| Tensile Strength. | Final Length—i.e., Elongation + 1. | Tensile Product. | ||
| 30 | × | 4 | = | 120 |
and for smoked sheet
| 15 | × | 8 | = | 120 |
The difference in properties between crepe and sheet may probably be attributed to the heavier rolling of the crepe; which compacts the rubber. But if the crepe is rolled too much, the tensile strength falls, and there is no increased elongation to compensate. For the same reason, crepe which has been rerolled in this country is inferior to crepe as received direct from the plantation. At the most it is permissible to unite two or three layers of thin crepe to a thicker one by a single passage through even speed rollers, if the physical properties of the original rubber are to be conserved.[44]
[44] Bulletin R.G.A., February, 1922, p. 64.
Attempts to prepare crepe for use in a raw state, by rerolling uneven or irregular surfaced crepe in this country, only result in a rubber with inferior physical properties. Nor can sheet be rerolled to give crepe of good physical properties. The power required to break down the sheet and the heat developed, even on cold rollers, are an indication of physical properties destroyed. For subsequent vulcanisation this is not a matter of importance, because the vulcanising process restores to the rubber the properties which are lost in the process of rolling and milling or mastication.
Raw rubber has been used to some extent for proofing purposes, as for the manufacture of material for hoods of motor-cars. In this case no attempt is made to preserve the physical properties. The rubber is masticated, mixed, taken up with solvent and spread on the cloth exactly as if it were to be vulcanised.
Vulcanised Rubber.—We have already explained that the properties of vulcanised rubber are dependent, to some extent, on the specific nature of the raw rubber, or what De Vries terms the “inner qualities.” That is to say, differences appear on vulcanising which are not apparent from the tests made on the raw rubber. Indeed, no investigation or analysis of the raw rubber can enable one to foresee exactly how the rubber will behave on vulcanisation. This illustrates the deficiency in our knowledge of vulcanisation. When dealing with soft, resinous, or decomposed rubbers, it is safe to anticipate a weak vulcanised product; but when we come to deal with a number of samples of “standard” crepe or sheet—i.e., sheet or crepe passing a certain standard of appearance—it is found that differences in vulcanising properties cannot be foreseen. This matter is, however, not so great a drawback as might be imagined, for reasonably well prepared consignments of standard crepe or sheet differ but little from one another, and the difference is mainly in the ease with which they break down, or the rate or speed with which they vulcanise, and not with the properties of the vulcanised product. Many of the plantation scrap grades are equal to or nearly equal to “standard“; but some of these, as also the rubber produced by native holders, show appreciable variation, and are the source of most of the complaints which emanate from manufacturers. We shall consider in turn the different grades and the effect of the usual surface defects, such as mould, spots, etc.
Crepe Rubber.—Oil marks and tackiness are the most serious defects from the manufacturing standpoint. In the first part of this book we have shown that damage caused by the so-called oil marks is not due to the oil, but to traces of copper from the bearings of the machines. There are several metallic compounds which cause deterioration of rubber both raw and vulcanised, but copper is the most deadly, and rubber showing signs of deterioration is rightly rejected by the manufacturers.
The only other defect of crepe rubber which has any bearing on its use in manufacture is mould. Crepe rubber very seldom shows the ordinary surface moulds not uncommon in sheet-rubber. There are, however, microscopic growths which cause the development of coloured spots referred to in detail in the earlier part of this book. The rubber hydrocarbon itself does not appear to be affected by the moulds, but some of the serum constituents are altered, with the result that the rubber vulcanises more slowly than it otherwise would do. For this reason, crepe rubber with coloured spots may give rise to trouble in the factory.