SIS 



THE INDIA RUBBER WORLD 



August 1, 1921 



ness may be cured to a large extent by soaking tluiii in wati-r and 

 hanging them to dry. 



As far as is known, greasiness has no marked inriuoncc on the 

 inner properties. A direct deterioration of the rubber or real 

 harm to the inner properties therefore is not to be feared. As 

 this defect is so often coupled with a large tendency to mouldi- 

 ness, it is quite reasonable that greasy sheets arc not classed under 

 first quality and are not accepted without protest. 



BUETIKESS 



"Rust" is the term applied to a thin, invisible film on the sheets 

 which breaks and becomes visible when the sheet is stretched. On 

 unsmoked sheets it has a yellowish-white color, on smoked sheets 

 it becomes brown, or rust-colored. This film is formed by the 

 growth of micro-organisms in the wet layer on the outside of 

 freshly rolled sheets. These organisms decompose some of the 

 serum constituents — probably the sugars — and form a very volum- 

 inous, jeily-like substance which dries to an invisible, cohesive 

 film. 



The organisms are aerobic, that is to say, they grow only when 

 sufficient air is present. They prefer dilute solutions of scrum. 

 Soaking the sheets in water after milling does not hinder the 

 rubber from becoming rusty. On the contrary, such sheets may 

 show rustiness to a very large degree, if the organisms have time 

 to develop. 



It is worth w'hile to state clearly that rustiness is caused by a 

 decomposition of serum-substances by micro-organisms, while 

 greasiness arises when itndecoinposcd, hygroscopic serum-sub- 

 stances dry up on the sheet. Scaking the fresh sheets in water 

 may help to prevent greasiness but does not help against rusti- 

 ness. ■ 



Rustiness itself produces no changes in the inner properties of 

 the rubber. The thin layer of harmless substances is found only 

 on the outer surface, since air is necessary to the growth of the 

 organisms. The interior of the sheets is not affected, and after the 

 sheets are once dry, the changes cease, while rusty sheets, probably 

 in connection with the changes by which rustiness is caused, are 

 not hygroscopic. 



As rustiness occurs only when the sheets are left in moist con- 

 dition for some time after milling — 24 hours or more, a beginning 

 of maturation of course takes place, so that rustiness is usuallv 

 accompanied by a somewhat greater rate of cure and often by a 

 higher viscosity. O. de Vries and H. J. Hellendoorn, who investi- 

 gated rustiness^ state as follows : 



"On keeping rusty sheets for several years, no deterioration 

 fakes place and the properties change in the same way as in the 

 non-rusty controls," 



Rustiness therefore must be classed among the harmless defects 

 and should not form the base of any claim. 



GAS BUBBLES 



Small, eventually microscopic bubbles that are sometimes found 

 in sheet are mostly called air-bubbles. It is, however, more to the 

 point to call them gas-bubbles, since bubbles really caused by air 

 may be regarded as rather an exception. 



In most instances the bubbles in sheet are filled with gases 

 which are formed in the latex and the coagulum, for instance by 

 micro-organisms ; therefore, in the first place, carbonic acid ; 

 further, probably nitrogen and some methane. On the composi- 

 tion of these gases very little is known as yet. 



The bubbles themselves, as inclusions of innocent gases, pro1)ably 

 have no effect at all on the inner properties of the rubber. The 

 processes by which they are produced — decomposition of scriuri- 

 constituents, maturing, etc, — generally cause a somewhat quicker 

 cure and higher viscosity so that one may expect these also in 

 sheets with gas bubbles. 



'Archief voor de Rubbcrcultuur. 2, 1918, 527 and 536, 



THE VARIABII^ITY OF CRUDE RUBBER' 



The author of this paper mentions the early observations and 

 explanations of the variability in the rate of cure of plantation, 

 rubber and summarizes the work of Katon, who concluded that 

 there are two agencies present in plantation rubber which act as. 

 accelerators in vulcanization. These are : 



(1) The vulcanization accelerating agent formed by the 

 biological degradation of proteins or organic nitrogenous 

 matter in the coagulum during the early stages of drying. 



(21 A vulcanization accelerating agent, performed in the- 

 • latex and retained by the dry rubber under certain conditions 



of preparation. The second substance may i)i)ssibly be 



identical with the first, although there are cerlani indications 



that they are different. 

 The accelerator formed by the degradation of the proteins con- 

 sists probably of an amine or amino acid, probably the former, 

 since it is known that putrescine, which is a degradation product 

 of animal proteins, behaves like an accelerator. 



Eaton and his coworkers arrive at the conclusion that the- 

 variability in crude rubber is the variability in the amounts of 

 accelerators which may exist before coagulation or may be formed 

 later, and which by the processes of washing and drying are 

 permitted to remain in the crude rubber. 



EFFECT OF ADDED ACCELERATORS 



The vast bulk of plantation rubber today is used in mixings 

 in which cither organic or inorganic accelerators are present in 

 sufficient quantity to produce a fairly rapid cure. For this reason, 

 it seems as though the work which has been done has been for 

 the benefit of a very small amount of plantation rubber, and does 

 not apply to the balance. We may divide the substances found in 

 crude rubber, which may influence vulcanization, into two classes : 



( 1 ) The accelerators formed in the latex or in the 

 coagulated rubber. 



(2) Retarding a.^ents which have been added to the latex 

 or coagulum (such as any coagulating agent whicli has not 

 been removed by washing), or substances in the .smoke which 

 are absorbed by the rubber, etc. 



These two classes of substances will always react one against 

 the other, as Eaton has pointed out. The balance between the 

 two will determine the rate of cure. These substances are neces- 

 sarily present in very small quantities, and consequently variations, 

 w-hich in themselves are small, will in the absence of fillers and 

 added accelerators produce considerable effect on the rate of vul- 

 canization and the tensile properties. When accelerators are 

 used these differences are of little importance, becau.se the amount 

 of accelerator which is added to a compound is sufficient in itself 

 In vulcanize the compound correctly, and the presence of these 

 minute amounts of accelerators foimd by Eaton will have little, 

 if any, effect on the vulcanization and tensile, properties of such 

 compounds. Not only arc these differences small, but they are 

 not necessarily indicative of the true qualit_\- of the rubber. 



The author at various times has tested rubber which had dif- 

 ferent rates of cure when rubber and sulphur only were used, 

 and found that in many cases these differences largely disappeared 

 with the addition of two to four per cent of litharge, or 0.5 to 

 cine per cent of the common accelerators, such as aniline, hexa- 

 methylene tetramine, etc. 



The whole point in discussion is that it is nut sufficient to bring 

 together rubber and sulphur, and assume the jiresence and action 

 of an accelerator, merely because one method of preparation pro- 

 duces a somewhat more rapid cure than annther. Results show 

 that with many organic accelerators it is necessary tn have the 

 proper environment in order to develop the maximum, or even 

 any accelerating action. 



For testing the rate of cure the proper procedure would be to 

 add to each mixture a sufficient quantity of zinc oxide to be cer- 

 tain that the vulcanization will take place in an alkaline medium. 



* .\bstr.-ict of paper presented by Jchn B. Tuttle before the Rubber Divi- 

 sion at tbe 5Ktb meeting of the -American Chemical Scciety, Philadelphia,. 

 Pennsylvania, September 2-6, 1919. 



