Patent Description:
Thermoplastic polymers with both high and low temperature resistance along with high chemical and weathering resistance are desirable in several applications in, for example, wire and cable, film, coatings, battery separators or binders, and tubing. For applications where melt-processing is required, it is a large advantage if the processing temperature and viscosity fall within a reasonable range such that the materials can be processed by common and well-known methods such as extrusion (film, wire, etc.), injection molding, blow molding, and 3D printing. Poly(vinylidene fluoride) (PVDF) exhibits very good weathering and processability, but is limited in terms of low temperature and high temperature performance. <CIT> discloses a coating composition of fluoropolymers with hydrophilizing effect,comprising A) a copolymer which is composed of tetrafluoroethylene or chlorotrifluoroethylene, a comonomer which contains -COOM groups (M = alkali metal, ammonium group) and, if desired, a fluorine-containing vinyl ether, dissolved in a mixture of an aprotic solvent and water, and mixed with B) a colloidal, aqueous dispersion of a fluoroolefinic or fluorochloroolefinic homo- or copolymer having a melting point of ≤<NUM>.

Poly(tetrafluoroethylene) (PTFE) and PTFE-based copolymers are known to have both high and low temperature performance, but often exhibit poor or difficult melt-processability. Furthermore, low molecular weight PTFE is known to fail (crack) when under mechanical stress and thermal stress simultaneously. There is a need for easily processable, high/low temperature resistant, and thermal stress crack resistant fluoropolymers.

Stress cracking is a phenomenon in polymeric materials where an external factor (solvent, heat, light, etc.) acting on said polymer can cause brittle failure at strains or stresses well below the yield point.

Document <CIT> discloses solid polymers consisting of <NUM>-<NUM> mol% perfluoroalkyl perfluorovinyl ether units, <NUM>-<NUM>% VDF and <NUM>-<NUM>% (-CFX-CFY-) repeat units for curable resins, but does not discuss viscosity or MW control, processing, or stress crack resistance.

Japanese Publication <CIT> teaches optical fibers and optical fiber cables having <NUM> to <NUM>% by mass VDF, <NUM> to <NUM>% TFE and <NUM> to <NUM> % fluorovinyl compound represented by CF<NUM>=CF-(OCF<NUM>CF(CF<NUM>))a-O-Rf2 and has a <NUM> to <NUM> refractive index. Rf2 is a C1 to C8 alkyl group, fluoroalkyl group, alkoxy alkyl group or fluoroalkoxy alkyl group and a is an integer of <NUM> to <NUM>. The polymer is used as the outer coating of an optical fiber.

Document <CIT> discloses a fluororesin that includes TFE, VDF and another ethylenically unsaturated monomer that has a storage modulus between <NUM>-<NUM> MPa at <NUM>. <CIT> requires a minimum quantity of TFE of <NUM> mol%.

The present invention relates to a tetrafluoroethylene (TFE), vinylidene fluoride (VDF) and fluorinated ether copolymer as described in appended claim <NUM>. Preferably the fluorinated ether is a perfluoroalkyl vinyl ether such as perfluoropropyl vinyl ether (PPVE). The composition can be processed by conventional means into wires, cables, films, and any other part or article made by a thermoplastic shaping process. The resulting articles have both high and low temperature resistance and are resistant to thermal stress cracking. The polymer of the invention has a high melting temperature (above 175C) and low temperature impact resistance below - 80C.

The polymer comprises from <NUM> - <NUM> mol% TFE, <NUM> - <NUM> mol% VDF and from <NUM> to <NUM> mol% fluorinated vinyl ether. The polymer is melt processable and exhibits stress crack resistance as measured by melt rheology and the method as described herein, respectively.

In the context of the present application, unless otherwise indicated, all viscosities are melt viscosities that are measured at a temperature of <NUM> and at a shear rate of <NUM>-<NUM>. More specifically, melt viscosities may be measured by a Dynisco LCR <NUM> capillary rheometer. Measurements are performed at <NUM> at shear rates from <NUM> to <NUM>-<NUM>, with the viscosity recorded at <NUM>-<NUM> using ASTM method D-<NUM>.

