Use of Sodium Bisulphite.—Some few years ago a demand for pale crepe rubbers sprang up, and this demand has been maintained. The total quantity of pale rubber put on the market previously could only have amounted to very little, and that little was obtained by luck and various tricks in manipulation. It must be premised that if coagulation is allowed to take place, either naturally or with the aid of acetic acid, the resulting rubber will almost inevitably oxidise on the surface, except in the cases of very dilute or young latices. Even supposing that this darkening of the surface does not take place in the wet stage, it is often found that a rubber expected to dry to a pale colour does not fulfil expectations, and a dull neutral shade results. This darkening of crepe rubber may be attributed to a slow process of oxidation, which continues until the rubber is dry. From these remarks it will be seen that the process of oxidation is a natural one, and that any pale rubber formerly shipped was the outcome of circumstances outside the control of the estate, except in such cases where boiling of the coagulum, etc., was resorted to. The fact that one rubber happened to be a shade darker than another was absolutely no criterion as to the value of the rubber, but apparently the market thought, and still thinks, otherwise, although the actual necessities of manufacturers for a pale crepe to be employed in special processes must be comparatively small.
The prevention of this natural oxidation was a problem which exercised the minds of all responsible for the preparation of pale rubbers, and much time and thought were expended upon it. Various theories were propounded, and the chief conclusion arrived at was that the darkening of rubber was to be prevented by excluding all the light possible from the drying houses. To this end windows were to be kept shut, or else they were provided with ruby-coloured glass, which incidentally kept out the air. In spite of these precautions, little success attended the expenditure of so much energy and thought. It was absolutely necessary that some chemical agent should be discovered which would make the preparation of pale crepe possible for any estate. This chemical would have to fulfil several requirements before it could become popular:
1. It must be a simple substance capable of being easily handled.
2. It must be very soluble, so that solutions could easily be made up by inexpert workers.
3. It must be cheap.
4. It must be quite innocent of any harmful effect upon the quality of the rubber.
After months of investigation into the properties of other chemicals the writers decided that the only one which satisfactorily answered all requirements was sodium bisulphite. The writers make no pretension to any claim of having discovered the properties of this substance, which was a common chemical, and widely known. Even its action on latex was suspected before they engaged upon the work. These matters are only mentioned because the credit, if any, should be given to the laboratories of the Rubber Growers’ Association.
As soon as it began to be known on the market that sodium bisulphite was being used in the preparation of pale crepe, a great outcry was made, and estates were warned that no more rubber prepared in this way would be accepted. It was said that the chemical would destroy the “nerve” of the rubber,[2] and it was definitely stated that rubber prepared with this chemical was brittle. It must be remembered that brokers had some legitimate excuse in raising objections to the introduction of new and strange chemicals for preparing rubber, as they were quite without means of judging whether the rubber had suffered harm or not. Still, on the other hand, private tests had been made in conjunction with Messrs. Beadle and Stevens for fully eight months before the name of the chemical was mentioned in reports, and they had decided from the results of vulcanisation tests that the chemical was quite innocuous. Then, and only then, did we consider it advisable to recommend the use of sodium bisulphite in general estate practice. Owing to the initial prejudice against rubber prepared with sodium bisulphite, the results of our preliminary work were published by permission of the Rubber Growers’ Association.[3] The original instructions to estates regarding the proper employment of this chemical were given in the private reports issued by the Rubber Growers’ Association in 1911. At the present time it is probably accurate to state that it is now used by all estates preparing fine crepes. Representatives of manufacturers have sometimes given us to understand that the question of paleness of colour in such rubber is of no such importance as is impressed upon us as producers. While we are prepared to believe, we can only plead that from our point of view the supply arises from the demand. Such are the conditions governing the sale of rubber that, irrespective of the requirements of the ultimate user, we have to market rubber which is valued almost completely upon its appearance at the time of sale.
[2] Williams, International Rubber and Allied Congress, London, 1914.