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INDUSTRIAL AND ENGINEERING CHEMISTRY
(16) Garvey and Forman, I b i d , 3 0 , 1 0 3 6 (1938). (16) Garvey and Thompson, Ibid., 25, 1292 (1933). (17) Geer, Zbid., News Ed., 15, 469 (1937); Rubber Chem. Tech., 11, 263 (1938). (18) Gehman and Field, Baltimore Meeting, Am. Chem. SOC.,1939. (19) Gibbons, Gerke, and Tingey, IND. ENQ.CHEM.,Anal. Ed., 5, 279-83 (1933). (20) . . Guth and Mark, Monatsh.. 65. 63 (1934): Naturwissenschaften, 25, 353 (1937); Hauk and Neuman, 2. phys. Chem., A182, 285 (1938). (21) Kuhn, Kolloid-Z., 87,3 (1939). (22) Mark, Chem. Rev., 25,121 (1939). (23) Messenger, Trans. Inst. Rubber Ind., 9,190 (1933). (24) Meyer and Hohenemser, Helv. Chim. Acto, 18,1061 (1935). (26) Meyer and Mark, Ber., 61,1939 (1928). (26) Meyer, Susich, and Valko, Kolloid-Z., 59, 208-16 (1932); Meyer, Chemistry & I n d u s t r y , 1938,439. (27) Midgley, Henne, and Renoll, J. Am. Chem. Soc., 53, 2733 (1931); 54,3343 (1932). (28) Midgley, Henne, and Shepard, Zbid., 56,1156 (1934). (29) Midgley, Henne, Shepard, and Renoll, Zbid., 57,2320(1935).
1397
(30) Ostwald and Riedel, Kolloid-Z., 59, 150 (1932); 70, 75-9 (1935). (31) Prins, Chem. Weekblad, 16,64 (1918). (32) Rankoff, Ber., 64,619 (1931). (33) Rossem, A. van, India-Rubber J., 92,845 (1936). (34) Scott and Sebrell, in Davis and Blake’s “Chemistry and Technology of Rubber”, A. C. S. Monograph 74, pp. 30431,New York, Reinhold Pub. Corp., 1937. (35) Smith, Seylor, and Wing, Bur. Standards J . Research, 10, 479 (1933). (36) Staudinger, “Der Aufbau der hochpolymerische Stoffe”, Berlin, Julius Springer, 1932. (37) Vonderbilt News, 4,31 (1934). (38) Weber, “Chemistry of India Rubber”, 1902. (39) Whitby, Trans. Inst. RubberInd., 5,184-95 (1929-30). (40) Williams, in Davis and Blake’s “Chemistry and Technology of Rubber”, A. C. S. Monograph 74, Chap. VI, New York, Reinhold Pub. Corp., 1937. (41) Williams. IND. ENQ.CHEX.26.746 (1934). (42) Williams; Proc. Rubber Tech.’ Coni., London, 1938, p. 304, Rubber Chem. Tech., 12,191 (1939).
X-Rav Structure of Vulcanized Rubber J
GEORGE L. CLARK University of Illinois, Urbana, Ill.
