Abstract:
A liquid coating applicator with a very precise means for controlling gap thickness as well as adapting to non-planar discontinuities in the substrate.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims benefit of the provisional application 60/860,230 filed on 21 Nov. 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention refers to the field of formation of thin-film coatings using flowable substances. More specifically, the invention refers to facilities for obtaining thin films or coatings possessing anisotropic physical properties. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various types of wet film applicators, in particular, those used for testing paints, are known from the prior art. For the correct determination of some special properties of coatings such as color, transparency, luster, strength, resistance to weathering and chemical factors, etc., it is necessary to ensure that the test coatings applied in sequential runs would have the same preset thickness. In addition, it is desired that the applicator device would be adjustable so as to obtain the films of the desired thickness from various substances having varied physical properties. 
         [0004]    One wet film applicator known from the prior art comprises a pair of wedge-shaped elements, which are parallel to each other and bear a transverse plane blade forming the coating. A gap between the bottom edge of the blade and the base plane (substrate) determines the thickness of the applied coating. The thickness of this gap is varied when the blade is moved along the wedge-shaped elements. Once the required gap thickness is set, the mutual arrangement of parts in the device is fixed. The blade is oriented perpendicularly to the direction of application and forms a film of desired thickness when the applicator is moved relative to the substrate surface. This device is quite universal and provides the level of accuracy that is sufficient for the formation of usual paint, lacquer, and other wet film coatings. In the clamping mechanism, the tightening screws directly presses against the blade which imparts a twisting motion to the blade However, neither this accuracy of this device nor (which is more important) the mode of device interaction with the applied liquid are sufficient for the formation of high-quality optically anisotropic films and coatings, especially such as are employed in modern multilayer interference devices. 
         [0005]    Thin films with anisotropic optical properties, which are formed using liquid-crystalline solutions of organic dyes, are now widely used in science and technology. The molecules of these organic compounds have planar configuration and form orientation-ordered supramolecular complexes in solution. When a solution of these organic molecules is applied onto a substrate surface in the presence of an external orienting action (alignment), the resulting coating acquires a macroscopic orientation (optical anisotropy), which is not only retained in the course of subsequent drying but can even increase as a result of crystallization. The polarization axis is oriented along the direction of the aligning action, which coincides with the direction of application of the coating. Specific structural features of such optical films determine the need for developing special coating devices capable of forming precise thin layers with the required molecular orientation. 
         [0006]    There are various known methods for the formation of optically anisotropic films and, accordingly, various devices which implement these methods. For example, liquid-crystalline solutions can be applied using a drawing plate or a wiper (squeegee), which can be of a blade (sheet) or cylinder type. The application of a liquid-crystalline solution onto a substrate surface can be performed simultaneously with the orientation of supramolecular complexes in a required direction. However, devices known in the prior art do not ensure the formation of highly anisotropic films with reproducible characteristics, which is explained by unavoidable disruption of the oriented molecular structure (defect production) during the film formation. In addition, the technology of film formation using the known devices requires prolonged preliminary work for determining the optimum application conditions for every batch of the initial raw materials. 
         [0007]    Attempts at solving the aforementioned problems led to the creation of rather complicated devices, in particular, those containing liquid feed channels of special shapes, additional smoothing elements, etc. 
         [0008]    Applicators known in the prior art also include devices of the slot-die coating system type such as the Sony setup (Alabama, USA), Cambridge Shearing System (Linkam Scientific, UK), sliding plate rheometers (FMR-MIT, USA), etc. 
         [0009]    Patents depicting various devices of the prior art are U.S. Pat. No. 4,299,789, November 1981, Giesbrecht; 
         [0010]    U.S. Pat. No. 4,869,200, September 1989, Euverard; U.S. Pat. No. 6,174,394, 16/2001, Gvon et al.; WO 02/087782, July 2002, Lazarev et al.; and WO 02/056066, July 2002, Lazarev et al. 
         [0011]    Despite the existing solutions, problems are still encountered that are related to the need for combining the necessary properties in one device, including high accuracy, simple adjustment, control over the film parameters (in particular, thickness), and the possibility to improve the quality of applied coatings by compensating for substrate unevenness. 
