US3012928A - Low resistance conductive paper and method of making the same - Google Patents

Description

Dec. 12, 1961 WHITMAN 3,012,928

R. M. LOW RESISTANCE CONDUCTIVE PAPER AND METHOD OF MAKING THE SAME Filed Feb. 19. 1958 ATTORNEYS Unite States Patent 3,012,928 LOW RESISTANCE CONDUCTIVE PAPER AND METHOD OF MAKING THE SAME Robert M. Whitman, Riegelsville, Pa., assignor to Riegel Paper Corporation, New York, N.Y., a corporation of Delaware Filed Feb. 19, 1958, Ser. No. 716,142 13 Claims. (Cl. 162-135) The present invention relates to paper and paper making, and more particularly to an improved, low resistance electrically conductive paper and to an improved method of making such paper.

Conductive papers are, in general, well known and are used extensively in communication work, for example, wherein conductive paper, coated with suitable materials, forms an advantageous medium for the simultaneous reception and reproduction of messages, charts, photographs, etc. A commonly utilized form of conductive paper comprises a base web of cellulosic fibers impregnated or filled with carbon and coated with electro-sensitive material. The carbon-filled web is electrically conductive, and when current is passed therethrough, as by means of a scanning stylus, the electro-sensitive coating is altered or destroyed, so that an image is produced on the paper.

In accordance with current communications technology, it has been found desirable to utilize conductive paper having a low electrical resistance, per unit of surface area. Thus, while in the past there has been some demand for so-called type H paper (500-600 ohms) and type M paper (200-250 oh-ms), the primary demand is presently for so-called type L paper (30-40 ohms). Paper resistance, expressed in ohms, is the resistance through the paper when a milliarnpere direct current is passed through the sheet, using a flat polished brass plate as the bottom contact and a polished brass ring of outside diameter and /2" inside diameter as the top contact, the top contact having a weight of 500:5 grams. In general, the electrical resistance of conductive paper may be reduced to the desired low range by sufliciently increasing the proportion of carbon (usually in the form of carbon black) to fibers. However, the addition of enough carbon to achieve low electrical resistance (i.e., below 100 ohms), causes a significant decrease in web strength, to the extent such that, when satisfactory resistance is achieved, the web may be weaker and limper than is desirable. In this respect, web strength does not appear tov be solely or directly related to the mere volume of carbon black required to attain low resistance, but also varies rather widely with the properties of the particular carbon black used. It is believed, although it has not been definitely confirmed, that in order to reach the low electrical resistance required for type L paper, in accordance with known practice, the fibers must be so completely covered with carbon black that fiberbonding is seriously impaired and web strength is accordingly reduced.

Heretofore, it has been necessary to supercalender the web,for example, to lower its resistance to the type L range of 30-40 ohms. This is not entirely satisfactory, however, in that expensive equipment is required for supercalendering and, in anyevent, substantial diificulties may be experienced'in handling the web priorto and during the supercalendering operation. Moreover, the final product continues to be relatively weak, at least where the paper resistance is lowered to the type L range.

In' accordance with the present invention, the proportion of carbon in-the paper web may be significantly increased without loss of web strength, to such an extent, even, that neither supe-rcalendering or regular machine calendering are required. The improved result is realized by initially forming the paper web with only as much carbon as does not reduce web strength below a desirable value, and passing the formed web through a size press or the like wherein sufficient additional carbon is added to provide the desired resistance characteristics, the subsequently added carbon having, however, no tendency to reduce significantly the strength of the web.

More specifically, the invention involves the preparation of paper making stock, with a portion only of the total carbon desired, the making of a paper web with the stock thus prepared, and the addition of further carbon, advantageously together with binder, to the preformed web. The first quantity of carbon may be added in the beater, in amounts insufiicient to reduce seriously the strength of the web, and the second quantity of carbon is advantageously added at a size press or the like, in amounts suflicient to impant the desired electrical characteristics of the paper.

