Curable fluoroelastomer compositions having improved compression set after curing are provided, as well as compositions that are of relatively high purity. These compositions include a hydrogen-containing fluoroelastomer having interpolymerized units derived from a cure-site monomer, a curative (optionally with a co-agent), and an organo-onium. When the fluoroelastomer contains units derived from vinylidene fluoride, the composition is essentially free from an inorganic acid acceptor. Methods of improving the compression of cured fluoroelastomer and cured fluoroelastomers having improved compression set are also provided.

The present invention relates to fluoropolymers, especially elastomeric fluoropolymers (i.e., fluoroelastomers), compositions incorporating such fluoropolymers, articles employing such fluoropolymers, and methods of making and using such fluoropolymers.
Cured compositions containing fluoropolymers are desirable for a number of applications. For example, they can be used as gaskets and O-rings. Such shaped articles having good compression set (i.e., little or no deformation of the cured article after a load is applied and removed) are particularly desirable.
Also, high purity fluoropolymers are used in a number of industries. They are especially preferred for use in the electronic, semiconductor, optical, medical, and pharmaceutical industries, to name a few. These polymers have a relatively low level of water extractable metals and metal compounds.
Inorganic acid acceptors are often added to fluoroelastomers utilizing peroxide cure systems. Optionally, at least one metal compound selected from divalent metal oxides or divalent metal hydroxides is frequently blended with the fluoroelastomer during preparation or before it is cured. While the presence of such compounds improves the heat aging resistance and thermal stability of the polymer (see, for example, U.S. Pat. No. 5,077,359), the addition of such acid acceptors has a very detrimental effect on the content of ions and extractables of the resulting elastomer composition. EP Pat. No. B-708 797 discloses the use of peroxide-curable fluoroelastomers with a fluoropolymer micropowder filler for semiconductor applications. There is no disclosure of how the materials were purified; however, due to the fact that this process requires acid acceptors the overall ion content seems to be unsatisfactorily high. Other fluoroelastomers that include such inorganic acid acceptors include EP 0 140 207 and U.S. Pat. Nos. 4,233,421 and 4,912,171, for example.
The present invention provides curable fluoroelastomer compositions, cured compositions, which can be secured to (e.g., coated on) a substrate, cured articles (e.g., shaped articles), and methods. Preferably, the compositions of the present invention are particularly useful as shaped articles such as gaskets and O-rings. In certain aspects of the invention, the fluoroelastomer compositions have improved compression set after being cured. In certain other aspects of the invention, the fluoroelastomer compositions are of a relatively high purity.
Preferably, the curable fluoroelastomer compositions of the present invention have improved compression set after they are cured. These curable compositions include a fluoroelastomer having units derived from a cure-site monomer and capable of peroxide cure, a peroxide curative for the fluoroelastomer, optionally a co-agent for the curative, and a non-fluorine-containing organo-onium. The improvement in compression set can be demonstrated by a comparison of cured fluoroelastomers with and without the organo-onium present. Those fluoroelastomers that contain the organo-onium have surprisingly better compression set properties than those of a similar or the same fluoropolymer composition but not employing the organo-onium. That is, there is little or no deformation of the cured article after a load is applied and removed using the test procedure described in the Examples Section when the fluoroelastomers are made using an organo-onium compared to when they are not. Preferably, the curative used in this embodiment is a peroxide.
This improvement is believed to be achieved whether or not the fluoroelastomer is of relatively high purity, although high purity can be desirable. Thus, in one aspect, the present invention provides a relatively high purity polymer that is essentially free of inorganic acid acceptors, which are typically bases such as oxides and hydroxides of calcium, magnesium, zinc, lead, etc. In an alternative embodiment, a relatively high purity polymer is essentially free of all ions other than NH4+, H+, and OHxe2x88x92. In these embodiments, the fluoroelastomer includes one or more units derived from a hydrogen-containing monomer and one or more units derived from a cure-site monomer. When the hydrogen-containing monomer is a vinylidene fluoride, the curable fluoroelastomer composition is essentially free from one or more inorganic acid acceptors. The preferred high purity elastomeric fluoropolymer is a peroxide-curable elastomer, optionally having pendant nitrile groups.
In one embodiment, the present invention provides a curable fluoropolymer composition that includes: a hydrogen-containing fluoroelastomer having units derived from a cure-site monomer, wherein the fluoroelastomer is capable of peroxide cure; a peroxide curative; optionally, a co-agent for the curative; and a non-fluorine-containing organo-onium; with the proviso that when the hydrogen-containing fluoroelastomer includes one or more units derived from vinylidene fluoride, the composition is essentially free from one or more inorganic acid acceptors.
In another embodiment, the present invention provides a process for improving compression set of a cured fluoroelastomer. The method includes providing a curable fluoroelastomer composition and curing the curable composition. The curable fluoroelastomer composition includes: a fluoroelastomer having units derived from at least one monomer including a carbon-bonded hydrogen and units derived from a cure-site monomer, wherein the fluoroelastomer is capable of peroxide cure; a peroxide curative; optionally, a co-agent for the curative; and a non-fluorine-containing organo-onium; with the proviso that when the hydrogen-containing fluoroelastomer includes one or more units derived from vinylidene fluoride, the composition is essentially free from one or more inorganic acid acceptors.
In yet another embodiment, the present invention provides a cured fluoroelastomer that includes the reaction product of a curable composition including: a hydrogen-containing fluoroelastomer having units derived from a cure-site monomer, wherein the fluoroelastomer is capable of peroxide cure; a peroxide curative; optionally, a co-agent for the curative; and a non-fluorine-containing organo-onium; with the proviso that when the hydrogen-containing fluoroelastomer includes one or more units derived from vinylidene fluoride, the composition is essentially free from one or more inorganic acid acceptors.
As used herein, the terms xe2x80x9ca,xe2x80x9d xe2x80x9can,xe2x80x9d xe2x80x9cthe,xe2x80x9d xe2x80x9cone or more,xe2x80x9d and xe2x80x9cat least onexe2x80x9d are used interchangeably.
The present invention provides curable fluoropolymer compositions that include a hydrogen-containing fluoroelastomer having units derived from a cure-site monomer, a curative, and an organo-onium. Optionally, the compositions can include a co-agent for the curative and a partially crystalline fluoropolymer (e.g., fluorothermoplast or other fluoropolymer such as polytetrafluoroethylene) as a filler. Certain embodiments of the present invention are directed to polymers that have improved compression set and certain embodiments of the present invention are directed to polymers that have relatively high purity (i.e., polymers that are essentially free of ions other than NH4+, H+, and OHxe2x88x92 and/or essentially free of inorganic acid acceptors).
The cured fluoroelastomer preferably has improved compression set relative to the same cured fluoroelastomer made without the organo-onium. Herein, an xe2x80x9cimprovementxe2x80x9d is a decrease in the residual deformation reported as a percentage compression set as determined by the test method described in the Examples Section. Preferably, the compression set is less than about 20%, more preferably, less than about 15%, even more preferably, less than about 10%, and most preferably, zero.
In cases where improved compression set is desired, the addition of organo-onium compounds are beneficial to improve properties in formulations that do not contain inorganic acid acceptors; however, the improvement in compression set may be achieved in the fluoroelastomers even if they are not essentially free of inorganic acid acceptors and ions different than NH4+, H+ and OHxe2x88x92.
Fluoropolymers
The fluoroelastomers used in the invention are not perfluorinated. That is, they include units derived from at least one monomer with hydrogen bonded to a carbon atom such that they are xe2x80x9chydrogen-containing.xe2x80x9d They include interpolymerized units derived from a cure-site monomer. The fluoroelastomers may or may not include one or more units derived from vinylidene fluoride. Preferably, the fluoroelastomers are essentially free of units derived from vinylidene fluoride. By xe2x80x9cessentially free from units derived from vinylidene fluoridexe2x80x9d it is meant that the fluoroelastomer contains less than 10%, preferably less than 5%, more preferably 0%, by weight of such units. Furthermore, the composition may or may not include one or more inorganic acid acceptors. If the fluoroelastomer includes one or more units derived from vinylidene fluoride, the composition is essentially free from one or more inorganic acid acceptors.
The fluoropolymer (including flouroelastomers and fluorothermoplasts), preferably a fluoroelastomer, is typically a polymerized product of one or more fluoroolefin monomers and optionally one or more hydrocarbon olefin monomers. Generally, the fluoroolefin monomers have from 2 to 8 carbon atoms. Examples of such fluoroolefin monomers include tetrafluoroethylene (TFE), vinylidene fluoride (VF2), hexafluoropropylene (HFP), trifluoroethylene, chlorotrifluoroethylene (CTFE) and fluorinated ethers such as perfluoroalkyl vinyl ethers (VE). Examples of useful hydrocarbon olefins include ethylene and/or propylene.
