VULCANIZABLE COMPOSITIONS OF FLUOROELASTOMERS EXHIBITING IMPROVED PROCESSABILITY AND CHEMICAL STABILITY

Abstract: Vulcanizable compositions of fluoroelastomers based in vinylidene fluoride and one or more monomers containing an ethylenic unsaturation and at least one fluorine atom, exhibiting improved processability and chemical stability and comprising, as a processing coadjuvant, a mono- or dihydroxypolyfluoroether having an average molecular weight >/= 400 (monohydroxypolyfluoroethers) and >/=2500 (dihydroxypoly­ fluoroethers), respectively.

Description: The present invention relates to new processing coadjuvants for improving the processability and chemical stability of vulcanizable compositions of fluoroelastomers. In particular, the invention relates to the use of mono-and di-hydroxypolyfluoroethers as plasticizers in vulcanizable compositions of fluoroelastomers based on vinylidene fluoride. Vulcanized elastomers based on vinylidene fluoride copolymers, in which vinylidene fluoride is copolymerized with one or more monomers containing an ethylenic unsaturation and at least one fluorine atom are well-known and are widely utilized for a range of applications which require an exceptional chemical stability towards solvents, lubricants, fuels, acids and the like. The vulcanized articles obtained from such elastomeric polymers are most suitably utilized as sealing gaskets in general, both in static and dynamic conditions, in the automotive, aeronautic, missile, naval, mechanical and chemical sectors. They are also used to make various supports impermeable, such as protective clothes against aggressive chemical agents. According to the most advanced prior art, the vulcanizable compositions of fluoroelastomers based on vinylidene fluoride comprise: (A) A copolymer of vinylidene fluoride with at least one other fluorinated monomer containing an ethylenic unsaturation; (B) A vulcanization accelerator selected from quaternary ammonium salts, quaternary phosphonium salts and phosphoranamine derivatives; (C) One or more basic acceptors (basic agents which are acceptors of acid) such as, for example, MgO and Ca(OH)₂; (D) A polyhydroxylated cross-linking agent. Examples of these compositions are described in GB-A-1 356 344 and US-A-3 876 654 and 4 259 463. The following requirements have recently gained importance in the field of fluorinated elastomers: 1. A general improvement of the rheological properties and processability of the mixtures. 2. A reduction of the hardness of the mixtures (with respect to the value of Shore hardness A - 75, reported by Kirk Othmer - III ed. (1980), Vol. 8, page 509, as being typical for a fluoroelastomer). 3. An easy extrusion of articles with a complicated profile and of considerable size. 4. The production of vulcanized articles with improved chemical stability towards methanol and fluid lubricants. 5. An improvement of the low temperature properties without affecting the thermal stability at high temperatures. The use of processing coadjuvants in order to overcome poor extrudability of the mixtures and to avoid their adhesion to the mould and the soiling of the moulds is well-known. The processing coadjuvants are products acting as plasticizers such as vegetable waxes, low molecular weight polyethylenes, stearates, polyesters, and the like. By using such plasticizers, it is possible to achieve the aims 1 to 3 mentioned above. However, these plasticizers of the art exhibit various drawbacks such as impairment of the elastic properties, lower heat stability and high compression set values. It has now, surprisingly, been found that, with compositions according to the present invention, aims 1 to 5 can be achieved. Thus, it is the object of the present invention to provide additives of the mono- and di-hydroxypolyfluoroethers class, comprising fluorooxyalkylene repeating units selected from the following classes: Mono-hydroxypolyfluoroethers: Fluoropolyethers having a molecular weight ranging from 400 to 10 000. 