CA2090277A1 - Mixture capable of free radical vulcanization on the basis of fluorine-containing rubber and acrylate rubber - Google Patents

Abstract

Mixture capable of free radical vulcanization on the basis of fluorine-containing rubber and acrylate rubber A B S T R A C T

A novel rubber mixture capable of free radical vulcani-zation, comprising about 35 to 98 parts by weight of a fluorine-containing rubber capable of peroxidic vulcani-zation and about 2 to 65 parts by weight of an acrylate rubber, as well as optionally vulcanizing, processing and modifying aids and fillers, the acrylate rubber comprising a partly cross-linked acrylate rubber with a gel content between about 20 and 99 wt.-% and a particle diameter (d50-value) from about 60 to 800 nm.
The mixture can be molded and vulcanized, and its composition permits incorporation of large amounts of fillers while still producing satisfactory products.

Description

209~277 Mixture capable of free radical vulcanization on ~he basis of fluorine-containing rubber and acrvlate rubber Fluorine-containing rubbers (FKM) based on vinylidene ~ fluoride, hexafluoropropene and optionally tetrafluoro-ethylene yield vulcanized materials with satisfactory mechanical properties and high resistance to heat, oil, ozone and irradiation. Thanks to these characteristics, fluorine-containing rubbers have opened up areas of application into which no other type of rubber has been able to penetrate in the past. However, fluorine-con-taining rubbers have the considerable disadvantage that the vulcanized materials possess extremely poor flexi-bility at low temperatures. In this respect fluorine-containing rubbers are far inferior to commercial acrylate and silicone rubbers, which, as regards hea~ageing and ozone resistance! are closest to ~he flu-orine-containing rubbers. A further disadvantage is the low degree to which fluorine-containing rubbers can be filled with reinforcing fillers. Highly ac~ive fillers give rise to poor processing characteristics, i.e. they impar~ a high Mooney viscosity ~o the rubber. Hence, such fillers as are added to fluorine-containing rubbers are predominantly of inactive type or fillers of low activity only, said fillers being added in a proportion of preferably up to 30 parts by weight, and rarely up to 60 parts by weight, in relation to 100 parts by weiaht of the fluorine-containing rubber.

~5 Le A 28 870-Foreign Countries 2~9~2~7 An improvement in ~he low temperature flexibility, the amount of filler which could be added to, and the pro-cessability of a fluorine-con~aining rubber composition should be achievable by blending the said fluorine-con-taining rubbers with other, cheaper rubbers. In order ~o achieve these enhancements in properties, it is necessary ~hat the blending partner be finely and evenly dis~ributed in the fluorinated rubber. Furthermore, a homogeneous co-vulcanization is necessary in order to achieve synergy as regards the mechanical properties of the co-vulcanized composition, i.e. in order that the profile of characteristics of the vulcanized blend combines the desirable characteristics of the individual blend par~ners: such desired blending of the fluorine-containing rubbers with other blending partners cannot however normally be achieved.

As regards the potential blending characteristics, fluorine-containing rubbers and acrylate rubbers appear to be excellent blending partners.

There have already been proposals to the effect that both acrylate rubber and fluorine-containing rubbers should be vulcanized with ~he aid of bisamine. Whereas this enables an acceptable profile of mechanical characteristics to be achieved, the ageing character-~ istics, especially in aggressive oils, of these rubbersvulcanized with the aid of bisamine are inferior (Rubber Chem. Technol. 63 (1990) 516-522).

Le A 28 870 - 2 -20~0277 Other cross-linking systems common with acrylate rubbers are not compatible with those of fluorine-containing rubbers. In the process of co-vulcanization there are always gradients in cross-linking density resulting in poor mechanical characteristics (.e.g. low ultimate ~ensile strength values).

