Curable perfluoroelastomer composition

Details Curable perfluoroelastomer composition Curable perfluoroelastomer composition

1999-08-04

2001-08-28

Wilson, Donald R. (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...

C525S342000, C525S354000, C525S370000, C525S374000, C525S375000, C525S378000, C525S380000, C525S381000, C525S382000

Reexamination Certificate

active

06281296

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to perfluoroelastomer compositions having enhanced cure rate.
BACKGROUND OF THE INVENTION
Perfluoroelastomers have achieved outstanding commercial success and are used in a wide variety of applications in which severe environments are encountered, in particular those end uses where exposure to high temperatures and aggressive chemicals occurs. For example, these polymers are often used in seals for aircraft engines, in oil-well drilling devices, and in sealing elements for industrial equipment used at high temperatures.
The outstanding properties of perfluoroelastomers are largely attributable to the stability and inertness of the copolymerized perfluorinated monomer units that make up the major portion of the polymer backbones in these compositions. Such monomers include tetrafluoroethylene and perfluoro (alkyl vinyl) ethers. In order to develop elastomeric properties fully, perfluoroelastomers are typically crosslinked, i.e. vulcanized. To this end, a small percentage of cure site monomer is copolymerized with the perfluorinated monomer units. Cure site monomers containing at least one nitrile group, for example perfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene, are especially preferred. Such compositions are described in U.S. Pat. Nos. 4,281,092; 4,394,489; 5,789,489; and 5,789,509.
Curing systems that incorporate tetraphenyltin have been successfully used to vulcanize nitrile-containing perfluoroelastomers. However, in certain circumstances the cure rate of such compositions is too slow for economically effective commercial production of perfluoroelastomer articles. Logothetis and Schmiegel, in U.S. Pat. No. 5,677,389, disclose ammonium salts as accelerators in combination with a variety of curing systems that crosslink perfluoroelastomers. Curable compositions containing the ammonium salts exhibit enhanced cure rate, but in some instances certain ammonium salt accelerators are not effective because of insolubility in the polymer. Saito, et al., in U.S. Pat. No. 5,565,512, disclose the use of ammonium salts of organic or inorganic acids as curing agents for perfluoroelastomers. Such compositions exhibit the same solubility problems that occur with compounds in which ammonium salts are used as accelerators. It would be beneficial to have a means for curing or for accelerating the cure rate of perfluoroelastomers that does not depend on the use of ammonium salts.
SUMMARY OF THE INVENTION
The present invention is directed to a curable perfluoroelastomer composition which contains a compound that decomposes to produce ammonia. More specifically, the present invention is directed to a curable composition comprising
A. a perfluoroelastomer comprising copolymerized units of
(1) tetrafluoroethylene, (2) a perfluorovinyl ether selected from the group consisting of perfluoro(alkyl vinyl) ethers, perfluoro(alkoxy vinyl) ethers, and mixtures thereof, and (3) a cure site monomer selected from the group consisting of nitrile-containing fluorinated olefins and nitrile-containing fluorinated vinyl ethers; and
A. a compound, other than an ammonium salt of an organic or inorganic acid, that decomposes at temperatures between 40° C. and 330° C. to produce ammonia.
The compound which produces ammonia acts as a curative for the perfluoroelastomer, if no other curative is present. However, when another curative is present, the compound that produces ammonia acts as a cure rate accelerator.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of the present invention are based on elastomeric perfluoropolymers (hereinafter “perfluoroelastomers”), that is, substantially fully fluorinated fluoropolymers which, when cured, exhibit an elastomeric character. The perfluoroelastomers contain nitrile groups that render the polymers crosslinkable.