Although it is not intended that the scope of the invention or of the patent be limited by this theory, it is believed that improved web strength is realized, in accordance with the invention, by limiting the amount of carbon added prior to web formation to such an extent that proper bonding of the fibers is not unduly interfered with. In a subsequent operation, sufiicient additional carbon is added to achieve the desired resistance characteristics. However, the subsequently added carbon does not seriously affect the previously bonded fibers.

The new product is characterized by substantial improvement in strength in relation to its low electrical resistance, and by its improved feel and scuif resistance.

For a better understanding of the invention, reference should be made to the following description and to the accompanying drawing, which is a simplified, schematic, flow-sheet representation of the new method.

Manufacture of the new product, in accordance with the invention, may be carried out with generally conventional paper making equipment. Thus, with reference to the drawing, the equipment may comprise a beater 10 for preparing stock to be delivered to the headbox 11 of a paper machine 12. The paper machine 12 may be of any suitable type, such as a regular Fourdrinier or a socalled triangular Fourdrinier, for example. The paper web formed on the paper machine is passed through a conventional dryer 13, after which the dried web is passed through a size press 14 or similar apparatus, as will be more fully explained. The web is then passed from the size press 14 to a dryer 15, following which it may be calendered, if desired, and wound into rolls or otherwise disposed of.

In accordance with the invention, in the manufacture of conductive paper having a predetermined carbon content, only a portion of the carbon is added prior to web formation, such portion being limited to that amount which will not seriously redu-ce web strength. Thus, it has been found that the carbon content of the paper making stock should not be substantially in excess of 25% of the fiber, by dry weight.

A carbon-free paper stock is first prepared, using any suitable (usually cellulosic) fiber, such as unbleached kraft, or bleached alpha, for example. Advantageously, the carbon-free stock is processed in the beater 10, by beating the pulp sufiiciently to attain good hydration. A freeness of about 200-300 cc. Canadian'Standard (as described in TAPPI method T-22 7m.-50) is satisfactory. When the heating is substantially complete, carbon black is added to the stock, and, advantageouly, little or no further beating takes place after the addition of the carbon black. The carbon may conveniently be in the form of a slurry of so-called fiulfy carbon black; and, in this respect, a carbon black manufactured by Godfrey L. Cabot, Inc. and sold under the trade name Vulcan XC-72 has been found quite satisfactory. By way of the carbon black onto the fibers.

example, the carbon black may comprise 10% of the furnish, by dry weight.

To minimize losses of carbon black in the paper making process, alum, sodium aluminate, or an equivalent composition should be added to the stock to precipitate For beset results, the precipitant should be added to the stock prior to the addition of the carbon. The addition of alum 'in the amount of approximately 4% of the furnish, by dry weight, has been found to be satisfactory.

After the addition of'the carbon black, the stock is directed to the headbox 11 of the paper machine, the stock being there diluted to a consistency of approximately /z% solids, for example, by the addition of water from the so-called White water system. If desired, additional carbon black may be added to the stock at the headbox, although any carbon thus added should be in amounts small enough to properly disperse in the stock, and in no case should the amount so added be such that the total carbon in the stock is in excess of that amount which causes significant loss of strength in the paper web. In addition, it may be desirable to add, at the headbox, a suitable wet strength resin. The use of a melamine wet strength resin in amounts of about 4% of the furnish, by dry weight, substantially increases (i.e., three times) the wet tensile strength of the web. 7

The stock in the headbox 11 is flowed onto the screen of the paper machine, to form a web in the usual manner. Water is drained from the web during its passage over the screen, and the moist web is eventually removed from the screen and passes through the dryer 13.

The paper Web emerging from the dryer 13 may have a relatively high electrical resistance, due to an insufiiency of carbon. However, the fibers in the web are well bonded so that the web has adequate strength.

In accordance with he invention, the resistance of the paper is lowered to the desired point by adding to the web a predetermined amount of additional carbon. This is accomplished by passing the web through a water suspension of carbon black, and, to this end, a conventional size press 14, size tub or the like may be located after the dryer 13, in a manner such that the web leaving the dryer may be passed through a carbon black suspension therein.

