Covulcanizable compositions of fluoroelastomers having improved chemical stability

Abstract: Covulcanizable compositions comprising (A) 55-100% by weight of an elastomeric copolymer of vinylidene fluoride with perfluoropropene and, optionally, tetrafluoro­ ethylene and containing from 0.001-2% by weight of bromine; (B) 0-45% by weight of a copolymer of tetra­ fluoroethylene and propylene; (C) 0.7-2.5 parts per 100 parts of (A)+(B) of a bisphenol compound; (D) 0.3-1.5 parts per 100 parts of (A)+(B) of an organic quaternary compound of ammonium, phosphonium or aminophosphonium; (E) 0.4-5 parts per 100 parts of (A)+(B) of an organic peroxide; (F) 1-5 parts per 100 parts of (A)+(B) of a peroxide crosslinking coagent.

Description: The present invention relates to covulcanizable compositions of fluoroelastomers having an improved chemical stability. Fluoroelastomers based on vinylidene fluoride and vulcanized using a bisphenol or dihydroxyl system (Kirk Othmer, Encyclopaedia of Chemical Technology, Vol. 8, page 506, 1979) are excellent products which are utilized in various fields of industry, in particular in the sectors of motor vehicles, of oil and of energy in genera!. In the automotive sector, the fluoroelastomers are mainly utilized in the fuel circulation system or as components of parts of the motor and of parts of the gearbox-differential-driveshaft aggregate. In the first case there may be cited, as examples of appliances, diaphragms for feed pumps, valve seats, check valves, flexible hoses and, generally, sealing gaskets for the fuel. In the second case, analogous examples are gaskets for gears, sealing jackets for cylinders in diesel motors, gaskets for the crankshaft, check valves for the exhaust gases, gaskets such as oil seal rings. In the oil and energy field the fluoroelastomers are utilized for appliances which are in direct contact with the crude oil, particularly at high temperatures and pressures, such as e.g. seals, valves and pumps or parts thereof. In this field, an important utility for the fluoroelastomers is represented by the expansion joints used in the discharge lines of flue gases in thermoelectrical stations, where a high resistance to high temperatures and to the corrosive action of the acid gases is required. In these sectors, as well as in others, such as the electrical and electronic field, the fluoroelastomers are utilized because of their high thermal stability and chemical resistance even at high temperatures. In the last years, however, and in specific fields of use, particular technological requirements have arisen, which cannot be met by the presently available articles vulcanized by means of a bisphenol or dihydroxyl system. Particularly involved is the chemical stability under very severe operating temperature conditions in the presence of vapours, of mineral acids, of lubricating fluids characterized by a high aggressivity, such as the oils of series SF containing compounds which exhibit a high aggressivity towards the fluoroelastomers based on vinylidene fluoride. Furthermore, a general improvement of the processibility characteristics of the blends is required, in order to permit the molding of articles such as oil seal rings, and the possibility of easily extruding, even with complex profiles, the blends for obtaining suitable preforms. The problem of the chemical stability has been solved by the use of peroxide vulcanization. This vulcanization, however, does not meet the above-mentioned processibility requirements, in particular as regards molding and extrusion. Furthermore, the processibility itself proves to be less safe (Rubber Chemistry and Technology, Vol. 55, page 906, 1982;. and the results in the compression set test are also much poorer (Kirk Othmer, Encyclopaedia of Chemical Technology, Vol.8 page 510, 1979). EP-A-136 596 discloses covulcanized compositions which permit certain improvements but, nevertheless, under certain conditions exhibit less than optimum chemical, thermal and mechanical properties. Thus, it is an object of the present invention to provide a covulcanizable composition of fluoroelastomers which overcomes the