The effect of peptizer on carbon black-rubber interaction in the internal mixer.

Reinforcement, the improvement of the service life of rubber goods, is dependent on the behavior of the carbon black-elastomer interface. It is generally believed that the interaction between the filler and the matrix involves physical and chemical forces (ref. 1). The addition of carbon black significantly improves the physical and mechanical properties of elastomers, i.e., increasing modulus, tensile strength, tear strength, fatigue resistance and wear resistance. A new model and theory have been proposed for the reinforcement of elastomers, based on the perspective of dynamic mechanics and mechanism of the reinforcement, using finite element method (FEM) stress analysis (ref. 2). The reinforcement of elastomers by carbon black is governed by the morphology of the black and its physical and chemical interactions with the polymer. In modern rubber-grade carbon blacks, strong bonding of the polymer to the carbon black surface is affected by several mechanisms, but surface chemical differences between blacks are relatively small, so that the dominant characteristic becomes the morphology (ref. 3). A higher bound rubber in cases of higher surface area carbon blacks was reported in the literature (ref. 4). Tan delta and resilience are

mainly determined by the distance between aggregates (ref. 5). Carbon black reinforces ultimate properties of rubber by tear deviation. This can occur at the colloidal level, as the tear is forced to pass around carbon black aggregates, thus increasing the area of the torn surface. Colloidal tear deviation may be the cause of the increase in threshold tearing energy, which appears to be a small but significant component of reinforcement. In order to be effective, in this or other ultimate properties, the carbon black must not debond; i.e., failure must go through the rubber rather than through the debonded rubber-filler interface. Reinforcing carbon blacks do not debond (from diene rubbers) under normal conditions and so do not require further strengthening of the rubber-filler bonds (ref. 6).

Natural rubber requires controlled reduction in molecular weight, a process called mastication, which is required for consistent mix quality. Reducing viscosity strictly by mechanical means has an inherent limitation. When the rubber is cold, the mixer provides high shear, and polymer breakdown occurs rapidly. However, mixing creates heat. As the mix warms, thermal softening provides less shear, and the breakdown of molecular weight becomes slower. Chemical additives called peptizers have been developed to hasten this molecular cleavage. As with all chemical reactions, the reaction of peptizers with NR occurs more rapidly as the temperature is increased. With the use of chemical peptizers, one generally sees a u-shaped curve of activity versus temperature (ref. 7). At the lowest temperature, the rate of breakdown is rapid because of high mechanical shear. As the temperature increases, the rate of breakdown declines to a minimum. At still higher temperatures, the chemical activity of the additive becomes the dominant effect and the rate of breakdown increases again. Internal mixers are very sensitive to changes in mixing time or rotor speed. High speed mixing reduces viscosity quite rapidly, but the batches may dump in a very sticky and non-uniform state. This problem may be partly due to inadequate dispersion of the peptizer. It is recognized that breakdown at the more elevated temperatures is due to oxidative scission and that, in the lower temperature range and under the stresses set up during mastication, mechanical rupture of primary bonds is possible. The free radicals thus produced become stabilized by addition of oxygen. When mastication is carried out in the absence of oxygen, e.g., in an inert atmosphere (either in an internal mixer or on a closed-in mill), little or no breakdown occurs. The general occurrence of mechanico-degradation when polymers undergo mastication or milling has been noted (ref. 8). Under these conditions, radicals resulting from mechanical shear recombine. The low-temperature breakdown of rubber is attributed to interaction of rubber radicals with oxygen to give oxygen-terminated fragments of lower molecular weight. Other radical acceptors can be expected to promote breakdown of rubber in an inert atmosphere and, moreover, do so in a manner capable of quantitative interpretation.

