Tetrafluoroethylene thermoprocessable copolymers

Abstract: Tetrafluoroethylene (TFE) thermoprocessable copolymers comprising: (A) from 0.1 to 15% by moles of a fluorodioxol of formula: wherein R F is a perfluoroalkyl having from 1 to 5 carbon atoms; X 1 and X 2 , equal to or different from each other, are -F or -CF 3 ; Z is selected from -F, -H, -Cl. (B) from 0 to 12% by moles of a perfluorinated monomer selected from hexafluoropropene (HFP) and perfluoropropylvinylether (PPVE), or their mixtures; (C) TFE forming the remaining part to 100%; with the proviso that the total amount of the monomers (A) and (B) is lower than or equal to 20% by moles.

Description: The present invention relates to tetrafluoroethylene thermoprocessable copolymers. More particularly, the present invention relates to tetrafluoroethylene thermoprocessable copolymers with a fluorodioxole having improved mechanical properties, in particular at high temperatures, combined with superior optical properties. It is known that polytetrafluoroethylene (PTFE) shows a very high melting viscosity, whereby it cannot be worked according to the techniques usually employed for thermoprocessable polymers (extrusion, molding, injection, etc.). To obviate this drawback it is known to copolymerize tetrafluoroethylene (TFE) with hexafluoropropene (HFP) (see for instance US patent 2,946,763 ), or to copolymerize TFE with a fluorodioxol and with perfluoromethylvinylether (PPVE) (see for instance the European patent application No. 94109782,6). In order to succeed in combining acceptable mechanical properties with a melting low viscosity and therefore with good processability, it is necessary to introduce high HFP amounts, generally around 7-11% by moles. However, so high amounts of HFP cause a significant fall in the second melting temperature and consequently a clear worsening of properties at high temperature, in particular tensile and creep properties. Therefore the continuous working temperature falls from values of about 260°C for PTFE to about 200°C for TFE/HFP copolymers. Other TFE thermoprocessable copolymers are those wherein TFE is copolymerized with a perfluoroalkylvinylether (PAVE), and in particular with perfluoropropylvinylether (PPVE) (see for instance US patent 3,635,926 ). Such copolymers allow to obtain a satisfactory balance between mechanical properties and processability with PPVE amounts around 2-3% by moles. Finally to try to obviate the drawbacks described above for the TFE/HFP copolymers, terpolymers have been suggested wherein TFE is copolymerized with 4-12% by weight of HFP and 0.5-3% by weight of PPVE (see US Patent 4,029,868 ). Even though the employment of PPVE has substantially raised the TFE thermoprocessable copolymers properties, in particular with reference to the maximum working temperature and to the processability, thus greatly enlarging the a-pplication field, it is however felt the need on the one hand to further broaden the properties range and on the other hand to make easier the conditions of the copolymer synthesis. It is indeed known that PPVE shows poor reactivity and needs complex processes of recovering of the unconverted monomer (see for instance GB patent 1,514,700 ). Fluorinated dioxols of various types are known, having general formula: wherein Y 1 , Y 2 are -H, -F or -Cl: Y 3 , Y 4 are -F or -CF 3 (see for instance US patents 3,865,845, EP 76,581, EP 80,187, EP 95,077, EP 73,087 ). Such compounds can be used for preparing homopolymers or copolymers with other fluorinated monomers. In particular both amorphous and crystalline copolymers are described and prepared between the above mentioned dioxols and TFE (see for isntance EP Patents 73,087, EP 95,077, US 4,558.141). In European patent application No. 94109782.6 (EP-A-0663251) in the name of the Applicant new fluorodioxols of formula: are described, wherein R F is a perfluoroalkyl having from 1 to 5 carbon atoms; X 1 and X 2 , equal to or different from each other, are -F or -CF 3 . Such dioxols can be used for preparing homopolymers and copolymers with other fluorinated monomers. In particular TFE thermoprocessable terpolymers are described wherein TFE is copolymerized with a fluorodioxol of formula (I) and with