Effect on gel time

The experimental result reveals that pH value has great effect on gel time. From the result shown in Table 3, it can be seen that gel time increases with the fall of pH value. When pH value is equal or down to 3, it is not gel body but white precipitate that is obtained when mixing solution is heated in water-bath at 343K for 8 h. When the pH value is 4 5, transparent gel can be obtained. 

Figure 11.18 Effect on gel times of 10% acrylamide grout of various catalyst and activator concentrations. (Courtesy of American Cyanamid, Wayne, NJ.)...

Our work (26) shows that carboxylic acids have no unusual catalytic effect on gel times when gel time versus pH (pH was obtained with a... 

Our work [26] shows that carboxylic acids have no unusual catalytic effect on gel times when gel time versus pH (pH was obtained with a glass electrode in the reaction mixture) was plotted over the range of pH = 0 — 7 for various catalysts (Figure 48.2). The reaction conditions of Mackenzie and coworkers [22,27] were used (TEOS ethanol water acid = l 4 4 var monomer and alcohol were mixed 30 min prior to water addition). The discrepancy in the catalytic effect of carboxylic acids results from Mackenzie and coworkers comparison of acids at similar concentrations, which is difficult with the wide range of p T values. Acetic acid is weaker than HCl and thus provides a higher solution pH at the same concentration. This property is accentuated in alcohol because acetic acid acts as a weaker acid in alcohol (versus water) whereas HCl maintains a more constant acid strength [28]. The sinusoidal curve obtained for gel time versus pH plots for each of the acids is consistent with the results of Pope and Mackenzie [22] for HCl-catalyzed sols and with Iler for gelation of Si(OH)4 [23]. Some differences in gel times were observed near pH 2,... 

All fillers will increase the viscosity of resins most fillers also influence the gel time. Gelation is normally retarded, but alumina and some types of china clay have the reverse effect. It is difficult to predict the precise effect on gel time of particular filler and this should be determined by experiment in each case. Fillers also reduce the volumetric shrinkage of resins when curing, as well as peak exothermic temperature. 

For any of the sol-gel parameters discussed so far, its effect on gel properties can often be followed by an experimental observable known as gel time. Gel time is defined as the time it takes for a solution to undergo a rapid rise in viscosity that corresponds to the transition from a viscous fluid to an elastic gel [9]. At the gel point there is a continuous solid phase containing a structure that reflects the formation and branching of particles under specific growth conditions. This particular phase is important because it is the genesis of structural evolution that takes place in all subsequent processing steps [9, 12]. 

The significant effect of NaOH/phenol ratio on gel time is plotted in Figure 5. The fastest gelation rate occurred between NaOH/phenol molar ratios... 

Figure 5. Effect of NaOH/phenol ratio on gel time of phenolic resins.

The gel time of a grout solution can also be affected by chemicals dissolved in formation groundwater or by contact with the formation itself. (The effects of groundwater on gel time can be canceled by using groundwater to mix the grout solution. This is discussed in greater detail... 

Gel permeation columns were selected for this task because (1) the highly polar nature of the analytes precludes adsorption chromatography (the chlorosilanes and their tin analogues are too strongly adsorbed) and (2) because the required removal of water from solvents would have much less effect on retention times. 

FIGURE 4.1 Influence of modifier HUM-01 (trimethyl-hexamethylene-diamine + propylene carbonate) on gel time of epoxy composition based on D.E.R. 331. (Reprinted from O. Figovsky, O. Birukov, L. Shapovalov, and A. Leykin, Hydroxyurethane Modifier as Effective Additive for Epoxy Matrix, Scientific Israel Technological Advantages 13, no. 4 (2011) 122-128. With permission.)... 

Probably the most widely studied and important reaction variables are pH and catalyst. Pope and Mackenzie (22) studied the effects of catalysts on gel times of TEOS reactions. Such studies combine information on catalytic effects for both hydrolysis and condensation. The gel time of HCl-catalyzed TEOS reactions plotted as a function of pH is a sigmoidal shape (Figure 1). This plot is consistent with Iler s results for polymerization of Si(OH)4 (23). Further, Pope and Mackenzie concluded that gel times depend on the catalytic mechanism of the catalyst as well as the pH, because sols catalyzed by HF and acetic acid have surprisingly short gel times (Table I). 

Di(tert Butyl)p-cresol techn. pure powder 40% solution in xylene (SETA flash point 30) Prolongs (up to weeks/months) shelf life - gel time of resin + peroxide - at ambient temperature effect on cure times diminishes with rise in temperature efficient with many types of resin and peroxide fgHI... 

Table 5.3 Effect of CTPEHA modification on gel time, T and T of modified epoxy cured with DETDA [64] ...

Figure 3.4. Effect of pH on gel time at 25°C of silicic acid sols of different concentrations made from HjSO< and sodium silicate Curve 1 1.09 A/, 65.4 gl" SiOj. Curve 2 1.33 A/. 79.8 gl SiOj. Curve 3 1.78 M, 106.8 gl" Si02. [From Vysotskii and Strazhesko (49).]...

Figure 3.14. Effect of fluoride ion on gel time of silicic acid sols line H represents effect of hydroxyl ion (Her). Below the isoelectric point, pH 2, the gelling rate (inverse of gel time) is proportional to H ion concentration and also to F ion concentration, provided the concentrations of AP and Fe impurities remain constant. Thus the catalyst is HF. [From Her (89).]...

With particles of a given size (14 nm) at 25 C and in the presence of 0.1 NaCl, the relation between gel time and silica concentration is not simple. As shown in Figure 4.14, the effect of silica concentration on gel time is not the same at various pH values. 

The effects of monovalent electrolytes on gel time have been reviewed by Mookeijee and Niyogi (219), who reported results on silicic add sols at pH 2.7 where the salt e ect is less marked, whereas most previous work was done at higher pH. The effects of cations to accelerate gelling increased in the order Li, Na, and K and of anions in the order NOj, Cl, Br, and I. For some unexplained reason I" had about fivefold the effect of NOj. The effects were directly proportional to... 

---