Gelation time, reduction

Figure 20.6. Left Gelation time reduction by pH adjustment by adding diluted base for sonosols submitted to an energy dose Us — 682 J/cm at 50 W. The pH increase is effective for the sonosol since it is poorly polycondensed. Right Calculated equivalence between pH adjustments for supplied dose of energy and Us. Lines are visual guides only. Adapted with permission from [39].

The effects of both pH and temperature of aluminum alkoxide hydrolysis on gelation is shown in Eigure 8. Addition of acid into the mixture hydrolyzed at 90°C, and by consequence reduction of pH, reduces the gelation time of the samples, whereas in mixtures hydrolyzed at room temperature, acidic addition increases gelation time. 

None of the Cr(III) products from Equations 6 or 7 are effective crosslinkers since a chromic aqua ion must be hydrolyzed first to form olated Cr to become reactive. Colloidal and solid chromium hydroxides react very slowly with ligands. In many gelation studies, this critical condition was not controlled. Therefore, both slow gelation times and low Cr(VI) Cr(III) conversion at high chromate and reductant concentrations were reported (9,10). 

In the absence of any salt, the electrostatic repulsion between the particles leads to stable sols without the formation of chains or aggregates. The addition of salt (e.g., NaCl) leads to a reduction in the thickness of the electrical double layer (Chapter 4), thereby reducing the stability of the sol and producing a dramatic reduction of the gelation time (Fig. 5.6). 

To determine whether pre-polymerisation has taken place after storage at elevated temperature, the gelation time can be determined. The powder from the physical stability test is used for this. From practical experience it is known that for an epoxy powder a reduction in gelation time of 10-15% at 40 C and 25% at 50 C as compared with freshly prepared powder may be considered acceptable. 

At x=l, the redox reaction was very slow and the UV-VIS absorption showed no change with time. The orange - yellowish color persisted for weeks, indicating that there was little or no reduction (i.e., poor conversion) of Cr(VI). At x=4 or more, development gf blue color occurred instantly, which is evidence of CrCHgOJg production via the acidic redox mechanism. At x=2 to 3, the green color of dated Cr(III) developed in minutes, followed by gelation of the polymer. 

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