JP4187593B2 - Liquid crystal display - Google Patents

Description

（ここで、Ｒは−Ｈまたは−ＣＯＣＨ₃である）
【００２３】
アセチル化度の測定は、本発明では、Ｔ．Ｓｅｉ、Ｋ．Ｉｓｈｉｔａｎｉ，Ｒ．Ｓｕｚｕｋｉ，Ｋ．Ｉｋｅｍａｔｓｕ Ｐｏｌｙｍｅｒ Ｊｏｕｒｎａｌ １７． １０６５−１０６９（１９８５）に記載の方法で¹³Ｃ−ＮＭＲスペクトルにより決定した。
【００２４】
セルロースアセテートのアセチル化度が２．９を超えたものをフィルム化し、一軸延伸すると、一軸延伸方向とは垂直方向が遅相軸となる。すなわち、負の光学異方性を有する位相差板となる。一方、アセチル化度が２．８以下では延伸方向が遅相軸となる正の光学異方性を有する位相差板となるが、このアセチル化度が小さくなるほど、測定波長４５０ｎｍ，５５０ｎｍのΔｎ・ｄの比であるΔｎ・ｄ（４５０）／Δｎ・ｄ（５５０）は大きくなる傾向を示す。
【００２５】
上記実験結果から、セルロースアセテートのアセチル基の数によって複屈折の分散が制御可能であることが分かったが、本発明の目的を達成するにはそのアセチル化度が２．４〜２．９、好ましくは２．５〜２．８であることにより達成することができる。アセチル基は分極率の大きいＣ＝Ｏ二重結合を含んでおり、このＣ＝Ｏ結合はセルロース環主鎖に対して略垂直に配向するものが多いと思われる。複屈折は、延伸されたフィルムの主鎖方向とそれに垂直な方向の分極率の差と相関するものである。アセチル基の数によってこの分極率の差が変化し、また、この分極率の差は波長によって異なることから、アセチル基数が２．５〜２．８のときに特に本発明の目的と一致するものと考えられる。
【００２６】
セルロースアセテートの製造方法は、公知の方法であるセルロースを一度完全に酢化してトリアセチルセルロースとした後、加水分解することにより目的のアセチル化度を得る方法が好ましい。
【００２７】
また、セルロースアセテートの粘度平均重合度は１２０以上であることが好ましく、さらに好ましくは１３０〜５００である。
【００２８】
本発明における位相差板を得るためにセルロースアセテートをフィルム化するためには、公知の方法である溶液キャスト製膜であることが好ましい。有機溶剤としては例えばメチレンクロライド、メチレンクロライド／メタノール（重量比９／１等）、ジオキソラン等公知の溶剤を挙げることができる。
【００２９】
得られたフィルムはついで延伸等により配向フィルムとすることができる。ここで配向フィルムは、５９０ｎｍでの位相差が２０ｎｍ以上であることが好ましい。
【００３０】
延伸方法も公知の延伸方法を使用し得るが、好ましくは縦一軸延伸である。延伸性を向上させる目的で、公知の可塑剤であるジメチルフタレート、ジエチルフタレート、ジブチルフタレート等のフタル酸エステル、トリブチルフォスフェート等のりん酸エステル、脂肪族二塩基エステル、グリセリン誘導体、グリコール誘導体等が用いられる。先述のフィルム製膜時に用いた有機溶剤をフィルム中に残留させ延伸しても良い。この有機溶剤の量としてはポリマー固形分対比１〜２０ｗｔ％であることが好ましい。
【００３１】
さらに、フェニルサリチル酸、２−ヒドロキシベンゾフェノン、トリフェニルフォスフェート等の紫外線吸収剤を位相差板中に添加しても良い。
【００３２】
本発明における位相差板は透明であることが好ましく、へーズ値は３％以下、全光線透過率は８５％以上であることが好ましい。また、ガラス転移点温度は１００℃以上であることが好ましい。
位相差板の膜厚としては１μｍから４００μｍであることが好ましい。
【００３３】
上記高分子配向フィルムからなる位相差板は、波長５５０ｎｍにおける位相差Δｎ・ｄ（５５０）が、該波長の四分の一であるとき、λ／４板として使用することができる。測定波長５５０ｎｍの四分の一の波長とは１３７．５ｎｍであるが、好ましくは１３７．５ｎｍ±２０ｎｍより好ましくは１３７．５ｎｍ±１０ｎｍの範囲であれば、λ／４板として十分機能を発揮しうる。
【００３４】
本発明における位相差板のうち、Δｎ・ｄが可視光において最も視感度の高い波長５５０ｎｍの四分の一波長のものを、偏光板一枚だけを使用し裏面電極を反射電極と兼ねた構成である反射型液晶表示装置に用いることにより、画質に優れた反射型表示装置を得ることが可能である。また、ゲストホスト型の液晶層の観測者に対して裏面側にこの位相差板を用いることも可能である。これらの場合の位相差板の役割は、直線偏光を円偏光に、円偏光を直線偏光に可視光領域において変換することであるが、本発明における位相差板はこのような目的を満足させることが可能である。
【００３５】
また、これらのフィルムを上記液晶表示装置の液晶層を挟持するガラス基板の代わりに用いて、基板兼位相差板の役割を持たせても良い。
【００３６】
また、左右どちらか一方の円偏光のみを反射するコレステリック液晶等から構成される反射型偏光板の円偏光を直線偏光に変換する素子としても、同様に使用することが出来る。
【００３７】
本発明における位相差板を四分の一波長板として用い、偏光板に貼り合わせたものは自然偏光を円偏光に変換できる円偏光板となる。これはプラズマデイスプレイ等の前面板における反射防止フィルムとして利用した場合、反射光の色付きを低減することが可能である。また、タッチパネル等の反射防止にも利用することが可能である。
