Novemher, 1926
I N D U S T R I A L A N D ENGTNEERING CHEMISTRY
1157
Optimum Cure Criteria in Vulcanized Rubber’ By W. B. Wiegand BINNBY& SMITH Co.. NEW YORKN. Y.
OST of the difficulties experienced by workers in connection with optimum cure arise from a lack of definition of this term. We therefore propose at the outset to distinguish between “optimum cure” and “technical cure.” By “optimum cure” we mean that cure a t which some numerically measurable property of the rubber, which may be agreed upon as the most suitable, reaches a fixed point, preferably its mathematical maximum. By this definition the factor of selection is eliminated, whether from viewpoint of stability on aging or suitability for any given industrial purpose. We define as “technical cure” that cure which brings about a state of vulcanization selected by the technologist because best for the particular mixing and the particular article involved.
M
Optimum Cure
COEFFICIENTOF VULCANlZATION-Beginnillg with the optimum cure as defined, and considering the various properties which might be eligible as criteria, we have, on the chemical side, the so-called coefficient of vulcanization, a function of the state of cure. Thoroughly studied by Stevens and others over ten years ago,2 it has assumed a definite place as a useful index under certain conditions. The fact that it passes through no maximum, involves analytical difficulties and delays, is relatively nonsensitive to cure, but extremely sensitive to type of m i ~ i n g or , ~ even of crude rubber employed, has prevented the coefficient from enjoying any general acceptance as a criterion of optimum cure. ELOXGATION AT RUPTURE-on the physical side we have first the elongation a t rupture, which, although measuring perhaps the most striking physical property of rubber, has never been regarded as a suitable index, (1) because its maximum (except in cases of reversion) almost always occurs in the region of definite undercure, (2) because of its lack of sensitiveness, and (3) because of its ambiguity in lowsulfur mixings where reversion may occur. TENSILE STRENQTHAT RUPTURE--Next we have tensile strength a t rupture, a property which no experienced technologist would ignore in his selection of optimum cure. The “tensile” is sensitive t o state of cure in its most important region, does pass through a maximum in that region, and vitally affects the utility of manufactured goods, no matter what the character of mixing. Thus, the tensile strength maximum should be seriously considered in all cure testing, whether theoretical or practical, of any rubber specimen. To be sure, there is the difficulty of ’irregular breaks. Such difficulties, however, can in large measure be surmounted by the expedient of breaking enough test pieces to secure duplicate highest values. The arithmetic mean, while the simplest, is by no means the most accurate way to arrive a t tensile strength values. Tensile strength a t rupture has often been damned with faint praise largely because of the limitations inherent in ring testing. The worker with Schopper rings has been appalled by the amount of material and trouble required to pick the best two or three 1 Presented under the title “The Physical Properties of Crude Rubber Compared with Those of Compounded Vulcanized Rubber.” Stevens, India Rubber J . . S I , 401, 679,794 (1916); I S , 220 (1917); I . SOC.Chem Ind., S I , 872, 1142 (1916); 87, 2SOT (1918). Cranor, I n d i a Rubber World, 61, 137 (1919).