Perfluoroelastomers are polymeric compositions having copolymerized units of at least two principal perfluorinated monomers. Generally, one of the principal comonomers is a perfluoroolefin, while the other is a perfluorovinyl ether. Representative perfluorinated olefins include tetrafluoroethylene and hexafluoropropylene. Suitable perfluorinated vinyl ethers are those of the formula
CF
2
═CFO(R
f′
O)
n
(R
f″
O)
m
R
f
  (I)
where R
f′
and R
f″
are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and R
f
is a perfluoroalkyl group of 1-6 carbon atoms.
A preferred class of perfluoro(alkyl vinyl) ethers includes compositions of the formula
CF
2
═CFO(CF
2
CFXO)
n
R
f
  (II)
where X is F or CF
3
, n is 0-5, and R
f
is a perfluoroalkyl group of 1-6 carbon atoms.
A most preferred class of perfluoro(alkyl vinyl) ethers includes those ethers wherein n is 0 or 1 and R
f
contains 1-3 carbon atoms. Examples of such perfluorinated ethers include perfluoro(methyl vinyl) ether and perfluoro(propyl vinyl) ether. Other useful monomers include compounds of the formula
CF
2
═CFO[(CF
2
)
m
CF
2
CFZO]
n
R
f
  (III)
where R
f
is a perfluoroalkyl group having 1-6 carbon atoms,
m=0 or 1, n=0-5, and Z=F or CF
3
. Preferred members of this class are those in which R
f
is C
3
F
7
, m=0, and n=1.
Additional perfluoro(alkyl vinyl) ether monomers include compounds of the formula
CF
2
═CFO[(CF
2
CFCF
3
O)
n
(CF
2
CF
2
CF
2
O)
m
(CF
2
)
p
]C
x
F
2X+1
  (IV)
where m and n independently=1-10, p=0-3, and x=1-5.
Preferred members of this class include compounds where n=0-1, m=0-1, and x=1.
Examples of useful perfluoro(alkoxy vinyl) ethers include
CF
2
═CFOCF
2
CF(CF
3
)O(CF
2
O)
m
C
n
F
2n+1
  (V)
where n=1-5, m=1-3, and where, preferably, n=1.
Mixtures of perfluoro(alkyl vinyl) ethers and perfluoro(alkoxy vinyl) ethers may also be used.
Preferred perfluoroelastomers are composed of tetrafluoroethylene and at least one perfluoro(alkyl vinyl) ether as principal monomer units. In such copolymers, the copolymerized perfluorinated ether units constitute from about 15-60 mole percent of total monomer units in the polymer.
The perfluoropolymer further contains copolymerized units of at least one cure site monomer, generally in amounts of from 0.1-5 mole percent. The range is preferably between 0.3-1.5 mole percent. Although more than one type of cure site monomer may be present, most commonly one cure site monomer is used and it contains at least one nitrile substituent group. Suitable cure site monomers include nitrile-containing fluorinated olefins and nitrile-containing fluorinated vinyl ethers. Useful nitrile-containing cure site monomers include those of the formulas shown below.
CF
2
═CF—O(CF
2
)
n
—CN  (VI)
where n=2-12, preferably 2-6;
CF
2
═CF—O[CF
2
—CFCF
3
—O]
n
—CF
2
—CF(CF
3
)—CN  (VII)
where n=0-4, preferably 0-2;
CF
2
═CF—[OCF
2
CF(CF
3
)]
x
—O—(CF
2
)
n
—CN  (VIII)
where x=1-2, and n=1-4; and
 CF
2
═CF—O—(CF
2
)
n
—O—CF(CF
3
)CN  (IX)
where n=2-4.
Those of formula (VIII) are preferred. Especially preferred cure site monomers are perfluorinated polyethers having a nitrile group and a trifluorovinyl ether group. A most preferred cure site monomer is
CF
2
═CFOCF
2
CF(CF
3
)OCF
2
CF
2
CN  (X)
i.e. perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene) or 8-CNVE.
The perfluoroelastomers suitable for use in the present invention may contain any of a variety of end groups as a result of the use of varying initiators or chain transfer agents during polymerization. For example, the polymers may contain sulfonate groups, sulfonic acid groups, carboxylate groups, carboxylic acid groups, carboxamide groups, difluoromethyl groups or trifluorovinyl groups.
The first embodiment of this invention is a curable composition comprising A) a perfluoroelastomer as defined above and B) a compound, other than an ammonium salt, that decomposes at temperatures

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