QUALITY OF CO ITS ADAPTA J. H. Fielding' The Goodyear Tire & Rubber Company, Akron, Ohio Cold rubber is an improvement over GK-S in most of its laboratory properties. I t should make better wearing tires in those sizes now made w-ith synthetic trcads and possibly in some of the tires made with natural rubber treads. It is not expected that cold rubber will be used in larger tires. at least for general service, nor in carcasses. Cold rubber comes nearer to the desired goal for synthetic tires than does GR-S. I t is not, however, the equal of natura1 rubber and cannot be expected to do away with the need for natural rubber.
T
HE qualities of cold rubber which are important in the manufacture of tires have been reviewed critically to evaluate the usefulriess of this rubber as compared to natural rubber and standard GR-S. The term cold rubber aq used here refers to a copolymer of butadiene arid styrene in the standard monomer ratio, made in a redox system a t 41" F., t o a conversion of about BO'%, and an average viscosity (Mooney-large rotor) of 55. Because many individuals and corporations as well as the Office of Rubber Reserve have made important contributions to the development of this polymer, there are a host of recipes, differing in seemingly minor respects, which have not been thoroughly evaluated against each other. This work w-as based on several of these recipes but no distinction was made among them,
Physical Properties Standard GR-S is a relatively inferior material. I t differs from natural rubber in many respects and these differences can be demonstrated in the laboratory. I t is of interest now to see in what respects cold rubber is superior to GR-S and in what way its properties approach or surpass those of natural rubber. Table I contains typical test data comparing natural rubber, standard GR-S, and cold rubber. The compounding formulas are not important, except that where reference is made t o a gum mixing, no reinforcing pigment is used and where reference is made to a channel black stock, 50 parts of easy processing black were used, In all cases the acceleration was such that a satisfactory state of vulcanization was obtained a t the same temperature in the same time. The first group of properties designated "strength" has to do with the resistance of the material t o rupture under several sets of conditions. I n the nonreinforced gum stock, GR-S has only about one tenth the tensile strength of natural rubber. It has been suggested before that the tendency of stretched rubber to crystallize may strengthen it in the direction of stress. The low tensile of GR-S may then be ascribed to its lack of any tendency to crystallize. Cold lubber is better than GR-S in its gum tensile but by only a barely measurable amount. When reinforced by high quality channel black the tensile of natural rubber is increased t o slightly over 4000 pounds per square inch when measured a t room temperature. GR-S under the same conditions is improved t o nearly 3000 pounds per square inch. Cold rubber, on the other hand, is considerably better than G1Z-S and neaily as good as natural rubber in this respect. One of the striking weaknesses of standard GR-S is its tendency to lose strength at higher temperatures. This is not a matter of degradation with time (aging) but purely a lack of strength at 1
higher temperatures. Natural rubber loses only about 25% of its tensile in going from room temperature to 200" F. Both GR-S and cold rubber lose nearly 66y0of their tensiles, dropping to 1100 and 1500 pounds per square inch, respectively. At higher temperatures rubber increases in its ultimate elongation also whereas both GR-S and cold rubber decrease. I n a gum stock or in a loaded stock a t 200" F., the dirference in tensile strength between GR-Sand coldrubberisnotsignificant, and both are infeiior to natural rubber. However, at room teniperature and in a reinforced compound, cold rubber approaches the tensile of natural rubber and is outstandingly better than GR-S. The properties which represent flex life have to do with rupture under repeated stresses, either in stretching or bending, and relate to trcad cracking and radial cracking of sidewalls. Both GR-S and cold rubber are noticcably poorer than crude although in every case cold rubber has a definite but moderate flex life superiority over GR-S. The tendency of GR-S and cold rubber t o Jveaken a t higher temperatures is evident so that although cold rubber may improve in its comparison with GR-S a t 200" F., i t suffers in its comparison with natural rubber a t this temperature. In this group of flex properties cold rubber seems to be better than GR-S but both are poorer than natural rubber by a wide margin. Abrasion data are reported because of their relation to wear data. The abrasion test must be thoroughly understood to be of value. I n this case, it was affected so much by differences in fatty acid contcnt that the effect of the polymer is masked and does not correlate with road experience. ~~~~~~~
~
Table I.
