Patent Application
20070276080
This invention relates to the melt fabrication of melt-fabricable fluoropolymer containing a high proportion of fiber.
Glass fiber is added to melt-fabricable fluoropolymer to increase the rigidity (modulus) of articles molded from the resulting composition, typically by such processes as extrusion, injection molding, and compression molding. In molding, mechanical pressure is applied to cause the molten composition to flow sufficiently to form the desired shape of the article. It has been customary to feed the composition to these processes as melt-formed pellets, typically formed by melt extrusion of the composition into a strand and chopping up the strand, wherein the resulting pellets are about 3000 to 4000 micrometers (118 to 157 mils) in diameter and about 2000 to 3500 micrometers (80 to 140 mils) in length. As the glass fiber content of the composition increases to increase the rigidity and dimensional stability of the molded article, e.g. at least 15 wt %, often at least 30 wt % glass fibers, both based on the combined weight of the glass fibers and melt-fabricable fluoropolymer, the melt flow of the composition decreases, making it difficult to extrude the composition as a strand. Canadian Patent 900075 discloses for ethylene/tetrafluoroethylene copolymer (ETFE), that the melt viscosity increases from 1.8×104 poises (no glass fiber filler) to 6.49×104 poises (26 wt % glass fiber filler). This difficulty carries over into the molding process, which manifests itself as an incomplete filling of the mold in the case of injection molding, longer cycle time for compression molding to avoid porosity in the molded article, and slow extrusion rate.
The present invention overcomes this difficulty by the process comprising molding a molten composition comprising glass fiber and melt-fabricable fluoropolymer to form an article, said molten composition being obtained by melting melt-formed particles of said composition, said particles having a width of no greater than about 70 mils (1784 micrometers). This process of incorporating glass fiber into such small fluoropolymer particles is also applicable to other fibers having a melting temperature above that of the fluoropolymer and above the melting temperature used to carry out the incorporation. The fibers used in the present invention can be inorganic, such as glass fibers, or organic, such as polymer fibers, such as aramid fiber and PTFE (polytetrafluoroethylene) fiber. Carbon fiber can also be used; this is considered an organic fiber because of its usual derivation from hydrocarbon polymers. The use of these fibers other than glass fiber provides advantage similar to when glass fiber is used in the molding process.
Surprisingly the small highly fiber-filled particles of the present invention, which resemble coarse sand particles, especially as the width of the particles are made even smaller, can be formed by melt-extrusion through a very small diameter extrusion orifice. This is surprising because the fibers do not plug up the small diameter extrusion orifice necessary to produce the small-width particles, as would be expected with the large amount of fibers present in the composition, e.g. at least 15 wt %, based on the combined weight of the fibers and melt-fabricable fluoropolymer or at least 10 vol % based on the combined volume of the fiber and fluoropolymer. All of the wt %s and vol %s disclosed hereinafter are on this same basis unless otherwise indicated. Indeed, observation of the extrusion orifice producing these small particles reveals that the orifice is quite small, leading to the expectation that the fibers would plug the extrusion orifice. A special extrusion die design is provided that avoids this result, as will be explained hereinafter.
The improvement arising from having these very small fiber-filled particles as the feed to the process of molding a molten composition comprising fiber and melt-fabricable fluoropolymer to form said composition into an article is to minimize the decrease in melt flow that otherwise occurs when the larger fiber-filled pellets of the composition are used. This effect on melt flowability is with respect to the melt-fabricable fluoropolymer by itself subjected to the same molding conditions. The improvement obtained by the present invention manifests itself in the molding result, complete filling of the mold in the injection molding process, faster extrusion rate, faster cycle time in compression molding to produce articles free of porosity. The molding processes of the present invention all involve the application of mechanical pressure to the molten composition to form it into the article shape desired. In injection molding, this mechanical pressure is applied by a screw pump, in compression molding, by a ram, and in extrusion, by an extrusion screw. This application of mechanical pressure to the molten composition formed from the particles described above causes the more fluid composition of the invention to give the improved molding result. The contribution to injection molding is especially noteworthy, because it enables the molding of articles that would otherwise not be melt fabricable by injection molding by virtue of the mold cavity not filling completely with the fluoropolymer/glass fiber composition. The conventional size of the pellets in Canadian Patent 900075, i.e. 3200 micrometers in diameter×3200 micrometers in length, sometimes called molding granules and molding powder therein, leads to the melt-fabrication of ETFE/glass fiber composition only by compression molding in the Examples.
