US3508952A - Coated paper article and process therefor - Google Patents

Description

United States Patent Olhce 3,508,952 Patented Apr. 28, 1970 3,508,952 COATED PAPER ARTICLE AND PROCESS THEREFOR Robert William Eykamp, Silver Spring and Charles C. Kirk, Laurel, Md., assignors to W. R. Grace & Co., New York, N.Y., a corporation of Connecticut No Drawing. Filed June 28, 1967, Ser. No. 649,475 Int. Cl. B44d 1/16; D21h 1/22; B32h 29/04 U.S. Cl. 117-76 10 Claims ABSTRACT OF THE DISCLOSURE The process of forming an impermeable composition which comprises imparting directly onto a smooth cellulosic substrate 0.5 to lbs. per 3000 ft. of said substrate of a plate-like clay coating material having an axial ratio in the range 20300:1 and a maximum length of about 30 microns and the composition therefor. The clay is pretreated with a salt of a member of the group consisting of chromium, zirconium and mixtures thereof. Additionally, if desired, the thus treated substrate can be top coated with conventional polymeric compositions.

This invention relates to a novel and useful composition, a process utilizing the composition and the product resulting from the process. More particularly it is directed to coated cellulosic substrates, a process for making said substrates and the gas impermeable cellulosic substrate resulting from said process.

Heretofore it was not thought possible to be able to coat directly onto a cellulosic substrate with a plate-like filler since the wicks, hills and valleys, i.e. unevenness of the cellulosic substrate afforded too much disorientation to the coating material to yield good barrier properties. Thus, the present state of the art teaches coating the cellulosic substrate first with a polymer coating, and thereafter coating it with a plate-like filler to otbain good barrier properties. Surprisingly we have now found that coating directly onto the cellulosic substrate with a platelike coating material affords good gas barrier properties.

One object of this invention is to provide a process for decreasing the gas permeability of cellulosic substrates. Another object is to provide a gas impermeable cellulosic substrate. Other objects and advantages will become apparent from a reading of the following description of the invention.

The aforestated objects are accomplished by the present invention which provides a composition comprising:

(a) A cellulosic substrate and (b) 0.5-5 lbs. per 3,000 sq. ft. of said cellulosic substrate of a plate-like coating material having an axial ratio in the range 20-30021 and a maximum length of about 30 microns.

The present invention also provides a process for forming a coated cellulosic substrate which comprises:

Coating a cellulosic substrate with 0.5-5 lbs. per 3,000 sq. ft. of cellulosic substrate of a plate-like coating material having an axial ratio in the range 20-300:1 and a maximum length of about 30 microns at a temperature in the range 0-100 C. and thereafter drying the coated substrate by heat, evaporation or other conventional means.

It is also possible by the instant invention to top coat the coated substrate with film-forming polymers. Filmforming polymers which exhibit excellent results include but are not limited to homopolymers of vinyl chloride, copolymers of vinylidene chloride with monomers selected from the group consisting of methyl acrylate, ethyl acrylate, acrylamide, and acrylonitrile, homopolymers of vinyl acetate and copolymers of vinyl acetate with monomers selected from the group consisting of methyl acrylate, ethyl acrylate, acrylamide and acrylonitrile. Excellent re sults have also been obtained using copolymers in which polyvinyl chloride was the principal constituent. When using copolymers of vinylidene chloride, it is preferred to use those in which polyvinylidene chloride constitutes the major component; however, excellent results with copolymers of vinylidene chloride in which the vinylidene chloride is not the major component having also resulted in satisfactory top coats. When using the copolymers of vinyl acetate, it is preferred to use those in which the polyvinyl acetate is a major component, however, top coats wherein the polyvinyl acetate is present in minor amounts are also operable in the instant invention.

Numerous other top coating polymers and copolymers which will give excellent results in the process of this invention will, as a consequence of the disclosure contained herein, be readily apparent to those skilled in the art.

Superior top coats have been obtained from the use of aqueous dispersions of film-forming polymers, i.e., filmforming latexes, in the process of the instant invention. Excellent results have been obtained with such latexes in which the polymer particles have diameters ranging from about 200 A. (Angstrom units) to about 100,000 A.; however, it is preferred to use polymer particles ranging in diameter from about 700 A. to about 4500 A. The amount of topcoat employed can -very between wide limits. Thus amounts of polymer in the range of 1 to 50 pounds/ 3000 ft? of substrate are operable in the present invention. Other operable top coating materials such as lac quers, extrusions, and hot melts will be mentioned here inafter.

