Coating processes – Solid particles or fibers applied – Uniting particles to form continuous coating with...
Reexamination Certificate
1999-05-21
2004-03-09
Rickman, Holly (Department: 1773)
Coating processes
Solid particles or fibers applied
Uniting particles to form continuous coating with...
C427S386000, C525S526000
Reexamination Certificate
active
06703070
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a powder coating system in which an epoxy resin and a low temperature curing agent are extruded or otherwise melt-mixed as one component and pulverized to form a low temperature curable coating powder. The powder cures at a temperature of from about 225 to about 300° F. and produces a coating having an exceptionally smooth surface with either a low or high gloss. This invention also relates to the electrostatic coating of massive metal articles and wood and low temperature curing to achieve a pleasing smoothness. It also relates to a method for compounding a coating powder by extruding a mixture comprising an epoxy resin having a low melt viscosity and a low temperature curing agent at a temperature of from about 160° F. to about 230° F. (about 71-110° C.).
Traditionally, coating powders have been made by the extrusion of a mixture of resins and curing agents to obtain a homogeneous mixture and then grinding the extrudate and screening the comminuted product to obtain the desired particle sizes and particle size distribution. The powder is then electrostatically sprayed onto a substrate, traditionally a metal substrate, and cured at temperatures much higher than 200° F. Achieving a powder coating composition that will cure on heat sensitive substrates at less than 300° F. in less than 5 minutes has long been a goal of the industry. The curing of powder coatings on heat sensitive materials such as wood, plastic, and the like has been limited by the fact that the extrusion of a mixture of a resin and a low temperature curing agent, i.e., one that is active at 250° F. or less, would cause the coating powder to gel in the extruder because the extrusion typically generates enough heat to raise the temperature to 200° F. or higher.
This problem has been avoided by extruding the resin and a small amount of catalyst or low temperature curing agent, grinding the extrudate, and then blending the resultant powder with an additional amount of the curing agent in powder form, according to the disclosure in co-pending, commonly assigned application Ser. No. 917,043, now U.S. Pat. No. 6,077,610 filed Aug. 19, 1997.
The high temperatures that are generated by the extrusion of a curing agent with a resin are caused in part by the fact that the temperature must be sufficient to facilitate the mixing of resins having initially high melt viscosities. Even higher temperatures are produced by the friction arising from the mixing of the still highly viscous molten resins with curing agents that are solid at room temperature.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a method for extruding a one-component low temperature curable coating powder.
It is a related object of this invention to provide a one-component low temperature curable coating powder that has satisfactory blocking resistance during storage at normal temperatures.
It is a related object of this invention to provide a one-component low temperature curable coating powder for heat sensitive substrates and massive metal articles.
It is another object of this invention to provide a low temperature process for producing a smooth, high gloss coating on wood.
These and other objects of the invention which will become apparent from the following description are achieved by a method for preparing a thermosetting coating powder comprising extruding or otherwise melt-mixing an epoxy resin having a melt viscosity of from about 200 to about 3000 centipoise at 150° C. and a curing agent which is latent at a temperature of from about 160° F. to about 230° F., cooling the mixture, and comminuting it.
DETAILED DESCRIPTION OF THE INVENTION
For the purpose of describing the proportions of components in the compositions of this invention, the term resin includes the resin per se and the curing agent but not the catalyst. Also, the term “one component coating powder” means that the powder is fully formed by grinding and screening only one extrudate of a mixture of the resin, curing agent, catalyst, and additives.
Extrusion is but one form of melt-mixing known to the powder coating art for the thorough mixing of components that is necessary in the manufacture of the powder. It is, however, a quite convenient and efficient form of melt-mixing, i.e., mixing of the powder components while in the molten state, and is preferred in this invention. In either instance, the molten material is cooled and ground to obtain an average particle size of from about 10 to about 100 &mgr;.
Epoxy resins which are suitable for the purposes of this invention have an equivalent weight of from about 100 to about 700. Mixtures of such epoxy resins may be used. A suitable mixture may comprise an epoxy resin having an equivalent weight between about 100 and 400 and one having an equivalent weight between 400 and about 700 in a weight ratio of from 1:99 to 99:1. The resins are exemplified by, but not limited to, those produced by the reaction of epichlorohydrin and a bisphenol, e.g., bisphenol A and bisphenol F. The low melt viscosities of these resins facilitate the extrusion of them in admixture with a curing agent, additives and pigments at about 160-220° F. The preferred melt viscosity is from about 300 to about 1000 centipoise. The melt viscosity of resins having a low T
g
, i.e., from about 35° C. to about 65° C., is suitable for the purposes of this invention. Epoxy resins known as EPN (epoxy phenol novolac) and ECN (epoxy cresol novolac) resins and those made by the reaction of isopropylidenediphenol (bisphenol A) with epichlorohydrin are suitable for the purposes of this invention. Bisphenol A epoxies are sold under the trademarks ARALDITE GT-7071, GT-7072, EPON 1001 and EPON 2042. ARALDITE GT-6259 is the trademark for an ECN resin. Epoxy resins suitable for the purposes of this invention are further exemplified by the triglycidylisocyanurate (TGIC) resins, the glycidyl methacrylate resins and glycidyl resins containing a hydrogenated bisphenol A moiety, all of which are useful when coatings having an improved UV stability are desired.
The use of a crystalline epoxy resin may improve the flow characteristics of the fused coating powder and, therefore, the smoothness of the fused and cured coating. A particularly desirable flow is achieved when a crystalline epoxy constitutes from about 5 to about 20% by weight of the total amount epoxy resin used in the formulation of the powder. The performance of a coating powder of this invention deteriorates as the level of crystalline epoxy resin therein is increased because of the relatively low equivalent weights of such resins and the preferred amount of such a resin is about 10% or less for that reason. A crystalline epoxy resin having a melting point between about 80° C. and about 150° C. is preferred. A crystalline epoxy resin having an equivalent weight of about 185 and sold by Shell under the trademark RSS 1407 is suitable for the purposes of this invention.
When resins having a T
g
of from about 35° C. to about 40° C., are used in this invention, sintering of the powder is avoided by allowing the temperature in the extruder to rise to activate the low temperature curing agent for a time sufficient to raise the extrudate's viscosity beyond the sintering point and then cooling the extrudate rapidly to about 10-20° C. (about 50-70° F.) before chipping and grinding it and storing the powder at such temperature to avoid a further viscosity build-up by continued curing. Another way to avoid sintering of the powder when low T
g
resins are used is to pre-mix the resin with a crystalline or non-crystalline curing agent powder having an average particle size of about 5 microns that does not liquefy in the extruder. The powdered agent does not react with the resin so readily as does the same curing agent in flake form. A specific example of a curing agent that may so used in the powdered form is sold under the trademark ANCAMINE 2441.
The low temperature curing agent of this invention is one that will be active at a temperature of from abo
Muthiah Jeno
Ruth William G.
Schlessman Jacquelyn M.
Seelig Carryll A.
Teti Jeremiah J.
Morton International Inc.
Rickman Holly
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