Patent Publication Number: US-2007107834-A1

Title: Method of making a polarizing sheet

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to the manufacture of a polarizing sheet and, more particularly, to a method for manufacturing a LCD polarizing sheet having improved resistance to heat and moisture.  
      2. Description of the Prior Art  
      Films based on polyvinyl alcohol (PVA) containing iodine or dichroic dyestuffs as polarizing agents are known in the art. Polarizers or polarizing sheets based on PVA dyed with iodine have high polarization characteristics and are widely used in production of liquid-crystal display devices for cell phones, watches, calculators, personal computers, monitors, electronic clocks, word-processors, automobiles, liquid crystal televisions, etc. Besides, more and more LCD devices are now used in relatively severer environments such as, for example, outdoors commercial displays, in-car GPS screens, navigation systems of vehicles, or satellites. The market demand for polarizing sheets with high polarization performance has increased along with the use of such liquid-crystal display devices.  
       FIG. 1  is a schematic, cross-sectional diagram demonstrating a polarizing sheet  10  according to the prior art. Typically, the polarizing sheet  10  includes an adhesive release film  12 , a pressure-sensitive adhesive film  14 , a polyvinyl alcohol (PVA) film  18  sandwiched between triacetyl cellulose (TAC) films  16  and  20 , and a protective film  22  laminated on the TAC film  20 . In other cases, an anti-glare coating, an anti-reflection coating, or an hard-coating may be employed on the TAC film  20 . The iodine-type polarizing sheet is more prevalent in the industry than other types because of its high optical performance and because it is cheaper.  
      Generally, an iodine-type polarizing sheet is produced by lamination of TAC films  16  and  20  on both sides of the PVA film  18 . Prior to the lamination, the PVA film  18  undergoes pre-treatment such as swelling, dyeing, and re-stretching. The pre-treated PVA film  18  is laminated with the TAC films  16  and  20 , which are also pre-treated with alkaline solution such as sodium hydroxide or potassium hydroxide, by applying aqueous hydrogel containing dissolved PVA powders.  
      However, the preparation of the aforesaid aqueous hydrogel containing dissolved PVA powders is troublesome and tedious. To dissolve PVA powders, a great deal of time (at least 3-4 hours) and vigorous stir are required. While stirring, the prepared aqueous solution is heated to 80-90° C. Besides, the PVA-containing aqueous hydrogel in prior arts has to be used in one or two days because its stability is not good and aggregation may occur. It is desired to reduce the time required to prepare the aforesaid PVA-containing aqueous hydrogel and to increase its stability.  
      Another drawback of the iodine-type polarizing sheet in prior arts is that its resistance to heat and moisture is not satisfactory. In some severe environments, the adsorbed molecular iodine decays due to its volatile nature, thus adversely affecting its optical performance. From this aspect, it is desired to improve both the optical performance and durability of the polarizing sheet such that the polarizing sheet can withstand severe environments.  
     SUMMARY OF THE INVENTION  
      It is one object of the present invention to provide a method for manufacturing a LCD polarizing sheet having improved resistance to heat and moisture.  
      According to the claimed invention, this invention provides a method of making a polarizing sheet. The invention starts with swelling an un-stretched PVA film in pure water. The swelled PVA film is dipped into an iodine-containing solution to adsorb iodine. The dyed PVA film is then dipped into a dicarboxylic acid-containing solution while uniaxially stretching the dyed PVA film. The dicarboxylic acid-containing solution further contains boric acid. The stretched PVA film is then laminated with etched TAC films. Just before the lamination of the stretched PVA film and the TAC films, a catalytic aqueous solution containing a strong protonic acid and a Lewis acid such as ZnCl 2  is applied to the stretched PVA film.  
      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  
      The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
       FIG. 1  is a schematic, cross-sectional diagram demonstrating a polarizing sheet according to the prior art;  
       FIG. 2  is a schematic diagram showing the process of manufacturing a polarizing sheet according to one preferred embodiment of this invention;  
       FIG. 3  demonstrates the structural formula of adipic acid;  
       FIG. 4  is a schematic diagram demonstrating the surface of the PVA film after stretching; and  
       FIG. 5  illustrates the results of resistance test. 
    
    
     DETAILED DESCRIPTION  
      As previously mentioned, prior to the lamination of the prior art polarizing sheet, the TAC films are pre-treated with alkaline solution such as sodium hydroxide or potassium hydroxide to render the surface of the TAC films hydrophilic. The pre-treated TAC films are then laminated on both sides of the PVA film by applying PVA-containing aqueous hydrogel.  
