128
INDUSTRIAL AND ENGINEERING CHdMISTRY POWER CONSUMPTION AND TEMPERATURE
SURFACE AREA. The relation of power consumption and temperature to the surface area of black in GR-5 is shown in Figure 3. An increase in surface area is accompanied by a corresponding increase in power consumption and temperature development. CHANNELAND FURNACE BLACKS. Figure 4 and Table IX compare the behavior of easy-processing channel and semireinforcing furnace blacks in six rubbers. Assigning an arbitrary value of 100 to the power consumption of easy-processing channel black in smoked sheet, the corresponding ratings in
the synthetics will be as high as 135 for Buna N and as low as 89 for Butyl. Similarly, assigning a n arbitrary rating of 100 for the power consumption of semireinforcing furnace black in smoked sheet, corresponding ratings in the synthetics range from 136 for Buna N to 95 for Butyl. ACKNOWLEDGMENT
The authors wish to express their appreciation to the United Carbon Company’s laboratory personne1 for assistance in assembling the rubber test data and to s. W. Nourse for preparing the charts.
THE ROLE OF CAR ON BLACK SURFACE CflEMISYWY The presence of surface oxygen on the carbon black particle appears to increase the secondary coherence forces between Butyl polymer molecules. I n an uncured state, the result i s an increase in the bound polymer per volume of carbon pigment or a decrease in the solubility of Butyl-carbon black mixtures. In the vulcanized state, the increase i n bound Butyl, due to carbon black surface oxygen, is reflected i n superior tear resistance and minimum hysteresis losses for a given pigment size. The advantages of superior tear resistance and minimum hysteresis losses are most apparent in Butyl polymers of higher chemical unsaturation as exemplified by Butyl B-3. With polymers of lower chemical unsaturation such as Butyl B-1.5, the relatively l o w states of cure minimize the reinforcing actions of the oxidized pigment surfaces. Improving tear resistance of a Butyl 8-3 polymer by decreasing the state of cure results i n impairment of hysteresis properties. Therefore reinforcement of tear resistance i n combination with higher states of cure becomes more dependent upon pigment surfaces as the chemical unsaturation of the polymer i s increased. From the standpoint of practical compounding, the large-particle carbon with a surface pH of 4 i s slightly superior to a finer-particle carbon with n o surface oxidation in the reinforcing of tear resistance of a Butyl B-3 polymer. This slightly superior tear resistance, over the cure range, i s obtained i n conjunction with the superior over-all hysteresis properties of larger-particle pigment.
H E N pigments or fillers are incorporated in a vehicle, three properties of the pigment that may affect the overall properties of the mixture are particle size, particle shape, and chemical nature of the particle surface. For all carbon blacks i t is generally assumed that the particle shape is spherical ( I ) , so that the colloidal properties of carbon black may be reduced to two variables. The effects of carbon black properties on the reinforcement of natural rubber are well known from the recent discussions by Wiegand (9),Fielding (a),and many other investigators. Drogin (9) and Turner et al. (8) have shown how the properties of the general types of carbon blacks specifically affect Butyl rubber vulcanizates; Haworth and Baldwin (6) described the effect of Butyl rubber unsaturation on some carbon black compounds. The purpose of the present work was to isolate the role of the surface chemistry of carbon pigments on the reinforcement of Butyl polymers. The reinforcing action of a pigment surface is attributed to the increase in the secondary coherence forces between the polymer molecules. Thus, according to Houwink ( 6 ) ,
Vol. 36, No. 2
R. L. ZAPP, Esso Laborat Standard Oil Development C Elizabeth, N. tlizabeth, PI. J.
14, ?
