Photopolymer coatings for optical discs

The application of polymer coatings in optical disc (OD) production was first mentioned at the beginning of the 1980s [ 11, when the manufacture of laser video discs by photopolymerization (the 2P process) commenced. The method involves the conveying of information codified into the microstructure of the mould on to the covering polymer layer of a relief-forming composition followed by photopolymerization. In comparison with other methods of replication (injection molding, pressing, etc.), this method eliminates any thermal deformation of the mould and substrate, thus improving the precision of microsubstrate transference and achieving a high polymer purity. In summary, all these features contribute to make the process a successful method for short series and mould replication. The replicate, which is a thin polymer coating, is made by coating the metal mould with a thin (20-50 pm) layer of a photopolymerizable composition. This is exposed via the substrate to ultraviolet light which leads to polymerization. After polymerization is complete, the photopolymer layer and substrate are removed from the mould, the replicate is coated with a reflective material and a protective coating is finally applied. The ultraviolet-curable compositions employed must be capable of providing the final coating with high optical and mechanical properties (mainly hardness) and adhesion to the reflective material. At present, relief-carrying photopolymerizable materials (PPMs) are used for the replication of video [ 1, 21, compact discs [3] and digital ODs for