Curing epoxy ring

Epoxy/Carboxy Cure Sites. Epoxy/carboxy cure sites probably represent the most important alternative to labile chlorine containing monomers. There has been increasing interest in them due to the discovery of the highly efficient quaternary ammonium salt-based accelerators (29—34). The reaction between the epoxy ring and carboxyUc acid can happen in the following three ways ... 

They will be formed as epoxy rings are opened during cure. 

Biernath et al. concluded that phenolic novolac and epoxidized cresol novolac cure reactions using triphenylphosphine as the catalyst had a short initiation period wherein the concentration of phenolate ion increased, followed by a (steady-state) propagation regime where the number of reactive phenolate species was constant.85 The epoxy ring opening was reportedly first order in the steady-state regime. 

For commercial application, diepoxides such as those derived from bisphenol A are employed, and they are cured via ring-opening crosslinking reactions, into which the epoxy group enters readily. Bisphenol A is so-called because it is formed from two moles of phenol and acetone (Reaction 1.7). 

The ion thus produced may itself react with another epoxy-group in a process which forms the first crosslink (Reaction 4.16). This reaction may occur at both ends of the molecule of the diglycidyl ether, so that a crosslinked stmcture can easily be built up from these substances. Reaction becomes complicated by the fact that the epoxy-group may also react with the hydroxy-groups that form as the epoxy ring opens up during cure. Thus the finished resin may contain a complicated array of structures within the three dimensional network. 

Properties of Epoxy Resins Cured with Ring-Alkylated m-Phenylene Diamines... 

Inertness The cured epoxy resins are chemically inert. The ether groups and benzene rings are virtually non-vulnerable to caustic attack and are extremely resistant to acids. Chemical inertness of an epoxy resin is enhanced by the dense and closely packed structure which is extremely resistant to solvent action. 

The specimen used for ITS testing can be any section of the composite, provided that after mounting and polishing, fibers normal to the polished surface can be found. A section of composite approximately 1 cm x 1 cm was cut from the laminates previously prepared. The sections were embedded in 1 in. diameter phenolic ring mounts with room temperature-cured epoxy mounting media. The sections were oriented so that the fiber ends were normal to the face of mount to be polished. The initial steps of the polishing were accomplished on a Struers Abramin polisher and the final step was a on Beuhler Vibromet I vibrating lap polisher. The steps used are listed in Table 2. 

The polyaddition reaction involves epoxy groups reacting with primary and secondary amines. Amines are the most commonly used curing agents/hard-eners for epoxides. One epoxy ring reacts with each aminoproton ... 

Crosslinking has no specific direct effect on thermal degradation crosslinks can be either weak points (e.g., tertiary carbons in polyester or anhydride-cured epoxies) or thermostable structural units (e.g., trisubstituted aromatic rings in phenolics, certain epoxies, or certain thermostable polymers). Indirect effects can be observed essentially above Tg crosslinking reduces free volume and thus decreases 02 diffusivity. It also prevents melting, which can be favorable in burning contexts. 

Reactive groups other than epoxy rings may be present in the molecule. Hydroxyl and olefinic groups are the most common. These can affect reactivity as well as the course and sequence of the polymerization reactions. Their number and location could accelerate or retard the overall reaction rate and lead to significantly different three-dimensional cured polymeric structures. Catalytic sites (e.g., tertiary nitrogen) on the epoxy molecule can also influence the reactivity and favor certain reactions. 

In the well-recognized epoxy-aliphatic amine reaction, the primary or secondary amine adds to the epoxy ring, forming a tertiary amine, as shown in Fig. 5.2 (top). The formed hydroxyl groups accelerate the amine curing, and with excess epoxy present, the secondary hydroxyl groups can also add to the epoxy ring, as shown in Fig. 5.2 (bottom). 

Pyromellitic dianhydride (PMDA) is a solid having a melting point of 286°C. It contains two anhydride groups symmetrically attached to a benzene ring. Because of the compactness of the molecule, PMDA achieves very high crosslink densities and, therefore, high heat and chemical resistance. PMDA cured epoxy adhesives have a heat distortion temperature on the order of 280 to 290°C. 

The effect of solvent type on the curing rate of epoxy reactions has been well defined. Hydroxyl compounds, such as alcohols, act as catalysts and accelerate curing. However, these solvents are not serious competitors with amines for reacting with the epoxy ring. Water, functioning as a hydroxyl compound, also accelerates the reaction, even more than alcohols. Aprotic solvents, such as aromatic hydrocarbons or mineral spirits, have no effect on the amine-epoxy resin and behave as inert diluents. Carbonyl solvents, such as acetone and methyl ethyl ketone, retard the reaction. 

The isocyanate reaction is several orders of magnitude faster than the epoxy reaction. Hence, attempts to cure mixtures of epoxy and isocyanate resins must provide for essentially complete consumption of all -NCO groups before any of the epoxy rings can react. 

Figure 1 shows kinetic curves of the DGER-mPhDA (P = 1) cure reaction at different T.ure. From these curves, the existence of the conversion at which the reaction stops due to diffusion control, adlf, can be seen. The measurement of the total reaction heat Q at different Tcure and its comparison with the estimated value based on the specific heat of epoxy ring opening give the values of adif for any T 16). 

Hydrogen bonds are one of the most representative structural features of epoxy polymers. Each step of the cure reaction involving epoxy ring opening forms one OH group, whose concentration in a cured polymer is high. 

Another curing mechanism is operative in the curing reaction initiated by a polymerization catalyst for epoxy ring opening, where anionic catalysts such as tertiary amines 1S-l9) and imidazols 20 21> and cationic catalysts such as amine complexes of... 

Epoxidized oils such as soybean oil or linseed oil, which are made by oxidizing the internal unsaturation in the fatty acid chain, find much use as plasticizers, particularly in vinyl resins. An internal epoxy ring is much less reactive than a terminal epoxy ring and usually does not take part in typical epoxy curing reactions under mild conditions. These materials can be used as plasticizers in epoxy adhesives but cannot really be considered reactive diluents. 

A typical step-growth reaction is one in which a single epoxy ring reacts with the active hydrogen of the curing agent. The general reaction is ... 

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