LIQUID PACKAGES HAVING IMPROVED LEAKER FREQUENCY PERFORMANCE

Abstract: A pouch for containing a flowable material, said pouch being made from a film in tubular form and having transversely heat-sealed ends, said film comprising at least one layer made from a material comprising from about 70 to about 100 parts of at least one interpolymer of ethylene and a C3-C20 alpha-olefin having a density of from 0.840 to 0.940 g/cm<3> and a melt index of 0.01 to 2.0 dg/min, and from about 0 to about 30 parts by weight of at least one high-pressure polyethylene having a density of at least about 0.916 g/cm<3> and a melt index of from about 0.01 to about 2 dg/min, said film having an overall melt strength of from about 10 to about 15 centiNewtons as determined using a Goettfert Rheotens unit at 190 DEG , and the transverse seal integrity of the pouch is such that leaker frequency rate for pouches, as measured in terms of 10,000 pouches produced, is substantially reduced as compared with the leaker frequency rate for pouches produced from films having a melt strength outside the specified range. Also described is a process for making such pouches.

Description: TITLE Liquid Packages having Improved Leaker Frequency Performance FIELD OF THE INVENTION This invention is directed to the area of liquid packaging, namely to the manufacture of liquid containing pouches. In particular this invention relates to pouches for flowable materials made from films formed principally from ethylene-alpha-olefm interpolymers that demonstrate superior performance in pouches for foods, such as milk. The fluid containing disposable pouches may be manufactured on form, fill and seal equipment, preferably employing impulse sealing techniques. The interpolymers are selected so that the film exhibits melt strengths in the range of from about 10 to about 15 centiNewtons. Packages made from such films exhibit superior leaker frequency performance in use. BACKGROUND OF THE INVENTION U.S. Patent No. 4 521 437 (Storms) (the disclosures of which are incorporated herein by reference) discloses film for making pouches for containing liquids, which comprise a linear polymer of ethylene and an α-olefin with at least one other polymer selected from linear low density copolymers of ethylene and a C4-C10 α- olefin and high pressure low density polyethylene. The film is selected based on its performance in the M-Test, which results are superior to a known film standard. For many years, this test has been a good indicator of film performance in a pouch package. Over the past 20 or more years, the measurement of Hot Tack Strength using a DuPont developed method, was considered the best predictor of film seal performance on DuPont Prepac™ liquid packaging machines. While evaluating various films for use in the manufacture of liquid packages, the typical focus has been upon films having curves of improved hot tack strength vs. heat seal temperature, on the basis that this property was an indicator of good performance. However, leaker frequency trials in the field have revealed that this property is not a good or consistent indicator of field behavior. On the basis of a superior Hot Tack Strength Curve, it was expected that a film would demonstrate improved seal performance in milk pouch packaging trials. In field trials of hybrid film (see U.S. Patent Application No. 09/135,187 filed August 17, 1998 (Climenhage and Chow) and based upon U.S. Provisional Patent Application Serial No. 60/056,900, filed on August 22, 1997, the disclosures of which are incorporated herein by reference) versus the industry standard film, SCLAIRFILM™ SM3, hybrid film was found to have roughly double the frequency of horizontal seal failures compared to SM3 in spite of its improved hot tack strength. This result was very surprising and led to the theory that some other "melt" characteristic, not understood at the time could be critical to superior performance. Hybrid film is obtained from resin that is produced using a combination catalyst that includes metallocene catalysts and Zeigler-Natta catalysts. Processes for producing such films are described in U.S. Patent No. 5,844,045, issued December 1, 1998 to Kolthammer et al (the disclosures of which are incorporated herein by reference). SCLAIRFILM™ SM3 is a blend of linear low density ethylene-octene copolymer resin and high pressure low density polyethylene resin and its use in pouches is described in U.S. Patent No. 4,521,437 to Storms. U.S. Patent No. 5,879,768 issued March 9, 1999 to Falla et al describes an environmentally friendly polymer film pouch made from a polyethylene film structure for the packaging of flowable materials, for example milk, including, for example, a pouch made from a monolayer or multilayer film structure such as a two-layer or a three-layer coextruded film containing at least one layer of a blend of a substantially linear ethylene polymer or a homogeneously branched ethylene polymer and a high pressure low density polyethylene as a seal layer. Also described is a process for making the pouch for packaging flowable materials using a film structure described hereinabove. In this patent, the pouch is formed from a film structure that has one seal layer that includes a high pressure low density polyethylene that has a melt strength greater than 10 cN as determined using a Gottfert Rheotens unit at 190°C. The melt strength of the polymeric composition is described as being greater than 5 cN, ranging from 5 to 70 cN, 10 to 70 cN, preferably from about 15 to 70 cN and most preferably from 15 to 50 cN. In PCT Patent Application WO 97/12755, Falla et al disclose an environmentally friendly polymer film pouch made from a polyethylene film structure for the packaging of flowable materials, for example, a pouch made from a monolayer or multilayer film structure such as a two-layer or a three-layer coextruded film containing at least one layer of a blend of a