Radical acceptors (ref. 9) that promote breakdown of rubber are not to be confused with chemical plasticizers (peptizers). The former are only effective under conditions of cold mastication and do not cause breakdown above 100 [degrees]C in the absence of oxygen. As the temperature reached in a mass of rubber during mastication is increased, the rubber becomes progressively softer, and the rate of mechanical rupture is correspondingly reduced. Hence, the breakdown efficiency in air or with radical acceptors under nitrogen falls with increasing temperature. A second type of breakdown due to conventional oxidative scission becomes apparent above 80 [degrees]C, and this increases rapidly with temperature. Interaction between carbon black and polymer starts during the mixing process. A primary agglomerate is formed, the composition of which is dependent upon the structure. Structure and specific activity determine incorporation time and further dispersion. During mixing, bound rubber is formed which is used as a measure of specific surface activity. In the final vulcanizate, the filler-polymer interaction is evident through reduced swelling in solvents like benzene, chloroform and cyclohexane (ref. 10). The higher [[gamma].sup.d.sub.s] (surface energy parameter) of carbon black causes strong filler-polymer interaction, which is reflected in a higher bound-rubber content of the compounds and higher moduli of the vulcanizates at high elongation (ref. 11). With regard to the effect of filler on the dynamic properties of a given polymer and cure system, filler networking, both its architecture and strength, is the main (although not only) parameter to govern the behavior of the filled rubber. From the ther modynamic and kinetic points of view, filler network formation is especially related to filler-filler and polymer-filler, as well as polymer-polymer interactions (ref. 12).

Factorial designs are widely used in experiments involving several factors where it is necessary to study the joint effect of the factors on a response. By factorial design, it means that in each complete trial all possible combinations of the levels of the factors are investigated (refs. 13 and 14). The factors at two different levels are commonly used. Normally, it is assumed that the response is approximately linear over the range of the factor levels chosen. In a [2.sup.k] factorial design, it is easy to express the results of the experiment in terms of a regression model. This model is natural and intuitive. The two level factorial design experiments are carried out to ascertain the trend direction of the response. Rubber-carbon black interaction competes with rubber-peptizer reaction. This is a practical situation. Additionally, the state of natural rubber molecules is altered with the addition of aromatic oil and influenced by the solubility match. In this state, it is interesting to study how the rubber would respond to the filler interaction and almost simultaneously with the peptizer. There seems to be no published information on this three-factor influence as of this date. In the present study, a two level, three factorial design is used to understand this situation through the assessment of cure and physico-mechanical characteristics of the rubber compositions.

Experimental

The selection of the formulations was done based on typical NR formulations. The standard design matrix and the compounding scheme are shown in tables 1 and 2. The test formulations and their mixing sequences are illustrated in table 3. An internal mixer (1.5 1) was used for both (master and final) stages of mixing. Final sheeting was done on an open 6" diameter and 13" width two-roll mill. Curing of the compounds was done in a 180 mt hydraulic press using hard chrome plated molds of 152 x 152 x 1.90 mm size as per ASTM D 412.

The following is the equipment used for characterizing compound properties: Mooney viscometer (MV 2000), rheometer (MDR 2000), hardness tester (IRHD), tensile tester (Z010), rebound resilience tester, dispersion rating tester (Dispergrader 1000 GT), specific gravity tester and abrasion tester.

Test procedures conform to the corresponding standards.

The variables that were planned for study are:

* Factor A - The peptizer content (0.1 and 0.3 phr);

* Factor B - The amount of process oil (0 phr and 10 phr); and

* Factor C. The peptizer's order of addition (with rubber and with carbon black).

In order to obtain all vital information in the study, including interactions, and at the same time with a minimum required number of experiments, a two level, three factorial design of experiments was followed. In order to minimize experimental bias and to avoid wrongly attributing the variations in test data to the factor influence, the sequence of mixing was randomized and performed in replication, respectively.

Results and discussion

The eight mixes are shown as eight comers of a cuboid with the factors (peptizer, oil and order of addition) forming the sides (figure 1). The properties of the rubber vulcanizates are described in table 4. The structure of the basic response sheet is represented in table 5. The "average," "effect" and "regression coefficient" values are derived and taken from the respective response sheets of properties.

The approach to deduce the regression equation (ref. 14) has been explained in detail in an earlier manuscript (ref. 15). The conformity of the deduced value with the observed value is described below.

