Measurements with a bomb calorimeter have been made of the heats of combustion of samples of rubber purified by various methods, and of compounds of rubber and sulphur containing up to 32 percent sulphur. In the calorimetric combustion experiments on rubber-sulphur compounds, an amount of aqueous Na 2 C0 3 or NaOH more than sufficient to react with the products of combustion containing sulphur was placed in the bomb before each experiment in order to obtain a definite final state for the system. The average value obtained for the heat of combustion of ether-soluble rubber in gaseous oxygen to form gaseous carbon dioxide and liquid water at a temperature of 30°C and a constant pressure of one atmosphere is 45,207 international joules per gram (weight in vacuo). The estimated uncertainty of this value is 0.2 percent. The average values obtained for ether-insoluble and total rubber are lower by 0.9 and 0.4 percent, respectively, than the value for ether-soluble rubber.
Evolution of hydrogen sulphide from vulcanized rubber containing 8 to 32 per cent sulphur has been measured when the temperature of the specimens was raised step by step from 105°to 265°C " and the time intervals kept equal. The same sample of each compound was employed throughout the whole temperature range.The rate of decomposition increases as the temperature is raised, except in the case of ihe compounds containing the higher percentages of sulphur, when it passes through a maximum.The loss of hydrogen sulphide has also been determined for four different compounds containing 4, 10, 18, and 32 per cent sulphur when samples were heated for 200 hours at 136°C, and also when other samples of these compounds were maintained for the same length of time at 220°C . In each case, the rate of decomposition decreases rapidly at first, but after several days' heating, the decrease becomes relatively slow. In general, the rate increases with temperature and with increasing sulphur content. Other products evolved are moisture and organic compounds.The data presented in this paper supplement and confirm previous results on the evolution of hydrogen sulphide from vulcanized rubber and indicate at what temperatures direct thermal decomposition of different rubber-sulphur compounds will become significant. No mechanism to explain the chemical changes involved is suggested.This question is worthy of further investigation.
The work described in this paper is part of a series of investigations at the National Bureau of Standards to determine the properties of purified rubber and of compounds of purified rubber and sulfur. Values of the heat of combustion of rubber have been reported by a number of observers, and values of the heat of combination of rubber and sulfur, determined as the difference in the heats of combustion of vulcanized and unvulcanized rubber-sulfur mixtures, have been reported by several observers. The results obtained by the various investigators are not in satisfactory agreement. The values reported in the literature for the heat of combustion of rubber vary over a range of about 4.5 per cent, and the two recent sets of data on the heat of combination of rubber and sulfur differ radically, as may be seen from Fig. 2. It was thought that these differences might be due, in part at least, to differences in the composition of the rubber, and that more consistent results might be obtained by using purified rubber. The data presented in this paper were obtained on samples of purified rubber and on compounds of purified rubber and sulfur prepared in connection with an investigation of the electrical properties of rubber and rubber-sulfur compounds by Scott, McPherson, and Curtis.
From the results obtained it is concluded that the boiling water hydrolyzes some of the esters in the acetone extract. In the case of direct extraction of the rubber with water, hydrolysis of esters must take place in the rubber, forming water-soluble acids; or else, although the esters are practically insoluble in the water, it extracts a little ester each time the extraction thimble empties (Bailey-Walker apparatus), carrying the ester down to be hydrolyzed in the boiling water below. Temperature and time of heating play a prominent part. The temperature must be close to the boiling point and the acidity increases progressively with time of heating. The fact that temperature is so important in producing this effect accounts for the erratic values obtained by direct water extraction. On the electric hot plates used water did not always boil steadily, and that contained in the thimble might vary considerably in temperature, much of the time not being hot enough to hydrolyze or extract any ester. Therefore, to avoid any difficulty in the determination of water-soluble acids in rubber, extract first with acetone, then digest this extract with water on a boiling water bath until no increase in acidity of the water extract is obtained. This is a part of the procedure developed at the Netherlands Government Institute by Van Rossem and Dekker.
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. CONTENTS Page
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