Peroxide cross-linking is a vulcanizing system which in the case of fluorine-containing rubbers resultsin especially high-grade vulcanization. For a mixture with a rubber not containing fluorine (e.g. acrylate rubber) to be sui~able for co-vulcanization, the rubber not containing fluorine must also be capable of peroxidic cross-linking, and its reactivity in regard to the cross-linking system must be similar to that of the fluorine-containing rubber. Peroxidic vulcanization of the rubber not containing fluorine by abstraction of atoms or molecule groups which can be easily split off (e.g. H-abstraction) generally presupposes more stringent vulcanization conditions than are necessary with fluorine-containing rubbers and is frequently accompanied by chain termination and decay. Special acrylate-containing rubbers capable of co-vulcanization with fluorine-containing rubbers are copolymers con-taining conjugated dienes capable of 1,4-polymeriza-tion such as butadiene, isopropene, etc., and are mentioned., e.g., in EP-A 163 971 and US-A 4 251 399.
However, such rubbers are subject to the disadvantage of low ageing stability owing to the double bonds remaining in the backbone of the polymer chain, the Le A 28 870 - 3 -20~77 proportion of such double bonds being reducible only in par~ by hydrogenation. When mixing soluble rubbers based on acrylate, i.e. rubbers in which cross-linking has not taken place, with fluorine-containing rubbers it is disadvantageous that heterogeneous phases are formed, the particle size distributions of which are difficult to reproduce and which may, in addition, change again in the process of cross-linking.

EP-A 424 347 and 424 348 suggest dispersing, in a first stage, a non-cross-linked copolymer based on ethyl acrylate in the fluorine-containing rubber and to effect, in a second stage, dynamic vulcanization by means of a vulcanization system specific to the acrylate rubber. The fluorine-containing rubber is subjected to peroxide vulcanization in a third stage. This gives rise to a true solid dispersion of vulcanized acrylate rubber particles by way of dispersed phase within the vulcanized fluorine-containing rubber by way of con-tinuous phase. Interpenetration of the blending partners is not possible. Moreover, owing to the different vul-canization systems, the matrix is only inadequately coupled with the dispersed phase. With these mixtures which have been dynamically vulcanized in two stages the level of the mechanical characteristics in considerably poorer than with pure vulcanized fluorine-containing rubber.

Le A 28 ~70 - 4 -.. ~ . . .. .... . .. . . . .

2090~77 I~ has now been found ~hat considerablv bet~er mu~ual interpenetration and coupling of fluorine- and acrylate-containing rubbers can be achieved if use is made, by way of acrylate rubber. of an acrylate copolymer con-taining 0.05 to 5 w~.-% of copolymerized units of a compound con~aining at least 2 but preferably at least 3 double bonds which readily lend themselves to polymer-ization. The acrylate copolymers are used in the formof partly cross-linked particles with particle diameters from 60 to 800 nm. According to M. Hoffmann et al., Polymeranalytik I and II, Georg-Thieme-Verlag, Stuttgart 1977, the partial cross-linking is characterized by determining the gel content. The gel contents of the acrylate copolymer (acrylate rubber) are preferably within the range between 20 and 99 wt.-%. The swelling index as measured in dimethyl formamide is prefer-ably higher than lO.

Accordingly, it is the obJect of the present invention to provide rubber mixtures capable of vulcanization by radicals and containing 35 to 98 parts by weight of a fluorine-containing rubber capable of peroxidic vulcan-ization and 2 to 65 parts by weight of an acrylate rub-ber as well as optionally vulcanization aids, processing aids and fillers. which are characterized in that the acrylate rubber is a partly cross-linked acrylate rubber with gel contents between 20 and 99 wt.-% wi~h particle diameters (d50-values) from 60 to 800 nm.

The mixture should preferably contain 45 to 95 oarts bv weight of fluorine-containing rubber and 5-55 parts by weight of acrylate rubber.

Le A 28 870 - 5 -, .. ... . .. ..

2~3~2~7 The gel content of the acrylate rubber should preferably amount to between 40 and 98 wt.-% and in particular between 50 and 95 wt~-%

The particle diameter of the partly cross-linked acrylate rubber is preferably between 80 and 600 nm, and particulary prefered not larger than 350 nm.

