CA1052488A - Fluoroelastomer latex - Google Patents

Abstract

Abstract of the Disclosure A fluoroelastomer latex which remains useful after being stored for a long period of time and which can be produced by a process employing a relatively brief residence time in the reactor can be prepared by (A) forming an aqueous emulsion of monomers capable of being copolymerized into a fluoroelastomer, said emulsion comprising (1) vinyl-idene fluoride plus at least one other fluorine-containing monomer, (2) a specified amount of lithium ion, and (3) an inorganic free-radical initiator; (B) subjecting said emulsion to copolymerization to form a fluoroelastomer latex; and (C) adding a creaming agent to the latex; and optionally carrying out creaming of the latex to form a layer of creamed (high solids) latex, followed by isolating the creamed latex.

Description

~2~8 Background of the Invention .. . ~ _ .
This invention relates to a fluoroelastomer latex and a process for pr~paring such a latex.
The industries which manufacture and use fluoro-elastomer latex compositions are in need of a fluoroelastomer latex which is capable of remaining useful after being stored for a long period of time, and capable of being formed during a relatively brief residence time (e.g., ahout 5-15 minutesl in an emulsion polymerization reactor. What is especially desired in certain applications is a fluoro-elastomer latex which is still useful after at least twelve months (preferably at least eighteen months) of storage at about room temperature. -`
of the Invention Th~ pre~ent invention provides a process for pre-paring a 1uoroelastomer latex which comprises (A) forming an aqueous emulsion of monomers capable of being copolymerized into a fluoro-elastomer, said emulsion comprising (1~ vinylidene fluoride plus at least one other fluorine-containing monomer in an amount such that the total monomer con-tent of the emulsion is about 10-35% by weight, 52) about 30 to 100 parts by weight of lithium ion per million parts of the emulsion, and (3) an inorganic free-radical ini-tiator in an amount which permits the free-radical emulsion copolymerization of

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the monomers, `
(B) subjecting said emulsion to free-radical emulsion copolymerization in a reaction zone at ~:-a temperature of about 50-135C. and a pressure of about 7-140 kg./cm.2 to form a fluoroelastomer latex, and ~ .
(C) adding a creaming agent to the result- -ing fluoroelastomer latex in an amount equal to about 0.07-3.0 grams of creaming agent per 100 grams of fluoroelastomer present. `
The process optionally includes the staps of creaming (causing a layer of creamed or high-solid~ latex to form), and isolating the creamed latex. ~.
This invention also provides a novel fluoro~
elastomer latex which i~ an aqueous dispersion of an elastomeric copolymer of vinylidene fluoride and at least one other fluorine~containing monomer, said dispersion having la) a content of said copolymer of about 10-75% by weight;
(b) a lithium ion content of a~out 30-400 parts per million, (c) a pH of about 3-7, and (d) a creaming agent content of about 0.07-3~0 grams per 100 grams of fluoroelastomer present. ~ :
De~cription o~ Preferred Embodiments The fluoroelastomer component of the present latex -~
is an ela~tomeric copolymer of vinylidene fluoride and at least one other fluorine-containing monomer. The fluoro- :
elastomer preferably has a vinylidene fluoride content of .~ .

