INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952
convenient to consider butadiene to be the principal hydrocarbon reactant (3,8),although the same conclusions can be reached with butenes. Assuming then that substitution of the hydrogen atoms in butadiene by sulfur is stepwise and rate determining, and that at high temperatures all six hydrogens are substituted a t approximately equal rates (a), a possible mechanism can be formulated as shown in Figure 2. It is assumed, too, that formation of a 5-membered ring is the most likely ~yclizationreaction and that two sulfur atoms will not remain on one carbon atom during ring closure. These assumptions are based on the absence of 2-thiophenethiol and 6-membered rings containing a disulfide linkage. ’ Moreover, substitution of hydrogen atoms on the thiophene ring by sulfur is thermodynamically unfavorable and all attempts to bring this about under the conditions of the reaction have failed. The mechanism, therefore, conforms with the experimental data on thiophene and thiophenethiol and lends support t o the selection of a 3,4-isomer of thiolanedithione. Furthermore, an estimation of the product distribution can be made, assuming random substitution of the hydrogen atoms of butadiene by sulfur. In order to form thiophene it is necessary for sulfur to attack the hydrogens on carbon atoms 1 or 4 (total hydrogens equal four) as shown in Figure 2. Thiophenethiol formation requires an initial attack at carbons 2 or 3 (total hydrogens equal two). This probability indicates a mole ratio of the precursors of thiophene and thiophenethiol of 2 to 1, I n order to form thiophenethiol, it is necessary for sulfur to attack the hydrogens on carbon atoms 1 or 4 (total hydrogens equal four) of the intermediate (IX). The formation of 3,4thiolanedithione requires substitution of the hydrogen atom a t carbon 3. Hence, the probable mole ratio of the precursors of 3-thiophenethiol and 3,4-thiolanedithione is 4 to I and the overall mole ratio of these three major products is 10 to 4 to 1 ( 4 ) . Compound Thiophene 3-Thiophenethiol 3,4-Thlolanedithione
Calculated Mole % Weight % 66.7 68 26.7 a2 6.6 10
Aptual
Weight % 67
23
10
125
ACKNOWLEDGMENT
The authors wish to express their gratitude to A. N. Sachanen, H. D. Hartough, and H. E. Rasmussen for their assistance and advice during this investigation, and to F. P. Hochgesang for the infrared data reported here. LITERATURE CITED
(1) American Petroleum Institute Research Project 44, ”Ultraviolet Spectral Data.” Serial No. 349,357,Carnegie Institute of Technology, Dec. 31, 1950. (2) Bost, R. W., Turner, J. O., and Norton, R. D., J . Am. Chem. Soc., 54, 1985 (1932). (3)Brooks, J. W.. Howard, E. G., and Wehrle, J. J., Ibid., 72, 1289, (1950). (4)Crowley, D. J., private communication. (.5) Hansford, R. C., Rasmussen, H. E., Myers, C. G.. and Sachanen, A. N. (to Socony-Vacuum Oil Co.),U. S. Patent 2,450,658 (1 948). (6) Hansford, R. C.,Rasmussen, H. E., and Sachanen. A . N. (to Socony-Vacuum Oil Co.), U. S. Patent 2,450,659(1948). (7) Hartough, H. D.,and Conley, L. G., J . Am. Chem. SOC.,69,3096 (1947). (8)Horton, A. W., J . Org. C h m . , 14,761 (1949). (9) Luthy, M. A., and Lardy, G. C., C m p t . rend., 176, 1547-8 (1923). (10) McMurray, H.L.,J . Chem. Phys., 9, 231,241 (1941). (11) Meyer, V., and Kreis, H., Ber., 17,1558 (1884). (12) Minnis, W., “Organic Syntheses,” Collective Vol. 11, p. 357, New York, John Wiley & Sons, 1948. (13) Mozingo, R., Wolf, D. E., Harris, S. A., and Folkers, K., J . Am. C h m . SOC.,65, 1013 (1943). (14)Paolini and Sjlbermann, Guzz. chim. ital., 45, 11, 385 (1915). (15)Rasmussen, H. E.,Hansford, R. C., and Sachanen, A. N., IND. ENQ.CHEM.,38, 376 (1946). (16)Rasmussen, H. E.,and Ray, F. E., Chem.. I d s . , 60,593, 620 (1947). (17) Rinkes, I. J., Rec. trav. chim., 53, 643 (1934). (18) Steinkopf, W., “Die Chemie des Thiophens,” p. 106, Dresden and Leipzig, Verlag von T. Steinkopff, 1941. (19)Steinkopf, W., and Ohse, W., Ann., 437, 14 (1924). (20) Steinkopf, W.,and Schmitt, H.F., Ibid., 533, 264 (1938). (21)Steinkopf, W., Schmitt, H. F., and Fiedler, H., Ibid., 527,237 (1937). RECEIVED January 6, 1951.