【００３８】
液晶表示装置、反射型偏光板等において用いられる位相差板の要求特性として、位相差板に入射する角度が正面入射から斜め入射に変化しても位相差が変化しないことが要求される場合がある。この場合には、三次元屈折率ｎｘ、ｎｙ、ｎｚで表される下記式（３）
【００３９】
【数８】
Ｎｚ＝（ｎｘ−ｎｚ）／（ｎｘ−ｎｙ） （３）
においてがＮｚが０．３〜１．５の間であることが好ましい。特にＮｚ＝０．５のとき、位相差板に入射する角度が正面入射から変化してもほとんど位相差が変化しない。この三次元屈折率は位相差板を屈折率回転楕円体と仮定し、位相差の入射角依存性を測定することにより得られる。
【００４０】
また、本発明における位相差板は、粘着層、接着層を介して偏光板と貼り合わせて円偏光板としたり、また、位相差板上に何らかの材料をコーテイングして湿熱耐久性を向上させたり、耐溶剤性を改良したりしても良い。
【００４１】
【実施例】
以下に実施例を挙げて本発明をより詳細に説明するが、本発明はこれらに限定されるものではない。
【００４２】
（評価法）
本明細書中に記載の材料特性値等は以下の評価法によって得られたものである。
【００４３】
（１）セルロースアセテートのアセチル化度測定法
Ｔ．Ｓｅｉ、Ｋ．Ｉｓｈｉｔａｎｉ，Ｒ．Ｓｕｚｕｋｉ，Ｋ．Ｉｋｅｍａｔｓｕ Ｐｏｌｙｍｅｒ Ｊｏｕｒｎａｌ １７． １０６５−１０６９（１９８５）に記載の方法で１３Ｃ−ＮＭＲスペクトルから測定した。装置は日本電子（株）製商品名『ＪＮＭ−Ａ６００』を用いた。スペクトルの例は図２を用いて説明する。２位の置換度は、図２中のセルロースアセテートのグルコース環の１位の炭素シグナル（ａ＋ｂ）のうち２位がアセチル基で置換されたもののシグナル（ｂ）の面積の割合から計算した。３位の置換度はグルコース環の４位の炭素のシグナルのうち３位が未置換のもののシグナル（ｃ）の面積を２，３，４，５位の４炭素分のシグナル（ｃ＋ｄ＋ｅ＋ｆ＋ｇ＋ｈ＋ｉ＋ｊ）の全面積の４分の１の値で割って、１からこの値を引いて計算した。６位の置換度は６位の炭素のシグナル（ｋ＋ｌ）のうち、アセチル基で置換されているもののシグナル（ｋ）の面積の割合から計算した。本発明ではこれら２，３，６位の置換度の和をセルロースアセテートのアセチル化度とした。
【００４４】
（２）位相差（Δｎ・ｄ）の測定
分光エリプソメータである日本分光（株）製の商品名『Ｍ１５０』により測定した。
【００４５】
（３）平均屈折率分散の測定
透明フィルムを作製し、測定波長５９０ｎｍでΔｎ・ｄを１０ｎｍ以下の状態としてアッベ屈折計にて測定した。アッベ屈折計は（株）アタゴ製の商品名『アッベ屈折計２−Ｔ』を、分光光源装置としては同じく（株）アタゴ製の商品名『ＭＭ−７０１』を用いた。
【００４６】
（４）Ｎｚの測定
先述の分光エリプソメータである日本分光（株）製の商品名『Ｍ１５０』を用い、サンプルの位相差入射角度依存性を測定することにより、三次元屈折率を求めた。その際、サンプルの屈折率異方性に対しては屈折率回転楕円体を仮定し以下の式より求めた。なお、以下の式における平均屈折率は上記（３）の測定で得られる平均屈折率を使用した。膜厚ｄはアンリツ（株）製の電子マイクロメータを用いた。三次元屈折率から位相差視野角特性である。
【００４７】
【数９】
Ｎｚ＝（ｎｘ−ｎｚ）／（ｎｘ−ｎｙ）
を算出した。
Ａ．回転軸が進相軸の場合
【数１０】
Δｎ＝ｎｘｎｚ／｛（ｎｘ²−ｎｚ²）ｓｉｎ²θ／ｎ²＋ｎｚ²｝^0.5−ｎｙ
Ｂ．回転軸が遅相軸の場合
【数１１】
Δｎ＝ｎｘ−ｎｙｎｚ／｛（ｎｙ²−ｎｚ²）ｓｉｎ²θ／ｎ²＋ｎｚ²｝^0.5
Δｎ・ｄ（θ）＝Δｎｄ／（１−ｓｉｎ²θ／ｎ²）^0.5
平均屈折率ｎ＝（ｎｘ＋ｎｙ＋ｎｚ）／３
θ；傾斜角（θ＝０゜で正面入射）
ｄ：膜厚（ｎｍ）
【００４８】
（５）全光線透過率及びヘーズの測定
日本工業規格ＪＩＳ Ｋ７１０５『プラスチックの光学的特性試験方法』に準じ積分球式光線透過率測定装置により測定した。評価装置としては、日本電色工業（株）製の色差・濁度測定器（商品名『ＣＯＨ−３００Ａ』）を用いた。
【００４９】
［参考例１］
和光純薬工業（株）より入手した極限粘度［η］＝１．３３５、アセチル化度２．９１７のセルローストリアセテート１００重量部を塩化メチレン５００重量部に溶解させた。これに９６％酢酸水溶液１０００重量部を加え、減圧により塩化メチレンを除去しながら、７０℃で１００分間、酢酸と水による三酢酸セルロースの加水分解を実施した。反応物を大過剰の水により沈殿、洗浄し、乾燥することにより、アセチル化度２．６６１のセルロースアセテートを得た。このポリマー１００重量部及び可塑剤であるフタル酸ジブチル３重量部を塩化メチレン／メタノール（重量比９／１）混合溶媒７００重量部に溶解させた。この溶液から溶媒キャスト法によりフィルムを製作しさらに、このフィルムを温度１７０℃、１．５倍に一軸延伸した。表１に光学特性測定結果をまとめる。また、そのフィルムの位相差Δｎ・ｄ及び平均屈折率の波長分散を図１に記す。このフィルムは、測定波長が短波長ほど位相差が小さいが、逆に平均屈折率は長波長ほど小さくなることを確認した。
【００５０】
［参考例２］
