It has been known for some years that certain sols, on standing, form gels which can be reliquefied by shaking. This phenomenon was first discovered by Schalek and Szegvary (13) in 1923 for ferric oxide sols to which a small amount of electrolyte, for example, sodium chloride, had been added. Since then many cases of this isothermal reversible sol-gel transformation, which Freundlich has termed "thixotropy," have been reported.Bentonite, a clay-like material, probably of volcanic origin and found largely in Wyoming, exhibits thixotropy to a marked degree in aqueous suspensions. Suspensions of suitable concentration will set to a gel in a few seconds, although a number of other factors materially influence this setting time. Particle size undoubtedly plays a great part, since centrifuged suspensions will give dispersions showing thixotropy down to at least a concentration of 1 per cent, whereas ordinary bentonite suspensions require a concentration of over 4 per cent.s The hydrogen-ion concentration of the suspension, as might be expected, was also found by Freundlich, Schmidt, and Lindau (5) to be of great importance. Acidwashed bentonite or electrodialyzed bentonite suspensions do not show thixotropic behavior. In this paper the effects of temperature and concentration upon gelation are described. No attempt has been made to determine the influence of pH, the hydrogen-ion concentration being kept m constant as possible. METHODS OF MEASUREMENTThe investigation of thixotropy is greatly hampered by the lack of an adequate method of measurement. The inverted-tube method first introduced by Schalek and Szegvary (14) has been most commonly used. The sol is introduced into a tube, which is stoppered or preferably sealed 1 Presented before the Thirteenth Colloid Symposium, held a t St.
THAT vulcanization of rubber with sulfur always involves a chemical reaction consisting in the addition of sulfur to the double bonds of the rubber molecule has been conclusively established (18, 28). The facts indicate that this addition of sulfur to rubber is an irreversible reaction (31). The temperature coefficient of the reaction is high, increasing about 2.65 fold per 10° C. at ordinary curing temperatures (31). Furthermore, the reaction is apparently exothermic (4, 24). It is noteworthy that catalysts are apparently necessary, since synthetic rubbers prepared from pure materials add sulfur slowly, if at all. The proteins and perhaps the resins in natural rubber undoubtedly serve as accelerators. The curves for combined sulfur vs. time of cure for typical mixes are shown in Figures 1 and 2. Figure 1 is taken from the data of Kratz and Flower (16); the composition and temperature of cure for this mix are shown in Cranor's Table I (9). Figure 2, curve 1, is from Table I of Eaton and Day (10), and curve 2 from data obtained in this laboratory (27, Table I). Superficial inspection of these curves shows extraordinary divergence of type. Figure 1 is a typical fadeaway curve, characteristic of most chemical reactions, where the reaction rate decreases with decreasing concentration of the reacting materials. Curve 1, Figure 2, is an entirely different type, where the rate of sulfur addition is constant until nearly 70 per cent of the initial sulfur has reacted. Curve 2, Figure 2, shows even more complex behavior. Again the rate is constant in the initial portions of the cure. However, following this period, the rate increases markedly but later falls off, approaching zero, to give an S-shaped eurve.
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