US3585278A

US3585278A - Electrical bushings having insulating means including a cured elastomer containing mineral oil

Info

Publication number: US3585278A
Application number: US863827A
Authority: US
Inventors: James F Quirk
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1969-10-06
Filing date: 1969-10-06
Publication date: 1971-06-15
Anticipated expiration: 1988-06-15
Also published as: DE2050232B2; DE2050232A1

Abstract

An electrical bushing having insulating means surrounding an electrical conductor. At least a portion of the insulating means is formed of gelled transformer or mineral oil, held in solution by a thermoset elastomeric binder, which adheres tenaciously to the structural members of the bushing, and has excellent electrical insulating qualities. A method of constructing a bushing is also disclosed in which the thermosettable elastomeric material containing the mineral oil is disposed between a solid insulating body portion of a bushing formed of a synthetic resin, and a weather shed formed of porcelain, with the elastomeric material being cured to form a bond between the elastomeric material and the porcelain, and between the elastomeric material and the solid insulating body portion of the bushing.

Description

United States Patent 72) Inventor James F. Quirk Monroeville, Pa.

[21] Appl. No. 863,827

[22] Filed Oct. 6, 1969 [45] Patented June 15, 1971 [73] Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

[54] ELECTRICAL BUSHINGS HAVING INSULATING MEANS INCLUDING A CURED ELASTOMER CONTAINING MINERAL 01L 7 Claims, 4 Drawing Figs. 7

[51] Int. Cl 1101b 17/26 [50] Field olSearch 174/18, 31,

[56] References Cited UNITED STATES PATENTS 2,089,284 8/1937 Milliken 174/76 X 2,906,810 9/1959 DAscoli 174/76 X 3,001,005 9/1961 Sonnenberg l 74/ l 42 3,242,446 3/1966 Leute 174/152 X FOREIGN PATENTS 1,559,676 2/1969 France 174/76 948,159 I 1964 Great Britain 174/31 Primary Examiner- Laramie E. Askin Attorneys-A. T. Stratton, F. E. Browder and D. R. Lackey formed of a synthetic resin, and a weather shed formed of p01'-' celain, with the elastomeric material being cured to form a bond between the elastomeric material and the porcelain, and between the elastomeric material and the solid insulating body portion of the bushing.

, 40CURED ELASTOMER INCLUDING MINERAL on.

PATENTED JUN] 51% SHEET 1 [IF 2 INVENTOR Jo mes F Qu! rk BY ,2 Jana/a6 ATTORNEY ELECTRICAL BUSHINGS HAVING INSULATING MEANS INCLUDING A CURED ELASTOMER CONTAINING MINERAL OIL BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to electrical bushings, and methods of constructing same, for electrical apparatus such as transformers and reactors.

2. Description of the Prior Art Condenser bushings of the prior art are constructed with a capacitor section surrounding the conductor stud, and a porcelain or ceramic weather housing surrounding the capacitor section. The capacitor or condenser section for many years was constructed of layers of oil impregnated paper, interleaved with layers of metallic foil, with the space between the condenser section and porcelain housing being filled with oil. The oil, however, presents certain disadvantages, such as being a fire hazard, and it also limits the mounting angle of the bushing from the vertical, due to the possibility of oil leakage.

A condenser section construction which eliminates the oil has been used, in which a thermosettable synthetic resin, such as an epoxide, is cast about the conductor stud, with the condenser plates being placed in the desired position within the casting mold prior to the introduction of the pourable resin system therein; or, layers of paper impregnated with a synthetic resin are wound about a conductor stud, interleaved with layers of metallic foil, with the resin being cured to form a laminated solid insulating system. While weather sheds may be integrally cast with the condenser section in the former construction, in certain instances it is desirable to enclose at least the weather end of the cast condenser section in a porcelain weather housing. In the latter construction, a weather enclosure, such as a porcelain housing, is required. However, when a porcelain weather housing is used, means must be included to seal the interface between the condenser portion and the porcelain housing and supplant any air therein, and to allow thermal expansion of the bushing components without danger of cracking the porcelain housing. Discrete resilient members and adhesives have been proposed for this purpose, but all have had certain disadvantages. For example, they may be relatively costly, they may not have the requisite electrical strength, they may not completely supplant the air between the condenser section and porcelain housing, or they may not be resilient enough to accommodate differential movement of the condenser section andporcelain housing during thermal changes, introducing excessive stresses into the porcelain housing.

