Vinylidene fluoride polymer and process for production thereof

Details Vinylidene fluoride polymer and process for production thereof Vinylidene fluoride polymer and process for production thereof

2002-01-25

2004-02-03

Reddick, Judy M. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S206000

Reexamination Certificate

active

06686427

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a vinylidene fluoride polymer (i.e., a homopolymer or a copolymer of vinylidene fluoride) having excellent thermal stability and a process for production thereof. More specifically, the present invention relates to a vinylidene fluoride polymer having excellent high-temperature coloring or discoloration resistance, i.e., remarkably less coloring of polymer or a shaped product thereof compared with a conventional vinylidene fluoride polymer obtained through suspension polymerization, when exposed to high temperature, and a process for production thereof.
A vinylidene fluoride polymer is a crystalline polymer and is used for providing various shaped products as a polymer having good mechanical strength. In this instance, for the purpose of providing such a shaped product with a good size stability suitable for the intended use, it has been generally practiced to subject the shaped product before the use thereof to a sufficient heat treatment (hereinafter sometimes called “aging”) so as to remove a strain during the shaping and cause additional crystallization. After the aging, however, the shaped product is liable to color into yellow or brown sometimes, thus lowering the commercial value thereof. Accordingly, a hardly colorable vinylidene fluoride polymer resin has been desired, and some commercial products of improved colorability have been put on the market, but further improvements have been desired.
As processes for producing vinylidene fluoride polymers, there have been known solution polymerization, emulsion polymerization and suspension polymerization. Solution polymerization is generally performed at a polymerization temperature of 20-80° C. and at a relatively low pressure of, e.g., at most 1.5 MPa, for providing a polymer used for paint having a relatively low molecular weight of, e.g., a number-average molecular weight of at most 10
5
corresponding to at most ca. 0.5 dl/g in terms of an inherent viscosity described hereinafter, and other polymerization processes are used for producing vinylidene fluoride polymers of higher molecular weights used for shaped products, etc. Further, because of a strong hydrogen-abstracting power of growing vinylidene fluoride radicals, the solvents usable therefor are restricted, and ketone solvents or acetate ester solvents (Japanese Laid-Open Pat. Appl'n (JP-A) 59-135257), chlorofluoroethane solvents (JP-A 6-322028), etc., have been used.
In emulsion polymerization, a chemically stable fluorine-containing emulsifier or buffer agent is used for the polymerization to obtain a polymerizate comprising a latex having small particle sizes of 0.2-0.5 &mgr;m, which is then treated with a flocculant, etc., for precipitation and particle formation to obtain polymer particles. Emulsion polymerization has an advantage of a rapid polymerization speed, but is accompanied with difficulties in convenience and production cost due to the use of an expensive emulsifier, and also difficulties, such as powder recovery through agglomeration and precipitation, and removal of ionic impurities originated from the flocculent.
Suspension polymerization is a process which has been practiced since development of initiators exhibiting polymerization initiation power at low temperatures so as to make possible polymerization at a temperature below 30.1° C. (i.e., critical temperature of vinylidene fluoride monomer). Vinylidene fluoride monomer alone or together with a copolymerizable monomer is dispersed within water, and the resultant monomer droplets are subjected to polymerization in the presence of a polymerization initiator. It is also possible to start the suspension polymerization at a temperature below 30.1° C. and then continue the polymerization by raising the temperature after formation of the particles. The suspension polymerization allows easy post-processing of the resultant polymer and is most suited for providing a vinylidene fluoride polymer excellent in processability, mechanical properties and thermal stability.
It is well known that the selection and used amount of a suspension agent, a chain transfer agent and a polymerization initiator have influences on the coloring characteristic of a vinylidene fluoride polymer obtained in the polymerization and a shaped product thereof. For example, Japanese Patent Publication (JP-B) 3-48924 has proposed a suspension polymerization process for a vinylidene fluoride polymer less liable to be colored by using a specific chain transfer agent (chain-adjusting agent), such as bis(ethyl) carbonate. However, even in the case of using such a chain transfer agent, the resultant vinylidene fluoride polymer has left a room for improvement regarding the durability, and also difficulties, such as the necessity of a high polymerization pressure and scaling on the gaseous phase wall in the polymerization vessel due to gaseous-phase polymerization.
On the other hand, JP-A 6-136008 has proposed a process for suspension-polymerizing a fluorine-containing copolymer in the presence of a halogenated hydrocarbon solvent. When this process is applied to suspension polymerization of a vinylidene fluoride polymer, the coloring resistance of the resultant vinylidene fluoride polymer cannot reach a satisfactory level.
SUMMARY OF THE INVENTION
In view of the above-mentioned problems of the prior art, a principal object of the present invention is to provide a vinylidene fluoride polymer having a high-temperature coloring resistance which has been further improved than ever.
Another object of the present invention is to provide a commercially feasible process for producing such a vinylidene fluoride polymer.
In the course of our study for achieving the above objects, we have got the following knowledge. First of all, we started to investigate causal substances of coloring in a vinylidene fluoride polymer during the high-temperature aging, and we now believe that they include decomposition products originated from additives, such as a polymerization initiator, a chain transfer agent and a suspension stabilizer, used in the suspension polymerization, low-molecular weight organic substances formed from low-molecular weight radicals by hydrogen abstraction, and oligomers formed by reaction between such additives and vinylidene fluoride monomer. It is true that such coloring-causal substances can be reduced by post-treatments, such as repetitive washing with water and drying, and also shaping at 200° C. or higher, after the suspension polymerization, and low-molecular weight organic substances that cannot be removed even by such post-treatments are believed to function as a causal substance for high-temperature coloring of vinylidene fluoride polymer. Further, we have found that such a causal substance for high-temperature coloring of vinylidene fluoride polymer is represented by an elutable total organic carbon content (hereinafter sometimes called a “TOC”) measured by dipping a shaped piece of vinylidene fluoride polymer in pure water at 95° C., which provides a good correlation with the high-temperature colorability of a product vinylidene fluoride polymer. Accordingly, a vinylidene fluoride polymer having a reduced TOC provides a vinylidene fluoride polymer of excellent high-temperature coloring resistance, which is a principal objective of the present invention.
Based on the above finding, the vinylidene fluoride polymer according to the present invention is characterized by an elutable total organic content in pure water at 95° C. of at most 1.1 &mgr;g/cm
2
.
According to our study, it has been also found very effective to perform a suspension polymerization of a vinylidene fluoride monomer in a mixture solvent comprising an aqueous medium and an appropriate amount of a specific halogenated hydrocarbon solvent that has a good dissolving power to both the vinylidene fluoride monomer and a polymerization initiator, and also little chain-transfer effect. Based on such finding, the process for producing a vinylidene fluoride polymer according to the present inv

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