Package making – Methods – Conforming by stretching or shrinking of cover over contents
Reexamination Certificate
2003-05-16
2004-05-11
Pyon, Harold (Department: 1772)
Package making
Methods
Conforming by stretching or shrinking of cover over contents
C428S034100, C428S034800, C428S035200, C428S035700
Reexamination Certificate
active
06732493
ABSTRACT:
The present invention relates to a thermoformable, multilayer, co-extruded adhesive-free and EVOH-free film containing polyamide, polyolefins and/or copolymers based on olefins. This film is particularly suitable for the packaging of products that have been subjected to a heat treatment in thermoformed films. The films are characterised by an outer layer of polyamide and a second outer layer of polyolefins or copolymers based on olefins, wherein the film composite does not delaminate even at temperatures between 50° C. and 150° C.
In order to simplify the discussion of the prior art and the invention, the polymers that are used for films in the field of the present invention are discussed in more detail hereinafter, and abbreviations that are used for these polymers in the further description are introduced. Insofar as specific polymers are named here, these are suitable for the corresponding layers of the film according to the invention.
The term polyamide (abbreviation “PA”) is understood to mean in the widest sense polymeric compounds that are coupled to one another by the acid amide group R—NH—CO—R'. A distinction is made between two groups of polyamides: those built up from one building block by polycondensation of &ohgr;-aminocarboxylic acids or polymerisation of their lactams to form the polyamide-6 type, and those built up from two building blocks, namely diamines and dicarboxylic acids, by polycondensation to form the polyamide-66 type. Other conventional polyamides are also copolyamides formed by a combination of the aforementioned types of polyamides.
“PO” is a generic abbreviation for polyolefins. Examples of polyolefins are low density polyethylenes (LDPE), high density polyethylenes (HDPE), low density linear polyethylene (LLDPE), polypropylene (PP), polyisobutylene, polybutylene and all further polymers that are formed from olefins. In addition the expression PO covers olefin copolymers consisting of olefins and other monomers, in whose composition olefins predominate. Such compounds include for example ethylene copolymers with unsaturated esters (e.g. vinyl acetate), ethylene copolymers with unsaturated carboxylic acids or their salts.
“PP” denotes polypropylene, irrespective of the spatial arrangement of the side-position methyl groups. “EPC” denotes copolymers of propylene with 1 to 10 mole % of ethylene, the ethylene being statistically distributed in the molecule.
“LPDE” denotes low density polyethylene that is in the density range from 0.86 to 0.93 g/cm
3
. The LDPE molecules are characterised by a high degree of branching.
“LLDPE” denotes low density linear polyethylenes that contain, in addition to ethylene, one or more &agr;-olefins with more than 3 C atoms as comonomers. Examples of &agr;-olefins that may be mentioned here include butene-1, hexene-1, 4methylpentene-1 and octene-1. The molecular structure typical of LLDPE, which is characterised by a linear main chain with side chains attached thereto, is produced by the polymerisation of the aforementioned compounds. The density varies between 0.86 and 0.935 g/cm
3
. The melt flow index MFR is normally between 0.3 and 8 g/10 min. In some publications the linear ethylene-&agr;-olefin copolymers are subdivided according to their density into VLDPE or ULDPE. Since however according to Gnauck/Fründt (Einstieg in die Kunststoffchemie, Hanser Verlag 1991, p.58) the properties, processing and use of these copolymers largely correspond to those of ethylene homopolymers, a more precise differentiation will not be adopted here.
“HDPE” denotes high density linear polyethylenes that have only a small degree of branching in their molecular chains. The density of the HDPE may vary between 0.93 g/cm
3
and up to 0.97 g/cm
3
.
