OF NATURAL AND SYNTHETIC RUBBER STOCKS IRVEN B. PRETTYMAN
,The Fitatone T i n and Rubber Company,
A h , Ohio
r
*.
*
Flexing Mechniarn on the Groove Gacking Machine
c ma tory tread cracking machinr i a d e r r i b d which mmploya a grooved tnt piece and
e flexing cycle, knrion, compmnaion, and bending in the b.n 01 the groove. cured in special mold or obtained horn the tread of a cured tire. Tnta of eithn initlation (normal mmpln) or crack growth (precutaampln) may be made. The d a t i v e merits ru*.Qpr nmaui;g the amount of cracking in a given flexing time and the time necnury ck a gi n amount Groove uacking tnta of varioua blends in bead atocka are prnenkd. The Hnea-nclaim atocka dectease in Rexing life with inuease in reclaim for both crack initiation angeack growth. In the crack initiation kats the HereaGR-S, guayule4R-S, and reclaimR-S docks insrum in cracking rniatance with increase in OR-S but d-an in crack growth teb. growth k s h d HCVM and G R S curn are compand with knaile atmngth measurements. For both atockr the cute lor the optimum "asking life i a lna than that lor optimum knaile alxength.
belts (a.14) sod tire trsads (16) have hean lked by weaving on, pulleye and thereby produoing a cycle which involvea both e o n and co&on. Tirea themaelves have been flexed under lcad on rotating drums (10). The etrsins whiuh produce cracking dong the leu& of a tire groove are m m p k (9, 11). They occur on and near the surface in the bsse of the goove. For the purpcse of this discussion, theJl prsdehedas the atraina at right aIlgleQ tothegoovelength and in the pbne ofthe groove base. To deviea a flexingmachine
senger tire6 under static load by means of cament caete of the -vas in which wire brads were mbedded BB markera (133. When the treed section in question leaves the mad surface, tension is introduced as a result of the beding caused by cantrifugal force. Thus the rubber in the base of the groove psases from compression, through zero strain, mto tension. An additional tension is probably introduced by the outward displacement of the tread, observed BB a bulge noti& at high apeeaS at a point immediately adjacent to the oontaot m a . In neither cam is this tension depndent to any aienifioapt extent on the tresd stock although it inoreases with inin tire speed. Thus, for any given tire speed, it may be c o r d e d to be of the oolletant amplitude type. The predomhance of the constant amplitude type of atrains justifiesthe we of a constent amplitude flexingtest for labomtoryevduation. The inclusion of compmaion and sero atrain to tension in the k i n g cycle of tread type stocka affeata the flexing resistauce (46); this indicatea their desirability in a laboratory evaluation te$. The camp-on element of the cycle slm nerves to eliminate the change in the flexing aycle esused by permmat set (18) during the progrees of a test involving tension only. Thus the groove crscking machine used in this work wan developed to produce, with a simple teat piece, a flexing cycle including both tension and camprwion. The maximum tensiCn WBB in-
which repodurn these strains quditatively, one must consider the~actingtopthem. The f o m acthgupvsrdon the ribs ofthe t i tread inthe contaot mgion bcmds the tresd to an extsnt determind primarily by the load on the tire.the Mation p m , the molded tire shape, and the fabric structure. This bending is concentrate3 to a brga a t in the groove h, with its magnitude a u M a n U y -of ths spead ofthe tire and of the tread stock used; and a rmnpHasion strain of coq&+nt ainplitude is produced. Thia upnard force slm deforms the ribs and thus contributes a alight a d d i t i d campreasion strain of the constant force type. Thin pmaWea litUe difioaenco in the campreusion strain between difC6mt stocb, sspeoiaur since all n o d tire tregds have moduli io Um ~ Y D Bgeneral range. Compoeasion strains of the order d .%%have bsrm fauud in the base of the groonm of p s s
as
30
INDUSTRIAL AND ENGINEERING CHEMISTRY
creased somewhat over that normally encountered in a tire to reduce the flexing time to a reasonable value.
TABLE I. BASICFORMULAS (PARTSBY WEIGHT)
TENSION-COMPRESSION FLEXING
Pigment
The test piece normally used in the groove cracking machine is shown in Figure 1. I t was one of four obtained from a cured slab by slicing in a direction perpendicular to the molded groove. Twelve such test pieces could be flexed in the machine a t once in six independent units holding two test pieces each. The six units with the test pieces clamped in place are shown in the photograph on page 29. For smooth operation, the six units were arranged in a balanced "firing" order. The test pieces were clamped in place one unit a t a time.
