US2927865A

US2927865A - Coated heat-sealable wrapper and process of making same

Info

Publication number: US2927865A
Authority: US
Inventors: Jr Dan Lewis; John S Barton
Original assignee: Crown Zellerbach Corp
Current assignee: James River Corp of Nevada
Priority date: 1956-02-24
Filing date: 1956-02-24
Publication date: 1960-03-08
Anticipated expiration: 1977-03-08

Description

March 8, 1960 D. LEWIS, JR., ETAL 2,927,865

COATED HEAT-SEALABL'.: wRAPPER AND PRocEss oF MAKING SAME Filed Feb. 24, 1956 INVENT0R.$` JOHN S. BARTON BYDAN LEWIS JR.

ATTORNEY COATED HEAT-SEALABLE WRAPPER AND PnocEss or MAKING SAME Dan Lewis, Jr., land John S.vBarton,l Camas, Wash., as-

signors to Crown Zellerbach Corporation, vSanlirzan- This inventionrelates to a wrapping material especially adapted for frozen foodstuffs and other products which, while wrapped, are to be kept in below freezing temperatures. l More specifically, this invention relates to the production of a paper wrapperwhich has been rendered substantially moistureproof by waxing, which is capable of being heat-sealed, but which, when heat-sealed, will maintain a strong bond where such heat-sealing has taken place regardless of subsequent prolonged exposure to sub-freezing temperatures.

While various types of waxed paper have long been employed as food wrappers, and, under certain conditions, have proved thoroughly satisfactory for such use due to the fact that they are capable of being heat-sealed and provide a protective and moistureproof covering as long as the seal is maintained, such wrappers have been found to be very unsatisfactory inthe case of foods which must be kept in freezers, or in other refrigerating means, under below freezing temperatures, due to the fact that the wax seal then in all likelihood will fail or delaminate during the handling of the packaged foodstuif. Various waxes and mixes of waxes have been tried out in an attempt to produce a heat-sealable wax paper wrapper in which the seal will resist delamination under exposure to low temperatures, but, to the best of our knowledge, thus far without success.

An object of the present invention is to provide an improved wrapper for foods and the like which will be capable of being heat-sealed and which will maintain such seals under temperatures as low as F.

Another object of theinvention is to provide an improved frozen food wrapper which will be substantially moistureproof and thus, when sealed, will enablepthe food incased therein to maintain its original moisture content while being protected from outside moisture, vapor, or dust as long as the wrapper remains sealed.

An additional object of the invention is to provide an improved wrapper which will have a surface especially adapted for receiving printing ink and thus suitable for printing.

A further object of the invention is to provide an improved wrapper, particularly adapted for frozen foods, which can be produced easily and economically and which will require only-conventional equipment in its manufacture.

Brieily, we attain these objects and other advantages by first coating at least one face of a sheet of paper, of`

suitable weight for use as a wrapper, with a iluid, pigmented coating composition comprising, as its essential ingredients, a finely divided coating pigment and a polyvinyl acetate resin. The coated paper then, after being substantially dried, is supercalendered, with the result that the treated 4surface is rendered smooth and is particularly receptive to printing ink. Consequently the wrapper at this stage presents a surface which is very suitable `for printing. As ya final step in the production of the '.wrapper, the coated paper (after being printed,

mired States Patent if a printed wrapper is desired), is given a treatment of wax, either on the coated face alone orV on both faces. The resultant coated and waxed sheet presents a finished, glossy surface which is heat-scalable, and, when heatsealed, Vwill maintain its seal even though exposed over a prolonged period to temperatures as low as 10 F. The important and novel feature of the invention is the fact that, due to some physical interaction between the particular pigmented polyvinyl acetate resin and the film of wax on the bonded surfaces of the base paper sheet, which occurs under the heat-sealing, a tenacious bond is set up between the heat-sealed surfaces and this bond will not delaminate, even under low temperatures, and can be broken only by rupture of the paper sheet..

Preferably, in the carrying out of the invention, the paper sheetfor the wrapper should have a basis weight of from about 15 pounds to 150 pounds per ream of 3,000 square feet, and a basis weight of from 20 pounds to 35 pounds. per ream is considered most satisfactory since. the paper then will be heavy enough for customary wrapper use Without being unnecessarily heavy. Any of the conventional beater additives, such as sizing agents, wet-strength resins, fillers, opacifying pigments, etc., may, if desired, be incorporated in the pulp furnish from which the paper sheet is made. It is desirable that the sheet be fairly well sized in order to prevent excessive penetration by the pigmented polyvinyl acetate resin. A pulp furnish consisting of about 8O percent bleached softwood suliite and about 20 percent hardwood soda pulp has been found to be very suitable for the manufacture of the paper sheet to be employed in the carrying out of this invention.

