INDUSTRIAL A N D ENGINEERING CHEiWISTRY
March. 1923
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Persistence of Calender Grain after Vulcanization‘ By W. B. Weigand and H. A. Braendle AMESHOLDEN MCCREADY, LTD.,MONTREAI,, CANADA
HE STUDY of the
and to confirm in a quantiNot the least fascinating property of rubber is the anisotropy in its “grain” effect in untative way the fact well physical properties induced by mechanical working. The “grain” vulcanized rubber known in the art that eflect so called is of course met with daily by the works manager in forms a part of the daily many articles of vulcanized practically all classes of rubber manufacturing. Its complete eluciactivity of any rubber-facrubber-such as inner tubes, dation has, however, a much broader significance since many of the tory millroom, the foreman rubber footwear, solid tires, problems in protoplasmic structure are akin to “grain” in rubber. of which is fully familiar etc.,-give conclusive eviThus, for example, the electrical phenomena associated with the with the so-called “crawl” dence of the persistence rhythmic contractions of the human heart seem allied to similar pheof calendered sheet, its of “grain” after vulcaninomena accompanying the mechanical working or stressing of the temporary prevention by zation. On theoretical rubber colloid. The present wort$ is a very fragmentary study of prompt chilling either by grounds it was conjectured some of the “grain” phenomena persisting ajter fixation of the colmeans of a cold-bottom roll, that the ‘‘grain” effect loid through vulcanization. cooling drums, a water might well bedual in origin, bath, or by wrapping into a part being attributable to liner, and, on the other hand, its neutralization by shrinking the rubber phase and part to any pigment phase present in on a hot plate or in a hot-water bath under conditions per- the mixing. It was also thought that if vulcanization were carried out under controlled conditions, it might be shown that mitting free adjustment. persistence of “grain” was a function of the degree of freedom PREVIOUS INVESTIGATIOXS of the sample during cure. Five compounds in continuous use in works practice were Braendle2 studied the “grain” effect in crude rubber from the point of view of swelling in a solvent, and showed that after selected.
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equilibrium the swelling was much greater across the “grain” than with it, also that the elastic modulus was much greater with the grain than across it. Skellona drew attention t o the well-known difference in tear with and across the “grain.” Van Rossem4 has recently published some interesting quantitative studies on the anisotropy in the mechanical properties of sheets calendered-first, under conditions of free adjustment, and second, under conditions of restraint both by chilling and by wrapping into a liner. Van Rossem confirmed the disappearance of “grain” by heat treatment and attributes it to the Joule effect. He also found the “grain!’ effect to disappear during hot vulcanization. Wiegandz showed that “grain” could be obliterated, either by heat treatment in the well-known manner, or with even greater facility by immersion in a rubber solvent. He also took issue with Van Rossem in ascribing the heat retraction of unvulcanized rubber to the Joule effect, pointing out that it was essentially a condition of thermodynamical equilibrium implying reversibility, a condition not obtaining in the case of a chilled calendered sheet of uncured rubber. He put forward the suggestion that the phenomenon was essentially a temperature-viscosity condition. I,unnGsupports Wiegand’s opinion in regard to the roIe played by viscosity, and offers a very lucid visualization of the phenomena in terms of what may be called spheroidal components of rubber structure, assuming that the chief structural components consist of spheres of viscous liquid contained within more rigid and elastic membranes. He also suggests that the absence of “grain” in vulcanized rubber as claimed by Van Rossem may be due to conditions of vulcanization. The Bureau of Standards’ has published stress-strain data for various compounds showing definitely that even after vulcanization there remain significant differences between the tensile and elongation properties with and across the “grain” and that samples cut with the “grain” show stiffer curves than those cut across the “grain.”
EXPERIMEKTAL The present communication describes a few experiments designed to reconcile, if possible, the conflicting evidence of workers such as Van Rossem and the Bureau of Standards, 1 Presented before the Division of Rubber Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, P a , September 4 to 8, 1922. 2 Whitby, “The Testing of Rubber,” p 484. a Rubber Age, 1 (1921), 544. 4 India Rubber J., 62 (1921), 343. 6 Ibid., 62 (1921), 7 3 3 . e I b i d , 62 (1921), 831. 7 Bur. Standards, Bull. 38 (1921).
