Polyolefin composition, comprising in percent by weight: 1) 75-85% of a copolymer of propylene, said copolymer containing up to 15% by weight, of ethylene and/or C4-C10 α-olefin(s) and having a fraction soluble in Xilene at room temperature lower than 6% by weight, and 2) 15-25% of a copolymer of ethylene with one or more C4-C10 α-olefin(s) containing from 10 to 25% by weight of said C4-C10 α-olefin(s); said composition having the value of MFR, measured at 230° C., 2.16 kg, of less than 2.5 g/10 min, the total content of ethylene of from 14 to 22% by weight, the total content of C4-C10 α-olefin(s) of less than 4.5% by weight, the ratio of the total content of ethylene to the total content of C4-C10 α-olefin(s) equal to or higher than 4, the value of the intrinsic viscosity of the total fraction soluble in xylene at room temperature equal to or less than 1.5 dl/g, and the ratio of the amount of component 2) and the amount of C4-C10 α-olefin(s) in component 2) equal to or higher than 0.80.