By simply lying embedded in finely divided sulphur at a temperature of 233° F. (112° C.) as much as ten per cent. of sulphur may be absorbed by thin sheet rubber. This is one of the processes peculiarly suited for work on the small scale. It may be used instead of the Parkes process next to be described.
Chloride of sulphur is an orange red mobile liquid of a peculiar and disagreeable odor. It boils at 276° F. (136° C.). It dissolves both sulphur and chlorine so that it is not easy to obtain it in a pure state. If unmixed india rubber is exposed to its action it will quickly become vulcanized. At ordinary temperatures the mixing action takes place, though it is much accelerated by a slight application of heat.
It is quite possible that this action may be of use to the reader in his manipulation of india rubber. Thin sheet may be vulcanized by being immersed in a solution of this substance in bisulphide of carbon followed by slight heating. The thin layer of caoutchouc left by evaporation of the chloroform solution of india rubber may thus be vulcanized so as to become comparatively strong and elastic. Where the same solution has been used as a cement or for patching overshoes and finishing the patch, a vulcanization can thus be given to it.
The process is known as Parkes’ cold curing process.
A solution of one part of chloride of sulphur in forty parts of bisulphide of carbon is of good strength for rapid work. A thin article needs but an instant of immersion. It then is placed in a box or tray upon some talc powder and is heated to about 104° F., (40° C.). One minute of curing will suffice. It is advisable to wash off the articles afterwards in water or in weak lye to remove any traces of acid.
Petroleum naptha can be used as the solvent instead of bisulphide of carbon. The latter substance has an exceedingly disagreeable odor, and its vapors must be considered rather injurious especially to those who are not accustomed to them.
When thick articles are to be cured by this process a much more diluted solution is used. One per cent. or less of the chloride of sulphur is the proportion used. The object of this is to enable a longer immersion to be employed so that the interior will be affected before the outer layers become too much charged with the vulcanizing material.
In this short description of the Parkes curing process hints for a useful method may be found. The process is beyond doubt by far the simplest known for treatment of india rubber. Exactly what reaction takes place is unknown. Whether the sulphur or the chlorine is the acting vulcanizer has not as yet been determined.
Its defect is that it produces surface action, analogous to casehardening. One method of avoiding this is to remove the articles from the sulphur chloride bath and at once to immerse them in water. This prevents the rapid volatilization of the solvent and an equalizing of the absorption ensues.