WO2004010441A2

WO2004010441A2 - High-temperature stable magnetic masking material

Info

Publication number: WO2004010441A2
Application number: PCT/US2003/021472
Authority: WO
Inventors: Jon B. Schneider; Orval Daine Ogden, Jr.
Original assignee: Magnum Magnetics Corporation
Priority date: 2002-07-17
Filing date: 2003-07-10
Publication date: 2004-01-29
Also published as: WO2004010441A3; WO2004010441B1; AU2003259101A1; AU2003259101A8

Abstract

A sheet material comprising magnetizable particles dispersed in a flexible synthetic resin matrix that does not melt or become distorted at fusing or curing temperatures of a coating or powder, the magnetizable particles when magnetized having a Curie temperature capable of withstanding the fusing or curing temperatures for a sufficient period of time to avoid demagnetization.

Description

HIGH-TEMPERATURE STABLE MAGNETIC MASKING MATERIAL

BACKGROUND OF THE INVENTION Field of the Invention:

This invention relates to a high-temperature stable magnetic sheet and claims benefit of the 7/17/02 filing date of provisional application P60/396,128. More particularly, the invention relates to a high-temperature stable magnetic sheet suitable for use as a masking material in the application of powder coatings to substrates . Brief Description of the Prior Art:

In modern industrial coating processes coatings applied to metallic substrates are frequently dried and/or cured at elevated temperatures. In particular, powder coating, using a high-temperature fusible powder, has come to be widely used because of its convenience and environmental advantages . In powder coating processes a fusible coating material in the form of electrically charged powder particles is sprayed onto a grounded substrate. The charged powder particles are attracted to the substrate and adhere thereto to form an even coating. Subsequently, the substrate is 'heated to a temperature at which the adhering powder fuses to form a smooth coat on the substrate. The substrate is then cooled to provide a coated object. In coating metal objects, such as automobile body parts, automobile wheels, computer enclosures, appliances, and the like, by a powder coating process it is often necessary to provide a patterned multi-colored coating on the substrate. Such patterned coating requires repeated coating steps, wherein portions of the substrate surface are masked during each separate coating step to create the desired pattern. Hitherto masks have typically been made of a thin, flexible sheet material, e.g., polyester or polyimide, provided with an adhesive backing for adhering to a substrate. However, masking of patterned substrates by applying adhesive masks to the surface is a time-consuming process because of the accuracy required and the difficulty of repositioning adhesive masks if they are not accurately positioned initially. Furthermore, the adhesive-backed masks are typically used only once.

Magnetic masks, made from flexible sheets of rubber or a synthetic resin containing magnetized particles, are more easily applied to magnetic metal substrates, and may be easily removed and reused. However, hitherto magnetic masks could not be used with powder coatings because the matrix material for the magnetic particles has typically been a thermoplastic material that will melt, or at least become deformed, at the temperatures used to fuse and cure the powder coating.

Similar considerations apply to more traditional coatings with paints, enamels, lacquers, and the like. Such coatings are generally applied by spraying liquid droplets of pigment in a vehicle and/or solvent carrier to form a thin liquid coating layer on a substrate and allowing the layer to dry or cure to a hard coating. In order to accelerate the drying and/or curing the coated substrate is frequently heated to a relatively high temperature, greater than conventional magnetic masking materials can withstand.

Accordingly, there is a need for a magnetically adherable masking material that can be used in industrial coating processes, in particular in powder coating processes where the masking material can withstand high fusion or curing temperatures of the powder coating without losing its magnetic characteristics and without having the synthetic resin matrix in which the magnetic material is dispersed melt or become distorted.

SUMMARY OF THE INVENTION

This need is met by the invention magnetic masking material which is a magnetizable layer comprising magnetic particles dispersed in a synthetic resin matrix having a softening point high enough to avoid substantial deformation when contacted with a heated surface at a temperature appropriate for drying and/or curing a liquid coating material or for fusing and curing a powder coating material. One object of the invention is to provide a magnetic masking material.

Another object of the invention is to provide a magnetic masking material suitable for use in industrial coating processes that can withstand high temperatures without losing its magnetic characteristics.

A further object of the invention is to provide a magnetic masking material suitable for use in powder coating processes that can withstand high temperatures without losing its magnetic characteristics.

