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
  • B05D1/00—Processes for applying liquids or other fluent materials
  • B05D1/18—Processes for applying liquids or other fluent materials performed by dipping
  • B05D1/22—Processes for applying liquids or other fluent materials performed by dipping using fluidised-bed technique
  • B05D1/24—Applying particulate materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
  • B05B5/00—Electrostatic spraying apparatus; Spraying apparatus with means for charging the spray electrically; Apparatus for spraying liquids or other fluent materials by other electric means
  • B05B5/08—Plant for applying liquids or other fluent materials to objects
  • B05B5/087—Arrangements of electrodes, e.g. of charging, shielding, collecting electrodes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C—APPARATUS FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C19/00—Apparatus specially adapted for applying particulate materials to surfaces
  • B05C19/02—Apparatus specially adapted for applying particulate materials to surfaces using fluidised-bed techniques
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C—APPARATUS FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C19/00—Apparatus specially adapted for applying particulate materials to surfaces
  • B05C19/02—Apparatus specially adapted for applying particulate materials to surfaces using fluidised-bed techniques
  • B05C19/025—Combined with electrostatic means
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C—APPARATUS FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C9/00—Apparatus or plant for applying liquid or other fluent material to surfaces by means not covered by any preceding group, or in which the means of applying the liquid or other fluent material is not important
  • B05C9/08—Apparatus or plant for applying liquid or other fluent material to surfaces by means not covered by any preceding group, or in which the means of applying the liquid or other fluent material is not important for applying liquid or other fluent material and performing an auxiliary operation
  • B05C9/14—Apparatus or plant for applying liquid or other fluent material to surfaces by means not covered by any preceding group, or in which the means of applying the liquid or other fluent material is not important for applying liquid or other fluent material and performing an auxiliary operation the auxiliary operation involving heating or cooling
• • 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
  • B05D3/0254—After-treatment
  • B05D3/0272—After-treatment with ovens
• • 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
  • B05D3/14—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 electrical means
  • B05D3/141—Plasma treatment
  • B05D3/145—After-treatment
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
  • B05B5/00—Electrostatic spraying apparatus; Spraying apparatus with means for charging the spray electrically; Apparatus for spraying liquids or other fluent materials by other electric means
  • B05B5/025—Discharge apparatus, e.g. electrostatic spray guns
  • B05B5/047—Discharge apparatus, e.g. electrostatic spray guns using tribo-charging
• • 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

Definitions

• the present invention relates to a method of covering an object with a film originating from the melting of a thin layer of powder previously deposited on the object and the apparatus for implementing this method.
• the first method is electrostatic powdering, it consists of charging the powder with static electricity and putting it in contact with the object to be covered connected to a zero potential.
• the powder is injected into an electrostatic gun which will charge the said powder by Corona effect (also called crown effect), by triboelectrification or by a combination of the two.
• Corona effect also called crown effect
• the powder thus charged is sprayed onto the object to be covered, connected to a zero potential.
• the object connected to zero potential is soaked in a fluidized bed of charged powder. Inside the fluidized bed is powder with which we want to cover the object.
• This powder is in the form of small solid particles, for example between 0.01 and 1 mm, of any shape, which are in a state of fluidization inside the bed in the presence of air or any other gas.
• electrodes for charging the powder by corona effect or a device for charging it by triboelectric effect In the fluidized bed there are electrodes for charging the powder by corona effect or a device for charging it by triboelectric effect.
• the object covered with powder is then placed in an oven at a temperature sufficient to ensure a coating by fusion of the powder causing the filmification. For example, for a polyamide 11 powder, it suffices to heat to 220 ° C.
• the second method consists in preheating the object to be covered to a temperature higher than the melting temperature of the powder. Once hot, the object is immediately immersed in a fluidized bed of the powder, the powder melts on contact with the hot object and films. A solid covering is thus ensured. In this process, a hot object is soaked in a fluidized bed cold and to combat heat loss we need an oven at a temperature higher than that necessary for filming, which leads to increased energy consumption. In addition, if the object consists of parts having very different thermal inertias, these different parts do not have the same temperature and therefore the thickness of the film is not uniform.
