Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-01-31
2003-04-15
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C525S199000, C525S200000, C526S242000, C526S247000
Reexamination Certificate
active
06548612
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/suitable 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 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
-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 (Modern 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. Unfortunately, these 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 polyetheretherketone and polyphenylene sulfide (WO 97/43102) and polyacetal (DE 41 12 248 A1). The latter method compromises important physico-chemical properties of the resulting composition, when compared to neat PTFE, or requires uneconomical and cumbersome removal of the host material.
There exist PTFE grades of low molecular weight and of low viscosity. These grades, which are often are referred to as micropowders, commonly are used as additives in inks, coatings and in thermoplastic and other polymers to impair, for example, nucleation, internal lubrication or other desirable properties that, in part, stem from the unique physico-chemical properties of the neat PTFE. Low molecular weight PTFE grades, in their solid form, unfortunately, exhibit extreme brittleness and, according to at least one of the suppliers, these PTFE grades . . . “are not to be used as molding or extrusion powders” (Du Pont, Zonyl® data sheets and url:http://www.dupont.com/teflon/fluoroadditives/about.html—Jul. 7, 1998).
Thus, a need continues to exist to develop melt-processible, thermoplastic poly(tetrafluoroethylene)s to exploit the outstanding properties of this polymer in a wider spectrum of product forms, as well as to enable more economical processing of this unique material.
SUMMARY OF THE INVENTION
Surprisingly, it has been found that poly(tetrafluoroethylene)s of a particular set of physical characteristics provide a solution to the above, unsatisfactory situation.
Accordingly, it is one objective of the present invention to provide melt-processible, thermoplastic PTFE compositions of good mechanical properties comprising PTFE grades that are characterized as having a non-zero melt-flow index in a particular range. As used hereinafter, the indication “good mechanical properties” means the polymer has properties suitable for use in thermoplastic applications, preferably including applications such as melt-processed thermoplastic formed into unoriented, solid fibers or films exhibiting an elongation at break of at least 10%, determined under standard ambient conditions at a rate of elongation of 100% per min.
Yet another object of the present
Bastiaansen Cees
Smith Paul
Tervoort Theodorus
Visjager Jeroen
Eidgenössische Technische Hochschule Zürich
Pezzuto Helen L.
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