A device for applying a liquid coating, a process for coating a surface of a substrate and a process for producing sandwich panels

Abstract

The invention relates to a device for applying a liquid coating on a surface (49) of a substrate (51), comprising a tube (33) for feeding the liquid coating, and a pad (43) made of a porous material, wherein the tube (33) comprises at least one opening (35) which extends parallel to the main axis of the tube (33) and / or wherein the tube (33) comprises a plurality of openings being arranged in a row parallel to the main axis of the tube (33), through which the liquid coating can be fed into the pad (43) and a metering device for feeding the liquid coating into the tube (33) with a predefined pressure, wherein the pad is disposed in a trough (37), the tube (33) being arranged below the trough (37) and the at least one opening (35) or the plurality of openings in the tube (33) being in fluid contact with an opening in the bottom of the trough so that the liquid coating can flow from the tube into the trough and the pad (43) can be uniformly soaked with the liquid coating. The invention further relates to a process for coating a surface by using the application device (17) and a process for producing sandwich panels, where a primer is applied on the surface of a sheet by the process for coating a surface.

Description

[0001] A device for applying a liquid coating, a process for coating a surface of a substrate and a process for producing sandwich panels

[0002] Description

[0003] The invention relates to a process for producing sandwich panels, comprising a first sheet and a second sheet and a foam layer applied between the first sheet and the second sheet, by applying a primer on an inner surface of the first sheet, applying a liquid reaction mixture comprising reactants for producing a polymer and a blowing agent onto the primer on the inner surface of the first sheet, wherein after application, the reaction mixture starts to react and expand, thereby forming the foam layer, applying a primer on an inner surface of the second sheet, and feeding the second sheet such that the primer on the inner surface of the second sheet is placed on the foam layer, thereby forming the sandwich panel.

[0004] Sandwich panels produced by the process for example are used as facade elements for buildings, in particular in the commercial sector, such as warehouses or industrial buildings, for instance workshops, sports halls, department stores or assembly shops. Besides the use as facade elements, the sandwich panel also may be used as insulating boards, in cold stores, climate chambers or truck-buildups. The sandwich panels are often also referred to as composite elements or sandwich elements.

[0005] The sandwich panels usually are produced in a continuous process, in which the first and the second sheet are fed into an application device in which a primer is applied to the inner surface of the first layer for a better adhesion of the foam layer on the inner surface of the first sheet. Processes for applying the primer on the first sheet are described for example in WO-A 2006 / 120234, EP-A 2412 526, WO-A 20010 / 060864 or EP-A 1 593438. In these processes, the primer is only applied to that sheet which forms the outside of the building. These sheets forming the outside of the building usually are exposed to greater stress than the sheet which forms the inside of the building due to wind load or heating and thermal expansion due to solar irradiation.

[0006] However, the main reason to apply the primer only on the first sheet was the difficulty to uniformly apply the primer onto the inner surface of the second sheet, because during the production process this inner surface shows downwards.

[0007] To apply primer to the inner surface of the second layer, which is arranged opposite of the inner surface of the first layer, WO-A 2015 / 155138 a rotating roller or a rotating brush is used, by which primer is transferred from the inner surface of the first layer on the inner surface of the second layer. After application of the primer to the inner surfaces of the first and second sheet, the reaction mixture is applied and the compound comprising the first and second sheets coated with the primer on their inner surfaces and the reaction mixture applied thereon is fed into a double belt in which the reaction mixture reacts and expands, thereby forming the foam layer. Besides using a rotating roller or rotating brush for application of the primer on the second sheet it is also known to apply the primer by using a tube with openings to which small hoses are connected and the primer is applied to the inner surface of the second sheet through the hoses which are in contact with the inner surface of the second sheet.

[0008] However, it is a disadvantage of all processes in which the primer is applied to the second sheet, that due to the position of the inner surface of the second sheet facing downwards, that the primer may be applied unevenly or that primer drips off, resulting in pollution of the inner surface of the first sheet. Additionally, particularly when applied by using the tube with the hoses connected to the tube, it is not possible to apply an even layer of the primer.

[0009] Therefore, it is an object of the present invention to provide a device for coating a surface of a substrate by which the surface can be coated with a uniform coating even if the surface does not run horizontally or is a bottom surface of the substrate, a process for coating the surface and a process for continuously producing sandwich panels, in which a primer is applied on the inner surface of the second sheet forming an even coating without dripping off.

[0010] This object is achieved by a device for applying a liquid coating on a surface of a substrate, comprising a tube for feeding the liquid coating, and a pad made of a porous material, wherein the tube comprises at least one opening which extends parallel to the main axis of the tube and / or wherein the tube comprises a plurality of openings being arranged in a row parallel to the main axis of the tube, through which the liquid coating can be fed into the pad and a metering device for feeding the liquid coating into the tube with a predefined pressure, wherein the pad is disposed in a trough, the tube being arranged below the trough and the at least one opening or the plurality of openings in the tube being in fluid contact with an opening in the bottom of the trough so that the liquid coating can flow from the tube into the trough and the pad can be uniformly soaked with the liquid coating.

