US2879323A

US2879323A - Electrical insulating cement

Info

Publication number: US2879323A
Application number: US473696A
Authority: US
Inventors: Frank S Nichols; Jr Sidney R Smith
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1954-12-07
Filing date: 1954-12-07
Publication date: 1959-03-24
Anticipated expiration: 1976-03-24
Also published as: JPS336391B1; CH362133A; GB818038A

Description

Marh 24, 1959 F. s. NICHOLS ErAL 2,879,323

Y ELECTRICAL INSULATING CEMENT Filed Dec. 7, 1954 l2,879,2.zs ELECTRICAL INSULATING CEMENT Frank S. Nichols, Pittsfield, and Sidney R. Smith, Jr.,

Stockbridge, Mass., assignors to General Electric Company, a corporation of New York Application December 7, 1954, Serial No. 473,696

5 Claims. ,(Cl. 174-177) The present invention relates to cement materials.

More particularly, the invention relates to a synthetic resin cement serving as an electrical insulating bond between ceramic, especially porcelain, insulator mem-bers `they are usually made of ceramic materials such as porcelain, or, less frequently, from certain types of glass. These materials are practically the only ones which in the course of many years of service have been demonstarted to combine good insulating properties with the required resistance to prolonged exposure to weathering 'under high voltage stress. Certain plastics and synthetic 'resins have been found to be excellent insulating materials' and to withstand long exposure to atmospheric conditions.l However, under the combined effect of weather ,and high voltage stress they are subject to tracking'the formation of a conducting carbon path which lquickly results in failure of the insulator.

In, the-various applications of electrical insulating material such as mentioned, the porcelain is requiredv to lic structure such as mounting brackets or tanks. A problem'which'has'always been present in mounting the insulator is the proper attachment of the insulator to such `metal parts, since it is not usually practical to cast in Ythese or other metal parts for their attachment during manufacture of the porcelain. It is the usuall practice to vsecure the metal parts to the porcelain insulator by means of a cement, which is used either to bond the metal part inserted in a recess in the porcelain'or to bond a projection on the porcelain which extends into a opening in the metal part.-

fBonding materials which have been commonly used heretofore for this purpose include sulfur cement, Portland cement and solders formed' of metal alloys, 'but these cement is subject to excessive shrinkage often necessitating more than one filling, and is highly corrosive to many metals, the fumes evolved during handling tending to corrode silver contacts in its vicinity. Portland cement it poorly adapted for high speed insulator mounting processes. Metal solder alloys are generally expensive and because of their high melting point the application of such alloys inmolten state-to the porcelain subjects the porcelain to excessive thermal shock. Also, they have no electrical insulating properties.

insulate a-high voltage conductor from a grounded metalcorresponding V bonding materials-have several disadvantages. Sulfurz ve5 requires a long, carefully controlled `steam cure making ,r 2,879,323 Patented Mar. 24, 1959 It is an object, therefore, of the present invention to provide a cement which is suitable for use with electrical insulators of the above type and which avoids the disadvantages of insulating cement materials heretofore used.

It is another object of the present invention to provide an insulator structure comprising a ceramic especially porcelain to metal joint wherein the bonding material is characterized by ease of handling, rapidity of hardening, low application temperature, relatively low shrinkage on hardening, non-corrosion of metals, high mechanical strength, good resistance to weathering, and high resistance to arcing and tracking.

A further object of the present invention is the provision of a composite insulating structure of the above type consisting of the ceramic insulator and cement wherein the cement material has a dielectric constant at least as high as the ceramic insulating material which it bonds to the metal hardware.

Another object of the invention is the provision of an electrical structure having the cement and ceramic of the composite insulation arranged in series between electrical conductors bonded thereto.

In accordance with the invention, an insulating structure is provided which comprises a ceramic body, a metallic body and a cement material rmly bonding the ceramic and metallic bodies together, the cement having a dielectric constant at least equal to the dielectric constant of the ceramic body and comprising a reactive polymerizable synthetic resin selected from the group consisting of polyester resins and epoxy resins, an inorganic filler, and a polymerizing agent for polymerization of the synthetic resin.

