ONWARD
The latest ad vances in the NORMAN A. SHEPARD field of rubber comThe Firestone Tire and pounding are reviewed Rubber Company, and discussed briefly, with Akron, Ohio special emphasis on vulcanizing agents, acidic compounding ingredients, softeners, organic accelerators, antioxidants, and reinforcing powders. Progress in the use of rubber as a raw material for the manufacture of synthetic products is described. New rubber-like materials recently developed as substitutes for, or adjuncts to natural rubber, receive attention, as does reclaimed rubber. Some of the newer applications of latex are illustrated, as well as some of the more imporN A previous paper (.%), tant new uses of rubdelivered in the fall precedber a n d n e w d e ing “A Century of Progress,” signs for rubthe writer reviewed the outstanding advances in ber prodthe rubber industry during. the Drevious centurv. ucts. Cited as the most -important discoveries since tiat
-
MARCH .
OF
RUBBER
of vulcanization in 1839 were the method of reclaiming vulcanized rubber scrap (1899), the value of acidic materials in compounding (1904), organic accelerators (1906), reinforcing pigments (1912-1915), and antioxidants (19181923). It will be the purpose of this paper to cover briefly the present status in these fields, and to point out some of the more recent advances in other fields related to rubber.
unchanged. The oils soften and swell the rubber; the caustic disintegrates the fabric. It was formerly thought that the caustic also removed what free sulfur remained in the scrap, but it is now known (34) that the high temperatures used in reclaiming rubber combined the free sulfur with the rubber before the caustic penetrates sufficiently to dissolve any appreciable amount of it. Recent advances in this field consist largely in processing to obtain smoother products of greater uniformity that will disperse more readily in the new rubber to which they are added when fabricated into new rubber articles.
Vulcanizing Agents Sulfur continues to be the chief vulcanizing agent; very few commercial rubber products are compounded without it. There are some exceptions-for example, certain virgin crepe soles, which are used in the unvulcanized state, and thin articles vulcanized with sulfur chloride. The thiuram disulfide accelerators effect vulcanization without the addition of any free sulfur. Various investigators have assumed, probably correctly, that these catalysts of vulcanization split off sulfur during the cure, thus supplying the necessary vulcanizing agent. For reasons of economy these thiuram disulfides are ordinarily used in conjunction with some free sulfur; otherwise a high concentration of the expensive organic compound must be used. Two other elements of the same group in the periodic system as sulfur-selenium and tellurium-are finding application in conjunction with sulfur, the former in the insulation of wire and the latter, very recently, in fast-curing rubber compounds where unusual aging properties are desired. The use of both these elements appears to be increasing, that of tellurium a t a somewhat faster rate than selenium. However, the total tonnage of these used is relatively small as compared with the enormous volume of sulfur consumed by the rubber industry.
Acidic Compounding Ingredients There has been much activity in the past few years in the search for cheaper acidic compounding materials, which are required for bringing out the highest activity of certain organic accelerators and enhancing the quality of the product, for softening the rubber to the point necessary for satisfactory dispersion of the reinforcing powders, and for imparting the desired workability, molding, and tubing properties to the unvulcanized batch.l Stearic acid from animal fats has been almost universally used and still remains the most important material for this purpose, but large quantities are now obtained from sources other than animal fats, and other fatty acids of the same homologous series are finding application. Examples are lauric acid from coconut oil, the acids obtained from hydrogenated fish oils, the mixed acids from cottonseed foots, and also press-cake acids-that is, solid acids obtained by chilling the mixed acids from various plant fats and oils. Mixed acids obtained from petroleum by 1 I n this connection, the paper by I. Williams and C. C. Smith, “Hydrazines as Rubber Softeners” [IND. ENO.CHEM.,27, 1317-21 (1936); U. s. P a t . 2,018,643(Oct. 22, 1935)1,should be mentioned. Methods of reducing the amount of mechanical working t h a t must be done i n processing rubber are of vital interest to the rubber chemist. However. the use of a hydrasine i n rubber compounding i s not new [R. B. Naylor. U. S.Patent 1,418,825 (1922); S. M .Cadwell, I b i d , , 1,566,415(192611. T h e toxicity of this class of compounds (Beilstein, Handbuoh der organischen Chemie, 4th ed., Vol. XV, p . 106, Berlin, J. Springer, 1932) and their cost have, up t o the present time, limited their practical application.
