THERMOFORMABLE SEALANT MATERIALS

Abstract: A sealing material for a thermoplastic packaging film for applications such as meat, poultry and sausage packages that have to be subject to moderate heat treatments common in processes such as pasteurization or sterilization. The sealing material includes a composition of a copolymer of ethylene and an alpha , beta -unsaturated C3-C8 carboxylic acid, optionally neutralized and optionally containing an acrylate and a semicrystalline and/or amorphous polyamide.

Description: TITLE THERMOFORMABLE SEALANT MATERIALS FIELD OF THE INVENTION The present invention relates to ethylene copolymer compositions that are useful for fabricating films used as sealant materials that have to sustain elevated temperatures. More particularly, the ethylene copolymer compositions of the present invention can be used as sealant materials in flexible film structures used for example for packaging of meat and poultry items that have to be subjected to elevated temperatures for a limited time while in the package as this is common for pasteurization and sterilization applications. BACKGROUND OF THE INVENTION Ionomers are thermoplastic resins that contain metal ions in addition to organic- chain molecules. Ionomers have solid-state properties characteristic of cross- linked polymers and melt-processing properties characteristic of uncrosslinked thermoplastic polymers : see for example US Patent 3,262,272. Major applications of ionomers are in the area of packaging. In particular ionomers have found widespread use for applications such as meat packaging where low sealing temperature, high seal strength and high hot tack strength is important for high packaging speed and high sealant reliability. In particular for meat packaging another feature of ionomers, meat adhesion, is used to provide additional benefit in terms of water retention in the meat. This is in particular desirable in order to maintain a good looking meat or sausage package. One of the main drawbacks of ionomers is also their advantage. Combined with the low sealing temperature they are also characterized by a low melting temperature and are therefore restrained to those packaging applications that do not require exposure to elevated temperatures such as commonly used in sterilization and pasteurization treatments where the package including the content is heated up to temperatures between 70-110°C, more preferably between 75-90°C for times ranging from 5 to 20 minutes. Under these conditions the seal strength usually suffers and meat adhesion is totally lost. This invention relates to a method and composition that overcomes these restrictions. As disclosed in JP 05193081 to Mitsubishi ionomers can be used in blends with polyamides for packaging films. These alloys are characterized through improved pin hole resistance and oxygen barrier. This is due to their excellent compatibility with polyamides. In this way it is possible to improve certain mechanical properties and still maintain most of the favorable properties of ionomers which are well known to those skilled in the art such as low seal temperature, high seal strength and hot tack strength as well as good meat adhesion. However, thermal properties, such as Vicat temperature are usually not improved making these blends. JP08332702 A to Sumitomo claims a compounded multilayer film comprising a sealing layer made of approximately equal parts of ionomer, polyamide, polypropylene and acid copolymer. Such a composition, in addition to containing four difficult to mix components, has poor optical properties and tends to delaminate due to insufficient compatibility between polypropylene and the other components JP 07276591 A to Mitsubishi Corp. claims a laminated film where the sealing layer consists of a blend of polyamides with up to 10% of an ionomer. Although having the required temperature resistance, this type of composition has an elevated sealing temperature due to the fact that the polyamide forms the continuous phase. It was , therefore, surprising and unexpected to discover that packages with such sealing layers which are made up of blends of the current invention and subject to the above mentioned heat treatments will very well sustain such treatments and maintain superior meat adhesion although the continuous phase in these blends is the ionomer. SUMMARY OF THE INVENTION The present invention provides a sealing material for a thermoplastic packaging film in particular for application such as meat, poultry and sausage packages that have to be subject to moderate heat treatments common in processes such as pasteurization or sterilization. The sealing material includes a composition comprising 90 to 40 wt.