US 2412592 A
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VARNISH PAPER LINER FACING MATERIAL Curtis E. Maier, Elmhurst, Ill., assignor to Continental Can Company, Inc., New York, N. Y., a corporation of New York No Drawing. Application July 21, 1943, Serial No. 495,659
6 Claims; i
This invention relates to a coated paper liner facing material which is suitable for use in contact with foodstuffs for sealing paper, glass and metal containers and which is particularly valuable as a component of crown caps, screw caps and similar closures as a material for resilient discs of cork, cardboard, etc., employed therein, and as a facing material for the inside of paper containers.
Such closures have been made with a resilient disc of cork or cardboard to permit accommodation to all irregularities of the container surface which is to be sealed, but because of the lack of moisture resistance of cardboard and to prevent contamination of the foodstuff by foreign flavors imparted by the cardboard or cork, it is advantageous to employ a protective facing. Various materials have been used heretofore as facings, but none of those used has been satisfactory in all respects. Some have given satisfactory results with a limited number of food products, thus requiring the manufacture and segregation of different types of facings for different food products. For example, relatively expensive aluminum foil has been extensively used as a facing in crown caps for beer, but since aluminum is lacking in resistance to acids such as those contained in citrus products, ginger ale and the like, and to mild alkalis such as those present in certain mineral waters, aluminum foil has attained only limited application as a facing material. I
Tin foil has been applied as a facing for closures, especially for the more acid and the more alkaline beverages, but it is more expensive than aluminum and like aluminum tends to contaminate the foodstuff with corrosion products that adversely affect flavor and color of some foodstuffs.
varnished paper has been employed as a facing for closures and in many respects has superior qualities. It is cheap compared to metal foils and can be made at high speed with great uniformity on comparatively simple roller coater and baking equipment. Varnished paper has excellent moisture resistance and resistance to acids and alkalis of the strengths ordinarily contained in foods, and despite its tendency to impart oily or varnish-like flavors to delicately flavored foods, enjoys very wide application in the closure industry due to its relatively low price. It has been found to be impossible to produce a varnished paper free from this objectionable flavor-imparting characteristic largely because the baking temperature used to cure the varnish is limited by the tendency of even the best grades of paper to become'brittle at baking oven temperatures. White sulphite papers may not be baked above 300 degrees F. without an appreciable loss in strength, which causes cracking or splitting of the coated paper in the closing or crowning operation. Even the special grades of heat resistant papers such as kraft and Drab Express stock begin to embrittle at baking temperatures slightly in excess of 350 degrees F. Therefore, even the high quality varnishes that produce almost flavor-free films when applied to metal surfaces such as steel, aluminum or tin plate and baked at 400 degrees F., as in the manufacture of metal containers and closures, cannot be made free from off-flavor imparting constituents when coating paper because of this baking temperature limitation.
Still another type of coated paper used as a facing for closures consists of a plasticized film of vinyl chloride-vinyl acetate coploymer calendered to a paper backing with heated rolls. While this type of coated paper has good flavor characteristics, good chemical resistance to a variety of products and fair resistance to moisture and carbon dioxide gas transmission, it is used to a much lesser extent than varnished paper primarily because of its greater cost. The high cost is due in part to the relatively high cost of the vinyl type resins and in part to the fact that the calendering operation, even with the use of the finest types of the very complicated and expensive equipment available for this purpose, cannot produce a uniform and continuous vinyl resin film in thicknesses below 0.002 of an inch. Furthermore, the unmodified vinyl copolymer of, for example, 87 percent vinyl chloride and 13 percent vinyl acetate composition, having a fairly good moisture resistance, cannot be heat-calendered. The addition of a softening agent is also essential to the satisfactory service of the vinyl resin calendered coated paper liner, since in films 0.002 of an inch thick or thicker, the non-softened vinyl copolymer is too hard and unyielding to give a'satisfactory seal between the vinyl film and the glass or metal sealing surface of the container. Such softening plasticizers are commonly added in quantities equal to 15 to 20 percent of the resin. The use of softening plasticizers is objectionable in liners for closures for food containers because of their tendency to impart foreign flavors to foods and because plasticizers tend to reduce the general chemical, water and gas resistance of the resin. Also, in times of national emergency, the few plasticizers that are least objectionable may become unavailable.