A still further object of the invention is to provide a magnetic masking material that does not melt or experience significant distortion at temperatures used to fuse and cure industrial powder coating materials. A yet further object of the invention is to provide a magnetic masking material that does not melt or experience significant distortion at temperatures greater than about 104°C.

A further object yet still of the invention is to provide a magnetic masking material that does not melt or experience significant distortion at temperatures greater than about 175°C.

A further object of the invention is to provide a magnetic masking material that does not melt or experience significant distortion when contacted with a surface heated to a temperature as high as about 235°C

Other objects and advantages of the invention will become apparent from the description which follows.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The magnetic masking material of the invention comprises magnetizable particles in a matrix capable of withstanding temperatures used in fusing and curing powder coating materials for a period of time long enough for the powder coating to cure .

The magnetic sheet comprises magnetic particles dispersed in a synthetic resin binder and may be any convenient thickness suitable for use as a coating mask. For masking flat surfaces that mask may be relatively inflexible, provided that it will lie flat on the surface. Typically, the magnetic sheet is thin enough to be flexible and able to conform to a curved substrate, as well as a flat substrate. Generally, the thickness of the magnetic sheet ranges from about 4 mils to about 60 mils, preferably from about 8 mils to about 30 mils. The magnetic sheet will typically be thick enough for convenient handling but not so thick that it will not conform to the substrate to be masked. The magnetizable sheet must be capable of withstanding temperatures it will experience during drying and/or curing of a liquid coating material or during fusing and curing of the powder coating process. Accordingly, the magnetizable sheet comprises a suspension of magnetic particles having a Curie temperature such that when dispersed in a synthetic resin matrix, is capable of withstanding the curing temperatures without demagnetization and without the resin melting or becoming distorted. Typically, drying and curing of a liquid coating material uses a temperature above about 104°C (220°F) , or preferably above about 121°C (250°F) . The curing step of a powder coating process typically operates at a temperature range of from about 175°C to about 235°C (350°F to 455°F) . Accordingly, any magnetizable particles having a Curie temperature above the temperature at which the sheet will be exposed in the drying or curing step are suitable. The Curie temperature is that temperature at which magnetized particles are completely demagnetized due to thermal^' agitation. Barium and strontium ferrites are usable as the magnetic particles in the magnetizable sheet. Preferred magnetic particles are strontium ferrite.

The synthetic resin matrix in which the magnetic particles are dispersed must be capable of withstanding the temperatures to which the substrate is heated in the drying step of a liquid coating process or the curing step of the powder coating process. The matrix material must not melt, soften excessively, or become distorted at the drying or curing temperature. Furthermore, it must remain sufficiently flexible at room temperature for easy application to curved substrates and the like.

Any synthetic resin material fulfilling these requirements is suitable for the matrix of the magnetic sheet of the invention. Any curable rubber or synthetic resin material that in its cured form has a softening temperature high enough to fulfil the above requirements is satisfactory. Suitable materials include synthetic resins and elastomers having a sufficiently high softening point such as nylon, polyester resins such as poly (ethylene terephthalate) , fluorinated polymers, silicone rubber, melt-processable rubbers, and the like. The material can be adapted to the particular coating process in which the mask is to be used. Thus, the softening temperature of a mask to be used in a liquid coating process may be not less than about 104 °C, preferably not less than about 121 °C. For use in a powder coating process the magnetic material should have a softening temperature not less than about 175 °C, preferably not less than about 200°C, and more preferably not less than about 235°C. The skilled practitioner will recognize that synthetic resins of the type used in the magnetic sheet material of the invention are polymers of high molecular weight and, as such, do not have sharp melting points, but gradually soften and become fluid over a range of temperatures. However, a temperature at which a particular material becomes too weak, soft, or fluid to serve as an effective mask in a powder coating process is readily determined by observing the behavior of a sheet of a particular resin when contacted with a heated substrate. Accordingly for purposes of the invention the term melting point means the temperature at which a sheet of a particular masking material becomes too weak, soft, or fluid to serve as an effective mask, i.e., the material is incapable of shielding a selected area of the substrate from the powder coating and producing an acceptably sharp border between coated and uncoated regions of the substrate. When using such a curable resin matrix, the magnetic particles are mixed with the uncured resin, and the mixture is extruded, calendered, or otherwise formed into a sheet, and the sheet so formed is cured. A preferred embodiment of the matrix material for use in the magnetic layer of the invention is a melt-processable rubber or high temperature synthetic resin. Such materials can be formed by extruding, calendering, molding, or other forming processes, conducted at a temperature above their softening temperature and then fixed in their final form simply by cooling below their softening temperature. Such materials are especially useful because, unlike cured materials, they can be reprocessed simply by remelting. A preferred material for use as the matrix of the magnetizable layer is a partially cross- linked halogenated olefin interpolymer alloy. A specific example of such a matrix material is a melt-processable rubber product supplied by Advanced Polymer Alloys, Inc., Wilmington, Delaware, under the registered trademark ALCRYN^® (an amorphous halogenated polyolefin thermoplastic elastomer) . Another preferred matrix material is silicone rubber.