Oxidation of Unvulcanized Cold Rubber INFLUENCEOFADSORPTIONBYCARBONBLACK C. W. SWEITZER AND FRANCIS LYON Columbian Carbon Co., New York, N . Y .
C
ARBON black gel (Y), the benzene-insoluble carbon-rubber
complex developed in unvulcanised mill-mixed tread compounds, t o which has been ascribed a fundamental role in the reinforcement of rubber, usually contains 30% or more of insolubilized rubber associated with the carbon, the exact percentage depending on the polymer, the carbon, and the mixing conditions employed. The development of this carbon gel complex, of which a typical example is shown in the electron photomicrograph in Figure 1 (S), results from a combination of factors during the mixing stage, including the adsorptive activity of the carbon, the oxidative reactions on the polymer and the mechanical shearing of the polymer. To date no satisfactory approach had been found for following the progressive growth of this carbon gel complex, as a ’means for assessing the role played by these various factors, and
for determining the mechanism whereby this high percentage of insolubilized rubber is developed. Attempts to approach this level of insolubilized rubber by dilute adsorption tests have failed, with values in the range of 1 to 3% by weight on carbon invariably resulting (7). This level is much lower than that found in the carbon gel complex of millmixed compounds, as shown in Figure 2. This range of values has not been altered significantly by varying such factors as the type of polymer, the grade of carbon, the solvent, the ratio of carbon to rubber, the temperature, or the tumbling time (9,7). Adsorption from dilute solution therefore appears inadequate as a method for following the progressive development of the carbon gel complex, through failure to bridge this gap between 3 and 30% insolubilized rubber.
INDUSTRIAL AND ENGINEERING CHEMISTRY
126
I n vivw t u f these liinitittionfi the ni:ed h r a n e w xdsorpt.iun procudure WBY recognized, D procedure whicli would permit. intimate ~ ~ n t a coft ,carbon and ruhl,cr, without the intwferencc either of d v c n t a8 in the dilute adsorption test o r of mechanical shear a8 in stsndavd mising procedures. Sucli a teat method should mnke it pmsil,le to follriw the pmgwssive devv1opmt:ot of the mrhon gel compIox, to appmiac t,he role o f adsorption in this rlcvoloprnent,
Vol. 44, No. 1
One-tcnt.b gram or more of pigment, depending on the pigment to rubber ratio desirpd, is accurately weighed into B 250.ml. Erlenmeyer flnsk. Ten rnilliliten of the GR-S X-478 benzene solutio? are pipetted into the flask. Concentretionis spproximntely2gramsof polymer per iw mi. oreoiution. Th? Hask is gently swirled in B current of warm iiir until drynese is approached. At this point vacuum is applied to ~emovethe last trt~reuofsolvent. The residue appear8 en a filmon the hottorn of t,hc Hask. The RRsk is heated st the desired temperature for 1 hour in the air oven. After eoolin the sample to room temperature, 200 m1. of benzene are addejand the flask is stoppered socurt4y. The flask ia set aside fw 3 d a y and shaken occenionally to ensure solution equilibrium. The contents o f the flask arc filtered and the concentration of the clear filt,rate is determined gmvimetrically. Results are expressed as per cent insolubilizod rubher, either by weight on pigment or original rubber, as desired. CARBON-RUUUER RATIO
Uaing thie new te8t procedure, the effect of the enrboo-ruther rat.io on adsorption was rletcrmined with thc results presented i n Table I1 r~ndFigure 4. I>con,asing the carbon-rubbcr ratio from 20 to 1 tb the trmd rstio of 0.5 to I raised the Ievd o l adsorhed rubber to approximately 10% by weight on the earhon. The films in each instance WI:I(E heated at 85" C. for I hour t o remove all traces of solvent. This drying temperabure resulted in a higher level of srlsorption than given by the mom temperature dryink employed previoulily in Talde I.
