1. Field of the Invention
The present invention relates to organopolysiloxane compositions which can be converted into silicone foams having improved burn resistance, and, more especially, to compositions comprising a mixture of organopolysiloxane polymers bearing the reactive groups SiH, SiOH, and Si-vinyl; they are converted into foams and cured by platinum catalysis.
2. Description of the Prior Art
Organopolysiloxane compositions which can be converted into foams and are catalyzed by a platinum derivative are known to this art. They are described in the patent and other literature, especially in U.S. Pat. Nos. 3,923,705; 4,189,545; 4,418,157, and British Pat. No. 2,065,661.
In U.S. Pat. No. 3,923,705 a process is described for the preparation of silicone foams which consists of mixing an organohydropolysiloxane containing at least three SiH groups per mole, a hydroxylated organopolysiloxane containing more than one and up to 2.5 hydroxyl radicals per mole and a platinum catalyst in a proportion of 5 to 200 parts of platinum per million parts of the composition, with the organohydropolysiloxane and the hydroxylated organopolysiloxane present in sufficient quantities to provide a ratio of the SiH groups to the SiOH groups of 2.5 to 40. A triorganosiloxy-blocked diorganopolysiloxane containing, on average, two vinyl radicals per mole may be added to the composition.
U.S. Pat. No. 4,189,545 relates to a composition for a silicone foam which resists burning, comprising 100 parts of a triorganosiloxy-blocked diorganopolysiloxane containing 0.0002 to 3% of vinyl radicals, 0 to 200 parts of a filler, 100 to 15,000 parts per million of water, 1 to 50 parts of a diorganopolysiloxane bearing SiH groups, having a viscosity of 5 to 100 mPa.multidot.s at 25.degree. C., and 1 to 250 parts per million of a platinum catalyst.
British Pat. No. 2,065,661 describes a silicone composition which can be converted into a foam which resists burning, comprising 100 parts of a triorganosiloxy-blocked diorganopolysiloxane containing from 0.0002 to 3% of vinyl radicals, 1 to 10 parts of an organopolysiloxane containing 2 to 10% of hydroxyl radicals, having a viscosity of 10 to 100 mPa.multidot.s at 25.degree. C., 0 to 200 parts of a filler, 1 to 50 parts of a diorganopolysiloxane bearing SiH groups, having a viscosity of 5 to 100 mPa.multidot.s at 25.degree. C., and 1 to 250 parts per million of a platinum catalyst. The composition may additionally contain 10 to 100 parts of a copolymer based on triorganosiloxy and SiO.sub.2 units, or of another copolymer based on triorganosiloxy, diorganosiloxy and SiO.sub.2 units, the ratio of the triorganosiloxy units to the SiO.sub.2 units in the two copolymers being 0.5 to 1, with the diorganosiloxy units representing 1 to 10% of all of the units in the second copolymer; 2.5 to 10 of the silicon atoms in the two copolymers bear vinyl radicals.
U.S. Pat. No. 4,418,157 relates to a process for the preparation of a silicone foam of reduced density, which consists of mixing a composition which can be converted into a foam (based on a diorganopolysiloxane, an organohydropolysiloxane and a platinum catalyst) with an effective amount, in order to reduce foam density, of a copolymer consisting of triorganosiloxy and SiO.sub.2 units or of another copolymer consisting of triorganosiloxy, diorganosiloxy and SiO.sub.2 units, the ratio of the triorganosiloxy units to the SiO.sub.2 units being 0.25 to 0.8 and the ratio of the diorganosiloxy units to the SiO.sub.2 units being 0 to 0.1.
Compositions prepared according to the aforementioned patents produce foams of good quality, that is to say, having the required density and quite good burn resistance. However, these compositions are sometimes difficult to prepare and to use because they require a highly active platinum catalyst and/or scrupulous metering of the distribution of their components.
Furthermore, the foams obtained do not always have sufficient burn resistance. Excellent flame resistance is necessary, however, because silicone foams are often used to protect essential components such as electrical cables or electronic relays which are attached, for example, to devices for processing hazardous products, or to devices for transporting human beings. In the event of a fire, the poor working state of these components could interfere with the operation of warning or safety systems.