Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1997-06-19
2003-05-20
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...
Reexamination Certificate
active
06566471
ABSTRACT:
The present invention relates to new peroxide curable fluoroelastomers particularly suitable for manufacturing O-rings.
Various types of fluoroelastomers are known in the art, widely used in all those fields where superior elastic properties associated with high thermochemical stability are required. For a wide survey on such products see for instance “Ullmann's Encyclopedia of Industrial Chemistry”, vol. A-11, pag. 417-429 (1988, VCH Verlagsgesellschaft).
It is also known that for manufacturing sealing elements, particularly O-rings, it is necessary to use elastomers endowed with particularly low compression set values. In fact, O-ring sealing effectiveness is as better as the article, upon compression, is able to recover initial dimensions. Since the fluoroelastomers are used in a wide temperature range, compression set values should be low not only at low temperatures, but also at high temperatures. Compression set values (measured at 200° C. for 70 hours, according to ASTM Standard D395, Method B) lower than 25% are generally required. More particularly, military specifications (MIL-R-83248B) asks for O-rings having a maximum compression set value of 20% (measured at 200° C. for 70 hours as well).
Fluoroelastomers which can meet such requirements are those curable ionically, which need addition of curing agents (for instance polyhydroxy compounds, such as Bisphenol AF or Bisphenol A), of suitable accelerators (for instance ammonium, phosphonium or amino-phosphonium salts), and of divalent metal oxides and/or hydroxides (for instance MgO, Ca(OH)
2
). Elastomers of this type are described for instance in patent application EP-525,685. However, ionic curing shows some drawbacks, among which the fact that a post-curing treatment is needed, generally carried out at 200°-260° C. for 12-24 hours, in order to complete curing, and to eliminate volatile by-products, so as to improve and stabilize mechanical and elastic properties. This implies a remarkable increase in processing times and costs and therefore strongly limits the possibilities of large scale production.
As described in U.S. Pat. No. 4,243,770, fluoroelastomers can be crosslinked also by means of peroxides. To such purpose it is necessary to carry out the polymerization in the presence of suitable iodinated chain transfer agents, which introduce into the macromolecules iodine atoms in terminal position: in the presence of radicals deriving from a peroxide said iodine atoms act as cure sites in consequence of homolitic breakage of the C—I bonds. Fluoroelastomers of this type generally do not require long post-curing treatments: in some cases it is sufficient a post-curing in air at about 200°-230° C. for 1-4 hours. However, such products do not meet the specifications indicated above for O-ring manufacturing: the compression set value is indeed usually high, at least equal to 28-30% or higher.
The Applicant has now found a new class of fluoroelastomers as defined hereinunder, which, upon peroxide curing, need extremely short post-curing treatments (around 30-60 minutes at 180°-230° C.) and are characterized by compression set values lower than 25% (measured on O-rings at 200° C. for 70 hours according to ASTM Standard D395 Method B).
Therefore, a first object of the present invention are peroxide curable fluoroelastomers, having iodine atoms in terminal position, and monomeric units in the chain deriving from an iodinated olefin of formula:
CHR═CH—Z—CH
2
CHR—I (I)
wherein: R is —H or —CH
3
; Z is a C
1
-C
18
(per)fluoroalkylene radical, linear or branched, optionally containing one or more ether oxygen atoms, or a (per)fluoropolyoxyalkylene radical.
Further objects of the present invention are the iodinated olefins of formula (I), and the preparation process thereof, as described hereinunder.
As regards formula (I), Z is preferably a C
4
-C
12
perfluoroalkylene radical, or a (per)fluoropolyoxyalkylene radical of formula:
—(Q)
p
—CF
2
O—(CF
2
CF
2
O)
m
(CF
2
O)
n
—CF
2
—(Q)
p
— (II)
wherein: Q is a C
1
-C
6
, preferably C
1
-C
3
, alkylene or oxyalkylene radical; p is 0 or 1; m and n are numbers such that the m
ratio is from 0.2 to 5 and the molecular weight of said (per)fluoropolyoxyalkylene radical is from 400 to 10,000, preferably from 500 to 1,000. Preferably, Q is selected from: —CH
2
O—; —CH
2
OCH
2
—; —CH
2
—; —CH
2
CH
2
—.
