StPTEMBEH 1. 1910. 



THE INDIA RUBBER WORLD 



657 



curing crepe, compared with the low nitrogen content of fast- 

 caring crepe from a slab rubber, can only be explained on the 

 theory that, in the slab rubber, decomposition of the protein 

 or nitrogenous substance takes place. A soluble portion is 

 washed out during crepeing, and the insoluble residue, or part 

 of it, is presumably the substance causing acceleration in rate of 

 cure in the case of slab rubbers. 



In view of the vulcanizing results obtained in Parts I and 

 II, the authors have analyzed a large number of their samples 

 which are tabulated below, together with the rate of cure de- 

 termined by the load-stretch curve method. 

 Per Cent Opti 



Nitrogen 

 in Dry 

 Sample. 



0.31 ... 



0.26 ... 



0.19 ... 



0.16 ... 



0.17 ... 



0.17 ... 



0.18 ... 



0.18 ... 



0.33 ... 



0.30 ... 



0.16 ... 



0.11 ... 



0.13 ... 



0.13 ... 



In every case, the amount of nitrogen in a slow-curing rubber, 

 is about SO to 100 per cent greater than the amount of nitrogen 

 contained in a fast-curing slab rubber, the amount of nitrogen 

 being determined on all samples after conversion to crepe and 

 drying. 



On the other hand, the amount of nitrogen in samples of 

 rubber prepared by the evaporation of thin layers of latex, or by 

 pouring out the latex into thin layers, after addition of acid 

 co^^lant, and allowing the thin sheets thus obtained to dry 

 rapidly, is high, and amounts in some cases to 0.5 per cent. Such 

 samples are rapid curing, although the percentage of nitrogen in- 

 dtcates that no decomposition of the protein or nitrogenous con- 

 stituents of the rubber has taken place, the factor deciding 

 rapidity of cure 'being apparently, in this case, the unknown sub- 

 stance present in the evaporated serum after removal of the major 

 portion of the protein. 



SUMM.AKY. 



1. The experiments and results of Part I show that one factor 

 which causes variability in respect of rate of cure in plantation 

 Para rubber is produced during the first six days after coagula- 

 tion and that the change which takes place in the coagulum 

 is progressive during this period, while after this period, no 

 further change, under ordinary conditions, takes place. 



2. The action of antiseptics, such as formalin, as well as heat 

 and cold, are also shown to inhibit this change, while soaking 

 of the fresh coagulum in running water considerably retards 

 the rate of cure. 



3. The action of formalin is also shown to be partly, tliough 

 not to any great extent, an action on the accelerating agent after 

 its formation. 



4. Experiments on the cold storage of freshly coagulated rub- 

 ber show that while the change which produces rapidity of cure 

 is inhibited as long as the coagulum remains in cold storage, if 

 the rubber is removed again and allowed to remain, without 

 machining, for a further period (13 days or possibly less) at 

 ordinary atmospheric temperatures (about 85 degrees F. in the 

 Federated Malay States), rapidity of cure is again brought aljont. 



5. All the experiments of Part I suggest that the change which 

 produces rapidity of cure in the rubber is caused by biological 

 agencies, that is to say, micro-organisms entering the latex after 

 collection and remaining in the coagulum, and that the change 

 is probably a decomposition of the protein or nitrogenous sub- 



stances firesent in tlie coagulum, producing an accelerating agent 

 which is a decomposition product of the proteins. 



6. The experiments of Part II, on the slow-curing rubbers 

 of various proteins and nitrogenous substances, and their de- 

 composition products, including the proteins from the latex serum, 

 decomposed by suitable methods after separation from the serum, 

 confirm the conclusions from the experimental evidence con- 

 tained in Part I, and show that the original proteins have little 

 or no effect under the conditions employed, while the decom- 

 posed proteins have a marked eflfect. 



7. Experiments with undecomposed evaporated scrum, after 

 separation of the proteins coagulated by heat, suggest the pres- 

 ence of a second factor which accelerates the rate of cure, and 

 is due to some substance originally present in the latex. 



8. In the case of the authors' so-called "slab" rubbers, possibly 

 both factors are responsible for the acceleration of the rate of 

 cure, and it would appear that the second factor may be re- 

 sponsible for the actual superior tensile properties of the rubber. 

 Some evidence to this effect is contained in the comparatively 

 poor quality of the rubbers to which the protein decomposition 

 product has been added, in which the second factor has been re- 

 moved, and also in the good quality in the case of the evaporated 

 latex samples and the rubber to which the evaporated serum has 

 been added. Further experiments are, however, necessary to con- 

 firm this. 



9. Experiments with evaporated latex, which contains all the 

 serum constituents and is dried with sufficient rapidity to prevent 

 decomposition of the proteins, also confirm the presence of this 

 ,-econd factor. 



10. The nitrogen figures given in Part III still further con- 

 firm the decomposition theory, that is to say, the production of 

 some substance from the protein which accelerates the rate of 

 cure, the nitrogenous portion which becomes soluble in water 

 and is removed on crepeing being non-essential. The high nitro- 

 gen content, on the other hand, in rapidly curing rubbers pro- 

 duced by evaporation of the latex, without decomposition of the 

 protein, again confirms the evidence obtained as to a second fac- 

 tor which is probably of a non-nitrogenous nature. 



11. These experiments and results also show why it has not 

 been possible hitherto to connect the nitrogen content of a rub- 

 ber with its rate of cure, since a rapidly curing rubber may have 

 either a low or high nitrogen content, and indicate how previous 

 workers have gone astray, or not gone sufficiently far in their 

 investigations, in connection with the protein or nitrogenous 

 constituents of latex and rubber. 



12. Many other experiments on nearly 1,000 samples of rubber 

 all confirm the above results and conclusions. 



13. A further investigation is now being made as to the exact 

 nature of the protein decomposition product, which accelerates 

 the rate of cure, and as to the nature of the second factor re- 

 s[ionsible for acceleration, together with the numerous subsidi- 

 ary factors which influence the rate of cure, a number of wliich 

 have already been investigated. 



The authors have since found that the protein left in sheet of 

 average thickness can be decomposed and so produce a more 

 rapidly curing sheet rubber by simply rolling up the sheets after 

 machining in order to retain sufficient moisture content for the 

 bacterial decomposition. This demonstrates that the rapidity of 

 cure of the so-called slab rubber is largely due to the decompo- 

 sition of protein nominally retained by the rubber, even after 

 rolling to sheet form. These results also show the importance ot 

 the rate of drying during early stages, as a factor in the prepar- 

 ation of sheet rubber, in order to have a uniform rate of cure. 



It is reported from France that the incendiary li^zenges and 

 asphyxiating gases the German army has been using so 

 freely are due to the work of Dr. Wolfgang Ostwald, the well- 

 known Herman chemist and editor of the "Kolloid-Zeitschrift." 



