Peelable heat seals for packages

Abstract

Methods and compositions are disclosed for providing a sealed non-leaking, non-contaminating flexible package for articles to be subsequently used, with the package being comprised of at least one uncoated layer of paper and one layer of a material other than paper, and with the seal for the package being readily peelable, substantially in the absence of any destruction of the matrix of the layers previously joined by the seal. The above is achieved by heat sealing said paper layer to a film of ethylene-vinyl acetate copolymer, having a content of vinyl acetate within the range of between about 9 and 28%. The ethylene-vinyl acetate copolymer film may be a self-sustaining film layer or may be laminated to one or more different backing or supporting layers prior to heat sealing to the paper.

Description

Generally speaking, this invention is directed to the preparation of packages from plies of materials normally used in packaging various articles for commerce. More particularly, this invention relates to methods and compositions for the preparation of such packages in which one of the facing layers utilized for sealing the contents of the package is an uncoated paper and the other layer is ethylene-vinyl acetate copolymer, with the two layers heat sealed together in a manner whereby the contents of the package are effectively maintained in a non-leaking and non-contaminating environment until such time when the articles are to be used. The seal is readily peelable when the package is to be opened without substantial destruction of the matrix of the paper and/or the copolymer film joined together by the seal.

With the ever increasing use of light, flexible packaging for a variety of items used in commerce, there is a corresponding concern with the ability to package such items rapidly in a manner whereby they are effectively sealed against failure of the seal and loss and/or contamination of the articles, simultaneously with making the packages readily openable by the user without the package having to be totally destroyed in order for the user to gain access to the contents of the package. This is particularly important in such instances where the contents of the package are to be used in emergency situations, where access must be gained rapidly. Even in those instances where the contents of the package are not to be used in such emergency situations, consumers demand that articles which are to be dispensed in such packages are readily available to them without unnecessary pulling or tugging in order to open the package.

In many instances, it is appropriate and desirable to utilize paper as one of the layers of such flexible packages, not only because it is relatively inexpensive for mass production techniques, but also for specialized applications in which the relative porosity of the paper, as compared with a variety of different resin products utilized for such flexible packaging, is desirable for certain sterilization techniques for sterilizing the contents of the package after the package has been sealed.

In those instances when sterilization of the contents is important, the seals joining the various plies of the package together must be taken into consideration because the environment of the sterilization procedures may destroy the sealing effect of the seal prior to the time when the contents of the package are to be utilized. A number of different materials have been proposed for use in the plies of such flexible packages in order to provide appropriate leak-proof seals, including flexible packages of plies of two different plastic films and/or laminations heat sealed together. Certain difficulties arise, however, with the use of resin thermoplastics for the entire flexible package in that the thermoplastics are not always porous enough for the utilization of such sterilizing gases as ethylene oxide or propylene oxide, which are particularly appropriate for use in sterilizing surgical instruments or bandages, etc.. Such contents must be maintained in their sterilized condition up to the time when the packages are opened and the contents are to be utilized. Even in those instances where certain of the thermoplastics are desirably porous for such applications, their cost makes them less than desirable for mass production purposes.

In order to overcome this difficulty, flexible packages have been proposed utilizing as one of the plies therefor, paper, which is sufficiently porous for the sterilizing gases to pass through to the contents of the package after the package is sealed. However, certain problems may arise with the utilization of paper, in that effective seals cannot always be achieved between the paper and the resin ply heat sealed to it. Certain proposals have been made for overcoming this difficulty, including coating the paper with a clay type material which is itself permeable to the sterilizing gases. However, this procedure involves an extra treatment of the paper ply utilized in the package, thus again increasing the cost of such packages for mass production uses. Furthermore, particularly in such instances as the use of articles in a surgical environment, it is important that the package be readily openable be peeling the seal without ripping or destroying the paper matrix because the small fibers ripped or pulled from the surface of the paper ply will contaminate the contents of the package, thus reducing the effectiveness of the previously sterilized contents for such uses.

