RUBBER 671 



Tackiness. As far as our present knowledge goes, 

 tackiness is due to one of three causes : (a) Action of 

 certain salts, e.g., salts of copper and iron; (b) sunlight, 

 which may be due to the heat effect, or to the light effect 

 due to the actinic rays; (c) direct heat. 



Salts of copper are known to be very deleterious in 

 their action on both raw and vulcanized rubber; the 

 action is an oxidation process, and the rubber increases 

 in weight, softens, and eventually becomes brittle. On 

 this account copper or copper-covered rollers, such as 

 are often found on hand machines, should be avoided, 

 as, unless kept scrupulously clean, " verdigris " forms 

 and may be incorporated with the rubber. Oil containing 

 brass from old or overheated bearings may constitute 

 a similar source of danger, but not a very probable one. 

 Tropical sunlight causes tackiness in rubber, as experi- 

 ments by the author have proved, but whether this was 

 due to the actinic rays or to the heating effect was not 

 definitely proved : this point will be tested later, also the 

 action of sunlight and heat in the presence of inert gases, 

 such as carbon dioxide and hydrogen. That tackiness 

 may be caused by heat alone can easily be shown, and it 

 has been known to occur in rubber hung near a boiler 

 in an estate factory. Vacuum-dried rubber and rubber 

 dried in hot-air chambers at about 130 F. exhibit tacky 

 surfaces, and the deleterious effect produced by these 

 methods of drying are invariably caused by the excessive 

 heat. 



Spot Disease. Spot diseases have been proved by a 

 number of investigators to be due to micro-organisms, 

 fungoid or bacterial in nature, and are of sufficient in- 

 terest and importance to merit separate treatment, but 

 in a paper of this nature they can only be discussed 

 somewhat briefly. The cause of such spots are air- 

 borne spores of the micro-organisms which gain entrance 

 into the latex in the cups in the field, or may be deposited 

 on the wet, freshly machined rubber in the factory or 

 drying room. Any process which tends to retard the 

 drying of the rubber is conducive to the development of 

 the spores, several of which are chromogenic fungi and 

 bacteria, causing yellow, black (blue in transmitted light), 

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