium condition between the colloidal processes of disaggregation and those which later cause gelation. The curves of Figure 3 have a bearing on this subject. They represent measurements on a dilute solution of sol rubber irradiated with ultraviolet light
I
I
TEMPERATURE COEFFICIENT OF VULCANIZATION IN SOLUTION
\
SAME SOLUTIONS A S IN FIGURE 4
I
I
I
in the absence of air. The course of the light scattering curves is similar t o those of Figure 2 during the early stages of vulcanization. Although the light scattering measurements indicate a close similarity between the disaggregation caused by the irradiation and the initial disaggregation caused by the vulcanizing ingredients, the previous irradiation has the effect of delaying the vulcanization, as Figure 1 showed. This is evidence that the early stages of vulcanization r e p resent not merely a disaggregation but an equilibrium between the disaggregating and gelling processes from the start. More evidence for this view is given in Table I. The accelerator, accelerator-sulfur, and accelerator-activator were added to benzene solutions of crepe rubber. After standing
FEBRUARY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
285
fore defined as the time required after the curing ingredients are added for the original viscosity of a solution to increase by a factor of 2. A11 of the curing times plotted in subsequent figures were obtained from viscosity curves similar to those of Figure 4. Figure 5 is a plot of the logarithm of curing time against temperature. The temperature coefficient of vulcanization is defined as the ratio of the curing times for a 10' C. temperature interval. Figure 5 shows that this coefficient is practically independent of the rubber concentration for the range investigated, 4 to 14 per cent concentration. Its value for the 8 per cent solution is 1.91 per 10' (2. I n a summary of the results of various workers on solid cures Eliel (6) gives values ranging from 2.52 to 1.82, based on modulus and depending upon the accelerator used. The value found for the solutions is thus very close to the values for solid stocks. This gives confidence that a study of the gelation of the solutions is a valid method for making fundamental investigations on the colloidal nature of vulcanization. SOLUTIONS A S I N F I G U R E 4
SAME
2
IO
6
b
\
14
FIG. 7 EFFECT OF CONCENTRATION
\ \
24' C\
--Originally AddedZn pro-Added after 17 Hr.P. P. D. pionate Sulfur Zn propionate Sulfur
%
%
%
%
%
Total Time for Gelation Hours
OF CURING AGENTS
\
\
'
\ ' 0
\ \
\ \ \
50'C.
OF MILLEDCREPE TABLEI. TENPERCENTSOLUTIONS RUBBERIN BENZENE
\O
O N THE SOLUTION
O N THE RUBBER A ' 2%aP.D. 3 $ Z N PROPIONATE
\yI
4 flSULr;UR \
\ \ \
I
I
5
IO RUBBER CONC.
h I
($1
15
for 17 hours, the time required for gelation to start when the whole curing formula was added a t once, the ingredients missing from the complete formula were added to the respective tubes. A comparatively long time was required in each case for gelation to occur.
Temperature Coefficient of Vulcanization in Solution The curves of Figure 4 show that after the viscosity starts to increase, it rises with extreme rapidity so that relative curing times can be defined in terms of the viscosity changes. This makes possible a more accurate experimental measurement than would be possible by trying to observe times of gelation. The time for vulcanization for this work is there-
Figure 6 shows that a linear relation exists, except a t the lower concentrations, between the logarithm of the curing time and the concentration of the solutions. Three points are given for low concentrations of sol rubber in benzene using the same vulcanizing system. These points lie on the extension of the straight line for the milled crepe solution. The solutions of sol rubber had much greater viscosities than corresponding solutions of milled crepe rubber. The rapid increase in the time of cure a t lower concentrations may therefore be partly due to the thermal agitation of the colloidal units which makes i t difficult to build up a gel structure. This idea is also supported by the fact that for 50" C., the linear relation begins to fail a t 8 per cent concentration, but a t 24" C. it extends to 6 per cent. The curing time is related in a simpler way to the concentration of rubber than it is t o the initial viscosities of the solutions. From this it follows that the chemical reaction with the hydrocarbon is more of a controlling factor for the time of cure than the initial colloidal condition. Figure 7 shows that when the concentration of curing ingredients per cubic centimeter of solution is maintained constant and the concentration of rubber varied, the plot of the logarithm of the curing time against the rubber concentration is a straight line over the range investigated. The dashed curves are taken from Figure 6 for comparison. The fact that the use of a higher concentration of curing agents in the solution extends this linear relationship to lower rubber concentrations casts some doubt upon the adequacy of thermal agitation as an explanation for the long curing times for dilute solutions. At any rate, the controlling factor could not be the thermal agitation in the rubber solution before the curing agents are added. As an analogy to the temperature coefficient of vulcanization i t is possible to define a concentration coefficient of vulcanization for these solutions. This may be taken as the ratio of the times required to cure solutions which differ by one per cent in rubber concentration. This is approximately
INDUSTRIAL AND ENGINEERING CHEMISTRY
286
VOL. 32, NO. 2
-501
\1 BASIC FORMULA 2 3 4
%
F! P.D.
