Abstract:
A liquid crystal alignment process comprises steps of: providing a first substrate and a second substrate to form a liquid crystal accommodating space therebetween; pouring a liquid crystal composition into the liquid crystal accommodating space, the liquid crystal composition comprising liquid crystal molecules, a first monomer material, and a second monomer material; applying a voltage difference to the first and second substrates for arranging the liquid crystal molecules at a pre-tilt angle; and exposing the liquid crystal composition by mixed multi-spectrum rays for polymerizing the first monomer material and the second monomer material to form at least one type of liquid crystal alignment polymer on opposite surfaces of the first and second substrates. The liquid crystal alignment process is capable of improving the efficiency of exposure procedure, reducing time to manufacture products, and is capable of solving the problems of high costs and waste pollution.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a process for a liquid crystal panel, and more particularly, to a liquid crystal alignment process. 
     BACKGROUND OF THE INVENTION 
     Controlling the alignment of liquid crystal molecules is one of the most important technologies of manufacturing a liquid crystal display. The image quality of the liquid crystal display is related to the alignment of liquid crystal molecules. High-quality images can be presented only when the liquid crystal molecules are aligned steadily and uniformly in the display panel. A thin layer which is utilized for directionally arranging the liquid crystal molecules is generally called a liquid crystal alignment layer. 
     According to different applications and principles, liquid crystal alignment technologies can be classified into a multi-domain vertical alignment (MVA) technology and a polymer sustained alignment (PSA) technology, for example. 
     Taiwan Patent No. I325081 discloses a display panel utilizing the MVA technology. Please refer to  FIGS. 1A and 1B .  FIG. 1A  is a diagram showing a pixel disposed in a traditional liquid crystal display panel utilizing the MVA technology.  FIG. 1B  is a sectional view of the pixel shown in  FIG. 1A . As shown in  FIG. 1B , the traditional MVA liquid crystal display panel includes a first substrate  12 , a second substrate  14  parallel to the first substrate  12 , and liquid crystal molecules  15  disposed between the first substrate  12  and the second substrate  14 . As shown in  FIG. 1A , the first substrate  12  has pixel areas  100  which are defined by scan lines  122  and  122 ′, and data lines  124  and  124 ′. Each pixel area  100  includes a storage capacitor bus line  126  which is parallel to the scan lines  122  and  122 ′, and the storage capacitor bus line  126  is set across the pixel area  100 . 
     As shown in  FIGS. 1A and 1B , the traditional MVA liquid crystal display panel utilizes bumps  125  which are disposed in the pixel  100  for aligning the liquid crystal molecules  15 . The bumps  125  are arranged in different regions and the respective planes of the bumps are inclined so that the liquid crystal molecules  15  are tilted along different directions, and therefore the pixels  100  can form multiple display regions so as to accomplish a feature of wide viewing angle. However, in the traditional MVA technology, the display aperture ratio is affected by the bumps  125 , resulting in decrease of the penetration rate. Moreover, the traditional MVA technology has drawbacks of dark fringes in a bright state and light leakage in a dark state, leading to degradation of the image quality. 
     U.S. Pat. No. 6,903,787 discloses a display panel utilizing the PSA technology. Please refer to  FIGS. 2A to 2C , which are diagrams showing a flow scheme of a conventional PSA process for aligning the liquid crystal molecules with a liquid crystal alignment polymer. As shown in  FIG. 2A , two parallel substrates, i.e. a first substrate  22  and a second substrate  24 , are provided. A first conductive layer  221  and a second conductive layer  241  are respectively disposed on opposite surfaces of the first and second substrates  22  and  24 . The first and second conductive layers  221  and  241  are coated respectively with polyimide (PI) alignment films  223  and  243 , in advance. Each polyimide molecule has an imide radical which makes the main chain possessing remarkable rigidity and strong molecular interaction so that the PI alignment films  223  and  243  are able to be utilized for auxiliary alignment. Next, liquid crystal molecules  252  and a monomer material  254  are poured into a liquid crystal accommodating space  25  which is confined by the first and second substrates  22  and  24 , and more specifically, located between the two PI alignment layers  223  and  243 . As shown in  FIG. 2B , a voltage source  261  is connected to the first conductive layer  221  on the first substrate  22  and the second conductive layer  241  on the second substrate  24 . The voltage source  261  applies a voltage difference to the first and second conductive layers  221  and  241 . The voltage difference makes the liquid crystal molecules  252  twisting at a pre-tilt angel. Moreover, an exposure procedure is performed with an ultraviolet light (UV)  262  to polymerize the monomer material  254 . As shown in  FIG. 2C , after the monomer material  254  is polymerized, polymer alignment layers  228  and  248  are respectively formed on the first substrate  22  and the second substrate  24 . The polymer alignment layers  228  and  248  have a function of aligning the liquid crystal molecules  252 . 
