894 



THE INDIA RUBBER WORLD 



September 1, 1921 



What the Rubber Chemists Are Doing 



PROPERTIES AND VALUATION OF PLANTATION RUBBER' 



TESTIN'G a colloid like rubber, especially as to its mechanical 

 properties, is complicated by the fact that no absolute but 

 only relative values can be obtained, dependent upon many 

 details of the method chosen. Average values obtained are 

 determined to form a base of comparison, and the ordinary varia- 

 tion of plantation rubber has to be determined to ascertain what 

 should be called normal, and what abnormal values. The results 

 of any single test should then be compared with these average 

 and normal values, and the rubber judged accordingly. 



STRENGTH OF UlTVULCANIZED RUBBER 



The strength of unvulcanized rubber, tested in its crude form 

 is certainly not independent of the method of preparation. Data 

 on this point are rather scarce, as the rubber is not in suitable 

 form for accurate tests. 



More important for the real quality of the rubber is the re- 

 sistance to stretching in the vulcanized state, especially at high 

 elongations. This property finds expression in the slope of the 

 stress-strain curve, that is, the ratio of stress to strain. It has 

 hitherto attracted less attention than it deserves. Apart from 

 its own importance, its close relationship to such characteristics 

 as deformation or set, and thereby to the intimate structure of 

 the rubber, should give it a prominent place in rubber testing. 



The slope is influenced by factors originating in the latex as 

 well as in preparation. Young trees or heavily tapped trees give 

 a vulcanizate which is less resistant to stretching, and in such 

 cases the composition of the latex must be the cause. Use of 

 sodium sulphite, bisulphite or other chemicals which prevent 

 oxidation of serum substances, and also maturation of coagulum 

 which is accompanied by a decomposition of serum substances, 

 cause an improvement in slope which must probably be ascribed 

 to a change in the non-rubber substances. On the otlier hand, 

 all the factors which cause a real deterioration (oxidation or 

 decomposition) of the rubber are accompanied by a higher figure 

 for slope, that is, a decrease in quality. It seems, therefore, that 

 slope is a rather complicated property which depends both on 

 the caoutchouc hydrocarbon and on the non-rubber substances. 



Besides tensile strength and slope which express the properties 

 of the rubber after vulcanization, the rate of cure is also an 

 important factor upon which attention has mainly been focused. 

 Differences in rate of cure are in the majority of cases, if not 

 always, caused by the non-rubber substances, which act as 

 catalyzers of the vulcanization process, and often in very small 

 amounts. Many of the chemicals which have been tried either 

 as coagulants or for other purposes, have an accelerating or re- 

 tarding eflFect ; but the powerful catalysts have to be looked for 

 among the substances present in the latex or formed during 

 preparation. 



It has been proved that nitrogenous bodies, amines or amino- 

 acids, which have a part in the building up or decomposition of 

 proteins, are, in the majority of cases, responsible for differences 

 in rate of cure. Which amines are actually present in latex and 

 rubber, and in what amounts they work is not yet known. Es- 

 pecially in simple rubber-sulphur mixtures the rate of cure is 

 very sensitive to small differences in content of these by-sub- 

 stances, so that nearly every change, either in the condition of 

 treatment of the trees or in method of preparation, makes itself 

 felt in the rate of cure. 



The subject has been studied systematically and the influence 

 of most factors is known, so that the properties of plantation 

 rubber in this respect are now well in hand. Methods of prep- 



•-Abstract of a paper by Dr. O. de Vries. director of the Central Rubber 

 Station, Buitenzorp. Java, read at the Fifth International Rubber Exhibi- 

 tion, London, England, June 3-9, 1921. 



aration have been worked out which, even with the variations 

 unavoidable in actual practice, assure a sufficiently uniform prod- 

 uct. 



EXAMPLE FROM ACTUAL rRACIICE 



On most plantations the trees are regularly tapped the whole 

 year around, until lately every day, nowadays often on alternate 

 days. The tree adapts itself to the treatment it is subjected to 

 and regularly yields a latex of fixed composition, which repre- 

 sents an equilibrium between what is taken away in the form of 

 late-x and bark and what the tree can produce during the period 

 between two tappings. No doubt the regularity in tapping 

 operations together with the adoption of very similar and simple 

 tapping systems, has in later years done much to insure a larger 

 degree of uniformitj' in the late.x and in the output of estates in 

 general. 



This regularity in the daily routine was sometimes broken up 

 by the resting of the trees, for instance, during the dry months, 

 at which time Hevea drops its leaves and develops a new crown 

 of foliage. This practice was rather exceptional in Java but 

 seems to have been applied more often in other rubber-producing 

 countries. Of late, resting the trees has been resorted to as a 

 general measure on account of low prices and restriction of out- 

 put. 



The physiological condition of the tree, of course, changes 

 when the regular loss of latex and bark comes to a stop, and 

 the consequence is that when tapping is restarted, a latex of 

 deviating composition is obtained which yields rubber of very 

 abnormal vulcanizing properties. This makes itself felt, espe- 

 cially in the first days or weeks after starting tapping. Gradually, 

 on continued tapping, the condition of the tree changes, imtil 

 after three or four weeks a state of equilibrium between the con- 

 dition of the tree and the tapping system is reached again and 

 the usual regularity is restored. The abnormal composition of 

 the latex after resting the trees is evident at once from its color, 

 which is yellow instead of bluish-white. This color remains very 

 evident in the rubber when it is prepared in the form of crepe, 

 which then shows a deep yellow, often saffron-yellow color. In 

 smoked sheet this irregularity is practically invisible, as it is 

 covered up by the dark-brown color of the smoke. 



Another abnormal property of the latex is shown in the rubber 

 content, which on the first two or three days may amoimt to 45 to 

 50 per cent or even more, and very gradually drops to 35 or 30 

 per cent, which corresponds with the state of equilibrium already 

 mentioned. The amount of latex, which on the first tapping is 

 rather small, quickly increases from day to day by virtue of 

 that remarkable property of the Hevea tree, which forms one 

 of the principal pillars of its culture, namely, the wound response. 

 In two weeks the yield of latex reaches its maximum and then 

 drops again somewhat to reach its normal level. 



MINOR DEVIATIONS 



These are the easily visible signs of the large deviations found 

 when tapping a tree that has had a rest, and it is not to be 

 wondered at that the composition of the latex is also abnormal. 

 This is very much so as regards its content of natural accelera- 

 tors, which determine the rate of cure of the rubber. The rub- 

 ber obtained from the first day's tapping vulcanizes with ab- 

 normal slowness. Taking 100 as the nonnal figure for time of 

 cure, a figure of 150 to 160 may be expected for the above ab- 

 normal rubber, and even 180 is no exception. 



These are values that are never met with in ordinary plantation 

 rubber which on the above scale seldom cures more slowly than 

 in 130 minutes. Therefore, tapping trees that have had a rest 

 means a serious disturbance of the general uniformity of planta- 

 tion rubber. A yellow color in crepe may also be obtained in 



