BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements.
FIG. 1 is a cross-sectional view of a prior art belted tire, such as a steel belted radial tire.
FIG. 2 is a cross-sectional view of a steel belted radial tire illustrating oxidation shield strips in accordance with embodiments of the invention.
FIGS. 3A-3D are photo spectrographs illustrating exemplary simulations for thermal oxidation of a tire, including results for steady state oxygen profiles and oxidation rates with and without an oxidation shield strip of the present invention being provided in a wedge region of the tire.
FIG. 4A-4D show the effect on oxidation rate of changing the width, L, of an oxidation shield strip used as an oxidation barrier, using Kapton® material properties.
FIG. 5 is an exemplary plot of the mean oxidation rate in the wedge region versus the width, L of an oxidation shield strip.
FIG. 6 illustrates exemplary simulation results for oxidation shield strips, of varying diffusion coefficients, being placed in the wedge region of a tire.
DETAILED DESCRIPTION OF THE INVENTION
To mitigate oxidative aging, the present invention increases durability of a tire by inserting one or more strips of material in the vicinity of critical areas acting as a local diffusion barrier or shield to oxygen. These strips are referred to herein as oxidation shield strips. These strips can reduce the overall oxidative aging process in critical areas of the tire by (1) reducing the molecular oxygen flux (rate of oxygen transport) to the region of interest, and (2) reducing the molecular oxygen concentration due to the reduced flux. This would desirably reduce the chemical kinetic rate because less oxygen is available to react. Examples of critical areas that could benefit from local oxidation protection include, but are not limited to, the tire wedge, the tire apex, and the lower, mid, and upper side walls.
FIG. 2 illustrates exemplary embodiments of the present invention. A cross section of an exemplary tire having treads and belts is shown. To simplify the drawing, the body plies from FIG. 1 have been omitted, with the understanding that at least one body ply is preferably present between the butyl liner and the belt(s). The tread increases tire traction and the belts help prevent puncturing of the tire. A sidewall extends from each side of the treads. The tire is also shown to have a butyl liner which, as stated above, is provided at the inner wall of the tire to reduce oxidation caused by inflation air. The wedges of the tire are located in between and adjacent to the edges of the belts. They act as energy absorbers and help prevent belt edge separation (BES). The apexes of the tire are located at distal ends of the side walls. As can be seen, at least one strip S1 of material acting as an oxidative shield (due to reduced oxygen permeability), can be provided in the vicinity of the wedge, preferably on each side of the tire. The strips S1 are preferably provided to be symmetrical about the axis of the cross section of the tire. The present invention contemplates providing any number of strips in the vicinity of the wedge. Like the butyl layer, the oxidation shield strips S1 reduce oxidation caused by inflation air, but do so in a localized manner in the vicinity of a critical area of the tire.
Alternatively or in addition, at least one strip S2, can be provided in the vicinity of the tire's apex to locally reduce oxidation caused by inflation air. The strips S2 are preferably provided to be symmetrical about the axis of the cross section of the tire. The present invention contemplates providing any number of strips in the vicinity of the apex.
The present invention also contemplates providing strips in other areas of the tire that would enhance tire durability, such as within the side walls of the tire.
The present invention contemplates oxidation shield strips having varying widths and thicknesses, depending on the diffusivity of the material being used, along with other factors such as cost and overall tire performance. The strip may, for example, comprise Kapton® and have a width of a few centimeters and a thickness of less than a millimeter. The present invention contemplates that a strip has a width that is preferably less than would extend through both the tread and side walls of the tire and is capable of providing localized results.
The oxidation shield strips S1 and S2 are preferably provided within the layers of the tire, but may also be placed along the inner wall of the tire to reinforce the butyl layer. In a particularly preferred embodiment, the oxidation shield strips S1 and S2 are provided between the butyl layer and the belts.
Each oxidation shield strip preferably has, as one of its properties, a low permeation rate (or diffusion coefficient D) for oxygen. With such a property and placement as described, the overall oxidative durability of a tire can be greatly improved.