perfluoromethylvinylether. In examples 9 and 10 crystalline copolymers are moreover prepared from TFE and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxol, the latter being present in amounts equal to 1.1% and 3.3% by moles respectively. The copolymerization process was carried out in solution in oganic solvent (CCl 2 FCF 2 Cl). The copolymers thus obtained have shown a very high melting viscosity, with a Melt Flow Index value unmeasurable and therefore not thermoprocessable. No characterization was supplied as regards the optical properties of such copolymers. The Applicant has now surprisingly found that the copolymerization of a fluorodioxol as hereinunder defined with TFE, in association with HFP and/or a perfluoroalkylvinylether, for instance perfluoropropylvinylether and perfluoromethylvinylether, allows to obtain TFE thermoprocessable polymers with improved mechanical properties, in particular at high temperatures. Such properties are combined with superior optical properties, as shown by radiation transmission measurements (transmittance and haze). This combination of properties results unexpectedly higher than the one obtainable with known modifying comonomers, in particular with respect to the known fluorodioxols. The fluorodioxols of the present invention determine, with the same amount introduced, a more marked fall of the second melting temperature surprisingly combined with a maintenance of the tensile properties and of resistance to brittle fracture at high temperatures. Conversely, experiments carried out by the Applicant have unexpectedly shown that the fluorodioxols of the present invention have a higher modifying power than the known fluorodioxols of the prior art, and this allows to prepare copolymers having a lower content of modifying comonomer, which, the second melting temperature being equal, show superior engineering properties, in particular yield stress and improved heat creep values. The reactivity of the fluorodioxols of the present invention is so high that there is no need of complex processes for recovering the monomer. Subject matter of the present invention are therefore tetrafluoroethylene (TFE) thermoprocessable copolymers comprising: (a) from 0.1 to 15%, preferably from 0.5 to 9%, by moles of a fluorodioxol of formula wherein R F is a perfluoroalkyl having from 1 to 5 carbon atoms; X 1 and X 2 , equal to or different from each other, are -F or -CF 3 ; Z is selected from -F, -H, -Cl; (b) from 0 to 15%, preferably from 0 to 10% by moles of a perfluorinated monomer selected from hexafluoropropene (HFP) and perfluoroalkylvinylethers of formula CF 2 =CF-OR' f , wherein R' f is a perfluoroalkyl C 2 -C 4 , or their blends; (c) TFE, forming the remaining part to 100%; with the proviso that the total amount of the monomers (a) and (b) is lower than or equal to 20% by moles, preferably lower than or equal to 12% by moles and excluding the value O for the amount of the monomer b. Preferably, in formula (I) X 1 , X 2 and Z are -F, R f is preferably -CF 3 , -C 2 F 5 , or -C 3 F 7 . Fluorodioxols of formula (I) wherein R f is -CF 3 or -C 2 F 5 and X 1 , X 2 and Z are -F, are particularly preferred. Among the perfluoroalkylvinylethers of formula CF 2 =CF-OR' f , perfluoropropylvinylether (PPVE) is particularly preferred. The fluorodioxols of formula (I) are described in European patent application No. 94109782.6 in the name of the Applicant. In case Z is -F, they can be prepared by the following process, comprising: (a) to react at a temperature comprised between -140° and +60°C (preferably between 110° and -20°C) a dioxol of formula: wherein X 1 and X 2 have the meaning indicated above, with a fluoroxy-compound of formula R f OF, wherein R f has the meaning indicated above, thus obtaining a dioxolane of formula: (b) to dehalogenate the dioxolane (III) according to known techniques, by reacting it with a metal in an aprotic dipolar solvent. The dioxols of formula (II) are known compounds; they can be prepared for instance according to EP Patent application 460,946 . Also fluoroxycompounds R f -OF are known products. CF 3 OF can be prepared, for instance, according to what is described by G.H.Cady and K.B.Kellogg in J. Am. Chem. Soc. 70, 3986, 