加水分解条件を７０℃で２００分間とすること以外は参考例１と同様にして、アセチル化度２．５３４のセルロースアセテートを得た。このポリマー１００重量部を塩化メチレン／メタノール（重量比９／１）混合溶媒７００重量部に溶解させた。この溶液から溶媒キャスト法によりフィルムを製作しさらに、このフィルムを温度１７０℃、１．５倍に一軸延伸した。表１に光学特性測定結果をまとめる。このフィルムは、測定波長が短波長ほど位相差が小さいが、逆に平均屈折率は長波長ほど小さくなることを確認した。
【００５１】
［参考例３］
加水分解条件を７０℃で６０分間とすること以外は参考例１と同様にして、アセチル化度２．７２７のセルロースアセテートを得た。このポリマー１００重量部及び可塑剤であるフタル酸ジブチル３重量部を塩化メチレン／メタノール（重量比９／１）混合溶媒７００重量部に溶解させた。この溶液から溶媒キャスト法によりフィルムを製作しさらに、このフィルムを温度１７０℃、１．３倍に一軸延伸した。表１に光学特性測定結果をまとめる。このフィルムは、測定波長が短波長ほど位相差が小さいが、逆に平均屈折率は長波長ほど小さくなることを確認した。
【００５２】
［参考例４］
加水分解条件を７０℃で３００分間とすること以外は参考例１と同様にして、アセチル化度２．４２１のセルロースアセテートを得た。このポリマー１００重量部及び可塑剤であるフタル酸ジブチル３重量部を塩化メチレン／メタノール（重量比９／１）混合溶媒７００重量部に溶解させた。この溶液から溶媒キャスト法によりフィルムを製作しさらに、このフィルムを温度１７０℃、１．５倍に一軸延伸した。表１に光学特性測定結果をまとめる。
【００５３】
表１から０．６＜Δｎ・ｄ（４５０）／Δｎ・ｄ（５５０）＜０．９かつ１．０５＜Δｎ・ｄ（６５０）／Δｎ・ｄ（５５０）＜１．３５を満足することが出来ないことが判った。さらに、このフィルムを後述の実施例１で用いた液晶表示装置の位相差板として用いたが、電圧オン時の黒表示の色味が実施例１よりやや劣った。
【００５４】
［比較例１］
参考例１のアセチル化度２．９１７のセルローストリアセテートを参考例１と同様にフィルム化し、このフィルムを温度１７０℃、１．４倍に一軸延伸した。表１に光学特性測定結果をまとめる。また、そのフィルムの５５０ｎｍで規格化したΔｎ及び平均屈折率の波長分散を図１に記す。このフィルムは短波長ほど位相差が大きく、かつ、延伸軸方向の垂直方向が遅相軸となる負の光学異方性を有するフィルムであることが判った。
【００５５】
［比較例２］
粘度平均分子量３８０００のビスフェノールＡを繰り返し骨格とするポリカーボネートからなるフィルムを溶液キャスト法により作製し、温度１５５℃、延伸倍率１．１倍にて一軸延伸した。このフィルムの光学特性を表１に示す。さらに、表１から０．６＜Δｎ・ｄ（４５０）／Δｎ・ｄ（５５０）＜０．９かつ１．０５＜Δｎ・ｄ（６５０）／Δｎ・ｄ（５５０）＜１．３５を満足することが出来ないことが判った。さらに、このフィルムを後述の実施例１で用いた液晶表示装置の位相差板として用いたが、電圧オン時の黒表示の色味が実施例１よりかなり劣ることが判った。
【００５６】
【表１】

【００５７】
［実施例１］
参考例１で作製したフィルムを一枚偏光板反射型液晶表示装置に組み込み評価した。その構成は観測者側から、偏光板／参考例１で作製した位相差板／ガラス基板／ＩＴＯ透明電極／配向膜／ツイストネマチック液晶／配向膜／金属電極兼反射膜／ガラス基板である。各層間の粘着層は省略してある。電圧オフ時に白表示となるような貼り合わせ角度にして、目視にて色味の評価を実施した。特に黒表示時における着色が少なく、それによりコントラストが高く視認性に優れることが確認できた。
【００５８】
［参考例５］
参考例１で作製したフィルムをコレステリック液晶からなる反射型偏光板上に設置して、市販のバックライト／コレステリック液晶層／参考例１のフィルム／偏光板の構成にて色味を評価した。そのフィルムの遅相軸と偏光板の偏光軸のなす角を４５゜とした。偏光板から出射された光は着色の少ない白状態であった。
【００５９】
【発明の効果】
以上説明したように、本発明によりセルロースアセテートのアセチル化度を制御することにより、フィルム１枚だけでも、測定波長が短波長ほど複屈折が小さい位相差板を得ることが可能となった。そのような複屈折波長分散性を有し、かつ、測定波長５５０ｎｍにおける位相差を四分の一波長にした位相差板は、広い波長領域において円偏光を直線偏光に、直線偏光を円偏光に変換する位相差板として機能するので、偏光板一枚型やゲストホスト型の反射型液晶表示装置、そして片方の円偏光だけ反射するような反射型偏光素子に応用することにより、画質に優れる液晶表示装置や高性能の反射型偏光素子を生産性良く提供することが出来る。
【図面の簡単な説明】
【図１】参考例１において作製した位相差板の位相差（Δｎ・ｄ）及び平均屈折率（ｎ）波長分散特性を示す。
【図２】参考例１における¹³Ｃ−ＮＭＲスペクトルを示す。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device using a retardation plate that is used in an optical element such as an antiglare film and has a retardation value that is smaller as the wavelength is shorter at a measurement wavelength of 400 to 700 nm.
