Fluoroelastomers

The present invention relates to VDF-based fluoroelastomers having a Tg (glass transition temperature) lower than −35° C. and an improved combination of mechanical and compression set properties in a wide range of temperatures, at high and at low temperatures.

More specifically the present invention relates to fluoroelastomers having a Tg lower than −35° C. and substantially —COF end group free, said end groups being undetectable with the method indicated hereinafter. The fluoroelastomers of the present invention show an improved molecular weight as shown by the improved intrinsic viscosity and are obtainable by a polymerization process with an improved productivity.

It is well known that fluoroelastomers are polymers particularly useful in the fields of the automotive, aerospace, oil, petrochemical and electronic industry due to their combination of thermal and chemical resistance and of maintenance of good mechanical and compression set properties. However it is necessary that these polymers have an improved combination of the above properties in a wide temperature range, at high and at low temperatures.

Various fluoroelastomers have been suggested in the prior art to obtain the combination of the above properties, highly requested by fluoroelastomer users. However the —COF end group values in fluoroelastomers are not reported in the prior art. The Applicant, after deep researches, has found that, if the polymerization brings to polymers having —COF end groups, the fluoroelastomers obtainable therefrom do not show high mechanical and elastic properties.

Various fluoroelastomers wherein the glass transition temperature (Tg) values are reported are known in the prior art. However in the prior art the combination of a low Tg and of improved mechanical and elastomeric properties at high and at low temperatures is not obtained.

U.S. Pat. No. 3,132,123 describes the preparation of perfluoroalkylvinylethers, their homopolymers and copolymers with TFE. The homopolymers are obtained under extreme experimental conditions, by using polymerization pressures from 4,000 to 18,000 atm. The general formula of the described vinylethers is the following:

CF₂═CFOR⁰_F

wherein R⁰_F is a perfluoroalkyl radical preferably from 1 to 5 carbon atoms. Tests carried out by the Applicant have shown that the homopolymer Tg is not very low and is of the order of −6° C.

U.S. Pat. No. 3,450,684 relates to vinylethers of formula:

CF₂═CFO(CF₂CFX⁰O)_n, CF₂CF₂X⁰

wherein X⁰=F, Cl, CF₃, H and n′ can range from 1 to 20. Also the homopolymers obtained by UV polymerization are reported. The exemplified copolymers are not characterized with their mechanical and elastomeric properties at low temperatures.

U.S. Pat. No. 3,635,926 relates to the emulsion copolymerization of perfluorovinylethers with TFE. It is stressed that the presence of —COF end groups makes the polymers unstable. The same was already reported in U.S. Pat. No. 3,085,083 in the polymerization systems of perfluorovinylethers in solvent.

U.S. Pat. No. 3,817,960 relates to the preparation and polymerization of perfluorovinylethers of formula:

CF₃O(CF₂O)_n″CF₂CF₂OCF═CF₂