Suitable fluoroelastomers capable of psroxidic cross-linking are such as contain the units of vinylidene fluoride and at least one further fluoroolefin which can be copolymerized therewith. The other fluoroolefin may be tetrafluoroethylene, chlorotrifluoro-ethylene, hexafluoro-propene, hexafluoro-isobutylene, perfluoro-alkyl-vinyl ether, etc, The fluorine-containing rubber may also contain units of monomers not containing fluorine such as propene, ethylene, vinylalkylether and vinyl ester. Such fluorine-containing rubbers are in principle known and disclosed e.g. in US-A 4 981 918 and DE-A 4 038 588. In addition, fluorine-containing rubber muxt possess reactive positions for peroxidic cross-linking. These may be both bromine- or iodine-, bromine-and iodine substituents, and pendent double bonds. Theintroduction of such reactive positions into the fluorine-containing rubber is effected, in the case of bromins and iodine substituents, according to known processes in which either bromine- and/or iodine-con-taining vinyl compounds are copolymerized in smallamounts with the fluoromonomer; see e.g. US-A 3,351,619, US-A 4,035,565, US-A 4,214,060, US-A 4 831 085. More-over, polymerization may take place in the presence of Le A 28 870. - S -2~9027 7 sa~ura~ed iodine- and/or bromine-containing compounds;
see e.g. US-A 4 243 770, US-A 4 748 22~. Optionally, both possibilities may be combined; see e.g. EP-A
407 937 or US-A 4,948,852. Copolymerization of fluoromonomers with small amounts of alkenyl iso-cyanurates, alkenyl cyanurates andlor non-conjugated dienes gives rise to a fluorine-containing rubber with pendent double bonds; see DE-A 4 038 588 and DE-A

4 114 598.

Suitable acrylate copolymers are at least partly cross-linked rubber-type copolymers consisting of one or several not less than C3-alkyl acrylates, and in particular C3-C8-alkyl acrylates but preferably not less than C4-alkyl acrylates and a polyfunctional polyvinyl-or allyl-compound capable of copolymerization preferably triallylcyanurate, vinyl ether of polyols, vinyl- or allyl ether, polyfunctional carboxylic acid, bis-acryl-amine of diamines such as divinyl benzene, glycol-bis-acrylate, bis-acrylamide, triallyl phosphate, triallyl citrate, allyl acrylate or methacrylate or allyl maleate. Particularly preferred are triallyl cyanurate and triallyl isocyanurate.

The alkyl acrylate can be substituted up to 40 wt.-% by acrylonitrile, or in particular C1-C3-alkyl methacrylate or a mixture thereof. Preferred substances are acrylo-nitrile and/or alkyl methacrylate.

Le A 28 870 - 7 -2~02~7 The polyfunc~ional polyvinyl- or allyl cGmpounds capable of copolymerization are used in quantities from 0.05 to 5 wt -% in relation to the acrylate copolymer. The acrylate cc,polymer can be produced in a known manner by free radical aqueous emul 5 ion polymerization in the presence of anionic surfactants at 40 to 95C but preferably at 50 to 80C.

In order to initiate the free radical copolymerization use in particular is made in a known manner of water-soluble inorganic per-compounds such as persulfates, perborates, percarbonates, e.g., generally in the form of their sodium-, potassium- or ammonium-salts.

The acrylate copolymer thus produced in the form of at least partly cross-linked particles in aqueous dispersion may be used in this form for blending with fluorine-containing rubbers to create a rubber mixture.
However, it is also possible first to coagulate the partly cross-linked particles, to separate them from the aqueous solution, to dry them and to mix the blending partners in dry compact form, e.g. in a roller device.
Coagulation is effected by acidification with diluted sulphuric acid to a pH value of about 2, this being followed by precipitation using a 4% aqueous magnesium sulfate solution.

The preferred process for producing the rubber mixture according to the invention consists in mixing the fluorine-containing rubber and the acrylate copolymer Le A 28 870 - 8 -. r ~

2~90277 rubber with one another, whereby each of them is in the form of an aqueous dispersion, and to coagulate and precipitate the resulting mixed emulsion before further processing.

Coagulation is brought about by acidification of the mixed emulsion with dilute sulphuric acid to a pH-value of about 2, precipitation with the aid of a 4% aqueous magneslum sulphate solution applied in a quantity of about 3,500 ml per S00 g solid rubber, separation from the emulsion fluid, washing with water and drying.

