Resin composition for a coating for an optical fiber, and coated optical fiber and optical fiber unit using the same

A resin composition for a coating for an optical fiber comprising a radical polymerizable oligomer (A) and a radical polymerizable monomer (B), wherein said radical polymerizable oligomer (A) comprises

• a radical polymerizable oligomer (A1) with a number average molecular weight of 600 to 1600 formed by reacting compounds (1) to (3) below:
• (1) an aliphatic polyol compound (a1) with a chain structure which may contain branch chains, with a number average molecular weight of 50 to 600,
• (2) a polyisocyanate compound (a2) with a ring structure, and
• (3) a compound (a3) with a hydroxyl group and a radical polymerizable unsaturated group within a single molecule;
• and a radical polymerizable oligomer (A2) formed by reacting compounds (4) to (6) below:
• (4) an aliphatic polyol compound (a4) with a chain structure which may contain branch chains, with a number average molecular weight of 800 to 10,000,
• (5) a polyisocyanate compound (a5), and
• (6) a compound (a3) with a hydroxyl group and a radical polymerizable unsaturated group within a single molecule;
• a mass ratio (A1)/ (A2) of said radical polymerizable oligomer (A1) and said radical polymerizable oligomer (A2) is within a range from 20/80 to 80/20, and a concentration of urethane linkages within said resin composition for a coating for an optical fiber is within a range from 1.85 to 3.00 mol/kg.