CO-VULCANIZABLE COMPOSITIONS OF FLUOROELASTOMERS HAVING IMPROVED CHEMICAL STABILITY

Abstract: Co-vulcanizable compositions comprising (A) 55-85% by weight of a fluoroelastomeric copolymer of vinylidene fluoride with perfluoropropene and possibly tetrafluoro­ ethylene; (B) 45-15% by weight of a copolymer of tetra­ fluoroethylene and propylene; (C) 0.7-2.5 parts, per 100 parts of (A)+(B) blend, of a bisphenolic compound; (D) 0.3-1.5 parts, per 100 parts of (A)+(B) blend, of a quaternary ammonium, phosphonium or aminophosphonium organic compound; (E) 0.4-5 parts, per 100 parts of (A)+(B) blend, of an organic peroxide; (F) 1-5% by weight, as referred to (A)+(B), of a peroxidic cross-­ linking coagent.

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 combined 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 problem involves in particular the chemical stability under severe operating conditions characterized by high temperatures 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 peroxidic vulcanization process. However, this type of vulcanization does not fulfil the above mentioned requirements as for processability, in particular by moulding and extrusion; furthermore, the same processability results to be 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). In European patent application No. 0 136 596, co-vulcanized compositions have been prepared and disclosed, which have allowed certain improvements in performance to be achieved, but which still show poor processing reliability, as well as a slight decrease of chemical-thermal properties. The Present Invention 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 based on vinylidene fluoride, which allows an improved processing reliability and a better chemical and thermal stability to be achieved. The Applicant has now surprisingly found, and is the object of the present invention, that these and further purposes are achieved by means of a co-vulcanizable elastomeric composition comprising: A) 55-85% by weight of a fluoroelastomer composed by 40-85 mol % of vinylidene fluoride, 15-35 mol % of perfluoropropene and 0-30 mol % of tetrafluoroethylene; B) 15-45% by weight of a copolymer constituted by 40-60 mol % of tetrafluoroethylene and 40-60 mol % of propylene; C) 0.7-2.5 parts, per 100 parts of (A)+(B) blend, of a dihydroxy organic compound; D) 0.3-1.5 parts, per 100 parts of (A)+(B) blend, of a quaternary ammonium, phosphonium or aminophosphonium compound; E) 0.4-5 parts, per 100 parts of (A)+(B) blend, of an organic peroxide; F) 1-5% by weight, as referred to 100 parts of (A)+(B) blend, of a peroxidic cross-linking coagent. 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 found on 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 found on the market under the tradename AFLAS. The dihydroxy organic compounds as of item (C)are compounds known in the cross-linking of elastomers based on vinylidene fluoride, and can be selected among di-, tri-, and tetra-hydroxy-benzene, di-, tri- and tetra-hydroxy-naphthalene, di-, tri- and tetrahydroxy-anthracene and bisphenols of general formula wherein A is a difunctional aliphatic, cycloaliphatic or aromatic radical having a number of carbon atoms comprised within the range of from 1 to 13, or a thio, oxy, carbonyl, sulphinyl or sulphonyl radical, and wherein A can be optionally substituted with at least one chlorine or fluorine atom; p can be either 0 or 1; n can be either 1 or 2. Any aromatic ring of polyhydroxy compound can be optionally substituted with at least a chlorine, fluorine, bromine atom, with a -CHO group, or with a carboxylic or carbonyl radical of general formula -COR, with R being equal to -OH or to an alkyl, aryl or cycloalkyl radical with a number of carbon atoms comprised wihin the range of from 1 to 8. In formula (1), the -OH groups can be bound in any position on both rings. Examples of products of formula (1) are described in U.S. Pat. 3,876,654. Preferred products for the elastomeric composition of the present invention can be hydroquinone and bisphenol AF (hexafluoroisopropylidene-bis(4-hydroxyphenol). The compounds as of item (D) are known as accelerator agents in the cross-linking of elastomers based on vinylidene fluoride, and can be: D1 - quaternary ammonium compounds selected among those having general formula [N R₁ R₂ R₃ R₄] q ⁺ X (-)q (2) wherein R₁, R₂, R₃ and R₄, equal to or different from each other, can be an alkyl, fluoroalkyl, aralkyl, polyoxyalkenyl