(Basic equation)

Yi = yo + [a.sub.1][x.sub.1] + [a.sub.2][x.sub.2] + [a.sub.3][x.sub.3] +
[a.sub.12] [x.sub.1][x.sub.2] + [a.sub.13]
[x.sub.1][x.sub.3] + [a.sub.23][x.sub.2][x.sub.3]

Regression equations deduced are:

Mooney viscosity as observed parameter:

[Y.sub.ML4] = 94.44 - 3.35 [x.sub.1] - 8.95 [x.sub.2] - 1.45 [x.sub.3]
- 0.25 [x.sub.1]
[x.sub.2] - 2.05 [x.sub.1] [x.sub.3] - 0.15 [x.sub.2] [x.sub.3]
  a) of higher range
[Y.sub.6] = 94.44 -3.35(-1) -8.95(-1) -1.45(+1) -0.25(-1)(-1) -2.05
(-1)(+1) -0.15(-1)(+1) = 107.24 (vs. observed value = 108.2)
  b) of lower range
[Y.sub.3] = 94.44 -3.35(+1) -8.95(+1) -1.45(-1) -0.25(+1)(+1) -
2.05(+1)(-1) - 0.15(+1)
(-1) = 85.54 (vs. observed value = 84.4)

MH as observed parameter:

[Y.sub.MH] = 17.49 - 0.3 [x.sub.1] - 1.4 [x.sub.2] - 0.2 [x.sub.3] - 0.1
[x.sub.1] [x.sub.2] - 0.2 [x.sub.1] [x.sub.3]
- 0.1 [x.sub.2] [x.sub.3]

  a) of higher range
[Y.sub.6] = 17.49- 0.3 (-1)- 1.4 (-1)- 0.2 (+1)- 0.1 (-1) (-1)- 0.2
(-1) (+1) - 0.1 (-1) (+1) = 19.19 (vs. observed value = 19.15)
  b) of lower range
[Y.sub.1] = 17.49- 0.3 (+1)- 1.4 (+1) - 0.2 (+1) - 0.1 (+1) (+1) - 0.2
(+1) (+1) - 0.1 (+l)(+1) = 15.19 (vs. observed value = 15.14)

300% modulus as observed parameter:
[Y.sub.M300%] = 15.88 - 0.1 [x.sub.1] - 2.05 [x.sub.2] + 0.35 [x.sub.3]
- 0.25 [x.sub.1] [x.sub.2] - 0.25
[x.sub.1] [x.sub.3] - 0.3 [x.sub.2][x.sub.3]
  a) of higher range
[Y.sub.8] = 15.88 - 0.1 (+1)- 2.05 (-1) + 0.35 (+1)- 0.25 (+1)
(-1) - 0.25 (+1)(+1) - 0.3 (-l)(+l) = 18.48 (vs. observed value = 18.57)
  b) of lower range
[Y.sub.1] = 15.88 -0.1(+1) -2.05(+1) +0.35(+1) -0.25(+1)(+1) -0.25
(+1)(+1) - 0.3 (+1)(+1) = 13.28 (vs. observed value = 13.62)

Tensile strength as observed parameter:
[Y.sub.T] = 23.42 - 0.2 [x.sub.1] + 0.2 [x.sub.2] + 0.2 [x.sub.3] - 0.55
[x.sub.1][x.sub.2] + 0.1 [x.sub.1][x.sub.3]
- 0.05 [x.sub.2] [x.sub.3]
  a) of higher range
[Y.sub.5] = 23.42 - 0.2 (-1) + 0.2 (+1) + 0.2 (-1) -0.55 (-1)(+1) + 0.1
(-1)(-1) - 0.05 (+1)(-1) = 24.32 (vs. observed value = 24.44)
  b) of Iower range
[Y.sub.3] = 23.42 -0.2 (+1) + 0.2 (+1) + 0.2 (-1) - 0.55 (+1)(+1) +
0.1 (+1)(-1) - 0.05 (+1)(-1) = 22.62 (vs. observed value = 22.46)

Abrasion loss as observed parameter:

[Y.sub.A] = 156.8 +4.5 [x.sub.1] + 17.5 [x.sub.2]- 0.2 [x.sub.3] + 4.8
[x.sub.1][x.sub.2] + 4.7 [x.sub.1][x.sub.3]
+ 2.7 [x.sub.2] [x.sub.3]
  a) of higher range
[Y.sub.1] = 156.8 + 4.5 (+1) +17.5 (+1) - 0.2 (+1) + 4.8 (+1)(+1) +
4.7 (+1)(+1) + 2.7 (+1)(+1) = 190.6 (vs. observed value =
190)
  b) of lower range
[Y.sub.6] = 156.8 + 4.5 (-1) + 17.5 (-1) - 0.2 (+1) + 4.8 (-1)(-1) + 4.7
(-1)(+1) + 2.7 (-1)(+l) = 132.0 (vs. observed value = 131.5)

