January i, 1908.] 



THE INDIA RUBBER WORLD 



105 



Rubber Insulation for Electrical Conductors. 



THE general subject of the use of india-rubber for the in- 

 sulation of electric wires and cables has been treated at 

 some length recently in The Electrical World (New 

 York), by Fred J. Hall,* in n manner which brings out some 

 practical suggestions of more than ordinary interest. Referring 

 to grades of rubber, he mentions that Para was specified uni- 

 formly for insulation work at a time when Africans and some 

 Other sorts came to market in a very crude form, being dirty 

 and improperly cured. These defects have been largely over- 

 come, and to-day rubber compounds can be made containing no 

 Para that will have as long a life and give as good practical re- 

 sults as an all-Para compound. 



The most important properties of rubber, considered from 

 the standpoint of insulation, may be grouped under the heads 

 (l) nonhydroscopic; (2) dielectric; (3) mechanical strength; 

 and (4) elasticity. AH these, except the nonhydroscopic prop- 

 erty, are materially affected by processes of manufacture, and 

 each one is developed more or less at the expense of the others. 

 In selecting a rubber compound for insulation the manufacturer 

 must consider many points, but as a rule he has no difficulty 

 in determining the quality of the compound and the method of 

 treating it so as to produce the best results at any given price. 

 The stumbling block appears when he tries to harmonize his 

 ideas, based on practical experience, with the theories of the 

 purchaser as expressed in the specification. 



The purchaser really has no interest in the composition of the 

 rubber compound or in the factory method of handling it. His 

 sole interest is to secure an insulation of high practical efficiency 

 for the conditions of service — 1. e., to purchase cables that will 

 work for a long time and give a minimum amount of trouble. 

 The purchaser may be convinced that this result can be ob- 

 tained by the use of fine Para rubber and draw a specification 

 intended to preclude the use of anything but fine Para. But he 

 may subject himself to useless expense and force the manufac- 

 turer to produce an article which is higher in price and no 

 more effective for the condition of service — perhaps less so — 

 than could be produced if the manufacturer was allowed to 

 exercise some discretion in the matter. 



The writer next considers the various tests by which the pur- 

 chaser endeavors to secure what he considers the best insulation. 

 When a specific compound is called for it is usually one con- 

 taining 30 per cent, of Para rubber, and in considering various 

 tests the writer has such a compound in mind. Para rubber 

 being the first to become well known, it was made the standard, 

 and the insulating value of other rubbers was fixed by the extent 

 to which they were shovifn to possess chemical and physical prop- 

 erties identical with Para. It has been assumed, for instance, 

 that unvulcanized rubbers containing a higher percentage of 

 extractive matter (oily or resinous substance) soluble in heated 

 acetone are of less value as insulators, but this is erroneous. 

 The percentage of this resinous or extractive matter varies 

 widely in different brands, nor is the quantity constant in the 

 same brand. As pointed out by Weber, the presence of these 

 resins in crude rubber does not appear to affect its stability in 

 an appreciable degree, though it docs affect vulcanization when 

 it exists in larger quantities than is generally found in fine 

 Para. In such cases more sulphur as well as a change in the prac- 

 tice of time and heat may be necessary to prevent under-vulcaniza- 

 tion. Hence a rubber containing a higher percentage of resin than 

 Para may be as effective an insulator if properly vulcanized. 



It is a mistaken idea that the resin in rubber is highly volatile 



*Mr. Hall has for a number of years been connected in a managerial 

 capacity with the well known India Rubber and Gutta Percha Insulating Co. 

 (Dr. Habirshaw's works), at Yonkers, New York, and has therefore an 

 intimate acquaintance with the general subject of which he treats. — The 

 Editor. 



and that when this disappears the rubber is lifeless and brittle. 

 In fact, the resin is not volatile; the hardening of the rubber is 

 due to the drying out of the rubber and oxidation. Dr. Esch 

 says that the best rubber articles are not made from pure 

 caoutchouc, and calls attention to the increase "as to elasticity 

 and resistance to tearing asunder" in tire inner tubes when 0.5 

 per cent, of pitch is added to the rubber. Now there is prob- 

 ably no article into which rubber enters where it is more im- 

 portant to increase the strength of the rubber and to prevent 

 hardening than tire tubes, and it has been proved that this is 

 best done by the addition of an ingredient possessing a high per- 

 centage of extractive matter of the same general character as 

 that obtained from the rubber Vulcanized rubber always shows 

 an increase in extractive matter over the pure unvulcanized 

 gum, due to chemical changes the nature of which is not fully 

 known. Sulphur is also soluble in acetone, and the free sulphur 

 will appear in the acetone extract. 



The writer regards the worthlessness of acetone tests as fre- 

 quently made, and the injustice that may be done to a manu- 

 facturer by them, as obvious. He would consider that if other 

 tests were satisfactory there would be no danger of over-adulter- 

 ation in a compound showing 8 per cent, of extractive matter, 

 including free sulphur, although purchasers are inclined to 

 specify fine Para with the idea that the unvulcanized rubber 

 contains but i or 2 per cent, of resin, but it should be borne 

 in mind that such results are obtamed from specially selected 

 samples. Nor is the ash test as ordinary practiced of more real 

 value. The writer has seen variations of 6 per cent, in the ash 

 obtained from the same compound due to inaccuracy in weigh- 

 ing and carelessness in burning. 



The writer next treats of the stretch test for rubber insula- 

 tion. Elasticity being the most obvious property of crude rub- 

 ber, it has been assumed that only good rubber compounds are 

 elastic, and the nonelastic are poor, but precisely the same com- 

 pound, by changes in the quantity of sulphur and method of 

 vulcanization, can be made highly elastic or brittle as glass. 

 Lack of elasticity in a vulcanized rubber compound therefore 

 is no proof of inferiority. The writer presents considerations 

 which lead to the conclusion that a very low grade rubber com- 

 pound cannot be vulcanized so as to meet the ordinary stretch 

 test, from which it follows that the stretch test does prevent 

 the use of low grade compounds. But such compounds do 

 not meet the maximum acetone test, nor would they be likely 

 to meet the electrical tests. That is, there are tests besides that 

 for elasticity which insure the purchaser against the excessive 

 use of low grade rubbers. Hence it is unwise to insist upon 

 a test that forces the manufacturer to sacrifice qualities in his 

 compound to a high factor of safety. The factors of time and 

 heat in vulcanizing should be regulated by the mass to be vulcan- 

 ized. When the surface of the outer layer of a thickly insulated 

 conductor has been vulcanized to a point where the elastic prop- 

 erty is greatest, the inner layers, particularlj' the portion lying 

 next to the conductor, are undervulcanized and a compound 

 undervulcanized is highly perishable. 



The principal contributors to the factor of safety to rubber 

 insulation for high tension are firmness and density, both of 

 which are antagonistic to stretch. Specifying a stretch test for 

 high pressure insulation invariably means reducing the factor 

 of safety. [The editor of The Electrical World says in com- 

 menting on this point : "One might as well endeavor to de- 

 termine the breaking strength of the Brooklyn bridge by loading 

 it until it broke."] 



It has been mentioned already that rubber compounds of high 

 efficiency can be made in which lower priced rubbers than Pari 



