US3643054A

US3643054A - Microwave heating apparatus

Info

Publication number: US3643054A
Application number: US40911A
Authority: US
Inventors: Eric O Forster
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1970-05-27
Filing date: 1970-05-27
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15

Abstract

A microwave heating unit for curing wire coatings comprising a microwave generator, a waveguide coupled to said generator and means for coupling the coated wire to said waveguide whereby said wire becomes the center conductor of a coaxial line. The outer conductor of the coaxial line is preferably circular in cross section. Propagation of microwave energy between the inner and outer conductors of the coaxial line necessarily passes through the wire coating, thereby curing said coating.

Description

United States Patent Forster [54] MICROWAVE HEATING APPARATUS [72] inventor: Fxic 0. Forster, Scotch Plains, NJ.

[73] Assignee: Fsso Research And Engineering Company 22 Filed: May 27, mo

[21] Appl.No.: 40,911

Related Us. Application 11111.

[63] Continuation-impart of Ser. No. 684,139, Nov. 20,

1967, Pat. No. 3,551,199.

52 u.s.c1. ....219/10.ss,117/227 [51] 1111. C1. H05b9/06 [5a] misuse-r611 ..219/10.55

[56] Relerences Cited UNI'I'EDSTATESPATENTS 3,457,385 7/1969 Cumming ..219/10.61 3,461,261 8/1969 Lewisetal... .....219/1o.55 3,535,482 10/1970 Kluck... ..219/10.55 8/1962 Schmidt ..219/10.55

[ 1 Feb. 15, 1972 3,478,188 11/1969 White ..219/10.55 2,640,142 5/1953 Kinn ..219/10.61 X 3,551,199 12/1970 Forster ..219/l0.55 X

FOREIGN PATENTS OR APPLICATIONS 1,452,124 8/1966 France ..2l9/l0.6l

Primary Examiner-R. F. Staubly Assistant Examiner-Hugh D. .laeger Attorney-Chasm and Sinnock and Anthony Lagani, Jr.

[ ABSTRACT A microwave heating unit for curing wire coatings comprising a microwave generator, a waveguide coupled to said generator and means for coupling the coated wire to said waveguide whereby said wire becomes the center conductor of a coaxial line.

The outer conductor of the coaxial line is preferably circular in cross section. Propagation of microwave energy between the inner and outer conductors of the coaxial line necessarily passes through the wire coating, thereby curing said coating.

7 Chins, 4 Drawing Figures PATENTEDFEB 15 I972 $543,054

sum 1 BF 2 ,VIS

FIG. JIo

5.0. Fqrs/er INVENTOR (lap/A7143 BY PATENT ATTORNEY PATENTEDFEB 15 I972 643 O54 SHEET 2 [IF 2 FIG. JIb

FIG. 110

5.0. Forster INVENTOR i I,

BY PATEN ATTORNEY MICROWAVE HEATING APPARATUS CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of copending application Ser. No. 684,139, filed Nov. 20, 1964, now U.S. Pat. No. 3,551,199.

BACKGROUND OF THE INVENTION t Wire is coated with electrical insulation by various'means. Typically a thermoplastic such as polyethylene may be extruded onto a wire using a cross head extrusion die. Alternately, a vulcanizable rubber suchas Ethylene 'PropyleneDiene Monomer (EPDM) may be blended with curatives and cross head extruded over the wire to be insulated. The extrusion Process may be Performed ate-temperature insufficient to acto cure vulcanizable polymers. For example, ferromagnetic or electrically conductive particles of less than 100 microns in size are blended into a synthetic rubber and cured by'induction heating at a frequency of about 1 MI-I2. Induction heating, 7

as the name implies, operates by inducing eddy currents, utilizes electromagnetic coupling, in the ferromagnetic or electrically conductive particles thereby heating the particles. The

surrounding rubber matrix is heated and cured by conduction from the particles, e.g., see U.S. PatJNo. 3,249,658.

