Method of making a field control element
This is a continuation of application Ser. No. 07/975,992, filed Nov. 13, 1992, now abandoned, which is a continuation of application Ser. No. 599,989, filed Oct. 9, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a field control coat of polymer material having electrically-conductive additives for medium voltage fittings. 
     2. Description of the Prior Art 
     Numerous field-controlling elements for cable fittings are known. A plurality of fillers are thereby employed as principal additives that form the field coating element in combination with a suitable polymer matrix and, potentially, other additives. For example, such fillers are inorganic substances having a high dielectric constant such as titanium (IV) compounds (titanium dioxide, barium titanate), electrically-conductive additives such as silicon carbide, carbon black, metal powder, as well as metal oxides, and semimetal oxides (zinc oxide, aluminum oxide, boron oxide, iron (II, III) oxide). Approximately 70% iron (II, III) oxide is therefore often employed as a principal additive, this being embedded in a polymer matrix of butyl rubber. This product is applied as an inner coating in a heat-shrinkable tube. Such a combination of materials, however, exhibits disadvantages given long-term use, particularly after the appearance of high transient overvoltages. The requirements thereafter are frequently no longer met. 
     A sealing compound of the above-mentioned type is provided as a field-controlling element for electrical field control in medium voltage fittings, this sealing compound being utilized as an inner coating in a shrink hose which, due to its composition, is in the position to avoid a disadvantageous stressing at the cable end by excessively-high field strengths. 
     This field-controlling coating must meet the following requirements: 
     For the purpose of a coating to be simply implemented, the field control coat must be formulated as a solvent-containing compound. The field-controlling coating forms from the coating in the hose by evaporation of the solvent that preferably has a low boiling point or, respectively, a high evaporation rate for this purpose; 
     At least at the shrinking temperature, the field-control coating must have or develop adequately-high stickiness in order to guarantee an adhesion to the cable surface. The activation temperature of the coating should lie at 90°-100° C.; 
     During the shrinking process, the compound must have adequate plasticity with good flow properties, so that the formation of bubbles or cavities can be prevented and a form-fitted connection to the substrate surface can be guaranteed; 
     The current-voltage characteristics should be non-linear with a steep rise of the curve at approximately 4 kV/cm, with the following guide lines 
     
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Voltage (kV)   Current (μA)                                            
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1              0.1                                                        
3-4            1-2                                                        
8              up to 60;                                                  
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     In particular the field-control coating, as a component part of a fitting, must pass the electrical tests of DIN 57 278/VDE 0278 (for example, for a 20 kV system) as set forth below 
     Nominal power frequency withstand voltage (one minute withstand voltage of 55 kV), 
     DC test (96 kV over 30 minutes), 
     Nominal withstand surge voltage test (125 kV, 10 surges each of both polarities), 
     Long-term AC voltage tests given cyclical current load (126 heating cycles, dry), 
     Partial discharge measurement (free of partial discharge at 24 kV (≦1 pC), and 
     Long-term tests (load change at 30 kV). 
     SUMMARY OF THE INVENTION 
     The object of the present invention, therefore, is to provide a field-control coating for medium voltage fittings that, in addition to the required conditions, also has a failure-free long-term behavior, whereby the electrical requirements as well as the softening behavior, the flow capability, the tackiness and the solubility should be improved on the basis of favorable raw material combination. 
     The above object is achieved, according to the present invention, in a field-control coating in that the polymer material is formed of a viscous, highly-sticky ethylene propylene &#34;diene&#34; terpolymer (EPDM) that is dissolved in a slightly unpolar solvent, in that the mean molecular weight of the EPDM amounts to 1000-15000, preferably 6000-8000, in that the diene part lies between 0 and 13, preferably between 4.5 and 9.5 weight percent, in that the ethylene-to-propylene ratio in the ethylene-propylene part lies between 40/60 and 75/25 weight percent, preferably between 43/57 and 54/46 weight percent, and in that the electrically-conductive additive is granulated silicon carbide having a grain size of F400-F320, whereby the weight part of the polymer material, without solvent, amounts to 10-40% and that of the electrically-conductive additive correspondingly amounts to 60-90%. 
     The selection of the raw materials for developing a field-control coating in accordance with the present invention including the inventive combination with a heat-shrinkable element for the formation of a field-controlling medium voltage fitting requires a careful matching. Thereby in the foreground, according to the object of the invention, is that the recipe must be simple to produce and with few constituents in order, among other things, to be able to also guarantee a reliable reproducibility in manufacture. 
