PROCESSING AATDTIRE S CHARACTERISTIC COLD RUBB
VICE
Irvin J. Sjothun and Otis D. Cole The Firestone Tire & Rubber Company, Akron, Ohio Because of the considerable interest in "cold rubber" (polymerized a t 41" F.) and the expansion of low temperature production facilities in the industry, a summary of the passenger tire test results obtained by this laboratory is presented. Commercial cold rubber polymers gave 10 to 2070 improvement over standard GR-§ in tread wear. The replacement of channel by various fine furnace blacks gave an additional 10 to 25% improvement. Tread craclring resistance of cold rubber was a t least equal if not superior to that of GR-S. Artificially aged tires also showed the low temperature polymer about equal to GR-S €or aging as measured b y tread wear, tread cracking, and chipping. Traction on ice and snow was equal to t h a t of GR-S from 20' to 30' F. hut inferior at 0' F.; traction
c
ONSIDERABLE time and money have been spent on synthetic rubber development by both govwnmcnt and private industry. It mag further these programs t o summarize the evaluation woik carried out by the Firestone tire division, particularly relative to low temperature polvmers. If others present similar reports, sound average answers may be developed. This is particularly true because the improvement in wear of the so-called '.cold rubber" is produced by a combination of three simultaneous changes in compounding rather than a single change in the manufacture of the rubber: polymerization temperature, Rlooney viscosity, and type of carbon black. The effect of polymerization temperature in altering the seivice value of GR-S had been established in a preliminary way before the outbreak of World K a r I1 ( 5 ) . This work indicated that the value of butadiene-styrene copolymers in tires was not materially altered by relatively small changes in polymerization tempei ature. Temperature levels originally set for the manufacture of standard GR-S represented the best balance among this background and the economies in manufacturing, and more particularly, the availability of construction materials during a period of extreme shortages due to war conditions. These conclusions were later substantiated by an extremely elaborate tire test carried out on the government test fleet (Test U). The question as to whether drastic temperature reduction might bring about appreciable improvement was ever present from the time of the first meetings of the industry technical committees. I n 1943, road tests were conducted on rubber polymerized a t 20" C., in which the best activating techniques known a t that time mere used. The improvement obtained was indicated to be of the order of 5 to 10%. Considering the long reaction times, the relatively small improvement obtained compared with the large improvement required for army truck tires, and the fact that the total synthetic rubber requirements of the w-ar effort were increasing, pursuit of this attack toward developing an improved GR-S did not seem opportune. It was not until after the war, when the much more effective activators under investigation in Germany ( 4 ) became available, that polymerizations in these low temperature ranges began to appear practical. Likewise the shift in the major requirement
on w-et pavement was equal. For the most part synthetic polymers were up to 15q0 poorer than natural rubber for traction on ice and snow, but up to 1070 better on wet pavements a t temperatures above 40" F. Low temperature polymers have generally been produced a t higher viscosity levels than GR-S, but found satisfactory for processing. Fine furnace blaclcs were an aid in providing smoother and more uniform extrusion. Other processing characteristics appeared satisfactory. Building tack was no better than that of GR-S-10. The variability of the polymer has been greater than that of standard GR-S. 3$05t physical properties of the low temperature polymers have fallen between GR-§ and natural rubber, particularly tensile, elongation, heat generation, and resilience.
for GR-S from army truck tires to civilian passenger tires for use under commercial conditions and in competition with natural rubber made an improvement of this order of greater importance. Laboratory data in the hands of German investigators indicated the possibility of making polymers of high molecular weight having fast mill breakdown and, therefore, adaptable t o processing on present equipment. Their vulcanizates showed markedly improved tensile and elongation and particularly improved hysteresis. Results t o date with polymers made in this country show only partial success in duplicating these advantages. Early in the prosecution of the synthetic rubber program it was found that the tread wear produced by treads made from GR-S tkpe polymers was closely related t o their &looney viscosities, the higher viscosity polymers giving better wear. Because of the absolute necessity of producing a rubber that could be easily processed in existing equipment, a polymer of 50 Mooney (ML 4/212) was chosen for production, even though some tread wear was sacrificed by so doing. I n work on lorn temperature polymers early in 1946 a high Mooney polymer polymerized in a redox system (unmodified) a t 50" F. showed indications of being more easily broken down, sheeted out, and handled on a cold mill than high Mooney polymers made in the GR-S systcni a t
Table I.
