JP4287016B2 - Reversible recording medium - Google Patents

Description

（式中Ｚ′、Ｙ′は各々独立してジハイドロコレステリル基、水素原子又は
アルキル基、式中ｍ、ｎは各々独立して１以上の整数、Ｚ′およびＹ′の
少なくともいずれか一方はジハイドロコレステリル基を表す。）
【０００８】
以下、本発明の可逆記録媒体、および該可逆記録媒体を使用した可逆的書き換え方法および可逆記録装置を図面に基づいて説明する。
１．本発明の可逆記録媒体の基本構成
図１に本発明の可逆記録媒体の基本的構成を示す。支持体１上に可逆的光吸収層、例えば電子供与性呈色性化合物（以下、発色剤とも言う）と電子受容性化合物（以下、顕色剤とも言う）を含む組成物から成る可逆的熱発色層を形成してある。更にその上に、コレステリック液晶相あるいは等方相を形成するサーモトロピック液晶性化合物を含む可逆的コレステリック反射層３（以下、コレステリック反射層と略す）を形成する。
必要に応じて、光吸収層２とコレステリック反射層３の間に中間層４を、コレステリック反射層３の表面に表面保護層５を、支持体１と可逆的光吸収層２の間に下地層６などを設けても良い。
【０００９】
支持体１としては、たとえば、紙、合成紙、ＰＥＳ、ＰＥＴ、ポリエーテルサルフォン、ポリエーテルイミドなどのプラスチックフィルムあるいはこれらの複合体、ガラス板などを用いることが出来る。支持体１は透明でも良いが、反射型でペーパーライクな可逆記録媒体とする場合は白色が好ましい。ペーパーライクな媒体とする場合の支持体１の厚さは通常、５０〜５００μｍ、好ましくは１００〜３００μｍ程度とする。その他のディスプレイ装置とする場合は板状の剛体でも良く、支持体の厚さは特に限定されない。
【００１０】
表面保護層としては、ガラス基板や、透明性と耐熱性に優れるポリエーテルサルフォン、ポリエーテルイミドなどのプラスチックフィルムが好ましいが、これらに限定されない。コレステリック反射層の上に光硬化性樹脂などを塗布した後、硬化処理を行ない、表面保態層を形成しても良い。光硬化性樹脂としては、透明性が高く、機械強度に優れる紫外線硬化樹脂が好ましい。また、紫外線硬化樹脂中にシリカ微粒子などのフイラーを添加し、さらに機械的強度を向上させることも出来る。
記録媒体の表面側からサーマルヘッドのような接触式加熱装置記録する場合、表面保護層の厚さは１μｍから３０μｍ程度が好ましい。これ以下の薄さであると、機械的強度が不足して表面保護層の破損が生じ、また、これ以上に厚いとコレステリック反射層と光吸収層への熱伝達効率が悪化し好ましくない。
【００１１】
この可逆記録媒体にサーマルヘッドなどで表面側から熱書込みを行なうと、コレステリック反射層３はコレステリック相で固定化され選択反射画像を形成する。また、下層の光吸収層２も熱書込みによる加熱により発色温度に到達した場合には、上層の選択反射画像とほぼ相似形の発色画像を形成する。この発色画像が、コレステリック反射色を観測するために必要な光吸収層として作用する。
【００１２】
図中に光を矢印で示したように、熱書込み部では、コレステリック反射層３で特定の波長光（但し、右または左旋光性の一方）が反射され、残りの波長成分は光吸収層２に入射する。発色が黒色の場合、入射した波長成分は全て吸収されるため、コレステリック反射層３での反射光のみが観察され、鮮やかな選択反射色の画像が得られる。発色が黒では無く青色など着色している場合には、光吸収層２からの反射光とコレステリック反射層３での反射光の加法混色による色の画像が観察される。また、熱書込み条件を適宜設定してコレステリック反射層３を透明状態に記録し、光吸収層２に発色画像を形成することで、黒色や青色などの発色画像がそのまま観察される。
このように、選択反射画像の可逆記録と同時に、それに対応した部分に光吸収画像を可逆的に形成することで、下地部とのコントラストが大きな多色画像を可逆的に記録出来る。
【００１３】
２．可逆的コレステリック反射層の詳細
可逆的コレステリック反射層としては、メモリー性があり、ガラス転移温度以上に加熱し、この加熱の加熱温度に応じて現れたコレステリック液晶相の選択反射状態あるいは等方相の透明状態をガラス転移温度以下に固定できるサーモトロピック液晶化合物を有して構成されるものであればいずれも使用が可能である。
【００１４】
記録速度向上と安定性向上のために、本発明では、分子量が９００以上１万以下、好ましくは１０００以上２０００以下、で分子量分布を持たないコレステリック液晶性化合物あるいはその混合物（以後、中分子コレステリック液晶とも記す）を用いることが好ましい。該中分子コレステリック液晶化合物は、等方相あるいは高温でのコレステリック液晶相とした後、ガラス転移温度以下、あるいは常温まで急冷すると、前記加熱状態のコレステリック化合物はその分子配列を保持したままガラス状固体（以下、コレステリックガラス相と呼ぶ。）になり、選択反射色が観測されるものである。
【００１５】
中分子コレステリック液晶化合物の分子量が９００より小さいと急冷により結晶化が起こってコレステリックガラス相が固定されない。これは、急冷に伴う分子の再配向が早いためと考えられる。また、分子量が１万より大きいと１画素が数百ミリ秒程度以下での実用的な記録や消去が困難になる。
前記中分子コレステリック液晶化合物は、等方相を示す温度まで到達させた後、次に該層をコレステリック反射色を示す温度まで冷却して前記コレステリック反射色を固定させた後、再び加熱し少なくともその一部を結晶化させるか、あるいは、等方相を示す温度まで到達させた後、急冷して少なくとも一部を非晶質状態とすることにより、白色表示ができる。
【００１６】
液晶記録材料として用いるコレステリック液晶化合物は、その分子量が２０００以下で、ガラス転移温度が３０℃以上で、ガラス転移温度以上でコレステリック液晶相を示し、さらにそれ以上の温度で等方相を示すものである。
特に下式（Ｉ）で示されるコレステリック液晶化合物（Ｄｉｃｈｏｌｅｓｔｅｒｙｌ １０，１２−Ｄｏｃｏｓａｄｉｙｎｅｄｉｏａｔ）が固定化されたコレステリックピッチ反射色による安定性が高いので好ましい。
【化３】

