Abstract:
An electrostatic dissipative footwear comprises a conductive insole and a conductive outsole spaced by a non-conductive mid sole. An electronic device extends through the mid sole for connecting the insole and the outsole in electrically conductive relationship. The electronic device includes a resistor for precisely controlling current flow between the insole and the outsole, and an active electrical device, such as a fuse, for protecting the wearer against electrical shocks.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to footwear and, more particularly, to a safety footwear which is adapted to dissipate electrostatic charges while at the same time protecting the wearer against electric shocks. 
     2. Description of the Prior Art 
     It is known to wear static dissipative shoes, for instance, to prevent electrostatic charges build up upon the body and clothing from being discharged to an electrostatic sensitive device being handled. Such static dissipative shoes are widely used in the electronic and computer industries to prevent damaging of electronic components, such as integrated circuits, by electrical discharges. 
     U.S. Pat. Nos. 4,366,630 and 4,785,371 respectively issued on Jan. 4, 1983 and Nov. 15, 1988 to Bloom and Edwards both disclose an electrostatically dissipating shoe having a composite sole including a conductive insole and a conductive outsole of rubber or plastic materials doped with an electrically conductive substance and connected together in electrically conductive relationship. U.S. Pat. No. 4,785,371 also teaches that the overall resistance across the composite sole should be in a range of about 10 6  to 10 8  ohms to not expose the wearer to the undue risk of injury from electrical contact, as would conventional conductive footwear exhibiting an overall resistance from zero to about 10 4  ohms. 
     U.S. Pat. No. 2,712,099 issued on Feb. 23, 1952 to Legge discloses a footwear having metallic grounding members for conducting electrostatic charges from the body of the wearer to the ground. Resistors are provided as safety device to prevent the passage of a current of high amperage from the grounding members to the wearer&#39;s body. 
     Although the electrostatically dissipating shoes described in the above mentioned patents are effective, it has been found that there is a need for a new electrostatically dissipating footwear having a relatively low electrical resistance between the wearer&#39;s foot and ground, while still providing protection against electrical shocks. 
     SUMMARY OF THE INVENTION 
     It is therefore an aim of the present invention to provide a composite sole for a safety footwear which is, adapted to dissipate electrostatic charges while at the same time protecting the wearer against potential electric shocks. 
     It is also an aim of the present invention to provide a safety footwear which is comfortable to wear. 
     It is a further aim of the present invention to provide an electrostatically dissipating sole having a relatively low electric resistance. 
     It is still a further aim of the present invention to provide a new method of manufacturing an electrostatically dissipating sole for a safety footwear. 
     Therefore, in accordance with the present invention, there is provided a composite sole for use in a safety footwear, comprising an insole and an outsole, said insole and said outsole being at least partly made of electrically-conducting material, an insulator provided between said insole and said outsole, and an electronic device integrated between said insole and said outsole for dynamically controlling current flow therebetween, said electronic device including a resistor for setting the electrical resistance of an electrically conductive path define by said insole, said outsole and said electronic device; and a microfuse for protecting the wearer against electrical shocks, wherein said microfuse includes a fusible wire having a cross-sectional area smaller or equal to about 1×10 −11  m 2 , said fusible wire being adapted to melt at a predetermined amperage to break said electrically conductive path, thereby protecting the wearer against electrical shocks. 
     In accordance with a further general aspect of the present invention, there is provided a composite sole for use in an electrostatic dissipative footwear, comprising an insole and an outsole spaced by a mid sole, said insole and said outsole being at least partly made of electrically-conducting material, whereas said mid sole is made of a substantially non-conductive material, and an electronic device extending through said mid sole for connecting said insole and said outsole in electrically conductive relationship, said electronic device including a resistor for controlling current flow between said insole and said outsole, and a fuse for protecting the wearer against electrical shocks, wherein said fuse is embedded in said mid sole which acts therefor as an insulator. 
     In accordance with a further general aspect of the present invention there is provided a method of manufacturing an electrostatically dissipating sole for a safety footwear, comprising the steps of: providing a conductive insole and a conductive outsole, providing an electronic device in a free space between said insole and said outsole for controlling current flow therebetween, connecting said insole and said outsole in electrically conductive relationship with said electronic device, and structurally joining said insole to said outsole by injecting a substantially non-conductive material in said free space to form a mid sole between said insole and said outsole with said electronic device at least partly embedded in said mid sole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawing, showing by way of illustration a preferred embodiment thereof, and in which: 
     FIG. 1 is a longitudinal cross-sectional view of a safety footwear in accordance with a first embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to FIG. 1, a safety footwear embodying the elements of the present invention and generally designated by numeral  10  will be described. 
     More particularly, the safety footwear  10  includes an upper  12  secured to a composite sole  14 . The composite sole  14  includes an electrically conductive insole  16 , an electrically insulating mid sole  18 , an electrically conductive outsole  20 , and an electronic device  22  embedded in the mid sole  18  for dynamically controlling the electric flow between the insole sole  16  and the outsole  20 . 
