Adhesives for optical films require durability so that foaming, lifting, peeling, etc. of optical films should not occur even in a high-temperature high-humidity atmosphere when optical members are bonded to adherends, and require light leakage prevention property so that the adhesives may flexibly follow dimensional changes of optical films in a high-temperature atmosphere so as not to cause light leakage in the case where two polarizing films that are optical films are bonded with the adhesive in such a manner that their polarization axes are at right angles to each other and allowed to stand at high temperature or at high temperature and high humidity. Further, rework property is also required in order that an optical member (member having an adhesive on an optical film) may be re-released from an adherend such as a liquid crystal panel without contaminating the adherend even in the case where the optical film is bonded to the adherend at the deviated position.
As such adhesives for optical films, acrylic-based adhesives have been mainly used in the past, and as an adhesive satisfying the above requirements, an acrylic-based adhesive obtained by blending an acrylic polymer of high-molecular weight with a medium- or low-molecular weight substance and crosslinking the resulting blend is known. In this acrylic-based adhesive, by crosslinking the acrylic polymer of high-molecular weight, cohesive force is enhanced to impart durability so as not to cause lifting and peeling, and by introducing the medium- or low-molecular weight substance, occurrence of light leakage is inhibited.
However, with recent increase of sizes of displays, light leakage attributable to dimensional changes of optical films, particularly polarizing films, in a high-temperature atmosphere becomes a more serious problem, and the adhesives for optical films have been desired to have higher flexibility. In severe conditions, further, there occurs a problem that the low-molecular weight substance added for enhancing light leakage prevention property bleeds out to contaminate the adherend in the re-release process or to cause peeling.
For example, technique to increase a ratio (Mw/Mn) of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) to not less than 4 in order to improve heat durability and stress relaxation property has been disclosed (patent document 1). However, by merely increasing the Mw/Mn ratio to not less than 4, the stress relaxation property required for large displays is insufficient.
It is also known to enhance adhesion to plastics in a high-temperature high-humidity atmosphere by using a mixture of a (meth)acrylic-based polymer having a weight-average molecular weight of not less than 200,000 and a (meth)acrylic-based polymer having a weight-average molecular weight of less than 200,000 (patent document 2). In this method, however, the component having a lower weight-average molecular weight bleeds on the surface, so that problems of contamination in the re-release process and peeling under severe working conditions could not been solved.
Further, it is also known to user as an adhesive for optical films, a polymer in which an aromatic group-containing monomer is copolymerized (patent document 3 and patent document 4). However, its object is to control the refractive index of the adhesive in order to inhibit interfacial reflection between an optical member, such as a polarizing plate or a retardation plate, and the adhesive and between the adhesive and an adherend. Since the adhesive is designed so as to exhibit strong adhesion immediately after bonding, there is a problem that the adhesive is poor in a property that re-bonding is possible even if wrong bonding is made (rework property).
Moreover, it is also known to use, as an adhesive for lowly polar films, a high-molecular weight polymer containing an alicyclic monomer or an aromatic group-containing monomer as a copolymer component (patent document 5). The adhesive, however, is designed so that the value (Mw/Mn) obtained by dividing the weight-average molecular weight of the copolymer by the number-average molecular weight thereof may become relatively small, and therefore, when the adhesive is used for a polarizing film having a large bond area or the like, there resides a problem that a stress due to dimensional change of the polarizing film or the like cannot be completely relaxed, and hence, sufficient light leakage prevention property is not obtained. Further, even if the alicyclic monomer is copolymerized, the light leakage prevention property is insufficient.
Patent document 1: Japanese Patent Laid-Open Publication No. 341141/2002
Patent document 2: Japanese Patent Laid-Open Publication No. 107507/2002
Patent document 3: Japanese Patent Laid-Open Publication No. 173656/2002
Patent document 4: Japanese Patent Laid-Open Publication No. 13029/2003
Patent document 5: Japanese Patent Laid-Open Publication No. 053976/2005
It is an object of the present invention to provide an acrylic-based adhesive composition for optical films, which not only relaxes a stress due to dimensional change of a member having a particularly large bond area, such as a polarizing film, a retardation film or an elliptically polarizing film, and prevents contamination in the re-release process and peeling or foaming under high-humidity high-temperature conditions but also inhibits color nonuniformity due to light leakage, and to provide an optical member using the adhesive composition.