As will be understood, papers of the type discussed herein must provide an effective seal with the resin ply to which it is sealed, so that the seal does not fail prior to use even though the contents of the package may be relatively heavy and cumbersome and have a tendency to jolt in shipment against the seal and break it, thus exposing the contents to contamination and/or loss. Thus, the problem arises in the use of paper plies for flexible packages to provide an effective seal between that paper ply and the resin ply to which it is joined, while at the same time making the seal readily peelable so that the user of the contents can rapidly gain access to the contents. This must be achieved in a manner whereby the matrix of the paper ply is not damaged to the extent where the individual fibers thereof may contaminate the environment in which the package is opened. Simultaneously, such packages must be inexpensive enough to compete in mass production applications with more rigid packaging structures.

By contrast, and quite unexpectedly, it has now been found in accordance with this invention that by utilizing in a flexible package, one ply of conventional packaging papers and heat sealing it to a second ply, the facing surface of which is ethylene-vinyl acetate copolymer, effective seals are achieved between the paper matrix and the copolymer (EVA). Such seals are effectively leak-proof and are simultaneously peelable by the user at the time when the contents are to be used, substantially in the absence of any destruction of the paper matrix. There is no tearing or destruction of the two layers which were previously sealed. With such an arrangement, therefore, the user may gain access to the package without the nuisance of having to rip apart the package and in those instances where it is important, no loose fibers are torn from the paper matrix which would ordinarily contaminate a surgical environment. Furthermore, with the invention herein, effective heat sealed packages can be achieved over a wide range of sealing temperatures and other operating conditions for sealing, even on conventional relatively old sealing equipment.

With the foregoing and additional objects in view, this invention will now be described in more detail and other objects and advantages will be apparent from the following description and the appended claims.

Before describing this invention in more detail, it may be well to note that this invention has been found applicable to the use of a wide variety of papers, including standard sterilizable kraft paper, wet strength strongly bonded sterilizable kraft paper, glassine, parchment, a food grade cardboard, Red Wallet type mailing paper, conventional pouch paper and Laundry Tag type waterproof paper. Furthermore, in order to reduce the amount of EVA copolymer utilized, the copolymer film may be first laminated to a supporting backing layer in a conventional manner, with such backing layer being selected depending upon the contents to be sealed in the package and the appearance thereof, including additional layers of paper, foil, polyolefin films, such as polypropylene and polyethylene, polyvinyl chloride, chlorotrifluoroethylene and cellulose acetate. The EVA copolymer film may be first formed and then subsequently adhesively adhered to the backing layer, it may be coextruded with a second resin film backing layer or it may be extrusion coated onto the backing layer.

For example, satisfactory results have been achieved in accordance herewith, and under satisfactorily and economically attractive conditions on commercial scale operations by first preselecting a backing substrate, such as a bleached or unbleached kraft paper or polypropylene film, and then extrusion coating the backing with a film of EVA, having a vinyl acetate content within the range of between about 9-28%, thereafter heat sealing the EVA film in conventional manner at temperatures within the range of between about 150.degree.-500.degree.F. to a paper ply for a period of time to heat seal the EVA copolymer film to the paper. This results in an effective leak-proof seal which is readily peelable subsequently when the contents of the sealed package are to be removed, substantially in the absence of any fiber tear or fiber pick of the paper ply.

In this connection, it should be understood that the vinyl acetate content of the EVA copolymer will be selected depending upon such conditions as the type of contents to be sealed in the package, the bulkiness or weight thereof and the degree of seal desired. For example, a heat seal measuring 75-100 gms per inch will be effective for such contents as a bandage but a much higher range of, for example, 350-450 gms. per inch might be necessary for a heavier object, such as a surgeon's scalpel.

By the use of the term "heat sealing" herein, it is meant that two surfaces are to be joined by utilizing a combination of heat and pressure. It is sufficient for purposes of this invention to include any of the techniques normally used in elevating the temperature at the joining interface, whether the techniques involve driving heat through the full thickness of one or both webs, from one or more outside sources.

Heat sealing may be carried out by conventional sealers such as the Sentinel sealer or a Bartelt sealer, in which the jaws thereof are configured to provide the appropriate configuration of seal between the plies being joined together. Generally, that portion of the seal of the package to be initially opened (the top) will be curved, chevron shaped or similarly configured so as to provide an easily grasped starting point for opening the package. By contrast, the opposite end of the package (the bottom) will be squared off or be in the form of a more gradual curve, all in well known manner.

