MXPA00011955A - Heat-sealable multilayer polyolefin films - Google Patents

Abstract

A multilayer polyolefin film of the type suitable for packaging application in which heat seals are formed, and in its preparation the multilayer film comprises a flexible substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interface surface of the substrate layer and is formed of a syndiotactic propylene polymer effective to produce a heat seal with itself at a sealing temperature of less than 110 DEG C. The surface layer has a thickness which is less than the thickness of the substrate layer. The heat-seal layer can be formed of syndiotactic polypropylene polymerized in the presence of a syndiospecific metallocene catalyst and having a melt flow index of less than 2 grams/10 minutes. The multilayer film can take the form of a biaxially-oriented film. In the production of the multilayer film incorporating a substrate layer and a heat-sealable surface layer, a crystalline thermoplastic polymer is ext ruded and formed into a substrate layer film. A second polymer comprising a syndiotactic propylene polymer which is effective to form a heat-sealable surface layer is extruded to form a surface layer that is bonded to the interface of the substrate layer at a temperature within the range of 150-260 DEG C.

Description

SEALABLE FILMS WITH HEAT DESCRIPTION OF THE INVENTION This invention relates to polyolefin multilayer films of the type suitable for packaging applications involving heat sealing. The polyolefin multilayer films incorporate a base or a substrate layer of a stereoregular crystalline thermoplastic polymer and one or more surface folds that can be formed on one or both sides of the base layer. Isotactic polypropylene is one of a number of crystalline polymers that can be characterized in terms of the stereoregularity of the polymer chain. Various stereospecific structural relationships, mainly denominated in terms of syndiotacticity and isotacticity, may be involved in the formation of stereoregular polymers of various monomers. The stereospecific propagation can be applied in the polymerization of ethylenically unsaturated monomers such as C3 + alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinyl compounds, such as vinyl aromatics, for example, styrene or vinyl chloride, chloride of vinyl, vinyl ethers such as such alkyl vinyl ethers, for example, isobutyl? rinylether, or even aryl vinyl ethers. The propagation of the stereospecific polymer is probably of the greatest importance in the production of polypropylene of isotactic or syndiotactic structure. Isotactic polypropylene is conventionally used in the production of relatively thin films in which the polypropylene is heated and then extruded through molds and subjected to biaxial orientation by dilation of the film in a longitudinal direction (referred to as the direction of the film). machine) and in a transverse or lateral direction, sometimes referred to as the ^^ 10 direction "of tender". The structure of the isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units located on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being up or down the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula: CH3 H CH3 H CH3 H CH3 H CH3 H CH3 IIIIIIIIIII - - - c - c - c - c - c - c - c - c - c - c - c - - lllll HHHHH Stereorregular polymers, such as isotactic and syndiotactic polypropylene, can characterized in terms of Fisher's projection formula. Using Fisher's projection formula, the The stereochemical sequence of the isotactic polypropylene as shown in Formula (2) is described as follows: J_L (2) Another way to describe the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentade is ... mmmm ... with each "m" representing a divalent "meso", or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain decreases the degree of isotacticity and crystallinity of the polymer. In contrast to the isotactic structure, the syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of sucrose monomeric units in the polymer chain are located on alternating sides of the plane of the polymer. Using Fisher's projection formula, the structure of the syndiotactic polypropylene can be shown as follows: Syndiotacticity can be characterized in terms of the group of five syndiotactic rrrr in which each "r" represents a racemic dyad. The syndiotactic polymers are semicrystalline and, like the isotactic polymers, are essentially insoluble in xylene. This crystallinity distinguishes the syndiotactic and isotactic polymers from an atactic polymer, which is not crystalline and highly soluble in xylene. An atactic polymer does not have a regular order of repeated unit configurations in the polymer chain and essentially forms a waxy product. For many applications, the preferred polymer configuration will be a predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins are described in U.S. Patent Nos. 4,794,096 and 4,975,403, for E in. These patents disclose chiral stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As described, for example, in the As mentioned in US Pat. No. 4,794,096, the stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between the cyclopentadienyl groups. Stereoregular hafnium metallocenes that can be described specifically in this patent are characterized by the following formula: ***** R "(C5 (R ')) 2HfQp (4) In formula (4), (C5 (R ') 4) is a substituted cyclopentadienyl or cyclopentadienyl group, R' is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and R "is a structural bridge that is extends between the cyclopentadienyl rings, Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1-20 carbon atoms and p is 2. Metallocene catalysts, such as those described above, can be used either as the so-called "neutral metallocenes" in which case an alumoxane, such as methylalumoxane is used as a catalyst, or they can be used as the so-called "cationic metallocenes" which incorporate a stable non-coordinating anion and usually do not require the use of an alumoxane For example, the syndiospecific cationic metallocenes are described in US Pat. No. 5,243,002 for Razavi. eno is characterized by the cationic metallocene ligand having sterically unequal ring structures that bind to a positively charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Similar relationships can be established for isospecific metallocenes. The catalysts used in the polymerization of alpha-olefins can be characterized as supported catalysts or unsupported catalysts, sometimes referred to as homogeneous catalysts. Metallocene catalysts are subsequently used as unsupported or homogeneous catalysts, although, as described below, they can also be used in supported catalyst components. Traditional supported catalysts are the so-called "conventional" Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride as described, for example, in US Patent Nos. 4,298,718 and 4,544,717, both for Mayr. et al. A supported catalyst component, as described in the '718 Mayr patent, includes titanium tetrachloride supported on an "active" anhydrous magnesium dihalide such as: magnesium dichloride or magnesium dibromide. The catalyst component supported in the 718 of Mayr is employed in conjunction with such a co-catalyst and an alkylaluminum compound, for example, triethylaluminum (TEAL). The '717 patent of Mayr describes a similar compound that can also incorporate an electron donor compound which can take the form of various amines, phosphenes, esters, aldehydes and alcohols.

