ANTISTATIC THERMOPLASTIC HOSE

Abstract: A plastic hose of limited conductivity designed to bleed static electric charges such as encountered in paint spraying. The hose has a conventional tube (2), reinforcement (3 and 4), and cover (5). The novel element is a static electric drain element (6) mounted between two components of the hose comprised of a tape or beading made from a beat sinterable thermoplastic material filled with carbon black.

Description: Antistatic Thermoplastic Hose Technical Field This invention relates to antistatic hose which has sufficient conductivity to prevent the accumulation of static electric charges . A limited degree of conductivity is achieved by incorporating within the hose a sintered plastic strip filled with carbon black. The hose of this invention is especially suitable for use in paint spray applications . Definitions The term "tube as used herein refers to a flexible polymeric inner tubular component of a hose that serves as an impervious conduit for the transmission of fluids. The term "reinforcement" as used herein refers to one or more plies of high tenacity synthetic yarns or monofilaments that are applied in tensioned relationship (as by spiral wrapping, braiding, knitting or the like) over the tube to increase the burst strength and kink resistance of the tube. The term "cover" as used herein refers to a con¬ tinuous protective layer of polymeric material that envelops the reinforcement. The term "thermoplastic hose" as used herein refers to a composite structure adapted for the transmission of fluids under pressure, which includes a tube, a reinforcement, and a cover. The term "bonding" as used herein refers to methods for adhering the several components of the hose to each other as through the use of heat, solvents, or adhesives . Background Art There are many applications in which flexible high pressure thermoplastic hose is used for the transmission of volatile or combustible fluids. Since most thermo¬ plastic materials are essentially electrically non- conductive, it is a common practice in these applications to incorporate an element of at least limited conductiv¬ ity-within the body of the hose to permit grounding of the hose and the dissipation of static electric charges. A typical application is a paint spray hose in which paints or lacquers are pumped through a flexible hose to a spray gun at relatively high pressures. In some recently developed commercial installations the pressures may reach as much as 700 kg/cm 2 . The flow of fluid materials through the hose may cause static electric charges to build up which, if not discharged to ground, can lead to an explosion of the volatile solvents or other paint vehicles. To avoid the danger of explosions or fire when inflammable fluids are transmitted through thermoplastic hoses, it is conventional to incorporate a conductive element within the wall of the hose. These conductive elements variously have been in the form of sinuous wires, both single strand and braided, helically wound conductive wires, flexible conductive elements made by interweaving elastic yarns and copper threads and, most recently, patent 4,059,847 discloses the use of a conductive layer of carbon particles laid down between overlying braided reinforcements. None of these constructions has proven entirely satisfactory in use primarily because the high pressures, repeated pressure pulses, tight bends and flexing which occur in use, may cause the grounding element to break from stress fatigue, overstraining and the like. Since the conductive element is usually imbedded within the hose, a discontinuity in the conductive element is not observable and unless the resistance of the hose is periodically checked, the fact that the conductive element is no longer functional can be overlooked. A less serious problem results from the fact that the free ends of a ruptured wire, if used as the conductive element, may penetrate the cover of the hose and cut the hand of a worker. Another fault with antistatic hoses of the prior art is that they sometimes utilize good electrically _ OMPI_ WIP conductive materials, such as a copper wire, for the grounding element. While good conductors function well in bleeding the static electric charges , they introduce a possible hazard in that the hose can conduct a sub- stantial current if contact is made with an electrical device. Disclosure of the Invention Accordingly, it is an object of this invention to provide a thermoplastic hose that may be grounded to prevent the accumulation of static electric charges . It is a further object of this invention to provide an antistatic thermoplastic hose that has a flexible conductive element that will withstand repeated bending and pressure