* *
out of five or, if need be, seven breaks. For this reason, he has fought shy of tensile at rupture and sought to establish criteria based upon stresses sustained a t intermediate elongations. MODULUS-Usually thought of in classical mechanics as Young’s modulus, a constant characteristic of a material obeying Hooke’s law, modulus becomes, in the greatly expanded elastic scale of rubber, a variable. Therefore the term has by usage come to signify the average ratio of stregs a t some defined value of the latter, rather than the inclination of the tangent to the stress-strain curve a t any point. Thus loosely used, one may refer to the modulus a t 300, 500, or 890 per cent, according to the character of the mixing and the particular information desired. For the purposes of plantation testing, it is clearly advantageous to push the reference point as far as possible toward the breaking point, thus opening out the scale of modulus fluctuations with consequent increase in sensitiveness. With a given type of crude in a given mixing-the stress-strain curves rotating with advancing cure, and without intersection-one determination of modulus clearly fixes the position of the curve. Free from any error due to defective test pieces, modulus can be measured without recourse to multiple tests. Again, since under the conditions just mentioned the rate of rotation of the curves, with increasing cure, is quite regular, the time necessary to attain any standard position of the curve can be approximately calculated from a Ringle preliminary test. I t is no wonder, then, that the singlepoint method has found favor as a quick control of successive deliveries of crude rubber. To object to this method on the ground that it does not include a factor representing the aging qualities of the rubber, seems not only academic but beside the point. We go further and say that to seek to inject into the optimum-cure criterion factors having to do primarily with the selection of a technical cure not only complicates but also confuses the issue. The items entering into the selection of a technical cure are so varied that it would seem by far the best policy, since they cannot all be put in, to leave them all out. So much for modulus as a rough control. However, as a general criterion of optimum cure, it has grave imperfections. I n the first place, the moment the grade of crude rubber is changed, so also is the course of the stress-strain curve. The family of curing curves of one crude rubber may and does intersect that of another. When this occuw, the time of cure required to attain standard stress a t standard strain carries an entirely new significance, thereby vitiating the rate-of-cure comparisons between different rubbers. As used by de Vries, the modulus criterion also entails extrapolation with all its classical limitations. Since, dealing with so di/ficile a colloid as rubber, most of us commit a sufficient number of sins when we interpolate, he who extrapolates is “stretching” things too far. Again, modulus passes through no mathematical maximum within striking distance of good cures. LastIy, it is ambiguous, because when we enter the range of lower sulfur percentages we encounter increasing possibility of reversion. Any given value of modulus may then designate two vastly different states of cure. A point to which we do not find any reference in the litera-
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Vol. 18, No. 11
INDLTSTRIAL AND ENGINEERING CHEMISTRY
_
TENSILES AT BREAU
PALE
CREPE
STRESS STRAIN C U R E S
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Figure 4
ture is that, in a family of curves rotating with advancing cure, the best technical cures frequently occur a t a point where the modulus begins to slow down. This means that &h,e modulus is least sensitive where most needed. In general, like tensile strength, it moves upward long after technical cure has been passed-generally, in fact, even after tensile maximum has been passed. For these reasons modulus or “standard curve” methods, although of prime importance and convenience as a plantation control, cannot be regarded as generally acceptable criteria of optimum cure. We pass next to the Schidrowitz-Goldsbrough-Hatschek miterion. A brief analysis of this method of cure has faheady been p ~ b l i s h e d ,and ~ a t the time of writing additional comments are being published on both sides of the Atlantic. Regardless of the outcome, it is a most sugtgestive piece of work, and cannot but promote a closer knowledge of the mathematical relations of the rubber curve. At the moment it does not seem to be within the realm of practical testing methods. The tear criterion, of which we hear more every day, cannot vet be admitted as a practical criterion for the optimum cure as defined, and this for the reason that it has not been brought to the necessary convenience and precision. TENSILE PRoDucT-Employed about a decade ago by Eaton and Grantham as the product of tensile strength .and find lengthi and called by Stevens the “tensile product,” this property, or rather composite of perhaps the two most jfnportant, properties of rubber, was featured prominently in the published work of Eaton and Grantham. These
authors, while appreciating its value, regarded it, from the physical point of view, as signifying only the tensile strength, referred not to the original but to the ultimate cross section. This physical significance seems never to have impressed engineers or rubber workers generally as adding anything vital to its meaning. The rubber engineer, like any other engineer, has to deal with the tensile strength of materials as they are, and not as they might be after stretching five or ten times their original length. From the point of view of strictly plantation control of curing rates, the tensile product figures, as de Vries has pointed out, do not offer any palpable advantages over those arrived a t by standard curve or modulus methods. Thus, the tensile product criterion has hitherto made no great impression upon plantation curing procedure. In the technical laboratories’of the rubber manufacturer, however, this factor has quietly, but definitely, assumed a dominating position. The increasing significance attached to proof resilience, or energy of resilience, as that single index of rubber quality most intimately related to pigment reenforcement and also to wear resistance has reminded technologists that tensile product may be most usefully regarded, not only as a corrected tensile figure, but as itself an energy index. When a rubber specimen obeys Hooke’s law its tensile product, measured in energy units, comes to exactly twice its proof resilience. Where, as in most cases, the curve does not follow Hooke’s law, its degree of concavity, measured in terms of’energy, has been described as the concavity factor.5
C a n . Chem. J
, 4, 160
(19241).