     In addition, Taiwan Patent Publication No. 200944901 discloses an alignment technology utilizing one monomer material to form a polymer for aligning the liquid crystal molecules. In this prior art, two procedures are adopted to expose the monomer material. In order to avoid destroying the liquid crystal molecules, rays of which wavelengths are longer than 290 nm are selected in a first exposure procedure for polymerizing a part of the monomer material to form two polymer steady alignment layers. Rays of which wavelengths lie between 290 nm and 340 nm are selected in a second exposure procedure for polymerizing the remaining monomer material. The &#39;901 TW published patent is capable of solving the problem of poor performance of image sticking test for the liquid crystal display panel caused by a great residual amount of the monomer material. 
     Compared to the MVA technology, the PSA technology can make the liquid crystal molecules aligned much steadily. Also, bump structures are not required in the PSA technology, and therefore the problems of dark fringes in a bright state and light leakage in a dark state do not exist. Therefore, the PSA technology is capable of increasing the penetration rate for the display panel and decreasing brightness for the dark state. 
     However, the liquid crystal alignment process disclosed by the &#39;787 US patent needs to coat the PI alignment films in advance. A PI coating generally at least requires equipments such as a coating machine, a baking machine, a heating plate, and a cooling plate. The PI coating results in requirement of expensive equipments so that the manufacturing cost is hard to be cut down and reduced. Moreover, the PI coating needs to use a PI cleaner, and high-temperature gas exhausted from the process will cause a great harm to the environment. Furthermore, the liquid crystal alignment process disclosed by the &#39;901 TW published patent adopts two exposure procedures to polymerize the monomer material, in which two UV tubes of different spectrums, together with filters, are respectively used in the two procedures. Therefore, the time for manufacturing products is lengthened and the whole manufacturing cost will be increased. 
     Therefore, how to solve the problems of high costs and waste pollution caused by coating the PI alignment films in advance in the conventional PSA technology, and how to decrease the manufacturing cost due to exposing the monomer material by multiple procedures are important issues in this technical field. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a liquid crystal alignment process for improving the efficiency of exposure procedure and reducing time to manufacture products. 
     Another objective of the present invention is to provide a liquid crystal alignment process for solving the problems of high costs and waste pollution caused by manufacturing liquid crystal alignment layers. 
     According to the above objective, the present invention provides a liquid crystal alignment process comprising steps of: providing a first substrate and a second substrate to form a liquid crystal accommodating space therebetween; pouring a liquid crystal composition into the liquid crystal accommodating space, the liquid crystal composition comprising liquid crystal molecules, a first monomer material, and a second monomer material; applying a voltage difference to the first and second substrates for arranging the liquid crystal molecules at a pre-tilt angle; and exposing the liquid crystal composition by mixed multi-spectrum rays for polymerizing the first monomer material and the second monomer material to form at least one type of liquid crystal alignment polymer on opposite surfaces of the first and second substrates. 
     In another aspect, the present invention provides a liquid crystal alignment process comprising steps of: providing a first substrate and a second substrate to form a liquid crystal accommodating space therebetween; pouring a liquid crystal composition into the liquid crystal accommodating space, the liquid crystal composition comprising liquid crystal molecules, a first monomer material, a second monomer material, and a polymerization initiator; applying a voltage difference to the first and second substrates for arranging the liquid crystal molecules at a pre-tilt angle; and exposing the liquid crystal composition by mixed multi-spectrum rays for polymerizing the first monomer material and the second monomer material, and the second monomer material being polymerized to form a second polymer alignment layer on at least one opposite surface of the first and second substrates as well as the first monomer material being polymerized to form a first polymer alignment layer on a surface of the second polymer alignment layer. 
     Compared to expose monomer materials by single-spectrum rays, the present invention does not need to substitute ultraviolet (UV) tubes and filters of different spectrums, and therefore is able to improve the efficiency of exposure procedure and reduce time to manufacture products since the mixed multi-spectrum rays are utilized to perform the exposure procedure for polymerizing the first monomer material and the second monomer material to form the first polymer alignment layer and the second polymer alignment layer. Moreover, the polymer alignment layers formed according to the present invention can maintain the quality of aligning the liquid crystal molecules. The liquid crystal molecules are aligned steadily and securely. 
