WO1998014517A1

WO1998014517A1 - Fluroelastomer alloy compositon having excellent low temperature properties

Info

Publication number: WO1998014517A1
Application number: PCT/US1997/017607
Authority: WO
Inventors: Ronald D. Stevens
Original assignee: Dupont Dow Elastomers L.L.C.
Priority date: 1996-10-01
Filing date: 1997-10-01
Publication date: 1998-04-09

Abstract

Peroxide curable fluoroelastomer alloy compositions are provided which, when cured, produce seals having excellent low temperature sealing performance and resistance to volume swell in fuels.

Description

TITLE

FLUOROELASTOMER ALLOY COMPOSITION HAVING EXCELLENT LOW TEMPERATURE PROPERTIES

FIELD OF THE INVENTION

This invention relates to alloy compositions of two or more fluoroelastomer copolymers which are co-curable with peroxides and, in particular, to seals made from these compositions.

BACKGROUND OF THE INVENTION

Elastomeric copolymers having interpolymerized units of vinylidene fluoride (VF₂) and comonomers such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), and perfluoro(alkyl vinyl) ethers (PAVE) are well- known, commercially available materials which exhibit outstanding chemical, thermal and oxidative stability. Fluoroelastomer copolymers which additionally contain iodine or bromine end-groups or copolymerized iodine-containing or bromine-containing cure site monomers are also known. The iodinated or brominated fluoroelastomers may be cured with organic peroxides to produce vulcanizates having excellent tensile strength and compression set properties (see e.g. U.S. Patent No. 4,035,565 and U.S. Patent No. 4,973,633).

The major end use for fluoroelastomers is seals. Because of their chemical stability, fluoroelastomer seals are useful in demanding environments under conditions which cause degradation of seals made from less stable elastomers. For example, shaft seals or o-rings for internal combustion engines must be capable of withstanding exposure to harsh solvents, such as Fuel C and Fuel C/ethanol mixtures, with minimal swelling. Further, leakage must be prevented over temperature ranges which reach at least as low as -40°C. It is difficult to produce a single fluoroelastomer composition economically which has both the required resistance to fuel swell and low temperature leak properties necessary for such end uses. SUMMARY OF THE INVENTION

The present invention provides fluoroelastomer alloy compositions which may be cured with organic peroxides in the presence of coagents to produce seals which have a leak temperature of less than or equal to -40°C (as described in the Test Method hereinafter) and a fuel swell (168 hours at 23°C in Fuel C) of less than 7 volume percent (as described in the Test Method hereinafter).

The fluoroelastomer composition of this invention comprises a curable alloy formed by blending: a) at least two fluoroelastomers, each fluoroelastomer having copolymerized units of vinylidene fluoride and tetrafluoroethylene, and at least one of the fluoroelastomers having copolymerized units of perfluoro(methyl vinyl) ether, wherein the total weight percentage of vinylidene fluoride units, based on the total weight of fluoroelastomers, is 50 to 55; the total weight percentage of tetrafluoroethylene units, based on the total weight of fluoroelastomers, is 10 to 16; and the total weight percentage of perfluoro(methyl vinyl) ether units, based on the total weight of fluoroelastomers, is 20 to 30; each fluoroelastomer having 0.1 to 1.0 weight percent of a halogen selected from the group consisting of bromine and iodine; b) 0.5 to 10 weight percent, based on the total weight of fluoroelastomers, of an organic peroxide; and c) 0.5 to 10 weight percent, based on the total weight of fluoroelastomers, of a peroxide curing coagent; thereby producing a composition which, when formed into seals and cured, said seals have a leak temperature of less than or equal to -40°C and a fuel swell, in Fuel C for 168 hours at 23°C, of less than 7 volume percent.