Test NO.^ E120W
Summary of Early Tests on High Mooney Polymers hlooney Viscosity 81 140 150+ 104
Polymerization Temperature,
F.
122 122 122 E120AC 122 A 151 50 E120M 51 41 ElPOAF 60 41 EIZOAE 60 41 E120N 153 41 1.53 E120N 41 Tests designated bv E numbers conducted by fleet: other test by Fircstone fleet. b EPC black used in all stooks.
1564
E120B E120E
Tread Wear b , GR-S EPC-100 108 116 133 138 141 107 111 123 140 156 government tire t e s t
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1949
122' F. This polymer also gave 41% better tread wear than that of regular GR-S in tread stocks containing E P C black, Following this lead, it was found t h a t partially modified low temperature polymers which had Mooney values somewhat higher than GR-S could be processed as easily and gave higher tread wear. Table I summarizes these preliminary tests. The tread wear values in these tests were determined by the press-formed tread technique described by Harrison (a), using polymers made in small quantities in the laboratory and pilot plant. These data indicate that tread wear improves with increasing Mooney viscosity. Several different low temperature polymers were tested in large scale tire test comparisons: 41' F. CHP
41 F. K oleate
41' F. TDN
x-435
Butadiene-styrene (70/30) copolymer made in rosin soap-cumene hydroperoxide redox recipe at 41' F., 60% conversion, salt acid coagulation, and 55-60 Mooney Butadiene-styrene (70 /30)copolymermade at 41 OF. in redox-activated potassium oleate system, 60% conversion, salt acid coagulation, and 55-60 Mooney Butadiene-styrene (71.6 /28.5) in Dresinate 731, (p-tolyldiaza-P-thionaphthalene)recipe, at 41 O F., 60% conversion, salt acid coagulation, and 60-65 Mooney GR-S made at 41' F. with cumene hydroperoxideactivated recipe with rosin soap emulsification, 60% conversion, 60 - 5 Mooney, salt acid coagulation, short-stopped with Santovar 0 and sodium nitrite Like X-435, but short-stopped with dinitrochlorobenzene, in place of Santovar 0
+
X-485
The tread wear results shown were obtained on tires made from different lots of polymer and carbon black, and were tested a t different seasons of the year in different tire sizes and under different testing conditions. This may explain, in some measure, the rather wide variations that were obtained in the tests.
Comparison of GR-S-AC and Low Temperature Polymers A summary of tire tests in which GR-S-AC and several low temperature polymers were compared in E P C (easy processing) black stocks is shown in Table 11. I n general, the small size tires showed greater improvement in tread wear through the use of low temperature polymers than the large sizes. An over-all rating of 15y0improvement in tread wear over that of GR-S-AC was assigned to the low temperature polymers on the basis of these results. Table 11. Polymer Comparison Using EPC
Test No.a E140 E140
B
E141 C-1 (unsged tires) C-2 (tires aged 4 days a t 180° F.)
Tire Size 6.00-16 6.00-16 6.70-15 6.00-16 8.20-15
Av. Miles 12,000 12,000 12,000 12,300 20,000
Relative Tread Wear GR-SAC LTP 100 110 100 161b 100 133 100 120 100 108
Polymer Used 41' C H P 41° Koleate X-435 41° T D N X-435
8.20-15 20,000 100 1Q4 X-435 Over-all rating 100 115 a Tests designated by E numbers conducted by government tire test fleet; other tests by Firestone fleet. 6 S o t included in average.
