Abstract:
A liquid crystal display device with plastic substrates is disclosed, which comprises: a subsidiary substrate, upper and lower plastic substrates including edge grooves and being fixed on the subsidiary substrate by heat resistant tapes and being joined with liquid crystals sealed there-between, and alignment layers formed on the surfaces which face one another of the upper and lower plastic substrates.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device equipped with plastic substrates, which has a light weight and can be prevented from breakage occurring through being dropped while in use. 
   2. Description of the Prior Art 
   As generally known in the art, a liquid crystal display (LCD) device is generally utilized as a screen for displaying information in a mobile terminal, following the development of wireless mobile telecommunication. 
   The liquid crystal display device employs a structure, in which a pair of glass substrates is generally joined together with liquid crystals being sealed there-between. However, recently it has become required that a substrate which is employed in a liquid crystal display device for a mobile terminal should be light in weight for ease in use, so that glass substrates have been changed to plastic substrates. 
   Highly polymerized compounds with a heat resistant property for temperatures up to 150 to 200° C., such as polycarbonate, polyimide, PES (polyether sulfone), PAR, PEN (polyethylene), and PET (polyether), are used as materials for plastic substrates. 
   A method for manufacturing a liquid crystal display device using a conventional plastic substrate is explained below with regard to  FIGS. 1A  to  1 C. 
   Referring to  FIG. 1A , a plastic substrate  10  with desired lower layer patterns (not shown), including a TFT and pixel electrodes, is provided. The plastic substrate is placed on a stage  1  with a vacuum hole  2 , and fixed on the stage  1  with vacuum pressure. 
   Referring to  FIG. 1B , an alignment material is coated on a fixed plastic substrate  10  by means of an offset printing method using a transfer plate  3 . In  FIG. 1B , the numeral  5  in the drawing refers to a coating roll. 
   Referring now to  FIG. 1C , the coated alignment material is rubbed by a rubbing rag  6 , to result in producing an alignment layer  11   a  on the substrate  10 , which controls initial alignment of liquid crystal molecules. The numeral  7  in the drawing refers to a rubbing roll. 
   Next, although not shown in the drawings, lower substrate and upper substrate, to which alignment layers are formed through the above processes, are joined by sealing materials, and then liquid crystal molecules are filled in a space made between the substrates, resulting in the production of a liquid crystal display device incorporating plastic substrates. 
   However, according to the above explained prior art, a plastic substrate is transformed into a non-homogeneous shape owing to a curling property of the substrate, which occurs during fixture to the stage, and the vacuum pressure applied to fix the substrate. As stated above, a plastic substrate curls up away from the stage in undesired place including an edge thereof. 
   Accordingly, as the applied pressure is different in different regions owing to variation in height of the surface of the plastic substrate, the alignment materials are coated non-uniformly, and also the alignment layer, which is produced by rubbing of the alignment materials, is subjected to non-uniform rubbing over the whole substrate owing to variation of the friction between the rubbing rags and the alignment materials, produced by the vacuum pressure applied in the rubbing process and the non-uniformly coated alignment materials. 
   Further, as the non-uniform rubbing of the alignment layer produces a non-uniformity in cell gaps, the picture quality of a liquid crystal display device is reduced owing to the non-uniform cell gap. 
   Conventionally, to solve the above problems of the prior art, it had been proposed that the plastic substrate be fixed to the stage using a subsidiary substrate such as glass substrate and a thermal resistant tape. 
   However, although this prior art can prevent the plastic substrate from being non-desirously transformed, a non-uniformity of rubbing is produced in the fixing tape, which corresponds to the height of the surface of the tape, when the alignment materials are subjected to rubbing. As shown in  FIG. 2 , a non-uniformity of cell gaps is produced to a height of about 30 to 100 μm which corresponds to the height of the stacked fixing tape  26 , when the plastic lower substrate  22  and upper substrate  24  are joined. 
   In  FIG. 2 , reference numeral  21  denotes a subsidiary substrate,  28  denotes a sealing material, d 1  denotes the height of the stacked fixing tape  26 , which is approximately 60 to 200 μm, and d 2  denotes a cell gap, which is approximately 3 to 8 μm. 
