Covulcanizable compositions of fluoroelastomers based on vinylidene fluoride and tetrafluoroethylene-propylene copolymers.

Within the development ambit of the fluorinated elastomers, lately, well definite needs have been emphasized, i.e.: <ul id="ul0001" list-style="none"><li>1) the chemical stability, also under strict conditions of working temperatures, in the presence of lubricating fluids characterized by a high aggressiveness, such as the oils of SF series, to which amino compunds have been added having a high swelling power towards the fluoroelastomers based on vinylidene fluoride;</li><li>2) a general inprovement in the processing characteristics of the mixes for the molding of manufactured articles such as oil retainer rings;</li><li>3) possibility of extruding the mixes easily, in order to obtain suitable premolds.</li></ul>

The contemporaneous carrying out of these objects can achieved according to the present invention, by covulcanization of an elastomeric composition comprising: <ul id="ul0002" list-style="none"><li>1) a fluoroelastomer (A) based on vinylidene fluoride (40-83% by moles), perfluoropropene (15-35%) and tetrafluoroethylene (0% - 30%) in an amount equal to 42% - 70% by weight of the total elastomeric composition;</li><li>2) an amino compound (B) consisting of a Schiff base or of a carbamate obtained from a diamino compound, in an amount equal to 1-4% by weight of the total elastomeric composition;</li><li>3) a copolymer (C) of tetrafluoroethylene (40-60% by moles) and propylene (60%-40%), in an amount equal to 22-51% by weight of the total elastomeric composition;</li><li>4) an organic peroxide as vulcanizing agent in an amount equal to 0.4-5% by weight of the whole.</li></ul>

Suitable amino compounds are the ones already known for the vulcanization of the fluoroelastomers: among others, they are described in Rubber World 141 (1960) page 827; for instance we can cite: hexamethylenediamine carbamate, the Schiff base from cinnamic aldehyde and hexamethylenediamine, propanediamine carbamate.

The abovesaid composition also contains preferably a vulcanizing coagent known in the vulcanization with peroxides of the fluoroelastomers, consisting of a polyfunctional compound such as triallylisocyanurate, triallylcyanurate, divinylbenzene, etc. Such a coagent is used in the ratio of 2%-4% by weight on the whole.

In the covulcanization of the abovesaid composition a vulcanizate is deemed to form, having a network system like the one described as "interpenetrating polymer network" (see Enc. of Polymer Sci. and Technol. suppl. volume 1, page 288, 1977).

The network of both polymers, type A by ionic way through amino compound and type C by radicalic way through peroxide, introduces a new structure, although according to a method different from the ones generally described for this class of vulcanizates having an interpenetrating network (see above cited literature).

The elastomeric vulcanizable composition, according to the invention, contains furthermore the usual additives for such elastomers, in particular basic substances such as MgO, PbO, calcium hydroxide and carbon black.

The vulcanization working conditions fall substantially within the ones used for the fluoroelastomers: generally a first heating is carried out in press, followed by a heating of the manufactured article in oven for a longer period of time.

For purposes of example, vulcanizable compositions were prepared, according to the invention, and for purposes of comparison vulcanizable compositions were prepared according to the art, starting from the following elastomers: <ul id="ul0003" list-style="none"><li>Tecnoflon NM: a copolymer vinylidene fluoride (79% by moles) with hexafluoropropylene (21%).</li><li>Tecnoflon TN: a terpolymer vinylidene fluoride (65% moles) with hexafluoropropene (20%) and tetrafluoroethylene (15%).</li><li>Polymeric composition AB.30: a mixture of 70 parts by weight of Tecnoflon NM and of 30 parts by weight of AFLAS 150 (R), a copolymer containing equimolecular amounts of tetrafluoroethylene and propylene, produced by Ashahi Glass.</li><li>Polymeric composition AB.40: the hereinbefore cited mixture, containing 40% of AFLAS 150.</li><li>Polymeric composition AB.20: the hereinbefore cited mixture, containing 20% of AFLAS 150.</li></ul>

Other ingredients used for the preparation of the vulcanizable compositions: <ul id="ul0004" list-style="none"><li>- triallylisocyanurate (TAIC)</li><li>- triallylcyanurate (TAC)</li><li>- benzyl-tris (dimethylaminophosphonium) tetrafluoroborate (BTPBF₄)</li><li>- Luperco .101 XL: 2,5-dimethyl-2,5-di-ter.butylperoxy- hexane, in the form of a mixture with an inert filler, with 40% of peroxide</li><li>- Tecnocin A: a Schiff base from cinnamic aldehyde and hexamethylendiamine.</li></ul>

The data relating to three compositions according to the invention (tests 2, 3 and 4) and for purposes of comparison vulcanizable composition according to the art (tests 1,5 and 6) are recorded on the following Table. The amounts are expressed as parts by weight. <tables id="tabl0001" num="0001"><img file="EP0136596A2_D0001.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0136596A2_D0002.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0136596A2_D0003.tif" /></tables>