Sulfur-containing coupling agents used for mineral-filled elastomers involve silanes in which two alkoxysilylalkyl groups are bound, each to one end of a chain of sulfur atoms. The two alkoxysilyl groups are bonded to the chain of sulfur atoms by two similar, and in most cases, identical, hydrocarbon fragments. The general silane structures just described, hereinafter referred to as “simple bis polysulfide silanes,” usually contain a chain of three methylene groups as the two mediating hydrocarbon units. In some cases, the methylene chain is shorter, containing only one or two methylenes per chain. The use of these compounds is primarily as coupling agents for mineral-filled elastomers. These coupling agents function by chemically bonding silica or other mineral fillers to polymer when used in rubber applications. Coupling is accomplished by chemical bond formation between the silane sulfur and the polymer and by hydrolysis of the alkoxysilyl groups and subsequent condensation with silica hydroxyl groups. The reaction of the silane sulfur with the polymer occurs when the S—S bonds are broken and the resulting fragment adds to the polymer. A single linkage to the polymer occurs for each silyl group bonded to the silica. This linkage contains a single, relatively weak C—S and/or S—S bond(s) that forms the weak link between the polymer and the silica. Under high stress, this single C—S and/or S—S linkages may break and therefore contribute to wear of the filled elastomer.
The use of polysulfide silanes coupling agents in the preparation of rubber is well known. These silanes contain two silicon atoms, each of which is bound to a disubstituted hydrocarbon group, and three other groups of which at least one is removable from silicon by hydrolysis. Two such hydrocarbon groups, each with their bound silyl group, are further bound to each end of a chain of at least two sulfur atoms. These structures thus contain two silicon atoms and a single, continuous chain of sulfur atoms of variable length.
Hydrocarbon core polysulfide silanes that feature a central molecular core isolated from the silicon in the molecule by sulfur-sulfur bonds are known in the art. Polysufide silanes containing a core that is an aminoalkyl group separated from the silicon atom by a single sulfur and a polysulfide group and where the polysulfide group is bonded to the core at a secondary carbon atom are also know in the art. As well as core fragments in which only two polysulfide groups are attached to the core.
When the polysulfide groups are attached directly to an aromatic core, the reactivity with the polymer (rubber) is reduced. The aromatic core is sterically bulky which inhibits the reaction. Compositions in which the polysulfides are attached directly to cyclic aliphatic fragments derived by vinyl cyclohexene contain more than one silated core and form large rings. The cyclohexyl core is sterically more hindered than the aromatic core and is less reactive. Although these compositions can form more than one sulfur linkage to the polymer rubber for each attachment of the coupling agent to the silica through the silyl group, their effectiveness is low due to the low reactivity.
The low reactivity is due to the attachment of the polysulfide to the secondary carbon of cyclic core structure. The positioning of the polysulfide group is not optimal for reaction with the accelerators and reaction with the polymer.
The present invention overcomes the deficiencies of the aforementioned compositions involving silane coupling agents in several ways. The silanes of the present invention described herein are not limited to two silyl groups nor to one chain of sulfur atoms. In fact the molecular architecture in which multiple polysulfide chains are oriented in a noncollinear configuration (i.e. branched, in the sense that the branch points occur within the carbon backbone interconnecting the polysulfide chains) is novel.
The silanes of the present invention have advantages over that in the prior art by providing a means to multiple points of sulfur attachment to polymer per point of silicon attachment to filler. The silanes described herein may be asymmetric with regard to the groups on the two ends of the sulfur chains. The silyl groups, rather than occurring at the ends of the molecule, tend to occur more centrally and are chemically bonded to the cyclic core through carbon-carbon, carbon-sulfur and carbon-silicon bonds. The carbon-sulfur bonds of the thio ester linkages (sulfide) are more stable than the sulfur-sulfur bonds of the disulfide or polysulfide functional groups. These thio ether groups are therefore less likely to react with the accelerators and curing agents or to decompose when subjected to high shear or temperatures normally associated with the mixing and curing of rubber compounds. The thio ether linkage also provides a convenient synthetic route for making the silanes of the present invention. The cyclic core also contains multiple polysulfide groups that are attached to ring by means of a divalent, straight chain alkylene group. The attachment of the polysulfide group to the primary carbon of the alkylene group decreases significantly the steric hinderance of the core, and increases the reactivity of the polysulfides with the polymer. In addition, the cyclic core orients these alkylene chains containing the polysulfide groups away from each other to further reduce the steric hindrance near the polysulfide groups. This distinction is what allows silica to become and remain bonded (through the intermediacy of a sequence of covalent chemical bonds) to polymer at multiple points using the silanes of the present invention.
The examples presented herein demonstrate that the silanes of the present invention impart a desirable balance of physical properties (performance to mineral-filled elastomer compositions) and better wear characteristics to articles manufactured from these elastomers. Improvements in rolling resistance are also apparent for elastomers used in tire applications.