EP1375014B1 - Coating process with a fluid film - Google Patents

Abstract

The method involves forming and pouring a curtain (5) of coating fluids (1-4) including an organic liquid or a mixture of coating material and organic liquid, from a nozzle (11) to the surface of a web material (10) conveyed at right angles to the curtain. The organic liquid has a surface tension of at most 40 mN/M. A fluid coating (6) is solidified on the surface of the web material.

Description

The invention relates to a liquid film coating method for single or multi-layer coating of an article, in particular a web-like substrate, such as a paper or cardboard material, a plastic film, a metal foil or tape or of web-like composite materials. However, the object to be coated may also be a general cargo.

The liquid film coating process, also known as curtain coating process, has been known for almost 100 years. In the beginning it was used for single-layer coating of piece goods, such as chocolates or furniture parts, as described in DE 145 517 A. Since about 1960, the process has also been used for the coating of endless belts, for example cardboard materials and aluminum foils, and has been used for example by C.C. Poirier in "Curtain Coating of Corrugated Paper Board" TAPPI 49 (10): 66A-67A. The first technically advanced curtain process for applying one or more layers to an endless substrate, particularly for photographic films and papers, is described in U.S. Patent No. 3,508,947. An example of the coating for producing magnetic recording materials is known from US Pat. No. 5,044,305, and the coating for producing a thermal recording material or an ink jet recording material is described in DE 100 33 056 A1.

Common to most processes is that for the coating fluid or fluids, water-based carrier fluids for each Coating material can be used. In order to adjust the surface tension of the carrier liquid and thereby the surface tension of the respective coating fluid as desired, wetting agents, ie the aqueous carrier liquids, are added to the coating fluids. The wetting agents are intended to reduce the surface tension of the carrier liquid. For example, DE 100 33 056 A1 proposes that the surface tension of a carrier fluid of a coating fluid which forms the lowest layer of the coating on the article to be coated should have a surface tension of 18 to 45 mN / m. DE 100 33 056 A1 further teaches that within the multiple layers of coating fluids, a surface tension gradient should prevail and that a coating fluid forming an upper layer of the coating on the article should have a lower surface tension than a coating fluid that will impinge on the article forms lower layer of the coating.

WO 96/23595 relates to a method and apparatus for applying thin fluid coatings. A system for coating a substrate with an ultrathin layer includes passing a substrate through a coating apparatus and forming a composite layer including a coating fluid and a carrier fluid. The composite layer flows at a rate high enough to form a continuously flowing fluid bridge of the composite layer to the surface of the substrate and to contact the substrate with the flowing composite layer to bring the coating layer between the substrate and the carrier fluid. The carrier fluid is removed while the coating fluid remains on the substrate as a coating layer.

DE 100 33 056 A1 relates to a method for producing an information recording material. Disclosed is a method for producing information recording materials, such as. A thermal recording material and an ink jet recording material which are excellent in coating properties and various other properties in good productivity.

The method comprises forming a coating composition film of a plurality of coating color layers on a substrate by curtain coating and drying the coating composition film to form part or all of layers for forming an information recording material, and either (1) adjusting the viscosity of each coating color for a plurality from coating color layers to at least 100 mPaZs and controlling the surface tension of the lowermost layer coating color to 18 to 45 mN / m, or (2) controlling the density of the topcoat layer coating color to be not more than 140% of the density the coating color is for a layer to be positioned adjacent thereto.

Experience has shown, however, that the known coating processes are problematic with respect to the stability of the curtain, in particular in the case of a multilayer curtain, but also in the case of a single-layered curtain. The problems increase with decreasing layer thickness and with decreasing conveying speed of the object to be coated and in particular at the meeting of low layer thickness and low conveying speed. The curtain layered from the coating fluid or fluids has a tendency to crack under production conditions.

It is therefore an object of the invention to improve the curtain stability of a liquid film coating process.

