Also to improve the elastomeric properties of such compositions a partially cured monoolefin copolymer rubber (such as EPM, ethylene-propylene copolymer rubber or EPDM, ethylene-propylene-non-conjugated diene rubber) has been blended with a polyolefin resin, such as polyethylene or polypropylene (U.S. Pat. No. 3,758,643).