WO2001028306A2

WO2001028306A2 - Process for preparing an aqueous powder coating dispersion

Info

Publication number: WO2001028306A2
Application number: PCT/EP2000/010424
Authority: WO
Inventors: Martinus Adrianus Anthonius Maria Koenraadt; Gerard Henk Beijers; Johan Eric Borgstrom; Kent Raabjerg Sorensen
Original assignee: Akzo Nobel N.V.
Priority date: 1999-10-19
Filing date: 2000-10-17
Publication date: 2001-04-26
Also published as: WO2001028306A3; AU2835101A

Abstract

A process is disclosed for preparing an aqueous powder coating dispersion from a solid powder coating composition and an aqueous medium, said composition comprising 1) a homogeneous mixture of a resin, a cross-linker, and, optionally, a pigment, a catalyst, an initiator, and/or one or more additives, or 2) a radiation-curable resin and, optionally, a cross-linker, a pigment, a catalyst, an initiator, and/or one or more additives. The process is characterised in that it comprises the steps of: a) elevating the temperature of solid powder coating composition such that a molten-like substance is obtained the temperature of which is above the melting point or the glass transition temperature (Tg) of the solid powder coating composition, b) optionally cooling down the molten-like substance, c) dispersion said substance, in the cooled down state or not, in the aqueous medium.

Description

PROCESS FOR PREPARING AN AQUEOUS POWDER COATING DISPERSION

The invention pertains to a process for preparing an aqueous powder coating dispersion from a solid powder coating composition and an aqueous medium, said composition comprising 1) a homogeneous mixture of a resin, a cross- linker, and, optionally, a pigment, a catalyst, an initiator, and/or one or more additives, or 2) a radiation-curable resin and, optionally, a cross-linker, a pigment, a catalyst, an initiator, and/or one or more additives.

Solid powder coating compositions comprising a suitable resin/cross-linker combination are commonly used for coating a substrate with a cross-linked film, by evenly distributing the composition over the surface of the substrate by spraying and curing the resulting layer on the substrate. Curing on a metallic substrate, for example in industry in general, most commonly takes the form of applying an elevated temperature, for example from 160 to 200°C. However, curing can also take the form of applying radiation/light, for example ultraviolet light or electron beam radiation, a process which is especially suitable for coating substrates that cannot withstand high temperatures (plastics, wood, etc.).

When a solid powder coating composition is used to coat a substrate, special application equipment is required. Furthermore, it is difficult to obtain a thin cross-linked film with a uniform thickness and appearance on the substrate, due to the relatively large size and irregular shape of the particles in the powder coating composition. Reducing the particle size in such a composition, e.g. by jet milling, is not only costly but often leads to caking and other handling/ application problems of the resulting fine powder. For example, it is difficult to charge the very small particles during electrostatic application, and elaborate steps need to be taken to cope with dust when such a composition is applied on a substrate. Further, once a solid powder coating composition is prepared, due to its solid character there is no flexibility to adapt properties like for example colour, gloss etc.

Hence there are considerable advantages to applying a particulate powder coating as a dispersion for coating a substrate. No dust is formed when the dispersion is spread on the surface of the substrate, and it was found that the particles in the dispersion can be much smaller than in a solid powder coating composition without creating handling/application problems. Also, in the case of a liquid dispersion it is possible to use equipment commonly used for handling/ applying conventional wet paints, while special equipment is needed for handling/applying solid powder coating compositions. Application on non- conductive substrates like wood and plastics is also easier. In general, the T_g of the solid powder coating composition which is used for preparing the aqueous powder coating dispersion can be much lower than in solid powder coatings, because there is no need for high T_g binders that are used in a standard (solid) powder coatings to ensure sufficient physical stability. The possibility to incorporate binders with much lower T_g's increases the freedom of binder formulation. This additional degree of freedom in binder formulation has a positive effect on final film properties like flow, appearance etc.

Furthermore, paint additive(s) can be added to a final dispersion without any problems, making the preparation of a coating formulation from a dispersion much more flexible. It is also relatively easy to filter a dispersion in order to remove impurities/large particles.