-4
c'
3"
'B
Figure 1.
1 , +
Y
7 i
Test Specimen
To clamp the samples, the unit to be loaded was secured in its closed position by a locking pin in the flywheel (covered in the picture) a t the left-hand end of the cam shaft. The test piece was positioned over centerin pins located in the lower jaw, The Allen screws were then &awn down with alternate one-quarter turns until they were tight against the upper boss. The screws forced the pivoted upper-jaws down on the faces of the test piece, reducing the 0.5-inch (1.27-cm.) thickness of the test piece to 0.4 inch (1.02 cm.) in the clamped region. With the standard test piece this procedure introduced a compression of 43% in the base of the groove. Although, by inserting the pin joining the connecting rod to the vibrating jaw mechanism in different positions, any one of five flexing cycles could be obtained, the machine was so designed that the closed position remained the same. Thus only the maximum tension component of the flexing cycle varied, the maximum compression component remaining at 43%. This change in amplitude of the flexing cycle of the machine corresponds closely to a change in speed of a tire in service where the maximum tension changes but the maximum compression remains relatively constant. The maximum strains of the five settings are as follows: Setting A
B
C
Tension, % 21.5 30.0 44.6
Setting D
E
Tension, % 64.6 93.0
Because of the slight bending given the test piece as a result of the pivoted motion of the vibrating jaw, the bottom of the test iece received slightly less maximum tension than the base o f the groove. The flexing cycles of the twelve-station machine were selected to give reasonable testin times for all types of tread stocks, natural and synthetic. ?Preliminary studies of flexing cycles had been conducted on an experimental two-station machine in
Haves Smoked Sheet
GR-S (75 Butadiene-2.5 Styrene)
Guayule, Underesinated
Reclaim, Whole Tire
TABLE 11. CURESUSEDIN STOCK COMPARISONS 7
Hevea 100 50 20 10
Parts by Weight of Polymer Guayule GR-S
.
... ... ...
...
60
... ...
. I
Reclaim
...
... ... ...
...
.60. .
60 60 90 90
...
...
...
Cure at 137.8' C . (280' F.), Min.
...
80 90 100 50
QlQ
'4
Vol. 36, No. 1
... ...
120 90
60 40 40
20 50
90 100 50
40 40 60
which the amplitude and pivot position of the vibrating jaw could be adjusted to any value desired.) The cam shaft, operated in an oil bath, was driven by one of two motors. The main motor in line with the shaft operated at 1160 r.p.m., the normal flexing speed. A lower motor-gear reducer assembly rotated the shaft at a speed of 57 r.p.m. Upon completion of construction of an oven to enclose the vibrating mechanism it is planned to use this motor to flex the samples slowly during the warm-up period in elevated ambient temperature tests to ensure equal conditioning of all samples. All tests here reported were made at a room temperature of 28" * 2' C. Both precut and uncut samples were tested. The precut, 0.2 inch long and 0.047 inch deep (0.508 X 0.149 cm.), wa? placed in the base of the groove, parallel to it, and midway from its ends. To make the cut, a knife was drawn the length of a slotted guide with the shoulders of the knife in conI tact with the SETTING A guide, and with the guide firmly pressed against the base of the SETTING 0 groove. Water was used to lubricate the knife. To prevent cracks from starting at the ends of the groove of an uncut sample, the ends were seared with a hot iron rod (slightly cooler than dull red) drawn across the upper edge of the groove base t t an SETTING E angle of 45 to the base. This technique eliminated the discrep40 60 60 I( ant results somePER CENT MAXIMUM TENSION times found for the flexing of unFigure 2. Effect of Amplitude on Crack cut samples. Growth (Time to Crack 0.3-lnch~ Sample Similar erratic reof 50 Hevea-50 GR-S Tread Stock) sults have been reDorted bv others
I
I
(3%)*
Two types of cracking tests resulbed from the use of the two kinds of test pieces-crack growth tests from the precut samples and crack initiation tests from the uncut samples. I n the crack initiation tests, measurements were made a t the beginning and at the completion of cracking-i.e., the point a t which the block
January, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