Figure 1. Carbon Gel H A F in -Id
rvbber m i x e d et 416Mepnifioatioo. x 15,aO
F.
to c ~ ~ l u sthe t e nccompmying oxidative naetions, and to provide infurmution o n the nature of tbe carbon-rubber bond. These were the objective, of t h e present, mtuci? w i t h 41" F. G R S X-478 (rnld rulrbcr). O E V E W P M E N 1 OF T H E U R Y ADSORlTION TEST
In i~rdcrto eliminate solvent interference in the adsorption test a serics wa-3 run in which the bensene solvent WFLE evaporated at room trmperat,ure in LL succession of step8 up to and including B state of dryneas. The concentrates from those successive steps were rediluted to thelr original volume with benzene and adsorp tion was then determined, with the remlts presented in Table I and graphed in Figure 3. The results showed no significant increme in adsorption until the last few milliliters of solvent hrhd been renraved at which p i n t a pronounced increase developed. Since in these tests the rediluted solution8 were always nllowed to reach pquilibriurn before analyses were made, the higher vnllies given by t,hr cmnplet.ely dried sample indicate a stronger carbonNhhW bond. In this ease advorplion had hecome irreversible, in contrast to the rrvcrnihility in dilute systems.
EFFECT OF TEMYERATURC ON A I > S U R l T l O N
Using the dry adsorption pmcedunL-a drying temperature of 85" C. and t~ cnrhon-rubber ratio in the tmwd range--gavc nn ndmrption value of lo%, which is mow than t,lirer times the
30
L
8 a
s 25 2.
0
c
I
I?
y 20 2 ar a
15
m
a 3 0
210 110 20
LO 0
2!U 2iu 210 2,O 210
2 5
2 3 2 4
2 5 3 4
w N
2
m
IO
i z 5
This new adsorption approsch ofiemi promise aa a means for bridging this 3 to 30% inmlubilized rubber gap and for following the development of the carhon gel complex. After confirmatory tests had established the utility of this approach, the following standardized dry adsorption test prooedure was used for all the xtudies reported in this paper.
Figure 2. CR-S X-478 Inwllubilieed in Dilute Sofution and Mill-Mixed Stocks
January 1952
value of 3% from dilute adsorption tests but still only one third of the value of 30% obtained in mill-mixed stocks. In the next phase of these studies the effect of temperature was investigated. Initial studies were limited to the range of 25' to 135' C.,the latter temperature lying just below the temperature a t which polymer gel is formed. Subsequently the study was extended to temperatures above 135' .C-within the region where gelation of polymer occurs. In all cases the time of heating was I hour.
TABLE 111. EFFECT OF TEMPERATURE ON INSOLUBILIZATIO OF GR-S X-478 BY CARBON BLACK Ail Films Heated 1 Hour in Air Oven GR-S X-478 Insolubilized, % ' C. Control5 50 phr Statex R 100 phr Statex R 0 1.8 6.0 0 3.3 8.6 7.5
Dry Adsorption Procedures GR-S X-478 Adsorbed, % by Weight on Carbon Carbon-Rubber Ratiob
20:o 10:1 5:1 2:l 1:l 0.5:l.tread ratio All films heated for 1 hour a t 85' C. in air oven. Carbon, Statex K.
