Sulfur Vulcanization of Vinyl-Substituted Polysiloxanes KEITH E. POLMANTEER and ROBERT J. KOCH Silastic Research Laboratory, Dow Corning Corp., Midland, Mich.
F A polysiloxane having all alkyl side groups such as methyl, ethyl, or propyl groups i s vulcanizable with certain peroxides but not with a sulfurvulcanization system. Introducing olefinic side groups in place of some alkyl side groups renders the polysiloxane susceptible to sulfur vulcanization. The effect of substitution of vinyl groups for some of the alkyl groups i s discussed.
1954 ( 7 7 ) . The complexity of sulfur vulcanization is well known and the number of combinations of sulfuraccelerator-activators is almost infinite. It is therefore necessary to confine this discussion to relating the effects of sulfurvulcanizing compounds more widely used in common sulfur-vulcanizable polymers such as natural rubber or
GR-S. A segment of a polysiloxane chain may be pictured schematically as shown below.
THE
principal method for crosslinking polysiloxane polymers to an elastic state has used peroxides such as benzoyl peroxide. The use of sulfur and accelerators as curing agents for silicone polymers was reported before a general professional group first in
Experimental
=
The polymers used in this work were prepared by mixing the desired molar ratios of the cyclic tetramers of dimethylsiloxane (MezSiO) and methylvinyl-
800
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v) 0
500 t LLJ
z W c LL
400 W
-I
z
VI
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t
300
I
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I
1
2 PT. SULFUR 0.5 PT. ALDEHYDE-AMINE E R I M E N E BASE)
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VULCANIZATION TIME
Figure 1.
- MIN.
siloxane (MeViSiO) and heating with potassium hydroxide as recommended by Hyde (9). The properties of (MeViwere published (70) in 1955. The physical properties of (MeViSi0)d used in this work are: boiling point, 222' C. a t 760 mm.; ng, 1.4325; d25, 0.9828; viscosity, 3.48 centistokes a t 2.5' C. All polymers were of comparable average molecular weight within the range of 449,000 to 571,000. The average molecular weights were calculated from intrinsic viscosity measurements using Barry's ( I ) formula,
AT E O 0 C
Effect of vinyl concentration on development of tensile strength
2
x
1 0 - 4 ~ 0 6 6
A silica filler, Hi-Si1 X303, and the vulcanizing agents were added to the polymers on a laboratory two-roll mill having roll diameters of 4 inches. Thirty parts by weight of silica based on 100 parts by weight of polymer were used in most of the sample stocks. Conventional rubber grade chemicals were used, with the exception of the metal oxides (zinc, lead, magnesium, calcium), zinc sulfide, and powdered zinc, which were reagent grade. Each individual series of formulations was very carefully prepared using a base mix technique. Curing agents were weighed on an analytical balance and added to portions of the base mix. The resulting compounds were vulcanized within 24 hours after the base mix was prepared, to guard against possible irregularities due to the age of the mix. The samples were vulcanized in a steam-heated press which had been preheated to the desired temperature. The vulcanization time was measured from the instant of insertion of the sample in the press to the time when cooling was started by flushing the platens with cold water. The cooling cycle required 19 seconds to cool from 160' to 100' C. and a total of 40 seconds to cool below 50' C., as indicated by thermocouples embedded in the press platens. The cooling times for vulcanization temperatures below 160' C. were correspondingly shorter. I t was considered that very little additional vulcanization took place below 100' C. and because the cooling cycle was short compared with the total vulcanization times used in these studies, the error introduced VOL. 49, NO. 1
JANUARY 1957
49
