Translucent polycarbonate compositions include an aromatic polycarbonate, a cycloaliphatic polyester, and a polyolefin. The compositions have excellent optical characteristics and superior physical properties, particularly low-temperature impact strength, compared to translucent polycarbonate-polyester blends requiring inorganic fillers.

The present invention relates to a thermoplastic composition comprising aromatic polycarbonate. In particular, the invention relates to a translucent polycarbonate composition having excellent physical properties.
Aromatic polycarbonates are engineering thermoplastics that combine desirable mechanical, optical, thermal, and electrical properties. When extruded in sheet form, aromatic polycarbonates have high transparency and excellent impact strength, making them ideal for a variety of glazing applications including roofs, greenhouses, sunrooms, and swimming pool enclosures. For structures in hot climates or for the southern exposure of structures in various climates, it is often desirable to use polycarbonate sheets having reduced light transmission in the form of opal whiteness and/or translucency.
It is known in the art that polycarbonate resins can be rendered translucent by the use of one or more inorganic additives such as titanium dioxide, zinc oxide, zinc sulfide, lead carbonate, and barium sulfate (see, for example, U.S. Pat. No. 4,252,916 to Mark, and Japanese Unexamined Patent Publication Nos. JP 06-306266 and JP 09-048911). It is also known to make polycarbonate translucent via addition of a partially fluorinated polyolefin (see, for example, U.S. Pat. No. 4,252,916 to Mark), a polyolefinic resin in combination with a plasticizer (see, for example, Japanese Unexamined Patent Publication JP 05-017599A), poly(dimethylsiloxane) gum in combination with finely divided silica (see, for example, U.S. Pat. No. 3,933,730 to Hoogeboom), poly(methyl silsesquioxane) (see, for example, European Patent No. 604,130 to Ohtsuka et al.), or spherical transparent thermoplastic particles (see, for example, International Publication No. WO 00/27927).
Of the above methods, the addition of light-scattering pigments, such as barium sulfate or calcium carbonate, is presently favored for commercial production of translucent polycarbonate compositions. However, the addition of such inorganic pigments adversely affects the physical properties of the sheet, especially its low temperature impact strength.
There remains a need for translucent polycarbonate formulations with improved low temperature impact strength.
A translucent polycarbonate composition comprises:
about 60 to about 99.8 weight percent of an aromatic polycarbonate;
about 0.1 to about 30 weight percent of a cycloaliphatic polyester;
about 0.1 to about 8 weight percent of a polyolefin;
wherein the molded or extruded composition has a transmission of about 15% to about 65% at a thickness of 2.0 mm according ASTM D-1003; and wherein all weight percentages are based on the weight of the total composition.
Translucency and high impact strength are provided by a thermoplastic composition comprising:
about 60 to about 99.8 weight percent of an aromatic polycarbonate;
about 0.1 to about 30 weight percent of a cycloaliphatic polyester;
about 0.1 to about 8 weight percent of a polyolefin;
wherein the molded or extruded composition has a transmission of about 15% to about 65% at a thickness of 2.0 mm according ASTM D-1003; and wherein all weight percentages are based on the weight of the total composition.
The inventors have discovered that polyolefins are especially suited for reducing the transparency of blends of aromatic polycarbonate and cycloaliphatic polyester while improving the low temperature impact strength of those blends compared to formulations relying on inorganic pigments for translucency. While polyolefins have sometimes been employed as impact modifiers in polycarbonate compositions, it should be noted that other types of impact modifiers, including methacrylate-butadiene-styrene (MBS) copolymers and styrene-(ethylene-butylene)-styrene block copolymers, have been found by the present inventors to be unsuitable for the present compositions because they do not provide the necessary reduction in transmittance at a similar concentration. It is therefore especially surprising that the desirable combination of properties is provided by the blends comprising polyolefins.
As used herein, the terms xe2x80x9cpolycarbonatexe2x80x9d and xe2x80x9caromatic polycarbonatexe2x80x9d include compositions having structural units of the formula
in which at least about 60 percent of the total number of R1 groups are aromatic organic radicals and the balance thereof are aliphatic, alicyclic, or aromatic radicals. Preferably, R1 is an aromatic organic radical and, more preferably, a radical of the formula
xe2x80x83xe2x80x94A1xe2x80x94Y1xe2x80x94A2xe2x80x94
wherein each of A1 and A2 is a monocyclic divalent aryl radical having from 6 to 12 carbon atoms and Y1 is a bridging radical having one or two bridging atoms that separate A1 from A2. In an exemplary embodiment, one atom separates A1 from A2. Illustrative, non-limiting examples of radicals of this type are xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94S(O)xe2x80x94, xe2x80x94S(O)2xe2x80x94, xe2x80x94C(O)xe2x80x94, methylene, cyclohexyl-methylene, 2[2,2,1]-bicycloheptylidene, ethylidene, isopropylidene, neopentylidene, cyclohexylidene, cyclopentadecylidene, cyclododecylidene, and adamantylidene. The bridging radical Y1 can be a hydrocarbon group or a saturated hydrocarbon group such as methylene, cyclohexylidene or isopropylidene.
Preferred dihydroxy compounds include those in which only one atom separates A1 and A2. As used herein, the term xe2x80x9cdihydroxy compoundxe2x80x9d includes, for example, bisphenol compounds having the formula
wherein Ra and Rb each represent a halogen atom or a monovalent hydrocarbon group and may be the same or different; p and q are each independently integers from 0 to 4; and Xa represents one of the groups of the formula
wherein Rc and Rd each independently represent a hydrogen atom or a monovalent linear or cyclic hydrocarbon group and Re is a divalent hydrocarbon group.
Some illustrative, non-limiting examples of suitable dihydroxy compounds include dihydric phenols and the dihydroxy-substituted aromatic hydrocarbons disclosed by name or formula (generic or specific) in U.S. Pat. No. 4,217,438 to Brunelle et al. A nonexclusive list of specific examples of the types of bisphenol compounds includes the following: 1,1-bis(4-hydroxyphenyl)methane; 1,1-bis(4-hydroxyphenyl)ethane; 2,2-bis(4-hydroxyphenyl)propane (hereinafter xe2x80x9cbisphenol Axe2x80x9d or xe2x80x9cBPAxe2x80x9d); 2,2-bis(4-hydroxyphenyl)butane; 2,2-bis(4-hydroxyphenyl) octane; 1,1-bis(4-hydroxyphenyl)propane; 1,1-bis(4-hydroxyphenyl)-n-butane; bis(4-hydroxyphenyl)phenylmethane; 2,2-bis(4-hydroxy-1-methylphenyl) propane; 1,1-bis(4-hydroxy-t-butylphenyl)propane; bis(hydroxyaryl)alkanes such as 2,2-bis(4-hydroxy-3-bromophenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclopentane; and bis(hydroxyaryl)cycloalkanes such as 1,1-bis(4-hydroxyphenyl)cyclohexane; and the like, as well as combinations comprising at least one of the foregoing.
Aromatic polycarbonate resins typically are prepared by reacting the dihydroxy compound with a carbonate precursor, such as phosgene, a haloformate or a carbonate ester and generally in the presence of an acid acceptor and a molecular weight regulator. These aromatic polycarbonates can be manufactured by known processes, such as, for example, by reacting a dihydroxy compound with a carbonate precursor, in accordance with methods set forth in the literature including the interfacial polymerization and melt polymerization processes. Generally in the melt polymerization process, the dihydroxy compound is reacted with a diester carbonate such as diphenyl carbonate, whereas in the interfacial polymerization the dihydroxy compound is reacted with a carbonyl chloride such as phosgene.
It is also possible to employ aromatic polycarbonates resulting from the polymerization of two or more different dihydric phenols or a copolymer of a dihydric phenol with a glycol or with a hydroxy- or acid-terminated polyester or with a dibasic acid or with a hydroxy acid or with an aliphatic diacid in the event a carbonate copolymer rather than a homopolymer is desired for use. Generally, useful aliphatic diacids have from 2 to about 40 carbons. A preferred aliphatic diacid is dodecanedioic acid. Polyarylates and polyester-carbonate resins or their blends can also be employed. Branched polycarbonates are also useful, as well as blends comprising a linear polycarbonate and a branched polycarbonate. The branched polycarbonates may be prepared by adding a branching agent during polymerization.
These branching agents are well known and may comprise polyfunctional organic compounds containing at least three functional groups, which may be hydroxyl, carboxyl, carboxylic anhydride, haloformyl and mixtures comprising at least one of the foregoing. Specific examples include trimellitic acid, trimellitic anhydride, trimellitic trichloride, tris-p-hydroxy phenyl ethane, isatin-bis-phenol, tris-phenol TC (1,3,5-tris( (p-hydroxyphenyl)isopropyl)benzene), tris-phenol PA (4(4(1,1-bis(p-hydroxyphenyl)-ethyl) alpha,alpha-dimethyl benzyl)phenol), 4-chloroformyl phthalic anhydride, trimesic acid and benzophenone tetracarboxylic acid, and the like. The branching agents may be added at a level of about 0.05 to about 2.0 weight percent. Branching agents and procedures for making branched polycarbonates are described in U.S. Pat. Nos. 3,635,895 and 4,001,184. All types of polycarbonate end groups are contemplated as being within the scope of the present invention.
