INDUSTRIAL AND ENGINEERING CHEMISTRY
1526
S'ol. 45, No. 7
DISCUSSION OF RESULTS
TABLEIv.
HEATSO F BONE CHAR
Table V summarizes the results for the two adsorbente. The absolute values, when the relatively small number of observations Temperatures of Adsorbent, F. ~ Measurement Initial Final and~ thel possibility of systematic errors are taken into account, are No. (in furnace) (in calorimeter) probably accurate within 3= 10%. The ratios, however, involving Bone C h a r the same syst,ematic errors for both adsorbents, are probably 1 482 82 0.195 reliable to within 2%. 2 568 88 0.206 Evidently there is no significant difference between t.he ad85 0.214 3 651 45 732 85 0.221 sorbents with respect to specific heat and thermal conductivity, 88 0.239 813 6 887 88 0.246 but, the heat of wett,ing of Synthad is much smaller than t.hat of Av. 0.219 bone char. It has been suggested by de Whalley and Dickinson Synthad (4)that, the heats of wetting of bone chars with similar histories 1 504 83 0.195 might be used as measures of their relative areas. It is, however, 2 545 82 0.218 3 544 83 0.213 not surprising that this concept does not appear t o be applicable 4 63 1 82 0.215 t o the Synthad and bone char used for these measurements be5 795 87 0.215 6 865 85 0.242 cause, in spite of their very similar properties, they are not 7 967 87 0.244 Av. 0 , 2 2 0 chemically identical. The area - of the Synthad was approximately 90 square meters per gram T A B L E v. PHYSICAL PROPERTIES O F SYXTHAD C-38 4 S D B O X E CHAR and that of the bone char Temp. Ratio of about 120, indicating, on the Range of ddsorbents Results Results, Property a n d Neasurements, Used for Bone Synthad Synthad/Bone de Whalley and Dickinson hyGnits Reported F. Measurement char C-38 Char pothesis, a ratio of heats of wet70-72 H e a t of wetting, calories/ New adsorbents heated 15.9 9.3 0.58 ting of about 0.75, the actual t o 1100O F.and cooled gram i n d r y atmosphere ratio was about 0.6. 90-210 T h e r m a l conductivity. Average of samples i n t o 0.100 0.101 1.01 SPECIFIC
O
SYKTH.4D Av. Specific H e a t from 1 ~ i ~ to i ~~ 1 i Temp., B.t.u./Lb. F. -4XD
O
B.t.u./ft. hr.
F.
Specific h e a t , B.t.u./lb. 0 P.
80-900
cycles 1, 10, 15, 20 of refinery test New adsorbents dried pulverized, a n d evacdated in quartz bulb
ACKNOWLEDGRZENT 0.24
0.24
1.00
The authors Fish to thank Baurrh and Sons Co. and Mellon Institute for permiseion to publish this material. I
100' F. over the range from 500" to 900' F. A t the Revere Sugar Refinery, where char is reburned in thin-tube alloy steel char temperature in reburning is cusretorts, the tomarily between 850" and 900" F. If this is taken as representative of maximum temperatures for refineries in general, it Seems that a specific heat of 0.24 may approximate more closely to working conditions than one of 0.21.
LITERATURE CITED I X D . ENG.C H E W , 43,639 (1961). Barrett, Joyner, and Halenda, Ibid., 44, 1827 (1952). ~~i~~ and ~ ~ bIbid.,i 40,~ 1073 ~ (1948,, ~ ~ , de Whalley and Dickinson, Intern. Sugar. J . , 48, 73 (1946). Robinson, Proc. Tech. Session Bone Char, 1949, 300.
(1) Barrett, Brown, and Oleck, (2) (3) (4) (5)
RECEIVED for review Xovember 14, 1952.