It has been found that substantially any substance-mineral, inorganic, or organic-existing in the form of platelets or particles having a plate-like structure (i.e. existing in the form of platelets having an axial ratio of about 20-30021 and a maximum length of about 30 microns), can be used with excellent results as a plate-like coating material in the present invention. Thus, by the term plate-like coating material, is meant at least one substance having a plate-like structure selected from the group consisting of mineral, inorganic or organic materials which, when applied as a coating in the instant system renders cellulosic substrates substantially impermeable to gases such as oxygen, nitrogen, air, carbon dioxide and the like. Said substances, when mineral, can be naturally occurring mineral, a synthetic mineral or a chemically modified mineral. Examples of such materials which can be used as plate-like coating material in the instant invention include, but are not limited to, graphite platelets, platelets of metals such as aluminum, platelets of alloys such as bronze, and organic substances having a plate" like structure (e.g. N-benzoyl acetamide), and the like. It has also been found that minerals having the above defined plate-like structure (e.g., platy clays, mica, vermiculite, kaolinite, montmorillonite, platy talc and the like) are operable as plate-like coating materials in the instant invention. In addition, the immediate aforesaid plate-like coating materials (i.e., minerals, especially' those having exchangeable cations) can be modified (i.e., by treating them with ions selected from at least one member of the group consisting of aluminum (III), positive ions of transition elements and mixtures thereof). By transition elements, are meant elements having atomic numbers 21-32, 39-51 and 57-83. Generally such modification is provided by treating said minerals with iron (III), chromium (III), and zirconium (IV) ions. This latter treatment With ions makes the aforestated minerals less susceptible to hydration and the disintegration action of water vapor present in the atmosphere, thereby rendering the coated substrate more impermeable. Other wellknown plate-like coating materials are obvious to one skilled in the art.

- The amount of plate-like coating material 'used a coating in the instant invention is between 0.5- lbs. per 3,000"sq. ftl of cellulosic substrate. it is also possible the instant invention to adrnix the plate-like coating inaterial with polymeric material and form a matrix structure, which can then be used as the coating. Conventional polymers which are -operable in the instant 'invention 'are well known to those skilled in the art, and memes; but are not limited to, polyvinyl acetate, polyvinyl methyl "ether, vinylidene chloride copolymers'su ch' as vinylidehe 'chloride-vinyl chloride, styrene-butadiene copolymers and acrylic polymers The amount of polymer operable in the matrix'structur'e as a coating'in the instant inventionis in the range "OJ- 60%, preferably 540% by Weight based on the weight of the plate-likefiller-polymer coatingco mpositio npiel'the matrix structureif I It is also operable, and sometimes desirable, ,in the; instant invention to employ the plate-like material per be as a coating material without the addition of polymer. In 'such. cases 'it 'is preferred toapply the plate-like; sjtructure as a dispersion'or suspension to coatthe cellulosic substrate. s

' When using clay-like minerals which have exchangeable cations (i.e. vermiculite, mica, talc, clays and the like having a plate-like structure) as a coating for a cellulosic substrate, it has been found that the permeability to humid, or to moist air of said substrate can be greatly decreased by modifying said plate-like materials. As 'used herein the term modifying clay-like minerals means to treat said minerals with at least one salt of at least one element selected from the group consisting of aluminum, the transition elements and mixtures thereof, whereby the prmeability to humid air of cellulosic substrates coated with the thus treated minerals is substantially less than that of similar substrates coated with the same minerals which have, not been treated with at least one salt of are ininum or a transition element. The amount of ions of aluminum, transition element or mixtures thereof added to treat the mineral is an amount sufiicient to satisfy 100% of the cation exchange capacity of the mixieral.

Cation exchange capacity of the mineral as used herein 'means the number of milliequivalents per unit weight of mineral of exchangeable cations in the mineral. The term modified clay-like material means claylike minerals which have been subjected to the above mentioned inedifying treatment.