Preferred aromatic polycarbonates are based on bisphenol A, in which each of A1 and A2 is p-phenylene and Y1 is isopropylidene. Preferably, the weight average molecular weight of the polycarbonate is about 5,000 to about 200,000, more preferably about 10,000 to about 100,000 and still more preferably about 15,000 to about 35,000.
The aromatic polycarbonate may be present at about 60 to about 99.8 weight percent, based on the total weight of the composition, with about 85 to about 98 weight percent being preferred, and about 90 to about 97.5 weight percent being more preferred.
The cycloaliphatic polyester resin comprises a polyester having repeating units of the formula
where at least one Rf or Rg is a cycloalkyl containing radical.
The polyester is a condensation product where Rf is the residue of an aryl, alkane or cycloalkane containing diol having 6 to 20 carbon atoms or chemical equivalent thereof, and Rg is the decarboxylated residue derived from an aryl, aliphatic or cycloalkane containing diacid of 6 to 20 carbon atoms or chemical equivalent thereof with the proviso that at least one Rf or Rg is cycloaliphatic. Preferred polyesters of the invention will have both Rf and Rg cycloaliphatic.
The present cycloaliphatic polyesters are condensation products of aliphatic diacids, or chemical equivalents and aliphatic diols, or chemical equivalents. The present cycloaliphatic polyesters may be formed from mixtures of aliphatic diacids and aliphatic diols but must contain at least 50 mole % of cyclic diacid and/or cyclic diol components, the remainder, if any, being linear aliphatic diacids and/or diols. The cyclic components are necessary to impart good rigidity to the polyester and to allow the formation of transparent blends due to favorable interaction with the polycarbonate resin.
The polyester resins are typically obtained through the condensation or ester interchange polymerization of the diol or diol equivalent component with the diacid or diacid chemical equivalent component.
Rf and Rg are preferably cycloalkyl radicals independently selected from the following formulas:
The preferred cycloaliphatic radical Rg is derived from the 1,4-cyclohexyl diacids and most preferably greater than 70 mole % thereof in the form of the trans isomer. The preferred cycloaliphatic radical Rf is derived from the 1,4-cyclohexyl primary diols such as 1,4-cyclohexyl dimethanol, most preferably more than 70 mole % thereof in the form of the trans isomer.
Other diols useful in the preparation of the polyester resins of the present invention are straight chain, branched, or cycloaliphatic alkane diols and may contain from 2 to 20 carbon atoms. Examples of such diols include but are not limited to ethylene glycol; propylene glycol, i.e., 1,2- and 1,3-propylene glycol; butanediol, i.e. 1,3- and 1,4-butanediol; diethylene glycol; 2,2-dimethyl-1,3-propane diol; 2-ethyl, 2-methyl, 1,3-propane diol; 1,3- and 1,5-pentane diol; dipropylene glycol; 2-methyl-1,5-pentane diol; 1,6-hexane diol; dimethanol decalin, dimethanol bicyclo octane; 1,4-cyclohexane dimethanol and particularly its cis- and trans-isomers; 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCBD), triethylene glycol; 1,10-decane diol; and mixtures of any of the foregoing. Preferably a cycloaliphatic diol or chenical equivalent thereof and particularly 1,4-cyclohexane dimethanol or its chemical equivalents are used as the diol component. Chemical equivalents to the diols include esters, such as dialkylesters, diaryl esters and the like.
When using cycloaliphatic diol components, a mixture of cis- and trans isomers may be employed, the ratio ranging from about 1:1 to about 1:5, and further a high trans isomer content ( greater than 70%) is most preferred. Mixtures of diols or chemical equivalents of the diols including esters and ethers, such as dialkyl esters, diaryl esters, and the like can also be useful.
The diacids useful in the preparation of the aliphatic polyester resins of the present invention preferably are cycloaliphatic diacids. This is meant to include carboxylic acids having two carboxyl groups each of which is attached to a saturated carbon. Preferred diacids are cycloaliphatic or bicycloaliphatic diacids having from 6 to 20 carbon atoms, for example 1,4-cyclohexanedicarboxylic acid and especially is trans-1,4-cyclohexanedicarboxylic acid or chemical equivalent. Other cycloaliphatic acids include decahydronaphthalene dicarboxylic acid, norbornene dicarboxylic acids, bicyclooctane dicarboxylic acids. Mixtures of cycloaliphatic diacids and linear aliphatic diacids are also useful provided the polyester has at least one monomer containing a cycloaliphatic ring. Illustrative examples of linear aliphatic diacids are succinic acid, adipic acid, dimethyl succinic acid, and azelaic acid.
Cyclohexanedicarboxylic acids and their chemical equivalents can be prepared, for example, by the hydrogenation of cycloaromatic diacids and corresponding derivatives such as isophthalic acid, terephthalic acid or naphthalene dicarboxylic acids in a suitable solvent such as water or acetic acid at room temperature and at atmospheric pressure using suitable catalysts such as rhodium supported on a suitable carrier of carbon or alumina. See, Friefelder et al., Journal of Organic Chemistry, volume 31, pages 3438 ff. (1966); and U.S. Pat. Nos. 2,675,390 and 4,754,064. They may also be prepared by the use of an inert liquid medium in which a phthalic acid is at least partially soluble under reaction conditions and a catalyst of palladium or ruthenium in carbon or silica. See, U.S. Pat. Nos. 2,888,484 and 3,444,237.
Typically, during hydrogenation, two or more isomers are obtained in which the carboxylic acid groups are in cis- or trans-positions. The cis- and trans-diastereomers can be separated by crystallization with or without a solvent, for example, n-heptane, or by distillation. The cis-isomer tends to blend better; however, the trans-diastereomer has higher melting and crystallization temperatures and is especially preferred. Mixtures of the cis- and trans-diastereomers are useful herein as well, and preferably when such a mixture is used, the trans-isomer will comprise at least about 70 parts by weight and the cis-isomer will comprise the remainder based upon 100 parts by weight of cis- and trans-isomers combined. A mixture of diastereomers or more than one diacid may be used in the cycloaliphatic polyester resins of this invention.
Chemical equivalents of these diacids include esters, alkyl esters (e.g. dialkyl esters), diaryl esters, anhydrides, acid chlorides, acid bromides, salts, and the like. The preferred chemical equivalents comprise the dialkyl esters of the cycloaliphatic diacids, and the most preferred chemical equivalent comprise the dimethyl ester of the acid, particularly dimethyl-trans-1,4-cyclohexanedicarboxylate.
Dimethyl-1,4-cyclohexanedicarboxylate can be obtained by ring hydrogenation of dimethyl terephthalate, and two diastereomers having the carboxylic acid groups in the cis- and trans-positions are obtained. The diastereomers can be separated, the trans-isomer being especially preferred. Mixtures of the isomers are suitable as explained above and preferably in the ratios as explained above.
A preferred cycloaliphatic polyester is poly(cyclohexane-1,4-dimethylene cyclohexane-1,4-dicarboxylate) also referred to as poly(1,4-cyclohexane-dimethanol-1,4-dicarboxylate) (PCCD) which has the formula:
With reference to the previously set forth general formula, for PCCD, Rf is derived from 1,4 cyclohexane dimethanol; and Rg is a cyclohexane ring derived from cyclohexanedicarboxylate or a chemical equivalent thereof. The favored PCCD has a cis/trans formula.
Preferred cycloaliphatic polyesters will have weight average molecular weights (determined by gel permation chromatography using polystyrene standards) of about 30,000 to about 150,000 atomic mass units (amu), with about 60,000 to about 100,000 amu being preferred, and about 65,000 to about 95,000 amu being more preferred. Preferred cycloaliphatic polyesters will also have viscosities of about 500 to about 25,000 poise, with about 1,000 to about 20,000 poise being preferred, and about 2,000 to about 4,000 poise being more preferred.
The preferred aliphatic polyesters used in the present transparent/translucent molding compositions have a glass transition temperature (Tg) which is above 50xc2x0 C., more preferably above 80xc2x0 C. and most preferably above about 100xc2x0 C.
Cycloaliphatic polyesters are commercially available from, for example, Eastman Chemical. Alternatively, cycloaliphatic polyesters can be synthesized following the teachings of, for example, U.S. Pat. Nos. 2,465,319, 5,986,040, and 6,084,055. The reaction is generally run in the presence of a suitable catalyst, such as tetraisopropyl titanate or tetrakis(2-ethyl hexyl)titanate, in a suitable amount, typically about 50 to 200 ppm of titanium based upon the final product.
The cycloaliphatic polyester may be present at about 0.1 to about 30 weight percent, based on the total weight of the composition, with about 1 to about 15 weight percent being preferred, and about 1 to about 10 weight percent being more preferred.
Polyolefins suitable for the composition include homopolymers and copolymers. Preferred polyolefins include those derived from monomers containing from 2 to about 10 carbon atoms. Some illustrative non-limiting examples of these polyolefins include polyethylene, polypropylene, polybutylene, polyhexene, polyisobutylene, and ethylenepropylene copolymer.
In one embodiment the polyolefin is free of fluoride substituents. In another embodiment, the polyolefin is free of all halide substituents (that is, all substituents from Group VIIA of the periodic table).
