Abstract:
A method and an apparatus for embossing a pattern into a varnish on a sheet-like substrate are provided. The method applies a planar stamp using a roller, while the apparatus includes a roller having a planar stamp.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/EP2014/064961 filed Jul. 11, 2014, which claims benefit under 35 U.S.C. §119(a) of German Application No. 10 2013 107 909.1 filed Jul. 24, 2013, the entire contents of all of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method and an apparatus for embossing a pattern on a sheet-like substrate coated with a varnish. More particularly the invention relates to embossing patterns on glass or glass ceramic sheets which are coated with a curable polysiloxane or sol-gel varnish system. 
         [0004]    2. Description of Related Art 
         [0005]    Methods for introducing patterns on glass or glass ceramic substrates which are coated with a sol-gel varnish system or a polysiloxane varnish are known. 
         [0006]    Document WO 2011/085997 A1 discloses embossing of a pattern by means of a planar stamp on a glass or glass ceramic substrate. 
         [0007]    In this case, a varnish system curable by UV light is applied. Then, a stamp having the desired pattern is pressed on the varnish. The varnish system comprises a component which is curable by UV radiation and is cured by radiation transmitted through the stamp, i.e. polymerization of the varnish starts. 
         [0008]    This causes the varnish system to be pre-cured at least to such an extent that the pattern retains its shape after the stamp has been pulled off. The varnish can then be cured in further processing steps, for example by heat in a range from 150° C. to 800° C. to obtain its ultimate strength. In particular, it is intended to burn off organic components of the varnish in this final curing step. This applies in particular to curing temperatures of more than 300° C. 
         [0009]    The applied pattern may serve for visual appearance of the composite material so produced. For example, in combination with another layer it is possible to produce composite materials which have the appearance of a brushed stainless steel surface. 
         [0010]    However, patterning for haptic features such as to obtain a gripping surface are also conceivable. 
         [0011]    Equally feasible is the manufacturing of optical components such as diffractive optical elements, e.g. gratings and the like, or refractive elements, e.g. light guiding patterns and the like. 
         [0012]    A problem with such known methods is that air bubbles may be produced. Furthermore, when applying the stamp, a non-homogeneous layer thickness distribution may result, in particular due to different contact pressure forces, thereby causing defects in the patterned layer. Such different contact pressure forces may especially be caused by unevenness of the substrate to be embossed. 
         [0013]    Furthermore, it may happen that the employed polymer stamps do not remain dimensionally stable, for example experience elongation, which also leads to defects or inaccurate transfer of the pattern from the master to the varnish. 
         [0014]    Document DE 10 2006 004 644 B4 discloses a method in which a stencil web is moved over the substrate by means of two rollers, similar to a tracked vehicle. The curable material applied on the substrate is cured through the stencil web by means of a UV lamp. 
         [0015]    This procedure, too, causes defects in the pattern, at least in case of highly viscous sol-gel or polysiloxane-based varnishes, since the pattern might be blurred due to the movement and due to the length of the stencil web. 
         [0016]    Also known from practice of applying are thermal nanoimprint lithography, step-and-flash imprint lithography, photolithography, and electron or ion beam lithography. All these methods have in common to enable to apply micropatterns on substrates with relatively high precision. However, because of the necessary processing steps and also because of the equipment employed for this purpose these methods are expensive and usually unsuitable for large substrates. 
       SUMMARY 
       [0017]    Therefore, the invention is based on the object to provide a method and an apparatus which permit to easily pattern, by means of a stamp, varnish systems applied to a sheet-like substrate, while eliminating the risk of defects or inclusion of bubbles in the pattern. In particular, it should be possible to pattern substrates having even large surface areas, in particular greater than 25 cm, at high speed, i.e. with high throughput in the apparatus. 
         [0018]    The object of the invention is already achieved by a method and an apparatus for embossing a pattern on a sheet-like substrate coated with a varnish as disclosed herein. 
