February, 1925
IATDUSTRIAL AND ENGIhTEERING CHEMISTRY
In comparing results obtained for rubber as compounded with the value stated by the compounder to be present, the calculated and determined specific gravities should also be compared. Any errors introduced in compounding the samples or in their analyses would be indicated by this comparison. Thus, if a sample has a calculated gravity of 1.5888 and is said to contain 40 per cent of rubber, and if the determined specific gravity is l .607, then the sample contains more inorganic materials than was planned and is therefore
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lacking in rubber content. The case given is that of 206-25 and is also true of 206-45. A modification of the method given above has been found useful in this and in other laboratories for quantitatively separating rubber compounds from frictioned rubber goods, such as rubberized cloth and asbestos materials. It makes possible an examination of the fibers used in weaving the fabric and an analysis of the rubber compound.
A Remarkable Property of Selenium' By Ernest C. Crocker ARTHUR D. LITTLE, INC., CAMBRIDGE, MASS.
URING a recent investigation2 to find new ways to flameproof insulated electric wire, it was discovered quite empirically that selenium had great value. The customary method for flameproofing telephone switchboard wire consists in treating the cotton braid with fusible materials, such as borates, phosphates, tungstates, molybdates, and even silicates, together with some salts which evolve ammonia or other gas, and some water on heating. While this method secures a degree of flameproofness under adjusted test conditions, which pass the best and refuse the poorer, such flameproofing is of very limited real value in service. In particular, while the specifications state that a single wire will not continue to burn in a horizontal position after removal of the igniting flame, a bundle or loose cable of such wires may and usually does burn freely. In trying out numbers of different classes of materials for leads which would indicate different methods of attack of the flameproofing problem, it was noted that selenium, which is itself combustible (though not free burning), imparted remarkable flameproofness. So far as known this is the first instance of a completely combustible material being used as a flameproofing agent. An outstanding feature of the flameproofing ability of selenium is that the larger the bundle of wires the more secure is the flameproofness. Another and perhaps no less valuable property is that when used no electrolyte is applied to the insulation of the wire to cause appreciable conduction during humid weather. Both of these features are highly desirable in flameproofed wire for use on the back of telephone switchboards. There a slight fire, if it spreads, can ruin the service of thousands of subscribers; and surface conductivity of the wire, such as is now unfortunately usual in damp weather, lowers the efficiency of all service, makes for cross talk, and frequently causes the sound of bell ringing in nearby wires. One part by weight of selenium protects 3.6 parts of cotton braid and 10 parts of rubber covering in the presence of 10 parts of copper conductor, constituting the usual No. 22 switchboard wire. Three milligrams of selenium thus protect a centimeter of length of wire, or one pound protects nearly a mile. The protection referred to is freedom from spreading of the flame along the wire held horizontally when the Bunsen burner, candle, or other igniting agent is removed. This much protection is secured by the usual processes only when the wire is practically crusted with salts. On application of the flame much difficulty is experienced in getting the wire to ignite a t all. When ignited, however, and the igniting flame is removed, the flame usually goes out without appreciable spreading. The selenium imparts a strong blue color to the border of the flame and particularly to that
part close to the wire. Just before the flame goes out the luminous part seems to rise upon a bank of blue flame with a separation of 6 mm. or more between the wire and flame a t the moment of extinction. It appears that the concentration of selenium vapor is highest close to the wire, where it forms a blanket which dilutes the otherwise flammable gases which get through it, and serves to insulate and thus prevent the heat of the flame from going down and making more gas out of the cotton and rubber below. It is interesting indeed to find as we do here, that when one heavily treated wire is braided or twisted into a cable with two untreated wires, the resulting cable is completely flameproofed. For some reason not yet apparent selenium combined with the rubber covering of the wire3 by vulcanization (in place of sulfur) is not effective for flameproofing, even though the amount of selenium be considerable. The selenium is most effective when applied to the outside of the cotton braid, but also works very well when applied to the outside of the rubber, under the braid. The complete story of why selenium is so effective in preventing combustion is yet to be told. There is possibly an action analogous to the combustion-retarding effect of tetraethyl lead or diethyl selenide on gasoline vapors. Selenium appears to be unique in possessing flameproofing power of the first magnitude. Arsenic oxide has some of the same action, though of a lower order, and molybdenum compounds a trace. The element tellurium is practically without action, as are also iodine, tin, bismuth, and sulfur. Compounds of selenium such as selenious and selenic acids, sodium selenite, and selenium oxychloride have been found to possess flameproofing power in high degree. In general, however, the use of the element selenium is preferable. It is perhaps best applied in powdered form, with an adhesive, to the rubber covering of the wire before the cotton is braided on. When selenium-treated wire is burned there is a strong odor reminiscent of both sulfur dioxide and garlic. Although unpleasant and noticeable over a wide area, no ill effects due to its inhalation have been noted. At any rate, during fires its fumes are far less objectionable and perhaps less poisonous than those from carbon tetrachloride. Selenium may be used to flameproof paper, scenery, and other diffuse materials, but the amount needed is relatively large. The effect appears to depend on surface action and the surface area is here quite great. Electric wire and probably other comparatively concentrated masses of combustible material are, however, economically protected. This is indeed fortunate, since the usual flameprookg, which works fairly well on the more diffuse materials, is but poorly suited for insulated wire.
Received December 8, 1924. This investigation was conducted by Arthur D . Little, Inc., for the Simplex Wire & Cable Co., Cambridge, Mass. The results are published with the kind permission of Everett Morss, president of the Simplex Company.
a This selenium-vulcanized rubber was made more than seven years ago by C. R. Boggs, superintendent of the Simplex Wire & Cable Company. It had retained the greater part of its original elasticity. See THISJOURNAL, 10, 117 (1918).
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