【００１７】
本発明で使用するコレステリック液晶化合物としては、前式（II）で示される液晶記録材料コレステリック液晶化合物があげられるが、該コレステリック液晶化合物を用いた可逆的コレステリック反射層は、コレステリックガラス相を高温下に放置した時の保存安定性が向上する。具体的にはコレステリックガラス相の結晶化が抑制される。これは、記録媒体を夏季の車内など高温になる場所に放置した場合などに画像が劣化するのを防止することができる。
本発明で使用する前記コレステリック液晶化合物は、その複数種類を混合して用いても良い。
【００１８】
なお、コレステリック反射層は、選択反射を示す液晶性化合物だけで構成することが好ましいが、バインダー樹脂と、例えば液晶性化合物を適当な溶媒に溶かして塗布した後、溶媒を蒸発させて反射層を形成させることができる。また、コレステリック反射層は、コレステリック液晶化合物、バインダー成分のモノマー成分あるいはプレポリマーおよび重合開始剤を混合して、重合、例えば光重合あるいは熱重合によりバインダー成分を重合させて形成しても良い。
スペーサー粒子としては、一般的な液晶ディスプレイ用に用いられている球状粒子や円柱状粒子などが使用できるが、これらに限定されない。
【００１９】
前記等方相を得るための加熱手段は、特にその種類は制限されるものではないが、必要とする加熱速度に応じて、適宜、適当な加熱手段および加熱条件を選択できるが、加熱手段としては、例えばホットプレート、レーザー光、サーマルへッドなどを使用することが出来る。ただし、加熱手段はこれらに限定されるものではない。
【００２０】
前記コレステリック反射色を固定する急冷手段も、特にその種類は制限されるものではないが、例えば所望するコレステリック反射色に応じて必要とする冷却速度に応じて、適宜、適当な冷却手段および冷却条件を選択できる。冷却手段としては、例えば水冷、空冷、金属板、ガラス板等などを使用することが出来る。ただし、冷却手段はこれらに限定されるものではない。
【００２１】
選択反射波長は、通常、４００〜７００ｎｍ程度の可視光領域に存在することが好ましく、この場合、人間が視認することができる。ただし、機械により読み取る場合などは、紫外領域や赤外領域に選択反射波長が存在していてもよい。
コレステリック反射層の厚さは、０．５〜５０μｍ、好ましくは１〜２０μｍの範囲から適宜選択すればよい。コレステリック反射層が薄すぎると最大反射が得られる波長における反射率が低くなるため表示画像のコントラストが低下し、厚すぎるとコレステリック反射層内での光吸収が多くなって表示画像のコントラストが低下する。また、厚すぎると下層の光吸収層２への熱伝達が悪くなる。
【００２２】
３．可逆的光吸収層
可逆的光吸収層２は、何らかの外部刺激によって、コレスリテック反射層を透過してきた光を吸収する機能を有するものであればすべて用いることが出来る。例えば加熱温度や冷却速度によって発色と消色を繰り適すことが出来る感熱記録材料、電界や熱の有無によって二色性色素分子の配向を切り替えるゲストホスト型液晶材料（特開平１０−１８３１１７）、磁界の方向によって平板状の磁性粒子の向きを切り替えて光シャッターとする磁気記録材料（特開平９−０９０８８７）などを用いることが出来る。コレステリック反射層と光吸収層のそれぞれの記録原理が異なる場合は、それぞれの層の記録動作が確実に行なわれるように記録層の構成を最適化する必要がある。また、いずれも材料でも光吸収（発色）状態と透過（無色）状態をエネルギー無しで保持できるメモリー性を有することが好ましい。
【００２３】
コレステリック反射層の記録動作が感熱記録の場合は光吸収層も感熱記録であることが好ましい。感熱記録装置により光吸収（発色）状態と透過（無色）状態を可逆的に保持出来、エネルギー無しで保持できるメモリー性を有する方式としては、顕色剤と発色剤の混合物を用いることが出来る。
可逆的光吸収層２は、顕色剤と発色剤の組合せだけによって達成されるものが好ましいが、顕色剤と発色剤の組合せによる方式だけに限らない。発色と消色が熱的に可逆変化する材料系ならば用いることが出来る。
ここで用いられる顕色剤は、その分子内に顕色能を持つ構造と長鎖の脂肪族基（以下、長鎖構造と言う）を持つものである。可逆的熱発色性の組成物では、顕色剤に対し、適当な発色剤の組合せが用いられる。たとえば、この組合せは、両者を加熱溶融し急冷して得た発色状態試料を示差走査熱量分析または示差熱分析したとき昇温過程において発熱現象を示すか否かによって選択され、発熱現象を示すものであれば本発明に適用可能なものである。この可逆的熱発色性の組成物は、顕色剤と発色剤を混合溶融し発色する温度以上に加熱し、急冷することにより発色状態をとることができる。これを再び昇温していくと、発色温度より低いある温度ですみやかにその発色体の消色が起きる。このような発色、消色現象を起こす可逆的熱発色性の組成物に対して消色促進剤を添加しても良い。
【００２４】
可逆的光吸収層に含まれる顕色剤は、基本的に分子内に発色剤を発色させるこ
とができる顕色能を示す構造と、分子間の凝集力をコントロールする長い脂肪族鎖構造部分を合わせ持つ化合物であり、炭素数１２以上の脂肪族基を持つ有機リン酸化合物、芳香族または脂肪族カルボン酸化合物あるいはフェノール化合物等である。脂肪族基は、直鎖状または分枝状のアルキル基、アルケニル基が包含され、ハロゲン、アルコキシ基、エステル基等の置換基を持っていてもよい。具体的には特開平１０−１５１８５９号公報に記載される化合物が例示できるが、これらの化合物には限定されない。
【００２５】
また、可逆的光吸収層に含まれる発色剤は電子供与性を示すものであり、それ自体無色あるいは淡色の染料前駆体であり、特に限定されず、従来公知のもの、たとえば、フルオラン系化合物、フェノチアジン系化合物、ロイコオーラミン系化合物、フタリド系化合物、アザフタリド系化合物などが用いられる。具体的には特開平１０−９５１７５号公報に記載される化合物が例示できるが、これらの化合物には限定されない。
【００２６】
更に、可逆的光吸収層には、消色促進剤として低融点化合物または高融点化合物などを添加することができる。その例として次のような物質が挙げられる。脂肪酸、脂肪酸誘導体または脂肪酸金属塩、ワックスおよび油脂、高級アルコール類、リン酸エステル類、安息香酸エステル類、フタル酸エステル類、オキシ酸エステル類、シリコーンオイル、液晶化合物、界面活性剤など長鎖炭化水素基をもつ化合物を用いることができる。
【００２７】
可逆的光吸収層を構成する発色剤と顕色剤の割合は、使用する化合物の物性によって適切な比率を選択する必要がある。その範囲はおおむね、モル比で発色剤１に対し顕色剤が１から２０の範囲であり、好ましくは２から１０の範囲である。顕色剤がこの範囲より少ないと消色性が不十分となり、また、多すぎると発色濃度が不十分となる。
【００２８】
光吸収層２は、層としての形態をとらせるために、必要に応じてバインダー樹脂を用いて顕色剤と発色剤を保持することができる。バインダーとしては、たとえばポリ塩化ビニル、ポリ酢酸ビニル、塩化ビニル−酢酸ビニル共重合体、ポリスチレン、スチレン系共重合体、フェノキシ樹脂、ポリエステル、芳香族ポリエステル、ポリウレタン、ポリカーボネート、ポリアクリル酸エステル類、ポリメタクリル酸エステル類、アクリル酸共重合体、マレイン酸共重合体、ポリビニルアルコールなどがある。
【００２９】
顕色剤および発色剤は、マイクロカプセル中に内包して用いることもできる。顕色剤、発色剤のマイクロカプセル化は、コアセルベーション法、界面重合法、インサイチュ重合法など公知の方法によって行うことができる。光吸収層２の形成は、従来公知の方法に従い、発色剤および顕色剤をバインダーと共に水または有機溶剤により均一に分散もしくは溶解して、これを支持体上に塗布・乾燥することによって行う。またバインダーを用いない場合、顕色剤と発色剤を混合・溶融して膜とし、冷却して光吸収層２とすることができる。光吸収層のバインダー樹脂の役割は、発色・消色の繰り返しによって可逆的熱発色性の組成物を均一に分散させた状態を保持することにある。特に、発色時の熱印加で組成物が集合して不均一化することが多いから、バインダー樹脂は耐熱性の高いものが好ましい。
【００３０】
４．可逆記録特性
本発明の可逆記録媒体を用いて着色画像を形成させるためには、いったんコレステリック反射層と光吸収層２を記録温度以上に加熱したのち急冷されるようにすればよい。具体的には、たとえばサーマルヘッドやレーザー光で短時間加熱すると記録媒体が局部的に加熱されるため、直ちに熱が拡散し急激な冷却が起こり、着色状態が固定できる。一方、消色させるためには適当な熱源を用いて比較的長時間加熱し冷却するか、消色温度に一時的に加熱すればよい。長時間加熱すると記録媒体の広い範囲が昇温し、その後の冷却は遅くなるため、その過程で消色が起きる。この場合の加熱方法には、熱ローラ、熱スタンプ、熱風などを用いてもよいし、サーマルヘッドを用いて長時間加熱してもよい。また、レーザー装置を用いることも出来る。高出力の炭酸ガスレーザーなどを用いれば、可視光の吸収層を用いること無く発熱させることが出来る。コレステリック反射層および光吸収層を消色温度域に加熱するためには、例えばサーマルヘッドへの印加電圧やパルス幅を調節することによって、印加エネルギーを記録時よりやや低下させればよい。この方法を用いれば、サーマルヘッドだけで記録・消去ができ、いわゆるオーバーライトが可能になる。もちろん、熱ローラ、熱スタンプによって消色温度域に加熱して消去することもできる。また、消色時にも光源装置を用いることが出来る。この場合、着色部は特に光を吸収し易いため、効率よく発熱させることが出来る。
【００３１】
まず、光吸収層２の発色・消色現象について説明する。図２は前記した可逆的熱発色性組成物の発色濃度と温度の関係を示す。この図の横軸は温度を示し、縦軸は濃度を示している。図中Ａは常温で消色状態にある組成物を示し、Ｂは加熱・溶融状態にあり発色した状態の組成物を示す。また、Ｃは常温で発色状態にある組成物を示す。組成物Ａを昇温していく場合、発色開始温度Ｔ３で発色し始め、発色状態濃度が飽和する温度Ｔ４（以後、発色飽和温度Ｔ４と記す）で組成物Ｂとなる。この組成物Ｂを急冷すると、発色状態を維持したまま、常温に戻り組成物Ｃに変化する（図中の実線の経路）。発色状態の組成物Ｃを再び昇温していくと、消色開始温度Ｔ１で濃度が低下し始める組成物Ｄとなり、ついには消色温度Ｔ２でほぼ消色状態となり組成物Ｅに変化する。組成物Ｅを冷却し降温するとそのまま消色状態の組成物Ａに戻る（図中の鎖線の経路）。ここで、消色温度Ｔ２から発色開始温度Ｔ３までの温度が組成物の消色温度領域となる。本組成物が示す発色・消色現象の特徴は、溶融して発色する温度より低い温度領域に消色温度範囲があり、組成物を常温発色状態からこの範囲に加熱すると消色することである。また、発色と消色の現象は繰り返しが可能である。なお、図２は本発明の組成物の代表的な発色と消色の仕方を示したものであり、各温度は用いる材料の組合せで異なる。また、溶融して発色している状態の組成物Ｂの濃度と、その状態から冷却して得た発色状態の組成物Ｃの濃度は必らずしも一致するものではなく、異なる場合もある。
【００３２】
次に、コレステリック反射層の反射・透過現象について説明する。図３に、中分子コレステリック液晶性化合物を用いた場合の可逆的コレステリック反射層の選択反射色と温度の関係を示す。この図の横軸は温度を示し、縦軸は選択反射色を示している。図中で、常温での結晶状態ａを昇温していくと、等方相転移温度Ｔ６以上で溶融状態の等方相ｂに変化する。この等方相ｂからガラス転移温度Ｔｇ（図示せず）以下まで急冷すると、常温で透明状態の非晶質相ｃに固定化される。高温の等方相ｂからコレステリック液晶相ｅの温度領域に徐冷すると、温度に応じた選択反射色（ｅ_Ｒ、ｅ_Ｇ、ｅ_Ｂ）を示す。前式（Ｉ）で示される１０，１２−ドコサジインジカルボン酸ジコレステリルエステル（分子量：１０９９．８、ガラス転移温度：８０℃、以下ｃｈｏｌ−８−ｄｉｙｎｅ−８−ｃｈｏｌと記す）では、１１５℃程度で４３０ｎｍの青色、８７℃程度で６１０ｎｍの赤色を示す。この状態からガラス転移温度Ｔｇ（図示せず）以下まで急冷すると、室温で選択反射状態が固定化されたコレステリックガラス相（ｆ_Ｒ、ｆ_Ｇ、ｆ_Ｂ）が得られる。あるいは、等方相ｂでの急冷開始温度や急冷速度を変えることで、一段階的に非晶質相ｃやコレステリックガラス相（ｆ_Ｒ、ｆ_Ｇ、ｆ_Ｂ）を得ることも出来る。
【００３３】
非晶質相ｃあるいはコレステリックガラス相ｆを再度加熱していくと、結晶化温度Ｔ５で結晶化が始まり白濁状態ｄとなる。ここで冷却すると結晶状態ａに戻る。また、コレステリック液晶相ｅから徐々に冷却すると、結晶化状態ａが得られる場合と、透明状態（図示せず）が得られる場合がある。この原因は明らかではないが、ガラス相の再加熱による液晶化と、液晶相からの徐冷による液晶化では、液晶構造が異なり光散乱能力が異なるためと考えられる。なお、図３は本発明の液晶性組成物の代表的な記録特性を示したものであり、等方相転移温度Ｔ６や結晶化温度Ｔ５などは用いる材料で異なる。
【００３４】
可逆記録媒体全体としての初期化あるいは記録消去プロセスは、支持基体１が白色の場合は、光吸収層２を消色温度Ｔ２以上に加熱した後冷却する。これは、コレステリック反射層が選択反射状態を示していても、光吸収層２が確実に消色していれば、選択反射色は観察されず白色表示が得られるためである。支持基体１が透明の場合は、コレステリック反射層を結晶化温度Ｔ５以上、かつ、光吸収層２を消色温度Ｔ２以上に加熱した後冷却する。これは、透過型の場合は、コレステリック反射層も透明あるいは僅かな白濁状態にする必要があるためである。ここで、コレステリック反射層の結晶化温度Ｔ５が光吸収層の発色開始温度Ｔ３よりも大きい場合、記録画像を消去するためにコレステリック反射層を結晶化温度Ｔ５以上まで加熱すると、光吸収層が再発色してしまう場合がある。したがって、Ｔ３＞Ｔ５となるように材料を組み合わせる必要がある。
【００３５】
サーマルヘッドなどの加熱装置によって画像を熱書込みした場合、画像エッジ部（輪郭部）の加熱温度と冷却速度にムラが発生する場合がある。したがって、コレステリック反射層の下層全面に光吸収層があるような場合には、記録した画像の輪郭部の選択反射色が異なって観察される場合がある。この輪郭部の色違いを防止するためには、輪郭部の選択反射色を観察出来ないようにすれば良い。すなわち、コレステリック反射層の記録画像よりも、光吸収層の記録画像を一回り小さく形成すれば、画像輪郭部の選択反射色は観察されず、均一な色の部分の画像のみが観察される。
【００３６】
このように、コレステリック反射層の記録画像よりも光吸収層の記録画像を小さく形成するためには、発色開始温度Ｔ３が等方相転移温度Ｔ６以上（Ｔ３≧Ｔ６）である必要がある。本発明の可逆記録媒体を表面側からサーマルヘッドなどで加熱した場合、上層のコレステリック反射層内の温度よりも、下層の光吸収層内の温度の方が低くなる傾向がある。図４および５の（ａ）に可逆記録媒体断面の温度分布のモデル図を示す。図中の点線は比較的高温部の分布を、実線の曲線はＴ６の分布を、破線の曲線は比較的低温部の分布を示している。コレステリック反射層ではＴ６以上に加熱・急冷された部分がコレステリックガラス相として固定化されるが、輪郭部では温度分布による色ムラが発生し易い。ここで、光吸収層の発色開始温度Ｔ３が比較的低いＴ３＜Ｔ６の場合、図４の（ａ）のように光吸収層は破線曲線部まで発色し、発色部の面積が比較的大きくなる。特に発色開始温度Ｔ３と発色飽和温度Ｔ４の差が小さい場合、高濃度に発色した部分の面積も大きくなる。すると選択反射色の輪郭部も観察され易くなるため、図４の（ｂ）のように画像輪郭部の色ムラが発生してしまう。そこで、Ｔ３≧Ｔ６の条件にすれば、図５の（ａ）のようにコレステリック反射層内の記録部面積よりも、光吸収層内の発色面積の方が確実に小さく出来る。したがって、輪郭部の色ムラの部分には光吸収層が無いため選択反射光は観察されず、図５の（ｂ）のように輪郭部の色ムラの無い均一な画像のみが観察される。
【００３７】
【実施例】
以下に本発明の実施例を示す。
【００３８】
実施例１
顕色剤として、下式（III）で示されるＫ１を用いた。
【化４】

