I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
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Vol. 16, No. 10
Recent Developments in Rabber Accelerators' By Julian F. Smith TIIS B. F. GOODRICH Co.,AKRON,OHIO
HIS discussion is a nontechnical summary of various
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uses of accelerators, intended to give a point of view to those not actively engaged with rubber chemistry. From the discovery of vulcanization until organic accelerators were introduced, no contribution of fundamental significance was added to the heritage left by Goodyear and his contemporaries. Inorganic accelerators were known from the start; Goodyear used white lead in his first recipe for vulcanized rubber and magnesium oxide in later experiments. The commercial use of organic accelerators originated with the Diamond Rubber Company in 1906, when Marks and Oenslager found that aniline would hasten the cure.
PURPOSE OF ACCELERATORS The primary purpose in using accelerators was originally to shorten the curing time. But the change in method led to other advantageous results, so that accelerators are now used for the threefold purpose of improving the mechanical properties, shortening the time of cure, and improving the aging properties. The effect of accelerators on mechanical properties may be visualized by comparing the tensile strength
not exert their maximum effect on the cure until a relatively high temperature is reached; but even these exert some curing effect a t low temperatures. The low-temperature accelerators are those which begin to exert their maximum effect a t relatively low temperatures. Thus, Twiss finds that aldehyde-ammonia will cure a t 95' C. and the Bayer Company claims that piperidine will cure a t 105" C. This may be contrasted with hexamethylenetetramine, a typical hightemperature accelerator, which has little action below about 130' C. and is most active a t 150' C. The so-called ultra-accelerators constitute a recent improvement in the low-temperature accelerators. Certain of the xanthates and the dithiocarbamates will cure fairly rapidly a t ordinary temperatures. The temperature of maximum effect is somewhat higher (50" C. or above), but the action is rather rapid at room temperature. Most organic accelerators belong in one of the following groups: 1-Organic bases, or compounds which form bases during vulcanization. 2-Carbosulfhydryl derivatives (thioureas, dithiocarbamates, thiurams, disulfides derived from mercaptans, dithio acids, xanthates, thiazole derivatives). These compounds either contain the grouping C-SH or undergo reactions by which it is formed. 3-Nitroso compounds.
Accelerators of the first two classes generally require the presence of an activator, usually zinc oxide, to bring out their maximum curing effect. This activation is attributed to the formation of a metallic salt of the accelerator or of one of its reaction products. Certain zinc salts, such as the stearate or oxalate, function in the same way as zinc oxide.
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FACTORS IKFLUENCING CHOICEOF ACCELERATOR 2000
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curves for the same stock when vulcanized with and without an accelerator (Fig. 1). The increased strength and elasticity may be attributed in many cases to the fact that a shorter time or lower curing temperature is possible when an accelerator is used; by decreasing the effect of heat there is less disaggregation and the cured product is therefore stronger. The effect on aging properties is due to retardation by the accelerator of the slow atmospheric oxidation of rubber. NQ accelerator totally inhibits the oxidation, but most accelerators retard it and thus lengthen the life of the cured article. KINDSOF ACCELERATORS Although it is common to speak of high-temperature and Sow-temperature accelerators, there is no sharp line of demamation. High-temperature accelerators are those that do 1
Received July 29, 1924. 1)
I n choosing an accelerator to be used for a particular purpose, the following factors must be considered: Cos-Both purchase price and the result obtained enter into this factor. Thus, an expensive accelerator of high power may be more economical to use than a cheaper lowpowered accelerator. EASEOF HAKiDr21NG-This factor is important from the cost standpoint, as well as for other reasons. Some liquid accelerators are volatile or do not mill into the rubber readily; some solids tend to escape as dust or do not disperse into the rubber. For accelerators that are difficult to disperse a master batch or a flux may be necessary. VARIABILITY-This is to be avoided; the accelerator for a given mixing should be uniform in composition and amount from batch to batch. The amount can be controlled readily enough, provided such losses as evaporation of volatile substances or dusting away of powders are avoided. When the accelerator is a single chemical individual, variations in composition can be prevented by controlling the purity; but in the case of resinous and amorphous substances consisting of mixtures of two or more constituents variations can be prevented only by carefully controlled standard procedure in manufacture. ToxrcITy-Aside from any effect on the value of the finished goods, there is a serious disadvantage in the use of poisonous or irritating substances. The added burden of maintaining industrial hygiene may even be sufficient to switch
INDUSTRIAL A N D ENGINEERING CHEMISTRY
October, 1924