The expression 100% reactive polymerizable synthetic resin as used herein is intended to cover compositions of matter which are polymerizable materials substantially free of inert, volatile solvents and which by the incorporation of a suitable polymerizing agent may be caused to polymerize to form substantially infusible and insoluble materials without the necessity of taking up oxygen from the air and without forming volatile products. Further, the expression polymerizing agent as used herein is intended to cover a catalyst or a crosslinking reactant which participates in the reaction bringing about polymerization of the polymerizable materials, examples of such catalysts and reactants being given below.

The expression polyester resins as used herein is intended to denote synthetic resins produced by polymerization of polybasic acids with polyhydric alcohols, either or both the acids and alcohols being unsaturated, with co-polymerization of an admixed vinyl monomeric compound.

The invention will be better understood by referring to the accompanying drawings in which:

Fig. 1 is a cross-sectional view of an enclosed type cutout embodying a cement in accordance with the invention; and

Fig. 2 is a cross-sectional view of a suspension insulator utilizingthe present cement.

' Referring now to the drawings, and particularly to Fig. l, there is shown an enclosed type electrical cutout of known construction, the cutout having a housing 1 made Aof porcelain or the like in which combination terminal clamp and contact devices 2, 2' are mounted by metal brackets 3,' 3 cemented to housing 1. The terminals of the power line in which the fuse is in circuit are connected to the opposite terminals of the fuse assembly 5 by means of the clamp and contact devices 2,2. Brackets 3, 3' are secured to porcelain housing 1 by means of cement joints '4, 4a, the composition of which is a feature of the present invention and which is more fully described fbelow. The door 7 of the cutout carries the fuse as-r sembly 5, and in the embodiment shown is mounted for turning about trunnion structure 8 secured to the cutout housing 1 by cement joint 4b. Mounting bracket 6 which'attaches cutout housing 1 to a pole or similar support is likewise secured to housing 1 by a cement joint 4c of similar composition. The metallic parts which are joined to the porcelain Iby the present cement composition are usually made of galvanized steel, but it is to be understood that any other electrically conductive metals could be effectively bonded to ceramic insulation by the present cement material.

TheI conditions of operation of the illustrated type of cutout cause considerable stresses to be placed on the various cement joints described. For example, the type of fusey illustrated is commonly designed to produce a gas when the fuse is blown for extinguishing the yresulting arc, and the expulsion of the gas from the fuse tube exerts a thrust on the latch mechanism associated with `the upper terminal device 2', which in turn exerts `a force o n the cement joint 4ay of the upper bracket 3. The same thrust force may also be exerted on the lower cement joint 4b through the trunnion structure. The droppingout of the fuse door 7 and its associated fuse assembly 5 with the latter members bearing on trunnion structure S in a somewhat modified type of cutout arrangement (not shown), which provides for such action upon blowing ofthe fuse, also subjects the cement joints to a rathersevere mechanical shock. Further, the cement joints may be subjected to strong mechanical stress due to tension exerted by the power lines attached to the terminal mounting brackets. -J

Fig. 2 illustrates a type of known suspension insulator device which may include stacked interconnected insulating members arranged with adjacent memberszlinked together by aconnecting pin 10. The enlarged end of pin 10 ts into and thereby holds in suspension a cap 11 made of steel or the like into which fits a projecting portion of a glazed porcelain insulating member'12, cap 11, and insulating member 12 being joined by a cement bond 13 having a composition in accordance withlthe present invention. The porcelain insulator member 12 in turn carries suspension link 14 by means of a cement joint 13a also corresponding to the present composition, the

4suspension link 14 being connected by means of an eye at its lower end to suspension clamp 15.'v A power line 1 6 is carried and clamped by suspension clamp 15 in the known manner. It is clear that in such' an arrangement the weight of the power line exerts a fconsiderable tensile stress on the cement joints referred to above tending to separate the component parts of the insulator support. y l

ln the operation of the insulator devices such as described and illustrated, the cement and porcelain serve as dielectric media arranged in series between two conductors, e.g., in the Fig. l cutout, the power line terminalsV and the cutout mounting bracket, across which there is a high voltage. Under these conditions itis known that the voltage stress between the conductors distributes itself in inverse proportion to the dielectric constants of the two dielectrics, that is, the voltage stress is concentrated on the dielectric having the lower dielectric constant.