It is obvious that in preparing the seals, in accordance herewith, the temperature used may vary depending upon the pressure and the dwell times, as well as the vinyl acetate content of the EVA copolymer being used and the particular paper ply being sealed to the copolymer film. There is, of course, an infinite number of different combinations of temperature, dwell times and pressure, depending upon the various combinations which can be selected, as noted above. Thus, for example, to obtain the same seal strength at 20 lbs. per square inch (psi) with a 0.25 second dwell time as would be obtained at 40 psi with a 0.50 second dwell time, it would be necessary to increase the sealing temperature by perhaps 25.degree.-50.degree..

In addition, the effectiveness of the various seals, in accordance herewith, are measured in gms. per inch. Various samples were prepared at 25.degree.F. or 50.degree.F. intervals with a fixed pressure of 40 lbs. psi and generally with a fixed dwell time normally of about 0.50 second, unless otherwise noted below. Each test specimen was evaluated in an Amthor tensile testing machine with one leg of the specimen in a fixed jaw of the machine, with the other leg in a moveable jaw. These jaws are separated at a constant rate, normally about 12 inches per minute and the force required to cause the seal to peel apart or to rupture is recorded in grams per inch of seal width.

As noted above, the EVA copolymer film may be a self-supporting film. However, it is preferred, in accordance herewith, to use a backing support. The backing support may be a bleached or unbleached kraft paper, also. Examples of other backing materials include foil, a polypropylene, saran-coated polypropylene film, polyvinyl chloride, chlorotrifluoroethylene, cellulose acetate or polyethylene terephthalate.

A preferred cycle of operation, in accordance herewith, for formulating the peelable seals of this invention between a paper ply and a film of EVA copolymer includes first preselecting a backing film for the EVA copolymer, preferably one mil uncoated polypropylene, and thereafter extrusion coating a film of EVA copolymer on the backing film. The thickness of the EVA copolymer film will generally be in the range of between about 0.5 mil - 5 mils. As noted above, thicknesses in the upper part of this range would be a self-sustaining EVA copolymer film. Preferably, however, the EVA copolymer film will range in thickness between 0.33 mil - 2 mils.

Thereafter, the paper ply will be selected depending on a number of factors, including the porosity desired for subsequent gas sterilization, for example, the smoothness of the paper surface and the quality of the paper which ultimately will depend upon the use for which the package is being formulated. Preferably, the paper ply will be comprised of 100% bleached wet strength Southern Kraft, having a minimum basis weight of 33.2 lbs., 24 .times. 36/500, having a porosity of 125, regular Gurley-Sec., a minimum density of 8.3 lbs. per mil and a smoothness of 275/295 Sheffield, F/W. Thereafter, the EVA copolymer is heat sealed to the paper ply utilizing conventional sealer apparatus, as noted above, at a temperature within the range of 150.degree.-500.degree.F., and preferably 200.degree.-300.degree.F. at a pressure within the range of between about 10 psi and 60 psi, for a period of time necessary to bring about appropriate sealing between the EVA copolymer ply and the paper ply, and preferably within the range of between about 0.03 - 0.7 seconds. In this connection, it should be understood that commercial machines will vary with pressure, time and temperature and these conditions will be selected depending upon the percent of vinyl acetate content in the EVA copolymer being utilized, as well as the particular type of paper being utilized to seal to the EVA copolymer. Typically, with a Sentinel sealer, for example, when sealing at temperatures within the range of 225.degree.-325.degree.F., the time utilized will be within the range of between about 0.4 - 0.7 seconds, while if the temperature utilized is in a higher range of, for example, 300.degree.-425.degree.F., the time involved will be within the range of between about 0.25 - 0.50 seconds.

It should be noted, further, that it is within the purview of this invention that the backing layer for the EVA copolymer film may be a laminate, including such materials as unbleached kraft laminated to a polyethylene film, which is in turn laminated to a foil. Generally, selections for the backing materials, if more than one is to be utilized, will depend upon the function of the final package, its desired appearance, and the weight and/or bulkiness of the item being packaged. Furthermore, the paper ply which is sealed to the EVA copolymer may also be comprised of a laminate of a variety of materials, so long as the particular papers noted above to be sealed with the EVA copolymer film face that film during the sealing procedures.

