GB1601379A - High voltage electric insulators made of resin-bonded glass-fibre and organic material and process for manufacturing same - Google Patents

Abstract

The electrical insulator for high tension has a central support of vitreous resin (1) and finning of EPR elastomer (5, 7), in which the fins are applied to the support: globally, in a single body, or individually or in groups and directly, or with interposition of a tubular sleeve (3) of said EPR elastomer. The parts are assembled into a single whole by means of an appropriate olefin polymer-based self-vulcanising adhesive mixture. The pertinent production process provides for the formation and vulcanisation of the fins (5, 7) and of the tubular sleeve (3) according to choice: separately - with subsequent assembly onto the support (1, or 1 + 3) - or directly onto the actual support (1, or 1 + 3). The insulators obtained in this manner are characterized by a very low weight/mechanical strength ratio, and by there ability to tolerate the intense operational vibrations, as compared with the conventional porcelain insulators. Moreover, they exhibit substantial properties of resistance to aging and to surface discharges, and high auto-extinction capability, as compared with insulators made from organic material of the prior art. <IMAGE>

Description

(54) HIGH VOLTAGE ELECTRIC INSULATORS MADE OF RESIN-BONDED GLASS-FIBRE AND ORGANIC MATERIAL, AND PROCESS FOR MANUFACTURING SAME (71) We, FIDENZA VETRARIA S.p.A. an Italian Company of 31, Foro Buonaparte, Milan, Italy, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to electric insulators for average, high and very high voltages, made of resin-bonded glass-fibre and equipped with a ribbed coating made of an ethylene-propylene elastomer having high physical properties and in particular high dielectric properties, as well as to a process for manufacturing the same.

Electric insulators composed of resinbonded glass-fibre (r.b.g.f.) with a ribbed coating made of organic materials are already known and broadly utilized in the field of electric outdoor overhead average and high voltage lines and in the field of electric traction. The main advantage offered by insulators made of resin-bonded glass-fibre and organic material in respect of the conventional porcelain or glass insulators resides in the particularly low i'weight/mechanical strength" ratio.

In fact. the conventional porcelin or glass insulator of the pin and cap type, when meant for outdoor overhead high and very high voltage lines involving high mechanical loads on the cables, must be equipped with metal caps of large dimensions. Thus, the weight of the whole insulator chain is high and is no longer negligible with respect to the weight of the conductor supported by the chain.

In the compound insulators made from resin-bonded glass-fibre and organic material. the mechanical supporting function is left just to the r.b.g.f. central body, which is generally manufactured as a solid bar according to the "pulltrusion" method (i.e.

extrusion by pulling or traction) (either continuously or discontinuously) when glass fibres arranged in filaments parallel with one another and parallel with the direction of the mechanical stress to which the supporting central body will be subjected during operation are used or when felted glass fibres are used, i.e. glass fibres cut and arranged incoherently (chopped strand), or woven glass fibres are used. The central body is manufactured in the form of a hollow cylindrical body by the filament winding method when continuous fibres are used, which in general are spirally arranged. For manufacturing the central body in the form of a hollow cylinder, glass-fibre fabrics may also be utilized. In both cases (solid bar or hollow cylinder) the glass fibres are impregnated with a resin having good electrical properties (such as a cycloaliphatic epoxy resin).

Such bodies exhibit a mechanical strength of around that of steel, but have a specific gravity corresponding to about one third of the specific gravity of steel.

In the case of electric railway and tramway traction, the r.b.g.f. insulators offer another considerable advantage in comparison with the conventional porcelain insulators, as in this field it is of the utmost importance that the insulation can successfully withstand the strong vibrations to which it is subjected during operation. This may be easily achieved by means of r.b.g.f. insulators, unlike those made of porcelain.

However, the manufacturing of r.b.g.f.

insulators which are fully satisfactory in every respect has met with a few serious obstacles. At first it was very difficult to find suitable materials for preparing the generally ribbed coating of the insulator, i.e. those which were resistant to the combined action of ageing and of surface electric discharges, the latter involving undesired tracking and/ or erosion phenomena.

Once resistance to ageing and to surface discharges had been achieved thanks to the utilization of suitable materials e.g. cycloaliphatic epoxy resins, silicone elastomers, ethylene-propylene elastomers and fluorinated resins (PTFE), there was still the difficulty of coupling the r.b.g.f. central body (solid bar or hollow cylindrical body) to the ribbed coating material to be overlaid.

