|Title of Invention||
METHOD OF MAKING AN AQUEOUS DISPERSION OF FLUOROPOLYMERS
|Abstract||The invention provides a process for removing steam-volatile fluorinated emulsifiers in their free acid form, from aqueous fluoropolymer dispersions. The process comprises adding a nonionic emulsifier to the aqueous fluoropolymer dispersion and, at a pH-value of the aqueous fluoropolymer dispersion below 5, removing steam-volatile fluorinated emulsifier by distillation until the concentration of steam-volatile fluorinated emulsifier in the dispersion reaches the desired value.|
|Full Text||METHOD OF MAKING AN AQUEOUS DISPERSION OF FLUOROPOLYMERS
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 Polytetrafluoroethylene (PTFE) dispersions are widely used in the coating industry, since the coatings have a unique combinafion 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 alkylaryipolyethoxy 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. upconcentration.
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 havmg 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 US-A-3 316 201, decanting (US-A-3 037 953) and ultrafiltration (US-A-4 369 266 and US-A-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", SO3', O-SO3") to stabilize the latex particles. Acidified aqueous dispersions can directly be distilled to obtain elastomer dispersions with significantly reduced APFO-levels (e.g., 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 lOO^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 (US-A-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 emuisifier is required which does not disrupt the polymerization by chain transfer. These emuisifiers are termed nontelogenic emuisifiers (US-A-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 emuisifiers, as long as they are steam-volatile, i.e., capable of being distilled. The content of this emuisifier is generally within the range from 0.02 to 1% by weight, based on the polymer.
Occasionally, other fluorinated emuisifiers are used. For example, EP-A-822 175 describes the use of salts of CHi-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.
US'-A-2 559 752 describes other fluorinated emuisifiers, 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 emuisifier 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 (US-A-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 emuisifier.
For example, in the ultrafiltration of US-A-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 emufsifier 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 by distillation is unsuitable, because thermal upconcentration proceeds at pH values > 7. In fact, the dispersions would be colloid chemically unstable at pH values During subsequent use of PFOA containing 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 are Icnown
to contribute to the global-warming effect ("greenhouse effect").
The process of the present invention can produce coagulate-free high-solids dispersions which are substantially free from PFOA. This can be achieved by removing the PFOA from fluoropolymer dispersions in the presence of nonionic emulsifiers, for example when thermally upconcentrating the dispersion at a pH Fluoropolymer dispersions that can be used in the process of the invention include dispersions of homopolymers or copolymers made from one or more fluorinated monomers, such as TFE, VDF or CTFE, or from other fluorinated olefins having from 2 to 8 carbon atoms, from perfluorinated olefins having from 2 to 8 carbon atoms, such as HFP, from fluorinated ethers, in particular from perfluorinated vinyl allcyl ethers having allcyl moieties of from 1 to 6 carbon atoms, for example PPVE or perfluoro (methyl vinyl) ether. Other conomomers which may be used are nonfluorinated olefins, such as ethylene or propylene. The invention is intended to include dispersions of this type irrespective of whether the fluoropolymer is processable from the melt or not.
In raising the concentration of aqueous fluoropolymer dispersions, any slight tendency to coagulate should be avoided, since coagulum unacceptably impairs the industrial coating of final products, such as glass fabric or kitchen equipment.
According to one embodiment of the invention it has been found that adding from 0.5 to 10% by weight of nonionic emulsifiers, based on the actual solids content, at pH ~ 1 allows the concentration to be raised industrially by thermal processes without formation of coagulate. It is preferable to add the nonionic emulsifiers to the dispersion before lowering the pH. The low pH can be produced by adding customary strong mineral acids, such as HCl, H2S04, HCIO4, or HNO3. HNO3 is preferred, since the NH4N03 that may be formed in a subsequent neutralization of the dispersion with NH3 is noncorrosive, has adequate volatility and does not interfere with the sintering of the fluoropolymer. A cation-exchange process, as described in US-A 5 463 021 for example, is also suitable for setting the desired pH and for preparing particularly pure dispersions,
A great advantage of the novel process is that it is very easy i.e. requires no significant capital expenditure, to integrate removal of the PFOA into thermal processes for raising the concentration.
A particular advantage of the process is that the surprisingly high volatility of the PFOA to be removed during the thermal process of raising concentration means that it is removed "before the water" during distillation of the dispersion whose concentration is to be raised. It appears that under these conditions PFOA forms an "azeotrope" with water which boils at about 99° C at atmospheric pressure. This may be a PFOA hydrate which has low water-miscibility. This means that a dispersion substantially free from PFOA can be prepared by distilling off only from about 5 to 10% of the amount of water to be removed. A very high concentration of PFOA is present in this two-phase mixture which passes over first. This two-phase mixture forms with identical concentration ratios in each of the two phases, independent of the distillation pressure. At relatively low cost, therefore, the PFOA can be recycled and reused in the emulsion polymerization.
