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Fluoropolymer blending processRelated Patent Categories: Synthetic Resins Or Natural Rubbers -- Part Of The Class 520 Series, Natural Rubber Compositions Having Nonreactive Materials (dnrm) Other Than: Carbon, Silicon Dioxide, Glass Titanium Dioxide, Water, Hydrocarbon, Halohydrocarbon, Ethylenically Unsaturated Reactant Admixed With A Preformed Reaction Product Derived From: (a) At Least One Polycarboxylic Acid, Ester, Or Anhydride; (b) At Least One Polyhydroxy Compound; And (c) At Least One Fatty Acid Glycerol Ester, Or A Fatty Acid Or Salt Derived From A Naturally Occurring Glyceride, Tall Oil, Or A Tall Oil Fatty Acid, At Least One Solid Polymer Derived From Ethylenic Reactants Only, Polymer Mixture Of Two Or More Solid Polymers Derived From Ethylenically Unsaturated Reactants Only; Or Mixtures Of Said Polymer Mixture With A Chemical Treating Agent; Or Products Or Processes Of Preparing Any Of The Above Mixtures, Solid Polymer Derived From Fluorine-containing Ethylenic ReactantFluoropolymer blending process description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070117930, Fluoropolymer blending process. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates to the blending together of polytetrafluoroethylene and melt-fabricable perfluoropolymer. [0003] 2. Description of Related Art [0004] US 2004/0242783 A1 discloses a blend of tetrafluoroethylene/hexafluoropropylene copolymer, commonly called FEP, and polytetrafluoroethylene (PTFE), the PTFE imparting the improved extrusion property of reduced cone breaks during melt draw down extrusion coating of wire. The FEP composition by itself is selected to provide good crack resistance for molded articles, and the PTFE has a high enough molecular weight so that the molded article does not have poor crack resistance. The PTFE content of the blend is disclosed to be 0.03 to 2 parts by weight based on 100 parts by weight of the copolymer. When the amount of PTFE is more than 2 parts by weight, two disadvantageous results are disclosed: the melt viscosity of the blend increases significantly and the molded article tends to become brittle [0027]. The FEP and PTFE are blended together by melt kneading. Before kneading, the copolymer and PTFE can be pre-mixed to improve the degree of dispersion of the PTFE [0042]. In Example 1, powders of the PTFE and copolymer are mixed together, followed by kneading in a twin-screw extruder to produce molding pellets, which are then melt-extruded, using a single screw extruder, as a coating onto wire. The PTFE powder has an average particle size of 450 micrometers. The particle size of the copolymer is not disclosed in Example 1, but the aqueous emulsion polymerization to obtain this copolymer is disclosed. The copolymer is recovered from emulsion polymerization by coagulation, which provides a dry powder particles having an average size about the same as the PTFE powder. SUMMARY OF THE INVENTION [0005] It has been discovered that much greater proportions of PTFE can be blended not only with FEP but with melt-fabricable perfluoropolymers in general without the resultant blend losing its melt fabricability and without causing articles molded from the resultant melt blend to be brittle. The only limitation on the greater proportion of PTFE that can be incorporated into the blend is that the PTFE is the disperse phase in the melt blend and the melt-fabricable perfluoropolymer is the continuous phase. [0006] The present invention can be described as the process for melt fabricating perfluoropolymer, comprising forming a mixture of submicrometer-size particles of both non-melt-flowable polytetrafluoroethylene (PTFE) and melt-fabricable perfluoropolymer, melt blending said mixture to form a dispersion of said particles of non-melt flowable PTFE in a continuous phase of said melt-fabricable perfluoropolymer, and molding the resultant melt blend into an article. Preferably, the non-melt flowable PTFE constitutes at least about 0.1 wt %, preferably at least about 0.5 wt %, based on the combined weight of the non-melt flowable PTFE and melt-fabricable perfluoropolymer to obtain appreciable advantage from the PTFE component. Good physical properties can exist when the non-melt flowable PTFE constitutes up to about 75 wt % based on the aforesaid combined weight. This composition and others disclosed herein for use in the present invention apply both to the mixture of polymer particles and to the melt blend, wherein the melt fabricable perfluoropolymer is the continuous phase and therefore is no longer in particulate form. The composition used to form the mixture is considered to be the same as the composition of the melt blend and thus of the article molded therefrom. [0007] In a preferred embodiment, the forming of the mixture of submicrometer-size particles is done by mixing together an aqueous dispersion of submicrometer-size particles comprising the PTFE and an aqueous dispersion of submicrometer-size particles comprising the melt-fabricable