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.

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—

and for smoked sheet

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]

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.

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.

“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.

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.

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.”

There is a second method of determining the rate of cure—namely, by analysing a vulcanisate produced under standard conditions, and determining the amount of sulphur which has entered into chemical combination with the rubber. For this purpose the weighed sample is cut thin or creped thin, and exhaustively extracted with acetone to remove any “free” sulphur—that is, sulphur not in combination with the rubber. The sulphur remaining is then determined and calculated as a percentage of the raw rubber contained in the sample taken. This gives the so-called coefficient of vulcanisation.

If we compare the coefficient with the time of cure at a constant temperature for an ordinary sample of plantation rubber, they are found to be approximately proportional, so long as the sulphur is in sufficient excess. The amount of combined sulphur is, therefore, an index of the time vulcanisation has been in progress (under standard conditions of temperature, etc.), and, therefore, the coefficient is a measure of the rate of cure.

The change in position of the load-stretch curve is not directly proportional to the time of heating, and it therefore follows that it is also not directly proportional to the coefficient. For ordinary samples of crepe and sheet the relationship is, however, not very far removed from proportionality. This applies particularly to sheet rubber. The relationship is readily seen on plotting one against the other and tracing the curves. For sheet we get an almost straight line; for crepe there is some curvature.[49] For ordinary estate samples of sheet and crepe rubber the maximal breaking strain is obtained when the coefficient reaches approximately five units, so that this corresponds to the elongation of 850 per cent. at a load of 130 kilos.

Either physical or chemical methods may, therefore, be used for determining the rate of cure of ordinary sheet or crepe rubber, but great care must be taken when interpreting the results obtained with rubber prepared in an unusual manner. The rate of cure may be expressed in terms of the time taken to vulcanise the rubber at a constant temperature (in our case 138° C.), so as to give an elongation of 850 per cent. at a load of 130 kilos, or to give a coefficient of five units. The higher the figure so obtained, the slower curing the rubber. To express the results more directly as rate of cure, we have adopted the plan of taking an average crepe rubber, calling the rate of cure 100 units, and expressing the rate of cure of other samples in these terms. Thus, a sample which gave a coefficient of four only, in the time taken by the standard to give a coefficient of five, would have a rate of cure four-fifths of the standard, that is, 80; or if a sample takes only two hours to give an elongation of 850 per cent., whereas the standard takes three hours, the rate of cure of the sample will be 3/2 of standard or 150.[50]

As stated, the coefficient is approximately directly proportional to the time of cure; it is also independent of the proportion of sulphur, if in fair excess, and in the presence of inert ingredients. It is also independent of the amount of mastication given to the original raw rubber, however great. On the other hand, the position of the load-stretch curve is variously modified by these factors—in some respects, therefore, the coefficient is a more reliable index. However, the coefficient is influenced by accelerators, so that here also great care must be exercised when interpreting results. For the purpose of detecting variations in rate of cure, it is best to choose a mixing which is particularly sensitive. In the first place, there must be an ample excess of sulphur; and in the second place, no ingredient should be added which will complicate the load-stretch curves, and no accelerators should be present which may possibly tend to obscure the vulcanising properties of the rubber itself. It has been found, therefore, that the best mixing to use consists of rubber with an excess of sulphur—say, in the proportion 9:1 without other ingredients. The rate of cure of a specimen of plantation rubber is attributed to the presence of certain natural vulcanising catalysts, because it is found that carefully purified raw rubber (that is, with the resinous and nitrogenous constituents removed) vulcanises very slowly or hardly at all, but that on replacing the extracted matter the rate of vulcanising is restored. The natural catalysts contained in the extracted matter are influenced to a varying degree by some of the common ingredients of manufactured rubber articles. This applies particularly to litharge (oxide of lead), to which reference has already been made. Thus, acetone extraction of raw rubber to remove resinous matter has but little effect on the vulcanising properties of a mixture of rubber and sulphur. But if litharge be a constituent, it is found that acetone-extracted rubber will hardly vulcanise at all. From this, it follows that a rubber giving a low acetone extract may be found to vulcanise exceptionally slowly in a mixing containing litharge, whereas it shows no such defect when compounded with sulphur only.[51] Litharge is used to a very large extent, as it has a balancing effect in a rubber compound—that is to say, it allows of appreciable variation in vulcanising conditions, without corresponding alteration in the state of cure.[52]

Influence of Various Factors in Raw Rubber Preparation on the “Rate of Cure,” or “Rate of Vulcanisation.”—As the capacity of a rubber for vulcanisation depends on the presence of small quantities of accessory substances in the serum which act as catalysts, the rate of vulcanisation (or curing) will depend on the nature and quantity of such substances present in the rubber. A very small quantity of these substances has a considerable influence on rate of vulcanising, and as the substances are difficult to isolate and identify, our knowledge of their formation and chemical nature is not as definite as is desirable. Substances have been isolated having the characteristics of “simpler bases.” Bodies of this class are formed by putrefaction of organic matter, and can be separated in much larger quantity from coagulated latex, which has been allowed to putrefy before working up than from such which has been worked up without giving time for an appreciable amount of putrefaction to take place. Further, rubber from putrefied coagulum vulcanised much faster than that ordinarily prepared, so that we are justified in connecting the putrefaction bases with the rate of vulcanisation. Moreover, it has been shown that any treatment of the latex or coagulum which inhibits the development of putrefactive organisms also prevents the rubber vulcanising as fast as would otherwise have been the case.[53] Also, the crude bases isolated from fast vulcanising rubber have the power of increasing the rate of vulcanisation when added to ordinary slow vulcanising rubber.[54]

[53] Eaton and Co-workers: See Bulletin No. 27, F.M.S. Department of Agriculture.