Processes for converting recovered PFOA into "polymerization grade" PFOA are described in US-A-5 312 935, US-A-5 442 097 and US-A-5 591 877.
Nonionic emulsifiers are described in detail in '"Nonionic Surfactants", M. J. Schick (ed.), Marcel Delkker, Inc., New York 1967.
The selection of the nonionic emulsifier is not critical. Examples of useful nonionic emulsifiers include alkylarylpolyethoxy alcohols, alkylpolyethoxy alcohols or any other nonionic emulsifier. This is a great advantage, since the formulation of the dispersions used remains substantially unaltered during the removal of PFOA from marketable dispersions.
No differences were found between various nonionic surfactants, such as those of , alkylarylpolyethoxy alcohol type or of alkylpolyethoxy alcohol type, with regard to the effectiveness of PFOA removal by distillation.
The removal of PFOA is preferably carried out using crude or raw dispersions directly from the polymerization. Nonionic emuJsifier is added to the crude dispersions, which generally have a solids content of from 15 to 30% by weight, the amount added being sufficient to give the dispersion stability during the subsequent upconcentration. An amount of from 0,5 to 15% by weight, preferably from 1 to 5% by weight of nonionic emulsifier is generally sufficient for this purpose. The percentages given are based on the solids content of the dispersion. If required, an appropriate addition of acid is then made to acidify the dispersion, which is preferably set to a pH of about 2. The setting of the pH must be below S, and is preferably in the range from 1 to 3. The higher the pH, the greater the amount of water which has to be distilled off to remove the PFOA completely. Relatively high pH values may therefore in particular be used during upconcentration of the dispersion.
The removal of PFOA by distillation according to the invention may also readily be carried out using dispersions whose concentration has already been raised to a desired level, for example by ultrafiltration.
The removal of PFOA from the fluoropolymer dispersion is generally followed by a readjustement of the pH. Typically, the pH is increased after the treatment, by way of ammonia or other bases, such as NaOH, to give a pH > 7, preferably 9. This may considerably increase the salt content of the dispersion and the result can adversely affect processing properties and the quality of coatings produced with the dispersion. It is therefore advantageous to use nitric acid for lowering the pH of the dispersion for the removal of PFOA because, as mentioned, the resultant salts from a subsquent readjustment of the pH of the dispersion, do not interfere.
The process of the invention may moreover be used for any crude fluoropolymer dispersion, e.g., dispersions directly from the polymerization process reactor.
The fluoropolymer dispersions produced with the process of the invention can be used in any coating application in which fluoropolymers have been used. In particular, the fluoropolymer dispersions produced with the process of this invention can be used to coat substrates,e.g, the dispersions can be used to coat metals including cookware and bakeware, fabrics, in particular glass fabrics and to coat chemical apparatus.
The examples below describe the invention in more detail without the intention to limit the invention thereto.
All percentages given are based on weight unless otherwise indicated.
Determination of PFOA
PFOA content of an anion-exchange dispersion can be determined quantitatively by the method of "Encyclopedia of Industrial Chemistry Analysis", Vol. 11, pp. 336 - 343, Interscience Publishers, New York, NY, 1971 and EP-A-194 690. In another method employed, the PFOA is converted into the methyl ester and the ester content analyzed by gas chromatography using an internal standard. In the latter method the detection limit for PFOA is 5 ppm. This method was employed in the examples below.
The nonionic surfactants used here were as follows;
NIS 1: octylphenoxypolyethoxyethanol (commercially available product TRITON X 100).
NIS 2: ethoxylate of a long-chain alcohol (commercially available product GENAPOL X 080).
Removal of PFOA:
Removal by distillation was carried out in a laboratory standard circulatory or loop evaporator. This apparatus permits the preparation of about 15 kg of dispersion with a solids content of from 50 to 60% in 6 hours. The evaporator was operated at atmospheric pressure, i.e. not in vacuo. Various fluoropolymer dispersions were tested at various pHs produced by appropriate addition of acid. The amount of PFOA in the dispersion was measured after the concentration of the dispersion had been raised. Finally, the dispersion with its concentration
thus raised was utilized with NH3 and studied for the presence of coagulate. None of the experiments shown in Table 1 revealed coagulate in amounts which adversely affect quality.
