Basic knowledge of PI guide film
As the main material for anchoring liquid crystal molecules, the application of the guide film is a major development from the simple ionic bond force to the intermolecular force. The application of the guide film not only has the ionic bond force, but also has molecules The force of the key, coupled with the continuous development of surface treatment technology and the continuous improvement of processing machines, in addition to the requirements for improving the output and improving the viewing angle, the guide film material is almost finalized except for further research and trial use.
It is very strange that as the guide film material used for ordinary TN-LCD, it is more difficult to manufacture in some LCD production plants than STN-LCD. In fact, due to the low contrast and wide viewing angle of STN-LCD products, it has masked some defects in the electrical properties of the guide film. Unless there is a serious collapse of the pretilt angle, the defects of the guide film will not cause customers to pay special attention to the display effect of STN-LCD products. However, in the production of TN-LCD products, the performance migration of the guide film often directly reflects the display effect of the product.
As an anchor material for liquid crystal molecules, the pretilt angle of the guide film plays almost the same role on TN-LCD and STN-LCD products. However, compared with STN-LCD, because the movement mode of liquid crystal molecules is different, the two types of guide film products are quickly differentiated. The guide film material for STN-LCD is made more severe when it is produced. Environment and raw material purity. In addition, some manufacturers use equal-length and secondary chain technology on the molecular branches to make the molecular branches more stable and the pretilt angle is more consistent. At the same time, this structure is mutually exclusive due to the intermolecular forces between the secondary branches The branch chain structure has a certain self-repairing function, which makes the compensation for the lack of machine performance in the guidance process certain, because theoretically, the mechanical accuracy can never meet the accuracy requirements of the pre-tilt angle treatment of the guidance film. The guide film made in this way is used in products with low drive path count, small area and wide line spacing, its unique performance may not be reflected, but in the production of high drive path count, large area and micro line pitch When it comes to products, in terms of improving product yield, improving product stability and product repeatability, its excellent performance is unmatched by other guide films that are not made with this technology.
The guide film for TN-LCD products is restricted by the viewing angle width, and it is impossible to adopt the above-mentioned structure, and it is not necessary to do so due to the manufacturing cost. Moreover, due to the improvement of the process and the improvement of the performance of the machine equipment, the low-cost guide film materials used to be unstable due to the process and rejected by everyone, but now it is because it can have different effects under different processes. On the contrary, it is more favored by some TN-LCD manufacturers, especially on some semi-automatic production lines with easy process adjustment.
1. Characteristics of PI guide film:
The composition of PI guide film:
For PI that is cured by heating polymerization, the components of the PI-directed membrane stock solution are polyimide and DMA, NMP, or BC solvents.
For PI which is polymerized and cured by ultraviolet rays, the components of the PI guiding film stock solution are polyimide and solvent with ultraviolet light-sensitive groups. This PI has a fast production efficiency and operation because the irradiation direction is the liquid crystal alignment direction. Convenience, but because it is often inadequate when it is aggregated, the products made using it currently have a high single-point defect rate during display, and are only suitable for the production of some light conversion curtain walls. They are rarely used in LCD displays. The following are not Describe again.
Characteristics of PI guide film:
The solid content of the PI guide film used in LCD is a small molecule compound in the original liquid, which produces polymerization reaction at high temperature to form a long-chain macromolecular solid polymer polyamide with many branches. The angle between the branch chain and the main chain in the polymer molecule is the so-called pretilt angle of the guide layer. These polymers have a strong interaction force between the branched chain groups and the liquid crystal molecules, and have an anchoring effect on the liquid crystal molecules, so that the liquid crystals can be arranged in the direction of the pretilt angle.
The PI stock solution or small molecule polyimide that has not been completely polymerized is sol-like when combined with water molecules. It will inhibit the polymerization reaction of polyimide, and the complete main chain cannot be obtained, and the branch chain will lose the original The arrangement direction cannot obtain the pretilt angle required for LCD production. Therefore, PI stock solution must be protected from moisture, and the operating environment must be strictly controlled in humidity.