[0002]
[Prior art]
The retardation plate is used in an STN (super twisted nematic) system of a liquid crystal display device, and is used for solving problems such as color compensation and viewing angle expansion. Generally, polycarbonate, polyvinyl alcohol, polysulfone, polyethersulfone, amorphous polyolefin, or the like is used as a material for a phase plate for color compensation, and a polymer in addition to the above-described materials as a phase difference plate material for viewing angle expansion. Liquid crystals, discotic liquid crystals, and the like are used.
[0003]
A quarter-wave plate, which is a kind of retardation plate, can convert circularly polarized light into linearly polarized light and linearly polarized light into circularly polarized light. This is because a liquid crystal display device, in particular, a reflection type liquid crystal display device with a polarizing plate having an electrode on the back side as a reflection electrode when viewed from the observer side, or a combination of a polarizing plate and a quarter-wave plate. And a reflection type polarizing plate that reflects only right or left-handed circularly polarized light made of cholesteric liquid crystal or the like.
[0004]
The retardation plate used in the above-described single-polarization type reflective liquid crystal display device and reflective polarizing plate has a linearly polarized light and a circularly polarized light at a measurement wavelength of 400 to 700 nm, preferably 400 to 780 nm in the visible light region. It is necessary to have an action of converting polarized light into linearly polarized light. If this is to be realized with a single retardation plate, it is ideal that the retardation plate has a retardation of λ / 4 (nm) at a measurement wavelength λ = 400 to 700 nm, preferably 400 to 780 nm.
[0005]
Generally, as the quarter-wave plate, the above-described retardation film material for color compensation is used, and these materials have wavelength dispersion in birefringence. In general, the birefringence of a polymer film is larger as the measurement wavelength is shorter, and is smaller as the wavelength is longer. Therefore, it is difficult to obtain a film having a smaller birefringence as the measurement wavelength is shorter as in the ideal quarter-wave plate described above at a measurement wavelength λ = 400 to 700 nm with only one polymer film. there were.
[0006]
In order to obtain a film in which the birefringence becomes smaller as the measurement wavelength is shorter, such as an ideal quarter-wave plate, JP-A-10-68816 discloses a quarter-wave plate and a half-wave plate. A technique in which the films are bonded together at an appropriate angle and a technique in which two retardation plates having different Abbe numbers are stacked are disclosed in Japanese Patent Laid-Open No. 2-285304.
[0007]
In addition, it is generally known to use a cellulose acetate material as a retardation plate material, but as will be described later, cellulose acetate has different wavelength dispersion characteristics of birefringence depending on the degree of acetyl substitution, and unless the degree of acetyl substitution is optimized, It has not been known that a retardation plate in which birefringence becomes smaller as the measurement wavelength is shorter, such as an ideal quarter-wave plate, cannot be obtained. In addition, a cellulose acetate film having a specific birefringence wavelength dispersion that optimizes the degree of acetyl substitution has a function of reflecting a reflective liquid crystal display device with a single polarizing plate and either circularly polarized light on the left or right. At present, it is not known to function effectively as a quarter-wave plate in a reflective polarizing plate or the like.
[0008]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-68816
[Patent Document 2]
JP-A-2-285304
[0009]
[Problems to be solved by the invention]
In order to obtain a film in which the phase difference becomes smaller as the measurement wavelength is shorter, such as the ideal quarter-wave plate described above, in the current technology, two films must be used. There are problems such as an increase, an increase in cost, and an increase in optical design load. The present invention solves such problems and provides a liquid crystal display device by realizing a retardation plate in which the retardation becomes smaller as the measurement wavelength is shorter in a single film.
[0010]
[Means for Solving the Problems]
  In order to solve the above-mentioned problems, the inventors have intensively studied a polymer material for a retardation plate., SpecialIn addition, it was found that by optimizing the degree of acetylation of cellulose acetate, the shorter the measurement wavelength, the smaller the phase difference, making it a suitable material for the ideal quarter-wave plate described above.
[0011]
Further, as the material whose phase difference becomes smaller as the measurement wavelength is shorter, it can be selected whether a material having a larger average refractive index is used for a longer wavelength or a material for which a shorter wavelength is used. In general, a material having an average refractive index that is larger toward the longer wavelength side in visible light must have an absorption maximum in the near-infrared region around visible light and / or 700 to 900 nm. In general, such materials are substantially limited to dye materials such as anthraquinone dyes, and when such materials are used as retardation plates, for example, there is a problem that they are colored, so that they are transparent to visible light. Is not suitable as a required retardation plate material. That is, it has been found that a material having a smaller phase difference as the measurement wavelength is shorter is required to have a larger average refractive index as the measurement wavelength is shorter, as opposed to the phase difference.