SUMMARY OF THE INVENTION porcelain housing, and it allows differential thermal expansion of the bushing components without cracking the porcelain. The elastomeric material obtains its low cost and good electri- .cal insulating qualities by gelling transformer or mineral (petroleum) oil. The mineral oil is held in solution by an elastomeric binder which is cross linked with suitable crosslinking means. i

A new and improved method of constructing an electrical insulating bushing is also disclosed, wherein a chemical bond is obtained between the elastomeric material and the cast solid insulation system. The cast solid insulation system and the cross-linking means for the elastomeric binder are both epoxides in this embodiment, with the curing of the elastomeric material and the cast solid insulation system being accomplished in the same step of the new and improved method.

, BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is an elevational view, in section, of a cast type capacitor or condenser section of a high voltage bushing, which illustrates a step in the method of constructing an electrical bushing according to the teachings of the invention;

FIG. 2 is an elevational view, partially in section, which illustrates the steps of disposing a porcelain weather housing over the weather end of the capacitor section shown in FIG. I, and introducing an elastomeric material between the capacitor section and weather housing;

FIG. 3 is an elevational view, partially in section, which illustrates a completed electrical bushing constructed according to.the teachings of the invention, mounted through an opening in electrical apparatus; and

FIG. 4 is an elevational view, in section, of an electrical bushing constructed according to another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, FIGS. 1 and 2 illustrate steps in a new and improved method of constructing a new electrical bushing 10 shown in FIG. 3. Bushing 10, shown in FIG. 3, is of the capacitive type, having a capacitor section 12 shown in section in FIG. I, and a porcelain housing or weather shed 14, shown in section in FIGS. 2 and 3.

More specifically, FIG. 1 is an elevational view, in section, of a capacitor section 12 of the cast type, including a conductor stud 16 having first and second ends Hand 19, respectively, which may be threaded as shown, a cast solid resinous insulation system disposed about the conductor stud 16, and a plurality of capacitor plates 20 which are embedded in the solid insulation 18 and which encircle the conductor stud 16 in predetermined spaced relation, calculated to enforce a more uniform distribution of electrical stresses across the solid insulation when the conductor stud 16 is connected to an electrical potential.

The capacitor section 12 may be constructed by any suitable method. For example, as disclosed in U.S. Pat. No. 3,394,455, which is assigned to the same assignee as the present application, it may be formed in a two step casting procedure wherein a first body portion is cast about the conductor stud, with the outer diameter being stepped" to receive tubular capacitor plates, such as electrically conductive foils. The next casting step then. forms the configuration shown in FIG. 1, embedding the plates 20 within the completed cast solid insulation system, after the cast resin system applied in the second casting step is gelled and cured to a solid. The solid insulation 18 may have an integral flanged portion 22 disposed at a predetermined location between the

ends

17 and 19 of the conductor 16, which divides the weather and encased

ends

21 and 23, respectively, of the capacitor section. The flange portion 22 may have a plurality of openings therein, such as opening 24, for receiving mounting bolts, and a circumferential groove 26 which is cast or machined into the underside of the flanged portion 22, for receiving a resilient gasket member for sealing an opening in the casing of its associated electrical apparatus when it is mounted therein.

The solid insulation system 18 may be formed of any suitable type of resin, preferably thermosetting, but thermoplastic resin'systems may be used if their softening temperatures are above the maximum operating temperature of the bushing. As will be hereinafter disclosed, a specific desirable embodiment of the invention uses resin systems of the epoxy type.

The resin system selected may be filled with finely divided inorganic fillers to reduce shrinkage upon curing of the resin,

to closely match the coefficient of thermal expansion of the resin system with that of the metallic inserts, and/or to provide nontracking characteristics. Examples of suitable fillers which may be used for one or more of these functions are quartz, silica, and alumina trihydrate (AI O BH O). US. Pat. No. 3,433,893, assigned to the same assignee as the present application, discloses a resin system of the epoxy type which has been found to be excellent for electrical bushings.

