March 1, 1916.] 



THE INDIA RUBBER WORLD 



What the Rubber Chemists Are Doing. 



VULCANIZATION EXPERIMENTS. 



DURING the past year B. J. Eaton and J. Grantham, of the 

 department of agriculture of the Federated Malay States, 

 have continued their experiments in vulcanization on the 

 extent of variabilitj- caused by lack of uniformity in factory 

 methods, which are reported in the "Agricultural Bulletin.' 



EFFECT OF CREPING. 



Samples of rubber were prepared, vv^hich had been creped as 

 follows: A, 5 times; B, 10 times; C, 15 times; D, 20 times; E, 

 25 times. These were mixed and cured under uniform condi- 

 tions, time of cure in all the experiments being the only variable. 

 The resulting physical tests gave no indication of any differences 

 between the rubbers due to the amount of creping they had 

 undergone. 



Although the differences in creping in this experiment were 

 insufficient to produce any effect, it was thought a more severe 

 treatment of the rubber would produce deterioration. A sam- 

 ple of crepe treated in the normal matter, and one severely 

 macerated on the creping machine, when compared gave physical 

 test figures which revealed no effect as regards the mechanical 

 results. The effect of very severe maceration was merely to 

 slightly retard the rate of its cure. 



It was therefore concluded that the effect of creping on rubber, 

 both as regards optimum mechanical properties and rate of 

 cure, is negligible unless excessive. The slight effect in retard- 

 ing the rate of cure in excessively worked rubbers is probably 

 due to removal of traces of a catalytic substance or something 

 necessary to its formation ; in other words, to over-washing 

 rather than over-working. 



EFFECT OF EXCESS OF .\CID. 



Two samples were prepared from bulked latex to ascertain the 

 effect of adding an excess of acid when coagulating the latex. 

 Sample A was coagulated by 3 ounces of 5 per cent acetic acid 

 per gallon of latex, and Sample B 15 ounces of 5 per cent acetic 

 acid per gallon of latex. The subsequent manipulation was kept 

 as constant as possible. 



The results showed that the use of an excess of acetic acid 

 for coagulation of the latex has a well marked effect in re- 

 tarding the rate of cure. Probably the acid has the effect of re- 

 ducing the amount of the catalytic substance finally present in 

 the dry rubber. The mechanical properties are not affected. 

 EFFECT OF SODIUM BISULPHITE. 



No effect was produced by sodium bisulphite on crepe rubbers. 



There is reason to believe that the use of sodium bisulphite 



in the preparation of sheet rubliers, and other forms in which 



the bisulphite is not afterwards completely removed, is injurious. 



PREPARATION OF THIN CREPE. 



Samples were prepared on an estate normally turning out 

 thin, pale crepe, and each represented a stage in the ordinary 

 process adopted by the estate. The samples were : 



.A, The coagulated slab allowed to drain. 



B. The coagulum rolled once. 



C. Rough crepe. 



D. Thick crepe. 



E. Thin crepe. 



The samples were kept for 20 days, and then .V, B and C 

 were made into thin crepe and dried. IJ and E were dried 

 without subsequent creping. .Ml the samples contained sodium 

 bisulphite. The tests showed time of satisfactory cure by load- 

 stretch curves as follows: A, lyi hours; B, 2 hours; C, 2!/2 

 hours; D, 2^ hours, and E, 3 hours. 



UNIFORMITY. 



.\n experimental study was made to ascertain how far uni- 

 formity is possible under ordinary estate conditions. The latex 

 was bulked in a large tank and coagulated by the addition of 

 3 ounces of 5 per cent acetic acid per gallon of latex. The 

 rubber was creped the same day, an hour or two after coagula- 

 tion. It was found necessary at this time to add a small quantity 

 of formalin to the latex in order to prevent spotting. The re- 

 sulting thm crepe was of a golden color, and brought top prices. 



The experiments on thin crepe and uniformity of preparation 

 are thus summarized : The effect of each stage in the preparation 

 of a thin crepe is to retard the rate of cure. It has previously 

 been shown that passing a thin crepe a number of times through 

 a creping machine, beyond that necessary first to produce thin 

 crepe, had little or no effect. When the retardation of the rate 

 of vulcanization reaches a limit, as in the preparation of thin 

 crepe machined on the day of coagulation, a very uniform rubber 

 is obtained, although the rate of cure is very slow and the 

 mechanical properties suffer slightly. 



PREPARATION OF BLOCK RUBBER. 



The process of preparing block rubber is to thick crepe the 

 coagulum on one machine and thin crepe it on another, and then 

 "worm" it by passage through a perforated plate by pressure. 

 The wormed rubber is then placed in hot air driers for 7 to 8 

 hours at about 130 degs. F. until cpmpletely dry. It is then 

 blocked while still warm in a hydraulic press. 



Twelve samples were taken representing the stages from 

 coagulum to the final block, mixed, vulcanized and tested in two 

 sets, the samples of corresponding stages in each set varying 

 as regards age and the use of sodium bisulphite. 



These experiments indicated a convenient method of producing 

 a rapidly curing rubber with good physical properties, namely, 

 the slab from the coagulum, with or without sodium bisulphite, 

 should be left ten days to "mature" or develop a rapid rate of 

 vulcanization, previous to being converted into block form. 

 The uniformity of the rate of cure thus produced remains to be 

 tested. 



MIXING RAW RUBBERS. 



The effect of mixing raw rubbers having different rates of 

 cure was demonstrated, and showed a gradation in rate of cure 

 according to the proportion of fast or slow curing rubber in 

 the mixture. These results are of importance as confirming 

 the probable reason for the uniformity of fine hard Para, 

 namely, that it is due to averaging of latex collected and cured 

 on different days through long intervals. 



A method of averaging could be adopted on any estate to ob- 

 tain similar results, and would be of marked advantage, but the 

 problem of sampling the mixed rubber for testing would prob- 

 ably be complex. 



VELOCITY OF SOLUTION OF LIQUIDS IN RUBBER. 



From available data relating to the velocity of the absorption 

 of various liquids by rubber, P. Bary concludes that for any given 

 liquid, the weight, p, of the liquid contained in a unit weight of 

 the rubber («. e., rubber plus absorbed liquid) after absorption 

 has proceeded for t minutes is given by the formula, 

 tp. 



p = 



(A + 

 in which p, is the value of p corresponding with the saturation 

 of the rubber with the liquid, and A is a constant depending upon 

 the nature of the liquid and the conditions of experiment, particu- 

 larlv the thickness of the rubber sheet. 



