Cast films

Details Cast films Cast films

2002-12-09

2004-03-30

Teskin, Fred (Department: 1713)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

C526S348000, C526S348100, C526S348200

Reexamination Certificate

active

06713189

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to cast films made from pseudohomogeneous linear low-density polyethylene.
BACKGROUND OF THE INVENTION
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.
Cast films prepared from homogeneous resins typically have excellent dart impact strength. However, these films also typically have very poor tear strength in comparison to films made from conventional (Zieglar Natta catalyzed) resins.
SUMMARY OF THE INVENTION
The present invention provides a monolayer cast film having 1) a dart impact strength as determined by ASTM D1709 of greater than 200 grams per mil; and 2) a slow puncture resistance of greater than 100 Joules per millimeter wherein said cast film is prepared from a pseudohomogeneous linear low density polyethylene having a COHO ratio of from 3.5/1 to 19/1 and a stress exponent of less than 1.29.
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 narrow 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 cast film made from these resins has a very surprising and highly desirable balance of puncture resistance and tear strength properties which are particularly well suited for mono- or multilayer film constructions for packaging goods of unusual size.
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).
DETAILED DESCRIPTION
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 8.0. Stated alternatively, this means that the pseudohomogeneous resins of the present invention are from about 78% copolymer (corresponding to a COHO ratio of 3.5/1) to 95% copolymer (corresponding to a COHO ratio of 19/1) with the balance being homopolymer. Similarly, the preferred copolymers are from 80% copolymer to about 89% copolymer with the balance being homopolymer. In contrast, conventional LLDPE resins of the same density (made for example, with conventional Ziegler Natta catalysts in conventional polymerization reactors) generally have a COHO ratio of less than 4 (with a COHO ratio of from 2 to 3 being common) and “homogeneous” resins (i.e. resins made with a so-called single site catalyst, such as a metallocene catalyst) are effectively 100% copolymer (i.e. with no appreciable amounts of homopolymer being detectable by conventional analytical techniques).
The LLDPE resins used in the present invention are preferably prepared in using a highly efficient Ziegler Natta catalyst and in a very well mixed solution polymerization reactor.
A. Description of Ziegler Natta Catalyst
The term “Ziegler Natta catalyst” is well known to those skilled in the art. A Ziegler Natta catalyst may be used in this invention. Ziegler Natta catalysts comprise at least one transition metal compound of a transition metal selected from groups 3, 4 or 5 of the Periodic Table (using IUPAC nomenclature) and an organoaluminum cocatalyst which is defined by the formula:
Al(X′)
a
(OR)
b
(R)
c
wherein: X′ is a halide (preferably chlorine); OR is an alkoxy or aryloxy group; R is a hydrocarbyl (preferably an alkyl having from 1 to 10 carbon atoms); and a, b or c are each 0, 1, 2 or 3 with the provisos that a+b+c=3 and b+c≧1.
It is preferred that the transition metal compounds contain at least one of titanium or vanadium. Exemplary titanium compounds include titanium halides (especially titanium chlorides, of which TiCl
4
is preferred); titanium alkyls; titanium alkoxides (which may be prepared by reacting a titanium alkyl with an alcohol) and “mixed ligand” compounds (i.e. compounds which contain more than one of the above described halide, alkyl and alkoxide ligands). Exemplary vanadium compounds may also contain halide, alkyl or alkoxide ligands. In addition vanadium oxy trichloride (“VOCl
3
”) is known as a Ziegler Natta catalyst component and is suitable for use in the present invention.
As will be appreciated by those skilled in the art of ethylene polymerization, conventional Ziegler Natta catalysts may also incorporate additional components such as an electron donor (for example an amine or an ether) and/or a magnesium compound (for example a magnesium alkyl such as a butyl ethyl magnesium). A halide source (which is typically a chloride such as tertiary butyl chloride) is typically used when a magnesium compound is present.
Such components, if employed, may be added to the other catalyst components prior to introduction to the reactor or may be directly added to the reactor.
It is highly preferred that the Ziegler Natta catalyst contains a titanium compound, a magnesium alkyl compound and a chloride compound and that an aluminum alkoxide is used as the cocatalyst.
The Ziegler Natta catalyst may also be “tempered” (i.e. heat treated) prior to being introduced to the reactor (again, using techniques which are well known to those skilled in the art and published in the literature). Particularly preferred Ziegler Natta catalysts and methods of preparing them are described in U.S. Pat. Nos. 5,492,876; 5,519,098; and 5,589,555.
B. Description of Solution Polymerization Process
Solution processes for the copolymerization of ethylene and an alpha olefin having from 3 to 12 carbon atoms are well known in the art. These processes are conducted in the presence of an inert hydrocarbon solvent typically a C
5-12
hydrocarbon which may be unsubstituted or substituted by a C
1-4
alkyl group, such as pentane, methyl pentane, hexane, heptane, octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. An example of a suitable solvent which is commercially availabl

Affiliated with

Also associated with

Rating

Cast films does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Cast films, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cast films will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3204223