substantially linear ethylene polymer and a high pressure low density polyethylene as a seal layer. Also disclosed is a process for making the pouch for packaging flowable materials using the film structure described. In PCT Patent Application WO 97/20693, Falla et al disclose an environmentally friendly polymer film pouch made from a polyethylene film structure for the packaging of flowable materials, for example, milk. The film may be a monolayer or multilayer structure, such as a two-layer or a three-layer coextruded film containing at least one layer of a blend of an ultra low density polyethylene and a high pressure low density polyethylene as a seal layer having high melt strength. Also disclosed is a process of making the pouch for packaging flowable materials using a film structure as described. In Falla PCT Patent Application WO 98/34844, there is disclosed an environmentally friendly polymer film pouch made from a polyethylene film structure for the packaging of flowable materials, for example milk, including, for example, a pouch made from a monolayer or multilayer film structure such as a two-layer or a three-layer coextruded film containing at least one layer of a blend of a linear ethylene interpolymer and a high pressure low density polyethylene as a seal layer. Also disclosed is a process for making a pouch for packaging flowable materials using a film structure of a blend of a linear ethylene interpolymer and a high pressure low density polyethylene. All of these references appear to teach that the melt strength of the polymeric composition is preferably greater than 5 cN and most preferably above 15 cN. While Falla et al were concerned with reducing the high incidence of "leakers" , that is seal defects such as pinholes which develop at or near the seal through which flowable material, for example milk, escapes from the pouch, they place emphasis on higher hot tack strength and lower hot tack and heat seal initiation temperatures in order to improve the processability of the film and to improve pouches made from the films. Falla et al, in the aforementioned references, have indicated that their film structures have improved melt strength and heat seal strength, particularly the end-seal strength. Falla et al claim that use of films for making pouches using form, fill and seal machines leads to machine speeds that are higher than those currently obtainable with the use of commercially available film and to pouches with fewer leakers. However, the machines used in their examples, while commonly used are not the latest generation of machines which operate at significantly increased speeds. Falla et al appear to believe that the use of LDPE having high melt strength in a film structure for pouches is the key ingredient responsible for the benefits associated with their proposed film structures. In the patent and applications, Falla et al teach that LDPE can be combined with an extremely wide range of linear and substantially linear ethylene interpolymers to achieve high melt strength. In the Examples of the above referenced patent and applications, a very small sample (20) of pouches was tested and/or examined for "on-line leakers", "subjective seal strength" and "end-seals". This sample was quite small and did not involve any actual field leaker frequency data. It is important to appreciate that when making liquid pouches, the process involves sealing through the liquid being packaged, which makes the sealing problematic by itself. However, given that the pouch is produced on a vertical fill machine, and the film is in molten condition when sealed, obtaining a superior seal is a challenge. Ensuring that the melt strength of the film is at the levels set out herein produces a seal that not only meets high speed manufacturing requirements for new machines, but also the rigorous requirements of the field. SUMMARY OF THE INVENTION We have now found that by ensuring that the melt strength of the final film structure is in the range of about 10 to about 15 centiNewtons, pouches may be obtained for containing flowable materials, i.e. liquids, that exhibit significantly improved leaker frequency rates in actual use. Melt Strength or Melt Tension is defined as the stress or force (as applied by a wind-up drum equipped with a strain cell) required to draw a molten extrudate at some specified rate above its melting point as it passes through the die of a standard plastometer such as that described in ASTM D1238-E. Melt strength values are determined using a Gottfert Rheotens at 190°C. In general, for ethylene α-olefin interpolymers and high pressure ethylene polymers, melt strength tends to increase with increased molecular weight, or with broadening of the molecular weight distribution and/or with increased melt flow ratios. Long chain branching also contributes to increasing melt strength. All of Falla et al patents and applications teach that the pouch leaker rate consistently decreases as the melt strength of the polymer composition increases. Surprisingly, the inventors have found that melt strengths greater than 15 cN are deleterious to pouch leaker rates. Operating the horizontal sealing jaws on the VFFS equipment at high speeds presents problems. The latest generation of pouch packaging machines (Prepac™ IS7E) operates at 120 pouches per minute (ll/3 1. size) compared to 90 pouches per minute for the Prepac™ IS-6, which is currently widely used in Canada. At a speed of 120 pouches per minute, the impulse sealing element has difficulty pressing through and cutting off cleanly. This results in the seal being partially attached when the jaw opens. The seal then will peel off and remain attached to one of the pouches resulting in a missing section of seal bead or a "Detached Seal" . This may leak immediately or lead to a high frequency of horizontal seal leakers. For this reason, experience and data show the maximum benefit of increasing melt