Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1999-12-20
2002-08-06
Hess, Bruce H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S500000, C428S525000
Reexamination Certificate
active
06428901
ABSTRACT:
SUMMARY
This invention relates to blends of polyethylene with polypropylene and to films made therefrom having improved tear strength.
BACKGROUND
Plastic films have found utility in a wide variety of packaging applications such as bags, pouches, tubes, trays, and the like. In many film packaging applications it is desirable for the film to possess good physical and mechanical properties such as resistance to tearing, tensile strength, and processabilty in high speed equipment.
Linear low and medium density polyethylene copolymers (LLDPE) produced with conventional Zeigler-Natta catalysts are widely used commercially in films which are conventionally prepared by blown film extrusion. Such films have generally good properties, but often exhibit undesirably low stiffness for many uses. Some such films are also difficult to process and consume a great deal of power to extrude and draw down the film. To address such shortcomings, many have developed blends with other polymers to tailor the balance of properties.
For example, U.S. Pat. No. 4,565,847 describes films formed from blends of LLDPE, polypropylene and ethylene-propylene rubbers having improved stiffness and good tear strength. U.S. Pat. No. 4,929,681 discloses films containing LLDPE having high modulus and good tear strength. The high modulus and improved processing conditions are achieved by blending minor amounts of polystyrene (or other aromatic polymer) and polypropylene with the LLDPE. In both of these inventions, the addition of a minor amount of polypropylene to LLDPE produced with conventional Zeigler-Natta catalysts showed an improvement in the modulus of the film but a decrease in the MD tear strength. The further addition of EPDM or polystyrene was required to achieve a desired balance of properties.
Some of the deficiencies of polymers produced with Zeigler-Natta catalysts are improved in polymers produced with metallocene catalysts. Some of these improvement result from the ability of metallocene catalyst to control certain structural variables in polyolefins which affect the ultimate properties of the polymer. Some of the most important structural variables are composition distribution (CD), molecular weight, and MWD. CD refers to the distribution of comonomer between copolymer molecules. This feature relates directly to polymer crystallizability, optical properties, toughness and many other important use characteristics. MWD plays a significant role in melt processability as well as in physical properties. Broadly speaking, weight average molecular weight (M
w
) is strongly related to melt viscosity and the ultimately desired physical properties of the polymer. All of these structural features are readily controllable through the use of metallocene catalysts as exemplified in U.S. Pat. No. 4,937,299, which is fully incorporated by reference herein for purposes of U.S. patent practice.
Three distinctive features of films produced from copolymers produced with metallocene catalysts relative to conventional Ziegler-Natta copolymers are their low extractables (and associated low reblock), their good clarity, and their unusual tensile properties at low elongation. It would be desirable to produce a film having improved balance of stiffness, tear strength, clarity, and low extractables. Although polymers produced with metallocene catalysts provide many property enhancements over those produced with Zeigler-Natta catalysts, a film property where metallocene-produced copolymers do not compare as favorably to those made with conventional Ziegler-Natta catalysts is tear strength in the machine direction.
In view of the ongoing need for films of optimum quality for growing market needs, it would be desirable to provide compositions that utilize the ability of metallocene catalysts to tailor polymer structure for an improved balance of properties while using as few blend components and processing steps as possible.
Accordingly, the present invention relates to blends of polyethylene having those desirable qualities resulting from production with a metallocene catalyst system and a polypropylene component having a high melt flow rate (MFR). The blends of the present invention are particularly useful in film applications requiring tear strengths and stiffness as high or higher than those produced with Ziegler-Natta catalysts without compromising clarity, low extractables content, and other attractive attributes of films formed from polyethylene polymers produced using metallocenes or other single-site catalyst systems.
SUMMARY OF THE INVENTION
According to one embodiment of this invention there are provided polymer blend compositions comprising: (a) an ethylene polymer component, having a melting temperature greater than or equal to 75° C. and crystallinty as measured by differential scanning calorimetry (DSC) of 10% or more, and (b) a propylene polymer component, having a melting temperature greater than or equal to 125° C. and a MFR of 500 dg/min. or more at 230° C. Preferably, both polymer components are produced with a metallocene catalyst system. In the polymer blend, component (a) is preferably about 90 to about 99.9 weight percent of the blend based on the total weight of (a) and (b). The polymer blend is formed by mixing blend components (a) and (b) under high shear mixing conditions. A unit such as a twin-screw extruder would be an example of a suitable piece of mixing equipment. Other means to achieve a well mixed blend will be apparent to those skilled in the art.
Another embodiment of this invention comprises film or sheet articles formed from a polymer blend as described above. According to yet another embodiment of this invention there are provided multiple layer films comprising at least one layer formed from the polymer blend as described above.
DESCRIPTION OF THE INVENTION
The polymer blends of this invention include, and preferably consist essentially of (a) an ethylene polymer component produced with a metallocene catalyst; and (b) a propylene polymer component having a high melt flow rate and preferably produced using a metallocene catalyst. All references to metallocene catalyst shall include other catalysts (e.g. single-sited catalyst) capable of producing polymers having properties the same as or similar to metallocene-produced polymers (e.g. narrow MWD, narrow CD, low extractables content, good optical properties). Such blends can optionally include additives well known to those skilled in the art.
The ethylene polymer component can be a single polyethylene polymer or a blend of two or more polyethylenes. The ethylene polymer component has a melting temperature greater than or equal to 75° C. and crystallinty of 10% or more DSC. If the ethylene polymer component is a blend of more than one ethylene-based polymer, preferably each ethylene polymer so blended will also have a melting temperature greater than or equal to 75° C. and crystallinty of 10% or more as measured by DSC.
Polyethylene, as used herein, can be a homopolymer or a copolymer and includes plastomers, VLDPE, LLDPE, LDPE, and HDPE. Plastomers, as used herein, refers generally to a class of ethylene based copolymers with density of less than about 0.900 g/cc (down to bout 0.865 g/cc) and having a weight average molecular weight (M
w
) greater than about 20,000 (about 200 MI lower). Plastomers have an ethylene crystallinity between plastics (i.e. linear low density and very low density polyethylenes) and ethylene/alpha-olefin elastomers. VLDPE is very low density polyethylene, typically having a density in the range of from 0.90 to 0.915 g/cc. LLDPE is linear low density polyethylene, typically having a density in the range of from 0.915 to 0.930 g/cc. LDPE is low density polyethylene, typically having a density in the range of from 0.915 to 0.930 g/cc. HDPE is high density polyethylene, typically having a density in the range of from 0.930 to 0.960 g/cc. Densities above about 0.90 g/cc were measured using standard accepted procedures. At densities below about 0.90 g/cc, the samples are preferably conditioned by holding them for 48 hours
Agarwal Pawan K.
Dekmezian Armenag H.
ExxonMobil Chemical Patents Inc.
Hess Bruce H.
Reid Frank E.
Runyan, Jr. Charles E.
Shewareged B.
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