発色剤として２−アニリノ−３−メチル−６−ｎ−ジブチルアミノフルオランを用い、これら顕色剤と発色剤を塩化ビニル−酢酸ビニル共重合体とともにメチルエチルケトン中で分散混合し、透明ＰＥＴフィルム上に塗布乾燥して膜厚約６μｍの光吸収層を形成した。
このフィルム上に、前式（Ｉ）で示されるコレステリック液晶（ｃｈｏｌ−８−ｄｉｙｎｅ−８−ｃｈｏｌ）系化合物を適量載せて１３０℃に加熱したホットプレート上に３０秒間載せて充分に加熱溶融させた。
【００３９】
溶融状態で厚さ２５μｍのＰＥＳフィルムを被せ、１３０℃に加熱した対向ホットプレートを載せて１ｋｇ／ｃｍ^２の圧力で均一に加圧しながら膜厚約１０μｍに伸ばした後、５℃／ｍｉｎの速度で徐々に冷却し、選択反射層を形成した。
その結果、光吸収層の記録特性が消色開始温度Ｔ１＝５５℃、消色温度Ｔ２＝９０℃、発色開始温度Ｔ３＝１３０℃、発色温度Ｔ４＝１５０℃で、選択反射層の記録特性が結晶化温度Ｔ５＝９７℃、等方相転移温度Ｔ６＝１２０℃の透過型の可逆記録媒体を得た。初期化時の冷却条件として徐冷しているため、光吸収層は無色状態、選択反射層も透明状態で固定化された。
【００４０】
サーマルヘッド装置で記録媒体の保護層表面に画像情報を熱書込みした。ドット密度６ｄｏｔ／ｍｍ、印加電力０．２Ｗ／ｄｏｔ、書込み速度５ｍｍ／ｓｅｃの条件で使用し、駆動パルス幅を数ｍｓｅｃから数十ｍｓｅｃの範囲で変化させて、単位面積当たりの記録エネルギーを制御した。常温環境下で、画像状に６５ｍＪ／ｍｍ^２のエネルギーで記録し、サーマルヘッド出口に冷風を送風して強制的に冷却したところ、光吸収層はほぼ無色のままで、コレステリック反射層にはコレステリックガラス相が得られた。このコレステリックガラス相は赤色波長の選択反射を示しており、透過光としては補色のシアン光が目視観測された。同様に画像状に８５ｍＪ／ｍｍ^２エネルギーで記録し、サーマルヘッド出口にファンで送風して冷却したところ、今度は光吸収層も黒色に発色し、コレステリック反射層にはコレステリックガラス相が得られた。このコレステリックガラス相は緑色波長の選択反射を示しており、光吸収層での補色光の光吸収により、反射光では目視で緑色の記録画像が観察された。この部分は光吸収層で透過光が吸収されるため、透過光では黒色が観察される。記録後のシートをＯＨＰで投影したところ、白色表示の下地の中にシアン色と黒色の画像が表示され、コントラストが高い多色表示が得られた。また、このシート全体をホットプレート上で１２５℃まで加熱して、５℃／ｍｉｎの速度で徐々に冷却したところ、光吸収層は消色し、コレステリック反射層は透明状態になった。ＯＨＰで投影したところ全面白色表示が得られ、記録画像が消去できた。
【００４１】
実施例２
顕色剤として下式（IV）の化合物を用い、また、発色剤として２−（ｏ−クロロフェニル）アニリノ−６−ジブチルアミノフルオランを用い、これら顕色剤と発色剤を塩化ビニル−酢酸ビニル共重合体とともにメチルエチルケトン中で分散混合し、白色ＰＥＴフィルム上に膜厚約６μｍの光吸収層を実施例１と同様形成した。
【化５】

【００４２】
このフィルム上に実施例１で使用したコレステリック系液晶化合物を適量載せて１３０℃で加熱溶融させた。実施例１と同様に、溶融状態で厚さ２５μｍのＰＥＳフィルムを被せて均一に加圧し、膜厚約１０μｍに伸ばして冷却し、コレステリック反射層を形成した。その結果、光吸収層の記録特性が消色開始温度Ｔ１＝６２℃、消色温度Ｔ２＝８０℃、発色開始温度Ｔ３＝９２℃、発色温度Ｔ４＝１００℃で、コレステリック反射層の記録特性が結晶化温度Ｔ５＝９７℃、等方相転移温度Ｔ６＝１２０℃の反射型の可逆記録媒体を得た。初期化時の冷却条件として徐冷しているため、光吸収層は最初の発色状態から無色状態に変化し、コレステリック反射層も透明状態で固定化された。
【００４３】
実施例１と同じサーマルヘッド装置で記録媒体の保護層表面を画像状に６５ｍＪ／ｍｍ^２エネルギーで記録したところ、光吸収層は黒色が固定化され、コレステリック反射層には赤色波長の選択反射を示すコレステリックガラス相が固定化された。光吸収層の黒色部が光吸収層として働くため、白色の下地の中に目視で赤色の記録画像が観察された。同様に画像状に８５ｍＪ／ｍｍ^２エネルギーで記録しサーマルヘッド出口にファンで送風して冷却したところ、光吸収層は黒色が固定化され、コレステリック反射層には緑色波長の選択反射を示す液晶ガラス相が固定化された。同様に画像状に１００ｍＪ／ｍｍ^２エネルギーで記録し、サーマルヘッド出口にファンで送風して強制的に冷却したところ、光吸収層は黒色が固定化され、コレステリック反射層には青色波長の選択反射を示すコレステリックガラス相が固定化された。同様に画像上に１１０ｍＪ／ｍｍ^２エネルギーで記録した、サーマルヘッド出口にファンで送風して強制的に冷却したところ、光吸収層は黒色が固定化され，コレステリック反射層には透明状態の非晶質相が固定化された。以上のようにして、ペーパーライクな白色表示の下地の中に赤、緑、青、黒の多色画像が表示できた。なお、サーマルヘッド出口での送風強度や空気温度などの冷却条件は，記録条件に応じて適宜変化させた。ここで，緑色画像の輪郭部を拡大観察したところ、赤色が記録されている部分もあった。これは、Ｔ４＜Ｔ６で発色温度Ｔ４が比較的低いため、発色部の画像面積が比較的大きくなり、コレステリックガラス部の輪郭部に発生した固定化状態のムラの選択反射色をそのまま表示しているためである。しかし，この輪郭部の色ムラは実用上問題無いレベルであった。
次に、ホットプレート上で、シート全体を結晶化温度Ｔ５より僅かに高い１００℃まで加熱して急冷したところ、コレステリック反射層は結晶化して僅かに白濁化したが、Ｔ３＜Ｔ５であるため、画像は一度消色したものの全面が再発色してしまった。そこで，再度ホットプレート上で１３０℃まで加熱して徐々に冷却したところ、光吸収層は消色し、コレステリック反射層は透明状態になった。
【００４４】
実施例３
顕色剤として下式（Ｖ）の化合物を用い、発色剤として２−アニリノ−３−メチル−６−ｎ−ジブチルアミノフルオランを用いた以外は、実施例２と同様にした。その結果、光吸収層の記録特性が消色開始温度Ｔ１＝７０℃、消色温度Ｔ２＝９０℃、発色開始温度Ｔ３＝１４０℃、発色温度Ｔ４＝１５０℃で、選択反射層の記録特性が結晶化温度Ｔ５＝９７℃、等方相転移温度Ｔ６＝１２０℃の反射型の可逆記録媒体を得た。Ｔ３＞Ｔ５およびＴ４≧Ｔ６の条件を満たしている。
【化６】

