Tear string assembly for containers

BACKGROUND OF INVENTION

This invention relates to a tear string assembly for a pouch construction and to a method of making the same. In particular, this invention relates to a fibrous tear string impregnated with dewaxed shellac which string has improved adhesion to the polyethylene lamina of a flexible container in which the string is embedded and to the method for making such an assembly.

It is well known to include a tear string in flexible foil pouch structures coated in their interior with an extruded polyethylene lamina. Such a structure permits easy opening of the heat sealed pouch. See, for example, U.S. Patent No. 3,057,539 to Leary, Ir. Most of such pouches have required that the string be buried between the polyethylene coating and the substrate to which the extruded polymer is applied. The polyethylene lamina usually faces the interior of the pouch. The drawback to such a structure is that when the string is pulled to open the pouch the polyethylene coating remains intact and has to subsequently be pulled apart by hand in order to get to the contents of the pouch. Attempts were made to eliminate this problem by laying the string on the exposed side of the extruded coating. Previous to this invention such attempts did not meet with a satisfactory degree of success because of the inability of those skilled in the art to secure proper adherence of the string to the polyethylene.

It has been discovered that if the tear string is first impregnated with a dewaxed shellac, such as dewaxed orange shellac, then embedded in the polyethylene while the latter is in a molten condition, and if the composite is then cooled, that the string will securely adhere to the polyethylene. The string may now be laid on the face of the polyethylene exposed to the interior of a container. In addition, the other face of the polyethylene may be laminated to materials such as metal foil, thermoplastic material, paper, etc. or to combined laminates of these materials, for the purpose of producing a pouch better suited for packaging a particular product.

The inherent advantages and improvements of the present invention will become more readily apparent upon considering the following detailed description of the invention and be reference to the drawings in which:

DETAILED DESCRIPTION

FIG. 1 is a view in cross section of one embodiment of the product of this invention, with some parts broken away.

FIG. 2 and FIG. 3 are views in cross section of two additional embodiments of the products of this invention, with some parts broken away.

FIG. 4 is a top view of a container embodying a product of this invention.

FIG. 5 is a schematic flow plan of the process of this invention.

FIG. 6 is a view in cross section of one product and some of the apparatus used in the process of this invention, with some parts broken away.

Referring now to FIG. 1, the most basic product of this invention is illustrated. A cross sectional view of a polyethylene lamina is shown containing a securely embedded fibrous string. The string is impregnated with dewaxed shellac.

FIGS. 2 and 3 illustrate further embodiments of the product of this invention. In FIG. 2 a substrate material has been securely laminated to the composite article of FIG. 1. The fibrous string and substrate are attached :to opposite faces of the polyethylene lamina. The substrate material could advantageously be, for instance, metal foil. FIG. 3 shows lamina of thermoplastic material and of paper securely laminated to a composite article similar to that of FIG. 2 in which the substrate is of metal foil. The additional laminae have been added to the composite article of FIG. 2 adjacent the lamina not containing the fibrous string.

In FIG. 4 a container is illustrated embodying one of the products of the invention. The container is shown for purposes of illustration as a fin sealed pouch 10, but the invention is equally useful with other types of containers. Pouch 10 is constructed of two rectangular sheets of composite laminae laid face to face and sealed together only at their perimeters. Each sheet of composite laminae may be of the type illustrated in FIG. 3, i.e. paper, thermoplastic material, metal foil and polyethylene, securely laminated together. Commonly two such sheets are laid polyethylene face to polyethylene face and then heat sealed together at their perimeters 12. The unsealed area between the sheets is used to contain a product which is inserted therein before the pouch is completely sealed.

The sheet forming the top wall 14 of pouch 10 has been provided with a tear string 16. Tear string 16 is a fibrous string such as cord or thread which has been impregnated with dewaxed shellac and securely embedded in the polyethylene lamina of top wall 14 in the arrangement shown in FIG. 3. When top wall 14 and bottom wall 18 of pouch 10 are heat sealed together string 16 is securely sealed to both sheets for the portion of its length intersecting the sealed perimeters 12 of said sheets. The remaining length of tear string 16 is embedded only in the interior facing polyethylene lamina of top wall 14.

Slits or notches 20 have been provided at one perimeter 12 of pouch 10 on both sides of string 16. These facilitate the opening of the container which is accomplished as follows. The portion 22 of perimeter 12 between notches 20 is grasped with the fingers of one hand and pulled across the top face of pouch 10 while the pouch itself is held secure against movement. This causes the portion 22 of perimeter 12 to tear away from pouch 10, taking with it a small portion 24 of top wall 14 as well as tear string 16. Continued pulling on portion 22 causes tear string 16 to completely sever through and across all of the various laminae of top wall 14, thus providing easy access to the interior of pouch 10. FIG. ⁴shows pouch 10 partially torn open.

A schematic flow plan of the method of this invention is shown in FIG. 5. The first step in the method is to impregnate a fibrous string with a dewaxed shellac.

