GB2090191A - Heat sealable sheet materials and packages - Google Patents

Abstract

Packages formed by heat sealing the edges of confronting sheet panels (19, 21) together use multiple layer sheet material having a heat seal layer which is a blend of EVOH (ethylene vinyl alcohol copolymer) and a normally heat sealable polymer. The said blend forms peelable heat seals with itself, and comprises by weight 15 to 80% EVOH, the balance being the said polymer. When the EVOH is present in an amount between 50% and 80% by weight, it serves a second primary function in providing an effective barrier to transmission of gaseous oxygen into the package. In a package, the blend forms an innermost layer (12) of each panel (19, 21), each panel having an outside layer (16) comprising a low density polyethylene and an intervening bonding layer (14) between the layers (12 and 16). <IMAGE>

Description

SPECIFICATION Heat sealable sheet materials and packages The present invention relates to heat sealable sheet materials and packages.

More particularly, this invention contemplates improved heat sealed packages and improved sheet packaging materials, of the type incorporating ethylene vinyl alcohol copolymer (hereinafter referred to as EVOH), preferably as a barrier to transmission of gaseous oxygen. A widely used packaging method to which this invention is applicable uses multiple layer sheet material disposed about the product to be packaged and heat sealed itself to enclose the product.

It is often desirable to provide a fin sealed package structure, and some means for opening the package without seriously damaging it. It may be further desirable to provide facile means for opening the package, not only for the convenience, but also for the improved control of the package and contents when the opening is made. Thus the package should open smoothly, without sudden or jerky operation of the opening means, so that the contents of the package are not spilled. Concurrently, and especially in packaging that requires a substantial degree of protection against gas infusion into the package, such conventional opening features as slits, perforations, and other lines of weakness are unacceptable because they necessarily weaken or penetrate the gas barrier material incorporated in the packaging material.

The packaging art has developed, in a heat sealable layer on the interior of the package, special polymeric materials which form heat seals of substantial strength, and which heat seals can subsequently be peeled apart to open the package.

It is often desirable, also, to provide a substantial barrier to transmission of oxygen through the package material. Packaging material containing a continuous layer of EVOH is known to possess excellent oxygen barrier qualities.

Conventional heat sealable package structures containing EVOH are, however, susceptible to permeation of oxygen through the package structure at the heat seals, since conventional heat seal materials provide poor protection against oxygen ingress.

According to the present invention, there is provided a heat sealable multiple layer sheet material, the layer of sheet material which forms one of the surfaces thereof comprising a blend, by weight, of (a) 15% to 80% ethylene vinyl alcohol copolymer and (b) 20% to 85% of a normally heat sealable polymer, wherein the minimum heat sealing temperature of the normally heat sealable polymer is at least about 5 degrees Fahrenheit (2.8 degrees Celsius) less than the minimum heat sealing temperature of the ethylene vinyl alcohol.

A preferred sheet material comprises a blend of (a) 50% to 80% EVOH and (b) 20% to 50% of the normally heat sealable polymer. The normally heat sealable polymer is preferable selected from the group consisting of ionomer, ethylene acrylic acid copolymer, ethylene methacrylic acid copolymer, ethylene vinyl acetate copolymer, and low density polyethylene (hereinafter referred to also as LDPE).

The invention also provides a heat sealable package comprising opposed panels of multiple layer sheet material heat sealed to each other in confronting relationship when the package is completely formed and sealed, the layers of the top and bottom panels which comprise the confronting surfaces in the formed and sealed package being composed of a blend, by weight, of (a) 15% to 80% ethylene vinyl alcohol copolymer and (b) 20% to 85% of a normally heat sealable polymer, the minimum heat sealing temperature of said normally heat sealable polymer being at least about 5 degrees Fahrenheit (2.8 degress Celsius) less than the minimum heat sealing temperature of said ethylene vinyl alcohol, and the heat seals between the confronting layers being peelable.

When the preferred sheet material comprises 50% to 80% EVOH in the blend layer, the package is relatively impermeable to the passage of oxygen through the heat seals. The heat seals between the confronting layers are peelable.

The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: FIGURE 1 is an enlarged cross-section through a typical multiple layer oxygen barrier sheet material packaging structure of the prior art, FIGURE 2 is an enlarged cross-section of a typical multiple layer oxygen barrier sheet material packaging structure according to this invention, FIGURE 3 is a cross-section of the sheet material of FIGURE 2 folded into package form, and showing the heat sealable layers closely spaced in the seal areas, FIGURE 4 is a perspective view of a partially-formed heat sealed package with one side of the package open and available for filling with product, FIGURE 5 is a perspective view of a filled and sealed package, and FIGURE 6 is an enlarged cross-section taken along the lines 6-6 of FIGURE 5.