Using Different Fine Furnace Blacks. Table I11 shows the results of individual tread wear tests on GR-S-AC and several low temperature polymers with two fine furnace blacks, HAF (high abrasion furnace) and VFF (very fine furnace). All the results are referred to those of GR-8-AC control stocks containing E P C black. Here too, the smaller sizes generally showed better wear than the larger sizes with both GR-S-AC and low temperature polymers. Both furnace blacks produced im-
1565 ~-
Table 111. Polymer Comparison
Tire Siae
Test N o G a
Av.
Miles
Relative Tread Wear GR-SACwith EPC-100 L T P
Polymer Used
I-IAF black E140
6.00-16 14,500 241b 2740 41' C H P 6.70-15 12,000 137 188 x-435 C-1 (unaged tires) 8.20-15 20,000 118 119 x-435 C-2 (tires aged 20,000 111 118 4 d a y s a t 180°F.) 8.20-15 x-435 D 8.20-10 16.000 .. 125 X-485 Av. 122 138 Av. rating 100 113 V F F black E140 6.00-16 14,500 . 23OC 41° CHI' B 6.70-15 12,000 176 x-435 E141 6,00-16 12,300 1'1'4 132 41° T D N C-1 (unaged tires) 8.20-15 20,000 105 117 x-435 C-2 (tires aged 4 days at 180' F.) 8.20-15 20,000 118 x-435 E 8.20-15 21,600 115 x-435 F 18.000 6.70-15 121 X-485 l.i. ' o. i__. m Av. Av. rating 100 118 a Tests designated by E numbers conducted by government tire test fleet. other tests by Firestone fleet. Not included jn rating. Not included in average but included in average rating.
B
.
.. ..
provement in tread wear with both types of polymer. The HAF black caused a greater improvement in wear with both types of polymer. The results on the polymer and black tests are summarized in Table IV. The average rating of the improvement in wear caused by the low temperature polymer is not changed by the fine furnace blacks over that in the case of EPC black. All these data indicate that three factors influeneed tread wear: Mooney viscosity, temperature of polymerization, and type of carbon black.
Table IV.
Summary of Polymer and Black Tests of Relative Tread Wear
Black EPC HAF VFF
GR-S-AC 100 122 110
LTP 115 138 130
Cracking and Chipping with Different Carbon Blacks. A summary of the observations on cracking and chipping of treads comparing GR-S-AC and X-435 polymers and several blacks is given in Table V. Tests were made on both unaged tires and those t h a t had been aged 4 days at 180' F. No definite conclusions can be drawn from these data as to the effect of the type of black used upon the tendency toward tread cracking. I n the polymer comparison, however, X-435 showed a very slight tendency toward poorer resistance to cracking with I l A F than with YPF black. I n chipping, a very slight improvement was shown f o r VFF over the other blacks. Other tests have shown that the
Table V.
Black
X-435 Polymer and Black Comparison of Cracking and Chipping (Tests C-1 and C-2) Tread Cracking Chipping GR-S-AC LTP GR-S-AC LTP
EPC HAF VPF
Slight Slight Severe
EPC HAF VFF
Slight Slight Slight
Normal Severe Moderate Slight
NO NO NO
KO
NO NO
Oven Aged 4 Days a t 180° F. Moderate Moderate Slight
Severe Severe Severe
Severe Severe Slight
INDUSTRIAL AND ENGINEERING CHEMISTRY
1566
Test NO.
E140
B E141 a
(Mile&tu failure1 GR-e(,R-S-
BC
Rubber
EPC
EPC
~ - 4 r
LTP ~IAF
2423
1826 1558
21811
2058 1974
c-1
2813 Av. 2618 Indoor step-speed test.
Table VII.
t o that of natura1 rubber, as drivcrs remembered it, has becu i~ source of much comment. Extensive tests have shown that GR-S
Indoor High Speed Tesisa
Table VI.
