It is easy to see that the whole may be so arranged that the screw handle or pressure nuts of the mould will rise above the liquid. In this case the press can be screwed down while the article is heating.
Instead of glycerine a strong solution of some salt in water has been recommended. A solution of calcium chloride, or some other salt can be substituted. Either are very cheap and will be quite satisfactory.
Another treatment which applies also to the mixing operation is by the sulphur bath. Sulphur is melted in an iron vessel and brought to a temperature of 248° F. (120° C.). A piece of unmixed pure caoutchouc immersed in this bath will gradually absorb sulphur. The case is almost parallel with the absorption of water or benzole by the gum. The piece swells and thickens as it is acted on and eventually will contain enough sulphur for vulcanization. It may absorb as much as fifty per cent. The point of proper absorption must be settled more or less empirically or by successive trials.
After enough has been taken up the piece is removed and dipped into cold water, which cracks the adherent sulphur so that it can be brushed or rubbed off. This gives a piece of mixed rubber ready for moulding and curing. It can be heated and moulded and may be cured as desired, in a liquid bath, hot press or vulcanizer.
It will be observed that this provides for the admixture of sulphur only; no talc or other solid can be thus introduced. The addition of these solids tends to make the rubber of a more attractive color and their use is not to be deprecated in all cases. Hence the sulphur bath process is not to be considered a perfect one.
In the sulphur bath the mixing and curing processes can be combined. If the liquid sulphur is heated to the vulcanizing temperature, 284° F. (140° C.), a thin strip of gum immersed in it will be vulcanized completely in a few minutes. A heating of several hours at the lower temperature will effect the same result.
The sulphur bath processes must be regarded as unsatisfactory. It is not easy to feel that any dependence can be placed upon them as regards reliability or constancy of product. The sulphur also will mostly effect the surface. Thin pieces may be satisfactorily treated, but the same confidence cannot be felt as is experienced when specific amounts of ingredients have been mixed in with pure caoutchouc in a regular mixing machine.
The sulphur bath is of value to the experimenter, enabling him to do his own mixing without expensive apparatus.
Bromine, iodine, chlorine and nitric acid are vulcanizers. A piece of sheet rubber dipped into liquid bromine is instantly vulcanized. Iodine and nitric acid have also been used in commercial work.
Alkaline or alkaline earth sulphides can be employed in solution under pressure for vulcanizing. At a vulcanizing temperature their solutions will answer for thin sheet very well. Polysulphides of calcium have thus been employed.