THE COMPOUNDER IN THE RUBBER INDUSTRY B. S. GARVEY, JR. Sharples Chemicals, Jnc., Wayne, Pa.
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HE place of the comful application frequently T h i s paper reviews the scope of the field of compoundpounder in the indusdepends on compounding ing, the role of the compounder i n the industry, the mantry can be shown by his development. Thus, the ner in which he works, and the materials he employs. Its relation to the flow of macompounder cooperates purpose is to give a picture of the broad field of compoundterials and to the flow of with, guides, and judges ing as a perspective for the more important papers which ideas. the technical developments follow. T o accomplish this purpose, the place of the comFigure 1 is a rough illusof the industry. pounder i n the rubber industry, what the compounder tration of the flow of maHis control of the flow does, and the tools he has available to accomplish his job of materials and the twoterials from the forests, will be discussed. plantations, and mines to way flow of ideas gives the the tires, belts, girdles, golf collective compounder a balls, and the thousands of rubber products which are esposition of tremendous responisibility to all phases of the rubsential to modern civilization. ber industry. All the raw materials, from accelerators to zinc oxide, flow WHAT THE COMPOUNDER DOES into the stream of rubber manufacturing through - the medium of the factory recipe. Only after it has been incorporated in The compounder’s problem is to furnish a rubber compound some factory recipe does any which can be manufactured, a t the lowest possible cost, into an item which will do a specific job. material begin to move in sigFirst he must analyze the use requirements of the item to be nificant volume. The bemade. This analysis must show the combination of properties havior of each material must be revealed by extensive required for the successful functioning of the product. It must compounding studies before also show how these properties must be balanced, because there are times when improvement in one respect can be obtained only i t can be used. I n the rubber by sacrifice in another respect. I n some cases, as for certain factory such studies are basic types of bumper, this analysis is simple, while in others such ae for the control of uniformity, for tires, it is complicated. cost, and quality. To the This analysis of use requirements demands an accurate knowlsupplier they are the only Figure 1. Flow of Materiedge of what the rubber item must do and of the service condichannel through which his als in the Rubber Industry tions to which it r i l l be exposed. This information is usually product can be sold. Obobtained from the customer through the sales department. The viously some factory recipe is customer, the salesman, and the compounder are allies. Tough the objective in both cases. Thus, the work of each indiproblems require the best efforts of all three both in the originaI vidual compounder is a part of the integrated job of what may analysis and in the evaluation of results. 0l”ten the requirebe called the collective compounder or just the compounder. ments are too complex t o perFigure 2 is an attempt to show graphically the technical mit resolution into a group of structure of the rubber industry and the flow of ideas through properties which can be measit. For simplicity the materials production side is illustrated ured accurately in the laboraby a single example, the production of synthetic polymers. tory. While simulated scrvFrom the activities represented in the upper part of the chart ice tests are often of great flow ideas intended to improve methods of manufacture and the value there are many case3 quality of the product. From the activities a t the bottom of the where the real answer depend< chart come the final evaluation of these ideas, suggestions for on extensive field tests, Unimproving them, the analysis of service requirements, and sugder these conditions the best gestions for new materials. It is the int,eraction of these two results are obtained when the countercurrents of thought and its effect on the materials stream customer is sympathetic, the shown in Figure 1 which is responsible for much of the progress of Figure 2. Flow of Ideas i n salesman is technically comthe industry. the Rubber Industry petent, and the compounder Compounding is the backbone of this chart. The qualities is alert and well informed demanded for service or for manufacture are revealed in requests Next, the compounder must analyze the processing methods for compounds having special characteristics. If the comwhich can, or must, be used to make the article: mixing, calenpounder cannot meet the demands, he must pass the problem on dering, extruding, coating, building, metal adhesion, etc. This to development and research groups. The probabilities for a is important because the cost of manufacturing is just as desuccessful solution depend in a large measure on the accuracy pendent on the processing properties of the unwlcanized comwith which the problem is analyzed and stated. An idea, pound as the use is on the properties of the vulcanized coniwhether i t leads to a new theory or a new material, requires pound. compounding studies to check itsvalidityand usefulness. Success796
