Clear organosiloxane compositions

Claim 1

In an improved organosiloxane composition curable by a platinum-catalyzed hydrosilation reaction and exhibiting cohesive bonding to substrates formed from organic polymers, said composition comprising a first polydiorgano­siloxane gum containing at least two ethylenically unsaturated hydrocarbon radicals per molecule, an organo­hydrogensiloxane containing at least two silicon-bonded hydrogen atoms per molecule and a platinum-containing hydrosilation catalyst, the improvement comprising the presence in said composition of a second polydiorganosiloxane gum containing a concentration of silanol groups sufficient to impart to the mixture of said first and second gums an activity number of at least 40, where said activity number is a function of the increase in viscosity with respect to time of a solubilized curable composition consisting essentially of said mixture, a stoichiometric excess of ethyl ortho­silicate, a curing catalyst and a solvent, where said activity number is calculated using the formula (600,000)(Log T e1 /T e2 / T, where T e1 and Te₂ each represent the time interval, in seconds, required for elution of said solubilized composition from a capillary type viscometer at 25°C., T e1 begins ten minutes following formation of said solubilized composition, T e2 begins 20 minutes following formation of said solubilized composition and T represents the difference, in minutes, between a) the time interval between formation of said solubilized composition and the end of time interval T e2 and b) the time interval between formation of said solubilized composition the end of time interval T e1 .