- Dielectric heating has been used to heat nonconductors having polar molecules. For example, polyvinyl chloridemay bepressed into molding preforms" and preheatedby dielectric heating prior to introduction into a compression mold. This heating technique relies on .the high polarity of the molecule to induce a heating: effect. The material to be'heated is.placed between two plates which form a capacitance inan electronic circuit. Thepol'arity of the, plates israpidlyreversed at a'frequency in the range of about 1 to about 300 MHz. Heat is causedby the rapid vibration of the polar molecules attempting to align themselves with the constantly changing field.

The degree of heating is relatedzto the die'lectric loss of-the material, that is, the energy dissipated :Ilt the dielectric. In general, the higher the frequency at which the dielectric heating is accomplished, the greater the lossiness of the material and consequently, the more efficient the energy conversion to heat. Additionally at the higher frequencies, the requirement for shielding is reduced. 1

These advantages: to very 'high frequency dielectric heating have stimulated much research in the use of a technique termed microwave heating. Microwave heating is based on the principle that electromagnetic waves interact with a dielectric material, some of the energy associated with these waves being stored and some being dissipated. The-heatingeffect is a result of the dissipated energy (dielectric loss). The dielectric loss is caused by the frictional drag associated with permanent or induced dipole orientation in the alternating electric field.

The term microwave heating" as used throughout the specification and claims means. heating with electromagnetic radiation at about'800to 30,000 MI-Iz; preferably'900 .to about 8,600 MHz; most-preferably at about 915 to about 2,450 MHz. At these'frequencies, it is no longer necessary to confine the material to be heated between'plates of a capacitor. The electromagnetic radiation may be conducted, much like any other fluid, by means of waveguides to the heating zone.

Though all polymer' molecules exhibit some polarity, with few exceptions, however, the synthetic elastomers are essentially nonpolar and hence have a low. dielectric loss atthe lower frequencies. In the microwave range it ispossibl'e to accomplish some heating due to increased lossiness at the higher frequency. "Thus, it is possible to cure natural or synthetic rubber as itleaves an extruderhead by passing it through a;

microwave oven. The material is partially cured by being passed through the center of a helical metal waveguide which is connected to a microwave generator running at 300 to 30,000 MHz. Curing is-completed by passing the material through a conventional heater, e.g., see British Pat. No. 1,065,971.

In order to effect a complete cure of such essentially nonpolar synthetic polymers by microwave heating, it is generally necessary to use large amounts of inert tillers such as carbon black, the actual heating being accomplished primarily by thermal conduction from the fillers which are readily heated by electromagnetic radiation. v

' So far as this inventor is aware, it has not been heretofore possible to apply microwave heating to wire coating operations for several reasons. The nonuniform size of the inert fillers results in nonuniform heating which causes hot spots and bum-out of the relatively thin insulation coating. Furthermore, thewire, being a' conductor, acts as an antenna and transmits energy along its length resulting in high energy losses. Thus, the energy available for heating is reduced to insignificant levels.

SUMMARY OF INVENTION It has now been found that synthetic polymers can be successfully cured in wire coating operations by filling the polymer with finely divided metal particles and using coaxial line/waveguide coupling techniques to heat the polymer to cure temperatures with-microwave heating.

The filler material particle size is critical and must be less than 10 microns.

, The wire to'be coated acts as the center conductor of the coaxial line. Toreduce energy losses along the center conductor, the coaxial line, which extends to either side of the waveguide, is equipped withcavities or baffles which attenuate the losses to tolerable limits.

DETAILED DESCRIPTION Any vulcanizable, extrudable elastomer may be used in the practice of this -invention. Typical of such vulcanizable elastomers are natural rubber, butyl rubber, halogenated butyl rubber, Ethylene Propylene Diene Monomer (EPDM) and Neoprene rubbers.

Saturated peroxide crosslinkable polymers such as polypropylene, polyethylene and ethylene propylene rubber may also be cured by the technique of this invention.

In addition to solid elastomers and polymers, plastisols may be used as the coating material, for example, a plastisol of PVC having a curing temperature of about 330 F. and a room temperature viscosity in the uncured state of 160,000 c.p.s.,

having suspended therein the metal fillers of this invention, may be coated on a wire and cured by microwave heating.

