Dispersibility of Gas Black I. Methods of Measuring Gas Black Dispersion RAYMOND P. ALLENAND FRANK K. SCHOENFELD, The B. F. Goodrich Company, Akron, Ohio
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H E need of a method by which the degree of dispersion may be measured has long been felt by those who mix gas b l a c k i n t o r u b b e r . Degree of d i s p e r s i o n may be defined as the completeness of wetting of the pigment by the rubber or, to e x p r e s s it more exactIy, the ratio of the pigmentrubber i n t e r f a c e to t h e total available pigment surface. In this paper t h e d i s p e r s i o n of channel gas black only will be considered.
EXAMINATION OF SURFACES
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Previous investigators have used the nature of the surface, the appearance of microsections, and various physical properties of the stock as a measure of the degree of dispersion of gas black in rubber. Here it is shown that a close relation exists between the nature of a torn surface of gas black stocks and Ihe appearance of microsections of the stocks. The relationship is so close that the degree of dispersion which is basically shown by microsections m a y be measured by examining the surface. As a practical working tool standard samples of gas black stocks have been prepared and rated in percentage dispersion. I t is also shown that the character of gas black agglomerates in rubber varies f r o m a hard, rigid type to a soft, easily dispersed variety. A microscopic study of those agglomerates aids in understanding the properties of stocks.
As mixing proceeds and more a n d m o r e p a r t i c l e s are wet by rubber, the surface appearance of the stock will gradually change from the dry, dull appearance of aas black to the dossv appearance of masticated rubber. The high surface gloss is enhanced by the intense opacity of a gas black stock and, because the reflection comes only from one plane, the surface appears almost metallic ( 2 ) . I n the early days of the rubber industry and persisting to the present day, practical mill room men have judged the perfection of mixing by the sheen of the surface of the stock. This criterion applies particularly to gas black stocks. If the surface of the stock appears glossy and black, like a highly polished shoe, it is judged that the pigment is well dispersed. This practical method of judging the degree of dispersion is useful and reasonably accurate, although it does not tell the whole story; it has been mentioned occasionally in the literature. Wiegand (18) states: “In some cases, practical operatives have been able to recognize even small departures from proper dispersion by the appearance of the calendered or extruded sheet. There is, namely, a loss in smoothness and shine. This test is, however, not unambiguous and may easily lead to false conclusions.” I n attempting to determine the distribution of the black inside of a piece of stock by examination of the surface, gloss is a valuable criterion, but there is another equally important factor-name1 y, the number and size of undispersed or almost completely undispersed lumps of black. The glossiness is a measure of the completeness with which gas black has been wet by the rubber; the undispersed chunks, large and small, are a measure of the amount of gas black which is still not at all or only slightly covered by rubber. It is not accurate to judge dispersion in any piece of stock on the basis of either of these factors alone. For example, a piece of stock may have a high gloss but still contain many undispersed chunks of black. This indicates that part of the gas black has been thoroughly wet by the rubber but much of it has only barely started to disperse. I n other instances a piece of stock may have a dull surface which close examination will show is full of small lumps of black, all of which, it may be imagined, have started to disperse. Only a small propor-
t i o n of t h e b l a c k has b e e n thoroughly wet by the rubber, but a large proportion has been partially wet. Many others have recognized the i m p o r t a n c e of the undispersed or only partially dispersed chunks of gas black in judging dispersion. Wiegand (18) says that “even moderate departures from uniform d i s p e r s i o n can generally be detected with a 20power hand magnifier on a torn surface of the mixed stock-that is, before the cure.” Grenquist (11)studied the surface lumps and related their size and frequency to significant physical p r o p e r t i e s of the stock. He states: “The dispersion was determined by means of the microscope on freshly cut surfaces of mixtures vulcanized and unvulcanized * * *. Magnified about 300 times, the aggregates of gas black appear like a nonhomogeneous black mass, while on the smoother and more homogeneous surface of the rubber the reflection is so increased that the field remains lighted.” Grenquist cut his samples “with very sharp scissors,” which in the authors’ opinion does not give a surface as satisfactory as one made by tearing. Grenquist points out that the examination of the surface is a rapid method of judging dispersion and that a large surface can be studied; however, he says that “it is evident that this method is not of use in the study of the actual dispersion of the particles, but it appears to be excellent for the study of the mixing process.” Grenquist characterized the dispersions as: very bad, bad, passable, good, very good, excellent. Shepard (15) also discusses the surface examination of gas black stocks. He states that “the microscope is of distinct value in determining the suitability of a black for rubber reenforcement. Owing to the difficulty in preparing rubber sections for observation with transmitted light, the color tones have found little application in controlling the quality of blacks. “Surface examination by reflected light * * * has been found of distinct value in estimating the dispersibility of a black and in evaluating various mixing procedures.” Shepard shows two photomicrographs, taken by H. P. Coats, showing poor and good dispersion. The references which have been given show that many people who have worked with gas black stocks have seen the possibility of judging dispersion by studying the character of the surface. By visual or microscopic examination they have used the surface qualities of gloss and lumps as a measure of dispersion.
