Electroconductive articles and a method of preparing them

Abstract

 Electroconductive articles such as paper have a coating consisting essentially of (1) a barrier coating comprising a copolymer of ethylene and vinyl acetate, a styrene- butadiene latex, sodium alginate or a fluorocarbon with starch or a modified starch; and (2) an electroconductive coating. Such articles are made by applying the barrier coating to a substrate, allowing it to dry, applying the electroconductive coating over the barrier coating and allowing it to dry.

Description

[0001] This invention relates to electroconductive paper and other electroconductive articles.

[0002] In the coating art, as currently practised for electroconductive papers, barrier coatings usually consist of only starch, starch that has been chemically modified in a known manner,or a combination of starch and a resin, such as a copolymer of styrene and maleic anhydride. The barrier-coated sheet is subsequently overcoated with a cuating that has electroconductive properties and is then referred to in the trade as conductive base stock. Using the preceding manufacturing process, frequent problems occur with the base stock in obtaining the required degree of solvent holdout, conductivity and low tackiness. Particularly at high relative humidities, tackiness of the conductivized paper causes numerous production problems on the coating machine and calender stacks, resulting in reel- blocking and processing difficulties.

[0003] Our unique barrier coating for electroconductive paper, when used in combination with a suitable conductive polymer formulation, will provide a finished conductive base stock that has improved solvent holdout and conductivity, and a low degree of tackiness at high relative humidities.

[0004] In accordance with the present invention, a barrier coating consists essentially of from 5 to 50 percent by weight of a copolymer of ethylene and vinyl acetate, a styrene-butadiene latex, sodium alginate or a fluorocarbon, together with from 95 to 50 percent by weight of a modified starch. This coating may be applied to the paper raw stock by conventional coating techniques in amounts ranging from 0.8 to 3.5 g/m². The conductive coating may contain from 10 to 90 percent by weight, preferably 70 to 90 percent by weight, of an electroconductive polymer and from 10 to 90 percent by weight of a combination of pigments and/or binders. This coating may also be applied to one or both sides of the barrier-coated paper by conventional coating techniques, such as blade, air knife or reverse roll methods in amounts ranging from 0.8 to 6.0 gfm2 per side of the paper.

[0005] The preferred copolymer of ethylene and vinyl acetate is an aqueous latex product marketed by Air Products and Chemicals, Inc., as Airflex 110 and the preferred modified starch is a hydroxyethylated, acetylated, oxidized or cationic starch.

[0006] The nature of the electroconductive polymer component of the improved coating formulations of this invention is not critical. Any of a variety of electroconductive polymers, both cationic and anionic, may by employed provided that the conductive polymer selected is capable of imparting adequate surface resistivity to the base raw stock. As cationic electroconductive polymers, there may be employed any water-soluble cationic polymer containing quaternary ammonium functional groups. Included in such cationic polymers are those of the formula:

wherein:

R stands for hydrogen or lower alkyl;

R₁ represents a member of the class composed of

R₂ stands for

wherein, in turn, A represents a lower alkylene, an hydroxy-lower alkylene or lower- alkyl-substituted lower alkylene group, and R₃ stands for a lower alkyl group. These polymers include those wherein the quaternary ammonium functional group is carried as a pendant group to the principal polymer chain, such as, for example, polyvinyl benzyl trimethyl ammonium chloride, poly-[alpha-(methylene trimethyl ammonium chloride)ethylene oxide] and poly(methacryl- oyloxyethyltrimethyl ammonium chloride). Also useful are those polymers wherein the quaternary ammonium functional group is incorporated in a cyclic structure which comprises a portion of the polymer backbone, such as, for example, polymers containing repeating units of the formula:

where R is an alkyl group of 1 to 18 carbon atoms and R₁ is R or β-propionamido and A is an anion. A preferred polymer of this class is poly-(dimethyl- diallylammonium chloride); and those wherein the quaternary ammonium functional group forms a part of the polymer chain, such cationic polymers being commonly designated as "ionenes".

