Substrate for surface protective sheet and surface protective sheet

1. A surface protective sheet, comprising a pressure sensitive adhesive part on a substrate, the substrate comprising a support film and an antistatic layer provided on one face of the support film, wherein:

the pressure sensitive adhesive part is provided on a face on the side of the substrate on which the antistatic layer is provided;

a stress relaxation rate of the substrate is 60% or more, as determined by extending the substrate at a rate of 200 mm/min at a temperature of 23° C., and measuring a stress A at a time of 10% extension and a stress B one minute after stopping the extension, wherein the stress relaxation rate is given by an expression of (A−B)/A×100(%);

the antistatic layer is formed by curing an antistatic layer-forming composition comprising a curing component and a metal filler;

a content of the metal filler is 55 mass % or more relative to a total mass of the curing component and the metal filler; and

the curing component comprises a urethane acrylate oligomer.

2. The surface protective sheet according to claim 1 , wherein the urethane acrylate oligomer is selected from the group consisting of a urethane acrylate oligomer having a polyester structure and a urethane acrylate oligomer having a polyether structure.

3. The surface protective sheet according to claim 1 , wherein the support film is a cured urethane film obtained by curing a urethane-containing curing resin composition.

4. The surface protective sheet according to claim 3 , wherein the urethane-containing curing resin composition comprises a urethane acrylate oligomer.

5. The surface protective sheet according to claim 1 , wherein the metal filler is a metal oxide filler.

6. The surface protective sheet according to claim 5 , wherein the metal oxide filler is a particle of phosphorus-doped tin oxide.