Differential scanning calorimetry (DSC) is used to determine melting point of the inventive polymer. The DSC is run with a heating/cooling rate of <NUM>/min. The ΔH of formation is defined by integrating the endothermic area in the heat flow curve during the second heat (Tm) or exothermic area during the first cool (Tc) and dividing by the temperature ramp rate and sample mass using ASTM <NUM>.

Thermal stress cracking is performed by molding <NUM> thick rectangular strips of the copolymers, pre-soak at the test temperature for <NUM>, impart stress by bending to a known radius, then visually checking for cracks at each time point. The failure is quantified by the fraction of parts that have cracked at a given time. The results reported here are reported as a simple pass or fail. A pass indicates that the polymer parts show no cracking after three days of testing, and a fail indicates that at least one part shows a crack within <NUM> days of testing. More specifically, the parts are molded to be <NUM> thick, <NUM> long, and <NUM> wide. The outer diameter of which they are wrapped for testing is <NUM> giving a calculated strain of <NUM>% on the outer polymer surface, and the testing temperature was at <NUM>.

The materials described herein are melt processable semicrystalline thermoplastic polymers. Melt processable herein means that the polymers can be melted shaped (through an extruder or injection molder, for example) and then cooled to provide a shaped article while retaining shape with no cracking. Additionally, herein a melt viscosity at or above <NUM> Pa*s (<NUM> kP) at <NUM> at <NUM>-<NUM>, as determined by capillary rheometry described above, is not melt processable.

Solid-state Fluorine NMR analysis is used to determine chemical composition. Using powder samples packed into <NUM> Bruker zirconia rotors, the <NUM>F solid-state spectra can be acquired on the Bruker A VIII <NUM> WB (<NUM> T) spectrometer equipped with a <NUM> CP MAS probe at room temperature with a spinning speed of <NUM>. In order to suppress background contributions, <NUM>F spectra of the empty rotors can be subtracted from the samples spectra.

The composition is a melt-processable fluoropolymer composition comprising <NUM> - <NUM> mol% TFE, <NUM> - <NUM> mol% VDF and <NUM> - <NUM> mol% fluorinated ether. When more than one ether is used, the total mol% ether present in the polymer is from <NUM> to <NUM> mol %. Preferably the ether comprises perfluoropropyl vinyl ether (PPVE).

Fluorinated ethers include but are not limited to, fluorinated or perfluorinated vinyl ethers such as perfluoromethyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), perfluoro-n-propyl vinyl ether (PPVE), perfluoroisopropyl vinyl ether (PiPVE), perfluoro-<NUM>-propoxypropyl vinyl ether, perfluorobutylvinyl ether (PBVE), longer chain perfluorinated vinyl ethers and combinations thereof.

The fluorovinyl ether may preferably be represented by CF<NUM>=CF-(OCF<NUM>CF(CF<NUM>))a-O-R. R is a C1 to C8 alkyl group, fluoroalkyl group, alkoxy alkyl group or fluoroalkoxy alkyl group and a is an integer of <NUM> to <NUM>. The fluorovinyl ether may have one of the following formulas: CF<NUM>=CF-O-(CH<NUM>)n-(CF<NUM>)m-(CF<NUM>) n, m are an integers of <NUM> to <NUM>; CF<NUM>=CF-O-(CH<NUM>)n-(CH<NUM>) n is an integer of <NUM> to <NUM>; CF<NUM>=CF-O-(CF<NUM>)n-O-CF<NUM>, n is an integer of <NUM> to <NUM>.

The inventive copolymer exhibits impact resistance of greater than <NUM> J/m, greater than <NUM> J/m, greater than <NUM> J/m at negative <NUM> as measured by izod impact testing ASTM D256.

The inventive copolymer exhibits stress crack resistance as determined by the method described herein.

The melting point of the inventive copolymers is between <NUM> to <NUM>, preferably between <NUM> to <NUM> as determined by DSC.

