For various applications utilizing rubber which requires high strength and abrasion resistance, particularly applications such as tires and various industrial products, sulfur cured rubber is utilized which contains substantial amounts of reinforcing fillers. Carbon black is commonly used for such purpose and normally provides or enhances good physical properties for the sulfur cured rubber. Particulate silica is also often used for such purpose, particularly when the silica is used in conjunction with a coupling agent. In some cases, a combination of silica and carbon black is utilized for reinforcing fillers for various rubber products, including treads for tires.
Carbon black is often considered a more effective reinforcing filler for rubber products, and particularly for rubber tire treads than silica if the silica is used without a coupling agent, or silica coupler as it may be sometimes referred to herein.
Various materials have been used as silica couplers, sometimes also known as coupling agents or adhesives, to overcome such deficiencies of silica for a purpose of reinforcing rubber compositions. Generally such silica couplers are compounds having a capability of reacting with both the silica surface and with a sulfur vulcanizable rubber elastomer molecule. A sulfur vulcanizable rubber is normally considered an elastomer which contains carbon-to-carbon unsaturation which will conventionally undergo sulfur vulcanization because of such unsaturation, normally through a carbon atom adjacent to a carbon atom which is double bonded to another carbon atom. It is believed that such vulcanization is well known to those skilled in such art.
The silica coupling agents may, for example, sometimes be premixed, or pre-reacted, with the silica particles or added to the rubber mix during the rubber/silica processing, or mixing, stage. If the coupling agent and silica are added separately to the rubber mix during the rubber/silica processing, or mixing, stage, it is considered that the coupling agent then combines in situ with the silica and with the rubber.
In one aspect, such coupling agents may be composed of an organosilane polysulfide which has a constituent component, or moiety, (the silane portion) capable of reacting with the silica surface and, also, a constituent component, or moiety, (the polysulfide portion) capable of reacting with the rubber, particularly a sulfur vulcanizable rubber which contains carbon-to-carbon double bonds, or unsaturation. In this manner, then the coupler acts as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement aspect of the silica.
In another aspect, the silane of the coupling agent apparently forms a bond to the silica surface, rather quickly during the rubber/silica mixing process and the rubber reactive component of the coupling agent combines with the rubber at a much slower rate. The rubber reactive component of the coupler is generally temperature sensitive and tends to combine with the rubber rather slowly during the rubber mixing steps and more completely during the higher temperature sulfur vulcanization stage.
An example of such coupling agents for use in combining silica and rubber is, for example, an organosilane polysulfide such as bis-(3-trialkoxysilylalkyl) polysulfide where the sulfide bridge contains 2 to 8 connecting sulfur atoms in which the average polysulfide bridge contains about 4.5 to about 5.5 sulfur atoms so that the polysulfide may be more generally referred to as a tetrasulfide and, further, in which not more than 25 percent of the polysulfide bridge portion contains 2 or less sulfur atoms. In other words, such polysulfide predominately 3 or more connecting sulfur atoms in its polysulfide bridge portion. An example of such commercially available silica coupler is Si69 manufactured by the Degussa AG company.
Various, although not exhaustive, patents relating to silicas and silica reinforced tire treads include U.S. Pat. Nos. 3,451,458; 3,664,403; 3,768,537; 3,884,285; 3,938,574; 4,482,663; 4,590,052; 5,089,554 and British 1,424,503.
U.S. Pat. No. 4,513,123 discloses a rubber composition containing dithiodipropionic acid with natural rubber, or blends of natural and synthetic rubbers, 30-80 parts carbon black, sulfur and organo-cobalt compound for use as skim stock for brass-plated steel. It relates that the rubber composition can contain other additives such as fillers such as clays, silicas or calcium carbonate, process and extender oils, antioxidants, cure accelerators, cure activators, cure stabilizers and the like.
In sulfur curable rubber compositions, the ingredients are conventionally blended in several stages, referred to as "non-productive mix stage(s)" followed by a final "productive mix stage" in which curatives such as sulfur and cure accelerators are added. The non-productive mix stage(s), which are conventionally 1 or 2 to 4 or more sequential mix stages when more than one stage is utilized, are typically conducted at temperatures in a range of about 140.degree. C. to 190.degree. C. and the productive mix stage may conventionally be conducted at temperatures in a range of about 100.degree. C. to 130.degree. C. The aforesaid curatives are conventionally only added in the final, lower temperature, mix stage to keep the rubber from prematurely curing at the aforesaid elevated mix temperatures of the non-productive mix stages.
While, conceivably, a cure accelerator might be added in a non-productive stage, the inventors are not aware of any circumstance where a cure accelerator such as 2-mercaptobenzothiazole or cure accelerator containing 2-mercaptobenzothiazole such as, for example, sulfenamides, had been added to a rubber/silica/silica coupler blend in a non-productive mix stage prior to a productive mix stage where the sulfur is added.
The term "phr" if used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber, or elastomer".
In the description of this invention, the terms "rubber" and "elastomer" if used herein, may be used interchangeably, unless otherwise prescribed. The terms "rubber composition", "compounded rubber" and "rubber compound", if used herein, are used interchangeably to refer to rubber which has been blended or mixed with various ingredients and materials and such terms are well known to those having skill in the rubber mixing or rubber compounding art.