Ju.VE 1, 1921 



THE INDIA RUBBER WORLD 



659 



What the Rubber Chemists Are Doing 



SPONTANEOUS COAGULATION^ 



SfONTA.NEOUS coagulatioii in the air gives a slimy .yellow to 

 brown surface layer, which, during crcping, causes a loss 

 in rubber and makes the crope streaky and somewhat dis- 

 colored. The coagulation is sometimes incomplete and the serum 

 milky. Coagulation with sugar, and spontaneous coagulation in 

 the absence of air have been proved by many investigators. 



1. Comparative experiments in which the portions of undi- 

 luted latex were weighed and the rubber collected showed close 

 average percentages of air-dried crepe when coagulated by various 

 means, that is: by acetic acid; spontaneously in air; the same in 

 absence of air; and by sugar in air. 



These rubbers showed tensile strength and slope practically the 

 same. The sgontaneously coagulated rubber cures somewhat 

 faster and shows a somewhat higher viscosity. The samples 

 coagulated in the air had two days to mature as against one 

 day for the others, cured much faster and showed a higher vis- 

 cosity and somewhat smaller slope. 



2. Spontaneous coagulation proceeds well in latex from trees 

 that have had a period of rest and therefore give a latex of high 

 rubber content, and a slow-curing rubber. The properties of 

 the rubber lie in the above-mentioned direction, but the rate of 

 cure, when creping after 24 hours, increases much more than in 

 normal latex, so that the abnormally slow rate of cure of ordi- 

 nary crepe disappears to some extent. 



3. From a series of experiments on heavily tapped trees under 

 varying conditions it is clear that heavy tapping causes the sub- 

 stances (sugars?) that are responsible for the progress of spon- 

 taneous coagulation to decrease, so that coagulation is less easy 

 and complete. The substances, however, that have an influence 

 on rate of cure (natural accelerators in the latex, decomposition 

 products formed by maturation) are not so affected by heavy 

 tapping that this can be detected in the properties of the spontane- 

 ously coagulated rubber, which are the same for light or heavy 

 tapping. 



4. Experiments on an estate with spontaneous coagulation in 

 the absence of air showed that coagulation proceeds well in un- 

 diluted and 15 per cent late.x, but the crepe was somewhat dull 

 and the color not so light as the market demands. 



^By De \'ries and Spoon. Archief voor de Rubbercultuiir, July, 1920. 

 Commiinic.ition of th^ Central Rnbber Station. 



ON THE DRYNESS OF PLANTATION RUBBER i 



There are irregularities in raw rubber which make their ap- 

 pearance in the masticating and mixing operations of the rubber 

 manufacturer. It is obvious that a soft resinous rubber such as 

 an African grade will break down easily in the mill and requires 

 but little power to reduce it to a suitable consistency for the in- 

 corporation of the mineral ingredients. A tougher and higher 

 grade of rubber will naturally require more power for this pur- 

 pose. Plantation rubber, as a class, takes more power than wild 

 rubber, even the best wild rubber such as fine iiard Para. Manu- 

 facturers complain of irregularity in plantation rubber in this re- 

 spect, and assert that some of it is excessively "dry," that is, it 

 cannot be made sufficiently plastic to take up the minerals ex- 

 cept as a result of a very prolonged treatment. The resulting 

 compound is also affected, being unusually hard and tough. 



Thus far the author has not succeeded in devising a really 

 satisfactory test by which the dryness of rubber can be measured. 

 An interesting series of experiments was carried out on five speci- 

 mens of plantation crepe variously prepared and including one 

 of unknown origin which was found to be particularly difficult 



to break down on full-sized mills in the factory. Each sample 

 was masticated to the same state of plasticity. After three 

 months, solutions were prepared in xylol and the viscosity of a 

 0.25 per cent solution determined and compared with tlie viscosity 

 of the rubber before mastication. 



From the results obtained it appears that a rough grading of 

 raw rubbers as to plasticity may be obtaired by an experienced 

 operator by masticating small quantities on an experimental mill, 

 also that the viscosity of the raw rubber is an indication of plas- 

 ticity, but it must not be assumed that in all cases high viscosity 

 is necessarily accompanied by low plasticity. In the past there 

 has been a tendency to associate high viscosity with "nerve" or 

 high quality, but it has been found that tlie relationship between 

 viscosity and the properties after vulcanization is more compli- 

 cated and less concordant than was at first supposed. 



It is, however, asserted that viscosity tests, if combined with 

 vulcanization tests, give valuable indications as to the quality 

 or "nerve" of the rubber. From the foregoing experiments it 

 would appear, however, that high viscosity is not of itself a 

 desirable property, as it appears to be accompanied by low plas- 

 ticity and the most desirable type of rubber from the manufac- 

 turer's point of view would appear, therefore, to be one giving a 

 low viscosity but high figures after vulcanization. 



We are ignorant of the cause of "dryness' in plantation rub- 

 ber. The acetone extract, resin content, does not vary 

 sufficiently to explain the difference between the samples. It is 

 possible that the nature and proportion of the coagulant may 

 have an influence. Certain batches of sulphuric-acid-coagulated 

 rubber were found to work "dry," and were difficult to break 

 down, but we failed to show that sulphuric-acid-coagulated rub- 

 ber always works dry. In any case, for other reasons sulphuric 

 acid should not be used for coagulation. Another possible cause 

 of dryness is insufficient washing on the mill. Crepe which is 

 lightly rolled and full of holes is said to be difficult to break 

 down. Occasionally rubber is not used for a year or more after 

 arrival. Such rubber freezes in the winter. When thawed it is 

 still harder and tougher than before, and requires more power 

 to break down than if it had been used fresh. Freezing and care- 

 ful thawing of rubber considerably increases the tensile strength 

 in the unvulcanized condition, and, therefore, probably aflFects 

 the plasticity. 



THE CAOUTCHOUC MOLECULE i 



Harries in his work on caoutchouc" regards the caoutchouc 

 molecule as a polymerized assemblage of units consisting of the 

 basal hydrocarbon CmHm, of such a nature that depolymerization 

 may occur with great ease. Presumably, as when his earlier 

 formula was enunciated, he considers the polymerization as due 

 to auxiliary valences of the nature of Thicle's partial valences. 



In one respect Harries' newer formulation of the caoutchouc 

 molecule approaches Pickles' formulation. The 20-carbon ring 

 wliich he adopts is identical with the ring proposed by Pickles, 

 if the number of isoprcne units included in the latter is limited 

 to five. Pickles, however, regarded the ring as including at least 

 eight such units. The essential difference between the two views 

 remains. The former regards the caoutchouc molecule as consist- 

 ing of a number of 20-carboii rings polymerized ; the latter as 

 consisting of a single larger ring. 



The chief considerations which Harries urges against the view 

 that the molecule consists of a single large ring are: (1) the 

 readiness with which caoutchouc depolymerizes ; (2) the oc- 



'Ry Henry P. Steven:;. lUdictin of tbe Rubber Growers' .Association, 

 Janu.-iry. 1<)21, page 43. 



'Dr. G. StsfVord Whitby. The India-Rubbcr Jturnal. February 12, 1921, 

 patre 313. 



'Untcrsuchuntten Ueber die Natiirlichen und Kiinstlichsn Kautschukarten, 

 Berlin. J. Sprii.aer. 1V19. 258 pp. 



