MXPA96005833A - Resin of thermo-sealed copolymer of improved propylene and articles of the mi - Google Patents

Abstract

The temperature at which propylene copolymers produced by metallocene catalysts form a term full strength seal is found to be lower than similar melting point copolymers correspondingly produced by copolymers catalyzed by Ziegler-Natta catalysts. At similar melting points, films made of metallocene-catalyzed copolymers were also found to exhibit higher values of tension modulus relative to films of polypropylene copolymers catalyzed with Ziegler-Natta catalysts. Films and articles made in accordance with the present invention offer superior stiffness properties, but are sealable at lower temperatures than currently available.

Description

TERMO-SEALED RESINS OF IMPROVED PROPYLENE COPOLYMER AND ARTICLES THEREOF Field of the Invention This invention relates generally to thermo-sealed articles. More specifically, this invention relates to articles produced from metallocene-catalyzed propylene copolymers and at least one other monomer designed to take advantage of the advantageous characteristics of heat sealing, hot tack and stiffness relative to articles produced from polypropylene copolymers. catalyzed with Ziegler-Natta catalysts. BACKGROUND OF THE INVENTION A variety of plastic films are used in packaging applications, such as bags, covers, tubs and trays. In many of these applications, it is important that the plastic film be easily thermo-sealable and possess other physical and mechanical properties such as tear strength, tensile strength, and processing capability in high speed equipment. Oriented polypropylene films are useful and widely accepted packaging films, due to their good moisture barrier, stiffness, high strength and optical properties. However, in general, polypropylene films do not exhibit good heat-sealing properties, which is an important consideration in packaging applications. This is generally because the polypropylene films have narrow ranges of sealing temperature and melting at high temperatures. The heat sealing behavior of polyolefins, and particularly polypropylene copolymers, has been studied extensively in a few recent years. The seal start temperature (SIT) and platform start temperature (PIT) of polypropylene films have been correlated with the melting point behavior. It is well known that the melting point of polypropylene can be reduced by the addition of co-monomer during the polymerization reaction. This phenomenon is true regardless of the polymerization catalyst used, ie Ziegler-Natta or metallocene catalyst. See documents EP-A-318 049 and EP-A-495 099. EP-A-495 099 discloses random copolymers of propylene and alpha-olefin having from 1 to 10 mol% alpha-olefin. Polypropylene copolymers catalysed with Ziegler-Natta and metallocene catalysts having the same or similar co-monomer content were compared and evaluated for melting point decrease. Figure 9 of EP-A-495 099 illustrates a linear relationship between the melting point of the polypropylene copolymer and the co-monomer content. It is illustrated that the melting point decreases linearly as the co-monomer content increases. The slope of the line shown for metallocene catalyzed polypropylene is substantially identical to that of polypropylene catalyzed by Ziegler-Natta catalyst; however, it is shown that the metallocene line is shown to be about 10 to 20 ° C lower than the polypropylene line catalyzed by Ziegler-Natta catalyst. EP-A-495 099 does not teach or suggest any distinction between a particular co-monomer and the effects of the melting point. EP-A-0 538 749 discloses a film made of physical mixtures of propylene copolymers catalyzed with metallocene with propylene copolymers catalyzed with Ziegler-Natta catalysts. EP-A-0 538 749 suggests that films made primarily of metallocene-catalyzed propylene copolymer have a surface roughness such that they are rendered unusable as films. It is desirable to have films that can be sealed at as low a temperature as possible. Being able to operate a packaging line at even a few degrees less than the current temperature of the line results in savings due to increased productivity. It would be desirable to develop a polypropylene film, or article, that can be sealed at lower temperatures but that maintains all other commercially attractive physical properties.

SUMMARY OF THE INVENTION This invention relates to the finding that the metallocene catalyzed polypropylene copolymers have lower platform start temperatures than the corresponding similar melting point copolymers having the same comonomer, produced by conventional Ziegler-Natta catalysts. . At similar melting points, it was found that the metallocene-catalyzed copolymers also exhibit higher stress modulus values relative to the polypropylene copolymers catalyzed with corresponding Ziegler-Natta catalysts. This yields important process and product benefits. An advantage of the films and articles made in accordance with the present invention includes a better stiffness character, but sealable at lower temperatures than those currently available with current films and articles. It has also been found that the PIT for the metallocene copolymers described herein is about 10 to 25 ° C less than the melting temperature (Tm) of the copolymer. An embodiment of this invention relates to a method for producing a thermoseal seal, comprising the steps of: a) having a film of at least one layer, made of a copolymer comprising propylene and at least one co-monomer , preferably an olefinic monomer, having between 2 and about 20 carbon atoms, excluding 3 carbon atoms (propylene monomer), said propylene copolymer produced by a metallocene catalyst; and b) applying a heat source to at least a portion of the film at a temperature in the range of about 10 to 25 ° C below the melting point of the copolymer. Preferably, the temperature of the heat source does not exceed about 10 ° C below the melting point of the copolymer. In a preferred embodiment, the comonomer content is in the range of about 0.5 to 10% by weight relative to propylene. If a physical polypropylene mixture is used, the content of co-monomer relative to polypropylene is preferably in the range of about 0.5 to about 15% by weight. Another embodiment of the invention relates to a method for producing a thermo-sealed article, and articles produced by this method, comprising the steps of: a) having a film of at least two layers, at least one layer made of a copolymer comprising propylene and at least one co-monomer, preferably an olefinic monomer, having between 2 and about 20 carbon atoms, excluding 3 carbon atoms, said propylene copolymer produced by a metallocene catalyst; and b) placing at least a portion of the propylene copolymer film layer in contact with a surface or other layer; - • c) raising the temperature at the point of contact in the range of about 10 to about 25 ° C below the melting point of the copolymer. The metallocene is preferably dihalide or dialkyl derivative of a bis-cyclopentadienyl transition metal of groups 4, 5 or 6, bridged. Even more preferred metallocenes include bridged group 4 metal bisindenyl dihalide derivatives. Specific metallocene catalysts known to be useful for producing isotactic polypropylene are desired. The metallocene is preferably supported in an inert carrier and is optionally prepolymerized with an