Heat-shrinkable laminate film and process for producing the same

Claim 2

A heat-shrinkable laminate film having heat resistance, oil resistance, gas-barrier property and a heat-shrinkability of not less than 15% at 95°C which comprises: (A) a core layer of a vinylidene chloride copolymer resin; (B) outer surface layers of a poly-a-olefin resin selected from the group consisting of a-olefin polymers having crystalline melting points of not lower than 110°C and mixtures each comprising not less than 20% by weight of one of said a-olefin polymers and not more than 80% by weight of an a-olefin polymer having a crystalline melting point of from 80° to 110°C; and (C) an intermediate layer of a thermoplastic resin having a crystalline melting point of from 70° to 100°C interposed between the core layer (A) and at least one of the surface layers (B), the sum of the thicknesses of the layers (B) being from 1/5 to 1/2 of the total thickness of the layers (B) and (C).

Claim 3

A laminate film as claimed in claim 1, wherein the layers (B) comprise an a-olefin polymer having a crystalline melting point of not lower than 110°C.

Claim 4

A laminate film as claimed in claim 1, wherein the layers (B) comprise a mixture of not less than 20% by weight of an a-olefin polymer having a crystalline melting point of not lower than 110°C and not more than 80% by weight of an a-olefin polymer having a crystalline-melting point of from 80° to 100°C.

Claim 5

A laminate film as claimed in claim 1, 2 or 3, wherein the a-olefin polymer having a crystalline melting point of not lower than 110°C is an ethylene resin.

Claim 6

` A laminate film as claimed in claim 4, wherein the a-olefin polymer having a crystalline melting point of not lower than 110°C is selected from the group consisting of polyethylenes having densities of not higher than 0.94 g/cc, copolymers of ethylene as the principal constituent and a-olefins and mixtures thereof.

Claim 7

A laminate film as claimed in any of claims 1 through 5, wherein the a-olefin polymer having a crystalline melting point of from 80° to 110°C is selected from the group consisting of copolymers of ethylene and vinyl acetate and alkyl acrylates and ionomer resins.

Claim 8

A laminate film as claimed-in any of claims 1 through 6, wherein the thermoplastic resin constituting the layer (C) is a resin having a crystalline melting point of from 70° to 100°C and is selected from the group consisting of ethylene-vinyl acetate copolymers, copolymers of ethylene and alkyl acrylates having.l to 6 carbon atoms in the alkyl group, isoprene-styrene block copolymers, 1,2-polybutadienes, ionomer resins, and mixtures thereof.

Claim 9

A laminate film as claimed in claim 7, wherein the thermoplastic resin constituting the layer (C) is an ethylene-vinyl acetate copolymer containing 5 to 20% by weight of vinyl acetate.

Claim 10

A laminate film as claimed in any of claims 1 through 8, wherein the thickness of said film is 20 to 100 µ.

Claim 11

A laminate film as claimed in any of claims 1 through 9, wherein said film is in tubular form.

Claim 12

ll. A process for producing a heat-shrinkable laminate film as claimed in any of claims 1 through 10 which comprises: laminating by extrusion (A) a core layer of a vinylidene chloride copolymer resin, (B) outer surface layers of a poly-a-olefin resin selected from the group consisting of a-olefin polymers having crystalline melting points of not lower than l10°C and mixtures each comprising not less than 20% by weight of one of said a-olefin polymers and not more than 80% by weight of an a-olefin polymer having a crystalline melting point of from 80° to 110°C, and (C) an intermediate layer of a thermoplastic resin having a crystalline melting point of from 70° to 100°C interposed between the core layer (A) and at least one of the surface layers (B),, so that the sum of the thicknesses of the layers (B) will be from 1/5 to 1/2 of the total thickness of the layers (B) and (C); quenching the laminate thus obtained; and then biaxially stretching the laminate at a temperature ranging from (T B H B + T C H C )/(H B + H C ) °C, wherein T B and T C are the crystalline melting points of the a-olefin polymer (B) and the thermoplastic resin (C) respectively expressed in °C and H B and H c are the thicknesses of the same polymer (B) and resin (C) respectively expressed in p, to a temperature 15°C lower than said temperature.

Claim 13

A process as claimed in claim 11, which comprises co-extruding the layers (A), (B) and (C) into concentric tubes thereby to produce heat-shrinkable laminate film of tubular form.

Claim 14

A process as claimed in claim 11 or 12, which comprises stretching the laminate film with a stretch ratio of at least two in each of the two directions in the biaxial stretching step.