      However, the preparation of the PVA-containing aqueous hydrogel is tedious. Further, there is a strong need to improve the stability and adhesive ability of the prior art PVA-containing aqueous hydrogel.  
      This invention provides a method of making polarizing sheets without using the aforesaid PVA-containing aqueous hydrogel by modifying the stretching process. A composition of PVA-free aqueous solution that is specifically formulated to fit the modified process is employed to replace the PVA-containing aqueous hydrogel in prior arts. The advantages of this invention at least comprise much simplified manufacturing process and better productivity and yield.  
      Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram showing the process of manufacturing a polarizing sheet according to one preferred embodiment of this invention. As shown in  FIG. 2 , the process generally includes the following steps:  
      Step  102 : swelling;  
      Step  104 : dyeing;  
      Step  106 : stretching;  
      Step  108 : drying;  
      Step  110 : lamination; and  
      Step  112 : drying.  
      It is one feature of the present invention that in the process of manufacturing a polarizing sheet according to one preferred embodiment of this invention, no PVA-containing hydrogel as the prior art is employed.  
      Starting with Step  102 , an un-stretched PVA film  50  is dipped in pure water  52  and swells. In Step  104 , the swelled PVA film  53  is then dipped into dye solution  54  containing iodine to adsorb iodine. The dye solution  54  basically contains molecular iodine and potassium iodide with an iodine concentration of about 0.01 wt. % to 0.5 wt. %.  
      In Step  106 , the dyed PVA film  55  is stretched. Step  106  is a uniaxially stretching process. The dyed PVA film  55  is dipped into cross-linking solution  56  containing boric acid and potassium iodide with a boric acid concentration of about 1 wt. % to 10 wt. %.  
      According to the preferred embodiment of this invention, the stretched PVA film  57  has a total stretching ratio of about 4 to 7. It is one feature of this invention that the cross-linking solution  56  further comprises dicarboxylic acid such as adipic acid (HOOCC 4 H 8 COOH), glutaric acid or succinic acid, preferably adipic acid. The structural formula of adipic acid is demonstrated in  FIG. 3 .  
      It is believed that the dicarboxylic acid added in cross-linking solution  56  reacts with surface hydroxyl groups during stretching in Step  106 . According to the preferred embodiment of this invention, the concentration of the dicarboxylic acid ranges between 1 wt. % and 5 wt. %.  
      Please refer to  FIG. 4 .  FIG. 4  is a schematic diagram demonstrating the surface of the PVA film  57  after treated with Step  106 . By way of example, in Step  106 , adipic acid is added as a component of the cross-linking solution  56 . As can be seen in  FIG. 3 , each adipic acid has two carboxyl groups at its two ends. One of the carboxyl groups of adipic acid reacts with hydroxyl group  572  on the surface of the PVA film  57 , thus producing chemical bonding structure  574 . This reaction is also referred to as esterification.  
      Moreover, the boric acid in the cross-linking solution  56  also reacts with the surface of the PVA film  57  to form hydrophobic structure  576 , which prevents the absorbed iodine  578  from diffusing out of the PVA film  57 .  
      Subsequently, as shown in  FIG. 2 , after stretching, Step  108  is carried out to dry the PVA film  57 . The dried PVA film  58  is then subjected to aqueous catalytic solution  70 .  
      In Step  110 , triacetyl cellulose (TAC) films  59  are laminated on both sides of the PVA film  58  to form TAC-PVA-TAC film stack  60 . The TAC films  59  are pre-treated with alkaline solution. The other carboxyl group of the adipic acid on the PVA film  58  reacts with hydroxyl group of the TAC films  59  (esterification catalyzed by aqueous catalytic solution  70 ) to form chemical bonding between the PVA film  58  and the TAC films  59 , such that the PVA film  58  and TAC films  59  are tightly bonded together. The aqueous catalytic solution  70  may be dropped on the PVA film  58  in Step  110 , or between Step  108  and Step  110 .  
      According to the preferred embodiment of this invention, the aqueous catalytic solution  70  comprises metal salts and strong protonic acid such as hydrochloric acid, sulfuric acid or nitric acid. The aforesaid metal salts may comprise inorganic salts containing zinc or magnesium, for example, ZnCl 2 , MgCl 2 , ZnBr 2  or ZnI 2 , preferably ZnCl 2 . The concentration of the metal salt ranges between 1 wt. % and 5 wt. %. The concentration of the strong protonic acid ranges between 0.1 M and 0.5 M.  
      Zinc chloride is a known Lewis acid and is sometimes used as a catalyst in other fields. Further, to speed up the following dry process, it is recommended to add volatile and hydrophilic solvent such as methanol into the aqueous catalytic solution  70 .  
