Device and method for covering a substrate

Abstract

A device for covering a substrate (10) with a coating comprises an inlet (11) and an outlet (12) for the substrate and transport means (15) for displacing the substrate therebetween along a substrate track at a track speed. At least one nozzle (30) is provided to receive the coating in liquid form and direct a mist thereof toward the substrate track. The nozzle (30) is provided with a drive (35) which carries the nozzle alternately in an oscillating movement. A potential plane (45) is provided on a side of the substrate track remote from the nozzle. A potential source (40) is connected between the nozzle (30) and the potential plane (45) in order to maintain a potential difference V(t) therebetween. The potential source can be modulated and the potential difference is adjusted to the track speed of the substrate and / or a position of the nozzle.

Description

[0001] Device and method for covering a substrate

[0002] The present invention relates to a device for covering a flat substrate with a coating, particularly a sheet-like substrate, more particularly a plastic sheet, comprising an inlet and an outlet for a flat substrate and transport means for displacing the substrate therebetween along a substrate track at a track speed and comprising coating means with a feed for a coating liquid, which coating means are able and configured to apply the coating liquid to the substrate over at least substantially a whole surface.

[0003] The invention also relates to a method for covering a flat substrate with a coating, particularly a sheet-like substrate, more particularly a plastic sheet, wherein a coating liquid is applied to the substrate in liquid form while the substrate is carried along a substrate track.

[0004] A device and method of the type described in the preamble are applied on a large scale for coating the whole surface of a substrate with an optionally wholly transparent coating. With a view to a high production speed this procedure is preferably performed in a fully continuous process, wherein the substrate is guided at a constant track speed while the coating is being applied. A known device comprises for this purpose a steel transfer roller which is fed with a coating liquid over a surface thereof on one side and is in rolling contact with the substrate on the other. The coating is thus transferred via the roller surface from the roller onto the substrate over a full width.

[0005] A drawback of this known device is that a width of the steel roller must always be adapted to a width of a substrate to be covered therewith. For relatively large substrate widths this requires a correspondingly large investment and heavy suspension, which will often be unprofitable or even unfeasible from an economic viewpoint. This technique is not suitable either for thick viscous liquids and multi-component varnishes requiring premixing of the components contained therein before being applied.

[0006] The present invention has for its object, among others, to provide a device and method for coating a substrate which obviate these drawbacks and can realize a high production speed while maintaining quality.

[0007] For this purpose a device of the type described in the preamble has the feature according to the invention that the coating means comprise at least one nozzle which is able and configured to receive the coating in liquid form and direct a mist thereof toward the substrate track, that the nozzle is provided with a drive which carries the nozzle alternately in an oscillating movement which crosses the substrate track over a full width thereof, and that provided between the transport means of the substrate and the drive means of the nozzle is a control which activates the drive of the nozzle subject to an activation of the transport means.

[0008] In order to achieve the stated object a method of the type described in the preamble has the feature that the coating liquid is atomized over the substrate with a nozzle in an oscillating spraying movement, which spraying movement alternately crosses the substrate transversely at an at least substantially constant speed Vs while the substrate is carried longitudinally at an at least substantially constant track speed Vb.

[0009] The nozzle is thus in each case guided reciprocally transversely of the substrate in an oscillating movement while the coating is being sprayed onto the substrate. This procedure is performed in a fully continuous process, wherein the substrate is preferably guided along under the nozzle at a constant track speed. Owing to the continuously advancing substrate, the coating is thus applied thereto in successive oblique lines in a zig-zag pattern. The individual lines cross or overlap each other, but have ultimately been found to be visually undetectable or hardly detectable in the overall view, or at least do not disrupt an appearance of the substrate to unacceptable extent. In this respect a preferred embodiment of the device has the feature according to the invention that the drive carries the nozzle alternately in an oscillating movement which crosses the substrate track beyond the full width thereof, wherein a turning point of the nozzle lies outside the substrate track on either side. Because both turning points are thus located outside the substrate, any accumulations of coating liquid likewise lie outside the substrate track.

[0010] Thickened portions in the coating pattern are thus avoided at the edges of the substrate.

