Processing effects on NR aging characteristics.

The aging characteristics of natural rubber (NR) under high temperature conditions are well known, but it has been observed that the aging characteristics of NR compounds mixed in the latex phase are different from those from NR compounds mixed using traditional dry mix technology.

This article discusses the differences in aged performance between latex-phase NR compounds and dry mixes in a number of key performance areas, which include tear, flexing, wire adhesion and dynamic properties. These properties were tested on similar dry mix and liquid phase mixed NR compounds featuring a range of cure systems.

In general, NR compounds mixed in the latex phase have improved heat resistance and result in better heat-aged performance over the range of compounds, compared to equivalent solid-phase mixed compounds. This may be due to differences in crosslink density, molecular weight and polymerfiller interaction during the mixing process, which results in a more thermally stable compound. The practical implication of these benefits is an improvement in part life in various applications including tires, vibration isolation parts and molded goods.

Thermo-oxidation in NR

Since the early days of the rubber industry, there have been innumerable studies to understand the mechanisms and effects of aging on NR. Thanks to the efforts of industry pioneers such as Hoffman and Spiller, Bevilacqua, Bateman and Watson, and Shelton (refs. 1-3) there is substantial agreement on the mechanisms and effects of thermo-oxidative degradation on the world's most widely used elastomer.

The effects of thermo-oxidative degradation are generally accepted to occur due to the following mechanisms:

A. In the raw or uncured state, fundamental polymer units of polyisoprene are particularly susceptible to atmospheric oxidation because the fundamental units of these polymers contain C-H and C = C groups, which are vulnerable to attack by alkylperoxyl radicals (ref. 4).

B. Chain scission--all hydrocarbon polymers undergo scission as a consequence of thermal oxidation. Thermal oxidation of NR is always accompanied by scission, and the formation of volatile oxidation products (ref. 4).

C. NR vulcanizates are subject to thermal and oxidative breakdown. In addition to the vulnerability of the polymer chain, thermal decomposition is influenced by changes in the crosslink structure (mono, di- and polysulfidic crosslinks) and changes in the crosslinking characteristics that occur during thermo-oxidative conditions, especially during service (ref. 5). In addition, sulfurization may occur in the main polymer chain, causing poor resistance to cut growth and fatigue.

An entire industry has grown up around remediating the deficiencies caused by these aging characteristics. Use of amine-type antioxidants prevents peroxide initiation by decomposing hydroperoxides to form stable products, rather than flee radicals (ref. 3). Use of certain mercaptans of sulfonic acids and alkyl sulfides are capable of decomposing peroxides and other products of polymers, forming stable compounds (ref 6). Nickel and zinc chelates of dialkyl dithiophosphoric acids are capable of scavenging alkylperoxy radicals during hydrocarbon autoxidation (ref. 4). And, modified EV, semi-EV and sulfur donor vulcanization systems are routinely used to adjust the ratios of the various crosslink structures to provide improved performance.

Thermo-oxidation and compound performance

Bateman and Watson best described the effects of these thermoxidation processes on compound performance: "Vulcanization represents the conversion of the chain-like polymer into a crosslinked network, but the nature, number and distribution along the chain of the crosslinks, and their variation with curing conditions and subsequent aging, are largely matters of speculation. In fact, one of the major impediments to scientific rationalization of rubber technology is the inability to interpret common physical observations, e.g., on plasticity, state of cure, deterioration on aging and cracking, in terms of molecular structure" (ref. 2). Over the decades, rubber technologists have learned empirically how to develop formulations and materials that give commercially desirable properties that function in applications.

Compound performance, as related to thermo-oxidation, is difficult to predict theoretically due to a number of important factors, not the least of which is that the fundamental problem is that the two mechanisms (A) and (B) above have effects on the finished compound that are in the opposite direction as mechanism (C). In addition, there are a number of other factors that have dramatic effects on thermo-oxidation. The presence of impurities such as iron, copper, manganese and cobalt have a catalytic effect on oxidation. The use of carbon black as a reinforcing material is known to dramatically increase compound oxidation, due to the presence of peroxide groups on the carbon black surface (ref. 6). Polymer chain scission and auto-oxidation are also opposite in effect to storage hardening, which is a phenomenon of NR well known to technologists, and which greatly handicaps efforts to measure the effects of oxidation during processing (ref. 7).