The expression butyl rubber as employed in the specification and claims is intended to include copolymers made from a polymerization reactant mixture having therein about IO-99.5 percent by weight of an isoolefin which has about four to sevencarbon atoms and about 30-0.5 percent by weight ofa and a Wijs Iodine'No. of about 0.5t5-aboutl 50; preferably 1 to 15. The preparation of butyl rubber is'descfibed3.in .U.S. Pat. No. 2,356,128, which is incorporated herein by reference.

' Theterm EPDM is used in the sense of its definition as foundin ASTMD-14l8-64 and is intended to mean a terpolymer containing ethylene and propylene in thebackbone and a diene in the side chain. Illustrative methods for'producing these terpolymers are found in US. Pat. No. 3,280,082

and British Pat. No. 1,030,989; which are incorporated herein by reference.

Any EPDM may be used in the practice of this invention. The preferred polymers contain about 50 to about 70 wt.

ethylene and about 2.0 to about 5 wt. of a diene monomer,

the balance of the polymer being propylene. Preferably, the polymer contains about 50 to about 60 wt. ethylene, e.g., 56 wt. and about 2.6 to about 4.0 wt. diene monomer, e.g., 3.3 wt.

The'diene monomer is preferably a nonconjugated diene. Il-

' lustrative of these nonconjugated diene monomers which may be used in the terpolymer (EPDM) are hexadiene, dicyclopentadiene, ethylidene norbornene, methylene norbomene, propylidene norbornene and methyl tetrahydroindene. The

particle diene used does not form a critical part of this in'vention and any EPDM fitting the above description may be used. Atypical EPDM is Vistalon 4,504 (Enjay Chemical Comparty.) a polymer having a Mooney viscosity at 212 F. of about 40, prepared from a monomer blend having an ethylene content of about 56 wt. and a nonconjugated diene content of about 3.3 wt.

Neoprene rubbers are described in a text entitled The Neoprenes by Murray and Thompson; DuPont, March 1963. There are two general types of Neoprenes, G-type and W- type. The G-types differfrom the W-type in that the former are interpolymerized with sulfur and contain thiuram'disulfide, whereas the W-type Neoprenes contain no elemental sulfur, thiuram disulfide or other compound capable of decomposing to yield either free sulfur or a vulcanization accelerator.

The term plastisol as used in this specification means a dispersion of finely divided resin in a plasticizer. When the plastisol is heated, the plasticizer solvates the resin particles, and the mass gels. With continued application of heat, the mass fuses to become a conventional thermoplastic material. The preparation of plastisols is well known to the artand will not be discussed in detail.

Illustrative of a plastisol suitable for use in this invention is 100 parts by weight of a dispersion grade polyvinyl chloride dispersed in 65 parts by weight based on the PVC of a plasticizer such as diethylhexyl phthalate.

Though this invention is directed toward the curing of coatings which have been extruded on a wire, it is not in tended to be limited solely to that method of coating application. a

The term curable as used in the specification and claims is intended to cover the range of materials described above whichmaybe vulcanized, cross-linked by peroxide cures or cured in the sense of converting a plastisol to a thermoplastic.

It will be readily evident to one skilled in the art that ex pandable polymer formulations may be used in the practice of this invention. The preparation of expandable preparations is Decomposition temperature of available blowing agents is known to those skilled in theart; hence, the preparation of these compositions will not be discussed in detail.

In addition to being used to cure polymers coated on electrical conductors, the process disclosed herein is suitable for use in drying paper wrapped conductors. In some electrical products, e.g., transformers, the wire is wrapped with paper or other cellulose insulators from which moisture must be removed. The apparatus and method of this invention may be used to remove that moisture.

Illustrative of the metals suitable for use in the practice of this invention are iron, aluminum, copper, nickel, tin, zinc,

and alloys of nickel w'ith cobalt. In principle, any metal may be used. Such metals asthenoble metals, however, are excluded purely by economic considerations though' they would form operable embodiments of this invention.

