Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
  • B05D3/06—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by exposure to radiation
  • B05D3/061—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by exposure to radiation using U.V.
  • B05D3/065—After-treatment
  • B05D3/067—Curing or cross-linking the coating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D1/00—Processes for applying liquids or other fluent materials
  • B05D1/02—Processes for applying liquids or other fluent materials performed by spraying
  • B05D1/04—Processes for applying liquids or other fluent materials performed by spraying involving the use of an electrostatic field
  • B05D1/06—Applying particulate materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
  • B05D3/02—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by baking
  • B05D3/0209—Multistage baking
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
  • B05D3/02—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by baking
  • B05D3/0218—Pretreatment, e.g. heating the substrate

Definitions

• the present invention relates to a method for powder coating and a plant for carrying out the method.
• Powder coating is a well known method for coating of objects.
• the starting material is a powdered coating, which is electrically charged and sprayed against the surfaces of the object, and which material is finally adhered and converted to a solid state by heating to its melting point.
• the powder consists of a plastic which is cured by heating, this must be brought to a comparatively high temperature, in the order of 200 °C.
• the coating method may well be performed on objects having good heat resistance and a conductive surface. If the surface is non-conductive, implying that the object cannot be earthed or supplied with a charge of an opposite polarity to the charge of the powder, difficulties arise in getting the powder to adhere during the time period between the spraying and the heating to the melting temperature.
• the addition of water may impair the adhesion of the powder coating and damage the object by confining the applied water under the coating.
• the object of the invention is to achieve a method which may be applied to powder coating of objects which are not suitable for heating to a high temperature, which may be limited to approx. 100 °C and also below.
• the method may be carried out without the need for any varnishing with a conductive varnish or any addition of moisture.
• the method is therefore suitable when coating wooden objects, such as furniture, and objects made of a plastic which, for example for reasons of tenacity or cost, is chosen from a type providing the finished object with a surface having a different look than the one possible with the construction plastic itself.
• the coating may be a clear varnish which allows the structure of the wood to stand out.
• the method comprises the following main steps:
• a powder for the coating Preparing a powder for the coating, said powder having a low melting point, approximately 60-100 °C and consisting of a polymer being curable by electromagnetic radiation, and in particular radiation by ultraviolet light.
• Heating to cause the powder particles to melt to a levelled layer and adhere to the surfaces of the object may be done by heating the object or the surrounding atmosphere, so that the application of the powder and the heating take place in the same operation, whereby special heating after the application of the powder is unnecessary.
• the method may be carried out without creating any opposite polarity between electrostatically charged powder and the object. Such a polarity difference may, however, occur and is valuable in order to get the powder distributed to all surfaces of the object, especially when of a complicated configuration.
• the method does not require, though does not exclude, any form of charging or neutralisation of the object, for instance when objects made of non-conductive material are concerned, through the addition of any method disclosed by way of introduction, coating with a conductive varnish or moistening.
• an electrostatic charge is attained in certain materials when they are heated, a fact which may be utilised in certain circumstances.
• the invention also comprises a powder for use in the method.
• the method comprises a number of main steps. These will now be described in greater detail for a certain embodiment.
• the main steps have been complemented by a number of sub-steps in order to adapt the method to the special requirements of the embodiment.
• the powder is composed of a polymer and may be pigmented for a coloured coating or non-pigmented for a clear coating which renders the underlying surface visible. This is something which is often aimed at when wooden objects are concerned.
• a principal property is that the powder should have a melting point which is lower than the temperature to which the objects, which are to be coated with the powder, should be heated.
• This temperature limit is partly decided by the properties of the material of the object, since the structure of certain materials changes at a temperature, which may be fairly low, already below 100 °C when certain thermoplastics are concerned. Said temperature limit is also partly decided by the sensitivity of the object in question to deformation when heated.
• This sensitivity depends on the construction of the object, an object having a compact form is not as easily deformed as disc-formed or long slender objects - and also depends on how homogenous the material in the object is; certain wood species are very sensitive to deformation when heated.
• As a principal region for the melting point or the softening point of the powder 60-100 °C may be specified.
• the object it is not necessary for the object to be through-heated to the melting temperature of the powder, but only its surface, however to such a depth that the temperature is fairly uniformly distributed in the object, and in such a way that the temperature is retained until the powder is applied on its surface. It is not intended by the expression "the melting temperature of the powder” that the powder material has to have become fluid, but in many cases it is sufficient that it has reached such a degree of softening that it sticks to the surface intended to be coated.