% of a copolymer of ethylene and an α,β-unsaturated C3 - C8 carboxylic acid 1-30% by weight of acrylic acid or methacrylic acid that is optionally neutralized with at least one metal ion and 10 to 60 wt.% of a semicrystalline and/or amorphous polyamide. The copolymer may further contain an acrylate, and may be processed by e.g. extruding or casting a sheet or blowing a film. The ethylene copolymer composition of the present invention overcomes the aforementioned problems of insufficient temperature resistance by providing a means of superior stability surprisingly obtained by a blend with a minor portion of a higher melting polymer , which forms the discontinuous phase in the aforementioned blend while still maintaining all sealing advantages of the ionomer or acid copolymer itself. DETAILED DESCRIPTION OF THE INVENTION This invention consists of the applications of blends of ionomers or acid copolymers and amorphous and / or semicrystalline polyamides as sealant layers for shrinkable bags for meat packaging with improved temperature resistance. It has been found that for this application a suitable composition is comprising 90 to 40 wt.% of a copolymer of ethylene and an α,β-unsaturated C3 -C8 carboxylic acid 1-30% by weight of acrylic acid or methacrylic acid that is optionally neutralized with at least one metal ion and 10 to 60 wt.% of a semicrystalline and/or amorphous polyamide. The copolymer may further contain an acrylate, and may be processed by e.g. extruding or casting a sheet or blowing a film. The components of the blend are described in more detail below. Acid Copolymer and Ionomer The acid copolymers and ionomers of the present invention are derived from direct copolymers of ethylene and ,β-ethylenically-unsaturated C3- C8 carboxylic acid ("ethylene-acid copolymers") by neutralization with metal ions. By "direct copolymer", it is meant that the copolymer is made by polymerization of monomers together at the same time, as distinct from a "graft copolymer" where a monomer is attached or polymerized onto an existing polymer chain. Methods of preparing such ionomers are well known and are described in U.S. Patent No. 3,264,272 which is herein incorporated by reference. Preparation of the direct ethylene-acid copolymers on which the ionomers are based is described in U.S. Patent No. 4,351 ,931 which is also incorporated by reference herein. The ethylene-acid copolymers used to make the ionomeric copolymer of this invention can be E/X/Y copolymers where E is ethylene; X is a softening comonomer and Y is the α,β-ethylenically-unsaturated C3- C8 carboxylic acid, particularly acrylic or methacrylic acid. Preferably, however, the ethylene-acid copolymer is a dipolymer (no softening comonomer). The preferred acid moieties are methacrylic acid and acrylic acid. By "softening", it is meant that the polymer is made less crystalline. Suitable "softening" comonomers (X) are monomers selected from alkyl acrylate, and alkyl methacrylate, wherein the alkyl groups have from 1 - 12 carbon atoms which, when present, may be up to 30 (preferably up to 25, most preferably up to 15) wt.% of the ethylene-acid copolymer. A wide range of percent acid moiety in the ethylene-acid copolymer may be used. The acid moiety may be present in a range of about 1 to 30 weight percent of the acid copolymer, preferably in a range of about 5 to 25, alternatively about 10 to about 20. The ethylene-acid copolymers with high levels of acid are difficult to prepare in continuous polymerizers because of monomer-polymer phase separation. This difficulty can be avoided however by use of "cosolvent technology" as described in U.S. Patent No. 5,028,674 which is also incorporated herein by reference or by employing somewhat higher pressures than those at which copolymers with lower acid can be prepared. The ethylene-acid copolymers are partially neutralized (10 to 99.5 percent) with metal cations, particularly monovalent and or bivalent metal cations. Preferably about 10-70% and more preferably 25 to about 70 of the acid is neutralized. Preferred metal cations include lithium, sodium, and zinc, or a combination of such cations. Zinc is most preferred in blends with polyamide. Preferred ethylene-acid dipolymers are ethylene/acrylic acid and ethylene/methacrylic acid. Specific other copolymers include ethylene/n-butyl acrylate/acrylic acid, ethylene/n-butyl acrylate/methacrylic acid, ethylene/iso-butyl acrylate/methacrylic acid, ethylene/iso-butyl acrylate/acrylic acid, ethylene/n-butyl methacrylate/methacrylic acid, ethylene/methyl methacrylate/acrylic