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active tillers as well as the action of cooli bring rubber into a physical state akin to that of vulcanization. Any conclusions about pigment reinforcement must of nccessity take into consideration the state of cure of the compJund and the effect of pigmentation upon the state of cure. In this paper the state of cure will be indicated by moduli at 300% elongation. Results are shown with a polymer of relatively low chemical unsaturation designated as Butyl B-1.5 and with a polymer of higher unsaturation designated as Butyl B-3. (Butyl B-3 was one of the products made during the development of the Butyl rubber process. Although not strictly representative of the currently manufactured Butyl known as GR-I, it is sufficientIy close to that product in physical properties to allow the observations, reported for Butyl B-3, to become applicable to GR-I.) Butyl R-3 is capable of yielding higher states of cure, presumably as a result of the ability of the increased unsaturation to give more sulfur cross linkages. I n studying the variations of physical properties with respect to the surface chemistry of carbon pigments, special attention has been given to tear resistance and hysteresis combinations and, more specifically, to the maintenance of good tear resistance over long curing times in combination with good hysteresis. These subjects are important factors in the consideration of synthetics for pneumatic tire construction. CARBON BLACK SURFACE CONSIDERATIONS
For a given carbon black, the most important surface property, the one that largely governs its adsorptive capacities, is the amount of chemically combined oxygen on the particle surfare. If a carbon particle has a high degree of combined oxygen on the surface, it possesses a high volatile content, a low or acidic pH in a water slurry and a high absorptive capacity especially for alkaline materials (IO). On the other hand, a carbon black with little or no combined oxygen on the surface has a volatile content measured in tenths per cent, a high p H in a water slurry, and a lowered adsorptive capacity. According to these adsorption concepts an oxidized surface could be considered an active carbon while the unoxidized surface could be considered inactive. Accordingly, such a range or a series of degrees of surface oxidation was studied in
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1944
Y
RUBBER IN BENZENE 1.2
1.0
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-
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65 OCTANE:
1 0 9 8 7 6
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I
5
4
3
Of CeRBON ,SC(RFACI 614 e:? io.9
66 VOLATILE ON CARBON SURFACL
x
Figure 1. Effect of State of Carbon Black Oxidation on Bound Butyl and Bound Rubber after 80-Hour Extraction
Butyl rubber, keeping the particle size of the carbon black constant. The properties of the two carbon black series studied are given in Table I. As a basis for comparison, a rubber-grade channel black falls within the oxidized surface range of pH .,4to 5 and volatile content of 5 to 6%; a furnace carbon (although not necessarily the same in particle size) is in the unoxidized surface range with a pH of 9 and volatile content of 0.5 to 1%. These experimental carbons were in the particle size range of 28-30 mp which is similar to easy-processing channel blacks.
BOUNb BUTYL AS INDICATION O F REINFORCEMENT
To anyone familiar with natural rubber, the fact that carbon blacks, especially channel blacks, tend to make uncured rubber insoluble is a well known phenomenon (8,7). As stated before, this is a state akin to vulcanization, and the extent or degree to which a pigment insolubilizes a polymer is an indication of its reinforcing or bonding power. “Bound Butyl” is the term applied to the unextractable polymer in an uncured Butyl-carbon mixture and is expressed in volumes of polymer per volume of pigment. The greater the bound Butyl, the less soluble is the mixture, and this is indicative of greater coherence forces between the polymer and the particle surface. I n order to show the relative differences in bound polymer, bound rubber is compared with bound Butyl in Figure 1. The extraction was conducted in Soxhlet extractors for 80 hours in a mixture of 100 parts polymer and 50 parts carbon black. The choice of solvents was made on the basis of best solubility for each polymer. The 65-octane gasoline used for Butyl was approximately 96% aliphatic, and bound rubber was determined with benzene as-the solvent. The conditions of the bound Butyl tests are arbitrary but were based on some preliminary investigations a t this laboratory (4). It is recognized that choice of solvent and time of extraction exert an effect upon the actual values; however, it is felt that the comparison of bound polymer with respect to the carbon surface oxidation is of aid in interpreting the carbon reinforcement data. The increase in bound polymer due to increased surface activity of the carbon is far more pronounced in
TABLE I. SURFACE CHARACTERISTICS OF CARBON BLACKS Surface pH 9.6 6.9 4.6
4.2 2.9
2.6
Series-1
% volatile 0.8 2.8 4.8 6.4 8.7
10.9
Seriee 2 Snrfaoe pH % volatile 8.8 1.0 6.9
4.9
3.4 2.8
2.6
5.6
7.8
9.5
129
Butyl than in natural rubber; the rise of bound Butyl from 0.3 to 1.0 volume of polymer per volume of pigment is compared to the rise in bound rubber from 1.16 to 1.55 as the carbon surface activity increases. Although there is a difference in the absolute position of the two bound polymer curves, the important consideration is the relative rise in each bound polymer with respect to an increase in carbon surface oxidation. When carbon surface oxidation is increased, the adsorptive capacity for accelerators becomes greater; however, if accelerator dosages are adjusted, reinforcement advantages of the higher bound Butyl should be realized. CARBON SURFACE OXIDATION IN BUTYL B-3 TREAD STOCKS
The series of carbons of varying state of surface oxidation was first studied in Butyl B-3 and then comppunded in a polymer of lower chemical unsaturation, Butyl B-1.5. Reinforcement effects in Butyl B-3 are studied in three concentrations of accelerator. The tread type recipes used for both polymers are shown in Table 11.