Metals such as tin or zinc may be reduced to the proper particle size by mechanical grinding in methanol at 40 C. (Heusler technique). l

The other metals, iron, for example, may be reduced to the proper particle size by preparing alloys with other elements which are removable by water, weak acids or alkali. Examples of such other elements are aluminum, silicon, sodium, calcium or magnesium.

For example, an alloy of nickel and aluminum containing about 50 to about 70 percent aluminum is prepared, annealed at about 800l,000 C. for several hours, and quenched to prevent a phase separation of the two metals. The alloy is then converted to a powder (ca. 200-325 mesh). The aluminum is dissolved in alkali; usually a 20 percent water solution of NaOH, the powder reacting readily with the cold solution with liberation of hydrogen. The heat generated by the reaction brings the liquid to a boil. The water, escaping as'steam, is replaced in order to maintain the volume of solution. Finally, the mass is digested for several hours at l l 8-] 20 C.

The nickel sludge in the tank is washed free of lye with cold water. The finely divided nickel is then dried by adding an organic solvent or oil, heating and agitating until the drying is completejlf desired, the oil may be a process oil which would ordinarily be used in compounding the elastomer to be coated onto the wire.

In the practice of this invention, the metal particle size is of critical importance and must be less than 10 microns, i.e., 0.5 to about 10 microns.

; It is quite surprising that the products of the present invention are nonconductors of electricity. It has been foundthat withirilimits, the dielectric constant of the metal-filled polymer increases with increasing metal filler particle size and yet the prior art products which make use of large particle size metal fillers are excellent electrical conductors; indeed, the prior art'products havebeen suggested for use as the plates of capacitors and not as the dielectric medium between the plates, see Metal-filled Plastics, page 195, by John Delmonte, Reinhold Publishing .Corp. (New York, 196i Furthermore, it would be expected that the electrical conductivity of the metal-filled polymers would rise sharply with in-' creased loadings of metal particles, elg., see U.S. Pat. No. 3,21 1,584, issued Oct. 13, 1965, to J. E. Ehrreich. However, it has been unexpectedly found that the electrical conductivity of the present metal-filled polymers rises imperceptibly in contrast to the almost asymptotic rise in the dielectric constant at high levels of metal loadings.

If the metal particles are substantially greater than microns, there will be metal to metal contact and the elastomer filled'with the metal will be conductive and useless as a wire insulation. Between about 20 to 100 microns the metal particles 'are sufficiently large to act as antennae. Hence, they will reirradiate substantial amounts of the electromagnetic energy absorbed, thereby reducing the heating efficiency to a very low level. Between about 10 to about 20 microns the metal particles do not act as antennae but their area to mass ratio is such that heat transfer to the surrounding elastomer matrix is slow. Hence, overheating of the metal particles results with subsequent hot spots and burn-out of insulation. Below 10 microns, on the other hand, the area to mass ratio is such that heat transfer to the surrounding matrix is rapid enough to result in uniform heating of the elastomer without localized overheating in the vicinity of the metal particles. Preferably, the particles have a particle size range of 0.5 to about 10 microns with an average particle size of about 5 microns.

An additional advantage of the small particle size is that the metal acts as a reinforcing filler in much the same manner as inorganic or carbon black reinforcing fillers.

In its more preferred embodiment the metal fillers of this invention have a particle size rangeof less than 1.5 microns. Preferably, the particles are made up about one-third of particles in the 1-1.5 micron range, one-third in the0.5'l micron range and one-third in the 0.5 micron range.

Metal particles of such small particle size are notoriously pyrophoric and are normally handled under water or solvent. In order to utilize such conventional blending means as rubber mills, banking mixers, etc., to incorporate the metal tillers into the polymers, it is necessary to treat the metal particles to eliminate pyrophoric tendencies.

It has been found that treating the metal particles with organosilanes, in particular vinyl silanes, results in a silane coating on the particle which allows it to be safely handled in air. Metal particles so treated may be readily blended into polymers on conventional equipment without any danger.