A review is given of the significant x-ray results on the structure of rubber since the original work of Katz on stretched rubber in 1925. This seryes as a basis for comparison of results on vulcanized rubber, for a critical consideration of structural models proposed for rubber, and for the role of sulfur as a vulcanizing agent. The most important observations are: (a)The spacings and structure for stretched rubber are identical before and after vulcanization; (b) a three-times greater extension is required after vulcanization to produce a crystal fiber pattern; (c) remarkable hysteresis effects are observed in the appearance and disappearance of crystal interferences as tension is applied and released; ( d ) unvulcanized sol rubber produces no crystal fiber patterns even a t 1000 per cent elongation but after vulcanization gives a pattern a t 200 per cent elongation; (e) the contention of many years, that the fiber spots for stretched rubber after
T
HE important contributions of the x-ray diffraction method for the solution of the complex and difficult problem of the nature of rubber are so well known and so generally accepted, that even the briefest r6sum6 may seem unnecessary. However, in attempting to answer the question, “what do x-rays tell us about the process of vulcanization?” it is essential to compare in every possible manner the diffraction patterns of rubber, unvulcanized and vulcanized. Consequently, it is desirable to summarize first the experimental data and structural interpretations for native crude rubber, a highly polymerized hydrocarbon. If sulfur or other agents whi,h produce the characteristic, easily observed, physical changes in this hydrocarbon associated with vulcanization, cause any structural change, this must be indicated on the diffraction pattern. When unstretched rubber is examined by x-rays, it produces a blurred ring, the halo which is typical of the amorphous or liquid state. Its behavior when stretched was first reported by Katz (9) who observed interference spots a t 80 per
once appearing do not change in sharpness or position but merely increase in intensity as new crystal individuals are formed with increasing elongation, is disproved by patterns characterized by long arcs instead of sharp spots; (f)there is some evidence of sharper interferences after vulcanization, possibly indicating an enlarged crystallite size, by means of sulfur linkages. There is some support for the two-phase theory of rubber structure, for the existence of crystallites rather than singly acting molecules, and for a physical rather than chemical interpretation of vulcanization, since chemically bound sulfur or bridge formation might reasonably be expected to exert some structural effect which should be apparent on diffraction patterns; however, the very useful x-ray results cannot be said to answer unequivocally either what rubber is, or what vulcanization is.
cent elongation. Their intensity increased with increasing elongation, and at 400 per cent a definite fiber diagram was observed. Consequently, rubber when extended was considered to be crystalline, and the Joule effect to result from an actual formation of crystals. When Katz heated stretched rubber, the interference spots vanished; The identity period along the direction of stretch was 8 A,, and the dimensions assigneg to the unit cell were 8 X 6.5 X 6.5 A. with a volume of 338 A.3 The behavior of stretched rubber between 80 and 1000 per cent elongations was investigated by Hauser and Mark (6) who made a more accurate study of the positions and intensities of the interferences. The positions of the interferences were found to be independent of the degree of stretching, but their intensities increased proportionally with it. The position of the amorphous ring remained unchanged during extension, but its intensity decreased with continued elongation. Therefore, an amorphous or liquid phase in unstretched rubber was supposed to be changed to a crystalline phase
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when it was stretched. The positions of the interferences, which depend on the dimensions of the unit cell, did not change with continued stretch, and consequently a definite space lattice was indicated. Other evidence showed that new crystalline units were constantly produced during elongation, and that the major axis of the crystalline phase was oriented parallel to the direction of stretch. The interferences in stretched rubber disappeared a t 60" C. When rubber was maintained in an extended condition for some time, the interference spots vanished. If rubber was milled or swollen by solvents before stretching, the interference spots did not appear. An orthorhombic configuration was assigned to the unit cell, but the possibility of a monoclinic ostructure was admitted. Its dimensions, 8.0 X 8.6 X 7.68 A., correspond to a cell volume of 529 There were 4.12 molecules of CaHs per unit cell. The interferences obtained by Mark and von Susich ( 1 1 ) were better defined, and better measurements were possible. A rhombic cell was assumed and the dimensions given to it were 8.3 X 8.1 X 12.3 * 0.1 A. The volume of the unit cell was reported to be 830*30 A.3, with 7.1 molecules and not 8, presumably because of poor density values. With the Weissenberg apparatus, Mark and von Susich proved that the indices (111) and (200) of Hauser and Mark are really (100) and (010). They observed that very thin specimens of rubber showed fibering and also three-dimensional orientation. So a distribution of carbon atoms was proposed which aims to reconcile the orientation of the primary valence chains in the direction of stretch, atomic spacings for single and double-linked carbon atoms, and the observed intensities. By topochemical reactions they attempted to convert stretched rubber into derivatives retaining the structure, but x-ray patterns of the derivatives indicated an amorphous condition. On the basis of a single experiment, Ott ( l a ) assigned a volume of 259 to the largest possible cell. By the use of Bragg's valuesofor the radius of carbon (0.77 A,) and of hydrogen (0.73 A.) and on the assumption of close packing, the volume of CSHS was computed to be 43.1 x ml. or 43.1 A.a On this basis the maximum number of C5Hs groups in Ott's cell is 6. A RECENT paper by Lotmar and Meyer (10) reports accurate measurements of the structure of crystallized rubber. The unit cell was derived by the graphical method of Sauter, is monoclinic, and has the following axes: a = 8.54*0.05
A.