In fact, the physical and in particular the mechanical properties of the r.b.g.f. body are such as not to allow reliable coupling with materials reacting inelastically to deformations caused by mechanical stresses.

With a view to attempting to obviate these drawbacks, various methods of coupling have been studied: for example, by means of elastic resins or silicone greases, or rubber mastics, or even by simple mechanical forcing of the ribbed coating material onto the r.b.g.f. body.

However, none of the above-mentioned methods was capable of solving this problem satisfactorily. In fact, every method produced a coupling which turned out to be the weak point of the insulator, from the electrical viewpoint.

It should be borne in mind that the outside configuration of an average, high, and above all, very high voltage insulator, whose ribbings have considerable widths and thicknesses, has been studied for the purpose of presenting to an electric discharge between the insulator's metal terminals (if any, or in any case between the live conductor and earth) a path which is as long as possible, i.e.

a particularly high surface creeping line.

"Surface creeping line" means the path of an electric discharge over a surface (and therefore, following the insulator ribbing) between a live electrode and earth (i.e. in particular between a live conductor and any part of the earth-connected supporting structure, or more generally any part of such structure having the same potential as the earth). In general, a surface creeping line is deemed high enough when the ratio of said line to the straight line distance, i.e. the distance in a straight line between a live conductor and earth (as defined hereina bove), is at least equal to 2, or preferably 3 or even higher than 3, when taking into consideration insulators intended for normal uses and insulators utilized under polluted and very polluted conditions, respectively.

It is therefore evident that any electric discharge generated in the gap between the r.b.g.f. central body and the ribbed coating would ruin the insulation, which would fall below the intended level and would further deteriorate within a short time to even lower levels as a result of damage chiefly caused by the first discharge. Further worsening may be caused by other successive discharges.

The methods of coupling the r.b.g.f. body with the ribbed coating practiced so far according to the art and mentioned hereinbefore exhibit the above-described undesirable drawbacks as they are not capable of preventing electric discharges in the gap between body and coating.

For instance, the silicone greases are subject to a pumping effect due to the elongation of the r.b.g.f. central body under the action of mechanical tensile stress and to the consequent compression exerted by the ribbed coating on the body. In this way the silicone grease may be expelled from the gap between the central body and the coating and will not be sucked in again when, as a result of reduction or elimination of the tensile stress, the compression exerted by the coating decreases. Narrow voids or cavities then occur which easily allow moisture infiltrations, so that undesirable electric discharges occur along the body (in the form of a bar or of a hollow cylinder) and inside the ribbed coating.

Conversely, as some types of coupling resins couple two materials different from each other (i.e. the r.b.g.f. body and the ribbed coating which, for example, may be made of PTFE) and from the resins themselves, they are subjected to a shear stress along the body that also causes voids or cavities which attract pollution and lead to electric discharges.

The formation of small voids in the gap between body and coating permits (as already pointed out) moisture incursion or diffusion as well as generation of partial localized electric discharges. These drawbacks, in their turn, involve as a final consequence the striking of a direct, continuous and total discharge between the metal terminals of the insulator (or anyhow between a live conductor and earth) inside the insulator, along the body (a bar or a cylindrical pipe). The body is thus irreparably damaged and in consequence puts the insulator out of service immediately or within short time.

Moreover, not even the flame resistance characteristic (i.e. the self-extinguishing power) of the insulator should be neglected.

Such characteristic must be as good as possible. However, the organic materials used so far in the art have been relatively unsatisfactory, so that the resistance to flames, if any, was nearly ineffective.

The present invention in one aspect provides an electric insulator for average, high or very high voltages which comprises a resin-bonded glass-fibre longitudinally ex tending supporting central body having a ribbed coating completely surrounding and being attached to the central body and comprising an ethylene-propylene (EPR) elastomer, the ribs being: either moulded or bonded directly onto the central body; or moulded or bonded to a sleeve of the said elastomer surrounding and attached to the central body: the bonding being by means of a self-curing blend which comprises a low unsaturation olefinic polymer and is compatible with the ethylene-propylene elastomer.

Thus, there may be provided an electric insulator for average, high and very high voltages, of the compound type, consisting of a r.b.g.f. body and of a ribbed coating made from a suitable elastic organic material. In such an insulator the generation of tensile shearing stresses between the supporting central body and ribbed coating is prevented by virtue of the high elasticity of the ribbed coating.

