SOLVENT SLURRY PROCESS FOR PRODUCING HIGH SOLIDS

FLUOROPOLYMERS

Background

Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), copolymers of TFE and perfluoro vinyl ethers (PFA polymers), copolymers of TFE and ethylene (ETFE polymers), terpolymers of TFE, HFP, and vinylidene fluoride (VDF) (THV polymers) and polymers of VDF (PVDF polymers). Commercially employed fluoropolymers also include fluoroelastomers and thermoplastic fluoropolymers.

The making of such fluoropolymers generally involves the polymerization of gaseous monomers, that is monomers that under ambient conditions of temperature and pressure exist as a gas. Several polymerization methods are known to produce fluoropolymers. Such methods include suspension polymerization, aqueous emulsion polymerization, solution polymerization, polymerization using supercritical CO ₂ , and polymerization in the gas phase Solvent based polymerization processes for making fluoropolymers have been disclosed. For example, chlorofluorocarbons (CFCs), as exemplified by trichlorotrifluoroethane (Fl 13), have been used. Due to the Montreal Protocol, however, Fl 13 is now under ban and is not a commercially viable polymerization solvent.

Summary

In one aspect, the present invention relates to a process for producing fluoropolymers by solvent slurry polymerization comprising introducing at least one fluorinated monomer into a reactor containing a hydro fluorocarbon (HFC) solvent and water and polymerizing the monomer to produce a fluoropolymer slurry, and maintaining a shear rate in the reactor above the yield point of the reaction slurry at 10% by weight fluoropolymer solids, wherein the amount of fluoropolymer solids produced in the

polymerization is greater than 10% by weight based on the amount of water and fluorinated liquids.

"Solvent slurry polymerizations" are defined as polymerizations carried out in two or more phases of water and solvent and may comprise a low-telogenic HFE, at least one fluorinated monomer, water, and an initiator.

"Fluorinated liquids" are defined as liquids under the conditions in the polymerization reactor and contain at least one fluorine atom in their structure.

Brief Description of the Drawing FIG. 1 is a perspective cut-away view of an exemplary reactor system of the present invention; and FIG. 2 is an enlarged perspective view of the coaxial agitation system of FIG. 1.

Detailed Description The present applicants have recognized a need for an economical and environmentally friendly polymerization process for fluoropolymers in high concentrations. Frequently, fluoropolymers are produced by an emulsion polymerization process using a fluorinated surfactant, particularly a perfluoroalkanoic acid or salt thereof as a surfactant. These surfactants are typically used because they provide a wide variety of desirable properties such as high speed of polymerization, good copolymerization properties of fluorinated olefins with comonomers, desirable stability, and good polymerization yields, that is a high amount of solids can be produced. However, environmental concerns have been raised against these and other fluorinated surfactants and moreover these surfactants are generally expensive. Chlorofluorocarbons (CFC) have been used as solvents but have unacceptable global warming effects.

The present applicants have investigated processes for fluoropolymer production with the use of more environmentally friendly HFCs, liquefied monomers (such as HFP, PMVE, PPVE, VDF or other monomers) and water, without the need for added surfactants, emulsifϊers, or CFC solvents. Prior polymerizations attempted by others encountered a variety of problems such as noted in Kasahara et al., Reports Res. Lab.,

Asahi Glass Co. Ltd., 52 (2002). There, the authors describe that thixotropic conditions are encountered in the polymerization process and special mixer technology is required.

Kasahara et al. report using an improved helical PTFE scraper double helical ribbon impeller system to produce ETFE. The described polymerization, however, yields only a 9 wt. % ETFE concentration.

The present applicants have carried out various such solvent slurry polymerizations with various agitation systems including one similar to that of Kasahara et al. When using the Kasahara agitation system the present applicants have surprisingly found that the polymerizations suddenly stop at relatively low concentrations of fluoropolymer. These low concentrations of fluoropolymer are considered unacceptable for large scale fluoropolymer production. The present applicants have found that higher concentrations of fluoropolymers may be attained by using improved agitation systems.