   However, this technique can not finally solve the problems in the prior art whereby the picture quality of the liquid crystal display device is decreased due to the non-uniformity of cell gaps. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a liquid crystal display device using a plastic substrate, which can improve non-uniformity of the plastic substrate, non-uniformity of the coating in the alignment materials, non-uniformity in the rubbing of the alignment materials, and non-uniformity in the cell gap. 
   Another object of the present invention is to provide a liquid crystal display device which has improved picture quality by way of securing uniform cell gaps. 
   In order to accomplish these objects, there is provided a liquid crystal display device with plastic substrates comprising: a subsidiary substrate, upper and lower plastic substrates including edge grooves and being fixed on the subsidiary substrate by heat resistant tapes, the upper and lower plastic substrates being joined with liquid crystals sealed there-between, and alignment layers formed on the surfaces which face one another of the upper and lower plastic substrates. 
   The upper and lower plastic substrates have a thickness of 200 to 700 μm, and the edge grooves of the plastic substrates have a depth of 100 to 350 μm. 
   Also, the edge grooves are alternatively formed along the long sides, along the short sides or along all four sides of the upper and lower plastic substrates, and are formed in a continuous or interrupted groove configuration. 
   Further, the subsidiary substrate is made of any one material selected from a group including glass, acrylic materials, and metals, and has a size equal to or bigger than the depth of the plastic substrates. 
   The heat resistant tape has a thickness thinner than the depth of the edge grooves in the plastic substrates. 
   The upper and lower plastic substrates with edge grooves are fabricated by a roll compression method employing a main roll with convex groove patterns and a subsidiary roll below, or a press method with a mold frame having convex groove patterns. 
   Also, a radius r of the main roll in the roll compression method satisfies the relation 2πr=L, in which L is a length of the plastic substrate. 
   Further, the convex groove patterns have a width two times larger than that of the edge grooves of the plastic substrate, and a length equal to that of the edge grooves of the plastic substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1A  to  1 C are views showing each step of a method for fabricating a liquid crystal display device using conventional plastic substrates; 
       FIG. 2  is a sectional view showing a conventional liquid crystal display device using plastic substrates; 
     FIG.  3 A and  FIG. 3B  are a planar view and a sectional view, respectively showing a plastic substrate incorporated in a liquid crystal display device in accordance with the present invention; 
       FIGS. 4  to  6  are views showing a method for fabricating a plastic substrate according to one embodiment of the present invention; 
       FIGS. 7A  to  7 C are views showing a method for fabricating a liquid crystal display device using plastic substrates according to one embodiment of the present invention; 
       FIG. 8  is a sectional view showing a plastic substrate fixed to a process table in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. 
   Referring to  FIGS. 3A and 3B , a plastic substrate  30  includes a thin edge groove  31  along edges thereof. The substrate  30  has a thickness t 1  of 200 to 700 μm, and the edge groove  31  has a depth t 2  which is a half of the thickness of the substrate  30 . In this embodiment, the depth t 2  of the edge groove  31  should be bigger than the thickness of the heat resistant fixing tape or the substrate fixing device which will be explained below. The edge groove  31  can be formed at any side of the substrate  30  including long sides and short sides, and can be formed at all four sides. 
   Further, as shown in  FIG. 3A , the edge groove  31  can be formed at an edge of the substrate  30  in a continuous groove configuration, and as shown in  FIG. 3B , can be formed at a desired portion of the edge in the substrate  30  in an interrupted groove configuration. 
   The edge groove  31  has a width W 1  enough to safely fix the plastic substrate  30  by the fixing tape or substrate fixing device, and for example, the width W 1  may be of 0.3 to 1.5 mm. 
     FIGS. 4  to  6  are views showing a method for fabricating a plastic substrate in accordance with one embodiment of the present invention. 
   Referring to  FIGS. 4 and 5 , the plastic substrate  30  can be fabricated by means of a roll compression method employing a main roll  40  with a convex groove pattern  41  on a surface thereof and a subsidiary roll  42  attached below the main roll. 
   As shown in  FIG. 4 , the convex groove pattern  41  of the main roll  40  can be formed in a continuous stripe configuration which corresponds to  FIG. 3A , and can be formed in an interrupted shape as shown in FIG.  3 B. 
   The convex groove pattern  41  of the main roll  40  has a width W 2  of 0.6 to 3 mm, which is two times the width of the edge groove in the substrate  30 . W 3  denotes a width of the groove  31  made in a circular plastic substrate  30 , and W 3  is the same as the width W 2  of the convex groove pattern  41 . 