     The present invention utilizes monomer materials to be polymerized to form structures for aligning the liquid crystal molecules instead of coating a PI alignment film. The expensive equipments generally used in the PI coating procedure are not required in the present invention. Therefore, the present invention can simplify the manufacturing process and speed up the production, and therefore is able to reduce the manufacturing cost. Also, the preset invention does not need to use organic solvents to clean APR plates, and therefore complies with energy saving and carbon reduction which are emphasized in modern life. In addition, the present invention does not need to dispose bump structures in the pixel areas of the display panel to align the liquid crystal molecules, and therefore is capable of achieving the optical properties such as high brightness and high contrast (without light leakage in a dark state) for the display panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram showing a pixel disposed in a traditional liquid crystal display panel utilizing a multi-domain vertical alignment (MVA) technology. 
         FIG. 1B  is a sectional view of the pixel shown in  FIG. 1A . 
         FIGS. 2A to 2C  are diagrams showing a flow scheme of a conventional polymer sustained alignment (PSA) process. 
         FIG. 2A  is a diagram showing a step of providing liquid crystal molecules and a monomer material in a liquid crystal accommodating space. 
         FIG. 2B  is a diagram showing a step of applying a voltage difference to two substrates and a step of performing an exposure procedure. 
         FIG. 2C  is a diagram showing a step of polymerizing the monomer material to form a polymer alignment layer for aligning the liquid crystal molecules. 
         FIGS. 3A to 3D  are diagrams showing a flow scheme of a liquid crystal alignment process implemented according to the present invention. 
         FIG. 3A  is a diagram showing a step of providing liquid crystal molecules, a first monomer material, and a second monomer material in a liquid crystal accommodating space. 
         FIG. 3B  is a diagram showing a step of applying a voltage difference to two substrates and a step of performing an exposure procedure. 
         FIG. 3C  is a diagram showing a step of polymerizing the second monomer material to form a second polymer alignment layer. 
         FIG. 3D  is a diagram showing a step of polymerizing the first monomer material to form a first polymer alignment layer on a surface of the second polymer alignment layer, the first polymer alignment layer being utilized for aligning the liquid crystal molecules. 
         FIG. 4A  is a diagram showing a mixed tube device capable of emitting rays of two different UV wavelengths (or spectrums). 
         FIG. 4B  is a diagram showing a mixed tube device capable of emitting rays of three different UV wavelengths (or spectrums). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Please refer to  FIGS. 3A to 3D  which are diagrams showing a flow scheme of a liquid crystal alignment process implemented according to the present invention. As shown in  FIG. 3A , a first substrate  32  and a second substrate  34  parallel to the first substrate  32  are provided. The first substrate  32  can be a thin-film transistor (TFT) array substrate. The second substrate  34  can be a substrate opposite to the TFT array substrate, or specifically, a color filter substrate (CF substrate). A first conductive layer  321  and a second conductive layer  341  are respectively disposed on the opposite surfaces of the first substrate  32  and the second substrate  34 . The conductive layers  321  and  341  can be transparent indium tin oxide (ITO) films. A liquid crystal accommodating space  35  is formed between the first and second substrates  32  and  34 , and more specifically, is formed between the first and second conductive layers  321  and  341 . 
     As shown in  FIG. 3A , a liquid crystal composition  350  is poured into the liquid crystal accommodating space  35 . The liquid crystal composition  350  includes liquid crystal molecules  352 , a first monomer material (A)  354 , and a second monomer material (B)  356 . The liquid crystal composition  350  may also include a polymerization initiator (not shown). That is, the polymerization initiator may also be poured into the liquid crystal accommodating space  35  together with the monomer materials  354  and  356 , and the liquid crystal molecules  352 . The polymerization initiator is capable of accelerating polymerizing reactions of the first monomer material (A)  354  and the second monomer material (B)  356 . 
     As shown in  FIG. 3B , a voltage source  361  is connected to the first conductive layer  321  and the second conductive layer  341  on the substrates  32  and  34 . The voltage source  361  can apply a voltage difference to the first conductive layer  321  on the first substrate  32  and the second conductive layer  341  on the second substrate  34  for arranging the liquid crystal molecules  352  at a pre-tilt angle. The voltage difference makes the liquid crystal molecules  352  twisting at the pre-tilt angel. 
     As shown in  FIG. 3B , mixed multi-spectrum rays  362  are utilized to expose the liquid crystal composition  350  in at least one exposure procedure so as to polymerize the first monomer material (A)  354  and the second monomer material (B)  356 . The mixed multi-spectrum rays  362  include rays of at least two spectrums. Said two spectrums may be overlapped, or independent to each other, or their wavelengths are discontinuous. In addition, said rays of two spectrums may be emitted out respectively from two independent light sources. Alternatively, rays of two distinguishable spectrums emitted from one light source can be implemented as well. Specifically, the mixed multi-spectrum rays  362  may include rays of an ultraviolet spectrum. In one embodiment, the first monomer material (A)  354  and the second monomer material (B)  356  are irradiated by the mixed multi-spectrum rays  362  at the same time. That is, the monomer materials  354  and  356  receive rays of at least two distinguishable spectrums at the same time. In addition, the polymerization initiator may also be irradiated by the mixed multi-spectrum rays  362 . In one embodiment, the first monomer material (A)  354  and the second monomer material (B)  356  are exposed to be polymerized in one exposure procedure. That is, polymerizing reactions of the monomer materials  354  and  356  are accomplished by only one exposure procedure. 
