BITUMINOUS filling compounds for the insulation of electrical apparatus (excluding cable boxes) are divided into five classes; one being a fluid grade compound with a dropping point less than 40 C., and the others having dropping points up to 120 C. "Oil Resisting" compounds are not included.
The requirements of compounds for filling electrical gear and the tests to which they should be subjected are stated; the methods adopted for carrying out the following tests are described, and contraction and brittleness are discussed briefly as follows: Appearance and Composition, and effect on fibrous materials and metals, are specified.
Solubility in Carbon Disulphide must be at least 99%, and is determined by the usual method. The use of other solvents has so far not been investigated.
Viscosity~The British Road Tar Viscometer is employed, with a tolerance of 25%.
Dropping Point.-The Ball and Ring test is used, but with an initial temperature of the sample of 15 C. The Krämer-Sarnow test is not liked.
Pouring Point.-This should, generally, not be more than 75 C. above the dropping point. It is the temperature at which 50 ml. of the sample flow into a receptacle in 240 sees., and is determined in a "Solid Cup Viscometer." Electric Strength.-After much investigation, a proof test rather than a break-down test has been adopted, of 10,000 or 20,000 volts at 60 f 1 C. or 90 f 1 C., according to the class of compound.
Flash Point~The Pensky-Martens and the open-cup apparatus were found to be unsuitable; so this property is determined by sparking in an air bath during a regular rise in temperature.. Mineral Matter.-No special remarks.
Acidity and Alkalinity.-A solution is made under a reflux condenser in equal parts of methylated spirit and benzole, and is titrated with N110 sodium hydroxide with phenol phthalein or alkali blue.
Water-soluble acids and bases are titrated with N/100 sodium hydroxide or sulphuric acid. The pa value may also be determined electrometrically or colorimetrically.
Resistance to Moisture.-Methods and conditions for determining this all-important property are still under discussion; but for the time being, a portion of the sample is boiled in water, which should remain clear and colourless.
Freedom from Harmful Sulphur.-A polished copper strip is immersed in a solution of the sample in chloroform (or carbon tetrachloride or methylene chloride), and should not be appreciably blackened.
Tendency to Emulsification is ascertained by treatment with 10% aqueous sodium hydroxide.
Adhesivenem.-See separate paper. Penetration.-The Standard I.P.T. method is used. Contraction-This depends on the temperature of the pour point, and can be calculated from the coefficient of cubic expansion of 000061 per C. The true contraction can be obtained from the specific gravity at several temperatures