• the present invention relates to the electrostatic process in which the object is soaked in a fluidized bed of charged powder.
• US Pat. No. 3,248,253 describes electrostatic fluidized beds in which are placed electrodes brought to very high potentials.
• the particles are charged by the corona effect which consists in ionizing the air in the vicinity of a point and therefore in electrically charging the particles in this zone.
• Objects to be coated attached to a conveyor system and connected to the earth, pass over the fluidized bed and become covered with powder by electrostatic attraction. According to a variant, the objects to be coated are immersed in the fluidized bed.
• a similar process is described in US Patent 4,381,728. Good recoveries are obtained in such beds but they present a certain danger due to the presence of electrodes carried at high potential which can give rise to electric arcs with the object to be covered.
• Patent WO 0076677 describes a process for covering an object with a film originating from the fusion of a thin layer of powder in which:
• the object covered with the powder is then placed in an oven at a sufficient temperature until the coating film is obtained by melting the powder. It is an electrostatic fluidized bed tribocharged essentially using a device other than the walls of the tank.
• the powder is tribocharged, thus creating a high volume density of charge inside the fluidized bed.
• the powder is loaded and fluidized. If we immerse in the charged bed an object to be covered connected to a zero or sufficient potential, we will be in the presence of an electric field created by the volume of charged powder. This will contribute to good electrodeposition on the grounded object.
• the object can be at a positive or negative polarization or zero.
• the powder is tribocharged, that is to say charged by contact or friction.
• the friction is provided by the air or the fluidizing gas which entrains the powder particles and allows them to come into contact with the tribocharging systems which will be described later.
• the charging system described in the present application is autonomous and does not require any energy input other than the gas ensuring the fluidization of the powder.
• a "honeycomb” is used as the tribocharging device. It is a structure made up of geometric elements whose section can go from any type of polygon (the elements are then prisms) up to the circle (the elements are then tubes). These elements are hollow, arranged vertically and preferably have a thickness of between 1 and 10 mm; their length is for example between 15 and 25 cm. These tubes are joined to each other so as to constitute a solid and homogeneous whole. The interstices between tubes are plugged by any means such as aluminum foil. Although any type of polygonal section can be envisaged, the cylindrical structure is preferred. A cylindrical geometry is preferred so as to allow homogeneous fluidization.
• Edge effects will be limited by an adapted length of the tubes constituting the honeycomb, that is to say that these tubes are advantageously greater than 15 cm in length.
• This "honeycomb" is placed at the bottom of the bed. Sufficient space must be left at the top of the bed to immerse the object and there must be around the said object a volume density of charge sufficient to ensure electrodeposition.
• the “honeycomb” is placed as low as possible in the bed, so as to optimize the contact in the tubes without however disturbing the fluidization.
• the diameter of the tubes is chosen as small as possible in order to increase the contact surface, but it is nevertheless necessary to ensure that the tubes will not get blocked and are therefore large enough to ensure correct fluidization.
• tubes 25 mm in diameter and 150 mm in length can be used. They are advantageously made of PVC or PTFE.
• Form circulation is understood to mean that part of the content of the fluidized bed, that is to say a mixture of fluidizing gas and powder, is withdrawn and using a pump or an equivalent device such as a gas ejector (preferably operating with the same gas as the fluidized bed) and passes through one or more tribocharging tubes.
• a pump or an equivalent device such as a gas ejector (preferably operating with the same gas as the fluidized bed) and passes through one or more tribocharging tubes.
• the pump and the tribocharging tube are outside the bed so as not to disturb its operation, the gas and the tribocharged powder after passing through the tribocharging tube are returned to the fluidized bed.
• the principle of tribocharging powders using a gun in which the powder to be charged is circulated under the effect of a gas flow is known from US Pat. No. 4,399,945.
