Rubber composition containing solid magnetizable particles of greater stiffness than the rubbery compounds

1. Field of the Invention

The present invention relates to rubber compositions and methods of making. More particularly, the present invention relates to rubber compositions including solid magnetizable particles bonded to the rubber matrix and methods of making.

2. Description of Background Information

The present invention is especially directed to a further improvement upon the invention described in U.S. Pat. No. 5,913,240 entitled “Method and Device for Controlling Slip and/or for Determining the Longitudinal Force or Flex Work-Proportional Parameter, and Vehicle Tire Therefore”, assigned to Continental AG and incorporated herein by reference. Further, the present invention is preferably combined with the disclosure of DE 196 46 251 of the same assignee. U.S. Pat. No. 5,895,854, which is also incorporated herein by reference, claims priority of this German application.

Beside the patents mentioned above and incorporated herein, we became aware of German patent applications DE 195 03 468 C1 and DE 195 03 469 C1, both being published May 30, 1996 and citing U.S. Pat. Nos. 4,161,120; 4,948,277; 5,017,868; 5,026,178; 5,131,763; 5,261,752; and 5,261,753.

Further, we became aware of European patent applications EP 0 890 752 A2 and EP 0 890 753 A2, both published Jan. 13, 1999; and EP 0 892 185 A2, published Jan. 20, 1999.

It is an object of this invention to detente or defuse the present conflict in measuring out the best dose of solid magnetizable particles to be imbedded in a polymer matrix or network, this conflict being recognized by us, the inventors, between:

a first goal of attaining high fatigue life and high crack growth resistance, both demanding a low or null dose of solid magnetizable particles, and

a second goal of attaining a strong magnetic signal by passing discrete magnetized areas of a circumferential row of magnetized areas along a sensor, demanding a high dose of solid magnetizable particles.

In an environment—such as a vehicle—there exist many low magnetic influences. In view of the relatively great distance (ca. half an inch for passenger cars) between magnetized areas of a tire and a sensor, it is a further object of the invention to generate the intended sequence of signals by strong magnetic areas, avoiding misrecognition of the passing times of magnetic areas and thus avoiding mismeasurements of forces acting on a tire and missignals given to brake controllers or brake actuators.

We, the inventors, have recognized that in those rubber compositions known in the art, the solid magnetizable particles function as flaw sites to initiate cracking and component failure when subjected to repeated deformation. Evidence of this is provided in FIGS. 1 and 2, showing fatigue life and crack growth resistance of a standard black tire sidewall composition versus a composition A containing 200 phr of a ferrite filler.

The primary reason for this unintended effect is the great difference between the stiffness of the matrix and that of the solid magnetizable particles; unfortunately this primary reason is unavoidable. But with a stronger bonding between the matrix and the solid magnetizable particles, the loss of fatigue resistance and crack growth resistance will be reduced. Therefore we focussed our labor on enhancing bonding between solid magnetizable particles and surrounding molecules of the rubbery compounds.

As a solution, we provide a chemical coupling agent and also a special mixing technique to provide a chemical bond between the magnetized particles and the polymer matrix, thereby allowing additional mechanisms for fatigue and tearing energy dissipation. Such chemical coupling agents can be, but are not limited to, organo-functional silane materials. This invention allows the filling of a rubber composition with a high level of solid magnetizable particles, also described as “magnetic fillers”, without diminishing too much fatigue resistance and crack growth resistance. Thus, magnetic fillers can be incorporated successfully in rubber compositions which are used for high flex components, especially for tire sidewalls.

Advantageously, our invention is facilitated by the fact that such organo-functional silane materials may be used which are already well-known in the rubber industry. However, up-to-date organo-functional silane materials are used only in rubber compositions for tire treads and not in compositions for tire sidewalls. In these tread rubber compositions, organo-functional silane materials are combined with silica and provide bonding of the silica to the polymer matrix; they are not combined with any solid magnetizable particle and therefore do not provide or disclose any bonding of such particles to the polymer matrix.

Beside the solid magnetizable particles, the rubber composition of the present invention contains molecules of natural and/or artificial polymer and/or other fillers such as carbon black, silica and/or other compounds such as sulfur, as a chain-building ingredient, vulcanizing accelerators, vulcanizing retarders and so on. These surrounding molecules are a matrix in which the solid magnetizable particles are imbedded.

Differences between this invention and other known arrangements include the use of chemical coupling agents to bond magnetic powder to the polymer matrix, and the special mixing technique to maximize reaction between coupling agent, magnetic powder, and polymer matrix.

