V O L U M E 21, NO. 4, A P R I L 1 9 4 9 (27) Ramsey, L. L., and Patterson, W. I., J . Assoc. Ofic. Agr. Chemists, 28, 644-56 (1945). (28) Roberts, J. D., and Green, C., IND. ENG.CHEW,ANAL.ED., 18, 335 (1946). (29) Rogers, A, F., and Kerr, P. F., “Thin-Section Mineralogy,” New York, McGraw-Hill Book Co., 1933. (30) Roth, W. L., DeWitt, T. W., and Smith, A. J., J. A m . Chem. SOC.,69, 2881-5 (1947).
46 1 (31) Wilson, J. B., and Keenan, G. L., J. Assoc. Ofic. Agr. Chemists, 27,446-8 (1944). (32) Winchell, A. N., “Microscopic Characters of Artificial Inorganic Solid Substances or Artificial Minerals,” New York, John Wiley & Sons, 1931. (33) Winchell, A. N., “Optical Properties of Organic Compounds,” Madison, University of Wisconsin Press, 1943. RECEIVED November 15, 1948.
chem2cd microecoply sgmposium
Resinography of Some Consolidated Separate Resins T. G. ROCHOW AND F. G . ROWE Stamford Research Laboratories, American Cyanamid C o m p a n y , Stamford, Conn. An unstressed single physical-chemical phase of any resin is characteristically without structure under a light microscope of even the highest resolving power. Generally, only polyphased resin systems manifest light-microscopical structure. Confronted with light-microscopically homogeneous resins, a ttention was paid to electron microscopical techniques. Replicas of either polished or molded surfaces were more characteristic of the method of preparation than of the specific resin. Therefore, the cold-embrittled resin was fractured under standardized conditions and electron micrographs were taken of replicas of the fracture surfaces. The resins examined
0
NE common characteristic of a separate resin is its optical homogeneity. In its consolidated state a resin appears to be structureless to the unaided eye. Even under the highest resolving power of the light microscope (approximately 0.2.4 a resin possesses inadequate variations in refraction, reflection, or absorption of visible light for the perception of structure within the boundaries of the resin. Therefore, it is classified as a single physical phase. Under the usual classification, a resin does not manifest the geometric configuration of its natural surfaces and internal graininess which are characteristic of a polycrystalline phase. Only “incompatible” resins in admixture (polyphased resinous systems) usually manifest structure under the light microscope. Possibly because of their very optical heterogeneity, polyphased resins are not used commercially as much as single-phased resins. Occasionally, however, heterogeneous resins are encountered by the resinographer-for example, phenol-formaldehyde resin plus butadiene-styrene elastomer (GR-S). Such a mixture was submitted (by D. JT. Toung, Esso Laboratories, Standard 011Development Company, Bayonne, N. J ) as a test bar 6 inches long and 0.5 inch by 0.5 inch in cross section. The bar 7%-assmoothed in cross section, flattened on abrasive papers, and polished on a cloth lap with water and magnesium oxide (“free of sulfate,” Merch). The two-phased system appeared by vertically reflected light as shown in Figure 1 at 1OOX. The phenolic resin is identified as the discontinuous phase because this IS lighter in shades of gray, corresponding with the higher refractive index A few of these areas are marked P in Figure 1. Thus the continuous phase (shown black in the photomicrograph) is the GII-S elastomer. Because the elastomer is the continuous phase, it is free to be distended and the mixture is therefore resilient. A resinographic examination of this type of plastic reveals the number of phases, their mode of association, particle sizes and
were RIelmac, polyacrylonitrile, a 3 to 1 mixture of the two, copolymer, Lucite, polystyrene, and Polythene. All the resins manifested different structures. It is tentatively concluded that these structures are typical and characteristic. Most of the fundameFta1 units are round particles only hundreds of Angstroms in diameter, approaching molecular dimensions. More empirical data are probably needed for a general theoretical explanation but, in the meantime, this resinographic method should be of practical value in the study of both accidental and experimental cracks and breaks among commercial resins and their plastics.
