Method and device for drying a fluid film applied to a substrate

Abstract

A method for drying a fluid film, which is applied to a surface of a substrate and includes a vaporizable liquid, includes following steps: transporting the substrate on a transport surface of a transport device along a transport direction through a drying device; vaporizing the liquid by way of a heat source having a heating surface, wherein the heating surface is disposed at a distance of 0.1 mm to 5.0 mm opposite to a surface of the substrate; and removing a vaporized liquid in a direction of the heat source.

Claims

1. A method for drying a fluid film, which is applied to a surface of a substrate and includes a vaporizable liquid, comprising following steps: transporting the substrate on a transport surface of a transport device along a transport direction through a drying device; vaporizing the liquid by way of a heat source having a heating surface, wherein the heating surface is disposed at a distance of 0.1 mm to 15.0 mm opposite the surface of the substrate, wherein a heat is essentially transmitted from the heating surface to the fluid film by way of direct heat conduction; removing a vaporized liquid in a direction of the heat source; and heating the transport surface by way of an additional heat source, wherein in the step of vaporizing the liquid by way of the heat source, the heating surface has a first temperature T.sub.G, and the first temperature T.sub.G of the heating surface is controlled as a function of an interface temperature T.sub.I of the fluid film, wherein the interface temperature T.sub.I of the fluid film is detected by an infrared measuring device, and wherein in the step of heating the transport surface, the transport surface has a second temperature T.sub.H, and the second temperature T.sub.H of the transport surface generated by the additional heat source is controlled as the function of the interface temperature T.sub.I of the fluid film.

2. A method according to claim 1, wherein the first temperature T.sub.G of the heating surface is controlled in a range of 80° C. to 200° C.

3. A method according to claim 1, wherein the second temperature T.sub.H of the transport surface is controlled so that a following relationship is met:
T.sub.H=T.sub.I+ΔT, where T.sub.I ranges from 5° C. to 40° C. and ΔT ranges from 5 to 10° C.

4. A method according to claim 1, wherein in the step of vaporizing the liquid, the liquid is carried out in a nitrogen or carbon dioxide atmosphere.

5. A method according to claim 1, wherein in the step of vaporizing the liquid, the heating surface facing the substrate is disposed at a distance of 0.2 mm to 5.0 mm opposite the surface of the substrate.

6. A method according to claim 1, wherein the second temperature T.sub.H of the transport surface is controlled so as to always be lower than the first temperature T.sub.G of the heating surface.

7. A method according to claim 1, wherein a temperature difference between the first temperature T.sub.G of the heating surface and the second temperature T.sub.H of the transport surface is controlled so that the temperature difference between the first temperature T.sub.G of the heating surface and the second temperature T.sub.H of the transport surface changes along the transport direction.

8. A method according to claim 1, wherein one heat source through which a flow is possible is used as the heat source, and in the step of removing the vaporized liquid, the vaporized liquid is removed through the one heat source.

9. A method according to claim 1, wherein in the step of vaporizing the liquid, an electrical heating source is used as the heat source.

10. A method according to claim 1, wherein in the step of vaporizing the liquid, a heat exchanger is used as the heat source.

11. A method according to claim 1, wherein in the step of transporting the substrate, the transport device including at least one rotatable roller, a lateral face of which forms the transport surface, is used.

12. A device for drying a fluid film, which is applied to a surface of a substrate and includes a vaporizable liquid, comprising: a transport device for transporting the substrate on a transport surface along a transport direction; a heat source that is provided opposite to the substrate and has a heating surface, which is disposed at a distance of 0.1 to 15.0 mm opposite the surface of the substrate so that a heat is essentially transmitted from the heating surface to the fluid film by way of direct heat conduction; and a device for removing a vaporized liquid in a direction of the heat source, wherein a first controlling device is provided for controlling a first temperature T.sub.G generated by the heating surface as a function of an interface temperature T.sub.I of the fluid film, wherein the interface temperature T.sub.I of the fluid film is detected by an infrared measuring device, and wherein a second controlling device is provided for controlling a second temperature T.sub.H of the transport surface as the function of the interface temperature T.sub.I of the fluid film.

