(c) Occasionally by abnormally wet weather.
(d) Allowing piles of crepe to remain too long before hanging.
(e) Using excessive quantities of deteriorated sodium bisulphite. In short, any factor contributing towards a retarded rate of drying may be responsible for the appearance of surface mildews. The last mentioned cause is of not infrequent occurrence. Knowing the chemical to be of poor quality, relatively more is used to produce the desired anti-oxidant effect. Unless the rubber is particularly well washed on the rolls, there remains within it a residue of sodium bisulphate, an oxidation product of the bisulphite. This is hygroscopic to some degree—i.e., it takes up moisture from the atmosphere. Hence drying is delayed, and even should mildews not develop the chemical may sometimes be seen on the surface of the rubber as a whitish “bloom.”
The enumeration of the possible causes of mildews on crepe rubber is sufficient to indicate the necessary precautions to be taken, and the discussion will not be extended further.
Tackiness in Rubber.—“Tackiness” is a term used to denote a deterioration of rubber which renders it sticky, and, beyond this, implies that some physical and chemical change in the nature of the substance has taken place. In fact, it is no longer “rubber,” but an oxidation product containing much resinous matter. It does not behave as rubber, and hence its value is much depreciated.
With modern ideas of erection of factories to guard against the introduction of direct sunlight, it was hoped that this defect had practically ceased to exist. In one grade of rubber it would be expected that tackiness would continue to appear. Earth-rubber, often exposed to direct sunlight for a week, would naturally become tacky, and this tackiness cannot be avoided unless the earth-scrap is to be collected more frequently. But in many cases even the higher grades of rubber show signs of tackiness. Experiments have been carried out at various times and in various places to determine the cause of tackiness. For some time the theory of bacterial origin was in favour, but none of the experimental results was convincing. Bacteria may be present in tacky rubber; but, on the other hand, many cases of bacteria in rubber have been observed in which there was no tackiness. Experiments were made by one of us some years ago with a view to testing the bacterial theory by inoculating latex with small pieces of tacky rubber. In opposition to the results which were stated to have been obtained, there was no spread of tackiness. Other investigators have obtained similar results. One writer proposed to explain tackiness as caused by excess of moisture. This perfectly simple explanation unfortunately displays only a profound ignorance of the subject, and does not take into account the fact that tackiness is incident in rubber after dryness has been reached. It need not be pointed out to planters in Malaya that wet sheets of rubber are often exposed to direct sunlight by workers of native holdings, with no resulting harm as long as plenty of moisture is present in the rubber.
Tackiness the Result of a Slow Process of Change.—As stated above, tackiness does not appear until the rubber is dry, and even then it is to be noted that it is possible for tackiness to appear in rubber arriving in London, which showed no indications of tackiness when packed for shipment.
Tackiness caused by Traces of Copper Salts.—Spence, as the result of investigations, has pointed out that none of the various theories put forward to account for tackiness—viz., the action of bacteria, premature putrefaction, oxidation, excess of moisture, the action of enzymes, etc.—have any basis in scientific proof, and believes that the cause of tackiness cannot be directly attributed to bacteria. It has been stated that the only known way of causing rubber to become tacky is to expose it to sunlight or heat. While agreeing that in the ordinary way this statement is correct as far as one rules out the employment of chemical substances, it must be pointed out that tackiness of the worst degree may be caused by the presence of traces of copper or copper salts. This point has already been touched upon in a preceding paragraph dealing with the defect of “green streaks” in pale crepe rubber.
In the course of laboratory experiments tackiness has often been induced by the use of traces of copper salts. The rate at which tackiness is induced appears to be dependent upon the amount of copper salt used, but once it begins, the rubber molecule is very rapidly broken down, and resins are formed. As the formation of resins is accompanied by the inclusion of oxygen in the chemical constitution, it would be expected that dry rubber becoming tacky should increase in weight. This is found to be the case, and to give an idea of how this weight increases with the progress of tackiness, the results below may be studied.
It will be seen that the maximum quantity of copper sulphate used amounted to 0·025 per cent, (approx.) upon the weight of latex taken. Now it is highly probable that only a fraction of this quantity was retained in the rubber on coagulation, the remainder being in solution in the serum. Furthermore, as the rubber was well washed and worked down to thin crepe, the total quantity of copper salt remaining in the dry crepe must have been exceedingly small. Yet the effect is most marked and should impress upon all managers the necessity for guarding against any possible contamination caused by brass or copper.