[0007] Included in this group, for example, are ionene polymers prepared from halo alkyl dialkyl amine monomer units, such as 3-ionene(poly-(dimethyl propyl)-ammonium chloride), prepared by the polymerization of 3-chloropropyl dimethyl amine, and ionene polymers prepared from di-tertiaryamines and dihalides, such as 3,4-ionene which is prepared from 1,3-bis-dimethyl- amino propane and 1,4-dichlorobutene. Other ionene polymers, of course, which are prepared similarly, may be employed as the electroconductive component of the coating formulations of this invention.

[0008] In addition to the cationic electroconductive polymers mentioned above, water-soluble cationic phosphonium and sulfonium polymers also may be employed as the electroconductive component in the coating formulations of this invention. Included among these are polymers, such as, for example, poly-(2-acryloxyethyldimethyl sulfonium chloride) and poly-(glycidyltributyl phosphonium chloride) and the like.

[0009] It should be noted that the typical cationic and anionic polymers mentioned above may contain one or more other mer units. For example, copolymers such as the copolymer of dimethyl diallyl ammonium chloride and diacetone acrylamide or the reaction product of dimethyl diallyl ammonium chloride and the copolymer of styrene and maleic anhydride also can be used as the electroconductive component of the coating formulations of this invention. The ratio of mer units in such copolymers will be determined by the quantity of cationic or anionic necessary to impart the desired surface resistivity to the base sheet.

[0010] Although any of the electroconductive polymers noted above, or other electroconductive polymers capable of imparting the necessary degree of surface resistivity to the base sheet, may be employed as the electroconductive component in the improved coating formulations of this invention, the preferred electroconductive polymers are the cationic polymers and copolymers and especially cationic quaternary ammonium polymers and copolymers. Of these, the most preferred polymers are poly-(di- methyldiallylammonium chloride), copolymers of dimethyl diallyl ammonium chloride and diacetone acrylamide containing from 70 to 98% diallyl monomer, polyvinylbenzyl trimethyl ammonium chloride, poly-methacryloyloxyethyl trimethyl ammonium chloride, polymethacryloyloxytrimethylammonium methosulfate polyepiclorohydrin 80 to 100% quaternized with trimethylamine, copolymers of acrylamide and methacryloyloxyethyl trimethyl ammonium chloride containing from 90 to 99.5% methacryloyloxyethyl monomer units, poly-methacryloyloxyethyl dimethyl hydroxyethyl ammonium chloride) and poly-(dimethylpropylammonium chloride).

[0011] As noted above, the binders employed in the improved coating formulations of this invention can be of great variety and do not constitute a critical aspect of the instant invention. Any of the water-soluble, film-forming polymers conventionally employed for this purpose may be used in the coating formulations of this invention. Suitable binders will include, for example, polyvinylalcohols, polyvinyl acetates, styrenebutadiene latices, polyethylene-polyvinyl acetate copolymers, unmodified starches, acetylated starches, hydroxyethylated starches, enzyme converted starches, oxidized starches, proteins, caseins, and the like or mixtures thereof. Similarly, any of the variety of pigments conventionally employed in coating formulations may be employed in the improved coating formulations of this invention including commercially available calcium carbonates, kaolin clays, titanium dioxides, aluminas or combinations of these materials.

[0012] The electroconductive coating fromulation utilized in this invention may also contain certain mono-and bis-(lH,lH,2H,2H-perfluoroalkyl)-phosphate esters, when incorporated into electroconductive coating formulations in the quantities specified below, are effective in imparting to such formulations improved solvent holdout properties. In general, useful perfluoroalkyl phosphate esters will have the formula,