[0012]
  The present invention has been made on the basis of such new findings, and in order to solve the above problems, specifically, the average refractive index is as high as the shorter wavelength at a measurement wavelength of 400 to 700 nm, preferably 400 to 780 nm. A film obtained by orienting a molecular film by stretching or the like is a film composed of a polymer oriented film having a birefringence Δn that is longer as the wavelength is longer in the measurement wavelength region, and more specifically, the polymer oriented film.2.4~2.9It is preferable to use a stretched film of cellulose acetate having a degree of acetylation as a retardation plate. In addition, by using such a retardation plate for a liquid crystal display device, particularly a polarizing plate single reflection type liquid crystal display device, a display device with excellent image quality can be obtained. Furthermore, in a reflection type polarizing plate that reflects only one of the left and right circularly polarized light, if the circularly polarized light is used as an element for converting to linearly polarized light, a good linearly polarized light can be obtained in a wide band.
[0013]
That is, the present invention is a retardation plate composed of a polymer oriented film having a birefringence Δn at a wavelength of 400 to 700 nm that is larger as the wavelength is longer, the polymer oriented film comprising an organic acid ester of cellulose, and at the wavelength. This is a liquid crystal display device using a phase difference plate, which is an oriented film of a polymer film having a larger average refractive index as the wavelength is shorter.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in detail.
The retardation plate of the present invention uses a polymer film having an average refractive index that is larger for shorter wavelengths at a measurement wavelength of 400 to 700 nm, preferably 400 to 780 nm. It is a polymer oriented film showing refraction Δn. Here, the orientation of the polymer oriented film refers to a state in which polymer molecular chains are arranged in a specific direction, and this state can be measured by measuring the phase difference (Δn · d) of the film. The term “orientation” refers to, for example, Δn · d of 20 nm or more at a measurement wavelength of 590 nm. Δn · d is the product of birefringence Δn and film thickness d. Orientation usually occurs by stretching the film. Here, the average refractive index refers to a value measured with an Abbe refractometer using a polymer film having a measurement wavelength of 550 nm and an Δn · d of less than 20 nm (refer to the examples for the apparatus).
[0015]
As described above, the polymer oriented film having a larger birefringence Δn as the longer wavelength at a measurement wavelength of 400 to 700 nm needs to have a larger average refractive index on the short wavelength side as opposed to birefringence. Street. That is, a material having a light absorption edge at a measurement wavelength of less than 400 nm is preferable. Further, in such a polymer and a polymer oriented film in which the stretching direction is a slow axis, Δn is expressed by a difference nx−ny between refractive indices nx and ny in the slow axis and fast axis directions. These nx and ny are considered to have larger values as the wavelength is shorter, like the average refractive index.
[0016]
The birefringence Δn needs to be larger as the wavelength is longer in the measurement wavelength region. More specifically, the retardation of the polymer alignment film at the measurement wavelengths of 450, 550, and 650 nm is expressed by Δn · d (450) and Δn · d, respectively. (550), Δn · d (650),
[0017]
[Expression 4]
0.6 <Δn · d (450) / Δn · d (550) <0.97 (1)
And
[Equation 5]
1.01 <Δn · d (650) / Δn · d (550) <1.35 (2)
It is preferable that When deviating from these values, for example, in the case of using as a λ / 4 plate, when a linearly polarized light of 400 to 700 nm is incident on this film, the obtained polarization state is completely circularly polarized at a specific wavelength. Although it is obtained, there is a problem that it is greatly deviated from circularly polarized light at other wavelengths. More preferably
[0018]
[Formula 6]
0.6 <Δn · d (450) / Δn · d (550) <0.90 (1-1)
And
[Expression 7]
1.05 <Δn · d (650) / Δn · d (550) <1.35 (2-1)
It is.
[0019]
The polymer oriented film includes a film having a positive refractive index anisotropy in which the stretching direction is a slow axis having a large refractive index, and a negative refractive index difference in which the stretching direction is a fast axis having a small refractive index. Although there are films having anisotropy, any film can be used if it satisfies the above-mentioned characteristics. Although it is currently difficult to accurately measure the absolute value of refractive index chromatic dispersion in both axial directions from the fourth decimal point to the fifth digit, the retardation plate in the present invention is a slow axis having a large refractive index. Is considered to be smaller than the refractive index wavelength dispersion of the fast axis having a small refractive index. The term “refractive index wavelength dispersion” as used herein means a state in which a change in refractive index due to wavelength is small in a polymer oriented film having a larger refractive index as the measurement wavelength is shorter.
[0020]
  Average refractive indexButIn the measurement wavelength range of 400 to 700 nm, the shorter the wavelength, the larger the birefringence ΔnButAs a material that gives a polymer oriented film having a longer wavelength in the measurement wavelength region, cellulose acetate, SpecialThe degree of acetylation of cellulose acetate2.4~2.9WhatButThe object of the present invention can be achieved.
[0021]
The term “acetylation degree” as used herein refers to the number of OH groups attached to carbons at 2, 3, and 6 positions in the cellulose skeleton of the chemical formula (1) substituted with acetyl groups. The acetyl group may be biased to any of the carbons at 2, 3, and 6 positions in the cellulose skeleton, or may exist on average. Furthermore, what blended the cellulose acetate from which acetylation degree differs may be sufficient, and the said acetylation degree should just be satisfied as a bulk average in that case.
[0022]
[Chemical 1]

(Where R is —H or —COCH_ThreeIs)
[0023]
In the present invention, the degree of acetylation is measured according to T.P. Sei, K .; Ishitani, R .; Suzuki, K .; Ikematsu Polymer Journal 17. 1065-1069 (1985)¹³Determined by C-NMR spectrum.
[0024]
When cellulose acetate having a degree of acetylation exceeding 2.9 is formed into a film and uniaxially stretched, the direction perpendicular to the uniaxial stretch direction is the slow axis. That is, the retardation plate has negative optical anisotropy. On the other hand, when the degree of acetylation is 2.8 or less, a retardation plate having positive optical anisotropy in which the stretching direction is the slow axis is obtained. The smaller this degree of acetylation, the smaller the Δn · of measurement wavelengths 450 nm and 550 nm. The ratio of d, Δn · d (450) / Δn · d (550), tends to increase.