broke completely in two. By means of this latter method of measurement, a combination of crack initiation w l I and crack growth was ob100 120 20 40 60 $0 xained. The shape of the test p i e c e made possible the flexing of samples obtained directly from tires. To make stock comparisons indep e n d e n t of groove s h a p e , the groove shape of the tire sample was duplicated in a laboratory control sample; a plaster of Paris cast of the tire sample groove was obtained, which, in turn, was used to form a plaster of Paris mold. From this mold a tin ribbed insert could becast for use in an auxiliary rubber mold of the same dimensions as the prepared tire sample. Studies of v a r i o u s cp groove shapes E could be made 5 by using a test -J U. sample mold in whi,ch ribs of "0 40 80 I20 160 200 various shapes CURE AT 137.8*C.(28Oo6) - M I N U T E S were inserted. All cracking Figure 3. Effect of Cure of Hevea and GR-S Stocks on studies reported Elongation at Break, Tensile Strength, and Crack Growth were conducted (Time to Grow 0.3-lnch, Precut Samples) on the basis of the time necessary to produce Figure 4. Physical Properties of Blended Stocks a selected degree of c r a c k i n g . This tvne of mea suiement placed the results on a semiquantitative basis. In routine testing, where qualitative results were usually sifficient and the time consumed by the operator became a factor, the amount of cracking in a given time was used as the index of evaluation. This latter test was not quantitative in the case of wide1 divergent stocks in which one block cracked in two before any perceptixle growth had taken place in the other. G R O O V E C R A C K I N G OF TREAD STOCKS
The increasing interest in the blends of various natural and synthetic rubber polymers suggested the desirability of studying the cracking resistance of tire tread stocks made from them. Blends of Hevea plus GR-S, guayule plus GR-S, reclaim plus GR-S, and Hevea plus reclaim were selected. Four basic formulas were used for these blends (Table I). Master batches of each of these stocks were mixed, with the sulfur omitted. The master batches were blended in the proportions desired, based on the weight of the polymer. (The proportions of reclaim were based upon the rubber hydrocarbon present. For the reclaim used, analyais showed 192.3 parts by weight to contain 100 parts of rubber hydrocar bon.) Sufficient sulfur was added to equal the amount of sulfur which would have been present had the total stocks been mixed and blended in similar proportions. It should be noted that the mixing
31 I
I
-
e l1000 ooo/, I
I
I
I
I 40 20 0 CRACK GROWTH OF B L E N D E D STOCKS; TIME TO $ROW 0.3 INCH i PRE-CUT SAMPLES; SETTING C 60
I
loooloo
8?
60 1
PARTS BY WEIGHT 60 OF
40 I
20 I
0 1
FIRST 40 NAMED WLYMER 0
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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 more thorough than that usually obtained from normal laboratory or factory procedure. The flex cracking tests should be interpreted accordingly, particularly in the case of GR-S crack initiation tests, since good channel black dispersion may materially aid flexing resistance. Tensile tests were made over a range of cures t o select the optimum (Table 11) for use in stock comparisons T o compare the effect of the five amplitudes of vibration of the machine ( A to E ) on crack growth, the 50 Hevea-50 GR-S stock was studied. The flexing time necessary for the precut sample to grow 0.3 inch (0.72 cm.) in length is plotted against the maximum tension in Figure 2. For convenience the flexing times of all groove cracking tests are plotted on a logarithmic scale. Figure 2 shows that i t was possible to obtain a wide range of flexing times on the machine and that the decrease in flexing time with increase in maximum tension proceeded a t an orderly but decelerating rate. (Of interest in this connection was an unpublished cracking test, conducted by the author several years ago, of a typical Hevea tread stock, using the original twostation experimental machine with a specimen somewhat larger than the present one. Precut samples were flexed through three different cycles: 65 to 22% tension, 65% tension to 8% compression, and 65% tension to 26% compression. The relative rates of crack growth were 9, 61, and 270, respectively. Taken together, these tests show that a decrease in flexing time, for a given crack growth, may be obtained by increasing the amplitude at either end of the flexing cycle. They also indicate the significant effect on the flexing life of allowing the flexing cycle to pass through zero strain.) Since the temperature of the test