         [0019]    The invention relates to a method for embossing a pattern on a sheet-like substrate coated with a varnish. 
         [0020]    The invention in particular relates to glass or glass ceramic sheets which are coated with a polysiloxane varnish or a sol-gel-based varnish. 
         [0021]    The method is particularly used for coating sheets such as those used for building glazing, as cover glazing for photovoltaic modules, for kitchen appliances, or for optical applications, for example. 
         [0022]    The embossed pattern may solely be intended to provide a specific visual appearance of the composite material so produced, but may also improve the optical properties of the composite material, especially if it is transparent, in particular with regard to an injection or emittance of light, or may basically provide for optical features on simple substrates. 
         [0023]    According to the invention, a planar stamp is applied to the substrate, which stamp is fixed on one side of the substrate and is lowered until contacting the substrate in a direction towards another side of the substrate by means of a roller. 
         [0024]    Thus, the stamp in form of sheet material is fixed at one end relative to the substrate, i.e. either attached to the substrate or to the apparatus on which the substrate is supported, and is rolled onto the substrate by means of a roller. 
         [0025]    Then, the stamp may rest on the substrate. If appropriate, it is fixed by a bracket so as to avoid slipping. 
         [0026]    While the stamp is resting on the substrate, the stamp is able to emboss a pattern into the varnish provided on the substrate. 
         [0027]    By the application of the stamp by being lowered until contacting the substrate in combination with being fixed at one end, a flow front is formed during the application, whereby inclusions of air are largely avoided. 
         [0028]    The substrate preferably rests horizontally on a support plate. 
         [0029]    The stamp is preferably greater than the substrate or the area to be patterned on the substrate. According to one option for applying the stamp, the stamp is first applied in a front region of the substrate. Then the stamp is lowered from above and at the same time pressed onto the substrate by a roller with an advancing movement. This ensures that the stamp does not shift or slip during the further applying process. 
         [0030]    According to another embodiment, the stamp is applied in front of the surface to be patterned and is progressively contacted with the varnish, while being bent around the roller, at least partially. 
         [0031]    In a preferred embodiment of the invention, the angle between the stamp area not yet applied and the substrate is more than 45°, preferably more than 75°, and more preferably more than 80°. This improves the generation of a flow front during the complete application of the stamp. 
         [0032]    According to the invention, application rates of more than 5 mm/s, preferably more than 30 mm/s, and most preferably more than 50 mm/s are possible. In particular, stamp application rate is between 5 mm/s and 500 mm/s, preferably between 15 mm/s and 250 mm/s, and more preferably between 25 mm/s and 100 mm/s. 
         [0033]    Once placed, the stamp preferably is at rest for at least 0.1 to 10 seconds, and the varnish is cured by light or by heat until the pattern is stable. 
         [0034]    Preferably, curing is effected by radiation transmitted through the stamp, in particular by UV light. 
         [0035]    It will be understood that further curing steps may be performed subsequently, in particular thermal curing steps. 
         [0036]    Furthermore, it is conceivable to pre-dry the varnish prior to the embossing, in order to expel any solvent that is still present on the one hand, or to slightly pre-polymerize the varnish, in particular by means of an infrared line emitter or by a short UV treatment. Other means/ways for pre-drying are known in the art. For example, it is possible in this way to remove solvents from the varnish, at least partially. 
         [0037]    The curing through the stamp may in particular be performed directly after application thereof. For this purpose, for example, a radiation source, in particular a line emitter may follow the roller with a time-shift and at the same or a slower speed. 
         [0038]    In particular, a UV lamp may be used which emits ultraviolet radiation. The wavelength may in particular be between 270 and 405 nm. 
         [0039]    In the range of wavelengths in which curing is effected, the stamp preferably has a transmittance of more than 50%, preferably more than 70%, and most preferably more than 85%. 