While metallocene catalysts are generally proposed for use as homogeneous catalysts, they are also known in the art to provide supported metallocene catalysts. As described in U.S. Patent Nos. 4,701,432 and 4,808,561, both for lborn, a metallocene catalyst component can be employed in the form of a supported catalyst. As described in the '432 patent of elborn, the support can be any support such as talc, an inorganic oxide, or a resinous support material, such as polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesium, zirconia and the like. Non-metallocene transition metal compounds such as titanium tetrachloride are also incorporated into the supported catalyst component. The '561 patent of Welborn describes a heterogeneous catalyst which is formed by the reaction of a metallocene and an alumoxane in combination the support material. A catalyst system comprising a homogeneous metallocene component and a heterogeneous component which can be a "conventional" supported Ziegler-Natta catalyst, for example, a supported titanium tetrachloride, is described in US Pat. No. 5,242,876, to Shamshoum et al. . Various other catalyst systems that involve metallocene catalysts • -.fi supported are described in US Patents Nos. 5,308,811 to Suga et al, and 5,444,134 to Matsumoto. The polymers normally used in the preparation of biaxially oriented polypropylene films are usually those prepared through the use of conventional Ziegler-Natta catalysts of the type described, for example, in the patents mentioned for Mayr et al. Thus, U.S. Patent No. 5,573,723, to Peiffer et al, describes a process for producing biaxially oriented polypropylene film having a base layer formed of an isotactic polypropylene homopolymer or propylene-ethylene copolymer. Other copolymers of propylene and alpha-olefins having 4-8 carbon atoms can also be used in the Peiffer process. Thus, the base layer can take the form of a mixture of isotactic polypropylene or propylene ethylene copolymers with resin polymers such as styrene homopolymers having a softening point of about 130-180 ° C. The surface layer or layers can in the same way take the form of a propylene or copolymer homopolymer of the same type used in the base layer. Processes for biaxially oriented polypropylene film preparation employ polymers produced by the use of isospecific metallocenes that involve di or tri-substituted indenyl groups are described ^^^^ ^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^ J ^^^^ ^^^^^ in Canadian Patent Application No. 2,178,104. Four isotactic polymers described therein are based on the polymerization of propylene in the presence of strongly substituted bis (indenyl) ligand structures. In each case, the metallocene used was a bis (indenyl) zirconium di or tri dichloride substituted with silicon bridges. More specifically, the metallocene catalysts used are identified in the aforementioned Canadian patent as racmethylsilanediethyl bis (2-methyl-4,6-diisopropyl-l-10 indenyl) zirconium dichloride, rac-climethylsilandiethyl bis (2-methyl) dichloride. -4, 5-benzo-l-indenyl) zirconium, 3-rac-dimethylsilandiethyl bis (2-methyl-4-phenyl-1-indenyl) zirconium dichloride and 4-rac-dimethylsilandiyl bis (2-ethyl-4-) dichloride phenyl-1-indenyl) zirconium. The various polymers produced by these metallocene catalysts are characterized in terms of molecular weight, molecular weight distribution, melting point, melt flow index, average length of the isotactic block, and isotactic index as defined in terms of triads mm. The polymers produced had isotactic indices, as defined thus, of about 97-98% contrasted with an isotactic index of 93% for a commercial polypropylene compared to a conventional Ziegler-Natta catalyst and molecular weight distributions ranging from about 2.0 to 3.0 in contrast to a molecular weight distribution of 4.5 for polypropylene '^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^ &ij ^^^^^^^^^^^^^^^^^ j ^^^^^^^ j ^^^^^^^^^^ produced by the catalyst Ziegler-Natta conventional. Similarly, as in the case of the aforementioned patent for Peiffer et al, the Canadian application describes multilayer films in which the base layer and one or two upper folds can be formed of the same or different propylene polymers including homopolymers or copolymers or terpolymers of propylene. Where a propylene homopolymer is employed in the upper layer, it is described as having a melting point of at least 140 ° C.

