Toughening principes

One of the principal weaknesses of pure polystyrene is its low impact resistance. To counteract this problem, we toughen it with various types of rubber. This is most effective when a portion of the rubber is chemically grafted to the polystyrene. The rubber forms small inclusions within a matrix of polystyrene. The presence of rubber also improves polystyrene s extensibility, ductility, and resistance to environmental stress cracking. 

An electron micrograph of a fracture surface of a CTBN-toughened epoxy resin is shown in Figure 5. This CTBN is particular in that the in situ formed particles are less than 0.5 /zm in diameter. A tensile bar of this system also shows shear deformation which indicate that the small particles have not interfered with the shear deformation characteristic of the unmodified resin. The deformation bands are nearly parallel to the planes of maximum shear stress—i.e., roughly at 45° to the principal... 

Thermal analysis as well as particle morphology and redispersion of precrosslinked poly(organosiloxane) microparticles proved to be promising with regard to an application as potential toughening agents. This caused us to investigate the principal suitability of silicone particles as modifiers for thermoplastic polymers. 

A variant of rubber toughening involves the use of preformed core-shell rubbers comprising a highly cross-linked polybutadiene core with a grafted shell of a vinylic polymer. In this case, the particles are small, typically ca. 0.1 pm, and thus have little effect on the observed viscosity of the epoxy. One of the principal advantages of this over simple rubber toughening is the ability to produce predetermined controllable morphology in the cured polymer. " ... 

Shear yielding is well established as the principal deformation mechanism and source of energy dissipation in both uiunodified and rubbo -toughened epoxy resins [2,3,27,83,121]. As molecular mobility in the epoxy resin network chains decreases, the ability of the matrix to deform by shear yielding is reduced. This is the reason why epoxy resins become both more brittle and more difficult to toughen as the epoxy resin crosslink density increases and/or as the network chains increase in rigidity, e.g. by use of highly aromatic epoxy resin monomers (see Section 19.7.1.1). 

As one might intuitively expect, the incorporation of rubber particles within the matrix of brittle plastics enormously improves their impact resistance. Indeed, the impact resistance imparted by the rubber is the principal reason for its incorporation (Rosen, 1967) in rubber-plastic blends and grafts. Toughening in such polymers is also observed under other loading conditions, such as simple low-rate stress-strain deformation and fatigue. It is believed that several deformation mechanisms are important in all such cases, though their relative importance may depend on the polymer and on the nature of the loading. 

The principal weakness of ceramics is their brittleness and toughening can be achieved by... 

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