WO1994000769A1

WO1994000769A1 - Apparatus for measuring electrical surface resistivity of a moving web

Info

Publication number: WO1994000769A1
Application number: PCT/US1993/005903
Authority: WO
Inventors: Richard Matthew Cribb; James Dennis Capistran; Egwu Eric Kalu
Original assignee: Monsanto Company
Priority date: 1992-06-26
Filing date: 1993-06-18
Publication date: 1994-01-06

Abstract

Apparatus for measuring electrical surface resistivity of a moving web comprising: (a) a non-conductive roller for carrying a conductive web; (b) at least four rolling electrodes, e.g. wheels having conductive rims wherein each wheel is electrically insulated from each other wheel and is free to rotate on an axle secured to an arm pivotably mounted on an elongated support disposed along the axis of said roller and each of said arms is biased to hold said wheels in aligned contact with said roller; (c) means for passing a known electrical current I through said web between an outer set of two of said rolling electrodes separated by an inner set of at least two other rolling electrodes; and (d) means for measuring the voltage drop V in said web between the inner set of electrodes; wherein the resistivity R of said web between said inner set of electrodes is determined from the algorithm R = (V/I) (C), where C is a calibration factor which accounts for edge effects. The calibration factor is determined from a relationship among the distance the web extends from the line of the electrodes in both directions along the line of travel, the width of the web and the distance between electrodes.

Description

APPARATUS FOR MEASURING ELECTRICAL SURFACE RESISTIVITY OF A MOVING WEB

Disclosed herein are apparatus and methods for measuring the electrical surface resistivity on a moving web, e.g. of metal-coated textiles. Such apparatus is useful for on-line continuous measurements that allow for the production of metallized web having a high degree of uniformity in electrical properties.

Metal-coated textiles, e.g. woven or non- woven fabrics, are useful for shielding electro¬ magnetic radiation, e.g. electromagnetic waves are reflected from a metal-coated textile due to an impedance difference between the surrounding medium and the metal-coated textile. A lower resistivity metal-coated fabric leads to a larger impedance mismatch and provides better shielding, other factors being equal. The effectiveness of metal-coated textiles in electromagnetic shielding applications is enhanced by the uniformity of metal coating and the continuity of the metal film on the fibers of the fabric. Such qualities of metal-coated textiles can be indicated by electrical resistivity. More particularly, on-line measurement of electrical resistivity of metal-coated textiles as it is being produced gives early warning of a potential quality defect and allows corrective action to be taken much sooner than if the product is tested off-line after a considerable quantity had been made. On-line testing also tests much more frequently than it would be practical to do manually off-line.

A variety of electrode designs have been proposed for determining a variety of electrical properties of materials. See, for instance, U.S.

Patent 3,995,213 which discloses a probe design for determining the properties of stationary metal sheets; U.S. Patent 3,456,186 which discloses the use of a four probe system for measuring sheet resistivity; and U.S. Patent 2,184,511 discloses the use of circular conductive rings on generally cylindrical body parts, e.g. arms and legs, to measure the impedance of internal tissue. More specific electrodes and procedures are disclosed in ASTM F 390-78 a Standard Test Method for sheet resistance of thin metallic films with a collinear four-probe array. In general such standard methods are tedious and result in a long lag time between measurements and the opportunity for corrective action.

The use of conductive rings on rollers has been applied to solving a variety of problems in measuring properties of moving webs. See, for instance, U.S. Patent 2,653,298 in which such a roller is used to detect moisture content of a moving web of tobacco rag and leaf, and U.S. Patent 3,541,437 in which such a roller is used to detect transverse motion of a moving web. Alternatively, U.S. Patent 3,384,815 discloses the use of segmented conductive discs arranged on a roller to measure the moisture content of polymeric films by applying an electrical potential in the range of 600-1500 volts. See also U.S. Patent 4,027,238 which discloses the use of three adjacent electrodes on a roller to compensate for induced voltage from A.C. motors when measuring the high resistance, e.g. about 10¹² ohms, of extremely dry material. The use of axially disposed conductor strips on a roller is disclosed in U.S. Patent 3,944,354 where the conductor strips are used for voltage measurement on sheets via a capacitance technique and in U.S. Patent 4,862,065 where such a roller is used to measure the resistivity of a high resistance (e.g. at least 10⁷ ohms, a moving web. Spiral-shaped conductive strips for rollers are disclosed in U.S. Patent 2,659,048 for measuring the moisture across the width of a moving web and in U.S. Patent 3,704,412 for measuring the impedance across the width of a moving web.