     As shown in  FIGS. 3C and 3D , after performing the exposure procedure with the mixed multi-spectrum rays  362 , the first monomer material (A)  354  and the second monomer material (B)  356  are polymerized to form at least one liquid crystal alignment polymer  389  on opposite surfaces of the first and second substrates  32  and  34 . The liquid crystal alignment polymer  389  includes first polymer alignment layers  390  and  390 ′, and second polymer alignment layers  380  and  380 ′. The liquid crystal alignment polymer  389  has a function of aligning the liquid crystal molecules  352 . As shown in  FIG. 3C , the second polymer alignment layers  380  and  380 ′, generated by polymerizing the second monomer material (B)  356  is formed on opposite surfaces of the first and second substrates  32  and  34 , respectively. As shown in  FIG. 3D , the first polymer alignment layers  390  and  390 ′, generated by polymerizing the first monomer material  354  is formed on the surfaces of the second polymer alignment layers  380  and  380 ′, respectively. 
     Compared to expose monomer materials by single-spectrum rays, the present invention does not need to substitute ultraviolet (UV) tubes and filters of different spectrums, and therefore is able to improve the efficiency of exposure procedure and reduce time to manufacture products since the mixed multi-spectrum rays  362  are utilized to perform the exposure procedure for polymerizing the first monomer material (A)  354  and the second monomer material (B)  356  to form the first polymer alignment layers  390  and  390 ′, and the second polymer alignment layers  380  and  380 ′. Moreover, the polymer alignment layers formed according to the present invention can maintain the quality of aligning the liquid crystal molecules. The liquid crystal molecules are aligned steadily and securely. 
     In one embodiment, the first monomer material (A)  354  and the second monomer material (B)  356  are two monomer materials respectively having a hydrophilic structure and a lipophilic structure. In the procedure of exposing by the mixed multi-spectrum rays  362 , the second monomer material (B)  356  having the lipophilic structure is polymerized gradually to form the second polymer alignment layers  380  and  380 ′, on the surface layers of the first and second substrates  32  and  34 , respectively. The second polymer alignment layers  380  and  380 ′ are steadily adhered to the first and second substrates  32  and  34 , respectively, and are interfered with the liquid crystal molecules  352  by the Van Der Waals force for auxiliary alignment. The function of the second polymer alignment layers  380  and  380 ′ is similar to that of traditional PI alignment films. The first monomer material (A)  354  having the hydrophilic structure is polymerized to form the first polymer alignment layers  390  and  390 ′ on the surfaces of the second polymer alignment layers  380  and  380 ′, respectively. The first monomer material (A)  354  has a side chain, and the polymerized first polymer alignment layers  390  and  390 ′ are constituted by a plurality of side chain structures which have a characteristic of joining the second polymer alignment layers  380  and  380 ′, and the liquid crystal molecules  352 . Said side chain structures are utilized for confining the liquid crystal molecules  352  to be arranged at the pre-tilt angel. 
     In another embodiment, the second monomer material (B)  356  is a non-polar monomer material having a lipophilic structure and a long chain. The long chain has a PI group in one end and an interfering group in the other end. That is, the long chain is bifunctional. The first monomer material (A)  354  is a polar monomer material of a short chain having a hydrophilic structure. The first monomer material (A)  354  also has a side chain. In the procedure of exposing by the mixed multi-spectrum rays  362 , the PI group of the second monomer material (B)  356  is polymerized gradually to form the second polymer alignment layers  380  and  380 ′, on the surface layers of the first and second substrates  32  and  34 , respectively. The second polymer alignment layers  380  and  380 ′ are steadily adhered to the first and second substrates  32  and  34 , respectively. The side chain of the first monomer material (A)  354  joins the liquid crystal molecules  352  and the interfering group of the second monomer material (B)  356 . The first monomer material (A)  354  is polymerized to form the first polymer alignment layers  390  and  390 ′, on the surfaces of the second polymer alignment layers  380  and  380 ′, respectively. It is noted that the first polymer alignment layers  390  and  390 ′ are not solid layers. The first polymer alignment layers  390  and  390 ′ confine the liquid crystal molecules  352  by a non-contact force, i.e. electromagnetic force, to arrange the liquid crystal molecules  352  at the pre-tilt angel. 
     In the present invention, the first monomer material (A)  354  and the second monomer material (B)  356  can be implemented respectively as follows: 
                                