Such an insulator avoids any other drawback depending on the low compatability existing in the prior art between the r.b.g.f. body and the ribbed coating.

The insulator of the mentioned type prefeably has a ribbed coating made of a material selected from among the ethylenepropylene elastomers having a proper antitracking and anti-erosion formulation, and high characteristics of elasticity, ageing resistance and opposition or resistance to flame propagation (self-extinguishing power), besides high impermeability and water repellency.

The invention in another aspect provides a process for producing an electric insulator for average. high or very high voltages which comprises a resin-bonded glass-fibre longitudinally extending supporting central body having a ribbed coating completely surrounding and being attached to the central body and comprising an ethylene-propylene (EPR) elastomer, the process comprising: either moulding or bonding the ribs directly onto the central body. or moulding or bonding the ribs to a sleeve of the said elastomer surrounding and attached to the central body, the bonding being by means of a selfcuring blend which comprises a low unsaturation olefinic polymer and is compatible with the ethylene-propylene elastomer.

There may be obtained by means of the invention an electric insulator for average, high and very high voltages, of the compound type. consisting of a supporting central body made from resin-bonded glassfibres equipped with a ribbed coating made from organic material and with metal connections, in such an insulator the r.b.g.f.

central body usually being a solid bar or hollow cylinder according to the uses the insulator is meant for. being thoroughly coated with an ethylene-propylene elastomer (EPR) in the form of ribs preferably supported by a tubular sleeve, it too being prepared from an ethylene-propylene elastomer (EPR) of quality and characteristics identical with those of the EPR elastomer the ribs are made of. such tubular sleeve being preferably integral with such ribs, the granting of the ribbed elastomeric coating on the r.b.g.f. central body, as well as of the ribs on such sleeve and of the ribs with one another being preferably attained by using a blend self-curing at room temperature and based on low-unsaturation olefinic polymers, similar to and compatible with the ethylenepropylene blend (EPR) the ribbed coating is made of.

The electric insulator of the present invention is advantageously manufactured according to a process comprising: preparing a mould for moulding the whole elastomeric ribbed coating associated with a longitudinally extending supporting central body made from resin-bonded glass fibres; treating the surface of the central body, in the form of a solid bar or of a hollow cylinder, depending on the uses the insulator is meant for, according to a conventional technique selected from sandblasting, rasping or rubbing with glass paper, smearing with a suited adhesion-promoting mastic (the so-called "primer"); placing the body into the mould as prearranged, the body being supported in the mould at its ends; then moulding and curing in the mould the entire ribbed coating on the central body according to one of the known methods of the art selected from the transfer method and the injection method, by employing, as moulding material, an ethylene-propylene elastomer having particularly high characteristics; and finally withdrawing the whole thus moulded insulator from the mould.

Alternatively the ribbed coating may be made separately followed by assembling the insulator. In such a case the rubberizing of the central body with the ethylene propylene elastomer and .the separate moulding of the ribs with the same ethylene-propylene elastomer is carried out first, and the assembling of the insulator thereafter.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, wherein: Figure 1 is a partially cutaway view of an electric insulator according to the present invention, having a supporting central body in the form of a solid bar, provided with a sleeve-shaped rubberizing previously applied onto such bar, and with ribs successively added onto the sleeve; Figure 1 bis is a partial cutaway view of an electric insulator according to this invention, having a supporting central body in the form of a solid bar, the entire ribbed coating being moulded in one piece directly on the bar;; Figure 2 is a partially cutaway view of an electric insulator according to the present invention, having a supporting central body in the form of a hollow cylinder, provided with an inside and outside sleeve-shaped rubberizing previously applied onto the hollow cylinder, and with ribs successively added onto the outside sleeve; Figure 2 his is a partial cutaway view of an electric insulator according to this invention having a supporting central body in the form of a hollow cylinder. the entire ribbed coating being moulded in one piece directly on the hollow cylinder. and the sleeveshaped inside rubberizing being successively applied inside the hollow cylinder.

