Multilayer structure

1. A multilayer structure comprising:

(i) a first polyethylene layer as a first external layer, said first polyethylene layer being oriented in at least machine direction;

(ii) a second polyethylene layer as a second external layer;

(iii) a layer made of a copolymer of ethylene and vinyl alcohol (EVOH) between the first external layer and the second external layer; and

(iv) a tie layer on each side of the EVOH layer, characterised in that the tie layers comprise one or more copolymers of ethylene,

wherein the first polyethylene layer (i) is a multi-layer film,

wherein the first polyethylene layer (i) is oriented only in a machine direction and the second polyethylene layer (ii) is unoriented.

2. The multilayer structure according to claim 1 , wherein the tie layers comprise modified ethylene homo- or copolymers comprising ethylene units and units selected from the group consisting of organic acids, organic acid derivatives and mixtures thereof.

3. The multilayer structure according to claim 2 , wherein the organic acid and organic acid derivative is selected from the group consisting of acrylic acid, methacrylic acid, fumaric acid, maleic acid, nadic acid, citraconic acid, itaconic acid, crotonic acid, the anhydrides thereof, metal salts thereof, esters thereof, amides thereof, imides thereof, and the mixtures thereof.

4. The multilayer structure according to claim 1 , wherein the structure comprises from 95 to 100% by weight of the structure of homopolymers of ethylene, copolymers of ethylene and mixtures thereof.

5. The multilayer structure according to claim 4 , wherein the structure comprises from 98 to 100% by weight of the structure of homopolymers of ethylene, copolymers of ethylene and mixtures thereof.

6. The multilayer structure according to claim 1 , wherein the first polyethylene layer (i) is formed from at least three layers.

7. A process for producing a multilayer structure according to claim 1 , said process comprising the steps of: (I) providing a substrate, wherein the substrate is a polyethylene film which has been oriented in at least machine direction; (II) coextruding onto the substrate (II-1) a layer of a copolymer of ethylene and vinyl alcohol; (II-2) a second polyethylene layer; and (II-3) tie layers on both sides of the layer of the copolymer of ethylene and vinyl alcohol, wherein the layer of the copolymer of ethylene and vinyl alcohol is between the substrate and the second polyethylene layer, thereby producing the multilayer structure, wherein the substrate is a multilayer polyethylene film which has been oriented in at least machine direction.

8. The process according to claim 7 , further comprising the step of extruding the layer of the copolymer of ethylene and vinyl alcohol at a temperature within the range of from 200° C. to 240° C.

9. The process according to claim 7 , further comprising the step of extruding the tie layers at a temperature within the range of about 200° C. to about 280° C.

10. The process according to any one of claim 7 , further comprising the step of extruding the layer of polyethylene at a temperature within the range of from 275° C. to 330° C.

11. The process according to claim 7 , wherein the substrate is a multilayer polyethylene film which is formed from at least three layers.

12. A process for producing a multilayer structure according to claim 1 , said process comprising the steps of: (a) providing a first polyethylene film which first polyethylene film has been oriented in at least machine direction; (b) providing a second polyethylene film; (c) coextruding (c-1) a layer of a copolymer of ethylene and vinyl alcohol between the first polyethylene film and the second polyethylene film; and (c-2) tie layers on both sides of the layer of the copolymer of ethylene and vinyl alcohol; and

(d) pressing the first polyethylene film and the second polyethylene film together thereby producing the multilayer structure, wherein the first polyethylene film is a multilayered polyethylene film which has been oriented in at least machine direction.

13. The process according to claim 12 , further comprising the step of extruding the layer of the copolymer of ethylene and vinyl alcohol at a temperature within the range of from 200° C. to 240° C.

14. The process according to claim 12 , further comprising the step of extruding the tie layers at a temperature within the range of about 200° C. to about 280° C.

15. The process according to claim 12 , further comprising the step of extruding layers of polyethylene between the first polyethylene film and the tie layer, and the second polyethylene film and the tie layer.

16. The process according to claim 15 , further comprising the step of extruding the layers of polyethylene at a temperature within the range of from 275° C. to 330° C.

17. The process according to claim 12 , wherein the first polyethylene film is formed from at least three layers.

18. The multilayer structure of claim 1 , wherein the first polyethylene layer measured has a tensile modulus at room temperature of at least 1500 MPa.

19. The multilayer structure of claim 1 , wherein the multi-layer film consists of a layer A and a layer B,

wherein layer A is made of a high density polyethylene (HDPE), a medium density polyethylene (MDPE) or a linear low density polyethylene (LLDPE) or blends with high pressure low density polyethylene (LDPE) or with an ethylene based plastomer or a blend of an ethylene based plastomer with high pressure low density polyethylene (LDPE); and

wherein layer B is made of high density polyethylene (HDPE) having a density of 940 up to 970 kg/m3 or of a medium density polyethylene (MDPE) having a density of 925 up to 940 kg/m3 or a linear low density polyethylene (LLDPE) having a density of 910 up to 950 kg/m3 or blends thereof or with plastomers.

20. The multilayer structure of claim 19 , wherein the layer A is HDPE and the layer B is LLDPE.