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United States Patent [w]
Thakker et al.
US006017615A [ii] Patent Number:  Date of Patent:
 FILM PRODUCT COMPRISING NOVEL POLYOLEFINS
 Inventors: Mahendra T. Thakker; Dharmendra Jani, both of Odessa, Tex.
 Assignee: Huntsman Polymers Corporation,
 Appl. No.: 08/918,064  Filed: Aug. 25, 1997
 Int. CI.7 B32B 27/32
 U.S. CI 428/213; 428/215; 428/220;
428/515; 428/516; 428/520
 Field of Search 428/220, 213,
428/215, 515, 516, 520; 526/348, 348.2, 348.5, 351; 525/240
 References Cited
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FOREIGN PATENT DOCUMENTS
0 611 801 8/1995 European Pat. Off. .
Primary Examiner—Paul Thibodeau
Assistant Examiner—D. Lawrence Tarazano
Attorney, Agent, or Firm—Russell R. Stolle; Ron D. Brown;
Christopher J. Whewell
Multi-layer film products comprising an inner or core layer made from polyolefin polymers produced by polymerizing an a-olefin in the presence of a catalyst including: a procatalyst having a magnesium halide; an aluminum halide; a tetravalent titanium halide, ;an electron donor; and a silane having the formula R1R2Si(0R3)(0R4), wherein Rt and R2 are each an H, C-^galkyl, aryl, C5_12cycloalkyl, each of which may be unsubstituted, mono- or di-substituted, and R3 and R4 are H, Cj.galkyl, or a mono- or di-substituted C-^galkyl; and a co-catalyst comprising an organometallic compound, or reaction products of the pro-catalyst and the co-catalyst. The film products have excellent clarity, superior dart impact, resistance to tear and favorable sealibility characteristics.
25 Claims, 1 Drawing Sheet
FILM PRODUCT COMPRISING NOVEL
Inventors: Mahendra T. Thakker and Dharmendra Jani
This invention relates generally to multi-layer polyolefin film products which are adapted for use in the packaging industry. The films have improved physical properties over prior art films and preferably comprise at least three layers of two outer layers and an intermediate or core layer sandwiched therebetween wherein all of the layers are chlorine-free, exhibit improved clarity, autoclavability, gloss, and dart impact properties relative to all other chlorine-free polymeric multi-structure film composites in prior art. The films of this invention are especially wellsuited for the packaging of medical products and foodstuffs, and are readily recycled.
BACKGROUND OF THE INVENTION 20
Multi-layer polymeric films must in general be possessive of several qualities in order to be useful in the various packaging industries, particularly where it is desired to employ the films to food product usage. In general, the films 25 must have a high degree of clarity and gloss in order to impart a positive cosmetic appearance to the food product packaged, must have a high degree of resistance to tearing and/or puncture as measured by the dart impact test, should have a high resistance to deformation at elevated tempera- 30 tures in order to assist in processibility and storage in various environments, should have sufficient tensile strength, tensile yield, hot tack strength, moisture impermeability, a wide heat seal range, high seal strength, good low temperature sealability, relatively low density, and should preferably be 35 completely recyclable. From the recyclibility standpoint, it is highly desirable that the films be free from chlorine, and are comprised of materials compatible with a range of polymers.
The most popular materials conventionally employed as 40 films in the packaging industry have included linear polyethylenes, high density polyethylenes, high molecular weight high density polyethylenes, ethylene vinylacetate copolymers, polyvinylchloride (PVC), isotactic and syndiotactic polypropylenes and interpolymers of the aforesaid 45 either alone or in combination with one another as in, for example a polymer blend in either single-layer or multiplelayer structural arrangements.
