206 ANNUAL REPORT SAHTHSONIAN INSTITUTION, 19 40 



certain of the newer accelerators it was highly desirable to add 

 additional fatty acid. This discovery is of gi'eat practical im- 

 portance, as it tends to eradicate one of the troublesome variables 

 in the supply of crude rubber — that is, the variability in curing rate. 



COMPOUNDING AND REINFORCEMENT 



The earliest workers were accustomed to blend rubber with other 

 ingredients. Although these materials were added for various rea- 

 sons, in many cases the purpose was admittedly to cheapen the mix- 

 ture. Rubber at that time was high in price, and neither the 

 manufacturer nor the consumer had developed tests and specifications 

 for quality. Furthermore, the crude rubber obtained from the 

 Tropics was in many cases of low quality and, either through igno- 

 rance or desire, was mixed with adulterants. According to an early 

 handbook (15), Rousseau proposed the following definition: "The 

 manufacture of India rubber is the art of incorporating with it 

 cheap substances without too far diminishing its particular proper- 

 ties." While this criticism may have been harsh in some cases, it 

 was probably justified in many others. 



As it became necessary to develop rubber compounds to withstand 

 severe operating conditions, rubber manufacturers became interested 

 in the effect of various so-called fillers on such properties as abrasion 

 resistance and resistance to stretch. It was found that if used in 

 sufficient amount, zinc oxide had the property of actually increasing 

 the tensile strength of a rubber compound, although it was not 

 recognized at that time that part of this effect was due to a favorable 

 chemical action on the vulcanization phenomenon. With the in- 

 creasing importance of the automobile tire, rubber manufacturers 

 adopted zinc oxide as the standard filling ingredient for tire-tread 

 compounds. For many years white tires were the rule, and the best 

 tires were those which owed their whiteness to a sizable portion of 

 zinc oxide. Those materials which, when added to rubber increased 

 its tensile strength and elastic modulus, were termed "reinforcing 

 agents," in distinction to those substances which acted merely as 

 inert fillers. 



The liistory of the modern method of compounding and reinforce- 

 ment, therefore, dates from the first discovery that if certain sub- 

 stances, such as zinc oxide, are used in sufficient quantities, they will 

 produce a reinforcing effect; the quantities to produce this effect are 

 greatly in excess of those used when color is the only consideration. 



A few years before the World War it was observed that carbon 

 black showed reinforcing effects in rubber. Wliile some pneumatic 

 tires using carbon-black treads were made in this country before the 

 war, the white tread continued in general use for several years. 