13. The device according to claim 12, wherein an additional heat source is provided for heating the transport surface.

14. A device according to claim 12, wherein a temperature difference between the first temperature T.sub.G of the heating surface and the second temperature T.sub.H of the transport surface is controlled by way of the first controlling device and/or the second controlling device so that the temperature difference between the first temperature T.sub.G of the heating surface and the second temperature T.sub.H of the transport surface changes along the transport direction.

15. A device according claim 12, wherein a device for rinsing a housing surrounding the transport device with a nitrogen or carbon dioxide atmosphere, is provided.

16. A device according to claim 12, wherein the heating surface facing the substrate is disposed at a distance of 0.2 mm to 5.0 mm opposite the substrate surface.

17. A device according to claim 12, wherein one heat source through which a flow is possible is used as the heat source so that the vaporized liquid can be removed through the one heat source.

18. A device according to claim 12, wherein the heat source is an electrical heating source.

19. A device according to claim 12, wherein the heat source is a heat exchanger.

20. A device according to claim 12, wherein the transport device comprises a rotatable roller, a lateral face of which forms the transport surface.

Description

(1) The invention will be described in more detail hereafter based on the drawings: In the drawings:

(2) FIG. 1 shows a schematic illustration to explain the variables used in the formulas;

(3) FIG. 2 shows the interface temperature as a function of the gas temperature at a predetermined transport surface temperature;

(4) FIG. 3 shows the interface temperature as a function of the transport surface temperature at a predetermined gas temperature;

(5) FIG. 4 shows the mass diffusion rate as a function of the gas temperature at a predetermined transport surface temperature;

(6) FIG. 5 shows the mass diffusion rate as a function of the transport surface temperature at a predetermined gas temperature;

(7) FIG. 6 shows the drying duration as a function of the gas temperature at a predetermined transport surface temperature;

(8) FIG. 7 shows the drying duration as a function of the transport surface temperature at a predetermined gas temperature;

(9) FIG. 8 shows a schematic sectional view through one exemplary embodiment of a diffusion dryer according to the invention;

(10) FIG. 9 shows a schematic detailed view according to FIG. 8; and

(11) FIG. 10 shows a schematic sectional view through another exemplary embodiment of a diffusion dryer according to the invention.

(12) The theoretical principles of the method according to the invention will be briefly described hereafter based on one-dimensional equations for the diffuse mass transport as a function of the temperature.

(13) The variables used in the following equations are essentially apparent from FIG. 1.

(14) The temperature gradient in the air gap above the interface of the fluid film fulfills the energy equation, which can be stated as follows for the gas phase:

(15) $\frac{d^{2}T}{d{y}^2}-\left(\frac{\stackrel{.}{m}{C}_P}{{\lambda }_G}\right)\frac{dT}{dy}=0$

(16) Upon solving this diffusion equation, the following general solution is obtained:

(17) $T=c_1+c_2\exp \left(\frac{\stackrel{.}{m}{C}_P}{{\lambda }_G}y\right),$
where c.sub.1 and c.sub.2 represent two constants of integration still to be defined. These can be determined via suitable boundary values. These boundary values are as follows:

(18) $y=0\frac{dT}{dy}{.Math.}_{I/G}=\frac{\left(1-f\right)*\left(T_H-T_I\right)}{\left(\frac{{\mu }_G\Delta h_{\mathrm{LH}}}{2T_I}-{\lambda }_G\right)*\left(\frac{H}{{\lambda }_S}+\frac{h}{{\lambda }_L}\right)}$$y={\delta }_G,T=T_G$
If the above equations are solved by inserting the boundary values according to c.sub.1 and c.sub.2, values are obtained for these variables which allow the temperature profile in the gas phase to be indicated as follows:

(19) $T=T_G-\frac{\left(1-f\right)*\left(T_H-T_I\right)*\left\{\exp \left(\frac{\stackrel{.}{m}{C}_P}{{\lambda }_G}{\delta }_G\right)-\exp \left(\frac{\stackrel{.}{m}{C}_P}{{\lambda }_G}y\right)\right\}}{\stackrel{.}{m}{C}_P*\left(\frac{{\mu }_G\Delta h_{\mathrm{LH}}}{2{\lambda }_G{T}_I}-1\right)*\left(\frac{H}{{\lambda }_S}+\frac{h}{{\lambda }_L}\right)}$

(20) For y=0, T=T.sub.1 is obtained. This allows the interface temperature T.sub.1, which is to say the temperature on the free surface of the fluid film, to be calculated as follows:

(21) $T_I=T_G-\frac{\left(1-f\right)*\left(T_H-T_I\right)*\left\{\exp \left(\frac{\stackrel{.}{m}{C}_P}{{\lambda }_G}{\delta }_G\right)-1\right\}}{\stackrel{.}{m}{C}_P*\left(\frac{{\mu }_G\Delta h_{\mathrm{LH}}}{2{\lambda }_G{T}_I}-1\right)*\left(\frac{H}{{\lambda }_S}+\frac{h}{{\lambda }_L}\right)}$

(22) The mass diffusion rate per unit area can be calculated as follows based on the temperature gradient that is present on the free surface:

(23) $\stackrel{.}{m}=\frac{\left(1-f\right)*{\mu }_G*\left(T_H-T_I\right)}{\left({\mu }_G\Delta h_{\mathrm{LH}}-2{\lambda }_G{T}_I\right)*\left(\frac{H}{{\lambda }_S}+\frac{h}{{\lambda }_L}\right)}$

(24) The drying time for the material to be coated can be calculated as follows:

(25) $t_d=\frac{M}{\stackrel{.}{m}}=\frac{{\rho }_L*h*\left({\mu }_G\Delta h_{\mathrm{LH}}-2{\lambda }_G{T}_I\right)*\left(\frac{H}{{\lambda }_S}+\frac{h}{{\lambda }_L}\right)}{\left(1-f\right)*{\mu }_G*\left(T_H-T_I\right)}$

(26) Using the above set of equations, the one-dimensional diffusion heat transfer problem and the problem of the associated release of mass and of the mass transport can be solved analytically.

(27) Using the boundary values described below, the mass diffusion rate of the vaporized liquid and the drying time were calculated. The calculation was made under the following assumptions:
H=300 μm, h=10 μm, δ.sub.G=300 μm
f=0.2, T.sub.G=350 K, T.sub.H=295 K

(28) The following material properties were assumed to be constant, despite the temperature changes:
μ.sub.G=1.8×10.sup.−5 kg/(ms), λ=0.024 W/(mK), C.sub.P=1.012 KJ/(KgK)
λ.sub.L=0.6 W/(mK), ρ.sub.L=1000 kg/m.sup.3, Δh.sub.LH=2260 KJ/Kg λ.sub.S=0.12 W/(mK)

(29) The drying of the fluid film according to the invention is essentially determined by controlling the second temperature T.sub.H on the transport surface and by the first temperature T.sub.G of the heat source. The heat source is provided at a distance δ.sub.G from the interface of the fluid film facing the gas phase.

(30) FIG. 2 shows the interface temperature T.sub.I as a function of the first temperature T.sub.G of the heat source or gas phase. FIG. 3 shows the interface temperature T.sub.I as a function of the temperature T.sub.H of the transport surface.

(31) As is apparent in particular from FIGS. 3 to 5, the mass diffusion rate can be achieved by increasing the first temperature T.sub.G. It is also apparent that an increase in the second temperature T.sub.H causes a decrease in the mass diffusion rate.

(32) As is apparent in particular from FIGS. 6 and 7, a reduction in the drying time can only be achieved when the second temperature T.sub.H is selected to be low and the first temperature T.sub.G is selected to be high. Both temperatures T.sub.G and T.sub.H can be set so that T.sub.I can be controlled. For example, T.sub.I can be kept at room temperature.

