200844503 九、發明說明 【發明所屬之技術領域】 本發明係關於一種製備聚乙烯醇偏光膜之方法及使用 該方法所製備之聚乙烯醇偏光膜，更特定而言，本發明係 關於一種製備具有高偏光度之聚乙烯醇偏光膜的方法，該 方法係藉由使用溶液-鑄膜程序及乾燥該聚乙烯醇偏光膜 而製得朝厚度方向具有高均勻性的聚乙烯醇偏光膜。 【先前技術】 近年來，資訊科技（IT)工業已有顯著進展，因此在IT 工業中屬於先進部份的液晶顯示裝置（LCD)也開發爲傳遞 現代工業社會之各種資訊及前進到高資訊導向之社會的媒 體。 具有偏光膜之偏光片係黏附在平面顯示器液晶晶元（ 如LCD或有機EL)的兩側，所以係作用爲液晶顯示裝置 面板的零件。液晶顯示裝置面板具有偏光片黏附在液晶晶 元之兩側的構造。圖1係示意性顯示習知之偏光片。參考 圖1，偏光片1包含排列在中央區域的偏光器6 ;及經由感 壓黏合劑層4，5黏附在偏光器6兩側的保護膜2，3。保護 膜2，3排列在偏光器6兩側係爲了扮演防止偏光器6受損的 角色。 上保護膜2(分離膜）在貼到液晶晶元之前即被剝離丟 棄，所以曝露之感壓黏合劑層4係作用爲偏光器6與液晶晶 元之間的黏附介質。下保護膜3在黏附到液晶晶元後也可 -5- 200844503 剝離拋棄。此時，感壓黏合劑層5也因黏附著保護膜3而一 起被剝離。保護膜2，3可由酯樹脂（如PET)形成。 排列在保護膜2，3之間的偏光器6包含偏光膜1 0及藉 使用黏合劑層9而黏附在偏光膜1 〇兩側的保護層7，8，該 黏合劑層9係插入保護層7，8與偏光膜1 0之間。本文中， 偏光膜1 〇通常係由經碘或重鉻染料染色的聚乙烯醇聚合物 所製成，而保護層7，8係由三乙醯基纖維素（TAC)等製造 〇 上述聚乙烯醇偏光膜之製造係藉由將聚乙烯醇聚合物 、增塑劑及表面活性劑溶解於水中而製備聚乙烯醇聚合物 水溶液，接著對該聚乙烯醇聚合物水溶液進行溶液-鑄膜 、乾燥及熱處理而製備聚乙烯醇膜，及染色該聚乙烯醇膜 。然而，若聚乙烯醇膜係藉由使用溶液-鑄膜法而製備， 則很難形成表面及內部皆具有均分結構的偏光膜，此乃因 爲該偏光膜在乾燥及熱處理期間朝厚度方向具有皮芯結構 〇 也就是說，經由溶液-鑄膜而從模具中卸下之聚乙烯 醇聚合物水溶液在溶劑、水自該膜中流出時，會讓聚乙烯 醇聚合物轉移到表層。通常，聚合物在上述乾燥及熱處理 步驟期間很少在該膜內流動，但乾燥低分子量之水時由於 聚乙烯醇聚合物與溶劑、水分子的強烈交互作用，該聚乙 烯醇聚合物會轉移到該膜的表層。因爲此一現象，聚乙烯 醇聚合物在該膜表層的密度增加，所以在熱處理程序期間 當結晶形成時，該膜之表層可能會有比核心層還高的結晶 -6 - 200844503 度。 結構中朝厚度方向的不均勻性會阻礙在製備偏光膜時 之染色程序期間碘或重鉻染料滲透到該膜的內部深處，因 此該膜朝厚度方向無法均勻地被染色，其偏光度也會惡化 。同時，該膜之核心層有較差的強度，所以，若製造厚度 爲5 0微米或更小之薄偏光膜時，其機械特性就不好，阻滯 現象可能在偏光膜中出現，導致拉伸加工性的困難。 【發明內容】 因此，本發明係設計爲了解決先前技術的問題，所以 本發明的目標之一係提出一種製備朝厚度方向具有均勻之 聚合物密度之乙烯醇偏光膜的方法。 同時，本發明的另一目標係提供一種藉使用該製備方 法所製備的聚乙烯醇偏光膜。 爲了完成上述目標，本發明提供一種製備聚乙烯醇偏 光膜的方法，該方法包含： (a) 將聚乙烯醇聚合物、水溶性交聯劑、增塑劑及表 面活性劑溶解於水中而製備聚乙烯醇聚合物水溶液； (b) 對該聚乙烯醇聚合物水溶液進行溶液-鑄膜、乾燥 及熱處理而製備聚乙烯醇膜；及 (c) 染色該聚乙烯醇膜。 . 同時，本發明係提供一種藉使用該製備方法所製備之 聚乙烯醇偏光膜。 如上所述，聚乙烯醇分子具有與溶劑、水的優異相容 200844503 性。據此，對聚乙烯醇聚合物水溶液進行溶液-鑄膜，然 後乾燥及熱處理，該自聚乙烯醇聚合物中製得之聚乙烯醇 膜的表面區域具有比其核心區域還高的聚合物密度，此乃 因爲水蒸發期間聚乙烯醇聚合物轉移到該膜表面之故。據 此，當聚乙烯醇膜被染色及拉伸而製備偏光膜時，由於該 膜之表層與核心層間的物理特性不同將難以進行隨後程序 〇 因此，爲了使聚乙烯醇聚合物的轉移減至最小，本發 明人頃發現，藉由在聚乙烯醇聚合物水溶液的溶液-鑄膜 步驟之前將水溶性交聯劑加到聚乙烯醇聚合物水溶液中， 可在染色步驟中誘發交聯反應，其中該交聯劑能使聚乙烯 醇聚合物互相交聯。結果，該聚乙烯醇聚合物之轉移在染 色及熱處理步驟期間減至最小，如此分子密度在該膜之表 層不會增加。所以，本發明係以上述之發現爲基礎而設計 〇 據此，由於根據本發明之製備方法所製備的聚乙烯醇 偏光膜朝厚度方向具有均勻結構，在隨後之程序如染色及 拉伸程序中的染色特性及拉伸性獲得改善’且使用該製備 方法所製備之偏光膜具有改善之機械特性及偏光度。 本發明較佳具體實施例的這些及其他特點、方面及優 點將參照附圖於下文中作更爲詳細的說明。 【實施方式】 本文以下將詳細說明本發明之較佳具體實施例。 -8 - 200844503 首先，係說明根據本發明之聚乙烯醇偏光膜的製備方 法。根據本發明製備聚乙烯醇膜的方法包含： (a) 將聚乙烯醇聚合物、水溶性交聯劑、增塑劑及表 面活性劑溶解於水中而製備聚乙烯醇聚合物水溶液； (b) 對該聚乙烯醇聚合物水溶液進行溶液-鑄膜、乾燥 及熱處理而製備聚乙烯醇膜；及 (c) 染色該聚乙烯醇膜。 下文將更加詳細地以逐步驟方式說明本發明之較佳具 體實施例。 (a)製備聚乙烯醇聚合物水溶液 此步驟係將聚乙烯醇聚合物、水溶性交聯劑、增塑劑 及表面活性劑溶解於水中而製備水溶液。 在根據本發明之聚乙烯醇偏光膜的製備方法中，該含 有乙烯醇作爲重複單元之聚乙烯醇聚合物可藉由使乙烯酯 聚合物（此聚合物係使乙烯酯單體聚合而製得）水解，接著 使乙烯酯單元皂化爲乙烯醇單元而製備。舉例之，此乙烯 酯單體包括甲酸乙烯酯、乙酸乙烯酯、丙酸乙烯酯、戊酸 乙烯酯、月桂酸乙烯酯、硬脂酸乙烯酯、苯甲酸乙烯酯、 新戊酸乙烯酯、非塞酸（vasatate)乙烯酯，等。在這些當 中以乙酸乙烯酯較適當。不用說，只要不違背本發明之精 神及範圍，欲使乙烯酯單體聚合時，也可使其他已知之單 體（如乙烯及丙烯）共聚。聚乙烯醇聚合物較佳地具有95% 或以上的皂化度，以99%或以上更佳，99.5%或以上最佳 200844503 。若皂化度小於9 5 %，該膜的耐熱性會惡化。同時’在考 慮到該所製備之膜的耐熱性、溶液-鑄膜特性等’聚乙烯 醇聚合物之平均聚合度較佳地在2,000至7,000範圍內’以 2,600至 5,000更佳，。 此步驟中，在考慮到該所製備之膜的均勻性及可成形 性時，聚乙烯醇聚合物較佳地具有以該聚乙烯醇聚合物水 溶液之總重量計爲10至60重量°/〇的含量，以15至45重量％ 更佳，2 0至4 0重量％最佳。 其間，在本發明之製備方法中，本發明所屬之技藝中 已廣泛得知的水溶性交聯劑都可在本文中使用。水溶性交 聯劑之特定實例包括，但不受限於硼酸、硼酸鹽及磷酸， 彼等可單獨使用或以組合方式使用。 在此步驟中，水溶性交聯劑的含量以水溶液中1 〇〇重 量份之聚乙烯醇聚合物計較佳地係在0.001至2重量份範圍 內，以0.005至1重量份更佳。若水溶性交聯劑之含量小於 0.001重量份，對聚乙烯醇聚合物的交聯效果會降低，同 時，若該含量超過2重量份，則拉伸程序期間拉伸性會惡 在本發明之製備方法中，若製備聚乙烯醇偏光膜時需 用到增塑劑，通常可在聚乙烯醇聚合物水溶液中使用，該 增塑劑之添加係欲改善聚乙烯醇的染色特性。多元醇可使 用作爲增塑劑，特別佳的是使用甘油，因其與聚乙烯醇聚 合物的相容性及其低光學擴散特性之故。在考慮到聚乙烯 醇膜之染色特性、膜強度、及捲繞筒中膜之間的黏附性時 -10- 200844503 ，增塑劑之含量以水溶液中1 00重量份之聚乙烯醇聚合物 計較佳地係在5至20重量份範圍內，以7至15重量份更佳。 在本發明之製備方法中，當膜在鑄膜程序後欲自鼓中 剝離，爲了使張力減至最小可將表面活性劑加到該水溶液 中。在本發明中，若製備聚乙烯醇偏光膜時需用到表面活 性劑，則非離子性表面活性劑、陽離子性表面活性劑、陰 離子性表面活性劑及其類似物都可不受限制地使用。可用 於本發明之表面活性劑的特定實例包括經聚丙烯酸酯改質 之化合物、經聚矽氧烷改質之化合物、聚伸氧乙基醚化合 物等。在考慮到該膜之剝離特性及表面粗糙度等，表面活 性劑之添加量以1 〇〇重量份聚乙烯醇聚合物計較佳地爲 0.001至2重量份。 (b)形成聚乙烯醇膜 此步驟係對上述步驟（製備聚乙烯醇聚合物水溶液之 步驟）所製備之水溶液進行鑄膜、乾燥及熱處理，而形成 聚乙烯醇膜。 此步驟之溶液-鑄膜及乾燥程序可在本發明所屬技藝 中已廣泛得知的條件下進行，但本發明並不受限於這些條 件。參考溶液-鑄膜及乾燥程序的具體實施例之一，步驟 (a)中所製備之聚乙烯醇聚合物水溶液係在80至100 °C溫度 下經由一具有80至100°C溫度的T-模具在鼓上進行鑄膜， 藉此形成膜。此時，T-模具與鼓之間的距離較佳地係維持 在10公釐或更小，以5公釐或更小較佳。 -11 - 200844503 若該膜係於鼓上形成，該膜需自鼓中剝離並通過多個 具有70至90 °C溫度的乾燥滾輪而形成兩面均勻的聚乙烯醇 膜。也就是說，由於聚乙烯醇聚合物係藉由水溶性交聯劑 (在製備該鑄膜水溶液時所添加）而互相交聯，所以乾燥期 間聚合物轉移到該膜之表層將於該膜內被抑制住。因此， 可製得具有高均勻性且聚合物密度朝該膜之厚度方向沒有 差異的膜。 稍後，對經由上述步驟所形成之聚乙烯醇膜進行熱處 理程序，如此可確保機械特性。在考慮到該膜之機械特性 及拉伸性時，熱處理較佳地係在100至160 °C溫度下進行， 而以1 2 0至1 4 0 °C更佳。 此步驟中，熱處理方法並無任何限制，但若習知之熱 處理方法在本發明所屬技藝中已用過，則可在本文中使用 。至於熱處理方法的實例之一，可使用讓薄膜與高溫滾輪 接觸而對該膜進行熱處理的方法，或以高溫空氣（如溫度 爲100至160 °C之流動方式的熱空氣）對該膜進行熱處理之 方法。爲了均勻地對薄膜進行熱處理，該膜較佳地係在空 氣流動方式中進行熱處理。 經熱處理之聚乙烯醇膜較佳地係進一步進行濕度調節 的步驟。濕度調節步驟之進行係爲了排除乾燥及熱處理程 序期間所引起的受熱歷程’藉以改善拉伸性及偏光性。