In the accompanying drawings, which are more or less diagrammatic, -Figure 1 is a sectional elevation, drawn to a considerably enlarged scale, of a portion of a `frozen food wrapper embodying the present invention;

' and Figure 2 is a similar sectional elevation of a food wrapper also made in accordance with the invention, but with both faces of the wrapper waxed.

In the drawings the reference character 10 indicates the paperrsheet from which the wrapper is made. The pigmented polyvinyl acetate resinl coating applied to the paper sheet is indicated by the reference character 11. After the coating has dried, and the coated surface preferably has been supercalendered and printed (assuming that a printed wrapper is desired), the .coated sheet is given a wax treatment. In Figure 1 the lm of wax which is applied to the coated face of the paper is indicated by the reference character 12. In Figure 2 both the coated and uncoated faces of the paper have received. a thin lm of wax, as indicated by the

reference characters

13 and 14, respectively. rIn some instances the waxing of both sides of the sheet may be preferred and this will be a convenience, for example, when the .waxing can be accomplished most conveniently by dipping the, coated paper in the melted wax. In such case, since both faces of the paper are waxed, the iilm of waxk on each face is preferably slightly thinner than when the wax is com'ned only to the coated face.

It would, of course, also be possible to carry out the invention by coating both faces of the paper sheet with the pigmented polyvinyl acetate resin and then waxing both coated faces. However, since this is not necessary, thisY consequently is not the preferred manner of performing the invention.

TheY essential constituents of the pigmented coating composition are a pigment and a polyvinyl acetate resin, adhesive. The materials which comprise these particular constituents for the carrying` out of the invention will now be described more fully.

For the pigment constituent several kinds of white combinations thereof may be used. Examples of these are barium sulfate, calcium: carbonate, calcium sulte', clays, titanium dioxide, etc. If desired, colors or dies customarily used in color mixes may be admixed therewith to impart to the pigmented composition a particular desired shade. In selecting a particular pigment or pigments, such factors as brightness, opacity, printing qualities of the coated sheet, as well as the compatibility with the polyvinyl acetate resin adhesive must be considered. A combination o-f 60 to 85 parts by Weight of titanium dioxide with 15 to 40 parts by weight of coating clay has been found to be highly satisfactory for employment in this invention. A small amount of dispersing agent should be added for the pigment. Some of the satisfactory dispersants are tetrasodium pyrophosphate, sodium hexametaphosphate, sodium silicate, etc. The proportion of the dispersant should not be too high to prevent flocculation of the pigment in the liquid medium.

The other and essential constituent of the coating composition, namely the polyvinyl acetate resin, is available on the market in form of liquid emulsions of dispersions. We have found that particularly suitable resins for use in combination with the above mentioned pigments are the aqueous polyvinyl acetate emulsions having an average particle size in the range of 0.5-5.0 microns,V and preferably l to 3 microns, viscosity in the broad range of 5 to 2000 centipoises, and preferably 50G-1200 centipoises at 25 C. Such resins provide a strong bond between the heat-sealed areas of the iinal wrapper upon its exposure to a temperature range of from about F. to about 90 F., the heat sealing operation being carried out at a temperature above the softening point of the polyvinyl acetate resin. Examples of highly satisfactory polyvinyl acetate aqueous emulsions are: Elvacet 81-900, produced by E. I. du Pont de Nemours and Co., Inc., Vinac XX, produced by Colton .Chemical Co., Cleveland, Ohio, and Resyn l0 K 04,V produced by National Starch Products Co., San Francisco, California.

If desired, a secondary adhesive, a plasticizer for polyvinyl acetate, and a liquid medium may also be used as auxiliary constituents in the coating composition. Any starch or a proteinaceous adhesive customarily used in the coating of papers may be used as a secondary adhesive. Thus unmodified starches, as well as enzymeconverted, oxidized, or otherwise modified starches are satisfactory. A starch particularly suitable for use in admixture with polyvinyl acetate emulsion is a hydroxyethyl ether derivative of corn starch supplied by the Penick & Ford Company, Cedar Rapids, Iowa, under the trademark Penford Gum 280. The protein adhesives usually employed in paper coating formulations include casein, animal glue, soya, etc. Alpha-protein (isolated soya protein), which is available 0n the market from many sources, has been found to be a highly satisfactory proteinaceous adhesive for use in the coating composition of this invention.