perfluoropolymer and separating the resultant mixture of said submicron particles from the resultant mixture of said aqueous dispersions, i.e. from the aqueous media of the combined dispersions. The separation step is conveniently carried out by co-coagulating the mixed-together aqueous dispersions and drying the resultant mixture of submicrometer-size particles. The mixture of submicrometer-size particles remains after the separation step. If co-coagulation of the mixed dispersions is used as the separation step, the resultant agglomerates contain particles of both polymers, i.e. particles of both polymers are agglomerated together. Upon drying, the agglomerates become a powder, which is typically referred to as secondary particles and which upon melt mixing, disperses the primary particles of the non-melt flowable PTFE into the continuous phase of the melt-fabricable perfluoropolymer formed during the melt mixing. The 450 micrometer average particle size for the PTFE of Example 1 of US 2004/0242783 A1 is a typical secondary particle size. In the melt mixing, the PTFE particles retain their particulate identity, while the perfluoropolymer particles melt and flow together to lose their particulate identity, to form the continuous phase of the melt blend. The continuous phase being the melt flowable perfluoropolymer is confirmed by the melt fabricability of the melt mixed composition. Articles molded from the composition are transparent to translucent, rather than opaque as are articles molded from PTFE. [0008] The melt fabrication process of the present invention preferably starts with the mixing together of the primary dispersion-polymerized particles of the two polymers. In contrast, US 2004/0242783 A1, practices the mixing together of secondary particles of each polymer. The kneading in a twin screw extruder as required in '783 is not required in the present invention. Melt blending can be carried out in a single screw extruder that would be used for extrusion or for polymer melting in injection molding. The significance of the difference between these procedures is the ability to incorporate a greater proportion of the PTFE into the melt blend with the perfluoropolymer to obtain surprisingly advantageous results. This is true even for PTFE contents of at least about 4 wt % based on combined weight non-melt flowable PTFE and melt-fabricable perfluoropolymer. [0009] With respect to melt properties, the melt blend produced in accordance with the present invention is thixotropic, i.e. the melt viscosity of the blend decreases (becomes more fluid) with increasing shear. In this regard, the mixture of submicrometer-size particles of non-melt-flowable PTFE and melt-fabricable perfluoropolymer is preferably characterized by a reduction in melt viscosity upon increasing shear rate from about 10 s.sup.-1 to about 100 s.sup.-1 that is at least about 10% greater than the reduction in melt viscosity at the same shear rates for the melt-fabricable perfluoropolymer by itself, as determined by the capillary rheometer method described hereinafter. [0010] With respect to physical properties, the absence of brittleness in articles melt fabricated in accordance with the present invention is indicated preferably by the mixture of submicrometer-size particles of non-melt-flowable PTFE and melt-fabricable perfluoropolymer being characterized by an elongation at break of at least about 200%, preferably at least 250%, as determined by tensile testing in accordance with ASTM D 638-03 as further described hereinafter. More preferably the elongation at break is at least 75% of that of the melt-fabricable perfluoropolymer by itself, more preferably at least 85% thereof. As determined by the same ASTM test, the tensile strength of the mixture is preferably at least about 75% of that of the melt-fabricable perfluoropolymer by itself, more preferably at least about 85% thereof. As shown in the Examples, mixtures containing much greater amounts of the PTFE component than 2 parts by weight of PTFE/100 parts of the FEP exhibit elongation at break and/or tensile strength that is at least as high as that for the perfluoropolymer composition by itself. [0011] Contrary to the expectation from US 2004/0242783 A1, these thixotropy and elongation attributes exist for compositions containing at least about 4 wt % of the PTFE component, based on combined weight as described above, as well as for lesser amounts, e.g. as little as 0.5 wt % PTFE. The maximum amount of PTFE in the composition at which these attributes will exist will depend on the particular melt-fabricable perfluoropolymer, and will extend up to at least about 15 wt % of the PTFE component, more preferably up to about at least about 25 wt %, and most preferably, at up to least about 30 wt % of the PTFE component, based on combined weight. DETAILED DESCRIPTION OF THE INVENTION [0012] The present invention starts with creating a mixture of the two components: submicrometer-size particles of non-melt flowable PTFE and submicrometer-size particles