acid, ethylene/methyl acrylate/acrylic acid, ethylene/methyl acrylate/methacrylic acid, ethylene/methyl methacrylate/methacrylic acid, and ethylene/n-butyl methacrylate/acrylic acid. Suitable ionomers for use in the present invention are commercially available under the trademark SURLYN®; suitable ethylene acid copolymers are available under the trademark NUCREL®; both from the DuPont Company. Amorphous Nylon Amorphous polyamides are well known to those skilled in the art (particularly as selected from among those described in U.S. Patents 4,952,628 and 4,990,562). "Amorphous polyamide," as used herein, refers to those polyamides which are lacking in crystallinity as shown by the lack of an endothermic crystalline melting peak in a Differential Scanning Calorimeter ("DSC") measurement (ASTM D-3417), 10°C/minute heating rate. Examples of amorphous polyamides are those prepared from the following diamines: hexamethylenediamine, 2-methylpentamethylenediamine, 2,2,4- and 2,4,4-trimethylhexamethylenediamine, bis(4-aminocyclohexyl)methane, 2,2- bis(4-aminocyclohexyl)isopropylidine, 1,4- and 1,3 diaminocyclohexane, meta- xylylenediamine, 1 ,5-diaminopentane, 1,4-diaminobutane, 1,3-diaminopropane, 2-ethyldiaminobutane, 1,4-diaminomethylcyclohexane, p-xylyenediamine, m- and p-phenylenediamine, and alkyl-substituted m- and p-phenylenediamine. These examples are prepared from the following dicarboxylic acids: iso- and tere- phthalic acid, alkyl-substituted iso- and tere-phthalic acid, adipic acid, sebacic acid, butane dicarboxylic acid, and the like. Specific examples of the amorphous polyamides that can be used include hexamethylenediamine isophthalamide/terephthalamide having iso/terephthalic moiety ratios of 100/0 to 60/40, mixtures of 2,2,4- and 2,4,4- trimethylhexamethylenediamine terephthalamide, copolymers of hexamethylene diamine and 2-methylpentamethylenediamine with iso-or terephthalic acids, or mixtures of these acids. Polyamides based on hexamethylenediamine iso/terephthalamide containing high levels of terephthalic acid moiety may also be useful provided a second diamine such as 2-methyldiaminopentane is incorporated to produce a processible amorphous polymer. Amorphous polyamides may contain, as comonomers, minor amounts of lactam species such as caprolactam or lauryl lactam, even though polymers based on these monomers alone are not amorphous as long as they do not impart crystallinity to the polyamide. In addition, up to about 10 wt. % of a liquid or solid plasticiser such as glycerol, sorbitol, mannitol, or aromatic sulfonamide compounds (such as "Santicizer 8" from Monsanto) may be included with the amo hous polyamide. The amorphous polyamide may also be selected from those containing less than about 100 milli-equivalents of terminal carboxyl groups per kilogram of polyamide, as disclosed in U.S. Patent No. 5,126,402. Preferably, those amorphous polyamides having less than 85 or less than 55 milliequivalents per kilogram may be used. Semicrystalline Nylon Semicrystalline polyamides used in the present invention are well known to those skilled in the art. Semicrystalline polyamides suitable for this invention are generally prepared from lactams or amino acids, such as nylon-6 or nylon- 11, or from condensation of diamines such as hexamethylene diamine with dibasic acids such as succinic, adipic, or sebacic acid. Copolymers and terpolymers of these polyamides are also included. Preferred semicrystalline polyamides are polyepsiloncaprolactam (nylon-6), polyhexamethylene adipamide (nylon-66), most preferably nylon-6. Other semicrystalline polyamides useful in the present invention include nylon- 11 , nylon- 12, nylon- 12, 12 and copolymers and terpolymers such as nylon-6/66, nylon-6/10, nylon-6/12, nylon-66/12, nylon-6/66/610 and nylon-6/6T. In the three-component blend, nylon 6 is the most preferred semicrystalline polyamide. In the framework of this invention we have surprisingly found that although the ionomers in the aforementioned composition form the continuous phase of the film and although their melting temperature does not surpass 100°C, the temperature performance of sealant films having the above compositional range is largely superior to the ionomer itself and can sustain typical sterilization and pasteurization conditions. The advantage of this improvement can be manifold in various applications but is mainly intended for sealants in applications preferably but not exclusively in food packaging where heat treatments as mentioned above are a requirement. Examples Materials used: Ionomer 1 10% MAA, MFI=5 (190C/2.16kg) Ionomer 2 15% MAA, MFI= 14 (190C/2.16kg) Ionomer 3 20% MAA, MFI = 2.5 (190C/2.16kg) Ionomers 1,2,3 can be commercially obtained from E. I. du Pont de Nemours and Company under the tradename Surlyn® 1652, 1702, 8140. PA6 , which was commercially obtained from BASF as Ultramid B36 characterized by a MVR of 36 respectively. Bynel® 41E623 is a tie layer commonly used for pasteurizable meat structures. It can be commercially obtained from DuPont. Subsequently these materials were prepared as a salt an pepper blend and fed to a 50 mm Reifenhaeuser extruder. This extruder was part of a 3 layer Reifenhauser blown film line equipped with a BARMAG 3 layer die, where the following 3 layer structure was prepared. PA6/Tie/Sealant PA6 fed to extruder 1 Ultramid B36 Tie fed to extruder 2 Bynel 41E623 Sealant fed to extruder 3 Described in E The temperature Profile that was established was as follows: Die Extruder 1 220 220 230 240 240 240 Extruder 2 170 180 190 200 210 220 Extruder 3 180 190 200 210 220 230 The so obtained three layer blown film having a diameter of ca 6cm was subsequently filled with processed meat to form a typical sausage. The packages were subsequently cooked in water at a temperature of 95 °C for 10 Minutes. The following results were determined thereafter: Melt Temperature of the sealant was measured with DSC according to ASTM 1238. Meat Adhesion was measured by pulling the film off the meat after cooking and subsequently cooling down of the package to room temperature. The sausage was opened and the package removed. Any processed meat retained on the packaging film was removed with a knife from the empty package, weighed and the weight divided by the packaging film surface recorded. Examples: Ionomer PA6 Melt Temp Meat Adhesion Comp EX l 100% Ion 3 0 80 0 EX 1 70% Ion3 30% 80 0.5 (g/dm2) Ex 2 80% Ion2 20%PA6 85 0.3 EX3 85% Ion3 15%PA6 95 0.2 It was surprisingly found that although the melting temperature of these blends is similar to the ionomers forming these blends the meat adhesion increases significantly through the addition of low levels of PA. This is in line with an increase of temperature stability which is important for applications being subject to heat treatment. Such sealant materials can therefore be used as sealing materials in, optionally thermoformable, shrink films which can optionally be also oriented or bi-oriented, together with at least one other structural or barrier layer and tie layer. Examples include structures such as PA/Sealant, LDPE/tie/ENOH tie/Sealant, Sealant tie/EVOH tie/Sealant or PA/tie/EVOH/tie/Sealant. ENOH is ethylenecovinylalcohol, a common oxygen barrier layer used in meat packaging instead of the more commonly used polyamide.

Claims: We Claim 1. A flexible packaging film with at least two layers of which one layer is an acid copolymer or an ionomer sealant material suitable for packaging food that can sustain temperatures 70-110°C 2. A flexible packaging film with an acid copolymer or an ionomer sealant according to claim 1 where the ionomer sealant is made up of a blend of 90- 40% of a copolymer of ethylene and 5-30% alpha-beta unsaturated C3-C8 carboxylic acid, wherein the acid groups are optionally neutralized from 10- 99.5%) with metal ions chosen from Na, K, Li, Mg, Ca, Zn or other cations, preferably with Zn and 10-60% of a polyamide or a copolyamide. 3. A flexible packaging film according to claim 2 where the acid copolymer or ionomer contains an acrylate as softening comonomer. 4. A flexible packaging film according to claim 2 that contains 65-90% of the acid copolymer or ionomer and 10-35% of the Polyamide. 5. A flexible packaging film according to claim 2 where the alpha-beta unsaturated C3-C8 carboxylic acid is present in an amount of from 10-25%. 6. A flexible packaging film according to claim 2 where the alpha-beta unsaturated C3-C8 carboxylic acid is present in an amount of from 10-20%. 7. A flexible packaging film according to claim 2 wherein the acid groups are optionally neutralized from 20-80%). 8. A flexible packaging film according to claim 2 wherein the acid groups are optionally neutralized from 25-70%. 9. A flexible packaging film according to claim 1 with an acid copolymer or ionomer sealant that is used in thermoformed bags, shrinkable bags or sausage casings that have to be subject to heat treatments (cooking and shrink temperatures) between 70-110°C after packaging and maintain meat adhesion. 0. A flexible packaging film according to claim 1 that is oriented or bi-oriented.

Inventors: FLIEGER HANS DIETER, HAUSMANN KARLHEINZ, FLIEGER, HANS, DIETER, HAUSMANN, KARLHEINZ
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1999
Main classification: B32B27
IPC codes: B32B27