Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Agitating to form larger particles
Reexamination Certificate
2000-01-18
2002-07-09
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Agitating to form larger particles
C264S235000, C264S346000, C526S247000, C528S50200C, C528S503000
Reexamination Certificate
active
06416698
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of finishing processes for melt-fabricable fluoropolymer resins, particularly for making beads from aqueous dispersions of such fluoropolymers.
BACKGROUND OF THE INVENTION
Melt-fabricable fluoropolymers are well known. Such polymers include vinylidene fluoride (VF
2
) homopolymers and copolymers, copolymers of ethylene with tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE), and copolymers of TFE with hexafluoropropylene (HFP) and/or perfluoro(alkyl vinyl ether) (PAVE). They are considered to be melt-fabricable because they can be fabricated into useful articles by conventional methods such as melt extrusion and injection molding. Since a large fraction of melt-fabricable fluoropolymers is processed by such techniques, such fluoropolymers are most commonly sold as cubes suitable for feeding to extruders and injection molding machines. Cubes, typically having dimensions of the order of 3 mm, are usually prepared by melt extruding raw polymer into coarse strands and then cutting the strands.
Such cubes are not suitable for all uses, and other physical forms of melt-fabricable fluoropolymer such as powders and aqueous dispersions are also supplied. One use for which extrusion cubes are not suitable is rotational casting (rotocasting), which encompasses rotational molding (rotomolding) and rotational lining (rotolining). Rotocasting requires a combination of good solid-state flow of the fluoropolymer particles to facilitate uniform distribution of resin in the final article, and a spherical shape and small particle size, in the range of 0.1-0.5 mm, to promote a smooth profile on the free surface of the article. The free surface is that surface not in contact with the surface of the mold or with the surface of the article to be lined. A convenient product form for this purpose is a bead or granule such as that disclosed by Buckmaster et al. in Canadian Patent 1,248,292. Such beads can be prepared, as therein disclosed, by agitation of aqueous dispersion of fluoropolymer particles in the presence of electrolyte and water-immiscible organic liquid, a process known as solvent-aided coagulation or as solvent-aided pelletization (SAP). Optionally, after isolation from the liquid, the resultant beads can be subjected to treatments to harden them and/or to stabilize any unstable end groups present on the fluoropolymer.
Water-immiscible organic liquids that can be used in SAP processes include the aliphatic hydrocarbons such as hexane, heptane, gasoline and kerosene, aromatic hydrocarbons such as benzene, toluene and xylene, halogenated derivatives such as carbon tetrachloride, monochlorobenzene, the trichlorotrifluoroethanes, difluorotetrachloroethanes, and liquid oligomers of chlorotrifluoroethylene as disclosed in Canadian Patent 1248292, in which 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) is used in the examples. Hydrocarbons are hazardous because of their flammability, while chlorocarbons and CFCs present environmental problems. Alternative water-immiscible liquids for use in SAP processes include the hydrofluorocarbons (HFC) disclosed by Takakura and Funagi in Japanese Patent Application Publication (Kokai) H07-2783 14. While such HFCs have low ozone depletion potential (ODP), they do have global warming potential (GWP). For example, one isomer of C
5
H
2
F
10
(HFC 43-10) has GWP of about 1300. Furthermore, HFCs can be sensitive to alkaline gelation agents, which restricts their application to acidic gelation processes. Additional environmentally friendly solvents are needed for SAP.
The SAP process is complicated in the sense that many variables are involved. Any change, e.g., a new solvent, may require adjustment of other variables. A process for preparing beads that has a broader operating window, i.e., is less sensitive to the variables, is desired.
