WO2007107367A1

WO2007107367A1 - Improved method and apparatus for drying water-containing coatings

Info

Publication number: WO2007107367A1
Application number: PCT/EP2007/002548
Authority: WO
Inventors: Pieter Jozef Luypaert; Carlo Groffils
Original assignee: MEAC NV
Current assignee: MEAC NV
Priority date: 2006-03-22
Filing date: 2007-03-22
Publication date: 2007-09-27
Anticipated expiration: 2008-09-22
Also published as: GB0606195D0

Abstract

A method and apparatus for drying water-containing coatings above 80 microns using microwave or RF ovens.

Description

IMPROVED METHOD AND APPARATUS FOR DRYING WATER- CONTAINING COATINGS

Field of invention

The present invention is broadly concerned with an improved apparatus and a method for solidifying and/or drying a coating on a porous substrate especially the drying or solidifying of thick water-containing coatings.

Background of the invention

When a solvent loaded coating is to be solidified or dried, organic or inorganic solvents, such as alcohols and/or water need to be removed from the coating. For some methods of application such as casting, these solvents are necessary to handle the coating products before the actual coating or necessary to spread the coating over the substrate that is being coated. A coating will usually be called "dry" once its solvent, e.g. water, content approaches 10-12%.

In conventional infra-red, UV or hot air drying of coatings on substrates, the temperature obtained during the drying process at the surface of the coating is usually higher than the temperature obtained in the layer of the coating closest to the substrate. In the case of hot air, IR or UV heating, this is mainly due to the heat energy impinging directly on the surface as well as the non- transparency of most coatings to the relevant wavelengths and in the case of hot air, this is mainly due to the fact the air is flowing over the surface of the body. This can lead to the drying of this surface prior to the drying of the bulk of the coating and therefore to the formation of a skin of a few microns or less at the surface of the coating. The presence of this skin has two major drawbacks. The first one is that it slows down the evaporation of the solvent(s) and/or suspending liquid(s) present in the bulk of the coating. The second one is that the pressure exerted on the skin by the solvent and/or suspending liquid vapours erupting from within the coating leads to surface defects such as bubbles or craters formed by burst bubbles. In the case of porous solid substrates such as e.g. wood, the drying process is complicated by the absorption of solvents and/or suspending liquid(s) by the substrate. This absorption is often responsible for the formation of a skin at the surface of the coating even prior to heating.

Summary of the invention

An object of the present invention is to provide an improved method and apparatus for drying water-containing coatings on porous substrates. An advantage of the present invention is that it permits an accelerated, defect-free drying of coatings on porous substrates.

Broadly speaking, the invention is based on the unexpected finding that a coating and in particular a water-containing coating applied on a non-porous or porous substrate can be dried in a fast and efficient way, without introducing surface defects or by reducing them, by the use of microwave or RF energy. For example, microwave energy applied to a porous substrate such as wood, pre-heats the substrate and drives moisture the surface. The vapour pressure of this moisture is preferably greater than the vapour pressure of the water in the coating to thereby prevent water from entering the substrate. The coating may be dried or solidified in any suitable way, e.g. by microwave or RF energy.

An embodiment of the present invention relates to a method for drying a water- containing coating on a porous substrate comprising the steps of: (i) creating one or more means at or in the surface of the porous substrate for preventing the exchange of water between the coating and the porous substrate (ii) applying the water-containing coating on the porous substrate in a thickness of 80 microns or more; and (iii) heating up the coating, wherein at least one of said one or more means for preventing the exchange of water is a raised relative water content at or in said surface of said porous substrate. In other words, an embodiment of the present invention relates to a method for drying a water-containing coating on a porous substrate comprising the steps of: (i) applying the water-containing coating on the porous substrate to a thickness of at least 80 microns, and

(ii) heating up the water-containing coating, wherein one or more means for preventing the exchange of water between the water-containing coating and the porous substrate are created at or in the surface of the porous substrate prior to step (i), wherein at least one of the one or more means for preventing the exchange of water is a raised relative water content at or in said surface of said porous substrate.

This embodiment has the advantage to efficiently prevent the formation of a dry skin on the surface of the coating. It therefore permits a fast and defect- free drying.

