Toughening of Thermosets

In Secs. 13.2 13.3 the principles of toughening of thermosets by rubber particles, and the role of morphologies, interfacial adhesion, composition, and structural parameters on the toughening effect are analyzed. Section 13.4 is devoted to the use of initially miscible thermoplastics for toughening purposes. The effect of core-shell rubber particles is discussed in Sec. 13.5 and, in Sec. 13.6, miscellaneous ways of toughening thermosets (liquid crystals, hybrid composites, etc.), are analyzed. 

Epoxy networks are the most widely studied materials, due to their well-known chemistry. Consequently, many studies are devoted to epoxy networks as model networks, although the principles and models developed can be applied to other thermosets. 

The principles of toughening have been described by Kinloch (1989), Miilhaupt (1990), Huang et al. (1993b), and McGarry (1996). The roles of particles during both the initiation and propagation of the crack may be analyzed separately. 

1 Role of the Inclusions in the Initiation Step (Before the Appearance of an Intrinsic Defect or Crack) 

For a single rubber particle in an infinite uniaxial tensile stress field, it was demonstrated that there is a stress concentration effect with a factor around 2, at the particle equator (Fig. 13.1). 

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Solvent-modified and macroporous epoxies prepared via the CIPS technique are ideal materials to verify these predictions and to throw some light on the ongoing discussion on the role of the second phase and cavitation for the toughening of thermosets. 

Thermosets are generally used in advanced composites due to their excellent thermal and dimensional stability, high modulus, and good mechanical properties. Because thermoset resins are inherently brittle, however, some applications require improved fracture resistance. Toughening of thermosets has been achieved through various methods, such as incorporation of reactive liquid rubber [1-9], elastomer [10], or rigid thermoplastics [11-25], and IPN formation with ductile component [26]. 

The major limitation of rubber toughening of thermosets results from the fact that the increase in toughness can be achieved only at the expense of high-temperature performance or of mechanical properties, e.g., a decrease in modulus and yield stress. This can be unacceptable for structural and long-term applications (see Fig. 13.7). A second limitation is the lack of significant success in the toughening of high-Tg networks (see Fig. 13.8). 

Specific quantitative models have been developed for the TP toughening of thermosets (Yee et al., 2000). 

To balance some of the drawbacks produced by the rubber toughening of thermosets, inorganic fillers that increase modulus and yield stress can be added to generate hybrid composites. Inorganic fillers such as glass beads, alumina, or silica - with high values of modulus and strength - are frequently included in thermoset formulations. 

It has been noted above that phase separation in thermoplastics is a common occurrence when two or more polymers are mixed and that miscibility is the uncommon event. This is exploited in toughening of thermosets by elastomers when phase separation occurs during the reaction that leads to three-dimensional network formation. If macroscopic phase separation is not desired then it is possible to achieve a different microscopic morphology and in some cases maintain some features of miscibility... 

The rubber toughening of thermoset polymers will be discussed in Section 11.C.8.C. 

Current Modifications of Epoxies. Particulate Toughening of Thermosets. . Rubber Toughening of Thermosets. . . Thermoplastic toughening of thermosets Epoxy Fibre Composites. ... 

Rubber toughening of thermosets can lead to a significant increase in toughness, but this method usually leads to a decrease in the material s stiffness and strength. 

Some authors have refused to accept the role of interfacial adhesion on the toughening of thermoset resins. Lavita and co-workers [190] reported that non-reactive rubber can toughen BPA-modified epoxy, but the mechanism was not fully discussed. Huang and co-workers [194] showed that when the second phase consists of micron-size rubber particles, the interfacial bonding has only a modest effect on the fracture properties of blends. 

In Chapter 4, toughening of thermoset resins was discussed in a general way. Among the commercially available thermosetting resins, epoxy resins have been the most extensively studied, and toughening technology has been exploited in the field of adhesive and fibre-reinforced composites. This is due to the inherent ductility of the cured epoxy resins and versatile epoxy resin chemistry. In this chapter, the toughening of epoxy resin will be discussed specifically. 

Kim J K and Robertson R E (1992) Toughening of thermoset polymers by rigid crystalline particles, J Mater Sci 27 161-174. 

Kim N H and Kim H S, Micro-void toughening of thermosets and inter-void distance, ACUN-5 International Composites Conference, Developments in Composites Advanced, Infrastructural, Natural and Nano-composites, UNSW, Sydney, Australia, July 11-14, 2006, pp. 217-222. 

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