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Per cent COMPOUND CONSTITUENTS by Volume Pure gum tube compound Rubber 97 High-grade zinc oxide tube comRubber 92 DOund Zinc oxide 4 Rubber 9 Red antimony tube compound Antimony containinz40 per cent calcium *sulfate 10 Rubber 73 18 First-grade shoe-upper compound 3 2.5 Gas black Rubber 52 Whiting 35 Shoe soling compound Litharge 2 Gas black 3
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In all cases sheets were taken from regular factory runnings which had been calendered in such a way as to arrest crawl either by chilling over a bottom roll and cooling drums, or by winding into a fabric liner substantially in accordance with calendering schemes 2 and 3 as classified by Van Rossem.4 Vulcanjzation of these samples was carried out under four conditions designated as follows: D. MAXIMUM DISTORTION-CUR carried out in standard laboratory molds with exceptionally heavy overflow, as, for example, sheets plied up to 150 mils amd molded down to 95 mils. This cure was designed to induce complete elimination of “grain.” F. MAXIMUM FREEDOM-sheets were cured on carefully soapstoned tin plates in live steam. This cure was designed to permit free retraction, with a view to the elimination of that part of the “grain” effect attributable to the rubber phase. Ta. IMPRISONED BY FABRIC-In the tube compounds thi3 cure was carried out on regular tube mandrels with a straight jacket and cross wrapper under standard works conditions. In the shoe compounds samples were cured an a wooden pole covered with holland, the ends of the sample sheets being tied together by lap joints. Ib. IMPRISONED I N MOLDCAVITY-stock was cut to exact dimensions of the mold cavity, overflow being, therefore, extremely light, and in no case more than 5 mils on a sample originally 100 mils thick. This cure was designed to inhibit crawl, and so preserve the “grain” effect due both to the rubber phase and the pigment phn’se.
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FIG. C CALENDER GRAININ VULCANIZED RUBBER
The foregoing samples were cured to standard technical optimum in each case, dumb-bell test pieces cut with and across the “grain” and tested in aScott machine at room temperature.
the anisotropy of mechanical properties was measured in terms of the energies of resilience and of what we shall call displacement. We have chosen three criteria-X, Y, and Z.
CHOICEOF CRITERIONFOR EXPRESSING “GRAINEFFECT” CRITERIONX : ROTATION-The rotation of a stress-strain curve from the elongation axis is commonly measured by the An examination of the curves shows at once the trend of tensile at some fixed elongation-e. g., 300 or 700 per cent. On account of the wide range of stocks covered, no one elongation the “grain” effect as affected by compound and cure, but the is suitable for this purpose; hence it was considered preferable choice of criteria for quantitative expression of the “grain” t o use an integral of all the rotation elements-viz., effect was attended with some difficulty owing, in the first place, to the variety of stocks used and to the accidental T (with grain) dE errors introduced by careless handling on the part of factory x 100 % help. The experiL-X mental error of testT (with grain) dE ing technic was taken care of by the proper choice of scale in f i T ’ dE plotting-vie., 1000 = - x 100 % lbs. tensile = l/2 in. (with grain) dE = 100 per cent elongation, in accordance with Wiegand’s stand- viz., the excess resilience of “with the grain” over “across the grain.” ard graph form.a Since as a rule no pair of curves terminates at either the same The criteria for tensile or elongation, the upper limit of integration was fixed at measuring t h e 90 per cent of the average breaking elongation with and across. CRITERION Y : ENERGY AT RuPTuRE-’l’his is the energy of “grain” effect are at rupture expressed again, as the percentage difference reduced to the per- resilience of “with” over “across” and mathematically is centage deviation of Rupture Rupture the “across grain” FIG. 2 (with grain) dE (across grain) dE from the “with grain.” Instead of showing conditions of tensile strength Rupture x 100 % and elongation a t rupture which are inherently inaccurate, J‘? (with grain) dE
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INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1923
The energy was preferred to other criteria a t rupture as being the most comprehensive single index to the characteristic properties of a rubber sample. CRITERION z : DISPLACEMENT-This criterion was developed on the theoretical hypothesis that at least as far as the rubber phase is concerned, the “with grain” samples had traversed as a result of the mechanical working a part of their stress-strain curves before vulcanizaton. Owing t o the flat character of the early part of the curve, this displacement can be approximated by the lateral shifting of one curve to coincide with the other. The displacement is measured in absolute percentage elongation, and on the spheroidal hypothesis of Lunne the Criterion Z would be equal to twice the distortion of the component spheroidal structural units.
