L* M m Whits U n i t e d S t a t e s Rtcbber C o m p a n y , Passuic, N . J. Continual reduction in the temperature of polymerization from 50" to -10" 6. appears to result in a linear change in properties affected by temperature of polymerization. I t is not possible to determine with certainty whether this linear change continues a t temperabures below -10" C. The following properties improve significantly with reduction in the temperature of polymerization from 50' to -10" C., the magnitude being indicated for the 60' C. reduction: Tensile strength at break, a t room temperature, a t 200" to 212' F., and after aging 24 hours a t 212' I?., 20 to 30Yo higher. Elongation a t break, a t room temperature, a t 200' to 212' F., and after aging 24 hours a t 212' F., 10 to 15% higher. Cut growth rate, unaged, a t least SOYO lower; aged 24 hours a t 213" F,, 40% lower. Tire tread wear, 25% better, Tire tread cracking rate, 50% lower. Hysteresis properties improve somewhat
HE major improvements in the quality of GR-S resulting from a large reduction in manufacturing temperature have been discussed (1-3, 6, 6'). All these publications have dealt with a relatively linlited amount of data or have discussed in only qualitative terms the effects on quality of reduction in polymerization temperature. The present paper is based on a survey of the many data collected during the past several years ( d ) , and is an attempt t o illustrate the quantitative effects on quality resulting from reduction in polymerization temperature. Because a large portion of the data is the result of commercial scale manufacture and processing experimental programs, the results of this survey should be representative of the changes in quality t o be expected by users of these new types of GR-8, This survey has been restricted to Consideration of types of GR-8 made with 20 t o 30% bound styrene and compounded in tire tread formulations.
Method of Review and Analysis TOaccomplish the objective of this review, the writer has endeavored t o use all the experimental data in final or summary reports issued by cooperating organizations t o Rubber Reserve. Information in review papers published in technical journals has not been included in the analysis, as such d a t a might duplicate those already reported t o Rubber Reserve. Several hundred reports were selected as a basis for this review and from these about one hundred were chosen for abstracting the detaiIed d a t a used in this analysis. A great aid in the analysis of the data was the fact t h a t most investigators have used GR-S made a t 50" C. as primary control in evaluations. Therefore, the simplest and probably the most logical method for arriving at a n average effect on each property caused by changes in t h e temperature of polymerization was t o determine in each investigation the differences between the values for low temperature polymers and for their respective 50" C. controls. These differences were then pooled and the average difference considering all investigations was determined. The average value for the physical property of t h e 50" C. GR-S con-
through reduction from 50" to -PO" C, : heat development, 15% lower; rebound, 10% higher. Compared to unaged
room temperature tests the retention of tensile strength and elongation. a t break, tested unaged a t 200' to 212" F. and after aging 24' hours a t 212" F., are not changed b y reduction in the temperature of poly meriaation from 50 * to -10" C, As the temperature of polymerization is reduced from 50" to -10" 6. the ratio of compound viscosity to polymer viscosity increases 20Y& increase in stress a t 200qo elongation on aging 24 hours a t 212' E?, i s 15% greater, and while both unaged and aged cut growth rates are reduced substantially, the ratio of aged to unaged cut growth resistance decreases by a t least 2 5 q ~ . These disadvantages may be due to incomplete stoppage of polymerization rather than to any basic changes in polymer structure,
trols was determined by averaging the values obtained by d l investigators. The average difference resulting from polymerization at temperatures other than 50" C, was added algebraically t o the average for all 50" 6 . controls t o obtain the values used in the tables and graphs. The absolute magnitudes ~f the properties were used in calculations of polymer viscosity, increase in viscosity on compounding, tensile strength and elongation a t break (at room temperature, a t 200' to 212" F., and after aging 24 hours a t 2121' F,),stress at 300% elongation, increase in stress a t 200y0 elongation aging, and rebound. Because of variations in methods of testing. relative ratings necessarily were used for heat development, cut growth, tread wear, and tread cracking. I n deducing these relative ratings, the 50' C, control was given arbitrarily a value of 100, the percentage change resulting from a change in t h e temperature of polymerization was determined, and these percentage changes were used in the calculations. I n plotting the data, circle diameters were made proporltioaajl to the square root of the number of values used in calculating the averages-that is, larger circles were given more weight than smaller circles in determining line position. I n some cases a statistical test of the significance of observed differences was made. The results of these tests are indicated by the notations ns and ss on the graphs under the points in question, ns denotes t h a t the odds were less than 20 t o 1 that the average differed from that of the 50" C, control, on the basis of the data used in the analysis, ss denotes that the odds were greater than 20 t o 1. I n selecting the data used in this analysis, the following general rules and limitations were imposed:
Conversions must be between 60 and as'%. Polymer viscosity must be between 45 and 75 Mooney, Control and experimental polymers must have the same kind of black and. loading of black. Changes in level of accelerator were disregarded. I n about of the tire tread comparisons of 50" C. control and lower temperature types of GR-S, &he softener level was higher in the lower temperature polymers. I n the interest of using these additional performance data in the
P 594
1555
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August 1949
75
25 ss
-25
IO Reaction Temp.
