Process for preparing high melt strength polypropylene and crosslinked prepared therewith

A process for preparing polypropylene and its copolymers having high melt strength, wherein said process comprises the steps of:

• Irradiating with the aid of low dose ionizing radiation, a polymer of polypropylene and its copolymers in the presence of a reactive atmosphere, containing at least one crosslinking promoting gas, so as to obtain a polypropylene that is crosslinked and grafted with high molecular weight chain portions;
• In a first treating step, herein called recombination step, keeping the so-irradiated polypropylene and its copolymers at a temperature, for a period of time, that makes possible to recombine most of the free radicals still remaining;
• In a second treating step, herein called termination step, annihilating any remaining free radicals from the crosslinked and grafted polypropylene and its copolymers.