Core-Shell Particle Toughening

Preformed particles are incorporated into the epoxy matrix by simple mechanical mixing. The dispersibility of the particles can be improved by 1) introducing crosslinking into the shell or 2) using comonomer-like acrylonitrile or GMA, which increases the interfacial adhesion by polar or chemical interaction [96, 97]. Quan and co-workers [98] reported that for poly (butadiene-co-styrene) core poly (methyl methacrylate) (PMMA) shell particles, the cluster size reduces from 3-5 pm to 1-3 pm as a result of using 5 wt% crosslinker (divinyl benzene). They also found that the cluster size could be further reduced to 1-2 pm by using a methyl methacrylate-acrylonitrile (MMA-AN) or methyl methacrylate-glycidyl methacrylate (MMA-GMA) copolymer shell composition. 

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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. 

One way of overcoming this is to have the second phase already separated before cure, i.e. the toughener is insoluble in the epoxy resin before cure. Examples are found in toughened thermoplastics, where core-shell particles that optimize adhesion and compatibility and have a particle-size distribution to maximize toughness have been synthesized. These particles can be added at the desired volume fraction to achieve toughness without compromising performance rather than relying on the phase trajectory to achieve the desired... 

Pearson and Lee (1991) examined the effects of particle-size and particle-distribution effects on rubber-toughened epoxy resins. They examined a variety of CTBN liquid rubbers and a methacrylated butadiene styrene core-shell particle in a DGEBA-piperidine system. They found that the toughening mechanism for small particles was internal cavitation of the... 

It is postulated that the role of the HBP in toughening will be to act similarily to a coreshell particle that is, the core of the HBP will act to cavitate and promote shear yielding, and the shell will be able to be tailored to control aggregation and interactivity with the epoxy-resin matrix. Increases in core should promote cavitation, and shell-chemistry functionalization should increase dissolution and reactivity with the epoxy resin. However, unlike with the core-shell particles, the inherently greater number of shell sites and low viscosity of the HBP will enable the toughening to occur without deleterious effects on other properties. 

Cavitation was also identified as an active mechanism in systems where a mbberlike phase (particles or interphase) is susceptible to implode under the effect of the hydrostatic stress induced by the applied tension. Fond (36) has recently revisited the critical conditions under which this form of damage becomes energetically favorable. In the case of core-shell rubber-toughened PMMA, he ascribed the extensive whitening under tension at room temperature to the profuse formation of voids in the mbber shell of the toughening particles. 

Table 19.2 Comparison of low-rate values of fracture toughness (A"ic) and fracture energy (Gic) froni literature on the toughening of epoxy resins using core-shell particles. (The best increase in toughness is given in each case.)... 

Use has been made of the miscibility of polycarbonate and PMMA to assist in toughening poly(buiylaie terephthalate) with core-shell particles which have a PMMA shell [138]. By addition of a small amoimt 10 wt%) of polycarbonate, the immiscibility of PMMA and poly(butylene terephthalate) is overcome and good particle dispersions are achieved. The polycarbonate simply acts as an interfacial agent. The mechanism of toughening in this system has been studied... 

In addition to the systems described in the preceding sections, a number of other polymers and polymer blends have been rubber-toughened using core-shell particles. 

Various compatibilizing agents have been used to assist in the toughening of polyamides by core-shell particles Qontaining PMMA shells they include copolymers of styrene with maleic anhydride [150] and with acrylic acid [151]. Good particle dispersions and interfaces are achieved because the copolymers are miscible with the PMMA shells of the particles and react with the polyamide matrix. Blends of polyamides with ABS have been extensively studied, with copolymers of styrene and maleic anhydride again used as compatibilizers [152-156]. 

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. 

A very effective way of toughening is the use of core-shell particles instead of homogeneous rubber particles. The core-shell particles were commercially introduced as PVC impact modifiers in 1958 and since that time, their use has continuously expanded into new toughening applications, which now include a wide variety of engineering polymers (Cruz-Ramos 2000). In contrast to other impact... 

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