Reclaimed Rubber In reclaiming rubber from used rubber articles, the original method of Marks ($1)) which consists in heating the rubber scrap with caustic soda a t high temperatures and pressures, continues to be the basic process. Higher steam pressures are used in certain cases (14), and numerous oils and softeners have been introduced, but basically the process is 281
282
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 28, NO. 3
Much progress has been made in the use of organic accelerators. Where formerly one organic cIiernica1 was ordinarily used to accelerate R rubber batch, now two materials are often employed, obtaining the joint effects of the two accelerators not only as regards acceleration itself, but as regards cured properties, for different accelerators have specific effects on certain physical properties of the vulcanized product. In some cases one accelerator activates another; that is, the result is not purely additive. Such combmations may lower acceleration costs and produce peculiarly valuable properties.
Reinforcing Powders The search for new and better reinforcing agents occupies much of the time and thought of the rubber chemist. Carbun black still holds first position among reinforcing powders, and much effort is being expended in learning how to use more effectively its enormous surface activity in order to improve the wear resistance of tire treah. Keeping pace with the rapidly increasing demands of the automotive engineer requires continuous intense activity on the part of the research and development staffs of the rubber industry. .LW1OT*I*L"L 0,L"LIIc.E The producers of rubb,er blacks, under pressure from the T I I IRELATED AZOLL COMPOUNDS consumers of their products, are making blacks far superior FIQURE 1. M E R C ~ ~ ~ . O R E N ~ ~ AND to those produced in the t.wenties. New standards of uniU0En A0 ORGANIC ACCELXERATOas OF TAB VULCANIZATION OF RURBER formity and freedom from grit have resulted. They have also materially assisted the rubber factories in improving the a p pearance of their mill rooms by largely removing the dust oxidation are also now available to the rubber compounder. nuisance during the incorporation of the black in the rubber. Many of these products can replace animal stearic acid in The past few years have seen the introdnction and adoption certain rubber batches with entirely satisfactory results and of the so-called dustless blacks (9, 3s). One of these is a considerable lowering of costs. form of black densified by a violent mechanical action wbich frees it from a considerable portion of its air and rolls it into Organic Accelerators small spherical pellets sufficiently compact to be relatively dustless (Figure 2). In the past few years no new class of organic accelerators Perhaps the most interesting of the newer reinforcing has been discovered; or, if any has been, the discovery has not agents are the new whitings (7, $8). Calcium carbonate is been made public nor has the product been made available precipitated in the presence of a protective colloid, under to the industry. Mercaptobenzothiaeole or its derivatives, such carefully controlled conditions of concentration and together with the substituted aryl guanidines and the aldetemperature that it comes down in a very fine state of sub. hyde amines, still constitute the main classes of accelerators division and the particles do not tend to grow. One of them of moderate speed; and the dithiocarbamates and thiuram is so fine that almost all of it is 0.5 fl or less in particle size mono- and disulfides are still the chief ultra-accelerators. (Figure 3), while ordinary whiting runs from 2.0 to 4.0 p , The very recent additions to the commercial accelerators with agglomerates as large as 20 fl (Figure 4). They promhe (Figure 1) have been chiefly derived from mercaptoto find a very definitc place in the economy of the rubber benzothiazole, I-for example, the zinc salt, 11; formaldehyde compounder. derivative (80, 4f), 111; the reaction product with forrnMent.ion should also be made of certain zincated barium aldehyde and aniline (9),IV; the benzoyl compound (151, sulfates which have recently been offered. V; and the oxidat,ion product (8.9), VI.
PIQURE2. R m m n CHANNEL BLACK,PELLXTED TO RENDER I T RELATIVELY DUSTLESS
FIGDRE3. SPECIAL W H ~ T I Y ~ ~ Frorrx~.4.