In addition, various studies, as well as much of the empirical experience noted above, have suggested that variation in compound processing, testing method and conditions, operating conditions in the application and such issues as sample and flaw geometry, compound properties such as dispersion, variations in rubber source and crystallinity, part design and variations in service exposure may all affect part life (refs. 8-11).

The processing effects on aging properties in NR are particularly interesting, and are the focus of this research effort. Two papers from the early days of industrial-scale robber production are important in this respect: Hastings (ref. 10) noted with interest that the technique used in drying the NR latex had a significant effect on long-term product degradation. A difference existed between air-dried and smoked sheets. Vulcanizates were prepared from smoked sheets and air-dried sheets, exposed to oven aging for 72 hours, and tested for tensile strength. The smoked sheet specimens retained approximately 80% of their tensile strength, compared to approximately 20% for the air dried. Dufiaisse and Viellefosse studied the effects of mastication on the oxido-aging of rubber (ref. 11). In this study, it was determined that the tendency for raw robber samples to show oxidation effects was influenced greatly by milling conditions. Various samples of latex were coagulated and milled under various conditions, then samples were dissolved in a solvent and the resulting solution was measured for viscosity over various periods of time to determine oxidative effects. In this experiment, milling under "cold" conditions had little effect on the rate of oxidation. However, milling under "hot" conditions (80[degrees]C) had an effect on the order of 24 times the rate of oxidation. The effect was diminished considerably when the sample was tested in a vacuum, i.e., outside the presence of oxygen. The researchers concluded that mastication indeed increased the oxidizability of the raw rubber, and noted that mastication in air and heating had an effect independent of one another, that can multiply the effects (ref. 11).

Practical implications

Since the time of these experiments, however, non-technical factors (such as economics) have resulted in the development of rubber industry practices as follows:

* About 75% of all natural rubber commercially used is airdried;

* carbon black, known to be a significant contributor to oxidation in NR, has become the dominant reinforcing agent in the industry: and

* mastication is an essential step in the rubber manufacturing process in order to facilitate compound forming and downstream processing. In particular, hot mastication in air has been utilized in internal mixing technology so as to attempt to disperse the carbon black, whose useful properties of compound reinforcement are undeniable. This process is energy-intensive, and complete dispersion is difficult to completely achieve: and it has caused a variety of hygiene issues with which we are all familiar. Notably, the use of peptizers to promote viscosity reduction and ease of downstream processing give the precise opposite of the desirable properties of long-term polymer chain stability that enhance some aspects of compound performance.

Since these industry practices were entrenched, rubber technologists have learned through the use of various antioxidants and other additives to produce functional, but probably sub-optimal compounds.

Natural rubber elastomer composites (EC)

The elastomer composite (EC) process is the first truly continuous commercial process for mixing carbon black and natural rubber in the liquid phase. In this process, a carbon black slurry is introduced under shear conditions into a stream of field latex. This allows the simultaneous coagulation of the latex and dispersion of the carbon black. The coagulation and dispersion occur at ambient temperature. The compound is de-watered and mechanically dried, and other compounding ingredients may be added continuously. Accelerators and curatives are added in a conventional mixer (figure 1).

[FIGURE 1 OMITTED]

Compounds produced using the elastomer composite process have the following characteristics:

* Excellent carbon black dispersion, much better than normally experienced in conventional dry-phase mixing (figure 2). This characteristic is independent of carbon black type.

[FIGURE 2 OMITTED]

* Excellent distributive mixing, in which the distribution of the carbon black in the polymer matrix is much more uniform than in dry mixing (figure 3).

[FIGURE 3 OMITTED]

* Molecular weight preservation--since the dispersive process happens in the liquid phase and the compound is subjected to a minimum of shear force during mixing and de-watering, elastomer composite materials of a given loading have greatly higher polymer molecular weight than equivalent dry mixes, while retaining the dispersion advantage.

* Intimate interaction between polymer and filler. Bound rubber values for elastomer composite compounds are typically 20-50% higher than for equivalent dry mixes.

* Efficient, low temperature mixing is possible with minimal mastication.

