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Influence of carbon black, process oil and antidegradant in a NR/BR blend.

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. 1 and 2). 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.

A tire sidewall must have good resistance to fracture over a range of conditions. A vulcanizate based on a blend of NR and cis-BR seems to meet this requirement. Lee and Donovan (ref. 3) observed that carbon black facilitated strain-induced crystallization and also increased the size of the crystallized zone at the stressed crack tip of carbon black filled NR. Both effects were major reinforcement mechanisms that increased the fracture resistance of the reinforced NR. It is proposed that the tightly linked gum does not strain-crystallize appreciably during stretching, but that its filled counterpart does. Carbon black appears capable of inducing crystallization in a network that alone remains amorphous during extension (ref. 4). Filled vulcanizates of various crosslink densities have similar tensile strengths. Furnace blacks invariably come with a certain amount of grit. Hoffmann (ref. 5) found that the presence of such gritty/sand particles, at a level greater than 0.01 pbw, reduced the cut growth resistance by 25%.

It was also proposed that a blend ratio of 2:3 for BR:NR would provide the best cut growth resistance. In one of the studies on a NR/cis-1,4-polybutadiene (BR) blend, N550 carbon black was added to the blend using different mixing techniques (ref. 6). When the black and polymers are mixed simultaneously, the black tends to distribute itself uniformly between the polymers. Physical test results on a typical sidewall compound indicated that the location of the carbon black has a significant effect on physical properties, especially cut-growth resistance. Hamed and Al-Sheneper (ref. 7), in their study of the effect of carbon black concentration on cut growth in NR vulcanizates, noted that modulus increased by seven-fold, elongation at break decreased by half, while tensile strength was unaffected by varying the carbon black from 0 to 70 phr. Thornley (ref. 8) in his detailed work on the mechanism on ozone cracking of rubber made an important observation stating that a low carbon black content would improve the resistance to radial cracking. In other words, the carbon black stayed in the phase to which the carbon black was added initially in the mixing cycle, and it had its proportional effect on the overall blend. At the same time, hysteresis and the hysteresis-bearing polymers were found to have a positive influence on fatigue crack growth. Crack growth resistance depends on the depth of cut and the carbon black (N115) content. With incremental loading of carbon black, crack growth resistance improves up to 15 phr of carbon black. Beyond that, and at least up to 45 phr, the crack growth resistance is poor. For a 50 phr loading of carbon black, the crack growth resistance is the best due to crack splitting. The addition of carbon black is expected to increase the chain scission, due to higher mixing temperature and increased shear stress, and this is countered by increased carbon black/rubber interaction. The crack growth resistance with carbon black passes through a maximum with crosslink density.

The standard test methods for measuring flex fatigue actually give different results. Improving the dispersion with longer mixer cycles improves DeMattia crack growth resistance. In the case of blends of polymers, the role of phase mixing is intriguing. It was found by Massie, et al, that the carbon black did not have any preference to a particular phase (ref. 6). However, if carbon black is present more in the NR phase in NR blends, then the crack growth resistance is better. For example, crack growth slowed if more carbon black is forced into the SBR phase in a SBR/BR blend using phase mixing techniques and into the NR phase in blends of NR/BR/EPDM, NR/BR or NR/chlorobutyl (ref. 9). The quality of dispersion is also important. Carbon black agglomerates with domain size larger than 6 [micro] can act as failure-initiating flaws and are responsible for the decrease in ultimate tensile strength, tearing energy, fatigue resistance, etc. Whereas a better fatigue life is reported in the case of larger particle size and lower structure blacks (example HAF-LS, N326) with even distribution in a NR/BR blend. Interestingly, in an N299 loaded NR/BR blend, fatigue was found to improve with a higher BR component (ref. 10). All this amounts to conclude that the blend composition and the morphology of the carbon black are equally important in the fatigue study. The nature of mixing also has a bearing on the crack growth resistance. For example, with a higher dump temperature (more than 163[degrees]C) in SBR (as a result of higher gel content), the crack growth resistance was found to decrease. Studebaker and Beatty (ref. 11) observed that poor dispersion, in terms of un-dispersed agglomerates, might interfere with the propagation of the cut and might lead to knotty tear. BR of higher molecular weight was reported to give a higher fatigue life. Maybe because of the requirement of higher molecular weight, in the case of NR it is strongly recommended to avoid peptizer (ref. 12). However, this may increase the heat build-up and decrease the blow out time. In the case of SBR, a higher bound styrene improves fatigue life and the tearing resistance (ref. 13), which means that a similar mechanism is operative in both cases. The antiozonant has a strong positive influence on fatigue performance and related properties like abrasion resistance. However, this was not revealed in the experimental data reported in certain cases (ref. 14).