[0011] The object further is achieved by a process for coating a bottom surface of a substrate with such a device for applying a liquid coating on a rectangular surface of a substrate, comprising:

[0012] (i) feeding a liquid coating from the tube through the at least one opening into the trough for soaking the pad with the liquid coating; and

[0013] (ii) passing the surface of the substrate along a surface of the pad under a predefined pressure so that liquid coating is applied to the surface of the substrate.

[0014] By using the device for applying the liquid coating, it is possible to achieve a uniform coating on the whole surface of the substrate. The liquid coating is applied to the surface of the substrate as long as a pressure acts on the surface of the pad which is in contact with the surface of the substrate. As soon as there is no more pressure on the pad, the delivery of the liquid coating stops. Disposing the pad in the trough has the additional advantage that no liquid drips off by penetrating through the sites of the pad and pollutes the application device or the inner surface of the first sheet.

[0015] For soaking the pad uniformly with the liquid coating, the at least one opening preferably is arranged such that the liquid coating can flow into the pad along its entire length. For this purpose, the at least one opening may be in the form of a slot which extends along the main axis of the tube. Alternatively, a plurality of openings may be provided. In this case, the plurality of openings is arranged in a row parallel to the main axis of the tube. Further, it is preferred that the distance between the openings is constant. If a plurality of openings in row is provided, the opening may have any cross sectional shape, for example circular, oval, rectangular, square or polygonal with any number of vertices. However, in this case for manufacturing reasons, it is particularly preferred if the openings are circular or slots.

[0016] By using a pad made of a porous material, it is possible to meter the liquid coating such that it is only dispensed from the porous material as long as the surface of the substrate is in contact with the pad. The dispensing of the liquid coating is terminated as soon as there is no contact between the surface of the substrate and the porous material of the pad. This can be achieved, for example, by using a compressible porous material which can be compressed under pressure and restores its initial shape as soon as the pressure acting on the porous material is interrupted. In this case, the porous material for impregnating the surface of the substrate is pressed against the porous material. To achieve this the compressible porous material is embedded in the trough which keeps the liquid in and avoids that the fluid drips off. To allow pressing the surface of the substrate against the compressible porous material, the sidewalls of the trough needs to be lower than the height of the compressible porous material when it is uncompressed. The liquid coating to be coated onto the surface of the substrate is released by the compression of the porous material. In order for liquid coating to be able to exit the porous material, it is preferable that the porous material is saturated with the liquid coating at least in that region against which the surface of the substrate is pressed. The volume is reduced as the surface of the substrate is pressed against the porous material and by way of the associated compression, and the liquid coating exits the porous material in a uniform way to be applied to the surface of the substrate and to form a uniform coating. As soon as no more pressure is applied to the porous material, the porous material restores its original shape again, and no further liquid coating exits as it is kept within the trough.

[0017] Alternatively, liquid coating can also be transferred by capillary forces from the porous material to the surface of the substrate to be coated by way of contact with the surface of the substrate to be coated, and liquid coating is replenished on account of the dispensing of liquid coating by virtue of the capillary forces within the porous material. Also in this case, liquid coating is dispensed from the porous material only when the porous material is in contact with the surface of the substrate to be coated. To enable transportation of the liquid coating to the surface of the pad which is in contact with the surface of the substrate to be coated, it is preferable if the porous material permits a transportation of liquid along the length by way of capillary forces. In this case, the cross-sectional shape of the porous material perpendicular to the feeding direction of the substrate may have any arbitrary shape. Preferably, the cross-sectional area of the pad perpendicular to the feeding direction of the second sheet is circular, rectangular, or square.

[0018] If the liquid coating is applied to the surface of the substrate by pressing the surface of the substrate against the porous material, it is necessary to provide a device by which the surface of the substrate is pressed against the porous material. Such a device for example may be a roller which acts on the substrate, thereby pressing the surface of the substrate to be coated against the porous material.

[0019] The porous material may be any of kind which allows a transfer of the liquid coating to the surface of the substrate, for example a felt, a woven fabric, a nonwoven fabric, a perforated elastic material or an open-cell foam. Particularly preferably, the porous material is an open-cell foam, having a cell size with a mean diameter in a range from 70 to 1500 pm, more preferred in a range from 70 to 800 pm, more preferred in a range from 70 to 500 pm and particularly in a range from 70 to 250 pm. The porous material is particularly preferably a foam which is composed substantially only of cell webs. “Composed substantially only of cell webs” in the context of the present invention means that more than 90 % of the foam, preferably more than 95 % of the foam and particularly more than 98 % of the foam has exclusively cell webs and no cell walls.

[0020] Suitable materials from which the porous material can be produced are all material that do not react with the liquid coating. If the porous material is an open-cell foam, the material the porous material is made of, can be any material that does not react with the liquid coating and can be formed into an open-cell foam, preferably a polymer. Suitable polymers, the porous material can be made of are for example polyolefins, in particular polyethylene or polypropylene, polyurethane, or melamine resins. Open-cell polymer foams from polyurethane or melamine resins are particularly preferable as the porous material.