The invention relates to a liquid-film multilayer coating method in which a curtain of coating fluids is poured onto an article conveyed across the curtain, thereby forming a fluid coating on the article. The direction of conveyance of the object to be coated can be normal to the curtain or even deviate from the normal exhibit. The fluid coating formed on the article is then solidified.

According to the invention, the coating fluid is based on an organic liquid. The organic liquid may be solidifiable by a chemical reaction and itself form a coating material or a part of a coating material, or it may be a volatile carrier liquid which itself is not part of the coating material. In the context of the invention, the term coating material refers to that material which, after solidification, forms a coating or a single layer of a coating. The coating material may for example consist of binders, pigments and additives or may be another material serving a desired function. The coating fluid is therefore in many cases a mixture of different substances, according to the invention, at least one of these substances is an organic liquid. This organic liquid may in turn form part of the coating material or be a carrier liquid for a coating material. The mixture may be a solution or a dispersion, for example a suspension or emulsion.

The organic carrier liquid of a coating fluid formed as a mixture and, if an organic liquid forms or co-forms the coating material, this organic liquid has a surface tension of at most 40 mN / m, that is, at most 40 × 10 ^-3 N / m. Hereinafter, organic liquids solidifiable by chemical reaction and organic carrier liquids are referred to as organic liquids. The term "organic carrier liquid" is used only when the organic liquid in question is removed from the coating fluid after coating, for example by evaporation.

The use of an organic liquid makes it possible in particular to dispense with the use of wetting agents which, when using water or aqueous Solutions serve as a carrier liquid of the lowering of the surface tension. Accordingly, in a particularly preferred embodiment of the invention, the organic liquid is wetting agent-free.

Organic liquids, for example ketones, alcohols, esters, aliphatic and aromatic hydrocarbons and ethers, have the inherent surface tension required by the invention, so that when using a plurality of coating fluids an excellent uniformity of the layers in the film and on the article is established. The uniformity is satisfactory even for thin film thicknesses at low web speeds. Compared to aqueous carrier fluids, this means a significant increase in the operating window for the curtain coating process.

The organic carrier liquid may be a carrier liquid evaporating upon heating so that a single-layered coating or the layer of a multi-layered coating formed from the coating fluid is solidified by heating, namely by removing the carrier.

Alternatively, the coating fluid can also contain an organic liquid without volatiles which can be solidified by introduction of energy and which directly forms the coating material, in particular a monomer or oligomer, that is to say a reactive organic liquid. In this embodiment of the invention, the organic liquid may be a heat-polymerizable liquid to solidify the coating or only the relevant layer (s) by heating and polymerizing the organic liquid. The organic liquid may advantageously also be a polymerizable liquid by irradiation, preferably by UV irradiation or electron irradiation, so that the single-layer coating or the layer formed from the coating fluid of a multilayer coating can be polymerized by appropriate irradiation and thereby solidified.

The invention is particularly suitable for forming multilayer coatings. All or part of the multiple coatings may each be formed from a different coating fluid. Also, two or more of the layers may be formed by the same coating fluid. However, the organic liquid which forms or co-forms the coating fluids is more preferably an organic liquid having a surface tension of at most 40 mN / m. In other words, each of the coating fluids is formed according to the invention.

The coating fluids forming the curtain can be formed with different coating materials. The easiest way to meet the requirement of the same or at least approximately the same surface tension in mixtures by using each of the same organic liquid.

Fig. 1

die Beschichtung eines Bahnmaterials im Wege eines Mehrschichtvorhangverfahrens,

Fig. 2

die dynamische Oberflächenspannung eines wässrigen Trägermittels über dem Oberflächenalter eines Beschichtungsfluids,

Fig. 3

die Vorhangfallzeit in Abhängigkeit von der Vorhanghöhe, und

Fig. 4

die Oberflächenspannung in ein- oder mehrschichtigen Flüssigkeitsfilmen und vorhängen von wässrigen Lösungen mit Netzmitteln unter Verwendung einer Kaskadendüse.