【００４５】
実施例２と同様な方法で熱記録したところ、良好な多色画像を形成することが出来た。緑色画像の輪郭部を拡大観察したが、輪郭部に赤色は観察されなかった。これは、等方相転移温度Ｔ６が発色温度Ｔ４よりも小さいため、コレステリック反射層の記録面積よりも光吸収層の発色面積の方が一回り小さくなり、コレステリックガラス部の輪郭部の選択反射色を表示できなくなったためである。但し、発色画像の面積が僅かに小さくなるため線が細くなったが、実用上問題無いレベルであった。
次に、サーマルヘッド装置を用いて、シート表面をＴ５以上の１１０℃になるように５５ｍＪ／ｍｍ^２のエネルギーで加熱したところ、光吸収層内部はＴ２まで達成し消色した。ここでは、Ｔ３＞Ｔ５で発色開始温度Ｔ３が十分に大きいため光吸収層の再発色は発生しなかった。また、コレステリック反射層もＴ５で結晶化して僅かに白濁化したため、全面白色表示が得られ、記録画像の消去ができた。
【００４６】
比較例１
可逆的熱光吸収層を設けず、黒色ＰＥＴフィルム上に実施例１のコレステリック系液晶化合物を適量載せて１３０℃で加熱溶融させた。溶融状態で厚さ２５μｍのＰＥＳフィルムを被せて均一に加圧し、膜厚約１０μｍに伸ばして冷却し、コレステリック反射層のみを形成した。コレステリック反射層の記録特性が結晶化温度Ｔ５＝９７℃、等方相転移温度Ｔ６＝１２０℃の反射型の可逆記録媒体を得た。初期化時の冷却条件として徐冷しているため、コレステリック反射層は透明状態で固定化された。
実施例２と同様な方法で熱記録したところ、黒色の下地の中に赤、緑、青の多色画像が表示できた。また、黒色の下地の中に白色の画像を記録するために、画像状に４５ｍＪ／ｍｍ^２のエネルギーで記録したところ、結晶化により白濁したが、膜厚が薄いため十分な白色表示は得られなかった。
【００４７】
実施例４
実施例２で得られた記録画像を５５℃下で２４時間放置したところ、コレステリックガラス相が部分的に結晶化した。光吸収層の黒色濃度には変化がなく、画像としては黒点状の部分が発生した。そこで、コレステリックガラス相の高温下での保存安定性を向上させるために、コレステリック反射層として実施例１で用いた前式（Ｉ）で示される化合物と、前式（II）においてｍ＝ｎ＝８、Ｚ′＝Ｙ′＝ジハイドロコレステリル基で示される化合物とを９：１の重量比で混合したコレステリック系液晶系化合物を用いた。それ以外はすべて実施例２と同様にして画像記録を行った。コレステリック反射層の材料組成が変化したものの、記録エネルギーに対する記録色の特性はほぼ同様であり、ペーパーライクな白色表示の下地の中に赤、緑、青、黒の多色画像表示ができた。記録画像を５５℃以下で２４時間放置してもコレステリックガラス相の結晶化は発生せず、コレステリックガラス相の保存安定性が向上していることが確認された。
【００４８】
【効果】
１．請求項１
支持基体と可逆的コレステリック反射層の間に可逆的光吸収層を設けているので、選択反射現象だけでは表示が困難な色を表示することができ、かつ、非画像部とのコントラストが向上した可逆記録媒体が得られる。
２．請求項２〜３
中分子コレステリック液晶化合物を用いているので、高速に記録・消去ができ、かつ多色表示が可能な可逆記録媒体が得られる。
【図面の簡単な説明】
【図１】 本発明の可逆記録媒体の基本的構成を示す模式的断面図である。
【図２】 本発明の可逆記録媒体の発色性組成物の発色濃度と温度の関係を示す図である。
【図３】 中分子コレステリック液晶性化合物を用いた場合の可逆的コレステリック反射層と温度の関係を示す図である。
【図４】 Ｔ６＞Ｔ３の場合の可逆記録媒体断面の温度分布のモデル図（ａ）および該媒体の輪郭部の色ムラの発生状態を示す図（ｂ）である。
【図５】 Ｔ３≧Ｔ６の場合の可逆記録媒体断面の温度分布のモデル図（ａ）および該媒体の輪郭部の色ムラの発生状態を示す図（ｂ）である。
【符号の説明】
１ 支持基体
２ 可逆的光吸収層
３ 可逆的コレステリック反射層
４ 中間層
５ 保護層
６ 下地層
Ａ 常温で消色状態にある組成物
Ｂ 加熱・溶融状態にあり発色した組成物
Ｃ 常温で発色状態にある組成物
Ｄ 加熱により消色し始めた状態にある組成物
Ｅ Ｃの再加熱によって形成された消色状態の固体組成物
ａ 常温での結晶状態
ｂ 等方相
ｃ 常温で透明状態の非晶質相
ｄ 白濁状態
ｅ_Ｂ 青色の選択反射色を示すコレステリック液晶層
ｅ_Ｇ 緑色の選択反射色を示すコレステリック液晶層
ｅ_Ｒ 赤色の選択反射色を示すコレステリック液晶層
ｆ_Ｂ 青色のコレステリックガラス相
ｆ_Ｇ 緑色のコレステリックガラス相
ｆ_Ｒ 赤色のコレステリックガラス相
Ｔ１ 消色開始温度
Ｔ２ 消色温度
Ｔ３ 発色開始温度
Ｔ４ 発色状態濃度が飽和する温度
Ｔ５ 結晶化温度
Ｔ６ 等方相転移温度[0001]
BACKGROUND OF THE INVENTION
  The present invention can be rewritten using a cholesteric liquid crystalline material.Reversible recording mediaIn particular, the present invention relates to a reversible recording medium having a high contrast and a high whiteness display in the case of multicolor display.
  The recording material can be applied to rewritable full color recording and multi-value recording media.
[0002]
[Prior art]
  Examples of conventional techniques related to the technical field of the present invention include the following.
1. JP-A-6-92016 (multicolored structure of leuco dye)
  A liquid crystal that causes light interference is formed on the surface of the temperature-sensitive ink coated with a leuco dye, a developer, and a desensitizer, to combine the color erasing and light interference red of the leuco dye. However, there is no memory.
2. JP-A-6-313880 (rewritable display medium)
  On the surface of the substrate, a microcapsule layer in which scale-like magnetic powder is dispersed in a liquid and a polymer liquid crystal layer exhibiting selective reflection are provided in this order. Reversible display of single color selective reflection color is possible.
3. JP-A-6-273707
  A cholesteric polymer liquid crystal can fix its cholesteric reflection color at room temperature by rapidly cooling the liquid crystal state that appears due to temperature rise to a glass transition temperature (Tg) or lower. By heating the cholesteric polymer liquid crystal to an isotropic phase transition temperature or higher to make the recording layer transparent and rapidly cooling, the recorded image can be erased and reversible recording is possible.
[0003]
4). N. Tamaoki, A .; V. Parfenov, A.M. Masaki, H .; Matsuda, Adv. Mater. 1997, 9, 1 102-1104
  Dicholesteryl 10,12-docosadiindioate is fixed in a solid state exhibiting a cholesteric reflection color when rapidly cooled from a temperature of 87-115 ° C. showing a cholesteric phase to 0 ° C. The cholesteric reflection color fixed by changing the temperature at which the rapid cooling starts changes continuously from blue to red, and the color is stable for more than half a year at room temperature. Further, the cholesteric reflection color disappears by heating to 119 ° C. or higher, and another cholesteric reflection color can be fixed by quenching from another cholesteric liquid crystal temperature. This recording material is capable of rewritable full color recording.
[0004]
5). Polymer, 47, October, 760
  A recording material made of a cholesteric liquid crystal compound having a molecular weight of 2000 or less and a glass transition temperature of 35 ° C. or higher or a material containing the compound has a longer cholesteric liquid crystal phase reflection color at room temperature by quenching from the cholesteric liquid crystal phase state. There is known a recording medium that can be stored for a long time and can be repeatedly written by reheating to return to the liquid crystal phase.
  Although the recording materials 3 to 4 have promising features as rewritable full-color recording and recording materials for multi-value recording media as described above, the display contrast is small because selective reflection is used. There are challenges. For example, in the case of a transmissive recording medium, there is a limit in reducing the transmittance due to white turbidity of the recording medium, and improvement in black display is required.
  Further, the reflective recording medium using the recording material requires a black layer that absorbs excess light on the back surface of the recording layer, and there is a limit to obtain a white display due to white turbidity of the recording medium. For this reason, the cholesteric liquid crystal recording material has a problem that it has a high whiteness like paper and it is difficult to display with high contrast.
[0005]
6). JP-A-6-79970
  A reversible multicolor thermal recording medium and display medium capable of obtaining a multicolor image even on a reversible recording medium using a color developer such as a leuco dye and a developer have been proposed.
  The recording medium of the present invention. Equipped with a recording layer or an image forming layer containing a color former and a developer on the support, color develops by heating and melting to form an image, and the image disappears when heated to a temperature lower than the color development temperature. A reversible thermosensitive recording medium or display medium for forming a state is characterized in that the recording layer or image forming layer comprises a plurality of layers or a plurality of types of microcapsules having different color tone and decoloration start temperature. Since this material system takes a colored state and a transparent state of the dye, an image like a color hard copy can be obtained by using a white support. However, in order to obtain a full-color image, it is necessary to match the coloring characteristics and decoloring characteristics of the three kinds of materials showing the three primary colors, but it is difficult to realize the optimum material characteristics.
  Furthermore, in the above-mentioned invention of Japanese Patent Laid-Open No. 6-79970, in order to obtain a multicolor image, it is necessary to match the coloring characteristics and decoloring characteristics of a material exhibiting multiple colors. Since it is difficult to do so, there is a problem of providing a multicolor image display recording medium that can easily realize matching of the material characteristics.
[0006]
[Problems to be solved by the invention]
  The present invention can display black or white, which is difficult to display only by using selective reflection, in a reversible recording medium using a cholesteric liquid crystalline compound, improving display contrast and easily matching material characteristics. Multi-color display that can be realizedProvision of reversible recording mediaWith the goal.
[0007]
[Means for Solving the Problems]
  The first aspect of the present invention is a reversible cholesteric reflective layer capable of reversibly showing a selective reflection phenomenon caused by a helical molecular arrangement of a cholesteric liquid crystal phase on a support substrate.(A)And a reversible light absorbing layer between the supporting substrate and the reversible cholesteric reflective layer.(B)It is configured with
  The reversible light-absorbing layer (B) is composed of a solid composition that is colorless at room temperature containing at least an electron-donating color-forming compound and an electron-accepting compound, and the colorless solid composition has a melting temperature higher than its melting temperature. When heated, a colored body of a colored state of the electron-donating coloring compound and the electron-accepting compound is formed, and when the colored body of the colored state is rapidly cooled, it becomes a solid composition at room temperature while maintaining the colored state, When the solid composition is reheated to a temperature lower than the melt coloring temperature, the electron-accepting compound is separated from the color former to form a decolored solid composition, and when the decolored solid composition is further cooled, it becomes colorless. A solid composition, and a color former by being reheated above the melting temperature, and
  In the reversible cholesteric reflective layer (A), a part or the whole of the layer is heated to a temperature showing an isotropic phase or a high temperature cholesteric liquid crystal phase, and then the glass transition temperature Tg or less from the isotropic phase. When cooled to room temperature, it is fixed in an amorphous phase that is transparent at room temperature, and when it is slowly cooled from the high temperature isotropic phase to the temperature range of the cholesteric liquid crystal phase, it shows a selective reflection color according to the temperature, and from this state the glass transition When rapidly cooled to a temperature Tg or less, a cholesteric glass phase in which the selective reflection state is fixed at room temperature is obtained, and further, the cholesteric glass phase in which the selective reflection state is fixed is reheated to a crystallization temperature. The cholesteric liquid crystal compound has a molecular weight of 2000 or less and a glass transition temperature of 30 ° C. or more. It is click liquid crystal compound, and,
The reversible cholesteric reflective layer (A) is formed by mixing the cholesteric liquid crystal compound with a prepolymer of a binder component and a polymerization initiator, and polymerizing the binder component.
  In a reversible recording medium.
  First of the present invention2The cholesteric liquid crystal compound constituting the reversible cholesteric reflective layer (A) is dicholesteryl-10,12-docosadiindioate.1In the reversible recording medium described.
  First of the present invention3The cholesteric liquid crystal compound constituting the reversible cholesteric reflective layer (A) is at least one compound represented by the following formula (II):1In the reversible recording medium described.
[Chemical formula 2]

