Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-01-05
2003-01-21
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S078000, C526S087000, C526S201000, C524S458000
Reexamination Certificate
active
06509429
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a novel process for preparing a fluorine-containing polymer by an emulsion polymerization method
BACKGROUND ART
Fluorine-containing elastomers such as elastomeric vinylidene fluoride/hexafluoropropylene (VdF/HFP) copolymer and elastomeric tetrafluoroethylene (TFE)/perfluoro(vinyl ether) copolymer exhibit excellent chemical resistance, solvent resistance and heat resistance and therefore have been widely used for O-ring, gasket, hose, stem seal, shaft seal, diaphragm, etc. in the fields of automobile, semiconductor industry and chemical industry which are used under tough environment.
Example s of the fluorine-containing rubber used in those applications are fluorine-containing elastomers having iodine which have highly active iodine atom at a molecular end thereof. The fluorine-containing elastomer having iodine makes it possible to give a good crosslinking efficiency by iodine atom at a molecular end thereof and is excellent in vulcanizability. Therefore a chemical substance having metal component need not be added to the elastomer and the elastomer is subjected to peroxide vulcanization to give a molded article. As a result, a vulcanized article having more excellent tensile strength, elongation at break and compression set as compared with other vulcanization system can be obtained.
The fluorine-containing elastomer having iodine is prepared by emulsion polymerization such as iodine transfer polymerization described in JP-B-63-41928. However in order to attain a high iodine transfer rate, it is necessary to reduce an amount of polymerization initiator (Masayoshi Tatemoto, Control of Polymer Structure in Radical Polymerization, p 19, 86/6, Micro Symposium of The Society of Polymer Science, Japan (1986)), and accordingly productivity cannot be increased. In a polymerization system where there is no restriction on an amount of polymerization initiator, a polymerization rate can be easily raised by increasing an amount of polymerization initiator. However in the iodine transfer polymerization system, since a concentration of polymer end derived from the initiator has a great effect on physical properties of final products, it is impossible to expect that an amount of initiator is increased.
In order to enhance productivity, various proposals have been made. For example, in JP-A-3-33108 and JP-A-3-221510, a method for enhancing productivity by conducting emulsion polymerization continuously is proposed, but good tensile strength and compression set which are characteristics of the fluorine-containing elastomer having iodine cannot be obtained.
Also in JP-A-5-222130, a method for polymerizing at high pressure of not less than 1.7 MPa (gauge pressure, hereinafter the same) is proposed, but a high pressure equipment is required. Further in JP-A-63-8406, micro emulsion polymerization method is proposed, but since use of fluorine-containing oil, etc. is initially necessary for forming micro emulsion and the fluorine-containing oil, etc. remain in a product and become contamination sources, it is necessary to remove the oil, etc. by washing.
In order to merely stabilize a polymerization system and increase a polymerization rate, an amount of an emulsifying agent may be increased, but since the emulsifying agent itself causes a vulcanization failure, it is also necessary to remove the emulsifying agent by washing.
As mentioned above, there was no method for preparing a fluorine-containing elastomer having iodine which can make productivity and maintenance of properties thereof compatible with each other.
With respect to the fluorine-containing rubbers, in addition to the above-mentioned fluorine-containing elastomer having iodine, there are some elastomers which are required to have a reduced amount of ionic functional groups at its molecular end which are derived from a water soluble radical polymerization initiator, from the viewpoint of characteristics of the elastomer at vulcanization and physical properties of vulcanized molded article. Examples thereof are fluorine-containing elastomers which are prepared in combination of aromatic polyhydroxy compounds with quaternary phosphonium salt, quaternary ammonium salt, and the like and are used for so-called polyol vulcanization. It is proposed that those fluorine-containing elastomers are prepared by two-staged emulsion polymerization method (WO96/17876).
In the two-staged polymerization method, at first in the first stage, several percent of polymer to a total yield is polymerized by using a water soluble radical polymerization initiator to give seeds of emulsified particles, and then in the second stage, the seed particles are grown by using an organic peroxide and most of remaining polymer of not less than 90% are synthesized. However in that polymerization method, there are limits in amounts of emulsifying agent and water soluble radical polymerization initiator which are used in the first stage and have an adverse effect on the vulcanization rate, and the number of seed particles cannot be increased. Therefore a final size of emulsified particles exceeds 500 nm and becomes unstable, sticking of polymer inside a polymerization tank is much, washing thereof is complicated, much loss of product arises and productivity is poor.
The inventors of the present invention have made intensive studies to enhance productivity of the fluorine-containing elastomer with maintaining characteristics of the elastomer without modifying existing emulsion polymerization facilities and have found that the fluorine-containing elastomer having uniform particle size can be obtained efficiently with maintaining inherent characteristics by separating emulsion polymerization into two stages, in which in the first stage polymerization, many polymer particles are synthesized by using relatively much amount of emulsifying agent and then the obtained emulsion is diluted to reduce concentrations of polymer particles and emulsifying agent, and in the second stage, polymerization is carried out by using the diluted emulsion.
Further it was found that when the two-staged polymerization including the diluting step is applied to preparation of other fluorine-containing polymer, excellent effects are exhibited, and thus the present invention was completed. For example, when a tetrafluoroethylene/hexafluoropropylene copolymer is prepared by emulsion polymerization, it was found that by employing the two-staged polymerization method of the present invention, a polymerization rate can be increased by four times or more even at the same concentration of emulsifying agent as compared with usual one-staged polymerization.
By employing the emulsion polymerization method of the present invention, a particle size of final emulsified particles after the polymerization can be easily reduced to not more than 500 nm, particularly not more than 400 nm without producing an adverse effect on vulcanizability of the obtained fluorine-containing elastomer and properties of the obtained vulcanized article, stability of an emulsion is greatly enhanced and sticking of polymer inside a polymerization tank can be reduced.
DISCLOSURE OF INVENTION
The present invention relates to the process for preparing a fluorine-containing polymer comprising:
(a) a first polymerization step for emulsion-polymerizing at least one fluorine-containing monomer or a mixture of at least one fluorine-containing monomer and at least one non-fluorine-containing monomer in an aqueous medium in the presence of a radical polymerization initiator and an emulsifying agent,
(b) a dilution step for diluting the obtained aqueous emulsion of polymer particles with water to reduce the number of emulsified polymer particles per unit amount of aqueous medium in the emulsion and a concentration of emulsifying agent, and
(c) a second polymerization step for emulsion-polymerizing at least one fluorine-containing monomer and/or at least one non-fluorine-containing monomer in the obtained diluted emulsion.
According to the preparation process of the present invention, a particle size
Aihara Hiroshi
Irie Sadashige
Kishine Mitsuru
Kitaichi Masanori
Maruya Yoshiki
Daikin Industries Ltd.
Egwim K C
Sughrue & Mion, PLLC
Wu David W.
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