It will be understood by those skilled in the :art that in choosing a secondary adhesive, its compatibility with the polyvinyl acetate resin and with the pigment, its ease of dissolving or forming homogeneous pastes, its ilmforming qualities, and other factors must be taken into consideration. We have found that the polyvinyl acetate resin solids in the adhesive composition may be reduced up to about 30% and the secondary adhesive (solids basis) substituted in place thereof without affecting adversely the seal strength of the finished wrapper. At the same time, the secondary adhesive improves to a certain degree, the rub resistance of the pigmented composition coating on the sheet in addition to reducing its cost.

The addition of a certain amount of a plasticizer to the polyvinyl acetate emulsion has been found to improve its effectiveness, mainly in enhancing the smoothness of the coated face of the paper after supercalendering, and hence its printing qualities. Among the many plasticizers suitable for admixture with Vthe polyvinyl acetate are: 2- ethyl hexyl diphenyl .phosphate and ethyl phthalyl ethyl Vglycollate, both supplied by Monsanto Chemical Co.

TABLE I Broad Preferred Range Range l0-90 75-87 10-60 13-25 Secondary adhesive 0-20 0-5 Plasticizer 0-10 1.2-2.0 Dieper-sant for pigment. 0. 01-1 0.1-0.3 Liquid medium Sufficient to attain desired viscosity As previously mentioned, the inclusion of a secondary adhesive or of a plasticizer is optional.` It should be noted, however, that when a secondary adhesive is used to replace a portion of the'polyvinyl acetate, the amount of such adhesive in the broad range of Table I should not exceed 20 parts by weight (solids basis) of the total composition solids, in which case the total proportion of the polyvinyl acetate resin and of the secondary adhesive would not bey higher-than 60 parts Yby Weight (solids basis); in the preferred range of Table I, the amount of the secondary adhesive would not behigher than 5 parts by weight (solids basis),y the total proportion of the polyvinyl acetate resin and of the secondary adhesive then being not higher than 25 parts by weight (solids basis).

In preparing the pigmented coating composition, it has been found preferable to dissolve the dispersant in water irst, and then to add the pigment so as to produce an aqueous pigment dispersion having a solids content from about 40% to 70%. The mixing should be carried out with vigorous agitation until a homogeneous mixture is obtained. A predetermined quantity of polyvinyl acetate emulsion is then added slowly to the aqueous pigment slurry and mildly stirred until a uniform mixture is produced. When a plasticizer is used, it is admixed first with the polyvinyl acetate. Water or other suitable liquid media may be added to the tluid pigmented coating mixture so that its nal viscosity will be adjusted within a broad range of -5000 centipoises, and preferably between 250 and1500 centipoises at 25 9C;

The secondary adhesive, if used, is either dissolved or dispersed in a liquid medium, e.g. water, with heating and/ or addition of alkali, in accordance with conventional procedures employed Vin Vthe preparation of starch or protein adhesives used in paper coatingoperation. It is preferred that the quantity of water used be such as to produce a concentration of` solids in the range of 10% to 25%. The secondary adhesive may be either mixed with the polyvinyl acetate resin or added directly to the pigment slurry. v

The pigmented coating composition may be applied to the paper by any conventional paper coating procedure, such as kiss-roll, air-knife, brush, size-press,y doctor-blade, etc. Care shouldibe taken, however, that the coating be applied over the entire surface of the paper in such a manner as to produce a continuous film of uniform weight ranging broadly` from 0.5 to 6 lbs/roam, and preferably 1 .5 Ito 3,011.55, (solids basis) per ream of 3000 square'feet.

During the coating operation it may be desirable toreduce the' foaming of the coating composition. Accordingly, a small amount of a suitable defoamer, usually about 0.05% based on the solids of the composition may be incorporated thereto.v Satisfactorydefoamers are: octyl alcohol, silicones, esters of fatty acids, etc.

