Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2003-06-25
2004-06-15
Woodward, Ana L. (Department: 1711)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S098000
Reexamination Certificate
active
06749912
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a polymer blend comprising a coherent phase made from an aliphatic and/or semiaromatic (co)polyamide and, dispersed therein, a phase made from an olefinic polymer. The present invention also relates to stretched flat or tubular films produced therefrom. The films are particularly suitable for packaging of food or drink materials, specifically as a synthetic casing for sausages.
2. Description of Related Art
Films based on polyamide are used widely as packaging materials for foods or consumable liquids. Such films generally have high mechanical strength (tensile strength and puncture resistance), as well as good dimensional stability when hot, thereby enabling heat-treatment of the food or drink materials while they are in a packaged condition (for example cooking or heat-sterilization). Once such films have absorbed sufficient moisture they generally have high mechanical flexibility and are therefore suitable for processing operations employed in automatic filling and packaging plants. They conform (or adhere) closely to the surface of the contents, and also provide a good barrier to oxygen and to flavors. In addition, such materials typically do not require use of plasticizers or other additives, thus dispensing with the inclusions constituents which are volatile or can migrate. Exemplary films can be produced by various methods, for example by film casting or film blowing, and each of these films may then also be mono- or biaxially oriented.
Nowadays, large quantities of polyamide (PA) are being processed using orientation processes, so to produce biaxially oriented tubular films. The orientation raises the strength of the film and improves its recovery performance. In addition, oriented films tend to shrink on exposure to heat as long as they have not been filly heat-set. Examples of such processes can be found, for example, in DE-C-28 50 182 and 28 50 181, and also in EP-A 325 151, each of which is incorporated herein by reference. In each case, biaxially oriented sausage casings based on aliphatic polyamides are described.
DE 28 50 181 and EP-A 325 151 describe casings made from a polyamide and from at least one other polymer. The other polymers disclosed in these two documents are an ionomer and/or a modified ethylene-vinyl acetate copolymer, (preferably a quaternary copolymer having units of ethylene, isobutylene, an aliphatically unsaturated carboxylic acid, and of an ester thereof as described in DE-A 28 50 181, or a combination of a polyalkylene terephthalate with a semiaromatic polyamide as described in EP-A 325 151).
The combination of aliphatic polyamide with one or more of such polymers can have an effect on the properties of the films produced therefrom. These properties include tear propagation resistance (which also affects the roll-back capabilities of sausage casings), elastic recovery performance, and the permeability of the films to oxygen and/or water vapor.
In the industrial processing of plastics, it is generally known that these types of polymers are generally added in relatively small proportions (up to about 20% by weight), since steady-state, (i.e. continuous, film-orientation) processes often place high requirements on the homogeneity of the material. Since the foreign polymers are incompatible with the polyamide at a molecular level, and therefore as the proportion of foreign polymer rises the content of disperse particles of foreign polymer in the polyamide phase become continues to increase as well. However, if the diameter of the disperse particles reaches an order of size comparable with the thickness of the film, the particles then penetrate the entire film and weaken its ability to tolerate mechanical stresses. Moreover, linking between the polyamide and the foreign polymer is restricted entirely to physical interactions. Thus the interfaces between these materials are areas of mechanical weakness. By their very nature, orientation processes induce high mechanical stress in the film and therefore require good mechanical coherence of the material. If this is not present, the film tears.
A characteristic property of all polyamides is (he marked dependence of their mechanical properties on moisture level. For example, anhydrous nylon-6 has a modulus of elasticity of about 3 GPa. The modulus of elasticity falls steeply under moist conditions and at 100% relative (ambient) humidity, reaches a value of only about 900 MPa. Other polyamides perform similarly. The “stiffness” or “flexibility” of polyamide is therefore highly dependent on ambient humidity.
WO 93/00404 (incorporated herein by reference) discloses a polyamide-based film whose properties are less markedly affected by moisture content. This result is achieved by blending the polyamide with a modified polyolefin. The modified polyolefin is prepared by reacting a crystalline or crystallizable homo- or copolymer made possessing and/or prepared using ±-olefin units having 2-6 carbon atoms, with unsaturated carboxylic acids or with anhydrides of the same. Examples of suitable polyolefins given are polyethylenes (specifically HDPE, LDPE, and LLDPE), polypropylene, and polybutylene. Examples of the unsaturated carboxylic acids or anhydrides disclosed are maleic acid, maleic anhydride, fumaric acid, crotonic acid, citraconic anhydride, or itaconic anhydride. The proportion of the modified polyolefin disclosed is from 5 to 50% by weight, based on the total weight of the polymer mixture. The modified polyolefin also may have been blended with an (unmodified) thermoplastic elastomer, as mentioned in Examples 2 and 3 of the above-mentioned WO specification. Examples of the thermoplastic elastomer given are an ethylene/propylene copolymer, an ethylene/1-butene copolymer, poly(1-butene), polybutadiene, a styrene/butadiene block copolymer, polyisoprene, and polyisobutylene. Films made from a mixture of polyamide with such a modified polyolefin are softer and more flexible than films made from pure polyamide. However, tubular films made from this material, are still not sufficiently elastic to be suitable as synthetic casings for sausages.
Flexibility is particularly important for packaging application involving moist food or consumable liquids and/or for packaging applications which may require heating of the packaging in water (vapor). The general desire is that at the moment of filling, the film should have high flexibility or low modulus of elasticity. In contrast, good dimensional stability of the film is desirable during any heating operation which follows. During the cooling process which then takes place, the contents shrink again, and as such, it is desirable that the film “responds”, (i.e. as nearly as possible, has perfect elastic recovery).
Naturally, the recovery force is identical with the force with which the film opposes further extension. That being said, the recovery stresses produced in a dry, stiff film are higher than those produced by a moist, flexible film.
Consequently, polyamide films in a dry state fresh from production are not very suitable for the applications mentioned above. That is, polyamide films lack flexibility during filling/packing and are capable of little elastic extension. Once thoroughly moistened by the contents, polyamide films then become soft, but their recovery potential, which is in any case weak, becomes still further reduced. The end result is a “slack” or wrinkled package, and/or a packed product whose appearance is not very attractive to the consumer.
In order to make polyamide films more flexible in moist applications, they are almost always softened (conditioned) in water prior to processing. However, this operation has a number of disadvantages. First, softening always carries an inherent risk to hygiene, i.e. there is a danger of contamination or microbial infestation of the film surface. Second, before the softening process can be conducted, which in practice takes at least 30 min, the total amount of film needed must be determined in advance. Once films have been
Delius Ulrich
Fritz Hans-Gerhard
Lue Runsheng
Connolly Bove & Lodge & Hutz LLP
Kalle GmbH & Co. KG
Woodward Ana L.
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