As an additional feature of the present invention the water-containing coating can be more than 80 micron thick, for example the coating can be 120 micron thick or thicker or 350 micron or thicker. Due to the controlled drying of the coating, thick coatings can be efficiently dried.

As an additional feature of the present invention the water-containing coating may be applied in a single pass. Due to the controlled drying of the coating, the need for multiple thinner coatings is reduced.

As additional feature of the present invention the surface of the substrate may be structured, e.g. include a three-dimensional pattern or ornamentation such as waves, wavelets, grooves, ridges, knurling, etc. The structured surface can have a depth of 1 mm or more, for example 0.5 mm or more or 1 cm or more. The structure has peaks and valleys. By depth is meant that the distance from an envelope of the surface which touches the upper peaks of the structure to the valleys of the structure. Due to the rapid drying of the coating, there is reduced sag, slip or displacement of the coating material while stiii wet even when applied to structured surfaces.

As an additional feature of the present invention, at least one of the one or more means for preventing the exchange of water may be a primer. The use of a primer is advantageous because it assures a very efficient barrier against water transfer from the coating to the substrate.

As another additional feature, the raised relative water content is at least equal to the relative water content of the water-containing coating at or in the surface of the porous substrate. The use of a raised relative water content as a means for preventing the exchange of water is particularly advantageous as it is an inexpensive, ecological and efficient barrier against the transfer of water from the coating to the substrate.

As another additional feature, the raising of the relative water content is at least partially due to internal water migration from within the porous substrate as the result of an elevation of the temperature of this porous substrate, wherein said elevation is due to microwave or RF heating, preferably microwave heating. This is advantageous because the microwave or RF heating raises the vapour pressure of the water at the surface of the porous substrate that acts as a barrier to the water entering the porous substrate from the coating. This is better than IR or hot air heating for instance, because these dry the outer surfaces.

Preferably the temperature of the substrate exceeds the temperature of the coating after this elevation of temperature. Most preferably, the temperature of the porous substrate exceeds the temperature of the coating by at least 10 ⁰C after this elevation. In this particularly advantageous feature of the present invention, the water used to create the barrier at the surface of the substrate is taken from the bulk of the substrate itself which is both practical and inexpensive. As another additional feature, the raising of the relative water content at the surface of the substrate is at least partially obtained by applying water on the porous substrate prior to the application of the coating. This is both economical and ecological since it relies only on the use of water, i.e. a commonly available material.

As yet another additional feature of the present invention, the heating up of the coating after its application may be performed by microwave or RF heating, This is advantageous because microwave or RF heating permits to obtain a higher temperature close to the substrate than close to the surface of the coating. This reduces the tendency of a skin to form on the surface of the coating.

As yet another additional feature of the present invention, the heating up of the water-containing coating using microwave or RF energy to solidify or dry the water-containing coating follows immediately after the coating applying step.

The present invention also provides a system for drying a water-containing coating on a porous substrate comprising: means for raising the relative water content at or in the surface of said porous substrate, means for applying said water-containing coating on said porous substrate to a thickness of at least 80 microns, and means for heating up said water-containing coating.

As an optional feature of the system of the present invention, the means for applying the water-containing coating enable the applying of the water- containing coating to a thickness of at least 80 microns in a single pass.

The present invention will now be described with reference to the following drawings. Brief description of the drawings

Figure 1 is a schematic view illustrating the skin formation at the surface of a non-porous substrate as known from the prior art.

Figure 2 is a schematic view illustrating the use of microwaves to avoid skin formation at the surface of a coating on a non-porous substrate as known from the prior art.

Figure 3 is a schematic view illustrating a skin formation at the surface of a coating on a porous substrate as known from the prior art.

Figure 4 is a schematic view illustrating an embodiment of the present invention.

Figure 5 is a schematic view illustrating another embodiment of the present invention.

Figure 6 is a schematic view illustrating another embodiment of the present invention.

Figure 7 is a schematic view of a system for treating coatings in accordance with an embodiment of the present invention.

Detailed description of the illustrative embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. Any reference signs in the claims shall not be construed as limiting the scope. The drawings described are only schematic and are non- limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term « comprising » is used in the present description and/or claims, it does not exclude the presence of other elements or steps.