The term organosilane compound as employed herein includes silanes, silanols (the corresponding partially or completely hydrolyzed forms of silane), siloxanes (the corresponding condensation product of the silanols) and mixtures thereof. The organosilane compound may be represented by the formula:

wherein R is a C C, radical containing vinyl-type unsaturation selected from the group consisting of alkenyl, styryl, alkenoylalkyl and alkenoyloxyalkyl; X is selected from the group consisting of hydroxyl, alkoxy and acyloxy, R and R are independently selected from the group consisting of hydroxyl, methyl, alkoxy, acryloxy and R Nonlimiting useful compounds which may be employed are the following: vinyl tri(beta-meth-oxyethoxy )-silane, vinyl triethoxy silane, divinyl diethoxy silane, allyl triacetoxy silane; and in place of the vinyl and allyl groups of the above-named compounds, the corresponding styryl, acryloalkyl, methacryloalkyl, acryloxy propyl and methacryloxy propyl compounds. All of the silanes are convertible into the useful corresponding silanols by partial or complete hydrolysis with water. The preferred organosilanes of choice are gamma-methacryloxypropyl trimethoxy silane and vinyl tri-(beta-methoxyethoxy)-silane.

The silanes are applied to the metal particles in solution. After preparation of the metal particles by either Raney or Heusler techniques, the particles are transferred to an organic solvent such as C -C normal alkanes, e.g., hexane heptane, C -C aromatics, e.g., benzene, toluene, xylene, carbon tetrachloride, trichloromethane or mineral spirits. The silane is then added to the solution with constant stirring for about 1 to about 10 minutes. Preferably, about 0.01 to about l wt. silane based on the metal is used; more preferably, about 0.05 to about 0.5 wt. most preferably about 0.1 to about 0.2 wt. e.g., 0.1 wt. After silane treatment, the metal may be removed from the solvent by filtration, centrifugation or other mechanical means and safely handled as a powder. The organosilane attached itself to the metal surface, prevents reaction of the metal powder with air.

It has been found that at least 0.1 part of metal powder must be blended into 100 parts of elastomer in order to effectively cure the elastomer by microwave heating. Preferably 0.2 phr. to about 18 phr., based on the elastomer, of metal powder is blended into the elastomer; more preferably about 0.5 phr. to about 5 phr.; most preferably about 0.5 phr. to about 1.5 phr., e.g., 1 phr.

In addition to the metal powder, various curatives, compounding aids and extender oils may be incorporated into the elastomer. Any curative known to the art may be used for the various wire coating materials suitable for use in the practice of this invention.

For example, elastomers such as butyl rubber or EPDM may be sulfur cured by such curatives as heavy metal dialkyl dithiocarbamates and quinoid compounds. Typically, in the vulcanizing of EPDM suitable sulfur cure may be obtained by the use of certain heavy metal dialkyl dithiocarbamates in conjunction with a thiourea, a metal oxide and mercaptobenzothiazole as cure activators.

Typical of the metal oxide cure activators which may be used are ZnO, Pb0 and MgO. Preferably the metal oxide cure activator is used at about 2.5 to about 10 phr., based on the rubber, more preferably about 4 to about 6 phr., e.g., 5 phr.

The heavy metal thiocarbamates usable in this invention have the general formula:

wherein R is an alkyl group having from one to four carbon atoms and preferably one to two carbon atoms; R is an alkyl, aryl, alkaryl or cycloparaffin group having from one to 10 carbon atoms and is preferably an alkyl group having one to four carbon atoms; x is the valence of the heavy metal and can be an integer of two to four; the heavy metal is selected from those elements in groups I-B, II-B, IV-A, VI-A and VIII of the Periodic Chart of the Elements as published on pgs. 56 and 57 of the Handbook of Chemistry by Lange, eighth edition, 1952.

The dithiocarbamate salt may be a single salt or a mixture of salts, e.g., zinc dimethyl dithiocarbamate (methyl zimate) may be combined with tellurium diethyl dithiocarbamate (Tellurac). Other dithiocarbamates that are suitable for the purposes of this invention include selenium diethyl thiocarbamate, lead dimethyl dithiocarbamate, tellurium benzyl dithiocarbamate, zinc butyl dithiocarbamate, etc. For best results, the thiocarbamate portion of the blend should comprise either the zinc or tellurium salt alone or a combination of these salts.