It has been discovered that all of the desirable characteristics previously described for varnished paper can be retained and the flavor imparting tendency overcome by applying a thin film of vinyl chloride-vinyl acetate copolymer over a varnished paper base, thus producing a closure facing material superior to any heretofore known to the industry. This desirable result is secured by virtue of the fact that in thin films of the order of one to five ten-thousandths (0.0001 to 0.0005) of an inch thickness (2 to 10 mg. per sq. in.) the vinyl resin has adequate inherent plasticity without the addition of any plasticizer whatever, to give a satisfactory seal between the vinyl film and the sealing surface of the glass or metal container, and in addition this very thin film of vinyl resin completely prevents the transfer to the product of the oily, off-fiavor-producing constituents of the varnish film. A film of about 5 milligrams per square inch of surface is presently preferred where the coated paper is non-rigid or where sharp folds and bends are necessary in forming the material or applying the seal.
In the practice of this invention, bleached sulphite paper may be used where whiteness is especially desired, but the more highly heat-resistant kraft papers. and particularly the special grade of unsized highly calendered kraft paper made from spruce and known as Drab Express stock is preferred. A paper thickness of four and a half to five thousandths (0.0045 to 0.005) of an inch is preferred when the coated paper is intended for use in crown and screw caps although lighter or heavier gages may obviously be employed for pan ticular purposes. Any sizing upon the paper should not be swellable by water when wet food products or the like are to be contacted, because inferior adhesion of the varnish to the paper stock as well as reduced resistance to moisture vapor penetration would then result.
The chosen paper stock is varnished, for example, by means of the usual roller coating machine, and the varnish is cured in a festoon or a tower-type baking oven. Several different math ods of varnishing, as described below, are prac ticed commercially and any of these may be employed without departing from the principle of this invention, as long as a waterand gas impermeable varnish film is produced having ade uate acid and alkali resistance to permit its use in contact with beverages, other foods and cert in industrial products.
iifhree methods of varnishing that may be employed are as follows:
1. STOP COAT METHOD temperature-induced oxidation and polymerination cause the oleoresinous varnish film to become hard and tacit-free. A second coat of varnish is then applied to the same side of the aper and baked in the same manner. Stop coat methods permit use of higher resin: oil ratios than saturating methods, since the coating does not produce brittleness to the extent of such saturatin coa ings; and the stop coating penetrates into paper base only to the extent necessa y 0 Qbtci. strong adhesion to the paper. a
3. SEEUL'I'ANEOEIB Two In this method of varnishing, the paper is dipped in or passed through a saturating or stop coat type oleoresinous varnish so that both sides of the paper are coat-ed siiiiiii -aiieoitsly. After passing through the baiting oven, the two-side varnished paper is varnished second time as before, and re-baired.
The preferred thickness of 2"" .ouble coating of oleoresinous varnish, upon the paper, is from to 40 milligrams per square Satisfactory varnishes io sired water, water vapor as impermeable varnished paper may be pro Iced from various drying oils in combination separately with various types of com tible resins, and having vari ous proportions o. 7 to drying oil. The dryhi z oils preierred e oi their suitability are tung oil, deh d caster oil and alkali isomer rorlucing the de :der
ized linseed of increasing the hardness, iernical resistance of those oils include nitecl to, phenolic (phcmoi-aldehyde) dined phenolic resins, fossil .ytlride glycerol asters of rosin.
. wishes suitsblnii these methods oi 5.
ion are as follows:
[to iizmyile No. Pounds Phenolic resin (Iriialaelite idfiriii il 100 Tune oi 4.00 Manganese resinate a- 4 Xyloi 1175 Example No. 2
Pounds Modified phenolic resin 1G0 iileliydrated caster 01L--. -u 4.98 ha up, a A manganes iimmc,t 3.-. -..-i.-c- -a-... a; liiyiol 1175 Example L};
" Lodiried nialeic resin (Lniberol 600) lilil .mcriaed linseed oil. (Kelliuol) act.