2013 1489 1722
.IC'
!89R
1SLi w2.i
GK-SAC T'TF
..
iii4
2078 1420
1949
1740
LTP VFr 2214 2133 2210 1673 2060
Indoor FIigh Speed Summary
AT. mles t o f ailui e Av, failing speed, miles w r hour
Vol. 41, No. 8
Kllhbrx
CTR-S-AC
LTI'
2600
1800
2000
1110
90
D5
fuiiiace blacks, when used in eithei lox temperatuie polymers or GR-S, tend to cause more chipping than does EPC black in the same polymers. I n general, the aging of the low temperature polymer, as measured by tread cracLing and chipping, was equal to that of GR-S-AC.
Polymers and Blavks in High Speed Indoor Tests Much can be leained about the serviceability of tire treads, where wear is not a factor, b> mean9 of an indoor high speed test. By this test, where the failuie is tiead separation oi cio-n break, the various factors involved in high speed seivice, such as heat build-up, adhesion to the tire body, etc., can be evaluated. Several such tests were made on tires manufactured to the same specifications as those used in the road test repoited above. These tests were made by the strp-spced method used by the Firestone Devclopmenr Laboratories. The conditions were as follows: 1. Test wheel, 10-foot dluin 2. Ambient temperature, 100' F 3. Inflation and load, normal 4. Starting speed, 75 miles per hour 5. Speed regulatlon, increased $5 miles per hour aftel cach 6 hours' duration of test
The results of the individual test., expressed in teims of miles of failuie, are given in Table VI, ahlch includes two tests made on a standard natural lubber tread containing EPC black for purposes of comparison. These iesults indicate that the furnace blacks improved the performance of GR-S-BC in treads over that produced by EPC. I n the furnace black compaiison, the VFF black gave slightly better performance in low temperature polymer, while HAF black was definitely better in GR-SAC. A summary of the results of the indoor high speed tests on the basis of the polymer used, averaging all the blacks, is given in Table VII, which shows the results on the basis of both average miles t o failure and average failing speed. The low temperatuie polymer gave mileages t o failure and failing speeds between those of GR-S-AC and natural rubber. Although the failing speeds indicate that low temperature polymri was midway between GR-SAC and natural rubber, closer analysis based on the mileages t o failure shows that the difference between the t v o synthetics was less than that indicated by the failing speeds. This is explained by the fact that the tires containing low temperature polymer failed soon after starting the 95-miles-perhour period (which started a t 1980 miles), whereas those containing GR-S-AC failed in the latter part of the 90-miles-perhour period. The natural rubber tires failed in the eaily part of the lWmiles-per-hour period (n-hich started a t 2550 miles).
Traction Tests on Wet Pavement, Snow, and Ice Since the advent of tire treads made from GR-S early in F o i l d R'ar 11, their traction on wet pavement, snow, and ice compared
is slightly better than natural rubber for traction on wet pavement, but poorer on snow and ice. The results of static and dynamic traction tests on rubber, GR.-S-IC, and low temper,> ture polymer treads with EPC, HBF. and T'FF blacks on wet pavement are shown in Table VIII. The static tests s h o w d GR-S to be slightly better on wet pavement than natural rubber and low temperature polymer. The fine furnace blacks were slightly better than EPC in GR-S-AC, but the reverse mas true in lorn temperature polymer. The dynamic tests showed the low temperature polymer to be better than natural rubber and GR-8-XC. I n this caie, the fine furnace blacks -rere better in the low temperature polymer t b m in GR-BhC.
Table VIII.
Laboratory Tests of Traction on Ice GR-S-AC F. 20' F.
Rubber O 0 F.
Black
ZOO
I-.
0'
LTP 20'
F.
0'
r.
Static EPC K.4F VI'F
108
*. ..
106
.. ..
100n 100 100
100n
87
104 104 100
86
10s 96 96
102
103
77 90
100" 105 105
78 75
Dynamic EPC 109 131 HAF .. .. VFF Controls for various tests.
