Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1995-02-21
2003-05-06
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S215000, C053S277000, C264S173140, C264S173150, C264S173190
Reexamination Certificate
active
06558808
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat-sealable film comprising a polyolefinic base layer and at least one outer layer. An aspect of this invention relates to a process for the production of the heat-sealable film. A further aspect of this invention relates to the use of the film.
DESCRIPTION OF THE PRIOR ART
Polyolefin films have been widely used in the past as packaging films. The success of these materials is based on the good optical and mechanical properties and the simple weldability of the films. The simple “welding” process for sealing film packages requires a heating device, for example in the form of an incandescent wire, which is brought into contact with the film layers and results in complete softening of these film layers. When the molten region cools, the weld seam forms. In order to achieve good welding, it is necessary that the surfaces pressed onto one another in the weld seam achieve the temperature of the crystallite melting point of the polyolefin used.
In addition to welding, heat-sealing of films has increased in importance. In this method, the film layers are laid one on top of the other and only warmed from 10 to 20° C. below the crystallite melting point, i.e. the films are not completely melted. The adhesion achieved is significantly less than in welding of the same material, but is sufficient for many applications (Kunststoff-Handbuch, Volume IV, Carl Hanser Verlag Munich, 1969, pages 623 to 640).
The literature discloses multilayer films which are provided with heat-sealable layers of random copolymers or terpolymers of propylene and other &agr;-olefins or corresponding polymer blends.
These heat-sealable layers enable the use of films on modern packaging machines. The operating speeds of these machines have been constantly increased in the past, meaning that the requirements of the film properties have also become greater and greater. In particular, the heat-sealing and hot-tack properties of the film are particularly crucial, since the strength of the seal seam must not be impaired for relatively short sealing times. The film must run well over the machine at high speeds and must not block or stick to the sealing jaws or other machine parts in the region of the seal zone.
Furthermore, packaging films should be readily printable and readily metallizable. Additional requirements arise from the new EC directives on foodstuff packaging, which relate to migration of oligomers or additives present in the raw material. Finally, there is a basic need for packaging films to be produced as cheaply as possible.
These multivarious requirements of heat-sealable materials have in the past resulted again and again in new improved heat-sealable raw materials, such as, for example, copolymers and terpolymers of ethylene and/or propylene.
For example, it is known that the minimum heat-sealing temperature can be reduced by increasing the comonomer content.
However, this is technically complex and expensive for a number of reasons.
1. With the increase in the comonomer content, in particular the C
2
content, the probability of incorporation of the ethylene in blocks increases. On the other hand, only the comonomers incorporated randomly predominantly contribute to the reduction in the minimum heat-sealing temperature.
2. High incorporated comonomer contents require a high comonomer supply in the polymerization reactor, which cannot be achieved inexpensively in all polymerization technologies.
3. Random copolymers having a high comonomer content, in particular a C
2
content, exhibit a strong tendency toward sticking due to low-molecular-weight components inevitably formed during the polymerization. These result in coating s formation in the polymerization reactor and must be removed in a second process step by solvent expansion. Both make the product more expensive.
DE-A-16 94 694 describes heat-sealable layered materials comprising a polypropylene base film with a heat-sealable thermoplastic layer strongly adhering thereto. This layer comprises a copolymer of from 98 to 94% by weight of propylene and from 2 to 6% by weight of ethylene. The film described has significantly better heat-sealing properties than polypropylene films or known heat-sealable coatings, such as PE, PVC or PVA. According to the examples, the minimum heat-sealing temperature of these heat-sealable layers is above 126° C.
EP-A-0 318 049 describes crystalline copolymers of propylene with ethylene and/or &agr;-olefins which contain from 2 to 10 mol % of ethylene and/or &agr;-olefins. The melting point of these copolymers is in the range from 110 to 140° C. The solubility in xylene is less than 10% by weight at 25° C., and the intrinsic viscosity in tetralin at 135° C. is greater than 0.2 dl/g. The copolymers have improved mechanical properties and improved welding properties and are therefore suitable for the production of films.
EP-A-0 484 817 describes a propylene film having outer layers of syndiotactic polypropylene. These films are said to have a balanced property profile, in particular good heat-sealing properties and good optical properties, but problems frequently occur during stretching of this material, for example caused by cracking in the syndiotactic outer layers or by delamination of the outer layers from the isotactic core layer. These problems are due firstly to the lower stretchability of the syndiotactic polypropylene and secondly to the lower compatibility of syndiotactic and isotactic polypropylene.
One of the most important objectives of the present invention is to avoid the disadvantages of the films previously described in the prior art. In particular, the objective is to provide a multilayer film which is distinguished by a combination of the following properties:
high gloss
low haze
a low minimum heat-sealing temperature
good seal seam strength
good hot-tack properties
low migration values
good corona resistance
low production costs.
SUMMARY OF THE INVENTION
According to the present invention, a highly useful combination of properties is achieved in a film, having a plurality of layers, in which an outer layer comprises a propylene-olefin copolymer prepared by means of a metallocene catalyst. The core or base layer is preferably a propylene polymer containing at least 90% propylene units and having a melt flow index of from 0.5 g/10 min. to 8 g/10 min., measured in accordance with DIN 53 735, and a melting point of from 150 to 170° C., The aforementioned outer layer is preferably an essentially random copolymer containing from 0.1 to 10% by weight, preferably 1 to 7% by weight, of an olefinic co-monomer of the formula RCH═CHR′, where R and R′ are H or C
1
-C
10
-alkyl or alkenyl (straight- or branched-chain), the total number of carbon atoms in the co-monomer typically being not more than 10. In a preferred embodiment, at least one of R and R′ is H and preferably both are H. The mean molecular weight, both weight average (M
w
) and number average (M
n
) of the essentially random copolymer is preferably at least 5·10
4
, and the molecular weight dispersity (“polydispersity” or M
w
/M
n
) is less than 7, preferably <5, optimally <3. Other preferred properties of the essentially random copolymer include the following:
DSC melting point:
100 to 146° C.
Melt flow index (MFI), measured
0.5 to 100 g/10 min
in accordance with DIN 53 735:
(230° C., load = 21.6 N)
Hexane-extractable content, measured
<3% by weight
in accordance with FDA 1771520:
Viscosity number (VN):
50 to 1000 cm
3
/g.
It is preferred that the essentially random copolymer possess as many of these properties as possible, at least two of the above-described properties being preferred, a typical combination of these properties being: a random monomer content within the stated range along with a melting point within the stated range and preferably also a melt flow index and/or molecular weight dispersity within the stated ranges.
Preferred films of this invention are multilayer (e.g. ≧3-layer) coextrudates in which an outer layer has a thickness of fr
Dries Thomas
Spaleck Walter
Winter Andreas
Basell Polyolefine GmbH
Connolly Bove & Lodge & Hutz LLP
Tarazano D. Lawrence
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