US 3832598 A
An economical, disposable, flexible, slip or skid resistant, electrically conductive strip device is formed which is useful for temporary attachment to shoes, booties, or the like to provide for the dissipation of static electricity from the bodies of personnel in environments containing explosive or inflammable materials. The strip device comprises a flexible, skid resistant, conductive strip having at one end a footwear-sole-engaging portion provided with pressure sensitive adhesive for attachment to the personnel's footwear and means at the other end for temporary electrically conductive attachment to the body of the personnel.
Description (OCR text may contain errors)
United States Patent [191 Oehmke et al.
[ ELECTRICALLY CONDUCTIVE TAPE DEVICE  Inventors: Richard W. Oehmke, Hudson Twsp., St. Croix Cty., Wis.; Paul H. Schertler, St. Paul, Minn.
 Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.
 Filed: Oct. 2, 1972  Appl. No.: 294,075
 U.S. Cl. 317/2 B, 174/117 A  Int. Cl. A61n 1/14, HOlb 7/08  Field of Search 317/2 R, 2 B; 174/117 A  References Cited UNITED STATES PATENTS 2,785,344 3/1957 Hines I 317/2 B 2,955,234 10/1960 Price 317/2 B Wilhelm 174/117 A 1 1 Aug. 27, 1974 3,132,204 5/1964 Giellerup 174/117 A 3,358,188 12/1967 Zimmon 317/2 B 3,505,144 4/1970 Kilduff et a1 174/1 17 A Primary ExaminerL. T. I-Iix Attorney, Agent, or FirmAlexander, Sell, Steldt & DeLahunt ABSTRACT An economical, disposable, flexible, slip or skid resistant, electrically conductive strip device is formed which is useful for temporary attachment to shoes, booties, or the like to provide for the dissipation of static electricity from the bodies of personnel in environments containing explosive or inflammable materials. The strip device comprises a flexible, skid resistant, conductive strip having at one end a footwearsole-engaging portion provided with pressure sensitive adhesive for attachment to the personnels footwear and means at the other end for temporary electrically conductive attachment to the body of the personnel.
' 8 Claims, 4 Drawing Figures ELECTRICALLY CONDUCTIVE TAPE DEVICE BACKGROUND OF THE INVENTION In one aspect this invention relates to flexible electrically conductive strip devices. In yet another aspect, this invention relates to shoes suitable for dissipating static electricity from the body of personnel.
It is common practice in many areas where there are stored flammable or explosive liquids or dust which could be ignited by an electric spark to provide some means of dissipating static electricity before it can accumulate to the point where an electric spark could form. When personnel are working in the area generally the static charges are dissipated by providing an electrically conductive path from the body of the personnel to the ground. The ground can be any electrically conductive mass of material such as a metal pipe which is in intimate contact with the earth and will provide a means of dissipating the static charge.
To dissipate static charges that may accumulate on the body of personnel in solvent storage areas, hospital operating rooms, flour mills, and the like where electrical discharge is a great hazard, the workers use slippers, overshoes, gloves, disposable booties and other clothing which is electrically conductive and has one end of a flexible conductive strap attached to the clothing by stitching, solvent adhesives, or other means of making a coherent electrical contact. The other end of the electrically conductive strap is wrapped about the ankles, tucked into the socks, or otherwise attached to the personnel to provide a conductive path from the body of the personnel to the ground.
Presently available devices are exemplified by foot coverings such as those disclosed in U.S. Pat. Nos. 3,281,624 and 3,605,291, wherein an electrically conductive strip is attached to a disposable foot covering having an electrically conductive sole. The attached strip is wrapped about the ankle or tucked into the socks of the wearer thereby forming an electrically conductive path from the body of the wearer to the surface on which he is standing.
Although the disposable shoes and the like described hereinabove for dissipating static electricity from the body of personnel in hazardous areas are satisfactory from safety point of view, they are expensive in that they constitute a special shoe or shoe covering which is of use only in certain areas. Also the prior art devices are special service items which have only limited utility despite their high cost.
