Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
2001-01-31
2002-01-22
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S516000, C428S910000, C526S348100
Reexamination Certificate
active
06340532
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to shrink films made from pseudohomogeneous linear low density polyethylene.
BACKGROUND OF THE INVENTION
The term “shrink film” refers to a plastic wrapping film which has the characteristic of shrinking when it is heated to near the melting point of the film. These films are commonly manufactured from plastic resins such as polyvinyl chloride (PVC); polypropylene (PP); linear-low density polyethylene (LLDPE); low density polyethylene (LDPE); high density polyethylene (HDPE); copolymers of ethylene and vinyl acetate (EVA); copolymers of ethylene and vinyl alcohols (EVOH); ionomers (e.g. Surlyn™); copolymers of vinylidene chloride (e.g. PVDC, SARAN™); copolymers of ethylene acrylic acid (EAA); polyamides (PA); among others.
End uses of these films include food packaging (for example, oxygen and moisture barrier bag for frozen poultry, primal meat cuts and processed meat and cheese products for preservation of freshness and hygienics) and non-food packaging (for example, “overwraps” for protecting goods against damage, soiling, tampering and pilferage) during transportation, distribution, handling and display. An example of end use is found in retail sales where the films are wrapped air-tight around single or multiple items of compact disks, audio/video tapes, computer software boxes, magazines, confectionery, boxed products, single serve bowls, etc. Another example of end use is found in wholesale retailing where multiple containers of bottled and canned goods such as beverages, condiments and personal hygiene products are sold in bulk. Yet another example is found in courier shipping where singular items of shrink-wrapped sporting goods and household appliances are now safely transported without the need for bulky protective cardboard cartons.
Films of the present invention must be made from linear low density polyethylene. Linear low density polyethylene (LLDPE) is conventionally prepared by a polymerization process using a so-called Ziegler Natta catalyst. It is well known to those skilled in the art that the conventional LLDPE resins prepared with Ziegler Natta catalysts do not have a uniform structure or composition. In particular, these conventional resins typically contain a minor amount of a very low density copolymer; a major portion of the “copolymer” having the desired molecular weight and density; and a large “homopolymer” fraction (which does not contain a meaningful amount of the desired comonomer). This lack of polymer homogeneity is associated with several disadvantages; for example 1) “organoleptic” problems caused by the low molecular weight material; and 2) suboptimal impact strengths which are believed to be caused by the crystallinity of the homopolymer fraction. The development of “homogeneous polyethylene” resins has mitigated these disadvantages.
Homogeneous resins may be prepared with the so-called metallocene catalysts which are well known and widely described in the literature.
The resulting “homogeneous” resins have a very uniform composition as evidenced by the substantial absence of very low molecular weight/low density fractions and/or homopolymer fractions. These homogeneous resins exhibit excellent organoleptic properties and impact strength properties. However, films made with homogeneous resins often have deficient tear properties. In addition, shrink properties may become “unbalanced” (with, for example, a greater shrink force in the machine direction then the transverse direction (TD) or even a negative TD shrinkage) when shrink films are made with highly homogeneous resins.
U.S. Pat. Nos. 5,591,390 and 5,595,705 (the disclosures of which are incorporated herein by reference) describe the preparation of shrink films made with a sub group of homogeneous resins, namely homogeneous resins having long chain branching. However, such resins are comparatively expensive.
The present invention provides a shrink film which mitigates certain of the disadvantages of films made from either “conventional” and “homogeneous” polyethylene resins.
SUMMARY OF THE INVENTION
The present invention provides a monolayer shrink film having, in non-crosslinked form, 1) a dart impact strength as determined by ASTM D1709 of greater than 450 grams per mil; 2) a tear strength as determined by ASTM D1922 of greater than 25 grams per mil in the transverse direction; and 3) a shrink tension of greater than 400 pounds per square inch (psi) in the machine direction; wherein said shrink film is prepared from a pseudohomogeneous linear low density polyethylene having a COHO ratio of from 3.5/1 to 19/1.
While not wishing to be bound by any theory, it is believed that the excellent properties of the films of this invention are attributable to two essential characteristics of the LLDPE used to make them, namely a combination of (a) a comparatively broad molecular weight distribution and (b) the presence of a pseudohomogeneous comonomer distribution (as evidenced by the copolymer/homopolymer or COHO ratio).
It is believed that the “pseudohomogeneous” LLDPE resins of the type described above have not been heretofore commercially available. We have discovered that shrink film made from these resins has a very surprising and highly desirable balance of strength properties and shrink properties which are particularly well suited for mono- or multilayer film constructions for food or non-food packaging.
Shrink films must have excellent strength properties and puncture resistance in addition to the obvious need for shrink characteristics to conform to the shape of the packaged goods and therefore allow for air-tightness and attractive shelf appearance.
It is well known to “crosslink” the films (for example, by irradiation) and/or to prepare multilayer shrink films in order to improve these “strength” and “puncture” properties as well as to improve barrier properties. The films of the present invention have excellent strength and puncture properties in the form of an uncrosslinked, monolayer film. However, it is also with the scope of the invention to further improve the films of the present invention via crosslinking and/or “multi-layering” technique.
Multilayer films are most commonly prepared by coextrusion (as opposed to lamination). Such films are available in constructions of from two to eleven layers. Films having between three and nine layers are typical at the present time. The use of the film of the present invention as the “core layer(s)” (i.e. one or more of the non-surface layers) of a multilayer construction is particularly preferred.
It is also within the scope of the present invention to prepare a multilayer film having at least one layer of film according to the present invention and one or more layers prepared from completely different plastic resins (such as LDPE, PP or EVA).
Shrink film is prepared by a blown film extrusion process (as opposed to a “cast” film process). The blown film process is well known to those skilled in the art.
DETAILED DESCRIPTION
Shrink film is now typically produced using two different processes known to those skilled in the art as 1) the “tenter framing” or 2) the “double bubble” process. In the double bubble process, an initial tube is extruded and then cooled or quenched (often using a water cooling system) before being subsequently expanded (and oriented in both the machine and transverse direction) through a heating oven.
The preparation of a multilayer shrink film is described in more detail in U.S. Pat. No. 4,551,380 (Shoenberg) and the preparation of a monolayer shrink film is described in U.S. Pat. No. 4,760,116 (Roberts). The detailed descriptions which are provided in these patents, and the aforementioned U.S. Pat. Nos. 5,591,390 and 5,595,705, are incorporated herein by reference.
As previously noted, the films of the present invention must be prepared using a pseudohomogeneous linear low density polyethylene resin. As used herein, the term “pseudohomogeneous” means that the resin has a copolymer/homopolymer (or “COHO”) ratio of from 3.5/1 to 19/1 and is preferably from 4.0 to 6.0. S
Auger James Arthur
Huang James Chin-Kuang
Kale Lawrence Thomas
Sipos Peter Andrew
Thomas Patrick R.
Johnson Kenneth H.
Nakarani D. S.
Nova Chemicals (International) S.A.
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