~
~
~
ComDarison of GR-S and Cold Rubber with Natural Rubber Natural Rubber
GR-S
Cold Rubber
2100 800 180
200 310
250 40?
4
J
4100 3100
2900 1100
3800
600 680
600 480
650 550
250 300
400
250
500 300
175
50 60 35 200 21 5.8
60 90 70 400 30 7.8
77
68
74 81
55 64 71 147
67 66 66 155
STRENGTH Gum mixing Tensile (room temp.) Elongation (room temp.) Tear resistance (200' F.) Channel black mixing Tensile Room temp. 200° F, Elongation Room temp.
ZOOo F.
Tear resistance Room temp. 200' F. Flex life" 667a perforated (room temp.) 35% c u t strip (room temp.) 20% cut atrip (2000 F,) P.G.b (room temp.) P.G: (200' F.) Abrasion loss (angle)
-100
500 1200 700 6 5
1500
RESILIENCE
Gum mixing % Rebound Room temp. ZOOo F. Channel black mixing % Rebound Room temp. 2000
88 90 71 81
F.
Goodrich Flex A t Goodyear Flex A t
28 '
71
I n all flex tests figures are minutes required t.0 produce a standard desree of growth. P.G. refers t o a De?rfattia type test in which the groove is pierced by a needle. a
*
Present address, Armsti ong Rubber Company, West Kaven, Conn.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Several properties related t o resilience are shown. They are important in a tire as a less resilient compound will consume more energy as the tire rolls; it will deform less readily and will be dynamically harder, tending t o put more burden on the fabric of the tire or a t the points where one component adheres t o another. It will develop more heat which will increase the running temperature of the tire, resulting immediately in less fabric strength and less resistance of the compounds to rupture, and resulting after a time in accelerated degradation of both the fabric and the compounds. Experience also tells us that tire wear is faster a t higher temperature. In this important property cold rubber is only moderately better than GR-S, and it is here that cold rubber makes its weakest bid as a replacement for natural rubber. I t s advantage over GR-S in resilience is greatest in a gum stock, but in a more practical loaded compound, the difference almost disappears. Optimistically it might be said that cold rubber has traveled 25% of the distance from GR-S t o natural rubber in this respect. Resistance t o weathering was demonstrated by static exposure outdoors, kinetic exposure outdoors, and static exposure to dilute ozone (sufficiently concentrated to cause cracks to appear in the rubber in 1 minute). I n all cases the tapered strips were stretched 15% over-all so that the range of elongation was from 10 to 2201,. The kinetic strips were stretched 60 times per minute. The data indicate generally that both synthetics have less resistance than natural rubber and cold rubber less than GR-S, but the tendency to crack a t low elongations is greatest in natural rubber and least in GR-S. Also, in the ozone test the first cracks appeared in rubber over the whole range of elongation in about 1 minute whereas no cracks appeared in either synthetic for 3 minutes. Summarizing the basic comparisons of these polymers, cold rubber appears better than GR-S in most important laboratory properties. Compared to rubber, however, both synthetics are weaker in their resistance to various types of rupture, particularly a t high temperature, and both are less resilient and more prone to generate heat.