         [0012]    The uniqueness of the device according to the present invention is the ability to obtain coatings of large areas at a high rate of application, low consumption of the raw material, and high-precision control over the film thickness and optical parameters (Mueller matrix, alignment, etc.). Additional important advantage of the proposed device is a sufficiently large size of the zone of shear action. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention refers to devices intended for controlled coating substrate surfaces with liquid (flowable) substances and forming the desired material structure due to the shear between two planes sliding relative to each other. The aim of this invention is to obtain films with improved physical characteristics and increased reproducibility of results, not only over the area of single coating, but within a series of films formed from a single stock solution of coating liquid as well as from one batch to another. 
         [0014]    A liquid coating device according to the present invention comprises 
         [0015]    (i) an applicator assembly, and a 
         [0016]    (ii) a compliant assembly for holding the applicator and compensating for unevenness in the surface of the substrate. 
         [0017]    Though the combination of the two components above produces the best thin film coatings, it is possible to produce a thin film coatings with just the applicator assembly as discussed below 
         [0018]    The above system is typically used in conjunction with an essentially planar substrate, and a substrate holder with a means of linear transportation of the compliant assembly/liquid film applicator relative to the substrate holder. For purpose of identifying various degrees of freedom,  FIG. 2  identifies the proper orientation of the three axis Tx, Ty, and Tz with respect to the coating device and the three rotations about these axis: Rx, Ry, and Rz. The arrows indicate translation or rotation in the positive direction. The compliant assembly permits motion of the liquid film applicator in only three degrees of freedom, which are translation in the plus and minus Tz direction and rotation in the plus and minus directions: Rx and Ry. 
         [0019]    The liquid film assembly is designed to be in direct physical contact with the substrate by way of the two parallel rails mounted on opposite sides of the bridge. A sample of the liquid to be coated is placed on the substrate along the front edge of the bridge. As the coating assembly is translated relative to the substrate, the coating liquid is drawn into the gap formed by the lower planar surface of the bridge and the substrate. Because of the compliant nature of the compliant assembly, the liquid film applicator will ride on the surface of the substrate and follow the minute variations in the surface of the substrate, limited by the three degrees of freedom discussed above. 
         [0020]    This method makes possible the compensation of the linear and angular displacements arising during the system operation and ensures high homogeneity and smoothness of the applied film even on a rough (wavy) substrate. In the case of thin optical films, deviations of the substrate surface from the horizontal plane (waviness) can be comparable with the film thickness, which frequently leads to distortions and detrimentally influences the optical device performance. Retention of specific degrees of freedom described above in the Compliant Assembly  110  design allows the Applicator Assembly  120  to follow the substrate surface, thus increasing the uniformity of coating. 
         [0021]    A liquid film applicator according to the present invention comprises
       (i) at least two longitudinal wedge-like rails with their bases occurring in the same plane called the base plane;   (ii) a bridge which spans the two side members, which has at least one flat face and is in contact with each rail in at least one point; and   (iii) a clamp system ensuring strict fixation of the bridge at any preset position in relation to the rails, such that a gap thickness of the desired dimension can be obtained       
 
         [0025]    The bridge can be moved along both rails so that the flat face of the bridge makes a certain constant dihedral angle within 0-10 arc minutes with the base plane, and the gap between this face and the base plane has a thickness from about 0 to about 100 microns. The bridge makes contact with each rail along the upper surface of the rail. The front flat face of the bridge makes a smooth continuous curved transition to the lower planar shear face with said transition typically being a one quarter circular arc having a radius of R. 
