Introduction of friction cloth for LCD alignment process

发布时间：2020-06-03 发布人：

Alignment process introduction

The LCD alignment process (Alignment) is mainly to arrange the liquid crystal molecules in a specific direction to facilitate the control of the display operation. The steps of the alignment process are as follows. First, apply the alignment solution to the inside of the upper and lower electrode substrates of the liquid crystal display, and then perform the rubbing process. The surface of the alignment film will be piled by the rubbing clothes on the alignment roller. The grooves in a certain direction are brushed out by friction, and the liquid crystal material on the alignment film will achieve the alignment effect due to the intermolecular force. If you want to control the liquid crystal molecules to be arranged at a specific angle, you can use the rubbing direction of the alignment roller to form a predetermined Pre-tile Angle, which will benefit different drive modes of the display.

Introduction to alignment principle

The principle of the alignment process is mainly to use the force between the pile tip of the friction cloth and the alignment film mainly composed of polyimide (PI) to make the surface configuration of the PI molecule (Conformation) change. As shown in Figure 1: (a) is the original structure of PI molecules, (b) is the extension of PI molecules after friction and changes in structure; by the configuration change of PI molecules, the liquid crystal molecules on it have an alignment effect, such as Figure 2 (a), (b) shows.

Source: Institute of Materials Science and Technology

 

Fig. 1 PI configuration change caused by the directional brushing

Source: Institute of Materials Science and Technology

 

Figure 2 Alignment of liquid crystal molecules produced by the rubbing action

Introduction of friction cloth

 

The equipment of the alignment process is mainly composed of the alignment roller (Rubbing Roller), the friction cloth, the lifting arm, and the substrate stage. Generally speaking, the parameters affecting the alignment process include: the moving speed of the stage, the radius of the alignment shaft, and the Rotational speed, alignment times and alignment pressure and other factors; among them, rotation speed and moving speed are process control parameters, and the selection of friction cloth material is closely related to the substrate yield after alignment.

 

The material of the friction cloth can be generally divided into cotton, cotton, rayon, nylon, etc. The main evaluation points include: high density and uniform brush tip, chemical resistance, pressure resistance, elasticity, static electricity Good performance, not easy to shed hair and other factors; among them, static electricity and shed hair are important factors that affect the yield of the process. Static electricity accumulation (ESD) will cause damage to the TFT layer and also easily cause the adsorption of dust particles, which will affect the cleanliness And the falling of the debris caused by the friction process is even the enemy of the environment. Although the subsequent process includes the action of cleaning, it is not easy to solve this problem if the material of the friction cloth cannot be basically improved. The comparison of various friction cloths is shown in Table 1.

 

summary

The current alignment methods include vacuum evaporation, traditional friction method and optical alignment, etc. In terms of the uniformity of large-size panels, the traditional friction method has the advantages of mature and continuous production technology due to the current process technology and relatively low process cost. Therefore, it is still the mainstream method in the market.

In the future, with the requirements of wide viewing angles, panel manufacturers have gradually developed non-contact photo-alignment film materials based on photosensitive polymers to comply with specially designed display modes such as MVA. Therefore, the friction process will be gradually replaced. The static electricity and swarf caused by the traditional friction method are easy to cause LCD panel defects, such as alignment film coating spots, peeling and swarf adhesion. The friction cloth supplier should redesign the fiber weave or modify the fiber composition to reduce Only when the defect rate is generated can we continue to have a place in the market.