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(35) Meyer and Hohenemser, Helv. Chim. Acta, 18, 1061 (1935); Rubber Chem. Tech., 9,201 (1936). (36) Midgley, Henne, and Shepard, J . Am. Chem. SOC.,56, 1156 f 1934). \ - - - - I
(37) Ibid., 56,1326 (1934). (38) Midgley, Henne, Shepard, and Renoll, Ibid., 56, 1325 (1934). (39) Nisson and Mandelbaum, U. S. Patent 1,836,183(1931). (40) Ostromislensky, J . Russ. Phys.-Chem. SOC.,44, 204 (1912); I n d i a Rubber World, 80,No.3,55(1929); Rubber Chem. Tech., 2. 489 (1929): ChaD. 7 in Davis and Blake’s “Chemistrv and Technology’o’f RubLer”, A. C. S. Monograph 74, New kork, Reinhold Pub. Corp., 1937. (41) Ostromislcnsky, J. Russ. Phys.-Chem. SOC., 47, 1467, 1904 (1915); India Rubber World, 81, No. 3. 55 (1929); Rubber Chem. Tech., 3, 195 (1930). (42) Parkes. British Patent 11,146(1846). (43) Peachey and Skipsey, J . SOC.Chem. Ind., 40,5T (1921). (44) Posner, Ber., 38, 646 (1905). (45) Rossem, van, I n d i a Rubber J., 92, 845 (1936). (46) Rossem, van, Dekker, and Prawirodipoero, Kautschuk, 7, 202, 220 (1931);Rubber Chem. Tech., 5,97 (1932). (47) Smallwood, H.M.,personal communication.
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(48) Spence and Ferry, J . Am. Chem. Soc., 59, 1648 (1937). (49) Spence and Ferry, J . SOC.Chem. Ind.,56,246T (1937). (50) Staudinger and Fritschi, Helv. Chim. Acta, 5,785 (1922). (51) Stevens, J . SOC. Chem. Znd., 36, 107 (1916). (52) Tanaka, Kambara, and Hirakawa, J. SOC.Chem. I n d . (Japan), 40,263 (1937);Rubber Chem. Tech., 10,708 (1937). (53) Whittelsey and Bradley, U.S. Patent 1,607,331(1926). (54) Williams, Chap. 6,in Davis and Blake’s “Chemistry and Technology of Rubber”, A. C. S. Monograph 74,p. 237,New York, Reinhold Pub. Corp., 1937. (55) Williams, J. IND. EKQ.CEEM.,15,1020 (1923). (56) Williams, Proc. Rubber Tech. Conf., London, 1938,304; Rubber Chem. Tech., 12,191 (1939). (57) Wright, Trans. Inst. Rubber Ind., 12, 183 (1936): Rubber Chem. Tech.. 11. 131 (1938). (58) Wright and Davies, Trans. Inst. Rubber I n d . , 13, 251 (1937); Rubber Chem. Tech., 11, 319 (1938). (59) Zhavaronok, J . Applied Chem. (U. S . S . R.), 9, 1290 (1936); Chimie & industr.ie, 37,742 (1936).
PARTof this article was also presented before the Rochester Section of the Ameriaan Chemical Society.