The coated paper is then substantially dried at a -temperature between 212 F. and 300 F., -preferably between 250 F. and 275 F. by hot air or by passing it over one or more drum driers, or in any other suitable manner. Care should be exercised, however, that the period of time necessary for drying and the drying temperature be so adjusted as to produce a coated paper having from about 4% to 6% moisture content.-

After the coated paper is dried, it is subjected to a calendering operation to impart thereto a smooth surface desirable for printing. This operation is carried out by first dampening the paper uniformly by anysuitable means, such as a spray of water, to increase its moisture content to about- 7%12%, and preferably 8%-10%, and then passing it through either the entire conventional calender stack or a part thereof if a finish requiring a lesser calendering action is desired. The calender rollsV may be either cold or internally heated to a surface temperature up to about 300 F., but preferably not exceeding about 200 F. During its passage through the calender stack, the paper is subjected to a controlled pressure which may vary from about 500 to 2,000 pounds per linear inch, depending on the desired ultimate characteristics of the sheet. Satisfactory results were obtained by supercalendering a 30 lb./ream coated sheet of paper with a pressure between 1200 and 1300 pounds per linear inch.

The coated and supercalendered paper may be next printed, if desired, by any of the conventional printing processes, such as rotogravure, letter-press, multicolor offset, etc. The coated surface of the paper ishighly satisfactory for printing operation, as it is smooth and has a good aiiinity for printing inks.

The coated paper, eitherl printed or unprinted, is then waxed by any of the well-known procedures of making papers waxed on either one side or both sides thereof. As indicated above, the wax may be applied only to the coated surface of the paper. This may be accomplished by depositing a lm of wax from aqueous emulsion, solvent solution, or from molten bath. Preferably, and generally more conveniently, however, the coated paper is made to pass through a bath of molten wax, then between a pair of squeeze rolls to remove excess `of wax,

and finally through a chilling bath, and as a result both sides of the paper will be covered witha thin iilm of Wax. The amount of wax necessary to obtain a film of adequate thickness for satisfactorily carrying out ythe invention may vary from about 8 pounds to 25 pounds per ream (3000 square feet) for both sides of paper, depending on the type of paper used and its basis weight. For a paperV having a basis weight of 20 to 35 pounds/ ream, the amount of Wax ranging from 10 to l5 pounds/ream for both sides of the paper, is generally sufficient.

The term wax, as used herein, includes such waxes or wax-like materials which give a flexible, non-blocking, heat-scalable film characterized by va good'moisture-vapor transmission resistance, and which are capable of producing 'in conjunction with the hereinbove described pigrnented coating a strong heat-seal bond, resistant at subfreezing temperatures, as low as 10 F.

ParticularlyY suitable in achieving a bond of such strength are .wax compositions containing a` relatively small amount of polyethylene. As is well known, polyethylene is produced commercially byfpolymerization of ethylene at high or low pressure. Polyethylene is also available as an irradiated or chlorosulfonated material.I Although the molecular weight of' the polyethylene may vary to a large extent, a material satisfactory for incorporation into Vthe wax compositions `of this invention may have a molecular weight in the range of 1,50035,000, `and preferably between 3,000 and 20,000.

The two types of waxes which are customarily used in the I nannfacture of waxed papers are the parain wax having a melting point range in the broad range of about F.200 F., and the preferred range 130 F.160 F., andthemicrocrystalline wax having a broad melting pointrange of about 120 F.-170 F., and preferred range FL- 160 F. The combinations thereof in various proportions are also widely employed.

Although other-'wax materials possessing the hereinabove specifiedcharacteristics are equally satisfactory for application'over the surface of the paper in the carrying out of this invention, the following Table IIv shows the most satisfactory wax compositions and proportions of their constituents expressed in percentages by weight. i

In preparing the wax compositions containing the polyethylene, the wax is usually melted and the predetermined quantity of polyethylene, preferablyy in finely divided form is added thereto and stirred until dissolved. The resultant Awax composition visthen ready for use in the waxing operation. The coated and waxed paper may be wound in roll form for ease in handling, shipping, and application.