strength is achieved between 10 and 15 cN and increasing the melt strength above 15 cN results in a high leaker frequency unless machine speed is slowed down rendering the film/machine combination very unfavourable. The above results were observed with a film composition having a melt strength of only 16 cN. This is a surprising indication that the melt strength optimum is in the narrow range of 10 to 15 cN for pouch operating speeds above 100/min., in particular 120/min. The "Detached Seal" effect was so pronounced that films above 15 cN could not safely be field tested. When the field data was plotted, it revealed that there is little benefit to further increases in melt strength above 15 cN because the horizontal seal frequency curve approaches the vertical and closes in on a frequency of zero. Thus, the present invention provides in one aspect a pouch for containing a flowable material, said pouch being made from a film in tubular form and having transversely heat-sealed ends, said film comprising at least one layer made from a material comprising from about 30 to about 100 parts of at least one linear or substantially linear interpolymer of ethylene and a C 3 - o alpha-olefin having a density of from about 0.840 to about 0.940 g/ cm 3 and a melt index of about 0.01 to about 2.0 dg/min, and from 0 to 70 parts by weight of at least one high-pressure polyethylene having a density of at least about 0.916 g/cm 3 and a melt index of from about 0.01 to about 2 dg/min, said film having an overall melt strength of from about 10 to about 15 cN as determined using a Goettfert Rheotens unit at 190°C, so that the horizontal or transverse seal integrity of the pouch exhibits a leaker frequency rate, as measured in terms of 10,000 pouches produced, that is substantially reduced as compared with the leaker frequency rate for pouches produced from films with melt strengths outside the specified range. The substantial decrease in leaker frequency rates has been found to be in the range of about a four to five-fold decrease. This is very significant since this represents a result in the field as opposed to theoretical laboratory measurements. Preferably, the interpolymer comprises from about 75 to about 90 parts by weight, most preferably 85 parts by weight with the high pressure low density polyethylene comprising about 10 to about 25 parts by weight, most preferably 15 parts by weight. In the event multi-layer films are desired, it is preferable that the melt strength of each polymer layer be greater than about 7 cN in order to produce a pouch film of the required melt strength and to achieve the field results set out herein. Most preferably the individual layers must have a melt strength of greater than 10 cN. The film preferably has an overall density that ranges from about .900 to about .930 g/cc, more preferably from about .915 to about .925 g/cc, and the melt index ranges from about .01 to about 2.0 dg/min. More preferably, the overall density of the film ranges from about .916 to .924 g/cc. and the melt index ranges from about 0.1 to about 1.0 dg/min. The melt index of the high-pressure, low-density polyethylene ranges preferably from about 0.1 to about 2 dg/min. The interpolymer may be selected from single-site catalyst, multi-site catalyst or mixed catalyst polymers. There are many examples of such polymers known in the art and it is expected that any of these would be suitable as long as the melt strength values set out herein are met. Preferably, the interpolymer is selected from the following: (a) from about 4 to about 100% by weight of the total parts by weight of the linear interpolymer of an ethylene C3-C20 α-olefin having a density of from about 0.900 to about 0.940 g/cc and a melt index of from about .01 to about 2.0 dg/min, and (b) from about 0 to about 96% by weight of the total parts by weight of the linear interpolymer of an ethylene C 3 -C 2 o α-olefin having a density of from about 0.840 to about 0.915 g/cc and a melt index of from about .01 to about 2.0 dg/min. More preferably, the interpolymer is selected from the following: (a) from about 40 to about 100% by weight of the total parts by weight of the linear interpolymer of an ethylene C 3 -C 2 o α-olefin having a density of from about 0.915 to about 0.925 g/cc and a melt index of from about 0.1 to about 1.0 dg/min, and (b) from about 0 to about 60% by weight of the total parts by weight of the linear interpolymer of an ethylene C3-Q0 α-olefin having a density of from about 0.900 to about 0.910 g/cc and a melt index of from about 0.1 to about 1.0 dg/min. The preferred amounts for (a) and (b) interpolymers will vary depending on the individual polymers and may be selected in accordance with known combinations, bearing in mind the melt strength requirements of the film and the individual polymer components as set out herein. Numerous patents and patent applications are referenced hereinafter that set out guidelines for selecting such polymers and these may be referred to and the choices easily made by a person skilled in the art. The Falla and Falla et al references mentioned earlier may also be referred to for suitable formulations. The disclosures of all patents and applications mentioned are incorporated herein by reference. Three methods for controlling melt strength have been used in the present description, but other methods known in the art may be employed. Melt strength of a film can be increased by increasing molecular weight of either the linear interpolymer component or the high pressure polyethylene component, by broadening the molecular weight distribution of the linear interpolymer component, or by selecting a polymer with a greater frequency of long chain branching. Typical polymers with long chain branching are high pressure low density polyethylenes, but substantially linear ethylene interpolymers are also available which have a limited amount of long chain branching. Increasing