the choice to a nontoxic but otherwise less desirable accelerator. AcTIvm-This is exerted in a variety of ways. The rate of cure may be slow or rapid, and may or may not be uniform; that is, there may be a rapid initial action followed by a slow final cure, or vice versa. The effect on tensile strength or modulus of elasticity may be great or small. An accelerator that gives a high tensile strength may give a high or a low elongation. Again, activity is manifested in the temperature and time required for curing. All these phases of activity need to be considered in adapting the accelerator to the desired result. C O L OEFFEcTs-some ~L accelerators cause discoloration and hence are unsuited for white or very light colored compounds. Paraphenylenediamine is an example of this. This disadvantage is not always limited to light-colored stocks; even the blaclrest compound may be thrown off color. GEKERALArrLIcABILITY-other factors being equal, the choice of an accelerator for a particular purpose will go to that one whivh can be used for the greatest number of other purposes. SHAPEOF CURIKG CuRvE-The shape of the curve obtained by plotting tensile strength against time of cure (under constant conditions of temperature and composition) is an important factor. Choosing, for example, a simple mix containing rubber 100, ZnO 10, and sulfur 4 parts, to be cured a t 287” F. with 1 per cent of accelerator, the curve may show for one acceleral or a rapid rise to a maximum tensile strength, quickly followed by a rapid fall. Another accelerator may give a slower rise and a nearly or quite horizontal curve a t the maximum. This latter feature, known as the “plateau effect,” is desirable because it avoids overcuring (a falling curve). These types of curves, as compared with a control (no accelerator), are represented by A and B in Fig. 1. A variety of shapes can be obtained with the same accelerator, however, keeping all factors constant except the accelerator-sulfur ratio. I’or example, some accelerators give a series of curves such as 1hose in Fig. 2, where A represents B represents C represents D represents
ACCELERATOR High Low High Low
SULFUR High High Low Low
Fig. 2 illustrates how curing curves can be altered to produce predetermined effects. Similarly, various shapes of curing curves can be obtained by varying only the temperature and the rate at which heat is applied. HOT-CUREMETHODS
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Most of the products of a modern rubber factory are cured by one of several hot-cure methods. These include: OPENSTEAhf-Packing, sheeting, and the like are generally cured by open steam under pressure. This is a cheap method which permits large output for a given amount of equipment. It requires careful temperature control, especially in large heaters subject to regional variations of temperature. The contents must also be protected from drippings of condensed steam, which would cause discoloration, and undercuring due to local cooling. The open-steam cure does not impart an attractive finish to the goods. It is common to modify the open-steam cure by wrapping the articles in cloth, which retains th(1ir shape. I n this case the finish shows the marks of the cloth. Accelerators for open or wrapped steam cures should preferably be not too easily soluble in hot water and not decomposed by steam. Diphenylguanidine is commonly used. PRESS OR MoLo--T\Iany articles are cured in the mold which gives then1 their final shape. This is true of pneumatic tires
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(with a suitable arrangement inside the tire for expanding i t against the mold) and of heels, hard rubber pieces, and various shaped articles. Some items, such as belts, are cured in a press. I n either case the source of heat is steam in a press or autoclave. On account of the cost of mold equipment and the relatively slow turnover, the overhead charges are high and this method of curing is comparatively expensive. It has, however, the advantages of high quality and attractive finish of the goods produced. 5000
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FIG.2
Nearly all the common accelerators are applicable to some of the mold cures. Two of the old favorites are hexamethylenetetramine and thiocarbanilide. The amines and aldehyde amines are also used, as are some of the more recently developed accelerators, such as the thiuram derivatives. HOTWATER-Dark-colored sheeting or packing and hardrubber sheet are the chief items that are cured in hot water. Hard rubber is coated on both sides with tin, and other goods are wrapped in cloth before immersing. Heat is supplied directly or indirectly by steam. This permits a gradual and uniform temperature rise and even distribution of heat, but the method is too slow to permit rapid turnover and the hot water tends to discolor the goods. Diphenylguanidine is often used in hot-water cures. Some of the aldehyde amines are also used. DRY HEAT---Rubberized fabrics are frequently cured in dry heat in air or an inert gas a t or above atmospheric pressure, with or without mechanical circulation of the gas. This method has the disadvantages of slowness and great difficulty of maintaining a uniform temperature. It does, however, permit large-scale production, and is applicable to sheeting of any color. Dry heat places a limitation on the choice and use of accelerators, because many rubberizing compounds contain litharge, which has a tendency to inactivate many accelerators. The inactivation is due to formation of unstable lead salts of the accelerators. This limitation does not apply to accelerators that do not form unstable salts of the metals, and it is even possible sometimes to overcome the difficulty and make use of compounds which do give unstable salts. Thus some of the xanthates and dithiocarbamates have been used, and thiocarbanilide and some of the thiuram disulfides have also found application in dry heat cures. S O A P S T O N E - H O ~articles, ~ O ~ such as tubing, bulbs, and various druggists’ sundries, are embedded in powered soapstone and heated by steam to the desired temperature. This permits even distribution of heat, provided the soapstone layers are not unduly thick, and the powder retains
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
heat well. It is a slow cure, however, and necessitates particular care in cleaning the articles after curing. Any of the accelerators found satisfactory for dry heat will be suitable for a soapstone cure.
