DE2265612C2 - Process for the preparation of aqueous dispersions of polytetrafluoroethylene - Google Patents

Description

The polymerization of tetrafluoroethylene in emulsion im a well known process that is the manufacture of dispersions of polytetrafluoroethylene in water which have found numerous technical applications, in particular for coating Metals and other substrates for impregnation of fibers. Fabrics and fabrics, for the production of foils and lacquers and for the through Coagulation production of powders for extrusion with lubricants.

In the previously known processes, the dispersions of polytetrafluoroethylene are produced, by placing the gaseous monomers in a pressure-tight autoclave or in the case of the production of Copolymers with a mixture of monomers an aqueous solution is brought together, which is an initiator, which at the Polvnierisierungstempcratur Free radicals / u are able to form, optionally a buffer, which the mixture at a certain pH value holds, and contains an emulsifier. Serves as an emulsifier generally an alkali or ammonium salt Carboxylic acid ι »the sulfonic acid. their carbon chain contains only Flunraiomc and possibly chlorine atoms. Under these conditions the obtained Dispersions only have a very low stability, with it is hardly possible. a concentration of polytetrafluoroethylene to exceed by 10 to 15%.

A substantial improvement was made according to d <? R French patent specification 10 1 »» 304 achieved by that the Polymeris.itionsgcmisch as a stabilizer 0.1 to 12 wt. 'Vo (based on the aqueous dispersion) of a saturated hydrocarbon containing more than 12 Contains carbon atoms and under the Polymerisaiionsbedingungen is liquid. can be added. However, this method has the disadvantage that the purification of the Polymerization autoclave is difficult. whereby there is a loss of time, which with a large-scale Production, especially when using a paraffin carbonate that is solid at ambient temperature “Toffs. is not in line. Further, if the hydrocarbon used is liquid, its requires complete removal of the dispersion time-consuming decanting, which in practice results in the loss of a does not result in an insignificant amount of polytetrafluoroethylene. Finally, when the removal of the If the hydrocarbon is incomplete, residual trace amounts of the hydrocarbon can become contaminants in the end product.

In GB-PS 11 78 170 is the polymerization of

ίο Tetrafluoroethylene in aqueous emulsion with the help of Initiators, emulsifiers in the presence of oligomers of tetrafluoroethylene and large amounts of saturated Hydrocarbons are described as stabilizers. The branched unsaturated fluorinated compounds are difficult to manufacture and make the process considerably more expensive. Because of the high proportion of additional Stabilizers, it is also necessary to add the dispersion resulting from the polymerization coagulate and separate the polymer before it can be fed to further use.

Another improvement known from Belgian patent 6,78,637. then exists that as Stabilizer is a halogenated paraffinic hydrocarbon with 1 to 3 carbon atoms, in particular 1,1.2-trifluoro-1.2.2-trichloroethane. is used. However, the use of this method is subject to restrictions imposed by the Necessity at polymerization temperatures between 5 and 30 "C to work, and by the need to have large amounts of the chlorinated and fluorinated hydrocarbon, namely 50 to 500 g per liter of the aqueous dispersion to be used, so that a recovery of the stabilizer by decanting the dispersion is necessary.

The use of C; FjCI) as a stabilizer for the

Emulsion polymerization is also described in US Pat. No. 3,345,317. The stabilizer must first be subjected to an essential cleaning process in order to remove all traces of unsaturated olefinic impurities. Furthermore, considerable amounts of stabilizer are required in order to achieve a sufficient effect, so that recovery and renewed work-up are necessary. to limit costs. Because of the volatility of the stabilizer, only low polymerization temperatures are possible and since the large amount present also slows down the polymerization, periods of up to 50 hours are required in order to obtain about 30 weight percent dispersions.
It is the object of the present invention. To create a process for the production of aqueous dispersions of polytetrafluoroethylene by emulsion polymerization which avoids the disadvantages of the known procedures, operates more economically and delivers a purer product. that can be used immediately

This object is achieved by the method according to the patent claim

It has now been found that concentrated dispersions made of polytetrafluoroethylene of very high purity and excellent stability can, if used as a stabilizer, extremely small amounts of the non-copolymerizable with tetrafluoroethylene links

C ^ -CH = CHi ₁ CF ₁ , -CH = CH ₂ or

C _S F,; - CH = CH ₂

or mixtures of these products can be used.

These hydrocarbons are known to be effective in the polymerization uls chain transfer agents, which Lower the molecular weight of the polymers formed. Surprisingly, it has been found that in the Emulsion polymerization of tetrafluoroethylene by introducing an extremely small amount of it The molecular weight of polytetrafluoroethylene and the stability of the derivatives practically do not decrease obtained dispersions is increased quite considerably.