Fluoropolymer dispersions studied in Table 1:
1. PTFE dispersion with a solids content of 20% and a PFOA content of about 1500
ppm. (Examples 1-4)
2. FEP dispersion is an emulsion-polymerized fluoropolymer dispersion with a solids
content of 25% and a PFOA content of about 2200 ppm. The fluoropolymer is a
copolymer made from TFE and HFP at a level of 12% HFP. (Example 5)
3. PFA dispersion is an emulsion-polymerized fluoropolymer dispersion with a solids
content of 22% and a PFOA content of 1800 ppm. The fluoropolymer is a copolymer
made from TFE and PPVE at a level of 3.9%o PPVE. (Example 6)
4. THV dispersion is an emulsion-polymerized fluoropolymer dispersion with a solids
content of 29%o and a PFOA content of 1900 ppm. THV is a terpolymer made from
TFE, HFP and VDF in a weight ratio of 20/25/55. (Example 7)
Examples 8 and 9
A commercially available PTFE dispersion which is marketed and used for meta! coating, with a solids content of 59%, NIS 1 content of 9% and PFOA content of 1500 ppm, was subjected to a first stage of distillation in a simple laboratory distillation apparatus, that is to say that about 50 ml of water were "distilled off per kg of dispersion. The pH was set at ~2 by adding H2SO4 (Example 8) and HNO3 (Example 9). The distillation was carried out at atmospheric pressure at 100°C (Example 8) and at WC and a correspondingly reduced pressure (Example 9). The measured PFOA content of the dispersions treated in this way was in all cases Example 10 and Comparative Examples A and B
The experiments in the simple laboratory distillation apparatus of Examples 8 and 9 were carried out using the PTFE dispersion and varying the pH of the dispersion. The results are given in Table 2.
Under these conditions a pH
An APS-initiated polymer dispersion (the dispersion originally has 3600 ppm PFOA) of TFE (57%), Propylene (14%) and VF2 (29%) having a solid content of 25%, Mooney viscosity ML (1-10 @ 121° C)=45 was run through a cation exchanger; the pH of the dispersion was pH=1.5. This dispersion was distilled in a standard distillation apparatus under normal pressure until a solid content of 50%) was obtained. The dispersion showed no coagulum and the .APFO-level was
1. A process for removing steam-volatile fluorinated emulsifiers in their free acid
form, from aqueous fluoropolymer dispersions, said process comprising adding a nonionic emulsifier to said aqueous fluoropolymer dispersion and, at a pH-value of said aqueous fluoropolymer dispersion below 5, removing steam-volatile fluorinated emulsifier by distillation until the concentration of steam-volatile fluorinated emulsifier in the dispersion reaches the desired value.
2. The process as claimed in claim 1, wherein said pH-value of said aqueous fluoropolymer dispersion is from 1 to 3.
3. The process as claimed in claim 1 or 2, wherein a strong mineral acid is used to set said pH.
4. The process as claimed in claim 3, wherein the acid is nitric acid.
5. The process as claimed in claim 1 or 2 wherein said pH is set through cation-exchange.
6. The process as claimed in any of the preceding claims, wherein said aqueous fluoropolymer dispersion is a crude dispersion obtained from the polymerization process producing the fluoropolymer.
7. The process as claimed in any of the preceding claims, wherein water is removed from the dispersion by distillation, thus raising the concentration of the dispersion.
8. The process as claimed in any of the preceding claims wherein said process
comprises increasing the pH of said aqueous fluoropolymer dispersion to more than 7
subsequent to said removal of steam-volatile fluorinated emulsifier.
9. The process as claimed in claim 1 wherein the fluoropolymer is a fluoroelastomer.
10. The process as claimed in claim 9 wherein the fluoroelastomer has a molecular weight less than 150,000.
11. The process as claimed in claim 10 wherein the nonionic emulsifier is omitted from the aqueous fluoropolymer dispersion.
12. The process as claimed in claim 10 wherein the molecular weight of the fluoroelastomer is less than 100,000.
13. An aqueous dispersion made according to the process as claimed in claim 9 wherein the dispersion contains less than 50 ppm fluorinated emulsifier.
14. A curable composition comprising a fluoroelastomer made as claimed in claim 9.
15. An article comprising a cured fluoroelastomer as claimed in claim 14.
16. A process as claimed in claim 1 wherein the aqueous fluoropolymer dispersion has been upconcentrated prior to removal of fluorinated emulsifiers.
17. A method of treating a substrate to provide a feature thereto selected from the groups consisting of antistickiness, weatherability, and noninflammability comprising contacting the substrate with a fluoropolymer prepared according to claim 1.
18. A method as claimed in claim 17 wherein the substrate is metal.
19. A method as claimed in claim 17 wherein the substrate is fabric.
|Indian Patent Application Number||IN/PCT/2002/1682/CHE|
|PG Journal Number||38/2007|
|Date of Filing||11-Oct-2002|
|Name of Patentee||3M INNOVATIVE PROPERTIES COMPANY|
|Applicant Address||3M Center, Post Office Box 33427, Saint Paul, MN 55133-3427 (US)|
|PCT International Classification Number||C08J3/03|
|PCT International Application Number||PCT/US01/11628|
|PCT International Filing date||2001-04-10|