As one of the primary raw materials of liquid crystal displays (LCD), polarizers account for about 20% to 30% of their manufacturing costs. This article takes the polarizer for TN LCD as an example, and introduces some issues that many LCD polarizer users are more concerned about.
Polarizer is a kind of optical film generating polarized light, which is compounded by multi-layer polymer materials. According to the different use in the LCD screen, it can be roughly divided into two types: face sheet (also called transparent sheet ) and negative sheet (also called reflective sheet).
Polarizing layer: It is made of PVA (polyvinyl alcohol) film after dyeing and stretching. This layer is the main part of the polarizer, also known as the original polarizing film. The polarizing layer determines the polarization performance and transmittance of the polarizer, and it is also the main part that affects the color tone and optical durability of the polarizer. The basic processing technology of the polarizing layer can be divided into two series of dye series and iodine series according to the dyeing method. According to the stretching process, it can be divided into two series of dry stretching and wet stretching. Changing its material and processing technology can achieve the right Adjustment of polarization, transmittance, hue and optical durability.
TAC layer: The polarizing layer made of PVA film is easy to absorb water, fade and lose its polarization performance. Therefore, it is necessary to use a TAC (triacetate cellulose ester) film with good optical uniformity and transparency on both sides to isolate moisture and air , Protect the polarizing layer. Using TAC film with UV CUT and Anti-Glare functions can be made into anti-ultraviolet polarizers and anti-glare polarizers.
Adhesive: It can be divided into reflective film side adhesive and peeling film side adhesive. The role of the reflective film side adhesive is to firmly bond the reflective film to the TAC film, and its process requirements do not allow re-peelability. The peeling film side adhesive is a pressure-sensitive adhesive, which determines the adhesion performance and patch processing performance of the polarizer. Its performance is one of the most concerned issues for LCD polarizer users. Separate film: It is a PET (ethylene terephthalate) film coated with a silicon coating on one side. It mainly protects the pressure-sensitive adhesive layer. At the same time, its peeling force is very important for the LCD patch. The workability has a certain impact.
Protective film: It is a PE (polyethylene) film coated with an EVA layer (ethylene vinyl acetate copolymer) on one side. It has low viscosity and protects the surface of the TAC film.
Reflective film: It is a single-side PET film with aluminum vaporized. At present, most of the non-directional reflective vaporized aluminum film is used. If the reflective film is replaced with a transflective film, a transflective polarizer can be made. In addition, various gold-plated, silver-plated, and laser films can also be used as reflective films to obtain various background colors and mirrors.
Generally, TN-type LCD polarizer has a fixed size and a fixed absorption axis. For a special sized LCD polarizer, manufacturers need to measure its absorption axis . At this time, we need to apply a special optical instrument- Optical Absorption Axis Measuring instrument (OI-OAM)
It can be divided into three items: single, parallel, and crossed. It is usually measured with an integrating sphere spectrophotometer in accordance with JIS-Z-8701. Among them, the single transmittance refers to the transmittance of a single polarizer, the parallel transmittance (H0) refers to the transmittance of two polarizers with parallel absorption axes, and the orthogonal transmittance (H90) refers to The transmittance of two polarizers with orthogonal absorption axes. Among the three indicators, H0 and H90 affect the brightness (H0) and contrast (H0/H90) of the LCD screen, which is very important to LCD manufacturers. Importantly, in order to obtain a good display effect with high brightness and high contrast, it is desirable that H0 be as high as possible and H90 as small as possible.
It is represented by a and b values, which are usually measured with an integrating sphere spectrophotometer. A and b values are color coordinate values in the CIE (International Commission on Illumination) Lab color system. The approximate color corresponding to a set of a and b values can be found from the color coordinate chart.
It is a calculated value to indicate the overall efficiency of polarized light produced by the polarizer. The formula can be transformed into H0/H90 = (1+ν2)/ (1-ν2). It can be seen that the closer the ν value is to 100%, the higher the contrast will be (H0/H90).
It is divided into three items: the peeling force of the protective film, the peeling force of the peeling film, and the peeling force against the glass substrate. The measurement of the three kinds of peeling force is carried out by the tensile testing machine with reference to JIS-C-2107 standard. The peeling force of the protective film and the peeling force of the peeling film are measured in the 180° direction, and the glass substrate is in the 90° direction. Peel off. For LCD manufacturers, the peeling performance of the polarizer to the glass substrate is very important. If it is difficult to peel off within a short time (4-6 hours) after pasting or there is residual glue on the glass plate after peeling, the polarizer will have poor reworkability, and poor placement will cause the entire LCD screen to be scrapped. However, if the peeling force is small, it is easy to cause the durability and moisture resistance of the pressure-sensitive adhesive to decrease after the polarizer is laminated on the glass substrate, and the surface of the peeling film is dented, which affects the performance of the polarizer.
The durability test is to peel off the peeling film and the protective film of the polarizer and attach it to the glass substrate. After defoaming under pressure, it is placed in a constant temperature and humidity box to observe the changes before and after the experiment. Among them, the foaming and peeling index is mainly to assess the durability of the adhesive, and the optical change index is to assess the durability of the PVA layer. The durability requirements of the polarizer should be determined according to the design requirements (use environment) of different types of LCD products.
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