5
Wiegand, Tma JOURNAL, 17, 623 (1925)
INDUSTRIAL AND ENGINEERING CHEMISl'RY
November, 1926
Thus, in general, we have the following connection between proof resilience and tensile product: C X TP E y = ___ 24
The concavity factor, although highly responsive to pigmenhtion, is influenced only slightly (in a positive sense) by advancing cure. Thus, we should expect, on theoretical grounds, that t,he position of the energy maximum would occur at a cure very close to that producing maximum tensile product. That this is the case will be illustrated by actual examples in our figures. STRESS STRAIN (URVES
TENSILE8 AT BREAK
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60
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20
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Figure 5
By definition tensile product represents a balance between elongation and tensile strength, the two dominating qualities of rubber, giving each equal weight. Used as a cure criterion, it carries with it the proof resilience maximum, thus compressing into one parameter a wealth of important practical information as to net quality. We repeat that the time of cure necessary to develop the mathematical maximum in tensile product marks a t the same time the development of the maximum (immediate) net quality of the specimen. Later we shall show how the technical cure often requires a curing period less than that required to develop the tensile product maximum and, in some cases, more. But it can readily be shown that such departures always originate either in the necessity of paying a premium for future quality or of attaining certain special properties, such as freedom from bloom, exceptionally high modulus, etc. For these reasons we advocate the mathematical maximum of tensile product as the state of cure to which the rate of cure may most usefully be referred, and by which measured. It is universally applicable to all mixings and to all grades of rubber; it is sensitive to cure; it is unambiguous; it is easy of measurement and of calculation; finally, it marks the point of optimum immediate quality. With modern mixings it tends to coincide, as well, with a very large proportion of technical cures. Let us now examine some possible objections to this criterion. Firstly, it may be said that its dependence upon breaking values marks it as inherently inaccurate. To this we reply that the inaccuracy can in large measure be eliminated by the method of duplicate highest tests instead of the arithmetic mean. More important still, we feel that the physical properties u p to rupture, and no others, can present a true and complete picture. The wearing down of a rubber overshoe or a tire tread is governed primarily not by some intermediate value of the stress-strain curve, but by its end points. Next, it may be suggested that in low-sulfur mixings where
1149
reversion occurs at longer cures (or even sometimes a t short cures) the tensile product does not represent a true balance between tensile strength and elongation, because the increasing elongations due to reversion tend to compensate for the diminishing tensiles. To this we agree, at the same time pointing out that such compensation is not sufficient, at least in the numerous cases we have examined, to distort the smooth convexity of the tensile product curing curve. Undoubtedly, this compensation delays the fall in tensile product with overcures and thus, in effect, flattens its peak. Since, however, reversion occurs only in certain mixings and gives ready warning of its onset (convergence of tensile and elongation curves), the practical compounder will select his cure on the near or under sides of the tensile product maximum. We cannot see that the flattening due to the compensation mentioned above detracts from the validity of the tensile product criterion. Finally, objection may be taken to the tedium of the long series of curves required to develop the tensile product maximum curve. Our answer to this is that we do not suggest the necessity of working out this curve as a routine item in the control of plantation crudes for which the single point method may in most cases suffice. Certain it is that no proper understanding of the properties of a crude or of a mixing can be attained without the completion, however tedious, of a complete range of cures. CHOICEOF Coupoum-For the purpose of determining the time of cure to the optimum, now defined as the tensile product maximum, it is of advantage toadopt a test mixing that will develop a highly convex tensile product curve, thus sharpening the maximum and rendering it more