The fluoropolymers are also derived from a cure-site monomer. Examples of useful xe2x80x9ccure site monomersxe2x80x9d include bromine, iodine, or nitrile groups, which can provide xe2x80x9ccure sitesxe2x80x9d for curing the fluoropolymer.
In case of peroxide-curable fluoroelastomers, bromine-containing cure site comonomers are preferred such as a bromine-containing olefin, preferably containing another halogen such as fluorine. Examples are bromotrifluoroethylene, 4-bromo-3,3,4,4-tetrafluorobutene-1 and a number of others noted in U.S. Pat. No. 4,035,565. Brominated fluorovinyl ethers useful in the invention include CF2Brxe2x80x94Rfxe2x80x94Oxe2x80x94CFxe2x95x90CF2, wherein Rf is a fully fluorinated alkylene of up to 4 carbons atoms like xe2x80x94CF2xe2x80x94, such as CF2BrCF2OCFxe2x95x90CF2, cited in U.S. Pat. No. 4,745,165 and of the type ROCFxe2x95x90CFBr or ROCBrxe2x95x90CF2 where R is a lower alkyl group or fluoroalkyl group each having up to 4 carbon atoms, such as CH3CFxe2x95x90CFBr or CF3CH2OCFxe2x95x90CFBr, cited in U.S. Pat. No. 4,564,662. The choice of bromine-containing units is based on copolymerizability with the major monomers and low branching tendency, in addition to cost and availability (U.S. Pat. No. 5,077,359).
Nitrile-containing cure site monomers may also be used. Preferred examples are nitrile-containing fluorinated olefins and nitrile-containing fluorinated vinyl ethers, such as CF2xe2x95x90CFO(CF2)mCN, CF2xe2x95x90CFO[CF2CF(CF3)O]q(CF2O)sCF(CF3)CN, CF2xe2x95x90CF[OCF2CF(CF3)]rO(CF2)tCN, where, in reference to the above formulaes m=2 to 12, q=0 to 4, r=1 to 2, s=0 to 6 and t=1 to 4. Representative examples of such a monomer include perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene), CF2xe2x95x90CFO(CF2)5CN, CF2xe2x95x90CFO(CF2)3OCF(CF3)CN, CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2)4xe2x80x94Oxe2x80x94CF(CF3)CN, and CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2)3xe2x80x94CN.
The optional partially crystalline fluoropolymer can be used as a filler. In contrast to amorphous fluoroelastomers, partially crystalline fluoropolymers are polymers with melting points (typically, from 100xc2x0 C. to 340xc2x0 C.). The partially crystalline fluoropolymers used in the invention can be used in the form of their dispersions, i.e., the mixtures of the fluoroelastomers and fillers may be prepared by blending the latexes of the materials. Such dispersions can contain low molecular weight polytetrafluoroethyene (PTFE), the so-called micropowders or waxes (as described in U.S. Pat. No. 3,956,000) optionally modified with HFP and/or VE. The dispersions comprise melt-processable copolymers of TFE and perfluoro(propyl-vinyl) ether known as xe2x80x9cPFAxe2x80x9d, melt processable copolymers of TFE and HFP, which is known as xe2x80x9cFEPxe2x80x9d, also partially crystalline polymers of TFE and ethylene (ET), which is known as xe2x80x9cETFExe2x80x9d, or TFE, HFP and VF2, which is known as xe2x80x9cTHVxe2x80x9d. These copolymers are extensively described in xe2x80x9cModern Fluoropolymersxe2x80x9d, High Performance Polymers for Diverse Applications, edited by John Scheirs, John Wiley and Sons (1997), especially pages 223-270, 301-310, and 373-396. Preferably, the partially crystalline fluoropolymers used as fillers have melting points higher than the processing temperature of the mixture.
Highly Pure Fluoropolymer Compositions
One aspect of the invention provides curable fluoroelastomer compositions and cured polymers that are preferably essentially free of inorganic acid acceptors and salts from coagulation. As used herein, xe2x80x9cessentially freexe2x80x9d means less than 1000 parts per million (ppm), and preferably less than 500 ppm. Such curable compositions and the resultant cured products have a relatively high purity.
In another aspect of the invention, there is provided curable compositions and cured products that are essentially free of all water extractable ions other than NH4+, H+ and OHxe2x88x92 and more preferably, essentially free of all water extractable ions except H+ and OHxe2x88x92, which are typically present in the latex as a result of the polymerization process. Examples of such ions include perfluorooctanoate, sulfate, chloride, fluoride, etc. As used in this context, xe2x80x9cessentially freexe2x80x9d means that less than 1000 ppm, preferably less than 500 ppm, anions and cations (other than NH4+, H+ and OHxe2x88x92) are found in the fluoropolymer (before adding cure chemicals and other ingredients). Such curable compositions and the resultant cured products have a relatively high purity.
Typically, inorganic acid acceptors are not added to the process, and thus do not need to be removed. Removal of ions can be performed by dialysis, preferably, ion exchange. An ion exchange process is disclosed in, e.g., U.S. Pat. Nos. 4,282,162 and 5,463,021 and DE-A-20 44 986. Other processes are disclosed in WO-A-99/62830 and WO-A-99/62858. A detailed discussion of certain purification processes is presented below.
A preferred process of manufacturing fluoroelastomer compounds that are essentially free of ions includes: purifying an aqueous fluoroelastomer latex (or a blend of aqueous fluoroelastomer latex and a latex of a partially crystalline fluoropolymerxe2x80x94if a fluoropolymer filler is required) by using separately a cationic and subsequently an anionic exchange treatment, or vice versa, and coagulating the purified latex by using salt-free methods.
Preparation of Fluoropolymers
The fluoropolymers (e.g., fluoroelastomers and partially crystalline fluoropolymers), preferably fluoroelastomers, can be prepared by known methods, preferably, by an aqueous emulsion polymerization process. In this process, an aqueous colloidal dispersion (i.e., latex) is obtained by polymerizing fluorinated monomers in an aqueous medium containing a relatively high amount of fluorinated emulsifiers such as salts of perfluoro octanoic acid and the like with mild agitation. This process is described in detail in xe2x80x9cModem Fluoropolymersxe2x80x9d, High Performance Polymers for Diverse Applications, edited by John Scheirs, John Wiley and Sons (1997), especially pages 233-237.
Emulsion polymerization is to be distinguished from the suspension polymerization process. The latter method uses little or no emulsifier and requires vigorous agitation that results in a granular product. Usually emulsion polymerization is carried out within a pressure range of 5 bar to 30 bar (5xc3x97105 to 3xc3x97106 Pa) and within a temperature range of 5xc2x0 C. to 100xc2x0 C. Generally, emulsion polymerization processes use significant amounts of adjuvants, such as emulsifiers, initiators, buffers, etc., although a preferred polymerization process includes a minimum level of adjuvants like buffers (and preferably, no buffer).
It is generally accepted that a prerequisite for an aqueous emulsification is the use of a non-telogenic emulsifier (U.S. Pat. No. 2,559,752). As noted above, fluorinated emulsifiers, most often perfluorinated alkanoic acids, are used. Generally, they are used in an amount of from 0.02% by weight to 3% by weight with respect to the polymer (i.e., based on the total weight of the polymerizable composition).
A further material used in the polymerization process is a water soluble initiator to start the polymerization. Commonly, salts of peroxo-sulfuric acids are applied, often in the presence of further co-agents like bisulfites or sulfinates (U.S. Pat. Nos. 5,285,002 and 5,378,782) or the sodium salt of hydroxymethane sulfinic acid (available under the trade designation RONGALIT from BASF, Ludwigshafen, Germany). All of these initiators and the emulsifiers have an optimum pH-range where they show most efficiency. For this reason, buffers are typically used. The buffers include phosphate, acetate, or carbonate buffers, or any other acid or base such as ammonia or alkali metal hydroxides. The concentration range for the initiators and buffers can vary from 0.05% by weight to 5% by weight, based on the aqueous polymerization medium.
Removal of Ions
The removal of the ions from the corresponding dispersions used during polymerization is preferably achieved by using cation and anion exchangers. It is a preferred way to remove first the anions (such as perfluorooctanoate, sulfate, chloride, fluoride, etc.) from the dispersions. The removal of the anions via anion exchangers is an important step for the following reasons: the latex particles have a submicroscopic diameter of less than 400 nm; the latex particles are anionically stabilized in the sense of colloid chemistry; the anionic stabilization is provided by anionic endgroups, mostly xe2x80x94COOH and xe2x80x94OSO3H groups, and by the adsorbed anionic emulsifier such as PFOA. Such anionically stabilized dispersions tend to coagulate readily in an anion exchange bed and thus jam the ion exchange bed. Therefore, the treatment of an anionically stabilized dispersion with an anion exchanger is considered to be technically not feasible, in particular at higher concentrations.
