Properties of the Curing Epoxy System

It is evident that relaxation studies in the solid state can look at the motions which are responsible for the mechanical properties of the cured epoxy systems 43). Therefore, Garroway, Moniz and Resing continued to do relaxation studies 61). Garroway, et al. looked at four epoxy polymers based on the DGEBA resin. Two of the epoxy resins were cured with amines and the other two were cured with anhydrides. Proton enhanced spectra of the epoxy systems were generated. The solid state spectra were compared to the solution spectra of the unreacted epoxy. The< epoxy resin of interest was again DGEBA which was reacted with ... 

Fig. 16 Schematic representation of an interface-induced segregation scenario. As long as the viscosity of the crosslinking system is low enough, segregation of epoxy resin and curing agent may occur, driven by the polar surface of the Cu component. Conservation of mass requires a depletion zone close to the zone of enrichment. Via the network structure the concentration profile is reflected by the local mechanical properties of the cured epoxy system...

Commercial products are available where n varies from 2 to 7. The flexible epoxy resins based on polyglycol also make excellent reactive diluents because they have a viscosity of 100 cP at 25°C. Table 4.4 shows the effect of two polyglycol diepoxides on the physical properties of a cured epoxy system. Another type of flexible epoxy resin is derived from dimerized unsaturated fatty acid, cashew nut oils, and other vegetable oils. Other flexible epoxy resins can be made with thiols, aliphatic acids, and hydroxyl-terminated compounds. Applications where flexible epoxy resins are valued include... 

Diluents will also affect the performance properties of the adhesive. Diluents generally lower the degree of crosslinking and degrade the physical properties of the cured epoxy. This reduction in crosslink density increases the resiliency of the adhesive, but it also reduces tensile strength as well as heat and chemical resistance. These effects are more pronounced at elevated temperatures than at room temperature. The degree of these effects will depend on whether the diluent has epoxy functionality (reactive diluents) or whether the diluent is incapable of reacting with the epoxy system (nonreactive diluents). 

This is a simplification of the process occurring in a curing resin-hardener system and a detailed discussion may be found in Pascault et al (2002), Williams et al (1997) and Inoue (1995). The main parameter that it is important to control in the reactive phase separation is the diameter of the elastomer particle. This is because the toughness of the resulting network is controlled by the energy-absorbing mechanisms such as particle cavitation and rubber bridging of cracks. Also of importance is the limitation of the effect of the rubber dispersed phase on the critical properties of the cured epoxy resin such as the stiffness and Tg. This will be affected by the extent to which the rubber dissolves in the matrix-rich phase. 

Versatility - numerous curing agents for the epoxies are available, and the epoxies are compatible with a wide variety of modifiers hence, the properties of the cured epoxy-resin system can be engineered to widely diverse specifications ... 

Diluents. Diluents affect the properties of the cured resin system and, in particular, lower the viscosity in order to improve handling and wetting characteristics. They are often used in the range of 2-20 wt% based on the epoxy resin. Diluents can be classified into reactive and nonreactive types. 

Vrana, M.A., Dillard, J.G., Ward, T.C., Rakestraw, M.D. and Dillard, D.A., The influence of curing agent content on the mechanical and adhesive properties of dicyandiamide cured epoxy systems. J. Adhes., 55, 31-42 (1995). 

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. 

The vapor pressure of an epoxy resin system will have an indirect effect on the final properties of the cured adhesive. However, vapor pressure of the epoxy adhesive and its components may have a direct effect on the health and safety of those who manufacture or apply these products. 

Some nonreactive diluents have been used to impart special properties on the cured epoxy in addition to lowering the viscosity of the uncured system. For example, chlorinated diluents have been used with antimony oxide to impart flame resistance to cured epoxy systems. A typical formulation of this type based on DGEBA employs about 15 pph chlorinated... 

Although the processing and final physical properties of epoxy-curing agent systems depend primarily on their chemical composition and degree of cure, the corresponding relations are often empirical or semiempirical and are not well understood. The tie between the cure chemistry and structure and properties of the cured resins consists in the theoretical and experimental study of network formation as a function of the depth of cure. 

An explanation is apparent by examining the cure curve. Because the diluent reduces the crosslinking density, it allows the epoxy to react more completely. The greater extent of reaction tends to compensate for the otherwise possibly deleterious effect of the diluent on the properties of the cured system. The use of diluent also provides the practical advantage of lowered viscosity in a formulated system. Since excessive use of diluents can lead to poor chemical resistance and reduced mechanical strength, recommended amounts should not be exceeded. 

Table III shows the reactivity properties of the molding powder and the physical properties of the cured compound. This table indicates there are virtually no differences in gel times, flow properties, flexural properties, moisture absorption, flame retardancy, and glass transition temperatures between the compound based on stable bromine CEN and the compound based on the standard system of CEN and the epoxy of TBBA.

A number of antioxidants were examined for solubility in the monomer, effect upon the photoinitiator, UV stability, effect upon physical properties of the cured coating and their effectiveness as stabilizers. The addition of hydrogen donors, HD-2 or HD-3, or a hindered amine light stabilizer, HALS-1, improves the stability of a cured epoxy acrylate system with a minimal change in the physical properties of the resin. DSC and oxygen absorption measurements show that the secondary hindered amine, HALS-1, imparts the best stability to the resin. 

The main attributes of properly cured epoxy systems are outstanding adhesion to a wide variety of substrates, including metals and concrete ability to cure over a wide temperature range very low shrinkage on cure excellent resistance to chemicals and corrosion excellent electrical insulation properties and high tensile, compressive, and flexural strengths. 

As discussed earlier, the use of unmodified Lewis acid catalysts as epoxy curatives has been restricted due to the extremely rapid rate of cure of such systems, and the poor physical properties of the cured resins. Lewis acids are frequently modified by the formation of heat-activatable complexes with amines or glycols. Such complexes display moderate room temperature... 

Important fiber materials are surface-treated glass, boron, graphite (carbon), and aromatic polyaramides (eg, DuPont s Kevlar). In most composites the reinforcement constitutes ca 65% of the final mass. Orientation of the fibers is important in establishing the properties of the laminate. Unidirectional, bidirectional, and random orientations are possible. The characteristics of the cured resin system are extremely important since it must transmit the applied stresses to each fiber. A critical region in a composite is the resin-fiber interface. The adhesive properties of epoxy resins make them especially suited for composite applications. 

The three most often used epoxy resins for monolithic surfacings are the bisphe-nol A, bisphenol F (epoxy novolac), and epoxy phenol novolac. These base components are reacted with epichlorhydrin to form resins of varying viscosity and molecular weight. The hardening system employed to effect the cure or solidification will determine the following properties of the cured system ... 

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