5ZN
PROPIONATE
5 SULFUR
FIG. 9
\
10% MILLED CREPE RUBBER IN BENZENE
\\
\\ I
% Pqo.
37- Z N PROPIONATE SULFUR VARIABLE
3
z
P
3
5-
VARIABL
25
?P,D. 3 % Z N PROPIONATE SULFUR VARIABLE
Z N PROPIONA1 SULFUR
25% PPD. % Z N PROPIONATE
3
I
I
0
2
m
\\
\\
I
I
4 CONC. OF VARIABLE
I
I
\\
(6)
I
6
2 4 SULFUR CONC. ($)
EFFECT OF VARIATION IN CURING FORMULA
1 FIG. I I
FIG. I O
\ \\
6
10% MILLED CREPE RUBBER I N BENZENE
5 % ?FD. .I7 SULFUR Z N PROF! VARIABLE
1:
4 %SULFUR
Z N PROP, VARIABLE
Z N PROP. VARIABLE
ZN
I
I
1
2
4
6
PROPIONATE CONC.
(%,
independent of the temperature. For the linear portion of the curves of Figure 6, this coefficient is 1.16; for the solid curves of Figure 7 it is 1.10. A complete description of the effect upon the curing time of variations in a formula containing three ingredients (accelerator, sulfur, and activator) would require a system of surfaces in space for graphical representation. The curves of Figure 8 show the effects when one ingredient in the formula 2 per cent P. P. D., 3 per cent zinc propionate, and 4 per cent sulfur is held constant and the other two are varied. If the percentage of sulfur exceeds 1.5, further increase has
1
0
I
2
PFID. CONC.
I
I
4
6
($1
small effect on the curing time. The curing times with the zinc propionate variable show a minimum. Increase in accelerator content, however, decreases the curing time continuously for the range studied. Figure 9 gives curves for which the zinc propionate was held constant at 3 per cent. These curves do not favor the idea of a reaction between the sulfur and accelerator as a controlling factor for the time of vulcanization since the break in the curves is shifted to lower sulfur content as the accelerator content increases. The curves of Figure 10, in which the break is shifted to
FEBRUAR'Y, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
lower zinc propionate values as the P. P. D. is decreased, may be interpreted as showing that a higher zinc content is required to make the higher accelerator content effective in reducing the time of cure. This hints of a reaction between the P. P. D. and zinc propionate. There is also evidence for such a reaction from rubber compounding data ( 5 ) . It is curious that an excess of zinc propionate actually retards the cure. The curves in Figure 11 bear out the above interpretations in a general way. If the zinc content is low, a high P. P. D. content can actually retard the gelation. This may be attributed to the colloidal effect on the hydrocarbon of the excess of the accelerat,or. Without meaning to suggest that the above results can be generalized to other curing systems, the data favor the idea that there is a reaction between the zinc salt and the P. P. D. rather than between the P. P. D. and the sulfur. The difficulty in applying the theory of reaction rates is apparent when it is realized that vulcanization results from a complicated series of reactions, the final step of which should probably be considered as a catalyzed polymerization. The intimate connection of these chemical and colloidal changes is most clearly demonstrated by such studies of vulcanization in solution.