by the method used for the measurement of vulcanization time was negligible. The test moldings were cut into tensile bars and tested on a Scott L-6 tensile testing machine according to ASTM designation D 412-5lT. There are several ways to evaluate the extent of vulcanization. Swelling measurements, bound sulfur determinations, and modulus measurements are most commonly used. Some of these methods can be combined to give a more detailed picture of the result of vulcanization. For example, Zapp and others (74) did an excellent job of combining swelling measurements with organically bound sulfur content to determine the average number of sulfur atoms per cross link. Tensile strength values may also be used to compare the relative extent of vulcanization, although the method is limited to the time region below the time required to develop the optimum tensile strength. This is referred to as the interval of constructive vulcanization processes. The drop in mechanical strength during the vulcanization process beyond the instant of optimum vulcanization is connected with a n increase in cross-link density to a point at which orientation of the molecular chains becomes difficult. Dogadkin (4) and others presented data which gave a n excellent comparison of bound sulfur determinations, swelling measurements, and tensile strengths as a function of vulcanization time. Their data substantiate the utility of tensile strength measurements as a means of following vulcanization rates within the previously mentioned limits. Becaus:: of the ease of obtaining data, the tensile strength method for following vulcanization during the constructive vulcanization processes was chosen as the chief method used in this work to follow vulcanization. Tensile strength was plotted as a function of vulcanization timefor most of the curing recipes studied. Values of the average molecular weight between cross links (M,) for a number of unfilled samples were determined by swelling measurements. The swelling measurements were carried out a t 25' =t O.OIo C. using toluene as the solvent. A p value of 0.465 as determined by Bueche (2) was used in conjunction with Flory's (5) equilibrium swelling relationship for calculating M,.
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80
I
I
I
1
100
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140
I
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IO0
0
Relation
180
200
TIME - MIN. AT 160" C
VULCANIZATION
Figure 2.
160
of ultimate elongation to vinyl concentration I t is apparent from Figures 1 and 2 that the vinyl content of the polymer is a determining factor in not only the combination of physical properties but also the stability of the properties. For example, a polysiloxane should have a vinyl content of less than 1 mole "/c if it is to be subjected to temperatures that will cause excessive cross linking by means of sulfur vulcanization or oxidation.
tration for vinyl content ranges of 0.33 to 1.Oand 1.Oto 4.0 mole respectively. The effect of vinyl content on the sharpness of the tensile strength peak would suggest that the optimum average number of units between cross links lies somewhere between 100 and 300 units, assuming that two vinyl groups are involved in a cross link. Figure 2 gives the elongations for the same samples. The elongations follow the expected trend of leveling off when the vinyl groups have been utilized in the vulcanization process. This same result was also found for butyl (13) with a low degree of unsaturation, and the limiting value indicated by physical properties was verified by swelling measurements. The reciprocal of the elongations a t the ultimate state of cure varies linearly with the vinyl content.
?A,
Sulfur-AcceleratorActivator Combinations
Polysiloxane polymers containing high concentrations of vinyl groups gave poor vulcanizates with sulfur alone, and as the vinyl content was decreased, all evidence of vulcanization disappeared. Figure 3 shows the effect of a variety of
700
600
506
400
Vinyl Group Concentration
Polymers having 0.33, 1.00, and 4.00 mole yo of methylvinylsiloxane units were compounded using 30 parts of silica, 2 parts of sulfur, and 0.5 part of Trimene base (aldehyde-amine) based on 100 parts of polymer, to determine the effect of varying the vinyl content (Figure 1). The vulcanization time varied inversely with approximately the 0.8 and 0.5 powers of vinyl concen-
50
INDUSTRIAL AND ENGINEERING CHEMISTRY
c
200
IO0
0
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VULCANIZATION
Figure 3.
I20
TIME
140
MIN. AT
I60
160°C
Relative activity of accelerators
180
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600
-
v)
a
500
r
co
$
400
v)
W
Z!
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iW -
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IO0
0
Figure 4.
