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US-12617181-B2 - Multilayer structure, and packaging material for retort using same

US12617181B2US 12617181 B2US12617181 B2US 12617181B2US-12617181-B2

Abstract

There is provided a multilayer structure comprising at least a barrier resin layer (X) and an inorganic barrier layer (Y) with a thickness of 500 nm or less which is adjacent to the barrier resin layer (X), wherein the layer (X) is made of a resin composition (x) comprising an ethylene-vinyl alcohol copolymer (A) and a polyamide (B) in a mass ratio (A/B) of 55/45 to 98/2, and the ethylene-vinyl alcohol copolymer (A) has an ethylene content of 20 to 46 mol % and a saponification degree of 90 mol % or more. Thus, there can be provided a multilayer structure having excellent gas barrier properties and appearance even after being subjected to stretching process or retorting process followed by bending process, and a packaging material for retort therewith.

Inventors

  • Kentaro Yoshida

Assignees

  • KURARAY CO., LTD.

Dates

Publication Date
20260505
Application Date
20210422
Priority Date
20200424

Claims (18)

  1. 1 . A multilayer structure comprising at least a barrier resin layer (X) and an inorganic barrier layer (Y) with a thickness of 500 nm or less which is adjacent to the barrier resin layer (X), wherein the layer (X) comprises a resin composition (x) made from an ethylene-vinyl alcohol copolymer (A), a polyamide (B), and a multivalent metal ion (C), in which a mass ratio of A to B (A/B) is 55/45 to 98/2; the resin composition (x) comprises from 20 to 400 ppm of the multivalent metal ion (C); the multivalent metal ion (C) comprises at least one multivalent metal ion selected from the group consisting of magnesium, calcium and zinc; the multivalent metal ion (C) is contained as a hydroxide; and the ethylene-vinyl alcohol copolymer (A) has an ethylene content of 20 to 46 mol % and a saponification degree of 90 mol % or more.
  2. 2 . The multilayer structure according to claim 1 , wherein the ethylene-vinyl alcohol copolymer (A) is a modified ethylene-vinyl alcohol copolymer (A2) represented by Formula (I); content rates (mol %) of a, b and c to the total monomer units satisfy Formulas (1) to (3); and a saponification degree defined by Formula (4) is 90 mol % or more: wherein R 1 , R 2 , R 3 and R 4 independently of each other represent a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and the alkyl group can comprise a hydroxy group, an alkoxy group or a halogen atom; X, Y and Z independently of each other represent a hydrogen atom, a formyl group or an alkanoyl group having 2 to 10 carbon atoms: 20≤ a≤ 46 (1) 0.3≤ c≤ 10 (2) [100−( a+c )]×0.9≤ b≤[ 100−( a+c )] (3) Saponification degree=[(the total molar number of X,Y and Z as a hydrogen atom)/(the total molar number of X,Y and Z )]×100 (4).
  3. 3 . The multilayer structure according to claim 1 , wherein the multivalent metal ion (C) comprises magnesium.
  4. 4 . The multilayer structure according to claim 1 , wherein the barrier resin layer (X) is a monolayer film with a thickness of 8 to 40 μm.
  5. 5 . The multilayer structure according to claim 4 , wherein the monolayer film is biaxially oriented.
  6. 6 . The multilayer structure according to claim 1 , wherein the barrier resin layer (X) is one layer of a coextruded film consisting of 2 or more layers comprising the barrier resin layer (X); a thickness of the coextruded film is 8 to 120 μm; and a thickness of the barrier resin layer (X) is 0.5 to 20 μm.
  7. 7 . The multilayer structure according to claim 6 , wherein the coextruded film is biaxially oriented.
  8. 8 . The multilayer structure according to claim 6 , wherein layers other than the barrier resin layer (X) in the coextruded film consists of layers comprising a polypropylene resin as a main component.
  9. 9 . The multilayer structure according to claim 1 , wherein the multilayer structure does not comprise a layer comprising a resin having a melting point of lower than 140° C. as a main component, a layer comprising a resin having a melting point of 240° C. or higher as a main component, nor a metal layer with a thickness of 1 μm or more.
  10. 10 . The multilayer structure according to claim 1 , wherein a ratio of the total thickness of the layers comprising a polypropylene resin as a main component to the total thickness of the multilayer structure is 0.75 or more.
  11. 11 . The multilayer structure according to claim 1 , wherein the multivalent metal ion (C) comprises calcium.
  12. 12 . The multilayer structure according to claim 1 , wherein the resin composition (x) further comprises a phosphoric acid compound.
  13. 13 . A packaging material for retort comprising the multilayer structure according to claim 1 .
  14. 14 . The packaging material for retort according to claim 13 , wherein at least one barrier resin layer (X) is outer than the inorganic barrier layer (Y).
  15. 15 . The packaging material for retort according to claim 13 , wherein an oxygen transmission rate (under the conditions of 20° C. and 65% RH) after a retorting process at 125° C. for 60 min is less than 10 cc/(m 2 ·day·atm).
  16. 16 . The packaging material for retort according to claim 13 , wherein a light transmittance at a wavelength of 600 nm after a retorting process at 125° C. for 60 min is 80% or more.
  17. 17 . A recovered composition comprising a recovered material of the multilayer structure according to claim 1 .
  18. 18 . A method for recovering a multilayer structure comprising pulverizing the multilayer structure according to claim 1 followed by melt molding.