In one embodiment, a flow field is developed in the reactor using a mixer that imparts both a tangential and an axial flow component.

In another embodiment, the amount of fluoropolymer solids produced in the polymerization is greater than 20% or even greater than 30% by weight based on the amount of water and fluorinated liquid.

In a further embodiment, the reactor agitator may be selected from a double planetary mixer and a coaxial turbine double helical ribbon mixer. In the case of the coaxial mixer, the double helical ribbon agitator may operate at a slower speed (as measured in revolutions per minute) than that of the turbine agitator. The double helical ribbon agitator and the turbine agitator of the coaxial mixer may operate with turbine and double helical ribbon agitators in opposed rotation to produce flow patterns of the solvent and water from each of the agitators in opposed directions. The coaxial turbine agitator may have multiple stages. The coaxial turbine double helical ribbon mixer may have a double helical ribbon agitator diameter to reactor internal diameter ratio of greater than 0.90 or even greater than 0.99.

In another embodiment, the fluoropolymer slurry may be thixotropic or shear thinning. In other embodiments, the process may include separating the fluoropolymer and water from the HFC solvent, agglomerating the fluoropolymer, drying the fluoropolymer and reusing the solvent for another process of the claimed invention.

In one embodiment the HFC solvent may be a hydrofluoroether (HFE) solvent. The HFE solvent may have a boiling point of from 5O ⁰ C to 200 ⁰ C.

In further embodiments the HFE solvent may be a blend of two or more different HFEs . The amount of water present may be from about 1 : 10 to 10:1 based on the total weight of the monomers added to the polymerization medium. The amount of solvent may be from about 1 :20 to 20: 1 both based on the total weight of monomers added to the polymerization medium.

In some embodiments the fluorinated monomer may be selected from TFE, trifiuoroethylene (TrFE), chlorotrifiuoroethylene (CTFE), HFP, VDF, vinyl fluoride (VF) and perfluoro(alkylvinyl) ether (PAVE) or the polymerization process may further comprise ethylene or propylene monomers or combinations thereof. In further embodiments the fluoropolymer may be semicrystalline with a melting point above 6O ⁰ C. In yet further embodiments, the fluoropolymer may be selected from the group consisting of PTFE, ETFE, FEP, PFA, PVF, THV, PVDF and combinations thereof.

The processes described herein can be used to produce any of the known fluoropolymers, that is polymers that have a partially or fully fluorinated backbone. In particular, the processes can be used to produce homo- and copolymers of fluorinated olefmic monomers such as tetrafluoroethylene, vinylidene fluoride and chlorotrifiuoroethylene. Suitable comonomers include fluorinated monomers such as hexafluoropropene, perfluoro vinyl ethers including perfluoro(alkylvinyl) ethers such as perfluoro(methylvinyl)ether and perfluoro-(n-propylvinyl)ether and perfluoro(alkoxy vinyl)ethers such as those corresponding to formula (II):

CF ₂ =CFO(R ¹ O) _n (R ² O) _m R3 (II) wherein RI and R^ are each independently selected from a linear or branched perfluoroalkylene group having from 1 to 6 carbon atoms, m and n are each independently from 0 to 10, and R^ is a perfluoroalkyl group of from 1 to 6 carbon atoms. Combinations of any of the above-named fluorinated monomers are also contemplated. The processes of the invention can be used to produce polytetrafluoroethylene, fluoroelastomers as well as fluorothermoplasts.

Non-fluorinated monomers that can be used as comonomers include alpha-olefms, for example ethylene and propylene.

The polymerization is generally initiated through the use of free radical generating initiators. Typically, such free radical sources include organic compounds capable of

thermally decomposing to free radical species, and in aqueous systems (as described below) redox type initiators are often used. However, any source of appropriate free radicals may be used in the process. For instance, one class of appropriate initiators includes organic compounds that thermally decompose or decompose on exposure to ultraviolet light.