With reference to the Figures, a high voltage electric insulator made from resinbonded glass-fibre and ethylene-propylene elastomers consists of a r.b.g.f. supporting central body of the solid bar type I (Figures 1 and l bis ). or of the hollow cylinder type 2 (Figures 2 and 2 his) to which the ribbed coating is applied. The coating may be in one piece. as coating 7 in Figure 1 bis and coating 8 in Figure 2 his. or may consist separately of a sleeve-shaped rubber coating such as coating 3 in Figure 1 and coating 4 in Figure 2, and of ribs 5 and 6 respectively, the ribs being successively applied onto the sleeveshaped rubber coating 3, 4.Furthermore, if the supporting central body made of resinbonded glass-fibre consists of a hollow cylinder 2. as shown in Figures 2 and 2 bis (this construction being usually adopted when the insulator is meant to be utilized as a bushing insulator), a sleeve-shaped rubberizing or rubber coating 10 in the inside of hollow cylinder 2 is used. Such inside rubberizing is connected without break in continuity in zone 14, precisely at the cylinder ends. with the external sleeve-shaped rubber coating 4, in order to ensure a complete coating, and, in consequence, full protection of r.b.g.f. body 2.

Generally, two metal terminals 9 complete the insulator at the supporting central body ends. The terminals 9 are fixed to the central body according to one of the known clamping methods of the art (e.g. conical ends; inserted wedge; outside notch; compression clamping; application of outside and inside cones; threading; etc). However, when the ribbed coating is moulded in one piece, a different type of clamping can be adopted, since in such a case it is possible to prepare r.b.g.f. bodies with enlarged ends, as described in more detail later on.

A preferred process for manufacturing the embodiments of the insulator illustrated hereinbefore comprises the following steps.

The process starts with a r.b.g.f. supporting central body (previously manufactured according to conventional methods) either of the solid bar type 1 or of the hollow cylinder type, 2 depending on the final type of insulator to be made.

In both cases, prior to any other operation, the body is treated. In particular it is sandblasted or rasped or rubbed with glass paper according to the usual techniques of the art for the purpose of increasing the contact surface and of obtaining a good adhesion of the coating blend. As an alternative to the sandblasting or rasping it is possible to spread over the surface of the body (a bar or a hollow cylinder) an adhesion promoter, a so-called "primer". After this treatment, the organic elastomeric material is applied. In particular the body is now provided with a ribbed coating prepared from an ethylene-propylene elastomer (EPR). The coating, as explained hereinbefore, is either in one piece 7 or 8. or consists separately of a sleeve-shaped rubber coating 3 or 4. with ribs 5 or 6 successively added thereto.

In case of the one-piece coating (Figures 1 his and 2 bis ), a mould is used to mould the entire ribbed coating 7 or 8 associated with the central body I or 2.

The central body is put into the mould, supporting it at both ends; the entire elastomeric ribbed coating is then moulded and cured-on the central body in the mould, according to the transfer method or to the injection method. Finally, the whole insulator so moulded is withdrawn from the mould.

For an insulator whose central body is of the solid bar type, the manufacturing process is concluded at this stage. Conversely, for an insulator whose central body is of the hollow cylinder type, the sleeve-shaped rubber coating of the inside surface must then be applied according to the procedure described hereafter, where the coating preparation in consecutive steps is described. The rubberizing of the inside surface may be carried out before moulding on the whole ribbed coating, if desired.

Conversely, in the case of a separately prepared coating, the central body is first coated with a tubular layer or sleeve of EPR elastomer, such operation being hereinafter called for the sake of brevity rubberizing.

When the central body consists of a solid bar, the sleeve-shaped rubberizing may be carried out according to one of the following methods: a) simultaneous moulding and curing by compression; b) simultaneous moulding and curing by transfer process; c) simultaneous moulding and curing by injection; d) extrusion from a T-head extruder'and successive steam-curing in an autoclave or in a liquid bath, for example in a liquid bath of molten salts.

Conversely, when the central body consists of a hollow cylinder, the elastomeric tubular layer (rubberixing) can be prepared according to any of the methods listed hereinbelow, in a manner analogous to the corresponding methods employed for the solid bar: b) simultaneous moulding and curing by transfer process; c) simultaneous moulding and curing by injection.

When using either method, both the inside and the outside surfaces of the hollow cylinder are simultaneously rubberized.