Various observations have indicated that the long-term heat aged mechanical properties of elastomer composites are superior to those of conventional dry-mixed compounds (figure 4). The intent of this experiment was to further quantify the performance advantages obtained through use of this mixing process, and to explore what differences in processing between the conventional process and the elastomer composite process are responsible for the performance differences.

[FIGURE 4 OMITTED]

Experimental

A formulation consisting of 60 phi N351 carbon black was prepared using the elastomer composite continuous mixing process. This formulation was compounded into final mixes, using three different sulfur levels ( 1.8, 1.3 and 0.8 phr) and three different TBBS levels (0.5, 0.8 and 1.0 phr) for a total of nine mixing variations. In addition, two control samples were produced using the conventional dry mix method, featuring two cure systems, a conventional cure system consisting of 1.0 phr TBBS and 1.8 phr sulfur, and a "reduced" cure system consisting of 0.8 phr TBBS and 1.3 phr sulfur. The compound formulations are presented in table 1.

The test batches were produced using a 1,600 cc internal mixer, per the procedure given in table 1. Since the carbon black was pre-dispersed, it was possible to mix the elastomer composite batches using a reduced, low temperature mixing cycle. This enabled the batches to be mixed at reduced temperature (up to 20[degrees]C cooler) and for a shorter time (up to one minute reduced mixing time).

The batches were put through a battery of tests, both before and after heat aging. In addition to conventional viscoelastic and stress-strain properties, the batches were tested for dynamic properties, and two important performance indicator properties, crack growth resistance and wire adhesion. The test results are presented in table 2.

Direct comparison of processing methods The test results from the experiment are depicted in figures 5-10. As expected, the elastomer composite samples had superior hysteresis performance, improved tensile strengt, and improved tear strength. Noteworthy is the observation that for both of the cure systems, the elastomer composite sample gave lower 100% modulus, but higher 300% modulus than the equivalently compounded dry mix. In performance properties, the elastomer composite samples gave as much as a 50% improvement in flex life, and as high as 100% improvement in wire adhesion. Even more improvement in these two properties was obtained by reduction in the sulfur and accelerator levels (figures 11-12). In some cases, greater than 500% improvement in aged DeMattia and 60cA improvement in aged wire adhesion wre obtained through the use of elastomer composites processing technology and an adjusted cure system.

[FIGURES 5-12 OMITTED]

Crosslink, density

Figure 13 depicts the crosslink density values for the direct comparison between dry mix technology and the elastomer composite. For the unaged samples, the crosslink densities obtained using the two processing methods (elastomer composites and dry mixing) were comparable. The reduced cure system, naturally, gave lower crosslink density overall. As expected, crosslink density increased after aging in all of the samples, but the rate of increase of the elastomer composite samples was much less than that of the dry mix in both the conventional and reduced cure system samples. Figure 14 is a three-dimensional chart showing the change in crosslink density for all of the elastomer composite samples in the experiment. As expected, the high sulfur level samples gave the highest percentage change in crosslink density, and notably, the difference in crosslink density increase, between the dry mix and the elastomer composite, is on the same order of magnitude as the variations within sulfur level in this experiment.

[FIGURES 13-14 OMITTED]

Molecular weight

The importance of maintaining high molecular weight was classically noted by Johnson (ref. 16), who associated this property with low hysteresis, resistance to cracking, high tensile strength and field performance in various applications. However, as pointed out by Kim (ref. 15), the measurement of this to the extent of being able to predict performance on this basis is difficult due to the complexities of the compound network. From the starting molecular weight of the material when it leaves the tree, one must first subtract contamination from non-rubber solids, the effects of the presence of any nonreactive short chains, the effects of "anomalous bonds," branching, chain ends that crosslink inefficiently, and gel that does not crosslink, to arrive at the amount of effective "available" polymer for filler and curative networking. From this, one must further subtract the net degradation from mixing and processing, and in addition consider the substantial difference in compound dispersion that is unattainable with dry mix technology, and also, add the ability of the polymer to form bound rubber, which varies substantially depending on compound and processing techniques (ref. 17). From there, the finished compound must be molded at high temperature (another opportunity for degradation) (ref. 18), and then subjected to various aging conditions to arrive at a net effective molecular weight, at some level of filler interaction, and at some relative level of efficient crosslinking, to arrive at the ability of the product to withstand the forces it encounters during service. The authors consider it evident that:

* The molecular weight of the elastomer composite is typically approximately three times higher than a black-incorporated dry mix masterbatch at the same stage of mixing (ref. 13).

