Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
2001-11-20
2004-04-27
Jackson, Monique R. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S516000, C428S517000, C428S519000, C428S910000
Reexamination Certificate
active
06727000
ABSTRACT:
This invention relates to multilayer, essentially three-layer heat-shrinkable sealable films in which the skin layers comprise a propylene polymer or polymer composition having defined crystallinity and melting characteristics and a core layer comprises a heterophasic composition comprising a crystalline propylene polymer and an elastomeric olefin polymer. An optimum balance of mechanical properties, processability and sealability at low temperatures characterizes the films.
Three-layer heat-shrinkable films are usually prepared by a coextrusion process in which the main polymer component forming the core layer is fed to the central extruder and a suitable polymer component forming the skin layers and improving film properties, in particular its workability, is fed to the lateral extruders. Depending on the technology used a flat or tubular primary film is obtained which is then oriented in a biaxial direction by the known tenter frame or twin bubble methods. Three-layer heat-shrinkable films usually consist of a core layer essentially made up of linear low-ensity ethylene polymer (LLDPE) modified with 1-octene and two outer layers intended to improve film processing. It is known in fact that certain aspects of the production of heat-shrinkable films based on LLDPE are critical because the temperature at which the orientation process takes place is close to the temperature at which the polymer melts. There may thus be problems such as tearing of the film and instability of the bubble when the film is produced by the twin bubble method.
Examples of heat-shrinkable films are given in U.S. Pat. No. 4,532,189. This patent describes films with 3 or 5 layers in which the middle layer is made up of linear low- or medium-density ethylene copolymers with the possible addition of amorphous ethylene/propylene copolymers (EPC), ethylene/vinyl-acetate copolymers (EVA) or low-density polyethylene (LDPE) and the outer layers are primarily made up of EPC. The film is reported to have good physical mechanical characteristics.
Patent application EP-A-586160 describes heat-shrinkable multi-layer films in which the middle layer is made up of LLDPE and the outer layers may be made up of a propylene/butene copolymer, or blends of EPC with polybutene, or blends of propylene homopolymer or copolymer with a propylene/butene copolymer. The patent application reports that the film has good lap seal strength characteristics.
Patent application EP-A-595252 describes three-layer heat-shrinkable films in which the middle layer is made up of LLDPE to which additives such as hydrogenated hydrocarbon resins, polyethylene or polypropylene waxes. etc. are added. The addition of these additives is claimed to give improved physical mechanical characteristics and improved lap seal strength to the films. The outer layers are made up of polypropylene or ethylene propylene copolymers, also with the additives mentioned above.
The films of the known art present various problems however depending on the composition of the various layers. If the outside layers are made up of polypropylene and/or ethylene propylene copolymers, the film can only be heat-sealed at relatively high temperature. In addition, the temperature range suitable to the orientation of the film without tearing is restricted and shifted towards relatively high temperatures. The use of propylene butene copolymers in the outer layers of the film is claimed to lower the sealing temperature. However, a copolymer containing large amounts of butene has the disadvantage of increasing the percentage of polymer extractable in xylene to levels that are not acceptable for applications of the film in the food sector. In all cases, the use of polyethylene-based layers coupled with polypropylene-based layers can cause problems of delamination of the resultant film, because of the poor compatibility between the various layers.
Now it has been found that it is possible to prepare quite effective heat-shrinkable sealable films wherein the skin layers comprise a propylene polymer or polymer composition having suitable values for crystallinity—expressed as percentage of material melted at a certain temperature—melting point and xylene-insoluble fraction, and the core layer comprises a heterophasic polyolefin composition made up of a crystalline propylene polymer and an elastomeric olefin polymer. The films of the invention do actually have an optimum balance of physical mechanical properties, processability and sealability over a wider and lower temperature range in comparison with the multilayer films of the prior art. Moreover the films of the invention have a good compatibility between the layers and in particular good heat-shrinking properties.
The object of the invention is therefore a multilayer heat-shrinkable sealable film wherein the skin layers A essentially consist of a polyolefin selected from:
A1) a composition containing
i) 25-45% by weight of a random copolymer of propylene and ethylene having 2-5% by weight of ethylene units, and
ii) 55-75% by weight of a random terpolymer of propylene, ethylene and one or more C
4
-C
10
alphaolefins having 2-5% by weight of ethylene units and 6-12% by weight of alphaolefin units;
A2) a random copolymer of propylene and one or more C
4
-C
10
alphaolefins or a blend of random copolymers of propylene and one or more C
4
-C
10
alphaolefis with different composition, said copolymer or blend of copolymers containing 4-14% by weight of alphaolefin units;
said polyolefin having a xylene-insoluble fraction greater than 85% by weight, a maximum melting peak at temperatures above 130° C. and a crystallinity such that at 90° C. the percentage of material melted is greater than 15% by weight, and a core layer B essentially consists of a heterophasic composition comprising:
a) 20-60% by weight of a propylene homopolymer with an isotacticity index value—determined as percent by weight of the polymer insoluble in xylene at 25° C.—higher than 80 or of a propylene crystalline copolymer with ethylene and/or C
4
-C
10
alphaolefins containing at least 85% by weight of propylene units and having an isotacticity index value of at least 80 or of mixtures thereof, and
b) 40-80% by weight of an ethylene copolymer with propylene and/or C
4
-C
10
alphaolefins and possibly a diene, containing 20-60% by weight of ethylene units and completely soluble in xylene at 25° C.
The maximum melting peak and the crystallinity level at a given temperature are determined by differential scanning calorimetry (DSC).
The polyolefin forming the skin layers A preferably has a crystallinity such that at 100° C. the percentage of material melted is greater than 20% and at 110° C. the percentage of material melted is greater than 30% by weight.
The polyolefin forming layers A preferably consists of a composition A1 of 30-40% by weight of a random copolymer of propylene and ethylene having 3-4% by weight of ethylene units and 60-70% by weight of a random terpolymer of propylene, ethylene and one or more C
4
-C
10
alphaolefins having 3-4% by weight of ethylene units and 8-10% by weight of alphaolefin units. The alphaolefin is generally chosen from 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene, and preferably is 1-butene.
Other preferred embodiments of the polyolefin forming layers A are the random copolymers A2 of propylene and 1-butene with a total content of 4-10% by weight of 1-butene units, more preferably as a blend of copolymers having a different content of 1-butene units.
The polyolefin compositions used to form layers A can be produced by mixing the components in the molten state, for example in a mixer having a high homogenizing power or in an extruder. Preferably, however, said compositions are obtained directly by synthesis using a sequential polymerization process using stereospecific Ziegler-Natta catalysts as described in the granted European Patents 400333 and 472946.
The heterophasic composition used to prepare the core layer B comprises a crystalline propylene polymer or copolymer and an elastomeric olefin copolymer as specified in detail abov
Beccarini Enrico
Joly Anne-Marie
Basell Poliolefine Italia S.p.A.
Jackson Monique R.
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