| PATENT ASSIGNEE'S COUNTRY | Germany |
| UPDATE | 11.00 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 21.11.00 |
| PATENT TITLE |
Coating of porous, hydrophobic substrates with thermoplastic fluoropolymers |
| PATENT ABSTRACT |
If hydrophobic substrates are coated--if appropriate under pressure--with an aqueous dispersion of a thermoplastic fluoropolymer having a number-average particle size of up to 100 nm and the fluoropolymer is fixed on the substrate thus impregnated by heat treatment, firmly adhering coatings are obtained. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 22.02.94 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT REFERENCES CITED |
Translation of DE 41 24730, Jan. 1993. Encyclopedia of Polymer Sci. and Eng., Second Edition, vol. 16, 577-648, 1989 |
| PATENT CLAIMS |
What is claimed is: 1. A process for coating porous hydrophobic substrates with fluoropolymers, which comprises bringing a porous hydrophobic substrate into contact with an aqueous dispersion of a thermoplastic fluoropolymer having a number-average particle size of up to 100 nm and said thermoplastic fluoropolymer being a copolymer containing a sufficient amount of co-monomer other than tetrafluoroethylene in order to provide its thermoplastic properties, so that said thermoplastic fluoropolymer is processable from a melt, impregnating the substrate with the dispersion forming an article, and then after-treating the article thus impregnated with heat to adhere the fluoropolymer to the substrate. 2. The process as claimed in claim 1, wherein the impregnating of the substrate with the dispersion takes place under a pressure which is increased above atmospheric pressure. 3. The process as claimed in claim 1, wherein the after-treatment with heat is carried out to form a coating on the substrate, first at a temperature of 100 to 150.degree. C. and then at a sufficiently high temperature to ensure sintering or fusion of the fluoropolymer and to densify said coating. 4. The process as claimed in claim 1, wherein the number-average particle size in the fluoropolymer dispersion is up to 50 nm. 5. The process as claimed in claim 1, wherein the number-average particle size in the fluoropolymer dispersion is up to 30 nm. 6. The process as claimed in claim 1, wherein the fluoropolymer is a copolymer with units of tetrafluoroethylene and a fluoroalkyl perfluorovinyl ether. 7. The process as claimed in claim 6, wherein the copolymer additionally contains units of hexafluoropropene. 8. The process as claimed in claim 6, wherein said thermoplastic fluoropolymer contains more than about 3% by weight of perfluoropropyl perfluorovinyl ether units. 9. The process as claimed in claim 1, wherein said substrate comprises carbon, a carbide, boride, or silicide. 10. The process as claimed in claim 4, wherein the number-average particle size in the fluoropolymer dispersion is from about 1 to 40 nm. 11. The process as claimed in claim 10, wherein the number-average particle size of the fluoropolymer dispersion is from 10 nm to 30 nm. 12. The process as claimed in claim 11, wherein the fluoropolymer contains tetrafluoroethylene units and comonomer units of the formula X--(CF.sub.2).sub.n --O--CF.dbd.CF.sub.2 in which X is hydrogen, chlorine or fluorine and n is a number from 1 to 8. 13. The process as claimed in claim 12, wherein X is fluorine and n is a number from 1 to 3. 14. The process as claimed in claim 2, wherein said pressure is from about 10 to 100 bar. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
DESCRIPTION The invention relates to a process for coating porous, hydrophobic, metallic and non-metallic substrates with thermoplastic fluoropolymers, i.e. fluoropolymers which can be processed from the melt, and to the coated substrates. DE-A 41 24 730 discloses a process in which fluoropolymers, in a form in which their particles or those of a precursor are smaller in at least one dimension than the diameter of pores or capillaries of a microporous oxidic surface which has been produced by anodic oxidation of objects of aluminum, magnesium or alloys thereof, are embedded in this surface. The preferred fluoropolymer is polytetrafluoroethylene, which, as is known, cannot be processed from the melt. Suitable fluoropolymers which are mentioned are the polymers and copolymers of tetrafluoroethylene, hexafluoropropene, vinylidene fluoride, vinyl fluoride and trifluorochloroethylene, in particular homopolymers thereof and tetrafluoroethylene copolymers. The polymer particles or the precursors thereof should have a particle size of 1 to 50 nm, preferably 1 to 10 nm. The powders or suspensions, which are not commercially available, are to be obtained by colloid