CA2345461A1

CA2345461A1 - Method for producing a fast crosslinkable fluororubber

Info

Publication number: CA2345461A1
Application number: CA002345461A
Authority: CA
Inventors: Ralf Kruger; David Bryan Harrison
Original assignee: Individual
Current assignee: Bayer AG
Priority date: 1998-09-28
Filing date: 1999-09-15
Publication date: 2000-04-06
Also published as: WO2000018811A1; AU6083699A; JP2002525401A; JP5031943B2; EP1129114A1; DE19844188A1

Abstract

The invention relates to a method for producing a fast crosslinkable fluororubber having exclusively hydrogen, alkyl and/or or alkoxy groups in addition to vinylic end groups. The invention also relates to the mixtures of fluororubber obtained therefrom and to their use in the production of all sorts of shaped articles.

Description

A method of nroducin~ a rapid-crosslinkin~ fluorinated rubber The present invention relates to a method of producing a fluorinated rubber, which apart from vinyl terminal groups exclusively comprises hydrogen, alkyl and/or alkoxy terminal groups.
According to the prior art, fluoroelastomers are preferably produced by aqueous emulsion or suspension polymerisation (I111mann's Encyclopedia of Industrial Chemistry', Vol. A-11. VCH Verlagsgesellschaft, Weinheim 1988, pages 417 et seq.).
Inorganic. water-soluble peroxides, such as persulphates for example, are generally used as initiators and as emulsifiers or suspension stabilisers in processes such as these. In order to obtain the desired molecular weight. the addition of chain transfer agents such as tetrachloromethane. acetone, diethyl malonate and methanol is customary. In this manner, ionic or polar terminal groups are introduced into the polymer chain. such as -COI~, -COOH, -COOR or OH groups for example. These impair flowability by increasing the viscosity due to intermolecular interactions.
There is therefore a need for rapid-crosslinking fluorinated rubbers which have a low mscositv.
~0 In order to overcome the problems described above, molecular weight regulators are used which do not give rise to any ionic, polar or hydrolysable terminal groups; see US-P-5 256 745 and US-P 516 863. For the most part. however, regulators such as these also act as terminators if the radical fragment which remains after transfer is not 2~ capable of adding fluoromonomers and thus of starting a new chain. Thus in order to start a new chain an initiator radical is again necessary, from which an ionic terminal group is consequently formed again, however.
Initiators which do not give rise to ionic terminal groups, such organic peroxides or >0 azo initiators for example, are difficult to handle in the preferred aqueous systems due to their insolubility in water.

