Plastic and nonmetallic article shaping or treating: processes – Forming electrical articles by shaping electroconductive...
Reexamination Certificate
2001-09-17
2004-03-09
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming electrical articles by shaping electroconductive...
C264S132000, C264S311000, C156S240000
Reexamination Certificate
active
06702968
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to molded polyethylene parts and, in particular, to a method for permanently coating polyethylene surfaces with electrically conductive circuits.
2. Brief Statement of the Prior Art
Surfaces of polyolefins, particularly polyethylene, are very non-receptive to coatings such as paints, inks and the like. Consequently, it is very difficult to apply a layer of electrically conductive elements in a permanently secure manner to the polymer surface.
The polyolefins, particularly polyethylene, are ubiquitously used for rotational molding of large, hollow-form parts such as tanks, boxes, children's toys, outdoor signs, etc. In some of these applications, it is desirable to include electrically conductive elements on the surfaces of the molded part. In one application for liquid storage tanks, electrically conductive, adhesive tapes have been applied along the vertical axis of the tanks to incorporate a liquid level sensing system. The use of adhesive tape for this application has not been successful since none has been found that will permanently bond to the polymer surface.
U.S. Pat. No. 4,352,762 describes and claims a method in which decorative or alphanumerical indica are applied as a viscous oil suspension to the interior surface of a rotational mold by silk screen printing for transfer to the molded part during molding. Further developments of this approach have included using transfers having indica contained in a mixture of waxy material and polyolefin powders printed on a carrier sheet to apply the indica onto the interior surface of the mold; see U.S. Pat. No. 4,519,972. U.S. Patent discloses and claims a method for the application of indica to the surface of a molded polyolefin part using a similar transfer. These developments spurned other developments such as disclosed in U.S. Pat. Nos. 5,648,030 and 5,498,307.
None of the aforementioned approaches has addressed the problems and objectives in permanently coating polyethylene surfaces with electrically conductive circuits which would be useful for a variety of applications such as level sensors for liquid storage tanks, static charge reducers, etc. These applications typically require low voltage circuitry and comparatively high conductivities.
OBJECTIVES OF THE INVENTION
It is an objective of this invention to provide a method for the permanent application of electrical circuit elements to polyolefin, particularly polyethylene, parts.
It is an additional objective of this invention to apply the method for the permanent application of electrical circuit elements to polyolefin parts during rotational molding.
It is a further objective of this invention to provide the method for the permanent application of electrical circuit elements to polyolefin parts during their fabrication in a rotational molding cycle.
It is also an objective of this invention to provide electrical circuit elements which are integrated into the outer skin of polyolefin, particularly polyethylene, parts.
Other and related objectives will be apparent from the following description of the invention.
BRIEF DESCRIPTION OF THE INVENTION
Electrical circuit elements such as electrically conductive strips are permanently applied to the surface of polyolefin parts by use of a transfer which can be applied to the molded part after its formation by various conventional molding operations. In a specific application to rotational molding, the transfer can be applied to the interior surface of the rotational mold at the start of the molding cycle. The transfer comprises a carrier sheet on which the conductive circuit elements are printed with a mixture of wax or polyethylene powders containing from 30 to about 60 weight percent of conductive particles, preferably metal flakes. After application, the carrier sheet is removed and the conductive circuit elements are permanently incorporated into the surface of the polyethylene part by heating, either during the rotational molding cycle or by heating of the molded polyolefin part.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is intended for incorporation of printed circuit elements in the surface of polyolefin parts. The invention can be applied as a post molding treatment of polyolefin parts which have been formed in conventional molding operations such as rotational molding, thermoforming, infection molding or blow molding. The invention can also be applied during a rotational molding cycle. In rotational molding, hollow-form plastic parts are formed by charging polyolefin particles to a rotational mold, closing and heating the mold to the molding temperature of the polyolefin while being rotating the mold about its major and minor axes for a time sufficient to form the molded part. The mold is then cooled to a demolding temperature, opened and the molded product is ejected.
The electrical circuit elements are printed on a carrier sheet to form a transfer using a heated metal screen and standard hot screen printing techniques. The carrier sheet useful for the transfer of this invention can be any flexible, dimensional stable paper such as parchment paper, or plastic film.
A mixture of conductive powders with wax particles or high density polyethylene powders is used to print the circuit elements onto the carrier sheet. Preferably the mixture contains these components in proportions suitable for screen printing such as from 45 to 70 weight percent wax or polyethylene powders and from 30 to 55 weight percent conductive powders. When the transfer is to be applied to the inside surface of the rotational mold, it is preferred to use a mixture of 45 to 55 weight percent conductive powders in microcrystalline wax. When the transfer is to be applied to the surface of a folded polyethylene part, it is preferred to use a mixture of 30 to 40 weight percent conductive powders in high density polyethylene powder. If desired, viscosity additives such as silica or silicates can be added in minor amounts to provide an optimum viscosity (100 to 1000 centipoise) for screen printing.
The conductive particles can be metal particles or metal coated cores of glass or ceramic particles, or can be carbon or graphite particles. Useful metals include tin, copper, nickel, silver and salts or oxides of the metals such as antimony doped tin oxide coated onto ceramic particles. Other specific examples include silver coated nickel particles, silver and silver chloride coated glass particles, etc. The particles should have a size range no greater than about 100 microns, preferably no greater than about 10-50 microns. The shape of the particles can be spherical, e.g., glass beads, however flake shapes are preferred. Most useful from a cost and efficiency standpoint are nickel flakes with a thickness of about 1 micron and 98 percent passing a 400 mesh screen.
The wax can be a hydrocarbon wax which is preferably transparent or lightly colored so as to avoid any coloration or shading. Examples of suitable waxes include natural waxes, paraffin wax, synthetic wax, microcrystalline wax, etc. Microcrystalline waxes are refined petroleum waxes that have been crystallized from solvents used to extract wax from highly paraffinic petroleum stocks. A very suitable wax is a microcrystalline wax having a melting point from 90 to 300 degrees F., preferably from 110 to 250 degrees F. and a molecular weight from 500 to 1000, preferably from 600 to 750. Plastic waxes are less refined and contain branched chain and cyclic hydrocarbons. Typically plastic waxes have hardness values and crystallinity less than those of microcrystalline waxes.
Paraffin wax comprises chiefly n-paraffin hydrocarbons having from 16 to 38 carbons with limited quantities of branched chain paraffins, monocyclic and polycyclic paraffins.
Synthetic hydrocarbon waxes are obtained by the polymerization and copolymerization of hydrocarbon olefins such as ethylene and propylene. Typically these synthetic waxes have molecular weights from 400 to about 3,000 with a narrow molecular weight distribution.
The high densi
Reeves Robert Alan
Stevenson Michael J.
Eashoo Mark
Stevenson Michael J.
Strauss Robert E.
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