US3522081A - Wax-coated fibrous materials and process for producing same - Google Patents

Description

United States Patent 3,522,081 WAX-COATED FIBROUS MATERIALS AND PROCESS FOR PRODUCING SAME Hallard C. Moyer, Homewood, Ill., assignor to Sinclair Research, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed May 1, 1967, Ser. No. 634,844

Int. Cl. D21h 1/36; B44d 1/44 US. Cl. 117119.2 23 Claims ABSTRACT OF THE DISCLOSURE Wax-coated paper or other fibrous, rollable sheet material having low water vapor permeability and high gloss is obtained by applying to the sheet a coating composition comprising about 4075 weight percent of a linear parafiin wax, about 10-30 percent of an ethylene-vinyl acetate copolymer, about 10-30 weight percent of an amorphous resin, and at least about 1 percent of polyethylene. Additionally, it is necessary to apply the coating composition at a temperature not greater than about 200 F., preferably not greater than about 190 F., and to cool the coating by contacting the uncoated side of the paper with a cooling roll maintained at about 4080 F. until the temperatureof the coating is reduced to less than about 100 F. before removing the cooling contact.

This invention relates to wax-coated fibrous materials, and, more particularly, to a process for applying a defined wax coating to fibrous, rollable sheet material such as paper, so that products having high gloss and high water vapor barrier characteristics are obtained. This invention further relates to novel wax coating compositions characterized by high gloss and superior Watervapor barrier properties.

-Major amounts of parafiin-wax coating compositions are consumed by the food industry as a material for coating, impregnating or laminating paper, paperboard or other fibrous sheet materials. These paper materials are usually used as wrappers, bags, cartons, cups, tubs or other style of container or covering for perishable goods. In order to perform effectively in these various specialized end uses it is highly desirable that the composition of the waxes be so controlled that a wax intended for a specific usewill afford the properties required. For example, a wax coating composition may be used to coat the .outer 3,522,081 Patented July 28, 1970 ice onto paper or other suitable substrates so as to obtain very low water vapor permeability (WVP) as measured on specimens not creased or wrinkled, but it has not been possible to achieve a high permanent gloss with such coatings. These coatings have not produced either the very high initial gloss or the desired resistance to dulling caused by aging, scufling, and handling. In searching for ways to improve the gloss of paraflin wax coatings, it has become common practice to use additives, particularly microcrystalline petroleum wax or polyethylene or both. The microwaxes have proved effective for improving initial gloss and have been used in amounts ranging from minor proportions to total replacement of the paraffin wax. Polyethylene, which is of value for improving gloss stability, has usually been used in minor amounts of about /2-5 percent. Polyethylene is usually effective in improving gloss stability both by reducing the normal loss of gloss often exhibited by parafiin wax coatings on aging, and by reducing the gloss reduction encountered in straight wax coatings from scufiing and rubbing incident in normal handling and usage. However, these additives do not of themselves produce the desired gloss. To obtain the improvements it is also necessary to sharply chill the coating from a molten state to a low temperature. This quenching is ordinarily done by running the freshly coated paper web directly into a cold water bath or contacting the hot coating directly with the surface of a wrapper on a carton of food which is offered for sale in a retail food store, and the waxed wrapper usually serves several functions. A major goal is to inhibit the passage of water vapor into or out of the packaged food, thus prolonging storage life of useful shelf life. The printing on the wrapper identifies the product. Gloss imparted by the wax film enhances the appearance of the product. Good gloss increases the attractiveness and sales appeal of such diverse items as waxed paperboard cartons for butter and bacon, paper bags for potato chips, and waxed paper for general household purposes. It is usually important that the wax film have certain other properties, including adequate resistance to blocking and scuffing, and good sealing strength. Due to the wide-spread recognition of the importance of visual impact and sense stimulation cold, smooth metal roll. It is also necessary that such a coating composition be applied while hot enough that the ingredients are in reasonably complete and uniform solu-' tion. For example, if polyethylene is used as an additive it is preferable to have the coating temperature well above the cloud point; otherwise the polyethylene will not contribute its full gloss stabilizing effect.