of melt-fabricable perfluoropolymer. [0013] With respect to the non-melt flowable PTFE component, the non-melt flowability aspect of the PTFE can also be characterized by high melt creep viscosity, sometimes called specific melt viscosity, which involves the measurement of the rate of elongation of a molten sliver of PTFE under a known tensile stress for 30 min., as further described in and determined in accordance with U.S. Pat. No. 6,841,594, referring to the specific melt viscosity measurement procedure of U.S. Pat. No. 3,819,594. In this test, the molten sliver is maintained under load for 30 min, before the measurement of melt creep viscosity is begun, and this measurement is then made during the next 30 minutes of applied load. The PTFE preferably has a melt creep viscosity of at least about 1.times.10.sup.6 Pa.circle-solid.s, more preferably at least about 1.times.10.sup.7 Pa.circle-solid.s, and most preferably at least about 1.times.10.sup.8 Pa.circle-solid.s, all at 380.degree. C. This temperature is well above the first and second melt temperatures of PTFE of 343.degree. C. and 327.degree. C., respectively. The difference between non-melt flowability of the PTFE core and the melt flowability of the melt-fabricable perfluoropolymer shell is apparent from the melt flow rate (MFR) test procedure of ASTM D 1238-94a. In this procedure, the MFR is the rate in g/10 min at which perfluoropolymer flows through a defined orifice under a specified load at a specified temperature, usually 372.degree. C. Since the PTFE used in the present invention has no melt flow in general and when subjected to the procedure of ASTM D 1238-94a, has a zero MFR, the melt characteristic of the PTFE is not determined by this ASTM procedure. The high melt creep viscosity of the PTFE present in the core of the core/shell polymer also means that the PTFE is sinterable, i.e. a molded article, unsupported by the mold (free-standing), of the PTFE can be heated above the melting point of the PTFE to coalesce the PTFE particles together without the molded article flowing to lose its shape. The PTFE used in the present invention is also often characterized by standard specific gravity (SSG), which is the ratio of weight in air of a PTFE specimen prepared in a specified manner to an equal volume of water at 23.degree. C. as further described in U.S. Pat. No. 4,036,802 and ASTM D 4894-94. The lower the SSG, the higher the molecular weight of the PTFE. The specimen preparation procedure as disclosed in ASTM D4894-94 includes compression molding the test specimen, removing the compression molded test specimen from the mold, and sintering the specimen in air, i.e. free standing, at 380.degree. C. The non-melt flowability of the PTFE enables this sintering to be carried out without the test specimen losing its compression molded shape and dimensions. [0014] The PTFE can be the granular type or the fine powder type, made by suspension or aqueous dispersion polymerization, respectively. The PTFE can be homopolymer of tetrafluoroethylene or a copolymer thereof with a small amount of comonomer, such as hexafluoropropylene or perfluoro(alkyl vinyl ether) wherein the alkyl group contains 1 to 5 carbon atoms, that improves the sinterability of the TFE, to obtain such improvement as reduced permeability and greater flex life, as compared to the TFE homopolymer. This type of PTFE is sometimes referred to as modified PTFE. Examples of modified PTFE are disclosed in U.S. Pat. Nos. 3,142,665, 3,819,594, and 6,870,020. For simplicity and because the modified PTFE exhibits the same non-melt flow, high melt creep viscosity of PTFE homopolymer, this type of PTFE is included in the term polytetrafluoroethylene or PTFE used herein. [0015] The non-melt flowable PTFE used in the present invention is to be distinguished from low molecular weight PTFE, which because of its low molecular weight has melt flowability but not melt-fabricability. This melt flowable PTFE, which has an MFR that is measurable by ASTM D 1238-94a, is obtained by direct polymerization under conditions that prevent very long polymer chains from forming, or by irradiation degradation of non-melt flowable PTFE. Such melt flowable PTFE is commonly called PTFE micropowder. It is not considered as being melt fabricable because the article molded from the melt is useless, by virtue of extreme brittleness. Because of its low molecular weight (relative to non-melt-flowable PTFE), it has no strength. An extruded filament of the PTFE micropowder is so brittle that it breaks upon flexing. [0016] With respect to the perfluoropolymer component of the mixture used in the present invention, as indicated by the prefix "per" in perfluoropolymer, the monovalent atoms bonded to the carbon atoms making up the polymer are all fluorine atoms. Other atoms may be present in the polymer end groups, i.e. the groups that terminate the polymer chain. The perfluoropolymer is a perfluoroplastic, not a perfluoroelastomer. The melt flow rate (MFR) of the perfluoropolymers used in the