b = 8.20t0.05A. (fiber axis) e = 12.65f0.05 A. p = 83'21 Volume of unit cell = 880 -&.a approximately
They report 7.6 molecules per unit cell and this value is based on the highest value of density (0.965) reported in the literature. Eight molecules are assumed to be present. By an elimination of possible space groups, there remains the probable one, CS2h, and the chains are presumed to have the symmetry of a twofold screw axis. The crystallite is said to be a molecular racemate of right and left spiral molecules. Sauter (IS) in Staudinger's laboratory disagreed with the assignment of indices to interferences in the patterns of Lotmar and Meyer, as was also the case with cellulose. Sauter assigned the following values for the rubber unit cell: a = 8.91
b = 8.20 (fiberaxis)
c = 12.60
6
=
90' (orthorhombic)
This proposal was disproved in the laboratory of Meyer in Geneva. The correctness of the Lotmar and Meyer pro-
VOL. 31. NO. 11
posed structure for rubber has been verified in the writer's laboratory by. the same conclusive methods as employed in the test of the alternative structures proposed for cellulose (4). Crystal interferences in frozen, unstretched smoked sheet were found by Hauser and Rosbaud (6). They were indicated by Debye-Scherrer rings. Later von Susich (14) constructed a melting curve from the behavior of patterns of frozen rubber a t different temperatures. With unstretched rubber the powder pattern disappeared completely a t approximately 35" C. and with stretched rubber at about 90" C. Above 90"the pattern was that of an amorphous material. Indices were assigned to four rings in the frozen-rubber pattern, but no spacings or calculations were mentioned. Subsequent to the work of Lotmar and Meyer, Barnes (1) examined two samples of frozen crude rubber, one of which had remained frozen for 22 years and the other for at least 11 and probably for 30 years. They lost their opacity a t approximately 41' C. The measurements of Barnes agree excellently with those reported by Lotmar and Meyer for stretched rubber. Identical patterns were obtained from each specimen.
IN COOPERATIVE researches between the National Bureau of Standards and the x-ray laboratory a t the University of Illinois, patterns have been made under extraordinarily careful conditions for sol, gel, and total rubber, both unstretched and stretched, and when crystallized by freezing and from solutions (3). For the latter specimens temperatures below -80" C. were maintained in the x-ray cameras during exposures. Total rubber when stretched and exposed to an x-ray beam produced the characteristic crystal fiber pattern. Stretched sol rubber produced no evidence of this pattern even at 1000 per cent elongation. With stretched gel rubber, the pattern was formed above 100 per cent elongation, and a t 200 per 5ent was sharp and intense. A large interplanar spacing of 54 A. (or 2 X 54 = 108 A,) found in the unstretched gel, was absent in the sol. The patterns obtained with stretched gel (crystal fiber), with frozen sol, gel, and total rubber (DebyeSchemer rings indicating small randomly oriented crystals), and with gel and sol crystals produced from solution all gave the same crystal structure results, in excellent agreement with those reported by Lotmar and Meyer. The general conclusion from all these experiments and many others not specifically mentioned is that rubber consists of long chains of isoprene units. I n the stretched state these molecules are extended and oriented in parallel fashion, and thus produce crystal fiber patterns. In the nonextended state rubber behaves as a liquid and its diffraction pattern is that of a liquid. But the question remains unanswered as to whether the molecules themselves are folded or like springs in the unstretched state so that the extensibility of rubber is the result of the extensibility of these springs, or whether they are fully extended and extensibility results from the bending and flexing of a tangled aggregate of chains. Another related question is whether molecules act separately or in bundles or crystallites. Simple spirals, distorted spirals, or some form of folding have been proposed frequently and have principal support from the analogy of proteins shown by Astbury to exist in folded, extended, and supercontracted forms. Thus folding of protein chains accounts for the noncrystalline diffraction halos for globular proteins which in the process of denaturation tend to unravel and unfold and approach the condition of elongated molecules. During elongation these molecules must first orient properly in order to extend in the direction of elongation. A considerable difficulty arises in explaining on this basis how the spirals become elongated in unstretched frozen rubber to form definite randomly oriented crystals with the same fundamental lattice dimensions as those derived from the fiber crystal patterns of stretched rubber.