• the object having been covered with powder it is taken out of the fluidized bed and it is heated so that the powder melts and forms a film on the object.
• the electrodes in the oven these electrodes are brought to a high electrical potential in order to cause a corona effect which compensates for the relaxation of the charge of the powder particles during their heating. This maintains the charge of the particles and therefore they remain on the object and can thus form the film by fusion. This has not been described in the prior art.
• the corona effect is used to electrically charge the powder in the bed while in the present invention the powder has been tribocharged, has deposited on the object and the corona effect is not used only to hold the powder on the object while it melts to form the film.
• the present invention is a method of covering an object with a film originating from the fusion of a thin layer of powder in which:
• the oven comprises electrodes brought to a high electrical potential in order to cause a corona effect which compensates for the relaxation of the powder particles during their heating. This maintains the charge of the particles and therefore they remain on the object and can thus form the film by fusion.
• the present invention also relates to the apparatus for implementing the method.
• the objects to be coated they can be of any kind provided that they can be immersed in the fluidization tank and withstand the temperature of the oven.
• metals such as aluminum, aluminum alloys, steel and its alloys.
• powders they consist of a substance which, when heated, will form a protective film for the object.
• Polyamide means condensation products:
• amino acids such as aminocaproic, amino-7-heptanoic, amino-11-undecanoic and amino-12-dodecanoic acids of one or more lactams such as caprolactam, oenantholactam and lauryllactam;
• diamine salts or mixtures such as hexamethylene diamine, dodecamethylenediamine, metaxylyenediamine, bis-p aminocyclohexylmethane and trimethylhexamethylene diamine with diacids such as isophthalic, terephthalic, adipic, azelaic, azelaic, dodecanedicarboxylic; or mixtures of several of these monomers which leads to copolyamides.
• polyolefins means polymers comprising olefin units such as, for example, ethylene, propylene, butene-1 units, etc. By way of example, mention may be made of:
• thin layer of powder means a thickness of up to 2 mm and advantageously between 0.1 and 0.6 mm.
• the fluidized bed it is dimensioned so as to completely immerse the part to be covered. Its shape does not matter as long as it contains the necessary volume of powder, that the part to be covered can be completely submerged and that the fluidization is correct.
• the forced circulation tribocharger device and first of all the material which will correctly tribocharge the powder a first choice can be made by comparing the working functions of the powder and of the material envisaged. This can be done by looking at the values of the work functions in electronvolts of the two species concerned and their respective positions in a triboelectric series. The more the difference:
• Ft denotes the function of work, these values are read in tables of triboelectric series such as for example ELECTROSTATICS of JA CROSS, IOP Publishing, 1987. Lower values can be considered, while knowing well that consequently the tribocharged will be less good and therefore the recovery less efficient.
• the powder is charged by triboelectrification, that is to say by friction or contact with a good tribocharger material.
• the tribocharger material is chosen according to the criteria previously defined. If the powder to be loaded is made of polyamide, the tribocharging material is advantageously made of PTFE (polytetrafluoroethylene). This principle of tribocharged is known in itself, this is what is used in the tribocharging pistols of the prior art.
• part of the powder and of the gas from the fluidized bed is removed and it is forced to pass through tubes of the tribocharging material and returns to the fluidized bed or it remixes with the powder present in the bed. It is not necessary to pass all of the gas and powder from the fluidized bed through the tubes of the tribocharger material.
• the gas flow containing the powder which is forced to pass through the tubes of the tribocharging material can be from 0.5 to 20% of the fluidization gas flow and advantageously from 1 to 10%. We can also define this flow of gas containing the powder which is passed through the tubes of the tribocharging material by its hourly flow which is a fraction of the volume of the fluidized bed.
• This fraction can be any, but advantageously it is from 0.5 to 30% of the volume of the fluidized bed.
• the tubes of the tribocharger material can be arranged in a spiral around a cylindrical part. Inside the tubes, charges will be created due to triboelectrification between the powder and the tribocharging material.