A series of spaced apart insulators on a conductive roller is disclosed in U.S. Patent 3,636,442 for detecting defects in a moving conductive web and in U.S. Patent 3,657,645 for measuring the thickness of conductive coatings on polymeric films.

Although rollers have been designed with a variety of electrode configurations for conducting different types of measurements, none of the prior art practitioners has assembled an apparatus that could be used to provide a reliable in-line measurement of electrical surface resistivity of a highly conductive moving web, e.g. of metal-coated fabric.

SUMMARY OF THE INVENTION This invention provides apparatus and methods that allow in-line and/or continuous measurement of electrical surface resistivity of a highly conductive moving web such as a metallized film or metal-coated fabric. These methods and apparatus allow for the on-line and continuous monitoring of such webs providing feedback for modifying the production of metallized webs to correct deviations in electrical properties along the width or length of the web. One aspect of this invention provides roll goods of metal-coated fabric having highly uniform electrical resistivity of a standard deviation of less than 0.005 ohms/square.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 are isometric views of the apparatus of this invention for measuring the resistivity of moving webs. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS This invention provides an apparatus for measuring electrical surface resistivity of a moving web comprising: (a) a non-conductive roller for carrying a conductive web such as metallized fabric or film; (b) means for applying at least four rolling electrodes to the conductive web on said non- conductive roller; (c) means for passing a known electrical current I through said web between an outer set of two of said rolling electrodes separated by an inner set of at least two other of said rolling electrodes; and (d) means for measuring the voltage drop V in said web between said inner set of electrodes; wherein the surface resistivity R of said web between said inner set of electrodes is determined from the algorithm R = (V/I) (C) , where C is a calibration factor which accounts for edge effects. When the electrodes are equidistantly spaced apart, C is determined from the algorithm 1/C = (1/JΓ) [7rs/d + ln(l-e^{"4τs d}) - ln(l-e^{" τs d}) +

_(e-2τ(a-2s)/d₎ (i.g-OTs dj ₍₁__e-2τs/d_{)/ (1+e}-2τa/d_{) +}

0.5(e-*^τ(a-^{2s) d}) (l-e^"12*^{s d}) (l-e^"4*^{s d})/ (l+e^{"4τa d}) ] where "s" is the distance between equidistantly-spaced apart electrodes, "d" is twice the distance from the line of the rolling electrodes to the closest end of the moving web in the direction of web motion and "a" is the width of the web transverse to the direction of web motion. Preferably, the contact area of the rolling electrodes is small, e.g. less than 1% of the width of the web, and the distance between electrodes,

"s", is less than 0.05 times the width of the web, i.e. a/s > 20.

When there is more than four equally spaced apart rolling electrodes, it is possible to electrically configure any four adjacent wheels for a resistivity measurement. Such configuration can be effected sequentially to measure resistivity at incremental positions across the web.

In one aspect of this invention the means for applying rolling electrodes comprises an elongated axle having raised electrically-conductive rims which are electrically insulated from each other. In a preferred aspect of this invention the means for applying rolling electrodes comprises at least four wheels having conductive rims, where each wheel is electrically insulated from the other wheels, is free to rotate on an axle secured to an arm which is pivotably mounted on an elongated support and is aligned with the non-conductive roller to contact the web as it is carried by the non-conductive roller. Each of said arms is biased to hold said wheels in aligned contact with said roller. The wheels are biased into contact with the fabric on the roller by a force which can be provided by a spring or by gravity acting on a cantilevered assembly of a wheel mounted at the end of a pivot arm.