Not all free radical sources will polymerize any particular fluoromonomer or combination of monomers. Free radical sources effective with various fluoromonomers and monomer combinations are described, for example, in J. C. Masson in J. Brandrup and E. H. Immergut, Ed., Polymer Handbook, 3 ^rd Ed., John Wiley & Sons, New York, 1989, p. II/1-II/65, and C. S. Sheppard and V. Kamath in H. F. Mar. et al, Ed., Encyclopedia of Chemical Technology, 3 ^rd Ed., vol. 13, John Wiley & Sons, New York, 1981, p. 355-373.

Typical organic compounds that thermally decompose that are useful for at least some fluoromonomers are organic peroxides such as t-butylpivalate, t-butylperoxy-2- ethylhexanoate, azobisisobutyronitrile, azobisisovaleronitrile, acetyl peroxide, (CF ₃ CF ₂ CF ₂ [CF(CF ₃ )CF ₂ O] _X CF(CF ₃ )COO) ₂ where x is 0 or an integer of 1 to 20, CF ₃ CF ₂ CF ₂ O[CF(CF ₃ )CF ₂ O] _X CF(CF ₃ )COOF where x is 0 or an integer of 1 to 20, [CF ₃ (CF ₂ ) _n COO] ₂ , HCF ₂ (CF ₂ ) _n COOF, HCF ₂ (CF ₂ ) _n COOF, and ClCF ₂ (CF ₂ ) _n COOF, all where n is 0 or an integer of 1 to 8. Redox type free radical sources include, but are not limited, to potassium persulfate, or a combination of persulfate and bisulfite (usually as alkali metal salts). Ionic species are especially useful in aqueous systems. Fluorinated sulfmates, such as those described in US 5,378,782 and US 5,285,002, can be used as well.

The amount of initiator employed is typically between 0.01 and 5 % by weight, preferably between 0.05 and 1 % by weight based on the total weight of the polymerization mixture. The full amount of initiator may be added at the start of the polymerization or the initiator can be added to the polymerization in a continuous way during the polymerization until a conversion of 70 to 80% is achieved. One can also add part of the initiator at the start and the remainder in one or separate additional portions during the polymerization. The polymerization system may further comprise other materials, such as emulsifiers, buffers and, if desired, complex-formers or chain-transfer agents. Adding a compound having chain transfer ability may allow control of the molecular weight of the

polymer produced. With regard to a compound having a large chain transfer constant, only a small amount is required for adjusting the molecular weight. Further, it is preferred that the chain transfer agent (CTA) has small ozone destruction ability. A CTA, which meets such requirements, may, for example, be an HFC such as CF ₂ H ₂ , a hydrochlorofluorocarbon (HCFC) such as CF3CF2CHCI2, a ketone such as acetone, an alcohol such as methanol or ethanol. Other CTAs include alkanes such as ethane, propane, pentane, hexane or ethers such as dimethylether. The amount of CTA added varies depending upon the chain transfer constant of the CTA used. However, the amount is usually from about 0.01 wt% to about 5wt% based on the weight of the polymerization medium.

Solvents used in the instant process can perform one or more than one function. They may be used as solvents for one or more of the constituents such as a monomer or free radical source, since adding such ingredients as solutions may be more convenient and/or accurate and may greatly facilitate a good dispersion of the reaction medium components (for example, initiators, CTAs, and comonomers) employed. The solvent may also function as a solvent for the polymer that is made in the process. The solvent may act as a swelling agent (which means that the polymer formed is not necessarily readily soluble in the solvent). Blends of solvents are also envisioned as suitable for the practice of this invention. A blend may, for example, be intentionally made or the result of an industrial process.

Mixed media of solvents and water can be used as a polymerization medium. The amounts of water can vary from about 1 : 10 to 10:1 based on the total weight of monomers added to the polymerization medium. Generally the amount of solvent will vary from 1 :20 to 20: 1 based on the total weight of monomers added to the polymerization medium. Additionally, mixtures including super critical monomers or supercritical fluids comprising materials such as HFP or CHF ₃ can be used.

A wide range of polymerization conditions can be employed without particular restriction. The polymerization temperature can be from 0 ⁰ C to up to 150 ⁰ C or even from 20 ⁰ C to 100 ⁰ C, depending mainly on the used initiators. The polymerization pressure is usually from 2 bar up to 40 bar, or even from 5 to 30 bar. The present invention can be performed as batch process as well as continuously.