ACCEPTEDMarch 2 2 , 1953.
tobenzothiazole Vulcanization Using Sulfur-35 IRVING AUERBACH The Goodyear Tire and Rubber Co., Akron 16, Ohio
S
INCE the discovery of the accelerating properties of mercapto-
benzothiazole (MBT), a number of mechanisms have been suggested to account for its role ( 3 , 5 , 6, 8). The formulation of these mechanisms was hampered from the very beginning, because the process of vulcanization itself was not well understood. Furthermore, intermediates were suggested whose presence was not assured. I n the light of newer information on the mechanism of vulcanization and because of the possibilities opened up by tracer techniques, a re-examination of this problem was undertaken. EXPERIMENTAL WORK
B y using either radioactive elementary sulfur or mercaptobenzothiazole in which the mercapto group was tagged, the analytical procedures were greatly simplified and very rapidly performed. Thus, it was possible t o extract vulcanized samples
containing tagged mercaptobenzothiazole with alcohol, determine consumed mercaptobenzothiazole, and then re-extract the same samples with chloroform to determine mercaptobenzothiaaole combined as the zinc mercaptide. Similarly, combined sulfur was determined by using radioactive sulfur in the vulcanization formula. Extracting with acetone and determining the residual activity in the vulcanizate gave the total combined sulfur. By re-extracting with an alcohol-ether solution of hydrochloric acid, zinc sulfide was removed. The residual activities of the samples then indicated organically combined sulfur, The term "consumed mercaptobenzothiazole" refers to mercaptobenzothiazole products not soluble in alcohol and combined sulfur includes rubber-sulfur compounds and zinc sulfide. The test specimens were prepared in the following manner. GR-S, containing 4.78% fatty acid, or unextracted pale crepe
July 1953
*
s
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
was dissolved in benzene with the aid of heat and stirring. Zinc oxide which had been dispersed by milling into raw stock (5050) was then suspended in this solution. Sulfur and mercaptobenzothiazole dissolved in benzene were added to the cool suspension. After stirring for about 30 minutes, the cement was poured on aluminum foil, the benzene was allowed to evaporate, and the compounded rubber was stripped from the foil. Stripping was facilitated by coating the foil with a very thin soap film before pouring the cement. The amount of consumed mercaptobenzothiazole, or more strictly the consumed sulfur from the mercaptobenzothiazole mercaptan group, was measured by curing duplicate samples containing radioactive mercaptobenzothiazole in mold cavities 1.5 inches in diameter at ca. 20,000 pounds per square inch for various time intervals; 30 to 45 seconds was allowed for the sample to come to press temperature. Then 13/8-inch disks were stamped out. These were approximately 0.025 inch thick and corresponded to infinite thickness for sulfur radioactivity measurements. Radioactivity measurements were made on both sides in a windowless proportional flow counter and the four values averaged. The samples were then extracted with absolute alcohol for 24 hours. The radioactivity of the rubber was redetermined and the per cent consumed mercaptobenzothiazole calculated from these values. Combined sulfur was determined by making additional cures under identical conditions with a recipe containing radioactive free sulfur. These were extracted with acetone. The per cent combined sulfur was determined from activities before and after extraction. All cures were performed a t 140' C. unless otherwise stated. This experimental procedure possesses several advantages. It is simple, rapid, and nondestructive, thereby allowing several different analyses with the same sample. Furthermore, it allows for the determination of combined sulfur which originated from the free sulfur independent of t h a t from the accelerator sulfur. FREE SULFUR-ACCELERATOR SULFUR INTERCHANGE
"I
The analytical results are of significance only if there is no appreciable exchange between mercaptobenzothiazole sulfur and free sulfur. This was shown in several experiments. Hevea stock containing zinc oxide, sulfur, and mercaptobenzothiazole in the same amounts used for the work was cured for various time intervals up t o 120 minutes. Combined sulfur was complete after about 60 minutes. The mercaptobenzothiazole was extracted and the specific activity determined. The average specific activity of the extracted mercaptobenzothiazole was 1.45 f 0.08 X lo4counts per minute per mg. The activity of the original mercaptobenzothiazole was 1.43 X 104 counts per minute per mg. No trend in specific activity with curing time was noted, indicating t h a t the recovered mercaptobenzothiazole had not undergone exchange.