To support the oxidative shielding concept for an oxidation shield strip placed in the region of the tire wedge, a tire oxidative aging model was used to simulate the steady state oxidative profile for a tire rolling at 40 mph with loading of 1500 pounds and an inflation pressure of 35 psi. The test tire geometry used was a Michelin P235/75 R15. The results are shown in FIGS. 3A through 3D, with oxygen content on the left and oxidation rate on the right. FIGS. 3A and 3B show results without an oxidation shield in the area of the wedge and FIGS. 3C and 3D show results when an oxidation shield is placed in the area of the wedge. The material selected for the oxidation shield strip in the simulation was a two centimeter wide strip of 0.1 millimeter thick Kapton® film (a polymer film), but the present invention contemplates oxidation shield strips comprising a variety of materials with low permeation rates, such as other polymer films, including nylon and polyester films. The shield may also comprise a combination of the above-listed materials. The present invention also contemplates the strip comprising a woven or non-woven material, as long as the overall permeability of the oxidation shield remained suitably low. The woven or non-woven material may be used in addition to or instead of a film.
As can be seen in FIGS. 3A-3D, simulation results show that oxygen concentration drops in the tread region close to the wedge, which is a result of limited replenishing of oxygen that is consumed in the oxidation process in the thick region of the tire. At a location near the center of the wedge, the simulation test results show that the molecular oxygen concentration was reduced from 68.4 e-9 mole/cm3 to 9.92e-9 mole/cm3, and the oxidation rate was reduced from 11.6e-12 mole/cm3/s to 2.41e-12 mole/cm3/s. The latter constitutes a reduction by a factor of five in the rate of oxidation in the wedge region, which implies a reduction in oxidative aging by the same factor.
FIGS. 4A-4D show the effect on oxidation rate of changing the width, L, of an oxidation shield strip used as an oxidation barrier, again using Kapton® material properties. It should be noted that there is a maximum oxidation rate close to the outside of the tire. This location coincides with the maximum temperature developed during driving conditions, but is offset toward the outside edge of the tire. The higher temperatures result in higher kinetic rates to cause the maximum oxidation rate, but also increase the oxygen diffusion coefficient in the tire's natural rubber, causing the shift in location of the maximum oxidation rate. It should also be noted that the lighter regions of the contours, representing lower oxidation rates, slowly engulf the wedge region as the insert width is increased from 0.5 centimeters to three centimeters. This indicates that the increased protection of the wedge from the transport of oxygen from within the tire results in a lower oxidation rate.
The effect is shown more clearly in FIG. 5, which is an exemplary plot of the mean oxidation rate in the wedge region versus the width of the insert strip. FIG. 5 clearly shows a reduction in the benefit of increasing the strip width beyond two centimeters with an inflection occurring near one centimeter. The inflection represents the maximum change in benefit for a given width, indicating that one centimeter is the preferred minimum strip width used given the assumed film thickness and properties. The results, of course, will likely vary depending on the characteristics (e.g., the diffusivity and thickness) of the strip material.
FIG. 6 illustrates the change in the mean oxidation rate for a tire's wedge region, given oxidative shielding having a variety of diffusion coefficients D. Thus, FIG. 6 shows that a wide range of materials may be selected for use as an oxidative shield, allowing flexibility in mechanical properties for the oxidation shield, economy, and shielding effect. The highest value used for the test simulation was 4.5×10−6 cm2/s, which is the diffusivity of a tire's rubber composition, and therefore the equivalent of having no oxidation shield. It should be noted that the mean oxidation rate increases with increasing diffusivity values, having an approximate exponential behavior with plateaus at the two extremes.
The embodiments of the invention set forth above are exemplary only. One skilled in the art would understand that the invention as claimed below can encompass known variations of the above embodiments and remain within the scope of the claims. For example, oxidation shield strips can be provided throughout the tire, to act locally within the wedge, the apex, and the side walls. The tire construction need not be steel belted as shown. The oxidation shield strip would also work well in the critical areas of most other types of tires, including but not limited to bias ply and bias belted tires.
Patent Application
20070277918
