High conductivity launder resistant grounding tape

A fabric grounding tape and method of making same is disclosed. In accordance with the method the tape is woven on a needle loom using polyester and stainless steel warp yarns. The yarns are maintained under tension during and after the weaving process and are subjected to a heat setting procedure while still under tension. The resultant tape has the hand or feel of a conventional all polyester tape and a low ohmic resistance, i.e. 10 ohms or less per inch. The tape can sustain a multiplicity of laundering cycles without significant increase of resistivity.

BACKGROUND AND FIELD OF THE INVENTION
 The present invention is directed to a high conductivity grounding tape
 used in connection with garments worn in so-called "clean rooms".
 It is well recognized in the industry that in the manufacture of certain
 sensitive electronic components i.e. "chips", that even low voltage
 charges can permanently damage the chips at certain stages of their
 manufacture. By way of example, operators of manufacturing equipment can
 produce several hundred volts of static electricity merely by minor
 movements of their arms.
 In order to dissipate voltages as low as those noted, it is imperative that
 the grounding, voltage dissipating conductor have a high conductivity
 factor.
 While a pure wire conductor will provide the requisite low resistance, it
 is desirable, and in most instances imperative that the grounding
 conductor exhibit the "hand" or feel of a conventional fabric.
 Numerous attempts have been made to create a fabric-like tape which will
 exhibit the necessary high conductivity and "hand" demanded by industry
 and which also will maintain these characteristic through multiple
 laundering cycles. By way of example, clean room, static dissipating
 garments are expected to be capable of withstanding a hundred or more
 washings in deionized and hence highly reactive water, without material
 increase in the resistance of the grounding tape.
 Attempts to provide a tape having the above desired characteristics have
 included incorporating in the fabric metal clad polymer yarns i.e. yarns
 coated with silver, copper, etc. The tapes fabricated from clad yarns fail
 to satisfy the laundering requirements since the ohmic resistance
 exhibited increases progressively from washing to washing.
 Tapes have been formed of yarns impregnated with conductive materials such
 as carbon. However, carbon filled yarns possess an electrical resistance
 thousands of times greater than that required in many "clean room"
 environments.
 PRIOR ART
 Without conceding the relevance thereof to the instant invention, there is
 cited below patent references located in the course of studies.
 U.S. Pat. No. 4,664,158 assigned to the applicant herein is directed to a
 grounding fabric for a wrist strap wherein conductive components are
 raised from the surface of the fabric. A similar disclosure is embodied in
 4,577,256.
 U.S. Pat. No. 4,557,968 discloses an electrostatic dissipating fabric for
 making into garments, the conductivity being afforded by carbon doped
 yarns.
 U.S. Pat. No. 3,288,175 discloses a textile material incorporated
 conductive yarn ends.
 U.S. Pat. No. 3,422,460 disclosures a static inhibiting cloth which avoids
 clinging to the wearer.
 U.S. Pat. No. 3,479,565 discloses a circuit board for electronic components
 made of a woven matrix.
 U.S. Pat. No. 3,586,597 discloses a cloth for use in garments employing a
 conductive fiber having finely divided articles of conductive material
 therein.
 U.S. Pat. No. 3,706,195 teaches a synthetic yarn having conductive carbon
 which is longitudinally extendible to a high degree before breaking.
 U.S. Pat. No. 3,851,456 is directed to anti-static yarn comprised of a
 mixture of metallic and non-metallic fibers, the metallic fibers including
 stainless steel.
 U.S. Pat. No. 3,882,667 is directed to a composite textile yarn formed of
 non-metallic yarns and a blended yarn plied with non-metallic yarns, the
 blended yarn being formed of metallic and non-metallic fibers.
 U.S. Pat. No. 3,971,202 is directed to an anti-static yarn particularly
 useful to form carpets, the yarn being obtained by cobulking
 non-conductive and conductive yarns.
 U.S. Pat. No. 3,986,530 is directed to a knitted or woven cloth having anti
 static properties including electrically conductive threads composed of
 metal plated natural or synthetic fibers and metallic filaments.
 U.S. Pat. No. 4,045,949 is directed to an integral electrically conductive
 textile filament including a multiplicity of electrically conducting
 longitudinally directed strata of fiber-forming polymer material.
 Conductivity is provided by carbon black.
 U.S. Pat. No. 4,064,075 is directed to a highly conductive extrudable
 material embodying electrically conductive carbon black.
 U.S. Pat. No. 4,145,473 relates to an anti-static filament having a
 polymeric sheath and a conductive polymer core.