the molecular weight of linear or high pressure polyethylenes will reduce the melt index. Molecular weight of a polymer is generally indicated by its melt index. In another aspect, the invention provides a process for making a pouch for containing a flowable material using a vertical form, fill and seal machine, in which process each pouch is made from a flat web of film by forming a tubular film therefrom with a longitudinal seal and subsequently flattening the tubular film at a first position and transversely heat sealing said tubular film at the flattened position, flattening the tubular film above the predetermined quantity of flowable material at a second position, the improvement comprising using the film of the present description. The preferred type of form, fill and seal equipment is that which produces a melt through seal and this seal is provided preferably by impulse sealing means. More preferably, the invention provides a process for making a pouch for containing a flowable material using a vertical form, fill and seal machine, in which process each pouch is made from a flat web of film by forming a tubular film therefrom with a longitudinal seal and subsequently flattening the tubular film at a first position and transversely heat sealing said tubular film at the flattened position, flattening the tubular film above the predetermined quantity of flowable material at a second position, the improvement comprising using the film as defined previously and operating the process at machine speeds above 100 pouches per minute. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings which are used to illustrate the present invention, Figure 1 illustrates graphically hot tack strength vs. heat seal temperature curves for typical films of the present invention used to manufacture liquid packages; and Figure 2 is a plot of melt strength vs. horizontal seal leaker frequency for films and pouches made therefrom in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION Examples of polymers that may be selected for the film used to make the pouches of the present invention are described hereinafter. In all instances the melt strength of these polymers may be modified as required for the present invention in accordance with known methods for producing films of the required melt strength. We believe that the present invention may be equally applied to all of the olefin copolymers currently used to make film for liquid pouches; namely linear low density polyethylene interpolymers (LLDPE), very low density polyethylene interpolymers (VLDPE), metallocene (single site catalyst) interpolymers, multi-site catalyst interpolymers and hybrid single site catalyst (metallocene) - multi-site catalyst, e.g. Zeigler-Natta (Z-N) interpolymers. Metallocene ethylene-octene, ethylene-hexene, ethylene-butene-hexene and ethylene-butene interpolymers are available commercially in the density range from 0.80 to .915 gm/cc. Films of these densities generally are too low in modulus to be used alone to make pouches for flowable materials. The metallocene interpolymers must be either blended with Zeigler-Natta interpolymers or used in co-extrusions to form liquid pouches. As noted, the development of single site catalyst (SSC) or metallocene technology has brought about an improved class of ethylene interpolymers ranging from crystalline to elastomeric materials. These ethylene interpolymers have features such as improved impact strength and toughness, better melt characteristics, because of the control over molecular structure, and better clarity. Exxon and Dow have developed SSC or metallocene ethylene interpolymers and each has the benefit of a number of patents relating to these polymers. Exxon is said to use mono- and bis-cyclopentadienyl metallocenes, while Dow's focus is on titanium cyclopentadienyl metallocenes, which it calls "constrained geometry catalysts" . In practice, Exxon produces ethylene-butene, ethylene-hexene copolymers and ethylene-butene-hexene terpolymers, while Dow makes ethylene-octene copolymers of the metallocene or SSC type. Dow claims that its metallocene (or SSC) polymers are different as it has uniformly introduced long chain branching that improves the processability in otherwise linear polymers. Examples of polymers of Exxon are found in the following patents and applications, the disclosures of which are incorporated herein by reference. U.S. Patent No. 5,382,630 issued January 17, 1995 to Stehling et al and WO 93/03093 published February 18, 1993 to Meka et al. These resins are available commercially from Exxon under the Brand name EXACT™. Examples of polymers of Dow are found in the following U.S. patents: U.S. Patents Nos. 5,508,051 issued April 16, 1996 to Falla et al, 5,360,648 issued November 1, 1994 to Falla et al, 5,278,272 issued January 11, 1994 to Lai et al, and 5,272,236 issued December 21, 1993 to Lai et al. These resins are available commercially under the Brand name AFFINITY™. In DuPont Canada Inc. 's PCT International Publication WO 95/10566 published April 20, 1995, there are disclosed pouches for flowable materials wherein the sealant film is made from a SSC linear copolymer of ethylene and at least one G- Cio alpha-olefin. Blends of these SSC interpolymers with at least one polymer selected from multi-site catalyst linear interpolymers of ethylene and at least one G- Cio alpha-olefin, a high pressure polyethylene and blends thereof. In DuPont Canada Inc. 