Method of making coextruded, cross-linked polyolefin foam with TPU cap layers
The present disclosure is directed to a physically crosslinked, closed cell continuous multilayer foam structure comprising at least one foam polypropylene/polyethylene layer with a TPU cap layer. The multilayer foam structure can be obtained by coextruding a multilayer structure comprising at least one foam composition layer with 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 structure comprising:
coextruding:
a first layer comprising:
polypropylene, polyethylene, or a combination of polypropylene and polyethylene;
a chemical foaming agent; and
a second layer on a side of the first layer, the second layer comprising:
at least 55 wt % thermoplastic polyurethane (TPU); and
20-25 wt. % polyether amine modified polypropylene, polyether amine modified polyethylene, or a combination of polyether amine modified polypropylene and polyether amine modified polyethylene;
irradiating the coextruded layers with ionizing radiation; and
foaming the irradiated, coextruded layers.
2. The method of claim 1 , wherein the first layer comprises 5-20 wt % TPU.
3. The method of claim 1 , wherein the first layer comprises at least 70 wt % polypropylene, polyethylene, or a combination of polypropylene and polyethylene.
4. The method of claim 1 , wherein the first layer comprises additives in an amount of 1-15 wt %.
5. The method of claim 1 , wherein the second layer comprises additives in an amount of 1-8 wt %.
6. The method of claim 1 , wherein the amount of chemical foaming agent in the first layer is 5-10 wt %.
7. The method of claim 1 , wherein the chemical foaming agent comprises azodicarbonamide.
8. The method of claim 1 , wherein the first layer comprises a crosslinking agent.
9. The method of claim 8 , wherein the amount of crosslinking agent in the first layer is 1-3 wt %.
10. The method of claim 1 , wherein the ionizing radiation is selected from the group consisting of alpha, beta (electron beams), x-ray, gamma, and neutron.
11. The method of claim 1 , wherein the coextruded structure is irradiated up to 4 separate times.
12. The method of claim 10 , 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 multilayer foam structure has a density of 20-250 kg/m3.
17. The method of claim 1 , wherein 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.