..
..
100' 85
109
The rebults of dynamometer tractioii tests on the samc tires using packed snow a t temperatures ranging from IO" to 25' F. for the road surface are sholvn in Table IS. There was only a slight difference between the two synthetic polymers, and both o€ them TTere poorer than natural rubber. Because these tests werc run out of doors, it W R S difficult to control the temperatures a t \yhich the tests were made. As the weather was euceptionallv mild, it was not possible t o inakr tests on ice or snow at temperatures lower than IO" F. Therefore, in order to obtain data a t controlled temperatures as low as 0" F., laboratory tests were run using the apparatus described by Liska ( 5 ) and Conant and Liska ( 1 ) . Because stocks prepared from all polymers normallv showed their poorest coefficient of friction (traction) on ice at temperatures from 20" to 32" F., tests were also run a t 20" F. The results of these static arid dynamic tests me s h o ~ in n Table X. In this table, the traction of each stock TWS referred t o that of GR-S-AC containing EPC black a t each temperat>ure. At 20" F. there \+as little, if any, difference between GR-SAC and low temperature polymer in either static or dynamic traction, although both were poorer than natural rubber. At 0" F., however, the Iow temperature polymer n as much poorer than
Dynamometer Traction Tests on Packed Snow at 10" to 25" F.
Table IX.
Black
EPC HAF VFF
GR-S-AC 100 97
Rubber 113
.. .
I
96
LTP 100
102
105
Table X. Traction on Wet Pavement Black
Rubber
GR-8-AC
LTP
100 109 105
101
Static XPC
HAF
97
VFF
1 06
94
Dynamic EPC HAF VL'F
101
100 110 94
106
113 122
August 1949 Table XI.
INDUSTRIAL AND ENGINEERING CHEMISTRY Processing C h a r a c t e r i s t i c s of R a w P o l y m e r X-485 VFF
GR-S-AC EPC Average viscosity (ML/4/212) Viscosity range
50 46-54
54 50-60
Plastication, average results 50 Crude 44 1st pass 40 2nd pass 37 3rd pas6 Satisfactory Storage Cold flow Handling Factory control testing Mixing Master batch Mixing time Cycle Dumping temp. Final Mixing time Cycle Dumping temp. Plasticitv ~
Appearance Shrinkage Uniformity Speed Building Forming Curing Cured hardness
54 52 51 50 Requires more lubrication Comparable Comparable Satisfactory Considerable variation in modulus Same Same Comparable Same Same Comparable Comparable Comparable Comparable Comparable Satisfactory Better Satisfactory Smoother Satisfactory Equal to slightly better Comparable Comuarable Comparable Satisfactory Better splice adhesion Comparable Comparable
are used with it. The use of fine furnace blacks which are alkaline (high p H ) tends t o compliment the slow curing properties of X-485, results in a n over-all improvement, and lowers the cost of acccleration. There appears t o be a greater variation from lot to lot of X-4&5than of GR-SAC, although when the variation shown in early lots of GR-S and the improvement made in its control are considered, the outlook for t h e future appears promising.
Physical Properties Although i t is well not t o rely too heavily upon the ordinary physical properties of synthetic in predicting the road wear of those polymers in comparison with t h a t of natural rubber, i t nevertheless is of some interest t o compare these properties. Such a comparison of X-485 containing EPC, HAE', and V F F blacks with natural rubber and GR-S-AC, both containing EPC, is shown in Table X I I . These properties, in general, fall between those of natural rubber and GR-S-AC. The physical properties appear, in several tests, t o have been improved by processing t h e polymer a t temperatures in excess of 360" F., either by preplastication or during the mixing of master batches. The tear resistance of X-485 with the fine furnace blacks was inferior to t h a t with E P C black. The aging of X-485 compounds was apparently equal t o t h a t of GR-S-AC compounds.
Table XII.
