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 process for making a porous ultra high molecular weight polyethylene (UHMWPE) membrane comprising:
lubricating an ultra high molecular weight polyethylene polymer having a molecular weight of at least about 500,000 g/mol and only a single endotherm with a melt enthalpy of at least about 190 J/g to form a lubricated polymer;
subjecting said lubricated polymer to pressure at a temperature below a first melt temperature of said ultra high molecular weight polyethylene polymer to form a tape; and
expanding said tape at a temperature below the first melt temperature of said ultra high molecular weight polyethylene polymer to form a porous membrane having a structure of nodes interconnected by fibrils,
wherein said porous membrane has a porosity greater than about 25%,
wherein said porous membrane has an additional, elevated endotherm from about 145° C. to about 155° C. that is associated with said fibrils in said porous membrane and that is above said first melt temperature of said ultra high molecular weight polyethylene polymer, and
wherein said subjecting step comprises calendering said lubricated polymer at a temperature from 120° C. to about 135° C.
2. The process of claim 1 , wherein said ultra high molecular weight polyethylene polymer has a molecular weight from about 500,000 g/mol to about 10,000,000 g/mol.
3. The process of claim 2 , wherein said ultra high molecular weight polyethylene polymer has a molecular weight from about 2,000,000 g/mol to about 4,000,000 g/mol.
4. The process of claim 1 , further comprising removing said lubricant from said tape prior to expanding.
5. The process of claim 1 , wherein said first melt temperature of said ultra high molecular weight polyethylene polymer is from about 139° C. to about 143° C.
6. The process of claim 5 , wherein said melt temperature of said ultra high molecular weight polyethylene is about 140° C.
7. The process of claim 1 , wherein said membrane has a porosity of at least about 60%.
8. The process of claim 1 , wherein said membrane has a percent porosity of at least about 75%.
9. The process of claim 1 , wherein said membrane has a percent porosity of at least about 85%.
10. The process of claim 1 , wherein said polymer includes from about 0.001 mol % to about 10 mol % of a comonomer.
11. The process of claim 10 , wherein said comonomer is an alpha-olefin or cyclic olefin having 3 to 20 carbon atoms.
12. A process for manufacturing thin, strong porous membranes, comprising subjecting a lubricated ultra high molecular weight polyethylene (UHMWPE) polymer having a molecular weight of at least about 500,000 and only a single endotherm with a melt enthalpy of at least 190 J/g to pressure and heat below a first melt temperature of said UHMWPE polymer to form a tape, followed by expansion of said tape below the melt temperature of said UHMWPE polymer to form a porous membrane,
wherein said porous membrane has a porosity greater than about 25%, and
wherein said porous membrane has, an additional, elevated endotherm from about 145° C. to about 155° C. associated with fibrils in said porous membrane, said additional, elevated endotherm being above said first melt temperature of said UHMWPE polymer.
13. The process of claim 12 , wherein said UHMWPE polymer has a molecular weight from about 500,000 g/mol to about 10,000,000 g/mol.
14. The process of claim 13 , wherein said ultra high molecular weight polyethylene polymer has a molecular weight from about 2,000,000 g/mol to about 4,000,000 g/mol.
15. The process of claim 12 , further comprising calendering said tape at a temperature from about 120° C. to about 135° C.
16. The process of claim 12 , wherein said subjecting step comprises ram extruding said lubricated UHMWPE polymer.
17. The process of claim 16 , further comprising removing said lubricant prior to said expansion below the melt temperature of said UHMWPE polymer.
18. The process of claim 12 , wherein said first melt temperature of said UHMWPE polymer is from about 137° C. to about 143° C.
19. The process of claim 12 , wherein said membrane has a porosity of at least 60%.
20. The process of claim 12 , wherein said membrane has a thickness less than about 1 mm.
21. The process of claim 12 , wherein said membrane has a thickness about 0.05 mm to about 0.5 mm.
22. The process of claim 12 , wherein said UHMWPE polymer includes from about 0.001 mol % to about 10 mol % of a comonomer.
23. The process of claim 22 , wherein said comonomer is an alpha-olefin or cyclic olefin having 3 to 20 carbon atoms.