Photoprotective zinc oxide coatings on polyethylene terephthalate films

Zinc oxide coatings have been deposited on polyethylene terephthalate by r.f. magnetron sputtering from a ceramic target in different argon–oxygen mixtures. The influence of the sputtering parameters on the properties of the coatings was studied. It was found that ZnO deposits crystallize in the würtzite phase and that the crystallinity decreased when the oxygen partial pressure increased. The composition and the optical constants (refractive index, extinction coefficient, optical band gap) of the ZnO films were varied over a wide range depending on sputtering parameters. In addition, the ZnO films grow under a state of compression, which is mainly due to the internal defects. The stress increased with the sputtered particles energy. The better adhesion between the coating and the polymer induced by cold plasma treatments and the influence of the sputtering conditions were also studied. The permeability to oxygen was limited and the PET photo-oxidation rate was strongly reduced once the polymer had been coated with ZnO layers prepared under optimal conditions.