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LLASTOMERS-Compounding The analysis of processing requirements demands a thorough knowledge of the equipment available as well as of the types of compound which work best in each process. In this case, the allies of tho compounder are the men in the processing divisionfrom the supeiintendent to the mill or press man. Mutual qpreciation of the possibilities and limitations of compounds and equipment may result in the solution of apparently insoluble problems. These two steps show the compounder the over-a11 combination of properties he must have and how they must be balanced. He is now ready to start writing recipes to develop a compound v hich will do the job. I n writing these recipes, he draws on the store of fundamental compounding data on the materials which he must use These data are made available through studies designed to show the characteristic compounding behavior of the different materials and are supplemented by the past experience of the individual compounder. Uniformity of materials is essential to uniformity in production. To maintain this uniformity all m8terials are constantly tested in control recipes which are designed to accentuate any variability. For the evaluation of all these recipes laboratory tests are essential. There are available a great many established test procedures; others are constantly being proposed and evaluated. Some of these have been developed by compounders and others by engineers in the testing departments. I n Committee D11 of the American Society for Testing Materials, these tests are standardized, improved, and adopted. Although great strides have been made in the precision of these tests, the rubber labora-. tories are handicapped in this respect by the large number of variables, both objective and subjective, in the preparation of samples. While there is available a large body of knowledge of methods of evaluation, its usefulness is dependent to a great extent on the skill with which the compounder selects his tests and interprets his results. The best of the prelimha& recipes, as judged by laboratory evaluation, are then subjected to factory trials which show how the compound works on production equipment and which give finished items for service tests. On the basis of these factory trials, the most satisfactory recipe is selected for production use. The extremes in this general scheme of operation are, first, the case where one recipe is written and is put in production the next day, and, a t the other end, the case where a group of compounders works year after year to maintain and improve the quality of Figure 3. The Recipe in the compounds for specific Rubber Industry products such as tires. In brief, what the compounder does is write recipes and evaluate compounds. How this work permeates the technology of the industry is illustrated in Figure 3. The central line is the work of the factory compounders in the rubber manufacturing plants. The research and development compounders operate in the research laboratories, in the plants of both the rubber manufacturers and the suppliers of raw materials, and in academic institutions. The control compounders work in the factories of the rubber manufacturers and of the suppliers. I n every cwe, the essence of the job is the writing of recipes for a definite purpose. The immediate objective varies with the work of the individual compounder but the final objective is always a production compound. April 1952
MATERIALS
The Recipe. Since the essence of compounding is the writing of recipes, a typical recipe which will serve as a basis for the discussion of the materials used in compounding is as follows: Rubber Sulfur Accelerator Zinc oxide Steario acid Antioxidant Softener Pigment
Parts 100 3 (0.5-50.0) l(0.3-3.0) 5 (2.0-10.0) 1 (0-4.0) 1 (0-3 .O) 5 2 50.0) 50 {20-300) 186
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This shows the accepted form for writing recipes, the types of material included in the great majority of rubber compounds, and specific ratios for a type compound such as tread The figures in parenthesis show the ranges in the amounts of materials which are commonly used. Elastomers. Here the base elastomer is natural rubber. Fifty years ago only wild rubber was available in a variety of grades. Twenty years ago 95% of all crude rubber was one of the various grades of plantation sheets or crepe, although the situation had become slightly complicated because of the extensive use of reclaimed rubber. Aside from the small amount of synthetic methyl rubber used by the Germans in World War I, the choice was crude natural rubber or reclaim. Then came Thiokol, neoprene, the styrene rubbers, and the nitrile rubbers. The range of rubbers or vulcanizable elastomers now available are, in the approximate chronological order of their appearance: wild rubber, reclaim, plantation rubber, Thiokol, neoprene, nitrile rubber, styrene rubber, Butyl rubber, polvbutadiene, silicone rubber, and acrylate rubber. Of these, the polymers and copolymers of butadiene have a general similarity to natural