[42] Hatschek Journal Soc. Chem. Ind. 1921; Trans., p. 251.
CHAPTER XXIII
THE PROPERTIES OF RUBBER
This section, like the last, is divisible into two subsections. The first deals with [raw rubber], the second with [vulcanised rubber].
We have already explained that, until recently, rubber was not used in the unvulcanised condition, but that the excellent physical properties of plantation rubber have made this possible. It is interesting to compare the physical properties of raw rubber with that vulcanised with sulphur. A compact sample of crepe as received from the East will give breaking strain of over 30 kilos per sq. cm. and over 300 per cent. elongation. When mixed with sulphur and vulcanised, a breaking strain of 150 kilos and elongation of 1,000 per cent. are not unusual. It is possible that crepe rubber would give higher figures if it could be prepared in the form of a compact ring, as used for tests on vulcanised rubber. In any case, the figures for vulcanised rubber are much in excess of those for raw crepe rubber. It must also be remembered that a breaking strain of 150 kilos is not permanent with vulcanised rubber, for reasons which will be explained later.[43] To obtain a reasonably permanent vulcanised product, the vulcanisation would not be carried further than to give a figure of 100 kilos. On the other hand, raw rubber is remarkable on account of its great permanency, although subject to some physical changes at ordinary atmospheric temperatures. Tensile tests, although valuable, do not tell us all about the physical properties of a sample of rubber. Abrasion tests, or tests designed to measure resistance to wear and tear, would be more valuable, but, unfortunately, these properties do not lend themselves to simple tests. There are grounds for believing that raw rubber is superior in some respects to fully vulcanised rubber, if prepared without the addition of finely divided mineral substances which exert a toughening effect.
[43] Journal Soc. Chem. Ind., 1916, p. 872.
Sheet rubber gives results in some ways inferior to compact crepe rubber when subjected to physical tests. Tensile strength seldom exceeds 15 kilos, but the elongation is usually higher—up to 600 or 700 per cent. That is to say, it stretches more, but breaks more easily. If, however, we take into consideration the diminution in sectional area of the test piece during stretching, it will be seen that crepe and sheet rubber have compensating properties.
As this matter of sectional area reduction during stretching is important, both for raw and vulcanised rubber, it may be briefly referred to here. When rubber is stretched, the volume does not appreciably alter—at any rate, as regards uncompounded rubber. Hence the reduction of sectional area on stretching bears a simple relationship to the amount of stretching. If we double the length of the test piece, we halve the sectional area; if we treble the length, we reduce it to one-third, and so forth. Hence, if we multiply the breaking strain by the final length (i.e., length at break, taking the original length = 1), we obtain a figure, the “tensile product,” which embodies both breaking strain and stretching capacity. In effect it gives us the breaking strain calculated on the sectional area at the moment of rupture of the test piece. Adopting this formula, we obtain for crepe—