In improving the gloss and gloss stability of petroleum wax based coatings in this manner, the desired low water vapor permeability is lost. Subsequent efforts to get both very low water vaporpermeability and very high gloss with a wax or wax-polymer coating have not been succes s ful. The comparatively random oriented small crystal structure formed by shock chilling whicli is normally necessary for high gloss, is not an effective barrier for water vapor. Indeed, it has appeared that it might never be possible to achieve both of these properties in a single coating'In order to obtain wax coatings of both high gloss and good'water vapor resistance, the art has been compelled to re ly on extensive and costly processes such as that illustrated in US. Pat. 2,882,182, wherein paper is coated on two sides with a wax composition and one side is quenched by shock cooling to provide a coating of high gloss while the coating on the opposite side of the paper is cooled slowly in order to provide a high water vapor resistance.

It has now been found that wax coating having both very high permanent gloss and very low water vapor permeabilit-y can be obtained by utilizing a particular "corn bination of coating materials and by adhering to certain prescribed conditions in applyingthe coating. Thus, the

present invention is embodied in both a particular composition suited for producing high gloss-low water vapor permeability coatings and a method by which such coatings are obtained. The composition of the present invention is composed of from about 40 to 75 weight percent of a linear paraffin wax, about to 30 weight percent, and often about -25 weight percent, of an ethylenevinyl acetate copolymer, about 10-30 weight percent, and often about 15-25 weight percent, of an amorphous resin and about 1-10 weight percent of polyethylene.

The linear paraffin wax suited for use in the composition of the present invention should have a melting point in the range of about 135-150" F. as measured by ASTM D-87 and have an n-paraflin content of at least about 90 weight percent, preferably about 95 weight percent or more, as measured by mass spectrometric or gas chromatographic analysis. It should further have a relatively low oil content, that is, less than about 0.5 weight percent and good hardness, typical of fully refined waxes of this general type.

The ethylene-vinyl acetate copolymer of my composition is a wax-compatible polymer of ethylene and vinyl acetate containing from about -30 weight percent, preferably about 24-29 Weight percent, of vinyl acetate and can be of the general type described, for instance, in US. Pat. 2,200,429. Exceptionally low water vapor permeability and a reduced viscosity of the molten composition can be achieved by using a combination of ethylene-vinyl acetate copolymers having varying melt indexes.

The amorphous resin in the composition of this invention is wax-compatible and transparent and has a ring and ball softening point of about 80 C. or more. These resins include, for example, terpene resins, petroleum resins, esterified rosin and esterified hydrogenated rosin obtained from the reaction of a polyhydric alcohol with a rosin or a hydrogenated rosin. Suitable polyhydric alcohols for this reaction are those containing from about 3 to 4 hydroxyl groups and having about 3 to 6 carbon atoms such as, for example, pentaerythritol and glycerol. The rosin esters and hydrogenated rosin esters can have an acid number of 2 to 16 and often about 6 to 14. Additionally, such resins as alpha-methyl styrene-vinyl toluene copolymers as described in US. Pat. 3,000,868, and mixtures of these resins, are also highly effective in the composition.

The polyethylene suitable for use in the present composition is a solid polymer of ethylene having a specific gravity of from about 0.985 to 0.930, a softening point of from about 200 to 240 F. and an average molecular weight of from about 1500 to 25,000. Such polymers are normally prepared by the high pressure polymerization of ethylene or by such polymerization followed by partial depolymerization. Such polymerization processes provide a relatively highly-branched form of polyethylene. Not suitable for use in this invention are the high density, linear polyethylenes having a specific gravity of more than about 0.940. As most commonly produced commercially, these linear polyethylenes are of very high molecular weight, that is, greater than about 50,000. I,