's PCT International Publication WO 95/21743 published August 17, 1995, there is disclosed an ethylene copolymer film of improved stiffness for use in the manufacture of fluid containing pouches. Typically, the structure comprises an interposed layer of polyethylene having a thickness in the range of 5 to 20 microns and a density of at least 0.93 gm/cc and a melt index of from about 1 to 10 dg/minute, with the outer layer being a SSC or metallocene polyethylene/alpha-olefin film which may have a density in the range of 0.88 to 0.93 gm/cc. The only requirements placed on the stiffening interposed layer are that it be of a particular thickness and density. These properties are greater in the stiffening layer than in the metallocene or SSC layer(s). This application indicates that the stiffening layer is included in order for the fluid containing pouch to stand up properly so that fluid may be poured from it when the pouch is placed in a supporting container. DuPont Canada Inc . ' s U . S . Patents Nos . 4 , 503 , 102(Mollison) and 4,521,437(Storms), disclose a polyethylene film for use in a form, fill and seal process for the manufacture of a disposable pouch for liquids such as milk. U.S. Patent No. 4,503,102 discloses pouches made from a blend of a linear ethylene copolymer of ethylene and a C 4 -Go α-olefin and an ethylene-vinyl acetate polymer copolymerized from ethylene and vinyl acetate. The linear polyethylene copolymer has a density of from 0.916 to 0.930 g/cm 3 and a melt index of from 0.3 to 2.0 dg/min. The ethylene-vinyl acetate polymer has a weight ratio of ethylene to vinyl acetate from 2.2: 1 to 24: 1 and a melt index of from 0.2 to 10 dg/min. The blend disclosed in Mollison U.S. Patent No. 4,503,102 has a weight ratio of linear low density polyethylene to ethylene-vinyl acetate polymer of from 1.2: 1 to 24: 1. U.S. Patent No. 4,503,102 also discloses multi-layer films having as a sealant film the aforementioned blend. U.S. Patent No. 4,521,437 (Storms) describes pouches made from a sealant film which is from 50 to 100 parts of a linear copolymer of ethylene and octene-1 having a density of from 0.916 to 0.930 g/cm 3 and a melt index of 0.3 to 2.0 dg/min and from 0 to 50 parts by weight of at least one polymer selected from the group consisting of a linear copolymer of ethylene and a G-Go-alpha-olefin having a density of from 0.916 to 0.930 g/cm 3 and a melt index of from 0.3 to 2.0 dg/min, a high-pressure polyethylene having a density of from 0.916 to 0.924 g/cm 3 and a melt index of from 1 to 10 g/ 10 minutes and blends thereof. The sealant film disclosed in U.S. Patent No. 4,521,437 is selected on the basis of providing (a) pouches with an M-test value substantially smaller, at the same film thickness, than that obtained for pouches made with film of a blend of 85 parts of a linear ethylene/butene-1 copolymer having a density of about 0.919 g/cm 3 and a melt index of about 0.75 dg/min and 15 parts of a high pressure polyethylene having a density of about 0.918 g/cm 3 and a melt index of 8.5 dg/min, or (b) an M(2)-test value of less than about 12% , for pouches having a volume of from greater than 1.3 to 5 litres, or (c) an M(1.3)-test value of less than about 5% for pouches having a volume of from 0.1 to 1.3 litres. The M, M(2) and M(1.3)-tests are defined pouch drop tests for U.S. Patent No. 4,521,437. The pouches may also be made from composite films in which the sealant film forms at least the inner layer. In Falla et al US Patent No. 5,288,531, there is described the use of polymers in the manufacture of films used to make fluid containing pouches. These films are characterised as ultra low density linear polyethylene ("ULDPE") and are sold commercially as ATTANE™ by Dow. They are described as a linear copolymer of ethylene with at least one α-olefin having from 3 to 10 carbon atoms, for example, the ULDPE may be selected from ethylene- 1-propylene, ethylene- 1-butene, ethylene- 1-pentene, ethylene-4-methyl-l-pentene, ethylene- 1-hexene, ethylene- 1- heptene, ethylene- 1-octene and ethylene- 1-decene interpolymers, preferably ethylene- 1-octene copolymer. In Meka et al WO 93/03093 published February 18, 1993, there are described metallocene polymers useful for making sealed articles, comprising ethylene interpolymers having a CDBI of at least 50% and a narrow molecular weight distribution or a polymer blend comprising a plurality of said ethylene interpolymers as blend components. There are now available commercially, metallocene/Zeigler-Natta (Z-N) hybrid interpolymers of ethylene and G to Go alpha-olefins that offer just the right combination of property improvements to boost performance of liquid pouches compared to those made with conventional (Z-N) LLDPE or pure metallocene interpolymers. The hybrid interpolymer may be obtained from a multiple catalyst polymerization process. The process may comprise at least two reactors in series or in parallel or both, with each reactor having at least one catalyst selected from metallocene catalysts or at least one catalyst selected from Zeigler-Natta catalysts, and the process utilizes both types of catalysts. Alternatively, both types of catalysts may be used in a single reactor. The process may be set up to produce desired weight fractions of polymers in accordance with known methods in the art. In the present instance, the polymer resins produced may comprise preferably from about 20 to about 80 wt. % of metallocene catalyst derived polymer and from about 80 to about 20 wt. % Zeigler-Natta catalyst derived polymer. More preferably the mixture of weight fractions may comprise from about 40 to about 60 wt. % of metallocene catalyst derived polymer with from about 60 to about 40 wt. % Zeigler- Natta catalyst derived polymer. Most preferably, the proportion is about 50:50 wt. % . An example of a commercially available resin which may be modified to produce film for pouches in accordance with this invention is the Dow series of ELITE™ Brand resins. As already stated the film used to make the pouch of this invention may be a monolayer or a multi-layer film. In multi-layer structures, the individual polymers preferably have melt strengths in the required range. The previously referenced patents provide many examples of suitable multi-layer structures, which may be used to make the pouches of the present invention. In a preferred form of the invention, there