The sleeve rubberizing of the central body when in the form of a hollow cylinder can also be carried out in two steps: rubberizing of the outside surface according to any of the methods listed hereinabove at a) to d) for the case of a solid-bar body; and --rubberizing of the inside surface according to the following method: a sufficiently thick pipe, previously prepared from a crude blend and extruded is introduced into the cylinder cavity, whereupon the pipe is adhered to the inside surface of the r.b.g.f. cylinder by means of an inflatable air tube of suitable shape, which is introduced into the crude blend pipe and is inflated during curing in a furnace or in a liquid bath at a suitable temperature or in a steam autoclave.Alternatively, the pressure required inside the blend pipe for making it adhere to the inside surface of the r.b.g.f.

cylinder can be obtained by means of a suitable substance (e.g. sodium bicarbonate) which evolves gases at the curing temperature and in consequence causes the pressure to rise.

The rubberizing blend for forming the elastomeric tubular layer that coats the solid bar, as well as the blend utilized for the two elastomeric tubular layers that coat the inside and outside surfaces of the hollow cylinder, have the same composition as the blend used for moulding the ribs, such moulding being described hereinbelow.

The moulding of the ribs may be effected according to two different methods: as a separate moulding of single ribs or as a moulding of groups of a number of units forming a whole with one another. Figures 1 and 2 show only single ribs 5 and 6, there being separation lines Il and 12 between one rib and the successive one.

When manufactured as single ribs, moulding can be carried out according to any of the following processes: a) simultaneous moulding and curing by compression: b) simultaneous moulding and curing by transfer process: c) simultaneous moulding and curing by injection.

In any case, for process economy, each mould has multiple impressions: i.e. in every mould there is more than one impression of a single rib. When the ribs are manufactured as groups of more than one unit in regular succession forming a whole with one another, either method described hereinbelow is followed for the moulding, quite analogously with the moulding of the single ribs: b) simultaneous moulding and curing by transfer process; c) simultaneous moulding and curing by injection.

At the conclusion of curing, the ribs (if manufactured individually) or the groups of ribs in one piece, are drawn out from their respective moulds.

Different methods may also be followed for assembling the insulator.

According to a first method, the premoulded ribs, either single ribs or in multiple groups, are slipped, preferably driven by force onto the r.b.g.f. central body previously treated (i.e.: sandblasted, or rasped, or smeared with a primer) as already described and smeared with a bland which is selfcuring at room temperature. The blend is based on low-unsaturation olefinic polymers, similar to or compatible with the ethylenepropylene blend (EPR) employed for rubberizing the body and for moulding the ribs.

The self-curing blend must be applied also on the surfaces (which are of a very limited area and generally have the shape of a circular ring) over which the ribs--either individual or in multiple groups-are in contact with one another, surfaces whose intersection lines with the drawing plane are indicated at 11 and 12 respectively, In Figures 1 and 2.

More particularly the self-curing blend employed includes a low-unsaturation amorphous olefinic terpolymer consisting of ethylene, an alpha-olefin, a cyclic or acyclic polyene having non-conjugated double bonds; a reinforcing filler; and as a curing agent, an organic hydroperoxide, preferably antioxidizers, pigments and other additives are also incorporated. This blend is prepared according to Italian Patent No.

780,429-Montedison S.p.A. (British Patent 1097415).

According to another method, the premoulded ribs, either single ribs or in multiple groups, are slipped (as mentioned hereinbefore), preferably driven by force onto the r.b.g.f. central body. However, unlike the previously described method, the central body used in this case is already rubberized, i.e. already covered with a tubular elastomeric coat; and it is this tubular coat or sleeve that is smeared with the self-curing blend, so as to adhere the ribs (single ribs or groups ribs) to the tubular coat. This occurs in the same way as the same blend secures direct adhesion between the ribs and the treated, but not rubberized, central body, in the first method.

Finally, according to a third method of assembly, use is made of pre-moulded ribs, either single or in multiple groups, and which are only partially cured, more precisely cured to an extent of 50%-. The ribs are slipped, preferably driven by force, onto the r.b.g.f.

central body (a bar or a hollow cylinder) which has been provided with a rubber coating which has also been cured only partially to an extent of 50. The unit so assembled is then placed into a proper moulding-press to complete the curing. This is attained by radial compression and by heating in the mould to a temperature ranging from 160 to 210cm, preferably from 160 to 180"C By employing this third assembling method it is possible to achieve excellent results as regards the adhesion between the various assembled parts without using selfcuring adhesives. as is foreseen conversely by the two previously described assembly methods.