The impairing or clogging of the anion exchange bed is already observed at solid contents 1000 times lower than those of the raw polymer dispersions, i.e., the dispersion after polymerization. This coagulation does not occur in the presence of a nonionic emulsifier usually in the range of 0.001 to 3.0% by weight of solid content as described in International Publication Nos. WO 99/62830 and WO 99/62858. Nonionic emulsifiers are described in detail in xe2x80x9cNonionic Surfactantsxe2x80x9d edited by M. J. Schick, Marcel Dekker, Inc., New York, 1967.
The choice of the nonionic emulsifier is not critical. Alkyl aryl polyethoxy alcohols, alkyl polyethoxy alcohols, or other nonionic emulsifier may be used. Preferred nonionic surfactants are alkyl aryl polyethoxy alcohol type such as that available under the trade designation TRITON X 100 from Rohm and Haas, or alkyl polyethoxy alcohol type such as that available under the trade designation GENAPOL X 080 from Clariant GmbH.
The choice of the ion exchange resin is not very critical. Usable anion exchange resins include those commercially available under the trade designations AMBERLITE IRA 402 and AMBERJET 4200 from Rohm and Haas, PUROLITE A 845 from Purolite GmbH, LEWATIT MP-500 from Bayer AG, and DOWEX 1X-2X series from Dow Chemical.
The specific basicity of the anion exchanger used is not very critical. Weakly, medium and strongly basic resins can be used. Preferably, the ion exchange resin is transformed to the OHxe2x88x92 form.
The flow rate is not very critical, standard flow rates can be used. The flow can be upward or downward. The ion exchange process can also be carried out as a batch process by mildly stirring the dispersion with the ion exchange resin in a vessel. After this treatment the dispersion is isolated by filtration.
The removal of the anions is preferably carried out with raw dispersions from the polymerization. Such dispersions generally have a solid content of 10% by weight to 40% by weight, to which is added sufficient nonionic emulsifier to provide dispersion stability and, if necessary, to decrease the solid content to less than about 20%. In a subsequent step the cations are removed by using readily available resins like LEWATIT SP 112 (Bayer AG), preferably in the H+-form. The use of mixed ion exchange resins (which have anion and cation exchange groups) is a possibility, too.
Preparation of Fluoropolymer Latex
Following removal of the ions, the fluoropolymer is preferably coagulated from the latex without the addition of ions. This can be effected by the known freeze-coagulation process as described in U.S. Pat. No. 5,708,131. During this process the whole aqueous latex is frozen and the polymer is coagulated. This process can be performed batchwise or continuously. While the isolated polymer is already clean some subsequent washing steps with water may be beneficial.
Another process for coagulation without the addition of ions is the so-called mechanical coagulation process, which is disclosed in U.S. Pat. No. 5,463,021 for fluorothermoplasts. The fluorothermoplast dispersions are first compressed to pressures up to 200 bar to 400 bar (2xc3x97107 to 4xc3x97107 Pascals) and then decompressed through nozzles or slits and thereby the coagulation is achieved. The high pressures are produced by a so-called homogenizer. This technology does not work for elastomer dispersions due to clogging and jamming the equipment. As suggested in DE 100 04 229.5 (filed Feb. 1, 2000) these difficulties can be overcome by generating high pressures with pressurized gases from 50 bar to 400 bar (5xc3x97106 Pa to 4xc3x97107 Pa). In contrast to the process disclosed in U.S. Pat. No. 5,463,021 the coagulation by this technique appears to be brought about by the rapid expansion of the dissolved gas, preferably via a nozzle. The preferred gases for use with this process are nitrogen, air, or CO2.
In yet another coagulation process a volatile water-miscible organic solvent effects the coagulation. The solvent is selected from the group consisting of alkanols of 1 to 4 carbon atoms and ketones of 2 or 3 carbon atoms. Such solvents should not have a significant swelling effect on the polymer to prevent the coagulated or agglomerated polymer from becoming too sticky and difficult to process.
Mechanical and thermal methods for coagulating polymers are disclosed in U.S. Pat. No. 5,463,021, and EP Pat. Nos. B-0 084 837, B-0 226 668, and B-0 460 284.
After being coagulated the fluoropolymer may be dewatered. One method of dewatering the polymer is by mechanical dewatering. This process is described in U.S. Pat. No. 4,132,845. The wet polymers can be dried, usually at a temperature within a range of 110xc2x0 C., preferably 150xc2x0 C., to 250xc2x0 C., in the presence of a carrier gas like air or nitrogen.
The elastomers with incorporated polymer fillers can be prepared by blending the solids as described in EP-B-0-708 797 or by blending latices or rubbers or both either before ion exchange treatment or thereafter. The latex blend method ensures the most uniform distribution of the polymer filler and avoids any contamination as compared to dry blending. After latex blending, the blends may be coagulated, dewatered, and dried as described above.
It is important to avoid the known coagulation chemicals like salts such as sodium or magnesium chloride or mineral acids since especially with regard to fluoroelastomers the coagulated products tend to be sticky and to clump together. Therefore, despite thorough washing the dried product still contains significant amounts of such added coagulation chemicals and of salts present in the polymerization recipe.
Curable Fluoropolymer Compositions
The present invention provides fluoropolymer compositions for coating articles by contacting the article with a coating composition containing the fluoropolymer and immediately after this coating step or at some later time optionally the coating will be cured to yield an article with excellent surface properties. The invention also provides shaped articles prepared from the fluoropolymers and especially fluoroelastomers, in the latter case by subsequent curing of the article.
A preferred curable fluoropolymer composition includes a peroxide-curable fluoroelastomer essentially free of ions other than NH4+, H+ and OHxe2x88x92, and, based on the elastomeric polymer less than 5.0 parts per one hundred parts of fluoropolymer resin (phr) of an organic peroxide, less than 10.0 phr of co-agent, and less than 50 phr of a partially crystalline fluoropolymer essentially free of ions.
Another preferred curable fluoropolymer composition includes a peroxide-curable fluoroelastomer and from 0.5 phr to 3 phr of an organic peroxide, from 1 phr to 7 phr of a co-agent, and from 10 phr to 40 phr of a partially crystalline fluoropolymer.
Yet another preferred curable fluoropolymer composition includes: a peroxide-curable nitrile group containing fluoroelastomer essentially free of ions other than NH4+, H+ and OHxe2x88x92, and, based on the elastomer polymer, less than 5 phr (preferably from 0.05 phr to 2 phr) of an ammonia generating cure system, and less than 50 phr (preferably from 10 phr to 40 phr) of a partially crystalline fluoropolymer essentially free of such ions.
For a peroxide-curable fluoroelastomer, then the cure-site monomer includes the bromine- or iodine- or nitrile-containing monomers described above. A peroxide-curable fluoroelastomer is especially useful in a curing composition containing an organic peroxide, a co-agent and, optionally, a fluoropolymer filler.
When the peroxide-curable fluoroelastomer contains a nitrile cure-site monomer, it may be cured by using the techniques described in U.S. Pat. Nos. 5,677,389, 5,565,512, and 4,281,092, and International Publication No. WO 00/09603. If the fluoropolymer contains a nitrile group it is especially useful in a composition containing an ammonia generating catalyst. See, for example, U.S. Pat. Nos. 5,677,389 and 5,565,512, and International Publication No. WO 00/09603.
Additional cure systems which may be used with copolymers containing the above-mentioned nitrile cure-site monomers include those described in copending applications with U.S. Ser. Nos. 60/265,498, 60/233,386 and 60/233,383. These may be used as a replacement for or in combination with the ammonia generating cure system discussed above.
A widely used cure-system comprises polyol compounds in the presence of onium compounds (U.S. Pat. Nos. 4,233,421, 4,912,171, and 5,262,490), which adds further ion loadings to the finished article. For the manufacture of electronic components, such as semiconductor devices, unusually stringent requirements exist for sealing compounds. Apparently, fluoroelastomers cured by known processes cannot meet such requirements.
The peroxide-curable elastomers are cured by a free radical process. A curable composition includes a fluoropolymer and a peroxide to generate free radicals at the desirable curing temperatures. A dialkyl peroxide, which decomposes at a temperature above 50xc2x0 C., is especially preferred when the composition is to be processed at elevated temperatures before it is cured. In many cases one will prefer to use a di-tertiarybutyl peroxide having a tertiary carbon atom attached to peroxy oxygen. Among the most useful peroxides of this type are 2,5-dimethyl-2,5-di(tertiarybutylperoxy)-hexyne-3 and 2,5-dimethyl-2,5-di(tertiarybutylperoxy)-hexane. Other peroxides can be selected from such compounds as dicumyl peroxide, dibenzoyl peroxide, tertiarybutyl perbenzoate, and di[1,3-dimethyl-3-(tertiarybutylperoxy)-butyl]carbonate.
