THE RUBBER INDUSTRY — GIBBONS 205 



amount of lead sulfide was formed; accordingly, the only colors 

 which could be produced were black or dark brown. 



The long heating practices and other conditions necessary to vul- 

 canize rubber in the earlier period imposed certain other serious 

 limitations on the product. There was a great variability in re- 

 sponsiveness of crude rubber to vulcanization. Modern accelerators 

 have made a great improvement in these conditions and have also 

 increased the strength, elastic properties, and durability of rubber. 



The practical effect is that an immense amount of money has been 

 saved to the rubber industry and to the public in increased output of 

 plants; and what is probably more important, the usefulness of 

 rubber has been greatly expanded. Old products have been im- 

 proved and made suitable to modern working conditions, and many 

 new products have been produced which would hardly have been 

 jjossible with the older methods. 



Although an enormous advance has been made in the technology 

 of accelerators, we are still without a fully satisfactory theory to 

 explain their action. Possibly there is no one theory which could 

 adequately explain their action, because so many different classes 

 of chemicals have accelerating power. Certain chemicals, such as 

 the thiuram and xanthic disulfides, are capable of vulcanizing rubber 

 without the inclusion of any free sulfur in the formula. Vulcani- 

 zation in such cases takes place at a rapid rate. The current opinion 

 is that the accelerator forms an intermediate compound with the 

 free sulfur and that the sulfur is then split off in active form, but 

 exact information about the way various accelerators do this is 

 lacking. 



NONRUBBER INGREDIENTS 



From the earliest time it was obsei-^^ed that commercial rubber 

 contained substances other than the so-called rubber hydrocarbons. 

 Some of these which were soluble in acetone were called "resins." 

 It was also found that there was a great difference in the susceptibil- 

 ity of various types of crude rubber to vulcanizing conditions. The 

 reason was for many years a mystery. Whitby, Bedford, and oth- 

 ers found that this so-called acetone extract or resin ingredient was 

 of great importance in the vulcanization process, particularly under 

 the conditions in use before the discovery of organic accelerators. 

 Whitby showed that in Hevea rubber, such as fine Para or plan- 

 tation rubber, the acetone extract consists largely of fatty acids. 

 These fatty acids have an important role in reacting with the basic 

 mineral materials such as lead oxide and zinc oxide, and if these 

 fatty acids are not present in sufficient amount, vulcanization is 

 seriously impaired. It was found that this deficiency could be made 

 up by the addition of fatty acid such as stearic, and that with 



