MOISTURE-INSENSITIVE ELECTRICALLY-CONDUCTIVE PAPER

Abstract: Paper which is electrically conductive by virtue of containing carbonaceous fibers is made from wood pulp and synthetic wood pulp, the ratio between the two types of pulp being so selected that the eletrical conductivity of the paper does not change appreciably with changes in ambient moisture, whether or not the paper is encapsulated within an envelope which is resistant to water-vapor transmission.

Description: MOISTϋRE-INSENSITIVE ELECTRICALLY-CONDUCTIVE PAPER Technical Field The invention concerns paper containing electrically-conductive fibers such as carbonaceous fibers. Background Art U. S. Patent No. 3,367,851 (Filreis et al.) discloses a paper which is electrically-conductive by virtue of its carbonaceous fibers. Such a paper provides inexpensive heating panels and food warming trays. Although not mentioned in the patent, the electrical conductivity of the paper decreases with increased moisture content. It is theorized that the moisture causes the paper to swell, thus tending to break the paths of electrical conductivity through the carbonaceous fibers. To protect it from moisture, a conductive paper may be encapsulated within an electrically-insulating moisture barrier while keeping the moisture content of the paper at a controlled low level. Especially useful in this respect is a laminate of biaxially-oriented polyethylene terephthalate film and polyethylene film. The polyethylene faces of two pieces of the laminate are bonded to the electrically conductive paper and to each other beyond the perimeter of the paper, as shown in U. S. Patent No. 3,774,299 (Sato et al.) Because that laminate is only partially resistant to water-vapor transmission, changes in ambient humidity can eventually produce changes in the moisture content of the paper, with corresponding changes in its electrical conductivity. Disclosure of nvention The present invention provides a novel electrically-conductive paper which experiences little change in electrical conductivity with changes in humidity. Like the paper of the Filreis patent, the novel paper comprises a mixture of wood pulp of paper-making grade plus electrically-conductive carbonaceous fibers in an amount of from 1% to 35% of the weight of the paper. By "carbonaceous' is meant that the fibers have a carbon content of at least 80% by weight. As disclosed in the Filreis patent, the carbonaceous fibers preferably have an average length from 0.15 cm to 1.1 cm and a tex (g/km) from 0.03 to 0.5. The novel paper differs from that of the Filreis patent in that it also comprises synthetic wood pulp (SWP), and the SWP/wood pulp ratio is from 4:1 to 49:1 when the paper is unencapsulated and from 1:1 to 9:1 when the paper is encapsulated in an envelope which transmits less than 15 grains of water per square meter per 24 hours at 38°C and 95% relative humidity. Preferably the SWP/wood pulp ratio for unencapsulated paper is from 8:1 (below which its electrical conductivity tends to decrease with increased humidity) to 25:1 (above which its electrical conductivity tends to increse with increased humidity). There is very little change in electrical conductivity at ratios from 12:1 to 20:1. Preferably the SWP/wood pulp ratio is from 5:3 to 4:1 for paper encapsulated in an envelope which transmits less water than 15 g/m 2 /24 hrs at 38 β C and 95% RH. If the barrier transmits water at about 4 g/m 2 /24 hrs under such conditions, the SWP/wood pulp ratio is preferably about 7:3. SWP is made by treating polyolefin fibers to make them readily dispersible in water. The following U.S. patents assigned to Crown Zellerbach Corporation disclosed various processes: 3,743,570; 3,787,256; 3,848,027 and 3,891,449. Crown Zellerbach currently sells SWP in several commercial grades such as E-400, E-600, etc. OMPI Preparation of Electrically Conductive Paper For ease of manufacturing an electrically- conductive paper on a commercial scale, a good basis weight _, is 85 g/m 2 . A typical requirement for specific electrical resistance is 27 ohms per square. In order to make 5 electrically-conductive paper having such basis weight and specific electrical resistance on a laboratory basis, add 2.083 g of SWP (Crown Zellerbach E-400) to 400-500 ml water and disperse in a blender at high speed for one minute. Add 0.893 g of bleached kraft softwood pulp and operate the 10 blender for ten seconds; then add 0.525 g of carbonaceous fibers and operate the blender for an additional ten seconds. Pour the dispersion into a hand-sheet mold fixture previously partially filled with water and add sufficient water to fill the fixture. Stir by hand with a 15 comb-like paddle until the fibers are uniformly dispersed and then drain the water. Remove the hand-sheet formed on the fixture's screen and dry in a paper dryer to provide an electrically-conductive paper. To make test samples of a suitable size, cut the 20 dried sheets into rectangles 8.9 x 11.4 cm. Using a heated-platen press, adhere a 68-micrometers-thick copper foil, 1.3 cm in width along each of the 8.9-cm edges using a conductive thermoplastic adhesive. Thus there is a 8.9-cm-square area between the copper-foil electrodes. By 25 applying a temperature of 150°-175°C and a pressure of 240-690 kPa, the heated-platen press also fuses the SWP fibers of the SWP-containing conductive papers to produce an integral, semi-transparent, film-like structure. The same press conditions are suitable for then encapsulating 30 the conductive papers within a laminate of polyethylene film and polyethylene terephthalate film. "Such a laminate may transmit water at about 4 g/m 2 /24 hr. at 38°C and 95% relative humidity. Detailed Description A number of encapsulated and unencapsulated electrically-conductive papers were prepared as described above except at various ratios of SWP to wood pulp. These papers were tested for changes in electrical resistance, first by testing in air at ordinary room temperature and 45% relative humidity and then retesting following 96-hour exposure to salt spray [ASTM B-117-73] . The percent changes in resistance, averaged from a large number of samples in each case, are reported in the table. Ratio of Water-Vapor Trans¬ % Change in SWP to mission of Envelope Electrical Wood Pulp (g/m /24 hr ' at 38 β C) Resistance Sample 1 50:50 10.9 6.4 Sample 2 70:30 do 0.06 Sample 3 90:10 do -2.2 Sample 4 50:50 3.9 4.4 Sample 5 70:30 do 0.14 Sample 6 90:10 do -4.5 Sample 7 100:0 do -10.7 Sample 8 50:50 0.3 1.2 Sample 9 70:30 do 0.39 Sample 10 90:10 do -0.78 Sample 11 50:50 unencapsulated 27.3 Sample 12 70:30 do 4.9 Sample 13 90:10 do 2.6 Sample 14 100:0 do -7.4 The electrically-conductive papers of the foregoing samples consisted only of mixtures of carbonaceous fibers and either SWP or SWP and wood pulp. However, other materials may be additionally incorporated into the novel conductive paper, as is known in the art, e.g., glass, asbestos or other inorganic fibers, various polymeric fibers, mica flakes and carbon particles in such quantities as do not interfere with the attainment of a Y/IP ^ R T A desired value of electrical conductivity or render the paper unduly sensitive to changes in humidity. By the same * token, the novel conductive paper may be impregnated with nonconductive resins such as phenolic and epoxy resins which can afford added strength and integrity without interfering with the desired unchanging electrical conductivity over the full area of a piece of the novel conductive paper. However, to the extent that such impregnating is a barrier to water vapor, the proportion of SWP to wood pulp should be adjusted as indicated by the data reported for the examples. An experimental use for the electrically- conductive paper of the invention is to heat liquid crystal displays which are used outdoors. If those displays are to be back-lighted, the paper should be reasonably translucent. Since higher proportions of SWP to wood pulp have better translucency, it is preferred for such use that the SWP/wood pulp ratio be at least 2:1.

