430 



THE INDIA RUBBER WORLD 



March 1, 1921 



The study of the colloid character of caoutchouc has led 

 Hoehn and Ostwald to doubt the chemical nature of vulcaniza- 

 tion. . Removal of the uncombincd sulphur in vulcanized rubber 

 by prolonged acetone extraction shows that some of the sulphur 

 has been absorbed and not simply dissolved by the caoutchouc. 



Certainly absorption plays a considerable part during the 

 changes of the vulcanization. It is still accepted as true that 

 the sulphur is chemically united to the vulcanized material. The 

 absorption of sulphur does not cease when the first chemical 

 action has been completed. All of the caoutchouc in the vul- 

 canized material loses its original character and cannot be re- 

 garded merely as a mixture of vulcanized and unvulcanized 

 caoutchouc. It is conceivable that the part of the caoutchouc 

 solution which has combined with the sulphur has been absorbed 

 irreversibly by another caoutchouc part which would bring about 

 a change in the colloidal condition and possibly also a change in 

 the polymerization. Vulcanized caoutchouc can still be regarded 

 as an emulsoid in a form more permanent and stabilized than 

 raw caoutchouc. Vulcanized caoutchouc does not retain the 

 adhesive character of raw caoutchouc, which by milling on rollers 

 will form plastic sheets, increasing in softness and plasticity by 

 further mechanical treatment. This treatment changes the struc- 

 ture of the emulsoid. Regeneration of old rubber consists in 

 restoring vulcanized caoutchouc to a condition such that its char- 

 acteristic colloids will be modified and assume the viscous con- 

 dition of raw caoutchouc. 



CAOUTCHOUC DIALYSIS AND COMPOSITION 



A caoutchouc membrane can be used for the purpose of making 

 dialyses of raw caoutchouc solutions. If a membrane is made 

 from a slightly vulcanized caoutchouc disk and on one side of 

 it is placed a sulphur-caoutchouc solution, and the pure solvent 

 on the other, the sulphur will pass through the membrane. By 

 renewing the solvent all sulphur can be extracted from the 

 caoutchouc solution. The old vulcanization method of Hancock 

 is based on the diffusion of sulphur in caoutchouc. 



Notwithstanding general scientific advancement, specific knowl- 

 edge of colloids is still very limited and the nature of caoutchouc 

 and other natural colloids is still not completely solved. The 

 colloid characteristic of coautchouc is a considerable obstacle 

 in the production of synthetic caoutchouc. The present system 

 of coagulation followed upon the plantations facilitates the en- 

 zymic actions. During vulcanization no special transformation 

 can be traced ; the only sign of the formation of a new product 

 is given by the chemical combination of the caoutchouc with 

 sulphur according to the formula C,oH„S;. 



Graphical representation of the caoutchouc molecule, vulcanized 

 or unTulcanized, is possible by a cyclic formula. The sulphur 

 certainly is bound to the caoutchouc, at least in part. Vulcaniza- 

 tion is a weak exothermic action. The exothermal heat of the 

 vulcanized caoutchouc is somewhat less than that of all its com- 

 ponents. It is generally admitted that the speed of diffusion of 

 a gas through caoutchouc depends upon the solubility of the gas 

 in caoutchouc and the specific speed of the gas has little in- 

 fluence upon the result. Caoutchouc cannot be regenerated after 

 vulcanization by mere dialysis. The combined sulphur does not 

 dialyzc, neither can a solution of the vulcanized material be 

 effected without the employment of very energetic means, whicli 

 entirely changes the nature of the caoutchouc. 



THE INTERPRETATION OF RUBBER ANALYSESi 



In the analysis of vulcanized rubber the principal tests applied 

 at present are: acetone extract, chloroform extract, alcoholic 

 potash extract, ash, total sulphur, free sulphur, mineral and 

 organic fillers, and special tests for the detection and determina- 

 tion of such substances as parafline, oils, glue, etc. 



» "The Interpretation of Rubber Analyses," by John B. Tuttle. The 

 Chemical Bulletin, Volume VII, No. 12, December, 1920, page 323. 



ACETONE EXTEACT 



The acetone extract may contain resinous matter from the 

 crude rubber, free sulphur, oils and waxes. The rubber resins 

 average between two and four per cent in high-grade rubber, and 

 the presence of more than these amounts suggests the use of low- 

 grade or reclaimed rubber, 



CHLOBOFOBM EXTRACT 



The chloroform extraction is made for the purpose of detecting 



bituminous substances such as the so-called mineral rubbers. It 

 is only qualitative, for a large part of these substances are soluble 

 in acetone, and cannot be isolated. The brownish solution in 

 chloroform is not to be niisiaken, and even as little as one per 

 cent is readily detected. 



ALCOHOLIC POTASH EXTEACT 



.'Mcoholic potash (or soda) dissolves the so-called oil substitutes. 

 There is always a small amount of extract from the rubber, but it 

 should never exceed one per cent of the amount of rubber present. 

 More than this amount is a pretty fair indication of the use of oil 

 substitutes. 



FATTY OILS 



Fatty oils, such as palm oil and cottonseed oil, were probably 

 added for their softening effect, but mineral oils usually denote 

 the presence of reclaimed rubber. Parafline wax is desirable in 

 insulated wire in order to close up the minute pores in the rubber. 

 It is easily isolated and determined with great accuracy, while 

 from the qualitative point of view its presence is easily discernible 

 by the while flakes which separate, on cooling, from the acetone 

 solution. 



SULPHtTE 



Sulphur is not only added as such for the purpose of effecting 

 vulcanization, but is contained in reclaimed rubber, oil substitutes, 

 accelerators and fillers, and the total sulphur determination gives 

 only the sum of the sulphur content of these various substances. 



In special cases, such as high-grade insulated wire, and wher- 

 ever the sulphur might have a deleterious effect on materials with 

 which it may come in contact, the free sulphur should be kept as 

 low as is consistent with safe manufacturing practice, but beyond 

 this, it is in itself harmless. However, it has been observed that 

 low-grade crude rubbers and reclaimed rubber require a larger 

 excess of sulphur for vulcanization than do the best grades of 

 either wild or plantation rubber, and a high free sulphur content 

 suggests the use of these cheaper grades. 



ASH 



The ash is the mineral residue left on ignition. It should be 

 remembered that some fillers change composition on heating, 

 others react during vulcanization to form new compounds, while 

 some fillers are more or less volatile. Hence the value of this 

 determination is problematical and depends largely on the material 

 being tested. 



EUBBER 



Probably the most important determination is one which is sel- 

 dom made, and that is the percentage of rubber present. The 

 usual practice is to calculate this quantity by difference after 

 determining all other constituents. A vast amount of work has 

 been done to develop methods for the direct determination of 

 rubber, but only a few of these have any merit whatever, and 

 these are usually time-consuming, require special equipment or 

 have some other drawback which prevents their widespread adop- 

 tion. The percentage of rubber is important because experience 

 has shown that in order to obtain reasonable service a certain 

 minimum amount of rubber is necessary. Rubber being normally 

 one of the more expensive components of rubber articles, it is 

 only natural that manufacturers should seek to use no more than 

 that actually needed. 



CONCLUSION 



The interpretation of the chemical analyses of rubber articles 

 should be approached with a consideration of the methods used 

 in making the analyses, the probable error of these methods, the 



