The actual vulcanising consists of heating the mass of mixed rubber for a definite time and at a definite temperature, each “heat” being chosen to suit the particular mixture. These data are arrived at empirically—that is, by trying a number of “heats” and choosing that which appears the most suitable. The suitability will depend on the nature of the article, the service to which it is to be put, and the time it is intended to last. All vulcanised rubber goods, whatever the process, have a limited life or period during which they can be relied on to give useful service. After a time, vulcanised rubber tends to harden, cracks appear on the surface when the article is bent or stretched, and eventually the rubber becomes rotten and “perished.” This tendency varies with the quality of the original raw rubber and the conditions of vulcanising. Before plantation rubber was available, the manufacturers were dependent on inferior wild grades for a great part of their output, and, consequently, the goods made from these inferior rubbers never showed very good mechanical properties and soon deteriorated. The severest critics of plantation rubber have admitted the advantages to the manufacturers of the replacement of the lower wild grades by plantation rubber.[36] But even the best grades give a vulcanised product which rapidly deteriorates if the vulcanisation is carried too far. This results from too long heating, or too high a temperature, and the product is termed “overvulcanised” or “overcured.”[37] The appearance of the product is deceptive, as the physical properties are remarkably good if the overvulcanising is not more than 50 to 100 per cent. in excess of the normal cure. Only in the case of very much overvulcanised rubber do we obtain a product which is brittle from the beginning.
[36] See Williams, “The Rubber Industry,” 1914, p. 284. It must also be remembered that the inferior wild grades were derived from latices often containing a large proportion of “resinous” matter, and which could not yield a really high grade of vulcanised rubber whatever the care and skill employed in preparation.
[37] The terms “curing” and “vulcanising” are generally employed as if synonymous. Twiss has suggested that the former be applied in regard to a change in physical properties, and the latter to the chemical change whereby sulphur is combined with the rubber. The term “curing” is also applied to the process of preparation of raw rubber. This must be kept in mind so as to avoid confusion.
The degree of vulcanising will vary with the type of article to be produced, and where a long life is desired, the tendency will be to “undervulcanise”; but if the best mechanical properties are desired, the tendency will be towards “overvulcanising,” or, more correctly, “fully” vulcanising. These considerations are aptly illustrated by reference to pneumatic tyres. The inner tube need not possess high tensile strength, provided that it is easily distensible, for the reason that, during use, it is protected by the casing of the tyre proper, which confines and supports it against the air-pressure applied. Inner tubes are therefore cured to give a long life without developing the maximal physical properties. On the other hand, the casing and tread of the tyre are required to withstand severe mechanical conditions—particularly the constant flexing of the cover, and the abrasion of the road surface. Tyres are not stored for any long period, and, when put into service, have a limited period of useful life. Consequently it is needful to develop maximal mechanical properties, and vulcanisation is therefore carried further than in the manufacture of inner tubes.
The rate of cure is controlled by a number of factors in addition to the period and temperature of vulcanisation, in particular by the proportion and nature of the other ingredients, especially sulphur and accelerators, and also by the rubber itself. The main complaint as regards plantation rubber is that it varies excessively in this respect. This matter will not be discussed here, but is only introduced in order to explain the importance of a constant rate of vulcanising to the manufacturer. Plantation rubber should, therefore, be prepared so as to be as uniform as possible in this respect, and the earlier part of this book gives full details of the precautions advised, and in many cases adopted on the plantations. Unfortunately, it is impossible to secure uniformity of methods among all producers, even when they are Europeans, to say nothing of the native producers, who account for perhaps one-third of the output. Hence the importance of branding the rubber whenever possible, so that the manufacturer may identify the rubber he purchases. If found satisfactory, he can then secure further supplies from the same estate.
CHAPTER XXII
TESTING OF PLANTATION RUBBER
This subject may be subdivided into (a) Tests on the raw rubber; (b) tests on the vulcanised rubber.