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.
[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 quantities 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 pronounced 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 quantities 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 coarse 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 masses 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 mass. 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 circumstances 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.