indiscriminate use of the coating composition as described above, does not insure successful preparation of a high gloss coating of low water vapor permeability. In fact, as one normally uses sucha coating composition most effectively according to the prior art, the desired high gloss and 10W water vapor permeability are not obtained. By such procedures to insure that all components are in complete solution and the viscosity of the melt is as low'as possible so as to facilitate application to the paper web, coating temperatures in the range of about 250 to 300 F. or even higher would beemployed. Contrarily it has now been found that the applications of the compositions of the present invention are most effective when coated at lower temperatures, that is, not higher than about 200 F. and preferably not higher than about 190 F. The temperature is sufiicient to melt the composition and bring it to application viscosity, and is often at least about 175 F. Thus, the relatively low viscosity of the coating formulation of this invention, even at these reduced coating temperatures, is highly desirable and avoids the necessity of, handling high viscosity blends which has been a major, deterrent to commercial usage by paper convertors of the wax-polymer hot melt coatings of this general type. 1

Another significant feature of the method of the present invention is the manner of cooling of the blend after it has been properly applied to the paper. It has been found that the coating must not be cooled too rapidly or too slowly. Rapid cooling such as by direct quenching with cold Water or cooling directly against chilled rolls gives a high initial gloss but rather poor gloss stability. Even more adversely, 7

rapid cooling gives a high water vapor permeability. If the cooling is very slow, as by allowing the molten coating to cool by exposure to ambient air alone, gloss is not good and more surprisingly the water vapor permeability does not attain the very low values attained under the proper conditions of the method of this invention.

In order to attain both the desired very high gloss and gloss stability, and very low Water vapor permeability, it has been found necessary to cool the coating at a controlled rate by contacting the uncoated side of the paper or other fibrous, rollable substrate with a metal roll, maintained at about 40-80 F. This contact is held long enough, either on a single roll or by successive contact with a plurality of such rolls, that the coating is brought to a temperature of less than about 100 F. before removing the reverse side of the substrate from contact with the last cooling roll.

Thus, it may be seen that the method of the present invention differs considerably from the prior art. Firstly, in that the prior art has taught that quenching or rapid chilling is necessary to obtain a high gloss, and that very slow cooling, as by exposure to ambient air conditions, is necessary to obtain low water vapor permeability. The term water vapor permeability as used herein refers to the resistance of the coated material to the passage of water vapor and canbe measured as the quantity of water in grams passing through 100 square inches of the coated material in 24 hours from at atmosphere of about percent humidity'to an atmosphere of about 10 percent humidity, the temperature being maintained at F. Gloss rating, as measured on a 20 gloss meter, can be translated into descriptive terms from the values using a scale wherein a rating of over 55 is excellent, 45-55 is good, 30-45 is fair and below.30 is poor.

The invention is further illustrated by the following specific examples. In the table Rosin Ester A refers to a pentaerythritol ester of a partially hydrogenated rosin, with a softening point of 104 C. supplied by Hercules Powder Company. Resin Bis a copolymer of alpha-methyl styrene and vinyl toluene having a softening point of 100 C. and supplied by Penn Industrial Chemical Company. E.V.A. Copolymer Afis a copolymer of ethylene and vinyl acetate containing'25 percent vinyl acetate and having a melt index of 3, supplied by E. I. Du Pont Company.

E.V.A. Copolymer B is an ethylene-vinyl acetate copolymer containing 25percent vinyl acetate and having a, melt index of 400, also supplied by E. I. Du Pont Company. E.V.A. Copolymer C is an ethylene-vinyl acetate copolymer containing 28 percent vinyl acetate, having a melt indexv of 5 and supplied byv E. I. Du Pont Company.

I Polyethylene A is a polyethylene having an average molecular weight of about 4000and a. specific gravity of 0.899 Polyethylene B hasan average molecular weight of about 8000 and .a specific gravity of 0.908. Polyethylene C has an average molecular weight of 1500 and a specific gravity of 0.924. Polyethylene D has an average molecularweight of 12,000 and a specific gravity of 0.912 and polyethylene E has an average molecular Weight of greater than 50,000 and a specific gravity of 0.953.

