The new catalysts are known as "single site", "constrained geometry", or just "metallocenes". Regardless of the nomenclature, the catalysts behave similarly and they differ significantly in structure and reactivity from the conventional Ziegler-Natta catalysts.
One unique feature of metallocene catalysts is that since they are "single site catalysts", the orientation of each monomeric unit is precisely controlled as it is added to the polymeric chains, i. These new catalysts can be rationally tailored to produce different structures of thermoplastics such as polyethylene, polypropylene and others. Another advantage is the extremely high productivity of metallocene systems.
The super-high activity of the metallocene systems will result in product costs lower than Ziegler-Natta catalysts. In addition, the very high catalyst productivity leads to a very low contamination level of the resin with catalyst residue. The FDA has approved the use of some of the resins and film made from these resins for food contact and medical applications. With the old Ziegler-Natta catalysts, ethylene and propylene would totally predominate over heavier olefins.
The content of these comonomers vary from 1 to 10 mol. This leads to a large number of grades with different density, crystallinity, and accordingly, to a very wide range of physical and mechanical properties. The new type of LLDPE resins, produced with metallocene catalysts, introduced by Exxon Chemical Company in , have a predominantly uniform compositional distribution uniform branching distribution.
All copolymer molecules in such resins have approximately the same composition. In contrast, conventionally manufactured LLDPE resins have pronounced nonuniform branching distribution and they differ significantly in physical and mechanical properties.
In addition, it is important to mention that as a rule, LLDPE resins do not contain long-chain branches. However, some copolymers produced with metallocene catalysts in solution processes can contain about 0.
Copolymers with nonuniform compositional distribution are in effect a mixture containing copolymer molecules with a broad range of compositions, from practically linear macromolecules which usually have higher molecular weights to short macromolecules with quite high olefin contents. Melting of such mixtures is dominated by their low-branched fractions which are highly crystalline.
If an LLDPE resin is compositionally uniform, all its macromolecules crystallize equally poorly due to the branching. They form very thin lamellae. Such materials have very low rigidity low modulus and are very flexible.
A number of polymer materials with a very attractive combination of properties not available before high tacticity, very uniform comonomer distribution along the molecular chains, precise MW, narrow MWD, relatively low melting point, low temperature impact properties, extremely high clarity, etc. For example, a new generation of polyethylene with substantially different properties, made by the use of metallocene catalysts has been commercialized.
These newly developed plastomers and elastomers, polymerized using metallocene catalyst systems, offer the possibility of obtaining unique combinations of properties required for sealing and wrapping applications. There is a need for a film which is crystal clear and can be stretched without tearing to create a leak proof seal and which has excellent resistance to a wide range of chemicals.
This is especially true in clinical and industrial research and in process applications requiring a custom seal or closure. It is therefore an objective of this invention to provide a sealing and wrapping film product having improved properties, such as, higher clarity, excellent tackiness at elevated, ambient and very low cryogenic temperatures, high chemical resistance, and whose cost to the user will be comparable to currently known seal and wrapping films.
The present invention is directed to blends of metallocene catalyzed olefin polymers which are processed into films having these desired properties for sealing and wrapping applications. The polymer compositions of the present invention are blends of metallocene catalyzed polyethylene plastomers component A having a density of at least 0. These optimal ratio ranges between the two resins in the blend assure an excellent quality product having the desired characteristics of stretchability, high elongation at break, relatively low modulus, high tear resistance, high clarity, very low glass transition point; tackiness to various surfaces at elevated, ambient and cryogenic temperatures, and high chemical resistance which are required for sealing and wrapping applications in food, medical and other industries.
The method of producing these films comprises selecting a metallocene catalyzed thermoplastic polyethylene plastomer having a density of at least 0. The compositions and resultant films may further comprise other metallocene catalyzed olefin polymers and copolymers either, in place of, or, in addition to, the polyethylene plastomers and elastomers as well as pigments, dies, colorants, nucleants, U.
It is odor free and waterproof since the water absorption after 24 hours is less than 0. Metallocene polymers are understood to be manufactured from the same monomers used to obtain conventional olefin polymers and copolymers, but using a metallocene catalyst.
This produces very low density polymers that have attractive properties. It is understood that olefin homopolymers are referred to as "plastomers" and those with a copolymerized monomer of the type that would usually yield an elastomer such as a diene are referred to as "elastomers". PE elastomers are defined as completely amorphous thermoplastics with densities as low as 0. The plastomer component A, according to the present invention, provides high clarity, low modulus and excellent stretchability of film.
Other metallocene catalyzed olefin polymers and copolymers may be substituted for or added to the composition of the metallocene catalyzed polyethylene plastomers and elastomers of the present composition. Additives such as pigments and dies to produce a colored film, nucleation agents to stabilize the submolecular structure of the film, UV blocking agents, stabilizers, etc. The film product, according to the present invention, can be manufactured using conventional film manufacturing technologies, both casting extrusion and blown extrusion can be used.
The resin can be produced by dry blending of pellets of the two components in conventual mechanical mixers. The well known state-of-the-art melt compounding technique using a twin screw extruder can be implemented if an additional improvement of the mixing uniformity is desired, or to improve the additives distribution in the polymer matrix.
However, this additional technological stage is not required to produce a high quality product of blends of plastomers and elastomers for general applications. Generally the blends of metallocene plastomers and elastomers of different grades are made by combining them in a dry mechanical mixer of any type for about min. Even a simple tumbling in a plastic bag is acceptable for mixing of two polymers if a mechanical mixer is not available.
A melt mixing process can also be used. In this case, the polymer components can be premixed in a dry mechanical mixer and loaded into the twin-screw extruder, molten in there, extruded into strands and cut pelletized into uniform pellets. However, this is not required to obtain a good quality film product due to the acceptable mixability of the two metallocene polymers in one stage, i.
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