“mPE” denotes in this context an ethylene copolymer that has been polymerised by means of metallocene catalysts. An (&agr;-olefin with four or more carbon atoms is preferably used as comonomer. The polymers produced using conventional Ziegler-Natta catalysts frequently contain in the low molecular weight fractions relatively high concentrations of the &agr;-olefins. As a consequence of the very uniform catalytically active metallocene centres, one finds narrow molecular weight distributions and, under fractionation, a very uniform incorporation of the &agr;-olefins in both the high molecular weight and low molecular weight fractions. The density is preferably between 0.900 and 0.930 g/cm
3
. The molecular weight distribution M
w
/M
n
is less than 3.5, preferably less than 3.
“EAA” denotes copolymers consisting of ethylene and acrylic acid and“EMAA” denotes copolymers consisting of ethylene and methacrylic acid. The ethylene content is preferably between 60 and 99 mole %.
“I” denotes copolymers based on olefins whose molecules are crosslinked via ionic bonds. The ionic bonds are reversible, which means that the ionic crosslinking is broken at the conventional processing temperatures (180° C. to 290° C.) and are reformed in the cooling phase. Copolymers of ethylene and acrylic acids that are crosslinked with sodium or zinc ions are normally used as polymers.
“EVA” denotes a copolymer consisting of ethylene and vinyl acetate. The ethylene content is preferably between 60 and 99 mole %.
“HV” denotes co-extrudable, coupling polymers. In contrast the term adhesives denotes non-metallic materials, including polymers, that can join two film layers by surface adhesion and internal strength without substantially altering the structure of the joined film layers. In contrast to coupling agents, adhesives are not co-extrudable but have to be applied separately by surface application, e.g. by lamination or liquid application.
Coupling agents are preferably modified polyolefins, such as for example LDPE, LLDPE, mPE, EVA, EAA, EMAA, PP, EPC, which are grafted with at least one monomer from the group comprising &agr;,&bgr;-singly unsaturated dicarboxylic acids, such as for example maleic acid, fumaric acid, itaconic acid or their acid anhydrides, acid esters, acid amides and acid imides. As extrudable coupling agents there may be used in addition copolymers of methylene with &agr;,&bgr;-singly unsaturated dicarboxylic acids, such as acrylic acid, methacrylic acid and/or their metal salts with zinc or sodium and/or their alkyl (C
1
-C
4
) esters, which may also be grafted with at least one monomer from the group comprising &agr;,&bgr;-singly unsaturated dicarboxylic acids, such as for example maleic acid, fumaric acid, itaconic acid or their acid anhydrides, acid esters, acid amides and acid imides. In addition there may also be used polyolefins such as for example polyethylene, polypropylene, ethylene/propylene copolymers or ethylene/&agr;-olefin copolymers, which may be grafted with copolymers of ethylene with &agr;,&bgr;-singly unsaturated dicarboxylic acids, such as acrylic acid, methacrylic acid and/or their metal salts with zinc or sodium and/or their alkyl (C
1
-C
4
) esters. Particularly suitable as bonding agents are polyolefins, especially ethylene/&agr;-olefin copolymers with grafted-on &agr;,&bgr;-singly unsaturated dicarboxylic acid anhydrides, in particular maleic anhydride. The HV may also contain an ethylene/vinyl acetate copolymer, preferably with a vinyl acetate content of at least 10 wt. %.
Different layers of a film are separated by “/”. Mixtures of polymers of a layer are characterised by round brackets and joined by a “+”. Thus, the film structure “. . . /LLDPE/(mPE+LDPE)” describes a multilayer film, one outer layer containing a mixture of mPE and LDPE. An inner layer consists of LLDPE. The remaining layers of the film are denoted by“. . . ”.
EP 0 490 558 describes a process for heating packaged foodstuffs. The thermoformable tray film consisting of (glycol-modified polyethylene terephthalate +polycarbonate)/HV/EVOH/HV/LLDPE in thicknesses of 200/10/15/10/100 &mgr;m is thermoformed, filled with food, and sealed with a microperforated cover film consisting of polyamide and polyethylene. The holes in the cover film have a diameter of ca. 85 &mgr;m. The pack is deep froze
Aughenbaugh Walter
Norris & McLaughlin & Marcus
Pyon Harold
Wolff Walsrode AG
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