[0021] If the porous material is a felt, a woven fabric or a nonwoven fabric, any material which forms a soft surface that does not abrade material from the surface of the substrate can be used for the fibers, the felt, the woven fabric or the nonwoven fabric is made of. In this case, suitable fibers may be polymer fibers or natural fibers. Suitable polymers for the fibers for example are polyolefins, polyurethane, or polyamides. Suitable natural fibers for example are wool, cotton, linen or hemp.

[0022] If the porous material is a perforated elastic material, the elastic material for example may be synthetic or natural rubber, a silicone or leather. Besides the materials mentioned above, it is also possible to use a porous material made of a metal, glass or ceramic, for example a felt or nonwoven fabric made of metal fibers or glass fibers or a sintered body made of a metal or a ceramic. However, in this case to avoid damaging the surface of the substrate, it is preferred to attach a soft material, for example a soft felt, nonwoven fabric, knitted fabric or woven fabric or alternatively a small layer of an open-cell polymer foam on the surface of the porous material through which the liquid coating is applied to the surface of the substrate.

[0023] In particular when the liquid coating is to exit the porous material by virtue of the pressure by the surface of the substrate, and no more liquid coating is to be released once the pressure ends, it is advantageous to use an elastic open-cell polymer foam which restores its original shape after the pressure ends.

[0024] In order to be able to continuously coat very long sheets, it is necessary for sufficient liquid coating to be provided. However, in order for the liquid coating only to exit the porous material when the surface of the substrate is in contact with the porous material, the porous material must not be oversaturated with the liquid coating. Saturation is achieved when the porous material can no longer receive any further liquid coating without liquid coating being dispensed at an arbitrary location on the surface. This requires that liquid coating can be re-metered into the porous material during coating the surface of the substrate. To this end, the porous material is preferably connected to a storage container in which the liquid coating is located. The liquid coating can then be fed to the porous material by using a suitable metering device, for example a metering pump. Alternatively, it is also possible for the liquid coating to be allowed to simply continue flowing, for example under the influence of gravity. To this end, a buffer container can be used above the porous material for example, said buffer container being filled with the liquid coating up to a predefined filling level. On account thereof, the hydrostatic pressure at which the liquid coating is transported into the porous material is kept largely constant. It is furthermore also possible for the connection from the storage container in which the liquid coating is stored to the porous material to be filled with a further porous material such that the liquid coating is directed through the connection line into the porous material by way of capillary forces.

[0025] To dispose the pad made of the porous material in a suitable trough, has the advantage to avoid dripping from the porous material when coating bottom surfaces. The trough may be made of any material which is inert to the liquid coating, for example a metal, a polymer, glass or a ceramic. For feeding the liquid coating into the porous material, a tube is arranged below the trough, the tube having an opening on the upper side which is in fluid contact with an opening in the bottom of the trough. The liquid then flows through the openings in the tube and the trough from the tube into the trough. For this purpose, a channel may be provided connecting the openings in the tube and the trough or the tube is directly attached to the trough, so that the openings in the tube and the trough are at the same position. In this case the tube may be fixed to the trough by welding, soldering or gluing so that a fluid tight connection is achieved and no liquid coating can exit between the tube and the trough. To allow pressure being applied to the porous material, the sidewalls of the trough have a lower height than the height of the uncompressed pad. In this context, the height of the pad corresponds to the extension perpendicular to the surface of the substrate to be coated, the length corresponds to the extension transversely to the running direction of the substrate and the width corresponds to the extension in the running direction of the substrate. On the other hand, the length of the substrate or of a surface of the substrate corresponds to the extension in the running direction of the substrate and the width of the substrate or of a surface of the substrate corresponds to the extension transversely to the running direction of the substrate.

[0026] Particularly if it is intended to coat the surface of the substrate in a continuous process, it is further preferred, if the edges of the substrate to be coated running transverse to the coating direction are parallel.

[0027] The surface of the pad being in contact with the surface of the substrate during application of the liquid coating may be a flat surface or may be curved. Further, it is also possible that the surface comprises protrusions along the width of the pad, the protrusions being in contact with the surface of the substrate during applying the liquid coating. The protrusions may have for example a circular, curved or triangular cross section perpendicular to the surface of the substrate. For applying the primer uniformly in a thin coating, it is only important, that the width, the pad is in contact with the surface of the substrate is constant along the whole width of the surface of the substrate to be coated. Ideally the pad should have an equal thickness along the whole width of the surface of the substrate to be coated.

[0028] If the surface of the substrate to be coated has a structure which runs parallel to the running direction of the substrate during coating, for example if the substrate has an undulated or zigzag surface structure or has any other surface structure with protrusions and recesses running parallel to the running direction of the substrate during the coating process, it is preferred that the pad comprises a structured surface having a structure which corresponds to the structure on the surface of the substrate so that during application of the primer the whole structured surface of the pad is in contact with the surface of the substrate to be coated.

