FREE EDGE-SEALED FILM ARTICLES AND PROCESS OF MAKING SAME

Abstract: Free edge-sealed film articles (10) and a process for making such articles, wherein the articles comprise a plurality of film portions (12 and 14) and contain a free edge-seal between two of such portions (12 and 14) with one or both of the two free edge-sealed portions (12 and 14) consisting of a plurality of layers (18 and 20) of diverse thermoplastic materials which are so ordered that on sealing the two portions (12 and 14) together, the layered film portion (12) is curled toward the other of the two free edge-sealed portions (14).

Description: FREE EDGE-SEALED FILM ARTICLES AND PROCESS OF MAKING SAME The present invention relates to that class of film articles which will for present purposes b'e termed the "free edge-sealed" film articles and to processes for making such articles. Many useful film articles are currently made by processes involving the application of heat, solvents or the like to adjacent film portions to effect a seal therebetween, wherein at the point of application the adjacent film portions are fundamentally unsupported and thus possess some freedom of movement to respond to the application of such influences. Manufacturing processes which can be characterized in this fashion are described, for example, in United States Patents No. 3,508,378 to Fehr et al., 3,736,219 to McTaggart, 3 . 9.7,198 to Hutt, .,055, -52 to Carlisle, and .,502,906 to Young et al. Each of these processes involve the application of some film-sealing medium or means across a gap between two film plies, wherein the film plies are not pinched or pressed between a pair of heated surfaces in sealing the plies together but in a sense are sealed between two "free" edges or portions of the plies. The "free edge-sealed" articles to which the present invention relates will, then, be understood to encompass the broad spectrum of film articles made by the processes characterized according to the preceding paragraph. In previous constructions of free edge-sealed film articles wherein diverse thermoplastic materials have been used, the materials have been used in a single blend layer or in a layer structure within a layered film portion which is balanced in terms of the moduli of the layers in the structure. These articles have been so constructed at least in part because of concerns over the curling at room temperature of plural layer films of the various diverse materials. Each of the film portions in the single blend layer and balanced layer constructions handles and seals essentially in the fashion of a layer of a single material, with the seal strength of a seal between adjacent film portions being dependent generally on the same factors known to be implicated in determining the seal strength of free edge-sealed film portions comprised of a single material. These factors include whether the film portions to be joined are properly clamped, the gauge of the film portions to be joined, and the tension placed on the film portions. Manufacturers of free edge-sealed film articles have because of these factors experienced significant difficulties in the past in achieving consistent seals, and these poor or inconsistent seals can limit the barrier or structural properties which can be achieved by the use of diverse and perhaps more costly materials, in addition to reducing the appeal of the articles to consumers. It would be desirable, then, if free edge- sealed articles could be made wherein an improvement in properties realized by a combination of diverse materials in the articles and the consumer appeal of such a combination were not limited or even overcome by the formation of weak or incomplete seals between film portions in a significant number of the articles. The present invention fulfills this desire by providing a free edge-sealed film article which -is comprised in general terms of a plurality of film ' portions and which contains a free edge-seal between two of the plurality of film portions. At least one of the two free edge-sealed film portions consists of a plurality of layers of diverse thermoplastic materials and is curled toward the other of the two film portions along at least a portion of the free-edge seal in the process of free-edge sealing the film portions together. Thus, it may be that only one of a respective pair of free edge-sealed film portions is layered, with the other film portion being comprised of a single layer only and only the one layered film portion curling toward the single layer portion. Or, both of the free edge-sealed film portions may be layered, but with one of the two film portions having a balanced structure such that again only one of the two layered film portions is curled toward the other. Or, both of the two film portions may be layered, and the layers within each ordered so that on sealing both film portions are curled toward the other. The curling of a given film portion toward a second film portion during the free edge-sealing of such film portions enables the formation of a superior seal between the two film portions on a more consistent basis, as particularly evidenced by a lesser likelihood of leaks in certain reclosable plastic containers constructed in accordance with the present invention. Fig. 1 is a fragmentary cross-sectional view of apparatus for free edge-sealing two layered film portions together. Fig. 2, taken along line 2-2 of Fig..1, is an enlarged fragmentary cross-sectional view of the two layered film portions to be free edge-sealed together. Fig. 3 is an enlarged fragmentary cross-, sectional view of a typical bead as formed by a single layer film portion on contact with a hot sealing wire in one free edge sealing method. Fig. 4 is an enlarged fragmentary cross- sectional view of a typical hot wire, free edge seal between two single layer film portions of the type shown in Fig. 3. Fig.5 is an enlarged fragmentary cross- sectional