As purely illustrative of the results achieved, in accordance herewith, utilizing a variety of different paper plies, one may note the results in Table I below, in which relatively poor grade paper plies were utilized for sealing to EVA copolymer films, including a food grade cardboard, "Red Wallet" type mailing paper and "Laundry Tag" type waterproof paper. It should be understood that each of these poor grade papers are comprised of relatively loose fiber type papers. Nevertheless, appropriate peelable seals were achieved utilizing a variety of different sealing temperatures.

More specifically, the specimens were sealed in a Sentinel sealer, which was set at 46 psi, with the actual pressure being 40 psi. The dwell time in each case was 0.3 seconds and the EVA copolymer utilized was U.E. 643, a product of U.S. Industries containing 18% vinyl acetate with a melt index of 9. It is to be understood, however, that these examples are being presented with the understanding that they are to have no limiting character on the broad disclosure of the invention as generally set forth herein, and as directed to men skilled in the art.

TABLE I __________________________________________________________________________ Food Grade "Red Wallet" Type "Laundry Tag" Type Example* Temp. .degree.F. Cardboard Mailing Paper Waterproof Paper __________________________________________________________________________ 1 175.degree. very tight peel tight peel-very soft tight peel (130-175 gms.) peel (180-200 gms.) (55-75 gms.) 2 200.degree. very tight peel tight peel-very soft tight peel (140-180 gms) peel (200-220 gms.) (90-120 gms.) 3 225.degree. very tight peel tight peel-soft peel tight peel (140-225 gms.) (230-310 gms.) (120-150 gms.) 4 250.degree. very tight peel tight peel-soft peel tight peel-some (170-280 gms.) 240-290 gms.) fiber tear (210-230 gms.) 5 275.degree. very tight peel tight peel-soft peel tight peel-some (180-275 gms.) fiber tear fiber tear (250-380 gms.) (250-300 gms.) 6 300.degree. very tight peel tight peel-soft peel tight peel-soft some fiber tear fiber tear peel (320-340 gms.) (180-240 gms.) (250-380 gms.) __________________________________________________________________________ *Average of Ten Specimens

As can be seen from Table I, even at relatively low sealing temperatures of 175.degree.F., a very tight peel was obtained with the food grade cardboard, as well as with the Red Wallet mailing paper. Furthermore, with the food grade cardboard there was no fiber tear until a temperature range of 300.degree. was reached, even with this very poor grade of paper. Also, as can be noted in Example 3, a tight peel was achieved at 225.degree.F. with the Laundry Tag type waterproof paper without any fiber tear, even with this extremely rough surfaced paper.

Further, as illustrative of the enhanced results achieved in accordance herewith, one may note the comparative results of the tests noted below in Table II, in which the paper ply utilized was KR-35YJ, a 35 lb., 24 .times. 36/500 white, machine finish paper of the Riegel Products Corporation, manufactured from 100% bleached Southern Kraft, having no internal additives other than a melamine wet strength additive and the addition of HCL during formulation to control the PH to within the range of 4.5 - 5. The wet strength additive was added at the rate of 2.25% based on the fiber content of the paper. The EVA copolymers used in the first 3 examples were products of U.S. Industries, containing varying percentages of vinyl acetate, and with varying melt indexes, as noted. The fourth example was an EVA copolymer product of Union Carbide Company. A Sentinel sealer was used again with a 0.3 second dwell time and an actual pressure of 40 psi. The values shown are in gms./inch Amthor bond.

TABLE II __________________________________________________________________________ % of Peel Strengths Ranges at Various Temperatures in gms/inch Ex- EVA Co- Vinyl Melt ample* polymer Acetate Index 200.degree.F 225.degree.F 250.degree.F 275.degree.F 300.degree.F __________________________________________________________________________ 1 UE-633 18% 20 50-120 300-350 350-450 400-500 450-550 fiber fiber pick tear 2 UE-635 9% 9 20-30 110-130 150-160 260-280 220-300 3 UE-643 18% 9 200-300 300-400 350-450 400-500 430 fiber fiber pick pick 4 DQDA-3131 20% 30 60-80 110-140 160-180 180-210 200-220 __________________________________________________________________________ *Average of Ten Specimens