SUMMARY OF THE INVENTION It has been found that the electrically conductive strip made in accordance with this invention provides a conveniently used, easily applied, disposable means for adapting shoes, slippers, overshoes, booties and the like into skid resistant, electrically conductive grounded clothing. The strip of this invention is a flexible, disposable, electrically conductive strip comprising a elongated flexible, skid resistant, electrically conductive material having a footwear-sole-engaging portion at one end thereof provided with a pressure sensitive adhesive over at least a portion thereof and a body contacting portion distally to said footwear engaging portion adapted for temporary attachment to the leg adjacent to said footwear in electrical contact with the skin of the wearer and in non-interfering relationship as re gards locomotion.
BRIEF DESCRIPTION OF THE DRAWING A further understanding of the invention can be had by referring to the accompanying drawing in which like reference numbers refer to like parts in the several views and in which:
FIG. 1 is'a side view in section of one embodiment of the flexible conductive strip of this invention;
FIG. 2 is a side view in section of another embodiment of this invention having an electrically conductive adhesive on one end thereof;
FIG. 3 is a side view in elevation illustrating the strip device of FIG. 2 attached to a shoe; and
FIG. 4 is a perspective view of a jumbo roll of mate rial suitable for being converted to the strips of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS tive strip to the sole of a shoe or other piece of apparel which is to be grounded. Pressure, sensitive adhesive 12 is protected by a release liner 13 which may be a piece of silicone-treated paper or other similar release liner known in the art for protecting adhesives. When the electrically conductive strip 10 is attached to a shoe or other apparel desired to be grounded, the distally opposed free end 14 of the strip can be wrapped about the body of the wearer in contact with the skin or tucked into a sock or wristlet in contact with the skin of the wearer.
Another embodiment of the invention, as shown in FIG. 2, has a strip of conductive material 11 having on one end thereof a pressure sensitive adhesive 12 covered by release liner 13 similar to the configuration shown in FIG. 1. The electrical strip device 16 has on the opposite end of conductive material 11 a coating of pressure sensitive adhesive 17, covered by release liner 18, which is electrically conductive, adhesive 17 covering the width of the material for about 2 to 5 centimeters. Adhesive 17 being conductive provides a means for attaching the conductive strip to the body of the wearer providing a firm, electrically conductive connection between the body of the wearer and the flexible conductive strip 11. When the electrically conductive device of this invention 16 is to be used, the release liners l3, 18 are removed and the pressure sensitive adhesive 12 applied to the sole of a shoe 19 or other footwear as shown in FIG. 3. The resulting configuration has a substantial portion of the electrically conductive v strip 11 in contact with the ground, forming grounding means, and the remainder of the strip can be brought into electrical contact with the ankle 21 by means of the pressure-sensitive adhesive 17.
It is also convenient to produce a jumbo roll of conductive material suitable for producing numerous strips as shown in FIG. 4. The conductive material 11 is produced in the form of an elongated sheet which has a pressure-sensitive adhesive 12 and a release liner 13 along one edge of the elongated sheet. The embodiment in FlG. 4 has a strip of conductive adhesive 17 protected by a release liner 18. When strips are cut from the jumbo roll 22 across the width as shown by the dotted lines 23 the resulting strips will be similar in configuration to FIG. 2. Of course the conductive adhesive could be omitted and a conductive strip similar to FIG. 1 formed.
The flexible, electrically conductive material useful in the strip device of this invention may be prepared from numerous flexible, electrically conductive compositions. Suitable conductive compositions include rubbery polymeric materials containing an electrically conductive powder that can be blended to a uniform composition andextruded, calendered or otherwise formed into sheets of a thickness of about 0. 1-1 .25 millimeters. Optionally, the polymeric composition may be impregnated, such as by pressure lamination to a textile fabric, to provide greater dimensional stability to the sheeted composition.
