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
1999-05-13
2001-10-23
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...
C525S240000, C428S035700
Reexamination Certificate
active
06306969
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A “MICROFICHE APPENDIX”
Not Applicable
FIELD OF THE INVENTION
This invention relates to an improved shrink film-obtained by selectively controlling and optimizing the density differential between at least two polyolefin polymer components to provide narrow density splits. One aspect of the invention relates to a biaxially oriented polyolefin shrink film having balanced properties and comprising a polymer composition, the polymer composition comprising and made from (A) a first ethylene polymer component having a single differential scanning calorimetry (DSC) melting peak and a single Analytical Temperature Rising Elution Fractionation (ATREF) peak and (B) a second ethylene polymer component having one or more DSC melting peaks, wherein the density differential between component(A) and component (B) is in the range from 0 to 0.03 g/cc. Another aspect of the invention relates to an oriented shrink film having improved toughness and comprising a polymer composition, the polymer composition comprising and made from at least one lower density, homogeneously branched ethylene polymer (C) and at least one higher density, higher molecular weight ethylene polymer (D) wherein the density differential between the two polymer components is in the range of 0.001 to 0.05 g/cc. This invention also relates to a biaxial orientation method of making a shrink film having balanced properties and a method of making an oriented shrink film having improved toughness.
DESCRIPTION OF THE RELATED ART
Food items such as poultry, fresh red meat and cheese, as well as nonfood industrial and retail goods, are packaged by various heat shrink film methods. Heat shrink films can be monoaxial or biaxial oriented and are required to possess variety of film attributes. For example, in addition to a high shrink response, for successful use in hot-fill or cook-in applications, shrink films must also possess a relatively high softening point.
There are two main categories of heat shrink films—hot-blown shrink film and oriented shrink film. Hot-blown shrink film is made by a hot-blown simple bubble film process and, conversely, oriented shrink film is made by elaborate biaxial orientation processes known as double bubble, tape bubble, trapped bubble or tenter framing. Both amorphous and semi-crystalline polymers can be made into oriented shrink films using elaborate biaxial orientation processes. For amorphous polymers, the orientation is performed at a temperature immediately above the glass transition temperature of the polymer. For semi-crystalline polymers, the orientation is performed at a temperature below the peak melting point of the polymer.
Shrink packaging generally involves placing an item(s) into a bag (or sleeve) fabricated from a heat shrink film, then closing or heat sealing the bag, and thereafter exposing the bag to sufficient heat to cause shrinking of the bag and intimate contact between the bag and item. The heat that induces shrinkage can be provided by conventional heat sources, such as heated air, infrared radiation, hot water, hot oil combustion flames, or the like. Heat shrink wrapping of food items helps preserve freshness, is attractive, hygienic, and allows closer inspection of the quality of the packaged food item. Heat shrink wrapping of industrial and retail goods, which is alternatively referred to in the art and herein as industrial and retail bundling, preserves product cleanliness and also is a convenient means of bundling and collating for accounting and transporting purposes.
The biaxial heat;shrink response of shrink film is obtained by initially stretching fabricated film to an extent several times its original dimensions in both the machine and transverse directions to orient the film. The stretching is usually accomplished while the fabricated film is sufficiently soft or molten, although cold drawn shrink films are also known in the art. After the fabricated film is stretched and while still in a stretched condition, the stretching or orientation is frozen or set in by quick quenching of the film. Subsequent application of heat will then cause the oriented film to relax and, depending on the actual shrink temperature, the oriented film can return essentially back to its original unstretched dimensions, i.e., to shrink relative to its stretched dimension.
The orientation window and shrink response of oriented films is affected by-resin properties and fabrication parameters. The orientation window depends upon the broadness of the resin melting range and, as such, relates directly to the short chain branching distribution of the resin. In general, ethylene alpha-olefin interpolymers having a broad short chain branching distribution and broad melting range (e.g., heterogeneously branched ultra low density polyethylene resins such as ATTANE™ resins supplied by The Dow Chemical Company) exhibit a wide orientation window compared to ethylene alpha-olefin interpolymers characterized as having a narrow short chain branching distribution and narrow melting range (e.g., homogeneously branched linear ethylene polymers such as EXCEED™ and EXACT™ resins supplied by Exxon Chemical Corporation).
Polyolefin film shrinkage depends on shrink tension and film density. Film shrinkage is decreased as the orientation temperature is increased due to lower shrink tension. Film shrinkage is increased at lower density (lower crystallinity) because crystallites provide topological constraints and, as such, hinder free shrinkage. Conversely, for a given draw ratio, shrink tension depends on the crystallinity of the resin at the orientation temperature.
While the temperature at which a particular polymer is sufficiently soft or molten is a critical factor in various orientation techniques, such temperatures are ill-defined in the art. Disclosures pertaining to oriented films that disclose various polymer types (which invariably have varying polymer crystallinities and melting points), simply do not define the stretching or orientation temperatures used for the reported comparisons. U.S. Pat. No. 4,863,769 to Lustig et al., WO 95/00333 to Eckstein et al., and WO 94/07954 to Garza et al. are two examples of such disclosures.
The direct effect of density or crystallinity on shrink response and other desired shrink film properties such as, for example, impact resistance, are known, for example, from WO 95/08441, the disclosure of which is incorporated herein by reference. That is, even where the orientation temperature is presumably constant, lower density polymer films will show a higher shrink response and improved impact resistance. However, the effects of density and other resin properties on the orientation temperature is not well-known. In the prior art, there are only general rules of thumb or generalized teachings relating to suitable stretching or orientation conditions. For example, in commercial operations, it is often said that the temperature at which the film is suitably soft or molten is just above its respective glass transition temperature, in the case of amorphous polymers, or below its respective melting point, in the case of semi-crystalline polymers.
While the effects of density and other resin properties on the optimum orientation temperature of polyolefins are generally unknown, it is clear that heterogeneously branched ethylene polymers such as ATTANE™ resins and DOWLEX™ resin have a relatively broad orientation window (i.e., the temperature range at which the resin can be substantially stretched when molten or softened). It is also clear that softening temperatures and other film properties such as, for example, secant modulus, tend to decrease at lower polymer densities. Because of these relationships, films with high shrink responses, wide orientation windows, high modulus and high softening temperatures (i.e., shrink films with balanced properties) are unknown in the prior art. That is, polymer designers invariably have to sacrifice high softening temperatures and high modulus to provide
deGroot Jacquelyn A.
Patel Rajen M.
Nutter Nathan M.
The Dow Chemical Company
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