Peroxide masterbatch comprising: —15-55 wt % of one or more organic peroxides, —15-45 wt % of at least one copolymer of (i) ethylene or propylene and (ii) 1-butene and/or 1-octene, and 13-45 wt % of at least two types of fillers—filler type 1 and filler type 2—each having a different BET surface area: (i) filler type 1 being present in the masterbatch in a concentration of 3-15 wt % and having a BET surface area of more than 100 m2/g; (ii) filler type 2 being present in the masterbatch in a concentration of 10-30 wt % and having a BET surface area of 100 m2/g or less.