Very often, because of higher carbon black loading and possibly from different origin and the method of manufacture, some of the variations in the rubber properties are attributed to the carbon black both in tire and non-tire applications. It is partly correct, allowing for the fact that the carbon black has a surface chemistry on its own. At the same time, no less important is the role of either the process oil or the chemically active antioxidant and/or antiozonant that is invariably added to the rubber mix. Though designed experiments and the role of anti-degradants have been studied in case of fatigue cracking, as of this date no work seems to have been done in applying the orthogonal matrix principle on the:

* Level of carbon black;

* process oil; and

* antioxidant, and this is examined in this article.

Experimental

The selection of the formulations has been done based on typical formulations. The polymers and the compounding ingredients are of standard quality and obtained from relevant sources. The N550 carbon black was from Birla carbon. Details of the orthogonal matrix and basic design matrix for the compounding study are given in tables 1 and 2. A two-roll mill of 6" diameter and 13" width was used for mixing of the com pounds. The trial recipes and the mixing cycle are given in table 3. A reference compound with no antioxidant (F9) was also added for internal comparison. Carbon black was added to the pre-blended mixture of NR and BR. 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 lists the equipment used for characterizing compound properties: Mooney viscometer, MDR rheometer, hardness (IRHD), tensile tester, and rebound resilience (Zwick), crack growth (DeMattia) and crack initiation (DeMattia) testers. A higher test temperature was chosen in order to bring out the influence of thermo-oxidative degradation on the flex property.

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.

The approach to deducing a regression equation (ref. 2) is given below step by step.

1. Start with the response sheet of a particular property (see table 5 example).

2. Average the observed values (call it as [Y.sub.O]).

3. Average the responses corresponding to the particular factor [oil ([X.sub.1]), filler ([x.sub.2]), antioxidant ([x.sub.3])] at the levels +1 and -1. Similarly, do it for the interaction terms (oil and filler), (oil and a/o), (filler and a/o) such as [X.sub.1][X.sub.2], [X.sub.1][X.sub.3] and [X.sub.2][X.sub.3] (let us denote them as xij).

4. Express the effect as the difference in response at +1 and at-1 levels, i.e., (average of observed value at + 1 level) - (average of observed value at -1 level) for a given factor.

5. Regression coefficient for a given factor = effect/2.0, i.e., the value deduced from the step (4) divided by 2.0 and label them as [a.sub.i]. i can be 1, 2 or 3, and in this case: 1 for oil, 2 for filler and 3 for anti-degradant (a/o) are assigned, aij are also obtained, where [a.sub.12], [a.sub.13], [a.sub.23] stand for the corresponding interactions.

6. Regression equation for a given rubber property (call it [Y.sub.i]): [Y.sub.i] = [Y.sub.o] + [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]

7. Verify the validity of this equation as follows: For a given property, [a.sub.i]s & [a.sub.ij]s are fixed as deduced (step nos. 4 to 6)

For a given standard order no. (refer to table 5 example response sheet), the [x.sub.i]s and [x.sub.ij]s will take either +1 or -1 according to where the observed test values are entered (NE means no entry to the column).

