OTR tires: challenges for CB and silica.

A blend of carbon black and silica with or without silane is commonly used for off-the-road tire tread compounds for earthmover tires or tires for mining vehicles. This is due to the improved cut and chip (C&C) behavior observed by adding silica at certain amounts to carbon black filled compounds.

The aim of this study was to check if the C&C behavior could be predicted in a more accurate way by using the tear analyzer instead of standard lab methods. It was also investigated to what extent silicas with varying specific surface areas influence the C&C behavior, as well as what effect the added silane has on the carbon black/silica compound performance. With a better knowledge of filler influence on the cut and chip behavior, special carbon blacks and silicas can be developed much faster, offering the customers new opportunities for their development of tires.

Background

In a previous study on cut and chip, a precise definition of the phenomenon was given, and it was shown that a low heat build-up of a compound is as essential as high tensile strength, high elongation at break and good tear resistance (ref. 1). Investigations of E-SBR and NR compounds have shown that tensile strength, as well as elongation at break, strongly depend on the testing temperature, and thus on the heat generation during the testing of a compound, respectively for a tire, on the heat generation during the service of a tire. Since both properties, tensile strength and elongation at break, have a crucial influence on tear, as well as on the C&C behavior, the heat build-up should be added to the list of the relevant in-rubber properties. Furthermore, a model was elaborated in this study (ref. 1) which describes the influence of the particle size of fillers on the tear performance independently of the used polymer. For the mentioned investigations, carbon black compounds were mostly referred to.

For the sake of completeness, different definitions of cut and chip and chunk include:

* Michelin definition (Michelin Factbook):

--Chipping: Flaking or tearing away small bits of tread rubber.

--Chunking: Tearing or breaking away pieces of tread rubber. Those definitions describe the appearance of the broken pieces of tread rubber, but do not give any indication of the causes. Therefore, the attempt was made to give a definition which also described the causes of these phenomena.

* New approach:

--Cutting takes place when the tire tread strikes sharp objects, which cause repetitive high pressures to the tread surface. If the forces are high enough, the tread surface will be penetrated or even cut.

--Chipping can follow cutting because of tractive and braking forces on sharp objects, causing tearing of the rubber, usually perpendicular to the direction of the cut (figure 1--left picture).

--Chunking: When hot tires experience rough roads, localized high pressures can result in overstraining of the material up to fractures inside the compound due to the reduction of elongation at break at high temperature. A further consequence is the removal of discrete chunks by fatigue fractures because of extensive heat generation at high-speed service (figure 1--right picture).

[FIGURE 1 OMITTED]

Thus it can be stated: Chunking occurs as a consequence of extensively localized heat generation.

Experimental

General description

For a first analysis, the carbon blacks Corax N 115 and Ecorax 1990 were used as well as silicas Ultrasil VN 2 GR, Ultrasil 7000 GR and Ultrasil 7005 (micro-granules), respectively. Typical analytical data ate shown in table 1. The influence of the specific surface area of the silica as well as the fundamental difference between carbon black and carbon black/silica compounds, are the main topics of the investigation.

Usually natural rubber (NR) was used as the polymer. For only a few investigations, NR was replaced by emulsion styrene-butadiene rubber (E-SBR). The degree of loading of the pure carbon black compounds amounted to 55 phr and 52 phr, respectively (table 3). The filler loading of the carbon black/silica compounds amounted to 52 phr for the carbon black, and for silica between 9 phr to 13 phr, depending on the different silica specific surface areas, to secure the same free silica surface area for all compounds. The compounds were produced in a three-stage mixing procedure. All compounds were prepared in a 1.5-1iter laboratory internal mixer according to the mixing procedure described in table 2.

Testing of the physical properties

Standard ISO, DIN or ASTM test procedures were applied for the evaluation of the in-rubber properties. Stress-strain properties were performed using ring specimens (ISO 37). The dynamic testing was conducted on a servo-hydraulic MTS Model 831 with 50 N pre-load, 25 N dynamic-load and 16 Hz frequency. The loss factor tan [delta] (60[degrees]C) was also analyzed with the MDR at 1.6 Hz and 0.5[degrees] shear/torsion after vulcanization in the die. The heat build-up testing was carried out with a Goodrich flexometer applying an amplitude of 0.175 inch and a cycle time of 120 minutes at room temperature. The Graves (with incision), as well as the Die C (without incision), tests were used for the analysis of the tear properties. The DeMattia test with an incision of 2 mm was performed according to ASTM D 813. The filler dispersion was evaluated by measuring the surface roughness of a vulcanizate using a topographic method (ref. 2).

