Patent Publication Number: US-2007113873-A1

Title: Method for cleaning optical films

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to a method for cleaning optical films, and more particularly, to a method for cleaning alkalized optical films.  
      2. Description of the Prior Art  
      A polarizer is an essential element used to transform unpolarized light into polarized light in a liquid crystal display (LCD). An LCD can control light flux to display different color signals by utilizing polarized light, the rotational character of liquid crystal molecules, and a color filter.  
      Please refer to  FIG. 1 , which is a schematic cross-sectional diagram demonstrating a polarizer. A polarizer  10  typically includes a protection film  100  such as a polyethylene film, triacetyl cellulose (TAC) films  102  and  106 , a polyvinyl alcohol (PVA) film  104 , a thermal bonding film  108 , and a release film  110 . The TAC films  102  and  106  are used to support and protect the PVA film  104 . Furthermore, the polarizer can be coated with a wide-view film or a brightness enhancing film. The TAC films  102  and  106  can be cross-linked with liquid crystal molecules on one of their surfaces to form a super wide-view TAC (SWV TAC) or an anti-glare TAC (AG TAC) for particular purposes.  
      Generally, TAC films are pre-treated with an alkaline solution before coating to the PVA film. After alkalizing, TAC films need to undergo a cleaning process to remove the remaining alkaline solution. Then, TAC films can be stored. Please refer to  FIG. 2 , which is a schematic drawing of a conventional alkalization and cleaning processes for TAC films. As shown in  FIG. 2 , a TAC film  20  is loaded into an unwinding machine  200  and driven forward by a rewinding machine  212 . The TAC film  20  is alkalized by an alkaline solution such as potassium hydroxide (KOH) or sodium hydroxide (NaOH) in an alkalization tank  202 . Then a rinsing process is performed in a first rinsing tank  204  to remove the alkaline solution from the surface of the TAC film  20 . Another rinsing process is then performed in a second rinsing tank  206  to further remove the remaining alkaline solution from the surface of the TAC film  20 . In addition, sulfuric acid can be added into the second rinsing tank  206  to neutralize the remaining alkalinity, and therefore the second rising tank  206  is called a neutralization tank. Thereafter, the TAC film  20  undergoes another rinsing process in a third rinsing tank  208  to clean the surface of the TAC film  20  with pure water. Finally, the water on the surface of the TAC film  20  is removed by an oven  210  and the TAC film  20  is stored up by the rewinding machine  212  for subsequent processes.  
      However, there are some unexpected problems that often happen in the conventional pre-treatment of TAC films, such as white spots resulting from sodium acetate formed in alkalization or from dried alkaline solution or water marks left after the oven when the temperature is not high enough.  
      Furthermore, please refer to  FIG. 3 , which is a schematic cross-sectional diagram demonstrating a SWV TAC film. As shown in  FIG. 3 , a SWV TAC film  30  is made by coating the TAC film  302  with an alignment layer  304  and cross-linking with a liquid crystal molecule layer  306  on a surface of the TAC film  302 . Because the alignment layer  304  is easily damaged by the alkaline solution in the alkalization process, the liquid crystal molecule layer  306  must entirely cover the alignment layer  304 . In addition, the margin of the liquid crystal molecule layer  304  is patterned for storing the SWV TAC film  30 . Due to poor adhesion between the TAC film  302  and the pattern section  308  of the liquid crystal molecule layer  306 , the liquid crystal molecules in the pattern section  308  are easily affected by temperature or pressure from the roll of the unwinding machine and are apt to peel off under certain conditions. Therefore the SWV TAC film  30  is not only susceptible to the aforementioned white spot or water mark problems, but also to a problem caused by liquid crystal molecules peeling from the surface of the liquid crystal molecule layer  306 .  