[0025]
From the above experimental results, it was found that the dispersion of birefringence can be controlled by the number of acetyl groups of cellulose acetate, but in order to achieve the object of the present invention, the degree of acetylation is 2.4 to 2.9, Preferably it can be achieved by being 2.5-2.8. The acetyl group contains a C = O double bond having a high polarizability, and this C = O bond is likely to be oriented substantially perpendicular to the cellulose ring main chain. Birefringence correlates with the difference in polarizability between the direction of the main chain of the stretched film and the direction perpendicular thereto. The difference in polarizability varies depending on the number of acetyl groups, and the difference in polarizability varies depending on the wavelength, so that the object of the present invention is particularly coincident when the number of acetyl groups is 2.5 to 2.8. it is conceivable that.
[0026]
The method for producing cellulose acetate is preferably a known method in which cellulose is completely acetylated once to give triacetyl cellulose and then hydrolyzed to obtain the desired degree of acetylation.
[0027]
Moreover, it is preferable that the viscosity average polymerization degree of a cellulose acetate is 120 or more, More preferably, it is 130-500.
[0028]
In order to form cellulose acetate into a film in order to obtain the retardation plate in the present invention, it is preferably a solution cast film formation which is a known method. Examples of the organic solvent include known solvents such as methylene chloride, methylene chloride / methanol (weight ratio 9/1, etc.) and dioxolane.
[0029]
The obtained film can then be made into an oriented film by stretching or the like. Here, the alignment film preferably has a retardation at 590 nm of 20 nm or more.
[0030]
Although a known stretching method can be used as the stretching method, longitudinal uniaxial stretching is preferred. For the purpose of improving stretchability, known plasticizers such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate and other phthalic acid esters, tributyl phosphate and other phosphoric acid esters, aliphatic dibasic esters, glycerin derivatives, glycol derivatives, etc. Used. The organic solvent used at the time of film formation may remain in the film and stretched. The amount of the organic solvent is preferably 1 to 20 wt% relative to the polymer solid content.
[0031]
Furthermore, you may add ultraviolet absorbers, such as a phenyl salicylic acid, 2-hydroxy benzophenone, a triphenyl phosphate, in a phase difference plate.
[0032]
The retardation plate in the present invention is preferably transparent, the haze value is preferably 3% or less, and the total light transmittance is preferably 85% or more. The glass transition temperature is preferably 100 ° C. or higher.
The film thickness of the retardation plate is preferably 1 μm to 400 μm.
[0033]
The retardation plate made of the polymer oriented film can be used as a λ / 4 plate when the retardation Δn · d (550) at a wavelength of 550 nm is a quarter of the wavelength. The quarter wavelength of the measurement wavelength of 550 nm is 137.5 nm, but if it is preferably in the range of 137.5 nm ± 20 nm, more preferably 137.5 nm ± 10 nm, it will function sufficiently as a λ / 4 plate. sell.
[0034]
Among the retardation plates in the present invention, Δn · d is a quarter wavelength of the wavelength 550 nm having the highest visibility in visible light, and only one polarizing plate is used, and the back electrode is also used as a reflective electrode By using the reflection type liquid crystal display device, it is possible to obtain a reflection type display device with excellent image quality. It is also possible to use this retardation plate on the back side with respect to the observer of the guest-host type liquid crystal layer. The role of the retardation plate in these cases is to convert linearly polarized light into circularly polarized light and circularly polarized light into linearly polarized light in the visible light region. However, the retardation plate in the present invention satisfies such a purpose. Is possible.
[0035]
In addition, these films may be used instead of the glass substrate that sandwiches the liquid crystal layer of the liquid crystal display device to serve as a substrate and retardation plate.
[0036]
Further, it can be used in the same manner as an element that converts circularly polarized light of a reflective polarizing plate composed of cholesteric liquid crystal or the like that reflects only either left or right circularly polarized light into linearly polarized light.
[0037]
The retardation plate in the present invention is used as a quarter-wave plate, and the one bonded to the polarizing plate becomes a circularly polarizing plate capable of converting natural polarized light into circularly polarized light. When this is used as an antireflection film on a front plate such as a plasma display, it is possible to reduce coloring of reflected light. It can also be used for preventing reflection of a touch panel or the like.
[0038]
There are cases where required characteristics of retardation plates used in liquid crystal display devices, reflective polarizing plates, etc. require that the phase difference does not change even if the angle of incidence on the retardation plate changes from front incidence to oblique incidence. is there. In this case, the following formula (3) represented by a three-dimensional refractive index nx, ny, nz
[0039]
[Equation 8]
Nz = (nx-nz) / (nx-ny) (3)
It is preferable that Nz is between 0.3 and 1.5. In particular, when Nz = 0.5, the phase difference hardly changes even if the angle of incidence on the phase difference plate changes from front incidence. This three-dimensional refractive index is obtained by measuring the dependence of the phase difference on the incident angle, assuming that the retardation plate is a refractive index spheroid.
[0040]
In addition, the retardation plate in the present invention is bonded to a polarizing plate through an adhesive layer and an adhesive layer to form a circular polarizing plate, or some material is coated on the retardation plate to improve wet heat durability. The solvent resistance may be improved.
[0041]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.
[0042]
(Evaluation method)
The material characteristic values and the like described in the present specification are obtained by the following evaluation methods.