濕 度調節方法並無任何限制’但此濕度調節方法可在本發明 所屬技藝中廣泛使用的濕度調節條件下進行。舉例之，濕 度調節步驟可在溫度爲8 〇至1 5 0 °C及濕度5 0至8 0 %的條件 -12- 200844503 下進行。 根據上述方法所製備之聚乙烯醇膜可使用本發明所屬 技藝中已知的慣用技術來捲繞。特別地，爲了解決捲繞筒 中膜之間黏附之問題所添加的抗阻滯劑不應在本發明之例 子中添加。這是因爲該膜內之水溶性交聯劑具有抑制膜之 間黏附的效果，該水溶性交聯劑係藉由將該膜表面的反應 率調整到一適當數値。 (C)染色聚乙烯醇膜 此步驟係染色根據上述步驟所製備之聚乙烯醇膜。 在本發明中染色程序並無任何限制，但本發明所屬技 藝中常用的染色程序可以碘或重鉻染料來進行，且聚乙烯 醇膜較佳地係在碘與硼酸水溶液中被染色，而更佳地係將 碘化鉀添加到該水溶液中。舉例之，可藉由在含有0.03至 0.2 %碘與0.5至5 %碘化鉀的水溶液中浸漬該聚乙烯醇膜3至 10分鐘。 該依此製備之聚乙烯醇膜係經由拉伸程序而完成聚乙 烯醇偏光膜。拉伸程序可在染色聚乙烯醇膜之前/之後進 行，或在染色步驟的相同時間下進行。也就是說，可先染 色聚乙烯醇膜再拉伸，及拉伸後再染色，或也可同時染色 並拉伸。拉伸程序並無任何限制，但本發明所屬技藝中常 用的拉伸程序也可在本文中使用。舉例之，聚乙烯醇膜係 在40至80°C溫度下預熱30秒至5分鐘，然後在40至80°C溫 度（45至60°C爲較佳地）下拉長1.1至7倍。 200844503 根據本發明之製備方法所製備的聚乙燃醇偏光膜朝厚 度方向具有均勻結構。同時’該聚乙烯醇偏光膜也具有非 常高的偏光度，此舉顯示和習知之聚乙烯醇偏光膜比較時 ，本發明之聚乙烯醇偏光膜具有改善的偏光度。 爲了能更加明瞭，下文將說明本發明之較佳具體實施 例，但本文所提及之說明只是基於解說目的之較佳實施例 ，並不意圖限制於本發明之申請專利範圍。 實施例1 混合具有平均聚合度3 200及皂化度99.9莫耳％的27重 量％的聚乙烯醇，並將之溶解於水中。同時，將以水溶液 中1 0 0重量份之聚乙烯醇計〇 · 1重量份的硼酸、1 2 · 4重量份 甘油及〇. 1重量份表面活性劑、聚伸氧乙基醚化合物加到 該水溶液中，而製備含有30.4重量％非揮發性化合物的水 溶液。此溶解步驟係在溫度爲1 3 0 °C或更高的槽中進行， 以便抑制不溶解物質的形成。對所製備之水溶液進行除泡 程序，然後經由齒輪泵及微過濾器將之移轉到T-模具中 ，經由9 5 °C之T -模具將該水溶液塗覆於9 5 °C的鼓上而形 成薄膜。此時，T-模具與鼓之間的距離係維持在3公釐範 圍內。自鼓中剝離該膜，然後在8 5 t的乾燥條件下讓該膜 通過1 1個乾燥滾輪而形成濕度含量爲8 %的聚乙燒醇膜。 將該依此形成之膜通過溫度1 2 0 °C的空氣流動機械而形成 結晶。接著，對該膜進行濕度調節程序而製備厚度7 5微米 之聚乙烯醇膜。 -14- 200844503 使該製備之聚乙烯醇膜膨脹並在含有0 · 0 5 %碘及0 · 3 % 碘化鉀的水溶液中染色，然後在溶解有3%硼酸的50 °C浴 中將之拉伸5倍而製得聚乙烯醇偏光膜。接著’在8 0 °C溫 度下以空氣流動方式乾燥該聚乙烯醇而製備最終之偏光膜 實施例2 混合具有平均聚合度2600及皂化度99· 95莫耳％的30.0 重量％聚乙烯醇，並將之溶解於水中。同時，將以水溶液 中100重量份之聚乙烯醇計〇.〇1重量份的硼酸、12.4重量 份甘油及〇 . 1重量份聚伸氧乙基醚化合物加到該水溶液中 ，而製備含有3 3.7重量％非揮發性化合物的水溶液。依實 施例1之相同方式進行隨後程序而製備聚乙烯醇偏光膜。 實施例3 依實施例1之相同方式重複此實施例而製備聚乙烯醇 偏光膜，但除外的是，在水溶液製備程序中加入以1〇〇重 量份聚乙烯醇計1.0重量份的硼酸而製備水溶液。然而， 該所製備之聚乙烯醇的厚度爲50微米。 比較性實施例1 依實施例1之相同方式重複此一比較性實施例而製備 聚乙烯醇偏光膜，但除外的是，在水溶液製備程序中並沒 有添加硼酸而製得水溶液。然而，在捲繞程序之前先以澱 -15- 200844503 粉塗覆該膜以防止捲繞筒中膜之間的黏附。此類膜係在 3%硼酸水溶液中交聯，0.05%碘與0.3%碘化鉀水溶液中染 色，及在溶解有3 %硼酸的5 0 °C浴中拉伸5倍而製得聚乙烯 醇偏光膜。 性能評估 染色及拉伸性 利用光學顯微鏡（Lei ca DM數位顯微方法）評估實施例 1及比較性實施例1中製備之偏光膜的染色及拉伸均勻性。 結果各別地顯示於圖2及圖3。 偏光度 在本發明中，偏光度係根據下列方程式藉由測量兩個 重疊偏光膜的光透射曲線而測得。所用之測量機械係取自 N&K Technology 公司的 n&kl280分析器。 方程式1 _ 偏光度χίοο 在方程式1中，H i表示藉由測量水平重疊之偏光膜所 得的光透射率，而H2表示藉由測量垂直重疊之偏光膜所 得的光透射率。測量中所用之光具有400至700奈米的波長 ，在偏光度測得後可算出相同波長的偏光度平均値。 根據實施例1至3及比較性實施例1之結果係列於下表1。 -16- 200844503 表1 實施例 比較性實施例 1 2 3 1 水溶液之比較 聚乙烯醇(重量％) 27 30 27 27 硼酸(重量份） 0.1 0.01 1.0 0 甘油(重量份） 12.4 12.4 12.4 12.4 表面活性劑(重量份） 0.1 0.1 0.1 0.1 非揮發性化合物(重量％) 30.4 33.7 30.4 30.4 染色/拉伸性 硼酸(％) 一 3 碘(％) 0.05 0.05 0.05 0.05 拉伸性(倍數） 5.2 5.4 5.0 5.2 偏光特性 偏光度(％) 99.7 99.5 99.3 98.5 從表1之結果可看出，根據本發明之實施例所製備的 聚乙烯醇膜具有優異的染色特性及拉伸性。染色特性及拉 伸性之結果可自圖2及圖3的光學顯微照片圖中確定，圖2 及圖3係取自實施例1及比較性實施例1所製備的偏光膜。 圖2係顯示根據本發明實施例1之聚乙烯醇偏光膜的染色特 性之光學顯微鏡照片，而圖3係顯示根據本發明比較性實 施例1之聚乙烯醇偏光膜的染色特性之光學顯微照片。圖2 中透露出該膜的中央區域係朝膜的厚度方向均勻地被染色 ，但如圖3所示，該膜的某些中央區域並沒有朝膜的厚度 方向染色。因此，其透露出根據本發明實施例1所製備之 偏光膜具有優異的染色特性及拉伸性，然而比較性實施例 1之偏光膜卻具有較差的染色特性及拉伸性。 從表1之結果中可顯示出，和比較性實施例1相較時根 據本發明之偏光膜具有優異的偏光度。 -17- 200844503 工業應用 根據本發明之製備方法，聚乙烯醇聚合物在水蒸發時 轉移到薄膜之表層的現像可在對聚乙烯醇聚合物水溶液進 行溶液-鑄膜及染色的步驟中獲得改善，此乃因爲聚乙烯 醇聚合物係以所添加的水溶性交聯劑予以交聯。因此，在 隨後步驟中之染色特性及拉伸性可獲得改善，且該所製備 之偏光膜具有改善之機械特性及偏光度。 如上文所述，本發明之較佳具體實施例已詳加說明。 然而，應明瞭的是，詳細之說明及特定實施例在表示本發 明之較佳具體實施例的同時只當作解說性質，因爲本發明 之精神及範圍內的各種變異及修正可讓熟諳此藝者自此詳 細說明中顯見。 【圖式簡單說明】 圖1係顯示設置有聚乙烯醇偏光膜之習知偏光板的橫 切面。 圖2係顯示根據本發明實施例1之聚乙烯醇偏光膜的染 色特性之光學顯微照片。 圖3係顯示根據本發明比較性實施例1之聚乙烯醇偏光 膜的染色特性之光學顯微照片。 【主要元件符號說明】 1 :偏光片 -18- 200844503 2 :保護膜 3 :保護膜 4 :感壓黏合劑層 5 :感壓黏合劑層 6 :偏光器 7 :保護層 8 :保護層 9 :黏合劑層 1 〇 :偏光膜200844503 IX. Description of the Invention [Technical Field] The present invention relates to a method for preparing a polyvinyl alcohol polarizing film and a polyvinyl alcohol polarizing film prepared by using the method, and more particularly, the present invention relates to a preparation having A method of a high degree of polarization polyvinyl alcohol polarizing film which is obtained by using a solution-casting film process and drying the polyvinyl alcohol polarizing film to obtain a polyvinyl alcohol polarizing film having high uniformity in the thickness direction. [Prior Art] In recent years, the information technology (IT) industry has made significant progress, so the advanced part of the liquid crystal display device (LCD) in the IT industry has also been developed to convey the information of the modern industrial society and advance to the high information orientation. The media of society. A polarizer having a polarizing film adheres to both sides of a flat-panel liquid crystal cell (such as an LCD or an organic EL), and thus functions as a component of