is provided a pouch formed from a film material which comprises from about 70 to about 100, preferably 80 to about 100, more preferably 85 or 90 parts by weight of the hybrid interpolymer and from about 0 to about 30, preferably from 0 to about 20, more preferably, 10 or 15 parts by weight of the high pressure polyethylene. In another preferred form of the invention, the α-olefin for the interpolymer is selected from the group consisting of 1-propylene, 1-butene, 1-isobutylene, 1- hexene, 4-methyl-l-pentene, 1-pentene, 1-heptene and 1-octene. Typical structures for the films of this invention are those known in the art and which will suit the packaging application. The previously referenced patents and applications describe the various processes, which may be used to manufacture the pouches of this invention. Preferably, vertical form, fill and seal apparatus is used to make the pouches envisaged herein. A flat web of film is unwound from a roll and formed into a continuous tube in a tube forming section by sealing the longitudinal edges together by either a lap seal or a fin seal. This tube is pulled vertically towards a filling station and is then collapsed across a transverse cross section of the tube, the position of which section coincides with a sealing device below a filling station. A horizontal or transverse heat seal is made at the section providing an air and liquid tight seal across the tube. The material to be packaged enters the tube above the horizontal seal, the tube drops a predetermined distance under the influence of gravity on its load. The sealing device is operated again, and a second horizontal seal is made together with a cut through the tube and then through the liquid or fluid being packaged in the pouch. Thus in this operation, the pouch which has an elongate pillow shape is formed, filled and sealed in a rapid sequence of steps. Many variations of this process are possible and are apparent to those skilled in the art. Examples of typical liquid packaging apparatus used for this type of manufacture are made by Hayssen, Thimonnier and Prepac. The term "flowable materials" as used herein encompasses materials which flow under gravity or which may be pumped. Gaseous materials are not included in this definition. The flowable materials include liquids, for example, milk, water, fruit juice and oil; emulsions, for example, ice cream mix and soft margarine; pastes, for example, meat pastes and peanut butter; preserves, for example, jams, pie fillings, marmalade, jellies and doughs; ground meat, for example, sausage meat; powders, for example, gelatine powders and detergents; granular solids, for example, nuts, sugar and like materials. The pouch of the present invention is particularly useful for liquids, for example, milk. The resins used to make the film of this invention are preferably extruded in known ways, although other suitable methods may be used, such as those involving laminates, coatings and the like. When blends are used, these may be made by blending the components prior to or at the time of extrusion just prior to remelting in the extruder. A film extruder may be used and the film made using known techniques. An example of a blown film process is found in Canadian Patent No. 460,963 issued November 8, 1949 to Fuller. Canadian Patent No. 893,216 issued February 15, 1972 to Bunga et al describes a preferred method using an external or internal cooling mandrel in the blown film process. Additives, known to those skilled in the art, such as anti-block agents, slip additives, antioxidants, UV stabilisers, pigments and processing aids may be added to the polymers from which the pouches of the present invention are made. Typically these may comprise up to about 5% by weight of total resin components. As stated previously, the film of this invention may be used in packaging applications where sealing properties, particularly hot tack strength is important. Reference may be had to The Wiley Encyclopaedia of Packaging Technology, 1986, John Wiley & Sons, Inc., under the heading Heat Sealing, the disclosures of which are incorporated herein by reference. Descriptions are found here for all types of heat sealing including bar, band, impulse, wire or knife, ultrasonic, friction, gas, contact, hot melt, pneumatic, dielectric, magnetic, induction, radiant and solvent sealing. Any of these techniques that lend themselves to packaging materials incorporating the film of this invention fall within the scope of this disclosure. Most preferred are packages made by impulse sealing. MELT STRENGTH MEASUREMENT Melt strength determinations are made at 190°C using a Goettfert Rheotens and an Instron capillary rheometer. The capillary rheometer is aligned and situated above the Rheotens unit and delivers, at a constant plunger speed of 25.4 mm/min, a filament of molten polymer to the Rheotens unit. The Instron is equipped with a standard capillary die of 2.1 mm diameter and 42 mm length (20: 1 L/D). The Instron delivers the filament to the toothed take-up wheels of the Rheotens unit rotating at 10 mm/s. The distance between the exit of the Instron capillary die and the nip point on the Rheotens take-up wheels is 100 mm. The experiment to determine melt strength begins by accelerating the take-up wheels on the Rheotens unit at 2.4 mm/s 2 , the Rheotens unit is capable of acceleration rates from 0.12 to 120 mm/s 2 . As the velocity of the Rheotens take-up wheels increases with time, the draw down force is recorded in centiNewtons (cN) using the Linear Variable Displacement Transducer (LVDT) on the Rheotens unit. The computerized data acquisition system of the Rheotens unit records the draw down force as a function of take-up wheel velocity. The actual melt strength value is taken from the plateau of the recorded draw down force. The velocity at filament break is also recorded in mm/s as the melt strength break speed. EXAMPLE Over a period of