Sulfur and Accelerator Variations in Cold ubber Tread Stock EFFECTS UPON PHYSICAL PROPERTIES L. R . SPERBERG', Phillips Chemical
Company, RartZesvitle, OkZa.
Effects of sulfur and accelerator variations over a wide curative range are investigated in two cold rubber tread stocks reinforced with Philblaclr A and with channel black. Sulfur variations range from 0.5 to 3.0 parts per hundred of rubber and accelerator variations from 0.75 to 3.0 parts per hundred of rubber. Complete physical properties on original and oven-aged stocks are presented. The data are plotted in a series of isopleths (a graph showing the occurrence of any phenomenon as a function of two variables). Changes in properties resulting from
variations in curative dosage may be determined instanti? b y reference to these isopleths. 4 few 3-dimensional graphs plotting accelerator versus sulfur versus curati+e are shown. The abrasion loss isopleths of natural rubber and GR-S as well as cold rubber, show no degradation in this property as the curative level is lowered (within the commercial usage range), although most of t h e other properties show evidence of so-called accelerator starvation. Road tests on both truck and passenger tires in natural and cold rubber confirm the laboratory abrasion results.
I
it was decided to use the method employed by Sperberg, Bliss, and Svetlik ( 2 ) in portraying the effccts of variations of black and softener loadings upon physical properties. This method depends upon a systematic coverage of the variables concerned with a subsequent construction of isopleths of the various physical propert,ies. The following basic rccipe was employed :
N T H E field of research compounding probably no single subdivision receives as much attention as that covering studies of t,he effects of curative variations upon ultimate physical properties of vulcanizates. Of the thousands of curative studies inaugurated yearly by development and research laboratories of the rubber industry only a few find their way into the technical literat,ure. The chief reason for t,his is probably because the majority of such studies are designed primarily for product development purposes and are generally quite limited in scope. \\-hen curative data that do find their way into the literature are studied, it is noted that while different authors generally concur as t,o the over-all effects of sulfur or accelerator variations upon physical properties, the magnitude of trends or variations may differ widely from one investigator to the next. Many reasons exist for these differences but most of them are probably due t,o minor or major variat,ions in the basic recipes employed. As a result, the effects of sulfur and/or accelerator variations may have been det,ermined a t different cure staies and it is not unreasonable to assume that the magnitude of these effects would vary as a consequence. I n view of these discrepancies in published data on the over-all effects of sulfur and accelerator variations, it appeared desirable to reinvestigate them t.horoughly in different elastomers. This was done for natural rubber, GR-S, and cold rubber using HMF (high modulus furnace) and EPC (easy processing channel) black reinforcement. This paper reports t,he results of the work with c o l d r u b b e r a l t h o u g- h s o m e r e f e r e n c e will be made to the other studies involving nat,ural rubber and GR-S. EXPERIMENTAL
I n o r d e r t o cover a broad range of both sulfur and accelerator variations and then to report the results in a readily comprehensible manner, 1 Present address, J. iM. Huber Corporation Borger, Tex .
Cold rubbera 100 HMF blackb 50 6 Asphalt N o . 6 Zinc oxide 3 Stearic acid 1.5 Sulfur 0,5, 1.0,2.0,or3.0 .V-cyclohexyl-2-benaothiazolesulfenaniide C 0.75, 1 . 3 7 , 2 . 0 , o r 2 . 7 3 a R33, a Philgrene h 41' F.butadiene-styrene synthetic rubber. b Philblack A. 6 Santocure.