The invention will be explained below with reference to figures and diagrams. Obviously, features which become apparent form the objects of the claims individually and in each feature combination. Show it:

Fig. 1

the coating of a web material by means of a multi-layer curtain method,

Fig. 2

the dynamic surface tension of an aqueous carrier over the surface age of a coating fluid,

Fig. 3

the curtain fall time depending on the curtain height, and

Fig. 4

the surface tension in single or multi-layer liquid films and curtains of aqueous solutions with wetting agents using a cascade nozzle.

Fig. 1 shows a coating apparatus for a liquid film coating by means of a multilayer curtain of coating fluids.

The coating apparatus comprises a nozzle device 11 for the production of the curtain from the plurality of coating fluids, in the exemplary embodiment of the four different coating fluids 1 to 4. However, the following explanations also apply to curtains of less than four coating fluids. They also apply to curtains with more than four different coating fluids. Such a curtain can be formed with ten or more layers. The number of different coating fluids in the curtain can be just as large.

The nozzle device 11 is a multilayer cascade nozzle, but could for example also be formed by a multi-slot slot nozzle. The coating fluids 1 to 4 are fed to the nozzle device 11 separately and metered in each case at a feed rate corresponding to the consumption. Such nozzle devices are known and therefore do not require a more detailed description.

Below the nozzle device 11, a material web 10 to be coated, with its surface to be coated facing the nozzle device 11, is conveyed endlessly by a conveying device in a conveying direction F. The material web 10 may be, for example, a paper web, a plastic film or a metal foil.

The nozzle device 11 forms transversely to the material web 10 in parallel, in the conveying direction F successively arranged slots through which the coating fluids 1 to 4 emerge and flow after their exit on a side facing away from the material web 10 top of the nozzle device 11 in the conveying direction F. The arrangement of the slots in the conveying direction F in succession, a multilayer liquid film is produced on the upper side of the nozzle device 11. The liquid film on the upper side of the nozzle device 11 already has the layer structure of the later coating of the material web 10. This multilayer liquid film flows on the upper side of the nozzle device 11 in the conveying direction F of the web 10 up to a front nozzle lip 15 of the nozzle device 11. After detachment from the nozzle lip 15 of the multilayer liquid film falls in free fall to form the curtain 5 substantially perpendicular to the Material web 10 conveyed through below the die lip 15 impinges on its upper side and forms a multilayered fluid coating 6. The width of the curtain 5 measured transversely to the conveying direction F is predetermined by a lateral boundary 7 on both sides.

The fluid coating 6 formed on the material web 10 is subsequently solidified, for example by drying.

The coating device further comprises a casting roll 12 arranged below the nozzle device 11, around which the material web 10 is deflected. The curtain 5 impinges on the upper side of the material web 10 in a web region which runs away from the casting roll 12 but is still supported by the casting roll 12. On the outside of the casting roll 12 upstream of the line of impact of the curtain 5 on the material web 10, a collecting trough 13 with integrated suction device 14 is further arranged. The drip pan 13 serves to fill and clean the nozzle device 11 and to form the liquid curtain. During these working phases, the nozzle device 11 is moved back horizontally into a parking position such that the coating fluids separating from the nozzle lip 15 fall into the directly underlying trough 13.

The casting roll 12 opposite wall of the tub 13 is also designed so that a narrow, precise and concentric gap between the trough 13 and the casting roll 12 and the casting roll 12 wraps around web 10 forms. The width of this gap must be small, preferably 0.5 mm, so that the trough 13 with its underside achieves a squeegee effect with which a substantial part of the air boundary layer entrained by the uncoated material web 10 is removed, i. is doctored off. With the consisting of a relatively narrow slot and a relatively large channel suction device 14 (vacuum cleaner principle) is also a part of the boundary layer air, which enters the concentric gap between trough 13 and casting roller 12, sucked by means of an external vacuum source. The scraping and suction of a substantial portion of the boundary layer air causes the curtain process can be operated even at high speeds of the web 10, without the remaining amount of boundary layer air along the transverse to the web 10 wetting line of the impinging curtain 5 between the curtain 5 and web 10th is withdrawn.

