June 1, 1921 



THE INDIA RUBBER WORLD 



663 



weaving, skipped threads, knots, etc. This cloth is applied wring- 

 ing wet and great care is exercised to lay it taut and smooth and 

 free from wrinkles or folds in every direction. The length of the 

 rubber sheet is 60 to 65 yards. The width is determined by the 

 calender rolls and varies in ditTerent factories from 36 to 60 

 inches, or even wider. 



After wrapping, the entire roll on the drum is cross-wrapped 

 or spirally bandaged under tension by a woven strip of strong 

 cotton webbing to insure positive solidity of the rubber mass and 

 to prevent tlie formation of air pockets between the plies. The 

 drum upon which this winding occurs is about three feet in diam- 

 eter and the precaution is observed of not bringing the first end 

 of the rubber sheet into contact with the metal of the drum, but 

 tlie drum is first wound or padded for several turns with wet, 

 woven cotton sheeting. The reason for this is to prevent over- 

 vulcanizing of any one part of the rubber sheet by metallic con- 

 tact and to ensure uniformity of vulcanizing conditions throughout 

 the entire thickness or mass. The drum is then placed vertically 

 m a vertical vulcanizer, the drum resting on end on a false per- 

 forated metal bottom placed several inches above the true bottom 

 of the vulcanizer. Water is then admitted to the vulcanizer to a 

 height to completely submerge the drum but not to completely fill 

 the vulcanizer. A foot or more of space is left at the top of the 

 vulcanizer above the water level to provide for expansion and for 

 a body of steam. The cover of the vulcanizer is bell shaped. 

 Steam is admitted through the bottom of the vulcanizer until the 

 temperature of the water is raised to 280 or 285 degrees F. and 

 brisk circulation of steam is maintained by means of a freely 

 opened pet-cock or valve at the top of the vulcanizer. This is 

 of vital importance. The water must be kept agitated, otherwise 

 vulcanization is never uniform, and soft, partially cured spots 

 show under later test. 



The time required to effect vulcanization varies from 65 to 85 

 minutes for a compound of — in this country — one ounce of sul- 

 phur to one pound of fine Para, and in England, of two ounces of 

 sulphur to one pound of fine Para. There is a curious feature 

 here presented, namely, that of climatic and atmospheric condi- 

 tions as affecting vulcanization. Where the atmospheric humidify 

 is normally heavy, as in Manchester, England, more sulphur is 

 required to effect vulcanization than in Massachusetts, where the 

 air is normally lighter and dryer; and also the time required to 

 effect vulcanization varies with the seasonal changes from extreme 

 sharp cold in winter to extreme humid heat in summer, and this 

 is quite aside and distinct from any differences whatsoever in the 

 strength or nerve of various lots of rubber. 



When the sheet of vulcanized rubber is unwound from the drum, 

 its color has changed from amber to dirty green and it is saturated 

 with water. Contrary to the opinion usually held by laymen, rub- 

 ber docs absorb water, and in the form of freshly vulcanized sheet 

 for thread as described, it contains from 18 to 20 per cent by 

 weight. The sheet is next suspended on racks in a drj'ing room 

 which is metal-sheathed throughout, to radiate heat from steam 

 pipes. The temperature of the room is from 100 to 110 degrees F. 

 Within 24 hours the moisture is evaporated and the sheet assumes 

 the gray color usually associated with pure vulcanized rubber 

 which has not been subjected to any alkaline desulphurizing proc- 

 ess. (Some thread is later desulphurized, some is not, but, as 

 Kipling says, "That's another story.") Whether the original man- 

 ufacturers of rubber thread — whoever they may have been — found 

 by experience that necessity compelled them to employ water as 

 a vulcanizing medium, or whether it just naturally occurred to 

 them in the first mstance, I cannot say, but it is a fact that no 

 other way of vulcanizing rubber sheets has ever been found to 

 equal it. Water is an ideal heat conductor, and sheet vulcanized 

 as described possesses a degree of permanent elasticity, due no 

 doubt in part to the practically spontaneous conducting throughout 

 the mass of a uniform temperature and perhaps in part to a 

 beneficent chemical action which sheet vulcanized in any other way 

 never possesses. Sheet so vulcanized and thread cut from it have 



a quality of something so lastingly virile, so velvety, in brief, so 

 rubber-like, that there is nothing to touch it where elasticity is 

 the main objective. 



