Method of making coextruded crosslinked polyolefin foam with polyamide cap layers
Described herein are physically crosslinked, closed cell continuous multilayer foam structures that includes a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene and a polyamide cap layer. The multilayer foam structure can be obtained by coextruding a multilayer structure comprising at least one foam composition layer and at least one cap composition layer, irradiating the coextruded structure with ionizing radiation, and continuously foaming the irradiated structure.
1. A method of forming a multilayer foam structure comprising:
coextruding:
a first layer comprising:
a first polypropylene, a first polyethylene, or a first combination of polypropylene and polyethylene;
a chemical foaming agent; and
a crosslinking promoter in an amount of 0.5-5 wt. %; and
a second layer on a side of the first layer, the second layer comprising:
60-85 wt. % polyamide; and
10-40 wt. % of a second polypropylene, a second polyethylene, or a second combination of polypropylene and polyethylene;
irradiating the coextruded layers with ionizing radiation; and
foaming the first layer of the irradiated, coextruded layers.
2. The method of claim 1 , wherein the first layer comprises at least 70 wt. % of the first polypropylene, the first polyethylene, or the first combination of polypropylene and polyethylene.
3. The method of claim 1 , wherein a thickness of the second layer is less than 1 mm.
4. The method of claim 1 , wherein the first layer comprises additives in an amount of 1-20 wt. %.
5. The method of claim 1 , wherein the second layer comprises additives in an amount of 1-10 wt. %.
6. The method of claim 1 , wherein the first or second polypropylene has a melt flow index of 0.1-25 grams per 10 minutes at 230° C.
7. The method of claim 1 , wherein the first or second polyethylene has a melt flow index of 0.1-25 grams per 10 minutes at 190° C.
8. The method of claim 1 , wherein an amount of the chemical foaming agent in the first layer is 3-15 wt. %.
9. The method of claim 1 , wherein the chemical foaming agent comprises azodicarbonamide.
10. The method of claim 1 , wherein the ionizing radiation is selected from the group consisting of alpha, beta (electron), x-ray, gamma, and neutron.
11. The method of claim 1 , wherein the coextruded structure is irradiated up to four separate times.
12. The method of claim 1 , wherein the ionizing radiation is an electron beam with an acceleration voltage of 200-1500 kV.
13. The method of claim 12 , wherein an absorbed electron beam dosage is 10-500 kGy.
14. The method of claim 1 , wherein the ionizing radiation crosslinks the extruded structure to a crosslinking degree of 20-75%.
15. The method of claim 1 , wherein foaming comprises heating the irradiated structure with molten salt and radiant heaters or a hot air oven.
16. The method of claim 1 , wherein the density of the multilayer foam structure is 20-250 kg/m 3 .
17. The method of claim 1 , wherein the first layer of the multilayer foam structure has an average closed cell size of 0.05-1.0 mm.
18. The method of claim 1 , wherein the multilayer foam structure has a thickness of 0.2-50 mm.