TABLE 6 about 20 to 30 weight percent vinyl acetate, about to 30 weight percent of a wax-compatible, transparent, amorphous resin, and about 1 to 10 weight percent of a hight pressure polyethylene having a specific gravity of about 0.895 to 0.930, a softening point of about 200 to 240 F., and an average molecular weight of about 1,500 to 25,000, said composition being applied at a temperature of about 175 to 200 F., and contacting the un- Examples Composition, Wt. percent 143 M.P. n-Paraffinic Wax.-. 153 M.P. Semi-Micro Wax Rosin Ester A. Resin B E.V.A. Copolymer A- E.V.A. Copolymer B- E.C.A. Copolymer'O Polyethylene A, 4000 M01. Wt., 0.899 Sp. Gr"..- Polyethylene B, 8,000 Mol. Wt., 0.908 Polyethylene 0, 1,500 M01. Wt., 0.924 Sp. r. Polyethylene D, 12,000 M01. Wt., 0.912 Sp. Gr Polyethylene E, 50,000 Mol. Wt., 0.953 Sp. Gr- Test Properties of Coating:

WYP, uncreased, at 100 'F., 90% vs. 10% R.H., grams/100 sq.

1n./day Gloss, Glossmeter: Initial/7 .day

Examples Composition, Wt. percent 143 M.P. Parafiinic Wax 153 M.P. Semi-Micro Wax Rosin Ester A Resin B E.V.A. Copolymer C Polyethylene A, 4,000 Mol. Wt., 0.899 Sp. Gr" Polyethylene B, 8,000 M01. Wt., 0.988 Sp. Gr- Polyethylene 0, 1,500 Mol. Wt., 0.924 Sp. Polyethylene D, 12,000 M01. Wt., 0.912 Sp. Gr Polyethylene E, 50,000 Mol. Wt., 0.953 Sp. Gr- Test Properties of Coating:

WVP, uncreased, at 100 F., 90% vs. 10% R H gms./

100 sq. m./day Gloss, 20 Glossmeter: Initial/7 day 1 In Example 15, the coating was applied at about 210 F.

It may be seen from the table that when paper or other rollable web is coated in accordance with the invention, a superior wax-polymer coating is obtained. When other compositions similar to but outside the limitations of the invention are used, gloss or water vapor barrier characteristics, or both, sufier.

Note that Example 3 illustrates the effect when the amorphous resin is omitted from the composition. Both gloss and water vapor permeability are inferior. In Example 4, a semi-micro wax was substituted for the nparaflinic wax of the invention. Again, the properties are inferior. In Example 8, too little polyethylene was used; in Example 11, too much was used; in Examples 12 and 13, the polyethylene was omitted altogether; and in Example 14, a high molecular weight, linear polyethylene with a specific gravity of 0.953 was utilized in place of the required low molecular weight, branched polyethylene. In each case, inferior gloss and high water vapor permeability resulted. In Example 15, it is evident that the poor gloss is due to the high temperature of applying the coating to the Web. The composition, being the same as in Example 6 where excellent results were obtained, is within the limitations of the invention, but the process conditions are not. Thus, in order to obtain the superior coating characteristics, it is necessary to rely upon both the composition and the process of the invention.

What is claimed is:

1. A process for preparing a coated fibrous material having a wax coating on one side of high gloss and high water vapor resistance comprising applying to one side of a fibrous rollable sheet material a film of a molten composition consisting essentially of about 40 to 75 weight percent of a linear parafiin wax having a melting point of about 135 to 150 R, an n-paraflin content of at least about 90 percent, and less than about 0.5 weight percent oil content, about 10 to weight percent of one or more ethylene-vinyl acetate copolymers containing coated side of the fibrous sheet material with a cooling surface maintained at about 40 to F. and maintaining cooling contact until the coating is brought to a temperature of less than about F.