The thiocarbamates are used in a range of about I to about 5 phr. based on the EPDM, preferablyabout 2 to about 5 phr. and most preferably about 3 to about 4 phr. Typically, mixtures of dithiocarbamates comprising about 0.5 to about 1.5 phr. of tellurium diethyl dithiocarbamate, e.g., 0.8 phr., may be used in conjunction with about 2 to about 4 phr., based on the EPDM, e.g., 3 phr., of zinc dimethyl dithiocarbamate.

Illustrative of the thioureas which may be used in the practice of this invention are thiocarbanilide (A-l), 1,3-diethyl thiourea (Pennzone E), 1,3-dibutyl thiourea (Pennzone B). Preferably, the thioureas are used in the range of about 1 to about 5 phr. based on the EPDM, more preferably about 2 to about 5 phr., e.g., 3 phr.

Mercaptobenzothiazole is also used as a cure activator. Typically, the mercaptobenzothiazole is used at about 0.5 to about 3 phr. based on the EPDM; preferably about 1 to about 2 phr., e.g., 1.5 phr.

Other additives which may advantageously be used in the practice of this invention are various conventional rubber processing aids and plasticizers such as paraffinic or naphthenic process oils, microcrystalline waxes, tributyl ethyl phosphate (KP methyl hydroxy stearate (Paricin-l and vulcanized vegetable oil such as that produced by the reaction of soya oil with sulfur monochloride (Factice 57-5). The term microcrystalline wax" as used in this specification means petroleum derived waxes characterized by the fineness of their crystals in distinction to the larger crystals of paraffin wax.

The term quinoid compound" means any dinitroso compounds, dioximes and similarly related compounds having an ortho or para quinoid aromatic nucleus or compounds which can be converted into such structure. Illustrative of these quinoid compounds are poly-p-dinitrosobenzene (Polyac), N- methyl-N,4-dinitrosoaniline (Esastopar), N-(2-methyl-2- nitrosopropyl) 4-nitrosoaniline (Nitrol) and p-quinone dioxime (GMF).

The saturated synthetic polymers such as ethylene propylene rubber, polypropylene and polyethylene are peroxide curable. Suitable peroxides are those well known in the art for cross-linking these materials such as di-tert.-butyl peroxide, 2,5-dimethyl-2,5-bis-(tert.butyl peroxy) hexane, 2,5- dimethyl-2,5-bis-(tert.-butyl peroxy) hexyne-3, benzoyl peroxide, di-tert.-butyl-diperphthalate, tert.-butyl per-,

benzoate, dicumyl peroxide, cumene hydroperoxide, 2,4-di- (tert.-butyl peroxyisopropyl) benzene, tert.-butyl cumyl peroxide, etc. and mixtures thereof.

It is common practice to propagate electromagnetic radiation in a waveguide by means of coaxial line/waveguide coupling. The outer conductor of the coaxial line is connected to an outer wall of the waveguide while the center conductor protrudes into and terminates in the free space within the waveguide. Similarly, the coaxial line can act as a pickup for microwaves flowing through the waveguide. The waveguide is closed at the coupling end and the center conductor of the coaxial line is located about one-fourth wavelength of the microwaves being transmitted, from the closed end of the waveguide.

Within the coaxial line radiation is between the center conductor and the outer conductor of the coaxial line in a radial direction. By this manner, microwaves may be transmitted in muchthe same way a current is transmitted along an electrical conductor.

In the practice of this invention, a modified coaxial line/waveguide coupling unit is utilized to heat the aforementioned vulcanizable polymer. For example, the coaxial line may be extended to either side of the waveguide, the wire to be coated acting as the center conductor of the coaxial line. To avoid large power losses, it is necessary to attenuate or suppress the radiation traveling along the coaxial line.

