Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1998-10-13
2000-10-24
Michl, Paul R.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C08L 300
Patent
active
061368938
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for concentrating aqueous dispersion of fluorine-containing polymer such as polytetrafluoroethylene (PTFE) by micro filtration membrane.
BACKGROUND ART
An aqueous emulsion of PTFE is prepared by emulsion-polymerizing a tetrafluoroethylene monomer in the presence of an aqueous polymerization initiator and fluorine-containing emulsifying agent (for example, U.S. Pat. No. 2,559,752). Fluorine-containing polymers other than PTFE are also prepared by nearly the same process, and a solid content thereof is usually from 5 to 45% by weight. In case of industrial uses such as coatings and binders for batteries, an aqueous dispersion having a high concentration (for example, about 60% by weight) is demanded. However it is difficult to obtain a high concentration simply by adding an additional amount of fluorine-containing polymer, and such a high concentration is usually achieved by concentrating procedure.
For example, U.S. Pat. No. 2,478,229 discloses a method for concentrating aqueous dispersion of fluorine-containing polymer by using anionic surfactant. However that method has not been practically adopted since there is a problem that when making the surfactant insolubilized by adding a large amount of electrolyte, a part of PTFE particles causes an avoidable irreversible coagulation.
Also a method of using nonionic surfactant disclosed in U.S. Pat. No. 3,037,953 can give an aqueous dispersion having a relatively high concentration without causing coagulation of PTFE particles and has been commercialized. However the defects of that method are such that since a large amount of a specific surfactant having the limited cloud point range (commonly 20.degree. to 80.degree. C.) is used and after the concentration, the surfactant (hydrophilic portion) having a deviated molecular weight distribution has to be wasted from a supernant liquid, a large material cost is necessary in raw materials, and such that since a large heat energy consumption and long-time process steps are required, the efficiency of the process is not good.
Further in a method of concentration by evaporation disclosed in U.S. Pat. No. 3,316,201, since the resulting concentrated dispersion contains an aqueous polymerization initiator and fluorine-containing emulsifying agent at high concentration which are used in polymerization reaction, there is a defect that viscosity of the aqueous dispersion is sensitively changed with its temperature. In addition, in case of an aqueous dispersion having poor dispersion stability such as aqueous dispersion of PTFE, primary particles easily cause coagulation during the concentrating step to break emulsion state.
In an electric decantation method disclosed in GB 642,025, since the resulting coagulated particles adhere to an electrode to obstruct the passing of electric current, concentration effect is very low with a large amount of electric energy consumption. Thus that method is not suitable for practical use.
On the contrary, there is proposed a membrane separation method which employs an ultrafilter membrane (UF membrane) having a pore size up to 0.5 time the minimum particle size of fluorine-containing polymer particles (JP-B-2-34971, U.S. Pat. No. 4,369,266). This membrane separation method employing UF membrane has been used for filtration of high molecular weight materials, and also in the concentration of an aqueous dispersion of fluorine-containing polymer, has merits such as low energy consumption and low cost, use of various surfactants and a reduced amount of the surfactant.
However in the membrane separation method employing UF membrane, there are problems to be solved such that: 1 since a separable molecular weight of UF membrane is as small as from about 1,000 to hundreds of thousands (not more than 0.01 .mu.m in pore size), desired filtrating performance cannot be obtained unless filtration pressure is adjusted to about 1 MPa. Also filtration speed is low and when liquid flow rate is increased, the UF membrane is clogged with the fluori
REFERENCES:
patent: 4369266 (1983-01-01), Kuhls
Hayashi Tadao
Hosokawa Kazutaka
Kawachi Shoji
Miura Toshiro
Murakami Shinji
Daikin Industries Ltd.
Michl Paul R.
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