Summary

The present disclosure relates to a cure system for fluoro elastomer gum. The present disclosure specifically relates to curable fluoropolymer compositions and cured fluoroelastomers prepared from such compositions.

In one aspect, the present disclosure provides a curable fluoropolymer composition comprising an amorphous perfluoropolymer; a nucleophile selected from alkoxides, thiolates, and combinations thereof; and an adjuvant selected from dipolar aprotic solvents, or crown ethers, and combinations thereof; where the amorphous perfluoropolymer comprises at least one cure site selected from a CF ₂ I cure site, a CF ₂ Br cure site, a CF ₂ Cl cure site, and combinations thereof.

In another aspect, the present disclosure provides a cured fluoroelastomer comprising crosslink units represented by the formula E-CF ₂ -A-Q-(A-CF ₂ E) _n wherein E is a fluoroelastomer backbone, A is selected from O or S, and Q is a multivalent organic linking group, n is an integer of at least one.

In yet another aspect, the present disclosure provides a cured fluoroelastomer comprising crosslink units represented by the formula E-Q-CF ₂ -CF ₂ -Q-E wherein E is a fluoroelastomer backbone and Q is a divalent organic linking group.

The terms "comprises" and variations thereof do not have a limiting meaning where these terms appear in the description and claims. The terms "a", "an", and "the" are used interchangeably with "at least one" to mean one or more of the elements being described.

As used herein, the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

The term "and/or" means one or all of the listed elements or a combination of any two or more of the listed elements.

The terms "polymer" and "polymeric material" refer to both materials prepared from one monomer such as a homopolymer or to materials prepared from two or more monomers such as a copolymer, terpolymer, or the like. Likewise, the term "polymerize" refers to the process of making a polymeric material that can be a homopolymer, copolymer, terpolymer, or the like. The terms "copolymer" and "copolymeric material" refer to a polymeric material prepared from at least two monomers. The terms "room temperature" and "ambient temperature" are used interchangeably to mean temperatures in the range of 2O ⁰ C to 25 ⁰ C.

Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numbers set forth are approximations that can vary depending upon the desired properties using the teachings disclosed herein.

The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The description that follows more particularly exemplifies illustrative embodiments. In several places throughout the application, guidance is provided through lists of examples, which can be used in various combinations. In each instance, the recited list serves only as a representative group and should not be interpreted as an exclusive list.

Detailed Description Perfluoroelastomers containing CF ₂ X cure sites can be cured using bisalkoxide salts (e.g., the dipotassium and sodium salts of bisphenol AF).

Conventional cure systems for fluoropolymers include peroxide cure. Perfluoroelastomers containing CF ₂ X cure sites (e.g., CF ₂ Br or CF ₂ I cure sites) are typically cured by peroxide cure systems. The peroxide cured network, using conventional coagents, such as triallyl isocyanurate (TAIC) typically show excellent chemical resistance but also suffer from poor high temperature resistance. The presently disclosed cure system provides CF ₂ X type cure sites that can react with bisphenol AF. The resultant crosslinks are expected to exhibit both enhanced high temperature resistance and excellent chemical resistance. Bisphenol AF has been used as a crosslinker for perfluoroelastomers using specialty pentafluorophenyl cure sites, which allow for crosslinking through aromatic nucleophilic substitution as disclosed in U.S. Pat. No. 3,682,872, the entirety of which is hereby incorporated by reference. However, the resulting crosslink is not resistant to nucleophilic attack (e.g., high temperature water and steam). Bisphenol AF also has been used as a curative for hydrogen containing perfluoroelastomers, again based on cure chemistry that relies on dehydrofluorination followed by nucleophilic attack of the resulting fluorinated olefin cure site again resulting in a cross link that is susceptible to nucleophilic attack. The use of bisphenol AF as a curative for hydrogen containing perfluoroelastomer is disclosed in U.S. Pat. No. 5,478,902, which is hereby incorporated by reference in its entirety.