.
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Another proposal by Hermann and Gerngross (7) considered rubber to be bundles of exbended molecules which run almost parallel over part of their length as a network and through curved and disorganized fringes in other parts. Elasticity and pliability would reside in these fringes. This allows for preexisting crystallites or bundles, though not detected by x-rays in unstretched rubber except after standing at low temperatures. This picture was recently applied with great power to the structure of cellulose by Mark, but of course cellulose is cwstalline and composed of ex-
Unatretohed
Frosen
Vuloenired rubber
Unvvtoaniecd Vuloanised Stretobcd and froseo rubber
FIGURE1. CoMPARLSoN OF X-RAYPATTERNS
sol phase cannot denied. The chief objection which has been raised to this rubber model is that it begs the issue as to what is the cause of long-range extensibility in tho solid network, though nothing is to prevent slipping of adjacent crystallites in the semifluid phase as a lubricant. The observation by Clark, Warren, and Smith (8) that pure sol rubber prepared at the National Bureau of Standards and protected against oxidation by extraordinary precautions fails to produce a fiber pattern even at 1000 per cent elongation, while gel rubber and total rubber indicate crystalline organization at less than 200 per cent elongation, seems a significant point in favor of the two-phase concept. If crystallites or micelles exist in rubber then i t should be possible to ascertain something as to the size and shape from tho measurement of tho breadth of the x-ray crystal interferences. It is well known that these interferences broaden as particle size decreases in the colloidal range, and that Scherrer, van Lane, Brill, and others have provided the necessary fundamental formulas which are theoretically sound and have been verified by direct experiment. By this means Hauser and Mark (6)first determined that the length of the lattice in the fiber axis and thus the average length of crystallites was 3M)-600 A;, and that the breadth and thickness were as high as 500 A,, in contrast with the in ccllulose; thus they correspond narrower bundles (50 1.) to 80,0MI-150,000 isoprene units in a crystallite. The breadths do not change (sharpen) with increasing elongation and intensity, which proves that particles do not enlarge but that new crystallites are formed with increasing extension.
W E MAY now turn to vulcanization and ask the following questions, particularly for the case of low percentages of sui-
OF
RUBRETIS
fur and the retention of distinctly elastic properties in the product: 1. Is the presence of sulfur or a vulcanizing agent indicated directly on patterns? 2. Is the same difference between nnstretched and stretched samples maintained BS in unvulcanized rubber? 3. Is there any change in the positions of interferences when a crystal fiber pattern appears? 4. What relation exists between elongation and the appearance of fiber maxima after vulcanization? 5. Is there any difference in the breadths of intprfereneesand thus in the crystallite sise as compared with values for crude rub-
bery 6. Are an? hysteresis effectsdetected bv x-rays which mixht indicate somi effect of sulfur? 7. How do vulcanized sol and gel rubber compare with the I
two
nntrcated phases?