1
1
1
1
1
l
1
1
1
1
1
TABLE IV.
Intrinsic VSscositya, 30" C. 50 phr Micronex 100 phr W-6 Statex R 1.98 ... 1.02 2,Ol 110 1.99 120 ... 2.03 125 1.80 1.98 135 1.48 1 98 140 1.12 ... 1 96 0 Intrinsic viscosity determined on filtrates from films heated 1 hour in air oven. b No gel formed in control at any of the temperatures. Temperature, C. 25 85
1
1
x
t
3
r" 3.0
ae I
V
0
20,
I
r
t
t
l
l
l
l
l
l
l
l
l
I
EFFECTOF HEATING IN AIR ON INTRINSIC VISCOSITY POLYMER AND CARBON-RUBBER FILMS
OF
3
5 F n 8 8 *
...
'
...
19 25 32
The intrinsic viscosities of the filtrates from these adsbrption tests were measured to determine if variability in the adsorption rate of GR-S X-478 by carbon, in the temperature range from 25' t o 135" C.,was due to oxidative changes occurring in the polymer in this temperature range. A polymer control and a second carbon-rubber system were included. Results are set forth in Table I V and Figure 7.
4.0
&
12
if
EFFECT O F TEMPERATURE ON OXIDATION O F GR-S X-478
The initial studies, within the temperature range of 25" t o 135" C., were carried out with Statex R high abrasion furnace black (HAF) in 41 O F. GR-S X-478 a t two loadings-50 and 100 parts per hundred rubber. Results are given in Table I11 and Figure 5. The level of adsorption increases with temperature, reaching a value of 20% for the loading of 100 parts per hundred rubber a t 135' C. The rate of adsorption increase, however, is seen to change significantly with temperature. I n the temperature range of 85" to 120' C.,the rate is approximately five times greater than that below 85" C. Since Figure 5 shows no polymer insolubility developing throughout this temperature range to explain this variability in adsorption rate, the explanation probably lies in oxidative changes in the polymer. Before investigating oxidative changes occurring in the polymer, the range of temperature treatment was extended from 135" to 180" C. (356' F.), in which range appreciable polymer gel is formed by this method. The results are presented in Figure 6. The polymer is seen to develop insoluble gel rapidly in this temperature range reaching 80% a t 180' C. This polymer gel formation has a significant effect on the adsorption curves; the second rapid increase noted in insolubilized rubber was due to in-
1
... 9.1 ...
soluble polymer gel. Since the rate of increase for insolubilized rubber in the presence of carbon is not as great as the rate of increase for the polymer, it must be assumed that the carbon acted to repress the formation of polymer gel.
3.0 4 3 5.8 6.5 8.4 9.5
~~
1
Temperature, O 25 85 110 120 0 135 0 140 0 160 14 180 80 a Control contains no carbon.
..
O F CARBON-RUBBER RATIOON ADSORPTION TABLE 11. EFFECT OF GR-S X-478 AFTER SOLVENT REMOVAL
a 6
127
INDUSTRIAL AND ENGINEERING CHEMISTRY
l
Controlb 2 05 2 03 1.84
...
... ... ...
