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.
[49] Bulletin R.G.A., June, 1921, p. 246, October, 1921, p. 398.
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]
[50] _Journal Soc. Chem. Ind._, 1918, p. 280.
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 const.i.tuents 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 const.i.tuent, 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]
[51] _Journal Soc. Chem. Ind._, 1916, p. 874.
[52] _Ibid._, 1915, p. 524.
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 quant.i.ties of accessory substances in the serum which act as catalysts, the rate of vulcanisation (or curing) will depend on the nature and quant.i.ty of such substances present in the rubber. A very small quant.i.ty 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 cla.s.s are formed by putrefaction of organic matter, and can be separated in much larger quant.i.ty 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.
[54] _Journal Soc. Chem. Ind._, 1917, p. 365.
On the other hand, there are one or two facts which are difficult although not impossible to fit in with theory. Thus, although the putrefaction bases are very easily soluble in water and acetone, they cannot be removed by washing on the creping rollers, or by acetone extraction. This may be due to the power of colloidal substances to retain other crystalloidal substances, such as the bases, which, in consequence, cannot be washed out.
A parallel case is the retention of small quant.i.ties of water soluble substances in the soil. Also, the theory does not explain why rubber obtained by evaporation of latex at relatively high temperatures is fast vulcanising, although the possibility of putrefaction is excluded.
As regards practical results, it follows that the rate of vulcanisation (or cure) of a sample of rubber will depend on the time allowed to elapse between the collection of the latex and treatment till the rubber is dry, as also on atmospheric conditions. Thus, slow drying will result in an increased rate of cure, for it gives an opportunity for putrefactive organisms to play a part. The results will, however, be influenced by the extent to which the rubber was washed previous to hanging, and so forth.
Smoking is an antiseptic process and will, therefore, tend to inhibit the action of micro-organisms and produce a slower vulcanising rubber. On the other hand, sheet contains more serum than crepe, so that there is more food material for growth of micro-organisms. The net result is to give a rubber (sheet) which usually vulcanises a little faster than crepe.
Among other factors controlling the rate of cure, special mention should be made of the nature and amount of coagulants. Weak "organic" acids, such as acetic, lactic, tartaric, etc., used in the minimal proportions (1 to 1,200 of standardised latex in the case of acetic acid), give the fastest vulcanising rubber; "strong" mineral acids, such as sulphuric acid, even when used in the minimal proportions (1 to 2,000), yield slower vulcanising rubber. Acid salts, such as alum, are intermediate in effect. Increased proportions of coagulant cause a reduction in rate of vulcanising with all coagulants, and the effect is least noticeable in crepe rubber, intermediate in sheet rubber, and most p.r.o.nounced in "slab" rubber (discussed below).[55]
[55] Bulletin R.G.A., July, 1919, p. 39; September, 1920, p. 343; November, 1920, p. 433; October, 1921, p. 393; March, 1922, p. 134.
OTHER TYPES OF PLANTATION RUBBER.--We have up to now confined our attention to ordinary thin air-dried crepe and smoked sheet, as almost all plantation rubber is now marketed in one or other of these two forms. There are, however, other types, to which reference has been made. Of these, the most important is the thick blanket crepe, made chiefly in Ceylon by rolling together thin crepe, which has been artificially dried (Colombo drier or vacuum drier). The heat of the driers causes a surface stickiness, which is got rid of by rolling several thin layers together to give one thick one.