New Uses for Sulfur in Industry’ By W.H. Kobbe
COLD-CURE METHODS Cold curing methods are used much less than the hot cures, for various reasons. The principal processes in this group are: ACIDCURE-A solution of sulfur chloride, usually in carbon disulfide, is the curing agent. Goods dipped in the solution are almost instantaneously vulcanized a t the surface. The method is comparatively cheap, but has several drawbacks. The lack of penetration of the curing agent limits the application to very thin articles, such as toy balloons, dental dam, dress shields, and the like. The irritating and toxic nature of sulfur chloride necessitates care in handling. The use of carbon disulfide involves a fire hazard. The choice of coloring agents is greatly limited because most organic dyes are attacked by sulfur chloride. Finally the mechanical and aging properties of an acid-cured article are inferior to those of a like steamrcured article. VAPORCum-The sulfur chloride may be applied as vapor instead of in solution. Except for the absence of solvent, the foregoing remarks concerning acid cure are equally applicable. Owing to the high speed of these cures, there is no need for using accelerators to shorten the time. Adding accelerators for any other purpose is useless, because organic accelerators are decomposed by sulfur chloride. Hence it is not customary to use any accelerator for a n acid or vapor cure. AIR CuREs-Certain so-called ultra-accelerators, among which are some of the xanthates and dithiocarbamates, will cure rubber at ordinary temperature if allowed to stand. This method is too slow, however, for any process in which large production is wanted.
NEWVULCANIZATION PROCESSES Among the recent innovations which show signs of promise but have not yet become thoroughly established in the industry may be mentioned the Peachey process and the Schidrowitz method of vulcanizing latex. Peachey’s process consists in exposing the rubber as such or in solution alternately to sulfur dioxide and hydrogen sulfide as gases or in solution in benzene or a like solvent. The reaction yields sulfur in an active form, which gives a rapid cure. The method makes all colored effects possible, since organic dyes are not attacked. Like the sulfur chloride cures, it is limited to very thin goods. It als6 involves the difficulty of handling the gases, or their solutions, and controlling the concentrations. Quinine is recommended by Peachey as an accelerator for vulcanizing rubber in solution by his method.2 The Schidrowitz method consists in mixing a suitable amount of sulfur and accelerator with latex (with or without coloring agents and other compounding ingredients), and vulcanizing in an autoclave. The vulcanized latex can then be used for rubberizing fabrics and for other purposes. The use of organic solvents is thus avoided, but this advantage is largely offset by the necessity of curing in an autoclave. Piperidine and aldehyde-ammonia are mentioned by Schidrowitz as accelerators in his p r o c e ~ s . ~ These processes show a trend of thought in rubber chemistry and may indicate directions of future progress. Meanwhile, the chemistry of accelerators is making steady progress along the established lines of the prevailing hot-cure methods. 2
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British Patent 190,051 (December 14, 1922). British Patent 193,481 (June 19, 1922).
Vol. 16, No. 10
TEXAS GULFSULPHUR Co.,NEWYORK,N . Y.
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HE very large supplies of crude sulfur developed in the United States during recent years have encouraged search for new applications of this interesting substance especially in the nonchemical field. Sulfur, being of comparatively low fusibility, acid-proof, wet-resistant, of high dielectric strength and low heat conductivity, would seem to be an ideal material for many industrial purposes and even in special construction work. It is available in almost unlimited quantity and at less than one cent per pound. On the other hand, its flammability and extreme brittleness in the crystalline state preclude its use in the fields mentioned. I n other words, pure sulfur as such is entirely unsuited for structural purposes, acid-proof tanks, or similar applications. I n general, there are two ways of utilizing the valuable characteristics of sulfur and a t the same time largely modifying its flammability and brittleness. One method is to impregnate other substances with sulfur, and the other is to admix certain materials to form various sulfur compositions. I n both methods advantage is taken of the comparatively low fusibility of sulfur. Many materials have been impregnated with molten sulfur by simple immersion in a bath of this substance, and among these may be mentioned diatomaceous earth, concrete, building brick, fibrous materials, various asbestos products, sandstone, fiber board, and many others. A few of the results achieved will be briefly described. CONCRETE Cured Portland cement concrete absorbs approximately 17 per cent by weight of molten sulfur. This treatment greatly increases its strength, makes it impervious to moisture, and improves its resistance to destructive agencies. The unexpected and surprising strength developed in preliminary experiments with impregnated concrete encouraged an extensive series of tests. This increase in strength is especially interesting when it is considered that the tensile strength of cement mortar briquets is increased from five to ten times. When the voids in a substance are eliminated by filling with another material, the compressive strength of the mass is naturally improved. These impregnated briquets, however, show a truly remarkable increase in tensile strength, which is not so easily explained, especially when it is considered that the tensile strength of sulfur is very low, being approximately 200 pounds to the square inch. A lean mix of cement and sand breaking under a tension of 150 to 200 pounds is frequently increased to 2000 pounds by this simple treatment. Many hundreds of tests have been made and few of the treated samples failed a t less than 1200 pounds per square inch. Many of them went as high as 1700 to 1800 pounds per square inch and a few resisted a tension of 2000 pounds. The weaker mixes show a greater proportionate increase in strength than those of a richer composition. A 1 : 5 mix after treatment compares favorably with 1: 2 mix, although very much weaker before being impregnated with sulfur. The following figures from the Bureau of Standards and other sources indicate the effect of impregnating concrete : 1 Presented under the title “Nonchemical Uses of Sulfur,” before the New York Sectmn of the American Chemical Society, May 2, 1924.