olefin monomer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates depression of melting point of polypropylene by co-monomer incorporation. Figure 2 illustrates a general curve of seal resistance versus seal temperature for polymers. Figure 3 illustrates the voltage modulus versus the DSC peak melting point of polypropylene copolymers. Detailed Description of Preferred Embodiments Introduction Thermo-sealing is widely used in the packaging industry to bond polymer films. There are many different types of heat sealing techniques, including jaw type, bar sealants, rotating sealants, band sealants, impulse sealers, bead sealers, hot blades, side welding, radio frequency, hot air and sonic wave sealants, to name a few techniques. In heat sealing techniques, at least two films, or portions of film, are pressed together between hot plates or dies to achieve fusion at the interface between the films. To achieve high production speeds in commercial practice, the contact time between the plates and the films, ie the residence time, is short, generally of the order of one second or preferably less. The seal strength, toughness, failure mode and the appearance of such seals after cooling to room temperature are important properties of the seal. The sophistication of packaging materials and the need for improved productivity in recent years have generated an increased interest in improved sealing resins. It has been found that the heat-sealing behavior of the films can be controlled by the comonomer content. It has further been found that metallocene catalysts produce polypropylene copolymers having a lower platform start temperature, and higher values of modulus and hot tack than conventional polypropylene of similar melting point and co-monomer content. Although it is known that by increasing the co-monomer levels the melting points of polypropylene are reduced, a correlation has been found between the type of co-monomer and the amount used and its impact on the melting point, the behavior of the thermo -seal, hot stickiness and module. Generally, it has been found that ethylene does not lower the melting point of polypropylene as significantly as higher alpha-olefins (HAO) when polymerized by metallocene catalysis. HAO are defined as olefins having 5 carbon atoms or more. As seen in Figure 1, which graphs the depression of the melting point versus the level of co-monomer, the slope of the line varies based on the particular comonomer used. A greater reduction is observed when 1-hexene is incorporated into polypropylene, compared to the decrease observed when ethylene is incorporated into polypropylene. For the incorporation of ethylene, there was no significant difference observed in the magnitude of melting point reduction based on whether the polypropylene was conventional or metallocene catalyzed. There was a considerable differential of observed depression based on hexene-1 and polymerization catalyst. A greater reduction for hexene-1 incorporated in metallocene-catalyzed polypropylene compared to hexene-1 incorporated in conventionally catalyzed polypropylene is observed. Additionally, the higher the co-monomer content, the lower the molar amount necessary to reduce the melting point of the polypropylene. For a given melting point, a lower molar percentage of hexene-1 co-monomer is necessary to reduce the melting point of polypropylene compared to the mole percentage necessary for ethylene co-monomer. Generally, the resins of the present invention employ propylene and one or more co-monomers, the co-monomers preferably being an alpha-olefin having from 2 to about 20 carbon atoms (excluding propylene), a diolefin, a monomer ethylenically unsaturated or a cyclic olefin. Ethylene is defined herein as an alpha-olefin. The polypropylene copolymers produced with metallocene can be a physical mixture of polypropylene and another or other polymers with different properties. Applications of these polypropylene compositions having advantageous properties, as described herein, include diapers, medical gowns, packaging for snacks and tobacco, wraps for food, especially for temperature sensitive foods such as chocolate, where a low PIT is beneficial. For the purposes of this invention, conventional polypropylene is that polymer produced from Ziegler-Natta catalysts having a broad molecular weight distribution and broad composition distribution.; Metallocene polypropylene is that polymer produced from a single site or transition metal catalysts derived from cyclopentadienyl, which commonly produce a polymer with a narrow molecular weight distribution, narrow distribution of the composition, and -. narrow distribution of tacticity. Polypropylene refers to isotactic polypropylene copolymers or physical mixtures thereof. Copolymers refers to polymers based on propylene prepared from propylene and one or more other monomers. The module refers to the tension module as measured on compression molded samples and tested using a modification of ASTM D638. The principles embodied in the present invention are applicable to most processes in which low temperature sealing is valuable. Almost any thermoplastic manufacturing process can benefit from these findings. Examples of melt forming processes that can benefit from reduced seal or bond temperatures include profile extrusion, sheet extrusion (optionally followed by thermoforming), film extrusion and the like. Examples of uses for oriented film products made in accordance with the present invention include oriented film products for tobacco packaging, snack foods or other uses of food wrapping. Polypropylene of the Present Invention In the preferred embodiment of the present invention, the copolymers include polypropylene and at least one other co-monomer (or alpha-olefin), wherein the co-monomer has between 2 and about 20 carbon atoms ( excluding propylene co-monomer). The copolymer has a lower melting point than copolymers produced from conventional catalysts having a similar content of co-monomer. Exemplary co-monomers include ethylene, butene-1, pentene-1, hexene-1, octene-1,4-methyl-1-pentene, and the like. Particularly preferred co-monomers include ethylene, butene-1, hexene-1 and octene-1, the most preferred co-monomer being hexene-1. The copolymers generally have a melting temperature in the range of about 110 to about 135 ° C, preferably about 112 to about 125 ° C, and most preferably about 115 to about 123 ° C. In a preferred embodiment, the resins generally have a platform start temperature of about 10 to about 25 ° C less than the Tm of the copolymer. Preferably, the PIT is from about 10 to about 20 ° C and more preferably, the PIT is from 10 to about 15 ° C less than the Tm of the copolymer. In one embodiment of the present invention, the propylene copolymers employed have a comonomer content in the range of about 0.5 to about 10% by weight. The preferred types and levels of co-monomers depend on the desired application, which in turn depends on the platform start temperature, and the desired module. Physical mixtures or mixtures of polymers containing two or more polymers can also be used. Exemplary physical blends include copolymer of propylene and alpha-olefin physically mixed with polyethylene, or a copolymer of butene-1, or an ethylene-propylene elastomer. Generally, the preferred types and levels of co-monomer for the copolymers include ethylene, butene-1, hexene-1 and octene-1. The co-monomers are generally in the range of 0.5 to 15% by weight, preferably 1 to about 7% by weight, most preferably around 4 to about 6.5% by weight, and most preferably around 4.5 to about 6.0% by weight, to achieve a platform start temperature of at least about 10 to about 25 ° C below the melting point of the resin. The present invention is discussed in terms of weight percentage; however, the examples are illustrated in molar percentage. A person skilled in the art will easily be able to convert the molar and weight percentage values. Instead of having the polypropylene produced by a metallocene catalyst, polypropylene can be produced by other known catalysts, such as certain Ziegler-Natta catalysts that produce polymers having a narrow composition distribution, as determined by temperature rise elution fractionation (TREF), as described in ild and collaborators, J. Poly Sci., Polv. Phys. Ed .. 