THE RUBBER INDUSTRY — GIBBONS 207 



However, the shortage of zinc during the war period caused manu- 

 facturers to employ carbon black in increasing amounts. This 

 change, which was probably in many cases a matter of necessity 

 rather than choice, was destined to have a far-reaching effect on the 

 performance of rubber products, particularly tires. Carbon black 

 had been known and used for many years by the rubber industry but 

 principally as a coloring agent, and it was therefore employed in 

 relatively small amounts. There was little or no publication of 

 scientific data on carbon-black compounds during the war, but a 

 number of articles have appeared since that time, beginning with 

 the work of Wiegand (21). These studies have demonstrated the 

 remarkable properties of carbon black in rubber. 



Most of these properties are favorable. In addition to increased 

 tensile and higher modulus, carbon-black compounds are tough and 

 give a higher resistance to abrasion. 



A great deal of work has been done in an endeavor to explain the 

 properties of carbon-black compounds. Carbon black possesses a 

 number of properties which differentiate it from other materials in 

 powder form, and there has been a tendency to hold all of these 

 differences responsible for the peculiar properties of carbon-black-rub- 

 ber mixtures. Of these various factors, rubber technologists are in 

 general agreement that fine particle size is the most important. This 

 is supported by the observation that fillers of entirely different 

 chemical composition have been made of varying particle size and 

 that they have shown a tendency to give increased reinforcement as 

 the particle size diminishes. 



ANTIOXIDANTS AND IMPROVEMENT IN AGING RESISTANCE 



From the earliest time it was noted that rubber and its products 

 deteriorated. Most of us today have little idea how serious this 

 problem was to the rubber industry 100 years ago. The extreme 

 variability of the wild rubber and the unsatisfactory methods used 

 to produce it at that time have been mentioned. In addition, for 

 most articles it was necessary to reduce the rubber to a softened con- 

 dition in order to form the article. One way of doing this was by 

 the use of a mill and calender. Another method was to disperse the 

 rubber in a solvent and then spread the resultant dough on fabric. 

 This was the method used by Mackintosh, whose name was applied 

 to the waterproof coats made by this process. 



There have been several cases in the liistory of rubber where good 

 fortune seemed to attend the inventor and the manufacturer. Mac- 

 kintosh, however, was not entirely fortunate. He selected turpentine 

 as his solvent, which, according to present knowledge, was about as 

 poor a choice as could have been made. The tendency of turpentine 