 U.S. Pat. No. 4,332,082 relates to an anti-electrostatically grounded
 suiting material. Conductivity is provided by carbon black coated
 monofiliment fibers.
 U.S. Pat. No. 4,296,855 is directed to a conductive fabric especially
 adapted for use as a controlling antenna on a conveyor belt. Carbon
 particles provide conductivity.
 U.S. Pat. No. 4,357,390 is directed to an anti static polyester fiber, the
 fiber being hollow.
 SUMMARY OF THE INVENTION
 The present invention may be summarized as directed to a method of
 manufacturing a tape and to the resulting tape characterized in that the
 tape has a low ohmic resistance, e.g. less than 2 ohms per inch, exhibits
 the "hand" or feel of a conventional textile, and which may be subjected
 to a multiplicity of washings (well over 100) without any significant loss
 in conductivity.
 It is noted that attempts have been made by applicant on an experimental
 basis to weave tapes comprised of polymeric yarns and continuous filiments
 of metal i.e., stainless steel. Initial such attempts have proven to be
 failures. More particularly, the ohmic resistance of the tape would
 increase dramatically in the course of handling and washing.
 We have discovered, that the loss of conductivity of conventionally woven
 tapes comprised of polymeric yarn ends and stainless ends is a result of a
 rupture of the stainless ends, the rupture being occasioned by a
 differential shrinkage of the polymer and stainless yarns. More
 particularly, microscopic examination of such tapes reveal the formation
 of a multiplicity of kinks in stainless yarns. We have theorized that the
 kinking results from the fact that polymer yarns stretch by a factor of 40
 or more times the stretch exhibited by stainless yarns when the yarns are
 woven together under normal tension parameters.
 We have discovered that the kinking with consequent loss of conductivity
 can be avoided by a unique departure from conventional weaving procedures,
 namely the polymer and stainless yarns are woven under tension and are
 removed from the loom while still under tension. The tension in the yarns
 is continuously maintained after weaving and is maintained until the tape
 is subjected to a heat setting procedure during which the tendency of the
 polymer yarns to shorten is removed. The resulting tape evinces the hand
 or feel of comparable tapes woven entirely of polymer yarns. The tape has
 been subjected to well over 100 cycles of washing with no significant loss
 of conductivity. In this connection it is noted that clean room garments
 are washed in deionized water. This material is more reactive as respects
 silver or copper clad yarn ends and accounts for the rapid loss of
 conductivity observed in grounding tapes employing clad yarns. It is noted
 that conventional weaving procedure is essentially the opposite of the
 practice described in patents such as U.S. Pat. No. 4,664,158 wherein
 conductive yarns are woven together with distended elastic yarns. Upon
 release of tension the conductive yarns are deflected away from the
 surface of the woven tape to ensure contact with the skin of the user as a
 result of foreshortening of the elastic yarns.
 It is accordingly an object of the invention to provide a highly conductive
 tape having the "hand" of a conventional tape, the tape being susceptible
 of a multiplicity of laundering cycles without material increase in the
 ohmic resistance of the tape.
 A further object of the invention is the provision of a method of
 manufacturing a conductive tape having the above described
 characteristics.

DETAILED DESCRIPTION OF DRAWINGS
 The schematic of FIG. 1 represents an essentially conventional needle loom
 device. Polyester warp yarn ends 20 are fed from beams 21, 22 through a
 conventional lease rod station 23. The beams 21, 22 are mounted for
 rotation, the beams being subjected to a frictional drag through the use
 of beam brakes 24. The brakes may comprise cords or ropes riding in
 grooves on the side of the beam.
 Stainless steel yarns ends 25 are fed from spools 26 to the lease rod
 station 23. The stainless yarn ends are fed through tensioners 27 which
 exert a frictional drag on the yarns.
 The stainless and polymer yarns are fed through harnesses 27a whereat
 filling yarns F are inserted through the fell 28 in a conventional manner.
 The woven tape 30 is maintained under tension, passing through a pressure
 roller box 31 schematically illustrated as incorporating mating drive
 rollers 32, 33. The formed tape is wound under tension on positive take up
 roll 34. Importantly, the tape is maintained under tension throughout the
 process, it being preferable to maintain the tape on the take-up roller 34
 under tension for subsequent processing. It is also feasible for the tape
 30 be fed directly from the loom to the heat setting device 35.