As concerns the curing times, it must be borne in mind that in any process step the employed ethylene-propylene elastomer (EPR) blend takes 5 to 45 minutes, more particularly 10 to 40 minutes. to become thoroughly cured. Therefore, curing conducted up to 50% in a first step, as illustrated in the third insulator assembling method, needs 3 to 22 minutes. more particularly 5 to 20 minutes. As many minutes are also needed for the second step, in which the curing of the entire assembled insulator is brought from 50 to 100%.

The curing temperatures to which the blend is subjected during the above-specified time-periods are the same for all the abovedescribed methods, as indicated in connection with the already mentioned third insulator assembly method, i.e. 160 to 210"C, preferably 160 to 180 C.

The adhesive blend is self-curing: therefore as mentioned above it cures at room temperature. The widest curing temperature range is from 5 to 60"C. The times required to complete the process vary from 20 to 48 hours or at the most from 20 to 96 hours.

The metal terminals are fastened to thesupporting central body according to any of the already known conventional clamping methods, but when the ribbed coating is moulded in one piece it is possible to initially prepare the supporting central body with both its ends shaped as a head which is oversized with respect to the central body and having undercuts which are properly arranged to provide an anchorage which prevents the slipping-off of the insulator's suspension metal connections 9.

In fact, by moulding the ribbed coating in one piece directly on the supporting central body it is no longer necessary to slip or drive the ribs onto the central body. Thus, it is possible to use a previously manufactured central body with oversized heads. This represents a further advantage of the process for moulding the ribbed coating in one piece.

The process for moulding the ribbed coating may be practised, according to a further variant. by coating the supporting central body with a tubular elastomeric layer or sleeve, i.e. by effecting the rubberizing as in the case of the separately prepared ribbed coating, but keeping the extent of curing to a rather low degree; then placing the already rubberized manufactured central body into a suitably prearranged mould and by moulding therein all the ribs in one piece. directly on the previously rubberized central body similarly to the case when the entire ribbed coating is constituted as a whole.

WHAT WE CLAIM IS: 1. An electric insulator for average, high or very high voltages which comprises a resin-bonded glass-fibre longitudinally extending supporting central body having a ribbed coating completely surrounding and being attached to the central body and comprising an ethylene-propylene (EPR) elastomer, the ribs being: either moulded or bonded directly onto the central body; or moulded or bonded to a sleeve of the said elastomer surrounding and attached to the central body; the bonding being by means of a self-curing blend which comprises a low unsaturation olefinic polymer and is compatible with the ethylene-propylene elastomer.

2. An electric insulator according to Claim 1, wherein the longitudinally extending supporting central body is a solid bar.

3. An electric insulator according to Claim 1, wherein the longitudinally extending supporting central body is a hollow cylinder.

4. An electric insulator according to any of Claims 1 to 3, wherein the ribs are integrally formed in one or more groups.

5. An electric insulator according to any of Claims 1 to 3, wherein the ribbed coating comprises a sleeve-shaped tubular layer intimately coupled with the central body and supporting the ribs, the ribs being either individual ribs or ribs arranged in multiple groups.

6. An electric insulator according to any of Claims 1, 3 and 5, wherein the hollow cylinder is internally lined with another tubular sleeve of the said ethylene-propylene elastomer, which in the proximity of the hollow cylinder ends forms a continuation of the outside tubular sleeve.

7. An electric insulator according to any of the preceding claims, wherein the blend is self-curing at room temperature and comprises a low unsaturation amorphous olefinic terpolymer of ethylene, an alpha-olefin, a cyclic or acyclic polyene having non-conjugated double bonds; a reinforcing filler; a curing agent; an organic hydroperoxide; and optionally an antioxidiser, and pigments.

