​Structure of liquid crystal polarizer and performance analysis of polarizer for LCD

The structure and main performance analysis of polarizer for LCD. Polarizer, as one of the main raw materials of liquid crystal display (LCD), accounts for about 20% of its manufacturing cost ~ 30%, however, because the manufacturing technology of polarizer has always been monopolized by Japan, South Korea and other countries, so there is very little information about introducing polarizer. Taking the polarizer for tn LCD as an example, this paper introduces some issues that many LCD polarizer users are concerned about.
Structure of polarizer
Polarizer is a kind of optical film which is composed of multi-layer polymer materials and has the function of generating polarized light. According to its different application positions on LCD, polarizer can be generally divided into surface sheets (also called translucent sheets) and negatives (also called reflectors ).
Material and main functions of each layer
Polarized layer: it is made of PVA (polyvinyl alcohol) film after dyeing and stretching. This layer is the main part of polarizer, also known as polarizer original film. The polarized layer determines the polarized performance and transmittance of the polarizer, and it is also the main part that affects the tone and optical durability of the polarizer. According to the dyeing method, the basic processing technology of polarized layer can be divided into dye series and iodine series, and according to the stretching process, it can be divided into two series: dry stretching and wet stretching, changing its material and processing technology can realize the adjustment of polarization, transmittance, hue and optical durability. TAC layer: the polarized layer made of PVC film is easy to absorb water, fade and lose the polarized property. Therefore, it is necessary to use a layer of TAC (cellulose triacetate) with good optical uniformity and transparency on both sides) membrane to isolate moisture and air and protect polarized layer. Using TAC film with uv cut and Anti-Glare functions can be made into Anti-UV polarizer and Anti-Glare polarizer.
Adhesive: it can be divided into reflective film side adhesive and stripping Film side adhesive. The function of the side adhesive of the reflective film is to firmly adhere the reflective film to the TAC film, and its technological requirements do not allow re-stripping. The adhesive on the side of the stripping film is a layer of pressure sensitive adhesive, which determines the adhesive performance of polarizer and the processing performance of the patch. Its performance is one of the most concerned issues for LCD polarizer users. Separate film: it is a PET (ethylene terephthalate) film coated with silicon on one side, which mainly plays a role in protecting the pressure sensitive adhesive layer, at the same time, the peeling force has certain influence on the workability of LCD patch.
Protective film: it is PE (polyethylene) film coated with EVA layer (ethylene and ethylene acetate polymer) on one side, which has low viscosity and plays a role in protecting the surface of TAC film.
Reflective film: it is a pet film of unilateral aluminum evaporation. Currently, most of them use non-directional reflective aluminum evaporation film. If the reflective film is replaced with semi-permeable and semi-reflective film, it can be made into semi-permeable and semi-reflective polarizer. In addition, various gold-plated, silver-coated and laser films can also be used as reflective films to obtain various effects such as background color and specular reflection.
Main performance indicators of polarizer
Table 1 is a typical performance table of polarizer, which is described one by one in the following aspects:
1. Direction of light absorption axis generally, the sizes of the polarizer for tn LCD sold in the market mainly include the following two types:
The direction of the light absorption axis is shown in the figure. If the polarizer with special size and shape is provided to the LCD manufacturer, the light absorption axis should be marked or explained.
2. Transmittance the Transmittance index of polarizer can be divided into three parts: single, Parallel and Crossed. Generally, integral sphere spectrophotometer is used to measure it according to JIS-Z-8701. Among them, the monomer transmittance refers to the transmittance of a single polarizer, and the parallel transmittance (H 0) refers to the transmittance of two polarizers with parallel optical absorption axes after superposition, and the direct cross transmittance (H 90) refers to the transmittance of two polarizers with direct cross optical absorption axes after superposition. Among these three indicators, H 0 and H 90 affect the brightness (H 0) and contrast (H 0/H 90) of LCD screen ), it is very important for LCD manufacturers. In order to obtain a good display effect with high brightness and contrast ratio, we hope that H 0 is as high as possible and H 90 is as small as possible.
3. (Hue) Hue is represented by a and B values, which are usually measured by integrating sphere spectrophotometer. The values of a and B are the color coordinate values in the Lab color system of CIE (international lighting Commission). The approximate colors corresponding to a and B values can be found from the color coordinate diagram.
4. Polarizing Co-Efficiency Polarizing is a calculated value, which is used to indicate the comprehensive Efficiency of Polarizing produced by polarizer, the formula can be changed into H 0/H 90 = (1 + ν 2)/(1-ν 2), it can be seen that the ν value is closer to 100%, the higher the contrast ratio (H 0/H 90) is.
5. Peeling force the peeling force of polarizer can be divided into three parts: peeling force of protective film, peeling force of peeling film and peeling force of glass substrate. The three kinds of peeling forces are measured by tensile testing machine with reference to JIS-C-2107 Standard, among which the peeling force of protective film and peeling force of peeling film are measured by peeling along the direction of 180 °, while the glass substrate is stripped along the 90 ° direction. For LCD manufacturers, polarizer is very important for the stripping performance of glass substrate. Such as short time after pasting (4 ~ 6 hours) if it is difficult to peel off or there is residual glue on the glass plate after peeling off, the reworking property of the polarizer will be poor, and the bad patch will cause the whole LCD screen to be scrapped. However, if the peeling force is very small, it is easy to cause the loss of durability and moisture resistance of the pressure sensitive adhesive after the polarizer is attached to the glass substrate, as well as the sunken surface of the peeling film, which will affect the performance of the polarizer. 6. Durability test of the polarizer is to peel off the stripping film and the protective film of the polarizer and then attach it to the glass substrate. After pressure defoaming, put it into a constant temperature and humidity box to observe the changes before and after the experiment. The foaming stripping index is mainly to assess the durability of adhesive, and the optical change index is to assess the durability of PVA layer. The durability requirements of polarizer should be determined according to the design requirements (using environment) of different types of LCD products.
The full name of polarizer should be polarizer. Those who have studied physics should know what polarized light is. The imaging of liquid crystal display must rely on polarized light. As for why, I haven't figured it out either, all the liquid crystals have two polarized light sheets closely attached to the liquid crystal glass, forming a liquid chip with a total thickness of about 1mm. without any polarizer, the liquid wafer cannot display images.
The reason why the polarizer is replaced is that the polarizer facing the eyes of the ordinary liquid crystal display is frosted to dissipate the surface reflection and scatter the light to increase the viewing angle of the liquid crystal display. for the projector, any scattering will cause the loss of light. The ideal state of the liquid wafer used in the projector should be 0 degree angle of view, that is, if you look at the liquid wafer from the vertical direction, there will be no light. Of course, this is impossible to achieve, but the closer the angle of view is to 0 degree, the higher the utilization rate of light is. Therefore, after replacing the frosted polarizer with a flat light polarizer, the brightness of the projector to the wall will be greatly improved, my personal estimation is that it can increase by 50-80%, which is the reason why polarizer should be changed.