* The bound rubber measurements for elastomer composites are typically much higher than for equivalent dry mix (table 2).

* The dispersion of the elastomer composite samples is excellent despite minimal degradation of the polymer chains due to mixing.

* The elastomer composite may be processed suitably in a shorter time (table 1).

Some percentage of the performance benefits shown above is attributable to advantages in molecular weight associated with the use of the elastomer composite. It will be left to further research to determine the exact nature and mechanism of this advantage.

Heat aging and crosslink density, effects on performance

Increases in the crosslink density due to either aging or deliberate adjustment by use of additional sulfur have similar effects on the compound performance, as described in the literature. Lee and Kim (ref. 15) have identified the effects of heat aging on both overall crosslink density and crosslink type. For conventional cure systems, heat aging results in an increase in overall crosslink density and a shift from a high ratio of polysulfidic crosslinks toward a high percentage of mono and disulfuric crosslinks. As a result of these changes, compound properties are changed accordingly (modulus increases, tensile and elongation decrease, tan delta increases and there is a corresponding decrease in flex life). The advantage of the semi-EV and EV cure systems is that they contain a greater proportion of monosulfidic crosslinks at the outset and change less during heat aging. The disadvantage has been that these systems typically result in poor resistance to fatigue and crack growth (refs. 5 and 15).

Meinecke (ref. 19) and others have made the observation that the modulus (E') of a crosslinked elastomer is proportional to the concentration of the crosslinks. Also, the degree of crosslinking does not affect loss modulus (E"). Since tangent delta is expressed by E"/E', the higher the degree of crosslinking, the lower the tan delta. This is illustrated in a plot of the experimental data of tan delta maximum 60[degrees]C (strain sweep) values versus crosslink density for aged, unaged and process-specific samples (figure 16). Interestingly, the aged and unaged elastomer composite samples lie clearly on a different line from the dry mix samples ([R.sup.2] = 0.8 for the elastomer composite samples only). This shows that for a given crosslink density, the tan delta maximum of the elastomer composite sample is lower. This suggests that either the elastomer composites samples are taking advantage of the molecular weight effects discussed above, or that for a given cure system, vulcanization in the elastomer composite samples is more efficient.

[FIGURE 16 OMITTED]

The relationship between crosslink density and flex life, shown in figure 17, is not as strong ([R.sup.2] = 0.58), and the dry mix samples are more in line with the elastomer composite. The plot of crosslink density with wire adhesion suggests the independence of these two variables (figure 18). This is not unexpected, since wire adhesion is probably more strongly related to crosslinking between the compound and the wire. However, this brings up the opportunity to design a compound with improved wire adhesion as well as lower tan delta by selecting a cure system with a given level of sulfur and accelerator, using the elastomer composite-processed material (figure 19 and table 3). An added advantage is that in each of these cases, the aged flex life was equal to or better than the control sample.

[FIGURE 17 OMITTED]

Summary

The effects of processing technique on the aging performance of NR have been shown to be significant, in that equally compounded elastomer composite materials produced using latex-phase mixing have been shown to provide improved aging resistance and improved performance properties such as aged hysteresis and aged flex life. Excellent carbon black dispersion and high molecular weight are probable contributors to this, but it has been shown that at various sulfur and accelerator levels, elastomer composite batches have substantially better crosslink stability and lower aged crosslink density than equally compounded conventional dry mixes. It is possible, by adjusting sulfur and accelerator levels, to develop an improved compound with improved hysteresis and equal or better aged performance properties compared to the conventional dry mix formulation.