8. A process for producing an electric insulator for average, high or very high

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (22)

**WARNING** start of CLMS field may overlap end of DESC **. preferably driven by force, onto the r.b.g.f. central body (a bar or a hollow cylinder) which has been provided with a rubber coating which has also been cured only partially to an extent of 50. The unit so assembled is then placed into a proper moulding-press to complete the curing. This is attained by radial compression and by heating in the mould to a temperature ranging from 160 to 210cm, preferably from 160 to 180"C By employing this third assembling method it is possible to achieve excellent results as regards the adhesion between the various assembled parts without using selfcuring adhesives. as is foreseen conversely by the two previously described assembly methods. As concerns the curing times, it must be borne in mind that in any process step the employed ethylene-propylene elastomer (EPR) blend takes 5 to 45 minutes, more particularly 10 to 40 minutes. to become thoroughly cured. Therefore, curing conducted up to 50% in a first step, as illustrated in the third insulator assembling method, needs 3 to 22 minutes. more particularly 5 to 20 minutes. As many minutes are also needed for the second step, in which the curing of the entire assembled insulator is brought from 50 to 100%. The curing temperatures to which the blend is subjected during the above-specified time-periods are the same for all the abovedescribed methods, as indicated in connection with the already mentioned third insulator assembly method, i.e. 160 to 210"C, preferably 160 to 180 C. The adhesive blend is self-curing: therefore as mentioned above it cures at room temperature. The widest curing temperature range is from 5 to 60"C. The times required to complete the process vary from 20 to 48 hours or at the most from 20 to 96 hours. The metal terminals are fastened to thesupporting central body according to any of the already known conventional clamping methods, but when the ribbed coating is moulded in one piece it is possible to initially prepare the supporting central body with both its ends shaped as a head which is oversized with respect to the central body and having undercuts which are properly arranged to provide an anchorage which prevents the slipping-off of the insulator's suspension metal connections 9. In fact, by moulding the ribbed coating in one piece directly on the supporting central body it is no longer necessary to slip or drive the ribs onto the central body. Thus, it is possible to use a previously manufactured central body with oversized heads. This represents a further advantage of the process for moulding the ribbed coating in one piece. The process for moulding the ribbed coating may be practised, according to a further variant. by coating the supporting central body with a tubular elastomeric layer or sleeve, i.e. by effecting the rubberizing as in the case of the separately prepared ribbed coating, but keeping the extent of curing to a rather low degree; then placing the already rubberized manufactured central body into a suitably prearranged mould and by moulding therein all the ribs in one piece. directly on the previously rubberized central body similarly to the case when the entire ribbed coating is constituted as a whole. WHAT WE CLAIM IS:

1. An electric insulator for average, high or very high voltages which comprises a resin-bonded glass-fibre longitudinally extending supporting central body having a ribbed coating completely surrounding and being attached to the central body and comprising an ethylene-propylene (EPR) elastomer, the ribs being: either moulded or bonded directly onto the central body; or moulded or bonded to a sleeve of the said elastomer surrounding and attached to the central body; the bonding being by means of a self-curing blend which comprises a low unsaturation olefinic polymer and is compatible with the ethylene-propylene elastomer.

2. An electric insulator according to Claim 1, wherein the longitudinally extending supporting central body is a solid bar.

3. An electric insulator according to Claim 1, wherein the longitudinally extending supporting central body is a hollow cylinder.

4. An electric insulator according to any of Claims 1 to 3, wherein the ribs are integrally formed in one or more groups.

5. An electric insulator according to any of Claims 1 to 3, wherein the ribbed coating comprises a sleeve-shaped tubular layer intimately coupled with the central body and supporting the ribs, the ribs being either individual ribs or ribs arranged in multiple groups.

6. An electric insulator according to any of Claims 1, 3 and 5, wherein the hollow cylinder is internally lined with another tubular sleeve of the said ethylene-propylene elastomer, which in the proximity of the hollow cylinder ends forms a continuation of the outside tubular sleeve.

7. An electric insulator according to any of the preceding claims, wherein the blend is self-curing at room temperature and comprises a low unsaturation amorphous olefinic terpolymer of ethylene, an alpha-olefin, a cyclic or acyclic polyene having non-conjugated double bonds; a reinforcing filler; a curing agent; an organic hydroperoxide; and optionally an antioxidiser, and pigments.

8. A process for producing an electric insulator for average, high or very high

voltages which comprises a resin-bonded glass-fibre longitudinally extending supporting central body having a ribbed coating completely surrounding and being attached to the central body and comprising an ethylene-propylene (EPR) elastomer, the process comprising: either moulding or bonding the ribs directly onto the central body, or moulding or bonding the ribs to a sleeve of the said elastomer surrounding and attached to the central body, the bonding being by means of a self-curing blend which comprises a low unsaturation olefinic polymer and is compatible with the ethylenepropylene elastomer.

