Oxidation kinetics in beltcoat compound.

Because tires spend most of their lives at ambient temperature, knowledge of oxidation kinetics in this temperature regime is very important. Oxidation has been shown to be a key degradation mechanism in beltcoat compounds governing belt edge durability in passenger tires. The ultra-sensitive oxygen consumption technique developed by K. Gillen (ref. 1) was used to measure oxidation kinetics of a passenger tire beltcoat compound over a wide temperature range, including ambient temperature. This technique allows accurate measurements in this lower temperature regime, from which accurate time-temperature lifetime oxidation master curves can be obtained. In this way, it is not necessary to extrapolate to ambient temperatures to predict lifetime at service temperatures.

Strategy for determining aged property retention at ambient temperatures

Although the measurement of property decay at elevated temperatures and subsequent extrapolation using an Arrhenius fit has been used to predict property retention at ambient temperatures, it is generally not satisfactory (ref. 2). The purpose of this work was to find another avenue to determine aged property retention of beltcoat compound at ambient temperatures. The oxidation kinetics were studied using the ultra-sensitive oxygen consumption technique developed by K. Gillen (ref. 1). This technique has been shown to be able to accurately measure oxidation kinetics at ambient temperatures (refs. 2-4). These kinetic results were subsequently used in combination with physical property data to predict aged property retention at ambient temperatures. The aged property retention was studied using tensile properties on aged thin sheets in an oven under accelerated aging conditions. Thin samples were used in each case to ensure that diffusion limited oxidation (DLO) conditions did not exist during the experiment. Experiments were conducted on passenger beltcoat/wedge compound extracted from the belts at the belt edge region. The tire was a passenger tire after 1.5 years of service with size P215/ 75R15.

Oxidation kinetics

Oxidation kinetics were measured over a range of temperatures from 20[degrees]C to 80[degrees]C. The isothermal results are shown in figure 1. A typical way to determine activation energy is an Arrhenius type plot, as shown in figure 2. The results show that the activation energy for oxidation of the beltcoat compound above 50[degrees]C was 100 kilojoules per mole. This activation energy would be considered to be in the expected range for oxidation (90-100 kilojoules/mole). Below 50[degrees]C, there was a deviation from the linear data expected by Arrhenius type analysis. In the same measurements, carbon dioxide and carbon monoxide generation were measured. The carbon dioxide and carbon monoxide generation rates are shown in figures 3 and 4, respectively. The carbon dioxide and carbon monoxide generation rate data appear to be linear in an Arrhenius type analysis. Their activation energies were 88 kilojoules per mole and 96 kilojoules per mole, respectively. By repeated ultra-sensitive oxygen consumption measurements on the same sample, the integrated oxygen consumption for each temperature was obtained (figure 5). Time temperature superposition was used to generate a master curve through empirical shift factors (figure 6). Subsequently, the shift factors were plotted as a function of temperature in an Arrhenius type plot (figure 7). The shift factors were linear above 50[degrees]C with activation energy of 100 kilojoules per mole. The key point is that high temperature (above 50[degrees]C) oxidation kinetics showed the expected trends in kinetics and mechanism (activation energy). Below 50[degrees]C, on the other hand, the kinetics are not following expected trends; but rather, are oxidizing significantly faster than expected by Arrhenius type analysis. Furthermore, it appears that the ultra-sensitive oxidation method with time temperature superposition has generated the necessary shift factors in the ambient temperature region to predict physical property decay at ambient temperatures. So, the next step was to measure elongation to break for various temperatures, generate a decay master curve, and then shift it to ambient (service) temperatures of interest.

[FIGURES 1-7 OMITTED]

Tensile properties

Tensile properties were measured over a range of temperatures from 60[degrees]C to 100[degrees]C (figure 8). Time temperature superposition

of the elongation to break data yields a master curve (figure 9) with shifted aging time at 80[degrees]C on the abscissa. A plot of shift factors as a function of temperature was linear with an activation energy of 92 kilojoules/mole. Using the shift factors from ultra-sensitive oxygen consumption, the elongation to break master curve was shifted to ambient temperatures (figure 11). Elongation retention is shown as a function of aging time at 20[degrees]C. The plot provides property changes at ambient temperature (20[degrees]C). When in service, a tire beltcoat compound at the belt edge may see higher temperatures when rolling. Also, the compound may see higher oxygen partial pressures (above atmospheric). These are further refinements that could be made to this elongation retention curve, leading to improvements in service life predictions. The key ingredient for life predictions and the basis of the master curve is the accurate oxygen consumption measurements at ambient temperatures to obtain the shift factors at low (service) temperatures.

[FIGURES 8&9,11 OMITTED]

Mechanism of oxidation

Figure 12 shows a comparison of shift factors from the various tests, elongation, oxygen consumption, carbon dioxide and carbon monoxide generation. All of the methods agree above 50[degrees]C, with activation energy at 92 kilojoules/mole. Below 50[degrees]C, the oxygen consumption results deviate from linearity. The carbon dioxide and carbon monoxide generation did not appear to deviate from linearity. This suggests that there is a mechanism change below 50[degrees]C for this beltcoat compound. Understanding the details of the low temperature oxidation mechanism is a key technical development to improve tire performance and durability. For the time being, the kinetics of oxidation and property decay could be incorporated in typical tire design strategy.

[FIGURE 12 OMITTED]

Summary and conclusions

Extrapolations from high temperatures to lower temperatures can lead to invalid estimates of oxidation rates. To rank oxidation resistance in field tires, it is preferable to measure oxidation kinetics at service temperatures.

The ultra-sensitive oxygen consumption technique allows accurate measurement of oxidation kinetics at temperatures below 50[degrees]C in a reasonable amount of experimental testing time (months instead of years).

At temperatures below 50[degrees]C, there was a change in the mechanism of oxidation in the beltcoat compound. Degradation was ten (10) times faster than extrapolated values from elongation to break data.

Elongation to break for various temperatures generated a property decay master curve. It was subsequently shifted onto ambient (service) temperatures of interest. The ultra-sensitive oxidation method with time temperature superposition has generated the necessary shift factors in the ambient temperature region to predict physical property decay at ambient temperatures.

References

(1.) J. Wise, K.T. Gillen, and R.L. Clough, Polymer Deg. Stab. 49, 403 (1995).

(2.) K.T. Gillen, M. Celina, R.L. Clough and J. Wise, Trends in Polymer Science 5, 250 (1997).

(3.) K.T. Gillen, R. Bernstein and D.K. Derzon, Polymer Deg. Stab. 87, 57 (2005).

(4.) K.T. Gillen, R. Bernstein and M. Celina, Polymer Deg. Stab. 87, 335 (2005).