• a person skilled in the art can adjust the flow rate of gas charged with powder in the tribocharger device by observing the quantity of powder which is deposited on the object to be coated and which is soaked in the bed.
• the tubes made of tribocharging material can for example be between 1 and 6 m in length and between 4 and 15 mm in diameter, advantageously between 4 and 8 mm.
• the tubes having a relatively small diameter the powder-tribocharger material contacts will be numerous and the triboelectrification will be all the better. Similarly, triboelectrification will be all the better as the tubes of tribocharging material are longer. It is recommended to remove the charges generated by the triboelectrification which are inside the teflon tubes. For this, we are based on the same principle as the tribogun (or tribocharger gun). There are several solutions for removing these charges.
• the outside of the tubes made of tribocharging material is covered with a conductive paint. After three or four hours of operation the charges accumulate.
• This accumulation creates a discharge which causes the appearance of a conductive charred part, which allows the evacuation of the charges.
• Charges develop inside the tubes of tribocharging material. After three to four hours of operation, sparks appear and a "breakdown" occurs. This breakdown shows that a conductive part between the interior (non-conductive) and the exterior (conductive) of the tubes appears. The charges can then flow through this conductive part.
• the conductive parts are "artificially" created by drilling a hole with a needle in the tribocharger tube. This hole is covered with a layer of conductive paint based on nickel. Holes can be made every 10 or 30cm. The tube is then covered on the outside of Aquadag®, a conductive paint. It is also recommended to connect the cylindrical part around which the tribocharger tube is wound.
• this forced circulation it is for example a pump or an equivalent device such as a gas ejector (preferably operating with the same gas as the fluidized bed). At the outlet of the tribocharger device, the gas and the charged powder are remixed with the fluidized bed.
• This tribocharging device with forced circulation can be placed inside the fluidized bed but it risks disturbing the operation of the bed.
• the tribocharging tubes must be electrically isolated from the fluidized bed by covering them with rubber or with an insulator while removing the charges from the tribocharging tube towards the outside of the bed. It is much simpler for this device to be outside the bed.
• the pump (or the forced circulation means) sucks powder and gas through tubes which pass through the wall of the fluidized bed and sends them into the tribocharging tube (s) then through another orifice drilled in the wall of the fluidized bed returns the powder and gas to the bed.
• the pump can be replaced by a gas ejector preferably operating with the same gas as that which is in the fluidized bed.
• the sampling in the fluidized bed of the gas loaded with powder and its return to the bed after having passed through the tribocharging tubes must be done as much as possible without disturbing the fluidization and therefore the deposition of powder on the object to be covered.
• One can for example use low flows in the tribocharger tube have one or more compartments in the fluidized bed or even carry out the return in the fluidized bed by a system of cyclones and / or compartments.
• the electric charge of the powder in the fluidized bed increases with the flow of powder in the tribocharger.
• the thickness of the powder layer on the object which is soaked in the fluidized bed increases with the electric charge of the powder.
• Fig 1 shows an embodiment of the invention.
• the air or the chosen fluidizing gas is injected in 3 in a box under the bed.
• the air then passes through a porous, or a grid or a perforated metal plate, the pressure drop of which is chosen so as to correctly fluidize the powder.
• the pump is shown, in 2 the tribocharger tube (we used PTFE sold under the brand Teflon® by Dupont) and in 4 the fluidized bed.
• a pre-surface treatment is carried out on the object before it is brought into the bed.
• These are the conventional pre-treatments used in the plastic coating industry: phosphating, degreasing, shot blasting, application of liquid or powder primer, etc. This list is not exhaustive.
• the objects to be covered are brought by a grounded conveyor.
• the powder is then loaded into the tribocharged bed described above. During the soaking, the electroplating is done.
• a tacking system makes it possible to remove the excess powder at the outlet of the object from the fluidized bed.
• the covering powders which require a primer it can be applied beforehand on the object before dipping it in the fluidized powder tank, it can be a liquid or solid primer.