9. A process according to Claim 8, which comprises the steps of: providing a mould for moulding the whole elastomeric ribbed coating onto the central body; roughening the surface of the central body by sandblasting, rasping or rubbing with glass paper, or smearing with an adhesion-promoting mastic; placing the central body into the mould, the central body being supported in the mould at its ends; moulding and curing the whole ribbed coating onto the central body by the transfer method or the injection method, utilizing as moulding material the ethylene-propylene elastomer; and withdrawing the insulator from the mould.

10. A process according to claim 8, which comprises the steps of pre-arranging a mould for moulding the ribs either individually as single ribs. or in integral groups of two or more ribs; roughening the central body: optionally coating or rubberizing the central body with the ethylene-propylene elastomer to the shape of a tubular sleeve: by simultaneously moulding and curing by compression; simultaneously moulding and curing by the transfer process: simultaneously moulding and curing by injection: or extrusion through a T-head extruder and successive steam-curing in an autoclave or in a liquid bath of molten salts; moulding and curing in the mould the coating ribs by any of the methods: for the separate moulding of single ribs: simultaneously moulding and curing by compression. simultaneously moulding and curing by the transfer process: or simultaneously moulding and curing by injection; and for the moulding of groups of two or more ribs: simultaneously moulding and curing by the transfer process; or simultaneously moulding and curing by injection: in each case, as moulding material of the ribs, the ethylene-propylene elastomer being used: withdrawing the ribs from the mould; and finally assembling the insulator.

I 1. A process according to claim 9 or 10, wherein the central support is a hollow cylinder, which further comprises rubberizing the inside surface of the said cylinder with the ethylene-propylene elastomer, the said rubberizing being carried out, optionally as initial or final step of the process, by introducing into the hollow cylinder a tubular sleeve made of a crude EPR blend; applying the sleeve to the said inside surface by either: 1) during curing inflating an air tube placed inside the sleeve or 2) creating the necessary pressure inside the said tubular sleeve by means of a suitable substance which gasifies at the curing temperature; and finally curing the inner tubular EPR sleeve by heating either in a furnace, or in a liquid bath, or in a steam autoclave.

12. A process according to claim 10, wherein the insulator is assembled by slipping the ribs, either single or in integral groups of more than one unit directly onto the roughened central support which has been smeared with a self-curing blend of low-unsaturation olefinic polymers, the said blend being applied also onto the surfaces of the ribs or the groups of ribs intended to remain in contact with one another, and allowing the blend to cure.

13. A process according to claim 10, wherein the insulator is assembled by slipping the ribs, either single or in groups onto the roughened central support, rubberized, and smeared with the said blend self-curing at room temperature, the same blend being applied also onto the surfaces of the ribs or groups of ribs, intended to remain in contact with one another.

14. A process according to claim 10, which further comprises stopping the curing, of both the rubberizing of the central body and of the separately moulded ribs, when the curing has reached substantially 50%; assembling the insulator by slipping the ribs, either single or in groups, onto the central body previously treated and rubberized; providing a suitable mould for containing the entire assembled insulator; placing the insulator into the mould; completing the curing of the whole insulator by radial compression and heating in the mould; and withdrawing the entire assembled and cured insulator from the mould.

15. A process according to any of claims 9, 10 and 14, wherein curing is conducted at a temperature between 160 and 210"C.

16. A process according to either of claims 9 and 10, wherein curing is completed in from 5 to 45 minutes.

17. A process according to claim 14, wherein curing is brought to 50% in from 3 to 22 minutes and is then completed in from 3 to 22 minutes.

18. A process according to any of claims 9 to 17, wherein the said self-curing blend cures at a temperature between 5 and 60"C and in between 20 and 96 hours.

19. A process according to any of the claims 8 to 18, wherein the central support provided with ends having a configuration suited to house metal connections by any of the following methods: notching; threading; wedging in; application of at least one outside cone; and application of an inside cone.

20. A process according to claim 8.

wherein prior to any other process step, the central body is provided with ends having a configuration suited to house the metal connections by any of the following methods: notching; threading; wedging in; application of at least one outside cone: and application of an inside cone: and each end being shaped as an oversized head and with undercuts for the anchorage of the said suspension metal connections.

21. An electric insulator according to claim 1, substantially as herein described with reference to, and as shown in, any of the figures of the accompanying drawings.

22. A process for producing an insulator according to claim 1 substantially as herein described with reference to the accompanying drawings.