specimen is known (6) to affect greatly its flex cracking resistance, measurements of the temperature build-up were obtained for the five machine settings, using uncut samples of the 20 Hevea-80 GR-S stock. These measurements were made during flexing with a No. 36, B. & S. copper-constantan thermocouple pressed against the base of the groove by means of a thin Bakelite rod wrapped with friction tape. The equilibrium temperature rise for each setting, established after 10 minutes of flexing with the thermocouple in place, was as follows: A , 45" C.; B, 58"; C, 70"; D, 85'; E , 101'. The increase in temperature rise found with increase in flexing amplitude undoubtedly contributed to the large decrease in crack growth resistance shown in Figure 2. The effect of cure on elongation at break, tensile strength at break based on the originalsection, and crack growth were studied for Hevea and GR-S stocks (Figure 3). For both stocks, the cure for optimum resistance to crack growth was found to be lower than that for optimum tensile strength. The physical properties obtained on the blended stocks are presented in Figure 4. Temperature rise values for test specimens of the four basic stocks (setting C) were as follows: Hevea, 68" C.; GR-S, 76"; guayule, 86"; reclaim, 63". The results of these blend comparisons may be summarized as follows: The Hevea-reclaim stocks decreased in flexing life with increase in reclaim for both crack growth and crack incipience tests. I n crack growth tests, the Hevea-GR-S, guayule-GR-S, and, t o a lesser extent, reclaim-GR-S stocks decreased in cracking resistance with increase in GR-S. I n crack incipience tests, these same stocks increased in crackingresistance with increase in GR-S. Minor deviations from these generalizations occurred: The crack growth of reclaim-GR-S stocks remained constant between 50 and 100 parts of GR-S; and an optimum cracking life existed for the 10 Hevea-90 GR-S blend in crack incipience tests. When the crack initiation tests were continued to completion of cracking, the results were found t o be similar t o those of the same blocks at incipience of cracking, except that the Hevea and guayule increased in cracking resistance t o values approximately equal to that of GR-S. I n other words, in the case of Hevea and guayule, a considerable flexing interval was found between the
Vol. 36, No. 1
time small cracks first appeared and the time the blocks were completely cracked. I n the case of all other stocks tested, the blocks broke in two very soon after cracks were first observed. Tensile strength was found to correlate with neither crack initiation nor crack growth. Inspection of the cracks in Hevea and GR-S revealed a fundamental difference in appearance similar to that observed in tear tests (4, 16). The cracks in the Hevea were forked and curled, typical of the condition described as knottiness. The cracks in the GR-S were straight and regular, with no apparent tendency toward knotting. IMPLICATIONS OF RESULTS
It was apparent from these results that a marked difference existed between tests of crack initiation and crack growth of stocks containing GR-S. This may resolve apparently contradictory statements in the literature (18, 20, 22) and explains why GR-S stocks resist cracking in some types of service but succumb to i t in others. Since, in most types of service, minute cuts and abrasions normally are introduced into the base of the grooves of a tire, it is suggested that crack growth tests on the groove cracking machine provide the more reliable index of tire groove cracking. From an analysis of the crack growth tests of the blended stocks, certain suggestions may be ventured regarding the probable effect of mixing various polymers: 1. The addition of a small amount of Hevea to a GR-S tread stock (say 10yo) appreciably increases flex cracking life. Conversely, the addition of a similar amount of GR-S to a Hevea tread stock decreases cracking life. 2. Similar results may be anticipated where guayule is substituted for Hevea, or reclaim for GR-S. 3. Reclaim and GR-S may be blended in any proportion with relatively little change in flex cracking resistance. Some superiority in the case of high-percentage reclaim stocks is indicated. 4. The order which might be anticipated for cracking of tires made from the single polymer stocks studied, in order of decreasing merit, is: Hevea, guayule, reclaim, and GR-S, with little choice between the latter two.