Another material which is usually blended with the composition before it is made into end products is a co-agent composed of a polyunsaturated compound that is capable of cooperating with the peroxide to provide a useful cure. These crosslinking co-agents can be added in an amount equal to 0.5 to 10%, preferably about 1 to 7%, by weight of the copolymer content, and may be one or more of the following compounds: triallyl cyanurate; triallyl isocyanurate; tri(methallyl)-isocyanurate; tris(diallylamine)-s-triazine; triallyl phosphite; N,N-diallyl acrylamide; hexaallyl phosphoramide; N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl terephthalamide, N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl malonamide; trivinyl isocyanurate; 2,4,6-trivinyl methyltrisiloxane; and tri(5-norbornene-2-methylene) cyanurate. Particularly useful is triallyl isocyanurate (see, U.S. Pat. No. 5,077,359).
As discussed above, inorganic acid acceptors are often added during the peroxide cure step to improve the heat aging resistance and thermal stability of the polymer; however, the addition of such acid acceptors would have a very detrimental effect to the ion content and extractables of the resulting elastomer composition. Thus, for the curing of the peroxide-curable fluoroelastomers of the present invention, with or without polymeric filler, it is not necessary to add further any kind of acid acceptors for obtaining excellent cure characteristics and physical properties and articles with low ion contents.
In cases where improved compression set is desired, the addition of organo-onium compounds are beneficial to improve properties in formulations that do not contain inorganic acid acceptors; however, the improvement in compression set may be achieved in the fluoroelastomers even if they are not essentially free of inorganic acid acceptors and ions different than NH4+, H+ and OHxe2x88x92. The organo-onium compounds can be selected from a large variety of compounds in such a way to meet the specific requirements of various application fields.
As noted above, the fluoroelastomers utilized in these curable compositions not only contain a cure-site monomer, they are also preferably essentially free from units derived from vinylidene fluoride units. In achieving this improvement in compression set the fluoroelastomer is combined with (a) an appropriate curative in amounts and types such as is described above, optionally (b) a co-agent for the curative in amounts and types such as is described above, and (c) an organo-onium in amounts and types such as described hereinafter.
Organo-onium compounds represent one class of useful additives to the fluoroelastomer compositions of the invention. Suitable organo-onium compounds are known in the art, generally as vulcanization accelerators for the elastomers cured by dihydroxy-containing curing agents. As it is known, an organo-onium is the conjugate acid of a suitable Lewis-base (e.g., phosphine, amine, ether, and sulfide) and can be formed by reacting said Lewis-base with a suitable alkylating agent (e.g., an alkyl halide or aryl halide). The organo-onium compounds contain at least one heteroatom such as N, P, S, or O bonded to organic or inorganic moieties. Preferably, they do not include fluorine atoms (i.e., they are non-fluorine-containing organo-oniums). One particularly useful class of the quaternary onium compounds broadly comprises relatively positive and negative ions wherein phosphorus and nitrogen generally comprise the central atom of the positive ion and the negative ion may be an organic or inorganic anion (e.g., halide, sulfate, acetate, phosphate, hydroxide, alkoxide, phenoxide).
Organo-onium compounds, preferably, non-fluorine-containing organo-onium compounds, suitable for use in the compositions of the present invention are described in U.S. Pat. Nos. 4,233,421; 4,912,171; and 5,262,490. Examples include triphenylbenyl phosphonium chloride, tributyl alkyl phosphonium chloride, tributyl benzyl ammonium chloride, tetrabutyl ammonium bromide, and triarylsulfonium chloride. Another class of organo-onium compounds are represented by the following formula:
wherein
Q is nitrogen or phosphorus;
Z is a hydrogen atom or is a substituted or unsubstituted, cyclic or acyclic alkyl group having from 4 to about 20 carbon atoms that is terminated with a group of the formula xe2x80x94COOA where A is a hydrogen atom or is a NH4+-cation or Z is a group of the formula CY2xe2x80x94COORxe2x80x2 where Y is a hydrogen or halogen atom, or is a substituted or unsubstituted alkyl or aryl group having from 1 to about 6 carbon atoms that may optionally contain one or more catenary heteroatoms and where Rxe2x80x2 is a hydrogen atom, a NH4+-cation, an alkyl group, or is an acyclic anhydride, e.g. a group of the formula xe2x80x94COR where R is an alkyl group or is a group that itself contains organo-onium (i.e. giving a bis-organo-onium); preferably Rxe2x80x2 is hydrogen; Z may also be a substituted or unsubstituted, cyclic or acyclic alkyl group having from 4 to about 20 carbon atoms that is terminated with a group of the formula xe2x80x94COOA where A is a hydrogen atom or is a NH4+-cation;
R1, R2 and R3 are each, independently, a hydrogen atom or an alkyl, aryl, alkenyl, or any combination thereof; each R1, R2 and R3 can be substituted with chlorine, fluorine, bromine, cyano, xe2x80x94ORxe2x80x3 or xe2x80x94COORxe2x80x3 where Rxe2x80x3 is a C1 to C20 alkyl, aryl, aralkyl, or alkenyl, and any pair of the R1, R2 and R3 groups can be connected with each other and with Q to form a heterocyclic ring; one or more of the R1, R2 and R3 groups may also be a group of the formula Z where Z is as defined above;
x is an organic or inorganic anion (e.g. halide, sulfate, acetate, phosphate, phosphonate, hydroxide, alkoxide, phenoxide or bisphenoxide); and
n is a number equal to the valence of the anion X.
Also useful as additives to the fluorinated elastomeric composition are phosphates, phosphine oxides, and amine oxides. These compounds include, for example, alkyl and aryl phosphate, triaryl phosphine oxides, trialkyl phosphine oxide, triarylamine oxide and trialkyl amine oxide. Such compounds include those of the formula PR3O and NR3O where each R substituent is, independently, a linear or branched alkyl or aryl group.
Nitrile-containing polymers may be cured by a catalytic interaction of alkyl tin compounds with the nitrile group, thereby creating a triazine crosslinked structure (xe2x80x9cModern Fluoropolymersxe2x80x9d, High Performance Polymers for Diverse Applications, edited by John Scheirs, John Wiley and Sons (1997), especially page 351). The addition of tin compounds, however, also detrimental to the ion level of the elastomer. It is an aspect of the invention to preferably cure the purified nitrile elastomer blends in the presence of ammonia-generating compounds that are solid or liquid at ambient conditions and generate ammonia under curing conditions. Such compounds include, for example, hexamethylene tetramine (urotropin), dicyan diamid, and substituted and unsubstituted triazine derivatives represented by the formula:
wherein R is a hydrogen or a substituted or unsubstituted alkyl, aryl or aralkyl group having from 1 to about 20 carbon atoms. Specific useful triazine derivatives include hexahydro-1,3,5-s-triazine and acetaldehyde ammonia trimer.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.
Description of Materials Used
Anionic exchange resin, AMBERLITE IRA 402 (chloride form) was supplied by Rohm and Haas. The resin was put into the OH-form with a 5% NaOH solution. After the elution of the NaOH solution, the column is flushed with deionized water (DI).
Cationic exchange resin, LEWATIT SP 112 (NH4+ form) was supplied by Bayer AG. It was put into the H+-form by an aqueous mineral acid such as sulfuric or hydrochloric acid. After treatment of the resin with the acid solution the column is rinsed with excess DI.
Dispersion Preparation
The fluoroelastomer and/or the partially crystalline fluoroplastic dispersion were prepared separately by emulsion polymerization. The desired mixtures thereof were prepared by blending the thus obtained dispersions. The solids content was adjusted to 20%. The requested amount of a nonionic surfactant is added in the form of a diluted solution. Practical levels of nonionic surfactant are in the range of 10 to 30,000 ppm of surfactant based on total weight of the dispersion mixture. Preferred ranges are from 10 to 100 ppm. Typical nonionic surfactants are, for example TRITON 100 X or GENAPOL X 080.
Column Ion Exchange Process
The cationic and anionic exchange processes are similar. The polymer latex is ion exchanged by passing the dispersion through a column packed with the desired exchange resin (dimension of the column: diameter 6 cm, height 30 cm). The latex can be delivered to the column by any means typical for a chromatographic procedure, e.g. gravity feed, static siphon or an automatic pumping system. The particular method used to pass the dispersion through the column is not critical. The elution rate should not exceed 3 times the bed volume/hour.
Analytical Test Methods
PFOA concentrations were determined from dispersion samples taken before and after ion exchange. The PFOA level was determined by gas chromatography according to standard methods (WO-A-99/62830 and WO-A-99/62858). The concentrations of Fxe2x88x92, Clxe2x88x92, Brxe2x88x92, SO42xe2x88x92, PO43xe2x88x92 were determined by ion chromatography of the xe2x80x9cmother liquorxe2x80x9d which is the particle free aqueous phase of the dispersion as obtained by freeze coagulation. The cation contents of the samples were determined by ion conductive plasma (ICP) of the polymer samples. Samples were treated with HNO3 followed by pyrolysis at 550xc2x0 C. for 10 minutes in a sealed pyrolysis bomb before subjecting them to ICP.