Where an indefinite article is used when referring to a singular noun e.g. « a », « an » or « the », this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in the description and/or in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Solidification of a coating, which has been applied by a solvent coating technique, refers to forming a mechanically stable coating which will no longer move or drip under its own weight. It is useful to solidify the coating as quickly as possible when the coating is applied to a substrate with a structured surface. Drying is reduction of the solvent in the coating to a level such that it can be stored, e.g. less than 12% by weight of solvent in the coating, particularly less than 12% of the solvent, e.g. water, in any part of the coating.

The present invention does not exclude the use of additional means for delaying film formation on the surface of the coating, e.g. by use of specific chemicals as additives.

In accordance with some embodiments of the present invention, one way to shorten the drying time is to use microwave or RF heating instead of traditional infra-red, UV or hot air heating. As the wavelengths are higher, the transparency toward microwaves or RF is usually much better. It has been found surprisingly that in microwave or RF heating of wet coatings which are thicker than 80 micron, the total mass of the coating is heated and, contrarily to traditional drying methods, it is close to the substrate that the coating's temperature is usually the highest. The result is that water is forced towards the exposed surface of the coating from within the coating. Because of this, the surface has a lesser tendency to dry and to form a skin. This enables a faster evaporation of the solvent(s) and/or suspending liquid(s) such as e.g. water and therefore a faster drying. It also reduces the formation of defects as the softer fluid surface of the coating allows release of the water vapour with less defects generated. It has been found that in the case of porous solid substrates such as e.g. wood, the drying process for a coating especially a thick coating such a one greater than 80 micron, is complicated by the absorption of solvents and/or suspending liquid(s), such as e.g. water, by the substrate. This absorption towards the substrate increases the drying of the exposed surface of the coating and is often responsible for the formation of a skin at the surface of the coating even to prior heating.

Without being limited by theory, the following explanation is given relating to aspects of the present invention. As soon as a water-containing coating is applied to a substrate the water begins to evaporate and a skin starts to form on the surface. If the substrate is porous the water may also be drawn into the substrate thus removing water from close to the surface of the coating and accelerating the formation of a skin. The absorption in the substrate depends upon the diffusion of the water through the substrate and the time. Initially, the water concentration in the coating is uniform. As time progresses the water is drawn from the surface into the surrounding air and is drawn into the substrate. This can result in a thick skin forming which impedes further release of the water and can result in the formation of bubbles. If these bubbles burst, craters are formed. The creation of a relative water content at or in the surface of the porous substrate, as taught in embodiments of the present invention, solves this problem. The method can also include creating one or more additional means at or in the surface of the porous substrate for preventing the exchange of liquid between the coating and the porous substrate. Hence, an embodiment of the present invention relates to a method for drying a water-containing coating on a porous substrate comprising the steps of: (i) applying said water-containing coating on said porous substrate to a thickness of at least 80 microns, and (ii) heating up said water-containing coating, characterised in that one or more means for preventing the exchange of water between said water-containing coating and said porous substrate are created at or in the surface of said porous substrate prior to step (i),

Wherein at least one of said one or more means for preventing the exchange of water is a raised relative water content at or in said surface of said porous substrate.

The coating can be more than 80 micron thick, for example the coating can be 120 micron thick or thicker, e.g. 350 micron or more or 800 micron or more. The thickness of the coating may only be limited by the power of the microwave or RF apparatus that is used.

The coating may be applied in a single pass. Due to the controlled drying of the coating, the need for multiple thinner coatings is reduced.

The surface of the substrate may be structured, e.g. include a three- dimensional pattern or ornamentation such as waves, wavelets, grooves, ridges, knurling, etc. The structured surface can have a depth of 1 mm or more, for example 0.5 mm or more or 1cm or more. The structure has peaks and valleys. By depth is meant that the distance from an envelope of the surface which touches the upper peaks of the structure to the valleys of the structure. Due to the rapid drying of the coating, there is reduced sag, slip or displacement of the coating material while still wet even when applied to structured surfaces.