The tear analyzer (refs. 3-5) is well suited for the quantitative determination of crack propagation rates of filled elastomers under dynamic load. For the dynamic deformation, typically a pulse mode is applied, which simulates the deformation of a small virtual rubber piece of a tire tread during the rolling of a tire better than it would be done by a sinusoidal deformation (ref. 6). A CCD-video camera system records very precisely the crack contour length c(n) unaffected by the way the cut grows in the compound (figure 2). Rubber strips with dimensions of 60 x 15 x 1.5 mm, with a lateral incision of 1 mm, were used as test specimens.

[FIGURE 2 OMITTED]

The crack growth rate:

D(ln c(n))/dn=1/[c.sup.*] dc(n)/dn

with: n = number of cycles; c(n) = crack contour length; and dc(n) = crack growth is determined by differentiating the logarithmic crack contour length ln c(n) with respect to dn.

The test conditions, as proposed by Bayer, are as follows: Test temperature of 23[degrees]C in a normal atmosphere, pulse load with a recovery sequence of 4 Hz; a pulse width of 30 ms and a strain deformation of 10% up to 30%.

Results

In-rubber data

The compounds 1-3 in table 3 ate pure carbon black compounds and represent the references. The filler loading of compounds 5 and 7 was adapted in the way that the total surface area of the silica added to the rubber compound remains constant (compound 4 being the reference). The filler loading for both carbon black and silica is kept constant in the compounds 4, 6 and 8.

Taking those parameters into account which have been found to be crucial for an improved tear resistance, the following trends can be observed for the compounds 4-8:

* Elongation at break is similar for all compounds.

* No difference in tensile strength can be observed.

* In the Graves test (adjusted silica filler loading--compounds 4, 5 and 7), a trend to higher and thus better values by using high surface silica is recognizable.

* In the Graves test (constant silica filler loading--compounds 4, 6 and 8), a slight improvement can be observed by using high surface area silica.

* For heat build-up (adjusted silica loading--compounds 4, 5 and 7), the heat build-ups of U 7000 GR and U 7005 are exactly on the same level and on a lower level compared with VN 2 GR.

Concerning the standard rubber testing, a significant difference can only be seen regarding the heat build-up. Differences in tearing are visible but not significant.

Additionally to the above mentioned tear tests, DeMattia testing with incision was also applied to check the tear performance. The testing was carried out knowing that it might show very strong variations. Thus, not every single value should be taken into account, only trends should be deduced. Table 4 gives an overview of the obtained DeMattia data regarding tear resistance behavior.

Considering the measured values for the pure carbon black compounds compared to the carbon black/silica blends, a trend to higher tear resistance values for the blends can be clearly seen.

Regarding the tear behavior, which certainly strongly influences the cut and chip behavior, it can be stated: Blends with carbon black and silica show a clear advantage in this respect compared to the pure carbon black filled compounds.

To test the compound performance under more realistic conditions, which means more tire-relevant conditions, the tear analyzer was applied. In a first study, the crack propagation rate for different NR and E-SBR compounds filled with the same filler systems was analyzed.

Figure 3 shows the logarithmic crack growth rate versus the logarithmic deformation amplitude of different NR and ESBR compounds. The testing temperature was 23[degrees]C. It can be seen very clearly that the crack growth rate for all natural rubber compounds is lower than for the E-SBR compounds, which is very well-known from practice.

[FIGURE 3 OMITTED]

The influence of different fillers on the tear resistance properties cannot be determined very accurately in figure 3, even by using the tear analyzer. A glance at the E-SBR compounds shows that E 1990 at a filler loading of 55 phr achieves the best performance regarding cut growth, followed by E 1990 with a filler loading of 52 phr. The addition of silica results in a rather worse performance. Among the NR compounds, the blend of E 1990 with VN 3 GR (filler loading 52/10 phr) is the only one which is noticed in a negative way, again a worse performance of a silica blend. These results are contradictory to those obtained by the Graves test and well known from practice.