Test Methods
In the following examples, indicated results were obtained using the following test methods:
Press-cure samples. Unless otherwise noted, 150xc3x97150xc3x972.0 mm sheets were prepared by pressing at about 6.9 Mega Pascals (MPa) for 10 minutes at 177xc2x0 C. for measuring physical properties.
Post-cure samples, unless otherwise noted, were prepared by placing a press-cured sample in a circulating air oven. The oven was maintained at 232xc2x0 C. and the samples treated for 16 hours.
Tensile strength at break, elongation at break, and modulus at 100% elongation were determined using ASTM D 412-92 on samples cut from the press-cure or post-cure sheet with ASTM Die D. Units reported in MPa.
Hardness was determined using ASTM D 2240-85 Method A with a Type A-2 Shore Durometer.
Compression set was determined on O-rings using ASTM 395-89 Method B. The O-rings had a cross-section thickness of 3.5 mm (0.139 inch). After post-curing, the O-rings were compressed for 70 hours at 200xc2x0 C. Results are reported as a percentage of permanent set. The melt flow index (MFI) was determined according to ISO 12086. Mooney viscosities were measured according to ASTM D-1646. All percentages are by weight unless otherwise stated.
Procedure for Anionic Exchange
Five kilograms (kg) of a dispersion with a solids content of 28.5% is diluted to 18% solids with deionized water (DI). Seventy-five grams (g) of a 20% solution of GENAPOL X 080 in DI is added to this mixture and stirred slowly over night. The dispersion is subjected to anionic exchange by passing the dispersion through a 600 ml column packed with 400 ml of the anionic exchange resin prepared as described above. The elution rate is adjusted to 600 ml dispersion/hour.
Procedure for Cationic Exchange
The dispersion after anionic exchange is treated in a similar manner with 400 ml of the cationic exchange resin. The polymer is isolated from the dispersion by freeze coagulation.


In a magnet system for MRI imaging comprising a superconducting magnet mounted within a cryogen vessel, apparatus is provided for controlling egress of cryogen gas from the cryogen vessel. The apparatus comprises a controlled valve linking the interior of the cryogen vessel to a gas exit path; and a controller arranged to control the valve. The valve is arranged such that a gas pressure in the cryogen vessel exceeding a gas pressure in the gas exit path acts on the valve to open the valve and allow venting of cryogen gas. The valve is also arranged such that a gas pressure in the cryogen vessel inferior to a gas pressure in the gas exit path acts on the valve to urge it closed, so restricting flow of gas into the cryogen vessel.

FIG. 1 schematically shows a cross-section of an MRI imaging magnet housed within a cryostat. As is well known in the art, such arrangements typically comprise a set of superconducting coils 10 mounted on a former (not shown), suspended in a cryogen vessel 12 which is partially filled with liquid cryogen 14. The liquid cryogen is selected such that its boiling point is below the superconducting transition temperature of the wire used in the coils 10. An outer vacuum container OVC 16 surrounds the cryogen vessel. The space 18 between the inner surface of the OVC and the outer surface of the cryogen vessel is evacuated, to reduce heat influx to the cryogen vessel by convection. One or more thermal radiation shields 19 may be provided in the evacuated space, to reduce thermal influx to the cryogen vessel by radiation. A solid thermal insulating layer such as aluminium coated polyester sheets 19a may also be provided within the evacuated space, to further reduce thermal influx. Careful design of support and suspension members 20 reduces heat influx to the cryogen vessel by conduction.
The coils 10 are provided with electrical current by current leads 22 leading into the cryogen vessel through an access turret 24. The access turret typically also provides a venting path 25 for cryogen gas to escape. It is necessary to allow cryogen gas to escape for several reasons, depending on the state of operation of the magnet which comprises coils 10. The present invention relates to the equipment provided to allow venting of cryogen gas. Some examples of situations which require the venting of cryogen gas are as follows. During operation, the cryogen vessel 12 must remain sealed against air ingress, yet the gas pressure within the cryogen vessel must be accurately controlled to maintain the correct thermal environment for the superconducting coils.
Control of cryogen gas venting during all normal operating conditions (cryogen fill, ramp, and operation at field) may be achieved using a direct-acting mechanical valve. Achieving the required control precision with such mechanical valves has proved difficult and expensive. Consequences of this poor control include less-than-ideal coil temperatures during ramping, with consequently increased risk of quench, and increased cryogen losses.
The mechanical vent valve depends on the balance of gas pressures and spring forces to modulate the opening of the valve's plate. The operating forces in a valve of this type are small, and consequently performance is sensitive to small changes in spring force, friction, operating temperature, and a range of manufacturing tolerances. Expensive calibration and conditioning techniques are used to reduce these effects, but despite this, pressure control performance is only just adequate for the application and reliability is poor.
Examples of such valves include a simple spring-loaded valve which opens when the pressure inside the cryogen vessel exceeds the pressure on the other side of the valve by an amount sufficient to overcome the bias force provided by the spring. The valve is opened by an excess of pressure on the cryogen-vessel side, and is closed by a spring acting against the pressure of the cryogen vessel. In the case of over-pressure within the cryogen vessel, the pressure acts on the valve against the spring to vent cryogen from the cryogen vessel. Should the pressure within the cryogen vessel fall below the pressure on the other side of the valve, typically atmospheric pressure, then that pressure will act on the valve in support of the spring to hold the valve firmly shut and prevent unwanted ingress of air to the cryogen vessel. A passive over-pressure and under-pressure protection is accordingly provided.
The internal cryogen vessel pressure required to open such valves may vary according to external influences. For example, the absolute cryogen vessel pressure required to open the simple spring loaded valve will vary with atmospheric pressure acting on the "downstream" side of the valve. While one may have designed and intended such mechanical valves to operate to maintain a certain absolute pressure within the cryogen vessel, variations in the absolute pressure required to open the valve will occur. This effect may be reduced by providing a selection of slightly different versions of the valves, and a precise valve type may be chosen from a range of such valves according to the expected atmospheric pressure and ambient temperature to be experienced by the cryogen vessel in operation. This is of course inconvenient, requiring multiple valve types and appropriate arrangements to select the correct valve for installation.
In summary, despite significant development efforts in the past, existing direct-acting vent valves (wherein the valve plate is directly operated by the pressure within a vessel or a spring, or similar) do not provide accurate or optimised control of cryogen vessel pressure for superconducting magnets for MR imaging, and similar apparatus. Due to demanding calibration requirements, such valves are also expensive to manufacture and unreliable in operation.
In known MRI imaging systems and the like, and as shown in FIG. 1, it is customary to provide a magnet supervisory system 30 which receives data from numerous sensors 32, 34; which is in control of electrical current flow in the magnet, and which controls the operation of the magnet system for optimal performance at all times: during ramp-up, in steady state operation; during imaging and during ramp-down.
It has been found difficult to provide a mechanical valve system with effective and reliable on/off operation. Even a small amount of contamination on the valve element or valve seat may cause the valve to leak in the closed position. On the other hand, contamination may prevent the valve from opening completely. In either case, the valve may not maintain the required pressure within the vessel or permit the required gas flow rate from the vessel.
Accordingly, it has been proposed to control venting by a valve 40 fitted with an actuation device, which is controlled using an intelligent controller such as magnet supervisory system 30, which has access to data defining the magnet operational status and existing conditions within the cryogen vessel. In the example illustrated in FIG. 1, sensors 32 and 34 provide data to the magnet supervisory unit 30 indicating the pressure within the cryogen vessel and the flow rate of gas venting from the cryogen vessel. With such an arrangement it is possible to optimise the operating pressure in the cryogen vessel and the vent gas flow rate from the cryogen vessel to suit the magnet operating conditions and/or existing conditions within the cryogen vessel, with consequent advantages including reduced quench risk and reduced losses of cryogen.
The controlled valve 40 may be operated whereby pressure within the cryogen vessel may be reliably maintained within a desired range of values, and/or a venting of gas may be operated when an internal absolute or gauge pressure reaches a certain value. An accurate and predictable control of vent gas flow rate from the cryogen vessel may be maintained as required.
To avoid the risk of air/ice contamination of the cryogen vessel, the pressure within the cryogen vessel 12 is normally maintained above atmospheric by magnet supervisory control system 30; for example by controlling an associated cryogenic refrigerator in accordance with pressure measurements provided by an absolute or gauge pressure transducer 32. However; during ramping, further pressure rise within the cryogen vessel must be strictly limited in order to maintain acceptable magnet temperatures by allowing increased boil-off of the liquid cryogen. As a result of these conflicting requirements, very precise control and measurement of cryogen vessel pressure is required to be provided by the vent valve 40 and the pressure control system, typically included within controller 30.
