Gelation stages

In general the amine-epoxy resin curing reactions show complex kinetics typified by an initial acceleration due to autocatalysis, while the later post-gelation stages may exhibit retardation as the mechanism becomes diffusion-controlled. However some workers 72 80) have found that over a limited range of conversion the kinetic data may be described by the simple models of Eq. (2-6) or (2-9). 

The ease with which plasticiser is combined with PVC is a measure of processing characteristics critical in the dry blend mixing operation of S-PVC or the gelation stage of plastisols. Predictive equations have been developed (349). 

Modification of organosilicon sorbents by metallic complexes in the gelation stage leads to the formation of a separate group of metal-containing FPS. Depending on the nature of the complex and a number of other factors the central metal atoms may or not become incorporated in the siloxane chain. In the first case a siloxane chain will contain chemically bonded heteroatoms, and the sorbents themselves can be of types A and B. In the second case, the nature of binding of the complex in a sorbent body may be quite different. 

Interesting enough, each of the above materials behaves as a thermoplastic at elevated temperatures, i.e., they are capable of being processed. Combinations of these materials form the basis for the newest category of IPNs thermoplastic IPNs. Obviously, they offer features heretofore unavailable in IPNs. While the SINs could be processed up to the gelation stage, the thermoplastic IPNs can be processed or reprocessed at any time. 

It is increasingly important to separate spatially (in the MF channel) the emulsification and gelation stages, so that buildup in viscosity does not influence emulsification. Emulsification of polymer solution with a subsequent removal of the solvent is the most straightforward method to form polymer particles. Importantly, in comparison with other two methods described above, this method allows generation of particles with dimensions significantly smaller than precursor droplets. In this case, the diameter of the resultant beads is determined by the volume of droplets and the concentration of the polymer. 

Our approach to the problem of gelation proceeds through two stages First we consider the probability that AA and BB polymerize until all chain segments are capped by an Aj- monomer then we consider the probability that these are connected together to form a network. The actual molecular processes occur at random and not in this sequence, but mathematical analysis is feasible if we consider the process in stages. As long as the same sort of structure results from both the random and the subdivided processes, the analysis is valid. 

The three isomeric diallyl phthalates are colorless Hquids of mild, odor, low volatiUty, and relatively slow polymerisation ia the early stages. At ca 25% conversion, the viscous Hquid undergoes gelation and polymerisation accelerates however, the last monomer disappears at a slow rate. 

Multicomponent sol—gel fibers have been successfuUy developed (1,52). The early stages of sol formation and gelation are cmcial for controlling the fiber microstmcture. Aluminosilicates, zirconates, and aluminates (1,18,52) can be prepared by sol—gel methods. MuUite [55964-99-3] 3AI2 O3 -2Si02, fibers... 

FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy technique. (1) Spherical particles tens of nanometers across can be seen in a colloidal silica sol. (2) Addition of a concentrated salt solution initiates gelation. (3) The gelled sample, after drying under the electron beam of the microscope, shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L. Mecartney, University of Minnesota. 

The metal ions are often added as salts of organic compounds, which form chelates. This causes a delayed gelation. Likewise, the components of the gelling agent are pumped down in two stages. Some metal cations cannot be used with brines. On the other hand, brines are often produced from wells, and it is desirable to find uses for them to avoid disposal processes. 

---