In order to produce the elastomers, i.e. the vulcanized rubber, from the rubber mixtures~the latter are mixed in conventional manner with radical initiators as well as with other auxiliaries and additives such as co-cross-linking agents, acid acceptors, fillers, reinforcing agents, plasticizers, lubricants, processing aids, pigments, etc., in conventional mixing devices such as twin-roller rubber mixers and, after shaping, cross-linked by high-energy irradiation or thermal meanS-Preferred radical initiators are peroxides which attemperatures above 100C possess a decay half-life of not less than 5 minutes as is the case with e.g.
dibenzoylperoxide, t-butylperoxybenzene, bis(t-butyl-peroxyisopropyl)-benzene, 2,5-bis-(t-butylperoxy~-2,5-dimethylhexane or 2,5-bis(t-butylperoxy)-2,5-dimethyl-hexane-(3). The radical initiator is used in quantities from 0.5 to 10 parts by weight but preferably 1 to 5 parts by weight in relation to 100 parts of polymer mixture.

Le A 28 870 - 9 -2~277 With a view to achieving better vulcanization character-istics, in particular with curing in a state of com-pression so as to improve the mechanical character-istics, it is possible to incorporate additional co-cross-linking agents. Suitable by way of co-cross-linking agents are in particular compounds with several double bonds such as triallyl cyanurate, triallyl iso-cyanurate, tri(meth)allyl-lisocyanurate, tetramethyl-or tetravinyl-cyclotetrasiloxane, triallylphosphite and N,N -m-phenylene-bis-maleimide, Co-cross-linking agents may be incorporated in quantities from 0.1 to 15 parts by weight but preferably 0.5 to 10 parts by weight, in each case in relation to 100 parts of polymer mixture.
It is also possible to incorporate, by way of acid acceptors, oxides or hydroxides of metals such as magne-sium, calcium, lead, zinc, barium, etc., or a basic salt with an organic acid residue, e.g., stearate, magnesium oxalate or carbonates, or basic lead phosphate, etc.,into the mixture capable of vulcanization. Different acid acceptors may be used in combination. The total proportion of acid acceptors shall not exceed 15 parts by weight in relation to 100 parts of polymer.

Due to the fact that the rubber mixtures according to the invention can, by comparison with pure fluorine-containing rubbers, be filled to a high degree, it is possible, by the addition of fillers, reinforcing agents, pigments, plasticizers, lubricants and otherprocessing aids, to achieve vulcanized materials in which the characteristics cover a wide range.

Le A 28 870 - 10 -2~90277 Thermal vulcaniza~ion is brough~ abou~ in a known manner at 120 to 180C, initially under pressure and then without pressure by post-curing in an air-circulation furnace.

The invention is explained in greater detail with reference to the following examples:

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2~90277 Example 1 a) Production of the fluorine-containing rubber 110 g vinylidene fluoride, 110 g hexafluoropropene, 230 g of an aqueous solution of 5 9 potassium persulfate (pH 11), 230 g of an aqueous solution of 2.3 g lithium perfluoro-octylsulfonate and 3 g triethanol amine (pH
10.5) as well as 8 ml of a solution of 2.5 g triallyl-isocyanurate in methyl acetate were continuously pumped every hour into a continuously operated reaction vessel with stirrer and with a volume of 6 1, the temperature in the vessel being maintained at 55C and subject to a pressure of 63 bar. After reaching the state of stable equilibrium, an emulsion with a proportion of solids from between 23 and 24 wt.-% is continuously extracted through a pressure release valve. The emulsion obtained was used in this form for producing the rubber mixture (below under c)).

In order to determine the characteristics of the fluorine-containing rubber thus obtained, a small quantity of the emulsion extracted was acidified with dilute sulphuric acid to a pH value of about 2 and precipitated using a 4% aqueous magnesium sulfate solution. The solids were separated from the emulsion fluid, washed with water and then dried. By this means a rubber-type terpolymer of vinylidene fluoride, hexa-fluoropropene and triallyl-isocyanurate was obtained. The molar ratio between vinylidene fluoride and hexafluoro-propene in the copolymer was determined by 19F-nuclear Be A 28 870 - 12 -.. ...... . . . .. . ... . . .