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, ' ' '' . '' ~5Z~IS 8 about 25-70% by weight. The "other fluorine-containing monomer" is usually an ethylenically unsaturated monomer containing at least one fluorine atom substituent on each double-bonded carbon atom. The copolymer is preferably com-posed of at least one of the following: copolymers of vinylidene fluoride and hexafluoropropylene or pentafluoro-propylene; copolymers of vinylidene fluoride, tetrafluoro-ethylene and hexafluoropropylene or pentafluoropropylene;
and copolymers of vinylidene fluoride, a perfluoroalkyl per-fluorovinyl ether and tetrafluoroethylene. Especially pre-ferred is a copolymer which contains, as interpolymerized units, about 25-70% by weight of vinylidene fluoride, 19-63 by ~eight of hexafluoropropylene and 3-35% by weight of tetrafluoroethylene. Also very useful is a copolymer which contains, as interpolymerized units, about 30-70% by weight of vinylidene fluoride and about 70-30~ by weight of hexa-fluoropropylene.
Copolymers of vinylid~ne fluoride and hexafluoro-propylene are described in U.S. Patent 3,051,677 issued to Rexford. Copolymers of vinylidene fluoride, tetrafluoro-ethylene and hexafluoropropylene are described in U.S. Patent 2,968,649 issued to Pailthorp and Schroeder. Copolymers of vinylidene fluoride and pentafluoropropylene are descrihed in U.S. Patent 3,331,823 issued to Sianesi et al.; and co-polymers of these two components with tetrafluoroethylene are described in U.S. Patent 3,335,106 issued to Sianesi et al. Copolymers of vinylidene fluoride, a perfluoroalkyl perfluorovinyl ether (having 1-5 carbon atoms in the alkyl group) and tetrafluoroethylene are described in U.S~ Patent 3,235,537 issued to Albin and Gallagher.

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The novel latex has a copolymer (fluoroelastomer) content based on th~ weight of the latex, of about 10-75%.
A copolymer content of about 10-35% is preferred in some applications; and a copolymer content of about 40-75% is preferred in other applications. `~
The latex has a pH of about 3-7, preferably about 4.5-605. Also, the latex has a lithium ion content of about 30 to 400 parts by weight tpreferably about 40 to 225 :
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parts) per million parts by weight of the latex. In a typical composition, one will find that less of the lithium ion is needed in order to get good results when the fluoro- ^
elastomer has a relatively high molecular weight than when the fluoroelastomer has a ~omewhat lower molecular weight.
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The lithium ions are present in the latex as the result of adding lithium hydroxide or another suitable lithium compound during the preparation of the latex as de-scribed below.
The latex also has a creaming agent content of about 0.07-3 grams per 100 grams of fluoroelastomer present.
The creaming agent is discussed below.
The latex can also contain one or more additives known to be useful in fluoroelastomer latex compositions, for example, fillers, coloring agents, surfactants and curing agents. When the latex is to be used for the manu~acture of :.
crack-free fluoroelastomer films, it preferably contains a ;~
suitable anionic surfactant such as a mixture of mono and di esters of phosphoric acid with nonylphenoxypolyethylene- ;
oxyglycol having an average molecular weight of about 4,000-6,000. The weight ratio of the mono ester to di ester com-ponents of such a mixture i5 usually about 40:60 to 60:40.

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A sm~ll amount of a nonionic surfactant (e.~., said oxyglycol) can also be used along with said anionic surf~ctant~ for example, one can add about 0~1-5~ of nonlonic surfactant based on the weight of the anionic surfactantO The surfac-tant can be added at any convenient time during the prepara-tlon of the latex by the process described herein, for example, during or after any step of the process, pre~erably after step (B~. A u~eful anionic surfactant content i8 illustrated by about 3-10 parts by weight ~or each 100 part~ ~
by weight of fluoroelastomer presen~ in the latex. However, ~ .
in many applicatlons~ the l~tex i~ very useful even when no ~urfactant at all is ad~ed; this is æhown below in Example 1.
In carrying out the process of thl~ inventlon, one flrst prepares an aqueous emulsion of monomers capable of being copolymerized to ~orm a ~luoroelastomer. This i~
step (A) of the proceæs as described above. This emulsion contains the required fluorine-contalnin6 monomers in an amount such that the total ~onomer content of the emulsion is about 10-35~ by weightO The emulslon al80 contains an lnorganic free-r~dical initiator in an amount whlch permlts the free~radical emulsion copolymerization of the monomers.
In some applications3 the initiator i~ used in combination with a reducing agentO one skilled in ~he art will haYe no difflculty in selecting a suitable type and amount of initiator to fit the needs of a particular appllcation in view of the teaching ln such prior art as the U.S. Patentæ
mentioned above in reference to the known vinylidene fluor-ide copolymers which are useful as fluoroelastomers; also in vlew of the teachln~ of U.S0 Patent 3,839,305 lssued to Ao Lo Moore on October 1, 19740 I~ desired, the initlator ,, , .