[0011] It is a challenge to nevertheless achieve full coverage despite the advancing substrate. For this purpose the spray pattern of successive movements of the nozzle must preferably connect to each other without interruption. In this respect a particular embodiment of the device has the feature according to the invention that a potential plane is provided on a side of the substrate track remote from the nozzle, that connected between the nozzle and the potential plane is a potential source which is able and configured to maintain a potential difference therebetween during operation, and that the potential source can be modulated and is provided with a control whereby the potential difference is adjustable during operation to the track speed of the substrate and / or a position of the nozzle. With said object in mind, a method of the type described in the preamble has the feature according to the invention that a modulable potential difference V(t), which is adjusted to the track speed of the substrate and / or a position of the nozzle, is maintained between the nozzle and the substrate.

[0012] According to the invention, a potential difference can thus be maintained between the nozzle and the substrate. The atomized liquid is guided to the substrate under the influence of this potential difference and becomes more directional as this potential difference increases. The diameter of the spray pattern can hereby thus be varied subject to more or less potential being applied. By modulating the potential difference and thereby adjusting the diameter of the spray pattern to the track speed of the substrate in relation to the displacement speed of the nozzle an accurate adjustment between successive spraying movements and the line width created thereby can be achieved. In practice this additional control of the spraying process under some circumstances is valuable for the quality and conformity of the substrate coverage.

[0013] With a view to an adequate effect of the potential difference applied to the spray pattern, a particular embodiment of the device has the feature according to the invention that the potential source is able and configured to maintain a static potential difference in the order of 10 kV between the nozzle and the potential plane. The static potential difference between the nozzle and the substrate is thus in the order of around 10 kV during operation, wherein modulation takes place around this value in order to adjust the spray pattern as required.

[0014] Although the potential plane can be realized in various ways, a preferred embodiment of the device has the feature according to the invention that the potential plane is formed by a flat metal plate on a side of the substrate track remote from the nozzle. This enables a particularly powerful coupling from the potential plane to the substrate by providing the substrate track of the substrate at a short distance from the metal plate and, particularly, by guiding the substrate along the metal plate in contact therewith.

[0015] A particularly practical embodiment of the device has the feature according to the invention that the nozzle is moveable along a linear guide, which guide extends transversely of the substrate track. The guide can here be formed by a linear actuator or by a straight rail along which the nozzle is guided by the drive, such as for instance an electric motor. In both cases the at least one nozzle follows a fixed, defined route transversely of the course of the substrate which is imposed by the drive and guide. By thus making use of such a linear guide transversely of the track direction of the substrate a relatively simple mathematical relation is obtained between the position of the nozzle during a movement in the guide relative to a corresponding position of the nozzle in a previous movement, while the substrate moved relative to the guide at a track speed vb. With this in mind, a preferred embodiment of the method has the feature according to the invention that the substrate is carried along the nozzle at a constant track speed vb, that the nozzle crosses the substrate reciprocally over a width b at an at least substantially constant speed Vs.

[0016] At moderate to medium track speed of the substrate a wholly uniform, continuous coverage can be realized with a particular embodiment of the method according to the invention, characterized in that the potential difference V(t) is kept constant and imposes a radius rson the mist at the position of the substrate, for which: rs> b.vb / vs. The radius of the spray pattern is thus adjusted to the track speed by reducing the potential difference as the track speed increases. According to the stated formula, the resulting greater spray diameter suffices to ensure an overlap with a subsequent spraying movement. A preferred embodiment of the method according to the invention has in this respect the feature that the potential difference V(t) imposes a radius rson the mist at the position of the substrate, for which: rs= b.vb / vs. Successive spraying movements thus connect seamlessly to each other without or almost without overlap.

[0017] A further particular embodiment of the method according to the invention is characterized in that the potential difference in a movement of the nozzle over the substrate in each case decreases from a starting value to an end value. Thus reducing the potential difference during a movement of the nozzle creates a fanned-out spray pattern which widens gradually. Despite the absence of overlap between successive spraying movements, a so-called full coverage can thus nevertheless be realized at higher track speeds of the substrate, wherein the stated widening fills any gap wholly or at least largely. Flowing out of the still wet coating then provides for a surprisingly uniform coverage.