287
Literature Cited (1) Boiry, F., I n d i a Rubber J . , 68, 651, 887, 727, 771, 807 (1924). ( 2 ) Bourbon, A., Rev. g6n. caoutchouc, 9, 9 (1932). (3) Brown, J. R., and Hauser, E. 9., TND. ESG. CHEM.,30, 1291 (1938). (4) Bruni, G., Rev. g6n. caoutchouc, 8, 19 (1931). (5) Dinsmore, R. P., IND. ENG.CHEY.,21, 722 (1929). (8) Eliel, K. W., T r a m . I n s t . R u b b e t I n d . , 12, 161 (1936:1. ( 7 ) Garvey, B. S., IXD.EXG.CHEM.,29, 208 (1937); I n d i a Rubber World, 98, 37 (1938). (8) Gehman, S. D., and Field, J. E., ISD.ESG. CHEM.,29, 793 (1937). (9) Ibid., 30, 1031 (1938). (10) Krishnan, R. S., Kolloid-Z., 84, 2, 8 (1938). (11) Lewis, W. K., Squires, L., and Nutting, R. D., IND. ENG.CHBY., 29, 1135 (1937). (12) Lotmar, W., Helv. Chim. Acta, 21, 791, 953 (1938). (13) Mueller, H., Proc. Roy. Soc., A166, 425 (1938). (14) Rossem, A. van, I n d i a Rubber J., 92, 845 (1936). (15) Staudinger, H., Trans. I n s t . R u b b e r l n d . , 10, 263 (1934). (18) Thiollet. R., Rev. g6n. caoutchouc, 9, 5 (1932). (17) Williams, Ira, IND.ESG.CHEM.,26, 746 (1934). (18) Ibid., 26, 1190 (1934). (19) Williams, Ira, Proc. Rubber Tech. Conf.,London, 1938, 304. PRESENTBD before the Division of Rubber Chemistry a t the 97th Meeting of the American Chemical Society, Baltimore, Md.
Benzene and Toluene Isopiestic Liquid-Vapor Equilibrium Data FLOYD TODD Lankenau Hospital Research Institute, Philadelphia, Penna.
OR the past twenty-five years investigators have used the benzene and toluene isopiestic liquid-vapor equilibrium data of Rosanoff, Bacon, and Schulze ( 5 ) as the most accurate data available for the evaluation of the height of the equivalent theoretical plate of fractionating columns and for other studies in fractional distillation. I n connection with a rather comprehensive and critical study of fractionating columns, the writer is desirous of using the most accurate data on tJhe benzene-toluene system for standardizing certain phases of the work. A review of much of the literature on this system showed that the data of Rosanoff and co-workers (5) are the most accurate available. The final results of their data are calculated mathematically and are based on the benzene and toluene vapor-pressure data of Regnault (0, Young ( 6 ) ,and Kahlbaumn (8). Although Rosanoff et al. have the greatest confidence in the accuracy of their calculated results, it is believed that in the light of the latest vapor pressure data their results may be recalculated to give a more accurate set of benzene and toluene equilibrium data. The original calculations ( 5 ) on the benzene and toluene system are given in Table I with the writer's corrected values. Since benzene and toluene can be prepared pure and be made gravimetrically to accurate molar concentrations, there can be no doubt as to the high degree of accuracy of the molar concentrations of benzene in toluene as shown in Table I. The data of these investigators are given a t 750 mm. For conventional reasons it was necessary to convert their data
F
to 760 mm. The boiling point-pressure variation constants for benzene and toluene were calculated by the rule of Ramsay and Young ( 3 ) . Crafts (1) used this rule and also confirmed its accuracy experimentally for a large number of compounds. These and several other reliable sources show that the boiling point of toluene varies by 0.460 O C. per 10.0-mm. variation in pressure in the vicinity of the boiling point of toluene. This would make the boiling point of toluene 110.05O C. a t 760 mm. (at 0 mole per cent benzene in Table I). A large number of reliable references showed that the average boiling point of pure toluene is 110.56° C. a t 760 mm. This is a variation of 0.51" C. which must be added to the boiling point of toluene as given by Rosanoff and co-workers (6) to yield a more generally accepted value. TABLE I. CALCULATIONS ON BENZENE-TOLUENE SYSTEM -Mole yo BenzenIn 19 liquid vapor 0 0.0 10 20.8 20 37.2 30 50.7 40 61.9 50 71.3 60 79.1 70 85.7 80 91.2 90 95.9 95 98.0 100 100.0
7 -
A t 750 mm. (6) 109.59 104.85 101.00 97.55 94.60 91.85 89.30 86.85 84.55 82.25 81.00 79.70
Boiling Point, a C. A t 760 mm. At 760 rnrn. (6) (aor.) 110.05 110.56 105.31 105.71 101.46 101.78 98.00 98.25 95.05 95.24 92.30 92.43 89.74 89.82 87.29 87.32 84.99 84.97 82.68 82.61 81.43 81.34 80.13 80.01
