Cross-linking density epoxy resin

As previously stated, to increase the cross-link density, multifunctional resins are mixed with these high molecular weight linear resins. An alternative to that approach is to add a multifunctional resin to the advancement process, thus synthesizing branched high molecular weight epoxy resins. 

In the formulation of resins such as phenol-formaldehyde or epoxy resins, stoichiometric requirements call for a 2+ l mole ratio of reactants to achieve a high cross-link density. UF resin can be prepared at mole ratios on the order of 1 1.10 with little problem. 

Figure 6. The effect of cross-link density on the yield stress of an epoxy resin. Circles are experimental data3.

The multifunctionality contributes higher reactivity and cross-link density. These factors are especially critical when formulating systems that require improved thermal performance over conventional epichlorohydrin—bisphenol A systems. The melt viscosity of these resins, which are solids at room temperature, decreases sharply with increasing temperature. This affords the formula tor an excellent tool for controlling flow of molding compounds, and facilitating the incorporation of ECN resins into other epoxies, eg, for powder coatings. 

It is interesting that, upon rubber modification, the CET resin matrix can no longer form dilatation bands (18). Only rubber-particle cavitation and matrix shear yielding are detected. This observation implies that a dilatational stress component is required to trigger the formation of dilatation bands. In other words, upon rubber-particle cavitation, the dilatational stress component in the matrix is reduced. This suppresses the formation of dilatation bands. This conjecture finds support in the work of Glad (27), who investigated thin-film deformation of epoxy resins with various cross-link densities and could not find any signs of dilatation bands in his study. 

Nonetheless, for the more than 50 years since the first publication in this field, NIPUs still do not have sufficiently broad application. This can be explained by certain features of these materials. Cyclic carbonate (CC) groups interact with aliphatic and cycloaliphatic polyamines at ambient temperatures more slowly than isocyanates with hydroxyl groups. The rate of this reaction is comparable to the rate of curing epoxy resins (ER) with amines. At the same time, the CCs react only with primary amino groups, in contrast to the ERs, which react with primary and with secondary amino groups. This results in a decrease in cross-linking density of the polymer network. 

As mentioned earlier, UV-curable resin formulations are very attractive for fiber coating because of the rapid cross-linking rates that are achievable. Most commonly, epoxy- or urethane-acrylate resins are employed (18-22), and viscosity and cross-link density are controlled through the addition of reactive diluents. With these systems work has focused on producing low modulus, low T properties (20-22) through the incorporation of appropriate chemical constituents to enhance higher chain flexibility, for example, ether linkages. 

The properties of epoxy resin was not seriously affected by the addition of 5% RVP grafted AC 5120. However, the properties decreased as the concentration of modifier increased. But the decrease was due to the lower cross-link density of the cured blend since lesser amount of hardener HHPA (70 parts) was used in the blends instead of 80 parts in the neat resin. 

This paper presents characterization studies performed on amine cured epoxy resins. Particular emphasis is placed on the characterization of the cross-link density, and on its influence on physical properties, especially the fracture toughness. 

Gel Tg. the temperature at which gelation and vitrification occm simultaneously Tgo. the Tg of the mixed reactants, corresponding to a minimiun cure In isothermal cures, at temperatures between the gel Tg and Tgo, the resin undergoes gelation followed by vitrification. Since Tg is a function of both the degree of cure and the cross-link density, it increases to a point at which aU polymer reaction sites have been consumed. At that point, the Tg reaches a plateau and does not increase further with temperature until the decomposition temperature where a char region occurs (Fig. 2.17). An example of the effect of cure conditions on Tg and CTE values was shown by Konarski " who varied the cure cycles of an anhydride-cured epoxy from 130 to 175 °C as shown in Table 2.10. The value of Tg continued to... 

Heat distortion temperature (HDT) or deflection temperature (DT) is a measure of the tendency of cured product to soften when heated. It is a feature of the inherent thermo-plasticity in cured epoxy compounds as a result of the relatively low cross-linking density, and may be any value from below 50°C to about 250°C, depending on formulation and cure cycle. Resins and hardeners of high functionality tend to have higher HDT. Postcuring at elevated temperature can increase HDT significantly. 

Although DGEBA resins provide the backbone of most epoxy formulations, they may be blended with other types to achieve modifications. Epoxy novolacs, having higher functionality, increase the cross-linking density, which improves heat resistance but decreases impact resistance. Incorporation of epoxidized oils increases flexibility at the expense of heat and chemical resistance. Low-viscosity polyfunctional epoxies based on polyols or polyhydric phenols reduce viscosity and can increase functionality without impairing cured properties. Monofunctional reactive diluents will also decrease viscosity and form part of the polymer backbone, to impart a measure of flexibility without the possibility of migration. Properties of commercially available epoxy resins and diluents from various suppliers are listed in Table 1. 

---