Similarly, as in the case of the aforementioned patent for Peiffer, et al, the Canadian application describes multilayer films in which the base layer and one or two top doubles can be formed of the same or different propylene polymers including homopolymers of propylene or copolymers or terpolymers. Where a propylene homopolymer is employed in the top layer, it is described as having a melting point of at least 140 ° C and a melt flow index of 1 to 20 grams / 10 minutes. In the '104 Canadian application, a typical movie structure, The base layer is characterized as providing at least 40% and typically 50-98% of the total film thickness with the outer layer or layers providing the remainder of the film thickness. The specific total film thickness described in the Canadian application '104 varies from 4 to 100 microns and more specifically from more than 6 to 30 micras with the base layer aam & AU «- * i faith varying specifically from 1.5 to 10 microns and the outer layer from 0.4 to 1.5 micras. In accordance with the present invention, a multilayer polyolefin film of the type suitable for packaging applications in which heat seals are formed is provided. The multilayer film comprises a flexible substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the surface of the substrate layer. The surface layer is formed of a syndiotactic propylene polymer which is effective to produce a heat seal with itself at a sealing temperature of less than 110 ° C. The surface layer has a thickness which is less than the thickness of the substrate layer. Preferably, the substrate layer has an average thickness within the range of 5-150 tiicras, and the surface layer has a thickness that is not more than half the thickness of the substrate layer, preferably not more than 1/3 of the thickness of the substrate layer and has a thickness within the range of 0.3-50 microns. Preferably, the heat seal layer is formed of polymerized syndiotactic polypropylene in the presence of a syndiospecific metallocene catalyst and has a melt flow rate of less than 2 grams / 10 minutes. Preferably, the multilayer film is a biaxially oriented film.

; In a further aspect of the invention, there is provided a process for the production of a multilayer film incorporating a substrate layer and a heat sealable surface layer. In carrying out the invention, a crystalline thermoplastic polymer was extruded and formed into a substrate layer film. A second polymer is employed comprising a syndiotactic propylene polymer which is effective to form a heat sealable surface layer. The propylene polymer is extruded to form a surface layer that binds to the interface of the substrate layer at a temperature within the range of 150-260 ° C. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration in isometric view of a stretched frame system which can be used to form biaxially oriented multi-layer films according to the present invention. • Figure 2 is a schematic illustration of a stretched frame process that incorporates systems for the coextrusion coating or extrusion of surface layers bonded to a substrate layer to produce multilayer films according to the present invention. Figure 3 is a graphic presentation of the maximum resistance of the seal against the temperature of the seal . for heat seal films formed of various polymers. Figure 4 is a graphical illustration of average resistance of the seal against the sealing temperature for the polymers illustrated in Figure 3. Figure 5 is a graphic illustration of heat seal resistance near the term as a function of the temperature of sealing for the various polymers depicted in Figure 3. Figure 6 is a graphic illustration of the strength of the heat seal coitio a function of the sealing temperature for the various polymers depicted in Figure 3. The sealable multilayer polyolefin films by heat, such as those used in the packaging of food items and the like, are generally formed by biaxial orientation procedures, and the invention will be described with respect to biaxially oriented films. However, it must be recognized that the invention will find Application in other multilayer polyolefin films in which enhanced heat seal and hot bonding characteristics are desirable. Biaxially oriented films can be characterized in terms of certain well-defined characteristics that refer to their stereoregular structures and physical properties, which w & amp; amp; ? i, í * * * j. ..--,. ^. *. , ^ i ^ .. (, * «.» _ »« «., .-.« -, s * ». ^. they include melting temperatures and shrinkage characteristics, as well as relatively low coefficients of friction and relatively high tensile moduli and relatively low oxygen and water permeation rates. Biaxially oriented films of the type embodying the present invention are formed with a heat sealable surface layer incorporating a particular syndiotactic propylene polymer as described in greater detail below and using any suitable oriented film production technique, such as the process of stretched frame conventionally used. In general, such biaxially oriented film production can be carried out by any oriented technique, such as described in the aforementioned Canadian Patent Application No. 2,178,104, to Peiffer et al. As described in the Peiffer application, the polymer or polymers used to make the film are melted and then passed through an extruder to a slit mold mechanism after which it is passed over a first roll, characterized as a cold roll, which tends to solidify the film. The film is then oriented by stretching it in a longitudinal direction, characterized as the direction of the machine, and in a transverse direction to arrive at a film which can be characterized in terms of orientation relationships, sometimes also . *** a ^ - * * ¿. * t * .y§ 1 £ ^ á £? ¿¿* *. referred to as dilatation relations, in both longitudinal and transverse directions. The orientation in the direction of the machine is achieved through the use of two sequentially arranged rollers, the second or fast roller 5 operating at a speed in relation to the slowest roller that corresponds to the desired orientation ratio. This can be achieved alternatively through a series of rollers with increasing speeds, sometimes with additional intermediate rollers for temperature control and other functions. After the film has been stretched in the machine direction, it is cooled again and then pre-heated and passed into a lateral expansion section, for example, a stretched frame mechanism, where it is stretched again , this time in the cross direction. The orientation in the transverse direction is often followed by a tempering section. Subsequently, the film cools down later and • may be subjected to additional treatment, such as corona treatment or flame treatment, as described for example, in the aforementioned Canadian Patent Application No. 2,178,104 or in US Patent No. 4,029,876 to Beatty, the descriptions of which they are incorporated herein by reference. The film may also be metallized as described in US Patent No. 25 4,692,380 to Reid, the complete description of which is a ^ j * ^^^^^ g ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ iM ^^^^^^^^^ ^^^^^^^^^^^^ _ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ While the corona and flame treatment typically occurs immediately after orientation and prior to the initial lamination, plating is typically performed at a separate time and location. Orientated multilayer films comprise a substrate layer, sometimes referred to as a "base layer" or "core layer", formed of a stereoregular propylene polymer, typically isotactic polypropylene homopolymer, selected for good stiffness and other physical properties with one or more thinner surface layers used to seal with heat as well as to provide other properties such as improved slip or barrier qualities, etc. There are numerous methods for producing multi-ply films that include co-extrusion, extrusion coating, extrusion laminate, or standard laminate techniques. Heat sealing can be achieved by placing the heat sealing surface layer in contact with a corresponding layer that normally has the same or similar chemical appearance as the heat sealing layer and using a combination of heat and pressure to create a selo that joins the two corresponding layers together. The heat seal layer will be a surface layer to be able to contact and seal with another layer (with a different section thereof).