         [0040]    After curing, the stamp is lifted from the substrate. This step is also referred to as demolding. 
         [0041]    Preferably, the pulling off is effected in a direction opposite to the application direction. Surprisingly, it has been found that in this manner deformations of the embossed pattern are largely avoided. 
         [0042]    However, the stamp may likewise be pulled off in the direction of application. 
         [0043]    When being lifted, the path of the stamp preferably corresponds to a reverse application process. The lifting process may be accomplished without contact to the pressure roller. 
         [0044]    The lifting rate is preferably the same, more preferably 1.5 times, most preferably 2 times that of the application operation. 
         [0045]    Thus, the invention relates to a step-and-repeat method in which the stamp is firmly clamped in the apparatus. 
         [0046]    It has been found that the stamp can be used for at least 1,000 embossing operations without cleaning and maintenance steps. 
         [0047]    As a material for the stamp, in particular a polymer can be used, preferably a silicone, in particular a two-component silicone rubber. This material can be cast and vulcanized at room temperature and exhibits high tear strength. 
         [0048]    Use of a polyolefin elastomer is also conceivable. 
         [0049]    In a preferred embodiment of the invention, the varnish has a viscosity of more than 500 mPa·s in its non-cured condition, i.e. in the condition in which the embossing operation takes place. 
         [0050]    Preferably, viscosity is between 600 and 1500 mPa·s, more preferably between 800 and 1100 mPa·s. 
         [0051]    In particular, in case of sol-gel-based varnishes it is necessary to pre-dry the varnish before embossing the pattern, in order to adjust the desired viscosity. 
         [0052]    For this purpose, infrared radiation sources may be used, for example. 
         [0053]    This may for instance be effected by passing the coated substrate with the solvent-containing varnish below the radiation source, at a rate from 1 to 30 mm/s, more preferably at a rate from 5 to 25 mm/s. 
         [0054]    The power output of the infrared module, based on the length of the radiation source, may be from 15 to 100 W/cm, most preferably from 60 to 80 W/cm. 
         [0055]    The line width of an infrared line emitter is preferably from 5 to 30 mm, more preferably from 15 to 20 mm. The radiation source is preferably spaced from the substrate by a distance from 50 to 60 mm. 
         [0056]    The peak of emitted wavelengths of the infrared emitter is preferably between 750 nm and 5 μm, more preferably a short-wave infrared radiation with a wavelength from 2,000 to 2,200 nm is used. An infrared emitter with a housing, most preferably with a focusing reflector housing is used. 
         [0057]    For curing the varnish, it is as well conceivable to use an IR emitter array consisting of a plurality of line emitters, which covers at least the same surface area as the substrate. 
         [0058]    In this case the substrate may be pre-dried statically over its entire surface, as well as in a continuous mode, using the power outputs as mentioned above. 
         [0059]    When curing using UV radiation, a line emitter with a reflector is preferably used, which may have a line width from 5 to 20 mm, preferably from 10 to 15 mm. 
         [0060]    The wavelength of the UV light is preferably between 200 and 400 nm. 
         [0061]    Depending on the varnish used, it is also possible to employ a mercury lamp with an intensity peak between 250 and 350 nm. 
         [0062]    Depending on the varnish system used, emitters with other intensity peaks may also be used, for example at 
         [0063]    300±20 nm, or 370±10 nm, which is in particular provided by iron-doped mercury lamps. 
         [0064]    Also, a plasma-based UV lamp may be used, which allows for rapid switching on and off of the lamp as well as constant power output throughout the lifetime of the lamp. UV LED arrays with wavelengths of 385 nm and 395 nm may also be used for curing, whereby the heat input into the substrate, the varnish, and into the stamp during the curing process is reduced. 
         [0065]    The curing step is preferably performed with a time delay to the application of the stamp, in particular from 0.1 s to 100 s, preferably from 0.3 s to 10 s, most preferably with a time offset from 0.5 to 5 s. 
         [0066]    The curing area or the curing front produced by the radiation emitter is preferably substantially in parallel to the main extension direction of the roller. 
         [0067]    The ratio of application rate to curing rate may range from 20:1 to 5:1.5. 