  Wherein Z ′ and Y ′ are each independently a dihydrocholesteryl group, a hydrogen atom or
    An alkyl group, wherein m and n are each independently an integer of 1 or more, Z ′ and Y ′
    At least one of them represents a dihydrocholesteryl group. )
[0008]
  Hereinafter, a reversible recording medium, a reversible rewriting method and a reversible recording apparatus using the reversible recording medium of the present invention will be described with reference to the drawings.
1. Basic configuration of reversible recording medium of the present invention
  FIG. 1 shows the basic configuration of the reversible recording medium of the present invention. A reversible heat absorbing layer comprising, for example, a reversible light-absorbing layer on the support 1, for example, a composition comprising an electron donating color developing compound (hereinafter also referred to as a color former) and an electron accepting compound (hereinafter also referred to as a color developer). A coloring layer is formed. Furthermore, a reversible cholesteric reflective layer 3 (hereinafter abbreviated as a cholesteric reflective layer) containing a thermotropic liquid crystalline compound that forms a cholesteric liquid crystal phase or an isotropic phase is formed thereon.
  If necessary, an intermediate layer 4 is provided between the light absorption layer 2 and the cholesteric reflection layer 3, a surface protective layer 5 is provided on the surface of the cholesteric reflection layer 3, and an underlayer is provided between the support 1 and the reversible light absorption layer 2. 6 or the like may be provided.
[0009]
  As the support 1, for example, a plastic film such as paper, synthetic paper, PES, PET, polyethersulfone, polyetherimide, a composite thereof, a glass plate, or the like can be used. The support 1 may be transparent, but white is preferable in the case of a reflective and paper-like reversible recording medium. When the paper-like medium is used, the thickness of the support 1 is usually 50 to 500 μm, preferably about 100 to 300 μm. When other display devices are used, a plate-like rigid body may be used, and the thickness of the support is not particularly limited.
[0010]
  The surface protective layer is preferably a glass substrate or a plastic film such as polyethersulfone or polyetherimide having excellent transparency and heat resistance, but is not limited thereto. After applying a photocurable resin or the like on the cholesteric reflective layer, a curing process may be performed to form a surface maintaining layer. As the photocurable resin, an ultraviolet curable resin having high transparency and excellent mechanical strength is preferable. In addition, a filler such as silica fine particles can be added to the ultraviolet curable resin to further improve the mechanical strength.
  When recording with a contact-type heating device such as a thermal head from the surface side of the recording medium, the thickness of the surface protective layer is preferably about 1 μm to 30 μm. If the thickness is less than this, the mechanical strength is insufficient and the surface protective layer is damaged, and if it is more than this, the heat transfer efficiency to the cholesteric reflective layer and the light absorption layer is deteriorated.
[0011]
  When thermal writing is performed on the reversible recording medium from the surface side with a thermal head or the like, the cholesteric reflective layer 3 is fixed in a cholesteric phase to form a selective reflection image. When the light absorption layer 2 in the lower layer also reaches the color development temperature due to heating by thermal writing, a color image that is substantially similar to the selective reflection image in the upper layer is formed. This color image acts as a light absorption layer necessary for observing the cholesteric reflection color.
[0012]
  As indicated by arrows in the figure, in the thermal writing unit, light of a specific wavelength (however, one of right or left optical rotation) is reflected by the cholesteric reflective layer 3, and the remaining wavelength component is the light absorbing layer 2. Is incident on. When the color is black, all the incident wavelength components are absorbed, so only the reflected light from the cholesteric reflective layer 3 is observed, and a vivid image of selective reflection color is obtained. When the color is not black but is colored blue or the like, an image of a color due to the additive color mixture of the reflected light from the light absorbing layer 2 and the reflected light from the cholesteric reflective layer 3 is observed. Further, by setting the heat writing conditions as appropriate and recording the cholesteric reflective layer 3 in a transparent state and forming a color image on the light absorption layer 2, a color image such as black or blue is observed as it is.
  Thus, simultaneously with the reversible recording of the selective reflection image, a multi-color image having a large contrast with the base portion can be reversibly recorded by forming the light absorption image reversibly at the corresponding portion.
[0013]
2. Details of reversible cholesteric reflective layer
  The reversible cholesteric reflective layer has a memory property and is heated above the glass transition temperature, and the selective reflection state of the cholesteric liquid crystal phase that appears according to the heating temperature of this heating or the transparent state of the isotropic phase is below the glass transition temperature. Any one can be used as long as it has a thermotropic liquid crystal compound that can be fixed to the surface.
[0014]
  To improve recording speed and stability,In the present invention, it is preferable to use a cholesteric liquid crystalline compound having a molecular weight of 900 or more and 10,000 or less, preferably 1000 or more and 2000 or less and having no molecular weight distribution, or a mixture thereof (hereinafter also referred to as medium molecular cholesteric liquid crystal). When the medium molecular cholesteric liquid crystal compound is converted into an isotropic phase or a cholesteric liquid crystal phase at a high temperature and then rapidly cooled to a glass transition temperature or below or to room temperature, the heated cholesteric compound retains its molecular arrangement and remains a glassy solid. (Hereinafter referred to as a cholesteric glass phase), and a selective reflection color is observed.
[0015]
  If the molecular weight of the medium molecular cholesteric liquid crystal compound is less than 900, crystallization occurs due to rapid cooling and the cholesteric glass phase is not fixed. This is thought to be due to the rapid reorientation of molecules accompanying rapid cooling. On the other hand, when the molecular weight is larger than 10,000, it is difficult to practically record and erase in one pixel of about several hundred milliseconds or less.
  The medium molecular cholesteric liquid crystal compound is allowed to reach a temperature showing an isotropic phase, and then the layer is cooled to a temperature showing a cholesteric reflection color to fix the cholesteric reflection color, and then heated again to at least its A white display can be achieved by crystallizing a part or reaching a temperature exhibiting an isotropic phase and then rapidly cooling to at least a part to be in an amorphous state.
[0016]
  A cholesteric liquid crystal compound used as a liquid crystal recording material has a molecular weight of 2000 or less, a glass transition temperature of 30 ° C. or higher, a cholesteric liquid crystal phase above the glass transition temperature, and an isotropic phase above that temperature. is there.
  Especially shown by the following formula (I)The cholesteric liquid crystal compound (Dicholesteryl 10,12-Docosadiynedioat) is preferable because of its high stability due to the cholesteric pitch reflection color to which the cholesteric liquid crystal compound is immobilized.
[Chemical 3]

