26 



THE INDIA RUBBER WORLD 



lOcTdllKR 1, 1918. 



What the Rubber Chemists Are Doln^. 



STANDARD PLANTATION RUBBER. 



A RESUME of tlic results of the investigation of Eaton and 

 Grantham on the variability of plantation rubber has been 

 published in the "Journal of the Society of Chemical In- 

 dustry," by M. Barrowcliflf, F. I. C, supplemented by a discussion 

 of the M. C. T. process of coagulation. Details of this process 

 will be found in The India Rubber World of April 1, 1918. The 

 principal features of this paper are quoted below. 



The reason for the obvious partiality shown by the manufac- 

 turer for Para is that plantation rubber, even of the highest 

 grades, varies in quality, the variations being considerably greater 

 tlian those found in different samples of fme hard Para. Much 

 experimental work, dealing with the causes of this variability 

 has been published, notably by B. J. Eaton and J. Grantham. It 

 was found that it is almost solely in respect of the time of heat- 

 ing required to ensure correct vulcanization, that is, to afford a 

 vulcanized rubber having the best mechanical properlies, that the 

 variations occur. Samples of plantation rubber, prepared by the 

 different methods in general use, were found to require for the 

 correct "cure" widely varying times of heating, at a constant tem- 

 perature. Further, rubber made by the same process, on differ- 

 ent estates, and at different times on the same estate, was found 

 to vary greatly in this respect. When, however, the proper time 

 of heating had been found and applied, relatively small differ- 

 ences were noted in the mechanical properties possessed liy the 

 vulcanized samples. 



The fundamental fact established by Eaton and Grantliam is 

 that the same factor for correct vulcanization varies inversely 

 according to the degree in which the rubber coagulum has been 

 allowed to remain, subject to coagulation, unworked, and in con- 

 tact with its serum. The longer this period the shorter the 

 length of heating required to produce a correct cure. 



Bearing this in mind, also the fact that the present custom 

 is to machine the rubber on the day following coagulation, the 

 influences exerted by the various factors introduced, in practice, 

 into the manufacture of rubber from latex can be accurately fore- 

 told. The greater the extent to which the coagulum is ma- 

 chined, and the more completely the serum is consequently 

 washed out, the slower the rate of "cure" of the rubber pro- 

 duced. This "crepe" rubber, as now marketed, is the slowest 

 to vulcanize. "Sheet" rubber, which receives a much less drastic 

 machining and retains a considerable quantity of serum that 

 drains out relatively slowly, cures in a much shorter time. "Slab," 

 which receives no mechanical treatment, and in which 12 to 15 

 per cent of the moisture is allowed permanently to remain, vul- 

 canizes still more quickly. 



In the' preparation of "smoked" rubber two variables are con- 

 cerned: (a) the action of the smoke products; (b) the tempera- 

 ture of drying. The former retards the rate of cure, while the 

 latter, within the limits attained in practice, accelerates it. Hence, 

 ■"smoked" sheet may cure either faster or slower than similar 

 smoked sheet, depending on the balance of these factors in the 

 earlier stages of drying. 



In the case of unsmoked rubber, the rate of drying similarly 

 affects the vulcanization time factor. The same plantation dry- 

 ing-room may produce on different days, according to the tem- 

 perature and atmospheric humidity, rubber having different rates 

 of "cure," even if the moisture content of the wet rubber hap- 

 pens on each occasion to be the same. 



The amount of serum left in "sheet" rubber depends on the 

 machining it has been given. Eaton found that the more con- 

 centrated the latex, the more rapid is the rate of cure of the 

 "sheet" afforded by it. The coagulum from the undiluted latex 

 being thicker and harder, more serum is retained in it after the 



■ roihng" process; the rate oi dr\ni«, cuii-c(iuently is slower than 

 in the case of the thinner sheet afforded by the diluted latex, af- 

 fecting in the way expected, the optimum time required for the 

 "cure." Similarly, "differential" rolls and rolls of equal size 

 and speed will produce sheet rubber requiring differing times of 

 "cure," other factors being constant. The effect of employing 

 acetic acid to coagulate the latex is to retard markedly the rate 

 of cure. Little difference is occasioned, however, between acid- 

 coagulated rubbers by variations in the amount of acid that has 

 been used, within the limits met with in practice. 



.■\s is obvious from the foregoing, to produce a uniform rubber 

 without some radical change in method would involve almost 

 insuperable difficulties. One type of rubber only could be per- 

 mitted. The way out of this difficulty is clearly indicated by 

 Iiaton and Grantham's further work. 



It was found that the change wliich takes place in the rubber 

 coagulum and causes the diminution in the time of heating re- 

 quired for vulcanization is completed in the course of a few days. 

 .Vlthough the maximum effect appears to be attained about the 

 -eventh day, there is little change in the rate of vulcanization 

 after three days, when the vulcanized rubber possesses the best 

 mechanical properties. The slower progress of the changes in 

 the coagulum, as compared with the slab is probably due to putre- 

 factive changes having occurred. To obtain the best results the 

 coagulum should be left unworked for three days. It is then im- 

 material to what extent the rubber is worked, in what form it is 

 linished, or how quickly or slowly it is dried, the time of "cure" 

 not being affected to any significant extent. 



If, however, coagulation is carried out in the usual way with 

 acetic acid in open vessels the rubber coagulum undergoes 

 changes if kept such a length of time, which render the method 

 unsuitable. Putrefactive action sets in, accompanied by darken- 

 ing of the coagulum. while spot diseases which have been shown 

 to cause an extraordinary retardation of "cure" are likely to de- 

 velop during the subsequent treatment. 



.\ new procedure is required, therefore, and seems to be fur- 

 nished by the method introduced by M. Maude and W. S. Crosse, 

 known as the M. C. T. process. It is based on the fact that in 

 closed vessels latex coagulates completely without the addition 

 of acid and without at the same time undergoing putrefactive 

 changes, .'^s now carried out in practice, large cement tanks 

 provided with heavy water-sealed covers are used. These are 

 filled with the bulked latex, leaving the smallest possible air 

 space. The covers are affixed and sealed and left undisturbed 

 until the following daj-. when the coagulation is found to be 

 complete. The resullinc: coagulum is converted into "crepe" in 

 the usual manner. 



Addition may be made to each 100 gallons of latex of a quan- 

 tity of a soluble calcium salt, namely, calcium acetate or sulphate 

 (gypsum), corresponding to four ounces of calcium. The effect 

 of this is to accelerate the coagulation. A beneficial effect on the 

 properties of the rubber may also be produced. In this connec- 

 tion the fact should be noted that whereas the soils of the 

 Amazon region are well supplied with lime, those of the Malay 

 peninsula are remarkably deficient in this respect. Further, the 

 Para method of coagulation favors the retention of calcium com- 

 pounds in the rubber, while acid coagulation tends to remove 

 them. These points may be not without bearing on the causes 

 of such differences as may exist between Para and plantation 

 rubber. 



Regarding his comprehensive series of tests of crepe, unsmoked 

 sheet, smoked sheet, and slab rubber made by the M. C. T. process, 

 B. J. Eaton states : "All the samples possess excellent tensile 

 properties, equal to the best samples of plantation sheet and 



