CO-VULCANIZABLE COMPOSITIONS OF FLUOROELASTOMERS

Abstract: Co-vulcanizable compositions comprising a fluoroe­ lastomer (A) composed of from 40% to 85% by mol of vinyl­ idene fluoride, from 15% to 35% by mol of perfluoropro­ pene and from 0 to 30% by mol of tetrafluoroethylene; a copolymer (B) containing from 40% to 60% by mol of tetra­ fluoroethylene and from 40% to 60% by mol of propylene; and (C) a compound belonging to the class of bis-peroxy­ dicarbamates.

Description: Background of the invention 1. Field of the Invention The present invention relates to co-vulcanizable compositions of fluoroelastomers. More particularly, the present invention relates to co-vulcanizable compositions of fluoroelastomers, having improved chemical stability. 2. Description of the Prior Art The fluoroelastomers on the basis of vinylidene fluoride, vulcanized according to the bisphenolic or dihydroxy systems (Kirk-Othmer, Encyclopaedia of Chemical Technology, vol. 8, page 506, 1979), are very valuable products which are used in the industrial world, in particular in the fields of car and oil industry, and in general in the energy industrial field. In the automobile field, fluoroelastomers are used mainly in the fuel circulation system, or as components of engine parts and of parts of gearbox-differential-transmission unit. In the first case, as application examples, membranes for fuel pumps, valve seats, non-return valves, flexible hoses and, in general, seal gaskets for fuel can be mentioned. In the second case, analogous examples are gaskets for gear wheels, seal liners for cylinders in diesel engines, gaskets for the crankshaft, non-return valves for the exhaust gases, gaskets of the type of shaft seals, and still others. In the oil and energy industrial field, fluoroelastomers are used for applications wherein they come into direct contact with the crude oil, above all at high temperatures and pressures , such as, for example, for gaskets, valves and pumps, or parts thereof. In this field, an important application for fluoroelastomers is represented by the expansion joints used in the exhaust ducts for the flue gases in thermoelectric power plants, wherein resistance to high temperatures associated with resistance to the corrosive action of acidic flue gases is required. Fluoroelastomers are used in these , and in other fields, such as in the electric and electronic field, thanks to their properties of high thermal stability and chemical resistance, even at high temperatures. But during the last years, and in some application types, precise technological requirements have arisen, which the manufactures of the prior art, vulcanized by the bisphenolic or dihydroxy systems, are not able to fulfil. The problematic parameter is in particular the chemical stability under severe operating temperature conditions in the presence of steam, of mineral acids, of highly aggressive lubricating fluids, such as, e.g., the oils of SF series, containing as additives compounds having a high swelling capacity for fluoroelastomers based on vinylidene fluoride. Furthermore, a general improvement is required as regards the processability characteristics of the mixes, for the moulding of such articles as the shaft seals, as well as the possibility to easily extrude the mixes to produce suitable semifinished articles having even complex profiles. The problem of the chemical stability has been solved thanks to the use of the vulcanization process by peroxides. This vulcanization type does not however allow the above mentioned requirements as for processability, in particular by moulding and extrusion, to be fulfilled; moreover, the same processability results less reliable (Rubber Chemistry and Technology, vol. 55, page 906, 1982), and on compression set test much poorer results are obtained (Kirk-Othmer, ibid., page 510). EP-0,136,596 discloses a composition comprising a copolymer based on vinylidene fluoride and a copolymer of tetrafluoroethylene and propylene. Unlike the present invention, the vulcanization is obtained by using a curing system comprising a ionic agent for the vinylidene fluoride copolymer and a peroxidic agent for the tetrafluoroethylene/propylene copolymer. It is well known in the art that,in order to vulcanizate vinylidene fluoride copolymers with peroxides,it is