Referring to FIGURE 2, a sheet material 10 according to this invention is comprised of three layers 12, 14 and 16. Exterior layer 12 is composed of a blend of ethylene vinyl alcohol copolymer and ethylene vinyl acetate copolymer. Exterior layer 1 6 is low density polyethylene. Intermediate layer 1 4 is a polyethylene modified with polar carbonyl groups, and serves as an adhesive between the two exterior layers 12 and 14.

Sheet material 10 can be made into a flexible pouch type package by folding the sheet material so that two layers 1 2 of sheet material 10 are in confronting relationship. FIGURE 3 shows the confronting relationship and exaggerates the closeness of the confronting layers 1 2 in the heat seal areas. Heat seals are formed about part of the periphery of the confronting layers of sheet material to form a containing packet or bag, having a top panel 1 9 and a bottom panel 21, as shown in FIGURES 3 and 6.

After the package is filled with product 24, further heat sealing together of the layers 1 2 is performed to complete package closure and sealing as shown in FIGURE 5.

Ethylene vinyl alcohol copolymers suitable for use in this invention are available commercially.

One such suitable copolymer, sold by Kuraray, Japan, under the tradename "EP-F," has 32% to 36% ethylene, a molecular weight of 29,500, a glass transition temperature, TG, of 1 560F (690C), and a melting point of 3560F (1 800C). Another commercially available suitable copolymer has about 29% ethylene, a molecular weight of 22,000 a TG of 1 490F (650C), and a melting point of 3650F (1 850C).

Still another suitable copolymer has about 40% ethylene, molecular weight of 26,000, TG of 1 F (690C), and a melting point of 3270F (1 640C). The film structures and packages reported in the tables which follow can be made using these commercially available ethylene vinyl alcohol copolymer resins.

The first primary function of the ethylene vinyl alcohol copolymers used in this invention is in the provision of peelable heat seals. In order to ensure peelability, the EVOH must be present in the blended heat seal layer in an amount of at least about 15% by weight, and not more than about 80% by weight.

At least 80% of the vinyl component is present in the hydrolyzed alcohol form. The EVOH copolymer, when constituting at least 50% by weight of the blend layer, serves a second primary function in the package by also providing an effective barrier to transmission of gaseous oxygen through the package enclosure. In fulfillment of this function, the EVOH copolymer should contain at least 50% by weight of the vinyl component, and preferably has at least 95% of that vinyl component present in the hydrolyzed, or alcohol form.

Layer 12 of the structure is the layer that may serve the dual primary role of providing a barrier to oxygen transmission and of providing a seal layer capable of forming peelabie heat seals. Peel able seals are defined as seals between confronting surfaces of sheet material, wherein the force required to separate the surfaces from each other is less than the force required to tear one of the confronting layers when the package is opened. In opening the package, unsealed portions of the opposing sheets are grasped and pulled apart. If the seal is peelable. the sheets separate at the seal. If, on the other hand, the seal is not peelable, there will be no separation thereat. Rather, the sheet materials may fail internally, frequently by tearing between the seal and the point of force application; and the package may not be opened.

While sealant layer 12 may comprise as little as about 15% EVOH to obtain peelable seal properties, in achievement of its preferred dual function as a barrier to transmission of gaseous oxygen, it comprises a blend of (1) at least 50% to about 80% ethylene vinyl alcohol, and (2) about 20% to about 50% of a normally heat sealable polymer. In the embodiments illustrated, the normally heat sealable polymer has a minimum heat sealing temperature, T min, at least about 5 degrees Fahrenheit (2.8 degrees Celcius) less than the T min of the EVOH, as measured by ASTM M-463. The normally heat sealable polymer may be a homopolymer, copolymer or blend thereof. Illustrative of suitable heat sealable polymers are polyethylene, polypropylene, ethylene propylene copoiymer, ethylene vinyl acetate, ethylene acrylic acid, ethylene methacrylic acid, and ionomers.

In preparation of the blend of layer 12, solid peilets of ethylene vinyl alcohol are physically blended with solid pellets of the selected normally heat sealable polymer. The physically blended pellets are then extruded in a conventional plastics extruder to form a sheet of plastic film. Preferably the blend is extruded as part of a coextrusion process. The multiple layer structure is usually produced most economically by that process.