This rubber vulcanises at about the same rate as ordinary thin crepe, for the relatively high temperature of drying does not appear to influence the rate of cure. The rubber is generally softer than air-dried crepe, and is easily "let down" in naphtha; it is, therefore, suitable for some solution work. Generally speaking, the properties of blanket crepe do not differ materially from ordinary thin crepe. Another type of rubber seldom met with is matured slab or crepe, prepared from it. This type of rubber is being made in small quant.i.ties on one or two estates, who supply direct to the manufacturer. The method of preparation has already been described. It is unsuitable for sale in the open market, as it contains a variable amount of moisture, has the various surface defects such as slime, mould, and "rust,"
and there is the additional disadvantage that it is not easy to judge of its cleanliness or freedom from coa.r.s.e impurities by inspection. If the slab rubber be creped and air-dried on the spot, the product is of satisfactory appearance, except that it is of low colour and may be streaked. As the crepe so produced vulcanises almost as fast as the original slab, the crepe embodies all the advantages of a fast curing rubber with few of the disadvantages of the slab itself. We have made experiments from time to time, and found that by a judicious use of sodium bisulphite it is possible to produce a fast vulcanising crepe rubber sufficiently even and light in colour to satisfy the Standards Committee.
A fast curing raw rubber is not necessarily a desirable type for all manufacturing purposes. In the vulcanising of large ma.s.ses of rubber, a slower rather than a faster vulcanising rubber may be desirable, so as to give ample time for the heat to penetrate and spread evenly throughout the ma.s.s. But for many purposes a fast curing rubber enables a larger output to be obtained, so that artificial organic accelerators are coming more and more into use. The addition of such accelerators might be obviated, if a suitable fast curing rubber were available, but it is essential that such rubber should be uniform. It is just in this respect that slab rubber or crepe made therefrom is found to be deficient.[56] The rate of cure depends on the functions of wild bacteria, which are naturally sensitive to changes of conditions, such as temperature, etc. The coagulated rubber depends on chance circ.u.mstances for infection, and, as a natural result, the activity of the bacteria and the nature and amounts of active vulcanising agent produced will vary and be difficult to control. Consequently, the rate of cure of slab rubber shows considerably greater variation than ordinary crepe or sheet.[57] This, in our opinion, is the main difficulty of utilising "slab," or crepe prepared from it. Experience in other industries, using micro-organisms, has shown that the only method of control has been to replace the wild growths by cultures of some particular strain, as, for instance, in yeasts for brewing. To control the rate of cure of slab, it might be possible to use a special culture for the purpose.
[56] Bulletin R.G.A., January, 1920, p. 6; January, 1921, p. 47.
[57] _Ibid._, January, 1920, p. 68.
Other less usual methods of preparation, referred to in the earlier part of this book, do not call for particular mention, as the properties of the rubber do not differ much from ordinary sheet or crepe. It is mainly a matter of variation in rate of cure.
This short account of the vulcanising properties of plantation rubber would not be complete without a reference to Fine Hard Para, the premier rubber of the Amazon. This rubber has come to be regarded as the standard high-grade product with which plantation rubber may be compared, and many manufacturers are still of the opinion that it is unsurpa.s.sed by any plantation product. Yet, when subjected to the ordinary vulcanising tests, we find that samples of Fine Hard Para give figures very similar to average plantation rubber; indeed, it is not difficult to find specimens of plantation rubber which give appreciably higher figures on testing. It is claimed, however, that Fine Para is more uniform than plantation rubber, and can be relied on always to give the same results. Yet tests on a series of Fine Hard Para specimens gave variations in rate of cure similar to those found for plantation. Some figures were published, which tended to show that the variation was smaller for Fine Para, but it turned out that each of the samples taken for examination consisted actually of a number of slices cut from different b.a.l.l.s, so that greater uniformity was not unexpected.[58] The superiority of Fine Para is, therefore, somewhat of a mystery. It is probable that some manufacturers prefer to use it because they feel safer with it, and know actually how it will behave from long experience. In one respect Fine Para is possibly superior to most plantation rubber--that is, for the preparation of raw rubber solution for sticking the seams of waterproof garments, and for similar purposes. The method of preparation may well influence the strength of the raw rubber when used for this purpose. Plantation rubber has been prepared in the same manner as Brazilian Para, in particular on an estate in Java. The product resembles Brazilian Para in appearance. Vulcanising tests gave satisfactory figures, but, as already stated, this would not serve to show that the rubber was equal to Brazilian Para from the manufacturer"s standpoint.
[58] Bulletin R.G.A., September, 1920, p. 347.