Each of the coating fluids 1 to 4 is or contains an organic liquid which simultaneously constitutes the coating material or a part thereof, or a mixture of a coating material and an organic carrier liquid which also refers to a solvent in the case of a solution of the coating material in the carrier liquid can be. All coating fluids 1 to 4, if they are mixtures, can be dispersions, for example emulsions or suspensions, or solutions. Mixed forms in which one or more of the coating fluids 1 to 4 is a solution and another or several other of the coating fluids 1 to 4 are a dispersion are also possible. However, it is essential that the liquid is organic in nature to ensure the stability of the curtain 5.

An important aspect of the curtain coating process, as already mentioned, is the stability of the curtain. A liquid curtain, for example, the curtain 5, is called stable, if it can not be torn open over long periods of operation, that is, over several hours of constantly present disturbing influences. Such a curtain is also referred to as suitable for production, that is, it can be used for industrial coating.

The invention has recognized that a low surface tension of at most 40 mN / m of the liquids used promotes curtain stability, especially when dynamic processes can be reduced or ideally avoided altogether. Such dynamic processes are flow processes and diffusion processes within the fluid layers and between the fluid layers of the curtain. Such dynamic processes are inventively reduced to practical insignificance in that the liquid in the case of a multilayer curtain is organic in nature. If the different coating materials of a multi-layer coating permit it, for mixtures with each other coating material, even a single organic liquid should be used for such different coating fluids of the curtain to obtain surface tension identity.

The problem of time-dependent flow and diffusion processes in liquid curtains is basically known, for example from "Surfactants: Static and Dynamic Surface Tension", YM Tricot, in Liquid Film Coating, Chapter 4, Chapman & Hall, London. In aqueous solutions, the surface tension of newly formed surfaces is a time-dependent and thus dynamic material property, because surface-active molecules contained therein first from the inside of the liquid diffuse the surface and be adsorbed there before lowering the surface tension. Above all, the diffusion process is strongly influenced by the viscosity of the liquid and by the local flow conditions.

Fig. 2 shows the relationship depending on the age of the free liquid surface, the characteristic time of the diffusion process and the wetting agent concentration. It is known that the characteristic time of the diffusion process depends on the type of wetting agent used, that is to say on its molecular structure, and on the wetting agent concentration. The characteristic time can take up to several seconds. With sufficiently long diffusion time, the equilibrium surface tension, that is, the static surface tension, is achieved. Until the static surface tension is reached, aqueous solutions mixed with wetting agents therefore have a changing surface tension.
It can be seen in particular from FIG. 2 that a low surface tension of 40 mN / m and preferably less with a low surface age of the aqueous solution in question can only be achieved if large amounts of a fast-diffusing wetting agent are added. Although adding wetting agents may improve curtain stability, it is problematic or even banned in many potential applications. In packaging materials such as food or drug wetting agents are undesirable or are not allowed by the relevant regulatory authorities, such as the FDA in the US, when the surfactant molecules come into contact with the product to be packaged and there is a risk that the product properties by on the Filling material passing wetting agent molecules could be changed. For materials to be printed after coating, such as in an ink-jet printing process, wetting agents can also be problematic because they can adversely affect wetting and bleeding of the ink on the material to be printed. The invention provides a remedy, especially when it is completely dispensed with the use of wetting agents, for which the invention just creates the best conditions. The invention thus opens up for mass production Particularly suitable curtain coating process new applications, namely in particular the above.

For the analysis of curtain stability, the characteristic diffusion time will be compared with the characteristic fall time of the curtain.