shortcomings described above. More particularly, it is an object of the present invention to provide a covulcanizable composition of fluoroelastomers based on vinylidene fluoride which permits a higher processing safety, a higher chemical and thermal stability and better mechanical characteristics. In accordance with the present invention these and other objects are achieved by a covulcanizable elastomeric composition comprising: A) 55-100% by weight of a fluoroelastomer consisting of 35-80 mol % of vinylidene fluoride, 15-35 mol % of perfluoropropene, 0-30% of tetrafluoroethylene, and containing from 0.001 to 2% by weight of bromine; B) 0-45% by weight of a copolymer consisting of 40-60 mol % of tetrafluoroethylene and 40-60 mol % of propylene; C) 0.7-2.5 parts per 100 parts of (A)+(B) of an organic polyhydroxy (preferably dihydroxy) compound; D) 0.3-1.5 parts per 100 parts of (A)+(B) of a quaternary compound of ammonium, phosphonium or of aminophosphonium; E) 0.4-5 parts per 100 parts of (A)+(B) of an organic peroxide; F) 1-5 parts per 100 parts of (A)+(B) of a peroxide crosslinking coagent, a composition consisting of (a) 100 parts by weight of a copolymer consisting of 35% by weight of vinylidene fluoride, 34% by weight of -hexafluoropropylene 29% by weight of tetrafluoroethylene and 2% by weight of bromotetrafluorobutene; (b) 1.7 parts by weight of a composition consisting of 33% organophosphonium salt plus 67% fluoroelastomer; (c) 4.0 parts by weight of a composition consisting of 50% dihydroxy aromatic compound plus 50% fluoroelastomer; (d) 6 parts by weight of ZnO; (e) 2 parts by weight of Ca(OH) 2 ; (f) 60 parts by weight of MT black; (g) 25 parts by weight of silica; (h) 1.3 parts by weight of process aids; (i) 3 parts by weight of triallylisocyanurate; and (j) 3 parts by weight of 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane being excluded. The elastomers A) are known products and may be prepared according to the methods described in Kirk Othmer, Encyclopaedia of Chemical Technology, Vol. 8, page 500 ff, 1979. Bromine is introduced into the copolymer by using either a comonomer containing bromine such as bromotrifluoroethylene or ethylbromovinylether, or by using, as a chain transfer agent, a product of the general formula: C a W b Br c in which W may be fluorine or chlorine a is an integer from 1 to 4 c is an integer from to (2a + 2) b is equal to (2a + 2 - c). The chain transfer agent is added during the reaction in amounts ranging from 0.01 to 0.3 mol % based on the total number of moles of the components. The preferred chain transfer agent is CF 2 Br 2 . The copolymers B) are known in literature and may be prepared according to the methods described in Rubber Chemistry and Technology, Vol. 50, page 403, 1977, or are commercially available under the trade name AFLAS. The organic polyhydroxy or dihydroxy compounds C) are known compounds for the crosslinking of elastomers based on vinylidene fluoride and can be selected from di-, tri- and tetrahydroxy derivatives of benzene, naphthalene, or anthracene and bisphenols of the general formula wherein A is a difunctional aliphatic, cycloaliphatic or aromatic group containing up to 13 carbon atoms, or a thio, oxy, carbonyl, sulphinyl or sulphonyl group and wherein A may be optionally substituted by at least one chlorine or fluorine atom; p is 0 or 1; and n is 1 or 2. Any aromatic ring of the polyhydroxy compound can be optionally substituted by at least one chlorine, fluorine or bromine atom or by a group -CHO or by a carboxy or acyl group of general formula -COR, with R being OH or an alkyl, aryl or cycloalkyl group having up to 8 carbon atoms. In formula (1) the groups OH can be present in any position in both rings. Examples of products of formula (1) are described in US-A-3 876 654. Preferred compounds are hydroquinone and bisphenol AF. The compounds D) are known as accelerating agents for the crosslinking of elastomers based on vinylidene fluoride and can be, e.g.: D1 - quaternary compounds of ammonium selected from those having the general formulae: [ N R 1 R 2 R 3 R 4 ] + q X (-)q wherein R 1 , R 2 , R 3 and R 4 are