         [0026]    The coating device according to the disclosed invention is a universal setup, which ensures excellent results at a relative simplicity of adjustment and high convenience in use. The disclosed coating device is capable of forming high precision coatings
       at shearing speeds of up to 1000 mm/sec;   with very low coating liquid material consumption (less than 1 cc)   Precisely controlled gap thickness (in the range 0-100 microns) and   Long length of shearing zone (up to 30 mm)       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Having just described the invention in general terms, other and further objects, features, and advantages of the invention will be made more explicit from the following detailed description taken with reference to the drawings, which are not necessarily drawn to scale, and wherein: 
           [0032]      FIG. 1  is a perspective view of one embodiment the Coating Device of the present invention; 
           [0033]      FIG. 2  is a perspective view of one embodiment of the Compliant Assembly; 
           [0034]      FIG. 3  is a perspective view of one embodiment of the Applicator Assembly; 
           [0035]      FIG. 4  is a sectional view of one embodiment of the Applicator Assembly; 
           [0036]      FIG. 5A  is side view of the bridge, the substrate and a sample of the liquid coating; 
           [0037]      FIG. 5B  is side view of the bridge showing the showing a Read Edge with a different theta angle; 
           [0038]      FIG. 5C  is a side and end view of the bridge and rails showing the gap d and wedge angle a; 
           [0039]      FIG. 6  is a wire frame model of showing a line of contact between the rails, the bridge and the substrate; 
           [0040]      FIG. 7  is a sectional view of an alternative embodiment of the bridge; and 
           [0041]      FIG. 8  is a sectional view of various configurations of the contact surfaces between the bridge the upper surfaces of the rails. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    Referring now primarily to  FIG. 1  a Coating Device  100  according to the disclosed invention comprises an Applicator Assembly  120  ( FIG. 3 ), and a Compliant Assembly  110 . 
         [0043]    The Applicator Assembly  120  is securely attached to one end of the Compliant Assembly  110  and the other end of Compliant Assembly is secured to fixed a mechanical anchor (not shown) via Clamp  160  ( FIG. 2 ). Further details of both the Complaint Assembly and the Applicator Assembly will be discussed below. 
         [0044]    The Coating Solution that is to be formed into a thin film is placed on the Substrate  105  just in front of Bridge  210 . Then Substrate  105  is moved from left to right, causing Coating Solution  108  to be dragged under Applicator Assembly  120  causing Thin Film Coating  109  to be formed. 
         [0045]    All references to motion and direction of motion of the substrate are to be understood to be relative to the coating device. It is possible to have the coating assembly fixed to an anchor and the substrate move. Alternatively it is possible to have the substrate fixed and move the coating assembly by attaching a mechanical means of motion to the coating device, preferably by way of the Clamp  160 . A third possibility would be have both the substrate and the coating device actually moving. All three possibilities will be understood to be encompassed within references to motion of the substrate of the Coating Device  100 . 
         [0046]    Compliant Assembly  110 , shown in both  FIGS. 1 and 2 , is attached to the Applicator  120 . Compliant Assembly  110  provides a means to position the Applicator  120  relative to the Substrate  105 . The transportation is usually performed using a worm mechanism with a step motor. However, embodiments of the present invention are not restricted to mechanisms of this type. Any means that ensures the smooth transport of the Coating Device  100  relative to Substrate  105 , with the required velocity can be employed. The optimum relative velocity has to be selected taking into account the Theological properties of particular coating liquid. 
         [0047]    The second important assembly of Coating Device  100  is the Applicator  120  (shown as part of the overall Coating Device  100  in  FIG. 1  and by itself in  FIGS. 3 and 4 ), which includes 
         [0048]    (i) at least two longitudinal Rails  220 A and  220 B. The lower portion of the Rails  220 A/B make contact with the Substrate  105  along Contact Surfaces  225 A and  225 B. The Contact Surfaces  225 A and  225 B are narrower than the overall width of Rail  220 A and  220 B and thus form Rail Offsets  230 A and  230 B the upper surfaces of which are at a distance D 1  ( FIG. 4 ) from the Contact Surfaces  225 A and  225 B. The Rail Offsets also have a horizontal dimension D 2  ( FIG. 4 ). The distances D 1  and D 2  are preferably 0.02 to 0.5 inches. 
         [0049]    The Rail Offsets  230 A and  230 B hinder the migration of Coating Liquid  108  which leaks from each edge of the Planar Shear Surface  250  facing each of the Rails  220 A and  220 B so that the Coating Liquid  108  tends to cling to these edges by capillary action and does not reach the interfaces between Contact Surfaces  225 A and  225 B and Substrate  105 . If any Coating Liquid did get underneath the Contact Surfaces, the ability of the Applicator  120  to properly follow the surface of Substrate  105  would be compromised and thus the quality of the Coating  109  formed. 