GR-SAC. This undoubtedly was caused by the greater tendency for the low temperature polymer stock t o crystallize a t this lower temperature. Only one reversal in trend due t o the black used was observed-in the case of H A F and V F F blacks in GR-S-AC, tested a t 0" F., HAF black lowered the traction and V F F black increased i t t o equal that of natural rubber.
General Processing characteristics of X-485 Compared to Gr-S-Ac General observations made during the processing of approximately 500,000 pounds of low temperature polymer (chiefly X-485) are summarized in Table XI. There appears t o be no advantage ih preplastication of X-485 before mixing, but GR-SAC is improved. Plasticated or massed X-485 polymer appears to adhere more tightly t o skids or racks after storage than does GR-S-AC. Most of the observations were made on X-485 stocks containing V F F black. These were compared with those made on GR-SAC containing EPC black. Additional milling of the X-485 stocks containing VFF does not increase the plasticity nor improve the processing. The authors have found no indication of polymerization (hardening) or reversion (excessive softening) of X-485 caused by either plastication or high temperature mixing. T h e times required for mixing are the same for X-485 as for GR-S-AC. The tread extrusion characteristics of X-485 are somewhat better than those of GR-S-AC. Treads extruded from X-485 containing V F F black show no more scorch and slightly less shrinkage than those from GR-SAC containing E P C black. They also have a smoother appearance. During the forming and bagging operation, the X-485 treads show better adhesion a t t h e splices than do GR-8-AC treads. Preliminary d a t a indicate t h a t X-485 requires less power for mixing than GR-SAC, b u t causes a slightly greater peak load during the mixing operation.
General Observations on Compounding X-485 X-485 is slower curing and requires higher acceleration than GRS-AC. This acceleration is reduced considerably if the X-485 is d u m coagulated. Because of its slow curing characteristics, X-485 does not produce the optimum in tread compounding when the channel blacks, which are inherently low in pH,
1567
C o m p a r i s o n of Physical Properties
Normal tests. cured 60 minutes a t 280' F. 300% modulus Tensile Elongation Running temperature, E". Heat aged 4 days a t 212' F. Tensile Elongation yotensile product retained Oven aged 14 days a t 158O F. Tensile Elongation % tensile product retained
Rubber EPC
GR-SAC EPC
X-485 LTP EPC
X-485 LTP H.4F
X-486 LTP VFF
1450 4250 580 220
1000 2600 540 278
1000
3300 580 255
1400 3600 550 252
1200 3600 580 250
2200 310 28
1900 230 31
2400 210 26
2800 200 28
2800 240 32
3400 360 50
2250 350 56
2600 280 38
3000 300 45
3400 330 53
Summary Low temperature polymers give 15% improvement over GR-S iii tread wear. The fine furnace blacks give 10 t o 25% improvement in wear over channel black in both the low temperature polymers and GR-S. A11 processing and other general service characteristics of low temperature polymers compare favorably with those of GR-S. The physical properties of low temperature polymer stocks, in general, fall between those of GR-S and natural rubber.
Acknowledgment The writers wish t o thank the members of the Firestone technical departments who cooperated in obtaining the data presented in this paper. They especially wish t o acknowledge the helpful advice given them by E. €3. Babcock, J. N. Street, H. J. Niemeyer, and F. W. Stavely. They thank the Fireston: management for permission t o publish these results.
Lfteratare Cited (1) Conant, F. S., and Liska, J. W., J. Applied Phys., 15, 767 (1944). (2) Harrison, S. R., presented before the Division of Rubber Chemistry a t 112th Meeting of AM. CHEM.SOC., New Pork, N. Y., 1947. (3) Liska, J.W., IND. ENG.CHEM.,36,40(1944). (4) Marvel, C. S., PB 11,193 (FIAT Final Rept. 618). ( 5 ) Shearon, W. H., Jr., MoKenzie, J. P., and Samuels, M. E., IND. ENG.CHEM.,40, 769 (1948). RBCBIVBD May 26, 1949.
