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% - 30% of its manufacturing cost. However, because the manufacturing technology of polarizer has been monopolized by Japan, South Korea and other countries, there is little information about polarizer. Taking the polarizer for TN LCD as an example, this paper introduces some problems concerned by many LCD polarizer users.
Structure of polarizer
Polarizer is a kind of optical film composed of multi-layer polymer materials, which has the function of producing polarized light. According to its use position in the liquid crystal screen, it can be generally divided into two types: patch (also known as transmission film) and negative (also known as reflection film).
Material and main functions of each layer
Polarizing layer: it is made of PVA (polyvinyl alcohol) film after dyeing and stretching, which is the main part of polarizing film, also known as polarizing film. The polarizing layer determines the polarizing 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 the polarizing layer can be divided into two series: dye series and iodine series. According to the drawing technology, it can be divided into two series: dry drawing and wet drawing. Changing its material and processing technology can realize the adjustment of the polarizing degree, transmittance, tone and optical durability TAC layer: the polarizing layer made of PVA film is easy to absorb water, fade and lose polarizing performance. Therefore, a layer of TAC (cellulose triacetate) film with good optical uniformity and transparency is needed on both sides to isolate water and air and protect the polarizing layer. TAC film with UV cut and anti glare functions can be used to make UV and anti glare polarizers.
Adhesive: it can be divided into reflective film side adhesive and peeling film side adhesive. The function of the adhesive on the side of the reflective film is to firmly bond the reflective film to the TAC film, and its process requirements do not allow re peeling. Adhesive is a layer of pressure-sensitive adhesive, which determines the adhesion and processing performance of polarizer. The performance of adhesive is one of the most concerned problems for LCD polarizer users Separate film: it is a pet (ethylene terephthalate) film coated with silicon on one side, which mainly protects the pressure-sensitive adhesive layer. At the same time, the size of its peeling force has a certain impact on the workability of LCD chip.
Protective film: PE (polyethylene) film coated with EVA layer (vinyl acetate copolymer) on one side, with low viscosity, plays the role of protecting the surface of TAC film.
Reflective film: it is a PET film with one side evaporated aluminum. At present, most of the films are of non directional reflective type. If the reflective film is replaced by a semi transparent and semi reflective film, a semi transparent and semi reflective polarizer can be made. In addition, various gold-plated, silver plated and laser films can be used as reflective films to obtain various background colors and mirror reflection effects.
Main performance index of polarizer
Table 1 is a typical polarizer performance table, and the following items are described one by one:
1. There are two main sizes of TN type LCD polarizer sold on the market in the direction of optical absorption axis:
The direction of the optical absorption axis is as shown in the figure. If a polarizer of special size and shape is provided to the LCD manufacturer, the optical absorption axis shall be marked or described.
2. The transmittance index of the polarizer can be divided into three parts: single, parallel and crossed. It is usually determined by integrating sphere spectrophotometer according to jis-z-8701. Among them, monomer transmissivity refers to the transmissivity of a single polarizer, parallel transmissivity (H0) refers to the transmissivity after superposition of two polarizers whose optical absorption axis is parallel, and direct cross transmissivity (H90) refers to the transmissivity after superposition of two polarizers whose optical absorption axis is direct cross. Among these three indicators, H0 and H90 affect the brightness (H0) and contrast (H0 / H90) of LCD screen, which is very important for LCD manufacturers, In order to get a good display effect of high brightness and high contrast, we hope that h 0 is as high as possible and H 90 is as small as possible.
3. (hue) color is represented by a and b values, and is usually measured by integrating sphere spectrophotometer. A and b values are the color coordinate values in CIE (International Commission on lighting) lab color system. The approximate colors corresponding to a group of a and b values can be found from the color coordinate map.
4. Polarizing co efficiency (V) is a calculated value, which is used to express the comprehensive efficiency of polarizing light produced by the polarizer. The formula can be transformed into H0 / H90 = (1 + V 2) / (1-V 2). It can be seen that the closer the V value is to 100%, the higher the contrast (h 0 / h 90).
5. The peeling force of the polarizing plate can be divided into three parts: the peeling force of the protective film, the peeling force of the peeling film and the peeling force of the glass substrate. Three kinds of peel force were measured by tensile testing machine according to jis-c-2107 standard, in which the peel force of protective film and peel force of peel film were measured along 180 ° direction, while the peel force of glass substrate was measured along 90 ° direction. For LCD manufacturers, polarizers are very important for the peeling performance of glass substrates. If it is difficult to peel off within a short period of time (4-6 hours) after pasting or there is residual glue on the glass plate after peeling off, the polarizer has poor rework, and poor patch will lead to the scrapping of the whole LCD screen. However, if the peeling force is very small, it is easy to cause the poor durability and moisture resistance of the pressure-sensitive adhesive after the polarizer is bonded on the glass substrate, as well as the surface depression of the peeling film, which will affect the use performance of the polarizer
6. The durability test of the durable polarizer is to peel off the peeling film and the protective film, a
nd then attach them to the glass substrate, and then put them into the constant temperature and humidity box after pressure defoaming, and observe the changes before and after the experiment. Among them, the foam peeling index is mainly to examine the durability of adhesive, and the optical change index is to examine the durability of PVA layer. The durability requirements of polarizers shall be determined according to the design requirements (service environment) of different types of LCD products.
