Vitrification effect

Finally, we noticed that in the last stages of the reaction, in contrast to the neat systems for which vitrification limited the conversion to about 0.9 at Tx = 180 °C, the conversion of all the rubber-modified systems easily reached 0.95 or more, a result indicating that the vitrification effect was delayed and suggesting that part of the rubber dissolved in the matrix and decreased its Tg(t). The same conclusion could be drawn from the glass-transition temperatures of the final networks (Tg00), which are displayed in Table III. Such an effect was not observed for the Unilink 4200-DPEDC mixture (without rubber). 

Partial vitrification is also observed in isothermally cured epoxy systems. However, the effect is less pronounced since the glass transition domain at goo is narrower for these networks [80]. An example is given in Figure 2.14 for the system DGEBA-MDA T oo = 102°C). At 80°C, a stepwise decrease in Cp and a relaxation peak are observed. At 100°C, the system is partially vitrifjdng and the phase angle remains in the relaxation regime at the end of cure. At 120°C, no vitrification effect is noticed anymore, neither in Cp, nor in heat flow phase. 

Polymerization proceeds quite rapidly in the first step, then slows down markedly because of the vitrification effect (Tg of cured fihn= 190 °C) as a consequence a large amoimt of epoxy groups remain trapped into the glassy polymer network because of the diffusion prevention. 

It is possible to explain these results by taking into accoimt that the FOX monomer, being a monofunctional additive, induces a decrease of the crosslinking density of the network. As a consequence, the network structure is more flexible and the mobility of the reactive species is retained so that the polymerization can be completed. In other words, the decrease of the functionality of the reactive system delays the vitrification effect. 

In contrast to the shift of the sulfur monomer/polymer floor temperature to higher temperatures, recent work from our laboratory shows that nanoconfinement shifts equilibrium in free radical methyl methacrylate (MMA) polymerization towards monomer [93]. Results are shown in Fig. 11.9 again for MMA polymerized by 0.5 wt% AIBN, where the equilibrium conversion is plotted versus temperature. Conversions at low reaction temperatures are less than 100 % due to the effect of vitrification (i.e., the reaction cannot reach equilibrium because the reaction mixture turns to a glass and the reaction stops prior to 100 % conversion being reached). As the reaction temperature approaches and rises above the glass transition of the neat polymer (Tg p 110 °C), vitrification effects disappear and the conversion reaches the maximum value of 1.0. At higher temperatures, conversion again decreases. 

Poly(ethylene oxide). The synthesis and subsequent hydrolysis and condensation of alkoxysilane-terniinated macromonomers have been studied (39,40). Using Si-nmr and size-exclusion chromatography (sec) the evolution of the siUcate stmctures on the alkoxysilane-terniinated poly(ethylene oxide) (PEO) macromonomers of controlled functionahty was observed. Also, the effect of vitrification upon the network cross-link density of the developing inorganic—organic hybrid using percolation and mean-field theory was considered. 

Some of the most common stabilization—soHdification processes are those using cement, lime, and pozzolanic materials. These materials are popular because they are very effective, plentiful, and relatively inexpensive. Other stabilization—soHdification technologies include thermoplastics, thermosetting reactive polymers, polymerization, and vitrification. Vitrification is discussed in the thermal treatment section of this article and the other stabdization—soHdification processes are discussed below. 

Vitrification is effective at destroying and immobilizing hazardous materials, but it is very energy intensive and thus expensive. Consequently, it is used primarily where wastes are difficult to treat or destmction—immobilization of contaminants is very important such as with radionucHdes. 

Effective produces a high rate of vitrification and remediation. 

Influent particle sizes must be less than 600 xm in diameter. Additives are required for the effective vitrification of some wastes. The quality of the produced glass product depends on the distribution of glass formers and fluxes in the feed material. 

The use of inorganic ion exchangers to solidify liquid radioactive waste followed by pressure sintering to produce a ceramic waste form appears to be a viable alternative to calcina-tion/vitrification processes. Both the process and waste form are relatively insensitive to changes in the composition of the waste feed. The stability of the ceramic waste form has been shown to be superior to vitrified wastes in leaching studies at elevated temperatures. Further studies on the effects of radiation and associated transmutation and the influence of temperature regimes associated with potential geologic repositories are needed for a more definitive comparison of crystalline and amorphous waste forms. 

Mendel, J. E., Palmer, C. R., and Eschback, E. A., "Preliminary Assessment of Potential Effects of Alternate Fuel Cycles on High-Level Waste Vitrification Processing," Symposium on Waste Management and Fuel Cycles 78," Edited by R.G. Post and M. E. Wacks, Tucson, AZ, March (1978). 

The vitrification of liquids and polymers can be effected not only by the decrease of temperature but also by the increase of pressure [4]. Shishkin and Novak [5] obser ved the dependence of free hydroxyl concentration upon pressure. This effect can be described by Eq. (6) or (8) in the same way as the course of vitrification upon decreasing temperature. 

Permeability and hydraulic conductivity are the controlling factors in the effectiveness of in situ treatment technologies. The ability of soil flushing fluids to contact and remove contaminants can be reduced by low soil permeability. Low permeability can lessen the volatilization of VOCs in soil vapor extraction or limit the effectiveness of in situ vitrification by slowing vapor releases. 

The construction of a mold-filling model has been considered in the theory of thermoplastics processing. A rapid increase in viscosity also occurs in the flow of these materials, but the effect is different than in flow during reactive processing. The increase in viscosity of thermoplastic polymer materials is due to physical phenomena (crystallization or vitrification), while the increase in viscosity of reactive liquids occurs due to chemical polymerization reactions and/or curing. This comparison shows that the mathematical formulation of the problem is different in the two cases, although some of the velocity distributions may have similar features. 

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