Referring now to the drawing, in particular FIG. I, numeral 11 designates a spool of electrical conducting wire, l2. The wire, 12, is passed through a cross head extruder die, 13, wherein the wire, 12, is coated with the curable composition of this invention,'said coated wire, 14, being passed through a rectangular waveguide, 15, having an enclosed end, 16. Extending to either side of the waveguide, 15, and concentric with the coated wire, 14, are outer conductors, l7 and 18, of a coaxial line. The coated wire acts as the center conductor of the coaxial line. A microwave generator, 19, is coupled with the waveguide, 15, through an isolator, 20, and variable attenuator, 21, and propagates a wave of electromagnetic radiation through the waveguide, 15, toward the coated wire, 14.

The portion of the coated wire within the waveguide is heated directly by the electromagnetic radiation which is also picked up by the center conductor wire, 14, of the coaxial line. Radiation between the center conductor, 14, and the outer conductors, l7 and 18, of the coaxial line continues to heat and cure the coating throughout the external (to the waveguide) heating zones, 22 and 22, until said radiation is suppressed or attenuated by the baffles, 23, or resonating cavi-' ties, 24.

The baffles attenuate the electromagnetic radiation by creating'a mismatch between the center and outer conductors of the coaxial line. The resonating cavities impede or suppress the propagation radiation by absorption of energy. In practice, it is preferred to combine both suppression means by utilizing alternating cavities and baffles.

Referring to the drawing, in particular FIG. Ila, numeral 17 designates the outer conductor of the coaxial line. Connected to the conductor, 17, is a series of resonating cavities, 24. The width of the cavity, 25, is M4, i.e., one-quarter of the wavelength of the electromagnetic radiation. The inner diameter of the cavity can be equal to or greater than that of ing. The diameter of the cavity, 24, is 2r where r=2C. The

distance between cavities, 28, is not critical.

Baffles are somewhat similar to the resonating cavities discussed above. Referring to the drawing, in particular FIG. Ilb, numeral 18 designates the outer conductor of a coaxial line. Connected to the conductor, 18, is a series of baffles, 23. The width of the baflle, 29, is M4, i.e., one-quarter of the wavelength of the electromagnetic radiation. Where the resonating cavities, 24, provide a space within the system, the baifles, 23, present a restriction and impede radiation in a manner similar to throttling the flow of fluid in a line. The baffles are connected by sections of insulating material, 30, e.g., Teflon, the diameter of which, C, is the diameter of the finished wire. The baffle diameter is 2r, where r=2C.

In its preferred embodiment the radiation suppression means constitutes a series of alternating bafi'les, 23, and cavities, 24, as shown in FIG. llc. Transition sections, 31, the dimensions of which are not critical, are required between the resonating cavities and baffles.

It will be obvious to those skilled in the art that a single microwave generating waveguide may be adapted to operate more than one curing unit. In such a case, the waveguide is modified to a T' transition section, the original waveguide forming the root of the T. The outwardly extending portions of the T are waveguides each of which is coupled to a coaxial line. The individual coaxial lines should be isolated from one another by conventional isolation means in order to avoid feedback from one unit to the other. The waveguide may be divided in a similar manner to operate a multiplicity of coaxial lines.

The following examples serve to illustrate the manner in which the process of this invention may be carried out and the benefits derived therefrom.

EXAMPLE 1 A butyl rubber composition having the formulation shown below was prepared using conventional blending techniques.

Component Parts by Weight Enjay Butyl O35 I00 Calcined clay l 10 Iron particles (av. 5p.) 4

w o, a I 5 Paraffin wax 5 LM polyethylene 5 Quinone dioxime 1.5 Mercaptobenzothiazole 4 (I) Butyl rubber having 0.8 mole unsaturation and a Mooney viscosity at 2 l 2 F. of about 4 1-49.

(2) A petroleum derived paraffin wax having a melting point of F.

(3) A low molecular weight polyethylene having a molecular weight of about 20,000 to 50,000. (Weight average).

This composition is extruded at about 200 F. through a cross head extruder die having a %-inch diameter bore onto a /;-inch diameter wire. The coated wire is passed through theunit described above wherein 2L (FIG. I) is 4 feet and C (FIG. II) is 0.275 inches.