[0029] The device for coating a surface of a substrate may be used for example in the production of sandwich panels, or for applying an adhesive on a surface. If the device is used in a production process for sandwich panels, the liquid coating usually is a primer for improving the adhesion of the foam layer on the outer sheets.

[0030] A process for producing sandwich panels, comprising a first sheet and a second sheet and a foam layer applied between the first sheet and the second sheet, comprises:

[0031] (a) applying a primer on an inner surface of the first sheet; (b) applying a liquid reaction mixture comprising reactants for producing a polymer and a blowing agent onto the primer on the inner surface of the first sheet, wherein after application, the reaction mixture starts to react and expand, thereby forming the foam layer;

[0032] (c) applying a primer on an inner surface of the second sheet by the process for coating a surface; and

[0033] (d) feeding the second sheet such that the primer on the inner surface of the second sheet is placed on the foam layer, thereby forming the sandwich panel.

[0034] In the context of the present invention, the terms “inner surface of the first sheet” and “inner surface of the second sheet” each mean the surface which is showing into the sandwich panel and which is in contact with the foam layer. During the production process, the first sheet and the second sheet usually are fed horizontally into a double belt. In this context, the first sheet is the lower sheet and the second sheet the upper sheet so that the inner surface of the first sheet is the upper surface and the inner surface of the second sheet is the bottom surface.

[0035] If the sandwich panel is used as a facade element for a building, it is preferred that at least one of the first sheet and second sheet is made of a metal, wherein if only one sheet is made of a metal, the sheet being made from metal preferably is the sheet which points outwards after assembly. In a preferred embodiment, the first sheet and the second sheet are made of a metal.

[0036] As the metals for the first sheet and the second sheet, if produced from a metal, aluminum, steel, stainless steel or copper are usually used. The use of steel or stainless steel is particularly preferred. The metal of the first and / or the second sheet can be coated or uncoated on the outer surface (The surface pointing outwards of the sandwich panel). Further, the metals of the first and / or second layer may be pretreated, for example with corona treatment, plasma treatment, flame impingement or other standard methods. In this case it is possible to use different materials for the first and the second sheet. Preferably, however, the same material is used for the first and second sheets.

[0037] Depending on the use of the sandwich panel, besides sheet metal, also metal films or polymer films or organic sheets, for example multi-layer composite films, may be used as first as second sheet of the sandwich panel. Further, it is also possible to use bitumen nonwoven fabric as material for the sheets.

[0038] The first sheet and the second sheet, if produced from metal and used as a facade element for a building, usually have a thickness in a range from 0.25 to 1 .5 mm. If steel is used as material for the first and / or second sheet, this preferably has a thickness in a range from 0.25 to 0.88 mm, and particularly preferably in a range from 0.4 to 0.75 mm. A sheet of stainless steel preferably has a thickness in a range from 0.3 to 0.9 mm, and particularly preferably in a range from 0.4 to 0.6 mm. The thickness of a sheet of aluminum is preferably in a range from 0.3 to 1 .5 mm, and particularly preferably in a range from 0.5 to 0.8 mm, and the thickness of a sheet of copper is preferably in a range from 0.3 to 1 mm and particularly in a range from 0.4 to

[0039] 0.7 mm. The surface of the first and second sheet may have any structure, though preferably the surface is plane or has only a light structure, a so-called microprofilation. This means that the elevations or depressions in the surface, which are present as a result of the structure, are preferably no deeper than 2 mm, preferably no deeper than 1 mm. The structure may be present only on the outer surface of the first and / or second sheet or on the outer surface and the inner surface.

[0040] Sandwich panels which are used for the facade of a building may have any lengths and widths which are customary for such sandwich panels. In general, the sandwich panels have a length from 2.5 to 30 m, preferably a length within a range from 3 to 24 m, and in particular within a range from 5 to 20 m. The width of the sandwich panel preferably is within a range from 0.5 to 1 .25 m, and particularly within a range from 0.9 to 1.2 m.

[0041] For a uniform application of the primer on the inner surface of the second sheet, the pad preferably has a length which is at least the same as the width of the inner surface of the second sheet. However, preferably, the length of the pad is larger than the width of the inner surface so that the pad extends beyond the width of the second sheet during application of the primer.

[0042] The foam layer of the sandwich structure is usually made of a rigid foam on an isocyanate base, for instance a polyurethane (PUR) foam, a polyisocyanurate (PI ) foam or a poly(urethane-iso- cyanurate) foam (PU / PIR). For this purpose, the reaction mixture applied on the primer on the inner surface of the first sheet contains reactants for producing a polyurethane foam, a polyisocyanurate foam or a poly(urethane-isocyanorate) foam. Such reaction mixtures are well known to the skilled person and described for example in WO-A 2010 / 060864.

