Core-shell particles, toughness enhancement

This investigation of silicone-modified hybrid composites demonstrates that compatibilized, segmented liquid rubbers can be tailored to promote the formation of colloidal rubber dispersions, which enhance the toughness of the epoxy matrix. Furthermore, such segmented liquid rubbers can in situ-modify filler surfaces to form core-shell particles with a hard filler core and a thin... 

Toughness enhancement can be realized by homogeneous rubber particles, core-shell particles, or heterogeneous modifier particles in the so-called disperse systems. The rubber content varies usually between 5 and 30 wt.%. An alternative is a network arrangement of the rubber in inclusion systems or network systems. 

Often, very small rubber particles or modifier particles are used to enhance the toughness, for instance, of PMMA or PP at lower temperatures. Figure 5.12 shows schematically one example with PBA core shell particles they consist of a hard core of PMMA (diameter about 180 nm) and a rubbery shell of poly(bu-tyl acrylate-co-styrene) (PBA) (approximately 40 nm thick). An outer PMMA shell increases compatibility between particles and matrix. The particles were preformed and possess spherical shapes with a narrow size distribution. Under load, the plastic deformation starts in the particles with cavitation and fibrillation of the rubbery shell. The second step is deformation in highly stressed zones between the particles in the form of crazes or homogeneous yielding. 

The thin inner shell of the core-shell polymers contained less than 20% rubber by weight, with 60% by weight of a plastic core in the particles. It is unusual to enhance the toughness of the normally brittle epoxy-resin system at such a low level of rubber content. We believe that the unique design, the use of a multilayer toughener with a thin, elastomeric inner layer, and the large plastic core together produced behavior like that of solid rubber particles and, hence, enhanced crack resistance. 

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