Yielding and Fracture of Toughened Networks

In the previous chapter, the relationships between structure and mechanical and fracture properties of neat thermosets were analyzed. It was shown that an increase in thermal resistance (Tg or HDT, heat deflection temperature) and yield stress leads to a decrease in toughness (Kj ), and in impact or fatigue resistance. So the challenge is how to increase toughness without sacrificing thermal and mechanical properties  

The two methods of improving the macroscopic toughness of thermosets are similar to those used for amorphous or semicrystalline thermoplastics (i) plasticization and (ii) amplification of deformation mechanisms via the generation of a heterogeneous structure. 

The plasticizer addition is a relatively simple technique. A miscible low-Tg compound is added to the formulation, so as to produce a decrease in both the glass transition temperature and the yield stress, and a corresponding improvement in the fracture resistance. These drawbacks are very severe for thermosets, and generally this method is not used for toughening purposes. 

The most frequently applied methods for improving toughness are the addition of preformed particles or the in-situ formation of dispersed rubbery or thermoplastic particles in the thermoset matrix (Chapter 8). 

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