Another embodiment of the present invention is the small fiber-filled particles themselves, which are preferably melt-formed particles of composition comprising fibers and melt-fabricable fluoropolymer, said fibers constituting at least 15 wt % of the combined weight of said fibers and said melt-fabricable fluoropolymer, at least 80 wt % of said particles having a width no greater than about 70 mils (1784 micrometers) and length no greater than about 80 mils (2400 micrometers). Just as the TFE copolymer minicubes of U.S. Pat. No. 6,632,902 useful for rotomolding can have some variability in size (length and width), so do the fiber-filled particles of the present invention. The extrusion of TFE copolymer in '632 is disclosed to produce die swell, i.e. the minicube has a larger diameter than the diameter of the extrusion orifice (sentence bridging cols. 2 and 3). The effect of the fiber in the composition melt-formed by extrusion of the composition as a strand in the present invention, followed by chopping up the strand into particles, essentially prevents die swell, and results in particles having a smaller width, e.g. diameter, than that of the extrusion orifice. By way of example, an extrusion orifice of 64 mils (1631 micrometers) typically produces smaller diameter glass-filled/melt-fabricable fluoropolymer particles, e.g. as small as 45 mil (1147 micrometers). This reduction in diameter of the particles from the diameter of the extrusion orifice is surprising because the presence of the fibers in the extruded strand reduces the melt strength of the molten extrudate. The melt-formed particles of the present invention exhibit surprising increase in melt flowability as compared to the same composition provided in the conventional pellet sizes disclosed above. It is unexpected that better mold filling, and thereby improved injection moldability, arises from decreasing the particle size of the fluoropolymer/fiber composition from the 3000 to 4000 micrometer diameter/1000 to 2000 micrometer length pellet sizes.
The photographic views of
It is apparent from the photographic views of
While the die design of
The chopping up of the solidified strand obtained from the die design of
Surprisingly, the resultant molten mass has good melt flowability, better than the molten mass obtained from the larger fiber-filled/melt-fabricable fluoropolymer pellets, and which facilitates the molding of articles by the application of pressure to this molten mass. This improvement manifests itself, i.e. is especially visible, in the injection molding of intricate articles, such as those containing a thin wall. For such articles, when the use of the large-size fiber-filled/melt-fabricable pellets results in incomplete filling of the mold, sometimes, or all the time, use of the small-size fiber-filled/melt-fabricable fluoropolymer particles according to the present invention, results in consistent filling of the mold.
Any glass fiber can be used in the present invention. Such glass fiber is high temperature resistant, such that it does not melt or soften to lose the fiber shape at the melt processing temperature for the particular fluoropolymer being used. A preferred glass from which the fiber if made is E-glass, which is a low alkali borosilicate glass. The denier of the glass fiber is fine enough that the cross-sectional area of all the fibers present in the particle is no greater than about 50% of the cross-sectional area of the extrusion orifice. Typically, the glass fiber will have a diameter of 5 to 50 micrometers, more preferably 5 to 20 micrometers. The invention is not limited to any particular finish (coupling agent) on the glass fiber. Generally, the glass fiber, with or without finish, is incompatible with the fluoropolymer, i.e. is not wet by the molten fluoropolymer. This is in contrast to hydrocarbon polymers, such as polyolefins and polyamides, that will adhere to glass fiber coated with a finish (coupling agent). Preferably, the glass fiber is free of coupling agent.
The thermal requirements for the glass fiber are the same for other inorganic fibers and for organic fibers, such as those of carbon (such as from Toray Carbon Fibers, Decatur Ala. USA), aramid (such as Kevlar® and Nomex®, available from DuPont, Wilmington Del. USA), and PTFE fiber (such as TFA Teflon® PTFE fibers available from Toray Fluorofibers (America), Decatur Ala. USA).
The fluoropolymers used in the present invention are preferably partially crystalline and are melt-fabricable, which means that they are sufficiently flowable in the molten state (heated above melting temperature) that they can be fabricated by such pressure applying molding processes such as extrusion, injection molding and compression molding. The melt flowability of the fluoropolymer can be described in terms of melt flow rate as measured in accordance with ASTM D-1238, and the fluoropolymers used in the present invention preferably have a melt flow rate (MFR) of at least 1 g/10 min, determined at the temperature which is standard for the particular fluoropolymer; see for example, ASTM D 2116a and ASTM D 3159-91a. Melt viscosity (MV) in poises is calculated from the measured MFR as follows: MV=53170/MFR in g/min, as disclosed in U.S. Pat. No. 4,380,618 (col. 3, I. 50-52). Thus, literature reported melt viscosities can be back-calculated to MFR. For example, the melt viscosities of 3.04×104 poises and 4.3×104 poises disclosed in Examples 2 and 3 of Canadian Patent 900075 correspond to MFRs of 17.5 and 12.4 g/10 min, respectively (calculation: MFR in g/10 min=531700/3.04×104 poises). The method of measuring of MFR for fluoropolymers is unique to (especially for) fluoropolymers because of the high melt viscosity of fluoropolymers as compared to hydrocarbon-based polymers. Polytetrafluoroethylene (PTFE) is generally not melt processible, i.e. it does not flow at temperatures above its melting temperature, whereby this polymer is not melt-fabricable. Low molecular weight PTFE is available, called PTFE micropowder, the molecular weight being low enough that this polymer is flowable when molten, but because of the low molecular weight, the resultant molded article has no strength. The absence of strength is indicated by the brittleness of the article. If a film can be formed from the micropowder, it fractures upon flexing. In contrast, the melt-fabricable fluoropolymers used in the present invention can be formed into films that can be repeatedly flexed without fracture. This flexibility can be further characterized by an MIT flex life of at least 500 cycles, preferably at least 1000 cycles, and more preferably at least 2000 cycles, measured on 8 mil (0.2 mm) thick compression molded films that are quenched in cold water, using the standard MIT folding endurance tester described in ASTM D-2176F.
The preferred melt-fabricable fluoropolymers for use in the present invention comprise one or more repeat units selected from the group consisting of —CF2—CF2—, —CF2—CF(CF3)—, —CF2—CH2—, —CH2—CHF— and —CH2—CH2—, these repeat units and combinations thereof being selected with the proviso that the fluoropolymer contains at least 35 wt % fluorine, preferably at least 50 wt % fluorine. Thus, although hydrocarbon units may be present in the carbon atom chain forming the polymer, there are sufficient fluorine-substituted carbon atoms in the polymer chain to provide the desired minimum amount of fluorine present, so that the fluoropolymer exhibits chemical inertness. The fluoropolymer preferably also has a melting temperature of at least 150° C., preferably at least 200° C., and more preferably at least 240° C.
Examples of perfluoropolymers, i.e., wherein the monovalent atoms bonded to carbon atoms are all fluorine, except for the possibility of other atoms being present in end groups of the polymer chain, include copolymers of tetrafluoroethylene (TFE) with one or more perfluoroolefins having 3 to 8 carbon atoms, preferably hexafluoropropylene (HFP). The TFE/HFP copolymer can contain additional copolymerized perfluoromonomer such as perfluoro(alkyl vinyl ether), wherein the alkyl group contains 1 to 5 carbon atoms. Preferred such alkyl groups are perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether). Typically, the HFP content of the copolymer is about 7 to 17 wt %, more typically 9 to 17 wt % (calculation: HFP Index (HFPI)×3.2) and the additional comonomer when present constitutes about 0.2 to 3 wt %, based on the total weight of the copolymer. The TFE/HFP copolymers with and without additional copolymerized monomer is commonly known as FEP.
Examples of hydrocarbon/fluorocarbon polymers (hereinafter “hydrofluoropolymers”) that can be used in the present invention include vinylidene fluoride polymers (homopolymers and copolymers), typically called PVDF, copolymers of ethylene (E) with TFE, typically containing 40 to 60 mol % of each monomer, to total 100 mol %, and preferably containing additional copolymerized monomer such as perfluoroalkyl ethylene, preferably perfluorobutyl ethylene. These copolymers are commonly called ETFE. While ETFE is primarily composed of ethylene and tetrafluoroethylene repeat units making up the polymer chain, it is typical that additional units or from a different fluorinated monomer will also be present to provide the melt, appearance, and/or physical properties, such as to avoid high temperature brittleness, desired for the copolymer. Examples of additional monomers include perfluoroalkyl ethylene, such as perfluorobutyl ethylene, perfluoro(ethyl or propyl vinyl ether), hexafluoroisobutylene, and CH2═CFRf wherein Rf is C2-C10 fluoroalkyl, such as CH2═CFC5F10H, hexafluoropropylene, and vinylidene fluoride. Typically, the additional monomer will be present in 0.1 to 10 mol % based on the total moles of tetrafluoroethylene and ethylene. Such copolymers are further described in U.S. Pat. Nos. 3,624,250, 4,123,602, 4,513,129, and 4,677,175. Additional hydrofluoropolymers include EFEP and the copolymer of TFE, HFP and vinylidene fluoride, commonly called THV. Preferably the MFR of ETFE is no greater than about 10 g/10 min. Notwithstanding this low MFR, the molding process of the present invention and the small particle melt-formed particles used therein provide improved melt flowability as compared to the conventional glass fiber-filled ETFE pellets used heretofore.
