February 1, 1920.] 



THE INDIA RUBBER WORLD 



289 



Wliat the Rubber Chemists Are Doing. 



VARIABILITY IN PLANTATION RUBBER.' 



AT THE Central Rubber Station a number of samples of the 

 ordinary output of estates in the Netherland Indies are 

 regularly tested. The review of this work by O. de Vries 

 anil W. Spoon is outlined in abstract as follows. 

 TENSILE STRENGTH. 



The differences in tensile strength between smoked sheei 

 and crepe are inconsiderable. A real intrinsic difference in ten- 

 sile strength between sheet and crepe, tested in a strictly com- 

 parable manner, is hardly to be expected, as the difference' 

 in their preparation does not affect the tensile strength. Even 

 the greater degree of milling in the preparation of crepe in 

 ordinary estate practice does not affect the tensile strength, the 

 young rubber, still wet, regaining its strength completely, in 

 contrast with excessive plasticising of older, dry rubber which 

 often becomes heated in the process and loses tensile strength. 



There is a marked difference in tensile strength in rubbers 

 from different estates. As to the causes of the deficiency in 

 this respect, our knowledge is still incomplete. The already 

 known factors in preparation (excessive heat; tackiness, caused 

 by copper salts ; sunlight or lower organisms ; growth of molds 

 resulting in violet-colored spots in crepe ; use of some chemicals, 

 as sulphuric acid or alum in coagulation) cannot give an ex- 

 planation in such cases. Other known causes, such as latex 

 from young trees, or excessive tapping of trees that will have 

 to be thinned out, also do not always offer a sufficient explana- 

 tion. It may be possible that soil, climate, or hereditary factors 

 (strain of seed) play a role — factors which it would be very 

 difficult to eliminate, so that a uniform high tensile strength for 

 all estates cannot be reached at will. 



SLOPE. 



"Slope" is the stretch produced by a certain load at high 

 elongations (determined after the method of Schidrowitz). A 

 low figure for slope denotes a rubber with a great resistance to 

 stretching at high elongations. The slope shows a relation to 

 permanent set.' The average figures for slope of the samples 

 tested in 1917 and 1918 are: 



36.; 



Smoked sheet averages somewhat greater slope than crepe. 

 The variation in both grades is not large, nearly all samples 

 falling between 34 and 39, while tlie ordinary figures for crepe 

 are 35-37 and for smoked sheet, 36-38. Whether this difference 

 is of any practical importance in the comparison of first-grade 

 samples will have to be investigated. The large deviations in 

 slope are found in the lower grades, where the characteristic 

 becomes typical. 



BATE OF CTTRE. 



The authors" "standard time of cure" in testing is the time 

 necessary to bring the stress-strain curve to standard position 

 (length 900 per cent at 1.30 kilograms load). 



The rate of cure is the property that has drawn the greatest 

 amount of attention from most investigators, and certainly there 

 was reason for it. It should, however, be stated clearly that the 

 quality of the product in the strict sense of the word is ex- 

 pressed by tensile strength and slope which represent intrinsic 

 properties of the rubber, while rate of cure is a property of more 

 or less accidental nature. Faults in this respect cause trouble 

 to the manufacturers, but are not irreparable, while shortcomings 

 in tensile strength (in the sense of maximum tensile strength 

 as wc determine it) or slope cannot be remedied. 



W. Spoon. "Archicf ■■oor de Rubber CulUi 



'O. de Vries 

 1919. page 266. 



="Tournal of the Society of Clicmical Industi 



July. 



For 1917 and 1918 the average figures and the normal values 

 of smoked sheet and first-quality crepe have remained practi- 

 cally the same. The time of cure for smoked sheet is nearly 

 20 minutes shorter than that for crepe, while the variability is 

 1J4 to twice as great. The same difference in standard time of 

 cure (20 minutes) was found for smoked sheet and crepe pre- 

 pared from one lot of bulked latex, while for unsmoked sheet 

 the time of cure was 29 minutes shorter. 



The factors causing variability of line are: (1) composition 

 of latex; (2) chemicals added; (3) changes in the non-rubber 

 constituents ; (4) smoke constituents and decomposition products 

 formed by heating. The adoption of standard methods of rubber 

 production is most important. Bulking of the largest possible 

 lots of latex is one of the first needs to obtain a uniform 

 product. 



VISCOSITY. 



As our knowledge now stands, viscosity determination in gen- 

 eral cannot be used to judge of the properties after vulcaniza- 

 tion, but supplies very useful figures to give an indication as 

 to the cause of deviations. A fairly uniform rate of cure may 

 be accompanied by rather large oscillations in viscosity. The 

 explanation is that the estate prepares smoked sheet after 

 standard methods but uses a so-called Barker smoke-house in 

 which temperature may vary considerably, causing deviations 

 in viscosity. Uniformity in viscosity does not insure uniformity 

 in rate of cure and tensile strength, owing to factors causing 

 a greater rate of cure and higher viscosity by incipient 

 maturation. 



A change in viscosity that accompanies a deviation in rate 

 of cure may indicate whether the cause is to be looked for in 

 errors or irregularities in the preparation, or in changes in the 

 latex (condition of the trees) and may give very useful indica- 

 tions for controlling the methods of preparation on the estates 

 and the properties and uniformity of their output. 



EFFECT OF SOAKING COAGULUM IN WATER. 



Dr. 0. Dc Vries has reported in the Communications of the 

 Central Rubber Station^ the results of his investigation of the 

 effect produced on the properties of the rubber by soaking the 

 rolled or unrolled coagulum in water. 



(1.) The general effect of soaking the coagulum, rolled or 

 unrolled, in water is, in the first place, to cause an extraction of 

 scrum substances, a loss in weight in the dry rubber, and a 

 decrease of cure. When kept in water a longer time, maturation 

 sets in, which causes a further loss in weight, while the rate of 

 cure increases and the original retardation diminishes or changes 

 into an acceleration. The accelerators form in spite of the 

 fact that a considerable part of the serum substances has been 

 first removed by the water. 



2. ) The loss in weight by soaking amounts to 0.2 to 0.4 

 per cent for crepe ; J^ to 3 per cent for sheet, and 0.2 to 2 per 

 cent for unrolled coagulum, according to hardness of coagulum, 

 duration and intensity of extracting, etc. 



(3.) Crepe rubber that has so abundantly been treated with 

 water during rolling, shows practically no change when soaked 

 in water for a few hours after rolling. 



(4.) Soaking freshly rolled sheet rubber in water has a 

 more marked effect, which is considerable when the sheets are 

 kept in water from Yi to five hours. Extraction for i/^-hour 

 gave, on the average, an increase of 13.45 per cent in time of 

 cure, while for one to five hours gave average figures from 

 17;^ to 25 per cent. Keeping in water a night causes the rate 



"Archief 



de Rubber Cultuur," September, 1919, page 369. 