1527
after which i t is once more consumed by the rubber in a manner paralleling the sulfur reaction. Several relationships are apparent. Thus the times at which the minimum occurs and the sulfur reaction starts are approximately the same. Secondly, in Figure 3 where 5 parts of sulfur
I 5a3 4
-3
2 a
W
z -2
%0 0
-I
' 0
50
100 150 MINUTES
200
250
o'
+
5
i/
Figure 1. Consumed Mercaptobenzothiazole and Combined Sulfur Formulation.
GR-S 100, zinc oxide 5, sulfur 3, MBT 1 0 Consumed MBT 0 Combined sulfur
per 100 parts of rubber is present, the minimum occurs at approximately 8 minutes, whereas in Figure 2 where 1part of sulfur is present the minimum occurs at 60 minutes. Thirdly, Figures 1 and 3, where 3 and 5 parts of zinc oxide per 100 parts of rubber were used, show t h a t the quantity of consumed mercaptobenzothiazole a t zero time is greatly enhanced b y the presence of the larger quantity of zinc oxide. I n Figure 4 where 5 parts of mercaptobenzothiazole is present the reaction is very rapid, the sulfur having reacted completely after 45 minutes. It is believed t h a t the induction period for the sulfur reaction and the minimum for the mercaptobenzothiazole reaction are absent for this reason.
0.4r
I
INITIAL VULCANIZATION REACTIONS
The effect of variations in the mercaptobensothiazole, sulfur, and zinc oxide concentrations on the sulfur combining rate was determined. A base formula of 100 parts of G R S , 3 parts of zinc oxide, 3 parts of sulfur, and 1 part of mercaptobenzothiazole was adopted. The sulfur, mercaptobenzothiazole, and zinc oxide concentrations were then varied individually. Figures 1 to 4 give curves which are representative of the results obtained. They show the variation of consumed mercaptobenzothiazole and combined sulfur with time and in two cases the variation of the zinc mercaptide of mercaptobenzothiazole with time. I n these figures the reacted quantities of vulcanization ingredients are given as the per cent of the formula weight. With the exception of Figure 4, the results show a marked induction period for the sulfur reaction. The consumed mercaptobenzothiazole is regenerated rapidly and goes through a minimum,
MINUTES Figure 2.
Consumed Mercaptobenzothiazole and Combined Sulfur
Formulation.
GR-S 100, zinc oxide 3, sulfur 1, MBT 1 0 Consumed MBT X Combined sulfur
I n the tests illustrated in Figures 3 and 4 the vulcanized samples were re-extracted with chloroform to remove any zinc mercaptide of mercaptobenzothiazole. Figure. 3 shows t h a t no mercaptobenzothiazole zinc mercaptide is present beyond the minimum until after 150 minutes' curing time. However, when 5 parts of mercaptobenzothiazole per 100 parts of rubber was used, Figure 4, a large quantity of the mercaptide was present. This information suggests that the primary mercaptobenzothiazole product is the zinc mercaptide, since a n increase in zinc
1528
INDUSTRIAL AND ENGINEERING CHEMISTRY
oxide from 3 Do 5 parts would be expected to give 3.34-fold increase of alcohol-insoluble mercaptobenzothiazole. The observed increase was 3.38-fold. The figures also suggest that a reaction takes place between the mercaptobenzothiazole zinc mereaptide and sulfur, as increasing the sulfur concentration correspondingly decreases the time a t which the minimum is reached. Thus for the experiments involving 1, 3, and 5 parts of
5 $
I
B
0!
60
120
180
240
2
MINUTES Figure 3.