view of the formation of the seal between two layered film portions constructed and stacked as in Figures 1 and 2, and sealed in the same fashion as the stacked single layer film portions in Fig. .. Fig. 6 is an enlarged fragmentary cross- sectional view of a completed free-edge seal between the two layered film portions in Fig. 5. Referring now to the drawings, and more particularly to Fig. 6, a preferred construction 10 of the free edge-sealed film articles of the present invention is depicted. In this preferred construction, the film article is comprised of a plurality of film portions, with two of these film portions being the film portions 12 and 14 variously shown in Figures 1, 2, 5 and 6. Referring now to Fig.1, the film portions 12 and 14 are shown being clamped by elements 11 and 13 in position to be sealed together in a free edge-sealing apparatus 15, the free edge seal to be formed approximately along the line 2-2 by free edge sealing means such as a hot wire 16. As has been previously noted and as is well known in the art, a gap 17 is often formed between the film portions 12 and 14 in a clamped condition which must be bridged in order to obtain an effective seal between the film portions 12 and 14. This gap 17 and the preferred construction of the layered film portions 12 and 14 are evident in Fig.2. In the preferred construction of the film portions 12 and 14 shown in Fig.2, the film portions 12 and 14 are substantially identically constructed of layers 18 and 20 of two diverse thermoplastic materials. The "diverse thermoplastic materials" are for present purposes those materials which are characterized by differing amounts of frozen-in stress or differing melt properties generally, whereby on melting in the presence of the hot wire 16 the layers 18 and 20 retract at faster and slower rates, respectively, from the hot wire 16. The "frozen-in stress" mentioned above can be considered a residual level of stress contained in layers 18 and 20 primarily as a function of the character of the materials comprising those layers. For example, the extrusion and drawing down in a conventional film manufacturing process of low density polyethylene(LDPE) can be seen to produce a level of residual or frozen-in stress in the resulting film layer due to the long side chains (comprising fifty or more carbon atoms) in the LDPE being bent and stretched along the backbone of the LDPE molecules. By the same token, the extrusion of linear low density polyethylene(LLDPE) (with side chains of fifteen and fewer carbons, generally) can be seen to result in comparatively lower amounts of frozen-in stress in the resulting film layer. The stresses involved are "frozen-in" in the sense that the materials in layers 18 and 20 are at least temporarily fixed in a different spatial configuration than the lowest potential energy configuration to which they would naturally be inclined in a molten state. With a proper selection of materials and layer thicknesses for the layers 18 and 20 formed from these materials, the differing rates of retraction experienced by layers 18 and 20 on free edge sealing can create a net bending moment in film portions 12 and 14, so that the ends of the film portions 12 and 14 are curled toward one another in forming a seal as shown in Figures 5 and 6 and described below. This tendency to curl toward one another on sealing, it will be appreciated, will be advantageous in bridging the gaps 17 which might be formed between layered film portions 1? and 14 when clamped together in the sort of free edge sealing apparatus 15 depicted in Fig. 1. A free edge-sealed reclosable plastic storage bag having substantially identically constructed film portions 12 and 14 which consist of a low density polyethylene layer 18 and a linear low density polyethylene layer 20, for example, demonstrates a tendency for the two film portions 12 and 14 to curl together where the linear low density polyethylene selected has a density of from about 0.912 to about 0.941 grams per cubic centimeter, and further where the linear low density polyethylene layer 20 comprises o.n . the average from 30 to 70 percent of the thickness of film portions 12 and 14. In contrast is the behavior of film portions 12a and 14a which are constructed from a homogeneous blend of materials, from a single material, or as a balanced set of layers of diverse materials when sealed in the manner suggested by Fig.1. As can be seen from Figures 3 and 4, film portions 12a and 14a retract from a wire 16 uniformly and without curling, while showing however a slight inclination in the direction of movement of the wire 16. The free-edge seal between such portions 12a and 14a results from the portions 12a and 14a being clamped closely enough, given the size of the beads formed in portions 12a and 14a, for the beads to run together. Thus, the formation of an acceptable seal between such film portions 12a and 14a is dependent as mentioned previously on such factors as whether the film portions to be joined are properly clamped, the gauge of the film portions to be joined, and the tension placed on the film portions. On the other hand, as may be seen with particular reference to Figure 5, when the layered film portions are properly constructed and arranged in accordance with the present invention, the beads formed by the layered film portions are predisposed to form a seal with a respective film portion by expanding or curling (in the direction of arrows 22) toward that film portion during the free-edge sealing process. While a preferred embodiment of the free edge- sealed articles of the present invention is as described above, a number of other embodiments are possible and may be employed if desired. For example, the film portions 12 and 14 can individually contain or can each contain any number of layers of any number of diverse thermoplastic materials, so long as the cumulative effect of the layers within at