The present invention relates to fluorine containing thermoplastic vulcanizate (“TPV”) compositions comprising a continuous thermoplastic fluorocarbon resin phase and a dispersed amorphous vulcanized fluorine containing elastomer phase, which is useful as a melt formable material having rubber elasticity. Further it relates to a process for producing a fluorine containing thermoplastic vulcanizate composition, which comprises melt blending the above mentioned thermoplastic fluorocarbon resin and the non-vulcanized amorphous fluorine containing elastomer, followed by dynamically vulcanizing this blend to form chemically cross-linked elastomer particles dispersed in the thermoplastic fluorocarbon resin.
The compositions of this invention have the highly desirable property of low compression set, that is when articles made of the composition are compressed for long periods of time, even at high temperatures, they have a strong tendency to return to their original size and shape. Another advantage is that articles made from the composition of this invention are highly fluid resistant, even to fluids containing chemically basic components. Articles molded from the composition of the present invention can find use as seals and gaskets in applications where high temperatures and harsh chemical environments are common, for example in certain types of automotive or aerospace applications.
BACKGROUND OF THE INVENTION
A two phase composition comprising a continuous phase thermoplastic material and a disperse phase elastomer, produced by dynamically vulcanizing the elastomer while the discrete phase elastomer is dispersed in the continuous phase thermoplastic material, is known, for example, in Coran et al. U.S. Pat. Nos. 4,348,502, 4,130,535, 4,173,556, 4,207,404 and 4,409,365.
Fluorocarbon resins and fluorine containing elastomers are excellent in heat resistance, and a two phase blend obtainable by a combination of these materials, is considered to be excellent in heat resistance as well. Such a fluorine containing two phase blend is discussed by Pazos and Rees in EP patent 168020. The elastomer used is substantially a vinylidene fluoride/hexafluoropropylene elastomer, and as its vulcanization method, polyol vulcanization by a combination of bisphenol AF, an acid receiving agent and an onium salt, or peroxide vulcanization by a combination of an organic peroxide and a polyfunctional unsaturated compound, is employed.
The compounds disclosed in EP 168020 require oven vulcanization (post cure) following dynamic vulcanization. By only dynamically vulcanizing the materials the mechanical properties tend to be inadequate, particularly the permanent strain. In addition, following dynamic vulcanization, the compositions also tend to powder, rendering them extremely difficult to subsequently melt process in standard thermoplastic processing equipment. Finally, the compounds of EP 168020A are inherently susceptible to chemical attack and degradation by chemically basic moieties.
To address some of these problems, Kamiya and Saito in U.S. Pat. No. 5,354,811 describe alternate two phase dynamically vulcanized compounds based on fluorocarbon resin continuous phase and a dispersed fluorocarbon elastomer dispersed phase. The fluorine containing elastomers of U.S. Pat. No. 5,354,811 have vulcanizable sites selected from the group consisting of epoxy groups, carboxylic acid groups, carboxylic acid derivative groups, sulfonic acid groups and sulfonic acid derivative groups. These chemically functional cure sites allow vulcanization to proceed quickly and fully in a short period of time. Such materials were shown to have adequate properties without the need of a post cure.
SUMMARY OF THE INVENTION
A major advantage of the present invention is that it operates to compose and produce fluorocarbon based TPV's which solve problems such as TPV powdering upon synthesis, poor melt processing, and poor physical and mechanical properties, without subjection to a post cure processing cycle. Furthermore, it is the intent of the present invention to overcome these problems without resorting to chemically modifying standard commercial fluoroelastomers, or producing fluoroelastomers from their monomer constituents, with special cure site monomers. This invention provides a two phase composition comprised of fluorinated polymers, that is useful as a thermoplastic vulcanizate which is excellent in mechanical properties, excellent in heat resistance, excellent in fluids resistance, including fluids containing basic moieties, and easily melt processible. In addition, a process for its production is disclosed.
It has been found that the desired two phase composition can be obtained by using as the fluoroelastomer component a blend of two or more fluoroelastomers with at least one of the fluoroelastomers being a terpolymer of vinylidene fluoride/tetrafluoroethylene/propylene. The present invention has been accomplished based on this discovery.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides a fluorine containing thermoplastic vulcanizate composition comprising a continuous phase of at least one melt formable thermoplastic fluorocarbon resin and a dispersed vulcanized elastomer phase containing a blend of two or more fluoroelastomers with at least one of the fluoroelastomers being a terpolymer of vinylidene fluoride/tetrafluoroethylene/propylene. The vinylidene fluoride/tetrafluoroethylene/propylene fluoroelastomer comprises between 5 and 95 weight percent of the total dispersed elastomer phase, while the total dispersed elastomer phase comprises from between 40 and 90 weight percent based on the total amount of the continuous phase and the disperse phase combined.
