Epoxy resin curing agents

See also curing agent epoxy resin curing agent... 

Epoxy resin curing agents EPOXY RESIN HARDENER W. Germany 1,033,047 1958 Badische Anilin... 

The crosslinking reactions are illustrated in Reaction 1.8, and they demonstrate that, in principle, only a trace of curing agent is necessary to bring about cure of epoxy resins. Selection of curing agent depends on various considerations, such as cost, ease of handling, pot life, cure rates, and the mechanical, electrical, or thermal properties required in the final resin. 

By contrast with tertiary amines used in catalytic quantities, primary and secondary amines or acid anhydrides may be used to bring about the cure of epoxy resins by reaction in stoichiometric proportions. A typical amine curing agent used at this level is diaminodiphenylmethane (DDM), which reacts with an individual epoxy-group in the way shown in Reaction 4.17. 

Cyanamide and its aromatic derivative, such as 4,4 -methylene bis(phenyl cyanamide), were reported to cure an epoxy resin at elevated temperatures.(4) It is also well known that the dimer of cyanamide (dicyandiamide) is the most important epoxy curing agent in one-package epoxy compounding.(5) Unfortunately, this dimer precipitates from the dispersion causing uneven mixing upon standing. 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

Transformation of epoxy resins, which are viscous liquids or thermoplastic solids, into network polymers is a result of interaction with alkali or acid substances by means of to polyaddition and ionic polymerization mechanisms.10 A resin solidified by to the polyaddition mechanism, is a block copolymer consisting of alternating blocks of resin and a hardener or curing agent. A resin solidified by the ionic mechanism is a homopolymer. Molecules of both resin and hardener contain more than one active group. That is why block copolymer formation is a result of multiple reactions between an epoxy resin and a curing agent.11... 

Figure 3.10. Dependences of the parameters T and dT/dt(tf) on time during curing of epoxy resin by triethanol aminotitanate. To = 120°C. Content of curing agent 40 wt% (curves 1 and 4) or 15 wt% (curves 2 and 3).

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. 

The viscosity of the epoxy resin is only one factor in determining the final viscosity of the formulated system. Some curing agents and resinous modifiers produce little effect on mixed viscosity however, others can have a significant effect by either increasing or decreasing viscosity. Fillers, in general, increase viscosity in direct relationship to their concentration in the system. Diluents can be used to decrease viscosity. 

In one-component epoxy adhesives, the curing agent and resin are compounded together as a single product by the adhesive formulator. The curing agent system is chosen so that it does not react with the resin until the appropriate processing conditions are applied. 

The main mechanism in all these methods is the physical separation and restriction of molecular mobility of the epoxy resin and the curing agent that are imposed by the solid state of the product. These adhesive systems generally provide a shelf life of up to 6 months at room temperature depending on the reactivity of the curing agent and resin. All these products require elevated temperatures to liquefy and crosslink. 

DADPS also provides excellent high-temperature properties and chemical resistance. Of the amine curing agents, DADPS provides the best retention of strength after prolonged exposure to elevated temperatures. It melts at 135°C and can be cured with epoxy resins at 20 to 30 pph with cure temperatures ranging from 115 to 150°C. Because of the low reactivity of this system, an accelerator, such as BF3-MEA, is usually employed at about 1 pph. 

Fig. 2 Structures of selected catalysts and curing agents commonly used for the cure of epoxy resins.

However, it was also shown that the CL-time profile was sensitive to the purity of the resin and the amine, DDS, which was found to contain a quenching agent, so the quantum yield of emission was reduced in the fluid state (Equation (3.28)). It is therefore concluded that simple CL experiments are not genemlly applicable as a method for monitoring the cure of epoxy resins. In addition to the self-probe fluorescence of TGDDM, the CL measurements rely on absolute emission intensities, which, in the absence of an internal standard for calibration, is unreliable. 

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