Hardness as observed parameter:

[Y.sub.H]=71.4-0.6 [x.sub.1] - 1.9 [x.sub.2] + 0.1[x.sub.3] + 0.1
[x.sub.1][x.sub.2]-0.4 [x.sub.1]
[x.sub.3] - 0.1 [x.sub.2] [x.sub.3]
  a) of higher value
[Y.sub.6] = 71.4- 0.6 (-1) -1.9 (-1) + 0.1 (+1) + 0.1 (-1)(-1) - 0.4
(-1)(+1) - 0.1 (-1)(+1) = 74.6 (vs. observed value
= 75)
  b) of lower range
[Y.sub.3] = 71.4 - 0.6 (+1) - 1.9 (+1) + 0.1(-1) + 0.1 (+1)(+1) - 0.4
(+1)(-1) - 0.1 (+1) (-1) = 69.4 (vs. observed value = 69)

Important findings (on interaction of order of addition, peptizer and oil)

Effect of increase in peptizer on viscosity:

* Addition of peptizer with carbon black, viscosity decreases by two to five units; and

* addition of peptizer with rubber, viscosity decreases by 12 units.

The increase in peptizer causes a viscosity reduction. This is more pronounced if it is added with the rubber. This observation indicates that the peptizer activity is stronger than the rubber-filler interaction. The schemes of the two competing reactions (ref. 16) are shown below.

R--.+ XSH (peptizer) [right arrow] R--H + XS. (1) (Rubber chain radical)

The peptizer activity (equation 1) is more prevalent than carbon black addition (equation 2).

* Elongation at break: Highest elongation if peptizer is at 0.1 phr and with 10 phr oil; lowest elongation if peptizer is added with rubber and withholding oil. Longer chains in a greater number in the plastic field would ensure higher elongation (ref. 17). At the same time, even with a given number of longer chains, if the plastic field is absent, the elongation suffers.

* 5% modulus: Highest in case of 0.1 pht peptizer with rubber and without oil addition; lowest in case of 0.3 phr peptizer added with carbon black and with 10 phr oil. The observation that the highest initial modulus (5% mod.) occurs with a controlled peptization of rubber in the absence of the oil concurs with general theory. At the same time, a higher peptizer (0.3 phr) going with carbon black and with 10 phr oil shows on one hand that such a procedure is detrimental to the modulus. On the other hand, a controlled peptization is also required, maybe to ensure a better alignment of rubber chains.

* 300% modulus: Highest if peptizer at 0.3 phr is added with rubber and withholding any oil; lowest with oil at 10 phr, and 0.3 phr peptizer is added with rubber. A higher 300% modulus is possible only with higher peptizer going with rubber. It also requires a tighter network without oil. The 300% modulus, being a measure of strain-induced crystallization, responds to higher degree of peptization. At the same time, the trend is reversed the moment the network is opened with oil addition.

* Tensile strength: Highest in case of peptizer at 0.1 phr added with carbon black and with 10 phr oil; least in case of peptizer at 0.3 phr added with carbon black and with 10 phr oil. In the case of tensile strength, a moderate opening of the network (10 pht oil added) with a controlled molecular breakdown on peptization is advantageous. This is contrary to the trend in initial modulus (5% modulus) and implies that the trends in tensile strength and in initial modulus are not identical.

* Abrasion resistance: Best if 0.1 phr peptizer added with rubber and without oil; poor if 0.3 phr peptizer added with rubber and with 10 phr oil. The vulcanizate offers the best abrasion resistance with a moderate reduction in molecular weight (peptization), and in a tighter network (without oil), in line with what was observed for the 5% initial modulus. The abrasion resistance is poor for a condition where 5% modulus is also low (0.3 phr peptizer + 10 phr oil). This is an important observation leading one to conclude that the conditions that favor the highest 5% modulus will also favor the best abrasion resistance (ref. 18).