105;~4B8 can be used in combination with a reducing agent and/or a chain transfer agent as taught in said prior art including said Moore patent. ~
The emulsion of step (A) also has a lithium ion `
content of about 30 to 100 parts by weight (preferably about 30 to 60 parts) per million parts by weight of the emulsion. A preferred way to provide the lithium ions is to mix enough lithium hydroXide with the emulsion so that the emulsion has a pH of about 2-7 9 preferably about 3-5. How-ever, the required lithium ion content can also be provided by adding any other suitable known lithium compound, prefer- -ably a water-soluble lithium salt; such a salt can be formed insitu. Useful lithium compounds other than lithium hydroxide include lithium acetate, lithium chloride, lithium bromide, lithium formate, lithium fluoride, lithium fluoro-sulfonate, lithium citrate, lithium sulfate and the like.
Sodium hydroxide or other suitable alkaline compounds known to be useful for adjusting emulsion p~l can be used in com-bination with the lithium compound to provide an emulsion pH

within the range mentioned earlier in this paragraph.
In step (B), the emulsion is subjected to free~
radical emulsion copolymerization in a reaction zone by maintaining the emulsion at a temperature of about 50-135C.
and under a pressure of about 7-140 kg./cm.2 to form a fluoroelastomer latex. It is not necessary to carry out the step (B) reaction in the presence of an inert gas.
One will often prefer to carry out the process so that the emulsion components of step (A) are continuously fed to the reaction zone, and the step ~B) copolymerization is carried out as a continuous process wherein the emulsion . . .
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has an average residence time in the reaction zone of about 5-3U minutes, preferably about 8-13 minutes. The average residence time can be calculated by dividing the reactor volume by the volum~ of water fed to the reactor per hour.
After step (B), the pH of the latex is preferably adjusted to about 4.5-7. This is preferably done by mixing lithium hydroxide with the latex. Another method i5 to add sodium hydroxide or other suitable alkaline compound in com-bination with lithium chloride or another lithium compound as indicated above in the discussion of the step (A) emulsion. -~
In the embodiments wherein an anionic surfactant is added as described above, the surfactant is preferably added prior to such pH adjus ment.
In step (C), a creaming agent is added to the latex in an amount equal to about 0.07-3 grams (preferably about 0.1-0.6 gram) of creaming agent per 100 grams of fluoroelastomer present. This is preferably done after the pH adjustment described in the previous paragraph. A pre-ferred creaming agent is ammonium alginate; however, one can also use sodium alginate or other known creaming agents.
The latex resul~ing from step (C) has a relatively low fluoroelastomer content (e.g., about 10-35% by weight), ~-and it is useful for forming applications without further modification. ~ i When a latex product of the present invention i5 desired which has a relatively high fluoroelastomer content (e.g., about 40~75%, preferably about 55-70%, by weight), one can prepare such a latex by carrying out the additional steps of creaminy the fluoroelastomer latex resulting from step (C) to form a creamed latex having a ~luoroelastomer - 8 ~

