JP2004029648A - Light diffusing sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an easily manufacturable light diffusing sheet for emitting uniform diffused light without generating wrinkles even when it is heated for a long period of time by a light source such as the back light unit of a liquid crystal display. <P>SOLUTION: For the sheet 1 composed of light transmissive resin, recesses and projections are formed on both surfaces 1a and 1b of the sheet and the average surface roughness of one surface 1a to be a light entry surface is higher than the average surface roughness of the opposite surface 1b to be a light exit surface. The sheet 1 contains 0.1-50 wt% of a light diffusing agent 1c having the average particle diameter of 0.5-50 μm. It is preferable that the light diffusing agent contains 10-40 wt% of talc whose average particle diameter is 1.0-15 μm. <P>COPYRIGHT: (C)2004,JPO

Description

【００１２】
式中、Ｒａは平均面粗さ、Ｓ_０　は測定面の基準面積、Ｆ（Ｘ，Ｙ）はＪＩＳ
Ｂ　０６０１で定義されているｆ（ｘ）を面に展開した粗さ曲線、Ｚ_０
は基準面の高さを示す。
【００１３】
この光拡散シートは、シートの入射面となる片面の平均面粗さが０．２〜５．０μｍで、出光面となる反対面の平均面粗さが上記片面のそれより小さく０．２〜２．０μｍの範囲であるので、この光拡散シートを例えばバックライトユニットの導光板の上に重ねると、導光板の内部を適度に反射しながら進む光の大部分が、シートの片面全体から略均等にシート内へ入るので、光損失は少なく入光量の部分的なバラツキも殆ど生じない。更に、片面から光拡散シート内へ入った光が光拡散剤によっても拡散されるため、光拡散作用が向上する。そして、この反対面の凹凸によっても光を均一に充分拡散させながら輝度のバラツキのない拡散光を放出することができる。しかも、シートの熱伸縮が光拡散剤によって小さくなり、光源からの光で熱せられてもシートに皺が発生しないので、均一な拡散光を安定して放出することができる。
【００１４】
上記の光拡散剤は、０．５〜５０μｍの平均粒径を有し、透光性樹脂中に０．１〜５０重量％含有されていると、光の透過が阻害されることなく拡散が一層良好となる。また、光源からの熱によるシートの伸縮が減少して皺の発生を充分抑えることができ、安定した拡散光を得ることができる。
【００１５】
上記の光拡散シートの片面の表面積率が１．００１〜１．１５０、反対面の表面積率が１．０１０〜１．２５０であると、更に入射光が均一となるうえに、拡散光もより輝度のバラツキのないものとすることができるので好ましい。また、前記片面の平均面粗さが１．０〜３．０μｍ、前記反対面の平均面粗さが０．５〜１．５μｍであり、前記片面の表面積率が１．０３０〜１．１００、前記反対面の表面積率が１．０５０〜１．２００であると、上記効果がさらに向上するので好ましい。
【００１６】
ここで、「表面積率」とは、測定面が平坦面であると仮定したときの面積Ｓ_０に対する実際の表面積Ｓの割合（Ｓ／Ｓ_０
）をいう。
【００１７】
また光拡散剤として１．０〜１５μｍの平均粒径を有するタルクを１０〜４０重量％透光性樹脂中に含有させると、長期間に亘る光源からの光によって熱せられても皺が発生しないので好ましい。
また、タルクとその他の光拡散剤とからなる光拡散剤を１０〜４０重量％含有し、前記タルクが透光性樹脂中に５〜３０重量％含有され且つ全光拡散剤の５０重量％以上であっても、長期間に亘る光源からの光によって熱せられても皺が発生しないので好ましい。
【００１８】
【発明の実施の形態】
以下、図面に基づいて本発明の具体的な実施形態を説明する。
【００１９】
図１は本発明の一実施形態に係る第１の光拡散シートの断面図である。
【００２０】
この光拡散シート１は、シート両面１ａ，１ｂに凹凸が形成された光拡散剤含有透光性樹脂からなるシートである。透光性樹脂としては、全光線透過率の高いポリカーボネート、ポリエステル、ポリエチレン、ポリプロピレン、ポリオレフィン共重合体（例えばポリ−４−メチルペンテン−１等）、ポリ塩化ビニル、環状ポリオレフィン（例えばシクロポリオレフィン等）、アクリル樹脂、ポリスチレン、アイオノマーなどの熱可塑性樹脂が好ましく使用され、特にポリプロピレンは耐熱性が良好で柔らかいから、液晶ディスプレイに組み込まれたとき、光源の放熱に対しての変形を少なくすることができるし、上側のレンズフィルム（プリズムシート）を傷付けたりすることがないので好ましく用いられる。また、環状ポリオレフィンは、透明性が非常に良好であるうえ、線膨張率が小さいので、光学特性に優れた熱変形の少ないシートを得ることができ、好ましく用いられる。なお、透光性樹脂とは、０．１ｍｍの厚さの樹脂シートでの全光線透過率（ＪＩＳ
Ｋ　７１０５による）が５０％以上である樹脂をいう。
【００２１】
この光拡散シート１の入光面となる片面（下面）１ａに形成された凹凸は、出光面となる反対面（上面）１ｂに形成された凹凸よりも高低差が大きく分布密度が粗くなっている。即ち、この片面１ａは、平均面粗さＲａが反対面１ｂのそれよりも大きく、０．２〜５．０μｍの範囲にしてあり、表面積率（Ｓ／Ｓ_０
）が反対面１ｂのそれよりも小さく、１．００１〜１．１５０の範囲にしてある。
【００２２】
これに対し、出光面となる反対面（上面）１ｂに形成された凹凸は、上記片面１ａに形成された凹凸よりも細かく密に分布させてあり、平均面粗さＲａが片面１ａのそれよりも小さく０．２〜２．０μｍの範囲にしてあり、表面積率（Ｓ／Ｓ_０　）が片面１ｂのそれよりも大きく１．０１０〜１．２５０の範囲にしてある。
【００２３】
そして、この光拡散シート１に含有される光拡散剤１ｃは、光の拡散性を向上させると共にシート１の熱伸縮を抑制して皺の発生をなくすためのものであって、透光性樹脂と光屈折率が異なる透光性合成樹脂のビーズや、透光性樹脂の屈折率との差がないか或いは屈折率差が０．１以下であるか或いは光の波長より大きく且つ光の透過を阻害しない程度の粒径を有する無機質粒子や、微粉末の金属粒子が使用される。かかる光拡散剤１ｃとしては、例えばアクリルビーズ、スチレンビーズ、ベンゾグアナミン等の有機ポリマー微粒子、シリカ、マイカ、合成マイカ、炭酸カルシウム、炭酸マグネシウム、硫酸バリウム、タルク、モンモリロナイト、カオリンクレー、ベントナイト、ヘクトライト、ウィスカー、ガラスビーズ等の無機粒子、酸化チタン、酸化亜鉛、アルミナ等の金属酸化物微粒子などが、それぞれ単独で又は二種以上組み合わせて使用される。
【００２４】
上記の光拡散剤１ｃは、その平均粒径が０．１〜１００μｍ、好ましくは０．５〜５０μｍ、最も好ましくは１〜１５μｍであるものが使用される。粒径が０．１μｍより小さいと、凝集しやすいために分散性が悪く、均一に分散できたとしても光の波長の方が大きくて散乱効率が悪くなる。そのために０．５μｍ程度以上の、更には１．０μｍ以上の大きさの粒子が好ましいのである。また、粒径が１００μｍより大きいと、光散乱が不均一になるし、光線透過率の低下や粒子が見えたりするので好ましくないし、粒子の数が少なくなり拡散が悪くなる。そのため、５０μｍ以下の大きさの、更には１５μｍ以下の大きさの粒子が好ましいのである。