The carbon black suspension applied at the size press may contain up to 20% carbon black, in water. However, it has been found that satisfactory results are achieved in most cases with from 2% to 6%carbon (solids) in the suspension. A satisfactory commercial preparation for use in the carbon suspension is Aquablak l manufactured by Colombian Carbon Company. Aquablak 15 is a 30% colloidal dispersion of carbon black, and the desired concentration of carbon in the final suspension may be readily achieved by diluting the commercial preparation with an appropriate quantity of water.

Passing the carbon-filled web through the carbon suspension in the size press greatly reduces the electrical resistance of the paper, and sufficient amounts of carbon can be incorporated in the paper to reduce the resistance to the desired value. However, the surface carbon is in some cases easily smudged. Accordingly, in accordance with one aspect of the invention, a binder is incorporated in the size press suspension to harden the surface of the paper and insure proper bonding of the surface carbon. Advantageously, the binder may be in the form of a solution of oxidized starch having concentration, in the size press suspension, of approximately 6% a suitable commercial product for this purpose being known as Clearsol gum, made by Penick and Ford, Ltd. However, it may be found desirable to use, as a binder, casein, natural or synthetic gums or resins or natural or synthetic latices.

When the web is passed through the size press suspension, a certain amount of the binder is absorbed by the paper, along with the additional carbon black. .This

is advantageous in that not only is smudging of the carbon avoided but the strength of the paper is improved and the paper is rendered scutf resistant. The paper also acquires an improved feel and rattle characteristic of bond paper.

The web leaving the size press is passed through a dryer 15, which may be of a conventional type. The paper emerging from the dryer 15 may be an acceptable finished product and have the resistance and physical characteristics desired. When calendering or supercalendering is desired to improve the surface of the paper and/ or its receptivity to coatings, this action will result in a lowering of the resistance by 1020 ohms. However, it is to be understood that no calendering or supercalendering is required to achieve the desired resistance characteristics and web strength.

Although the various constituents in the final product may be varied somewhat, depending on the characteristics sought, a representative final product will comprise from 10% to 25% carbon, 1% to 8% (advantageously 3% to 6%) starch binder and 1% alum, the balance being substantially comprised of cellulosic fibers.

Although neither the invention nor the patent are to be thus limited, it has ben observed that satisfactory type L paper may be produced by adding, at the size press, approximately one-third the total amount of carbon in the final product, the remaining two-thirds being incorporated in the paper web during the formation thereof on the paper machine. This is merely a rule of thumb, however, and in no case should the amount of carbon in the initially formed web be such as to prevent adequate bonding of the fibers and materially weaken the web.

The new method is a substantial improvement over methods heretofore known in that low resistance papers may be readily manufactured without sacrificing web strength and without resorting to procedures such as supercalendering. Perhaps the most important advantage is the improved strength of the paper, which eliminates substantial handling problems heretofore encountered and results in a product more suitable for its intended purpose. By this new method it is also possible to make highly conductive papers of less weight per unit area or of less thickness than by methods previously known. It is further possible to make papers of lower resistance, than by the former, less etfective methods. Another advantage of this method isthat the electrical resistance of the paper can be conveniently and accurately controlled at a point close to the end of the process by varying the amount of carbon added at thesize press. For example, it has been found that when a sheet was made from unbleached kraft pulp beaten to 200 cc. Canadian Standard freeness which then had 12.5% of Vulcan XC-72 carbon black (made by Godfrey L. Cabot, Inc.) added to it; the resistance of the resulting sheet was 65 ohms. The bursting strength (TAP'PI Method 403 m53) was 12 pounds per square inch and the tearing resistance (done in accordance with TAPPI T.-4l9m49) was 48/52 g. Whenthe amount of the same carbon black, was only 10.6%, the resistance was 240 ohms, the bursting strength 16 lbs. per square inch and the tearing re: sistance 88/99 g. When this stronger sheet wasfurther treated with an aqueousdispersion containing 3.9% carbon black and 5.2% starch, the resistance dropped to 40 ohms and the bursting strength increased to 34 lbs. per square inch. The tearing resistance remained about the same (/94 g.). I

The invention resides, in part, in the new paper product characterized by having high web strength in combination with low ohmic resistance. It is believed, although it is not certain, that the improved characteristics of the product result from the fact that, of the total carbon in the paper, only a portion thereof is present when the fiber bonding action takes place. Accordingly, the web acquires a certain strength, which i ot i ifi.

cantly afl ected when additional carbon is subsequently added to the web. It is possible that the physical structure of the new product is analogous to a laminant, in which there is a center stratum comprising a felted web impregnated with carbon particles, and outer strata, on opposite sides of the felted web, comprising, largely, particles of carbon.