         [0068]    The velocity of the lamp for curing may be between 1 and 100 mm/s, preferably between 3 and 30 mm/s, and most preferably between 5 and 20 mm/s. 
         [0069]    It is as well conceivable to already partially cure the varnish by means of UV prior to the embossing operation. 
         [0070]    Furthermore, it is conceivable to additionally perform light-based curing through the substrate, provided that the substrate is transparent to the employed radiation. 
         [0071]    The rate at which the stamp is lifted is preferably from 10 to 100 mm/s. 
         [0072]    The total dwell time of the substrate in the apparatus, i.e. for applying the stamp and UV irradiation, is preferably between 1 and 500 s, more preferably between 10 and 250 s, and most preferably between 15 and 100 s. 
         [0073]    In particular substrates are coated which have a size between 0.001 and 5 m 2 , preferably between 0.04 and 1 m 2 . Operation may be performed in so-called “single piece flow”, i.e. one substrate per embossing process, as well as in “multi piece flow”, i.e. a plurality of substrates per embossing process. Furthermore, it should be noted that both final sizes and storage sizes that are cut to the final size at a later time, may be embossed. 
         [0074]    According to a refinement of the invention, the roller is actively driven during application. That means the roller is not simply entrained to rotate during the application, but is provided with an own drive. It has been found that in this manner deformation of the stamp can be largely avoided. 
         [0075]    In particular, it is possible to drive the roller such that the peripheral speed of the roller is different from the advance rate at which the stamp is applied. 
         [0076]    The roller may in particular be rotated at a lower peripheral speed than the application rate, whereby any elongation of the stamp may be compensated for and better pattern fidelity is preserved. 
         [0077]    The invention further relates to an apparatus for embossing a pattern on a sheet-like substrate coated with a varnish, in particular by a method as described above. 
         [0078]    The apparatus includes a table for supporting the substrate, a planar stamp, which is fixable or fixed at one end relative to the table, and a roller by means of which the stamp can be applied onto the substrate. 
         [0079]    This is in particular an apparatus adapted to process sheet-like substrates in a step-and-repeat process. 
         [0080]    For this purpose, the table is preferably displaceable for feeding substrates to the apparatus. To cover different substrate sizes and thicknesses, different inserts are conceivable for this table. Substrates in a range from 0.005 mm-50 mm can be patterned. Furthermore, in a particular embodiment the table may be equipped with a fixture for simultaneously embossing a plurality of final sizes (multi-piece flow). 
         [0081]    Thus, the substrate is fed into the apparatus on the table, the stamp is applied, the varnish is cured, and the stamp is pulled off. 
         [0082]    During application and curing, the table is locally fixed and the substrate rests on the table. 
         [0083]    Only when the stamp has been lifted, the substrate on the table is moved out of the apparatus and may then be subjected to further processing steps, in particular a thermal curing process. 
         [0084]    The roller used for applying the stamp is preferably configured so as to be replaceable, in particular in order to be able to use different rollers depending on the stamp or varnish that is employed, in particular rollers having a different weight and/or different diameter and/or made of a different material, depending on the stamp material chosen. With respect to the latter, in particular the coefficient of friction of the roller and the stamp has to be taken into account. It is important to ensure that the coefficient of static friction takes a value such that any slippage of the stamp relative to the roller is prevented during the application process. This is particularly the case when the coefficient of static friction is at least 0.4. 
         [0085]    In a particular embodiment of the apparatus, the latter is combined with an upstream coating apparatus, so that in particular with application of the stamp and UV curing at the same time, a very high throughput and minimized processing time in the sense of an inline production is achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0086]      FIG. 1  schematically illustrates a side view of an apparatus for embossing a pattern; 
           [0087]      FIG. 2  is a perspective view of one of the two levers on which a roller is mounted; and 
           [0088]      FIG. 3  is a perspective view of a roller with gears. 
       
