三醋酸纤维素TAC

三醋酸纤维素TAC(三醋酸纤维素,Triacetyl Cellulose)，液晶显示器生产过程中的重要材料。

主要用于保护LCD偏光板。

酯化纤维素薄膜应用历史超过一世纪，原料来自木材纤维素，为造纸工业之延伸，目前LCD偏光板用之保护膜主要成份为TAC(三醋酸纤维素,Triacetyl Cellulose)，其组成非常复杂，其中包含可塑剂、助溶剂、润湿剂、滑剂以及抗紫外线剂等等，TAC 以溶剂铸膜加工成膜，至今仍是穿透度最高之高分子材料之一。

虽然在偏光板发展历史中，只要有透明塑料出现即尝试是否可以取代TAC，但是均无法超越TAC 93%以上之光穿透度，且TAC本身即是一片负型之C-plate，不同之配方与酯化程度影响相位差值，目前相位差值约为30~200nm之间，对于液晶显示器具有特定之补偿能力，所以虽然TAC有吸水率高、尺寸安定性与表面特性易受环境影响缺点，但均无法被其它材料所取代。

FujiFilm、Konica-Minolta等TAC制造商为巩固市场，均致力于：开发性质更稳定、加工性更好之配方；开发厚度更薄之薄膜，目前主流厚度为80μm，有部分产品使用40μm厚度；开发宽度更宽(1330mm→1470mm)、长度更长(3900m/roll)之薄膜成形技术，降低后续加工成本；引入相位差之功能，使其不单是保护膜也是补偿膜，如日本Konica所开发之N-TACTM，为一光轴属于Biaxial-plate特性之保护膜，应用于液垂直配向(MVA)液晶显示器补偿色偏及视角。

近来快速发展之光学材料COP，最有机会取代TAC保护膜之角色，因其光学特性不输TAC，而机械性、耐温性及耐候性远超过TAC，目前问题在于价格约为TAC 三倍而未能普及，不过值得期待。

偏光片是以聚乙烯醇（PVA）拉伸膜和醋酸纤维素膜（TAC）经多次复合、拉伸、涂布等工艺制成的一种复合材料，可实现液晶显示高亮度、高对比度特性。

本文以TN型LCD用偏光片为例偏光片的结构偏光片是一种由多层高分子材料复合而成的具有产生偏振光功能的光学薄膜，按其在液晶屏的使用位置不同，大体上可分为面片（又称透过片）和底片两种（又称反射片），下图是典型TN型偏光片的面片和底片剖面结构示意图：各层的材质和主要功能偏光层：是由PVA（聚乙烯醇）薄膜经染色拉伸后制成，该层是偏光片的主要部分，也称偏光原膜。

偏光层决定了偏光片的偏光性能、透过率，同时也是影响偏光片色调和光学耐久性的主要部分。

偏光层的基本加工工艺按染色方法可分为染料系和碘系两大系列，按拉伸工艺可分为干法拉伸和湿法拉伸两大系列，改变其材料和加工工艺可实现对偏光度、透过率、色调和光学耐久性的调整。

TAC层：由PVA膜制成的偏光层易吸水、褪色而丧失偏光性能，因此需要在其两边用一层光学均匀性和透明性良好的TAC（三醋酸纤维素酯）膜来隔绝水分和空气，保护偏光层。

采用具有紫外隔离（UV CUT）和防眩（Anti-Glare）功能的TAC膜可制成防紫外型偏光片和防眩型偏光片。

粘着剂（adhesive）：可分为反射膜侧粘着剂和剥离膜侧粘着剂。

反射膜侧粘着剂的作用是将反射膜牢固地粘合在TAC膜上，其工艺要求不允许有再剥离性。

剥离膜侧粘着剂是一层压敏胶，它决定了偏光片的粘着性能及贴片加工性能，其性能优劣是LCD偏光片使用者最为关心的问题之一。

剥离膜（separate film）：为单侧涂布硅涂层的PET（对苯二甲酸乙二醇酯）膜，主要起保护压敏胶层的作用，同时其剥离力的大小对LCD贴片时的作业性有一定影响。

保护膜（protective film）：为单侧涂布EVA层（乙烯醋酸乙烯共聚物）的PE（聚乙烯）膜，具有低粘性，起保护TAC膜表面的作用。

反射膜（reflective film）：为单侧蒸铝的PET膜，目前大多使用无指向反射型蒸铝膜。

如将反射膜更换为半透半反膜，则可制成半透半反型偏光片，此外也可使用各种镀金、镀银膜、镭射膜作为反射膜，以获得各种底色和镜面反射等效果。

偏光片的主要性能指标表1是一份典型的偏光片性能表，下面对其各项目指标逐一进行说明：1、吸光轴方向通常市面销售的TN型LCD用偏光片的尺寸主要有以下两种：其吸光轴方向如图所示，如果提供给LCD厂商特殊尺寸形状的偏光片，应对吸光轴加以标记或说明。