1948, the higher homologues by the process described in US Patent 4,827,024 . An alternative process to the preceding one for preparing fluorodioxols of formula (I) wherein Z is -F, described in European patent application No. 94109782.6 as well, comprises: (a) to react, at a temperature comprised between -140° and +60°C (preferably between -110° and -20°C), an olefin of formula: R f O-CCl = CFCl wherein R f is defined as above, with a bis-fluoroxy-compound having the formula: CX 1 X 2 (OF) 2 thus obtaining the dioxolane of formula (III); (b) to dehalogenate the dioxolane (III) as described above. The olefin (IV) can be obtained by reacting CCl 2 =CCl 2 with R f OF so as to obtain the compound R f -OCCl 2 -CFCl 2 , which, by dechlorination reaction with powdered zinc in organic solvent, supplies the olefin (IV). A further process for preparing fluorodioxols of formula (I) when Z is -F, described in European patent application No. 94109782.6 as well, comprises reacting at a temperature comprised between 50° and 150°C a dioxolane of formula: optionally in admixture with a dioxolane of formula: wherein R f is defined as above, with KOH in the solid state, with consequent dehydrochlorination and formation of fluorodioxol. The dioxolane (VI) can be obtained by reacting R f OF with trichloroethylene so as to obtain the R f O-CHCl-CFCl 2 compound, which is then dechlorinated with powdered zinc in organic solution. The olefin of formula R f -OCH=CFCl is thus obtained, which is finally reacted with CX 1 X 2 (OF) 2 , as described above for the olefin (IV), obtaining the dioxolane (VI). The dioxolane (VII), in admixture with the dioxolane (VI), can be prepared as follows. The olefin CHCl=CHCl is reacted with CX 1 X 2 (OF) 2 , obtaining the dioxolane having the formula: which, by dehydrohalogenation with solid KOH, gives the dioxol of formula: This is at last reacted with R f OF, thus obtaining a mixture of the dioxolanes (VI) and (VII). The synthesis of fluorodioxols (I) can utilize, similarly to the processes already shown, the dichloroethylene or trichloroethylene, which are reacted with hypofluorites of formula: R f -OF and CX 1 X 2 (OF) 2 , so as to obtain, by alternating reactions of dechlorination and dehydrochlorination, the various reaction intermediates described above. Similarly to what described above for Z = -F, when Z is -Cl or -H, the fluorodioxols of formula (I) can be prepared according to the following method. 1,1-dichloroethylene CH 2 =CCl 2 is reacted with hypochlorite R f OCl and the reaction product is dehydrochlorinated as described above, so that the olefin of formula R f -OCCl=CHCl is obtained. This last is reacted with CX 1 X 2 (OF) 2 , as described above for the olefin (IV), obtaining the dioxolane having the formula: The dioxolane (X) can be dehydrochlorinated with solid KOH by supplying the fluorodioxol (I) with Z = -Cl, or it can be dechlorinated with powdered Zn in organic solvent to give the fluorodioxol (I) with X = -H. The copolymers of the present invention show a melting viscosity such as to make them thermoprocessable according to conventional techniques, with measurable Melt Flow Index (MFI) values. In particular MFI, measured according to standard ASTM D 1238-52T, is generally comprised between 0.5 and 50 g/10', preferably between 1 and 30 g/10'. The copolymers of the present invention can be prepared according to known techniques, by copolymerization of the corresponding monomers, in suspension in organic medium or in aqueous emulsion, in the presence of a suitable radicalic initiator, at a temperature comprised between 0° and 150°C, preferably between 20° and 100°C. The reaction pressure is generally comprised between 0.5 and 100 bar, preferably between 5 and 40 bar. Among the various radicalic initiators can be used in particular: inorganic peroxides soluble in water, such as for instance persulphates and ammonium or alkaline metal perphosphates, in particular ammonium or potassium persulphate; organic or inorganic redox systems, such as ammonium persulphate/sodium sulphite, hydrogen peroxide/aminoiminomethansulphinic acid; bis-acylperoxides of formula (R f -CO-O) 2 , wherein R f is a (per)haloalkyl C 1 -C 10 , or a perfluoropolyoxyalkylenic