THE INDIA RUBBER WORLD 



[October 1, 1919. 



time (It heating was too short and the temperature of the steam 

 digester could not be raised sufficiently rapidly to enable accurate 

 measurements to be made of the time of heating. Therefore a 

 lower temperature was chosen. Specimens consisting of 90 

 parts of raw rubber and 10 parts of sulphur were vulcanized 

 for 30. 40, 50, and 60 minutes at 125 degrees C, at which 

 temperature the rate of vulcanization was reduced to about one- 

 seventh of that at 145 degrees C. 



The following figures were obtained after exhaustive extrac- 

 tion of the vulcanized specimens with acetone at the boling 

 point: — 



Minutes Vulcan- 

 ized at 125 Sulphur 

 Degrees C. Per Cent. 



(1)30 0.27 



(2)40 0.39 



(3) 50 0.45 



(4) 60 0.54 



These figures show that the percentage of combined sulphur 



is approximately proportional to the time of heating, as in the 

 case of more fully vulcanized rubber. 



.\fter standing 24 hours, small pieces of the vulcanizates were 

 allowed to swell in benzene overnight. On shaking, (1) dis- 

 solved readily, giving an apparently homogeneous solution; (2) 

 appeared to be only partly dissolved and the solution was ropy ; 

 (3) gave a swollen mass which was broken up on shaking, 

 gelatinous lumps remaining visible ; (4) was unaffected by shak- 

 ing, the swollen mass remaining unbroken. We have therefore 

 the transition states between solubility and insolubility of the 

 vulcanizate in benzene, and according to Harries, (1) might be 

 taken as the unvulcanized or metastable form, while (4) cer- 

 tainly represents the stable or vulcanized form. On this basis 

 about ^ per cent, of combined sulphur is sufficient to confer 

 the property of insolubility in benzene. 



(To Be Continued) 



SWELLING OF RUBBER IN SOLVENTS.i 



For rubber, the rate of swelling in a solvent depends on the 

 nature of the liquid used ; the origin and purity of the rubber ; 

 the coefficient of vulcanization, and the temperature. The rate 

 and extent of swelling are believed to provide a more rapid and 

 reliable indication of the "nerve" of rubber that is given by 

 viscosity measurements. Most of the materials present in 

 technical raw rubbers, such as resins, do not interfere with 

 the "turgescence" curve, but the natural proteins retard arrival 

 at the critical point at which the rubber loses its tenacity and 

 resistance to stretching, and it is claimed that their quantity 

 can be estimated from this effect. 



With vulcanized rubber, the greater the proportion of mineral 

 fillers and of factice the more rapidly this "critical point of 

 turgescence" is attained. Decrease in the proportion of rubber 

 and increase in that of sulphur diminishes the rate of swelling. 

 The rate of swelling may be measured gravimetrically or voiu- 

 metically. The latter being more convenient. A modification 

 of Justin-Mueller's apparatus for the examination of cotton 

 during mercerization "Journal of the Society of Chemical In- 

 dustry," 1914. page 1201) is recommended for this purpose. 



The order of various solvents in their effect on vulcanized 

 rubber has been found experimentally to be tetrachloroethane, 

 carbon disulphide, carbon tetrachloride, petroleum spirit (boil- 

 ing at 158-212 degrees F.), and benzene. The rate of swelling 

 in the boiling solvents or their vapors is much greater, but the 

 results for the various solvents are less comparable, due to 

 the diflferences in temperature. The advantage of tetrachloro- 

 ethane over carbon disulphide at the same temperature is only 

 slight by volume, but much greater by weight. The relative 

 positions of petroleum spirits and benzene are reversed if the 

 swelling is measured by increase in weight. 



The values of the swelling constant, x, calculated by Kirchoff's 



formula Q = KS.r, where S is one hundred times the specific 

 gravity of the solvent, K tlie volume before swelling, and Q the 

 maximum volume, were found to be as follows : 



Solvent. Swelling Constant. 



Tetrachloroethane 2.107 



Carhon tetrachloride 1.872 



Carbon disulphide 1.747 



Henzene 1.672 



Petroleum spirit 1.245 



Heptane 1.727 



Tetrachloroethylene 2.072 



Pcntachluroethanc 1.987 



PREPARATION OF RAW RUBBER. 



In reviewing recent investigations on the production of raw 

 rubber, E. de Wildeman finds that fine hard Para is generally 

 not superior to plantation rubber. In the preparation of planta- 

 tion rubber it is advisable to avoid excessive dilution of the 

 latex; use the least quantity of coagulant; use bisulphite, and 

 smoke the rubber at not exceeding 55 degrees C. (133 degrees 

 F.). Smoking should begin one day after milling the rubber 

 and continue for two weeks after the rubber is dry. Sheet 

 rubber is superior to crepe, and the latter is better thick than 

 thin. It IS recommended that as few forms of rubber as possible 

 be made and preference is given to smoked sheet. {"Le 

 Caoutchouc ct la Gutta-Pcrcha," volume 16, 1919, pages 9826-29.) 



COAGULATION OF HEVEA LATEX.- 



Reviewing the researcnes on latex coagulation, G. Vernet 

 shows that the results obtained with latex preserved by the 

 addition of ammonia may be very misleading, and that it is 

 necessary to use fresh latex. This may account for some 

 of the results by the advocates of the enzyme theory of coagula- 

 tion. 



Regarding the various theories advanced by others, the 

 author concludes that the function of the protein constituents 

 of the latex is of first importance in coagulation. All the exper- 

 iments made in this connection can be explained by the reac- 

 tions of these proteins. Coagulation results from an insoluble 

 condition of the proteins. Drying and centrifugal separation 

 are not alone able to produce coagulation, but may assist sepa- 

 ration in presence of ordinary coagulants. Without the use of 

 coagulants these processes simply increase concentration of the 

 latex and gradual coalescence of the rubber globules, leaving 

 the proteins entirely reiriovable by washing. While the possible 

 activity of enzymes during spontaneous coagulation of latex 

 cannot be denied, it is significant that their presence has not 

 been directly demonstrated and that coagulation can be ex- 

 plained satisfactorily without assuming their existence. 



Latex below 39.2 degrees F. can be preserved perfectly for 

 more than a month, and by tapping with careful exclusion of 

 micro-organisms and collecting in sterilized glass tubes it is 

 possible to obtain samples which remain liquid a month or 

 more. 



Spontaneous coagulation can be satisfactorily obtained with 

 ordinary latex by adding one to ten grams of sugar per liter, 

 excluding air during coagulation and maintaining the tempera- 

 ture at 86 to 113 degrees F. Serum from a previous coagula- 

 tion or a selected growth of micro-organisms may be employed 

 as a further aid. Latex so treated will coagulate with perfect 

 evenness if diluted with several times its bulk of water. 



CHEMICAL PATENTS. 



THE UNITED STATES. 



PRESERVATIVE COMPOSITION for treating rubber fabric, compris- 

 ing a mixture of tar, pitch, resin, rubber cement, fish glue, 

 glycerine, and turpentine. (Richard E. Thierfelder and John 

 Schmaelzle, Jr., Milwaukee, Wisconsin. United States patent No. 

 1,312,007.) 



Process for Vulcanizing Rubber, which comprises mixing an 

 organic vulcanizing agent and red lead with rubber and vulcan- 



"Le Caoutchou 



la Cutta-Percha," 1919, pages 9835-9844. 



