Thermoplastic Toughening of Thermosets

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). 

Following the requests to increase toughness by keeping a high Tg, for several applications (the aerospace industry in particular), high-Tg or semicrystalline thermoplastics (TP) can be used instead of rubbers to modify thermosetting polymers (Hedrick et al., 1985 Pearson, 1993 Hodgkin et al., 1998 Pascault and Williams, 2000). 

The thermoplastic-rich phase may be separated in the course of polymerization (Sec. 13.4.2) or can be incorporated as a dispersed powder in the initial formulation (Sec. 13.4.3). A strong drawback of the in situ-phase separation for processing purposes is the high viscosity of the initial solution which results from the much higher average molar mass of the TP compared with the liquid rubbers. Also, for the same reason, the critical concentration 0crit has a smaller value (phase inversion is observed at smaller concentrations of modifier). 

Different TPs have been used to modify thermosets, such as poly(ether sulfone) (PES), polysulfone (PSF), poly(ether ketone) (PEK), polyether imide (PEI), poly(phenylene oxide) (PPO), linear polyimides, polyhydan-toin, etc. (Stenzenberger et al., 1988 Pascal et al., 1990, 1995 Pascault and Williams, 2000). 

The major trend observed is a modest increase in Kjc or by the introduction of initially miscible thermoplastics. This improvement is obtained without any loss in stiffness and thermal properties. Some very high improvements in Kic, claimed by some authors, are due to phase inversion, leading to a thermoplastic matrix with thermoset particles. 

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

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. 

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... 

Diez-Pascual AM, Shuttleworth PS, Gdnzalez-Castillo El, Marco C, Gomez-Fatou MA, Gary Ellis G. Polymer blend nanocomposites effect of selective nanotube location on the properties of a semicrystalhne thermoplastic-toughened epoxy thermoset. Macromol Mater Eng 2014 299 1430-44 [10.1002/mame.v299.12]. 

The modulus of thermosets, such as phenolic plastics, is much greater than that of thermoplastics because of the high cross-link density present in thermosets. Since these network polymers are brittle, they are usually toughened by the addition of fibrous reinforcements. 

In the case of thermosets toughened with thermoplastics particles (Sec. 13.4), this mechanism may be of a considerable importance because of the intrinsic toughness and/or ductility of these particles. 

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. 

Within the past several years, improvements in the toughening of high-temperature epoxies and other reactive thermosets, such as cyanate esters and bismaleimides, have been accomplished through the incorporation of engineering thermoplastics. Additions of poly(arylene ether ketone) or PEK and poly(aryl ether sulfone) or PES have been found to improve fracture toughness. Direct addition of these thermoplastics generally improves fracture toughness but results in decreased tensile properties and reduced chemical resistance. 

It was noted in the previous section that the carboxyl end groups on the CTBN elastomer affected the final performance of the material as a toughener since these groups would co-react with the epoxy resin and facilitate stress transfer from the brittle matrix to the phase-separated elastomer. Without this adhesion the particles could debond prematurely, which would lead to poor dissipation of the energy of the growing crack. It has also been noted that excessive adhesion between an epoxy resin and a thermoplastic could be deleterious to the performance (Williams et al, 1997). The process of toughening of a thermoset... 

Developing biphasic materials in order to improve the fracture toughness of thermoset resins is now a common practice. Thermoplastics that have a high glass-transition temperature (Tg) are used as tougheners in preference to low-Tg elastomers because of their insignificant effect on the thermal and modulus properties. 

A variant of thermosetting has been described in that PPE resins are dissolved in epoxy resins. A variety of polymers can be dissolved in epoxy resins." In order to facilitate the processability of PPE, the PPE is dissolved in an epoxy resin as processing aid. After processing by kneading, the epoxy resin is cured. In contrast to other approaches where the thermoplastic polymer acts as a toughener for the epoxy matrix, the amount of epoxy resin added can be adjusted so that the PPE will form the continuous phase in the final state. 

Several books have been devoted to the toughening of thermoplastic and thermosetting polymers [2-4]. 

As technology advances, materials have been developed which evade the above classification. Mixtures of thermoplastics and thermosetting resins have been developed in which the thermoplastics material acts as a toughening agent for the thermosetting resin. Alloys or blends of two or more thermoplastics, or of plastics and rubbers, are becoming commonplace. 

Gop Gopala, A., Wu, H., Harris, F., Heiden, P. Investigation of readily processable thermoplastic-toughened thermosets. I. BMIs toughened via a reactive solvent approach. J. Appl. Polym. Sci. 69 (1998) 469 77. 

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