[0043] The primer which is applied onto the inner surface of the first sheet and on the inner surface of the second sheet usually is an adhesive or a bonding agent and well known to the skilled person. Suitable primers are described for example in WO-A 2006 / 120234, EP-A 2 412 526, WO- A 20010 / 060864 or in WO-A 2015 / 155138. Particularly preferably, the primer is a one-component bonding agent being an isocyanate prepolymer with free isocyanate groups.

[0044] It is possible to use the same primer on the inner surface of the first sheet and on the inner surface of the second sheet or to use different primers on the inner surface of the first sheet and on the inner surface of the second sheet. However, it is preferred to use the same primer on the first sheet and on the second sheet.

[0045] The primer used preferably is a one-component system to allow application with one pad. If it is intended to apply a primer comprising two components or two different primers onto the inner surface of the second sheet, the primer comprising two components or the two different primers are applied to the inner surface of the second sheet by applying the first component or the first primer with a first pad and the second component or the second primer with a second pad so that the second component or the second primer is applied on the first component or the first primer. Mixing of the two different primers or of the two components in this case is performed directly on the inner surface of the second sheet by applying the second primer or the second component onto the first primer or the first component before the first primer or the first component starts to cure. Further, it is possible to apply the same primer by at least two pads arranged in series to achieve a larger thickness of the primer coating on the inner surface of the second sheet. However, usually it is preferred to apply the primer as thin as possible to have only little extraneous material in the sandwich panel but thick enough that the primer is still liquid when coming into contact with the expanding reaction mixture which forms the foam layer.

[0046] The thickness of the primer coating on the inner surface of the second sheet, and if the pad made of porous material also is used for applying the primer coating onto the inner layer of the first sheet, also the thickness of the primer coating on the inner surface of the first sheet may be set by the pressure, the porous material is pressed against the inner surface of the second sheet and / or of the first sheet, the application width of the porous material, the kind of material used for the porous material, the backpressure set by a reducing valve if the primer is fed into the porous material in a pump circuit, the pressure height, if the primer is fed into the porous material under gravity and or the velocity the inner surface of the first and / or second sheet passes the porous material.

[0047] The sandwich panel may be produced in a continuous process or discontinuously.

[0048] For continuously producing the sandwich panel, the first sheet preferably is fed from a coil to a device for applying the primer on the inner surface of the first sheet. The device for applying the primer on the inner surface of the first sheet may be any device known to a skilled person, for example a rotating disc as described for example in n WO-A 2006 / 120234, EP-A 2 412 526, WO-A 20010 / 060864 or in WO-A 2015 / 155138. However, besides such a rotating disc, any other suitable device for applying the primer on the inner surface of the first sheet may be used. Besides the well-known devices, it is also possible to use a pad as described above for applying the primer on the inner surface of the second sheet. Particularly if the first sheet is a profiled sheet, particularly if the first sheet has an undulated or zigzag cross sectional shape or a cross sectional shape with recesses and protrusions with any shape, it is preferred to use the pad as described above with a profiled surface which corresponds to the profile of the inner surface of the first sheet for applying the primer onto the inner surface of the first sheet for achieving a uniform coating over the whole inner surface

[0049] After application of the primer on the inner surface of the first sheet, the reaction mixture for producing the foam layer is applied on the primer on the inner surface of the first sheet. For applying the reaction mixture, also any suitable device known to the skilled person may be used.

[0050] Usually the first sheet is fed horizontally to the devices for applying the primer and for applying the reaction mixture. Thus, the primer and the reaction mixture are applied from above on the inner surface, which forms the upper side, of the first sheet. The second sheet is placed above the first sheet and preferably also fed horizontally and from a coil. Onto the inner surface of the second sheet which points downwards and is opposite the inner surface of the first sheet, the primer is applied with the pad as described above.

[0051] After application onto the inner surface of the first sheet, the reaction mixture starts to react and, due to the blowing agent contained in the reaction mixture, to expand, forming a foam. After application of the primer onto the inner surface of the second sheet, the second sheet is transported into the direction of the first sheet until the distance between the inner surfaces of the first and the second sheet corresponds to the intended thickness of the foam layer. Due to expansion of the reaction mixture, the foam extends upwards into the direction of the inner surface of the second sheet and comes into contact with the primer, thereby achieving a stable connection of the foam layer with the inner surface of the second sheet.

[0052] To avoid corrugations and deviations in the thickness of the foam layer which would result in an uneven thickness of the sandwich panel, the first and second sheets are fed into a double belt after application of the reaction mixture, wherein in the double belt the reaction and expansion of the reaction mixture takes place. Further, the double belt is used for transport of the first and second sheet and, after reaction and expansion of the reaction mixture of the sandwich panel produced.

[0053] Usually each belt of the double belt comprises a link conveyor which is guided around rollers. To move the belts, at least one of the rollers of each belt is a driven roller. The link conveyors are arranged in parallel having a distance which corresponds to the thickness of the sandwich panel to be produced. During expansion of the reaction mixture and forming the foam, the first sheet is pressed against the bottom belt and the second sheet against the upper belt of the double belt. Due to the pressure against the belts, a constant thickness of the sandwich panel is achieved. To avoid an expansion of the sandwich panel after leaving the double belt, the double belt preferably has such a length that the reaction of the reaction mixture is completed and a rigid foam is formed when the sandwich panel leaves the double belt.

