SE504784C2 - Process for powder coating and powder for carrying out the process - Google Patents

Abstract

A method and a powder for powder coating in which objects (2) which are to be powder coated are prepared in order to temporarily retain the powder, whereafter the powder is applied onto the object in a layer which is retained on the object until the powder, by means of melting and transformation to a solid state by means of curing, is caused to form a coating layer on the surfaces of the object. The powder is prepared in order to have a low melting and softening temperature below about 100 DEG C and preferably 60-100 DEG C, and comprises an initiator system arranged to bring the powder material to curing under the influence of electromagnetic radiation. The powder prepared in this way is heated on the object to a temperature such that it melts and forms an adherent layer on the coated surfaces of the object. Thereafter, this layer is subjected to the radiation, thus curing it to a coating layer covering the powder-coated surfaces.

Description

504 784 2 10 15 20 25 30 certain processes solved by either varnishing the object with a conductive varnish or exposing it to water in such a way that a conductive moisture layer is formed on the surface.

However, these methods have been of limited use due to disadvantages such as the fact that the coating involves an extra operation and an extra material addition and can also give a poorer adhesion than powder coating on the clean surface and in clear lacquer also a discoloration.

Water supply, on the other hand, can impair the adhesion of the powder paint and damage the object by trapping the applied water inside the paint layer.

A further method of causing the powder to adhere to the surface of the non-conductive object is described in DE, A1, 3 211 282 (August Albers). The object, which has good heat resistance, glass objects are mentioned, is heated to a temperature of 400-900 ° C. This leads to the powder grains hitting the object melting and sticking to the surface, so that the curing process for transferring the high temperature required for this method, the powder to homogeneous solid form can not be completed. objects are treated that are at risk of deforming or otherwise changing even at lower temperatures. The method in question can therefore not be applied, for example, to objects made of wood or plastic. 10 15 20 25 30 3 504 784 DISCLOSURE OF THE INVENTION; The object of the invention is to provide a process which can be applied for powder coating of articles which are not suitable for heating to any higher temperature, it should be possible to limit it to about 10 ° C and also below. The procedure can be carried out on objects with a non-conductive surface without the need for any varnishing with conductive varnish or the addition of moisture. It is hereby suitable for use in varnishing wooden objects such as furniture and objects of a plastic, for which, for reasons of strength or cost, for example, a type is chosen which gives a surface which the finished object wishes to give a different appearance. than the construction plastic itself allows. In the case of wooden objects, the coating can be a clear varnish, which allows the structure of the wood to appear.

According to the invention, the method comprises the following main steps: I. Preparation of a powder for the coating, which has a low melting point, approximately 60-10 ° C and consisting of a polymer with the property of being initiated for curing ultraviolet radiation.

II. Preparation of the object so that the powder can be retained on its surface until a permanent retention is achieved by melting and curing the powder. This can be accomplished in various ways depending on the material and design of the article, such as: N 15 20 25 30 30 a) Heating the object to the melting temperature of the powder, so that the powder particles adhere to the surface during melting. This can be done regardless of whether the object has a conductive surface or not and with the powder composition specified here at a low temperature. b) Spraying the powder in such a heated atmosphere that it reaches its melting temperature and in the molten state adheres to the surface of the object. c) Retention of the powder by electrostatic forces, thus by giving the powder an electric potential and the object a potential of opposite polarity. This can be achieved with objects with a conductive surface and with objects of non-conductive material, the surface can, as stated in the introduction, be made conductive by a conductive varnish or wetting. Other ways of creating the opposite polarity of the object may also be applied. 10 15 20 25 30 5 504 784 III. Application of the powder, preferably by spraying while the powder particles are electrostatically charged, good spreading in the space. However, application to others so that they are obtained in this way is not excluded, for example, dipping in a fluidized powder bed may occur.

IV. Heating, so that the powder particles melt and adhere to the surfaces of the object. As shown in (II a and b) above, attachment of the powder to the surface of the object can be done by heating the object or ambient atmosphere so that the application and melting of the powder takes place in the same operation, whereby special heating after application of the powder is not required.

V. Exposure of the object to ultraviolet radiation, so that the curing process is initiated.

Here it appears that the process can be carried out without creating any opposite polarity between electrostatically charged powder and the object. However, this can occur and is valuable for getting the powder spread to all surfaces of the object, especially if it has a complicated configuration. The method thus does not require, but does not exclude any form of charging or neutralizing the object, in the case of objects of non-conductive material, for example by adding any method initially described, coating with conductive varnish or wetting . In some materials, an electrostatic charge is triggered during heating, a condition that can be used in certain circumstances.