30
50 Figure 1.
- 10
0
- "C.
50 $0
20 5 Reaction Temp.
Increase in Viscosity Due to Compounding
Figure 2.
-10 -18
- "C.
Polymer Viscosity
I 0 Y
30
0------
-2 C
0
w
50
30
-
.
-
Reaction Temp. Figure 3.
.
L
.
50
- "C.-10 Figure 4.
Tensile at Room Temperature
analysis, differences in softener levels in these few tests were dis. regarded. It was recognized that HAF (high abrasion furnace) and EPC (easy processing channel) blacks, the two most commonly used, would impart different performance characteristics. It was impossible, however, to obtain separate analyses of compounds involvin the two types of black, because in many cases the investigator di! not state explicitly which type of black was used, but mere1 stated that the same type and loading of black were used in 50' control and lower temperature types. Comparison of oIymers was made only when the black was constant in type an$ loading within the comparison. No differentiation was made between %ol mers made at 35', 41",and 50" F.; all were considered as 5 polymers. In a given set of results, an average for each property was obtained from two cures representing the two best cures in the data reported, and was considered as a single test.
c?I
6
Discussion of Resnlts Processing. Reduction in the temperature of polymerization results in a large increase in the work required t o mix the polymer with reinforcing blacks and other compounding ingredients ordinarily used in tire tread stocks. A common method of showing this change in work required to mix is to relate the increase in Mooney viscosity on compounding to the temperature of polymerization. The average increase obtained is shown in Table I and plotted in Figure 1. This method exaggerates the actual change in work required to mix. The increase in work to mix is probably something less than 50q6,as the ratio of compound viscosity to polymer viscosity increases only 15 to 20%.
Table I.
Average Viscosities
Temperature of polymerization, C. Polymer viscosity, M p (ML) Compound viscosity, iMc (ML) Mc Mp Number of tests
-
50 52 61 9 56
40
30
20
5
59
..
71 84
60
55
13 8
55
76' 16
79 24 22
71 12 7
..
.. ..
-10
,
-18 51 69
18 3
. . 30
,
, 3s
10 Reaction Temp.
.
,
,
-10
- "C.
Elongation at Room Temperature
The increase in the difficulty of working as represented by this Mooney viscosity increase on compounding is amply illustrated b y qualitative observations on both factory and laboratory scales. GR-S made at temperatures lower than 50" C. was reported by most investigators t o break down more slowly during plastication, require more power to mix in black, develop higher temperatures during mixing, and develop higher tubing temperatures. As far as can be ascertained from the data available on change in viscosity on compounding, the difficulty in mixing GR-Sincreases linearly as the temperature of polymerization is reduced from 50" t o -10' C. It is evident from comparing the increases in viscosity on compounding with the initial viscosities of the base polymers that the increase in viscosity is not the result of a proportionate increase in initial viscosity. The bar graph of average polymer viscosities in Figure 2 and data given in Table I indicate, for example, that -10" C. GR-S averaged only three points higher initial Mooney but about fifteen points higher increase in viscosity on compounding. Thus the small changes in initial viscosity had at most only secondary effects on the magnitude of the increase in viscosity on compounding. Factors t h a t may be responsible in part for the observed inorease in difficulty of mixing as the temperature of polymerization is reduced include decrease in breakdown rate, inadequate short-stopping, and more interaction between polymer and reinforcing black. The lower rate of breakdown with reduction in the temperature of polymerization is well substantiated by the experimental data. It may be due not t o inherently greater resistance to the type of oxidation resulting in chain scission but t o inadequate shortstopping which could result in cross linking during mixing sufficient to offset the effect of oxidative scission. If inadequate short-stopping is the more important factor, the remedy is likely to be simpler than if the basic structure is more stable t o oxidative scission.