SUMMARY OF THE INVENTION
This invention provides a process comprising forming beads of melt-fabricable fluoropolymer resin by solvent-aided pelletization of aqueous dispersion of particles of said resin, wherein said solvent-aided pelletization is carried out using fluorinated solvent containing oxygen in which the oxygen is present only as ether oxygen. Fluorinated solvents containing ether oxygen include (a) perfluorinated cyclic amines of formula (I) hereafter, (b) hydrofluoroethers having the formula R—O—R
f
, wherein R is alkyl having 1-3 carbon atoms and R
f
is linear or branched fluoroalkyl having 2-7 carbon atoms and containing no halogen other than fluorine and at most one terminal hydrogen atom, and (c) hydrofluoroether having the formula R
4
—O—R
5
, wherein R
4
is a fluoroalkyl having 1-6 carbon atoms and at least one hydrogen atom and R
5
is linear or branched fluoroalkyl containing no halogen other than fluorine and having 1-7 carbon atoms and optionally containing ether oxygen. Preferred solvents include hydrofluoroethers.
The process of the invention has environmental advantages, and also yields small well-formed beads over a broad range of solvent-to-polymer ratio, extending to low solvent-to-polymer ratios.
DETAILED DESCRIPTION
It has been discovered that organic liquids having oxygen present only as ether oxygen can be used in SAP processes for agglomeration of melt-fabricable fluoropolymer particles in aqueous dispersion. Preferably, the organic liquids have one ether oxygen per molecule.
Organic liquids that can be used to form beads of melt-fabricable fluoropolymer resin by the SAP process of the present invention include perfluorinated cyclic amines having a nitrogen atom in the ring and having an ether oxygen in the ring. These compounds have no ODP.
Among the perfluorinated cyclic amines useful in this invention are those having the general formula
in which R
1
is a linear or branched saturated perfluorocarbon group having 1-4 carbon atoms, R
2
and R
3
are linear or branched saturated perfluorocarbon groups having, independently, 1-5 carbon atoms, and the total number of carbon atoms in the molecule of formula (I) is 3-10.
Examples of compounds of formula (I) include perfluoro-N-methylmorpholine (PFNMM) and perfluoro-N-isopropylmorpholine. Preferred compounds of formula (I) are those in which the total number of carbon atoms is 4-8 and the number of atoms bonded in the ring is 5-6. Especially preferred compounds are those in which R
2
and R
3
are —CF
2
CF
2
—, and R
1
has 1-3 carbon atoms. PFNMM and perfluoro-N-isopropylmorpholine are available commercially (Fluorinert® PF-5052 and FC-6003, respectively, 3M Company). PFNMM is most preferred.
Organic liquids that can be used also include hydrofluoroethers (HFE) having the formula R—O—R
f
, wherein R is alkyl having 1-3 carbon atoms, preferably 1-2 carbon atoms, and R
f
is linear or branched fluoroalkyl, preferably linear, containing no halogen other than fluorine and having 2-7 carbon atoms, preferably 4-5 carbon atoms, and at most one terminal hydrogen atom. Preferably, R
f
is perfluorinated. Examples of HFE that can be used include perfluorobutyl methyl ether (CH
3
—O—C
4
F
9
) and perfluorobutyl ethyl ether (C
2
H
5
—O—C
4
F
9
). These are commercially available as HFE-7100 and HFE-7200 respectively, from 3M Company, St. Paul Minn. USA. Other suitable HFE have the formula R
4
—O—R
5
, wherein R
4
is a fluoroalkyl having 1-6 carbon atoms, preferably 1-3 carbon atoms, and at least one hydrogen atom, preferably on the carbon adjacent to the oxygen atom, and R
5
is linear or branched fluoroalkyl, preferably linear, containing no halogen other than fluorine and having 1-7 carbon atoms, preferably 2-4 carbon atoms and optionally containing ether oxygen. Preferably R
5
is perfluorinated. Examples include CF
3
—CHF—O—CF
2
—CF
2
—CF
3
and CF
3
—CHF—O—CF
2
—CF(CF3)—O—CF
2
—CF
2
—CF
3
, the preparation of which is described in Preparation, properties, and industrial applications of organofluorine compounds, by R. E. Banks, John Wiley and Sons, NY, 1982, page 100.
Among organic liquids having oxygen present only as ether oxygen, the HFE described above are preferred.
The process of this
E. I. du Pont de Nemours and Company
Vargot Mathieu D.
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