US 3268766 A
Description (OCR text may contain errors)
Aug. 23, 1966 E. AMOS 3,268,766
' s APPARATUS FOR REMOVAL OF ELECTRIC CHARGES FROM DIELECTRIC FILM SURFACES Filed Feb, 4, 1964 FIG. 2
I/I/I/I/I/I/l/I I/I/I/ll3 INVENTOR STEPHEN EARL AMOS BY f/z zz ATTORNEY United States Patent APPARATUS FOR REMOVAL OF ELECTRIC CHARGES FROM DIELECTRIC FILM SUR- FACES Stephen Earl Amos, Circleville, Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Feb. 4, 1964, Ser. No. 342,378 Claims. (Cl. 317-2) This invention relates to anti-static devices, and, more particularly to apparatus for the removal of surface charges of static electricity and of dust particles from running continuous webs of dielectric material.
A major problem in the manufacture, processing, and use of web materials, especially webs of high purity synthetic organic materials which have high electrical resistivity, is the accumulation of surface charges of static electricity. These charges, which usually are of positive polarity, attract negatively charged dust, cause poor roll formation and result in poor operation on converting machinery or slitting machines. Further, such charged webs have a poor hand in the case of textile fabrics. This problem has been increasingly acute in the manufacture and handling of thin gauge films of high resistivity organic polymers. The surface charges often are so great that extremely high potentials are reached which coupled the very small mass of the film, makes handling of the web a virtual impossibility. Further, accumulation of dust particles on thermoplastic webs during processing permits the particles to be embedded, or cause embossing, and thereby impairs the clarity and utility of the product.
The presence of and circumstances under which these surface charges are produced is well known to those who are experienced in the handling of continuous film and like dielectric web materials, although the ultimate source of the charges or the mechanism of their deposition may be quite obscure. It is known, for example, that simply rolling or unrolling these high electrical resistivity webs, such as polyethylene terephthalate, polyvinylidene chloride, polyethylene and the like, will produce charges of such magnitude as to make the film unmanageable, and result in poor r-oll formation. Once these charges have been deposited on the surface, the high surface electric resistivity does not permit them to flow from it. The result is that many coulombs of charge accumulate as a web passes through various steps, producing potentials which may be in excess of 30,000 volts. Grounded idler (undriven) rolls, as a means of removing charges as they accumulate are not satisfactory, since elongation of thin gauge films would occur, as well as slippage which would mar the surface. Grounded driven rolls, as the many points where it may be desirable to eliminate static charges would be prohibitively expensive, since the peripheral speed must be regulated to correspond precisely to the speed of the running web. Further, grounded rolls or bars are usually not completely effective in removing surface charges; these charges are reduoed to the extent where they possess a potential of the order of 1000 volts, but this residual charge still causes dust particles to cling to the film and causes poor running characteristics on slitting and converting machinery.
A number of non-contacting devices have been employed, in the prior art, for removal of static charges, but for one reason or another these have not been altogether satisfactory or universally adaptable. -Most of these involve the passage of the web through a cloud of gaseous ions which are produced by radiation from a radioactive material or exposure to an ionizing electrical discharge. Generally, the drawbacks of these are cost, fire and radiation hazard, and the adverse effects on the film surface resulting from the active gases produced by electrical discharges. One example of difliculties in discharging static electricity from the film by ions from an electrical discharge is the difficulty of control. The magnitude of the charge on the film is a function of atmospheric conditions, e.g., relative humidity, etc.; as this charge varies, the concentration of ions in the air surrounding the film should be controlled so as to assure discharge without depositing a charge of the opposite sign on the film. This necessitates control apparatus which adds to the cost of each discharge element.
It is, therefore, an object of this invention to provide simple apparatus for the removal of static electricity charges from dielectric web surfaces, which apparatus is free of the foregoing drawbacks. It is a further object of this invention to provide apparatus effective to remove contaminating dust as well as accumulated static charges from the surface of dielectric webs. Still another object is to provide apparatus for smoothing a thin dielectric web and producing an improved roll structure. The foregoing and related objects will more clearly appear from the detailed description which follows.
These objects are realized by the present invention which, briefly stated, comprises.