The copolymer of the present invention is generally insoluble at ambient temperature in solvents that are generally used for PVDF copolymers. Such solvents include polar aprotic solvents such as dimethylsulfoxide, n-methylpyrrolidone (NMP), N,N'-dimethylformamide (DMF), N,N'-dimethylacetamide (DMAc), ketones such as acetone, methyl ethylketone (MEK), methyl isobutylketone (MIBK), and ethers such as tetrahydrofuran (THF) and methyl t-butylether (MTBE). The copolymer of the present invention is also generally insoluble in solvents that PVDF and PVDF copolymers are also insoluble in such as water and aqueous solutions, aliphatic alcohols, alkyl and aromatic hydrocarbon, and chlorinated aliphatic and aromatic solvents.

The TFE/VDF/Ether polymer of the invention is synthesized with <NUM>-<NUM> mol% based on the total moles of monomer chain transfer agent (CTA), relative to total monomer used in the polymerization. To obtain the polymer of the invention both a fluorinated ether and the presence of a CTA in the polymerization reaction are required. The polymer of the invention is melt processable and is stress crack resistant.

The copolymers are prepared in via emulsion or suspension polymerization as known in the art and described in <CIT> incorporated herein by reference. For emulsion polymerization a pressure range of <NUM> MPa to <NUM> MPa and a temperature range of <NUM> to <NUM> is often used with initiating systems consisting of inorganic peroxides, redox systems, and organic peroxides as known in the art. With the TFE/VDF comonomer pair as the majority components in the structure, the reactivity ratios are such that the monomers are distributed randomly throughout the resulting polymer backbone giving a homogeneous composition throughout the bulk of the material.

A general procedure that can be used for the polymerization reaction is: tetrafluoroethylene (TFE) is purified by passing through an activated carbon bed and mixed via recirculation in a holding tank with vinylidene fluoride (VDF) in the desired ratio. A reactor is charged with deionized water in the desired amount and surfactant. The water charge is deoxygenated by pressurization of the reactor with nitrogen, holding at the pressure (e.g. for at least <NUM>) with agitation, then venting to <NUM> psig. This cycle can be repeated. The chain transfer agent (CTA) is injected to the reactor along with the prescribed amount of fluorinated ether. The reactor contents are agitated, heated and pressurized with the TFE/VDF mixture. Initiator is added to start the polymerization. TFE/VDF mixture is then added to maintain the pressure. Additional aliquots of initiator solution are added to maintain monomer uptake at the desired rate. Reaction is completed once the prescribed amount of TFE/VDF mixture has been added to the reaction, at which point, TFE/VDF mixture feed is stopped and pressure is allowed to decrease autogenously. The reaction is cooled to room temperature, vented and product discharged through the reactor bottom port. The solid content of the latex product is determined by drying a known mass of latex to constant weight then using the mass difference to calculate the percent solids of the latex product.

The inventive polymer is synthesized with <NUM>-<NUM> mol%, preferably <NUM>-<NUM> mol%, and most preferably <NUM>-<NUM> mol% based on the total moles of monomer of a suitable chain transfer agent. Chain-transfer agents are added to the polymerization to regulate the molecular weight of the product. They may be added to a polymerization in a single portion at the beginning of the reaction, or incrementally or continuously throughout the reaction. The amount and mode of addition of chain-transfer agent depend on the activity of the particular chain-transfer agent employed, and on the desired molecular weight of the polymer product. Examples of chain transfer agents useful in the present invention include, but are not limited to oxygenated organic compounds such as alcohols, carbonates, ketones, esters, and ethers may serve as chain-transfer agents; halocarbons and hydrohalocarbons, such as chlorocarbons, hydrochlorocarbons, chlorofluorocarbons and hydrochlorofluorocarbons; ethane and propane. Specific examples of the chain transfer agent(s) tha can be used include, but are not limited to, hydrocarbons such as ethane, propane, butane, isopentane, n-hexane and cyclohexane; aromatic compounds such as toluene and xylene; ketones such as acetone; acetic acid esters such as ethyl acetate and butyl acetate; alcohols such as methanol and ethanol; mercaptans such as methyl mercaptan; and halogenated hydrocarbons such as carbon tetrachloride, chloroform, methylene chloride and methyl chloride, and polyacrylic acid.

