Epoxy compounds, polyaddition

First let us consider the initiation process in the presence of proton-donor compounds specially introduced into the system. Two contradictory viewpoints about the reaction mechanism may be distinguished in this case. One of them 138,139f l45,154) presupposes a molecular mechanism of the reaction, i.e. a stepwise polyaddition of the epoxy compound to the alcohol group, e.g. according to Scheme (32)... 

A multistep reaction, polyaddition, is a polyreaction of at least two bifunctional or higher functional compounds. Polyaddition can result in either linear polymers (thermoplastics) or cross-linked plastics (duroplastics), depending on the specific functionality. Cross-linked products are obtained by means of a reaction of a bifunctional reactant with a trifunctional one. The more polyfunctional the reactant, the more closely meshed the cross-linking will be. That is why the polyols in polyurethane or epoxy resin production are frequently replaced by polyesters and polyethers containing large numbers of OH groups. Polyaddition, like polycondensation, is a multistep reaction. Fig. 4. Important polyadducts include linear and cross-linked polyurethanes as well as epoxy resins (see [2]). 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

FIGURE 2.11 Polyaddition reaction of an epoxy resin with an active hydrogen compound. 

Curing agents or hardeners are chemically active compounds which convert epoxy resins into hard, infusible thermosets. They promote the cross-linking reaction either by polyaddition or by homopolymerisation. The curing reaction occurs without the formation of any by-product. 

Basically, there are two general types of epoxy curing agents active hydrogen compounds that react with the epoxies to cure them by polyaddition reaction and ionic catalysts. Among the first group are the aliphatic polyamines and their derivatives. 

The synthesis of elevated-molecular-weight (E-M) epoxy resins (solid epoxy resins) based on polyaddition of bisphenol-A and lower-molecular-weight (L-M) epoxy resin under microwave irradiation was recently described [117]. Usually, E-M epoxy resins are manufactured by the melt process in which liquid epoxy resins and organic compounds that contain two active hydrogen atoms (e.g., bisphenol... 

The main reactions arising from the epoxy groups are polyaddition, resulting from reaction with compounds containing labile hydrogen atoms such as polyamines, polyacids, polythiols, polyphenols etc, according to reaction (55), and polymerization of the epoxy groups via an ionic mechanism. 

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