When a frozen food article is to be wrapped and sealed, a sheet of the coated and waxed paper of adequate size for the package is used and placed so that the coated and waxed side of the sheet will form the outside of the package. The overlapping marginal portions of the wrapping sheet and the closing folds in the same are so arranged that the coating and the wax -will be in betweenthe plies to be sealed. The sealing is then accomplished by the application of light pressure and heatfor a short period of time suficientto produce a strong'bond. Although what actually transpires to produce theresulting strong bond with the heat-sealing is not entirely understood, it is evident vthat some physical' interaction of the pigmented coating composition with the wax composition and with the two opposed paper surfaces takes place under the influence of the sealing heat and pressure. It has been found that the heat-sealing operation should be carried out at a temperature above the cloud point (ASTM designation D9'7-47) of the wave composition, preferably at least 10 F. above such cloud point of the wax composition, under which conditions the polyethylene, if present in the wax composition, will be substantially dissolved therein. Furthermore, it has been found to be important that the temperature at which the heat-sealing takes'place should be above the-softening point of the polyvinyl acetate resin. This is further evidence that some physical interaction between the constituents does occur and is an important factor. Extensive tests have shown that the strength of the bond produced by such heat-sealing is such that it is impossible to pull the bonded areas apart except by rupturing the paper. In other words, whatever the exact nature of the interaction between the constituents may be, no delamination occurs after such interaction is produced by the heat-sealing. Such sealed packages have been tested by being stored for prolonged periods in temperatures around 10 F. without any delaminating or failure of the seals taking place as a result of pulling the bonded areas apart.

'The following are some specific examples of the carrying-out of the invention. In these examples the proportions are given as parts by weight, solids basis. IIt is to beiunderstood, of course, that the invention is not limited entirely to these .specific examples.

The pigment was composed of 33.5 parts by weight of titanium dioxidefTitanox RA-SO, supplied by Titanium Pigment Corp., New York, New York, and 16.5 parts by weight of coating clay -X43, supplied by J. M. Huber Corp., New York, New York, The polyvinyl acetate resin was Vinac XX, supplied by Colton Chemical Co., Cleveland, Ohio, in form of an aqueous emulsion containing 55-57% solids, having a mean particle size of 1 micron, viscosity at 25 C. in the range of 900-1200 centipoises, and specific gravity at 25 C. of 1.11. (The temperature at which a dry film of this polyvinyl acetate resin begins to soften is 185 F. to 195 F.) The dispersant was tetrasodium pyrophosphate, readily available on the market from many sources.

The dispersant was first dissolved in the 4l parts of water and the two pigments were added thereto gradually and stirred vigorously until a homogeneous dispersion was obtained.V The polyvinyl resin was then added slowly to the slurry'and stirred mildly until a uniformV mixture was produced. The iinal viscosity of the adhesive was 1200 centipoises at 25 C.

The -paper used had a basis weight of 301pounds per ream (3000 square feet) and was made of a pulp furnish composed of 80% coniferous bleached suliite1and20% deciduous soda pulp, to which pulp furnish were added about of titanium dioxide filler and 1.1% rosin size precipitated onto the fibers with papermakers alum, both based on the dry weight of pulp. This paper was uniformly coated on one side thereof with the above coating composition. The coated paper was then passed through a hot-air oven at about 260 F. in such a manner as to leave therein about 5% moisture content. The lweight of the dry coating film was 2.5 pounds per ream of paper.

The paper was next supercalendered, by rst dampening it with a water mist to increase its moisture content to about 9%, and then passing it between the calender rolls heated to about 190 F., the pressure in each nip being about 1250 pounds per linear inch. The supercalendered'paper was then printed on the coated side by the conventional rotogravure process.

Finally the paper was waxed on both sides in a conventional wax coating equipment using a wax composition consisting of 60% of a parain wax having a melting point in the range of about 140 F.-l45 F., 35% of a microcrystalline wax having a melting point in the range of about '145 F.-l50 F., and 5% of a polyethylene polymer having a molecular Iweight of about 19,000. After the two waxes had been melted, the polyethylene, in finely divided form, was added thereto and the mixture was stirred until the polyethylene was dissolved. The cloud point of this wax composition was 188 F.-l90 F. The paper having a iilm of molten wax on each side thereof, was then passed through a chilling bath'to solidify the wax. total amount of wax applied on both sides of the paper was about 12 pounds per ream.

A sheet of this finished wrapper was employed for wrapping a package of frozen foodstuff, the coated and printed side of the wrapper constituting the outside surface of the wrapped package. The marginal portions andthe endfolds of the wrapper were arranged in such a manner that the surface coated-with the pigmented coating and wax was in opposed relationship with the surface having only the wax thereon. The heat-sealing operation was carried out using a conventional sealer, the platens of which were heated to 225 F. Slight pressure was applied between the hot platens for :a very brief period of time (actually about 2 seconds?) sufficient to produce a strong .bond between the heat-sealed areas of the wrapper.

