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Chemical and Physical Tests of Solid Tires.

12th November 1908
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Page 4, 12th November 1908 — Chemical and Physical Tests of Solid Tires.
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Which of the following most accurately describes the problem?

By Clayton Beadle and Henry P. Stevens.

Tern hie-, new, to Table D, which shows the figures for stretch in the transverse and longitudinal directions. It will be noted that the longitudinal stretch is almost universally less than that in the transverse direction, the greatest dif

TABLE D. — Showing the Transverse and Longitudinal Stretch of Solid Cab Tires, and Ratios between the two.

ferenee being in tire " 7," in which the ratio is 119.5 per cent. In tire " 9," the difference is practically nil, the others ranging between these figures.

The moist heat test is one which is favoured by the War Office and Admiralty in their specifications. It consists in an exposure of the samples of vulcanised rubber to a temperature of 32o' F., for four hours, in steam under pressure. It is obvious that so drastic a treatment will have a considerable effect on the quality of the rubber, and will impair its physical qualities as determined by tensile strength and elongation. This deterioration will be all the more noticeable, should the rubber contain anything but a very small proportion of rubber substitute and saponifiable oily matter.

The moist heat test was applied to the samples included in Table C, after which they were again tested for physical qualities with the results indlieated (Table E).

In order to determine to what extent the samples have suffered by the " moist heat '' test, it is advisable to place the figures for the mean strength before the application of the test against the figures after the test, and to calculate therefrom the percentage loss in strength, in each case, that has resulted from this trcattrient. The same ii 10(11(3 TABLE E.—Strength and Stretch of Solid Cab Tire Rubbers, after application of "Moist Heat" test.

operandi can be adopted with the figures for the elongation, etc. This has been done in tables F and G.

We notice that the reduction in strength due to the " moist heat " test (as shown on Table F, last column), is greatest with sample " 5 " and least with sample " 9." Sample "6," which has the greatest initial strength, loses 39 per cent, of its strength under this test, On turning to Table G, it will be noted that sample " 6," which has also the greatest initial strength as well as stretch, has actually increased in stretch by 15 per cent., whereas sample " 5 " has only decreased 7 per cent. in stretch. These results would seem In indicate that sample " ti " was under-vulcanised. It should be noted, also, that " 7," " S " and " 9," which lost the least strength (see last column Table F), have lost the most in stretch by the test.

Determination of Percentage Rubber.

We now pass to the chemical investigation, and we shall endeavour to explain how these figures are obtained, and what significance they possess as a guide to the determination of the value of a rubber tire.

The percentage of rubber is usually determined by difference, that is to say, the various other constituents are determined, and what remains over is reckoned as rubber. Attempts have been made to estimate rubber directly, but up to now no satisfactory method has been found. The figure for the percentage rubber, therefore, really represents rubber and any other organic matter that may be present which is not removed or allowed for in the determination of the other constituents. The simplest method of all is, first, to determine the ash in the residue after treatment with alcoholic potash, which removes rubber substitutes and most other organic non-rubber substances, and to deduct the weight of ash from the weight of residue; this gives the rubber proper, plus the sulphur of vulcanisation, provided that there is no organic or other matter unacted upon in alcoholic potash. When the sulphur of vulcanisation, i.e., combined sulphur, has been determined, this should, strictly speaking, be deducted from the above figure for rubber, in order to arrive at the anmunt of rubber proper. In the case, TABLE F.—Reduction in Strength of Rubber used for Solid Cab Tires due to "Moist Heat" test.

however, where such bodies as fibre and lamp black are present (as they often are in these solid tires), it is necessary to earl-

, the analysis a stage further, and here serious difficulties will be encountered. The method adopted maybe outlined as follows :

The rubber, after treatment in alcoholic potash and drying, is treated with a solvent. Two solvents which have been recommended are nitrobenzene and nitronaphihalene. In the course of our work, we have found that, although the solvent readily dissolves the vulcanised rubber with decomposition, it is extremely difficult to manipulate a solvent such as nitronaphthalene, which is solid at the ordinary temperatures, and we have found it quite impossible to filter and obtain a clear solution in the presence of lamp black, etc., even when the solution is largely diluted with benzene. Lamp black consists of carbon in such a finely-divided form that it is not retained by filter papers, and we have abandoned the method as impracticable. As an alternative. we have tried to separate the lamp black by allowing the solution to settle in tall glass tubes; owing to the low specific gravity