Substituting these +1 s and -1 s in the regression equation, one can get Yr, where r can be the standard order number like 1, 2, 3 ... 8 for a given property.

Results and discussion

The entire scheme of the factorial design to evaluate the influence of carbon black, oil and antidegradant is described in figure 1. The mixes were prepared on a two-roll mill in order to avoid any dispersion-related influence on the fatigue behavior (ref. 11). The properties of the robber vulcanizates both replicate I and replicate II are given in tables 4a and 4b, respectively. A study by the MRPRA (ref. 15) on a NR/BR blend with N330 carbon black held that open mill mixing was inferior, particularly for abrasion resistance. Further, it was found that antiozonant level did not affect the abrasion resistance. Because of this, although a possible application of this study could be in tire sidewalls, abrasion resistance was not included. The structure of a response sheet is illustrated in table 5.

[FIGURE 1 OMITTED]

The conformity of the deduced value with the observed value is described below.

Regression equation deduced is:

(Basic equation) [Y.sub.i] = [Y.sub.o] + [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]

Mooney viscosity as observed parameter:

[Y.sub.ML4] = 30.35 - 9.3 [x.sub.1] + 10.45 [x.sub.2] - 0.7 [x.sub.3] - 2.5 [x.sub.1][x.sub.2] - 0.5 [x.sub.1][x.sub.3] + 0.15 [x.sub.2][x.sub.3]

a) of higher range

[Y.sub.7] = 30.35 - 9.3 (-1) + 10.45 (+1)- 0.7 (-1)- 2.5 (-1) (+1) - 0.5 (-1) (-1) + 0.15 (+1) (-1) = 53.65 (vs. observed value = 52.8)

b) of lower range

[Y.sub.4] = 30.35- 9.3 (+1) + 10.45 (-1) -0.7 (-1)-2.5 (+1)(-1) -0.5 (+1) (-1) + 0.15 (-1) (-1) = 14.45 (vs. observed value = 14.6)

Carbon black and the oil have comparable and opposite influence. The interaction and the antidegradant effects are weak.

Maximum torque (MH) as observed parameter: [Y.sub.MH] = 9.39- 2.4 [x.sub.1] + 2.8 [x.sub.2] + 0 [x.sub.3]- 0.9 [x.sub.1][x.sub.2] + 0 [x.sub.1] [x.sub.3] + 0 [x.sub.2][x.sub.3]

a) of higher range

[Y.sub.5] = 9.39 - 2.4 (-1) + 2.8 (+1) + 0 - 0.9 (-1) (+1) = 15.49 (vs. observed = 15.50)

b) of lower range

[Y.sub.4] = 9.39 - 2.4 (+1) + 2.8 (-1) - 0.9 (+1) (-1) = 5.09 (vs. observed = 5.05)

Carbon black and the oil have comparable and opposite influence. The interaction and the antidegradant effects are weak.

300% modulus as observed parameter:

[Y.sub.M300] = 6.495 - 2.0 [x.sub.1] + 3.2 [x.sub.2]- 0.1 [x.sub.3] - 0.9 [x.sub.1][ x.sub.2] + 0 + 0

a) of higher range

[Y.sub.7] = 6.495- 2(-1) + 3.2 (+1)- 0.1(-1) - 0.9 (-1) (+1) = 12.69 (vs. observed = 12.71)

b) of lower range

[Y.sub.4] = 6.49- 2 (+1) + 3.2 (-1)- 0.1 (-1)- 0.9 (+1) (-1) = 2.29 (vs. observed = 2.39) Same comments as the preceding properties.