1982, vol. 20, p. 441 and US Patent No. 5,008,204, a narrow molecular weight distribution (MWD), as determined by gel permeation chromatography, and a narrow tacticity distribution, as is commonly described by 13 C NMR analysis of the polymer. The MWD is defined as the ratio of the average heavy molecular weight (Mw) to the average numerical molecular weight (Mn). For the purposes of this invention, MWD is generally in the range of about 1 to about 5, the preferred MWD is in the range of about 1.5 to about 3.5, the most preferred being in the range of about 1.6 to around 2.5, and the most, most preferred is in the range of about 1.8 to about 2.2. Additives may be included in the polymer compositions. These can be selected from additives commonly used with plastics, such as fillers and / or reinforcers, strengthening fibers, plasticizers, dyes, pigments, flame retardants, anti-oxidants, anti-blocking agents, dyes, release agents, drip retardants. , and the like, in conventional quantities. Effective amounts are selected, typically varying from about 0.05 to 0.2% by weight of the polymer. The polypropylene copolymers of the present invention will generally exhibit melting points in the range of about 110 to about 135 ° C, although one skilled in the art will appreciate that the melting point depends in part on the particular comonomer or comonomers and the amount of these used with polypropylene. Films prepared in accordance with the present invention will typically exhibit extractable basses with n-hexane, generally less than 10% by weight and preferably less than about 4% by weight, and most preferably less than 2.5% by weight, and by Both are desirable for products used in food and medical applications. It was found that the conventional polypropylene / ethylene melting temperature of 134 ° C has 5.6% extractables with n-hexane at 50 ° C, while polypropylene / ethylene catalyzed by metallocene having a melting point of 135 ° C only They had 0.5% extractables at 50 ° C. Extraction tests were conducted in accordance with the provisions of 21 CFR 177.1520 (d) (3) (ii). Useful melt flow rates (MFR), as measured by ASTM D-1238, of the polymers of the present invention are in the range of about 0.1 to about 200. In a preferred embodiment, the flow rates melt ranges from about 0.5 to about 50. The preferred MFR range for extrusion and molding applications is from about 1 to about 20, the most preferred being from about 1 to about 10. The oriented fibers produced by fibrillation or oriented film grooving they have MFR ranges of about 1 to about 10, and more preferably a range of about 1 to about 5. Polypropylene copolymers or physical blends can be produced by conventional means such as process of gaseous phase, slurry, bulk, solution or high pressure polymerization, using metallocene catalyst; Multiple catalysts (two or more metallocenes, or metallocene and one or more conventional catalysts) may also be employed. The copolymers can be produced in gas phase reactors of fluidized or stirred bed, slurry reactors or bulk tank or loop type reactors, and any other process put into practice for the polymerization of propylene. The polymers can also be produced by the use of multiple reactors of the type described herein. Preferably, a supported catalyst (metallocene plus some activating component on a support) is employed in a slurry or gas phase reactor to produce the polypropylene copolymer. Polymerization can occur under standard conditions. In a preferred embodiment, the copolymers are produced with supported metallocenes under slurry polymerization conditions. The supported metallocene is preferably pre-polymerized with olefinic monomer, more preferably polymerized with ethylene monomer under standard pre-polymerization conditions. Useful Metallocenes in Preferred Embodiments The invention is useful with any class of metallocenes, including mono, di or tri-cyclopentadienyl radical systems, or derivatives thereof. Monocyclopentadienyl fractions include, for example, those described in EP-A-129 368 or US 5,055,438, all incorporated herein by reference for purposes of United States patent practice. The metallocenes described in US 4,808,561, 5,017,714 and 5,296,434, all incorporated herein by reference for the purposes of US patent practice, may also be employed in the present invention. In the most preferred embodiment, the polypropylene copolymer employed is produced from at least one dihalide or dialkyl derivative of a bis-cyclopentadienyl transition metal of groups 4, 5 or 6, bridged. Even more preferred metallocenes include bridged silicon group 4, bisindenyl or bisinde-nyl dihalide derivatives. Specific bisciclopentadienyl metallocene catalysts (or derivative thereof), which are known as useful for producing isotactic polypropylene, are discussed in EP-A Nos. 485,820; 485 821; 485 822; 485 823; 518,092; and 519 237; and US Nos. 5,145,819; 5,296,434. The preferred metallocene employed according to this invention is chiral and is used as a racemate for the preparation of isotactic poly-1-olefins. Illustrative but non-limiting examples of metallocenes include: dimethylsilylbis (2-methylindenyl) zirconium dichloride, dimethylsilylbis (2-ethyl-4-phenylindenyl) zirconium dichloride, dimethylsilylbis (2-methyl-4-phenylindenyl) zirconium dichloride, dimethylsilylbis (2-methyl- 5-isobutylindenyl) zirconium dichloride, dimethylsilylbis (2-methyl-4,5-benzindenyl) zirconium dichloride, and dimethylsilylbis (2-methyl-4,6-diisopropylindenyl) zirconium dichloride. The most preferred specific metallocene is dimethylsilylbis (2-methyl-4,5-benzindenyl) zirconium dichloride. Generally, metallocenes are prepared by a multi-step process involving repeated deprotonations / metalations of the aromatic ligands and the introduction of the bridge and the central atom by their halogen derivatives. It is suggested to refer to Oraanometallics. 