Coating processes – Solid particles or fibers applied – Fluidized bed utilized
Reexamination Certificate
1998-10-05
2001-09-04
Parker, Fred J. (Department: 1762)
Coating processes
Solid particles or fibers applied
Fluidized bed utilized
C427S314000, C427S318000, C427S459000
Reexamination Certificate
active
06284311
ABSTRACT:
BACKGROUND OF THE INVENTION
Described herein is a process for coating a substrate with a polymer by immersing a heated substrate in a fluidized bed of polymer particles. After removal of the coated substrate from the fluidized bed, additional heat can be applied to level the coating and, if the polymer is thermosetting, to effect cure.
The coating of substrates, such as metals, is useful for aesthetic purposes and for practical purposes such as corrosion protection. Many types of coating materials and processes for utilizing these coating materials are known in the art. For environmental reasons, there is a trend to using coating materials that emit low levels of organic volatiles, and preferably no volatiles at all, during the coating process.
One method which creates low levels of volatiles in the coating process is powder coating applied by fluidized bed. One drawback to the process as it is currently practiced is that relatively thick coatings are produced because of the lack of appreciation of how to control coating thickness to consistently obtain thinner coatings. In order to overcome this shortcoming, electrostatic spraying is sometimes used. However, the electrostatic process requires elaborate equipment, and does not typically coat all surfaces within an object.
Descriptions of typical powder coating methods are found in Jilek, “Powder Coatings”, Federation of Societies for Coating Technology, Blue Bell, Pa., U.S.A., October 1991, pages 7 to 35; Landrock in Encyclopedia of Polymer Science and Technology, Vol. 3, McGraw Hill Book Co., New York, 1965, pages 808 to 830; Landrock in Chem. Eng. Progress, Vol. 63, No. 2, pages 67 to 73; Richart, Plastics Design and Processing, July 1962, pages 26 to 34; and Kroschwitz, Ed., Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 6., John Wiley & Sons, New York, 1993, pages 635 to 661. Fluidized beds are well-known in the art, see for instance, Elvers, et al, Ed., Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. B4, VCH Verlagsgesellschaft mbH, Weinheim, 1992, pages 240 to 274. With respect to making spherical particles of copolymer, see U.S. Pat. No. 3,933,954 and U.S. Pat. No. 4,056,653.
None of these references describes a fluidized bed process into which is dipped a substrate, heated just to the temperature at which it causes tackiness of the polymer particles that contact the substrate, or modestly higher, together with control of the particle size. By heating the substrate significantly above the melting point of the polymer, the art regularly achieves coating thicknesses exceeding what is useful in certain practical applications. For instance, typical procedures taught in the art produce coatings too thick for automotive applications, as well as other applications where thicknesses of 150 micrometers, even significantly below 150 micrometers, are desired. This deficiency has been a primary factor in slowing the growth of powder coating applications.
SUMMARY OF THE INVENTION
This invention concerns an improvement in a process for coating a substrate with a polymer comprising immersing a heated substrate into a fluidized bed of particles of the polymer, coating the substrate with the polymer and removing the coated substrate from the fluidized bed; the improvement comprising:
i) heating the substrate to a temperature sufficient to tackify the polymer particles upon contact with the substrate;
ii) maintaining particle temperature in the fluidized bed below that at which the particles tackify;
iii) covering substantially uniformly all surfaces of the substrate;
iv) optionally heating the coated substrate to level the coating and to cure the polymer if it is thermosetting; and
v) controlling the coating thickness, per unit time, in this manner:
(a) to obtain relatively thin coatings of up to about 150 micrometers, heat the substrate such that the coating temperature is within the tack temperature gradient but below Tm and maintain particle sizes so that at least 80 weight percent are between 10 to 80 micrometers;
(b) to obtain thicker coatings, heat the substrate above the tack temperature gradient, employ larger particle sizes than described immediately above, or both.
The buildup in coating thickness is believed to result primarily from substrate heating profiles above the tack temperature gradient of the polymer. By “tack temperature” (Tt) is meant the substrate temperature just high enough to cause the polymer particles to adhere thereto. The “tack temperature gradient” comprises a temperature range whose lower limit is the tack temperature and whose upper limit is about 75° C. higher, provided it remains below Tm (melt temperature). One skilled in the art will appreciate that Tm has relevance with respect to crystalline and semicrystalline polymers, not amorphous polymers. Accordingly, when an amorphous polymer has been selected as the coating, the important considerations, so far as temperature is concerned, are Tt and tack temperature gradient.
It is a preferred embodiment of this invention to control coating thickness as described in paragraph v above to obtain thicknesses of 150 micrometers or less. The preferred process involves steps i) through v)(a).
This invention also concerns preferred embodiments wherein the process is operated to coat a galvanized steel substrate, treated or untreated; a substrate having a curved shape with recesses; a substrate which is an automobile body or component thereof; in which the polymer is semicrystalline thermoplastic or semicrystalline thermosetting or amorphous thermoplastic or amorphous thermosetting. When the polymer is thermosetting, the substrate to be coated is immersed into the fluidized bed at a temperature that is controlled so as to effect adherence of the polymer but without substantial crosslinking while the substrate is within the bed.
It is a preferred aspect of this invention to coat a substrate of a vehicle body or component thereof having a curved shape and recesses comprising:
i) applying a coating to the substrate by immersing the heated substrate into a fluidized bed of particles and adhering the particles substantially uniformly to all surfaces of the substrate to produce a coating with an average thickness not exceeding about 150 micrometers;
ii) optionally applying a pigmented basecoat or monocoat to the substrate coated in step i); and
iii) optionally applying an unpigmented topcoat to the substrate coated in steps i) and ii).
A preferred basecoat comprises water-borne or solvent-borne polymer; a preferred clear topcoat comprises water-borne, solvent-borne or powder polymer. The invention also concerns optionally pre-treating or post-treating the coated substrate with a primer-surfacer and/or post-treating with a colored basecoat and/or a clear topcoat.
Preferred elements of the claimed process comprise one or more of the following: using fumed silica as a component of the fluidized bed at weight percentages typically between about 0.1 to 0.5 percent; vibrating the part exposed to the fluidized bed to facilitate even coating; and employing spherical particles which have been found to produce the best coating quality.
One of the strategies to obtain the best coatings is to control all variables so that the derived coating in the targeted thickness is deposited independently of dwell time of the substrate in the fluidized bed.
DETAILS OF THE INVENTION
The material coated on the substrate is a polymer powder which is crystalline or amorphous. By crystalline is meant that the polymer has a heat of melting of at least 2 J/g, preferably at least 5 J/g when measured by the Differential Scanning Calorimetry (DSC) using ASTM D3417-83. Such crystalline polymers often contain considerable amounts of amorphous (uncrystallized) polymer. The Tg referred to herein is measured by the method described in ASTM D3417-83 and is taken as the middle of the transition. The Tg described is the highest Tg for the polymer, if the polymer has more than one Tg. If the Tg is undetectable by DSC, Thermomechanical Analysis can be used to determine the Tg,
Gregorovich Basil Volodymyr
Kodokian George Kevork
E. I. Du Pont de Nemours and Company
Parker Fred J.
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