【００２５】
光拡散剤１ｃの含有量は０．０５〜５０重量％、好ましくは０．１〜５０重量％、最も好ましくは１０〜４０重量％程度とするのが良いが、使用する光拡散剤の種類と光拡散シートに要求される品質に応じて適宜変更される。０．０５重量％より少なくなると、光拡散効率が期待できないし、熱伸縮抑制効果も期待できなくなる。そのため、０．１重量％以上の、更には１０重量％以上の含有量が好ましいのである。一方、含有量が５０重量％より多くなると、粒子による吸収・反射で光の透過量が少なくなり、光拡散シートを通して表示が見えにくくなったりして機械的強度等の品質の低下を生じ、使用に耐えなくなる。そのため、４０重量％以下にするのが好ましいのである。
【００２６】
光拡散剤１ｃとしてシリカを使用する場合は、その平均粒径が０．５〜５０μｍのシリカを０．１〜４０重量％、好ましくは平均粒径が１〜１５μｍのシリカを３〜３５重量％均一に含有させるのが好ましい。このシリカを含有した光拡散シート１は、その全光線透過率が光拡散剤を含まないシートと略同じとなり、且つ、ヘーズ値が高くなり、光を良く透過する隠蔽性に優れた光拡散シートとして、液晶ディスプレイのバックライトユニットや電飾看板やアーケードや採光板やバルコニーの目隠し板として使用できる。
【００２７】
また、光拡散剤１ｃとしてタルクを使用する場合は、その平均粒径が０．５〜５０μｍのタルクを５〜５０重量％、好ましくは平均粒径が１〜１５μｍのタルクを１０〜４０重量％均一に含有させるのが好ましい。このタルクを含有した光拡散シート１０は、その熱伸縮が大幅に抑制されて、光源４の熱により光拡散シート１が熱せられても伸びが小さくなり、たとえ光拡散シート１を固定して使用しても長期間にわたり皺の発生を抑えることができる。ただ、ヘイズ値がシリカを含有したシートと比較すれば若干劣るが、液晶ディスプレイのバックライトユニットの光拡散シートとしてはなんら支障はない。
【００２８】
また、光拡散剤１ｃとしてタルクとシリカとを併用する場合は、その合計含有量が５〜５０重量％、好ましくは１０〜４０重量％となるようにすると共に、合計含有量に占めるタルクの割合が５０重量％以上となるように５〜３０重量％含有させるのが好ましい。この併用する光拡散剤１ｃは、平均粒径が０．５〜５０μｍのタルクを５〜３０重量％及び平均粒径が０．１〜５０μｍのシリカを２〜１５重量％、好ましくは平均粒径が１．０〜２０μｍのタルクを１０〜２０重量％及び平均粒径が１〜２０μｍのシリカ拡散剤を５〜１２重量％均一に含有させることが好ましい。このタルクとシリカを共に含有する光拡散シートは、タルクにより熱伸縮が大幅に抑制されると共にシリカにより光拡散効果が相乗されて、全光線透過率とヘイズ値が良好で且つ熱伸縮が小さなシートとなり、光源４の熱により熱せられても伸びが小さくて、たとえ光拡散シート１が固定されていても皺の発生をなくすことができる。
【００２９】
また、光拡散剤１ｃとしてタルクと炭酸カルシウムとを併用する場合は、その合計含有量が５〜５０重量％、好ましくは１０〜４０重量％となるようにすると共に、合計含有量に占めるタルクの割合が５０重量％以上となるように５〜３０重量％含有させるのが好ましい。この併用する光拡散剤１ｃは、その平均粒径が０．５〜５０μｍのタルクを５〜３０重量％及び平均粒径が０．１〜５０μｍの炭酸カルシウムを２〜１５重量％、好ましくは平均粒径が１〜２０μｍのタルクを１０〜２５重量％及び平均粒径が０．５
〜２．０μｍの炭酸カルシウムを５〜１２重量％均一に含有させることが好ましい。このタルクと炭酸カルシウムを含有した光拡散シート１は、タルクにより熱伸縮が大幅に抑制されると共に炭酸カルシウムにより成形性が向上し、その相乗効果によって熱伸縮が小さく皺の発生し難いシートを生産性良く製造することができる。
【００３０】
このような光拡散シート１は、その全光線透過率が９０％以上（厚さ１３０μｍ）、ヘイズ値も９０％以上となり、光を良く拡散させながら透過させるシートとすることができる。なお、全光線透過率及びヘイズ値は日本工業規格ＪＩＳ
Ｋ　７１０５に基づいて測定した値である。
【００３１】
斯かる光拡散シート１は、例えば、原料の熱可塑性樹脂（必要に応じて各種添加剤を配合したもの）に光拡散剤１ｃを添加した配合組成物をフィルムないしシート状に押出成形したのち、シボの細かさが異なる上下のシボ付けロールでシート両面に凹凸を形成する方法により、効率良く製造することができる。その他、フィルムやシートを凹凸を有するプレス板で押圧して形成したり、塗料の塗布後に該塗料層を上下のシボ付けロールに挟んで凹凸を形成したりする等、公知の方法が採用される。尚、シートの厚さは限定されないが、液晶ディスプレイのバックライトユニットに用いる光拡散シートの場合は、０．０２５〜１ｍｍ程度の厚さに成形することが好ましい。
【００３２】
上記の光拡散シート１を図１に示すように導光板２の上に重ね、その上にレンズフィルム３を重ねて、導光板２側部の光源４から光を導光板２に入射すると、光拡散シート１の片面１ａの凹凸が入光に適した高低差（大きさ）及び分布密度になっているため、導光板２の内部を適度に反射、出光しながら進む光がシート１の片面１ａ全体から略均等に光拡散シート１内へ入り、光損失が少なくなると共に、入光量の部分的なバラツキも殆ど生じなくなる。そして、光拡散シート１内に入射した光は、その内部に含有された光拡散剤１ｃにより更に均一に拡散されつつシート１内を進む。反対面１ｂに達した光は、その反対面１ｂの凹凸が、上記片面１ａの凹凸よりも細かく密に分布して光の散乱に適した凹凸の大きさ及び分布密度となっているため、この反対面１ｂの凹凸によって充分に拡散され、より均一な拡散光がレンズフィルム３の方へ放出される。従って、導光板２表面のドットが見えたり、部分的な輝度のバラツキを生じることはなくなる。なお、５は反射シートであって、導光板２から下方に出光する光を再度導光板２内に入光させるためのものである。そして、光拡散シート１は、光源からの光により熱し続けられるが、その内部の光拡散剤によりシートの線膨張率が低下し、剛性が向上しているので、例えシート１を固定していても伸縮による皺が発生せず、長期に亘り安定した拡散光が放出される。
【００３３】
なお、シート片面１ａの平均面粗さＲａが反対面１ｂのそれより小さくなって０．２μｍを下回り、シート片面１ａの表面積率（Ｓ／Ｓ_０
）が反対面１ｂのそれより大きくなって１．１５０を越える場合は、シート片面１ａでの乱反射が増して導光板２端面からの光の散逸などが起こり、シート１への入光量が減少して、輝度が低下する。また、シート片面１ａの平均面粗さＲａが０．２μｍを下回り、表面積率（Ｓ／Ｓ_０
）が１．００１を下回る場合は、該片面１ａと導光板２との界面の空気層が極めて薄くなり、光源４から導光板２へ進んだ光がほとんど正反射せず、正反射によって遠方へ伝播されず、また光の干渉や回折などによる光学欠陥が生じるので、導光板２の光源４に近い部分から多くの光がシート１に入光してその部分の輝度が高くなるが、逆に、導光板２の光源から遠い部分からは僅かの光しかシート１に入光せずその部分の輝度が低下するため、全体に亘って輝度のバラツキを生じる。さらに導光板と密着しすぎるので、光の干渉などによる色のにじみなど表示品位の低下が起こる。
【００３４】
一方、シート１の反対面１ｂの平均面粗さＲａが２．０μｍより大きくなり、表面積率（Ｓ／Ｓ_０　）が１．０１０より小さくなると、光拡散される起点が少なくなるため、拡散光成分の偏在が起こり均一な面発光が難しくなる。
【００３５】
また、上記光拡散シート１の表面の凹凸により、シート１に含有される光拡散剤１ｃが脱落し難くなっている。即ち、光拡散剤１ｃは凹凸の傾斜する側面より例え突出していても、レンズフィルムや導光板には凸頂部が接して光拡散剤が直接接することがないために、脱落することがないのである。
【００３６】
図２は本発明の光拡散シートの他の実施形態のシートの断面図である。
【００３７】
この光拡散シート１０は、光拡散層１２の表裏両面に透光性樹脂表面層１１、１１を積層一体にしたものである。光拡散層１２は前記実施形態の光拡散シート１と同じく、透光性樹脂に光拡散剤１ｃを含有させてなるものであるが、その光拡散層１２の表面に凹凸が形成されていない点で異なる。その他は光拡散シート１と同様であるので、その説明を省略する。
【００３８】
