TFE polymerization process

Details TFE polymerization process TFE polymerization process

1999-06-30

2001-10-02

Zitomer, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S078000, C526S206000, C526S209000, C526S909000, C526S911000

Reexamination Certificate

active

06297334

ABSTRACT:

The invention relates to a process for preparing poly-tetrafluoroethylene (PTFE) aqueous dispersions having sizes in the range 5-60 nanometers, for at least 60% by weight of the dispersion particles.
More specifically the invention relates to a tetrafluoroethylene (TFE) radicalic polymerization process in aqueous emulsion, in the presence of an inert fluorinated phase previously mixed with a fluorinated surfactant and with water in ratios such as to give a microemulsion.
As it is well known, by microemulsion a system is meant wherein a phase with non reactive terminals is solubilized from a surfactant aqueous solution to give a monophasic solution which is stable in a long term and is obtained by simple mixing all the components.
The microemulsions are stable isotropic mixtures of oil, water and surfactant which are spontaneously formed when the ingredients are put together. Other components, such as salts or co-surfactants (alcohols, amines or other amphiphilics) can be a part of the microemulsion. Oil and water reside in distinct domaines separated by a surfactant-rich layer. Since these domains are small the microemulsions visually appear transparent or translucid. The microemulsions can show a variety of microstructures, mainly depending on their compositions and temperature. The common feature is the presence of a film or a surfactant rich interlayer which interposes among domains rich in water or oil. There are three structures among the most common ones. One is the so called microemulsion water in oil, wherein water is contained within distinct domains (droplet) surrounded by an oil-rich continuous phase. A second one is the microemulsion oil in water wherein oil is contained within distinct domaines surrounded by a continuous phase water rich. The third one is a bicontinuous structure in which zones of oil and water interlaced each other are present, the one separated from the other by surfactant rich layers.
The polymerization in the presence of microemulsion is known and shows a series of advantages in comparison with the conventional emulsion polymerization. Emulsions are opaque, while microemulsions are usually transparent or translucid, so as to be more suitable for photochemical type initiation. Moreover the possibility to have a higher number of particles per litre of water allows to increase the polymerization rate and to incorporate in the polymeric chains not very reactive monomers, without dramatic losses of the reaction yield.
In the U.S. Pat. No. 4,864,006 it is foreseen that the microemulsion can be diluted at the time when it is introduced in the reaction medium maintaining the above mentioned characteristics to guide the polymer particle nucleation and to determine the number thereof. In this patent no PTFE homopolymer polymerization examples are given; tests carried out by the Applicant (see the Examples) have shown that by operating in the conditions indicated in said patent, in the PTFE polymerization case, no dispersions of the order of nanometers as defined above are obtained.
In the U.S. Pat. No. 5,523,346 the choice of the oil phase to be used for obtaining the polymerizable unsaturated liquid monomer microemulsion is instead limited, and the microemulsion dilution method in the reaction medium, as indicated in the U.S. Pat. No. 4,864,006, is not used. In the U.S. Pat. No. 5,616,648 it is shown that in order to obtain fluoropolymer particles having the nanometer sizes as those of the present invention, the microemulsion use is necessary, the oil phase of which is the TFE polymerizable liquid monomer. It is well known that operating under these conditions is extremely dangerous due to the TFE explosiveness. According to this patent it is necessary that the weight ratio between surfactant and monomer is higher than 1.17 to obtain the polymer particles having the nanometer diameter as defined in this invention.
The substrata coating with fluorinated polymers is well known. In order to produce uniform films having a very low thickness (for example lower than 1 &mgr;m) using fluorinated solvents, it is necessary to operate through expensive processes with not negligible environmental impact. Moreover with the conventional fluoropolymer dispersions, formed by particles of 0.1-1.0 micron size, it is difficult to obtain uniform coatings on substrata showing microporosity. Besides, the large size particles can obstruct the submicronic porous structures, what is undesirable in some applications. In other applications the chemical and thermal stability given by the fluoropolymers in general and by the TFE homopolymers having a medium-high molecular weight in particular, is important.
The need was felt to have available polymerization techniques capable to give PTFE dispersions with very small particle size as defined above (from now on indicated as nanoemulsions), preferably characterized by an high thermal stability and by a suitable molecular weight.
An object of the present invention is a process for the preparation of dispersions based on tetrafluoroethylene (TFE) homopolymers, or based on its copolymers with one or more monomers containing at least one unsaturation of ethylenic type in amounts from 0-6 by moles, preferably 0 up to 3% by moles, more preferably from 0 to 1% by moles, having a particle fraction equal to at least 60% by weight, preferably 70% by weight, with diameter average sizes in the range 0.005-0.06&mgr;m (5-60 &mgr;m), preferably 0.01-0.05 &mgr;m, comprising:
a) preparation of an aqueous perfluoropolyether (PFPE) microemulsion having non reactive terminals, preferably (per)fluorinated terminals, optionally the terminals containing one or more H atoms, Cl instead of fluorine;
b) microemulsion feeding into the polymerization reactor, in amounts so that the microemulsion perfluoropolyethereal oil phase is present in a concentration higher than 2 ml per litre of reaction medium, preferably from 2.2 ml up to about 50 ml per litre, still more preferably from 3 to 30 ml per litre of reaction medium;
c) feeding of the reaction medium in the polymerization reactor, reactor degassing, reactor pressurization with gaseous TFE, optional addition of surfactants, stabilizers, comonomers, chain transfer agents;
d) initiator addition and optionally during the polymerization addition of further amounts of surfactants, stabilizers, comonomers, chain transfer agents;
e) discharge of the polymeric latex from the reactor.
The microemulsion feeding mentioned at point b) can also be carried out after feeding of the reaction medium and the other ingredients mentioned at point c).
Besides the components mentioned at points c) and d) other components commonly used in the TFE polymerization can be added. Among these, polymerization inhibitors, buffering agents, etc. can be mentioned.
During the polymerization additional initiator and the other components indicated in c) and in d) can be added even though they have already been introduced in the reactor at the reaction beginning.
A particular case of delayed feeding of comonomers is that aimed to alter and to control the surface properties of the PTFE particles. For instance a polar comonomer, such as VDF (vynilidene fluoride), CTFE, or non polar comonomers such as C
2
H
4
or C
3
H6, can be added at the final stage of the polymerization reaction in order to achieve a polymer particles surface with a higher surface tension.
This modification of the particles surface energy can be useful when a better blendability with materials like, for instance, polyacrylates, polystyrene or engineer plastics, is desired. Another application of this technique is the use of PTFE nanoemulsions as an ingredient in the polymerization of hydrogenated monomers (e.g. seed polymerization).
The discharged latex from the reactor can be subjected, if desired, to the usual post-treatments in connection with the specific uses. For example the discharged dispersion can be coagulated to obtain fine powders having a very high specific surface. The latex concentration, carried out for example, by ultrafiltration, can also be mentioned. Anothe

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