ianganese resinate Sror Corie No. 6
Ph nolic resin hhafifihmwsuii'ung' oi rnganese resinatesww Example No. 5
rounds 0 Modified pi oiic resin (M-Bil) 10G Dehydrated costar oil Manganese resinate 1 Kyle! 2.00
The manganese resinate is a drier, and may be omitted or replaced by another drier, depending hleoiteslh ch 0; surface and, *npai'tihg characteristics, w red 01 a high quality plied, e. by means of a very thin film 0f vinyl ,lly effective in prea shy film weight of 2 er square inch 01 surface the axes; type 01 vinyl e1" about one to Gll005) of an are not comthe Olly and .ars, hs 3.01385) of an inch, in the abtlelzei'. begin to show poorer gas efficiency due to their stiffness ul: to imperfections in the r ce. In addition, no 301- 30 by using quantities of the resins in thicknesses eludes those resins n of compounds ldene group, more s tired from vinyl res thereof with acrylic acids and their ciht interpolymers of the percent of vinyl chloride Any} acetate, or similar yinyliclehe halide Wlth VYHH (a of vinyl chloride" curpcse are the meleic resin which ayell names Sill, and 61/ A. illustr tion icllcws:
Example Na. 7
Pounds Vinyl chloride-acetate polymer resin 15.3
Maleic anhydricle glycerol rosin esters 1.?
Methyl ethyl ketone 37.35 Methyl isobutyl ketone 8.3 T011101 37.35
In the event that extremely high holding power against carbon dioxide gas pressure is re quired, as when carbenated-type beverages to be sealed, up t0 16 percent of the weight (A l the vinyl resin may be replaced with a scal pai-ilcle, non-toxic, llavor-lree high-boiling emf; non-Water-reactive modifying agent such as butyl Cellosolve phthalate, methyl phthelyl, ethyl glycollate, or diamyl phthalate. Two examples of this type of formulation are:
Example No. 8
For certain applications where the typical yellowbrown varnish color is undesirable, the e13- pearance of the coated paper may he enhances by the addition of suitable pigments. White and aluminum pigments are particularly edvante genus. 'l'fypical examples 0f these are:
Example Na. 16
@eumls Titanium diemzle pigment m. 7.14 Vinyl chloride-acetate polymer resin Diamyl phthalate Methyl lsobutyl ketcne Methyl ethyl ketone m. 3%..29 Tolucl isms.
Example No, 21
Phases Aluminum pewcler 1.23.9 Vinyl chlorlde-ecetate polymer resin n lfifil Methyl isobutyl ketcne 8.36 Methyl ethyl ketone 37.35 Telucl 37.35
The ketones and toluol in the examples provide an crganic solvent for the vinyl resin, and for the modlfylng agent whexe employed; anal may be replaced by another organic solvent which is with exhibition of good chemical and physical properties even when all the solvent has not been eliminated, can be secured by the addition of a tripolymer resin containing maleic anhydride, vinyl chloride and vinyl acetate, such as that which is commercially available under the designation Vinylite VMCH, which contains approximately 2 parts of maleic component copolymerized with about 87 parts of vinyl chloride and 13 parts of vinyl acetate. Tests have shown that a replacement of 20 percent of the vinyl chloridevinyl acetate resin by such a tripolymer, as exemplified by the following formula, is to be preferred although considerable deviations from this basic formula can be made.
Example No. 12
Pounds Vinyl chloride-acetate polymer resin 13.6 Trlpolymer resin from maleic anhydrlde,
vinyl chloride and vinyl acetate (Vi- In baking the vinyl resin top coat a minimum temperature of 285 degrees F. is required to release the solvents and to develop satisfactory adhesion of the top coat to the underlying oleores inous varnish. A maximum baking temperature limitation is set by the tendency of vinyl resins to decompose at highentemperatures; however, it has been determined that the practical maximum is fixed by the tendency of the preferred paper base to become embrittled at temperatures exceeding 350 degrees F. Within the range of 300 to 350 degrees F.. elimination of solvent from the vinyl resin and the development of adequate adhesion to the varnish are accomplished.
As a specific example of the practice of this invention, a Drab Express paper was stop-coatvarnished with a dehydrated castor oil varnish (Example No. 2 above) and baked at 350 degrees F. for 40 minutes. The resulting paper had a very high moisture-vapor permeability and therefore was unsuited for use as a cap liner. A second coat of varnish was applied and baked at 350 degrees F. for 40 minutes. The double-coated paper then had a sufiiciently low moisture vapor transmission value as shown in Table I below but, when tested in a crown cap in contact with beer for ten days, transmitted to the beverage an objectionable oily flavor. Two portions of the same lot of double varnished paper were re-lacquered with a vinyl resin lacquer (Example No. 6 above) in thicknesses of deposit which produced, after baking at 300 degrees F. for 3 minutes, dry film weights of vinyl resin of two and of five milligrams per square inch of surface, respectively. These triple coated papers, as indicated in Table I. had even lower moisture vapor transmission values and when flavor-tested with beer, were found to impart no foreign flavors whatever to the beverage.