rubber in compounding. However, this similarity may be a trap for the unwary because the differences are sufficient t o demand significant variations in compounding. Some of the polymer characteristics, such as oil resistance and brittle point, depend almost entirely on composition-e.g., the type and amount of comonomer. Others such as procesmbility and tensile strength appear to depend chiefly on chain structure and are controlled by the method of polymerization. In the neoprenes the diene itself has been changed by replacing the methyl group of isoprene by an atom of chlorine. Here, not only are the properties changed but the method of vulcanization is different. The other rubbers are not even polydienes. The thiokols are polysulfides with a distinctly different basis for cure and processing. The acrylate rubbers are saturated polyesters and have their own peculiar properties and methods of vulcanieation. With silicone rubbers the boundary between organic and inorganic chemistry is crossed-they may be considered as hybrids between glass and rubber. In addition to these vulcanizable elastomers there are the flexible plastics which are thermoplastic but do not vulcanize. The more important of these are polyvinyl chloride, polyvinyl butyral, polyethylene, Teflon, and Kel-F. Just as the chemical rubbers are both competitive and supplementary to natural rubber, so the flexible plastics are both competitive and supplementary to the vulcanizable elastomers. Each class of synthetic elastomer represents a group of materials rather than an individual. The characteristics of each individual polymer are dependent on the principal monomer, on the comonomer, on the ratio of the two, and on the method of, polymerization. Thus, the modern compounder is faced with a truly bewildering array of elastomers from which he must choose the one best suited to his needs. Without coordinating theories, this would be an almost impossible job. Consequently the compounder
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LLASTBMERS-Compounding needs a working knowledge of the current theories which relate the chemical composition and structure of polymers with the chemical and physical properties of his compounds. Vulcanizing Agents. Sulfur has remained the principal vulcanizing agent since its early use by Goodyear and Hancock. Several other materials can be used as vulcanizing agents for natural rubber and the butadiene polymers. Some are of academic interest only, while others such as the thiuram disulfides are used t o obtain special qualities such as heat resistance. With other types of elastomer other vulcanizing agents are used-for example, metallic oxides with neoprene, and benzoyl peroxide with some of the saturated polymers. The better known vulcanizing agents are sulfur, sulfur monochloride, selenium, tellurium, thiuram disulfides, p-quinone dioximes, polysulfide polymers, alkyl phenol sulfides, zinc oxide, magnesium oxide, and benzoyl peroxide. Accelerators. Accelerators are a contribution of the organic chemists to the rubber industry. They are used to increase the rate of cure, improve the physical properties, and improve the aging of rubber compounds. Of the thousands of compounds tried, perhaps 50 have stood the test of competition. The more important ones fall into the class of aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, or thiazoles. Of these the thiazoles are by far the most important. In this group are mercaptobenzothiazole (MBT), benzothiazyl disulfide (MBTS), and several activated thiazoles. With these accelerators or combinations of them, it is possible to vulcanize rubber at almost any desired time and temperature. Many important physical properties can be controlled to a considerable extent by the choice of the accelerator and the sulfur-accelerator ratio. Among the important characteristics of accelerators arc the scorch time, the vulcanization time, the cure time, and the plateau time. These are illustrated in Figure 2, where tensile strength is the property measFigure 4. Accelerator Characured. Other important teristics Illustrated with Tensile characteristics are the Strength effect of temperature variations on these times over a wide temperature range and the quantitative level of the plateau for various properties. These characteristics vary with the accelerator, with the sulfur and accelerator ratios, with the type of rubber, and with some of the other components in the compound. Accelerator Activators. Most accelerators require both zinc oxide and fatty acid in order to develop the best quality in the compound. Hence i t is almost universal practice to add zinc oxide and stearic acid to rubber compounds. Further activation and improved properties can often be obtained by the use of additional activators such as litharge, magnesium oxide, amines, or amine soaps. Antioxidants. Antioxidants are another contribution ’of the organic chemist to the rubber industry. They are used to retard oxidation of the rubber and hence its deterioration. While all have antioxidant activity some are more effective than others under such special conditions as flexing, heat, or copper contamination. Those, which are most widely used and are most eTfective stain and discolor in sunlight. However, recent developments in this field indicate that good, nondiscoloring antioxidants are on the way, if not already here. Pure Gum Compound. The first six parts of the recipe are for the pure gum compound. Such compounds are basic because all 798
