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
2001-03-22
2003-08-26
Wu, David W. (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...
C525S372000, C525S384000, C524S430000
Reexamination Certificate
active
06610790
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fluoroelastomers that are capable of being crosslinked with polyhydroxy compounds to produce cured compositions having excellent processability, low temperature properties and resistance to bases.
BACKGROUND OF THE INVENTION
Elastomeric fluoropolymers (i.e. fluoroelastomers) exhibit excellent resistance to the effects of heat, weather, oil, solvents and chemicals. Such materials are commercially available and are most commonly either dipolymers of vinylidene fluoride (VF
2
) with hexafluoropropylene (HFP) or terpolymers of VF
2
, HFP, and tetrafluoroethylene (TFE). While these di- and terpolymers have many desirable properties, including low compression set and excellent processability, their low temperature flexibility is not adequate for all applications.
It is known that incorporation of perfluorinated vinyl ether monomer units into vinylidene fluoride elastomers improves low temperature properties. For example, Carlson, in U.S. Pat. No. 5,214,106 discloses that when perfluoro(methyl vinyl) ether (PMVE) is substituted for HFP, the resultant VF
2
/PMVE/TFE copolymers have glass transition temperature (T
g
) values which are 10°-20° C. lower than those of the corresponding VF
2
/HFP/TFE copolymers. T
g
is often used as an indicator of low temperature flexibility because polymers having low glass transition temperatures maintain elastomeric properties at low temperatures.
Kruger, in U.S. Pat. No. 5,696,216, discloses PMVE-containing fluoroelastomers that are similar to those disclosed by Carlson. Those disclosed by Kruger contain copolymerized units of VF
2
, TFE, at least one fluorinated propene and or fluorinated methyl vinyl ether, at least one perfluoro(polyoxyalkyl vinyl) ether, and a crosslinking site.
The compositions of Carlson and Kruger are most effectively crosslinked through use of peroxide cure systems. However, when compression molding equipment is used with peroxide curable VF
2
/PMVE copolymers the compositions generally exhibit a tendency to stick to and foul the mold.
Tetrapolymers of VF
2
, HFP, TFE and perfluoro(vinyl ethers) (PVE) other than PMVE are also known to exhibit improved low temperature properties compared to terpolymers of VF
2
, HFP and TFE. For example, Arcella, et al. in U.S. Pat. No. 5,260,393 disclose a tetrapolymer comprising copolymerized units of 48-65 wt. % VF
2
, 21-36 wt. % HFP, 3-9 wt. % PVE, and 0-17 wt. % TFE. The compositions can be cured using a bisphenol curing system and do not exhibit the mold fouling problems associated with peroxide cures of VF
2
/PMVE copolymers. Similarly, British Patent 1,296,084 discloses fluoroelastomeric tetrapolymers containing copolymerized units of 48-65 wt. % VF
2
, 8-23 wt. % HFP, 4-15 wt. % TFE, and 17-30 wt. % PVE. Such compositions have good low temperature properties and are curable with bisphenols or amines. Although these tetrapolymers exhibit good low temperature properties, many applications require improved low temperature and processability performance.
Merely raising the PVE content while lowering the HFP content is not a solution to the problem of improving low temperature performance of VF
2
/HFP/PVE/TFE tetrapolymers. This is because polymers wherein the level of HFP is below about 8-10 mole N, percent do not contain sufficient copolymerized monomer sequences consisting of HFP units flanked by VF
2
units to permit efficient crosslinking with bisphenols. As is well known in the art, efficient curing of VF
2
/HFP-containing fluoroelastomers with a bisphenol/accelerator system is possible only when a —CH
2
— group in the polymer backbone is flanked by two perfluorinated carbons (e.g. CF
2
CF(CF
3
)CH
2
CF
2
CF
2
), rendering the hydrogens acidic enough to be abstracted by base. The dehydrofluorinated polymers are easily crosslinked by bisphenols. Furthermore, as discussed by W. W. Schmiegel, in
Angewandte Makromolekulare Chemie,
76/77, 39 (1979), completely eliminating HFP to form VF
2
/TFE/PMVE terpolymers results in formation of monomer sequences consisting of TFE/VF
2
/TFE; TFE/VF
2
/PMVE; PMVE/VF
2
/PMVE; and PMVE/VF
2
/TFE. Although such sites readily undergo elimination of HF and trifluoromethanol in the presence of base, the double bonds thus formed are not easily crosslinked by bisphenols or any other traditional crosslinking agents.