After sealing, it is possible that the sealed structure may constitute an inner layer in a yet more complex multilayer film or multilayer film composition. The efficiency of a heat seal layer can be characterized in terms of the heat seal resistance of the product, the seal start temperature (SIT) and the so-called "hot bond" characteristic, that is, the resistance of the seal of heat as measured shortly after the formation of the laminated film layer. Typically, the resistance of the heat seal will be measured at 250 milliseconds after the formation of the surface layer substrate bond and in a subsequent time interval of 500 milliseconds. The start temperature of the seal is the temperature at which the junction of the surface layer to the corresponding layer begins to occur. The failure of a heat seal can occur through a number of mechanisms that can be characterized in terms of "peeling", "membrane dilatation" or "tearing failure". The failure of a heat seal due to peeling is characterized by a peeling of the seal apart at the interface. Failure due to membrane dilation occurs due to a differential resistance between the membrane and the heat seal. The failure due to tearing involves a tear in the membrane itself at the edge of the seal.

The heat-sealable surface layer is typically formed by coextrusion, the surface layer polymer with the substrate layer polymer. The co-extrusion can be carried out by simultaneously co-injecting the polymer of the heat seal layer and the polymer of the substrate layer through a slit mold system to form a film formed of an outer layer of the polymer of the heat seal and the substrate layer of the core polymer. The additional layers can also be co-extruded, either as an additional heat seal layer on the other surface of the substrate layer, or layers serving other functions, such as barriers, anti-block layers, etc. Alternatively, a heat seal layer can be coated by extrusion then in the process of manufacturing the movie. Also, other layers can be added to create a more complex film later or contemporaneously with the formation of the basic heat seal layer for the core layer ratio. The advantages of the present invention remain as much as the heat seal layer is contiguous with, and bonded with, the substrate layer. Turning now to Figure 1, there is shown a schematic illustration of a suitable "Laying Frame" illustration process that can be used to produce biaxially oriented polypropylene film in accordance with the present invention. More particularly and with reference to Figure 1, a molten polymer source is supplied from a hopper 10 to an accessor 12 and thence to a grid mold 14 which produces a relatively thick flat film 16 at its outlet. The film 16 is applied on a cooled roller 18, and is cooled to a suitable temperature within the range of about 30 ° -60 ° C. The film is taken out of the roller 13 cooled to a dilation section 20 in which orientation in the machine direction occurs by idle rollers 22 and 23 leading to preheated rollers 25 and 26. As the film is taken out of the cooled roll 18 and passed over the idle rolls, it is cooled to a temperature of about 30 ° -60 ° C. Upon dilation of the film in the machine direction it is heated by pre-heated rolls 25 and 26 to an increased temperature increase of about 60 ° -100 ° C, and then passed to the slow roll 30 of the longitudinal orientation mechanism . The slow roller can be operated at any suitable speed, usually about 20-40 feet per minute. The quick roller 31 is operated at a suitable speed, typically about 150 feet per minute, to provide a surface velocity in the circumference of about 4-7 times that of the slow roller to orient the film in the machine direction. As the oriented film is removed from the roller »^^ fast, it is passed on roller 33 at ambient temperature conditions. From there, the idle rollers 35 and 36 are passed in tandem to a lateral expansion section 40, where the film is oriented when it dilates in the transverse direction. The section 40 includes a preheating section 42, which comprises a plurality of tandem heating rollers (not shown) is reheated again at a temperature within the range of 130 ° -180 ° C. From the preheating section 42 the tenter frame, the film is passed to a dilation or shot section 44 where it is progressively expanded by means of tenter tapes (not shown) which grip the opposite sides of the film and progressively dilate it until it reaches its maximum lateral dimension. The proportions of lateral expansion are typically greater than the dilatation ratios in the machine direction and can often vary from 5-12 times the original width. Normally, lateral dilatation ratios of 8-10 times are preferred. The ending portion of the lateral expansion phase includes a tempering section 46, such as a homemade heater, where the film is heated to a temperature within the range of 130 ° -170 ° C for a suitable period and time, approximately 1-10 seconds Tempering time helps control certain properties, and increased tempering can be used specifically to reduce shrinkage. The biaxially oriented film is taken out after the tenter frame and passed over a cooled roll 48 where it is reduced to a temperature of less than about 50 ° C and then applied to take spools 5 on a picking mechanism 50. From the above description, it will be recognized that the initial orientation in the machine direction is carried out at a somewhat lower temperature than the orientation in the lateral direction. For example, the film exiting the preheating rollers is dilates in the machine direction at a temperature of approximately 120 ° C. The film can be cooled to a temperature of about 50 ° C and subsequently heated to a temperature of about 160 ° C before it is held in the orientation of the dimension progressive side in the tenter section. From the above description it will be recognized that the biaxially oriented film can have a number of properties for its advantage during and after the machine processing steps. A coefficient of friction Relatively low is desirable, during the biaxial orientation process and in the end-use applications of the biaxially oriented film, finally produced. A relatively high stiffness, as indicated by the tension module in the machine direction and in the direction Cross section is usually advantageous. Relatively low ^^^^ g ^^ jjjp ^^^^ gj ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ j ^^^^; Agi ^^.

