Kaolin Clay in Silica Tread Compounds Technical Field

The invention relates to a method for decreasing dynamic modulus without decreasing hardness so as to improve ride quality while maintaining wet and dry handling characteristics, in silica tread compounds and tires generally for passenger vehicles made in accordance with this method. The present invention also relates to the composition made by this method. This result is achieved by using a kaolin clay as a partial replacement for silica in a performance-enhancing system added to the silica tread compounds.

The object of the present invention is to provide a tread rubber composition having a base tread rubber combined with a system or package of performance-enhancing agents comprising silica, coupling agent, carbon black, and kaolin clay.

A wide variety of factors are taken into consideration in the art and science of formulating tire tread compositions. For example, the tires are formulated to achieve specific wet and dry handling characteristics, provide traction under snowy conditions, resist abrasion, and provide a quiet, comfortable ride while achieving suitable tire wear. Performance-enhancing agents are added to the base elastomer to help achieve the desired characteristics. Sometimes combinations of such agents can act synergistically and can function unexpectedly in collaboration.

It has been customary in the past to incorporate silica in tire tread compositions as a filler and also to enhance the performance characteristics of the tire, such as low rolling resistance, while improving adherence to wet and snow-covered ground, improving wear and aging properties while reducing noise.

While numerous patents relate to the state of the art with respect to the base polymer for tire treads, U.S. Pat. No. 5,227,425, to Rauline, relates more specifically to all-season, high-performance tires utilizing a specific type of silica to improve performance characteristics of the tires.

U.S. Pat. No. 4,522,970, to Scriver et al., is an example of a patent relating to sulfur cured, rubber tread compositions formulated to meet certain defined performance characteristics.

The present invention relates to a method of achieving unexpected stress/strain characteristics in tread compositions so as to reduce the low strain modulus of a tire tread without reducing the hardness or high strain modulus to the same degree. This contrasts with the customary relationship between these characteristics. More specifically, the compositions maintain wet and dry handling, abrasion resistance, and snow traction of a silica tread compound while improving ride comfort and impact damping.

The present invention is particularly suitable for tire treads intended for passenger cars, all-terrain vehicles, pick-up trucks, and motorcycles.

The above objects are met by the partial replacement of silica with a kaolin clay in conjunction with a silane coupling agent as a performance-enhancing system in an elastomeric tread composition.

In the method of the present invention low strain modulus is reduced without reducing the hardness or high strain modulus to the same degree in silica tread compounds by replacing from about 20 to about 40% of the total silica with an equal amount of kaolin clay to result in a performance-enhancing package of about 5 to 20 parts by weight of clay, about 20-50 parts by weight of a high structure, high surface area silica which preferably includes a silane coupling agent, and about 0-60 parts of highly structured, tread-grade carbon black, all per 100 parts of base elastomeric composition.

In accordance with the present invention, this performance-enhancing system can be used with various tread compositions. Such tread compositions are primarily based upon a mixture of natural rubber, styrene butadiene rubber, and butadiene rubber and/or blends thereof.

The kaolin clay of the present invention is also known as hydrated aluminum silicate and can be air-floated or water-washed (having a median particle size of from about 0.2 microns). The kaolin clay is substituted on a 1:1 weight basis for from about 20 to about 40 percent of the silica. More broadly, the kaolin clay may be provided in a weight ratio of from 0.1:1 to 1:1 parts by weight, and preferably 0.2:1 to 0.4:1 part by weight, of clay to silica.

The kaolin clay is enhanced by the presence of a silane coupling agent which is for example Bis(3-triethoxysilylpropyl)-tetrasulfane (such as sold under the name Si69.RTM. from Degussa). The coupling agent may be reacted in situ with the clay (as well as the silica), although alternatively clay pretreated with the coupling agent may be used. The coupling agent is provided for the clay at the same ratio that it is provide for the silica, i.e., from about 6 to 12 weight percent of the silica.

Silica is also used in the high-performance package and it includes high-structure, high surface area silica, i.e., having a nitrogen surface area of from about 190 to about 220 m.sup.2 /g, for a total of not more than about 50 parts; i.e. from about 20 to about 50 parts; preferably not more than about 45 parts; i.e. from about 25 to about 45 parts; and most preferably not more than about 40 parts; i.e. from about 30 to about 40 parts of silica; all parts based on 100 parts of base rubber.

Carbon black is also used: preferably high structured, tread-grade carbon black at from 0-60; preferably 15-45; and most preferably 35 to 40 parts, all parts based on 100 parts of base rubber.

In accordance with the present invention specific mixing sequences follow in which the first step rubber and silica are mixed with the carbon black and the clay at a temperature of from about 300.degree. F. to about 390.degree. F. and preferably about 320.degree. F. to about 380.degree. F., i.e., a relatively high temperature. In a second stage, coupling agent is added and the temperature is held between about 260.degree. F. and about 300.degree. F. for mixing and to react the coupling agent with the silica and the kaolin clay, which temperature is held below the cross-linking initiation temperature. In an optional third stage, the compound is re-melted at a temperature of from about 220.degree. F. to about 260.degree. F. to improve the processing. In the final stage, curatives are introduced and mixing continues at a temperature of about 180.degree. F. to about 220.degree. F. The mixing occurs in an internal mixer such as a Banbury.RTM. or Intermix.RTM..

In accordance with this invention, two rubber tread compositions were compounded having the formulas listed in Table I. Specifically, the rubber and silica were mixed at the percentages listed in the control and with kaolin clay in the parts listed under Formula A at a temperature of about 340.degree. F. In a second stage, the silane coupling agent was added and the temperature was held at about 290.degree. to react the coupling agent respectively with the silica or the kaolin clay. The oil and carbon black were added in the first stage. In the final stage, state-of-the-art sulphur cure system was used at a typical ratio at a temperature of about 190.degree. F. The parts are listed in the Table so that the pertinent ingredients are based on 100 parts of rubber as is known to those skilled in the art of rubber compounding. The additional components above 100 are components which are not rubber, such as extenders, fillers, and the like.

In a second example, a different tread composition was formulated in accordance with Table II corresponding to the process described for Example 1. Once again, kaolin clay was added to Sample B and omitted from the control. The amount of the silane added in both systems was the same relative to the amount of silica; however, since there was less silica in the samples, the cumulative amount of silane was lower. It is believed that this lower amount is still effective since the clay has smaller surface area than the amount of silica which it replaces, i.e. enough residual silane coupling agent is present to also couple the kaolin clay in the system.

After compounding, appropriate test specimens were formed of the compositions and physical properties were tested for Examples 1 and 2. With respect to the test, the low strain modulus was considered herein to be 0 to 25 percent deflection, while the high strain modulus is 100 percent or greater deflection. The results of these tests are set forth in Tables III and IV. Test equipment and methods are listed in Table V. The test results for the controls were normalized to 100 and the test data normalized to show improvement as a higher number relative to the control. The Tables indicate improvement in dynamic modulus, tangent delta, 5 percent modulus, while maintaining a relatively similar, or better 300 percent modulus and Shore-A Durometer hardness. An unexpected rolling loss improvement was demonstrated which results in a projected fuel economy for the tire vehicle system. Moreover, there was some improvement in snow and ice traction demonstrated at the dynamic modulus at -20.degree. C. for Example 1.

While in accordance with the patent statutes the best mode and preferred embodiment has been set forth, the scope of the invention is not limited thereto, but rather by the scope of the attached claims.