In some embodiments, it is desirable that the solvent be readily removed from the polymer once the polymerization is completed. Solvents may be removed by distillation or evaporation. Accordingly, the solvent may be volatile. During the evaporation or distillation, agglomeration of the fluoropolymer may occur with the continued application of stirring and heat. Typically free flowing, easy to handle agglomerates are obtained.

The boiling point of the pure solvent at atmospheric pressure of the solvent described herein is no higher than about 200 ⁰ C, no higher than 150 ⁰ C or even no higher than 100 ⁰ C. Conversely, the solvent should not have a very low boiling point. Solvents that boil well below process temperature add their vapor pressure to the total pressure generated in the process, which may lead to the need for more expensive process equipment capable of holding higher pressures, or could inadvertently evaporate leaving possibly dangerous residues (e. g., peroxide residue if peroxide is used as the initiator). Thus, the solvent has an atmospheric pressure boiling point of about 0 ⁰ C or higher, or even about 20 ⁰ C or higher. Particular ranges of solvent boiling point include from about 0 ⁰ C to about 200 ⁰ C, from about 20 ⁰ C to about 120 ⁰ C, or even 50 ⁰ C to about 200 ⁰ C. The solvent used in the polymerization may be easily recovered and recycled.

Due to the high viscosity of the reaction medium obtained during the suspension polymerization of fluorinated monomers in the presence of a fluorinated liquid it may be difficult to mix the reaction medium well enough during the course of polymerization to achieve high conversion of monomer to polymer solids. Sufficient mixing should be used to remove the heat of polymerization and to deliver monomers fed to the reactor to the locus of polymerization.

Usually, the viscosity of a reaction medium increases with increasing polymer content. Depending on the polymer content of the reaction medium, the chemical nature of the obtained polymer and the fluorinated solvent utilized, the rheo logical properties of the polymer suspension might change drastically. If, for instance, only the fluorinated solvent used is changed, the rheo logical properties of the polymer suspension can change from a Newtonian fluid to a thixotropic fluid. Examples of thixotropic fluids comprising ETFE copolymer and a fluorinated solvent are described by Kasahara et al. (Reports Res. Lab., Asahi Glass Co. Ltd., 52; 2002). The flow curve diagrams described in this report (see Fig. 4-1 and 4-2) show a marked hysteresis indicating that these polymer suspensions have a reversible thixotropy. The thixotropic nature of these polymer suspensions implies

that the viscosity (given in physical units of Pa* s) changes drastically with the shear stress (given in physical units of Pa) applied. The viscosity drops dramatically once a certain value for the shear stress is exceeded (for example see Figure 13.22 in M. Pahl, W. GleiBle, H. M. Laun in "Praktische Rheologie der Kunststoffe und Elastomere" VDI- Verlang GmbH, Dϋsselldorf 1995). The threshold value for the shear rate (given in physical units of 1/s) is herein referred as yield point. The mixing of a reaction medium comprising a fluoropolymer, a fluorinated liquid, and optionally water, is greatly facilitated, if the yield point of the polymer suspension/slurry is exceeded by the mixing apparatus used. Typically, the yield point is exceeded at shear rates of 10 to 1000 1/s. The agitation system installed in the polymerization reactor may generate fluid motion not just in the horizontal direction (axial mixing or perpendicular to the agitator shaft). If fluid motion is only applied in horizontal direction, the liquid moves as in a merry-go-round, and there is limited mixing because liquid is not forced sideways or vertically (for example see Fig. 2 in Kirk-Othmer: "Encyclopedia of Chemical Technology"; Vol. 13, Mixing and Blending). The applicants have found that conventional mixing technology such as the double ribbon helical impeller described by Kasahara et al. (Reports Res. Lab., Asahi Glass Co. Ltd., 52; 2002) is inadequate to provide sufficient mixing in an axial or vertical direction (that is, parallel to the agitator shaft) when the polymer content of the reaction medium reaches about 10%. As a result, the monomer uptake suddenly stops at a certain monomer feed due to insufficient mixing.

