A polarizer used in a polarizing plate is an optical device for converting natural light or randomly polarized light into light polarized in a certain direction, and it has been widely used in display devices such as liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs). As a polarizer commonly used in such display devices, a polyvinyl alcohol (PVA)-based polarizing film containing an iodine compound or a dichroic dye and having molecular chains oriented in a predetermined direction has generally been used.
The PVA-based polarizing film may be manufactured by dyeing a PVA-based film with iodine or a dichroic dye and stretching and crosslinking the film in a predetermined direction. In this case, the stretching process may be performed through wet stretching in a solution such as an aqueous boric acid solution or an aqueous iodine solution, dry stretching in the atmosphere, or the like. In the aforementioned manufacturing process according to the related art, the PVA-based film needs to have a thickness exceeding 60 μm, prior to the stretching thereof, in order to perform stretching thereon without the occurrence of breakage. In a case in which the thickness of the PVA-based film is equal to or less than 60 μm prior to the stretching process, the degree of swelling in the PVA-based film may be increased, and a modulus of such a thin film per unit area may be increased during the stretching process, whereby breakage of the film may easily occur.
Meanwhile, in accordance with the recent trend for the thinning of display devices, polarizing plates also need to have a reduced thickness. However, in the case in which a PVA-based film having a thickness exceeding 60 μm is used according to the related art, there has been a limitation on reducing the thickness of a polarizer. Therefore, research into manufacturing a polarizer having a reduced thickness has been carried out.
Korean Patent Laid-Open Publication No. 2010-0071998 discloses a method for manufacturing a thin polarizing plate using a laminate fabricated by coating a base layer with a hydrophilic polymer layer, or co-extruding a base layer-forming material and a hydrophilic polymer layer-forming material. However, in the case of using the coating or co-extruding method, the PVA layer and the base layer may not easily be separated from each other after stretching. Since a high degree of peel strength is required for the separation therebetween, defects such as damage to or a deformation of the PVA layer may occur during the separation process, resulting in a deterioration of optical properties of the PVA-based film, such as the degree of polarization and the like. In addition, in the case of using the coating or co-extruding method, since the PVA-based film is manufactured by melting a PVA resin and then co-extruding the PVA layer and the base layer, or by preparing the PVA resin as a coating solution and then applying the coating solution to the base layer, the physical properties of the manufactured PVA film may be easily changed, depending on extruding conditions, coating conditions or film forming conditions. Thus, the physical properties of the resultant PVA film may deteriorate and uniformity thereof may not be obtained.
Therefore, a polarizing plate including a thin film polarizer having superior optical properties needs to be manufactured.