         [0050]    In the alternative, it is possible to have an equivalent offsets formed within each side of Bridge  210 . Offsets located within the Bridge would function in exactly the same manner as the offsets shown in  FIG. 4 . 
         [0051]    Located along the upper portion of each Rail are Rail Wedge Surfaces  235 A and  235 B respectively. This surface is angled with respect to the Contact Surfaces  225 A and  225 B. This angle is referred to as “a” and is preferably in the range of 1 minute to 60 minutes. 
         [0052]    (ii) Clamp  200  is designed for two primary functions. The first is to securely hold Bridge  210  and Rails  220 A and  220 B. Bridge  210  is mounted between the Rails  220 A and  220 B. Rails  220 A and  22 B make contact with Clamp Contact Surfaces  285 A and  285 B. Bridge  210  is positioned between the two Rails. All clearances are such that the Rails and the Bridge form a snug, but adjustable fit within Clamp  200 . Once the Rails and Bridge are properly positioned (discussed below) they are securely tightened within Clamp  200 . This is accomplished by tightening Clamp Screws  260  in Threaded Hole  263  which then pushes against Clamp Flex Member  280 . Clamp Slit  270  weakens the structure just enough to allow Clamp Flex member  280  to be biased against the Rails and the Bridge, thus holding them securely in place. 
         [0053]    (iii) Bridge  210  is T-shaped structure that contacts the Rails along the inner parallel face of each of the Rails as well as along the upper Rail Wedge Surfaces  235 A and  235 B. Bridge  210  has two Bridge Wedge Surfaces  240 A and  240 B which are the surfaces which contact the Rail Wedge Surfaces  235 A and  235 B. The Bridge Wedge Surfaces  240 A and  240 B have the same slope as the Rail Wedge Surfaces  235 A/ 235 B. Thus when the two Rails are moved relative to the Bridge, they are urged in a vertical direction with respect to the lower flat surface of the bridge, Planar Shear Surface  250 . Typically the Rails are adjusted so that they extend slightly beyond Planar Shear Surface  250 . When the Applicator Assembly is placed on Substrate  105 , this difference causes Gap  237  ( FIG. 4 ,  FIG. 5A  and  FIG. 5C ) to be formed between Planar Shear Surface  250  and Substrate  105 . Gap  237  has a thickness d when measured from the mid-point of the Planar Shear Surface to the Substrate  105 . 
         [0054]    It should be noted that it is critical that each mating pair of Bridge Wedge Surfaces  240 A/Rail Wedge Surface  235 A and Bridge Wedge Surfaces  240 B/Rail Wedge Surface  235 B have the same angle, but it is not critical that each pair has the same angle as the other pair. 
         [0055]    Because the whole Applicator Assembly rides the substrate on the two Rails, the Planar Shear Surface  250  will be positioned above the surface of the Substrate  105 . This gaps controls the thickness of the Coating  109 . 
         [0056]    The width of the bridge  212  ( FIG. 4 ) is determined by the required width of the coating, while the length of the Shear Zone  217  ( FIG. 5 ) is based in part upon the Theological properties of the coating liquid and relative velocity between Substrate  105  and Planar Shear Surface  250 . The extended length of Shear Zone  217  is a significant feature and provides an important means of adapting the Applicator Assembly to Coating Liquids  108  having a wide variety of properties. 
         [0057]    The Front Face  247  of the bridge makes a smooth continuous curved Transition Surface  245  to the Planar Shear Surface  250  with a curvature radius R of sufficient size to uniformly pull the Coating Liquid  108  into the gap and cause its homogeneous spreading under the Planar Shear Surface  250 . The size of the radius R is dependant in part on the rheological properties of the Coating Liquid  108  and the relative velocity between the Planar Shear Surface  250  and the Substrate  105  and is typically greater than 50 microns. Though the smooth curved transition is shown in this embodiment as a ¼ radius circle, there is no requirement that the curved transition be circular, and other shapes and curvatures may be employed as the characteristics of the liquid solution dictate. 
         [0058]    The Shear Zone  217  extends from point where the Transition Surface  245  meets the Planar Shear Surface  250  and the Rear Edge  255 . 
