been found serviceable. Still another heat cure, now but little used, is that of solarization, whereby the fabrics, coated with a thin skin of rubber, are exposed to the sun's rays for vulcaniza- tion. In very exacting work, such as the vulcanizing of hard- rubber sheets, curing is effected by immersion of the material in hot water. In the cold cure either the acid or the vapour process is employed. In the former, goods are dipped in solu- tion of chloride of sulphur dissolved in bisulphide of carbon, after which they are given an alkali wash. For the vapour cure, rubber goods are suspended in a heated compartment in which the fumes of chloride of sulphur pass freely over the surfaces to be vulcanized. Over-curing is checked by the admission of ammonia fumes.
Since Charles Goodyear (see 12.240) in 1839 discovered, and in 1844 patented, his process for vulcanizing rubber with sulphur by means of heat, numerous attempts have been made radically to im- prove on his method and material ; but the bulk of the rubber goods produced is still cured by the sulphur and heat method. In the long train of experiments, many of which have led to important results, the curing of rubber has been effected by the use of sulphates, sul- phides, chlorides, nitrates, fluorides, bromides, iodidesand phosphorets of nearly all the common earths and metals, as well as chlorine, sul- phurous acid and various gases. The Russian chemist Ivan Ostro- mislensky, in later experiments, succeeded in vulcanizing rubber with trinitrobenzene and other nitro-compounds so as to impart all the qualities given it by sulphur, effecting the curing more rapidly than with sulphur, and with but one-twelfth of the material, while a lower temperature was maintained during the cure. Victor Henry, a French chemist, also reported in 1909-10 that he had effected the vulcanization of thin layers of rubber solutions by means of the ultra-violet rays, and others have made similar researches along the same line that have much scientific if not practical interest. The period of vulcanizing ranges from a few minutes to many hours, de- pending on the degree of heat employed, the nature of the compound, the thickness of the goods, etc. Factors which affect the rate of cure, as shown in England by Dr. Philip Schidrowitz, are the amount of protein or nitrogen in the crude gum, its stay in storage, its den- sity, the amount of smoke, formalin or other preservatives used, quantity of acid used in coagulating the latex, time in drying, age of latex-yielding tree, etc. An important vulcanization development is the chemical process of vulcanization described by S. J. Peachey, the English chemist, in 1918, after an investigation of the behaviour of rubber towards the various allotropic forms of sulphur. Unlike the Parkes process, which yields an addition-product of both sulphur and chlorine, this leads to the formation without the aid of heat of a sulphur addition equal to that produced by the hot-curing process. By it rubber, alone or compounded with fillers and pigments, is exposed successively to the action of two gases, sulphur dioxide and hydrogen sulphide. Diffusing through the rubber and interact- ing, the gases produce an especially active form of sulphur capable of combining with and vulcanizing the rubber at the ordinary tem- perature, and more thoroughly than by either the hot process or the sulphur-chloride cure. A density is acquired, it is said, unattainable by the older methods. The dual gas treatment can be used either for rubber in its original solid form or liquefied with a solvent. In the latter case the gases effect a complete pectization of the solution, forming a ielly which, on evaporating the solvent, is found to be fully vulcanized rubber. One of the advantages claimed is that fabrics, as well as organic fillers such as leather waste, sawdust, woodmeal, etc., that would be more or less decomposed by the hot cure or the sulphur-chloride cure, could be used with rubber for a wide variety of new and useful purposes, as in the making of fine or heavy re- formed leathers, linoleums, etc. It may also effect a considerable improvement in the waterproofing of cloth. Rubber footwear, it is said, may be produced by the new process without either the heat or pressure hitherto deemed essential, and without special machines for stitching and riveting, thus greatly cheapening the product. An additional advantage pointed out is that this process makes it possible to use both natural and coal-tar colouring-matter in rubber, so as to obtain both deep shades and delicate tints impossible with the old methods of vulcanizing.