Consumed Mercaptobenzothiazole and Combined Sulfur
Formulation. GR-S 100, zinc oxide 3, sulfur 5, MBT 1 0 Combined sulfur 0 MBT n o t extracted by alcohol X MBT not extracted by chloroform
sulfur the minima were obtained a t approximately 60, 18, and 8 minutes. No minimum was obtained, however, for the case where 0.5 part of sulfur per 100 parts of rubber was used. This initial reaction may be accounted for by the formation of hgdrogen sulfide in sufficient quantities t o inhibit the sulfur-rubber reaction. It would be removed by reaction with the mercaptide to regenerate mercaptobenzothiazole ( 5 ) . 4 s the mercaptobenzothiazole zinc mercaptide reacts so readily, it might be anticipated t h a t its concentration would be very small, even though mercaptobenzothiazole is regenerated and probably reacts with more zinc oxide or zinc stearate. This was confirmed when the vulcanized samples from the compounded mixture of 100 parts of GR-S, 3 parts zinc oxide, 5 parts of sulfur, and 1part of mercaptobenzothiazole (Figure 3) were re-extracted with chloroform, in which the mercaptide is soluble, after the alcohol extraction. Figure 3 shows t h a t consumed mercaptobenzothiazole is the same for both the alcohol- and chloroformextracted samples, except for the last few cures. Thus the mercaptide reacted as rapidly as i t was formed, and did not accumulate to any extent until after 150 minutes, a point beyond which there is very little sulfur. For the 180- and 240-minute cures only 0.18 and 0.04 formula yo sulfur remained. Similar results were obtained with a formula containing 0.5 part of sulfur per 100 parts of rubber. I n Figure 4, where 5 parts of mercaptobenzothiazole was present, the decrease in mercaptide concentration was very rapid, and because of the large quantity originally present it was never depleted. KINETICS O F VULCANIZATION REACTIOIVS
The effect of variations in the mercaptobenzothiazole, sulfur, and zinc oxide concentrations on the initial sulfur combining rate was determined. The same base formula of 100 parts of GR-S, 3 parts of zinc oxide, 3 parts of sulfur, and 1 part of mercaptobenzothiazole was adopted and the sulfur, mercaptobenzothiazole, and zinc oxide concentrations were varied individually as before. The initial sulfur combining rates were then measured from the plots of combined sulfur us. time, These values are given in
Vol. 45, No. 7
T#ABLE I. EFFECT O F REACTANT CONCENTRATION SULFUR COMBININQ RATE
INITIAL
ON
(Combining rate X 10-8 mole/100 grams/minute) Phr MBT Zinc Oxide Sulfur 0.5 1
i:ig
3 5
6:79
0,052
1:iz
0.158
1.05
3.20
1.19
1.19
Table I, as moles of combined sulfur per 100 grams of vulcanizate per minute. The concentrations of the various reactants are listed, for convenience, as parts per 100 grams of GR-S. Variation in the zinc oxide concentration does not affect the rate a t which sulfur reacts over this range. This would seem to indicate that this reactant is not involved in the rate-determining step a t these concentrations. The effect of the mercaptohenzothiazole on the sulfur combining rate is linear. A fivefold increase in the mercaptobenzothiazole concentration increased the rate approximately five times. A linear relationship mas also noted in data obtained by Adams (1)with natural rubber. With respect to sulfur, however, the rate is proportional to the square of the sulfur concentration. This is shown graphically on Figure 5. That mercaptobenzothiazole is consumed during the vulcanization process has also been observed by othera (3, 7 ) . Some of the consumed mercaptobenzothiazole is in a form which is insoluble not only in alcohol but also in acetone and chloroform. This suggests that this mercaptobenzothiazole might be chrmically bound to the rubber. T h a t the ratio of consumed mercaptobenzothiazole to combined sulfur is constant was made evident when the consumed mercaptobenzothiazole was plotted against combined sulfur. Straight lines were obtained in all cases except where curing proceeded rapidly and most of the data obtained consisted of values after curing was almost complete.
5,
I
0
60
120
180
I
240
MINUTES Figure 4.
Consumed Mercaptobenzothiazole and Combined Sulfur
Formulation. GR-S 100, zinc oxide 3, sulfur 3, MBT 5 0 Combined sulfur MBT n o t extracted by alcohol A MBT not extracted by chloroform
If the consumed mercaptobenzothiazole and combined sulfur are divided by their respective molecular and atomic weights and these values are plotted, the slopes of the straight lines obtained indicate the number of atoms of sulfur which combine with the rubber or zinc per molecule of mercaptobenzothiazole consumed. Typical plots are shown in Figure 6 and the slopes are given in Table 11. Where curing had proceeded rapidly, the ratios mcre obtained by averaging the ratios for the individual cures. The ratio of combined sulfur to consumed mercaptobenzothiazole is independent of the temperature. This is brought out clearly in the case of GR-S, (Table 111). D a t a were obtained at three temperatures with the following formulation: 3 parts of
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1953
sulfur, 3 parts of zinc oxide, and 1 part of mercaptobenzothiazole per 100 parts of rubber. Curing proceeded slowly enough so that sufficient data could be obtained with pale crepe at 120"C . At higher temperatures, however, the curing was practically complete within the first few minutes. The data could not be averaged as in the case of GR-S, since mercaptobenzothiazole continued to combine with the rubber even after all the sulfur had reacted (Figure 6).
