Time of gelation

Theories of Gelation. The classical or mean field theory of polymeri2ation (4) is useful for visuali2ing the conditions for gelation. This model yields a degree of reaction, of one-third at the time of gelation for chemical species having functionaUty equal to four. Two-thirds of the possible... 

Stein166 has indicated that the reactivity of the terminal double bond of the macromonomer (112) is 80% that of VAc monomer. The kinetics of incorporation of 112 have also been considered by Wolf and Burchard175 who concluded that 112 played an important role in determining the time of gelation in VAc homopolymerization in bulk. 

Fig. 7. An optical transmitted light micrograph of a single H micron carbon fiber in an epoxy matrix. The fiber had not been degassed before encapsulation. The volatile material on the surface vaporized around the time of gelation creating large voids around the fiber...

The time of gelation changes significantly with sol-gel chemistry. One method of measuring determines the viscoelastic response of the gel as a function of shear rate. 

The adhesive properties of epoxy resins coupled with their dielectric behavior have made them attractive to the electronic industry. The evaluation of thermally cured rubber modified epoxy thermosets has been the subject of recent studies (1, 2), which dealt with the dependence of morphology on the curing parameters, e.g., catalyst, cure schedule, time of gelation, etc. This work utilizes one of the new series of photocationic initiators (PCI) developed by Crivello, et al (3) which are presently commercially available. These onium salts initiate the reaction by absorbing the actinic radiation, generating radicals and producing a protonic acid. The radicals can lead to polymerization of olefinic moieties (4) while the acid initiates the polymerization of the epoxy groups (3). 

Selection of the proper catalyst and the amount to be used for any application depends on the resin, the temperature at which the resin is to be cured, the required working or pot life, and the time of gelation. No catalyst is available that can meet all the requirements. Therefore, combinations of catalysts, or of catalysts and accelerators, must be used to obtain the best results. 

The maximum exothermic temperature reached, the time required for the reaction to attain peak exothermic temperature, and the time of gelation are important factors to be considered when selecting a resin. Reactivity tests provide a method for determining the behavior, uniformity, and curing characteristics of a resin. The use of a resin for a specific application often depends on the reactivity of the resin. Measurements of reactivity are helpful in the evaluation of accelerator, catalysts, and other materials that must be considered for the correct use of the resin. 

Kozlov, G. V. Malkanduev, Yu. A. Afaunova, Z. I. Zaikov, G. E. Hie time of gelation in radical polymerization of dimethyl diallyl ammonium chloride fractal analysis. J. Balkan Tribological Association, 2002,8(1), 35-39. 

Times of gelation and point of phase separation can be described mathematically by the relation (23) t = fcV" (10)... 

Time of Gelation Plotted Versus Functionality of the Novolac Synthesized from 4,4 -(l -Methylethylidene)bisphenol Reacting with 2,2 -[(l-Methylethylidene)bis-(4,l-phenyleneoxy-methylene) bis-oxirane] (DGEBA) (Epoxy Groups to Hydroxy Groups = 2 1 0.65 mole-% Imidazole Relative to Epoxy groups) at Different Crosslinking Temperatures (data from ref. [33])... 

Influence of the Functionality of Epoxidized 4,4 -(l-Methylethylidene) bisphenol Novolac Prepolymers on the Time of Gelation during Crosslinking with 4,4 -Diaminodiphenyl-methane at Different Temperatures (epoxy groups amino hydrogen=l) (data from ref. [91 ])... 

The suggestion is that if a mixture of monomers are simultaneously pol3nnerizing, then that portion of the material already incorporated in the network at the time of gelation may tend to be more continuous in space and dominate the properties. That material polymerized later in time, statistically, may form less continuous domains and act like filler to a greater or lesser extent. Partial confirmation of this concept has already been obtained in this laboratory (29,30). 

IMC also affects the timing of gelation and the properties of the developed gel. It consumes the chromium crosslinker that would otherwise be available for formation of intermolecular crosslinks, and thereby delays the development of the gel network. It is also suspected that IMC may be the primary factor in preventing a mechanically degraded Cr(III)-PAAM gel from reforming, as observed in our laboratory. 

The timing of gelation, physical properties of developed gels, and gelation behavior in the reservoir can be significantly affected by IMC, when it occurs to an appreciable degree. 

The profiles of gelation curves determined for amine-cured epoxy resins correspond to the variation of CL intensity on time and the narrow differences between the times of gelation obtained from chemiluminescence experiments and rheological analysis were reported (Fig. 53)[95K1]. 

Mix of solutions Concentration of DHA-24, % Concentration of MCTS, % The time of gelation, sec... 

Sample Ratio of moles DHA-24 MCTS-2 Time of gelation, sec. 

Figure 3. Evolution of the SAXS intensity during a one-stage polymerization of the DGEBA-D2000-TEOS hybrid. O reaction mixture, dilute solution reaction time with respect to time of gelation tgei t/tgei = 0.24 (1), 0.32 (2), 0.48 (3), 0.72 (4).

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