292 



THE INDIA RUBBER WORLD 



The Expansion of Rubber Compounds During Vulcanization/ 



Bv C. W. Sanderson. 



TiiK txTE.NT lo wliicli ruUlicr cumpounds will expand when sub- 

 jected to the heat of vulcanization is of importance in the 

 molding of rubber goods. As far as published results go. 

 however, there are no available data on the value of the coeffi- 

 cient of expansion of rubbers of different compositions. 



Ftc. 1. RunBER Exp.\xsiox App.\r.\tus. 



The work was undertaken by us primarily from a practical 

 point of view, with the object of determining whether or not 

 use could be made of an expansion test in distinguishing between 

 stocks which move freely and mold well and those which do not 

 mold well and are characterized as being "dead." Other possi- 

 bilities suggested themselves and although the results are by no 

 means complete or beyond question, they are set forth as we 

 have found them. 



APPARATUS. 



A special apparatus was designed for the investigation (Fig. 

 1). It consists of a hollow steel cylinder which may be filled, 

 with rubber and which is surrounded by a steam-jacket. The 

 top surface of the rubber is the only surface free to move when 

 the rubber is heated, and it acts against a piston and spring. The 

 motion is transmitted through a magnifying lever to a recording 

 pencil. The piston, spring and recording mechanism were taken 

 from a Crosby steam indicator gage and made over to fit the 

 apparatus. .Xbovc the piston is a place for a spring to keep 



Fig. 2. CrRVE Showinti Effect of Rl^bber on R.^te of Exp.\n- 

 siox 1698 Fine Para vs. 1698 Brown Crepe. 



pressure enough on the rubber so that it will not "blow." Ten, 

 50 and 100-pound springs were used. The ten-pound spring was 

 hardly strong enough to prevent blowing. The use of the 50 and 

 100-pound springs will be mentioned later. 



In making the determinations of the expansion the rubber 

 was heated at a constant rate, usually 20 degrees per five minutes. 

 The recording cylinder was advanced at each interval and from 

 the values on this graph smooth curves could be drawn. 



The dimensions of the steel cylinder were : diameter, 0.7978- 

 inch ; height, 2 inches. 



These dimensions were chosen so that at room temperature 

 (80 degrees F.) the volume of the contained rubber was one 

 cubic inch. The motion of the recording pencil was reduced 

 to the motion of the piston by dividing by six. 



The cubical coefficient of expansion, derived from the regular 

 forms, may be expressed in simplified form if we neglect the 

 expansion of the steel shell and approximate. 



Thus, a = • 



12/ 

 where t = temperature change and 

 tlie chart. 

 It was found that the differences in the results obtained with 



expansion recorded on 



ruRVE Showing Effect oi 

 Expansion. 



Milling on Rate of 



the exact and witli the approximate formula were less than the 

 experimental errors. The approximate formula was therefore 

 used in our work. 



Use. 

 Tul)e. 

 Tube. 



Pneumatic tread. 

 Semi-hard. 

 Pneumatic tread. 

 Sidewall. 

 Solid tire. 

 Pneumatic tread. 

 Hard rubber. 



10—=. 



Coeff.= 

 4.680 X 10—' 

 4.023 X 10—* 

 3.365 X 10—* 

 3.322 X 10—* 

 3.000 X 10—* 

 2.807 X 10—* 

 2.828 X 10—* 

 2.73 X 10—* 

 1.121 X 10—* 



Raw. 



0.89 

 0.95 



Sp. Gr. 

 Cur. 

 0.96 



^Published by courtesy of the 

 before the Rubber Divisic " ' 

 phia, Pennsylvania, September 4, 1919. 



lical Society. 





^Ebonite (Smithsonian table). 4.6 X 10—=. 



In general the results show that the higher the rubber content, 

 the higher the coefficient of expansion. It is, however, difficult to 

 show any definite relation because, as will be shown later, other 

 factors enter in. 



NATURE OF THE EXPANSION. 



The graphs obtained showed that after the first fifteen minutes 

 the expansion curve was practically a straight line. The begin- 

 ning of its curve showed a lower scope because of the fact 

 that the rubber was not thoroughly heated through or perhaps 