a liquid crystal display panel. The liquid crystal display panel has a structure in which polarizers are adhered to both sides of the liquid crystal cell. Fig. 1 is a schematic view showing a conventional polarizer. Referring to Fig. 1, a polarizer 1 comprises a polarizer 6 arranged in a central region; and a protective film 2, 3 adhered to both sides of the polarizer 6 via a pressure-sensitive adhesive layer 4, 5. The protective films 2, 3 are arranged on both sides of the polarizer 6 in order to prevent the polarizer 6 from being damaged. The upper protective film 2 (separation film) is peeled off before being attached to the liquid crystal cell, so that the exposed pressure-sensitive adhesive layer 4 functions as an adhesion medium between the polarizer 6 and the liquid crystal cell. The lower protective film 3 can also be peeled off after being adhered to the liquid crystal cell, -5- 200844503. At this time, the pressure-sensitive adhesive layer 5 is also peeled off by the adhesion of the protective film 3. The protective films 2, 3 may be formed of an ester resin such as PET. The polarizer 6 disposed between the protective films 2, 3 includes a polarizing film 10 and a protective layer 7, 8 adhered to both sides of the polarizing film 1 by using the adhesive layer 9, and the adhesive layer 9 is inserted into the protective layer. 7,8 and the polarizing film 10 between. Herein, the polarizing film 1 is usually made of a polyvinyl alcohol polymer dyed with iodine or a heavy chromium dye, and the protective layer 7, 8 is made of triethylenesulfonyl cellulose (TAC) or the like. The alcohol polarizing film is prepared by dissolving a polyvinyl alcohol polymer, a plasticizer and a surfactant in water to prepare an aqueous solution of a polyvinyl alcohol polymer, followed by solution-casting and drying the aqueous solution of the polyvinyl alcohol polymer. And a heat treatment to prepare a polyvinyl alcohol film, and dye the polyvinyl alcohol film. However, if the polyvinyl alcohol film is prepared by a solution-casting method, it is difficult to form a polarizing film having a uniform structure on both the surface and the inside because the polarizing film has a thickness direction during drying and heat treatment. The sheath-core structure, that is, the polyvinyl alcohol polymer aqueous solution which is removed from the mold through the solution-casting film, causes the polyvinyl alcohol polymer to transfer to the surface layer when the solvent and water flow out from the film. Generally, the polymer rarely flows in the film during the drying and heat treatment steps described above, but when the low molecular weight water is dried, the polyvinyl alcohol polymer is transferred due to the strong interaction of the polyvinyl alcohol polymer with the solvent and water molecules. To the surface layer of the film. Because of this phenomenon, the density of the polyvinyl alcohol polymer in the surface layer of the film is increased, so that when the crystal is formed during the heat treatment process, the surface layer of the film may have a higher crystal -6 - 200844503 degrees than the core layer. The unevenness in the thickness direction of the structure hinders the penetration of iodine or heavy chrome dye into the inner depth of the film during the dyeing process in the preparation of the polarizing film, so that the film cannot be uniformly dyed in the thickness direction, and the degree of polarization is also Will deteriorate. At the same time, the core layer of the film has poor strength. Therefore, if a thin polarizing film having a thickness of 50 μm or less is produced, the mechanical properties are not good, and the retardation phenomenon may occur in the polarizing film, resulting in stretching. Difficulties in processing. SUMMARY OF THE INVENTION Therefore, the present invention has been devised in order to solve the problems of the prior art, and therefore one of the objects of the present invention is to provide a method of preparing a vinyl alcohol polarizing film having a uniform polymer density in the thickness direction. Meanwhile, another object of the present invention is to provide a polyvinyl alcohol polarizing film which is prepared by using the preparation method. In order to accomplish the above object, the present invention provides a method for preparing a polyvinyl