[0018] Because a reduction of the potential difference results in a wider spray pattern, a degree of coverage in terms of quantity per unit of surface area will decrease when the supply of the coating liquid remains unchanged. In order to avoid this, if desired, a further particular embodiment of the method has the feature according to the invention that the coating is supplied to the nozzle at a flow rate, which flow rate increases from a starting value to an end value during each movement of the nozzle over the substrate as the potential difference decreases. The liquid flow rate can thus be increased as the sprayed area increases as a result of a reduction of the potential difference.

[0019] The method according to the invention is particularly suitable for a multicomponent polyurethane coating. Any desired material can in principle be used for the substrate, although the method is particularly suitable for an embodiment characterized in that the substrate comprises a plastic sheet, particularly a tarpaulin, more particularly a tarpaulin of polyvinyl chloride.

[0020] Many coating liquids inevitably comprise a considerable proportion of volatile substances such as solvents, precursors and the like, with which the substrate will therefore come into contact during the coating process. Under some circumstances this can have adverse effects on the integrity of the substrate if it is insufficiently resistant to an effect of such components. Solvent-based plasticizers in a plastic, such as for instance PVC, may particularly leach out, which may result in undesirable wrinkling in the substrate. In order to prevent this a preferred embodiment of the method has the feature according to the invention that the coating is applied successively in successive layers, particularly an underlayer with a relatively small layer thickness followed by a subsequent layer with a greater layer thickness. The underlayer is here preferably applied relatively thinly, with a correspondingly low content of solvents, precursors and other volatile components. These volatile substances will already largely evaporate during a mild drying, before reaching the substrate. The thus formed base layer seals off the substrate and protects the substrate against the effect of volatile substances in the subsequent layer or layers. The next layer is preferably applied here before a preceding layer has fully hardened, so that the layers fuse together and transpose into each other seamlessly without any noticeable boundary layer therebetween.

[0021] With such a double-layer coverage in mind, a particular embodiment of the device has the feature according to the invention that a similar further device for covering the substrate with a further coating is provided downstream of the nozzle and that provided between the nozzle and the further device are heating means which are able and configured to enter into heat-exchanging contact with the substrate track downstream of the at least one nozzle. The further coating can here have a different composition, but is particularly identical to the first coating in respect of composition.

[0022] The heating means bring about the above described drying of the base layer before the substrate is covered with a subsequent layer downstream thereof in the further device. In a further particular embodiment the device is characterized here in that the heating means comprise an infrared radiator, particularly an infrared panel extending at least substantially over a full width of the substrate track.

[0023] Given a relatively small layer thickness of the base layer, a correspondingly low output of such a radiator suffices, which radiator can thereby act rapidly and effectively on the substrate in order to expel therefrom any volatile components present in the coating without exceeding a maximum allowable substrate temperature, which maximum allowable substrate temperature is preferably always maintained as threshold value.

[0024] A preferred embodiment of the method according to the invention has the feature here that each layer is in each case followed by a drying below a threshold value of a temperature of the tarpaulin. The method particularly has the feature here according to the invention that the drying comprises of a first drying of a relatively thin base layer followed by a second drying following application of a subsequent layer with a greater layer thickness.

[0025] An advantage of applying an infrared radiator for the purpose of the first drying in particular is that the infrared radiation will particularly be absorbed at a boundary surface between the substrate and the base layer, so that this will initially close there and will seal off the substrate from migration of solvents from the coating. This is contributed to further by a further preferred embodiment of the device according to the invention which is characterized in that the infrared panel is arranged on a side of the substrate track remote from the nozzle. The heat will thereby reach said boundary surface particularly effectively, while the panel under the substrate is moreover protected from the mist distributed by the nozzle.

[0026] In the device and method according to the invention use is particularly made of a substrate which is wound from roll to roll and therebetween has been subjected to the above described coating. For this purpose the device can advantageously be expanded at the inlet and beyond the outlet with respectively an (un)winding station for unwinding the substrate to be coated from a roll and a winding (up) station for winding the substrate onto a roll again once it has been coated. In this latter step it is of great importance that the coated substrate has been subjected to sufficient drying so that no adhesion occurs between successive windings in the roll.

