Microscopic Examination of Rubber and Other Solid Technical Products FRANK H. RONINGER, JR.,~ Firestone Tire & Rubber Company, Akron, Ohio
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HE preparation of mate-
need no further hardening treatA method for preparing sections for microrials used or made in a ment, but if they contain textile scopic examination is described. The method is rubber factory for microfibers, these can be more successof general application to raw materials and semiscopic e x a m i n a t i o n presents f u l l y p o l i s h e d if t h e y a r e finished or finished products of the rubber innumerous difficulties. A part of supported and made brittle by dustry, and is of especial value for preparing secthe raw materials and nearly all the m o l t e n s u l f u r cure. To of the semi-finished and finished avoid blowing, it is best to give tions of composite materials, whose constituents goods are t o u g h and e l a s t i c , uncured s t o c k s a preliminary diJer widely in hardness. The samples are opaque, and composite in strucquick press cure such as that hardened by curing and impregnation with molten ture. Rubber itself, cured or unused for gravity samples. This sulfur and are then polished by variations of the cured, is very difficult to section preliminary press cure does not usual metallographic methods and examined by and when combined with copper seem to affect the d i s p e r s i o n or iron wire, cotton, asbestos, or materially, but it will seriously reflected light. An application of the method to coarse mineral fillers, it is almost disturb directional effects where the quantitative estimation qf the degree of disimpossible to prepare. Allen ( 1 ) these are of importance. persion of pigments in rubber stocks is outlined. has reviewed previous work and The samples to be hardened Illustrations show some applications of the method detailed his own method. Steele are cut with one suitable face of examination to a variety of products. ( l a ) ,Pohle ( 7 ) ,and Grenquist (ti) r o u g h l y p l a n e and are then have made additional contribuimmersed in a bath of molten tions. All these methods now in general use employ either sulfur a t 135”C. for 12 to 24 hours. Excessive temperatures extremely thin transparent sections that are of limited size must be avoided to prevent blowing of the stock and distillaor rough opaque sections that are unsuitable for even moder- tion of volatile constituents. The cold sample, after a proper ate magnification. cure, will have a surface layer several millimeters thick, of I n a series of papers published in 1905 (S),Breuil suggested, a tough hornlike consistency too hard to be dented with the among other methods for examining rubber, an adaptation fingernail. of metallographic methods. Because of inferior equipFor the grinding and polishing of the hardened samples in ment, his results were not very useful, and he abandoned this the present work, an automatic polishing machine after the method in favor of transparent sections. The present paper design of Epstein and Buckley (5) was used. Where such a is a description of a somewhat more successful attempt to machine is not available, entirely satisfactory results can apply metallographic methods to rubber fabrications. be obtained on a simple variable-speed polishing lap, or by The general methods for preparing, examining, and photo- an adaptation of Pulsifer’s hand-block methods (8). graphing opaque materials, as described by Chamot and The samples are first flattened on a coarse loose-bonded Mason ( d ) , have already been applied to a limited extent to silicon carbide grinding wheel (Norton’s 3760551’-2) thortechnical products (9, 10, 11). The procedure developed by oughly flushed with water and operating a t about 300 r. p. m. the author for rubber and similar materials is much the same, The very important rough polishing is accomplished on a but requires a method for the preliminary hardening of the wheel covered with wool broadcloth charged with a paste of sample that will not disturb its structure. The most suit- “600” silicon carbide in water a t 300 r. p. m. After a thorough able hardening method has been found to be a cure in molten rinsing to remove any trapped coarse material, the sample is sulfur, which thoroughly impregnates any porous structures given its final polish on a wheel covered with cotton-backed as well as hardens the rubber structure itself. silk velvet charged with a small amount of a mecia1magnesium oxidk (Shamgva) in PREPARATION OF water. The samples SAMPLES are f i n i s h e d off by flooding this last lap The sample to be with q u a n t i t i e s of examined may be of water, which removes any size n e c e s s a r y , all the p o w d e r film but for convenience and leaves the sample in manipulation, large with a clean, bright, s u r f a c e s should be easily dried surface. cut i n t o b l o c k s no The fisished larger on t h e face samples can’ be exto be polished than amined and photo2 em. t o t h e s i d e . graphed in a manner Hard rubber and identical with t h a t hard asphaltic comused for metal specipositions usually mens (d), wing any FIGURE 1. TIRE TREADSTOCKS, CONTAINING (1) GOOD-DISPERSIXG AND 1 Present address, (2) POOR-DISPERSING GAS BLACK( x 64) good vertical illumi17832 Clifton Blvd., Lakenator and shortwood, Ohio. Black spots are polishing pits in the surface 251
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FIGURE 2. GAS-BLACK STOCK WITH CROSS-LINE MICROMETER ( X 600). SURFACE OF GAS-BLACK STOCK SHOWING GRANULAR MATRIX( x 1350). BOUNDARY OF P U R E G U M AND RECLAIM CARCASS STOCKS ( x 600)
mount objectives. Well-corrected lenses i n d careful adjustment of the illumination are essential, because of the absence of color and the low contrast in the specimens. Magnifications up to 600 can easily be utilized and useful observations have been made a t 2300, using well-corrected oil-immersion objectives. Auxiliary methods of illumination that have been found useful in interpreting structures are polarized reflected light, filtered colored light, and fluorescence from ultra-violet light.