The means for passing electrical current and for measuring voltage drop is conveniently provided by a brush, e.g. carbon brushes, in contact with the rolling electrodes, e.g. the rotating conductive rims on each wheel. A preferred brush of low internal and contact resistance comprises a silver-loaded carbon. The apparatus and method of this invention utilize what is known as the "four probe" method as distinguished from a "two probe" method where voltage drop is measured across the two electrodes (probes) delivering the current to the sample. In a "four probe" method a known current is caused to flow through the sample between two outer electrodes and voltage drop is measured in the path of the current by two intermediate electrodes. Because current flow is divided into a portion flowing between the two intermediate electrodes and a portion bypassing the intermediate electrodes, a correction factor is needed to determine the actual surface resistivity from the apparent surface resistance. For the case where d/a >3 and a/s > 20, C approaches 4.5 as a limiting value. For instance, in the case of a 1 meter wide web and an inter-electrode distance "s" of 5 centimeters, C is conveniently 4.5.

With reference to Figures 1 and 2, an electrically conductive web 1 is supported by a non- conductive roller 2 while the web passes under electrode assemblies 3 which comprise an arm 4 pivotably mounted on a support axis 5. Rolling electrodes are provided at the end of each arm by a wheel having a conductive rim 6, e.g. preferably a corrosion resistant metal such as stainless steel, mounted on a non-conductive hub 7, e.g. comprising a plastic material such as fluorocarbon polymer or PVC which insulates and supports the conductive rims. Bolt 8 can be conveniently used to rotatably secure the wheel to the arm. Electrical contact between a conductive rim 6 and apparatus for supplying current and/or measuring voltage drop is made through wire 10 connected to spring 11 which forces brush 12 into contact with the rotating conductive rim 6; the spring is mounted to insulated support 13 which can be securely bolted to arm 4.

As illustrated in Figure 2 a preferred apparatus and method of this invention employs at least four wheel-electrodes. In some case, e.g. for narrow width webs or for laboratory use, a four wheel apparatus is appropriate. In other cases, e.g. for wide width webs, more than four wheel-electrodes are preferred. When there are more than four wheel- electrodes, it is relatively simple to electrically connect them for measurements at various locations on a moving web to achieve a high degree of sampling. The methods of this invention provide for measuring the electrical resistivity of a moving web of metal-coated fabric or film by: (a) carrying the web over a support roller; (b) passing a known electrical current I through the web between the an outer set of two rolling electrodes, (c) measuring the voltage drop V in the web between an inner set of rolling electrodes; and (d) determining the surface resistivity R of the web between said inner wheels from the algorithm R = (V/I) (C) , where C is as defined above is a calibration factor for edge effects of current flow.

The surface resistivity of a web, e.g. of metal-coated fabric, is an indication of the quality of metal deposited on the fabric. The ability to continuously monitor the resistivity advantageously allows for corrective action to compensate for deviations in the deposited metal. Thus, another aspect of this invention provides a method for controlling the amount of metal deposited onto a moving web. The method comprises measuring surface resistivity, sequentially or randomly, across incremental areas of a moving web, and correcting for deviations in the resistivity by adjusting process variables for deposition of metal onto moving webs. Such process variables, e.g. for electroless or electrolytic or vapor deposition of metal, may include web speed, plating bath composition and temperature, the length of time the web is exposed to the deposition environment and the deposition driving force, e.g. current density in the case of electrolytic deposition.

Thus this invention also provides methods for controlling the amount of metal deposited onto a moving web by: (a) measuring surface resistivity across incremental areas of a moving web and (b) correcting for deviations in the surface resistivity by adjusting deposition process variables.

Yet another aspect of this invention provides roll goods of metal-coated fabric at least 100 meters in length, prepared by depositing metal onto a fabric substrate where the deposition process was controlled in response to feedback of the electrical surface resistivity of said metallized fabric using an apparatus of this invention wherein the standard deviation of electrical surface resistivity of said metallized fabric is less than 0.005 ohms/square.

While specific embodiments have been described herein, it should be apparent to those skilled in the art that various modifications thereof can be made without departing from the true spirit and scope of the invention. Accordingly, it is intended that the following claims cover all such modifications within the full inventive concept.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for measuring electrical surface resistivity of a moving web comprising:

(a) a non-conductive roller for carrying a conductive web;

(b) means for applying at least four rolling electrodes to the conductive web carried on said non- conductive roller;

(c) means for passing a known electrical current I through said web between an outer set of two of said rolling electrodes separated by an inner set of at least two other of said rolling electrodes; and

(d) means for measuring the voltage drop V in said web between the rolling electrodes of said inner set; wherein the surface resistivity R of said web between said inner set of rolling electrodes is determined from the algorithm R = (V/I) (C) , where C is a calibration factor which accounts for edge effects.