wherein m is an integer betwen 4 and 10, n is an integer between 1 and 11, y is 1 or 2 and M is a water-solubilizing cation, such as, for example, an alkali metal (Li, K, Na and the like), ammonium or substituted ammonium including methylamine, dimethylamine, diethylamine, monoethanolamine, diethanolamine, triethanolamine or morpholine and the like. Preferred salts generally are the diethanolamine salts. Desirably, C and C taken together, constitute a straight chain of at least 8 carbon atoms. Such perfluoroalkyl phosphate esters are well-known materials and are available commercially or readily prepared by methods fully described in the art. Particularly preferred is the perfluoroalkyl phosphate ester manufactured by E. I. du Pont de Nemours Company, Inc., Wilmington, Delaware, under the Trademark, ZONYL RP, which contains diethanolamine salts of mono- and bis-(lH,lH,2H,2H-perfluoroalkyl) phosphates where the alkyl group is even numbered in the range C₈-C₁₈ and the salts have a fluorine content of 52.4% to 54.4% as determined on a solids basis or ammonium bis(N-ethyl-2-perfluoro- alkylsulfonamido ethyl) phosphates, containing not more than 15% ammonium mono(N-ethyl-2-perfluoroalkyl- sulfonamido ethyl) phosphates, where the alkyl group is more than 95% C₈ and the salts have a fluorine content of 50.2% to 52.8% as determined on a solids basis. Materials of this type are sold by 3M Co., Inc., under the Trademark SCOTCHBAN.

[0013] In order to illustrate the advantages derived from the present invention, the barrier coating and conductive coating formulations were applied to raw stock by drawdowns with the appropriate wire-wound rod and also ran a Keegan Pilot Coater. The coated sheets were conditioned overnight at 20% and 50% relative humidity (RH) and 72°F. after which they were weighed to obtain coatweight and evaluated for conductivity by using a Keithley 610B Electrometer to measure surface resistivity. Solvent holdout was determined by measuring dyed toluene penetration after 10 seconds contact time. Tackiness of the paper was determined by hand feel. The results of these tests are set forth in the following tables.

[0014] 

Claims

1. An electroconductive article having improved solvent holdout and a low degree of tackiness, said article comprising a substrate coated on at least one side with a barrier coating and an electroconductive coating overlying the barrier coating, characterized by the barrier coating's consisting essentially of from 5 to 50 percent by weight of a copolymer of ethylene and vinyl acetate, a styrene-butadiene latex, sodium alginate or a fluorocarbon, together with from 95 to 50 percent by weight of a modified starch.

2. An article as claimed in Claim 1 in which both sides of the electroconductive article have an electroconductive coating.

3. An article as claimed in Claim 1 or 2 in which the electroconductive coating is poly-(dimethyl diallyl ammonium chloride), a copolymer of dimethyl diallyl ammonium chloride and diacetone acrylamide containing from 70 to 98% of diallyl monomer units, polyvinylbenzyl trimethyl ammonium chloride, polymeth- acryloyloxyethyl trimethyl ammonium chloride, polymethacryloyloxy- ethyl trimethyl ammonium methosulfate, polyepichlorohydrin 80 to 100% quaternized with trimethylamine, a copolymer of acrylamide and methacryloyloxyethyl trimethyl ammonium chloride containing from 90 to 99.5% methacryloyloxyethyl monomer units, poly-(methacryloyloxyethyl dimethyl hydroxyethyl ammonium chloride), or poly-(dimethyl propyl ammonium chloride).

4. An article as claimed in Claim 1, 2 or 3, characterized by the electroconductive coating's also containing a fluorosur- factant of the formula :

where m is an integer between 4 and 10, n is an integer between 1 and 11, y is 1 or 2 and M is an alkali metal, ammonium or substituted ammonium cation, or an ammonium bis(N-ethyl-2-per- fluoroalkylsulfonamido ethyl) phosphate containing not more than 15% ammonium mono(N-ethyl-2-perfluoroalkylsulfonamido ethyl) phosphates where the alkyl group is more than 95% by weight C₈ and the fluorine content is 50.2 to 52.8% by weight as determined on a solids basis.

5. A method of preparing an article as claimed in any one of Claims 1 to 4, comprising applying the barrier coating to at least one side of the substrate, drying the barrier-coated substrate, applying the conductive coating over the barrier coating and drying the conductive coating.