Physical Changes Induced by Vulcanization W. W. VOGT The Goodyear Tire & Rubber Company, Akron, Ohio
N T H E period during which Charles Goodyear was con-
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ducting his history making experiments which led to the discovery of the “change” which is called vulcanization, it seems on retrospect that the chief property of rubber which it was thought most desirable to exploit was its waterproofness. T o make practical use of this property, Goodyear was seeking a method of stabilizing rubber against the ravages of heat and sunlight which softened the rubber, and the effect of low temperatures which hardened and embrittled it. Today many a rubber article or structure depends for its success on this same property of waterproofness, but a far greater tonnage of rubber is employed in the construction of articles in which its physical properties of strength, resilience, and extensibility are paramount. Is it not then a most happy circumstance that the process of vulcanization not only achieved the objective sought by Charles Goodyear-namely, the stabilization of rubber with respect to temperature effects-but also enhanced or modified in a desirable direction those other properties of strength, resistance to plastic flow, wear and tear resistance, which have led to the ever-expanding applications of this substance to the service of mankind? Rubber technologists are familiar with the changes which ’ vulcanization produces and, knowing them, are apt to accept them without further thought, but it is interesting to imagine what might have been the course of events had the process of vulcanization produced no effect other than to stabilize rubber against the effects of temperature. Imagine a pneumatic tire with all of the physical properties which it had in the unvulcanized state, except that by the process of vulcanization it had been converted into a structure
immune to change by temperature alone. Its carcass plies would have little or no resistance to separation, its tread would flow out of shape as a result of pressure and the driving torque, and there would be a minimum of resistance to wear. It is unnecessary to enumerate further similar examples. Suffice i t to say that the modern automobile would be impossible. Rubber would still be valued as a waterproofing material, possibly in considerable use by virtue of its electrical insulating properties and in general as a space-filling material. The development of the whole technology of organic accelerators and the use of fillers and reinforcing pigments would have been unnecessary or impossible. The innumerable applications as a structural material in the engineering field would never have materialized, and the scope and tempo of our modern civilization would have been undeniably altered to an astonishing degree. Possibly it somewhat strains our credulity to imagine that the final result which we know as vulcanization might have been achieved only through the intervention of two separate and independent processes, one of which imparted the improvement in the physical properties, the other of which conferred the resistance to temperature changes. We instinctively shun this idea as having any possibility of being true, but I am afraid that we shun it because, after the fact, we know that it is not so. Before vulcanization was discovered, it would have taken a brave man to predict that a process which conferred resistance to the effects of temperature would of necessity also improve the mechanical properties. As a rather weak analogy to this line of speculation, one might consider the question of antioxidants for rubber. Did any of the early inventors in this field predict or claim in their specifications that, a priori, a material which would make rubber more resistant to the action of oxygen might also improve its resistance to mechanical flexing? Happily, how ever, these inventors and their predecessor, Charles Goodyear, found that, regardless of the object of their experiments, the materials or the process did have other and perhaps even
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more valuable qualities; in Charles Goodyear's case the single, simple process of combining sulfur with rubber a t high temperatures extended not only the useful temperature range of the material, but also vastly improved its mechanical properties. Let us examine in a somewhat general way the changes in physical properties induced by vulcanization. The subject should be treated broadly without hairsplitting differentiations, and in the case of the vulcanized rubber, no particular emphasis should be laid on the exact state of vulcanization or the progressive changes in physical properties with the state of cure. Suffice it to say that any reasonably well-vulcanized specimen will differ sufficiently from any sample of raw rubber or unvulcanized mixture to make clear the changes induced by vulcanization. First of all there are a few physical constants which are little, if any, affected by vulcanization; what effect there is, is probably due to the slight volume effect of the combined sulfur. For example, density, compressibility and expansivity, thermal properties of conductivity, diffusivity and specific heat, Poisson's ratio, and electrical conductivity are virtually unaffected by vulcanization. On the other hand, the resistance to swelling by solvents is increased tremendously-one might be justified in using the word "infinitely". The property called tackiness or adhesiveness which is of the greatest importance in the raw stage virtually disappears during vulcanization. Between these extremes lie the great majority of the mechanical properties, the extent of the change of which varies for each property studied and according to the method of test employed. Let it be clearly recognized that vulcanization creates not a single new property, and the changes that ensue are of degree only. The most valuable property of rubber is its long-range reversible extensibility. This, coupled with adequate tensile strength, gives it a relatively enormous energy storage capacity. This is the fountainhead from which flows wear and tear resistance, cushioning, flexibility. Important and indispensable as are these qualities, they would be largely nullified unless they were preserved over a considerable temperature range; lastly, but no less important, plastic flow must be almost completely suppressed. These, then, are the essentials of vulcanization-enhancement of the strength and reversible extensibility, suppression of plasticity, and relative immunity to temperature change.
