THE INDIA RUBBER WORLD 



293 



liail not been put in so as entirely to fill its cavity. Therefore, 

 all measurements taken to determine the value of the coefficient 

 were taken over the range of 100 degrees to 300 degrees. Some 

 of the lower grade compounds, especially those with high sulphur 

 contents, showed a lessening of the slope and even a falling off 

 in the curve after 300 degrees had been reached. 

 INFLUENCE OF THE RUBBER. 

 Two samples were made up from the same formula, using in 

 one fine hard Para and in the other a soft grade of brown crcpc 

 (Fig. 2). Each was subjected as nearly as possible to the same 

 amount of milling. The Para sample gave a coefficient of 

 2.337 X 10-', while the brown crdpe gave 4.49 X 10-'. 

 INFLUENCE OF THE MILLING. 

 Other parts of the same batch with the Para rubber were sub- 

 jected to further milling and a sample gave a coefficient of 

 4.680 X 10"*, while another sample which had been milled to 

 excess on hot rolls gave a value of 4.786 X 10-* (Fig. 3). 

 Then it is seen that milling increases the expansion and brings 

 the value of Para up to that for the brown crepe. 



VOLUME CHANGE AT CONSTANT TEMPERATURE. 

 One of the first questions which arose in connection with the 

 expansion of the rubber was whether or not there was a break in 

 the curve or, in other words, a volume change at the point of 

 vulcanization. We know- that a physical change takes place and 

 that the specific gravity increases and therefore it was only 

 natural to look for a drop in the expansion. The curves ob- 

 tained with a rising temperature failed to show any drop, so 

 samples were tried using a constant temperature o. , in other 

 words, subjecting the sample to the same condition as an ordinary 

 cure (Fig. 4). The graphs failed to show any drop with the 

 exception of the cases already mentioned. Even in those cases 



the drop was very gradual and did not start at the point of vul- 

 canization. Therefore, the method could not be used as a method 

 of regulating or determining the cure of a sample of rubber. 



Since the curves failed to show any volume contraction during 

 the heating process, the increase in specific gravity which always 



occurs i»ist come as the result of the pressure and the subse- 

 quent contraction on cooling. Since rubber is nearly incom- 

 pressible, the decrease must come as the result of the squeezing 



of air out of the raw rubber. If this be true, then a compound 



Fig 5. Curve Showing Effect of Different Pressures on Two 



Rubbers, One of Which Shows a Large and the 



Other a Small Difference Between Raw and 



Cured Specific Gravities. 



showing a big difference between raw and cured specific gravity 



will expand more against a slight spring than against a strong 



one. This was tried out and found to be the case (Fig. 5). 



Expansion against Spring. 

 Stock. 50-lb. 



355 



355 



10—* 



100-Ib. 



3.81 X ; 



1.36 X 1 



Exp. Apparatu 



R.\\v. 



0.95 

 1.457 



0.957 



SO-lb. spring. 100-lb. spri 

 0.956 0.956 



1.568 1.577 



EXPANSION VS. CONTRACTION. 



The increase in specific gravity shows that the contraction is 

 greater than the expansion. Therefore the question arises 

 whether or not the values obtained apply to the cured rubber. 

 In order to determine this, measurements were made on sam- 

 ples cured in a disk mold. 



Aluminum 

 Diameter o 



mold 1595 stock 



f mold hot 3'Vm inches 



f rubber cold 3^/ 32 inches 



• u 



0.0625 



L„ / 39063 (218> 



.7 = 0.00007339 

 <i (cubical) = 2.20 X 10^' 

 This compares with 2.8 X 10^* obtained the other way. This 

 is a check considering the fact that the rubber was of a different 

 sample and was from mill stock, while the expansiometer sam- 

 ple was from calendered stock. The conclusion may be drawn 

 then that the coefficient determined between 200 degrees and 300 

 degrees be taken as sufficiently accurate for the coefficient for 

 vulcanized rubber from the same sample. 

 CONCLUSION. 

 -Mthough it is not yet possible from the results obtained with 

 this apparatus to state definitely whether or not a stock will 

 mold well in factory practice, it does give us a means of dis- 

 tinguishing between batches of the 'same stock which may have 

 different properties, due either to rubber milling or other con- 

 ditions. 

 The results may be summarized as follows : 



I. The values of the coefficient of cubical expansion for differ- 

 ent grades of rubber have been determined. 



II. The higher the rubber content the greater the expansion. 



III. The harder the crude rubber the less the expansion. 