The four different sulfur lcvels with each of four accelerat,or quantities result, in sixteen compounds. The low and high levels of sulfdr and accelerator were chosen so that extremes of both undrrand over-acclerated compounds would be studied. Vulcanization was performed a t 3 0 i o F. Complete physicid properties were determined both before and aftcr oven aging foi, 2.1 hours a t 212" F. DISCUSSION O F RESULTS
After all of the physical properties had been determined, t h ( 3 tabular data were transformed to isopleths by the method illupt.rated in Figure 1. \Vorking graphs of the Lypc illustrated in Figure 1 are prepared after the rcgular time versus property curves are drawn for each compound. If any points are obviously in erroi', variations from standard p a t t e r n s become quite pronounced and the timccure curves can be altered so that a uniform pattern is obtained. (About the only properties where any PHR-SULFUR PHR-DANTOCURE PUR-SULFUR alteration from actual exp e r i m e n t a l data is enFigure 1. Illustration of Isopleth Preparation 1412
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1413
that the relaxed compression set test measures something closely akin to, if not actual, state of cure. The reversion or retarding effect of higher sulfur levels in the natural rubber system is quite pronounced by both the compression set and relative flee sulfur values. The free sulfur values plotted in Figure 2, b and d, are constant percentages of the original variable total sulfur loadings in the compounds. Any reference to state of cure made in this paper will be based upon relaxed compression set data. After all of the sulfur has been combined the compression set values still continue to decrease. Thus, if the natural rubber compound containing 3 parts per hundred of rubber sulfur and 0.5 part per hundred of rubber Santocure is examined, it will be noted that all of the sulfur has been combined after 45 minutes of cuie and that the relaxed compression set at this time is 15%. After 75 minutes of cure the relaxed compression set has decreased to 10%. SCORCH DATA. Mooney scorch data were determined at 280" F. (Figure 3, a). The scorch point is taken as the total elapsed time required for thc Mooney viscosity to decrease to a minimum and then to increase 2 points above the minimum value. Previous work has shown that a 2-point rise corresponds to the point a t which the first visible signs of roughness become perceptible when the Mooney scorch specimen is sheeted out on the laboratory mill. The slope of the lines of Figure 3, a, indicates that variation in the acceleration has only a slight effect upon the scorch point and that any major improvement in scorch time would have t o be obtained by varying the sulfur level. Data determined a t 307" F. show similar trends, although the scorch times are obviously shorter. STATE OF CURE. The relaxed compression set isopleths (35Oj, deflection for 2 hours a t 212" F. plus 1-hour relaxation a t 212" F.) show state of cure to inFigure 2. Compression Set and Free Sulfur Isopleths crease as both sulfur and accelerator levels are increased (Figure 3, 6 ) . The change in slope of the countered are tensile strength, elongation, flex life, and abrasion curves indicates the decreased activating effect of sulfur as the dosage is increased from 0.5 to 3 parts per hundred of rubber. For resistance and even here changes are generally small.) The property values a t specific cure times are then read from the the cold rubber-furnace black system, most commercial coincurves and are used for plotting the working graphs as illustrated pounds will give relaxed compression set values of approximately by Figure 1. The conversion from the working graphs to the final 25 =t1 0 ~ o .Commercial cold rubber-furnace black tread-type isopleth form needs no further explanation since a study of the compounds give set values of about 20 * 5Y0at the technical cure. figure will reveal the mechanics of the transition. It is the author's belief that the many possible sulfur-accelerator STATEOF CURE. The T-50 test is not applicable for determincombinations forming the 20% set curve at 30 minutes a t 307 ' F. ing state of cure of synthetic rubber, nor can it be used with are all at the same state of cure, although the other physical natural rubber when variations of sulfur content are encountered. properties may vary widely along the isopleth. The determination of combined or free sulfur offers a partial soluStress-strain isopleths for the STRESS-STRAIN PROPERTIES. tion to the problem but the test is time consuming and further30 minutes a t 307' F. cure are shown in Figures 3, c, d , and e. more is useless as a guide to the vulcanization reaction which As sulfur and accelerator levels are lowered, modulus, tensile strength, and elongation are also reduced. The "island" formastill continues after all of the sulfur has been Combined. I t is nevertheless a valuable referee method. Since the data retion of the tensile strength graph is interesting because it shows ported in this paper involve only sulfur and accelerator variations that the tensile strength first increases, then decreases as the another test obviously would have to be employed to measure sulfur level is increased. Similarly, tensile strength increases state of cure if comparisons at equal state of cure were desired. then decreases as the accelerator level is increased at the 2 parts For the past 5 years the author has used the relaxed compression per hundred of rubber of sulfur level. This same effect is shown set test (357, deflection for 2 hours a t 212" F. plus 1-hour relaxaafter oven aging, although the effect of accelerator variation is tion a t 212" F.) for measuring state of cure, using as a basis for somewhat reduced. The isopleths of Figure 3 are all a t the conthe method Williams' definition of vulcanization to produce soft stant cure time of 30 minutes a t 307" F. I n order t o show the rubber as ' I . any treatment which maintains the elasticity effects of curing time upon several of the more important physical of the rubber while its plasticity is decreased" (1). The similarproperties, additional isopleths were prepared for the 20-, 45-, ity of relaxed compression set and free sulfur curves for natural and 75-minute cure times. From these isopleths 3-dimensional rubber and GR-S Philblack A systems is shown graphically in plots were then constructed. The 3-dimensional characterizaFigure 2. Similar data for cold rubber systems are not available, tions of modulus, tensile strength, and elongation are shown in hence the reporting of the natural rubber and GR-S data to show Figures 4 and 5. Also shown is a 3-dimensional plot of the 2070
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INDUSTRIAL AND ENGINEERING CHEMISTRY
compression set surface. Aglance at the modulus, tensile strength, and elongation plots shows that variation of cure time from 20 to 75 minutes has a rather slight effect upon these properties, but that the compression set decreases sharply with an increase in cure time. RIBILIENCE. Resilience isopleths of Figure 3, f, show the resilience to decrease as the sulfur and accelerator levels are lornr-
Figure 3.
