US2686744A - Heat sealable wrapping material - Google Patents

Description

Patented Aug. 17, 1954 HEAT SEALABLE WRAPPING MATERIAL Ralph T. K. Cornwall, Rosemont, Pa., assignor to American Viscose Corporation, Wilmington, Del., a corporation of Delaware No Drawing. Application October 11,1951, Serial No. 250,960

This invention relates to a heat-scalable wrapping material comprising a hydrophilic base or support and a polyethylene which is firmly anchored to the support by a thermosetting resin in the thermoset condition. A

The properties of polyethylene, such as its resistance to chemicals and solvents, resistance to the permeation of fluids and ases, and its heatsealing properties are desirable in a packaging material. However, unsupported films or sheets of polyethylene have not proved to be entirely satisfactory for wrappin purposes, especially where a strong heat-sealed bond is required and it is desired to perform the heat-sealing operation on conventional wrapping machinery.

Unsupported polyethylene films lack stiffness and are diflicult to handle on the automatic wrapping machine in general use. When it is attempted to heat-seal the unsupported film to itself on the wrapping machine, unless the condi tions are carefully controlled the polythene melts, creating a messy condition and resulting ina weak heat-sealed bond or no heat-sealed bond at all. It is, therefore, desirable to associate the polythene film with a reinforcin base or support which permits of obtaining a strong heatsealed bond when the composite sheet is used as a wrapping material and heat-sealed on a basedetermines the strength of the heat-sealed bond, the better the adhesion, the stronger the bond. That is why, although cellophane and paper are widely used as Wrapping materials, they are not suitable as such for use as a reinforcing support for a polythene film or coating. Cellophane and paper are both hydrophilic and the polythenes, which are of hydrophobic character,

do not adhere to the hydrophilic materials with the requiredtenacity evenunder' conditions considerably less severe than the conditions to which wrapping. material is normally exposed in practical use.

Testshave established that when polythene is deposited ou thesurfaceof a cellophane sheet 11 Claims. (Cl. 154-50) which is then pressed and cooled, the polythene is not permanently bonded to the cellophane and is readily separated from it by a light pulling force. The adhesion between the cellophane and polythene is so poorthat the cellophane carrying a the polythene can be used in transfer coating, the polythene being placed in contact with another base to which it is to be transferred by heat, and the cellophane being easily stripped off, leaving the polythene in contact with the other base.

Numerous problems arise, also, when it is attempted to coat paper with a polythene or laminate it with a polythene film. The polythenes are not easily soluble to produce solutions from which they can be applied, they are, per Se, viscous materials which do not penetrate the fibers of the paper and, being of non-polar type, they exhibit little inherent tendency to bond with the paper. The bond between the polythene and paper is not as strong as is required for practical use. These difiiculties are recognized and in an attempt to overcome them it has been suggested to modify the polythene with paraffin Wax before it is applied to the paper, Such modification is not a simple matter, since the product obtained by mixing parafiin wax with the polythene is very difficult to work with.

Some attempt has been made in the past to improve the adhesion of polythene to hydrophilic bases such as cellophane. For instance, it has been proposed to coat the cellophane with a solution of'a material which functions as an anchor for the polythene, dry thefilm and evaporate the solvent, apply a solution of the polythene to the dried film, and again dry the film and evaporate the solvent. That procedure is tedious, introduces additional steps into the process of preparlng cellophane sheets, and adds to the ultimat cost of the wrapping material.

An object of this invention is to provide a superior, heat-sealab1e wrapping material compris ing a hydrophilic base havin a polythene coating or film tenaciously associated with it, the base and polythene film or coating being inseparable under the conditions to which wrappin materials are normally subjected in manufacture or use. Another object is to provide such aheat-sea1able Wrapping material which is easily handled and heat-sealed on conventional wrappin machines to obtain an extremely strong heat-sealed bond. A further object is to provide a wrapping material comprising the hydrophilic base and substantially permanently adhered polythene film or coating which can be made without fundamental change in the methods and apparatus conventionally used in the manufacture of nonfibrous pellicles, or in conventional paper-making methods or equipment.

