本發明之實施形態之偏光元件保護膜之製造方法包括：對含有丙烯酸系樹脂及核殼型粒子之組合物進行膜形成及對所獲得之膜進行延伸。 A.丙烯酸系樹脂 A-1.丙烯酸系樹脂之構成 作為丙烯酸系樹脂，可採用任意適當之丙烯酸系樹脂。丙烯酸系樹脂代表而言含有作為單體單元之(甲基)丙烯酸烷基酯作為主要成分。本說明書中之所謂「(甲基)丙烯酸」意指丙烯酸及/或甲基丙烯酸。作為形成丙烯酸系樹脂之主骨架之(甲基)丙烯酸烷基酯，可例示直鏈狀或支鏈狀之烷基之碳數1～18者。該等(甲基)丙烯酸酯可單獨使用或組合使用。進而，可藉由共聚將任意適當之共聚單體導入至丙烯酸系樹脂中。此種共聚單體之種類、數量、共聚比等可根據目的適當設定。以下一面參照通式(2)一面描述丙烯酸系樹脂之主骨架之構成成分(單體單元)。 丙烯酸系樹脂較佳為具有選自由戊二醯亞胺單元、內酯環單元、馬來酸酐單元、馬來醯亞胺單元及戊二酸酐單元中之至少一種。具有內酯環單元之丙烯酸系樹脂例如記載於日本專利特開2008-181078號中，該公報之記載以參考之方式援引入本說明書中。戊二醯亞胺單元較佳為由下述通式(1)表示： [化1]通式(1)中，R1
及R2
分別獨立地表示氫原子或碳數1～8之烷基，R3
表示氫原子、碳數1～18之烷基、碳數3～12之環烷基或碳數6～10之芳基。通式(1)中，較佳為R1
及R2
分別獨立地為氫原子或甲基，R3
為氫原子、甲基、丁基或環己基。更佳為，R1
為甲基，R2
為氫原子，R3
為甲基。 上述(甲基)丙烯酸烷基酯代表而言由下述通式(2)表示： [化2]通式(2)中，R4
表示氫原子或甲基，R5
表示氫原子，或可經取代之碳數1～6之脂肪族或脂環式烴基。作為取代基，例如可列舉：鹵素、羥基。作為(甲基)丙烯酸烷基酯之具體例，可列舉：(甲基)丙烯酸甲酯、(甲基)丙烯酸乙酯、(甲基)丙烯酸正丙酯、(甲基)丙烯酸正丁酯、(甲基)丙烯酸第三丁酯、(甲基)丙烯酸正己酯、(甲基)丙烯酸環己酯、(甲基)丙烯酸氯甲酯、(甲基)丙烯酸2-氯乙酯、(甲基)丙烯酸2-羥乙酯、(甲基)丙烯酸3-羥丙酯、(甲基)丙烯酸2,3,4,5,6-五羥基己酯及(甲基)丙烯酸2,3,4,5-四羥基戊酯。通式(2)中，R5
較佳為氫原子或甲基。因此，特別較佳之(甲基)丙烯酸烷基酯為丙烯酸甲酯或甲基丙烯酸甲酯。 丙烯酸系樹脂可僅包含單一戊二醯亞胺單元，或者可包含通式(1)中之R1
、R2
及R3
不同之複數個戊二醯亞胺單元。 上述丙烯酸系樹脂中之戊二醯亞胺單元之含有比率較佳為2莫耳%～50莫耳%，更佳為2莫耳%～45莫耳%，進而較佳為2莫耳%～40莫耳%，特別較佳為2莫耳%～35莫耳%，最佳為3莫耳%～30莫耳%。若含有比率少於2莫耳%，則存在源自戊二醯亞胺單元而表現之效果(例如，高光學特性、高機械強度、與偏光元件之優異之接著性、薄型化)未充分地表現出來之虞。若含有比率超過50莫耳%，則存在例如樹脂之耐熱性、透明性不充分之虞。 上述丙烯酸系樹脂可僅包含單一之(甲基)丙烯酸烷基酯單元，或者可包含通式(2)中之R4
及R5
不同之複數個(甲基)丙烯酸烷基酯單元。 上述丙烯酸系樹脂中之(甲基)丙烯酸烷基酯單元之含有比率較佳為50莫耳%～98莫耳%，更佳為55莫耳%～98莫耳%，進而較佳為60莫耳%～98莫耳%，特別較佳為65莫耳%～98莫耳%，最佳為70莫耳%～97莫耳%。若含有比率少於50莫耳%，則存在源自(甲基)丙烯酸烷基酯單元而表現之效果(例如，高耐熱性、高透明性)未充分地發揮之虞。若上述含有比率多於98莫耳%，則存在樹脂變脆並且易於斷裂，無法充分地發揮高機械強度，從而生產性較差之虞。 上述丙烯酸系樹脂可包含戊二醯亞胺單元及(甲基)丙烯酸烷基酯單元以外之單元。 於一實施形態中，丙烯酸系樹脂可包含例如0～10重量%之不參與後述分子內醯亞胺化反應之不飽和羧酸單元。不飽和羧酸單元之含有比率較佳為0～5重量%，更佳為0～1重量%。若含量在此種範圍內，則可維持透明性、滯留穩定性及耐濕性。 於一實施形態中，丙烯酸系樹脂可包含上述以外之可共聚之乙烯基系單體單元(其他乙烯基系單體單元)。作為其他乙烯基系單體，例如可列舉：丙烯腈、甲基丙烯腈、乙基丙烯腈、烯丙基縮水甘油醚、馬來酸酐、衣康酸酐、N-甲基馬來醯亞胺、N-乙基馬來醯亞胺、N-環己基馬來醯亞胺、丙烯酸胺基乙酯、丙烯酸丙基胺基乙酯、甲基丙烯酸二甲基胺基乙酯、甲基丙烯酸乙基胺基丙酯、甲基丙烯酸環己基胺基乙酯、N-乙烯基二乙胺、N-乙醯基乙烯胺、烯丙胺、甲基烯丙胺、N-甲基烯丙胺、2-異丙烯基噁唑啉、2-乙烯基噁唑啉、2-丙烯醯基噁唑啉、N-苯基馬來醯亞胺、甲基丙烯酸苯基胺基乙酯、苯乙烯、α-甲基苯乙烯、對縮水甘油基苯乙烯、對胺基苯乙烯、2-苯乙烯基-噁唑啉。該等可單獨使用或併用。較佳為苯乙烯、α-甲基苯乙烯等苯乙烯系單體。其他乙烯基系單體單元之含有比率較佳為0～1重量%，更佳為0～0.1重量%。若含量在此種範圍內，則可抑制不期望之相位差之表現及透明性之降低。 上述丙烯酸系樹脂中之醯亞胺化率較佳為2.5%～20.0%。若醯亞胺化率在此種範圍內，則可獲得耐熱性、透明度及成形加工性優異之樹脂，並可防止膜成形時發生燒焦或機械強度降低。於上述丙烯酸系樹脂中，醯亞胺化率由戊二醯亞胺單元與(甲基)丙烯酸烷基酯單元之比表示。該比可由例如丙烯酸系樹脂之NMR譜或IR譜等獲得。於本實施形態中，醯亞胺化率可藉由使用1
H-NMR BRUKER AvanceIII(400 MHz)藉由樹脂之1
H-NMR測定來求出。更具體而言，將於3.5 ppm至3.8 ppm附近之源自(甲基)丙烯酸烷基酯之O-CH3
質子之峰面積設為A，將於3.0 ppm至3.3 ppm附近之源自戊二醯亞胺之N-CH3
質子之峰面積設為B，藉由如下式而求出。 醯亞胺化率Im(%)＝{B/(A＋B)}×100 上述丙烯酸系樹脂之酸值較佳為0.10 mmol/g～0.50 mmol/g。若酸值在此種範圍內，則可獲得耐熱性、機械物性及成形加工性之平衡優異之樹脂。若酸值過小，則有產生由於用於調整為所需酸值之改性劑之使用而引起之成本增加，由於改性劑之殘留而引起之凝膠狀物之產生之問題。若酸值過大，則於膜成形時(例如，熔融擠出時)易於引起發泡，有成形品之生產性降低之傾向。於上述丙烯酸系樹脂中，酸值係該丙烯酸系樹脂中之羧酸單元及羧酸酐單元之含量。於本實施形態中，酸值可藉由例如WO 2005/054311或日本專利特開2005-23272號中所記載之滴定法而計算出。 上述丙烯酸系樹脂之重量平均分子量較佳為1000～2000000，更佳為5000～1000000，進而較佳為10000～500000，特別較佳為50000～500000，最佳為60000～150000。重量平均分子量可用例如凝膠滲透色譜(GPC系統，東曹公司製造)，藉由聚苯乙烯換算而求出。再者，可將四氫呋喃用作溶劑。 上述丙烯酸系樹脂之Tg(玻璃轉移溫度)較佳為110℃以上，更佳為115℃以上，進而較佳為120℃以上，特別較佳為125℃以上，最佳為130℃以上。若Tg為110℃以上，則包括由此種樹脂獲得之偏光元件保護膜之偏光板容易具有優異之耐久性。Tg之上限值較佳為300℃以下，更佳為290℃以下，進而較佳為285℃以下，特別較佳為200℃以下，最佳為160℃以下。若Tg在此種範圍內，則可使成形性優異。 A-2.丙烯酸系樹脂之聚合 上述丙烯酸系樹脂可藉由例如以下方法製造。該方法包括：(I)使與通式(2)所表示之(甲基)丙烯酸烷基酯單元對應之(甲基)丙烯酸烷基酯單體與不飽和羧酸單體及/或其前驅物單體共聚來獲得共聚物(a)；以及(II)用醯亞胺化劑處理該共聚物(a)，以於共聚物(a)中進行(甲基)丙烯酸烷基酯單體單元與不飽和羧酸單體及/或其前驅物單體單元之分子內醯亞胺化反應，從而將由通式(1)表示之戊二醯亞胺單元導入共聚物中。 作為不飽和羧酸單體，例如可列舉：丙烯酸、甲基丙烯酸、巴豆酸、α-取代丙烯酸、α-取代甲基丙烯酸。作為其前驅物單體，例如可列舉：丙烯醯胺、甲基丙烯醯胺。該等可單獨使用或併用。較佳之不飽和羧酸單體為丙烯酸或甲基丙烯酸，較佳之前驅物單體為丙烯醯胺。 可使用任意適當之方法作為用醯亞胺化劑處理共聚物(a)之方法。作為具體例，可列舉使用擠出機之方法、及使用分批式反應槽(壓力容器)之方法。使用擠出機之方法包括使用擠出機加熱熔融共聚物(a)，並用醯亞胺化劑處理該共聚物。於該情形時，可使用任意適當之擠出機作為擠出機。作為具體例，可列舉：單軸擠出機、雙軸擠出機、及多軸擠出機。於使用分批式反應槽(壓力容器)之方法中，可使用任意適當之分批式反應槽(壓力容器)。 只要可產生由通式(1)所表示之戊二醯亞胺單元，則可使用任意適當之化合物作為醯亞胺化劑。作為醯亞胺化劑之具體例，可列舉：甲胺、乙胺、正丙胺、異丙胺、正丁胺、異丁胺、第三丁胺、正己胺等含脂肪族烴基之胺；苯胺、苄胺、甲苯胺、三氯苯胺等含芳香族烴基之胺；以及環己胺等含脂環式烴基之胺。進而，例如亦可使用藉由加熱產生此種胺之脲系化合物。作為脲系化合物，例如可列舉：脲、1,3-二甲基脲、1,3-二乙基脲、1,3-二丙基脲。醯亞胺化劑較佳為甲胺、氨、環己胺，更佳為甲胺。 於醯亞胺化中，根據需要，除上述醯亞胺化劑外亦可添加閉環促進劑。 醯亞胺化中之醯亞胺化劑之使用量相對於共聚物(a)100重量份，較佳為0.5重量份～10重量份，更佳為0.5重量份～6重量份。若醯亞胺化劑之使用量少於0.5重量份，則很多情形未達成期望之醯亞胺化率。其結果，存在所獲得之樹脂之耐熱性變得極不充分，引起成膜後之燒焦等外觀缺陷之情形。若醯亞胺化劑之使用量超過10重量份，則存在醯亞胺化劑殘留於樹脂中，藉由醯亞胺化劑而引起成形後之燒焦等外觀缺陷或發泡之情形。 除了上述醯亞胺化以外，本實施形態之製造方法亦可根據需要包括採用酯化劑之處理。 作為酯化劑，例如可列舉：碳酸二甲酯、2,2-二甲氧基丙烷、二甲基亞碸、原甲酸三乙酯、原乙酸三甲酯、原甲酸三甲酯、碳酸二苯酯、硫酸二甲酯、甲苯磺酸甲酯、三氟甲磺酸甲酯、乙酸甲酯、甲醇、乙醇、異氰酸甲酯、對氯苯基異氰酸酯、二甲基碳二醯亞胺、二甲基第三丁基甲矽烷基氯、乙酸異丙烯酯、二甲基脲、四甲基氫氧化銨、二甲基二乙氧基矽烷、四-N-丁氧基矽烷、亞磷酸二甲基(三甲基矽烷)酯、亞磷酸三甲酯、磷酸三甲酯、磷酸三甲苯酯、重氮甲烷、環氧乙烷、環氧丙烷、環氧環己烷、2-乙基己基縮水甘油醚、苯基縮水甘油醚、苄基縮水甘油醚。該等之中，就成本、及反應性等觀點而言，較佳為碳酸二甲酯。 可設定酯化劑之添加量，使丙烯酸系樹脂之酸值為期望之值。 A-3.其他樹脂之併用 於本發明之實施形態中，可併用上述丙烯酸系樹脂及其他樹脂。即，可將構成丙烯酸系樹脂之單體成分與構成其他樹脂之單體成分共聚，將該共聚物供於C項中下文描述之膜形成；或者將丙烯酸系樹脂與其他樹脂之摻合物供於膜形成。作為其他樹脂，例如可列舉：苯乙烯系樹脂、聚乙烯、聚丙烯、聚醯胺、聚苯硫醚、聚醚醚酮、聚酯、聚碸、聚苯醚、聚縮醛、聚醯亞胺、聚醚醯亞胺等其他熱塑性樹脂，酚系樹脂、三聚氰胺系樹脂、聚酯系樹脂、矽酮系樹脂、環氧系樹脂等熱硬化性樹脂。併用之樹脂之種類及調配量可根據目的及獲得之膜所需之特性來適當地設定。例如，苯乙烯系樹脂(較佳為丙烯腈-苯乙烯共聚物)可併用作相位差控制劑。 於併用丙烯酸系樹脂及其他樹脂之情形時，丙烯酸系樹脂與其他樹脂之摻合物中之丙烯酸系樹脂之含量較佳為50重量%～100重量%，更佳為60重量%～100重量%，進而較佳為70重量%～100重量%，特別較佳為80重量%～100重量%。於含量未達50重量%之情形時，存在未充分反映丙烯酸系樹脂所固有之高耐熱性、及高透明性之虞。 A-4.添加劑 丙烯酸系樹脂於聚合時亦可根據目的添加任意適當之添加劑。作為添加劑之具體例，可列舉：紫外線吸收劑；受阻酚系、磷系、硫系等抗氧化劑；耐光穩定劑、耐候穩定劑、熱穩定劑等穩定劑；玻璃纖維、碳纖維等補強材料；近紅外吸收劑；磷酸三(二溴丙基)酯、磷酸三烯丙基酯、氧化銻等阻燃劑；陰離子系、陽離子系、非離子系之界面活性劑等抗靜電劑；無機顏料、有機顏料、染料等著色劑；有機填料或無機填料；樹脂改質劑；有機填充劑或無機填充劑；塑化劑；以及潤滑劑等。添加劑可於丙烯酸系樹脂聚合時添加，或者可於膜形成時添加。添加劑之種類、數量、組合、添加量等可根據目的適當設定。再者，添加劑可於C項中後述之膜形成時添加於組合物中。 B.核殼型粒子 核殼型粒子代表性而言具有包含橡膠狀聚合物之核及包含玻璃狀聚合物且被覆該核之被覆層。核殼型粒子可具有一層以上包含玻璃狀聚合物之層作為最內層或中間層。然而，於本發明之實施形態中，於將核殼型粒子分散於組合物中之步驟中，存在被覆層與組合物中之樹脂成分相容，無法視覺(包括經由顯微鏡等之情形)識別被覆層之情形。 形成核之橡膠狀聚合物之Tg較佳為20℃以下，更佳為-60℃～20℃，進而較佳為-60℃～10℃。若構成核之橡膠狀聚合物之Tg超過20℃，則存在丙烯酸系樹脂之機械強度之提高不充分之虞。構成被覆層之玻璃狀聚合物(硬質聚合物)之Tg較佳為50℃以上，更佳為50℃～140℃，進而較佳為60℃～130℃。若構成被覆層之玻璃狀聚合物之Tg低於50℃，則存在丙烯酸系樹脂之耐熱性降低之虞。 核殼型粒子中之核之含有比率較佳為30重量%～95重量%，更佳為50重量%～90重量%。核中之玻璃狀聚合物層之比率，相對於核之總量100重量%，為0～60重量%，較佳為0～45重量%，更佳為10～40重量%。核殼型粒子中之被覆層之含有比率較佳為5重量%～70重量%，更佳為10重量%～50重量%。 核之平均粒徑較佳為70 nm～300 nm。若為此種平均粒徑，則可藉由後述之延伸扁平化成期望之長度及厚度(因此為長度/厚度之比)。 上述組合物較佳為含有核殼型粒子7重量%～30重量%，更佳為含有8重量%～25重量%。若核殼型粒子之含量在此種範圍內，則可實現極其優異之與偏光元件之密接性及極其優異之耐彎曲性。 構成核殼型粒子之核之橡膠狀聚合物、構成被覆層之玻璃狀聚合物(硬質聚合物)、該等之聚合方法及其他構成之詳細情況，例如記載於日本專利特開2016-33552號公報中。該公報之記載以參考之方式援引入本說明書中。 核殼型粒子之扁平化(補強粒子之形成)，與延伸相關，於D項中後述。 C.膜形成 可採用任意適當之方法，作為由上述組合物形成膜之方法。作為具體例，可列舉：澆鑄塗佈法(例如，流延法)、擠出成形法、射出成形法、壓縮成形法、轉移成形法、吹塑成形法、粉末成形法、FRP成形法、壓延成形法、熱壓法。較佳為擠出成形法或澆鑄塗佈法。其原因在於，提高所獲得之膜之光滑性，可獲得良好之光學均勻性。尤其較佳為擠出成形法。其原因在於，無須考慮由殘留溶劑引起之問題。其中，就膜之生產性及以後延伸處理之容易性之觀點而言，較佳為使用T型模頭之擠出成形法。成形條件可根據所使用之樹脂之組成或種類、所獲得之膜期望之特性來適當地設定。 D.延伸 可採用任意適當之延伸方法、延伸條件(例如延伸溫度、延伸倍率、延伸速度、延伸方向)作為延伸方法。作為延伸方法之具體例，可採用自由端延伸、固定端延伸、自由端收縮、固定端收縮。該等可單獨使用，亦可同時使用，或者可逐次使用。 根據目的可採用適當之方向作為延伸方向。具體而言，可列舉：長度方向、寬度方向、厚度方向、傾斜方向。延伸方向可為一方向(單軸延伸)，亦可為兩方向(雙軸延伸)，或者可為三方向以上。於本發明之實施形態中，代表而言可採用長度方向之單軸延伸、長度方向及寬度方向之同時雙軸延伸、長度方向及寬度方向之逐次雙軸延伸。較佳為雙軸延伸(同時或逐次)。其原因在於，容易控制面內相位差，容易實現光學各向同性。 於採用雙軸延伸之情形時，延伸模式可為同時雙軸延伸，或者可為逐次雙軸延伸。同時雙軸延伸由於無輥延伸步驟，因此膜表面幾乎難以劃上傷痕，就膜之外觀而言，與逐次延伸相比具有優勢。相對於此，逐次雙軸延伸由於分為縱向延伸步驟與橫向延伸步驟，因此膜難以破裂，就生產性之觀點而言，具有優勢。於逐次雙軸延伸中，從向延伸或橫向延伸中之任一者可先進行。於逐次雙軸延伸中，較佳為縱向延伸及橫向延伸依次進行。 延伸溫度可根據偏光元件保護膜期望之光學特性、機械特性及厚度、所使用之樹脂之種類、所使用之膜之厚度、延伸方法(單軸延伸或雙軸延伸)、延伸倍率、延伸速度而發生變化。本發明之實施形態中，延伸溫度如上所述為Tg＋20℃～Tg＋55℃，較佳為Tg＋30℃～Tg＋50℃，更佳為Tg＋35℃～Tg＋50℃。於採用同時雙軸延伸之情形時，延伸溫度較佳為Tg＋30℃～Tg＋55℃，更佳為Tg＋40℃～Tg＋55℃，進而較佳為Tg＋40℃～Tg＋50℃。於採用逐次雙軸延伸之情形時，延伸溫度較佳為Tg＋20℃～Tg＋55℃，更佳為Tg＋30℃～Tg＋55℃，進而較佳為Tg＋35℃～Tg＋50℃。藉由於此種溫度下進行延伸，可獲得具有適當之特性之偏光元件保護膜。具體之延伸溫度例如為140℃～175℃，較佳為155℃～175℃。根據本發明之實施形態，藉由將延伸溫度、延伸倍率及延伸速度組合最佳化，可獲得耐彎曲性及與偏光元件之密接性均優異之偏光元件保護膜。再者，此處之Tg係組合物之樹脂成分之Tg。 與延伸溫度同樣，延伸倍率亦可根據偏光元件保護膜期望之光學特性、機械特性及厚度、所使用之樹脂之種類、所使用之膜之厚度、延伸方法(單軸延伸或雙軸延伸)、延伸溫度、延伸速度而發生變化。於採用雙軸延伸之情形時，於一個方向上之延伸倍率與另一個方向上之延伸倍率之比較佳為1.0～1.5，更佳為1.0～1.4，進而較佳為1.0～1.3。於一實施形態中，上述一個方向與上述另一個方向正交。例如，兩個方向中之一個方向可為長度方向(MD：縱向)，另一個方向可為寬度方向(TD：橫向)。於採用雙軸延伸之情形之面倍率(一個方向上之延伸倍率與另一個方向上之延伸倍率之乘積)如上所述為2.0～6.0，較佳為3.0～6.0，更佳為4.0～5.9。於採用同時雙軸延伸之情形時，面倍率較佳為2.0～5.0，更佳為2.0～4.5，進而較佳為3.0～4.5。於採用逐次雙軸延伸之情形時，面倍率較佳為2.0～6.0，更佳為3.0～5.9，進而較佳為4.0～5.9。根據本發明之實施形態，藉由將延伸溫度、延伸倍率及延伸速度組合最佳化，可獲得耐彎曲性及與偏光元件之密接性均優異之偏光元件保護膜。 與延伸溫度同樣，延伸速度亦可根據偏光元件保護膜期望之光學特性、機械特性及厚度、所使用之樹脂之種類、所使用之膜之厚度、延伸方法(單軸延伸或雙軸延伸)、延伸溫度、延伸倍率而發生變化。延伸速度如上所述為3%/秒～130%/秒，較佳為4%/秒～120%/秒，進而較佳為5%/秒～100%/秒。於採用同時雙軸延伸之情形時，延伸速度較佳為3%/秒～15%/秒，更佳為4%/秒～12%/秒，進而較佳為5%/秒～10%/秒。於採用逐次雙軸延伸之情形時，延伸速度較佳為3%/秒～130%/秒，更佳為4%/秒～110%/秒，進而較佳為5%/秒～100%/秒。於採用雙軸延伸之情形時，一個方向上之延伸速度與另一個方向上之延伸速度可相同或不同。於採用同時雙軸延伸之情形時，一個方向上之延伸速度與於另一個方向上之延伸速度之比較佳為1.0～1.3，更佳為1.0～1.2，進而較佳為1.0～1.1。例如於依縱向延伸及橫向延伸之順序採用逐次雙軸延伸之情形時，縱方向之延伸速度與橫方向之延伸速度之比(縱向/橫向)較佳為7.0～17.0，更佳為8.0～15.0，進而較佳為9.0～13.0。根據本發明之實施形態，藉由將延伸溫度、延伸倍率及延伸速度組合最佳化，可獲得耐彎曲性及與偏光元件之密接性均優異之偏光元件保護膜。 藉由上述此種延伸，將核殼型粒子適當地扁平化(以下將經扁平化之核殼型粒子稱作補強粒子)。 補強粒子之長度/厚度之比較佳為7.0以下、更佳為6.5以下、進而較佳為6.3以下。另一方面，長度/厚度之比較佳為4.0以上，更佳為4.5以上，進而較佳為5.0以上。若長度/厚度之比在此種範圍內，則可一面維持因調配粒子所引起之優異之耐彎曲性一面可顯著提高偏光元件保護膜與偏光元件之密接性。本說明書中所謂「厚度/長度之比」意指補強粒子之平面視形狀之代表長度與厚度之比。此處，所謂「代表長度」係指於平面視形狀為圓形時之直徑，於平面視形狀為橢圓形時之長徑，以及於平面視形狀為矩形或多邊形時之對角線長度。該比可藉由例如以下順序來獲得。用透射電子顯微鏡(例如，加速電壓80 kv，RuO4
