METHOD FOR SELECTIVE HEAT SEALING OR JOINING OF MATERIALS

3,528,867

Patented Sept. 15, 1970

3,528,867

METHOD FOR SELECTIVE HEAT SEALING OR JOINING OF MATERIALS Alfred F. Leatherman, Columbus, Ohio, and William C. Heller, Jr., 3521 N. Shepard Ave., Milwaukee, Wis. 53211; said Leatherman assignor to said Heller Filed Aug. 15, 1966, Sen No. 572,580 Int. CI. B29c 19/02 U.S. CI. 156—272 8 Claims

ABSTRACT OF THE DISCLOSURE

A method for selective heat sealing materials at predetermined interfacial surface areas comprises the steps of positioning the material surfaces in opposing abuttable relation, applying a susceptor material to a nonopposing surface of the material in registration with the predetermined interfacial areas, and inducing heat in the susceptor material to seal the opposing surfaces at the interfacial areas.

This invention relates, in general, to the art of joining of non-metals with the aid of heat, and relates especially to improved means by which a seal, bond, or joint can be obtained directly between the materials, such as thermoplastic films, being joined.

This invention permits bonding to be accomplished with or without an adhesive agent, on a continuousprocess basis if desired, without the need for contact between the materials and the energy source, and with improved freedom in the size and shape of the seal. At the same time, in certain embodiments the method is used to prevent heat sealing at selected areas.

The method is applicable to joining of diverse covering materials in a variety of forms and dimensions, such as fabrics, sheets, films, webs, plates, bars, etc., with relative insensitivity to gage variation, and is applicable to materials varying in composition such as oriented or unoriented materials, joining dissimilar materials to one another, and to various processes such as preheating, melting, and laminating in addition to sealing of bags, carton liners, bag closures, carton overwraps, coated folding cartons, and the like.

One of the most common present practices for heat processing, such as the heat sealing of thermoplastic films, is to employ heated-element devices which are energized by electric self-resistance heating such as in the "impulse" type of sealer. Such equipment requires direct connection to a source of electrical energy. This requirement means that if continuous processing is required by this conventional method, it is necessary to mechanically translate the heating element in step with the work during sealing. This calls for complicated machinery, electrical slip rings, etc., to maintain the proper forces on the materials being sealed and to maintain electrical connection to the heating elements.

The method of the present invention permits the source of heating energy and applicators, etc., to remain stationary, if desired, while the work being heated or sealed can move smoothly in a continuous process. This is accomplished in some embodiments by means of heat-generating agents, referred to herein as susceptors, which are deposited, printed, extruded, laminated, doctored onto or otherwise made a part of the materials being sealed. The heat-generating agent, or susceptor, thus moves with the work during processing and creates the heat needed for sealing upon passing near an appropriate stationary source of energy such as an induction coil, pair of dielectric heating electrodes, microwave source, radiant or laser energy source, ultrasonic or electron beam, etc.

In other embodiments, the process is performed in a stationary arrangement or "batch" method, and in other forms the energy source can be moved while the materials being sealed remain stationary. Also, in general, the sealing material need not necessarily be preattached to the work but can be fed or wound into the process simultaneously as a filament or tape, etc., or can be dusted into place, metered on, printed on. In many cases, it is reasonable to reclaim the heat-generating material for re-use with economic advantages while the finished sealed product then does not contain the heat-generating material. In other cases, by leaving the heat-generating material in the product, at least temporarily, subsequent additional seals can be made in the same area thus making a package closure or adding new layers of material to the product, or the like.

The conventional heating element method of heat sealing also is restricted by the need to employ a heating element device having the same size and shape as the desired seal area, since all portions of the work exposed to the heating element normally become sealed. Furthermore, when it is necessary in the conventional method to change the shape of the sealed area, substantial, expensive, and time-consuming changes can be necessary in changing the heating element to a new shape to provide the new desired shape of sealed area.

