140 



THE INDIA RUBBER WORLD 



[Decemulk ], 1919. 



of 120, 180, 240 and 300-minute cures at 292 degrees F. on the rub- 

 ber with 6 per cent sulphur and 1 per cent zinc oxide, these being 

 represented by the dotted line curves A, B, C and D, respectively. 

 On the same graph there is also indicated the stress-strain curves 

 corresponding to 1, 2, 3, 4 and S-niinute cures at 292 degrees F. 

 in case of the same mixing containing a H of one per cent ad- 

 dition to the powerful catalyst, these being the solid lines E, 

 F, G, H, J and K, respectively. Addition of 14 and -^ of 1 per 

 cent were not sufficient to produce good vulcanization at a temper- 

 ature of 292 degrees in periods ranging from 2 to 5 minutes. 

 These results are in general similar to those obtained by adding 

 the accelerator to the smoked sheet, but are more notable, inas- 

 much as the brown crepe was altogether incapable of being well 

 vulcanized in its natural state, whereas through the use of the 

 accelerator, samples having good physical properties are pro- 

 duced in extremely short vulcanizing periods. 



It is to be noticed that the sample in Table VI cured 240 

 minutes and represented by Curve C in Figure 3, has all the 

 characteristics of a technically undercured sample, although the 

 coefficient amounts to 3.45, which is well above the 3 per cent 

 limit mentioned for first-grade rubbers. On the other hand, 

 the sample cured for 300 minutes is undoubtedly both overcured 

 and overheated. The best cure would, therefore, fall between 

 these points, but it is evident that no good cure is possible with 

 this rubber except through treatment with chemical accelerating 

 agents. 



.Although more results would be required before any definite 

 conclusions should be drawn, the writer would summarize as 

 follows : 



SUMMARY. 



(1) Although average good quality plantation Hevea rubbers 

 may require a combination of sulphur of about 3 per cent to 

 effect the proper or optimum cure, such rubbers can be vulcan- 

 ized in a small fraction of the original time through the use of 

 very powerful catalysts, together with zinc oxide, in which case 

 excellent physical properties are obtained in samples, the vulcan- 

 ization coefficient of which may be in the neighborhood of one. 



(2) Inferior types of plantation rubbers which are incapable 

 of being transformed into satisfactory vulcanizates through the 



500 1000 1500 ?000 JSOO 3000 3i00 



Load in Pounds per Square Inch 



Fig. 3. Complete Stress-Strain Curves. 

 agency of sulphur alone may be readily vulcanized in very short 

 curing periods and at normal vulcanizing temperatures through 

 the use of powerful accelerators in the presence of small quan- 

 tities of oxide of zinc. ,\lthough such rubbers may be under- 

 vulcanized when as much as 3M; per cent of sulphur is brought 

 into combination through prolonged heating, good mechanical 



properties may be found in samples having abnormally low vul- 

 canization coefficients when the vulcanization is carried out with 

 minimum heating through the use of powerful accelerators. 



(3) No definite figure can be established for tne vulcanization 

 coefficient of properly vulcanized soft rubber goods in general, 

 or for the optimum cure of any mixing in particular, for the 

 reason that the mechanical properties, as well as the chemical 

 combination of sulphur is influenced by the time and degree of 

 temperature used for vulcanizing. 



(4) It would appear probable, however, that in the case of 

 any definite compound, the vulcanizing conditions could be 

 standardized by the use of an accelerator, and that the degree 

 of vulcanization could then be checked most accurately through 

 determinations of the coefficient of vulcanization. 



SOME METHODS OF TESTING THE HARDNESS OF 

 VULCANIZED RUBBER." 



5.V H. P. Gumcy. 



USTTAl QUALITATITE METHODS. 



HPhe most n.\tural way for an experienced rubber man to 

 •^ estimate the hardness of a piece of cured rubber is to note the 

 force required to indent it with the point of a pencil or sim- 

 ilarly convenient means. Although practical rubber workers are 

 thus able to flistincui^h very accurately between different grades 



I II XSIMETER. PlASTOMETER. DuROMETER. 



of rubber coverings, the information acquired is of no value as 

 a matter of record since it cannot be designated quantitatively. 



aUANTITATIVE METHODS. 



Spring Type Apparatus. The use of any of these dififerent 

 instruments for obtaining a figure expressing the hardness of 

 rubber surfaces affords an accurate method for the comparison 

 and recording of the quality of hardness by an arbitrarily chosen 

 number. 



In instruments of the spring type, the force is applied by means 

 of a spring and the rubber is indented by a point of specified 

 shape and dimensions. 



The Durometer is an extremely convenient pocket instrument 

 of this class. On its scale the hardness of glass or steel registers 

 100, while that of soft rubber good may register as low as 40. 



At the Boston Belting Corp.'s factory, a so-called "Densimeter" 

 has been in use for many years. This instrument is also of the 

 spring type. A perfectly hard object has a densimeter reading 

 of 1000, while softer objects may run as low as 250. There is 

 one marked disadvantage in the use of the spring type of hard- 



of the Ame 



Chemical So 