The physica! construction of the packages according to this invention may be done by any conventionally practiced method of forming packages having heat seals. In a preferred method, the sheet material 10 is folded over onto itself to form top and bottom package panels 1 9 and 21 with the layer 12 on the inside of the fold, as shown in FIGURE 3. Conventional heat sealing devices are then used to form heat seals on three sides of what will be finally formed as a four-sided rectangular package or sachet. The package in this stage of construction is shown in FIGURE 4, and is bag-shaped with the three sealed sides designated 1 8. After the product is introduced into the package, the fourth and final side 20 is closed and heat sealed. The completed package is as shown in FIGURE 5. In the completed package, the confronting layers 12, are heat sealed to each other as shown in FIGURE 6. Preferably a portion 22 of the outer edges of the confronting layers 12 is left unsealed as shown in FIGURE 6. These unsealed edges are conveniently grasped by the user for peeling the package open.

EXAMPLE 1 Pellets of "EP-F" brand ethylene vinyl alcohol copolymer were physically blended with pellets of 3130 ethylene vinyl acetate, available from the DuPont Company to make a blend that was 60% by weight EVOH, 40% EVA. This blend was coextruded with Plexar, described in United States patents 4,087,587 and 4,087,588, and available from Chemplex Company, and low density polyethylene to make a three-layer structure; with an intermediate layer of Plexar. The EVOH-containing blend and low density polyethylene were positioned as external layers on opposing surfaces of the Plexar. The EVOH blend, Plexar, and polyethylene layers were 0.3, 0.3, and 1.6 mils thick respectively, for an overall film thickness of 2.2 mils total.The resulting multiple layer structure was formed into a rectangular package measuring 8 inches by 16 inches on a model 8000 vertical form, fill and seal machine made by MIRAPAK Company, Houston, Texas, with the EVOH blend layer on the inside of the package at representative seal jaw temperatures. The peel strengths of the resulting seals were measured according to ASTM D-882 on an Instron testing machine with a head traversing speed of 5 inches per minute (2 mm/sec). The averages of the resulting peel strengths were as shown in Table 1 below, and represent separation of the confronting sealed layers of the EVOH blend at the seals, rather than internal failures of the multiple-layer sheet material, or of one of the layers thereof.

TABLE 1

Sea: Jaw Temperature Peel Strength 1900F 240 grams/inch (94.5 grams/cm) 2100F 850 grams/inch (334.6 grams/cm) 2300F 500 grams/inch (196.8 grams/cm) EXAMPLES 2-12 Additional structures were made in similar manner, and tested. The structures and averages of their peel strengths, in grams per inch, were as shown in Table 2. (Peel strengths in grams/cm are given in brackets.) Peel strengths from Example 1 are repeated in Table 2, for ease cf comparison. Oxygen permeability data, also, are shown for Examples 6 and 7.Oxygen permeability data were not determined for Examples 1-5 and 8-12, but are believed to be realistically represented by Examples 6 and 7, as well as following Examples 13-1 5.

TABLE 2 Peel Strength at Oxygen Blend Seal Jaw Temperature Permeability Example EP-F Heat Sealable* cm3-ml No. EVOH Polymer 1900F 2100F 2300F m2day 1 60% 40%3130 240(95) 850(335) 500(197) NM 2 60% 40% 3120 680 (268) 640 (252) 995 (392) NM 3 40% 6O%3120 525(207) 1050(413! 710(280) NM 4 60% 40% 3165 345(136) 515(203) 735(289) NM 5 40% 60%3165 75(30) 420(165) 565(222) NM 6 80% 20%1707 210(83) 310(122) 325(128) .15 7 60% 40%1707 0 210(83) 375(148) .46 8 40% 60%1707 350(138) 480(189) 390(154) NM 9 80% 20%1650 0 115(45) 1000 (394) NM 10 40% 60% 1650 35 (14? 125 (49) 260(102) NM 11 50% 50% 455 195(77) 630(248) 790(311) NM 12 30% 70% 455 395(156) 690 (272) 700 (276) NM * 3120 = EVA, 7.5% ethylene 3165=EVA, 18% ethylene 3130 = EVA, 12% ethylene 1650 = EVA, a Zinc ionomer 1707 = a Sodium ionomer 455 = an ethylene acrylic acid NM = Not measured It will be appreciated that this invention provides the user with a number of advantages when embodied in package structures, as compared with prior art structures incorporating EVOH as a separate layer, as in FIGURE 1. One advantage is derived from the simplification involving fewer layers required in the present structure. As shown in FIGURE 1, the typical prior art structure has at least five layers.These layers, as in FIGURE 1 are, from top to bottom; polypropylene/Plexar/EVOH/Plexar/EVA (sealant). The EVOH layer is enclosed within the structure, and has special adhesive layers on both its faces. The complete five layer structure has a sealant layer on the face of one adhesive layer, and usually has a protective outer layer on the face of the other adhesive layer. Restated another way, the prior art structure has a three-layer core containing EVOH and adhesvie tie layers, and inner and outer layers on the exterior faces of the adhesive layers. To make this structure, one requires rather complex five-layer coextrusion equipment. To perform the adhesive function, two layers of adhesive must be extruded in the coextrusion. Further, coextruding the five layer structure results in an amount of scrap related to the complexity of the structure being extruded.