Table I Moisture vapor transmission 1 gms./100 sq. inches/24 hours Drab Express paper with stop coat and top coat of dehydrated castor oil vamish Drab Express paper with stop coat and top coat of dehydrated castor oil varnish, plus a 2 mg./sq. in. coat of vinyl resin Drab Express paper with stop coat and top coat of dehydrated castor oil varnish, plus a 5 mzJsq. in. coat of vinyl resin 1.91
Calendered Vlnylite coated paper 8.91 At 98 defilees F. with anhydrous calcium chloride on one side of ii percent humidity on other.
As another specific example, a Drab Express paper was coated with a stop coat and a top coat oi a tung oil varnish (Example No. 1 above) and baked for 40 minutes at 350 degrees F. following each application. As shown in Table II below, the double varnished paper had a lower moisturevapor transmission value than the above calendered Vinylite coated paper and hence in this respect, at least, was superior to the letter. However, when crown caps having a 0.85 inch diameter circular spot facing of this varnished paper adhered to the corlr were tested in contact with beer, a faint but umistakable oily flavor was discerned in the beverage by expert taste testers. This oil-flavor was less objectionable than in the case of the dehydrated castor oil varnished paper heretofore described, but was sufficiently noticeable to prevent the utilization of the tung oil varnished paper for this purpose.
A quantity of the same double-varnished paper was re-lacquered with a. vinyl resin lacquer (Example No. 6 above) producing alter a three minute bake at 300 degrees F., a dry vinyl lacquer film weight of 5 mg./sq. in. The completed paper showed only a very slightly greater resistance to moisture-vapor transmission, but expert flavor testers pronounced the triple coated paper perfectly free from foreign flavor imparting characteristics after a prolonged storage period in contact with beer, thus proving that the thin coat of vinyl resin acts as an impermeable membrane to the oft-flavor imparting components of the varnish film.
Table I! Moisture water transmission 2 gms./ sq. inches/24 hours Drab Express paper with a stop coat and a top coat or tung oil varnish Drab Express paper with a stop coat and a top coat of tung oil varnish, plus a 5 mg./sq. in. coat of vinyl resin 2.88
At 98 degrees F. with anhydrous calcium chloride on one side of film, 95 percent humidity on other.
The completed vinyl resin coated varnished paper produced according to this invention may be fabricated in the usual ways, without any special requirements or limitations. For example, it may be laminated to cardboard or other backing material, punched into discs and inserted in caps on a single machine such as is used in the industry, or the discs cut with a simple die may be inserted by hand or by any other convenient means.
For use with cork in crown caps, it is perfectly feasible to apply a thermoplastic cement to the reverse side oi the paper and to cut discs of the desired diameter, which may be somewhat less than the diameter of. the cork liner, and to insert and adhere the coated paper spot to the cork on one o! the special machines employed for this 9 purpose. Suitable adhesives are gutta percha and alkyd resin base thermoplastic adhesives.
It is obvious that the invention may be employed in many ways within the scope of the appended claims.