To be able to easily recognize measurement fluctuations, as well as differences between various compounds, diagrams with linear scaling of crack growth rate and strain were used in the following instead of double-logarithmic plots. Two pure carbon black compounds with E 1990 and N 115, as well as the corresponding blends with 10 phr VN 3 GR, are depicted in figure 4. For all further investigations, NR was taken as the polymer.

[FIGURE 4 OMITTED]

It can be observed that the measurement variations increased starting at deformation amplitudes larger than 25%, and the differences between the compounds regarding the differential cut growth rate became larger.

As mentioned before, the heat build-up plays an important part regarding tear resistance according to the previous investigations. But since no noticeable heat build-up takes place in the dynamic deformation of strip specimens, neither by the Graves testing nor by the tear analyzer, the influence of the heat build-up is not taken into consideration in these investigations.

Therefore, a crucial factor of the determination of the tear resistance in practice is missing. The object of the following investigations is to analyze the influence of the heat build-up on the tear resistance of a compound using the tear analyzer.

Tear analyzer results at varying test temperatures

To determine the influence of the heat build-up during a dynamic deformation, the testing temperatures 80[degrees]C, 100[degrees]C and 120[degrees]C were added to the standard test temperature of 23[degrees]C. These ate temperatures that can also be measured directly on a tire tread during use.

Figure 5 shows for the E 1990 the differential cut growth rate at different temperatures.

[FIGURE 5 OMITTED]

The strong influence of the higher testing temperatures on the tear resistance behavior can be depicted very clearly. The higher the testing temperature, the higher is the cut growth rate and, respectively, the poorer is the tear resistance behavior. This leads to the conclusion that it is absolutely necessary to also consider the heat build-up to make a statement concerning the tear resistance of compounds. Furthermore, higher measurement variations can be observed from 25% deformation amplitude on.

Additionally, it was investigated if higher testing temperatures offer better possibilities in determining the difference between a pure carbon black compound and a blend of carbon black and silica regarding the tear resistance. Figure 6 displays the results of this investigation.

In figure 6, the trend to an improved tear resistance behavior can be observed for the blend of carbon black and silica in comparison to a pure carbon black compound. However, it must be noticed that the difference could only be seen at amplitude greater than 25%, where higher measurement variations can also be observed. The fact that the difference between the two compounds is not more substantial might be due to the fact that the silica was added additionally to the carbon black in the compound, and thus the total loading amounted to 68 phr instead of 55 phr. In practice, the carbon black share should be decreased in ratio to the added silica. However, such a compounding was not taken into account in these investigations.

[FIGURE 6 OMITTED]

In figure 7, three types of silica are compared which have different specific surface areas. The filler loading of the silicas VN 2 GR, U 7000 GR and U 7005 was adjusted to their specific surface area to get the same total silica surface area in the compound. A clear trend to a better tear resistance behavior can be seen with decreased particle diameter of the silica used. The best performance regarding tearing resulted from the high specific surface area silica (HSA-silica) U 7005.

[FIGURE 7 OMITTED]

Figure 8 displays, in comparison to figure 7, two additional curves for U 7000 GR and U 7005, both with 13 phr of filler loading, in order to investigate the influence on the cut growth rate at equal silica loadings. The use of HSA-silica at similar filler loadings instead of a low specific surface area silica (LSA-silica) becomes less advantageous and even disadvantageous for U 7005 in comparison to VN2 GR. Furthermore, the influence of the higher heat build-up with the usage of HSA-silica was not taken into account for those compounds (table 3).

[FIGURE 8 OMITTED]

In order to simulate the influence of the heat build-up on the tear resistance behavior, the cut growth rate over the deformation amplitude was fitted with an exponential function (figures 7 and 8), and the cut growth rate was calculated for an amplitude of 22%. This specific amplitude was chosen because it presents a good compromise of measurement accuracy and differentiation between various compounds. In figure 9, the calculated crack growth rate versus the measurement temperature is plotted for two silica compounds: E 1990 + 13 phr of VN 2 GR and E 1990 + 9 phr U 7005.

[FIGURE 9 OMITTED]

The advantages of the HSA-silica compared to the low specific surface area silica (LSA-silica) can be seen clearly in this graph. For the judgement of the tear performance, it is now possible to take into account the special heat build-up of a compound measured by Goodrich flexometer. This way, the cut growth rate can be determined depending on the heat build-up that was measured under well-defined conditions. After entering those values in figure 10 (as black circles), an even more considerable difference between the compounds containing the two different types of silica can be observed. By using this method, it becomes possible to combine the heat build-up with the cut growth tate and to simulate more accurately the tire-specific conditions.

