Homopolymerization epoxy

Recently, FTIR spectroscopy studies have been reported which support the above observations. Moacanin et al 3) concluded that two reactions dominate the TC3fDA/DDS cure epoxy-primary amine addition is the principal reaction occurring during the early stage of cure followed by the epoxy-hydroxyl addition reaction. Indeed they find that the rate of epoxy-hydroxyl addition is at least an order of magnitude slower than for the epoxy-primary amine reaction at 177 C. Furthermore, Morgan et al (4) report that the epoxysecondary amine addition and epoxy-epoxy homopolymerization reactions also occur at 177°C but at rates that are approximately 10 and 200 times slower, respectively, than the epoxy-primary amine react ion. 

Chemical clusters can be obtained also with two monomers, when two reaction mechanisms are in competition, favoring formation of regions of higher and lower crosslink densities. This situation is more complex and more difficult to control. It is certainly the case for dicyanodiamide (Dicy)-cured epoxies with this hardener an accelerator is always used and a competition between step (epoxy-amine addition) and chain (epoxy homopolymerization) occurs (Chapter 2), leading to inhomogeneous networks. 

Tetraglycidyl ether of tetraphenolethane is an epoxy resin that is noted for high-temperature and high-humidity resistance. It has a functionality of 3.5 and thus exhibits a very dense crosslink structure. It is useful in the preparation of high-temperature adhesives. The resin is commercially available as a solid (e.g., EPON Resin 1031, Resolution Performance Polymers). It can be crosslinked with an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur. 

Since epoxy homopolymerization may be neglected in the absence of catalysts (T), the major cure reactions can be assumed to be the reactions between epoxide and amine groups as expressed in Scheme I. 

Theoretical treatment of this polymerization is difficult because of the presence of both primary and secondary amine reactions as well as tertiary amine catalyzed epoxy homopolymerization. To obtain kinetic and viscosity correlations, empirical methods were utilized. Various techniques that fully or partially characterize such a system by experimental means are described in the literature ( - ). These methods Include measuring cure by differential scanning calorimetry, infra-red spectrometry, vlsco-metry, and by monitoring electrical properties. The presence of multiple reaction mechanisms with different activation energies and reaction orders (10) makes accurate characterizations difficult, but such complexities should be quantified. A dual Arrhenius expression was adopted here for that purpose. 

Fig. 17 Proposed ctalytic effect of the organoclay on epoxy homopolymerization [58]...

In amine cured systems, the hardener is used at near stoichiometric quantities, but since a tertiary amine has a weak catalytic effect, slightly less than theoretical amounts should be used. In formulations containing anhydrides, less than stoichiometric quantities of curing agent are used, since acid formation is an intermediate stage of the cure that will produce some epoxy homopolymerization. With strong dibasic chlorinated anhydrides, like chlorendic anhydride, the optimum ratio is about 0.50, whilst with the weaker anhydrides, like hexahydrophthalic anhydride, the optimum ratio is in the range 0.70-0.85. 

L6WiS BaS6S. Lewis bases contain an unshared pair of electrons in an outer orbital and seek reaction with areas of low electron density. They can function as nucleophilic catalytic curing agents for epoxy homopolymerization as cocuring agents for primary amines, polyamides, and amidoamines and as catalysts for anhydrides. Tertiary amines and imidazoles are the most commonly used nucleophilic catalysts. Several different mechanisms are possible ... 

Gagnebien proposes three possible side reactions and mechanisms other than the main reaction. The first is epoxy homopolymerization as initiated by the fi ee tertiary amine. The initial step is the formation of a zwitterion, Z, in Rxn 27. The zwitterion then attacks another epoxy ring, which forms a larger zwitterion (Rxn 28). This process could repeat ad infinitum until all the epoxy is incorporated into the zwitterion ... 

The second side reaction proposed is a branching reaction, and is more complex than the epoxy homopolymerization ... 

The uncatalyzed ECN-PN reaction is very slow compared to the catalyzed epoxyphenol reaction. The uncatalyzed equimolar epojq -phenoUc reaction was also found to be slu sh by Shechter. A cure carried out at 200 C required 12 hours to consume 90% of the epoxy. In addition, the catalyzed epoxy homopolymerization reaction (Rxns. 27 and 28) was found to be very slow by Shechter and by Narracot. It was actually necessary to add alcohol in order to trigger any reaction, and then reaction was observed between only epoxy and the alcohol. It was concluded that the zwitterion formed by the ring opening reaction between the tertiary amine and the epoxy (Rxn. 27) is unable to react with epoxy. This was beUeved to be due to the proximity of the zwitterion s positive and negative charges." ... 

These cases indicate that neither epoxy homopolymerization nor uncatalyzed epojQ -phenol reaction can play a significant role in the catalyzed epojqr-phenol reaction in the short timescales (i.e., 20-40 minutes) during which the catalyzed reaction occurs. 

The fusion process has many desirable features. For example, it is not necessary to handle the relatively volatile and toxic ECH. Nor are there volatiles or inorganic salts produced that must be removed, and all materials charged to the kettle end up as final product. There have, however, heen serious drawbacks to the use of the fusion process, arising primarily from the catalysts heretofore available. These basic catalysts, while very effective in promoting the desired epoxy/ phenolic hydroxyl condensation reaction, are also effective to a varying degree in promoting one or both of the known side reactions, namely, the epoxy/epoxy homopolymerization and the epoxy/aliphatic hydroxyl condensation. 

These high performance resins can be cross-linked with either an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur. 

Catalytic curing agents are a group of compounds which promote epoxy to epoxy reactions without being consumed in the process. A typical epoxy homopolymerization using a tertiary amine is shown below ... 

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