For example, in WO 96/37561 a wet milling process is disclosed for the production of an aqueous powder coating dispersion, in which process a solid powder coating composition is wet milled in an aqueous medium. However, the powder coating dispersion obtained has a comparatively wide particle size distribution (3 - 20 μm), and the particles are still irregularly shaped and relatively large. Preparation of smaller particles than described by traditional wet-milling processes is in general very difficult and economically not attractive. Larger particles in powder coating dispersions have a strong tendency to sediment which is undesirable because this limits the stability of the final dispersion. The sediments formed are normally hard to re-disperse and in practice additional measures have to be taken to prevent sedimentation for example by adjusting the rheology of the coating formulation by adding thickeners or other rheology modifiers. A disadvantage of adding thickeners in general is that the solids content of the final coating formulation decreases which is not desirable. Also the film formation is negatively influenced by the presence of bigger particles which is observed by phenomena like mud- cracking, poor flow, no uniform layer thickness etc. Furthermore, the process cannot be carried out continuously, and obtaining particles with the above- mentioned size requires a comparatively long milling time (about 2 hours).

Similar sized/shaped particles are obtained in US 5,379,947, where a jet milling process for the production of an aqueous powder coating dispersion is disclosed. Powder coating flakes are jet milled, and the resulting powder is dispersed in an aqueous medium to obtain a dispersion. Such a process cannot be carried out continuously either, and jet milling until particles of the desired size are formed also requires a comparatively long period of time. Other disadvantages of such a jet milling process are the requirement to use expensive equipment and a relatively high energy consumption.

It is the objective of the present invention to provide a process for preparing an aqueous powder coating dispersion from a solid powder coating composition and an aqueous medium, which dispersion is suitable for coating a substrate with a thin cross-linked film without having the above-mentioned disadvantages.

A process was found for preparing an aqueous powder coating dispersion from a solid powder coating composition and an aqueous medium, said composition comprising 1) a homogeneous mixture of a resin, a cross-linker, and, optionally, a pigment, a catalyst, an initiator, and/or one or more additives, or 2) a radiation-curable resin and, optionally, a cross-linker, a pigment, a catalyst, an initiator, and/or one or more additives. The process is characterised in that it comprises the steps of: a) elevating the temperature of the solid powder coating composition such that a molten-like substance is obtained the temperature of which is above the melting point or the glass transition temperature (T_g) of the solid powder coating composition, b) optionally cooling down the molten-like substance, c) dispersing said substance, in the cooled down state or not, in the aqueous medium.

For the purpose of the present invention, a dispersion comprising a binder/crosslinker mixture wherein the binder and/or the crosslinker have a T_g, softening or a melting temperature above 30 °C is considered as a powder coating dispersion.

Without being bound to any theory, the formation of the dispersion in the process according to the present invention can be described as a process whereby starting with a powder coating composition as a continuous phase under certain conditions by the addition of water the continuous phase inverts into a phase where the aqueous medium becomes the continuous phase resulting in the aqueous powder coating dispersion. This process is known as indirect emulsification or phase inversion emulsification.

An advantage of the process according to the invention over the wet and jet milling processes described above is that it is not necessary to reduce the particle size of the solid powder coating composition used as raw material, thereby eliminating the need for an energy-consuming milling step and a time- consuming classification step. The solid powder coating composition can be dosed into the process as a powder, flakes, granulates, or in any other physical form suitable for dosing. Furthermore, the process can be carried out continuously.

In WO 97/45476 a process is disclosed for the preparation of a powder coating dispersion wherein a molten-like substance comprising a resin and a cross- linker is dispersed in a liquid medium under extrusion conditions. In this process the raw materials such as resin(s) and cross-linker(s) are fed individually for the preparation of the molten-like mixture, while in the process according to the invention an already prepared solid powder coating composition is used as raw material. According to its examples, the process of WO 97/45476 seems only capable of preparing dispersions with relatively large particles (average particle size 1 ,1 - 22 μm) and relatively low solids content (22 - 33 wt.%).