1) (C₃F₆O) (CFXO) where X is -F or -CF₃; said units being statistically distributed along the fluoropolyoxyalkylene chain; 2) (C₃F₆O); 3) (CH₂CF₂CF₂O) Di-hydroxypolyfluoroethers: Fluoropolyethers having a molecular weight from 2 500 to 10 000. 4) (C₂F₄O), (CF₂O), said units being statistically distributed along the fluoropolyoxylalkylene chain; 5) (C₃F₆O), (C₂F₄O), (CFXO), where X is -F or -CF₃, said units being statistically distributed along the fluoropolyoxyalkylene chain; 6) (CH₂ - CF₂ - CF₂ -O), said units being linked within the fluoropolyoxyalkylene chain in the following manner: -(O-CF₂-CF₂-CH₂) p - O - R′ f - O -(CH₂CF₂-CF₂-O) q - where R' f is a fluoroalkylene group of 1 to 8 carbon atoms and p and q are integers, p + q having a value of 9 to 38; 7) said units being linked within the fluoropolyoxyalkylene chain in the following manner: where R f is a fluoroalkylene group of 1 to 8 carbon atoms, x is 0 or 1, a + b are integers, a + b having a value of 12 to 50. The hydroxylated compounds of the indicated classes may also be present as mixtures. Monohydroxypolyfluoroethers of class 1 are, in particular, selected from those having the following general formula: CF₃-(C₃F₆O) m' (CFXO) n' - CFX -ZH with where D and E are the same or different and are selected from -H, -CH₃ and -CF₃, X is -F or -CF₃ and m' and n' are positive integers having values of 1 to 40 such that the m'/n' ratio ranges from 1 to 20. These compounds may be prepared by the process described in US-A-3 513 203 and 3 847 978, the -COF end group being converted into the -OH group according to US-A-3 810 174. Monohydroxypolyfluoroethers of class 2) are, in particular, selected from those having the following general formula: wherein r' is an integer having a value of 1 to 45 and Z is as defined above. These products are described in US-A-3 242 218, wherein the group -COF is converted so as to obtain the end group -OH, as is described in US-A-3 847 978 and 3 810 874. Monohydroxypolyfluoroethers of class 3) are, in particular, selected from those having the following general formula: F(CH₂CF₂CF₂O) s' -CH₂CF₂ - ZH where s' is an integer having a value of 2 to 50 and Z is as defined above. These products are described in EP-A-148 482 in which the group -COF is converted so as to obtain the end group -OH, as is described in US-A-3 847 978 and 3 810 874. Fluoropolyoxyalkylene diols belonging to class 4) are, in particular, selected from those having the following general formula: HZ-CF₂O (C₂F₄O) m (CF₂O) n CF₂-ZH wherein Z is as defined above, m and n are integers such that (m + n) has a value of 24 to 100 and the ratio m/n ranges from 0.5 to 1.5, with the oxyfluoroalkylene units being statistically distributed along the chain. These compounds may, for example, be prepared according to the methods described in US-A-3 810 874. Fluorianted diols belonging to class 5) are, in particular, selected from the compounds having the following general formula: HZ-CF₂-(C₃F₆O) r (C₂F₄O) s (CFXO) t -CF₂-ZH wherein X is -F or -CF₃, r is an integer having a value of 2 to 8, s is an integer having a value of 10 to 40, t is an integer having a value of 10 to 40 and Z is as defined above. As described in US-A-3 665 041, such compounds are obtainable by photooxidation of mixtures of C₃F₆ and C₂F₄ and by subsequent conversion of end groups -COF into groups containing the end group -OH, said conversion being accomplished according to the known methods described in US-A-3 847 978 and 3 810 874. Fluorinated diols belonging to class 6) are, in particular, selected from the compounds having the general formula: HZ-CF₂CH₂(OCF₂CF₂CH₂ ) p -OR' f -O-(CH₂CF₂CF₂O) q CH₂CF₂-ZH wherein Z is as defined above, and are described in EP-A-148 482. The end groups -COF may be converted to end groups -OH as is described in US-A-3 847 978 and 3 810 874. Fluorinated diols belonging to class 7) are, in particular, selected from the compounds having the