Broadly stated, the objects of the invention are accomplished by providing a wrapping material comprising a hydrophilic base containin a substantially uniformly distributed thermosetting resin in the thermoset state which conditions the base to receive and bond with polythene, and a film or coating of a polythene which is firmly anchored to the base by the thermoset resin In preparing the wrapping material, the hydrophilic base is conditioned to receive the polythene by pre-treatment with the thermosetting resin which is brought to the thermoset condition in or on the base, the polythene is deposited on the prepared base either in the molten state or in the form of a hot, preformed film, and the base carrying the polythene is pressed and cooled. It is found that the polythene bonds tenaciously with the base containing the thermoset resin and that when the resulting product is used as a wrappin material it can be heat-sealed on the wrappin machine to obtain a heat-sealed bond which is not ruptured by the forces to which the wrapped article is normally exposed. The improvement in the adhesion of the polythene to the base is particularly advantageous in the case of cellophane and paper, both of which are made available for use as commercially acceptable supports for the polythenes by the present invention.

The proportion of thermosetting resin incorporated with or applied to the hydrophilic base may be varied somewhat, but preferably the resin is present in an amount of about 0.5% to 2% by weight.

The hydrophilic base may be a non-fibrous pellicle produced from any suitable water-insoluble hydrophilic film-forming material, such as regenerated cellulose, Whether formed from viscose solutions, cuprammonium cellulose solutions or solutions of cellulose in inorganic or organic solvents, gelatin, casein, deacetylatecl chitin, Water-insoluble polyvinyl alcohol, cellulose ethers which are insoluble in but swollen by water, and water-insoluble alginate salts. The pellicles may be in the form of films, sheets, tubing, bands or shaped hollow bodies. Or the hydrophilic base may be paper of any type, including heavy duty kraft paper, formed from or comprising cellulose fibers.

The hydrophilic base is prepared and conditioned to receive the polythene by pi e-treating it with an aqueous solution or dispersion of the unpolymerized or partially polymerized thermosetting resin. There may be used for this particular purpose the water-soluble precondensates of melamine formaldehyde, urea formaldehyde, ketone-formaldehyde and phenol-formaldehyde resins, and mixtures of the several resin types. In general, the water-soluble precondensates of those thermosetting resins which have been recommended for use with paper to improve its wet strength are satisfactory.

In the preferred method of preparing the new Wrapping material, the base is pre-treated with an acidic aqueous colloidal solution containing from about 0.5 to by weight of a partially polymerized melamine-formaldehyde condensation product which is in a state of polymerization less than that characterizing a gel, said solution being water-dilutable, colloidal in nature, and havin positively charged, hydrophilic particles of less than about one micron in diameter. Solutions of this type are fully described in U. S. 2,394,009. The aqueous acid solutions are aged, after preparation, as set forth in the above patent, or in U. S. 2,345,543.

When the .hydrophilic base is a non-fibrous pellicle such as is obtained by extruding a filmforming material into an appropriate bath, it is preferably pre-treated with the thermosetting resin as a step in the manufacture of the pellicle by treating the latter, in the wet gel state, with the aqueous solution or dispersion of the unpolymerized or partially polymerized resin. For example, in conditioning cellophane to receive and hold the polythene, the Wet regenerated cellulose gel obtained by extruding viscose into a coagulatin and regenerating bath through a suitable film-forming device and subjecting it to the usual after-treating liquids, may be treated with the aged aqueous acid colloidal solution of the melamine-formaldehyde precondensate suitably diluted, prior to passing it through the usual aqueous bath containing a softening agent or plasticizer. Since regenerated cellulose normally carries a negative electrical charge, the positively charged resin particles in the aged dispersion or colloidal solution are preferentially absorbed by the cellulose. After heating and washing the pellicle, it is treated with the aqueous medium containing the softening or plasticizing agent (which may be glycerol, ethylene glycol, sorbitol, ethanolamine lactate, ammonium sulfamate or another hydroscopic substance), washed, and dried. In the final, dried pellicle the melamineformaldehyde resin exists in the fully condensed, thermoset state. The softening or plasticizing bath may also contain the partially condensed melamine-formaldehyde resin, or the resin may be applied after the softening bath.

The final, dried pellicle containing the thermoset melamine-formaldehyde resin is dyed uniformly throughout by a stain which is selectively absorbed by the resin but which does not affect the regenerated cellulose, indicating that the resin either unites chemically with the regenerated cellulose or is dispersed throughout the film in such small particles and so uniformly that variation in the stain is not detectable by ordinary instruments.