Suitable polymeric materials for use in the electrically conductive strip of this invention include natural and reclaimed rubber, butyl rubber, polyisoprene, cisbutadiene, styrene-butadiene rubbers, ethylenepropylene-polyene terpolymer rubbers commonly known as EPDM" rubbers, polymers and copolymers of chloroprene, polyacrylates, polyvinyl alkyl ethers, and the like. The elastomeric polyurethanes and polyureas provide still other known elastomers that are useful in the practice of this invention. The preferred source of polymeric material for use in the flexible electrically conductive strip of this invention is scrap and recovered vulcanized elastomeric material resulting from the molding and forming of automobile tire products. Particularly preferred is the rubber scrap generated during the skiving and retreading of automobile tires. For use, the vulcanized scrap and recovered elastomeric materi' als are comminuted by cutting or grinding, blended with a thermoplastic binder containing an electrically conductive powder and formed into sheets by calendering or other means known in the art.
Suitable thermoplastic binders which may be used in the practice of this invention include the natural and synthetic elastomers capable of being vulcanized that are mentioned hereinbefore and thermoplastic polymers such as polyvinyl halides, e.g. polyvinyl chloride and plasticized polyvinyl chloride; copolymers of vinly chloride and vinly acetate; vinylidene chloride polymers and copolymers of vinyl chloride, vinyl acetate, acrylonitrile; styrene polymers and copolymers as for example, styrene copolymers with ethylene, butylene, butadiene, methylacrylate, and the like; polyolefins, e.g., polyethylene and polypropylene; and halogenated polyolefins, e.g. chlorinated polyethylene. Preferred binders are the copolymers of ethylene and copolymerizable olefinically unsaturated monomers such as described in U.S. Pat. No. 3,489,710 and the ABA block polymers such as described in U.S. Pat. No. 3,231,635.
The amount of particulate elastomer used in the conductive composition of this invention can vary from about 0.5-9 parts by weight of elastomer per part of thermoplastic binder with a preferred composition being about 3 to 6 parts by weight of particulate elastomer per part of thermoplastic binder.
Suitable electrically conductive powder for use in the flexible electrically conductive material includes finely divided electrically conductive metals, e.g., silver, gold, aluminum, copper or zinc and conductive forms of carbon, e.g. graphite and gamma-carbon black. The average particle size of the conductive powder and the amount used in the conductive material will vary with the conductive material used. For example, where nickel particles are used it is preferred that the conductive powder have an average particle size of about 2-4 microns and that the nickel particles be present in the composition from about 1 to 25 percent by volume. Where a conductive carbon black is used, it is preferred that the average particle size be from 2080 millimicrons and be present in amounts from about 1 to 50 percent by volume. Another useful conductive powder is a silver-coated ceramic particle available as Sil- Clad-Microspheres" (a registered trademark of Sigmatronics Company). When such coated particles are used, it is preferred that the average particle size be of about 5-40 microns and be present in the composition from about 560 percent by volume. In addition to the conductive powder or coated microspheres, the conductive composition may contain additives such as fillers, e.g. silica, clay, talc; antioxidants; vulcanizing agents, e.g. sulfur, peroxides; dyes; pigments, e.g. zinc oxide, titanium oxide; plasticizers and lubricants.
The pressure sensitive adhesive suitable for use for attaching the conductive tape device of this invention to shoes or other articles of clothing may be any pressure sensitive adhesive having a peel adhesion of at least 0.14 kilograms pre centimeter width as determined by ASTM Dl000-6 1. Examples of suitable pressure sensitive adhesives include adhesives composed of rubbery polymeric material such as those polymeric materials mentioned hereinbefore for use in the electrically conductive composition. The polymeric materials are modified according to the procedures well known in the adhesive art by the addition of plasticizers, tackifiers, fillers, solvents and stabilizers. Preferred pressure sensitive adhesives are the copolymers of alkyl acrylates and acrylic or methacrylic acid, the copolymers of alkylacrylates and maleic anhydride and the copolymers of alkyl acrylates and acrylimide. Of these copolymers the most preferred pressure sensitive adhesive is the copolymer of isooctylacrylate and acrylic acid because of its availability and ease of handling.