[0054] In the continuous process, after leaving the double belt, the sandwich panel produced is cut into elements of a predefined length.

[0055] If the sandwich panels are produced in a discontinuous process, the primer and the reaction mixture are applied by moving the application devices, i.e. the pad made of the porous material for application of the primer and the device for application of the reaction mixture along the first sheet and the application device for the primer along the second sheet to apply the primer onto the whole inner surface of the first and second sheets and the reaction mixture onto the primer applied to the inner surface of the first sheet and subsequently place the second sheet into its place, so that the sandwich panel is obtained. Besides the use as a facade element, in particular in the commercial sector, such as warehouses or industrial buildings, for instance workshops, sports halls, department stores or assembly shops, sandwich elements produced by the inventive process may be used as insulating boards, as walls or doors of cold stores or climate chambers, or as walls or doors of truck-build- ups.

[0056] Embodiments of the invention are shown in the figures and explained in more detail in the following description.

[0057] In the figures:

[0058] Figure 1 shows a schematic view of a device for producing sandwich elements;

[0059] Figure 2 shows a pump circuit for feeding liquid coating into an application device with a pad made from a porous material;

[0060] Figure 3 shows a three-dimensional view of a feeding part of an application device;

[0061] Figure 4 shows a cross sectional view of an application device;

[0062] Figure 5 shows a pad made from a porous material having a structured surface;

[0063] Figure 6 shows an application device for application of primer on a top surface of a sheet;

[0064] Figure 7 shows an application device for application of primer on a bottom surface of a sheet.

[0065] Figure 1 shows a schematic view of a device for continuously producing sandwich elements.

[0066] A device 1 for producing sandwich panels comprises a first application device 3 to apply a primer onto an inner surface 5 of a first sheet 7. In a continuous process, the first sheet 7 usually is uncoiled from a coil and fed into the device 1 for producing sandwich elements. If the process is carried out discontinuously, it is possible to feed a first sheet 7 having the dimensions of the sandwich panel to be produced into the device 1 for producing the sandwich panel. By feeding the first sheet 7 below the first application device 3 with a constant velocity, the primer is applied in a uniform coating.

[0067] A second application device 9 to apply a reaction mixture is located downstream the first application device 3. The reaction mixture comprises reactants for forming a polymer and a blowing agent, so that during reaction a foam layer 11 is formed. To achieve a stable connection of the foam on the inner surface 5 of the first sheet 7, the reaction mixture is applied onto the inner surface 5 of the first sheet 7 before the primer is cured. Above the first sheet 7, a second sheet 13 is fed into the device 1 for producing sandwich panels. As the first sheet 7 and the second sheet 13 are connected by the foam layer 11 , the velocity, the second sheet 13 is fed is the same as the velocity of the first sheet 7.

[0068] T o achieve a stable connection of the foam layer 11 on the inner surface 15 of the second sheet 13, a primer is applied by an application device 17 for a liquid coating. Due to the expansion of the reaction mixture, the foam obtained from the reaction mixture comes into contact with the primer on the inner surface 15 of the second sheet 13. As for the first sheet, a stable connection of the foam layer 11 and the second sheet 13 is achieved, if the foam layer 11 contacts the primer before the primer is cured.

[0069] For transport of the first sheet 7 and the second sheet 13 and to produce a sandwich panel having a uniform thickness, a double belt 19 is arranged downstream the first and second application devices 3, 9, and the application device 17 for a liquid coating.

[0070] The double belt 19 comprises a first conveyor belt 21 below the first sheet 7 and a second conveyor belt 23 above the second sheet 13. During production of the sandwich panel, the first sheet 7 is pressed against the first conveyor belt 21 and the second sheet 13 is pressed against the second conveyor belt 23 by the expanding foam. Due to the conveyor belts, the foam cannot lift the second sheet 13 above the distance between the first and second conveyor belts 21 , 23 so that a constant thickness of the sandwich element is achieved. For this purpose, length of the conveyor belts 21 , 23 is selected such that the first and second sheets 7, 13 are in contact with the conveyor belts until the reaction mixture is cured and a stable foam is produced.

[0071] To apply a uniform coating of the primer onto the inner surface of the second sheet 13, the application device 17 for the liquid coating comprises a pad made of a porous material which is pressed against the inner surface 15 of the second sheet 13.

[0072] In a continuous process, it is necessary to continuously feed primer into the application device 17 for the liquid coating. A pump circuit for feeding the primer into the application device 17 for liquid coating is shown in figure 2.