The main advantage of applying an already prepared solid powder coating composition in the process according to the invention is that the solid powder coating composition already comprises a homogenous mixture of the above- mentioned ingredients. Therefore, when heating the powder coating composition less time, a lower process temperature, and no or less intensive mixing are needed to obtain a molten-like substance wherein all the ingredients are homogeneously distributed. It was found that a homogeneous distribution of the ingredients in a final powder coating dispersion produced from the powder coating composition is very important for obtaining a cross-linked film with a uniform appearance and the desired properties on a substrate. The appearance of the cross-linked film is greatly influenced by the distribution of the optionally present pigment(s). It was found that the process according to the invention is highly suitable for obtaining a pigmented powder coating dispersion wherein the pigment is homogeneously distributed.

The application of an already prepared powder coating composition in the process according to the invention also has storage, logistic, sales, and other production advantages. For example, a produced powder coating composition in the form of ftakes or granulates can be stored. These flakes or granulates can either be milled to obtain a solid powder coating composition suitable for coating a substrate directly (or for dosing into the process according to the invention), or the flakes or granulates can be dosed in the process according to the invention to obtain a powder coating dispersion, and so are adaptable to market needs and efficient utilisation of the available equipment in a factory.

It is preferred in the process according to the invention to keep the time during which the cross-linker is in contact with the resin at relatively high temperature as short as possible, either by dispersing the molten-like substance in an aqueous medium having a lower temperature than the substance as soon as possible, or by cooling down said substance in a different way before dispersing it, in order to prevent cross-linking reactions between these components during the process.

Preferably, the molten-like substance is cooled down, if necessary, to such an extent that when said substance is dispersed in the aqueous medium, the temperature in the forming dispersion does not exceed the boiling point of the aqueous medium, in order to prevent as fully as possible the evaporation of water and/or build-up of pressure in the process equipment.

Furthermore, it is preferred in the process according to the invention that the dispersing is carried out at atmospheric pressure, in order to facilitate process control without the need to use equipment that can withstand high pressures.

A suitable amount of an aqueous medium for obtaining a homogeneous dispersion can be applied in the process. Furthermore, it is possible to dose an additional amount of aqueous medium during or after dispersing, in order to obtain an aqueous powder coating dispersion with the desired temperature, viscosity, and/or content of solids. All steps in the process according to the invention are preferably carried out in one apparatus, preferably an extruder, more preferably a twin-screw extruder (for example a Berstorff twin-screw extruder, type ZE 25, 43 UD, or a Leistritz twin-screw extruder, type micro-18 GL 40D), in order to allow rapid heating of a solid powder coating composition, rapid cooling down of the obtained molten- like substance when required, and dispersing of the substance in an aqueous medium under ideal mixing conditions, thereby enabling the formation of an aqueous powder coating dispersion comprising small, spherical-like particles with a narrow particle size distribution. When an extruder is used as the apparatus to prepare the aqueous powder coating dispersion in accordance with the present invention, the process can be also referred to as 'phase inversion extrusion', abbreviated as 'PIE process'.

The desired average particle size can be obtained by choosing the right conditions (mixing speed, number of mixing and/or transporting elements in the apparatus, additives, temperature, etc.). The average particle size may be between 0.05-100 μm. The process according to the invention is highly suitable for obtaining a powder coating dispersion comprising very small, spherical-like particles with a narrow particle size distribution. The average size of the particles is preferably from 50 to 2,000, more preferably from 80 to 800, most preferably from 100 to 500 nm. Particles in the range below 800 nm have the advantage that they do not have the tendency to sediment in dispersions which ensures a very good stability of the final dispersion. In general, sedimentation can be prevented by adjusting the rheology of the dispersion by addition of for example thickeners or other rheology modifiers. However, this has a negative effect on the final solids content of the dispersion (which will decrease by adding these constituents) and can have a negative effect on the properties of a film prepared from such dispersion. The particles in the aqueous powder coating dispersion prepared according the present invention can be so small (smaller than 800 nm) that no additional measures are needed to control sedimentation which obviates the necessity to add rheology modifiers and results in coating formulations with higher solids content and better coating performance.

