The invention relates to rubber mixtures which crosslink with moisture and which comprise a niobium compound or tantalum compound and polymers having alkoxysilyl terminal groups.
Silicone rubber mixtures which can be stored with exclusion of water and on ingress of water crosslink at room temperature to give elastomers have been known for a long time, and this particularly applies to single-component silicone rubber mixtures (RTV-1). Large amounts of these products are used by way of example as jointing compositions in the construction industry. RTV-1 systems generally comprise a polymeric, mostly linear siloxane, a compound having crosslinking action and comprising groups that are readily hydrolysable, a plasticizer, and optionally other additions, such as a crosslinking catalyst, processing aids, pigments, and fillers. Crosslinking catalysts mainly used currently are Sn catalysts, e.g. dibutyltin diacetate or dibutyltin laurate. However, the recent toxicological classification of these tin derivatives has resulted in increasing criticism of these compounds, and the amounts permitted for use have been reduced accordingly. The use of Sn catalysts is therefore forbidden in elastomers which come into contact with foods, pharmaceuticals, or prostheses. There is a possibility that Sn-containing catalyst systems may be banned completely in the medium or long term. There is therefore a requirement for systems which can operate without tin catalysts but which at the same time have hardening performance comparable with that of conventional tin systems.
Various Sn-free catalyst systems have already been disclosed in the patent literature, especially using Ti compounds and Zr compounds. By way of example, U.S. Pat. No. 3,334,067 describes the use of catalysts such as Ti(OiPr)2(acac)2 or diisopropoxytitanium bis(ethyl-acetoacetate) which have improved shelf life when compared with standard Sn catalysts. Other titanium chelate compounds are mentioned inter alia in U.S. Pat. No. 3,542,901. Disadvantages of these types of catalyst derive from yellowing and incompatibility with aminic coupling agents. EP 102,268 claims the use of titanium-siloxy compounds, such as Ti(OSiMe3)4 or Ti(OSiMe3)2(On-Bu)4 and, respectively, Zr alkoxides, such as Zr(On-Bu)4. Siloxy-substituted Zr compounds are also used as condensation catalysts in JP 2004-238617A. JP 2007-131799 A describes niobium- and tantalum-alkoxy compounds of the general formulaM(OR)5 M=Nb, Ta R=alkylwhich are extremely susceptible to hydrolysis.
A feature common to all of said Sn-free compounds is that, when comparison is made with the familiar Sn systems, there are disadvantageous properties, for example coloring of the rubber compositions, or excessive susceptibility to hydrolysis, or inadequate shelf life.
The present invention is based on the object of providing Sn-free alkoxy-crosslinking rubber mixtures which are not toxicologically hazardous and which give sufficiently high speed, and tack-free hardening.