The olefins of formula (I) can be prepared starting from compounds of formula I—Z—I according to the following process:
(1) adding ethylene or propylene to a compound of formula I—Z—I, thus obtaining a diiodinated product of formula:
I—CHR—CH
2
—Z—CH
2
—CHR—I (III)
where R and Z are defined as above;
(2) partially dehydroiodinating the product of formula (III) with a base (for instance NaOH, KOH, tertiary amines, etc.), so as to obtain the iodinated olefin of formula (I).
As to step (1), the addition of ethylene or propylene is usually carried out in the presence of suitable catalysts, such as redox systems, for instance CuI or FeCl
3
, in solution in an organic solvent, for instance acetonitrile. The addition reaction between a perfluoroalkyliodide and an olefin is described, for instance, by M. Hudliky in “Chemistry of Organic Fluorine Compounds” (2nd Edition, Ellis Horwood Ltd., Chichester, 1976), and by R. E. Banks in “Organofluorine Chemicals and Their Industrial Applications” (Ellis Horwood Ltd. Chichester, 1979), or in J. Fluorine Chemistry, 49 (1990), 1-20 and in J. Fluorine Chemistry, 58 (1992), 1-8.
The dehydroiodination reaction of step (2) can be carried out either without any solvent, or by dissolving the diiodinated product in a suitable solvent (for instance a glycol such as diethylenglycol, or a long chain alcohol). To maximize the iodinated olefin yield, avoiding as far as possible a further dehydroiodination reaction with formation of the corresponding bis-olefin of formula CHR═CH—Z—CH═CHR, it is possible:
(a) to employ the base in defect with respect to the stoichiometric amount, with a molar ratio base/diiodinated compound preferably from 1.5 to 0.5, and then separating the iodinated olefin from the bis-olefin by fractional distillation; or
(b) to carry out the dehydroiodination reaction at reduced pressure, so as to remove she iodinated olefin from the reaction mixture as it forms, taking advantage of the fact that the latter has a boiling point lower than that of the starting diiodinated product; in such a case the reaction is preferably carried out without any solvent.
Alternatively, it is possible to carry out step (1) in defect of ethylene or propylene, to favour as much as possible formation of mono-addition product I—Z—CH
2
—CHR—I (which can be separated from the di-addition product by fractional distillation); the mono-addition product is then dehydroiodinated as described above, with formation of olefin I—Z—CH═CHR, which is finally submitted to a further addition of ethylene or propylene to give the iodinated olefin I—CHRCH
2
—Z—CH═CHR.
When Z is a (per)fluoroalkylene radical, optionally containing one or more ether oxygen atoms, the starting diiodinated compound I—Z—I can be obtained by telomerization of a (per)fluoroolefin C
2
-C
4
or of a (per)fluorovinylether C
3
-C
8
(for instance tetrafluoroethylene, perfluoropropene, vinylidene fluoride, perfluoromethylvinylether, perfluoro-propylvinylether, or mixtures thereof), by using a product of formula I—(R
f
)
k
—I (where k=0, 1; R
f
=C
1
-C
8
(per)fluoroalkylene radical) as telogenic agent. Telomerization reactions of this type are described, for instance, by C. Tonelli and V. Tortelli in J. Fluorine Chem., 47 (1990), 199, or also in EP-200,908.
When Z is a (per)fluoropolyoxyalkylene radical, the preparation of the products I—Z—I is described, for instance, in U.S. Pat. No. 3,810,874.
The amount of monomeric units deriving from the iodinated olefin of formula (I) present in the fluoroelastomers object of the present invention is generally from 0.01 to 1.0 moles, preferably from 0.03 to 0.5 moles, even more preferably from 0.05 to 0.2
Albano Margherita
Arcella Vincenzo
Brinati Giulio
Tortelli Vito
Arent Fox Kintner Plotkin & Kahn
Ausimont S.p.A.
Zitomer Fred
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