The next steps of the method are illustrated in FIG. 2, with the porcelain housing 14, which has a plurality of corrugations or weather sheds 28 on its external surface, being disposed over the weather end 21 of the capacitor section 12. Housing 14 has an opening 30 therein, which extends between the first and second ends 31 and 33, respectively, of the housing, with the opening being sized to closely conform to the outer surface of the cast capacitor section 12, but spaced therefrom by at least 0.030 inch, to provide a tubular shaped space 32 between the outer surface of the weather end 21 of the capacitor section 12 and the internal surface of the weather housing. End 33 of the housing is butted against the upper surface of the flange portion 22, and the interface is sealed with a gasket member 34 disposed in a complementary groove formed in either the end 33 of the porcelain as shown, or in the upper surface of the flanged portion 22.

The next step of the method, also indicated in FIG. 2, is to apply a downward force against the first end 33 of the housing 14, with the force being indicated generally by arrow 36, to insure that the gasket member 34 is compressed and the associated joint sealed, Then, a liquid, pourable elastomeric material 40,'shown in FIG. 3, is introduced into the space 32 between the capacitor section 12 and the housing 14, with the method of introducing the elastomeric material being indicated by the spout 38 shown in FIG. 2.

The elastomeric material 40 should have an excellent electrical strength, it must be fluid enough to insure that it will completely fill. the space 32 without leaving voids, 18 must firmly bond to both the capacitor section 12 and to the housing 14 when cured,.and it must be relatively inexpensive. It has been found that transformer or mineral oil, gelled or thickened by an elastomeric binder, and cross-linking means for the binder, provides superior results in this application, combining an excellent electrical strength with a low cost, and providing a superior bond to both the inside of the porcelain housing and to the weather end of the capacitor section 12.

The binding means must be an elastomer which is a viscous liquid at room temperature. The binding means, mineral oil and cross linking means are combined in random order and thoroughly mixed to provide a liquid, pourable solution of the mixed materials. The mineral oil is added in sufficient concentration to make the mixture very fluid, to insure that it will completely fill the space 32, with the assembly being placed in a vacuum chamber which is evacuated to and millimeters of mercury for 5 minutes, to remove trapped air and insure that the fluid mixture will completely penetrate and fill the space 32, without leaving voids therein. The assembly may then be heated in an oven to a predeterminedtemperature for a predetermined time, with the temperature and heating cycle depending upon the specific components used.

A composition which has been found to provide excellent results in this application was prepared according to the following formulation:

Binder, Carboxyl terminated polybntndiene (B. F.

Goodrich llycar 2.000 x 162) Cross-linking means. epoxy resin, E.E.W. 180-200 Jones Dabney Enl-Rez 510) 4 Mineral oil. transformer oil (Esso-Unlvolt 33) Accelerator 4 O .2

Curing the elastomeric material having the above specified composition, after it is disposed in the space between the cast solid insulation and the porcelain housing, provides a voidless thermoset material 40 in the space 32 which has a resilient or rubbery composition, and which tenaciously'adheres to both the porcelain l4 and capacitor section 12. The composition will cure when heated to about 150 C. for l to 2 hours. The large percentage of transformer oil in the composition substantially reduces the cost of the system, and surprisingly adds greatly to the electrical strength of the elastomer, with the electrical strength being in the order of 850 volts per mil. The excellent electrical strength may be due to the fact that the transformer oil is dispersed homogeneously, without chemical change, throughout the elastomer, obtaining the maximum electrical strength of the transformer oil. Transformer oil has an excellent electrical strength when finely divided, but drops in electrical strength as the dimension of the oil space is increased. The disclosed composition of gelled transformer oil results in the oil being homogeneously dispersed throughout the elastomer binder, which has the effect of breaking the oil into a large plurality of very small oil gaps, providing excellent electrical strength without the disadvantage of obtaining different dielectric constants across the material, such as when transformer oil is physically separated by discrete insulating barriers. Thus, there is no transferal of electrical stress from areas of high dielectric constant to areas of lower dielectric constant, as the disclosed formulation of elastomeric material has a uniform dielectric constant of about 3.8. It is desirable to utilize as much transformer oil as possible in the formulation without deleteriously affecting the adhesive characteristics of the cured system. The concentration of the transformer oil listed in the composition is the preferred percentage, but it will be understood that it may be varied considerably, especially in the direction of less transformer oil, and still obtain excellent results. For example, the transformer oil may be in a range of about 10 to percent by weight of the overall elastomeric system.