The heat-sealed 'package produced in this way was stored in a freezer at a `temperature Iof 10 F. for a period of live weeks. At the lend of this period, the bond between the heat-sealed areas was still very strong, and no delamination of these areas occurred.

Example 2 The vpigmented coating .had the following composition:

Parts Pigmentv l 8l -Po`lyvinyl acetate resin 17 Plasticizer 1.7

Dispersant 0.24

Water Yto `adjust Afinal viscosity to .200 centipoises at 25 C. Y

The pigment was .composed of the Asame titanium dioxide and coating clay, as in Example 1, but used in manner as in Example l, however, the .plasticizer was added to the resinfirst, and the plasticized resin was then mixed with the pigment slurry.

The same paper as described in Example .1 was coated with the above composition, then dried, .supercalendered, printed, and waxed in the manner set forth in Example 1. The vproportions of the ingredients in the -wax composition were: 40% paraffin Wax,'50% microcrystalline wax, and 10% polyethylene.

The wrapper formed in accordance with this example proved Vhighly satisfactory for packaging frozen foodstuffs, as .the strength of its heat-.sealed .bond had .essentially the .same characteristics 4as that 'of the wrapper of Example l.

Water to adjust final viscosity to 750 rcentipoises .at 25 C.

The pigment and the .dispersant `were .the -same as in Example 2. The polyvinyl 'acetate resin was Elvacet 81-900, supplied by the E. I. du Pont de Nemours-Co., in form of an aqueous emulsion containing 55*57% solids, having a mean particle size of 3 microns,viscosity Iat 25 C. in the range of G-.1000 centipoises, and `specific gravity at 25 C. of 1:19. The secondary Vadhesive was alpha-protein (isolated soya protein) solution, prepared by adding 4 parts `of the 4protein to .sufficient water :to forma 15% aqueous slurry, heating the'slurryzto 38 C., adding 0.5 parts .of 28% vammonium yhydroxide tsolution to solubilize the protein, and mixing :until the yprotein was dissolved. 'The plasticize'r .was butyl stearate.

In preparing vthe `coating composition, the protein .solution was added to the aqueous pigment dispersion, then the polyvinyl acetate emulsion .containingzthefplasticizer was added thereto and mixed until a uniform `mixture was obtained.

The -sarne paper fas described in YExample 1 lwas coated with the above composition, lthen dried, `supercalenclered, printed, and waxed in the .same lmanner as .set forth sin ...9, Example l. The proportions of theingredients of the wax composition were slightlyv modified, however, as'follows: 60% paraffin wax, 32.5% microcrystalline wax and 7.5% polyethylene. f I

The wrapper produced according to this example had the saine excellent properties as that of Example 1.

Water to adjust final viscosity to 2500 centipoises at 25 C.

The pigment consisted of titanium dioxide and coating clay as described inExample 1, but employed in the ratio of 4:1 respectively. The polyvinyl acetate resin was ,Vinac XX, the plasticizer was Santicizer 141, and the dispersant wasthe tetrasodium pyrophosphate, all previously mentioned. The secondary adhesive was a hydroxyethyl ether derivative o f corn starch, `produced by the Penick & Ford Co., under the trademark Penford Gum 280. It was used in form oa-solution prepared by admixing the 3.4 parts of the starch in sulicient water to produce about 25% solids content, and heating at 180 F. with agitation until dissolved. L

The above ingredients were intermixed in the manner described in Example 3. The same paper as described in Example 1 was coated with the above coating composition, then dried, supercalendered and waxed in the same manner as set forth in Example l. The wax composition used in this example consisted of 85% paratiin wax, 13.5% microcrystalline wax, and 1.5% polyethylene, the characteristics of the three ingredients having been previously mentioned.

The coated and waxed wrapper had the same properties as that of Example l.

In Example 1 the platens with which the heat-sealing of the wrapper was accomplished were heated to 225 F. and the time during which the wrapper was in contact with the heated platens was about 2 seconds. The platens could be heated to a higher temperature and the contact time correspondingly reduced. It will be understood, of course, that the temperature of the platens does not mean the temperature to which the pigmented coating and wax are raised during the heat-sealing since the contact period must be of sutriciently short duration to prevent excessive heating of the coating on the wrapper. Thus in commercial operation it has been found to be practical to have the temperature of the platens as high as from 360 F. to 440 F., but the contact time is then carefully restricted to about one-half of a second.

