Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
1998-11-27
2001-04-17
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S068000, C428S142000, C428S156000, C428S213000, C264S001600, C264S001700, C264S001240
Reexamination Certificate
active
06217995
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical sheet and an optical part made of transparent resin. More particularly, the present invention relates to an optical sheet and an optical part made of transparent resin obtained by subjecting a transparent resin to a specific heat treatment. The optical part of the present invention can be prepared by an efficient method. The product thus obtained has a light weight and excellent heat resistance, light resistance and impact resistance. The product is also advantageous in that if used in a planar form, it has a reduced thickness.
2. Description of the Related Art
An inorganic glass has heretofore predominated in the materials to be used in the preparation of products in the optical art. This is attributed to the fact that an inorganic glass inherently has an excellent light transmittance over a wide wavelength range, excellent heat resistance, chemical resistance, water resistance, surface hardness and abrasion resistance and a wide range of linear expansion coefficient.
However, the foregoing-inorganic glass is disadvantageous in that it requires a polishing step and much time to be finely worked on the surface thereof, if necessary. Thus, the foregoing inorganic glass cannot be fairly mass-produced and cannot be efficiently produced in a small amount for each of many varieties.
Further, the foregoing inorganic glass is required of low birefringence index if used for some glass parts. However, the foregoing inorganic glass cannot always attain birefringence as desired.
Moreover, an attempt has been made to apply resins such as polymethyl acrylate, polycarbonate, polyarylate and cyclic polyolefin to the art of optical parts taking advantage of its characteristics and transparency. An example of this attempt is to develop the application of such a resin to optical disc by reducing its optical distortion.
As such an example there has been noted the development of the application of these resins to the art of parts requiring a low birefringence index such as liquid crystal display board and liquid crystal projection optical parts. In these uses, these resins are required of birefringence index of not more than 15 nm, preferably not more than 10 nm, more preferably not more than 5 nm, most preferably not more than 2 nm, as calculated in terms of retardation value in single pass. These resins are required of good retention of shape at a temperature of 100° C. to meet requirements in various processings or practical use.
In an attempt to solve the foregoing problems, JP-A-60-222241 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes a process for solvent-casting a heat-resistant resin such as polyether sulfone. However, this process can difficultly allow the solvent to evaporate while maintaining desired surface precision on an industrial basis if the product has a thickness excess 100 &mgr;m. Thus, a sheet or molded product having a thickness of greater than 100 &mgr;m suitable as liquid crystal display board or optical member cannot be obtained from the standpoint of self-supporting properties and gas barrier properties.
Contrary that, in the case of using a thermoplastically forming method such as a extruded-molding or injection molding, the optical distortion of the sheet or molded product grow large at a stage immediately after forming, and it is impossible to obtain the sheet or molded having the desired quality from the standpoint of optical properties.
JP-A-7-126375 proposes a method which comprises heating an extruded sheet laminated with a protective film at a temperature of not lower than its glass transition temperature under an ordinary pressure as a method for eliminating the optical distortion of a thermoplastically formed flat board.
However, the foregoing heat treatment at a temperature of not lower than the glass transition temperature of the extruded sheet can difficultly maintain the flatness of the flat board which has been provided during forming at a good reproducibility because the frozen strain developed at forming disappears with a dynamic change. As a result, the optical distortion of the extruded flat board cannot be eliminated, making it impossible to obtain a substrate having the desired quality from the standpoint of optical properties or precision in surface flatness, as described later in the comparative examples.
SUMMARY OF THE INVENTION
An object of the present invention is to provide optical sheet or optical part having a good productivity and a necessary low birefringence index.
The inventors made extensive studies of the foregoing problems. As a result, it was found that optical sheet or optical part obtained by subjecting a specific material to a specific heat treatment has a low birefringence index and properties sufficient for optical part. Thus, the present invention has been worked out.
The essence of the present invention lies in an optical sheet made of transparent resin having a visible light transmittance of not less than 80%, a glass transition temperature of not lower than 150° C. and a thickness of from 0.2 to 1.5 mm, which exhibits a birefringence index of not higher than 15 nm, a surface roughness Ra of not more than 0.1 &mgr;m and an impact strength of not less than 600 gf·cm as calculated in terms of dropping ball breakdown energy determined by Du Pont process impact resistance test.
The essence of the present invention also lies in an optical part made of a transparent resin having a visible light transmittance of not less than 80% and a glass transition temperature of not lower than 80° C., which exhibits a birefringence index of not higher than 15 nm, a surface roughness Ra of not more than 0.1 &mgr;m.
The optical sheet and optical part according to the present invention can be used in any form and thus can be applied to optical communications parts such as light amplifier, light waveguide and optical fiber, non-linear optical elements, lens, dark room fluorescent materials, etc. The resin to be used as starting material in the present invention exhibits a good light transmittance in visible light range and thus can be used also as lens for visible light to advantage.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be further described hereinafter.
(Optical Sheet)
The optical transparent resin sheet of the present invention is made of a transparent resin having a visible light transmittance of not less than 80% and a glass transition temperature of not lower than 150° C. The thickness of the sheet is defined to be from 0.2 to 1.5 mm from the standpoint of the nerve of the hot-pressed sheet or the reduction of the weight and thickness of the resin substrate of liquid crystal display. The sheet thus obtained exhibits a birefringence index of not more than 15 nm, a surface roughness Ra of not more than 0.1 &mgr;m and an impact strength of not less than 600 gf·cm as calculated in terms of breakdown energy determined by Du Pont process dropping ball impact resistance test.
If the transparent resin has too low a glass transition temperature, it cannot attain heat resistance required at the process for the preparation of liquid crystal display elements. The glass transition temperature of the transparent resin is preferably 160° C., more preferably 170° C. higher than required at the foregoing preparation process.
If the thickness of the foregoing sheet (hereinafter occasionally referred to as “resin substrate”) is too small, the gap between the substrates cannot be kept constant, disadvantageously causing color stain, if used in a liquid crystal display form having an area exceeding that with a diagonal line having a length of 2 inch. Further, the resin substrate thus obtained lacks self-supporting properties if used in a liquid crystal display form having a large area. Moreover, the resin substrate thus obtained exhibits a low gas barrier properties, possibly causing the deterioration of the liquid crystal enclosed therein after a prolonged use and other troubles. Fro
Dixon Merrick
Mitsubishi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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