582 



THE INDIA RUBBER WORLD 



protect the rubber substance from the effects of liglit and may 

 possibly retard oxidation. 



Whatever may be the true state of affairs as to the irre- 

 placeability of carbon black in rubber, an enormous amount is 

 now consumed by the rubber companies, probalily 20,000,000 

 pounds annually, and 10,000,000 pounds exported for foreign 

 trade in normal times. 



TESTING METHODS. 



The final test of the suitability of a black for a given purpose 

 is the actual trying out of the working qualities. In rubber 

 making a sample mix is made and the finished piece tested for 

 tensile strength, per cent elongation, toughness, and resistance 

 to abrasion. 



There are, however, a number of laboratory tests which are 

 of use in matching a standard sample. The tests most com- 

 monly employed are for tinting strength, color, and grit. It is 

 also desirable to determine moisture, ash, and acetone extract. 



TINTIITG STRENGTH. 



According to the American Society for Testing Materials, 

 tinting strength is "the power of coloring a given quantity of 

 paint or pigment selected as a medium or standard for esti- 

 mating such power." Tinting strength, then, as applied to car- 

 bon blacks is the measure of the ability of the black to im- 

 part a color to a definite weight of standard white. It depends 

 on the size of the particles and the specific gravity of the 

 black. In making the test, the black is always compared with 

 a standard black. 



Weigh out accurately an a sen.^itive balance, 0.100-gram of the 

 black to be tested and 10.0 grams of a standard zinc white kept 

 especially for the purpose. Transfer to a glass or marble slab and 

 add from a burette exactly 3.5 cc. refined linseed oil. Mix with 

 a palette knife and rub out thoroughly with the palette knife (or, 

 better, a glass muller) until no streakiness or difference of color 

 is observed, when successive small portions are spread out on a 

 clean piece of window glass and viewed from the upper side. It 

 is important that the rubbing out be thorough ; 10 minutes are 

 usually sufficient. Follow the same procedure with the standard 

 black. Then spread a small amount of each mix side by side on 

 a clean glass (a microscope object glass serves the purpose very 

 nicely). Examination of the samples from the other side of the 

 glass, particularly at the line where they overlap, will show the 

 difTcrence in tinting strength. 



To make a quantitative estimation of the tinting strength of the 

 sample as compared to the standard, more white is added to the 

 stronger mix until the colors match. A new sample of the 

 stronger black is then weighed out, using the calculated amount 

 of zinc white, and the process repeated until mixes of the same 

 color are obtained. If, for example, it was necessary to mix IS 

 grams of zinc white with 0.1-gram of the standard to match a 

 mixture of 10 grams zinc white and 0.1-gram of the sample, the 

 latter has two-thirds the strength of the standard. 



COLOR. 



By this term is meant the relative blackness of the material 

 when mixed in oil. 



To 0.3 gram of each of the blacks to be compared add 1.3 cc. 

 of refined linseed oil from a burette. Mix thoroughly with 

 the palette knife and spread side by side on a slip of glass and 

 compare the relative color by viewing from the upper side of 

 the glass. 



GRIT. 



Presence of gritty matter is determined by rubbing a por- 

 tion of the black under the finger or by placing a small amount 

 on the tongue and rubbing between the tongue and palate. 



CHEMICAL TESTS. 



It is occasionally desirable to make a few quantitative chemi- 

 cal tests of carbon black. A black containing more than 0.2 per 

 cent ash is probably adulterated with mineral black or charcoal. 

 An acetone extract over 0.1 per cent indicates adulteration with 

 a poorly calcined lampblack. Too great a percentage of mois- 

 ture is undesirable from the point of view of working qualities. 

 Certain blacks will absorb as much as 15 per cent of their weight 

 of moisture, making a total moisture content of 20 per cent 



or more. More blacks for ink making contain from 2 to 4 

 per cent of moisture, although certain blacks may contain as 

 high as 7 per cent. 



Moisture." A 1-gram sample of the black is placed in a 

 weighed porcelain crucible and heated for one hour at 105 

 degrees C. in a constant temperature oven in circulating dry air. 

 The crucible is then removed from the oven, covered, and cooled 

 in a desiccator over sulphuric acid. The loss in weight multiplied 

 by 100 is recorded as the percentage of moisture. 



Ash." The crucible containing the residue from the moisture 

 determination is heated gradually with a Meker burner (for 

 better, in a muffle furnace) to cherry-red (about 750 degrees C). 

 Ignition is continued until all the particles of carbon have 

 disappeared. The crucible is then cooled in a desiccator and 

 weighed, after which it is heated again for 15 minutes, cooled 

 in a desiccator, and reweighed. If the change in weight is 

 more than 0.0002-gram, the process is repeated until successive 

 weighings are constant to this figure. The weight of the crucible 

 and ash minus the weight of the crucible is taken as the weight 

 of the ash. 



Acetone Extract. A 2-gram sample is weighed into an 

 alundum or paper extraction thimble of 20 cc. capacity and the 

 extraction carried out for one hour, using any standaid ap- 

 paratus of the Soxhlet type. The weight of the residue after 

 evaporation of the acetone is taken as the acetone extract. The 

 extract for a pure carbon black is usually zero. 



SPECIFICATIONS. 



The Bureau has- received a great many inquiries in regard to 

 tests which a carbon black must meet to be suitable for use 

 in printing ink or rubber. 



The following specification represents the requirements for 

 the rubber trade. It should be realized that there are no hard 

 and fast specifications for carbon black, and that the test on 

 which a black stands or falls is the practical test. 



Carbon Black for Rubber Co.mpounding. 

 Chemical Tests. 

 Moisture — Less than 4 per cent. 

 Acetone Extract — Less than 0.5 per cent. 

 Ash — Less than 0.25 per cent. 



Physical Tests. 

 Grit — None. 



Tinting Strength — Not less than 90 per cent of the strength of 

 standard. 



Practical Tests. 

 Rubber mixes are made up containing equal weight of the 

 sample to be tested and of the standard. Mixes are cured under 

 exactly the same conditions. The finished sheet is tested for 

 tensile strength, per cent elongation, toughness, and resistance to 

 abrasion. 



CHEMICAL ANALYSIS. 



Carbon blacks consist of from 85 per cent to 95 per cent 

 amorphous carbon, 1 per cent to 7 per cent water, 0.5 per cent 

 to 0.8 per cent h3drogen, and from 2 per cent to 8 per cent 

 oxygen, present partly as CO and CO2, and partly as fixed 

 oxygen. 



BENZOL POISONING. 



Benzol or benzene is one of the most important industrial 

 poisons. Poisoning due to benzene may occur in the manufac- 

 ture of aromatic hydrocarbons, or in the technical use of prod- 

 ucts containing these hydrocarbons, for example, in the re- 

 moval of grease from goods, and in the manufacture of varnish 

 and rubber. The poisoning may be mild or severe, acute or 

 chronic, and may have fatal results. When man is exposed to a 

 mixture of benzene and air, he absorbs 80 per cent of the ben- 



