Epoxide resins hardening systems

Fig. 143. Kinetics of the accumulation of systems with conjugated bonds in the decomposition of epoxide resins hardened with maleic anhydride (a) and polyethylenepolyamine (b). 1 and 4) 350 C 2 and 5) 375 C 3 and 6) 405 C.

In a study of the thermooxidative destruction of epoxide resins (hardened with maleic anhydride and polyethylenepolyamine) [12], it was shown that the oxidation process is characterized by the presence of induction periods (Fig. 144), which decreased regularly with increases in the initial oxygen pressure (P) in the system and with increasing temperature (Fig. 145), in accord with the formula... 

Epoxidized phenol novolak and cresol novolak are the most common curing agents. The composition of the resin and hardener system is optimized for each specific appHcation eg, incorporating phenol novolaks in the matrix resin can iacrease cure speed. 

Comparison of hardening systems Miscellaneous Epoxide Resins... 

Fig.51 Effect of the hardening system in an epoxide resin powder coating on the coloristic properties of the paint. Pigment Dibromoanthanthrone (Pigment Red 168). Cross-linking conditions 15 minutes at 180°C, respectively.

Two-layered GRPs for copper clad laminates are obtained with one layer consisting of the three-component system (e.g. BPA/DC, BMI, brominated epoxide resin, Zn octoate and triethylenediamine in methylethylketone). The other layer has the usual epoxy matrix (brominated epoxide resin, dicyandiamide as a hardener and 2-ethyl-4-methylimidazole as curing accelerator) [119]. As similar two-layered laminate contains BPA/DC, BMI, epoxynovolak resin, Zn acetate and triethylenediamine in the first layer and BPA/DC only with the same catalysts in the second layer [120]. 

Today the epoxide-resin base is almost exclusively epoxidized cresol-novolak, especially when performance at elevated temperatures is important. Hardened by an aromatic amine, the cured material has a heat destortion temperature 50-60°C higher than found for similar cured bisphenol A based systems. 

The activation energies were calculated for the processes of gas evolution, loss in weight of the residue, and rate of accumulation of systems with conjugated bonds in the thermal destruction of hardened epoxide resins. 

In order to impart these properties to the final product, very careful selection of the epoxide resin and hardener system, as well as all the other components of the formulation, is vitally important. As the cured material is crosslinked, the structure and properties of the original resin will be completely changed. The material will no longer be soluble without decomposition and its chemical resistance will depend as much upon the hardener selected as upon the epoxy resin. As an illustration of the dependency of the cured system upon the hardener, acid anhydride based hardeners offer excellent resistance to acid environments, whilst amine-type hardeners offer maximum resistance to alkaline environments. 

Even though there are literally hundreds of different epoxide resins and hardeners commercially available, giving countless formulations with a diversity of cured properties, further modifications of a material are sometimes still required. These modifications can be achieved by incorporating a range of compounds known as diluents, flexibilisers and plasticisers into the epoxide resin system during the formulation process. The properties of some typical diluents, flexibilisers and plasticisers are now discussed. 

Epoxy Contain epoxide resin cures by chemical reaction two-part system (hardener and resin) excellent adhesion to most substrates, tough and durable films good resistance to solvents, water, chemicals, and abrasion poor resistance to sunlight (chaUdng breakdown process) which limits outdoor use to primers and intermediate coats most commonly used are polyamide- and amine-cured modified epoxies can be used in steel concrete, flooring and lining applications. 

The methylated maleic acid adduct of phthalic anhydride, known as methyl nadic anhydride VI, is somewhat more useful. Heat distortion temperatures as high as 202°C have been quoted whilst cured systems, with bis-phenol epoxides, have very good heat stability as measured by weight loss over a period of time at elevated temperatures. The other advantage of this hardener is that it is a liquid easily incorporated into the resin. About 80 phr are used but curing cycles are rather long. A typical schedule is 16 hours at 120°C and 1 hour at 180°C. 

When cured with room temperature curing system these resins have similar thermal stability to ordinary bis-phenol A type epoxides. However, when they are cured with high-temperature hardeners such as methyl nadic anhydride both thermal degradation stability and heat deflection temperatures are considerably improved. Chemical resistance is also markedly improved. Perhaps the most serious limitation of these materials is their high viscosity. 

One can rationalize a need for small rubber inclusions in some of the newer approaches to waterborne and high solids epoxy coating systems. Water-thinned epoxy coating compositions are described (48) where the two-component system consists of a nitrile rubber modified epoxy resin in the epoxide component and a styrene/ butadiene/methylmethacrylate latex modifier for an emulsion-based polyamide hardener component. Showing improved adhesion, impact and water resistance, the paint has good wetting characteristics and can be formulated to a high solids content at low viscosity. 

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