other compounds are modifications of them. I n the pure gum recipe the type of rubber is selected and this automatically sets limits to the degree in which various properties can be reached The time and temperature of vulcanization are set within comparatively narrow limits by the choice of the amount of sulfur and the type and amount of accelerator. On the other hand, only a small percentage of commercial compounds are of the pure gum type, because the variation ira combinations of specific properties is quite limited and because those combinations which can be obtained are generally not the best for any particular commercial use. While certain properties, such as oil resistance, are set within narrow limits by the choice of the rubber, other properties, such as tensile strength, are as dependent on the pigments and softeners as they are on the rubber. Pigments. Pigments are finely divided powders which are used in rubber to improve properties, to change the balance of properties, and to lower cost. Some of them, such as zinc oxide, magnesia, lime, and litharge, are either accelerators or activators for organic accelerators. The principal types of pigment together with the variations in process, raw material, or treatment which result in the different grades are shown in the following table: Zinc oxide
Carbon black Clay Calcium carhonate Titanium dioxide
French procem American process Palmorton process Gas Oil
Lead content Coated Channel l’urnace Thermal
Air-floated Water-washed Water-washed. obemicallv treated Dry ground Wet ground Precipitated Coated Rutile Coprecipitated Anatave
Of these the carbon blacks are the most important and the one8 which have been studied most thoroughly. The effects of a pigment in a compound depend on particle size, particle shape, the chemical nature of the surface, the degree of dispersion in the rubber, and the tendency to form certain types of agglomerate or structure These pigment characteristics are governed by the raw materiaIs used and by careful control of the details of manufacture. In addition to these groups of pigments which are used in large volume, there are a number of other pigments which are used in smaller volumes and many color pigments which are used in small amounts solely for color. Softeners. The term “softener” is a sort of catch-all expression which is a hangover from a simpler era. It was formerly used for miscellaneous oils and pitches, such as palm oil, coal tar, and pine tar, which were used principally to get better processing through improved softness and tack. Other materials which required similar handling in the compound room were thrown into the same classification. With the advent of the flexible plastics and synthetic rubbers the number of materials has been increased and the scope of their functions enlarged. Today the term includes a great number of materials which are used for a variety of purposes. Hence, i t has become necessary to reclassify the group. The six main purposes for which these materials may be used are processing aids for uncured stock, softeners for cured stock, elasticizers, freezing point depressants, organic reinforcing agents, and extenders. Softeners which are used as processing aids for uncured stock may be classified as plasticizers used for softness, for retentivity or flatness, and for thermoplasticity; lubricants; tackifiers; and dispexsing aids. A number of developments have contributed to the increasing importance in compounding of the broad field of softeners--the development of the synthetic rubbers especially of the oil-resistant
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
RLASTOMERS-Compoundins-, type, the growing use of the flexible plastics in the rubber factory, shortages of rubber, and military demands for low temperature service. Miscellaneous Materials. In addition to the classes of materials in general use, there are a number of materials, such as antiseptics, blowing agents for sponge rubber, odorants, peptizing agents, and retarders, which are used in special cases. I n latex compounding all materials must be used as solutions or dispersions in water. This requires not only knowledge of colloidal Dhenomena but also a number of special types of materials. Some bf the more important types are antifoam agents, coagulants, dispersing and wetting agents, gelling agents, and preservatives.
SUMMARY
Briefly, the collective compounder is largely responsible for the cost and quality of all rubber products, he controls the sale of raw materials to the rubber manufacturer, and he coordinates the technical progress of the industry. He has available a host of materials, a number of rather well-developed theories, and phases Of the It is the support Of people in UP t o the individual compounder t o use these resources with skill, judgment, and tact in order to get the best results on his own particular problems. RECEIVED for review September 17, 1951.