The electrically conductive pressure-sensitive adhesive useful in one embodiment of this invention may be formed from the pressure sensitive adhesives hereinbefore mentioned to which has been added one of the aforementioned conductive powders. The presently preferred electrically conductive pressure sensitive adhesive is a composition composed of a soft, highly conformable adhesive, a plurality of insoluble soft spherical domains, and a conductive powder such as those described in copending patent application Serial No. l73,857 filed Aug. 23, i971.
The electrically conductive strip of this invention can be manufactured by blending the desired rubbery polymeric material, conductive powder and additives on a rubber mill to form a flexible electrically conductive composition. Generally, mixing times from about 3 to 30 minutes or longer are sufficient to thoroughly mix the ingredients with the shorter times being preferred since electrical resistivity increases after optimum mixing with increased mixing times. The resulting composition is then calendered, extruded or otherwise molded to form a sheet of flexible electrically conductive material having a thickness of about 0.1 to 1.25 millimeters which can be cut into widths of 30 to 75 centimeters and wound onto a large roll. Where particulate elastomeric material such as reclaimed rubber particles is used it is also convenient to blend the particulate elastic material, the binder, conductive powder and additives and then to mold the composition into a bun or other configuration followed by skiving the resulting mass into sheets of flexible conductive material which can be wound into rolls.
A pressure-sensitive adhesive is applied to the sheet of flexible conductive material by techniques known in the art, such as solvent-casting the pressure-sensitive adhesive along a portion of one side of the sheet material. When solvent-casting of this adhesive is used, the solvent is removed and a strip of release liner applied over the pressure-sensitive adhesive to protect the adhesive, and the sheet material may be rewound for convenient storage. Alternatively the pressure-sensitive'adhesive can be applied by laminating a transfer adhesive and release liner to a portion of one side of the conductive sheet material.
If desired, an electrically conductive pressure sensitive adhesive can be applied to the other side of the sheet material in a manner similar to and simultaneously with the pressure sensitive adhesive designed to attach the electrically conductive strip to the shoes of the personnel to be protected.
The flexible conductive strip devices can be obtained from the sheet of material by use of a guillotine cutter or similar device forcutting strips perpendicular to the width of a large sheet of material or roll of material which is being unwound. For use in making a large quantity of strips or booties having strips attached, the conductive material is formed into a long sheet and pressure-sensitive adhesive is applied along one edge. The material can then be cut into strips having a pressure-sensitive adhesive on a portion of the strip suitable for attaching to a shoe or the like.
Further understanding may be had by reference to the following nonlimiting examples in which all parts are by weight unless otherwise specified.
EXAMPLE 1 A thermoplastic binder was formed by mixing parts of styrene-butadiene-styrene. block copolymer wherein the styrene blocks have a molecular weight of about 14,000 and the butadiene block has a molecular weight of about 80,000 on a rubber mill at l25-l 50C. for about 20 minutes. Fifteen parts of ground rubber particles, (obtained from an automobile tire recap operation), having a particle size such that substantially all the pieces passed through a US. Sieve No. 30 was blended into the thermoplastic mixture for 20 minutes. The composition was sheeted from the mill and pressed at 150C. in a hydraulic press having heated platens to form a sheet 0.75 mm. thick.
The surface resistance of the resulting sheet when measured by a Simpson volt-ohm meter with razor blade electrodes was 4,000-7,000 ohms per square. Physical properties of the material were an ultimate tensile strength of 665 pounds per square inch and an elongation at break of 290 percent.
The flexible electrically conductive sheet material was cut into strips 1.5 cm. in width by 60 cm. in length and one end of the strips was knife coated on one side with a thin layer of an acrylic pressure sensitive adhesive comprising a 20 percent solution of a isooctylacrylate: 5 acrylic acid copolymer in isopropanol and heptane for a distance of about 5 cm. and the adhesive allowed to dry. The adhesive was protected by covering with a piece of silicone treated release liner to provide a conductive tape device similar to the configuration shown in FIG. 1.