Effect of thiuram disulfide concentration in nonsulfur system
accelerators on sulfur vulcanization. Two facts are apparent from this graph. First, accelerators can make the difference between suitable vulcanization and no apparent vulcanization. Secondly, the specific accelerator plays an important part in determining the rate of vulcanization. The control for the series contained 2 parts of sulfur and after 180 minutes a t 160' C. showed no signs of vulcanization. The sample with 5 parts of calcium oxide in addition to the 2 parts of sulfur vulcanized in 180 minutes at 160' C. to a fairly good
rubber. A sample with 5 parts of zinc oxide showed no vulcanization after 200 minutes at 160' C., while 5 parts of lead oxide gave slight vulcanization after 180 minutes at 160' C. A sample with 5 parts of magnesium oxide developed optimum tensile strength in approximately 40 minutes a t 160' C. This difference in activity of the metal oxides is probably due to their relative basicity. The bivalent metal dithiocarbamates accelerate sulfur vulcanization of vinylcontaining polysiloxanes. The activity of these dithiocarbamates decreases in the
800
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2I
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8
b z E
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t 0 U -I
5 300 z U I-
30PT SILICA
2PT.SULFUR 5PT. M9O
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IO0
'Irp 30
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VULCANIZATION TIME- MIN. Figure 5.
Effect of temperature
I50
order of zinc, lead, and copper. The difference in vulcanization rates for these dimethyldithiocarbamates was small. O u t of this group, the results for copper dimethyldithiocarbamate (Cu DMDC) are given in Figure 3. Tetramethylthiuram disulfide (TMTD) showed considerable activity as an accelerator for sulfur vulcanization. I t reached its optimum properties after 30 minutes a t 160' C. Other accelerators showing approximately the same activity were its ethyl analog (TETD) and selenium and tellurium diethyldithiocarbamates. T h e guanidines such as diphenylguanidine gave the second fastest acceleration to sulfur in this polysiloxane system. One part of diphenylguanidine (DPG) with 2 parts of sulfur reached optimum properties in approximately 10 minutes a t 160' C. The fastest acceleration was accomplished with strong organic bases such as tetraethylenepentamine, triethylenetetramine, Trimene base (diethylamineformaldehyde reaction product), Beutene (butyraldehyde-aniline reaction product), and Hepteen base (heptaldehyde-aniline reaction product). The results with Trimene base are given in Figure 3. I t was difficult to classify these organic base compounds among themselves because of the difficulty involved in resolving the differences in the short vulcanization times obtained with the different compounds mentioned above. These strong basic accelerating compounds appeared to reach their optimum properties in approximately 7 minutes a t 160' C. The results of many additional compounding studies with vinyl-containing polysiloxane polymers using cure systems containing elemental sulfur may best be summarized by the following statements : The amount of a n accelerator such as Trimene base (aldehyde-amine type) is important in determining the vulcanization rate. I n general, zinc oxide and litharge retard vulcanization in this silicone system to varying degrees, depending on tlie type of acceleration used. For example, zinc oxide greatly retards the sulfur-diphenylguanidine system, but only slightly retards the sulfur-mercaptobenzothiazole (MBT) system. Contrary to the action of zinc oxide and litharge, both magnesium oxide and calcium oxide slightly activate sulfuraccelerator cure systems. I n a TMTD-accelerated system the oxides of zinc and lead form their less active dithiocarbamates at the curing temperatures. Organic acids such as stearic or benzoic acid retard and in many cases also inhibit vulcanization in this vinyl-containing polysiloxane system. I n a sulfurmagnesium oxide accelerated system, the effect of organic acids was less severe VOL. 49, NO. 1
JANUARY 1957
51
'
than in a sulfur-TMTD-zinc oxide recipe. Carbon black may be used as filler in sulfur-vulcanizable polysiloxanes. T h e alkaline furnace blacks give slightly faster vulcanization rates than a corresponding silica-filled stock. Acidic channel black-filled stocks give slopver vulcanizaticn rates than silica-filled stocks. Vulcanization of a carbon black-filled vinylcontaining polysiloxane is retarded by zinc oxide in much the same way as silica-filled stocks. The degree of retardation and inhibition exhibited by the presence of zinc oxide in carbon blackfilled polysiloxanes depends also on the sulfur-accelerator combination. For example, the vulcanization of a furnace black-filled vinyl-containing polysiloxane is retarded less by zinc oxide in a sulfur-Santocure recipe than in a sulfurT M T D recipe.