2. The process of claim 1 wherein said composition is applied at a temperature of not more than about 190 F.

3. The process of claim 1 wherein the ethylene-vinyl acetate copolymer contains about 24 to 29 weight percent vinyl acetate.

4. The process of claim 1 wherein the amorphous resin is a member selected from the group consisting of terpene resins, petroleum resins, polyhydric alcohol esters of rosin and hydrogenated rosin, a-methyl styrene-vinyl toluene copolymers, and mixtures thereof.

5. The process of claim 1 wherein the molten composition consists essentially of about 55 to about 59 weight percent of the linear paraflin wax, about 15 to 25 weight percent of the amorphous resin, about 15 to 25 weight percent of the ethylene-vinyl acetate copolymer, and about 1 to 5 weight percent of the polyethylene.

6. the process of claim 5 wherein the linear parafiin wax has a melting point of about to F.

7. The process of claim 5 wherein the ethylene-vinyl acetate copolymers comprise a mixture of about 15 weight percent of a first copolymer containing about 25 mole percent vinyl acetate and having a melt index of about 3, and about 5 weight percent of a second copolymer containing about 25 mole percent vinyl acetate and having a melt index of about 400.

8. The process of claim 5 wherein the polyethylene has a molecular weight of about 1500 to 12,000 and a specific gravity of from about 0.899 to 0.924.

9. The process of claim 5 wherein the amorphous resin is a member selected from the group consisting of terpene resins, petroleum resins, polyhydric alcohol esters of rosin and hydrogenated rosin, a-methyl styrene-vinyl toluene copolymers, and mixtures thereof.

10. The process of claim 5 wherein the amorphous resin is an ester of a polyhydric alcohol and a member selected from the group consisting of rosin and hydrogenated rosin.

11. The process of claim 10 wherein the polyhydric alcohol contains about 3 to 4 hydroxyl groups and about 3 to 6 carbon atoms.

12. The process of claim 11 wherein the polyhydric alcohol is pentaerythritol.

13. The process of claim 11 wherein the polyhydric alcohol is glycerol.

14. The process of claim 11 wherein the cooling surface is a metal roll.

15. The process of claim 1 wherein the fibrous rollable sheet material is paper.

16. The process of claim 15 wherein the cooling surface is a metal r011.

17. The product produced by the process of claim 1.

18. The product produced by the process of claim 5.

19. The product produced by the process of claim 6.

20. The product produced by the process of claim 7.

21. The product produced by the process of claim 8.

22. The product produced by the process of claim 9. 23. The product produced by the process of claim 10.

References Cited UNITED STATES PATENTS Butler 99-171 Weisgerber et al. 117-158 X Apikos et a1 260-28 Zaayenga 260-28.5 Mathew 26028.5 Apikos et al 260285 Pullen et al 260-28.5

Hammer 260-28.5

Cox 26028.5

Mase et a1. 26028.5 Eihl et a1. 26028.5

WILLIAM D. MARTIN, Primary Examiner 20 M. R. LUSIGNAN, Assistant Examiner U.S. Cl. X.R.

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(SEAL) Attest:

Patent No.

3522081 Dated July 28, 1970 Inventor(s) EDWARD M.FLETGHER,JR. Attesting Officer H. C. Moyer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

line 5, change "theerwith" to therwithline 49, change "0.985" to --0.895-- Table, line 7 change "E.C.A." to -E.V.A.

line 4, change hight" to -high-- the first letter of that line line 59, in Claim 6,

should be --T-- lines 64 and 66, change "mole" to -weight*'-- Signed and sealed this 23rd day of March 1 971 WILLIAM E. SGH'UYLER, JR. Commissioner of Patents FORM Po-wso (10-69) 1 U5COMM-DC 603764 59 Q 5, GOVERNMENT PIQHTIRG OFFICE "II 0-355-314