Gums, Resmx, 



284 



[Oct. 1906. 



Toys and other small objects are vulcanised in moulds, hollow articles 

 usually having a little water or ammonia placed inside them in the mould, so that 

 when heated the elastic force of the vapour compresses the rubber against the walls 

 of the mould. Another method of vulcanising employed for flat objects such as shoe- 

 soles, belting sheet, &c, is by the employment of screw or hydraulic presses some- 

 what similar to a letter-press, the two plates of which are hollow and can be steam- 

 heated from an ordinary boiler. The lower plate is fixed in the screw-presses, but 

 movable in those worked by hydraulic power. Objects made of mixed rubber are 

 placed in moulds between the two plates of the press, which are strongly compressed 

 and steam admitted to a pressure of 2| to 4 atmospheres, corresponding to 

 a temperature of 128 to 144° C. The length of time required varies according 

 to the proportion of sulphur, the steam-pressure in the plates and the kind 

 of mixture, but rarely exceeds two hours, and this is often reduced to one or 

 even half-an-hour. In the case of vulcanising by Si 01.2, this is dissolved in carbon 

 bisulphide and the objects are immersed in it in the cold. The next process to 

 mastication is the incorporation of sulphur or the solid sulphides required for 

 vulcanisation, and if necessary the various colouring and mineral or other ingre- 

 dients employed for modifying the rubber according to the uses to which it is to be 

 put. For the operation to be successful the mixing must be perfect, and the mass 

 must form one homogeneous whole. The masticated and dried rubber is put 

 through the mixer with 7 to 10 per cent, of sublimed sulphur, though the range is 

 sometimes from 2i- to 25 per cent. In the latter case the excess can only be con- 

 sidered as an inert addition giving a fictitious weight to the manufactured article. 

 9 to 11 lb. of the rubber are passed repeatedly through the hot rolls, diminishing the 

 space between them as the operation proceeds. As the sheet issues from the roll, 

 it is dusted for the first time with sublimed sulphur and rolled upon itself, and 

 again passed through the mixer. This is repeated until the requisite amount of 

 sulphur, &c, has been added and thoroughly incorporated into a homogeneous 

 mass. In this state, the mass which is still nothing but a simple mixture, is 

 wrought in the ordinary way for its conversion into threads, sheets, tubes, shoes 

 or any other object of definite shape. It is only now that vulcanisation is 

 effected. 



TREATMENT OP RAW RUBBER. 



The rubber as imported into England in the various forms now to be seen in 

 the Exhibition first undergoes a process of softening, washing and mastication to 

 remove solid and other impurities. This is effected by immersing in hot water in 

 wooden vats for 12 to 24 hours, and then by passing it in small quantities at a time 

 through very powerful rollers revolving at different speeds, which tear and com- 

 press the rubber, while a stream of water from above washes away all the impurities 

 in the form of wood, bark, stone, &c. The rollers are made of hardened cast iron 

 and are usually grooved spirally or in the form of lozenges so as to facilitate the 

 shredding and mastication. Their distance apart can be adjusted by tightening 

 screws, and beneath them is a wrought-iron collecting tank covered by a perforated 

 plate. The rubber after being passed through several times appears in the form of 

 a long strip of lace-work similar to that so largely manufactured in the Malay 

 States. This process requires much power — a machine capable of working 20 to 30 

 lb. of rubber at a time, requiring at least 15 to 20 H.P. actual. Rubbers vary 

 as to the ease with which they can be washed, but Para is the best in this repect, 

 as it contains fewest impurities. The rubber has now to be dried, which is effected 

 by spreading on iron wires or in stoves capable of being heated to 50° or 60° O. 

 12°-140° Fahr. ), care being taken to dry the greasy and pitchy rubbers at as Iowa 

 temperature as possible. In many cases the sheets soften and fall to the ground 

 in lumps, from which the moisture can only be evaporated with great difficulty. 