Although the thermosetting resin may be applied to paper, when the latter forms the hydrophilic base, after the paper has been formed, it is preferred to incorporate an aqueous solution or dispersion of the thermosetting resin with the slushstock either at the beater or at some other point of the machine system up to and including the machine headbox, and to sheet the resin with the stock. For example, the aqueous acid colloidal solution of the melamine-formaldehyde precondensate described above may be added to the paper-making stock in the beater and slieeted onto the paper-making wire or other forming means, the product'or article so obtainedbeing dried to obtain a sheet or other shaped article comprising paper-making fibers having associated therewith the melamine-formaldehyde resin in fully condensed, insoluble condition.

The polythene may be deposited on the melamine-formaldehyde resin-treated base in any manner. For example, a preformed layer or film of polythene may be anchored to the base by subjecting the base and film to heat and pressure. Or solid, powdered polythene may be applied to the base which is then passed through a heating zone, the polythene being melted by means of a heated doctor knife which spreads acsame it on the base. Again, e powdered polythene may be sprayed on the basethrough a. flame whichmelts it before it impinges on the surface of the base. Or the polythenermay be associated with the base by forming a layer of polythene ona metal surface, pressing the baseagainst the layer while the polythene is molten, cooling the polythene layer until its surface incontact with the metal is solid, and removing the base to which the polytheneis firmly bonded from the metalisurface. In the preferred embodimerit, ahot filmiof the polythene is applied directly touthebase. Preferably, the cellophane comprising the thermoset. melamine-formaldehyde resin, or the paper comprising the thermoset resin, is carried underan extruding device for the hot polythene supported above the nip between chilled polished rolls and the hot polythene film falls downwardly onto the base which thenpasses into the nip between the rolls. The polythene lays; evenly on the surface of the base and is pressed onto the base between the chilled rolls so that the sheet leaving the nip of the rolls is. a composite sheet comprising a polythene film or coating which is tenaciously anchored to the base by the thermoset resin. lIhe hot polythene film maybe stretched between the extruding device and the nip of the rolls to vary the thicknesslof the polythene film or coating bonded to. the base. The sheet is cooled as rapidly as possible while under the pressure of the rolls. This cooling or chilling sets the melamine formaldehyderesin-impregnated hydrophilic base and the polythene inthe bonded relation, and increases the toughness, transparency,

jandgloss of the polythene, the more rapid the cooling the greater the transparency. The finish of the surface of the. cooling roller will be reproduced inversely on the surface of the polythene, and the surface of the roller may be plain or it may be engraved or embossed to produce composite sheets having decorative surface effects. The film or coating of polythene on the hydrophilic base having the thermoset resin intimately associated with it preferably has a thickness of from 0.1 mil. to 1 mil.

The polytheneswhich are useful for the present purpose are those resinous polythenes having a. molecular weight between 10,000 and 38,000 and a softening point between 100 and. 112 C., preferably a molecular weight between 10,000 and 19,000 and a softening point between 100 and 108 C.

Two or more of the polythenes having molecular weights between 10,000 and 38,000 and softening points between 100-112 C. may be used in admixture, if desired. 1 The molten polythene may be modified by the addition of other ingredients for special purposes. For example, natural and synthetic plastic materials including natural or synthetic uncured rubber, waxes, resin, bitumin, polyisobutylene and other synthetic resins, dyes and pigments, and mineral fillers such as finely divided calcium carbonate or titanium dioxide may be used as modifiers.

The following examples in which parts are by weight unless otherwise indicated illustrate some embodiments of the invention.

EXAMPLE I Three moles of melamine mixture were mixed with moles of aqueous formaldehyde to give a solution having a pH of 9.0. The solution was heated for thirty minutes, cooled, and. the

6'. resulting crystallinecondensate was separated and dried. Fifty parts by weight of the condensate were dissolved in an aqueous solution containing 27.6 parts of 18 B hydrochloric acid and: 125 parts of water, thus giving a solution containing 25% by weight of the resin. The solution was diluted to 14% by Weight of the resin and allowed to age at room temperature for 24 to 28 hours. A bluish haze developed and the solution then exhibited the Tyndall efiect thus indicating that the resin particles had a diameter between 0.1 and 1.0 micron. The parti'cles migrated toward thecathode when a direct current was passed through the solution.

A sheet of washed regenerated cellulose in the wet gel state was passed through a solution obtained by diluting the melamine-formaldehyde resin solution prepared as above to a resin content of 3%. In the contact time of 201 seconds, sufficient resin solution penetrated the film to give a concentration in the film of. about 1% by weight of resin.