Some STN and TFT-specific PIs will form sub-branches on the basis of branches after polymerization and curing. The angle between the sub-branches and the branches matches or is close to the end structure of the liquid crystal molecules, so it has more Strong anchoring effect. Due to the sub-branched chain, if the contact area between the liquid crystal molecules and the guide layer is increased, some processing defects of the guide layer are compensated to some extent. This PI makes the display more uniform on a large area display. At the same time, it also avoids damage to the relatively weak CF layer and TFT generator in order to obtain higher friction density under high-strength friction in the friction treatment process.
The PI guide film that has been polymerized and cured also easily absorbs water and will decompose in water. The PI layer after friction treatment is also more fragile. When exposed to the air for a long time, it will combine with the water and carbon dioxide in the air, thereby disrupting the original branch chain arrangement state, making the pretilt angle uneven, and even producing The phenomenon of pre-tilt angle collapse.
PI classification:
PI is divided into:
1 ~ 3 degree low pretilt TN type PI and 4 ~ 9 degree high pretilt STN type PI.
The main parameters affecting the performance of PI guide film: solid content; viscosity; Na ion content; thermal decomposition temperature; dielectric constant; volume resistance; transmittance; water absorption; refractive index;
3. Factory adaptive testing method and judgment standard of PI guide film:
Viscosity:
A. Test method: use a viscometer to test the viscosity of the PI guide film to be measured.
B. Judgment standard: The viscosity value of the test result is consistent with the parameters provided by the supplier.
Additive drying performance (pre-baking performance)
A. Test method: According to the thickness requirements of the production process, coat the PI guide film on the ITO conductive glass, and heat and bake according to the temperature and time parameters provided by the supplier.
B. Judgment standard: Under the parameter temperature and time, the PI oriented film additive is completely dried.
Curing performance:
A. Test method: The ITO conductive glass with the PI-directed film additive completely dried is cured separately according to the production process requirements and the parameters provided by the supplier.
B. Judgment standard: curing is complete. After filling the LCD into the box, it can display normally.
Wear resistance:
A. Test method: The ITO conductive glass with the PI-directed film additive completely dried is subjected to directional rubbing five times according to the parameters required by the production process.
B. Judgment standard: LCD can be displayed normally after filling liquid crystal into a box.
Photoelectric performance and reliability:
A. Test method: According to the current production process parameters, the glass made of the PI guide film to be tested and the glass made with the PI guide film currently produced are selected to be made into finished products and tested for their photoelectric and reliability performance
B. Judgment standard: The photoelectric performance and reliability test results are equivalent to the results of the glass produced with the PI guide film currently produced, and are within the scope of the test product model requirements.
4. Selection rules of PI guide film:
Generally, the corresponding pre-tilt angle PI is selected according to the LCD display type: TN-LCD selects a low pre-tilt angle PI, and STN-LCD selects a high pre-tilt angle PI.
For some LCDs with additional layers with special processing technology under the PI guide layer, the characteristics of the additional layer should be considered when selecting PI.
For COLOR-STN and BLACK MASK products, select a PI whose curing temperature is less than the decomposition temperature of CF. For LCDs made with flexible conductive films, the curing temperature should be less than the PI of the softening point temperature of the flexible substrate.
5. How to use PI guide film:
Manufacturing method of PI guide film:
Spin coating method; screen printing coating method; offset printing method first thaw at room temperature to room temperature, and then open the bottle cap to avoid moisture absorption, PI guide film is recommended to open, prepare, use in a thousand-level clean room environment .
It is generally diluted with NMP, DMA, BC and other solvents. It is best to filter the solvent once under normal pressure before use.
For the coating thickness of PI guide film, TN-LCD recommends about 400 ~ 600 angstroms, STN-LCD recommends about 800 ~ 1000 angstroms.
6. Storage and handling methods of PI guide film:
PI guide membrane should be stored frozen. The storage temperature is less than 4 ℃, which can guarantee the storage period of more than 6 months. If a longer storage period is required, it should be stored below -10 ℃. Solvents are flammable, do not approach open flames.
Control according to chemical regulations when handling.
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