Time of solidification

Gel for which the time of solidification after discontinuation of a relatively high shear rate, is reduced by applying a small shear rate. 

Thomasow (1968) used a thrombelastograph for measuring the clotting time of milk. This device also measured the elasticity of the coagu-lum as it is formed, and gives information about the time of solidification of the curd. Clotting times measured with this device are longer than those determined visually. 

Thixotropic gelation may be considered as a form of flocculation and is sensitive to all kinds of additions. The time of solidification (T) is strongly dependent on the concentration (c) of the electrolyte, as seen in the empirical equation given by Schalek and Szegvari (1923),... 

It has been shown that thixotropic gelation is a slow coagulation. Time of solidification is also changed with a change in the pH of the solution as mentioned by Freundlich. It is seen that alcohol promotes the thixotropic gelation and after evaporating the alcohol, the sol returns to the gel state. 

As seen from Schalek and Szegvari s experiments, the increase of temperature will decrease the time of solidification. 

From the front side we now have exact solidification ages for the rocks of the major maria. The numbers in Table 11 refer to the most recent lava flows detected and represent the time of solidification of the outermost layer. 

Fig. 7. Dynamic-static crystallization of cocoa butter. Effect of the shear time, on A. finish time of solidification, B. density of nuclei sites. Dynamic stage is at 600 rpm, and large dots are the results for purely dynamic experiments.

There may be certain lubricants (with very high aromatic content) which when mixed with equal volume of aniline may remain completely miscible and separation into different phases may not be observed even at the time of solidification. For determining such low aniline imiiits, 1 volume of sample is mixed with 2 volumes of aniline and 1 volume of a suitable diluent (n-hexane or -heptane). Addition of the diluent lowers the miscibility of aniline with the sample and so with decrease in Icmjrerature, separation of phases can be easily observed. The equilibrium solution temperature observed under these conditions is known as Mixed Aniline Point, which can be used in the. same way as the Standard Aniline Point. 

A quantitative estimate of thixotropy should at least contain two different kinds of information, one about the strength of the gel formed and the other about the rate of solidification. In the determination of the time of solidification, introduced by Schalek and Szegvari and much used by FREunoLiCH, the rate of gelation is measured, but ail different samples are compared at one and the same gel strength. 

Fig. 11. Time of solidification of an iron oxide sol as a function of the concentration of electrolytes.

On raising the temperature the time of solidification is shortened as is the time of coagulation. The energy of activation of 22f,000 cal. calculated form Schalek and Ss SGYARi s experiments would correspond to an energy barrier of about 35 kT, which is rather larger than the expected value. 

Paints and printing inks arc usually thixotropic. If the pigment were completely stabilized on standing it would form a very compact sediment, which is not easily redispersed. Moreover, after brushing the paint on a vertical surface, it would have a tendency to flow off. On the other hand of course, the system should be capable of being brushed out and therefore behave like a liquid. Its time of solidification should be just long enough to allov/ the strokes of the brush to be levelled out. A time of solidification zero is an undesirable property in a paint (hence the name false body for this property). 

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