-7_ EP-A-796 877 describes a 2-stage aqueous method in which a seed latex is produced in the 1 st stage by means of a water-soluble initiator and is further polymerised in the 2nd stage by means of an organic peroxide which is insoluble in water.
All fluoropolymers produced by known methods contain terminal groups which are ionic at least in part and which originate from the initiator. Moreover, when certain molecular weight regulators are used, such as diethyl malonate for example, there is a risk of the ester terminal groups undergoing saponification during production (including work-up) and thus of furnishing an additional proportion of ionic terminal groups.
EP-A-739 911 describes fluoroelastomers which are "substantially free'" from ionic terminal groups. These fluoroelastomers are produced in an emulsion polymerisation process in water by means of UV irradiation. A homogeneous product cannot be 1 ~ obtained here. because homogeneous UV irradiation is difficult to accomplish industrially in large polymerisation vessels. Moreover, the peroxides used are only soluble with difficulty in the aqueous medium and there is always the risk of ionic terminal groups being introduced into the polymer by the protic solvent. In addition, the tluoroelastomers described there do not contain a vinyl terminal group.
~0 Of the nonaqueous methods, polymerisation processes conducted in the pure liquefied fluoromonomer have proved to be disadvantageous, since the polymers formed are mostly insoluble therein and also only swell to a slight extent. Moreover, it is not possible by this route to conduct polymerisation reproducibly with good heat-and 2 ~ mass transfer and therefore with acceptable space-time yields.
In contrast, fluoromonomers can be polymerised well in the presence of certain tluorine-containing solvents: see US-4 243 770 and DE-A-196 40 972.1, for example.
US-~ 182 342 describes the use of fluorohydrocarbons in the presence of up to _s0 water as polymerisation media which fulfil certain criteria with regard to the F/H ratio and with regard to the position of hydrogen. With all compounds of this type, which contain hydrogen and which possibly also contain chlorine in addition, there is always the problem that they are capable of taking part in transfer and/or termination reactions.
In WO 98/15 X83, 1,1,2-trichlorotrifluoroethane is used as a polymerisation medium.
S However. compounds of this type (chlorofluorocarbons) exhibit a significant ozone-damaging potential. For this reason, the industrial use thereof is already prohibited in many industrialised countries. The fluorinated rubbers described in the above patent contain 0.5 to 2.5 % by weight of iodine terminal groups.
In a previous Application, namely DE-197 40 633.5, liquid fluorinated rubbers are produced in inert solvents of the RF-SO~F or perfluoroalkylsulphone type in the presence of a molecular weight regulator. The fluorinated rubbers described there likewise comprise iodine or bromine terminal groups.
Surprisingly, it has now been found that fluorinated rubbers which apart from vinyl terminal groups exclusively comprise hydrogen, alkyl and/or alkoxy terminal groups can be produced by the polymerisation of least one fluoromonomer by means of organic peroxides as initiators in inert fluorine-containing solvents and in the absence of water and molecular weight regulators, wherein the solvent is selected so that the monomers are soluble therein but polymers with molecular weights higher than 25 to kg/mol are no longer soluble.
The present invention therefore relates to a method of producing a fluorinated rubber which apart from vinyl terminal groups exclusively comprises hydrogen, alkyl and/or 25 alkoxy terminal groups, characterised in that at least one fluoromonomer is polymerised by means of organic peroxides as initiators in at least one inert fluorine-containing solvent in the absence of water and molecular weight regulators, wherein the solvent is selected so that the monomers are soluble but polymers with molecular weights higher than 25 kg/mol are no longer dissolved.
_~ 0 The monomers which can be used for the fluorinated rubbers according to the invention comprise fluorinated ethylenes, which are optionally substituted, and which apart from fluorine may contain hydrogen and/or chlorine, such as vinylidene fluoride.
tetrafluoroethylene and chlorotrifluoroethylene for example, fluorinated I -alkenes containing 2 to 8 carbon atoms, such as hexafluoropropene, 3,3,3-trifluoropropene.
chloropentafluoropropene and hexafluoroisobutene for example, and/or perfluorinated vinyl ethers of formula CFZ=CF-O-X where X = a Ci-C3 perfluoroalkyl or -(CF=-CFY-O)°-RF, wherein n = I-4, Y = F or CF3 and RF = a C~-C3 perfluoroalkyl.
A combination of vinylidene fluoride and hexafluoropropene and optionally of tetrafluoroethvlene and/or of perfluorinated vinyl ethers. such as perfluoro(methvl-I 0 vinyl ether) for example, is preferred.
The following composition is particularly preferred:
40 to 90 11101 °'° vinylidene fluoride to 15 mol % hexafluoropropylene 1 ~ 0 to 25 mol % tetrafluoroethylene 0 to 25 mol % of perfluorinated vinyl ethers of formula CFZ=CF-O-X, where X =
a Ci-C, pertluoroalkyl, or -(CFA-CFY-O)"-RE:, wherein n = I-4. Y = F or CF, and RF =
a C,-C; perfluoroalkyl.
~'0 In addition, it is also possible to use copolymerisable monomers which contain bromine, such as bromotrifluoroethylene, 4-bromo-3,3,4,4-tetrafluorobutene-1 as described in US-A-4 035 565, or I-bromo-2,2-difluoroethylene for the production of fluorinated rubbers which can be crosslinked by peroxides.
~5 Apart from vinyl terminal groups, the terminal groups are exclusively hydrogen, alkyl or alkoxy groups, and are preferably methyl groups in addition to other alkyl groups depending on the organic peroxide used, e.g. 2-ethyl-pentyl or isobutyl radicals as well as the t-butoxy radical.
s0 The number average molecular weights fall within the range from 25 to 100 kg/mol, preferably from 40 to 80 kg/mol, with molecular weight distributions, defined as M~,,/M", within the range from 1.5 to 3.5. The Mooney viscosities ML,+io at 120°C
fall within the range from 1 to 40, preferably from 4 to 20. The Mooney viscosity is determined according to DIN 53 523.
Radical polymerisation is effected in the presence of at least one peroxide compound as an initiator.
Organic or fluorinated organic dialkyl peroxides. diacyl peroxides, dialkyl peroxydicarbonates, alkyl peresters and/or perketals are used as initiators, e.g. tert-butyl peroxypivalate. tert-butyl peroxy-2-ethyl-hexanoate, dicyclohexyl peroxy-dicarbonate, bis(trifluoroacetyl peroxide) or the peroxide of the hexafluoropropene oxide dimer ;CF,CF.,CFZOCF(CF3)COO},.
The type and amount of initiator used depend on the reaction temperature concerned.
I ~ The peroxides which are selected preferably have half-lives between 30 and minutes. Accordingly, amounts between 0.05 and 1.0 parts by weight of peroxide per 100 parts by weight of monomers to be converted are preferably required.
The molecular weights and thus the viscosities of the target products are exclusively ?0 determined by the amount of initiator and by the solubility of the polymer chains in the solvent. To a first approximation, the desired molecular weight is set by the ratio of monomer conversion to peroxide conversion. Amounts within the range from 1 to 10 mmol peroxide per 100 mol monomer are accordingly reacted. The use of regulators is completely dispensed with.
~' S
The inert fluorinated solvent which is used is characterised in that it enters into no significant transfer reactions under the reaction conditions, no longer homogeneously dissolves the resulting rubber above a molecular weight of 25 kg/mol, and exhibits no ozone-damaging potential. Certain fluorocarbon compounds or fluorohydrocarbon _~ 0 compounds which contain fluorohydrocarbons or hetero atoms are suitable as inert fluorine-containing solvents, such as 1,1,1,3,3-pentafluoropropane, 1,1,1,2,3,3-hexafluoro-propane, 1,1,2,2,3,3-hexafluorocyclopentane, 1,1,2,2-tetrafluorocyclo-butane, 1-trifluoromethyl-1,2,2-trifluorocyclobutane, 2,3-dihydrodecafluoro-pentane, 2,2-bis(trifluoromethyl)-1,3-dioxolane, perfluoro(tripropylamine), methoxy-2-hydro-hexafluoropropane, methoxynonafluorobutane, perfluorobutane sulphofluoride or perfluorosulpholane, and also the compounds of formulae (I) or (II) R~-SOZ-Rz (I) ( 11).
(CF~)~
which are cited in the prior Application DE-197 40 633.5.
wherein Ri represents a fluorine atom or a perfluoroalkyl radical comprising 1-4 C
atoms, 1 ~ R, represents a perfluoroalkyl radical comprising 1-4 C atoms, and n = 4 or ~. particularly perfluorobutane sulphofluoride and perfluorosulpholane.
1,1.1,3.3-pentafluoropropane, perfluorobutane sulphofluoride and perfluoro-~0 sulpholane, individually or in admixture, are preferred.
It is advantageous to ensure that the solvents have low boiling points, in order to facilitate ease of separation of the solvent from the fluorinated rubber after the completion of polymerisation. On account of their low boiling points between 15 and ~ 70°C and their low enthalpies of evaporation, the compounds cited as preferred can readily be separated by volatilisation from the rubber after polymerisation.