pulsations without loss of conductive continuity. A further object of this invention is to provide a conductive element adapted to be incorporated into a high pressure hose that is nonmetallic and which is not subject to fatigue f ilures. Another object of this invention is to provide a hose that has sufficient electrical conductivity to dissipate static electric charges but sufficient resist¬ ivity to prevent high current flows . These and other objects of this invention are achieved by incorporating a tape of limited conductivity within the sidewalls of a thermoplastic hose. The tape is made from a heat sinterable plastic filled with finely divided carbon particles . In the preferred embodiment of this invention, the heat sinterable plastic is pol tetrafluoroethylene, ultrahigh molecular weight polyethylene or other thermoplastic having a high melt viscosity. The preferred carbon is a carbon black having a mean particle size of less than a micron and more preferably less than 1/10 of a micron. The conductive tape (which can either be prepared in the form of a tape or small diameter beading) is located within the hose either between the cover and the outer reinforcement o?._rι layer, between the tube and an inner reinforcement layer, or, when one or more layers of reinforcement are used, between the several reinforcement layers. The antistatic element can be made by blending finely divided carbon black with a powdered heat sinter¬ able plastic and forming a billet, as by compression molding or ram extrusion. It is preferred that the melt viscosity of the plastic be sufficiently high so that it will not flow above its melting point and encapsulate the carbon particles. It has been found that a product prepared by using a thermoplastic that has a high melt viscosity has a higher electrical conductivity than can be achieved through the use of a plastic that flows above its melting since a low melt viscosity plastic encapsulates the carbon particles. When a heat sinterable plastic having a high melt viscosity is used, however, there is a greater liklihood that some of the conductive particles will remain in electrically conductive, touching relation¬ ship with each other. A hose (or any other article) is generally considered sufficiently conductive to dissipate static electric charges if its total resistance is less than 10**- * ohms. While the length of hose will vary from application to application, ordinary industrial uses usually do not require a hose longer than 30 meters and, therefore, a resistance less than 10 6 ohms per linear 30 centimeters is an acceptable value for most applications. The resistance of the preferred conductive element of this invention can be altered either by changing the amount of carbon black in the tape or the cross sectional area of the tape. In a preferred embodiment in which about 10% by weight carbon black is used and the conductive tape is dimensioned about 0.1 mm thick and 13 mm wide, a resistance of about 1 to 2 x 10^ ohms per 30 cm is obtained. This value is sufficiently low to permit the use of lengths of hose in considerable excess of 30 meters and maintain a total resistance of less 10*-** ohms. The cross-sectional configuration of the conductive element of this invention can vary from a small diameter bead to a thin strip but the configuration of a thin, flat tape is preferred. The reason for this is that a thin tape will best preserve the concentricity of a hose and have the least tendency to develop to weak spots in the hose. There are other subsidiary advantages to using a flat tape as, for example, a comparatively wide tape is more difficult to cut accidentally than is the case of a small diameter bead. Also, a tape may be more efficient in gathering static electric charges because of its larger contact area. The Drawings The drawing is a side view of a portion of an anti¬ static hose, partially broken away, assembled in accordance with this invention. The hose 1 is comprised of a tube 2, a first braided layer 3, and a second braided layer 4. A protective cover 5 is extruded over the second braided layer 4. A conductive tape 6 is incorporated in the hose 1 between the first braided layer 3 and the second braided layer 4. For purposes of clarity, the conductive tape 6 is illustrated in the drawing covering approximately a quarter of the circumference of the hose but it should be noted that even wider tapes can be used. As discussed above, it is advantageous to use a conductive tape having a high width to thickness ratio to prevent distortion of the concentricity of the hose. Best Mode for Carrying Out the Invention 0.73 kg of carbon black (Shawinigan acetylene black) was blended into 8.3 kg of finely divided polytetra- fluoroethylene. The