Aqueous dispersions of fluoropolymers

Details Aqueous dispersions of fluoropolymers Aqueous dispersions of fluoropolymers

2001-08-03

2004-12-21

Wu, David W. (Department: 1713)

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

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S544000, C524S546000, C523S310000, C523S332000, C525S482000

Reexamination Certificate

active

06833403

ABSTRACT:

The invention is concerned with aqueous dispersions of fluoropolymers being essentially free of fluorine-containing emulsifiers, a process for preparing such dispersions and their use. “Essentially free” means a content of less than 100 ppm, preferably less than 50 ppm, especially less than 25 ppm and in particular less than 5 ppm.
Polyfluoroethylene dispersions find wide application in the coating industry due to the unique performance of the coatings in respect of e.g. release properties, good weathering resistance, and flame retardancy. They are mainly used for coating kitchenware, chemical apparatus and glass fabrics. In many such applications, the dispersions are applied at relatively high solids contents, e.g., up to 70% by weight. These concentrated dispersions are mainly stabilized by nonionic emulsifiers such as alkylarylpolyethoxy alcohols and alkylpolyethoxy alcohols, using colloid-chemistry methods.
There are in principle two different polymerization processes for preparing fluoropolymers, namely suspension polymerization leading to polymer granules and, on the other hand the process known as emulsion polymerization, leading to an aqueous colloidal dispersion. This invention concerns emulsion polymerization, the resultant dispersions and their use.
The manufacturing of such dispersions involves in principle the two processing steps polymerization and concentration.
Polymers which are obtainable by aqueous emulsion polymerization are firstly homopolymers not processible from the melt, for example PTFE, secondly “modified” polymers, for example a polymer with more than about 99 mol % of tetrafluoroethylene (TFE) and an amount of comonomer(s) which is so low that the product retains its “not processible from the melt” character and thirdly low-molecular weight “micropowder” dispersions which are processible from the melt, and fourthly copolymers, for example fluorinated thermoplastics or fluoroelastomers. The fluorinated thermoplastics include copolymers which are composed mainly of TFE and the amount needed of one or more comonomers to make the product processible from the melt, for example from 1 to 50 mol %, preferably from 1 to 10 mol %. Customary fluoromonomers, besides TFE, are vinylidene fluoride (VDF) other fluorinated olefins, such as chlorotrifluoroethylene (CTFE), in particular perfluorinated olefins having from 2 to 8 carbon atoms, such as hexafluoropropene (HFP), fluorinated ethers, in particular perfluorinated vinyl alkyl ethers whose alkyl moieties have from 1 to 6 carbon atoms, for example perfluoro (n-propyl vinyl) ether (PPVE). Other comonomers which may be used arc nonfluorinated olefins, such as ethylene or propylene. The resultant dispersions of polymers which are processible from the melt or not processible from the melt generally have a solids content of from 15 to 30% by weight. To achieve the abovementioned high solids content for application as a coating, and advantageously also for storage and transport, the solids content has to be increased by raising the concentration. Examples of methods used for this are raising the concentration thermally as in U.S. Pat. No. 3,316,201, decanting (U.S. Pat No. 3,037,953) and ultrafiltration (U.S. Pat. No. 4,369,266).
The known emulsion polymerization mostly takes place within a pressure range from 5 to 30 bar and within a temperature range from 5 to 100° C. as described in EP-B-30 663, for example. The polymerization process for preparing PTFE dispersions substantially corresponds to the known process for preparing fine resin powders, known as paste product (U.S. Pat. No. 3,142,665). The polymerization process for preparing copolymers, such as dispersions of fluorinated thermoplastics, corresponds to the process for preparing these materials in the form of melt pellets.
In all of these emulsion polymerizations an emulsifier is required which does not disrupt the polymerization by chain transfer. These emulsifiers are termed nontelogenic emulsifiers (U.S. Pat. No. 2,559,752). Use is mainly made of perfluorooctanoic acid (PFOA for example n-PFOA, CAS No. 335-67-1) in the form of ammonium and/or alkali metal salts. However, the abbreviation PFOA when used in the text below is not intended to exclude other fluorinated emulsifiers. The content of this emulsifier is generally within the range from 0.02 to 1% by weight, based on the polymer.
Occasionally, other fluorinated emulsifiers are used. For example, EP-A-822 175 describes the use of salts of CH
2
-containing fluorocarboxylic acids for the emulsion polymerization of TFE. WO-A-97/08214 describes the use of 2-perfluorohexylethanesulfonic acid or salts thereof for TFE polymerization.
U.S. Pat. No. 2,559,752 describes other fluorinated emulsifiers, but these have not been widely used since their volatility is low. These chemicals can cause discoloration of the final products at high processing temperatures.
One of the greatest advantages of PFOA is its high volatility. PFOA is a very effective emulsifier and is practically indispensible due to its inertness in the polymerization reaction. However, PFOA is not biodegradable and has recently been classified as hazardous to the environment.
However, it is known that PFOA can be removed from exhaust gases (EP 731 081), and moreover advantageous processes for removing PFOA from wastewater have been described (U.S. Pat. No. 4,282,162 and the as yet unpublished German Patent Applications 198 24 614.5 and 198 24 615.3 filed on Jun. 2, 1998).
In the techniques listed above for raising concentration, the majority of the PFOA remains in the polymer dispersion, even in the case of ultrafiltration or removal by decanting using a 100-fold excess of the nonionic emulsifier.
For example, in the ultrafiltration of U.S. Pat. No. 4,369,266 about 30% of the initial PFOA content remains in the marketable dispersions. In specific cases the residual PFOA content can be reduced to below 10%, but the process is generally not cost-effective: achieving a reduction of this type requires addition of water and of a nonionic emulsifier to the dispersion whose concentration is to be raised. This gives unacceptably long process times.
During subsequent use of these dispersions, PFOA can pass into the environment, for example with the wastewater inevitably arising from cleaning the equipment, and into the atmosphere as aerosol. The latter emission is still more pronounced when coatings are produced, since PFOA and its ammonium salt are highly volatile. In addition, PFOA and its salts decompose by decarboxylation at the sintering temperatures normally employed, from 350 to 450° C., to give fluorinated hydrocarbons, which have a major global-warming effect (“greenhouse effect”).
The present invention provides high solid dispersions essentially free of PFOA. In this invention, “essentially free” means a content of less than 100 ppm, preferably less than 50 ppm, especially less than 25 ppm and in particular less than 5 ppm. These values are based on the entire dispersion, and not just the solids content. This is achieved by removal of fluorinated emulsifiers, e.g. PFOA, from fluoropolymer dispersions, such as PTFE, fluorothermoplast or fluoroelastomer dispersions, via anion exchange, namely by adding a nonionic emulsifier to the fluoropolymer dispersion and contacting this stabilized dispersion with a basic anion exchanger. This process works without jamming or clogging the ion exchange bed by coagulated latex particles. The resulting dispersion may optionally be upconcentrated.
Fluoropolymer dispersions useful in this inventions include dispersions of homopolymers and copolymers of one or more fluorinated monomers, such as TFE, VDF or CTFE or other fluorinated olefins of 2 to 8 carbon atoms, perfluorinated olefins of 2 to 8 carbon atoms, e.g. HFP, fluorinated ethers, especially perfluorinated vinyl-alkyl ethers with alkyls of 1 to 6 carbon atoms, such as perfluoro-(n-propyl-vinyl) ether and perfluoro-(methyl-vinyl) ether. Useful comonomers also include non-fluorinated olefins, such as ethylene and propylene. The invention is intended to inclu

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