In summary, it can be stated that a HSA-silica shows clear advantages in the crack propagation compared to a LSA-silica. This fact can also be explained on the basis of the reported model (ref. 1).

Virtual experiment

Taking two spheres with a radius of R for the one and of r = 0.5 R for the other sphere, four spheres with radius r are needed to obtain the same surface.

4 [pi] [R.sup.2] =4 [pi] [(2r).sup.2] = 4 (4 [pi] [r.sup.2])

However, the path length of the spheres with the small radius r is twice the length of the one of the sphere with large radius R (figure 11).

[FIGURE 11 OMITTED]

As a consequence, even if a HSA-silica (small primary particle size) is added to a compound in a dosage adapted to the same total surface area as a LSA-silica (large primary particle size), the larger path length of the HSA-silica provides a more pronounced filler network. This leads to the situation depicted in figure 12.

[FIGURE 12 OMITTED]

Figure 12 shows the influence of a stronger filler network (second picture). The stronger the filler network, the greater is the probability for a cut to meet a filler particle, and thus the cut has to stop or to run around the particle.

Influence on tear resistance by the usage of silane

The table 5 comparisons of compounds 2 with 3 (13 phr of U 7000 GR with and without silane) and 4 with 5 (13 phr of U 7005 with and without Si 69) show that the addition of Si 69 silane results in a considerably higher 300% modulus, but an almost unaltered elongation at break. The hardness level was increased, and the abrasion loss level (according to DIN) was improved, which was expected due to the coupling of the silica to the rubber matrix by the silane. Regarding the dynamic properties, the following can be stated: The heat build-up showed a clear decrease, the ball rebound at 60[degrees]C was higher and the tan [delta] 60[degrees]C values were lower. The tear propagation properties, determined with the Graves test at 100[degrees]C, displayed slightly better values when silane was added. The tear resistance without incision tested with Die C (100[degrees]C) remained constant.

Comparing compound 6 with 7 (which contain a reduced amount of U 7005 vs. compounds 4 and 5), it can be noticed that with the reduced silica amount (dosage is adjusted to the specific surface area of VN 2 GR), the addition of the same silane quantity results only in small improvements regarding 300% modulus and tan [delta] 60[degrees]C. The tear resistance properties for the compounds 6 and 7 tested with Die C and Graves are on the same level.

Until now, nothing suggests that the tear resistance behavior deteriorates by adding silane to the compounds. Regarding cut and chip behavior, clear improvement should be even observed since, as reported, a lower heat build-up leads to an improved C&C behavior.

Looking to the results achieved by the DeMattia test, which means cut propagation testing, the situation is quite different. Comparing the cycles necessary to create a cut length of 8-12 mm, it is obvious that the addition of silane to the compound significantly deteriorates the tear resistance behavior (figure 13). Consequently, the compound with the high specific surface area U 7005 at a dosage of 9 phr without silane will give the optimum performance regarding cut and chip and chunk.

[FIGURE 13 OMITTED]

The reason for the discrepancy between the tear resistance results obtained with the Graves test and the DeMattia test is not clear up to now. It can be suspected that the Graves test is strongly influenced by the maximum energy at break because it reflects the maximum tear energy that is necessary for the propagation of a crack. The energy at break is improved by adding silane to a carbon black/silica compound (table 5), because tensile strength and elongation at break stay constant but 300% modulus is increased. On the other side, the DeMattia test investigates more the fatigue resistance, which means the crack propagation caused by each bending of the specimen. Consequently, the stresses which are applied to the specimens differ very strongly. In reality, both kinds of stresses are applied to a tire during service: a) rolling of a tire--repeated bending of the damaged part; b) turning on the spot--single deformation of the damaged part with very high stresses. Consequently, it can be stated that if both tests give positive results and show the same trend, the probability of also finding positive results on tires is very high.

The first results obtained with the tear analyzer confirmed the results received with the DeMattia test. Figure 14 depicts very clearly the negative trend with 2 phr of Si 69 compared to the carbon black/silica blend without silane regarding the crack propagation rate.