The magnetizable layer can be made by conventional procedures. For example, ferrite particles and the matrix resin, e.g., a partially cross-linked halogenated olefin interpolymer alloy, may be mixed together and the mixture may be formed into a thin sheet by conventional methods such as extrusion or calendering. The proportions of ferrite and matrix are conventional, and typically amount to about 23-29 parts by weight of synthetic resin to 100 parts by weight of ferrite. If the matrix resin is a curable resin, the magnetic layer may then be cured, e.g., by cross-linking. If the matrix resin is a melt-processable material, the mixing will be conducted above the melting temperature of the resin and the magnetizable sheet will be solidified by cooling after it has been formed by, e.g., extrusion or calendaring and then magnetized by conventional procedures to a sufficient intensity for good adhesion to magnetic substrates. The sheet material may be distributed in large sheets and cut to an appropriate outline at the point of use. In use, the magnetic sheet of the invention is applied to a magnetic metal, e.g., iron, steel, or the like, to prevent selected areas from being covered by the powder coating. The magnetic sheet is cut to the desired shape to mask the selected portions of the substrate and applied to the substrate in the desired location. A plurality of such masks may be applied if necessary. The metal substrate is then grounded and a spray nozzle attached to a source of electric charge is used to spray a powder coating material onto the substrate . The electric charge imparted to the powder by the spray equipment assures that a generally uniform coating of the powder adheres to the exposed surfaces of the metal substrate. The substrate with adhered powder is then heated, e.g., in an oven, to a temperature- at which the powder fuses and cures to form a continuous hardened coating on those portions of the substrate that have not been masked. After the powder coating has cured the article is allowed to cool and the masking sheets can be easily removed. The process can be repeated as many times as necessary to provide a patterned coating on the substrate.

When used with coatings applied in the liquid state which are flash dried or cured by heating to relatively high temperatures, the procedure is essentially similar. The masking material is applied to selected areas of the substrate before the coating is applied. A liquid paint, enamel, lacquer, or the like is then applied by any conventional technique such as spraying, brushing, immersion, electrolyte deposition of a colloidally dispersed pigment and vehicle from a bath, or the like. The coated substrate is then heated to an elevated temperature at which the coating dries and/or cures to form a hard protective and/or decorative coating.

Because the magnetic mask of the invention is not adhesively bonded to the substrate it leaves no residue on the substrate and does not need to be recoated with adhesive before it is reused. The magnetic mask of the invention may be simply removed from a coated substrate and immediately applied to a new substrate to be coated. Accordingly the magnetic masking material of the invention provides enhanced convenience and economy in powder coating operations. While the invention has been described via specific illustrations and embodiments it is to be understood by those skilled in the art that many variations thereof may be practiced without departing from its spirit and scope of the invention and that the description is illustrative only and not restrictive, and many changes which come within the meaning and range of equivalence of the claims are intended to be embraced herein.

Claims

WE CLAIM :

1. A sheet material comprising magnetizable particles dispersed in a flexible synthetic resin matrix that does not melt or become distorted at fusing or curing temperatures of a coating or powder, said magnetizable particles when magnetized having a Curie temperature capable of withstanding said fusing or curing temperatures for a sufficient period of time to avoid demagnetization

2. The sheet material of claim 1 wherein said flexible synthetic resin matrix has a melting point greater than about

104°C.

