Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1999-10-12
2001-05-29
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S195000, C427S202000, C427S270000, C427S369000, C427S370000
Reexamination Certificate
active
06238750
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to an improved technique for coating substrates using powdered coating materials. The technique is particularly useful for coating substrates which are heat sensitive, such as cellulosic or plastic substrates. In preferred embodiments, it enables the use of powdered coating materials in manufacturing processes, such as the membrane press coating process and roll coating processes, wherein such powder coating materials have not been successfully used in the past. The invention is particularly useful for the production of coated wood articles, such as medium density fiberboard and particle board panels.
2. Description of Related Art
The application of dry coating materials on manufactured articles has become increasingly important because of their significant environmental advantage over the use of liquid coating materials, such as paints. These advantages principally involve avoiding, or minimizing, the use of volatile organic solvents, and thereby avoiding the air pollution and health concerns associated with such solvents.
Dry coating materials have generally been applied as powders or as films. Dry powder coating methods have involved depositing a dry, free flowing powder on a substrate and then heating the powder to cause it to fuse and cure. Since the heating step has generally required exposing the substrate to temperatures which cause deterioration of heat sensitive materials, such as those based on wood and/or plastic materials, the use of such dry powder coating methods has been primarily directed to coating metal articles. Recently, dry coating powder materials which are capable of fusion and curing at temperatures consistent with their use on wood based substrates have been introduced. Examples of such lower temperature coating materials are described in commonly assigned U.S. Pat. Nos. 5,714,206 and 5,721,052 and in U.S. patent applications Ser. Nos. 09/191,398 [3477-05-99] and 09/316,545. The commonly owned patents and applications referred to throughout this application are hereby incorporated by reference in this application. While these dry coating methods and materials have produced excellent textured coatings on wood based substrates, it has been difficult to produce smooth high gloss coatings with these methods and materials. Moreover, relatively thick coatings, approximately 5 mils thick, have been required to provide coatings with good moisture resistance and other barrier properties.
Membrane pressing is an important commercial process for laminating sheets on composite wood panels, such as medium density fiberboard (MDF) panels. The process involves vacuum forming a thermoplastic sheet on a MDF profile/substrate and activating a preapplied glue to bind the sheet to the profile. The technique is generally limited commercially to laminating vinyl sheets on relatively smooth and flat profiles, or substrates. If the profile is irregular, having grooves or other surface effects, the laminated film tends to not be uniformly bound to the profile. If the profile is not finished to a suitable degree of smoothness, surface irregularities appear through the laminated film. Moreover, the laminated film may exhibit irregularities, such as bubbles or orange peel surface texture, caused by gases trapped, or released from volatile components, between the sheet and the profile. A further problem occurs when localized bonding defects result in delamination, or peeling, of the film from the profile.
SUMMARY OF THE INVENTION
The present inventive coating process provides an improved coated product while minimizing or eliminating the previously noted problems. Moreover, the process permits cost savings by requiring fewer manufacturing steps, and by requiring less coating material to provide equivalent barrier protection and finish, than has been generally required in prior membrane press coating processes.
The process broadly involves providing a layer of a powder of dry curable material on a substrate, melting the powder to provide a layer of molten curable material, compressing the layer of molten material and then fully curing the compressed layer to provide a continuous cured coating on the substrate. The layer of molten material is generally compressed by a pressing means exerting pressure on the surface of the layer causing it to be compressed against the underlying substrate. It is believed that such compression of the layer causes macroscopic voids in the layer to be closed, or at least minimized, whereby comparable barrier properties of the layer, such as moisture resistance, are achieved with thinner layers. Compression of the layer also controls and/or introduces surface texture and appearance properties by appropriate selection of the surface of the pressing means. Whereas some surface finishes, such as high gloss, have only been possible with relatively thick coatings in the past, compression of the molten layer with an appropriate pressing means can provide equivalent high gloss finishes in relatively thin coatings. Thinner coating layers, of course, provide an important commercial advantage since less dry coating material and less processing time is required.
A preferred aspect of the invention involves partially curing the molten layer prior to compressing it. Partial curing increases the viscosity of the layer whereby the material is less capable of migrating from its deposited location on the substrate. This is particularly advantageous where the initially melted molten coating material is sufficiently flowable that it tends to run or be squeezed from its deposited location during the compression step.
The powder layer may be melted and, optionally, partially cured as soon as the powder is applied to the substrate. Initially, the melted curable material wets the substrate providing intimate contact capable of developing into a strong bond. The material is then partially cured to raise its viscosity sufficiently that it will not drip or otherwise migrate from its deposited location on the substrate when it is subsequently compressed. In most cases, the partial curing step does not cure the material past a condition wherein it is capable of deforming to reduce any macroscopic voids (a) at its interface with the substrate, (b) throughout the body of the layer, or (c) at its exterior surface. In those situations where modification of the surface finish or texture is the primary desired objective of the compression step, such compression may be applied at any time prior to reducing the coating temperature beneath the coating material's glass transition temperature, even if the coating is previously cured past a condition wherein it is capable of deforming to reduce voids.
The layer is then compressed against the substrate by a pressing means applied at its surface. The pressing means may be any conventional pressing device, for instance, a platen press using a pressure plate, a press using a rolling pressure plate, or opposed rolls. The process is well adapted for use with a membrane press wherein an inflatable membrane is deployed over and caused to press against the surface of the layer. Sufficient pressure is applied to cause the partially cured material to reduce any voids existing throughout its body or at its surfaces. The surface finish of the cured layer may be controlled by the pressing means, the pressing surface of which may be selected to provide a glossy, textured, matte or even an embossed surface on the coating.
Final curing of the layer may be heat activated or it may be radiation (i.e. ultraviolet or electron beam) activated. If the final curing is heat activated, portions of the required heat may be delivered by preheat stored in, and/or heating means provided in, the pressing means.
A preferred embodiment of the process employs a membrane press to form a coating on a heat sensitive substrate, such as a wood, particleboard or MDF substrate, from a dry curable powder. While membrane presses have been extensively used to form laminates of a vinyl sheet
Correll Glenn D.
Daly Andrew T.
Horinka Paul R.
Muthiah Jeno
Parker Fred J.
Rohm and Haas Company
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