[FIGURE 14 OMITTED]

The actual reason for the significant differences regarding crack propagation between compounds containing silane and compounds without silane could be a chemical one. It is well known that the silica surface adsorbs and deactivates accelerators. Consequently, it can be deduced that the crosslinking density close to the silica surface is reduced, resulting in a localized high elongation at break, which is advantageous for avoiding crack propagation. By adding silane to a compound, additional crosslinkages between the silica surface and polymer chains are introduced into the system and also bridging between silica particles can occur. Both processes result in a localized increased crosslinking density (multi-crosslinking-sites), which is not advantageous for tearing. This happens on a microscopic scale and can not be detected on a macroscopic scale by stress-strain tests because the silica loading is relatively low.

Conclusion

In analyzing tear properties of compounds, first of all it is necessary to distinguish between testing the initiation of a cut (e.g., by Die C) and testing the propagation of a cut (e.g., by Graves). Standard lab test methods for measuring crack propagation like the Graves test (specimens with incision) and DeMattia (specimens with incision) display significant differences only if the compounds show very large variations regarding tearing, e.g., difference between natural rubber and emulsion styrene-butadiene rubber compounds. For both methods, it must be stated that single measurement results should be handled very carefully. It is advisable to look for trends, but it should not be argued with single results. By using the tear analyzer it is, for the first time, possible to measure the crack propagation quantitatively by exactly monitoring the crack contour length during dynamic deformation of a rubber strip. The advantages regarding crack propagation of carbon black/silica blends compared with pure carbon black filled compounds could be displayed clearly. Furthermore, the improved crack propagation could be depicted by using a high specific surface area silica, like the highly dispersible U 7005, instead of low specific surface area silica. In particular, this was possible by testing the crack propagation rates at different elevated temperatures and combining the results with the heat generation of the compounds. A model was given to explain the improved tear properties by a high specific surface area silica. The addition of silane to carbon black/silica compounds results in a significant deterioration of the tear properties tested with DeMattia, as well as with the tear analyzer. Consequently, the most suitable compound for outstanding performance regarding cut and chip and chunk is a blend of a high specific surface area carbon black like E 1990 (ref. 1) in combination with a high specific surface area highly dispersible silica like U 7005.

This article is based on a paper presented at the ITEC 2004 Conference.

References

(1.) W. Niedermeier, "New insights into the tear mechanism," Tire Technology International, 96 (2003).

(2.) A. Wehmeier, "Filler dispersion analysis by topography measurements," technical report from Degussa (TR 820).

(3.) U. Eisele, S. Kelbch and H.-W. Engels, Kautschuk Gummi Kunstst. 45, 1,064 (1992).

(4.) S.A. Kelbch, U. Eisele and H. Magg, "A new predictive crack growth test for v-belt compounds," ACS Rubber Division meeting, October 11-14, 1994.

(5.) A.J.M. Sumner, S.A. Kelbch and U. Eisele, "Crack growth performance of tire compounds," ACS Rubber Division meeting, October 11-14, 1994.

(6.) H. Deckmann, "Dynamic testing as a means to decrease drum dependence," Tire Technology International, 88 (2003).

Table 1--typical data of the silicas (GR: granules)
and carbon blacks used

                      VN2     VN 3    7000    7005 *  N 115    E 1990
                      GR *    GR *    GR *

CTAB [[m.sup.2]/g]    125     165     160     175      129      135
BET [[m.sup.2]/g]     125     175     170     185      136      140
DBP [ml/100g]         210     200     240     245      115      104

* (Free of water)

Table 2--natural rubber tread formulation

1st mixing cycle      Phr    2nd mixing cycle     Phr

NR                    100    Batch stage 1
Carbon black        52-55
Silica               0-13    3rd mixing cycle     Phr
Oil                     5
Stearic acid            2    Batch stage 2
ZnO                     5    TBBS 80%             1.2
6PPD                    3    Sulfur               1.5
Wax                   1.5    PVI                 0.15

Table 3--relevant in-rubber data of carbon black and carbon
black/silica compoundsE

Compound no.                                      1        2        3
Filler                                        N 115        E        E
                                                        1990     1990