13 (3), 1994, pp. 954-963, and EP-A 320 762, for preparation of the metallocenes described. Oraanometallics and EP-A 320 762 are hereby incorporated by reference in their entirety for purposes of United States patent practice. Although the silyl bridge and the zirconium transition metal are specifically disclosed, one skilled in the art would be able to appreciate that other types of bridging systems and transition metals can be employed. The metallocene employed is preferably supported in an inert carrier and optionally is pre-polymerized. Numerous support techniques are known in the art. Most preferred is the technique employed in accordance with US Pat. No. 5, 240,894, incorporated herein by reference for purposes of United States patent practice. As previously disclosed, a preferred embodiment employs a pre-polymerized supported metallocene. The pre-polymer can be any alpha-olefin, preferably polyethylene, polypropylene, or polybutene-1, or mixtures thereof, most preferably polyethylene prepolymer. The metallocene is preferably used during polymerization in the form of a metallocene complex with an activator. The activators can be alumoxane, as is well known in the art, or ionic activators such as those disclosed in US patents 5,198,401 or 5,278,119. It is believed that any compound that serves to activate the metallocene to a catalytic state is applicable to this invention. Films and Sheets The present invention will find its most common application in thermo-sealable films, whether oriented or non-oriented. The films can be produced by techniques known to those skilled in the art. For example, blown films produced with an annular die and air cooling, or forged film using a slot die and a quench roller for cooling are acceptable techniques. Films are generally in the range of about 0.2 to about 10 thousandths of an inch (5 to 254 microns); however, the total thickness may vary based on the desired application. The sheets are a precursor of the films and significantly thicker than the films. The sheets can be prepared by conventional techniques such as extruding a substantially flat profile from a die. Before stretching and thus forming the films, the sheets will generally have a thickness of about 10 to about 75 mils (254 to 1,905 microns), although they may be substantially thicker. The films of the present invention may be mono- or multilayer (composite) in shape, preferably in the form of multilayers for use in thermo-sealed articles. The composite materials would include at least one first epidermal layer and at least one other layer. The composite materials can be formed (1) by co-extrusion, followed by orientation, (2) film orientation followed by lamination, or (3) film orientation followed by extrusion coating or (4) co-extruded by wrought. Articles Extruded or molded articles can be manufactured by conventional techniques such as profile extrusion, injection molding, injection blow molding, extrusion blow molding, rotational molding, compression molding, or foam molding. There are parts of many thicknesses, generally around 500 microns (20 thousandths) or greater. It is important that the resin be heated considerably above the melting point to give the molecules a random character. The resins of the present invention allow lower temperatures for this heating process than conventional resins. Additionally, the increased modulus observed for metallocene catalyzed polypropylene polymer allows an erect, stiffer article or film than those produced by conventional catalysts. Thermo-Sealing and Platform Start Temperature Polypropylene copolymers are commonly used as one or more seal layers on oriented polypropylene. The advantages of these copolymers as seal layers compared to very low or low density polyethylene resins are the excellent inter-layer adhesion that can be obtained without the use of tie-in resins. An important characteristic for a thermo-sealable film is the temperature at which the sealing begins, that is to say the temperature of initiation of heat-sealing. The start temperature of the seal is defined as that temperature at which the resistance of the seal is 200 g / in (1.1 N / 15 mm). Films or sheets produced within the scope of the present invention can be sealed at lower temperatures than currently known with conventional polypropylene. A lower sealing temperature has advantages in multi-layer films, where the sealing layer is produced from a polymer melting at a lower temperature than the other layer or layers. To estimate the temperature of film sealing, a graph of seal resistance versus temperature for the polymer composition used is useful. Figure 2 is an illustration of seal strength versus temperature for polymers. The resistance of the seal is a function of temperature. The platform start temperature (PIT) is generally defined as the temperature at which the platform region appears, where the resistance of the seal maximizes in the curve, or the temperature at which the resistance of the seal becomes independent of the temperature sealing. Generally, the PIT corresponds to the temperature at which the tear failure occurs. Where a platform is not well defined, the platform start temperature is the temperature at which a change in slope of the seal resistance versus seal temperature occurs. A point is reached in the sealing temperature at which the film, usually the core layer in a multi-layer film, it melts. Generally, this is observed at high sealing temperatures (in the figure, this is the downward curve on the extreme right side). When the SIT is determined, the seal fails due to detachment or delamination. At the PIT, all the resistance of the seal is measured or a value just below it, and it is said that the fullest development of the seal is present. Just below or near the PIT, the seal resistance test tears the film and retains the actual integrity of the seal. The PIT can thus be defined by proximity to the platform region and by the film failure mode when the resistance of the seal is measured. Due to variations in commercial thermal bar temperatures during operation, it is difficult to know precisely at what temperature the film is sealed. Accordingly, it is advantageous to use heat-sealing resins having a wide heat-sealing platform. By operating at a midpoint on the platform, minor variations in sealing temperature can be accommodated without materially affecting the strength of the seal. Table 1 illustrates the comparison of heat seal resistance of metallocene catalyzed copolymers and conventionally. The SIT is the one stipulated at 200 g / in (1.1 N / 15 mm). ZN is copolymer catalyzed with Ziegler-Natta or conventional catalysts. MC is the metallocene catalyzed copolymer. Commercially available polypropylene PD9282-E2 was obtained from Exxon Chemical Company, Baytown, Texas, United States, which is a copolymer containing about 5% by weight of ethylene. This commercially available product is representative of the random copolymer polypropylenes widely used in the industry today as thermosetting layer resins for polypropylene films. The resulting copolymer was compared with metallocene-catalyzed polypropylene copolymers. Copolymers having between about 1 and about 4% by weight of co-monomer and having similar melting points and prepared under similar processing conditions were compared. With respect to the melting point and the PIT, it is observed that the polymers produced with metallocene exhibit PIT values in the range of about 10 to about 25 ° C less than the melting points. C2, polypropylene film or oriented, co-extruded (OPP), having a melting point of around 132 ° C, was found to possess a PIT of around 120 ° C, a difference of 12 ° C. A oriented, co-extruded polypropylene film, ZN, having a melting point of about 133 ° C and prepared under similar processing conditions was found to have a PIT of about 130 ° C, a difference of only 3 ° C. Comparing forged films, co-extruded conventional polypropylene versus