Effect of Vulcanization on Tensile Strength At normal temperatures raw, smoked sheet rubber has a maximum tensile a t break of say 50 kg. per sq. cm. or 700 pounds per square inch a t an elongation of the order of 1000 to 1200 per cent. Accelerated pure-gum type vulcanizates have been known to have tensile strengths approximating 400 kg. per sq. cm. (5600 pounds per square inch) a t an elongation of about 700 per cent. Hence vulcanization easily increases tensile strength, say eight fold, and energy storage capacity, say five fold. At extremely low temperatures (by immersion in liquid air a t -185" C.) both raw and vulcanized rubber exhibit phenomenally high tensile values approximating 530 kg. per sq. cm. (7500 pounds per square inch) at virtually zero elongation. And a t -70" C . both raw and vulcanized rubbers become hard, inextensible, and inelastic, and show substantially the same tensile values. It is a t this temperature that a sharp discontinuity in the internal structure of raw rubber occurs, as shown by heat capacity and volume expansity determinations. It is when the temperature of testing is raised above normal that the effect of vulcanization becomes most pronounced.
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Raw smoked sheet a t 80" C. shows tensile values of the order of 2 to 5 kg. per sq. cm. (30 to 70 pounds per square inch), whereas pure gum-type vulcanizates at the same temperature exhibit tensile strengths of 200 kg. per sq. cm. (2800 pounds per square inch). Here we have a fortyfold difference. The comparative elongations a t break are approximately 2000 per cent for the raw rubber compared with about 900 per cent for the vulcanized. But this comparison is deceiving in that the vulcanized rubber exhibits a high degree of reversible extensibility, whereas the extensibility of the raw rubber (although greater) is almost entirely nonreversible.
Changes in Plastic Flow Phenomena Plasticity is that property by virtue of which a material is able to accept and retain a change in dimensions. It is probably the most useful property of unvulcanized rubber. By virtue of it we can tube, calender, and mold. Under a quick, light impact a t normal temperatures, raw rubber is equal if not superior to vulcanized rubber in resilience. Hence vulcanization does not increase resilience or elasticity. But as the applied force, the time through which it acts, and the temperature of test are increased, plastic flow phenomena become of increasing significance, and thus we obtain a more striking demonstration of the effect of vulcanization. The process of mastication increases the plastic nature of raw rubber but never completely obliterates its elasticity. Oppositely, vulcanization decreases the plastic properties but never succeeds in eliminating them. Dead milled rubber will bounce, and on the other hand, the most perfectly vulcanized sample will still exhibit creep or flow. Elasticity resides in the rubber molecule; and the implications of the work of Wiegand and Snyder on the dynamic stress-strain curve and of Williams on the composite nature of the stress-strain curve indicate that there is a reversible flow starting at 200 to 400 per cent elongation, which has its origin within the molecule. Even though this flow is reversible, in that no permanent set remains, yet it involves a considerable consumption of energy to cause the contraction, and hence it appears that perfect elasticity can never be achieved. And even though this type of flow is reversible, it is a flow and will manifest itself as increase in length a t constant load or decay of tension a t constant length in exactly the same fashion as does the irreversible flow, except that on release of the applied force there will be no permanent change of dimensions, which is the true criterion of plasticity. Present-day opinion seems to be almost unanimously in favor of the view that vulcanization builds up a three-dimensional network of the long, tangled, and possibly kinked rubber molecules. Such a structure seems to be necessary to prevent unlimited slippage between molecules (the cause of unlimited plastic flow in raw milled rubber). Perfect vulcanization might conceivably achieve a perfect three-dimensional tie-up of every molecule. According to this theory, irreversible plastic flow would then no longer be present. It might therefore be expected that, if rubber were worked under conditions of relatively low strain, creep and flow phenomena would be entirely absent. There would be no need for compression-set tests. Gaskets, shock absorbers, and rubber mountings of various kinds would stay put. Needless to say, that condition has not as yet been perfectly achieved. However, if it is desired to employ rubber more efficiently so that greater loads may be carried by smaller units, it would appear that the next obstacle will be the reversible flow. Since the evidence supports the view that this is a property resident within the molecule itself, its final solution might involve a molecular change which would greatly modify the other physical properties as we now know them.