The present invention is further directed to seals such as, but not limited to, shaft seals and o-rings made from the cured fluoroelastomer alloy compositions. Such seals may find use in internal combustion engines. DETAILED DESCRIPTION OF THE INVENTION

The fluoroelastomer component of the alloys of the present invention comprises at least two fluoroelastomer copolymers. Each of the copolymers includes copolymerized units of vinylidene fluoride and tetrafluoroethylene. In addition, at least one of the copolymers has copolymerized units of perfluoro(methyl vinyl) ether (PMVE). Alloys containing fluoroelastomers having copolymerized units of higher perfluoro(alkyl vinyl) ethers, such as perfluoro(ethyl vinyl) ether and perfluoro(propyl vinyl) ether, exhibit unsatisfactory low temperature sealing performance when formulated into seals. Fluoroelastomers containing higher perfluoro(alkyl vinyl) ethers are therefore not suitable for use in the present invention. The copolymers suitable for use in the present invention may also include copolymerized units of other fluorinated comonomers, including hexafluoropropylene (HFP), 1 -hydropentafluoro- propylene, 2-hydropentafluoropropylene, and chlorotrifluoroethylene.

Each of the fluoroelastomer copolymer components must be peroxide curable. That is, each fluoroelastomer copolymer must contain about 0.1 to about 1 weight percent bromine or iodine, either as a polymer endgroup or incorporated into the polymer in a cure site monomer, or both. Examples of suitable brominated and iodinated cure site monomers and modifiers include 4-bromo- 3,3,4,4-tetrafluoro-butene-l (BTFB); bromotrifluoroethylene; 2-bromoperfluoro- (ethyl vinyl) ether; methylene iodide; 1,4-diiodoperfluorobutane and the like (e.g. U.S. Patent Nos. 5,032,655 and 4,035,565). The composition of each fluoroelastomer copolymer and the relative amount of each copolymer used in the alloy is chosen so that the fluoroelastomer component of the resulting alloy will have a total of 50 to 55 weight percent units of vinylidene fluoride (all percentages are based on the total weight of the fluoroelastomers unless otherwise indicated), 10 to 16 weight percent units of tetrafluoroethylene, and 20 to 30 weight percent units of perfluoro(methyl vinyl) ether. That is, according to the present invention, it is the total amount of vinylidene fluoride units, tetrafluoroethylene units, and perfluoro(methyl vinyl) ether units present in the fluoroelastomer component of the alloy which is important to attainment of low temperature properties and low fuel swell, not the particular comonomer content of each fluoroelastomer present in the alloy. Preferably the alloy will contain a fluoroelastomer having copolymerized units of hexafluoropropylene, which will be present in amounts of from 5 to 14 weight percent, based on the total weight of the fluoroelastomers. While it is necessary that each fluoroelastomer copolymer contain units of vinylidene fluoride and tetrafluoroethylene, it is not necessary that each copolymer contain hexafluoropropylene or perfluoro(methyl vinyl) ether. Such fluoroelastomer copolymers, and their method of preparation are well known in the art, (see for example, U.S. Patent Nos. 4,035,565; 4,214,060; 4,973,633; 5,032,655; 4,948,852; and 5,077,359, the disclosures of which are hereby incorporated by reference).

The organic peroxide used as a curative in the alloys of the present invention is selected from those peroxides which generate free radicals at curing temperatures. A dialkyl peroxide which decomposes above 50°C is especially preferred. In many cases one will prefer to use a di-t-butylperoxide having a tertiary carbon atom attached to a peroxy oxygen. Among the most useful are 2,5- dimethyl-,2,5-di(t-butylperoxy)hexyne-3 and 2,5-dimethyl-2,5-di(t-butylperoxy)- hexane. Other peroxides can be selected from such compounds as dicumyl peroxide, dibenzoyl peroxide, t-butyl perbenzoate, and di[l,3-dimethyl-3- (t-butylperoxy)butyl]carbonate. Between about 0.1 to 10 wt.%, based on the total weight of the fluoroelastomers, of organic peroxide is used in the compositions of this invention. Preferably, 1 to 3 wt.% is utilized. Between about 0.5 - 10 wt.% (preferably 1-7 wt.%), based on the total weight of the fluoroelastomers, of a peroxide curing coagent which is capable of cooperating with the peroxide to effect a useful cure, such as triallylisocyanurate (TAIC), trimethylallylisocyanurate (TMAIC), tris(diallylamine)-s-triazine, triallyl phosphite, and the like (U.S. Patent No. 5,032,655) is additionally present in the compositions of this invention. Other useful coagents include hexaallyl phosphoramide, N,N,-diallyl acrylamide; N,N,N'N'-tetraallyl tetraphthalamide; N,N,N',N'-tetraallyl malonamide; trivinyl isocyanurate; 2,4,6-trivinylmethyl- trisiloxane; and tri(5-norbornene-2-methylene)cyanurate. TAIC and TMAIC are preferred coagents.