染色超薄切片法)拍攝所獲得之膜剖面，自所得之照片中存在之補強粒子中依序抽出30個最長粒子(獲得接近代表長度之剖面者)，計算(長度之平均值)/(厚度之平均值)，獲得該比。 核之厚度較佳為20 nm～100 nm。該核之代表長度較佳為200 nm～600 nm。於核之代表長度過短之情形時，存在所獲得之膜之機械強度之提高不充分之情形。於核之厚度過厚之情形或核之代表長度過長之情形時，存在所獲得之膜與偏光元件之密接性受損之虞。 以如上方式，可形成偏光元件保護膜。如此獲得之偏光元件保護膜含有丙烯酸系樹脂及分散於該丙烯酸系樹脂中並具有扁平形狀之補強粒子。 E.利用上述製造方法獲得之偏光元件保護膜及其特性 偏光元件保護膜較佳為實質上具有光學各向同性。本說明書中所謂「實質上具有光學各向同性」意指面內相位差Re(550)為0 nm～10 nm，厚度方向之相位差Rth(550)為-20 nm～+10 nm。面內相位差Re(550)更佳為0 nm～5 nm，進而較佳為0 nm～3 nm，特別較佳為0 nm～2 nm。厚度方向之相位差Rth(550)更佳為-5 nm～+5 nm，進而較佳為-3 nm～+3 nm，特別較佳為-2 nm～+2 nm。若偏光元件保護膜之Re(550)及Rth(550)在此種範圍內，則可防止將包括該偏光元件保護膜之偏光板應用於圖像顯示裝置之情形時對顯示特性之不良影響。再者，Re(550)係用23℃下之波長550 nm之光測定之膜之面內相位差。Re(550)係由公式：Re(550)＝(nx－ny)×d來求出。Rth(550)係用23℃下之波長550 nm之光測定之膜之厚度方向之相位差。Rth(550)係由公式：Rth(550)＝(nx－nz)×d」來測定。此處，nx為面內折射率最大時之方向(即，遲相軸方向)上之折射率，ny為面內與遲相軸正交之方向(即，進相軸方向)上之折射率，nz為厚度方向上之折射率，d為膜之厚度(nm)。 偏光元件保護膜之厚度80 μm下之380 nm下之光線透射率較佳為越高越好。具體而言，光線透過率較佳為85%以上，更佳為88%以上，進而較佳為90%以上。若光線透過率在此種範圍內，則可確保期望之透明度。藉由利用上述製造方法將補強粒子之長度/厚度之比如上述範圍最佳化，不僅可實現優異之耐彎曲性及與偏光元件之優異之密接性，而且可實現此種光線透過率。光線透過率可利用例如根據ASTM-D-1003之方法進行測定。 偏光元件保護膜之霧度越低越佳。具體而言，霧度較佳為5%以下，更佳為3%以下，進而較佳為1.5%以下，特別較佳為1%以下。若霧度為5%以下，則可賦予膜良好之清晰感。進而，即便於圖像顯示裝置之視認側偏光板中使用上述膜之情形時，亦可良好地視認顯示內容。藉由利用上述製造方法將補強粒子之長度/厚度之比如上述範圍最佳化，不僅可實現優異之耐彎曲性及與偏光元件之優異之密接性，而且可實現此種霧度。 偏光元件保護膜之厚度80 μm下之YI較佳為1.27以下，更佳為1.25以下，進而較佳為1.23以下，特別較佳為1.20以下。若YI超過1.3，則存在膜之光學透明性不充分之情形。藉由利用上述製造方法將補強粒子之長度/厚度之比如上述範圍最佳化，不僅可實現優異之耐彎曲性及與偏光元件之密接性，而且可實現此種YI。再者，YI可藉由使用例如高速積分球光譜透射率測量機(商品名DOT-3C：村上色彩技術研究所製造)之測定所獲得之顏色之三色激勵值(X、Y、Z)，由以下所示之公式來求出： YI＝[(1.28X－1.06Z)/Y]×100 偏光元件保護膜之厚度80 μm下之b值(與亨特色彩系統相一致之色相之量度)較佳為未達1.5，更佳為1.0以下。於b值為1.5以上之情形時，有出現不期望之色調之情形。再者，b值可藉由例如將偏光元件保護膜樣品切割成3平方厘米，用高速積分球光譜透射測量機(商品名DOT-3C：村上色彩技術研究所製造)測定其色相，並根據亨特色彩系統評價該色相來獲得。 偏光元件保護膜之透濕度較佳為300 g/m2
·24 hr以下，更佳為250 g/ m2
·24 hr以下，進而較佳為200 g/m2
·24 hr以下，特別較佳為150 g/m2
·24 hr以下，最佳為100 g/m2
·24 hr以下。若偏光元件保護膜之透濕度在此種範圍內，則可獲得耐久性及耐濕性優異之偏光板。 偏光元件保護膜之延伸強度較佳為10 MPa以上且未達100 MPa，更佳為30 MPa以上且未達100 MPa。於延伸強度未達10 MPa之情形時，存在無法表現出充分之機械強度之情形。若延伸強度超過100 MPa，則存在膜之加工性不充分之虞。延伸強度可根據例如ASTM-D-882-61T進行測定。 偏光元件保護膜之延伸伸長率較佳為1.0%以上，更佳為3.0%以上，進而較佳為5.0%以上。延伸伸長率之上限例如為100%。於延伸伸長率未達1%之情形時，存在膜之韌性不充分之情形。延伸伸長率可根據例如ASTM-D-882-61T進行測定。 偏光元件保護膜之拉伸彈性模數較佳為0.5 GPa以上，更佳為1 GPa以上，進而較佳為2 GPa以上。拉伸彈性模數之上限例如為20 GPa。於拉伸彈性模數未達0.5 GPa之情形時，存在膜無法表現出充分之機械強度之情形。拉伸彈性模數可根據例如ASTM-D-882-61T進行測定。 易接著層可形成於偏光元件保護膜之一個面上。易接著層例如含有水系聚胺基甲酸酯及噁唑啉系交聯劑。 F.偏光元件保護膜之用途 藉由本發明之製造方法獲得之偏光元件保護膜可適用於偏光板。偏光板代表性地具有偏光元件及配置於該偏光元件之至少一側之偏光元件保護膜。偏光元件由於採用業界公知之構成，因此省略詳細之說明。偏光板可適用於圖像顯示裝置。作為圖像顯示裝置之代表例，可列舉：液晶顯示裝置、有機電致發光(EL)顯示裝置。圖像顯示裝置由於採用業界公知之構成，因此省略詳細之說明。 [實施例] 以下藉由實施例來具體說明本發明，但本發明不受該等實施例之限定。各特性之測定方法如下所述。再者，只要未特別說明，則實施例中之「份」及「%」為重量基準。 ＜實施例1＞ (偏光元件保護膜之製作) 將MS樹脂(MS-200；甲基丙烯酸甲酯/苯乙烯(莫耳比)＝80/20共聚物，新日鐵化學股份有限公司製造)以單甲胺醯亞胺化(醯亞胺化率：5%)。所獲得之醯亞胺化MS樹脂具有由通式(1)表示之戊二醯亞胺單元(R1
及R3
為甲基，R2
為氫原子)、由通式(2)表示之(甲基)丙烯酸酯單元(R4
及R5
為甲基)及苯乙烯單元。再者，於上述醯亞胺化中使用口徑15 mm之相互嚙合型同向旋轉式雙軸擠出機。將擠出機之各溫度控制區之設定溫度設為230℃，並將其螺桿轉數設定為150 rpm，以2.0kg/hr供給MS樹脂，相對於MS樹脂100重量份，將單甲胺之供給量設為2重量份。自料斗投入MS樹脂，藉由捏合塊將樹脂熔融及充滿後，自噴嘴注入單甲胺。於反應區之末端插入密封環將樹脂充滿。將通風口之壓力減壓至-0.08 MPa，使反應後之副產物及過剩之甲胺脫揮。於水槽中冷卻自配置於擠出機出口之模頭中作為線料排出之樹脂，然後用造粒機將其顆粒化。所獲得之醯亞胺化MS樹脂之醯亞胺化率為5.0%，酸值為0.5 mmol/g。Tg為120℃。 將90重量份之上述獲得之醯亞胺化MS樹脂及10重量份之核殼型粒子(由Kaneka Corporation製造，商品名「KANE ACE M-210」)投入單軸擠出機中，於260℃下熔融擠出以獲得厚度120 μm之膜。將所獲得之擠出膜於延伸溫度160℃(Tg＋40℃)下沿其長度方向及寬度方向分別進行雙軸延伸各兩次。延伸速度於長度方向及寬度方向均為10%/秒。藉由該延伸使核殼型粒子扁平化，從而形成補強粒子。補強粒子之長度/厚度之比為6.3。以如此方式，製作偏光元件保護膜。所獲得之偏光元件保護膜之厚度為40 μm，面內相位差Re(550)為2 nm，厚度方向相位差Rth(550)為2 nm。將所獲得之偏光元件保護膜供於上述(2)之評價。結果示於表1。 (偏光板之製作) 1.偏光元件之製作 藉由利用輥式延伸機對厚度30 μm之聚乙烯醇(PVA)系樹脂膜(可樂麗製造，商品名「PE3000」)之伸長輥一面於長度方向上進行單軸延伸，以使其為5.9倍，一面進行膨潤、染色、交聯、洗淨處理，最後，對輥進行乾燥處理而製作厚度12 μm之偏光元件。 具體而言，膨潤處理係一面於20℃下之純水中進行處理一面將輥延伸至2.2倍。繼而，染色處理係於以所獲得之偏光元件之單體透射率為45.0%之方式調整碘濃度之碘與碘化鉀之重量比為1：7之30℃之水溶液中一面進行處理一面將輥延伸至1.4倍。進而，交聯處理係採用兩階段之交聯處理，第一階段之交聯處理係於40℃下之溶解有硼酸及碘化鉀之水溶液中進行處理一面將輥延伸至1.2倍。將第一階段之交聯處理之水溶液之硼酸含量設定為5.0重量%，並將碘化鉀含量設定為3.0重量%。第二階段之交聯處理係一面於65℃之溶解有硼酸及碘化鉀之水溶液中進行處理一面將輥延伸至1.6倍。將第二階段之交聯處理之水溶液之硼酸含量設定為4.3重量%，並將碘化鉀含量設為5.0重量%。又，洗淨處理係於20℃之碘化鉀水溶液中進行處理。將洗淨處理之水溶液之碘化鉀含量設為2.6重量%。最後乾燥處理係於70℃下乾燥5分鐘，獲得偏光元件。 2.偏光板之製作 於上述偏光元件之單側經由聚乙烯醇系接著劑貼合上述獲得之偏光元件保護膜，獲得偏光板。將所獲得之偏光板供於密接性之評價。將結果示於表1中。再者，密接性之評價係以如下方式進行。對所獲得之偏光板進行剝離試驗，評價偏光元件保護膜與偏光元件之密接力。具體而言如下所述。將偏光板剪切成為偏光元件之吸收軸方向上為200 mm、與吸收軸正交之方向上為15 mm之大小。於保護膜與偏光元件之間用切割刀刻入切口，將其貼合於玻璃板上。利用萬能拉力機以剝離速度為300 mm/min於90度方向上將保護膜及偏光元件剝離，測定其初期剝離強度(N/15 mm)。利用以下之基準進行評價。 ○：初期剝離強度為1.0(N/15 mm)以上 ×：初期剝離強度未達1.0(N/15 mm) ＜實施例2＞ 除將延伸溫度設為165℃(Tg＋45℃)以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例3＞ 除將延伸溫度設為170℃(Tg＋50℃)以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例4＞ 除將延伸溫度設為175℃(Tg＋55℃)以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例5＞ 除將延伸溫度設為155℃(Tg＋35℃)以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜比較例1＞ 除將延伸溫度設為180℃(Tg＋60℃)以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例6＞ 除將寬度方向之延伸倍率設為1.5倍，將表面倍率設為3.0以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例7＞ 除將寬度方向之延伸倍率設為2.6倍，將表面倍率設為5.2以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例8＞ 除將延伸速度於長度方向及寬度方向上均設為3%/秒以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例9＞ 除將延伸速度於長度方向及寬度方向上均設為5%/秒以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例10＞ 除將延伸速度於長度方向及寬度方向上均設為15%/秒以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜比較例2＞ 除將延伸速度於長度方向及寬度方向上均設為1%/秒以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例11＞ 除將延伸溫度設為170℃(Tg+50℃)，寬度方向之延伸倍率設為1.5倍，使表面倍率設為3.0，以及使延伸速度於長度方向及寬度方向上均設為3%/秒以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例12＞ 除將延伸速度於長度方向及寬度方向上均設為5%/秒以外，以與實施例11同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例13＞ 除將延伸速度於長度方向及寬度方向上均設為10%/秒以外，以與實施例11同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例14＞ 除將延伸速度於長度方向及寬度方向上均設為15%/秒以外，以與實施例11同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例15＞ 除將延伸溫度設為175℃(Tg＋55℃)，寬度方向之延伸倍率設為1.5倍，使表面倍率設為3.0，以及使延伸速度於長度方向及寬度方向上均設為3%/秒以外，以與實施例1同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例16＞ 除將延伸速度於長度方向及寬度方向上均設為5%/秒以外，以與實施例15同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例17＞ 除將延伸速度於長度方向及寬度方向上均設為10%/秒以外，以與實施例15同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例18＞ 除將延伸速度於長度方向及寬度方向上均設為15%/秒以外，以與實施例15同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜比較例3＞ 於長度方向上，以延伸溫度為160℃、延伸速度為100%/秒進行延伸之後，於寬度方向上以延伸溫度為136℃、延伸速度為12%/秒、表面倍率為5.8倍進行延伸，製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例19＞ 除將寬度方向之延伸溫度設為140℃以外，以與比較例3同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例20＞ 除將寬度方向之延伸溫度設為160℃以外，以與比較例3同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例21＞ 除將寬度方向之延伸溫度設為170℃以外，以與比較例3同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜比較例4＞ 除將長度方向之延伸速度設為40%/秒，寬度方向之延伸速度設為2%/秒以外，以與實施例20同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜實施例22＞ 除將長度方向之延伸速度設為60%/秒，寬度方向之延伸速度設為4%/秒以外，以與實施例20同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 ＜比較例5＞ 除將表面倍率設為7.0以外，以與實施例20同樣之方式製作偏光元件保護膜及偏光板。將所獲得之偏光板供於與實施例1同樣之評價。將結果示於表1。 [表1]
＜評價＞ 由表1明確，本發明之實施例之偏光元件保護膜之製造方法能夠以優異之生產性獲得與偏光元件之密接性之平衡優異之偏光元件保護膜(作為結果為偏光板)。 [產業上之可利用性] 藉由本發明之製造方法獲得之偏光元件保護膜較佳用於偏光板。偏光板較佳用於圖像顯示裝置。圖像顯示裝置可用於個人數字助理(PDA)、智能電話、行動電話、時鐘、數碼相機、便攜式遊戲機等便攜式設備；個人計算機監視器、筆記型個人計算機、複印機等OA設備；攝像機、電視機、微波爐等家用電器；倒車監視器、汽車導航系統用之監視器、汽車音頻等車載用設備等；數字標牌、商業店鋪用信息監視器等展示設備；監視用監視器等警備設備；以及護理用監視器、醫療用監視器等護理/醫療裝置；等各種用途。The method for manufacturing a polarizing element protective film according to an embodiment of the present invention includes: forming a film on a composition containing an acrylic resin and core-shell particles, and extending the obtained film. A. Acrylic resin A-1. Composition of acrylic resin As the acrylic resin, any appropriate acrylic resin can be used. The acrylic resin typically contains an alkyl (meth) acrylate as a monomer unit as a main component. The "(meth) acrylic acid" in this specification means acrylic acid and / or methacrylic acid. Examples of the (meth) acrylic acid alkyl ester that forms the main skeleton of the acrylic resin include those having 1 to 18 carbon atoms in the linear or branched alkyl group. These (meth) acrylates can be used individually or in combination. Further, any appropriate comonomer can be introduced into the acrylic resin by copolymerization. The type, amount, and copolymerization ratio of such comonomers can be appropriately set according to the purpose. The constituent components (monomer units) of the main skeleton of the acrylic resin will be described below with reference to the general formula (2). The acrylic resin preferably has at least one selected from the group consisting of a glutarimide unit, a lactone ring unit, a maleic