Epoxy networks, toughened

Bec Becu-Longuet, L., Bonnet, A., Pichot, C., Sautereau, H., Maazouz, A. Epoxy networks toughened by core-shell particles Influence of the particle structure and size on the rheological and mechanical properties. J. Appl. Polym. Sci. 72 (1999) 849-858. 

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. 

Semicrystalline polyamide fine powders have been used as toughening agents for epoxy networks. The powders can be obtained by grinding granules, or directly by anionic polymerization of lactams, 6 or 12, in an organic solvent from which the formed semicrystalline polymer precipitates. Microporous powders with an average particle size in the range of 10 pm and a narrow particle-size distribution, are obtained. 

Figure 13.6 Relative contributions (%) of the different toughening mechanisms in epoxy networks versus temperature ( ) rubber bridging ( ) shear yielding and (A) cavitation. (From the results of Huang et a ., 1993b.)... 

Regarding the toughening of epoxy networks, it may be stated that... 

As in the case of rubber particles it was demonstrated that the fracture energy is roughly proportional to the volume fraction of TP rich phase, both in the case of epoxy/PEI networks (Bucknall and Gilbert, 1989) and bismaleimide networks toughened with various TPs (Stenzenberger et al., 1988). This improvement was evidenced in the range of volume fractions below [Pg.415]

Figure 13.10 Toughness (K C) and toughening mechanisms of epoxy networks modified with 10 phr CSR versus molar mass between crosslinks, Me (O) neat systems ( ) with 10 phr CSR. (Adapted from Lu, 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. 

Becu, L. Sautereau, H. Maazouz, A. Gerard, J.F. Pabon, M. Pichot, C. Synthesis and structure-property relationships of acrylic core-shell particle-toughened epoxy networks. Polym. Adv. Technol. 1995, 6, 316-325. 

Hed Hedrick, J. L., Yilgor, I., Jurek, M., Hedrick. J. C., Wilkes, G. L., McGrath, J. E. Chemical modification of matrix resin networks with engineering thermoplastics 1. Synthesis, morphology, physical behaviour and toughening mechanisms of poly (arylene ether sulfone) modified epoxy networks. Polymer 32 (1991) 2020-2032. 

Figure 4.1 SEM photographs of a CTBN-toughened epoxy network showing two-...

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