Furthermore, the small amounts of stabilizer do not reduce the polymerization, so that shorter ones Reaction times than with the known methods are possible. According to the invention in small amounts Stabilizers to be used do not limit the upper temperature range of the reaction strong, like the substances and quantities used in the known processes.

The amounts of the stabilizers to be used depend on the transfer agent used and on the polymerization temperature. however, 10 to 10% are advantageous, preferably 20 to 500 parts per Use a million parts of the aqueous medium. These amounts are so small that it's not even necessary is. to recover the stabilizer after polymerization.

The loss of this product is immaterial and lies well below the loss that occurs when the known processes much larger amounts of paraffinic or chlorinated and fluorinated hydrocarbons be used.

Apart from the stabilization conditions, the emulsion polymerization according to the invention is in an carried out in a known manner

The polymerization is triggered by free radicals, which decompose a water-soluble Peroxides, e.g. B. succinyl peroxide. or an alkali or ammonium sulfate. The Persulfai becomes at a Polymerisaiionstcmperatur about 50 ("alone or at polymerization temperatures below 50 C in combination with a reducing agent. z. B. a ferrous salt. .Silver nitrate or sodium bisulfite used. The pH can be adjusted with a buffer. z. B. an alkali or ammonium phosphate, pyrophosphate or borate. be kept in the alkaline range.

Sodium or in particular are used as emulsifiers Ammonium salts of perfluoromonocarboxylic acids with b to 11 carbon atoms. / B ammonium perfluoroclanoate. used. The emulsifiers are in aqueous solution either all at once at the beginning of the polymerization or added gradually during the course of the polymerization, as in the French patent 15 50 055 of the applicant is described.

The polymerization temperature is generally between 5 and 100 ° C. and the pressure between 1 and 100 bar, preferably between 10 and 25 bar.
The mechanical stability of the dispersions according to the invention can be determined by placing the dispersions previously diluted to 10% polytetrafluoroethylene at room temperature with a four-blade stirrer in a cylindrical container provided with four counter-blades (corresponding to that of JF Lontz in Industrial and Engineering Chemistry, 1952, Volume 44, page 1805, described apparatus) are stirred. The total duration of stirring at certain speeds, which is necessary to achieve coagulation, allows the stability of the dispersion to be assessed on the following arbitrary scale:

Stirring time stability 0 to 5 minutes at 250 rpm small amount 5 to 1N minutes at 250 rpm mediocre 10 to 30 minutes at 250 rpm high 30 minutes at 250 rpm + 0 to
30 minutes at 350 rpm very high No coagulation after 30 minutes
at 250 rpm + 30 minutes
at 350 rpm + 30 minutes
at 450 rpm out of the ordinary
high

The following example illustrates the process for preparing the aqueous dispersions of polytetrafluoroethylene according to the invention.

example

A polymerization experiment is carried out in a 3-liter autoclave carried out, the l.b i water. 0.J2 g ammonium persulphate and 6.4 g of ammonium perfluorooctanoate. which is added during the entire polymerization, contains. 0.5 ml of the stabilizer mentioned in the table below is used. The polymerization is carried out in each case at 65''C under a pressure of 20 to 22 bar until a concentration of polytetrafluoroethylene between 25 and 30% is achieved. The values in the table below illustrate the particularly favorable results obtained with a stabilizer used according to the invention will hold.

stabilizer

concentration
the dispersion

Polymer
in powder form

stability

the dispersion

CV ₁ -CH = CH,

28.2
19.5

20 g
300 g

high
small amount

Claims (1)

Palent claim: Process for the preparation of aqueous dispersions of polytetrafluoroethylene by emulsion polymerization of tetrafluoroethylene in one aqueous medium containing a water-soluble peroxide or an alkali or ammonium persulfate as Initiator, a salt of a fluorocarboxylic or chlorofluorocarboxylic acid as an emulsifier, a m't tetrafluoroethylene unsaturated fluorocarbon which is not copolymerizable under the polymerization conditions used and optionally reducing agents and / or containing buffer substances, d a through marked that one C ₄ F ^ -CH = CH ,, CF ₁ , -CH = CH, or
CbF ₁₇ -CH = CH ₂ or a mixture of these fluorocarbons in an amount of 10 to 1,000 parts by weight per Million parts by weight of the aqueous medium used and in the absence of saturated Polymerized hydrocarbons as stabilizers.