Abstract:
A polarizing substrate is placed into a dyeing tank for dyeing. A concentration of a dyeing agent in the dyeing tank is less than 800 ppm, and a dyeing period is greater than 200 seconds, so as to improve the dyeing of the polarizing substrate by lengthening the dyeing period and reducing the dyeing agent concentration.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is based on, and claims priority from, Taiwan Application Serial Number 93134019, filed on Nov. 8, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety.  
       BACKGROUND  
       [0002]     1. Field of Invention  
         [0003]     The present invention relates to a method for manufacturing polarizers. More particularly, the present invention relates to a method for dyeing polarizers.  
         [0004]     2. Description of Related Art  
         [0005]     Liquid crystal displays (LCD) have many advantages over conventional types of displays including having high display quality, having small volume, being lightweight, and having low driving voltage and low power consumption. Hence, LCDs are widely used in small portable televisions, mobile telephones, video recording units, notebook computers, desktop monitors, projector televisions and so on, and they have gradually replaced the conventional cathode ray tube (CRT) as a mainstream display unit.  
         [0006]     Polarizers are main components of a liquid crystal display. Traditionally, a polarizing substrate is dyed, extended and corrected first, and then is adhered onto a protection film to form a polarizer. The polarizing substrate is made of polyvinyl alcohol (PVA) molecules, which are distributed irregularly at any angle in the beginning. After extending, PVA molecules are gradually turned toward the extending direction, and iodine ions adhered on the PAV molecules are also arranged in the extending direction. Thus, the polarizer can absorb an amount of light parallel to the extending direction and only allow an amount of light perpendicular to the extending direction to pass through.  
         [0007]      FIG. 1  is a schematic view of a conventional dyeing apparatus for polarizers. As illustrated in  FIG. 1 , a polarizing substrate  110  is dyed as mentioned above in a dyeing tank  102 . A common dyeing agent is an iodine solution  112 , and the iodine ions thereof are diffused into the polarizing substrate  110  by soaking the polarizing substrate  110  in the iodine solution  112 . As production capacity gradually increases, the rolling speed of polarizers while manufacturing, i.e. the machine speed, also becomes faster. When the machine speed is fast, the period of the polarizing substrate  110  soaking in the iodine solution  112  is short, thus causing insufficient dyeing.  
         [0008]     Therefore, the prior art generally enlarges the size of the dyeing tank  102  or increases the concentration of the iodine solution  112  to obtain better dyeing results. However, a larger dyeing tank or a higher concentrated dyeing agent requires a greater amount of chemicals and increases manufacturing cost. Moreover, the dyeing result of a higher concentrated dyeing agent is hard to control, easily causing uneven dyeing, which decreases the quality and affects the optical performance of polarizers.  
       SUMMARY  
       [0009]     It is therefore an aspect of the present invention to provide a method for dyeing polarizers, of which the dyeing period is increased and the concentration of the dyeing agent is decreased, to reduce manufacturing cost and enhance polarizer quality, especially for a polarizer manufacturing process with high machine speed.  
         [0010]     In accordance with the foregoing and other aspects of the present invention, a method for dyeing polarizers is provided. A polarizing substrate is placed into a dyeing tank for dyeing. A concentration of a dyeing agent in the dyeing tank is less than 800 ppm, and a dyeing period is greater than 200 seconds, so as to improve the dyeing of the polarizing substrate by lengthening the dyeing period and lowering the dyeing agent concentration.  
         [0011]     According to one preferred embodiment of the present invention, a dyeing path of the polarizing substrate is extended to increase the dyeing period by use of a plurality of rollers configured in the dyeing tank or by increasing a size of the dyeing tank. The dyeing agent comprises iodine, and a material of the polarizing substrate is polyvinyl alcohol.  
         [0012]     The present invention increases the dyeing period and decreases the concentration of the dyeing agent of the dyeing process and thus prevents uneven dyeing and improves the dyeing quality. Moreover, decreasing the dyeing agent concentration also reduces the amount of chemicals used and thus decreases manufacturing cost.  