DESCRIPTION OF THE FIGURES: In the accompanying drawings, a schematic representation shows a plant for carrying out the method according to the invention.

PREFERRED EMBODIMENT: A brief description of the procedure according to the invention has been provided in the introduction to the description. According to this, the procedure comprises a number of main steps. These will now be described in more detail in a specific embodiment. In this case, the main steps have been supplemented with a number of sub-steps for adapting the method to the special requirements of the embodiment.

Step I: Preparation of powder The powder consists of a polymer, and can be pigmented for a colored coating or unpigmented for a clear coating, which makes the underlying surface visible, something that is often sought after in wooden objects. A main property is that the powder should have a melting point which is lower than the temperature to which the objects intended to be coated with the powder should be heated. This temperature limit is determined partly by the properties of the material in the object, certain materials change to their structure at a temperature which can be relatively low, at certain thermoplastics already below 100 ° C, and partly by the sensitive sensitivity to deformation during heating. This sensitivity depends on the structure of the detail; objects with a compact shape are not deformed as easily as disc-shaped or long slender objects - and also on how homogeneous the material in the part is; some woods are very sensitive to deformation when heated. As the main range for the melting point or softening point of the powder can be stated 60-100 "As is understood from the following description, the object does not need to be heated to the melting temperature of the powder but only its surface, but to such a depth that the temperature is fairly uniform the temperature is maintained until the powder is laid on its surface. By melting temperature for the powder is not meant that the powder material needs to have become runny, but at least in many cases it is sufficient that it has achieved such a softening that it adheres to the intended coating surface.

The fact that only the surface needs to be heated and that the temperature can be kept low is advantageous when powder coating objects which, although they can withstand higher temperatures, but which it is disadvantageous to heat to a high temperature. This is the case, for example, with objects with a large mass, where heating to a higher temperature gives a large energy consumption. This is especially the case with objects with conductive materials, where the heat spreads quickly inwards. As an example may be mentioned solid cast iron objects, which require a considerable heating time with high energy consumption if methods other than the present are applied.

Another main property that the powder material should have is that its curing can be initiated by ultraviolet radiation.

Low melting temperature can be obtained by the powder consisting at least in part of polymers such as polyester and also flow agents.

Curing by ultraviolet radiation in the wavelength range 350-400 nm can be achieved if polymers are added in a known manner with initiators.

These are only examples of how said properties can be achieved and there are also other compositions of the powder which can give it the desired properties. Without pigmentation or other staining, a clear layer of polymer powder is obtained after curing, which does not obscure the underlying surface. If an opaque layer, pigment or dyes are desired. such as opaque, white, black or colored are added It is also possible to control the gloss of the coated surface through additives. If the additives give changes in the mentioned, necessary properties, low melting point and possibility of UV curing, this must be taken into account in the composition of the powder and possibly also in the design of the process.

Preferred composition of polymer powder for the process described here: The main component of the powder is 50-polyester, amorphous or crystalline. In addition, a hardener is preferably included in order to obtain increased crosslinking during the curing process. This hardener can be an aromatic urethane diacrylate oligomer, a triacrylate of trishydroxyethyl isocyanurate, a vinyl ester, an obligate acrylic urethane or something else similar to 15-50%. the hardening process. This additive may vary. An additive of a photoinitiator is required for between 1-3%. For a clearcoat it is good to use 1-hydroxy-cyclohexyl-ketone as photoinitiator and for white pigmented systems 2,4,6-trimethylbenzoyldiphenylphosphine oxide can be used. However, this is given by way of example only and completely different photoinitiators may be needed for special purposes. An addition of a flow agent is also required. 1-3% of this is recommended.

As a floating agent, e.g. acrylates are used. A large number of other additives can also be added, for example to mattify the paint, avoid degassing problems from the part to be painted or the like.

Basic recipe for a composition that is to pre- (clear lacquer) leaching after melting at low temperatures and soak for varnishing wood and which provides good solvent resistance. 504 784 W 10 15 20 25 30 Unsaturated polyester 70-85% Hardener 15-30% Photoinitiator 1-3% Liquid 1-3% The melting temperature of the powder should not exceed 80-90 ° C in order to be sure that a wooden hoist is not damaged during the melting phase. The melting should be done using IR heat or a combination of IR and convection heat. This means that the melting phase at such relatively high temperatures as this does not have to last for a particularly long time because IR heats up the wood details quickly to the desired temperature. A few minutes can be assumed to be what is needed, but is very dependent on the material to be painted. Some wood materials are very sensitive to rapid heating and give strong degassing, this may mean that you have to choose a slower and more careful heating method.