FIG. 1 shows the results of tests of fatigue until failure at different extensions “Ext”. There is compared a prior art sample called “Standard” made of a vulcanized standard rubber composition for sidewalls without magnetizable particles and a prior art sample A made of nearly the same standard vulcanized rubber composition but additionally comprising 200 phr of a ferrite filler;

FIG. 2 shows the results of the well-known DeMattia test with pierced samples made of the same materials as in FIG. 1 . “KC” denotes kilocycles, i.e., units of a thousand cycles of uploading and downloading. This number is divided by the length of crack growth, measured in millimeters, resulting in the dimension “KC/mm”;

FIG. 3 shows in a way analogous to FIG. 1 the results of tests of fatigue until failure with samples of a vulcanized Standard rubber composition for sidewalls and vulcanized rubber compositions A to E according the invention, disclosed in detail in Table B;

FIG. 4 shows in a way analogous to FIG. 3 the results of the DeMattia test with pierced samples made of rubber compositions A to E of FIG. 3;

FIG. 5 shows in a way analogous to FIG. 1 and FIG. 3 the results of tests of fatigue until failure with samples of a vulcanized Standard rubber composition for sidewalls and vulcanized rubber compositions A, F and G according the invention, disclosed in detail in Table C; and

FIG. 6 shows in a way analogous to FIG. 2 and FIG. 4 the results of the DeMattia test with pierced samples made of rubber compositions A, F and G of FIG. 5 .

The disclosure of the preferred embodiments contains three parts. First, the reaction mechanism is modeled to provide an understanding of the nature of the chemical bonds between the magnetic filler, coupling agent, and polymer network. Second, experimental laboratory studies were performed to demonstrate benefits of using the coupling agents and the special mixing technique. Third, there is disclosed in detail the best way of processing such a rubber composition containing 2 to 8 phr by weight of an organosilane polysulfide.

Reaction Mechanism

It is proposed that the organo-functional silane coupling agents will have potential bonding sites at the strontium ferrite molecule as shown below. The organo-functional silane may bond to the ferrite in a similar mechanism as with silica.

Experimental Description

A standard black sidewall recipe was modified to include from 50 phr up to 250 phr magnetic filler. Here for the experimental laboratory study, 200 phr of a strontium ferrite is used. Table A shows the standard black sidewall recipe and the magnetic ferrite sidewall compound A containing 200 phr of strontium ferrite.

FIGS. 1 and 2 show the lower fatigue and crack resistance of compound A, despite the polymer blend modification to enhance fatigue life.

Table B shows ferrite sidewall A modified, named ferrite sidewalls B, C and D, having 4, 6 and 8 phr of TESPT (triethoxy silyl propyl tetrasulfide), respectively, and ferrite sidewall E having 4 phr of TESPD, (triethoxy silyl propyl disulfide), both organo-functional silane coupling agents.

FIG. 3 shows significant improvements in fatigue life with use of organo-functional silane coupling agents. FIG. 4 shows that there is not much improvement in crack growth resistance using TESPT, but a significant improvement can be realized using the TESPD material.

Table C displays the last study in this invention description utilizing 4 and 6 phr of TESPD coupling agent and a mixing technique to optimize the reaction of the coupling agent with the magnetic filler and polymer matrix. The mixing technique, shown in Table D, employs an internal rubber mixer. Half of the strontium ferrite and all of the TESPD are added together early in the first pass to begin the reaction sooner and to allow more time for completion of the reaction for greater bonding to the rubber matrix.

As the third part of this disclosure, Table D shows the progressive new procedure according to the invention to produce a vulcanizable rubber composition.

FIG. 5 shows that fatigue life is significantly improved, actually slightly better than the standard tire black sidewall, with TESPD at either 4 or 6 phr and using the mixing technique to optimize reaction between the magnetic filler and the polymer matrix. FIG. 6 shows that crack growth resistance is significantly improved over ferrite compound A and only slightly less than the standard tire side wall, but still a satisfactory production compound.

The preferred versions of this invention are as follows:

Magnetic filler type: strontium ferrite

Magnetic filler loading range: 50 phr to 250 phr

Coupling agents: TESPT and TESPD

Coupling agent loading range: 2 to 8 phr

Natural rubber phr range: 20 to 70 phr

Polybutadiene phr range: 20 to 70 phr

Emulsion SBR phr range: 10 to 30 phr

Carbon black phr range: 25 to 60 phr

Carbon black type: N 660, N 339, N 550

Processing oil phr range: 8 to 20 phr

Processing oil type: aromatic and naphthenic

Tackifying resin phr range: 0 to 6 phr

Tackifying resin types: phenolic and hydrocarbon

Antidegradants phr range: p-phenylenediamine class

Micro crystalline paraffm wax phr range: 2 to 6 phr

Zinc oxide phr range: 2 to 6 phr

Stearic acid phr range: 1 to 4 phr

Accelerator type: TBBS and CBS

Accelerator phr range: 0.3 to 1.5 phr

Sulfur type: soluble sulfur and/or 20% oil treated crystex

Sulfur phr range (soluble sulfur): 1.5 to 2.5 phr

Sulfur phr range (oil treated crystex): 1.88 to 3.13 phr

All measurements given in “phr” units are by weight.

This invention allows high loadings of non-reinforcing or low reinforcing magnetic powder in the tire sidewall compound for the Sidewall Torsion Sensor system while maintaining the proper sidewall compound performance.

The present invention can be applied to all tire types using the Sidewall Torsion Sensor system and to all other rubber articles generating magnetic signals as—by way of example—driving or conveyor belts for the purpose of counting the number of revolutions executed.