shapes, and relative reflectivity (refractivity). Separate chemical tests and some physical tests (such as impression and scratch hardness) may often be made on the separate phases. Another example of light-microscopical heterogeneity shows how differences in refractive index are perceptible pictorially by reflected light. Figure 2 is one photograph of the molded surface of two laminates placed side by side, taken by unidirectional, oblique, reflected light a t lox. The two laminates are a pair which was produced under identical experimental conditions. R e are concerned with two layers: a surface (protective) layer composed of a thin sheet of paper impregnated with a Melmac resin and, i n the place usually occupied by the layer carrying the design or decoration, a sheet of RIelmac-resin impregnated black paper for the experimental purpose of showing scattered light t o the best advantage. On the left the protective layer is satisfactorilj transparent, for it displays the “design” layer in almost its original degree of blackness. On the right, the protective layer scatter? so much light that it almost entirely obscures the black design layer. The only difference between the two samples was in the. fiber composition of the paper in the protective layer: viscose fibers (satisfactory) versus cellulose acetate fibers (unsatisfactory). Figure 3 shows each entire protective layer in polished cros:: sect’ion, at 500X by vertically reflected light. Figure 3 (left) corresponds to Figure 1 (left) and shows the viscose fibers (medium gray) as a discontinuous phase in close contact with the continuous phase of Melmac resin, The measured average refractive index of the viscose fibers was 1.545 and that of the hlelmac resin, was 1.652. These refractivities were sufficiently close to transmit light satisfactorily to the design layer (Figure 2, left) but sufficiently different for microscopical differentiation in polished section (Figure 3, left). In Figure 3 (right), the cross section of the unsatisfactory layer, not only are the cellulose acetate fibers shown in darker gray (average measured refractive index 1.470) but they are surrounded by black bands, assumed to be spaces of air (refractive index 1.000). Probably a t the airfiber and air-resin interfaces so much light is scattered that the
ANALYTICAL CHEMISTRY
462
~
-.
i
~compleXplastie, ~ ~ ~ phenolicR&n l .
+ GR-S. loox
All dieeontinuoua area5 like P, ere phheuoli- resin. Black continuis GR-S elastomer. Photomiorowaphofpolished section by v s r t i o d y reflected light
Figure 3. Proteotive Layer in Polished cross Section. 500X Left. Viscose fibers (medium gray) in cloas confa~fwith rosin (light gray). Right. Cellulose acetate fibera (dark gray) separated from reain (light p a y ) by air (black). Vcrtioslly reflected (bright-field) ill",..i..ltiO..
positively, as follows: Each specimen was covered with two t o four layers of polystyrene tape, a glass plate, anda4-pound weight. The assembly was transferred to an oven and heated to 160" C. After cooling, the polystyrene was stripped from the specimen and silica was evaporated onto the rephca. side of the polystyrene in a vacuum evaporator. The polystyrene replica was removed by placing the composite polystyrene-silica replica in ethyl bromide. Micrographs of the silica. (positive replica) were taken in the electron microscope. In addition to using polished surfaces, in some instances, the molded surface was replicated in the d u d manner described above.
Figure 2.
Transparent Surface Layer (Left) a n d Lightwing Surface Layer (Right). 1OX >ft w o laminates of resin-impregnatrdpaper, differe of fibers in pvotsotive fop layer. Oblique reflected (dark-field) illumination
fihers obscure all the areas of black paper which they cover (Figure 2, right). The explanation given t o the laminate experimenter was that the cellulose acetate fihers were not wetted by the Melmae resin. U W ~ U U C ~W ~XWE
ELECTRON MICROSCOPICAL APPEARANCE OF RESINS
Polished and Molded Surfaces. Whereas single, unstressed resins do not manifest light-microscopical structure, the authors hoped to show structure in such resins h s examining their replicas under the electron microscope. They began by polishing sections of moldings of Melmac resin, couolvmer resin. exuerimentsl polvacrvlonitrile. and commercial Lieit;? (polymethylhethacryla
