Analysis of the Structure of Liquid Crystal Polarizers and the Performance of Polarizers for LCD Applications

Structure and main performance analysis of polarizing film for LCD. Polarizers, as one of the main raw materials for liquid crystal displays (LCDs), account for about 20% to 30% of their manufacturing costs. However, due to the fact that the manufacturing technology of polarizers has always been monopolized by countries such as Japan and South Korea, there is very little information about polarizers. This article takes TN type LCD polarizer as an example to introduce some issues that many LCD polarizer users are concerned about.

Structure of polarizing film

Polarizing film is an optical thin film composed of multiple layers of polymer materials with the function of generating polarized light. According to its different usage positions in liquid crystal screens, it can be generally divided into two types: surface film (also known as transparent film) and back film (also known as reflective film).

The materials and main functions of each layer

Polarization layer: It is made by dyeing and stretching PVA (polyvinyl alcohol) film, which is the main part of the polarizer, also known as the polarizing film. The polarizing layer determines the polarizing performance and transmittance of the polarizer, and is also the main part that affects the color tone and optical durability of the polarizer. The basic processing technology of polarizing layer can be divided into two series: dye series and iodine series according to the dyeing method, and can be divided into two series: dry stretching and wet stretching according to the stretching process. Changing its material and processing technology can achieve adjustment of polarization degree, transmittance, color tone, and optical durability. TAC layer: The polarizing layer made of PVA film is prone to water absorption, fading, and loss of polarizing properties. Therefore, a layer of TAC (cellulose triacetate) film with good optical uniformity and transparency is needed on both sides to isolate moisture and air and protect the polarizing layer. The use of TAC film with UV CUT and anti glare functions can produce anti UV polarizer and anti glare polarizer.

Adhesive: It can be divided into reflective film side adhesive and release film side adhesive. The function of the adhesive on the reflective film side is to firmly adhere the reflective film to the TAC film, and its process requirements do not allow for re peeling. The adhesive on the peeling film side is a layer of pressure-sensitive adhesive, which determines the adhesion performance and surface mount processing performance of the polarizer. Its performance is one of the most concerned issues for LCD polarizer users. Separate film: A PET (ethylene terephthalate) film coated with silicon on one side, mainly used to protect the pressure-sensitive adhesive layer, and its peeling force has a certain impact on the workability of LCD mounting.

Protective film: A PE (polyethylene) film coated with an EVA layer (ethylene vinyl acetate copolymer) on one side, which has low viscosity and serves to protect the surface of the TAC film.

Reflective film: It is a PET film with single-sided aluminum evaporation, and currently most use non directional reflective aluminum evaporation films. If the reflective film is replaced with a semi transparent and semi reflective film, a semi transparent and semi reflective polarizing film can be made. In addition, various gold-plated, silver plated films, and laser films can be used as reflective films to obtain various base colors and mirror reflection effects.

Main performance indicators of polarizing film

Table 1 is a typical performance table of polarizing film, and the following will explain each item index one by one:

1. The size of polarizing film for TN type LCD commonly sold in the market for the direction of the absorption axis is mainly divided into the following two types:

The direction of its absorption axis is shown in the figure. If a polarizer with a special size and shape is provided to the LCD manufacturer, the absorption axis should be marked or explained.

2. The transmittance index of polarizing film can be divided into three categories: single, parallel, and crossed. Usually, an integrating sphere spectrophotometer is used to measure it according to JIS-Z-8701. Among them, monomer transmittance refers to the transmittance of a single polarizer, parallel transmittance (H0) refers to the transmittance of two polarizers with parallel absorption axes stacked together, and orthogonal transmittance (H90) refers to the transmittance of two polarizers with orthogonal absorption axes stacked together. Among these three indicators, H0 and H90 affect the brightness (H0) and contrast (H0/H90) of LCD screens, which are very important for LCD manufacturers. In order to achieve good display effects with high brightness and high contrast, it is hoped that H0 is as high as possible and H90 is as small as possible.

3. Hue is represented by the values of a and b, and is usually measured using an integrating sphere spectrophotometer. a. The b value is the color coordinate value in the CIE (International Commission on Illumination) Lab color system, and the approximate color corresponding to a set of a and b values can be found from the color coordinate graph.

4. Polarizing Co Efficiency: Polarizing Co Efficiency is a calculated value used to represent the overall efficiency of a polarizer in producing polarized light. This formula can be transformed into H 0/H 90=(1 × 2)/(1- × 2). It can be seen that the closer the value of V approaches 100%, the higher the contrast (H 0/H 90).

5. The peeling force of polarizing film can be divided into three categories: protective film peeling force, peeling film peeling force, and peeling force on glass substrate. The determination of three types of peeling forces was carried out using a tensile testing machine in accordance with JIS-C-2107 standard. The determination of protective film peeling force and peeling film peeling force was carried out along the 180 ° direction, while for glass substrates, it was carried out along the 90 ° direction. For LCD manufacturers, the peeling performance of polarizing films on glass substrates is crucial. If it is difficult to peel off within a short period of time (4-6 hours) after pasting, or if there is residual glue on the glass plate after peeling, the polarizer has poor reworkability, and poor surface mounting will result in the entire LCD screen being scrapped. But if the peeling force is very small, it is easy to cause a decrease in the durability and moisture resistance of the pressure-sensitive adhesive after the polarizer is attached to the glass substrate, as well as multiple defects such as surface depressions on the peeling film, which affects the performance of the polarizer.

6. The durability test of polarizing film is to peel off the peeling film and protective film of the polarizing film and stick it to a glass substrate. After pressure degassing, it is placed in a constant temperature and humidity box to observe its changes before and after the experiment. The foaming peeling index mainly assesses the durability performance of the adhesive, while the optical change index assesses the durability performance of the PVA layer. The durability requirements of polarizing film should be determined according to the design requirements (usage environment) of different types of LCD products.

The full name of polarizing film should be polarizing film. Those who have studied physics should know what polarized light is. The imaging of liquid crystal displays must rely on polarized light. As for why, I haven't figured it out yet. All liquid crystals have two polarizing films tightly attached to the liquid crystal glass, forming a liquid crystal film with a total thickness of about 1mm If any polarizer is missing, the LCD panel cannot display images

The reason for replacing the polarizer is that the polarizer facing the eyes on a regular LCD display is frosted to dissipate surface reflections and scatter light to increase the viewing angle of the LCD display For projectors, any scattering will cause a loss of light. The ideal state of the LCD panel used in projectors should be a 0-degree viewing angle, which means that if the LCD panel is viewed from a vertical direction, there will be no light. Of course, this is impossible to achieve, but the closer it is to the 0-degree viewing angle, the higher the utilization rate of light. Therefore, replacing the frosted polarizer with a flat polarizer will significantly increase the brightness of the projector on the wall. My personal estimate is that it can increase by about 50-80%. This is why it is necessary to replace the polarizer.