February 1, 1917.] 



THE INDIA RUBBER WORLD 



265 



timum" cure should never greatly exceed the figures we have 

 given. It should be pointed out, however, that if the time of cure 

 required to produce the "optimum" result is extended, the chances 

 are an increase in the amount of the combined sulphur at the 

 "optimiun" point over the figures we have given will be found; 

 depolymerization, requiring an increase in cure to bring the rub- 

 ber up to apparent physical "optimum" leads to an increase in 

 the combined sulphur considerably over the amount which we 

 have given. The rubber in this case is nevertheless over-cured, 

 and where the vulcanization of the rubber is carried out scienti- 

 cally, in a minimum of time, and with the least possible injury 

 to the molecule, the combined sulphur at "optimum" cure will 

 never be found to exceed 3 per cent. 



Whether these figures obtain for rubbers of different botanical 

 origin or not we have not sufficient analytical evidence at present 

 to say. The constancy of this result is deduced from experi- 

 ments made on Hevea Brasiliensis rubber only. The relation 

 between the rubber and combined sulphur at correct cure is so 

 constant that it is regarded as representing a more or less defi- 

 nite compound of rubber and sulphur to which a formula may be 

 assigned on the assumption that partial valencies of the rubber 

 aggregate have not all the same aflinity for sulphur. 



It may be of interest to record the fact that we have 

 observed that the point at which the physical properties of 

 pure balata on vulcanization suddenly change to more nearly 

 resemble those of rubber, corresponds very closely with a 

 combined sulphur content of 3 per cent. If pure balata is mixed 

 with a little sulphur and a suitable catalyst, which is essential 

 to its proper vulcanization, it will be found that when about 3 per 

 cent of sulphur has combined with the balata, the physical prop- 

 erties of the vulcanized balata change from those of a hard in- 

 elastic product, more like hard rubber, to a pliant, semi-elastic 

 product, more nearly resembling soft vulcanized india rubber. 

 This phenomenon is exceedingly remarkable and interesting, as 

 the transition point in the physical characteristics of balata on 

 vulcanization occurs at about the same degree of chemical vul- 

 canization as corresponds to the "optimum" cure of vulcanized 

 india rubber. This has given rise to a number of experiments 

 by us, with a view to converting balata into rubber and vice versa, 

 some of which have led to exceedingly interesting results. 



METHODS OF ANALYSIS. 



EXAMINATION OF VULCANIZED RUBBER GOODS. 



' I 'HE following methods are standard for the examination of 

 ^ vulcanized rubber goods as specified by the Board of 

 Estimate and Apportionment of New York City. 



The methods for free sulphur, mineral fillers, vulcanized rub- 

 ber gum by weight, and vulcanized rubber gum by volume, have 

 already been given in this department (.^.ugust 1, 1915), and are 

 here omitted. 



PREPARATION OF SOFT RUBBER FOR ANALYSIS. 



A sample of not less than 25 grdins is made by taking pieces 

 from various parts of the original sample. The backing of fire' 

 hose is buffed off before grinding ; in all other hose separate 

 samples of tube and cover are made without removing the back- 

 ing or the friction compound. Other rubber goods containing 

 frictioned fabric and rubber layers are ground up without re- 

 moval of the adhering friction. 



The sample is cut into small pieces and run through the 

 grinder, taking for analysis only such material as will pass a 20- 

 tnesh sieve. The grinder should not become appreciably warm 

 during the grinding. If the material gums together and cannot 

 be sieved, it is simply passed twice through the grinder and all 

 the material taken for the final sample. Crude rubber is cut with 

 scissors. 



A strong magnet is passed through the sample to remove any 

 metal from the grinder and the thoroughly mixed sample is put 



in tightly-stoppered bottles and kept not exposed to sunlight or 

 heat. 

 Hard rubber is prepared for analysis by rasping. 

 ACETONE EXTRACTION. 