The conductive strip device of this invention could be readily attached to footwear such as shoes, slippers, paper booties and the like by removing the release liner and attaching the conductive strip to the sole of the footwear by means of the adhesive. When such footwear was worn by personnel and the unattached end of the strip tied around the ankle of the personnel, static electrical charges did not accumulate on the body of the personnel wearing the altered footwear but were dissipated from the body of the person to the ground.
EXAMPLE 2 The flexible, electrically conductive composition of Example 1 was mixed in a Banbury mixer until the temperature had risen to C. and then formed into a 1 millimeter thick sheet on a calendering machine. The composition as formed had a surface resistivity of about 1,000,000 ohms per square. Strip devices were formed as in Example 1 from the sheet material of this example and when attached to footwear as in Example 1 dissipated the static electricity from the body of personnel wearing such altered footwear and prevented the buildup of static charges on the body of personnel so protected.
EXAMPLE 3 A thermoplastic binder was formed from 8 parts of styrene-butadiene-styrene block copolymer such as used in Example 1, 24.5 parts of mineral oil and 8.2 grams of conductive carbon balckcommercially available from Cabot Co. under the tradename XC- 72Rmixed on a rubber mill at l25l50C. for 20 minutes. To the thermoplastic mixture 59.4 parts of ground tire rubber scrap such as that used in Example 1 was mixed and the mixture mixed for 10 minutes. The mixture was sheeted off the mill and pressed between steel plates at 150C. to form a sheet having a thickness of 0.5 mm. Surface resistance of the material was ohms per square. The ultimate'tensile strength of the resulting material was 195 psi and the elongation at break was 100 percent.
The sheet material was cut into strips 1 cm. wide and 35 cm. long. On one end of each strip a thin layer of To use the strip device of this invention the release I liners were removed from the pressure sensitive areas and the end of the strip device having the nonconductive adhesive was applied to the sole of the footwear of personnel and the conductive adhesive was applied to the ankle. When used in this manner, electrical charges did not accumulate on the bodies of personnel but were dissipated from the body of the persons wearing such conductive strips.
EXAMPLE 4 The flexible electrically conductive composition of Example 1 was compression molded using a hydraulic press having heated platens to form a block of material 60 cm. long by 30 cm. wide by 0.3 mm. thick. Pressure sensitive adhesive was applied to a cm. wide strip along one edge as in Example 1 and a release liner used to cover the adhesive. The sheets were then cut into strip devices 60 cm. long by 2 cm. wide and were useful for dissipation of static electricity when tested using the procedure of Example 1.
EXAMPLE 5 A flexible electrically conductive composition was prepared by mixing on a rubber mill 9 parts of smoked natural rubber, 1 part of polybutene oil, and 10 parts of ground tire scrap. After mixing for 30 minutes, 3.5 parts of conductive carbon black was milled in and the material sheeted from the mill and pressed between two steel plates at 150C. to form a sheet 1.0 mm. thick. Surface resistivity, measured as in Example 1, was 5,500-7,500 ohms per square. The ultimate tensile strength was 690 psi, the elongation at break was 225 percent. The conductive sheet composition was cut into strips 30 cm. long by 2 cm. wide and pressure sensitive adhesive applied as in Example 2. The resulting conductive tape was effective to prevent the accumulation of static electricity on the body of personnel wearing such a strip attached to the ankle and to their footwear.
EXAMPLE 6 A flexible electrically conductive composition was prepared by mixing on a rubber mill 10 parts of an ethylene-ethylacrylate copolymer containing about to percent ethylacrylate, 2 grams of dibutylphthalate and 3.5 parts of conductive carbon black. After mixing for about 15 minutes, 30 grams of finely shredded rubber tire scrap was milled into the mixture and the material sheeted from the mill and pressed between two steel plates at 300C. to form a sheet 1 mm. thick. The surface resistivity of the conductive sheet as measured in Example 1 was 500 ohms per square. The ultimate tensile strength was 500 psi and elongation at break was 50 percent. Conductive strip devices prepared as described in Example 2 were readily attached to footwear and allowed for the dissipation of static electricity from the body of personnel wearing the adapted footwear.
