360 



THE INDIA RUBBER WORLD 



[March 1, 1920. 



for the comparison of the tendency to soften with rise of tem- 

 perature. Probably the method most commonly applied is that of 

 G. Kramer and C. Sarnow, in which a core of the bituminous 

 material in a glass lube is submitted to the pressure of a drop 

 of mercury of definite weight and the temperature is measured 

 at which the mercury forces its way through the material. This 

 method is probably the most satisfactory but possesses some dis- 

 advantages for the avoidance of which the arrangement repre- 

 sented in Fig. 1 is very convenient. The apparatus consists of a 

 U-tube M one arm of which is connected by capillary tubing 

 with a gun-metal tap T. The central plug of this tap, which 

 forms the most important part of the apparatus, has a conical or 

 tapered bore '4-inch in length and '/» and 1/16-inch in diameter, 

 respectively, at the two ends; the bore is terminated at each 

 end by a flat groove (see Fig. 2) to which its axis is perpen- 

 dicular; the capillary metal tube of the tap is connected with the 

 glass capillary by means of a well sealed metal sleeve. 



Fig. 1. Testing Softening 

 P01NT.S OF Asphalts. 



Fig. 2. Det.^ils of Tap T. 



To perform a test the sample of bituminous material and the 

 clean plug of tap T are warmed in a steam oven for about 10 

 minutes so that the material becomes somewhat plastic ; a piece 

 of the softened material is then pressed into the wider end of the 

 bore of the warm tap plug, as expeditiously as possible, with a 

 small spatula, until the bore is filled throughout and a little ex- 

 trudes from the other end; the tap plug is then allowed to cool 

 and the excess of material removed carefully so as to leave 

 the exposed surfaces of the material flush with the metal at 

 the end of the bore. The plug is then refitted into its seating in 

 the barrel, being previously lubricated if necessary with a smear 

 of glycerin. From the principle of the test it is essential that 

 the narrower end of the bore should face downwards and be 

 directly above the free opening of the barrel of the tap. Suffi- 

 cient mercury is present in the U-tube to reach approximately 

 to the equator of the bulb A when the pressure is the same in 

 each limb. Air is then forced gently through the tap S until 

 the U-tube, acting as a manometer, indicates an excess internal 

 pressure cf IJ^ inches of mercury between the two limbs. The 

 tap S is then closed, when, if the apparatus is properly fitted, 

 the internal pressure remains constant. On warming the me- 

 dium in the bath, with the usual precautions, a temperature is 

 finally attained at which the air pressure is sufficient to cause 

 the complete extrusion of the core of bituminous material 

 through the narrower end of the bore of the tap plug; the at- 

 tainment of this temperature is indicated sharply by the sudden 

 rise of the level of the mercury above B, and the reading of the 

 thermometer is recorded as the softening point of the material. 



The flat grooves cut at the two ends of the tapered bore 

 of the plug of the tap T not only facilitate the filling of the 

 bitumen and enable a considerable degree of accuracy in fixing 

 the length of the bituminous core, but also, at the narrower 

 end, provide a convenient space to receive the extruded bitumen 



so that the subsequent removal of the plug from the barrel is 

 possible without difficulty. 



The bulbs A (diameter approximately 1'4 inches) and B (di- 

 ameter approximately H inch) are so arranged that any expan- 

 sion of the air enclosed between T and B, due to heat received 

 from the bath despite the interposed screen, causes no appreciable 

 alteration in the difference between the mercury levels. The 

 pressure, therefore, is practically constant until the extrusion 

 of the bitumen at the end of the determination. For a second 

 test it is merely necessary to remove the plug of tap T and to 

 clean it with a camel-hair brush (or a piece of filter paper) 

 moistened with carbon bisulphate; the apparatus can therefore 

 be kept fitted up ready for immediate use. Any gradual dis- 

 coloration of the heating medium is of no consequence to the 

 performance of the test. The construction of the tap T in metal 

 facilitates the transference of heat to the central core of bitumen 

 and so reduces the "lag" of its temperature behind that recorded 

 by the thermometer. For the heating medium in the bath, gly- 

 cerin is generally convenient. As the commercial products of 

 the asphalt type have generally been already well mixed when 

 in a fluid condition, the smallness of the sample tested is not 

 detrimental. 



The results obtained with the apparatus described above are 

 generally higher than those obtained with the Kramcr-Sarnow 

 method, and the essential difTerence between the two methods 

 of testing the softening is reflected in the fact that, although 

 both methods give concordant results, we have found the differ- 

 ence between the results of the two methods for various ma- 

 terials to range from 5 degrees to 30 degrees C. 



In the following table is given the range of the readings 

 obtained with various commercial samples which were tested 

 repeatedly with the described apparatus and by the Kramer- 

 Sarnow method ; the first four samples were probably of 

 gilsonite, whilst the fifth was of a coal-tar pitch. 



Softening Points. 



Above Extrusion Kramer-Sarnow 



Sample. Method. Method. 



Degrees C. Degrees C. 



1 155—157 138—140 



2 145—147 127—129 



.1 146—147 124—126 



4 179— ISI 148—150 



5 94— 95 87— 88 



It is evident that the apparatus described above will also be 

 of very considerable utility for the comparison of the softening 

 points of other materials, such as gutta percha, balata, etc., 

 which exhibit a similar gradual softening when heated. The 

 relative behavior of various grades of gutta percha and balata 

 towards heat is of great importance for some purposes. On 

 account of the lack of adhesion between glass and gutta the 

 Kramer-Sarnow method is not satisfactorily applicable, whereas 

 our experiments using the method described above have given 

 clear indication of its trustworthiness for this additional pur- 

 pose. The significance of the test is manifest from the fact 

 that although cons'stcnt results are obtainable with various com- 

 mercial samples, the softening temperature observed ranged 

 from 101 degrees C. for a sample of washed raw balata, to 

 190 degrees C. for a commercial sample of so-called "pure gutta." 

 It is essential, however, that the portions used for the test should 

 previously be rendered air-free and dry. In making these experi- 

 ments the same "head" of mercury was used as was mentioned 

 earlier for asphalt materials, but in the comparative examination 

 of balatas or guttas it might be advisable in some cases to apply 

 a greater pressure. 



Dr. L. J. H. Stadhouder has discovered a method of co- 

 agulating latex without making use of any coagulating mate- 

 rial. Samples of his rubber are being tested scientifically at the 

 Central Rubber Station at Buitenzorg and also at the Nederland- 

 Indisch Caoutchouc Fabrik at Bandoeng. 