equal to or different from each other and represent alkyl, fluoroalkyl, aralkyl, polyoxyalkene or polyoxyfluoroalkene groups containing up to 20 carbon atoms, provided that at least two out of R 1 , R 2 , R 3 and R 4 are an alkyl or aralkyl group; R 5 is an alkyl group containing 1 to 20 carbon atoms; X is an anion selected from hydroxyl, halide, sulphate, sulphite, carbonate, pentachlorothiophenolate, tetrafluoroborate, hexafluorosilicate, hexafluorophosphate, didmethylphosphate of a carboxylate or dicarboxylate with an alkyl or aralkyl or aryl group having up to 20 carbon atoms; q is 1 or 2 and represents the valence of X. D2 - quaternary phosphonium compounds of the general formula [P R 6 R 7 R 8 R 9 ] + Z -(r) wherein P may be, besides phosphorus, also arsenic and antimony, and wherein R 6 , R 7 , R 8 and R 9 are selected from an alkyl, aryl, arylalkyl, alkenyl group having up to 20 carbon atoms, chlorine, fluorine, bromine, cyano, -OR 10 and -COOR 10 or analogous substituted groups, R 10 being an alykl, aryl, arylalkyl or alkenyl group having up to 20 carbon atoms; Z is an anion selected from halide, sulphate, sulphite, carbonate, pentachlorothiophenolate, tetrafluoroborate, hexafluorosilicate, hexafluorophosphate, dimethyl phosphate, or a carboxylate or dicarboxylate with an alkyl or aralkyl or alkenyl group containing up to 20 carbon atoms; r is 1 or 2 and represents the valence of Z. D3 - aminophosphonium compounds of the general formula wherein R', R" and R"' are equal to or different from each other and represent alkyl, cycloalkyl, aryl, arylalkyl, oxyalkyl or polyoxyalkyl groups with a free or etherified hydroxy end group containing up to 18 carbon atoms and optionally containing, as substituents, a halogen, cyano, hydroxy or carboalkoxy group, wherein R' and R" may be linked to each other by means of a nitrogen atom to form a heterocyclic ring; s is an integer from 1 to 4, m is an integer from 1 to 3, equal to the valence of ion Y, and Y is an organic or inorganic anion having the valence m. The products of general formulae (2) to (4) are described in more detail in GB-A-1 356 344; the compounds of formula (5) are described in US-A-3 876 654 and the compounds of formula (6) are described in GB-A-2 010 285. The organic peroxides E) are generally used for the peroxide crosslinking of elastomers. They are described along with suitable peroxide crosslinking coadjuvants F), such as triallylisocyanurate (TAIC), in Rubber Chemistry and Technology, Vol. 51, page 940, 1978. Examples of organic peroxides utilizable in the compositions of the present invention are 2,5-dimethyl-2,5-di-t-butylperoxy-hexane and the corresponding 3-hexine derivative. The working conditions for preparing the blends and for vulcanizing them are substantially the same as those usually employed for fluoroelastomers, such as described in Kirk Othmer Encyclopaedia of Chemical Technology, Vol. 8, page 500 ff., 1979. The covulcanizable compositions of the present invention exhibit an excellent extrudabilitiy, an excellent stability to the oils SF and an excellent stability to water vapour and water glycol mixtures at high temperatures and, when injection-molded, they do not give rise to any problems connected with demoulding. The following examples are given in order to further illustrate the present invention without, however, limiting it in any way. Example 1 A fluoroelastomer (A) consisting of 50 mol % of vinylidene fluoride, 25 mol % of perfluoropropene, 25 mol % of tetrafluoroethylene and containing 0.12% by weight of bromine (Mooney viscosity ML (1+4) at 100°C = 90) was mixed in a cylinder mixer with a copolymer (B) containing equimolar amounts of tetrafluoroethylene and propylene. The weight ratio of fluoroelastomer (A) to copolymer (B) was equal to 70/30. During the mixing step, the following vulcanizing agents were added: Parts by weight Fluoroelastomer 70 Copolymer 30 Bisphenol AF 1.15 Accelerating agent 0.51 2,5-dimethyl-2,5-di-t-butylperoxy hexane 2 Triallylisocyanurate 2 Ca(OH) 2 3 MgO 5 Carnauba wax 1 Black MT 30 As an accelerating agent an aminophosphonium salt of formula was used. The mixture thus