The use of polypropylene as a multi-layer film material has been known for quite some time. The films are favorable 50 from several standpoints, particularly of strength, scratchresistance, and optical properties. However, the films exhibit very poor heat-sealing properties. To obviate the disadvantages inherent in polypropylene, outer skin or sealing layers comprising typically, for example, random copolymers of 55 propylene with other alpha olefin monomers, are coextruded onto one or both sides of the base layer. However, these sealing layers generally have the disadvantage that often impair the optical properties of the film composite as a whole. Additionally, the higher melting points of the outer 60 sealing layers can create difficulty when the temperature of the heat sealing device is set to the melting point of the outer layer insomuch as the excessive heat causes the inner layer(s) to pit or severely deform thus causing weakness along the seal. Alternatively, difficulty is seen when the heat 65 sealing temperature is set for the melting point of the inner layers in that the outer layers do not become sufficiently
fused to form a homogeneous seal. Machines for forming seals on polyolefin films typically have forming collars or bars which shape and hold a flat piece of film in a desired position and actuate hot metal sealing bars or jaws to contact the film, thus causing a seal to be formed. It is important that the outside of the film has a higher melting temperature than the inside of the film so that when the j aw re-opens or the bar is removed, the outside of the film which has not melted is not stuck to the sealing jaw or bar. When using multi-layer films the temperatures which must be applied for sealing are so high that it is difficult to achieve the proper balance of heat necessary between the layers, and severe shrinkage in the sealing zone is often a resultant problem. The finished packages have a less than optimal appearance and an inferior sealed-seam strength.
Another difficulty often encountered by pioneering film products intended for use in packaging foodstuffs are the regulations set forth by the US Food and Drug Administration (FDA) at 21 C.F.R. 177.1520 (d)(iii) and (d)(iv) related to the content of the films which are soluble in xylene and n-hexane.
Accordingly, it is an object of this invention to provide a flexible film product comprising only polymers of propylene wherein the sealing temperatures of the outer layers are close enough to that of the core layer to avoid processing difficulties affecting seal integrity.
It is a further object of this invention to provide a multi-layer film product having superior optical properties over similar films of prior art.
It is a further object to provide film products having the aforesaid qualities which is also suitable for use in the foodstuff packaging industries.
It is a further object of this invention to provide a multi-layered film structure which is free from chlorine.
It is a further object still of this invention to provide a multi-layered film product which can meet high heat resistance specifications required for high-speed processing of film products.
It is a further object of this invention to provide a multi-layered film product which is sufficiently tear and abrasion resistant to withstand conditions normally experienced by film products in the packing industry and end uses, and which meets FDA regulations for use with foodstuffs, by film products in the packing industry and end uses, and which meets FDA regulations for use with foodstuffs.
SUMMARY OF THE INVENTION
The film products of this invention are comprised of a novel polymeric material as a core layer and conventionaltype polypropylene as outer or "skin" layers. It has been unexpectedly found that the film structure combination herein disclosed inherently satisfies the critical requirements of film products used in the packaging industry in general and the food packaging industry in particular.
The novel polymeric materials suitable as an inner layer in the multi-layer film products of this invention are composed predominantly of polypropylene. It is well known that crystalline polypropylene generally has an isotactic or syndiotactic structure and that amorphous polypropylene generally has considerable atactic structure. U.S. Pat. Nos. 3,112,300 and 3,112,301, for example, describe isotactic polypropylene and provide structural formulae for isotactic and syndiotactic polypropylene. Conventional polymers of this type typically have a crystallinity, or heat of fusion, of 75 Joules per gram (J/g) or higher, and more typically 90 J/g or higher.
In order to produce polyolefins, a monomeric alpha olefin raw material is generally contacted with a suitable catalyst under conditions of pressure and temperature sufficient for causing a polymerization of the monomer. Great volumes of investigation in the field of polymerization catalysis have 5 yielded a multitude of polymeric products having a wide range of physical and chemical properties. By modifications to the catalyst and reaction conditions it is possible in some instances to produce materials especially suitable for a particular application. The inner layer polymeric material 10 embodied in the instant disclosure is but one example of such.
Of the various known catalysts used for polymerizing olefins, the prior art patents forthwith presented disclose one type of catalyst used in the formation of such polymers. :5 They include generally a pro-catalyst that is typically formed from the reaction product of a magnesium alkoxide compound of the formula MgR1R2 where Rt is an alkoxy or aryl oxide group and R2 is an alkoxide or an aryl oxide group or halogen, and a tetravalent titanium halide wherein the 20 reaction takes place in the presence of an electron donor and, preferably, a halogenated hydrocarbon. These include: U.S. Pat. Nos. 5,118,768; 5,164,352; 5,089,573; 5,118,649; 5,118,767; 5,294,581; 5,118,768; 5,164,352; 5,438,110; 4,990,479; 5,218,052; 5,182,245; 5,153,158; 4,990,477; and 25 in European Patent 475,307. The present invention employs a core layer having unique physical properties which is produced from a catalyst material not found in any prior art.