表面層１１、１１は、その厚みを１〜１０μｍとされ、光拡散層１２に使用したのと同じ透光性樹脂或いは他の透光性樹脂が用いられる。そして、表面層１１には光拡散剤が全く含有されていないことが好ましいが、光拡散剤１ｃを光拡散層１２の含有量より少なくなるように含有させても良い。
【００３９】
そして、この表面層１１の表面１ａ、１ｂには凹凸が形成されていて、その凹凸は前記実施形態と同様の平均面粗さと表面積率とを有している。即ち、入射面となる片面１ａの平均面粗さＲａを出光面となる反対面１ｂの平均面粗さＲａよりも大きくし、且つ、該片面１ａの表面積率（Ｓ／Ｓ_０
）を反対面１ｂの表面積率（Ｓ／Ｓ_０　）よりも小さくしてある。この片面１ａの平均面粗さＲａは０．２〜５．０μｍ、反対面１ｂの平均面粗さＲａは０．２〜２．０μｍ、片面の表面積率（Ｓ／Ｓ_０
）は１．００１〜１．１５０、反対面１ｂの表面積率（Ｓ／Ｓ_０　）は１．０１０〜１．２５０としてある。
【００４０】
このような光拡散シート１０を作製するには、透光性樹脂と光拡散剤１ｃとよりなる光拡散層用配合物と透光性樹脂よりなる表面層用配合物を共押出し成形技術を用いて共押出しし、シボの細かさが異なる上下のシボ付けロールでシートの両表面に凹凸を形成する方法により、効率良く製造することができる。
この光拡散シート１０は、その押出し成形時に、共押し金型の吐出口の汚染がしにくいために、効率良く生産することができる点で優れている。
【００４１】
また、この光拡散シート１０の表面には、光拡散剤１ｃが全く含まれないか或いは光拡散層より少ないので、光拡散剤の脱落をなくすことができる。この効果は、表面層１１の凹凸により更に助長される。
【００４２】
なお、図２において、表面の凹凸は表面層１１にのみ形成されているが、表面層１１の厚みが薄く光拡散剤１ｃの粒径が大きい場合は、表面層１１は勿論のこと、光拡散層１２にまで達する凹凸となることは当然である。
【００４３】
次に、本発明の更に具体的な実施例を説明する。
【００４４】
〔シボ付けロールの決定〕
ポリプロピレン樹脂に対して、平均粒径が８μｍのシリカ光拡散剤（富士シリアル化学（株）製、サイロフォービック４００４）を１０．０重量％添加し均一に混合した後、暑さ１１０μｍのシート状に押出し成形し、シボの細かさが異なるシボ付けロールを用いて、シートの両面に異なる凹凸を有するシートを作製した。
【００４５】
このシートについて、ヘイズメーター［スガ試験機（株）株式会社製　ＨＧＭ−２ＤＰ］を用いて全光線透過率とヘイズ値を測定したところ、全光線透過率は１００％、ヘイズ値は９２．３％であった。また、ＷＹＫＯ表面形状測定装置ＮＴ−２０００［ＷＹＫＯ（株）製］を使用し、２３０．６×１７５．４μｍの測定範囲で中心線平均面粗さＲａを測定したところ、片面の平均面粗さは１．５０３、反対面の平均面粗さは１．１２６であった。さらに、プローブ顕微鏡［セイコーインスツルメンツ（株）製のＮａｎｏｐｉｃｓ１０００コントローラとＮＰＸ１００ヘッドを使用］を用いて、上記のシートについて、４００×４００μｍの測定範囲で表面積を測定したところ、片面の表面積率は１．０５６、反対面の表面積率は１．１５３であった。
【００４６】
そして、該シートを液晶ディスプレイ用のバックライトユニットの導光板上に載置し、さらに液晶パネルを載置した。光源を点灯し、導光板裏面のドットが隠蔽されるかどうかも目視で観察したところ、ドットを観察することはできなかつた。
【００４７】
この結果より、該シートは、光拡散シートとしての平均面粗さと表面積率とを有することがわかる。従って、本テストに用いたシボ付けロールは光拡散シートを作製するために有用であることがわかった。
【００４８】
［実施例１、２］
ポリプロピレン樹脂に対して、平均粒径が８μｍのシリカ光拡散剤（富士シリシア化学（株）製、サイロフォービック４００４）を、下記の表１に示すように１６重量％、２１重量％添加し均一に混合した後、厚さ１３０μｍのシート状に押出成形し、上記シボ付きロールの決定に用いたのと同じシボ付けロールを用いて、シート両面に凹凸を有する光拡散シートを作製した。
【００４９】
上記の光拡散シートについて、ヘイズメーター［スガ試験機（株）製　ＨＧＭ−２ＤＰ］を用いて全光線透過率とヘイズ値を測定し、その結果を表１に記載した。
【００５０】
さらに、その線膨脹率（２０〜２５℃）を理学電機（株）製の熱機械分析装置ＴＭＡ−８１４０Ｃにて測定（荷重５ｇの引張りモードで、５℃／分の昇温速度）すると共に、２０℃での引張り弾性率を測定し、その結果を表１に併記した。この引張り弾性率は、セイコーインスツルメンツ（株）製の動的粘弾性装置ＤＭＳ６１００で測定した貯蔵弾性率を示す。
【００５１】
また、各光拡散シートを液晶ディスプレイ用のバックライトユニットの導光板上に載置し、光源を点灯し、光拡散シートから２０ｃｍの距離にミノルタ（株）製の輝度計ｎｔ−１°ｐを置いて輝度を測定した。同時に導光板裏面のドットが隠蔽されるかどうかも目視で観察した。また、各光拡散シートを一定寸法（１９．６ｃｍ×１４．０ｃｍ）に切断し、その一側部の幅方向中央と他側部の幅方向両端の３点を固定した状態で、温度６０℃、湿度９０％の条件に保たれた恒温恒湿装置内に放置し、光拡散シートの状態を２日毎に目視で観察して皺が発生した時間を求めた（皺発生時間試験）。それらの結果について、下記の表１に併記する。
【００５２】
［比較例１］
実施例１と同じポリプロピレン樹脂を使用し、光拡散剤を全く含まない同じ厚さのシートを実施例１と同様に作製して、比較例１とした。この比較例１についても、実施例１と同様に、全光線透過率、ヘイズ値、線膨張率、輝度、ドットの隠蔽性、皺発生時間試験、引張り弾性率の各測定を行なった。その結果を表１に併記する。
【００５３】
［実施例３〜５］
実施例１で使用したポリプロピレン樹脂を用い、これに対して、平均粒径５μｍのタルク（日本タルク株式会社製、ミクロエース
Ｋ−１）を３０重量％添加し均一に混合した配合物、及び平均粒径８μｍの前記タルクを１５重量％、３０重量％それぞれ添加し均一に混合した配合物を、それぞれ厚さ１３０μｍのシート状に押出成形し、実施例１で使用したシボ付きロールを用いて、シート両面に凹凸を有する光拡散シートを作製した。
これらの光拡散シートの全光線透過率、ヘイズ値、線膨張率、輝度、ドットの隠蔽性、皺発生時間試験、引張り弾性率を、実施例１と同様に行なった。その結果を表１に併記する。
【００５４】
［実施例６、７］
実施例１で使用したポリプロピレン樹脂を用い、これに対して、実施例４で用いた平均粒径８μｍのタルクを１６重量％と実施例１で用いた平均粒径８μｍのシリカを８重量％をそれぞれ添加し均一に混合した後、厚さ１３０μｍのシート状に押出成形し、実施例１で使用したシボ付きロールを用いて、シート両面に凹凸を有する光拡散シートを作製し、実施例６とした。
また、実施例１で使用したポリプロピレン樹脂を用い、これに対して、実施例４で用いた平均粒径８μｍのタルクを１０重量％と平均粒径０．８μｍの炭酸カルシウムを５重量％それぞれ添加し均一に混合した後、厚さ１３０μｍのシート状に押出成形し、実施例１で使用したシボ付きロールを用いて、シート両面に凹凸を有する光拡散シートを作製し、実施例７とした。
これらの光拡散シートの全光線透過率、ヘイズ値、線膨張率、輝度、ドットの隠蔽性、皺発生時間試験、引張り弾性率を、実施例１と同様に行なった。その結果を表１に併記する。
【００５５】
【表１】

【００５６】
この表１を見ると、全光線透過率は実施例１〜７の光拡散シートも比較例１の光拡散シートも１００％と同じ値を示した。一方、ヘイズ値は、比較例１のシートが８５．５％であるのに対し、実施例１、２、３、６、７の各シートは９２．８〜９３．８％で、７．３〜８．３％も高くなっており、光を良く透過する隠蔽性に優れた光拡散シートであることがわかった。また、実施例４、５はヘイズ値が他の実施例と比較して低いが、実施例４は比較例１より５％も高く、実施例５は比較例１と略同じヘイズ値を示し、光拡散剤の添加によるヘイズ値の低下は見られなかった。
【００５７】
