_TESTING OF PLANTATION RUBBER_
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.
[38] Bulletin Rubber Growers' Association, January, 1921, p. 43; August, 1921, p. 340.
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.
[39] "Plantation Rubber and the Testing of Rubber."
[40] "Estate Rubber."
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 hyperbolae.
[41] Bulletin R.G.A., October, 1921, p. 397.
[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--
_Tensile _Final Length--i.e., _Tensile Strength._ Elongation + 1._ 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.
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."
[48] Bulletin R.G.A., June, 1921, p. 246.
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.