US2304287A - Manufacture of grease-resistant paper - Google Patents

Description

- Patented Dec. 8, 1942 UNITED STATES- PATENT OFFICE Milton 0. Schur, Berlin, N. 11., assignor, by mesne assignments, to Reconstruction Finance Corporation, Washington, D. 0., a corporation oi." the United States of America Application March 23, 1939, Serial No. 263,664

13 Claims.

This invention relates to the manufacture of grease-resistant paper and more especially of paper such as is to serve in the food-wrapping and food-packaging art for such purposes as lining bags, boxes, or other containers for greasy or oily comestibles of the nature of coffee, cocoa, peanuts, potato chips, cookies, crackers, etc. The grease-resistant or greaseproof paper product hereof may be used to advantage as a liner in an ordinary paper bag or in a box or container of the usual paperboard variety. It may also be used by itself, that is, without an external wrapping, bag, or container, as a food-wrapping or food-packaging medium.

Such grease-resistant papers as have heretofore been available for food-wrapping or foodpackaging purposes are quite expensive. This is especially true of parchment paper and is also true of glassine paper, which latter paper is so brittle and deficient in flex-endurance and tearduring machine-fabrication or filling of the package or once the package in which it serves as a liner has been opened. and the liner is exposed to handling,

The present invention provides at comparatively low cost a greaseproof paper product that is not only serviceable in lieu of glassine and parchment papers in the food-wrapping and food-packagin art but that is also characterf ized by much higher flex-endurance and tearresistance than glassine paper. As an illustration, the greaseproof paper product hereof may be supplied to bakeries, wherein it may advantageously replace the more expensive glassine paper for lining the paperboard boxes in which shortened crackers, cookies, and other bakery products are packed and sold, As another illustration, the greaseproof paper product hereof may be sold to cofiee-packing establishments, wherein it may be used instead of the more expensive greaseproof' papers heretofore used in making the liner bag of a two-ply paper bag for holding and vending coffee. Indeed, if the outer coffee bag is prepared from paper according to the present invention, the inner bag may be dispensed with, thereby lending a substantial saving in the cost of the package.

In producing. the greaseproof paper product hereof, an ordinary non-grease-resistant paper base, such as bleached or unbleached kraft paper, may serve as the starting material. The paper base, which may contain, if desired, a small amount of such usual sizing material as rosin, starch, or the like, is preferably primed on at least one surface with a composition containing bentonite and preferably also a suitable binder, such as cooked starch, casein, glue, etc. After the paper surface has been primed with an aqueous priming composition containing bentonite and water-soluble binder and the primed surface has been dried, the resulting primed surface is resistance as to be apt to be fractured or torn talc, calcium carbonate, or the like, it is much coated with rain or equivalent grease-resistant I prolamine in the form of a solution in a suitable volatile organic solvent, such as ethyl alcohol, and the coated surface dried. Only an extremely thin coating of the zein on the preferably bentonite-primed paper surface is necessary for developing a finished paper product of remarkably high grease-resistance.

The priming of the paper surface with bentonite composition is preferred in that it facilitates the realization of the desired grease-resistant or grease-proof paper product hereof.-

Thus, when priming of the paper surface is effected with other filling or priming compositions, such as compositions containing ordinary clay,

more difficult to produce a paper product of unimuch zein is subsequently applied as a coating to the primed surface as to render less feasible economically the manufacture of a paper product competitive with glassine paper. .It is evidently the case that the colloidal clay known as bentonite is outstanding in its ability to deposit within the pores on a normal paper surface a priming film of such tightness, continuity, or fine texture that it can take on a very thin zein coating practically free from such gaps, disruptions, orpinholes as would detract seriously from or vitiate the greaseproof quality desired in the finished paper product. Again, the desired greaseproof quality is attained in the finished paper product hereof while using only such small paper. Along with the bentonite as the priming substance, it is preferred to use cooked starch as the binder, for the combination of bentonite and cooked starch in the priming composition has been found to give excellent results. Thus,

' a priming composition containing bentonite and cooked potato starch has been found to be much better for the purposes hereof than a priming composition containing bentonite and such other binders as casein, glue, etc.

