Epoxy excess networks

The effect of the reactant ratio, A/E, on the physical properties and fracture behavior of epoxy systems has been the subject of many studies and the results have been inconclusive, This is due largely to the fact that network structure changes dramatically with changes in A/E, especially in epoxy excess (A/E < 1) and amine excess (A/E > 1) compounds. Comparison of different systems, therefore, must take into consideration whether the networks involved are amine... 

Epoxy resins used in composite manufacturing are intentionally prepared with an excess of epoxy compared to hardener. This is done to ensure that the hardener is completely reacted during cure and is not allowed to remain in the network and cause plasticisation. A mixture with an epoxy—hardener ratio of 1 1 would not be able to fully cure due to the mobility arguments given in this chapter. The epoxy excess means that there will be a significant number of unreacted and relatively polar functional groups present in the cured 3D network. 

Void-free phenolic networks can be prepared by crosslinking novolacs with epoxies instead of HMTA. A variety of difunctional and multifunctional epoxy reagents can be used to generate networks with excellent dielectric properties.2 One example of epoxy reagents used in diis manner is the epoxidized novolac (Fig. 7.34) derived from the reaction of novolac oligomers with an excess of epichlorohydrin. 

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. 

Figure 8.7 SEM photograph of a fully cured rubber-modified epoxy network. The rubber CTBN (26 wt% AN) is first pre-reacted with a large excess of diglycidyl ether of bisphenol A (DGEBA) to obtain an epoxy-terminated rubber. Then an equivalent of 15 wt% initial CTBN is introduced in DGEBA-4,4 -diamino diphenyl sulfone, DDS, system precured at 135°C (time > tgei) and then postcured at 230°C. Rubber-rich particles are spherical, D 2.8 0.5 gm, and well dispersed. (From LMM Library.)...

The rate of this reaction is about ten times smaller than that of Eq. (I) and under typical cure conditions it becomes noticeable only after the end of the main process. Scheme (II) changes the structure of networks by the formation of additional crosslinks of the ether type. This makes the total connectivity of the network higher. This structural change influences some properties of polymers in the glassy and rubbery state (see Sects. 4 and 5), but it is really pronounced in nonstoichiometric systems with an excess (P < 0.8) of epoxy components 19,22). Crosslinks of the ether type may principally appear in polymers due to a condensation reaction between OH groups. Under our conditions this process normally does not take place. 

Furthermore, the chemical structure of networks are changed by thermal oxidation reactions 17,23,24F These are rather important for epoxy networks with aliphatic amines since they usually take place in the presence of air at T 130 °C. In aromatic amine-based polymers this kind of reaction becomes important at T > 220° 240 °C 17-23>. The only exception are polymers with a large excess of epoxy groups in the initial mixture. For example, the polymer with P = 0.4 23) starts loosing its weight at 160 °C17 23,24). All polymers considered in this paper are prepared from mixtures with 0.6 P 1.6. Cure and post-cure treatment temperatures are below 190 °C. This means we may not consider thermal oxidation processes in our structural analysis of the networks. 

In conclusion to the short analysis on curing chemistry of epoxy-aromatic amine networks (for more detailed analysis see papers of K. Dusek and B. Rozenberg in this volume), one can say that the chemical structure of the polymers under consideration is mainly determined by the curing reaction in Eq. (I). Equation II becomes important only for polymers with an excess of epoxy groups at T 150 °C. This rather simple situation makes the analyzed polymers very suitable for basic investigations. 

In Sect. 5, the macroscopic extension at break of the glassy networks considered was shown to be s, 4-6% and only weakly dependent on the network s chemical composition. However, local plasticity markedly depends on the chemical composition of the network. L( in the samples with an excexs of amine (P = 1.3) is about 10 times larger than in the samples with an excess of epoxy groups (P = 0.8). This is an additional argument in favour of the assumption of the keyrole of 3-linked chemical crosslinks in network plasticity (see Sect. 5 and Fig. 20). It is evident that the excess of amine in the initial mixture leads to a relatively high concentration of 3-type crosslinks in cured resins. 

The shape of the dependence of G on r for excess amine and excess epoxide in epoxy-amine elastomeric networks predict by the theory has been well described by experiments both for r, > 1 and r < 1 (Refs, but the results have not... 

The butadiene acrylonitrile rubbers have clearly been successful to a considerable extent in improving the toughness characteristics of epoxy networks. However, since the butadiene component of the elastomer contains unsaturation, it would appear to be a site for premature thermal and/or oxidative instability. One would imagine that excessive crosslinking could take place with time which would detract from the otherwise desirable improvements accomplished with these structures. 

Epoxy resins are complex network polyethers usually formed in a two-staged process. The first stage involves a base-catalyzed step-growth reaction of an excess epoxide, typically epichlorohydrin with a dihydroxy compound such as bisphenol A This results in the formation of a low-molecular-weight prepolymer terminated on either side by an epoxide group. 

Cross-linking provides anchoring points for the chains, and these points restrain excessive movement and maintain the position of the chain in the network. This is not confined to elastomers, however, and the improved material qualities that result are also foimd in the cross-hnked phenol-formaldehyde, melamine, and epoxy resins. 

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