August 1, 1921 



THE INDIA RUBBER WORLD 



823 



may run as hiuli as 85 per cent. The specific gravity of this 

 stearin pitch will average 1.000 specific gravity. 



It has been found to be impractical to produce these pitches in 

 such grades that the melting point would be about 150 degrees C, 

 as the material in the stills is decomposed at a point far below 

 that temperature. One of the unique applications of stearin pitcli 

 has been in the manufacture of rubber-coated fabrics. Coal-tar 

 pitch if used alone would give a coating too brittle for practical 

 purposes, but if a certain percentage of stearin pitch be used in 

 the compound, this objection is overcome and the whole compound 

 can be worked out without much difiiculty. 



LABORATORY TESTS 



The laboratory tests used to determine the fitness of these mate- 

 rials for use in rubber compounding include a determination of : 



(1) the melting point; (2) penetration of a needle point; (3) non- 

 -volatile mineral matter ; (4) matter volatile below 290 degrees F. ; 



(5) specific gravity; (6) free carbon; (7) fi.xed carbon; (8) mat- 

 ter soluble in various organic solvents ; (9) saponifiablc matter. 



LABORATORY METHODS OF TESTING 



1. Melting Pni-VT (Bv the B.xli, .\nd Ri.\g Method). .\ ring 

 ■of metal, 6 millimeters thick and 16 millimeters in diameter is 

 filled to the level with solid asphalt. This can be done by filling 

 the ring rounding full with asphalt, cooling in cold water for a 

 few minutes, and then cutting off the excess with a hot knife. 

 There shall be no air bubbles inside the ring. A steel bicycle 

 iall 10 millimeters in diameter, weighing S grams, is then placed 

 in the center of the asphalt. The ring is hung on a level with the 

 rnercury bulb of a thermometer, the point being immersed in a 

 ieaker of water. For asphalt pitches melting below 50 degrees C. 

 the water should have a teniperature of about 5 degrees C. at the 

 start. For pitches melting below 65 degrees C, about 20 degrees 

 C. at the start. For pitches melting above 65 degrees C, about 

 25 degrees C. Heat the water and beaker at the rate of about 

 5 degreev C. per minute. Note the temperature at which the ball 

 and pitch begin to drop rapidly on leaving the ring. This is 

 visually about one centimeter below the bottom of the ring. This 

 temperature is recorded as the melting point or "dropping point" 

 of the pitch. 



Melting Point (Bv the Cube Method). This method was 

 ■developed in the year 1900 to he used on coal-tar pitches. It is 

 not well suited to pitches of the type of stearin pitch, which are 

 tacky, slimy or sticky at the point of melting. Cubes of pitch are 

 made in a mold having a hole one centimeter square. A beaker 

 ■of about 600-cc. capacity is used. The cube of pitch is suspended 

 by means of a wire, so that it is exactly 2 cm. from the bottom 

 of the beaker. The water is heated by a Bunsen burner at the 

 rate of about 5 degrees C. per minute. For high-melting-point 

 pitches the water is replaced with glycerol, cotton oil, or calcium 

 chloride solution. When the cube of pitch drops and just touches 

 the bottom of the beaker, the temperature of the surrounding liquid 

 is noted, and this is recorded as the melting point (by the cube 

 method ) . The thermometer should be so placed in the beaker that 

 the bottom of the mercury bulb is on a level with the cube of 

 pitch. 



2. Penetration. The consistency or hardness of pitch can be 

 determined by noting how far a point or surface of an instrument 

 will penetrate the pitch, provided the indenting object has a defi- 

 nite weight, the pitch has a definite temperature, and the point or 

 surface is allowed to act on the surface of the pitch for a given 

 number of seconds. The instruments used for this test are desig- 

 nated as penetrometers, and ditTercnt types have been devised by 

 Dow, and Bowen, and Richardson. This apparatus must not be 

 confused with tlie plastometer, which is used to measure the extent 

 to which a small flat or hemi-spherical surface will deform a sub- 

 stance which offers elastic resistance. The plastometer can be 

 used on vulcanized rubber goods. 



