Patent Publication Number: US-2007096367-A1

Title: Thin films and fabrication method therefor

Description:
BACKGROUND  
      The invention relates to a thin film, and more specifically to a thin polarization film and a method of fabricating the same.  
      In recent years, there has been an increasing demand for polarization films because of wide use of liquid crystal displays (LCD) in various apparatuses such as word processors, monitors, cell phones, and liquid crystal televisions. Specifically, larger LCD TVs show a growing trend due to increasing popularity of family entertainment systems. Thus, it is necessary to develop thin and wide polarization films to meet the market requirements.  
      Polyvinyl alcohol (PVA) polarization films are generally categorized into iodine-type films where iodine is adsorbed on a properly oriented PVA film, dye-type films where a dichroic dye is adsorbed rather than iodine, and polyene-type films obtained by partial dehydration.  
      Among these, iodine-type film is most widely used due to ease of manufacture and excellent optical performance. A polarizer generally comprises a central stretched PVA film and at least one protective layer, typically of triacetyl cellulose (TAC). The stretched and dried PVA film is a main polarizing element, with a thickness commonly around 20˜35 μm and a stretching ratio of about 5˜6, most often 5.8. The conventional abovesaid PVA polarization film, however, is thick, narrow, and has a low-stretching-ratio, resulting in low utilization yield thereof, even with an extremely high width ratio (original/stretched) of 2˜2.5, fully unsatisfied to the requirement of large-sized panels.  
      U.S. Pat. No. 6,760,156 and 6,855,276 provide methods for increasing optical properties and uniformity of films by dry processes. A stretching ratio of 5˜7 can be achieved. Few efforts, however, are put to increase the stretching ratio/width and decrease the thickness of the resulted polarizing element. Thus, a thin and high stretching ratio/width stretched PVA film is desired to achieve the maximum material utilization yield.  
     SUMMARY  
      The invention provides a method for fabricating a thin film, in which a thin film having a hydroxyl groups-containing surface is placed and stretched in a water tank containing dicarboxylic acid.  
      The invention also provides a thin film fabricated by the disclosed method.  
      A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
       FIG. 1  shows chemical bonding between dicarboxylic acid and boric acid and a polyvinyl alcohol film surface.  
       FIG. 2  shows a result of thermal mechanical analysis (TMA) of the polarizer fabricated by example 4 of the invention.  
       FIG. 3  shows an extending path of a polarization film in an extending tank of the invention.  
       FIG. 4  shows another extending path of a polarization film in an extending tank of the invention. 
    