almost two years a series of test films were formulated to have increased melt strength and then field tested to determine the horizontal seal performance to see if seal performance and melt strength of the films were related. To measure horizontal seal failure frequency, a dairy was selected that had two DuPont Prepac™ IS6 milk pouch packaging machines that were identical and operated side by side in the dairy. This allowed one line to operate using a standard SCLAIRFILM™ SM3 (85% Ethylene Octene LLDPE copolymer and 15% High Pressure polyethylene: the film contained slip, antiblock, and extrusion aid as part of its formulation) and the parallel line to operate using the test films designated as Film A, Film B, Film C and Film D. The code dates on the milk packaging were colour coded to allow identification of the line from which a returned package was produced and on what date. The dairy distributed much of its milk through its own dairy store outlets along with some regular supermarkets. Any leaking or damaged packages were returned from the outlet stores or supermarkets to the dairy for credits. For this reason, a very high proportion of leaking packages was returned to the dairy. By examining all returned packages from the test days, (as identified by their code date and code date colour) the cause and frequency of leakers could be determined for each of the test films to be evaluated. The cause and location of each leaker was identified and classified into three categories; horizontal seal leakers, vertical seal leakers and physical damage caused by a puncture or tear. The leaker frequency was calculated for each category per 10,000 pouches produced during the test period. This method of evaluating films is very laborious and time consuming but was the best way to determine comparative performance of films since it looked at all modes of failure. A film may have good sealing performance but may be more prone to physical damage or vice versa. A common goal in the industry is to develop films that perform well in all categories. In the past, pouch drop failure rate has been used to predict performance, and while it is a good general indicator, actual field performance is more definitive because it evaluates films under all misadventures that can cause leakers and not just severe impacts. For example, seal leakers may be caused by a failing seal element or during start-up when seal components are heating up, etc. Physical damage may be caused by abuse during distribution (transportation, loading and unloading, etc.) or packaging machinery used to put milk pouches in bags and then in dairy cases. In summary, this field trial was designed to evaluate films under the actual conditions of use, looking at the three main modes of failure. Values for the field trials appear in Table I hereafter. TABLE 1 EFFECT OF MELT STRENGTH ON HORIZONTAL SEAL LEAKER FREQUENCY * Earlier this Melt Strength value was estimated at 14 CN. Test results measured gave a value of 12 CN. The variance from predicted is due to variation in blend accuracy and measurement accuracy but does not change the conclusion of a correlation between Melt Strength and Seal Leakers. Results of the field trials were plotted in Figure 2. Film A with a melt strength of about 4 cN produced a horizontal seal leaker rate of 11.6 per 10,000 pouches produced. A modified Ethylene Octene Hybrid copolymer, Film B in which the melt strength was increased to about 6 CentiNewtons (cN) was found to produce a horizontal seal leaker rate of 8.6 per 10,000. A further modification resulted in test film C with melt strength increased to about 11 CN. Its horizontal seal leaker rate was reduced to 2.1 horizontal seal failures per 10,000 pouches produced. A further increase in melt strength to about 12 CN was evaluated in a test film designated Film D, demonstrating a horizontal seal failure rate of only 1.8 per 10,000 pouches made. In Figure 2, melt strength versus horizontal seal leaker failure rates were plotted. The surprisingly strong correlation between Rheotens melt strength and horizontal seal failure rate had never been observed and measured before. The correlation confirmed that melt strength is a critical and previously unknown parameter in formulation of films for liquid pouch application in which seals and cutoff are made through a liquid column. Three methods of controlling melt strength were used in the test Films A to D. Melt strength can be increased by increasing molecular weight of the interpolymer, by broadening the molecular weight distribution of the interpolymer or by selecting a fractional melt index high pressure polyethylene in the film blend. Molecular weight of a polymer is generally indicated by its melt index. The base resin used at about 85% in Film A had a melt index of 0.85 dg/min. Test film B contained a similar ethylene octene hybrid copolymer at the 85 % level in which the melt index was reduced to 0.5. The same high-pressure low density polyethylene (5.0 melt) was blended at 10% in Film A and Film B. In test film C the 5.0 melt index high pressure polyethylene was replaced by a 0.2 melt index high pressure polyethylene increasing its melt strength to about 11. The Rheotens Melt Strength and Horizontal Seal Performance of SCLAIRFILM™ SM3 was also plotted in Figure 2 and was found to fall close to the melt strength/seal performance curve for the hybrid copolymers. It appears that melt strength is independent of polymer used and must be carefully controlled whether using a conventional linear low density polyethylene or the metallocene-hybrid linear copolymer of ethylene. The invention may be varied in any number of ways as would be apparent to a person skilled in the art and all obvious equivalents and the like are meant to fall within the scope of this description and claims. The description is meant to serve as a guide to interpret the claims and not to limit them unnecessarily.