alcohol polarizing film, which comprises: (a) preparing a polypolyol polymer, a water-soluble crosslinking agent, a plasticizer, and a surfactant in water to prepare a poly An aqueous solution of a vinyl alcohol polymer; (b) preparing a polyvinyl alcohol film by solution-casting, drying and heat-treating the aqueous solution of the polyvinyl alcohol polymer; and (c) dyeing the polyvinyl alcohol film. Meanwhile, the present invention provides a polyvinyl alcohol polarizing film prepared by using the production method. As described above, the polyvinyl alcohol molecule has excellent compatibility with a solvent and water. Accordingly, the solution of the polyvinyl alcohol polymer aqueous solution is subjected to solution-casting, followed by drying and heat treatment, and the surface area of the polyvinyl alcohol film obtained from the polyvinyl alcohol polymer has a polymer density higher than that of the core region. This is because the polyvinyl alcohol polymer is transferred to the surface of the film during evaporation of the water. Accordingly, when the polyvinyl alcohol film is dyed and stretched to prepare a polarizing film, it is difficult to carry out the subsequent procedure due to the difference in physical properties between the surface layer of the film and the core layer. Therefore, in order to reduce the transfer of the polyvinyl alcohol polymer to Minimally, the inventors have found that a crosslinking reaction can be induced in the dyeing step by adding a water-soluble crosslinking agent to the aqueous solution of the polyvinyl alcohol polymer before the solution-casting step of the aqueous solution of the polyvinyl alcohol polymer, wherein The crosslinking agent enables the polyvinyl alcohol polymers to crosslink each other. As a result, the transfer of the polyvinyl alcohol polymer is minimized during the dyeing and heat treatment steps, so that the molecular density does not increase in the surface layer of the film. Therefore, the present invention has been designed based on the above findings, and since the polyvinyl alcohol polarizing film prepared by the production method according to the present invention has a uniform structure in the thickness direction, in subsequent procedures such as dyeing and stretching procedures. The dyeing property and the stretchability are improved' and the polarizing film prepared by the preparation method has improved mechanical properties and polarization. These and other features, aspects and advantages of the preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. [Embodiment] Hereinafter, preferred embodiments of the present invention will be described in detail. -8 - 200844503 First, a method of preparing a polyvinyl alcohol polarizing film according to the present invention will be described. The method for preparing a polyvinyl alcohol film according to the present invention comprises: (a) preparing a polyvinyl alcohol polymer aqueous solution by dissolving a polyvinyl alcohol polymer, a water-soluble crosslinking agent, a plasticizer, and a surfactant in water; (b) The polyvinyl alcohol polymer aqueous solution is subjected to solution-casting, drying and heat treatment to prepare a polyvinyl alcohol film; and (c) dyeing the polyvinyl alcohol film. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in more detail in a step-by-step manner. (a) Preparation of aqueous solution of polyvinyl alcohol polymer In this step, an aqueous solution is prepared by dissolving a polyvinyl alcohol polymer, a water-soluble crosslinking agent, a plasticizer and a surfactant in water. In the method for producing a polyvinyl alcohol polarizing film according to the present invention, the polyvinyl alcohol polymer containing vinyl alcohol as a repeating unit can be obtained by polymerizing a vinyl ester polymer obtained by polymerizing a vinyl ester monomer. Hydrolysis, followed by saponification of the vinyl ester unit to a vinyl alcohol unit. For example, the vinyl ester monomer includes vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, non- Vasatate vinyl ester, and the like. Among these, vinyl acetate is more suitable. Needless to say, other known monomers (e.g., ethylene and propylene) may be copolymerized when the vinyl ester monomer is to be polymerized without departing from the spirit and scope of the present invention. The polyvinyl