metallocene, it is found that for the metallocene copolymer there is a difference of 17 ° C at the melting point and the PIT is observed, while for the conventional copolymer, a difference is observed 8 ° C. Accordingly, an embodiment of the present invention relates to a method for producing a thermoset, comprising the steps of: a) having a film of at least one layer, made of a copolymer comprising propylene and at least one a co-monomer, preferably an olefinic monomer, having between 2 and about 20 carbon atoms, excluding 3 carbon atoms, said propylene copolymer produced by metallocene catalysts, and further having an MWD of less than about 5 carbon atoms. , preferably less than 3.5, and a propylene tacticity distribution of more than about 90% pentads, preferably greater than about 95% pentads, with greater preference greater than about 98% pentads, as determined by NMR analysis; and b) applying a heat source to at least a portion of the film at a temperature in the range of about 10 to about 25 ° C below the melting point of the copolymer. The heat source is typically a thermo-sealing bar; however, for the purposes of this invention, any source capable of heating the film to form a weld will suffice. The temperature of the heat source is preferably in the range of from about 10 to about 20 ° C, and more preferably from about 10 to about 15 ° C below the melting point of the copolymer. As an alternate embodiment, the heat source is 10 ° C below the melting point of the copolymer. In another embodiment, films according to the present invention have a platform start temperature of at least about 10 ° C or more, less than a similar processed film produced from copolymer catalyzed with Ziegler-Natta catalyst having the same comonomer and similar melting point. To determine the sealing range and the sealing strength of the preferred film, two strips of film (15 mm wide) are superposed with the surfaces of the epidermal layer in contact. A flat sealing bar, heated by means of an electric resistance, 5 mm wide, was used to press the sheets together against a backing surface with a pressure of about 0.5 N / mm2 for 0.5 seconds and the temperature was recorded of the sealing bar. The sample was then cooled to room temperature and aged. To determine the strength of the seal, the force to tear the seal is measured. A one-inch (2.54 cm) strip was cut perpendicular to and through the seal made by the bar, and the ends of the respective strips were placed on the jaws of an Instron test machine with the seal located approximately at the midpoint between grip jaws. Force was applied separating the jaws until the seal came off or ripped. Seals having a minimum strength considered sufficient for packaging applications were achieved by the preferred embodiment at a seal bar temperature as low as 104 ° C. The present invention exploits these attributes of the metallocene resin to define a heat sealing process capable of operating at lower temperatures than currently possible with current conventional resins, but yielding a product substantially equivalent to the best current film products. Hot Stickiness The hot tack resistance of the seal is the ability of the melted seal to withstand a load. It is defined as the resistance of the seal when it is tested immediately after the heat seal, before the sample cools.

The resistance to hot tack will therefore be less than the "cold seal" strength. This is a key property in some packaging applications, particularly that involve formation-fill-seal vertical packaging where the packaging material is placed in the package immediately after making the lower seal. Hot tack is important for manufacturing because the hot tack resistance of a heat sealable product is necessary in order to maintain the integrity of the melted seal when subjected to stress. Especially in high speed forming-filling and packing machines, the hot stickiness of a polymer at the sealing temperature is important to resist the stress while the seal is still in the melted / semi-melted state. Factors that contribute to hot stickiness include hot tack strength, film processing, co-monomer type and content. It has been found that the films produced according to the present invention have a hot tack of at least about 20%, or more, at the same temperature, higher than a catalyzed copolymer with Ziegler-Natta catalyst having the same co-monomer , similar melting point, and / or similar film processing. Table 2 illustrates a comparison of hot tack resistance for various examples of metallocene-catalyzed propylene-ethylene and propylene-hexene-1 copolymers and conventionally, processed as forged monolayer or co-extruded, forged multilayer films. It is noted that a forged monolayer film of conventional copolymer having a melting point of about 133 ° C was found to have a hot tack value at 100 ° C of about 9 g / in (0.55 N / 15). mm). A forged monolayer film of metallocene copolymer, having a melting point of about 132 ° C, had a hot tack value at 100 ° C of about 151 g / in. Comparing metallocene-catalyzed propylene copolymers and conventionally, co-extruded, forged, is a hot tack value at 100 ° C of 256 g / in (1.48 N / 15 mm) and 384 g / in (2.22 N / 15 mm), respectively. The increased values of hot stickiness translate into higher line speed because less cooling time is required before continuing the expansion of the packing and the loading of the product in a similar process. As can be seen, the metallocene catalyzed copolymers provide higher hot tack, compared to conventionally catalyzed copolymers. Upper Module The tension module is a measure of the inherent rigidity of a material. The tension versus temperature module for conventional and metallocene polypropylenes was measured and plotted in Figure 3. Compression-molded samples were evaluated for the tension modulus. The tension module was derived from the Instron test of compression molded samples, homogeneous under modified ASTM D638 test conditions. The modifications involved a jaw separation of 1.0 in (2.54 cm), and a cross head speed of 2.0 in / min (5.1 cm / min). The melting temperature for the polymers was derived from differential scanning colorimetry (DSC), a widely used tool for quantifying thermal transitions in polymers. The peak temperature of the melting endotherm was used to quantify the melting temperature. The data plotted in Figure 3 illustrates a trend observed between metallocene-catalyzed polymers and conventionally, and does not differentiate between the specific copolymers evaluated. The copolymers evaluated were propylene-ethylene and propylene-hexene-1 copolymers. The metallocene-catalyzed polypropylenes provided a different balance of modulus versus melting temperature, when compared with conventionally catalyzed polypropylenes. Figure 3 demonstrates this behavior. This different behavior achieves benefits in thermo-sealing application where seal capacity at a lower temperature, coupled with high rigidity for good "erect" packaging behavior, constitute a desired balance of properties. Also the higher stiffness indicates greater crystallinity, which provides superior barrier properties (for example, to oxygen, air, gases and the like). The data in Figure 3 indicates that at a melting temperature of about 135 ° C (typical for the current random copolymer, polypropylene / ethylene, used in sealing applications), the modulus, or