Toughening by Core-Shell Rubber CSR Particles

Core-shell rubber (CSR) particles are prepared by emulsion polymerization, and typically exhibit two or more alternating rubbery and glassy spherical layers (Lovell 1996 Chapter 8). These core-shell particles are widely used in thermoplastics, especially in acrylic materials (Lovell, 1996), and have also been used to modify thermosets, such as epoxies, cyanates, vinyl ester resins, etc. (Been et al., 1995). 

The emulsion free-radical polymerization carried out in different steps ensures a precise control of the particle size and particle size distribution. The particle diameter can be adjusted between 100 nm and 1000 nm, with a low polydispersity (generally less than 1.1) (Chapter 8). Rubber particles with sizes lower than 100 nm are ineffective for toughening purposes (Sec. 13.3.2b). 

The usual requirement is a rubbery core and an outer shell of a glassy polymer. The rubbery core is generally based on polybutadiene or butyla-crylate or copolymers, the choice affecting strongly the ductile- brittle transition temperature. 

The crosslinking of the rubber is important to maintain the structural integrity during the mixing and curing processes. In order to favor internal cavitation of the rubbery core and a good stress transfer, grafting of the inner shell with the rubbery core is necessary. 

The outer glassy shell is designed to prevent coalescence of rubbery particles during synthesis and drying, and to insure a good interface with the matrix. The shell is usually based on styrene/acrylonitrile or styrene/methyl methacrylate copolymers. Depending on the acrylonitrile or methyl methacrylate content, the compatibility between the shell and the matrix can be varied over a wide range. 

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