TABLE 11. BASEFORMULAS OF TREAD-TYPE STOCKS Butyl B-3 Butyl B-1.5 Zinc oxide Stearic acid Carbon block Sulfur Tetramethylthiuram disulfide Mercaptobensothiszole
100
... 5 3
50 2 1.0, 1 . 2 , or 1 . 6 0.5
...
100 5 3
56 2 1 . 0 or 1 . 2 0.5
TEAR-HYSTIRESIS COMBINATIONS AT EQUAL MODULI. I n Figure 2 the variations of physical properties with carbon surface activity, using the normal acceleration of 1.0 part of tetramethylthiuram disulfide, are shown and compared to two oxidized types of carbon surface compounded with 1.2 parts of the thiuram accelerator. Each curve shows the relation of the various physical properties of the vdcanizates with respect to the pH or volatile content of the compounded carbons for a certain cure time. The stress-strain and tear properties are given for 15-, 30-, and 60-minute cures a t 307” F.; hysteresis and rebound data are given for 30-, 60-, and 120-minute cures. On the right-hand side the physical properties of the compounds of increased acceleration are depicted by the short horizontal lines for the cure times indicated; the scale of the numerical values is the same and read from the ordinates at the extreme left. From the curves the following observations can be made: 1. With the normal 1.0 part of tetramethylthiuram disulfide, devolatilization (that is, reducing the volatile from 6.4%) increases the rate of cure; the state of cure, measured by modulus a t 300%, is progressively increased, presumably as the result of a decrease in the carbon adsorption of the accelerator. Continued reduction in carbon surface oxidation finally results in a decrease of tensile strength values which is more apparent in the longer cure times. 2. Tear resistance steadily decreases with reduction in carbon surface oxidation. To show that this effect is not all due to a state of cure, comparisons are made at equal moduli with the compound of increased acceleration, using a relatively active surface carbon of 6.4y0 volatile and 4.3 pH. This carbon is identical with the one yielding the greatest cure retardation in the normally accelerated series. The effect of increasing acceleration to 1.2 parts of thiuram is to increase the rate of cure and the state of cure as measured by modulus. It is also apparent, even though the atate of cure was increased by this method, that tear resistance (especially a t longer cure times) does not suffer the same degradation as it does in the high-modulus stocks obtained with the carbon surfaces of lower volatile and higher pH. This is graphically shown when a dotted line is drawn from the modulus lines to the modulus curves and then projected vertically to the corresponding points on the tear curves. At these points the tear resistance of the 60- and 30-minute cures are approximately 100 pounds per inch less than those obtained with equivalent moduli using au adjusted compound with a more oxidiaed carbon surface. This
Vol. 36, No. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
130 1.0 PART OF
A
THIURAM
1 2 PTS.
1.2 PARTS TETRAMETHYL THIURAM DISULFIDE
60060'
900 40030'
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Figure 2. Effect of Carbon Surface Oxidation on Butyl 8 - 3 Tread-Type Stocks Containing 1 and 1.2 Parts of Tetramethylthiuram Disulfide
flat cure with respect to tear resistance is the outstanding advantage of carbon surface oxidation; coupled with minimum hysteresis loss considerations, it is shown to occur in several types of compounding. 3. Hysteresis properties to be considered with the tear resistance just discussed were observed with the Goodyear-Healy pendulum and the Goodrich flexometer. When a carbon with an oxidized surface is compounded with adjusted acceleration, the good tear-resistant properties are coupled with minimum hysteresis losses as shown by temperature rise in the flexometer, and by the sum1 of the energy-loss variables a t constant load and constant deflection. (Other hysteresis considerations, such as per cent rebound resilience, constant-load and constant-deflection energy loss, are omitted here to simplify the graphical representations.) 4. The third column on the right-hand side of Figure 2 represents the highest state of carbon surface oxidation compounded with 1.2 parts of thiuram. In this case the low surface pH of 2.6 appears to warrant a greater increase in acceleration in order to realize the optimum properties. This stock exhibits retarded cure as shown by the continued increase of tensile strength with cure time and by the lowered moduli. From these observations it is concluded that an oxidized carbon surface will yield greater reinforcing properties with mini1 T h e use of t h e s u m or average of the two energy loss conditions a8 a n indioation of hysteresis in tire treads was arrived a t independently and in more detail b y S. D. Gehman in work as yet unpublished.