[0073] A pump circuit 25 for feeding liquid coating into the application device 17 for liquid coating comprises a storage container 27 in which the liquid coating is stored. From the storage container 27, the liquid coating is pumped with a pump 29 into the application device 17. Downstream the application device 17, the circuit comprises a valve 31 . By the valve 31 the backpressure of the liquid coating in the circuit is set and such the amount of liquid coating in the porous material of the application device 17 and the amount of liquid coating applied to the surface of a substrate.

[0074] A three dimensional view of a feeding part of an application device for the liquid coating is shown in figure 3 and a cross sectional view of an application device for a liquid coating comprising the feeding part is shown in figure 4. A feeding part for the liquid coating may comprise a tube 33, through which the liquid coating flows. The tube 33 for example may be part of the pump circuit as shown in figure 2, if the liquid coating is fed into the application device 17 by such a pump circuit.

[0075] On the upper side, the tube 33 has an opening 35 through which the liquid coating can flow into a trough 37 which is located above the tube and connected to the opening 35 in the tube by a suitable channel 39. Besides the channel 39 as shown here, it is also possible to directly connect the trough 37 with the tube 33. If a channel 39 is used for connecting the trough 39 to the tube 33, the channel preferably has a cross sectional shape which corresponds to the cross sectional shape of the opening 35 in the tube 33. To further allow the liquid coating to flow into the trough 37, the through 37 comprises an opening 41 at the bottom. The cross sectional shape of the opening 41 in the bottom of the trough 37 also corresponds to the cross sectional shape of the channel 39. The channel 39 is mounted on the tube 33 and on the bottom of through 37 in a fluid tight manner, so that no liquid coating can leak.

[0076] If no channel 39 is provided, the trough 37 is directly mounted on the tube 33. In this case it is preferred that the openings 35, 41 have the same cross sectional shape and are flush mounted. Independently of mounting the trough 37 on the tube 33 with the channel 39 or without the channel 39, a fluid tight connection may be achieved for example by welding, gluing or soldering.

[0077] As an alternative to the openings 35, 41 in the shape of a slot as shown here, it is also possible that a plurality of openings is provided in the tube 33 and / or in the bottom of the trough 37. If in this case the trough 37 is mounted to the tube 33 without the channel 39, it is preferred that one of the parts comprises an opening in the form of a slot and the other part a row of openings. In this case, after mounting the trough on the tube, the row of openings must be completely within the cross-sectional area of the slot and the weld-seam, soldered seam or the adhesive must surround the opening which is in form of a slot to avoid leakage of the liquid coating.

[0078] For coating a surface of a substrate with a liquid coating, the trough 37 contains a pad 43. The pad 43 is made of a porous material so that the liquid coating can flow from the tube 33 through the channel 39 into the pad 43 and can be released from the pad 43 to a surface of a substrate as soon as the surface of the substrate is in contact with a surface 45 of the pad 43. For fixing the pad 43 in the trough 37, suitable mounting devices 47 are provided. The mounting devices 47 for example are brackets or clamps with which the pad 43 is fixed in the trough 37. However, besides brackets or clamps any other mounting device for fixing the pad 43 in the trough 37 may be used. To allow a change of the pad 43, for example for cleaning the pad 43 or to remove used pads, it is preferred that the pad 43 is mounted releasably in the trough 37.

[0079] Using the pad 43 for applying a liquid coating on a surface 49 of a substrate 51 , has the advantage that it is possible to also apply a uniform coating layer on the surface 49 which has a structure. This is exemplary shown in figure 5 for a surface having protrusions 53 and recesses 55. For a uniform application of the liquid coating, the pad has a surface 45 with a structure that corresponds to the structure of the surface 49. This means that the surface 45 of the pad 43 has protrusions 57 at that positions the surface 49 of the substrate 51 has recesses 55 and recesses 59 at that position the surface 49 of the substrate 51 has protrusions 53. If the surface 49 has a different structure than shown here, for example an undulating structure or a zigzag structure, the surface 45 of the pad 47 has a respective undulating or zigzag structure.

[0080] Figure 6 shows a device for applying a liquid coating on an upper surface of a substrate.

[0081] For applying a liquid coating on an upper surface of a substrate, the surface 45 of the pad 43 points downwards and is in contact with the upper surface 49 during coating. Using the porous material for the pad 43 has the advantage that after releasing the surface 49 from the surface 45 of the pad the liquid coating stops flowing and does not drip from the pad 43.

[0082] The liquid coating may be fed to the pad 43 by a pump circuit as shown in figure 2 or, alternatively as shown here by using storage containers 61 for the liquid coating which are located at a higher position than the tube 33 and the pad 43. In this case the liquid coating flows into the tube 33 by virtue of gravity. Depending on the hydrostatic pressure of the liquid coating, the thickness and the pore size of the pad 43 it is possible to soak the pad 43 with the liquid coating but to avoid oversoaking so that after soaking the pad 43 with the liquid coating only liquid coating is fed into the pad 43 when liquid coating is dismissed by application onto the surface 49 of the substrate 51 to be coated.

[0083] Figure 7 shows a device for applying a liquid coating on a bottom surface of the substrate.