Permeabilities to gas and water are desirable in many applications. In addition, a high shrink factor of the processed film, while desirable in some cases, may be advantageous in other applications, such as where the film is used in the expanded wrapper of food products, electrical components and the like. Figure 2 is a schematic diagram illustrating a tenter frame process carried out with the coextrusion of one or two surface layers with a substrate layer. The main extruder 100 is flanked by two additional extruders 102 and 104. Through the operation of one of the supplementary extruders 102 or 104, a separate polymer or polymer mixture can be extruded to be in contact with the main polymer or polymer mixture forming the substrate supplied from the main extruder 100. If both supplementary extruders 102 and 104 are used, then a sandwich can be created with the main polymer forming the core or substrate layer, and the polymers extruded by the supplementary extruders 102 and 104 forming the surface layers. After extrusion and condition, the multilayer film continues through the section 106 for orientation in the machine direction, the preheating section 108, the orientation section 110 in the transverse direction, the hardening section 112, the section 114 of cooling, the corona treatment section 116, and finally the intake (118) section 118, in an alternative mode of operation, one or more surface layers may be added in the extrusion coating section 120, after the orientation in the direction of the machine, but before orientation in the transverse direction. In the extrusion coating section 120, additional material is extruded to coat one or both surfaces of the monoaxially oriented film arising from the orientation section 106 in the machine direction. The monoaxially oriented film, supplied to be coated by extrusion, can be a monolayer film generated by the main extruder 100 or it can be a multilayer film created by coextrusion by a combination of the main extruder 100 and the extruder 102 and / or 104 supplementary The syndiotactic propylene polymer used to form the heat seal layer of the present invention can be characterized by a low seal initiation temperature (SIT) of less than 100 ° C and effective seal strength characteristics at relatively low seal temperatures. of approximately 110 ° C or less. The seal initiation temperatures and the low temperature heat seal resistances are substantially lower than the corresponding values observed for the isotactic polypropylene conventionally used to form heat heat layers. In fact, the SIT and the strength characteristics of the heat seal, together with the properties of the hot bonding of the syndiotactic polypropylene, are generally better for the syndiotactic polypropylene film than for the corresponding films produced with ethylene-propylene copolymers. The syndiotactic polypropylenes used in the present invention are produced by the polymerization of propylene in the presence of a syndiospecific metallocene catalyst of the types described in U.S. Patent Nos. 4,892,851 to Ewen et al, 5,225,500 to Eider et al, and 5,243,002 to Razavi. Syndiospecific metallocenes can be used as homogeneous catalyst systems, or can be used as supported catalyst systems as described, for example, in US Patent No. 5,807,800, to Shamshoum et al. for a further description of suitable syndiospecific metallocene catalyst systems that can be employed in the polymerization of propylene to produce syndiotactic polypropylene, reference is made to the patents mentioned for Ewen et al, Razavi, and Shamshoum et al, the full descriptions of which are incorporated in the present for reference.