また、線膨張率は実施例１〜７の光拡散シートは比較例１と比較して３．１〜９．０×１０^−５／℃も大幅に低下し、熱で熱せられても伸びが小さく皺が発生しにくいシートであることがわかる。特に、タルクを使用した実施例３〜７は、シリカを使用した実施例１、２と比べて大きく低下しており、過酷な条件下においても皺が発生しにくいことがわかる。さらに、恒温恒湿装置での皺発生時間試験では、皺の発生時間が比較例１の２４時間に比べて９６〜４３２時間と大幅に遅くなっており、大きく改善されていることがわかる。そのうちでも、平均粒径が８μｍのタルクを用いた実施例４〜６は、線膨張率においても皺発生時間試験においても他の実施例より優れた数値を示しており、タルクが他の光拡散剤より優れた光拡散剤であることがわかる。特に実施例５の平均粒径８μｍのタルクを３０重量％含有したシートは４３２間放置した時に皺が発生しており、比較例１の約１８倍もの長時間の間、皺が発生しないことがわかる。従って、平均粒径８μｍのタルクを使用した実施例４〜６、特に実施例５はさらに過酷な実使用においても皺の発生がないことがわかる。
【００５８】
また、輝度は平均粒径が８μｍの光拡散剤を使用した実施例１、２、４、５、６は、比較例１と同じ程度の値を示していて、光損失の少ない明るいシートであることがわかる。
さらに、ドットの隠蔽性は、タルクを使用した実施例３〜７がドットの隠蔽性に優れているのに対して、比較例１、実施例１、２はドットの隠蔽性に劣り、光拡散剤の中でもタルクが隠蔽性に優れていることがわかる。
【００５９】
以上の結果より、光拡散剤の内でも、タルクを使用した光拡散シートは熱による伸縮が小さくて皺の発生がなく、隠蔽性にも優れたシートであり、液晶ディスプレイのバックライトユニットの光拡散シートとして特に有用である。また、シリカを使用したシートはヘイズ値が高く、光学特性を特に要求される光拡散シートに有用である。
【００６０】
【発明の効果】
以上の説明から明らかなように、本発明の光拡散シートは、全光線透過率が高くヘイズ値も高いので、光を良く透過する隠蔽性に優れたシートであるうえに、シートが熱せられても伸縮しにくくし皺の発生を抑制することができ、液晶ディスプレイのバックライトユニットなどに好適に使用できる。
特に、光拡散剤としてタルクを使用した光拡散シートは、長期間にわたり皺が発生せず、液晶ディスプレイのバックライトユニットの光拡散シートとして使用できる。
【図面の簡単な説明】
【図１】本発明の一実施形態に係る光拡散シートの断面図である。
【図２】本発明の他の実施形態に係る光拡散シートの断面図である。
【符号の説明】
１，１０　光拡散シート
１ａ　入光面となる片面（下面）
１ｂ　出光面となる反対面（上面）
１ｃ　光拡散剤
２　導光板
３　レンズフィルム（プリズムシート）
４　光源[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light diffusion sheet used for a backlight unit of a liquid crystal display, an illuminated signboard, a lighting cover, an arcade, a lighting plate, a blindfold plate of a balcony, and the like.
[0002]
[Prior art]
A general backlight unit of a liquid crystal display includes a light guide plate having dots for light diffusion printed on a back surface thereof, a light source (a cold cathode tube or the like) disposed on one or both sides of the light guide plate, and a light guide plate. It is composed of a light diffusion sheet stacked on top, a lens film (prism sheet) stacked on or above or below this light diffusion sheet.
[0003]
The light diffusion sheet incorporated in such a backlight unit uniformly diffuses light from the light guide plate, prevents dots from being seen on the display screen, suppresses light loss, and uniformly emits diffused light to the liquid crystal panel surface. It plays the role of doing.
[0004]
Examples of such a light diffusing sheet include (1) a sheet in which a light diffusing layer containing polymer beads or inorganic fine particles as a light diffusing agent is provided on at least one surface of a transparent substrate (Japanese Patent No. 2665301); One or both surfaces of the film are embossed to form irregularities, and a light diffusion layer containing fine particles is provided on one or both surfaces (JP-A-11-337711). (3) No light diffusing agent is contained. A sheet having random irregularities on its surface (Japanese Patent No. 2562265) is known.
[0005]
[Problems to be solved by the invention]
However, the light diffusing sheet of (1) is sharp because polymer beads and inorganic fine particles protruding from the surface of the light diffusing layer easily damage the lens film superimposed thereon or fall off from the light diffusing layer due to impact or the like. There were problems such as insufficient degree of display, deterioration of display quality, and poor production yield and high manufacturing cost.
[0006]
Although the light diffusing sheet of (2) improves the diffusivity by the unevenness formed on one or both surfaces, the light diffusing layer containing fine particles is provided on the surface, so the light diffusing sheet of (1) There is a problem similar to that of the sheet, and furthermore, there is a problem that the production cost is further increased because two steps of the step of forming unevenness by embossing and the step of forming the light diffusion layer are required.
