Fatigue is defined as decay caused by cyclic deformations at an amplitude less than necessary for fracture in one cycle. Such failures are initiated by flaws which act as stress concentrators. These flaws occur in the material either through mechanical or chemical action during service or through agglomeration of certain ingredients during mixing and fabrication. This paper deals with the latter process, where the nature and size of the flaws as well as the properties of the matrix are contingent on carbon black variables. Using the tearing energy concept of fatigue developed by Lake and Lindley, it was shown that the size of the flaw is primarily determined by carbon black particle size. On the other hand, the cut growth constant depends on carbon black structure. When translated to actual fatigue life using the Monsanto Fatigue-to-Failure Tester, these relationships mean that under constant strain conditions, compounds containing coarse carbons will have a significantly higher fatigue life than those with fine carbons. Under conditions of constant strain, higher structure carbons will impart a slight positive effect. However, under conditions of constant stress, the beneficial effects of structure become magnified. Other factors known to affect fatigue life were also considered. These are : set, stress relaxation, hysteretic energy dissipation, and flaw size distribution.

This paper has attempted to review current knowledge on mixing fundamentals from a carbon black standpoint. The mixing parameters discussed include volume fill factor, specific energy of mixing, temperature buildup, and flow characteristics. Together, these parameters define the economics of mixing in terms of mixing capacity, energy expenditure, limitations imposed by dump temperature on second-stage operations, and extruder flow requirements. The four parameters in turn were related to basic carbon black properties by way of mixing profiles. Ten currently available tread blacks which exhibit a wide range of tread wear resistance have been characterized in terms of the processability criteria. A processability rating system based on the most simple equipment requirement (a Banbury followed immediately by a second-stage mill or an extruder) was applied to the ten blacks. Comparison of the processability ratings with the tread wear ratings clearly demonstrates that a valid assessment of carbon black utility must include processing cost in addition to performance capability. As in many rubber properties, there is clearly an inverse relationship between performance quality and processability. High-structure, high-surface-area blacks that are desirable from a performance standpoint require the highest processing cost. The question of uniformity was also discussed, and it was concluded that the present specifications controlling carbon black surface area and structure are adequate in minimizing processing variations. The challenge to processing technology from a carbon black standpoint then involves the reduction in the cost of processing high-structure, high-surface-area blacks. At this stage, a quantitative measure of the processing characteristics of the currently available tread blacks at least allows a rational product selection in terms of optimum performance and processing cost. In the meantime, it is evident that significant advances in carbon black technology are still needed if performance optimization is to be reconciled with the economics of processing.

Further insight into the mixing process has allowed a definition of the intermediate objectives of processing, which relate to the different transformations that occur as mixing energy is added to a rubber formulation. These convert the ingredients into a coherent mass with specific flow characteristics and determine the efficiency of the next unit, therefore contributing to overall productivity. The total energy added to the batch is derived from a combination of Banbury, mill, and extruder. These processing units vary in the efficiency with which they achieve the required material transformations. Proper allocation of mixing energy to the most effective equipment, with a knowledge of the total energy required to achieve the desired quality, allows a rational optimization of productivity and product quality. Operating profiles for each unit have been constructed in order to aid in optimizing the process. Using a fixed total energy input, these profiles were used to estimate the productivity of each processing unit. The study shows that in a semicontinuous operation with laboratory-size Banbury, mill, and extruder, the extruder is the primary determinant of overall productivity. The study also shows that maximizing productivity in a single unit will not necessarily lead to the highest productivity along the equipment train. Material properties affect overall productivity in several ways. In this work, carbon black surface area determined the total energy required to attain the desired quality level, the flow rate in the extruder, and the energy required to attain the maximum flow rate. Future studies should focus on the derivation of operating equations for specific equipment and materials. These equations should quantify the interrelationships between the different engineering parameters (such as screw or rotor speeds and Banbury ram pressure), the processing parameters (such as mixing time, fill factor, and temperature), and material variables (such as Mooney viscosity, carbon black morphology, and black and oil loading).

1. Car, wheel position, driver, inflation pressure, and shoulder drop have a statistically significant effect upon wear loss and need to bo taken into consideration before material factors affecting wear can be studied. 2. Variations in macrostructure of the polymers are not found to have a significant effect on wear as compared to microstructure variations. 3. At least two material factors control wear loss of tire treads. 4. When polymers are tested near their glass transition temperature (within 80° C), wear loss is dominated by viscoelastic properties. Viscoelastic properties can be related to wear loss through Tg or the combined effect of the cis, trans, vinyl, and styrene content. 5. At higher test temperatures (over 100° C above Tg) wear loss is dominated by a material factor that has a positive correlation with temperature. This is particularly noticeable when treads are worn under mild conditions. However, there is evidence that this wear factor is present at the testing nearer to Tg but is masked by the dominant viscoelastic effect. 6. The combined effect upon wear of the different material factors leads to an optimum wear resistance for any polymer in the butadiene—styrene system in the range of 75°–105° C above the Tg for that polymer. 7. For polymers tested at the same ambient temperature, (T), the effect of viscoelastic properties decreases non-linearly as T−Tg increases. 8. In the range of test severity studied, severity has little effect upon the inter-relationship of material factors. 9. In the range of test temperature where Tg dominates wear loss, skid distance on wet asphalt pavement is inversely related to wear rating.

A principle of equivalent strain at equal stress has been applied to dynamic measurements of hysteresis. At 24°C and 25% ptp dynamic strain, the relation of hysteresis to strain amplitude and the equivalence principle lead to an expression which describes the measured hysteresis of SBR-1712 filled with a wide variety of carbon black types and loadings to within about ±25%. At 105°C and 25% strain, the experimental results are much larger than the theory predicts, but are better described by the theory if an empirically determined exponent n is introduced in the equivalence principle. At strain amplitudes below 10%, the experimentally determined hysteresis is much lower than the theory predicts, but this deviation is described by the change in elastic modulus with strain amplitude at both 24° and 105°C. The deviations of the results from the theoretical predictions which are not described by the rate of change of elastic modulus with strain amplitude appear to be a function of carbon black surface area, structure, and loading.