Methods for the preparation of the polyolefins are abundantly described in the literature and are well known to those skilled in the art. Polyethylene, for example, can be prepared by various procedures using cationic, anionic or free radical initiating catalysts, with conditions varied to produce a range of molecular weights and densities and various degrees of branching or non-branching. In one procedure, which involves free radical initiation, ethylene gas is polymerized in the presence of a peroxide initiating catalyst at a pressure between 15,000 and 40,000 pounds per square inch (psi) and a temperature between 100xc2x0 C. and 200xc2x0 C. to produce a relatively low density polymer, for example 0.90 to 0.94 g/cm3.
The polyethylene can also be prepared by low pressure processes to obtain a polymer of higher molecular weight and a higher density. In one such procedure, known as the Phillips process, ethylene is contacted in an inert solvent slurry of a catalyst such as chromium oxide supported on silica-aluminum, at pressures of 400 to 500 psi and temperatures of 130xc2x0 to 170xc2x0 C., followed by extraction of the polymer with hot solvent and purification, to produce a polyethylene product having a density between 0.96 to 0.97 g/cm3. Still other procedures are possible, such as emulsion polymerization in aqueous media in the presence of a peroxy compound, as well as suspension polymerization at low temperatures using a silver salt-peroxide redox system.
Another suitable polyolefin is polypropylene, a common commercial form of which is isotactic polypropylene. Such polymers can be prepared by anionically initiated reactions using Ziegler type catalysts, for example titanium halide such as TiCl4 in combination with an organometallic co-catalyst such as trialkyl aluminum halide. Polymerization proceeds readily at temperatures between 25xc2x0 C. and 100xc2x0 C. to yield a polymer in the form of a slurry of insoluble granular powder.
Copolymers of ethylene and propylene can be prepared using procedures similar to those for polyethylene and other polyolefins; for instance by the polymerization reaction of a mixture of ethylene and propylene in the presence of a Ziegler type catalyst or by free-radical initiation under high pressures.
Examples of higher polyolefins are polymers based on 2-methyl-1-butene, 1-pentene, 4-methyl-1-pentene, and the like. They can be prepared by known procedures including those described in Encyclopedia of Polymer Science and Technology, John Wiley and Sons, Inc., Vol. 9, pp. 440-460, 1965, incorporated herein by reference.
Particularly preferred polyolefins include linear low density polyethylenes (LLDPE). Linear low density polyethylenes are well known materials that are available commercially, for example, from Exxon under the tradename ESCORENE(copyright) or from Dow Chemicals under the tradename DOWLEX(copyright). Alternatively, they may readily be prepared by state of the art polymerization processes such as those described in U.S. Pat. Nos. 4,128,607, 4,354,009, 4,076,698, and European Patent Application No. 4645 (published Oct. 17, 1979). These polymers have a density between about 0.92 and 0.96 gram/milliliter. These linear low density polyethylene polymers are actually co-polymers of ethylene and a minor amount, typically less than 20 mole percent, preferably less than 15 mole percent, of an alpha olefin of 3 to 18 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 4 to 8 carbon atoms. These linear low density polyethylenes are distinguishable from polymers such as high pressure low density polyethylene and high density polyethylene made from Zeigler catalyst systems in that they are substantially free of side chain branching, having a controlled concentration of simple side chain branching as opposed to random branching.
The preferred linear low density polyethylene co-polymers are prepared from ethylene and one or more alpha olefins selected from the group consisting of propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, and 1-octene, most preferably 1-butene and 1-octene. Polymers of desired density may be obtained by controlling the co-polymerization ratio of ethylene to alpha olefin during co-polymerization. The addition of increasing amounts of the co-monomers to the co-polymers results in lowering the density of the co-polymer.
The melt indices of useful linear low density polyethylenes may vary widely. However, when using linear low density polyethylene derived from ethylene and a short chain monomer, for example 1-butene, it is preferred that such linear low density polyethylenes have melt indices of at least 5, preferably at least 10, more preferably at least about 12 gram/10 minutes.
With linear low density polyethylenes derived from longer chain monomers, for example 1-octene, the melt indices of the linear low density polyethylenes may be even lower.
In general, the co-polymerization of linear low density polyethylene can take place in either a gas phase fluidized bed reaction or liquid phase solution process reactor, preferably the former, at pressures ranging from normal to 5,000 psi, preferably less than 1,000 psi and at temperatures of from 20xc2x0 C. to 310xc2x0 C., preferably 30xc2x0 C. to 115xc2x0 C. in the presence of a suitable high activity catalysts. Typical catalyst systems comprise a transition metal complex catalyst, preferably comprising at least one compound of a transition element of group IVB, VB, or VIB having a halide and/or hydrocarbon group attached to said transition metal and a reducing component such as a metal halide or a compound having metal attached directly to carbon, for example metal alkyl. Highly satisfactory catalyst systems comprise (a) a Group IVB or Group VB metal compound bearing at least one halogen atom and (b) a lithium or aluminum or magnesium alkyl compound, especially LiAl(hydrocarbon)4. Such systems include, for example, TiC4 and LiAl(alkyl)4, VOCl3 and Li(alkyl), MoCl3 and Al(alkyl)3, TiCl4 and (alkyl)MgBr, and the like. Catalyst systems such as these as well as other useful catalysts systems are disclosed in U.S. Pat. Nos. 4,128,607, 4,354,009, 4,076,698, and European Patent Application No. 4645. Such catalyst systems are used in a molar ratio of ethylene to catalyst of about 35,000:1 to about 400,000:1.
The preferred linear low density polyethylene co-polymers so produced have an unsaturated group content of less than or equal to about 1, and preferably from about 0.1 to about 0.3 carbon-carbon double bonds per 1000 carbon atoms and a n-hexane extractables content (at 50xc2x0 C.) of less than about 3, preferably less than about 2, weight percent. Preferred materials include those made by the Unipol process, which is described in Chem. Eng., Dec. 3, 1979, pages 80-85.
Highly preferred polyolefins include polyolefin plastomers. Polyolefin plastomers are substantially linear ethylene/alpha-olefin copolymers. The alpha-olefin monomer preferably has from 4 to 8 carbon atoms. Polyolefin plastomers are characterized by a density of about 0.88 to about 0.92 g/ml, a molecular weight distribution Mw/Mn not greater than about 4 and a melt flow ratio I10/I2 of not less than about 7. The molecular weight distribution Mw/Mn is a ratio of the weight average molecular weight to the number average molecular weight of the copolymer, while the melt flow ratio I10/I2 is a ratio of the melt index at the 10 kg loading to the melt index at the 2.16 kg loading at 190xc2x0 C. according to ASTM D1238. The polyolefin plastomers are distinguished from other polyolefins, including LLDPE, in Mw/Mn and/or I10/I2.
Suitable polyolefin plastomers include those sold by ExxonMobil under the tradename EXACT(copyright) as, for example, the ethylene/butene copolymer EXACT(copyright) 4033 having Mw/Mn of about 1.8 and the ethylene/octene copolymer EXACT(copyright) 8201 having Mw/Mn of about 2.7; and by Dow Chemical Company under the tradename AFFINITY(copyright) as, for example, the ethylene/octene copolymer AFFINITY(copyright) PL1880.
Suitable polyolefins include graft copolymers of any of the above polyolefins with unsaturated acids or acid derivatives, including acrylic acid, maleic acid, maleic anhydride, itaconic acid, and the like. These acid-modified polyolefins may be prepared by known methods including those described in, for example, U.S. Pat. Nos. 3,404,135 to W. Tietz, 3,461,108 to Heilman et al., and 3,560,456 to Hazen et al. Especially preferred are the acid-modified polyolefins of linear low density polyethylene and maleic anhydride (LLDPE-g-MAH), which are commercially available from, for example, ExxonMobil under the tradename EXXELOR(copyright) (these materials include maleic anhydride-modified copolymers of (a) ethylene and butene, or (b) ethylene and octene); and the acid-modified polyolefin plastomers available from ExxonMobil as, for example, MDEX 95-2 and MDEX 96-2.
The total amount of polyolefin may be about 0.1 to about 8 weight percent, based on the total weight of the composition, with about 0.5 to about 4 weight percent being preferred, and about 1 to about 2.5 weight percent being more preferred. This total amount of polyolefin may preferably comprise 0 to about 100 weight percent, more preferably about 0 to about 50 weight percent, yet more preferably about 20 to about 40 weight percent, acid-modified polyolefin, with the remainder being unmodified polyolefin.
The composition may further comprise a catalyst quencher. Catalyst quenchers, as used herein, are agents that quench any residual polymerization catalyst remaining from the synthesis of the polycarbonate or the cycloaliphatic polyester resin. The residual catalyst needs to be quenched to prevent any transesterification reaction between the polycarbonate and the cycloaliphatic polyester resin. There is no particular limitation on the structure of the quencher. Suitable transesterification quenchers include mono-, di-, and tri-hydrogen phosphites and their metal salts; mono-, di-, and tri-hydrogen phosphates and their metal salts; mono- and di-hydrogen phosphonates and their metal salts; pyrophosphates and their metal salts; mixtures comprising at least one of the foregoing quenchers; and the like. The suitability of a particular compound for use as a transesterification quencher and the determination of how much is to be used may be readily determined by preparing a mixture of the cycloaliphatic polyester and the aromatic polycarbonate with and without the particular transesterification quencher and determining the effect on melt viscosity, gas generation or color stability or the formation of interpolymer.
The mono-, di-, and tri-hydrogen phosphites and their metal salts have the formula
P(OR1)a(OMn+1/n)3xe2x88x92a
wherein each R1 is independently C1-C12 alkyl, C1-C12 aryl, or C1-C18 alkylaryl; each M is independently hydrogen or a metal atom selected from Group IA, IIA, IB, or IIB of the periodic table; a is 0-2; and n is 1 or 2. Preferred compounds in this class include phosphorous acid, H3PO3.
The mono-, di-, and tri-hydrogen phosphates and their metal salts have the formula
Oxe2x95x90P(OR1)a(OMn+1/n)3xe2x88x92a
wherein R1, M, a, and n are as defined for the phosphites above. Preferred compounds in this class include those in which a=0 and M is a metal atom selected from Group IB or IIB of the periodic table. A preferred compound is mono zinc phosphate (MZP; ZnHPO4).
The mono- and di-hydrogen phosphonates and their metal salts have the formula
P(R1)(OR1)b(OMn+1/n)2xe2x88x92b
wherein R1, M, and n are defined as above, and b=0 or 1.
The pyrophosphates and their metal salts have the formula
MzxHyPqO3q+1
wherein M is as defined for the phosphites above, x is 1-12, y is 1-12, q is 2-10, and z is 1-5, with the proviso that the sum (xz)+y is equal to q+2. M is preferably a Group IA or IIA metal. Preferred compounds in this class include Na3HP2O7; K2H2P2O7; KNaH2P2O7; and Na2H2P2O7. The particle size of the polyacid pyrophosphate should be less than 75 micrometers, preferably less than 50 micrometers and most preferably less than 20 micrometers.
These and other quenchers, including quencher mixtures, are described, for example, in U.S. Pat. Nos. 4,401,804 to Wooten et al., 4,532,290 to Jaquiss et al., and 5,354,791 to Gallucci, 5,441,997 to Walsh et al., 5,608,027 to Crosby et al., and 5,922,816 to Hamilton.
As mentioned above, the determination of how much of a particular quencher is to be used may be readily determined by preparing the composition with and without the particular compound and determining the effect on melt viscosity, gas generation or color stability or the formation of interpolymer. Typical quencher amounts are about 0.001 to about 2 weight percent, preferably about 0.005 to about 1 weight percent, more preferably about 0.01 to about 0.5 weight percent.
Various other additives may be used alone or in combination. As used herein, additives may include such materials as whitening agents, thermal stabilizers, antioxidants, light stabilizers, plasticizers, colorants, impact modifiers, extenders, antistatic agents, mold releasing agents, additional resins, blowing agents, and processing aids. The different additives that can be incorporated in the compositions are commonly used and known to one skilled in the art. Illustrative descriptions of such additives may be found in R. Gachter and H. Muller, Plastics Additives Handbook, 4th edition, 1993.
Examples of thermal stabilizers include triphenyl phosphite, tris-(2,6-dimethylphenyl)phosphite, tris-(2,4-di-t-butyl-phenyl) phosphite, tris-(mixed mono-and di-nonylphenyl)phosphite, dimethylbenzene phosphonate and trimethyl phosphate. Examples of antioxidants include octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. Examples of light stabilizers include 2-(2-hydroxy-5-methylphenyl)benzotriazole, 2-(2-hydroxy-5-tert-octylphenyl)-benzotriazole and 2-hydroxy-4-n-octoxy benzophenone. Examples of plasticizers include dioctyl-4,5-epoxy-hexahydrophthalate, tris-(octoxycarbonylethyl)isocyanurate, tristearin and epoxidized soybean oil. Examples of the antistatic agents include glycerol monostearate, sodium stearyl sulfonate, and sodium dodecylbenzenesulfonate. Examples of mold releasing agents include pentaerythritol tetrastearate, stearyl stearate, beeswax, montan wax, and paraffin wax. Examples of other resins include but are not limited to polypropylene, polystyrene, polymethyl methacrylate, and polyphenylene oxide. Combinations of any of the foregoing additives may be used. Such additives may be mixed at a suitable time during the mixing of the components for forming the composition.
In one embodiment, the composition is substantially free of inorganic fillers. By this it is meant that the composition comprises less than 2 weight percent, preferably less than 1 weight percent, more preferably 0 weight percent, of inorganic fillers. Inorganic fillers may include particulate fillers, such as barium sulfate, and fibrous fillers, such as glass fibers. Various other fillers, including particulate, plate-shaped, and fibrous fillers, are described, for example, in U.S. Pat. No. 4,763,133 to Takemura et al. While the composition is generally free of inorganic materials that function primarily as fillers, it may comprise small amounts, typically less than about 1 weight percent, preferably less than about 0.5 weight percent, more preferably less than about 0.25 weight percent, of insoluble whiteners or colorants, which function primarily to adjust the color of the composition. Such insoluble colorants are described in R. Gachter and H. Muller, Plastics Additives Handbook, 4th edition, 1993.
In an alternative embodiment, inorganic fillers may be present in an amount that contributes significantly to reducing the transmission or increasing the haze of the composition.
The production of the compositions may utilize any of the blending operations known for the blending of thermoplastics, for example blending in a kneading machine such as a Banbury mixer or an extruder. The sequence of addition is not critical but all components should be thoroughly blended.
To prepare the resin composition, the components may be mixed by any known methods. Typically, there are two distinct mixing steps: a premixing step and a melt mixing step. In the premixing step, the dry ingredients are mixed together. The premixing step is typically performed using a tumbler mixer or ribbon blender. However, if desired, the premix may be manufactured using a high shear mixer such as a Henschel mixer or similar high intensity device. The premixing step is typically followed by a melt mixing step in which the premix is melted and mixed again as a melt. Alternatively, the premixing step may be omitted, and raw materials may be added directly into the feed section of a melt mixing device, preferably via multiple feeding systems. In the melt mixing step, the ingredients are typically melt kneaded in a single screw or twin screw extruder, a Banbury mixer, a two roll mill, or similar device.
In a preferred embodiment, the composition will have a melt volume rate measured at 300xc2x0 C./1.2 kg according to ISO 1133 of about 7 mL/10 minutes to about 13 mL/10 minutes, preferably about 9 mL/10 minutes to about 12 mL/10 minutes. In another preferred embodiment, the composition after molding or extruding will exhibit at least one of (1) a transmission measured on a 2.0 mm thick plaque according to ASTM D1003 of about 15% to about 65%, preferably about 20% to about 55%, more preferably about 30% to about 45%; (2) a haze measured on a 2.0 mm thick plaque according to ASTM D1003 not less than about 90%, preferably not less than about 95%, more preferably not less than about 98%; (3) a yellowness index measured for 2.0 mm samples according to ASTM D1925 not greater than about 35, preferably not greater than about 30, more preferably not greater than about 25; (4) an Izod notched impact measured at 0xc2x0 C. according to ASTM D256 of not less than about 500 J/m, preferably not less than about 700 J/m, more preferably not less than about 800 J/m; (5) an Izod notched impact measured at xe2x88x9220xc2x0 C. according to ASTM D256 of not less than about 150 J/m, preferably not less than about 200 J/m, more preferably not less than about 400 J/m; and (6) a Flexplate Impact energy at break measured at 23xc2x0 C. and 3.0 millimeter thickness according to ISO 6603-2 not less than about 100 J, preferably not less than about 110 J, more preferably not less than about 115 J.
The optical properties and impact resistance of the composition make it suitable for use in a variety of applications where translucency is preferred to transparency, including, for example, lighting fixtures, ornaments, and signs. The composition is particularly well adapted for use in a variety of glazing applications including, for example, roofs, greenhouses, sun rooms, swimming pool enclosures, and the like.
The compositions described above may be used in the fabrication of translucent sheets. Techniques for the extrusion of translucent sheets, including solid sheets, multi-wall sheets, and multi-wall sheets comprising hollow bodies, are known in the art and described in, for example, U.S. Pat. Nos. 3,476,627 to Squires, 3,565,985 to Schrenk et al., 3,668,288 to Takahashi, 3,918,865 to Nissel, 3,933,964 to Brooks, 4,477,521 to Lehmann et al., and 4,707,393 to Vetter. There is no particular limitation on the composition of additional layers used to form coextruded sheets. There is no particular limitation on the structure or geometry of the multi-wall sheets. The additional layers may comprise, for example, fluorescing agents to facilitate manufacturing and/or ultraviolet light absorbers to improve weatherability. The thickness of the multi-wall sheet is preferably about 4 mm to about 40 mm, while the thickness of the solid sheet is preferably about 1 mm to about 12 mm.

