Tetrafluoroethylene copolymer fine powder resin

Abstract

Good extrusion at high reduction ratios is obtained with a non-melt-fabricable tetrafluoroethylene fine powder resin whose particles have an inner portion and an outer portion, each portion being composed of a copolymer of tetrafluoroethylene and a selected comonomer, in which the comonomer content of the copolymer in the inner portion is greater than the comonomer content in the copolymer in the outer portion. Such particles are obtained by polymerizing tetrafluoroethylene and a desired amount of the selected comonomer in an aqueous dispersion medium, then lowering the amount of comonomer available, and continuing the polymerization.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: .[.1. An aqueous polymer dispersion comprising a dispersion of non-melt-fabricable tetrafluoroethylene polymer particles in water at a solids content of between about 10 and about 65 weight percent; said particles having an average size of between about 0.1 and 0.5μ; said particles having at least two portions, an outer portion and an adjacent inner portion, each portion consisting essentially of a copolymer of units of tetrafluoroethylene and at least one comonomer having the formula ##STR11## wherein R 1  independently if F or H; 
     
        the non-melt-fabricable nature of the polymer particles..]. .[.2.  The aqueous polymer dispersion of claim 1 wherein the comonomer is F 2  C═CFCF 3 ..]. .[.3. The aqueous polymer dispersion of claim 1 wherein the comonomer is F 2  C═CFOR F ..]. .[.4. The aqueous polymer dispersion of claim 1 wherein thee comonomer is F 2  C═CFCl..]. .[.5. The aqueous polymer dispersion of claim 2 wherein the copolymers of the inner portion and the outer portion combined contain between about 0.005 and about 2.0 weight percent comonomer units based on total weight of said particles..]. .[.6. The aqueous polymer dispersion of claim 1 wherein the tetrafluoroethylene in polymerized form in the inner portion comprises between about 25% and 85% by weight of the total tetrafluoroethylene polymerized in the inner and the outer portions..]. .[.7. The aqueous polymer dispersion of claim 1 wherein said particles contain a core of polytetrafluoroethylene of up to 15% by weight based on the weight of the particle..]. .[.8. The aqueous polymer dispersion of claim 7 wherein the comonomer is F 2  C═CFCF 3 ..]. .[.9. The aqueous polymer dispersion of claim 8 wherein the comonomer is F 2  C═CFOR F ..]. .[.10. The aqueous polymer dispersion of claim 8 
     
     
        wherein the comonomer is F 2  C═CFCl..]. 11. An agglomerated fine powder resin whose agglomerated particles have an average size of between about 350μ and 800μ and are composed of primary particles .[.defined as provided in the definition of the particles in claim 1.]. .Iadd.having an average size of between about 0.1 and 0.5μ; said particles having at least two portions, an outer portion and an adjacent inner portion, each portion consisting essentially of a copolymer of units of tetrafluoroethylene and at least one comonomer having the formula ##STR15## wherein R 1  independently is F; R 2  independently is F;   R 3  is Cl; said inner copolymer portion containing a higher percentage of said comonomer than the outer copolymer portion; the total comonomer content present in the particle, the amount of comonomer present in the copolymer of each portion, and the amount of each portion within such particles being sufficient to produce on AWG 22 wire a sintered coating having no more than 5 flaws per 100 meters of coated wire when said particles are paste extruded at a reduction ratio of 2840:1, the flaws being detected by subjecting the sintered coated wire to a high voltage spark tester at 2 KV and 300 Hz; the copolymers of the inner portion and the outer portion combined containing an amount of comonomer which is low enough to maintain the non-melt-fabricable nature of the polymer   
     
