Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1999-12-20
2002-04-23
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S185000, C427S195000, C427S385500, C427S458000, C427S508000, C427S553000, C427S557000, C427S559000
Reexamination Certificate
active
06376026
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to new materials for making coating powders. It also relates to a new method for applying a powder coating to an article. It further relates to a powder coating system wherein a macrocyclic oligomer in a coating powder is converted to a high molecular weight linear or crosslinked polymer as the coating powder is heated. It further relates to the coating powder, itself, and particularly to the fluidized-bed method for coating sheet metal and other articles of manufacture.
Traditionally, coating powders have been made by the extrusion of a mixture of resins, curing agents, pigments, fillers, etc. to obtain a homogeneous mixture, cooling and grinding the extrudate, and screening the comminuted product to obtain the desired particle size distribution.
It is well known that the physical properties of a polymer improve and that the melt viscosity of the polymer increases as the molecular weight increases. In the powder coating art, he viscosity of the binder resin should be low at the application temperatures but the resin must solidify at the end of the process. The molecular weight and melt viscosity of thermoplastic polymers are just as high after application as they are before. Thermosetting polymers may have very low melt viscosities when applied but do solidify when cured.
The binder resin in a coating powder should have a low melt viscosity, good adhesion to the substrate, good impact resistance, and toughness. The toughness of thermoplastic resins makes them attractive as coatings, especially when lightly crosslinked but still thermoplastic. The extremely high melt viscosity of high molecular weight thermoplastic polymers, however, has been compared to that of cold molasses. Cryogenic grinding of thermoplastic resins is generally necessary because of their very nature; they are otherwise difficult to grind because of the problem of heat dissipation. Attempts to improve their melt-flow characteristics by lowering their molecular weight or plasticizing them have harmed the physical properties of thermoplastic resins.
Most thermoplastic coating powders are applied at 200-300° C., short residence times permitting what otherwise might well be above their upper temperature limit. Polyesters and polyamides are among the principal thermoplastic resins used in coating powders. Typically, the polyesters melt or soften at 160-170° C. and substrates are preheated to about 300° C. Polyamides have higher melt temperatures (~186° C.) and application temperatures (~310° C.).
Thermosetting resins, on the other hand, are brittle solids having low melt viscosities. The low viscosity of thermosetting resins promotes good flow out of fused coating powders containing them. Smooth thin films result. Also, the brittle solids can be ground to fine powders without the cost of cryogenic grinding. Although the variety of thermosetting powders is large and growing, the variety is limited by the fact that they are usually cured by addition reactions rather than by condensation because of the voids in a cured film that arise from trapped by-products. The curing temperature of thermosetting resin based powder coatings ranges from 135 to 240° C.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a novel coating powder having the low melt viscosity and friability of a thermoset powder but which is converted at high fusing temperatures to a tough coating having the good impact resistance and good elongation of a thermoplastic powder.
It is a related object of this invention to provide a novel crosslinked powder coating.
It is a related object of this invention to provide a novel method for powder coating an article.
It is related object of this invention to provide a novel method for the fluidized-bed coating of sheet metal and other articles of manufacture.
DETAILED DESCRIPTION OF THE INVENTION
These and other objects which will become apparent from the following description are achieved by a method for coating an article comprising applying a coating powder comprising a macrocyclic oligomer selected from the group consisting of polyesters, polycarbonates, polyamides, polyimides, polyamideimides, a ring-opening polymerization agent, and optionally, a photoinitiator, to the article, fusing the powder and causing the fused powder to flow across the surface of the article, and converting the oligomer to a thermoplastic or thermosetting polymer.
Thus, the coating powder of this invention may contain a wide variety of macrocyclic oligomers, the structures, preparation, and polymerization of which are described hereinafter in the order given above.
For the purposes of this invention, coating powder means the material and powder coating means the process or the applied film as the context requires.
As used herein, the term ring-opening polymerization agent includes agents that act catalytically (i.e., catalysts) and agents that open a macrocyclic ring by reaction, e.g., a lactam and basic reagent for a macrocyclic polyamide, or a metal sulfide for a macrocyclic polyimide used in this invention.
As used herein, the term macrocylic oligomer includes compounds in which spiro(bis)indane groups are part of a larger ring structure. The mere fact that the spirobiindane moiety is itself cyclic is not significant to the macrocylic nature of the compound; rather, the presence of a larger ring structure is mandatory.
POLYESTERS
In one embodiment of this invention, the coating powder comprises a macrocyclic polyester oligomer and a ring opening polymerization catalyst, each as described hereinbelow.
Macrocyclic polyester oligomers suitable for this invention may be obtained in relatively high yields, typically 15-75%, by the reaction of a diol with a diacid chloride in the presence of a non-sterically hindered amine as a catalyst, under anhydrous conditions and at relatively low temperatures in the presence of a substantially water-immiscible organic solvent at a temperature of from −25° to +25° C. The macrocyclic polyester oligomers thus prepared have degrees of polymerization from 2 to about 12 and are usually predominantly dimer, trimer, tetramer and pentamer. Said oligomers comprise structural units of Formula I:
wherein R is an alkylene or mono-or polyoxyalkylene radical containing a straight chain of about 2-8 carbon atoms and A is an m- or p-linked monocyclic aromatic or alicyclic radical, or a saturated or unsaturated aliphatic radical having from 2 to 10 carbon atoms.
The diols are exemplified by ethylene glycol, propylene glycol, tetramethylene glycol, hexamethylene glycol, neopentylene glycol, diethyleneglycol and triethylene glycol. The A radical of the diacid chloride is exemplified by the m- and p-phenylene radicals, substituted derivatives thereof, cyclohexylene, cyclopentylene, ethylene, octylene, decylene, and ethenylene radicals. The acids are exemplified by fulmaric, maleic, octanoic, decanoic, and dodecanoic acids.
The amines are exemplified by N-methyl heterocyclic monoamines such as N-methyl-pyrrolidine and N-methylpiperidine, and polycyclic compounds with a tertiary nitrogen in the bridgehead position, as illustrated by quinuclidine and 1,4-diazabicyclo[2.2.2]octane (DABCO)
As organic solvents, various water-imnmiscible non-polar organic liquids may be employed. Illustrative liquids of this type are aromatic hydrocarbons such as toluene and xylene; substituted aromatic hydrocarbons such as chlorobenzene, o-dichlorobenzene and nitrobenzene; and chlorinated aliphatic hydrocarbons such as chloroform, methylene chloride, trichloroethane and tetrachloroethane; methylene chloride being particularly suitable. It is advantageous to employ, in combination with the water-immiscible solvent, a more polar combined oxygen-containing solvent such as tetrahydrofuran in which the diol is soluble to facilitate dissolution thereof.
The molar ratio of diol to acid chloride is about 1:1. The amine is employed in amounts approximating those of the diol and diacid chloride combined rather than in catalytic amounts. Formati
Correll Glenn D.
Tullos Gordon L.
Tullos Tina L.
Pianalto Bernard
Rohm and Haas Company
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