Conclusion

These highlights, drawn from the experimental findings, lead to certain important conclusions regarding the role of factors other than carbon black in influencing vital control parameters of rubber vulcanizates. The control parameters mean the attributes of the rubber vulcanizates in terms of measurable properties that in many cases do not truly represent the behavior of the vulcanizates under the service conditions. However, it can be considered as effective under laboratory conditions.

Therefore, one may end up optimizing the conditions, depending on the properties. For example, the findings suggest that the best abrasion resistance was obtained with peptizer at 0.1 phr, added with rubber, and withholding the process oil. And for high modulus peptizer at 0.3 phr, added with rubber and withholding the oil, is favored. For the tensile strength peptizer at 0.3 phr, added with the carbon black in the presence of 10 phr oil, was found to give the best result. The peptizer was found to have a greater influence on the rubber properties than the rubber-filler interaction.

References

(1.) Jean Le Bras and Eugene Papirer, "The filler-elastomer chemical link and the reinforcement of rubber, "Journal of Applied Polymer Science, Volume 22, Issue 2, February 1978, pp. 525-531.

(2.) Yoshihide Fukahori, "The mechanics and mechanism of the carbon black reinforcement of elastomers, '" Rubber Chemistry and Technology (2003), 76 (2), p. 548.

(3.) Gerard Kraus, "Reinforcement of elastomers by carbon black," Rubber Chemistry and Technology (1978), 51(2), p. 297.

(4.) Siegfried Wolff, Meng-Jiao Wang and Ewe-Hong Tan, "Filler-elastomer interactions. Part VII. Study on bound rubber," Rubber Chemistry and Technology (1993), 66 (2), p. 163.

(5.) Meng-Jiao Wang, Siegfried Wolff and Ewe-Hong Tan, "Filler-elastomer interactions. Part VIII. The role of the distance between filler aggregates in the dynamic properties of filled vulcanizates," Rubber Chemistry and Technology (1993), 66(2),p. 178.

(6.) A.I. Medalia, "Effect of carbon black on ultimate properties of rubber vulcanizates," Rubber Chemistry and Technology (1987), 60(1), p. 45.

(7.) R.E Ohm, "Additives that affect mixing," chapter 6, The Mixing of Rubber, ed. by Richard E Grossman, Chapman & Hall, 1997.

(8.) WE Watson, "Mechanico-chemical reactions of polymers, " abstract of Die Makromolekulare Chemic, Volume 34, Issue 1, October 1959, pp. 240-252.

(9.) GE Bloomfield, "Raw polymeric materials," chapter 4, part A, 4.1, p. 87, Rubber Technology and Manufacture, Second Edition, ed by C.M. Blow and C. Hepburn, Butterworth, London, 1982.

(10.) B.B. Boonstra, "Mixing of carbon black and polymer: Interaction and reinforcement," Journal of Applied Polymer Science, volume 11, issue 3, March 1967, pp. 389- 406.

(11.) Siegfried Wolff and Meng-Jiao Wang, "Filler-elastomer interactions. Part IV. The effect of the surface energies of fillers on elastomer reinforcement," Rubber Chemistry and Technology (1992), 65 (2), p. 329.

(12.) Meng-Jiao Wang, "Effect of polymer-filler and filler-filler interactions on dynamic properties of filled vulcanizates," Rubber Chemistry and Technology (1998), 71 (3), p. 520.

(13.) Douglas C. Montgomery, Design and Analysis of Experiments, Chapter 5, 5th edition, John Wiley & Sons, Inc. (2003).

(14.) Robert E.H. Lochner and J.E. Matar, Designing for Quality: An introduction to the best of Taguchi and Western of Statistical Experimental Design, Chapman & Hall, 1990.

(15.) B. Arun, V. Subrahmanian and V. Taneja, "Factorial design assisted experiment: Influence of carbon black, oil and anti-degradant in a NR/BR blend," Rubber World, Vol. 233 (2), November 2005, p. 28.

(16.) W.F. Watson, "'Chemical interaction of fillers and rubbers during cold milling," chapter 8, p. 248, Reinforcement of Elastomers, edited by Gerard Kraus, Interscience, New York, 1965.

(17.) Xuanying Zhang and Qiang Hu, Guoji LL "Oil-extended SBR for use in agricultural tire treads," abstract of Huaxue Shijie (1985), 26 (3), 92-6.