1529
TABLE11. RATIOOF COMBINEDSULFURTO CONSUMED MERCAPTOBENZOTHIAZOLE-GR-S Phr 0.5 1
ZnO
S
43.3
13'2
15.0 17.1
...
43.3 34.0
43.3
MBT
...
26.W
3 5
6.5 4 la
4
52 8
Values obtained with slower curing GR-S stock.
TABLE 111. EFFECTOF TEMPERATURE O N RATIO OF COMBINED SULFURTO CONSUMED MERCAPTOBENZOTHIAZOLE Temp., C. 120
Pale Crepe 165
130 140 150
GR-S 26:s 26.9 28.6
TABLEIV. MERCAPTOBENZOTHIAZOLE CONSUMEDPRIORTO THE
Phr 0 5 1 6
3
[MBT][S]b
9
IO
'
a
(Moles X 10-4) ZnO
LMBT
8
1.j
4 0
3.3 2 8
..
1 7 2 6
z
2 0'
3 5
12
SULFURREACTION
..
Slower curing GR-Sstock.
Figure 5. Effect of Sulfur Concentration on Initial Sulfur-Combining Rate
1
As temperature does not affect the ratio, it seems likely that both reactants are involved in the same reaction, possibly simultaneously but more probably in some sort of consecutive reactions. The data in Table I11 also show that pale crepe is a much more efficient medium for mercaptobenzothiazole vulcanization reactions than GR-S. A slower rate of mercaptobenzothiazole consumption might account for the large difference in the ratio of sulfur to mercaptobenzothiazole for pale crepe and GR-S. It was found, however, that mercaptobenzothiazole is consumed more rapidly with pale crepe. Deviations from straight lines were noted in several cases where one of the reactants had been depleted. When the pale crepe medium was used, all of the sulfur reacted; mercaptobenzothiazole, however, continued t o be consumed (Figure 6). In the series of cures with GR-S (Table 11) using 1 part of zinc oxide per 100 parts of rubber, all of the mercaptobenzothiazole was consumed, yet sulfur continued to react with the rubber. I n Figure 4, however, the sulfur and mercaptobenzothiazole reactions appear to have terminated at the same time. Of further interest is the observation that mercaptobenzothiazole is consumed before the sulfur reaction begins. This is indicated by the intercept of the lines in Figure 6. The intercept values are given in Table IV for the experiments conducted with GR-S. For the pale crepe stock the intercept occurred at 1.4 X 10-4 mole of mercaptobenzothiazole. These results indicate that some of the mercaptobenzothiazole forms an alcohol-insoluble compound in the initial stages of vulcanization before any sulfur has had a chance to react. The data correspond to the values of the consumed mercaptobenzothiazole a t the minima in Figures 1 to 3. From the data given in Table I a kinetic expression can be given for the sulfur combining rate which a t least empirically accounts for the initial vulcanization reaction. If a and b represent the initial mercaptobenzothiazole and sulfur concentrations and y and x reacted mercaptobenzothiazole and sulfur, respectively, then the eqpation relating them can be written as
dx/dt =
- y) ( b - z ) ~
~ I ( U
(1)
w0 I
MOLES OF MET PER 1000. OR-S X I O '
Figure 6. Variation of Combined Sulfur with Consumed Mercaptobenzothiazole X 120OC.; 0 , l40'C.i 0, 160'C. GR-S.' 0 \Ma C. rapid curing stock
Hevea
Composition.