present invention can vary widely, depending on the proportion of non-melt flowable PTFE component, the melt-fabrication technique desired for the mixture of polymer components, and the properties desired in the melt-fabricated article. Thus, MFRs for the melt-fabricable fluoropolymer can be in the range of about 0.1 to 500 g/10 min, but will usually be preferred as about 0.5 to 100 g/10 min, and more preferably 0.5 to 50 g/10 min., as measured according to ASTM D-1238-94a and following the detailed conditions disclosed in U.S. Pat. No. 4,952,630, at the temperature which is standard for the resin (see for example ASTM D 2116-91a and ASTM D 3307-93 that are applicable to the most common melt-fabricable fluoropolymers, both specifying 372.degree. C. as the resin melt temperature in the Plastometer.RTM.). The amount of polymer extruded from the Plastometer.RTM. in a measured amount of time is reported in units of g/10 min in accordance with Table 2 of ASTM D 1238-94a. The higher the MFR of the perfluoropolymer, the greater is the tendency to generate smoke when the polymer is subjected to the NFPA-255 burn test, thus failing such test. The perfluoropolymer component can have high MFR, e.g. greater than 20 g/10 min, without the article melt-fabricated from the polymer mixture used in the present invention failing the NFPA-255 burn test, because the presence of the PTFE component as dispersed particles in the continuous phase of melt-fabricable perfluoropolymer making up the molded article does not flow, and thus, does not drip to cause smoke generation. [0017] Examples of perfluoropolymers that can be used in the polymer mixture used in the present invention include the copolymers of tetrafluoroethylene (TFE) with one or more polymerizable perfluorinated comonomers, such as perfluoroolefin having 3 to 8 carbon atoms, such as hexafluoropropylene (HFP), and/or perfluoro(alkyl vinyl ether) (PAVE) in which the linear or branched alkyl group contains 1 to 5 carbon atoms. Preferred PAVE monomers include perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), perfluoro(propyl vinyl ether) (PPVE), and perfluoro(butyl vinyl ether) (PBVE). The copolymer can be made using several PAVE monomers, such as the TFE/perfluoro(methyl vinyl ether)/perfluoro(propyl vinyl ether) copolymer, sometimes called MFA by the manufacturer. The preferred perfluoropolymers are TFE/HFP copolymer in which the HFP content is about 5-17 wt %, more preferably TFE/HFP/PAVE such as PEVE or PPVE, wherein the HFP content is about 5-17 wt % and the PAVE content, preferably PEVE, is about 0.2 to 4 wt %, the balance being TFE, to total 100 wt % for the copolymer. The TFE/HFP copolymers, whether or not a third comonomer is present, are commonly known as FEP. TFE/PAVE copolymers, generally known as PFA, have at least about 2 wt % PAVE, including when the PAVE is PPVE or PEVE, and will typically contain about 2-15 wt % PAVE. When PAVE includes PMVE, the composition is about 0.5-13 wt % perfluoro(methyl vinyl ether) and about 0.5 to 3 wt % PPVE, the remainder to total 100 wt % being TFE, and as stated above, may be referred to as MFA. The low melt viscosity of these copolymers relative to the high melt creep viscosity of the PTFE, provides the melt flowability to the perfluoropolymer for its melt fabricability, and the perfluoropolymer composition itself provides the strength required for the practical utility of the article melt fabricated from the perfluoropolymer. The melt flow difference between the melt-fabricable perfluoropolymer being characterizable by melt viscosity and MFR and the non-melt flowable PTFE being characterizable by melt creep viscosity and SSG is great as indicated by the inability of the melt-fabricable perfluoropolymer to be characterizable by melt creep viscosity or by SSG. The melt-fabricable perfluoropolymer cannot be characterized by either melt creep viscosity or by SSG. In the melt creep viscosity test, the sliver of molten perfluoropolymer melts, flows, and breaks during the 30 minutes initial heating under load at 380.degree. C., so there is no sliver remaining for the melt creep determination during the second 30 minutes of heating. In the SSG test, the specimen melts and flows during the 380.degree. C. heating (sintering for non-melt flowable PTFE), undermining the integrity of the specimen for the SSG determination. Of course, the non-melt flowability of the PTFE used in the present invention, enables either of the melt creep viscosity or SSG determinations to be made on such PTFE. [0018] The perfluoroolefin or PAVE comonomer composition of the perfluoropolymer component is determined by infrared analysis on compression molded film made from the perfluoropolymer particles in accordance with the procedures disclosed in U.S. Pat. No. 4,380,618 for the particular fluoromonomers (HFP and PPVE) disclosed therein. The analysis procedure for other fluoromonomers are disclosed in the literature on polymers containing such other fluoromonomers. For example, the infrared analysis for PEVE is disclosed in U.S. Pat. No. 5,677,404. [0019] The combination of the non-melt flowable PTFE and melt fabricable perfluoropolymer components (submicrometer-size particles) used in the present invention results in a polymer mixture that is also melt fabricable. One attribute of melt flowability, enabling melt fabricability, is that the polymer mixture used the present invention exhibits a melt viscosity of preferably no more than about 5.times.10.sup.5 Pa.circle-solid.s, more preferably, no more than about 1.times.10.sup.5 Pa.circle-solid.s, and most preferably, no more than about 5.times.10.sup.4 Pa.circle-solid.s, all at a shear rate of 100 s.sup.