grinding processes or by correspondingly shortened polymerization times. The fluoropolymer precursor to be embedded can be a fluoromonomer or oligomer which is polymerized in the pores or on the coating. The surfaces to be coated according to DE-A 41 24 730 are preferably freshly produced oxidized surfaces which have a high chemical reactivity. In contrast, the invention relates to coating of hydrophobic surfaces of little or even no reactivity. Preferred substrates are materials which are subjected to severe chemical and/or thermal stresses and are employed, for example, in chemical apparatus construction, such as carbides, borides, suicides and similar materials. Preferred substrates are shaped articles of non-graphitic carbon, of graphite or of silicon carbide, which are porous in their starting state, in spite of their fine-grained structure. To render these shaped articles impermeable to liquid or gas, their pore system must be at least partly filled with synthetic resins. For many uses, it is also advantageous for these articles additionally to be provided with a surface film of synthetic resin. The use of the shaped articles in highly reactive, aggressive, for example oxidizing, media in particular requires impregnation, which not only at least largely fills up the pores, but also produces a firmly adhering coating on the surfaces. Fluoropolymers are known to be suitable for such coatings. However, the processes and coating agents known to date have not proved suitable for hydrophobic substrates, since these substances do not penetrate deeply enough into the pore system of the articles and the coatings are therefore not sufficiently anchored. If correspondingly finely divided dispersions are employed and the coated substrates are heated for fixing, fluoropolymers which cannot be processed from the melt, such as polytetrafluoroethylene, require such high temperatures that damage to the substrate may occur. These disadvantages are overcome by the process according to the invention, which also produces such effective coatings on porous, hydrophobic substrates that correspondingly treated shaped articles can be employed in chemical apparatus construction even in the presence of aggressive media. In the process according to the invention, a hydrophobic substrate is brought into contact with an aqueous dispersion of a thermoplastic fluoropolymer having a number-average particle size of up to 100 nm, if appropriate under pressure, and the article thus impregnated is after-treated with heat in order to fix the fluoropolymer. Preferred embodiments of this process are described in more detail below and defined in the patent claims. The aqueous suspension of the fluoropolymer preferably comprises particles having a number-average particle size of up to 50 nm, advantageously up to 40 nm, in particular up to 30 nm. The lower limit of the particle size is about 1 nm, preferably 10 nm. The choice of fluoropolymer depends on the requirements: if the chemical resistance requirements are high, a copolymer of tetrafluoroethylene with a fluoroalkyl perfluorovinyl ether, for example a bipolymer having units of the formula X--(CF.sub.2).sub.n --O--CF.dbd.CF.sub.2, in which X is hydrogen, chlorine or, preferably, fluoro and n is a number from 1 to 8, preferably 1 to 3, is chosen as the fluoropolymer. If the chemical resistance requirements of the coating are lower, less expensive fluoropolymers, such as copolymers of tetrafluoroethylene with ethylene, are suitable. The fixing conditions depend on the apparatuses available, and if necessary can easily be determined by simple preliminary experiments. In general, the shaped article is initially exposed to a vacuum and is then, while maintaining the reduced pressure, brought into contact with the aqueous dispersion, if necessary under pressure, for example under about 10 to 100 bar, and the excess aqueous dispersion is removed after the pressure has been released. Thermal fixing is preferably carried out in two stages, first at a low temperature of the order of 100 to 150.degree. C. and then at a sufficiently high temperature to ensure sintering or fusion of the fluoropolymer to form a coating which is as dense as possible. Preferred embodiments of the invention will be explained in more detail in the following examples. Examples 1 to 5 here are preparation examples for a preferred dispersion of a bipolymer of tetrafluoroethylene with perfluoro(propyl vinyl) ether, which is particularly suitable for coatings which are highly resistant to chemicals. |
| PATENT EXAMPLES | This data is not available for free |
| PATENT PHOTOCOPY | Available on request |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |