Ferruarv 



1904.] 



THE INDIA RUBBER WORLD 



153 



VULCANIZATION AND VULCANIZING TEMPERATURES.* 



IF it were not known, as the result of many chemical analy- 

 ses, that in vulcanization there is a chemical union of 

 Rubber and Sulphur, we would be justified in inferring 

 that the union is the result of a chemical process, from the 

 fact that it proceeds in accordance with rules which observers 

 have deduced from a large number of observations of chemical 

 processes in general. 



The Rubber hydrocarbon is composed of 10 parts of Carbon 

 and 16 parts of Hydrogen, or a multiple of these numbers, and 

 therefore belongs to that class of hydrocarbons that are able to 

 form combinations. Hydrocarbons having a much larger pro- 

 portion of Hydrogen do not form addititive compounds and 

 are therefore called saturated hydrocarbons. The union of 

 Rubber and Sulphur is brought about by the influence of heat 

 in the same manner as many other sulphides are formed. The 

 union also proceeds more rapidly with each increase of temper- 

 ature, and more slowly with each decrease of temperature, 

 which is a rule applicable to chemical processes in general. 

 Neither the Rubber that has been vulcanized nor the Sulphur 

 of combination is any longer soluble in their usual solvents. 

 The freezing point of vulcanized Rubber is very much lower 

 and the boiling point much higher than the freezing and the 

 boiling points of crude rubber — if Rubber can be said to have 

 a freezing or a boiling point. The lowering of the freezing 

 point of a substance and the raising of its boiling point is a re- 

 liable indication of a change in the chemical condition of the 

 substance. 



In the sense that the boiling point of a substance is the tem- 

 perature of ebulition, rubber has no boiling point. But in the 

 sense that the boiling point is the temperature at which the 

 substance decomposes, a sense in which it is often used, both 

 crude and vulcanized Rubber have boiling points. 



In the sense that water freezes and forms ice, crude Rubber 

 has no freezing point. But it stiffens as the temperature falls 

 near the freezing point of water and has all the physical ap- 

 pearances of having been frozen. But, being an uncrystalhzable 

 substance, it cannot crystallize as water does when it freezes. 

 When the temperature rises above its freezing point, there is 

 no change of form, as when ice turns to water, or when a metal 

 melts. It merely resumes its normal condition without having 

 its characteristics changed in the slightest as the result of the 

 freezing. This normal condition is retained, when subjected 

 to a rising temperature until, at a temperature no higher than 

 those employed in vulcanization operations, its structure 

 changes and the substance decomposes. 



As soon, however, as masticated Rubber is compounded with 

 a proper quantity of Sulphur and Litharge for the temperature 

 to which it is to be submitted, there is immediately a change in 

 its characteristics. It freezes at substantially the same tem- 

 perature as before, showing that in this respect, Sulphur has 

 yet brought about no change. The boiling point, however, 

 (temperature of decomposition) of the compounded Rubber is 

 immediately changed, and the compound cannot be injured by 

 any proper vulcanizing temperature to which it may be sub- 

 mitted. But if the masticated Rubber, before the addition of 

 Sulphur, be submitted to the same temperature, it decomposes 

 and is no longer, Rubber, whatever may be its constitution. 



The compounded Rubber, it is true, softens at first under the 

 influence of a rising temperature, but it does not decompose, 



* Copyrighted, 1904, by The India Rubber Publishing Co. 



and as soon as the heat is removed, it either assumes its nor- 

 mal condition or is partially vulcanized. If the heat be con- 

 tinued long enough at proper temperatures, the compound 

 gradually becomes vulcanized Rubber. But, at no time after 

 the Sulphur is added to the masticated Rubber until vulcaniza- 

 tion is complete, does the action of heat affect the compound 

 injuriously. 



Thus, Sulphur has a very marked effect on Rubber immedi- 

 ately on being incorporated with it ; or, in other words, imme- ' 

 diately on being brought into close contact with every portion 

 of it. What is the nature of this effect? What can it be, ex- 

 cept that the change which we call vulcanization begins with 

 the incorporation of the Sulphur? How else can the action 

 of the Sulphur be explained ? 



If Rubber vulcanizes at low temperatures, this effect of Sul- 

 phur is easily understood. But, if it only vulcanizes at high 

 temperatures, as some contend, the effect produced by Sulphur 

 at lower temperatures cannot be explained. Theoretically, 

 there is no reason to believe that Rubber does not vulcanize at 

 low temperatures. Because we have observed it to take place 

 only at high temperatures, is no proof that it does not take 

 place at lower temperatures. " We have in general no ground 

 for supposing that any chemical process which takes place at 

 a higher temperature cannot take place at a lower." To illus- 

 trate this, let us consider the familiar subject of combustion. 

 We observe it taking place rapidly at high temperatures. But 

 because we do not observe its progress at lower temperatures 

 we cannot say that combustion cannot then take place. The 

 fact is that " no temperature can be found at which combus- 

 tion just begins, and such that just below this point no com- 

 bustion takes place at all." And so it cannot be said that any 

 temperature has yet been found at which vulcanization just 

 begins and such that just below this point no vulcanization 

 takes place at all. 



It is therefore desirable to know at what temperatures 

 Rubber is commonly vulcanized on a large scale, and then to 

 ascertain by careful experiment the range of temperatures in 

 which it readily vulcanizes. Regarding the exact temperatures 

 at which Rubber is actually vulcanized on a commercial scale, 

 very little is known even by the most careful operator, when the 

 steam or the dry heat process is used. In vulcanizing by either 

 of these processes there is generally no definite relation at all be- 

 tween the temperature indicated by the thermometer and the 

 actual temperature of the Rubber undergoing vulcanization, 

 and from the nature of the case, as these operations are usually 

 conducted, there can be none. 



Such operations are carried on in large closed cylinders, often 

 60 feet long or longer, or in large close rooms often 12 feet 

 wide and 25 to 30 feet long. In the former case live steam is 

 admitted to the cylinders generally from two inlets, and in the 

 latter case the chamber is heated by coils of steam pipe which 

 are a little below the level of the floor. It is evident that one 

 or even two thermometers cannot indicate the correct temper- 

 ature of all parts of either these steam or dry heat vulcanizers, 

 unless the vulcanizing medium be kept in rapid circulation, even 

 "when no articles are being vulcanized. The loss of heat by con- 

 stant radiation requires a constant supply, which tends to main- 

 tain the unevenness of the temperature. But if the chambers be 

 filled with goods, say hose on hollow iron hose poles in the steam 

 vulcanizer and boots and shoes on wooden lasts in the dry heat 