in View of the superior electrical insulating and dielectric properties of the porcelain, it is desirable in accordance with the invention to concentrate the stress on the porcelain by providing a cement which has a dielec- Atric constant at least as great as and preferably greater Suitable for use in the invention includes the polyester4 resins which are formed from a mixture of an unsaturated alkyd resin and a vinyll monomer: The unsaturated alkyd resins are the reaction products of polyhydric alcohols, mixtures of polyhydric alcohols, or mixtures of polyhydric and monohydric alcohols, and one or more polycarboxylic acids, either the alcohols or the acids or 'both being unsaturated. Examples of suchl polyhydric alcohols are ethylene glycol, diand triethylene glycols, propylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, glycerine or pentaerythritol in combination with a monohydric alcohol. Examples of unsaturated polycarboxylic acids are maleic, fumarie, and itaconic acids. Anhydrides of polycarboxylic acids may also be employed. The term polycarboxylic acid as used herein is intended to includeI within its meaning the anhydrides of such acids. In addition to one or more of the unsaturated polycarboxylic acids, saturated polycarboxylic acids may also be present in the reaction mixture in the preparation of the resins referred to above. Examples of such saturated polycarboxylic acids are succinic, adipic, sebacic, and phthalic acids.

Of the vinyl monomers used in accordance with the invention to form the polyester resin cement,'styrene has proved particularly suitable. Examples of other'vinyl monomers which may be used include esters of unsaturated monohydric alcohols and carboxylic acids, including unsaturated carboxylic acids, halogenatedv` aromatic carboxylic acids and inorganic acids. Examples of-` such substances are diallyl phthalate, diallyl succinate, diallyl maleate, diallyl fumarate, diallyl itaconate, diallyl chlorophthalates, triallyl phosphate, and vinyl acetate. `Other substances which may be incorporated in these polymerizable liquids are esters 4of monohydric alcohols and unsaturated carboxylic acids which are capable of copolymerizing with unsaturated alkyd resins, such as, for example, dioctyl itaconate, dibenzyl itaconate, diethyl fumarate, dibenzyl fumarate, methyl acrylate and methyl methacrylate.

A suitable commercial polyester resin is Laminac 4128, which is a styrene-unsaturated alkyd resin manufactured by the American Cyanamid Company.

While the above polyester resins may be cured to an infusible state by means of heat alone, eg., at temperatures of -150 C., a small amount of catalyst is preferably added to the polyester resin mixture to facilitate polymerization. Among the substances whichymay be used as catalysts for this purpose are inorganic super oxides such as barium peroxide and sodium-peroxides; aliphatic peroxides such as acetyl peroxide, lauroylfperoxide, stearoyl peroxide and the like; aromatic acid peroxides such as benzoyl peroxide; and other mixed organic peroxides such as acetyl benzoyl peroxide and'ketone peroxides such as acetone peroxide and triacetone peroxide. A particularly suitable catalyst composition which is commercially available is a benzoyl peroxide suspension in tricresyl phosphate.

Instead of the polyester resins described, epoxy resins may be used, these resins being condensation products of polyhydroxy compounds, such as polyhydric phenols and polyhydric alcohol, and epichlorhydrin. Examples of polyhydroxy compounds which may be used as glycerol, diphenylol propane, and the mixed poly (hydroxyl phenyl) pentadecanes derived from cashew nuts. An example of a suitable epoxy resin,y is Araldite CNSOZ of the Ciba Company, which is the condensation product of epichlorhydrin clom-oH-CH and 2,2'lbis phenylol propane 'l @Ht In the case where epoxy resins. are used, the polymerizing agent employed is a cross-linking reactant, examples of which are polyamines, eg., diethylcne triamine, or dibasic anhydrides, e.g., phthalic anhydride.

-With respect to the inorganic ller materials which are used with the above resins to raise the dielectric constant of the cement, it has been found that in general the stannates and titanates of the alkaline earth metals, i.e., barium, strontium and calcium, may be used satisfactorily for the purposes desired, and of these barium titanate and barium strontium titanate have proved especially suitable. Further, titanium dioxide, e.g., in the form of rutile, and certain forms of impure alumina have been discovered to constitute very satisfactory inorganic fillers. Rullux #84 made by the National Lead Company is a granular rutile composition found to be satisfactory for the present cement.

