Refractive index of rubber

Abstract

The refractive index of rubber was measured on an Abbe refractometer by a method of total r efl ection. For plant a tion Hevea rubber and purified rubber average values of n~ are 1. 5188 and 1. 5190, respectively. The index of crude rubber is not altered by the introduction of insoluble fillers. Sulphur in solution increases nii of crude rubber by O. 0016 for each percent, and phenyl-l1naphthylamine, by O. 001 5 for each p ercent. The refractive index of vulcanized compounds of purified rubber and sulphur is given by, nb = 1. 5190 + 0. 00370 S,-O. 00035 (t-25) where S, represents the percentage of combined s ulphur, between 0 and 16, and t, the temperature in degrees centigrade, between 10 and 75 C. The curve relating refractive index to t emperature for a compound containing 19 percent of s ulphur undergoes a change in slope at about 17 C. Preliminary m easurements on compounds containing 19 to 32 percent of sulphur indicate tha t the transition from soft to hard rubber is accompanied by a decrease in the slope of the curve relating refractive index to t emperature.