As LCDs, one of many types of image displays, have been technically improved to provide wide viewing angles, high resolution, high response, good color reproduction, and the like, applications of LCDs are spreading from laptop personal computers and monitors to television sets. In a basic LCD structure, two flat glass substrates each provided with a transparent electrode are opposed via a spacer to form a constant gap, between which a liquid crystal material is placed and sealed to form a liquid crystal cell, and a polarizing plate is adhered on the outside surface of each flat glass substrates. In a conventional technique, a glass or plastic cover plate is attached to the surface of a liquid crystal cell in order to prevent scratches on a polarizing plate bonded to the surface of the liquid crystal cell because of low scratch resistance of the polarizing plate. However, the placement of such a cover plate is disadvantageous in terms of cost and weight. Thus, a hard-coating process has gradually been used to treat the surface of polarizing plates. The hard-coating process is commonly conducted in a way such that a hard coat film comprising a transparent plastic substrate on which a hard coat layer is formed is provided on a side of a polarizing plate.
The hard coat layer is formed as a thin coat film with a thickness in the range about from 2 to 10 μm on a transparent plastic film substrate using a thermoset resin or an ionizing radiation curable resin such as an ultraviolet curable resin. Since a thickness in the range is insufficient as a thickness of the hard coat layer, it has been common that even a hard coat resin having a characteristic of a pencil hardness of 4H or higher being coated on glass, suffers an influence of a transparent plastic film substrate, which is an underlying base, and a surface hardness of a hard coat layer formed on a transparent plastic film substrate is reduced to 2H or lower in pencil hardness.
LCD applications have come to include home television sets, and thus it is easily expected that the users of general home television sets should handle LCD television sets in the same manner as in the case of glass CRT television sets. Glass CRTs have a pencil hardness of about 9H, which significantly differs from that of current hard coat films. Thus, hard coat films have been required to have higher hardness, even if it cannot reach a pencil hardness of 9H.
Besides, in a case where a hard coat film is adhered onto various kinds of image displays, visibility of a display is reduced by light reflection on a display surface, which is on a polarizing plate surface. Hence, more improvement on visibility is required to a hard coat film.
It is thought simply to increase a thickness of a hard coat layer as a method increasing a hardness of a hard coat layer. Though hardness is increased in such a method, cracking and peeling in a hard coat layer occurs with ease and at the same time, curling due to cure shrinkage of a hard coat layer increases, which negates practicality of such a hard coat layer. In recent years, proposals have been offered of a method in which not only a high hardness of a hard coat film realized but also a problem of cracking or curling due to cure shrinkage in a hard coat layer is solved (JP-A No. 9-113728, JP-A No. 11-300873, JP-A No. 2000-52472 and JP-A No. 2000-112379).
A proposal has been offered in JP-A No. 9-113728 of a protective film for a polarizing plate comprising a transparent plastic film substrate on at least one side of which a cured coat layer (hard coat layer) is formed that is made from a composition containing an ultraviolet curable polyol acrylate-based resin. Dipentaerythritol hexaacrylate is mainly exemplified as an ultraviolet curable polyol acrylate-based resin. In a case where the resin is coated on a plastic film substrate, a thickness of a cured coat layer of 10 μm or more can secure a pencil hardness of 4H or higher, whereas curling due to cure shrinkage is hard to be suppressed at the same time.
A proposal has been offered JP-A No. 11-300873 of a hard coat film comprising a transparent plastic film substrate on at least one side of which a cushioning layer is formed that has a thickness in the range of 3 to 50 μm, constituted of a single layer or a multiple of layers and a hard coat layer is formed thereon that has a thickness in the range of 3 to 15 μm. Pencil hardness values of the transparent plastic film substrate, the cushioning layer and a hard coat layer increase in the order, with which structure the hard coat film is designed to have a pencil hardness in the range of from 4H to 8H as a whole. In JP-A No. 11-300873, however, a cushioning layer is required in addition to a hard coat layer and at least a two layer structure is required, leading to a fault to give a load in a fabrication process.
A proposal has been offered in JP-A No. 2000-52472 of a construction in which a cured resin layer containing inorganic or organic internally crosslinked ultrafine particles as a first hard coat layer is formed on at least one side of a transparent plastic film or a sheet substrate and thereafter, a thin film as a second hard coat layer is formed thereon that is made of a clear cured resin not containing inorganic or organic internally crosslinked particles. JP-A No. 2000-52472, however, also has, in a similar way as that in JP-A No. 11-300873, a fault that a load is given to a fabrication process because of a two layer structure.
A proposal has been offered in JP-A No. 2000-112379 of a construction in which a hard coat film is constituted of a transparent plastic film substrate on at least one side of which a hard coat layer having at least one layer is formed, wherein the hard coat layer forming material contains inorganic particles in the range of from 20 to 80 parts by weight relative to 100 parts by weight of a resin, a thickness of the hard coat layer as a whole is in the range of from 10 μm to 50 μm and a surface pencil hardness is 4H or higher. However, in a case where a hard coat layer with a thickness of 10 μm or more is formed on a transparent plastic film substrate with a hard coat forming material containing inorganic fine particles in the range relative to the resin such as polyester acrylate or polyurethane acrylate, which is used in JP-A No. 2000-112379, difficulty is encountered striking a balance between securement of a sufficient hardness and a suppression of curling due to cure shrinkage.