[0017]
  As the cholesteric liquid crystal compound used in the present invention, the above formula (IIThe reversible cholesteric reflective layer using the cholesteric liquid crystal compound is improved in storage stability when the cholesteric glass phase is left at a high temperature. Specifically, crystallization of the cholesteric glass phase is suppressed. This can prevent the image from deteriorating when the recording medium is left in a hot place such as in a car in summer.
  A plurality of the cholesteric liquid crystal compounds used in the present invention may be used in combination.
[0018]
  The cholesteric reflective layer is preferably composed only of a liquid crystalline compound exhibiting selective reflection,After the binder resin and, for example, a liquid crystal compound are dissolved in an appropriate solvent and applied, the reflective layer can be formed by evaporating the solvent. Also,The cholesteric reflective layer may be formed by mixing a cholesteric liquid crystal compound, a monomer component or a prepolymer of a binder component, and a polymerization initiator, and polymerizing the binder component by polymerization, for example, photopolymerization or thermal polymerization.
  As the spacer particles, spherical particles or cylindrical particles used for general liquid crystal displays can be used, but are not limited thereto.
[0019]
  The type of heating means for obtaining the isotropic phase is not particularly limited, but appropriate heating means and heating conditions can be appropriately selected according to the required heating rate. For example, a hot plate, a laser beam, a thermal head, or the like can be used. However, the heating means is not limited to these.
[0020]
  The type of quenching means for fixing the cholesteric reflection color is not particularly limited, but for example, appropriate cooling means and cooling conditions are appropriately selected according to the cooling rate required according to the desired cholesteric reflection color. Can be selected. As a cooling means, water cooling, air cooling, a metal plate, a glass plate etc. can be used, for example. However, the cooling means is not limited to these.
[0021]
  The selective reflection wavelength is usually preferably in the visible light region of about 400 to 700 nm, and in this case, it can be visually recognized by humans. However, the selective reflection wavelength may exist in the ultraviolet region or the infrared region when reading with a machine.
  The thickness of the cholesteric reflective layer may be appropriately selected from the range of 0.5 to 50 μm, preferably 1 to 20 μm. If the cholesteric reflective layer is too thin, the reflectance at the wavelength at which maximum reflection can be obtained will be low, and the contrast of the display image will be reduced. If it is too thick, the light absorption in the cholesteric reflective layer will increase and the contrast of the display image will be reduced. . On the other hand, if it is too thick, heat transfer to the lower light absorption layer 2 is deteriorated.
[0022]
3. Reversible light absorption layer
  Any reversible light absorbing layer 2 can be used as long as it has a function of absorbing light transmitted through the core reflection reflective layer by some external stimulus. For example, a thermosensitive recording material capable of repeating color development and decoloring according to heating temperature and cooling rate, guest-host type liquid crystal material (Japanese Patent Laid-Open No. 10-183117) that switches the orientation of dichroic dye molecules depending on the presence or absence of an electric field or heat, a magnetic field For example, a magnetic recording material (Japanese Patent Laid-Open No. 9-090887) or the like that switches the direction of the plate-like magnetic particles depending on the direction of light to form an optical shutter can be used. When the recording principles of the cholesteric reflective layer and the light absorbing layer are different, it is necessary to optimize the configuration of the recording layer so that the recording operation of each layer is performed reliably. Further, it is preferable that any of the materials has a memory property that can maintain a light absorption (coloring) state and a transmission (colorless) state without energy.
[0023]
  When the recording operation of the cholesteric reflective layer is thermal recording, the light absorption layer is also preferably thermal recording. A mixture of a developer and a color developer can be used as a system having a memory property that can reversibly hold a light absorption (coloring) state and a transmission (colorless) state by a thermal recording apparatus and can be held without energy.
  The reversible light absorbing layer 2 is preferably achieved only by a combination of a developer and a color former, but is not limited to a system based on a combination of a developer and a color developer. Any material system in which coloring and decoloring change thermally reversibly can be used.
  The developer used here has a structure having a developing ability and a long-chain aliphatic group (hereinafter referred to as a long-chain structure) in the molecule. In the reversible thermochromic composition, an appropriate color developer combination is used for the developer. For example, this combination is selected depending on whether or not a color development state sample obtained by heating and melting both samples is subjected to differential scanning calorimetry or differential thermal analysis, and whether or not it exhibits an exothermic phenomenon in the temperature rising process. If so, the present invention is applicable. This reversible thermochromic composition can take a colored state by mixing and melting the developer and the color former and heating them to a temperature higher than the temperature at which the color develops and then rapidly cooling. When the temperature is raised again, the colored body is quickly decolored at a temperature lower than the coloring temperature. A decolorization accelerator may be added to a reversible thermochromic composition that causes such color development and decoloration phenomena.
[0024]
  The developer contained in the reversible light-absorbing layer is basically capable of developing a color former in the molecule.
A compound having a structure capable of developing color and a long aliphatic chain structure part that controls the cohesive force between molecules, and an organophosphate compound having an aliphatic group having 12 or more carbon atoms, aromatic or An aliphatic carboxylic acid compound or a phenol compound. The aliphatic group includes a linear or branched alkyl group or alkenyl group, and may have a substituent such as a halogen, an alkoxy group, or an ester group. Specific examples include compounds described in JP-A-10-151859, but are not limited to these compounds.
[0025]
  Further, the color former contained in the reversible light-absorbing layer exhibits electron donating properties, and is itself a colorless or light dye precursor, not particularly limited, and conventionally known ones such as fluorane compounds, Phenothiazine compounds, leucooramine compounds, phthalide compounds, azaphthalide compounds and the like are used. Specific examples include compounds described in JP-A-10-95175, but are not limited to these compounds.
[0026]
  Furthermore, a low melting point compound or a high melting point compound can be added to the reversible light absorbing layer as a decoloring accelerator. Examples thereof include the following substances. Long chain carbonization such as fatty acids, fatty acid derivatives or fatty acid metal salts, waxes and fats, higher alcohols, phosphate esters, benzoates, phthalates, oxyesters, silicone oils, liquid crystal compounds, surfactants, etc. A compound having a hydrogen group can be used.
[0027]
  An appropriate ratio of the color former and the developer constituting the reversible light absorbing layer needs to be selected depending on the physical properties of the compound used. The range is generally in the range of 1 to 20 developer, and preferably in the range of 2 to 10, with respect to the color former 1 in molar ratio. If the developer is less than this range, the decoloring property is insufficient, and if it is too much, the color density is insufficient.
[0028]
  The light absorbing layer 2 can hold the developer and the color former using a binder resin as necessary in order to take a form as a layer. Examples of the binder include polyvinyl chloride, polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, polystyrene, styrene copolymer, phenoxy resin, polyester, aromatic polyester, polyurethane, polycarbonate, polyacrylate, Examples include methacrylic acid esters, acrylic acid copolymers, maleic acid copolymers, and polyvinyl alcohol.
[0029]
  The developer and color former can also be used by being encapsulated in microcapsules. The microencapsulation of the developer and color former can be performed by a known method such as a coacervation method, an interfacial polymerization method, or an in situ polymerization method. The light absorbing layer 2 is formed by uniformly dispersing or dissolving the color former and the developer together with a binder with water or an organic solvent according to a conventionally known method, and applying and drying this on a support. When a binder is not used, the developer and color former can be mixed and melted to form a film, and cooled to form the light absorption layer 2. The role of the binder resin in the light absorption layer is to maintain a state in which the reversible thermochromic composition is uniformly dispersed by repeated coloring and decoloring. In particular, since the composition often aggregates and becomes non-uniform by application of heat during color development, the binder resin preferably has high heat resistance.
[0030]
4). Reversible recording characteristics
  In order to form a colored image using the reversible recording medium of the present invention, the cholesteric reflective layer and the light absorbing layer 2 may be once heated to the recording temperature or higher and then rapidly cooled. Specifically, for example, when the recording medium is heated for a short time with a thermal head or laser light, the recording medium is locally heated, so that the heat is immediately diffused and abrupt cooling occurs, and the colored state can be fixed. On the other hand, in order to erase the color, it may be heated and cooled for a relatively long time using an appropriate heat source, or may be temporarily heated to the color erase temperature. When heated for a long time, the temperature of a wide range of the recording medium rises, and the subsequent cooling becomes slow. As a heating method in this case, a heat roller, a heat stamp, hot air, or the like may be used, or heating may be performed for a long time using a thermal head. A laser apparatus can also be used. If a high-power carbon dioxide laser or the like is used, heat can be generated without using a visible light absorbing layer. In order to heat the cholesteric reflection layer and the light absorption layer to the decoloring temperature range, for example, the applied energy may be lowered slightly from the time of recording by adjusting the voltage applied to the thermal head and the pulse width. If this method is used, recording / erasing can be performed only by the thermal head, and so-called overwriting becomes possible. Of course, the image can be erased by heating to a decoloring temperature range with a heat roller or a heat stamp. Moreover, a light source device can be used also at the time of decoloring. In this case, since the colored portion is particularly easy to absorb light, it can generate heat efficiently.
[0031]
  First, the coloring / decoloring phenomenon of the light absorption layer 2 will be described. FIG. 2 shows the relationship between color density and temperature of the reversible thermochromic composition. In this figure, the horizontal axis indicates temperature, and the vertical axis indicates concentration. In the figure, A shows a composition that is in a decolored state at room temperature, and B shows a composition that is in a heated and molten state and has developed color. C represents a composition in a colored state at room temperature. When the temperature of the composition A is increased, the color development starts at the color development start temperature T3 and becomes the composition B at the temperature T4 at which the color development state density is saturated (hereinafter referred to as the color development saturation temperature T4). When this composition B is rapidly cooled, it returns to room temperature and changes to the composition C while maintaining the colored state (the solid line path in the figure). When the temperature of the colored composition C is increased again, the composition D starts to decrease in density at the decolorization start temperature T1, and finally becomes decolored at the decolorization temperature T2 and changes to the composition E. When the composition E is cooled and cooled down, it returns to the composition A in a decolored state as it is (route of chain line in the figure). Here, the temperature from the decoloring temperature T2 to the color development start temperature T3 is the decoloring temperature region of the composition. A characteristic of the color development / decoloration phenomenon exhibited by this composition is that there is a decoloration temperature range in a temperature range lower than the temperature at which it melts and develops color, and the color disappears when the composition is heated from this temperature to this range. . In addition, the phenomenon of color development and color erasing can be repeated. FIG. 2 shows a typical coloring and erasing method of the composition of the present invention, and each temperature varies depending on the combination of materials used. Further, the concentration of the composition B in a state of being colored by melting and the concentration of the composition C in a colored state obtained by cooling from the state are not necessarily the same and may be different. .
[0032]
  Next, the reflection / transmission phenomenon of the cholesteric reflection layer will be described. FIG. 3 shows the relationship between the selective reflection color of the reversible cholesteric reflective layer and the temperature when a medium molecular cholesteric liquid crystalline compound is used. In this figure, the horizontal axis indicates the temperature, and the vertical axis indicates the selective reflection color. In the figure, when the temperature of the crystal state a at room temperature is raised, it changes to the isotropic phase b in the molten state at an isotropic phase transition temperature T6 or higher. When the isotropic phase b is rapidly cooled to a glass transition temperature Tg (not shown) or lower, it is fixed to the amorphous phase c that is transparent at room temperature. When the temperature is gradually cooled from the high temperature isotropic phase b to the temperature range of the cholesteric liquid crystal phase e, the selective reflection color (e_R, E_G, E_B). Previous formula (I10,12-docosadiindicarboxylic acid dicholesteryl ester (molecular weight: 1099.8, glass transition temperature: 80 ° C., hereinafter referred to as chol-8-diyne-8-chol), about 430 nm at about 115 ° C. Blue, shows a red color of 610 nm at about 87 ° C. When rapidly cooled from this state to a glass transition temperature Tg (not shown) or lower, a cholesteric glass phase (f) in which a selective reflection state is fixed at room temperature._R, F_G, F_B) Is obtained. Alternatively, by changing the quenching start temperature and quenching rate in the isotropic phase b, the amorphous phase c and the cholesteric glass phase (f_R, F_G, F_B) Can also be obtained.
[0033]
  When the amorphous phase c or the cholesteric glass phase f is heated again, crystallization starts at the crystallization temperature T5 and becomes a cloudy state d. If it cools here, it will return to the crystalline state a. Further, when the cholesteric liquid crystal phase e is gradually cooled, a crystallization state a may be obtained or a transparent state (not shown) may be obtained. The cause of this is not clear, but it is considered that the liquid crystal structure is different between the liquid crystal formation by reheating the glass phase and the liquid crystal formation by slow cooling from the liquid crystal phase, and the light scattering ability is different. FIG. 3 shows typical recording characteristics of the liquid crystal composition of the present invention. The isotropic phase transition temperature T6, the crystallization temperature T5, and the like differ depending on the materials used.
[0034]
  In the initialization or recording erasing process of the entire reversible recording medium, when the support substrate 1 is white, the light absorbing layer 2 is heated to the decoloring temperature T2 or higher and then cooled. This is because even if the cholesteric reflection layer shows a selective reflection state, if the light absorption layer 2 is surely decolored, the selective reflection color is not observed and a white display is obtained. When the support substrate 1 is transparent, the cholesteric reflective layer is heated to the crystallization temperature T5 or higher, and the light absorption layer 2 is heated to the decolorizing temperature T2 or higher, and then cooled. This is because in the case of the transmissive type, the cholesteric reflective layer also needs to be transparent or slightly cloudy. Here, when the crystallization temperature T5 of the cholesteric reflection layer is higher than the color development start temperature T3 of the light absorption layer, the light absorption layer reappears when the cholesteric reflection layer is heated to the crystallization temperature T5 or higher in order to erase the recorded image. It may be colored. Therefore, it is necessary to combine the materials so that T3> T5.
[0035]
  When an image is thermally written by a heating device such as a thermal head, unevenness may occur in the heating temperature and cooling rate of the image edge portion (contour portion). Therefore, when there is a light absorption layer on the entire lower layer of the cholesteric reflection layer, the selective reflection color of the contour portion of the recorded image may be observed differently. In order to prevent the color difference of the contour portion, the selective reflection color of the contour portion should not be observed. That is, if the recording image of the light absorption layer is formed slightly smaller than the recording image of the cholesteric reflection layer, the selective reflection color of the image contour portion is not observed, and only the image of the uniform color portion is observed.
[0036]
  As described above, in order to form the recording image of the light absorption layer smaller than the recording image of the cholesteric reflection layer, the color development start temperature T3 needs to be equal to or higher than the isotropic phase transition temperature T6 (T3 ≧ T6). When the reversible recording medium of the present invention is heated from the surface side with a thermal head or the like, the temperature in the lower light absorption layer tends to be lower than the temperature in the upper cholesteric reflective layer. FIGS. 4A and 4A show model diagrams of the temperature distribution of the cross section of the reversible recording medium. In the figure, the dotted line indicates the distribution of the relatively high temperature portion, the solid curve indicates the distribution of T6, and the dashed curve indicates the distribution of the relatively low temperature portion. In the cholesteric reflective layer, a portion heated and rapidly cooled to T6 or higher is fixed as a cholesteric glass phase, but color unevenness due to temperature distribution tends to occur in the contour portion. Here, when the color development start temperature T3 of the light absorption layer is relatively low T3 <T6, the light absorption layer develops color up to the broken line curve portion as shown in FIG. 4A, and the area of the color development portion becomes relatively large. . In particular, when the difference between the color development start temperature T3 and the color development saturation temperature T4 is small, the area of the portion colored at a high density also increases. As a result, the contour portion of the selective reflection color is easily observed, and color unevenness occurs in the image contour portion as shown in FIG. Therefore, under the condition of T3 ≧ T6, the color development area in the light absorption layer can be surely made smaller than the recording area in the cholesteric reflection layer as shown in FIG. Therefore, the selective reflection light is not observed because there is no light absorption layer in the color uneven portion of the contour portion, and only a uniform image without color unevenness of the contour portion is observed as shown in FIG.
[0037]
【Example】
  Examples of the present invention are shown below.
[0038]
Example 1
  As a developer, the following formula (III) Indicated by K) was used.
[Formula 4]

