Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Autogenously or by activation of dry coated particles
Patent
1999-01-19
2000-02-01
Foelak, Morton
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Autogenously or by activation of dry coated particles
521145, B27J 500
Patent
active
060199202
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to porous polytetrafluoroethylene (PTFE hereinafter) materials formed by fusion of granular-type PTFE resin particles thus creating a loose or open interconnection of fused particles.
BACKGROUND OF THE INVENTION
Porous PTFE articles have been produced by fusing PTFE resin particles together through application of heat at a temperature above the melt (or sinter) temperature of the PTFE.
For example, GB 2 242 431 B to Allan, et al. (W. L. Gore & Associates, Ltd.), describes a sintered porous PTFE structure used as a filter to filter solids or gases from liquids. The porous structures are formed by fusing together particles of granular-type PTFE to form an integral network of void spaces between the particles. The pores are created by the void space between the particles and results in tortuous paths of voids through the article. Because of the small pore size and the chemical inertness of the PTFE, these structures are useful as filters, fuel cell electrodes, photocopy webs or rolls, and the like. However, these structures tend to have weak inter-particle connections and are not as strong as needed for some applications.
U.S. Pat. No. 5,154,866 to Honda (Daikin), describes preparation of porous PTFE articles of sintered granular PTFE and fine powder PTFE used as filters for filtering out small particulate. The preparation comprises the steps of first gelling a PTFE powder prepared by suspension polymerization (which to one skilled in the art identifies the PTFE as granular-type PTPE) at a temperature not lower than the melting point of PTFE. Next the PTFE is pulverized and mixed with an aqueous dispersion of PTFE fine powder prepared by emulsion polymerization. The solids are then coagulated, ram extruded, and baked by heating above the melting point of the PTFE. The gelled PTFE particles are hard and resist compaction. This results in relatively large size pores and the articles are useful mostly for separating out relatively large size particulate.
U.S. Pat. No. 3,855,191 to Doughty, et al.(DuPont Co.) describes a modified granular-type PTFE in which the modification is in use of a modifier monomer which enters into the polymer chain to modify the homopolymer structure without changing the well known nonmelt processible characteristics of the polymer.
SUMMARY OF THE INVENTION
In this invention, strong porous PTFE articles are obtained that are useful as filters in filtration or in the metered dispensing of liquids, as for example in photocopying and in photocopier rolls or webs and in fuel cell electrodes.
The porous PTFE articles are prepared from an aqueous dispersion of particles of at least one non-melt-processible copolymer of granular-type PTFE modified by small amounts of comonomer, by laying down a coating of the dispersion followed by heating to drive off the aqueous liquid and to fuse together the particles into an open network of fused particles that define a tortuous network of interconnected voids through the article between the fused interconnected particles. Thus, the porosity is not due to stretching or expanding of PTFE as is the case in some unrelated PTFEs. On the contrary the granular particles are non-expanded.
As will be discussed below, the granular-type PTFE modified by a small amount of recurring units of a comonomer used herein is commonly referred to in the art as "modified granular PTFE" and this term will be used hereinbelow.
Thus, the product aspect of the invention is two-fold. In one aspect, the product is a porous sheet of particles of modified granular-type PTFE fused together in a manner which forms a strong network of fused interconnected particles, without the need for additives, e.g., fine powder PTFE, to strengthen the article. In the other aspect the product is the aqueous dispersion from which the fused porous sheet is prepared.
It has unexpectedly been found that when the so-called modified granular-type PTFE aqueous dispersions are employed, the resulting fused sheets are stronger than when the usual unmodified
REFERENCES:
patent: 3855191 (1974-12-01), Doughty, Jr. et al.
patent: 4186121 (1980-01-01), Gangal
Foelak Morton
Samuels Gary A.
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