l. A paper liner facing material comprising a paper base and a coating thereon comprising a paper-adherent layer of oleoresinous varnish sealing the paper base and providing a continuous outward surface, and a continuous top coating film having an unplasticized resinous base consisting for 80 to 100 percent by dry weight of vinyl polymer resin selected from the group consisting of polymerized vinyl and vinylidene hal-' ides and vinyl resins con-taining 40 to 95 percent by weight of vinyl and vinylidene halides as mixed and interpolymers with vinyl esters, acrylic 7 acid, esters of acrylic acid, and alkyl-substituted acrylic acids and their esters; the remainder of the dry weight of the resin base of said top coating film being a resin selected from the group consisting of maleic anhydride glycerol esters of rosin and the tripolymer resin made from maleic anhydride, vinyl halide and vinyl acetate; said top coating film being adherent to said oleoresinous layer and having a. thickness of substantially one to five ten-thousandths of an inch with a dry film weight of substantially 2 to 10 milligrams per square inch, said material being characterized by being flexible. having a tough andimpermeable exposed surface and having its several components inseparably bound together;
2. A paper liner facing material comprising a paper base and a coating thereon comprising a paper-adherent layer of oleoresinous varnish sealing the paper base and providing a continutough and impermeable exposed surface and having its several components inseparably bound ous outward surface, and a continuous top coat- I ing film having an unplasticized resinous base of which for 80 to 100 percent by dry weight conproximately 8'! percent vinyl chloride and 13 percent vinyl acetate, said top coating film being a thickness of substantially one to five ten-thousandths of an inch with a dry film weight of substantially 2 to 10 milligrams per square inch, said material being characterized by being flexible, having a tough and impermeable exposed surface and having its several components inseparably bound together.
3. A paper liner facing material comprising a paper base and a coating thereon comprising a paper-adherent layer oi oleoresinous varnishsealing the paper base and providing a continuous outward surface, and a continuous top coating film having a plasticized resin base consistsists of vinyl resin which is a copolymer of aping adherent to said oleoresinous layer and hav.-
ible, having a tough and impermeable exposed surface and having its several components inseparably bound together.
4. A paper liner facing material comprising a paper base and a coating thereon comprising a paper-adherent layer of oleoresinous varnish sealing the paper base and providing a continuous out-ward surface, and a continuous top coating film having a resin base-consisting essentially of a mixture containing 85 to 95 parts by dry weight of copolymer of vinyl chloride and vinyl acetate with the ratio of the vinyl chloride component to the vinyl acetate component being from 60:40 to 95:5; the remainder of the said top coating film mixture being 5 to 15 parts of maleic anhydride-glycerol ester of resin, being compatible with the said copolymer vinyl resin and free from off-flavor whenbaked therewith at a temperaturebetween 285 and 350 degrees F.; said top coating film being adherent to said oleoresinous layer and having a thickness of substantially one to five ten-thousandths of an inch with a dry film weight of substantially 2 to 10 milligrams per square inch; said material being characterized by being flexible, having a at least 80 percent dry weight of a vinyl polymer 1 resin selected from the group consistingof polymerized vinyl and vinylidene halides and vinyl resins containing 40 to 95 percent by weight, of vinyl and vinylidene halides as mixed and interpolymers with vinyl esters, acrylic acid, esters of acrylic acid, and alkyl-substituted acrylic acids and their esters; the remainder of the dry ing essentially of a mixture containing 80 percent by dry weight of copolymer of vinyl chloride and vinyl acetate with the ratio of the vinyl chloride component to the vinyl acetate component being from 60:40 to 95:5; the the remainder 01 said top coating film mixture being a tripolymer formed from substantially 2 parts by weight of maleic anhydride. substantially 8''! parts of vinyl chloride and 13 parts of vinyl acetate, being compatible with the said copoly- 'mer vinyl resin and free from off-flavor when baked therewith at a temperature between 285 and 350 degrees K; said top coating film being adherent to said oleoresinous layer and having a thickness of substantially one to five ten-thousandths of an inch with a dry film weight of substantially 2 to 10 milligrams per square inch; said material being characterized by being flexweight of the resin base of said top coating film being a resin selected from the group consistin: of maleic anhydride glycerol esters of rosin and the tripolymer resin made from maleic anhydride, vinyl halide and vinyl acetate; said material being ,characterized by being flexible, having a tough and impermeable exposed surface and having its several components inseparably bound to ether.
6. A paper liner facing material comprising a paper base and a coating thereon comprising a paper-adherent layer of oleoresinous varnish seaiingthe paper base and'providing a continuous outward surface, and a continuous top coating film having a thickness of substantially one to five ten-thousandths of an inch with a dry film weight of substantially 2 to logmilligrams persquare inch; said top coating film having an unplasticlzed resin base adherent to said oleoresinous layer, with said resin base consisting of unplasticized vinyl polymer resin which is a copolymer of approximately 87 percent of vinyl chloride and 13 percent of vinyl acetate; said material being characterized by being flexible,
having a tough and impermeable exposed surface and having its several components inseparably bound together.
\ CURTIS E. MAIER.