December i, 1903.] 



THE INDIA RUBBER WORLD 



75 



THE NATURE OF VULCANIZATION.* 



WHAT is Vulcanized Rubber? It is somewhat surpris- 

 ing that there is no established definite meaning 

 (or a term which is in such common use. The rea- 

 son (or this doubtless is that its meaning varies ac- 

 cording to the class of persons that uses it. To the general 

 public, it has no special meaning except that the rubber articles 

 so designated are adapted to the purposes for which they were 

 intended. To the dealer in such articles, it means scarcely 

 more. To the manufacturer, it means that these articles have 

 been subjected to the final step of a very complicated process, 

 and, as a result, possess certain physical qualities. To the 

 chemist, and to him alone, it means rubber that has become 

 chemically united with sulphur. He has in mind the chemical 

 change that has taken place during the vulcanizing operation. 

 The manufacturer has in mind only the physical properties be- 

 longing to the product. If a certain percentage of sulphur has 

 become chemically combined with the rubber, the chemist says 

 it is vulcanized, the manufacturer that it is not vulcanized un- 

 less it possesses certain physical properties. 



It is evident that there are great changes in the physical 

 properties of rubber that is well vulcanized, but it is not an 

 easy matter to define in simple terms what these changes are 

 It is the common belief that vulcanized rubber is stronger 

 more distensible, more elastic, and more durable than crude 

 rubber. This is not, however, necessarily the case. Rubber 

 freshly coagulated by the best methods is stronger, more dis- 

 tensible, and more elastic than almost any vulcanized rubber 

 to be found on the market, a fact that is well known to those 

 who are familiar with such crude rubber. Manufacturers are 

 familiar with the fact that the durability of vulcanized rubber 

 depends not so much upon the proper application of the vul- 

 canizing operation as on its proper previous manipulation. Dr. 

 C. O. Weber, who is probably the best authority on vulcaniza- 

 tion, says in his excellent work, "The Chemistry of India- 

 Rubber": "The physical state of the India-rubber colloid 

 while under vulcanization largely determines the physical con- 

 stants of the vulcanization product." 



It is practically impossible to judge of the durability of most 

 vulcanized rubber. Articles, to all appearances well vulcanized, 

 may have within themselves the seeds of decay, which may de- 

 velop in a few weeks, a few months, or not until after the lapse 

 of several years. Manufacturers of vulcanized rubber threads — 

 an article that probably requires more care in every step of the 

 manufacture than any other — are accustomed to preserve and 

 label one thread from each day's work for future reference. 

 Some of these samples will remain sound for an indefinite pe- 

 riod, others will begin to decay after five or six years, and 

 others after two or three years. It is very seldom that any of 

 the samples will show signs of decay sooner. And yet all have 

 been subjected to precisely the same vulcanizing process ; 

 all were made of the same kind of rubber, the best in the mar- 

 ket, and compounded precisely alike. These variations must 

 have occurred through very slight differences in the physical 

 condition of the samples at the time they were subjected to 

 the vulcanization operation— differences so slight that they 

 could not be detected by the most careful inspection, or the 

 most careful chemical analysis, and which were brought about 

 during the preparatory steps of the manufacture. 



On the other hand, there is no uncertainty as to the durabil- 



* Copyrighted, 1903, by The India Rubber Publishing Co, 



ity of crude rubber. Crude rubber of the best varieties will re- 

 tain all its useful properties for an indefinite period if preserved 

 from the action of sunlight and heat, which are fatal to both 

 crude and vulcanized rubber. Unvulcanized rubber shoes, 

 manufactured from Pard rubber, have been kept for more than 

 half a century without showing signs of decay. 



Perhaps the best general definition of the physical qualities 

 of Vulcanized Rubber is that given by Charles Goodyear in his 

 original patent of 1844 — that it is not affected by the ordinary 

 extremes of heat and ccld nor by the ordinary solvents of rub- 

 ber. But even this is not exact, for long continued heat, and 

 long continued subjection to the action of its ordinary solvents, 

 will affect it. 



It is now generally considered that there is a chemical union 

 of rubber and sulphur in vulcanized rubber and that the union 

 is brought about or assisted by the action of heat. This union 

 takes place only in the presence of the vapor of sulphur, and 

 proceeds more or less rapidly according as the temperature is 

 higher or lower. It is also considered that it is only dissociated 

 sulphur vapor that can thus unite with rubber. 



Sulphur, like camphor and iodine, can pass entirely into va- 

 por from the solid state at a temperature not much above the 

 ordinary. At ordinary temperatures its vapor has a percepti- 

 ble tension, which may explain what is said to be a fact that 

 thin sheets of rubber and sulphur become vulcanized when left 

 to themselves a long time. The union of rubber and sulphur 

 always requires time, as does the dissociation of sulphur. Rub- 

 ber is a colloid and, like all colloidal substances, it submits to 

 change but slowly. The union takes place throughout the sub- 

 stance of the rubber equally, if the temperature be equal 

 throughout ; that is to say, there is a gradual chemical union of 

 rubber and sulphur until the process is complete. We would 

 therefore expect to find, as is the case, the amount of combined 

 sulphur in the rubber sulphide to vary from a very minute per- 

 centage to the largest ever found, which, in the case of hard 

 rubber, is sometimes as high as 33 per cent. But the physical 

 qualities of what we call vulcanized rubber are not found un- 

 less at least from 2 to 2^ per cent, of sulphur has combined 

 with the rubber. 



With every increase of temperature the rate of vulcanization 

 increases rapidly, from the ordinary temperature to the highest 

 commonly used, which does not exceed 350 F. From 256° to 

 270° F. is, however, the usual range for soft rubber goods, 

 though many articles are vulcanized at lower temperatures. It 

 is not clear why the vapor of sulphur is dissociated at these 

 temperatures. The molecule of ordinary sulphur consists of 8 

 atoms of sulphur. At a temperature of 900° F. the molecule 

 consists of 6 atoms and at 1800° F. of 2 atoms, which is the 

 constitution of amorphous or colloidal sulphur. If such great 

 temperatures are required to change the sulphur molecules of 

 eight atoms to those composed of 6 and 2 atoms, why is it 

 that in the presence of rubber the sulphur is dissociated at so low 

 temperatures ? The answer doubtless is that at the higher tem- 

 peratures the sulphur molecules are all decomposed, while at 

 the lower temperatures used in vulcanizing there is a mixture 

 of undecomposed and decomposed molecules. But, in vulcan- 

 ization, not only is the combined sulphur changed to amorphous 

 or colloidal sulphur, but the uncombined sulphur of the com- 

 pound also. Is it not possible that the rubber itself assists the 

 dissociation of the sulphur ? We know that colloidal substances, 