alcohol polymer preferably has a degree of saponification of 95% or more, more preferably 99% or more, and most preferably 99.5% or more. If the degree of saponification is less than 95%, the heat resistance of the film may deteriorate. At the same time, the average degree of polymerization of the polyvinyl alcohol polymer is preferably in the range of 2,000 to 7,000, more preferably 2,600 to 5,000, in consideration of heat resistance, solution-casting property, and the like of the film prepared. In this step, the polyvinyl alcohol polymer preferably has a weight of from 10 to 60% by weight based on the total weight of the aqueous solution of the polyvinyl alcohol polymer, in consideration of uniformity and formability of the film to be prepared. The content is preferably 15 to 45% by weight, and most preferably 20 to 40% by weight. Meanwhile, in the preparation method of the present invention, a water-soluble crosslinking agent which is widely known in the art to which the present invention pertains can be used herein. Specific examples of water-soluble crosslinking agents include, but are not limited to, boric acid, borate, and phosphoric acid, which may be used alone or in combination. In this step, the content of the water-soluble crosslinking agent is preferably in the range of 0.001 to 2 parts by weight, more preferably 0.005 to 1 part by weight, based on 1 part by weight of the polyvinyl alcohol polymer in the aqueous solution. If the content of the water-soluble crosslinking agent is less than 0.001 parts by weight, the crosslinking effect on the polyvinyl alcohol polymer may be lowered, and if the content exceeds 2 parts by weight, the stretchability during the stretching process may be evil in the preparation of the present invention. In the method, if a polyvinyl alcohol polarizing film is used, a plasticizer is usually used, and it is usually used in an aqueous solution of a polyvinyl alcohol polymer, and the addition of the plasticizer is intended to improve the dyeing property of polyvinyl alcohol. The polyol can be used as a plasticizer, and it is particularly preferable to use glycerin because of its compatibility with the polyvinyl alcohol polymer and its low optical diffusion property. In consideration of the dyeing property of the polyvinyl alcohol film, the film strength, and the adhesion between the films in the winding drum, the content of the plasticizer is preferably 100 parts by weight of the polyvinyl alcohol polymer in the aqueous solution. The ground system is in the range of 5 to 20 parts by weight, more preferably 7 to 15 parts by weight. In the preparation method of the present invention, when the film is to be peeled off from the drum after the casting process, a surfactant may be added to the aqueous solution in order to minimize the tension. In the present invention, if a surfactant is used in the preparation of the polyvinyl alcohol polarizing film, nonionic surfactants, cationic surfactants, anionic surfactants and the like can be used without limitation. Specific examples of the surfactant which can be used in the present invention include compounds modified with polyacrylate, compounds modified with polyoxyalkylene oxide, polyoxyethylene ethyl ether compounds and the like. The surfactant is preferably added in an amount of from 0.001 to 2 parts by weight based on 1 part by weight of the polyvinyl alcohol polymer, in consideration of the peeling property and surface roughness of the film. (b) Formation of polyvinyl alcohol film This step is carried out by casting, drying and heat-treating the aqueous solution prepared in the above step (the step of preparing an aqueous solution of a polyvinyl alcohol polymer) to form a polyvinyl alcohol film. The solution-cast film and drying procedure of this step can be carried out under conditions which are widely known in the art to which the present invention pertains, but the present invention is not limited to these conditions. Referring to one of the specific examples of the solution-cast film and the drying procedure, the aqueous solution of the polyvinyl alcohol polymer prepared in the step (a) is passed through a T-temperature having a temperature of 80 to 100 ° C at a temperature of 80 to 100 ° C. The mold is cast on a drum to form a film. At this time, the distance between the T-die and the drum is preferably