Method for removing a liquid crystal alignment film

#The present invention relates to a method for removing a liquid crystal alignment film from a liquid crystal cell substrate and is intended to provide a method for removing only the alignment film under a condition as mild as possible without giving a damage to the substrate and thereby reclaiming an expensive electrode-formed substrate or a color filter substrate for a liquid crystal display device.

#A liquid crystal cell for a twisted nematic (TN) type liquid crystal display device or a super twisted nematic (STN) type liquid crystal display device is formed by coupling two sets of substrates each having a structure as shown in FIG. 1 or 2. In the case of a color display device, one of the substrates has a structure as shown in FIG. 3. In an active matrix (AM) type liquid crystal display device for color display, a liquid crystal cell is formed by bonding a substrate having semiconductor elements as shown in FIG. 4 with substrate of FIG. 3 (in a thin film transistor (TFT) type liquid crystal display device, the transparent electrode is not patterned), as shown in FIG. 4. These substrates are all provided with an alignment film made of e.g. polyimide as the top layer.

#Display failures of liquid crystal display devices may be caused by various factors. Among them, failures resulting from the process for forming an alignment film, are caused by many factors, such as coating irregularities in the alignment film, dusts having deposited on the film during the coating, drying or baking step, or local scraping, peeling or scratching of the film at the time of rubbing and constitute a substantial proportion in overall failures.

#Cyclobutane-1,2:3,4-tetracarboxylic dianhydride and 1,1,1,3,3,3-hexafluoro-2,2-bis[4-(4-aminophenoxy)phenyl] propane were subjected to an addition reaction at a molar ratio of 1:1 at room temperature for 8 hours to obtain a polyamic acid. An N-methyl-2-pyrrolidone solution containing 3% by weight of the obtained polyamic acid and 0.1% by weight of 7-(2-aminoethyl)aminopropyltrimethoxysilane as a silane coupling agent, was coated on a soda-lime glass substrate having an alkali-sealing film and having a transparent electrode and an electrode-protecting film thereon, followed by baking at 230.degree. C. for one hour to form an alignment film having a thickness of 920 .ANG. and thus to obtain a liquid crystal cell substrate having a structure as shown in FIG. 1. This-substrate was irradiated for 25 seconds by a high pressure ultraviolet ray irradiation apparatus (mercury lamp type: H03-L31W) manufactured by Aigraphic K.K. The dose of irradiated was 2000 mj/cm.sup.2 at 365 nm and 1140 mj/cm.sup.2 at 254 nm. The substrate was immersed in N-methyl-2-pyrrolidone at room temperature for 10 minutes while applying ultrasonic waves thereto, followed by washing with water for 2 minutes. After drying, the substrate was inspected by a microscope, whereby it was found that the alignment film was completely removed without any residual film, and the electrode-protecting film and the transparent electrode as underlayers were not damaged at all.

#A liquid crystal cell substrate was treated under the same conditions as in Example 1 except that the baking of the alignment film was conducted at 260.degree. C. for one hour, and the dose of the high pressure ultraviolet rays was changed to twice as much. Such a substrate was immersed in a solution of pH 13.2 obtained by diluting an alkaline cleaning agent (Homezaline F-236L-1, tradename, manufactured by Kao Corporation) with 20 times of pure water at room temperature while applying supersonic waves, followed by washing with water for 2 minutes. After drying, the substrate was inspected by a microscope, whereby it was found that the alignment film was completely removed without any residual film, and the electrode-protecting film, the transparent electrode and the alkali-sealing film as underlayers were not damaged at all.

#The liquid crystal cell substrate of Example 1 was irradiated for 2 minutes by a PL1-907 Model low pressure ultraviolet ray irradiation apparatus (mercury lamp type: SUV-90US) manufactured by Sen Engineering, while being covered by a filter capable of absorbing lights with wavelengths of 300 nm or more. The dose was 1200 mj/cm.sup.2 at a wavelength of 254 nm.