[0043]
(1) Method for measuring the degree of acetylation of cellulose acetate
T.A. Sei, K .; Ishitani, R .; Suzuki, K .; Ikematsu Polymer Journal 17. It measured from the 13C-NMR spectrum by the method of 1065-1069 (1985). As the apparatus, a product name “JNM-A600” manufactured by JEOL Ltd. was used. An example of the spectrum will be described with reference to FIG. The degree of substitution at the 2-position was calculated from the ratio of the area of the signal (b) of the carbon signal (a + b) at the 1-position of the glucose ring of cellulose acetate in FIG. 2 where the 2-position was substituted with an acetyl group. The substitution degree at the 3-position is the area of the signal (c) of the carbon signal at the 4-position of the glucose ring that is unsubstituted at the 3-position, and the total of the signals (c + d + e + f + g + h + i + j) of the 4-carbon at the 2,3,4,5-positions. It was calculated by subtracting this value from 1 by dividing by the value of one quarter of the area. The substitution degree at the 6-position was calculated from the ratio of the area of the signal (k) of the carbon signal at the 6-position (k + l) that was substituted with an acetyl group. In the present invention, the sum of the substitution degrees at the 2, 3, and 6 positions is defined as the acetylation degree of cellulose acetate.
[0044]
(2) Measurement of phase difference (Δn · d)
The measurement was performed with a trade name “M150” manufactured by JASCO Corporation, a spectroscopic ellipsometer.
[0045]
(3) Measurement of average refractive index dispersion
A transparent film was prepared, and measured with an Abbe refractometer at a measurement wavelength of 590 nm with Δn · d of 10 nm or less. The Abbe refractometer uses the product name “Abbe Refractometer 2-T” manufactured by Atago Co., Ltd., and the spectroscopic light source device also uses the product name “MM-701” manufactured by Atago Co., Ltd.
[0046]
(4) Measurement of Nz
Using the trade name “M150” manufactured by JASCO Corporation, which is the spectroscopic ellipsometer, the three-dimensional refractive index was determined by measuring the phase difference incident angle dependency of the sample. At that time, the refractive index anisotropy of the sample was calculated from the following equation assuming a refractive index spheroid. In addition, the average refractive index obtained by the measurement of said (3) was used for the average refractive index in the following formula | equation. An electronic micrometer manufactured by Anritsu Co., Ltd. was used for the film thickness d. From the three-dimensional refractive index, it is a phase difference viewing angle characteristic.
[0047]
[Equation 9]
Nz = (nx-nz) / (nx-ny)
Was calculated.
A. When the rotation axis is a fast axis
[Expression 10]
Δn = nxnz / {(nx²-Nz²) Sin²θ / n²+ Nz²}^0.5-Ny
B. When the rotation axis is a slow axis
[Expression 11]
Δn = nx−nynz / {(ny²-Nz²) Sin²θ / n²+ Nz²}^0.5
Δn · d (θ) = Δnd / (1-sin²θ / n²)^0.5
Average refractive index n = (nx + ny + nz) / 3
θ: Angle of inclination (front incidence at θ = 0 °)
d: Film thickness (nm)
[0048]
(5) Measurement of total light transmittance and haze
It was measured with an integrating sphere light transmittance measuring device in accordance with Japanese Industrial Standard JIS K7105 “Testing method for optical properties of plastics”. As an evaluation apparatus, a color difference / turbidity measuring instrument (trade name “COH-300A”) manufactured by Nippon Denshoku Industries Co., Ltd. was used.
[0049]
[Reference Example 1]
100 parts by weight of cellulose triacetate having an intrinsic viscosity [η] = 1.335 and an acetylation degree of 2.917 obtained from Wako Pure Chemical Industries, Ltd. was dissolved in 500 parts by weight of methylene chloride. To this, 1000 parts by weight of 96% aqueous acetic acid solution was added, and cellulose triacetate was hydrolyzed with acetic acid and water at 70 ° C. for 100 minutes while removing methylene chloride under reduced pressure. The reaction product was precipitated, washed with a large excess of water, dried, and dried to obtain cellulose acetate having a degree of acetylation of 2.661. 100 parts by weight of this polymer and 3 parts by weight of dibutyl phthalate as a plasticizer were dissolved in 700 parts by weight of a mixed solvent of methylene chloride / methanol (weight ratio 9/1). A film was produced from this solution by a solvent casting method, and the film was further uniaxially stretched at a temperature of 170 ° C. and 1.5 times. Table 1 summarizes the optical characteristic measurement results. In addition, the retardation of the film Δn · d and the wavelength dispersion of the average refractive index are shown in FIG. This film was confirmed to have a smaller retardation as the measurement wavelength was shorter, but the average refractive index was smaller as the wavelength was longer.
[0050]
[Reference Example 2]
Cellulose acetate having a degree of acetylation of 2.534 was obtained in the same manner as in Reference Example 1 except that the hydrolysis conditions were set at 70 ° C. for 200 minutes. 100 parts by weight of this polymer was dissolved in 700 parts by weight of a mixed solvent of methylene chloride / methanol (weight ratio 9/1). A film was produced from this solution by a solvent casting method, and the film was further uniaxially stretched at a temperature of 170 ° C. and 1.5 times. Table 1 summarizes the optical characteristic measurement results. This film was confirmed to have a smaller retardation as the measurement wavelength was shorter, but the average refractive index was smaller as the wavelength was longer.
[0051]
[Reference Example 3]
Cellulose acetate having a degree of acetylation of 2.727 was obtained in the same manner as in Reference Example 1 except that the hydrolysis conditions were set at 70 ° C. for 60 minutes. 100 parts by weight of this polymer and 3 parts by weight of dibutyl phthalate as a plasticizer were dissolved in 700 parts by weight of a mixed solvent of methylene chloride / methanol (weight ratio 9/1). A film was produced from this solution by a solvent casting method, and the film was further uniaxially stretched at a temperature of 170 ° C. and 1.3 times. Table 1 summarizes the optical characteristic measurement results. This film was confirmed to have a smaller retardation as the measurement wavelength was shorter, but the average refractive index was smaller as the wavelength was longer.