As can be seen from Table II, a variety of different peel strengths can be achieved in accordance herewith, at varying sealing temperatures, utilizing the same dwell time and pressure, as well as the same paper, by merely selecting different percentages of vinyl acetate content in the EVA copolymer. Obviously, the selection will be made depending upon the final use to which the sealed package is to be utilized and the weight and bulkiness characteristics of the material to be packaged. For example, in Example 1 with an 18% vinyl acetate content, peel strengths can vary between 50 and 450 gms. with no fiber pick, let alone fiber tear. If some fiber pick is satisfactory for the particular use to which the package is being made, (that is, non-sterilizable conditions), seals can be obtained at 275.degree.F. and with a peel strength up to 500 gms. Again, in Example 2 with a 9% vinyl acetate content, peel strengths range from 20 to 300 gms. at varying sealing temperatures.

As further illustrative of the results achieved herein, one may note Table III below, in which various effective ranges of seal strengths are noted which were achieved at different temperature intervals for paper - EVA copolymer bonds using EVA copolymer films having three different percentage contents of vinyl acetate. In each incidence, the paper used for heat sealing to the EVA copolymer was the same as that used in Table II above. The EVA copolymer films were produced from U.S. Industries EVA copolymers designated UE-633, UE-646 and UE-635. The readings are an average of ten specimens in grams/inch Amthor bond. A platen-type sealer was used in this case and the pressure was 60 psi and the dwell time was 0.5 seconds. T designates the top of the package, S the side seal of the package and B the bottom seal. In this instance, the configuration of the seal was a modified chevron-shaped top with straight sides and bottom. The UE-633 EVA copolymer film was extruded onto a backing of polypropylene film, the UE-646 EVA copolymer film was extruded onto a backing of unbleached Kraft paper, and the UE-635 EVA copolymer film was extruded onto a backing of unbleached Kraft paper. In each instance, the thickness of the EVA copolymer films was about 1.5 mils.

TABLE III ______________________________________ 18% Vinyl 28% Vinyl 9% Vinyl Ex- Temp. Acetate Acetate Acetate ample* .degree.F UE-633 UE-646 UE-635 ______________________________________ 1 175.degree. very slight tick T 260-300 No Seal B 290-350 S 210-585 2 200.degree. T 10-20 T 530- No Seal B 10-20 B 500-600 S 26.5-53.3 S 535-795 3 225.degree. T 180-200 T 650 Slight Tick B 150-160 B 800 S 210-265 S 795-135 4 250.degree. T 280-300 T 270-280 B 240-250 B 220-240 S 320-425 S 265-320 5 275.degree. T 210-220 T 350-370 B 240-250 B 350-360 S 290-345 S 425-505 6 300.degree. T 230-240 T 540-550 B 250-260 B 480-500 S 290-370 S 742-850 7 350.degree. T 260-270 T 460-470 B 360-370 B 670-675 S 370-450 S 900-1010 8 400.degree. T 290-300 T 580-585 B 340-350 B 510-520 S 400-475 S 795-900 Slight Fiber tear ______________________________________ *Average of ten specimens

As can be seen, a wide variety of peel strengths can be achieved with a wide range of sealing temperatures, depending upon materials being used, items being packaged and so forth. For example, in example 1 at a sealing temperature of 175.degree.F. very strong seals were achieved with a vinyl acetate percentage of 28%, while none was achieved for the 9%, and only slight ticking or adherence at 18%. By the same token, good seals were achieved with 9% and 18% content at a sealing temperature of 275.degree.F. but a peelable seal was not achieved at 28% content of vinyl acetate. In many instances, the choice of backing layer will be determinative of the ranges at any particular level of vinyl acetate content and more substantial backing layers such as polypropylene film will be chosen for higher peel strength ranges rather than a second paper backing for the EVA film.

Table IV below shows the results achieved under the same conditions with the same materials as in Table III above, except that the pressure was 40 psi and the dwell time was 0.3 seconds.

