Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-02-25
2002-01-29
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S485000, C427S521000, C427S598000
Reexamination Certificate
active
06342273
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for coating a substrate with a powder paint composition.
BACKGROUND OF THE INVENTION
In the powder paint industry, powder pats are usually electrically charged by means of a corona discharge or tribocharging. Then the powder paint is electrostatically applied to the object to be coated. In general, the electrostatic spraying technique involves use of corona and/or tribo charging guns as described in, for example, Misev “Powder Coatings Chemistry and Technology”, pages 324-344, John Wiley, 1991.
In industrial applications varnishing of, for example, cellulose based sheets and webs is usually carried out using solvent or water-based varnishes. The solvent-based varnishes have as a disadvantage the evaporation of solvents into the atmosphere. Because of environmental reasons emission of solvents has to be reduced drastically. The water-based varnishes have as a disadvantage that they need much energy for drying. To achieve the highest possible gloss the use of calendars is suitable. This separate step is costly and limits the process speed.
Alternatively, liquid UV-curable varnishes are used to overcome these difficulties. However, this group of materials is often rejected because of their sensibilizing and irritating character due to the applied monomers, which are the main components of these coating liquids. Another disadvantage of this coating technology can be that the smell makes the coating unsuitable for food packaging.
GB-A-2056885 discloses a process for coating a cellulose fiber substrate like cardboard with a powder paint composition and curing by radiant heat. Generally, a binder system for a powder paint composition consists of a resin and a crosslinked. A disadvantage of the process according to GB-A-2056885 is the limited capacity of the applied powder paint spray guns which imposes a restriction on the rate of application of powder paint to the substrate. Another disadvantage is that it is very difficult to achieve coatings having a layer thickness below 15 &mgr;m.
A further disadvantage is the major thermal load imposed on the heat-sensitive substrate during curing of the powder paint composition.
Powder paints are also being considered for use in the coil coating industry. Coil coating is a special application method in which coiled metal strips are unwound and then passed through pretreating, coating, and drying equipment before finally being rewound. Coil coating allows for very efficient coating of large surface areas in a short time at high throughput. However, the inherent limitation in powder paint spray gun capacity imposes restrictions on the rate of application of powder paints to the coil and on the reliability of the coil coating process. Hence, there is a strong need in this industry for a high speed but yet reliable continuous process for the application of powder coatings.
This appears, for example, from “Coil Coatings Strategies in change” by G. C. Simmons Polymers Paint Colour Journal, Jul. 28, 1993, page 373.
An object of the present invention is to provide an improved process for coating a powder paint on a substrate, at rapid rates (e.g. 100 metres of substrate/minute) while yielding a high quality coating.
The process must also offer the possibility to treat large surface areas simultaneously.
SUMMARY OF THE INVENTION
The invention is characterized in that powder paint particles are first charged by friction or induction in the presence of magnetic or non-magnetic particles, are next transported and then applied to the substrate, or applied to a transfer medium and subsequently transferred to the substrate, by means of an electric field between the substrate respectively the transfer medium and the means of transport, whereafter the powder paint composition is cured or fused into a powder coating and wherein the substrate is, for instance, metal, textile, plastic wood, board, or paper-like. The powder coating adheres to the substrate.
If a transfer medium is used, the powder paint particles are first applied to the transfer medium by means of an electric field, transported to the substrate by the transfer medium and then applied to the substrate by, for example, electrical, electrostatic or mechanical forces. Thermal processes can also be used in this application step.
The present process allows powder paint particles to be applied to substrates at rates up to, for instance, 200 meters of substrate/minute with film thicknesses of the cured film of between for example, 3 and 200 &mgr;m. Moreover, this process satisfies the need to eliminate multiple spray guns in a coil coating process.
The present process allows powder paint particles to be applied to coated or uncoated board- or paper-like substrates such as, for example, cardboard, hardboard, fiber board for example MDF, particle board, chipboard, plywood, veneer, block board (which board products can be paperfaced), wood, timber and paper at rates up to, for instance, 400 meters of substrate/minute with a film thickness of the cured film of between, for example 1 &mgr;m and 100 &mgr;m.
The process according to the invention results in a substantially (e.g. more than 90%) fully coated substrate. In contrast, in, for example, a printing process the coverage is, for example, only 10% of the substrate.
The charging of the powder paint particles by friction or induction, the transporting or conveying and the application to substrates can be effected using processes commonly known in photocopying technology or laser printer technology (which processes are elucidated in, for example, L. B. Schein, Electrography and, Development Physics, pages 32-244, Volume 14, Springer Series in Electrophysics 1988: the disclosure of which is incorporated herein by reference).
DESCRIPTION OF PREFERRED EMBODIMENTS
According to a preferred embodiment of the invention the powder paint particles are mixed with magnetic or non-magnetic carrier particles to obtain friction charging whereupon the mixture is transported and the powder paint particles are applied to the substrate by means of an electric field between the substrate and the means of transport.
The powder paint particles may be based on solid particles of a radiation curable binder system.
Suitable radiation curable binder systems include for example a polymer having unsaturated groups and a cationic curable system based on, for example epoxy resins.
Preferably, the radiation curable binder system comprises a polymer having unsaturated groups.
Preferably, said polymer having unsaturated groups has a degree of unsaturation between about 200 and about 1800 grams of polymer per mole of unsaturated group.
Depending on the conductivity of the substrate the electric field between the substrate and the transfer medium or means of transport can be applied with suitable methods like for instance a corona discharge or a moving or fixed counterelectrode (see for instance Schein, pp. 36-37 and 47).
Preferably, the polymer having unsaturated groups has a molecular weight (Mn) between about 800 and about 6800 grams/mole and a viscosity between 1 dpas and 800 dPas (measured with an Emila rheometer at 165° C.)
Preferred polymers are unsaturated polyesters, unsaturated polyacrylates and acrylated polyesters.
Preferably, the unsaturated polymer has a degree of unsaturation between 300 and 1800 grams per mole of unsaturated group (WPU), a molecular weight (Mn) between about 800 and about 6800 and a viscosity between 1 dpas and 400 dpas.
The preferred degree of unsaturation for an unsaturated polyester is between 500 and 1500 grams per mole of unsaturated group.
The molecular weight (Mn) is preferably between about 2500 and about 4500 grams/mole. 30 The unsaturated polyester can be (semi)crystalline or amorphous.
In general, the glass transition temperature (Tg) of the amorphous resin is higher than 40° C. Semicrystalline resins have a Tg and a melting point (Tm), and Tg is lower than Tm.
The melting point of the (semi)crystalline unsaturated polyester is between about 40° C. and about 180° C., and preferably, is betw
Buchweitz Jorg
Handels Johannes W. H.
Huber Burkard
Misev Tosko A.
Schulze-Hagenest Detlef
DSM N.V.
Parker Fred J.
Pillsbury & Winthrop LLP
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