The ratio of fluoromonomer (monomer) to solvent, as well as the reactor filling ratio, are preferably selected so that at the temperature of reaction the content of monomer in the liquid phase is at least 20 % by weight. The amount of monomer dissolved in the liquid phase can be determined from the mass balance by means of the partial ~ pressure of the monomer present in the gas phase, for example.
The temperatures of reaction fall within the range from -~?0 to 130°C, preferably from 0 to 80°C. Lower temperatures result in a prolongation of the time of reaction and in a considerable increase in the viscosity of the polymer. Moreover, it is not possible to I 0 achieve a significant increase in space-time yields by employing higher temperatures.
The pressure depends on the aforementioned conditions and on the composition of the monomer mixture; it preferably falls within the range from 10 to 100 bar, and is most preferably within the range from 15 to 50 bar.
1~
Polymerisation can be effected by a batch, continuous or batch-feed process in stirred tank reactors. wherein a batch-feed process is preferred.
After the completion of polymerisation, the reaction mixture can readily be discharged ~0 from or pushed out of the vessel via a bottom outlet or an ascending pipe.
The residual monomers and solvent can then readily be separated from the polymer by depressurisation.
The fluorinated rubbers according to the invention ~~re mainly suitable for the ~5 production of rapid-crosslinking fluorinated rubber compounds, particularly by means of ionic crosslinking agent systems consisting of a polyhydroxy compound, an onium salt and an acid acceptor.