Translucent polycarbonate composition, method for preparation thereof, and articles derived therefrom


An optical absorption measuring instrument for determining the percentage of a component in a fluid shall be improved such that changes in the measuring radiation caused by drift or temperature have only a limited effect on the measuring result. Provisions are made for deflecting the measuring beams (10, 11) from a radiation source (4) to a detector (5) by at least two plane mirrors (71, 72), which are positioned such that the measuring beams (10, 11) emitted by the radiation source (4) are deflected to the detector (5). The surface of each plane mirror (71, 72) is dimensioned to be such that its illuminated surface in the area of the detector (5) is larger than the receiving surface (12) of the detector (5).

The present invention pertains to an optical absorption measuring instrument for determining the percentage of a component in a fluid.
Optical absorption measuring instruments, e.g., in the form of infrared optical measuring instruments, measure the absorption of infrared beams at a wavelength specific of the component to be detected. The amount of the absorption is an indicator of the percentage of the component in the fluid. There are various designs of such measuring instruments with an infrared radiation source or with a plurality of infrared radiation sources, with the measuring beam passing through an open or closed measuring chamber once or several times. The measuring beam reaches a detector or a plurality of detectors, which are sensitive to the corresponding infrared wavelengths. If a plurality of detectors are used, one of which is a measuring detector and the other a reference detector, the measuring beam is distributed possibly uniformly among the detectors. So-called beam splitters are used to split the measuring beam.
An infrared optical measuring instrument of this type has been known from DE 37 36 673 C2. The measuring beam emitted by an infrared radiation source passes through a measuring chamber, in which the gas component, whose concentration is to be monitored, is located. A ray beam emitted by a radiation source is reflected back by means of two spherical mirrors, which are arranged opposite the radiation source, and is imaged as a focused luminous beam on a detector, which is located at the said limiting surface of the measuring chamber as the radiation source. The individual luminous beams now reach the detector at different reflection angles. The drawback of the prior-art device is that the optical imaging on the receiving surface of the detector is changed by a change in the geometry of the radiation source or due to changes in the position of the mirrors due to changes in temperature. Drift and temperature effects are thus superimposed to the measured absorption signal.
The basic object of the present invention is to improve a measuring instrument of the above-described type such that changes in the measuring radiation due to drift and temperature have only a limited effect on the measuring result.
According to the invention, an optical absorption measuring instrument is provided for determining the percentage of a component in a fluid. A radiation source emitting measuring beams and at least one detector with a receiving surface sensitive to the measuring beams is at a first limiting surface of a measuring chamber containing the fluid. At least two plane mirrors are at a second limiting surface of the measuring chamber. The second limiting surface is located opposite the first limiting surface. The plane mirrors are positioned at the second limiting surface such that the measuring beams emitted by the radiation source are deflected to the detector. The surface of each plane mirror is dimensioned to be such that its illuminated surface in the area of the detector is larger than the receiving surface.
The advantage of the present invention is essentially that by using at least two plane mirrors for deflecting the beam, the radiation source is not imaged on the receiving surface, but the receiving surface of the detector is illuminated two-dimensionally, and the surface of each plane mirror is dimensioned to be such that the illuminated surface in the area of the detector is larger than the receiving surface of the detector. Due to the overlap of the illuminated surface and the receiving surface, it is achieved that the full receiving surface is illuminated even in case of drift and temperature effects and no variations can be observed as a result in the intensity of the measuring radiation in the area of the detector in case of temperature-related changes in the position of the plane mirrors. The overlap between the illuminated surface and the receiving surface is dimensioned to be such that the measuring beam still reaches the full receiving surface even in case of the most unfavorable constellation of the parameters. The tolerance range for the overlap can be determined most simply experimentally by exposing the measuring chamber to different temperatures and checking at each temperature whether the receiving surface of the detector is fully illuminated.
The present invention is not limited to the pure gas analysis, but it is also suitable generally for the determination of the percentage of a component, gas or liquid, in a gas or liquid sample.
It is especially advantageous to arrange a reference detector at the first limiting surface of the measuring chamber in addition to the detector. The reference detector has a reference receiving surface, which is illuminated by one of the plane mirrors, while the other plane mirror deflects the measuring beam to the detector. The surface of the plane mirror directed toward the reference detector is dimensioned to be such that the illuminated surface in the area of the reference detector is larger than the reference receiving surface.
The overlap between the illuminated surface and the receiving surface in the area of the detector or the reference detector is advantageously greater than 10% relative to the respective receiving surface. Experiments revealed that an overlap of about 10% to 20% is sufficient to suppress drift and temperature effects.
It is especially useful to design the limiting contour of the plane mirrors such that it is adapted to the receiving surface of the detector or of the reference detector. Thus, a rectangular mirror shape is suitable for a rectangular receiving surface, a round plane mirror in case of a round receiving surface, and a hexagonal mirror shape, which can be advantageously arranged in a honeycomb-like manner at the second limiting surface of the measuring chamber, in the case of a hexagonal receiving surface.
It is especially advantageous to design the plane mirrors at the second limiting surface in the form of a plane mirror matrix, wherein a first group of plane mirrors is directed toward the detector and a second group of plane mirrors toward the reference detector.
Due to the use of a plurality of plane mirrors in the form of a plane mirror matrix, contamination effects can be compensated in an especially simple manner and, moreover, no beam splitter is necessary, which would split the measuring beam between the detector and the reference detector behind the measuring chamber, because the individual plane mirrors can be positioned in the case of the plane mirror matrix such that the measuring beams are reflected directly to the detector and the reference detector.
The infrared optical measuring instrument can be manufactured in an especially simple manner and inexpensively by means of a plane mirror matrix, because the plane mirror matrix can be manufactured in connection with the second limiting surface as a one-piece molding, e.g., in the form of a plastic injection-molded part.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.


A pump coupling or hydraulic actuator controlling one or more limited slip couplings of a distribution system for distribution torque between front and rear axles in all wheel drive vehicles and/or distribution of torque between left and right wheels in two or four wheel drive vehicles. The hydraulic actuator includes an electric motor, a hydraulic pump, driven by a drive shaft of the electric motor, and the one or more limited slip couplings. Each limited slip coupling includes a disc package and a piston acting on the disc package, which piston is actuated by a hydraulic pump. A centrifugal regulator is connected to a rotating part of the electric motor or the hydraulic pump, which centrifugal regulator controls a pressure overflow valve, connected to the oil outlet of the hydraulic pump. The pressure is controlled by current to the electric motor or by control of the motor speed.

In all wheel drive vehicles there is often a multiple disc clutch that is actuated for distribution of torque between front and rear axles. Such a multiple disc clutch may also be used in distribution of torque between left and right wheels. The multiple disc clutches are often actuated by some kind of hydraulic actuation. In EP 1 256 478 one example of such a hydraulic actuation is shown. It has a hydraulic pump, acting in opposite directions of speed, an oil pressure sensor, two bypass orifice valves, a non-return valve, a relief valve and a quick open valve.
There is a constant strive to reduce costs. This is often done by simplifying existing system and it could also be done by reducing the space needed for a specific system. Less space occupied will simplify adapting the system to different vehicles.
Thus, one object of the present invention is to reduce the number of the hydraulic components (no bypass orifice valves) for a robust design, more compact installation, lower weight and lower productions costs. Furthermore, in the present invention no pressure sensor is needed. A further general object is to arrange a simple but yet reliable actuation system for a multiple disc clutch in a torque distribution system for example for all wheel drive vehicles. Another object is that the actuation system should react promptly on regulation signals.


A seat latch mechanism accommodating a plurality of seatback positions, comprising an upper housing and a lower housing whereby the lower housing includes a recess for a striker to pass therethrough when the latch mechanism is in an unlatched position. Further, a latch assembly is provided and is operably supported by the lower housing, whereby the latch assembly is operable between a latched position and an unlatched position, and an actuation assembly operably supported by the upper and lower housings and in operable communication with the latch assembly to selectively toggle the latch assembly between the latched and unlatched position.

The present invention relates to latch mechanisms and, more particularly, to an improved latch mechanism for a seat assembly.
In automotive applications, it is increasingly desirable that a vehicle be capable of accommodating varying requirements, such as cargo carrying and the like. It is further desirable that a vehicle be capable of accommodating varying requirements with respect to occupant comfort. To that end, reconfiguration of the vehicle seating system plays a significant role.
Generally speaking, articulation of a seatback enables a vehicle interior to be configurable for accommodating cargo-carrying needs and to provide occupant comfort. Latching mechanisms are often used to selectively anchor a seatback to a stationary striker assembly, thereby securing the seatback to a vehicle structure. In such a system, a conventional latching mechanism cooperates with a striker to provide the seatback with one of either a latched position or an unlatched position. In the latched position the seatback is secured to the vehicle structure in a fixed relationship. In the unlatched position the seatback is free to be dumped forward or folded flat. Conventional latching mechanisms suffer from the disadvantage that they only allow for the seatback to be latched in a single position, thereby hampering occupant comfort.
In addition to limiting occupant comfort, conventional latch mechanisms cooperating with striker assemblies also limit the flexibility of a cargo area of a vehicle located behind a vehicle seat. Current seating systems accommodate large cargo by folding the seatback in a substantially fold-flat position such that the seatback is generally parallel to the seat bottom. In other known systems, the seatback is again folded flat, and together with the seat bottom, the seatback is articulated or dumped forward, thereby increasing the cargo area of the vehicle. Such systems, while adequately providing for increased cargo areas in a vehicle, do not provide for occupant use of a seat when increased cargo capacity is required.
One prior art latch system, as shown in FIGS. 1-3, provides a latch mechanism W cooperating with a plurality of strikers (not shown) to selectively position a seatback in a variety of positions relative to a seat bottom. The latch mechanism W provides a wheel X for receiving a striker and a cam Y to selectively lock the wheel X in one of a plurality of positions. An actuation handle Z is further provided to manipulate the cam Y between a locked and an unlocked position.
The wheel X of latch mechanism W receives a first striker which causes the wheel X to rotate about a central axis. Continued rotation of the wheel X will eventually cause the cam Y to engage the wheel X such that the wheel X is not permitted to rotate, thereby locking the wheel in a fixed position. In this manner, the seatback is latched in a predetermined position relative to the seat bottom due to the relationship of the first striker to the seatback. To position the seatback in a different position relative to the seat bottom, a force is applied to the actuation handle Z which causes the cam Y to disengage the wheel X and subsequently allows the wheel X to rotate. Sufficient rotation of the wheel X releases the first striker, thereby allowing the wheel X to receive a second striker and subsequently position the seatback in a different relationship relative to the seat bottom.
While this latch system adequately positions the seatback in a plurality of positions relative to the seat bottom, small variations between the striker and the latch mechanism components may allow the seatback to move a miniscule amount even when the mechanism is locked. These small variations become noticeable at the upper end of the seatback and can cause noise and discomfort to an occupant. For example, the seatback of an unoccupied seat may tend to vibrate when the vehicle encounters rough road conditions, thereby creating undesirable noise and rattling. This magnified play in a latch mechanism has been termed xe2x80x9cchuckingxe2x80x9d and refers to any manufacturing variation or play in the mechanism components or between the mechanism components and a striker that allows movement of the seatback while the mechanism is in a latched condition.
Therefore a latch mechanism in cooperation with a striker assembly that provides for angular adjustment of a seatback in a plurality of positions relative to the seat bottom and securely latches a seat to a vehicle structure when in a latched position is desirable in the industry. Additionally, providing increased cargo-carrying capability in the cargo area of a vehicle while still maintaining use of the vehicle seat is also desirable. Further yet, it is desirable to provide a latch mechanism that significantly reduces or eliminates chucking of a seat assembly.
Accordingly, the present invention provides a seat latch mechanism including an upper housing and a lower housing. The lower housing includes a recess for a striker to pass therethrough when a latch assembly is in an unlatched position. The a latch assembly is operably supported by the lower housing and is operable between a latched position and an unlatched position. An actuation assembly is operably supported by the upper and lower housings and is pivotably attached to the latch assembly to selectively toggle the latch assembly between the latched and unlatched position.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

Multi-position latch apparatus and method


Molybdenum metal powder. Molybdenum metal powder includes molybdenum metal particles having a surface-area-to-mass ratio of between about 1 m2/g and about 4 m2/g, as determined by BET analysis, and a flowability of between about 29 s/50 g and 86 s/50 g as determined by a Hall Flowmeter.