After melting, the curing procedure follows, see Step V.

It should be done at different UV wavelengths depending on how the varnish is pigmented and which photoinitiator has been added. A UV spectrum lying in the lower region, 200-350 nm is suitable, provided that a photoinitiator absorbing at this region is used.

In white pigmented varnishes, rutile titanium dioxide is used, which absorbs at these wavelengths. Thus, another photoinitiator must be used that responds to wavelengths that are not absorbed by the pigment, which requires a different lamp. There are lamps that have a maximum at 350- N U 20 25 30 u 504 784 400 nm and at 400-450 nm and there are also photoinitiators that absorb at these high wavelengths. However, one can pigment UV-curing powder paint in a variety of other ways. Each time it is important to match the pigments with the right photoinitiator and lamp.

High-intensity lamps can make it easier to cure thick layers with them and increase the curing speed. The part to be hardened does not have to be in focus, but the intensity at a certain distance may suffice. This is particularly noticeable with clearcoats, for pigmented systems it is more important that the intensity is as high as possible.

Basic recipe for a composition that better leaves (clear lacquer), provides good flexibility and good adhesion to metals. lucky. pairs for painting metal and which Should not be used at too low process temperature- Unsaturated polyester 80- Hardener O-20% Photoinitiators l-3% Liquid l-3% This formulation can also be pigmented. The photoinitiator should then be adapted.

What has previously been said about radiation, radiation data largely applies to the last given recipe.

It should also be mentioned that in some compositions, photoinitiators can be replaced by other radiation-sensitive initiator systems.

The recipes provided are given by way of example only and may be varied within wide limits as indicated and in such a way that they may be used in the process described. Thus, powder compositions for use in the invention may be based on main components other than polyester, such as epoxy, melamines, acrylates, urethanes, etc. Mixtures of different polymers can also be used.

Step II: Preparation of the object, so that the powder is retained on its surface In the embodiment, II az Heating of the coating object is applied. The object to be coated is assumed to have limited heat resistance; Typical are wooden objects, pressed objects such as plates of wood fiber or plastic objects and thereby also of plastics with reinforcement and / or a large mixture of fillers. The fact that a material has low heat resistance, such as wood and most plastics, generally also means that it is non-conductive. Materials with high heat resistance are typically structural metals, which are conductive. Conventional powder coating generally presupposes objects with a conductive surface, however, the present procedure is not limited to such objects but can advantageously also be applied to non-conductive surfaces without the need for any pre-treatment to achieve conductive properties. This makes the procedure particularly valuable. However, with advantage, however, the method, as mentioned, can also be applied to solid objects, e.g. cast iron bodies, to reduce energy consumption for heating.

The heating can take place in different ways: by convection by means of the flow of hot air, by infrared radiation or in exceptional cases such as plates, which are to be coated on one side only, by heating by conduction from heated surfaces. Particularly advantageous is a method in which simultaneous heating takes place by means of convection by air flow and by means of IR radiation. The IR radiation provides a rapid and relatively deep heating of surfaces that it hits and the air flow means that the temperature is distributed very uniformly over the object's surfaces even in objects with a complicated external shape and even where the IR radiation does not reach all surface sections. The heating is assumed to take place in a specially designed chamber in a plant where the objects to be coated can be moved between different workstations intended for carrying out the procedure steps. See further in the plant description.

Step III: Powder spraying In immediate connection with the heating being carried out, the respective object is moved to a space where the powder can be sprayed. This is conveniently done with spray guns arranged in such a way that the surfaces to be coated can be hit by the powder. It is then suitable that the guns are arranged to charge the powder with an electrostatic charge. It is known to use a charging unit driven by a high-voltage electric current or by charging the powder by friction against walls of suitable material during its path through the spraying equipment. Through the charge, the powder grains will repel each other whereby clouds of particles can be made to enclose the object.

When the particles hit the object, they come to a sticky state during the previously described adaptation between the heating temperature and abut against the object of the object and the melting temperature of the powder. Thus, the respective object obtains a covering, but uncured layer of the polymer-based coating material.

Step IV: Heating to the melting temperature of the powder As stated, such heating takes place in connection with the application of the powder.

Complementary steps: Intermediate temperature To obtain a finished coating, curing with UV radiation now remains. However, in at least some cases it may be appropriate to control the condition of sticky change of the layer occurs by temperature change, the applied, coating layer. Such as either cooling or heating.