The fall time depends on the curtain speed V _V , which in turn is approximately determined by the initial velocity V ₀ dependent on the nozzle type, the drop height x of the curtain and the gravitational acceleration g: $${\mathrm{V}}_{\mathrm{v}}\mathrm{=}{\mathrm{<figure-callout\; id="0"\; label="V"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathrm{0}}\mathrm{+}\sqrt{\mathrm{2}\begin{array}{}\end{array}\mathit{gx}}$$

In practice, the initial velocity V ₀ can generally be neglected, since its dependence on the type of nozzle is usually low and the initial velocity is usually much smaller than the gravitational term in equation (1).

FIG. 3 shows a typical example of the dependence of the curtain falling time on the drop height x. For curtain heights of 50 to 300 mm, the curtain downtime is between 50 and 200 ms. This is a very short time, especially compared to the diffusion time required to produce in aqueous solutions the surface tension of not more than 40 mN / m, and preferably less, due to the short surface age in the curtain.

As described in "Curtain Coating" by K. Miyamoto and Y. Katagiri, Chapter 11c in Liquid Film Coating, Chapman & Hall, London, the theoretical stability criterion for the curtain can be quantified by equation (2) below: $$\mathrm{We}=\frac{{\mathit{\rho QV}}_v}{\sigma }=\frac{{\mathit{\rho UH}}_F{V}_v}{\sigma }>2$$

We are the dimensionless Weber number, which describes the relationship between inertial and surface tension forces, ρ and σ are the density and surface tension of the coating fluid, Q is the ratio volumetric flow / width, V _V is the curtain velocity according to equation (1), U is the Speed of the web to be coated and H _F is the wet thickness of the coating on the web.

The curtain coating process belongs to the class of so-called pre-metered coating processes, in which only just the exact amount of coating fluid is pumped to the nozzle device. In contrast to other coating methods, such as roller, knife or air knife method, the curtain method is operated without excess liquid. On the basis of the law of conservation of mass, it can therefore be concluded that the volume flow V * and thus also the volume flow per width Q depend on the casting width W, the web speed U and the wet film thickness H _F according to equation (3) below: $$\mathrm{V}\mathrm{*}\mathrm{=}\mathrm{QW}\mathrm{=}{\mathrm{UH}}_{\mathrm{F}}\mathrm{W}$$

In particular, the volume flow per width Q is determined by the web speed U and the wet film thickness H _F as follows: $$\mathrm{Q}\mathrm{=}{\mathrm{UH}}_{\mathrm{F}}$$

From equation (2), it can be concluded not only that the curtain stability depends on the surface tension σ and the volume flow / width Q, but that the curtain stability is ensured even with small values of Q as long as the surface tension σ is sufficiently small. From equation (4) it can further be concluded that for a given web speed U, the wet film thickness H _F or for a given film thickness H _F the web speed U can be reduced all the more, the lower the surface tension σ is. The production of very thin layers and Coating at very low web speeds using the curtain method is also of industrial importance because it allows extremely high uniformity and therefore high quality of the coating to be produced.

It is mentioned in US Pat. No. 3,632,374 that the minimum volume flow per width Q for aqueous solutions is about 0.5 cm ² / s. With this minimum value for Q, however, the curtain stability can hardly be kept stable over long periods of operation. Rather, when using aqueous solutions as carrier liquid, the minimum of Q for industrial applications should be at least 1.0 cm ² / s. On the other hand, however, this large value makes it impossible to produce thin layers and above all thin layers at simultaneously low line speeds, which is often a disadvantage in industrial applications of the curtain coating method.

Furthermore, the invention has recognized that not only the lowest layer of a multilayer curtain should have a low surface tension, as required in DE 100 33 056 A1. The effect of rapidly adjusting low surface tension on curtain stability is due to the ability to avoid local cross flows, called Marangoni flows, on the curtain surface, which are created by local surface tension differences due to perturbations. On the one hand, a liquid curtain has two external surfaces on which faults can attack. On the other hand, in a multi-layer application, the layers between the outermost layers must also have the ability to rapidly reduce their surface tension to these levels so that curtain stability is maintained even if the outer layers can not withstand the disturbances. A multilayer curtain is thus industrially robust or stable, in particular, when all the layers of the curtain have a surface tension of at most 40 mN / m according to the invention. Preferably, the surface tensions of all layers are even lower.