The impregnated pellicle was passed through squeeze rolls to remove excess solution, heated, washed, and then passed into a plasticizing bath containing 3.5% to 4% of glycerine. The pellicle was dried in the usual manner.

Molten resinous polyethylene having a molecular weight of about 19,000 and a softening point of 108 C. was extruded downwardly onto the melamine-formaldehyde resin-treated regenerated cellulose film as the latter advanced to the nip of a pair of press rolls. The film carrying the molten polythene passed into the nip of the rolls and was subjected to a pressure of 50-100 lb./sq. in. The roll adjacent to the polythene film was cooled. A composite film or sheet the sheet components of which were firmly bonded together was obtained.

The composite sheet was cut into strips one inch in width and the force in grams required to pull the polythene sheet from the regenerated cellulose base was determined on the wellknown 1P4 Scott tester, and compared with the force required toseparate the sheet components of a composite sheet obtained by depositing the molten polythene on a regenerated cellulose pellicle which did not contain a thermoset resin.

A 500 gm. pull was required to separate the polyethylene film from the resin-treated cellophane, whereas the adhesionof the polyethylene to the cellophane which did not contain the thermoset resin was so weak or slight that it could not be measured on the 1P4 Scott tester. In other words, the adhesion of the polyethylene to the cellophane was increased to 500 gms. by the melamine-formaldehyde resin. The sheet to which the polythene did not adhere sufiiciently to permit measurement of the adhesion on the IP 1 Scott tester could be used in transfer coating processes.

EXAMPLE II Awell-beaten bleached kraft pulp which did not contain resin, alum, or other filler was prepared and after all refining thereof had been completed a solution obtained by diluting the melamine-formaldehyde resin solution of Example I with water to a resin content of 5% was added at the fan pump. The pulp was then sheeted in the usual way, and the sheet was finished and dried in accordance with conventional practice. The paper leaving the machine comprised the melamine-formaldehyde resin in fully cured condition.

7 Molten resinous polythene having a molecular weight of about 12,000'and a softening point of about 104 C. was extruded onto a surface of the paper as it advanced to the nip of a pair of press rolls. The paper carrying the polythene was passed into the nip of the rolls and subjected to a moderate pressure (50-100 lbs/sq. in.) and cooled. A composite sheet was obtained in which the polythene was firmly bonded to the kraft paper base. The composite sheet was out into strips one inch in width and the force in grams required to pull the polythene film from the paper base was determined on the 1P4 Scott tester and compared to the force required to separate the sheet components of a product obtained by depositing molten polythene on a sheet of Kraft paper obtained from a kraft pulp containing 1% alum but which did not contain a thermosetting resin, with the results shown in Table II. In the table, A represents the composite sheet the paper component of which was pre-treated with the melamine-formaldehyde resin and B represents the sheet the paper component of which was not pretreated with a thermosetting resin.

Table I Gms. G'ms.

750 340 730 260 680 200 660 160 610 200 680 (mean) 260 686 (average) 244 EXAlWPLE III To three furnishes comprising unbleached Swedish kraft pulp there was added, at the fan pump, an aqueous dilution of the urea-formaldehyde' resin available commercially under the trade designation P-682-35 Beckamine. The resin dilution was added to the pulps in an amount to obtain pulps containing, respectively, 0.5%, 1 and 2% of the resin, by weight. The pulps were sheeted, and the sheets were finished and dried in accordance with conventional practice. The paper leaving the machine comprised the urea-formaldehyde resin in the thermoset state.

A hot film of a polythene as characterized herein was applied to a surface of the paper under pressure, and with cooling. The composite sheets thus produced were cut into strips 1" wide and the force in grams required to pull the polythene film from the paper base was measured on the 1P4 Scott tester. The results are shown in the following table, which also gives the results of the same test applied to a paper made from a pulp of the same type but without the use of any thermosetting resin.

Grams Representmg Adhesion Percent Percent Resin Increase EXAMPLE IV To two furnishes comprising bleached southern kraft pulp there was added, at the fan pump, an aqueous dilution of the urea-formaldehyde resin of Example III in amounts to obtain pulps containing, respectively, 0.5% and 1% of the resin by weight. After the pulp had been sheeted, and the. paper had been finished and dried in the usual way, a hot polythene film was applied to each of the papers as in Example III. The papers were cut into 1" strips which were tested on the 1P4 Scott tester with the following results, shown in contrast to the results obtained by subjecting a paper made of pulp of the same kind, but without the use of a thermosetting resin, to the same test.

