Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-07-11
2003-11-11
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S078000, C526S087000, C526S247000, C526S249000, C526S250000, C526S255000, C526S296000
Reexamination Certificate
active
06646077
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to peroxide curable fluoroelastomers comprising copolymerized units of vinylidene fluoride or tetrafluoroethylene, at least one other fluorinated major monomer, and a cure site monomer having the general formula CH
2
═CH—(CF
2
)
n
I, where n is an integer between 2 and 8, and wherein said fluoroelastomer has iodine atoms at chain ends.
BACKGROUND OF THE INVENTION
Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF
2
) and units of at least one other copolymerizable fluorine-containing major monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a fluorovinyl ether such as a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether). Other examples of fluoroelastomers include the copolymers of tetrafluoroethylene with a perfluoro(alkyl vinyl ether) such as perfluoro(methyl vinyl ether) (PMVE).
In order to develop the physical properties necessary for most end use applications, fluoroelastomers must be crosslinked. A preferred curing system for many end uses is the combination of an organic peroxide and a multifunctional unsaturated coagent. The coagent forms crosslinks by reacting with cure sites on the fluoroelastomer polymer chain. A preferred cure site is an iodine atom bonded to a carbon atom on the fluoroelastomer chain.
The major challenge with the use of iodine-containing cure sites in fluoroelastomers is to balance sufficient heat resistance with acceptable processability and rheology. Sufficient heat resistance is conferred by synthesizing polymers with an average minimum of 2.5 iodine atoms per chain. Fewer than an average of 2.5 iodine atoms per polymer chain results in destruction of crosslink sites during heat aging, leading to excessive amounts of dangling polymer chains that do not contribute to network strength and weaken the tensile strength of the cured article (Flory, P. J.
Principles of Polymer Chemistry
, p 432, Cornell University Press, 1952).
One method of introducing iodine cure sites into the fluoroelastomer is by conducting the polymerization in the presence of a chain transfer agent containing iodine. In this manner, an iodine atom is attached to the resulting fluoroelastomer polymer chain at one or both terminal positions. Such chain transfer agents typically have the formula RI
n
, where R may be a C
1
-C
3
hydrocarbon, a C
1
-C
6
fluorohydrocarbon or chlorofluorohydrocarbon, or a C
2
-C
8
perfluorocarbon, and n is 1 or 2 (U.S. Pat. No. 4,243,770). However, use of an iodine-containing chain transfer agent alone in a fluoroelastomer polymerization will inevitably give rise to polymer chains containing less than two iodine atoms per chain. Therefore fluoroelastomers produced using this process have insufficient heat resistance for many applications.
A solution to this difficulty is to use large amounts of iodine-containing chain transfer agents together with monomers containing two olefin moieties (U.S. Pat. No. 5,585,449). The use of the diolefin-containing monomers crosslinks 2 short polymer chains during polymerization to give one larger chain that contains at least an average of 2.5 iodine atoms. However, the resulting polymer is branched and may show poor rheology and flow properties.
Another common method of introducing iodine atom cure sites onto a fluoroelastomer polymer chain is by copolymerizing a minor amount of an iodine-containing fluoroolefin or fluorovinyl ether cure site monomer along with the major monomers (e.g. VF
2
, HFP, TFE, PAVE, etc.). In this manner, cure sites may be randomly distributed along the resulting fluoroelastomer polymer chain (U.S. Pat. Nos. 4,529,759; 4,694,045). However, each iodine-containing cure site monomer that is introduced in this way itself acts as a chain transfer agent. During polymerization, these sites will serve as branch points. The resulting fluoroelastomers may be highly branched and show unacceptably poor rheology. In many cases the resulting fluoroelastomer will be branched to such a degree as to display insoluble gel (EP-A-0171290). While such polymers will display excellent physical properties such as compression set, their poor flow behavior precludes them from practical use and their gel content causes poor hot tear strength and poor demolding.
In addition, while iodine cure sites may be introduced both along the fluoroelastomer polymer chain and at terminal positions by a combination of the above methods (EP-A-0171290; and U.S. Pat. No. 5,717,036), a highly branched polymer may similarly be obtained, albeit with lower overall molecular weight. In particular, the fluoroelastomer molecular weight may be too low to displace trapped air during compression molding or may lead to mold fouling, poor tensile strength, or poor compression set.
Arcella et al. (U.S. Pat. No. 5,625,019) attempt to address this problem by use of a cure site in which the iodine is attached to a short length of hydrocarbon chain. In this case the cure site monomer does not serve as an active chain transfer agent during polymerization. However, during curing the reactivity of a —CH
2
CH
2
I site is less than that of a —CF
2
CF
2
I site and cure times are longer. In addition, the presence of a —CH
2
CH
2
I group during polymerization is highly retarding and excessive amounts of polymerization initiator are required in order to maintain a desirable polymerization rate.
Therefore it remains a general problem in the field of fluoroelastomers to provide a polymer that has high molecular weight, low or no branching, and contains sufficient iodine cure sites to confer adequate heat resistance.
A secondary problem in the field of fluoroelastomers is the use of monomers of the formula CH
2
═CH—(CF
2
)
n
I (n=2 to 10) as cure sites. These monomers are easily prepared as described in J. Org. Chem. 42,1985-90 (1977) and are therefore of interest as a cure site monomer. But as described in EP-A-0171290, their use in a continuous polymerization process leads to highly branched products and their use in a batch polymerization process leads to a non-uniform distribution along the polymer backbone and consequently poor physical properties. Another difficulty when using this class of cure site monomers in a batch process is that they are so highly retarding that polymerization times are prolonged to undesirable lengths.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides a peroxide curable fluoroelastomer having excellent processability and wherein the cured fluoroelastomer has excellent tensile properties. The fluoroelastomer comprises copolymerized units of
(A) a first fluoromonomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene;
(B) at least one second fluoromonomer, different from said first fluoromonomer;
(C) 0.05 to 4 weight percent, based on total weight of said fluoroelastomer, of a cure site monomer having the general formula CH
2
═CH—(CF
2
)
n
I, where n is an integer between 2 and 8; and
(D) 0.01 to 1 weight percent, based on total weight of said fluoroelastomer, of iodine bound at terminal positions of fluoroelastomer polymer chains.
Another aspect of the present invention is a curable fluoroelastomer composition comprising:
(A) the above fluoroelastomer;
(B) an organic peroxide; and
(C) a coagent.
Another aspect of the present invention is a semibatch polymerization process for the manufacture of the above fluoroelastomer comprising:
(A) charging a reactor with a quantity of an aqueous solution comprising a surfactant;
(B) feeding to said reactor a quantity of an initial major monomer mixture to form a reaction medium, said initial major monomer mixture comprising i) from 10 to 70 weight percent, based on total weight of said monomer
DuPont Dow Elastomers LLC
Zitomer Fred
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