         [0013]     It is to be understood that both the foregoing general description and the following detailed description are examples and are intended to provide further explanation of the invention as claimed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
         [0015]      FIG. 1  is a schematic view of a conventional dyeing apparatus for polarizers; and  
         [0016]      FIG. 2  is a schematic view of one preferred embodiment of the present invention 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
         [0018]     The present invention provides a method for dyeing polarizers, which improves the dyeing by using a long dyeing period and a dyeing agent with low concentration. The features of the present invention comprise controlling the dyeing period and the concentration of the dyeing agent using specific values. More particularly, the dyeing period is greater than 200 seconds and the concentration of the dyeing agent is less than 800 ppm, so as to obtain polarizers with good optical quality.  
         [0019]     According to a preferred embodiment of the present invention, a dyeing path in the dyeing tank can be extended to increase the dyeing period.  FIG. 2  is a schematic view of one preferred embodiment of the present invention. As illustrated in  FIG. 2 , the preferred embodiment adds rollers  202  in the dyeing tank  102 . The dyeing path in the dyeing tank  102  is extended to increase the dyeing period of the polarizing substrate  110 , such as a PVA substrate, soaked in the iodine solution  112 . Because the dyeing period is increased to be greater than 200 seconds, the concentration of the dyeing agent can be decreased to be lower than 800 ppm.  
         [0020]     In the preferred embodiment, the monomer transmittance of the polarizers can be over 43%; and the degree of polarization of polarizers can be greater than 99.90% when the dyeing temperature is between about 30° C. and 50° C., the dyeing time is greater than 200 seconds, and the concentration of the iodine solution is less than 800 ppm. The dyeing uniformity of the polarizer of the preferred embodiment is superior to the polarizer manufactured at the same dyeing temperature with a dyeing period of 80 seconds and the concentration of the iodine solution greater than 1200 ppm. Therefore, the preferred embodiment reduces the amount of chemicals, decreases the manufacturing cost and prevents uneven dyeing.  
         [0021]     An experiment is used to prove that the preferred embodiment reduces the amount of chemicals by extending the dyeing path. As in the foregoing embodiment, the dyeing path is extended by rollers  202  to increase the dyeing period of the polarizing substrate  110 . It is noted that, several dyeing paths with different lengths are provided by changing the number of the rollers  202  in this experiment. However, the hues of the obtained polarizers are the same, and thus the amount of the dyeing agent used for different dyeing paths are observed. Table 1 lists path lengths, dyeing periods and amounts of dyeing agents for different dyeing paths, which are formed by different numbers of rollers, all combinations of which obtain polarizers with the same hue.  
                                                                             TABLE 1                           A comparison of different dyeing paths       toobtain polarizers with the same hue.                Roller number                    5   3   2   1                            Path length (mm)   210   140   130   80           Dyeing period (sec)   225   150   140   80           Iodine stock solution (L)   3   3.6   3.6   4.9                      
 
         [0022]     The dyeing agent used here is an iodine solution diluted with other solvents and additives. For clarity, the iodine stock solutions, which are not diluted with other solvents and additives, for different dyeing paths are listed in Table 1 for easy and clear comparison. Because the size of the dyeing tank  202  and the volume of the iodine solution  112  therein are both fixed, the changes of the iodine stock solutions can be regarded as the changes of the concentrations of the iodine solutions. As indicated in Table 1, the preferred embodiment can certainly reduce the amount of dyeing agent used for dyeing by increasing the dyeing period, without affecting the desired polarizer hue.  
         [0023]     In addition, besides increasing the number of rollers to extend the dyeing path in order to increase the dyeing period as illustrated in the foregoing embodiment, the dyeing period can also be increased by changing the size and configuration of the rollers, increasing the size of the dyeing tank, and applying dyeing delay agents or other dyeing period extending methods known in prior art, according to other preferred embodiments.  
         [0024]     The present invention increases the dyeing period and decreases the concentration of the dyeing agent of the dyeing process, and thus prevents uneven dyeing and improves the dyeing quality. Moreover, decreasing the dyeing agent concentration also reduces the amount of chemicals used and thus decreases manufacturing cost.  
         [0025]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.