Oxalate ester blocked phenols

Details Oxalate ester blocked phenols Oxalate ester blocked phenols

1999-10-08

2002-03-19

Wilson, D. R. (Department: 1713)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C525S326300

Reexamination Certificate

active

06359166

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to curing agents for fluoropolymers and to curable fluoropolymer compositions. In another aspect, the present invention relates to delayed curing fluoropolymer compositions.
BACKGROUND OF THE INVENTION
Fluorocarbon elastomers are synthetic elastomeric polymers with a high fluorine content—see, for example, W. M. Grootaert et al., Fluorinated Elastomers, 8 Kirk-Othmer Encyclopedia of Chemical Technology 990-1005 (4th ed. 1993). Fluorocarbon elastomers, particularly the copolymers of vinylidene fluoride with other ethylenically unsaturated halogenated monomers such as hexafluoropropene (C
3
F
6
), have become the polymers of choice for high temperature applications, such as seals, gaskets, and linings. These fluoropolymers exhibit favorable properties against the exposure to aggressive environments such as solvents, lubricants, and oxidizing or reducing agents. Additionally, these polymers can be compounded and cured to have high tensile strength, good tear resistance, and low compression set.
Presently used curing agents for fluoropolymers include aromatic polyhydroxy compounds, such as polyphenols, used in combination with certain vulcanization accelerators such as ammonium, phosphonium, or sulfonium compounds. U.S. Pat. No. 4,882,390 (Grootaert et al.); U.S. Pat. No. 4,912,171 (Grootaert et al.); and U.S. Pat. No. 5,086,123 (Guenthner et al.), for example, describe these compounds.
In accordance with conventional curing processes, desired amounts of compounding ingredients and other conventional adjuvants or ingredients are added to unvulcanized fluorocarbon elastomer stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as Banbury mixers, roll mills, or other convenient mixing device. The components and adjuvants are distributed throughout the fluorocarbon gum during milling, during which period the temperature of the mixture typically will not rise above about 120° C. The curing process typically comprises either injecting (injection molding) the compounded mixture into a hot mold or pressing (compression molding) the compounded mixture in a mold, for example, a cavity or a transfer mold, followed subsequently by an oven-cure (post cure).
Many conventional fluoropolymer compositions tend toward “scorching” behavior, that is, the premature crosslinking or partial cure of the composition when exposed to elevated temperatures or conditions of high shear. This scorching behavior particularly is pronounced when the fluoropolymer is injection molded, wherein scorching is characterized by a premature cure initiation occurring prior to and during injection of the compounded composition into a mold. This can cause non-uniform curing of the fluoropolymer and results in poor physical properties.
The point of cure initiation for injection-molded fluoropolymers may be defined as the time after which the compounded fluoropolymer is subjected to injection-molding conditions (that is, upon introduction into an injection barrel at a temperature of approximately 70-90° C. and/or while injecting the compound into the mold under high shear at temperatures between about 180° C. and 200° C.) when the curing compound begins to gel or harden. Such a change in physical properties, particularly the corresponding viscosity increase, can greatly reduce processing efficiency by hindering the ability to inject the compounded mixture into a mold. Scorching phenomena also produce high levels of waste product; because a cured fluoropolymer is very difficult to reprocess, any fluoropolymer that cures outside the mold cavity must usually be discarded.
Thus, there exists a need for fluoropolymer curing agents that provide a composition having improved scorch safety and end-use products having improved physical properties.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides curable fluoropolymer compositions comprising the reaction product of: (a) fluorine-containing polymer or blend of fluorine-containing polymers each comprising interpolymerized units derived from one or more fluorine-containing ethylenically unsaturated monomers; (b) organo-onium compound; and (c) oxalate-blocked crosslinking agent.
In another aspect, the invention provides a method of curing a fluoropolymer comprising the steps of: (a) mixing organo-onium compound and oxalate-blocked compound into said fluoropolymer to form a curable fluoropolymer composition, said onium and oxalate-blocked compounds present in a sufficient amount to crosslink said fluoroelatomer to the desired degree; and (b) heating the curable fluoropolymer composition at a temperature of from about 180° to 210° C. for a sufficient time to crosslink said fluoropolymer.
In another embodiment, the invention provides a composition of matter comprising an oxalate-blocked compound having the formula:
wherein
Z is an aryl or polyaryl group;
R is an aryl group or an alkyl group; and
n and n′ each is independently selected as 0 or 1 with the proviso that when either n or n′ is 0, its corresponding portion of the Z moiety is terminated by hydrogen (that is, its corresponding terminal portion is —Z—OH) or is terminated by a metal or nonmetal cation.
The combinations of an organo-onium compound and the oxalate curing agents of the present invention provide increased processing control in the curing of fluoropolymer compositions, and in the formation of articles derived therefrom, without adversely affecting the physical properties of those cured compositions and articles.
The use of oxalate-blocked crosslinking agent in accordance with the teachings of the invention, either alone or in combination with one or more other crosslinking agents, yields improved scorch safety of curable fluoropolymers by providing a retarded cure at pre-molding temperatures below about 150° C. and a rapid cure at molding temperatures of about 180 to about 210° C. The ability significantly to retard this curing mechanism outside of the mold (where the temperature of the admixture typically do not exceed 150° C.) drastically reduces the probability of severe scorching behavior and consequently reduces attendant processing difficulties. For example, such ability allows for heating of the compound in a cold runner injection-molding process without scorching, thereby reducing the amount of waste generated while also reducing cycle times.
DETAILED DESCRIPTION OF THE INVENTION
Among the polymers that may be compounded in accordance with this invention are generally the fluoropolymers whose interpolymerized units are derived from one or more of the following fluoromonomers: vinylidene fluoride, vinyl fluoride, hexafluoropropene, chlorotrifluoroethylene, 2-chloropentafluoropropene, fluorinated vinyl ethers, fluorinated allyl ethers, tetrafluoroethylene, 1-hydropentafluoropropene, dichlorodifluoroethylene, trifluoroethylene, and mixtures thereof. Said fluoromonomers may also be copolymerized with other compounds such as with other cure-site monomers (for example, bromine-containing monomers or perfluorinated monomers such as perfluorobenzyl vinyl ether) or with non-fluorinated alpha-olefin co-monomers (for example, ethylene or propylene). Preferred fluoropolymers are copolymers of vinylidene fluoride and at least one terminally ethylenically-unsaturated fluoromonomer containing at least one fluorine atom substituent on each double-bonded carbon atom, each carbon atom of said fluoromonomer being substituted only with fluorine and optionally with chlorine, hydrogen, a lower fluoroalkyl radical, or a lower fluoroalkoxy radical.
Fluoropolymer copolymers according to the type described above are available commercially as copolymer gumstock under, for example, the “Fluorel” trademark by Dyneon LLC, Saint Paul, Minn. Suitable products of these lines include THV™ 200 and Fluorel™ FC-2230, FC-2145, FC-2178, and FC-2211. Other commercially available products include fluoropolymers sold under the “Viton” trademark.
The organo-onium compound which is admixed with the fluorine-containing polym

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