2. An apparatus according to claim 1 wherein said electrodes are equidistantly spaced apart and C is determined from the algorithm

1/C = (l/π)[πs/d + ln(l-e^{"4τs d}) - ln(l-e^{"2τs d}) +

_(e-2»(a-2β>/d_{) (1}__e-6«/d_{) (1}__e-2«/d_)/(1+e-ara/d_{) +}

0.5(e-^4τ(a-^2s)/d) (l-e^"12τs ) (l-e^{"4τs d})/ (l+e^"4*³"¹) ] where "s" is the distance between the rolling electrodes, "d" is twice the distance from the line of the rolling electrodes to the closest end of the moving web in the direction of web motion and "a" is the width of the web transverse to the direction of web motion.

3. An apparatus according to claim 1 wherein d/a > 3, a/s > 20 and C = 4.5.

4. An apparatus according to claim 1 comprising more than four rolling electrodes and means for sequentially measuring resistance at incremental positions across said web.

5. An apparatus according to claim 1 wherein said means for applying rolling electrodes comprises an elongated axle having raised electrically conductive rims which are electrically insulated from each other.

6. An apparatus according to claim 1 wherein said means for applying rolling electrodes comprises at least four wheels having conductive rims, wherein each wheel is electrically insulated from each other wheel and is free to rotate on an axle secured to an arm pivotably mounted on an elongated support disposed along the axis of said roller and each of said arms is biased to hold said wheels in aligned contact with said roller.

7. An apparatus according to claim 6 wherein each of said arms is biased into contact with said roller by the force of gravity.

8. An apparatus according to claim 6 wherein said means for passing an electrical current and said means for measuring a voltage drop comprises a brush in contact with the conductive rim on each of said wheels.

9. A method for measuring the electrical surface resistivity of a moving web of metal-coated fabric comprising:

(a) passing a said web over a non-conductive roller;

(b) passing a known electrical current I through said web between the points of contact with said web by an outer set of two rolling electrodes, wherein an inner set of two rolling electrodes is located between said outer set of electrodes and wherein the distance between adjacent electrodes is substantially the same and said electrodes are electrically insulated from each other; (c) measuring the voltage drop V in said web between the points of contact with said web by said inner set of rolling electrodes; and (d) determining the surface resistivity R of said web between said inner set of rolling electrodes from the algorithm R = (V/I) (C) , where C is determined from the algorithm 1/C = (1/π) [πs/d + ln(l-e^"4τs/d) - ln(l-e^{'2τs d}) +

_(e-2x⁽a-2s⁾/d_{) (1}__e-««/d₎ (l-e'²*^s/d)/(l+e^{'2τa d}) + 0.5(e-^4τ(a"2s) ) (l-e-^{12τs d}) (l-e^{"4τs d})/ (l+e^{"4τa d}) ] where "s" is the distance between equidistantly-spaced apart rolling electrodes, "d" is twice the distance from the line of the electrodes to the closest end of the moving web in the direction of web motion and "a" is the width of the web transverse to the direction of web motion.

10. A method according to claim 9 wherein d/a > 3, a/s > 20 and C = 4.5.

11. A method according to claim 9 comprising sequentially determining resistivity at incremental positions across said web.

12. A method according to claim 9 further comprising adjusting the amount of metal coated onto said web in proportion to deviations in the surface resistance, wherein said adjusting is effected by changing process variables for deposition of said metal, said variables being selected from the group consisting of web speed, plating bath composition, plating bath temperature and length of time the web is immersed in the plating bath.

13. Roll goods of metal-coated fabric at least 100 meters in length prepared by depositing metal onto a fabric substrate where the deposition process was controlled in response to feedback of the electrical surface resistivity of said metallized fabric using an apparatus according to claim 1 wherein the standard deviation of electrical surface resistivity of said metal-coated fabric is less than 0.005 ohms/square.