The applicants have found that this hurdle can be overcome, if the reactor shear rate operates above the yield point of the reaction slurry at 10% or greater fluoropolymer solids based on the amount of water and fluorinated liquid. The agitation system may include, for example, double planetary mixers of the type exemplified by "DPM 10" to "DPM 750", commercially available from Charles Ross and Company (Hauppaugee, NY) and a coaxial turbine double helical ribbon mixer of the type "KOAX 2035", commercially available from EKATO Ruhr- und Mischtechnik GmbH (Schopfheim, Germany).

Coaxial stirrers generally feature a close-clearance stirrer in anchor form and a highspeed coaxial, central agitator with separate drive. The close-clearance anchor stirrer produces the tangential component of the flow, while the central stirrer, which can be single or multiple stage, produces the axial component. Alternatively the reactor agitation system

may use non-impeller type shear such as pumps or tumbler mixers or combinations of these methods.

Fig. 1 illustrates one possible reactor design using a coaxial turbine double helical ribbon agitation system. Reactor (10) comprises double helical impeller (25) and motor (15), turbine multi stage agitator (30) and motor (20), high-pressure water sprayer for cleaning (35), opening for reactor additions (40), initiator feed (45), temperature control (50) and venting valve (55). Fig. 2 is an enlarged view of coaxial agitation system (27) shown in Fig. 1 comprising inner shaft (24) for turbine impellers (30), outer shaft (22) for double helical impeller (25), joined by bottom blade (23) and upper attachment (21), wherein upper attachment creates an angle α between upper attachment (21) and double helical impeller (25).

Useful solvents include but are not limited to RfOCH3 where Rf =C4 to C6,

RfOC ₂ H ₅ where Rf =C4 to C6, HCF ₂ -CF ₂ -O-CH ₃ , HCF ₂ -CF ₂ -O-CH ₂ -(CF ₂ VX or CF ₃ - CHF-CF ₂ -O-CH ₂ -(CF ₂ ) _n -X or (CFs) ₂ CH-CF ₂ -O-CH ₂ -(CF ₂ )D-X where X = H or F and n = 1-6, HCF2-CF2-OCH2-CF2-CHF-CF3, CF ₃ -CHF-CF ₂ -OCH ₂ -CF ₂ -CHF-CF ₃ , (CF ₃ ) ₂ CF- CF(OCH ₃ )-CHF-CF _{3 ,} (CF ₃ ) ₂ CHF-CF ₂ -O-CH ₂ -CF ₂ -CHF-CF ₃ , (CF ₃ ) ₂ CF-CHF- CF(OCH ₃ )-CF _{3 ,} C ₆ H ₅ O-CF ₂ -CF ₂ H, C ₆ H ₅ O-CF ₂ -CHF-CF ₃ , C ₆ H ₅ O-CF ₂ -CH(CF ₃ ) _{2 ,} Rf-CH ₂ -O-CF ₂ H where Rf is selected from a linear partially fluorinated alkyl group, a linear partially fluorinated alkyl group interrupted with one or more oxygen atoms, a branched partially fluorinated alkyl group, a branched partially fluorinated alkyl group interrupted by one or more oxygen atoms, and a perfluorinated alkenyl group, (CF ₃ ) ₂ CH- CF ₂ -O-CH ₃ , (CF ₃ ) ₂ C=CF-O-CH ₃ , CFsCF ₂ CF2θ[CF(CFs)CF2θ] _q CFHCFs, CH ₃ OCF ₂ CFHCF ₃ , CF ₃ (CF ₂ ) _x CFH(CF ₂ ) _y CF ₃ , CF ₃ (CF ₂ ) _x CH ₂ (CF ₂ ) _y CF ₃ , CF ₃ (CF ₂ ) _x CFHCH ₂ (CF ₂ ) _y CF ₃ , 1 , 1 ,2,2-tetrafluorocyclobutane, 1 -trifluromethyl- 1 ,2,2- trifluorocyclobutane, l,2-bis(perfluoro-n-butyl)ethylene and 2,2-bis(trifluoromethyl)-l,3- dioxolane, wherein q is 0-3; z is an integer of 2 to about 10; x is 0 or an integer of 1 to about 8; and y is independently 0 or an integer of 1 to about 8. Further HFC's comprise X- (CF ₂ ) _n -H with n = 2 to 14 and X=H or F, CF ₃ -(CF ₂ ) _n -H with n = 2 to 14, RKCH ₂ -CF ₂ ) _n -X with n = 1 to 4 and X = H and Rf-(CHF-CF ₂ ) _n -X with n = 1 to 4 and X=F or H. Preferred are HFE's due to their low ozone depleting potential. Furthermore, liquid or liquefied monomers can be used as reaction medium, for example HFP, VDF, vinyl/allyl ethers