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SZ~38 content of about 40-75% by weight, and isola~ing the creamed latex. ~he creaming procedure described by Buf~ington in U.S. Patent 2,878,196 illustrates a useful known way to cream a latex. After on~ has added a suitable amount of creaming agent, the mixtuxe is allowed to stand until the desired degree of crea~ing has taken plaae. At the end of the creaming step, the lower liquid phase is a concentrated dispersion of polymer (creamed latex), and the upper liquid phase is a serum containing water and various contaminantsO
In step (E) one separates or isolates ~he creamed latex formed in step (D). This can be done by me~hods known to be useful for separating one liquid phase from another, for example, by decantation. The resulting high-solids latex is more useful than a low-solids latex in appli-cations whexe one wishes to form relatively thick cast films, or where it is desired to minimize the cost of trans-porting the latex.
Fluoroelastomer latex compositions having excellent shelf life and a fluoroelastomer solids content within a broad range (from very high to very low solids content) can be prepared in accordance with the present inventio~ which have benefiaial utility in the manufacture of fluoroelastomer films, coa~ings and composite sheet materials containing fibers and a fluoroelastomer binder. The latex is very useful as a binder for heat-resistant fibers te-g~, asbestos, glass or polyimide fibers) in the production of resilient, heat-resistant gaskets and insulation products~ The process of this invention, because of the use of the lithium ion content as specified in combination with the othex ingredients :. ' ' . ., ,', '. :

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~S~2~L8~3 en~bles one to prepare ~uch fluoroelastomer la~ex compositions by using a relatively brief residence time in the polymerization reactor employed to form the copolymer;
this results in an important economic advantage. Moreover, the process enables one to prepare such latex compositions, even when no surfactant is added, which are capable of re-maining useful after being stored for long periods of time.
A typical latex of this invention can be stored for twelve months or even eighteen months or more at about room temper-ature and -qtill be useful for the applications mentioned above. This advantage is important to those who make, ship, buy or use fluoroelastomer latex compositions; and it is a surpri~ing advantage. It will be apparent that one can ex-perience a serious financial loss if a sizable quantity of fluoroelastomer latex becomes useless during storage before one is able or ready to use it for the intended purpose.

This example, and those that follow, illustrate the invention; all amounts are by weight unless otherwise indicated.

A fluoroelastomer latex having a relatively high fluoroelastomer content (69.1%~ is prepared by a process composed of ~he following operations:
(1) Continuou-~ly feeding the monomers described below to a l-liter stainless-steel pressure vessel reactor (polymerization reaction zone), while operating the stirrer of the reactor at 900 rpm for thorough mixing of the reactor con~ents, and while the content of the reactor are heated at 128C. under a pressure of 63 kg./cm.2 so that the reac-tion mix~ured form in operation ~2) below will undergo an ... .
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emulsion polymerlzation reaction as it passes through the reactor, the reactor residence time being about 9 minutes based on the ratio of the l-lit~r reactor to ~he emulsion output rate (or water input rate) of about 7 liters per hour, the monomer~ and the feed rate for each being 844 yrams per ::
hour of vinylidene fluoride, 617 grams per hour of hexa-fluoropropylene and 390 grams per hour of tetrafluoroe hylene;
(2) During operation (1), constantly feeding to the reactor through a first metering pump during each hour a solution composed of 3.92 grams of ammonium persulfate dis-solved in 3409 ml. of water (distilled), and simultaneously feeding to the reactor through a second metering pump during each hour a solution composed of 7.64 grams of lithium hy- .
droxide and 4.65 grams of sodium sulfite dissolved in 3670 ml. of water, the reaction mixture being at a pH of 3.3; the reaction mixture (emulsion) being formed in the reactor has a lithium ion content of about 43 parts per million parts of the emulsion;
(3) Continuously removing from the reactor the ~-resulting copolymer latex which is continuously formed during operations (1) and (2), the la~ex being passed first through a back-pressure regulating valve set to maintain the desired :~
reactor pre~sure of 63 kg./cm.2;
(4) After discarding the latex obtained during the first 5 residence times, collecting the desired quantity . .
of la~ex and mixing it for uniformi~y, the latex having a pE~ of about 3.3, a lithium ion content of about 43 parts per ;;
million, and a copolymer solid~ content o~ 20.45%;
(5) Pouring the latex through a 50-micron filter bag;