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2 3 4 5 6 7 8 9 VaLqTlLE ON CARBOP SURFACE, d9 4.'2 2:e pn OF CARBON SURFACE
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Figure 3. Effect of Carbon Surface Oxidation on Butyl 8-3 Tread-Type Stocks Containing 1.2 Parts of Tetramethylthiuram Disulfide
mum hysteresis losses when retardation of cure by accelerator adsorption is adjusted, so that comparison may be made at equal modulus values. INCREASED ACCELERATION OVER ENTIRE RANQEOF CARBON SURFACE OXIDATION.Figure 3 shows the effect of the degree of carbon surface oxidation on the physical properties of Butyl B-3 when an accelerator dosage of 1.2 parts of tetramethylthiuram disulfide is used throughout. I n this series a carbon of 2.9 pII. and 8,7y0volatile was used instead of the 2.6 pH and 10.9% volatile carbon, which proved a little too active for the 1.2-part accelerator loading. The graphical results may be interpreted as follows: 1. With the 1.2 parts of the thiuram accelerator, devolatilization of the carbon surface is accompanied by reduced tensiles; and as the devolatilieation becomes more drastic, the stocks show decided tendencies to overcure with respect to tensile strength. 2. As the carbon surfaces become less oxidized, modulus is increased a t the expense of decreased tear resistance. The better tear resistance that accompanies increased carbon surface volatility is attributed to better bonding, as noted in the tear-modulus projects of Figure 2 as well as to the fact that a lower state of cure exists. 3. The high tensile strength and tear resistance of the. more oxidized carbons is maintained over a long cure range wlth no sacrifice in over-all resilient or hysteresis properties; the unoxi-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
February, 1944
131
1.0 PARTS OF A THIUAAM I
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GOODRICH FLEXOMETER
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dized carbons, which yield highest modulus, tend toward overcuring with respect to hysteresis. Figure 4 gives the effect of the range of carbon surface conditions on Butyl B-3 vulcanizates in a compound accelerated with 1.6 parts of tetramethylthiuram disulfide. This relatively high concentration was used so that good cures could be obtained with the highest degree of carbon surface oxidation showing a 10.9% volatile and 2.6 pH. The results are summarized as follows: 1. Increasing acceleration t o 1.6 parts allows the use of the most highly oxidized black. This results in stocks of maximum tensile strengths and the highest and flattest tear resistance] with respect to the range of cures, of all the combinations of carbon surface and thiuram accelerator shown. 2. The hysteresis especially as measured by temperature rise, is still higher than whh the other carbons. However, the 8.7% volatile channel carbon with 1.6 parts of the accelerator shows a minimum hysteresis loss with better tear resistance than was obtained with those carbons of lower surface volatile content. 3. Other observations show, as in Figures 2 and 3, the overcuring with respect to tensile, tear resistance, and even hysteresis as measured here, when low activity carbons are compounded with high dosages of accelerator.
Therefore it can be stated that the channel black type surfaceLe., one of higher degree of oxidation-yields superior tear-hys-
i- 0 0 3
I Figure
,
,
,
,
,
GOODRICH FLEXOMETER 146 LBS.: 0.12S"STROKE
60
2.6
Figure 4. Effect of Carbon Surface Oxidation on Butyl B-3 Tread-Type Stocks Containing 1.6 Parts of Tetremethylthiuram Disulfide
3d
-
,
I
2 3 3 . 5 6 7 6.4 V0LATIL;L ON CqRBON SURFACL 9.6 69 4.8 4.2 4.2 pU W CARBON SURFACE
5.