N o nelementa I
SulfurCuring Systems
Certain sulfur-containing accelerators are capable of cross-linking sulfurvulcanizable organic polymers without the aid of elemental sulfur. Accelerators such as selenium or tellurium diethyldithiocarbamates and thiuram disulfides are of this type. These compounds are also capable of vulcanizing vinyl-containing polysiloxane stocks. Figure 4 shows the effect of different concentrations of T M T D on the rate of vulcanization. For concentrations of 3 parts of agent and below rhe vulcanization time varies inversely as the 2.3 power of the concentration. The results of many additional compounding studies Tvith vinyl-containing polysiloxane polymers using cure systems not containing elemental sulfur may best be summarized by the following statements: Based on equivalent dithiocarbamate content, selenium diethyldithiocarbamate (SeDEDC) gives slightly faster vulcanization rates than T E T D and approximately four times the rate of tellurium diethyldithiocarbamate (TeDEDC). As anticipated, the bivalent metal dithiocarbamates such as copper, zinc, and lead dimethyldithiocarbamates are inactive as vulcanizing agents for this polymer system ivhen used in the absence of elemental sulfur. The effect of tetramethylthiuram monosulfide ( T M T M ) on T M T D recipes is to give tighrer cures with very little change in the vulcanization rate. However: the tensile values are lowered from those of the control stock. This trend is different than for extracted pale crepe; Craig, Juve, and Davidson ( 3 ) found that T M T M both retarded and inhibited the T M T D cures. Zinc oxide inhibits vulcanization of a T M T D or a similar type of cure system. In fact when equivalent moles of zinc oxide (1.35 parts) to T M T D (4 parts) are used, vulcanization is completely inhibited over a 5-hour vulcanization span.
52
INDUSTRIAL AND ENGINEERING CHEMISTRY
I 30
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I 90 TIME
I 120
I I50
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- MIN.
Figure 6 . Effect of t e m p e r a t u r e
These results again suggest that the corresponding dithiocarbamates are formed, as zinc dialkyldithiocarbamate ill not vulcanize this polymer system in the absence of elemental sulfur. Finely powdered zinc and litharge both exhibit essentially the same inhibiting and retarding effects in the T M T D system as zinc oxide. Zinc sulfide, on the other hand, retards vulcanization by about a factor of 2 and shoil-s only very slight inhibiting effects. Acids such as stearic or benzoic acid inhibit vulcanization in disulfide or dithiocarbamate cure systems. For example, 2 parts of stearic acid effectively prevent suitable vulcanization with 3.0 parts of T M T D . Mercaptobenzothiazole slightly retards and inhibits vulcanization in this elemental sulfur-free curing system. Magnesium oxide, calcium oxide, and organic bases activate this elemental sulfur-free curing system. Temperature Coefficient of Rate of Vulcanization and Apparent Energy of Activation for Reaction
The practical value of knowing the remperature coefficient of the rate of vulcanization is well known to the rubber technologist. The temperature coefficients for rubber were summarized from published data for 1930 to 1939 by Gerke (6). T h e values ranged from 1.87-fold increase in rate per 10' C. increase in temperature to 2.67, depending largely upon the accelerators used. These values, as given in the literature, are dependent to a small extent upon the method used to determine the values -Le.: bound sulfur or modulus measurements. I t seemed worth while to study a variety of curing recipes in this
