High build dispersions

Abstract

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.

What is claimed is:  
     
         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 and a shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene, wherein 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 dispersion of  claim 1  wherein the average melt creep viscosity of the polytetrafluoroethylene of said core is greater than about 1.2×10 10  Pa·s.  
     
     
         3 . The dispersion of  claim 1  wherein the average melt creep viscosity of the polytetrafluoroethylene of said core is greater than about 1.3×10 10  Pa·s.  
     
     
         4 . The dispersion of  claim 1  wherein the average melt creep viscosity of the polytetrafluoroethylene of said core is greater than about 1.5×10 10  Pa·s.  
     
     
         5 . The dispersion of  claim 1  wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is greater than about 9×10 9  Pa·s and less than the average melt creep viscosity of polytetrafluoroethylene of said core.  
     
     
         6 . 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.  
     
     
         7 . 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.  
     
     
         8 . The dispersion of  claim 1  wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is about 9×10 9  Pa·s to about 1.3×10 10  Pa·s.  
     
     
         9 . The dispersion of  claim 1  wherein said shell comprises about 5 to about 30% by weight of said fluoropolymer particles.  
     
     
         10 . The dispersion of  claim 1  wherein at least 50% of said fluoropolymer particles are generally cylindrical having a length to diameter ratio of greater than about 1.5.  
     
     
         11 . The dispersion of  claim 1  wherein the molecular weight of the polytetrafluoroethylene or modified polytetrafluoroethylene in the shell is sufficiently low that said dispersion at about 60 weight % fluoropolymer and at about 6 weight % surfactant has a gel time greater than about 700 seconds.  
     
     
         12 . The dispersion of  claim 1  wherein the fluoropolymer is fibrillating.  
     
     
         13 . The dispersion of  claim 1  wherein the shell is polytetrafluoroethylene.  
     
     
         14 . The dispersion of  claim 1  wherein about 1.5 to about 25 weight % of said fluoropolymer particles comprise substantially rod-shaped particles having a length to diameter ratio greater than 5.  
     
     
         15 . The dispersion of  claim 1  wherein at least about 1.5 to about 20 weight % of said fluoropolymer particles comprise substantially rod-shaped particles having a length to diameter ratio of greater than about 5.  
     
     
         16 . The dispersion of  claim 1  wherein said fluoropolymer particles have a melt creep viscosity of greater than about 1.4×10 10  Pa·s.  
     
     
         17 . The dispersion of  claim 1  wherein said rod-shaped dispersion particles have a number average diameter of less than about 150 nm.  
     
     
         18 . The dispersion  claim 1  wherein said fluoropolymer particles have a number average length of about 220 to about 500 nm and a number average diameter of about 150 to about 300 nm.  
     
     
         19 . The dispersion of  claim 1  having an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the warp direction.  
     
     
         20 . The dispersion of  claim 1  having an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the fill direction.  
     
     
         21 . The dispersion of  claim 1  having an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the warp direction and an MIT Flex Life for coated glass fabric of greater than 10,000 cycles in the fill direction.  
     
     
         22 . A non-melt-processible fluoropolymer powder obtained by coagulating and drying the aqueous dispersion of  claim 1 .  
     
     
         23 . A coating composition comprising a concentrated dispersion of non-melt-processible fluoropolymer particles having an SSG of less than about 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 high molecular weight polytetrafluoroethylene and a shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene, wherein at least about 1.5 weight % of said fluoropolymer particles consists of substantially rod-shaped particles having a length to diameter ratio of greater than about 5.  
     
     
         24 . The coating composition of  claim 23  wherein said concentrated dispersion contains about 45 to about 65 weight % fluoropolymer.  
     
     
         25 . The coating composition of  claim 23  having a critical cracking thickness of greater than about 24 micrometers at about 60 weight % fluoropolymer and about 8 weight % surfactant.  
     
     
         26 . The coating composition of  claim 23  having a critical cracking thickness of greater than about 20 micrometers at 60 weight % fluoropolymer and 6 weight % surfactant.  
     
     
         27 . The coating composition of  claim 23  having a gel time of greater than about 700 seconds at about 60 weight % fluoropolymer and about 6 weight % surfactant.  
     
     
         28 . The coating composition of  claim 23  wherein the average melt creep viscosity of the polytetrafluoroethylene of said core is greater than about 1.2×10 10  Pa·s.  
     
     
         29 . The coating composition of  claim 23  wherein the average melt creep viscosity of the polytetrafluoroethylene of said core is greater than about 1.3×10 10  Pa·s.  
     
     
         30 . The coating composition of  claim 23  wherein the average melt creep viscosity of the polytetrafluoroethylene of said core is greater than about 1.5×10 10  Pa·s.  
     
     
         31 . The coating composition of  claim 23  wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is greater than about 9×10 9  Pa·s and less than the average melt creep viscosity of polytetrafluoroethylene of said core.  
     
     
         32 . The coating composition of  claim 23  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.  
     
     
         33 . The coating composition of  claim 23  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.  
     
     
         34 . The coating composition of  claim 23  wherein the average melt creep viscosity of the polytetrafluoroethylene or modified polytetrafluoroethylene of said shell is about 9×10 9  Pa·s to about 1.3×10 10  Pa·s.  
     
     
         35 . The coating composition of  claim 23  in the form of a baked layer.  
     
     
         36 . A substrate coated with the composition of  claim 35 .  
     
     
         37 . The substrate of  claim 36  wherein the substrate is metal.  
     
     
         38 . The substrate of  claim 36  wherein the substrate is glass fabric.  
     
     
         39 . A self-supporting film cast from the dispersion of  claim 1 .  
     
     
         40 . 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 1.2×10 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.  
     
     
         41 . The process of  claim 40  wherein said first amount of initiator produces polytetrafluoroethylene having an average melt creep viscosity greater than about 1.3×10 10 Pa·s.  
     
     
         42 . The process of  claim 40  wherein said first amount of initiator produces polytetrafluoroethylene having an average melt creep viscosity greater than about 1.5×10 10  Pa·s.  
     
     
         43 . The process of  claim 40  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.  
     
     
         44 . The process of  claim 40  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.  
     
     
         45 . The process of  claim 40  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.  
     
     
         46 . The process of  claim 40  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.  
     
     
         47 . The process of  claim 40  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.  
     
     
         48 . The process of  claim 40  wherein said second amount of initiator and said telogenic agent are added when at least about 70% of the total tetrafluoroethylene has been polymerized.