the inherent rigidity, of the product catalysed with Metallocene is greater than that of conventional polypropylene. Alternatively, the metallocene-catalyzed polypropylene allows, at the same modulus or rigidity, a substantial reduction in the melting temperature, which results in a decrease in the heat sealing temperature. Those skilled in the art will recognize the advantages in packing speed and the reduction of packing rejects (due to poor or inadequate sealing) achieved by the opportunity to reduce the sealing temperature. Miscellaneous Advantages It is expected that the oriented thermo-sealed films or products thereof, produced from metallocene catalysts, have properties of steam transmission (water) or moisture (MVTR) at least similar to those of products formed to from conventional catalyst resins. Steam or moisture transmission rates are indicators of the ability of the film to serve as a barrier to water or moisture. With the current processes to form thermo-sealed films and articles from conventional resins, the MVTR will deteriorate as the melting point of the resin is reduced. With the present invention, the lower melting point of the resin, and the reduced processing temperature are achieved without compromising the moisture or vapor barrier properties of the final film. Indicators are present that suggest that the improved module also leads to improved barrier properties in a sample. Method of Producing Thermo-Sealed Structures In one embodiment of the invention, a method of producing a thermo-sealed film or article comprises the steps of: a) having a film of at least one layer, made of a copolymer comprising propylene and at least one co-monomer, preferably an olefinic monomer, having between 2 and about 20 carbon atoms, excluding 3 carbon atoms, said propylene copolymer produced by a metallocene catalyst; and b) applying a heat source to at least a portion of the film at a temperature in the range of about 10 to about 25 ° C below the melting point of the copolymer. A modified embodiment of the invention relates to a method of producing a thermofixed article, and articles produced by this method, comprising the steps of: a) having a film as described herein; b) placing at least a portion of the propylene copolymer film layer in contact with a surface or other layer; and c) raising the temperature at the point of contact in the range of 10 to about 25 ° C below the melting point of the copolymer. The surface or layer can be a woven fiber or a non-woven layer, a metal foil, a cardboard surface, or the like. As discussed above, in the most typical form the polypropylene film will be in contact with a core or substrate, for example another film, a surface, or an article or object, so that once the heat source is applied to a particular point in the polypropylene film, a thermo-seal is formed at the point of contact. An application devised in this embodiment is the formation of packages for snacks where a film is folded, two points of the film are joined, and a seal is established to form a bag. Alternatively, a polypropylene film can be joined with a fabric and a seal formed at the point of contact, such as to form a diaper or a gown. The methods described above result in films or articles produced at lower temperatures than conventionally catalysed polypropylene films or articles. These items or films have good optical properties and PITs that are commercially attractive and suitable for high-speed online applications. EXAMPLES The following illustrative but non-limiting examples will further illustrate the invention. They should not be construed to limit the claims in any way. The metallocene used can be prepared by procedures published in the literature. Preparation of the Supported Metallocene Catalyst To an eight-liter flask, equipped with cooling jacket and efficient head agitator, methylalumoxane (30% by weight in toluene, 925 ml) was added. With stirring, a suspension of rac-dimethylsilandylbis (2-methyl-4,5-benzoindenyl) zirconium dichloride (5.0 g) in toluene (700 ml) was added N2 through a double-ended needle. After stirring for 10 minutes, dehydrated silica solution was added to the solution (Davison 948, dried at 800 ° C, 200 g) for 20 minutes. The slurry was stirred for 10 minutes and then, while a vacuum was applied from the top of the flask, a slight flow of N2 was added through the bottom. The mixture was heated to 70 ° C on evaporation of the solvent over a period of 9 hours. The dried solid was cooled to room temperature overnight. Isopentane (5 1) was added to put the solids in slurry and the mixture was cooled to 0 ° C. Ethylene was added to the stirred mixture by a drop tube at a rate of 0.003-0.06 SCF / minute until a total of 491 1 of ethylene had been added. The agitation was stopped and the solids were allowed to settle. The liquid was decanted from the solids, which were washed twice, each with 1.5 1 of isopentane. The wet, pre-polymerized solids were transferred to a dry box under N2 and filtered through a No. 14 sieve. The fine particles were filtered, washed with pentane (4 1) and dried in vacuo. Yield: 326 g. Laboratory Ethylene Copolymerization Experiment Preparation of Copolymers 1. A 2 1 autoclave reactor containing triethylaluminum (0.5 ml of a 1M solution in hexane) was pressurized to 6.9 bar (100 psi) with ethylene. After introducing the ethylene, 1,000 ml of propylene was added, and the reaction vessel was heated to a temperature of 45 ° C. A sample of the supported catalyst was put into slurry in 2 ml of hexane, was flooded in the reactor with an additional 250 ml of propylene. The reaction was run for 0.5 hours, after which the reactor was cooled, ventilated and purged with nitrogen for 20 minutes. After purging with nitrogen, the reactor was opened, and the propylene-ethylene copolymer product was collected and dried under vacuum for a minimum of two hours at 75 ° C. 2. A 2 1 autoclave reactor containing triethylaluminum (0.5 ml of a 1 M solution in hexane) was pressurized to 6.9 bar (100 psi) with ethylene. After introducing the ethylene, 1,000 ml of propylene was added, and the reaction vessel was heated to a temperature of 45 ° C. A sample of the supported catalyst was put into slurry in 2 ml of hexane, was flooded in the reactor with an additional 250 ml of propylene. The reaction was run for one hour, after which the reactor was cooled, ventilated and purged with nitrogen for 20 minutes. After purging with nitrogen, the reactor was opened, and the propylene-ethylene copolymer product was collected and dried under vacuum for a minimum of two hours at 75 ° C. 3. A 2 1 autoclave reactor containing triethylaluminum (0.5 ml in a 1 M solution in hexane) was pressurized to 3.5 bar (50 psi) with ethylene. After introducing the ethylene, 1,000 ml of propylene was added, and the reaction vessel was heated to a temperature of 55 ° C. A sample of the catalyst supported on slurry in 2 ml of hexane was flooded in the reactor with an additional 250 ml of propylene, the reaction was run for 0.5 hours, after which the reactor was cooled, ventilated and purged with nitrogen for 20 minutes. After purging with nitrogen, the reactor was opened, and the product was collected and dried under vacuum for a minimum of two hours at 75 ° C. All ethylene copolymers were combined with the same stabilizer, anti-blocker and neutralizer as the control propylene PD9282-E2.

