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...
Reexamination Certificate
1999-08-20
2001-09-18
Lipman, Bernard (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...
C524S545000, C524S546000, C525S326300, C525S384000
Reexamination Certificate
active
06291576
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fluoroelastomers that are capable of being crosslinked without sponging.
BACKGROUND OF THE INVENTION
Elastomers are typically compounded with fillers, crosslinking agents and other additives, followed by forming or shaping into a desired product configuration. This forming may be accomplished by calendering or extrusion, but most often the forming process is carried out by compression or injection molding. After shaping, the elastomer molecules are crosslinked through the action of crosslinking agents, thereby creating a three dimensional structure that provides strength and stability. Such crosslinking, or curing, is usually effected by heat and pressure. Fluoroelastomers (i.e. elastomeric fluoropolymers) are often press cured in a first step at elevated temperature and pressure while confined in a mold, and then post cured, after removal from the mold, at elevated temperatures and ambient pressures. The post cure step serves to complete the curing process. Certain fluoroelastomer compositions have a tendency to evolve undesirable quantities of gaseous byproducts during the press cure process, resulting in a sponged product, i.e., one that contains voids. This problem is described, for example, in U.S. Pat. No. 4,520,170 with respect to perfluoroelastomers.
SUMMARY OF THE INVENTION
The present invention provides a fluoroelastomer composition that has a reduced tendency to sponge during press cure. In particular, the present invention is directed to a curable fluoroelastomer composition comprising a fluoroelastomer, a curative for the fluoroelastomer, and a molecular sieve compound.
DETAILED DESCRIPTION OF THE INVENTION
Representative crosslinkable elastomers which can be used in the present invention include fluoroelastomers comprising copolymerized units of one or more monomers containing fluorine, such as vinylidene fluoride, hexafluoropropylene, 1-hydropentafluoropropylene, 2-hydropentafluoropropylene, tetrafluoroethylene, chlorotrifluoroethylene, and perfluoro(alkyl vinyl) ether, as well as other monomers not containing fluorine, such as ethylene or propylene. Elastomers of this type are described in Logothetis, Prog. Polym. Sci., Vol. 14, 251-296 (1989).
Specific examples of such fluoroelastomers include copolymers comprising units of vinylidene fluoride and hexafluoropropylene and, optionally, tetrafluoroethylene; copolymers comprising units of vinylidene fluoride, perfluoro(alkyl vinyl) ether and tetrafluoroethylene; copolymers comprising units of tetrafluoroethylene and propylene; and copolymers comprising units of tetrafluoroethylene and perfluoro(alkyl vinyl) ether, preferably perfluoro(methyl vinyl) ether. Each of the above fluoroelastomers may optionally also include a cure site monomer. Copolymers of ethylene, tetrafluoroethylene, perfluoro(alkyl vinyl) ether and, optionally, a bromine-containing cure site monomer, such as those disclosed by Moore, in U.S. Pat. No. 4,694,045 are suitable for use in the present invention. Copolymers of tetrafluoroethylene and perfluoro(alkyl vinyl) ether commonly contain fluorinated nitrile cure sites, for example perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) and others disclosed in U.S. Pat. No. 4,281,092 or U.S. Pat. No. 5,789,509.
Depending on their composition, fluoroelastomers are generally crosslinked by the action of curatives. Curatives include diamines; polyols, in conjunction with an accelerator such as a quaternary ammonium or phosphonium compound; or organic peroxides, in conjunction with a polyfunctional coagent. The amount of curative required will depend on the particular polymer and cure site which is present therein, but usually about 2-10 parts per hundred parts fluoroelastomer (phr) will be adequate for efficient cure. Generally about 2-15 phr of metal oxides or metal hydroxides are also present in the compounded elastomer. If fillers or carbon black are used, they are usually present in amounts of up to 65 phr.
The above described fluoroelastomers are generally cured using a two stage process. That is, the compositions are first cured in a closed mold, i.e. press cured. Then the compositions are post cured in an oven in an atmosphere of air or an inert gas. The initial cure usually takes place over a period of about 1-60 minutes at temperatures of about 145°-220° C., depending on the particular fluoroelastomer composition employed. During the initial cure, the fluoroelastomer composition is generally maintained under a pressure in the mold of about from 0 to 3500 MPa. The post cure is typically performed at ambient pressure, and temperatures of about from 125° to 300° C. for periods of about 1-24 hours.
The elastomer compositions of the present invention contain about from 0.5-15 parts per hundred parts fluoroelastomer (phr) of a molecular sieve additive. A preferred subclass includes zeolites. Molecular sieve zeolites are crystalline aluminosilicates of Group IA and Group IIA elements, such as sodium, potassium, magnesium, and calcium. They are represented by the following empirical chemical formula: M
2
O.Al
2
O
3
.ySiO
2
.wH:
2
O where y is 2 or greater, n is the cation valence, and w represents the water contained in the voids of the zeolite. Commercially available examples of such compositions include Molecular Sieve 3A, Molecular Sieve 4A, Molecular Sieve 5A, and Molecular Sieve 13X, all available from Aldrich Chemical Co., Inc. Milwaukee, Wis. Use of this class of additives prevents sponging and improves heat aging of vulcanizates upon press curing in many instances.
Other additives may be compounded into the fluoroelastomer to optimize various physical properties. Such additives include carbon black, stabilizers, plasticizers, lubricants, pigments, fillers, and processing aids typically utilized in perfluoroelastomer compounding. Any of these additives can be incorporated into the compositions of the present invention, provided the additive has adequate stability for the intended service conditions.
Carbon black is used in elastomers as a means to balance modulus, tensile strength, elongation, hardness, abrasion resistance, conductivity, and processability of the compositions. Carbon black is generally useful in amounts of from 5-60 phr.
In addition, or in the alternative, fluoropolymer fillers may be present in the composition. Generally from 1 to 50 phr of a fluoropolymer filler is used, and preferably at least about 5 phr is present. The fluoropolymer filler can be any finely divided, easily dispersed plastic fluoropolymer that is solid at the highest temperature utilized in fabrication and curing of the perfluoroelastomer composition. By solid, it is meant that the fluoroplastic, if partially crystalline, will have a crystalline melting temperature above the processing temperature(s) of the perfluoroelastomer(s). Such finely divided, easily dispersed fluoroplastics are commonly called micropowders or fluoroadditives. Micropowders are ordinarily partially crystalline polymers.
The molecular sieve compound, crosslinking agent, optional accelerator, metal oxide, and other additives are generally incorporated into the polymer by means of an internal mixer or on a rubber mill. The resultant composition is then cured, usually by means of heat and pressure, for example by compression molding.
The curable compositions of the present invention are useful in production of gaskets, tubing, and seals. Such articles are most commonly produced by molding a compounded formulation of the curable composition with various additives under pressure, curing the part, and then subjecting it to a post cure cycle. The cured compositions of the invention exhibit a marked decrease in the tendency to sponge, compared to compositions that do not include the molecular sieve compounds.
The invention is now illustrated by certain embodiments wherein all parts and percentages are by weight unless otherwise specified.
REFERENCES:
patent: 4214060 (1980-07-01), Apotheker et al.
patent: 4281092 (1981-07-01), Breazeale
patent: 4478965 (1984-10-01), Concannon et al.
patent: 4520170
DuPont Dow Elastomers L.L.C.
Lipman Bernard
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