These intrinsic viscosity data indicate that carbon black preferentially adsorbs the higher molecular weight fraction of the polymer a t temperatures up to about 100" C. Similar adsorption effects 1 1 1 1 1 1 have been noted previously with mill-mixed stocks (7). With the polymer, little change in intrinsic viscosity takes place up to 85' C., but t h e r e a f t e r the viscosity decreases rapidly, indicating that oxidative scission increases rapidly beyond 85" C. This sharp drop in the intrinsic viscosity of the polymer a p pears to be related t o the increased adsorption rate pointed out in connection with Figure 5. I n the pres0 ence of carbon black there is substantially no change in the intrinsic viscosity u p to 135' C. There are two suggested explanations for this effect of t h e c a r b o n - e i t h e r the low l 1 l l l l molecular weight scission products are being adsorbed or the oxidation i n t e r m e d i a t e s through which the scission reactions would p r o c e e d are being adsorbed and inactivated. It is also possible that both adsorption effects might take place
128
INDUSTRIAL AND ENGINEERING CHEMISTRY 40 14
t
Vol. 44, No. 1
-
35 -
a
30 -
W
E
tl
5 250"
v1
z
azo-
?-
P U)
15
-
IO
-
$
2t I
I
1
1
I
5 -
,
0 -
Figure 4. Effect of Carbon-Rubber Ratio on Adsorption of GR-S X-478 after Solvent Removal (Dry Adsorption) Carbon-rubber Statex K
(from eolution) heated 1 hour et 8S0 C.
and that the decreasing intrinsic viscosity of the polymer control could result from changes in molecular structure (1). To check the possibility of carbon adsorbing polymer selectively on the basis of molecular weight, tests similar t o those just described were run in an atmosphere of nitrogen, under conditions which promote cross-linking reactions, Results are presented in Figure 8. Under these conditions the intrinsic viscosity of the polymer is seen to increase considerably, whereas in the presence of carbon black this increase is markedly depressed, Again there are two possible explanations for this effect of the carbon-either the high molecular weight products are being adsorbed or the oxidation intermediates through which the crosslinking reactions proceed are being adsorbed and inactivated, It is concluded that the dominating role of carbon in these reactions is to curtail both polymer scission and cross-linking by adsorption of the activated intermediates through which both of these reactions would proceed. These results would not be expected on the basis of studies reported by Winn, Shelton, and Turnbull (9), in which they claim that carbon black is a catalyst for the oxidation of GR-S vulcanizates. They also showed that increased oxygen adsorption resulted in a correspondingly greater deterioration of physical properties. Mesrobian and Tobolsky ( d ) , on the other hand, reported that while tread stocks adsorb more oxygen in a given time interval than corresponding gum stocks, physical aging tests showed that there was very little difference between the aging of gum and tread stocks for a given polymer. ADSORPTION ISOTHERMS
The presence of carbon black curtailed the development of polymer gel. White et al. (8) found that easy processing channel black (EPC) prevented the build-up of substantial quantities of polymer gel with standard GR-S, an effect which they ascribed as possibly due to the adsorption by the carbon of gelation intermediates and the destruction or dispersion of the gel by the shearing stresses during mastication. The exact role of adsorption could not be evaluated because of mastication interference.
I
I
I
25
50
I
I
75 100 TEMPERATURE, 'C.
I
125
I
150
I
175
Figure 5. Effect of Temperature on Adsorption of GR-S X-478 by Carbon Black below Temperature of Polymer Gel Formation All carbon-rubber films heated 1 hour Statex R
By the dry adsorption approach described in this paper this mastication effect is eliminated. Following the procedure outlined for Figure 5, a series of adsorption isotherms was developed, using Micronex W-6 over a range of loadings with GR-S X-478, the temperature range extending well into the gel-forming region, The results of these tests are given in Table V and the isotherms are presented in Figure 9. Below the temperature of polymer gel formation (135' C.) the adsorbed rubber increases proportionately with carbon loading. Above the temperature of polymer gel formation, the carbon black represses gel formation, since the sum of adsorbed rubber PIUE polymer gel in a carbon-rubber film is significantly less than the polymer gel formed at that temperature in the control. This repression effect increases with loading until a maximum gel repression is reached-Le., minimum insolubilized rubber. Carbon loadings beyond this minimum again increase rubber insolubilbation proportional to the loading. With Micronex W-6 the maximum gel repression occurs a t about a BO part loading.
