Stock material or miscellaneous articles – Composite – Of polyamidoester
Reexamination Certificate
1996-09-24
2002-06-04
Cameron, Erma (Department: 1762)
Stock material or miscellaneous articles
Composite
Of polyamidoester
C428S704000
Reexamination Certificate
active
06399206
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for electrostatically painting polymers. This invention particularly relates to electrostatically painting polymers having urethane or urea groups.
It is well known that most painting methods are unlikely to be 100 percent efficient. For example, when paint is applied to an object as a spray, some fraction of the paint directed at the object may not be deposited on the object. In even more inefficient circumstances, the paint and the object can acquire a static charge of the same polarity resulting in the paint being partially repelled from the object. Another painting inefficiency commonly observed is that the paint layer on a painted object can be of inconsistent thickness. Yet another painting inefficiency commonly observed, particularly in spray painting objects having complex shapes, is that paint can tend to travel from a spraying apparatus to the object to be painted in a relatively straight line and may not cover surfaces not directly accessible to the spraying apparatus.
In painting applications requiring a very high quality paint finish, such as in the painting of automobile body parts, it is generally desirable to completely and evenly coat an object to be painted with a minimum expenditure of paint. This is desirable for several reasons. First, paint for such applications is often very expensive. Therefore, reducing paint consumption offers the immediate benefit of reduced paint cost. Second, paint which is not deposited upon the object to be painted can be lost to the environment and this loss can be environmentally undesirable. Therefore, reducing the quantity of paint lost to the environment in a painting process reduces the costs of disposing of the lost paint solids and reduces emissions of paint solvents. Third, while the layer of paint being slightly too thick on an object can sometimes be tolerated, the paint layer being too thin more often cannot. Besides improving the appearance of an object, modern paints often play a vital role in protecting a painted object from its environment. For example, paint can protect a metal object from corrosion or protect a plastic object from degradation by ultraviolet radiation. Therefore, failures of painted objects due to areas of too thin paint on the objects can be avoided by applying paint to objects wherein the layer of paint is of a consistent and sufficient thickness.
In order to minimize problems with inefficient painting, it is common practice in painting some materials to apply paint electrostatically. In electrostatic painting, a static electric potential is generated between paint and an object to be painted causing the paint to be attracted to the object. As a result of the electrostatic attraction, less paint can be lost to the environment and the paint can be more evenly applied to the object without the entire surface being directly accessible to the paint spraying apparatus.
But electrostatically painting objects is not always trouble free. In order to electrostatically paint an object, an electric charge potential must be generated between both the object to be painted and a paint to be applied to the object. If an object is either not conductive or of very low conductivity, it cannot be efficiently electrostatically charged and cannot, therefore, be efficiently electrostatically painted.
One means of electrostatically painting polymers is to first make them more conductive by preparing the polymers from formulations including conductive fillers. European Patent Application 0 363 103 to Suzuki, et al., discloses preparing a thermoplastic polymer having 2 to 50 percent by weight of a fibrous conductive filler such as carbon fibers, metallic fibers, metalized glass fibers, metal coated carbon fibers and conductive potassium titanate whiskers. The polymer is etched and then electrostatically painted. However, adding such large amounts of fibrous fillers to a polymer can adversely affect both the polymer's physical properties and paint finish. A separate etching step can also be undesirable.
JP II 2-180960 assigned to Kanto Auto Works discloses preparing polyurethane substrates having improved conductivity. The polyurethanes of this reference are prepared with ammonium salts such as n-alkyldimethyl ammonium sulphates. One problem with such additives are that they promote conductivity only when humidified. This could be a problem in a painting application located in a region wherein the ambient humidity is not constant. Alternatively, deliberately humidifying articles prior to electrostatically painting them could be time consuming and expensive.
Another means of electrostatically painting polymers is to first make them more conductive by applying thereto conductive agents called “prepcoats”.These prepcoats are conductive agents which adhere to the surface of the polymer. Such agents can include materials such as quaternary amines. One problem with such compounds is that they are often hydrophilic. In reaction injection molding (RIM) polyurethanes, adsorbed water can cause blemishes on the surfaces of the polymer. Hydrophilic coatings can aggravate the tendency of objects to adsorb water prior to painting, thus promoting the formation of blemishes and thereby be undesirable in applications requiring high quality finishes.
Yet another solution to the problem of electrostatically painting plastics is to first coat a plastic object with a conductive coating, and then to electrostatically paint the coated object. For example, U.S. Pat. No. 5,071,593 to Takahashi, et. al., discloses coating problematic plastics such as polyacetal and polyesters with a conductive agent prior to electrostatic painting. However, coating an object with a conductive agent can be inefficient. It requires a capital expenditure for additional painting equipment, additional processing time to apply the conductive agent, additional processing time to allow the conductive agent to cure and expenditures for the cost of the conductive agent.
It is known in the art that polymers, particularly reaction injection molded polyurethane/polyurea polymers, are useful materials for preparing automobile parts such as, for example, fascia and interior and exterior panels. In order to paint articles prepared from such polymers and produce a high quality paint finish, as is required in modern automobile manufacturing practices, it is known to electrostatically paint these articles after first applying a conductive primer thereto. Known processes for electrostatically painting polymer articles include at least the steps of: (1) preparing an object to be painted from a polyurethane/polyurea polymer formulation; (2) coating the object with a conductive agent (or conductive primer); (3) applying an electric charge of a first polarity to a paint; (4) applying an electric charge of second and opposite polarity to the object (or merely charging either the paint or the article relative to ground while leaving the other neutral to ground); and (5) discharging the paint from a painting apparatus onto the object.
It would be desirable in the art to efficiently electrostatically paint polymers without having to pretreat the polymer by priming or prep-coating the polymer with a conductive material. It would also be desirable in the art to more efficiently electrostatically paint a polymer which has been primed or prepcoated with a conductive substance. It would be desirable in the art to prepare a polymer with sufficient native conductivity to efficiently electrostatically paint the polymer. It would be desirable in the art that the polymer to be painted not require special treatment such as humidification. And it would also be desirable in the art that the painted polymer have a sufficiently high quality painted surface and sufficiently strong physical properties such that the polymer could be used in very demanding applications such as the manufacture of automobiles. Finally, it would be desirable in the art if objects of dissimilar composition, e.g., metal and plastic, can be painted as a unit rat
Carswell Robert
Cornell Martin C.
Groseth Cynthia K.
Porter James R.
Priester, Jr. Ralph D.
Cameron Erma
The Dow Chemical Company
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