(18.) B.B. Boonstra, "Reinforcement by fillers," chapter 7, section 7.2, p. 271, Rubber Technology and Manufacture, Second Edition, ed. by C.M. Blow and C. Hepburn, Butterworth, London, 1982.

V. Subrahmanian, B. Arun, V. Taneja and A. Nithya, Aditya Birla Fundamental Research Institute

Table 1--standard design matrix

Standard      Factor     Factor     Factor
order no.       A          B          C

1                1          1          1
2               -1         -1         -1
3                1          1         -1
4                1         -1         -1
5               -1          1         -1
6               -1         -1          1
7               -1          1          1
8                1         -1          1

Table 2--design matrix for the compounding study

Standard       Factor A      Factor B     Factor C
order no.     (peptizer)      (oil)       (pep. addition)

1                0.3            10        With rubber
2                0.1             0        With CB
3                0.3            10        With CB
4                0.3             0        With CB
5                0.1            10        With CB
6                0.1             0        With rubber
7                0.1            10        With rubber
8                0.3             0        With rubber

Table 3--test recipe details and mixing procedures

Ingredients                                 Phr
Formulation no.     F1     F2     F3     F4     F5     F6     F7     F8

1 NR STR 20        100    100    100    100    100    100    100    100
2 Accimel-PCTP     0.3    0.1    0.3    0.3    0.1    0.1    0.1    0.3
3 CB-N220           60     60     60     60     60     60     60     60
4 Zinc oxide         3      3      3      3      3      3      3      3
5 Stearic acid       1      1      1      1      1      1      1      1
6 Aromatic oil      10      -     10      -     10      -     10      -
7 Sulfur           1.5    1.5    1.5    1.5    1.5    1.5    1.5    1.5
8 CBS              1.5    1.5    1.5    1.5    1.5    1.5    1.5    1.5

Mixing condition

Start temp: 50[degrees]C, RPM 70, fill factor 80%

First stage:      Order of addition
0 - 01:00 m:s     Add rubber
01:01 - 02:00     1/2CB+ZnO+S.acid
02:01 - 03:00     Add remaining CB+oil
03:01 - 03:30     Sweep
03:40             Dump and sheet it out

Dump temperature: 125[degrees]C - 130[degrees]C

Maturation period: 2 hrs. at 25 [+ or -] 3[degrees]C

Second stage:     Order of addition
0 - 00:30 m:s     Warm masterbatch
00:31 - 01:30     Add curatives
01:31 - 01:40     Sweep
01:45             Dump and sheet it out

Cool at room temperature (25 [+ or -] 3[degrees]C)

Dump temperature: 95[degrees]C - 105[degrees]C

Table 4--rubber compound properties

SL                                                     Test
no.   Physico-chemical properties            UOM     method

1     ML(1+4) @           Viscosity          MU        ASTM
      100[degrees]C                                   D1646

2     Scorch @            [t.sub.5]          m:s      D1646
      125[degrees]C       [t.sub.35]         m:s

3     Rheological         MH                 lb.in    D6204
      properties:         ML                 lb.in
      145[degrees]C       [ts.sub.2]         m:s
                          [t.sub.50]         m:s
                          [t.sub.90]         m:s

4     Hardness            Durometer A                 D2240
                          IRHD                        D1415

5     Stress-strain       5% modulus         MPa       D412
      properties: cured   50% modulus        MPa
      @ 145[degrees]C     100% modulus       MPa
                          300% modulus       MPa
                          Tensile strength   MPa
                          Elong. brk         %

6     Rebound             DIN 53512, N = 5            D1054

7     Abrasion loss                          mg       D5963

8     Specific gravity                       gm/cc

9     Dispersion rating   "F" mode

SL
no.   Physico-chemical properties              F1      F2      F3

1     ML(1+4) @           Viscosity           78.3   105.7    84.4
      100[degrees]C

2     Scorch @            [t.sub.5]           4.34    4.17    4.42
      125[degrees]C       [t.sub.35]          5.29    5.07    5.39

3     Rheological         MH                 15.14   18.99   16.20
      properties:         ML                  2.73    3.60    3.12
      145[degrees]C       [ts.sub.2]          3.24    3.12    3.25
                          [t.sub.50]          4.44    4.45    4.41
                          [t.sub.90]          8.44    9.01    8.25