As described in greater detail below, the syndiotactic polypropylene used to form the heat seal layer of the present invention is characterized by a melt flow index which is smaller, usually substantially lower, than the various other useful crystalline or copolymer polymers. to form heat seal layers. As a practical matter, syndiotactic polypropylene is characterized by a melt flow rate of less than 3 grams / 10 minutes and preferably less than 2 grams / 10 minutes. The melt flow rate is characterized as the proportion of molten flow as determined in accordance with ASTM Standard D-1238 at 230 ° C using 2.16 kilograms of force. The heat seal layer (or layers) and the substrate layer are normally provided in configurations in which the surface layer has a thickness substantially less than the thickness of the substrate layer. For typical packaging applications, the substrate layer will have an average thickness within the range of 5-150 microns. The heat seal layer will have a thickness of not more than half the thickness of the substrate layer and normally less than 1/3 of the substrate layer. The surface layer will typically have a thickness in the range of 0.3-50 microns. _, * _ The substrate layer can be formed from various polymer or polymer blends as previously described. Isotactic polypropylene homopolymers or propylene / ethylene copolymers typically contain no more than 10% by weight of ethylene, can be used to form the substrate layer. A preferred substrate layer incorporating polypropylene has a very high isotacticity is defined in terms of meso pentadas and meso diadas but which also has irregularities in the polymer structure characterized in terms of inserts 2, 1 in contrast to the 1.2 prevalent insertions characteristic of isotactic polypropylene. Thus, the isotactic polypropylene polymer chain is characterized by intermittent head-to-head inserts to result in a structure of polymer as exemplified later. CH3 CH CH CH3 CH l l l l I - CH2 - CH - CH2 - CH - CH2 - CH - CH - CH2 - CH2 - CH2 ~ (5) As shown by the polymer structure represented by Formula (5), the occasional head-to-head insert resulting from the use of the substituted indenyl-2-alkyl group results in adjoining adjacent methyl groups separated by ethylene groups, resulting in a structure of the polymer which behaves as a bit in the form of a random ethylene-propylene copolymer and results at a variable melting point. This results in a polymer that can be advantageously used to produce a biaxially oriented film having good characteristics in terms of resistance in the directions of the machine and transverse directions, low coefficients of friction, and relatively low permeabilities to water and oxygen. At the same time, the biaxially oriented films thus produced have fog properties, usually less than 1%, and good gloss characteristics, greater than 90%. This polymer 10 can be prepared by the polymerization of propylene in the presence of a metallocene catalyst characterized by the formula rac-R'R "Si (2-RiInd) MeQ2 (6) In Formula (6), R ', R" are each independently a C1-C4 alkyl group or a group Phenyl; Ind is a substituted indenyl group in the proximal position by the substituent Rs and otherwise unsubstituted; Ri is an ethyl, methyl, isopropyl or tertiary butyl group; Me is a transition metal selected from the group consisting of titanium, zirconium, hafnium and vanadium; Y Each Q is independently a hydrocarbyl group or containing 1 to 4 carbon atoms or a halogen. As indicated by Formula (6) above, the silyl bridge can be substituted with various substituents in which R 'and R "are each independently a group ^^^^^^ Z ^ m ^ & amp; ^ methyl, an ethyl group, a propyl group (which includes an isopropyl group), and a butyl group (which includes a tertiary butyl group or an isobutyl group). Alternatively, one or both of R ', R "may take the place of a phenyl group The suitable bridge structures are dimethylsilyl, diethylsilyl and diphenylsilyl structures The substituent Ri at the 2-position (the position proximal to the carbon atom) bridgehead) may be a methyl, ethyl, isopropyl, or tertiary butyl, preferably, the substituent at the 2-position is a methyl group, as previously noted, the indenyl group is otherwise unsubstituted except that it may be a group Hydrogenated indenyl Specifically, the indenyl ligand may take the form of a 2-methyl indenyl ligand or a 2-methyl tetrahydroindenyl As will be recognized by those skilled in the art, the ligand structure must be a racemic structure to provide the control mechanism of the desired enantiomorphic site to produce the isotactic polymer configuration As previously described, the 2, 1 inserts produce "errors" "in the polymer structure which imparts the desired non-uniform melting point characteristics of the present invention. The corresponding film is characterized in terms of low water and oxygen permeabilities and low coefficients of friction as described below. The "errors" due to the inserts 2, 1 should however not be confused with the errors resulting in racemic insertions as indicated, for example, by the following polymer structure: CH3 CH3 CH3 CH3 - CH - CH2 - CH - CH - CH - CH2 - CH ~ (7) I CH2 As will be recognized, the structure (7) can be indicated by the mrrm pentad. The "errors" corresponding to the mechanism of head-to-head insertion involved in the present invention are not characterized or not necessarily characterized by racemic dyads. In experimental work carried out with respect to the present invention, the characteristics of seal strength and hot bonding of the films formed of syndiotactic polypropylene were evaluated against films formed of isotactic propylene homopolymers and propylene / ethylene copolymers. The syndiotactic polypropylene was prepared by the polymerization of propylene in the presence of a bridged syndiospecific metallocene of the type described in U.S. Patent No. 4,892,851 to Ewen. Examples of such metallocene catalyst systems are metallocenes based on cyclopentadienyl fluorenyl ligand structures such as isopropylidene (cyclopentadienyl fluorenyl) c irconium dichloride employed with a co-catalyst such as alumoxane. Such syndiotactic polypropylenes can also be prepared through the use of so-called "cationic" metallocenes that incorporate a stable non-coordinating anion and do not normally use the present of an alumoxane. The syndiospecific cationic metallocenes are described, for example, in the aforementioned US Patent No. 5,243,002. The syndiotactic polypropylene used in the experimental work discussed below has a melt flow rate of 1.5 g / 10 minutes, and has a microstructure characterized by approximately 80% syndiotactic pentada (rrrr). The isotactic polymers used in the experimental work include propylene homopolymers and propylene ethylene copolymers prepared by catalysis with isospecific metallocenes as described in the patents mentioned for Ewen and Ziegier-Natta catalysts as described in the patents mentioned for Mayr et al. The characteristics of metallocene-based propylene homopolymers (identified here as MP-1 to MP-4), the metallocene-based ethylene propylene copolymer (identified here as MEP-5), the Ziegler-Natta homopolymer (identified here as ZP-7), and the í. > Sk i s.