The method of the present invention can employ application of the heat-generating agent only at the areas desired to be sealed. Since areas of the work are not provided with the heat-generating agent of the present method are affected very little or not at all in passing near the source of energy, the size and shape of the desired seal are readily predetermined by the susceptor material and the seal can be restricted only to the intended dimensions.

In certain processes, susceptor materials of certain types have heretofore been used to accomplish some advantages of the present invention such as continuous processing, no contact with the heat source, and predetermination of the sealed area. However, these past methods illustrate the use of the heat-generating agent directly at the interface being sealed. In attempting to practice these past methods some difficulty has been found in obtaining full seal strength when the composition of the heat-generating agent directly at the seal area is not optimum with respect to the composition of the materials being joined.

However, we have discovered that good heat transfer to the bond area can be obtained by locating the agent near but not at the bond area. When suitable pressure, etc., is then applied at these heated regions, direct bonding of one workpiece to another occurs, providing a superior direct seal. Thus, with the method of the present invention, optimum material preparations can be made in advance and the final seal, such as in closing a package at a packing plant, can be made between naturally matched surfaces. Also, in this same process, the composition of the heat-generating agent can be purposely selected to be incompatible with the materials to be sealed. In the latter case, it is found that although the heat-generating agent supplies enough heat to seal materials near it, the composition is such as to prevent sealing immediately to the heat-generating agent itself, thus providing sealed and unsealed areas at the same portion of a multilayered product.

Since the heat is generated at one surface of a material in the present invention while heat sealing is accomplished at a different surface, means are provided by which decoration, inked copy, or the like, can be applied to surfaces remote from the greatest source of heat so as to reduce or avoid possible degradation of such printed matter by the affects of elevated temperature.

3

In other types of heat-sealing processes used in the past, problems have been encountered, for example, in attempting to heat seal, by dielectric heating, plastic films directly. Several of the most popular films such as polyethylene, polystyrene, and polypropylene offer such low dielectric loss properties that indirect methods have been required, such as the use of lossy buffers as in U.S. Pat. No. 2,667,437 to Zoubek, which are themselves heated by the dielectric energy and which transfer this heat to the work essentially as in the common heating-element meth- Jq od. Since contact with the work is required, prior methods such as this can be highly restrictive in attempting to perform continuous sealing.

On the other hand, the methods of the present invention can provide an arrangement with a heat-generating 15 agent that is selected to be receptive to dielectric heating and that can be a part of the work itself if desired, and move with it, thus eliminating a former problem of dielectric heat sealing in continuous processes.

In attempting to heat seal thermoplastic films by radiant 20 energy such as infrared rays, it is found that many popular transparent or translucent films transmit the energy without sufficient absorption to produce the desired heating action. The present invention provides an arrangement with a heat-absorbent or heat-generating agent that 25 is receptive to this type of energy, thus intercepting the rays and converting them to efficient heating action for use in sealing.

Therefore, it is an object of the present invention to provide a method of heating which permits a direct bond 30 between the natural surfaces of the parent materials being sealed.

Another object is to provide means to avoid the heat sealing of one layer of material, for example, while producing a heat seal at a different layer at the same time. 35

A further object of the present invention is to provide means to circumvent the exposure of decorating inks and the like to maximum heat sealing temperatures.

It is a further object of this invention to provide a method of heat sealing which is useful with many different 40 sources of energy and which voids the need for any direct contact between the energy source and the work, thereby permitting continuous motion of the work and avoiding intermittent stopping to make a seal, the method also being useful for noncontinuous or intermittent processing. 45

Yet another object is to provide a means whereby the area desired to be sealed can be restricted and predetermined prior to the sealing step and in which the area to be sealed is thus automatically limited even though the source of energy be applied to the entire body of the work 50 in process, whereby a single energy applicator can be employed for making an unlimited variety of shapes and sizes of heat seals without changing the applicator.