A 140' C. slower curing stock Sulfur 3, zinc oxide 3, MBT 1 per 100 parts of rubber
As the ratio of consumed mercaptobenzothiazole to combined sulfur is constant, as is represented in the following,
xly = T
(2)
Equation 1 can be changed into an equation with a single dependent variable and integrated. The integrated form of this equation, however, did not satisfactorily account for the data for an extended range of vulcanization beyond the initial stage. An equation which does reproduce the experimental results in this range is:
d x / d t = k2b ( a
- ?J)( b - x ) 1 / 2
(3)
Equation 3 was tested by graphical integration using experimental values of x and y and also by using the integrated form, Equation 4,after substituting x/r for y from Equation 2.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1530
(4) The values of k2 obtained by graphical integration are given in the fourth column of Table V. Values calculated from Equation 4 are given in columns two and three.
TABLE5'. EFFECTOF REACTANT CONCENTRATION O N RATE kz COKSTAKT,
(100 gram mole-a/2 minute-') MBT Zinc Oxide
Phr 0.5 1.0
I
.
..
.
5.78 3.735
5.58
Sulfur 5.32 5.03
3.0 5.0
5.78 5.89 6.93 6.29 7.25 3 . 4fja a Experiments conducted with a slower curing stock, emphasizing t h e independence of t h e r a t e constant on hIBT concentration.
Although the average deviations for the rate constants varied between 10 and 20%, they are not indicative of any trend in the rate constant with time. This is shown in Table VI, where the individual constants for the series of cures involving 5 parts of sulfur per 100 parts of rubber are given. This experiment was chosen because it covers the largest range of reacted sulfur, 4.8 to 99.1%. The curing times given have been corrected for the induction period.
FOR CURESINVOLVING 5 PARTS TABLE VI. RATE CONSTASTS OF SULFUR PER 100 PARTS OF RUBBER
h,100 Gram Mole-3/* h h - 1
Curing Time, Min.
9.61
4
6.48
13 28 43 73 103 133 163 223
6 38 6.51
7,50 7.12 6 05
8 40 7 18
The sulfur combining reaction follows Equation 1 initially and Equation 3 for the remainder of the reaction. This indicated that sulfur is being used up faster than would be predicted by Equation 1 and suggests that the reaction product of the sulfurrubber reaction is also reacting with sulfur. The rate of this secondary reaction is the difference in the rates obtained from Equations 1 and 3 for any given value of z. It is therefore possible to determine these rates, since kl can be obtained from Figure 5 and k2 from Table V. The rate equation for the secondary reaction can be expressed as dz/dt =
k3
(b
- 2)" xn
(5)
where m and n are unknown exponents. This equation predicts that the secondary rate should be initially zero, increase and go through a maximum, and then decrease t o zero again. The difference in the calculated rates of Equations 1 and 3 when x is zero obviously will not be equal. The differences do increase, however, go through a maximum, and decrease to zero. If Equation 5 is differentiated and set equal to zero, which is the condition for the maximum, the following relationship is obtained:
The quantity b/xmax.should be a constant.
This was found to
Vol. 45, No. 7
be true for the experiments in which the initial sulfur concentration varied. The average value of b/xmax. was 2.0. This indicated that 7n and n were equal. To determine m and 72, log ( d z / d t ) was plotted against log ( b - z)x. log ( d z l d t ) = m log ( b
- x)x
+ log k3
(7)
According to Equation 7 , m should be the slope and log k3 the intercept. Log ( d z l d t ) from all of the above experiments fell on a single straight line whose slope was 0.69 and whose interm was then cept indicated that the value of ka was 2.2 X assigned a value of 2/3. When the calculated values of ( d z l d t ) in Equation 5 were plotted against [ ( b - x ) x ] 2 / 3straight lines which went through the origin were obtained in all cases. The slope values, which corresponded to k3, were reasonably constant. The average value of k3 was 2.00 X IOWz(100 gram mole-'/a minute-'). This information permits an equation that more accurately characterizes GR-S vulcanization. dx/dt =
ki
(a - ?J)( b
-z
) j ~-k ) [ ( b - x ) s ] ~ / ~ (8)
The adequacy of Equation 8 was shown when values of t were obtained from corresponding values of x by graphical integration of Equation 8. These described a curve which coincided with that obtained from experimental values. A plot of the log of uncombined sulfur us. time gave straight lines for every series of cures performed with GR-S and Hevea when mercaptobenzothiazole was used. The rate constants varied, however, with time and with change in the initial sulfur concentration. I n a previous section i t was shown (Figures 1 to 3) that mercaptothiazole reacts initially to form the zinc mercaptide of mercaptobenzothiazole. -4 second reaction then takes place which is dependent on the sulfur concentration. These reactions take place before any sulfur has combined with the rubber. Once the sulfur-rubber reaction begins, mercaptobenzothiazole is once again consumed. The rate of this third mercaptobenzothiazole reaction is also proportional to the sulfur concentration. This information was obtained by determining the initial mercaptobenzothiazole consumption rates from the curves shown in Figures 1 t o 3. The data, therefore, are only approximate. They are given in Table VII.