[0084] The application device 17 shown in figure 7 differs from the embodiment shown in figure 6 in the position of the surface 45 of the pad 43 through which the liquid coating is applied to the surface 49 of the substrate 51. In the embodiment shown if figure 7, the surface 45 of the pad 43 points upwards so that during the coating process, the surface 49 of the substrate to be coated lies on the surface 45 of the pad. In this case, the pressure the liquid coating is fed into the tube 33 must be sufficiently high to completely soak the pad 43. When the surface 49 to be coated is in contact with the surface 45 of the pad, the liquid coating is applied to the surface 49 of the substrate 51 either by a defined pressure, the substrate 51 is pressed on the surface 45 of the pad 43 or by capillary forces. To achieve a sufficiently high pressure, either a pump circuit as shown in figure 2 may be used, or, alternatively, as shown here, storage containers 61 containing the liquid coating, which are at a position such that the surface of the liquid coating is above the surface 45 of the pad to achieve a sufficiently high hydrostatic pressure.

Claims

Claims1 . A device for applying a liquid coating on a surface (49) of a substrate(51 ), comprising a tube (33) for feeding the liquid coating, and a pad (43) made of a porous material, wherein the tube (33) comprises at least one opening (35) which extends parallel to the main axis of the tube (33) and / or wherein the tube (33) comprises a plurality of openings being arranged in a row parallel to the main axis of the tube (33), through which the liquid coating can be fed into the pad (43) and a metering device for feeding the liquid coating into the tube (33) with a predefined pressure, wherein the pad is disposed in a trough (37), the tube (33) being arranged below the trough (37) and the at least one opening (35) or the plurality of openings in the tube (33) being in fluid contact with an opening in the bottom of the trough so that the liquid coating can flow from the tube into the trough and that the pad (43) can be uniformly soaked with the liquid coating.

2. The device according to claim 1 , wherein the porous material is an open cell foam having a cell size with a mean diameter in a range from 70 to 1500 pm.

3. The device according to claim 1 or 2, wherein the pad (43) comprises a structured surface (45) having a structure which corresponds to a structure of the surface (49) of the substrate (51 ) so that during applying the liquid coating the whole structured surface (45) of the pad (43) is in contact with the surface (49) of the substrate (51 ) to be coated.

4. The device according to any of claims 1 to 3, wherein the opening (35) is a slot parallel to the main axis of the tube (33) or wherein a plurality of openings is arranged in a row parallel to the main axis of the tube (33).

5. The device according to any of claims 1 to 4, wherein sidewalls of the trough (37) have a lower height than the height of the uncompressed pad (43).

6. A process for coating a bottom surface of a substrate with a device according to any of claims 1 to 5, comprising:(i) feeding a liquid coating from the tube (33) through the at least one opening (35) into the trough (37) for soaking the pad (43) with the liquid coating; and(ii) passing the surface (49) of the substrate (51 ) along a surface (45) of the pad (43) under a predefined pressure so that the liquid coating is applied to the surface (49) of the substrate (51 ).

7. A process for producing sandwich panels, comprising a first sheet (7) and a second sheet (13) and a foam layer (11 ) applied between the first sheet (7) and the second sheet (13), the process comprising:(a) applying a primer on an inner surface (5) of the first sheet (7);(b) applying a liquid reaction mixture comprising reactants for producing a polymer and a blowing agent onto the primer on the inner surface (5) of the first sheet (7), wherein after application, the reaction mixture starts to react and expand, thereby forming the foam layer (11 );(c) applying a primer on an inner surface (15) of the second sheet (13) by a process according to claim 6; and(d) feeding the second sheet (13) such that the primer on the inner surface (15) of the second sheet (13) is placed on the foam layer (11), thereby forming the sandwich panel.

8. The process according to claim 7, wherein for applying the primer on the inner surface (5) of the first sheet (7), the inner surface (5) of the first sheet (7) passes over a pad (43) made of a porous material and soaked with the primer such that the inner surface (5) of the first sheet (7) is coated with the primer.

9. The process according to claim 7 or 8, wherein the first and the second sheets (7, 13) are made of a metal.

10. The process according to any of claims 7 to 9, wherein the reaction mixture contains reactants for producing a polyurethane foam, a polyisocyanurate foam or a poly(urethane-iso- cyanorate) foam.11 . The process according to any of claims 7 to 10, wherein the primer is an adhesive or a bonding agent.

12. The process according to any of claims 7 to 11 , wherein a primer comprising two components or two different primers are applied to the inner surface (15) of the second sheet(13) by applying the first component or the first primer with a first pad (43) and the second component or the second primer with a second pad (43) so that the second component or the second primer is applied on the first component or the first primer.

13. The process according to any of claims 7 to 12, wherein the process is carried out continuously.

14. The process according to claim 13, wherein after application of the reaction mixture, the first and second sheets (7. 13) are fed into a double belt (19), wherein in the double belt (19) the reaction and expansion of the reaction mixture takes place.

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