Since commercially available clay-like minerals contain impurities (such as silica, particles of rock and some very large particles of clay-likeminerals), it has been found desirable to refine such minerals before using them in the process of this invention. Although various methods to accomplish such' refining will be readily apparent to those skilled in the art, it is preferred to accomplish this by forming a suspension ofabout 01-10%. of the clay-like mineral in water, agitating the'resulting mixture vigorously and addingasmall amount (OJ-5% based on the we ght of the clay-like mineral present) of a dispersing andexfoliating agent such as sodiumpQlyphc-sphate, sodium hydroxide, sodium carbonate, sodium oxalate, sodium phosphate, or the like to the resulting slurry, thereby dispersing and exfoliating the clay-like mineral. After thorough mixing, the resultant'dispersion (or slurry) is separated by centrifuging or decanting same from the new remaining l'arge'particles. I Y

The dispersion is "now ready'for treatment with a solution of at least one salt of aluininumora transition element wherebythe properties of the clay-like minerals (especially resistance to water vapor) are improved...It is prferred to' conduct such treatment by addingto the dispersion of "clay-like mineral, a ,solution of at least one salt of; at least one element selected from thev group. consisting oflaluminum and the transition elements, in which saidelementis present asacation, while stirring the dispersionas the ,solutionfof said salt is added slOWlythereto. The dispersion becomes more flocculated as the solution of said salt is a'dded until flocculation is substantially complete. .The clay-like .mineral is, then separated by conventional means such as decantation, filtration or centre fugation and washed to remove any salts or other 1mpurities which are present in the mother liquor or adhering to the clay-like mineral. In another embodiment of this invention the clay-like mineral is "treated with t least one ion selected from the'group consisting of ons of aluminum and'the transition eleiri'ent's by passing a dispersionor slurry of said mineral throughan ion exchange column which has been prepared by pass ng acoucentra-ted aqueous solution of at least one" salt of. at least one element selected from the group consisting of alu- .a conventional ion exchange resinin the hydrogen form,

therebyv toconvert saidresin from the hydroge'n'form to ,the chromium form. The dispersion of clay1 lil e mineral is then passed through the ion exchange resin and the thus treated clay-like mineral is" recovered and washed. it has been found that passing the dispersion ofclaydiker'nnieral through a resin bed at a rate of about 01005-03 bed volume per minute yields excellent results. I The resulting modified clay-like mineral is separated, washed and redispersed as a slurry for use as a coating material in the process of the instant invention. When using graphite, aluminum platelets, bronze platelets orsimilar metal platelets and plate-like organic materials, it has not been found necessary to treat said graphite or metal or organic platelets with a compound of aluminum or a transition elements. Such latter materials can be made up ina dispersion, suspension or slurry, agitatedand then separated by decantation or centrifugation to remove any large particles which may be present. The resulting slurry is then ready for use in the process of the instant'invention. The above treatment of the clay-like minerals can be used whetheror not the clay-like mineral is employed per se as the coating or whether it is employed in combination with a polymer (a matrix structure) to coat the cellulosic substrate. r r p The resultant impermeable cellulosic substrate product ofthe present invention has many and varied uses. For example, it can be used most efiectively as a packaging "material for foodswhich require protection from air and moisture. That is, it can be used to packag e cheese, butter and snackfoods such as potatochips. r

The general procedure for forming the impermeable cellulosic substrate of the instant invention is to prepare the plate-like coating material as aforesaid to remove impurities and large particles therefroin When the platelike coating material is a clay-like mineral it can be modified if desired by dispensing same in water and treating said dispersion with as'olution ofat'least one salt of aluminum or a transition element in which said element is present as a cation while stirring the dispersion a s a solution of said salt is added slowly thereto a't' robin temperature. As the dispersion becomes substantially completely flocculated the clay-likemineralis separated therefrom'by conventional means "such addecantation,

filtration or centrifugation. .The separated clay-like material is then washedto'remove 'anysalts or other impurities "which arepresentinthe mother liquor or adhering'to the clay-like mineral. I