Molybdenum (Mo) is a silvery or platinum colored metallic chemical element that is hard, malleable, ductile, and has a high melting point, among other desirable properties. Molybdenum occurs naturally in a combined state, not in a pure form. Molybdenum ore exists naturally as molybdenite (molybdenum disulfide, MoS2).
Molybdenum ore may be processed by roasting to form molybdic oxide (MoO3), which may be further processed to form pure molybdenum (Mo) metal powder. In its pure state, molybdenum metal is tough and ductile and is characterized by moderate hardness, high thermal conductivity, high resistance to corrosion, and a low expansion coefficient. Molybdenum metal may be used for electrodes in electrically heated glass furnaces, nuclear energy applications, and for casting parts used in missiles, rockets, and aircraft. Molybdenum metal may also be used in various electrical applications that are subject to high temperatures, such as X-ray tubes, electron tubes, and electric furnaces.
Because of its desirable properties, molybdenum powders are useful in spray coating and powder injection molding applications. The utility of molybdenum powders may be enhanced through densification. Since the outcome of sensitive metallurgical processes may be affected by molybdenum powders of varying densities, there developed a need for a densification process that could be easily controlled to produce a flowable molybdenum powder of a desired density and flowability, given certain cost parameters.
In addition, because of the desirable properties of molybdenum powders made through known plasma densification processes, there developed a need to produce beneficial densified molybdenum powders through a cheaper and more efficient process than previously known.

This patent is subject to a terminal disclaimer.


A method for adaptively scheduling maintenance of a sensor (14) infers the need for maintenance of that sensor on the basis of an error signal indicative of a difference between a measurement signal generated by that sensor and an independently obtained reference signal that is correlated with the measurement signal (16). Although the reference signal is not identical to the measurement signal, the correlation between these two signals assures that when the sensor is functioning, the two signal will track each other over time. The method includes the step of generating this error signal and then monitoring that error signal to detect a triggering event indicative of a mismatch between the reference signal and the measurement signal. This mismatch is indicative of a probability that there exists a malfunction in the sensor. The value of this probability depends on the choice of the triggering event.

This invention relates to the field of scheduled maintenance in a process control system and, in particular, to a method and system for determining, on the basis of its performance, whether a system component requires maintenance.
In a process control system, a controller typically sets the value of one or more manipulated variables with the objective of maintaining the value of a controlled variable at a specified setpoint. For example, a controller can maintain a fluid level (the controlled variable) in a tank, by altering the flow rates (the manipulated variables) of fluid entering and/or leaving the tank. Since the appropriate value of the manipulated variable typically depends on the value of the controlled variable, a process control system almost invariably includes a measurement sensor for monitoring the value of the controlled variable.
The correct operation of the measurement sensor is of paramount importance in the operation of a process control system. It is easy to see, in the foregoing example, that a measurement sensor that under-represents the fluid level can cause the controller to increase the flow rate into the tank, thereby causing the tank to flood. It is thus desirable, in a process control system, for measurement sensors to function with utmost reliability.
Because of the nature of its function, a measurement sensor is typically in intimate contact with the process under control. In many cases, this results in exposure of the measurement sensor to extremes of pressure and temperature in the presence of corrosive fluids. In some cases, the mechanical agitation used to mix process constituents results in prolonged vibration of the measurement sensor. Indeed, the environment faced by a measurement sensor in its day-to-day operation can be more hostile than that faced by interplanetary probes.
Given the cumulative effect of the conditions under which they operate, it is not surprising that even the most rugged measurement sensors require periodic maintenance to assure their optimal functioning. This periodic maintenance generally entails shutting down the process and testing the operation of the measurement sensor. The loss of production time resulting from shutting down the process makes it desirable to avoid unnecessary maintenance of the measurement sensor.
What is difficult to determine, however, is when the periodic maintenance should occur. Periodic maintenance that occurs more frequently than necessary is economically wasteful. However, periodic maintenance that is too infrequent can lead to catastrophe. Because the maintenance requirements of a particular measurement sensor depend on its cumulative exposure to wear, it is difficult to predict with certainty when failure is likely to occur.
A particular difficulty with accounting for a measurement sensor""s cumulative exposure to wear is that, in some cases, the variable that the measurement sensor measures, is itself one of the factors contributing to wear on the measurement sensor. If, as is often the case, exposure of the measurement sensor to a hostile environment gradually reduces the accuracy of the sensor measurement, and if one is also relying on that sensor""s measurements to predict the need for maintenance of that sensor, then one""s predictions can be flawed.
For example, in order to determine when a pressure sensing device might need replacement, one can monitor the performance of the pressure sensing device. However, it is not generally possible to determine whether the measured pressure is the actual pressure, in which case the device is working correctly, or whether the measured pressure and the actual pressure differ, in which case the device may need maintenance. This poses a bootstrapping quandary, since the only way to know the actual pressure is to read the measured pressure using the very pressure sensing device whose accuracy is in question.
It is not a satisfactory solution to provide two measurement sensors since any discrepancy would simply indicate that at least one of the two sensors is unreliable without specifying which it is. Although additional measurement sensors could be provided, this would dramatically increase equipment costs.
A system incorporating the principles of the invention determines the probability that a measurement sensor, which generates a measurement signal, requires maintenance. The system includes a reference sensor adapted to generate a reference signal that is at least partially correlated with the measurement signal generated by the measurement sensor. This reference signal, together with the measurement signal generated by the measurement sensor, are provided to an error signal generator that generates, from the measurement signal and the reference signal, an error signal. A trend spotter, in communication with the error signal generator, monitors the error signal to detect the occurrence of a triggering event. This triggering event is pre-selected to be indicative of the probability that the measurement sensor requires maintenance.
The method of the invention infers the probability that a measurement sensor requires maintenance on the basis of an error signal indicative of a difference between a measurement signal generated by that sensor and an independently obtained reference signal that is correlated with, but distinct from, the measurement signal. Although the reference signal is not identical to the measurement signal, the correlation between these two signals assures that when the sensor is functioning correctly, the two signals track each other over time. The method of the invention monitors this error signal to detect a triggering event indicative of a mismatch between the reference signal and the measurement signal. The occurrence of the triggering event is indicative of a probability of a malfunction in the sensor and hence indicates a need for maintenance.
A xe2x80x9ctriggering eventxe2x80x9d is intended to include composite events formed by unions and intersections of events. For example, a triggering event can be that the reference signal and the measurement signal differ by more than a selected threshold. A triggering event can also be the event that the reference signal and the measurement signal differ by more than a selected threshold a pre-selected number of times during a pre-selected interval. In both cases, the triggering event is indicative of the probability that the measurement sensor is malfunctioning. By suitably defining the triggering event, the method of the invention reduces the likelihood that spurious differences between the measurement signal and the reference signal incorrectly indicate a need for maintenance.
The step of generating the error signal includes the step of generating a difference signal representative of the difference between the reference signal and the measurement signal. This is followed, in one embodiment, by the step of rectifying, or evaluating the absolute value of, the difference signal. The absolute value of the difference signal, also referred to as the xe2x80x9crectified difference signal,xe2x80x9d is then monitored to determine the extent to which it deviates from a constant value.
In another embodiment, the error signal is generated by passing the difference signal in parallel through two different first-order filters having different time constants. The outputs of these two first-order filters are then subtracted from each other and the resulting difference is used to generate the error signal. To the extent that the difference between the outputs of these two filters is close to zero, the method of the invention reports that the measurement sensor is functioning correctly. In this embodiment, a relatively constant non-zero or monotonically changing value indicates that one of the two inputs generating the difference signal is steadily moving away from the second input as a function of time.
In yet another embodiment, the error signal is the time rate-of-change of the rectified difference signal. This error signal is monitored to determine whether its value remains substantially equal to zero. To the extent that its value remains substantially equal to zero, the method of the invention reports that the measurement sensor is functioning correctly.
In another embodiment, the error signal is a cumulative sum of the time-derivative of the rectified difference signal. This error signal is monitored to determine whether its value remains substantially zero. To the extent that the error signal remains substantially zero, the method of the invention reports that the measurement Sensor is functioning correctly.

Statistical trend generator for predictive instrument maintenance


A method for transparently monitoring and/or modifying web browsing activities over an entire computer network does not require modification of client software and can be implemented on a single server. By rerouting HTTP requests through a centralized gateway server, the sequence and timing of URLs accessed by individual clients are recorded, providing the full information required to recreate a user session. The client HTTP requests are rerouted through the gateway server by modifying URLs within HTML documents. For each document sent to the client, any original URL which points to another server is modified so that it points to a CGI script program on the gateway. The gateway server then fetches the requested document from the other server, modifies the URLs within the document, and passes it on to the client. Thus, subsequent requests from the client will automatically be directed through the gateway server. The method permits tracking individual web sessions and provides valuable information about user behavior. This information can be used for general purpose evaluation of web-based user interfaces to information systems. The technique can also be used to modify a browsing experience by layering or stripping hypertext content or functionality.

In order to evaluate existing web site organization and improve web site design, there is a need to understand how users access and use hypertext resources. Current methods for monitoring the use of web resources, however, either require the modification of the client browser or are limited to tracking access to a single server. Modifying all client browsers is an expensive and impractical solution since all existing browsers would have to be modified. Present approaches to monitoring web browsing activity, therefore, have focused on the server. Although these techniques permit a server to record all requests which are directed to that particular server, once the user has browsed to another site, information about the browsing activity of the user is not available. Because a browsing session typically involves jumping between many different servers, no single server is able to accurately determine from its log what any given user has done. One possible solution is for sites to collaborate and attempt to determine user activity from a combination of their HTTP log files. This solution is impractical, however, and would still result in a very incomplete record of a user's browsing session. Another possible solution is to require that the client use a proxy server, as in the case of clients behind an internet firewall. This solution, however, requires special proxy servers and requires that the users manually configure their browsers to a particular proxy server to turn on tracking and then manually change the configuration to turn off tracking.

