Epoxy network, properties

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Other reports on the morphology and mechanical behavior of organosiloxane containing copolymeric systems include polyurethanes 201 202), aliphatic 185, 86) and aromatic117,195> polyesters, polycarbonates 233 236>, polyhydroxyethers69,311, siloxane zwitterionomers 294 295) and epoxy networks 115>. All of these systems display two phase morphologies and composition dependent mechanical properties, as expected. 

Pathak, S.K. and Rao, B.S. (2006) Structural effect of phenalkamines on adhesive viscoelastic and thermal properties of epoxy networks. 

While the surface modification is not effective to suppress cavitation, Yee and coworkers performed an experiment to suppress the cavitation mechanically in a rubber-modified epoxy network. They applied hydrostatic pressure during mechanical testing of rubber toughened epoxies [160]. At pressures above BOSS MPa the rubber particles are unable to cavitate and consequently no massive shear yielding is observed, resulting in poor mechanical properties just like with the unmodified matrix. These experiments proved that cavitation is a necessary condition for effective toughening. 

The catalytically functioning curing agents do not directly participate in the crosslinked network but promote reactions between epoxy groups themselves. Tertiary amines as well as boron trifluoride type complexes are effective catalytic agents. Excellent discussions of the specific curing agents, their reactivity with epoxy and their effect on epoxy mechanical properties are available in the literature 4 6I. 

As will be discussed, incorporation of siloxane oligomers modified the elastic moduli and the fracture properties of the crosslinked epoxy network. Previous work 15) indicated that the surface of these materials was rich in siloxane, which is believed to foster a low energy surface. These characteristic properties have led to our interest in the friction and wear of siloxane-modified epoxies. 

The dynamic mechanical properties of the siloxane-modified epoxy networks were also investigated. The DMTA curves for the control epoxy network exhibit the two major relaxations observed in most epoxy polymers 39 40,41>. A high temperature or a transition at 150 °C corresponds to the major glass transition temperature of the network above which large chain motion takes place. The low temperature or (5 transition is a broad peak extending from —90° to 0 °C with a center near —40 °C. It has been attributed predominantly to the motion of the CH2—CH(OH)—CH2—O (hydroxyether) group of the epoxy 39-40 2 ... 

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... 

The investigation of the gas permeation characteristics of epoxy-polyimide would also be interesting in noting the packing efficiency of the polyimide molecules and the effect of immobilization in an epoxy network would have on the permeation characteristics of various gases. The extent of epoxy crosslinking would also have an effect these properties of the composites. 

Cure of epoxy with combinations of monomeric curing agents and the use of polymers containing reactive groups to cure epoxy systems is rarely touched upon in the literature and could be investigated much further. Amides, polyesters, polyethers etc. used to cure the epoxy network could lead to the creation of materials having interesting and heretofore unfathomed properties. The fundamental kinetics of the reactive polymer epoxy reaction is an area of interest as well. 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Many different test methods are used to measure the reactivity or cure rate of the epoxy adhesive. Some of these, such as working life or pot life, are very practical and are used to plan the production process. Others, such as exotherm, are used to determine reaction kinetics. Still others are used to characterize the epoxy network as it cures for the purposes of determining the degree of crosslinking and the rheological properties of the curing adhesive. 

The latter result shows that to interpret the mechanical properties of networks we do not need to take into account the spatial inhomogeneities of crosslink distribution in a sample, at least in the rubbery state. The analysis of epoxy networks performed under the framework of a tree-like model and experiments 7,10-26) brought the... 

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 ... 

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. 

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. 

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... 

---