Further problems may arise when using the curtain coating method for aqueous solutions in combination with a cascade nozzle as they do is exemplified in Fig. 1. In particular, in the case of this nozzle configuration, an asymmetrical situation with respect to the surface tension results between the outer outer curtain surface which is in the conveying direction F of the material web 10 and the rear outer curtain surface.

As can be seen from FIG. 1, the origin of the front outer curtain surface at the slot exit of the uppermost layer 1 lies on the inclined plane of the cascade nozzle 11. Depending on the number of layers, this location may be far away from the nozzle lip 15. For example, when three fluid layers are formed, the distance from the slot exit to the nozzle lip 15 is about 150 mm. The age of the free film surface at the location of the die lip 15, that is, at the location where the curtain 5 begins, is determined by the flow rate of the multilayer fluid film on the inclined plane. The flow rate depends on the viscosities of the coating fluids, their densities and volumetric flows and on the angle of inclination. For a three layer fluid film having a viscosity of 50 mPas, the surface age at the die lip 15 is about 2 seconds. During this time, wetting agent molecules can diffuse to the liquid surface using aqueous carrier liquids and reduce the surface tension due to adsorption.

As shown schematically in FIG. 4, the front curtain surface at the beginning of the curtain, that is to say at the nozzle lip 15, therefore has a deep surface tension. If the flow time of the fluid film on the nozzle surface is long enough, the surface tension even reaches the equilibrium value corresponding to the static surface tension. Below the nozzle lip 15, the front outer curtain surface is stretched due to the gravitational acceleration, that is, there is a new liquid surface, which contains less wetting agent molecules immediately after their formation, so that the local surface tension increases again before they due to wetting agent and adsorption after a certain fall time decreases again. Fig. 4 shows this relationship and shows beyond also the profile of the surface tension as a function of the surface age for the rear outer curtain surface.

The origin of the rear outer curtain surface (back) is located on the nozzle lip 15. Also due to the gravitational acceleration, this surface is stretched immediately after their formation, so that the surface tension, as described above, decreases only relatively slowly. As a result, there is a surface tension difference Δσ between the front and rear outer curtain surfaces, which may be very large at the die lip 15 and decreases with increasing fall time, that is, depending on the curtain height. However, this surface tension difference Δσ between the two outer curtain surfaces leads to an influence on the curtain drop curve. In particular, the curtain can be bent directly to the rear immediately below the nozzle lip 15. This phenomenon is known by the term "tea pot effect" and leads to a reduction in curtain stability, because it becomes difficult to reconcile the falling curve of the curtain with the shape of the curtain side boundary 7. On the other hand, due to vortex formation along the trailing edge of the die lip 15, lines and streaks in the curtain may result, thereby adversely affecting the quality of the coating on the web 10.

According to the invention, the disadvantages described above are avoided by carrying out the curtain coating process with liquids based on organic substances, that is to say on an organic liquid. Many of these liquids have an inherently deep surface tension of less than 40 mN / m. A large number of organic carrier liquids industrially used in other applications, in particular solvents, have a surface tension in the range of 15 to 35 mN / m, as is preferred for the purposes of the invention. The problem of dynamic surface tension is eliminated in organic liquids, because the surface tension does not have to be reduced by dynamic effects such as diffusion and adsorption. The surface tension of such liquids is therefore small even for very short surface ages, and lower as the maximum value of 40 mN / m not to be exceeded according to the invention. This could be demonstrated by directly measuring the surface tension in the curtain using the Mach angle method as described in the article "Surfactants: Static and Dynamic Surface Tension", which is based on YM Tricot. It is also advantageous that the surface tension does not have to be reduced only by adding wetting agent molecules. In multi-layer applications, the requirement of a deep surface tension for all layers is met from the outset.

