1. Field of the Invention
The present invention relates to a cellulose acylate film showing favorable optical uniformity and a method for producing the same.
2. Description of the Related Art
It has conventionally been attempted to increase viewing angles of liquid crystal display devices by using a cellulose acylate film obtained by stretching a cellulose acylate resin so that the film should have an in-plane retardation value (Re value) and retardation value along the thickness direction (Rth value) as a phase difference film.
As methods for stretching a cellulose acylate resin for such a purpose, a method of performing the stretching along the longitudinal direction (for example, the longitudinal direction in a roll-to-roll process), a method of performing the stretching along the transverse direction (for example, the direction perpendicular to the longitudinal direction in a roll-to-roll process, width direction) and a method of performing the stretching along the longitudinal and transverse directions simultaneously (simultaneous stretching) are known.
Among these, the longitudinal stretching has conventionally been used in many cases, because it uses a compact apparatus. FIG. 2 shows an example of a conventionally used longitudinal stretching apparatus, and in this apparatus, a cellulose acylate resin (4) is heated to a temperature higher than the glass transition temperature (Tg) and stretched between two or more pairs of nip rollers (5, 5) by using a transportation speed of the rollers on the exit side faster than that of the rollers on the entrance side.
Furthermore, for example, as for a method of stretching a cellulose acylate resin, Japanese Patent Laid-open Publication (Kokai) No. 2001-42130 describes that the unevenness of slow axis angle can be improved by using reverse directions for casting for film formation and longitudinal stretching.
Further, Japanese Patent Laid-open Publication No. 2003-315551 describes a method of performing the stretching by using nip rollers installed in a stretching zone with a short span represented by a length/width ratio (L/W) of 2 or less so as to improve the orientation along the thickness direction (Rth value). The length/width ratio (LAN) used herein means a value obtained by dividing the interval of the nip rollers used for the stretching (L) with the width (W) of the cellulose acylate film to be stretched (the same shall also apply to the present invention).
However, if a stretched film obtained by any of the methods described in these patent documents is used as a phase difference film, fine planar unevenness and frame-like failure are generated, and improvement of these phenomena has been desired.