Tread Wear A road test is usually considered a more reliable indicator of tire quality than any group of laboratory tests. This is particularly true of tread wear which is easy t o determine on the road but difficult in the laboratory as there is no test which correlates t o a very high degree with road wear. Table I1 shows results of a group of 15 separate road tests involving a comparison of cold rubber with GR-S; compounding changes were made only when necessary to get the proper state of cure. Each test included 2 two-way tires making a total of 30 tires contributing to the average wear rating of 123. I n work of
Table 11. No. of Tiresa 2 2 2 2 2 2 2 2 2 2
2 2 2 2 2
Tread Wear Comparison of Cold Rubber and GR-S Size 6.00-16
7.10-15
Miles 4,823 2,575 3,726 3,735 5,408 5,408 4,612 6,241 15,072 7,001 2,952 6,241 6,002 18,089 9,050
Location Akron
Akron Texas Akron Texas
Inches Loss Cold rubber 0.188 0.095 0.154 0.123 0.136 0.141 0,137 0.148 0.168 0.174 0.077 0,142 0.065 0.175 0,136
GR-S 0.224 0.128 0.162 0,165 0.174 0,180 0.173 0.175 0,199 0.209 0.104 0.167 0,075 0.219 0.168
Cold Rubber Rating 119 135 105 135 128 128 127 118 120 120 135 118 115 126 123
Average 123 All tires were two-way. The loss i n design depth is t h e average of all grooves and is averaged for two tires. a
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this kind, there is usually some experimental error. A part of the error is chance and can be minimized by running a large number of tires. Another part is systematic in the sense that the tires built a t one time may tend t o give one average and those built a t another time may tend to give a slightly different average. In the present work both types of error should be low since many tires and many distinct tests are involved. The averages of the tests varied from a low of 105 to a high of 135 with an average of 123. This means that under the conditions of the test, cold rubber should give 23% more miles than GR-S for the same amount of wear. The development of high abrasion furnace blacks has been roughly concurrent with the development of cold rubber. The blacks and the polymer have been so closely associated that it is sometimes assumed that the good quality of cold rubber can be achieved only with high abrasion furnace black and vice versa. I n Table I1 no high abrasion black was used. The quality of high abrasion black in natural rubber is shown in Table 111; twelve separate tests were run comparing high abrasion, or a mixture of high abrasion and easy processing blacks, with easy processing black alone. The improvement in wear resistance of 11%is substantial.
Table 111. Tread Wear Comparison of Blacks in Natural Rubber Ratings
Inches Loss
NO.
of Tircs'
Size 7.00-15
Miles 5,000 17,000 13,298 5,534 7,200 12,000 6,,503 19,000 5,000 7,400 6,353 12,000
Location Akron Texas Akron Akron Akron Texas Akron Texas Akron Akron Akron Texas
onCon-
50 parts
EPC
50 parts
HAF
25 parts EPC and 26 parts
HAF
trol black black blacks 110 122 0.194 1 100 110 0.261 '1 100 95 b4 0.118 2 7.00-15 100 129 112 0.176 2 100 116 .. 0.220 2 100 107 .. 0.211 2 119 0,170 100 2 100 , . Ib'l 0.214 2 100 104 .. 0.228 1 100 100 .. 0.216 2 100 122 .. 0,157 2 100 101 .. 0.135 2 Averages 100 111 102 a All tires were two-way. number of tires refers t o each experiment-for example line 3 represents four tires. b EacA rating is t h e average of t h e number of tires indicated.
..
I n the same way various intercomparisons of easy processing, high abrasion furnace blacks, and blends in GR-S are shown in Table IV. I n analyzing these data it must be assumed that a blend of blacks will give the same result as the average of the individual blacks. Experience has indicated that this is not far from the truth, and the assumption appears reasonable in the light of present data. All the data in Table IV, whether single blacks or mixtures, result in an average rating of 111for high abrasion furnace blacks against 100 for easy processing blacks in GR-S. Finally a similar comparison of blacks in cold rubber is shown in Table V. Here the number of tires tested was not sufficient t o give an unrefutable rating, the results average 114 for high abrasion furnace black as against 100 for easy processing black. This average includes a lower rating for high modulus furnace blacks than is usual and it could be concluded, in view of this, that a higher value than 114 should be used for high abrasion black. However, other data (not reported here) indicate that insufficient data were available for this comparison and that a reasonable conclusion is t h a t high abrasion furnace black improves wear whether the medium is natural rubber, GR-S, or cold rubber and that the degree of improvement is substantially the same in each of these stocks. This phase of the work, then, has demonstrated first the superiority of cold rubber over GR-S, and secondly the superiority of
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INDUSTFIAL A N D ENGINEERING CHEMISTRY
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keep failures due to these other hazards a t a low level. Many of these other hazards are influenced by the properties of the coinpounds in the tire. It is conceivable that a particular compouqding change nught produce better resistance t o wear, but a t the same time make the tire vulnelable t o these othei hazards, so that fewer tires could attain their complete mileage.