The cross-linking means in the disclosed composition of elastomeric material is an 'epoxide, and the preferred method of constructing the bushing 10 shown in FlG. 3 is to also construct the capacitor section 12 of an epoxy resin. In this embodiment, instead of curing the epoxy resin system 18, it is merely gelled, such as by heating the mold and its contents for 2 to 4 hours at C. If the capacitor section is formed by a two part casting method, the resin added in the second casting step would be merely gelled, while the resin applied in the first casting step may be gelled and cured. The mold is then stripped from the gelled system, the porcelain housing placed over weather end 21 of the capacitor section, and the elastomeric material 40 poured into the space 32. After evacuation, the whole assembly is heated for 16 hours at C. to cure the epoxy resin system of the capacitor section, which also cures the elastomeric material 40. The step of simultaneously curing the solid insulation 18 and the elastomeric material 40 provides a chemical bond at the interface between the capacitor section 12 and the elastomeric material 40, which is superior to a physical bond obtainable when the solid insulation of the capacitor section 12 is cured prior to the curing of the elastomeric material 40. The interface between the epoxy solid insulation 18 and the elastomer material 40 is virtually eliminated, providing an integral outer resilient coating on the solid insulation system 18 which is physically bonded to the inner surface of the porcelain housmg.

The composition of the elastomeric material 40 hereinbefore specified, has a 60 Hz. power factor which increases with temperature. However, when the elastomeric material is used in thin sections, as illustrated in the embodiment shown in F lGS. l, 2 and 3, the overall power factor of the bushing is not adversely affected by this characteristic of the elastomer. If the elastomer is to be used in thicker sections where the increasing power factor of the material with temperature may be a disadvantage, elastomeric compositions utilizing transformer oil may be formulated which have a 60 Hz. power factor which is virtuallystable between 25 C. and l C. An example of such a composition is listed in the following formulation:

system Molecular sieve This composition provides a resilient rubbery consistency when cured or cross linked, which has an electrical strength about the same as that of an epoxy resin system, i.e., 400-450 volts per mil, a dielectric constant of 2.8, and a stable 60 Hz. power factor of about 0.3 across the temperature range of 25 C.- 100 C. The lower electrical strength of this composition compared with the first specified system, is probably due to the fact that less transformer oil may be introduced into this system. While the listed percentage by weight of the transformer oil is the preferred percentage, it may be in the range of about to 60 percent by weight of the composition, and still provide gelled transformer oil having excellent electrical strength and adhesive characteristics.

Any typical rubber vulcanizing system may be used to cross link the binder, such as 5 parts by weight of zinc oxide (ZnO), 1 part Stearic acid, 3.5 parts GMF (Quinone Dioxime), 8 parts lead oxide (Pb O and 5 parts Altax (bensothiazyl disulfide).

After the elastomeric material 40 is cured, the porcelain housing 14 is secured in assembled relation with the capacitor section 12 by disposing a metallic cap 42 over end 17 of conductor 16, with the cap being sealed to the of the housing 14 by a suitable gasket member 44, which may be disposed in a groove in the end 31 of housing 14, and the cap 42 is secured firmly to the housing 14 by a nut 46 which is threaded over end 17 ofconductor l6.

Bushing 10 may then be disposed through an opening 50 in the casing 52 of itsassociated electrical apparatus such as a transformer, and secured in sealed relation therewith by a gasket disposed in groove 26 and by the mounting nut and bolt assemblies 54. l

While the capacitor section 12 shown in H0. 1 has been described as being formed of a cast solid resinous insulation system, with the capacitor plates being embedded therein, it will be understood that it maybe constructed by any other suitable method. For example, it may be constructed by tightly wrapping resin impregnated paper about the conductor stud, with the capacitor plates being disposed at the desired locations during the winding of the capacitor section. Curing the resin then provides alaminated solid insulation system. In this embodiment, it may be desirable to provide a porcelain housing on the encased end of the capacitor section, as well as the weather end. Further, it will be understood that the body of solid insulation about the conductor stud need not have capacitor plates therein, in those applications which do not require them.