Effect of Carbon Surface Oxidation
on Butyl B-1.5 Tread-Type Stocks
teresis combinations (when properly accelerated) to a carbon with a less oxidized surface. When compounded to equal hysteresis, the oxidized carbon surface has the better tear; as will be shown in a study of state of cure, a low-volatile high-pH surface would yield poorer hysteresis if compounded to equal tear. EFFECT OF CARBON SURFACE ACTIVITY ON BUTYL B-1.5
Since it has been shown that state of cure of Butyl rubber is important in comparisons of reinforcement, variations of carbon surface activity were studied in Butyl B-1.5. On the basis of chemical unsaturation, this polymer would have about half the latitude in possible states of cure as compared to Butyl B-3. Since it is possible t o obtain only low states of cure, we presumed that in Butyl B-1.5 the tear-hysteresis reinforcement by the more oxidized carbon surfaces would not be realized. I n other words, low states of cure would tend to minimize the need for tear reinforcement. As Figure 5 shows, this presumption wm correct. Tensile and tear strengths are relatively the same over the range of carbon surface activities for the 30-, 60-, and 120-minute cures a t 307' F. Increased acceleration, with an oxidized carbon surface, does increase the modulus somewhat; 5ut projections of tear a t equal
INDUSTRIAL AND ENGINEERING CHEMISTRY
132
modulus, in contrast to Butyl B-3, do not yield the tear resistance reinforcement. Tear values remain fairly constant over all degrees of surface oxidation and accelerator dosages. CONC. OF TETRAMETHYL THIURAM DISULFIDE 1.0 BART
1.25 PARTS
Vol. 36, No. 2
eration of tear resistance and heat build-up in the Goodrich flexometer. A flat cure with respect to tear occurs a t a carbon pH of about 4.5; at slightly decreased pH (around 3.5) tear reqistance increases with respect t o cure. Attention is directed to this point of flat cure with respect to tear by arrows. If the hysteresis properties in this pH range of 4.5 to 3.5 are examined, a minimum in heat build-up and energy loss is noted at this point. This minimum is possibly due to a balance of vulcanization and pigment bonding, which means that the state of cure, coupled with the coherence force. of this type of carbon surface, results in a combination that yields minimum hysteresis properties under the conditions of the laboratory tests. As carbon suiface oxidation is decreased (Le., pH increases), overcuring with respect to tear is noted with no improvement in hysteresis.
OF CARCASS-TYPE Srocx TABLE 111. BASEFORMULA
Butyl B-3 Zinc oxide Stearic acid Carbon black Sulfur
SUM Of ENERGY
,
FLEXOYETER 148 LBS. 0.125"STR
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,
,
,
;
,
1 i"
COODR'ICH FLEXOMETLR
FLEXOMETER 148 LBS O4I25" STR
148LBS.
0.1 2 5"s T R
1
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Figure 6.
8
6
4
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m a 6 4 2 K, CARBON SURFACE
8
!
6
/
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l
100 5 3 27 2
Tetramethylthiuram disulfide aIercaptobenEothiazole Dibenzyl ether
0 . 7 5 , 1. 0 . or 1 . 2 5 0.5 7.5
This point of flat cure with respect to tear, in combination ivith minimum hysteresis loss, would appear to have practical significance in outer carcass plies and breaker or cushion stocks for pneumatic tires. If an inactive carbon surface were used ?nd stocks were compounded for minimum hysteresis, overcuring with respect to tear might take place during the tire curing cycle as shown by the curves a t carbon surface pH 8 or 9. ACCELERaTION 1.00 PAriT. The increased amount of tetramethylthiuram disulfide results in an increased rate of cure with respect to tensile strength and modulus for the more oxidized blacks. The carbons with high pH begin to overcure with respect to tensile. This trend toward overcuring is reflected t o a still larger extent with respect to tear resistance. The low-volatile high-pH blacks overcure a t 30 minutes, and the channel black range (pH 4.5) overcures in 60 minutes. At this accelerntor concentration (1.00 part of thiuram) the more acid surface carbons are needed to resist overcuring with respect to tear. In other words, the point of constant tear resistance over a long curing time is shifted to the more oxidized carbon Eurface range around pH 3, as noted by the arrows.
l
4
Effect of Carbon Surface Oxidation on Butyl B-3 Carcass-Type Stocks \
Hysteresis differences are also minimized, although increased acceleration with a regular channel-type carbon surface brings the 60- and 120-minute cures closer together on the Goodrich flexometer temperature rise. Therefore, it can be concluded that for polymers with only low possible states of cure where over-all hysteresis properties are poorer, improvement in and maintenance of good tear resistance over a long cure range are not critical problems. I t becomes incremingly more critical as higher states of cure are realized with polymers of increased chemical unsaturation.