such as CF ₂ =CF-O-Rf, CF ₂ =CF-O-Rf-SO ₂ -F or CF ₂ =CF-O-Rf-COOCH ₃ . Polyether HFE's may also include RfO(CF ₂ CFCF ₃ ) _n OCHFCF ₃ where n=0-4 and Rf is perfluoroalkyl;

CF ₃ CHFO(CF ₂ ) _n OCHFCF ₃ where n=l-6; HCF ₂ (OCF ₂ X ₁ OCF ₂ H where n=l-6; and HCF ₂ (OCF ₂ CF ₂ VOCF ₂ H where n=l-6. Regarding accessibility of the solvents, it is noted that they can be easily prepared by reacting an olefin such as TFE or HFP with an appropriate alcohol under basic conditions as disclosed in US 2,409,274 and J. Am. Chem. Soc. 73, 1785 ((1958). For example Rf-CH ₂ O-CF ₂ H is obtainable by the reaction of the corresponding alcohol and

R22 as described, for example, in J. of Fluorine Chem., 127, (2006), 400-404. Reactions of partially fluorinated alcohols with fluorinated olefins resulting in fluorinated ethers are described in Green Chemistry 4, 60 (2002). Branched fluroolefϊns like perfluoroisobutene or dimeric HFP can be converted to partially fluorinated ethers as well using alcohols under basic conditions. Such reactions are demonstrated in Russian Chem. Rev. 53, 256

(1984), Engl. Ed., and Bull. Chem. Soc. Jap. 54, 1151 (1981). HFEs may be made from the corresponding ketones or acid fluorides, for example using the methods described in WO/9937598, US 6,046,368, or J. Fluorine Chem. 126,

1578 (2006).

Tetrafluoroethyl ethers, carrying one or two -OCHFCF ₃ groups, can be made based on HFPO oligomers or HFPO addition products to ketones or acid fluorides as disclosed in Angwandte Chemie Int. Ed. Engl. 24, 161 (1985).

The synthesis of HCF ₂ O(CF ₂ ) _n OCF ₂ H with n being one or two is disclosed in EP

879839. Other fluorinated ethers carrying one or more -O-CF2H group(s) are made according to J. Fluorine Chem. 127, 400 (2005) by reaction of fluorinated alcohols with

CF ₂ ClH (R22) in the presence of base. Decarboxylation of primary carboxylic acids in the presence of proton donators results in the formation of CF ₂ H groups as shown in J. Am. Chem. Soc. 75, 4525 (1953).

This reaction creates fluorinated ethers as well as fluoroalkanes carrying -CF ₂ H groups. Tetrafluorocyclobutane and substituted partially fluorinated cyclobutanes are synthesized according to WO/0075092. 2,2-Bis(trifluoromethyl)-l ,3-dioxolanes can be made by methods described in J.

Fluorine Chem. 9, 359 (1977).

Partially fluorinated alkanes can be obtained by telomerisation of fluorinated alkyl iodides with for example vinylidene fluoride (Macromolecules 38, 10353 (2005)) and replacement or further reaction of the iodine atom.

