Aprjl 1, 1919.] 



THE INDIA RUBBER WORLD 



363 



plain the curved course of the binding of sulphur the supposition 

 could be made that the accelerating action diminishes on con- 

 tinued vulcanization, but it seems more plausible that the catalytic 

 acceleration surpasses by far the auto-catalytic one (here lYi per 

 cent of sulphur), the latter, therefore, acting only very slightly 

 on the process. 



In the remainder of his paper the author elaborates two work- 

 ing hypotheses on the vulcanization process. His suppositions 

 are as follows : 



FWsl. That mentioned originally by Bernstein" as a possi- 

 bility. According to that conception, the colloidal sulphur 

 formed on heating the sulphur solution, would unite to one com- 

 plex with the considerably depolymerized rubber during the 

 vulcanization. This complex would be insoluble in the ordinary 

 solvents for rubber. 



Second That an "active" sulphur modification exists and that 

 it combines chemically with a part of the rubber to a compound 

 insoluble in the usual solvents, and that the latter afterwards 

 combines "colloidally" with the remainder of the rubber to an 

 insoluble complex. 



For the author's development of these hypotheses the reader 

 must he referred to the original paper. 



SOME PROBLEMS OF THE PLANTATION RUBBER INDUSTRY. 



The relative advantages of the various methods for coagulat- 

 ing rubber latex employed in the plantation rubber industry are 

 discussed by H. P. Stevens under the above title in the "Journal 

 of the Society of Chemical Industry/' June 29, 1918, page 237r. 



The coagulation of latex was studied in Ceylon by Parkin 

 who recommended the use of acetic acid. His choice of an 

 organic acid in preference to a mineral acid was fortunate, but 

 he considerably overestimated the proportions of acid necessary 

 for coagulation. At this time some estates, for fear of using 

 too much acid, were doing without acid altogether and relying 

 on the so-called natural coagulation. This method of "natural" 

 or spontaneous coagulation yields a satisfactory product, but at 

 that time it was neither economical nor convenient. More re- 

 cently, the method of spontaneous coagulation has been revised 

 and modified by allowing the coagulation to be carried out under 

 anaerobic conditions. It is known that when latex is coagulated 

 spontaneously and exposed to the air the interior of the mass 

 acquires an acid reaction brought about by lactic acid fermenta- 

 tion, while the surface develops a yellowish alkaline slime in 

 which coagulation is very imperfect. 



According to the most recent work, carbon dioxide gas given 

 nfi from the mass is retained on the surface in the anaerobic 

 process, and this prevents the purtrefactive changes which give 

 'ise to the alkaline slime. The evidence at present appears to be 

 ill favor of an enzyme action as the primary cause of coagula- 

 tion, although sufficient lactic acid soon develops to bring about 

 coagulation in the ordinary manner. If the anaerobic process 

 can be relied on to produce regular and complete coagulation 

 within twenty-four hours, the process has a future and should 

 eventually displace coagulation by acetic acid, at any vate for 

 bulk coagulation in the manufacture of crepe rubber. 



The -author's original studies showed that rapid and complete 

 coagulation could be obtained with a very small proportion of 

 acetic acid — about one part per thousand of diluted latex. The 

 coagulation was clear and the resulting rubber pale and more 

 even in appearance than that produced by spontaneous coagula- 

 tion. With the same end in view, small quantities such as one to 

 two parts per 1.000 of sodium bisulphite are commonly added to 

 the latex which inhibits the action of the oxydase present. This 

 would otherwise produce a darkening of the surface of the coag- 

 ulum, and eventually dark streaks or patches in the finished rub- 

 ber. The bisulphite is without appreciable efifect on the quality of 

 the rubber, but it is questionable whether a pale, even-colored 



~i»''Kolloid-Zeitschrift," I9I3, page 273. 



rubber is the best that can be produced, whatever method be 

 adopted to prevent the darkening. This pale rubber cures more 

 slowly than a darker and less attractive-looking material which 

 has been prepared by setting the coagulum aside for a few days 

 to putrefy before making and creping. The putrefactive bases 

 so formed accelerate vulcanization. 



It is often stated that rapid-curing rubber is of better quality 

 than the pale-colored product. To produce the technical effect 

 of vulcanization it is necessary to heat the mixture of rubber 

 and compounding ingredients, and it is argued that the heating 

 detracts from the beneficial effect of vulcanization. The argu- 

 ment is based on the known fact that heating rubber by itself to 

 vulcanizing temperature damages it. If this is so, the shorter the 

 period of heating the better the product, provided an equal de- 

 gree of vulcanization is produced. There is some evidence of a 

 general nature to support this view, but conclusive experiments 

 to this end have not been published. 



Similar considerations may be applied to organic accelerators 

 added to the rubber in the course of manufacture. One authority 

 goes so far as to state that a good accelerating agent must not 

 only facilitate vulcanization but should also toughen rubber and 

 render it immune from deterioration. 



Variation in rate of cure of Hevea rubbers must certainly be 

 ascribed to a variation in the nature and proportion of the non- 

 caoutchouc ingredients. The insoluble nitrogenous matter com- 

 monly referred to as the protein matter facilitates the combma- 

 tion of the rubber and sulphur, as is shown by the effect of its 

 removal from the rubber. On the other hand, the putrefactive 

 bases are far more active. It has been suggested that the fresh 

 latex contains a substance having an accelerating action, pre- 

 sumably in addition to the insoluble nitrogenous matter and the 

 putrefactive bases. It has not, however, been isolated, and 

 nothing is known of its composition. 



The ever-increasing supplies of plantation rubber have ren- 

 dered the collection and marketing of the inferior grades of 

 wild rubber unprofitable, but the supphes of fine Para have not 

 diminished, and this rubber has for years commanded a pre- 

 mium over the best plantation grades. This is the more remark- 

 able as the former requires to be first washed and dried, in which 

 process it loses about 20 per cent of its weight. Standard plan- 

 tation, on the other hand, can be used for the majority of pur- 

 poses without preliminary treatment. Those who have made 

 comparative vulcanizing tests agree that fine Para does not give 

 a stronger vulcanized product than plantation, nor does it vul- 

 canize very rapidly. It is claimed that fine Para is less variable 

 than plantation, but this is open to question if fine Para be 

 compared with regular consignments of any of the best marks 

 of plantation. It is also admitted that fine Para shows some 

 variation. 



The explanation for the preference given to fine Para is prob- 

 ably to be found in hesitation on the part of manufacturers to 

 employ plantation rubber for purposes where the cost of the 

 rubber is a relatively small item in comparison with the value of 

 the product, as, for example, in the case of a submarine cable, on 

 the lasting qualities of which no chances can be taken. 



AZO PROCESS ZINC OXIDES. 



Lead-free zinc oxide is essential in the production of clear 

 white rubber goods although the presence of small percentages 

 of lead oxide is not objectionable in zinc oxide for rubber work 

 where color is not important. 



An American company has perfected a new process which 

 accomplishes results previously considered impossible in the 

 manufacture of zinc oxide, P.y this process zinc oxide of ex- 

 ceptional purity and physical properties is uniformly obtained. 

 Manufacturing facilities have recently been extended to permit 

 these lead-free and low-Uaded oxides to be offered to the 

 rubber trade. 