  Using 2-anilino-3-methyl-6-n-dibutylaminofluorane as a color former, these developer and color former were dispersed and mixed in a methyl ethyl ketone together with a vinyl chloride-vinyl acetate copolymer, on a transparent PET film. Then, a light absorption layer having a film thickness of about 6 μm was formed.
  On this film, the previous formula (IAn appropriate amount of a cholesteric liquid crystal (chol-8-dyne-8-chol) compound represented by () is placed on a hot plate heated to 130 ° C. for 30 seconds and sufficiently heated and melted.
[0039]
  Put a PES film with a thickness of 25 μm in a molten state and place an opposing hot plate heated to 130 ° C. to 1 kg / cm²The film was stretched to a film thickness of about 10 μm while being uniformly pressurized at a pressure of 5 ° C., and then gradually cooled at a rate of 5 ° C./min to form a selective reflection layer.
  As a result, the recording characteristics of the light-absorbing layer are the decolorization start temperature T1 = 55 ° C., the decoloration temperature T2 = 90 ° C., the color development start temperature T3 = 130 ° C., and the color development temperature T4 = 150 ° C. A transmission-type reversible recording medium having a crystallization temperature T5 = 97 ° C. and an isotropic phase transition temperature T6 = 120 ° C. was obtained. Since it was gradually cooled as a cooling condition at the time of initialization, the light absorption layer was fixed in a colorless state and the selective reflection layer was also fixed in a transparent state.
[0040]
  Image information was thermally written on the surface of the protective layer of the recording medium with a thermal head device. Used under conditions of dot density 6 dots / mm, applied power 0.2 W / dot, writing speed 5 mm / sec, and changes the drive pulse width in the range of several msec to several tens msec to control the recording energy per unit area did. 65mJ / mm in the form of an image at room temperature²When the recording head was forcibly cooled by blowing cool air to the outlet of the thermal head, the light absorbing layer remained almost colorless and a cholesteric glass phase was obtained in the cholesteric reflective layer. This cholesteric glass phase showed selective reflection of a red wavelength, and complementary cyan light was visually observed as transmitted light. Similarly, 85mJ / mm in the form of an image²Recording with energy, cooling with a fan at the outlet of the thermal head, this time, the light absorbing layer also turned black, and a cholesteric glass phase was obtained in the cholesteric reflective layer. This cholesteric glass phase showed selective reflection of a green wavelength, and a green recorded image was visually observed in the reflected light due to light absorption of complementary color light in the light absorption layer. In this portion, the transmitted light is absorbed by the light absorption layer, so that black is observed in the transmitted light. When the sheet after recording was projected by OHP, cyan and black images were displayed on the white display base, and a multi-color display with high contrast was obtained. Further, when the entire sheet was heated to 125 ° C. on a hot plate and gradually cooled at a rate of 5 ° C./min, the light absorption layer was decolored and the cholesteric reflective layer was in a transparent state. When projected with OHP, a white display was obtained on the entire surface, and the recorded image could be erased.
[0041]
Example 2
  The following formula (IV) And 2- (o-chlorophenyl) anilino-6-dibutylaminofluorane as a color former, and these developer and color former are dispersed in methyl ethyl ketone together with a vinyl chloride-vinyl acetate copolymer. A light absorption layer having a thickness of about 6 μm was formed on the white PET film in the same manner as in Example 1.
[Chemical formula 5]

