October 1, 1919.1 



THE INDIA RUBBER WORLD 



23 



final length (length at the moment of rupture) corresponds 

 throughout with the breaking strain and tends to give a lower 

 figure, the higher the breaking strain. It may be said that (1) 

 and (3) are quite typical of a moderately under-vulcanized rub- 

 ber. All vulcanized specimens, even those vulcanized for 19 

 minutes only, swell but do not dissolve in cold benzene. 



These analytical results show clearly that, within the limit 

 of accuracy of the sulphur estimations (the rubber being in the 

 form of thin sheet), the whole of the so-called free sulphur is ex- 

 tracted in two weeks. A further seven weeks' extraction did not 

 reduce the amount of combined sulphur. To judge from the 

 figures for the rubber A, the extraction of free sulphur is com- 

 plete in one week (say SO hours), and for technical work a 5 — 10 

 hours' extraction is usually regarded as sufficient. 



These results are in total disagreement with those obtained by 

 Harries and Fonrobert. All three specimens of raw rubber 

 contained over 1 per cent of combined sulphur after 9 weeks' 

 extraction when cured for 30 minutes at 145 degrees C. (45 

 pounds steam pressure), whereas Harries and Fonrobert ob- 

 tained only 0.29 per cent under less favorable conditions for the 

 e.xtraction of the free sulphur. When vulcanized for only 19 

 minutes, the figures for combined sulphur are more than double 

 those obtained by Harries and Fonrobert. One can only con- 

 clude that these investigators were working with an excessively 

 slow vulcanizing and inferior quality of rubber or that, owing 

 to the thickness of the disks used and the short time of vul- 

 canization, or for some other reason, the rubber was only sur- 

 face vulcanized. It appears therefore that the technical effect 

 of vulcanization has not been obtained without an appreciable 

 amount of sulphur becoming insoluble in acetone, that is pre- 

 sumably combined with the caoutchouc hydrocarbon. 

 Fart U. 

 the action of solvents on vulcanized rubber. 



Although the process of vulcanization is of enormous technical 

 importance, it is not possible to formulate a definition of vulcani- 

 zation which will enable a sharp distinction to be drawn between 

 a vulcanized and an unvulcanized rubber. Technically, vulcaniza- 

 tion produces a physically improved product, showing greater 

 indifference to changes of temperature and greater tensile 

 strength and elasticity. Of the ordinary soft vulcanized goods, 

 2 — 4 per cent of the sulphur cannot be extracted with acetone — 

 the so-called combined sulphur. By "undervulcanizing," the 

 amount is smaller and the product is softer and physically 

 weaker, as already shown in Part I. If the vulcanization be 

 carried to its limits, hard resilient solid (vulcanite or hard rub- 

 ber) is produced, and up to 32 or 33 per cent of sulphur may 

 remain undissolved after prolonged acetone extraction. 



Solvents such as benzene and carbon bisulphide are frequently 

 employed to differentiate between vulcanized and unvulcanized 

 rubber (compare Harries, Berlin, 1916. 49, 1196), on the assump- 

 tion that vulcanized rubber is insoluble and unvulcanized rub- 

 ber soluble, but tests with different solvents under different 

 conditions showed that the "solubility'' of vulcanized rubber in 

 organic solvents is dependent on (1) the nature of the solvent, 

 (2) the time of immersion in the solvent, (3) the temperature, 

 (4) the degree of vulcanization, and (5) the age of the vul- 

 canized specimen. I have already remarked on the progressive 

 solubility in benzene of rubber "vulcanized" with benzoyl per- 

 oxide ("Journal of the Society of Chemical Industry," 1917, 

 109), the rubber tending to insolubility in benzene with intensifi- 

 cation of the vulcanizing conditions as, for instance, by in- 

 creasing the proportion of benzoyl peroxide. Vulcanization 

 with sulphur and with benzoyl peroxide is therefore analogous 

 as regards the physical properties and solubility of the vulcani- 

 zate. 



