Flexible envelope for electronic component packaging
FIELD OF THE INVENTION 
     The invention relates to anti-static bags or envelopes utilized for the storage and shipment of electronic components, and more particularly, to a reusable anti-static envelope having an outer conductive, carbon-contained plastic layer integrally coextruded with an inner anti-static plastic layer while the plastics are in a liquid state. 
     BACKGROUND OF THE INVENTION 
     In recent times, plastic composites have been fabricated into anti-static bags and envelopes. These are utilized to protect sensitive electronic components and chips from electrical charges and static electricity, thus preventing their damage. While various laminates of plastic have been suggested as desirable anti-static composites, most have the drawback that the thin metal conductive layer of the composite tends to abrade or crack. Abrasion is a particularly troublesome problem, since it not only diminishes the effectiveness of the bag or envelope in protecting the components, but the &#34;flaking&#34; or &#34;crumbing&#34; of the metal forms a contaminate. These contaminates can damage rather than protect the electronic components contained in the envelopes. 
     Usually several layers of plastic are used to form these anti-static envelopes in order to add mechanical strength and durability to the package. However, most packages are vulnerable to breakdown of their protective metal layer. 
     Where plastic composites have been impregnated with carbon to provide a conductive property, they have been found to lack mechanical strength. Such layers are usually bonded to another layer by an aluminum paste adhesive, as taught in U.S. Pat. No. 4,337,285 issued June 29, 1982, U.S. Pat. No. 4,478,903, issued Oct. 23, 1984, a composite wrapping or packaging sheet having a carbon-impregnated conductive covering layer laminated by coextruding this layer on the face of a substrate, is suggested. However, this patent does not suggest that such a coextruded laminate can be formed into a flexible bag or envelope. The coextruded composite is formed from non-flexible plastics, such as styrene and an ABS copolymer. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention fabricates a flexible anti-static bag or envelope by the process of blow film coextrusion. The envelope has an inner anti-static polyethylene layer and an outer conductive carbon-containing polyethylene layer which is made integral with the inner layer by coextruding the polyethylenes in the liquid state. Thus, a single film layer is formed, having two distinctly different properties. 
     The inner anti-static layer has a surface resistivity of less than approximately 10 12  ohms per square to minimize triboelectric charge buildups, which avoids damage to electronic components. The outer conductive layer has a surface resistivity of less than approximately 10 3  ohms per square. 
     The thickness of each plastic layer is approximately from 2 to 4 mils. 
     The carbon contained in the outer polyethylene layer ranges from 15 to 40 percent by weight. The outer layer forms a faraday cage. 
     The plastics are coextruded into a tube or sheet at a temperature in the range of 320° F. to 350° F. The envelope is then formed by conventional polyethylene bag making equipment with heat sealing and cutting bars. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Although such novel feature or features believed to be characteristic of the invention are pointed out in the claims, the invention and the manner in which it may be carried, may be further understood by reference to the description following and the accompanying drawing. 
     FIG. 1 is a perspective view of an electronic anti-static package of this invention utilizing an outer conductive surface and inner anti-static surface. 
     FIG. 2 is an enlarged sectional view of the layers of the package of FIG. 1 illustrating the two distinctly different layers forming one integral film. 
     FIG. 3 is a schematic sectional view of the electronic anti-static film package of FIG. 1 being manufactured by a coextruder. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures in greater detail, where like reference number denote like parts in the various figures. 
     FIG. 1 is a view of an anti-static electronic package 10 having a tube 12 of a single film of composite material that is cut at 14 to length and heat sealed along the bottom edge 16 to form an envelope. The inner layer 20 of the envelope comprises an anti-static, heat sealable plastic, and the outer layer 22 comprises a conductive plastic. 
     Referring to FIG. 2, an enlarged sectional view of the tubular material 12 of the single film of the composite material envelope illustrated in FIG. 1. The inner layer 20 is comprised of a heat sealable anti-static polyolefin. The anti-static surface 20 may be made of many polymers which will accept the addition of a wide variety of internal anti-static, such as amines, anioncis and others at levels which result in surface resistivities of less than 10 12  ohms per square. The anti-static surface 21 will meet the electrical test of the federal test method STD 101, test method 4046, which is to dissipate a 5000 volt applied positive or negative charge to technical zero volts (50 volts or less) in less than two seconds when conditioned to 72° F. and 15 percent relative humidity. The outer layer 22 may be made from a number of polymers and copolymers which allow loading of conductive materials and fillers to levels in which surface 23 has a resistivity of less than 10 3  ohms (i.e. low resistance shunt path). Types of polymers can be low density polyethylene, linear low density polyethylene, high density polyethylene and many other polymers and copolymers. 
     Referring to FIG. 3, a schematic sectional view of a coextruder 24 (Kieffel Coextrusion Film Line), manufacturing a single film of two layers, each having two distinctly different surface characteristics. The material that forms the plastic inner layer 20 of FIGS. 1 and 2 leaves the coextruder die 26 (arrows 34) in liquid form 30 and cools to form the inner layer of a bubble of blown film 28. The material that forms the conductive plastic outer layer 22 of FIGS. 1 and 2, leaves the coextruder die 26 (arrows 36) in liquid form 32 to form the outer layer of the bubble of blown film 28. The bubble of blown film 28 is achieved by injecting or forcing gas or air, under pressure, into the center of the tube of coextruded materials 30 and 32, which can be coextruded in tube or sheet form. The two materials bond together while in the liquid state and form an integral film 28 with two distinctly different surfaces 21 and 23, respectively, when cooled. Characteristics of blow rate and melt index of each type of polymer varies, but most are easily extruded, due to the inner layer 30 which supports the outer layer 32 in the form of a bubble of blown film 28. 
     The terms and expressions which are employed are used as terms of description; it is recognized, though, that various modifications are possible. 
     It is also understood the following claims are intended to cover all of the generic and specific features of the invention herein described; and all statements of the scope of the invention which as a matter of language, might fall therebetween.