Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-10-13
2002-03-12
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S211000, C525S222000, C525S239000, C525S240000
Reexamination Certificate
active
06355733
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to polyethylene blends. The blends comprise a high molecular weight, medium density polyethylene (HMW MDPE) and a linear low density polyethylene (LLDPE). The invention also relates to films made from the blends.
BACKGROUND OF THE INVENTION
Polyethylene is divided into high density (HDPE, density 0.941 g/cc or greater), medium density (MDPE, density from 0.926 to 0.940 g/cc), low density (LDPE, density from 0.910 to 0.925 g/cc) and linear low density polyethylene (LLDPE, density from 0.910 to 0.925 g/cc). (See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials.) One of the main uses of polyethylene (HDPE, LLDPE, and LDPE) is in film applications, such as grocery sacks, institutional and consumer can liners, merchandise bags, multi-wall bag liners, produce bags, deli wrap, and shrink wrap. The key physical parameters of polyethylene film include tear strength, impact strength, tensile strength, stiffness and clarity. Tear strength is measured in machine direction (MD) and transverse direction (TD). Total tear strength (the product of MD tear and TD tear) is an indicator of overall tear properties. Critical processing properties on the film line include the output, bubble stability, gauge control (variability in film thickness), extruder pressure and temperature.
Film stiffness can be measured by modulus. Modulus is the resistance of the film to deformation under stress. It relates to its density. A higher density gives a higher modulus. A typical LLDPE film has a modulus of about 32,000 psi, while a HDPE film has a modulus of about 100,000 psi or higher. LLDPE film has higher impact strength and MD tear, while HDPE has higher stiffness and tensile strength. When LLDPE producers attempt to increase the density (thereby increasing the modulus of the film), they often encounter losses in impact strength and MD tear. Historically, blending LLDPE and HDPE has not achieved “breakthrough” success. The blends often give films that have improved stiffness and tensile properties, but the impact and tear properties are usually sacrificed. There are no straightforward methods or single resins that have the combined properties of both.
While there are few polyethylene films of modulus between about 40,000 psi and about 90,000 psi, there is an increasing demand for such films. For example, gardening has become one of the largest hobbies by dollars spent in the United States. To support gardeners, a variety of products need to be commercially available in large and small packages. Because consumer perception is important, the bags need to possess a high aesthetic appearance and excellent mechanical integrity. When consumers load 50-lb bags of fertilizer or pesticide into their cars, they need to feel comfortable and safe. This requires the bags to be easy to handle and stack, to resist puncture and tear propagation, to have good sealability and excellent seal strength, and to be glossy and printable. Existing films for these markets are primarily LLDPE resins. Although HDPE films are more similar to the paper packaging that they have replaced in these industries, HDPE films do not have the impact and tear properties essential for acceptable durability standards.
Recently, a high molecular weight, medium density polyethylene (HMW MDPE) has been developed (see copending appl. Ser. No. 09/648,303 (docket No. 88-1026A) filed on Aug. 25, 2000). The HMW MDPE has many unique properties and offers new opportunities for improvement of polyethylene films.
SUMMARY OF THE INVENTION
The invention is a blend comprising a high molecular weight, medium density polyethylene (HMW MDPE) and a linear low density polyethylene (LLDPE). The blend comprises from about 20 wt % to about 80 wt % of HMW MDPE. The HMW MDPE has a density from about 0.92 to about 0.944 g/cc, a melt index MI
2
from about 0.01 to about 0.5 dg/min, and a melt flow ratio MFR from about 50 to about 300. It has a multimodal molecular weight distribution comprising a high molecular weight component and a low molecular weight component. The low molecular weight component has an MI
2
from about 50 to about 600 dg/min and a density from about 0.94 to about 0.97 g/cc. The blend also comprises about 20 wt % to about 80 wt % of LLDPE. The LLDPE has a density within the range of about 0.90 to about 0.925 g/cc and an MI
2
within the range of about 0.50 to about 50 dg/min.
The invention also includes a film prepared from the blend and a method for making the film. We have surprisingly found that blending the HMW MDPE and a high performance, conventional or single-site LLDPE gives the film toughness and tear strength similar to LLDPE with stiffness and tensile properties similar to medium density HDPE films.
DETAILED DESCRIPTION OF THE INVENTION
The blend of the invention comprises from about 20 wt % to about 80 wt % of a high molecular weight, medium density polyethylene (HMW MDPE). Preferably, the blend comprises from about 30 wt % to about 70 wt % of HMW MDPE. The HMW MDPE has a density within the range of about 0.92 to about 0.944 g/cc. Preferably, the density is within the range of about 0.935 to about 0.944 g/cc. Preferred HMW MDPE is a copolymer that comprises from about 85 to about 98 wt % of recurring units of ethylene and from about 2 to about 15 wt % of recurring units of a C
3
to C
10
&agr;-olefin. Suitable C
3
to C
10
&agr;-olefins include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, and 1-octene, and the like, and mixtures thereof.
The HMW MDPE has an MI
2
from about 0.01 to about 0.5 dg/min, preferably from about 0.01 to about 0.3 dg/min, and an MFR from about 50 to about 300. Melt index (MI
2
) is usually used to measure polymer molecular weight, and melt flow ratio (MFR) is used to measure the molecular weight distribution. A larger MI
2
indicates a lower molecular weight. A larger MFR indicates a broader molecular weight distribution. MFR is the ratio of the high-load melt index (HLMI) to MI
2
. The MI
2
and HLMI can be measured according to ASTM D-1238. The MI
2
is measured at 190° C. under 2.16 kg pressure. The HLMI is measured at 190° C. under 21.6 kg pressure. The HMW MDPE has a considerably higher molecular weight (or lower MI
2
) and broader molecular weight distribution (or larger MFR) than conventional HDPE or LLDPE.
The HMW MDPE has a multimodal molecular weight distribution. By “multimodal molecular weight distribution,” we mean not only that the HMW MDPE has at least two different molecular weight components, but also that the two components differ chemically and structurally from each other. The low molecular weight component has an MI
2
within the range of about 50 to about 600 dg/min, while the high molecular weight component preferably has an MI
2
less than about 0.5 dg/min. The high molecular weight (low MI
2
) component gives the polyethylene superior bubble stability in a blown film process and the low molecular weight (high MI
2
) component gives the polyethylene excellent processability. Furthermore, the low molecular weight component has a density from about 0.94 to about 0.97 g/cc (i.e., in the range of conventional HDPE), while the high molecular weight component preferably has a density from 0.90 to 0.94 g/cc, more preferably from 0.91 to 0.94 g/cc, which is similar to the conventional LLDPE.
Copending appl. Ser. No. 09/648,303 (docket No. 88-1026A) filed on Aug. 25, 2000, the teachings of which are herein incorporated by reference, teaches preparation of the HMW MDPE by a multiple zone process with Ziegler catalysts. For example, a HMW MDPE can be produced by polymerizing an olefin mixture containing from about 85 to about 98 wt % of ethylene and from about 2 to about 15 wt % of a C
3
to C
10
&agr;-olefin in a first reaction zone to produce a first polymer, removing some volatile materials from the first polymer, and then continuing the polymerization in a second reaction zone by adding more of the olefin mixture.
The blend of the invention comprises from about 20 wt % to about 80 wt % of a lin
Chandrashekar Venki
Cribbs Leonard V.
Imfeld Stephen M.
Williams Kelly L.
Equistar Chemicals LP
Guo Shao
Nutter Nathan M.
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