Suitable organic carrier liquids are both low-boiling and high-boiling liquids, as long as the surface tension does not exceed the value of 40 mN / m. Suitable carrier liquids are in particular ketones such as acetone, butanone and cyclohexanone, further alcohols such as ethanol, butanol and cyclohexanol, further esters such as ethyl acetate, butyl acetate and finally also aliphatic and aromatic hydrocarbons such as benzine and toluene. Not to be forgotten are ethers such as tetrahydrofuran or those with other or more functional groups such as chlorobenzene or 2-methoxy-1-propyl acetate. Mixtures of the abovementioned carrier liquids can also be used in the context of the invention and are subsumed under the term carrier liquid.

The advantages according to the invention are achievable not only with physically-thermally drying systems containing volatile carrier liquids, but also with those which contain reactive organic liquids, including liquid mixtures, under the action of high-energy radiation, preferably UV rays or electron beams, or through the action of higher Polymerize temperatures and thereby cause the solidification of the fluid coating on the web.

When using low-boiling, evaporating organic carrier liquids, time-dependent problems such as changes in concentration, changes in viscosity, Foaming and the like which are typical in roll and doctor blade processes because, due to the closed metering system in the curtain coating process, the free liquid surface area is smaller. Accordingly, the fire and explosion hazards and the occupational hygiene problems are reduced due to the significantly reduced evaporation losses.

Claims (10)

1. Liquid film coating process, in which:

   a) a curtain (5) comprising a plurality of layers of coating fluids (1, 2, 3, 4) is poured on an object (10) being conveyed at right angles to said curtain (5) and

   b) a fluid coating (6) is formed on said object (10) as a result,

   c) wherein mixtures of a coating material and a carrier liquid which can be solidified on the object (10) are used as coating fluids (1),

   c1) wherein the coating material of at least one of the mixtures is different from the coating material of at least one other of the mixtures,

   c2) wherein a plurality of the coating fluids (1, 2, 3, 4) of the plurality of layers are a mixture of a coating material and the same carrier liquid, which carrier liquid is an organic liquid,

   d) wherein the layers of the curtain each have a surface tension of at most 40 mN/m,

   e) and wherein the fluid coating (6) is solidified.
2. Liquid film coating process in accordance with claim 1, characterised in that the organic liquid has a surface tension of at most 35 mN/m.
3. Liquid film coating process in accordance with one of the above claims, characterised in that the organic liquid is free from wetting agent.
4. Liquid film coating process in accordance with one of the above claims, characterised in that the organic liquid is removed from the fluid coating (6) formed from the mixture on the object (10) by evaporation in order to solidify said coating (6).
5. Liquid film coating process in accordance with one of the claims 1 to 3, characterised in that a liquid that can be polymerised by heating is used as the organic liquid in order to solidify the fluid coating (6) formed on the object (10) by polymerising the organic liquid.
6. Liquid film coating process in accordance with one of the claims 1 to 3, characterised in that a liquid that can be polymerised by irradiation, preferably UV irradiation or electron beams, is used as the organic liquid in order to solidify the fluid coating (6) formed on the object (10) by polymerising the organic liquid.
7. Liquid film coating process in accordance with one of the above claims, characterised in that the curtain (5) comprises the plurality of layers of coating fluids (1, 2, 3, 4).
8. Liquid film coating process in accordance with one of the above claims, characterised in that the coating material of each of the mixtures is different from the coating material of each of the other mixtures.
9. Liquid film coating process in accordance with one of the above claims, characterised in that at least one of the coating fluids (1, 2, 3, 4) of the plurality of layers is an organic liquid that can be solidified by introducing energy, preferably by polymerisation.
10. Liquid film coating process according to one of the above claims, characterised in that all coating fluids (1, 2, 3, 4) of the plurality of layers are organic liquids that can be solidified by introducing energy, preferably by polymerisation, wherein at least two of the organic liquids that are adjacent to each other are different.

Images