least one of the two film portions 12 and 14 is the curling toward the other of the film portions along at least a portion of the resulting free-edge seal between film portions 12 and 14. It will also be appreciated that a free edge- sealed film article constructed in accordance with the present invention may be comprised of a number also of other layered film portions to be free edge-sealed to a single layer film portion, or to a different layered film portion of the same or a different construction. It may be desirable for still other applications to create a localized zone of weakness in the seal between two film portions, at least one of which is layered in such a fashion that on free edge- sealing the two film portions together the layered film portion is curled toward the other film portion along at least a portion of the seal. A localized zone of weakness in a different portion of the seal can be achieved for example by altering the relative thicknesses of layers within a layered film portion so that it curls away from, rather than toward, the other film portion to which it is being sealed. In another 10 embodiment, seals between film portions may be made of different strengths, so that for instance a completely sealed plastic film container is preferentially ruptured along certain seals. m m Considering, however, more conventional uses of free edge-sealed film articles, it will normally be desirable to employ the teachings of the present invention to form uniformly stronger seals (in the sense of requiring a greater energy to break the seals) 0 between free edge-sealed film portions. For example, a most preferred embodiment of the articles of the present invention will be as reclosable plastic containers of the type having two flexible opposed free edge-sealed sidewalls and an interlocking closure mechanism which is 5 operable for being interlocked over a predetermined length and which is connected to the two sidewalls adjacent an open end of the container. More particularly, the most preferred 0 reclosable plastic containers of the present invention will be characterized by two opposed flexible sidewalls free edge-sealed along at least two edges, and an interlocking closure mechanism comprising a pair of opposed interlocking rib- and groove-type closure members which may be interlocked to substantially effect a sealing closure of the adjacent open end of the container. The opposed flexible sidewalls are preferably substantially identically constructed with each being from 1 to 4 mils (0.025 to 0.1 mm) in thickness and consisting of an inner layer 18 relative to the interior of the bag of low density polyethylene, and an outer layer 20 of a linear low density polyethylene having a density of from 0.917 to 0.935 grams per cubic centimeter. The outer LLDPE layer, further, should preferably comprise from 35 to 65 percent, and most preferably from 45 to 55 percent of the thickness of a respective sidewall on the average. The most preferred reclosable plastic containers should preferably have at least one and most preferably both of the sidewalls curl toward the other sidewall on sealing the same together, so that the reclosable plastic containers leak to a significantly lesser extent when blown up like a balloon and submersed underwater than reclosable containers which are substantially identically constructed, blown up and submersed but which comprise sidewalls of the same diverse materials in a blend. As a measure of this improved sealing, the reclosable plastic containers of the present invention should preferably have less than 20 percent of the leaks, and most preferably less than 10 percent of the leaks seen in the same container but with blended rather than layered sidewalls. A preferred process for making a free edge- sealed film article such as described herein would involve coextruding layers of diverse thermoplastic materials in a continuous film web, while ordering and dimensioning the layers according to differing levels of frozen-in stress, for example, to create a tendency to curl in the assembled layers in a free edge-sealing process. A first portion from the film web could then in one embodiment be inverted in stacked relationship to a second, preferably substantially identically constructed portion of the same or a different film web as shown in Fig.1, and sealed thereto. In a most preferred method of making the free edge-sealed film articles of the present invention, the layered first and second portions would comprise adjacent portions of the same continuous film web, and the inverting and stacking of the first portion, with respect to the second would be accomplished .simply • by folding the film web over on itself intermediate the first and second portions. Example 1 In this example, single layer film portions of an average thickness of about 1.7 mils were produced by blending 50 percent by weight of a low density polyethylene and 50 percent by weight of a linear low density polyethylene having a density of 0.935 grams per cubic centimeter, and extruding the substantially homogeneous blend in a conventional cast film process to a chill roll. These single layer film portions were observed microscopically to form a generally triangularly-shaped bead such as shown in Fig. 3 when exposed to a hot sealing wire. The slight upward tilt of the bead again typically reflects the direction of movement of the hot wire in a hot wire, free edge- sealing apparatus relative to the film portion. The single layer film portions when clamped and wire-sealed t^cet er on a manually controlled wire sealer produced a seal having the general appearance of two triangles joined along a side, such as shown in Figure 4. For comparison, layered film portions having discrete layers 18 and 20 formed from " the same LDPE and LLDPE, respectively, were made by coextruding the LDPE and LLDPE layers 18 and 20 to a chill roll in the same cast film process as employed