Filler content                     phr           55       55       52
Stress-strain, ring
Tensile strength                   MPa         24.8     25.8     22.7
100% modulus                       MPa          2.3      2.2      2.1
200% modulus                       MPa          6.3      6.6      5.8
300% modulus                       MPa         11.9     13.3     11.6
Elongation at break                 %           530      480      470
Hardness, duro. A                   --           69       66       64
DIN abrasion 10 N               [mm.sup.3]       96       82       93
Goodrich Flexometer
  Contact temp.                 [degrees]C       61       51       49
  Center temp.                  [degrees]C      106       89       86
Tear properties
Tear resistance Die C              N/mm         124      140      137
  (RT)
(Tear resistance Die C             N/mm          65       65       66
  (100[degrees]C)
Graves (RT)                        N/mm          65       75       81
Graves (100[degrees]C)             N/mm          65       58       46
Viscoelastic properties
Ball rebound 60[degrees]C           %          58.2     61.2     62.7
MTS E* 0[degrees]C                 MPa         18.1     18.3     16.0
MTS E* 60[degrees]C                MPa          9.6      9.0      8.2
MTS tan [delta] 60[degrees]C        --        0.151    0.125    0.117
MDR tan [delta] 60[degrees]C        --        0.226    0.186    0.175
Dispersion (Topo)/P.A.              %           2.1      4.6      5.1

Compound no.                                      4         5         6

Filler                                      E 1990/   E 1990/   E 1990/
                                                VN2    U 7000    U 7000

Filler content                    phr         52/13     52/10     52/13
Stress-strain, ring
Tensile strength                  MPa          22.3      22.5      21.7
100% modulus                      MPa           2.2       2.1       2.1
200% modulus                      MPa           6.2       5.9       5.9
300% modulus                      MPa          12.5      12.1      11.9
Elongation at break                %            460       460       470
Hardness, duro. A                  --            66        66        67
DIN abrasion 10 N              [mm.sup.3]       100        98        99
Goodrich Flexometer
  Contact temp.                [degrees]C        63        58        69
  Center temp.                 [degrees]C       117       104       128
Tear properties
Tear resistance Die C             N/mm          112       115       118
  (RT)
(Tear resistance Die C            N/mm           65        64        53
  (100[degrees]C)
Graves (RT)                       N/mm           47        75        51
Graves (100[degrees]C)            N/mm           58        64        55
Viscoelastic properties
Ball rebound 60[degrees]C          %           60.9      60.5      60.2
MTS E* 0[degrees]C                MPa          18.1      17.5      18.5
MTS E* 60[degrees]C               MPa           9.1       9.2       9.2
MTS tan [delta] 60[degrees]C       --         0.130     0.131     0.137
MDR tan [delta] 60[degrees]C       --         0.172     0.170     0.165
Dispersion (Topo)/P.A.             %            2.6       3.3       2.7

Compound no.                                      7         8

Filler                                      E 1990/   E 1990/
                                             U 7005    U 7005

Filler content                    phr          52/9     52/13
Stress-strain, ring
Tensile strength                  MPa          23.5      22.9
100% modulus                      MPa           2.1       2.0
200% modulus                      MPa           6.0       5.8
300% modulus                      MPa          12.2      11.9
Elongation at break                %            490       480
Hardness, duro. A                  --            66        65
DIN abrasion 10 N              [mm.sup.3]        93       100
Goodrich Flexometer
  Contact temp.                [degrees]C        58        78
  Center temp.                 [degrees]C       105       141
Tear properties
Tear resistance Die C             N/mm          100        98
  (RT)
(Tear resistance Die C            N/mm           53        61
  (100[degrees]C)
Graves (RT)                       N/mm           72        53
Graves (100[degrees]C)            N/mm           64        58
Viscoelastic properties
Ball rebound 60[degrees]C          %           60.6      60.3
MTS E* 0[degrees]C                MPa          17.5      18.7
MTS E* 60[degrees]C               MPa           8.9       9.0
MTS tan [delta] 60[degrees]C       --         0.131     0.141
MDR tan [delta] 60[degrees]C       --         0.167     0.169
Dispersion (Topo)/P.A.             %            3.5       3.7

Table 4--tear resistance tested by DeMattia with incision

Compound no.                       1      2      3         4

Filler                         N 115      E      E   E 1990/
                                       1990   1990       VN2

Filler content           Phr      55     55     52     52/13
Demattia with incision
Number of cycles,        MPa       2      2      2         3
  2-4 mm
Number of cycles,        MPa       9      9     16        29
  4-8 mm
Number of cycles,        MPa      21     22     46       107
  8-12 mm
Number of cycles,        MPa     191    222    567     1,915
  break

Compound no.                         5         6         7         8

Filler                         E 1990/   E 1990/   E 1990/   E 1990/
                                U 7000    U 7000    U 7005    U 7005