The controlled valve 40 may have a simple cyclic on/off function. Accurate pressure control is achieved by varying a duty cycle of the on/off state of the valve, eliminating the need for precise calibration of the valve. In such arrangements, the exact flow capacity of the valve is not particularly important, as the pressure measurement and duty cycle adjustment will compensate for minor variations. For example, a very simple control method may operate along the lines of:
(1) Set required absolute pressure within cryogen vessel=x;
(2) detect actual pressure p within cryogen vessel from sensor 32 conventionally provided;
(3) If p>x, increase "open" proportion of valve operation duty cycle; and
(4) If p<x, reduce "open" proportion of valve operation duty cycle.
Where control of gas egress flow rate is required, rather than gas pressure, the control method may resemble:
(1) Set required gas egress flow rate from the cryogen vessel=R;
(2) detect actual gas egress flow rate r from the cryogen vessel from a sensor 34 conventionally provided;
(3) If r<R, increase "open" proportion of valve operation duty cycle; and
(4) If r>R, reduce "open" proportion of valve operation duty cycle.
Suitable control signals and suitable arrangements for modifying the control signals to provide the required operation may be simply derived by those skilled in the art.
Various control strategies are possible for the valve and may be defined in the software of the control unit. An advantage of such arrangements is that imperfections in the valve hardware may be compensated for in the software of the magnet supervisory control system 30. The control unit conventionally receives data from various sensors fitted to the magnet and elsewhere within the cryogen vessel, indicating such parameters as absolute cryogen vessel pressure and liquid cryogen level. As will be apparent to those skilled in the art, one may employ data provided by such sensors to operate the actively controlled valve of the present invention to control the absolute pressure inside the cryogen vessel within a desired range of values as required for any of a number of operational situations and requirements. The required valve control may be based on measurements of cryogen vessel pressure. Such pressure measurements can be either absolute or gauge (that is, relative to atmospheric pressure), and the type of transducer used can be selected for use in different operating conditions if desired. By providing an atmospheric pressure sensor, the gauge pressure of the cryogen vessel may be controlled. Alternatively, the actively controlled valve may be operated to maintain a gas egress flow rate within a desired range of values.
Control signals for valves operated by stepper motors, generated by the magnet supervisory control system 30 may easily be derived by those skilled in the art.
For current MRI imaging magnet systems, it has been found that a cyclic valve on/off function at an operating frequency of below 1 Hz is quite sufficient, given the size of the system. Greater frequencies of valve operation may be found necessary, particularly for much smaller cryogen vessels 12.
Typically, the valve element is directly operated by a solenoid. FIG. 2 shows a conventional arrangement of a solenoid valve 42 provided to control cryogen vessel pressure, or rate of cryogen gas egress flow. In FIG. 2, the solenoid valve 42 comprises an actuating coil 44 and an armature 46 carrying, or serving as, a valve element. According to the magnitude and/or direction of electrical current supplied through leads 48 to the actuating coil 44, the armature 46 will move to bring the valve element into contact with, or away from, a valve seat 50. A weak spring 52 urges the valve element towards the valve seat to close the valve when the solenoid is not activated. While the spring is schematically illustrated as a coil spring, any known resilient member may be used as appropriate, such as a leaf spring, a conical spring, a helical spring, or resilient members such as a deformable rubber member. In FIG. 2, the valve is illustrated in a mid-way position, between a fully-open position and a fully-closed position. This is for illustration purposes only, and does not represent a stable position for a solenoid valve.
FIG. 3A shows the valve 42 in a fully open position, typical of the position of a solenoid valve in its "actuated" position—with an actuating current flowing in actuating coil 44.
FIG. 3B shows the valve 42 in a fully closed position, typical of the position of a solenoid valve in its "rest" position—with no current flowing in actuating coil 44. The valve element is urged into contact with the valve seat 50 by spring 52; possibly also by gravity.
In a system such as illustrated in FIG. 1, control system 30 controls the valve so as to maintain a pressure within the cryogen vessel 12 within a desired range of values, or a gas egress flow rate within a desired range of values by the controller changing the on-off time ratio (duty cycle) of cyclically opening and closing the valve 42 by appropriately controlling the current in actuating coil 44. By using a suitably dimensioned valve and operating frequency, the variation in pressure can be maintained within close limits.
In the case of an unexpected over-pressure within the cryogen vessel 12, while the valve is closed (FIG. 3B), the pressure will act on the "upstream" surfaces of the armature 46 and the valve element, to urge the valve tightly into its closed position, with the valve element on the valve seat. This will prevent or impede venting of boiled off cryogen gas from the cryogen vessel, possibly leading to an unwanted build-up of pressure within the cryogen vessel. Such an increase in pressure may lead to an increase in the temperature of the gas, and so raise the temperature of the coils, risking a quench.
Conversely, in the case of an under-pressure within the cryogen vessel 12, while the valve is closed (FIG. 3B), bringing the pressure within the cryogen vessel below the pressure external to the cryogen vessel, the pressure external to the cryogen vessel will act on a surface of the armature 46 and the valve element, to urge the valve open. This may allow the unwanted ingress of air or other contaminants into the cryogen vessel 12 and/or the venting path 25. Such leakage is to be avoided since the cryogen vessel is typically at a temperature well below the freezing point of the major components of air. The ingress of air or other contaminants may cause deposits of frozen contaminants such as nitrogen and water to form within the cryogen vessel 12 and/or the venting path 25. This may cause a blockage which may prevent or impede later venting of boiled off cryogen gas from the cryogen vessel, possibly leading to an unwanted build-up of pressure and an associated increase in temperature within the cryogen vessel at a later time. This may cause a quench, with cryogen unable to vent satisfactorily due to the blockage, causing dangerously high pressure within the cryogen vessel. Solid contamination of the valve may also make pressure control more difficult, while contamination of the magnet structure may cause quench events.
For these reasons, use of a controlled solenoid valve such as discussed above and illustrated in FIGS. 2-3B has been avoided in the prior art, and the use of direct-actuated mechanical valves has remained current, &spite their disadvantages discussed above.

Passive overpressure and underpressure protection for a cryogen vessel


In a feed screw device according to the present invention, a coil member is disposed on the outer periphery of a rotary shaft member. The coil member is formed of an elastic body such as a piece of piano wire. The coil member is mounted at least partially on the shaft member so as to have at least a portion remaining in a free state. The coil member is used to engage a nut placed on the outer periphery of the shaft member. When the shaft member is rotated, the coil member is also rotated together with the shaft member, so that the nut is moved in the axial direction of the feed screw. When the nut is brought into contact with a stopper by a driving force, the nut impactively strikes the stopper. However, since the coil member can be elastically deformed in the axial direction, the coil member is elastically deformed to absorb its impact. Also the "spring back" effect enabled by the deformable coil member serves as a contribution to the initiation of movement of the nut in the reverse direction. Thus, when the direction of the driving force is reversed, the nut can immediately be moved in the reverse direction by a slight driving force.

1) Field of the Invention
The present invention relates to a feed screw device for driving a reciprocating mover, and particularly to a feed screw device suitable for use in, for example, a cleaning apparatus for a wire-type static eliminator or discharger employed in an electrostatic recording apparatus, and the like.
2) Description of the Related Art:
Heretofore, feed screws have been known in which spiral grooves or projections are formed on a shaft-shaped member by machining such as cutting and rolling to be used as threaded portions.
In addition, simple feed screws with no need for accuracy are also known wherein wire members are spirally wound on shaft-shaped members and fixed thereon over the entire length thereof.
In the above-described conventional feed screws, the threaded portions are constructed as rigid portions. Thus, the concept that the threaded portions be deformed is contradictory to common sense.
However, one example is known where a reciprocating mover to be carried by a feed screw is caused to collide with a stopper at the stroke ends of a driving action or the like so as to stop its movement. In this case, threaded portions of the feed screw and of a feed nut bite into one another due to impact force produced by the collision, or an excessive force is exerted on a reduction mechanism used for rotating and driving the feed screw. By way of example, when a simple worm gear system is employed in such a reduction mechanism, a worm shaft or the output shaft of a motor connected directly to the worm shaft is brought into a state in which an excessive thrust load is applied thereon, thus leading to difficulty in a subsequent reverse startup of the motor.


A display apparatus and computer system employing the same include a dual mode television/monitor display which is operable to provide television display output in an interlaced mode of operation and computer output in a noninterlaced computer graphics mode of operation in a visually detectable manner and further includes a communication channel for transmitting commands from the host computer system to the display apparatus. The display apparatus contains a microprocessor for receiving commands from the host computer system for switching the display between the interlaced and noninterlaced modes of operation. In a further aspect, a method of transitioning the dual mode display between the television and computer graphics modes of operation via a host computer system is provided.