The coatings that can be solidified and/or dried by the method of any of the embodiments of the present invention are composed of at least one or more solvents or suspension liquids and one or more binders. Water must be present as one of the at least one or more solvents or suspension liquids. The water-containing coatings can optionally further contain oils or organic solvents The basic composition of such coatings can be a binder agent, a solvent comprising water, an optional pigment and optional fillers. These components forms a colloidal mass which can be applied in any suitable way such as spraying, flooding, casting, application using a doctor blade or squeegee, etc.

Elements that can be present in the coating are one or more pigments or dyes and one or more fillers. Additional optional components can be present in the coating as well such as but not limited to polymerization initiators, light- stabilizers, sensitizers, waxes and fluidizing agents among others. The one or more binders are preferable for their film forming ability and they play also an important role in holding together the optional one or more pigments and fillers. The binders can be either of the chemically drying kind or of the physically drying kind. Chemically drying binders, also known as drying oils, are oils which hardens to a tough, solid film after a period of exposure to air. They are not drying through the evaporation of water, but hardening through a chemical reaction in which oxygen is absorbed from the environment (oxidation). Heat and light speed up this reaction. Examples of commonly used drying oils include but are not limited to linseed oil, tung oil, poppy seed oil, walnut oil and the likes.

Physically drying binders are binders drying through the evaporation of water. They are in general either of polymeric or oligomeric nature. They may be either curable or not. Examples of physically drying binders include but are not limited to polyvinylbutyral resins, polyacrylate resins, polycarbonate resins, polyester resins, phenoxy resins, vinyl chloride-vinyl acetate copolymers, polyamide resins, acryl resins, polyacrylamide resins, polyvinylpyridine resins, cellulose resins, urethane resins, epoxy resins, polyvinyl alcohol resins, polyvinylpyrrolidone resins and the like. When these binders are curable, curable functional groups are present in these binders such as but not limited to cation-polymerizable groups or radical polymerizable groups. Groups capable of causing cationic polymerizations are unsaturated groups such as but not limited to alkenyl ether-type functional groups, e.g., vinyl ether group, propenyl ether group and isopropenyl ether group; styrene-type functional groups, e.g., styryl group, isopropenyl phenyl group, propenyl phenyl group and vinyl naphthyl group; diene-type functional groups, e.g., butadienyl group and hexadienyl- group; and alkene-type functional groups, e.g., vinyl group, propenyl group and isopropenyl group and heterocyclic groups such as but not limited to 2- to 5-membered ring cyclic ethers, e.g., glycidyl group, epoxy cyclohexyl group and tetrahydrofurfuryl; 3- to 5-membered ring cyclic sulfide groups, e.g., thiirane-type functional groups; and 3- to 4-membered ring cyclic imine groups, e.g., methyleneimine-type functional groups.

Groups capable of causing radical polymerization are such as but are not limited to acryloyl groups, e.g., (meth)acryloyloxy groups and

(meth)acryloylamide group; styrene-type functional groups, e.g., styryl group, isopropenyl phenyl group, propenyl phenyl group and vinyl naphthyl group; diene-type functional groups, e.g., butadienyl group and hexadienyl group; vinyl ester-type functional groups, e.g., vinyl ester group and isopropenyl ester group; and vinyl ketone-type functional groups, e.g., vinyl ketone group and isopropenyl ketone group.

Examples of solvents or suspension liquids that can optionally be used together with water include both inorganic and organic solvents as well as polar or non-polar solvents (in as far as they are partially miscible), e.g. toluene, xylene, acetone, methyl isobutyl ketone, methyl ethyl ketone, methanol, ethanol, isopropanol, n-propanol, n-butanol, benzyl alcohol, methylcellosolve, ethylcellosolve, cyclohexanone, methyl acetate, n-butyl acetate, dioxane, tetrahydrofuran, chlorobenzeneethylene glycol, monoethyl ether, VM and P naptha, mineral spirits, hexane, heptane and other aliphatic, cycloaliphatic, aromatic hydrocarbons, esters, ethers and ketones and the like. A necessary solvent or suspension liquid used either as sole solvent or suspension liquid or in combination with other solvents and/or suspension liquids is water.

