Heat-curable polyorganosiloxane composition and use thereof

A heat-curable polyorganosiloxane composition which comprises

• (A) an alkenyl group-containing linear polyorganosiloxane represented by the formula (I):
• wherein
• each Ra independently represents a C2-C6 alkenyl group,
• each Rb independently represents a C1-C6 alkyl group or a phenyl group,
• n1 is a number which results in a viscosity of (A) at 23° C. of 2000 cP or less;
• (B1) a linear polyorganohydrogensiloxane represented by the formula (II):
• wherein
• Rc is a hydrogen atom,
• Rd is a methyl group,
• n2 is a number which results in a viscosity of (B1) at 23° C. of 0.1 to 20 cP;
• (B2) a cyclic polyorganohydrogensiloxane comprising an Re2HSiO1/2 unit, wherein Re represents a hydrogen atom or a C1-C6 alkyl group, and an SiO4/2 unit, and having 3 or more hydrogen atoms bonded to a silicon atom in one molecule, wherein a ratio of the moles of the Re2HSiO1/2 unit to the moles of the SiO4/2 unit is 1.5 to 2.2 moles of the Re2HSiO1/2 unit per 1 mole of the SiO4/2 unit;
• (C) a platinum series catalyst;
• and
• (D) an adhesion promoter;
• wherein a first ratio (HB1+HB2)/ViA is 0.2 to 1.0 and a second ratio HB1/(HB1+HB2) is 0.5 to 0.8, ViA is a number of alkenyl group(s) of (A), HB1 is a number of the hydrogen atom(s) bonded to a silicon atom of (B1), HB2 is a number of the hydrogen atom(s) bonded to a silicon atom of (B2), and (D) is a reaction product of 1,1,3,5,7-pentamethylcyclotetrasilioxane and 3-methacryloxy-propyltrimethoxysilane, and (D) is in an amount of 0.1 to 2 parts by weight based on 100 parts by weight of (A), wherein, upon curing, the heat-curable polyorganosiloxane composition has an E hardness of 1 to 40 measured according to JIS K 6253 E.