In the drawings:

FIG. 1 is a fragmentary schematic view showing an 55 embodiment of the invention for dielectric heat sealing of thermoplastic films;

FIG. 2 illustrates an alternative embodiment of a portion of FIG. 1;

FIG. 3 is a fragmentary schematic view showing the 60 joint use of pressure rolls as electrodes;

FIG. 4 is a like view showing the collection of the susceptor material for re-use;

FIG. 5 is a fragmentary schematic view showing an arrangement of equipment using induction heating to pre- 05 pare a multilayered sheet containing susceptor material;

FIG. 6 is a similar view showing an embodiment of the invention useful for joining of materials one or more of which normally are not heat sealable, such as paperboard stock; 70

FIG. 7 is a like view showing an arrangement in which radiant energy is used for joining materials which themselves have not been specially prepared in advance;

FIG. 8 is a sectional view of a stationary induction heat sealing arrangement in which heat sealing is avoided 75

4

at one layer of an assembly of plastic-coated paperboard while being accomplished at two other layers;

FIG. 9 illustrates the accomplishment of three heat seals between plastic films while preventing heat sealing at two other areas of the assembly; and

FIG. 10 illustrates a combination with the features of FIG. 9 but which permits an additional heat seal to be made without using additional susceptor material.

Referring to FIG. 1, two heat-sealable thermoplastic films 11 and 12 are shown being advanced from left to right and being heat sealed in a continuous process according to the present invention. In this case, the films could be, if desired, of the popular type such as polyethylene which have certain properties that create problems in several types of conventional heat sealing. Such problems stem from the transparent or translucent character of such materials, their low dielectric loss, very low electrical conductivity, and very low magnetic loss properties. These properties make it generally impractical to heat such films directly by means of radiant heating, dielectric heating, eddy-current induction heating or induction-hysteresis heating methods. Therefore, in the past, it has been common practice to heat seal such films by contact heat transfer methods using an external heat source such as the hot-wire impulse type. Such transfer sealing methods often result in difficulties in operation of a continuous process. Also, difficulties are encountered in control of heat transfer to the films often resulting in warping, sticking, wrinkling, charring, and the like.

At the left-hand region of FIG. 1, the film 11 has special auxiliary material 14 applied thereto or as a part of it. The auxiliary material 14 is selected to be energizable by an appropriate energy source so as to serve as a source of heat for heat sealing. The auxiliary material serving as a source of heat when energized, is referred to herein as a susceptor. The susceptor 14 has been applied to, or made a part of, the film 11, such as by various printing methods including offset, gravure, flexography, silk-screen, magnetic printing, xerography, or the like, or has been applied by other coating processes such as doctoring, laminating, extrusion, pneumatic, or electrostatic jet. It is not necessary for the susceptor 14 of FIG. 1 to be tightly adhered to the film 11 so long as it remains in position until after sealing is accomplished. Further, if desired, the application of the susceptor 14 to the film 11 could be accomplished conveniently as part of a continuous or discontinuous process such as already might be in use in an existing production line for the decorating, perforating, slitting, treating, rewinding, or folding of the film 11.

Although the susceptor material 14 is shown in FIG. 1 as having been applied only to one film member 11, it is understood that technically the purpose of the susceptor material is to generate sufficient heat to accomplish heat sealing. In some instances, therefore, especially when very high sealing speeds are desired, it may be necessary or desirable to employ susceptor material on both films, perhaps at opposite locations as shown in FIG. 2 for the susceptor material 14a, 146 and films 11a and 12a. It is also noted in FIG. 2 that the susceptor materials may have different sizes and shapes. Other means by which cost savings can be realized by reducing or altering the arrangement of the susceptor material will become evident to those skilled in the art. Methods for reclaiming the susceptor are considered below.

At a later station in the process in FIG. 1, the susceptor 14 passes between conventional dielectric heating electrodes 16, connected to a source of high-frequency voltage 17, which could be operating at one of the popular commercial frequencies such as 27.5 or 41 megacycles. In this case, the films 11 and 12, and susceptor 14 are shown not touching the electrodes 16, but it is understood that the invention may also be practiced with contact to the electrodes. Also, films 11 and 12 are shown with a parallel space between them. The size of the space shown in FIG. 1 is exaggerated for purposes of clarity. Nor