Method for monitoring and/or modifying web browsing sessions

The Board of Trustees of the Leland Stanford Junior University


A method for producing low molecular weight organic compounds, said method comprising the step of passing a mixture of CO and H.sub.2 over palladium supported on a suitable acidic metal oxide at a temperature and pressure sufficient to form said organic compounds. The palladium is highly dispersed on the chosen support and has a crystallite size of less than 100 A, preferably less than 70 A, most preferably, less than 40 A. In this state, the surface Pd atoms have a much higher activity than surface Pd atoms of very large Pd crystallites. To insure maintenance of such high activity and small crystallite size for a substantial period during the course of the reaction, the synthesis reactions are run at reactor bed temperatures below which agglomeration of crystallites is avoided, i.e., at temperatures below 350.degree. C., preferably about 325.degree. C., most preferably 300.degree. C.

This invention pertains to improvements in the production of organic compounds from carbon monoxide and hydrogen. In one aspect, the invention relates to passing a feedstream containing hydrogen and carbon monoxide over a supported palladium catalyst in which palladium is highly dispersed and of small crystallite size, i.e. less than 100 A, preferably less than 70 A, most preferably less than 40 A and to a process for maintaining the palladium in the dispersed state.
In still another aspect, the invention is concerned with improvements in the production of methane from a feedstream containing H.sub.2 and CO.
Although a vast amount of research and development has been conducted over the past 50 years pertaining to the Fischer-Tropsch synthesis (the reaction between CO and H.sub.2 to form organic compounds) almost no effort has been made to study the capability of noble metals to catalyze this reaction.
Although the metals of the palladium group are all generally active as hydrogenation-dehydrogenation catalysts, they differ considerably in their ability to catalyze the various reactions between carbon monoxide and hydrogen. Ruthenium has been known for many years to be an effective catalyst in the Fischer-Tropsch synthesis and in the catalytic synthesis of methane from carbon monoxide and hydrogen, the reaction producing high molecular weight waxes at high pressures and large quantities of methane at atmospheric pressure. Suspensions of ruthenium oxides in a hydrocarbon solvent have been shown to possess exceptionally high activity, the CO--H.sub.2 reaction being measurable at 100.degree. C. By contrast, the other unsupported platinoid metals are much inferior catalysts for these reactions. Unsupported metals such as rhodium and osmium have been shown to exhibit some activity at elevated temperatures whereas palladium and platinum are generally inert for both the Fischer-Tropsch synthesis and the methanation reaction under these same conditions.
Of the nine Group VIII metals of interest, iron, cobalt and nickel have been extensively studied and large quantities of information exist in the literature. All three produce significant amounts of methane from a CO + H.sub.2 feedstream but nickel is the most selective toward methane and is presently the commercially used catalyst for the methanation reaction.
In an article authored by F. Fischer, H. Tropsch and P. Dilthey, Brennstoff-Chemie 6, 265 (1925), the methanation activities of a series of unsupported metals were studied to determine the order of their activity. The metals in order of decreasing activity were ruthenium, iridium, rhodium, nickel, cobalt, osmium, platinum, iron, molybdenum, palladium and silver. However, none of these activities was corrected for differences in metal surface areas. Surface area variations for the different metals could significantly alter the order of activities of the metals. Palladium was found to be the least active Group VIII metal giving no conversion at 300.degree. C and negligible conversion at 400.degree. C.
In another article by H. Pichler, Adv. in Catalysis, IV, 271 (1952), ruthenium was described as the most active catalyst for the low temperature, high pressure synthesis of high molecular weight materials. It was verbally stated that ruthenium and osmium gave appreciable amounts of liquid and solid products, platinum was "much less suitable" as a catalyst for this reaction, and palladium and iridium gave only traces of products.
In another article by McKee, D. W., J. Catal. 8 240 (1967), the interaction of CO + H.sub.2 at 1 atmosphere over various unsupported Group VIII metals was studied by the author. He found that Pd was completely inactive toward methane formation whereas rhodium and iridium produced small amounts of methane at 200.degree. C. and ruthenium was quite active at 100.degree. C.
U.S. Bureau of Mines Report #6974 (1967) discloses the only data concerning Pd placed on a metal oxide support (Pd/Al.sub.2 O.sub.3). Contrary to the findings of the present application, the Bureau of Mines found that a Pd/Al.sub.2 O.sub.3 catalyst was the least active of the noble metals.
However, no information is disclosed in this Report as to their pretreatment procedure of the catalyst. We have discovered that a careful pretreatment procedure of the catalyst is required for insuring that Pd/Al.sub.2 O.sub.3 possesses high activity as a catalyst in the methanation reaction and that such high activity can be maintained by running the process within certain operating parameters.
U.S. Pat. No. 3,625,664 to Padovani, teaches a process for the production of rich fuel to replace natural gas by means of catalytic hydrogasification under pressure of fluid hydrocarbons. This patent discloses that catalyst systems such as nickel, ruthenium, iron, cobalt, palladium, or platinum on a support such as alumina, and/or magnesia and/or silicon oxides and/or calcium oxides may be used to generate methane from CO and H.sub.2. This patent is not specifically directed to Pd on Al.sub.2 O.sub.3 or other acidic supports as is the instant invention and does not recognize or teach the criticality of the palladium being well dispersed and of small crystallite size on the support. Nor does the patent teach the necessity of the support being acidic or the temperature sensitivity of the system. Finally, the patent while broadly disclosing a supported palladium catalyst did not actually utilize such a catalyst, all working examples being Co, Ni, Mo catalysts and iron-zinc oxide catalysts.
Therefore, the understanding of the prior art was that palladium was not a particularly good catalyst for the production of methane from CO and H.sub.2. All of these authors who actually conducted experiments studied palladium in its unsupported state and employed it in its powder form, except for the U.S. Bureau of Mines, while U.S. Pat. No. 3,625,664 did not test palladium at all.
The present invention has found that if palladium is supported on various acidic metal oxide supports in a highly dispersed state, it has an especially high specific activity to catalytically produce low molecular weight organic compounds, especially methane, from a mixture of CO and hydrogen. This enhanced activity is 60-70 times greater than any enhancement due only to an increase in Pd metal surface area due to increased dispersion. Supported palladium catalysts have several advantages over commercial nickel catalysts which include (1) better activity maintenance, (2) better tolerance to sulfur, (3) better selectivity to methane and (4) more tolerance to higher CO pressures and lower temperatures as palladium does not form volatile metal carbonyl as nickel would do under similar conditions.
Briefly, the subject invention relates to a method for producing low molecular weight organic compounds, primarily methane, said method comprising the step of passing a mixture of CO and H.sub.2 over palladium supported on a suitable acidic metal oxide at a reactor bed temperature below which agglomeration of the crystallites is avoided for a reasonable period (i.e. for enough time to make operation of the catalyst economically feasible) at a temperature of between 100.degree.-400.degree. C., preferably 300.degree.-350.degree. C., more preferably at 300-325, most preferably at 300.degree. C., and pressure sufficient to form said organic compounds.
The instant invention teaches a process for maintaining the activity of a catalyst selected from the group consisting of palladium on alumina and palladium on hydrogen-y-zeolite used to produce low molecular weight organic compounds comprising substantially methane, said process consisting of the steps of passing a mixture of CO and H.sub.2 over palladium supported on an acidic metal oxide selected from the group consisting of alumina and hydrogen-y-zeolite wherein the catalytic activity is maintained by maintaining the palladium metal particle size at less than 100 A which is accomplished by running at a reactor bed temperature of between 100.degree. and 400.degree. C at a pressure ranging from 1 to 500 atmospheres and at a GHSV of from 1200-4600 hr.sup.-1. In the practice of the invention the H.sub.2 /CO ratio may range from 1 to 5 and the amount of palladium may range from 0.2 to 2.0 wt. % based on the total weight of the support plus the palladium. Preferably, the temperature of the reactor bed ranges from 300.degree. to 400.degree. C.
The catalyst used in accordance with this invention is formed by the steps of impregnating a suitable support material with a salt solution of the palladium followed by heat treating the impregnated support to form a chemical complex at the surface of the support and to drive off moisture. The complex is highly dispersed and the palladium has a crystallite size of less than 100 A, preferably less than 70 A, most preferably less than 40 A.
Palladium may be added to the catalyst in the form of a solution containing a soluble palladium salt or any other palladium salt which is soluble in the solvent used for the impregnation.
The impregnated support in powder or granular form is then treated by establishing time-temperature relationships suitable to produce a chemical change on the surface of the support and to remove water and adsorbed oxygen. Suitably, the impregnated support can be heated in air, in an inert atmosphere or in vacuum, e.g. 20-29 inches of mercury, and from about 150.degree. to about 650.degree. C., preferably from about 200.degree. to about 430.degree. C. for periods ranging from about 0.5 to about 4 hours, or preferably from about 1 to about 2 hours. On the other hand, the reaction between the salt and support can be accomplished by the elevated temperatures while moisture is stripped from the support with nitrogen or other nonreactive gases. If desirable, the impregnation and heat treating steps can be conducted in multiple stages, for example, the support can be impregnated and then dried, or partially dried at low temperature. Support can then be reimpregnated and again dried or partially dried. The heat treatment per se can also be conducted in multiple stages, if desired. The impregnated support, to facilitate handling, can thus be subjected to a first rather mild heat treatment to dry the support and thence in a second step, to a more severe treatment to produce the desired chemical change at the surface. This pretreatment procedure is employed to obtain small Pd crystallites and is similar to that employed by Aben and disclosed in the Journal of Catalysts 10, 224 (1968). The Pd metal dispersions of the instant invention are such that the Pd particle size is <100A, preferably less than 70 A, most preferably less than 40 A.
