204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



employed generally for a number of years. Thiocarbanilide and hex- 

 amethylenetetramine then came into use. 



The aldehyde-amines developed by Cadwell and others were an 

 important addition to the rapidly growing list of accelerators. Guan- 

 idines were also discovered to be quite effective and have had wide 

 usage. Mercaptobenzothiazole was reported to be an accelerator by 

 Bedford and Sebrell and also by Bruni in 1921. It has been em- 

 ployed in many classes of rubber goods. It was found that zinc 

 oxide increased the effectiveness of many accelerators. One of the 

 most spectacular discoveries, of which little commercial use has been 

 made, was the observation that in the presence of zinc oxide and 

 certain chemicals the rate of vulcanization is accelerated and vul- 

 canization will occur in a reasonable time at room temperature. 



The inorganic accelerators such as litharge, and the early organic 

 accelerators such as an aniline, were used principally to accomplish 

 more rapid vulcanization. This permitted the manufacturer to 

 realize considerable economies in the output of his plant equipment. 

 In order to secure a satisfactory state of cure, however, it was 

 necessary in many cases to use an excess of sulfur, with the result 

 that after the vulcanization was completed a substantial amount of 

 uncombined sulfur remained. Some of this appeared on the surface 

 as a powdery deposit. This was known as bloom and was a familiar 

 characteristic of rubber goods made during the period. Certain 

 difficulties in the control of the product were also introduced since 

 the amount of sulfur combined with the rubber was in direct rela- 

 tion to the time of heating, up to the point where all the sulfur was 

 combined. The common practice of the day required from 7 to 10 

 percent of sulfur in the mixture in order to secure from 2 to 5 

 percent of sulfur combined after vulcanization. It is evident, then, 

 that unless the temperature conditions were carefully controlled, one 

 might easily make a product with more than the desired amount of 

 combined sulfur. 



With the introduction of modern organic accelerators this diffi- 

 culty was largely corrected. It is now possible to vulcanize rubber 

 in such a way that all, or practically all, of the sulfur is combined. 

 If the manufacturer desires to have 3 percent of sulfur combined 

 with his rubber, approximately that amount is used in the formula. 

 The modem accelerator also permits the manufacture of goods of 

 any desired color without the later development of an unsightly 

 bloom of free sulfur. Further, it permits the vulcanization of 

 rubber goods of any color in an atmosphere of heated air. Before 

 the introduction of organic accelerators the only rubber goods w^hich 

 were vulcanized satisfactorily in hot air were those accelerated with 

 litharge or other lead compounds. During this operation a certain 