The present invention relates generally to computer systems and, more particularly, to display devices for use with computer systems such as personal computer systems.
Personal computer systems, such as IBM personal computer systems, have attained widespread use. Historically, such systems have been information handling systems designed primarily to give independent computing power to a single user (or a relatively small group of users in the case of personal computers which serve as computer server systems).
Convergence products are now beginning to form a relatively new category of entertainment products, based on the convergence of personal computer (PC) technology and traditional consumer electronics technology. With this trend towards convergence, or using a personal computer with a large monitor as the heart an entertainment system, it is likely that standards will eventually be developed to assure the compatibility and interoperability of products. However, conventional interlaced television (broadcast and cabled) is still currently a widespread viable medium. Even if such standards are eventually developed, there will exist a very large number of xe2x80x9clegacyxe2x80x9d consumer electronic devices that users may wish to continue using, along with a large number of existing articles containing analog video information in well-established formats. Therefore, even if a computer is employed in an entertainment system, it is likely that users will wish to continue to view conventional (interlaced) video from sources such as broadcast and cabled television, inputs from other analog video sources such as videotape recorders (VTRs), laser disk players, video camcorder devices, and so forth.
It is known to receive an interlaced video source and convert it to a noninterlaced VGA format for display on a computer monitor. In so doing, it is also known to switch the monitor between an overscanned xe2x80x9cTV modexe2x80x9d with increased brightness and altered color temperature for employing a computer monitor to view television content, and an underscannedxe2x80x9ccomputer graphics modexe2x80x9d which generally has decreased brightness and sharper focus for viewing computer graphics content. However, even in television mode, the interlaced signal is not displayed directly, but is converted to a VGA signal.
Combination television/computer monitors are also known which are capable of displaying both a VGA (noninterlaced) signal and which further contain standard television tuning electronics. Although such devices may be interfaced to a computer, they are switched between the television and computer graphics modes only via the television-user interface, i.e., buttons or switches on the television itself or via a remote control television interface. Similarly, once the television mode has been selected, there does not exist any method for controlling the traditional television features, such as channel selection, via the host computer. Therefore, there exists a need for a system and method for switching between the computer (noninterlaced) and television (interlaced) of such TV/monitor hybrid displays from a host computer system interface. There also exists a need for a method and system for controlling traditional television features from a host computer system interface.
In one aspect, the present invention provides a display apparatus for use with a host computer system, wherein the display apparatus is of the type capable of a television (interlaced) mode of operation mode and a computer (non interlaced) mode of operation. Displays capable of both types of operation are generally known in the art. As used herein, the terms television mode and interlaced mode will refer to an interlaced mode of display operation with any other modifications for optimizing the viewing of a television signal, such as overscanning, increasing brightness, changing the color temperature, and so forth. As used herein, the terms computer graphics mode and noninterlaced mode refer to a progressively scanned computer graphics mode of operation which is typically underscanned and generally is less bright and more sharply focused than the television mode of operation.
The display apparatus according to the present invention comprises a screen which is operable to display in a visually detectable manner output from the host computer system when operating in the interlaced mode of operation and which is further operable to display a television compatible signal such as an NTSC signal when operating in the noninterlaced mode of operation. The display apparatus further comprises a communication channel for transmitting commands from the host computer system to the display apparatus. The display apparatus contains a microprocessor for receiving commands from the host computer system, the microprocessor comprising control logic for switching the display between the interlaced and noninterlaced modes of operation.
In one embodiment, the communication channel between the display apparatus and the host system also allows the host system to control, in addition to the display mode, additional TV functions, such as changing channels, selecting a video source, making volume and picture adjustments, and so forth.
In another aspect, the present invention provides a computer system comprising a host computer system which comprises a processor, a memory coupled to the processor, a video controller coupled to the processor and memory, and the video display apparatus in accordance with the present invention as described herein.
In another aspect, the present invention provides a method of operating a computer system to control a display apparatus, the display apparatus coupled to a video controller of a host computer system. The computer system and display apparatus are coupled via a communication channel and the display apparatus comprises circuitry providing a first mode of operation and a second mode of operation, wherein the first mode of operation is one of an interlaced mode of operation and a noninterlaced mode of operation and wherein the second mode of operation is the other. The method according to the present invention comprises operating the display in the first display mode, receiving user input to change the display mode from the first mode to the second mode, sending a mode change command to the display apparatus in response to user input, and transitioning the display apparatus from the first mode to the second mode of operation.
In yet another aspect, the present invention provides a display apparatus capable of operation in both a television mode and a computer graphics mode and further operable to output an analog video signal to a host computer system when operating in the computer graphics mode. The host system is operable to send a command to switch the display between the television and computer graphics modes and the host system further comprises processing circuitry to digitize the analog video material and to display it in an overlay window as a part of the host system""s computer graphics mode output.

Method and apparatus for changing the mode of a display apparatus


A method of preparing aliphatic-aromatic polycarbonate resin is disclosed. Accordingly, the method entails reacting an oligocarbonate of diphenol having chlorocarbonic acid ester terminal groups, with an aliphatic diol, in a non-aqueous solvent. The amount of solvent is such that the oligocarbonate is about 10-40% relative to the total weight of solvent, oligocarbonate and diol.

The invention relates to the preparation of polycarbonate resins and more particularly to a process entailing reacting an oligocarbonate diphenol having chlorocarbonic acid ester terminal group with an aliphatic diol.
The present invention relates to a method of preparing aliphatic-aromatic polycarbonates in non-aqueous solution, which is characterized in that oligocarbonates of diphenols with chlorocarbonic acid ester terminal groups are reacted with aliphatic diols in non-aqueous, neutral solvents with cleavage of hydrogen chloride, optionally in the presence of catalysts. The molar ratio of oligocarbonate to diol is preferably stoichiometric, i.e. one OH group is reacted per chlorocarbonic acid group.
Monophenols may optionally be added as chain terminators in amounts from 0.5 mole % to 10 mole %, preferably from 1.5 mole % to 8 mole % based on the total carbonate groups.
When chain terminators are used, the usual branching agents for aromatic polycarbonates may be used in addition, namely, trisphenols, tetraphenols, etc., in particular, in amounts from 0.05 mole % to 2 mole % based on the total carbonate groups. Aliphatic polyhydric alcohols with 3, 4, 5, 6 or more aliphatic OH groups are also suitable branching agents.
The addition of branching agents without the simultaneous addition of chain terminators leads to cross-linked products which are undesirable in the present context.
The reaction of bischlorocarbonic acid esters of diphenols with aliphatic diols is known (see, for example, DE-PS 971,777 (Ue1692), GB-808,485, U.S. Pat. No. 3,136,741, and Schnell, Chemistry and Physics of Polycarbonates, 1964, Interscience Publishers, New York, London, Sydney, pages 55-58, particularly page 57). However, the use of chlorocarbonic acid esters of aromatic oligocarbonates is not mentioned in the above referenced documents.
Packaging films comprising polycarbonates are known from U.S. Pat. No. 2,964,797 (Ue1752); these may be synthesized from diphenols and aromatic diols (see column 1, lines 64-68 and column 3, lines 35-38 of U.S. Pat. No. 2,964,797). Bischlorocarbonic acid esters of dihydroxy compounds may also be used for the preparation of polycarbonates, the reaction being conducted in inert solvents and in the presence of media which react with acids, such as tertiary amines (column 5, lines 30-35 of U.S. Pat. No. 2,964,797). The above document makes no mention of the use of chlorocarbonic acid esters of aromatic oligocarbonates.
U.S. Pat. No. 3,161,615 describes polycarbonates of diphenols and aliphatic diols. Both random and block copolymers are described (column 1, lines 40-57 of the above-mentioned U.S. Patent). However, U.S. Pat. No. 3,161,615 contains no description of the subject of the present invention.
U.S. Pat. No. 3,069,385 describes the chain lengthening of aliphatic aromatic polycarbonates with phosgene. This U.S. patent contains no description of the subject of the present invention.
Mixed block polycondensation products are known from DE-AS 1,162,559, and are prepared from polycarbonate blocks containing OH groups and from bischloroformic acid esters of polyalkylene ether glycols or polyolefines, for example.
DE-OS 2,064,095 (LeA 13,461) mentions that glycols can be used as reaction additives in the preparation of aromatic polycarbonates from diphenols (pages 3 and 4 of DE-OS 2,064,095). The use of chlorocarbonic acid esters of aromatic oligocarbonates is not mentioned in this patent.
