Birefringent banknote film

A birefringent polyolefinic substrate 1 and a printable coating on at least one major surface 4 of the substrate 1, the printable coating 6 comprising a polymeric binder and an ethylenically unsaturated compound dispersed with or bonded to the binder, wherein the compound is able to covalently bond with an ink. The banknote is authentifiable by means of the retardation signal. The substrate 1 may comprises polyethylene, polypropylene, polybutylene and/or mixtures, blends of copolymers thereof. The substrate 1 may comprise a core 2 and one or more skin layers 3. The core 2 may comprise biaxially orientate polypropylene, the skin layers 3 may comprise propylene-ethylene-butylene terpolymer. The polymeric binder is preferably water dispersible polymer such as acrylics, acrylates, urethanes, urethane acrylates, styrene butadiene/maleic anhydride copolymers. The coating 6 may comprise a crosslinker such as carbodiimide, aziridine crosslinkers, or co-ordinating metal ligands. The substrate 1 may be pre-treated using corona discharge, flame or plasma treatment and/or oxidising chemical treatment. A primer layer 5, eg polyethylene imine or crosslinked polyurethane acrylates, may be provided between the substrate 1 and the coating 6. Coating 6 may carry an ink which is optionally UV or electron curable. Also included is a process for manufacture of a birefringent film as described.