Filler content           Phr     52/10     52/13      52/9     52/13
Demattia with incision
Number of cycles,        MPa         3         3         3         4
  2-4 mm
Number of cycles,        MPa        30        22        26        38
  4-8 mm
Number of cycles,        MPa       111        66        87       136
  8-12 mm
Number of cycles,        MPa     2,012       873     1,361     2,335
  break

Table 5--in-rubber data of carbon black/silica blends with and without
Si 69

Compound No                                     1         2         3

Filler                                          E   E 1990/   E 1990/
                                             1990       VN2       VN2

Filler content                    phr          55     52/13     52/13
Si 69 content                     phr                               2
Stress-strain, ring
Tensile strength                  MPa        26.0      23.3      23.8
100% modulus                      MPa         2.0       2.1       2.5
200% modulus                      MPa         5.7       6.0       7.4
300% modulus                      MPa        11.5      12.2      14.1
Elongation at break                %          530       490       460
Hardness, duro. A                  --          64        65        68
DIN abrasion 10 N              [mm.sup.3]      99       101        92
Goodrich Flexometer
  Contact temp.                [degrees]C      50        62        56
  Center temp.                 [degrees]C      79       102        91
Tear properties
Tear resistance Die C             N/mm         63        55        59
  (100[degrees]C)
Graves (100[degrees]C)            N/mm         69        50        64
Viscoelastic properties
Ball rebound 60[degrees]C          %         60.7      58.4      60.9
MTS E* 0[degrees]C                MPa        12.3      16.5      17.8
MTS E* 60[degrees]C               MPa         7.3       8.6       9.3
MTS tan [delta] 60[degrees]C       --       0.128     0.146     0.130
MDR tan [delta] 60[degrees]C       --       0.171     0.170     0.156
Dispersion (Topo) / P.A.           %          2.1       1.5       3.4

Compound No                                       4         5         6

Filler                                      E 1990/   E 1990/   E 1990/
                                             U 7005   U 7005     U 7005

Filler content                    phr         52/13     52/13      52/9
Si 69 content                     phr                       2
Stress-strain, ring
Tensile strength                  MPa          21.6      23.9      23.6
100% modulus                      MPa           1.8       2.4       2.0
200% modulus                      MPa           5.2       7.0       5.8
300% modulus                      MPa          11.0      13.8      12.1
Elongation at break                %            470       470       480
Hardness, duro. A                  --            63        67        65
DIN abrasion 10 N              [mm.sup.3]       111        93       103
Goodrich Flexometer
  Contact temp.                [degrees]C        81        57        56
  Center temp.                 [degrees]C       128        90        86
Tear properties
Tear resistance Die C             N/mm           58        60        62
  (100[degrees]C)
Graves (100[degrees]C)            N/mm           52        69        69
Viscoelastic properties
Ball rebound 60[degrees]C          %           58.5      61.1      60.5
MTS E* 0[degrees]C                MPa          15.1      16.8      14.2
MTS E* 60[degrees]C               MPa           7.8       9.1       8.0
MTS tan [delta] 60[degrees]C       --         0.150     0.137     0.137
MDR tan [delta] 60[degrees]C       --         0.160     0.151     0.156
Dispersion (Topo) / P.A.           %            3.9       2.6       3.1

Compound No                                       7

Filler                                      E 1990/
                                             U 7005

Filler content                    phr          52/9
Si 69 content                     phr             2
Stress-strain, ring
Tensile strength                  MPa          24.0
100% modulus                      MPa           2.3
200% modulus                      MPa           6.5
300% modulus                      MPa          13.0
Elongation at break                %            480
Hardness, duro. A                  --            66
DIN abrasion 10 N              [mm.sup.3]        98
Goodrich Flexometer
  Contact temp.                [degrees]C        53
  Center temp.                 [degrees]C        85
Tear properties
Tear resistance Die C             N/mm           65
  (100[degrees]C)
Graves (100[degrees]C)            N/mm           64
Viscoelastic properties
Ball rebound 60[degrees]C          %           61.9
MTS E* 0[degrees]C                MPa          15.2
MTS E* 60[degrees]C               MPa           8.5
MTS tan [delta] 60[degrees]C       --         0.128
MDR tan [delta] 60[degrees]C       --         0.147
Dispersion (Topo) / P.A.           %            2.3