obtained was examined by means of a Monsanto rheometer according to ASTM D 2705 at 180°C, arc 5, 100 Hz, without preheating, and the following results were obtained: Minimum torque 20 Ts 10 2 min 21 sec Maximum torque (after 5 min) 64 The mixture was vulcanized in a press, in the form of O-rings and plates, at 170°C for 10 minutes, starting pressure 2.5 MPa, final pressure 17.5 MPa, and was subjected to a postvulcanization in an oven at 250°C for 24 hours. The following mechanical characteristics were obtained: 100% modulus 6.6 MPa tensile strength 14.1 MPa elongation at break 272 % hardness 79 Shore A compression set (O-ring) 40 % The vulcanized test pieces were subjected to a chemical resistance test in BP Olex (oil SF) at 150°C for 21 days. The results were as follows: 100% modulus variation + 11.8 % tensile strength variation - 6 % elongation at break variation - 25 % hardness variation - 2 Shore A Example 2 A fluoroelastomer consisting of 50 mol % of vinylidene fluoride, 25 mol % of perfluoropropene, 25 mol % of tetrafluoroethylene and containing 0.12% by weight of bromine (Mooney ML viscosity (1+4) at 100°C = 90) was mixed in a cylinder mixer with the following vulcanizing agents: Parts by weight Fluoroelastomer 100 Bisphenol AF 1.15 Accelerating agent 0.51 2,5-dimethyl-2,5-di-t-butylperoxy hexane 2 Triallylisocyanurate 2 Ca(OH) 2 3 MgO 5 Carnauba wax 1 Black MT 30 The same product as in example 1 was utilized as an accelerating agent. The mixture thus obtained was examined by means of a Monsanto rheometer according to ASTM D 2705 at 100°C, arc 5, 100 Hz, without preheating and the following results were obtained: Minimum torque 18 Ts 10 2 min Maximum torque 74 The mixture was vulcanized in a press, in the form of O-rings and plates, at 170°C for 10 minutes, initial pressure 2,5 MPa, final pressure 17.5 MPa, and was subjected to a postvulcanization in an oven at 250°C for 24 hours. The following mechanical characteristics were obtained: 100% modulus 5.8 MPa Tensile strength 18.5 MPa Elongation at break 271 % Hardness 78 Shore A Compression set (0-ring) 38 % The vulcanized test pieces were subjected to a chemical resistance test in BP olex (oil SF) at 150°C for 21 days. The results were as follows: 100% modulus variation + 7 % Tensile strength variation - 14% Elongation at break variation - 30 % Hardness variation - 2 Shore A

Claims: Covulcanizable compositions of fluoroelastomers comprising: A) 55-100% by weight of a fluoroelastomer consisting of 40-85 mol % of vinylidene fluoride, 15-35 mol % of perfluoropropene, 0-30 mol % of tetrafluoroethylene, and containing from 0.001 to 2% by weight of bromine; B) 0-45% by weight of a copolymer consisting of 40-60 mol % of tetrafluoroethylene and 40-60 mol % of propylene; C) 0.7-2.5 parts per 100 parts of (A)+(B) of an organic polyhydroxy compound; D) 0.3-1.5 parts per 100 parts of (A)+(B) of a quaternary ammonium, phosphonium or aminophosphonium compounds; E) 0.4-5 parts per 100 parts of (A)+(B) of an organic peroxide; F) 1-5 parts per 100 parts of (A)+(B) of a peroxide crosslinking coagent, a composition consisting of (a) 100 parts by weight of a copolymer consisting of 35% by weight of vinylidene fluoride, 34% by weight of hexafluoropropylene, 29% by weight of tetrafluoroethylene and 2% by weight of bromotetrafluorobutene; (b) 1.7 parts by weight of a composition consisting of 33% organophosphonium salt plus 67% fluoroelastomer; (c) 4.0 parts by weight of a composition consisting of 50% dihydroxy aromatic compound plus 50% fluoroelastomer; (d) 6 parts by weight of ZnO; (e) 2 parts by weight of Ca(OH) 2 ; (f) 60 parts by weight of MT black; (g) 25 parts by weight of silica; (h) 1.3 parts by weight of process aids; (i) 3 parts by weight of triallyl isocyanurate; and (j) 3 parts by weight of 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane being excluded. A composition according to claim 1 wherein bromine has been introduced into the fluoroelastomer by using, as a chain transfer agent during copolymerization, a product of the general formula C a W b Br c wherein W is fluorine or chlorine a is an integer from 1 to 4 c is an integer from 