2~9~277 resonance spectroscopy, the proportion of triallyl-isocyanurate of the polymer being determined by elemental nitrogen analysis. The molar copolymer composition proved to be 78.9% vinylidene fluoride, 20.7% hexafluoropropene and 0.4% triallyl-isocyanurate.
The presence of free double linkages can be demonstrated by addition of iodine bromide. The iodine number according to HAN~S is 1.5 g iodine/100 g polymer. The Mooney value ML1o (100C) of the raw polymer is 96, The glass temperature determined by differential scanning colorimetry (~.S.C.) is in the region of minus 14C.
b) Production of the acrylate rubber A solution of 0.7 parts by weight Na-salt of C14-C18-alkyl sulfonic acids ( ~ ersolat K 30) and 1,545 parts by weight of water is introduced into a reactor subject to stirring and in a nitrogen atmosphere. After heating to 70C one adds 120 parts by weight of a solution of 0,47 wt,-% triallyl cyanurate in butyl acrylate. Then a solution of 4.5 parts by weight of potassium peroxidi-sulfate in 100 parts by weight of water is added in order to initiate polymerization. Once polymerization has started, further 1,380 parts by weight of a solution of 4,7 wt.-% triallyl-cyanurate in butyl acrylate and a solution of 30 parts by weight Na-salt of C14-C18-alkyl sulfonic acids in 1000 parts by weight of water are added at a constant rate and over a period of 5 hours at a constant temperature of 70C. Thereafter the temperature is maintained at 70C for 4 hours. This results in a latex with a proportion of polymer solids amounting to 35 wt.-%, which is used in this form for producing the rubber mixture (see below under c).

Le A 28 870 -- 13 -?0~0~77 The particle diameter (d50) of the acrylate copolymer is found to be 170 nm, The gel content (with dimethyl-formamide by way of solvent) at 23C of the acrylate copolymer is found to be 97 wt.-%.

c) Production of the rubber mixture The fluorine-containing rubber emulsion produced ac-cording to a) and the acrylate copolymer emulsion produced according to b) are mixed at a ratio of 80 parts by weight of fluorine-containing rubber: 20 parts by weight of acrylate copolymer (each in relation to the polymer solids concentration), This is followed by acidification with dilute sulphuric acid to a pH value of about 2 and precipitation using a 4% aqueous magne-sium sulfate solution ~3,500 ml~per 500 g solid rubber).
The solids are separated from the emulsion fluid, washed with water and dried.

d) Production of the vulcanization mixture and vulcanized material Using a twin-roller rubber mixer, 100 parts by weight of the rubber mixture produced according to c), 3 parts by weight calcium hydroxide, 30 parts by weight carbon black MT N 990, 2 parts by weight of 50% triallyliso-30 cyanurate in inactive fillers t~Percalink 301-50) and 3 parts by weight 2,5-dimethyl-2,5-bis(tert,-butyl)-hexane, 45% in inactive fillers (~Luperco 101 XL) were mixed ln.

Le A 28 870 - 14 -6, ~ ," . . ... .

2090,~77 The mixture was then subjected to vulcanization for 10 minutes at 170C and at a pressure of 200 bar and postcuring over a period of 24 hours at 180C in the air-circulation furnace.

It was found thet the tensile strength of the vulcanized material was 12.5 I~Pa. The tensile elongation was 340%.
The glass transition determined by temperature-dependent shear-module measurements ("Brabender" automatic torsion measuring apparatus) was found to be at minus 20C.

Example 2 The procedure was as in Example 1 with the difference that the rubber mixture was produced from 60 parts by weight fluorine-containing rubber and 40 parts by weight acrylate copolymer (determined in each case as polymer solids concentration).

The vulcanized material produced has a tensile strength of 11.5 ~Pa and a tensile elongation of 190%. The glass transition was found to be at minus 25C.

Example 3 The procedure was as in Example i with the difference that 70 parts by weight of fluorine-containing rubber and 30 parts by weight of acrylate copolymer (consisting of 10% by weight of 2-ethylhexylacrylate, 89,5% by weight of butylacrylate and 0,5% by weight of triallyl-cyanurate) were u~ed, the acrylate copolymer having a gel content of 97% by weight and a particle diameter of 185 nm.

Le A 28 870 - lS -20902~

The vulcanized material produced therefrom had a tensile strength of lOMPa, a tensile elongation of 200~/.. The glass transition was found to be at minus 21C.