EXAMPLE 7 A flexible electrically conductive composition was prepared by mixing on a rubber mill 20 grams of smoked natural rubber, 1.6 grams of coal tar oil, 1 gram zinc oxide, 0.6 grams stearic acid, 0.1 gram of benzothiazyl disulfide and 0.5 grams of sulfur. After mixing for 10 minutes, 6 grams of conductive carbon black were mixed in and the material sheeted from the mill and pressed between two steel plates at 145C. to form and cure a conductive sheet 1.0 mm. thick. Surface resistivity measured as in Example 1 was 60-150 ohms per square, and the resulting material had a tensile strength of 3200 psi and an elongation at break of mer, (Kraton" 1107), 4 parts of mineral oil, 45 parts 600 percent. Conductive strip devices prepared as described in Example 2 were readily attached to footwear and allowed for the dissipation of static electricity from the body of personnel wearing the adapted footwear.
EXAMPLE 8 The procedure of Example 7 was repeated using styrene-butadiene rubber (SBR-1500 available from Imperial Chemical Industries) in place of the smoked natural rubber. The flexible, electrically conductive sheet material had a surface resistivity of 700 to 2100 ohms per square, a tensile strength of 2660 psi and an elongation at break of 750 percent. Conductive strip devices prepared as described in Example 2 were readily attached to footwear and allowed for dissipation of static electricity from the body of personnel wearing the so adapted footwear.
EXAMPLE 9 Six parts of styrene-isoprene-styrene block copolyof nickel dust (having particle size of about 3 microns), and 4 parts of ground white wall rubber scrap (particle size less than 1 millimeter) were blended together on a rubber mill at 250F. The components were blended in sequence the total milling taking minutes.
The resulting mixture was formed into a 1.25 millimeter thich sheet by pressing between two sheets of silicone treated release paper.
The physical properties of the conductive material were resistivity about 2.5 l0 ohms per square, tensile strength 9.8 kilograms per square centimeter and elon gation at break percent. The conductive material produced was suitable for forming conductive strips.
EXAMPLE 10 The milling procedure of Example 9 was repeated substituting 30 parts of silver coated ceramic beads (available as Sil-Clad-Gl00, from Sigmatronics Laboratories Inc.). 1
The milled mixture was warm pressed to 0.75 millimeter thick between heated platens at 250F. and 1000 pounds per square inch.
The resistivity was about l-2 ohm per square. The tensile strength was 3.2 kilograms per square centimeter, and the elongation at break was percent.
It is possible to use a variety of conductive particles to form the conductive material useful in the practice of this invention.
What is claimed is:
1. A flexible, one-piece elastomeric skid resistant, disposable conductive strip for one-time use for electrically grounding the human body, comprising an elongated strip of flexible, electrically conductive elastomeric material having a foot wear-sole-engaging portion at one end thereof provided with pressure-sensitive adhesive having a peel adhesion of at least 0.14 kilograms per centimeter of width over at least a portion thereof and a body-contacting portion distally to said foot-engaging portion adapted for temporary attachment to the leg adjacent to said foot wear in electrical contact with the skin and in noninterfering relationship as regards locomotion.
2. The conductive strip of claim 1 having a conductive pressure-sensitive adhesive on the end distal to the pressure-sensitive adhesive on the footwear-soleengaging portion.
of pressure-sensitive adhesive, and a release liner surfacing said pressure-sensitive adhesive.
7. The material of claim 6 having in addition an electrically-conductive pressure-sensitive adhesive protected by a release liner on a portion of the sheet distal to the pressure-sensitive adhesive.
8. The material of claim 7 where said conductive sheetcomprises an elastomer, a thermoplastic binder,
and a particulate conductive material.