Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-09-26
2004-04-20
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S247000, C526S253000, C526S254000
Reexamination Certificate
active
06723812
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a continuous process for the high-pressure polymerization of 1,1-difluoroethylene (VF2) to give polyvinylidene fluoride (PVDF).
BACKGROUND OF THE INVENTION
The polymerization of 1,1-difluoroethylene is currently carried out industrially in an aqueous medium either in emulsion or in suspension. This type of polymerization thus corresponds to batchwise processes.
In the case of the emulsion, it is necessary to add emulsifiers to the polymerization medium in order to stabilize the PVDF latex particles formed. The emulsifiers must be removed in a subsequent step in order to ensure that the polymer is in a suitable purity. Furthermore, certain emulsifiers which are used in the emulsion process are perfluoro molecules. Studies have shown that these molecules have a tendency to accumulate in aquatic flora. Although considered as harmless, nothing is as yet known regarding the long-term impact of this bioaccumulation.
In the case of the suspension, protective colloids are incorporated in order to stabilize the PVDF particles.
Another process is currently under investigation in many laboratories. This continuous process differs from emulsion/suspension processes in that it is carried out in supercritical CO
2
(ScCO
2
) without stabilizing additive. During the polymerization, the PVDF which is insoluble in the ScCO
2
precipitates and forms a powder. However, it is necessary to add a powder-treatment step in order to convert it into PVDF granules since this type of process does not make it possible to obtain a powder of controlled and narrow particle size (as with the emulsion process).
Patent FR-A-1 260 852 discloses the polymerization of VF2 under pressure either in the presence of a neutral reaction medium such as water or in the absence of a reaction medium. According to the first form, deionized and deoxygenated water are loaded into an autoclave, followed by a peroxide and VF2, the proportion of peroxide being 0.8 g per 35 g of VF2. The autoclave is closed and heated, the pressure establishes at values of about 40 to 60 bar, and the reaction time is about 18 hours. Next, the autoclave is cooled and PVDF is recovered therefrom. According to the second form, the process is performed as in the first form, but without introducing water, the pressure and the reaction time being the same. It is stated that this second form lends itself to a continuous operation in which the autoclave is connected to a source of VF2 under pressure such that the fresh VF2 enters the autoclave when the conversion into polymer takes place. According to a variant, catalyst may be added continuously or in batchwise mode. This prior art is based on a fatal pressure (autogenous pressure) generated by the volume of the autoclave, the reagents loaded in and the temperature. The fact that it can be made continuous by injecting fresh VF2 to compensate for the VF2 which is polymerized has nothing to do with a process in which the reaction pressure is determined by the pressure supplied by the VF2 injection pumps. Nothing is stated regarding the oxygen content of the VF2.
Patent U.S. Pat. No. 2,435,537 discloses a process similar to the previous one but still in the presence of water, 50 parts per 40 parts of VF2, and the pressure may be 1000 bar. All the examples are in batchwise mode in an autoclave. It is stated that the process may be performed continuously; however, it is recommended always to use an inert medium such as water to disperse the catalyst and control the reaction by dissipating the heat. It is stated that oxygen has a harmful effect on the polymerization, but the oxygen content of VF2 is not specified and nothing is stated regarding the means for reducing it.
Patents FR-A-2 650 593 and FR-A-2 689 134 disclose processes for the high-pressure synthesis either of copolymers of VF2 and of ethylene or copolymers of VF2 and of fluoroacrylates.
Patent WO 98/28351 discloses the continuous polymerization of VF2 in supercritical CO
2
(75° C.—276 bar). PVDF copolymers may thus be manufactured. In one example, the flow rate of fluoro monomers is 200 g/h for a CO
2
flow rate of 518 g/h.
SUMMARY OF THE INVENTION
A high-pressure process exists, in which the VF2 is converted into PVDF solely under the effect of pressure and traces of a peroxide. The high-pressure polymerization makes it possible to overcome the problems mentioned in the other processes cited above:
the process does not require stabilizing additives (protective colloids or surfactants),
no treatment of the powders.
Another advantage of the high-pressure process is that it offers better production efficiencies than the continuous process in ScCO
2
.
The present invention relates to a process of this type. Particularly, a continuous process for preparing PVDF homopolymer or copolymer has now been found, the comonomer being a fluoro monomer rather than an acrylate, in which no organic solvent or water is used and which is not in ScCO
2
medium.
The invention is a continuous process for the manufacture of PVDF homopolymer or copolymer, the comonomer being chosen from compounds containing a vinyl group capable of being opened by the action of free radicals in order to polymerize, and which contains, directly attached to this vinyl group, at least one fluorine atom, a fluoroalkyl group or a fluoroalkoxy group in which:
(a) a flow of VF2, of optional comonomer and of radical initiator is introduced into a reactor maintained at a pressure of between 300 bar and 3000 bar, the reactor containing essentially VF2, an optional comonomer and PVDF;
(b) a flow of reaction mixture is removed from the reactor for step (a) and introduced into a separator;
(c) molten PVDF is recovered in the separator and purged continuously;
(c1) the flow of PVDF from step (c) is optionally introduced into a device to place it in the form of granules;
(d) VF2 and optionally comonomer are recovered in the separator and recycled into step (a).
According to one advantageous form of the invention, the fresh VF2, the other portion being recycled, which is introduced into step (a) contains less than 5 ppm of oxygen, preferably less than 1 ppm and better still between 0.1 and 0.8 ppm.
The process of the invention advantageously comprises an additional step consisting in deoxygenating the fresh VF2 before introducing it into step (a).
This step consists in placing the flow of VF2 in contact with (i) a sufficient amount of a catalyst whose active sites are elements belonging to groups 8 to 11 of the Periodic Table of the Elements and (ii) for a time which is sufficient to obtain the desired oxygen content.
The invention also relates to this isolated step of treatment of VF2 to reduce its oxygen content. This step advantageously precedes a process which is different from the preceding one in which the VF2 is polymerized or copolymerized.
This deoxygenation may also apply to mixtures of VF2 and of one or more comonomers and also to the VF2 comonomers alone.
The invention also relates to, as a product, VF2, the mixture of VF2 and of comonomer or the comonomer alone (or mixture of comonomers) containing less than 5 ppm of oxygen, advantageously less than 1 ppm and better still between 0.1 and 0.8 ppm.
The present invention also relates to a PVDF with an Mw/Mn ratio of between 1.5 and 1.9.
The process of the invention has many advantages:
there is no water or organic solvent: it is a clean process, not requiring the recycling/treatment of water or the recycling of solvent;
it is a <<dry >> process not requiring the removal of water or solvent from the polymer: it is less expensive in energy terms;
the polymer is cleaner since it is not soiled by the presence of surfactants or other additives;
there is no use of perfluoro surfactants which may bioaccumulate in the environment: the process is environmentally friendly;
there is no handling of powder as in the ScCO
2
process: this entails a simplification of the process, with no cyclones to be added, no step of uptake of the powder to convert it into granules, no problem of powder
Atofina
Lipman Bernard
Millen White Zelano & Branigan P.C.
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