' While not limited thereto, I shall now give a typical example of procedure leading to a greaseproof paper product which resists for very long periods of time penetration by grease, for instance, when put to such exacting service as the liner bag of a two-ply paper bag for holding and vending ground coffee. Such procedure will be described with reference at the appropriate time to the accompanying drawing, whereof Figure 1 illustrates diagrammatically and conventionally preferred instrumentalities, including a pair of rolls, for applying the zein solution to the primed paper sheet and Figure 2 represents a fragmentary section through the rolls as the paper sheetis passing therebetween and is receiving a coating of the zein solution.

The paper base used as the starting material may be a rosin-sized kraft paper sheet of the ordinary wrapping paper type composed of wellbeaten pulp and having a basis weight of, say, about to pounds (i. e., the weight in pounds of a ream of 480 sheets, 24:" x 36"). The expression well-beaten pulp as used herein means that, although the pulp has been beaten to a stage well beyond that customary in the manufacture of, say, ordinary wrapping paper, it is none the less at a stage of hydration or gelatinization far short of that developed in glassine paper manufacture. The sheet may be run continuously either from a'roll accumulation or from the dry end, of a papermaking machine through an ordinary size-press or tub containing the priming composition hereof. A satisfactory priming composition for the purposes hereof may consist of an aqueous suspension of about 3% bentonite and about 3% cooked potato starch. The size-press may be operated to apply the priming composition to either or both faces of the sheet. Ordinarily, however, it is desirable to apply the priming composition to both faces of the sheet, since the composition is comparatively inexpensive and only a very small amount need be deposited on the sheet and infused into its surface pores. In such latter case, the sheet may be progressively led through the bath of priming composition and thence into the nip of the press rolls forming part of the size-press. Pursuant to such operation, sheet as it issues primed on both its faces from the size-press may have taken up about 0.5% to 2% bentonite and about 0.5% to 2% starch, based on the dry weight of the starting sheet.

The primed sheet issuing from the size-press the sheet at the'nip, a small bank of solution being constantly maintained at the front or ingress of the nip. It is desirable to prevent a coating of dried zein from building up on the upper roll periphery by reason of evaporation of solvent from the film of zein solution remaining on such periphery as the sheet leaves it. While this may be done by housing the upper the v The dried, primed paper sheet may .paper surface.

roll in a suitable metal enclosure, it is accomplished simply and advantageously, as illustrated in Figure 1, by causing the paper sheet as it leaves the nip to wrap over the top roll for as much of its periphery as possible, preferably a peripheral surface of about ormore.

It will be observed from Figure 1 that the primed paper sheet S is progressively passed at an angle upwardly into the nip of the press rolls III in front of which is maintained a small bank B of the zein solution. The zein solution may be continuously fed onto the upper paper surface from a nozzle ll whose discharge end I! is disposedsubstantially centrally and immediately in front of the nip and slightly above the The solution may be carefully metered or regulated in its flow onto the paper so as to maintain the bank B of substantially constant size sufllcient to coat the paper sheet substantially uniformly throughout its entire width or upper surface and at the same time to avoid appreciable overflow or extrusion of solution beyond the paper edges. It is preferable that one of the two rolls in be peripherally covered with a yielding or resiliently soft substance, for example, soft rubber, and that the other roll present a hard peripheral surface, for example, a smoothly polished steel 'surface. As shown, the lower roll has a soft-rubber periphery "a,

whereas the upper roll has a smooth steel perlph cry. The upperroll may be urged or pressed against the paper by a dead weight or by springs in the usual manner.

The soft-roll periphery ensures uniform pressure between the paper sheet and the hard rollperiphery and thereby fosters the deposition of a substantially uniform coating of zein solution on the preferably primed paper sheet. When it is realized that a paper sheet normally varies in thickness at least a fraction of a thousandth of an inch and that the zein coating is of a thickness amounting to a fraction of a thousandth of an inch, the desirability of providing substantially uniform pressure over the entire upper sheet surface, as it is being coated with zein solution, is at once recognized.

It is highly desirable that the sheet entering the nip between rolls in, i0 wrap the lower rollv for a material distance so as to seal the edges of the paper sheet and thereby prevent the coated solution from running under the sheet edges. If such running of coating solution under the sheet edges were permitted, not only would needless waste of relatively expensive coating solution be occasioned, but the roll of paper sheet finally accumulated at the end of the zein-coating machine would lack uniformity in that the edge portions of the sheet would be much harder than the rest of the sheet; and the edge portions would tend to stick together or block" with attendant difliculty in unwinding the sheet from the roll.

As illustrated in Figure 1, the sheet S, as it is being coated with the zein solution, wraps thelower roll l0 so that the bank of coating solution II cannot creep' under the sheet edges; and such wrap of the lower roll by the sheet is over a substantial arc, say, upwards of about 10, the particular arcuate angularity or extent of the wrap depending upon the size of the roll. In order to realize maximum benefit from the wrap on the lower roll l0, it is desirable to station the two rolls with their axes in the same vertical plane or with the axis of the upper roll somewhat onset in the direction of traveLof the paper sheet through the nip. The long wrap of the paper 2,304,987 'sheet about the upper roll periphery avoids such long-time exposure of the residual film ofzein solution on such periphery as would be attended by drying and setting of such film.

Although zein solution as it is applied to the paper sheet may vary in its concentration and contain various volatile organic-solvents of zein, it preferably consists of a zein solution of about 20% to 25% strength or concentration in ethyl alcohol. Such a solution has a consistency or viscosity appropriate for coating at about room temperature the preferably primed paper sheet by the particular and advantageous method just described. The conditions of coating are prefer.- ably such as to deposit the zein on one face of the paper sheet in amount of only about 1 to 2 pounds per ream (2880 square feet) of paper. Indeed, with an appropriate base paper. sheet and an appropriate priming treatment, it is possible to arrive at the greaseproof quality desired in the finished paper product hereof with a zein coating composition applied to only one face and amounting, on a dry basis, to as low as about 0.4 pound of such composition per ream of paper.

The paper sheet thus thinly coated with zein solution may be progressively run through an airdrier, for instance, through a warm-air tunnel, to promote complete removal of the ethyl alcohol solvent. The resulting zein-coated paper sheet may then be wound in rolls and shipped to the consumer. It has been found that when such sheet is used as the liner bag of a two-ply paper bag for holding ground coffee, its greaseproofness is such that no stain is visible on the exterior surface of the liner even after its interior or zein-coated surfacehas been in contact with its ground coffee contents for many months. It

might be noted that the greaseproof paper product hereof can be handled in the usual bag-feeding,- bag-folding, an pasting equipment without the slightest difficulty; and the finished bag may without danger of tearing or other difilculty be filled with coffee or other comestibles by automatic bag-filling machinery. It will be appreciated, of course, that the paper product herein might be coated with zein on both its faces, but it is usually desirable from the standpoint of economy to coat with zein only the face of the paper product to come into contact with greasy or oily comestibles.

In one example of procedure, kraft. pulp was beaten to a Canadian freeness of 350 cc. and then was converted into 25'-pound paper (weight per 2880 sq. ft.) in the conventional way on a Four- I drinier machine. The paper showed an oil resistance of only 5 seconds on the wire side and seconds on the top or so-called web side. The sheet was then passed through a size press wherein it was treated with an aqueous mixture of bentonite and cooked potato starch containing 3% of each. After it had been redried it showed an oil resistance of 35 and 110 seconds on the wire and web" sides, respectively. The oilresistance values were determined by noting the time for lubricating oil of S. A. E. viscosity 30.

to break through the sheet over about 90% of the surface of 1 inch square specimens floated on the oil at .70 F. A zein solution was then applied to the web side of the bentonite treated sheet, and, although only 1.7 pounds of dry coating was deposited per ream (2880 sq. ft.), the sheet was substantially oil-and-turpentine-proof.

The web"- side of the sheet was coated, since this side usually gives the better result with regularity. In a second case, the pulp was beaten to being coated with zein.

