CHAPTER VII 



THE CHEMISTRY OE RUBBER 



Composition of Crude Rubber. — Commercial rubber con- 

 sists essentially of a hydi'ocarbon (caoutchouc), the 

 true rubber substance, together with varying amounts 

 of resinous and protein substances, mineral matter, and 

 moisture. Small quantities of carbohydrates also occur 

 in certain rubbers. 



The rubber collected from wild plants by natives 

 usually contams in addition more or less vegetable 

 matter, in the form of fragments of bark, etc., and 

 sometimes mmeral impurities as well. In badly prepared 

 specimens the amounts of these impurities are often very 

 considerable. 



Properties of Rubber. — In its purest form, rubber is a 

 practically colourless, highly elastic solid, which accord- 

 ing to Weber has a specific gravity of 0-911 at 17° C. It is 

 insoluble in water, but absorbs about one-fourth of its 

 weight of the liquid, swelling considerably. Alcohol and 

 acetone dissolve the resins from crude rubber, but are 

 without solvent action on the hydrocarbon, which, how- 

 ever, absorbs them as in the case of water, and swells. 

 Rubber may be dissolved by carbon disulphide, chloro- 

 form, benzene and its homologues, petroleum spirit and 

 turpentine, and usually to a greater or less extent by 

 ether. When rubber is treated with one of these solvents, 

 it swells up, forming a gelatinous mass which slowly 

 dissolves and yields a viscous solution ; rubber thus 

 behaves as a colloid substance. On dissolving crude 

 rubber a quantity of an insoluble constituent often re- 

 mains suspended in the solution, varying in amount 

 according to the variety of rubber. This constituent 

 was formerly termed " insoluble caoutchouc " on the 

 assumption that it is a hydrocarbon like the soluble 

 portion, but analyses made by Weber showed that the 



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