Ziegler-Natta propylene / ethylene copolymers (identified herein as ZEP-8 and ZEP-6) are indicated in Table I. In Table I, the polymers are characterized in terms of melt flow index, MFl (the proportion of molten flow in grams / 10 minutes), the ethylene content (EC) in% by weight, where applicable, the content of soluble in xylene (XS) of the polymers, and where available, the isotactic index I as it is indicated by the percent of meso pentadas. TABLE I In the experimental work, the characteristics of resistance of the seal and hot bonding of the syndiotactic polypropylene and in the previously identified polymers were evaluated on melting films of 50 microns. In carrying out the experimental work, the seal resistances were evaluated on films formed at temperatures that start below the initiation temperature and seal until temperatures indicate a plateau in the resistance characteristics of the seal. In Figure 3, the maximum seal resistance (SM) in newtons per centimeter a plot is plotted on the ordinate against the temperature of the seal T in degrees Celsius on the abscissa for some of the polymer systems described above. In Figure 3, the curves are designated by the reference numbers SP for the syndiotactic polypropylene and by the designations shown in Table 1 for the various isotactic propylene homopolymers or copolymers. Figure 4 shows the SA average seal strength graphs plotted on the ordinate versus the seal temperature in ° C plotted on the abscissa. From an examination of Figures 3 and 4, it can be seen that the syndiotactic polypropylene film and the films formed with the Ziegler-Natta, ZEP-6 and ZEP-8 based copolymers showed much lower seal initiation temperatures (SIT). than the Ziegler-Natta homopolymers. The syndiotactic polypropylene had an SIT of about 94 ° C, 6-7 ° below the ZEP-6 copolymer, the highest ethylene content copolymer tested. The Ziegler-Natta copolymer had an initiation temperature ^^ ^ * of somewhat higher seal of approximately 108 ° C, and as indicated, the seal temperatures of the remaining polymers were much higher, indicating SIT values of approximately 120-130 ° C or even higher. As further indicated by the resistance of the heat seal, the syndiotactic polypropylene develops a maximum seal strength on a relatively large plateau in the low temperature range. While temperatures within the range of 95-115 ° C can be used to achieve seals effective heat, maximum results are achieved within the range of 100-110 ° C. The hot bonding operation of the syndiotactic polypropylene and the other tested polymers is shown in Figures 5 and 6, which are graphs of the heat seal resistance Sh in newtons per centimeter plotted on the ordinate against the temperature of seal T in ° C plotted on the abscissa. In each of Figures 5 and 6, the curves showing the characteristics of hot sticking of the polymers are designated by the same reference characters as found in Table 1 and used in Figures 3 and 4. Figure 5 indicates the strength of the hot seal, as measured at 250 milliseconds after joining and Figure 6, the resistance of the seal hot to 500 milliseconds after the union. As can be seen from an examination of Figures 5 and 6, k ~ x * »* j & ** tíá sam¿ & j ^. . & . '.. ** ** -. *. *. . *. & * £. "» .... ^ * _, - •. * * ^ .. * "and * ^. < . -i .í. the syndiotactic polypropylene film demonstrated hot bonding performance that is substantially superior to Ziegler-Natta-based (and metallocene-based) propylene-ethylene copolymers. In the operation of hot gluing of the remaining polymers the MiPP homopolymers, as well as the Ziegler-Natta homopolymers, are indicated substantially lower in hot bonding operation. In fact, at a seal temperature of 105 ° C, where the propylene / ethylene copolymer based on Ziegler-Natta starts to show a greater seal strength than the syndiotactic propylene, the syndiotactic polypropylene film still shows an operation of Hot gluing very superior as measured in the short term and also long term. Having described the specific embodiments of the present invention, it will be understood that modifications thereof may be suggested by those skilled in the art, and attempts are made to cover all such modifications as falling within the scope of the appended claims.

Claims (20)

1. CLAIMS 1. In a multilayer polyolefin film of a type suitable for packaging applications, the combination comprises: (a) a flexible substrate layer formed of a crystalline thermoplastic polymer having an interface surface; and (b) a heat sealable surface layer bonded to the interfacial surface of the substrate layer formed of a syndiotactic propylene polymer and having a thickness which is less than the thickness of the substrate layer, the surface layer being effective to produce a heat seal with itself at a seal temperature less than 110 ° C.
2. 2. The combination according to claim 1, characterized in that the substrate layer is formed of a stereoregular propylene polymer.
