Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-01-27
2004-05-04
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C428S001310, C428S001330
Reexamination Certificate
active
06731357
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates particularly to a suitable film as the polarizing plate protective film of a liquid crystal display unit.
A liquid crystal display unit (LCD) can be operated at low voltage as well as low electric power consumption, can be directly connected to an IC circuit, and can be specifically produced in a shape having less thickness. Accordingly, the liquid crystal display unit has been widely employed as a display unit for word processors, personal computers, and the like.
Such an LCD is basically constructed in such a manner that, for example, both sides of a liquid crystal cell are provided with polarizing plates.
The polarizing plate transmits light having a polarization plain in the definite direction. Accordingly, the LCD plays an important role to visualize the variation of orientation of liquid crystal due to an electric field. Namely, the performance of the LCD largely depends on that of the polarizing plate.
FIG. 1
shows a common structure of the polarizing plate. In
FIG. 1
, reference numeral
1
is a polarizer, and both sides of said polarizer
1
are laminated with polarizing plate protective films
2
. The LCD is constructed by laminating a liquid crystal cell with the polarizing plate having such a structure.
Polarizer
1
is prepared in such a manner that iodine and the like are adsorbed onto a polymeric film and the resulting film is stretched. Specifically, after a solution called H ink containing a dichroic substance (iodine) is wet-adsorbed onto a polyvinyl alcohol film, the dichroic substance is allowed to orient in one direction by uniaxially stretching the resulting film.
Employed as the polarizing plate protective film
2
are cellulose resins, especially cellulose triacetate.
Said polarizing plate protective film
2
is produced employing apparatuses of a drum casting system shown in
FIG. 2 and a
belt casting system shown in FIG.
3
.
In the drum casting apparatus, casting die
12
is provided adjacent to casting drum
11
and tenter drying section
14
via peeling roll
13
, roll drying section
15
and winding roll
16
are provided. After a dope from the casting die is cast onto the casting drum, the resulting film is peeled from the casting drum
11
, employing the peeling roll
13
, is dried in a first, second, third, and fourth drying zones of the tenter drying section
14
, completely dried in the roll drying section
15
, and wound onto the winding roll
16
.
In the belt casting apparatus shown in
FIG. 3
, casting die
22
is provided adjacent to casting belt
21
, and roll drying section
24
via peeling roll
23
and winding roll
25
are provided. After a dope from the casting die
22
is cast onto the casting belt
21
, the resulting film is peeled employing the peeling roll
23
, completely dried in the roll drying section
24
, and then wound by the winding roll
25
. In this casting belt, the cast dope is dried at a high rate. As a result, the tenter drying section employed in the drum casting system is not required.
At present, the thickness of the polarizing plate protective film
2
is at least 80 &mgr;m, which is relatively thick.
Recently, from the viewpoint of optical properties such as transmittance and the like, a decrease in the thickness of the polarizing plate protective film has been demanded.
However, when the polarizing plate protective film having a thickness of below 80 &mgr;m is produced, it has been found that a number of wrinkles are formed in a film which is wound by the winding rolls
16
and
25
to markedly decrease the production yield.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a suitable film employed for a liquid crystal display member, which results in minimal wrinkles which are likely to be formed when said film has a thickness of below 80 &mgr;m, increases the production yield, enhances transparency as well as retardation due to a decrease in the thickness.
Furthermore, a decrease in the thickness of a visibility angle enlarging film provided in the entire surface of a liquid crystal display apparatus and the like has been demanded, and said film has had the same problems as those of the polarizing plate protective film.
As the results of investigation performed by the present inventors, it has been discovered that as variation in film thickness increases, more wrinkles are formed, while as variation of the same decreases, less wrinkles are formed.
Further, as the results of detailed investigation on the ratio of wrinkle formation as well as variation in film thickness, it has been discovered that at the film thickness of no more than 60 &mgr;m, when variation in film thickness is within ±3.0 percent of the standard thickness (average film thickness) of said film, is preferably ±2.9 percent, and is more preferably ±2.8 percent, wrinkles, which are not permitted as the polarizing plate protective film, are not formed.
Further, it has been discovered that when a film having a thickness of no more than 60 &mgr;m is comprised of a cellulose ester film, frequency of wrinkle formation largely depends on the degree of average substitution. The average degree of substitution as described herein is defined based on the average number of hydroxyl groups among three of them in a unit composing cellulose, substituted with other substituents.
As the results of detailed investigation on the frequency of wrinkle formation as well as the average degree of substitution (the average number in entire cellulose of hydroxyl groups of three of them in a unit of C
6
H
10
O
5
composing said cellulose substituted with other substituents), it has been discovered that at the film thickness of no more than 60 &mgr;m, if the average degree of substitution is preferably between 2.88 and 3.00, is more preferably between 2.90 and 3.00, and is most preferably between 2.91 and 3.00, wrinkles which are not permitted in the polarizing plate protective film, markedly decrease.
The present invention has been achieved based on the aforementioned discoveries, and the aforementioned problems are solved employing a film for a liquid crystal display unit, which has a film thickness of 20 to 60 &mgr;m and a variation in film thickness within ±3.0 percent of the standard film thickness. The variation in film thickness is preferably within ±2.9 percent of the standard film thickness, and is more preferably within ±2.8 percent.
When the variation in film thickness is controlled within ±3.0 percent of the standard film thickness, in the case of a thin film in the range of 20 to 60 &mgr;m, especially in the range of 25 to 55 &mgr;m, and further in the range of 30 to 55 &mgr;m, wrinkles are not substantially formed. Namely, wrinkles, which are not permitted in the film for a liquid crystal display unit, especially a polarizing plate protective film, are not substantially formed. For example, when a 5000 m long film is wound, wrinkles, which are not permitted in the polarizing plate protective film, are not formed. As a result, the production yield increases.
Further, when the film for a liquid crystal display unit is comprised of cellulose ester film, adjusting the average degree of substitution to the range of 2.88 to 3.00 effectively minimizes wrinkles.
Namely, when a film comprised of cellulose ester having an average degree of substitution of 2.88 to 3.00 is wound in a roll, the formation of wrinkles is markedly minimized as the film thickness decreases from the range of 20 to 60 &mgr;m via the range of 25 to 55 &mgr;m, to the range of 30 to 55 &mgr;m. Namely, when the average degree of substitution of cellulose ester is at least 2.88, is preferably at least 2.90, and is more preferably at least 2.91, the film comprised of said cellulose ester is not subjected to wrinkling even using the thin film having a thickness of no more than 60 &mgr;m., that is, wrinkles which are not permitted in the polarizing plate protective film are not substantially formed. For example, when a 5000 m long film was wound, wrinkles, which were not
Michihata Isamu
Nagayasu Koichi
Shimizu Kunio
Tachibana Noriki
Takada Masahito
Duong Tai
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Ton Toan
LandOfFree
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