or polyoxyfluoroalkenyl group with a number of carbon atoms comprised within the range of from 1 to 20, with the proviso that at least two of R₁, R₂, R₃ and R₄, are an alkyl or aralkyl group; R₅ can be an alkyl group with a number of carbon atoms comprised within the range of from 1 to 20; X can be a hydroxide, a halide, a sulphate, a sulphite, a carbonate, a pentachlorothiophenolate, a tetrafluoroborate, a hexafluorosilicate, a hexafluorophosphate, a dimethyl phosphate, or a carboxylate or dicarboxylate of an alkyl or aralkyl or aryl group having from 1 to 20 carbon atoms; q is either 1 or 2 and represents the valence of X. D2 - quaternary phosphonium compounds of general formula [P R₆ R₇ R₈ R₉] + / r Z (-)r (5) wherein P can be, besides phosphorus, also arsenic or antimonium, and wherein R₆, R₇, R₈ and R₉ can be selected among an alkyl, aryl, aralkyl, alkenyl group, with a number of carbon atoms comprised within the range of from 1 to 20, and possibly substituted with chlorine, fluorine, bromine, cyano, -OR₁₀ and -COOR₁₀ or similar substituents, wherein R₁₀ can be an alkyl, aryl, arylalkyl or alkenyl group with a number of carbon atoms comprised within the range of from 1 to 20; Z can be a halide, a sulphate, a sulphite, a carbonate, a pentachlorothiophenolate, a tetrafluoroborate, a hexafluorosilicate, a hexafluorophosphate, a dimethyl phosphate, or a carboxylate or a dicarboxylate of an alkyl or aralkyl group or of an alkenyl group having a number of carbon atoms comprised within the range of from 1 to 20; r is either 1 or 2 and represents the valence of Z. D3 - compounds of aminophosphonium of general formula [P (N R' R'') s R''' 4-s ]⁺ Y m- (6) wherein R', R'' and R''', equal to or different from each other, can be alkyl, cycloalkyl, aryl, aralkyl, oxyalkyl or polyoxyalkyl radicals with a free or etherified end hydroxy group, containing from 1 to 18 carbon atoms and optionally containing, as substituents, halogens, cyanides, hydroxy groups or carbalkoxy groups, and wherein R' and R'' can be linked to each other by a nitrogen atom, to form a heterocyclic ring; s is an integer comprised between 1 and 4, m is an integer from 1 to 3 equal to the valence of ion Y and Y is an either organic or inorganic anion having valence m. The products having general formula from (2) to (4) are described in greater detail in U.K. Pat. 1,356,344; compounds (5) are described in U.S. Pat. 3,876,654 and compounds of formula (6) in U.K. Pat. 2,010,285. The organic peroxides as of item (E) are those usually employed for the peroxidic cross-linking of elastomers; they are described together with the peroxidic cross-linking coagents (F), such as triallylisocyanurate (TAIC), in Rubber Chemistry and Technology, vol. 51, page 940, 1978. Another peroxidic cross-linking coagents is, e.g., tetramethyl-tetravinyl-cyclotetrasiloxane. Examples of organic peroxides which can be used in the compositions of the present invention comprise 2,5-dimethyl-2,5-di-tert.butylperoxyhexane and the corresponding derivative of hexine. The operating conditions for the mix preparation and vulcanization are within 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 and to the mixture water-ethylene glycol 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; AFLAS (R) 150, a product of Asahi Glass, a copolymer containing equimolecular amounts of tetrafluoroethylene and propylene; Bisphenol AF: hexafluoroisopropylidene-bis(4-hydroxyphenol); BDDPC1: benzyldiphenyldiethylaminophosphonium chloride; Luperco (R) XL 101 = 2,5-dimethyl-2,5-di-tert-butylperoxyhexane. TAIC = triallylisocyanurate. to which the conventional additives for these types of mixes, such as, e.g., magnesium or lead oxides, calcium hydroxide, carbon black, carnauba wax, are added. 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 10 minutes and in oven at 250°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 composition of fluoroelastomers comprising: A) 55-85% by weight of a fluoroelastomer constituted by 40-85 mol % of vinylidene fluoride, 15-35 mol % of perfluoropropene and 0-30 mol % of tetrafluoroethylene; B) 15-45% by weight of a copolymer constituted by 40-60 mol % of tetrafluoroethylene and 40-60 mol % of propylene; C) 0.7-2.5 parts, per 100 parts of (A)+(B) blend, of a dihydroxy organic compound; D) 0.3-1.5 part, per 100 parts of (A)+(B) blend, of a quaternary ammonium, phosphonium or aminophosphonium compound; E) 0.4-5 parts, per 100 parts of (A)+(B) blend, of an organic peroxide; F) 1-5% by weight, as referred to 100 parts of (A)+(B) blend, of a peroxidic cross-linking coagent. Co-vulcanizable composition of fluoroelastomers according to claim 1, wherein the dihydroxy organic compound is selected among di-, tri-, and tetra-hydroxybenzene, di-, tri- and tetra-hydroxy-naphthalene, di-, tri- and tetra-hydroxy-anthracene and bisphenols of general formula wherein A is a difunctional aliphatic, cycloaliphatic or aromatic radical having a number of carbon atoms comprised within the range of from 1 to 13, or a thio, oxy, carbonyl, sulphinyl or sulphonyl radical, and wherein A can be optionally substituted with at least one chlorine or fluorine atom; p can be either 0 or 1; n can be either 1 or 2, any aromatic ring of polyhydroxy compound can be optionally substituted with at least a chlorine, fluorine, bromine atom, with a -CHO group, or with a carboxylic or carbonyl radical of general formula -COR, with R being equal to -OH or to an alkyl, aryl or cycloalkyl radical with a number of carbon atoms comprised wihin the range of from 1 to 8. Co-vulcanizable composition of fluoroelastomers according to the preceding claims, wherein the quaternary ammonium compound is selected among those having general formula wherein R₁, R₂, R₃ and R₄, equal to or different from each other, can be an alkyl, fluoroalkyl, aralkyl, polyoxyalkenyl or polyoxyfluoroalkenyl group with a number of carbon atoms comprised within the range of from 1 to 20, with the proviso that at least two of R₁, R₂, R₃ and R₄, are an alkyl or aralkyl group; R₅ can be an alkyl group with a number of carbon atoms comprised within the range of from 1 to 20; X can be a hydroxide, a halide, a sulphate, a sulphite, a carbonate, a pentachlorothiophenolate, a tetrafluoroborate, a hexafluorosilicate, a hexafluorophosphate, a dimethyl phosphate, or a carboxylate or dicarboxylate of an alkyl or aralkyl or aryl group having from 1 to 20 carbon atoms; q is either 1 or 2 and represents the valence of X. Co-vulcanizable composition of fluoroelastomers according to claims 1 and 2, wherein the quaternary phosphonium compound is selected among those having general formula [P R₆ R₇ R₈ R₉] + / r Z (-)r (5) wherein P can be, besides phosphorus, also arsenic or antimonium, and wherein R₆, R₇, R₈ and R₉ can be selected among an alkyl, aryl, aralkyl, alkenyl group, with a number of carbon atoms comprised within the range of from 1 to 20, and optionally substituted with chlorine, fluorine, bromine, cyano, -OR₁₀ and -COOR₁₀ or similar substituents, wherein R₁₀ can be an alkyl, aryl, arylalkyl or alkenyl group with a number of carbon atoms comprised within the range of from 1 to 20; Z can be a halide, a sulphate, a sulphite, a carbonate, a pentachlorothiophenolate, a tetrafluoroborate, a hexafluorosilicate, a hexafluorophosphate, a dimethyl phosphate, or a carboxylate or a dicarboxylate of an alkyl or aralkyl group or of an alkenyl group having a number of carbon atoms comprosed within the range of from 1 to 20; r is either 1 or 2 and represents the valence of Z. Co-vulcanizable composition of fluoroelastomers according to claims 1 and 2, wherein the quaternary aminophosphoniun compound is selected among those of general formula [P (N R' R'') s R''' 4-s ] + / m Y m- (6) wherein R', R'' and R''', equal to or different from each other, can be alkyl, cycloalkyl, aryl, aralkyl, oxyalkyl or polyoxyalkyl radicals with a free or etherified end hydroxy group, containing from 1 to 18 carbon atoms and optionally containing, as substituents, halogens, cyanides, hydroxy groups or carbalkoxy groups, and wherein R' and R'' can be linked to each other by a nitrogen atom, to form a heterocyclic ring; s is an integer comprised between 1 and 4, m is an integer from 1 to 3 equal to the valence of ion Y and Y is an either organic or inorganic anion having valence m. Co-vulcanizable composition of fluoroelastomers according to any of preceding claims, wherein the organic peroxide is 2,5-dimethyl-2,5-di-tert.butylperoxyhexane or the corresponding derivative of hexine. Co-vulcanizable composition of fluoroelastomers according to any of preceding claims, wherein the peroxidic cross-linking co-agent is triallylisocyanurate.

Inventors: CIRILLO, GIANNA, COSTANZI, FABIO, GERI, SERGIO, MOGGI, GIOVANNI
Applicants: AUSIMONT S.P.A.
Application year: 1986
Main classification: C08L27
IPC codes: C08L27, C08K5, C08L33, C08L57, C08L23, C08L101, C08L1