July 1, 1921 



THE INDIA RUBBER WORLD 



r39 



What the Rubber Chemists Are Doing 



KEEPING QUALITIES OF PLANTATION RUBBER' 



THE conditions that have prevailed for a year in the crude 

 rubber market have resulted in the accumulation in storage 

 in the Far East, London, New York, and other points, of 

 enonnous stocks of plantation rubber, regarding the keeping quali- 

 ties of which expert opinion is desirable. 



The following expert views on this topic, therefore, have a 

 timely interest for crude rubber producers, dealers and rubber 

 goods manufacturers. It should be noted that the author's re- 

 marks apply more particularly to tropical conditions. 



Most plantation rubber, as such, has only a short existence. 

 Between harvest and vulcanization in the rubber factory, not 

 more than a few years elapse, as a rule, and little is heard of 

 serious deterioration during that time. The general conclusion is 

 that first-grade plantation rubber stands keeping for some years 

 very well ; yet there is no doubt that even excellent rubber may 

 completely deteriorate in the course of years. For instance, a 

 piece of smoked sheet which had been awarded a first prize at 

 the London Rubber Exhibition of 1911, after seven years' keeping 

 in the tropics, had become absolutely brittle and weak by 1918, 

 with a viscosity of only three units, while vulcanization gave an 

 unsatisfactory spongy mass. 



In actual practice the changes which take place in the product 

 during the first two or three years are the most important. In- 

 fonnation is still incomplete on many points, for instance, on the 

 difiference between keeping in subdued light, or in the dark, in a 

 wet or dry atmosphere, at tropical or European temperatures, no 

 accurate data are as yet available. 



From our experience with hundreds of samples, kept at a fairly 

 regular temperature of 27 degrees C (78.6 degrees F.) in cup- 

 boards, we may state that under such circumstances the rubber 

 does not show appreciable deterioration in appearance. The very 

 light color of pale crepe liecomes decidedly darker in all cases, but 

 no other visible change occurs, provided the samples remain dry 

 enough so that no mouldiness or spots appear. 



INNER PROPERTIES 



Tensile strength in many cases may remain constant for periods 

 of at least four years. We retested many samples which after 

 three to four years showed the same excellent strength as when 

 tested a few weeks after preparation. Other samples, however, 

 showed a gradual decrease. No apparent reason has as yet been 

 discovered as to why ordinarily prepared samples should in some 

 cases remain constant in properties, and in others show a slow 

 deterioration. 



TIME OF CURE AND SLOPE 



The time of cure in a number of cases showed a decrease of 

 five to ten minutes on keeping, but in others remained constant, or 

 increased. The causes of such changes are not yet cleared up. 

 Preliminary review of data indicates that where rate of cure was 

 retarded by some cause, a decrease in time of cure gradually sets 

 in, while rapid-curing rubber often showed an increase, so that in 

 both cases the rate of cure, on keeping, gradually returned to more 

 normal values. 



The slope generally shows no change, or only small variations 

 in which, so far, no regularity could be detected. 



VISCOSITY 



Viscosity is perhaps the figure in which a change is first to be 

 detected. While the properties after vulcanization often remain 

 the same for a year or more — and only begin to change by longer 

 keeping — the viscosity often begins to decrease sooner. Most 



' "Estate Rubber, Its Preparation and Testing.'* By Dr. O. de Vries, 

 19J0. Abstract from Chapter 12. 



first-grade samples, crepe or sheet, remain constant in viscosity 

 during several months or a year, but then gradually show a de- 

 crease which seems to continue slowly on further keeping. 



The above statements all relate to estate samples prepared in 

 the ordinary manner and taken at random from the output of a 

 large number of estates. Data on keeping qualities in general do 

 not point to a large influence of the method of preparation on 

 the keeping qualities of first-grade rubber. Traces of special 

 coagulants, such as sulphuric acid or alum, were not found to 

 cause larger changes than the ordinary dose of acetic acid ; neither 

 did exaggerated creping cause any marked or rapid deterioration 

 in the raw product. 



PACKING AND STORAGE 



Packing the rubber only when quite dry and in dry chests, tak- 

 ing care that the chests do not get wet during transportation or 

 in a damp shelter, and storing the rubber in a dry place with 

 sufficient ventilation, especially between chests and floor, are the 

 most necessary precautions for keeping rubber, whether in the 

 tropics or in storage in temperate climates. 



EFFECT OF MOULD ON RUBBER QUALITY' 



Factory experience has shown that wet, mouldy smoked sheet 

 rubber almost invariably retards the rate of cure and gives quite 

 an inferior vulcanized product. Light mould, on the other hand, 

 very often yields a normally cured rubber, but some samples have 

 been obtained in which even a light mould has a deleterious efifect. 

 The greatest drawback in having such material in the factory is 

 the fact that it necessitates extra work for inspection and hand- 

 ling in order that it may be segregated and separately tested be- 

 fore it can he safely used. Heavy wet mould is not often met 

 with, but a light mould is quite common. 



Testing the efifect of a heavy, wet mould presents some diffi- 

 cuhies, as one cannot say what the quality or rate of cure of 

 the rubber would have been had it been properly cured and free 

 from mould, but in the case of a light mould the effect can be 

 judged by mixing and vulcanizing separately the sound and 

 mouldy portions of the sheets. 



Results are here given on one sample which has been tested on 

 the separated sound and mouldy portions. 



Clean Mouldy 



Time of cure (minutes) 117 132 



Kate of cure (per cent of standard) 128 114 



It will be seen that there is a distinct difference in rate of cure 

 between clean and mouldy parts of the sheet. The clean part 

 vulcanizes faster than the mouldy part. The above figures repre- 

 sent the mean of two independent vulcanizations wliich were in 

 close agreement and are based on the coefficient of vulcanization 

 determined by analysis. So far as can be judged, without further 

 details of the physical properties of the clean and mouldy parts, 

 there is little or no difference in quality between the two por- 

 tions. 



Except for the mouldy patches, the rubber was of normal ap- 

 pearance, medium shade, glossy surface, and no defects. The 

 mouldy portions were sharply defined so that the clean and mouldy 

 parts were easily separated. It was noticed that the mouldy 

 parts after washing off the mould were darker in color than the 

 remainder. A sheet wiped clean showed irregular dark patches 

 exactly as described by Hartjens.' The patches were irregular in 

 size and shape, just as are the patches of mould growth, and 

 darker in color than the remainder of the rubber, but not less 

 translucent. 



'"Mouldv ."^heet and the Eftect of Mould on Quality." By H. P. Stevens. 

 Bulletin cit the Kubher Growers' Associatitn, \*ol. .1, No. 2. February, 1921. 

 ^"Spots on Sheets, " Arcliief voor de Rubbercultuur, April, 1920. 