Claims: WE CLAIM 1. A pouch for containing a flowable material, said pouch being made from a film in tubular form and having transversely heat-sealed ends, said film comprising at least one layer made from a material comprising from about 70 to about 100 parts of at least one interpolymer of ethylene and a G-Qo alpha-olefin having a density of from 0.840 to 0.940 g/cm 3 and a melt index of 0.01 to 2.0 dg/min, and from about 0 to about 30 parts by weight of at least one high-pressure polyethylene having a density of at least about 0.916 g/cm 3 and a melt index of from about 0.01 to about 2 dg/min, said film having an overall melt strength of from about 10 to about 15 centiNewtons as determined using a Goettfert Rheotens unit at 190°, and the transverse seal integrity of the pouch is such that leaker frequency rate for pouches, as measured in terms of 10,000 pouches produced, is substantially reduced as compared with the leaker frequency rate for pouches produced from films having a melt strength outside the specified range. 2. The pouch as claimed in claim 1 wherein the overall density of the film ranges from about .915 to about .925 g/cc and the melt index ranges from about .01 to about 0.8 dg/min. 3. The pouch as claimed in claim 1 wherein the melt index of the interpolymer ranges from about 0.1 to about 1.0 dg/min and the melt index for the high pressure low density polyethylene ranges from about 0.1 to about 2 dg/min. 4. The pouch as claimed in claim 1 wherein the melt index of the linear copolymer ranges from about 0.01 to 2 dg/min. 5. The pouch as claimed in claim 1 wherein the linear interpolymer is selected from single-site catalyst, multi-site catalyst or mixed catalyst polymers. 6. The pouch as claimed in claim 1 wherein the linear or substantially linear interpolymer comprises: (a) from 4 to 100% by weight of the total parts by weight of the linear interpolymer of an ethylene G-Qo α-olefin having a density of from about 0.900 to about 0.940 g/cc and a melt index of from about .01 to about 2.0 dg/min, and (b) from 0 to 96% by weight of the total parts by weight of the linear interpolymer of an ethylene G-Go α-olefin having a density of from about 0.840 to about 0.915 g/cc and a melt index of from about .01 to about 2.0 dg/min. 7. The pouch according to claim 1 wherein the film is a monolayer film. 8. The pouch according to claim 1 wherein the α-olefin used in the interpolymer is selected from the group consisting of 1-propylene, 1- butene, 1-isobutylene, 1-hexene, 4-methyl-l-pentene, 1-pentene, 1- heptene and 1-octene. 9. A process for making a pouch for containing a flowable material using a vertical form, fill and seal machine, in which process each pouch is made from a flat web of film by forming a tubular film therefrom with a longitudinal seal and subsequently flattening the tubular film at a first position and transversely heat sealing said tubular film at the flattened position, flattening the tubular film above the predetermined quantity of flowable material at a second position, the improvement comprising using the film as claimed in claim 1 and operating the process at machine speeds above 100 pouches per minute.

Inventors: CLIMENHAGE DAVID CHARLES, SMILLIE BENJAMIN ANDREW, CLIMENHAGE, DAVID, CHARLES, SMILLIE, BENJAMIN, ANDREW
Applicants: CLIMENHAGE DAVID CHARLES, DU PONT CANADA, SMILLIE BENJAMIN ANDREW, CLIMENHAGE, DAVID, CHARLES, DUPONT CANADA INC., SMILLIE, BENJAMIN, ANDREW
Application year: 1999
Main classification: C08L23
IPC codes: B65B9, B65D30, C08J5, C08L23