Rotomoulded structural parts having asymmetrical layering

1. A method for preparing a rotomoulded article having asymmetrical layering that comprises the steps of:

a) providing a mould for rotomoulding comprising 2 or more portions insulated from one another;

b) thermally insulating one or more portions of the mould, and leaving remaining one or more portions of the mould not thermally insulated, wherein thermally insulating the one or more portions of the mould is achieved by lining the one or more portions of the mould with a thermally insulating sheet that separates the one or more portions of the mould that are thermally insulated from the one or more portions of the mould that are not thermally insulated;

c) heating the one or more portions of the mould that are not thermally insulated;

d) injecting a first polymeric material in the mould and submitting the mould to rotation about two perpendicular axes, said first polymeric material constituting an outer layer of a finished rotomoulded article for the one or more portions of the mould that are not thermally insulated, and wherein said first polymeric material is a first composition comprising:

from 40 to 100 wt %, based on a weight of the first composition, of homopolymers or copolymers of ethylene and/or propylene, and/or polyamide, and/or functional fluoropolymer or a mixture thereof, and

from 0 to 60 wt % based on the weight of the first composition of a functionalised polyolefin (FPO) or of a polyolefin (PO) grafted to a non-polyolefin (NPO) in the form of a block copolymer ((PO)g-NPO) or of an ionomer or a combination thereof;

e) opening the mould and removing the thermally insulating sheet;

f) injecting a second polymeric material in the mould and submitting the mould to rotation about two perpendicular axes, wherein the second polymeric material is different from the first polymeric material of step d), wherein said second polymeric material constitutes an outer layer of the finished rotomoulded article for the one or more portions of the mould that were thermally insulated during the injection of the first polymeric material and said second polymeric material is a second composition comprising:

from 70 to 99 wt %, based on the weight of said second polymeric material, of a polyethylene resin; and

from 1 to 30 wt % of an ionomer, or from 1 to 30 wt % of a combination of an ionomer and a grafted polyolefin;

g) optionally repeating steps b) through f) or step f) alone; and

h) retrieving the rotomoulded article having asymmetrical layering.

2. The method of claim 1 wherein the first polymeric material is polyamide.

3. The method of claim 1 wherein the second polymeric material comprises from 70 to 99 wt %, based on the weight of said second polymeric material, of the polyethylene resin and from 1 to 30 wt % of the combination of ionomer and the grafted polyolefin.

4. The method of claim 1 , wherein the second polymeric material comprises a blend of from 40 to 60 weight percent of a first polyethylene and from 40 to 60 weight percent of a second polyethylene.

5. The method of claim 4 , wherein the first polyethylene has a melt index of at least 0.5 dg/min and at most 25 dg/min and a density of at least 0.920 g/cc and at most 0.965 g/cc; and

wherein the second polyethylene has a melt index of at least 1 dg/min and at most 30 dg/min and a density of at least 0.930 g/cc and at most 0.965 g/cc.

6. The method of claim 1 , wherein the polyethylene of the second polymeric material has a polydispersity of from 2 to 20.

7. The method of claim 1 , wherein the first polymeric material comprises polypropylene having a melt index of from 1.5 to 100 g/10 min determined in accordance with ASTM D 1238 at a temperature of 230° C. and a load of 2.16 kg.

8. The method of claim 1 , further comprising, prior to retrieving the rotomoulded article having asymmetrical layering, injecting a third composition comprising a polyethylene and a chemical blowing agent and submitting the mould to rotation about two perpendicular axes, wherein the retrieved rotomoulded article having asymmetrical layering has a foamed polyethylene layer as an innermost layer of the rotomoulded article.

9. The method of claim 1 , wherein the second composition has a melt index of from 0.5 to 30 dg/min determined in accordance with ASTM D 1238 at a temperature of 190° C. and a load of 2.16 kg.

10. The method of claim 1 , wherein the second composition has a density of from 0.920 to 0.965 g/cc determined in accordance with ASTM D 1505 at 23° C.

11. The method of claim 1 , wherein the first polymeric material is polyamide, and wherein the second polymeric material is a mixture of polyethylene, grafted polyethylene and ionomer.

12. The method of claim 11 , wherein the rotomoulded article is a car door having an outside part prepared from the outer layer, wherein the polyamide of the outer layer is painted, and wherein car door has an inside part prepared from the mixture of polyethylene, grafted polyethylene and ionomer.