vinyl-containing polysiloxane system The methylvinylsiloxane content was kept constant a t 4 mole yo throughout the different recipes, and the silica filler content was held constant a t 30 parts by weight. The temperature coefficients and activation energies for the different systems are given in Table I. Figure 5 shows the tensile strength values plotted as a function of vulcanization time a t a series of temperatures for a curing system of 2 parts of sulfur-5 parts of magnesium oxide. Figure 6 shows a similar plot for a curing system of 2 parts of sulfur-1 part of ThITD. Figure 7 gives a plot of tensile strength us time for a curing svstem of 2 parts of sulfur1 part of diphenylguanidine. Figure 8 gives the curves of tensile strength us time for a curing system of 2 parts of TMTD-1 part of MBT-3 parts of Trimene base. The temperature coefficients for the rate of vulcanization in this po1)siloxane system follow a very close parallel to organic rubber values For example, a rubber-sulfur system is usually quoted (6)as having a temperature coefficient of around 2.5 as compared with 2.54 for the sulfur-magnesium oxide combination in this system. The tempcrature coefficient values of T M T D in rubber ( 6 ) and in this polymer qstem are practicall} identical. The Arrhenius activation energy values ( E in kilocalories per mole) calculated from these data are verv close to values found in the literature for sulfur vulcanization of organic polymers. For the sulfur-magnesium oxide system a value of 31.5 kcal. was found. This value is in the expected range for heat of reaction of sulfur, which was calculated
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v) 0 I 500 r c m
z
p~ f M 7' MeViSiO 96 M % Me2 S i 0
W
E
ln
4oc
30 P T . S I L I C A 2PT. SULFUR I PT DPO
w
-I
G5 300
"
2 kW
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VULCANIZATION
- MIN.
Figure 7.
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Effect of temperature
by Powell and Eyring (72) to be 27.5 f 5 kcal. Gordon (7) determined a value for unaccelerated Buna vulcanization to be 30.5 to 35.0 kcal. and 23.0 kcal. for butyl ( 8 ) accelerated by tellurium dithyldithiocarbamate. Dogadkin and coworkers ( 4 ) obtained 31.5 kcal. for sulfur-zinc oxide-stearic acid combination in smoked sheet and from 22.9 to 24.5 kcal. for various accelerator combinations in the same system. Thus the values for activation energies for the various accelerator combinations in this vinyl-containing polysiloxane system seem in line with the values quoted in the literature for other polymers. Vinyl Groups in Cross links
I t seemed of interest to gain some insight as to the role played by the vinyl
I 0 ,
Figure 9. content
Effect of temperature
20
Figure 8.
O
groups in sulfur vulcanization of these polysiloxaneg. As in the case,of organic polymers, it was established that the presence of unsaturated groups was necessary to render the polymer vulcanizable with sulfur and related compounds. I t was found qualitatively by following progressive times of vulcanization with infrared curves that the number
Table I.
I I O
I 11
I 10 ME
M%
vi
1 5
~ 3 0
I I ,
4 0
Dl 0
Relation of (D.P.), to vinyl
of vinyl groups diminished as the degree of cross linking increased. This indicated that the vinyl groups were probably a part of each cross link. As an all-methyl substituted polysiloxane did not show evidence of vulcanization with sulfur under normal vulcanization temperatures, it seemed likely that methyl groups would not be involved in cross links in the vinyl-substituted polysiloxane. This would mean that the most probable cross link would include two vinyl groups joined with sulfur. This conjecture would be strengthened by determining the (D.P.), values (the average number of R & O units between points of cross linking) for a range of vinyl-content polymers. For example, if two vinyl groups are involved per cross link, polymers having molar contents of 0.5, 1.O, 2.0, and 4.0% would have (D.P.)c values of 200, 100, 50, and 25, respectively. Furthermore, if one vinyl group is involved per cross link, the corresponding (D.P.), values would then be 100, 50, 25, and 12.5. The above conjecture was tested by a series of experiments. First, a copolymer 2.0 MYo MeViSiO-98.0 M yo Me2Si0, was compounded with 2 parts of sulfur1 part of diphenylguanidine combination and 2 parts of sulfur-I part of T M T D
Temperature Coefficients and Activation Energies
Temp. Range,
Temp. Coef.