    
     DETAILED DESCRIPTION  
      The invention provides a method for fabricating a thin film, in which a thin film having a hydroxyl groups-containing surface is placed and stretched in a water tank containing dicarboxylic acid.  
      The thin film having a hydroxyl groups-containing surface may be a polyvinyl alcohol film.  
      The thin film may further be swollen and dyed with an iodine solution containing molecular iodine and iodide ions. The molecular iodine has a weight percentage of about 0.01˜0.1% and the iodide ions a weight percentage of about 0.1˜10%.  
      The dicarboxylic acid may have 4˜6 carbons, such as an adipic acid. The dicarboxylic acid has a weight percentage of about 1˜10%, preferably 2˜5%. In addition to dicarboxylic acid, transition metal ions such as zinc, hydrochloric acid, sulfuric acid, or boric acid, are further added to the water tank (extending tank). The transition metal ion has a weight percentage of about 0.1˜3% and the boric acid has a weight percentage of about 1˜3%. The extending tank has an operating temperature of about 40˜60° C.  
      In the invention, traditional boric acid bonded to the polyvinyl alcohol film surface is replaced by the dicarboxylic acid, as shown in  FIG. 1 . The hydrophilic carboxyl groups of the dicarboxylic acid easily bond to the hydroxyl groups of the polyvinyl alcohol film surface to form ester bonds therebetween. Compared to the rigid boric acid, the long-chain dicarboxylic acid having 4˜6 carbons is flexible, thus increasing tenacity and extensibility of the thin polarization film, without breakage during stretching. Also, to improve chemical reactivity and properties of the film stretched and avoid discoloration, additives such as transition metal ions, hydrochloric acid, sulfuric acid, or boric acid are added to the extending tank.  
      A thin, wide, and high-stretching-ratio polarization film is formed by adjusting various stretching parameters such as operating temperature, extending path, rotation rate of nip roller, and concentrations of dicarboxylic acid and additives.  
      After the polarization film is formed, at least one protective film (optical film), such as triacetyl cellulose (TAC), polyethylene terephthalate(PET), or polynorbornene, is laminated thereto.  
      The invention also provides a thin film fabricated by the disclosed method. The thin film is suitable for use in a variety of liquid crystal displays, for example, GPS system screens mounted in cars.  
      The stretched polarization film has a stretching ratio of about 6˜9, a thickness of about 10˜20 μm, a thickness ratio (original/stretched) of about 4˜8, and a width ratio (original/stretched)-of-about 1.5˜2.  
     EXAMPLE 1  
      A polyvinyl alcohol film with 650 mm width and 75 μm thickness was swollen in a water tank. Next, the polyvinyl alcohol film was dyed with an iodine solution. Next, referring to  FIG. 3 , the polyvinyl alcohol film  30  was stretched in an extending tank  10  containing 3% adipic acid, 1% boric acid, and 0.15% zinc ion to form an extended polyvinyl alcohol polarization film. The rotation rates of the nip rollers  20  were 1:1.5:1.55, respectively, and the extending path was shown in  FIG. 3 . The operating temperature was 53° C. Finally, a polarization film with a stretching ratio of 7.24, a thickness of 12 μm, and a width of 380 mm was obtained after being stretched and dried.  
     EXAMPLE 2  
      A polyvinyl alcohol film with 650 mm width and 75 μm thickness was swollen in a water tank. Next, the polyvinyl alcohol film was dyed with an iodine solution. Next, referring to  FIG. 3 , the polyvinyl alcohol film  30  was stretched in an extending tank  10  containing 3% adipic acid, 1% boric acid, and 0.15% zinc ion to form an extended polyvinyl alcohol polarization film. The rotation rates of the nip rollers 20 were 1:1.45:1.5, respectively, and the extending path was shown in  FIG. 3 . The operating temperature was 53° C. Finally, a polarization film with a stretching ratio of 6.77, a thickness of 17 μm, and a width of 342 mm was obtained after being stretched and dried.  
     EXAMPLE 3  
      A polyvinyl alcohol film with 650 mm width and 75 μm thickness was swollen in a water tank. Next, the polyvinyl alcohol film was dyed with an iodine solution. Next, referring to  FIG. 4 , the polyvinyl alcohol film  60  was stretched in an extending tank  40  containing 3% adipic acid, 1% boric acid, and 0.15% zinc ion to form an extended polyvinyl alcohol polarization film. The rotation rates of the nip rollers  50  were 1:1.5:1.5, respectively, and the extending path was shown in  FIG. 4 . The operating temperature was 53° C. Finally, a polarization film with a stretching ratio of 7.00, a thickness of 11.5 μm, and a width of 418 mm was obtained after being stretched and dried.  
     EXAMPLE 4  
      A polyvinyl alcohol film with 650 mm width and 75 μm thickness was swollen in a water tank. Next, the polyvinyl alcohol film was dyed with an iodine solution. Next, referring to  FIG. 4 , the polyvinyl alcohol film  60  was stretched in an extending tank  40  containing 3% adipic acid, 1% boric acid, and 0.15% zinc ion to form an extended polyvinyl alcohol polarization film. The rotation rates of the nip rollers  50  were 1:1.45:1.5, respectively, and the extending path was shown in  FIG. 4 . The operating temperature was 53° C. Finally, a polarization film with a stretching ratio of 6.77, a thickness of 14.5 μm, and a width of 385 mm was obtained after being stretched and dried.  
      The polarization film was then laminated with two TAC films (pre-etched with strong base) and dried at 70° C. to form a polarizer. Next, the polarizer was subjected to thermal mechanical analysis (TMA) at 80° C. for 30 min. Compared to similar samples on market (90˜110 μm), the contraction quantity thereof was 47.93 μm, as shown in  FIG. 2 .  
     COMPARATIVE EXAMPLE 1  
      A polyvinyl alcohol film with 650 mm width and 75 μm thickness was swollen in a water tank. Next, the polyvinyl alcohol film was dyed with an iodine solution. Next, the polyvinyl alcohol film was stretched in an extending tank containing 5% boric acid to form an extended polyvinyl alcohol polarization film. The rotation rates of the nip rollers were 1:1.4:1.33, respectively. The operating temperature was 53° C. Finally, a polarization film with a stretching ratio of 5.8, a thickness of 24 μm, and a width of 340 mm was obtained after being stretched and dried.  
      Stretching ratios, thicknesses, widths, thickness ratios (original/stretched), and width ratios (original/stretched) of various polarization films formed by examples 1˜4 and comparative example 1 are compared in Table 1.  
                                       TABLE 1                                   Stretching   Thickness                       ratio   (μm)   Width (mm)   R T     R W                                                              Example 1   7.24   12.0   380   6.25   1.72       Example 2   6.77   17.0   342   4.41   1.89       Example 3   7.00   11.5   418   6.52   1.56       Example 4   6.77   14.5   385   5.17   1.69       Comparative   5.80   24.0   340   3.13   1.91       Example 1                 R T  = thickness ratio (original/stretched)            R W  = width ratio (original/stretched)             
 
      The results indicate that the polarization film provided by the invention has a higher stretching ratio, thickness of only around 10˜20 μm, and increased width after being stretched, thus significantly improving material utilization yield.  
      While the invention has been described by way of examples and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.