1
Figure 1.
2
3
5
4
6
Kinetic arid Static Weather Checking
Kinetic:
1 = natural rubber, 2 Static: 4 = natural rubber, 5
= =
GR-S. 3 = cold rubber. GR-S, 6 = cold rubber
high abrasion fuinace black3 ovei the conventional blacks in iesistance t o tread wear. A4ddingt o these ratings a value for rubber which is based on data beyond the scope of this paper, ratings showing roughly what may be expected in ear with these vaiious combinations axe as follows:
S a t u r a l rubber
C'aihon B h r k , Pal ts I50 EPC
Rating 100 111 90
1
2
3
Figure 2.
4
5
6
Ozone Cracking
20-rninut~exposure: 1 = natural ruhber, ? ' = GR-S, 3 = cold rubber. 10-minute exposure: 4 = natura1 rubber. 3 = GR-5, 6 = coldrubher
Tread cracking is one of these hazards. I t is a complicated phenomenon, but fatigue and temperature seem t o play a large part GR-8 85 in it. Ozone may be involved in the initiation of cracks but prob100 ably a greater number are started by cuts. 9 compound which 150 EPC 111 Cold rubber 25 H I I F 104 has better flex life should crack less as a tread. Anything that 123 makes a tire run hotter should make cracking worse. It is quite There is no question, therfore, that cold rubber can provide betclear from the discussion of the physical data that cold rubber ter wear than has been experienced with GR-S and that there is a should be better than GR-S in this respect, It has better flex life chance of better wear than has been experienced with natural and better resilience, two contributing causes of tread cracking. rubber, but there remains a question as to what extent this imHowever, cold rubber is poorer than natural rubber in both of provement in wear be realized. these respects and, therefore, mould be expected to tread crack more than natural rubber. Adaptability to Tires Sidewall cracking (black tires) seems to be a complicatcd property of fatigue resistance and ozone resistance, and it is probably Most tires wear out, They are designed to do this because a modified to a large extent by oxidation, particularly as catalyzed failure from any other cause means a loss in the potential mileage by light. It seems contradictory that GR-S, which has less fatigue of the tire. However, there are many other hazards to which a resistance and less apparent ozone resistance than natural rubtire may be subjected and the tire manufacturer does his best to ber, would produce a sidewall which would crack less. However, this is the case. A possible explanation might be that GR-S is more resistant to Table IV. Tread Wear Comparison of Blacks in GR-S crack initiation. Both crack initiation and crack Rating b growth are influenced by ozone. Although GR-S 25 25 25 cracks more than rubber in ozone, it nevertheless parts parts parts EPC H A F EPC does not start, t>o crack as soon. Moreover a and and and Inches 60 50 25 25 25 greater elongation is required t o crack GR-S than KO. Loss parts parts HMF parts HMF parts rubber. I t may be that these two effects are of Locain EPC parts HAF HAfF Tiresa Size Miles tion Control black black black black black enough to offset the poorer flex life of GR-S and 3 6.00-16 12,000 Texas 0.178 107 100 103 give it greater resistance to cracking than rubber 2 7.00-16 8,400 Akron 0.190 Ib'O 117 .. .. .. .. under the conditions existing in a tire eidervall. 2 12,500 Texas 0.177 100 110 2 3,109 Akron 0.126 ,. 152 l'd0 ff7 1'2'8 If this is the explanation, it ~ o u l dbe reasonable 2 7,109 Akron 0.131 .. 101 100 105 111 2 6,281 Akron 0.174 . . 131 100 105 123 t,o predict that, cold rubber will be poorer t.han 2 7,293 Akron 0.180 . . 116 100 102 119 GR-S in this respect, but that i t might still be 4 6.70-15 7,162 Akron 0.166 , . 100 08 .. 2 7.10-15 2,100 Texas 0.086 100 109 ., better t.han rubber. 2 1,536 Akron 0.047 .. 108 100 113 fO5 2 6,384 Akron 0.141 ,. 110 100 103 114 Fabric fatigue is a complicated failure which is 2 0,200 Texas 0.153 .. 106 100 107 117 usually attributed to the fabric, but certainly Averages 100 112 .. arb See Table 111. 116 100 f05 117 lack of resilience of the carcass compound contributes to it, A great penalty in regard to resili150 H.IF 50 EPC
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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1563
Extent of Use in Tires Table V.