where R 1  and R 2  can be hydrogen, halogen, methyl group, or cyanic group, m, n, l are integers greater than or equal to 1, and A can be a group such as:
 
                                
where X is alkyl group or alkyl halide group in which C≧1. In addition, the polymerization initiator can be implemented as the following structure:
 
     
       
                 
         
             
             
         
      
     
     Above all, the first monomer material (A)  354  and the second monomer material (B)  356  are polymerized to respectively form the first polymer alignment layers  390  and  390 ′, and the second polymer alignment layers  380  and  380 ′. The function of the second polymer alignment layer  380  and  380 ′ is similar to that of traditional PI alignment films formed by coating. The present invention utilizes monomer materials to be polymerized to form structures for aligning the liquid crystal molecules  352  instead of coating a PI alignment film. The expensive equipments generally used in the PI coating procedure are not required in the present invention. Therefore, the present invention can simplify the manufacturing process and speed up the production, and therefore is able to reduce the manufacturing cost. Also, the preset invention does not need to use organic solvents to clean APR plates, and therefore complies with energy saving and carbon reduction emphasized in modern life. In addition, the present invention does not need to dispose bump structures in the pixel areas of the display panel to align the liquid crystal molecules  352 , and therefore is capable of achieving the optical properties such as high brightness and high contrast (without light leakage in a dark state) for the display panel. 
     The absorption wavelengths of the monomer material (A), the monomer material (B), and the polymerization initiator (C1 or C2) are respectively, for instance, 200-220 nm (A), 310-365 nm (B), 254-300 nm (C1), 254-320 nm (C2), as shown in Table 1. Generally, the absorption wavelengths of the liquid crystal molecules are less than 300 nm. When the liquid crystal molecules are irradiated by the UV light of short-spectrum, the original property of the liquid crystal molecules is easily to be destroyed. Traditionally, when irradiating with an UV light, a filter is utilized to filter out the wavelengths less than 300 nm for protecting the liquid crystal molecules from being destroyed. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Monomer 
                 UV absorption wavelengths (nm) 
               