general formula: wherein A is a group comprising the end group -OH, for example CH₂OH, and n is an integer equal to or greater than 1. Such compounds are described in EP-A-151 877. The fluoroelastomers constituting component (A) are, generally, copolymers of vinylidene fluoride with hexafluoropropene, optionally containing tetrafluoroethylene. It is also possible to utilize chlorotrifluoroethylene, 1-hydropentafluoropropene and 2-hydropentafluoropropene instead of hexafluoropropene. Mixtures of the above-mentioned comonomers may also be employed. The tetrafluoroethylene and/or hexafluoropropene units in the copolymers may be partially or fully replaced by perfluoroalkylvinylether units. Generally, the amount of perfluoroalkylvinylether units ranges from 0.5 to 15 mole%, based on the total amount of copolymer. Preferably, it is lower than 5 mole%. Other fluorinated monomers may also be employed as comonomers, provided they contain a double bond of the ethylene type and at least one fluorine atom. Compounds (A) may be prepared as described in the literature (Kirk Othmer, Encyclopaedia of Chem. Technology, Vol. 8, pages 500 ff., 1979) by preferably operating in an aqueous emulsion and, optionally, in the presence of chain transfer agents like those described in US-A-4 000 356. The copolymers in which the amount of vinylidene fluoride ranges from 40 to 85 mole%, the amount of perfluoropropene ranges from 15 to 30 mole% and the amount of tetrafluoroethylene ranges from 0 to 30 mole% are particularly preferred. Accelerator (B) belongs to the classes previously mentioned, the preferred compounds being: From among the quaternary ammonium salts: methyltrioctylammonium chloride, laurylpyridinium bromide and benzyltrioctylammonium chloride; From among the quaternary phosphonium salts: benzyltriphenylphosphonium chloride, benzyltriphenylphosphonium tetrafluoroborate, methyltrioctylphosphonium acetate and carbethoxymethyl-triphenylphosphonium bromide; From among the phosphoranamine derivatives or aminophosphonium compounds: 1-chloro - 1-benzyl - 1,1-diphenyl-N(diethyl)-phosphoranamine, 1-tetrafluoroborate - 1-benzyl-N,N′,N″-(hexamethyl)phosphorantriamine, 1-bromo - 1-benzyl-1-phenyl-N,N′-(tetraethyl)phosphorandiamine. Component (C) is an inorganic compounds such as an oxide of a divalent metal, for example, ZnO, MgO, PbO, CaO or a mixture of the oxides and hydroxides of said metals or a salt of a weak acid, as is described in US-A-3 876 654. Component (D) is a polyhydroxylated compound known as a vulcanizing agent for fluoroelastomers such as, for example, hydroquinone, resorcin, 2,2′-bis(p-hydroxyphenyl)-hexafluoropropane (known as bisphenol AF) and 2,2′-bis(p-hydroxyphenyl)-propane (known as bisphenol A). Examples of these compounds are listed in US-A-3 876 654. Generally, per 100 parts by weight of an elastomeric copolymer of vinylidene fluoride (A) and one or more fluorinated monomers as indicated above the following are used: From 1 to 40 parts by weight of basic acceptor (C), consisting of one or more basic oxides of divalent metals, as indicated above, optionally, in the form of cationic complexes or chelates, optionally, in the presence of 0 to 10 parts by weight of one or more basic compounds, selected from the group comprising the calcium, strontium and barium hydrates and the metal salts of weak acids such as carbonates, benzoates and phosphates of calcium, strontium, barium, sodium and potassium, optionally, in the form of complexes with the usual cationic chelates or complexing agents of type well-known to those skilled in the art; From 0.5 to 15 parts by weight of mono- or de-hydroxypolyfluoroether or the invention as coadjuvant; From 0.2 to 4 parts by weight of vulcanization accelerator (B) selected from the classes described above; From 0.3 to 6 parts by weight of compounds (D). The vulcanizable compositions of the above-mentioned type are vulcanized by means of a process wherein said compositions are first