A preferred fluoroelastomer blend component of the alloy is a mixture of a) 60-80 weight percent of a fluoroelastomer copolymer having copolymerized VF₂ units, TFE units, PMVE units, and BTFB units with b) 40-20 weight percent of a fluoroelastomer copolymer having copolymerized VF₂ units, TFE units, HFP units, BTFB units and iodine endgroups derived from the chain transfer agent CH₂I₂ which is present during polymerization. A particularly preferred blend is one wherein the first copolymer is a fluoroelastomer having about 54 wt.% VF₂ units, about 10 wt.% TFE units, about 35 wt.% PMVE units, and about 1.2 wt.% units of BTFB, and the second copolymer is a fluoroelastomer having about 50 wt.% VF₂ units, about 20 wt.% TFE units, about 29 wt.% HFP units, about 0.6 wt.% BTFB units and an iodine content of about 0.2 wt.%. The fluoroelastomer alloys of the present invention are prepared by blending together two or more fluoroelastomer copolymers, an organic peroxide curing agent, and a coagent for the peroxide. Blending will be carried out most generally in a Banbury mixer, internal mixer, or on a rubber mill.

The fluoroelastomer alloy composition may also include other well known additives useful in rubber compounding, for example fillers, such as carbon black, non-fibrillating fluoropolymers, clay, silica, talc, metal oxides, divalent metal hydroxides, calcium carbonate, pigments, antioxidants, stabilizers and the like; plasticizers such as dibutylsebacate, dioctylphthalate and the like.

In order to fully develop their physical properties, shaped fluoroelastomers are generally cured. Typically, curing is carried out by heating the composition for about 1-60 minutes at about 150°-200°C. Conventional rubber curing presses, molds, extruders, and the like provided with suitable heating and curing means can be used. Also, for maximum heat resistance and dimensional stability, it is preferred to carry out a post curing operation wherein the molded or extruded article is heated in an oven or the like for an additional period of about 1-48 hours, usually from about 180°-300°C. The fluoroelastomer alloy compositions of the invention are useful for producing o-ring seals, for use in fuel, lubricant and hydraulic systems, and in automotive shaft seals. They are particularly suited for formulation into seals which are used in low temperature environments. Cured seals produced from the fluoroelastomer alloy compositions of the present invention have a leak temperature of less than or equal to -40°C and a fuel swell, in Fuel C for 168 hours at 23 °C, of less than 7 volume percent, preferably less than 6.5 vol.%.

EXAMPLE Test Methods

The following test methods were used to measure various properties reported in the Example and in the claims.

Mooney Viscosity - ASTM D 1646 Tensile Strength - ASTM D 412

Elongation at break - ASTM D 412

Hardness, Shore A - ASTM D 2240

Compression set - ASTM D 395,

Method B, plied disks Fuel Immersion Volume Swell - ASTM D 471

TR-10°C and Leak Temperature - R.D. Stevens, E.W. Thomas, J.H. Brown and W.N.K. Revolta in SAE Technical Paper Series #900194 "Low Temperature Sealing Capabilities of Fluoroelastomers", 1990

EXAMPLE

Fluoroelastomer Copolymer A ("A") was a tetrapolymer containing copolymerized units of VF₂, TFE, PMVE, and BTFB units in a weight ratio of about 54:10:35:1.2 The copolymer was prepared according to the emulsion polymerization process disclosed in U.S. Patent 4,035,565 and had a Mooney viscosity, ML-10 (121°C) of approximately 85. Fluoroelastomer Copolymer B ("B"), was a tetrapolymer containing copolymerized units of VF₂, TFE, HFP, and BTFB units in a weight ratio of about 50:20:29:0.6 The copolymer contained about 0.2 wt.% iodine, had a Mooney viscosity, ML-10 (121°C) of 25, and was prepared according to the emulsion polymerization process disclosed in U.S. Patent 4,973,633.