ISOTHERMS TABLE V. GR-S X-478 DRYADSORPTION
Temperature, C. 26 85 110 125 135 165 180
All FiIms Heated for 1 Hour in Air Oven GR-SX-478 Insolubilized, % ' Carbon loading (phr) with Micronex Polymer W-6 control 10 25 50 75 100 0 2.1 a.o 4.4 4.7 7.8 9.5 0 .. 13 15 9.5 0 ,. 20 14 9.3 0 22 15 11. .. 0 19 37 ii 14 29 62 27 22 38 62 25 80
.. ..
The probable mechanism whereby the aggregative and di% aggregative reactions in the polymer are repressed involves selective adsorption by the. carbon black of the intermediates through
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
January 1952
129
To 80% at 2.I
4 I
2.0
i
I .9
Micronaa W-6 Filtrote 50 phr
e
1.8
I .7
&
2 1.6
rz
Y
>. 1.5
t v)
o 1.4
-::
' 1.3
0
2
1.2
GUS X,478/
Control
0 t-
-
z I .I I .o
0.9 0.8
Figure 6.
I
I
I
25
50
75
I
100 TEMPERATURE,'C.
I
125
I
*
I50
l7!5
0.7 1
I
Effect of Temperature on Insolubilization of GR-S X-478by Carbon Black
I
TEMPERATURE, OC.
Figure 7. Effect of Heating in Air on Intrinsic Viscosity of Polymer and Carbon-Rubber Films
All carbon-rubber 5lma heated 1 hour Statex R
Heating time 1 hour in all aama (air oven) on B l m e from dry adsorption p d u m
L."
,
I
2.4
2.3 2.2 2.1 ,J
x
'70O h
2.0
0
E
8
5 Y
E z
1.9 1.8
1.7
I .6 1.5 IA 1.3
P.2 1.1
'1.0 CIREKlN
COnoING ( p h r )
Figure 9. GR-S X-478 Dry Adsorption Isotherms
Fsgure 8. Effect of Heating in Air and Nitrogen on Intrinsic Viaeusity of P d p ~ e rand Gu4mm-Rubber Films
Filme heated I hour in air oven MIartmes W-6
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
130
VOl. 44, No. 1
80
70 Micronex W-6
60 0 W
N
i m
3
50
8 z
(0
I-
40
d
>i:
? a (3 30 ap
20
10
0 0
I
I
50
CARBON
I
100
200 PIGMENT
300 400 LOADING ( PHR
I
100
500
600
I
150
I
I
200
250
LOADING ( P H R )
Figure 11. Effect of Deoxidation of Micronex W-6 on Gel Repression of GR-S X-478 Polymer
Figure 10. GR-S X-478 Polymer Gel Repression for Various P i g m e n t s
Micronex W-6 deoxidized for 18 hours at 2 0 0 0 O F. Films heated 1 hour at 180° C. i n air oven
All films heated 1 hour at 180' C. in air oven
which these reactions would normally proceed. It appears on the basis of this isotherm study that for maximum adsorption of these intermediates a critical total carbon surface is required. If the carbon surface, which is a function of particle size and loading, is less than this critical amount, the adsorption of these intermediates will be incomplete and gelation will not be fully repressed. GEL REPRESSION BY VARIOUS RUBBER PIGMENTS
wide differencesin this gel repressionproperty which must be associated with total surface and chemistry of surface. Chemistry of surface undoubtedly determines the degree of selective adsorption; the greater this selectivity, the greater the gel repression and the lower the minimum in Figure 10. Since the fine thermal carbon, a coarse black which was benzene-extracted to remove oily matter, shows almost the same maximum gel repression as channel carbon, although differing radically in surface chemistry, there is apparently more than one mechanism involved in selective adsorption. The higher loading required by the fine thermal for t,his maximum gel repression is ascribed t o its coarse particle size. The HAF carbons do not repress gel formation to the extent of channel black and require a higher loading to reach their maximum repression effect. This shift to higher loading for the HAF carbons, in spite of an equivalent particle size to channel black, may possibly be ascribed to their lower activity towards gelation intermediates. This selectivity of adsorption does not necessarily provide a measure of the total rubber insolubilized by carbon.