Bowers and Schmiegel (WO 00/11050) have disclosed bisphenol cures of fluoroelastomers containing units of VF
2
, PMVE, 2-hydropentafluoropropene (2-HPFP) and, optionally, TFE. The 2-HPFP acts as a cure site monomer, causing the fluoroelastomer to react readily with bisphenol curative. However, 2-HPFP is not readily available in commercial quantities.
Fluoroelastomers which contain copolymerized units of TFE and a hydrocarbon olefin, e.g. ethylene (E) or propylene (P), are well known in the art to have better resistance to attack by bases than most other fluoroelastomers. Examples of such fluoroelastomers include TFE/P (Ito et al. U.S. Pat. No. 4,758,618; Grootaert et al. U.S. Pat. No. 4,882,390) and E/TFE/PMVE (Moore et al. U.S. Pat. No. 4,694,045). However, these polymers are relatively difficult to cure, especially with a polyhydroxy curative. For example, TFE/P polymers generally must contain about 30-35 mol % of copolymerized units of a cure site monomer such as VF
2
in order to render the polymers vulcanizable by a polyhydroxy curative. Unfortunately, such a high level of VF
2
adversely affects the resistance of the resulting fluoroelastomer to bases.
There thus exists an unfulfilled need in the art for a method of providing polyhydroxy-curable compositions of 1) copolymers of VF
2
, TFE, and PVE that maintain optimum low temperature properties, but which exhibit low mold sticking characteristics, improved processability and are easily curable; and 2) copolymers of TFE and a hydrocarbon olefin which maintain optimum resistance to bases.
Hauptschein et al. in U.S. Pat. No. 3,106,589 disclose a novel class of fluoroolefins of the formula R
f
—(CH
2
CF
2
)
n
—CH═CF
2
, wherein R
f
is a C
1
-C
6
perfluoroalkyl group or a C
1
-C
6
perfluoroalkoxy group and n is an integer from 1-25. Potential utilities mentioned for these fluoroolefins include intermediates for the preparation of fluorinated carboxylic acids, nitroalcohols and hydroxy acids. Hauptschein also suggests that these fluoroolefins may be copolymerized with other olefins, but it is not suggested that these compounds might be utilized, in minor amount, as cure site monomers for fluoroelastomers largely comprised of other fluoromonomers.
SUMMARY OF THE INVENTION
The present invention is directed to a curable composition comprising:
A. a fluoroelastomer consisting essentially of (1) copolymerized units of 0.05-4 mole percent of a fluoroolefin of the formula R
f
—(CH
2
CF
2
)
n
—CH═CF
2
, wherein R
f
is a C
1
-C
6
perfluoroalkyl group or a C
1
-C
6
perfluoroalkoxy group and n is an integer from 1-3; (2) 0-1 mole percent iodine chemically bound to fluoroelastomer chain ends; and (3) copolymerized units selected from the group consisting of
(i) 35-85 mole percent vinylidene fluoride, 10-60 mole percent perfluoro(vinyl ether), 0-35 mole percent tetrafluoroethylene;
(ii) 30 to 70 mole percent tetrafluoroethylene, 5-25 mole percent vinylidene fluoride and 15-55 mole percent of a hydrocarbon olefin; and
(iii) 10 to 40 mole percent of a hydrocarbon olefin, 2 to 60 mole percent tetrafluoroethylene and 20 to 45 mole percent of a perfluoro(vinyl ether);
B. a polyhydroxy crosslinking agent;
C. a cure accelerator; and
D. a metal oxide or metal hydroxide.
DETAILED DESCRIPTION OF THE INVENTION
The fluoroelastomers utilized in the curable compositions of the present invention are copolymers capable of undergoing crosslinking reactions with polyhydroxylic compounds to form cured elastomeric compositions that exhibit either unusually good low temperature properties or unusually good resistance to attack by base.
All the fluoroelastomers used in this invention contain copolymerized units of a certain fluoroolefin (FO1) cure site monomer whi
Hung Ming-Hong
Schmiegel Walter Werner
Dupont Dow Elastomers L.L.C.
Hu Henry S
Wu David W.
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