Grams Representmg Adhesion Percent The paper pulp to which the acidic colloidal solution of the melamine-formaldehyde resin is added may contain the usual paper-making sizing materials and fillers, such as rosin size, wax size, alum, starch, glue and clay. Although the molten polythene may be blended with parafiin wax for special purposes, before it is deposited on the paper base, the invention has the advantage that, because of the presence of the melamine-formaldehyde resin in the paper base, the parafiin wax is not required to insure penetration of the molten polythene into the paper or to facilitate adhesion of the polythene to the paper base, and may be omitted.

The hot polythene film may be applied to one or both sides of the cellophane, paper, or other support containing the thermoset resin.

The invention provides pellicles of regenerated cellulose having bonded thereto a thin film or sheet of polythene which are strong, tough, exhibit low permeability and resistance to creasing, and from which the cellulose base does not tend to separate. These sheets are useful for many purposes, for example, as liners for bottle tops, as backing for pressure-sensitive tapes, and as superior wrapping and packing materials for a wide variety of products including foodstuffs, tobacco, pharmaceuticals and other substances susceptible to deterioration by oxygen, both as the wrapping which contacts such products directly and as the exterior wrapping for the cartons or other containers in which the wrapped products are packaged for shipping. The composite sheets may also be used as linings for multi-wall bags for fertilizers and chemicals, as well as for dried mill: and other powdered foodstuifs. The composite sheets comprising a paper base having a polythene film anchored to it by the thermoset resin, particularly the melamine-formaldehyde resin specifically exemplified herein, are characterized by high wet strength as well as excellent dry strength and fold endurance. They may be used as camouflage strips and netting, in the manufacture of paper bags which must substi tute for burlap in times of emergency, and as shipping containers which do not fall apart in the presence of water. In all of these applications, the overlapping edges of the wrapping or bag may be tightly sealed by heat and moderate pressure. The strength of the heat-seal thus obtained is such that is not destroyed by a force less than the force required to destroy or partially destroy one or both components of the composite sheet.

Since various modifications and changes may be made in practicing the invention, without departing from the spirit and scope thereof, the

invention is not to be limited except as defined by the appended claims.

I claim 1. As an article of manufacture, a composite heat-scalable wrapping material which comprises a base comprising a normally hydrophilic sheet selected from the group consisting of regenerated cellulose pellicles and paper carrying a substantially uniformly distributed thermoset resin characterized in that in the form of a precondensate it is soluble in aqueous media, and a film Of a plythene havin a molecular weight between 10,000 and 38,000 and a softening point between 100 C. and 112 C., the polythene film being firmly anchored to the thermoset resin and through said resin to the normally hydrophilic sheet whereby the wrapping material is adapted to be heat-sealed on conventional wrapping machines without separation of the polythene film from the base.

2. As an article of manufacture a heat-sealable wrapping material as in claim 1 in which the base comprises a regenerated cellulose sheet.

3. As an article of manufacture, a heat-sealable wrapping material as in claim 1 in which the base comprises a paper sheet.

4. As an article of manufacture, a composite heat-scalable wrapping material which comprises a base comprising a normally hydrophilic sheet selected from the group consisting of regenerated cellulose pellicles and paper carrying a substantially uniformly distributed thermoset melamineiormaldehyde resin characterized in that in a state of partial polymerization less than that characterizing a gel it forms acidic, aqueous solutions of from 0.5% to 20% by weight concentration which are water-dilutable, colloidal in nature, and have positively charged, hydrophilic particles of less than'about 1 micron in diameter, and a film of a polythene having a molecular weight between 10,000 and 38,000 and a softening point between 100 C. and 112 C., the polythene film being firmly anchored to the thermoset melamine-formaldehyde resin and, through said resin, to the normally hydrophilic sheet whereby the wrapping material is adapted to be heatsealed on conventional wrapping machines without separation of the polythene film from the base.

5. As an article of manufacture, a heat-sealable wrapping material as in claim 4 in which the normally hydrophilic sheet comprises regenerated cellulose.

6. As an article of manufacture, a heat sealable wrapping material as in claim 4 in which the normally hydrophilic sheet comprises paper.