Vol. 42, No. 7
ered. This is in accord with other reported data. Figures 6 and 7 show %dimensional plots of resilience as a function of curative dosage and cure time. FLEXLIFE. Flex life (Figure 3, 8 ) varies in the manner in which it is generally assumed to change. It improves markedly as the sulfur level is lowered but is relatively unaffected bjr accelerator variations. The region of excellent flex life a t approximately 1.0 part per hundred of rubber of sulfur and from 1.25 to 1.75 parts per hundred of rubber of Santocure is interesting since this is at a relatively COLD RUBBER-PHILBLACK %* SYSTLM high state of cure. VOLUMESWELL. Volume swell was determined after 7-day immersion in A.S.T.M. reference fuel No. 1 a t room temperature (Figure 3, i ) . As the sulfur and accelerator are lowered, swelling tendencies become greater. The volume swell isopleths parallel almost exactly the modulus and hardness isopleths. HARDNESS. Shore, A.S.T.M., and deflection hardness isopleths all s h o v similar trends in the expected manner; the hardness is lowered as the sulfur and accelerator dosages are decreased (Figures 3, h, j , IC, 6, and 7). ABRASIONRESISTANCE. The abrasion resistance was determined on a modified Huber angle abrader using two different test procedures. The unextracted results are the average losses of eight test wheels run on each of eight mounts while the ethyl alcoholtoluene azeotrope extracted results are the average of two wheels rotated on the machine so that each test specimen was on each of the eight different mounts one eighth of the total test time. Abrasion loss isopleths are shown in Figure 3, 1. The results are rather surprising in that they show abrasion resistance t o improve as sulfur and accelerator levels are lowered, which results are in direct contrast t o the generally accepted ideas of the effect of curative dosage upon abrasion resistance. In order to interpret the abrasion results more accurately i t was decided to extract the abrasion wheels with ethyl alcohol-toluene azeotrope to remove any gummy resinous material which might be lubricating the abrasive surface by exuding from the test specimen. This was done by extracting the test specimens for 6 days with ethyl alcohol-toluene azeotrope, followed by 1-day extraction with ethyl alcohol. The wheels were then placed in a moving air stream (SO' F.) for 2 weeks, after which time they were tested, The abrasion results obtained with the extracted wheels are also shown in Figure 3, I , and show t h a t decreasing the curative level results in unchanged or slightly lowered abrasion resistance, although the degradation is very small and not of the order of magnitude that one would anticipate on the basis of most oral discussions. Because of the surprising results encountered with the cold rubber-Philblack A system, the abrasion data developed with the other elastomer systems and with EPC black were inspected, These isopleths are shown in Figure 8. The cold rubber-EPC black system shows a similar trend to that shown by cold rubber reinforced with Philblack A. Natural rubber very surprisingly shows the same degradation in abrasion resistance as sulfur and acceierator levels are increased, and this trend persists even with ETAextracted samples. The contours of the different PHR SULFUR isopleths indicate that a change of 1 part of sulfur while maintaining an equal cure state results Cold Rubber Isopleths of Various Properties a t 30 in only a small improvement or debasement in Minutes at 307' F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950 COLD R U B B E R . P H I L B L A C K * A ' SYSTEM
COLD RUBBER-PHILBLACK "A" SYSTEM
forced with EPC black resulted in the same genera1 trends as were obtained with HMF black, but for purposes of brevity the additional data are not reported in this paper. I n view of this general degradation of physical properties as curative level is lowered (while still maintaining essentially equal state of cure), it would be easy to postulate t h a t abrasion resistance would also decrease, but according to laboratory abrasion data such is not the case. The majority of t h e cold and natural rubber abrasion data show the wearing quality to improve as curative levels are lowered. This is particularly true in the case of the unextracted test specimens. The ETA-extracted specimens show varying and sometimes contradictory trends but i t may be stated with certainty t h a t the effects of curative variations upon abrasion resistance are minimized as a result of ETA extraction. Thus the various stocks show only minor degrees of improvement or degradation. If more credence is placed upon the ETA-extracted, oven-aged results, the abrasion resistance is only slightly degraded, if a t all, and there appears to be a possibility that the abrasion resistance in terms of actual road wear might actually be increased slightly, I n addition, the better ffex life offered by stocks containing smaller amounts of curative is a very definite advantage. The chief deficiency of the stocks containing lower sulfur and accelerator dosages appears to be their higher hysteresis.