anhydride unit, a maleimide unit, and a glutaric anhydride unit. An acrylic resin having a lactone ring unit is described in, for example, Japanese Patent Laid-Open No. 2008-181078, and the description of the publication is incorporated herein by reference. The glutariminium unit is preferably represented by the following general formula (1): In the general formula (1), R 1 and R 2 each independently represent a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, R 3 represents a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, and a ring having 3 to 12 carbon atoms. Alkyl or aryl having 6 to 10 carbon atoms. In the general formula (1), it is preferable that R 1 and R 2 are each independently a hydrogen atom or a methyl group, and R 3 is a hydrogen atom, a methyl group, a butyl group, or a cyclohexyl group. More preferably, R 1 is a methyl group, R 2 is a hydrogen atom, and R 3 is a methyl group. The aforementioned (meth) acrylic acid alkyl ester is represented by the following general formula (2): In the general formula (2), R 4 represents a hydrogen atom or a methyl group, R 5 represents a hydrogen atom, or an aliphatic or alicyclic hydrocarbon group having 1 to 6 carbon atoms which may be substituted. Examples of the substituent include a halogen and a hydroxyl group. Specific examples of the alkyl (meth) acrylate include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, Tert-butyl (meth) acrylate, n-hexyl (meth) acrylate, cyclohexyl (meth) acrylate, chloromethyl (meth) acrylate, 2-chloroethyl (meth) acrylate, (meth) ) 2-hydroxyethyl acrylate, 3-hydroxypropyl (meth) acrylate, 2,3,4,5,6-pentahydroxyhexyl (meth) acrylate, and 2,3,4, (meth) acrylate, 5-tetrahydroxypentyl ester. In the general formula (2), R 5 is preferably a hydrogen atom or a methyl group. Therefore, a particularly preferred alkyl (meth) acrylate is methyl acrylate or methyl methacrylate. The acrylic resin may include only a single pentamidine unit, or may include a plurality of pentamidine units that are different from R 1 , R 2 and R 3 in the general formula (1). The content ratio of the pentamidine imine unit in the acrylic resin is preferably 2 mol% to 50 mol%, more preferably 2 mol% to 45 mol%, and further preferably 2 mol% to 40 mol%, particularly preferably 2 mol% to 35 mol%, and most preferably 3 mol% to 30 mol%. If the content ratio is less than 2 mol%, the effects (for example, high optical characteristics, high mechanical strength, excellent adhesion to a polarizing element, and thinning) that are derived from a glutarimide unit are not sufficient. Be warned. If the content ratio exceeds 50 mol%, the heat resistance and transparency of the resin may be insufficient, for example. The acrylic resin may include only a single (meth) acrylic acid alkyl ester unit, or may include a plurality of (meth) acrylic acid alkyl ester units having different R 4 and R 5 in the general formula (2). The content ratio of the (meth) acrylic acid alkyl ester unit in the acrylic resin is preferably 50 mol% to 98 mol%, more preferably 55 mol% to 98 mol%, and still more preferably 60 mol. Ear mole% to 98 mole%, particularly preferably 65 mole% to 98 mole%, and most preferably 70 mole% to 97 mole%. If the content ratio is less than 50 mol%, there is a possibility that effects (for example, high heat resistance and high transparency) exhibited by the (meth) acrylic acid alkyl ester unit may not be sufficiently exhibited. If the content ratio is more than 98 mol%, the resin may become brittle and easily fracture, and high mechanical strength may not be sufficiently exhibited, which may result in poor productivity. The acrylic resin may include a unit other than a glutariminium unit and an alkyl (meth) acrylate unit. In one embodiment, the acrylic resin may include, for example, 0 to 10% by weight of an unsaturated carboxylic acid unit that does not participate in the intramolecular fluorination reaction. The content ratio of the unsaturated carboxylic acid unit is preferably 0 to 5% by weight, and more preferably 0 to 1% by weight. When the content is within this range, transparency, retention stability, and moisture resistance can be maintained. In one embodiment, the acrylic resin may include copolymerizable vinyl monomer units (other vinyl monomer units) other than those described above. Examples of other vinyl-based monomers include acrylonitrile, methacrylonitrile, ethacrylonitrile, allyl glycidyl ether, maleic anhydride, itaconic anhydride, N-methylmaleimide, N-ethylmaleimide, N-cyclohexylmaleimide, aminoethyl acrylate, propylaminoethyl acrylate, dimethylaminoethyl methacrylate, ethyl methacrylate Aminopropyl, cyclohexylaminoethyl methacrylate, N-vinyldiethylamine, N-ethylfluorenylamine, allylamine, methallylamine, N-methallylamine, 2-isopropene Oxazoline, 2-vinyloxazoline, 2-propenyloxazoline, N-phenylmaleimide, phenylaminoethyl methacrylate, styrene, α-methylbenzene Ethylene, p-glycidylstyrene, p-aminostyrene, 2-styryl-oxazoline. These can be used alone or in combination. A styrene-based monomer such as styrene or α-methylstyrene is preferred. The content ratio of other vinyl-based monomer units is preferably 0 to 1% by weight, and more preferably 0 to 0.1% by weight. If the content is within such a range, it is possible to suppress the expression of an undesired phase difference and a decrease in transparency. The fluorene imidization rate in the acrylic resin is preferably 2.5% to 20.0%. When the imidization ratio is within this range, a resin excellent in heat resistance, transparency, and moldability can be obtained, and scorch or reduction in mechanical strength during film formation can be prevented. In the above-mentioned acrylic resin, the fluorene imidization ratio is represented by the ratio of the glutariminium unit to the alkyl (meth) acrylate unit. This ratio can be obtained from, for example, an NMR spectrum or an IR spectrum of an acrylic resin. In this embodiment, the amidine ratio can be determined by 1 H-NMR measurement of the resin using 1 H-NMR BRUKER AvanceIII (400 MHz). More specifically, the peak area of the O-CH 3 proton derived from an alkyl (meth) acrylate in the vicinity of 3.5 ppm to 3.8 ppm is set to A, and the peak area derived from glutarin in the vicinity of 3.0 ppm to 3.3 ppm is set to A. The peak area of the N-CH 3 proton of fluoreneimine is set to B, and it is determined by the following formula.醯 Imidization ratio Im (%) = {B / (A + B)} × 100 The acid value of the acrylic resin is preferably 0.10 mmol / g to 0.50 mmol / g. When the acid value is within such a range, a resin excellent in a balance of heat resistance, mechanical properties, and moldability can be obtained. If the acid value is too small, there is a problem that the cost is increased due to the use of a modifier for adjusting to a desired acid value, and gelling is caused due to the residue of the modifier. If the acid value is too large, foaming tends to occur during film formation (for example, during melt extrusion), and the productivity of a molded product tends to decrease. In the above-mentioned acrylic resin, the acid value is the content of carboxylic acid units and carboxylic anhydride units in the acrylic resin. In this embodiment, the acid value can be calculated by a titration method described in, for example, WO 2005/054311 or Japanese Patent Laid-Open No. 2005-23272. The weight average molecular weight of the acrylic resin is preferably 1,000 to 2,000,000, more preferably 5,000 to 1,000,000, still more preferably 10,000 to 500,000, particularly preferably 50,000 to 500,000, and most preferably 60,000 to 150,000. The weight-average molecular weight can be determined, for example, by gel permeation chromatography (GPC system, manufactured by Tosoh Corporation) in terms of polystyrene. Furthermore, tetrahydrofuran can be used as a solvent. The Tg (glass transition temperature) of the acrylic resin is preferably 110 ° C or higher, more preferably 115 ° C or higher, even more preferably 120 ° C or higher, particularly preferably 125 ° C or higher, and most preferably 130 ° C or higher. If Tg is 110 ° C or higher, a polarizing plate including a polarizing element protective film obtained from such a resin is likely to have excellent durability. The upper limit of Tg is preferably 300 ° C or lower, more preferably 290 ° C or lower, even more preferably 285 ° C