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
1998-09-02
2001-03-06
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C526S348000, C526S352000, C526S351000
Reexamination Certificate
active
06197887
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to compositions having particular utility in the production of film. More particularly, the present invention relates to compositions comprising a blend of an ethylene polymer and a very low viscosity propylene polymer, having particular utility in the production of film, especially stretch wrap cling film.
BACKGROUND OF THE INVENTION
Linear low density polyethylene (“LLDPE”) has been found useful as the film-forming component of blown stretch wrap films since it is especially strong. However, a cling additive must be blended with the LLDPE prior to film formation since LLDPE does not exhibit a high level of cling. Stretch wrap film loses peel cling when stretched. It is important that blown stretch wrap film have an adequate amount of cling when stretched to lengths of at least 200 percent elongation, for typical stretch wrapping processes.
Amorphous polyolefins having a viscosity greater than about 2,000 mPa.s at 190° C. have been found to be useful as LLDPE “cast” stretch film tackifiers for providing film cling, but not useful as LLDPE “blown” stretch film tackifiers. The LLDPE film containing the amorphous polyolefins does not have adequate peel strength at 200 percent elongation.
U.S. Pat. No. 4,430,457 (Dobreski) discloses a cling additive for ethylene vinyl acetate (“EVA”), low density polyethylene (“LDPE”), or LLDPE film compositions comprising a low viscosity amorphous propylene-ethylene copolymer having about 1 to about 7 weight percent ethylene units. The particular propylene copolymer disclosed in the example in that patent is POLYPOL 19, produced and sold by Crowley Chemical Company. POLYPOL 19 copolymer has a viscosity of 50 to 120 mPa.s at 190° C., a ring and ball softening point (RBSP) of 77 to 99° C., and a cone penetration of 65 to 90 dmm at 21° C. (needle penetration estimated by applicant to be approximately 130 to 180 dmm at 21° C.) according to Crowley Chemical Company, Inc. Technical Bulletin (Jan. 10, 1994, POLYPOL 19). Dobreski discloses that blown film formed from those compositions have adequate peel cling when unstretched.
SUMMARY OF THE INVENTION
The present invention relates to a composition of matter comprising about 85 to about 99 weight percent based on the composition of a component selected from the group consisting of low density polyethylene, linear low density polyethylene, ethylene vinyl acetate copolymer, and mixtures thereof, and about 1 to about 15 weight percent, based on the composition, of a propylene polymer comprising 0 to less than about 1 weight percent based on the polymer, of an olefin comonomer unit having 2 to 10 carbon atoms, wherein said propylene polymer has a Brookfield Thermosel viscosity of about 1 to about 2,000 mPa.s at 190° C. and a needle penetration of about 5 to about 50 dmm, at 23° C.
DETAILED DESCRIPTION OF THE INVENTION
The applicant has unexpectedly found that, within the particular viscosity range of about 1 to about 2,000 mPa.s at 190° C., polypropylene homopolymer, and polypropylene copolymer comprising 0 to less than about 1 weight percent olefin comonomer, is a very good cling additive in polyethylene-based film compositions. It was surprising to find that the polypropylene of the present invention could provide cling properties to LLDPE, LDPE, and EVA since polypropylene homopolymer and copolymers having viscosities greater than about 2,000 mPa.s, at 190° C. do not generally provide cling in polyethylene-based blown film.
An unexpected benefit of the composition of the present invention is the improved packagability and processability. The composition can easily be formed into individual pellets that can be stored at room temperature for later use.
The composition of the present invention comprises about 85 to about 99 weight percent based on the total composition, of a component selected from the group of ethylene polymers consisting of low density polyethylene (“LDPE”), linear low density polyethylene (“LLDPE”), ethylene vinyl acetate (“EVA”), and mixtures thereof, and about 1 to about 15 weight percent based on the composition, of a propylene polymer comprising from greater than 99 percent to 100 weight percent based on the polymer, of propylene units and 0 to less than about 1 weight percent based on the polymer, olefin comonomer units having 2 to 10 carbon atoms. The propylene polymer has a Brookfield Thermosel viscosity of about 1 to about 2,000 mPa.s at 190° C. and a needle penetration of about 5 to about 50 dmm, at 23° C.
The ethylene containing polymeric component is a hompolymer of ethylene or a copolymer of ethylene with a comonomer copolymerizable therewith. More particularly, the ethylene containing polymeric component is selected from a hompolymer of ethylene, an ethylene alpha olefin copolymer, an ethylene vinyl acetate copolymer, and mixtures thereof, wherein the ethylene containing polymeric component has a density no greater than 0.94 g/cc. The ethylene containing polymeric component preferably has a density of from about 0.88 to about 0.93 g/cc, and more preferably, from about 0.89 to about 0.92 g/cc.
The alpha olefin comonomer of the ethylene alpha olefin copolymer preferably has from 3 to 20 carbon atoms per molecule and is present in an amount of from about 2 to about 20 weight percent based on the ethylene alpha olefin copolymer. Examples of the alpha olefin copolymer include 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, a terpolymer of ethylene, a terpolymer of 1-butene, a terpolymer of 1-hexene, and mixtures and derivatives thereof. In a preferred embodiment, the alpha olefin is selected from 1-butene, 1-hexene and 1-octene.
The ethylene containing polymeric component may be prepared readily by any conventional technique known in the art.
Stretch wrap film made from blown film processes is generally stronger than film made from casting. Since LDPE and EVA films are not as strong as LLDPE films, the preferred ethylenic component of the present invention is LLDPE. LLDPE has excellent elongation, good tear resistance, good tensile strength, and good puncture resistance. LLDPE is a copolymer of ethylene and an alpha olefin, with density from about 0.88 g/cm
3
to about 0.94 g/cm
3
and a melt index of about 0.4 to about 8 g/10 min. The LLDPE used is more preferably an ethylene-octene or ethylene-hexene LLDPE copolymer. The presence of up to about 20 weight percent LDPE, along with the LLDPE, improves processability, bubble stability, and optical properties. A minor amount of other polyolefins may be present in the ethylenic component.
The ethylene containing polymeric component, comprises about 85 to about 99 weight percent of the present composition and the propylene polymer comprises about 1 to about 15 weight percent. More preferably, the composition comprises about 90 to about 99 weight percent ethylenic component, and the propylene polymer comprises from about 1 to about 10 weight percent, based on the total composition.
The propylene polymer of the composition is a very low molecular weight polypropylene homopolymer or polypropylene copolymer having less than about 1 weight percent C2 to C10 alpha olefin comonomer unit content. The propylene polymer preferably has less than about 0.5 weight percent comonomer content. The homopolymer is still more preferred. The modifying alpha olefin comonomer units are preferably ethylene units.
The propylene polymer of the present invention is partially crystalline. That is, the propylene polymer has a measurable amount of crystallinity, as determined using a hexane extraction method, such as ASTM D5227. The propylene polymer more preferably has at least 1 weight percent crystalline content. The usefulness of polypropylene having the presence of a measurable amount of crystallinity is a further unexpected aspect of the present invention since previously known polypropylene tack additives generally have been amorphous.
The low molecular weight of the propylene polymer of the present invention is critical. The low molecular weight is evidenced
Choi Ling-Siu
Eastman Chemical Company
Gwinnell Harry J.
Wood Jonathan D.
Wu David W.
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