Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
1996-01-29
2001-12-25
Michl, Paul R. (Department: 1714)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S034200, C428S460000, C428S480000
Reexamination Certificate
active
06333086
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to multilayered polymer film structures for use as a barrier layer in packages. Such film structures comprise at least one first polymer layer having low permeability to moisture and, attached to the first layer, a second polymer layer having low permeability to gases, said first film comprising a isotropic polymer and said second film containing a liquid crystalline polymer.
The invention also concerns a process for preparing multilayered polymer films based on combinations of isotropic polymer layers and liquid crystalline polymer layers. Furthermore, the invention relates to laminates, comprising a substrate and at least one multilayered polymer structure coated on the surface of said substrate, wherein the polymer layer acts as a barrier to the transport of gases and moisture through the laminate.
BACKGROUND OF THE INVENTION
In food container laminates, aluminum foils have traditionally been used as sealing layers to protect the foodstuff from deterioration caused by contact with oxygen and moisture. Nowadays, the aluminum foils are to an increasing extent being replaced by various polymer films, which have good barrier properties, but which are more easily degradable in nature than aluminum. Since no single polymer can provide the same resistance to gas and water penetration as aluminum, the polymer-based laminates typically comprise multilayered polymer structures. Often, these structures incorporate thermoplastic polymers, such as polyolefins, which are heat sealable and thus make it possible to manufacture continuous structures. The polyolefins themselves are known to have excellent moisture and water vapor resistance, but they suffer from poor gas barrier properties. Therefore, the multilayered polymer structures usually include a specific gas barrier material, such as ethylene/vinyl alcohol (EVOH), a conventional polyethylene-EVOH-based multi-layer product having the following structure: PE/adhesive polymer/EVOH/adhesive polymer/PE).
It is known that thermotropic liquid crystalline polymers (LCP), have excellent barrier properties. However, the processing of them is difficult due to anisotropic behavior. Because the price of the liquid crystalline polymers is also rather high, LCP's have not been used as barrier materials in practical applications.
However, some LCP-based barrier structures are described in the prior art. Thus, EP Patent Application No. 0 503 063 discloses a composite film comprising a liquid-crystal polymer layer containing a thermotropic liquid crystal polymer and a thermoplastic polymer layer laminated on at least one surface of the liquid crystal polymer layer. The polymer layer is fixed on the liquid crystalline polymer through an adhesive layer. The use of such a layer can be avoided by functionalizing the thermoplastic in order to ensure proper attachment to the LCP film.
Similar structures are also described in JP Published Patent Applications Nos. 2 220 821, 2 253 949-2 253 951, and 2 261 456.
U.S. Pat. No. 5,084,352 describes a multilayered barrier film product, which includes a first polymer having low permeability to moisture and a heterogeneous polymer blend film containing a gas barrier polymer. The heterogeneous film comprises a barrier polymer, such as EVOH, and a second polymer, distributed within the barrier polymer. The second polymer consists of a polyolefin which is functionalized, so as to allow the heterogeneous polymer film to adhere to the first polymer film having low permeability to moisture. As an example of alternative barrier polymers, U.S. Pat. No. 5,084,352 also mentions thermotropic liquid crystal polymers. These polymers are, however, not suggested for use in packaging applications, nor are there any examples given on the actual use of a LC polymer in the described multilayered film product.
The main problem associated with the known barrier structures based on liquid crystalline polymers as a barrier to oxygen penetration resides in the high price of the LC polymer and the difficult processing of the LCP layer. Since only homogeneous LC layers have so far been used, the amount of the liquid crystalline polymer in relation to the other components of the polymer structure becomes rather high, which increases the total costs of the barrier structure. Furthermore, homogeneous LCP layers are brittle and difficult to process with traditional lamination techniques.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to eliminate the above problems and to provide a multilayered polymer structure essentially comprising at least a first layer comprising a thermoplastic polymer and a second layer, attached to the first layer, comprising a liquid crystalline polymer blended with the polymer forming the first layer.
It is another object of the present invention to provide a process for preparing multilayered LCP-containing barrier structures. A third object of the present invention is to provide laminates containing multilayered LCP/thermoplastic-film structures.
The present invention is based on the finding that the barrier properties of anisotropic liquid crystalline polymers are not to any significant degree impaired by blending them with a thermotropic polymer provided that the liquid crystalline polymer forms a continuous layer. Therefore, it is required that the second layer contain about 60 to 99% by volume of an LCP, 40 to 1% by volume of a thermotropic (isotropic) polymer, and 0 to 10% by volume of a compatibilizer. In a layer comprising a blend of at least one anisotropic rigid-rod liquid crystalline polymer and at least one flexible isotropic polymer, the former acts as a barrier material against gas penetration because of the rigid linear molecules, the packing density of which is high in solid state. The latter provides resistance to the penetration of water vapor. The isotropic polymer preferably comprises a thermoplastic polymer which enhances attachment to an adjacent thermoplastic layer. The isotropic polymer can be functionalized in order to improve attachment.
The oxygen transmission rate of the second layer is typically less than about 150 cm
3
/(m
2
d bar), determined according to the standard ASTM D 3985-81, and the water vapour transmission rate of the integral film structure is less than 10 g/(m
2
24 h) at RH 80% 23° C., determined according to ASTM F 1249-90.
The prerequisite for proper formation of a continuous liquid crystalline phase is that the blend is melt processed at laminar flow conditions so as to obtain a homogeneous structure. Therefore, the process of preparing the above defined monolayer polymer materials comprises the steps of providing at least one first polymer comprising a isotropic polymer and processing the first polymer into a film, providing a polymer blend containing about 60 to 99 parts by volume of an anisotropic liquid polymer, 1 to 40 parts by volume of an isotropic thermoplastic polymer, and 0 to 10 parts by volume of a compatibilizer which improves the interaction between the anisotropic and the isotropic polymers of the blend, melt processing the polymer blend at a ratio of the viscosity of the anisotropic polymer to the viscosity of the isotropic polymer [&lgr;=&eegr;
anisotropic polymer
/&eegr;
isotropic polymer
] which is in the range of about 0.5 to 5 in order to produce a polymer compound, and processing the polymer compound into a polymer film, and attaching the two films together to form a multilayered structure. Depending on the specific film forming techniques used, the formation of the polymer films and the formation of the multilayered structure may be carried out simultaneously or sequentially, preferably the coextrusion technique is employed.
The laminate suited for use, for instance, in food containers is comprised of a substrate, and at least one polymer layer coated on a surface of said substrate to act as a barrier to transport of oxygen and water vapour through the laminate. The polymer layer is comprised of a multilayered polymer structure having at least
Harkonen Mika
Kuusela Seppo
Laiho Erkki
Ora Marja
Suokas Esa
Cohen & Pontani, Lieberman & Pavane
Michl Paul R.
Optatech Corporation
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