Toughening, second phase

Solvent-modified and macroporous epoxies prepared via the CIPS technique are ideal materials to verify these predictions and to throw some light on the ongoing discussion on the role of the second phase and cavitation for the toughening of thermosets. 

The aim of toughening a matrix by adding second-phase particles is essentially to increase its strength. For cracks in homogeneous materials under uniform... 

Toughening mechanisms in a/p-sialon composites are similar to those operative in second-phase particle reinforced composites, but, rather than the deliberate addition of a second phase, a/P-sialon composites are fabricated by simultaneous crystallisation of the two solid solutions a- and P-sialon from a eutectic composition liquid. This requires careful design of the starting composition which is usually located within the (a + P)-sialon region of the a-sialon plane as illustrated in Fig. 18.1. 

A statistical framework has been established for describing the spatial dispersion of second phase particles in a continuous matrix. Based on this scheme a computerized image analysis method has been developed for characterizing the morphology of toughened plastics. 

Polypropylene homopolymer (PP) is a widely used thermoplastic material, despite its brittle behaviour at either low temperature or high loading rates. Improvement in the fi acture toughness of PP can be achieved by either modifying the crystalline structure, or addition of a second phase material [16], The toughening effect and mechanisms of different second phase materials such as stiff fibres, soft rubbery inclusions (EPR, EPDM), and some mineral fillers have been analysed. Recent developments concern the effect of hybrid system consisting of rigid and rubbery inclusions. 

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... 

We proposed a new test method to evaluate the thermal shock resistance of epoxy resin (7). This test method uses a notched-disk specimen, and the thermal shock resistance can be evaluated analytically on the basis of linear fracture mechanics (8). In our previous studies, we reported on the use of our proposed thermal shock test and evaluation methods (8, 11) to determine the thermal shock resistance of toughened epoxy with a soft second phase (9, 10), and also with hard particulates (11). 

McGarry, F. J. Wlllner, A. M. "Toughening of an Epoxy Resin by an Elastomeric Second Phase" R68-8, Massachusetts Institute of Technology, March, 1968. 

We can add a second phase to modify the mechanical properties of a ceramic. An example we discussed in Chapter 18 is the toughening of alumina using zirconia. In this application again, the materials should not react (which is why zirconia is used). 

Figure 10. Schematic presentation of toughening mechanisms. Frontal-wake mechanisms (a) dislocation glide, (b) microcracking, (c) phase transformation, (d) ductile second phase. Bridging mechanisms (e) grains, (f) fibers, (g) whiskers, (h) ductile second phase. Reproduced with permission of.(9).

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