Table 1--formulations and mixing procedures

Compound formulations (phr)

Method          Dry     Dry     EC      EC      EC
TBBS             1.0     0.8     1.0     0.8     1.0
Sulfur           1.8     1.3     1.8     1.3     1.3

SIR 20            100     100
N351               60      60
Latex MB/EC                     162.7   162.7   162.7
6PPD             1.2     1.2      0.5     0.5     0.5
Stearic acid        2       2      -       -        -
Zinc oxide          4       4       4       4       4
TBBS             1.0     0.8     1.0     0.8      1.0
Sulfur           1.8     1.3     1.8     1.3      1.3
Total          170.0   169.3   170.0   169.3    169.5

Mixing in 1,600 CC internal mixer

Stage 1--0.7 fill factor, 80 rpm

Mix time, sec.    175     165     100     110     105
KwH              0.43    0.48    0.39    0.42    0.42
Max. temp. C      165     163     143     147     143

Stage 2--0.65 fill factor, 60 rpm

Mix time, sec.     60      60      60      60      60
KwH              0.29    0.22    0.21    0.20    0.20
Max temp. C       111     110     107     103     104

Compound formulations (phr)

Method          EC      EC      EC      EC      EC      EC
TBBS            1.0     0.8     0.8     0.5     0.5     0.5
Sulfur          0.8     1.8     0.8     1.8     1.3     0.8

SIR 20
N351
Latex MB/EC     162.7   162.7   162.7   162.7   162.7   162.7
6PPD              0.5     0.5     0.5     0.5     0.5     0.5
Stearic acid       -       -       -       -       -        -
Zinc oxide          4       4       4       4      4        4
TBBS              1.0     0.8     0.8     0.5     0.5     0.5
Sulfur            0.8     1.8     0.8     1.8     1.3     0.8
Total           169.0   169.8   168.8   169.5   169.0   168.5

Mixing in 1,600 CC internal mixer

Stage 1--0.7 fill factor, 80 rpm

Mix time, sec.    108     105     107     115     105     100
KwH              0.41    0.41    0.41    0.41    0.42    0.43
Max. temp. C      141     143     144     147     145     148

Stage 2--0.65 fill factor, 60 rpm

Mix time, sec.     60      60      60      60      60      60
KwH              0.21    0.19    0.13    0.20    0.19    0.19
Max temp. C       105     103     108     104     107     106

Table 2--test results

Method                            Dry     Dry     EC      EC      EC
TBBS                               1.0     0.8     1.0     0.8     1.0
Sulfur                             1.8     1.3     1.8     1.3     1.3

Bound rubber, %                   39.1    39.3    43.6    42.4    42.8

ML (1+4) @ 100[degree]C           78.2    80.8    74.5    73.8    74.9
Scorch, t5 @ 125[degree]C         14.2    17.1    13.0    16.7    14.7

Moving die rheometer (60' @ 150[degree]C)
  Minimum torque, [M.sub.L]        8.1     8.5     7.7     7.3     7.6
  Maximum torque, [M.sub.H]       42.2    36.7    43.2    36.1    37.1
  Torque @ 60'                      39      34      38      32      35
  Reversion, %                     9.4     9.6    14.6    14.2     7.1
  ts2 (minutes)                    4.42   5.18     4.07    5.00    4.54
  t90 (minutes)                   10.0    11.6     9.6    10.8     7.1

Crosslink density, swelling method
  Crosslink density, mmol         0.151   0.118   0.149   0.116   0.124
  Aged 48 hrs./100[degree]C       0.196   0.152   0.182   0.138   0.160

Zwick rebound
  at room temp.                   52.1    51.2    57.5    55.4    56.1
  at 60[degree]C                  61.8    59.1    66.7    62.5    64.6
  RT--aged 48 hrs./100[degree]C   53.6    52.1    58.1    56.4    57.8

Rheometrics--strain sweep, 10 Hz, 60[degree]C
  Tan [delta]max                  0.146   0.161   0.130   0.138   0.139
  [G".sub.max] (MPa)              0.574   0.497   0.416   0.341   0.430
  [G'.sub.0.19%] (MPa)             6.80    5.38    5.43    4.16    5.38
  [G.sub.min] MPa)                 1.91    1.51    1.67    1.30    1.52
Payne effect, (MPa)                5.08    3.97    3.84    2.91    3.94
  Aged 48 hrs./100[degree]C
  Tan [delta.sub.max]             0.140   0.166   0.116   0.123   0.133
  [G".sub.max] (MPa)              0.832   0.745   0.543   0.448   0.487
  [G'.sub.0.19%] (MPa)             9.92    8.17    7.31    5.63    6.01
  [G.sub.min] MPa)                 2.57    1.97    2.38    1.98    1.73
Payne effect, (MPa)                7.66    6.39    5.11    3.74    4.52