RESULTS Instead of recording all the stress-strain and other data, we have condensed the results into a compact table showing merely the percentage differences with and across the “grain” in terms of the compound used, method of cure, and Criteria X, Y, and Z. STUDY1 (Figs. 1 and 2)-At the extreme bottom and right are shown the average values for the three criteria-first, for all five compounds under varying conditions of cure, and second, for all cures under varying conditions of compound. The “grain” effect is decisively evident as measured by Criteria X and Z, being positive in all cases and reaching percentages of over 25 per cent. The effect of the addition of compounding ingredients is most marked, and also the effect of conditions of cure. Criterion Y , the energy a t rupture, is not a significant index t o grain phenomena after vulcanization, figures being sometimes positive and sometimes negative, and showing no systematic drift. Hence, the rejection of all criteria at rupture. STUDY 2 (Fig. 3)-This is a separate study of Compound 3namely, the antimony tube stock in which three grades of antimony were used containing varying percentages of calcium sulfate. This series was carried out t o ascertain the effect of acicular crystal habit upon the “grain” effect. The results are quite interesting STUDY 3-Here is shown another study of Compound 3, red antimony tube, given a cure designated Ia-namely, on regular steel mandrels and designed to illustrate the effect of the method of preparing the sheet. Regular calendering shows, as would be expected, maximum “grain” effect, the extruded sample intermediate “grain” effect, and the sample prepared by crossing the grain in the well-known manner practically no “grain” effect.
In view of the well-known difficulties of securing minute p r e cision in rubber-testing data and in view also of the somewhat
haphazard character of the compounds chosen, we put forward the following general conclusions as suggestive rather than in any degree final.
GENERAL CONCLUSI o NS The “grain” effect may persist after vulcanization over a wide range of compounds, including pure gum. Jn all cases the stress-strain curve with the “grain” ie stiffer than across the “grain,” thus confirming the data of the Bureau of Standards and others. The magnitude of the “grain” effect after vulcanization can be artificially controlled by, and varies markedly according to, the curing conditions and also the compounds, the latter being the more influential. Thus, for example, averaging all the compounds cured in the four different ways, the “grain” effect by Criterion X varies from 9 to 16 per cent. Averaging all the cures over a range of five different compounds, the “grain” effect varies from 6 to 20 per cent. The “grain” effect must be regarded as dual in originthat due to the rubber phase and that due to the pigment phase. The former can be preserved or obliterated during vulcanization according as the sample is imprisoned or markedly distorted during the cure. The latter persists under all conditions although definiteIy influenced in a similar direetion by the curing method. In general and for all methods of cure, pigmentation increases the “grain” effect. This is particularly true of pigments showing acicular crystal habit as is proved by the data of Study 2, the incorporation of 3 per cent by volume of calcium sulfate more than doubling the “grain” effect as measured by the rotation X, and nearly quadrupling it as measured by the displacement Z. The displacement Criterion Z offers a measure of t h e distortion of the component structural units or spheres (according to the Lunn hypothesis) in terms of actual per cent elongation. Thus, with Stock 1, pure gum, cured under conditions of minimum distortion ( I b ) , the displacemenb figure would indicate that each component sphere retained after vulcanization an elongation of ‘/zX 30, or 15 per cenk of its original length. On theoretical grounds this would seem to throw an interesting light on the structure of rubber, indicating the probable presence and persistence through vul-
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canization of definite structural elements possibly along the lines referred to by Lunn, which serve to impede any readjustment of internal stresses during the early stages of vulcanization. It would be most stimulating to compare the displacement Criterion Z of a calendered sheet after vulcanization with the crawl or shrinkage of the same uncured sheet when exposed to intermediate temperatures, and also to rubber solvents.
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Further study of ‘(grain” phenomena after vulcanization would include a systematic variation of both the amount and degree of dispersion of the pigment phases in the compound. It would also seem that the search for the ideal rubber pigment should include a study of the “grain” effects after cure in terms of not only size but crystal habit of the particles. The whole “grain” problem is replete with theoretical and practical significance.