Recipe O C. 100 poly., 30 silica, ZS, 5 MgO 140-160 100 poly., 30 silica, ZS, 1 TMTD 130-160 100 poly., 30 silica, Z S , 1 DPG 130-160 100 poly., 30 silica, 2 TMTD, 1 MBT, 3 Trimene base 130-160 Polymer was a 4 mole % ' MeViSiO-96 mole % MezSiO copolymer.
VOL. 49, NO. 1
0
per l o o C. 2.54 2.15 2.11
1.85
E, Kcal./
Mole 31.5 26.4 25.1 20.3
JANUARY 1957
53
Table II.
No.
Recipe”
1
IOOC, 2S, 1 DPG IOOC, 2.3, 1 DPG lOOC, 25, 1 D P G IOOC, ZS, 1 DPG IOOC, 2S, 1 TMTD IOOC, ZS, 1 TMTD IOOC, 2S, 1 TMTD lOOC, 25, 1 TMTD IOOC, 4 TMTD IOOC, 6 TMTD IOOC, 10 TMTD 100A,25, 1 DPG IOOB, 25, 1 DPG IOOC, 2 5 , 1 DPG 100D, 4S, 1 DPG 100A, 2 5 , 1 TMTD IOOB, 2S, 1 TMTD lOOC, 2S, 1 TMTD IOOD, 45, 1 TMTD IOOA, 25, 5 MgO IOOB, 2S, 5 MgO IOOC, 25, 5 MgO IOOD, 4S, 5 MgO
2 3 4 5 6
7 8
9
10 11
12 13 14 15 16
17 18
19
20 21 22 23
A. B. C. D.
(D.P.), Values for Unfilled Vulcanizates
Copolymer-0.5 Copolymer-1.0 Copolymer-2.0 Copolymer-4.0
mole mole mole mole
% MeViSi0-99.5 % ’ MeViSiO-99.0 % MeViSiO-98.0 % MeViSiO-96.0
Vulc. Time, Hours a t lGOo C. 1
1.5 3 6 1 1.5 3 6 6
6 6 3 3 3
3 6
6 6 6 6 6
6 6 mole mole mole mole
(D.P.)c 246 139 63 59 224 112 75 63 258 121 69 262 115
63 32 232 93 63 34 668 183 107 52
% MezSiO. % MezSiO. % Me2SiO. % MezSiO.
of elemental sulfur follow very closely the list of compounds capable of vulcanizing organic polymers. Vulcanization time is dependent upon the concentration of accelerator. Zinc oxide, powdered zinc, lead oxide, and organic acids all gave marked retarding and inhibiting effects on systems using agents, such as T M T D or SeDEDC. Zinc sulfide showed only slight retarding and inhibiting effects. Magnesium oxide is a n activator in this system as in the elemental sulfur system. Strong organic bases greatly increase the rate of vulcanization. This vinyl-containing polysiloxane system gave temperature coefficients for rate of vulcanization ranging from 2.54 down to 1.85 per 10’ C. change in temperature. The Arrhenius activation energies for this system ranged from 31.5 to 20.3 kcal. per mole. Both the temperature coefficients and the activation energy values depended upon the type of accelerators present. Both agreed very well with literature values for sulfur vulcanization of organic polymers. (D.P.), data indicated that two vinyl groups are involved in each cross link. Acknowledgment
combination to determine the vulcanization time a t 160’ C. necessary to reach the respective ultimate states of cure. The results are given in Table 11. Three hours a t 160’ C. was considered adequate for the polymer-sulfur-diphenylguanidine system and 6 hours a t 160’ C. was necessary for the polymersulfur-TMTD system to reach ultimate states of cure. These vulcanization times were then used for subsequent work with copolymers containing 0.5, 1.0, 2.0, and 4 mole yo MeViSiO and 99.5, 99.0, 98.0, and 96.0 mole yo MezSiO, respectively. T o each of these copolymers were added 1 part of either DPG or T M T D and 2 parts of sulfur, with the exception of the 4 mole Yo MeViSiO-containing copolymers, to which 4 parts of sulfur were added. Another series was also evaluated having 5 parts of magnesium oxide as the accelerator along with the sulfur concentrations mentioned above. Data given in Table I1 and plotted in Figure 9 show how the experimental (D.P.)c values compare with the theoretical curve, which assumed that two vinyl groups would be involved in each cross link. The results give good evidence that the above conjecture is correct. I t is interesting to compare the similar results of the DPG- and the TMTD-accelerated stocks with the results of the magnesium oxide-accelerated stocks. I t would appear that fewer vinyl groups are destructively blocked as crosslink sites in the cases of DPG and T M T D than in the case of magnesium oxide.