TABLE G.—Reduction in Stretch of Rubber used for Solid Cab Tires, due to application of "Moist Heat" test.

of lamp black, this process is a very slow one. The tall tubes should be covered with watch glasses, or otherwise stoppered, to prevent evaporation of benzene and consequent crystallisation of the nitronaphthalene. When the

solution is sufficiently clear, it is carefully poured off, and the residue treated a few times to wash it free of nitronaphthalene, after which it is completely removed—by means of a little benzene— to a weighed dish or flask, and dried and weighed. This dried residue may be examined under the microscope, to see how far it contains fibre, lamp black, etc. The total residue, after drying down on a watch glass and weighing, 'nay be deducted from the weight of rubber after subtraction of the ash, and the net weight gives the rubber proper, but it should be noted that, here again, the rubber has to be determined by difference. The more direct methods for determination of the percentage of rubber are of a highly-technical nature, and space will not permit us to enter into them now.

Sample " 9 " (Table B, in first portion of this article). shows a very low percentage of rubber. It presents a ease where very little rubber is used, its place being taken by rubber substitute, etc., combined with a high percentage of oh. We consider this make to be useless for solid tires.

For fuller information on special points, readers should consult standard text books on the subject. Our remarks are merely an indication of the methods which may be employed, and of some of the difficulties which are met with in this class of work. Correct interpretation of the results, even when obtained, is, however, a more difficult matter than the determination of the analytical figures. For this purpose, some experience on the manufacturingside is necessary, and the chemist must constitute himself a rubber manufacturer on a small scale. In this way, with a certain amount of experimental machinery, which must include a small pair of power-driven masticating, mixing, and sheeting rolls, and a few moulds, vulcanising pan, etc., he can accumulate a large range of products of known composition and physical properties, which may serve him as a guide in any diagnosis of the composition and mode of manufacture of unknown articles that may be brought to his notice.

Estimation of Resins and Free Sulphur.

We will now proceed to examine the methods by which the other constituents of the tires are determined. Portions of the tire are first passed between the rollers (ride supra) until they are reduced to a fine powder or crumb. The rubber is generally used, in this form, for the chemical analysis. In the first place, the rubber is subjected repeatedly to the action of the solvent acetone : if the operation is continued long enough, all the resinous matters and free sulphur will be removed. With a view to the simplifying of this process, we cut very thin transverse sections of the rubber tires, as thin as a sheet of paper, and do not grind to powder and use a Soxhlet apparatus. We take a carefully-weighed flask of about suoc.c. capacity, in the neck of which are placed the weighed pieces of thin rubber, and into the flask is poured about 50 to 70c.c. of acetone. The flask is placed at an angle of 45 degrees, just dipping below the surface of the water in a water-bath, the water being of a temperature just sufficient to cause a very slight ebullition of the acetone, at the same time having the bulk of the acetone out of the hot water. This simple arrangement insures that the hot acetone vapour surrounds the rubber, and the condensation of the escaping' vapours.

The bulb tube is sufficient to condense the vapours, which, on condensation, drip gently from the tube upon the rubber held in the neck of the flask. The rubber is maintained at a temperature approximately that of the boiling-point of the acetone, and this is more effective than the Soxhlet apparatus, where the acetone which comes in contact with the rubber is much cooler. Sulphur is only slightly soluble in acetone, and more soluble in hot than in cold acetone, so that crystals of sulphur. in most cases, separate out from the acetone liquor, on the bottom and sides of the flask.

