An optical film has been used for a display screen such as a touch panel, a liquid crystal display (LCD), a cathode ray tube (CRT) for TV or computer, a plasma display, or the like.
The optical film requires excellent transparency and visibility and uses a biaxially stretched polyester film having mechanical characteristics and electrical characteristics as a base film.
The biaxially stretched polyester film is insufficient in terms of surface hardness, wear resistance, or scratch resistance and thus, the surface thereof is easily damaged by a friction and contact with an object. In order to prevent this, a hard coating layer is stacked on a film surface. In order to improve the adhesion between the base, that is, the polyester film and the hard coating layer, a coating layer as an intermediate layer is disposed on the polyester film.
As the intermediate layer for improving the adhesion between the base layer and the hard coating layer, acrylic resin or urethane resin has been generally used. However, when forming the coating layer using the above-mentioned resin, a refractive index is about 1.5, which shows a large difference from a surface refractive index 1.66 of the biaxially stretched polyester film. As a result, interference stain, that is, a rainbow phenomenon occurs due to external light reflection between the surface of the polyester film and the intermediate layer. The rainbow phenomenon deteriorates the visibility and causes an eye fatigue when the rainbow phenomenon is generated on a display member. Therefore, a need exists for a method for improving the rainbow phenomenon.