In one embodiment, the melt-processable fluoropolymer composition comprising <NUM> - <NUM> mol% TFE, <NUM> - <NUM> mol% VDF and <NUM> - <NUM> mol% PPVE is synthesized using <NUM>-<NUM> mol% ethyl acetate (based on the total moles of monomer) as the chain transfer agent.

The polymerization can use fluorosurfactant or non-fluorinated surfactant as an emulsifier as well known in the art. (<CIT>, <CIT> incorporated herein by reference).

The inventive polymer can be heat processed on normal processing equipment used to extrude or mold PVDF copolymers. The inventive polymer can be extruded or heat molded to any desired shape.

The applications for such a polymer could include jacketing for wires, cables, architectural films (greenhouse), coatings, battery binder, battery separator films or coatings, offshore pipes, dielectric films, piezoelectric films and sensors, pyroelectric films and sensors, chemical processing films, polymer processing aids, matrix for composites, and automotive tubing.

The procedure below is written using poly(tetrafluoroethylene-co-vinylidene fluoride-co-perfluoropropyl vinyl ether) (p(TFE-VDF-PPVE)) as the model copolymer. One of ordinary skill in the art could use the examples below, and teachings of the application to extend the invention to other fluoropolymers of the invention. Table <NUM> shows reaction parameters for the Examples.

Tetrafluoroethylene (TFE) is purified by passing through an activated carbon bed and mixed via recirculation in a holding tank with vinylidene fluoride (VDF) in the prescribed ratio. A <NUM>-volume autoclave equipped with internal cooling coils and mechanical agitation is charged with deionized water in the desired amount as well as <NUM> of (surfactant) perfluoro (<NUM>,<NUM>-dimethyl-<NUM>,<NUM>-dioxanonanoic acid). This water charge is deoxygenated by pressurization of the reactor to 60psig with ultra-pure nitrogen, holding at that pressure for <NUM> with agitation, then venting to <NUM> psig. This cycle is repeated an additional <NUM> times. At that point the ethyl acetate (EA) chain transfer agent (CTA) is injected to the reactor along with the prescribed amount of perfluoropropyl vinyl ether (PPVE). The reactor contents are agitated, heated to 80C and pressurized to <NUM>. 4MPa with the TFE/VDF mixture. <NUM> of an aqueous solution of <NUM>% potassium persulfate and <NUM>% dipotassium phosphate (KPS/K2HP) is added using a high-pressure syringe pump to start the polymerization. TFE/VDF mixture is then added in the same ratio as during pressurization to maintain the <NUM>. 4MPa pressure. Additional aliquots of KPS/K2HP solution are added to maintain monomer uptake at a rate greater than <NUM>/hr. The reaction is completed once the prescribed amount of TFE/VDF mixture has been added to the reactor, at which point, TFE/VDF mixture feed is stopped and pressure is allowed to decrease autogenously for <NUM> minutes. The reactor is cooled to room temperature, vented and product discharged through the reactor bottom port. The solid content of the latex product is determined by drying a known mass of latex to constant weight then using the mass difference to calculate the percent solids of the latex product.

Each run constitutes a single batch as described in the general procedure with the material quantities and process parameters as noted.

Melting temperature and melt viscosity was measured as described above.

Claim 1:
A fluoropolymer comprising <NUM> - <NUM> mol% TFE, <NUM> - <NUM> mol% VDF and <NUM> - <NUM> mol% fluorinated ether, wherein the fluoropolymer is melt processable and has a melt viscosity at <NUM>-<NUM> as measured according to ASTM D-<NUM> at <NUM> using a capillary rheometer at shear rates from <NUM> to <NUM>-<NUM> of from <NUM> to <NUM> Pa*s (<NUM> to <NUM> kP).