[0027] With an adequate drying of the coating in mind, a further particular embodiment of the device has the feature according to the invention that the further device comprises at least one further nozzle, and that further heating means are provided which are able and configured to enter into heatexchanging contact with the substrate track downstream of the at least one further nozzle. Depending on the substrate material, it must be ensured here that a determined maximum temperature is not exceeded. It is important particularly in the case of a plastic textile provided with a part of a print to be manufactured into a full image afterwards that separate parts of this image accurately align with each other. Uncontrolled shrinkage or other impediment to the substrate lying flat could be inconvenient, if not disastrous, in such a case. For manufacture of the tarpaulin after it has been provided with a coating it is important for the tarpaulin to lie as flat as possible, without having to be tensioned for this purpose.

[0028] In a particular embodiment the device according to the invention is therefore characterized in that the further heating means are able and configured to generate and maintain a heated airflow over the substrate track. These further heating means force a heated airflow over the substrate track in order to dry the coating. Such a drying with heated air can be thermally controlled with exceptional accuracy, wherein the hot air, and thereby the substrate, will never exceed a maximum allowable temperature, irrespective of the duration of the drying. Any flammable solvents can furthermore be expelled from the coating and out of the device by the airflow. All in all, the forced airflow provides a particularly effective and safe drying of the coating wherein almost all volatile substances are expelled therefrom. Shrinking or wrinkling of the substrate, particularly of a plastic textile, due to shortening of fibres as a result of this threshold value being exceeded can thereby be counteracted in an at least substantially complete drying of the coating.

[0029] The invention will be further elucidated hereinbelow with reference to an exemplary embodiment and an accompanying drawing. In the drawing: Figure 1 shows schematically an exemplary embodiment of the device according to the invention in a cross-section, transversely of a sheet flow;

[0030] Figure 2 shows the device of figure 1 in a longitudinal section in the direction of a sheet flow;

[0031] Figure 3A shows a top view of a number of successive spraying movements from left to right over the substrate in accordance with a first embodiment of the method according to the invention;

[0032] Figure 3B shows a top view of a number of successive spraying movements from right to left over the substrate in accordance with the first embodiment of the method according to the invention; Figure 3C shows a top view of an overall view of a substrate covering by the spraying movements of figure 3A alternated with the spraying movements of figure 3B in accordance with the first embodiment of the method according to the invention;

[0033] Figure 4A shows a top view of a number of successive spraying movements from left to right over the substrate in accordance with a second embodiment of the method according to the invention;

[0034] Figure 4B shows a top view of a number of successive spraying movements from right to left over the substrate in accordance with the second embodiment of the method according to the invention;

[0035] Figure 4C shows a top view of an overall view of a substrate covering by the spraying movements of figure 4A alternated with the spraying movements of figure 4B in accordance with the second embodiment of the method according to the invention;

[0036] Figure 5A shows a top view of a number of successive spraying movements from left to right over the substrate in accordance with a third embodiment of the method according to the invention;

[0037] Figure 5B shows a top view of a number of successive spraying movements from right to left over the substrate in accordance with the third embodiment of the method according to the invention; Figure 5C shows a top view of an overall view of a substrate covering by the spraying movements of figure 5A alternated with the spraying movements of figure 5B in accordance with the third embodiment of the method according to the invention; and

[0038] Figure 6 shows a schematic arrangement of a coating line according to the invention.

[0039] It is otherwise noted here that the figures are purely schematic and not all drawn to (the same) scale. Some dimensions in particular may be exaggerated to greater or lesser extent for the sake of clarity. Corresponding parts are designated in the figures with the same reference numeral.

[0040] Figure 1 and figure 2 show in respectively cross-section and longitudinal section a schematic representation of an exemplary embodiment of a device 1 according to the invention for covering a substrate. In this example the substrate comprises a tarpaulin 10 which is formed by a PVC (polyvinyl chloride) sheet with a width of about 320 centimetres. The sheet 10 comes from a roll (not shown) with an excess of length and is guided through the device as a continuous sheet at a constant track speed along a substrate track from an inlet 11 to an outlet 12 by transport means in the form of a set of conveyor rollers 15. The substrate crosses here a linear guide 20 in which a nozzle 30 is movably suspended. In this example a rotating nozzle 30 of the type Nordson® RA-20 is applied. The coating liquid is hereby entrained carried by a compressed air and spread over the substrate at a high number of revolutions in the order of 40,000 revolutions per minute. Both the compressed air flow, the liquid pressure and the number of revolutions of the nozzle are controllable. These parameters are controlled within narrow limits by a central control device (not shown).