COLD RUBBER.PH~LBLACK*A' SYSTEM
PWR-SULFUR
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COLD R U B B E R . P H I L B L A C K ~ A ~SYSTEM
PnR S U L F U R
Figure 4. Three-Dimensional Plots of StressStrain and Compression Set Properties
C O L D R U B B E R . PHlLBLACK"A' SYSTEM
2 75
abrasion resistance for the natural and cold rubber systems. GR-S, on the other hand, shows the familiar improvement in abrasion resistance as the sulfur level is increased. This was also found to be the case in the GR-S-EPC black system although t h e data are not reported. I n summarizing the data to this point the following observations may be made. The similarity between the modulus, elongation, resilience, hardness, and volume swell isopleths is worthy of note and shows quite clearly the interdependence of these properties with each other. If a constant cure line--e.g., 20% compression set curve-is inspected (Figure 3, b ) it will be noted t h a t even though the sulfur level is lowered rather markedly the amount of additional accelerator required to maintain a constant state of cure is relatively small. If this constant cure line were superimposed upon any of the isopleths, i t would show that as the over-all curative level decreased, the modulus, tensile strength, resilience, and hardness would decrease while the elongation, flex life, and volume swell properties would increase. These changes all reflect the phenomenon of curative starvation which other investigators have discussed at various times. A similar investigation of cold rubber rein-
2 2s
w $1 7 5
f 2s
;I
I 0
7s
0 25 0
PnR SULFUR
Figure 5. Three-Dimensional Plots of Stress-Strain Properties Oven-aged 24 hours et 212O F.
PHR
SULFUR
'1
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COLD RUBEER.PHILBLACK"A.SYSTEM
COLD RUEELR.PHILBLACK~A~ SYSTEM
COLD RUBBER.PHILBUCK'An SYSTEM
Figure 6.
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The results of the road wear testing are reported in Table I. When calculat'ing the relative abrasion resistance, compounds containing the highest curative level were arbitrarily pegged a t 100%. The cold rubber tires containing 0.90 part per hundred of rubber of Santocure were superior in abrasion resistance to the comparison compound containing 1.2 parts per hundred of rubber of Santocure. The magnitude of superiority decreased from 22 to 4YG as the test progressed to completion. Reference to Figure 3, 1, will show that the laboratory abrasion graphs indicate an improvement in road wear for such a change although it should be remembered that the comparable laboratory and tire tread compounds are not identical. The crack growth resistance of both tread compounds proved t,o be equal as indicated by the laboratory data. The natural rubber tires show all three compounds to be essentially equal in wear resistance. A reduction in sulfur level from 2.50 to 1.75 a t 0.50 accelerator results in equivalent tread wear and improved crack growth resistance. Similarly a reduction in accelerator content from 1.0 to 0.5 part per hundred of rubber a t 1.75 sulfur level shows equivalent wearing qualities with greatly improved crack growth resistance. KO tires failed during the test because of excessive heat build-up. Table I shows that all the tires traveled in excebs of 53,000 miles before wearing smooth. This is considered to be excellent mileage considering thz severity of the test. Further road tests were subsequently conducted on
Three-Dimensional Plots of Resilience and Hardness Properties Original COLD RUBBER-PHILBUCK 'k'5YSTEM
ROAD TESTS
These laboratory results point to improved crack growth resistance with little or no loss in abrasion resistance and only a small increase in hysteresis. Therefore, it was decided to compound tires embodying these ideas and subject them to actual road tests. However, the program was restricted to only five compounds because of the unit expense involved and the fact that the fundamental idea appeared to be contrary to the generally accepted ideas of the effect of curative variations upon abrasion resistance. The basic formula- that were used in performing these tests are shown belon.
Cold rubber S a t u r a l rubher Philhlack 0
Sulfur Santocure
100 ,
..
.
60 1.75
1.20
100
...
BO 1.75 0.90
,,
.