[0042]
  An appropriate amount of the cholesteric liquid crystal compound used in Example 1 was placed on this film and melted by heating at 130 ° C. In the same manner as in Example 1, a PES film having a thickness of 25 μm was applied in a molten state and uniformly pressed, and the film was cooled to a thickness of about 10 μm to form a cholesteric reflective layer. As a result, the recording characteristics of the light absorbing layer are the color erasing start temperature T1 = 62 ° C., the color erasing temperature T2 = 80 ° C., the color development start temperature T3 = 92 ° C., and the color development temperature T4 = 100 ° C. A reflective reversible recording medium having a crystallization temperature T5 = 97 ° C. and an isotropic phase transition temperature T6 = 120 ° C. was obtained. Since the cooling was carried out gradually as a cooling condition at the time of initialization, the light absorption layer changed from the first colored state to a colorless state, and the cholesteric reflective layer was fixed in a transparent state.
[0043]
  The surface of the protective layer of the recording medium is 65 mJ / mm in the form of an image with the same thermal head device as in Example 1.²When recorded by energy, the light absorbing layer was fixed in black, and the cholesteric reflective layer was fixed with a cholesteric glass phase exhibiting selective reflection of red wavelength. Since the black portion of the light absorption layer works as the light absorption layer, a red recorded image was visually observed in the white base. Similarly, 85mJ / mm in the form of an image²When recording with energy and cooling with a fan at the outlet of the thermal head, the light absorption layer was fixed in black, and the cholesteric reflection layer was fixed with a liquid crystal glass phase exhibiting selective reflection of the green wavelength. Similarly, 100mJ / mm in the form of an image²When recording with energy and forcibly cooled by blowing with a fan at the outlet of the thermal head, the light absorbing layer is fixed in black, and the cholesteric reflective layer is fixed with a cholesteric glass phase that exhibits blue wavelength selective reflection. It was. Similarly, 110mJ / mm on the image²When it was forcibly cooled by blowing with a fan at the outlet of the thermal head recorded by energy, the light absorbing layer was fixed in black, and the cholesteric reflective layer was fixed in a transparent amorphous phase. As described above, a multicolor image of red, green, blue, and black could be displayed in a paper-like white display background. The cooling conditions such as the blowing intensity at the outlet of the thermal head and the air temperature were appropriately changed according to the recording conditions. Here, when the outline portion of the green image was enlarged and observed, there was a portion where red was recorded. This is because when T4 <T6 and the color development temperature T4 is relatively low, the image area of the color development part is relatively large, and the selective reflection color of the unevenness of the fixed state generated in the contour part of the cholesteric glass part is displayed as it is. Because it is. However, the color unevenness of the contour portion is at a level that causes no problem in practical use.
  Next, on the hot plate, the entire sheet was heated to 100 ° C., which is slightly higher than the crystallization temperature T5, and rapidly cooled. As a result, the cholesteric reflective layer was crystallized and slightly turbid, but T3 <T5. Although the image was once erased, the entire surface was recolored. Therefore, when the mixture was again heated to 130 ° C. on the hot plate and gradually cooled, the light absorption layer was decolored and the cholesteric reflection layer became transparent.
[0044]
Example 3
  The following formula (V), And 2-anilino-3-methyl-6-n-dibutylaminofluorane was used as the color former. As a result, the recording characteristics of the light-absorbing layer are the decolorization start temperature T1 = 70 ° C., the decoloration temperature T2 = 90 ° C., the color development start temperature T3 = 140 ° C., and the color development temperature T4 = 150 ° C. A reflective reversible recording medium having a crystallization temperature T5 = 97 ° C. and an isotropic phase transition temperature T6 = 120 ° C. was obtained. The conditions of T3> T5 and T4 ≧ T6 are satisfied.
[Chemical 6]

[0045]
  When thermal recording was performed in the same manner as in Example 2, a good multicolor image could be formed. The contour portion of the green image was enlarged and observed, but no red color was observed in the contour portion. This is because the isotropic phase transition temperature T6 is smaller than the color development temperature T4, so that the color development area of the light absorption layer is slightly smaller than the recording area of the cholesteric reflection layer, and the selective reflection color of the contour portion of the cholesteric glass part. This is because no longer can be displayed. However, although the line became thin because the area of the color image was slightly reduced, it was at a level where there was no practical problem.
  Next, using a thermal head device, the surface of the sheet is 55 mJ / mm so that the sheet surface becomes 110 ° C. of T5 or higher.²As a result, the inside of the light absorption layer reached T2 and was decolored. Here, since T3> T5 and the color development start temperature T3 was sufficiently high, no recurrent color of the light absorption layer occurred. Also, the cholesteric reflective layer was crystallized at T5 and slightly turbid, so that a white display was obtained on the entire surface, and the recorded image could be erased.
[0046]
Comparative Example 1
  Without providing a reversible heat-light absorption layer, an appropriate amount of the cholesteric liquid crystal compound of Example 1 was placed on a black PET film and melted by heating at 130 ° C. In a molten state, a PES film having a thickness of 25 μm was covered and pressed uniformly, and the film was stretched to a thickness of about 10 μm and cooled to form only a cholesteric reflective layer. A reflective reversible recording medium was obtained in which the recording characteristics of the cholesteric reflective layer were a crystallization temperature T5 = 97 ° C. and an isotropic phase transition temperature T6 = 120 ° C. The cholesteric reflective layer was fixed in a transparent state because it was gradually cooled as a cooling condition at the time of initialization.
  When thermal recording was performed in the same manner as in Example 2, a multicolor image of red, green, and blue could be displayed on the black base. Also, in order to record a white image on a black background, the image shape is 45 mJ / mm.²As a result of recording with the energy of 1, turbidity was caused by crystallization, but sufficient white display could not be obtained due to the thin film thickness.
[0047]
Example 4
  When the recorded image obtained in Example 2 was allowed to stand at 55 ° C. for 24 hours, the cholesteric glass phase partially crystallized. There was no change in the black density of the light absorption layer, and a black spot-like portion was generated in the image. Therefore, in order to improve the storage stability of the cholesteric glass phase at a high temperature, the formula (1) used in Example 1 as a cholesteric reflective layer (I) And the formula (IIThe cholesteric liquid crystal compound in which m = n = 8 and Z ′ = Y ′ = dihydrocholesteryl group were mixed at a weight ratio of 9: 1 was used. Otherwise, image recording was performed in the same manner as in Example 2. Although the material composition of the cholesteric reflective layer was changed, the characteristics of the recording color with respect to the recording energy were almost the same, and a multicolor image display of red, green, blue, and black was made on the paper-like white display base. Even when the recorded image was allowed to stand at 55 ° C. or lower for 24 hours, crystallization of the cholesteric glass phase did not occur, and it was confirmed that the storage stability of the cholesteric glass phase was improved.
[0048]
【effect】
1. Claim1
  Since a reversible light absorption layer is provided between the support substrate and the reversible cholesteric reflective layer, it is possible to display colors that are difficult to display only by the selective reflection phenomenon, and the contrast with the non-image area is improved. A reversible recording medium is obtained.
2. Claim2-3
    Since a medium molecular cholesteric liquid crystal compound is used, a reversible recording medium capable of recording / erasing at high speed and capable of multicolor display can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing a basic configuration of a reversible recording medium of the present invention.
FIG. 2 is a graph showing the relationship between the color density and temperature of the color forming composition of the reversible recording medium of the present invention.
FIG. 3 is a diagram showing the relationship between a reversible cholesteric reflective layer and temperature when a medium molecular cholesteric liquid crystalline compound is used.
4A is a model diagram of a temperature distribution of a cross-section of a reversible recording medium when T6> T3, and FIG. 4B is a diagram illustrating a state of occurrence of color unevenness in a contour portion of the medium.
FIG. 5A is a model diagram of a temperature distribution of a cross-section of a reversible recording medium when T3 ≧ T6, and FIG. 5B is a diagram showing a state of occurrence of color unevenness in a contour portion of the medium.
[Explanation of symbols]
  1 Support base
  2 Reversible light absorption layer
  3 Reversible cholesteric reflective layer
  4 Middle layer
  5 Protective layer
  6 Underlayer
  A Composition in a decolored state at room temperature
  B Colored composition in the heated / molten state
  C Composition in a colored state at room temperature
  D Composition in a state of starting to be decolored by heating
  Decolored solid composition formed by reheating of E C
  a Crystallized state at room temperature
  b Isotropic phase
  c Amorphous phase that is transparent at room temperature
  d Cloudy state
  e_B  Cholesteric liquid crystal layer showing blue selective reflection color
  e_G  Cholesteric liquid crystal layer exhibiting green selective reflection color
  e_R  Cholesteric liquid crystal layer exhibiting red selective reflection color
  f_B  Blue cholesteric glass phase
  f_G  Green cholesteric glass phase
  f_R  Red cholesteric glass phase
  T1 Discoloration start temperature
  T2 color erasing temperature
  T3 Coloring start temperature
  T4 Temperature at which color density is saturated
  T5 Crystallization temperature
  T6 Isotropic phase transition temperature

Claims (3)

A reversible cholesteric reflective layer (A) capable of reversibly showing a selective reflection phenomenon caused by the helical molecular arrangement of the cholesteric liquid crystal phase is provided on the support substrate, and between the support substrate and the reversible cholesteric reflection layer. And a reversible light absorbing layer (B) .
The reversible light-absorbing layer (B) is composed of a solid composition that is colorless at room temperature containing at least an electron-donating color-forming compound and an electron-accepting compound, and the colorless solid composition has a melting temperature higher than its melting temperature. When heated, a colored body of a colored state of the electron-donating coloring compound and the electron-accepting compound is formed, and when the colored body of the colored state is rapidly cooled, it becomes a solid composition at room temperature while maintaining the colored state, When the solid composition is reheated to a temperature lower than the melt coloring temperature, the electron-accepting compound is separated from the color former to form a decolored solid composition, and when the decolored solid composition is further cooled, it becomes colorless. A solid composition, and a color former by being reheated above the melting temperature, and
In the reversible cholesteric reflective layer (A), a part or the whole of the layer is heated to a temperature showing an isotropic phase or a high temperature cholesteric liquid crystal phase, and then the glass transition temperature Tg or less from the isotropic phase. When cooled to room temperature, it is fixed in an amorphous phase that is transparent at room temperature, and when it is slowly cooled from the high temperature isotropic phase to the temperature range of the cholesteric liquid crystal phase, it shows a selective reflection color according to the temperature, and from this state the glass transition When rapidly cooled to a temperature Tg or less, a cholesteric glass phase in which the selective reflection state is fixed at room temperature is obtained, and further, the cholesteric glass phase in which the selective reflection state is fixed is reheated to a crystallization temperature. The cholesteric liquid crystal compound has a molecular weight of 2000 or less and a glass transition temperature of 30 ° C. or more. It is click liquid crystal compound, and,
The reversible cholesteric reflective layer (A) is formed by mixing the cholesteric liquid crystal compound with a prepolymer of a binder component and a polymerization initiator, and polymerizing the binder component.
A reversible recording medium characterized by the above. Reversible cholesteric cholesteric liquid crystal compound constituting the reflective layer (A) is Jikoresuteriru 10,12 docosapentaenoic diyne dioate reversible recording medium of claim 1, wherein. Cholesteric liquid crystal compounds constituting reversible cholesteric reflective layer (A) is at least one compound reversible recording medium of claim 1, wherein represented by the following formula (II).

Wherein Z ′ and Y ′ are each independently a dihydrocholesteryl group, a hydrogen atom or an alkyl group, wherein m and n are each independently an integer of 1 or more, and at least one of Z ′ and Y ′ is Represents a dihydrocholesteryl group.)

Images