[0007]
Further, in the light diffusing sheet of (3), if the unevenness of the surface formed on one or both sides is inappropriate, light scattering is insufficient or light scattering becomes uneven and brightness partially varies. However, there is a problem that dots on the light guide plate surface are visible. Further, there is also a problem in that the light diffuser sheet is heated by light and heat from the light source, causing wrinkles in the light diffuser sheet, resulting in non-uniform light diffusion and uneven brightness.
[0008]
Therefore, the present applicant sets the average surface roughness of one surface, which is a light incident surface, of the sheet larger than the average surface roughness of the opposite surface, which is a light output surface, and the surface area ratio of the one surface is larger than the surface area ratio of the opposite surface. Proposed a light diffusing sheet made of a light-transmitting resin, which solved the problem (3). However, when the sheet is used as a light diffusion sheet for a backlight unit of a liquid crystal display, a new problem arises in that the sheet is heated by the heat of the light source and the sheet is stretched and wrinkled.
[0009]
The present invention has been made in view of the above-described problems, and has as its object the purpose of preventing wrinkles from occurring even when heated by light and heat from a light source, reducing light loss, and ensuring stable and uniform light. An object of the present invention is to provide a light diffusion sheet for performing diffusion.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, the light diffusing sheet of the present invention has irregularities formed on both surfaces of the sheet, and the average surface roughness of one surface serving as a light incident surface is larger than the average surface roughness of the opposite surface serving as a light emitting surface. A sheet made of a light-transmitting resin containing a diffusing agent, wherein one surface has an average surface roughness of 0.2 to 5.0 μm, and the opposite surface has an average surface roughness of 0.2 to 2.0 μm. It is characterized by the following.
[0011]
Here, the “average surface roughness” is a three-dimensional extension of the center line average surface roughness Ra defined in Japanese Industrial Standards JIS B 0601, which can be applied to the measurement surface. This is a value obtained by averaging the absolute values of the deviations from the reference plane to the specified plane, and is calculated by the following equation.
(Equation 1)

[0012]
Where Ra is the average surface roughness, S₀Is the reference area of the measurement surface, F (X, Y) is JIS
A roughness curve obtained by developing f (x) on a surface defined by B 0601, Z₀
Indicates the height of the reference plane.
[0013]
This light diffusion sheet has an average surface roughness of 0.2 to 5.0 μm on one surface serving as an incident surface of the sheet, and an average surface roughness of 0.2 to 5.0 μm on the opposite surface serving as a light emitting surface. When the light diffusing sheet is overlaid on, for example, a light guide plate of a backlight unit, most of the light traveling while appropriately reflecting inside the light guide plate is substantially from one side of the sheet. Since the light enters the sheet evenly, light loss is small and there is almost no partial variation in the amount of incident light. Further, light entering the light diffusion sheet from one side is also diffused by the light diffusion agent, so that the light diffusion action is improved. And, even with the unevenness on the opposite surface, it is possible to emit diffused light without variation in luminance while uniformly and sufficiently diffusing light. Moreover, the thermal expansion and contraction of the sheet is reduced by the light diffusing agent, and the sheet does not wrinkle even when heated by the light from the light source, so that uniform diffused light can be stably emitted.
[0014]
The light diffusing agent has an average particle size of 0.5 to 50 μm, and when contained in the translucent resin in an amount of 0.1 to 50% by weight, diffusion of light is not hindered. It will be even better. Further, the expansion and contraction of the sheet due to the heat from the light source is reduced, and the generation of wrinkles can be sufficiently suppressed, and stable diffused light can be obtained.
[0015]
When the surface area ratio of one side of the light diffusing sheet is 1.001 to 1.150 and the surface area ratio of the opposite side is 1.010 to 1.250, incident light becomes more uniform and diffused light becomes more uniform. This is preferable because there can be no variation in luminance. The average surface roughness of the one surface is 1.0 to 3.0 μm, the average surface roughness of the opposite surface is 0.5 to 1.5 μm, and the surface area ratio of the one surface is 1.030 to 1.100. It is preferable that the surface area ratio of the opposite surface is 1.050 to 1.200, because the above-mentioned effect is further improved.
[0016]
Here, the “surface area ratio” refers to the area S assuming that the measurement surface is a flat surface.₀Ratio of the actual surface area S to (S / S₀
).
[0017]
When talc having an average particle size of 1.0 to 15 μm is contained in the light-transmitting resin as a light diffusing agent in 10% to 40% by weight, wrinkles do not occur even when heated by light from a light source for a long time. It is preferred.
Further, the light-transmissive resin contains 10 to 40% by weight of a light diffusing agent composed of talc and another light diffusing agent, and the talc is contained in 5 to 30% by weight of the total light diffusing agent. However, wrinkles do not occur even when heated by light from a light source for a long period of time, which is preferable.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.
[0019]
FIG. 1 is a sectional view of a first light diffusion sheet according to one embodiment of the present invention.
[0020]
The light diffusing sheet 1 is a sheet made of a light diffusing agent-containing translucent resin having irregularities formed on both sides 1a and 1b of the sheet. Examples of the light-transmitting resin include polycarbonate, polyester, polyethylene, polypropylene, polyolefin copolymer (eg, poly-4-methylpentene-1), polyvinyl chloride, and cyclic polyolefin (eg, cyclopolyolefin) having high total light transmittance. Thermoplastic resins such as acrylic resin, polystyrene, and ionomer are preferably used. In particular, since polypropylene has good heat resistance and is soft, when incorporated in a liquid crystal display, it can reduce deformation of a light source against heat radiation. However, it is preferably used because it does not damage the upper lens film (prism sheet). In addition, the cyclic polyolefin is very preferably used because it has excellent transparency and a small coefficient of linear expansion, so that a sheet having excellent optical properties and little thermal deformation can be obtained. The translucent resin refers to the total light transmittance (JIS) of a resin sheet having a thickness of 0.1 mm.
K (according to 7105) is 50% or more.
[0021]
The unevenness formed on one surface (lower surface) 1a serving as the light incident surface of the light diffusion sheet 1 has a greater difference in height than the unevenness formed on the opposite surface (upper surface) 1b serving as the light emitting surface, and the distribution density is coarse. I have. That is, this one side 1a has an average surface roughness Ra larger than that of the opposite side 1b and is in the range of 0.2 to 5.0 μm, and the surface area ratio (S / S₀
) Is smaller than that of the opposite surface 1b and ranges from 1.001 to 1.150.
[0022]
On the other hand, the irregularities formed on the opposite surface (upper surface) 1b serving as the light emitting surface are distributed more finely and densely than the irregularities formed on the one surface 1a, and the average surface roughness Ra is greater than that of the one surface 1a. And the surface area ratio (S / S₀) Is in the range of 1.010 to 1.250, which is larger than that of one side 1b.
[0023]
The light diffusing agent 1c contained in the light diffusing sheet 1 is for improving light diffusibility and suppressing thermal expansion and contraction of the sheet 1 to eliminate wrinkles. There is no difference between the refractive index of the light-transmitting resin and the light-transmitting resin, or the difference in the refractive index is 0.1 or less, or is larger than the wavelength of light, and the light transmission is different. Inorganic particles having a particle size that does not hinder the reaction, and fine metal particles are used. Examples of the light diffusing agent 1c include organic polymer fine particles such as acrylic beads, styrene beads, benzoguanamine, silica, mica, synthetic mica, calcium carbonate, magnesium carbonate, barium sulfate, talc, montmorillonite, kaolin clay, bentonite, hectorite, Inorganic particles such as whiskers and glass beads, metal oxide fine particles such as titanium oxide, zinc oxide and alumina are used alone or in combination of two or more.