,
100 50 2.50 0.50
,
iQo
00 1.7d 0 50
iQo 30
1.75 1 00
The experimental tread sections a ere fabricated into 10.00-20 size truck tires and were tested on gasoline transport trucks operating out of Kansas City, Mo. The testing was accomplished between December 1948 and September 1949. I n order to discount differences in truck severity the tires were fabricated using half and half treads-half' the tread being experimental stock and half control stock. Eight tires of each type were built making a total of 40 tires. Tires were tested in drive wheel service only.
Figure 7.
Three-Dimensional Plots of Resilience and Hardness Properties Oven-aged 24 hours a t 212' F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
1417
passenger car tires in which the following basic tread formulas were employed. Compound F G Cold rubbera Philblack 0 Sulfur Santocure
100
100
50 2.25 0.84
50 1.75 1 .oo
Experimental Philprene A rubbers differing in only minor details from commercial cold rubber.
-
The road wear results were developed on passenger cars running under fairly constant conditions of load, speed, temperatures, and road surface, using the same drivers. The results are reported in Table 11. No cracking of any of the tires was encountered. While this road test was designed with another objective in mind the road wear data do substantiate the compounding trends discussed previously since compound G would possess laboratory physical properties that show evidence of curative starvation when compared to compound F. CONCLUSIONS
The laboratory data trends have been substantially confirmed by the road wear results obtained from the relatively few tests completed. Reduction in curative level results in a general lowering of modulus, tensile strength, hardness, and resilience, but despite this, the abrasion resistance has not been degraded to any extent and the scorch resistance and the ffex crack growth characteristics have been improved. It should be emphasized again that the reduction in curative level in the tread compounds investigated was accomplished while still maintaining a high state of cure.
Figure 8.
Abrasion Isopleths of Cold, Natural, and GR-S Rubber Loss in c u b i c c e n t i m e t e r s
~~
ACKNOWLEDGMENT
TABLE I. TRUCK TIREROADWEARRESULTS % -
Compound
Rubber
Sulfur
Accel.
A B
Cold Cold Natural
1.75 1.75 2.50
1.20 0.90 0.50
100 122 100
Natural Natural Natural
1.75 1.75 1.75
0.50 1.00 0.50
100.5 100 103
C
D
E D
25
Abrasion Indexes, % of Tread Loss-50 75 io0 100 110 107 100 100
257, of Nonskid
Compound
a
Sulfur Accel. Slight A Cold 1.75 ’ 1.20 0.2 B Cold 1.75 0.90 0.5 2.50 0.50 1.5 C Natural D Natural 1.75 0.50 0 E Natural 1.75 1.00 0.6 1.75 0.50 D Natural 0 Expressed as the average number of cracks Rubber
98.2 100 99.5
’
98.8 100 98.5
Crack Growth R e s i s t a K 50% of Nonskid -75%
1-2 In. 2-4 In. Slight 1-2 in. 0.1 0 7.3 0 0 0 6.8 0.3 0.3 0 12.2 0.3 0 0 9.2 0.8 0 0 9.8 0.5 0 0 9.2 0.8 of specific sizes per tire.
2-4 In.
0 0
0 0 0.3 0.
TABLE 11. PASSENGER TIREROADWEARRESULTS Compound Sulfur F 2.25 G 1.75
7 -
Accel. 0.84 1.00
25 100 100.5
Abrasion Indexes % of Tread Loss 50 75 100 100 99.3 100.0
100 100 104 100 99.5 100 99.0
-100 100 103.6
2k::tEness 39,500
Slight
6.0 8.2 25.8 10.8 48.2 10.8
~~l~~ to Smoothness 57,950
53,300 55,900
of Nonskid
1-2 In.
2-4 In.
1.0 0.5 0 0.3 3.0 0 0.9 0.5 11.8 0 0.9 0.5
The author is indebted to the following individuals for the work which they performed in the development of the original data and the construction of the many isopleths: J. F. Svetlik, L. A. Bliss, H. E. Railsback, W. T. Cooper, A. H. Geib, W. S. Howard, M. L. Gallaugher, and W. I,. Gibson. The author wishes to acknowledge also C. C. Biard who determined the road wear results and J. A. Tallant and R. M. Leader for their many helpful comments in the preparation of the manuscript. LITERATURE CITED
(1) Davis and Blake, “Chemistry and Technology of Rubber,” A.C.S. Monograph 74, p. 237, New York, lteinhold Publishing Gorp., 1937. (2) Sperherg, Bliss, and Svetlik, IND.ENG.CHEM.,39, 5 1 1 (1947). RECEIVED September 26, 1949. Presented before the Division of Rubber Chemistry at the 116th Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
