The present application is a National Phase entry of International Application No. PCT/KR2019/004707 filed on Apr. 18, 2019, and claims priority to and the benefit of Korean Patent Application No. 10-2018-0044904 filed in the Korean Intellectual Property Office on Apr. 18, 2018, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to a retardation film, a polarizing plate comprising the same, and a liquid crystal display device comprising the same.

BACKGROUND

Recently, with the development of optical technology, display devices using various systems such as a plasma display panel (PDP), a liquid crystal display (LCD), and an organic light emitting diode (OLED) replacing a cathode-ray tube in the related art have been proposed and come into the market. Recently, characteristics required for these display devices tend to become highly advanced, and accordingly, characteristics required for peripheral parts, such as an optical film, applied to a display device also tend to be highly advanced. In particular, as the recent display devices have become thinner and lighter and the screen areas thereof have been enlarged, wider viewing angle, higher contrast, suppression of a change in image tone according to a viewing angle, and uniformity of screen display have become particularly important issues.

A retardation film is an optical film used in a display device for the purpose of improving viewing angle, display quality, and the like, and may be divided into retardation films having a forward wavelength dispersion property, a flat wavelength dispersion property, and a reverse wavelength dispersion property, in accordance with wavelength dispersion characteristics. A retardation film having the forward wavelength dispersion property means a retardation film having a characteristic in which the retardation value generated decreases as the wavelength of the incident light increases. A retardation film having a flat wavelength dispersion property means a retardation film having a characteristic in which a similar retardation value is generated regardless of the wavelength of the incident light. A e retardation film having the reverse wavelength dispersion property means a retardation film having a characteristic in which the retardation value generated also increases as the wavelength of the incident light increases.

Most retardation films developed so far have the forward wavelength dispersion property or the flat wavelength dispersion property. In the case of a retardation film having the forward wavelength dispersion property or the flat wavelength dispersion property, the degree of phase delay varies when the optical wavelength is changed, so that there is a problem in that it is difficult to obtain uniform color or visual sensation. In contrast, when a retardation film has a reverse wavelength dispersion property, the phase delay value also increases as the optical wavelength increases, so that the retardation film having the reverse wavelength dispersion property has drawn attention in that it is possible to implement a relatively uniform phase delay in a relatively wide band of optical wavelengths.

However, since the wavelength dispersion property is a characteristic exhibited inherently depending on a material for a retardation film, a new raw material needs to be found in order to manufacture a retardation film having the reverse wavelength dispersion property with one film, which is not actually easy. Accordingly, in the related art, a method of manufacturing a retardation film having the reverse wavelength dispersion property by laminating two or more retardation films having different wavelength dispersion properties using an adhesive agent or a bonding agent, a method of co-extruding a resin having a positive retardation value and a resin having a negative retardation value to form a laminate, and stretching the laminate, or the like, has been proposed.

However, the first method has a problem in that, when the optical axes of two retardation films to be laminated are not disposed exactly, the reverse wavelength dispersion property is exhibited, so that it is very difficult to manufacture the retardation film having the reverse wavelength dispersion property. The second method has a problem in that, when the glass transition temperatures of the resins forming the respective layers are not similar, the resins are not properly stretched, so that an available resin is limited.

In addition, a retardation film in which the wavelength dispersion property is improved into reverse wavelength dispersion or flat wavelength dispersion has a problem in that the glass transition temperature is significantly lowered, so that the heat resistance is weak.

Therefore, there is a need for developing a retardation film capable of implementing the reverse wavelength/flat wavelength dispersion property, which is easily manufactured and excellent in heat resistance.

Korean Patent Application Laid-Open No. 2005-0101743

SUMMARY

The present invention has been made in an effort to provide a retardation film, a polarizing plate comprising the same, and a liquid crystal display device comprising the same.

An exemplary embodiment of the present application provides a retardation film comprising a composition or a cured product thereof, the composition comprising: an acrylate-based resin having a glass transition temperature (Tg) of 120° C.; or more; a retardation adjusting agent comprising a styrene monomer; and a triazine-based birefringence adjusting agent, in which the triazine-based birefringence adjusting agent is included in an amount of more than 5 parts by weight and 15 parts by weight or less, based on 100 parts by weight of the acrylate-based resin.

Another exemplary embodiment of the present application provides a polarizing plate comprising: a polarizer; and the one or more retardation films according to an exemplary embodiment of the present application.

Still another exemplary embodiment of the present application provides a liquid crystal display device comprising: a liquid crystal cell; an upper polarizing plate provided on an upper part of the liquid crystal cell; a lower polarizing plate provided on a lower part of the liquid crystal cell; and a backlight unit provided on a lower part of the lower polarizing plate, in which at least one of the upper polarizing plate and the lower polarizing plate comprises: a polarizer; and the retardation film according to an exemplary embodiment of the present application, disposed on one surface of the polarizer.

The retardation film according to the present invention comprises: an acrylate-based resin having a glass transition temperature (Tg) of 120° C.; or more; a retardation adjusting agent comprising a styrene monomer; and a specific content of triazine-based birefringence adjusting agent, thereby providing a retardation film which is excellent in heat resistance.

Further, the retardation film according to the present invention may implement the reverse wavelength/flat wavelength dispersion property by using the specific content of triazine-based birefringence adjusting agent to change the wavelength dispersion property using a difference in refractive index from the acrylate-based resin, and the retardation film of the present invention has the reverse wavelength dispersion property in which as the optical wavelength increases, the phase delay value increases, thereby generating a relatively uniform degree of phase delay in a wide band of optical wavelengths. As a result, when the retardation film of the present invention is applied to a polarizing plate or a display device, the retardation film can implement excellent color, visual sensation, and optical characteristics as compared to the related art.

