Use of desiccant material in rubber compounds for reducing the cure time of tires

The present invention pertains to desiccant material and to methods of use thereof for reducing the curing time of tires cured under pressure.

Tires, including tire components, must typically be cured under pressure beyond what is known as the blow point, which is the point of cure of the rubber compound where blows no longer form. Blows generally are small or large bubbles, or blisters, that are formed in the rubber compound as a result of the formation and expansion of gases trapped therein during curing. In a typical pressure molding process, e.g. compression or injection molding, the external pressure from the mold helps prevent the gases from expanding. At a certain point during vulcanization or crosslinking of the rubber compound, i.e. the blow point, the modulus of the rubber compound prevents further expansion of the gases, without pressure from the curing mold. In typical rubber compounds, the blow point is a level of curing of those compounds that is usually at least 30% of full cure thereof. At this point, the tire can be removed from the mold.

In a tire, the cure time typically is set so that the point of least cure exceeds the blow point. The location of least cure is usually found at the thickest part of the tire. In order to increase the production rate of pressure-cured tires, it would be desirable to lower the curing time. Lowering the blow point will reduce the cure time of tires cured under pressure, allowing for increased tire production. As indicated above, the blow point is the extent of cure of the rubber compound of interest wherein gas expansion generated during pressure curing of the rubber compound no longer occurs. Normally, long cure times under pressure, e.g. 30% to 100% of the full cure of the rubber compound, are required for the elimination of blows in curing tires or tire components. In pressureless cures, such as high temperature salt baths, high levels of a desiccant, e.g. at least about 12 phr, are required for the elimination of blows. PHR is used herein to describe parts by weight per 100 parts by weight of rubber in the rubber compound.

Accordingly, there remains a need for an improved method for reducing the curing time of tires cured under pressure to increase the production rate thereof.

To increase tire production, the present invention provides for an improved method for reducing the curing time of tires cured under pressure by using low levels of desiccant material in the tire's rubber compounds, e.g. a tire tread. The low level of desiccant material reduces the blow point of the rubber compound by preventing blows earlier in the pressure curing process, thereby allowing for the reduced cure time of the tire under pressure.

In one embodiment, low levels of no greater than about 9 phr of desiccant material are provided in a rubber compound for use in a tire, such as a passenger or truck tire. The desiccant material may include generally a metal oxide, e.g. calcium oxide or magnesium oxide. In another embodiment, the desiccant material is an 80/20 blend by weight of calcium oxide in naphthenic oil present in low levels or a range of about 3 to about 6 phr of the rubber compound.

The rubber compound including the low levels of desiccant material can be cured, under pressure, at less than 30% of full cure of the rubber compound. The full cure of the rubber compound of the tire may be determined or measured by means known in the art, such as by use of cure rheometer and thermocouple data.

By virtue of the foregoing, there is thus provided an improved method for reducing the curing time of tires cured under pressure to increase the production rate thereof by incorporating low levels of desiccant material into the rubber compound(s) of these tires.

The features and objectives of the present invention will become more readily apparent from the following Detailed Description.

The present invention is directed towards the use of low levels of a desiccant material in a rubber compound, such as but not limited to use as a tire tread, to allow for reduced cure times of tires cured under pressure, thereby increasing the tire production rate. The resultant tires, or tire components, include substantially no blows.

In one embodiment, low levels of no greater than about 9 phr of desiccant material are provided in the rubber compound. In another embodiment, the desiccant material is present in an amount not greater than about 6 phr. In another embodiment, the desiccant material is present in an amount not greater than about 3 phr. In yet another embodiment, the desiccant material is present in a range of about 3 phr to about 9 phr. In another embodiment, the desiccant material is present in a range of about 3 to about 6 phr. It should be understood that the desiccant material may be present in different amounts in different areas of the tire itself corresponding to the different rubber compounds utilized in preparing the components of the tire. For example, if two different rubber compounds separately define a tread and the sidewalls, the sidewalls (a thinner portion of the tire) may include about 3 to about 6 phr of desiccant material uniformly distributed throughout that rubber compound and the tread (a thicker portion of the tire) may include about 6 to about 9 phr of desiccant material.

The desiccant material of the present invention may include generally a metal oxide, such as calcium oxide (CaO) or magnesium oxide (MgO). The metal oxide may be blended with oil, such as naphthenic oil, for addition to the rubber compound. In one embodiment, the desiccant material is an 80/20 blend by weight of calcium oxide in naphthenic oil.

At typical tire pressure curing temperatures of about 150° C. or higher, water, which is present in the rubber compound and has a boiling point of 100° C., evaporates into a gas to form blows in the rubber compound. With the addition of the desiccant material, water can react during the pressure curing process, such as with the calcium oxide, to form calcium hydroxide Ca(OH) 2 . The calcium hydroxide remains as a solid at the curing temperature, thus reducing the volume of gas which could form blows and decreasing the curing time of the tire under pressure, thereby allowing for an increased tire production rate.

