Method of making an aqueous dispersion of fluoropolymers

Details Method of making an aqueous dispersion of fluoropolymers Method of making an aqueous dispersion of fluoropolymers

2001-08-28

2004-09-21

Egwim, Kelechi C. (Department: 1713)

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

C524S462000, C524S544000, C524S545000, C524S546000, C526S911000

Reexamination Certificate

active

06794550

ABSTRACT:

The present invention relates to a process for preparing aqueous dispersions of fluoropolymers which are substantially free from fluorine-containing emulsifiers. For the purposes of the present invention, “substantially free” means a content of less than 100 ppm, preferably less than 50 ppm, particularly preferably less than 25 ppm, and in particular less than 5 ppm.
A process for preparing fluoropolymer dispersions of this type has been disclosed in WO 00/35971. In the process disclosed there, the fluorine-containing emulsifier is removed using anion-exchange resins. The emulsifier is practically quantitatively removed.
In the process according to the present invention, the high volatility of fluorinated emulsifiers is utilized in aqueous dispersions at pH<5, in particular at pH<4. At these pH values the emulsifier can be removed completely by steam distillation. High volatility is a feature in particular of fluorine-containing alkanecarboxylic acid emulsifiers, in particular perfluorooctanoic acid (PFOA), which is frequently used in aqueous emulsion polymerizations to produce fluoropolymers. Fluorinated emulsifiers which instead of the carboxyl group have other dissociable groups, such as SO
3
H and SO
2
H, are not sufficiently steam-volatile in the form of the free acid and cannot advantageously be removed by the novel process. To remove emulsifiers of this type the anion-exchange process is generally more suitable.
Polytetrafluoroethylene (PTFE) dispersions are widely used in the coating industry, since the coatings have a unique combination of performance characteristics, such as release properties, good weathering resistance and nonflammability. They are mainly used for coating kitchen equipment, such as cookware and bakeware, chemical apparatus and glass fabrics. In many applications of this type the dispersions are applied using relatively high solids contents, for example up to 70% by weight. These concentrated dispersions are mainly stabilized with nonionic emulsifiers used in colloid chemistry, such as alkylarylpolyethoxy alcohols and alkylpolyethoxy alcohols.
There are in principle two different polymerization processes for preparing fluoropolymers, namely suspension polymerization, which gives polymer granules, and emulsion polymerization, which gives an aqueous colloidal dispersion. The present invention relates to emulsion polymerization, and to the resultant dispersions and their use.
In principle there are two steps in the preparation of dispersions of this type, namely polymerization and raising the solids concentration, i.e. upconcentrition.
Polymers which are obtainable by aqueous emulsion polymerization include homopolymers not processable from the melt, for example PTFE; “modified” polymers, for example a polymer with more than about 99 mol % of tetrafluoroethylene (TFE) and an amount of one or more comonomers which is so low that the product retains its “not processable from the melt” character; low-molecular weight “micropowder” dispersions which are processable from the melt; and copolymers, for example fluorinated thermoplastics or fluoroelastomers. The fluorinated thermoplastics include copolymers which are composed mainly of TFE and one or more comonomers in an amount necessary to make the product processable from the melt, for example from 1 to 50 mol %, preferably from 1 to 10 mol %. Customary fluoromonomers, besides TFE, are trifluoroethylene, 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). VDF may also be polymerized as a homopolymer. Other comonomers which may be used are nonfluorinated olefins, such as ethylene or propylene. The resultant dispersions of polymers which are processable from the melt or not processable 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 (thermal upconcentration) 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 and U.S. Pat. No. 5,219,910).
In the case of fluoroelastomers or amorphous fluoropolymers, which have a molecular weight less than 150,000, preferably less than 100,000, there is no need to add a nonionic emulsifier because these polymers normally have enough polar endgroups (e.g., COO
−
, SO
3
−
, O—SO
3
−
) to stabilize the latex particles. Acidified aqueous dispersions can directly be distilled to obtain elastomer dispersions with significantly reduced APFO-levels (e.g., <50 ppm) and with solid contents of >50%. These dispersions can be used without modification, for example, for coating applications. The PFOA-reduced elastomer dispersions are also easier to coagulate, i.e., one needs less salt to coagulate in the work-up to obtain solid raw gums.
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 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, as long as they are steam-volatile, i.e., capable of being distilled. 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.
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.
It is known that PFOA can be removed from exhaust gases (EP 731 081), and removed from wastewater (U.S. Pat. No. 4,282,162, WO 99/62830 and WO 99/62858).
In the techniques listed above for raising concentration, a substantial quantity 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 dispersions as offered on the market. In specific cases the residual PFOA content can be reduced to about 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 solids concentration is to be raised. This gives unacceptably long process times.
Using previously known methods of thermal upconcentration of fluoropolymer dispersions, removal of PFOA

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