Addition polymerization gelation

Falling off of the log viscosity-time cure curve can also occur due to dewetting, or wall slip, at the cone-sample interface. This occurs rarely, in our experience, and probably more with highly filled samples as gelation is approached. A kinetic order greater than unity could also produce a curvature similar to C, but evidence of such kinetic orders for condensation polymerization reactions has not been reported from chemorheological studies, to our knowledge. (Certain types of addition polymerization reactions may show non-first-order viscosity kinetics.)(13)... 

In contrast, chemical gelation involves formation of covalent bonds and always results in a strong gel. There are three main chemical gelation processes condensation, vulcanization, and addition polymerization. 

Hiese, M. Martin, G. Gotro, J. Gelation in thermosets formed by chain addition polymerization. Polym. Eng. Sci. 1990, 30 (2), 83. 

Addition Polymerization. Gelation by addition polymerization involves a chain reaction in which a free radical is capable of reacting to open the double bond of a vinyl monomer resulting in polymerization by linear addition of these units. 

In the paste coating method, a PVC paste, which contains emulsion-polymerized PVC and additives, is appHed onto a substrate and heated to gelation before fusion to produce a coating layer. This method is employed for products with a thin layer, ie, of 0.007—0.05 mm thickness. For foamed vinyl-coated fabrics, a substrate is laminated onto a transfer paper on which a PVC paste containing a foam-blowing agent has been appHed and geUed. After removal of the transfer paper, the paste is blown. 

Addition of dialkyl fumarates to DAP accelerates polymerization maximum rates are obtained for 1 1 molar feeds (41). Methyl aUyl fumarate [74856-71-6] (MAF), CgH QO, homopolymerizes much faster than methyl aUyl maleate [51304-28-0] (MAM) and gelation occurs at low conversion more cyclization occurs with MAM. The greater reactivity of the fumarate double bond is shown in copolymerization of MAF with styrene in bulk. The maximum rate of copolymerization occurs from monomer ratios, almost 1 1 molar, but no maximum is observed from MAM and styrene. Styrene hinders cyclization of both MAF and MAM. 

In addition to acting as impact modifiers a number of polymeric additives may be considered as processing aids. These have similar chemical constitutions to the impact modifiers and include ABS, MBS, chlorinated polyethylene, acrylate-methacrylate copolymers and EVA-PVC grafts. Such materials are more compatible with the PVC and are primarily included to ensure more uniform flow and hence improve surface finish. They may also increase gelation rates. In the case of the compatible MBS polymers they have the special function already mentioned of balancing the refractive indices of the continuous and disperse phases of impact-modified compound. 

Polymerization inhibitors are key additives which prevent premature gelation of the adhesive. The foimulator must carefully balance shelf stability and the required cure on demand. Due to its high propagation rate, MMA is difficult to inhibit. Some comments on specific inhibitors follow. The most common inhibitor to be found in component monomers is 4-methoxyphenol, which is also called the methyl ether of hydroquinone. This inhibitor is effective only in the presence of oxygen. A mechanism has been proposed, and is illustrated in Scheme 13 [128]. 

In the absence of co-reactants, it is supposed that the polymerization is promoted by traces of water or other nucleophiles, since very pure monomer does not gel even after extended heating. Conversely, gelation may be accelerated by addition of phenols such as bisphenol A. Dynamic mechanical analysis of cured resins confirms that they are practically identical whether or not the phenol is added. 

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