maintained at 10 mm or less, preferably 5 mm or less. -11 - 200844503 If the film is formed on a drum, the film is peeled off from the drum and passed through a plurality of drying rolls having a temperature of 70 to 90 ° C to form a uniform polyvinyl alcohol film. That is, since the polyvinyl alcohol polymer is cross-linked to each other by a water-soluble crosslinking agent (added when preparing the aqueous solution of the casting film), the transfer of the polymer to the surface layer of the film during drying will be carried out in the film. Suppress. Therefore, a film having high uniformity and having no difference in polymer density toward the thickness direction of the film can be obtained. Later, the polyvinyl alcohol film formed through the above steps is subjected to a heat treatment process, thereby ensuring mechanical properties. In view of the mechanical properties and stretchability of the film, the heat treatment is preferably carried out at a temperature of from 100 to 160 ° C, more preferably from 1 20 to 140 ° C. In this step, the heat treatment method is not limited, but if a conventional heat treatment method has been used in the art to which the present invention pertains, it can be used herein. As one of the examples of the heat treatment method, the film may be heat treated by contacting the film with a high temperature roller, or may be heat treated with high temperature air (such as hot air at a temperature of 100 to 160 ° C). The method. In order to uniformly heat treat the film, the film is preferably heat treated in an air flow mode. The heat-treated polyvinyl alcohol film is preferably a step of further humidity adjustment. The humidity conditioning step is performed to eliminate the heat history caused during the drying and heat treatment process, thereby improving the stretchability and polarization. There is no limitation in the humidity adjustment method 'but this humidity adjustment method can be carried out under humidity conditioning conditions widely used in the art to which the present invention pertains. For example, the humidity adjustment step can be carried out under the conditions of a temperature of 8 Torr to 150 ° C and a humidity of 50 to 80% -12 - 200844503. The polyvinyl alcohol film prepared according to the above method can be wound using conventional techniques known in the art to which the present invention pertains. In particular, the anti-blocking agent added to solve the problem of adhesion between the films in the winding drum should not be added in the examples of the present invention. This is because the water-soluble crosslinking agent in the film has an effect of suppressing adhesion between the films by adjusting the reaction rate on the surface of the film to an appropriate number. (C) Dyeing polyvinyl alcohol film This step is a process of dyeing a polyvinyl alcohol film prepared according to the above steps. The dyeing procedure is not limited in the present invention, but the dyeing procedure commonly used in the art to which the present invention pertains may be carried out by iodine or a heavy chrome dye, and the polyvinyl alcohol film is preferably dyed in an aqueous solution of iodine and boric acid, and more preferably Preferably, potassium iodide is added to the aqueous solution. For example, the polyvinyl alcohol film can be immersed in an aqueous solution containing 0.03 to 0.2% iodine and 0.5 to 5% potassium iodide for 3 to 10 minutes. The polyvinyl alcohol film thus prepared is subjected to a stretching process to complete a polyvinyl alcohol polarizing film. The stretching procedure can be carried out before/after dyeing the polyvinyl alcohol film, or at the same time as the dyeing step. That is to say, the polyvinyl alcohol film can be dyed first and then stretched, and then dyed after stretching, or can be dyed and stretched at the same time. The stretching procedure is not limited in any way, but a stretching procedure commonly used in the art to which the present invention pertains can also be used herein. For example, the polyvinyl alcohol film is preheated at a temperature of 40 to 80 ° C for 30 seconds to 5 minutes, and then pulled down by 1.1 to 7 times at a temperature of 40 to 80 ° C (preferably 45 to 60 ° C). 