This can be satisfactorily accomplished in a continuous manner in an in-line operation by soaking the string in

3,494,538 a 1.2% solution of a dewaxed shellac, such as dewaxed orange shellac, in ethyl alcohol until the string is substantially saturated, followed by heating the wet string to about 175° F. to remove all the solvent alcohol. Such impregnation produces a “polarized” surface condition on the string which will enable it subsequently to “bite” into the polyethylene. In other words, the impregnation primes the string surface for better adherence to the polyethylene.

In another step of the process a molten polyethylene lamina is prepared. This may be accomplished by any of the processes known to the art, such as, for example, by extrusion of molten polyethylene at a temperature of about 600° F. through a narrow film or curtain-forming extrusion die. The next step involves embedding the dried 15 impregnated string in the molten polyethylene lamina. This can be done, as illustrated in FIG. 6, by calendering. The molten lamina and the freshly impregnated string is an in-line operation are continuously brought into contact in the nip of a pair of rotating pressure exert- 20 ing rolls. It is at this point that the improved ability of the string to “bite” into the polyethylene is utilized. A pressure of about 50 p.s.i between the rolls is sufficient to embed the string in the lamina.

If desired, other optional additional lamina may be lam- 25 inated to the polyethylene lamina while the component materials are between the pressure rolls. Lamina of metal foil, thermoplastic material, paper, and the like may be laminated singly or in combinations to the polyethylene lamina. Such additional laminae can be located on the 30 same or opposite face of the polyethylene lamina as that containing the embedded string.

As a final step in the process the thus produced composite article is cooled in any convenient manner to about room temperature. For example, the pressure exerting 35 roll which contacts the surface of the polyethylene into which the string is embedded can advantageously be a cooled chill roll. By this means the string is embedded and the composite product is cooled.

The illustration in FIG. 6 shows the calendering step 40 in one possible method for making the product of FIG. 3. Into the nip of pressure rolls 30 and 32 are fed a dried impregnated fibrous string 34, a molten polyethylene lamina 36 and a preformed laminate of metal foil 38, thermoplastic material 40 and paper 42. Roll 32 is a 45 cooled chill roll. The calendering operation laminates the preformed laminate to one side of the polyethylene lamina, embeds the fibrous string in the other side of the polyethylene lamina, and then cools the composite article. 50

The fibrous string used in the product and process of this invention preferably comprises polyester fibers in the form of a string, thread, cord, or the like having a round, flat or any other desired cross section.

The shellac referred to herein is an acetone or alcohol 55 solution of a natural resin secreted by the lac insect, most of which is imported from India. Such shellac is naturally orange in color, but can be bleached colorless by known techniques. It is necessary for purposes of this invention that the shellac be “dewaxed,” i.e. that its 2-4% 60 natural wax content be removed. Such wax acts as a lubricant which interferes with firm adherence of the shellac impregnated string to the polyethylene lamina into which it is embedded. A suitable dewaxed shellac is sold by F. H. Paul & Stein Bros., Inc., 235 5th Ave., New 65 York, N.Y. 10016.

Other materials besides dewaxed shellac have been found which improve fibrous string-polyethylene adherence. A 10% toluene solution of Du Point Elvax 260, which is a copolymer of ethylene vinyl acetate, has been 70 found satisfactory.

Adcote 312B, a polyester isocyanate, made by the Morton Chemical Co., may also be used.

In one experiment a Dasew V-14 shade white polyester string was treated with a solution of 1.2% dewaxed orange shellac in ethyl alcohol. The treatment was accomplished by means of a rotogravure printing press. The reservoir of the press normally filled with ink was filled in this case with the shellac solution. The polyester string was passed through this reservoir and then conveyed to the press’ drying oven. There the string was heated to 175° F. for a sufficient time to remove all of the solvent alcohol. A preformed laminate composed of paper (25 pound bond), polyethylene (½ mil thick) and aluminum foil (0.00035 in. thick), in that order, was prepared. A pair of calender rolls was provided, one of the rolls being a cooled chill roll. The pressure between these rolls at their nip was set at about 50 p.s.i. The preformed laminate was fed into the rolls’ nip with the aluminum foil surface facing the cooled chill roll. Low density (.914-.925 when compared with water equal to unity) polyethylene was extruded as a molten thin lamina at about 600° F. and of about 1 mil thickness. This molten lamina was fed into the nip of the calender rolls adjacent the aluminum foil surface of the preformed laminate. The freshly prepared impregnated string was fed into the rolls’ nip between the molten polyethylene and the cooled chill roll.

The composite article exiting from the calender rolls had been cooled to about room temperature by the cooled chill roll. It was observed that a securely adhered laminate had been produced containing a securely embedded string. The string was pulled, starting at one end, completely through the laminate to sever it into two sections without experiencing failure in this operation due to premature operation of the string from the polyethylene lamina.

While presently preferred embodiments of the invention have been illustrated and described, it will be recognized that the inventions may be otherwise variously embodied and practiced within the scope of the claims which follow.