PRECIPITATED IN THE PRESENCE OF A Paomcmvn COLLOID
ORDINARY ParcmWHITING
TATED
INDUSTRIAL AND ENGI[NEERING CHEMISTRY
MARCH, 1936
283
New Rubber Substitutes or So-called Synthetic Rubbers Spacc will permit no more than brief reference to these
*#I
products, however interesting and remarkable they may be. Nor is it necessary to give more t h m pas&! mention to DuPrene in order to recall the remarkable chemical engheering feat that this product represents. k e d on the purely scientific work of Nieuwland (W,a product (6, 6) has been developed so closely resembling rubber in its physical characteristics that it is easily the most outstanding recent scientific development in the field of rubber. It is a misnomer to call it "synthetic rubber," since it is not a hydrocarbon and differs considerably from rubber in behavior. In spite of its present high price, it is filling a real need in the rubber industry owing to its romarkably low absorption of petroleum products in the entire range from gasoline to lubricating oils, and is now going into oil-resistant gaskets, gasoline hose, and similar articles. Another product, also highly oil-resistant, is that produced by the action of polysulfideson ethylene dichloride, and known as Thiokol ($4). Recently, modifications have been made that are, to a considerable extent, freed from the highly obnoxious odor of the original product., and t,he cornmercial possibilities are thereby much extended. Other synthetic products are bidding for notice in this field-for example, a product made from benzene and ethylene dichloride in the presence of aluminum chloride (56). Wonas 6 . SEcoNnAaY Aaoar*TIc AUINESUsEn DANTS FOR VULCANIZED RUBBER
ASTIOXI-
AS
Antioxidants Practically all of the chemical compounds (Figure 5) used comnercially as oxygen inhibitors or antioxidants for vulcanized rubber are secondary aromatic amines, many of them closely related to diphenylamine, VI1. Among the most reoent ones are p-isopropoxy diphenylamine (%), VIII; the corresponding P,P'-diniethoxy compound, IX; and N,N'diphenyl-p-phenylene diamine (8, f0, 8f), X. Both the alpha- and beta-phenylnaphthylamines, XI1 and XIII) continue to be used in large quantities, as does also a mixture of diphenylamine with 5,5-dimethyIacridan (17, N),XIV, and other substances. This acridan and the diiiydroqninoline(%), X V , are acetone condensation products of diphenylamine and aniline, respectively. All of these compounds not only preserve the tensile strength and elasticity of rubber, but also definitely reduce the tendency for highly compounded stocks, such as tire tread stocks, to crack when subjected to repeated and severe flexing. Especially interesting from this latter standpoint is the A',??'-diphenyl-p-phenylene diamine. The beneficial influence of this material is illustrated in Figure 6 , which shows st,nps of tread stocks after severe %exine. A very r e c e n t c o n t r i b u t i o n to our knowledge of antiolodants (86) concerns mixtures of s e c o n d a r y amines with byd r o q u i n o n e or pyrogallol. Using -4 B c triethanolamine F l o n n ~A. S~orfis .~Trnx: TREAD with this combinaA . iqo antioridant. tiongsi,eenhanced B . 1 part of B stsndard antioxidant to 100 Part. of rubber resistance to flex C. 0.7 asrt of the 6 ~ m eantiarident with 0.3 w r t N "-diphen I P phenylene diamine cracking. to 1oob.rt. of &e;. ~~
~
~
~
~
~
Latex The current literature, both journal and patent, is redundant with reference to the direct use of rubber latex. Each year sees new use8 and applications of rubber in this form in which it comes from the rubber tree. Preserved with ammonia and shipped either in its original concentration (approximately 38 per cent rubber content) or concentrated either by centrifuging, creaming, or evaporating to approximately 60-70 per cent dry rubber content, this colloidal suspension offers seemingly unlimited possibilities in making unique and quality r u b h r products. In many cases-for example, in the manufacture of tire inner tul~cs-it has failed to compete with ordinary plantation rubber (acid-coagulated), owing to its higher price in this form, but only for this reason and the higher manufacturing costs, for ex-
281
INDIJSI'RIAI, A S D ENGIXEER.IKG CHEMISTRY
FIGURE 8. RUBBER ARIICLX: MADEB Y DIFPIN