-g_ Examples Example 1 ~ 620 ml perfluorobutane sulphofluoride (PFB°SF) were placed in a 4.1 litre autoclave.
The closed autoclave was evacuated twice whilst being cooled, was subsequently subjected to a nitrogen pressure of 3 bar and was slowly stirred for 10 minutes each time. 440 g vinylidene fluoride (VDF) and 880 g hexafluoropropene (HFP) were added to the evacuated autoclave and the reaction mixture was heated to 60°C with I 0 stirring . After this temperature had been reached, the autoclave internal pressure was 27 bar. Polymerisation was initiated by the addition of 4.25 g TBPPI-75-AL
(=tert.-butvl peroxypivalate as a 75 °,'° solution in aliphatic compounds, supplied by Peroxid-Chemie GmbH. peroxide content 47.1 %) dissolved in 20 g PFBSF. Polymerisation commenced within a few minutes, as could be identified by the pressure starting to I ~ decrease. During the polymerisation, a monomer mixture comprising 60 % by weight vinylidene fluoride and 40 % by weight hexafluoropropene was fed in under pressure so that the autoclave internal pressure was held constant at 26.8 ~ 0.2 bar.
In this manner, a total of 300 g vinylidene fluoride and 200 g hexafluoropropene were subsequently added over a time of reaction of 14 hours. After the completion of ~0 polymerisation the reaction mixture was cooled and the unreacted monomer mixture was removed from the reactor by depressurisation and evacuation. The remaining reactor contents were heated to 80°C with stirring. 15 minutes after switching off the stirrer, the reactor contents were discharged via a bottom outlet valve into a second pressure vessel situated underneath.
~' S
The product was separated from the PFBSF and dried, whereupon 450 g of a rubber-like copolymer were obtained.
The following copolymer composition was determined bv'9F NMR analyses (solvent:
30 acetone; standard: CFC13): 20.5 mol % hexafluoropropene, 79.5 mol %
vinylidene fluoride.

