Stock material or miscellaneous articles – Composite – Of polyamide
Reexamination Certificate
2001-04-10
2004-08-24
Seidleck, James J. (Department: 1711)
Stock material or miscellaneous articles
Composite
Of polyamide
C428S475800, C428S476100
Reexamination Certificate
active
06780522
ABSTRACT:
The present invention relates to a flexible single- or multi-layer film having at least one polyamide layer containing from 10 to 2000 ppm of a nano-disperse nucleating agent.
The film according to the invention is distinguished by good optical properties, high mechanical strength and good thermoformability. It can be produced particularly economically and reliably on conventional production installations, especially flat-film installations.
The film, as a single-layer film, may consist of only one PA layer or, as a multi-layer film, it may contain at least one PA layer nucleated and composed according to the invention.
Polyamide-containing films are widely used inter alia in the packaging of foodstuffs.
Advantages of the material polyamide are high mechanical strength, a good barrier against oxygen, carbon dioxide and other non-polar gases, and high temperature stability and scratch resistance. In addition, unstretched polyamide-containing films can be shaped thermally, that is to say deep awn into a shape suitable for accommodating goods placed therein. Films of polyamide are flexible and accordingly are able to adapt to the contours of the goods placed therein. That is important, for example, in the case of vacuum packaging.
Important properties of such films are a pleasing appearance, that is to say a high gloss and a low degree of cloudiness: in addition, high mechanical resistance, especially in the form of high resistance to damage by bending and folding, hereinafter referred to as bending strength, is required.
Polyamide is a semi-crystalline thermoplastic polymer. The polyamide structure that occurs in a film is dependent to a large degree on the processing conditions and on the composition of the polyamide. The slower the rate of cooling of the polyamide, the larger the crystalline structures that are able to form by means of crystallisation. Nucleation allows the rate of formation of nuclei in the crystallisation process to be increased and a more finely crystalline structure to be obtained.
Polyamide films may be produced by the flat-film or blown-film process. The flat-film process is in many cases preferred because it permits a markedly higher output per machine as compared with the blown-film process. Corresponding films can therefore generally be produced more economically.
However, thin polyamide films in particular require high casting roll temperatures for adequate web stability. A high degree of crystallinity thus occurs, which brings about the required strength. A disadvantage, however, is the cloudiness associated with the crystalline structure, as well as the relatively low gloss of the film.
For that reason, nucleated polyamides are used according to the prior art for the above-described applications. As compared with non-nucleated polyamide, such systems permit an improvement in the optical properties, the bending strength and the web stability of films produced therefrom as a result of the more rapid and more finely disperse crystallisation.
Application-related information from Bayer AG, as a well-known manufacturer of polyamide, summarises the relevant prior art in this field as follows [ATI KU 25304-9709 d,e/4332845, 1997, p. 3]:
“PA6 film products: [ . . . ] In order to take into account the increased demands of the market, it was necessary to develop tailored PA raw materials for film production. Accordingly, the following products inter alia have been developed in recent years: [ . . . ] Medium-viscosity PA 6 types having a relative solution viscosity in the range from 3.5 to 3.8, provided with processing and nucleating agents (crystal nucleus formers). The particular features of those products in comparison with non-nucleated, high-viscosity types are: [ . . . ]
Single-layer flat films in small thicknesses (from 15 &mgr;m) can be produced with greater web stability at higher takeoff speeds,
a higher degree of crystallinity with a particularly fine-grained and dense spherulite structure
better dimensional stability, less subsequent shrinkage as a result of crystallisation processes
less cloudiness at higher chill-roll temperatures [ . . . ]
higher bending strength [ . . . ]”
With the polyamides nucleated according to the prior art too, the production output in the case of thin flat films continues to be limited by inadequate strength. Likewise, corresponding films continue to have a high degree of cloudiness as well as a slightly matt appearance to their surface.
The use of conventional nucleating systems, especially in the form of dispersed finely divided inorganic solid particles, is state of the art. WO 8802763 mentions in this connection especially talcum, mica, kaolin and, less preferably, substances such as asbestos, aluminium, silicates, silver bromide, graphite, molybdenum disulfide, lithium fluoride, sodium phenylphosphinate, magnesium oxide, mercuric bromide, mercuric chloride, cadmium acetate, lead acetate, silver chloride, kieselguhr and the like. The mentioned systems are added in concentrations of from one thousandth of a percent to one percent, based on the total weight of the nucleated polymer.
The addition of solid particles having a size in the region of less than one micrometre to polymer matrices and, especially, polyamides has likewise been known for a relatively long time. Such systems are described in concentrations of approximately from 0.3 to 10 wt. %. Advantages that are achieved are higher rigidity owing to the reinforcing action of the fillers and, where the fillers used are plate-like in structure, also a higher oxygen barrier owing to extended diffusion paths through the polymer matrix.
A nucleating action of nano-scale fillers is not described.
EP 358415 discloses a film of a polyamide resin with layered silicate uniformly dispersed therein, wherein the individual layers of the layered silicate may have thicknesses of about 1 nm and lateral lengths of up to 1 &mgr;m The layers are present in the polyamide matrix separated by suitable opening and are at distances of about 10 nm from one another. Films produced using that material in concentrations of from 1.2 to 6.5 wt. % layered silicate are distinguished as compared with materials of pure polyamide 6 by a markedly increased oxygen barrier and rigidity. The frictional properties are improved. The transparency of single-layer amorphously quenched flat films as well as blown films with water cooling having the structure polyamide/adhesion promoter/PE-LD remains unchanged as compared with pure polyamide 6. By reference to given examples of PA6 films with a stepped content of layered silicate, the significant fall in bending strength and the increase in rigidity in the range up to 3.0 wt. % silicate becomes clear. Accordingly, such structures are generally not suitable for the demands of the present case.
WO 9304118, and WO 9311190 and WO 9304117 of the same Applicant, disclose polymer nano-composites likewise having plate-like particles with thicknesses in the region of a few nanometres, which are obtained not by introduction by polymerisation but by mechanical incorporation. There are described in particular composites of PA6 and montmorillonite and of PA6 and silicates having a filler content of from 0.27 to 9 wt. %. However, measurements carried out on rods of the corresponding material did not yield any increase in bending strength with a silicate content of 0.27%. Such materials can also be processed to films. A parallel orientation of the plate-like particles to the surface of the film is advantageous in this case. Applications as a single-layer film and also the possibility of producing multi-layer films are described. The films produced from that material may optionally be stretched in order to achieve even better orientation of the nano-particles. The main advantage of such films over those without nanoscale particles is a higher rigidity, which is always associated, however, with a markedly reduced stretchability. In view of the required high bending strength, therefore, such systems are generally a
Brandt Rainer
Eggers Holger
Eilers Bernd
Gasse Andreas
Kaschel Gregor
Bissett Melanie
Norris McLaughlin & Marcus PA
Seidleck James J.
Wolff Walsrode AG
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