-1 and melt temperature in the range of about 350.degree. C. to 400.degree. C. The determination of melt viscosities disclosed herein, unless otherwise indicated, is by dividing shear stress applied to the polymer melt by shear rate applied to the polymer melt as disclosed on p. 31 of F. N. Cogswell, Polymer Melt Rheology. A Guide for Industrial Practice, published by Woodhead Publishing (1996). As a practical matter, the equivalent melt viscosities are obtained simply by readout from the computer accompanying the rheometer used to determine shear rate and shear stress. The melt viscosity of the melt-fabricable perfluoropolymer by itself is such that the above mentioned melt viscosities for the polymer mixture are obtained. The melt viscosity of the perfluoropolymer component by itself can also be characterized by the above mentioned melt viscosities. [0020] As discussed above, the melt fabricability of the perfluoropolymer component can be characterized by its MFR. Although the presence of the non-melt flowable PTFE component may lower the MFR of the overall melt blend of the polymers as compared to the MFR of the perfluoropolymer by itself, and may even render the MFR not measurable by ASTM D 1238-94a, the thixotropy exhibited by the polymer blend when subjected to sufficient shear in the molten state, enables the resultant melt blend to be melt fabricated into articles by the typical melt fabrication techniques of extrusion and injection molding. The melt viscosity of the polymer blend, as discussed above, reflects the thixotropic effect, because of its determination at a much higher shear rate than is encountered in the MFR determination. The thixotropic effect extends over the entire range of polymer mixture compositions. At least about 0.5 wt % of the PTFE component is required before the thixotropic effect is appreciable. The maximum amount of PTFE component is preferably up to that amount beyond which the PTFE is no longer the dispersed phase when the polymer mixture is melt mixed (blended), such as occurs in extrusion or injection molding. Preferably the reduction in viscosity is at least about 100%, and more preferably at least about 500% greater than the viscosity reduction for the perfluoropolymer by itself when the shear rate is increased from about 10 s.sup.-1 to about 100 s.sup.-1. These shear rates are expressed in terms of "about", because of limitations in the operation of the rheometer used to measure them. The rheometer includes a variable speed piston that provides the volumetric flow rate (Q) of molten polymer through the rheometer orifice and various orifice sizes, the selection of which provides the radius r in the equation: shear rate (.gamma.)=4Q/.pi.r.sup.3. With particular rheometers it may be difficult to adjust the piston speed and orifice size such that the exact shear rates of 10 s.sup.-1 and 100 s.sup.-1 are obtained. The shear rates used in the Examples were 11.9 s.sup.-1 and 101 s.sup.-1. Typically, the rheometer can be operated so that the shear rates are 10 s.sup.-1.+-.3 s.sup.-1 and 100 s.sup.-1.+-.5 s.sup.-1. In absolute terms, the preferred reduction in melt viscosity by the polymer mixture used in the present invention is at least about 200 Pas, more preferably at least about 400 Pas at the shear rates specified above. [0021] The advantage of thixotropy discovered by the present invention extends to higher shear rates than 100 s.sup.-1 enabling the polymer mixture to be extruded at a faster rate by melt-draw down extrusion than the melt-fabricable perfluoropolymer by itself. Alternatively, the melt cone formed in melt-draw-down extrusion can have a lower draw-down ratio (DDR) than the usual DDR of 80 to 100:1, to improve concentricity of the wall thickness of the extrudate, applied for example as jacketing on FEP insulated communications cable, especially such cable used in plenums of buildings. DDR is the ratio of the cross-sectional area of the annular die opening to the cross-sectional area of the final shape and size of the extrudate, e.g. the plenum cable jacket just described. Continue reading about Fluoropolymer blending process... Full patent description for Fluoropolymer blending process Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Fluoropolymer blending process patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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