sensitive. Next, a mixture should be selected which isfreefrom reversion even a t excessive overcures. Clearly, to meet these conditions the sulfur ratio on the rubber in a pure gum mixing must not be less than 7.5 per cent. The standard formulas adopted by most of the plantation workers are suitable. If it is desired to ascertain the rate of cure with organic accelerators. some moportion such as six of sulfur with one of hexa'or four i f sulfur with one of A AGINGvs,O P T I M U M diphe n y 1g u a n id i n e A TECHNICAL C U R E would seem satisfact o r y , although the moment one gets away from pure rubber-sulfur mixings the issue becomes comp l i c a t e d . For instance, there is the question of o r g a n i c acid content in the crude; natural variat i o n s i n acidity are bound to show curing v a r i a t i o n s even in highly a c c e l e r a t e d 70 80 90 /00 90 compounds, and yet a HIGH INTERMEDIATE HIGH * s ACC. report to this effect -LOW LOW would be quite misACC. S leading to those, the CONVEX P L A TEA0 number of whom is Figure 6 rapidly i n c r e a s i n g , who automatically add some stearic or similar acid to their formulas. We therefore leave the matter of an organic, or more generally an accelerated, test mixing out of our purview. AGING-If rubber did not grow old, the selection of a
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I100
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Figure 10
that the course of the aged quality curve fully confirms the general trend described by Stevens. Maximum aged quality requires cures markedly shorter than those producing paximum green quality. In these cases the sulfur ratio is 10 per cent or more, and the tensile product curing curve is distinctly convex. Fortunately, the goods made from pure gum compounds of this type are not in general required to withstand abrasive or cutting impacts, so that the necessity of “paying the premium” for stability does not constitute a serious practical handicap. INTERMEDIATE SULFURAND ACCELERATOR RATIOS-LOW CONVEXCURVE (Figures 3 and 4)-This range of mixings is characterized by medium sulfur percentages, together with definite, although not the highest, addition of accelerators. The curve connecting tensile product with cure may be described as of “low convex” character. Of great importance is the concomitant change in the trend of the aged tensile product curve, which likewise flattens out. This phenomenon is of the highest significance to the technologist, enabling him to advance his cure to a point much nearer the green optimum without any loss in aged quality. The percentage figures run between 80 and 90. The premium paid for aging stability is thus reduced from around 30 to about 15 per cent. Low SULFUR-HIGH ACCELERATIOX-PLATEAU CURVE (Figure 5)-In this category is shown a tire-tread mixing, Aging as a Function of Nature of Mixing the sulfur ratio being 3.5 per cent. Note the flatness of HIGH SULFUR-LITTLE OR No ACCELERATION-HIGH the aged tensile product curve. With such a curve the CONVEX CURVE-Figure 2 includes the results of our time technologist has a much wider scope. He can choose as aging determinations on first latex rubber. It will be seen his technical cure a point practically up to the tensile product technical cure criterion would, for the most part, coincide with the optimum cure as outlined above. Unfortunately, the works technologist must take out an insurance policy against the senile decay of his goods. As premium he gives up a fraction of his “green” quality. As Stevens pointed out in 1916* for rubber-sulfur compounds, the tensile product maximum a t the end of one or two years does not occur a t the same time of cure as the green maximum, but lies markedly to the left of it. Clearly, the difference between the tensile product a t the cure yielding maximum aged results and the maximum developable tensile product (Figure 1) measures the discount upon present quality paid to attain maximum durability. If one wishes optimum quality in the future, one must be content with something less than optimum quality now. We have therefore chosen to express the state of cure of a specimen, whether cured on the under or on the over side, in terms of the tensile product developed a t the cure in question, expressed as a percentage (to the left or right) of the maximum tensile product. From the foregoing figure we see that the cure developing maximal properties after aging occurs a t a time when the tensile product is only about 70 per cent of its maximum. The premium paid for longevity was, in this case, nearly 30 per cent-a pretty high rate.