The relative content of water in the coating may amount to 5 wt% or more, 8 wt% or more, 11 wt% or more or 15 wt% or more. Examples of pigments or dyes includes but are not limited to kaolin, calcium carbonate, titanium dioxide, micronized titanium dioxide, iron oxide pigments, carbon black, azo, phthalocyanine, quinacridone or pyrrolopyrrole pigments or dyes, pyrene, perylene and higher rylene pigments or dyes and the likes, special effect pigments such as but not limited to metal pigments, for example, from aluminium or copper, interference pigments, such as, for example, aluminium coated with titanium dioxide, coated mica, graphite effect pigments and iron oxide laminae and the likes.

Examples of fillers include organic fillers of inorganic fillers, for example but not limited to ceramic particles or beads, glass particles or beads, silica (Siθ₂), tin oxide (SnO₂), titanium dioxide (TiO ₂), mica, clays, talcum powders, chalk, doped-metal oxides (e.g. InSbO₂, ZnSb₂O ₂ and the likes), nitrides (e.g. ZrN, TiN and the likes), carbides (e.g. silicon carbide, TiC, WC and the likes), borides (TiB₂, ZrB₂, NbB₂, TaB₂, TaB₂, CrB₂, MoB, WB, LaB₆ and the likes), AI₂O ₂, ZrO₃, In₂ O₂, MgO, BaSO₄, aluminium silicate and magnesium silicate and the likes.

Examples of porous substrates include but are not limited to wood, tarmac, asphalt, wooden materials (e.g. medium density fibre board), plywood, paper, cardboard, paperboard, paper, cellulose, lignin, fibrous materials, polyweave™, fabrics, woven fabric, carpets, cloth, leather, felt, porous plastics such as foamed thermosetting or thermoplastic polymers, porous thermal insulating materials, concrete, marble, limestone, granite, ceramics, sand- casting, and the likes.

An important aspect of the present invention is the creation of one or more means at or in the surface of a porous substrate for preventing the exchange of water between the substrate and the coating, e.g. the transfer of liquid (i.e. the solvent and/or the suspending liquid, preferably water) from the coating to the porous substrate.

A necessary means consists in raising the relative water, content at or in the surface of the substrate in order to form a water barrier between the bulk of the substrate and the coating. This water barrier prevents the transfer of water from the coating to the substrate. The relative water content at or in the surface of the porous substrate is preferably raised to at least the relative water content of the coating. One way to raise the relative water content of the surface of the porous substrate is to apply water on this porous substrate. Water can be applied as a liquid film, as a spray, as a vapour or by any other ways known to the person skilled in the art.

Another way to raise the relative water content at the surface of the porous substrate is to elevate the temperature of this porous substrate in order to provoke the internal migration of water from within the porous substrate to the surface of this porous substrate. This elevation of temperature can be obtained by any means known from the person skilled in the art such as but not limited to RF or microwave heating means. Good results are obtained with microwave heating. Preferably, the temperature of the substrate is raised above the temperature of the coating, most preferably at least 10⁰C above this temperature.

An optional additional means for preventing this transfer is a primer, i.e. a liquid-proof layer.

The microwave or RF heating for the substrate is preferably applied from 4 kWh/m², e.g. 6 kWh/m² or higher.

The method used to apply the water-containing coating to the porous substrate can be any methods known in the art such as but not limited to spraying, curtain coating, roller coating, doctor blade coating, dip coating, screen printing and the likes.

The coating can be performed either off-line or on-line.

In order to heat up the coating, in the last step of the method, the method of the present invention can make use of microwave or RF heating or a combination of those means. For each heating source, the method of the present invention permits to shorten the drying time usually obtained without the use of this method. Among the different possible energy sources usable with the present invention, microwave or RF heating is preferred.

For heating the coating alone a lower rate can be applied, e.g. from 1 kWh/m², e.g. 2 kWh/m² or higher.

Figure 1 illustrates one example of skin (5) formation at the surface of the coating (1 ) observed in the prior art. At the top of figure 1 , a schematic view of a coating (1 ) applied on a non-porous substrate (2) is shown. IR heating (3) leads to the formation of a temperature gradient (4) in a such a way that the temperature of the coating (1 ) is higher close to the surface than close to the substrate (2). At the bottom of figure 1 , the resulting skin (5) at the surface of the coating is displayed.