7. As an article of manufacture, a composite heat-scalable wrapping material which comprises a regenerated cellulose sheet carrying a substantially uniformly distributed thermoset melamine-formaldehyde resin characterized in that in a state of partial polymerization less than that characterizing a gel it forms acidic aqueous solutions of from 0.5% to 20% by weight concentration which are water-dilutable, colloidal in nature, and have positively charged, hydrophilic particles of less than about 1 micron in diameter, and a film of polythene having a molecular weight between 10,000 and 38,000 and a softening point between C. and 112 C., the polythene film being firmly anchored to the thermoset melamine-formaldehyde resin and through said resin to the regenerated cellulose sheet whereby the wrapping material is adapted to be heat-sealed on conventional wrapping machines without separation of the polythene film from the regenerated cellulose sheet.

8. As an article of manufacture, a. composite heat-scalable wrappingmaterial which comprises a base comprising a normally hydrophilic sheet selected from the group consisting of regenerated cellulose pellicles and paper carrying a substantially uniformly distributed thermoset resinous reaction product of an aldehyde with a substance selected from the group consisting of melamine, urea, ketone, phenol and mixtures thereof, and a film of a polythene having a molecular weight between 10,000 and 38,000 and a softening point between 100 C and 112 C., the polythene film being firmly anchored to the thermoset resinous product and through said resinous product to the normally hydrophilic sheet whereby the wrapping material is adapted to be heat-sealed on conventional wrapping machines without separation of the polythene film from the base.

9. As an article of manufacture, a composite heat-scalable wrapping material comprising a regenerated cellulose pellicle as a base sheet, said pellicle carrying on at least one surface thereof a substantially uniformly distributed thermoset resinous reaction product of formaldehyde with a substance selected from the group consisting of melamine, urea, ketone, phenol and mixtures thereof, and a film of a polythene having a molecular weight between 10,000 and 38,000 and a softening point between 100 C. and 112 C., the polythene resin being firmly anchored to the thermoset resinous product and through said resinous product to the normally hydrophilic sheet whereby the wrapping material is adapted to be heat-sealed on conventional wrapping machines without separation of the polythene film from the base.

10. An article of manufacture as defined in claim 9 in which the resinous reaction product is a melamine-formaldehyde resin.

11. An article of manufacture as defined in claim 9 in which the resinous reaction product is a urea-formaldehyde resin.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,175,125 Mack et al Oct. 8, 1939 2,219,700 Perrin et al Oct. 29, 1940 2,394,009 Pollard Feb. 5, 1946 2,559,220 Maxwell et al July 3, 1951 2,563,897 Wilson et al. Aug. 14, 1951 FOREIGN PATENTS Number Country Date 551,339 Great Britain Feb. 18, 1943

Claims (1)

1. 7. AS AN ARTICLE OF MANUFACTURE, A COMPOSITE HEAT-SEALABLE WRAPPING MATERIAL WHICH COMPRISES A REGENERATED CELLULOSE SHEET CARRYING A SUBSTANTIALLY UNIFORMLY DISTRIBUTED THERMOSET MELAMINE-FORMALDEHYDE RESIN CHARACTERIZED IN THAT IN A STATE OF PARTIAL POLYMERIZATION LESS THAN THAT CHARACTERIZING A GEL IT FORMS ACIDIC AQUEOUS SOLUTIONS OF FROM 0.5% TO 20% BY WEIGHT CONCENTRATION WHICH ARE WATER-DILUTABLE, COLLODIAL IN NATURE, AND HAVE POSITIVELY CHARGED, HYDROPHILIC PARTICLES OF LESS THAN ABOUT 1 MICRON IN DIAMETER, AND A FILM OF POLYTHENE HAVING A MOLECULAR WEIGHT BETWEEN 10,000 AND 38,000 AND A SOFTENING POINT BETWEEN 100* C. AND 112* C., THE POLYTHRENE FILM BEING FIRMLY ANCHORED TO THE THERMOSET MELAMINE-FORMALDEHYDE RESIN AND THROUGH SAID RESIN TO REGENERATED CELLULOSE SHEET WHEREBY THE WRAPPING MATERIAL IS ADAPTED TO BE HEAT-SEALED ON CONVENTIONAL WRAPPING MACHINES WITHOUT SEPARATION OF THE POLYTHENE FILM FROM THE REGENERATED CELLULOSE SHEET.