ACCEPTED February 7, 1952.
BREAKAGE OF CARBON-RUBBER NETWORKS BY APPLIED S T R E S S A. F. BLANCHARD AND D. PARKINSON Research Centre, Dunlop Rubber Co., Ltd., Birmingham, England
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changes in the stiffness of reA study of the softening of rubber by applied stress has inforced rubber by considerstiffened by the inincreased our theoretical knowledge of reinforcement. ing the extension a t an arcorporation of reinforcing A semiempirical relation has been derived which under bitrary stress and the obfillers and this aspect of recertain conditions expresses the stress-strain curve in vious alternative technique inforcement haa received terms of two parameters, G and p. Carbon black inof considering the stress at much attention. Most rubcreases the modulus, C, which is roughly equivalent to an arbitraryextension (modber technologists defme stiffthe modulus in the equation representing the statistical ulus) have not been adopted ness or “modulus” as the theory of rubber elasticity, and causes a deviation from in the present investigation. stress a t a given extension theoretical behavior which is determined for a given It was thought that a matheor compression. An imporextension by the parameter, p, and which becomes very matical relation between tant fact to be noted in large as the extension is increased. Softening by stress is stress and extension for preconsidering the stiffness of due primarily to breakage of attachments between filler stressed specimens would be reinforced rubber is that it , and rubber, and the linkages between rubber molecules more useful, if it could be obcan be softened drastically formed through carbon particles are of two kinds of which tained so that the stiffness by the application of a prethe stronger type is due to chemisorptive attachments characteristics could be repstress which exceeds the which remain unbroken by stressing. The strong linkresented by parameters stress attained during measages are relatively few, and confined to carbon reinforcewhich are independent of urement, This phenomment, but the distribution of strengths of the weaker the extension. The most enon was first described (van der Waals) type does not depend on the chemical promising way of achieving and studied in some detail nature or grade of filler, although differences are obtained this appeared to be through by Mullins (17) and has in the total number of such linkages. The strong type of a semiempirical solution of since been investigated and attachment is thought to contribute much to improved the problem which, if sucdiscussed in some of its abrasion resistance and to be among the factors influenccessful, should lead to valuaspects by the authors (8). ing tensile strength. Evidence of a considerable reduction able fundamental inforThe work described in the of abrasion resistance by applied stress indicates that the mation. present paper was underweaker bonds also influence abrasive wear; their inditaken in order to throw To be generally successful vidual contributions apparently depend on their strength. more light on the mechaany empirical equation for nism of reinforcement by a the stress-strain curve of study of the changes which occur when the rubber is stressed. reinforced rubber must continue to apply as the concentration of It was considered that there was a particular need for a method reinforcing material in the rubber is reduced to zero. This of quantitative description of the stress-strain properties of remeans that in certain conditions it must approximate the equainforced rubber which included the softening of the rubber by tion for rubber that has not been reinforced. Theoretical applied stress, It was hoped that this approach would promote treatments of the elasticity of cross-linked rubberlike polymers a clearer understanding of the nature and strength of the attachinvolving the statistics of long chain molecules have been made ments between carbon particles and rubber molecules and their by Kuhn ( I 4 ) , James and Guth (IS),Wall (,%4),Treloar (99, role in reinforcement. W), and Flory and Rehner ( 7 ) . Guth and James ( 1 1 ) and later Wall (,%,$), who used a different method from that of Guth and James, derived the following relation between stress and QUANTITATIVE DESCRIPTION OF STRESS-STRAIN PROPERTIES extension The approximately linear relation in certain experimental r conditions between the extension at an arbitrary stress and the F = G La magnitude of greater previously applied stresses which was obtained (,%)for a limited range of prestresses and for one type of where F is the force per unit area of original cross section; a! reinforcing black does not apply for prestresses much outside the is the extension ratio-i.e., the ratio of the extended length to the range 5 to 100 kg. per square cm. This method of studying
UBBER is markedly
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