     
        particles.Iaddend.. .[.12.  An agglomerated tetrafluoroethylene fine powder resin whose agglomerated particles have an average size of between about 350μ and 800μ and are composed of primary particles of claim 2..]. .[.13. An agglomerated tetrafluoroethylene fine powder resin whose agglomerated particles have an average size of between about 350μ and 800μ and are composed of primary particles defined as provided in the difinition of the particles in claim 3..]. .[.14. An agglomerated tetrafluoroethylene fine powder resin whose agglomerated particles have an average size of between about 350μ and 800μ and are composed of primary particles defined as provided in the definition of the particles of claim 4..]. .[.15. An agglomerated tetrafluoroethylene fine powder resin whose agglomerated particles have an average size of between about 350μ and 800μ and are composed of primary particles defined as provided in the definition of the particles in claim 5..]. .[.16. A process for preparing an aqueous dispersion of tetrafluoroethylene polymer particles which comprises 1. subjecting tetrafluoroethylene and at least one comonomer of the formula ##STR16## wherein R 1  independently is F or H;   R 2  independently is F or Cl;   R 3  can be Cl, --R F , --OR F , --R' F  H, --OR' F  H, --R' F  Cl, --OR' F  Cl, or ##STR17## wherein R F  is linear perfluoroalkyl of 1-5 carbon atoms, and R' F  is linear perfluoroalkylene (perfluorinated alkane diradical) of 1-5 carbon atoms in which the designated substituent is an omega substituent; and   when R 2  is F, R 1  and R 3  taken together can be ##STR18## or the formula ##STR19## wherein R 5  and R 6  independently are --CF 3  or --CCLF 2  ; wherein the mole ratio of the amount of the comonomer to the tetrafluoroethylene is between about 0.0005 and about 0.05, to polymerizing conditions to temperature and pressure in an aqueous medium having dissolved therein a free-radical initiator and a dispersing agent and at an agitation level of from between about 2 to 12 joules/sec.-l.until the polymer solids content is between about 20 and 50% of the weight of the resulting dispersion;   
     
     
       2. subjecting the aqueous dispersion obtained in step (1) to tetrafluoroethylene and said comonomer in a mole ratio of comonomer to tetrafluoroethylene of between about 0.0001 and 0.005, provided said ratio is less than that in step (1) under polymerizing conditions of temperature and pressure and at an agitation level of from between about 2 to 12 joules/sec.-l. until the solids content is between about 35 and 65% of the weight of the resulting dispersion and is at least about 15% greater than the solids content of the dispersion obtained in step (1)..]. .[.17. The process of claim 16 wherein the comonomer is F 2  C═CFCF 3 ..]. .[.18. The process of claim 16 wherein the comonomer is F 2  C═CFOR F ..]. .[.19. The process of claim 16 wherein the comonomer is F 2  C═CFCl..]. .[.20. The process of claim 16 wherein step (1) is carried out in the presence of particles of between 0.03 and 0.12μ of polytetrafluoroethylene in said aqueous medium..]. .[.21. The process of claim 16 wherein in step (2) the mole ratio of comonomer is 
     
     
        tetrafluoroethylene is between about 0.0002 and 0.005..]. .Iadd.22.  An aqueous dispersion of a tetrafluoroethylene polymer comprising colloidal particles containing at least 98% by weight of polymerised tetrafluoroethylene and polymerised therewith at least one copolymerisable monomer selected from perfluoroalkyl trifluoroethylenes and perfluoroalkoxy trifluoroethylenes each having from 3 to 7 carbon atoms, said particles having inner core and outer shell portions in which the weight concentration of copolymerised monomer present in the outer shell portion (relative to the weight concentration of tetrafluoroethylene with which it is polymerised) is less than half that of the inner core portion..Iaddend. .Iadd.23. An aqueous dispersion according to claim 22 in which the polymer forming the shell consists of at least 99.995% polytetrafluoroethylene..Iaddend. .Iadd.24. An aqueous dispersion according to claim 22 in which the copolymerised monomer is hexafluoropropylene..Iaddend. .Iadd.25. A process for the preparation of an aqueous dispersion of a tetrafluoroethylene polymer comprising tetrafluoroethylene and a copolymerisable comonomer selected from perfluoroalkyl trifluoroethylenes and perfluoroalkoxy trifluoroethylenes having from 3 to 7 carbon atoms, which process comprises: a. subjecting a mixture of tetrafluoroethylene and comonomer to polymerisation in the presence of an aqueous phase to produce an aqueous dispersion of a copolymer wherein said copolymer contains at least 98% of tetrafluoroethylene units   b. reducing the concentration of comonomer in the reactants and continuing the polymerisation to produce a final aqueous dispersion wherein the particles of the final dispersion comprise an inner core portion produced in (a) and an outer shell portion produced in (b), the concentration of comonomer in said shell portion is less than one half the concentration of the comonomer in said core portion..Iaddend. .Iadd.26. A method according to claim 25 in which the amount of comonomer is used in step (a) is between 27 parts per million and 10830 parts per million expressed relative to the weight of the aqueous phase..Iaddend. .Iadd.27. A method according to claim 26, in which the comonomer is hexafluoropropylene and the amount of comonomer used in step (a) is between 27 parts per million and 10830 parts per million..Iaddend. .Iadd.28. A method according to claim 25, in which the mole ratio of comonomer to tetrafluoroethylene used in step (a) is in the range 0.0005 to 0.05..Iaddend.