The glossy, heat-scalable, waxed surface of the wrapper of this invention is non-cohesive and non-tacky at normal room temperature, so that the wrapper may be handled or shipped in roll form without any special precautions. The superior printing qualities of the coated surface combined with the advantageous heat-sealing characteristics, imparted by both the coating and the wax film, give as a result a highly Asatisfactory frozen food paper wrapper possessing the desired feature of being capable of forming a bond at the heat-sealed areas of such a strength that it will not break or delaminate when exposed to sub-freezing temperatures as low as about F. Furthermore, the wrapper is pliable, has a very low moisture-vapor transmission rate, and good physical strength properties.

We claim:

l. The process of making a heat-scalable paper wrapper for use in wrapping frozen foodstuifs, which comprises coating an entire surface of a paper suitable for use as a .'wrapp'er.withzaquantity of a'liquid, pigmented'coating composition consistingaessentially of a hornogeneous'mixtureoffrom 40'to 90 parts by weight of a finely divided coating pigment and from l0 to 60 parts by weight, solids basis, of4 polyvinyl acetate resin, substantially drying the coating so asto producea continuous film ranging from 0.5 Vto 6 pounds per ream'of 3,000sq. ft. of the paper, supercalendering the coated paper, and .covering the entire coated surface of the paper with a exible, non-blocking film of. wax, the amount of applied wax being sufcient to: form with the ypigmented polyvinyl acetate coating, upon heat-sealing, a strong bond which will resist delaminationoat"'temperatures as low as about 10 F., when the..wrapper isY heat-sealed at a temperature high enough to melt said wax and soften the polyvinyl acetate resin. i

2. The process of claim 1 wherein thewax is selected from the4 group consisting of paraffin wax, microcrystalline wax, and combinations thereof, and contains up to 40% by weight of polyethylene.

3. The process of making a heat-,scalable'paper wrapper for use in wrapping frozen foodstuffs, which comprises coating an entire surface of a paper suitable for use as a wrapper with a quantity of a liquid, pigmented coating composition consisting essentially of a homogeneous mixture of from 75 to 87 parts by weight of a nely dividedV coating pigment and fromY 13 to 25 parts by weight, solids basis, of polyvinyl acetate resin, substantially-drying the coating so as to produce aA continuous tilmranging from 0.5 to 6 pounds per ream of 3,000 sq. ft. of the paper, supercalendering the coated` paper,.and covering the entire coated surface of the paper with a iiexible, nonblocking film of wax, the amount of applied wax being sufcient to form with the pigmented polyvinylv acetate coating, upon heat-sealing, a strong bond which will resist delamination at temperatures as low as about 10 F. when the wrapper is heat-sealed at a temperature high enough to melt said wax and soften the polyvinyl acetate resin.

4. The process of claim 3 wherein the pigmented coating composition also includes a plasticizcr for the polyvinyl acetate in the amount of from 1.2 to 2.0 parts based on the dry weight of said composition and the wax contains up to 15% by weight of polyethylene.

5. The process of making a heat-scalable paper wrapper for use in wrapping frozen foodstuffs, which comprises coating one entire surface of a paper suitable for use as a wrapper with an aqueous, pigmented coating composition, having a viscosity of 50-5000 centipoises and consisting essentially of a homogeneous mixture of from 40 to parts by weightof a finely divided white coating pigment and from 10 to 60 parts by weight, solids basis, of polyvinyl acetate resin, substantially drying the coating, dampening the paper to impart thereto a moisture content of 7%-12%, supercalendering the coated paper, printing the paper on the coated surface, and applying a flexible, non-blocking film of wax over both surfaces of the paper, the amount of applied wax being sufficient, in conjunction with said coating, to enable a strong bond to be produced between the exposed waxed areas of the paper with heatsealing, whereby, when such heat-sealing takes place at a temperature high enough to melt said Wax and soften the polyvinyl acetate resin, said coating and said wax will interact to produce a bond which will resist delamination at temperatures as low as 10 F.

6. The process of claim 5 wherein the wax is selected from the group consisting of parain wax, microcrystalline wax, and combinations thereof, and contains up to 15% by weight of polyethylene.

7. The process of claim 5 wherein the amount of pigment is from 75 to 87 parts, the amount of polyvinyl acetate is from 13 to 25 parts by weight, solids basis, and

the viscosity of the adhesive composition is 250-1500 centipoises.

