December 1, 1920 



THE INDIA RUBBER WORLD 



169 



The Viscosity of Rubber 



By A. M. Munro' 



IN OHUER, on the one hand, to secure adequate control over 

 the compounding and vulcanizing of manufactured rubbers, 

 and on the other, to be in a better position to trace to their 

 origin obscure differences in ihe physical and mechanical prop- 

 erties of linished goods, suitable methods are urgently needed 

 for the evaluation of raw rubber, as it enters the factory. 



At the present time the "variation" of raw rubber is one 

 of the most perplexing problems faced by the manufacturer and 

 the chemist. The problem is acknowledged to be, in the main, 

 a physico-chemical one, complicated by the existence in a sample 

 of raw rubber of an unknown luimber of colloidal aggregates 

 of widely different physical projjerties and subject to variation 

 in quality according to the amounl of mechanical "breaking 

 down" or other treatment which they may have received. 

 Again many factories have not yet standardized their buying 

 of crude rubber, with the result that the manufacturer is com- 

 pelled to wcrk in ignorance of the history of any particular 

 consignment of rubber; age of the trees, seasonal variations, 

 quality of the soil, method of coagulation, degree of milling 



a new viscometer. Frank employed xylene as a solvent and 

 his solutions contained 3 per cent of rubber. The apparatus 

 was standardized with pure glycerine. Schidrowitz and Golds- 

 borough in 1909 attempted to establish a relation between the 

 "nerve" of a rubber and the viscosity of its solutions. Their 

 experiments showed that a relationship does appear to exist 

 but that it is not direct, "nerve" being determined by two 

 different factors, one of a mechanical nature and the other 

 chemical (polymerization). Schidrowitz used the Ostwald type 

 of viscometer, employing 1 per cent benzene solutions and cal- 

 culating the viscosity in terms of the solvent as unity. The 

 rubber content of the solution was detennined by evaporation 

 of the solvent at the end of the experiment. For the sake of 

 comparison he made mechanical tests and found that the indica- 

 tions of the viscometer were of a comparative value from the 

 point of view of the determination of elasticity. 



It will be noticed in the above work that little has been done 

 to compare the suitability of the many rubber solvents for the 

 mcasurcinent of viscosity, to investigate the influence of varying 



P'iG. I. THKRMO.STAT I Ok Vl-'^tClSlTV W'oRK 



l-'ic. 2. Re.se.\rcii Bench. Showinc. Thermost.^t .axu I'ouer- 



tJRIVEN Sh.VKINC M.^CHINE 



and washing, length of storage before shipment, and a number 

 of other factors being unknown or, at least, uncertain quantities. 



However, in the face of these many and obvious difficulties 

 it appears to the writer that the desired goal of rubber evalua- 

 tion and standardization will eventually be reached along the 

 lines of physical and physico-chemical measurement, and with 

 this in view he has commenced a series of researches on the 

 viscosity of rubber. 



In this paper, which is of a preliminary nature, he ventures 

 to put forward a few of the results which have been obtained 

 up to the present date, together with a description of the ex- 

 perimental methods employed. 



HISTORICAL 



Axelrod, to whom the idea of studying the viscosity of rubber 

 first occurred, worked with benzene solutions and observed the 

 time taken by 100 cc. to run through a capillary from a con- 

 taining vessel. The ratio of this figure to 4.5, he called ihe 

 viscosity of the solution. He proposed at this time (1906) to 

 apply the method to the evaluation of rubbers. 



Schidrowitz, and later Frank, took up this question again, 

 and the latter put before the International Testing Committee 



temperature, or to correlate tlie viscosity value with the other 



chemical and physical constants of the sample. Most important 



of all, none of the results has been expressed in absolute units. 



APPARATUS 



In order to carry out an accurate series of measurements of 

 viscosity, the writer has set up the apparatus shown in Figs. 1 

 and 2. The thermostat employed for securing constant tempera- 

 tures consists of a glass tank, measuring 18 inches in length by 

 9 inches in width, by 13 inches in depth. The framework and 

 bottom of this tank are of sheet copper. The water contained 

 in this tank is stirred by a good sized propeller, driven from 

 overhead shafting, which power supplied by a '4-h.-p. electric 

 motor; while heating is effected by a gas burner placed below the 

 tank, which is itself ■supported on an iron stand. 



The gas supply is' regulated by a toluene-filled Ostwald thermo- 

 regiilator placed in the water. Temperatures are read by means 

 of a standard Centigrade thermometer, graduated in tenths of 

 a degree. In order to minimize loss of heat by radiation and 

 irregular cooling, the tank and stand are both covered with 

 asbestos cloth, holes being provided for observation of the 



1 Chief chemist, Dunlop Rubber Co. of .\uslralasia Ltd., MontaRiie, .\us 

 Iralia. 