carbon black had an electrical resistivity, at an apparent density of 0.7 grams/cc, of 0.05 ohm-inches and a bulk density at 50% compression of 0.1 grams/cc. The mean particle size of the carbon black was between .04 and .05 microns. The dry blended mixture was charged into a compression mold and placed in an oven at 370°C. for a period of 12 hours to obtain thermal equilibrium. The mold was removed from the oven and placed in a press at a pressure of 200 kg/cm^ until the mold cooled to room temperature. The molded billet, approximately 23 cm in diameter and 38 cm in length, was then skived and slit to provide a tape that was approximately 0.1 mm thick and 1.3 mm wide. The resistivity of the tape so prepared was approximately 33,000 ohms per meter, its density was about 2.1, its tensile strength 150 kg/cm 2 and its elongation 175%. A 6.4 mm OD nylon tube was extruded having a wall thickness of 7.6 mm. A layer of high tenacity nylon fibers was then braided as a reinforcement over the tube. The braid was solvent bonded to the tube using the method disclosed in U.S. Patent 2,977,839. A second layer of high tenacity nylon fibers was then braided over the first braid layer while inserting a continuous length of conductive tape made as disclosed immediately above. The hose construction was then com¬ pleted by applying a polyurethane cover of about .64 mm inches over the second braid and the cover was bonded to the braid using the method disclosed in U.S. Patent 3,489,631. The completed hose had good concentricity with no objectionable bulges. The resistance of the hose manufactured as above was measured to be between 33,000 and 66,000 ohms per linear meter. The hose was then tested using a standard pressure pulse test. In this test, fittings are mounted on a two foot long sample of the hose and the assembly is subjected to pressure pulses at the rate of 35 cycles per minute in which the pressure during each cycle varies from 3.5 kg/cm 2 to 320 kg/cm 2 . This test is run at a temperature of 50°C. A flex test was superimposed upon the above pressure pulse test by bending the length of hose being tested into a U-shape and changing the bend radius at a rate of 60 cycles per minute at the same time the hose was being pressure pulsed. The bend radius of the hose was changed by moving one fitting through a 20 cm stroke for each cycle in which one fitting was moved alternately from a point 15 cm from the fixed fitting to a position in which it was 36 cm from the fixed fitting. As the industry standard for pressure pulses only requires withstanding 150,000 cycles before failure, the test of the hose manufactured as above described was discontinued when no break or visible damage occurred after 300,000 pressure cycles (over 1/2 million flex cycles). At the end of this test, there was no appreciable reduction in the electrical resistance of the hose. This number of flex/pulse cycles without loss of electrical continuity is in substantial excess of the cycles that can be withstood by a more conventional hose incorporating a braided wire grounding element.

Claims: Clai s 1. An antistatic hose adapted for containing and transmitting combustible fluids under pressure comprising a tube, a reinforcement, a cover and a static electric drain element of limited electrical conductivity positioned between two of the other components of the hose, characterized in that the static electric drain element is a tape or beading comprised of a heat sinterable thermo¬ plastic material filled with carbon black. 2. An antistatic hose according to Claim 1 in which the electrical resistance of the hose is less than 1 x 10° " ohms per 30 centimeters. 3. An antistatic hose according to Claim 2 wherein the electrical resistance of the hose is greater than 33,000 ohms per meter. 4. A hose according to Claim 1 in which the heat sinterable material is filled with between about 5 and 15 weight percent carbon black. 5. An antistatic hose according to Claim 1 in which the thermoplastic heat sinterable material is polytetra- fluoroethylene or ultrahigh molecular weight polyethylene. 6. An antistatic hose according to Claim 3 in which the cross-sectional area of the tape is from about 0.003 to about 0.13 square centimeters. 7. An antistatic hose having an electrical resistance less than 1 x 10 6 ohms per 30 centimeters in which a tape made from carbon black filled polytetrafluoroethylene is incorporated into the hose. 8. An antistatic hose according to Claim 7 in which the hose is reinforced with a plurality of reinforcing layers and the tape is positioned between an adjacent pair of reinforcements.

Inventors: ALBERTS R, MCKENNA R
Applicants: POLYMER CORP
Application year: 1980
Main classification: H05F1
IPC codes: H05F3