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~6) Gradually adding to the resulting latex enough of a 10% aqueous lithium hydroxide solution to raise the latex p~ to 6.0;
(7) Mixing with the resulting latex 10 liters of a 1% aqueous solution of ammonium alginate (creaming agent) for each 100 liters of the latex; ~:
(8) Allowing the mixture to stand for 16 hours at 22C.; and ;
(9) Separating the creamed latex by decanting off the upper serum layer, and pouring the creamed latex thxough a chee~ecloth filter aft~r adjusting the latex pH with 10 lithium hydroxide solution to about 6.4.
The latex produc~ obtained in step ~9) has a fluoroelastomer content of 69.1%, a lithium ion content of ~ iabout 55 parts per million parts of latex, and a Brookfield viscosity of 133 cps. (No. 2 spindle at 30 rpm). The fluoroelastomer is a copolymer whose interpolymerized units .
are 45% vinylidene fluoride~ 30% hexafluoropropylene and 25%
tetrafluoroethylene.
All or part of the ammonium alginate used in step : -(7) can be replaced with sodium alginate in applications where it is not objectionable to obtain a somewhat darkex-appearing product.
For some applications, it will be desirable to ~.
add to the latex one or more curing agents, fillers and/or coloring agents known to be useful in fluoroelastomer latex compositions.
The Example 1 latex can be used in the manufacture of high quality fluoroelastomer reinforced asbestos sheet material having good strength, elongation and fluid resistance .

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~3S;~fl~8~3 properties by forming a 4~ aqueous slurry of asbesto3 fibers containing a small amount of a suitable qurfactant, mixing the latex (wi~h ox without dilution) with the asbes-tos slurry, reducing the pH of the mixture to 4.5 with aqueous alum solu~ion, forming the mixture into sheets in a manner known to be useful for preparing asbestos sheets, and drying the resulting sheets in a drum-dryer and pressing them to the desired thickness. In a typical application, enough of the latex is used so that the product contains 20 parts of the fluoroelastomer for each lO0 part o~ asbestos.

The Example l latex has ~he utility described in the paragraph preceding Example 1. Quite unexpectedly, the utility of the latex after it has been stored fox twenty-one months at 22C. is found to be about the same as that of the freshly-prepared latex. Any of the fluoroelastomer component which has settled out is easily and satisfactorily redis-persed by briefly stirring the latex. It is surprising that the fluoroelastomer is easily redispersed after such a long storage period.
When Example l is repeated except in tha~ ~he lithium hydxoxide is replaced with ~nough sodium hydroxide in steps (2~, (6) and (8) to provide the specified pH, and the `
resulting latex is examin~d after being stored for nine month~q at 22C., it is found that the latex is no longer useful because the fluoroelastomer at the bottom of the con-tainer cannot be redispersed with stirring to provide a useful latex.

A fluoroela~tomer latex having considerably lower fluoroelastomer content than the product of Example l (about - , .

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~524~313 18~) is pr~pared by repeating Example 1 except that steps (8) and (9) are omitted; and there is an additional StPp, (5a)~ immediately after step (5). Step (Sa) consists of adding an anionic surfactant in an amount equal to four parts of the surfactant for ea~h 100 parts of fluoroelastomer present; said surfactant is a mixture of mono and di esters of phosphoric acid with nonylphenoxypolyethyleneoxyglycol, the weight ratio of mono ester to di ester in said mixture being abou~ 50:50 and said mixture having an average molecular weight of about 5000~
The resulting fluoroelastomer latex is useful for the manufacture of crack-free fluoroelastomer films and coatings; and it can be used for the manufacture of fluoroelastomer-reinforced asbestos sheet material in about the same manner as described in Example 1.