with the single blend layer. The LLDPE layer 20 comprised roughly 50 percent of the thickness of the coextruded film, on the average, When the LDPE layer 18 of a given layered film portion was placed in a position to be exposed to the same hot sealing wire after the LLDPE layer, an upward' curving, generally oval-shaped edge bead was formed. Where the LDPE layer 18 was placed in a position to be exposed to the wire first, the edge bead still curved downward, even despite the inherent tendency of an edge bead to curve somewhat in the direction of movement of the hot sealing wire generally. A comparison of the seal shown in Fig. 4 and an enlarged cross-sectional view (Fig. 6) of the seal between two film portions 12 and 14 with layers 18 and 20 of LDPE and LLDPE arranged as in Figures 1 and 2 suggests that the edge beads formed in the layered film portions of the present invention cooperatively "knit" together rather than merely abutting one another as in the known art. The integrity of the seal in the free edge-sealed articles of the present invention are as a result less dependent on such factors as whether the film portions to be sealed are properly clamped or not, so that adequate seals are more consistently and reliably achieved. It should be noted that thinner gauge layered film portions were as a rule observed to curl even more strongly in a given direction than thicker gauge films. Whereas thinner gauge films of a conventional construction would form smaller beads, and thus would presumably be given to greater sensitivity to clamping defects and like difficulties, the comparative benefit provided in terms of sealing by a layered and ordered film portion as described herein is even more pronounced for films of an average thickness of 2.0 mils (0.05 mm) or less. Example 2 - .-. For this example, a number of sets of reclosable plastic storage containers of the type having two opposed flexible sidewalls of an average thickness of 2.0 mils (0.05 mm), and a pair of opposed rib- and groove-type closure members were constructed by standard commercial methods and equipment. The sidewalls of a container in a particular set were substantially identically constructed to one another, and to sidewalls of the other containers in a set, by extruding or coextruding in a cast film process a low density polyethylene and/or a linear low density polyethylene having a density of 0.935 grams per cubic centimeter in various combinations. The resulting films were: 100 percent low density polyethylene (set "A"); a blend of 70 percent by weight of low density polyethylene and 30 percent by weight of the LLDPE (set"B"); a coextruded film with a LDPE layer comprising 70 percent of the film thickness and with a LLDPE layer comprising the remaining 30 percent of the film thickness (sef'C"); a coextruded film having a LDPE layer and a LLDPE layer which each comprised 50 percent of the thickness of the coextruded film (set "D"); a coextruded film having a LDPE layer comprising 30 percent of the film thickness and a LLDPE layer comprising the other 70 percent of the film thickness on average (set'Ε"); and 100 percent of the LLDPE (sef'F"). Rib- and groove-type closure members were conventionally applied to the LDPE side of these films, if any, and the films folded over one another for wire sealing on a manually controlled wire sealer with the LDPE layers facing inwardly. One hundred containers were formed for each set by identically sealing the sidewalls together from the area of the fold completely through the closure members, so that the containers could thereafter be blown up, immersed and tested for leaks. To accentuate any differences between the various sets, several things were purposely done which are conventionally known to cause leaks along the wire seals of such containers. For example, the closure members were oriented upside down with respect to the notch placed in the hold-down bar to accommodate the closure members. The completed containers were then blown up, the closure members were engaged to seal off the respective open ends of the containers, and the containers immersed underwater. The containers for each set were observed for signs of leaks along the wire- sealed right and left side welds, in the right and left bottom corners, and adjacent the closure members at the right and left sides of the containers. Table 1 reports the number of leaks observed in the hundred containers of each set that were formed and tested, the location of the observed leaks, and the distribution of the leaks among the hundred containers within the set. TABLE Sets B and C from Table 1 particularly suggest that the reclosable plastic containers employing layered sidewalls which curl toward one another in the process of being free-edge sealed (as in Set C) are substantially less likely to leak along the wire seals thereof, than containers conventionally made from the same materials in a substantially homogeneous blend (Set B). It should be noted that the success of the present invention is not dependent on, nor does the present invention necessarily adversely affect, the room temperature curling properties and machinability of those layered film portions of which the free " edge- sealed articles of the present invention are to be comprised. In fact, some films having a lesser tendency to curl at room temperature have been found to curl to a greater extent in a free edge-sealing process, and to form better seals when stacked and arranged in accordance with the present invention, than other films which curl to a larger extent at room temperature. While preferred embodiments of the free edge- sealed articles of the present invention and of a method for making such articles have been described and exemplified herein, it will be appreciated that numerous changes can be made thereto without departing in scope or spirit from the invention disclosed herein and defined by the appended claims.