In accordance with the present invention, the use of the desiccant material allows for reduced curing time of a tire cured under pressure. In one embodiment, a tire with a rubber compound comprising the low levels of desiccant material is cured under pressure at less than 30% of full cure of the rubber compound. The full cure of the rubber compound of the tire may be determined or measured by means known in the art, such as by use of a cure rheometer and thermocouple data. For example, if it is determined, such as by use of a cure rheometer, that the full cure of the rubber compound involves a curing time of 20 minutes, the use of low levels of desiccant material would allow the rubber compound, or tire, to be cured under pressure at less than 30% of 20 minutes, i.e. in less than 6 minutes. In another embodiment, the rubber compound including low levels of desiccant material may be cured, under pressure, at less than about 20% of full cure. In yet another embodiment, the cure time is no less than about 10% and no greater than about 20% of full cure.

Non-limiting examples of various rubber compounds incorporating low levels of desiccant material in accordance with the present invention are disclosed in Table 1 below. The rubber compounds in Table 1 were prepared by methods known in the art.

Cure times for the rubber compounds disclosed in Table 1 were measured using a moving disc rheometer (MDR) as is known in the art. The data from these measurements are presented in Table 2 below.

The rubber compounds 1A, 1B, 1C, and 1D from Table 1 were cured at 150° C. using a bladder mold with a pressure of 100 psi. The rubber compounds were cured under pressure for 2, 4, 5, and 8 minutes. The pressure from the bladder was then released after these times, which represent times above and below the blow point determined for sample 1A, sample 1A being the control or rubber compound devoid of desiccant material. The blow point for 1A was determined by visual inspection to be at T25 or at about 7.4 minutes. The full cure of each rubber compound was determined to be at the T90 time. For example, the full cure of 1A was 13.93 minutes.

After 2 minutes, visual inspection of the rubber compounds revealed that rubber compounds 1A, 1B, and 1C included blows but 1D did not. After 4 minutes, only rubber compounds 1A and 1B included blows. After the 5-minute cure time, only rubber compound 1A included blows. Finally, at the 8-minute cure time no rubber compounds showed blows. Relative to the use of no desiccant, this data indicates that cure times can be reduced up to about 75% using 9 phr of desiccant material, up to about 50% using 6 phr of desiccant, and up to about 37% using 3 phr of desiccant material in the rubber compounds.

In addition, considering the full cure time at T90 for each of 1B, 1C, and 1D and the approximate blow points thereof as determined from the visual inspections, the pressure curing time of the rubber compound using 9 phr of desiccant material may be less than about 10% of the full cure thereof; the pressure curing time of the rubber compound using 6 phr of desiccant material may be less than about 20% of the full cure thereof; and the pressure curing time of the rubber compound using 3 phr of desiccant material may be less than 30% of the full cure thereof. In comparison, the curing time of the rubber compound of 1A under pressure should be no less than 57% of the full cure of the tire compound in order to surpass the blow point.

Other non-limiting examples of various rubber compounds incorporating low levels of desiccant material in accordance with the present invention are disclosed in Table 3 below. The rubber compounds in Table 3 were prepared by methods known in the art.

Cure times for the rubber compounds disclosed in Table 3 again were measured on a moving disc rheometer (MDR). The data from these measurements are presented in Table 4 below.

The rubber compounds 2A, 2B, 2C, and 2D in Table 3 were cured at 150° C. using a bladder mold with a pressure of 100 psi. The rubber compounds were cured under pressure for 4 minutes and 6 minutes. The pressure from the bladder was then released after these times, which represent times above and below the blow point determined for sample 2A, sample 2A being the control or rubber compound devoid of desiccant material. The blow point for 2A was determined by visual inspection to be at T25 or at about 5.6 minutes. The full cure of each rubber compound was taken to be at the T90 time. For example, the full cure of 2A was 9.14 minutes.

After 4 minutes, visual inspection of the rubber compounds of Table 3 revealed that only rubber compound 2A included blows while rubber compounds 2B, 2C, and 2D did not, thus, indicating that the cure time under pressure may be reduced up to at least 25% relative to the use of no desiccant. In addition, although visual inspections for blows in the rubber compounds of Table 3 did not occur earlier than 4 minutes, it is believed that the pressure curing time of 2B, 2C, and/or 2D may be completed at less than 30% of the full cure of the rubber compound. Also, it is noted that the curing time under pressure of the rubber compound of 2A should be no less than 61% of the full cure of the tire compound in order to surpass the blow point.

While the present invention has been illustrated by the description of the various embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of Applicants' general inventive concept.