Hexeno Copolymerization Experiment in Laboratory 4. A 2 1 autoclave reactor was treated with a desired amount of triethylaluminum solution (1 M solution of TEAL in hexane). Then, 50 ml of hexene-1 comonomer was added to the reactor. After hexene was introduced, 1,000 ml of propylene was added and the reaction vessel was heated to a temperature of 60 ° C. A sample of the supported catalyst was placed in slurry in 2 ml of hexanewas flooded in the reactor with an additional 250 ml of propylene. The reaction was run for the desired period of time, after which the reactor was cooled, vented and purged with nitrogen for 20 minutes. After purging with nitrogen, the reactor was opened and the propylene-hexene copolymer product was collected and dried under vacuum for a minimum of two hours at 75 ° C. Three runs, 1, 2 and 3, were physically mixed after drying and used as MC4. MC4 was combined with the same stabilizer, anti-blocker, and neutralizer as the PD9282-E2 control polypropylene.

Procedure for Continuous Copolymerization of Ethylene 5. The designated sample MC2 was produced in a bulk liquid phase polymerization process of a series reactor. The reactor was equipped with a stirrer and a jacket to remove the polymerization heat. The reactor temperatures were set at 59/54 ° C (previous / subsequent reactor temperatures), and the catalyst was fed only to the previous reactor at 6.0 g / hr of pre-polymerized supported catalyst. Propylene was fed to the reactors at a rate of 63.5 / 36.3 kg / hr (before / after), and ethylene at a rate of 0.45 kg / hr was fed to both reactors. Under these conditions, the total residence time of the catalyst in the process was 5.1 hours. The copolymer was produced at a rate of 7.7 kg / hr. It was determined that the product had an ethylene incorporation of 1.4% by weight, with a melting point of 132 ° C. MC2 was combined with the same stabilizer, anti-blocker and neutralizer as the control polypropylene PD9282-E2. ZN Control Copolymers 6. Conventional polypropylene commercially available from Exxon Chemical Company, Baytown, Texas, USA, PD9282-E2 and PD4254, was employed. The PD9282-E2 was combined with Irganox 1010 as a thermal stabilizer, Syloblock 48 as an anti-blocker, and DHT-4TA as a neutralizer. PD4252 contains Irganox 1010 as a thermal stabilizer and calcium stearate as a neutralizer. Polypropylene with three melt flow rates was used in the formation of the co-extruded forged film core layer. The heat seal layer control polymer used in Examples ZN 1 to 4 was polypropylene-ethylene, Escorene PD9282-E2, with a melting temperature of 133 ° C. Preparation of Film Samples Forged films in the following examples were prepared as monolayer (A) or co-extruded multilayer (AB) structures. The copolymers of the examples were formed as films cast by melt extrusion through a slit die followed by passage through a roller suddenly quenched with water at 90-100 ° F.

All the films were prepared in one of two movie lines. Films AB 19:90 with a width of 8"and thickness of 0.020 in. Before thermo-stretching / orientation (see procedure below) of Examples ZN 2 and MC 2 were prepared on a line of forged film, co-extruded , of three extruders from Killion Extruders.The resin was formed as co-extruded film AB after extrusion through a 1"screw (for the core layer of the film) and a 3/4" screw (for the thermostatic layer) The Killion screw was 24: 1 length / diameter with maximum outputs of 20 lbs / hr and 20 lbs / hr, respectively, a screw speed of 95-112 rpm and a temperature profile in 375-450 ° F ramp were employed Forged films of total thickness from 0.002 to 0.020 in were collected at about 15 ft / min Co-extruded films from Examples ZN 2 and MC 2 were stretched with heating to form films of oriented polypropylene, co-extruded, commonly referred to as p OPP films, 0.002 in. in a plastic stretcher T.M. Long before the thermofix formation and measurement. Forged films A and AB of Examples ZN 1, ZN 3, ZN 4, MC 1, MC 3, MC 4, MC 5 and MC 6, with widths of 2"and thicknesses of 0.020 in, were prepared in a Microtruder device from Randcastle Inc. (31 Hopson Ave., Little Falls, NJ 07424, United States), model RC-025.The Microtru-der device requires about 25-50 g of resin.The screws are 1/4"in diameter with a length / diameter ratio of 24: 1. Screw speeds of 5 to 65 rpm were employed, depending on the desired thickness of the individual core or seal layer. Samples for Testing Heat seal and hot tack evaluations were carried out after controlled heating in the DTC hot tack tester, model 52-D. The hot tack test was carried out in accordance with ASTM D 3706-88, flat spring test, after aging for at least 24 hours. The resistance to hot tack is measured directly with the DTC device model 52-D. Standard conditions were used in both procedures.

The conditions are indicated in the following table. Condition Thermo-Seal Hot Stickiness Sealing time 0.5 sec 0.5 sec Seal pressure at seal 0.5 N / mm2 0.5 N / mm2 on test strips 37.5 N 37.5 N Delay time N / D 0.4 sec Release speed N / D 200 mm / sec Width of the sample 15 mm 15 mm Width of the seal bar 5 mm 5 mm Temperature range 5 ° C (int.) 5 ° C (int.) They were aged for 48 hours, at controlled laboratory temperature and humidity, before measurement of resistance, "* ~ heat seal samples." An Instron model 1122, interfaced with a Compaq 386S computer, was used to determine the resistance, and an Instron crosshead speed of 130 mm / min was used. were determined by the following methods: 1. Melting point: DSC measurement, maximum melting curve, heating rate 10 ° C / minute 2. Platform start temperature: as indicated in the description. : ASTM D628 modified, as indicated herein 4. Chromatography of p Gel ermeation (GPC) is a liquid chromatographic technique widely used to measure the average heavy molecular weight and the molecular weight distribution or polydispersity of polymers. A Waters chromatograph model 150C, three columns Shodex AT-80M (mixed bed) and 1, 2, 4-trichlorobenzene (HPLC grade) as solvent was used at 145 ° C with a flow rate of 1 ml / min., For a run time of 60 minutes, and the total injection volume used was 300 microliters. 5. 13C NMR was used to quantify the comonomer level in the copolymer samples. This is a well-accepted spectroscopic technique, widely used in the field. Those skilled in the art will appreciate that modifications and variations of the present invention are possible in the light of the foregoing teachings without departing from the scope or spirit of the present invention. Therefore, it should be understood that changes may be made in the particular embodiments of the invention described, which will be within the intended full scope of the appended claims.