heated under pressure at temperatures from 130 to 230°C, preferably from 160 to 200°C, for 0.5 to 60 minutes and, preferably, from 1 to 20 minutes. The articles obtained are then post-vulcanized in an oven or in a furnace at atmospheric pressure, at temperatures from 130 to 315°C, preferably from 200 to 275°C, for 5 to 48 hours, preferably from 10 to 24 hours. According to the present invention, small amounts (0.5 to 4 parts) of polyfluorethers not containing functional groups corresponding to the classes indicated for the mono-and di-hydroxypolyfluoroethers may be added to the fluoroelastomers. It has been found that said compounds, which do not contain functional groups, are compatible with the compositions of the invention, while they would not be compatible in the absence of the coadjuvants used in accordance with the invention, resulting in blooming phenomena. The improved results obtainable with the coadjuvants used in the present invention as compared with those of the art are due to the fact that, during the vulcanization process, a chemical reaction between the monohydroxylated or dihydroxylated fluoropolyether of the invention and the elastomeric chain takes place. The coadjuvants is thereby prevented from blooming or migrating during vulcanization and post-vulcanization. In fact, extraction tests with 1,1,2-trichloro-1,1,2-trifluorethane (F 113) at 40°C showed that, prior to vulcanization the hydroxylated fluoropolyether could be recovered from the mixture nearly quantitatively. Identical extraction tests conducted after vulcanization did not lead to the isolation of the hydroxylated fluoropolyether. The vulcanizable compositions according to the invention may, in addition to the above-mentioned substances (A) to (D) and in addition to the coadjuvant, contain carbon black and white coloured fillers, while the used of plasticizers and lubricants of the conventional type such as stearates, arylphosphates, polyethers, polyesters, polyethylene, sulphones, sulphoxides and other known additives which cause the above-mentioned drawbacks may be avoided. The components of the vulcanizable compositions of the present invention can, readily, be incorporated into the vinylidene fluoride elastomeric copolymer, either individually or premixed. In this way, it is possible to attain good vulcanization rates at the normal processing temperatures without encountering any risk of scorching (prevulcanization) during the initial processing steps which precede the actual vulcanization step. A further advantage of the present invention resides in the complete elimination of the undesired phenomenon of "reduced mould shrinkage" of fluorinated articles, particlarly O-ring gaskets, during vulcanization in a closed mould and under pressure. The following examples merely serve to illustrate the invention and do not limit it in any way. EXAMPLES 1 to 12 In the examples reported in tables 1 and 2 the following products were used: Fluoroelastomer 1 = Copolymer CH₂ = CF₂/C₃F₆ in a molar ratio of 4/1, having a specific weight of 1.8 at 25°C, known as Tecnoflon NM® (Montedison S.p.A). Fluoroelastomer 2 = Terpolymer CH₂ = CF₂/C₃F₆/C₂F₄ in a molar ratio of 3/1/1, having a specific weight of 1.86 at 25°C and a Mooney viscosity ML (1 + 4) of 120 at 100°C. Accelerator 1 = 1-chloro-1,1-diphenyl-1-benzyl - N-diethylphosphoranamine. Accelerator 2 = Benzyltriphenylphosphonium chloride. Fluoropolyether 1 of formula: CF₃O(C₃F₆O) m′ (CF₂O) n′ CF₂CH₂OH having an average molecular weight (based on the hydroxyl titer) of 600. Fluoropolyether 2 of the same general formula as fluoropolyether 1 but with a mean molecular weight of 1 500. The results of the tests reported in tables 1 and 2 show that the compositions of the invention lead to a decrease in hardness, to better results in the extrudability test, to better properties in the low temperatures test (brittle point) and in the chemical stability test. Examples 1, 4 and 8 are comparative tests.