Since the elastomeric material 40 is rubbery" in character after curing, solid but possessing little structural strength, its application in bushings is limited to space filling, as it would have little value as a structural member. Thus, the elastomeric material containing the transformer oil may be used in any application of space filling in electrical bushings where it is desirable to provide a high strength, low-cost solid material with excellent adhesive properties.

HO. 4 is an elevational view, in section, of an electrical bushing 60 which functionally illustrates that the solid elastomeric material containing the transformer oil may extend from the conductor to the outer housing, where the structural characteristics of the bushing are not dependent upon the strength of this space filling material. More specifically,

bushing 60 includes a tubular housing 62, which may be metallic, or of any other suitable structural material, having a mounting flange 64 suitably attached thereto. Insulating end caps 66 and 68 are disposed to close the ends of the tubular housing, and a conductor 70 extends through the tubular housing and through openings in the end plate or cap members. The space between the conductor 70, the tubular housing 62 and the

end plates

66 and 68 is filled with gelled transformer oil 72, as hereinbefore described relative'to the other embodiments of the invention. The gelled transformer oil or elastomeric insulating material 72," when cured to a solid, bonds tightly to the conductor 70, to the

end plates

66 and 68, and to the inside surface of housing 62, to provide an electrical bushing assembly having excellent electrical strength, for relatively low cost. The elastomeric material holds the transformer oil in solution, to obtain the maximum insulating qualities of the transformer oil along with its advantage of low cost.

Electrical bushings were constructed similar to the embodiment of the invention shown in FIGS. 1, 2 and 3, wherein a cured solid epoxy resin system is disposed about an electrical conductor, with this assembly then being disposed within a porcelain housing. The space between the outer surface of the cast solid insulation and the inner surface of the porcelain housing was filled and sealed by a transformer oil filled elastomer, as taught by the invention. Prior to adding the elastomeric material, a first pair of electrically conductive tape electrode members were secured in predetermined spaced relation to the outer surface of the solid epoxy portion, and a second pair of electrically conductive tape electrodes were secured in spaced relation to the inner surface of the porcelain housing. The liquid elastomeric system was then added, a vacuum drawn to remove the air therefrom, and the elastomeric material was then oven heated to cure the system to a rubbery solid. The first and second pairs of electrodes were disposed 180 apart in the assembly, and were used to determine the degree of adhesion between the elastomer and the solid epoxy system, and between the elastomer and the porcelain housing. Measurements of the degree of interfacial adhesion were made immediately after the elastomeric material was cured, and during thermal cycling between C. and -40 C. In every instance, the AC surge flashover occurred between the electrodes of a pair exterior to the bushing, and not along the interface between the epoxy body and elastomeric material, or along the interface between the porcelain housing and the elastomeric material, indicating the initial adhesion or bond was excellent, and that it was not adversely affected by the thermal cycling.

ln summary, there has been disclosed a new and improved electrical bushing, and methods of constructing same, using gelled transformer or mineral oil to provide a low cost elastomeric bond between structural members of an electrical bushing. The gelled transformer oil has the electrical strength of finely divided oil gaps, providing a low-cost adhesive which bonds tenaciously to metal, porcelain, and cast solid resinous insulation systems.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative, and not in a limiting sense.

1 claim as my invention:

1. An electrical bushing comprising:

an enclosure having first and second ends, and an opening which extends between its ends, said enclosure defining at least a portion of the outer surface of the electrical bushing,

a conductor disposed through the opening in said enclosure,

and insulating meansdisposed to surround a predetermined longitudinal dimension of said conductor, extending from said conductor to said enclosure,

2. The electrical bushing of claim 1 wherein the mineral oil is in an amount ranging from to 75 percent by weight of the elastomeric material.

3. An electrical bushing comprising: 10

a conductor disposed through the opening in said enclosure,

and insulating means disposed to surround'a predetermined longitudinal dimension of said conductor, extending from said conductor to said enclosure,

said insulating means including at least one circumferential layer of a cured elastomeric material which comprises a mixture of mineral oil, a binder, and cross-linking means for said binder,

said binder being an elastomer prepared from isobutylene and isoprene.