In general, the apparatus used in the process according to the invention comprises a feeding port, an exit port, and options to add additional liquid or solid components between the feeding port and the exit port. Also a high flexibility is desired to control the different steps in the process (melting, cooling down, and dispersing).

The aqueous medium, e.g., tap water or demineralised water, used in the process according to the invention optionally comprises a dispersing agent in order to facilitate the formation of a homogeneous dispersion. Furthermore, said dispersing agent may contribute to the formation of particles with a uniform size and shape. The aqueous medium may also comprise thickeners and/or other additives.

Any suitable dispersing agent can be used in the process according to the invention, for example anionic, cationic, amphoteric, nonionic compounds or combinations thereof as described in C.R. Martens, Emulsion and Water- Soluble Paints and Coatings, Reinhold Publishing Corporation, 1965. The dispersing agent may also comprise functional groups capable of reacting with the resin and/or the cross-linker in order to form a cross-linked network comprising said agent. The dispersing agent should be selected carefully, because some types of dispersing agents may inhibit cross-linking reactions. Furthermore, the presence of relatively high amounts of non-reactive dispersing agents in a cross-linked film generally has a negative impact on the final properties of the film. Therefore, it is preferred:

1. to use dispersing agents with reactive groups (e.g., acid-functional group), and/or 2. to use limited amounts of non-reactive dispersing agents with high dispersing/stabilising properties, and/or 3. to use agents which can form ionic groups with functional groups (e.g., carboxylic groups) present in the resin and/or crosslinker. Typical examples of these neutralising agents are amines, ammoniak and alkylmetalhydroxides. Preferably volatile neutralising agents with a boiling point below the curing temperature of the solid powder coating composition are used. Organic amines, preferably tertiary amines, are highly suitable volatile neutralising agents, but there is no intention to limit the invention to these compounds only. It was found that the use of dispersing agents with reactive groups or the use of neutralising agents which can form anions with functional groups present on the binder and/or crosslinker enable the preparation of dispersions with an average particle size in the range from 50 to 800 nm and a solid content in the range of 30 to 70 wt.%, more particular in the range from 40 to 60 wt.%.

The solid powder coating composition used as raw material in the process according to the invention may have been prepared by applying well-known process steps, e.g. pre-mixing, melt-mixing, grinding, sieving/classifying, that are commonly used in the powder coatings industry.

Any suitable solid powder coating composition comprising a resin for coating a substrate with a cross-linked film can be used as raw material in the process according to the invention. Resin in this patent application means any resin that can be cured on a substrate. The resin in the solid powder coating composition used as raw material in the process according to the invention may be a thermally curable resin, i.e. a resin that can be cured by elevated temperature, or a radiation-curable resin, i.e. a resin that can be cured by using electromagnetic radiation with a wavelength of less than 500 nm, e.g., UV or electron beam radiation. A thermally curable resin is usually combined with a cross- linker in order to enable the formation of a cross-linked network. The resin and/or cross-linker in the solid powder coating composition used as raw material in the process according to the invention can be amorphous and/or (semi)-crystalline. Also combinations of an amorphous resin and a crystalline cross-linker are possible.

Furthermore, the resin and/or cross-linker in the solid powder coating composition may comprise functional groups that facilitate the dispersing of the molten-like substance obtained from said composition in the process according to the invention in the aqueous medium. Such functional groups may be regarded as self-emulsifyable groups. It was found that the use of resins and/or cross-linkers with such self-emulsifyable groups are in. particular favourable to obtain dispersions with particles having a small average particle size, i.e. an average particle size in the range from 50 to 800 nm. Examples of functional groups that can facilitate the dispersing of the molten-like substance in the process according to the present invention are carboxylic acid, sulphonic acid, and/or phosphonic acid functional groups.