The contrast between the 28040-1 superiority of the Hevea to
GR-S in crack growth tests and the 4-to-1 superiority of GR-S to Hevea in crack initiation tests measured at incipience of cracking is striking. I n view of the close similarity in appearance between test sample submitted to cracking and t o tear tests, it is suggested that the same physical properties of the stocks influence both. It seems clear that the physical properties producing knottiness in Hevea stocks are responsible, at least in part, for its superior crack growth resistance to that of GR-S. Although a short time and a moderate ambient temperature were used in obtaining tbe cracking data, the relatively high temperatures developed within the samples during flexing (approaching those in tire treads in service) suggest the possibility that oxidative scission influenced the results t o an appreciable extent. Experiments on aged and unaged samples over a range of smbient temperatures are contemplated to investigate this contingency. ACKNOWLEDGMENT
The author wishes t o thank F. S. Grover for his assistance in the design and construction of the original two-station experimental machine. The twelve-station machine was designed by H. G. Hager of the Engineering Laboratory. The guidance of J. H. Dillon and interest of J. N. Street throughout the course of this work is gratefully acknowledged. The permission of The Firestone Tire and Rubber Company t o publish this work is appreciated. LITERATURE CITED
(1) Anonymous, India Rubber World, 93,61 (Jan., 1936). ( 2 ) Anonymous, Rubbev Age (N. Y.), 26, 542 (1930).
JMUUY.
1941
I N D U S T R I A L A N D EN 0 INKERIN 0 C # E M I S T R Y
(€9 Booth. E. W.. I& Rubber W&, aS.M W..1930). (4) Buass. W. F.. IND. Exa. Cnmu.. 28,1194 (1934): Rubber CAm.
TML.8. iai ( 1 % ~ ) . (SI C d d . 8. M.. M d .R. A., Bloman. C. M.,and Yost, F. L.. h. EUQ.b. hfi. . ED.. 12. 18 (1940): Rubber C h T&. 13, 304 (1940). (6) C d d . MedU. Eloman. and Yo& h. Eaa. CRBU.,33,370 (1941): *R Ch-. "4. 14,. 878 (1941). nd N - h . W. J. 8.. Tlotu.I d .
Rv&w Chmm. Tkh.. 4, 184 (1931). Eochea, 8.. Gbn*M.4 , l (Jsn.-Feb.. 1940): R u b k CAm. Tkh., 13.688 (1940). 0r.y. H.. Kamb. A. 8.. and HUn. R. J.. IND.EUQ.*&mu., AXAL. ED., 6. a86 (1934). Gmta. H. W.,Rubber Aw (N. Y.),38,3%7(1936). Hopkim, 0. E., India R u b b Wmld. 88,34 (8ept.. 1983). Liaks. J. W.. Firestone Tim and Rubbe Co.. uoub. oommuni-
33
(14) Nul. A. M., .nd Northam, A. J.. h. ma.Carnu.. 23, 1449 (1831): RC A ~ T&., . 5. QO ( 1 9 3 ~ . (16) Nellsn, A. H.. RUMMI A@ (N. Y.),24,873 (1929). (16) Patrikssv. 0. A,. and MslniLov.A. I., C w and RVWV m.8.8.R.). De&. 1940. 1% R&a climr. Tkh.. 14 %S8 (1941). (17) Rehie& E. T..and Qeake. R. H., h. &a. Cnmu..&A&. ED., 7. 388 (1986): RUMMI C h . Ted., 9, 178 (1936). (18) &W.L. B.. and Dimmore, R. P.,Inlh Rubber World,10% 46 ( A i l . 1941). (19) 8omSrville. A. A,. T?-. I d . Rnbber I d . . 6, 130 (1890). (20) 8-t. J. N.. and E M H. L.. Rubber Chm. T d . , 14, 111 (1941). (B) Tomanoe, P. M..and Petamn, L. C.. Indk R u b b 82 (Juls, 1928); Rut
TEMPERATURE COEFFICIENT OF VULCANIZATION OF
*
Tempw tb of vulcanization been dcbnnimd fw tmfve Burr S rtodcr the temperature range 970' to 30% F. average values obtained from phyliaal test
Y
h
c r a ternperi re change of I O o C. a temperature change 01 10' F. n&ng values obtained from cornweIy 9.06 and 1.49, mweativdy.
Y The only ubliaheed mention of the &eot of temperatun on Buna 8 W M in a rshase fmm the office of the Rubber Dir+ar, giving tablea for wnvemion of cum to a atandsrd bemperatn#a (Sf). These tshlea w bMed on a t e m p a h dcisht of 1.Q per 10' F. The l o mof thia informaticm im not available, hqwsMI.. EftXClOFlEMPERA~
This investigation WM devotnd to a atudy of the eE& of bemm t u m on the rata of chemlaal combination M weU M the Mt4 bf change of physical p @ e a of three relate3 Buna 8 reoipS (Table IJ:
