Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-05-24
2003-10-14
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C264S239000, C264S299000, C264S310000, C264S312000, C526S247000
Reexamination Certificate
active
06632902
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for rotomolding tetrafluoroethylene/perfluoro(alkyl vinyl ether) (TFE/PAVE) copolymer with pellets prepared by melt-extrusion.
BACKGROUND OF THE INVENTION
Rotomolding, which as used here includes rotocasting and rotolining, is a melt-fabricable tetrafluoroethylene copolymer processing method especially suited for making large hollow articles in a heated rotating mold. In the case of rotocasting, after cooling of the mold, the hollow article is removed from the mold. In the case of rotolining, the hollow article remains as a lining for the mold. Thus, the mold can be a capped pipe tee, and the hollow article forms a lining for the tee. The rotomolding process is described in The Encyclopedia of Polymer Science and Engineering, Vol. 14, p. 659, John Wiley & Sons Inc., New York, 1988. Polymer for rotomolding is in the form of powder, sometimes called beads or granules, of roughly spherical shape. Although particle sizes up to 3000 &mgr;m are disclosed in the literature, in practice, the particle sizes are typically less than about 500 &mgr;m in size. Conventional standard melt-extruded polymer cubes, sometimes called pellets of the sort used as a feed for melt extrusion or injection molding have not been suitable for rotomolding, because the resultant rotomolded article has a rough interior, uneven thickness and contains bubbles within its thickness. In an attempt to avoid these deficiencies, rotomolding powder has been made by other processes, such as agglomerating polymer particles obtained from polymerization into granules as disclosed in U.S. Pat. No. 4,714,756 relating to the preparation of tetrafluoroethylene/perfluoro(alkyl vinyl ether) (TFE/PAVE) copolymer rotomolding powder. Because polymers such as TFE/PAVE are used both in melt extrusion and in rotomolding, and these require the polymer to be in different forms, separate polymer preparation processes and equipment must be maintained. Furthermore, powders must be classified to remove fines or dust. Settling and segregation in storage and shipping may require remixing to ensure uniform size distribution. Static electricity can make handling difficult. In other words, while the use of powder instead of melt-extruded cubes for rotomolding has produced a better rotomolded product, the use of the powder has had the disadvantage of increasing expense.
There is a need for a rotomolding process that does not require the use of polymer in powder form.
SUMMARY OF THE INVENTION
The present invention satisfies this need by providing smaller size melt-extruded cubes of melt-fabricable TFE copolymer. In particular the size range of the “minicubes” is that at least 80% are in the range of 200 to 1200 &mgr;m.
The process of the present invention is carrying out the rotomolding using these copolymer minicubes to form a hollow article of the copolymer.
DETAILED DESCRIPTION
Melt-fabricable TFE copolymer used in the present invention is preferably partially crystalline, and the preferred comonomers are at least one monomer selected from the group consisting of perfluoroolefin having 3 to 6 carbon atoms, preferably hexafluoropropylene (HFP); olefins, preferably ethylene and propylene, more preferably ethylene; and perfluoro(alkyl vinyl ether) (PAVE), wherein the alkyl group contains 1 to 8 carbon atoms, preferably 1 to 3 carbon atoms. The more preferred copolymers are perfluorinated. Among the perfluorinated copolymers are terpolymers of TFE/HFP/perfluoro(ethyl vinyl ether), such as that disclosed in U.S. Pat. No. 5,677,404. The most preferred copolymers are TFE/PAVE copolymers such as disclosed in U.S. Pat. No. 3,528,954. Preferred forms of TFE/PAVE copolymers are disclosed in U.S. Pat. No. 5,760,151, wherein the perfluoroalkyl group is ethyl (PEVE), and the copolymer contains at least about 3 wt %, preferably at least about 5 wt %, and more preferably at least about 7 wt % of the PVE comonomer, preferably the PEVE copolymer. The PEVE copolymers are called TFE/PEVE copolymers.
The rotomolding process involves the steps of feeding the copolymer minicubes described above to the hollow mold, rotating the mold, heating the mold while it is rotating, the temperature being above the melting point of the copolymer, with the heating and rotating being carried out for sufficient time for the minicubes to fuse together in the interior shape of the mold to form a hollow article conforming to this interior shape, and cooling the mold and the article contained in it. The rotation of the mold causes the minicubes to tumble over one another and the heating of the mold eventually causes these minicubes to soften, flow, and then fuse together. The only force exerted upon these minicubes is the centrifugal force resulting from rotation. The rotation of the mold is multiaxial, so that the entire exposed interior surface of the mold is covered with the molten copolymer. It is therefore important that the copolymer be sufficiently flowable at the temperature used so as to form the molded hollow article. Thus, the melt flow rate (MFR) of the copolymer is preferably in the range of about 2 to about 100 (g/min), more preferably about 5 to about 50, and even more preferably about 10 to about 5.
The melt-extruded particle described in this invention is referred to hereinafter as “minicube”, although as will be shown, it is not cubic in shape. Minicube is made by melting the TFE copolymer in a single screw or twin screw extruder, and extruding the melted polymer through a multihole die to yield minicubes in the form of small cylinders approximately 200 to 1200 &mgr;m in diameter, preferably about 500 to about 1200 &mgr;m in diameter, and melt-cutting the copolymer as it exits the die into lengths of about 200 to about 2000 &mgr;m, preferably about 500 to about 1500 &mgr;m. It has been found however, that cutting to these short lengths also produces longer cubes, e.g. up to 3000 &mgr;m, in a minor proportion (<30%). The small diameter of the minicubes, however, still enables the melt-extruded cubes to have an overall small size as determined by sieving as described hereinafter. The minicubes, insofar as dimensions are concerned, are used in the rotomolding process as-extruded and cut into lengths; they are unground, i.e. they are not subjected to comminution to smaller sizes.
To reduce the effect of shear on melted polymer, it has been found useful to put a melt pump, an example of which is a gear pump, between the extruder and the die. By this means, the pressure necessary to extrude the melt through the die is generated by the melt pump, in which shear forces are less than would be experienced if the extruder alone were used to generate the necessary pressure. Excessive shear can cause copolymer degradation and thus adversely affect copolymer properties. An example of a suitable die is one having about 700 holes (extrusion orifices), each about 30 mils (760 &mgr;m) in diameter. Because of the phenomenon known as “die swell” or “extrudate swell”, the minicubes so produced have a larger diameter than the extrusion orifices, whereby for a given minicube diameter and orifice selected to provide such diameter, there will be some variation in the minicube diameters obtained. The cylindrical shape of the minicubes may also vary, from perfect cylinders to cylinders having slightly flattened sides or elliptical cross-section. The diameter of the minicube is considered to be the longest dimension normal, i.e. perpendicular, to the length of the minicube.
The 760 &mgr;m diameter orifice provides minicubes having an average diameter of about 1000 &mgr;m. The extrudate is cut into short lengths. One means of doing this is through use of a conventional cutter operating across the outer face of the extrusion die, the cut minicubes falling into water for cooling. The preferred length is in the range of about 200 to about 2000 &mgr;m, more preferably about 500 to about 2000 &mgr;m. There will also be some variation in the minicube length for a given cutting length. The variability of the minicube diameter and length is
E. I. du Pont de Nemours and Company
Zitomer Fred
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