3. 3. The combination according to claim 1, characterized in that the substrate layer has an average thickness within the range of 5-150 microns and the surface layer has a thickness that is not more than half the thickness of the substrate layer. .
4. 4. The combination according to claim 1, characterized in that the surface layer is formed of syndiotactic polypropylene produced by the * £ * - U k, M jj «* ^ ^ AU ^^ * - polymerization of propylene in the presence of a syndiospecific metallocene catalyst.
5. 5. The combination according to claim 4, wherein the syndiotactic polypropylene is characterized by a melt flow index of less than 3 grams / 10 minutes.
6. 6. The combination according to claim 4, wherein the syndiotactic polypropylene is characterized by a molten flow rate of less than 2. 10 grams / 10 minutes.
7. The combination according to claim 4, characterized in that the substrate layer is formed of an isotactic propylene polymer.
8. 8. The combination according to claim 7, characterized in that the substrate layer is formed of an isotactic propylene polymer produced by the polymerization of propylene in the presence of an isospecific metallocene catalyst.
9. 9. The combination according to claim 1, characterized in that the substrate layer is formed of an ethylene propylene copolymer having an ethylene content of not more than 10% by weight.
10. The combination according to claim 1, characterized in that the multilayer film 25 is oriented in at least one direction. ¿¿¿¿G388 ».. * aeaiS ^ * * *? . «- ****. - - i * - *. *. . * ** .- *. * -M *. ^ -. ', - *. * Á? ^ * - * £ * &6te &g *
11. 11. The combination according to claim 1, characterized in that the multilayer film is biaxially oriented. The combination according to claim 1, characterized in that the multilayer film produces a maximum seal strength in the heat sealing of the surface layer with itself at a temperature within the range of 95-110 ° C of at least 4 newtons / centimeter The combination according to claim 12, characterized in that the multilayer film produces a maximum seal strength of at least 4 newtons / centimeter through a predominant portion of the 100-110 ° C range. 14. In a process for the production of a multilayer film having a substrate layer and a surface layer, the process comprises: (a) providing a first crystalline thermoplastic polymer; (b) extruding the propylene polymer and forming the polymer within a substrate layer; (c) providing a second polymer comprising a syndiotactic propylene polymer effective to form a surface layer of the multilayer film; (d) extruding the syndiotactic propylene polymer to form a surface layer; and (e) attaching the surface layer to the inferred surface of the substrate layer at a temperature within the range of 150 ° -260 ° C to form a multilayer film having a surface layer of the syndiotactic propylene polymer having a thickness which is less than the thickness of the substrate layer. 15. The process according to claim 10 14, characterized in that the first polymer is an isotactic propylene polymer. 16. The method according to claim 14, characterized in that the second polymer is syndiotactic ß polypropylene, produced by the polymerization of propylene 15 in the presence of a syndiospecific metallocene catalyst. 17. The method according to claim 16, wherein the syndiotactic polypropylene is characterized by a melt flow index of less than 3 grams / 10 minutes. 18. The method according to claim 16, characterized in that the cap >filmThe substrate is formed by orienting the shape of the substrate layer in at least one direction and thereafter forming the surface layer at ^^^^^^^^ fflife ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ syndiotactic on the oriented substrate layer film. The process according to claim 14, characterized in that the multilayer film is formed by coextruding the first and second polymers through a slotted mold system to form a multilayer film comprising a substrate layer of the first polymer and a layer surface of the second polymer and thereafter orienting the film in the machine direction followed by orienting the film in the transverse direction to form a biaxially oriented multilayer film. 20. In a process for the production of a multilayer film having a substrate layer and a surface layer, the process comprises: (a) providing a first polymer to form the substrate layer of a multilayer film; (b) providing a second polymer comprising a syndiotactic propylene polymer effective to form a heat-sealable surface layer of the multilayer film; and (c) coextruding the first and second polymers through a slotted mold system at a temperature within the range of 150 ° -260 ° C to form a film comprising a substrate layer of the first polymer and a surface layer of the second polymer of a thickness that is less than the thickness of the substrate layer. M-fe * A - ** - *. * SUMMARY A polyethylene polyolefin film of the type suitable for packaging applications in which heat seals are formed, and in its preparation the multilayer film comprises a flexible substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interfacial surface of the substrate layer and formed of an effective syndiotactic propylene polymer to produce a heat seal with itself at a seal temperature of less than 110 ° C. The surface layer has a thickness that is less than the thickness of the substrate layer. The heat seal layer may be formed of a syndiotactic polypropylene polymerized in the presence of a syndiospecific metallocene catalyst and having a melt flow index of less than 2 g / 10 minutes. The multilayer film can take the form of a biaxially oriented film. In the production of the multilayer film incorporating a substrate layer and a heat sealable surface layer, a crystalline thermoplastic polymer is extruded and formed into a substrate layer film. A second polymer comprising a syndiotactic propylene polymer which is effective to form a heat sealable surface layer is extruded to form a surface layer which is bonded to the interface of the substrate layer at a temperature within the range of 150-260. ° C.