Stress/strain (20"/min.)
100% modulus (MPa)                 5.8     4.6     5.4     3.8     4.1
300% modulus (MPa)                22.3    19.8    24.4    20.5    21.4
Tensile (MPa)                     27.0    26.2    28.3    28.0    28.7
Elongation (%)                     369     409     365     417     406
Hardness (dur. A2)                  69      68      67      65      66
Tear--die C (kNM)                   75      92      80      94      97
Aged 48 hrs./100[degree]C
  100% modulus                     8.5     6.3     6.9     5.1     5.8
  Tensile                         18.8    21.8    22.4    22.8    23.1
  Elongation                       207     287     246     307     295
  Hardness                          73      71      71      67      68
  Tear--die C                       31      30      28      32      37

DeMattia crack growth @ room temp., kilocycles (1,000 cycles/3.3
minutes)

Failure (0.8") @ kc                 105     135     160     220     220
Aged 48 hrs./100[degree]C            15     140      20     210     240

Wire adhesion, kg-force
Average, kg force                  27.4    34.4    58.2    65.2    30.4
Aged 48 hrs./100[degree]C          23.4    22.4    29.4    31.0    16.4

Method                            EC      EC      EC
TBBS                               1.0     0.8     0.8
Sulfur                             0.8     1.8     0.8

Bound rubber, %                   41.8    41.7    42.9

ML (1+4) @ 100[degree]C           73.0    74.6    75.9
Scorch, t5 @ 125[degree]C         16.7    14.4    18.7

Moving die rheometer (60' @ 150[degree]C)
  Minimum torque, [M.sub.L]        7.1     7.5     7.5
  Maximum torque, [M.sub.H]       31.5    40.4    30.0
  Torque @ 60'                      30      35      28
  Reversion, %                     6.1    16.4     8.9
  ts2 (minutes)                    5.02    4.42    5.69
  t90 (minutes)                    9.9    10.7    11.1

Crosslink density, swelling method
  Crosslink density, mmol         0.097   0.140   0.091
  Aged 48 hrs./100[degree]C       0.110   0.166   0.105

Zwick rebound
  at room temp.                   55.3    56.9    55.1
  at 60[degree]C                  62.4    66.3    61.3
  RT--aged 48 hrs./100[degree]C   55.9    57.1    56.1

Rheometrics--strain sweep, 10 Hz, 60[degree]C
  Tan [delta]max                  0.156   0.130   0.167
  [G".sub.max] (MPa)              0.396   0.380   0.457
  [G'.sub.0.19%] (MPa)             4.40    4.87    4.81
  [G.sub.min] (MPa)                1.22    1.59    1.20
Payne effect, (MPa)                3.25    3.36    3.72
  Aged 48 hrs./100[degree]C
  Tan [delta.sub.max]             0.136   0.118   0.149
  [G".sub.max] (MPa)              0.485   0.448   0.393
  [G'.sub.0.19%] (MPa)             6.16    5.91    4.15
  [G.sub.min] MPa)                 1.87    2.09    1.37
Payne effect, (MPa)                4.25    3.95    2.86

Stress/strain (20"/min.)
100% modulus (MPa)                 3.2     4.7     3.1
300% modulus (MPa)                18.2    22.3    18.5
Tensile (MPa)                     28.4    28.5    29.0
Elongation (%)                     452     392     468
Hardness (dur. A2)                  63      67      63
Tear--die C (kNM)                   91      91      92
Aged 48 hrs./100[degree]C
  100% modulus                     4.5     6.6     4.4
  Tensile                         24.5    22.3    24.2
  Elongation                       371     255     369
  Hardness                          66      69      65
  Tear--die C                       44      30      47

DeMattia crack growth @ room temp., kilocycles (1,000 cycles/3.3
minutes)

Failure (0.8") @ kc                 200     220     240
Aged 48 hrs./100[degree]C           240      20     250

Wire adhesion, kg-force
Average, kg force                 16.4    96.7    46.6
Aged 48 hrs./100[degree]C         14.9    55.0    20.8

Method                            EC      EC      EC
TBBS                               0.5     0.5     0.5
Sulfur                             1.8     1.3     0.8

