Stock material or miscellaneous articles – Composite – Of fluorinated addition polymer from unsaturated monomers
Reexamination Certificate
2000-02-16
2004-05-18
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Composite
Of fluorinated addition polymer from unsaturated monomers
C428S421000, C525S499000, C526S242000, C526S247000, C526S250000, C526S253000, C526S254000, C526S255000
Reexamination Certificate
active
06737165
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to melt-processible poly(tetrafluoroethylene) (PTFE), compositions thereof, articles formed therefrom, and methods for making the same. More particularly, the present inventions relates to a particular range of poly(tetrafluoroethylene) polymers which are readily melt-processible while maintaining good mechanical properties. Further, the present invention relates to products made of melt-processible, thermoplastic PTFE compositions.
BACKGROUND OF THE INVENTION
Poly(tetrafluoroethylene) (PTFE) is well-known for, among other properties, its chemical resistance, high temperature stability, resistance against ultra-violet radiation, low friction coefficient and low dielectric constant. As a result, it has found numerous applications in harsh physico-chemical environments and other demanding conditions. Equally well-known is the intractability of this important polymer. Numerous textbooks, research articles, product brochures and patents state that PTFE is intractable because, above its crystalline melting temperature, it does not form a fluid phase that is of a viscosity that permits standard melt-processing techniques commonly used for most thermoplastic polymers (Modern Fluoropolymers, J. Scheirs, Ed. Wiley (New York), 1997; The Encyclopaedia of Advanced Materials, Vol. 2, D. Bloor et al. Eds., Pergamon (Oxford) 1994; WO 94/02547; WO 97/43102). Suitability of a polymer for standard melt-processing techniques may be evaluated, for example, through measurement of the melt-flow index (MFI) of the material (cf. ASTM D1238-88). Melt-processible polymers should, according to this widely employed method, exhibit at least a non-zero value of the melt-flow index, which is not the case for common PTFE under testing conditions that are representative of, and comparable to those encountered in standard polymer melt-processing. The extremely high viscosity of PTFE, reported to be in the range of 10
10
14 10
13
Pa.s at 380° C., is believed to be associated, among other things, with an ultra-high molecular weight of the polymer, which has been estimated to be in the regime well above 1,000,000 g/mol and often is quoted to be of the order of 10,000,000 g/mol. In fact, it is claimed (Modem Fluoropolymers, J. Scheirs, Ed. Wiley (New York), 1997, p. 240) that “to achieve mechanical strength and toughness, the molecular weight of PTFE is required to be in the range 10
7
-10
8
g/mol . . . ”. Due to this high viscosity, common PTFE is processed into useful shapes and objects with techniques that are dissimilar to standard melt-processing methods. Rods, sheets, membranes, fibers and coatings of PTFE are produced by, for example, ram-extrusion, pre-forming and sintering of compressed powder, optionally followed by machining or skiving, paste-extrusion, high isostatic pressure processing, suspension spinning, and the like, and direct plasma polymerization.
Illustrative for the difficulties encountered in processing common PTFE are the complex and indirect methods by which fibers are produced from this polymer. Polytetrafluoroethylene fibers have been produced, as described in U.S. Pat. No. 3,655,853, by forming a mixture of viscose and PTFE particles in a dispersion, extruding the mixture through a spinneret into an acidic bath to form fibers consisting of a cellulosic matrix containing the PTFE particles. After washing and rinsing, the fibers are heated to a temperature of about 370° C. to 390° C. to decompose the cellulosic material and to melt and coalesce the polymer particles. The fibers are then drawn at a ratio of about 4:1 to 35:1 typically at a temperature between 370° C. and 390° C. The fibers produced by this relatively complex and expensive process may require further processing steps, such as bleaching to remove residual contaminants, which commonly lowers the tensile strength. Another method to produce fibers of PTFE is described in U.S. Pat. Nos. 3,953,566, 3,962,153, and 4,064,214. In this method a paste formed by mixing a lubricant, such as a mineral spirit, with a fine powder of PTFE produced by coagulation of an aqueous dispersion of PTFE particles, is extruded and formed to produce a tape, film or bead. The product thus formed, is slit to form fibers, is dried to remove the lubricant and subsequently stretched at a high rate, and at a temperature lower than the crystalline melt point of PTFE, to produce a porous article. The porous article is then heated while maintained in the stretched condition to a temperature above the melt point of crystalline PTFE, generally considered to be in the range 327° C. to 345° C., to increase strength. Alternatively, PTFE fibers are produced by first forming a solid preform by sintering the polymer for prolongued periods of time above the melting temperature of the polymer and cooling the mass down to room temperature, which is a process that may take as much as 48 hrs. Subsequently, PTFE fibers are cut from the preform by the well-know skiving method, typically yielding fibers of high denier (>>100).
Unfortunately, the above methods generally are less economical than common melt-processing, and, in addition, severely limit the types and characteristics of objects and products that can be manufactured with this unique polymer. For example, common thermoplastic polymers, such as polyethylene, isotactic polypropylene, nylons, poly(methylmethacrylate) polyesters, and the like, can readily be melt-processed into a variety forms and products that are of complex shapes, and/or exhibit, for example, some of the following characteristics: dense, void-free, thin, clear or translucent; i.e. properties that are not readily, if at all, associated with products fabricated from PTFE.
The above drawback of PTFE has been recognised virtually since its invention, and ever since, methods have been developed to circumvent the intractability of the polymer. For example, a variety of co-monomers have been introduced in the PTFE macromolecular chains that lead to co-polymers of reduced viscosity and melting temperature. Co-polymers are those that are polymerized with, for example, hexafluoropropylene, perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), perfluoro(propyl vinyl ether), or perfluoro-(2,2-dimethyl-1,3-dioxole), partially-fluorinated monomers and combinations thereof, in addition to the tetrafluoroethylene monomer. Several of the resulting co-polymers (for example, those referred to as FEP, MFA, PFA and Teflon® AF) provide improved processibility, and can be processed with techniques for common thermoplastic polymers (WO 98/58105). However, a penalty is paid in terms of some or all of the outstanding properties of the homopolymer PTFE, such as reduced melting temperature and thermal and chemical stability.
Additional methods to process the PTFE homopolymer include, for example, the addition of lubricants, plasticizers, and processing aids, as well as oligomeric polyfluorinated substances and hydrocarbyl terminated TFE-oligomers (for example, Vydax® 1000) (U.S. Pat. Nos. 4,360,488; 4,385,026 and WO 94/02547). The latter method, however, is directed to the improvement of the creep resistance of common PTFE which results in a bimodal morphology with two distinct melting temperatures, and generally does not lead to homogeneous PTFE compositions that can be melt-processed according to standard methods. For example, only a hot-compression molding method is heretofore known for mixtures of standard PTFE and Vydax® 1000, that preferably is carried out in the narrow temperature range between about 330° C. to 338° C. The other aforementioned additions of lubricants, plasticizers, and processing aids also do not yield truly melt-processible PTFE compositions. Solution processing, at superautogeneous pressure, of PTFE from perfluoroalkanes containing 2-20 carbon atoms has been disclosed in WO 94/15998. The latter process is distinctly different from melt-processing methods. Also disclosed is dispersion, and subsequent melt-processing of standard PTFE into thermoplastic (host-) polymers such as polyetheretherke
Bastiaansen Cees
Smith Paul
Tervoort Theodorus
Visjager Jeroen F.
Chen Vivian
Omlidon Technologies LLC
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