TABLE
VII. EFFECTO F SULFUR CONCENTRATION O N INITIAL MERCAPTOBENZOTHIAZOLE COXSUMPTION RATE P h r Sulfur 0.5 1
3 5
Initial AIBT Consumption R a t e X 10-j Mole/100 G GR-S/Mln: 0.196 1 26 2.69 6.99 ~
~~
EFFECT O F TEMPERATURE ON COMBINING RATE O F SULFUR
Combined sulfur was measured a t three temperatures for both Hevea and GR-S. The following formulation was used: polymer 100, zinc oxide 3, sulfur 3, and mercaptobenzothiazole 1. For GR-S, rate constants were calculated from Equation 4. A plot of log kz vs. the reciprocal of the absolute temperature gave a straight line. The activation energy was calculated from the slope of this line. For Hevea, the rate constant was reasonably constant a t 120" C. but not a t 140" and 160" C. The drop in value of the rate constant with time a t these higher temperatures presumably was due t o reversion. When the initial values of the rate constant were used and log ks was plotted against 1/T, a straight line was obtained. First-order plots of the uncombined sulfur give straight lines. If the values of kq obtained from these plots are used to determine the activation energy, a value identical to that obtained from the initial kz values of Equation 4 was obtained (Table VIII).
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1953
TABLE VIII. RATECONSTANTS AND ACTIVATION ENERGIES FOR GR-S AND HEVEA ka, Activation Energy, Temp., C. ka X 10-2 Min.-1 Koal. 25 9.80 1.90 120 140 54.4 9.40 160 226 35.2 GR-8 130 1 35 37 140 3.73 150 12 6 k2, oalcd. from Eq. 4. Units in 100 g. mole-8'2 minute-'. ka, first-order constant.
Polymer Hevea
1531
a t 140' C. for time intervals up t o 120 minutes. In all cases the amounts of consumed mercaptobenzothiazole were constant over the entire time range. Figure 7 shows that the presence of stearic acid markedly influences the amount of consumed mercaptobenzothiazole. It is believed that this is due to the competitive reaction of stearic acid for zinc oxide. VULCANIZATION WITH ZINC MERCAPTIDE OF MERCAPTOBENZOTHIAZOLE AND MERCAPTOBENZOTHIAZYL DISULFIDE
The zinc mercaptide of mercaptobenzothiazole and mercaptobenzothiazyl disulfide have been found in vulcanizates ( 2 , 4, 8). Either might be an intermediate in the curing process. The sulfur combining rates were therefore measured using these accelerators and compared with the rate obtained with mercaptobenzothiazole. The amounts of zinc, as the mercaptide or oxide, mercaptobenzothiazole, and sulfur were maintained constant in all the cures. The results are shown in Figure 8. 3.0, LT I) LL. LL: -I
2 5:
n 2.0 2 W
z
g
s ..o I
8 '
4
3
B
e
0
120
180
-1
240
MINUTES Figure 8.
