Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1998-06-30
2002-05-28
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192150
Reexamination Certificate
active
06395149
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides a method of making electrically conductive, light colored coated particles that are particularly useful for the manufacture of static dissipative compositions. The coated particles are useful for making static dissipative composites.
BACKGROUND OF THE INVENTION
Static electricity is a common problem. In industry, electrostatic discharge (ESD) events can be responsible for equipment failures, manufacturing defects and even explosions of solvents or flammable gases. One method of controlling static electricity is the use of static dissipative materials. Static dissipative materials are often required in manufacturing, the electronics industry and hospital environments. Examples of static dissipative materials include floorings in solvent handling areas and molded plastic trays for handling electronic components.
Static dissipative materials have electrical resistance between insulative and conductive materials. In general, materials that have a surface resistivity of more than 10
12
ohms per square and/or a volume resistivity of more than 10
11
ohm-cm are considered non-conductors, or insulators. Materials that have a surface resistivity of less than 10
5
ohms per square and/or a volume resistivity of less than 10
4
ohm-cm are considered conductive. Materials that have surface resistivities or volume resistivities in between these values are considered to be static dissipative. More specifically, static dissipative materials have surface resistivities between 10
5
and 10
12
ohms per square and/or volume resistivities between 10
4
and 10
11
ohm-cm. Some static dissipative applications require surface resistivity to be between 10
6
and 10
9
ohms per square and/or volume resistivity to be between 10
5
and 10
8
ohm-cm. (
ESD Association Advisory for Electrostatic Discharge Terminology
, ESD-ADV1.0-1994, published by the Electrostatic Discharge Association, Rome, N.Y. 13440.)
Surface resistivity is measured across the surface of a material. A typical method for measuring the surface resistivity of a material is to place electrodes on the surface, and then measure the resistance between the electrodes. The dimensions of the electrodes and distance between them is used to convert the resistance to surface resistivity in units of ohms per square.
Volume resistivity is measured through the bulk, or volume, of a material. A typical method for measuring the volume resistivity of a material is to place electrodes on the upper and lower surfaces of the material, and then measure the resistance between the electrodes. The area of the electrodes and thickness of the composite are used to convert the resistance to volume resistivity in units of ohm-cm.
Many commonly used materials are non-conductive. Examples of these are polymers, such as polyethylene or polysulfone, and epoxy resins, such as bisphenol A based resins. One method for making these materials static dissipative is to add conductive particles to them. Those non-conductive materials which are made static dissipative by adding conductive particles are called static dissipative composites. In order to make a non-conductive material static dissipative, conductive particles must be added in sufficient quantity to create a network of conductive paths through the material. These paths are formed by the conductive particles in electrical contact with each other. The level of conductivity depends on the number of conductive paths created by the particles. If there are too few particles, there will not be enough conductive paths to provide static dissipative properties to the composite.
Traditional conductive particles for static dissipative composites include carbon, graphite, and metal. These particles have several disadvantages. They are difficult to disperse and the static dissipative properties are strongly dependent on particle filling. This makes it difficult to produce composites within the desired conductivity range. These conductive particles are also dark in color and impart a dark color to the static dissipative composite.
JP SHO 53(1978) 9806 and SHO 53(1978) 9807 (Mizuhashi et al) teach glass microspheres with conductive indium oxide or tin oxide or indium tin oxide coatings. The object of JP SHO 53 (1978) 9806 is to produce glass microspheres with high conductivity without increasing the reflectivity of light too much. This reference teaches glass microspheres of transparent soda lime silicate glass, boron silicate glass, lead silicate glass, etc. with a low refractive index or high reflective index, or containing a coloring component. The manufacturing process includes a film formation process in which a solution containing a solvent, comprising water and/or a lower alcohol, a soluble-indium compound, and an organic thickener, is coated onto the surface of the glass microspheres to form a film on the surface of the glass microspheres. The next step is a drying process, in which the glass microspheres having a surface film formed from the above-mentioned solution are dried to evaporate the solvent in the film, and to form a film mainly composed of the above-mentioned indium compound and an organic thickener on the glass microspheres. This is followed by a baking process, in which the above-mentioned glass microspheres are baked in an oxidizing atmosphere at a high temperature to form a transparent coating mainly composed of indium oxide on the surface of the glass microspheres. A soluble tin compound can also be included with the soluble indium compound to form an indium tin oxide coating.
JP SHO 53 (1978) 9807 describes a method for making tin oxide coated microspheres that includes a solution production process in which an organic tin compound containing oxygen is dissolved in an organic solvent to produce a solution. The next step is a solution coating process in which the above-mentioned solution is coated onto the surface of glass microspheres to form a film on the surface of the glass microspheres. This is followed by a drying process in which the above-mentioned glass microspheres are dried under reduced pressure to form a resin-like film containing the organic tin compound on the surface of the glass microspheres. The final step is a baking process in which the above-mentioned glass microspheres are heated at a high temperature and under reduced pressure so that thermal decomposition of the organic tin compound is carried out to form a transparent tin oxide film on the surface of the glass microspheres.
Neither JP SHO 53(1978) 9806 nor JP SHO 53(1978) 9807 make reference to particles containing voids, such as hollow glass microspheres, nor do they disclose particles that have non-spherical shapes, such as glass fibers. These references also do not disclose the use of these particles for static dissipative composites. Both of these references declare that other methods for coating particles with diameters of 1 min or less, such as sputtering, vacuum deposition, and chemical deposition are “difficult to apply,” and state that “uniform formation of the film over the entire surface on the sphere is not possible,” and “production equipment becomes expensive.”
JP SHO 58(1983)-25363 (Tanaka) teaches pigment particles coated with indium oxide or tin oxide for conductivity. The particles are described as inorganic pigments. Inorganic pigment particles of the type listed in this reference are typically very small, on the order of several microns or less. This reference makes no reference to spherical particles, including those containing voids, such as hollow glass microspheres. Fibers other than asbestos are not taught. The objective of this invention is to provide a method of producing an inexpensive conductive pigment that can be used effectively as a recording material in electrophotographic or electrostatic recording systems or recording systems in which a color is formed by the passage of an electric current, and that can also be used to provide antistatic properties to polymer films, etc. This reference does not teach how to provide antistatic properties t
3M Innovative Properties Company
Cantelmo Gregg
Nguyen Nam
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