It is known from DE-OS 2,447,349 (LeA 16,012) that aliphatic diols can be reacted with bischlorocarbonic acid esters of diphenols to form polycarbonates (see also Houben-Weyl, Volume E20/II, pages 1443-1457, particularly page 1447). The use of chlorocarbonic acid esters of aromatic oligocarbonates is not mentioned in this patent.
DE-OS 2,619,831 (LeA 16,933) teaches that the reaction of polyhydric alcohols with chlorocarbonic acid esters of phenols in the presence of media reactive with hydrogen chloride has disadvantages (page 2 of this document). A special transesterification process is, therefore, preferred.
According to DE-OS 2,636,784 (LeA 17,025), the preparation of polyether polycarbonates is effected by first converting the polyalkylene oxide diols into bis-diphenol carbonates, so that they contain phenolic terminal groups which are subsequently polycondensed with other diphenols by the known phase boundary process. It is emphasized that the use of bischloroformic acid esters of polyalkylene oxide diols involves disadvantages (page 24, paragraph 3 of DE-OS 2,636,784).
The preparation of aliphatic-aromatic polycarbonates is known from DE-OS 2,827,325 (LeA 18,819); this preparation again proceeds via the bis-diphenol carbonate intermediate stage. This document also mentions (page 17, penultimate paragraph of DE-OS 2,827,325) that the use of bischloroformic acid esters is disadvantageous.
Accordingly, it was surprising that the method according to the invention, with the use of bischlorocarbonic acid esters of aromatic oligocarbonates, leads to aliphatic-aromatic polycarbonates of good quality and in good yield.
The bischlorocarbonic acid esters of aromatic oligocarbonates used according to the invention are known from the literature (see DE-AS 1,212,983, for example), and can be obtained by methods which are known from the literature (see EP 263,432, for example).
They preferably correspond to the formula (I) ##STR1## where n represents the mean value of a distribution and has a value from 4 to 20, preferably from 5 to 10, most preferably from 6 to 8, and where --O--Ar--O is a diphenolate radical with 6 to 30 C atoms, which may be mononuclear or polynuclear and if polynuclear may be linked by bridging elements such as alkylene radicals, alkylidene radicals, cycloalkylidene radicals, --CO-sulphide, oxide or sulphone, or by a single bond, for example, and may contain alkyl, chloro or bromo substituents. Preferred diphenols EQU HO--Ar--OH (II)
are those corresponding to formula (III) ##STR2## where X is a C.sub.1 -C.sub.8 alkylene, a C.sub.2 -C.sub.8 alkyl idene, a C.sub.5 -C.sub.10 cycloalkylidene, --S-- or a single bond, and where R.sub.1 to R.sub.4 may be the same or different and may represent CH.sub.3, Cl, Br or H.
Suitable diphenols comprise hydroquinone, resorcinol, dihydroxydiphenyls, bis-(hydroxyphenyl)-alkanes, bis(hydroxyphenol)-cycloalkanes, bis(hydroxyphenyl)sulphides, bis-(hydroxyphenyl)-ethers, bis-(hydroxyphenyl )-ketones, bis-(hydroxyphenyl)-sulphones, .alpha.,.alpha.'-bis-(hydroxyphenyl )diisopropylbenzenes, and their compounds with alkylated and halogenated nuclei, for example.
Preferred diphenols comprise 4,4'-dihydroxydiphenyl, 2,2-bis-(4-hydroxyphenyl)-propane, 2,4-bis-(4-hydroxyphenyl)-2-methylbutane, 2,2-bis-(3-methyl-4-hydroxyphenyl)-propane, 2,2-bis(3-chloro-4-hydroxyphenyl)-propane, bis-(3,3-dimethyl-4-hydroxyphenyl)-methane, 2,2-bis-(3,5-dimethyl -4-hydroxyphenyl)-propane, 2,4-bis-(3,5-dimethyl -4-hydroxyphenyl)-2-methylbutane, 2,2-bis-(3,5-dichloro-4-hydroxyphenyl)-propane, 2,2-bis-(3,5-dibromo-4-hydroxyphenyl) propane, 1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane and 1,1-bis-(4-hydroxyphenyl)-cyclohexane, for example.
Particularly preferred diphenols comprise 2,2-bis-(4-hydroxyphenyl)-propane, 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)- propane, 2,2-bis-(3,5-dichloro-4-hydroxyphenyl)-propane, 2,2-bis-(3,5-dibromo-4-hydroxyphenyl)-propane, 1,1-bis-(4-hydroxyphenyl)-cyclohexane, and 1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane, for example.
The oligocarbonates (I) may contain --Ar radicals which are either the same or different, and may also be based on the same or different diphenols HO--Ar--OH (II).
Preferred aliphatic diols are those with 2 to 50 C atoms, particularly preferred diols contain 2 to 20 C atoms, and the most particularly preferred diols contain 2 to 18 C atoms; the alkylene radicals of the diols may be linear or branched, and the diols may contain ether and/or thioether bridges. The OH substituents of the diols may be in the .alpha., .beta. or .alpha., .gamma. to .alpha., .omega. positions. Diols with .alpha.,.omega. OH groups are preferred.
The alkylene radicals of the aliphatic diphenols may additionally contain arylene radicals or cycloalkylene radicals, preferred arylene radicals comprising 1,4-phenylene or 1,3-phenylene radicals, and preferred cycloalkylene radicals comprising 1,4-cyclohexylene radicals.
Aliphatic diols which are suitable according to the invention also comprise cycloaliphatic diols, preferably with 5 to 10 C atoms, such as 1,4-cyclohexanediol, for example.
Examples of diols which may be used according to the invention include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,3-butylene glycol, 2,3-butylene glycol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, 1,12-dodecanediol, neopentyl glycol, cyclohexane diol, 1,4-bis-(hydroxymethyl)-cyclohexane, 2-methyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, diethylene glycol, thiodiglycol, triethylene glycol and tetraethylene glycol, and also polyethylene glycols, dipropylene glycols, polypropylene glycols, dibutylene glycol and polybutylene glycols. Preferred diols comprise ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,3-butylene glycol, 2,3-butylene glycol and 1,6-hexanediol.
Non-aqueous solvents which are suitable according to the invention comprise chlorinated hydrocarbons such as methylene chloride or chlorobenzene, aromatic hydrocarbons such as toluene or mesitylene, acetone or acetonitrile, or mixtures of these solvents, for example. The solvents are used in an amount such that the resulting reaction mixture contains 10-40% of oligocarbonate.
Catalysts which are suitable according to the invention are preferably tertiary aliphatic amines and nitrogen heterocycles. Examples include trimethyl amine, triethylamine, tripropylamine, tributylamine, and also pyridine, quinoline, isoquinoline, picoline, acridine, pyrazine, pyridazine, pyrimidine, oxazine, thiazine, benzimidazole or imidazole. Apart from the free bases the corresponding acid-addition salts may also be used, i.e. for example the trimethylammonium hydrochloride or the pyridine hydrogen sulphate. The catalysts are used in amounts from 0.5-5% based on the oligocarbonate.
The preferred embodiment of this method employs catalysts having a boiling point equal to or higher than that of the solvent used.
Suitable chain terminators comprise phenol, C.sub.1-10 alkylphenols such as p-tert.-butylphenol or p-iso-octylphenol, chlorophenols and bromophenols, for example.
Suitable branching agents are those cited in EP 480,237, for example.
The hydrochloric acid produced during the reaction is expelled from the reaction mixture in gaseous form.
The polycarbonates obtained according to the invention are isolated from the non-aqueous organic solution in the known manner, by washing the mixture until it is neutral and free from electrolytes, possibly after dilution with additional organic solvent, followed, for example, by isolation as a granular material using an evaporation extruder or by precipitation of the polycarbonate from organic solution followed by further isolation.
The polycarbonates obtained by the method according to the invention have molecular weights M.sub.w (weight average molecular weight; determined by gel chromatography after prior calibration) of 9000 to 190,000, preferably 19,000 to 65,000.
They exhibit a good overall spectrum of properties, namely good flowability in combination with toughness.
The polycarbonates obtainable according to the invention may be processed very satisfactorily to produce any desired molding on the usual machines, for example, extruders, injection-molding machines, etc., at temperatures of 250.degree. C. to 350.degree. C., preferably from 260.degree. C. to 300.degree. C., so that even complicated moldings can be manufactured with a high degree of dimensional accuracy. Dimensionally accurate moldings of this type are used in the electronics industry for example, and are used in particular in aircraft construction and in space travel.
The usual additives for aromatic polycarbonates, such as stabilizers, flow promoters, plasticizers, de-molding agents, fillers, reinforcing agents such as glass fibers, glass spheres, carbon fibers, kieselguhr, kaolin, powdered minerals and pigments may be incorporated in the usual manner in the polycarbonates obtainable according to the invention. Processing to form the desired moldings may be effected at the same time as or after the additives are incorporated.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.

Method of preparing aliphatic-aromatic polycarbonates