The presently disclosed cure system provides reaction of a CF ₂ X (CF ₂ Br, CF ₂ I, CF ₂ Cl, etc.) with a sodium salt of bisphenol AF via an SRN-I type mechanism. This type of reaction was published in the Journal of Fluorine Chemistry (2008), 129 , 991-993, which is hereby incorporated by reference in its entirety, specifically with regard to the reaction of CF ₂ Cl with a sodium salt of bisphenol A via an SRN-I type mechanism.

The fluoropolymer compositions described herein may further contain additives, such as carbon black, stabilizers, plasticizers, lubricants, fillers, fumed silicas (e.g., silicas available under the trade designation "Aerosil R-972" from Degussa, Parsippany, New Jersey), perfluoropolyethers, and processing aids, any of which are familiar to fluoropolymer compounding, provided that the fillers have adequate stability for the intended service conditions. Carbon black fillers generally may be used to affect properties such as modulus, tensile strength, elongation, hardness, abrasion resistance, conductivity, and processability of the compositions. Examples of carbon black include MT blacks (medium thermal black) designated N-991, N-990, N-908, and N-907; FEF N-550; and large particle size furnace blacks. When large size particle black is used, from 1 to 70 parts filler per hundred parts fluoropolymer is generally sufficient.

The fluoropolymer compositions described herein are curable, and can be prepared by mixing one or more fluoropolymers, a nucleophile (such as alkoxides, thiolates, and the like), and an adjuvant (such as dipolar aprotic solvents, crown ethers, and the like). Useful nucleophiles in the presently disclosed curable fluoropolymer composition include mono functional and multifunctional nucleophiles, such as multifunctional phenoxides, multifunctional thiophenoxides, and the like. Particularly useful nucleophiles in the presently disclosed curable fluoropolymer composition include sodium salt of bisphenol AF, sodium salt of bisphenol, A, bisphenol AF, bisphenol A, sulfonyl diphenol, and the like.

Dipolar aprotic solvents useful as adjuvants in the presently disclosed curable fluoropolymer compositions include HMPA, HMPT, dimethylsulfone, dimethylsulfoxide and tetramethylenesulfone, and the like.

Crown ethers, useful as adjuvants in the presently disclosed composition, include 18-crown-6, 15-crown-5, 12-crown-4, and the like. Crown ethers are heterocyclic chemical compounds having a ring containing several ether groups. Common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, for example, CH ₂ CH ₂ O. Important members of this series are the tetramer, the pentamer, and the hexamer. The term "crown" refers to the resemblance between the structure of a crown ether bound to a cation, and a crown sitting on a head. The first number in a crown ether's name refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are oxygen. Crown ethers are much broader than the group containing oligomers of ethylene oxide. For example, another important group is derived from catechol.

Crown ethers strongly bind certain cations, forming complexes. Oxygen atoms in the crown ethers are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in non-polar solvents. While not being bound by theory, this may be the reason crown ethers are useful in phase transfer catalysis. The denticity of the poly ether influences the affinity of the crown ether for various cations. For example, 18-crown-6 ether has high affinity for potassium cation, 15-crown-5 ether for sodium cation, and 12- crown-4 ether for lithium cation. Any other desirable additives may also be added. The mixing may take place in conventional rubber processing equipment. The desired amounts of compounding ingredients can be added to the nonvulcanized fluoropolymer and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as internal mixers, roll mills, or any other convenient mixing device. The temperature of the mixture during the mixing process typically should not rise above about 120. degrees C. During mixing, it is preferable to distribute the components uniformly throughout the composition. The fluoropolymer compositions of the present description include compositions comprising a mixture of fluoropolymer gums capable of being cured to give a fluoroelastomer, and fluoroplastic polymers having cure sites selected from halogen- containing cure sites, such as CF ₂ I cure site, a CF ₂ Br cure site, a CF ₂ Cl, and the like. In such embodiments, the fluoroplastic may comprise a these types of cure site and units derived from one or more fluorinated monomers. These compositions may be further used to provide cured, shaped articles and latex blends. The halogen-containing cure sites may be provided by a halogen-containing cure site monomer such as, for instance, those described above. In particular embodiments, the one or more fluorinated monomers are selected from the group consisting of perfluoroolefms (such as tetrafluoroethylene, hexafluoropropylene, and the like), perfluorovinyl ethers (such as perfluoroalkylvinyl ethers, perfluoroalkoxyvinyl ethers, and the like), and combinations thereof.