Resilience as observed parameter:

[Y.sub.R] = 51.8 - 3 [x.sub.1] - 6.3 [x.sub.2] -0.5 [x.sub.3] + 0.4 Xl[x.sub.2] + 0.2 Xl[x.sub.3] + 0.2 [x.sub.2] [x.sub.3]

a) of higher range

[Y.sub.2] = 51.8- 3(-1) -6.3(-1) -0.5(-1) + 0.4 (-1) (-1) + 0.2 (-1)(-1) + 0.2 (-1)(-1) = 62.4 (vs. observed = 62.2)

b) of lower range

[Y.sub.3] = 51.8- 3 (+1)- 6.3 (+1)-0.5 (-1) + 0.4 (+1) (+1) + 0.2 (+l) (-1) + 0.2 (+1) (-1) = 43.0 (vs. observed = 42.8)

Carbon black and the oil have similar influence. This may be attributed to effective reduction in the rubbery nature of the vulcanized network both with carbon black and oil addition. The interaction and the antidegradant effects are weak.

Tensile strength as observed parameter:

[Y.sub.T] = 11.19 - 2.3 [x.sub.1] + 2.8 [x.sub.2] - 0.2 [x.sub.3] + 0.3 [x.sub.1][1.sub.2] + 0 + 0

a) of higher range

[Y.sub.7] = 11.19 - 2.3 (-1) + 2.8 (+1) - 0.2 (-1) + 0.3 (-1) (+1) = 16.09 (vs. observed = 16.08)

b) of lower range

[Y.sub.6] = 11.19 - 2.3 (+1) + 2.8 (-1) - 0.2 (+1) + 0.3 (+1) (-1) = 5.59 (vs. observed = 5.52)

Carbon black and the oil have comparable influence, but in the opposite direction. The interaction and the antidegradant effects are weak.

Hardness as observed parameter:

[Y.sub.H] =45.12- 6.35 [x.sub.1] + 9.35 [x.sub.2] + 0.15 [x.sub.3]- 1.65 [x.sub.1] [x.sub.2] + 0.15 xl[x.sub.3] + 0.35 [x.sub.2][x.sub.3]

a) of higher range

[Y.sub.5] = 45.12 - 6.35 (-1) + 9.35 (+1) +0.15 (+1) - 1.65 (-1) (+1) + 0.15 (-1) (+1) + 0.35 (+1) (+1) = 62.82 (vs. observed = 63)

b) of lower range

[Y.sub.4] = 45.12 - 6.35 (+1) + 9.35 (-1) +0.15 (-1) - 1.65 (+1) (-1) + 0.15 (+l) (-1) + 0.35 (-1) (-1) = 31.12 (vs. observed =31)

The oil affects the hardness to a lesser degree as compared to the increase that is caused by the carbon black loading. The interaction effects are noticeable. But here also the antidegradant has only a weak effect.

Flex fatigue

The average of three tested specimen values is given in terms of crack growth resistance and resistance to crack initiation. For the crack growth test (pre-cut samples), the number of kilocycles required to generate 12 mm crack growth was measured. Compounds F 1, F3, F5, F7 and F9 attained 12 mm at 140, 120, 80, 80 and 50 kilocycles, respectively. The common feature in these mixes is higher carbon black loading, which is not adequately compensated by the process oil through softening the network. The F9 reference compound has poor crack growth resistance because of the absence of antidegradant and the test being done at 50[degrees]C.

Crack initiation was not noticed for the compounds F2, F4, F6 and F8 until 200, 520, 520 and 280 kilocycles, respectively. Similarly, after 200 kilocycles, crack growth measured for the compounds F2, F4, F6 and F8 were 4.8, 5.3, 4.5 and 5.1 ram, respectively. In both cases, testing was discontinued after the respective kilocycles.

In general, a fatigue failure originates at some flaw, such as a nick, inclusion, a microscopic region of under-curing or over-curing, or other stress raising discontinuity in the rubber matrix. Hence, some evidence of fatigue could be expected even in the best specimens made of reinforced rubber (ref. 16). Furnace blacks of low chemical reactivity inhibit polymer oxidation by functioning as free radical traps or by decomposing peroxides into inert non-radical products (ref. 17).