Before answering these questions it must be frankly admitted that in fourteen years of research not very much has been added to the original observations on vulcanized rubber made by Katz and Bing in 1925. 1. Crystalline sulfur or additional new crystalline compounds formed by the vulcanizing agent and rubber in adequately cured stock are not indicated on diffraction patterns by the appearance of new rings or spots. Even in hard rubber with high sullur content this is true. The only diffraction effects not due to rubber itself are due to cxces~ crystalline vulcanizing agent or to crystalline fillers. 2. Vulcanized rubber nnstretched produces an amorphous halo, and when sufficientlystretched, a crystal fiber pattern. 3. The positions of halos and of crystal interferences are identically the same for crepe or smoked sheets and for vulcanized rubber. Hundreds of mcasurements with the aid of
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the best microphotometers have detected no shift in positions, alterations in relative intensities, missing or additional interferences. This simply means that vulcanization produces no measurable changes in intermolecular spacings or lattice arrangement of polymerized isoprene molecules. There is no evidence of solid solution, compound formation, separate crystalline phase (if heated sufficiently to remove crystalline sulfur), or any other structural changes in terms of the unit crystal cell. 4. The crystal fiber pattern appears only after a considerably greater elongation after vulcanization than is required for crude rubber. For the latter the crystal pattern appears a t 80 per cent extension and increases in intensity with further extension. Crude rubber which has been subjected to mastication must be stretched above 80 per cent (depending on the extent of the treatment). Vulcanized rubber (about 5 per cent of sulfur) must be stretched to 225-250 per cent elongation. Thus there is considerably greater resistance in vulcanized rubber to the parallel arrangement of molecules or crystallites, but once formed the crystal pattern indicates a lattice structure identical with that of stretched crude rubber. In the ebonite or hard rubber state the pattern is obviously that of a nonparallel arrangement of molecular chains and crystallites in keeping with the very high viscosity and nonelasticity. Amix of rubber with 30 per cent sulfur and 1 per cent accelerator must be heated a t least an hour a t 132’ C. before all evidences of excess crystalline sulfur are removed. 5. The sharpening of interferences for vulcanized rubber, if observed, would mean an increase in crystallite size, as if more molecules were bonded together laterally as by bridges or lengthened by catalyzed chain polymerization. Increased breadth or diffuseness of interferences would mean breakdown of particles to smaller fragments and depolymerization. This, however, can also mean an increased distortion within particles of the same size. Katz and Bing observed that for stretched vulcanizates with 25 per cent sulfur a t 132’ C. for 120 minutes the interferences were diffuse and broad. However for ordinary vulcanized rubber of maximum elasticity and strength these breadths are at least as narrow and in many cases slightly sharper when compared with crude rubber patterns at high elongations. This observation has been made many times in the course of twelve years a t the University of Illinois. 6. Definite hysteresis effects are observed for vulcanized rubber. When a specimen is stretched rapidly to 250 per cent, crystal interferences appear a t once which are much more intense than the patterns for a specimen stretched 250 per cent very slowly (8). When this is allowed to retract slowly, the crystal interferences remain visible down to 130 per cent. Furthermore, photometric analysis proves that the amorphous halo which is perfectly uniform for unstretched samples shows differential intensity of the equator compared with the poles for elongations of 100 per cent. This indicates that even before reaching the critical elongations, rubber molecules bearing sulfur are being directed toward a parallel arrangement within the amorphous, mesomorphic, or liquid crystal state. 7. Experiments on vulcanized sol and gel rubber in the writer’s laboratory seem highly significant. Pure sol rubber does not tend to form crystal fibers even at 1000 per cent elongation, as though the material were a fluid or the molecules too short to maintain a preferred orientation in the fiber once formed. Total rubber behaves normally in that greater elongations are required; vulcanized gel is quite similar to the untreated gel on the elongation required; but vulcanized sol rubber crystallizes and forms fibers a t about 200 per cent, far below the value for the unvulcanized phase and nearly the same as the vulcanized gel. This seems to indicate that