Table 1. Comparison of Thermo-Seal Resistance of Random Copolymers Produced with Metallocene (CM) and Conventional Ziealer-Natta (ZN) Copolymers (S) is the temperature at which the seal strength is 200 g / in.

Table 2. Comparison of Hot Stickiness Resistance of Random Copolymers Produced with Metallocene (MC) and Conventional Ziegler-Natta (ZN) Copolymers (ZN) (1) Co-monomer = ethylene (2) Co-monomer = hexene-1

Claims (27)

1. CLAIMS 1. A method for producing a heat-seal, comprising the steps of: a) having a film of at least one layer made of a copolymer comprising propylene and at least one having 2 to 3 carbon atoms comonomer , excluding 3 carbon atoms, said propylene copolymer produced by a metallocene catalyst; and b) applying a heat source to at least a portion of the propylene copolymer film at a temperature in the range of 10 to 25 ° C below the melting point of the copolymer. The method of claim 1, wherein the temperature is in the range of 10 to 20 ° C, preferably 10 to 15 ° C below the melting point of the copolymer. The method of claim 1 or 2, wherein the content of co-monomer is in the range of 0.5 to 15% by weight. The method of any of the preceding claims, wherein the copolymer comprises at least one comonomer having between 2 and 10 (not 3) carbon atoms. The method of claim 4, wherein the comonomer is in the range of 1 to 7% by weight. The method of any of the preceding claims, wherein the metallocene comprises a bis (substituted indenyl) dihalide of transition metal of group 4, 5 or 6, bridged with silicon. 7. The method of claim 6, wherein the metallocene is selected from the group of dimethylsilylbis (2-methylindenyl) zirconium dichloride, dimethylsilyl (2-methyl-4, 5-benzindenyl) zirconium dichloride, dimethylsilyl (2-methyl-4, 6-diisopropylindenyl ) zirconium dichloride, dimethylsilylbis (2-methyl-4-phenylindenyl) zirconium dichloride. The method of claim 7, wherein the metallocene is dimethylsilylbis (2-methyl-4,5-benzindenyl) zirconium dichloride. The method of any of the preceding claims, wherein the metallocene is supported on a carrier. 10. A method for producing a heat sealing article, comprising the steps of: a) having a film of at least two layers, at least one layer made of a copolymer comprising propylene and at least one comonomer having from 2 and 20 carbon atoms, excluding 3 carbon atoms, said propylene copolymer produced by a metallocene catalyst; b) placing at least a portion of the propylene copolymer layer in contact with a surface or other layer; and c) raising the temperature at the point of contact in the range of 10 to 25 ° C below the melting point of the copolymer. The method of claim 10, wherein the surface or layer is a non-woven article. The method of claim 10, wherein the surface or layer is a woven article. The method of claim 10, wherein the temperature is in the range of 10 to 20 ° C, preferably 10 to 15 ° C below the melting point of the copolymer. 14. An article produced by heat sealing steps comprising: a) having a film of at least two layers, at least one layer made of a copolymer comprising propylene and at least one co-monomer having from 2 to 20 atoms carbon, excluding 3 carbon atoms, said propylene copolymer produced by a metallocene catalyst; b) placing at least a portion of the propylene copolymer layer in contact with a surface or other layer; and c) raising the temperature at the point of contact in the range of 10 to 25 ° C below the melting point of the copolymer. 15. Items produced by the method of claim 10. 16. The method of claim 10, wherein the article is a film. 17. The method of claim 16, wherein the film has a hot tack as the PIT, at least 20% greater than a copolymer catalyzed Ziegler-Natta catalyst having the same comonomer and a melting point Similary. 18. The method of claim 11, wherein the article has a modulus of tension at least two times greater than a copolymer catalyzed with Ziegler-Natta catalyst having the same comonomer and a similar melting point. 19. A method for producing a heat sealing article, comprising the steps of: a) producing a film having a layer of heat-seal, said heat-seal layer incorporating a copolymer of isotactic polypropylene comprising at least one co - C4 monomer to CIO, said copolymer having a MWD greater than or equal to 3.5; b) placing at least a portion of said thermo-seal layer in contact with another polymeric layer; and c) raising the temperature at the point of contact to a temperature in the range of 10 to 25 ° C below the melting point of said thermostatic layer, said temperature being maintained on the thermo-sealing platform for said layer of thermo-seal, whereby a variation in temperature at said point of contact within said stipulated temperature range does not materially affect the strength of said seal. The method of claim 19, wherein said thermostatic layer is at least 90% by weight of said isotactic polypropylene copolymer. The method of claim 19, wherein said film is a monolayer film. 22. The method of claim 19, wherein said co-monomer is selected from the group consisting of cyclic and non-cyclic olefins C6 to CIO. 23. The method of claim 19, wherein said temperature range is from 12 to 20 ° C below the melting point of said heat seal layer. The method of claim 19, wherein said temperature is in the range of 14 to 24 ° C below the melting point of said heat seal layer. 25. The method of claim 19, wherein said temperature is in the range of 16 to 24 ° C below the melting point of said heat seal layer. 26. The method of claim 19, wherein said other polymeric layer is substantially identical to said thermo-seal layer. 27. The method of claim 19, wherein said polymeric component of said heat seal layer is at least 98% by weight of said isotactic polypropylene copolymer.