Further, the present invention provides a process for producing a fluorine containing thermoplastic vulcanizate composition, which comprises a step of melt blending at least one melt formable thermoplastic fluorocarbon resin and at least one fluorine containing elastomer with at least one of the fluoroelastomers being a terpolymer of vinylidene fluoride/tetrafluoroethylene/propylene while exerting a mixing shear force at a temperature higher than the melting point of the thermoplastic fluorocarbon resins.
The present invention is now further described in detail with reference to the preferred embodiments and the best mode.
The thermoplastic fluorocarbon resin useful for the present invention is required to have thermoplasticity, i.e., it is required to be melt formable. Namely, it must be a resin whereby the melt flow or the volume flow rate described in ASTM D-1238, or ASTM D-2116, can be measured at a temperature higher than the melting point. It is preferably a thermoplastic fluorocarbon resin which can be melt formed at a temperature at which there is no problem of deterioration of the fluorine containing elastomer. Among thermoplastic fluorocarbon resins, all fluorocarbon resins except for polytetrafluoroethylene resins which cannot be melt formed, may be employed.
The thermoplastic fluorocarbon resin useful for the present invention is a thermoplastic fluorocarbon resin having a fluorine content of at least 35% by weight, which can be obtained by polymerizing an ethylenically unsaturated compound containing a completely or partially fluorinated fluoroolefin, preferably at least one fluoroolefin selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, vinylidene fluoride, vinyl fluoride, trifluoroethylene chloride and a perfluoroalkylvinyl ether (wherein the alkyl group has from 1 to 8 carbon atoms).
The ethylenically unsaturated compound may, for example, be a non-fluorinated olefin such as ethylene or propylene, an alkylvinyl ether, or a perfluoroalkyl ethylene, in addition to the above olefins.
Preferred among such polymers is a tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymer, a tetrafluoroethylene/ethylene copolymer, a tetrafluoroethylene/hexafluoropropylene copolymer, a tetrafluoroethylene/perfluoropropylvinyl ether copolymer, a trifluoroethylene chloride/ethylene copolymer, or a vinylidene fluoride polymer. Particularly preferred are a tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymer and a tetrafluoroethylene/ethylene copolymer. Most preferred is a tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymer. A plurality of fluorocarbon resins may be used in combination. These copolymers may have other copolymerizable components further copolymerized.
Suitable semi-crystalline fluorine containing thermoplastics are tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymers which are available under the tradename THV from Dyneon LLC (Oakdale, Minn.), such as grades THV 410, THV 500 and THV 610X, especially the latter. The monomer ratio affects crystallinity, mechanical properties and melt temperature. These grades have fluorine contents in the range of 70 to 76 weight percent and have crystalline melting points of 155° C., 165° C. and 185° C. respectively. As an example of the monomer ratios in these terpolymers, THV410 has as a ratio of tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride of 53/18/29 respectively. Generally, an increase in the tetrafluoroethylene in the monomer mix leads to an increase in the crystalline melting point.
The fluorocarbon resin is suitably selected also for the combination with the fluorine containing elastomer which will be described hereinafter. For example, when the fluorocarbon resin for the fluorine containing elastomer is a polymer having vinylidene fluoride units, a polarity will be formed. Accordingly, as a combination of the fluorocarbon resin and the fluorine containing elastomer, it is most desirable to select a combination of the resin and the elastomer both having vinylidene fluoride units to obtain a composition having uniform and excellent physical properties.
The fluorocarbon elastomers required for the present invention are blends of two or more fluorine containing elastomers where at least one of the elastomers is vinylidene fluoride/tetrafluoroethylene/propylene terpolymer. Small amounts of a fourth monomer, such as hexafluoropropylene, may or may not be present. The proportion of the respective monomer units in such terpolymers are optionally selected, taking into account various properties, such as the mechanical properties, heat resistance, low temperature resistance, chemical resistance, oil resistance, etc. For instance, the vinylidene fluoride-tetrafluoroethylene-propylene terpolymer preferably comprises from 2 to 50 mol % of vinylidene fluoride units, from 20 to 60 mol % tetrafluoroethylene units and from 20 to 50 mol % of propylene units.
Further, in such terpolymers, the proportions of the vinylidene units is preferably from 2 to 50 mol %, more preferably from 10 to 40 mol %. If the proportion is too high, there will be drawbacks with respect to the physical properties, such as a decrease in the alkali resistance or in the amine resistance of the terpolymer. On the other hand, if the proportion is too low, the formation of unsaturated bonds will be inadequate, and the effectiveness for the improvement of the curability deteriorates.