Another fine feature of water vulcanization is that it overcomes 

 the bothersome shrinking problem. As every rubber man knows, 

 sheet rubber cured in open steam or suspended from racks and 

 exposed to dry heat shrinks unevenly, thus altering the gage or 

 thickness as delivered from the calender. 



Manufacturers of elastic bands have found that the best results 

 are obtained from calendered sheet by suspending the tubes made 

 from the sheet from poles or racks submerged in water raised to 

 vulcanizing temperature. Wherever it is possible to employ water 

 vulcanization, it will be found to posses distinct advantages over 

 nearly every other form of curing. 



THE RELATIVE ACCELERATING ACTION OF DIF- 

 FERENT COMPOUNDS OF LEAD IN THE 

 VULCANIZING OF RUBBER 

 By J. M. Grove 

 INTRODUCTION 



THE more one studies the subject of organic accelerators 

 the more puzzling it becomes. Much has been learned 

 about their immediate effect, but much more remains to be 

 learned. Hence the numerous papers on the subject, and if, 

 finally, dicta may be laid down governing their immediate 

 action in varying proportions, there will still remain the ques- 

 tion of their action as affecting the ultimate life of vulcanized 

 rubber. 



It was while engaged in an extensive investigation of the 

 immediate action of various organic accelerators upon the 

 vulcanization of different compounds that the writer was 

 asked to determine the relative accelerating action of the 

 most widely used compounds of the known — the old lead group. 



So far as he was able to learn by reference to many com- 

 pounds laid before him for investigation, there appeared to 

 be no rule governing their use, although their use has for 

 many years proved beneficial where it is possible to use them 

 at all. Of course, their employment is prohibited wliere 

 colors other than gray or black must be obtained. 



METHOD OF EXPERIMENTATION 



The accelerating action of one per cent of litharge was 

 taken as the basis of comparison. The formula used con- 

 sisted by weight of: smoked sheet 80 per cent, sulphur 25^^ 

 per cent, zinc oxide 5 per cent, and varying percentages of 

 baryfcs, and the given lead compound under investigation to 

 make up 100 per cent by weight. As the percentage of the 

 lead compound introduced into the above formula was in- 

 creased, the percentage of barytes was proportionately de- 

 creased. Thus, the relative percentage of smoked sheet, 

 sulphur and zinc remained unchanged in all of the trials. 

 (See Table 1.) 



Table 1 

 Formulas or Compounds 



Ingredients 1 2 3 4 5 6 7 8 9 10 11 12 



Smoked 

 sheet ... 80 80 80 80 80 80 80 80 80 80 80 80 

 Sulphur .. 2'A 2yi 2'A 2K ^Vi iVi Z'A 2yi 2!^ 2^ 2'A 2J4 

 Zinc oxide 5555555S555S 



Barytes .. UK S'A 6'A 7'A 9'/, 8'A 6yi ny, lO'A 9A 7'A m 



I.ith.irge .1 



Suhlimed 



white Iciid .. 7 6 5 



B.isic lead 



carbonate 3 4 6 



Sublimed 



blue lead 1 2 3 S 7 



100 100 100 100 100 100 100 100 100 100 100 100 



The ingredients of the different compounds were very care- 

 fully weighed and mixed in the laboratory, under conditions 

 exactly alike. After a period of 24 hours' rest a sample from 

 each compound, Yt-inch in thickness, was cured in a press 