In some cases there may be a risk that the layer in its partially dissolved, sticky state and especially through the continued heating by conduction from the heated object, reaches such a thin liquid state that there is a risk of run-off and drip formation at the outlet. 25 30 15 504 784 sliding edges. To prevent this, a cooling can be effected so that the temperature required for melting the powder particles is lowered to a temperature where the formed layer acquires a more solid form.

Alternatively, depending on the temperature required by the powder used, if it is not suitable for heating the desired melting vessels, a post-spray heating for viscosity reduction may instead be of value. As a result, the incompletely molten powder grains can be made to flow together to form an even layer. If the temperature of the object has been kept low due to it not being exposed to a higher temperature, this post-heating must be carried out so that basically only the applied layer is heated but not the object below it. In this case, the heating can be carried out by a rapid process with heating by IR radiation, suitably in combination with the flow of hot air in a short process.

In many cases, such an intermediate tempering is not needed at all, and in that case this step is omitted.

Step V: Curing In conventional powder coating, the polymerization of the powder material mentioned by heating takes place, usually in a convection oven. The heating then first leads to melting of the material while the powder grains are initially held by electrostatic forces. Then the curing takes place, which is initiated by the heating.

The present process is directed to carrying out the process at such a low temperature that no curing can be achieved by the heating or would in any case require such a long time after initiation that it would be impractical in an industrial process.

Therefore, the curing must be performed in another way: by initiating the curing process with ultraviolet radiation. During Step I, it has been described how the powder material is prepared for such curing.

The UV radiation takes place in a specially arranged chamber to which the objects are fed after the powder spraying and the possible intermediate tempering. A number of UV radiators are arranged in the chamber, from which the radiation is to reach all the coated surfaces of the object. For some objects with complicated shapes and a coating on many sides, special arrangements may be required. Thus, it may be necessary to arrange a large number of UV radiators directed in different ways and they may also be supplemented with mirrors, which redirect existing radiation at new angles. You can also arrange movement of the UV rays around each object. Alternatively, the objects can be rotated or otherwise moved in front of the radiation sources. 10 15 20 25 30 _ obtained a cured coating. All advantages, ”504 784 When the radiation hits the coating layer, the initiator system of the material will start the polymerization.

It is then possible to implement this very quickly - times of up to 2 seconds are possible.

The specified intermediate temperature, especially cooling, can occur at the same time as the UV radiation. By means of an adapted cooling, the temperature can be prevented during the curing from reaching unfavorably high values by energy additions from the stream of heated objects and the UV radiation.

After Step V, the procedure is completed and the objects associated with powder coating, namely the possibility of achieving greater layer thicknesses and greater mechanical resistance than in wet coating, have thus been achieved.

The procedure is also very environmentally friendly; no solvents need be used and powders that did not hit the object during the spraying step can be collected in the spray chamber for reuse.

The attached figure schematically shows a plant in which the various process steps can be carried out in a rational, industrial process.

The plant shown in the figure is in the form of a tunnel 1 through which the objects 2 to be treated can be transported by means of a hanging conveyor 3, the conveying part of which runs in the direction from left to right in the figure. In the figure, the tunnel is shown opened in a longitudinal section 504 784 18 10 15 20 25 30. It appears that it is divided into four chambers, each set up for carrying out one of the Stage II-V production of the powder, Step I is not included in the plant - the powder is expected to be supplied in a prepared condition for use in the plant.

First comes a chamber 5 for Stage II, the heating.

This chamber has radiators 6 for infrared light and inlet openings 7 for heated air from a heating and fan unit.

This is followed by a chamber 9 for the spraying process. A number of spray guns 10 are inserted therein, which are connected via hose lines 12 to vessels 13 for the powder.

The spray guns can, as shown, be provided with several nozzles 15 each. By means of a compressed air driven system (not shown), the powder from the vessel 13 can be sucked up through the hose line 12 to resp. gun 10 for spraying with the nozzles 15. Here it is assumed that in the guns there are channels of a material, for example polytetrafluoroethylene, which in friction between the walls and the powder gives the same an electrostatic charge.

Alternatively or in addition, the guns can be provided with charging surfaces, which are supplied with high-voltage, electric current.

The next chamber 16 is arranged for the occasional post-tempering. It is provided with inlet openings 17 for either heated or cooled air and may also be provided with IR radiators for supplementary heating. This chamber can be excluded if no post-tempering is assumed in the intended processes.

A remaining chamber 18 is provided for step V, the curing step. In it is inserted a number of radiators 19 for UV radiation. As mentioned earlier, mirrors for redirecting radiation may also be present and suitably the walls of the chamber may be reflective.