APPLICATIONS The most useful application of this method of examining rubber has been in the quantitative estimation of the degree of dispersion of pigments in rubber mixes, particularly bread stocks. The term “degree of dispersion,” as used in the rubber industry, is rather indefinite in its connotation: For the purposes of the present work, it is taken to mean the degree of freedom from discontinuities visible t o the unaided eye or by the use of a moderately powerful microscope. These discontinuities may be sand, grit, wood, free pigment, or unblended master batch. They are arbitrarily considered significant if they are readily measurable a t 600 magnification, using “daylight” illumination and good optical equipment. The samples prepared as previously described are examined under the conditions specified above, using a net-micrometer in the ocular. The area of individual particles of undispersed material is estimated in terms of the squares of the micrometer, and the per cent of the area of the field occupied by undispersed material is calculated. This procedure is repeated on a number of fields distributed over the surface of the section, and the average value is found. For well mixed treads, the average value based on a measurement of forty fields gives a value reproducible t o a few per cent. For master
batches and similar mixes, the average value of ten fields is of sufficient accuracy. Other methods of areal analysis might be used ( d ) , but this has been found to be the most convenient. Alling and Valentine ( 2 ) have shown that when a sufficient number of plane sections of a solid are examined, the average areal percentage of any component is the same as the volume percentage of that constituent. This is the value designated as “per cent undispersed material by volume.” This method of estimating dispersion has been used in studying milling operations, in judging the dispersibility of pigments, and in rating the value of dispersing agents. This and other applications of the method are ilhstrated in the photomicrographs, Figure 1 shows the appearances of two tread stocks processed in the same way, but containing good-dispersing and poor-dispersing carbon blacks. The white areas are areas of poor dispersion. Figure 2 shows the surface of a gas-black stock as it appears in conjunction with the ocular net-micrometer for quantitative determinations of dispersion; the surface of a gas-black stock a t very high magnification; and the boundary between two stocks of different composition. Flow and mixing a t boundaries can be studied in this way. Figure 3 shows the application of the method to composite structures. The first photomicrograph, a t low magnification, shows the relative positions of the cords in a heavy tire carcass. The second is a portion of one of the same cords a t high magnification, showing the appearance of the cross section of the individual fibers. The third shows the possibilities of the method when applied to composite materials, the constituents of which differ widely in hardness. This tire bead section contains soft rubber, semi-hard rubber, cotton, and steel wires.
FIGURE 3. COMPOSITE STRUCTURES 1. *Cross section of tire carcass ( X 8). 3.
2. C f o s s section of tire cord showing individual cotton fibers ( X 600). Cross section of tire bead area ( X 8)
July 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4 is representative of the analytical possibilities of the method. It gives a photomicrograph of a f r a g m e n t of wood, identified by its cellular structure, embedded in a tire tread. Following this lead it was found possible to prepare sections of wood by this method, as in the second photomicrograph. Another highly complex subject is illustrated in Figure-5. Brake linings are complex fabrications of semi-hard rubber, asphaltic material, cotton, asbestos, and c o p p e r wire. The second photomicrograph shows the boundary between rubber and solid brass in a product depending on the adhesion of rubber to metal for its usefulness. These photomicrographs suggest the possibilities to which this method for the microscopic examination of technical prodficts may be put.