200844503 The polyoxyethylene alcohol polarizing film prepared according to the production method of the present invention has a uniform structure in the thickness direction. At the same time, the polyvinyl alcohol polarizing film also has a very high degree of polarization, which shows that the polyvinyl alcohol polarizing film of the present invention has an improved degree of polarization when compared with a conventional polyvinyl alcohol polarizing film. The preferred embodiments of the present invention are described in the following, and the description of the present invention is intended to be a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Example 1 27% by weight of polyvinyl alcohol having an average degree of polymerization of 3,200 and a degree of saponification of 99.9 mol% was mixed and dissolved in water. At the same time, 1 part by weight of polyvinyl alcohol in an aqueous solution, 1 part by weight of boric acid, 12.4 parts by weight of glycerin and 0.1 part by weight of a surfactant, a polyoxyethylene ethyl ether compound are added. In the aqueous solution, an aqueous solution containing 30.4% by weight of a nonvolatile compound was prepared. This dissolution step is carried out in a tank having a temperature of 130 ° C or higher in order to suppress the formation of insoluble matter. The prepared aqueous solution was subjected to a defoaming process, and then transferred to a T-die via a gear pump and a microfilter, and the aqueous solution was applied to a drum at 95 ° C via a T 5 mold at 95 ° C. And a film is formed. At this time, the distance between the T-die and the drum was maintained within 3 mm. The film was peeled off from the drum, and then the film was passed through a drying roller under a drying condition of 85 ° to form a polyethene film having a moisture content of 8%. The film thus formed was passed through an air flow machine at a temperature of 120 ° C to form crystals. Next, the film was subjected to a humidity adjustment procedure to prepare a polyvinyl alcohol film having a thickness of 75 μm. -14- 200844503 The prepared polyvinyl alcohol film was swollen and dyed in an aqueous solution containing 0. 05 % iodine and 0. 3 % potassium iodide, and then stretched in a 50 ° C bath in which 3% boric acid was dissolved. A polyvinyl alcohol polarizing film was obtained 5 times. Then, the final polarizing film was prepared by air drying the polyvinyl alcohol at a temperature of 80 ° C. Example 2 mixing 30.0% by weight of polyvinyl alcohol having an average degree of polymerization of 2600 and a degree of saponification of 99.95 mol%, And dissolve it in water. At the same time, 100 parts by weight of polyvinyl alcohol in an aqueous solution, 1 part by weight of boric acid, 12.4 parts by weight of glycerin, and 1 part by weight of a polyoxyethylene ethyl ether compound are added to the aqueous solution, and the preparation contains 3 parts by weight. An aqueous solution of 3.7% by weight of a non-volatile compound. A polyvinyl alcohol polarizing film was prepared in the same manner as in Example 1 to carry out a subsequent procedure. Example 3 This example was repeated in the same manner as in Example 1 to prepare a polyvinyl alcohol polarizing film, except that, in the aqueous solution preparation procedure, 1.0 part by weight of boric acid was added in terms of 1 part by weight of polyvinyl alcohol. Aqueous solution. However, the prepared polyvinyl alcohol has a thickness of 50 μm. Comparative Example 1 This comparative example was repeated in the same manner as in Example 1 to prepare a polyvinyl alcohol polarizing film, except that an aqueous solution was prepared without adding boric acid in the aqueous solution preparation procedure. However, the film was first coated with powder -15-200844503 prior to the winding process to prevent adhesion between the films in the winding drum. The film was crosslinked in a 3% aqueous solution of boric acid, dyed in 0.05% iodine and 0.3% potassium iodide aqueous solution, and stretched 5 times in a 50 ° C bath in which 3% boric acid was dissolved to obtain a polyvinyl alcohol polarizing film. . Performance Evaluation Dyeing and Stretching The dyeing and stretching uniformity of the polarizing film prepared in Example 1 and Comparative Example 1 were evaluated by an optical microscope (Lei ca DM digital microscopic method). The results are shown separately in Figures 2 and 3. Polarization In the present