The dielectric constant of a composite material is a logarithmic function of the dielectric constants of the constituents andof their proportions by volume. Since the' dielectric constant of the resin constituent of the present cement composition will =be only of the order of`3'to 5, the dielectric constant of the inorganic filler must be relativelyvery high in order to obtain the desired dielectric constant of the mixture. Therefore, the inorganic substance used to raise the dielectric constant of the cement composition should have a dielectric constant of not less than 30 and preferably of several hundred. In this connection, it is to be understood that the dielectric constant values as recited in the claims are as measured at 60 cycles.

In addition to the above inorganic materials, the inorganic filler may, if desired, contain a finely divided mineral substance such as ground silica, talc, clay, or mica, forthepurpose of controlling the viscosity of the cement, which is of particular advantage in proper application of the cement to the bodies to be bonded, and for imparting mechanical strength to the final joint struc ture.

`In the preparation of the cement, the amounts of ingredients as set forth herein are weighed out accurately, particular attention being paid to the amount of polymerization catalyst used. The resin material is first added to a mixer, the polymerizing agent then added and dissolved before addition of the inorganic fillers. Thoroughmixing is then carried out to ensure ease of ow of the mixture. Ordinarily 30 minutes of mixing are necessary to obtain good flow. It is desirable for the mixer apparatus to be equipped with a cold water jacket to dissipate the heat generated in mixing.

When polyamine cross-linking reactants are used with 'the epoxy resins, because of the high reactivity of these vreactants at room temperature, this procedure should be modified to allow the cross-linking reactant to be added at the end of the mixing period.

The cement made as described using the polyester resins cures hard in about 2 minutes at 125 C. If treated at 110 C., the cure is somewhat slower. Therefore, the cement isA preferably heated as rapidly as possible to 115 to 135 C. The' cement may be applied to the parts to be bonded at room temperature and the assembly heated to the required temperature. The time required to do thisl depends on the parts to be bonded andthe method of heating. Alternatively, the porcelain maybeA preheated to between 110 C. and 135 C. and

the cement is then applied tothe hot porcelain, the Aresidual heat of the porcelain being suicient to harden thecement. In some cases, it may be more convenient or otherwise desirable to preheat the hardware.

v,Asis known in the art, the curingtime and temperayture ofthe epoxy resins depend on the particular crosslinking reactant-used. For example, a polyamine cross- -linking reactant would permit the curing of epoxy resins at room temperature, whereas the use of dibasic anhy- 6 drides as cross-linking reactants would require cures at higher temperatures, e.g., at 150 C. for 6 hours.

The amount Vof resin used in the present cement composition whether the polyester or epoxy resin, ranges approximately from 10 to 80% by weight, while the inorganic filler for both types of resins ranges between 20 to 90%. The catalyst used for the polyester resin is present in the range of 0.255% by weight of the polyester resin and the amount of cross-linking reactant for the epoxy resin is about 5-45% by weight of the epoxy resin. The silica or other mineral substance used for viscosity control may vary between 5-50% of the total ycement composition.

Illustrative cement compositions in percent by weight are se't forth in the following examples, it being understood that the invention is not limited to the specific formulations given since considerable variations may be made depending on the type of materials to be bonded, the desired viscosity, and other factors:

Example 1 Percent Titanium'dioxide 76.4 Styrene-unsaturated alkyd resin mixture 12.9 Silica 10.2

Benzoyl peroxide (50% suspension in tri-cresyl phosl. inch of notch. Dielectric constant, 60 cycle:

25 C 16.4. C 19.5. Percent power factor, 60 cycle:

25 C 4.1. 100 C 10.2. Insulation resistance, ohm/ cm.

25 C 5.5 101.3. 100 C 2.8X1012. Dielectric strength, step by step, 60

cycle 96 VPM (0.25

' section). Arc resistance (ASTM) 207 secs. Shrinkage on setting:

Linear 0.27%. Volume 0.9%. Flammability, ASTM DD635 Self extinguishing. Specific gravity 2.89. Coefcient of thermal expansion 21.6 X 106. Moisture absorption, 24 hours immersion 0.4%. Oil resistance Unatected.

