An expandable, self-adjusting core plug which will maintain a fit between core plug and an out-of-round web roll core. The binder composition includes a solvent, a hyper-branched polymer and a polymer with high ion conductivity, wherein the hyper-branched polymer and the polymer with high conductivity of hydronium are distributed uniformly over the solvent, and the hyper-branched polymer has a DB of more than 0. The strap may be retracted by a torsion spring, and may be coiled in the pack by use of a rotating fork.