or lower, particularly preferably 200 ° C or lower, and most preferably 160 ° C or lower. When Tg is within this range, excellent moldability can be achieved. A-2. Polymerization of acrylic resin The above-mentioned acrylic resin can be produced, for example, by the following method. The method includes: (I) making an (meth) acrylic acid alkyl ester monomer corresponding to the (meth) acrylic acid alkyl ester unit represented by the general formula (2) and an unsaturated carboxylic acid monomer and / or a precursor thereof Copolymerization of monomers to obtain copolymer (a); and (II) treating the copolymer (a) with a sulfonimide to carry out alkyl (meth) acrylate monomer units in the copolymer (a) Intramolecular ammonium imidization reaction with an unsaturated carboxylic acid monomer and / or a precursor monomer unit thereof, thereby introducing a glutaridine imine unit represented by the general formula (1) into a copolymer. Examples of the unsaturated carboxylic acid monomer include acrylic acid, methacrylic acid, crotonic acid, α-substituted acrylic acid, and α-substituted methacrylic acid. Examples of the precursor monomers include acrylamide and methacrylamide. These can be used alone or in combination. The preferred unsaturated carboxylic acid monomer is acrylic acid or methacrylic acid, and the preferred precursor monomer is acrylamide. Any appropriate method can be used as a method for treating the copolymer (a) with the fluorinated imidating agent. Specific examples include a method using an extruder and a method using a batch reaction tank (pressure vessel). The method using an extruder includes heating the molten copolymer (a) using the extruder, and treating the copolymer with a fluorinated imidating agent. In this case, any appropriate extruder can be used as the extruder. Specific examples include a uniaxial extruder, a biaxial extruder, and a multiaxial extruder. In the method using a batch type reaction tank (pressure vessel), any appropriate batch type reaction tank (pressure vessel) can be used. As long as a glutariminium unit represented by the general formula (1) can be generated, any appropriate compound can be used as the fluorenimine. Specific examples of fluorene imidating agents include aliphatic amine-containing amines such as methylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine, isobutylamine, tert-butylamine, and n-hexylamine; aniline, Aromatic hydrocarbon group-containing amines such as benzylamine, toluidine, and trichloroaniline; and alicyclic hydrocarbon group-containing amines such as cyclohexylamine. Further, for example, a urea-based compound that generates such an amine by heating can be used. Examples of the urea-based compound include urea, 1,3-dimethylurea, 1,3-diethylurea, and 1,3-dipropylurea. The amidine imidating agent is preferably methylamine, ammonia, cyclohexylamine, and more preferably methylamine. In the fluorene imidization, a ring closure accelerator may be added in addition to the fluorene imidization agent as required. The amount of the fluorene imidating agent used in the fluorene imidization is preferably 0.5 to 10 parts by weight, and more preferably 0.5 to 6 parts by weight based on 100 parts by weight of the copolymer (a). If the amount of fluorene imidating agent used is less than 0.5 parts by weight, the desired fluorination rate of fluorene is not achieved in many cases. As a result, the heat resistance of the obtained resin may be extremely insufficient, causing appearance defects such as burnt after film formation. If the amount of the fluorinated imidating agent exceeds 10 parts by weight, the fluorinated imidizing agent may remain in the resin, and the fluorinated imidizing agent may cause appearance defects such as scorching after molding or foaming. In addition to the above fluorene imidization, the manufacturing method of this embodiment may include a treatment using an esterifying agent as required. Examples of the esterifying agent include dimethyl carbonate, 2,2-dimethoxypropane, dimethyl sulfene, triethyl orthoformate, trimethyl orthoacetate, trimethyl orthoformate, and dicarbonate. Phenyl ester, dimethyl sulfate, methyl tosylate, methyl triflate, methyl acetate, methanol, ethanol, methyl isocyanate, p-chlorophenyl isocyanate, dimethylcarbodiimide , Dimethyl tert-butylsilyl chloride, isopropenyl acetate, dimethylurea, tetramethylammonium hydroxide, dimethyldiethoxysilane, tetra-N-butoxysilane, dimethylphosphite (Trimethylsilyl) ester, trimethyl phosphite, trimethyl phosphate, tricresyl phosphate, diazomethane, ethylene oxide, propylene oxide, cyclohexane, 2-ethylhexyl shrink Glyceryl ether, phenyl glycidyl ether, benzyl glycidyl ether. Among these, dimethyl carbonate is preferable from a viewpoint of cost, reactivity, and the like. The amount of the esterifying agent can be set so that the acid value of the acrylic resin is a desired value. A-3. Combination of other resins In the embodiment of the present invention, the above-mentioned acrylic resin and other resins can be used in combination. That is, the monomer component constituting the acrylic resin and the monomer component constituting other resins may be copolymerized, and the copolymer may be supplied to the film formation described later in item C; or a blend of the acrylic resin and other resins may be supplied. In film formation. Examples of other resins include styrenic resin, polyethylene, polypropylene, polyamine, polyphenylene sulfide, polyether ether ketone, polyester, polyfluorene, polyphenylene ether, polyacetal, and polyfluorene. Other thermoplastic resins such as amines, polyethers and imines, and thermosetting resins such as phenol resins, melamine resins, polyester resins, silicone resins, and epoxy resins. The types and blending amounts of the resins used in combination can be appropriately set according to the purpose and characteristics required for the obtained film. For example, a styrene-based resin (preferably an acrylonitrile-styrene copolymer) can be used in combination as a retardation control agent. When the acrylic resin and other resins are used in combination, the content of the acrylic resin in the blend of the acrylic resin and other resins is preferably 50% to 100% by weight, and more preferably 60% to 100% by weight. , More preferably 70% by weight to 100% by weight, and particularly preferably 80% by weight to 100% by weight. When the content is less than 50% by weight, there is a possibility that the high heat resistance and high transparency inherent in the acrylic resin may not be sufficiently reflected. A-4. Additives The acrylic resin may be added with any appropriate additives according to the purpose during polymerization. Specific examples of the additives include ultraviolet absorbers; hindered phenol-based, phosphorus-based, sulfur-based antioxidants; stabilizers such as light-resistant stabilizers, weather-resistant stabilizers, and thermal stabilizers; reinforcing materials such as glass fibers and carbon fibers; Infrared absorbers; flame retardants such as tris (dibromopropyl) phosphate, triallyl phosphate, antimony oxide; antistatic agents such as anionic, cationic, and nonionic surfactants; inorganic pigments, organic Colorants such as pigments and dyes; organic or inorganic fillers; resin modifiers; organic or inorganic fillers; plasticizers; and lubricants. Additives may be added during polymerization of the acrylic resin, or may be added during film formation. The type, amount, combination, and addition amount of additives can be appropriately set according to the purpose. The additive may be added to the composition at the time of film formation described later in item C. B. Core-shell type particles The core-shell type particles typically have a core containing a rubbery polymer and a coating layer containing a glassy polymer and covering the core. The core-shell type particles may have one or more layers containing a glassy polymer as an innermost layer or an intermediate layer. However, in the embodiment of the present invention, in the step of dispersing the core-shell particles in the composition, the coating layer is compatible with the resin component in the composition, and the coating cannot be visually recognized (including through a microscope or the like). Situation. The Tg of the rubbery polymer forming the core is preferably 20 ° C or lower, more preferably -60 ° C to 20 ° C, and even more preferably -60 ° C to 10 ° C. If the Tg of the rubber-like polymer constituting the core exceeds 20 ° C, the mechanical strength of the acrylic resin may not be sufficiently improved. The Tg of the glassy polymer (hard polymer) constituting the coating layer is preferably 50 ° C or higher, more preferably 50 ° C to 140 ° C, and still more preferably 60 ° C to 130 ° C. When the Tg of the glassy polymer constituting the coating layer is lower than 50 ° C, the heat resistance of the acrylic resin may be reduced. The content ratio of the core in the core-shell particles is preferably 30% to 95% by weight, and more preferably 50% to 90% by weight. The ratio of the glassy polymer layer in the core is 0 to 60% by weight, preferably 0 to 45% by weight, and more preferably 10 to 40% by weight based on 100% by weight of the total amount of the core. The content ratio of the coating layer in the core-shell particles is preferably 5 to 70% by weight, and more preferably 10 to 50% by weight. The average particle diameter of the core is preferably 70 nm to 300 nm. If it is such an average particle diameter, it can be flattened to the desired length and thickness (hence the ratio of length / thickness) by stretching described later. The composition preferably contains 7 to 30% by weight of core-shell particles, and more preferably contains 8 to 25% by weight. When the content of the core-shell particles is within this range, extremely excellent adhesion to the polarizing element and extremely excellent bending resistance can be achieved. The details of the rubber-like polymer constituting the core of the core-shell particles, the glass-like polymer (hard polymer) constituting the coating layer, the polymerization method thereof, and other structures are described in, for example, Japanese Patent Laid-Open No. 2016-33552 In the bulletin. The contents of this publication are incorporated herein by reference. The flattening of core-shell particles (the formation of reinforcing particles) is related to elongation and will be described later in item D. C. Film formation can be performed by any appropriate method as a method for forming a film from the above composition. Specific examples include a casting coating method (for example, a casting method), an extrusion molding method, an injection molding method, a compression molding method, a transfer molding method, a blow molding method, a powder molding method, an FRP molding method, and a calendering method. Forming method and hot pressing method. An extrusion molding method or a casting coating method is preferred. The reason is that by improving the smoothness of the obtained film, good optical uniformity can be obtained. Particularly preferred is an extrusion molding method. The reason is that it is not necessary to consider the problems caused by the residual solvent. Among them, an extrusion molding method using a T-die is preferred from the viewpoint of the productivity of the film and the ease of subsequent stretching treatment. The molding conditions can be appropriately set according to the composition or type of the resin used and the desired characteristics of the obtained film. D. Extension Any appropriate extension method and extension conditions (such as extension temperature, extension magnification, extension speed, extension direction) can be adopted as the extension method. As specific examples of the extension method, free end extension, fixed end extension, free end contraction, and fixed end contraction can be used. These can be used individually or simultaneously, or can be used sequentially. An appropriate direction may be adopted as the extending direction according to the purpose. Specific examples include a length direction, a width direction, a thickness direction, and an oblique direction. The extending direction may be one direction (uniaxial extension), two directions (biaxial extension), or more than three directions. In the embodiment of the present invention, uniaxial extension in the longitudinal direction, simultaneous biaxial extension in the longitudinal direction and the width direction, and successive biaxial extension in the longitudinal direction and the width direction may be used. Biaxial extension (simultaneous or sequential) is preferred. The reason is that it is easy to control the in-plane phase difference, and it is easy to achieve optical isotropy. When a biaxial extension is adopted, the extension mode may be simultaneous biaxial extension, or may be sequential biaxial extension. At the same time, biaxial stretching has no roller stretching step, so it is almost difficult to scratch the surface of the film. As far as the appearance of the film is concerned, it has advantages compared with sequential stretching. On the other hand, since the sequential biaxial stretching is divided into a longitudinal stretching step and a transverse stretching step, the film is difficult to rupture, which is advantageous from the viewpoint of productivity. In the successive biaxial extension, either the longitudinal extension or the lateral extension may be performed first. In the successive biaxial extension, the longitudinal extension and the lateral extension are preferably performed sequentially. The stretching temperature can be determined according to the desired optical characteristics, mechanical characteristics and thickness of the protective film for polarizing elements, the type of resin used, the thickness of the film used, the stretching method (uniaxial or biaxial stretching), the stretching magnification, and the stretching speed. Changed. In the embodiment of the present invention, as described above, the elongation temperature is Tg + 20 ° C to Tg + 55 ° C, preferably Tg + 30 ° C to Tg + 50 ° C, and more preferably Tg + 35 ° C to Tg + 50 ° C. In the case of simultaneous biaxial stretching, the stretching temperature is preferably Tg + 30 ° C to Tg + 55 ° C, more preferably Tg + 40 ° C to Tg + 55 ° C, and still more preferably Tg + 40 ° C to Tg + 50 ° C. When using successive biaxial stretching, the stretching temperature is preferably Tg + 20 ° C to Tg + 55 ° C, more preferably Tg + 30 ° C to Tg + 55 ° C, and still more preferably Tg + 35 ° C to Tg + 50 ° C. By stretching at such a temperature, a polarizing element protective film having appropriate characteristics can be obtained. The specific elongation temperature is, for example, 140 ° C to 175 ° C, and preferably 155 ° C to 175 ° C. According to the embodiment of the present invention, by optimizing the combination of the stretching temperature, the stretching magnification, and the stretching speed, a polarizing element protective film having excellent bending resistance and adhesion to the polarizing element can be obtained. The Tg here is the Tg of the resin component of the composition. Like the stretching temperature, the stretching ratio can also be based on the desired optical characteristics, mechanical characteristics and thickness of the protective film for polarizing elements, the type of resin used, the thickness of the film used, the stretching method (uniaxial or biaxial stretching), The elongation temperature and elongation speed change. In the case of biaxial stretching, the ratio of the stretching magnification in one direction to the stretching magnification in the other direction is preferably 1.0 to 1.5, more preferably 1.0 to 1.4, and even more preferably 1.0 to 1.3. In one embodiment, the one direction is orthogonal to the other direction. For example, one of the two directions may be a length direction (MD: longitudinal direction), and the other direction may be a width direction (TD: lateral direction). The area magnification (product of the extension magnification in one direction and the extension magnification in the other direction) in the case where biaxial stretching is used is 2.0 to 6.0, preferably 3.0 to 6.0, and more preferably 4.0 to 5.9. In the case where simultaneous biaxial stretching is used, the area magnification is preferably 2.0 to 5.0, more preferably 2.0 to 4.5, and even more preferably 3.0 to 4.5. In the case of successive biaxial extension, the area magnification is preferably 2.0 to 6.0, more preferably 3.0 to 5.9, and even more preferably 4.0 to 5.9. According to the embodiment of the present