A cement of the above composition has been exposed tor weather under voltage stress for almost three years without visible deterioration. Flashover experiments on cutouts utilizing the above type of cement showed no carbonization of the cement nor fall of ashover voltage on subsequent flashes.

Example Il Percent Titanium dioxide 74.7 Styrene-unsaturated alkyd resin mixture 14.8 Benzoyl peroxide (50% suspension in tri-cresylphosphate) 0.6 Silica 9.9

The above cement when cured has properties similar to those of the cement of Example I, but flow of the uncured material is considerably greater.

7 Example III Percent styrene-unsaturated alkyd resin mixture 18.5 Ground' silica 40.5 Titanium dioxide 40.5 Benzoyl peroxide 0.5

The above cement composition has the following properties: i

Dielectric constant, 60 cycles, 25 C 7.8 Power factor, percent, 60`cycles, 25 C 2.6 Insulation resistance (ohm cms.) 9.6 1014= This cement cures hard in two to three minutes at 125 C.

Example IV Percent vstyrene-unsaturated alkyd resin mixture 23.8 Sand c, 38.0 Alumina (impure) 38.0 Benzoyl peroxide 0.2

The impure alumina referred to above has the following approximate composition in parts by weight, and in this connection it is to be understood that the expression impure alumina as used in the claims is construed as lhaving this composition:

Parts F6203 S102 14 Tio,l 11 CaO Na20 10 Loss on ignition 10 An impure alumina of the above type known as R20 Insulatng Powder is manufactured by the Aluminum Company of America.

The above cement composition has the following properties:

Dielectric constant, 60 cycles, 25 C 9.5 Power factor, percent, 60 cycles, 25 C 18.4 Insulation resistance (ohms cm.) 3.8 1013 Examplel V,

Percent Titanium dioxide 77.0 Silica v, 10.2 Epoxy resin (Ciba Araldite CN502) 11.8 Diethylene triamine 1.0

The above cement composition has the following properties:

Dielectric constant, 60 cycles, 25 C. 18.0. Power factor, percent, 60 cycles,

25 C. 4.7. Insulation resistance, 60 cycles, 25 C. 5)(10u ohms. Dielectric strength (step by step), 60

cycles, 25 C. 100 VPM on M4 thickness.

and metal hardware and particularly as utilized in electrical devices such as cutouts and insulators, it will be understood that other members may be effectively bonded by the use of thepresent cements and that the latter may be used in devices other than those shown and described. For example, the cement may be used for bonding glass insulators to metal, or joining ceramic insulators to each other. Therefore, the described embodiments are not to be construed as limiting the scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is: l. A ceramic to metal joint structure comprising a ceramic body; a metallic body; and a cement material rmly bonding said bodies together, said cement material having a dielectric constant at least equal to that of said ceramic body and being formed from a 100% reactive polymerizable synthetic resin selected from the group consisting of polyester resins and epoxy resins, a polymerizing agent for polymerization of said synthetic resin, andan inorganic iller material having a dielectric constant of at least 30.

2. A ceramic to metal joint structure comprising a v porcelain body; a metallic body, and an electrical insulating cement material firmly bonding said bodies together, said cement material having a dielectric constant at least equal to the dielectric constant of said porcelain body and 4being formed from a 100% reactive polymerizable synthetic resin selected from the group consisting of polyester resins and epoxy resins, a polymerizing agent for polymerization of said synthetic resin, and an inorganic ller material including an inorganic substance having a dielectric constant of at least 30 and a"finely divided mineral substance for controlling the viscosity of the cement composition, said inorganic substance being'selected from the group consisting of titanium dioxide, titanates of the alkaline earth elements,and impure alumina which consists essentially of oxides-'of aluminum, iron, silicon, titanium, calcium and sodium.

