Multilayered structure

Details Multilayered structure Multilayered structure

2000-06-09

2003-01-07

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Hollow or container type article

Polymer or resin containing

C428S036600, C428S215000, C428S216000, C428S516000, C428S520000, C428S910000

Reexamination Certificate

active

06503588

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multilayered structure where a resin layer made of an ethylene-vinyl alcohol copolymer and a polypropylene resin layer are laminated via an adhesive resin layer. In particular, the present invention relates to a multilayered structure that has excellent gas barrier properties, thermoformability and mechanical properties and is useful in a variety of fields, for example as a gas barrier film and a food container.
2. Description of the Related Art
In general, a polypropylene resin (hereinafter, referred to also as PP) can be easily processed, for example easily stretched, and a molded product thereof has excellent mechanical properties and a good appearance. Therefore, polypropylene resins are used in a wide range of fields including food packaging. However, one drawback of PP is its poor gas barrier properties.
On the other hand, ethylene-vinyl alcohol copolymer (hereinafter, referred to also as EVOH) is characterized by its excellent gas barrier properties to oxygen, carbon dioxide and nitrogen, transparency and oil resistance. Therefore, for example, a film made of EVOH is used as a material for packaging a product whose quality should be preserved, such as food and medicine. However, EVOH is disadvantageous in that the gas barrier properties are influenced significantly by humidity and become low under high humidity, and its impact strength is poor.
In this context, in order to compensate for the drawback of the low gas barrier properties of the PP and the drawbacks of the poor humidity resistance and the poor impact resistance of the EVOH, a multilayered film is produced where a polyolefin that has excellent humidity resistance and impact resistance, for example a PP, and an EVOH that has excellent gas barrier properties are laminated. Furthermore, a container obtained by thermoforming a multilayered structure where polypropylene and EVOH are laminated is used as a food container or the like.
However, the thermal processability of the EVOH generally is significantly low, compared with that of a polypropylene resin. For example, in the production of an oriented multilayered film by stretching a multilayered structure comprising an EVOH and a PP, the stretchability of the EVOH is significantly poor, compared with that of the PP. In the production of a container with a multilayered film by thermoforming, cracks, pinholes or local thickness deviations are generated in the EVOH layer, so that the gas barrier properties of the obtained container may be poor. Furthermore, cracks or wave patterns may be generated on the side face of the container, and this may prevent a thermoformed container from having a good appearance.
In the production of the oriented multilayered film, various methods are under development to provide the EVOH with sufficient stretchability.
For example, Japanese Patent No. 2679823 U.S. Pat. No. 4,880,706) discloses a biaxially oriented film formed of a multilayered structure of PP layer/adhesive layer/EVOH layer/adhesive layer/PP layer as a multilayered barrier film employing EVOH and polyolefin. This film is characterized by its composition and its production method. As the EVOH used for this film, an EVOH having an ethylene content of at least 45mol %, a hydrolysis degree of at least 99%, and an melt flow rate (hereinafter, may be referred to as MFR) of 14 to 18 g/10 min at 190° C. under a load of 2,160 g is preferable. In addition, as the polyolefin used for this film, a PP having an MFR of 2 to 6 g/10 min is disclosed in this patent. This film is obtained by producing a multilayered structure (multilayered coextruded film) of EVOH and polyolefin, immediately cooling the multilayered structure rapidly to around 50° C., preheating it in the range from 135 to 150° C., stretching it to 4 to 7 times larger in the longitudinal direction, preheating it in the range from 170 to 180° C., and stretching it to 7 to 10 times larger in the transverse direction in the range from 155 to 165° C.
Furthermore, for example, Japanese Laid-Open Patent Publication No. 60-187538 (U.S. Pat. No. 4,561,920) describes that it is preferable to use an EVOH having a MFR of at least about 8 g/10 min for a laminated film of EVOH and polyolefin. In this publication, the multilayered structure comprising the EVOH is cooled rapidly to prevent the crystallinity of the EVOH from exceeding about 25%.
All of the above publications use an EVOH having a MFR in a specific range or an EVOH having a high ethylene content in order to provide the EVOH with sufficient stretchability. These laminated films using such an EVOH do not necessarily have high gas barrier properties.
In the case of the production of a container with a multilayered film by thermoforming, various methods have been proposed to improve the thermoformability or the impact resistance, including incorporation of nylon into EVOH (U.S. Pat. No. 4,079,850). However, there are still problems to be solved, such as unsatisfactory thermoformability, reduced gas barrier properties, unsatisfactory thermal stability during film production, and possibly, low transparency, depending on the type of resin or the state of dispersion of the nylon in EVOH. Thus, sufficient improvements have not been fulfilled yet.
A recent tendency for a wider range of applications of a thermoformed container requires the formation of a complex shape having a deep drawing depth. In the production of a container with a multilayered film of the EVOH and PP by thermoforming, it is necessary for the EVOH to have sufficient moldability, especially in the thermoforming of a complex shaped container having a deep drawing depth. In this case, in order to provide the EVOH with sufficient moldability, an EVOH having a high ethylene content or a low saponification degree has to be used.
However, the gas barrier properties of the EVOH are such that the higher ethylene content or the lower saponification degree results in the poorer gas barrier properties. Thus, a multilayered structure for producing a container having a deep drawing depth, a complex shape and sufficient gas barrier properties has not been obtained yet.
As described above, the EVOH has poor moldability, and therefore providing the EVOH with sufficient moldability, namely, increasing the ethylene content in the EVOH, is under examination. However, it is well-known that the smaller the ethylene content is, the better the gas barrier properties of the EVOH are, except for under extremely high humidity conditions such as 100%RH. Thus, at present, an EVOH-PP multilayered structure that has sufficient moldability and that can provide a molded product such as a film or a container with sufficient gas barrier properties even under high humidity has not been obtained yet. Investigation regarding the PP to be laminated with the EVOH is also insufficient.
SUMMARY OF THE INVENTION
The present invention includes a multilayered structure where an A layer made of an ethylene-vinyl alcohol copolymer resin (A) and a C layer made of a polypropylene resin (C) are laminated via a B layer made of an adhesive resin (B); wherein the ethylene content of the ethylene-vinyl alcohol copolymer resin (A) is 20 to 60 mol %, and the saponification degree of the resin (A) is at least 90%; the melt flow rate (MFR) of the polypropylene resin (C) at 230° C. under a load of 2160 g is in a range from 0.1 to 100 g/10 min; and the stereoregularity index [M5] of the polypropylene resin (C) defined by Equation 1, with absorption intensities of Pmmmm and Pw in the
13
C-NMR spectrum of a boiling heptane insoluble component of the polypropylene resin (C), is at least 0.950,
[M
5]=[
Pmmmm]/[Pw]
  (1)
wherein [Pmmmm] is the absorption intensity derived from methyl groups of third propylene units in the polypropylene resin (C), each of the third propylene units being the third unit of five consecutive propylene units that are isotactically bonded, and [Pw] is the absorption intensity derived f

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