invention, the degree of polarization is measured by measuring the light transmission curves of two overlapping polarizing films according to the following equation. The measuring machine used was taken from N&K Technology's n&kl280 analyzer. Equation 1 _ Polarization χίοο In Equation 1, H i represents the light transmittance obtained by measuring the horizontally overlapping polarizing film, and H2 represents the light transmittance obtained by measuring the vertically overlapping polarizing film. The light used in the measurement has a wavelength of 400 to 700 nm, and the average degree of polarization of the same wavelength can be calculated after the degree of polarization is measured. The results according to Examples 1 to 3 and Comparative Example 1 are serially listed in Table 1 below. -16- 200844503 Table 1 Example Comparative Example 1 2 3 1 Comparison of aqueous solution Polyvinyl alcohol (% by weight) 27 30 27 27 Boric acid (parts by weight) 0.1 0.01 1.0 0 Glycerol (parts by weight) 12.4 12.4 12.4 12.4 Surface activity Agent (parts by weight) 0.1 0.1 0.1 0.1 Non-volatile compound (% by weight) 30.4 33.7 30.4 30.4 Dyeing/stretching boric acid (%) A 3 iodine (%) 0.05 0.05 0.05 0.05 Stretchability (multiple) 5.2 5.4 5.0 5.2 Polarization characteristic polarization (%) 99.7 99.5 99.3 98.5 As can be seen from the results of Table 1, the polyvinyl alcohol film prepared according to the examples of the present invention has excellent dyeing properties and stretchability. The results of the dyeing properties and the stretchability were determined from the optical micrographs of Figs. 2 and 3, and the polarizing films prepared in Example 1 and Comparative Example 1 were taken from Figs. 2 and 3. 2 is an optical micrograph showing the dyeing characteristics of the polyvinyl alcohol polarizing film according to Example 1 of the present invention, and FIG. 3 is an optical microscope showing the dyeing property of the polyvinyl alcohol polarizing film according to Comparative Example 1 of the present invention. photo. It is revealed in Fig. 2 that the central portion of the film is uniformly dyed in the thickness direction of the film, but as shown in Fig. 3, some central regions of the film are not dyed in the thickness direction of the film. Therefore, it is revealed that the polarizing film prepared according to Example 1 of the present invention has excellent dyeing properties and stretchability, whereas the polarizing film of Comparative Example 1 has poor dyeing properties and stretchability. From the results of Table 1, it can be seen that the polarizing film according to the present invention has an excellent degree of polarization as compared with Comparative Example 1. -17- 200844503 Industrial Applicability According to the preparation method of the present invention, the phenomenon that the polyvinyl alcohol polymer is transferred to the surface layer of the film upon evaporation of water can be improved in the step of solution-casting and dyeing the aqueous solution of the polyvinyl alcohol polymer. This is because the polyvinyl alcohol polymer is crosslinked with the added water-soluble crosslinking agent. Therefore, the dyeing property and the stretchability in the subsequent step can be improved, and the prepared polarizing film has improved mechanical properties and polarization. As described above, the preferred embodiments of the present invention have been described in detail. It should be understood, however, that the description of the preferred embodiments of the invention and the invention It is obvious from this detailed explanation. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a cross section of a conventional polarizing plate provided with a polyvinyl alcohol polarizing film. Fig. 2 is an optical micrograph showing the dyeing characteristics of a polyvinyl alcohol polarizing film according to Example 1 of the present invention. Fig. 3 is an optical micrograph showing the dyeing characteristics of the polyvinyl alcohol polarizing film of Comparative Example 1 according to the present invention. [Main component symbol description] 1 : Polarizer -18- 200844503 2 : Protective film 3 : Protective film 4 : Pressure-sensitive adhesive layer 5 : Pressure-sensitive adhesive layer 6 : Polarizer 7 : Protective layer 8 : Protective layer 9 : Adhesive layer 1 〇: polarizing film