a freeness of 225, the 28-pound paper made from it had an oil resistance of 25 seconds on the wire side and 90 seconds on the web side, prior to the bentonite-starch tubbing, and an oil resistance of 70 and 250 seconds on the corresponding sides after the tubblng and redrying. When the web side .was coated with only 1.2 pounds, dry basis, of a mixture of 3 parts of zein and 1 part of invert sugar deposited from an alcohol solution containing a small proportion of water, it was substantially oil-and-turpentine-proof. When to the bentonite-starch tubbing mixtures a little ammonia water or equivalent defiocculating agent is added, the oil resistance of the tubbed sheet is even higher than above indicated. In any case,it is significant that the ash-content of the paper was increased less than about 1%% when the sheet was primed with the bentonitestarch mixture, and thatno coating of bentonite or starch was visible on the paper prior to its' In lieu of bentonite, it is possible to use for the purpose ofpriming the paper surface an appropriately colloidal clay, preferably a colloidal clay that has been additionally peptized in water containing ammonia or equivalent clay defiocculating agent. The use for the paper-priming purposes hereof of the colloidal fraction of china clay, for instance, such fraction as is separated from the whole clay by an electrophoretic fractionating method, together with ammoniated water as the suspending medium for such fraction leads to results somewhat similar to those had from bentonite; and the term "bentonite as used in the appended claims is meant to include such colloidal clay fraction. However, bentonite as terized by the firmness or high tenacity of its I .etc.

such is generally to be preferred because of its lower.cost and the consistently excellent results realized therefrom. Again, it is possible to use in lieu of zein on the bentonite-primed paper surface such other grease-resistant, film-forming,

solid substances as nitrocellulose, acetylcellulose,

bond with the primed paper surface, insomuch thatsuch coating or film has practically no tendency to peal or break away from the paper base in th course of handling the greaseproof paper product hereof 'or of fabricating it into liner bags,

liners, etc.

Because of the extreme thinness of the zein coating solution on the preferably primed paper sheet hereof, the tendency for curlingof such paper sheet during its drying is min mized. Such minimization of curling is-especially important in the case of thin or lightwe ght paper bases,

whose res stance to curling is low, especially when the finished paper product hereof is intended to be cut into fiat sheets or blanks for lining paperboard boxes. -It is, moreover, possible to incorporate into the zein solution plasticizing agents, such as invert sugar, glycerine, ethylene glycol,

and other substantially non-volatile alcohols or glycols that serve to offset curling tendency in the finished paper product hereof.

The zein-coated paper product hereof lends itself particularly well to coating very thinly with wax, such as paraflln, to yield a paper product highly resistant to the passage of water vapor therethrough. Thus, the paper product hereof may be made remarkably moistureproof as well as greaseproof' by being coated on its zein-coated face with molten paraflin wax or its equivalent in amount as little as about 2 to 4 pounds of paraffin wax per ream. The reason why such small amounts of wax are effective in contributing high moisture-impermeability to the waxed paper product hereof is. because .the zein coating on such product keeps the paraffin wax practically entirely on the paper surface, that is, prevents the wax from being dissipated into the body of the paper sheet. In some instances, it

may be desirable to produce a paper product waxed on both its faces, as by running the zeincoated base' paper sheet through a bath of molten paraflin wax. In such instances, th preferably primed paper sheet-is advantageously coated with the zein composition on both its faces, as by" running the preferably primed paper sheet through a bath of the zein coating composition or by spreading such composition first on one face and then on the other. The resulting paper product waxed on both its faces may carry no more than about -2 to 4 pounds ofpara'flin wax per ream of paper in acquiring the desired high moistureproofness. It might be noted that the application of the zein composition and wax to both surfaces of the paper sheet minimizes tendency for the paper product to curl, since such shrinking or curling forces as may be incident to the application of the zein composition and/or wax to one face of the sheet are substantially neutralized by similar forces incident to the application of the zein composition and/or wax to the other face bf'the sheet. v

- The improvement in the oil or grease-resistance of the base paper effected by the bentonitestarch. composition or tubbing is important because it makes possible, by keeping the filmforming medium practically entirely on the surface, the attainment of substantially complete imperviousness with an exceedingly small quanprimed with the bentonite-binder composition, a

finished paper sheet of the desired greaseproof quality even when the greaseproofing zein coating composition is extremely thin or attenuated, for instance, is spread on one face of the paper sheet in amount ranging from only about 0.4 to 2 pounds of the dry zein composition per ream (2880 square feet) of paper. Because the dried The valuable factor of priming the base sheet need not, however, necessarily be utilized, provided that the factor of beating or hydrating the pulp entering into the paper sheet or base is ap propriately correlated with the aspect of the zein-coating composition to foster the greaseproof quality desired in the finished paper product along with such other qualities as the desired high flex-endurance and tear-resistance.

in realizing the highly greaseproof paper products hereof, spread more zein coating composition than that already indicated as yielding a very thin or highly attenuated film on the paper base,

' for instance, zein coating composition amounting on a dry basis to no more than about 0.4 to about 2 pounds per ream surface (2880 square feet). As already indicated, such a very thinly zeincoated paper product has the desired high flexenduring and tear-resisting qualities coupled with the desired greaseproofness.