Peroxide co-curable fluoroelastomer alloy compositions of this invention and controls of peroxide curable compositions of Fluoroelastomer Copolymer A (Control A) and of Fluoroelastomer Copolymer B (Control B) were prepared by mixing the ingredients shown in Table I in a B anbury mixer. The compositions of Table I were molded into 75 mil (1.9 mm) thick 3-inch x 6-inch (7.6 cm x 15 cm) slabs and press cured at 177° C for 10 minutes under greater than 20 tons (276 MPa) pressure, followed by a post cure at 232 °C for 24 hours in an air oven. The physical properties were measured by the above test methods and are reported in Table I. O-rings were prepared, in the same manner as the slabs, for TR-10 and leak temperature testing. As can be seen from the test results, the cured slabs prepared from samples 1-3 of the invention have excellent physical properties, particularly Fuel Volume swells of less than 7 vol.%. O-rings had leak temperatures of -40°C or less.

TABLE I

'Parts by Weight

"Triallylisocyanurate

³2,5-dimethyl-2,5-di(t-butylperoxy)hexane

Claims

CLAIMS:

1. A fluoroelastomer composition comprising a curable alloy formed by blending: a) at least two fluoroelastomers, each fluoroelastomer having copolymerized units of vinylidene fluoride and tetrafluoroethylene, and at least one of the fluoroelastomers having copolymerized units of perfluoro(methyl vinyl) ether, wherein the total weight percentage of vinylidene fluoride units, based on the total weight of fluoroelastomers, is 50 to 55; the total weight percentage of tetrafluoroethylene units, based on the total weight of fluoroelastomers, is 10 to 16; and the total weight percentage of perfluoro(methyl vinyl) ether units, based on the total weight of fluoroelastomers, is 20 to 30; each fluoroelastomer having 0.1 to 1.0 weight percent of a halogen selected from the group consisting of bromine and iodine; b) 0.5 to 10 weight percent, based on the total weight of fluoroelastomers, of an organic peroxide; and c) 0.5 to 10 weight percent, based on the total weight of fluoroelastomers, of a peroxide curing coagent; thereby producing a composition which, when formed into seals and cured, said seals have a leak temperature of less than or equal to -40°C and a fuel swell, in Fuel C for 168 hours at 23 °C, of less than 7 volume percent.

2. The composition of Claim 1 wherein at least one fluoroelastomer has copolymerized units of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene.

3. The composition of Claim 1 wherein the organic peroxide is 2,5-dimethyl-2,5-di(t-butylperoxy)hexane.

4. The composition of Claim 1 wherein the organic peroxide is present in an amount of from 1 -3 wt.%, based on the total weight of fluoroelastomers.

5. The composition of Claim 1 wherein the coagent is selected from the group consisting of triallylisocyanurate and trimethylallylisocyanurate.

6. The composition of Claim 1 wherein the peroxide curing coagent is present in the amount of from 1-7 wt.%.

7. The composition of Claim 1 wherein the fluoroelastomer component of the curable alloy is composed of a) 60-80 weight percent of a fluoroelastomer having copolymerized units of vinylidene fluoride, tetrafluoroethylene, perfluoro(methylvinyl) ether and 4-bromo-3,3,4,4-tetrafluoro- butene-l; and b) 40-20 weight percent of an iodinated fluoroelastomer having copolymerized units of vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, and 4-bromo-3,3,4,4-tetrafluorobutene-l.

8. The composition of Claim 1 wherein at least one of the fluoroelastomers comprises copolymerized units of a comonomer selected from the group consisting of 4-bromo-3,3,4,4-tetrafluorobutene- 1 ; bromotrifluoro- ethylene; and 2-bromo-perfluoro(ethyl vinyl) ether.

9. A seal formed from the cured alloy of Claim 1.