Description

TECHNICAL FIELD The present invention relates to a multilayer structure having at least a barrier resin layer made of a resin composition containing an ethylene-vinyl alcohol copolymer and a polyamide, and an inorganic barrier layer which is laminated in adjacent to the barrier resin layer; and a packaging material for a retort therewith; and a method for recovering the multilayer structure and a recovered composition comprising a recovered material of the multilayer structure. BACKGROUND ART Packaging materials for long-term storage of foods are often required to have gas barrier properties such as oxygen barrier ability. Oxidation of foods by oxygen and propagation of microorganisms can be suppressed by using a packaging material having higher gas barrier properties. As foods with a further extended edible period, retort foods have increased, which are prepared by filling a packaging material with foods and then sterilizing it by hot water under pressure (hereinafter, sometimes simply abbreviated as “retorting process”). A gas barrier layer for packaging materials for a retort is required to be resistant to hot-water treatment (hereinafter, sometimes simply abbreviated as “retort resistance”), and an aluminum foil or a gas barrier film in which a transparent vapor-deposited layer of silicon oxide or aluminum oxide is laminated on a highly heat-resistant polyester film is commonly used. When an aluminum foil is used, not only gas barrier properties but also shading performance can be imparted, and when a gas barrier film in which a vapor-deposited layer of silicon oxide or aluminum oxide is laminated is used, visibility of a content can be imparted (Patent References No. 1 and 2). An ethylene-vinyl alcohol copolymer (hereinafter, sometimes abbreviated as “EVOH”) which is widely used as a gas barrier resin in a packaging material, is crystallized and densified by hydrogen bonding between hydroxy groups in the molecule, exhibiting gas barrier properties. Furthermore, a resin composition prepared by blending EVOH and a polyamide (hereinafter, sometimes abbreviated as “PA”) has higher retort resistance than EVOH as a simple substance, so that it has been preferably used as a packaging material for retort (Patent Reference No. 3). Furthermore, it has been reported that a resin composition prepared by blending a particular modified EVOH and a polyamide has excellent retort resistance and bending resistance (Patent Reference No. 4). In recent years, demand for so-called post-consumer recycling (hereinafter, sometimes simply abbreviated as “recycling”) in which packaging materials consumed in the market are recovered and recycled, has been globally increased due to environmental problems and waste problems. Recycling is generally conducted by a process comprising cutting a recovered packaging material and, after, if necessary, sorting and washing, melt-mixing it using an extruder. PRIOR ART REFERENCES Patent References Patent Reference No. 1: JP 2002-331578 APatent Reference No. 2: JP 2002-308285 APatent Reference No. 3: WO 2015/174396 A1Patent Reference No. 4: JP 2015-151428 A SUMMARY OF THE INVENTION Problems to be Solved by the Invention However, while a gas barrier films with a laminated transparent vapor-deposited layer can maintain excellent gas barrier properties even after retorting process, there is concern that the gas barrier properties may be easily deteriorated due to stretching or bending processes, causing fluctuations in quality of a food to be packaged. In addition, although EVOH exhibits higher gas barrier properties in a dry state, the gas barrier properties are deteriorated due to relaxation of hydrogen bonding when the material absorbs moisture, such as after retorting process, and it may limit extension of an edible period of a retort food. Meanwhile, aluminum foils or polyester films which are widely used in conventional packaging material for retort are difficult to uniformly mix with other components in a melt-mixing process during recovery and recycling, which has been an obstacle to their recycling. In view of these circumstances, a first object of the present invention is to provide a multilayer structure having excellent gas barrier properties and appearance even after stretching or retorting followed by bending processes, and packaging material for retort therewith. A second object of the present invention is to provide a multilayer structure further having excellent recyclability in addition to the above properties, and a method for recovering the same. Means for Solving the Problems The inventors have found that a multilayer structure having a structure where a resin layer made of a resin composition containing a particular EVOH and PA and an inorganic layer are laminated by a method represented by vapor deposition such that these layers are adjacent to each other has excellent gas barrier properties and appearance even after stretching process or retorting process followed by bendi