Claims: Vulcanizable compositions of fluoroelastomers based on vinylidene fluoride exhibiting improved processability and chemical stability, comprising, as a processing coadjuvant, a mono- or di-hydroxypolyfluoroether having an average molecular weight of ≧ 400 (monohydroxypolyfluoroether) and ≧ 2500 (dihydroxypolyfluoroether) respectively, comprising fluorooxyalkylene repeating units selected from the following classes: Monohydroxypolyfluoroethers: 1) (C₃F₆O) (CFXO) wherein X is -F or -CF₃; said units being statistically distributed along the fluoropolyoxyalkylene chain; 2) (C₃F₆O); 3) (CH₂CF₂CF₂O); Dihydroxypolyfluoroethers : 4) (C₂F₄O), (CF₂O), said units being statistically distributed along the fluoropolyoxylalkylene chain; 5) (C₃F₆O), (C₂F₄O), (CFXO), where X is -F or -CF₃, said units being statistically distributed along the fluoropolyoxyalkylene chain; 6) CH₂ - CF₂ - CF₂ - O -, said units being linked within the fluoropolyoxyalkylene chain in the following manner: -(O-CF₂-CF₂-CH₂) p - O - R' f - O -(CH₂-CF₂-CF₂-O) q - wherein R' f is a fluoroalkylene group containing from 1 to 8 carbon atoms and p and q are integers such that (p+q) ranges from 9 to 38; 7) said units being linked within the fluoropolyoxyalkylene chain in the following manner: where R f is a fluoroalkylene group containing from 1 to 8 carbon atoms, x is 0 or 1 and a and b are integers such that (a+b) ranges from 12 to 50. Vulcanizable compositions according to claim 1, wherein the monohydroxypolyfluoroethers of class 1) are selected from those having the general formula: CF₃-(C₃F₆O) m' (CFXO) n' - CFX - ZH with where D and E are the same or different and are selected from -H, -CH₃ and -CF₃, X is -F or -CF₃ and m' and n' are positive integers of from 1 to 40 such that the m'/n' ratio ranges from 1 to 20. Vulcanizable compositions according to claim 1, wherein the monohydroxypolyfluoroethers of class 2) are selected from those having the general formula: where r' is an integer of from 1 to 45 and Z is defined as in claim 2. Vulcanizable compositions according to claim 1, wherein the monohydroxypolyfluoroethers of class 3) are selected from those having the general formula: F(CH₂CF₂CF₂O) s' -CH₂-CF₂ - Z-H where s' is an integer of from 2 to 50 and Z is as defined in claim 2. Vulcanizable compositions according to claim 1, wherein the fluoropolyoxyalkylene diols belonging to class 4) are selected from those having the general formula: HZ-CF₂O (C₂F₄O) m (CF₂O) n CF₂-ZH where Z is as defined in claim 2 and m and n are integers such that (m+n) is from 24 to 100 and the m/n ratio ranges from 0.5 to 1.5, with the oxyfluoroalkylene units being statistically distributed along the chain. Vulcanizable compositions according to claim 1, wherein the fluoropolyoxyalkylene diols belonging to class 5) are selected from those having the general formula: HZ-CF₂-(C₃F₆O) r (C₂F₄O) s (CFXO) t -CF₂-ZH where X is -F or -CF₃, r is an integer of from 2 to 8, s and t each are integers of from 10 to 40 and Z is as defined in claim 2. Vulcanizable compositions according to claim 1, wherein the fluoropolyoxyalkylene diols belonging to class 6) are selected from those having the general formula: HZ-CF₂CH₂(OCF₂CF₂CH₂) p -OR' f -O-(CH₂CF₂CF₂O) q CH₂CF₂-ZH where Z is as defined in claim 2. Vulcanizable compositions according to claim 1, wherein the fluoropolyoxyalkylene diols belonging to class 7) are selected from those having the general formula: where A is a group comprising the end group -OH, n is an integer equal to or greater than 1. Vulcanizable compositions according to any one of claims 1 to 8, wherein the fluoroelastomer is a copolymer of vinylidene fluoride, perfluoropropene and, optionally, tetrafluoroethylene. Vulcanizable compositions according to any one of claims 1 to 9, wherein the mono- or dihydroxypolyfluoroethers are present in amounts of from 0.5 to 15 parts by weight per 100 parts of fluoroelastomer. Vulcanizable compositions according to any one of claims 1 to 10, comprising polyfluoroethers not containing functional groups corresponding to the classes indicated for the mono- and dihydroxypolyfluoroethers, in amounts ranging from 0.5 to 4 parts by weight per 100 parts of fluoroelastomer.

Inventors: CIRILLO, GIANNA, DI FEDE, CINZIA, MOGGI, GIOVANNI, STREPPAROLA, EZIO
Applicants: AUSIMONT S.P.A.
Application year: 1986
Main classification: C08L27
IPC codes: C08L27, C08K5, C08L71