Epoxy-amine system properties

Low-molecular-weight model compounds such as phenylglycidyl or other mono-glycidyl ethers as well as primary, secondary and tertiary amines have been used for the study of the kinetics, thermodynamics and mechanism of curing. To reveal the kinetic features of network formation, results of studies of the real epoxy-amine systems have also been considered. Another problem under discussion is the effect of the kinetic peculiarities of formation of the epoxy-amine polymers on their structure and properties. 

The reaction of curing the epoxy-amine system occurring in the diffusion-controlled mode has little or no effect on the topological structure of the polymer 74> and on its properties in the rubbery state. However, the diffusion control has an effect on the properties of glassy polymers 76 78). 

Many dielectric studies of epoxy-amine systems have been reported. It appears that the change in dielectric properties during cure may be due to three main contributions ... 

Curing properties. Depending on the level of LP used and on the reactivity of the curative, it is possible to accelerate or retard the rate of cure of an epoxy-amine system (Figure 10.2). 

In order to elaborate the relationship between stoichiometric imbalance of the epoxy-amine system and its resulting elastic properties, a series of reference samples of well-defined concentration ratios r was investigated using dynamic mechanical analysis (DMA). A clear increase in the glassy epoxy modulus (as measured at 20 °C) with an increasing excess of epoxy (r< 1) was observed [32]. Notably, the observation of modulus enhancement with increasing excess of epoxy is consistent with the concentration and modulus maps of the PVP/epoxy IP. 

In addition, blending with epoxy/amine blends has been reported as a suitable route to improve the mechanical properties of phenolic resins and to reduce the cure temperature [a.368]. The results reported demonstrate that the epoxy/amine content should be kept below 15 wt% to avoid a significant reduction of the thermal stability of the blend. Flame resistance experiments identified the aromatic diglycidyl ether of bisphenol-A-based (epoxy equivalent of 190 g/mol) epoxy/amine system as one of the best epoxy resins to produce thermally stable blends with the phenolic resol. 

Epoxy-polysulfide systems can be formulated either as a liquid DGEBA epoxy mixed with liquid poly sulfide polymer or as an epoxy-terminated polysulfide polymer either may be cured with a tertiary amine such as DMP-30. Table 11.19 describes the formulation and shows the physical properties of these epoxy-polysulfide adhesives compared to an unmodified epoxy adhesive. 

The microstructure of epoxy thermosets can be complex, and both molecular and physical microstructures are presumed. Unfortunately, the intractable nature of these materials makes direct structural characterization extremely difficult. The most accessible technique for direct structural characterization is evaluation of epoxy rubber-like properties above Tg. Sometimes, indirect characterization of epoxy structure is possible due to the fact that the chemistry of several epoxy systems is well behaved (e.g., epoxy-amine chemistry). This permits epoxy network structure to be modeled accurately as a function of the extent of the crosslinking reaction(s). This approach has been developed extensively by Du ek and coworkers for amine-linked epoxies ... 

On the other hand, the glassy-state modulus was retained in all epoxy systems. The Tg improvement was caused by the promotion of the diffusion-controlled epoxy-amine reaction, hence, this promotion worked better in a network with a higher AT. Moreover, the addition of a small amount of POSS-triol ensured the consistency of the thermomechanical properties of epoxy networks with a high degree of steric constraints (high functionality of epoxy monomer), as evidenced by several parallel experiments. Because the addition of such a small amount of POSS-triol did not increase the viscosity of... 

To improve high temperature stability over amine cured systems and to give better physical and electrical properties above their heat distortion temperatures, it has been general practice in epoxy resin systems to use anhydride curing agents with DGEBA epoxy resins (8 ). Most anhydride formulations require elevated-temperature cures with the ultimate properties dependent on postcures at temperatures of 150 C or higher. 

A number of antioxidants were examined for solubility in the monomer, effect upon the photoinitiator, UV stability, effect upon physical properties of the cured coating and their effectiveness as stabilizers. The addition of hydrogen donors, HD-2 or HD-3, or a hindered amine light stabilizer, HALS-1, improves the stability of a cured epoxy acrylate system with a minimal change in the physical properties of the resin. DSC and oxygen absorption measurements show that the secondary hindered amine, HALS-1, imparts the best stability to the resin. 

Coating formulations and properties have been detailed in technical bulletins (31). The epoxy-polyamide system is popular because it provides an unusual degree of inherent corrosion resistance. This will be discussed in detail later. The system is unusually tolerant as compared to related systems, such as amine cured, since the ratio of components is not particularly critical. Tolerance is also demonstrated because it may be applied to wet surfaces and to surfaces with tightly bound rust. Indeed, formulations are available that may be applied under water to structures such as submarines and off-shore oil well riggings. Both the corrosion resistance and the tolerance relative to application on poorly prepared wet surfaces are believed to be functions of the surface activity of the polyamide resin. Related also to the surface activity are the unusually strong adhesive properties that the system demonstrates with a broad range of substrates. 

Epoxy-amine liquid prepolymers are extensively appHed to metallic substrates and cured to obtain painted materials or adhesively bonded stractures. Overall performances of such systems depend on the interphase created between the organic layer and the substrates. When epoxy-amine Hquid mixtures are appHed to a more or hydrated metaUic oxide layer (such as Al, Ti, Sn, Zn, Fe, Cr, Cu, Ag, Ni, Mg, or E-glass), amine chemical sorption concomitant with metaUic surface dissolution appear, leading to the organometaUic complex or chelate formation [1, 2]. Furthermore, when the solubility product is exceeded, organometaUic complexes may crystaUize. These crystals induce changes of mechanical properties (effective Young s modulus, residual stresses, practical adhesion, durability, etc.). 

Chem. Desaip. Poly condensated polyamine with benzyl alcohol Uses Hardener, curing agent for solv.-free epoxy flooring systems Features Solv.-free = 25 min. pot life 25 C Properties Gardner < 2 color sp.gr. 1.03 0.02 g/cm (20 C) vise. 75 25 mPa s amine no. 400 50 Staage 12 mos min. shelf life 23 C in original container WorlMDur H75 [Woriee-Chemiej Chem. Desaip. Mannich base-based... 

In contrast to epoxy-amines, water really participates in the cure chemistry of the epoxy-anhydride system and alters the living polymerisation mechanism because of interfering termination reactions [34]. A less dense network structure with altered properties results [35]. 

For the amine system under non-isothermal cure at 0.2°C min goo of 245 °C causes devitrification to occur at a temperature more than 150°C above vitrification. The importance of this extended mobility-restricted cure on the final material s properties should be emphasised. For this tetrafunctional epoxy-diamine system, an increase in Tg of ca. 170°C, corresponding with a residual cure of ca. 44% and a reaction enthalpy of more than 230 J g is caused by diffusion-controlled reactions and drastically influences the flnal network structure (crosslink density). 

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