BRIEF DESCRIPTION OF THE DRAWINGS 30
Two embodiments of the present invention are more particularly described with reference to the accompanying drawing of FIG. 1 and FIG. 2 wherein
FIG. 1 is a schematic cross-sectional view of a preferred 3J embodiment of a multi-layer film composite of this invention and
FIG. 2 is a schematic cross-sectional view of-an alternative embodiment of a multi-layer film product according to this invention. 40
DETAILED DESCRIPTION OF THE
INVENTION INCLUDING A PREFERRED
The present invention is a multi-layer film product which 45 comprises an inner core layer and one or more outer skin layers disposed on at least one side of the core layer. FIGS. 1 and 2 depict cross sectional views of the multi-layer film products according to this invention. In FIG. 1,24 represents the core layer; 17 and 86 each represent an outer skin layer. 50 In FIG. 2, the core layer is depicted by 24; 20 and 64 are representative of the outer skin layers; and 43 and 37 depict intermediate layers sandwiched between the skin layers and core layer. The compositions of materials suitable for the various layers described are set forth below. 55
The inner core layer is comprised of a polyalphaolefin prepared by polymerizing an a-olefin monomeric raw material in the presence of a catalyst comprising the reaction product produced from the combination of a first catalyst component and a second catalyst component. The first 60 catalyst component comprises an intimate anhydrous mixture of a magnesium halide, an aluminum halide, and a silane made preferably by ball milling the magnesium halide with the aluminum halide and then adding the silane component, and subsequently ball milling the resultant three 65 component mixture further. Subsequently, a titanium halide which is preferably tetravalent, is added along with a
nitrogen-containing organic compound and further ball milling is commenced which finally yields what is referred to by some in the art as a "pro-catalyst". The pro-catalyst is subsequently mixed with a second catalyst component which includes an organometallic compound, preferably tri-ethyl aluminum, to produce the final active catalyst material by which the polymeric core layer polymer of this invention is derived from polymerization of an alpha-olefin. Optionally, a second silane material may be added to the finished catalyst to assist in controlling the crystallinity of the polymeric core material products manufactured to a desired level.
The core layer of the film product according to this invention may be comprised of a polymer prepared from a wide variety of monomeric raw materials. For example, one or more monomers selected from the group of alpha olefins including propylene, ethylene, butene, pentene, octene, or mixtures thereof may be employed as feedstocks. When ethylene is selected, it is provided in the raw material feed in an amount of about 1 to 20 weight percent. It is also desirable to include hydrogen in the raw material feed in an amount not to exceed about 10 weight percent.
The multi-layer film product of this invention is produced by co-extrusion.
The Core Layer Composition
A new catalyst capable of producing a new class of polymers, (flexible polyolefins, or "FPO's"), has, by virtue of recent investigative efforts been created and discovered to yield a controllable degree of crystallinity in polyolefin polyjmers lower than that found in commercial isotactic polypropylene. This class of polymers has been unexpectedly found to yield a core layer for film products having enhanced physical properties over film products produced from prior art catalysts and methods.
The catalyst employed for the production of the FPO polymers used as the core layer in the instant invention includes a pro-catalyst component which is combined with a co-catalyst component in order to form a finished catalyst material to which an external modifier compound may preferably be added. The pro-catalyst includes a magnesium halide, an aluminum halide, a tetravalent titanium halide, a particular nitrogen-containing organic compound which functions as an electron donor, and an internal modifier that is typically a silane component. The co-catalyst is an organometallic compound. The external modifier is a second silane compound.
Catalyst preparation begins with the magnesium halide and aluminum halide being combined, preferably with some degree of mixing, for example, by ball milling. The mixing is carried out at about room temperature, although the exact temperature is not a crucial aspect of catalyst preparation. The silane component, or internal modifier, is typically a liquid and is added to these halides either before or after they are mixed.
The silane and halides are preferably mixed by pulverization in a ball mill to form a first mixture before additional components are added. Suitable milling of these pro-catalyst components is readily accomplished in a 1L stainless steel rotary mill pot filled to about 50 volume percent with stainless steel balls.
After the initial ball milling, the electron donor and a titanium halide are combined with the halide and silane mixture. The specific amounts added are determined relative to the other catalyst ingredients, and fall within the atomic ratio range described elsewhere herein. The donor and