A fluoroelastomer latex having a relatively high fluoroelastomer content (about 67%) and having utility as `
described in Example 2 is prepared by repeating Example 1 except that there is an additional step (5a) a~ described in Example 2 immediately after step (5) wherein an anionic surfactant is added to the latex. When a 1.2 mm. thick fluoroelastomer film is made by casting a layer of the Example 3 latex and drying it, the film is free of cracks.
It will be apparent that the process for preparing the latex materials of Examples 2 and 3 has beneficial utility when compared with another process whi~h can be used for the manufacture of a fluoroelastomer latex for making crack-free films wherein one must first prepare an organic `
solvent solution of the fluoroe:Lastomer followed by forming . . . . . .
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Claims (20)

WE CLAIM:

1. A process for preparing a fluoroelastomer latex which comprises (A) forming an aqueous emulsion of monomers capable of being copolymerized into a fluoro-elastomer, said emulsion comprising (1) vinylidene fluoride plus at least one other fluorine-containing monomer in an amount such that the total monomer content of the emulsion is about 10-35% by weight, (2) about 30 to 100 parts by weight of lithium ion per million parts of the emulsion, and (3) an inorganic free-radical initiator in an amount which permits the free-radical emulsion copolymerization of the monomers, (B) subjecting said emulsion to free-radical emul-sion copolymerization in a reaction zone at a temper-ature of about 50-135°C. and a pressure of about 7-140 kg./cm.2 to form a fluoroelastomer latex, and (C) adding a creaming agent to the resulting fluoroelastomer latex in an amount equal to about 0.07-3.0 grams of creaming agent per 100 grams of fluoroelastomer present.

2. A process according to claim 1 which also com-prises (D) creaming the fluoroelastomer latex resulting from step (C) to form a creamed latex having a fluoroelastomer content of about 40-75% by weight, and (E) isolating the creamed latex.

3. A process according to claim 2 wherein the step (A) emulsion has a pH of about 2-7.

4. A process according to claim 3 wherein the emulsion components of step (A) are continuously fed to the reaction zone, and the step (B) copolymerization is carried out as a continuous process wherein the emulsion has an average residence time in the reaction zone of about 5-30 minutes.

5. A process according to claim 4 wherein the step (A) emulsion has a pH of about 3-5.

6. A process according to claim 4 wherein said average residence time is about 8-13 minutes.

7. A process according to claim 5 wherein the pH
of the latex is adjusted to about 4.5-7 after step (B).

8. A process according to claim 7 wherein the pH
is adjusted by adding lithium hydroxide to the latex.

9. A process according to claim 8 wherein said creaming agent is ammonium alginate.

10. A process according to claim 9 wherein the amount of creaming agent added is about 0.1-0.6 gram per 100 grams of fluoroelastomer present.

11. A process according to claim 9 wherein the creaming agent is added after the pH is adjusted with lithium hydroxide.

12. A process according to claim 11 wherein about 3-10 grams of an anionic surfactant per 100 grams of fluoro-elastomer present are added to the latex before the pH is adjusted with lithium hydroxide.

13. A process according to claim 12 wherein said surfactant is a mixture of mono and di esters of phosphoric acid with nonylphenoxypolyethyleneoxyglycol.

14. As a novel fluoroelastomer latex, an aqueous dispersion of an elastomeric copolymer of vinylidene fluoride and at least one other fluorine-containing monomer, said dis-persion having (a) a content of said copolymer of about 10-75%
by weight, (b) a lithium ion content of about 30-400 parts per million, (c) a pH of about 3-7, and (d) a creaming agent content of about 0.07-3.0 grams per 100 grams of fluoroelastomer present.

15. A latex according to claim 14 wherein said copolymer contains, as interpolymerized units, about 25-70%
by weight of vinylidene fluoride, 19-60% by weight of hexa-fluoropropylene and 3-35% by weight of tetrafluoroethylene.

16. A latex according to claim 14 wherein said co-polymer contains, as interpolymerized units, about 30-70%
by weight of vinylidene fluoride and about 70-30% by weight of hexafluoropropylene.

17. A latex according to claim 15 wherein the lithium ion content is about 40 to 225 parts per million, and the pH
is about 4.5-6.5.

18. A latex according to claim 17 wherein said co-polymer content is about 10-35% by weight.

19. A latex according to claim 17 wherein said co-polymer content is about 40-75% by weight.

20. A latex according to claim 17 which contains about 3-10 grams of an anionic surfactant per 100 grams of fluoroelastomer present.