Examples

MEASUREMENT METHODOLOGY

The melt flow index (MFI), reported in g/10 min, was measured according to DIN 53735, ISO 12086 or ASTM D- 1238 at a support weight of 5.0 kg. A temperature of 297°C or 265°C was applied and a standardized extrusion die of 2.1 mm diameter and a length of 8.0 mm was used.

Melting peaks of the fluororesins were determined according to ASTM 4591 by means of Perkin-Elmer DSC 7.0 under nitrogen flow and a heating rate of 10°C/min. The indicated melting points relate to the melting peak maximum.

Comparative Example 1

A slurry polymerization process was conducted using a standard marine propeller agitation system. The polymerization kettle with a total volume of 48.5 1 (including feeding pipes) equipped with an impeller agitator system suitably optimized for emulsion polymerizations was charged with 19.0 1 of deionized water and 10.0 1 of "NOVEC HFE 7200" (3M Company, St.Paul, MN). The oxygen free kettle was then heated up to 60° C and the agitation system was set to 240 rpm. The kettle was charged with 413 g ethylene and 1484 g tetrafluoroethylene (TFE) to 15.0 bar absolute reaction pressure. The polymerization was initiated by the addition of 40 g tert-butylpilvalate (75% solution in n-decane, "TBPPI- 75-AL" from Akzo Nobel). As the reaction started, the reaction temperature was maintained and the reaction pressure of 15.5 bar absolute was maintained by the feeding TFE and ethylene into the gas phase with a feeding ratio ethylene (kg)/TFE (kg) of 0.280. When a total feed of 1265 g TFE was reached in 30 min, the reaction temperature could not be maintained and the monomer valves were closed. For an additional 10 min, the temperature was further raised to 92°C although no monomer was fed. The reaction pressure dropped until 3.3 bar absolute was reached.

The so-obtained reaction slurry formed a thick layer on the reactor wall (about 8 cm thickness) that could not be vented from the kettle. The temperature control problem most likely could be attributed to the fact that this polymer slurry layer covered the reactor wall. The reactor had to be opened and the polymer slurry had to removed manually from the reactor wall. The dried polymer showed a MFI (297/5) of 48 g/10 min and a melting point maximum of 269°C.

Comparative Example 2

A slurry polymerization process was conducted using the same polymerization kettle of comparative example 1. In this example, the standard impeller agitator was replaced by a double helical ribbon impeller system being similar to the setup disclosed by Kasahara et al. in Reports Res. Lab. Asahi Glass Co., Ltd., 52 (2002). The double helical ribbon impeller system used in this example had the following dimensions: 2 blades with blade dimensions 42x10 mm, blade to blade distance of 105 mm and spiral height of 197 mm. The oxygen free kettle was charged with 22.0 1 deionized water and 10.0 1 "NOVEC

HFE 7200" (3M company, St.Paul, MN) and was then heated up to 60° C. Initially, the agitation system was set to 80 rpm. The kettle was charged with 144 g ethylene and 1519 g tetrafluoroethylene (TFE) to 15.0 bar absolute reaction pressure. The polymerization was initiated by the addition of 8 g tert-butylpilvalate (75% solution in n-decane, "TBPPI-75- AL" from Degussa; Pullach/Germany), but no discernible reaction occurred for 30 min. After that period, the agitation was set to 110 rpm, which resulted in a vigorous monomer uptake. Subsequently the reaction temperature and reaction pressure of 15.5 bar absolute was maintained by the feeding TFE and ethylene into the gas phase with a feeding ratio of ethylene (kg)/TFE (kg) of 0.280. When a total feed of 1200 g TFE was reached in 80 min, the monomer uptake suddenly stopped and the monomer valves were closed. Within a time period of 50 min, the reaction pressure dropped to 5.5 bar absolute.

After the reactor was cooled down to room temperature, it was vented and flushed with N ₂ in three cycles. The so-obtained reaction slurry did not form any layer on the reactor wall and could be vented from the kettle easily. The slurry contained about 8 % fluoropolymer solids based on the amount of water.