The solid powder coating composition may comprise any suitable thermally curable resin/cross-linker combination. Suitable combinations are described for example in T.A. Misev, Powder Coatings Chemistry and Technology, John Wiley & Sons Ltd 1991 , like an acid/epoxy, acid anhydride/epoxy, epoxy/amino resin, polyphenol/epoxy, phenol formaldehyde/epoxy, epoxy/amine, epoxy/amide, isocyanate/hydroxy, carboxy/hydroxyalkylamide, or hydroxyl/- epoxy combination.

It is preferred to use a solid powder coating composition comprising a thermally curable resin and a cross-linker of which the glass transition temperature (T_g) or the melting point is below the temperature at which substantial cross-linking reactions between these components take place, because in the process according to the invention it is undesirable for such reactions to occur already during the formation of a molten-like substance. Even more preferably, the solid powder coating composition has a T_g or a melting point below 100°C, because then it is possible to obtain a molten-like substance with a temperature below 100°C, obviating the necessity to cool down the substance before it is dispersed in an aqueous medium at atmospheric pressure.

Examples of highly suitable thermally curable resin/cross-linker combinations are: acid/epoxy, isocyanate/hydroxy, and carboxy/hydroxyalkylamide. An example of a powder coating composition comprising an acid/epoxy combination is a hybrid powder coating, in which a carboxy-functional polyester is present as a thermally curable resin and for example Bisphenol-A epoxy resin is present as a cross-linker. Examples of commercial carboxy-functional polyesters are: Uralac P3560 (DSM Resins) and Crylcoat 314 (UCB Chemicals). Examples of Bisphenol-A epoxy resins are Epikote 1055 (Shell) and Araldite GT 7004 (Ciba Chemicals). A powder coating composition comprising an isocyanate/hydroxy combination is often called a polyurethane powder coating. A blocking agent, such as caprolactam, or an internally blocked isocyanate is required to prevent cross- linking reactions at relatively low temperatures, e.g., during the melt-mixing stage in the production of the powder coating. An example of an internally blocked isocyanate product is Vestagon EP-BF 1310 (Creanova). Examples of commercial hydroxy-functional polyesters are: Uralac P5504 (DSM Resins) and Alftalat AN 739 (Vianova Resins). A powder coating composition comprising a carboxy/hydroxyalkylamide combination is most commonly obtained from a carboxy-functional polyester and a hydroxyalkylamide such as Primid XL-552 (EMS). An example of a commercial carboxy-functional polyester resin is Crylcoat 76-17 (UCB Chemicals).

The solid powder coating composition that is used in the process according to the invention may comprise a radiation-curable resin and, optionally, a cross- linker, a pigment, a catalyst, a photoinitiator, and/or one or more additives. A radiation-curable resin is a resin that can be cured by using electromagnetic radiation with a wavelength of less than 500 nm, e.g., UV or electron beam radiation, without the need to apply an elevated temperature. A photoinitiator is an initiator that can be activated by using said radiation, thereby enabling initiation of the cross-linking of the radiation-curable resin and optionally the cross-linker when curing a thin layer of the dispersion on a substrate. Preferably, the initiator and the resin can be activated and cured, respectively, by using ultraviolet light, typically characterised by a wavelength from 50 to 400 nm. Suitable photoinitiators are described for example in Chemistry & Technology of UV & EB Formulations for Coatings, Inks & Paints, Volume 3: Photoinitiators for Free Radical and Cationic Polymerisation by P.K.T. Oldring.

Examples of suitable radiation-curable powder coating compositions are described in EP 636 669 and EP 702 040. In EP 636 669 radiation-curable resin compositions are disclosed which comprise an unsaturated polyester and/or an unsaturated polyacrylate, and a cross-linking agent having vinyl ether, vinyl ester or (meth)acrylate-functional groups. In EP 702040 radiation-curable unsaturated polyesterurethane acrylates are disclosed.

The main advantage of using said powder coating composition as raw material in the process according to the invention is that the step of heating the solid powder coating composition to obtain a molten-like substance is less critical, because no or hardly any undesirable initiation of cross-linking reactions occurs during the process when such a composition is used.