Place a two-gram sample, which has been ground not more 

 than 24 hours before, in an acetone-extracted paper thimble, and 

 insert in a glass syphon cup under a condenser. Dry and weigh 

 a clean 8 by l}4-inch test tube, weighing between 20 and 40 

 grams, by means of a wire loop; pour in SO cc. acetone, connect 

 the apparatus, and extract continuously for 8 hours in such man- 

 ner that the drops of condensed solvent will fall directly on the 

 ground rubber ; that the syphon cups will fill in 2H to 3J4 min- 

 utes; that the discharge of the syphon cup will not appreciably 

 interrupt the boiling; that the condensed solvent will filter rapidly 

 through the paper thimble, and that no fine particles of rubber 

 or fillers will be carried over. If the solution in the cup is 

 colored after eight hours' extraction, the extraction is continued 

 for four hours the next day. Evaporate the acetone from the 

 tube in a slanting position over live steam, wipe off the outside 

 with a clean linen cloth, and dry to constant weight in a water 

 oven at 95 to 100 degrees C, or until the weight increases, cool- 

 ing in a desiccator. Calculate and record as "total acetone ex- 

 tract." 



ORGANIC ACETONE EXTRACT. 



When waxy hydrocarbons are not to be determined, subtract 

 the percentage of free sulphur from the percentage of total ace- 

 tone extract above, and record the difference as "organic acetone 

 extract." 



TOTAL SULPHUR. 

 A half-gram sample with six grams of potassium carbonate 

 and four grams of sodium peroxide are mixed by rotating in a 

 crucible; cover, heat at a low temperature over an asbestos shield 

 to avoid sulphur fumes, until the mixture fuses, then bring to 

 quiet fusion for 15 to 20 minutes. Avoid rapid heating and ex- 

 plosions. Rotate the melt while solidifying. When cool, put 

 crucible and cover into a casserole containing 200 cc. of water, 

 add five to ten cc. of bromine, and boil until the melt is dissolved. 

 Allow to settle, decant, filter and wash through a thick filter with 

 hot water. Acidify the filtrate with hydrochloric acid, using 

 Congo red paper, make up the volume to 400 cc, and precipitate, 

 boiling with 10 cc. of a 10 per cent solution of barium chloride, 

 keeping the beaker covered with a watch glass. Allow the pre- 

 cipitate to stand over night, filter on an asbestos mat in a Gooch 

 crucible, wash with hot water, ignite, and weigh, cooling in a 

 desiccator. Calculate to sulphur (factor 0.1372) and record as 

 "total sulphur." 



OILY FOREIGN MATTER, 

 Where the presence of tar, pitch or asphalt is not indicated, 

 spread out the rubber residue from the acetone extraction and 

 dry in the water bath at 95 to 100 degrees C. until the odor of 

 acetone is no longer apparent. Transfer to a 100 cc. pressure 

 flask fitted with washers previously extracted with alcoholic pot- 

 ash. Add 50 cc. of alcoholic potash, stopper and heat in an air 

 oven kept between 105 and 110 degrees C, for four hours. Cool 

 the flask, filter and wash the residue with hot absolute alcohol 

 until the washings are no longer colored. Make the filtrate 

 strongly acid with concentrated hydrochloric acid to precipitate 

 potassium chloride, allow to settle, filter and wash with hot 

 chloroform into a small casserole. Place the casserole on the 

 steam bath and evaporate until the odor of hydrochloric acid 

 just disappears. Take up the residue with chloroform, filter and 

 wash with hot chloroform into a weighed beaker, evaporate the 

 chloroform on the steam bath and dry the residue in the water 

 oven at 95 to lOO degrees C, in 15-minute periods, until the 

 weight is constant, or increases, cooling in a desiccator. If the 

 residue is not oily to the touch no report shall be made. If 

 the residue is oily to the touch, subtract 2 per cent of the weight 

 of vulcanized rubber gum, as determined below, from the cal- 