[If desired'the plate like coating material can formed into a matrix structure by adding same to a' polymer. The

addition of the p late like coating materialto the polymer is preferably accomplished in an aqueous system whereby the plate-likecpating material is present. in concentrathe polymer in an aqueous dispersion in an amount ranging from 0.15 to solids if desired. The aqueous dispersion of the coating material is then applied to a cellulosic substrate, e.g., glassine on a coating machine. The plate-like material is applied from an aqueous suspension by means of a roller passing through said aqueous suspension which thereafter contacts the cellulosic substrate passing thereover. The excess coating is doctored off by conventional means, e.g., air knife, Mayer bar, Baker film applicator bar, or doctor bar in a conventional manner. The thus coated substrate is then passed to a heated oven wherein the coating is dried on the substrate at temperatures in the range ZOO-400 F. Thereafter, if desired, a top coat of polymer can be applied to the coated substrate to improve moisture barrier properties. The top coat also improves the mechanical properties and the abrasion resistance of the coatings. This is accomplished by running the coated substrate over rollers which have contacted either an aqueous latex of the polymer or a solvent solution of the polymer and thereafter drying the top coat by conventional means such as heating, evaporation, etc. It is also possible to apply the top coat by a hot melt extrusion of the top coat polymeric material onto the coated substrate or by calendering or other conventional means.

Throughout the instant invention the oxygen permea ability was measured under conditions specified in ASTM D1434 63. Oxygen permeability is the rate of flow of oxygen through a unit area of the material having unit thickness per unit pressure difference and time. Because the thickness of the plate-like coating material that functions as the gas barrier is defined by the coating weight per 3000 ft. the usual thickness dimension is not specified in our data. In all cases, measurements were made by exerting a gas pressure difference across the coated substrate and measuring the amount of gas Which difiuses through in a period of time. Units are:

cc.=cubic centimeters of diffused oxygen, reduced to standard temperature and pressure atm.=pressure difference in atmospheres m. =area of coated substrate in meters D=time in days.

Moisture vapor transmission was measured in accord with procedure A of ASTM E96-63T except that the humidity outside the dish is maintained at 75% RH.

The following examples are set out to explain but expressly not to limit the instant invention. Unless otherwise noted all parts and percentages are by weight.

For ease of explanation the coatings employed in the examples will be a modified clay material.

EXAMPLE 1 A suspension of about 2% of montmorillonite clay in water at about 70 C. was agitated with rapid stirring and treated with sodium polyphosphate as a dispersing and an exfoliating agent. About 0.5% sodium polyphosphate, based on the weight of the clay present was added. When dispersion was substantially complete after about 2 hours, stirring was discontinued and a small amount of larger unexfoliated clay particles and a few rticles of rock, silica and the like were allowed to settle out. The dispersion was decanted from the particles which had settled to obtain a dispersion of a purified clay-like material, i.e. purified montmorillonite clay. A one molar solution of chromium (III) chloride was slowly added to the suspension of purified clay while stirring said suspension. As the chromium salt was added, the clay flocculated slowly until flocculation of the clay was finally complete. The thus flocculated clay was separated from the supernatant liquor and washed with water, to yield a modified clay suitable for use as a plate-like coating material in the process of this invention. The thus Washed modified clay was redispersed in an aqueous solution of sodium polyphosphate (about 8.0% sodium polyphosphate, based on the weight of the modified clay) at about 70 C. with sufficient water to form a dispersion containing about 8% of the modified clay. The resulting dispersion of modified montmorillonite clay was labeled Dispersion No. 1.

EXAMPLE 2 A tray of Dispersion No. 1 of modified montmorillonite clay was positioned underneath a roller such that on rotation the roller picked up a coating of the clay dispersion and thereafter deposited same on a cellulosic substrate, i.e. 30 lb. Riegel glassine paper (12" width) passing over the roller at a speed of 50 ft. per minute. The excess of the coating was doctored off with an air knife so as to leave a coating of 1 1b. of solids per 3,000 sq. ft. of glassine paper. The thus coated paper, by means of rollers, was then passed through an oven at 300 F. for 10 seconds. The thus coated paper was collected on a takeup roll. On characterization the oxygen permeability at 25 C. was 233 cc./atm. m. day. A control run of the same glassine paper uncoated had an oxygen permeability at 25 C. of 2.4)(10 cc./atm. m? day.