Example 4 Example 3 was repeated with the difference that the acrylate copolymer consisted of 20% by weight of 2-ethylhexylacrylate, 79,5% by weight of butylacrylate and 0,5/. by weight of triallylcyanurate, having a gel content of 98% by weight and a particle diameter of 180 nm.

The vulcanized material produced therefrom had a tensile strength of 9,7 MPa, a tensile elongation of 190%. A
slightly broadened glass transition was found to be at minus 26C.

Example 5 The procedure was as in Example 1 with the difference that for the production of the acrylate copolymer a mixture of 50% butyl acrylate and 50% 2-ethylhexyl acrylate was used instead of butyl acrylate.

The acrylate copolymer had a gel content of 98 wt.-% and a particle diameter of 180 nm.

Le A 28 ~70 - 16 -, . . . .. . .. .. ... . ..

2090~7 The vulcanized material produced from the mixture had a tensile strength of 10 MPa and a tensile elongation of 280/.. Two broadened overlapping glass transition effects were found at minus 38C and minus 13C. This led to the conclusion that this was a multiphase vulcanized material.
Example 6 The procedure was as in Example 1 with the difference that use was made of an acrylate polymer according to Example 5 and that the quantitative ratio between the fluorine-containing rubber and the acrylate copolymer was as in Example 2.

The tensile strength of the vulcanized material was found to be MPa and the tensile elongation 160%. Two broadened and overlapping glass transition effects were found at minus 38 and minus 14C.

Example 7 The procedure was as in Example 1 with the difference that instead of butyl acrylate a mixture consisting of 78 wt.-% ethylhexyl acrylate and 22 wt.-% acrylonitrile was used in order to produce the acrylate copolymer.
Fluorine-containing rubber and acrylate copolymer were used at a quantitative ratio of 60 parts by weight fluorine-containing rubber to 40 parts by weight acrylate copolymer (in each case in relation to the proportion of polymer solids).

Le A 28 870 - 17 -2Q9V.''77 The vulcanization material had a tensile strength of 10 MPa and a tensile elongation of 200%. The glass transition was found to be at minus 9C.

Example 8 a) Fluorine-containing rubber Use was made of a commercially available rubber (~Viton GF). This is a copolymer consisting of 69 mol-% vinyl- :
idene fluoride, about 14 mol-% hexafluoropropene, about 17 mol-% tetrafluoroethylene and a bromine cure-site (about 0.6 wt.--/. bromine in the polymer). The raw poly-mer has, in its non-cross-linked state, a glass transition (measured by D.S.C,, temperature at half unit function response) of minus 4C and a Mooney value MLlo 20 (100C) of 103.

b) Acrylate copolymer An acrylate copolymer is produced according to Example 16, coagulated, precipitated and separated from an emulsion according to Example lb), washed and dryed.

c) Production of the mixture ~ To begin with, 90 parts by weight fluorine-containing rubber is applied to the roller of a twin-roller rubber mixer, and then 10 parts by weigh'. acrylate copolymer are incorporated.

Le A 28 870 - 18 -" . . . v . . , d) Produc~ion of vulcanization mixture and vulcanization material The Drocedure is as in Examole 1.

The vulcanization material has a tensile strength of 15 MPa and a tensile elongation of 300%. Two broadened overlapping glass transition effects were found at minus 20C and minus 1C.

Examole 9 The procedure was as in Example 8 with the difference that the mixture was produced from 60 parts by weight fluorine-containing rubber and 40 parts by weight acrylate copolymer.
The tensile strength of the vulcanized material was found to be 8 MPa and the tensile elongation 110%. Two broadened overlapping glass transition effects were found at minus 20C and minus 2C.
Examole 10 (comparative example) The procedure was as in Example 8 with the difference that only butyl acrvlate was used for oroducing ~he acrylate copolymer instead of a mixture of butyl acrylate with 0.47 wt.-% triallvl-isocyanura~e. The precipita~ed acrylate polymer was entirelv soluble in Le A 28 870 - 1~ -, . . . ~ ~ . ~ .. .. . . . . .