800 -
CARBON SURFACE ACTIVITY IN 6-3 CARCASS STOCKS
400-
One of the series of large-particle channel blacks, treated to obtain various stages of surface activity, was compounded in a carcass-type formula using dibenzyl ether as elasticator. This section shows the effects of the state of carbon surface oxidation when compounded in low loadings of black. Varying accelerator dosages were used in the general recipes of Table 111. The accelerator dosages shown in Figure 6 will be discussed separately before an over-all summary is made: ACCELERATION 0.75 PART. The addition of 0.75 part tetramethylthiuram disulfide is too low for the most highly oxidized carbons; that is, for those below a pH of 3 but with carbon surfaces of only slightly gieater pH, interesting results are obtained. It is well to keep in mind that the pH range 4 to 5 represents the usual grade of channel black. Of special interest is the consid-
I
,
I
FLEXOMETER 50
\ 0.125" STR, 30" C.
600-
I-
200
I
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/, 1 OF2 SULFUR 3 PTS.
I 4
u
'00
I 2 3 4 PTS. OF SULFUR
Figure 7.
Effect of State of Cure on Tear-Hysteresis Combinations with an Unoxidized Carbon Surface 54 p a d remirelnforcins furnace blsck (pH 9.8), 0.75 part thiuram.
Heat build-up properties are fairly constant over the range pH 3.4 to 8.8 and, as a whole, are slightly lower than the series containing 0.75 part of the accelerator, The sharper minimum in the series with 0.75 pert accelerator is not apparent at 1.00 part acceleration. Resilient properties as determined by the Goodyear-Healy pendulum are at an optimum in the active rang? of pH 3.
February, 1944
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY ACCELERATION
1.25 PARTS.
A concentration of 1.25 parts tetramethylthiuram disulfide appears to be too high for all but the most oxidized surface carbon black in the series. Overcures with respect to tensile strength are drastic as the volatile content of the carbon decreases. The same trend is noted in tear resistance. Resilience and heat build-up are not significantly improved over lower accelerator concentrations, so that this accelerator concentration can be considered of no interest from a tire carcass standpoint. In summing up the results on low loadings of pigment, 0.75 part of a thiuram with a carbon of channel black surface or a pH range of 4.5 30 SUM OF ENERGY to 3.5 is of interest in Butyl B-3 carcass stocks. This degree of carbon surface oxida3 2 tion yields flat cures with respect to tear while maintaining minimum hysteresis w 40 OMETLR. 148 LBS losses under the laboratory 40-C. conditions noted. Higher -.----.-caccelerator dosages with more acid carbon surfaces do not show sufficient decrease in 35 3s 75 MINUTES AT 307’F. over-all hysteresis to warrant immediate interest for exFigure 8. Comparison of perimental carcass construcan O x i d i z e d Large-Particle t i o n . I t s h o u l d b e reChannel Black (No. 1) with e m p h a s i z e d that possible an Unoxidized Fine-Particle Carbon Black (No. 2) overcuring with respect to in Butyl B-3 tear may occur when high pH carbon surfaces are used. The onlv - way- that overcuring with respect to tear can be eliminated with an unoxidized surface carbon is to reduce the state of cure of the Butyl vulcanizate, and this automatically increases over-all hysteresis losses as shown in the following section. m 300 J
2 s
EFFECT OF STATE OF CURE ON TEAR-HYSTERESIS
R
When the state of cure of Butyl B-3 is reduced, this type tends to duplicate the properties of a polymer with lower chemical unsaturation. The characteristic properties of a low state of cure, such as high elongation and low modulus, result in good retention of tear resistance over a long curing period. However, as shown by these curves, hysteresis as measured by the Goodrich flexometer is detrimentally affected. Figure 7 shows how the state of cure of a Butyl B-3 vulcanizate affects the previously emphasized tear-hysteresis combinations. The polymer was compounded with 54 parts or 30 volumes of a semireinforcing furnace black which has an unoxidized surface measured by a p H of 9.8. The acceleration was 0.75 part of tetramethylthiuram disulfide, and the state of cure was controlled by varying the sulfur from 0.5 to 4.0 parts. From these curves it is apparent that Butyl B-3 has a wide latitude in the possible state of cure. A high concentration of sulfur results in overcuring with respect to tensile strengths while low sulfur concentration maintains peak tensiles or optimum degree of molecular orientation upon stretch over a long cure range. Modulus curves rise regularly with increased concentration of sulfur. Tear resistance decreases rapidly as the state of cure is
