Core-shell fluoropolymer dispersions

1. A dispersion comprising non-melt-processible fluoropolymer particles having an SSG of less than about 2.225 in aqueous medium, said fluoropolymer particles comprising a core of high molecular weight polytetrafluoroethylene having an average melt creep viscosity greater than about 1.5×10 10 Pa·s and a shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene, said shell having an average melt creep viscosity greater than about 9×10 9 Pa·s and comprising about 5 to about 30% by weight of said particles, said fluoropolymer being fibrillating.

2. The dispersion of claim 1 wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is at least 0.1×10 10 Pa·s less than the average melt creep viscosity of polytetrafluoroethylene of said core.

3. The dispersion of claim 1 wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is at least 0.2×10 10 Pa·s less than the average melt creep viscosity of polytetrafluoroethylene of said core.

4. The dispersion of claim 1 wherein said shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene has an average melt creep viscosity of about 9×10 9 Pa·s to about 1.3×10 10 Pa·s.

5. The dispersion of claim 1 wherein said shell of said particles is polytetrafluoroethylene.

6. The dispersion of claim 1 wherein said particles have a number average particle size of about 250 to about 300 nm.

7. The dispersion of claim 1 having a gel time greater than about 700 seconds at about 60 weight % fluoropolymer and at about 6 weight % surfactant.

8. The dispersion of claim 1 wherein said fluoropolymer has a melt creep viscosity greater than about 1.4×10 10 Pa·s.

9. The dispersion of claim 1 wherein said shell comprises about 5 to about 25% by weight of said particles.

10. The dispersion of claim 1 wherein said shell comprises about 5 to about 20% by weight of said particles.

11. The dispersion of claim 1 having an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the warp direction.

12. The dispersion of claim 1 having an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the fill direction.

13. The dispersion of claim 1 having an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the warp and an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the fill direction.

14. A non-melt-processible fluoropolymer powder obtained by coagulating and drying the aqueous dispersion of claim 1 .

15. A coating composition comprising a concentrated dispersion of non-melt-processible fluoropolyrner particles having an SSG of less than 2.225 in aqueous medium containing a surfactant, said dispersion containing about 30 to about 70 weight % fluoropolymer, said fluoropolymer particles comprising a core of fibrillatable high molecular weight polytetrafluoroethylene having an average melt creep viscosity greater than about 1.5×10 10 Pa·s and a shell of lower molecular weight fibrillatable polytetrafluoroethylene or modified polytetrafluoroethylene, said shell having an average melt creep viscosity greater than about 9×10 9 Pa·s and comprising about 5 to about 30% by weight of said particles, said fluoropolymer being fibrillating.

16. The coating composition of claim 15 wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is at least 0.1×10 10 Pa·s less than the average melt creep viscosity of polytetrafluoroethylene of said core.

17. The coating composition of claim 15 wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is at least 0.2×10 10 Pa·s less than the average melt creep viscosity of polytetrafluoroethylene of said core.

18. The coating composition of claim 15 wherein said shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene has an average melt creep viscosity of about 9×10 9 Pa·s to about 1.3×10 10 Pa·s.

19. The coating composition of claim 15 wherein said shell of said particles is polytetrafluoroethylene.

20. The coating composition of claim 15 wherein said particles have a number average particle size of about 250 to about 300 nm.

21. The coating composition of claim 15 having a gel time of greater than about 700 seconds at about 60 weight % fluoropolymer and about 6 weight % surfactant.

22. The coating composition of claim 15 wherein said fluoropolymer particles have a melt creep viscosity greater than about 1.4×10 10 Pa·s.

23. The coating composition of claim 15 wherein said shell comprises about 5 to about 25% by weight of said particles.

24. The coating composition of claim 15 wherein said shell comprises about 5 to about 20% by weight of said particles.

25. A batch process for producing non-melt-processible fluoropolymer dispersion comprising polymerizing tetrafluoroethylene in an aqueous medium in the presence a dispersing agent to produce fluoropolymer having an SSG of less than 2.225, said polymerizing being carried out in first stage during which a first amount of free radical initiator is added and second stage during which a second amount free radical initiator and a telogenic agent are added, said first amount of initiator producing polytetrafluoroethylene having an average melt creep viscosity greater than about 1.5×10 10 Pa·s, and said second amount of initiator being at least about 10 times said first amount and being added before about 95% of the total tetrafluoroethylene has been polymerized, said second amount of initiator producing polytetrafluoroethylene or modified polytetrafluoroethylene having an average melt creep viscosity greater than about 9×10 9 Pa·s and less than the average melt creep viscosity of the polytetrafluoroethylene of said core.

26. The process of claim 25 wherein said second amount of initiator produces polytetrafluoroethylene or modified polytetrafluoroethylene having an average melt creep viscosity at least 0.1×10 10 Pa·s less than the average melt creep viscosity of the polytetrafluoroethylene produced during said first stage.

27. The process of claim 25 wherein said second amount of initiator produces polytetrafluoroethylene or modified polytetrafluoroethylene having an average melt creep viscosity at least 0.2×10 10 Pa·s less than the average melt creep viscosity of the polytetrafluoroethylene produced during said first stage.

28. The process of claim 25 wherein said second amount of initiator produces polytetrafluoroethylene or modified polytetrafluoroetbylene having an average melt creep viscosity of about 9×10 9 Pa·s to about 1.3×10 10 Pa·s.

29. The process of claim 25 wherein said first amount of initiator produces polytetrafluoroethylene having an average melt creep viscosity of greater than about 1.0×10 10 Pa·s before about 30% of the total tetrafluoroethylene has been polymerized.

30. The process of claim 25 wherein said fluoropolymer produced is fibrillating.

31. The process of claim 25 wherein polymerizing in said second stage produces polytetrafluoroethylene.

32. The process of claim 25 wherein said second amount of initiator and said telogenic agent are added when at least about 70% of the total tetrafluoroethylene has been polymerized.

33. The process of claim 25 wherein said second amount of initiator and said telogenic agent are added when at least about 75% of the total tetrafluoroethylene has been polymerized.

34. The process of claim 25 wherein said second amount of initiator and said telogenic agent are added when at least about 80% of the total tetrafluoroethylene has been polymerized.

35. The process of claim 25 wherein said polymerizing in said first stage is carried out without adding telogenic agent.

36. The process of claim 25 wherein said dispersing agent comprises a perfluorinated carboxylic acid containing 6 to 10 carbon atoms.

37. The process of claim 25 wherein said second amount of initiator is at least about 25 times the said first amount.

38. The process of claim 25 wherein said second amount of initiator is at least about 50 times the said first amount.

39. Fine powder comprising non-melt-processible fluoropolymer particles having an SSG of less than about 2.225, said fluoropolymer particles comprising a core of high molecular weight polytetrafluoroethylene having an average melt creep viscosity greater than about 1.5×10 10 Pa·s and a shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene, said shell having an average melt creep viscosity greater than about 9×10 9 Pa·s and comprising about 5 to about 30% by weight of said particles, said fluoropolymer being fibrillating.