EXAMPLE 3 Example 2 was repeated with the addition that the thus coated glassine was top coated by passing it through a 50% aqueous dispersion of a vinyl chloride acrylic latex sold under the trade name Geon '0X20 by B. F. Goodrich Co. The excess topcoat was doctored off with a #6 Mayer rod, and then passed through a heated oven for 10 seconds at 325 P. On characterization the top coated glassine paper had an oxygen permeability at 25 C. of 10.8 cc./atm. m. day and a moisture vapor transmission of 29.4 grams/m. day. A control run consisting of glassine paper with a coating of Geon 450X20 but without the coating of montmorillonite clay had an oxygen permeability at C. of 251 cc./atm. m? day and a moisture vapor transmission of 44.5 grams/m. day.

EXAMPLE 4 Example 3 was repeated except that the topcoat consisted of a vinyl acetate copolymer latex containing solids sold under the trade name Daratak B by Dewey & Almy Co. On characterization the montmorillonite coated glassine, topcoated with Daratak B had an oxygen permeability at 50 C. of 3.9 cc./atm. m. day. A control run of glassine and Daratak B top coating without montmorillonite showed an oxygen permeability at 25 C. of 91 cc./atm. m? day.

What is claimed is:

1. An article of manufacture having improved barrier properties consisting of (a) a paper substrate and adhering directly thereto,

(b) 0.5-5 pounds per 3000 square feet of said paper substrate of a plate-like clay coating material having an axial ratio in the range 20-300:1 and a maximum length of about 30 microns, said clay having been pretreated with a salt of a member of the group consisting of chromium, zirconium and mixtures thereof in an amount suflicient to satisfy 10l'00% of the cation exchange capacity of the clay at a temperature in the range 0-100" C.

2. The article of manufacture of claim 1 wherein the plate-like clay coating material has admixed therewith 0.1 to by weight of said plate-like clay coating material of a film-forming polymer.

3. The article of manufacture according to claim 1 wherein a film forming polymer is employed as a topcoat over the plate-like clay coating material.

I '4. The article of manufacture of claim 1 wherein the paper substrate is glassine and the plate-like clay coating material is montmorillonite, said montmorillonite clay having been pretreated with a salt selected from the group consisting of chromium, zirconium and mixtures thereof in an amount sufiicient to satisfy 10-100% of the cation exchange capacity of the montmorillonite.

5. The article of manufacture according to claim 2 wherein the polymer is a member of the group consisting of polyvinyl acetate, polyvinyl methyl ether, vinylidene chloride-vinyl chloride copolymer, styrene-butadiene copolymer and acrylic polymers.

6. The article of manufacture according to claim 3 wherein the film-forming topcoat polymer is a member of the group consisting of vinyl chloride; copolymers of vinylidene chloride with monomers selected from the group consisting of methyl acrylate, ethyl acrylate, acrylamide, vinyl chloride and acrylonitrile; homopolyrners of vinyl acetate and copolymers of vinyl acetate with monomers selected from the group consisting of methyl acrylate, ethyl acrylate, acrylamide and acrylonitrile.

7. A process for forming a coated paper substrate having improved barrier properties which comprises coating a paper substrate with 0.5 to 5 pounds per 3000 square feet of said paper substrate of a plate-like clay coating material in an aqueous dispersion, said clay coating material having an axial ratio in the range 20-300z1 and a maximum length of about 30 microns, said clay having been pretreated with a salt of a member of the group consisting of chromium, zirconium and mixtures thereof in an amount suflicient to satisfy 1'0-100% of the cation exchange capacity of the clay at a temperature in the range 0-100 C. and thereafter drying the coated paper substrate.

8. The process according to claim 7 wherein the coating consists of a plate-like clay coating material and 01-60% by weight of said plate-like clay coating material of a polymer selected from the group consisting of polyvinyl acetate, polyvinyl methyl ether, vinylidene chloride-vinyl chloride copolyrner, styrene-butadiene copolymer and acrylic polymers.

9. The process according to claim 7 wherein the coating is applied as a water dispersion.

10. The process according to claim 7 wherein the paper substrate is glassine and the plate-like coating material is a member of the group consisting of bentonite, and montmorillonite.

References Cited UNITED STATES PATENTS 2,341,884 2/1944 Sowa 11728 X 2,559,893 7/1951 Nadeau et a1 117-144 2,665,262 1/ 1954 Rolle et al. 26'041 X 2,721,150 10/1955 Grantham 117-145 2,795,568 6/1957 Ruehrwein 26041 WILLIAM D. MARTIN, Primary Examiner T. G. DAVIS, Assistant Examiner US. Cl. X.R. 117152,