    
    
     DETAILED DESCRIPTION 
       [0089]    The subject matter of the invention will now be described by way of the drawings of  FIGS. 1 to 3  with reference to an exemplary embodiment schematically illustrated therein. 
         [0090]      FIG. 1  schematically illustrates a side view of an apparatus  1  for embossing a pattern. 
         [0091]    The apparatus comprises a table  2  which is configured to be displaceable and can be moved into the apparatus from the right in this view. Means for displacing a table are known to persons skilled in the art and are not illustrated. 
         [0092]    A substrate  3 , in particular in the form of a glass or glass ceramic sheet, rests on table  2  and is coated with a varnish (not shown). 
         [0093]    For embossing a pattern, apparatus  1  comprises a planar, i.e. sheet-like stamp  4 . 
         [0094]    This view illustrates the position of stamp  4  at the start of the embossing operation. 
         [0095]    Stamp  4  is fixed at one end by means of a stamp holder  12  as a stamp fixture. It may for instance be clamped or may have recesses into which engaging members of the stamp holder (not shown) engage. 
         [0096]    During the embossing process, table  2  is locally fixed in the apparatus. Thus, stamp  4  is fixed at one end relative to substrate  3  by means of stamp holder  12 . 
         [0097]    The apparatus further comprises a roller  5  by means of which the stamp  4  may be applied onto the substrate  3 . 
         [0098]    For application of the entire surface of the sheet, a stamp holder  8  is provided which in this embodiment is equipped with a spring that allows for a movement of the stamp holder  8  in vertical direction. 
         [0099]    During an embossing operation, roller  5  is advanced to the right, in the horizontal direction, by a drive  6 . 
         [0100]    Stamp holder  8  is also moved to the right, in a manner so that the angle between the stamp surface area not yet applied and the substrate  3  remains constant during the application process. 
         [0101]    Arrow  9  symbolizes the movement direction of the advancing roller to the right. 
         [0102]    As the stamp is applied, stamp holder  8  is moved vertically downwards, in a movement direction symbolized by arrow  10 . 
         [0103]    The movement of the upper edge of stamp  4  resulting therefrom is symbolized by arrow  11 , running in a straight line towards a right edge of substrate  3 . 
         [0104]    Roller  5  can be lifted by means of a lifting mechanism  14 . 
         [0105]    A UV lamp which follows the movement of the roller is not illustrated in this view. 
         [0106]    Preferably, the varnish applied on substrate  3  is cured through stamp  4  once the stamp  4  has been applied. 
         [0107]    Then, roller  5  is lifted by lifting mechanism  14  not shown in more detail here, and the stamp is lifted substantially in the opposite movement direction, i.e. opposite to arrow  11 . 
         [0108]    Then, substrate  3  may be retracted from the apparatus by means of table  2  and may be subjected to further processing steps. 
         [0109]    Roller  5  is mounted on pivotable levers  7  which are connected to the apparatus or lifting mechanism  14  via a vibration damper  15 . 
         [0110]    Furthermore, the roller is driven by an active drive  13 . The peripheral speed of roller  5  is adjustable, in particular such that it might be slightly less than the advancing rate when applying the stamp  4 . 
         [0111]    Stamp  4  should be applied in a manner so that the angle of the not yet applied portion of stamp  4  (except for the portion that directly engages the roller) and the substrate  3  is more than 45°, preferably more than 75°. 
         [0112]    Preferably, the roller has a diameter between 3 and 50 cm. 
         [0113]    Details of the roller mechanism will now be explained with reference to  FIG. 2  and  FIG. 3  which are perspective views of components of the apparatus. 
         [0114]      FIG. 2  illustrates one of the two levers  7  on which the roller is mounted. 
         [0115]    It shows the shaft  16  of the roller and gears  17  and  18  through which the roller is actively driven. 
         [0116]    Moreover, a belt  19  can be seen, which serves to drive gear  18  by which the roller is driven through gear  17 . 
         [0117]    It goes without saying that the drive mechanism illustrated herein may as well be configured in any other manner. 
         [0118]    Levers  7  are pivotally arranged on a bearing  21  so that roller  5  can be raised to be relieved when the stamp is lifted. 
         [0119]      FIG. 3  shows roller  5  with gears  17 . 
         [0120]    Shaft  16  of roller  5  is mounted in ball bearings  20  which are inserted into the levers ( 7  in  FIG. 2 ). 
         [0121]    The invention permits patterning of highly viscous varnishes with high pattern fidelity, reduced number of defects, and high processing speed. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 LIST OF REFERENCE NUMERALS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Apparatus for embossing a pattern 
               
               
                 2 
                 Table 
               
               
                 3 
                 Substrate 
               
               
                 4 
                 Stamp 
               
               
                 5 
                 Roller 
               
               
                 6 
                 Drive 
               
               
                 7 
                 Lever 
               
               
                 8 
                 Stamp holder 
               
               
                 9 
                 Movement direction of advancing roller 
               
               
                 10 
                 Movement direction of stamp holder 
               
               
                 11 
                 Movement of stamp upper edge 
               
               
                 12 
                 Stamp holder 
               
               
                 13 
                 Active drive 
               
               
                 14 
                 Lifting mechanism 
               
               
                 15 
                 Vibration damper 
               
               
                 16 
                 Shaft 
               
               
                 17 
                 Gear 
               
               
                 18 
                 Gear 
               
               
                 19 
                 Belt 
               
               
                 20 
                 Ball bearing 
               
               
                 21 
                 Bearing