Epoxy resins, cured crosslink density

DSC may also be used to determine the degree of cure by measuring the Tg as a function of cure time at constant temperature. As an epoxy increases in crosslink density, its Tg increases to a maximum value. Figure 4.19 shows the increase in Tg as the resin continues to cure. DSC has also been used to measure changes in Tg with increasing moisture content of both uncured and cured resins. 

Figure 9 shows the density of the matrix of GMAEVC and epoxy resin cured at different curing conditions. The density of the matrix increased with increasing curing temperature. This result is consistent with the XPS results that show the cured at higher temperature contains the more abimdant tight crosslinks such as ether bonds in the network of the cured matrix rather than bulky ester bonds in the network of the matrix cured at lower temperature. 

The first moment of the distribution is Pt0T the total, cumulative molar concentration of polymeric material. As the molecular weight of polymeric species increases, branching and crosslinking reactions yield a thermoset resin. Chromatography analysis of epoxy resin extracts confirms the expected population density distribution described by Equation 4, as is shown in Figure 2. Formulations and cure cycles appear in Table II. 

Amines are one of the most important curing agents for epoxy resins. They provide fast cures with a relatively high crosslink density. Unmodified amine cured epoxy resins are generally too brittle for adhesive applications, and so there are many derivatives that have been developed. More information on amine curing agents can be found in Chap. 5. 

Crosslink density may be defined as the number of effective crosslinks per unit volume. The crosslink density is a key parameter in determining the properties of an epoxy resin after cure. It is dependent on the number of reactive sites (functionality), the molecular distance and chain mobility between functional sites, and the percentage of these sites that enter into reaction. Crosslink density is inversely related to the molecular weight between crosslinks Mc. 

The value of Tg is important, since rigidity decreases in the glass transition region, Tg is closely related to the structure and crosslinking density of the cured resin. The structure of the cured resin can be derived from the initial starting materials, the epoxy prepolymer and hardener, and reaction conditions. However, the structures of many cured resins is still unclear which prevents to establish the structure-mechanical properties relationships. Further studies are needed. Furthermore, the commercial epoxy formulations may contain several components and also diluents, plasticizers, liquid rubbers, etc. which makes a prediction of Tg and mechanical properties even more difficult. 

It can be shown that the specific diffusion control is not operative for the conventional mechanisms of curing of epoxy resins. The situation becomes somewhat complicated when high crosslinking densities are reached. 

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. 

---