[0024]
The light diffusing agent 1c has an average particle diameter of 0.1 to 100 μm, preferably 0.5 to 50 μm, and most preferably 1 to 15 μm. If the particle size is smaller than 0.1 μm, the particles are easily aggregated and thus have poor dispersibility. Even if the particles can be uniformly dispersed, the wavelength of light is large and the scattering efficiency is poor. Therefore, particles having a size of about 0.5 μm or more, and more preferably 1.0 μm or more are preferred. On the other hand, if the particle size is larger than 100 μm, light scattering becomes non-uniform, the light transmittance is reduced, and particles are seen, which is not preferable. For this reason, particles having a size of 50 μm or less, more preferably 15 μm or less, are preferred.
[0025]
The content of the light diffusing agent 1c is preferably about 0.05 to 50% by weight, preferably about 0.1 to 50% by weight, and most preferably about 10 to 40% by weight. It is appropriately changed according to the quality required for the light diffusion sheet. If the amount is less than 0.05% by weight, the light diffusion efficiency cannot be expected, and the effect of suppressing thermal expansion and contraction cannot be expected. Therefore, the content is preferably 0.1% by weight or more, and more preferably 10% by weight or more. On the other hand, if the content is more than 50% by weight, the amount of transmitted light decreases due to absorption and reflection by particles, and the display becomes difficult to see through a light diffusion sheet, resulting in a decrease in quality such as mechanical strength. Become intolerable. Therefore, the content is preferably set to 40% by weight or less.
[0026]
When silica is used as the light diffusing agent 1c, silica having an average particle size of 0.5 to 50 μm is 0.1 to 40% by weight, preferably silica having an average particle size of 1 to 15 μm is 3 to 35% by weight. It is preferred to make it uniformly contained. The light-diffusing sheet 1 containing silica has substantially the same total light transmittance as a sheet containing no light-diffusing agent, has a high haze value, and is excellent in concealing properties for transmitting light well. It can be used as a backlight unit of a liquid crystal display, an illuminated signboard, an arcade, a daylighting board, or a blindboard of a balcony.
[0027]
When talc is used as the light diffusing agent 1c, talc having an average particle size of 0.5 to 50 μm is 5 to 50% by weight, preferably talc having an average particle size of 1 to 15 μm is 10 to 40% by weight. It is preferred to make it uniformly contained. The light diffusion sheet 10 containing this talc has its thermal expansion and contraction greatly suppressed, and its elongation is reduced even if the light diffusion sheet 1 is heated by the heat of the light source 4. Even in such a case, generation of wrinkles can be suppressed for a long period of time. Although the haze value is slightly inferior to that of a sheet containing silica, there is no problem as a light diffusion sheet for a backlight unit of a liquid crystal display.
[0028]
When talc and silica are used in combination as the light diffusing agent 1c, the total content thereof is 5 to 50% by weight, preferably 10 to 40% by weight, and the proportion of talc in the total content. Is preferably 5 to 30% by weight so as to be 50% by weight or more. The light diffusing agent 1c to be used in combination is 5 to 30% by weight of talc having an average particle size of 0.5 to 50 μm and 2 to 15% by weight of silica having an average particle size of 0.1 to 50 μm, and preferably 2 to 15% by weight. However, it is preferable to uniformly contain 10 to 20% by weight of talc having a particle size of 1.0 to 20 μm and 5 to 12% by weight of a silica diffusing agent having an average particle size of 1 to 20 μm. The light diffusion sheet containing both talc and silica is a sheet that has a good total light transmittance and a haze value and a small thermal expansion because the thermal expansion and contraction is greatly suppressed by talc and the light diffusion effect is synergized by silica. Thus, even when heated by the heat of the light source 4, the elongation is small, and even if the light diffusion sheet 1 is fixed, it is possible to eliminate the generation of wrinkles.
[0029]
When talc and calcium carbonate are used in combination as the light diffusing agent 1c, the total content thereof is 5 to 50% by weight, preferably 10 to 40% by weight. It is preferable to contain 5 to 30% by weight so that the ratio becomes 50% by weight or more. The light diffusing agent 1c to be used in combination is 5 to 30% by weight of talc having an average particle size of 0.5 to 50 μm and 2 to 15% by weight of calcium carbonate having an average particle size of 0.1 to 50 μm, and preferably 2 to 15% by weight. 10 to 25% by weight of talc having a particle size of 1 to 20 μm and an average particle size of 0.5
It is preferred that calcium carbonate having a thickness of 2.0 μm is uniformly contained in a content of 5 to 12% by weight. The light diffusion sheet 1 containing talc and calcium carbonate produces a sheet in which thermal expansion and contraction is greatly suppressed by talc and the formability is improved by calcium carbonate, and the thermal expansion and contraction is small due to the synergistic effect and the wrinkles are hardly generated. It can be manufactured with good performance.
[0030]
Such a light diffusion sheet 1 has a total light transmittance of 90% or more (thickness 130 μm) and a haze value of 90% or more, and can be a sheet that transmits light while diffusing light well. In addition, the total light transmittance and the haze value are based on Japanese Industrial Standard JIS.
It is a value measured based on K 7105.
[0031]
Such a light diffusing sheet 1 is formed, for example, by extruding a blended composition in which a light diffusing agent 1c is added to a raw material thermoplastic resin (comprising various additives as necessary) into a film or sheet. Efficient production can be achieved by a method of forming irregularities on both sides of the sheet using upper and lower crimping rolls having different grain sizes. In addition, known methods such as forming a film or sheet by pressing with a press plate having unevenness, or forming unevenness by sandwiching the paint layer between upper and lower crimping rolls after applying the paint are adopted. . The thickness of the sheet is not limited, but in the case of a light diffusion sheet used for a backlight unit of a liquid crystal display, it is preferable to form the sheet to a thickness of about 0.025 to 1 mm.
[0032]
When the light diffusing sheet 1 is overlaid on the light guide plate 2 as shown in FIG. 1, a lens film 3 is overlaid thereon, and light from the light source 4 on the side of the light guide plate 2 is incident on the light guide plate 2. Since the unevenness on one side 1a of the diffusion sheet 1 has a height difference (size) and distribution density suitable for light input, light traveling while appropriately reflecting and emitting light inside the light guide plate 2 is transmitted to the one side 1a of the sheet 1. The light enters the light diffusion sheet 1 substantially uniformly from the whole, and the light loss is reduced, and the partial variation of the incident light amount hardly occurs. The light that has entered the light diffusion sheet 1 travels through the sheet 1 while being more uniformly diffused by the light diffusion agent 1c contained therein. The light that has reached the opposite surface 1b has irregularities on the opposite surface 1b distributed more finely and densely than the irregularities on the one surface 1a, and has a size and distribution density of irregularities suitable for scattering light. The light is sufficiently diffused by the unevenness of the opposite surface 1b, and more uniform diffused light is emitted toward the lens film 3. Therefore, the dots on the surface of the light guide plate 2 are not seen, and a partial variation in luminance does not occur. Reference numeral 5 denotes a reflection sheet for allowing light emitted downward from the light guide plate 2 to enter the light guide plate 2 again. The light diffusion sheet 1 is continuously heated by the light from the light source. However, since the linear expansion coefficient of the sheet is reduced and the rigidity is improved by the light diffusing agent inside, the sheet 1 is fixed. No wrinkles due to expansion and contraction occur, and stable diffused light is emitted over a long period of time.