-
INDUSTRIAL A N D ENGINEERING CHEMlSTRY
November, 1920 5000
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maximum, thus combining almost full immediate quality with full aged quality. The percentages are between 90 and 100 per cent. In this case the tread mixings were successfully used in the manufacture of tires, and the indicated technical cure was the one actually used for volume output. Examples of actual time aging of from one to two years are portrayed in Figure 6. This diagram is built up from only a few sets of green and aged data, and merely illustrates the principle governing the aging behavior of mixings ranging from pure gum-high sulfur, to modern, high “set-up” works mixings. Minute precision is expressly disclaimed. It may be of interest to plantation workers as illustrating the relationship between aging curves and types of ‘[vulcanizing skeletons,” and also the importance to the works technologist of flat curing curves both green and aged. With sulfur and accelerator ratios fixed, that crude rubber is clearly the best which will develop the flattest green and time aging curves. No doubt, this has had the attention of research workers in the East.
cures, as shown, are those selected in the works and confirmed by successful performance in service. TUBETYPEMIsIms-Figure 7 shows a red tube organically accelerated. An arrow shows the state of cure adopted in works production which gave satisfaction both in the green and aged condition. Curing curves are distinctly convex. Sulfur ratios are moderate, as also is the acceleration. As we have seen in discussing the aged behavior, this kind of curing set-up permits a technical cure of within 10 to 15 per cent of the green tensile product maximum, and yet because of the flat character of the aging curve, without danger of early perishing. In this tube there is slight bloom. Figure 8 shows a gray tube with a sulfur ratio of 6 per cent and lower acceleration. In this case the technical cure works out at 86 per cent. These few illustrations of tube results constitute the only available cases which fulfil the conditions laid down for this paper-namely, a record of continuous success in large-scale production. Undoubtedly, in many cases tube mixings are being used with more vigorous acceleration, and consequently very flat green and aged curves, and with little or no bloom. Doubtless, in such cases technical cure will be found to approach more closely to the tensile product maximum. TREAD TYPEComOuNDs-Figures 5 , 9, and 10 (treads), and 11 and 12 (cushions) all depict technical cures chosen by practical technologists, carried through into works production, and confirmed by service performance. Here the requirements of immediate resistance to abrasion, together with, in certain cases, the elimination of bloom, or else, as in cushions, the necessity of low hysteresis, have pushed the technical cure
Technical Cure
We pass now to other practical considerations, illustrating how the technical cure may require to be pushed still further to the right, sometimes in fact even beyond the green optimum. The mixings chosen for this purpose represent in every case practical works formulas. They also represent volume production, generally over a period of years. The technical
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1162
right up to the maximum tensile product. While this has unquestionably involved a certain sacrifice in aging, the flatness of both green and aged curves renders this sacrifice of less importance to the practical compounder than that of immediate quality. The cautious technician will view the state of cure he is selecting, not from the point of view of how far to the right he may safely go, but how close to the aging optimum he can adhere and still attain the necessary immediate properties. RUBBERFOOTWEAR-The technical cure in the case of rubber footwear mixings requires a very fine balance of judgment. (Figures 13 to 16, incl.) The sulfur ratio is always SOW
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low and the acceleration high, since for the most part these goods must show no bloom. The acceleration may be either by litharge, mild accelerators of the type of thiocarbanilide, or ultra-accelerators. I n the latter case the sulfur ratio may reach extremely low values ( 2 per cent or less). I n practically all of these cases, the technical cures may, and indeed must, come very close to the tensile product maximurn. When the condition of "no bloom" obtains, a footwear technologist will, in general, endeavor so to arrange his compounds that the standard heat used in the works lies just beyond the earliest nonblooming cure. He seeks to prevent the exhaustion of free sulfur and, on the contrary, aims at keeping the free sulfur content as high as the solubility will permit, making due allowance for errors in curing. If he pushes his cure too far to the right, he will incur inferior aging, and finally, reversion, the bete noire of low-sulfur compounding. I n the case of litharge formulas, the technical cure is seen to lie at, or sometimes slightly to the left of, the tensile product maximum. With mild organic accelerators, the nonblooming requirement brings the technical cure to the right of the tensile product maximum. With very low sulfurs and ultra-accelerators, on the other hand, conditions of nonbloom may occur before the tensile product maximum is reached, and this for the reason that the dissolved free sulfur, although not bloomed, constitutes a considerable fraction of the total sulfur used. What was said in respect to tire treads regarding the importance of not moving too far to the right, applies with equal force to footwear cure determinations. The extreme flatness or plateau character of footwear curing curves makes very careful control necessary. Although the essential character of the compound may be learned through laboratory press cures, they do not run strictly parallel to those obtained in dry heat, whether a t atmospheric or higher pressures. In addition to the point at which maximum tensile product is developed, the shoe