VOL. 31, NO. 11
the primary action of the vulcanizing agent is to solidify and strengthen the sol phase in which the gel phase may be dispersed. Under the same experimental conditions the vulcanized sol interferences are appreciably broader than those of vuloanized gel at the same elongation, which can mean that the degree of polymerization and the number of molecules in the crystallite are still below the values for the gel. Years ago Ostromislensky compared vulcanization with a swelling in which sulfur-containing rubber serves as the swelling medium for unchanged rubber and is mechanically stronger than rubber itself. And yet the reversible stretching still maintained after vulcanization demands that the inner crystallite structure and the ability of the molecules to fold u p again on release of tension must be retained, and so the crystal interferences are unchanged in position. After all, as Katz stated in his original paper, the remarkable fact is not that a crystal pattern is produced upon stretching, but that this pattern reverts immediately upon the release of tension to the liquid halo. NOW we ask from the viewpoint of x-ray data, is vulcanization physical or chemical? Are there sulfur bridges or may we agree with Ira Williams that with the highly nonuniform distribution of sulfur the evidence is against sulfur bridges? In total rubber we can state only that whatever the change (physical or chemical), the essential state of unstretched rubber and the crystalline organization which is gained upon stretching are quantitatively the same after vulcanization as before. The diffraction pattern pertains to the periodicity of a small unit crystal cell which is unaffected even though the lengths of the great chains that are not measured by the pattern directly may undergo profound changes upon vulcanization. Sulfur or any other vulcanizing agent leaves these small subunits of chains untouched. If they become chemically attached to chains, sulfur atoms must fit into lattice holes and affect even structure factors practically not a t all, a case which has few analogies. And yet the sulfur disappears in considerable quantity as such and registers no measurable effect on patterns, even granted that vulcanized rubber must be stretched farther than crude rubber before rubber crystallinity appears. The effort to measure long periodicities a t very small angles, in order to ascertain whe$her sulfur acts as a polymerization catalyst, yielded the 54 A. spacing distinguishing gel from sol rubber; but upon vulcanization this interference becomes much less certain. We have the slight evidence, which must be statistically verified, that interferences of some vulcanized specimens appear sharper than the corresponding untreated samples; the possibility is thus indicated of a growth of crystallites a t least laterally by a bonding of more molecules together. And there is the evidence that stretched sol rubber does not “crystallize” a t 1000 per cent elongation, although it does form crystals on freezing and from solution, whereas after vulcanization the typical crystal fiber pattern is obtained at 200 per cent elongation of specimens with greatly enhanced “nerve”. Thus vulcanization does seem to have primary effect on this ether-soluble phase in organizing and strengthening but not in changing it into gel, for the two phases still have widely different properties. New experiments with specimens maintained at -80” C. while x-ray patterns are photographed have been undertaken in the writer’s laboratory in the hope of finding additional effects of vulcanization. One of the most interesting results of these studies a t low temperatures concerns the sharpness of the crystal fiber interference maxima. Katz, Hauser and Mark, the writer, and other early investigators of rubber with x-rays all found that the crystal interferences which first appeared faintly above the critical elongation of rubber were sharp spots from the
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INDUSTRIAL AND ENG1NEERIN.G .CHEMISTRY
