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
2002-01-28
2003-09-02
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
06613841
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the preparation of uniaxially oriented polyethylene films. More particularly, the invention relates to the preparation of uniaxially oriented polyethylene films from a blend comprising a high molecular weight, medium density polyethylene (HMW MDPE) and a linear low density polyethylene (LLDPE).
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, shipping sacks, food packaging films, multi-wall bag liners, produce bags, deli wraps, stretch wraps, and shrink wraps. 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 an HDPE film has a modulus of about 100,000 psi or higher. LLDPE film has higher impact strength than 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.
Recently, a high-molecular weight, medium-density polyethylene (HMW MDPE) has been developed (see co-pending application Ser. No. 09/648,303, filed on Aug. 25, 2000). The HMW MDPE has many unique properties and offers new opportunities for improvement of polyethylene films. Co-pending application Ser. No. 09/688,314 teaches a blend comprising HWM MDPE and LLDPE. The blend provides films with significantly improved toughness and tear strength compared to MDPE or HDPE and high modulus compared to LLDPE.
Machine direction orientation (MDO) is known to the polyolefin industry. When a polymer is strained under uniaxial stress, the orientation becomes aligned in the direction of pull. Most commercial MDO films are produced by orienting cast extrusion films. When an HDPE film undergoes MDO, the resultant film usually shows improved gloss, clarity, tensile strength, modulus and barrier properties. However, the oriented film often shows greatly reduced machine direction tear strength (MD tear) and dart impact strength.
It would be desirable to prepare polyethylene films that have high modulus, high gloss, low haze, and relatively high MD tear and dart impact strength after MDO.
SUMMARY OF THE INVENTION
The invention is a process for preparing machine direction oriented (MDO) polyethylene films. The oriented film has high modulus, high gloss, low haze, and relatively high MD tear and dart impact. The process comprises blending from about 20 wt % to about 80 wt % of a high-molecular weight, medium-density polyethylene (HMW MDPE) and about 20 wt % to about 80 wt % of a linear low density polyethylene (LLDPE), converting the blend into a film, and orienting the film uniaxially in the machine direction. The HMW MDPE has a density from about 0.92 to about 0.94 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. The LLDPE has a density from about 0.90 to about 0.93 cc/g and an MI
2
from about 0.5 to about 50 dg/min.
DETAILED DESCRIPTION OF THE INVENTION
The process of the invention comprises blending a high-molecular weight, medium-density polyethylene (HMW MDPE) and a linear low-density polyethylene (LLDPE), converting the blend into a film, and orienting the film uniaxially in the machine direction.
The HMW MDPE has a density within the range of about 0.92 to about 0.94 g/cc. Preferably, the density is within the range of about 0.93 to about 0.94 g/cc. Preferred HMW MDPE is a copolymer that comprises from about 85 wt % to about 98 wt % of recurring units of ethylene and from about 2 wt % 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 a 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 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. The low-molecular weight component preferably has a density from about 0.94 to about 0.97 g/cc, which is in the range of the conventional high-density polyethylene (HDPE). 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.
Co-pending application Ser. No. 09/648,303, the teachings of which are herein incorporated by reference, teaches the preparation of HMW MDPE by a multiple-zone process with Ziegler catalysts. For example, an HMW MDPE can be produced by polymerizing an olefin mixture containing from about 85 wt % to about 98 wt % of ethylene and from about 2 wt % to about 15 wt % of a C
3
to C
10
&agr;-olefin in a first-reaction zone to produce a first polymer. Volatile materials are removed from the first polymer, and then the polymerization is continued in a second-reaction zone by adding more of the olefin mixture.
LLDPE can be produced by Ziegler catalysts or newly developed single-site catalysts. Ziegler catalysts are well known. Examples of suitable Ziegler catalysts for making LLDPE include titanium halides, titanium alkoxides, vanadium halides, and mixtures thereof. Ziegler catalysts are used with cocatalysts such as alkyl aluminum compounds.
Single-site catalysts can be divided into metallocene and non-metallocene. Metallocene single-site catalysts are transition metal
Equistar Chemicals LP
Guo Shao
Nutter Nathan M.
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