invention, by optimizing the combination of the stretching temperature, the stretching magnification, and the stretching speed, a polarizing element protective film having excellent bending resistance and adhesion to the polarizing element can be obtained. Like the stretching temperature, the stretching speed can also be based on the desired optical characteristics, mechanical characteristics and thickness of the protective film for polarizing elements, the type of resin used, the thickness of the film used, the stretching method (uniaxial or biaxial stretching), The stretching temperature and stretching magnification change. The elongation speed is 3% / second to 130% / second, as described above, preferably 4% / second to 120% / second, and more preferably 5% / second to 100% / second. In the case of simultaneous biaxial stretching, the stretching speed is preferably 3% / second to 15% / second, more preferably 4% / second to 12% / second, and further preferably 5% / second to 10% / second. When using sequential biaxial stretching, the stretching speed is preferably 3% / second to 130% / second, more preferably 4% / second to 110% / second, and further preferably 5% / second to 100% / second. In the case of biaxial extension, the extension speed in one direction and the extension speed in the other direction may be the same or different. In the case of simultaneous biaxial extension, the comparison of the extension speed in one direction and the extension speed in the other direction is preferably 1.0 to 1.3, more preferably 1.0 to 1.2, and even more preferably 1.0 to 1.1. For example, in the case of sequential biaxial extension in the order of longitudinal extension and lateral extension, the ratio of the extension speed in the longitudinal direction to the extension speed in the transverse direction (longitudinal / transverse) is preferably 7.0 to 17.0, and more preferably 8.0 to 15.0. , And more preferably 9.0 to 13.0. According to the embodiment of the present invention, by optimizing the combination of the stretching temperature, the stretching magnification, and the stretching speed, a polarizing element protective film having excellent bending resistance and adhesion to the polarizing element can be obtained. By such an extension described above, the core-shell particles are appropriately flattened (hereinafter, the flattened core-shell particles are referred to as reinforcing particles). The length / thickness ratio of the reinforcing particles is preferably 7.0 or less, more preferably 6.5 or less, and even more preferably 6.3 or less. On the other hand, the length / thickness ratio is preferably 4.0 or more, more preferably 4.5 or more, and even more preferably 5.0 or more. If the length / thickness ratio is within this range, the adhesion between the polarizing element protective film and the polarizing element can be significantly improved while maintaining excellent bending resistance caused by the blended particles. The "thickness / length ratio" in this specification means the ratio of the representative length to the thickness of the planar shape of the reinforcing particles. Here, the "representative length" refers to a diameter when the planar view shape is circular, a long diameter when the planar view shape is oval, and a diagonal length when the planar view shape is rectangular or polygonal. The ratio can be obtained by, for example, the following procedure. Use a transmission electron microscope (for example, accelerated voltage 80 kv, RuO 4 staining ultra-thin sectioning method) to take the obtained film cross-section, and extract the 30 longest particles in sequence from the reinforcing particles present in the obtained photo (to obtain a representative length For the profile), (average of length) / (average of thickness) is calculated to obtain the ratio. The thickness of the core is preferably 20 nm to 100 nm. The representative length of the core is preferably 200 nm to 600 nm. When the representative length of the core is too short, there is a case where the improvement of the mechanical strength of the obtained film is insufficient. In the case where the thickness of the core is too thick or the representative length of the core is too long, the adhesion between the obtained film and the polarizing element may be impaired. In this manner, a polarizing element protective film can be formed. The polarizing element protective film thus obtained contains an acrylic resin and reinforcing particles having a flat shape dispersed in the acrylic resin. E. The polarizing element protective film and its characteristics obtained by the above-mentioned manufacturing method are preferably substantially isotropic. The “substantially optically isotropic” in this specification means that the in-plane retardation Re (550) is 0 nm to 10 nm, and the retardation Rth (550) in the thickness direction is -20 nm to +10 nm. The in-plane retardation Re (550) is more preferably 0 nm to 5 nm, further preferably 0 nm to 3 nm, and particularly preferably 0 nm to 2 nm. The phase difference Rth (550) in the thickness direction is more preferably -5 nm to +5 nm, further preferably -3 nm to +3 nm, and particularly preferably -2 nm to +2 nm. If the Re (550) and Rth (550) of the polarizer protective film are within this range, it is possible to prevent adverse effects on display characteristics when a polarizing plate including the polarizer protective film is applied to an image display device. In addition, Re (550) is an in-plane retardation of a film measured with light having a wavelength of 550 nm at 23 ° C. Re (550) is obtained by the formula: Re (550) = (nx−ny) × d. Rth (550) is a retardation in the thickness direction of the film measured with light having a wavelength of 550 nm at 23 ° C. Rth (550) is measured by the formula: Rth (550) = (nx−nz) × d ″. Here, nx is the refractive index in the direction (that is, the direction of the late phase axis) when the in-plane refractive index is maximum, and ny is the refractive index in the direction that is orthogonal to the late phase axis (that is, the direction of the phase axis) in the plane , Nz is the refractive index in the thickness direction, and d is the thickness (nm) of the film. The polarizing element protective film preferably has a higher light transmittance at 380 nm at a thickness of 80 μm. Specifically, the light transmittance is preferably 85% or more, more preferably 88% or more, and even more preferably 90% or more. If the light transmittance is within this range, the desired transparency can be ensured. By using the above manufacturing method to optimize the length / thickness of the reinforcing particles such as the above-mentioned range, not only excellent bending resistance and excellent adhesion with the polarizing element can be achieved, but also such light transmittance can be achieved. The light transmittance can be measured by, for example, a method according to ASTM-D-1003. The lower the haze of the polarizer protective film, the better. Specifically, the haze is preferably 5% or less, more preferably 3% or less, still more preferably 1.5% or less, and particularly preferably 1% or less. When the haze is 5% or less, a good clear feeling can be imparted to the film. Furthermore, even when the above-mentioned film is used in the viewing-side polarizing plate of the image display device, the display content can be viewed well. By using the above manufacturing method to optimize the length / thickness of the reinforcing particles such as the above-mentioned range, not only excellent bending resistance and excellent adhesion with the polarizing element, but also such haze can be achieved. The YI at a thickness of the polarizing element protective film of 80 μm is preferably 1.27 or less, more preferably 1.25 or less, even more preferably 1.23 or less, and particularly preferably 1.20 or less. If YI exceeds 1.3, the optical transparency of the film may be insufficient. By using the above manufacturing method to optimize the length / thickness of the reinforcing particles such as the above-mentioned range, not only excellent bending resistance and adhesion with the polarizing element can be achieved, but also such YI can be achieved. Furthermore, YI can use the three-color excitation values (X, Y, Z) of the colors obtained by measurement using, for example, a