Combining Rate of Sulfur
FormulaFion. GR-S 100, accelerator (based on MBT) 1, zinc (as oxide and/or accelerator) 3, sulfur 3 0 MBT X Zinc mercaptide of MBT 0 MBTS
The sulfur-combining rate in the presence of the zinc mercaptide is approximately 1.7 times the rate obtained with mercaptobenzothiazyl disulfide and corresponds closely with the rate obtained for mercaptobenzothiazole and zinc oxide. It seems probable, therefore, that the zinc mercaptide of mercaptobenzothiazole is the intermediate involved in the acceleration process. MECHANISM OF MERCAPTOBENZOTHIAZOLE VULCANIZATION
cured for 60 and 240 minutes and
0
-
[1: I
I
J
The information presented indicates that the zinc mercaptide is the initial product of reaction. This reacts rapidly in the presence of sulfur. Furthermore, it can be assumed that as long as any mercaptobenzothiazole is present the zinc mercaptide is being formed because of the rapidity of this reaction. Under conditions of low mercaptobenzothiazole concentration, however, no zinc mercaptide is found except initially and near the end of the vulcanization reaction. This suggests that the zinc mercaptide of mercaptobenzothiazole is reacting as rapidly as it is formed to regenerate mercaptobenzothiazole. Since two hydrogen atoms are necessary in the conversion of the zinc mercaptide t o mercaptobenzothiazole, they must come from the rubber. The rubber in its activated form (minus the hydrogen atoms) can be assumed to react with sulfur. SUMMARY
A study has been made of the function of mercaptobenzothiazole in the vulcanization process. Radioactive sulfur-35 and mercaptobenzothiazole tagged with sulfur-35 were used for measuring the sulfur and mercaptobenzothiazole reaction rates.
1532
INDUSTRIAL AND ENGINEERING CHEMISTRY
The effects of variations in the zinc oxide, sulfur, and mercaptobenxothiazole concentration on the sulfur combining rates were determined. It was found that a marked induction period exists, during which the zinc me1 captide of mercaptobenzothiazole, the initial product, reacts readily with sulfur. The ratio of combined sulfur to consumed niercaptobenzothiazole is constant during the vulcanization process. A kinetic expression is given which accounts for the data. When polyisobutylene was substituted for GR-S or Hevea, the zinc mercaptide of mercaptobenaathiazole was found to be the initial product. A mechanism is postulated for the accelerating properties of mercaptobenzothiazole. I t involves the formation of the zinc mercaptide, which reacts with sulfur and rubber t o regenerate mercaptobenzothiazole and form sulfur-rubber compounds. ACKNOWLEDGMENT
The author wishes to el;press his sincere appreciation to earl Parks for experimental assistance and t o 8. D. Gehman for Buggestions and encouragement. He also wishes to thank H. J.
Vol. 45, No. 7
Ostcrhof and the Goodyear Tire and Rubber Co. for permission to publish this work. LITERATURE CITED
Adams, H. E., private communication, May 29, 1952. Clark, G. L., Le Tourneau, R. L., and Ball, J. M.,IND.ENG CHEM.,35,198 (1943). Davis, C. D., and Blake, J. T., “Chemistry and Technology of Rubber,” Chap. IX, Kevv York, Reinhold Publishing Corp., 1937.
Dufraisse, C., and Houpillart, J., Rea. g6n. caoutchouc, 19, 207 (1942). Petrov, K. D., J . A p p l . Chem. (U.S.S.R.),16, 214 (1943). Petrov, K. D., Rubber Chem. and Technol., 17, 56 (1944). Wistinghausen, L. V., Kautschuk, 5, 57, 75 (1929). Zeide, 0. A., and Petrov, K. D., Rubber Chem. and Technol., 11, 97 (1938). RECEIVEDfor review February 26, 1953. ACCEPTEDApril 3 , 1353. Presented before t h e Division of Rubber Chemistry. AMERICAN CHEMICAL SOCIETY,Buffalo, N. Y . , October 1352. Work performed as a p a r t of t h e research project sponsored b y t h e Reconstruction Finance Corp., Office of Synthetic Rubber, i n connection with the synthetic rubber program. Contribution 137, Research Laboratory, Goodyear Tire and Rubber Co.
olymerization of Alpha, BetaUnsaturated Ketones BENZALACETOPHENONE AND OTHER KETONES COPOLYMERIZED WITH BUTADIENE AND OTHER MONOMERS C. S. MARVEL, W. R. PETERSON, H. I