               
                   
                   
               
             
             
               
                   
                 A 
                 200-220 
               
               
                   
                 B 
                 310-365 
               
               
                   
                 C1 
                 254-300 (strong source &gt; 100 mW) 
               
               
                   
                 C2 
                 254-320 (weak source &lt; 20 mW) 
               
               
                   
                   
               
             
          
         
       
     
     In the procedure of exposing by the mixed multi-spectrum rays, the present invention can determine the exposure time interval and energy for each selected spectrum and then perform the exposure procedure by continuous exposure or intermittent exposure based on the premise that the liquid crystal molecules will not seriously damaged. Rays of which wavelengths are less than 300 nm can be selected for the exposure procedure but the exposure time interval and energy should be limited in order to avoid destroying the liquid crystal molecules. Since the exposure time interval and energy for each spectrum can be altered or adjusted, the aforesaid mixed multi-spectrum rays may have at least two spectrums of which irradiation time spans are different to each other. 
     In the present invention, the mixed multi-spectrum rays may comprise rays of which spectrum is within a range of absorption wavelengths of the monomer material (A), i.e. 200-220 nm, or comprise rays of which spectrum is within a range of absorption wavelengths of the monomer material (B), i.e. 310-365 nm, or comprise rays of which spectrum is within a range of absorption wavelengths of the polymerization initiator (C1 or C2), i.e. 254-300 nm or 254-320 nm. 
     As shown in Table 1, the absorption wavelengths of the monomer material (A) (200-220 nm) are very short, and therefore it is necessary to accompany with a high-reactive initiator (e.g. C1 and C2) which absorbs less UV energy. When using the polymerization initiator and radiating with UV rays of 254-300 nm or 254-320 nm, the polymerization initiator will speed up the releasing of electron pairs and therefore accelerate the polymerization reaction of the monomer material (A). Therefore, it can avoid using UV rays of 200-220 nm. A preferred spectrum combination for the mixed multi-spectrum rays may include rays of which spectrum is within a range of absorption wavelengths of the monomer material (A) (200-220 nm) and rays of which spectrum is within a range of absorption wavelengths of the polymerization initiator (254-300 nm or 254-320 nm). In other embodiments, a preferred spectrum combination for the mixed multi-spectrum rays may include rays of which spectrum is within a range of absorption wavelengths of the second monomer material and rays of which spectrum is within a range of absorption wavelengths of the polymerization initiator. 
     The monomer material (A), the monomer material (B), the polymerization initiator (C1 or C2), and the liquid crystal molecules are mixed together. Since the absorption wavelengths of each monomer material are different from each other, it has significant difficulty in UV exposure technology. A multi-spectrum UV light source can be manufactured in the present invention by combing a couple of different UV tubes, and each UV tube emits rays of a single spectrum (or a single wavelength) and low energy (below 60 mw). The two mixed tube devices respectively shown in  FIGS. 4A and 4B  are capable of emitting rays of two different UV wavelengths (or spectrums) and three different UV wavelengths (or spectrums), respectively. It is convenient to control the polymerization reaction or formation of the monomer materials induced by UV light when utilizing the mixed tube devices. Said devices also can avoid destroying the original property of the liquid crystal molecules when utilizing the liquid crystal alignment process of the present invention. The way to select UV spectrums, i.e. the selection of wavelengths or spectrums of UV1, UV2, and UV3, can be made according to Table 1. Different kinds of single-wavelength (or single-spectrum) rays can be selected and combined to fabricate a device capable of emitting rays of multiple wavelengths (or spectrums). 
     While the preferred embodiments of the present invention have been illustrated and described in detail, various modifications and alterations can be made by persons skilled in this art. The embodiment of the present invention is therefore described in an illustrative but not restrictive sense. It is intended that the present invention should not be limited to the particular forms as illustrated, and that all modifications and alterations which maintain the spirit and realm of the present invention are within the scope as defined in the appended claims.