One of the specific advantageous features of the invention resides in the addition, at the size press, along with the carbon black, of a binder. The effect of the binder is to improve the appearance and feel of the paper, prevent smudging of the surface carbon and to increase the strength of the web.

Inasmuch as certain departures may be made from the procedures and proportions specifically described herein without departing from the clear teachings of the disclosure, reference should be made to the following appended claims in determining the scope of the invention.

I claim:

1. The method of making low resistance, conductive paper which comprises preparing a furnish of fibers and carbon, the amount of carbon being not substantially in excess of 25% by weight of the dry weight of the fibers, forming from the furnish a base web of paper comprising felted fibers and carbon, and subsequently adding to said base web suflicient additional carbon to reduce the resistance of the paper.

2. The method of claim 1, in which the additional carbon is added by passing the base web through a suspension of carbon black.

3. The method of claim 2, in which the suspension of carbon black comprises approximately 2% to 6% carbon.

4. The method of claim 2, in which the carbon black suspension includes a binder.

5. The method of claim 1, in which the base web is prepared by beating a carbon-free furnish to a point of low freeness, adding carbon black after substantial completion of the beating, and forming the base web from the carbon-carrying furnish.

6. The method of claim 1, in which the additional carbon is added by passing the base web through a suspension of carbon black and a solution of binder.

7. The method of claim 6, in which the solution of binder contains about 6% oxidized starch.

8. The method of claim 1, in which the base web is at least partially dried in the period following formation of the base web and the adding of additional carbon.

9. The method of claim 1, in which the additional carbon added comprises approximately one-third the total carbon in the reduced-resistance paper.

10. The method of claim 1, in which the base web is prepared by adding carbon black to a furnish, directing the carbon-carrying furnish to the headbox of a paper machine and forming paper stock, adding carbon to the stock in the headbox, and flowing the carbon-carrying stock onto the wire of the paper machine.

11. The method of claim 1, in which the base Web is prepared by beating a carbon-free furnish, adding a precipitant to the furnish during beating, adding carbon black substantially at the end of beating, and thereafter forming a web with the furnish.

12. The method of claim 11, in which the precipitant is alum, and the alum is added in an amount of approximately 4%, by dry weight, of the furnish.

13. As a new product, a low resistance, conductive paper made in accordance with the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 1,758,055 Nanfeldt May 13, 1930 2,328,198 Davenport et a1. Aug. 31, 1943 2,359,858 Iler Oct. 10, 1944 2,554,017 Dalton May 22, 1951 FOREIGN PATENTS 524,073 Canada Apr. 17, 1956

Claims (2)

1. THE METHOD OF MAKING LOW RESISTANCE, CONDUCTIVE PAPER WHICH COMPRISES PREPARING A FURNISH OF FIBERS AND CARBON, THE AMOUNT OF CARBON BEING NOT SUBSTANTIALLY IN EXCESS OF 25% BY WEIGHT OF THE DRY WEIGHT OF THE FIBERS, FORMING FROM THE FURNISH A BASE WEB OF PAPER COMPRISING FELTED FIBERS AND CARBON, AND SUBSEQUENTLY ADDING TO SAID BASE WEB SUFFICIENT ADDITIONAL CARBON TO REDUCE THE RESISTANCE OF THE PAPER.

8. THE METHOD OF CLAIM 1, IN WHICH THE BASE WEB IS AT LEAST PARTIALLY DRIED IN THE PERIOD FOLLOWING FORMATION OF THE BASE WEB AND THE ADDING OF ADDITIONAL CARBON.

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