A method is disclosed for improving the gas and vapour barrier properties of a polymeric composition containing an inorganic platelet filler. A coupling agent is added to the composition, the coupling agent having a first chemical group which is reactive with the polymer and a second chemical group which is reactive with the platelet filler. The coupling agent may be a vinyl silane, preferably vinyl trimethoxysilane. A free radical initiator may also be added. Also disclosed are compositions manufactured by such methods, and flexible tube containers formed from such compositions.