     Provided within the safety footwear  10  and overlying the insole  16  is a removable foot bead  24  made of a conductive material, such as a woven material with conductive fibers. The removable foot bead  24  has a resistance of about 10 5  ohms or less. 
     The inner sole  16  forms the bottom portion of the upper  12  and is typically made of a conducting fiberboard material having a resistance of about 10 5  ohms or less. 
     According to the illustrated embodiment, the electronic device  22  defines with the insole  16  and the outsole  20  a pair of electrically conductive paths  26  for dissipating electrostatic charges from the feet of the wearer to the ground. The resistance of each path  26  is precisely set by a resistor  28  forming part of the device  22 . Each resistor  28  has a precise resistance selected in a range of about 0 to 10 6  ohms. It is understood that the resistance of the removable foot bead  24 , the insole  16  and the outsole  20  must be less than that of the resistors  28  to ensure that the overall resistance of the electrically conductive paths  26  be limited by the resistors  28  and, thus, set thereby. As opposed to conventional conductive soles solely constructed of doped rubber or plastic materials, the resistors  28  advantageously allows to precisely set the effective resistance of the footwear between the wearer&#39;s feet and the ground. 
     A microfuse  30  is inserted in each electrically conductive path  26  to prevent the passage of a current of high amperage from the outsole  20  to the insole  16  in order to protect the wearer against electrical shocks. The provision of such microfuse  30  advantageously allows to reduce the effective resistance of the safety footwear  10  between the wearer feet and the ground, thereby increasing the overall dissipating capacity of the safety footwear  10  and that without exposing the wearer to the undue risk of electrical shocks. 
     According to Canadian Standards (CSA), a person should not be exposed to a current exceeding 1 mA for a voltage of 18 kV. Commonly available safety fuses do not meet these standards for a circuit having a resistance of about 10 6  ohms or less. Applicant has found that an appropriate microfuse could be made for this particular application by using the techniques developed for manufacturing integrated circuits. For instance, thermal evaporation, e-beam or sputtering techniques could be used to obtain a length of fusible wire having the required dimensions to ensure melting thereof in the event that the current passing through the wire increases beyond 1 mA. According to a preferred embodiment of the present invention, each microfuse  30  could consist of an aluminum or lead wire having a maximum cross-sectional area of about 1×10 −11  m 2 . 
     Each microfuse  30  is connected to a ground engaging plug  32  forming part of the outsole  20 . One plug  32  could be provided at the heel region of the outsole  20  and the other one at the forepart region thereof. The ground engaging plugs  32  are made of a conductive material having a resistance of 10 4  to 10 6  ohms. For instance, the plugs  32  could be made of a rubber material containing an electrically conductive substance. The outsole  20  is preferably molded from a high density polyurethane having a resistance in a range of about 10 7  to 10 11  ohms. 
     Conductive washers  34  having sharp teeth  36  protruding at right angles from one side thereof can be used to anchor the resistors  28  and the microfuses  30  into the insole  16  and the ground engaging plugs  32 . Alternatively, a conductive adhesive could be used. 
     The insole  16  and the outsole  20  are structurally joined by the mid sole  18  which is formed by injecting low density polyurethane in the free space between the prefabricated insole  16  and outsole  20 . A conductive adhesive is used to secure the insole  16  to the mid sole  18 . The mid sole has a resistance in a range of about 10 8  to 10 11  ohms and, thus, acts as an insulating cartridge for the microfuses  30  which are completely embedded therein. 
     The use of a two density injection molding process to form the mid sole  18  and the outsole  20  allows to obtain a composite sole which is flexible and of a relatively lightweight construction. 
     During the manufacturing process of the safety footwear  10 , the outsole  20  is first molded and the insole  16  fabricated with the upper  12  using known techniques. Then, the electric circuits formed by the resistors  28  and the microfuses  30  are inserted between the insole  16  and the outsole  20  and connected thereto in electrically conductive relationship. Thereafter, the insole  16  and the outsole  20  are maintained in relatively close proximity in a mold and low density polyurethane is then injected in the free space between the insole  16  and the outsole  20  for forming the mid sole  18  in which the resistors  28  and the microfuses  30  are embedded. 
     It is noted that according to a further embodiment of the present invention the outsole could be solely molded from a rubber material containing electrically conductive substance. This would provide a continuous grounding surface of conductive material but would increase the weight of the safety shoe  10 . 
     According to further embodiments of the present invention which are not illustrated, the fuse could consist of a reversed biased diode or, alternatively, of an assembly of transistors. 
     According to another embodiment of the present invention, the plugs  32  could be electrically connected together with a single resistor and a single fuse completing the electric circuit between the insole and the outsole.