This gel repression effect was next investigated for a series of carbon and noncarbon pigments to determine first whether carbons were unique in this property and secondly whether the type of carbon surface was a factor. For this purpose three typical noncarbon rubber pigments and a variety of carbon pigments with widely differing surface chemistry were used. I n all cases the maximum temperature of the preceding study (Figure 7) was employed- Le., 180" C., which in the dry adsorption procedure gelled 80$70of the polymer. The results of this study are set forth in Table VI and Figure 10. All TABLEVI. GR-S X-478 POLYMER GEL REPRESSION FOR VARIOUS PIGMENTS the carbons show a pronounced gel All Films Heated for 1 Hour at 180° C. in Air Oven repression effect, whereas the noncarGR-S X-478 Insolubilized for Various Pigment Loadings (phr), % ' bon pigments exhibit little of this property, It would appear, therefore, Pigment 0 10 25 60 75 100 150 200 250 300 400 500 600 80 62 25 22 27 38 49 55 58 63 .. . . .. that the carbons as a group have a 77 .. .. 56 . . 35 40 46 52 55 .. . . .. g $ z n g W-6 selective adsorption activity toward ~-33= 80 . . . ,. 56 24 .. 24 26 28 29 80 . 72 78 . . 86 .. the oxidative intermediates while g%oxide 80 . . . . i82Q .. .. 71 70 77 83 87 1: .. the noncarbon pigments are relatively Hi Si1 82 . . . . 83 . . 75 68 67 67 68 .. . . . . nonselective in this adsorption aca Benzene extracted. tivity. The carbons display rather
.
.
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
January 1952 SCISSION REACTIONS
2.4
-
I
0’
c) 0
c
2.0
-
U
e EPC
5 w
eHAF
0
a W
v)
?
0 FT
a
k
’.5-
a
t-
OSilico
0 Control
0
cp
REACTIONS
:1
r
-8
TABLEVII.
Control
OSIllCO
2
0
b
P In
EFFECT OF CARBON AND NONCARBON PIGMENTS ON OXIDATIVE SCISSION OF G R S X-478 All Films Heated for 1 Hour at 135O C. in Air Oven Pigment Loading, phr Intrinsic Viscositya, 30° 0. Pol mer oontrolb ... 1.23 SbLx R 100 1.89 Micronex W-6 50 1.98 300 P-33(FT) 1.74 Silica 100 1.37 Zinc oxide 300 0.86 Intrinsic viscosity determinedon filtrates. No insolubilizedrubber formed in control.
In
W
K .W P
APPLICATIONS TO MILL MIXING
a
v)
r
GELATION
Zinc Oxide 2 v)
>
II
131
This inhibitory effect of carbon black on the various oxidative reactions in cold rubber would be expected to play an important role in mill-mixed Compounds. The importance of this role would vary as the conditions employed in mill mixing deviated from the conditions found to give maximum effects in this investigation. For example, with Micronex W-6 this maximum repressive effect occurs at the 50 part loading which is normal for mill-mixed compounds, whereas with P-33, maximum repression takes place at a loading above 200 parts, which is normally well above that for I mill-mixed compounds. Similarly, Statex R shows a maximum 170 180 BO TEMPERATURE, *C. repression at the 100 part loading which is also higher than the loading normally used in tread compounds. The 1-hour heating Figure 12. Effect of Carbon and Noncarbon Pigments time used in the present method and that used in mill mixing is on Oxidation of GR-S X-478at Maximum Gel Represanother important factor. Some confirmation of this difference sion Loadings between carbons has been obtained in preliminary tests on mill compounds mixed a t high temperatures in the polymer gel formEFFECT OF SURFACE CHEMISTRY ON GEL REPRESSION ing range. A 50 part M B of Micronex W-6 milled for 45 minutes at 365” F. developed substantially less total insolubilized rubber Channel carbon and fine thermal black inhibit gel formation to than a polymer control mixed under identical conditions; a 50 practically the same degree although differing radically in surface part M B of Statex R developed a higher level of total insolubilized chemistry. Channel carbon is a high volatile, low p H