N550 with a higher structure (120 cc/gm DBP) is not able to offer the best fatigue life through the phenomenon of knotty tearing. This indirectly means that more than the knotty tearing (a beneficial feature for fatigue life) the role of hysteresis enabled by the presence of aromatic oil is strongly felt. An intense rubber-filler interaction provides scope for resilient deformation in the network. This phenomenon is favorable in static strength-related issues like tension or tearing. But under repeated loading, if an alternate source of energy dissipation is available, then such a situation will result in higher fatigue life. A simple route in a general purpose rubber formulation is the addition of process oils of a suitable solubility match. The hysteresis promotes stress distribution. If[E.sub.0] is the bond energy, and hence the energy which thermal fluctuating must add to the bond to instigate its failure, then the probability at any instant that the bond will have sufficient energy to fail is proportional to the exponent -[E.sub.0]/kT (ref.16). If this energy is spent through viscous dissipation, then the fatigue life will be extended to that extent.

Compounds F4 and F6 gave the best resistance to flex cracking. These compounds have a high level of process oil (30 phr) and low carbon black content (30 phr). It is likely that these vulcanizates undergo viscous dissipation, resulting in higher hysteresis loss. The corresponding energy, thus, is not translated in terms of formation of new surface, i.e., flex cracking. With this background, if we look at the crack growth resistance, compound F6 gave a better result than compound F4. Compound F6 has a higher antioxidant content. This finding strongly supports the need for viscous dissipation and the presence of antidegradant at an appropriate dosage. At the same time, the network is weak with a low hardness, modulus and tensile strength.

Resistance to crack initiation was found to be the best at a low carbon black level, irrespective of the amount of antidegradant. The resistance to cracking was poor with higher carbon black (70 phr), even in the presence of an antidegradant. This leads to the conclusion that the carbon black plays a negative role on fatigue cracking that is greater than the positive influence by the antidegradant. This observation is interesting when it is generally held that a high structure carbon black imparts better fatigue, particularly at higher temperatures.

For cut growth resistance, the carbon black and oil levels need to be low (30 and 10 phr, respectively) irrespective of the antidegradant. On the mechanism side of fatigue life, it is held that NR is a strong candidate, with the reason being its ability to undergo strain-induced crystallization. There is a sustained research effort in BR polymerization, wherein an ultra high cis BR is said to match NR in this respect (ref. 13).

There is another dimension to this in the case of a carbon black loaded elastomeric network. The competing phenomena of strain-induced crystallization and the carbon black bound rubber interaction decide the fatigue life. A higher strain-induced crystallization is possible at a higher operating strain, whereas at low operating strains, a higher fatigue life is possibly enabled by carbon black bound rubber interaction, irrespective of the nature of the polymer.

This understanding is in line with the general preference to NR (crystallizing) for truck (higher operating stress-strain situations) and SBR+BR (non-crystallizing) for passenger tire applications. It does not in any way explain the role of the basic attributes of carbon black (like surface area, structure and aggregate size distribution) on fatigue-related tire tread wear. The unique applications of rubber products under cyclic loading are largely attributed to its flexibility without detrimental effect on the dimensional stability. Load bearing is less critical in such applications. Therefore, products like bellows, inner tubes, dust covers, etc., are designed in such a way that the rubber network is not unduly diluted either by the filler or by the process oil. In fact, this could be the reason for engine mounts being designed with less filler loading. At the same time, enough care is taken to maintain the stability of the network through expensive cure systems.

Conclusion

A study of this nature, which involves compounding variables with their corresponding effects on the rubber compounds and vulcanizate properties, is fulfilled only with a proper approach to designing the experiments. A two-level three-factorial design with an orthogonal matrix was found to be convenient in this study. The major findings include:

* It is apparent from the test data that NR+BR as the blend for tire applications requires antidegradant. The role of carbon black and the process oil on the various physico-mechanical properties are on the expected lines.

* With a limited number of experiments based on a factorial design approach, the interaction effects studied indicate that such influences are minimal.

* With respect to resistance to flex fatigue, the study indicates that the unique elastomeric network (with a propensity to strain-induced crystallization) needs to be preserved. Given the options, a better flex life is possible with low carbon black addition.