2、透过率（Transmittance）偏光片的透过率指标可分为单体（single）、平行（Parallel）、直交（Crossed）三项。

通常使用积分球式分光光度计依照JIS-Z-8701对其进行测定。

其中，单体透过率指单片偏光片的透过率，平行透过率（H0）指两片吸光轴平行的偏光片叠加后的透过率，直交透过率（H90）指两片吸光轴直交的偏光片叠加后的透过率，在这三项指标中H0和H90影响LCD屏的亮度（H0）和对比度（H0/H90），对LCD制造商十分重要，为了获得高亮度高对比度的良好显示效果，希望H0尽可能的高而H90尽可能的小。

3、（Hue）色相由a、b值来表示，通常使用积分球式分光光度计进行测定。

a、b值为CIE（国际照明委员会）Lab表色系统中的色坐标值，一组a、b值所对应的大致颜色可从色坐标图中查得。

4、偏光度（Polarizing Co-Efficiency）偏光度（ν）是一个计算值，用来表示偏光片产生偏振光的综合效率，该公式可变换为H0/H90=（1+ν2）/（1-ν2），可以看出，ν值越趋近100%，则对比度（H0/H90）越高。

5、剥离力偏光片的剥离力又分为保护膜剥离力、剥离膜剥离力、对玻璃基板剥离力三项。

三种剥离力的测定均使用拉力实验机参照JIS-C-2107标准进行测定，其中保护膜剥离力和剥离膜剥离力的测定是沿180°方向剥离，而对玻璃基板则是沿90°方向剥离。

对LCD制造商而言，偏光片对玻璃基板的剥离性能十分重要。

如粘贴后短时间（4～6小时）内剥离困难或剥离后玻板上有残胶，则该偏光片返工性差，贴片不良会导致整块LCD屏报废。

但如果剥离力很小，则易造成偏光片在玻璃基板上贴合后压敏胶耐久性和耐湿性能下降以及剥离膜表面凹陷等不良多发，从而影响偏光片的使用性能。

6、耐久性偏光片的耐久性测试是将偏光片剥去剥离膜和保护膜后贴合在玻璃基板上，经加压脱泡后放入恒温恒湿箱中，观察其实验前后的变化。

其中的发泡剥离指标主要是考核粘着剂的耐久性能，光学变化指标是考核PVA层的耐久性能。

偏光片的耐久性要求应根据不同类型LCD产品的设计要求（使用环境）而定China's State Council warned yesterday about the risks of inflation and pledged to curb rapid increases in housing costs as new figures showed that property prices were continuing to accelerate.In a statement released last night, the State Council said that the dramatic increase in property values in some cities had become a “prominent problem” after government figures released earlier in the day showed urban housing prices in March rose 11.7 per cent during the previous 12 months, up from 10.7 per cent the month before and the biggest increase since the index began nearly five years ago.The warning came ahead of the expected announcement today that the Chinese economy grew by close to 12 per cent in the first quarter of the year, the fastest rate in three years and underlining the risk that China's robust recovery from the global crisis could end in overheating. “Some factors that are pushing up prices have appeared, strengthening inflationary expectations,” the State Council said.If the gross domestic product data for the first quarter are as strong as expected, it will renew pressure on the Chinese authorities to begin to allow its currency to appreciate and to take other tightening measures such as raising interest rates.The tone of the State Council statement, which dropped previous language about the economy not yet having achieved a solid footing, suggested that new tightening measures could be afoot. “An array of signals indicates that the risk of overheating is much larger than that of a second dip,” says Ha Jiming, an economist at China International Capital Corporation.Reuters reported last night the economy grew by 11.9 per cent in the first quarter, while economists are forecasting a rise of 11.7 per cent. The figures for the first few months of this year are flattered in part by the base of comparison, given the Chinese economy slumped in the first quarter of last year and the government's stimulus measures did not start to kick in until April and May.The rebound in the economy has been partly driven by the property sector amid sharp price increases in housing and land markets in a number of cities, causing some analysts to warn of a bubble.The figures released yesterday by the National Bureau of Statistics suggested some cooling in demand for housing, with the 35.8 per cent on-year increase in sales in the first quarter down from the 50 per cent rate seen at the end of last year. However, separate figures prepared by consultancy Soufun indicated housing sales surged again in March by 90 per cent from the previous month, having fallen by 49 per cent in February and by 46 per cent in January from the month before.He Fan, analyst at Soufun, said the rise in March sales partly reflected the fact that January and February are slow months in the housing market. But he added that buyers were returning because expectations of further price rises remained strong. “I think that sales in April will continue to increase,” he said.In spite of accelerating growth and rising housing prices, officials recently have sounded more relaxed about the risks of inflation and overheating, in part because the pace of lending by banks has slowed sharply during the last month. Xu Lianzhong, an official at the National Development and Reform Commission, said on Monday that interest rates would not need to rise in the short-term, as inflationary pressures were easing.。