EXAMINATION OF MICROSECTIONS As a measure of judging the dispersion of gas black, the examination of microsections has also engaged the attention of many workers.
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This method is obviously limited to
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sniallcr sections and is subject to tlie criticism that large specificsurface,it could conceivably he used asameasureof disnumbers of observations must he made if representative re- persiou in which undispersed agglomeratee act as large particles. Hhepard ( f a ) points out that flex-cracking is related to disresults are to he obtained. Pohle (14) has done valuable work with microsections. Depew and Ruby (5) show photo- persion. This property is obviously most valuable as an micrographs of sections made by freeoiiig with carbon dioxide indication of poor dispersion. It may be concluded that many physical properties are reand cutting on a microtomc which illustrate good and poor lated to the denee of diwersion. and the disnersion of c a r b o n black. Hauser (15) . . mentions that in m i c r o s e c t i o n s made as proof of such a relationship would be of sciendescribed by Uannenherg (4)the degree of tific and practical intercut. Irrespective of the objectives sought by those who have predispersion may he measured either by countviously s u g g e s t e d physical properties as a ing the a g g r e g a t e s or in some cases by measure of dispersion, no one is interested in colorimetric methods. Grecn (7), in a paper on the stretching of measuring dispersion by first measuring some rubher stocks, gave some photomicrographs physical property, hut everyone is interostcd in first measuring the degree of dispersion and wliich showed the stretching of agglomerat.es from that concluding that the tensile, tear that w c ~ cpresurnahly of carbon black. In resistance, flex cracking, or tread wear will be a paper (8) on a met.liod of inirking microsntisfactory, if other factors are constant. st:ctions he showed photomicrographs of undisperscd carbon black. RELATIONBETWEEN MICROSCOPIC AND In another paper (0)Green stated, as a reb~ACROSCOPICAPPEAllANCE OF GAS sult of examining microsections, that “all gas BLACK STOCKS black contains large lumps of undispersihle material wliich are probably adamantine,” If small pieces are torn from two gas black and that “mor incoruoration of pas black will stocks and these torn surfaces are examined show as dense, black lumps.” F , 1. ~rronN ~s~~~~~~~ ~ ~ with the naked eye or under a low-power OP Two Gas BLACK Smms m i c r o s c o p e , a difFerence may he found in Spear and Moore (IO) also d c s c r i b e a method of making microsections and show ( X 2) their a p p e a r a n c e . Two such stocks, so p h o t o m i c r o g r a p h s of s o m e c a r b o n saleeted that there is a dilference, are shown black stocks, hut in their published pictures the agglomerates in Figure 1. One of these is extremely glossy and relatively are rather obscure. Grenquist (IO) showed some photomicro- free from large undisperscd agglomerates; the other has a dull graphs and discussed the dispersion of gas black in rubhcr, but surface, and on the surface are many undispersed lumps. he worked only with stocks containing a low concentration of Comparative examination of thin sections of these two black. In another paper (11) Grenquist describes micro- samples, magnified several hundred times, illustrates how sections made of stretched stock by freezing and cutting on a faithfully the cliaracters of the stocks are again shown. microtome, and he showed the formation of vacuoles chiefly Figure 2A is a microsection of the better dispersed stock. Almost no agglomerates of gas black are observed. I n the picaround aggregates of black or other pigments. ture of tlie section from tlie other stock (Figure ZR), many PROPERTIES INDICATWE OF DISPEKSXJN PBYSICXL agglomerates of gas black are present. From the general Aside from microscopic methods, certain physical proper- lack of smoothness which the stock of Figure 213 possesses, ties have heen recommended to indicate the degree of rlis- the impression prevails that even the gas black which is in persion. Wiegand (18) states: “I think the most sensitive part covered hy the ruhher still does not form a coherent index (for good dispersion) is tensile strength. Within the mixture with it; the mixture has an entirely different charpast year I have hnd personally an opportunity of ohsening acter from that shown in Figure 2A. This impression is fnrseveral cases where improved dispersion notably increased ther heightened by the appearance of the edges of the two the tensile, in some cases up to 25 per cent.” Grenqnist sections. I n Figure 2A the edges are smooth, straight, and (12) also related dispersion to tensile. Wiegand further coherent; in 213 the edges of the section are jagged, irregular, stated that “another way is throng11 irregularity in breaks. and give the impression that the stock is noncoherent. If the Altlrougli not a refined or quantitative mca.ns of measuring torn surfaces of other samples of stock are compared with bad dispersion, it is nevertheless an excellent qualitative microsections made from them, close relationship between the warning.” Endres (6) discussad the effect of agglomeration surface appearance and the appearance of microsections will on physical properties and showed some photomicrographs he found in each case. These sections were made hv the method develoued by one of snecial carhon blacks. of the authors-(l). -This Inspired hy a paper of method is used because it Stamberger (17) on the israpid and effective hut r a t e o f d i f f u s i o n and more p a r t i c u l a r l y besettling of different pigcause by its use the coments, Carson and herent character of the Sebrell (3) centrifuged stock and the nature of ruhher cements containthe agglomerates can be i n g d i f f e r e n t carbon better s h o w n than b y hlacks and found a relamethods which employ a tion hetween the amount microtome. of black left in suspension and the modulus at ESTABLrSHMEhT OF A 500 per c e n t . W h i l e QUANTITATIVE their m e t h o d required STANDARD Derfect disnersion of the Fmum 2. MICEOSECTION or THE Two ST~CKS OF FIGURE 1 ( x 200) T h e information blaeksandwasindicative A. Better dispersed stock B. stock black hggionieratM g a i n e d by the m i c r o only of particle size or of I
I ?i 1) C S T H I 4 1.