Claims: CLAIMS 1. Electrically-conductive paper comprising wood pulp and electrically-conductive carbonaceous fibers in an amount providing 1-35% by weight of the paper, characterized in that the paper comprises synthetic wood pulp (SWP) and the SWP/wood pulp ratio is 1:1 to 49:1. 2. Electrically-conductive paper as defined in claim 1, further characterized in that the SWP/wood pulp ratio is from 4:1 to 49:1 when the paper is unencapsulated and from 1:1 to 9:1 when the paper is encapsulated in an envelope which transmits less water than 15 g/m /24 hr. at 38°C and 95% relative humidity. 3. Electrically-conductive paper as defined in claim 2 which is unencapsulated and is further characterized in that the SWP/wood pulp ratio is from 8:1 to 25:1. 4. Electrically-conductive paper as defined in claim 3 which is further characterized in that the SWP/wood pulp ratio is from 12:1 to 20:1. 5. Electrically-conductive paper as defined in claim 2 which is encapsulated within an envelope which transmits less water than 15 g/m 2 /24 hr. at 38 β C and 95% relative humidity and is further characterized in that the SWP/wood pulp ratio is from 5:3 to 4:1. 6. Electrically-conductive paper as defined in claim 5 further characterized in that said envelope transmits water at about 4 g/m 2 /24 hr. at 38°C and 95% relative humidity and the SWP/wood pulp ratio is about 7:3. i O ?I " , WIΪO 7. Electrically-conductive paper as defined in claim 5 or claim 6 further characterized in that said envelope is provided by two pieces of a laminate of biaxially-oriented polyethylene terephthalate film and polyethylene film, the polyethylene layers being fused to the paper and to each other beyond the edges of the paper.

Inventors: DRESSLER DARYL D, DRESSLER, DARYL, D.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1982
Main classification: D21H1
IPC codes: B66F9, D21H13, H01B1, H05B3