[0041] The nozzle is provided with a drive 35 whereby the nozzle 30 can be set into an oscillating motion, alternately from left to right and from right to left, along the guide 20 at an almost constant speed vs. The guide 20, and thereby this movement, has a width b of 360 centimetres, whereby the nozzle in each case changes direction outside the surface of the substrate at a turning point KP and there undergoes an acceleration or deceleration. Nozzle 30 is connected to a volumetric feeder whereby a coating in liquid form is fed thereto at a controllable flow rate. In this case this is a two-component varnish of polyurethane and a precursor (hardening agent) which are guided to nozzle 30 separately and are combined and mixed therein. Nozzle 30 forms a mist 33 from these two components and directs it toward the substrate 10.

[0042] The device further comprises a potential source 40 which is connected between nozzle 30 and a metal plate 45. The metal plate 45 is situated on a side of the substrate track 10 remote from nozzle 30 and there generates a potential plane which serves as counter-potential to the potential experienced by nozzle 30, and thereby the mist 33. The mist is thus drawn electrostatically to the substrate under the influence of the potential difference V(t) which can be created and maintained with the potential source 40. This potential difference V(t) is in the order of 10 kV and thereby directs mist 33 toward substrate 10 to greater or lesser extent depending on higher or lower modulation of the potential. For this purpose the potential source 40 is controllable and connected to the central control of the device, which also controls and regulates the track speed vbof substrate 10, the speed vsof nozzle 30 and the liquid supply (flow rate) to nozzle 30. This control adjusts the potential difference V(t) to the track speed vbof substrate 10 and / or a position of nozzle 30 within its movement in guide 20 during operation. A spraying width is thus adjustable, varying from a diameter of about 200 millimetres to about 600 millimetres. A number of examples thereof follow below.

[0043] In a first exemplary embodiment a relatively high constant potential difference V(t) = T is created and maintained, whereby the mist is narrowed and will have at the position of the substrate a circular section with a radius rs, for which: rs> b.vb / vs, wherein b indicates the width of guide 20 along which the nozzle is in each case displaced alternately from left to right or from right to left at a substantially constant speed Vs. The track speed at which the sheet 10 is carried along under nozzle 30 is indicated by vb.

[0044] Figure 3A shows a number of spray lines 33L which were thus shot onto the substrate from left to right. These spray lines 33L were here in each case alternated with spray line 33R from right to left. These spray lines are shown separately in figure 3B. The width of the spray lines 33L, 33R i.e. twice the radius rsof mist 33, in this case suffices to provide in each case an overlap with a subsequent spray line 33L, 33R. This ensures full coverage of the substrate. In the shown example it is particularly the case for this width bsof the spray lines that: bs= 2.rs = 2.b.vb / vs, whereby in each case an overlap is obtained over half of the spray lines and the coating is applied in four layers with a substantially constant layer thickness. This is also contributed to in that the nozzle is guided beyond the edges of the substrate track on both sides. Hereby, the nozzle has turning points KP which will always be located outside the substrate. The nozzle can thus accelerate from standstill, a speed of vs=0, to the desired speed, nozzle speed vs, outside the substrate without adverse effects on the coating thickness.

[0045] In the exemplary embodiment of figures 4A-4C the track speed vbhas been increased while potential difference V(t) and nozzle speed vsremain the same, whereby successive lines will lie further apart as a result of the greater path length over which the substrate has been displaced in the meantime. In this case: rs= b.vb / vs. This makes the width of the lines 33R, 33L just great enough to touch each other without overlapping in any way, as shown separately in figure 4A for the lines from left to right and in figure 4B for the lines from right to left. The overall view of figure 4C is thereby still fully continuous, with a double-layer coverage with a substantially uniform layer thickness.

