June 1. 1920.] 



THE INDIA RUBBER WORLD 



581 



What the Rubber Chemists Are Doing. 



c 



CARBON BLACK.' 

 HETHOD OF MANUFACTURE. 



.ARBON BL.vCK. as knowii to the Amt-rican trade, is the fluffy, 

 elvety black pigment produced by burning natural gas 

 ■ilh a smoky flame against a metal surface. It is en- 

 tirely different in physical characteristics from lampblack, which 

 IS made by burning oil or other carbonaceous material with 

 insufficient air for complete combustion and collecting the smoke 

 in settling chambers. Lampblack is gray in contrast to the 

 deep black of carbon black, and often contains considerable 

 quantities of cmpyreumatic matter. 



The process of manufacture used to the greatest extent at 

 the present time is the so-called channel system, in which the 

 black is deposited on the smootli under-surface of steel chan- 

 nels by lava-tip burners set at a distance of three or four inches 

 below the channel. The channel irons are usually built up in 

 tables of eight, sometimes 100 feet long, and are given a slow 

 I reciprocating motion which scrapes the black deposited on 

 them into hoppers from which it is carried by screw conveyors 

 to the packing house, where it is bolted and sacked. The mech- 

 anism is enclosed in sheet-iron buildings in order that the 

 amount of air may be regulated. Varying the amount of air, 

 speed of scraping, and pressure of the gas controls the quality 

 of the product. The shape of the burner and distance from 

 the collecting surface also affects the quality of the black, but 

 these are constant for any one plant. Other similar processes 

 differ only in the nature of the collecting surface and burners. 



THEORY OF FORMATION OF CARBON BLACK. 



When natural gas burns in an incomplete supply of air. the 

 carbon is liberated, not as a result of preferential combustion 

 of hydrogen, but as a direct decomposition of unburned gas in 

 the heat of the flame. According to Bone' and co-workers, 

 combustion takes place in steps as a result of hydroxylation : 



A B o.xidalion via C 



uxidation H2:C:(0H): 

 CH. >'H3:C.0H >-H:!:C:0+H:.0 



-^30 



+ 59 



Q t C t 



c + m-- CO -f m-. CO + h.- 



I A' B' C 



It is evident that the tendency is always to run from A to C. 

 ^\'hen the proportion of methane to oxygen is CH, : O; the 

 reaction goes from A to B to C to C. If the ratio is 2 CH, : 

 Oj or higher, only a part of the methane can be oxidized through 

 the reaction A to C and part is decomposed at A by the heat 

 evolved in the A to C reactioii. The lowest per cent of 

 oxygen in which a methane flame will burn is 15.6 per cent. 

 Carbon will be evolved only in the inner part of the flame, 

 ' where the oxygen supply is low but where there is sufficient heat 

 to break up the methane, and the per cent of carbon to be ex- 

 pected by the incomplete combustion of methane is low. Gases 

 rich in ethane and the higher homologs produce greater yields 

 by this process. 



The function of the cold surface is to cool the liberated car- 

 bon in the flame sufficiently to prevent its combustion. This 



' "Carbon P.lack, Its Properties and ITses." By G. St. J. Perrott and 

 Reinhardt Thiessen. Chemical Kcsearcii Laljoratory, Bureau of Mines Ex- 

 lierimentation, Pittsburgh, Pennsylvania. Presented at the mcetinK of the 

 .\nierican Chemical Society, Philadelphia Pennsylvania. September 2 to 6, 

 1919. Published by courtesy of the American Chemical Society. 



-Transactions of the Royal Society. London, 215 (1915), 275. 



permits the use of a sufticiently hot flame to give carbon un- 

 contaminatcd with hydrocarbons or their partial oxidation 

 products and produces a Ihiely divided material which has been 

 prevented from agglomerating by the sudden cooling. It is 

 evident that there must be an optimum temperature and an 

 optimum position for the surface in the flame. Too cold a sur- 

 face may prevent the ma.ximum separation of carbon ; too hot 

 a surface will allow too much carbon to be burned and may 

 change the properties of the carbon which remains unburned. 

 The temperature of the channels in the present processes is 

 about 300 degrees C. 



USES OF CARBON BLACK. 



In order of importance the uses of carbon black are as fol- 

 lows : 



1— Printers' ink. 



2 — .-Kutomobile tires and other rubber goods. 



3 — Black and gray paint and enamel. 



4 — Stove polish. 



5 — Other products — such as phonograph records, carbon paper, 

 crayons, typewriter ribbons, black and gray paper, glazed paper, 

 tarpaulins, black leather, bookbinders' board, marking and stencil- 

 ing inks, rubber sheeting and clothing, hard rubber, anilicial stone 

 and black tile, Chinese and India inks, fireworks, insulating 

 materials, crucible steel, case hardening. 



The amounts used by these industries in 1918 were approxi- 

 mately as follows : 



Pounds. 



Printers' ink 10,tKX).OIX)~12.000.(M10 



Rubber goods 20.000,000- 



Stove polish • • 4,tX)0,000- 5.000.000 



All others 1.000,000- 



Besides this, in normal times probably 13,000,000 pounds are 

 exported. 



RUBBER TIRES. 



Prior to 1914 little carbon black was used by the rubber in- 

 dustrj- and then onlj- in small amounts as a coloring material. 

 Little distinction was made between carbon black and lamp- 

 black, the two compounds being used indiscriminately. At this 

 time, due partly to the stimulus aft'orded by the rising price 

 of zinc o.xide, it was found that carbon black could be used in 

 very large amounts as a filler for rubber, with a correspond- 

 ingly smaller amount of zinc oxide. Carbon black is used in 

 rubber in quantities of 3 per cent to 20 per cent by weight. 

 Many manufacturers claim very unusual properties for rubber 

 so compounded. It is said to increase the tensile strength very 

 greatly and to give increased toughness and resistance to 

 abrasion. It is believed by some authorities that the life of 

 the rubber is increased. Other rubber chemists are more con- 

 servative in their praises of carbon black and do not admit that 

 it possesses any properties which make it irreplaceable. 



From the point of view of cost, carbon black is much cheaper 

 than zinc oxide. Carbon black, absolute specific gravity 1.8, 

 suitable for compounding in rubber, can be procured for as low 

 as 10 cents a pound at the present time. Zinc oxide costs at 

 least as much and has a specific gravity of 5.8. Carbon black 

 evidently costs one-third as much as zinc oxide on a volume 

 basis, supposing that equal volumes were compounded with 

 the rubber, but in practice a greater volume of carbon black is 

 used than of zinc oxide so that the resulting mix with carbon 

 black contains less rubber per unit volume than the correspond- 

 ing zinc oxide mix. 



From a theoretical consideration, carbon black should be an 

 ideal filler for rubber on account of its extremely fine state of 

 division and the correspondingly large surface energy- devel- 

 oped when intiiuately mixed with the rublicr. It also serves to 