13. A method for preparing a rotomoulded article having asymmetrical layering comprising:

heating one or more portions of a mould that are not thermally insulated, wherein one or more portions of the mould are thermally insulated;

injecting a first polymeric material in the mould and submitting the mould to rotation about two perpendicular axes, wherein said first polymeric material forms an outer layer of the rotomoulded article for the one or more portions of the mould that are not thermally insulated, and wherein said first polymeric material comprises:

from 40 to 100 wt % of homopolymers or copolymers of ethylene and/or propylene, and/or polyamide, and/or functional fluoropolymer or a mixture thereof; and

from 0 to 60 wt % of a functionalised polyolefin (FPO) or of a polyolefin (PO) grafted to a non-polyolefin (NPO) in the form of a block copolymer ((PO)g-NPO) or of an ionomer or a combination thereof;

removing the thermal insulation;

injecting a second polymeric material in the mould and submitting the mould to rotation about two perpendicular axes, wherein the second polymeric material is different from the first polymeric material, wherein said second polymeric material forms an outer layer of the rotomoulded article for the one or more portions of the mould that were thermally insulated during the injection of the first polymeric material, and wherein said second polymeric material comprises:

from 70 to 99 wt %, based on the weight of said second polymeric material, of a polyethylene resin; and

from 1 to 30 wt % of an ionomer, or from 1 to 30 wt % of a combination of an ionomer and a grafted polyolefin;

retrieving the rotomoulded article having asymmetrical layering;

wherein the mould is heated through ducts designed to be run through by heat-regulating fluid, said ducts being realised by grooves on external surfaces of the mould and wherein the one or more portions of the mould that are not thermally insulated are heated whereas the one or more portions of the mould that are thermally insulated are cooled down.

14. The method of claim 13 , wherein the first polymeric material is polyamide, and wherein the second polymeric material is a mixture of polyethylene, grafted polyethylene and ionomer.

15. A method for preparing a rotomoulded article having asymmetrical layering comprising:

heating portions of a mould that are not thermally insulated, wherein one or more portions of the mould are thermally insulated;

injecting a first polymeric material in the mould and submitting the mould to rotation about two perpendicular axes, wherein said first polymeric material forms an outer layer of the rotomoulded article for the portions of the mould that are not thermally insulated, and wherein said first polymeric material comprises:

from 40 to 100 wt % of homopolymers or copolymers of ethylene and/or propylene, and/or polyamide, and/or functional fluoropolymer or a mixture thereof; and

from 0 to 60 wt % of a functionalised polyolefin (FPO) or of a polyolefin (PO) grafted to a non-polyolefin (NPO) in the form of a block copolymer ((PO)g-NPO) or of an ionomer or a combination thereof;

injecting a second polymeric material in the mould and submitting the mould to rotation about two perpendicular axes, wherein the second polymeric material is different from the first polymeric material, wherein said second polymeric material forms an outer layer of the rotomoulded article for the portion of the mould that was thermally insulated during the injection of the first polymeric material, and wherein said second polymeric material comprises:

from 70 to 99 wt %, based on the weight of said second polymeric material, of a polyethylene resin; and

from 1 to 30 wt % of an ionomer, or from 1 to 30 wt % of a combination of an ionomer and a grafted polyolefin;

retrieving the rotomoulded article having asymmetrical layering;

wherein thermally insulating the one or more portions of the mould is achieved by jacketing one or more portions of the mould and wherein the thermal insulation is removed after injection of the first polymeric material, or wherein thermally insulating the one or more portions of the mould is achieved by inserting one or more physical barriers into the mould that are molten after injection of the first polymeric material.

16. The method of claim 15 , wherein thermally insulating one or more portions of the mould is achieved by jacketing one or more portions of the mould wherein the thermal insulation is removed after injection of the first polymeric material.

17. The method of claim 15 , wherein thermally insulating one or more portions of the mould is achieved by inserting one or more physical barriers into the mould that are molten after injection of the first polymeric material.

18. The method of claim 17 , wherein inserting the one or more physical barriers into the mould that are molten after injection of the first polymeric material comprises lining the one or more portions of the mould with the one or more physical barriers that separate the one or more portions of the mould that are thermally insulated from the one or more portions of the mould that are not thermally insulated.

19. The method of claim 15 , wherein the first polymeric material is polyamide, and wherein the second polymeric material is a mixture of polyethylene, grafted polyethylene and ionomer.