To make certain that the operation is complete, it is as well tu substitute a second weighed flask for the first, with the addition of fresh acetone, and to note whether any more crystals are formed, or further resins removed. When the extraction process is ended, the acetone liquor, which contains the resin, can (when cold) be poured off into another weighed flask, the crystals of free sulphur being left behind, and these, after washing once or twice with fresh acetone to remove all resins, can be dried and weighed in the flask. The acetone liquid in the other flask is evaporated to dryness and weighed, its weight giving the amount of resin extracted, and the thin sections of the acetone-extracted rubber are dried in a current of coal gas at too degrees C., until the weight is constant. The loss in weight which the rub

ber sample suffers by the treatment represents the aeetoie extract, plus moisture. The moisture, however, is best determined on a fresh sample, by drying carefully in r current of coal gas. The above-mentioned method for the determination of free sulphur works admirably in certain cases, but, in others, the sulphur refuses to crystallise in a manner in which it can be removed by the method. In such cases, it is neceasary to evaporate the whole acetone extract to dryness, and to weigh, the weight representing the resin and sulphur, and then to remove the resins by repeated moistening with acetone, as recommended by Weber, and again to dry and weigh. This final residue is returned as sulphur which, deducted from the previous residue, gives the resins. If greater accuracy is required, an allowance must be made for the solubility of sulphur in the acetone which is used for 'removal of the resin from the sulphur. Properly speaking, the loss sustained by the sample on acetone extraction should be equal to the eesin, sulphur, and moisture combined, so that, by deduction of the residue obtained in the acetone extraction from the loss that the sample has sustained on extraction with acetone, one should have the moisture figure. Although this procedure is recommended in some text books, we have not found it reliable in practice. The moisture figure is, as a rule, something under o.5 per cent. in vulcanised rubber. For most practical purposes, the acetone extract is best expressed as such, without any attempt to differentiate the ingredients extracted.

The great advantage (If the process, when carried out as described, by placing sections in the neck of the flask, is that a large number of small flasks with bulb tubes can he placed together in a water bath ' • they require no attention, and give no trouble, provided that the water is kept at the right temperature, and the flasks equally immersed. For rougher determinations still, it is sometimes sufficient L6 place yerv thin sections of tires in boiling acetone under an inverted condenser, and, after eight hours' treatment, to evaporate the extract, and weigh. This method is not so accurate or so complete as the foregoing.

When the rubber in the form of powder is extracted in an ordinary Soxhlel, it is dried before weighing in a current of coal gas. Although the drying in the coal gas undoubtedly prevents oxidation and a consequent gain in weight therefrom, the rubber gains in weight very materially from absorption of the gases. It is, therefore, necessary to weigh the sample periodically, and to take the minimum weight recorded.

Estimation of Rubber Substitutes.

The next step in the analysis is to determine the india. rubber substitute; this, of course, is done by taking the above sample—after acetone extraction—and placing it in a flask, which contains a boiling solution of alcoholic potash, under an inverted condenser, for a sufficient time to render all the rubber substitutes soluble. After this treatment, the sample is washed until free from alkali and soluble products, and, after careful drying in a current of coal gas as above, is again weighed, the loss in weight representing indiarubber substitute.

Resins and free sulphur (acetone extract) and rubber substitutes, as determined by analysis, arc given in the next table : all these are expressed on the weight of the original sample. For purposes of comparison, the figures for indiarubber and combined sulphur are also given.

TABLE H.—Percentage Composition of Samples.

The figure for acetone extract may be of considerable value in arriving at the composition of tires. The figure for substitute speaks for itself. The combined sulphur . ; S a more useful figure when it is expressed as vulcanising coefficient, by its calculation upon the weight of actual indiarubher present. The extent of die vulcanisation may, in some cases, be judged by this " coefficient of vulcanisation," which represents the amount of combined sulphur in every Tao pme:s of rubber present. The following list gives the coefficient of vulcanisation fer each of the tires in question, beginning with the greatest, from which it will be seen that the coefficient of yulcanisadon varies between 0 and 2.3 :—No. 9, 6.o; O. 7, 3:9; No. 8, No. to, 3.6; No, 6, 3.2; No. F„ 2.3. In interpreting these figures, the possible or probable presence of reclaimed rubber must be taken into consideration, because all such rubbers will have sulphur in combination with them. Some

reclaimed rubber contains the whole of the combined sulphur, the free sulphur onlY having been removed, whereas it is claimed that other brands do not contain more than half of the combined sulphur originally present. Further, combination with sulphur may take place in the. process of vulcanisation. However, a high " coefficient of vulcanisation," on a sample possessinggood physical qualities, would suggest the presence of reclaim robber.