1 to (2a + 2) b is equal to (2a + 2 - c). A composition according to claim 2, wherein the chain transfer agent has been added during the reaction in an amount ranging from 0.01 to 0.3 mol %, based on the total number of moles of the components. A composition according to claims 2 and 3, wherein the chain transfer agent is CF 2 Br 2 . A composition according to any one of claims 1 to 4, wherein the organic polyhydroxy compound is selected from di- tri- and tetrahydroxy derivatives of benzene naphthalene or anthracene and bisphenols of the general formula wherein A is a difunctional aliphatic, cycloaliphatic or aromatic group containing up to 13 carbon atoms or a thio, oxy, carbonyl, sulphinyl or sulphonyl group and wherein A may be optionally substituted by at least one chlorine or fluorine atom; p is 0 or 1; n is 1 or 2; and any aromatic ring of the polyhydroxy compound may be optionally substituted by at least one substituent selected from chlorine, fluorine or bromine atoms or a -CHO group or a carboxy or acyl group of the general formula -COR, R being OH or an alkyl, aryl or cycloalkyl group having up to 8 carbon atoms. A composition according to any one of claims 1-5, wherein the quaternary ammonium compound is selected from those having the general formulae: [ N R 1 R 2 R 3 R 4 ] + q X (-)q wherein R 1 , R 2 , R 3 and R 4 are equal to or different from each other and represent an alkyl, fluoroalkyl, aralkyl, polyoxyalkylene or polyoxyfluoroalkene group having up to 20 carbon atoms, provided that at least two out of R 1 , R 2 , R 3 and R 4 are an alkyl or aralkyl group; R 5 is an alkyl group having 1 to 20 carbon atoms; X is an anion selected from hydroxy, halide, sulphate, sulphite, carbonate, pentachlorothiophenolate, tetrafluoroborate, hexafluorosilicate, hexafluorophosphate, dimethyl phosphate, or a carboxylate or dicarboxylate with an alkyl or aralkyl or aryl group having up to 20 carbon atoms; q is 1 or 2 and represents the valence of X. A composition according to any one of claims 1-5, wherein the quaternary phosphonium compound is selected from those having the general formula [P R 6 R 7 R 8 R 9 ] + r Z -(r) wherein P may be, besides phosphorus also, arsenic and antimony, and wherein R 6 , R 7 , R 8 and R 9 are selected from an alkyl, aryl arylalkyl, alkenyl group having up to 20 carbon atoms, chlorine, fluorine, bromine, cyano, -OR 10 and -COOR 10 or analogous substituted groups, R 10 being an alkyl, aryl, arylalkyl or alkenyl group containing up to 20 carbon atoms; Z is an anion selected from halide, sulphate, sulphite, carbonate, pentachlorothiophenolate, tetrafluoroborate, hexafluorosilicate, hexafluorophosphate, dimethyl phosphate, or a carboxylate or dicarboxylate with an alkyl or aralkyl or alkenyl group having up to 20 carbon atoms; r is 1 or 2 and represents the valence of Z. A composition according to any one of claims 1-5, wherein the quaternary aminophosphonium compound is selected from those having the general formula wherein R', R" and R"' are equal to or different from each other and represent an alkyl, cycloalkyl, aryl, arylalkyl, oxyalkyl or polyoxyalkyl group with a free or etherified hydroxy end group, containing up to 18 carbon atoms and, optionally, containing, as substituents, one or more halogen atoms, cyano, hydroxy or carboalkoxy groups, and wherein R' and R" may be linked to each other by a nitrogen atom to form a heterocyclic ring; s is an integer from 1 to 4, m is an integer from 1 to 3, equal to the valence of Y, and Y is an organic or inorganic anion of valence m. A composition according to any one of claims 1 to 8, wherein the organic peroxide is 2,5-dimethyl-2,5-di-t-butylperoxy hexane or the corresponding derivative of 3-hexine. A composition according to any one of claims 1 to 9, wherein the peroxide crosslinking coagent is triallylisocyanurate.

Inventors: ARCELLA VINCENZO, BRINATI GIULIO, CIRILLO GIANNA, COSTANZI FABIO, TOMMASI GIULIO, ARCELLA, VINCENZO, BRINATI, GIULIO, CIRILLO, GIANNA, COSTANZI, FABIO, TOMMASI, GIULIO
Applicants: AUSIMONT SPA, AUSIMONT S.P.A.
Application year: 1986
Main classification: C08L27
IPC codes: C08K5, C08F2, C08L27