Bound rubber, %                   42.1    41.9    42.7

ML (1+4) @ 100[degree]C           75.1    75.0    77.0
Scorch, t5 @ 125[degree]C         16.0    18.4    21.1

Moving die rheometer (60' @ 150[degree]C)
  Minimum torque, [M.sub.L]        7.8     7.7     7.8
  Maximum torque, [M.sub.H]       36.1    31.7    26.4
  Torque @ 60'                      31      28      24
  Reversion, %                    18.0    15.4    12.9
  ts2 (minutes)                    4.77    5.41    6.22
  t90 (minutes)                   13.3    13.2    13.0

Crosslink density, swelling method
  Crosslink density, mmol         0.113   0.095   0.073
  Aged 48 hrs./100[degree]C       0.140   0.112   0.084

Zwick rebound
  at room temp.                   55.6    54.8    53.2
  at 60[degree]C                  63.3    60.9    58.0
  RT--aged 48 hrs./100[degree]C   55.4    54.8    53.1

Rheometrics--strain sweep, 10 Hz, 60[degree]C
  Tan [delta]max                  0.132   0.146   0.174
  [G".sub.max] (MPa)              0.362   0.378   0.502
  [G'.sub.0.19%] (MPa)             4.36    4.30    5.00
  [G.sub.min] MPa)                 1.44    1.27    1.21
Payne effect, (MPa)                3.07    3.11    3.91
  Aged 48 hrs./100[degree]C
  Tan [delta.sub.max]             0.128   0.155   0.167
  [G".sub.max] (MPa)              0.381   0.461   0.459
  [G'.sub.0.19%] (MPa)             4.41    4.86    4.41
  [G.sub.min] MPa)                 1.74    1.49    1.29
Payne effect, (MPa)                2.72    3.46    3.20

Stress/strain (20"/min.)
100% modulus (MPa)                 3.9     3.5     2.6
300% modulus (MPa)                20.6    19.3    15.9
Tensile (MPa)                     27.8    26.9    25.7
Elongation (%)                     410     428     457
Hardness (dur. A2)                  64      62      60
Tear--die C (kNM)                  102      99      90
Aged 48 hrs./100[degree]C
  100% modulus                     5.5     4.4     3.6
  Tensile                         20.2    20.7    22.1
  Elongation                       269     314     397
  Hardness                          67      64      62
  Tear--die C                       22      25      65

DeMattia crack growth @ room temp., kilocycles (1,000 cycles/3.3
minutes)

Failure (0.8") @ kc                 340     420     380
Aged 48 hrs./100[degree]C           140     460     540

Wire adhesion, kg-force
Average, kg force                 90.3    100.3   105.2
Aged 48 hrs./100[degree]C         59.0     61.8    52.1

(Additional data available upon request)

Table 3--compound design-selection of
compound with equal or better hysteresis to
the dry mix, with better aged wire adhesion

Method                      Dry     EC      EC      EC      EC
TBBS                        0.8     0.8     0.5     0.5     0.5
Sulfur                      1.3     1.3     1.8     1.3     0.8

Zwick rebound
@ room temp.                51.2    55.4    55.6    54.8    53.2
@ 60[degree]C               59.1    62.5    63.3    60.9    58.0
Aged 48 hrs./100[degree]C
  @ room temp.              52.1    56.4    55.4    54.8    53.1

Rheometrics--strain sweep, 10 Hz, 60[degree]C
Aged 48 hrs./100[degree]C
tan [delta.sub.max]         0.166   0.123   0.128   0.155   0.167
[G".sub.max] (MPa)          0.745   0.448   0.381   0.461   0.459
[G'.sub.0.19%] (MPa)        8.17    5.63    4.41    4.86    4.41
[G'.sub.min] (MPa)          1.97    1.98    1.74    1.49    1.29
Payne effect, (MPa)         6.39    3.74    2.72    3.46    3.20

Stress/strain (20'%min.) @ RT

100% modulus (MPa)          4.58    3.81    3.92    3.53    2.60
300% modulus (MPa)          19.8    20.5    20.6    19.3    15.9
Tensile (MPa)               26.2    28.0    27.8    26.9    25.7
Elongation (%)               409     417     410     428     457

DeMattia crack growth @ RT, kilocycles (1,000 cycles/3.3 min.)

