Process for providing increased conductivity to a material

Details Process for providing increased conductivity to a material Process for providing increased conductivity to a material

2000-06-07

2002-05-28

Kopec, Mark (Department: 1751)

Compositions

Electrically conductive or emissive compositions

Elemental carbon containing

C427S097100, C427S122000

Reexamination Certificate

active

06395199

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a process for providing increased electrical and/or thermal conductivity to a material, and to the materials prepared by the process. More particularly, the invention relates to a process involving applying particles of expanded graphite to a substrate or material in order to increase the conductivity of the substrate or material. The particles of expanded graphite can be applied to the substrate or material through coating of the substrate with a composition comprising the expanded graphite particles, or by incorporating particles of expanded graphite into the substrate or material itself.
BACKGROUND OF THE INVENTION
There has been a longstanding need in industry to provide electrical conductivity to non- or insufficiently conductive materials. For instance, in the automotive industry, electrostatic painting is the most highly preferred method of painting component parts. Electrostatic painting offers several times the transfer efficiency of spray painting, providing improved quality and creating significant savings by allowing paint usage to be cut by up to 75%. This has the additional benefit of greatly reducing emissions of volatile organic compounds and other hazardous pollutants, as compared with spray painting. However, electrostatic painting requires a conductive surface for effectiveness. Conventionally, when the part to be painted is a plastic or other non-metallic material, the part first has to have a primer coating of a conductive paint applied, making the electrostatic painting process more involved and therefore less desirable than otherwise.
For instance, in U.S. Pat. No. 6,019,833, Hartman, Rei, Castagnone and Hamay describe the use of a primer coating for facilitating the electrostatic painting of a non-conductive article, the primer coating containing carbon fibrils. In an attempt to address this problem, U.S. Pat. No. 6,001,919 to Yen, Ingham and Bono, describes a molding composition having a conductive filler, specifically carbon black, to form an article having sufficient conductivity to support electrostatic painting;
Likewise, certain components, such as automotive fuel system components, must be treated to dissipate static electricity, to avoid accidental ignitions. For instance, certain fuel system components are being proposed, such as fuel filler components, which are made out of plastics like nylons. Such plastic components require improved static discharge properties.
Another, unrelated area in which improved electrical conductivity may be desired is in the manufacture of printed circuit boards. Conventionally, printed circuit boards are solid circuits formed from a conductive material positioned on opposite sides of an insulating material, or in layers with insulating material interposed between the layers of conductive material. In order to make electrical connections between the circuits on the circuit board, a hole is first drilled through the board, i.e., through the conducting sheets and the interposed insulator material. These holes are known in the art as “through holes.” A conductive pathway must then be formed to connect the respective circuits. Most commonly, that conductive pathway is formed by the electrolytic deposition of copper on the surfaces of the through holes. However, the presence of insulating material makes the electrolytic deposition of copper difficult and inconsistent. As a result, sufficient conductivity must be established on the through hole surfaces to permit the electrolytic copper deposition.
Several methods have been suggested for creating sufficient conductivity to permit the consistent electrolytic deposition of copper on through hole surfaces. One such method is through the use of so-called “electroless” copper, that is, copper that is chemically deposited on the through hole surfaces at a thickness sufficient to permit electrolytic deposition (but not thick enough to eliminate the need for electrolytic deposition). Although electroless deposition has proven effective, it has several commercial disadvantages. For instance, the electroless deposition process requires multiple steps prior to electroplating; involves a relatively long process time; uses multiple treatment baths; involves a complex chemistry that may require constant monitoring and individual ingredients which may require separate replenishment; uses various chemical agents that are considered carcinogens and/or are otherwise hazardous or include heavy metals, thus posing safety concerns and requiring extensive waste treatment; and utilize a multiplicity of rinse baths and thus may require large amounts of water.
In an attempt to avoid the disadvantages of the electroless deposition process, Minten and Pismannaya, in U.S. Pat. No. 4,619,741, describe coating the surfaces or walls of printed circuit board through holes with particles of carbon black, to provide sufficient conductivity to support electrolytic copper deposition. In a similar vein, Thorn, Polakovic and Mosolf, in U.S. Pat. No. 5,389,270, disclose coating the walls of printed circuit board through holes with particles of graphite having a mean particle size of from 0.05 to 50 microns. Although Thorn et al. suggest that particles of natural graphite can be used, they indicate that synthetic graphite is preferred.
Prior methods disclosed for providing conductivity to insufficiently conductive articles suffers from significant drawbacks. For instance, carbon fibers, carbon fibrils, nanotubes, nickel coated carbon fibers, steel fibers, carbon blacks, etc. have been proposed as conductive fillers, but the loading levels required in many prior art methods to provide the required degree of conductivity can be prohibitively high; likewise, consistency is often a problem, as is weight and density. Similarly, the cost of some of the prior art methods, such as carbon fibrils or nanotubes, can also be prohibitive.
Other relatively non-conductive materials, including greases and oils, can act as heat and electrical insulators, and thereby do not sufficiently dissipate heat and static electricity from, e.g., gear boxes and other moving metallic parts where friction can generate heat and static electricity. Failure to sufficiently dissipate the heat and static electricity can cause undue or early corrosion.
There are also many applications in which conductive adhesives may be desirable. For instance, adhesives are often used to bond materials such as heat sinks or thermal interfaces to a heat source. With insufficient conductivity, an adhesive can interfere with the desired thermal transfer, degrading the usefulness of the heat sink or thermal interface. In addition, a conductive adhesive can also function to facilitate grounding of an electrical device in which it is used.
What is desired, therefore, is a process for increasing the conductivity of an insufficiently conductive material, article or surface, which does not adversely affect the other desirable properties of the material, article or surface. Such a process is cost-effective, and provides sufficient conductivity without undesirably high loading levels or weight increase.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for increasing the conductivity of an insufficiently conductive material, article or surface.
It is another object of the invention to provide a process that increases the conductivity of a material, article or surface while preserving the desirable characteristics of the material, article or surface.
It is yet another object of the invention to provide an article having sufficient conductivity to permit the electrostatic painting of the article.
It is still another object of the present invention to provide a printed circuit board having through hole walls sufficiently conductive to permit the electrolytic deposition of copper on the walls.
It is a further object of the present invention to provide an oil or grease sufficiently conductive to at least partially dissipate the heat and/or static electricity generated by moving parts.
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