Figure 2 illustrate one example of the prior art in which a skin (5) formation at the surface of the coating (1 ) can be avoided for some types of substrate. At the top of figure 2, a schematic view of a coating (1 ) applied on a non-porous substrate (2) is shown. Microwave heating (6) leads to the formation of a temperature gradient (7) in such a way than the temperature of the coating (1 ) is higher close to the substrate (2) than close to the surface. At the bottom of figure 2, a resulting dried portion (8) of the coating is displayed at the interface between the bulk of the substrate (2) and the coating (1 ).

Figure 3 illustrates the process known from the prior art by which a skin (5) is formed at the surface of a coating (1 ) deposited on a porous substrate (9). At the top of figure 3, a schematic view of a coating (1 ) applied on a porous substrate (9) is shown. Water (10) is transferred by diffusion from the coating (1 ) to the porous substrate (9). No heating is applied. The bottom of figure 3 shows the resulting skin (5) formed at the surface of the coating (1 ).

Figure 4 illustrates one particular embodiment of the present invention. At the top of figure 4, a schematic view of a coating (1 ) applied on a porous substrate (9) is shown. A means (11 ) for preventing the transfer of water from the coating (1 ) to the porous substrate (9) is shown at the surface of the porous substrate (9). Water is shown to be unable to be transferred, i.e. to diffuse from the coating (1 ) to the porous substrate (9) through the means (11 ). The bottom of figure 4 shows that no skin is formed in these conditions.

Figure 5 illustrates one particular embodiment of the present invention. At the top of figure 5, the temperature of a porous substrate (9) is elevated due to microwave heating (6). As a result of this elevation of temperature, water migrates internally from within the porous substrate (9) to the surface of the porous substrate (9). The result of this migration is represented directly below where the same porous substrate (9) now presents a raised relative water content (11 ) at or in the surface of the porous substrate. Directly below is represented the same porous substrate (9) after that a coating (1 ) has been applied thereon. Water is shown to be unable to be transferred, i.e. to diffuse from the coating (1 ) to the porous substrate (9) through the raised relative water content forming a means (11 ) for preventing this transfer. The bottom of figure 5 shows that no skin is formed in these conditions.

Figure 6 illustrates one particular embodiment of the present invention. At the top of figure 6, water is applied on a porous substrate (9). The result of this appliance is represented directly below where the same porous substrate (9) now presents a raised relative water content (11 ) at or in the surface of the porous substrate. Directly below is represented the same porous substrate (9) after that a coating (1 ) has been applied thereon. Water is shown to be unable to be transferred, i.e. to diffuse from the coating (1 ) to the porous substrate (9) through the raised relative water content forming a means (11 ) for preventing this transfer. The bottom of figure 6 shows that no skin is formed in these conditions.

Figure 7 is a schematic drawing of a system in accordance with an embodiment of the present invention. A conveyor means (12) is adapted to convey a substrate through an optional microwave or RF oven (13), beneath a coating station (14) and an optional oven (15). The oven (15) may be a further microwave or RF oven. In the first oven (13) the substrate may be raised in temperature as explained above as a preheating step. The substrate plus coating may be dried in the oven (15), e.g. by microwave or RF heating as explained above.

The invention is by no means limited to the above-described embodiments given as an example and represented in the accompanying drawings; on the contrary, the methods according to the invention can be performed in all sorts of variants while still remaining within the scope of the invention.

Example 1

A pine wood substrate pre-heated in a microwave industrial oven MEAC DRY S4 (supplied by the applicant) at 60 ⁰C was subsequently coated with a 150 μm thick layer of a paint at room temperature (about 18 ⁰C) containing 22% of water. Low profile microwave energy (less than 2Kwh/m²) was then used to reduce the water content of the paint to 10% in 2 minutes. The surface quality of the dried paint was high and did not witness overheating of the radiation side or bubbling.

