Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-11-18
2001-04-24
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S326200, C525S340000, C525S368000, C525S369000, C525S378000, C525S384000
Reexamination Certificate
active
06221971
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fluoroelastomer curing processes. In particular, this invention relates to an improved process for curing elastomeric vinylidene fluoride copolymers.
BACKGROUND OF THE INVENTION
Elastomeric copolymers of vinylidene fluoride with other fluorinated comonomers, for example hexafluoropropylene, are well known, commercially available compositions which are resistant to chemical and thermal attack. As a result, they find particular utility as the polymeric components of o-ring seals for fuel and lubricant systems and shaft seals in automotive powertrain systems. The stability of these polymers is primarily due to their chemical structure, specifically to the high percentage of carbon-fluorine bonds which they contain. Carbon-fluorine bonds are relatively unreactive compared to bonds between carbon and other elements. Consequently, highly fluorinated vinylidene fluoride copolymers are less susceptible to chemical degradation than are non-fluorinated polymers of comparable structure.
In order to fully develop physical properties such as tensile strength, elongation, and compression set, elastomers must be cured, i.e. crosslinked. In the case of fluoroelastomers, this is generally accomplished by mixing uncured polymer (i.e. fluoroelastomer gum) with a polyfunctional curing agent and heating the resultant mixture, thereby producing chemical reaction of the curing agent with active sites along the polymer backbone or side chains. Interchain linkages produced as a result of these chemical reactions cause formation of a crosslinked polymer composition having a three-dimensional network structure. Commonly used curing agents for fluoroelastomers include difunctional nucleophilic reactants, such as polyhydroxy compounds or diamines. Alternatively, peroxidic curing systems containing organic peroxides and unsaturated coagents, such as polyfunctional isocyanurates, may be employed. Peroxide cure systems are most often utilized to crosslink those fluoroelastomers which contain copolymerized brominated or iodinated cure site monomers or brominated or iodinated end groups derived from chain transfer agents.
The polyhydroxy and peroxide cure systems were developed and optimized for use with copolymers of vinylidene fluoride and hexafluoropropylene (VF
2
/HFP copolymers). Such copolymers are the most widely used fluoroelastomers commercially. However, products made from other vinylidene fluoride-containing fluoroelastomers are sometimes preferred for use in unusally demanding environments. For example, products made from copolymers of vinylidene fluoride, tetrafluoroethylene, and propylene (VF
2
/TFE/P terpolymers) are often utilized in applications wherein resistance to low temperature and basic fluids is critical. In many cases the polyhydroxy and peroxide cure processes or curing agent formulations are unsatisfactory when used to crosslink these specialty fluoroelastomers. For example, it is known to cure elastomeric VF
2
/TFE/P terpolymers with either peroxide or polyhdyroxy cure systems as disclosed in U.S. Pat. Nos. 4,882,390; 4,912,171; and 4,910,260. However, when such compositions are cured using a combination of polyhydroxy compound and organo-onium compound, as taught in U.S. Pat. Nos. 4,882,390 and 4,912,171, the cured products exhibit high compression set. The peroxide cures disclosed in U.S. Pat. No. 4,910,260 are undesirable because the curable compositions which are initially formed are extremely scorchy and would therefore be unsuitable for commercial processes.
Polyhydroxy and peroxide cures of modified VF
2
/TFE/P terpolymers are also deficient. For example, unsaturated analogues of VF
2
/TFE/P terpolymers can be prepared in aqueous latex or solution as disclosed in U.S. Pat. Nos. 4,759,618; 4,645,799; and 4,742,126. However, base treatment in latex is cumbersome and uniform dispersion is difficult. Solvent processes are also undesirable on a commercial scale. In addition, polyhydroxy cures of the unsaturated VF
2
/TFE/P prepared by the latex routine require relatively large amounts of crosslinking agent.
Because of the broad utility of elastomeric vinylidene fluoride copolymers, it would be desirable to have available a cure system which would provide a more efficient and effective cure than the polyhydroxy and peroxide processes of the prior art. It would be particularly desirable to have an improved system adaptable to such copolymers which do not contain hexafluoropropylene as well as those which contain this comonomer.
SUMMARY OF THE INVENTION
The present invention provides an improved process for curing vinylidene fluoride copolymers. In particular, the present invention is directed to a process for curing an elastomeric vinylidene fluoride copolymer which comprises the steps of
a) mixing i) an elastomeric vinylidene fluoride copolymer gum with ii) a composition comprising an organic base and a metal oxide or hydroxide, said mixing step being conducted at a temperature of 150° C.-250° C., thereby producing an unsaturated elastomeric vinylidene fluoride copolymer;
b) mixing said unsaturated elastomeric vinylidene fluoride copolymer with a composition comprising at least one compound selected from the group consisting of polyhydroxy compounds, polyamines, and organic peroxides to form a compounded unsaturated fluoroelastomer; and
c) crosslinking said compounded unsaturated fluoroelastomer.
The invention is further directed to a curable unsaturated fluoroelastomer composition prepared by a process which comprises mixing i) an elastomeric vinylidene fluoride copolymer gum with ii) a composition comprising an organic base and a metal oxide or hydroxide, said mixing step being conducted at a temperature of 150° C.-250° C., thereby producing an unsaturated elastomeric vinylidene fluoride copolymer.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention is a sequential cure process for use with fluoroelastomers having copolymerized vinylidene fluoride units. In a first step, the fluoroelastomer is treated with a base at elevated temperatures to generate reactive sites, i.e. carbon-carbon double bonds, along the polymer backbone. The resultant unsaturated polymer is then mixed with a curative selected from the group consisting of polyhydroxy compounds, polyamines, peroxides and mixtures thereof, and crosslinked.
Fluoroelastomers suitable for use in the process are saturated copolymers of vinylidene fluoride with at least one other fluorinated comonomer capable of forming an elastomeric composition. Such comonomers include tetrafluoroethylene, chlorotrifluoroethylene, trifluoroethylene, vinyl fluoride, hexafluoropropylene, perfluoro(alkyl vinyl) ethers, perfluoro(alkoxy vinyl) ethers, perfluoro(alkoxyalkyl vinyl) ethers, ethylene, propylene, isobutylene, bromotrifluoroethylene, 4-bromo-3,3,4,4,-tetrafluorobutene-1, bromoperfluoro(ethyl vinyl)ether, and iodotetrafluorobutene. Preferred comonomers include tetrafluoroethylene, propylene, hexafluoropropylene, and perfluoro(methyl vinyl) ether. Preferred copolymers include vinylidene fluoride/tetrafluoroethylene/propylene terpolymers, vinylidene fluoride/tetrafluoroethylene/perfluoro(alkyl vinyl) ether terpolymers, vinylidene fluoride/hexafluoropropylene dipolymers, and vinylidene fluoride/hexafluoropropylene/tetrafluoroethylene terpolymers. The process is particularly useful for curing copolymers containing relatively low levels of copolymerized vinylidene fluoride units, i.e. those fluoroelastomers having less than 50 wt. % copolymerized vinylidene fluoride units, typically 2-45 wt. % copolymerized vinylidene fluoride units. Brominated or iodinated fluoroelastomers which are suitable for use in the invention may contain copolymerized brominated or iodinated comonomer units which have been introduced by copolymerization of brominated or iodinated monomers, for example 4-bromo-3,3,4,4-tetratetrafluorobutene-1 or bromoperfluoro(ethyl vinyl) ether. Alternatively, these halogens may be introduced by use of iodinated or brominated chain transfer agents such as methylene io
DuPont Dow Elastomers L.L.C.
Zitomer Fred
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