• the powder is at least partially composed of polymers such as polyester in addition to levelling agents.
• 350-400 nm may be attained if polymers in a known way are admixed with initiators, or in another manner are provided with a curing system which may be activated by radiation.
• the main component of the powder is 50- ⁇ 100% of an unsaturated, amorphous or crystalline, polyester.
• a curing agent is preferably included in order to obtain an increased crosslinking during the course of the curing.
• This curing agent may to 15-50% be an aromatic urethane diacrylate oligomer, a triacrylate of trihydroxyethyl-isocyanurate, a vinyl ester, an oligomer acrylo-urethane or the like.
• Addition of a photoinitiator is required in order to initiate the curing sequence. This addition may vary between 1-3%.
• the melting temperature of the powder should at the most be 80-90 °C in order to ensure that a wooden component is not damaged during the melting phase.
• the melting should be done by means of IR heat or with a combination of IR and convection heating. This implies that the melting phase, at such comparatively high temperatures as this, does not have to take place during a particularly long time since IR rapidly heats the wooden components to the wanted temperature. A few minutes may be assumed to be what is needed, but this is very dependent on the material which is to be coated. Certain wood materials are very sensitive to a rapid heating and may exhibit strong degassing. This may imply that a slower and more careful heating method has to be used.
• Step V After melting follows the curing procedure, see Step V. It should take place at different UV-wavelengths depending on how the varnish is pigmented and on the photoinitiator which has been added.
• rutile titanium dioxide In white pigmented varnishes, rutile titanium dioxide is used which absorbs at these wavelengths. Consequently, another photoinitiator, which reacts to wavelengths that are not absorbed by the pigment, must be used. This requires the use of another lamp. There are lamps which have a maximum at 350-400 nm and at 400-450 nm and there are also photoinitiators which absorb at these two high wavelengths.
• High intensity lamps may imply that it is easier to cure thick layers with these and that the curing rate may be raised.
• the component which is to be cured does not have to be in focus, but the intensity at a certain distance may be sufficient. This is especially noticeable when clear varnishes are concerned; for pigmented systems it is more important that the intensity be as high as possible.
• this formulation may be pigmented.
• the photoinitiator should be adapted accordingly.
• powder compositions for application in the invention may start from main components other than polyester, such as epoxy-, acrylates, urethanes, melamines and others. Also, mixtures of several different polymers may be used.
• Step II Preparation of the object in order to retain the powder on its surface
• Heating the object which is to be coated is assumed to have a limited heat resistance; typical of such are wooden objects, pressed objects such as woodfibre-board or plastic objects. This includes objects made of reinforced plastics and/or objects having a high addition of filler.
• a material has low heat resistance, as when wood and a majority of plastics are concerned, generally also implies that it is non ⁇ conductive.
• Materials of high heat resistance are typically construction metals which are conductive.
• Conventional powder coating generally presumes objects with a conductive surface, however the present invention is not limited to such objects but may advantageously be applied also when non-conductive surfaces are concerned, and no pretreatment in order to achieve conductive properties has to occur. This makes the method particularly valuable.
• the method may also, as earlier mentioned, advantageously be applied to solid objects, e.g. cast iron bodies, in order to reduce the energy consumption for heating.
• the heating may occur in different ways: through convection by means of heat air flow, through infrared radiation, or in exceptional cases, when for instance plates which are to be coated only on one side are concerned, through heating by conduction from heated surfaces.
• Particularly useful is a method in which simultaneous heating occurs by means of convection by air flow and by means of IR radiation.
• the IR radiation provides a rapid and comparatively deep heating of surfaces which are hit and the air flow results in the temperature being very uniformly distributed over the surfaces of the object. This even applies for objects having a very complicated outer shape and also when the IR radiation does not reach all surface sectors.
• the heating is presumed to occur in a chamber, set up for the purpose, in a plant where the objects which are to be coated may be transported between different work stations intended for carrying out the method steps. See the description of the plant.
• Step III Powder spraying
• the respective objects are transported to a location at which the powder may be sprayed on.
• This is conveniently accomplished by means of spray guns arranged in such a way that the surfaces which are to be coated may be impacted by the powder.
• the guns are arranged to charge the powder with an electrostatic charge. It is previously known to use a high voltage driven charging device, or that the powder, during its journey through the spraying equipment, is charged by friction against walls made of a material adapted to the purpose. The charge makes the powder granules repel each other, whereby clouds of particles are formed which encompass the object.