Aged 48 hrs./100[degree]C
  failure (0.8") @ kc       140     210     140     460     540
Wire adhesion, kg-force
Aged 48 hrs./10[degree]C
  average, Kg               22.4    31.0    59.0    61.8    52.1

References

(1.) E.M. Belivaqua, "The reaction of molecular oxygen with rubber," Rubber Chemistry and Technology, Vol. 30, Issue 2, p. 667 (1957).

(2.) L.C. Bateman and W.E Watson, "The polymeric structure of natural rubber," Rubber Chemistry and Technology, Vol. 27, Issue 2, p. 321 (1954).

(3.) J.R. Shelton, "Review of basic oxidation processes in elastomers," Rubber Chemistry and Technology, Vol. 45, Issue 2, p. 359 (1972).

(4.) J.A. Howard, "Autoxidation and antioxidants. Basic principles and new developments," Rubber Chemistry and Technology, Vol. 47, Issue 4, p. 976 (1974).

(5.) W. W. Paris and C. Doney, "Improved service life of automotive suspension bushings through the use of semi-efficient vulcanization systems," Rubber Chemistry and Technology, Vol. 53, Issue 2, p. 368 (1980).

(6.) A.S. Kuz'minskii, "Aging and stabilization of raw and cured rubbers," Rubber Chemistry and Technology, Vol. 39, Issue 1, p. 88 (1966).

(7.) M.S. Sambhi, "Degradative studies related to the plasticity retention index of the Standard Malaysian Rubber Scheme. L Kinetics of degradation," Rubber Chemistry and Technology, Vol. 55, Issue 1, p. 181 (1982).

(8.) G.R. Hamed, "Effect of crosslink density on the critical flaw size of a simple elastomer," Rubber Chemistry and Technology, Vol. 56, Issue 1, p. 244 (1983).

(9.) G.R. Hamed and N. Rattanasom, "Effect of crosslink density on cut growth in gum natural rubber vulcanizates," Rubber Chemistry and Technology, Vol. 75, Issue 2, p. 323 (2002).

(10.) J.D. Hastings, "The effect of the conditions" of drying on the aging properties of sheet rubber," Rubber Chemistry and Technology, Vol. 9, Issue 4, p. 545 (1936).

(11.) Charles Dufraise and Roger Vielleosse, "The influence of mastication on the oxido-aging of rubber. A comparison with the influence of heating," Rubber Chemistry and Technology, Vol. 9, Issue 2, p. 206 (1936).

(12.) M.A. Mabry; F.H. Rumpf J.Z Podobnik, S.A. Westveer, A.C. Morgan, B. Chung and M.J. Andrew, USP 6,048,923 (2000, to Cabot Corporation).

(13.) Meng-Jiao Wang, Ting Wang, James Shell and Khaled Mahmud, "NR/carbon black masterbatch produced with continuous liquid phase mixing," presented at a DIK International Seminar on "Continuous mixing," Hanover; Germany, December 10-11, 2001.

(14.) David Novakoski, James Shell, Ping Zhang and Steven Laube, "Getting more truck tire mileage front advanced filler technology," paper #48, Rubber Division, ACS, 163rd Meeting, April 28-30, 2003.

(15.) Sang-Goo Kim and Suck-Hyun Lee, "Effect of crosslink structures on the fatigue crack growth behavior of NR vulcanizates with various aging conditions," Rubber Chemistry and Technology, Vol. 67, Issue 4, p. 649 (1994).

(16.) B.L. Johnson, "Effect of molecular weight distribution on physical properties of" natural and synthetic rubbers," Rubber Chemistry and Technology, Vol. 21, Issue 3, p. 654 (1948).

(17.) E.M. Dannenberg, "Bound rubber and carbon black reinforcement," Rubber Chemistry and Technology, Vol. 59, Issue 3, p. 512 (1986).

(18.) C.G. Moore and M. Porter; "Structural characterization of natural rubber vulcanizates," Rubber Chemistry and Technology, Vol. 36, Issue 2, p. 547 (1963).

(19.) Eberhard Meinecke, "The effect of cart)on black loading and crosslink density, on the heat build-up in rubbers," Rubber Chemistry and Technology, Vol. 64, Issue 2, p. 269 (1991).