Rubber Toughening of Thermosets

1 Fracture Modeling of Rubber-Modified Epoxy Networks 

The fracture modeling of rubber-modified thermosets was developed by Huang and Kinloch (1992a), Kinloch and Guild (1996), Huang et al. (1993b), and Yee et al. (2000). 

Huang et al. (1993b) proposed a two-dimensional plane strain model, which was successfully used to identify the stress field around the rubbery particles and to simulate the initiation and growth of shear bands between rubbery particles. A model was proposed to quantify the different mechanisms. Gic of the rubber-modified network was written as 

AGr is proportional to the tearing energy (Ft) multiplied by the rubber volume fraction 0.  

AGj may be expressed as proportional to j , (compressive yield stress), Yi (fracture strain), the plastic zone size, and the square of the concentration factor, K. The influence of hydrostatic pressure was taken into account with a modified von Mises criterion (Chapter 12). 

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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). 

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. 

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. 

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]. 

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. 

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. 

Two chemically dissimilar polymers will naturally attempt to phase separate and the structure that is formed will reflect the way in which this process occurs and the driving forces associated with the process. Phase separation is used to achieve rubber toughening in thermoset resin systems. Low molar mass CTBN copolymer is soluble in the simple mixtures of monomers used to create amine-cured epoxy resins systems. However, as the molecular mass of the epoxy resin increases so the balance of entropy and enthalpy of mixing of these components changes and a driving force for phase separation is created. 

Recent systematic studies on the relation between network structure and substituents in kraft lignin, steam exploded, have shown that the lignin containing networks can be modified in new ways, cf. e.g. (80). Also the toughening of glassy, structural thermosets can be achieved by incorporating a variety of polyether and rubber-type soft segment components in the polymer network structure. 

An improvement in the toughness of thermosets can be favored by rubber or thermoplastic particles, which operate both in crack initiation and propagation mechanisms. The different toughening mechanisms can act simultaneously and can be modeled quantitatively. 

The importance of the science and engineering of toughened plastics is reflected in the successful series of symposia held on the topic under the auspices of the American Chemical Society. The first, on Rubber-Modified Thermoset Resins, was held in Washington, DC, in 1983 the papers from that conference were published in 1984 as Volume 208 of the Advances in Chemistry Series. The theme of the 1988 symposium, Rubber-Toughened Plastics, was broadened to cover both thermosets and thermoplastics. The papers from that symposium, held in New Orleans, LA, were published in 1989 as Volume 222 of the Advances in Chemistry Series. In 1990 the symposium returned to Washington, DC, and was titled Toughened Plastics Science and Engineering. The papers were published in 1993 as Volume 233 of the Advances in Chemistry Series. 

Hence a low molecular weight, reactive elastomer is normally used for impact modification of thermosets. The low molecular weight of the mbbery prepolymer aids its easy dissolution or dispersability in the thermosetting resin. The reactive functionality couples the rubber covalentiy to the growing polymer network during the curing reaction. Hence the rubber toughened thermosets may also be considered as co-reacted thermosets and not true blends. 

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