133
increased. The tear at the 60-minute cure begins to fall off a t concentrations above 0.5 part of sulfur, and the 30-minute cure begins to fall off a t 1.5 parts. The flatkear-cure relation, similar to these noted with oxidized carbon surfaces, occurs a t 0.5 part of sulfur where onIy low states of cure are possible. At this point of good tear retention, hysteresis loss, as determined by the Goodrich flexometer, is a t a maximum. It will be recalled that, when good tear retention was obtained in vulcanizates containing oxidized surface carbons, hysteresis losses were a t a minimum. The role, then, of the more oxidized surface carbons, when properly compounded, is t o shift the relative tear-hysteresis combination to a more favorable position as shown schematically by the dotted line and arrow. The oxidized carbon surfaces tend to shift the regular tear-hysteresis position, controlled only by state of cure, to this new relative poaition. OXIDIZED
us.
UNOXIDIZED CARBON OF SMALLER PARTKLE SIZE
The conclusions reached in the preceding sections were based upon experiments in which particle size remained constant and surface activity varied. This resulted in advantageous hysteresis-tear retention combinations for the more oxidized carbon surface in both high and low loadings of carbon black. From these results it could be postulated that, if an oxidized or low-pH carbon surface of definite particle sine possessed tear resistance equivalent to that of a carbon black of h e r particle sine and higher pH surface, then the lower-pH lwger-particle carbon black would be superior in hysteresis properties, This is essentially what takes place when two such carbon blaeks are compared in Figwc 8. Carbon 1 has an oxidized surface in the regular channel black range with a particle size of about 30 m p , a pH of 4.1, a volatile content of 6%, and a color index of 60. Carbon 2 is a devolatilized carbon black of smaller particle size; it has a particle size estimated by color or blackness t o be in the range of 22 m p , a pH of 9.4, a volatile of 0.4%, and a colo? index of 100. The effect of the oxidized carbon surface is to “temper” the vulcanization so that the physical properties are more constant over the curing time range. This is especially true in tear resistance and tensile properties. Although the fine-particle unoxidized carbon has a higher tear resistance a t low cure times, the tear resistance decreases more rapidly as state of cure increases so that eventually this smaller-particle carbon yields a lower tear resistance than one of large particle diameter. This better overall tear resistance for the larger particle with an oxidized surface is realized with better over-all hysteresis properties, as indicated by the sum of energy losses and temperature rise in the Goodrich flexometer. ACKNOWLEDGMENT
The writer wishes to express his indebtedness to L. H. Cohan of Continental Carbon Company and C. W. Sweitver of Columbian Carbon Company for the preparation of the series of carbon blacks used in this study, and to many of his associates for advice and assistance in the prepayation of the paper. LITERATURE CITED
(1) Columbian Carbon Co., Rubber C h m . Tech., 14,62-84 (1941). (2) Drogin, India Rubber World, 106,561 (1942). (3) Fielding, IND.ENG.CHEM.,29, 886 (1937). (4) Flory and Rehner, unpublished work. (5) Haworth and Bddwin,IND.ENG.CHEM., 34,1301 (1942). (6) Houwink, “Elasticity, Plasticity and Structure of Matter”, pg. 189-91 (1939). (7) LeBlanc, Kroger, and Kloi, KoZZ&dohm. Beihefte, 20,356 (1925). (8) Turner, Haworth, Smith, and Zapp, IND.ENO.CHEM.,35, 958 (1943). (9) Wiegand, Can. Chm. Process Ind., 25,579-81 (1941). (10) Wiegand, IND.ENG.CHEM.,29, 953 (1937). PREBENTBD before the fall meeting of the Division of Rubber Chemistry, AMERICAN CEITMICAL SOCIETY, in New York, N. Y.,1943.