8. A coated heat-scalable wrapper for use in wrapping frozen foodstuffs, comprising .a sheet of paper having on y an entire surface thereof `Aa pigmented coating consisting essentially lof a mixtureof fro1n'40 to 90 parts by weight of a finely divided vcoating pigment and from yto 60 parts, on a dry weight basis, of polyvinyl acetate resin, the amount of said coating being from 0.5 `to 6 pounds per ream 3,000 sq. ft. of .the paper, and having a exible, non-blocking film of wax covering the .entire coated ,surface of said sheet, the amount of Wax being suicient 'to interact with the polyvinyl acetate resin under Vheat-sealing to form a strong bond resisting delamination at tern peratures astlow as about 10" F.

9. A heat-sealable wrapper as described in claim 8, ythe wax in said lm on the coated sheet being selected fromr the group consisting `of paraffin Wax, -microcrystalline wax, and combinations thereof, and containing up to 40% by Weight of polyethylene.

10. VA coated, heat-scalable Wrapper for use in Wrapping frozen foodstuffs comprising Ia ysheet of paper having `a coating over one entire surface, said coating `consisting essentially of va mixture of from 75 to 87 Yparts of white coating pigment and vfrom V13 to 25 parts by Weight, solids basis, of polyvinyl acetate resin, the amount of vsaid coat.- ing benig from 0.5 to 6 pounds per ream V3,000sq. ft. of the paper, at least the coated surface of the paper being covered by a flexible lmof Wax selected fromthe group consisting of paraffin wax, microcrystalline Wax, and combinations thereof, and containing up to 15% by weight of polyethylene, said wrapper ibeing characterized by forming, upon heat-sealing, a bond resistingdelamination at temperatures as low as 10 F.

Y the paper.

References Cited in the le of this patent .UNITED STATES PATENTS 2,306,046 Duggan 'Dec. 22, 1942 2,394,254 -Noble r lFeb. 5, 11946 2,402,605 Cowen June 25, "1946 2,406,990 'Borden' Sept. v3, 11946 2,424,386 Czeczowitzka vJuly 22, 1947 2,520,900 Frost Sept. 5, 1-950 2,529,060 Trillich Nov. 7, 1950 2,609,317 Vogt Sept.V 2, 1952 2,656,286 Fisher` Oct. 23, 1953 2,720,467 Franke Oct.Y 1'1, 1955 2,720,496 Bushnell Oct. r11, 1955 2,733,225 Smith Jan. 3-1, 1956 2,791,570 Y Backlund May 7, 1957 2,803,612 Moyer Aug. 20, 1957 2,808,382

Iahoitis Oct. 1, 1-957

Claims

5. THE PROCESS OF MAKING A HEAT-SEALABLE PAPER WRAPPER FOR USE IN WRAPPING FROZEN FOODSTUFFS, WHICH COMPRISES COATING ONE ENTIRE SURFACE OF A PAPER SUITABLE FOR USE AS A WRAPPER WITH AN AQUEOUS, PIGMENTED COATING COMPOSITION, HAVING A VISCOSITY OF 50-5000 CENTIPOISES AND CONSISTING ESSENTIALLY OF A HOMOGENEOUS MIXTURE OF FROM 40 TO 90 PARTS BY WEIGHT OF FINELY DIVIDED WHITE COATING PIGMENT AND FROM 10 TO 60 PARTS BY WEIGHT, SOLIDS BASIS, OF POLYVINYL ACETATE RESIN, SUBSTANTIALLY DRYING THE COATING, DAMPENING THE PAPER TO IMPART THERETO A MOISTURE CONTENT OF 7%-12%, SUPERCALENDERING THE COATED PAPER, PRINTING THE PAPER ON THE COATED SURFACE, AND APPLYING A FLEXIBLE, NON-BLOCKING FILM OF WAX OVER BOTH SURFACES OF THE PAPER, THE AMOUNT OF APPLIED WAX BEING SUFFICIENT, IN CONJUCTION WITH SAID COATING, TO ENABLE A STRONG BOND TO BE PRODUCED BETWEEN THE EXPOSED WAXED AREAS OF THE PAPER WITH HEATSEALING, WHEREBY, WHEN SUCH HEAT-SEALING TAKES PLACE AT A TEMPERATURE HIGH ENOUGH TO MELT SAID WAX AND SOFTEN THE POLYVINYL ACETATE RESIN, SAID COATING AND SAID WAX WILL INTERACT TO PRODUCE A BOND WHICH WILL RESIST DELAMINATION AT TEMPERATURES AS LOW AS 10*F.