Suitable supports are the oxides in Groups IIIA and IVA, V-B and VI-B of the Periodic Chart of the Elements and are described in a Table found in the Handbook of Chemistry and Physics, Chemical Rubber Company, 45th ed., (1964) page B-2. Acidic metal oxides, particularly alumina, are essential to obtain maximum activity enhancement. Alumina supports in fact, are quite outstanding from a cost effect standpoint and are readily available. Silica-free alumina has been found especially suitable though silica-alumina is also highly active. Silica-alumina, i.e., acidic zeolites, certain natural acidic clays, diatomaceous earth, e.g., kieselguhr and other supports are also useful. Silicon dioxide, and mixtures of silicon dioxide and aluminum oxide are also used in this invention as well as the acidic zeolites. In fact, any acidic refractory oxide that will give well-dispersed palladium is suitable in the use of this invention, with alumina, silica-alumina (for example, acidic zeolites) being preferred.
Methane formation is the predominant reaction desired in the subject invention and the catalyst incorporates an acidic support, i.e. palladium-Al.sub.2 O.sub.3. The palladium loading ranges from 0.01 to 5 wt. % based on the total weight of the support + palladium and the preferred amount of palladium ranges from 0.2 to 2 wt. % based upon the total weight of the support + palladium. In the methane formation, the hydrogen to CO mole ratio ranges from 0.5 to 15 and is preferred from 1 to 5. The pressure ranges from 1 to 500 atmospheres and is preferred from 1 to 20 atmospheres; temperature of the reaction bed ranges from 100.degree. to 400.degree. C., preferably 300-350, more preferably 300.degree.-325.degree. C., most preferably about 300.degree. C.
Reaction bed temperatures of less than 350.degree. C. are necessary for the maintenance of catalytic activity during the course of the process, i.e. for a long enough time to make operation of the catalyst economically feasible. Exposure to reactor bed temperatures in excess of 350.degree. C. results in marked decreases in surface area (i.e. agglomeration of the palladium, evidenced by crystallite sizes greater than 100 A) which result in decreased activity. Highly dispersed Pd/Al.sub.2 O.sub.3 catalysts possess very high initial activity at 400.degree. C. with over 70% of the CO converted. These conversions are much higher than those obtained by the U.S. Bureau of Mines even though the temperature is 100.degree. C. lower and the space velocity is 14 times higher. However, at these high conversions and high temperatures, the catalysts deactivate rapidly and possess only about 1/10 their activity after 31/2 hours on-stream. As shown in Table I, a concomitant loss in metal surface area occurs and accounts for much of the loss in activity. At 350.degree. C. the catalyst still shows an activity decline although it is not so marked. The greater metal surface area is sufficient to give this catalyst a higher activity after 3 hours on-stream than the catalyst run at 400.degree. C. even though the temperature is 50.degree. C. lower. Finally, when run at 300.degree. C., the catalyst shows some activity decrease during the first hour but then stabilizes and retains a constant activity during the remainder of the test run. This demonstrates a necessary limitation in reactor bed temperatures such that at 300.degree. C. the catalyst is very active and stable, possessing at this temperature 50% the activity of the catalyst run at 400.degree. C., whereas at 350.degree. C. the activity continuously declines and at 400.degree. C. the activity declines very precipitously. The preferred temperature range is therefore 300.degree.-350.degree. C., with 325.degree. C. the more preferred temperature, and 300.degree. C. the most preferred temperature.
This behavior is more clearly evidenced in FIG. 1. The two catalyst samples run at 400.degree. C. have much higher initial rates, as expected due to the higher temperature, but a rapid decline occurs with increasing time on-stream. A less rapid decline occurs at 350.degree. C. while very stable activity maintenance exists at 300.degree. C. Therefore, between 300.degree. and 350.degree. C. the Pd/Al.sub.2 O.sub.3 catalyst can be utilized without deactivation. This figure not only shows a temperature window in which both high activity and activity maintenance can be obtained together, but also provides a clear explanation of the Bureau of Mines result. At their temperature of 500.degree. C., an even more rapid and severe activity decline is expected to occur creating a catalyst with very low activity. The exceedingly low activity is shown in Table II by calculating an activity from the Bureau of Mines data. Even at 500.degree. C. the activity per gram catalyst is lower than the high-activity Pd/Al.sub.2 O.sub.3 catalyst disclosed here. Since the activation energy for this reaction has been found to be -19,700 cal/mole over Pd/Al.sub.2 O.sub.3, the rate at 300.degree. C. can be calculated. If this is done and correction is also made for space velocity differences, the activity for the Bureau of Mines catalyst is only 0.044, more than 1000 times lower than the catalyst of the present invention. This result is also shown in Table II.
The data in Table I shows very clearly that increasing the reactor temperature decreases the available Pd surface area as indicated by the ratio of adsorbed CO to the total number of Pd atoms in the catalyst. The catalysts subjected to the highest temperatures show an order of magnitude drop in metal surface area while the decrease in the sample heated only to 300.degree. C is much less severe and still allows the catalyst to possess a very high activity.
One catalyst sample had 0.7 wt. % Na added to establish the effect of this component. Sodium clearly has a detrimental effect as it blocks part of the Pd surface to CO adsorption as shown in Table I by the CO uptake on the fresh reduced sample. A concomitant decrease in initial activity occurs as shown in FIG. 1. However, after several hours on-stream, both the final activities and CO uptakes are identical indicating that the Na has been removed from the Pd surface, possibly by a washing effect since large quantities of water are produced in the course of the reaction.
Table II demonstrates that these catalysts achieve significant conversions although the temperatures are 100.degree.-200.degree. C lower than the Bureau of Mines runs and the space velocities are 4 to 21 times higher.
TABLE I ______________________________________ Chemisorption Data for Used 0.5% Pd/Al.sub.2 O.sub.3 Catalysts Pd Crystal- Reaction CO Uptake lite Size Run Temp. (.degree. C) (.mu.mole/g) CO/Pd (avg.) (A) ______________________________________ 1a Fresh 46.5 0.99 <15 1b 400 3.8 0.081 130 2a Fresh 16.5 0.35 -- w/0.7% Na added 2b 400 3.8 0.081 130 w/0.7% Na added 3 350 7.5 0.16 70 4 300 11.6 0.25 40 ______________________________________
TABLE II __________________________________________________________________________ Kinetic Data for CO Conversion Temp. Pressure GHSV On-Stream CO Conversion Catalyst Activity Run (.degree. C (ATM) (hr.sup.-1) (hr) to CH.sub.4 (%) (.mu. mole CH.sub.4 /min .multidot. g cat) __________________________________________________________________________ 2 408 20.3 4500 1/2 71 1740 406 20.7 5300 3 1/2 18 258 3 402 20.7 3200 1 43 950 399 20.9 1200 3 1/2 30 262 4 352 21.1 6800 3/4 17 866 351 21.6 6900 5 1/2 5.0 259 5 301 21.0 3000 1/2 25 363 298 20.7 4500 1 4.3 142 304 20.7 4600 6 1/2 3.4 117 Bureau of Mines 499 21 318 -- 35.5.sup.a 56 (est.) " 300 21 4600 -- -- 0.004 (calc.) __________________________________________________________________________ .sup.a [H.sub.2 +CO] conversion to all hydrocarbon products.
The methanation reaction occurs very selectively over Pd and only very small amounts of other hydrocarbons are detected. In fact, it is surprising to note that Pd is the most selective methanation catalyst of the Group VIII metals. A portion of this invention is the discovery that the specific activity (rate per unit surface area of metal) of Pd can be enhanced by placing the metal on particular supports, especially Al.sub.2 O.sub.3, and pretreating this catalyst properly to maintain small Pd crystallites. Other supports will also enhance activity, but the greatest enhancements come from acidic supports, as indicated in Table III. The highest activity is achieved with Pd supported on the more acidic metal oxides, such as Al.sub.2 O.sub.3, and the smallest improvement in activity occurs when the least acidic support, SiO.sub.2 is used. That the rate per surface Pd atom is greatly increased is clear from a comparison of rates in column 4. The specific activity of Pd/Al.sub.2 O.sub.3 is 70 times greater than the rate of unsupported Pd.
The specific activity of a catalyst is represented by a "Turnover Number", defined as the number of CO molecules reacted per second per site on the metal surface. The number of sites is determined by selective chemisorption techniques using a gas such as carbon monoxide or hydrogen. Such gases are selectively chemisorbed on the metal surface and by employing justified, common assumptions such as 1 hydrogen atom adsorbs on every surface metal atom or 1 CO molecule adsorbs on every surface metal atom, the number of surface metal atoms, M.sub.s, can be calculated. This value then represents the total number of metal surface sites and can also be used to calculate the metal dispersion, D, defined by D = M.sub.s /M.sub.t where M.sub.t is the total number of metal atoms in the sample.
Such a representation of catalyst activity is very meaningful since it allows direct comparisons not only between different metals but also between different metal loadings of the same metal catalyst. Space velocity measurements do not correct for different metal loadings in different catalysts, or any differences in metal surface areas at the same loading, i.e. dispersion effects. A comparison of turnover numbers, therefore, already has normalized out differences in surface area due to dispersion effects. If conversion data is desired, it is easily calculated from the following formula which assumes no diffusional limitations: ##EQU1##
These supported palladium catalysts heretofore unrecognized as an active selective catalyst in the reaction between CO and hydrogen are useful in any process producing such a mixture such as gasification of coal, residuum, oil shale, tar sands, etc. All these processes can utilize these catalysts to produce methane.

Catalytic formation of hydrocarbons from CO, H.sub.2 mixtures and process for maintaining catalytic activity