* For a balance in mechanical strength and fatigue life, a moderate filler loading and corresponding process oil level are required for a given vulcanizate hardness.

* Irrespective of the hardness, the best fatigue life was possible with a liberal process oil loading (30 phr), and the mechanism suggested was viscous dissipation.

* Table 6 is a summary of the individual and the interactive effects of the N550, aromatic process oil and the antidegradant within the chosen dosages of (70 to 30), (30 to 10) and (1 to 2), respectively.

* It is widely held that the rubber-carbon black interaction is through a free radical mechanism. Similarly, the controlled molecular breakdown of the rubber, particularly NR with peptizer, is by a free radical route. Therefore, rubber-filler interaction could be effectively studied almost simultaneously with rubber-peptizer reaction. It will be the objective of further work to examine this and the effect of varying the order of addition using a factorial design of experiments approach.

References

(1.) Douglas' C. Montgomary, Design and Analysis of Experiments, 5th edition, Ch. 5, John Wiley & Sons (2003).

(2.) 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.

(3.) D.J. Lee and J.A. Donovan, "Microstructural changes in the crack tip region of carbon black-filled natural rubber," Rubber Chemistry and Technology, 60 (5), p. 910-923 (1987).

(4.) GR. Hamed and N. Rattanasom, "Effect of crosslink density on cut growth in black-filled natural rubber vulcanizates, " paper given at Rubber Division, ACS, Oct. 16-20, 2001.

(5.) J.F.J. Hoffmann, "Factors affecting the cut-growth resistance of vulcanizates, " Caoutchoucs & Plastiques, 43 (10), p. 1,315 (1966).

(6.) J. M. Massie, R.C. Hirst and A.E Halasa, "Carbon black distribution in NR/polybutadiene blends, "Rubber Chemistry and Technology, 66 (2), p. 276 (1993).

(7.) GR. Hamed and A.A. Al-Sheneper, "Effect of carbon black concentration on cut growth in NR vulcanizates, "paper given at Rubber Division, ACS, Apr. 29-May 1, 2002.

(8.) E.R. Thornley, "Relations between flex cracking and ozone cracking of rubber, "' Proc. Rubber Technol. Conf., 4th, (55), London (1962), 12pp.

(9.) D. G Young, "Dynamic property and fatigue crack propagation research on tire sidewall and model compounds," Rubber Chemistry and Technology, 58 (4), p. 785 (1985).

(10.) W. Hess, "Characterization of dispersions," Rubber Chemistry and Technology, 64 (3), p. 386 (1991).

(11.) M. Studebaker and J. Beatty, "The rubber compound and its' composition," Ch. 9, Science and Technology of Rubber, F. Eirich, Academic Press, 1978, p. 367.

(12.) J.S. Dick, Rubber Technology, Compounding and Testing for Performance, Hanser Publications, 2001, p. 166.

(13.) J. Zhao and G Ghebremeskel, "A review of some of the factors affecting fracture and fatigue in SBR and BR vulcanizates," Rubber Chemistry and Technology, 74 (3), p. 409 (2001).

(14.) See Toh Mook Sang, "Natural rubber--polybutadiene blending," Rubber Research Institute of Malaysia, 1983, p. 34.

(15.) ibid.

(16.) S.D. Gehman, "Mechanism of tearing and abrasion of reinforced elastomers," p. 25, Reinforcement of Elastomers, Gerard Kraus, ed., Interscience Publishers, 1965.

(17.) W.L. Hawkins and H.F. Winslow, "Antioxidant properties of carbon black," p. 571, Reinforcement of Elastomers, Gerard Kraus, 1 ed., Interscience Publishers, 1965.

B. Arun, V. Subrahmanian and V. Taneja, Aditya Birla Fundamental Research Institute, Tamil Nadu, India
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Author:Taneja, V.
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Date:Nov 1, 2005
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