High build dispersions
View Patent ↗An aqueous dispersion of non-melt-processible fluoropolymer, process for making same and concentrated dispersion and powder obtained from the dispersion. The fluoropolymer particles of the dispersion have a standard specific gravity (SSG) of less than 2.225 and comprise a core of high molecular weight polytetrafluoroethylene and a shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene. At least about 1.5 weight % of the fluoropolymer particles comprise substantially rod-shaped particles having a length to diameter ratio greater than about 5.
1. 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 long half life free radical initiator is added and second stage during which a second amount of long half life 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 1.2×10 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, and wherein said polymerizing in said first stage is carried out so that at least about 1.5 weight % of said fluoropolymer particles comprise substantially rod-shaped particles having a length to diameter ratio of greater than about 5.
2. The process of claim 1 wherein said first amount of initiator produces polytetrafluoroethylene having an average melt creep viscosity greater than about 1.3×10 10 Pa·s.
3. The process of claim 1 wherein said first amount of initiator produces polytetrafluoroethylene having an average melt creep viscosity greater than about 1.5×10 10 Pa·s.
4. The process of claim 1 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.
5. The process of claim 1 wherein said second amount of initiator produces 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.
6. The process of claim 1 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.
7. The process of claim 1 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.
8. The process of claim 1 wherein said second amount of initiator produces polytetrafluoroethylene or modified polytetrafluoroethylene having an average melt creep viscosity about 9×10 9 Pa·s to about 1.3×10 10 Pa·s.
9. The process of claim 1 wherein said second amount of initiator and said telogenic agent are added when at least about 70% of the total tetrafluoroethylene has been polymerized.
10. The process of claim 1 wherein said long half life free radical initiator is persulfate initiator.