In machinery for the processing of a travelling continuous web of dielectric material susceptible to the accumulation of charges of static electricity, the improvement which comprises a stationary web guide operative to remove accumulated charges of static electricity from the surface of the travelling web comprising a close tubular member disposed transversely of and below or adjacent the travelling web, and having an electrically conductive Wall pervious to gas in the area thereof immediately adjacent the travelling web, means for continuously feeding gas under pressure to the interior of said tubular member whereby gas is continuously forced thru said pervious area to maintain a blanket of gas of predetermined thickness between the surface of the tubular member and the web travelling thereover, and a low resistance electrical conductor connecting said electrically conductive wall to an earth ground.
Following is a description of a preferred embodiment of my invention, reference being had to the accompanying drawing wherein:
FIGURE 1 shows in perspective the apparatus of this invention positioned with respect to the travelling web; and
FIGURE 2 is a cross-sectional view of the tubular member on the line 22 of FIGURE 1.
Referring to the drawings, web 11 partially wraps tubular web guide 12 which is fixed and non-rotating. The portion of web guide 12 adjacent web 11 is provided with a sintered metal face 13. The walls of the tubular guide are impervious to gas except for porous section 13 and air inlet 15 which is coupled to flexible conduit 16, which is in turn coupled to a source of compressed air (not shown). Ground wire 17 couples the porous wall 13 of the guide to ground 18. It is essential that section 13 be of an electrically conductive material. Sintered bronze which is gas pervious is a preferred material for the wall 13; however, other porous sintered metals or porous carbons are useful. A critical feature of this invention is the manner of grounding, and the necessity for an extremely low resistance pathway from the mandrel to a good earth ground. This is essential in order that the conductor may carry a relatively large current under the driving force of a small potential difference. Ground wire 17 should be of at least No. B and S copper for distances of approximately 50 feet or less; of course for' greater distances, a larger size wire would be required. Earth ground 18 may be a water pipe to which the ground wire is attached. In any event, as stated above, this ground should provide a low resistance path to the moist earth, so as to conduct a relatively large current driven by a relatively low potential. The guide structure is supported on the appropriate machinery by trunnions 19 and 20 at each end and brackets (not shown).
In the use of air spreaders employing air emission from the gas pervious passageways in the wall of sintered metal mandrels to remove longitudinal wrinkles from running films, it was found that direct grounding of the mandrel with a very low resistance conductor caused static charges to be completely removed from the film. This complete removal of charges caused dust particles to no longer cling to the film, and thus facilitated their removal. It was found, however, that the usual ground achieved by mounting a conductive sintered bronze mandrel on the steel framework of the machine is not adequate to effectively neutralize the charge on the film and remove the dust particles. The charge on high resistance webs, for example polyethylene terephthalate, generally is at levels so great that the potential reaches 30,000 volts or above. Emission of air from the periphery of a sintered metal tubular guide which is grounded by being attached to the framework of the machine reduces this potential to values of the order of 1,000 volts, but this reduction is not adequate to permit easy removal of clinging dust particles, nor to materially improve poor handling properties resulting from static charges. The coupling of the sintered metal gas pervious wall to a good ground, such as a water supply line which passes through the ground, by means of a heavy ground wire as described above, reduced the voltage on the film to zero and caused dust particles to be propelled from the film surface and cling to nearby framework where they could be drawn away by a vacuum duct which may be situated nearby for disposal of the dust.