• the particles When the particles impact the object they will, when performing the aforementioned adaption between the heating temperature of the objects and the melting temperature of the powder, arrive in a sticky state and be deposited on the surface of the object. In this manner the respective objects receive a covering, but uncured layer of the polymer-based coating material.
• Step IV Heating to the melting temperature of the powder As already evident, such heating is performed in connection with the application of the powder.
• Complementary step Intermediate tempering Curing with UV radiation now remains in order to obtain a finished coating. However, it may, at least in certain cases, be convenient to regulate the condition of the applied, sticky coating layer. Such a change of the layer occurs by means of a temperature change, either cooling or heating. In certain cases there might be a risk that the layer, in its partly dissolved, tacky state and particularly through the continued heating by means of conduction from the heated object, reaches such a fluent state that there is a risk of running and drop-forming at protruding edges. In order to prevent this, cooling may be undertaken, thus lowering the temperature which was necessary for melting the powder particles to a temperature at which the formed layer obtains a more solid state.
• heating after the spraying may instead be valuable in order to lower the viscosity.
• the incompletely melted powder granules can be made to run together in order to form an uniform layer.
• this subsequent heating must be performed in such a way that, in the main, only the applied layer is heated but not the underlying object.
• the heating may be undertaken by means of a rapid process involving IR radiation, conveniently in combination with a heated air flow in a short process.
• Step V Curing
• the polymerization of the powder material occurs by heating, as a rule in a convection oven, when conventional powder coating is concerned. Accordingly the heating at first leads to a fusion of the material while the powder granules are initially retained by means of electrostatic forces. Thereafter the curing, which is initiated by the heating, occurs.
• the present method is aimed at carrying out the process at such a low temperature that no curing can be attained by means of the heating, or that in any case would require such a long time after initiation, that it would render it unfeasible in an industrial process.
• the curing must be accomplished in another way: by initiation of the curing process by means of ultraviolet radiation.
• Step I it has been described how the powder material is prepared for such a curing.
• the UV curing takes place in a specially adapted chamber into which the objects are brought after the powder spraying and the possible intermediate tempering.
• a number of UV radiators are arranged, from which the radiation should reach all coated surfaces of the object.
• special arrangements might be necessary.
• the objects may be rotated or moved in another way in front of the radiation sources.
• the initiator system of the material When the radiation impacts the coating layer, the initiator system of the material will start the polymerization. It is thereby possible to conduct this very rapidly - times down to 2 seconds are possible.
• the short processing time in relation to the time for heat curing gives important advantages when industrial production is concerned. On the one hand, a faster flow-through of work pieces and, on the other hand, a possibility to reduce the length of the plant in relation to what is required for a curing oven are attained.
• the earlier mentioned intermediate tempering, particularly cooling may take place simultaneously with the UV radiation. By means of an adapted cooling it may be prevented that the temperature during the curing reaches disadvantageously high values because of the energy contribution from the flow of heated objects and because of the UV radiation.
• Step V the method has been completed and the objects have obtained a cured coating. Accordingly, all advantages which are associated with powder coating, namely the possibility of obtaining greater layer thicknesses and higher mechanical resistance as compared to wet varnishing, have been attained.
• the method is also very environmentally friendly. This is because no solvents need to be used, and powder which in the spraying step did not impact the object, may be collected in the spraying chamber in order to be reused.
• the plant shown in the drawing is in the form of a tunnel 1 through which the objects 2 which are to be treated may be passed by means of a suspended conveyor 3, the transporting portion of which is travelling in a direction from the left to the right in the drawing.
• the tunnel is shown in an opened-up state along a longitudinal section. Accordingly it is evident that it is divided into four chambers, each being adapted for the realization of one of the Steps II-V - Preparation of the powder, Step I is not included in the plant - the powder is presumed to be added in a state of preparation, ready for use in the plant.
• This chamber exhibits both radiators 6 for infrared light as well as inlet openings 7 for heated air from a combined heating and blower set.
• a chamber 9 follows for the spraying process.
• spray guns 10 are inserted, which via hoses 12 are connected to a powder container 13.
• the spray guns may be provided with several spray nozzles 15.
• the powder may be sucked from the container 13 up through the hose 12 to the respective gun 10 in order to be sprayed out via the nozzles 15.
• the guns may be provided with charging surfaces which are supplied with a high voltage electrical current.
• the next chamber 16 is arranged for the occasionally occurring post-tempering. It is provided with inlet openings 17 for either heated or cooled air and may also be supplied with IR radiators for complementary heating. This chamber may be omitted if, in the processes concerned, no post-tempering is presumed.
• a remaining chamber 18 is adapted for step V, the curing step.