[0033]
In addition, the average surface roughness Ra of the sheet one surface 1a is smaller than that of the opposite surface 1b and is smaller than 0.2 μm, and the surface area ratio of the sheet one surface 1a (S / S₀
) Is greater than that of the opposite surface 1b and exceeds 1.150, diffuse reflection on one surface 1a of the sheet increases, light is scattered from the end surface of the light guide plate 2, and the amount of light entering the sheet 1 decreases. As a result, the brightness decreases. Further, the average surface roughness Ra of one side 1a of the sheet is less than 0.2 μm, and the surface area ratio (S / S₀
) Is less than 1.001, the air layer at the interface between the one surface 1a and the light guide plate 2 becomes extremely thin, and the light that has traveled from the light source 4 to the light guide plate 2 is hardly specularly reflected, and is distant by specular reflection. Since the light is not propagated and an optical defect occurs due to light interference or diffraction, a large amount of light enters the sheet 1 from a portion of the light guide plate 2 close to the light source 4 to increase the brightness of the portion. Since only a small amount of light enters the sheet 1 from a portion of the light guide plate 2 far from the light source, the brightness of that portion is reduced, so that the brightness varies over the whole. Furthermore, since it is too close to the light guide plate, display quality such as color bleeding due to light interference or the like is lowered.
[0034]
On the other hand, the average surface roughness Ra of the opposite surface 1b of the sheet 1 is larger than 2.0 μm, and the surface area ratio (S / S₀If) is less than 1.010, the starting point of light diffusion is reduced, and diffused light components are unevenly distributed, making uniform surface light emission difficult.
[0035]
Further, the light diffusing agent 1c contained in the sheet 1 is hard to fall off due to the unevenness of the surface of the light diffusing sheet 1. That is, even if the light diffusing agent 1c protrudes from the inclined side surface of the unevenness, the convex portion is in contact with the lens film or the light guide plate, and the light diffusing agent does not come into direct contact with the lens film or the light guide plate. .
[0036]
FIG. 2 is a sectional view of a sheet according to another embodiment of the light diffusion sheet of the present invention.
[0037]
The light diffusion sheet 10 is formed by integrally integrating light transmitting resin surface layers 11 on both front and back surfaces of a light diffusion layer 12. The light-diffusing layer 12 is made of a light-transmitting resin containing a light-diffusing agent 1c, similarly to the light-diffusing sheet 1 of the above-described embodiment, but the surface of the light-diffusing layer 12 has no irregularities. Different. The other parts are the same as those of the light diffusion sheet 1, and the description thereof is omitted.
[0038]
The surface layers 11 have a thickness of 1 to 10 μm, and the same light-transmitting resin as that used for the light diffusion layer 12 or another light-transmitting resin is used. It is preferable that the surface layer 11 does not contain any light diffusing agent, but the light diffusing agent 1c may be contained so as to be smaller than the content of the light diffusing layer 12.
[0039]
The surfaces 1a and 1b of the surface layer 11 are formed with irregularities, and the irregularities have the same average surface roughness and surface area ratio as those in the embodiment. That is, the average surface roughness Ra of the one surface 1a serving as the incident surface is made larger than the average surface roughness Ra of the opposite surface 1b serving as the light emitting surface, and the surface area ratio (S / S) of the one surface 1a is set.₀
) Is changed to the surface area ratio (S / S₀) Is smaller. The average surface roughness Ra of the one surface 1a is 0.2 to 5.0 μm, the average surface roughness Ra of the opposite surface 1b is 0.2 to 2.0 μm, and the surface area ratio of one surface (S / S₀
) Is 1.001 to 1.150, and the surface area ratio (S / S₀) Is 1.010 to 1.250.
[0040]
In order to produce such a light diffusion sheet 10, a compound for a light diffusion layer composed of a light-transmitting resin and a light diffusion agent 1c and a compound for a surface layer composed of a light-transmissive resin are co-extruded using a molding technique. The sheet can be efficiently manufactured by a method of co-extrusion and forming irregularities on both surfaces of the sheet with upper and lower crimping rolls having different grain sizes.
The light diffusion sheet 10 is excellent in that it can be efficiently produced because the discharge port of the co-pressing mold is not easily contaminated during extrusion molding.
[0041]
Further, since the light diffusing agent 1c is not contained at all in the surface of the light diffusing sheet 10 or less than the light diffusing layer, the light diffusing agent can be prevented from falling off. This effect is further promoted by the unevenness of the surface layer 11.
[0042]
In FIG. 2, the surface irregularities are formed only on the surface layer 11, but when the surface layer 11 is thin and the particle size of the light diffusing agent 1c is large, not only the surface layer 11 but also the light diffusion Naturally, the unevenness reaches the layer 12.
[0043]
Next, more specific examples of the present invention will be described.
[0044]
[Determination of textured roll]
After adding 10.0% by weight of a silica light diffusing agent (Silofovic 4004, manufactured by Fuji Serial Chemical Co., Ltd.) having an average particle size of 8 μm to the polypropylene resin, and mixing them uniformly, a sheet having a heat of 110 μm was formed. Then, a sheet having different irregularities on both sides of the sheet was prepared using a crimping roll having different grain fineness.
[0045]
The sheet was measured for total light transmittance and haze value using a haze meter [HGM-2DP manufactured by Suga Test Instruments Co., Ltd.]. The total light transmittance was 100% and the haze value was 92.3%. Met. The center line average surface roughness Ra was measured in a measurement range of 230.6 × 175.4 μm using a WYKO surface shape measuring device NT-2000 (manufactured by WYKO Co., Ltd.). Was 1.503, and the average surface roughness of the opposite surface was 1.126. Further, the surface area of the above sheet was measured using a probe microscope [using a Nanopics 1000 controller and NPX100 head manufactured by Seiko Instruments Inc.] in a measurement range of 400 × 400 μm, and the surface area ratio of one side was 1.056. And the surface area ratio of the opposite surface was 1.153.
[0046]
Then, the sheet was placed on a light guide plate of a backlight unit for a liquid crystal display, and a liquid crystal panel was further placed thereon. When the light source was turned on and the dots on the rear surface of the light guide plate were concealed visually, the dots could not be observed.
[0047]
From this result, it is understood that the sheet has an average surface roughness and a surface area ratio as a light diffusion sheet. Therefore, it was found that the embossing roll used in this test was useful for producing a light diffusion sheet.
[0048]
[Examples 1 and 2]
16% by weight and 21% by weight of a silica light diffusing agent (silophobic 4004 manufactured by Fuji Silysia Chemical Ltd.) having an average particle size of 8 μm were added to the polypropylene resin as shown in Table 1 below, and the mixture was uniformly mixed. Then, the mixture was extruded into a sheet having a thickness of 130 μm, and a light diffusion sheet having irregularities on both sides of the sheet was produced using the same embossing roll used in the determination of the embossed roll.
[0049]
For the above light diffusion sheet, the total light transmittance and the haze value were measured using a haze meter [HGM-2DP manufactured by Suga Test Instruments Co., Ltd.], and the results are shown in Table 1.
[0050]
Further, the coefficient of linear expansion (20 to 25 ° C.) was measured with a thermomechanical analyzer TMA-8140C manufactured by Rigaku Denki Co., Ltd. (5 ° C./min heating rate in a tensile mode with a load of 5 g). The tensile modulus at 20 ° C. was measured, and the results are shown in Table 1. The tensile elastic modulus indicates a storage elastic modulus measured by a dynamic viscoelasticity device DMS6100 manufactured by Seiko Instruments Inc.
[0051]
Each light diffusion sheet was placed on the light guide plate of a backlight unit for a liquid crystal display, the light source was turned on, and a luminance meter nt-1 ° p manufactured by Minolta Co., Ltd. was placed at a distance of 20 cm from the light diffusion sheet. The brightness was measured by placing the sample. At the same time, it was visually observed whether or not the dots on the back surface of the light guide plate were hidden. Further, each light diffusion sheet was cut into a fixed size (19.6 cm × 14.0 cm), and the temperature was 60 ° C. in a state where three points at the center in the width direction on one side and both ends in the width direction on the other side were fixed. The sheet was allowed to stand in a thermo-hygrostat maintained at a humidity of 90%, and the state of the light diffusion sheet was visually observed every two days to determine the time at which wrinkles occurred (wrinkle generation time test). The results are shown in Table 1 below.