[0046] If the track speed vbis increased still further, an overlap of successive lines 33L, 33R can no longer be achieved at the same line width. In order to nevertheless realize a full coverage the potential difference V(t) in a movement of nozzle 30 over substrate 10 decreases in each case from a starting value to an end value. The mist will hereby gradually be urged toward substrate 10 less forcefully, and instead diverge further. The spray pattern thereby fans out, as shown in figures 5A and 5B for respectively the movement 33L from left to right and for the movement 33R from right to left. The combined coating pattern of figure 5C leaves almost no gaps. A natural flowing out of the still wet coating will close any gaps and level local layer thickness differences. A uniform coverage can thus still be achieved. If desired, a liquid flow rate to nozzle 30 can be increased as the selected potential V(t) decreases. This can compensate for the increased area of the section of the mist 33 in order to maintain the same layer thickness.

[0047] The substrate 10 is preferably provided with a coating in two successive steps, wherein in a first device 1 a base layer with a relatively small layer thickness is applied and, after hardening partially, the substrate is covered with a second, thicker layer in a subsequent device 2. Figure 6 shows schematically an arrangement of an installation with which this can be achieved.

[0048] The installation comprises an (un)winding station 3 at an inlet of the first device 1 for coating a substrate 10. The substrate, in this case a roll of polyester- reinforced PVC tarpaulin with a width in the order of 3 metres, is received on a roll of several tens to more than a hundred metres in length. A sheet 10 is removed therefrom with unwinding means, which are not shown but are deemed sufficiently known to an average skilled person, and guided into the first device 1 along a substrate track. A transparent base layer of a two- component polyurethane coating c1 with a dry thickness of about 7 microns is here deposited over the whole surface of the sheet in the above described manner. Next, substrate 10 is guided through a drying station 60 in which heating means

[0049] 65 heat the substrate 10 from below in order to evaporate volatile substances, such as solvents and precursors, in coating c1 therefrom before they reach the PVC material of the substrate, The heating means 65 comprise here one or more electrically or gas-fired infrared panels which (together) extend over substantially a full width of substrate 10 on a side of substrate 10 remote from nozzle 30. It is ensured here that a substrate temperature does not exceed a threshold value in the order of 70°C so that the threads of a polyester mesh incorporated in substrate 10 are not affected thereby.

[0050] The substrate 10 is guided with the thus almost wholly dried yet still sticky base layer c1 into the second device 2 along the defined substrate track over a set of conveyor rollers 15. The transparent base layer c1 is here covered in the above described manner with a relatively thick top layer c2 of the same polyurethane liquid; now with a final (dry) thickness of about 18 microns. The sufficiently dried and therefore sealing base layer c1 now forms a barrier to the underlying PVC substrate 10 so that volatile substances, such as solvents and precursors, present in top layer c2 are unable to penetrate through to the substrate 10.

[0051] Next, the substrate 10 is guided through a second dryer 70 in order to fully harden and dry the assembly of the base layer c1 and the top layer c2. Use is for this purpose made of a system of hot air blowers 75, which each guide a forced airflow with a temperature in the order of 65°C over the substrate. Finally, the substrate 10 is received in a winding (up) station 4 in which substrate 10 with a desired substrate length is wound into a roll again after being separated from the initial roller 3. A relatively thick coating in the order of 25 microns or more was thus composed of a relatively thin base layer with a thickness of several microns, such as 7 microns here, and a thicker top layer with a thickness of up to several tens of microns, such as 18 microns here, without affecting the vulnerable substrate material. The rolled-up product 4 is now ready to be sent to a customer.

[0052] Although the invention has been further elucidated above with reference to only several exemplary embodiments, it will be apparent that the invention is by no means limited thereto. On the contrary, many variations and embodiments are still possible within the scope of the invention for a person with ordinary skill in the art.

Claims

Claims:

1. Device for covering a flat substrate with a coating, particularly a sheetlike substrate, more particularly a plastic sheet, comprising an inlet and an outlet for a flat substrate and transport means for displacing the substrate therebetween along a substrate track at a track speed and comprising coating means with a feed for a coating liquid, which coating means are able and configured to apply the coating liquid to the substrate over at least substantially a whole surface, characterized in that the coating means comprise at least one nozzle which is able and configured to receive the coating in liquid form and direct a mist thereof toward the substrate track, that the nozzle is provided with a drive which carries the nozzle alternately in an oscillating movement which crosses the substrate track over a full width thereof, and that provided between the transport means of the substrate and the drive means of the nozzle is a control which activates the drive of the nozzle subject to an activation of the transport means.