4     Hardness            Durometer A           64      69      64
                          IRHD                  69      73      69

5     Stress-strain       5% modulus          0.51    0.55    0.49
      properties: cured   50% modulus         1.41    1.70    1.43
      @ 145[degrees]C     100% modulus        2.59    3.11    2.62
                          300% modulus       13.62   16.54   13.79
                          Tensile strength   23.23   22.67   22.46
                          Elong. brk           479     393     468

6     Rebound             DIN 53512, N = 5    36.5    41.2    37.1

7     Abrasion loss                            190     148     177

8     Specific gravity                       1.125   1.134   1.129

9     Dispersion rating   "F" mode             6.8     7.0     7.2

SL
no.   Physico-chemical properties              F4      F5      F6

1     ML(1+4) @           Viscosity          103.7    89.7   108.2
      100[degrees]C

2     Scorch @            [t.sub.5]           4.13    5.20    3.33
      125[degrees]C       [t.sub.35]          5.07    6.12    4.29

3     Rheological         MH                 18.91   16.52   19.15
      properties:         ML                  3.51    3.25    3.71
      145[degrees]C       [ts.sub.2]          2.42    3.37    2.49
                          [t.sub.50]          4.13    4.53    4.17
                          [t.sub.90]          8.17    8.33    8.15

4     Hardness            Durometer A           69      65      70
                          IRHD                  73      70      75

5     Stress-strain       5% modulus          0.58    0.49    0.58
      properties: cured   50% modulus         1.92    1.40    1.82
      @ 145[degrees]C     100% modulus        3.73    2.58    3.56
                          300% modulus       18.02   13.79   18.55
                          Tensile strength   23.23   24.44   23.13
                          Elong. brk           383     487     370

6     Rebound             DIN 53512, N = 5    39.5    35.8    39.6

7     Abrasion loss                            140     170   131.5

8     Specific gravity                       1.137   1.128   1.137

9     Dispersion rating   "F" mode             6.1     6.5     6.0

SL
no.   Physico-chemical properties              F7      F8

1     ML(1+4) @           Viscosity           89.6    95.9
      100[degrees]C

2     Scorch @            [t.sub.5]           4.36    3.00
      125[degrees]C       [t.sub.35]          5.34    3.49

3     Rheological         MH                 16.41   18.58
      properties:         ML                  3.27    3.19
      145[degrees]C       [ts.sub.2]          3.24    2.14
                          [t.sub.50]          4.38    3.37
                          [t.sub.90]          8.14    7.22

4     Hardness            Durometer A           65      68
                          IRHD                  70      72

5     Stress-strain       5% modulus          0.52    0.56
      properties: cured   50% modulus         1.45    1.89
      @ 145[degrees]C     100% modulus        2.68    3.73
                          300% modulus       14.16   18.57
                          Tensile strength   24.31   23.84
                          Elong. brk           484     380

6     Rebound             DIN 53512, N = 5    36.8    40.5

7     Abrasion loss                            160     138

8     Specific gravity                       1.127   1.138

9     Dispersion rating   "F" mode             7.0     6.6

Table 5--basic response sheet structure

Standard        Observed    Peptizer       Oil        Order of
order no.        value                                addition

                               A            B            C

                             1    -1     1    -1        1     -1
                Property   0.3   0.1    10     0     With   With
                                                   rubber     CB
             xxx
1                                 NE          NE              NE
2                           NE          NE             NE
3                                 NE          NE       NE
4                                 NE    NE             NE
5                           NE                NE       NE
6                           NE          NE                    NE
7                           NE                NE              NE
8                                 NE    NE                    NE
Total
No. of values                4     4     4     4        4      4

Standard                        Interaction
order no.
                     A * B         A * C         B * C

                    1    -1       1    -1       1    -1

1                        NE            NE            NE
2                        NE            NE            NE
3                        NE      NE           NE
4                  NE            NE                  NE
5                  NE                  NE     NE
6                        NE      NE           NE
7                  NE            NE                  NE
8                  NE                  NE     NE
Total
No. of values       4     4       4     4       4     4

Average (y0)
Effect (+1) - (-1)
Regression coeff. = effect/2
(NE = no entry)