3. Non-volatile Mineral Matter. The ash contained in 

 pitches varies from about one per cent in pitches of hardwood. 



coal-tar and petroleum, to fwe per cent in stearin pitch and 22 

 per cent in Venezuela asphaltums. For making the test, a piece 

 of asbestos '^^-inch thick is provided with a hole large enough to 

 take a porcelain crucible. The pitch is weighed ofT in the crucible 

 and the mass is ignited with a small blue flame. 



4. .Matter Volatile Below 290 Degrees F. .^s the majority 

 of rubber compounds are vulcanized between 280 and 3(X) degrees 

 F. (138 and 149 degrees C), it is important that compounding 

 materials should contain nothing which will volatilize below those 

 temperatures. If a pitch has been "cut back" with a low-boiling 

 oil, the vapors of this oil may volatilize during vulcanization and 

 cause "blowing" or "blistering" of the rubber product. Effects 

 such as these are very much desired in the production of sponge 

 rubber, but for other articles they merely cause defects and prod- 

 ucts with blisters. 



5. Specific Gravity. The weight of the pitch per culjic foot is of 

 interest and importance to the rubber compounder, because pitches 

 of high specific gravity will naturally increase the weight of the 

 finished rubber product "per cubic foot." Generally, a pitch with 

 a specific gravity of less than 1.300 will 1^ preferred. The test 

 may be made in water kept at a temperature of 20 degrees C. By 

 determining the weight of a sample of about one cubic inch, in air 

 and then in water, one can by simple arithmetic calculate the rela- 

 tive weight of the pitch. 



6. Free Carbon. A sample of the pitch weighing less than 

 five grams is weighed off accurately, and placed in a Schleicher 

 and Schill fat-extraction thimble, in the Soxhlet cup of an Under- 

 writers' extraction apparatus. Pure toluene (toluol) is used for 

 the extraction. The process is carried on until the major por- 

 tion of the solubles has been extracted, and then the sample is 

 extracted with coal-tar benzene. When the benzene coming from 

 the thimble ie no longer colored, the extraction is complete. The 

 thimble is dried in a steam bath and finally in an oven at 110 de- 

 grees C, after which it is weighed. 



The amount of free carbon varies all the way from one per cent 

 in hardwood pitches to 40 per cent in coal-tar pitches. This is 

 an item that does not appear important at first thought. Now if 

 it were possible to offer to the rubber goods manufacturer a ma- 

 terial which contained, say, 50 per cent of a "fine carbon," to- 

 gether with other substances which would not detract appreciably 

 from the valuable physical properties of the rubber compound, 

 such a pitch product would be of considerable interest. It would 

 be a great step forward if "fine carbon" could be incorporated 

 with rubber compounds while in the form of a pitch. One of the 

 technical considerations noted by the producers of pitches has been 

 to produce a pitch of high carbon content which would at the 

 same time have a suitable melting point, as the pitches now being 

 made with melting points of 30O to 350 degrees F. are not actually 

 "high carbon" pitches. 



.Although benzene and toluene have been found to be the solvents 

 best suited for determining "free carbon" in coal-tar pitch, the 

 makers of stearin pitch usually use carbon disulphide for this test. 

 It has in fact been observed that various solvents act differently 

 on any one pitch, so that it becomes necessary to observe and note 

 the action of each. 



7. M.\TTER Soluble in Organic Solvent.s. The volatile or- 

 ganic liquid solvents which have been used at various times to 

 obtain a closer knowledge of the constituents of pitches include: 

 (1) benzene, (2) toluene, (3) xylene, (4) petroleum naphtha, 

 (5) chloroform, (6) carbon disulphide, (7) acetone. 



It will be found for example that about 92 per cent of a hard- 

 wood pitch is soluble in acetone. The matter soluble in carbon 

 disulphide is about 85 per cent of stearin pitch and 95 per cent 

 of I'tah gilsonite. The petroleum naphtha used for extraction is 

 that known as 85 to 90 degrees Be. (or 0.650 specific gravity). 

 By some chemists, the matter insoluble in the principal organic 

 solvents is recorded arbitrarily as "free carbon," as it is a well- 

 known fact that carbon is unaffected by practically all of the 

 solvents. 