Raw Rubber. It is generally assumed that raw rubber is 

 soluble in solvents such as benzene and carbon bisulphide, but 

 in many cases the solubility is only partial after months of 



immersion in the solvent. It is necessary to distinguish between: 



(a) Rubber obtained by evaporation or coagulation without 

 mechanical working— at the most, a pressing of the clot to 

 expel part of the mother liquor and facilitate drying. To this 

 class belong the so-called plantation sheet rubber, "fine Para," 

 and most wild sorts. 



(b) Rubber which has been worked (creped) after coagulation 

 in the moist state or by subsequent working (mastication) of 

 the dry rubber of the type described under (a). The working, 

 whether creping or mastication, is accomplished by passing the 

 rubber through differentially geared rollers. 



Rubber in category (a), w^hen immersed in a solvent, swells 

 considerably unless of low quality, i.e. degraded* rubber which 

 is soft, adhesive, or even semi-fluid, owing to unsatisfactory 

 treatment in preparation or preservation. Taking the case of 

 plantation sheet or dry Para rubber, the rubber swells enormous- 

 ly and gradually passes into solution, the mass of rubber 

 retaining its swollen skeleton form. After a long period and 

 treatment with fresh solvent, the skeleton may collapse, leaving 

 a slimy deposit rich in nitrogen. It is probable that the reten- 

 tion in shape and only gradual dissolution is caused by a network 

 of protein films formed when the rubber is coagulated. This 

 is particularly the case with rubber coagulated with tannin or 

 products containing tannin which act on and toughen the protein 

 films. The reticulated structure of these films is microscopically 

 visible if thin pieces of the rubber are swollen in benzene with 

 due care. 



Rubber in category (b) readily dissolves in benzene; the 

 amount of swelling depends on a variety of circumstances, 

 including the degree of working to which it has been subjected. 



Caspari ("Journal of the Society of Chemical Industry," 1913, 

 1041), separated samples of rubber into a soluble and a "pectous" 

 variety by prolonged extraction with petrol ether in the cold. I 

 have repeated Caspari's experiments but was unable to obtain 

 concordant results in repeat extractions. The proportion of 

 soluble to "pectous" appeared to depend on the period of 

 extraction. Moreover, I found that the "pectous fraction," if 

 allowed to stand sufficiently long in cold petroleum spirit, dis- 

 solved wholly with the exception of a small quantity of slimy 

 nitrogenous matter which settled to the bottom of the containing 

 vessel, so that the behavior of petroleum spirit as a solvent 

 differed from that of benzene in degree and not in kind. Petrol- 

 eum spirit is merely a less effective solvent than benzene. 



Raw and vulcanized rubber exhibit varying behavior when 

 immersed in a solvent. This should be noted as indicating the 

 degree of "solubility."' The most readily soluble rubber dis- 

 solves with little or no swelling, almost like a crystalloid. A 

 less easily soluble rubber swells before dissolving. As the 

 volume of the gel increases, more difficulty is experienced in 

 producing a permanent emulsion with excess of solvent. Vigor- 

 ous shaking is necessary and eventually one arrives at a .stage 

 when gelatinous flakes remain "undissolved." This marks the 

 limit of "solubility." Beyond this stage, the more fully the 

 rubber is vulcanized (i.e., the higher the percentage of combined 

 sulphur), the less the rubber swells in the solvent. 



VuLC.'VNizED Rubber. No investigations have been published 

 dealing with the solunility of vulcanized rubber in solvents such 

 as benzene and carbon bisulphide. The general impression is 

 that vulcanized rubber is insoluble in contrast to the solubility 

 of raw rubber. In connection with the previous paper it was 

 important to ascertain the lower limit of combined sulphur 

 which would confer the property of insolubility in organic solv- 

 ents. Preliminary experiments showed that the quantity of 

 combined sulphur required to confer insolubility as above de- 

 fined was very small, and it was therefore found impracticable 

 to vulcanize at the previous temperature of 145 degrees C. as the 



* The term "depolymerized" is ii.'sually cw 

 a molecular change for which proof is w.nnl 

 term "degraded." 