3. A ceramic to metal joint .structure comprising a porcelain body; a metallic body; and an electrical insulating cement material firmly bonding said bodies together, said cement material having a dielectric constant at least equal to the dielectric constant of said porcelain body and being formed from a reactive polymerizable synthetic resin comprising a styrene-unsaturated alkyd polyester resin, a polymerizing catalyst for polymerization of said synthetic resin, and an inorganic 'ller material including an inorganic 'substance having a .dielectric constant of atleast 30 and a finely divided 'mineral substance for controlling the viscosity of the cement composition, said inorganic substance being selected from the group consisting of titanium dioxide, titanates of the alkaline earth elements, and impure alumina which consists essentially of oxides of aluminum, iron, silicon, titanium, calcium and sodium.

4. A ceramic to metal joint structure comprising a porcelain body; a metallic body; and an electrical insulating cement material firmly bonding said bodies together, Vsaid cement material having a dielectric constant at least equal to the dielectric constant of said porcelain body and being formed from a 100% rective polymerizable synthetic resin comprising an epoxy resin obtained as the reaction product of epichlorhydrin and 2.2' ybis phenylol propane, a cross-linking reactant for polymerizing said epoxy resin, and an inorganic ller material including an inorganic substance having a dielectric constant of at least 30 and a finely divided mineral substance for controlling the viscosity of the cement composition.

5. An electrical structure comprising a plurality of electricalconducting members, a porcelain body, and an insulating cement material firmly bonding said electrical conducting members to said porcelain 4body andforming in series with said porcelain body a compositeflinsulating joint between said conducting members, saidY cement material having a ldielectric constant atl least equal to that of .said porcelain body and being formed from a 100% reactive polymerizable synthetic resin selected from the group consisting of polyester resins and epoxy resins, a polymerizing agent for polymerization of said synthetic resin, and an inorganic filler material including an inorganic substance having a dielectric constant of at least 30 and a finely divided mineral substance for controlling the viscosity of the cement composition.

References Cited in the le of this patent UNITED STATES PATENTS Naylor Aug. 20, 1929 Scott et al Oct. 10, 1939 Phillips July 4, 1950 Wiles Nov. 7, 1950 Simons et al. Jan. 9, 1951 Di Giacomo Dec. 4, 1951 10 FOREIGN PATENTS 706,067 Great Britain Mar. 24, 1954 716,987 Great Britain Oct. 20, 1954 OTHER REFERENCES Libbey-OWens-Ford Glass Company, Plaskon Division, Bulletin A-4, page 1, Properties of Plaskon Alkyd 420. Received U.S. Patent Oice March 13, 1952.

Libbey-OwensFord Glass Company, Plaskon Division,

10 Bulletin A-7, page 1, Properties and Uses of Molded Plaskon Alkyd. Received U.S. Patent Oce March 17, 1952.

Moss: British Plastics, November 1948, pages 521,

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Psi-item 'Noe 2,879,323 March 24, 1959 Ef-zfemk Nemle et al It is hereby certified that error' appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column ,fl-r, line' 62 fol'u "Egea, as" read w used, .are m; line 68, the ozcmula Should, appealnl as shown instead of as in tha' patent:

Sigmed and sealed thfs Stb day ai 19590 E; AXEINE l ROBERT C. WATSUN [Mieming Ocer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECHUN Patent No., 2,879,323 Merola QZT, 'i959 Frenk S., Nichole et al It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Golem .r, line 62y for "used as" read .fm-f used .are mu; line 68, the formule. yShould appear as shown 'below insteai oi as' in the' patent:

cieegmcameg Signed and sealed this. 8th dey o September 19590 Attest:

E; AXLINE A ROBERT C..WATSON Atteeting Officer a Commissioner of Patents

Claims

1. A CERAMIC TO METAL JOINT STRUCTURE COMPRISING A CERAMIC BODY; A METALLIC BODY; AND A CEMENT MATERIAL FIRMLY BONDING SAID BODIES TOGETHER, SAID CEMENT MATERIAL HAVING A DIELECTRIC CONSTANT AT LEAST EQUAL TO THAT OF SAID CERAMIC BODY AND BEING FORMED FROM A 100% REACTIVE POLYMERIZABLE SYNTHETIC RESIN SELECTED FROM THE GROUP CONSISTING OF POLYESTER RESINS AND EPOXY RESINS. A POLYMERIZING AGENT FOR POLYMERIZATION OF SAID SYNTHETIC RESIN, AND AN INORGANIC FILLER MATERIAL HAVING A DIELECTRIC