The solid powder coating composition that is used as raw material in the process according to the invention may also comprise a pigment or a mixture of pigments, a catalyst or a mixture of catalysts, an initiator or a mixture of initiators, and/or an additive or a mixture of additives and/or a filler or a mixture of fillers. One or more of these additional components may also be dosed individually during the process. Furthermore, it is possible to add one or more of these additional components to the final powder coating dispersion. By an additive is meant, any suitable additive that is usually present in a commonly applied coating composition for coating a substrate with a cross- linked film. Such an additive is selected for example from an auxiliary agent, a flowing agent, a stabiliser, a anti-settling agent, a surface-active agent, a de- gassing agent, a UV-absorber, an optical whitener, a radical scavenger, a thickener, and an anti-oxidant. Furthermore, any filler known to those skilled in the art can be present, e.g., barium sulphate, calcium sulphate, calcium carbonate, silicas or silicates (such as talc, feldspar, and/or china clay).

The process according to the invention is highly suitable for preparing an aqueous powder coating dispersion comprising a pigment. The pigment can be dosed individually during or after the process, or it can be introduced via a pigmented solid powder coating composition. It is preferred that the molten-like substance comprises the pigment before it is dispersed in the aqueous medium, thereby enabling the formation of an aqueous powder coating dispersion with small and spherical-like particles having a uniform size, wherein the pigment is homogeneously distributed. When said dispersion is used for coating a substrate, then a cross-linked film of uniform thickness and appearance is formed on the substrate.

A powder coating dispersion obtained in the process according to the invention may be applied directly for coating a substrate with a cross-linked film. It is also possible to add one or more of the above-described additional components to the dispersion in order to obtain a coating composition that can also be applied directly onto a substrate. Furthermore, the powder coating dispersion or coating composition may be dried, e.g., by spray-drying, in order to obtain a solid powder coating composition comprising small, spherical-like particles comparable with the particles in the original powder coating dispersion. The invention will be further illustrated with reference to the following non- limiting examples, wherein the following general conditions and procedures for preparing aqueous powder coating dispersions were applied:

Apparatus: Leistritz twin-screw micro-18-GL 40 D extruder, comprising the following zones, including type of screw element and temperature applied in each zone:

0 transport RT (feeding a powder coating composition)

1 melting / kneading 110°C

2 melting / kneading 110°C

3 transport 90°C 4 transport 90°C (dosing a dispersing agent and water) 5 kneading 90°C 6 kneading 90°C

7 transport 90°C (dosing water)

8 mixing 80°C

RT = room temperature.

The screw speed of the apparatus was set at 250 rpm.

A solid powder coating composition (1000 grams) in the form of flakes, comprising a homogeneous extruded blend of a curable resin, a cross-linker, and, optionally, one or more additives, was dosed into the intake feed zone (no. 0) of the extruder at a constant rate of 500 grams per hour using a gravimetric device, followed by heating at 110°C to obtain a molten-like substance. The substance was cooled down to 90°C before an aqueous solution containing 12.5% by weight of dimethylethanolamine was injected into the extruder (zone no. 4). The content of solids in the forming dispersion was adjusted to 50% by weight by adding water in zones no. 4 and 7, thereby obtaining the dispersion at approximately 1000 grams per hour. The content of solids in the dispersion was determined by air-drying in a circulated oven during 1 hour at 125°C. The average size of the particles and their standard deviation in the dispersion was measured using a Coulter LS230 particle sizer. The shape of the particles was evaluated by Scanning Electron Microscopy (SEM).

Furthermore, the ingredients that were present in the applied solid powder coatings as raw material are specified below:

Crylcoat 76-17 is a polyester resin comprising carboxylic functional groups and is obtainable from UCB Chemicals, Primid XL552 is a hydroxyalkylamide-based cross-linker obtainable from EMS,

Resiflow PV88 is a flowing agent obtainable from Worlee, Benzoine is a de-gassing agent obtainable from DSM, Kronos 2310 is a TiO₂ based pigment obtainable from Kronos, BYK 380 is a levelling agent obtainable from Byk, Rheoloate 1 is a polyacrylate thickener obtainable from Rheox,

Viaktin VAN 1743 is an unsaturated polyester resin obtainable from Vianova Resins,

Viaktin 03546 is cross-linker comprising unsaturated functional groups obtainable from Vianova Resins,

Crylcoat 109 is a flowing agent obtainable from UCB Chemicals, Irgacure 184 is a UV-photoinitiator obtainable from Ciba Chemicals.