Comparative Examples 3 - 5

A variety of TFE/HFP/VDF/PPVE-1 quad polymers were produced in slurry polymerization processes under various conditions. The same polymerization kettle equipped with the same double helical ribbon impeller system of comparative example 2 was used. In all cases, the kettle was charged with 22.0 1 demineralized water and 10.0 1 fluorinated liquid. The polymerization was initiated by the addition of 2.0 g tert- butylperoxy-2-ethylhexanoate ("TBPEH" from Akzo Nobel) dissolved in the fluorinated liquid. The reaction temperature was 60 ⁰ C in all cases. It was found that the polymerization rate was very sensitive to the agitation speed. A vigorous stirring usually increased the monomer uptake. The other reaction conditions are summarized in Table 1. In none of the cases was the formation of a polymer layer on the reactor wall observed. All polymer slurries could be vented from the kettle easily. The fluorinated liquid was removed from the polymer slurry by distillation. The physical characteristics of the dried polymers are also summarized in Table 1.

Table 1

Comparative Example 6

The reaction slurry from comparative example 2 was placed into a 20 1 reaction kettle made or boron silicate glass to allow visualization of the mixing. The reactor was equipped with a helical ribbon impeller agitation system with the dimensions 250 mm diameter x 400 mm height, commercially available from Ruhr- und Verfahrens-Technik (RVT), Tόring/Inn; Germany. The agitator was set to 89 rpm and the mixing performance of the agitator was observed visually. It was observed that the polymer slurry was only

transported in a horizontal or tangential direction. A material transport along the vertical or axial direction did not occur and the polymer slurry did not reach the water/air interface at the top. It was further seen that the surface of the vortex (which makes the water/air interface) was not exchanged with the bulk of the medium. In effect the vortex was "standing still". Without being bound to theory it is speculated that a monomer transport from the gas phase towards the polymer slurry (which is the locus of polymerization) is strongly impaired under these agitation conditions.

Example 1 The reaction slurry from comparative example 2 was placed into a 50 1 boron silicate glass reaction kettle of the dimensions 360 mm diameter x 400 mm height. The reactor was equipped with a coaxial turbine double helical ribbon mixer agitation system "KOAX 2035", commercially available from EKATO Ruhr- und Mischtechnik GmbH (D-79841 Schopfheim, Germany). The coaxial agitation system consisted of the following elements: an inner shaft with independent motor equipped with 2 marine type impellers ("EKATO- Viscoprop" with the dimensions: 2 blades each, 240 mm diameter, 53° angle; outer shaft with independent motors equipped with anchor or turbine type bottom stirrer ("EKATO- Bodenorgan"); double helical ribbon impeller agitator fixed at the anchor type bottom stirrer ("EKATO-Paravisc"). The outer Paravisc agitator was operated at 38 rpm (clockwise, material flow directed upwards) and the inner Viscoprop agitator was operated at 380 rpm (counter-clockwise, material flow directed downwards). The filling height under these conditions was 95%. The mixing performance of the coaxial agitator was also visually inspected. It was observed that the polymer slurry was efficiently transported in both horizontal and vertical directions. In addition, the slurry did not stick together at the bottom. The material transport along the vertical and horizontal direction was so efficient that the polymer slurry did reach the water/air interface at the top and it was seen that the surface of the vortex was continuously replaced. Without being bound to theory, it is speculated that a monomer transport from the gas phase towards the polymer slurry (which is the locus of polymerization) is greatly facilitated under these agitation conditions. This being the case it would be expected from someone skilled in the art of polymerization that the use of this type of mixer, a coaxial turbine double helical ribbon mixer, would suffice to allow continued polymerization beyond the point reached in comparative examples 2-5.

Comparative Example 7

The same reaction slurry from comparative example 2 and the same mixing reactor of comparative example 6 was used in the following experiment. In a deviation from comparative example 6, the Viscoprop marine type stirrer from the inner agitator section was removed. The outer Paravisc agitator again operated at 38 rpm. As in comparative example 6 it was observed that the polymer slurry was only transported in a horizontal direction and a material transport along the vertical direction did not occur. The mixing performance was not improved even when a baffle was installed. In all cases, the vortex was "standing still" and its surface was not exchanged with the bulk medium.