Heat-activable adhesive sheet of an adhesive comprising a blend of at least three synthetic nitrile rubber compositions S1, S2 and S3, each of which comprises at least one synthetic nitrile rubber, and at least one reactive resin which is capable of crosslinking with itself, with other reactive resins and/or with the nitrile rubbers of synthetic rubber compositions S1, S2 and S3, in which a) the blend of the heat-activable sheet being microphase-separated, characterized by at least three different glass transition temperatures in a DSC, b) at least one of said glass transition temperatures being greater than 10° C. and at least one of said glass transition temperature lower than −20° C., c) the nitrile rubber or rubbers S1 having an acrylonitrile fraction of greater than/equal to 35%, d) the nitrile rubber or rubbers S2 having an acrylonitrile fraction of greater than 25% and less than 35%, e) the nitrile rubber S3 having an acrylonitrile fraction of less than/equal to 25%.