Vol. 18, No. 11
technologist is therefore careful to observe the trend of every significant physical property, both green and aged, including flexing, sun-cracking, and wear tests. Figure 6 may now be reexamined in the light of the foregoing data regarding technical cure. Here again the figure is intended to be only diagrammatic, but will serve to bring out the important features of our discussion regarding curenamely, (1) the vital significance of the green and aged tensile product maxima as in general marking, respectively, the right and left boundaries of technical cure; ( 2 ) the advantages of modern, lom-sulfur, highly accelerated mixings in bringing these boundaries closer together and, cons? quently, permitting greater immediate quality without the sacrifice of aging quality. With the steady improvement in compounding science, it would thus seem that the tensile product maximum, as well as denoting maximum immediate quality, may one day likewise denote maximum aged quality. When this occurs, technical cure will, in general, coincide with optimum cure, the latter being defined as tensile product maximum. SECOKDARY CURE CRITERIA-SO far we have only indicated the general position of the technical cure, together with a few remarks about the special considerations which may influence its choice. With the modern type of compounds, characterized by very flat curing curves both green and aged, difficulties of picking the best cure are in many cases increased. The physical properties change slowly; the stress-strain curves do not rotate fan-like with the advancing cure; elongation may remain practically unchanged over a very wide curing range, likewise tensile strength. For this reason wherever possible the cautious works chemist employs other aids. SULFUR-while sulfur has divided its place with accelerators as protagonist of vulcanization, it can, rievertheless, play a very vital role as a critic in the wings. I n the case of slightly blooming mixings of intermediate sulfur and accelerator ratios, the technical cure can often be elegantly 5000
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controlled on the overcured side by the disappearance of bloom. Similarly, with low-sulfur, strongly set-up mixings, while useless on the overcured side, the appearance of sulfur clearly marks the undercure. In the latter case the determination of free sulfur, even though nonblooming, is a part of standard practice in most laboratories. It is indeed fortunate that with plateau type mixings the flattening out of the physical properties with consequent loss of sensitiveness to cure is compensated by increased sensitiveness of the sulfur-bloom criterion. The more the technical cure is pushed to the right of the point corresponding to maximum aging, the more vigilance
INDUSTRIAL A N D ENGINEERING CHEMISTRY
November, 1926
and attention must be shown to a criterion such as free sulfur. ARTIFICIAL AGING-The data we have shown as to the position of the time-aged tensile product maximum are not meant to serve as a practical guide in the selection of the best cure, but merely to indicate the general relation between the aging maximum and the vulcanizing skeleton. I n any given case the quick determination of the aging maximum can be made only on the basis of accelerated aging tests. Unfortunately, neither the heat aging test, nor the oxygen bomb, nor any other artificial aging test, can claim entire, or even approximately entire, correspondence with time aging. Thus, for instance, the mixing designed for dry heat curing will show up astonishingly well on a heat aging test, whereas another mixing, built for press curing, may fall down badly; and yet both may be equally proof against the ravages of time. Here the complexity of the technical cure reaches its climax. Some knowledge of the actual time aging behavior must be a t hand before the accelerated aging results for any given mixing can be safely applied. With such general knowledge the aging maximum can be determined quite closely by an artificially aged series. One of our correspondents6 has been kind eriough’to state a convenient short-cut to this determination as follows: The tensile figures obtained a t cures 20 per cent below and 20 per cent above the factory cure are expected to show equal values on aging. This, it will be observed, is merely a convenient way of determining the apex of the aged curve. It is of vital importance in each case to work out the time of cure corresponding to the aged maximum, since this may be regarded as the lower limit of technical curing. 6
J. W. Schade, The B.
F.Goodrich Co.,Akron, Ohio.