The number average molecular weight (membrane osmosis) was 68,900 g/mol.
M,v/M" was 2.3 as determined by GPC investigations.
A value of 16 was determined for the Mooney viscosity MLi+io at 120°C
(Table 1).
J
Example 2 Polymerisation was conducted analogously to Example l, except that 1,1,1,3,3-pentafluoropropane was used instead of PFBSF and 2.5 g tert.-butyl per-2-I 0 ethylhexanoate was used as the initiator instead of TBPPI-75-AL, the temperature of polymerisation was increased to 78°C and the initial pressure was accordingly 33.6 bar.
After a run time of 1 ~ hours, 541 g rubber were isolated.
IS
Example 3 Polymerisation was conducted analogously to Example l, except that 620 ml 1.1,1.3,3-pentafluoropropane was used as the polymerisation medium instead of ?0 PFBSF and the initial amount of HFP was increased to 1026 g. The initial pressure was accordingly 29 bar.
After a run time of 25 hours. 518 g rubber were isolated (Table 1:
Properties).
?5 Comparative Example 1 25.2 kg deionised water, 30.2 g lithium perfluorooctyl sulphonate and 29.3 g oxalic acid dihydrate were placed in a 36 litre autoclave, which resulted in a pH of 3 in the aqueous starting mixture as a whole. The closed autoclave was evacuated four times, 30 followed in each case by subjecting it to a nitrogen pressure of 3 bar and slowly stirring the contents for 10 minutes. 269 g vinylidene fluoride and 366 g hexafluoropropene were added to the evacuated autoclave and the reaction mixture was heated to 35°C with stirring. After this temperature had been reached, the autoclave internal pressure was 10.5 bar. Polymerisation was initiated by the addition of 53 ml of an aqueous solution which contained 20 g/1 potassium permanganate.
Immediately after this first addition, said solution was continuously metered in at a rate of 39 ml/hour. Polymerisation commenced after 20 minutes, as could be identified by the pressure starting to decrease. During the polymerisation, a monomer mixture comprising 60 % by weight vinylidene fluoride and 40 % by weight hexatluoropropene was fed in under pressure so that the autoclave internal pressure was held constant at 10.3 ~ 0.2 bar. After 250 g monomer had been converted, a total of 80 ml diethyl malonate were metered in at a rate of 20 ml/hour.
' In this manner. a total of 4641 g vinylidene fluoride and 3078 g hexafluoropropene were added over a time of reaction of 7.9 hours. In order to terminate the polymerisation. the addition of permanganate was stopped, the unreacted monomer 1 ~ mixture was removed from the reactor by depressurisation and evacuation and the remaining autoclave contents were cooled. The resulting latex was precipitated by adding it drop-wise to a well stirred receiver liquid consisting of 8000 ml of a 2 solution of CaCI~, and was subsequently washed with deionised water and dried for 24 hours at 60°C in a vacuum drying oven. 7.5 kg of a rubber-like copolymer were ~0 isolated in this manner.
The following copolymer composition was determined by '9F NMR analyses: 21.4 mol % hexafluoropropene, 78.6 mol % vinylidene fluoride.
2 ~ The number average molecular weight was 79,800 g/mol. M,~/M" was 1.97 as determined by GPC investigations.
A value of 34 was determined for the Mooney viscosity IVILi+io at 120°C
(Table 1).

Table 1 Example Example Example Comparative Example ML, _, at 120C 16 4 17 34 M (kg mol- ) 69 43 57 80 M~~/M" 2.3 2.0 2.8 2.0 VDF content (mol 79.5 79.1 79.5 78.6 %) Cl. Br or I terminalno no no no groups-Carbonyl terminal no no no yes ' ;
groups Hydroxyl terminal no no no no groups -CH=CFZ terminal yes yes yes no groups3 H. alkyl and alkoxyyes yes yes no terminal '~roups~

by ' ''F NMR
'~ by elemental analysis by IR analysis (High Polymers Vol. XXV: Fluoropolymers, ed. L. Wall, Wiley. New York. 1972, 336) by ' H NMR (Balague et al., J. Fluorine Chem. 70, (1995), 215) At comparable molecular weights (M°), the examples according to the invention have lower viscosities than the corresponding product produced by aqueous emulsion polymerisation.

Claims

Claims 1. A method of producing a fluorinated rubber, which apart from vinyl terminal groups exclusively comprises hydrogen, alkyl and/or alkoxy terminal groups, characterised in that at least one fluoromonomer is polymerised in at least one inert fluorine-containing solvent in the presence of organic peroxides and in the absence of water and molecular weight regulators, wherein the solvent is selected so that the monomers are soluble but polymers with molecular weights higher than 25 kg/mol are no longer dissolved.
2. A method according to claim 1, characterised in that the fluoromonomer is vinylidene fluoride or hexafluoropropylene.
3. A method according to claim 1, characterised in that an organic or fluorinated organic dialkyl peroxide, diacyl peroxide, dialkyl peroxydicarbonate, alkyl perester and/or perketal is used as the organic peroxide.
-1. A method according to claim l, characterised in that 1,1,1,3,3-pentafluoropropane, perfluorobutane sulphofluoride and perfluorosulpholane are used, individually or in admixture, as the inert fluorine-containing solvent A method according to claim 1, characterised in that polymerisation is conducted at a polymerisation temperature within the range from 0 to 80°C.
6. Fluorinated rubbers, which apart from vinyl terminal groups exclusively comprise hydrogen, alkyl and/or alkoxy terminal groups, which are produced from vinylidene fluoride, hexafluoropropylene and optionally from other fluoromonomers and which have a Mooney viscosity ML1+10 within the range from 1 to 40 as measured at 120 °C.
7. Fluorinated rubbers according to claim 6, characterised in that the vinyl terminal groups are -CH=CF2 terminal groups.