Epoxy network density

Void-free phenolic-epoxy networks prepared from an excess of phenolic novolac resins and various diepoxides have been investigated by Tyberg et al. (Fig. 7.37).93 -95 The novolacs and diepoxides were cured at approximately 200°C in the presence of triphenylphosphine and other phosphine derivatives. Network densities were controlled by stoichiometric offsets between phenol and... 

For imperfect epoxy-amine or polyoxypropylene-urethane networks (Mc=103-10 ), the front factor, A, in the rubber elasticity theories was always higher than the phantom value which may be due to a contribution by trapped entanglements. The crosslinking density of the networks was controlled by excess amine or hydroxyl groups, respectively, or by addition of monoepoxide. The reduced equilibrium moduli (equal to the concentration of elastically active network chains) of epoxy networks were the same in dry and swollen states and fitted equally well the theory with chemical contribution and A 1 or the phantom network value of A and a trapped entanglement contribution due to the similar shape of both contributions. For polyurethane networks from polyoxypro-pylene triol (M=2700), A 2 if only the chemical contribution was considered which could be explained by a trapped entanglement contribution. 

Fig. 36. Density of epoxy networks prepared via CIPS with cyclohexane before and after the drying procedure...

Fig. 48. Density of solvent-modified, semi-porous, and macroporous epoxy networks prepared via kinetically controlled CIPS with 1 wt % catalyst...

Due to the approach chosen for controlling the crosslink density and the curing conditions adopted, the considered systems can be viewed as model epoxy networks. 

In principle, these relationships open the way to a determination of fg which is found to decrease with crosslink density as well in ideal epoxy networks (Gerard et al., 1991), as in nonideal polyesters (Shibayama and Suzuki, 1965). However, it must be recognized that, in both series of data, it is impossible to have consistent values of, Cf, Cf, a, and fg except if BD varies with the structure, which can be considered as a serious argument against the free volume interpretation of WLF parameters. 

There are several ways to modify the crosslink density of ideal networks. The first one is the use of monomers with the same structure but with different molar masses. Many workers have reported on epoxy networks... 

A linear decrease of KIc with an increase in crosslink density was reported for model PU based on triisocyanate and diols of various molar masses (Bos and Nusselder, 1994), and for epoxy networks (Lemay et al., 1984). It was suggested that the dilational stress field at the crack tip may induce an increase in free volume and a devitrification of the material. A linear relationship between GIc and M XJ2 was verified for these systems, although other empiric equations were found in other cases (Urbaczewski-Espuche et al., 1991). 

Epoxy networks were synthesized using mixtures of amines of different functionality to vary Me and the average functionality, fE (Galy et al., 1994 Crawford and Lesser, 1998). In both studies, the increase in crosslink density increased both ay and Tg and produced a corresponding decrease in KIc. A linear relationship between KIc, measured at room temperature, and crosslink density is shown in Fig. 12.11 (Galy et al., 1994). 

Figure 12.11 Evolution o K C at 25°C for ideal epoxy networks based on DGEBA./3DCM/MCHA versus crosslink density. (+), theoretical (o), experimental. (see Table 12.1 for abbreviations). (By permission from Galy et al., 1994, Copyright 2001, ChemTec Publishing.)...

Grillet et al. (1991) studied mechanical properties of epoxy networks with various aromatic hardeners. It is possible to compare experimental results obtained for networks exhibiting similar Tg values (this eliminates the influence of the factor Tg — T). For instance, epoxy networks based on flexible BAPP (2-2 - bis 4,4-aminophenoxy phenyl propane) show similar Tg values ( 170°C) to networks based on 3-3 DDS (diamino diphenyl sulfone). However, fracture energies are nine times larger for the former. These results constitute a clear indication that the network structure does affect the proportionality constant between ay and Tg — T. Although no general conclusions may be obtained, it may be expected that the constant is affected by crosslink density, average functionality of crosslinks and chain... 

Epoxy networks may be expected to differ from typical elastomer networks as a consequence of their much higher crosslink density. However, the same microstructural features which influence the properties of elastomers also exist in epoxy networks. These include the number average molecular weight and distribution of network chains, the extent of chain branching, the concentration of trapped entanglements, and the soluble fraction (i.e., molecular species not attached to the network). These parameters are typically difficult to isolate and control in epoxy systems. Recently, however, the development of accurate network formation theories, and the use of unique systems, have resulted in the synthesis of epoxies with specifically controlled microstructures Structure-property studies on these materials are just starting to provide meaningful quantitative information, and some of these will be discussed in this chapter. 

A nodular epoxy network is thought to be a two-phase system in which regions of relatively high crosslink density are dispersed in a less crosslinked interconnecting matrix. If this is true, then the nodules should exhibit properties different from those of the matrix, e.g., a higher Tg, and a different specific volume. It is also reasonable to expect that the size and concentration of nodules should be sensitive to variations in the reactant ratio and cure conditions. 

Apparently Eqs. (1) and (2) do a reasonable job in describing the elasticity of epoxy networks, even at high crosslink densities. For weU-defined epoxies of... 

Incorporation of monofunctional epoxy POSS into an amine-cured epoxy network increased and broadened the Tg without changing the crosslink density and enhanced the thermal properties. Additionally, it was found that the thermal and thermal-mechanical properties of resultant styrene-POSS vinylester resin nanocomposites were dependent on the percentage of POSS incorporated into the resin [171]. Over a range of POSS incorporations, the Tg of the copolymers changed very little, but the flexural modulus increased with increasing POSS content. 

As an illustration of this transition let us examine in more detail the case where the connector chains are dPS-COOH-grafted to an epoxy network. At low areal chain density and for 412 < NdPS< 1478, where NdPS is here the degree of polymerization of the chains grafted to the epoxy, the measured fracture tough-... 

Improvement of adhesive and nonrigid properties of epoxy coatings cured by PMBs due to an increase of network density and a decrease of its inherent flaw. 

It is assumed that the irreversible plasticization is caused by the breaking of bonds under the influence of water. This hydrolytic aging process should reduce the crosslink density of the epoxy network. The hydrolysis does not deteriorate the entire network, however, because the irreversible plasticization comes to an end within the period of HTA we investigated. 

Controlling epoxy network cross-link density by varying the reactant ratio may result in changes in other structure variables as well, which may be observed by their effects on physical properties. 

Amine light stabilizer, melamlne-acryllo copolymer degradation, 281-83 Amine-cured epoxy networks cross-link density, 166-67,177-81 structure characterization, 172-73... 

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