The mixture may then be processed and shaped, such as by extrusion (e.g., into the shape of a film, tube, or hose) or by molding (e.g., in the form of a sheet or an o-ring). The shaped composition can then be heated to cure the fluoropolymer composition to form a cured article. The so-cured shaped composition may further be post-cured at a temperature above the curing temperature.

Molding or press curing of the compounded mixture is usually conducted at a temperature sufficient to cure the mixture in a desired time duration under suitable pressure. For instance, in the first curing step, the composition may be cured at a temperature that facilitates the formation of more nucleophile crosslinks than halogen cure site cross-links. A second heating may take place that initiates additional halogen cure site curing and nucleophile curing. The second heating may be, for instance, post-curing (e.g., in an oven) at a temperature and time sufficient to complete curing. The particular times, temperatures, and pressures can be selected by those skilled in the art and are not particularly limited.

In some embodiments, the perfluoroelastomers derived from curing the compositions described herein provide better heat and chemical resistance than standard peroxide-grade fluoroelastomers or even fluoroelastomers cured with bisphenol AF. EXAMPLES

The following examples are merely for illustrative purposes and are not meant to limit in any way the scope of the appended claims. All parts, percentages, ratios, and the like in the examples are by weight, unless noted otherwise.

Materials

These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims. All parts, percentages, ratios, and the like in the examples are by weight, unless noted otherwise. Solvents and other reagents used were obtained from Sigma- Aldrich Chemical Company; Milwaukee, Wisconsin unless otherwise noted.

EXAMPLES:

A copolymer of tetrafluoroethylene (67.9 mole%), MV-31 (CF ₂ =CFOCF ₂ CF ₂ CF ₂ OCF ₃ ; 21.3 mole%), perfluoromethyl viny lether (10.0 mole%) and

MV2Br (CF ₂ =CFOCF ₂ CF ₂ Br; 0.8 mole%) was investigated for curability. 100 g of a the aforementioned copolymer was combined with a freshly prepared solution of a disodium salt of bisphenol AF in methanol (4 millimoles). Investigation for cure using a moving die rheometer (MDR) showed no sign of torque increase even after 30 minutes at 188 ⁰ C. This compound was combined with 20 parts of MT N990 carbon black and 1.5 parts of 18- crown-6. This resulted in the compound showing clear signs of curing at 188°C. The MDR results were as follows: ML: 0.06 in.lbs; MH: 0.26 in.lbs; t50: 1.61 minutes; t90: 9.01 minutes.

A solution of 4 mmoles of dipotassium salt of bisphenol AF was prepared by reacting 8 mmoles of potassium methoxide and 4 mmoles of bisphenol AF in methanol followed by addition of 1.5 grams of 18 crown-6 and added this to the same polymer (100 g) and 15 parts of MT N990 carbon black. MDR rheology showed also a cure reaction even at 177 ⁰ C. The MDR results were as follows: ML: 0.29 in.lbs; MH: 0.59 in.lbs; t50: 0.91 minutes; t90: 7.44 minutes. Copolymers made with a higher cure site content (e.g., 2 mole%) are expected to show a higher state of cure. Cured articles derived from these copolymers are expected to provide improved steam resistance, high temperature resistance, and ethylene diamine resistance over cured articles prepared from known curing systems.

The complete disclosures of the patents, patent documents, and publications cited herein are incorporated by reference in their entirety as if each were individually incorporated. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.