54
Summary
The vinyl content of the polysiloxane polymer affects the rate of vulcanization in such a way that the vulcanization time varies inversely with the vinyl concentration raised to a power less than 1. Furthermore, this power appears to be dependent upon the vinyl concentration, so that a stationary value cannot be quoted. For recipes containing elemental sulfur, several conclusions may be made: Sulfur alone cross-links only polysiloxane polymers of high vinyl content. A 4 mole vinyl-containing polymer exhibited no vulcanization after 3 hours at l6Oo C. with sulfur alone. Vulcanization times in the same range as those found with sulfur-vulcanizing organic polymers were obtained in this polysiloxane system by the proper choice of accelerators. Starting with the most active, compounds for accelerating or activating this vinyl-containing polysiloxane-sulfur system were: strong organic bases (TEPA), DPG, thiuram disulfides (TMTD), magnesium oxide, bivalent metal dithiocarbamates, thiazoles (MBT), weak organic bases (aniline), calcium oxide. Zinc oxide and litharge are inhibitors in this system to varying extents: depending on the accelerator present. Organic acids such as stearic acid inhibit vulcanization in this system. For recipes void of elemental sulfur, the following conclusions may be made : Accelerators that vulcanize vinylcontaining polysiloxanes without the aid
INDUSTRIAL AND ENGINEERING CHEMISTRY
The authors wish to thank Gretchen Hutzel for carrying out the swelling measurements and 0. K. Johannson for determining the properties of [MeViSiO];. literature Cited
(1) Barry, A. J., J . Appl. P h j ~ .17, 1020 (1 946). (2) Bueche, A . A I . , J . Polymer Sci. 15, 97.103 (1955). (3) Craig, D., Juve, A . E., Davidson, W. L., Rubber Chem. and Technol. 24, 254 (1951). (4) Dogadkin, B., Karmin, B., Dobromyslova, A , Sapozkkova, L., Ibid., 23, 563 (1950). ( 5 ) Flory, “Principles of Polymer Chemistrv.” D. 579. Cornell Universitv Pre$s, Ifhaca, N. Y . , 1953. Gerke, R. H., IND.ENG. CHEM.31, 1478 (1939). Gordon, iM.,J . Polymer Sci. 3, 438 (1948). Gordon, M., Rubber Chem. and Techwl. 25, 1 (1952). Hyde, J. F., U. S. Patent 2,490,357 (Auril 24. 1946). (10) Kantbr, S. iV., Osthoff, R . C., Hurd, D. T., J . Am. Chem. SOC.77, 1685 (1955). ( 1 1 ) Osthoff, R. C., Hurd, D. T., Division of Rubber Chemistry, 126th meeting, ACS, New York, September 1954. (12) Powell, R. E., Eyring, H., J . Am. Chem. SOC. 65, 648 (1943). (13) Whitby, G. S., “Synthetic Rubber,” p. 856, Wiley, New York. 1954. (14) Zapp, R. L., Decker, R. H., Dyroff, M. S., Rayner, H. A , , Rubber Chem. and Technol. 24, 878 (1951). RECEIVED for review November 4, 1955 ACCEPTED May 14, 1956 Division of Rubber Chemistry, ACS, Philadelphia, Pa., November 3, 1955.
