January 1, 1921 



THE INDIA RUBBER WORLD 



245 



PREPARING SECTIONS 



The method finally developed consists in freezing the rnbber 

 by expansion of carbon dioxide on the stage of an ordinary 

 Spencer microtome. The sample is held tight on the stage by 

 means of a watcr-glycerol solution (90 : 10) which, on cooling, 

 solidifies to a white mass. This solution is used because it docs 



Kig. 3 contains ahuninium flake. The grain is shown very 

 decidedly, and it is not difiicult to believe that the tensile strength 

 and elongation will vary according to the direction of testing. 



Fig. 4 shows whiting. 



Fig. 5 is iron oxide. It is well dispersed but contains many 

 large particles. 



l'"ig. 6 was litharge when introduced and should have shown 



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A 



.*■■■',. 



r -VM .^IHMIt*. JK.'« 



I" I 



1 



Fig. 2 



Fig. 3 



not become very brittle at the low temperatures. The carbon 

 dio.xide does not cool the sample to a sufficiently low tempera- 

 ture, and a surface flash cooling is given by liquid air. The 

 liquid air can be obtained at a convenient liquid air manufactur- 

 ing plant and transported without difficulty in an "Icy-Hot" 

 bottle, which will keep liquid air for several days. The liquid 

 air may be ejected very satisfactorily from the bottle by intro- 

 ducing carefully a one-hole rubber stopper through which a glass 

 tube about 3 to 4 mm. in diameter is passed into the bottle. The 

 pressure developed by the continual evaporation will force a 

 stream of the liquid out, which can be directed on the sample. 

 A very small amotnit will freeze the sample sufficiently. 



clear centered particles when in focus, but the particles have 

 become black, due to a surface coating of sulphide. 



Fig. 7 is a well-known while tread with magnesia as an accel- 

 erator. Chunks of magnesia stand out in sharp contrast to the 

 uniform distribution of the zinc oxide. The individual particles 

 of zinc oxide do not show up well except near the edge, owing 

 to the thickness of the section which was necessary to show the 

 large particles of magnesia. 



Fig. 8 is a section from a widely advertised composition sole 

 bought from a shoe repairman. It shows iron oxide used in 

 coloring, zinc oxide used in reinforcing, and fiber used for stif- 

 fening The photograph is poor, but the section can be seen 

 more distinctly by direct observation with the microscope. 



Fig. 9 shows a zinc oxide tread with a very small amount of 



Fir.. 5 



Fk;. 6 



Fig. 7 



Fig. 8 



The section is then cut with the knife. Raising the stage is 

 best controlled by hand. The thin section is mounted in Canada 

 Ijalsam on a microscope slide and preferably examined visually at 

 1,500 diameters, using a Zeiss 2-mm. apochromatic oil-immersion 

 lens. For photographing, sharper negatives are obtained at 800 

 diameters. 



DESCRIPTION OF ILLUSTRATIONS 



In Fig. 1 zinc oxide is shown uniformly dispersed in rubber. 

 .\ typical triplet crystal can be noted and also the grain in the 

 rubber in the direction of working is shown by the definite aline- 

 ment of the needle-shaf)ed crystals. 



In Fig. 2 a good lithoponc is shown very well dispersed. 



lampblack to give a gray color. The lampblack is badly floc- 

 culated. 



I'ig. 10 shows a poorly dispersed carbon black. 



Fig. 11 shows a well-dispersed carbon black together with 

 zinc oxide. 



Fig. 12 a section from the sidewall containing zinc oxide and 

 whiting. 



Fig. 13 a section of a cushion from a tire showing reclaimed 

 rubber and a small amount of zinc oxide. 



CONCLUSIONS 



Microscopic examination affords the following possibilities : 

 (1) A knowledge of the dispersion of the pigment in rubber 