   The main roll  40  has a radius r satisfying the equation 2πr=L, in which L is a length of the substrate  30 . For example, when the length L is 470 mm, the radius r of the main roll 40 is 74.8 mm. 
   Referring to  FIG. 5 , the convex groove pattern  41  has a length l  2  that is the same as a length l  1  of the edge groove  31  in the groove  30 , which is a length enough to fix the substrate  30  stably by the heat resistant fixing tape or the substrate fixing device employed in a liquid crystal fabricating process, for example, the length l  2  is of 3 to 10 cm. 
   Referring to  FIG. 6 , the plastic substrate  30  of the present invention can be fabricated by means of a press method using a lower mold frame  62  in which a plastic solution  63  is filled and an upper mold frame  60  equipped with a convex groove pattern  61 . 
   In this case, the plastic solution  63  is first filled in the lower mold frame  62 , and then the plastic solution  63  is pressed by the upper mold frame  60  equipped with the convex groove pattern  61 , resulting in the formation of the plastic substrate  30  including the edge groove  31 . In this instance, the width W 4  of the convex groove pattern  61  is of approximately 0.3 to 1.5 mm, which is the same as the width W 5  of the edge groove of the substrate  30 . 
     FIGS. 7A  to  7 C are sectional views showing each step of the method for fabricating a liquid crystal display device using a plastic substrate in accordance with one embodiment of the present invention. 
   Referring to  7 A, the plastic substrate  30  is arranged on a subsidiary substrate  70 , and fixed on the substrate  70  by heat resistant fixing tape  71 . The subsidiary substrate  70  is made of glass, acrylic materials, or metals such as SUS, preferably made of glass substrate. The subsidiary substrate  70  is fixed to a processing table  72  by vacuum pressure. 
   Then, alignment materials are coated on the plastic substrate  30  by means of an off-set printing method using a transfer plate  73 . Tn this instance, the plastic substrate  30  is arranged on the subsidiary substrate  70 , and fixed by heat resistant tape  71 , thereby preventing the transformation into a non-uniform shape. Also, the alignment materials  74  can be coated uniformly, as they are coated on the even surface of the plastic substrate  30 . 
   Referring to  FIG. 7B , the alignment materials are subjected to rubbing by a rubbing rag  75 , resulting in an alignment layer  74   a  on the substrate  30  which controls the initial alignment of the liquid crystal molecules. In this instance, as the plastic substrate is fixed by the heat resistant tape  71 , the friction forces between the rubbing rag  75  and the alignment materials are equal during the time of rubbing, resulting in the formation of a uniformly rubbed alignment layer  74   a.    
   Referring now to  FIG. 7C , the lower and upper plastic substrates  30  on which the alignment layers are formed are joined by the sealing materials, and then liquid crystals (not shown) are filled in the space made between the substrates  30 , resulting in incorporating a liquid crystal display device having a plastic substrate in accordance with the present invention. In this instance, as the heat resistant tape  71  does not affect the cell gaps, the liquid crystal display device of the present invention has uniform cell gaps, resulting in improved picture quality. 
   Thereafter, although not shown in the drawings, a processing table and the subsidiary substrate and the heat resistant tape are removed. 
     FIG. 8  is a sectional view showing a plastic substrate fixed at a process table in accordance with another embodiment of the present invention. 
   According to this embodiment of the present invention, a plastic substrate  30  with an edge groove is attached at a process table  80  equipped with a substrate fixing device  82  without employing a subsidiary substrate or a heat resistant tape. The substrate fixing device  82  has a thickness t 4  which is thinner than the depth t 2  of the edge groove in the substrate  30  in order to prevent non-uniformity of rubbing at the time of rubbing alignment materials and non-uniformity of the cell gaps at the time of joining the substrates, and for example, the thickness t 4  may be 50 to 300 mm. 
   In accordance with the liquid crystal display device using a plastic substrate of the present invention, alignment materials are uniformly coated over an entire region of the substrate, the alignment materials can be uniformly subjected to rubbing, and uniform cell gaps can be obtained. Accordingly, the liquid crystal display device of the present invention can provide improved brightness characteristics and high picture quality. 
   Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.