Tread Wear Comparison of Blacks in CoId Rubber Ratingb 25 26 parts EPC
2 2 2
7.60-15
a,b
See Table 111.
7084 6000 6000
Akron
0.221 0.166 0.166
.. 1'3'5
100
100 100
107
.. ..
ence would be paid in changing from a natural rubber to a cold rubber carcass. Ply separation is another failure which has some connection with the characteristics of the compound in the tire. It is influenced greatly by a certain balance in dynamic properties between various components of the tire, but assuming that this balance is satisfactory, the strength of the compound a t the point of separation and at the temperature generated by the tire is important. Although better than GR-S in these respects, cold rubber should be poorer than rubber by a wide margin. The effect of resilience has been mentioned in connection with the influence of tire temperature on several types of failure. It has other aspects also. Lower resilience increases the rolling resistance of a tire and as a direct result gasoline consumption goes up. Although the maximum effect produced by a tire does not seem great, it is nevertheless considered important by automotive engineers. Furthermore, a tire containing nonresilient compounds is definitely harder riding and lacks the extreme cushioning demanded in the cars of today. Again, in these qualities related to resilience, cold rubber should be superior to GR-S but it is still inferior to natural rubber. The resilience of the synthetics may be even lower than expected. It will be recalled that neither synthetic attains any degree of strength without reinforcement by carbon black. Therefore, cold rubber in a carcass stock must have more black or finer black than rubber to provide the desired degree of strength. This places both synthetics at a great disadvantage in resilience as compared with natural rubber. It has been shown that tires with GR-S treads tend t o skid more than rubber tires under some conditions. The difference is not large, but it is measurable. Preliminary experiments have indicated that cold rubber is no better than GR-S in this respect.
Laboratory Low Temperature Polymerization Polymerization flasksare immersed in bath containing ethylene glycol antifreeze and operated at 1 4 O F. COURTESY PHILLIPS PETROLEUM COMPANY
There is no question as to the superiority of cold rubber over GR-S in its resistance to wear and in most of its other properties. It is superior to crude rubber in wear but inferior to it in most other properties. The high abrasion type of black is a better wearing black and its contribution to improved wear can be attained in rubber, iii in GR-S, or in cold rubber. Likewise cold rub.. ber can be used with conventional black or i t can be combined with high abrasion black t o give a greater improvement in wear. Complete replacement-of GR-S by cold rubber wherever GR-S is used in treads should produce a tire which is better in wear and possibly better in resistance to all other hazards. The replacement of GR-S by cold rubber in sidewalls is questionable and probably would not produce a better tire. Larger tires, a t present starting with the 7.10 size, have natural rubber treads. Added resistance to wear could be obtained by using cold rubber with or without high abrasion black. On the other hand, the matter of tread cracking and all the other hazards to which a tire is subjected become of serious concern. It is common experience that failures of this type tend to be more prevalent in the larger tires. A tread compound which gives satisfactory wear and is just passable on heat generation in the 6.70 size might give satisfactory wear in the 8.20 but would fail hopelessly from cracking or fatigue in a larger size. It might be argued t h a t if GR-S is satisfactory as a tread on the 6.70 from the standpoint of these other hazards, cold rubber, which is better than GR-S in these respects, should be satisfactory for the 7.10 size. This is probably so, but should be proved. For still larger sizes it becomes increasingly less probable that a cold rubber tread would be satisfactory. Most tires now have natural rubber carcasses. I n all of its qualities, cold rubber is poorer than natural as a carcass rubber. It is entirely possible that some use could be made of cold rubber in the smaller tires but it would not make a better tire. 26
Acknowledgment The author wishes to thank R. P. Dinsmore, vice president of The Goodyear Tire & Rubber Company for his interest in the subject and for permission to publish the results of this work, RECEIVED May 26, 1949.
