Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-09-04
2002-01-22
Mulcahy, Peter D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S429000, C524S904000, C525S934000, C209S001000, C209S047000, C209S054000
Reexamination Certificate
active
06340722
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the preparation of powder coatings, and more particularly, to the preparation of powder coatings by polymerizing the base polymer resin and blending the powder coating materials in a supercritical fluid. By using a supercritical fluid as the solvent throughout the initial polymerization process, the molecular weight range of the base polymer resin can be controlled and a polymer base resin having a polydispersity of about 2 or less can be obtained. By blending the powder coating materials in the supercritical fluid medium, a more homogeneously blended powder coating can be obtained. The process further provides for more effective control of the particle size of the powder coatings initially dissolved in the supercritical fluids.
BACKGROUND OF THE INVENTION
Powder coatings are polymer materials blended with other additives used to coat various substrates. Such powder coatings are today used in a variety of industries to provide protective barriers to the substrates that they coat. Powder coatings are particularly desirable today because they are environmentally friendly. They are typically applied without the use of toxic solvents, and overspray material from the application process can be reused because it has not been cured onto the substrate. To form optimum powder coatings, it is generally necessary to use polymers which have narrow molecular weight distributions. This, in turn, provides the polymer with a small polydispersity. It is also necessary to attempt to keep the polymer from curing prior to being used as a coating. In other words, it is important that minimal cross-linking take place during the blending of the coating materials. Finally, it is highly desirable that the resultant powder coating produced have a narrow particle size distribution on the order of about 15 to 40 &mgr;m.
Historically, powder coatings have been made by a multiple stage process in which each stage is a unique and separate operation. First, the polymer or base resin is polymerized in a solvent based reaction. The solvent is recovered, leaving a dry resin. Typical resins used to make powder coatings may include, but are not necessarily limited to, epoxy, polyester, acrylics, and mixtures thereof.
Next, the resin is premixed, to facilitate homogeneous mixing, with a cross-linking agent and additives such as flow agents, pigments, and interfacial agents. Subsequently, the mixture is processed through a twin screw extruder at approximately 160° C. to form a homogeneous powder coating material. The material is cooled prior to grinding the material into a powder. The powder is finally separated by size into its classification.
The drawbacks to this process are polymerizing a resin with a wider molecular weight distribution and a higher polydispersity (approximately 2.6-3.0). Premature curing caused by the high heat during extrusion is also a problem. Furthermore, the extrusion process does not produce a fully homogeneous dispersion. The high temperature of the extrusion process forces the manufacturer to use high temperature compatible cross-linking agents. Yet another drawback is that grinding produces dust and material losses and consumes energy, and creates a wider particle size distribution. Also, the process requires multiple stages that require large amounts of floor space and are dedicated to operating at a particular batch size.
Wider molecular weight distribution is undesired because the resin is less effective. For a particular application, a target molecular weight for the resin is optimal for that application. As the molecular weight diverges from the target molecular weight, the properties of the resin change and become less optimal. Staying narrowly close to the target molecular weight achieves the desired optimum properties for the resin. Molecular weight of the resin affects adhesion to the substrate and porosity of the coating which, in turn, affects the protection of the substrate. The molecular weight distribution is measured as polydispersity. Polydispersity is the weight average molecular weight divided by the number average molecular weight. As the polydispersity number decreases, the molecular weight distribution becomes narrower and closer to the average molecular weight of the resin. Historically, polydispersity has ranged from 2.6 to 3.0 for resins. Preferably, polydispersity is desired to be about 2 or less.
Analogous to molecular weight is particle size. A targeted particle size is optimal for a given application. As the particle size diverges from the target molecular weight, the properties of the resin change and become less optimal. Particle size affects the porosity of the coating and the ability to protect the substrate and the adhesion of the resin to the substrate.
The selection of cross-linking agents is limited by the high temperatures required for melt blending in an extruder. Cross-linking agents have to be selected that cure at a temperature above the operating temperature of the extruder to prevent premature curing of the cross-linking agent. The premature curing causes the resin to cross-link with itself and reduces the available cross-linking sites that would be available to cross-link with the substrate. This makes for a weaker coating and also reduces the gloss of the resin. Also, the high temperature cross-linking agent requires that high temperatures be used when curing the resin to the substrate. This adds additional energy costs. Lower blending temperatures would permit a larger number of cross-lining agents to be selected based on functionality and costs of the cross-linking agent, and lowers energy costs to cure the resin to the substrate.
Additionally, the extrusion process does not provide a uniform blending. This is because there will be zones of varying temperature which affect the melt of the resin and the fluidity of the melt. Uniform blending is desired to reduce the variation in the powder coating and increase the effectiveness of the additives in the powder coating and the powder coating itself.
The traditional powder coating production process comprises many dissimilar process operations. The first step is polymerizing the resin in a reactor. Once reacted, the resin is separated from its solvent and prepared for extrusion blending. Next, the resin is blended with the additives to make the powder coating material. After blending, the powder coating is processed through a grinder to form particles of a desired size, and finally sorted according to the desired size. Each of these steps is unique and requires different processing equipment. This increases the capital cost of purchasing these various types of equipment and the amount of floor space needed to store the equipment.
Additionally, these pieces of equipment are designed to operate with a predetermined batch size. Particularly, the extruder has a minimum amount of material required to reach a steady state operation. If a small batch of a particular powder coating product is desired, the amount of waste in proportion to the amount of product generated is high as compared to a large batch run. Alternatively, a large batch could be produced; however, storage costs would be incurred storing the excess, and if stored for too long, the quality of the product deteriorates.
Similarly, because of the dissimilarity between the processing steps, material has to be produced and stored from each step before being processed by the next step so that there is a surplus amount of material to continuously feed the process. This adds additional costs to store this intermediate material in terms of warehouse space and environmental controls to maintain the quality of the material.
The use of supercritical fluids as a solvent in various steps of this prior art process is well known. For example, U.S. Pat. No. 5,328,972 to Dada et al. discloses forming a reaction mixture of one or more polymerizable monomers, and a free radical initiator in supercritical carbon dioxide at an elevated temperature of at least 200° C. and an elevated pressure of at
Fullerton Kathy L.
Lanterman H. Bryan
Lee Sunggyu
Pettit, Jr. Paul
Mulcahy Peter D.
Renner Kenner Greive Bobak Taylor & Weber
The University of Akron
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