 Preferably, the tape is maintained under tension on roller 34 and
 subsequently processed to account for the differential in speed of the
 feed of the tape during weaving in the needle loom 29 and the speed of the
 feed within the heat setting device.
 EXAMPLES
 In order to comply with the best mode requirements of the patent laws there
 is noted below the specific parameters for the formation of a one-quarter
 inch and a three-eighths inch wide tape width.
 A quarter inch width tape was woven, the warp being comprised of 79 yarn
 ends namely 75 polyester ends and four stainless steel yarns.
 Suitable ground and filling yearns F surrounding the stainless comprised
 single ply polyester, yarn diameter 4 mils, 1/70/34.
 The stainless steel yarns were supplied by the Bekaert Corporation type NV
 14/2.times.90/175S/316T being 2 ply twisted yarn (175 Turns/meter) 14
 micron filament diameter, yarn diameter 10 mils.
 Optionally, certain regularly spaced filling years F' may comprises
 stainless yarns exposed, encompassed in, or plied with polyester, although
 this is a non-preferred embodiment which is necessary only in instances
 where the tape is subjected to extreme stresses and bends which might
 rupture the stainless warps.
 The yarns were woven on a conventional needle loom. The brake on the beams
 applies tension to the polyester yarn ends, the tension on the polyester
 yarns being effected by adjustment of the brake to provide tension of
 approximately 150 grams per yarn end. The yarn tensioners controlling the
 stainless yarns are adjusted to supply a tension of approximately 150
 grams to each of the stainless ends. It is noted that the tension applied
 can vary within a significant range it being desirable to apply just
 sufficient tension to maintain a clear shed at the fell of the loom.
 It is mandatory that the tension be maintained as the tape emerges from the
 loom. To maintain the tension, the tape is passed through the nip of
 rollers 32,33, of a pressure roller box. It is noted that the arrangement
 involves a modification of the tape take up device from a conventional
 negative feed to a positive feed.
 The fabric, is wrapped on positive take up roller 34 while still
 maintaining the tension in the fabric.
 It is noted that the total fabric tension in respect of quarter inch width
 fabric amounts to approximately 11.85 kilograms.
 The tension of the tape is maintained throughout the heat setting process.
 The tensioned tape on take up roll 34 is run at 30 yards per minute over
 heated cylinders at a surface temperature of 350.degree. Fahrenheit. The
 tape is exposed to surface heat until the tape reaches a temperature of
 225.degree. Fahrenheit. The tape is exposed to the heating process for 8.5
 minutes. The tape is then allowed to cool to ambient before the tension is
 released.
 The resulting tape (illustrated in FIG. 2 schematically) evinces a "hand"
 comparable to that of an all polyester tape. Examination of the tape
 demonstrates that the stainless yarns are free of kinks of the type which
 have rendered the use of stainless yarn ends heretofore known to be
 impracticable. The tapes have been laundered in deionized water for over
 100 cycles. The ohmic resistance of the heat set tape has been measured at
 approximately 0.5 ohms/inch. Following 100 washings in deionized water the
 ohmic resistance was measured at 2 ohms per inch.
 Example 2
 A 3/8 inch wide fabric was woven in the manner described above. The warp of
 the 3/8 inch fabric incorporated a total of 81 polyester and five
 stainless yarn ends. The fabric tension was maintained at 12.15 kilograms
 for the 3/8 inch width. The woven tape was subjected to heat setting as
 described above. The pre-washed ohmic resistance of the 3/8 inch tape was
 measured at 0.5 ohms per inch, the post 100 launderings measurement of the
 tape sample was 2 ohms per inch. Desirably, the ratio of polyester to
 stainless yarn ends is at least about 10 to 1.
 While it is feasible to lead the woven tape directly into a heat setting
 device, it has been found, as a practical matter, to be preferable to wind
 the tape under the tension on a take up roll (34) for subsequent
 processing. It is noted that due to the substantial tension in the fabric,
 it is necessary to employ a take up roll which is capable of resisting the
 cumulative compressive forces developed in the process.
 As will be apparent to skilled workers in the art, variations of the
 described procedure may be made without departing from the spirit of the
 invention. By way of example, it may be feasible to employ other means of
 avoiding shrinkage in the polymeric yarns beside heat setting procedures.
 It will be clear that the foregoing description of this invention is for
 purposes of explanation and illustration. It will be apparent to those
 skilled in the relevant art that modifications and changes may be made to
 the invention as above described without departing from its scope and
 spirit. Accordingly, the invention is to be broadly construed within the
 scope of the appended claims.