20~277 dime~hyl formamide. The gel content was accordingly O
S wt.-% Owing to the extrPme tackiness of the non-cross-linked butyl acrylate rubber used in this case, a rolled sheet could not be formed while it was being mixed with the fluorine-containing rubber on the roller. For this reason the rubber mixture and the vulcanization mixture were produced in an internal kneader.

The vulcanized material had a tensile strength of 8 MPa and a tensile elongation of 170%.

Example 11 (comparative example) The procedure was as in Example 10 with the difference that use was made of 80 parts by weight fluorine-containing rubber and 20 parts by weight butyl acrylate rubber. Also in this case processing on the twin-roller rubber mixer was not possible.

The tensile strength of the vulcanization material amounted to 6 MPa, and the tensile elongation to 110%.
Example 12 (comparative example) The procedure was as in Example 11 with the difference that use was made of a fluorine-containing rubber sold ~ commercially under the tradename ~Viton A, this being a copolymer consisting of about 78 mol-% vinylidene fluoride and 22 mol-% hexafluoropropene. The glass temperature in the non-cross-linked state is minus 16C, and the M~oney value ML1o (100C) to 72. The fluorine-~5 containing rubber is not capable of peroxidic cross-linking.

Le A 28 870 - 20 -... ~.. .. ..

209~277 Devia~ing further from Example 9. ~he vulcanization S mix~ure was accordingly structured as follows: 6 parts by weight calcium hydroxide~ 3 parts bv weight magnesium oxide. 30 parts by weight carbon black MT Black N 990, 2 parts by weight bisphenol AF 2,2-bis-t4-hydroxyphenyl~
hexafluoropropane) and 0.66 parts by weight benzyl-triphenyl phosphonium chloride were added to the rubbermixture consisting of 80 parts by weight fluorine-containing rubber and 20 parts by weight butyl acrylate rubber and mixed in an internal kneader.

The vulcanized material oroduced has a tensile strength of Z.5 MPa and an ultimate tensile elongation of 225%.

The specimens produced according to Example 10~ ll and 12 shrank in the course of oost-curing and lost their original shape (lOOxlOOxl mm 3 plates~ when molded by vulcanization in the compressed state.

It will be apprecia~ed that the instant soecification and c-laims are set forth by way of illustration and not limitation. and that various modifications and changes may be made without departina from the spirit and scooe of the present invention.

Le A 28 870 - 21 -

Claims (7)

1. A rubber mixture capable of free radical veulcaniza-tion, comprising about 35 to 98 parts by weight of a fluroine-containing rubber capable of peroxidic vulcanization and about 2 to 65 parts by weight of an acrylate rubber, as well as optionally vulcaniz-ing, processing and modifying aids and fillers, the acrylate rubber comprising a partly cross-linked acrylate rubber with a gel content between about 20 and 99 wt.% and a particle diameter (d50-value) from about 60 to 800 nm.

2. A rubber mixture according to claim 1, wherein the fluorine-containing rubber has a fluorine content from about 54 to 69 wt.%.

3. A rubber mixture according to claim 1, wherein the fluorine-containing rubber contains about 0.2 to 2.5 mol-% of copolymerized units of at least one of di- and triallyl(iso)cyanurate.

4. A rubber mixture according to claim 1, wherein the flourine-containing rubber contains about 0.2 to 2.5 mol-% of copolymerized units of a non-conju-gated diene.

5. A rubber mixture according to claim 1, wherein the acrylate rubber is a partly cross-linked rubber-type polymer of about 60 to 99.5 wt.-% C3-C8-Le A 28 870 - 22 -alkylacrylate, 0 to 40 wt.-% of at least one of acrylonitrile and C1-C6-alkylmethacrylate, and about 0.05 to 5 wt.-% of a polyfunctional poly-vinyl- or allyl compound capable of copolymerization.

6. A process for the production of a rubber mixture according to claim 1, comprising mixing an aqueous dispersion of the fluorine-containing rubber and the acrylate rubber, coagulating, precipitating and freeing the mixed dispersion of water.

7. In the production of an elastomer based on an acrylate rubber mixture optionally containing vulcanizing, processing and modifying aids and fillers by molding and subsequent free radical vulcanization, the improvement which comprises employing as said acrylate rubber mixture an acrylate rubber mixture according to claim 1.

Le A 28 870 - 23 -