group, such as for instance bis(perfluoropropionyl)peroxide; dialkylperoxides of formula (R f -O) 2 , wherein R f is a perhaloalkyl C 1 -C 10 , such as for instance diterbutylperoxide (DTBP). In case of copolymerization in suspension, the reaction medium is formed by an organic phase, to which water is usually added to favour the dispersion of the heat developing during the reaction. As organic phase, halogenated hydrocarbons, in particular hydrogen(chloro)fluorocarbons and chlorofluorocarbons; fluoropolyopxyalkylenes, can be employed. In case of the (co)polymerization in aqueous emulsion, the presence of a suitable surfactant is required. The most commonly used are fluorinated surfactants of formula: R f -X - -M + wherein R f is a (per)fluoroalkylic chain C 5 -C 16 or a (per)fluoropolyoxyalkylenic chain, X - is -COO - or -SO 3 - , M + is selected from: H + , NH 4 + , an alkaline metal ion. Among them, ammonium and/or sodium perfluoro-octanoate; (per)fluoropolyoxyyalkylene terminated with one or more carboxylic groups, can be mentioned. The process of the present invention can be advantageously carried out in the presence of perfluoropolyoxyalkylenes emulsions or microemulsions, according to US patents 4,789,717 and 4,864,006 , or also of fluoropolyoxyalkylenes microemulsions having hydrogenated end groups and/or hydrogenated repeating units, according to EP patent application 625,526. In order to check the molecular weight of the final product, and therefore the melting viscosity, suitable chain transfer agents are added to the reaction system, such as: hydrogen, hydrocarbons; optionally containing halogens, for instance methane, ethane, chloroform, methylenechloride, esters, ethers or aliphatic alcohols, for instance methanol, ethanol, diethylmalonate. The transfer agent is sent into the reactor at the beginning of the reaction, or continuously in discrete amounts during the course of the polymerization. The amount of the chain transfer agent utilized can range within rather wide limits, depending on the molecular weight desired, of the effectiveness of the transfer agent itself and of the reaction temperature.

Claims: Tetrafluoroethylene (TFE) thermoprocessable copolymers, comprising: (a) from 0.1 to 15% by moles of a fluorodioxol of formula wherein R F is a perfluoroalkyl having from 1 to 5 carbon atoms; X 1 and X 2 , equal to or different from each other, are -F or -CF 3 ; Z is selected from -F, -H, and -Cl; (b) from 0 to 15% by moles of a perfluorinated monomer selected from hexafluoropropene (HFP) and a perfluoroalkylvinylether of formula CF 2 =CF-OR' f wherein R' f is a perfluoroalkyl C 2 -C 4 or their mixtures; and (c) TFE forming the remaining part to 100%; with the proviso that the total amount of the monomers (a) and (b) is lower than or equal to 20% by mole, and excluding the value 0 for the amount of the monomer (b). Copolymers according to claim 1, wherein the pefluoroalkylvinylether is perfluoropropylvinylether. Copolymers according to anyone of the preceding claims, wherein the fluorodioxol of formula (I) is present in an amount comprised between 0.5 and 9% by moles. Copolymers according to anyone of the preceding claims, wherein the perfluorinated monomer (b) is present in amounts up to 10% by moles. Copolymers according to anyone of the preceding claims, wherein the total amount of the monomers (a) and (b) is lower than or equal to 12% by moles. Copolymers according to anyone of the preceding claims, wherein in the formula (I) of the monomer (a) X 1 and X 2 are both -F. Copolymers according to anyone of the preceding claims, wherein in the formula (I) of monomer (a) R f is selected from -CF 3 , -C 2 F 5 , and -C 3 F 7 . Copolymers according to anyone of the preceding claims, having a melting viscosity such as to obtain Melt Flow Index (MFI) values, measured according to standard ASTM D 1238-52T, comprised between 0.5 and 50 g/10'. Copolymers according to claim 8, wherein the Melt Flow Index (MFI) value is comprised between 1 and 30 g/10'.

Inventors: ABUSLEME JULIO A, COLAIANNA PASQUA, ABUSLEME, JULIO A., COLAIANNA, PASQUA
Applicants: AUSIMONT SPA, AUSIMONT S.P.A.
Application year: 1995
Main classification: C08F214
IPC codes: C08F14, C08F214, C08F24, C08F224