By having the EVOH-containing layer on the exterior surface of the film, as now proposed, provides a much simpler structure. Only one face of the EVOH-containing layer needs adhesive to bond it to the remainder of the structure, eliminating one adhesive layer. Since the EVOH-containing layer is located at the film exterior, and can serve both heat sealing and barrier functions, a separate barrier layer may optionally be eliminated. Alternatively, in comparing the present structure with the prior art, one may consider that the pure EVOH layer is eliminated. From whichever viewpoint, the resulting structure, having had two layers omitted, contains only three layers.

The simpler three-layer structure is very economical. First, cost is saved in that the use of expensive adhesive material is reduced from two layers to one. Secondly, the coextrusion process can be performed on less expensive three-layer equipment rather than five layer equipment. Thirdly, the probable amount of scrap generated in making a three layer structure is less than the amount generatec in making the more complex five-layer structure.

While the oxygen barrier efficiency of structures having the oxygen barrier material cnly in the blended sealant layer is somewhat less than the efficiency of prior structures having a discrete barrier layer composed 100% of EVOH, for many applications, the barrier efficiency with the 100% EVOH layer is substantially greater than required, even when the thickness of that layer is minimized to the extent allowed by known manufacturing equipment. The reduced barrier provided efficiency resulting from the blended sealant layer is, however, usually highly satisfactory when the EVOH content is at least 50% by weight. Within the blend range listed, the barrier can be specifically chosen according to the protection necessary.Thus, the package of EXAMPLE 1 has an oxygen permeability of 3.3 x 10-3 cm3/day as measured on an Oxtran 1050 at zero percent relative humidity and at 730F (22.7"C). Oxtran 1050 is a testing apparatus available from Modern Controls Incorporated, Minneapolis, Minnesota. A package of the same size made from prior art sheet material of similar thickness, constructed as in FIGURE 1, and having a 100% EVOH layer 0.3 mils thick, has a permeability of 3.0 x 10-3 cm3/day. Another similar prior art package, having a 0.3 mil thick oxygen barrier composed of a Saran (RTM) has a permeability of 5.1 cm3/day.

EXAMPLE 13-15 A multiple layer structure is made by coextruding a multiple layer film consisting of LDPE/LDPE/blend. LDPE is low density polyethylene supplied by Chemplex Company under the designation 3024. The blend layer is composed of EP-F EVOH blended with 31 30 EVA, in three illustrative ratios. Oxygen permeability of the resulting sheet material structure is shown in Table 3.

TABLE 3* Example Blend Ratio, Oxygen Permeability No. EVOH/EVA cm3/mil m2 day 13 80/20 0.6 14 65/35 0.9 15 50/50 1.2 CONTROL 100%EV0H .003 CONTROL 100% SARAN 15.

* 0.3 mil barrier layer Table 3 is indicative of the versatility provided by this invention, namely the ability to tailor the oxygen barrier, within a substantial range, to the barrier needs of the product to be packaged. It shows that the oxygen permeability may be adjusted by adjusting the percentage EVOH in the blend layer.

Permeability may also be adjusted by changing the blend layer thickness. Such versatility is highly advantageous and economical because it allows the user to maximize the efficiency of use of the most costly component of the sheet material - the EVOH - while directly benefitting from its dua! role to the extent it is used.

In the presently-unusual case where a barrier efficiency greater than that provided by the usual 100% EVOH layer is desired, the artisan conventionally would provide a thicker EVOH layer. Using this invention, the artisan may now choose to incorporate the peelable seal into the barrier calculation. For example, the presence of a 0.3 mil layer of the blend, of as little as 50% EVOH, as the sealant, in a structure such as in FIGURE 1, reduces the permeability by about an additional 12%, primarily by reducing oxygen permeation through the heat seals. Thus, the choice can then be made to adjust the thickness of either, or both, EVOH-containing layers, and, optionally, to adjust the EVOH content of the blend layer.

