Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
2001-10-17
2004-02-10
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C427S470000, C427S477000, C525S440030, C525S443000
Reexamination Certificate
active
06689457
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to compositions sprayed by internally charged electrostatic sprayers that harden to form conductive materials, and to articles comprising one or more layers of these conductive materials.
BACKGROUND OF THE INVENTION
Automobiles are painted by various methods and some of them include electrostatic sprayers having high voltage (50 to 140 kilovolt) sprayheads. Automobile bodies used in such processes are usually made of metal and are electrically grounded to create a voltage differential between the spray head and the automobile body. An electrode in the spray head charges droplets of paint, and a charged spray cloud from the spray head is attracted to the grounded automobile body. Paint transfer efficiency (i.e., the amount of paint transferred from a sprayer to a substrate surface) is increased as the paint to be sprayed becomes charged. Internally charged electrostatic sprayers typically have a tube inside the spray head and are able to charge approximately 90% of the paint within the tube. Externally charged electrostatic sprayers typically have external projections surrounding the spray head orifice and charges paint leaving the spray head. Externally charged electrostatic sprayers charge a smaller percentage of paint than do the internally charged sprayers and painting processes using externally charged electrostatic sprayers typically provide lower paint transfer efficiencies than processes using internally charged electrostatic sprayers.
Most automobile bodies also contain plastic parts, such as plastic exterior car body panels and plastic trim components. During electrostatic spray painting, electrostatic charges accumulate on surfaces of uncoated plastic parts and reduce the potential between spray heads and uncoated plastic surfaces. A reduced potential diminishes electrical forces acting on charged paint droplets. Accumulated surface charges may also result in an opposing electrical field on uncoated plastic surfaces repelling air-borne paint particles and resulting in low paint transfer efficiency when plastic surfaces are electrostatically sprayed.
Plastic surfaces coated with conductive primers are thought to reduce accumulation of surface charges during electrostatic spray painting. Coated plastic parts are attached to an automobile body prior to painting and can be grounded just like metal allowing the plastic to “look” like metal when subsequently electrostatically sprayed with topcoat, clearcoat, and/or other automotive finishes. Unfortunately, internally charged electrostatic sprayers are unable to spray compositions, which form conductive coatings with high efficiency because of track back. Track back refers to the ability of electricity to travel along the paint spray and return back to the electrostatic sprayer. Most paint compositions sprayed by an internally charged electrostatic sprayer are not conductive enough to result in track back when being sprayed. However, compositions which form conductive coatings, sprayed with an internally charged electrostatic sprayer, provide a path for electricity to track back and short out the sprayer. Therefore, airsprayers, or externally charged electrostatic sprayer, are usually used to coat plastic parts before automobile manufacture. Then the coated plastic parts are attached to the automobile body and then primed a second time (simultaneously with the application of a primer to metal parts) prior to being painted. Consequently, the surfaces of the plastic parts typically include two layers of primer.
Compositions, which form conductive coatings and are sprayable by an internally charged electrostatic sprayer onto both metal and plastic would be desirable. Both metal and plastic parts could be primed at the same time and the composition would eliminate a manufacturing step of pre-coating plastic parts prior to automobile body assembly. Using an internally charged electrostatic sprayer, to spray such a composition, would increase paint transfer efficiency compared to processes using externally charged electrostatic sprayers.
SUMMARY OF THE INVENTION
The present invention provides compositions comprising graphite, binder precursor, and low polarity solvent The compositions are capable of being sprayed by an electrostatic sprayer that is internally charged and that upon hardening forms a layer of electrically conductive material. Both metal and plastic parts of an automobile body may be electrostatically sprayed with compositions of the present invention so that plastic parts are primed only once during manufacturing reducing the costs of separately applying a layer of conductive material to a plastic part prior to automotive assembly. The compositions of the present invention may also be used as primers for metal parts that are applied prior to the electrostatic spraying of topcoat. Consequently, the conductive compositions of the present invention provide, in part, at least two functions: a primer for metal and a conductive surface to plastic. The conductive compositions of the present invention are capable of being sprayed through the spray head of an internally charged electrostatic sprayer without track back and shorting out the spray equipment (firefault limit is typically set at 180 microamp). The spray head of an internally charged electrostatic sprayer used to spray compositions of the present invention may have a voltage in the range of about 50 to about 140 kilovolts, preferably about 80 to about 90 kilovolts. By electrostatically spraying compositions a step is removed from an automobile manufacturing process. Automobile manufacturers may therefore obtain a cost savings. In addition the coatings of the present invention are chip resistant, impact resistant, have a good appearance and have good adhesion to metal and plastic.
The present invention also provides an article comprising a substrate attached to a layer of electrically conductive material. The compositions of the present invention harden to form a coating (a layer of electrically conductive material) on the surface of a substrate.
The present invention also provides a process of coating comprising the steps of spraying with an internally charged electrostatic sprayer a composition comprising graphite, binder precursor, and a low polarity solvent
As used herein, with respect to the present invention, the following shall apply:
“capable of being sprayed electrostatically” refers to compositions sprayed by an internally charged electrostatic sprayer without resulting in track back.
“composition” refers to the flowable state of a coating comprising binder precursor, graphite and a low polarity solvent.
“coating” refers to a hardened composition described above.
“pigment” refers to carbon black, graphite, TiO
2
or other particles that provide color to a composition.
DETAILED DESCRIPTION OF THE INVENTION
Compositions of the present invention contain a binder precursor including one or more resins that harden to form a binder. A binder precursor may include a hydroxy containing resin, such as polyester, acrylic modified alkyl, acrylic polyols, epoxy, isocyanate, acrylate, methacrylate or combinations thereof. Polyester resins are the preferred binder precursor and include polyester urethane, polyester epoxy, or combinations thereof. Preferably, a polyester used in the present invention has a hydroxyl number of about 100 to about 250, a number average molecular weight of about 500 to about 9000, and an acid number of 1 to about 40. A binder precursor of the present invention preferably comprises about 45 to 85 weight percent of a hydroxyl containing resin.
A binder precursor of the present invention also includes one or more cross-linking resins, preferably melamine resin. A melamine resin suitable for use in the practice of the present invention includes a non-alkylated melamine resin, a partially alkylated melamine resin, a completely alkylated melamine resin, a melamine formaldehyde resin or combinations thereof. Suitable partially alkylated melamine resin and/or fully alkylated melamine resin used
Chang David C. K.
Pickett Monica A.
Singer Debra Feldman
Sorser Aleksandr
Strickland Debra Sue
Benjamin Steven C.
E. I. du Pont de Nemours and Company
Rickman Holly
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