The microwave generator operates at 2,450 MHZ. and 5 kw. microwave power. The total curing time in the heating zone is about 20 seconds, i.e., line speed ca. 12 ft./min.

EXAMPLE 2 A l/l6-inch diameter wire is coated with the ethylene propylene rubber composition shown below by passing the wire through a cross head extruding die having a Va-inch diameter bore, the EPR being extruded at about 200 F.

Component Parts by Weigh! Enjay Vistalon 404") I00 Calcined clay 1 l0 Aluminum particles (av. Sn) 5 Dicumyl peroxide 2.8

Triallyl cyanurate 1.5

(l) Ethylene propylene rubber having an ethylene content of 40-46 wt. and a Mooney viscosity at 2! 2 F. of about 35-45.

The microwave unit of Example. 1 is used except that 2L equals 2 ft., C is 0.135 inches and the line speed is about 6 ft./sec., i.e., curing time seconds.

The coated wires produced by the method of Examples 1 V and 2 are equivalent in physical and electrical characteristics to conventionally coated wires.

Since it is readily evident that many different embodiments may be made without departing from the spirit of this invention, it is not intended to limit the scope thereof to the particular embodiments disclosed herein.

What is claimed is:

l. A microwave heating unit suitable for curing the insulating covering of an insulated electrical conductor which comprises:

a. a microwave generator operating at a frequency of about 800 to about 30,000 Ml-lz.;

b. at least one waveguide, said waveguide having at least one'end enclosed and having two aligned diametrically opposed openings in said waveguide, said openings being sufficient to pass said insulated conductor therethrough and being located a distance equal to one-quarter wavelength of the operating frequency of the microwave generator from theclosed end of said waveguide, said waveguide being coupled to said microwave generator;

c. an outer conductor of a coaxial line concentric with said openings, communicating with and outwardly extending from the waveguide, said outer conductor being spacially oriented in such a manner that its central axis is coincid'ent with the center of both of said opening;

d. a center conductor of said coaxial line concentric with said outer conductor, said center conductor comprising an electrical conductor having a curable coating; and

e; means for suppressing microwave radiation transmitted,

via the waveguide, to a center conductor of the coaxial,

comprises at least three resonating cavities in series.

- 7. The apparatus of claim 1 wherein the suppressing means Q is an alternating combination of baffles and resonating cavities comprising at least one bafi'le and at least two resonating cavities.

Claims

1. A microwave heating unit suitable for curing the insulating covering of an insulated electrical conductor which comprises: a. a microwave generator operating at a frequency of about 800 to about 30,000 MHz.; b. at least one waveguide, said waveguide having at least one end enclosed and having two aligned diametrically opposed openings in said waveguide, said openings being sufficient to pass said insulated conductor therethrough and being located a distance equal to one-quarter wavelength of the operating frequency of the microwave generator from the closed end of said waveguide, said waveguide being coupled to said microwave generator; c. an outer conductor of a coaxial line concentric with said openings, communicating with and outwardly extending from the waveguide, said outer conductor being spacially oriented in such a manner that its central axis is coincident with the center of both of said opening; d. a center conductor of said coaxial line concentric with said outer conductor, said center conductor comprising an electrical conductor having a curable coating; and e. means for suppressing microwave radiation transmitted, via the waveguide, to a center conductor of the coaxial line wherein said center conductor is said electrical conductor whose covering is to be cured.

2. The apparatus of claim 1 wherein the waveguide is a rectangular waveguide.

3. The apparatus of claim 1 wherein the waveguide is a circular waveguide.

4. The apparatus of claim 1 wherein the microwave generator operates at a frequency of 2,450 MHz.

5. The apparatus of claim 1 wherein the suppressing means comprises at least three baffles in series.

6. The apparatus of claim 1 wherein the suppressing means comprises at least three resonating cavities in series.

7. The apparatus of claim 1 wherein the suppressing means is an alternating combination of baffles and resonating cavities comprising at least one baffle and at least two resonating cavities.