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
1556
-
50
IO
30
Reaction Temp. Figure 5.
60
-
-
ns
.
1
-10
50
"C.
Tensile at 200" to 212" F.
ss ns
38
10 Reaction Temp,
Figure 6.
-10
-
"C.
Elongation a t 200" to 212" F.
I 1
-
50
IO
30
-10
- "C.
Reaction Temp, Figure 7 .
yo
1
Tensile Retention a t 200" to 212" F.
2 3 4
I
&
50 Figure 8.
. . 50
,
30
.
ss IO
Reaction Temp. Figure 9.
Vol. 41, No. 8
. . -18
- "c.
Stress at 30070 and Room Temperature
Stress-Strain Properties. Tensile strength at break and elongation a t break at both room temperature and 200" to 212" F. increase substantially as the temperature of polymerization is reduced. As far as can be determined from the data available, the improvements are linear with reduction in temperature of polymerization from 50' t o - 10' C. T h e few data reported on GR-S made a t - 18' C. do not indicate t h a t further improvement in stress-strain properties would be obtained by reducing the temperature of polymerization below -10" C. The averages obtained are plotted in Figures 3 t o 6, and are given in Table 11. The percentage retention of tensile strength and elongation a t break when the temperature of testing is increased from room temperature t o 200' to 212' F. does not appear t o change when the temperature of polymerization is reduced from 50" t o 5" C. (Figures 7 and 8 and Table 11). Thus no appreciable improvcment in retention of stress-strain properties at high temperatures has been achieved. There is a slight indication, however, t h a t retention of stress-strain properties at elevated temperatures is improved somewhat by reducing t h e temperature of polymerization t o -10' and -18' C. It was impossible t o determine whether this was a statistically significant trend, and for t h a t
38 10 Reaction Temp.
- 10
- "C.
Elongation Retention at 200' to 212" F.
reason a dotted curve in this region is used to indicate lack of certainty as to the trend. Stress a t 300% elongation increases linearly as the temperature of polymerization is reduced (Figure 9 and Table 11). An analysis of the types of compounds used in the various investigations indicates that this Etress increase is due not t o compound changes employed but rather t o attributes of the polymers made a t t h e lower temperatures. Hysteresis Properties. Hysteresis properties were only slightly improved by reducing the temperature of polymerization from 50' t o - 18" C. The small amount of improvement appeared t o be linear with decreasing temperature of polymerization. Hysteresis properties were judged on the basis of measurements made by two general testing methods, heat development and rebound tests. The relationships between temperature of polymerization and the results of these two methods of estimating hysteresi. are shown in Figures 10 and 11 and Table 111. T h e comparison of heat development properties has been made on a relative basis, giving the 50" C. control arbitrarily a rating of 100.
Table 11.
Average Stress- Strain Properties
(Tests a t room temperature and 200' Temperature of polymerization, ' C . 30 40 30 Averanes for room temverat IGe Tensile strength, Ib./Bq. inch 2870 3310 3140 540 640 Elongation at. break, 7. 560 13 82 Number of tests 5 Stress a t 300% elongation, lh./sq. inch 1170 I350 s i i n 13 Number of tests 81 6 Ayerages for ZOOo t o 212' F. Tensile strength, Ib./aa. inch 1190 . . 1290 42 44 Retention, % 345 Elongation st'break, % .. 375 62 Retention, 70 .. 58 1 26 Kumber of tests
to 212' F.)
20
6
-10
-18
3720 590
3670 600 78
3660 600 31
3320 540
7
1270 31
1470 13
1650 45 445
1590
1430 7
1"?
..
1590 45
1440
..
62 2
370
10
42
370 62 17
73 17
13
48
385 73
10
150
1557
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
August 1949
1
80
=I 33
0 1
-.
50
-
ns .
- .
30
IO
Reaction Temp. Figure 10.
. _ -
50
30 IO Reaction Temp.
-io
- "c,
-10 "C.
-
Figure 11. Hysteresis-Rebound Tests
Hysteresis-Temperature Rise Tests
50 0
00
.-> 4,
Y
4,
oc
I . . 50
I
c
050° 1
0
0 1
ss 50
10
30
Reaction Temp.