Example 2

An afrormosia wood substrate was pre-heated in a microwave industrial oven, namely a MEAC DRY S4 to 60⁰C and subsequently coated with 80 micron thick layer of a water based paint at 20°C. Low profile microwave energy was then used to reduce the water content of the paint to less than 10% in one minute. The surface quality of the dried paint was high and did not show bubbling or craters.

Example 3

A padouk wood substrate was pre-heated in a microwave industrial oven namely a MEAC DRY S4 to 60⁰C and was subsequently coated with a 300 micron thick layer of a water based paint at 20°C. Low profile microwave energy was then used to reduce the water content of the paint to less than 10% in 3 minutes. The surface quality of the dried paint was good and did not show bubbling or craters.

Example 4

An afrormosia wood substrate pre-heated in a microwave industrial oven MEAC DRY S4 (supplied by the applicant) at 60 ⁰C was subsequently coated with a 80 μm thick layer of a paint at room temperature (about 18 ⁰C) containing 50% of water. Low profile microwave energy (less than 2Kwh/m²) was then used to reduce the water content of the paint to 10% in 2 minutes. The surface quality of the dried paint was high and did not witness overheating of the radiation side or bubbling.

Example 5 A padouk wood substrate was pre-heated in a microwave industrial oven, namely a MEAC DRY S4 to 60⁰C and subsequently coated with 150 micron thick layer of a water based paint (50% of water) at 20°C. Low profile microwave energy was then used to reduce the water content of the paint to less than 10% in one minute. The surface quality of the dried paint was high and did not show bubbling or craters.

Example 6 A pine wood substrate was pre-heated in a microwave industrial oven namely a MEAC DRY S4 to 60⁰C and was subsequently coated with a 300 micron thick layer of a water based paint (60% of water) at 2O⁰C. Low profile microwave energy was then used to reduce the water content of the paint to less than 10% in 3 minutes. The surface quality of the dried paint was good and did not show bubbling or craters.

Claims

Claims:

1. A method for drying a water-containing coating (1 ) on a porous substrate (9) comprising the steps of: (i) applying said water-containing coating (1 ) on said porous substrate (9) to a thickness of at least 80 microns, and

(ii) heating up said water-containing coating (1 ) by microwave or RF heating, characterised in that one or more means (11 ) for preventing the exchange of water between said water-containing coating (1 ) and said porous substrate (9) are created at or in the surface of said porous substrate (9) prior to step (i), wherein at least one of said one or more means (11 ) for preventing the exchange of water is a raised relative water content (11 ) at or in said surface of said porous substrate (9).

2. A method according to claim 1 , characterised in that said raised relative water content (11 ) is at least the relative water content of the water- containing coating (1 ) that will be applied in step (i).

3. A method according to claim 1 or claim 2, characterised in that said raising of the relative water content is at least partially due to internal water migration from within said porous substrate as the result of an elevation of the temperature of said porous substrate, wherein said elevation is due to microwave heating (6).

4. A method according to claim 3, characterised in that said temperature of said porous substrate (9) exceeds the temperature of said water-containing coating (1 ) after said elevation.

5. A method according to claim 4, characterised in that said temperature of said porous substrate (9) exceeds said temperature of said water- containing coating (1 ) by at least 10 ⁰C after said elevation.

6. A method according to any preceding claims, characterised in that said raising of the relative water content is at least partially obtained by applying water on said porous substrate (9) prior to said application of said water- containing coating (1 ).

7. A method according to any preceding claims, characterised in that said water-containing coating (1 ) is applied in a single pass.

8. The method of any of claims 1 to 7, wherein the heating up of said water- containing coating (1 ) using microwave or RF energy to solidify or dry said water-containing coating follows immediately after said coating applying step.

9. The method of any of claims 1 to 8, wherein said water-containing coating (1 ) contains at least 5% of water.

10. A system for drying a water-containing coating (1 ) on a porous substrate (9) comprising: means (13) for raising the relative water content at or in the surface of said porous substrate (9), means (14) for applying said water-containing coating (1 ) on said porous substrate (9) to a thickness of at least 80 microns, and means (15) for heating up said water-containing coating (1 ).

11. A system according to claim 10, wherein said means (14) for applying said water-containing coating (1 ) enable the applying of said water-containing coating (1 ) to a thickness of at least 80 microns in a single pass.