The terms "greaseproof or grease-resistant," as used in the foregoing description and in the appended claims in characterizing a paper product hereof, mean that such product shows no sign of oil-staining or oil-leakage when such product is subjected to the so-called turpentine test. This test is conducted by supporting the paper product specimen with its zein-coated face up on a sheet of white absorbent paper, placing on such upper face a. small mound of fine sand, and adding to the sand a slight excess of moisture-free turpentine containing a small amount of a suitable dye. After about 4 to 6 hours, the specimen is lifted from the underlying absorbent paper support. Should the absorbent 'paper support display no sign of staining by the dyed turpentine, the specimen is considered to be greaseproof. It might be noted, however, that the greaseproof or grease-resistant paper products hereof usually are vastly more grease-resistant or greaseproof than that necessary to satisfy such conventional test. Indeed, the products hereof usually show no transmission therethrough of turpentine even after a. test period of about 12 to 16 hours.

I claim: I

1. A grease-resistant paper product primed on its surface with bentonite and cooked starch and filmed on such primed surface with grease-resistant prolamine.

zein composition exists in such attenuated or thin 2. A grease-resistant paper product comprising a substantially non-grease-resistant paper base primed on its faces with a mixture of bentonite and binder and filmed on at least one primed face with grease-resistant prolamine.

-3. A grease-resistantpaper product primed on its surface with bentonite and cooked starch and filmed on such primed surface with zein in amount of only about 0.4 to 2 pounds per ream surface (2880 square feet).

4. A grease-resistant paper product comprising a substantially non-grease-resistant paper base primed on its faces with a mixture of bentonite and cooked starch and filmed on at least one primed face with a film containing greaseresistant prolamine as an essential constituent,

said film being present in amount of only about 0.4 to 2 pounds per ream surface (2880 sq. ft.).

5. A grease-resistant paper product primed on its surface with bentonite-binder composition and filmed on such primed surface with zein, said zein containing a plasticizing agent.

6. A grease-resistant paper sheet primed on its surface with bentonite-binder composition and filmed on such primed surface with zein, said zein containing invert sugar.

'7. A paper sheet of high grease-resistance and low water-vapor penetrability, said sheet being primed on its surface with bentonite-binder composition, being filmed on such primed surface with zein, and being waxed over said zein.

8. A paper sheet of high grease-resistance and low water-vapor penetrability, said sheet being primed on its surface with bentonite and cooked starch, being filmed on such primed surface with zein in amount of only about 0.4 to 2 pounds per ream surface (2880 square feet), and being waxed over said zein with only about 2 to 4 pounds of wax per ream surface.

9. A process of producing a grease-resistant paper product, which comprises priming the surface of a paper base with an aqueous composition containing bentonite and water-soluble binder, drying the primed surface, filming the primed surface with an alcohol solution of zein, and drying the filmed surface.

10. A process of producing a grease-resistant paper product, which comprises priming the sur, face of a paper base with an aqueous composition containing bentonite and cooked starch, drying the primed surface, filming the primed sur- 5 face with an alcohol solution containing zein as an essential constituent in amount to deposit only about 0.4 to 2 pounds of filming substance per ream surface (2880 square'feet) and drying the filmed surface. g

11. A process of producing a grease-resistan paper product, which comprises priming the surface of a paper base with an aqueous composition containing bentonite and cooked starch, drying the primed surface, filming the primed surface with an alcohol-water solution of zein and invert sugar, and drying the filmed surface.

thoroughly beaten state greater than that of the pulp used in kraft wrapping paper manufacture but far short of the gelatinized state of pulp entering into glassine paper manufacture, said base being of a weight ranging from about 15 to pounds per ream surface (2880 square feet) and being filmed with prolamine in amount of only about 0.4 to- 2.0 pounds per ream surface.

said product exhibiting flex-enduring and tearresisting qualities markedly superior to those of glassine paper of similar basis weight.

MILTON 0. SCHUR.

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