[0052]
[Reference Example 4]
A cellulose acetate having a degree of acetylation of 2.421 was obtained in the same manner as in Reference Example 1 except that the hydrolysis conditions were changed to 70 ° C. for 300 minutes. 100 parts by weight of this polymer and 3 parts by weight of dibutyl phthalate as a plasticizer were dissolved in 700 parts by weight of a mixed solvent of methylene chloride / methanol (weight ratio 9/1). A film was produced from this solution by a solvent casting method, and the film was further uniaxially stretched at a temperature of 170 ° C. and 1.5 times. Table 1 summarizes the optical characteristic measurement results.
[0053]
From Table 1, 0.6 <Δn · d (450) / Δn · d (550) <0.9 and 1.05 <Δn · d (650) / Δn · d (550) <1.35 are satisfied. I can't do it. Furthermore, although this film was used as a phase difference plate of the liquid crystal display device used in Example 1 described later, the color of black display when the voltage was turned on was slightly inferior to Example 1.
[0054]
[Comparative Example 1]
Cellulose triacetate having a degree of acetylation of 2.917 in Reference Example 1 was formed into a film in the same manner as in Reference Example 1, and this film was uniaxially stretched at a temperature of 170 ° C. and 1.4 times. Table 1 summarizes the optical characteristic measurement results. Further, Δn normalized at 550 nm and the wavelength dispersion of the average refractive index of the film are shown in FIG. This film was found to be a film having a negative optical anisotropy in which the shorter the wavelength, the larger the phase difference, and the slow axis in the direction perpendicular to the stretching axis direction.
[0055]
[Comparative Example 2]
A film made of polycarbonate having bisphenol A having a viscosity average molecular weight of 38000 as a repeating skeleton was prepared by a solution casting method, and uniaxially stretched at a temperature of 155 ° C. and a stretching ratio of 1.1 times. The optical properties of this film are shown in Table 1. Further, from Table 1, 0.6 <Δn · d (450) / Δn · d (550) <0.9 and 1.05 <Δn · d (650) / Δn · d (550) <1.35 are satisfied. I found that I couldn't do it. Further, this film was used as a phase difference plate of a liquid crystal display device used in Example 1 described later, but it was found that the color of black display when the voltage was turned on was considerably inferior to Example 1.
[0056]
[Table 1]

[0057]
[Example 1]
The film produced in Reference Example 1 was incorporated into a single polarizing plate reflective liquid crystal display device for evaluation. The configuration is, from the observer side, polarizing plate / retardation plate prepared in Reference Example 1 / glass substrate / ITO transparent electrode / alignment film / twist nematic liquid crystal / alignment film / metal electrode / reflection film / glass substrate. The adhesive layer between each layer is omitted. The color was evaluated visually by setting the bonding angle so as to display white when the voltage was turned off. In particular, it was confirmed that there was little coloring at the time of black display, whereby the contrast was high and the visibility was excellent.
[0058]
[Reference Example 5]
The film produced in Reference Example 1 was placed on a reflective polarizing plate made of cholesteric liquid crystal, and the hue was evaluated by the configuration of a commercially available backlight / cholesteric liquid crystal layer / film of Reference Example 1 / polarizing plate. The angle formed by the slow axis of the film and the polarizing axis of the polarizing plate was 45 °. The light emitted from the polarizing plate was in a white state with little coloring.
[0059]
【The invention's effect】
As described above, by controlling the degree of acetylation of cellulose acetate according to the present invention, it is possible to obtain a retardation plate having a smaller birefringence as the measurement wavelength is shorter, even with only one film. A retardation plate having such birefringence wavelength dispersion and a phase difference of ¼ nm at a measurement wavelength of 550 nm makes circularly polarized light into linearly polarized light and linearly polarized light into circularly polarized light in a wide wavelength region. Since it functions as a phase difference plate to convert, it can be applied to single-polarized or guest-host type reflective liquid crystal display devices and reflective polarizing elements that reflect only one circularly polarized light. A display device and a high-performance reflective polarizing element can be provided with high productivity.
[Brief description of the drawings]
1 shows retardation (Δn · d) and average refractive index (n) wavelength dispersion characteristics of a retardation plate produced in Reference Example 1. FIG.
FIG. 2 in Reference Example 1¹³A C-NMR spectrum is shown.

Claims (5)

A liquid crystal display device using a phase difference plate made of a polymer alignment film having a birefringence Δn at a wavelength of 400 to 700 nm that is larger as the wavelength is longer,
The liquid crystal display device, wherein the polymer alignment film is an alignment film of a polymer film made of cellulose acetate having an average refractive index at the wavelength that is larger as the wavelength is shorter and has a degree of acetylation of 2.4 to 2.9 . The liquid crystal display device according to claim 1, wherein the phase difference plate satisfies the following formulas (1) and (2) in a phase difference ratio at a specific wavelength.

(Here, Δn · d (450), Δn · d (550), and Δn · d (650) are retardations of the polymer alignment film at wavelengths of 450 nm, 550 nm, and 650 nm, respectively). 3. The liquid crystal display device according to claim 1, wherein the retardation plate has a retardation Δn · d (550) at a wavelength of 550 nm of the polymer alignment film which is a quarter of the wavelength. When the three-dimensional refractive index of the polymer oriented film at a measurement wavelength of 590 nm is nx, ny, and nz, the retardation plate is represented by the following formula (3).

There the liquid crystal display device according to any one of claims 1 to 3, which is 0.3 to 1.5. Orientation film, a liquid crystal display device according to any one of claims 1-4 is obtained by stretching a polymer film.

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