RUBBER 675 



Drying in an atmosphere of carbon dioxide may also be 

 adopted to arrest the further oxidation which takes place 

 during the drying of the rubber. The present practically 

 universal method adopted in Malaya is the use of sodium 

 bisulphite. This is added to the latex and thoroughly 

 mixed with it immediately before the addition of the acid 

 coagulant. The use of sodium bisulphite powder or very 

 strong solutions not properly mixed with the latex will 

 cause streakiness or yellow and white patches. 



The proportion of sodium bisulphite required varies 

 somewhat with the latex, since some latices, especially 

 from older trees, oxidize rather rapidly and to a greater 

 extent. The amount usually found sufficient is T J o oz. per 

 gallon of latex, or, say, 2 oz. of a 5 per cent, solution 

 per gallon. The chief disadvantage of this chemical is 

 that it retards the drying of the rubber, due probably 

 to the formation of the hygroscopic magnesium salt 

 formed by interaction with the magnesium salts present 

 normally in latex; this retardation is not of great import- 

 ance in the case of thin crepe. Sodium bisulphite in 

 smaller quantities is valuable in preventing the rough 

 surface on sheet due to oxidation films and thus improves 

 the appearance of smoked sheet; the sheet takes longer 

 to darken, and thus the appearance of "over-smoking" 

 is also avoided by using small quantifies of this salt 

 in the preparation of smoked sheet. According to vul- 

 canizing experiments and tests carried out by the chemists 

 of the Rubber Growers' Association there is no dele- 

 terious action on the rubber in using this salt. 



Oil Marks. Oil marks are not an uncommon fault, 

 especially in crepe rubber, the oil being taken up from the 

 sides of the rollers or from the trays beneath; this may 

 be due to careless lubrication of bearings on the part of 

 the operator, or excess of oil on the bearings, which 

 drops into the trays beneath, which are usually made too 

 wide, extending not only the width of the rollers, but 

 also partly under the bearings. An excellent method, 

 by which only the central portion of the rollers form the 

 working part, is to have movable guides placed over the 

 top, which fit over the rollers, such as are found on 