TABLE IV ______________________________________ 18% Vinyl 28% Vinyl 9% Vinyl Ex- Temp. Acetate Acetate Acetate ample* .degree.F UE-633 UE-646 UE-635 ______________________________________ 1 175.degree. No Seal Slight Tick No Seal 2 200.degree. Very Slight T 440-450 No Seal Tick B 490-500 S 695-740 3 225.degree. T 25-40 T 500-510 Slight Tick B 30-50 B 580-600 S 53-60 S 1065-1600 4 250.degree. T 90-100 T 700 T 20-25 B 120-130 B 700 B 30-40 S 160-185 S No reading S 26.5-60 obtainable 5 275.degree. T 130-140 T 600 T 210-220 B 130-140 B 610-620 B 190-200 S 160-210 S 1385 S 185-210 6 300.degree. T 170-180 T 660 T 250-260 B 160-170 B 650 B 280-300 S 160-210 S No reading S 210-265 obtainable 7 350.degree. T 200-210 T 740 T 340-350 B 210-220 B 830 B 330-340 S 210-265 S 2125 S 320-420 8 400.degree. T 240-250 T 780 T 460-465 B 270-290 B 830 B 470-480 S 320-400 S 2080 S 740-850 ______________________________________ *Average of Ten Specimens

As can be seen from Table IV, decreases in temperature and dwell times decreases the seal strength at given temperatures and vinyl acetate levels.

Table V below gives further results using the same three EVA copolymer films on the same backing materials bonded to the same paper as in Tables III and IV above. In these examples, however, a Sentinel sealer was used and comparisons were made for each example at different pressures and dwell times as noted in the table.

TABLE V __________________________________________________________________________ 18% Vinyl 28% Vinyl 18% Vinyl Ex- Temp. Acetate Acetate Acetate ample* .degree.F. Conditions UE-633 UE-646 UE-635 __________________________________________________________________________ 1 175.degree. .5 sec 40 psi 9-14 90-120 No Seal 2 175.degree. .5 sec 60 psi 15-22 330-360 No Seal 3 175.degree. .3 sec 40 psi 9-10 40-60 No Seal 4 175.degree. .3 sec 60 psi 4-7 50-70 No Seal 5 200.degree. .5 sec 40 psi 90-100 480 Slight Tick 6 200.degree. .5 sec 60 psi 100-120 540 Slight Tick 7 220.degree. .3 sec 40 psi 70-80 450 No Seal 8 200.degree. .3 sec 60 psi 50-60 500 No Seal 9 225.degree. .5 sec 40 psi 130-150 580 100-120 10 225.degree. .5 sec 60 psi 150-170 550 200-220 11 225.degree. .3 sec 40 psi 80-90 520-530 10-15 12 225.degree. .3 sec 60 psi 120-140 No Reading 40-80 13 250.degree. .5 sec 40 psi 120-140 FT 200-250 14 250.degree. .5 sec 60 psi 140-170 FT 250-270 15 250.degree. .3 sec 40 psi 80-90 FT 120-150 16 250.degree. .3 sec 60 psi 110-150 600 120-150 17 275.degree. .5 sec 40 psi 190-230 FT 260-320 18 275.degree. .5 sec 60 psi 200-230 FT 290-360 19 275.degree. .3 sec 40 psi 110-130 FT 80-120 20 275.degree. .3 sec 60 psi 150-170 FT 180-210 21 300.degree. .5 sec 40 psi 170-190 FT -- 22 300.degree. .5 sec 60 psi 220-250 FT -- 23 300.degree. .3 sec 40 psi 160-190 FT -- 24 300.degree. .3 sec 60 psi 190-220 FT -- 25 350.degree. .5 sec 40 psi 220-250 FT -- 26 350.degree. .5 sec 60 psi 280-310 FT -- 27 350.degree. .3 sec 40 psi 180-200 FT -- 28 350.degree. .3 sec 60 psi 180-220 FT -- 29 400.degree. .5 sec 40 psi 270-350 FT 800-870 30 400.degree. .5 sec 60 psi 350-390 FT 550-650 31 400.degree. .3 sec 40 psi 190-220 FT 280-320 32 400.degree. .3 sec 60 psi 230-260 FT 300-350 __________________________________________________________________________ *Average of Ten Specimens FT-Fiber Tear

In addition, packages were produced, in accordance with this invention, under commercial conditions on a Rotowrap machine. The size of the packages were 2 inches .times. 103/4 inches and 35 packages were produced per minute. The paper used for the paper ply was the same as that described in Table II above. UE-633 EVA copolymer film having a vinyl acetate content of 18% was extruded to a thickness of 1.5 mil on a backing of 100 gauge Hercules polypropylene film (B502 1 mil uncoated). The side seals were made at 340.degree.F., a dwell time of about 0.04 second, and the top and bottom seals were made at 270.degree.-290.degree.F. Bond strengths within the range of between about 300-400 gms/inch were achieved for the side seals and 400-480 gms/inch for the top and bottom seals.

