This subject may be subdivided into (a) Tests on the raw rubber; (b) tests on the vulcanised rubber.The tests on the raw rubber may be carried out (1) on the sample of sheet and crepe as received. For this purpose the rubber is cut into a strip, which is clamped between grips and gradually stretched to breaking-point. The ring testing machine can be adapted for this purpose by replacing the rollers with clamps. As the thickness of the samples to be tested will vary, it is advisable to cut the strips of such a width that the cross-sectional area of all test pieces is the same—say, 40 sq. mm. The method is applicable to both sheet and crepe rubber. (2) Tests may be made as to the behaviour of the rubber during milling or mastication. Small batches are milled under uniform conditions, preferably in an enclosed masticator such as Baker and Perkins supply. The power taken (as measured by the current taken to drive the motor actuating the machine) and the time are recorded. A further test may be applied to the milled or masticated rubber, to ascertain the amount and the time taken to incorporate a finely divided mineral matter, such as carbon black, zinc oxide, or one of the refined clays.[38] The results are not very exact, and the difference in plasticity and dryness noted are usually less than found when working with full-sized machines in the factory. (3) The rubber, either raw or masticated, may be “dissolved” in a “solvent,” such as benzene, and the viscosity of the “solution” measured. Generally speaking, the less viscous the solution, the more plastic the rubber.

The testing of vulcanised rubber has been treated in such detail in the recent works of Whitby[39] and De Vries[40] that a few special points only will be dealt with here. The preparation of samples for testing involves first the sheeting of the mixture of rubber, sulphur, and other ingredients, if any. The sheets may be 1 to 2 mm. thick. They are soft and adherent, and must be kept between layers of calico to prevent adhesion. A sheet of rubber is then built up by laying three or four sheets evenly upon one another, and pressing together to form a sheet 5 mm. thick. The thick sheet is then roughly cut to shape and vulcanised in a mould by heating in steam under pressure. From the vulcanised sheet so obtained the rings for testing are cut (45 mm. internal diameter. 5 mm. face, and 4 mm. thick). Rings obtained in this manner will not vary in diameter or thickness (reckoned as sections of a tube), as these are controlled by the size of the punch, but will vary a little in the face, as this is controlled by the thickness of the sheet, which depends on the completeness with which the mould is closed. More recently smaller moulds have been adopted, one mould for each ring, and an annular space for moisture to develop a pressure during vulcanising and prevent porosity. The moulds are vulcanised in an oil bath, or oven of some description, in which a constant temperature is maintained. I have adopted for some years a third method. The principle is that used in the factory for making annular-shaped rubber articles, such as washers, rings, elastic bands, etc. An aluminium mandrel, 45 mm. external diameter, is taken, and the thin rubber sheet is wrapped round this, so as to build up a tube about 4 mm. thick, the surplus rubber is cut off, and the edge bevelled with a wet knife. The manipulation will vary somewhat with the type of compound to be treated; thus, in some cases, it is sufficient to well roll the tube with a hand roller to secure adhesion. In other cases it is better to wipe the sheet of compound with a rubber solvent previous to rolling. In the latter case time must be given for the solvent to evaporate before vulcanising. The tube is next tightly wrapped in wet cloth, and is then ready for the vulcaniser. Or the tube may be enclosed in moulds which form an outer circular shell and take the place of the cloth, but for most purposes, and in particular for the rubber-sulphur mixing usually employed, it is sufficient to use cloth to obtain even and regular tubes. The tube, after vulcanising, is slipped on to a wooden mandrel and cut into rings on a lathe. Of these rings the internal diameter is constant, for this is formed on the mandrel, also the face, which can be cut accurately in the lathe, but the external diameter, and consequently the thickness, may vary a little.

It appears, therefore, that all methods result in rings of approximately the correct size, and it is usual to check, and, if necessary, make an allowance for variation in dimensions. It is not possible to do this, even approximately, with soft rubbers, as the rubber gives under the pressure of the micrometer. No doubt a photographic method would give more accurate results, but would take too long. I have found that a very close approximation is obtainable by weighing the rings as the specific gravity of the standard rubber mix is known. It is not necessary to weigh each ring, but the whole five or ten taken for testing may be weighed together.The next point that arises is the choice of a formula for the test mix. Practically all the work to date has been carried out on mixtures of rubber with 7 to 10 per cent. of sulphur. For some purposes—e.g., detecting variation in rate of cure—this mixing is satisfactory, but for other purposes it is not. Nor is the behaviour of a rubber-sulphur mixing a sure guide to the behaviour of one containing other ingredients, such as litharge. Thus, two samples vulcanised satisfactorily when mixed with sulphur only, but one of them gave unsatisfactory results in the presence of litharge. It has long been recognised that mineral ingredients may modify the product when vulcanised, but the modification is not necessarily uniform. Consequently, tests should also be made, when practicable, with vulcanised rubber containing other ingredients in addition to sulphur.As regards physical tests on the vulcanised products, these usually involve determination of breaking load and elongation at rupture (usually recorded as final length—that is, including the original length reckoned either as unity or as 100 units). Simultaneously a load-stretch curve is recorded on an autographic attachment. The type of curve varies with (1) state of cure, or degree to which the rubber is vulcanised; (2) proportion of sulphur and/or other ingredients; (3) specific nature of the rubber used. The last factor is almost negligible compared with the two former—at any rate for average quality rubber. As (2) is kept constant for any batch of tests, or even for every test, it follows that the load-stretch curve is mainly dependent on the state of cure, and the degree of vulcanising may be measured by comparing either the elongation produced at a given load or the load produced at a given elongation. Either set of figures is readily determined by measuring up the load-stretch diagram.

The peculiar type of the curves has long been a subject of comment and speculation. Special properties have been attributed to the “slope” or inclination of the upper and approximately straight portion of the curve. According to the writer’s investigations, the “slope” is largely dependent on the degree of vulcanisation, so that it is difficult to “place” as an index of the specific nature of a rubber.[41] Moreover, it has recently been shown that the peculiar type of curve given by vulcanised rubber is the result of plotting the load against the sectional area of the unstretched test piece, whereas this area decreases progressively as the test piece stretches. If this decrease be allowed for, the curve obtained is an equilateral hyperbola.[42] Preliminary experiments with rubber compounded with large proportions of finely divided mineral matter, such as carbon black, show that the load-stretch curves obtained autographically are likewise reducible to equilateral hyperbolÆ.