necessary the presence of cure sites (i.e.bromine atoms, double bonds, etc.), which are introduced into the polymeric chain by adding small amounts of another comonomer. In EP-0,136,596, where a vinylidene fluoride copolymer devoid of cure sites is used, the presence of the ionic agent is essential to obtain a vulcanization of the whole composition. GB-2,102,413 discloses bis-peroxy-dicarbamates as cross-linking agents. In this GB patent, it is stated that the bis-peroxy-dicarbamates are suitable as vulcanizing agents for unsaturated rubbers or diene rubbers. All the examples refer to non-fluorinated rubbers containing double bonds in the polymeric chain, such as natural rubber, polyisoprene rubber,etc. Also the curing mechanism proposed, presupposes the presence of insaturations along the chain. The only teaching deriving from the GB patent is that bis-peroxy-dicarmates could be used as alternatives for the peroxidic agents described in EP-0,136,596. No indication is given that these products can covulcanize also vinylidene copolymers devoid of cure sites,rendering useless the addition of ionic agents. Moreover, no indication is given that this particular kind of peroxides does not require the presence of a vulcanizing co-agent, which is usually used in the art when fluoroelastomers are vulcanized with peroxides. The Present Invention The purpose of the present invention is to provide a co-vulcanizable composition of fluoroelastomers, which allows the above described drawbacks to be overcome. More particularly, purpose of the present invention is to provide a co-vulcanizable composition of fluoroelastomers containing vinylidene fluoride units, which allows an improved processing reliability and a better chemical and thermal stability to be achieved. It has been now surprisingly found by the Applicant, and is the object of the present invention, that these and further purposes are achieved by means of a covulcanizable elastomeric composition comprising: A) 15-85% by weight of a fluoroelastomer composed by 40-85% by mol of vinylidene fluoride, 15-35% by mol of perfluoropropene and 0-30% by mol of tetrafluoroethylene; B) 15-85% by weight of a copolymer constituted by 40-60% by mol of tetrafluoroethylene and 40-60% by mol of propylene; C) 1-12 parts by weight, as referred to 100 parts of (A)+(B) blend, of a bis-peroxy-dicarbamate with vulcanizing accelerating action, of general formula R-OO-C(O)-NH-R₁-NH-C(O)-OO-R (1) wherein R is a hydrocarbon group containing a tertiary carbon atom, which is bound to aliphatic, alicyclic or aromatic substituents, said hydrocarbon group having a number of carbon atoms comprised within the range of from 1 to 25, and R₁ is a difunctional, i.e., divalent aliphatic, alicyclic or aliphatic-alicyclic radical with a number of carbon atoms comprised within the range of from 1 to 25. By an "aliphatic-alicyclic radical", we mean a radical which ends at one side with an aliphatic group, and at the other side with an alicyclic group. The group -C(O)-NH-R₁-NH-C(O)- can also be derived from a urethane prepolymer as described in Rubber World, May 1984, page 50. Co-vulcanizable elastomeric compositions containing from 5 to 10 parts by weight, as referred to 100 parts of (A)+(B) blend, of bisperoxydicarbamate are preferred in the practical embodiment of the present invention. Elastomeric compositions comprising from 30 to 70% by weight of a fluoroelastomer (A) constituted by 50-65 mol % of vinylidene fluoride, 15-20 mol % of perfluoropropene and 0-25 mol % of tetrafluoroethylene; and of from 70 to 30% by weight of a copolymer (B) constituted by 55-45 mol % of tetrafluoroethylene and 45-55 mol % of propylene are particularly preferred in the practical embodiment of the present invention. The elastomers as of item (A) are known from technical literature and can be prepared as described in Kirk-Othmer, ibid., pages 500-foll., or they can be obtained from the market under the tradenames VITON, TECNOFLON, FLUOREL, DAIEL. The copolymers as of item (B) are known from technical literature and can be prepared as described in Rubber Chemistry and Technology, vol. 50, pages 403, 1977, or they can be obtained from the market under the tradename AFLAS. The bisperoxydicarbamates as of item (C) can be prepared as described in Rubber World, May 