EXAMPLE I (thermally curable powder coating dispersion): A solid powder coating composition, comprising 932 grams of Crylcoat 76-17, 49 grams of Primid XL552, 11.8 grams of Resiflow PV88, and 7.2 grams of Benzoine, obtained by mixing the ingredients at about 110°C in an extruder, was used as a raw material for preparing a dispersion according to the above- described general conditions and procedure, with 194 grams of the aqueous solution (zone 4), 82 grams of water (zone 4), and 724 grams of water (zone 7) being dosed at a constant rate. A white, milk-like powder coating dispersion with a solids content of 50% was obtained, wherein the pH was 7.2 and the average size of the spherical-like particles was 125 nm (standard deviation 7 nm).

EXAMPLE II (thermally curable, pigmented powder coating dispersion): A solid powder coating composition, comprising 653 grams of Crylcoat V76-17, 34 grams of Primid XL552, 8 grams of Resiflow PV88, 5 grams of Benzoine, and 300 grams of Kronos 2310 as pigment, obtained by mixing the ingredients at about 110°C in an extruder, was used as raw material for preparing a pigmented dispersion according to the above-described general conditions and procedure, with 138 grams of the aqueous solution (zone 4), 136 grams of water (zone 4), and 726 grams of water (zone 7) being dosed at a constant rate. A white, milk-like powder coating dispersion with a solids content of 50% was obtained, wherein the pH was 7.1 and the average size of the spherical-like particles was 420 nm (standard deviation 18 nm).

EXAMPLE III (UV curable powder coating dispersion):

A solid powder coating composition, comprising 644 grams of Viaktin VAN 1743, 276 grams of Viaktin 03546, 50 grams of Crylcoat 109, and 30 grams of Irgacure 184, was used as raw material for preparing a dispersion according to the above-described general conditions and procedure, with 136 grams of the aqueous solution (zone 4), 84 grams of water (zone 4), and 780 grams of water (zone 7) being dosed at a constant rate.

A white, milk-like powder coating dispersion with a solids content of 50% was obtained, wherein the pH was 7.0 and the average size of the spherical-like particles was 179 nm (standard deviation 9 nm).

EXAMPLE IV (coating formulation based on example 1): A sprayable clear coating composition was prepared by mixing 1000 parts of the powder coating dispersion described in Example 1 with 550 parts of water, 8 solid parts of polyacrylate thickener Rheolate 1 and 3 parts of levelling additive BYK 380. After mixing thoroughly, 3 parts of 10 wt.% Dimethylethanolamine solution was added to adjust pH to 8.8.

Example V (coating formulation based on example 2):

A sprayable pigmented coating composition was prepared by mixing 1000 parts of the powder coating dispersion described in Example 2 with 570 parts of water, 5 solid parts of polyacrylate thickener Rheolate 1 and 2 parts of levelling additive BYK 380. After mixing thoroughly, 40 parts of 10 wt.% Dimethylethanolamine solution was added to adjust pH to 9.4.

EXAMPLE VI (comparative):

The suitability of coating formulations based on the described powder coating dispersions for coating steel panels was compared with the suitability of solid powder coating compositions by investigating the properties of the cross-linked films formed on the steel panels.

The coating formulations obtained in Examples IV and V were applied to steel panels (Q-panel R36) and cured at 180°C for 15 minutes. Furthermore, the solid powder coating compositions that were used as raw materials in Examples I and II were pulverised, sieved on a sieve having a mesh size of 100 μm, applied on steel panels by using standard powder coating application techniques, and cured under the same conditions as the coating formulations based on the dispersions.

