Val. ~17, No. 9
INDUSTRIAL A N D ENGINEERING CIIEMISTRY
936
Distribution of Carbon Black in Rubber Stocks' By Ellwood B. Spear and Robert L. Moore 'rzmMmoMzc
CAIIBOY c o . , I'ITTSDURGW, FA.
€$Eoptical deteriiiination
of the distribution of carbon bla& in tread stocks presents great difficulties because of the conipletcness wit,li which carbon black absorbs light rays. lleHe,cted light eaii be used to reveal structure only when the snrface of the object exliihits optical inhomogeiiciiy of sucli dimensions t,liat one particle or aggregate reflect,s light difrerciitly Eruni that of its immediate environment. Such cunditims, however, (io iiot exist in the case of tread storks coiitaiuing a relatively high prcentage of carbon black where the latter is w l l dislarsed. Time facts reduce our niethods to the employment oi traiisiiiit,ted light and, at best, an oil immersion objective. Uur difficulties arc by no ineaiis a t an end, however, for the thiiinest sections of blni:% stock trend obtainable with eveii the most efiicient iriicxotoiire are so opaque that little idea of the dist.ributioii of the carbon black is possible. Later in this article a niebliod ail1 be outliiied by which sections of tread stock may he obtained so thin that light can he transinit.ted througli and tile structure studied optically.
T
Properties of Carbun Black to BeKevealed by Microsections
Before taking up the method in detail let us discuss briefly what one would expect to see in thesc t.liin sections if we are to jndgc from our present knowledge of carbon blacks. Some of the carbon blacks most highly prized in the rubber iridustry for use in tread stocks have long been recognized to have a so-called reddish brown undertone. That is to say, if a white pigment such as zinc oxide i s tinted with the carbon black in an oil aiedium the resulting color is not a pure g a y , but rather a gray with a marked reddish brown tone. In the mauufacture OS printing inks it is often necessary to offset this brown undertone by the addition of a blue such as ultrainarine or Prussian blue. I,amphlacks, on the other hand, usually bare a blue or violetblue undertone and are frequently employed in rubber and paint vehicles where beautiful blue-gray efrects are desired. Bnother side OS the subject requires some comments at this point. It i s generally admitted that carbon black is a very finely di\-ided substance. It has been stated' that the probable average diameter of the particles is in the neighborhood of 0.1 to 0.2 &. Tliis means that a Parge portion of the partides cannot be sceii by means of the most efficient microscope, becanse their diameters are below t.he resolving power of the instrument. This stat.ement can be verified easily if the experimenter will but rub tliorougl~lllya small amount of moistened carbon black with tannic acid and suspend the resulting paste in water. A drop of the liquid thus produced appears almost optically clear when observed under a microscope using the oil immersion method and a magnification of 1500 to zoo0 diameters. A few particles can be seen, of course, showing that their diameters must be greater than 0.2 I.'. If, on the other hand, the same slide is observed by dark field illumination, the liquid will appear to be filled with tiny stars in rapid Brownian movement. This may be taken as evidence that in carbon black we have to do with particles varying greatly froni one another with regard to their re-
, Presented
belore the Division of Rvbbrr Cl~mislryst the 68th tin^ 01 the American Chemieal Society. Raltimore. Md., April tl to 10, l?l?i. 7 Endrcs, Tals J o v a a ~16, ~ . 1154 (18%).
spective diameters aiid that large nnrnbers of tliein tire iilt,ramicroscopicalin ske. The next question we have to deal with is t , l d of color. It seems difficult to conceive of carbon black as having any i:oIor except hlark. It 'is an experimental fact., howcver, that very thin sections of many 20 per cent mrlion Llacli tread stocks, where the pigment is well distributcd, are not blrtck at all by hnsmitted light. They Yary from reddish brown to yollow, rlrpending upon t,he thickness, the prevailing color being redrliili amber. The larger particles of the pignieut appear hlack, of course, inzause tliey completely :disorb nntl do not transmit light rays. The colloidal portion, on the i)tlier hand, transniits light and the resulting color i s yellow or reddish yellow. A microscopic slide of a sufficiently thin soctiori of tread stock should show black part,icles of carboil black scattered throughout a yellow, reddish yellow, or reddish brown matrix. This is illustrated in Figures 1 and 2. Tlie visible particles of carbon appear much mnre clearly defined in the microscope to the eye than they do in the pliotoniicrograph. The camera enlarges individual particles until they appear more like agpegates with somewhat indefinite boundaries. The hlack part,ieles are imbedded in a reddish yellow mass. It >vi11 be noted that there are comparatively I p w
Flsure 1
particles of carbon large enough to be resolved by the objective of the microscope. The magnification is 1500 diameters. Nolc-It m a y be of interest t o t h o s who do not hive access to firstCIPSP photomicmgraphie equipment thot the accompanying photographs were isken with a W,t. x +5/. %artman folding camera. The bellow were pulled out to give the correct focua and the lens of the cemrra was placed where the eye would ordinsrily be when obaerviilg an object in the microscope. The lens of the camera was attached t o the eyepiece of the microscope by means of an adaptor made of a piece of black i r e d ntock that had been cold-cured into the form of a suitable rubber band.
Figure 3 is a photomicrograph of a lampblack stock eontaining 14 volumes of lampblack on 100 volumes of rubber. Here particles, or possibly aggregates, of carbon can be Been separated by spaces that appear to he almost clear. It is
IhlD(iST:TtlIAL A N D ENGINBBRI NG CHEXISTR Y
Septcirihw, 19Z
937
~ltaget,lierpnrhnblc t,liiit these spaces are filled wit11 colloidal Inmpblack in rubber, nltl~ouglicxperimeuts with nionochromat.ic light are not sufficiently conclusive at present to remove all doubt as to the validity of t,hisst,atement. The undertone, or color of lampblack by transmitted light, is blue, and blue in thin layers does not show prominently under high ma,gni-
they have worked. This may also be said of the lampblack used, which was a11 expensive (21 cents per pound), highgrade type employed in the manufacture of first-class paints and enamels. Disregarding the color for the time being, the microscopic picture of carbon black stocks is very different from that of
Figure 2
Figure 3
Iii:ation FOTinstance, thin layers of blnc fountain pen ink :~ppearto be quite colorless a t 1000 to 1500 diameters when