(33) FIG. 8 shows a schematic sectional view of one exemplary embodiment of a diffusion dryer according to the invention. A supply roller 2, on which the substrate 3 to be coated is accommodated, is located in a housing 1. The substrate 3 is guided over first tension pulleys 4a, 4b onto a transport roller 5. A lateral or transport surface 6 of the transport roller 5 is surrounded by a drying device 7 in some regions, preferably over an angle of 180 to 270°. Upstream of the drying device 7, a slotted nozzle tool denoted by reference numeral 8 is provided for applying a fluid film F onto the substrate 3. At least one further tension pulley 9, over which the substrate 3 is rolled onto a roller 10, is located downstream of the drying device 7. Reference numeral 11 denotes a roller cleaning device, which is disposed downstream of the drying device 7 and upstream of the coating tool 8.

(34) The drying device 7 comprises an additional housing 12. The additional housing 12 is provided with suction devices 14, which are used to suction off a liquid vapor escaping from the fluid film F.

(35) As can be seen in particular in combination with FIG. 9, a heat source 13 accommodated in the additional housing 12 can be formed of resistance wires, for example, which are disposed in a grid-shaped manner. The heating wires form a heating surface G, which is disposed at a distance δ.sub.G of 0.1 mm to 1.0 mm, for example, opposite the interface I of the fluid film F. The suction devices 14, which are not shown in detail in FIG. 9, result in the formation of a flow, which develops essentially perpendicularly to the transport surface 6 and is identified in FIG. 9 by arrows. Advantageously a negative pressure is generated in the intermediate space between the interface I and the heating surface H by the suction devices 14. This prevents potentially flammable liquid vapors from escaping into the surroundings. The housing 1 can additionally be rinsed with a protective atmosphere so as to prevent a risk of fire or explosion by escaping flammable liquid vapors.

(36) The device according to the invention shown in FIG. 8 has a particularly compact design. Instead of one transport roller 5, it is also possible to use multiple transport rollers 5. A drying section can thus be enlarged, which makes it possible to dry relatively thick fluid films F as well. Moreover, the device according to the invention can be used in combination with conventional convection dryers. For this purpose, the device according to the invention is expediently used upstream of a conventional convection dryer. By using the device according to the invention in combination with a conventional convection dryer, the energy that is used to operate the conventional convection dryer can be drastically reduced.

(37) FIG. 10 shows a schematic sectional view through a further exemplary embodiment of a diffusion dryer according to the invention or of a further drying device 15. The substrate 3 is again accommodated on a supply roller 2 and is transported by a driven roller 16. Reference numeral 8 again denotes a slotted nozzle tool for applying a fluid film onto the substrate 3 and is disposed upstream of an additional drying device 15.

(38) The additional drying device 15 includes heating elements 17 in the transport direction T, which can be plate-shaped resistance heating elements disposed behind one another in the transport direction T. In this embodiment, the heating elements 17 form an essentially closed heating surface H and are disposed at a distance δ.sub.G of 2 to 10 mm from a substrate surface. The additional drying device 15 thus includes a rectangular channel K having the height δ.sub.G, through which the substrate 3 is guided in the transport direction T.

(39) At the upstream end of the additional drying device 15, air L is suctioned into the channel K by way of the suction device 14 and moved counter to the transport direction T in the direction of the suction device 14 in a counter flow. A flow velocity is 30 cm/s to 3 m/s, for example.

(40) An additional transport surface 18 of the additional drying device 15 is also designed to be planar here. It can likewise be designed to be heatable (not shown here).

LIST OF REFERENCE NUMERALS

(41) 1 housing 2 supply roller 3 substrate 4a, 4b tension pulley 5 transport roller 6 transport surface 7 drying device 8 slotted nozzle tool 9 additional tension pulley 10 roller 11 roller cleaning device 12 additional housing 13 heat source 14 suction device 15 additional drying device 16 driven roller 17 heating element 18 additional transport surface δ.sub.G distance F fluid film G heating surface I interface L air T transport device