The properties of the cross-linked films formed on the steel panels were measured after storage at room temperature for at least 24 hours (Table 1).

The film thickness was measured in accordance with ASTM D1186.

The Gloss 20° was measured in accordance with ASTM D523. The Impact resistance (Direct/Reverse) was measured in accordance with

ASTM D2794. The Cross-cut test was carried out in accordance with ISO 2409.

The Scratch resistance determination with was carried out in accordance with

ASTM D1474.

TABLE 1 : properties of coatings on steel

It was concluded that the films obtained from powder coating dispersions are thinner than the films obtained from solid powder coating compositions, and that the properties of films that have been obtained from the same basic raw materials (e.g. when comparing the pigmented powder coating dispersion paint of Example V with the pigmented solid powder coating composition of Example II) are on the same level.

Claims

1. A process for preparing an aqueous powder coating dispersion from a solid powder coating composition and an aqueous medium, said composition comprising 1) a homogeneous mixture of a resin, a cross-linker, and, optionally, a pigment, a catalyst, an initiator, and/or one or more additives, or 2) a radiation-curable resin and, optionally, a cross-linker, a pigment, a catalyst, an initiator, and/or one or more additives, characterised in that the process comprises the steps of: a) elevating the temperature of the solid powder coating composition such that a molten-like substance is obtained the temperature of which is above the melting point or the glass transition temperature (T_g) of the solid powder coating composition, b) optionally cooling down the molten-like substance, c) dispersing said substance, in the cooled down state or not, in the aqueous medium.

2. The process according to claim 1 wherein the molten-like substance is cooled down to such an extent that when said substance is dispersed in the aqueous medium, the temperature in the forming dispersion does not exceed the boiling point of the aqueous medium.

3. The process according to claim 1 or 2 wherein the dispersing is carried out at atmospheric pressure.

4. The process according to any one of claims 1-3 wherein the aqueous medium comprises a dispersing agent.

5. The process according to claim 4, characterized in that the dispersing agent is a neutralising agent which can form anions with the functional groups present in the resin and/or crosslinker.

6. The process according to claim 5 wherein the boiling point of the neutralising agent is below the curing temperature of the solid powder coating composition.

7. The process according to claim 5 or 6 wherein the neutralising agent is a tertiary amine or ammoniak.

8. The process according to any one of claims 1-7 wherein the solid powder coating composition comprises a radiation-curable resin and, optionally, a photoinitiator.

9. The process according to any one of claims 1-8 wherein all steps in the process are carried out in an extruder.

10. A process for preparing a coating composition by adding to an aqueous powder coating dispersion one or more additional components selected from suitable pigments, thickeners, catalysts, initiators, additives, and fillers, characterised in that the aqueous powder coating dispersion is obtained according to any one of claims 1-9 and that the dispersion is filtered before or after the addition of said additional components.

11. A process for preparing a solid powder coating composition, characterised in that the solid powder coating composition is prepared either by drying the aqueous powder coating dispersion obtained by the process according to any one of claims 1-9, or by drying the aqueous coating composition obtained by the process according to claim 10.

12. Aqueous powder coating dispersion comprising 1) a homogeneous mixture of a resin, a cross-linker, and, optionally, a pigment, a catalyst, an initiator, and/or one or more additives, or 2) a radiation-curable resin and, optionally, a cross-linker, a pigment, a catalyst, an initiator, and/or one or more additives, characterized in that the resin and/or the optionally present cross- linker have functional groups that may be regarded as self-emulsifyable groups.

13. Aqueous powder coating dispersion according to claim 12, characterized in that the resin and/or the optionally present cross-linker has carboxylic acid, sulphonic acid, and/or phosphonic acid functional groups.

14. Aqueous powder coating dispersion comprising 1) a homogeneous mixture of a resin, a cross-linker, and, optionally, a pigment, a catalyst, an initiator, and/or one or more additives, or 2) a radiation-curable resin and, optionally, a cross-linker, a pigment, a catalyst, an initiator, and/or one or more additives, characterized in that the average particle size of the particles present in the dispersion is in the range from 50 to 800 nm.