     Silicon carbide is then employed as a principal additive for the field-control coating of the present invention, this material not having any magnetic properties and thereby having a comparatively better voltage stability after transient overvoltages than the known iron (II, III) oxide. As anticipated, the quantity portion of the silicon carbide, as well as the grain size thereof, are critical for the function of the field-control coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other objects, features and advantages of the invention, its organization, construction and operation will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, on which: 
     FIG. 1 is a sectional view of a shrink hose before shrinking; and 
     FIG. 2 is a sectional view of the shrink hose after shrinking as employed as an end closure in medium voltage systems. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to the drawings, FIG. 1 illustrates a shrink hose 1 composed of a known shrink material at whose inner surface the field-control coating 2 of the present invention is applied, at least in the required, effective region. 
     FIG. 2 illustrates an end closure 4 for medium voltage systems, whereby the shrink hose 1 having the inwardly-disposed field-control coating 2 has been shrunk onto a cable end piece. A tracking-resistant adhesive 3, for example a commercially-available adhesive on the basis of polyisobutylene that is rendered tracking-resistant with suitable fillers, with which a sealing occurs is applied in the admission region onto the inside of the shrink hose 1. 
     A raw material that meets the following conditions is employed for the polymer matrix of the field-control coating 2 constructed in accordance with the present invention: 
     The polymer must be completely dissolved in commercially-available, relatively low-boiling solvent at room temperature; 
     The polymer must be capable of absorbing extremely great quantities of filler; 
     The highly-filled polymer must still develop adequate stickiness at 90°-100° C.; and 
     After evaporation of the solvent from the coating material, the material must be adequately flexible and form a smooth surface (no formation of tracks due to brittleness of the dry compound). 
     A viscous, highly sticky ethylene propylene diene terpolymer (EPDM) or ethylene propylene copolymer (EPM) is then selected, according to the present invention, as the polymer in the form of a resin that meets the above-recited requirements. This polymer can be easily dissolved in unpolar solvents such as, for example, petroleum ether (boiling range 40°-60° C.) or other hydrocarbon solvents or, respectively, in chlorinated hydrocarbons such as 1.1.1-trichloroethane. The EP (D) M resins which come into consideration have mean molecular weights of 1000-15000, preferably 6000-8000. Dicyclopentadiene (DCP), ethylidene norbornene (EN) or 1.4-hexadiene can be used as the diene constituent. The diene portion in the resin lies between 0 and 13 weight percent, preferably at 4.5-9.5 weight percent, whereas the ethylene-to-propylene ratio in the EP part lies between 40/60 and 75/25 weight percent, preferably within the range of 43/57-54/46 weight percent. It has proven that a silicon carbide part between 60 and 90 weight percent, preferably 80 weight percent, is required for meeting the electrical requirements. F400-F 320 are particularly suitable as grain sizes for the SiC. 
     A field-control coating constructed in accordance with the present invention has, for example, the following composition in the EP (D) M/SiC ratio: 
     40-10 weight percent Trilene 66; 
     60-90 weight percent silicon carbide &#34;Carbogran™ dark&#34; of Lonza-Werke GmbH, Waldshut, Germany, having the micrograin size F360 or F320. 
     The amount of solvent, for example 1.1.1-trichloroethane, is determined in accordance with the viscosity desired for the coating (approximately 30000 mPa·s/23° C.). 
     Coming into consideration as a polymer matrix in addition to pure EP (D) M liquid resin also mixtures thereof with other resins and/or rubbers that are compatible with trilene and are soluble in the mentioned solvents. Suitable polymers, for example, are other EP (D) M types with deviating molecular weight, polyisobutylene and butyl rubber, acrylate resins or, respectively, rubber, polybutylenes, atactic polypropylene, epichlorohydrine rubber. 
     Additional tackifying agents such as, for example, resin ester and derivatives thereof, tall resins, hydrocarbon resins and terpene-phenol resins can likewise be mixed into the polymer matrix. 
     In addition to the mentioned constituents, the field-control coating can contain one or more additives for improving the thermal-oxidative stability (antioxidants) such as, for example, Irganox 1010 or Irganox 1076. 
     According to the invention, the field-control coating is applied onto the inner surface of a shrink hose as set forth in greater detail in the two figures. 
     Although I have described my invention by reference to particular illustrative embodiments thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.