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IN TIRETREAD ( X 600). POLISHED SECTION FIGURE 4. WOODFRAGXENT OF SYCAMORE WOOD( X 64)
LITERATURE CITED (1) Allen. R . P.. IND. ENG.CHEM..Anal. Ed.. 2 311 (1930). Alling, H. L., and Valentine, W. G., Am. J . S a . , 14, 50 (1927). Breuil, P., Caoutchouc & gutta-percha, 2, 82, 118, 158, 197 (1906). Chamot. E. M.. and Mason. C. W.. "Handbook of Chekiical Microscopy," Vol. I, Wiley, 1930. Epstein, S., and Buckley, J. P., Bur. Standards J. Research, 3, 783 (1929). Grenquist, E. A , , Rev. gBn. caoutchouc, 7, No. 63, 17 (1930). FIGURE 5. CROSSSECTION OF FABRIC IN BRAKELINING ( x 64). CROSS Pohle, H . , " H a n d b u c h der K a u t s c h u k SECTION OF ADHESION ZONE,RUBBERTO SOLIDBRASS( X 600) Wissenschaft," ed. by K. M e m m l e r , S. Hineel, Leipeig, 1930. Pulsifer, H. B., Metals & Alloys, 2, 84 (1931). (11) Short, M. N., U. S. Geol. Survey, Bull. 825 (1931). Rousch, G. A , , J. IND.ENG.CHEM.,3, 368 (1911). (12) Steele, T. F., IKD.ENG.CHEM.,Anal. Ed., 2, 421 (1930). Seyler, C . A., and Edwards, W.J.,"Microscopical Examination of Coal," Dept. Sci. Ind. Research (Brit.), Fuel Research Board, RECEIVED March 30, 1933. Presented before the Division of Rubber ChemPhysical and Chemical Survey of Yational Coal Resources, 16 istry at the 85th Meeting of the American Chemical Society, Washington, (1929.) D. C., March 26 to 31, 1933.
Volumetric Potassium Bromate-Bromide Titration of Furfural Effect of Temperature 0. C. MAGISTAD Experimental Station, Association of Hawaiian Pineapple Growers, University of Hawaii, Honolulu, T. H.
I
(4) published a method for the electrometric titration of furfural by potassium bromate. Youngburg and Purcher ( 5 ) , Kline and Acree (S), and Iddles and Robbins (2) report difficulty in obtaining a good end point, or failure to check the method against other methods. Using fresh solutions of furfural in ethyl alcohol, the author was unable to obtain theoretical titrations, the titer being several per cent too great. Investigation disclosed that a t temperatures of about 18" C. theoretical values could be obtained, while a t 27" C., the laboratory temperature, more bromine was used than expected. Gortner (1) stated that the investigations of Pervier and Gortner were conducted during the winter a t 15" to 18 " C. and that the temperature coefficient was not investigated. Determinations of furfural made by the author by the electrometric method suggested by Gortner and Pervier gave an end point of 20.37 cc. a t 10" C., 20.75 cc. a t 20" C., 1; 1923, Pervier and Gortner
21.05 cc. a t 24" C., and 21.80 cc. a t 36" C., with the theoretical titer (0.09904 gram of furfural) of 20.62 cc., indicating that an increasingly greater titer is obtained as the temperature a t which the reaction takes place is increased. Kline and Acree (3) state that the end point is indefinite because of a second reaction involving the combination of another molecule of bromine with furfural. The extent of the secondary reaction is influenced by temperature, becoming especially marked a t temperatures around 50" C.
LITERATURE CITED (1) Gortner, R. A. Personal communication, April 20, 1932. (2) Iddles and Robbins, IND.ENG.CHEM.,Anal. Ed., 5 , 55 (1933). (3) Kline and Acree, Bur. Standards J . Research, 8 , 25 (1932). (4) Pervier and Gortner, IND.EXG.CHEW.,15, 1255 (1923). ( 5 ) Youngburg and Purcher, J . Bzol. Chem., 61, 741 (1924). RECEIVED March 11, 1933. Technical Paper 58 of the Experiment Station, Association of Hawaiian Pineapple Canners, University of Hawaii. Published with the approval of the director.