Rubber Softeners’ By Paul M. Aultman and C. 0.North LEE TIRE & RUBBERCo., COWSHOHOCKBN, P A . , AND RUBBERSERVICE LABORATORIES, APRON,OHIO
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literature The following article describes the authors’ inoestigations with aggregate is broken into on the subject of regard to rubber softeners. smaller fragments. This is They haw studied the comparatioe action of various Softeners a reversible action as the rubber compoundon oulcanized rubber at the uulcanizing temperature of 140” c., rubber slowly recovers aping is comprised chiefly of articles on three raw mateand ham found thaf sulfur and accelerators aid in the depolymproximately its former properizafion of rubber, that the more adoanced the state of cure the erties. It is obvious that rials: (a) crude rubber, @) slower the depolymerization rate becomes, and that depolymerizing otherwise unexplained difpigments, and ( e ) accelersubstances usually accelerate the combination of sulfur. ferences in crude rubber atom. It covers quite thoroughly the variations of can be understood best plantation and wild rubby a conception of varirtbers, and lately it has considered the volume relations tions in the value of rz. However, many softeners when of the pigments as well as giving us better working theo- added in such small quantities as one-eighth of one per ries on accelerators. One other class of substances, how- cent, not only help to make the rubber more plastic, but ever, has not received the attention of the rubber chemists even prevent it from going back to its former state. in proportion to the importance it assumes in most practical These softeners aid in the process of breaking down, and mixes. We will designate this class as “rubber softeners,” hence are depolymerizing agents. This depolymerizing which includes substances such as mineral rubber, oils, tars, action of the softeners is undoubtedly carried on into the pitches, resins, and gums. The organic accelerators com- aging of the rubber, where it prevents the formation monly used may also be classified in this group, as they of the larger aggregates, thus tending toward preventing generally soften or render more plastic the uncured rubber. shortness. We assume that aging consists in the formation of larger aggregates through the agency of sulfur and through NEED FOR RUBBERSOFTENERS a natural tendency of the rubber aggregate to revert to larger Rubber softeners are added to mixes usually for two major Wgregates. reasons-first, to facilitate the processes of milling, calenderIn the curing of rubber we must assume that there are ing, and tubing; and, second, to cheapen the stock. The two major factors-one a breaking down of the rubber compounding of these into rubber gives us a wide variation aggregate, and the other a combination of sulfur and brokenin the characteristics of both cured and uncured rubber. down rubber aggregates. These processes can well be termed In the milled batch the rubber becomes more plastic and more depolymerization and polymerization. Our interest in softeneasily worked, even when very small amounts of Some sub- ers lies primarily in their effect on depolymerization. Their stances are incorporated. In the cured product the most effect on the combination of sulfur can be considered merely notable effects usually are the lowering of the stress values as a disturbing factor. of the stress-strain curve and the increasing of the elongation In order to find out this comparative depolymerizing effect, more than warranted by either any retardation of sulfur it was realized that the method of incorporating into a base combination or mere filling action. There is some effect stock in equal proportions and comparing the physical propon the properties of the rubber mix diametrically opposed erties with those of the base stock, when both were wlcanto that of finer pigments when added to rubber, as the pig- ized in the same cure, was the ultimate method of attack. ments make the stock less plastic and raise the values of the However, this method, even though used with some intereststress-strain curve, as well as cutting down the elongation. ing results in the case of pigments, has one serious faultThis effect varies greatly, depending on the softener used, namely, that it does not bring into consideration another probably because softeners have some action on the rubber variable, that of a change of the state of c u e . For instance, glue acts as a mild acce1erator;according to our combined aggregate itself. Rubber is generally considered a polymer of CloHle, the sulfur results, while clays often act as retarding agents. formula usually being written (CloH&, where n is a variable Comparison of any one effect can only be made when one number which must be large. It may well be called the variable is present. Inasmuch as the stress-strain curve coefficient of polymerization. The processes of milling is affected, not only by the state of the cure, but also by the and calendering, as well as heat alone, cause changes in the rapidity of the cure, it would be necessary for purposes of plasticity of rubber which can be explained only by assuming comparison to have the true combined sulfur the same a t other words, the rubber the cures tested. Any comparison tests which do not conthat the value n decreases-in d e r the coefficient of vulcanization cannot be accurate. 1 Presented before the Division of Rubber Chemistry at the 64th These caused us to another more direct Meeting of the American ChemicaI Society, Pittsburgh, P a , September 4 to 8. 1922. method for discovering the comparative actions of the various