• a solid primer In the case of a solid primer, it can be applied by electrostatic powdering, Corona gun, tribo or both. You can also apply the primer with a tribocharged bed.
• the primer particles are very small, so the primer cannot be fluidized alone. But if the primer is mixed in a first bed with the powder which one wants to cover, a primer content of at least 1% by weight (relative to the weight of powder) is used, and preferably 5 to 10% by weight, then the fluidization of the small primary particles is ensured by the large particles of fluidization powder.
• This first tribocharged bed is of the same type as those described above. The charge acquired by a particle is more or less inversely proportional to its radius. The smaller, more charged primary particles will provide most of the electroplating.
• the object a solid primer.
• the object is then coated with a second layer in a tribocharged bed containing coating powder alone.
• a first firing of this primer it is also possible to avoid this intermediate firing and to carry out the second covering then to carry out an overall firing.
• the object is covered in the bed, it is brought into an oven where cooking is ensured.
• a convection, infrared or induction oven can be used. This step of the method is known in itself and has already been described in the prior art.
• the oven comprises electrodes brought to a high electrical potential in order to cause a corona effect which compensates for the relaxation of the charge of the powder particles during their heating. This maintains the charge of the particles and therefore they remain on the object and can thus form the film by fusion. This process is particularly useful when there is no primer on the object to be coated.
• a space charge generation system is placed inside the baking oven to prevent the powder from falling off the metal parts. Indeed, with certain powders and with the increase in temperature, the relaxation time of the powder decreases, thus the powder is detached from the metal part.
• a system generating a charge of the same sign as that of the powder is placed.
• an electrical potential is applied to a metal part isolated from ground. If this metal part contains pointed parts, a Corona effect appears.
• four vertical copper tubes of 1.2 cm in diameter are enough, placed on a metal plate inside in the lower part of the oven and in each corner. Each tube is blocked at its upper end by a convex convex bottom in order to avoid the unnecessary corona effect.
• the corona effect must be concentrated with the needles mounted on the copper tubes. These needles are the cause of the Corona effect.
• a potential electric is applied to the metal part, insulated by high temperature resistant insulators (ceramic insulators for example).
• the temperature is an important factor in the baking of a coating of a part.
• Polyamide 11 for example, melts at 186 9 C, the oven temperature is adjusted to 220 g C.
• the cooking time varies according to the oven temperature. In fact, the higher the oven temperature, the shorter the cooking time of the part. For an oven temperature of 220 -C and a powder of PA 11 the cooking time is generally between .6 and 10 minutes. In fact, the higher the temperature of the oven and the shorter the relaxation time of the powder charge, that is to say that the powder discharges faster when the temperature increases.
• Fig 2 shows an embodiment of the electrodes in the oven.
• 1 is the power supply, 2 the ceramic insulation, 3 a copper needle, 4 the electrical connection by a copper cable, 5 a copper tube and 6 the oven.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Electrostatic Spraying Apparatus (AREA)
• Coating Apparatus (AREA)

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• 2002
  • 2002-05-03 FR FR0205554A patent/FR2832653A1/fr active Pending
  • 2002-11-29 WO PCT/FR2002/004109 patent/WO2003045581A2/fr not_active Application Discontinuation
  • 2002-11-29 EP EP02801081A patent/EP1448315A2/de not_active Withdrawn
  • 2002-11-29 KR KR10-2004-7008295A patent/KR20040068565A/ko not_active Application Discontinuation
  • 2002-11-29 CN CNA028275594A patent/CN1617770A/zh active Pending
  • 2002-11-29 US US10/496,816 patent/US20050069652A1/en not_active Abandoned
  • 2002-11-29 AU AU2002364800A patent/AU2002364800A1/en not_active Abandoned
  • 2002-11-29 CA CA002468758A patent/CA2468758A1/fr not_active Abandoned
  • 2002-11-29 JP JP2003547073A patent/JP2005510351A/ja not_active Abandoned

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