The reason for the efficacy of this apparatus when employing a low resistance ground, as distinguished from the behavior if grounded through the machine, is not thoroughly understood. It is believed, however, that this may be attributable to the inability of the electrical pathway through the machine to conduct a sufficiently high current to produce a quantity of ions in the emitting gas adequate to neutralize the charge on the web. To demonstrate the flow of current from the ground to the mandrel, a resistor of 1000 ohms was inserted in the ground line of the guide and a current of 1.25 milliamperes was detected (LR. drop of 1.25 volts), running a web of polyethylene terephthalate 0.5 inch wide at a speed of 2500 feet per minute. This current of course will vary with the speed and the width of the film, atmospheric conditions (e.g., relative humidity), and the flow of air. It was found that a simple narrow (e.g., inch) slit in a pipe was partially as effective in removal of charge and in web spreading at slower speeds. When using a porous walled tubular guide as described in the preferred embodiment, it was found that the porosity was not especially critical; however, a porosity of the order of 10 micro inches was determined to be most effective. The tubular guide is shown in the drawing as cylindrical, which is preferred for ease of fabrication, since it is adapted to mounting in a lathe for turning down to produce a smooth face on the porous wall adjacent the web. Flat faces, however, can be used with appropriate modification as will be evident to those skilled in the art. No critical level for the rate of air flow was found; preferably it should be adjusted to float the film at a distance of 0.1 mil to 0.5 mil from the surface of the guide. The addition to the compressed gas of traces of electro negative materials, such as halogenated hydrocarbons (carbon tetrachloride or trichloroethylene) increased the current from the ground at low rates of air flow, but these may not be desirable for use under conditions of poor ventilation. With air alone it was found that 7 cubic feet per hour per square inch of porous opening is adequate to float the film around the mandrel and remove surface charges.
The apparatus of this invention is especially useful in the production of thin gauge dielectric films, but it is also valuable in the production of fabrics and threads, especially those of synthetic fibers, which also have-low conductivity, whereby the initial hand is improved. Further, adapted to tape recorders and playback apparatus it improves the running qualities of tapes at high speeds. Modification of this apparatus will be apparent to those skilled in the art and yet still remain within the spirit of this invention; for example, in the production of threads the porous guide member should preferably be trough-shaped in cross-section, and for narrow width thin gauge film the guide should be provided with porous side flanges which would be operative to maintain the travelling film in alignment.
The apparatus of the present invention has advantages over previously known devices for discharge of webs in three aspects, cost, convenience and safety. First of all, the ability to function without the consumption of power or the use of radioactive materials greatly reduces the initial cost of the apparatus. Further, without power consumption the operating cost is materially reduced. Secondly, without the need for special power sources this apparatus can be positioned on virtually any type of equipment and at virtually any location. For example, it may be employed at any location on a film manufacturing line where the web may be wrapped over one mandrel, or on alternate sides over two or more. This apparatus also removes longitudinal wrinkles from running webs. This apparatus is especially adapted to slitting machinery, where rolling and unrolling cause large accumulations of static electricity. This apparatus is especially advantageous in equipment for high speed handling of recording tapes, in that surface charges are removed without the adverse eifects of electric or radioactive discharges to produce electrical interference. And finally, this apparatus is particularly useful in locations where electrical discharge methods present a hazard in connection with solvents or other combustibles. This apparatus removes charges by innocuous means before sparking or other breakdown can occur. The ability to function without radioactive materials adds to the safety, since radiation hazards do not exit.
What is claimed is:
1. In machinery for the processing of a travelling continuous web of dielectric material susceptible to the accumulation of charges of static electricity, the improvement which comprises a stationary web guide operative to remove accumulated charges of static electricity from the surface of the travelling web comprising a closed tubular member disposed transversely of and below the travelling web, and having an electrically conductive wall pervious to gas in the area thereof immediately adjacent the travelling web, means for continuously feeding gas under pressure to the interior of said tubular member whereby gas is continuously forced thru said pervious area and against said web to maintain a blanket of gas of predetermined thickness between the surface of the tubular member and the web travelling thereover, and a low resistance electrical conductor connecting said electrically conductive wall to an earth ground.
2. The apparatus of claim 1 wherein the wall pervious to gas is a sintered metal wall.
3. The apparatus of claim 2 wherein said sintered metal is sintered bronze.
4. The apparatus of claim 1 wherein said tubular member is cylindrical.
References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES RCA Technical Notes, Floatation and Edge Guiding of Paper Web for Electrostatic Printing Press, Martin L. Levine and Paul L. Wright, September 1961, RCA TN 5 No. 457, sheet 1 of 2; sheet 1 of 3; and sheet 2 of 2.
if? 1 5? SAMUEL BERNSTEIN, J. A. SILVERMAN, Cichel1i 11111111111 34-156 x Assistant Exammm' Brooks 34-156 X 10 MILTON 0. HIRSHFIELD, Primary Examiner.