'
50
SS -
-
1
.
-10
SS
SS
. .
.
I
30 10 Reaction Temp.
-
I
,
-10
"c.
Figure 13. Aged C u t Growth
"C.
24 hours at 212' F.
Figure 12. Rate of C u t Growth
A lower value in this case means that less heat was developed and the sample was considered superior. Statistical analyses of the trends in the case of the heat development and rebound tests show that improvement in heat development is not highly probable. The increased rebound observed, however, was highly significant on the statistical basis, though the magnitude of the improvement with reduction in polymerization temperature was small. Another factor that must be considered in deciding whether reducing the temperature of polymerization has basically improved the hysteresis properties of GR-S is the sensitivity of most hysteresis tests to modulus of the compounds. I n the case of many methods of testing hysteresis, better performance-lower heat development and higher rebound-is shown if modulus is increased by an increase in the state of cure (density of cross links). As shown in Figure 9, the stress a t 300% increases linearly t o a moderate degree with reduction in temperature of polymerization. This generally higher modulus level of the lower temperature polymers could be responsible for the improved hysteresis properties. If higher modulus is the reason for better hysteresis performance, no basic improvement in hysteresis properties of GR-S is caused by reducing the temperature of polymerization. Although reduction in temperature of polymerization has not yielded appreciable improvement in hysteresis properties, the general properties of the polymer are so improved that it is better able t o stand the effects of temperature rise. It should not be
Table 111.
Average Hysteresis Properties
Temperature of polymerization, O C. Heat development (relative) Number of tests Rebound, % Number of tests
50
40
100 53 49 38
96
9
50 8
30 108 1 47 1
20 78 7 54 3
5
93 41 53 32
-10 89 21 54
17
-18 72 13 53 12
01
I
50 Figure 14.
.
,
.
,
ns
IO Reaction Temp.
30
. . .
-
-10 "C.
Rate of Tread Cracking
concluded that production of GR-S types with greatly reduced hysteresis is not possible. Several variations in the method of manufacture of GR-S show promise of larger improvements in hysteresis than those obtained by reduction of polymerization temperature alone, Cracking Resistance. Both laboratory cut growth tests, on unaged and aged stocks, and extent of cracking in tire tread tests show that cracking resistance is improved greatly by reduction in the temperature of polymerization (Figures 12 t o 14). I n the analysis of the data on laboratory cut growth, i t was found convenient t o convert all data t o a "rate of crack growth" basis. Lower values for the polymers made a t low temperatures mean that the lower temperature polymefs are more resistant t o the growth of cracks. As in the case of heat development, the 50" C. control is arbitrarily given a rating of 100. It is believed that the improvement in cracking resistance is actually somewhat better than is apparent from the curves by virtue of the sensitivity of cut growth rate to modulus. Cut growth rate increases rapidly as the modulus of the compound is increased. Modulus increases linearly as the temperature of polymerization is reduced. Had the lower temperature poly-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1558
Vol. 41, No. 8
cv I
2
-
X Y)
E 0)
I-
2o
1
ss .
.
50
.
I
.
ss .
.
.
-10
30 10 R e a c t i o n Temp, Figure 15. Tensile
20
Figure 16.
c al V L
0)
Q
30
10 Reaction Temp.
-10
- "C,
Figure 17. Tensile Retention Aged 24 hours at 212" F.
mers been compounded to give the same state of cure a3 the control, the relative cut growth rating would have improved even more. I n reviewing d a t a on rates of cut growth as obtained in laboratory testing, it was found t h a t some investigators det,ermined cut growth rate a t about 200" E'. whereas others used a room t'emperature test. I n the initial analysis of t.he data, average rates for the two temperatures were calculated separately. However, it was apparent t h a t the relative rat,e of cut growth obtained in a 200" F. test was, within experimental error, the same as t h a t obtained in the room temperature test. Therefore, in order t o simplify the presentation, the average obtained by combining t'he high temperature and room temperature tests was uscd (Figures 12 and 13). The similarity in results can be seen from the detailed data in Table IT'. The unaged and aged laboratory cut growth tests agree qualitatively with the results of tire tests. Comparison of the relat'ive rates obtained from laboratory testing. (Figures 12 and 13) wit,h those from the tire tests (Figure 14) shows that. t,he laboratory cut growth rating after aging 24 hours a t 212" F. more nearly predicts the road test results.