In the above illustrations, the invention has been described in terms of three and four layer structures made entirely of polymeric materials. It is known in the packaging art to make composite structures including metallic and/or cellulosic layers. The structures of this invention are entirely compatible with such composite structures. In either composite structures or 100 percent polymeric structures, this invention is highly adaptable to use in packaging structures having layers of packaging material in addition to those discussed.

In applications of this invention not shown, the sheet materials on opposite sides of the heat sealed interface may be structurally dissimilar. This application may find use, for example, where one of the sheet materials functions as a shaped receptacle and the other sheet material functions as a peelable lid. In such an application, it is entirely possible that the two dissimilar sheet materials may have substantially dissimilar rigidities, as well as other physical dissimilarities.

The earlier recited method of forming the blend by mixing pellets is illustrative only, as other known methods of blending the polymeric materials may be used.

Claims (14)

1. A heat sealable multiple layer sheet material, the layer of said sheet material which forms one of the surfaces thereof comprising a blend, by weight, of (a) 1 5% to 80% ethylene vinyl alcohol copolymer and (b) 20% to 85% of a normally heat sealable polymer, the minimum heat sealing temperature of said normally heat sealable polymer being at least about 5 degrees Fahrenheit (2.8 degrees Celsius), less than the minimum heat sealing temperature of said ethylene vinyl alcohol.

2. A heat sealable package, said package comprising opposed panels of multiple layer sheet material heat sealed to each other in confronting relationship when the package is completely formed and sealed, the layers of the panels which comprise the confronting surfaces thereof in the formed and sealed package being composed of a blend, by weight, of (a) 1 5% to 80% ethylene vinyl alcohol copolymer and (b) 20% to 85% of a normally heat sealable polymer, the minimum heat sealing temperature of said normally heat sealable polymer being at least about 5 degrees Fahrenheit (2.8 degrees CelsiusY4ass than the minimum heat sealing temperature of said ethylene vinyl alcohol, and the heat seals between the confronting layers being peelable.

3. A sheet material according to claim 1, wherein the blend comprises (a) 50% to 80% ethylene vinyl alcohol copolymer and (b) 20% to 50% of the said normally heat sea;able polymer

4. A sheet material according to claim 1 or claim 3, wherein the said normally heat sealable polymer is selected from ionomers, ethylene acrylic a & d copolymer, ethylene methacrylic acid copolymer, ethylene vinyl acetate copolymer, and low density polethylene.

5. A sheet material according to claim 1 or claim 3, wherein the said normally heat sealable polymer is either an ionomer or ethylene vinyl acetate copolymer.

6. A package according to claim 2, wherein the blend comprises (a) 50% to 80% ethylene vinyl alcohol copolymer and (b) 20% to 50% of said normally heat sealable polymer.

7. A package according to claim 2 or claim 6, wherein the said normally heat sealable polymer is selected from ionomers, ethylene acrylic acid copolymer, ethylene methacrylic acid copolymer, ethylene vinyl acetate copolymer and low density polyethylene.

8. A package according to claim 2 or claim 6, wherein the said normally heat sealable polymer is either an ionomer or ethylene vinyl acetate copolymer.

9. A package according to any of claims 2 or 6 to 8, wherein the confronting panels are of substantially identical construction.

10. A package according to any of claims 2 or 6 to 8, wherein the confronting panels are cf dissimilar construction.

11. A package according to any of claims 2 or 6 to 10, wherein the said panels have dissimilar rigidities.

12. A heat sealable package which, when closed and sealed, is substantially impermeable to passage of oxygen through the heat seals, the closed package comprising top and bottom panels of multi-layer sheet material sealed to each other in confronting relationship, the layers of each of the said top and bottom panels which comprise the confronting surfaces thereof being composed of a blend by weight of (a) 50% to 80% ethylene vinyl alcohol copolymer and (b) 20% to 50% normally heat sealable polymer, the minimum heat sealing temperature of the said normally heat sealable polymer being at least about 5 degreees Fahrenheit (2.8 degrees Celsius) less than the minimum heat sealing temperature of the ethylene vinyl alcohol.

13. A heat sealable multiple layer sheet material, substantially as herein described by way of example with reference to Figs. 3 to 6 of the accompanying drawings.

14. Heat sealable multiple layer sheet materials as set out in Examples 1 to 1 5 herein.

1 5. Heat sealable packages substantially as herein described by way of example with reference to Figs. 2 to 6 of the accompanying drawings.