      Finally, in Step  112 , the TAC-PVA-TAC film stack  60  is then subjected to a 50-80° C. drying process. Since the methanol is added, the time required for the drying process can be reduced. Further, to enhance the function of the polarizing sheet, the TAC films may coated with, for example, liquid crystal layer, protective layer, anti-glare layer, anti-reflection layer or anti-scratch or anti-smear layers prior to the alkaline treatment.  
      To more explain the features and advantages of this invention, one preferred example (best mode) and four comparison examples are demonstrated. In the preferred example, adipic acid is added into the cross-linking solution (Step  106 : stretching) and the aqueous catalytic solution contains no PVA (Step  110 : PVA-TAC lamination). In the first comparison example, the adhesive glue used during PVA-TAC lamination (Step  110 ) contains no PVA. The other three comparison examples use PVA-containing aqueous hydrogel glue. The four comparison examples use cross-linking solution that contains no adipic acid in Step  106 .  
     PREFERRED EXAMPLE (BEST MODE)  
      Weighted adipic acid was dissolved in the aqueous cross-linking solution. The dry PVA film was prepared and treated according to the processing steps mentioned above and in  FIG. 2 . Lamination of the polarizing PVA film and TAC films was performed using the catalytic solution which was prepared in advance by mixing methanol (MeOH), zinc chloride and hydrochloric acid. The combinatory effect of added adipic acid in the cross-linking solution and the content of the catalytic solution provides the required adhesive property. The TAC-PVA-TAC film stack was subjected to heat and moisture test performed in a 70° C. hot water bath.  
     FIRST COMPARISON EXAMPLE  
      Weighted adipic acid was first dissolved in hot water and the rest three elements including glyoxal, zinc chloride and hydrochloric acid were added. Clear and colorless glue was obtained after careful stirring, yielding the PVA-free chemical glue. The time required for preparation is less than ten minutes. The PVA-free chemical glue was used in the manufacture of polarizers according to typical procedures and likewise the formed TAC-PVA-TAC film stack is subjected to heat and moisture test performed in a 70° C. hot water bath.  
     SECOND COMPARISON EXAMPLE  
      Weighted PVA powders were dissolved in hot water, heated and stirred vigorously in order to prevent aggregation. The solution was cooled to room temperature under stirring conditions. Vaporized water is refilled and the solution was filtrated to remove aggregates, yielding a hydrogel with 5% PVA content. The time required for preparation was few hours. The PVA-type hydrogel was used in the manufacture of polarizers according to typical procedures and the formed TAC-PVA-TAC film stack is subjected to heat and moisture test performed in a 50° C. hot water bath.  
     THIRD COMPARISON EXAMPLE  
      Weighted PVA powders were dissolved in hot water, heated and stirred vigorously in order to prevent aggregation. The solution was cooled to room temperature, refilled with water to compensate lost weight during heating, and added with weighted additives including boric acid and zinc chloride which were added under stirring conditions. The mixture was filtrated to remove aggregates, yielding a hydrogel with 5% PVA content. The time required for preparation was few hours. The PVA-type hydrogel was used in the manufacture of polarizers according to typical procedures and the formed TAC-PVA-TAC film stack is subjected to heat and moisture test performed in a 50° C. hot water bath.  
     FOURTH COMPARISON EXAMPLE  
      Weighted PVA powders were dissolved in hot water, heated and stirred vigorously in order to prevent aggregation. The solution was cooled to room temperature, refilled with water to compensate lost weight during heating, and added with weighted additives including hydrochloric acid, glyoxal and zinc chloride which were added under stirring conditions. The mixture was filtrated to remove aggregates, yielding a hydrogel with 5% PVA content. The time required for preparation was few hours. The PVA-type hydrogel was used in the manufacture of polarizers according to typical procedures and the formed TAC-PVA-TAC film stack is subjected to heat and moisture test performed in a 50° C. hot water bath.  
      Resistance test:  
      The resistance to heat and moisture of the formed TAC-PVA-TAC film stack is presented in term of time of peeling as can be seen in the fourth column of comparison table in  FIG. 5 . In order to monitor the resistance to heat and moisture, the produced polarizers or TAC-PVA-TAC film stacks are exposed to severe environmental conditions. The polarizers were immersed into water bath at 70° C. for the preferred example and the first comparison example, while water bath at 50° C. for the third to fourth comparison examples. The resistance to heat and moisture of the TAC-PVA-TAC film stack made according to the preferred example is significantly improved.  
      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.