In a futher run under commercial conditions, a Packmaster rotary type Model No. 50 sealer was used. The paper used again was the same as that described in Table II above. UE-633 EVA copolymer film having a vinyl acetate content of 18% was extruded to a thickness of 1.5 mils on a backing of 50 gauge Mylar, a polyethylene terephthalate film supplied by du Pont. The packages produced were 4.25 inches machine direction and 6 inches crossweb direction and 46 packages per minute were produced. The temperature range was 275.degree.F.-300.degree.F. at 13-20 psi with a dwell time of about 0.08 seconds. The average seal strengths achieved in gms/inch were 410-445 for the top seal, 275-355 for the side seals and 300-320 for the bottom seals on the Amthor tester.

Accordingly, and as will be apparent from the foregoing, there are provided in accordance herewith, methods and compositions for achieving peelable seals for flexible packages over a wide range of operating conditions while simultaneously achieving seals which substantially eliminate fiber fuzz and/or loose fiber pick, thus making the invention herein particularly appropriate for those uses in which fiber contamination of the contents of packages is a factor. Furthermore, because one of the plies of the packages or pouches produced in accordance herewith is a porous paper, the packages can be readily sterilized by the application of sterilizing gases after sealing of the contents in the package. Furthermore, because of the wide range of operating conditions which may be utilized and the wide selection of materials which can be utilized, old sealing equipment may be used without the expense of undergoing alteration in that equipment prior to the use of the invention herein. Moreover, because the plies being sealed may be in turn laminated to other substrates, the packages and/or pouches produced, in accordance herewith, provide a wide selection of appearances for the final package. In addition, the materials utilized for the packages of the invention herein are within a range of cost of less than half the price of some materials utilized for previous sterilizable peelable type heat sealed packages providing comparable performance, thus making the compositions and formulations in accordance herewith, highly advantageous commercially. Even in those instances where cost advantages of materials used are not achieved over some prior art materials, the increased performance in accordance with this invention makes the use of this invention competitive over those cheaper materials.

While the methods and compositions herein disclosed form preferred embodiments of this invention, this invention is not limited to those specific methods and compositions, and changes can be made therein without departing from the scope of this invention, which is defined in the appended claims.

Claims

What is claimed is:

1. A readily openable flexible package for use in containing objects which may be sterilized after sealing of the package, comprised of two layers in facing contact sealed only along a continuous peripheral seal to form said package, said seal being readily rupturable in the absence of destruction of the matrix of said layers, the combination consisting essentially of a first layer of paper and a second layer of ethylene-vinyl acetate copolymer film having a vinyl acetate content within the range of between about 9 and 28%, said first and second layers being heat sealed together in facing contact to form an object containing pocket.

2. A flexible package as described in claim 1, in which said first layer is selected from the group consisting of standard sterilizable kraft paper, wet strength strongly-bonded sterilizable kraft paper, glassine, parchment, pouch paper, food grade cardboard, Red Wallet mailing paper, and Laundry Tag waterproof paper.

3. A flexible package as recited in claim 1, in which said second layer is a laminate comprised of said copolymer film laminated on the side opposite said first layer to a backing layer comprised of a member selected from the group consisting of paper, foil, polyolefins, polyesters, polyvinyl chloride, chlorotrifluoroethylene, cellulose acetate and mixtures thereof.

4. A flexible package as recited in claim 1, in which said first layer is a laminate comprised of said paper bonded on the side opposite said second layer to a layer of a material selected from the group consisting of foil, paper, polyolefin films and mixtures thereof.

5. A flexible package as recited in claim 1, in which said copolymer film layer has a thickness within the range of between about 0.5 and 5 mils.

6. A flexible package as recited in claim 3, in which said copolymer film has a thickness within the range of between about 0.33 - 2 mils.