high-speed integrating sphere spectral transmittance measuring machine (trade name DOT-3C: manufactured by Murakami Color Technology Research Institute) Calculated from the formula shown below: YI = [(1.28X－1.06Z) / Y] × 100 b value at a thickness of 80 μm of the polarizer protective film (a measure of hue consistent with the Hunter color system) It is preferably less than 1.5, and more preferably 1.0 or less. When the b value is 1.5 or more, an undesired color tone may appear. In addition, the b value can be measured, for example, by cutting a sample of a polarizing element protective film into 3 cm2, measuring its hue with a high-speed integrating sphere spectral transmission measuring machine (trade name DOT-3C: manufactured by Murakami Color Technology Research Institute), and measuring the The special color system evaluates this hue to obtain. The moisture permeability of the protective film for the polarizing element is preferably 300 g / m 2 ·24 hr or less, more preferably 250 g / m 2 ·24 hr or less, and further preferably 200 g / m 2 ·24 hr or less, particularly preferably It is 150 g / m 2 ·24 hr or less, and most preferably 100 g / m 2 ·24 hr or less. If the moisture permeability of the protective film for a polarizing element is within this range, a polarizing plate having excellent durability and moisture resistance can be obtained. The tensile strength of the polarizing element protective film is preferably 10 MPa or more and less than 100 MPa, and more preferably 30 MPa or more and less than 100 MPa. When the tensile strength is less than 10 MPa, there are cases where sufficient mechanical strength cannot be exhibited. If the tensile strength exceeds 100 MPa, the processability of the film may be insufficient. The tensile strength can be measured according to, for example, ASTM-D-882-61T. The elongation of the polarizer protective film is preferably 1.0% or more, more preferably 3.0% or more, and even more preferably 5.0% or more. The upper limit of the elongation at extension is, for example, 100%. When the elongation is less than 1%, the toughness of the film may be insufficient. The elongation at extension can be measured according to, for example, ASTM-D-882-61T. The tensile elastic modulus of the polarizer protective film is preferably 0.5 GPa or more, more preferably 1 GPa or more, and even more preferably 2 GPa or more. The upper limit of the tensile elastic modulus is, for example, 20 GPa. When the tensile elastic modulus is less than 0.5 GPa, the film may not exhibit sufficient mechanical strength. The tensile elastic modulus can be measured according to, for example, ASTM-D-882-61T. The easy-adhesion layer may be formed on one surface of the protective film for the polarizing element. The easy-adhesive layer contains, for example, an aqueous polyurethane and an oxazoline-based crosslinking agent. F. Use of protective film for polarizing element The protective film for polarizing element obtained by the manufacturing method of the present invention can be applied to a polarizing plate. The polarizing plate typically has a polarizing element and a polarizing element protective film disposed on at least one side of the polarizing element. Since the polarizing element adopts a structure known in the industry, detailed description is omitted. The polarizing plate is applicable to an image display device. Typical examples of the image display device include a liquid crystal display device and an organic electroluminescence (EL) display device. Since the image display device adopts a structure known in the industry, detailed description is omitted. [Examples] The following specifically describes the present invention through examples, but the present invention is not limited by these examples. The measurement method of each characteristic is as follows. In addition, "part" and "%" in the examples are based on weight unless otherwise specified. <Example 1> (Production of protective film for polarizing element) MS resin (MS-200; methyl methacrylate / styrene (Molar ratio) = 80/20 copolymer, manufactured by Nippon Steel Chemical Co., Ltd.) Monomethylamine hydrazone (hydrazone imidization rate: 5%). The obtained amidine imidized MS resin has a glutarimide unit (R 1 and R 3 are methyl groups, and R 2 is a hydrogen atom) represented by the general formula (1), and ( meth) acrylic acid ester unit (R 4 and R 5 is methyl) and styrene units. Furthermore, a 15 mm-diameter intermeshing co-rotating twin-screw extruder was used in the above-mentioned sulfonimidization. The setting temperature of each temperature control zone of the extruder was set to 230 ° C, and the number of screw revolutions was set to 150 rpm. MS resin was supplied at 2.0 kg / hr. With respect to 100 parts by weight of MS resin, The supply amount was set to 2 parts by weight. MS resin was charged from a hopper, and the resin was melted and filled by a kneading block, and then monomethylamine was injected from a nozzle. Insert a sealing ring at the end of the reaction zone to fill the resin. The pressure of the vent is reduced to -0.08 MPa, and the by-products and excess methylamine after the reaction are devolatilized. The resin discharged from the die disposed at the exit of the extruder as a strand was cooled in a water tank, and then pelletized with a pelletizer. The obtained amidine imidized MS resin had a amidine imidization rate of 5.0% and an acid value of 0.5 mmol / g. Tg is 120 ° C. 90 parts by weight of the fluorene imidized MS resin obtained above and 10 parts by weight of core-shell particles (manufactured by Kaneka Corporation, trade name "KANE ACE M-210") were put into a uniaxial extruder at 260 ° C It was melt-extruded to obtain a film with a thickness of 120 μm. The obtained extruded film was biaxially stretched twice each in the length direction and the width direction at an extension temperature of 160 ° C (Tg + 40 ° C). The extension speed is 10% / second in both the length and width directions. By this extension, the core-shell particles are flattened to form reinforcing particles. The length / thickness ratio of the reinforcing particles was 6.3. In this manner, a protective film for a polarizing element was produced. The thickness of the obtained polarizing element protective film was 40 μm, the in-plane retardation Re (550) was 2 nm, and the thickness direction retardation Rth (550) was 2 nm. The obtained polarizing element protective film was subjected to the evaluation of (2) above. The results are shown in Table 1. (Production of polarizing plate) 1. Production of polarizing element The length of one side of the elongation roller of a polyvinyl alcohol (PVA) resin film (made by Kuraray, trade name "PE3000") with a thickness of 30 μm is made by a roll stretcher. Uniaxial extension was performed in the direction so that it was 5.9 times, and swelling, dyeing, cross-linking, and washing treatments were performed on one side, and finally, a roll was dried to produce a polarizing element having a thickness of 12 μm. Specifically, the swelling treatment stretches the roll 2.2 times while processing in pure water at 20 ° C. Next, the dyeing treatment was performed in a 30 ° C aqueous solution whose weight ratio of iodine to potassium iodide was 1: 7 to adjust the concentration of the iodine in a manner such that the monomer transmittance of the obtained polarizing element was 45.0%. The roll was extended to 1.4 times. Furthermore, the cross-linking treatment is a two-stage cross-linking treatment. The first-stage cross-linking treatment is performed in an aqueous solution in which boric acid and potassium iodide are dissolved at 40 ° C. while extending the roll to 1.2 times. The boric acid content of the first-stage cross-linked aqueous solution was set to 5.0% by weight, and the potassium iodide content was set to 3.0% by weight. The cross-linking treatment in the second stage was carried out in a 65 ° C. aqueous solution in which boric acid and potassium iodide were dissolved, and the roller was extended to 1.6 times. The boric acid content of the second-stage cross-linked aqueous solution was set to 4.3% by weight, and the potassium iodide content was set to 5.0% by weight. The washing treatment was performed in a potassium iodide aqueous solution at 20 ° C. The potassium iodide content of the rinsed aqueous solution was set to 2.6% by weight. The final drying treatment was performed at 70 ° C. for 5 minutes to obtain a polarizing element. 