Method for producing porous articles from ultra high molecular weight polyethylene
Ultra high molecular weight polyethylene (UHMWPE) polymers that have an average molecular weight of at least 500,000 g/mol and an enthalpy of at least 190 J/g is provided. The UHMWPE polymer may include at least one comonomer. The UHMWPE polymer is used to form a membrane, that when expanded, has a node and fibril structure. The UHMWPE membrane has an endotherm of about 150° C. associated with the fibrils in the membrane. The membrane has a percent porosity of at least 25%, and in exemplary embodiments, the percent porosity is at least 60%. Additionally, the UHMWPE membrane has a thickness less than 1 mm. An UHMWPE membrane may be formed by lubricating the UHMWPE polymer, subjecting the lubricated polymer to pressure at a temperature below the melting point of the UHMWPE polymer to form a tape, and expanding the tape at a temperature below the melting temperature of the UHMWPE polymer.
1. A porous ultra high molecular weight polyethylene (UHMWPE) article comprising:
a porous membrane formed from a UHMWPE polymer having a molecular weight from about 2,000,000 g/mol to about 10,000,000 g/mol and a first endotherm from about 139° C. to about 143° C. and a melt enthalpy greater than about 225 J/g, said membrane having a structure of nodes interconnected by fibrils,
wherein said UHMWPE membrane has a second endotherm associated with said fibrils in the porous membrane,
wherein the second endotherm is from 145° C. to 155° C. such that the second endotherm is above said first endotherm of said UHMWPE polymer, and
wherein said UHMWPE membrane has a porosity of at least about 25%.
2. The porous article of claim 1 , wherein said second endotherm is about 150° C.
3. The porous article of claim 1 , wherein said UHMWPE polymer includes from about 0.001 mol % to about 10 mol % of a comonomer.
4. The porous article of claim 3 , wherein said comonomer is an alpha-olefin or cyclic olefin having 3 to 20 carbon atoms.
5. The porous article of claim 1 , wherein said UHMWPE membrane has a thickness less than about 1 mm.
6. The process of claim 1 , wherein said UHMWPE membrane has a thickness from about 0.08 mm to about 0.5 mm.
7. The porous article of claim 1 , wherein said UHMWPE membrane has a porosity of at least about 60%.
8. The porous article of claim 1 , wherein said UHMWPE membrane has a percent porosity of at least about 75%.
9. The porous article of claim 1 , wherein said UHMWPE membrane has a percent porosity of at least about 85%.
10. The porous article of claim 1 , wherein said UHMWPE polymer has a molecular weight from about 2,000,000 g/mol and about 4,000,000 g/mol.
11. The porous article of claim 1 , wherein said article is selected from a sheet, membrane, fiber, tube and three dimensional self-supporting structure.