Suitable amorphous vinylidene fluoride/tetrafluoroethylene/propylene terpolymers are available under the tradename BRE (Base Resistant Elastomer) from Dyneon LLC (Oakdale, Minn.), such as grades BRE 7131X, BRE 7132X and BRE 7231X, especially the first one listed. The fluorine content of these terpolymer elastomers is approximately 60 weight percent. Appropriate monomer ratios and the absence, or near absence, of hexafluoropropylene, renders the vinylidene fluoride/tetrafluoroethylene/propylene terpolymers chemically resistant to chemically basic moieties.
The vinylidene fluoride/tetrafluoroethylene/propylene fluoroelastomer comprises between 5 and 95 weight percent of the total dispersed elastomer phase, while the total disperse elastomer phase comprises from between 40 and 90 weight percent based on the total amount of the continuous phase and the disperse phase combined in the material of this invention.
The elastomeric components making up the balance of the dispersed phase are fluorine containing amorphous materials which are known in the art and are commercially available. Suitable elastomeric components include, but are not limited to, copolymers of vinylidene fluoride/hexafluoropropylene, copolymers of tetrafluoroethylene/propylene, copolymers of tetrafluoroethylene/perfluoroalkylvinyl ether, copolymers of vinylidene fluoride/perfluoroalkylvinyl ether, terpolymers of tetrafluoroethylene/vinylidene fluoride/hexafluoropropylene and terpolymers of tetrafluoroethylene/perfluoroalkylvinyl ether/propylene.
The elastomer phase of the composition of this invention may be cured with conventional curing systems known for curing fluoroelastomers, such as combinations of magnesium oxide/Bisphenol AF/organophosphonium salts, magnesium oxide/the dipotassium salt of Bisphenol AF/dicyclohexyl 18 Crown 6,2,5-dimethyl-2,5-diterbutylperoxyhexyne-3/meta phenylenebismaleimide or triallyl isocyanurate and cumene hydroperoxide/maleimide or triallyl isocyanurate.
The composition of the present invention may contain vulcanization accelerators, fillers, antioxidants, stabilizers, pigments, processing assistants, etc. in an amount at a level of common use.
The particle size of the dispersed phase of the present invention varies depending upon the components of the composition, the proportions of the respective components, the viscosities of the respective components, the production conditions, etc. However, the average particle size is: broadly stated, no larger than about 50 microns, preferably no larger than about 5 microns, and most preferably not larger than 3 microns.
The process of the present invention comprises melt blending the thermoplastic fluorocarbon resin and the fluorine containing elastomers at a temperature higher than the melting temperature of the fluorocarbon resin in either batch or continuous mixers, followed by vulcanizing the fluorine containing elastomer while exerting a mixing shear force. The temperature for mixing may be suitably selected depending on the types of thermoplastic fluorocarbon resin and the fluorine containing elastomer used.
It is critical to conduct the vulcanization while exerting a mixing shear force. As the vulcanization is conducted while a mixing shear force is applied, the thermoplastic fluorocarbon resin will form a continuous phase; and a disperse phase, composed of a cured fluorocarbon elastomer, will be uniformly dispersed in the continuous phase. Such a continuous phase will form even when the thermoplastic resin is not the major component. If the continuous and the disperse phase are reversed, that is if the thermoplastic fluorocarbon resin becomes the dispersed phase and the fluorocarbon elastomer and continuous phase, then the material is not melt processible.
The present invention provides a two phase composition comprising a continuous phase thermoplastic material and a disperse phase elastomer, produced by dynamically vulcanizing the elastomer—that is curing the elastomer while it is undergoing shear stress from mixing and after it has been melt mixed with the thermoplastic. A fluorocarbon resin is used as the thermoplastic phase and a blend of two or more fluorine containing elastomers are used as the dispersed elastomeric phase, where at least one of the elastomers is a fluorine vinylidene fluoride/tetrafluoroethylene/propylene terpolymer. The present invention is highly useful as a thermoplastic vulcanizate, excellent in moldability, mechanical properties and fluid resistance, especially to fluids containing chemically basic moieties. The present invention also provides a process for its production.
The following examples explain the present invention, however, in no way should they be taken to limit the scope of the present invention.
All data presented in the following examples were generated using standard ASTM test methods. The hardness test was performed according to ASTM D2240-97, the tensile properties according to ASTM 412-98a and the specific gravity according to ASTM D297-93. Fluid immersion testing was performed according to ASTM D471-97, and compression set testing according to ASTM D395—Method B.
In addition, test plaques used to die cut test specimens were all prepared by compression molding material for 5 minutes at 254° C. and then cooling under pressure for 10 minutes so the temperature falls below 100° C.