1984, pages 49-foll. According to a preferred embodiment of the present invention, bisperoxydicarbamates which can be used in the composition of the present invention can be: Their preparation is described in detail in the bibliografic reference quoted above. The elastomeric vulcanizable composition according to the present invention contains furthermore the conventional additives for such a type of elastomers, in particular such basic substances as MgO, PbO, CaO, calcium hydroxide and carbon black. The operating conditions for the mix preparation and vulcanization belong to those normally used for fluoroelastomers, and described in Kirk-Othmer, ibidem, pages 500-foll. The co-vulcanizable compositions of the present invention have optimum characteristics of extrusion, optimum stability to SF oils, and optimum strength to steam at high-temperature. To the purpose of better understanding the present invention, and of describing practical embodiments thereof, some illustrative, but not limitative Examples are reported hereunder. Example 1-4 By blending in cylinder mixer, elastomeric compositions have been prepared, by starting from the following products: Tecnoflon NM (R) : a product of Montefluos and constituted by a vinylidene fluoride (79% by mol) with perfluoropropene (21% by mol) copolymer; Blend A 30, constituted by a mixture of 70 parts by weight of Tecnoflon NM (R) and 30 parts by weight of AFLAS (R) 150, a product of Asahi Glass and constituted by a copolymer containing equimolecular amounts of tetrafluoroethylene and propylene; Blend 40: as A 30, but containing 40% of AFLAS 150; Blend 20: as A 30, but containing 20% of AFLAS 150; Bisphenol AF: hexafluoroisopropylidene-bis(4-hydroxyphenol); BDDPC1: benzyldiphenyldiethylaminophosphonium chloride; BPXC1: having the above reported formula. In Table 1 the data relating to three compositions according to the present invention (Tests 2, 3 and 4) and, to comparative purposes, the data relating to a vulcanizable composition of the prior art (Test 1) are reported. Examples 5 and 6 Two compositions of fluoroelastomers have been prepared with the same modalities as of the foregoing Examples (Test 6 is a comparative test); after vulcanization in press at 170°C for 8 minutes and in oven at 200°C for 24 hrs, two samples have been treated with steam at 160°C for 7 days. The physical properties of the two products are reported in Table 2.

Claims: Co-vulcanizable compositions of fluroelastomers comprising: A) 15-85% by weight of a fluoroelastomer composed by 40-85% by mol of vinylidene fluoride, 15-35% by mol of perfluoropropene and 0-30% by mol of tetrafluoroethylene; B) 15-85% by weight of a copolymer constituted by 40-60% by mol of tetrafluoroethylene and 40-60% by mol of propylene; C) 1-12 parts by weight, as referred to 100 parts of (A)+(B) blend, of a bisperoxydicarbamate with vulcanizing accelerating action, of general formula R-OO-C(O)-NH-R₁-NH-C(O)-OO-R (1) wherein R is a hydrocarbon group containing a tertiary carbon atom, which is bound to aliphatic, alicyclic or aromatic substituents, said hydrocarbon group having a number of carbon atoms comprised within the range of from 1 to 25, and R₁ is a difunctional aliphatic, alicyclic or aliphatic-alicyclic radical with a number of carbon atoms comprised within the range of from 1 to 25. Composition according to claim 1, wherein the bisperoxydicarbamate is: Composition according to claim 1, wherein the bisperoxydicarbamate is: Composition according to claim 1, wherein the bisperoxydicarbamate is: Composition according to claim 1, wherein the bisperoxydicarbamate is: Co-vulcanizable composition of fluoroelastomers according to one of preceding claims, comprising: A) 30-70% by weight of fluoroelastomer constituted by 50-65 mol % of vinylidene fluoride, 15-20 mol % of perfluoropropene and 0-25 mol % of tetrafluoroethylene; B) 70-30% by weight of a copolymer constituted by 55-45 mol % of tetrafluoroethylene and 45-55 mol % of propylene; C) 5-10 parts by weight, as referred to 100 parts of blends (A)+(B), of bisperoxydicarbamate of general formula (1).

Inventors: CHIODINI, GRAZIELLA, CIRILLO, GIANNA, GERI, SERGIO, MOGGI, GIOVANNI
Applicants: AUSIMONT S.P.A.
Application year: 1986
Main classification: C08L27
IPC codes: C08L57, C08K5, C08L27, C08L101, C08L1, C08L23