Table IV.
Average Relative Cracking Rates
Temperature of polyinerization, O C. Unaged, laboratory Tests a t room temp. (ielati7e) Tests a t 200-212° F. (relative) Over-all average (relative) S u m b e r of tests Aged 24 hours at 212' F laborat.ory Tests a t room temp. (rklative) Tests a t 200-212" F. (relative) Over-all average (relative) Number of tests Tire tests R a t i n g (relative) Number of testsa One test represents average result
50
40
100 100 100
..
46
100 100 100 24
30
ii3 6
..
5
43 60 i3 4 1 4 38
77 7
133
20
43
77
.. .. .. .,
83 72
72
85 3
83 2
7B 73 16
-10
-18
05
38
65 47 41 10
66
.. .. .. ..
42 20
64 11
50 14 33 24 20 2 1 .. 2 from 3 and in some cases 4 tires. 100
Elongation
Aged 24 hours at 21z3 F.
1
50
-10
- "c.
R e a c t i o n Temp.
Aged 21 hours at 2 1 2 O F.
Y
IO
30
50
- "c.
As far as can be determined from t,he available data, c u t growth resistance is a linear function of the t,emperature of polymerization except possibly in laboratory testing of unagcd stocks. The flattening of the curve betLwen 20" and -10" C. for unaged laborat,ory results (Figure 12) should not necessarily be interpreted as meaning that, no further improvements in unaged cut growth resistance are being made as the tcmperature is reduced below 20" C. It appears much more likely t h a t the flattening of the curve in this region is due t o an inability of the tests used to pick up further iinprovements-that is, the rates have become so low t h a t differences in rates cannot be measured with any precision. Bupport,ing this argument, aged laboratory cut gro1Tt.h follo~rsa linear pat.tern, the possible test inadequacies having been removed because the rates of cut growth of the better samples have been increased materially by aging. Aging Resistance. Some tests, but not all, show a gradual decrease in the aging resistance of GR-S as the temperature of polymerization is reduced. This conclusion is based on analysis of results obtained after aging 24 hours a t 212" F. in air.
Table V.
Average Stress-Strain Properties after Aging 24, Hours at 212" F.
Temperature of polymerization, C. Tenqile strength, lb./sq. inch Retention, % Elongation a t hreak, 70 Retention, T Number of tebts Increase in stress a t 200% elongation Average, lb./sq. inch Number of tests
50
40
30
20
5
-10
-18
2,580
2x10
2880 100
3290 94 400 67 7
3190 90 300 63 31
8230 87 410 73
2680 06
500
500
400
91 360 61 40 460 39
a5 3O : F3 9 520 8
410
63 2
.
,. .
a
33
12
12
410 76 2
. .
Tensile strength and elorigat'iori a t brcak aftcr aging 24 houi.s a t 212" F. (Figures 15 and 16) show the same linear increase with reduction of polymerization temperature as do unaged tensile and elongation. The percent'age retent'ion of tensile strengt,h and elongation at break after aging, however, does not change as the polymerization temperature is w r i e d (Figures 17 and 18). Thus from the point of view of retent'ion of these physical properties, there has been no basic change in quality due to reduced temperature of polymerization. As judged by increase in stress a t 200y0 elongation, or "marching modulus," reduction in the temperature of polymerization has produced a polymer less stable t o oxidation (Figure 19). This increase in stress a t 200% elongation on aging may be due t o basic structural changes or simply to pro-oxidant impurities, such as iron, in the lower temperature polymers. Increase in modulus on aging has been looked upon as a basic deficiency in GR-S type synthetics because of its unfavorable effect on import,ant practical properties such as cracking. From
August 1949
80
:X8 {
Y
c
L
1559
INDUSTRIAL AND ENGINEERING CHEMISTRY
60.
Q)
40. 55
50
-
10
30
Reaction Temp, Figure 18.
-10 "C.
30 IO Reaction Temp,
50
Elongation Retention
Figure 19.
Aged 24 hours a t 212' F.