         [0059]    The Planar Shear Surface  250  intersects with the smooth Rear Surface  248  at a sharp angle theta (See  FIGS. 5A and 5B ) forming sharp Rear Edge  255 A. ( FIG. 5A ) of about 90 degrees or greater (e.g. Rear Edge  255 B,  FIG. 5B ) such that a sharp Rear Edge  255 , which is devoid of irregularities, exists between said Planar Shear Surface  250  and said smooth Rear Surface  248  so as to avoid end-sticking of the wet layer to Rear Surface  248   
         [0060]    The plane of the Planar Shear Surface  250  is usually parallel to the base plane. However, depending on the rheological properties of the coating liquid and the required parameters of coating, the Planar Shear Surface  250  can make an angle β ( FIG. 7 ) that is typically within 10-30 arc minutes with the base plane (the front edge can be either higher or lower than the rear edge). By varying this angle, it is possible to control the shear stress on the Coating Liquid  108  lying with the gap and change the mode of application and release of this stress. The angle between the Planar Shear Surface  250  and the base plane is usually changed by replacing the whole Bridge  210 . 
         [0061]    The Planar Shear Surface  250  must be a smooth and have a mirror-like surface and flat to within 1-3 wavelengths over the entire surface (0.3-1 micron) 
         [0062]    Gap  237  between the Planar Shear Surface  250  and the Substrate  105  has a thickness d which is typically within the range of about 0 microns to about 100 microns. Thickness d of Gap  237  can be changed by precisely shifting the Rails with respect to the Bridge  210 . Because the Rails are longer than the depth of the Clamp and Bridge, the Rails can be positioned anywhere along their length. However, the Bridge  210  is typically centered, front to back within the Clamp  200 . The wedge angle a, must provide for the smooth control and precise setting of the gap thickness and with the required accuracy (typically, about 20 nm). When it is necessary to change the parameters and/or thickness of the applied coating, the Applicator  120  is removed from the Coating Device  100  and placed upside down with the Contact Surfaces  225 A/ 225 B and the Planar Shear Surface  250  facing upwards. 
         [0063]    Initially the rails are adjusted so that the Contact Surfaces  225 A/ 225 B and Planar Shear Surface  250  are all coplanar. Then because, the wedge angle a is known, the Rails  220 A/ 220 B can be moved a precise distance relative to the bridge, which translates into the desired change in the distance between the plane formed by the Contact Surfaces  225 A/ 225 B and Planar Surface  250 . 
         [0064]    The actual gap distance can be measured and confirmed by measuring an interference pattern that arises due to multiple reflection of a light beam between the Planar Shear Surface  250  and a glass plate used for the testing which rests upon the Contact Surfaces  225 A/ 225 B 
         [0065]    In one possible alternative embodiment ( FIG. 7 ), the Bridge  210  is made up of two wedge-like elements ( 215  and  216 ), which allow for a relative shift along the slippage plane, which is inclined relative to the base plane at an angle γ, which is smaller than angle a. This alternative design of the Bridge  210  member is convenient for the additional fine adjustment of the gap thickness d. 
         [0066]    In second alternative embodiment of the Bridge  210 , some or all of the material forming Bridge  210  can be a essentially transparent. 
         [0067]    Although most depictions of the Bridge  210  shown herein have the bridge made of a single monolithic member, it is within the scope of the invention that the Bridge  210  could be made of two or more elements as long as the assembly of these components provides the same functionality as a monolithic bridge. 
         [0068]    The site of the contact between the Bridge Wedge Surfaces and the Rail Wedge Surfaces must, on the one hand, ensure a reliable and strong structure of the Applicator Assembly  120  and, on the other hand, provide for their free and high-precision mutual displacement.  FIG. 8  shows a cross section of four alternative embodiments of the contact sites, which can provide for the required quality and properties of these contact surfaces. However, the possible embodiments are not restricted to these variants and admit any other structures which provide the needed physical requirements. 
         [0069]    The liquid film applicator according to the present invention usually employs two identical wedge-like rails. However, embodiments incorporating other configurations of wedge-like rails are possible as well. 
