Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-03-12
2003-06-17
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S133000, C525S153000
Reexamination Certificate
active
06579942
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to polytetrafluoroethylene compositions and the fabrication of articles therefrom.
BACKGROUND OF THE INVENTION
Polytetrafluoroethylene (PTFE) is known for its non melt-fabricability, i.e. it has a melt viscosity which is so high, e.g. at least 10
8
Pa·s at 380° C., that it does not flow in the molten condition. Consequently, the PTFE cannot be fabricated by the usual melt-fabrication process of extrusion, including injection molding. Instead, fabrication processes have been developed which do not involve melt flow. For the fine powder type of PTFE, wherein the PTFE is fibrillatible when subjected to shear, the PTFE is paste extruded, which is a low temperature (less than 75° C.) extrusion process carried out on a mixture of the fine powder type of PTFE and lubricating oil. The granular type of PTFE is fabricated by compression molding, which involves filling the mold with cold granular PTFE, and then subjecting it to compression by a ram and heating to form the molded article. Among the problems associated with paste extrusion is the need to remove the lubricant after paste extrusion. Compression molding of granular PTFE is limited to simple shapes. Neither type of PTFE is injection moldable, whereby when intricate shapes of the PTFE are desired, they have to be machined from compression molded blocks of PTFE.
SUMMARY OF THE INVENTION
The present invention provides polytetrafluoroethylene compositions which are melt fabricable by extrusion and thus by injection molding. The composition comprises at least 50 wt % polytetrafluoroethylene, at least 20 wt % of which has a melt viscosity of at least 10
8
Pa·s, and the remainder polyaryleneetherketone, to total 100 wt % of the combination of these two resins.
When extruded in the molten form, the composition has a matrix of PTFE and the polyaryleneetherketone is dispersed therein as fine particles (domains) which solidify from the melt. Thus, molded articles of the melt-fabricated composition have much of the characteristics of PTFE, e.g. high service temperature, chemical inertness, and lubricity (low friction). As will be explained hereinafter, the composition can contain at least 90 wt % of the PTFE, which furthers the attributes of the molded article being like pure PTFE.
DETAILED DESCRIPTION OF THE INVENTION
The polyaryleneetherketone is a well known family of resins which includes polyetherketone having the repeat unit
polyetheretherketone having the repeat unit
and polyetherketoneketone having the repeat unit
wherein the arylene group between the ketone groups of the polyetherketoneketone is typically a mixture of para- and meta-arylene groups, derived for example from isophthalyl and terephthalyl halides, present in the repeat units making up the resin. These resins are commonly known as PEK, PEEK, and PEKK, respectively, and are disclosed in one or more of the following U.S. Pat. Nos. 3,065,205, 3,441,538, 3,442,857, 5,357,040, 5,131,827, 4,578,427, 3,516,966, 4,704,448, and 4,816,556. The polyaryletherketones typically have a melting point which is at least 300° C. and high molecular weight, high strength and high modulus characteristic of engineering plastics. The inherent viscosity of the resin can for example be at least 0.4 measured on a 0.5 wt % solution in concentrated sulfuric acid at 30° C. PEKK is preferred because it can have low melt viscosity and slow crystallization, which facilitates mixing of the resin with the PTFE and providing melt flowability to the resultant composition. PEKK typically contains units derived from both terephthalyl (T) and isophthphalyl (I) halide in a ratio of 90:10 to 30:70, and more typically 80:20 to 60:40. As the proportion of T units decrease and I units increase, the crystallinity of the PEKK diminishes, until at 60:40, the PEKK crystallizes so slowly that it resembles an amorphous polymer except that it will exhibit a melting point. The present invention is applicable to all of the polyaryleneetherketone resins, i.e. they all impart melt-fabricability to the PTFE.
The PTFE is also a well known resin. The fine powder type is preferred and this type is prepared by aqueous dispersion polymerization. The resultant PTFE fine powder type is a fibrillatible resin. A non-fibrillatible, non-melt-fabricable PTFE resin, which is also prepared by aqueous dispersion polymerization as disclosed in U.S. Pat. No. 4,952,636 can also be used in the present invention. The PTFE can be homopolymer of tetrafluoroethylene or can be modified homopolymer, i.e. containing a small amount, e.g. less than 0.5 mol %, of copolymerized monomer which improves the film forming property of the resin, such as disclosed in U.S. Pat. No. 3,142,665. The PTFE will generally have a melt viscosity of at least 10
8
Pa·s at 380° C.
The PTFE and PEKK components can be melt blended as part of the extrusion process or can be premixed, followed by melt blending at temperature at which at least the polyaryleneetherketone is molten. In the case of injection molding, preferably the composition is first melt blended, using for example a twin screw extruder equipped with high shear screws, and formed into molding granules (pellets), followed by injection molding, which procedure promotes uniformity in the dispersion of polyaryleneetherketone domains within the PTFE matrix. Generally, the melt blending temperature will be at least 350° C. Under this condition, the polyaryleneetherketone fluidizes the composition and becomes uniformly dispersed as fine molten domains in the PTFE component, enabling the composition to be melt extruded. The resultant extrudate can be the final molded article, such as in the case of an injection molded article or an extruded tube, sheet or coating, or can be chopped into molding granules for subsequent melt processing into the article desired. The presence of the polyaryleneetherketone component as discrete particles (domains) within the PTFE matrix indicates the incompatibility between the two resins. Nevertheless, articles molded from compositions of the present invention have good properties
An amount of the polyaryleneetherketone which is effective to impart melt-extrudability to the composition while maintaining the PTFE as the matrix phase when molded is present in the composition. When the PTFE is the fine powder type, as little as about 20 wt % of the composition can be the polyaryleneetherketone is needed to impart melt fabricability to the resultant composition, whereby the composition will contain 20 to 50 wt % of the polyaryleneetherketone, preferably 20 to 30 wt %. The wt % of the components are based on the total weight of the PTFE and polyaryletherketone components unless otherwise indicated herein.
It has also been discovered that the amount of polyaryleneetherketone in the composition can be reduced while retaining melt fabricability. The addition of PTFE micropowder to the composition enables this result to be achieved such that as little as about 4 wt % of the polyaryletherketone is all that is necessary for imparting melt extrudability to the composition, the 4 wt % being based on the combined weight of the PTFE and polyaryletherketone components. In this embodiment, the PTFE component comprises both the high melt viscosity (non-melt-fabricable) PTFE and the PTFE micropowder. The PTFE micropowder is not merely a fine powder of PTFE. Instead, PTFE micropowder is a tetrafluoroethylene homopolymer or modified homopolymer which has a considerably lower molecular weight than the normal high melt viscosity PTFE, e.g. the PTFE fine powder described above, which enables the micropowder by itself to be melt flowable, the melt viscosity of the micropowder being 50 to 1×10
5
Pa·s as measured at 372° C. in accordance with the procedure of ASTM D-1239-52T, modified as disclosed in U.S. Pat. No. 4,380,618. Preferably the melt viscosity of the PTFE micropowder is 100 to 1×10
4
Pa·s at 372° C. PTFE micropowder is described further in Kirk-Othmer, The Encyclopedia of Chemical Technology, 4
th
Ed., pub. by John Wiley & Sons (1994) on
E. I. du Pont de Nemours and Company
Short Patricia A.
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