When carrying out the procedure in the described plant, the objects are suspended in turn on the transport part to the conveyor 3. First, the objects are moved in turn into the chamber 5. The conveyor then moves at a speed which is adapted to the required length of time. for the treatment step, so that the residence time in resp. chamber becomes sufficient. In the chamber 5 the objects are wrapped with heated air in a calm flow blown through the openings 6 and exposed. This leads to a heating well spread over the surface of the objects, which drives IR radiation from the rays 7. so far that the required heat can be maintained. for the next step.

In the chamber 9, the next step, the powder spraying, is performed.

It should have been apparent from the foregoing description how this is performed by means of the spray guns 10. These must generally be adapted to their positions and often also to design such as the number of nozzles, to existing objects. In some cases it may be necessary to make the spray guns movably suspended so that they carry out a movement scheme during spraying.

If required, a complementary heat treatment is performed in the chamber 16, either cooling to stabilize the layer on the heated objects or heating for better flowing out of the layer sticking on the objects.

Finally, in the chamber 18, the curing is initiated by radiation from the UV rays 19. After the irradiation or in connection with this, a certain curing time may be required and suitably the chamber 18 is so extended that the layer is stabilized when the objects leave the chamber.

Radiation equipment can then be differentiated along the extent of the chamber, but for example more intense radiation at the entrance end of the chamber than at its exit end.

The method and plant described here is described as a preferred embodiment. However, other forms may be included within the scope of the appended claims. It has been mentioned at the outset that in order to retain the applied powder on the surface of the objects, this can be done in other known ways than by preheating the objects. In cases where one works with objects with a conductive surface, the adhesion of powder to the objects can very advantageously take place by means of electrostatic forces and the melting of the powder necessary for carrying out the process then takes place by a reheating without pre- WU 20 25 21 504 784 the targets need to be preheated. Such an embodiment of the method then completely follows the specified main steps: Step II c, electrostatic charge or neutralization of the object; Step III, application of the powder; Step IV, Step V, curing. heating to the melting temperature of the powder; and Various methods have also been applied for applying the powder, of which spraying of the powder is the most useful method and has therefore been chosen in the preferred embodiment.

The procedure as well as the plant can therefore be adapted in a variety of ways to the requirements and type of objects to be treated and to the material therein. Common to all its embodiments, however, is that a melt of a low-temperature meltable powder is applied, so that a polymer layer is formed on the surface of the respective object to be coated, after which the curing takes place by radiation without a significant temperature increase. The process thus maintains a temperature that is significantly lower than what has previously been applied in the area.

Claims (5)

504 784 10 15 20 25 30 35 40 22 PATENT REQUIREMENTS: A method of powder coating in which objects (2) which are to be powder coated with their surface are physically affected to temporarily retain the powder, after which the powder is applied to the object as by spraying in a layer which is retained by said physical effect on the object, and within the time temporary retention of the powder by melting and transfer to a solid state by curing is caused to form a lacquer layer on the powder-coated surfaces of the article, characterized in that the powder is prepared during production to have a low melting and softening temperature, mainly below 100 ° C and preferably 60 ° C. -100 ° C and that a polymeric material in the powder comprises an initiator system, arranged to cure the polymer under the influence of electromagnetic radiation, and that said physical influence on the surface of the object (2) comprises heating at least the surface layer of the object to such a temperature that the object applied the powder according to said preparation reaches such a degree of melting or softening that it adheres to the surface of the object by sticking, and that the powder is further heated so that it melts and forms said homogeneous layers, after which the curing is carried out by radiation with ultraviolet light. 2. A method according to claim 1, characterized in that the object (2) and the powder are physically actuated in such a way that they obtain electrostatically deviating potential, preferably different polarity, that the powder is applied, preferably by spraying, so that it spreads to the surfaces of the object by electrostatic forces. A method according to any one of claims 1 or 2, wherein after the powder has reached its melting temperature, the object is kept by characterized by tempering at such a temperature that the melting temperature of the powder is not substantially exceeded or preferably undercut while the curing is performed by the radiation. Powder for use in the process according to any one of claims 1-3, characterized in that it is composed of at least one polymer as main component, a photoinitiator system for bringing the polymer to curing by ultraviolet radiation and a flowing agent for achieving of said low softening and melting temperature, substantially not exceeding 100 ° C and preferably in the range 60-100 ° C. Powder according to claim 4, characterized in that the main component, which is preferably an unsaturated polyester, is present at a percentage of substantially 70 to close to 100%, a hardener to substantially a maximum of 30%, photoinitiators to substantially 1 -3% and flow agents mainly 1-3%.