         [0070]    A schematic depiction of the minimum requirement for the Bridge and Rail contact surfaces is shown in  FIG. 6 . The minimum required contacts between Bridge  210 A and Rail  221 A and  221 B is shown as line TW 1  and TW 2 . Likewise the minimum required contact between Rail  221 A and  221 B and Substrate  105  are depicted as lines TS 1  and TS 2 . All references to contact surfaces and contact between contact surfaces contained herein shall be understood to include at least one line of contact between the surfaces. Though the contact surfaces as shown in the various embodiment contained herein are shown as flat surfaces, such contact surfaces may include any number of configurations as long as there is a single line of contact between the surfaces. 
         [0071]    Though the best films can be performed when the Compliant Assembly  110  “drags” the Applicator Assembly  120  across the substrate, it is possible to attach the Complaint Assembly  110  to Clamp  200  by rotating Compliant Assembly 180° around the Tz axis (from its position shown in  FIG. 1  where Compliant Member  140  is attached to Inner Face  147 ) and attaching Compliant Member  140  to Outer Face  145  ( FIG. 3 ) of Clamp  200 . Then Compliant Assembly  110  can “push” Applicator Assembly  120 . This configuration still allows the Complaint Assembly to control/limit the movement of the Applicator Assembly  120  to the three degrees of freedom previously discussed. It should be noted that the stress on the system in this configuration must be kept below the buckling limit of Flex Members  150 . 
       Applications: 
       [0072]    Preferred coating liquids for the formation of anisotropic optical films include liquid colloidal systems containing anisometric particles, in particular, lyotropic liquid crystals of organic dyes. Examples are offered by organic dyes such as indanthrone (Vat Blue 4), 1,4,5,8-naphthalenetetracarboxylic acid dibenzimidazole (Vat Red 14), 3,4,9,10-perylenetetracarboxylic acid dibenzimidazole, and quinacridone (Pigment Violet 19), and some other whose derivatives or their mixtures are capable of forming stable lyotropic liquid crystal phases. 
         [0073]    When such an organic compound is dissolved in an appropriate solvent, a colloidal system (liquid-crystalline solution) is formed, in which organic molecules combine to form c representing kinetic units of the colloidal system. A liquid-crystalline liquid is a preferred coating liquid, from which a desired anisotropic crystalline film (also called thin-film crystal) is formed in the course of application, orientation of the liquid-crystalline solution, and subsequent removal of the solvent. 
         [0074]    This colloidal system must possess the property of thixotropy, whereby the viscosity of the medium at a preset temperature and a given concentration of the dispersed phase can by changed by applying an external action. The type and degree of this action must be sufficient to provide that the kinetic units of the colloidal system could acquire the necessary orientation and form a base structure for the required film. The direct action upon the coating liquid and the formation of a wet film is performed by the liquid film applicator ( FIG. 3 ,  120 ) as it moves along the substrate ( 105 ). Special features of the liquid film applicator design allow this device to produce the necessary orienting action upon the material structure and to form an even wet layer of preset thickness with a smooth surface. 
         [0075]    Anisotropic optical films can also be formed using inorganic lyotropic liquid crystals, for example, based on iron oxohydroxide or vanadium oxide, which possess anisotropic electrical and magnetic properties. 
         [0076]    Use of the present invention is by no means restricted to the formation of coatings based on of liquid-crystalline and colloidal systems with anisometric particles. Any liquid capable of forming a coating on the given substrate can be applied using this system as well. 
         [0077]    The possible substrate materials are plastics, glass, and other materials, including polymeric films. Prior to film application, the substrate usually treated by certain means (e.g., corona discharge, surfactants, etc.) to render it homogeneously hydrophilic over the entire surface. A substrate holder may be employed, which is usually a vacuum table, which reliably ensures that the substrate is immobile during the film application and provides leveling of the substrate surface. 
         [0078]    To those skilled in the art it will be understood that there can be many other variations of the embodiments what have been described above while still achieving the same objectives of the invention. Such variations are intended to be covered by the scope of this invention. As such, the foregoing description of embodiments of the invention is not intended to be limiting. Accordingly, it is intended that the appended claims will cover all modifications of the invention that fall within the true spirit and scope of the invention.