impingerubber than Micronex W-6 over a range of conditions. ment black, while the fine thermal black is a low volatile, alkaline The polymer gel repression characteristic of carbon black may p H thermal decomposition black. Since the furnace carbons used modify or invalidate corrections, in which the gelation of a blank in this study did not show the maximum repressive effect of either raw polymer gum is subtracted from the bound rubber values the channel or thermal carbons, whatever surface chemistry is for similarly treated mixed stocks. required for maximum repression with these two carbons is This adsorption approach is applicable to the study of all present to a reduced degree on the furnace carbons. rubber-carbon systems and offers possibilities of providing addiTo test the effect of volatile on gel repression the Micronex tional information on the mechanism of the carbon-rubber bond. W-6 was deoxidized by heating in the absence of air for 18 hours at 2000” F. The gel repression of this deoxidized Mironex W-6 ACKNOWLEDGMENT was reduced with the minimum level of insolubilized rubber and the optimum carbon loading shifted to values slightly beyond the Grateful acknowledgment is made to W. B. Wiegand, vice HAF carbons (Figure 11). Volatile as present on Micronex, president and director of research, for permission t o publish this therefore, appears t o play an important role in repressing gel paper, to K. A. Burgess for many helpful suggestions in the prepaformation. Recently, Polley, Schaeffer, and Smith (6)have conration of this paper, and to Helen Astarita and Beatrice Schubert cluded from heat of adsorption data, that dihydromyrcene apwho carried out much of the experimental work. pears to undergo chemisorption on the surface of channel black. Preliminary tests with P-33 demonstrated that after treatment LITERATURE CITED with iodine monochloride, the gel repression effect was even more (1) Baker, W. O . , IND.ENO.CHEM.,41, 511 (1949). drastically reduced than in the case of deoxidized Micronex. (2) Kolthoff, I. M., and Kahn, Allan, J. PhY8. & Colloid Chem., 54, The mechanism of gel repression for P-33 therefore appears to 251 (1960). be associated with its unsaturated surface. Unsaturated carbon (3) Ladd, W. A., and Ladd, M., “Csr%on Gel as Revealed by the Electron Microscope,” paper presented before the Division of surfaces involving ethylenic double bonds have been previously reRubber Chemistry at the 117th Meeting of the American ported by Stearns and Johnson (6). The inhibiting effect of these Chemical Society, Detroit, Mich. carbons on the oxidation reaction may therefore result from free (4) Mesrobian, R. B., and Tobolsky, A. V.,J . Polyrter Sci., 2, 463 radical interaction with chemical groups on the carbon surface. ’ (1947). (6) Polley, M. H., Schaeffer, W. D., and Smith, W. R., J . Am. C h m . BOG.,73,2161 (1951). CARBON us. NONCARBON PIGMENTS (6) Stearns, R. S., and Johnson, B. L., IND.ENG.CHEM.,43, 146 (1951). Carbon pigments have been shown to repress polymer gel (7) Sweitzer, C. W., Goodrich, W. C., and Burgess, K. A., Rubber Age, formation, which is associated with their behavior in repressing (N.Y.),65,651 (1949). the oxidative scission reaction. Noncarbon pigments, on the (8) White, L. M., Ebera, E. S., Shriver, G. E., and Breck, S. J., IND ENG.CHEM.,37, 770 (1945). other hand, showed a t best only a minor gel repression effect and (9) Winn, H., Shelton, J. R., and Turnbull, D., Ibdd., 38, 1052 (1946). therefore would be expected to have little repressive effect on the oxidative scission reaction. This was confirmed by the data set RECEIVED March 2, 1951. Presented before the Division of Rubber forth in Figure 12 and Table VII. SOCIETY, Washington, D. C., 1951. Chemistry of t h e AMERICANCHEMICAL E
1.0-
OZinc Oxide
e HAF
