Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
1999-04-16
2001-07-31
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C210S702000, C210S703000, C210S705000, C210S708000, C210S723000, C526S242000, C528S50200C, C528S50200C
Reexamination Certificate
active
06268469
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a solid-liquid separation method of an emulsified dispersion of fluorine-containing polymer particles.
BACKGROUND OF THE INVENTION
A fluorine-containing polymer is prepared by emulsion polymerization method, suspension polymerization method or the like method. The fluorine-containing polymer prepared by emulsion polymerization is in a state that primary particles of the polymer having an average particle size of from several tens of nanometers to hundreds of nanometers are emulsified and dispersed in water. Since it is difficult to directly filtrate the fine primary particles in such an emulsified dispersion, those primary particles are coagulated into secondary particles having an average particle size of several microns to hundreds of microns and then filtrated.
Known methods for the above-mentioned coagulation are as follows.
(1) A method for coagulation by stirring an emulsified dispersion mechanically to apply shearing force to the dispersion and break an emulsified state.
(2) A method for coagulation by adding a coagulant and then stirring to break an emulsified state.
(3) A method for coagulation by adding a water-insoluble organic solvent to an emulsified dispersion and then stirring to coagulate fluorine-containing polymer particles in droplets of the organic solvent. After the coagulation, the organic solvent is removed (for example, JP-B-60-240713).
The both methods (1) and (2) are methods for breaking an emulsified state to coagulate polymer particles. By breaking an emulsified state of fluorine-containing polymer, coagulated secondary particles become water-repellent. The water-repellent fluorine-containing polymer particles are hardly redispersed, that is, hardly becoming in the form of slurry, which makes it difficult to feed the particles into a filtration step. Also water retained in the water-repellent coagulated particles is hardly released from the particles. Therefore it is difficult to decrease a water content greatly by the filtration method and it takes a long period of time for heating and drying. Further since a dried powder has a large space having been occupied by water, its apparent density becomes very small and handling property thereof is not so good, which results in occurrence of bridging in a powder storage tank and an increase in equipment size.
In conventional methods, primary dewatering of coagulated particles has been usually carried out by solid-liquid separation means and then heating and drying follow. Usually the solid-liquid separation of a dispersion of fluorine-containing resin is carried out by filtration, particularly by using filtration means which causes the filtrated cake to have a free surface. This is because when the fluorine-containing polymer particles are water-repellent, they are in a state of being floating on the separated water. Examples of such filtration means are Nutsche filtration, press filtration in which pressurized dispersion is fed onto the sole surface of filtration medium, and filtration under reduced pressure (suction) which utilizes the sole surface of filtration medium.
If it is intended to let a cake have a free surface, cracking occurs on the cake and water content cannot be lowered further. That phenomenon arises in case of the coagulation methods of above (1) and (2). If the water content is not lowered, in case where the obtained cake is heated and dried, a resulting powder has a small apparent density and poor handling property as mentioned above.
On the other hand, the above method (3) is a method for coagulating without breaking an emulsified state. Therefore particles do not become water-repellent and thus the above-mentioned problems in the filtration step do not arise. However since an organic solvent is used, an increase in cost is resulted and equipment for recovering the organic solvent is required and besides environmental problem arises due to exhaust water and gas.
SUMMARY OF THE INVENTION
The present invention relates to the solid-liquid separation method of an emulsified dispersion of fluorine-containing polymer particles which can greatly decrease a water content of coagulated particles of fluorine-containing polymer.
The solid-liquid separation method of the present invention comprises:
a step for adding a coagulant in an emulsified dispersion of fluorine-containing polymer particles to coagulate the fluorine-containing polymer particles with stirring and obtain a slurry of coagulated particles (coagulation step),
a step for filtrating the slurry (filtration step), and
a step for pressing the obtained cake (pressing step).
The washing step may be provided before the pressing step to wash the filtrated cake.
DETAILED DESCRIPTION
The solid-liquid separation method of the present invention is suitable particularly for solid-liquid separation of an emulsified dispersion of fluorine-containing polymer particles which is prepared by emulsion polymerization, in which the fluorine-containing polymer is tetrafluoroethylene (TFE) homopolymer (PTFE), vinylidene fluoride (VdF) homopolymer (PVdF), chlorotrifluoroethylene (CTFE) homopolymer (PCTFE) or a copolymer of at least two monomers selected from the group consisting of TFE, VdF, CTFE, hexafluoropropylene (HFP), perfluoro(alkyl vinyl ether) and ethylene.
Examples of perfluoro(alkyl vinyl ether) monomer are vinyl ethers represented by the formula (1):
CF
2
=CFO(CF
2
)
m
F (1)
in which m is an integer of 1 to 6, and the formula (2):
CF
2
=CF(O—CF
2
CF(CF
3
))
n
OC
3
F
7
(2)
in which n is an integer of 1 to 4.
Examples of the fluorine-containing copolymer are, for instance, TFE/VdF copolymer, TFE/HFP copolymer, TFE/perfluoro(alkyl vinyl ether) copolymer, TFE/ethylene copolymer, TFE/CTFE copolymer, TFE/HFP/perfluoro(alkyl vinyl ether) copolymer, and the like.
An average particle size of primary particles of the polymer is from 80 to 300 nm.
An emulsifying agent in the emulsified dispersion does not have a substantial effect on the method of the present invention.
The coagulation step of the method of the present invention is carried out by adding a coagulant with stirring under the condition of preventing the coagulated fluorine-containing polymer particles in a slurry from becoming water-repellent. Such a condition can be produced by, for example, using an inorganic acid as a coagulant and regulating a stirring speed and an adding amount of the coagulant.
The use of an inorganic acid as the coagulant is preferred from the viewpoint that a surface activating effect of the emulsifying agent being adsorbed on the surface of polymer particles is lowered and the particles are easily coagulated. Examples of the preferred inorganic acid are hydrochloric acid, sulfuric acid, nitric acid and the like. Particularly nitric acid is preferable from the viewpoint of handling property and corrosion of materials of each equipment. An adding amount of the inorganic acid differs depending on kind thereof, primary particle size of the polymer, and kind and amount of an emulsifying agent to be used. Basically from the viewpoint of preventing the coagulated particles from becoming water-repellent, it is preferable that the adding amount of the inorganic acid is from 0.1 to 20 parts (part by weight, hereinafter the same), particularly from 0.5 to 10 parts based on 100 parts of the fluorine-containing polymer particles. When the adding amount is too much, it causes the particles to become water-repellent. When too small, coagulation becomes insufficient and thereafter solid-liquid separation cannot be carried out easily.
In case of strong stirring, a shearing force to be applied becomes too big and the particles become water-repellent. A shearing force to be produced differs depending on kind of stirring apparatus, and a suitable degree of stirring which can be applied to any methods of stirring cannot be determined unconditionally. However the particles must not become water-repellent at least only by stirring. The stirring speed is determined experimentally in each practica
Hiraga Yoshiyuki
Imanishi Hiroyuki
Komatsu Satoshi
Noda Tomohisa
Daikin Insdustries, Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Zitomer Fred
LandOfFree
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