Gelation time

The free radicals initially formed are neutralized by the quinone stabilizers, temporarily delaying the cross-linking reaction between the styrene and the fumarate sites in the polyester polymer. This temporary induction period between catalysis and the change to a semisoHd gelatinous mass is referred to as gelation time and can be controUed precisely between 1—60 min by varying stabilizer and catalyst levels. 

Fig. 5. Influence of catalyst systems on cure rate gelation time is at 25°C as a function of the initiator concentration. A represents MEKP (1.0%) B, MEKP...

Gelation. A sol becomes a gel when it can support a stress elasticaUy, defined as the gelation poiat or gelation time, C A sharp iucrease ia viscosity accompanies gelation. A sol freezes ia a particular polymer stmcture at the gel poiat (27). This frozen-ia stmcture may change appreciably with... 

Fig. 2. Loss tangent as a measure of gelation time for a siUca sol (27) (a) loss (A) and storage (B) modulus as a function of aging time for H2O/TEOS mol ratio of 20 and HNO /TEOS mol ratio of 0.01 and (b) loss tangent as a function of aging time. To convert mPa to mm Hg, multiply by 7.50 x 10 . ...

The amount of water employed for hydrolysis also has a dramatic influence on gelation time. Eor an R, ie, mole ratio of water to siUcon alkoxide, of 2, is about 7 hs when the gelation process is held at 70°C and HF is present as catalyst for i = 8, / decreases to 10 min (3). For low water contents, an increase in the amount of hydrolysis water decreases the gelation time, although there is a dilution effect. For higher water content, the gelation time increases with the quantity of water. 

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. 

The end point may be ehecked by noting the extent of flow of a heated pellet down a given slope or by melting point measurements. Other control tests include alcohol solubility, free phenol eontent and gelation time with 10% hexa. 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

In order to increase the capacity of a production line especially by reducing the necessary press times, adhesive resins with a reactivity as high as possible should be used. This includes two parameters (1) a short gelation time and (2) a rapid bond strength increase, and this even at a low degree of chemical curing. 

The storage modulus (G ) was recorded at a frequency of IHz under 0.015 strain amplitude until stabilization of the protein network. In order to reduce stress in the sample, G recording started just before the gelation time which corresponds to the time at which G deviated from the baseline. Data were collected and rheological parameters were calculated using Carri-Med 50 software. For each system, the experiments were performed in triplicate. 

By endcapping PTMO with trifunctional silanes, the resulting PTMO molecules will have a functionality, f, of 6. This should reduce the gelation time considerably and potentially enhance the dispersion of the PTMO species. 

Occasionally it may be desirable to have a rapid crosslinking take place. Blends of chromium triacetate and hydrochloric acid have been used in this situation (212). Gelation time decreases substantially as applied shear increases (213,214). Thus, static laboratory gelation time experiments should not be used to predict gelation time in actual well treatments. 

Table II. Variation of Gelation Time by Different Cr(III) Sources...

Gelation time of a 2000 ppm Flocon 2% NaCl solution with 90 ppm Cr(III) according to Equation 5 was 2 weeks (Table II), which is in the range of the Cr colloid gelation discussed earlier. Based on the earlier discussion, the gelation reaction of redox generated Cr(III) can also be accounted for with the olation mechanism. However it is... 

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). 

One ofthe main reasons for the catalysis of reactions represented by Equations (2)-(4) is based on the fact that there was no j ellification ofthe solutions with biopolymers in the neutral region after a month. When the processes could also proceed with the addition of an acid or alkali in the absence of biopolymers, one could observe a drastic acceleration. The gelation time could be decreased from hours to a few minutes once polysaccharides and proteins were added. 

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