Thermoset Polycondensation Polymers

This approach is used in several instances to control the arms, molecular weight, and crystallinity of the resulting polymer [52], The convergent approach is illustrated elsewhere [53, 54], This subject is not reviewed here in detail because a whole chapter of this handbook (Chapter 30) is dedicated to it. 

This section focuses on polycondensation reactions to synthesize thermoset polymers. Specific condensation chemistries are studied here a more extensive treatment of the subject of thermoset polymers can be found in Chapter 28 of this handbook. 

The most common applications of these materials are as paints and coatings. However, their uses are extremely diversified and include boat and marine construction 

In general, polyester resins are synthesized by the reaction between carboxylic acids and alcohols, with three or more reactive groups. Recently, unsaturated polyesters were incorporated in various ways to produce terminal, pendant, and internal double bonds [57-59]. In the case of unsaturated polyesters, maleic anhydride is most commonly used to produce internal unsaturation. The double bond present on unsaturated polyester reacts with a vinyl monomer, mainly styrene, resulting in a 3D crosslinked structure. This structure acts as a thermoset. The crosslinking is initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide (Fig. 3.18). 

Epoxy resins are among the most important of the high performance thermosetting polymers and have been widely used as structural adhesives and matrix for fiber composites. Epoxy resins are characterized by the presence of epoxide groups before cure, and they may also contain aliphatic, aromatic, or heterocyclic structures in the backbone. The epoxy group can react with amines, phenols, mercaptans. 

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Many reactions familiar to organic chemists may be utilized to carry out step polymerizations. Some examples are given in Table 2.2 for polycondensation and in Table 2.3 for polyaddition reactions. These reactions can proceed reversibly or irreversibly. Those involving carbonyls are the most commonly employed for the synthesis of a large number of commercial linear polymers. Chemistries used for polymer network synthesis will be presented in a different way, based on the type of polymer formed (Tables 2.2 and 2.3). Several different conditions may be chosen for the polymerization in solution, in a dispersed phase, or in bulk. For thermosetting polymers the last is generally preferred. 

Polycondensations can be carried out in an aqueous or a solvent medium, or they can be performed while the reactions are in a liquid or in a molten state. In industry, reactions of polyfunctional monomers leading ultimately to the tridimenaonal-network molecules of thermosetting resins are usually interrupted at a stage where the polymers still are soluble and fusible. They then are shipped to the fabricators, who convert them by heat curing processes into the final thermosetting product. 

So, in this chapter the interest and importance of the polymer gel theory approach to the formulation of adhesives will be briefly shown and, in particular, we will limit this to thermosetting wood adhesives obtained by polycondensation such as phenol-formaldehyde-, urea-formaldehyde-, melamine-formaldehyde- and resorcinol-formaldehyde-based adhesives. The same approach is, however, very valid for other polycondensation resins, and also for adhesives obtained by routes other than polycondensation. 

First level every constituent is considered as monodisperse. For a modi-fied-thermosetting polymer undergoing polycondensation, this approximation level leads to a quasi-binary system consisting of... 

The FH model will be applied to a modified-thermosetting polymer undergoing polycondensation. First, both constituents will be considered as monodisperse (quasi-binary approach). Then the analysis will be performed taking polydispersity of both constituents into account. While the first analysis will enable us to discuss the most significant aspects of the phase separation process, the more refined model will provide an explanation of some experimental observations like the presence of a secondary phase separation process. 

At this point we are assuming a very fast phase separation rate such that the system attains the equilibrium condition for every conversion. At the conversion where the binodal curve is first attained (p p = cloud-point conversion), a differential amount of a new phase, with a composition located on the other branch of the binodal, appears in the system. In this sense, it is correct to assess that the new phase will be the dispersed one. However, as polycondensation of the thermosetting polymer proceeds, a macroscopic amount of the initially dispersed phase will be present in the system. At this stage, the distinction between continuous and dispersed phases will depend on other factors such as interfacial energies, viscosities, volume ratio of phases, etc. 

Reaction polymerization reactions of isocyanates with suitable monomers can he performed in an extruder or in a RIM machine. In the latter reaction thermosets (cross-hnked polymers) are produced. In an extruder usually linear polymers are manufactured. For example from methylene di-p-phenylene isocyanate (MDI), with some macroglycols and 1,4-hutanediol as extenders, segmented polyurethane elastomers are produced in an extruder (6). However, linear condensation polymers are also produced in a vented extruder. For example from MDI, with macrodicarboxylic acids and dicarboxyhc acids as extenders thermoplastic block copolyamide elastomers are produced. The by-product of the condensation reaction, carbon dioxide, is removed in the vented extruder. The polycondensation process can also be performed in solution. For example, MDI can be added to a solution of dicarboxyhc acids in tetramethylene sulfone, with simultaneous removal of the carbon dioxide. Tetramethylene sulfone is the solvent of choice for solution polymerization of isocyanates (7). In addition to dicarboxyhc acids trimellitic acid anhydride and benzophenonetetracarboxylic acid dianhydride (BTDA) are utilized as monomers for condensation polymers. With these monomers poly(amide imides) and poly(imides) are produced. The diisocyanate-derived commercial polycondensation products are listed in Table 1. 

Polycondensation of Phosphoryl Dihalides with Diols. Poly(arylene aryl phosphate)s can be made stepwise by first preparing a phenyl phosphorodichlori-date which reacts with a dihydric phenol, or in one step by the reaction of phosphorus oxychloride with a mixture of monohydric and dihydric phenols. Depending on the reactant ratio, the products can be oligomers or high molecular weight thermoplastic or thermoset polymers (129). 

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