2. Device according to claim 1, characterized in that the nozzle is moveable along a linear guide, which guide extends transversely of the substrate track.

3. Device according to claim 1 or 2, characterized in that the drive carries the nozzle alternately in an oscillating movement which crosses the substrate track beyond the full width thereof, wherein a turning point of the nozzle lies outside the substrate track on either side.

4. Device according to claim 1, 2 or 3, characterized in that a potential plane is provided on a side of the substrate track remote from the nozzle, that connected between the nozzle and the potential plane is a potential source which is able and configured to maintain a potential difference therebetween during operation, and that the potential source can be modulated and is provided with a control whereby the potential difference is adjustable during operation to the track speed of the substrate and / or a position of the nozzle.

5. Device according to claim 4, characterized in that the potential source is able and configured to maintain a static potential difference in the order of 10 kV between the nozzle and the potential plane.

6. Device according to claim 4 or 5, characterized in that the potential plane is formed by a flat metal plate on a side of the substrate track remote from the nozzle.

7. Device according to one or more of the preceding claims, characterized in that a similar further device for covering the substrate with a further coating is provided downstream of the nozzle and that provided between the nozzle and the further device are heating means which are able and configured to enter into heat-exchanging contact with the substrate track downstream of the at least one nozzle.

8. Device according to claim 7, characterized in that the heating means comprise an infrared radiator, particularly an infrared panel extending at least substantially over a width of the substrate track.

9. Device according to claim 8, characterized in that the infrared panel is arranged on a side of the substrate track remote from the nozzle.

10. Device according to claim 7, 8 or 9, characterized in that the further device comprises at least one further nozzle, and that further heating means are provided which are able and configured to enter into heatexchanging contact with the substrate track downstream of the at least one further nozzle.

11. Device according to claim 10, characterized in that the further heating means are able and configured to generate and maintain a heated airflow over the substrate track.

12. Method for covering a flat substrate with a coating, particularly a sheetlike substrate, more particularly a plastic sheet, wherein a coating liquid is applied to the substrate in liquid form while the substrate is carried along a substrate track, characterized in that the coating liquid is atomized over the substrate with a nozzle in an oscillating spraying movement, which spraying movement alternately crosses the substrate transversely at an at least substantially constant speed Vswhile the substrate is carried longitudinally at an at least substantially constant track speed vb.

13. Method according to claim 12, characterized in that a modulable potential difference V(t), which is adjusted to the track speed of the substrate and / or a position of the nozzle, is maintained between the nozzle and the substrate.

14. Method according to claim 13, characterized in that the potential difference V(t) is kept constant and imposes a radius rson the mist at the position of the substrate, for which: rs= b.vb / vs.

15. Method according to claim 14, characterized in that the potential difference V(t) imposes a radius rson the mist at the position of the substrate, for which: rs= b.vb / vs.

16. Method according to claim 13, characterized in that the potential difference in a movement of the nozzle over the substrate in each case decreases from a starting value to an end value.

17. Method according to claim 16, characterized in that the coating is supplied to the nozzle at a flow rate, which flow rate increases from a starting value to an end value during each movement of the nozzle over the substrate as the potential difference decreases.

18. Method according to one or more of the claims 12-17, characterized in that the coating comprises a multi-component polyurethane coating.

19. Method according to one or more of the claims 12-18, characterized in that the substrate comprises a plastic sheet, particularly a tarpaulin, more particularly a tarpaulin of polyvinyl chloride.

20. Method according to claim 19, characterized in that the coating is applied successively in successive layers, particularly an underlayer with a relatively small layer thickness followed by a subsequent layer with a greater layer thickness.

21. Method according to claim 20, characterized in that each layer is in each case followed by a drying below a threshold value of a temperature of the tarpaulin.

22. Method according to claim 21, characterized in that the drying comprises of a first drying of a relatively thin base layer followed by a second drying following application of a subsequent layer with a greater layer thickness.

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