2. Production of polarizing plate The above-mentioned polarizing element protective film was bonded on one side of the polarizing element via a polyvinyl alcohol-based adhesive to obtain a polarizing plate. The obtained polarizing plate was subjected to evaluation of adhesion. The results are shown in Table 1. The evaluation of the adhesion was performed as follows. The obtained polarizing plate was subjected to a peeling test, and the adhesion between the polarizing element protective film and the polarizing element was evaluated. Specifically, it is as follows. The polarizing plate was cut to a size of 200 mm in the absorption axis direction of the polarizing element and 15 mm in the direction orthogonal to the absorption axis. A cut is made between the protective film and the polarizing element with a cutting knife, and then it is bonded to a glass plate. The protective film and the polarizing element were peeled in a 90-degree direction at a peeling speed of 300 mm / min by a universal tensile machine, and the initial peeling strength (N / 15 mm) was measured. Evaluation was performed using the following criteria. ○: Initial peel strength is 1.0 (N / 15 mm) or more ×: Initial peel strength is less than 1.0 (N / 15 mm) <Example 2> Except for setting the elongation temperature to 165 ° C (Tg + 45 ° C), Example 1 A polarizing element protective film and a polarizing plate were produced in the same manner. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 3> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension temperature was 170 ° C (Tg + 50 ° C). The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 4> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension temperature was 175 ° C (Tg + 55 ° C). The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 5> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension temperature was 155 ° C (Tg + 35 ° C). The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Comparative Example 1> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension temperature was set to 180 ° C (Tg + 60 ° C). The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 6> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the stretching ratio in the width direction was set to 1.5 times and the surface magnification was set to 3.0. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 7> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1, except that the stretching ratio in the width direction was set to 2.6 times and the surface magnification was set to 5.2. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 8> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension speed was set to 3% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 9> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension speed was set to 5% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 10> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension speed was set to 15% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Comparative Example 2> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1 except that the extension speed was set to 1% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 11> Except that the extension temperature was set to 170 ° C (Tg + 50 ° C), the extension ratio in the width direction was set to 1.5 times, the surface magnification ratio was set to 3.0, and the extension speed was set in both the length direction and the width direction. Other than 3% / second, a polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 12> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 11 except that the extension speed was set to 5% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 13> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 11 except that the extension speed was set to 10% / second in both the length and width directions. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 14> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 11 except that the extension speed was set to 15% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 15> Except that the stretching temperature was set to 175 ° C (Tg + 55 ° C), the stretching ratio in the width direction was set to 1.5 times, the surface magnification was set to 3.0, and the stretching speed was set to 3 in both the length and width directions. Other than% / second, a polarizing element protective film and a polarizing plate were produced in the same manner as in Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 16> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 15 except that the extension speed was set to 5% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 17> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 15 except that the extension speed was set to 10% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 18> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 15 except that the extension speed was set to 15% / second in both the longitudinal direction and the width direction. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Comparative Example 3> After stretching in the longitudinal direction at an extension temperature of 160 ° C and an extension speed of 100% / sec, the extension direction in the width direction was at 136 ° C, an extension speed of 12% / second, and the surface magnification. 5.8 times the extension to produce a polarizer protective film and a polarizer. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 19> A polarizing element protective film and a polarizing plate were produced in the same manner as in Comparative Example 3 except that the extension temperature in the width direction was 140 ° C. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 20> A polarizing element protective film and a polarizing plate were produced in the same manner as in Comparative Example 3, except that the extension temperature in the width direction was 160 ° C. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 21> A polarizing element protective film and a polarizing plate were produced in the same manner as in Comparative Example 3 except that the extension temperature in the width direction was 170 ° C. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Comparative Example 4> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 20, except that the extension speed in the longitudinal direction was set to 40% / second and the extension speed in the width direction was set to 2% / second. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Example 22> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 20, except that the extension speed in the longitudinal direction was set to 60% / second and the extension speed in the width direction was set to 4% / second. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. <Comparative Example 5> A polarizing element protective film and a polarizing plate were produced in the same manner as in Example 20, except that the surface magnification was set to 7.0. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. [Table 1] <Evaluation> It is clear from Table 1 that the method for manufacturing a polarizing element protective film according to the example of the present invention can obtain a polarizing element protective film (as a result, a polarizing plate) having excellent balance with the adhesion of the polarizing element with excellent productivity. [Industrial Applicability] The protective film for a polarizing element obtained by the manufacturing method of the present invention is preferably used for a polarizing plate. The polarizing plate is preferably used for an image display device. Image display devices can be used in portable devices such as personal digital assistants (PDAs), smart phones, mobile phones, clocks, digital cameras, portable game consoles; OA equipment such as personal computer monitors, notebook personal computers, and copiers; video cameras, televisions , Microwave ovens and other household appliances; back-up monitors, monitors for car navigation systems, car audio and other automotive equipment; display equipment such as digital signage and information monitors for commercial shops; surveillance equipment such as surveillance monitors; and nursing equipment Nursing / medical devices such as monitors, medical monitors, etc .;