Stock material or miscellaneous articles – Composite – Of polyamide
Reexamination Certificate
2001-04-09
2004-05-25
Sergent, Rabon (Department: 1711)
Stock material or miscellaneous articles
Composite
Of polyamide
C428S476100, C428S349000, C428S347000
Reexamination Certificate
active
06740422
ABSTRACT:
The present invention relates to a flexible, multilayer film having an outer layer composed essentially of polyamide 6 that contains between 0.1 and 3.0% of a nanodisperse nucleating agent, and at least one further polyamide. The multilayer film according to the invention is notable for good optical properties accompanied by high strength and good slip. The multilayer film can be produced without difficulty, in particular on blown-film plants. The multilayer film can be processed without difficulty using a suitable heat-sealable coating on modern moulding, filling and sealing packaging machines at high packing speeds. The packages produced are optically attractive and resistant to mechanical stress. The invention also comprises the use of the multilayer film according to the invention as a container, in particular for the packaging of foodstuffs, and also the use as thermoformed film. In the context of the present invention, a nanodisperse filler is understood in this connection as meaning a filler whose smallest particles that form a rigid unit in the dispersion have a dimension of not more than 100 nm as a number-weighted average of all the particles in at least one direction that can be chosen arbitrarily for each particle.
Polyamide-based films are by far predominantly used to package perishable foodstuffs. The structure of suitable films is set out, for example, in The Wiley Encyclopedia of Packaging Technology (ed. M. Bakker and D. Eckroth; John Wiley & Sons, 1986) and also in Kunststoff-Folien by Joachim Nentwig (Carl Hanser Verlag 1994, Munich).
As barrier layer material in flexible multilayer films or composite films, usually in combination with low-density polyethylene (LDPE), polyamides have acquired increasing importance for packaging perishable foodstuffs over many years. In this field, polyamide 6 (PA 6) has acquired the greatest share. The annual consumption of PA 6 for this market is currently around 35,000 tonnes in Europe and about 85,000 tonnes world-wide; in addition, a few thousand tonnes of copolyamides are used.
By far the largest quantity of said multilayer films is produced using PA 6 types in, various flat-film processes. For blown-film composites, special copolyamides are usually used.
Copolyamides are almost exclusively processed by the blown-film coextrusion process. Their solidification rate or crystallization rate during the cooling phase is markedly slower than pure PA 6 products. As a result, despite the relatively low cooling rate of the molten tube through air cooling, it is possible to produce PA/PE composite films having a high transparency. The copolyamides that are conventional in this connection are nowadays also partly used with various modifications, such as nucleation agents and processing aids, for example an asymmetrical PE-X-PA blown coextrusion film (X=coupling agent) for the PA outer layer to improve free running in machines and dimensional stability. Small losses in transparency due to the crystallizing action of the nucleating agents are, as a rule, accepted because of the other advantages.
In many cases, films are necessary that have been produced as blown film. An advantage of a blown film is the better uniformity of the film roll compared with flat films as the result of a changing take-up. As a result, the blown film is given a flatness that is markedly superior to a flat film. This is advantageous for fast machine running.
According to the prior art, an outer polyamide layer is used because of the outstanding thermal stability and abrasion resistance.
The disadvantages that increase as a result of the use of copolyamides reside in the post-crystallization of the film. Because of the low crystallization rate of these systems, the crystallization is still not complete with the actual forming. On the contrary, subsequent to the actual production of the film, a post-crystallization takes place over a fairly long time interval, as a result of which, due to the dimensional change in the polyamide-containing film accompanying the post-crystallization, the multilayer film may curl up and/or may shrink considerably, resulting in flatness deficiencies on the roll, and is consequently no longer suitable for further processing.
In addition, because of the lower crystallinity compared with PA 6, copolyamides have the disadvantage of a markedly poorer surface slip. This is relevant in the case of the outer polyamide.
Not least of all, the production of the copolyamides even requires the use of mores expensive raw materials and polymerization procedures than the production of PA 6. In this respect, the use of copolyamides results in an even greater consumption of resources.
In this respect, there is a need, not covered at present, for blown films having a polyamide outer layer that have both good optical properties and good machine free running in the context of good surface slip. The best existing solution in the prior art is to use nucleated copolyamide. However, compared with the use of polyamide 6, this solution has a marked disadvantage because of the more exotic and expensive copolyamide raw material.
A further important property is a high mechanical resistance. This relates, in particular, to a high resistance to destruction as a result of buckling and bending, referred to below as loop strength, and also a high resistance to puncture by sharp objects, referred to below as puncture strength.
Nucleating agents, such as the systems mentioned above for use in copolyamides, have been prior art for a fairly long time. They also serve as crystallization nuclei during slight supercooling of the melt around which spherulitic structures form during the solidification process. Depending on dispersion and effectiveness, such nucleating agents achieve, in this way, the development of a crystal structure containing more and finer spherulites than in the unnucleated polymer, even in the case of slow cooling from the melt, as in the case of blown-film production. Post-shrinking as a result of post-crystallization can thereby be minimized. The loading of the moulding composition with nucleating agents is, however, restricted by the light-scattering properties of the nucleating agents themselves.
The addition of very fine-grain solid particles in the size range below one micrometer to polymeric matrices and especially polyamides has likewise been described for a fairly long time. Such systems are primarily used to increase the mechanical rigidity, the barrier to gases and the heat resistance, and also to reduce the cycle time, for instance, during injection moulding, the flammability or the moisture absorption in the case of hydrophilic systems. Systems that, in contrast to the abovementioned nucleated polyamides, retain their transparency despite greater additions of nanoscale particles have also been described.
EP-A-358415 describes a polyamide resin moulding composition containing sheet silicate uniformly dispersed in it, in which the individual layers of the sheet silicate may have thicknesses around 1 nm and side lengths of up to 1 &mgr;m. The layers are separated in the polyamide matrix by suitable breakdown and are at distances from one another of around 10 nm. Mouldings, such as, for example, films, produced with this material composed of polyamide 6 as base polymer are notable, compared with those of pure polyamide 6, for a significantly increased oxygen barrier and rigidity. To an equal extent, however, the loop strength decreases markedly. The transparency of single-layer amorphously quenched flat films and also water-cooled blown films having the structure polyamide mixture//coupling agent//LDPE remains unaltered compared with purepolyamide 6. Consequently, the structures used do not achieve the necessary property profile for the present application.
WO 93 04118 and WO 93 11190 and WO 93 04117 disclose polymer nanocomposites containing likewise platelet-shaped particles in the thickness range of a few nanometers. In particular, composites are described that are composed of PA 6 and montmorillonite or of PA 6 and silicates. Thes
Eggers Holger
Kaschel Gregor
Bissett Melanie
Norris & McLaughlin & Marcus
Sergent Rabon
Wolff Walsrode AG
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