Gums, Resins, 



210 



[Sept. 1900. 



venient in assisting and controlling coagulation, should be discontinued from fear 

 that such use will produce a rubber which will not stand the test of time, and which 

 will perhaps injure in the future the reputation of plantation grown and cured 

 rubber. 



DRYING RUBBER. 



15. Until the introduction of mechanical washing of coagulated rubber and 

 the formation of crepe, drying had been a troublesome operation in the preparation 

 of rubber for export. Artificial heat almost always led to the softening of the 

 rubber, and often through inefficient control of the temperature caused it to become 

 distinctly tacky. Crepe rubber dried easily and well if simply hung up in a dark bu^ 

 airy shed, and the preparation of rubber in this form appeared to have solved the old 

 difficulties associated with drying. There had been, however, suggestions and 

 proposals to introduce vacuum drying on estates, and consequently I paid special 

 attention to the modes of drying washed rubber in use in Great Britain. In a few 

 manufactories only I found vacuum had been introduced, but the great bulk of 

 washed rubber is still dried by hanging it up in dark warm rooms. A vacuum 

 drying chamber is a large iron box, of from 100 to 200 cubic feet capacity, fitted 

 inside with shallow iron trays with perforated bottoms, and heated with steam 

 pipes. The interior is connected by an iron pipe with an exhaust pump. Wet 

 rubber is placed in the iron trays, the doors are closed, and the temperature raised 

 to 120 to 130 degrees F., and the pump started. The air and water vapour that are 

 drawn out of the chamber are passed through a condensing cylinder, fitted with a 

 glass front, aud the condensed vapour is seen falling as a stream of drops of water. 

 After two or three hours this stream of drops of water ceases, and the rubber or 

 whatever material is being operated upon is then regarded as dry and ready for 

 removal. Rubber dried in this way is always softened by the heating which is 

 necessary if the drying is to be rapid, and in the opinion of manufacturers who have 

 not adopted the process the nerve and quality of the rubber are injured, though 

 with those who have adopted vacuum drying this is not regarded as very important, 

 chiefly because the cause of softening is known, aud it is regarded as only an 

 anticipation of the softening which always occurs in mastication of the rubber, the 

 next step in the process of manufacture. On the other hand, some forms of 

 rubber — such as very soft African rubbers — cannot be dried in this way at all 

 because the softening in their case proceeds too far. The sheets of rubber dried in 

 this way adhere to one another when packed and stored away, this is of no im- 

 portance in the factory, because the rubber is known to be clean and dry and ready 

 for use, but if the plantation rubber were offered in the same soft and adhesive 

 masses, objection, aud serious objection, would naturally be made. It would be 

 necessary to maintain a lower temperature in the vacuum chamber than is in 

 common use if this softening of the rubber is to be entirely avoided, and this would 

 seriously iuterfere with the efficiency of the machine. The rate of the evaporation 

 of the water and drying of the contents depends upon the difference between the 

 temperature of the vacuum chamber and the temperature of the condensing 

 chamber, as well as upon the absolute temperature of the vacuum chamber. In the 

 tropics it will not be possible to maintain the condenser below 80 deg. F., which is 

 20 degrees above the temperature of the condenser in England. This diminished 

 difference, together with the necessary lowering of the temperature of the vacuum 

 chamber itself, will seriously impair the efficiency of the vacuum drier, the only 

 advantage of which is the rapidity of drying. Taking into consideration the 

 further fact that plantation rubber is always inclined to be soft, I should certainly 

 not recommend any form of drying in which artificial heat is necessary, and which 

 involves the elaboration of machinery and increase in power in doing what, with 

 washed rubber, can be done in a more simple, safe and natural manner. 