      To solve the aforementioned problem, there are many methods provided by the prior art, such as raising the temperature of the neutralization tank or increasing the concentration of sulfuric acid to reduce the formation of the white spots, or raising the temperatures in each rinsing tank to over 35° C. to improve the cleaning effect. However, if the aforementioned solutions are applied to a SWV TAC film, they increase the amount of liquid crystal molecules that peel from the pattern section of the SWV TAC film. Furthermore, in order to prevent TAC film from forming water marks, the temperature of the oven is raised to over 95° C. in the prior art, but this high temperature can make the TAC film fragile.  
      Therefore the prior art provides another method to prevent the abovementioned problems, which is to attach a protecting film on the surface of the TAC film. However, this method is more complicated due to the added processes for attaching and removing the protecting film. In addition, the protecting film itself is a consumable material, and therefore the cost is increased.  
     SUMMARY OF THE INVENTION  
      It is one object of the present invention is to provide a method for cleaning optical films. The provided method is for cleaning an alkalized optical film and preventing such film from forming white spots and water marks. Furthermore, the provided method simplifies the cleaning processes.  
      According to the claimed invention, a method for cleaning an optical film after alkalization is provided. The method comprises performing a first rinsing process to clean the optical film, performing a second rinsing process comprising at least an organic solution to further clean the optical film, and performing a drying process to remove water remaining on optical film.  
      According to the method provided by the claimed invention, each rinsing process can be performed at room temperature. In addition, additional process for attaching and removing the protecting layers on each side of the optical film is economized, and therefore the processes are simplified and cost is decreased.  
      These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic cross-sectional diagram illustrating a polarizer.  
       FIG. 2  is a schematic drawing illustrating a conventional alkalization and cleaning processes for TAC films.  
       FIG. 3  is a schematic cross-sectional diagram illustrating a SWV TAC film.  
       FIG. 4  is a schematic drawing illustrating a preferred embodiment provided by the present invention. 
    
    
     DETAILED DESCRIPTION  
      Please refer to  FIG. 4 , which is a schematic drawing illustrating a preferred embodiment provided by the present invention. The method for cleaning an optical film provided by the present invention can be applied to every kind of optical film such as surface-untreated TAC film, SWV TAC on which liquid crystal molecules are attached, wide-view TAC, brightness enhanced TAC, or anti-glare TAC.  
      As shown in  FIG. 4 , an optical film  40  is loaded into an unwinding machine  400  and is driven forward by a rewinding machine  412 . The optical film  400  is first alkalized in an alkalization tank  402 . The alkalization tank  402  has an alkaline solution, such as potassium hydroxide solution (KOH) or sodium hydroxide (NaOH) having a concentration in a range of 5%-10%. A temperature of the alkalization tank  402  is about 45° C.-55° C. Then, a first rinsing process is performed in a first rinsing tank  404  which contains pure water. Because the first rinsing tank  402  is used to rinse away the alkaline solution remaining on the optical film  40 , the amount of time spent in the first rinsing tank  402  is adjustable depending on the requirement. In addition, for completely rinsing sodium acetate formed in the alkalization process and the remaining alkaline solution and preventing the optical film  40  from shrinking and expanding, the temperature of the first rinsing process is slightly lower than the temperature of the alkalization process and is, for example, 30° C.-40° C.  
      Next, to remove white spots formed by sodium acetate more effectively, the method for cleaning an optical film provided by the present invention further comprises a neutralization process. The neutralization process is performed in a neutralization tank  406  which contains at least an acidic solution such as sulfuric acid. The concentration of the sulfuric acid is less than 4%.  
      Thereafter, a second rinsing process used to further clean the optical film  40  is performed in a second rinsing tank  408  which contains at least an organic solution. The organic solution is a volatile organic solution comprising alcohol, ketone, or ester such as methanol, ethanol, propanol, isopropanol, acetone, methyl ethyl ketone, or ethyl acetate, or preferably isopropanol. The concentration of the organic solution is in a range of 1%-20% and preferably 5%-6%.  