Organic Accelerators in Vulcanization. The recent discover- ies of Dr. Spence and others that certain organic substances, termed accelerators or catalysers, mixed with rubber, notably hastened the process of vulcanization, have caused a revolu- tion in compounding and vulcanizing. Mineral or inorganic catalysers, such as litharge and magnesia, had been in use since the discovery of vulcanization. The organic type was, however, unknown until in attempts to vulcanize synthetic rubber it was found necessary to add organic accelerators to effect the union of sulphur and the rubber-like substance. It was but a short step to their use in connexion with natural rubber, and the re- sults were surprising. In fact, the time required in vulcaniza-
tion was reduced by one-half, thus doubling the vulcanizing output without extra heat or pressure. The theory of catalytic action, according to M. Andre Dubosc, an eminent French chemist, is explained as follows. He found that when a typical organic accelerator derived from an amine, such as hexamethy- lene tetramine, was mixed with sulphur, placed in a sealed tube, and heated to i3S-i45 C., not only carbon sulphide or hydrosulphuric acid but also sulphocyanic acid was evolved. At the vulcanization temperature, sulphocyanic acid separated, yielding hexavalent sulphur and cyanhydric acid. While the same temperature was maintained, this acid combined in the presence of ordinary divalent sulphur, producing unstable sul- phocyanic acid which by dissociation again furnished hexavalent sulphur. M. Dubosc holds that cyanhydric acid is the true active agent in such catalysis, the practical effect of which is the transformation of ordinary divalent sulphur into hexavalent sulphur. Assuming that vulcanization is the saturation by sulphur of a double bond in the rubber molecule, then by satur- ation of two such bonds the speed of the reaction between sulphur and rubber should be doubled, and by saturation of three bonds the speed would be tripled. Saturation is accom- plished with hexavalent sulphur generated during vulcaniza- tion through catalysing action of cyanhydric acid, evidently the true accelerator, and corresponding to Dr. Spence's " active principle." While a single molecule of rubber reacting with ordinary divalent sulphur will saturate only one double bond, hexavalent sulphur in vulcanizing may saturate three double bonds belonging to rubber with which it is in contact and during its polymerization, which M. Dubosc explains thus: (i) In the case of an aggregate of rubber molecules, the end molecules, which have a double bond, will be broken and give a molecule of rubber of which the four valences will be saturated. The aggregate will have its polymerization increased by one mole- cule and its resistance to break will be modified in a slight degree only. (2) In the case of vulcanization with hexavalent sulphur, saturation of the terminal free valences of three physical aggre- gates of rubber will take place. Polymerization will therefore be three times as great as that produced with ordinary vulcani- zations, because it acts on three aggregates instead of one. Resistance to break, dependent on polymerization, will there- fore be much increased and its theoretical tripling has been demonstrated experimentally. This theory would appear to apply not only to the amino (NH 2 ) or imino (NH) groups, but also to the nitroso compounds discovered by Peachey. Nitroso bodies decompose during vulcanization and generate cyanic acid. The latter, influenced by sulphur, yields sulphurous anhydride and sulphocyanic acid. The acid dissociates and leaves hexavalent sulphur, and the liberated cyanhydric acid again functions as a catalyser.
The most important organic catalysers are: (i) Aniline, exten- sively employed to quicken the combining of rubber and sulphur in vulcanization, particularly in the manufacture of tires and tubes, and obtained through a series of chemical transformations from coal-tar. It is an oily liquid boiling at 184-8 C. Special precautions are taken to carry off its noxious fumes and prevent contact of the oil with the skin of the workers. (2) Carbon bisulphide with aniline, diphenylthiourea or thiocarbanilide, melting at 154 C., used for quick-curing stocks. (3) Carbon bisulphide with dimethylamine; effects vulcanization within 15 minutes at 135 C. (4) Carbon bisul- phide with either dimethylamline, tetrahydropyrrole or dimethyl X methyl trimethylene amme. (5) Ammonium borate; effective but not practicable. (6) Aldehyde ammonia ; melts between 70 and 80" C. ; a very useful catalyser. (7) Quarternary ammonium bases; patented, rapid accelerators with aldehyde ammonia, para-phenyl- ene-diamine, sodium amide, bencylamine and naphthylenediamine. (8) Accelerene; widely used and powerful English catalyser. Used in one-third to one-half of i% reduces vulcanizing period to one- third of normal, and with quick repair compounds to one-eighth. It owes its activity to the presence of the nitroso group and adds notably to tensile strength of goods. (9) Para-phenylene-diamine; a very poisonous catalyser melting at 140 C., and subliming at 267 C., used with synthetic rubber. (10) Tetramethylenediamine; a substance formed from decomposing animal matter such as fish; called also putrescin. (u) Hexamethylene-tetramine ; known also as hexamethylenemine and formin; an accelerator "largely used; caution is required in its use as it is not only very soluble in water but vaporizes freely, irritating the exposed skin of the workmen. (12) Piperidine or aminopentane ; a liquid easily miscible in water, boiling