4. The electrical bushing of claim 3 wherein the mineral oil is in an amount ranging from l0 to 60 percent by weight of the elastomeric material.

5. An electrical bushing comprising:

a conductor disposed through the opening in said enclosure,

and insulating means disposed to surround a predetermined longitudinal dimension of said conductor, extending from said conductor to said enclosure,

said insulating means including a solid body member formed of an epoxy resin and at least one circumferential layer of a cured elastomeric material which comprises a mixture of mineral oil, a binder, and cross-linking means for said binder,

said elastomeric material being disposed between and bonded to the inner wall of the enclosure and the outer surface of said solid body member,

said binder being a carboxylic elastomer, and the cross-linking means being an epoxide.

6. The electrical bushing of claim 5 wherein the enclosure is formed of porcelain.

7. An electrical bushing comprising:

an enclosure having first and second ends, and an opening which extends between its ends, said enclosure defining at least a portion of the outer surface of the electrical bushmg,

a conductor disposed through the opening in said enclosure,

said insulating means including a solid body member and at least one circumferential layer of ,a cured elastomeric material which comprises a mixture of mineral oil, a binder, and cross-linking means for said binder,

said elastomeric material the enclosure disposed between and bonded to the inner wall of the enclosure and the outer surface of said solid body member,

said binder being an elastomer prepared from isobutylene and isoprene.

Claims

1. An electrical bushing comprising: an enclosure having first and second ends, and an opening which extends between its ends, said enclosure defining at least a portion of the outer surface of the electrical bushing, a conductor disposed through the opening in said enclosure, and insulating means disposed to surround a predetermined longitudinal dimension of said conductor, extending from said conductor to said enclosure, said insulating means including at least one circumferential layer of a cured elastomeric material which comprises a mixture of mineral oil, a binder, and cross-linking means for said binder, said binder being a carboxylic elastomer and said cross-linking means being an epoxide.

2. The electrical bushing of claim 1 wherein the mineral oil is in an amount ranging from 10 to 75 percent by weight of the elastomeric material.

3. An electrical bushing comprising: an enclosure having first and second ends, and an opening which extends between its ends, said enclosure defining at least a portion of the outer surface of the electrical bushing, a conductor disposed through the opening in said enclosure, and insulating means disposed to surround a predetermined longitudinal dimension of said conductor, extending from said conductor to said enclosure, said insulating means including at least one circumferential layer of a cured elastomeric material which comprises a mixture of mineral oil, a binder, and cross-linking means for said binder, said binder being an elastomer prepared from isobutylene and isoprene.

4. The electrical bushing of claim 3 wherein the mineral oil is in an amount ranging from 10 to 60 percent by weight of the elastomeric material.

5. An electrical bushing comprising: an enclosure having first and second ends, and an opening which extends between its ends, said enclosure defining at least a portion of the outer surface of the electrical bushing, a conductor disposed through the opening in said enclosure, and insulating means disposed to surround a predetermined longitudinal dimension of said conductor, extending from said conductor to said enclosure, said insulating means including a solid body member formed of an epoxy resin and at least one circumferential layer of a cured elastomeric material which comprises a mixture of mineral oil, a binder, and cross-linking means for said binder, said elastomeric material being disposed between and bonded to the inner wall of the enclosure and the outer surface of said solid body member, said binder being a carboxylic elastomer, and the cross-linking means being an epoxide.

7. An electrical bushing comprising: an enclosure having first and second ends, and an opening which extends between its ends, said enclosure defining at least a portion of the outer surface of the electrical bushing, a conductor disposed through the opening in said enclosure, and insulating means disposed to suRround a predetermined longitudinal dimension of said conductor, extending from said conductor to said enclosure, said insulating means including a solid body member and at least one circumferential layer of a cured elastomeric material which comprises a mixture of mineral oil, a binder, and cross-linking means for said binder, said elastomeric material the enclosure disposed between and bonded to the inner wall of the enclosure and the outer surface of said solid body member, said binder being an elastomer prepared from isobutylene and isoprene.