The molecule of rubber is unsaturated, and is able to enter into direct combination with different elements. This fact is expressed in its constitutional formula by the presence of double links between two pairs of carbon atoms in the ring. Among the addition products of rubber, those with sulphur possess the greatest technical importance. Rubber combined with sulphur is said to be vulcanised. We have already referred in Chapter I to the remarkable properties of resistance and durability possessed by vulcanised rubber.
Rubber is vulcanised by two entirely different processes. In hot vulcanisation the rubber is directly combined with sulphur under the influence of heat. In cold vulcanisation, rubber is combined, without heating, with chloride of sulphur dissolved in benzene or other solvent. The molecule of rubber vulcanised by the second method contains chlorine as well as sulphur.
Vulcanisation with hot sulphur was discovered by Nelson Goodyear in 1839. At the melting point of sulphur there is little or no action. Combination begins at about 120°C, and the temperatures used in practice range from 125°C upwards. According to the amount of sulphur employed, the time of action and the temperature, the final product ranges in character from soft elastic to hard vulcanite.
Weber considered that the process of vulcanisation was one of definite chemical combination. He believed that a series of sulphides of rubber were formed ranging from (C10H16)20S2 to the final product vulcanite, to which he attributed the formula C10H16S2. This opinion, which has been subjected to severe criticism, appears to be confirmed by the results of the most recent researches. The process is a reversible one, and the vulcanised rubber apparently always contains a certain amount of uncombined rubber and a certain amount of free sulphur. As a result, goods which have only been partially vulcanised become more fully vulcanised on keeping. It is therefore the practice to vulcanise the majority of goods rather less completely than is finally required.
Some recent writers still regard the process of vulcanisation as one of adsorption. That is to say they look upon the vulcanised rubber as existing in a condition of physical mixture rather than in one of true chemical combination. As previously stated, the whole problem is one of extreme difficulty, and we are perhaps still far from a final solution.
The physical condition of the rubber has a marked influence upon the final product, and the same sample of raw rubber, if differently worked before vulcanisation, requires to be vulcanised with a different quantity of sulphur and to a different extent in order to produce the same final result. After long continued kneading and mastication more sulphur is required in order to bring the vulcanised rubber to the same condition of physical consistency.
The process of cold vulcanisation with sulphur monochloride was discovered by Parkes in 1848. The action is exceedingly vigorous. Dilute solutions are therefore employed, and the period of contact is short The solvent almost universally employed is carbon bisulphide.
The action of sulphur monochloride on rubber, like that of sulphur, is an addition and not a substitution process. This is proved by the fact that no sulphuretted hydrogen is emitted during the process of combination. The action probably gives rise to a series of addition-products parallel with those arising during simple vulcanisation with sulphur. The series would then begin with (C10H16)20S2Cl2 and end with C10H16S2Cl2. The fact that the addition of large amounts of the pure solvent leads to the extraction of S2Cl2 from the vulcanised rubber is equally well explained on the hypothesis of a reversible chemical action as it is on the adsorption hypothesis.