1163
From this limit the technologist moves his cure toward the right to the minimum extent necessary to attain freedom from bloom, or exceptionally high modulus (soling and heels), immediate abrasive resistance, low hysteresis, etc. Until the technic of artificial aging comes closer to the effects of time, the practical compounder must continue to be ultra-conservative regarding the change of curing “skeleton.” Where he lacks all time aging data he must wait till these can be obtained before attempting a drastically new compounding set-up, based solely on artificial aging. TEAR AND TECHNICAL CURE-Tear has been briefly mentioned in connection with the standard or optimum cure. Tear is, however, already playing a useful role in certain types of technical cure determinations. An excellent beginning in the technic of tear testing is that of E. C. Zimmermann, which provides means of separating the true resistance to tear from the observed data. Tear is obviously a highly complex phenomenon. It is hoped that there will be an increasing volume of published data relating tear to other well-known physical properties both in the green and aged condition. Tear determinations become less sensitive to cure the higher the pigmentation, and also grain effects become increasingly disturbing. Tear deserves careful attention with a view to its refinement as a guide to a state of cure which lies within the limits we have prescribed as suitable for technical cure. The chief difficulty, a t present, seems to be ready and quantitative means for its determination and calculation. Acknowledgment The writer wishes to express his deep appreciation of the important part played throughout the preparation of this paper by his colleague, D. F. Cranor.
The Present and Future of Reclaimed Rubber By H. A. Winkelmann PHILADELPHIA RUBBERWORKSCo.,AKRON,OHIO
HE consumption of reclaimed rubber is greater today than ever before in the history of the industry. The volume consumed in rubber products is larger than that of any other compounding ingredient. The fact that products made with reclaimed rubber are capable of rendering service equivalent to those without it is resulting in its greater use. A survey of the literature on reclaimed rubber shows that, despite its importance, there has been very little work published on this subject, except in the patent literature. Any further increase in the consumption of reclaimed rubber above that which is due to the normal growth of the rubber industry will depend both upon the reclaimer and rubber manufacturer. In recent years we have learned a great deal about the use of reclaimed rubber through proper compounding, with the result that it is now being used with satisfactory results where formerly it was not used a t all, or only in small amounts. Improvements in its quality can only be attained through research and development. Utilization of scrap rubber is an economic necessity. The position of the rubber industry is rather unusual in that it produces a by-product for which it is the only outlet. At present the reclaiming industry is wholly dependent upon the rubber industry. The development of other outlets for scrap or reclaimed rubber, or products made from them, would tend to stabilize the industry.
T
The relation between crude and reclaimed rubber consumption for the United States from 1919 to 1926 is shown in Table I. At prese.nt 1 pound of reclaimed rubber is consumed for every 2 pounds of crude rubber. In 1921 and 1922, when the price differential between these two products amounted to only a few cents, 1 pound of reclaimed rubber was consumed for every 4.5 pounds of crude. The reason for the present use of reclaim in such volume is that there are certain advantages to be obtained thereby. There is a saving of labor and power due to its plasticity. Compounds containing it may be mixed in a shorter time and pigments can be incorporated into them more quickly than when no reclaimed rubber is present. For tubing machine work the presence of reclaim often gives a smoother product at an increased rate of speed. Table I-Relation
between Crude a n d Reclaimed Rubber Consumption in United States (From figures by Rubber Association of America) Consumotion -~
Year 1919 1920
1921
1922 1923 1924 1926 1926
Crude rubber consumed Pounds 406,231,000 373 507 000 383:OOO:OOO 631 680 000 683:200:000 750,400,000 863 600 000 (6 months) 406:300:000
~~
-RECLAIMEDRUBBERratio of reConsumed Produced claimed rubber Pounds Pounds to crude 169,504,000 0.42 179 980 000 0.48 85:OOO:OOO 0.22 133,870,000 0.21 173,000,000 0.25 189,300,000 0.25 307,600,000 0.35 206,400,000 0.50
