Sheet Rubber.—The commonest defect is mould.[45] This is usually of a light surface type, easily brushed off, and numbers of vulcanising tests failed to trace any reduction in rate of vulcanising or other defect due to this. In spite, however, of the harmlessness of light surface moulds, they are looked upon with suspicion by the manufacturer. Occasionally samples of smoked sheet are offered contaminated with a “heavy” type of mould. The sheet feels damp and “heavy” or flabby, and contains an excess of moisture; sometimes a moist exudation is noticeable on the surface, and “virgin” patches are present. Such sheet vulcanises more slowly than F.A.Q. samples, but does not necessarily show other defects after washing and drying.
[45] Bulletin R.G.A., February, 1921, p. 97; April, 1921, p. 190; June, 1921, p. 243; November, 1921, p. 472.
“Stretching rusty,” as already explained, is due to a dry film on the surface of the sheet, and according to a recent investigation, this film consists, not of serum substances, but of a microscopic mould growth, which presumably grows on the serum substances. A sample of sheet which stretches rusty gives the rubber a “dry” appearance, and for a long time manufacturers mistook the surface film for resin. On the assumption that such rubber was “resinous” they rejected it, and to this day it is regarded as a defect, although it has no influence on the vulcanising properties of the rubber.
It is hardly necessary to point out that defective appearance, such as is due to thickened edges, faint markings, bubbles, and so forth, have no effect on the vulcanising properties of the rubber. They only point to some irregularity or carelessness in preparation. The only justification for distinguishing between rubber of good and bad appearance is that the former bears the impress of careful preparation, and is therefore more likely to be uniform in rate of vulcanising.
Similar considerations apply to the colour of smoked sheet, which may vary from a pale yellow-brown, through various shades of red-brown to dark brown. There are various factors affecting the colour, but the buyer can see but one—viz., the “degree” of smoking—and the rubber, from his point of view, may be undersmoked or oversmoked. No doubt the degree of smoking affects the vulcanising properties, but to a less extent than was at one time imagined. In a recent paper[46] it has been shown that the average breaking strain and rate of cure of a number of samples of smoked sheets were practically the same for light as for dark sheets.
[46] Bulletin R.G.A., December, 1921, p. 521.
Variation in Physical Properties.—A very large number of tests on vulcanised specimens of plantation rubber have been carried out. The rubber was almost invariably mixed with 7 to 10 per cent. of sulphur, and no other ingredient, and vulcanised to give the maximal breaking load. Unfortunately, this determination is subject to a very appreciable experimental error, so that a large number of determinations are necessary to give a reliable figure. It is quite impracticable to make a large number of determinations in routine testing, on account of the labour involved. It is usual to make five, or possibly ten, determinations, although some investigators have been content with two. It is generally conceded that any exceptionally low figures should be ignored, as probably caused by some flaw or irregularity in the test piece. On the other hand, a study of actual determinations shows an occasional excessively high figure, and it is questioned whether this also should be left out of account. Others ignore all except the highest figure, and take this to represent the true breaking strain. As a consequence, the figures published by different workers show considerable variation. De Vries has analysed a large number of the figures obtained in systematic examination of estate samples, and has constructed curves to illustrate the results.[47] It is open to question how far the variations shown are attributable to experimental error. The figures show, however, that the variation in breaking strain is relatively small, and not very different for crepe and sheet rubber. In our opinion, undue importance should not be attached to very high or exceptionally high figures for breaking strain, which are occasionally met with. Provided the figure does not fall much below the average, the sample may be regarded as satisfactory. It is very seldom that any sample of first latex estate rubber does not show satisfactory figures.
[47] “Estate Rubber,” p. 466.
The Rate of Cure or Rate of Vulcanisation is subject to more exact measurement, whether this be based on the physical or the chemical properties of the rubber. If the testing machine be provided, as is usual, with an autographic attachment, the position of the curves traced on the recording paper gives a measurement of the rate of cure. These load-stretch curves, to which reference has already been made, take up a definite position in accordance with the physical properties; it is only the length of the curve, or the point where it terminates (which gives the breaking strain and elongation at break), which is largely fortuitous.
As a measure of rate of cure we may take the actual measurements made on the record.[48] It is convenient to measure the elongation produced by a load of 130 kilos per sq. cm., as all fully vulcanised rings of soft rubber should give higher breaking load figures. For less cured or weaker samples a lower figure may be taken, such as 60 kilos. We have found that when fully vulcanised to give the maximal breaking strain, the elongation at a load of 130 kilos is in the neighbourhood of 850 per cent. (final length 950 per cent.). This applies to ordinary samples of estate rubber under the conditions of testing indicated above. If, however, the proportion of sulphur be considerably reduced, or mineral ingredients in a fine state of division be added to the mixing, or accelerators, whether organic or inorganic, be employed, the above relationship no longer holds. Nor does it hold with regard to plantation rubber prepared in an exceptional manner, as, for instance, matured coagulum or “slab.”