-10
- "c,
Increase in 200Yo Stress
Aged 24 hours at 212' F.
the data available it appears that reduction in the temperature of polymerization has not remedied this deficiency and probably
9
'
has made it worse. A comparison of the relative improvements as judged by unaged and aged laboratory cut growth tests also indicates poorer aging resistance for the lower temperature polymers. For easy contrast the curve for unaged cut growth (from Figure 12) has been indicated by the dotted curve in Figure 13. It is apparent t h a t aging damages the cut growth resistance of the lower temperature polymers much more than it does t h a t of the 50' C. control. At least part of this damage t o aged cut growth resistance can be traced back t o the increased marching modulus of the lower temperature polymers. With the data available i t was impossible to determine whether the increase in marching modulus was the major factor in this loss of cut growth resistance on aging. As in the case of processing, these disadvantages in aging may be a result of inadequate stoppage of the polymerization reaction, rather than basic structural changes in the polymer. More experience in the manufacture and utilization of these lower temperature types of GR-S may partially or completely eliminate these disadvantages. Tread Wear Resistance. Tread wear resistance shows a large and highly significant linear increase as the temperature of polymerization is reduced from 50" t o -10" C. (Figure 20). The magnitude of the improvement is of the order of 25 to 30% at polymerization temperatures of 5" t o -10" C. The wear tests included both passenger and heavy service tires, the majority being passenger. I n a preliminary analysis, data on passenger and heavy service tires were considered independently but, within experimental error, the two types of tires gave substantially the same relative wear ratings. Thus the two groups of data were combined for plotting in Figure 20. T h e detailed averages are given in Table VI. Other Properties. Though no attempt was made t o obtain detailed data on the effect of reduction in the temperature of polymerization on the following miscellaneous properties, t h e general conclusions about some of these are: Increase in temperature of polymerization from 50" to 100" or 130" C. results in further reduction in quality as measured by stress-strain properties. Reduction of polymerization temperatures below - 18" C. does not give additional major improvements. However, very few data have been obtained.
. .
0
50 Figure 20.
ss
I
, IO
30 Reaction Temp.
,
, -10
I
- "c.
Resistance to Road Wear
The gum tensile properties of latex stocks are improved materially by a reduction in the temperature of polymerization. The tensile properties of milled gum stocks on the other hand have shown only minor improvements of no practical significance. When mineral fillers or nonreinforcing carbon blacks are used, some improvement is obtained in practical properties as polymerization temperatures are reduced. The low temperature properties of GR-S containing 20 to 3070 of styrene become slightly poorer as the temperature of polymeiii zation is reduced. Changes in quality similar to those shown in this review--e.g., tensile, cracking, wear, etc.-are obtained in polymers containing less than 20% styrene and in polymers containing other comonomers in place of styrene.
Acknowledgments This survey was carried out under the sponsorship of the Office of Rubber Reserve, Reconbtruction Finance Corporation, in connection with the government synthetic rubber program. T h e author wishes to thank the Offlce of Rubber Reserve for permission t o publish these results. The assistance of R. J. O'Brien and G. S. Mills of the U. S. Rubber Company in surveying reports and analyzing data is gratefully acknowledged.
Literature Cited (1) Dinsmore, R. P., and Fielding, J. H., India Rubber World, 1198 457 (January 1949). (2) Drogin, I., Bishop, H. R., and Wiseman, P., Rubber Age, 64, 309 (December 1948). (3) Howland, L. H., Messer, W. E., Neklutin, V. C., and Chambers, V. S., Ibid., 64,459 (January 1949). (4) Office of Rubber Reserve, Reconstruction Finance Corp., reports submitted by U. s. Rubber Co. and other organizations
cooperating in government synthetic rubber program. Individual references are not given in order to avoid disclosure of information that may be considered confidential by participating organizations. ( 5 ) Sperberg, L. R., Bliss, L. A,, and Biard, C. D., Phillips Petroleum Co., Philblack Bull., 15 (December 1948). (6) Sperberg, L.R., and Svitlik, J. F., India Rubber World, 119, 195 (November 1948). '
Table VI.
Average Wear Resistance
Temperature of polymerization, O C. ' Passenger tires (relative) Heavy service tires (relative) Over-all average (relative) Number of tests" a One test is average value for 3
50
40
30
20
5
-10
100 100 100
111 107
103
120
119
121 119
123 127 126
64
10
132 io3 126 54 3 2 6 and in some cases 4 tires.
10s
RECEIVED May 26, 1949. States Rubber Company.
Contribution 93, General Laboratories, United