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saipic rwtrniiiatimi of thin sections or by the cxamiuatioii of the s w f a c e of it piece of stock is siibjcctire arid not strictly quantitative. However, in practice and in the actual study of mixing proccdures, the appearance of either tom surfaces or microsections is a drpendablc nicthod in the hands of an expcrimceil oliserrer. In order io make blie metliotl of still more wrricr, it was i‘uund clesirahlc to express the surface appcar-
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sei.\ation of sectioiis of varying ttiickncss, it rvoulrl he conrludcd that the opaque areas contain from 50 to 90 per cent of channel black and from 50 to 10 per cent of rubber. (2) If tile ratio of areas is used as a hasis for quantitative measurement, thc importance of surface gloss is not sufficiently empliasized. When using these standards it lias been found tirat different observers can cheek each other with accuracy. Certain precautions are necessary in preparing the samples of stock which ate to be coinpar& ritli the standards: 1. The s;implr should 1,e tarn by gripping firmly on each side of a dit cut in t h e stock and pulling with little stretching as possible. This procedure is recommended hoeauve iii the P the charackr of tlre stock is showii hettrr t,han by 8 cut mriace. A cutting tool smears rubber over the surface o i gas black lumps and, fiirthcrmom, makes ii poor piree of rulrher sbock ilppear more cohcrent thsii
FIGURE
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ST.&NII.AHD S.AMPl.liS OF G A S I(1.1CK
STOCKs
axice in terms of actual numbers. For erarrrplc, tlie better dispersed ample of E’iwre 1 was rated 90 per cent, the poorer, 70 per cent. ;Uthougii it vias found p8sible for one observer, examining many samples each day, to duplicate and clreek his judgment, the need for a set iri standard samples in the interest of accuracy was apparent. The standards wliicli were prepared are niixturesof rubber and gas black only (65:35), prepared by mixing for different times in the improved Schiller mixer (Figwe 3). The sainples themselves are torn from the resulting stocks arid for permanency are sealed in nitrogen io small, hard-rubber boxes, one of wliich is shown in Figure 4. The face of the sample is irrcliried a t a slight angle to the glass top so that the retiectiim froin the glass will not interfere with the surface appearance of the sample itself. The objection may be raised that the percentage dispersion expreaed by tliesc nunrbers has no quantitative meaning. It is freely admitted that the quantitative basis is not as exact as one could wish. However, so far as is known, there is at present no rigidly accurate method of determiniug the amount of channel black which is completely covered by rubber. One method wlriclr lias h e n employed to measure more exactly the degree of dispersion of black fias been to determine the ratio of areas covered by opaque aggkimerates t o the total area under observation. This ratio has been determined for these standard samples, hut the prepared set does not follow these measurements exactly. For exarrr~ile,a drawing showing
tearing revesl* it to he. 2. The stock from which t,he sample is tom must not bc warm. 3. Tho sheet should preievably he 6 to 8 nun. thick. 4. TJiiifonn procedure should be followed in takin samples from the ruhher mill; for example, the samples shoufd always be taken from the mills at t,he same stage of the mixing operation.
The use of these standard samples as a means of giving a quantitative value to the degree of dispersion of stocks has proved useful in studying factory operations. In the next paper of this series some oi its uses in the study of experimental mixing will be discussed.
Froune 5. OOTLINESOF OPAQIJE Awx,oMen*TrSs IN 65 Pen CENT SIMPLE
(X 100)
TYPX.3 OF .~!XLoMB;RATXS I N
CHANNEL (;AS
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