[0052]
[Comparative Example 1]
Using the same polypropylene resin as in Example 1, a sheet having the same thickness and containing no light diffusing agent was produced in the same manner as in Example 1, and Comparative Example 1 was obtained. As in Comparative Example 1, each measurement of the total light transmittance, the haze value, the linear expansion coefficient, the luminance, the dot concealing property, the wrinkle generation time test, and the tensile elastic modulus was performed for Comparative Example 1. The results are also shown in Table 1.
[0053]
[Examples 3 to 5]
The polypropylene resin used in Example 1 was used, and talc having an average particle size of 5 μm (Microace manufactured by Nippon Talc Co., Ltd.) was used.
A sheet having a thickness of 130 μm was prepared by mixing 30% by weight of K-1) and uniformly mixing the mixture, and uniformly mixing 15% by weight and 30% by weight of the talc having an average particle diameter of 8 μm. A light diffusion sheet having irregularities on both sides of the sheet was produced using the roll with a grain used in Example 1.
The total light transmittance, haze value, linear expansion coefficient, luminance, dot concealability, wrinkle generation time test, and tensile modulus of these light diffusion sheets were performed in the same manner as in Example 1. The results are also shown in Table 1.
[0054]
[Examples 6 and 7]
Using the polypropylene resin used in Example 1, 16% by weight of talc having an average particle diameter of 8 μm used in Example 4 and 8% by weight of silica having an average particle diameter of 8 μm used in Example 1 were used. After each addition and uniform mixing, the mixture was extruded into a sheet having a thickness of 130 μm, and a light diffusion sheet having irregularities on both sides of the sheet was produced using the embossed roll used in Example 1. did.
The polypropylene resin used in Example 1 was used, and 10% by weight of talc having an average particle diameter of 8 μm and 5% by weight of calcium carbonate having an average particle diameter of 0.8 μm used in Example 4 were added thereto. After the mixture was uniformly mixed, the mixture was extruded into a sheet having a thickness of 130 μm, and a light diffusion sheet having irregularities on both sides of the sheet was prepared using the roll with embossments used in Example 1.
The total light transmittance, haze value, linear expansion coefficient, luminance, dot concealability, wrinkle generation time test, and tensile modulus of these light diffusion sheets were performed in the same manner as in Example 1. The results are also shown in Table 1.
[0055]
[Table 1]

[0056]
Referring to Table 1, the total light transmittance of the light diffusion sheets of Examples 1 to 7 and the light diffusion sheet of Comparative Example 1 showed the same value as 100%. On the other hand, the haze value of the sheet of Comparative Example 1 was 85.5%, while the sheets of Examples 1, 2, 3, 6, and 7 were 92.8 to 93.8%, and the haze value was 7.3. ８8.3%, which proved to be a light diffusing sheet that transmits light well and has excellent concealing properties. Examples 4 and 5 have lower haze values than the other examples, but Example 4 has a haze value that is 5% higher than that of Comparative Example 1, and Example 5 shows the same haze value as Comparative Example 1. No decrease in haze value due to the addition of the light diffusing agent was observed.
[0057]
The linear expansion coefficients of the light diffusion sheets of Examples 1 to 7 were 3.1 to 9.0 × 10 in comparison with Comparative Example 1.^-5/ ° C is also greatly reduced, indicating that the sheet is small in elongation and hardly wrinkled even when heated by heat. In particular, Examples 3 to 7 using talc are significantly lower than Examples 1 and 2 using silica, and it can be seen that wrinkles hardly occur even under severe conditions. Further, in a wrinkle generation time test using a thermo-hygrostat, the wrinkle generation time was significantly slowed down to 96 to 432 hours as compared with 24 hours in Comparative Example 1, indicating that the wrinkle generation time was greatly improved. Among them, Examples 4 to 6 using talc having an average particle diameter of 8 μm showed superior numerical values in both the linear expansion coefficient and the wrinkle generation time test than the other examples. It can be seen that the light diffusing agent is superior to the agent. In particular, the sheet containing 30% by weight of talc having an average particle size of 8 μm of Example 5 wrinkled when left for 432 hours, and it was found that wrinkling did not occur for about 18 times as long as that of Comparative Example 1. Understand. Therefore, it can be seen that Examples 4 to 6, especially Example 5 using talc having an average particle size of 8 μm do not have wrinkles even in severer actual use.
[0058]
Examples 1, 2, 4, 5, and 6 using a light diffusing agent having an average particle diameter of 8 μm show the same value as Comparative Example 1, and are bright sheets with little light loss. You can see that.
Further, the opacity of the dots was high in Examples 3 to 7 using talc, while the opacity of the dots was inferior in Comparative Example 1, Examples 1 and 2, and the light diffusion was low. It can be seen that talc is excellent in hiding properties among the agents.
[0059]
From the above results, even among the light diffusing agents, the light diffusing sheet using talc is a sheet that has a small degree of expansion and contraction due to heat, does not generate wrinkles, and has excellent concealing properties, and has an excellent light shielding property for the backlight unit of the liquid crystal display. Particularly useful as a diffusion sheet. Further, a sheet using silica has a high haze value and is useful for a light diffusion sheet particularly required for optical characteristics.
[0060]
【The invention's effect】
As is clear from the above description, the light diffusion sheet of the present invention has a high total light transmittance and a high haze value, and is a sheet excellent in concealing properties that transmits light well, and further, the sheet is heated. Can be hardly expanded and contracted to suppress wrinkles, and can be suitably used for a backlight unit of a liquid crystal display.
In particular, a light diffusion sheet using talc as a light diffusion agent does not wrinkle for a long period of time, and can be used as a light diffusion sheet for a backlight unit of a liquid crystal display.
[Brief description of the drawings]
FIG. 1 is a sectional view of a light diffusion sheet according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view of a light diffusion sheet according to another embodiment of the present invention.
[Explanation of symbols]
1,10 ° light diffusion sheet
1a: One side (lower surface) to be the light incident surface
1b 反 対 Opposite surface to be light emitting surface (upper surface)
1c @ Light diffuser
2 Light guide plate
3 Lens film (prism sheet)
4 Light source

Claims (6)

An uneven surface is formed on both surfaces of the sheet, and the average surface roughness of one surface serving as a light entrance surface is larger than the average surface roughness of the opposite surface serving as a light exit surface, and the sheet is made of a light-transmitting resin containing a light diffusing agent. A light diffusing sheet having an average surface roughness of 0.2 to 5.0 μm on one surface and an average surface roughness of 0.2 to 2.0 μm on the opposite surface. The light diffusing sheet according to claim 1, wherein the light diffusing agent has an average particle size of 0.5 to 50 µm, and is contained in the translucent resin in an amount of 0.1 to 50% by weight. The surface area ratio of the one surface is smaller than the surface area ratio of the opposite surface, the surface area ratio of the one surface is 1.001 to 1.150, and the surface area ratio of the opposite surface is 1.010 to 1.250. The light diffusion sheet according to claim 1 or 2, wherein The average surface roughness of the one surface is 1.0 to 3.0 μm, the average surface roughness of the opposite surface is 0.5 to 1.5 μm, and the surface area ratio of the one surface is 1.030 to 1.100. The light diffusion sheet according to any one of claims 1 to 3, wherein the surface area ratio of the opposite surface is 1.050 to 1.200. The light-diffusing agent is talc having an average particle size of 1.0 to 15 µm, and is contained in the translucent resin in an amount of 10 to 40% by weight. Light diffusion sheet. A light diffusing agent composed of talc and another light diffusing agent is contained in an amount of 10 to 40% by weight, and the talc is contained in a translucent resin in an amount of 5 to 30% by weight and 50% by weight or more of the total light diffusing agent. The light diffusion sheet according to any one of claims 1 to 4, wherein:

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