3. The sheet material of Claim 2, wherein said flexible synthetic resin matrix has a melting point greater than about 121 °C.

4. The sheet material of Claim 2, wherein said flexible synthetic resin matrix has a melting point greater than about 175°C.

5. The sheet material of Claim 2, wherein said flexible synthetic resin matrix has a melting point greater than about 200°C.

6. The sheet material of Claim 2, wherein said flexible synthetic resin matrix has a melting point greater than about 235°C.

7. The sheet material of Claim 2, wherein said magnetizable particles are magnetized.

8. The sheet material of Claim 2, wherein said synthetic resin comprises a partially cross-linked olefin interpolymer alloy.

9. The sheet material of Claim 6, wherein said magnetizable particles are magnetized.

10. The magnetic sheet material of Claim 2, wherein said synthetic resin comprises a silicone rubber.

11. The magnetic sheet material of Claim 8, wherein said magnetizable particles are magnetized.

12. A method for masking a magnetically attractive substrate to be coated with a fusible powder coating material, said method comprising: applying to a selected area of a ^•magnetically attractive substrate a magnetic sheet material comprising magnetized particles dispersed in a flexible synthetic resin matrix that does not melt or become distorted at fusing or curing temperatures of a coating or powder, said magnetized particles having a Curie temperature capable of withstanding said fusing or curing temperatures for a sufficient period of time to avoid demagnetization in the presence of said matrix having a melting point greater than about 175°C.

13. The method of Claim 12, wherein said flexible synthetic resin matrix has a melting point greater than about 200°C.

14. The method of Claim 12, wherein said flexible synthetic resin matrix has a melting point greater than about 235°C.

15. The method of Claim 12, wherein said synthetic resin comprises a partially cross-linked olefin interpolymer alloy.

16. The method of Claim 10, wherein said synthetic resin comprises a silicone rubber.

17. A method for applying a fusible powder coating material having a fusion temperature greater than about 175 °C to a selected area of a substrate comprising:

1) applying to a portion of a surface of a magnetically attractive substrate a mask comprising a magnetic sheet material comprising magnetized particles dispersed in a flexible synthetic resin matrix that does not melt or become distorted at fusing or curing temperatures of a powder coating, said magnetized particles having a Curie temperature capable of withstanding said fusing or curing temperatures for a sufficient period of time to avoid demagnetization in the presence of said matrix having a melting point greater than about 175°C;

2) applying said fusible powder coating material to said surface of said substrate;

3) heating said surface of said substrate to a temperature above said fusion temperature of said fusible powder coating material; 4) cooling said surface of said substrate to a temperature below said fusion temperature of said fusible powder coating material; and

5) removing said mask from said substrate.

18. The method of Claim 17, wherein said flexible synthetic resin matrix has a melting point greater than about 200°C.

19. The method of Claim 17, wherein said flexible synthetic resin matrix has a melting point greater than about 235°C.

20. The method of Claim 17, wherein said synthetic resin comprises a partially cross-linked olefin interpolymer alloy.

21. The method of Claim 17, wherein said synthetic resin comprises a silicone rubber.

22. A method for applying a coating to a selected area of a substrate comprising:

1) applying to a portion of a surface of a magnetically attractive substrate a mask comprising a magnetic sheet material comprising magnetized particles dispersed in a flexible synthetic resin matrix that does not melt, soften or become distorted at fusing or curing temperatures of a liquid coating material, said magnetized particles having a Curie temperature capable of withstanding said fusing or curing temperatures for a sufficient period of time to avoid demagnetization in the presence of said matrix having a melting point greater than about 104 °C;

2) applying a liquid coating material to said surface of said substrate; 3) heating said surface of said substrate to a temperature above about 175C, whereby said liquid coating is converted to a solid coating;

4) cooling said surface of said substrate; and

5) removing said mask from said substrate.

23. The method of Claim 22, wherein said flexible synthetic resin matrix has a melting point greater than about 121°C.

24. The method of claim 22, wherein said flexible synthetic resin matrix has a melting point greater than about 175°C.

25. The method of claim 22, wherein said synthetic resin comprises a partially cross-linked olefin interpolymer alloy.

26. The method of claim 22, wherein said synthetic resin comprises a silicone rubber.