• a number of radiators 19 for UV radiation are placed in the chamber.
• mirrors for re ⁇ directing of radiation may also be present and the walls of the chamber may conveniently be reflective.
• inlet openings 22 for air In order to enable the temperature to be kept constant or to even achieve cooling in this chamber, it is provided with inlet openings 22 for air.
• This air may be collected partly from a return line 23 from the chamber and partly from an inlet 24 from a source of air with a temperature corresponding to or lower than the lowest temperature which is assumed to be required from the cooling air through the openings 22.
• This source may be the ambient atmosphere if the ambient temperature is sufficiently low, or air from a refrigerating machine.
• outlet 25 for air from the outlet opening 26 in the chamber in case the discharged air is not completely going in return and in through the openings 22, but is completely or partially replaced by air from the inlet 24.
• the proportion between return air supplied through the openings 22 and fresh air from the inlet 25 is controlled by a thermostat-controlled throttle 27 in order to keep the temperature inside the chamber constant at the temperature most suitable for the process.
• the objects are in turn suspended on the transporting portion of the conveyor 3. Initially the objects are brought into the chamber 5 in turn.
• the conveyor moves with a speed adapted to the length of time required for the treatment step in order to obtain a sufficient retention time in the respective chambers.
• the objects are surrounded by heated air, blown through the openings 6 in a smooth flow, and are exposed to IR radiation from the rays 7. This leads to a heating, well distributed over the surface of the objects, which is driven far enough to make it possible to retain the heat required for the next step.
• the powder spraying In the chamber 9 the next step is performed, the powder spraying. It should be evident from the preceding description how this is performed with the aid of the spray guns 10. These generally have to be adapted to the object in question in terms of their positions and often also to their design, for instance the number of nozzles. In certain cases, it might be necessary to suspend the spray guns in a movable way, making them perform a movement pattern during the spraying.
• a complementary heat treatment is performed in the chamber 16, either cooling in order to stabilise the layer on the heated objects, or heating in order to achieve a better levelling-out of the layer sticking onto the objects.
• the curing is initiated in the chamber 18 by radiation from the UV radiators 19.
• a certain curing time may be required, and the chamber 18 is conveniently extended in such a way that the layer is stabilised when the objects leave the chamber.
• the radiation equipment may be differentiated along the extension of the chamber, for instance with a more intensive radiation at the inlet end of the chamber than at the outlet end.
• the method as well as the powder composition may be adapted in a multitude of different ways to the actual requirements and types of objects which are to be treated, and to the material thereof.
• melting of a powder which is fusible at a low temperature is applied, thus bringing about the formation of a polymer layer on the surface of the respective objects which are to be coated, whereafter the curing takes place by means of radiation without any substantial temperature increase. Throughout the process a temperature is thus maintained which is considerably lower than that which has previously been practised within the field.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Paints Or Removers (AREA)

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Citations (2)

• 1995
  • 1995-08-10 SE SE9502795A patent/SE504784C2/sv unknown
• 1996
  • 1996-08-09 ES ES96927229T patent/ES2154415T3/es not_active Expired - Lifetime
  • 1996-08-09 PL PL96324925A patent/PL182899B1/pl not_active IP Right Cessation
  • 1996-08-09 PT PT96927229T patent/PT846034E/pt unknown
  • 1996-08-09 DK DK96927229T patent/DK0846034T3/da active
  • 1996-08-09 CA CA002229174A patent/CA2229174A1/en not_active Abandoned
  • 1996-08-09 RU RU98103745/12A patent/RU2192315C2/ru not_active IP Right Cessation
  • 1996-08-09 WO PCT/SE1996/001003 patent/WO1997005963A1/en not_active Application Discontinuation
  • 1996-08-09 EE EE9800047A patent/EE9800047A/xx unknown
  • 1996-08-09 EP EP96927229A patent/EP0846034B1/en not_active Revoked
  • 1996-08-09 AU AU67119/96A patent/AU6711996A/en not_active Abandoned
  • 1996-08-09 CN CN96197273A patent/CN1108197C/zh not_active Expired - Fee Related
  • 1996-08-09 DE DE69610930T patent/DE69610930T2/de not_active Revoked
  • 1996-08-09 AT AT96927229T patent/ATE197414T1/de not_active IP Right Cessation
• 1998
  • 1998-02-09 MX MX9801095A patent/MX9801095A/es not_active IP Right Cessation
  • 1998-02-09 NO NO980547A patent/NO980547L/no not_active Application Discontinuation

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