A substantially planar self-supporting sheet has been patented. It has on at least one side a substantially continuous adherent coating to create a barrier to oxygen. The coating comprises biopolymer nanoparticles in an amount of at least 20% by dry weight of the coating. This forms a substantially continuous phase on the sheet. The biopolymer nanoparticles are obtained from a mechanical thermoplastic process comprising a cross-linking step using a cross-linking agent.