Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Reexamination Certificate
2000-01-04
2002-09-17
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
Reexamination Certificate
active
06452034
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of preparing polysulfide-functional silanes composed of novel sulfur rank distributions, i.e., distributions which do not form from the practiced prior art.
Sulfur-containing silane coupling agents are particularly useful in providing rubber tires, including automotive tires, with improved properties, generally by coupling inorganic fillers or fibers with the rubber matrix in a fashion which leads to the improved properties. The sulfur-containing silane coupling agents which have achieved commercial success to date have been produced by disadvantageous processes which involve large quantities of chlorine-containing by-products.
A number of variations have been proposed for the preparation of oligosulfide-functional silanes starting from the chloroalkyl silane starting materials and involving sulfur and/or a sulfur anion. This art is described in the following U.S. Patents: U.S. Pat. No. 3,842,111; U.S. Pat. No. 3,873,489; U.S. Pat. No. 3,946,059; U.S. Pat. No. 3,978,103; U.S. Pat. No. 3,997,581; U.S. Pat. Nos. 4,072,701; 4,129,585; U.S. Pat. No. and 4,507,490; U.S. Pat. No. 5,405,985 and U.S. Pat. No. 5,468,893. This art is limited to the use of an integral mole ratio of sulfur reacted to bis-silylalkyl oligosulfide product obtained. The art teaches that in this preparation, only a single oligosulfide species is obtained, in which the number of sulfur atoms present in each molecule is dependent on the ratio of sulfur to starting chloroalkyl silane used in the preparation. The art does not teach the generation of distributions of species of different molecular sulfur content. The art suggests that the exclusive formation of a specific oligomer can be controlled by controlling the amount of sulfur used in the preparation. Elemental analyses and NMR data are provided in the examples to support these conclusions.
Other art describes another method which yielded similar substances, but starting from alkenyl silane starting materials instead of the aforementioned chloroalkyl silane starting materials. This art is described in the following U.S. Patents: U.S. Pat. No. 4,384,132; U.S. Pat. No. 4,408,064; and U.S. Pat. No. 4,444,936. This art implies the formation of distributions of the corresponding oligosulfide silane derivatives.
More recently, art has been described which fully recognizes and embraces the notion that distributions of oligosulfide silanes of different individual molecular sulfur content are obtained in efforts to prepare any of the oligosulfide silanes, and that it is useful to regard this mixture from the standpoint of the average molecular sulfur content. This art was described in EP 0 773 224 A2 and in U.S. Pat. No. 5,674,932. It is clear from these disclosures that the total sulfur content of the oligosulfide silane mixtures can be controlled within a wide range by adjusting the amount of sulfur introduced into their preparation. What is difficult to control, however, is the way in which this sulfur becomes distributed among the individual oligosulfide silanes, hereafter referred to as “sulfur ranks”, in which this term is taken to mean the number of sulfur atoms linked by sulfur-sulfur bonds in a molecule of the oligosulfide silane. Any given ratio of total sulfur to silicon introduced into the preparation tends to yield a single or only narrowly variant specific sulfur rank distribution which is dependent only on the aforementioned ratios of reactants used and which is essentially independent of the way in which the reaction is carried out. Thus, no good method is described in the prior art which allows one to control the way the sulfur distributes itself among the individual oligosulfides. The total amount of sulfur can be controlled, but the system controls the way this sulfur is distributed.
The aforementioned inability to control the distribution of sulfur has a special commercial significance in the manufacture of disulfide-functional silanes for applications as coupling agents in filled elastomers. Recent disclosures in, DE 197 02 046 A1, U.S. Pat. No. 5,674,932 and EP 0 732 362 A1, teach that disulfide-functionalized silanes offer advantages over higher sulfur ranks for use in mineral-filled elastomer compositions. However, preparation of disulfide silane compositions by the methods known from the aforementioned prior art, in which the average sulfur rank is two, yields a substantial portion of the thioether silane. This is a disadvantage because this particular species has a sulfur rank of one, and is widely considered to be an inactive diluent. A second disadvantage is that to average a sulfur rank of two, some of the individual oligosulfide silane species must have a sulfur rank greater than two, which further detracts from the advantages reaped by use of the disulfide silane.
The preparation of pure or nearly pure disulfide compositions cannot be carried out by the processes described above. Even compositions which are not necessarily pure, but different from preparations which arise naturally by the aforementioned synthetic pathways, cannot be prepared either by those processes. The preparation of any such compositions would require more elaborate and more economically disadvantaged methods. Only examples of such methods limited specifically to the preparation of essentially pure disulfide silanes, are described in the prior art. Thus, the use of sulfuryl chloride or iodine to oxidize mercaptosilanes to the corresponding disulfide, is taught in DE-PS 2 360470 and EP 0 217 178 A1, respectively. These methods are disadvantaged. The mercaptosilane would have to be prepared by a method very similar to the aforementioned prior art for the preparation of oligosulfide silanes in a first step. A separate second step would then be needed, which requires the use of corrosive and/or expensive materials to convert the mercaptan to the disulfide, and which furthermore produces undesirable waste products. Thus, not only is a process required involving two separate steps, but there is also a necessity to deal with additional reagents, waste streams, and special hazards and inconveniences associated with the use of these materials. Other methods would include oxidizers based on oxygen, such as manganese dioxide, chromates, dichromates, or molecular oxygen optionally with an appropriate catalyst, but their use is precluded because water would be coproduced with their use.
A second type of a two-step method, again limited to the preparation of pure or nearly pure disulfide compositions, has recently been taught in EP 0 773 224 A2, in which an oligosulfide silane or oligosulfide silane mixtures containing components with a chain of more than two sulfur atoms is desulfurized to yield a product, all of whose components contain chains of a maximum of two sulfur atoms. In its simplest form, this document describes the desulfurization of a mixture of oligosulfide silanes to yield essentially pure disulfide, free of higher sulfur ranks. In its most general form, this amounts to the desulfurization of a mixture of oligosulfide silanes to yield a product mixture of oligosulfide silanes consisting only of the disulfide and thioether.
The procedures described in EP 0 773 224 A2 require desulfurization reagents such as cyanides, sulfites, and trivalent phosphorus compounds to remove sulfur from the oligosulfide silane to yield thiocyanates, thiosulfates, and the corresponding thionophosphorus derivatives, respectively. Although each of these types of reagents very effectively removes the excess sulfur, the use of each of them also presents some problems. Cyanides can be toxic. Sulfites do not dissolve readily in the preferred alcohol solvents, nor in any other readily used organic solvents, necessitating the use of water and with it, the need to take measures to prevent decomposing the hydrolyzable groups on silicon. Trivalent phosphorus compounds are expensive and can contaminate the final product. In addition to these shortcomings, only a single mole of sulfur is removed per mole of desulfurizing agent used. The
Crompton Corporation
Dilworth Michael P.
Shaver Paul F.
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