The fluoropolymers used in the present invention are all characterized by a high melting temperature, e.g. at least about 175° C., usually at least about 200° C. and most often at least about 225° C. The molding of these fluoropolymers is carried out at considerably higher temperatures, usually greater than 300° C., and most often greater than about 325° C. Even at these extremely high temperatures, the fiber remains incompatible with the fluoropolymer, decreasing the melt flow of the molten mixture obtained prior to formation of the fiber-filled particles.
The melt-formed fiber-filled/melt-fabricable fluoropolymer particles of the present invention can contain other ingredients, such as pigments in an effective amount to color the particle and thus the article molded therefrom. The pigment carbon black can also be present for colorant purpose or for the purpose of rendering the particles and the articles molded therefrom sufficiently electrically conductive that the article dissipates static electrical charge.
The fiber-filled/melt-fabricable fluoropolymer particles used in the present invention can be made by dry mixing fibers chopped to the length desired, e.g. 100 to 300 mils (2550 to 7650 micrometers) with melt-formed pellets, e.g. measuring 125 mils (3185 micrometers) in diameter and 175 mils (4460 micrometers) in length and then melt mixing the resultant composition and extruding it through the die design such as shown in
Preferably, the portions of fiber filling in the melt-fabricable fluoropolymer particles, when the fiber is glass is at least 15 wt % fibers, more preferably, at least 20 wt %, and even more preferably, at least 25 wt % fibers, the increasing amount of fibers serving to correspondingly increase the rigidity and dimensional stability of the article eventually to be molded from the particles. Generally, no more than 40 wt % of the particles will be glass fiber.
Other fillers such as aramid fiber have a lower density (g/cc) (note that this refers to the density of the material that the fiber is made of, and is not the bulk density of the fiber in whatever physical form it is being used), about 1.5, than the fluoropolymer (density of about 1.7 to 2.15) and glass (density of about 2.5), with the density of carbon fiber (about 1.85), and PTFE (density 2.15), being nearer to or identical with the fluoropolymer. Under this circumstance of differing densities for the filler fibers, the highly filled condition for all the filler fibers is better expressed as volume % rather than weight %. In this regard, the preferred vol % of filler fiber in the fluoropolymer particles is at least 10 vol %, preferably at least 15 vol %, more preferably at least 18 vol %, and even more preferably at least 22 vol %. Generally no more than 50 vol % of the fluoropolymer particle will be fiber filler. All volume percents are based on the total volume of the fluoropolymer plus fiber filler in the particles.
Preferably the width of the particles, which will be the diameter of the extruded/quench strand, is no greater than about 60 mils (1530 micrometers) and more preferably no greater than about 50 mils (1275 micrometers). The minimum width of the particle will depend on the wt % of fibers present, i.e. a smaller wt % fibers will enable smaller diameter strands to be extruded. Generally, the minimum width will be at least about 25 mils (637 micrometers), preferably at least 40 mils (1020 micrometers) and more preferably at least 45 mils (1147 micrometers, to facilitate the extrusion fabrication of the fiber-filled particles without plugging the extrusion orifice, especially as the loading of fibers in the fluoropolymer is increased. Where there are two width dimensions, e.g. in the case of the oval cross-section particle shown in
Preferably the, length of the fiber-filled/melt-fabricable fluoropolymer particles is no greater than about 80 mils (2400 micrometers), and more preferably no greater than about 70 mils (1784 micrometers). In actual operation of chopping up a molten/quenched extruded strand of the fiber-filled/melt-fabricable fluoropolymer composition, the length of the particles is also subject to variation even though the cutter is set for a single length. For example, the above described thirty particles exhibited lengths ranging from 62 to 69 mils (1590 to 1770 micrometers). Preferably, the fiber-filled fluoropolymer particles have a small aspect ratio (ratio of length to width dimensions), which facilitates the feeding of the particles to the molding equipment without interrupting the feed by bridging. Thus, the aspect ratio of the particles is preferably no greater than 2:1, preferably no greater than 1.5:1.
Because of this variation in particle size (width and length), the particle sizes expressed herein apply to at least 80 wt % of the particles, more preferably to at least 90 wt %, and even more preferably to all the particles. The number of particles exceeding the specified particle size can be estimated by comparing the count of specified particle size particles with the count of particles outside the specified size from a sample of at least thirty randomly selected particles, or by classification as described in the next paragraph. The weights of these counted particles can be compared for more precise determination.
The particle size, width and length can be determined by actual measurement, e.g. using a micrometer, or by measurement applied to magnified photographs of particles. The screening (classification) method disclosed in U.S Pat. No. 6,632,902 can also be used for preliminary determination, which can negate the need for direct measurement. In accordance with the screen method, if 80 wt % of a representative sample of a lot of particles pass through a sieve with openings of 70 mils (1784 micrometers) and is retained on a screen with openings of 25 mils (637 micrometers), then 80 wt % of these particles would be in the size range of 637 micrometers to 1784 micrometers). In accordance with this measurement method for determining particle size, it is preferred that at least 80 wt % of the glass-filled/melt-fabricable fluoropolymer particles of the present invention and used in accordance with the present invention are in the size range of 500 to 1800 micrometers, more preferably in the 500 to 1500 micrometer size range.
The fiber-filled/melt-fabricable particles described above are used in molding processes in the same manner as the larger pellets, to obtain improved results arising from increased melt flowability of the molten composition as compared to when the molding is carried out with the larger pellets.
The fluoropolymer used in these Examples is ETFE available from E.I. du Pont de Nemours and Company as TEFZEL® 200 ETFE fluoropolymer, having a melt flow rate of 7 g/10 min, determined at 297° C., and in the form of extrusion melt-formed pellets measuring about 125 mils (3185 micrometers) in diameter and 175 mils (4460 micrometers) in length. The fiber used in the Examples is glass fiber, but any of the other fibers described above could be used to obtain similar results. The glass fiber used is available from the St. Gobain/Vetrotex as grade 910 chopped E-glass fiber strand, the fiber length being about 188 mils (4.5 mm) and the fiber diameter being 10 micrometers. A dry-mixed composition is prepared containing 30 wt % glass fibers and 70 wt % of the ETFE pellets. This composition is feed to a single screw Brabender® mixer operating at 10 rpm and heated to a melt temperature of 585° F. (307° C.), which extrudes the molten composition through a die shown in
These particles are fed to an injection molding machine wherein the article to be formed is in the shape of a thin-walled cup having a hole in the bottom and an outwardly extending apron from the top of the cup shape. The diameter of the apron is about 4 mm, the outer diameter of the cup is about 2 mm, the depth of the cup is about the same as its outer diameter, and the wall thickness is about 2.5 mm. The mold is double gated on opposite sides of the apron and multiple molds are simultaneously filled with fluoropolymer/glass fiber composition for each cycle of injection molding. This means that the molten composition has to travel through the runners to each gate of each mold and then into each mold. The injection molding result is that each mold is completely filled, cycle after cycle, with the molten composition to give completely formed thin-walled articles. In addition to this complete filling of each mold, the resultant articles have a substantially uniform wall thickness.
When this injection molding is repeated with pellets of glass-filled ETFE (30 wt % glass fiber) of about the same size as the starting pellets (125 mils (3185 micrometers) in diameter and 175 mils (4460 micrometers) in length) obtained by Brabender processing and melt extrusion of the dry mixture of ETFE pellets with the glass fibers and through a 125 mil (3185 micrometers) diameter extrusion die of the design of
The better melt flowability of the glass-filled fluoropolymer particles used in accordance with the present invention not only provides better injection molded articles, but enables the number of articles to be molded per cycle of injection molding operation to be increased. Thus, for articles that can be molded using the larger-size glass-filled fluoropolymer pellets, use of the glass-filled fluoropolymer particles in accordance with the present invention enables a greater number of molds to be filled per cycle, thereby improving productivity of the injection molding operation. Similar results are obtained when such fibers as aramid fiber, carbon fiber, or PTFE fiber are substituted for the glass fiber.
| Number | Date | Country | |
|---|---|---|---|
| 60808437 | May 2006 | US | |
| 60839143 | Aug 2006 | US |