Claims: 1. A free edge-sealed film article comprised of a plurality of film portions and containing a free edge-seal between two of said plurality of film portions, wherein: • at least one of said two free edge-sealed film portions consists of a plurality of layers of diverse thermoplastic materials; and the plurality of layers are so ordered within a respective layered film portion that on sealing the two film portions together, the layered film portion is curled toward the other of the two film portions along at least a portion of the free-edge seal. 2. A free edge-sealed film article defined as in claim 1, wherein the diverse thermoplastic materials contain differing amounts of frozen-in stress. 3. A free-edge-sealed film article as defined in claims 1 or 2, wherein both of the two free edge- sealed film portions are layered and substantially identically constructed. 4. A free edge-sealed film article as defined in claim 3, wherein each of the two free edge-sealed layered film portions is less than 2.0 mils in thickness. 5. A free edge-sealed film article as defined in claims 3 or 4, wherein the two free edge-sealed layered film portions each consist of a low density polyethylene layer and a linear low density polyethylene film layer. 10 6. A free edge-sealed film article as defined in claim 5, wherein the linear low density polyethylene layer in each of the two layered film portions is made ' of linear low density polyethylene having a density - 5 ranging from 0.912 to 0.941 grams per cubic centimeter. 7. A free edge-sealed film article as defined in claim 6, wherein the linear low density polyethylene layer in each of the two layered film portions is from 0 30 to 70 percent of the thickness of a film portion. 8. A reclosable plastic container of the type including two flexible opposed free edge-sealed sidewalls and an interlocking closure mechanism 2 connected to the sidewalls adjacent an open end of the container and operable for substantially effecting a sealing closure of the open end of the container, wherein each of the two sidewalls consists of a low density polyethylene layer and a linear low density 30 polyethylene layer which are ordered within each sidewall so that on free edge-sealing the sidewalls curl toward each other. 9. A reclosable plastic container as defined in claim 8, wherein: the two sidewalls are substantially identically constructed; each sidewall is less than 2.0 mils in thickness; the linear low density polyethylene layer in each sidewall is made of a linear low density 0 polyethylene having a density in the range of from 0.917 to about 0.935 grams per cubic centimeter; and the linear low density polyethylene layer in each sidewall is from 35 to 65 percent of the thickness 5 on average of the respective sidewall. 10. A reclosable plastic container as defined in claim 9, wherein the linear low density polyethylene layer in each of the two opposed free edge-sealed 0 sidewalls is from 45 to 55 percent of the thickness on average of a respective sidewall. 11. A process for making free edge-sealed film articles, comprising the steps of: 5 coextruding layers of diverse thermoplastic materials to form a continuous planar film web; folding the resulting web over on itself, Q whereby from top to bottom of the folded-over web the layers of diverse thermoplastic materials are arranged to curl inwardly toward a central location; and free edge-sealing adjacent, folded portions of the web.

Inventors: MCCREE JOHN O, MILLER BURDETTE W, PRICE WILLIMAN D, MCCREE, JOHN, O., MILLER, BURDETTE, W., PRICE, WILLIMAN, D.
Applicants: DOWBRANDS INC, DOWBRANDS INC.
Application year: 1991
Main classification: B32B23
IPC codes: B29C65, B29L22, B32B1, B32B27, B65D33