      The cleaning effect of the method provided by the present invention is achieved by the concept of “like dissolves like”, with polar material attracting polar material and non-polar material attracting non-polar material. The organic solution added in the second rinsing process can dissolve in water and is not dangerous to the TAC film  40 . Furthermore, when the organic solution is applied in cleaning the SWV TAC film, peeled liquid crystal molecules are prevented from attaching to the surface of the SWV TAC film.  
      Please refer to  FIG. 3  again. As mentioned, the liquid crystal molecules of the liquid crystal molecule layer  306  cross-linked on a surface of the SWV TAC film  30  are easily peeled during the alkalization process and the rinsing processes. Such peeled liquid crystal molecules often attach themselves to the surface of the liquid crystal layer  306  and not on the other surface of the TAC film  302  which has not been cross-linked with liquid crystal molecules. When the organic solution added in the second rinsing process is applied to rinse the SWV TAC film  30 , it decreases the contact angle of the liquid crystal molecule layer  306  from 90 degrees to lower than 35 degrees. Due to the decrease of the contact angle of the liquid crystal molecule layer  306 , the surface of the liquid crystal molecule layer  306  can be wetted and separated from the peeled liquid crystal molecules by the organic solution. Therefore the peeled liquid crystal molecules are effectively prevented from attaching to the SWV TAC film  30 . In addition, because the organic solution comprises a hydrophobic base such as hydrocarbon radicals which have stronger van der Waal force with the peeled liquid crystal molecules, and a hydrophilic base such as hydroxyl radicals, which are easily dissolved in water due to the hydrogen bond formed in between, the organic solution therefore functions as a detergent: it readily removes the peeled liquid crystal molecules and itself is easily removed by water.  
      Please refer to Table 1 which illustrates a comparison between the method provided by the present invention and the prior art:  
               TABLE 1                          Comparison of the cleaning effects between the method       provided by the present invention and the prior art                             Organic solution/   Amount of attached           concentration   liquid crystal molecules                                     Prior art   none   4-6       Embodiment 1   Methanol/1-20%   2-3       Embodiment 2   Ethanol/1-20%   1-3       Embodiment 3   Propanol/1-20%   1-3       Embodiment 4   Isopropanol/1-20%   0-1       Embodiment 5   Acetone/1-20%   1-3       Embodiment 6   Methyl ethyl ketone/1-20%   1-3       Embodiment 7   Ethyl acetate/1-20%   0-3                  
 
      According to the observed data shown in Table 1, the method of cleaning an optical film provided by the present invention effectively solves the problem of the prior art. It is noteworthy that because the added organic solution improves the cleanliness and decreases the attachment of the liquid crystal molecules, the temperatures of the neutralization tank  406  and the second rinsing tank  408  are not raised, but remain at room temperature. Therefore the application of the organic solution on the SWV TAC film can further decrease the amount of peeled liquid crystal molecules in the pattern section. Furthermore, the addition of organic solution is not limited in the second rinsing process, but can be selectively added to the first rinsing process or the neutralization process depending on the requirements or considerations for any special products.  
      Please refer to  FIG. 4 . The present invention further provides a drying process. In the drying process, the optical film  40  is driven to pass through an oven  410 . Because the organic solution comprises alcohol, ketone, or acetate, which are volatile, the boiling point is reduced. Therefore the temperature of the oven  410  is set at between 70° C.-80° C. to dry out the water on the surface of the optical film  40 . Thus, the fragility problem of the optical film  40  caused by the prior art high temperature in the oven  410  is prevented. Finally, the optical film  40  is stored by the rewinding machine  412  for following processes.  
      As mentioned above, the method provided by the present invention is to add organic solution in the second rinsing process to rinse away white spots on the surface of the optical film after an alkalization process. Furthermore, according to the method provided by the present invention, the neutralization process and the second rinsing process are performed at room temperature, which can further reduce the amount of peeled liquid crystal molecules. In addition, the temperature of the oven in the drying process is deceased to 70° C., which can prevent the fragility problem. Therefore the method provided by the present invention not only effectively removes white spots and peeled liquid crystal molecules from the optical film, but also simplifies the cleaning processes for an optical film.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.