very beginning, and changed only in intensity with increasing elongation. This was considered important evidence that crystals when formed were already exactly oriented with respect to the direction of stretching, and that new individual crystals were formed with increasing elongation to account for increasing intensity. By contrast cellulose rayon filaments may be regenerated from viscose or cuprammonium solution under weak tension, in which case the cellulose interferences appear as long arcs, indicating a considerable divergence in orientation of cellulose crystallites around the mean preferred position; or under high tension these arcs may narrow down to sharp spots, indicating a nearly perfect fiber structure. I n many years of experience a pattern for rubber with arcs instead of sharp spots was not observed until a short time ago. Extended total, gel, and sol rubber strips were frozen and maintained until in many cases there was a secondary automatic extension of the strip so that it bulged between the two attached ends. Figure 1 shows a comparison of the usual sharp pattern of rubber quickly stretched a t ordinary temperatures and of a frozen stretched gel. This is a fiber pattern of arcs for a material apparently entirely different in its mode of formation and properties than that which produces the customary pattern. These arcs lie along the continuous, uniform Debye-Scherrer rings which appear for “frozen” unstretched rubber, and hence, just as with cellulose, result from pulling true crystals already existent from a random array into a preferred orientation. This looks much more like a state in which somewhat rigid long rods, instead of springs, embedded in a plastic matrix are pulled into parallel array. Vulcanization seems to play the part of stabilizing the true crystals and of affecting the nature and plasticity of the matrix. This interpretation may not be correct, but it is suggestive and consistent with the behavior of cellulose. Further intensive work on these phenomena is in progress. At this time it must be concluded that though the x-ray diffraction supermicroscope has made a brilliant contribution to the science and technology of rubber, it has not yet been able, alone or in combination with every other method of approach, to answer unequivocally the question, “What is vul-
1401
canization?” simply because the question, “What is rubber?” is still a challenging uncertainty. NOTE: Two important contributions in this field have just been reported. Hauser (4A) finds that when stretched rubber is vulcanized by the process of admixing sulfur and vulcanizing the stretched sample in boiling water, the fiber attern normally characteristic of the stretched sample is compretely destroyed. This would seem t o indicate, therefore, that the vulcanization process tends t o disarrange the parallel alignment of diffracting molecules at least under conditions of this experiment. Hauser also reported sharp maxima and minima in various curves, such as permanent set for specimens studied in this paper at about 600 per cent elongation. Further details of these experimente will be awaited with interest. Gehman and Field ( S A ) have proved definitely that vulcanization does not affect the ability of rubber samples t o take up “higher” or three-dimensional orientation, such that fiber diffraction patterns are obtained in three directions through a given specimen. As in the case with unvulcanized rubber, this higher orientation is observed when the per cent contraction in gage exceeds the per cent contraction in width. The variations in intensities of the two principal equatorial spots in the pattern offer the first definite opportunity t o observe the possible effects of sulfur in the vulcanization process.
Literature Cited (1) Barnes, W. H., Can. J.Research, 15,156 (1937). (2) Clark, G. L.,Warren, W. J., and Smith, W. H., Science, 79, 433 (1934). (3) Clark, G. L.,Wolthuis, E., and Smith, W. H., J . Research Natl. B u r . Standards, 19,479 (1937); Clark, G. L., Gross,S. T., and Smith, W. H., Ibid., 22, 105 (1939). (3A) Gehman and Field, J . AppZied’Phys., 10,564 (1939). (4) Gross, S.T., and Clark, G. L., 2. Krist., 99,357 (1938). (4A)Hauser, Div. Colloid Chemistry, A. C. S. Meeting, Boston, Mass. (5) Hauser, E. A,, and Mark, H., Kolloidchem. Beihefte, 22, 63 (1926). (6) Hauser, E. A., and Rosbaud, P., Kautschuk, 3, 17 (1927). (7) Hsrmann and Gerngross, Ibid., 8, 181 (1932). (8) Iguchi and Schossberger, Ibid., 12, 193 (1936). (9) Kata, J. R.,Kolloid-2.. 36,300; 37, 19 (1925). (10) Lotmar, W., and Meyer, K. H., Monatsh., 69,115 (1936). (11) Mark, H., and Susich, G. von, Kolloid-Z., 46,11 (1928). (12) Ott, E.,Naturwissenschaften, 14,320 (1926). (13) Sauter, E.,2. p h y s i k . Chem., B36, 405,427 (1937). (14) Susich, G. von, Naturwissenschaften, 18,915 (1930).
RECOVERY OF BLAST FURNACE FLUEDUST The gas washer water empties into this basin and sludge is pumped from it. This sludge passes into a Dorr thickener t h e overflow from whibh flows to an Oliver filter for removal of the very fine particles of flue dust. (See text on page 1366.)
Courtesy, CarnegieIZZinois Steel Company
