Effect toughening

At the present time it is generally accepted that the toughening effect is associated with the crazing behaviour.Because of the presence of the low-modulus rubber particles most of the loading caused when a polyblend is subject to mechanical stress is taken up by the rigid phase (at least up to the moment of... 

By plasticisation. This in effect reduces the Tg and in the case of nylon which has absorbed small quantities of water the toughening effect can be quite substantial. It should, however, be noted that in the case of PVC small amounts of plasticiser actually reduce the impact strength. 

Table 3.9 Examples of toughening effects on properties of polyamide and polyacetal...

D. Li, H. Xia, J. Peng, M. Zhai, G. Wei, J. Li, and J. Qiao, Radiation preparation of nano-powdered styrene-butadiene rubber (SBR) and its toughening effect for polystyrene and high-impact polystyrene, Radiat. Phys. Chem., 76(11-12) 1732-1735, November-December 2007. 

This substantial toughening effect of CTBN on the cycloaliphatic epoxide ERL-4221, coupled with significant increase of the strength of the resin without seriously lowering the heat distortion temperature, is quite unusual for thermosetting systems. 

In Secs. 13.2-13.3 the principles of toughening of thermosets by rubber particles, and the role of morphologies, interfacial adhesion, composition, and structural parameters on the toughening effect are analyzed. Section 13.4 is devoted to the use of initially miscible thermoplastics for toughening purposes. The effect of core-shell rubber particles is discussed in Sec. 13.5 and, in Sec. 13.6, miscellaneous ways of toughening thermosets (liquid crystals, hybrid composites, etc.), are analyzed. 

As discussed in the previous section, the toughening effect depends both on the matrix, where the shear bands are propagating, and the rubbery phase, which induces cavitation and crack bridging. 

It has been assessed that bimodal particle size distributions consisting of a population of small and large particles may exhibit a better toughening effect than unimodal ones, due to a synergistic effect (Chen and Yan,... 

The toughening effect at room temperature increases with a decrease of the matrix Tg. 

For some thermoplastics such as PEI, a toughening effect is observed even in the absence of reactive end groups (no covalent bonds are formed with the matrix). Possibly, physical interactions are strong enough to produce an adequate adhesion level for toughening purposes. 

As in rubber toughening, the cavitated particles induce shear yielding of the matrix, producing a plastic zone smaller than the cavitated zone. These mechanisms induce a significant toughening effect. Should matrix yielding precede the cavitation of CSR particles, the croiding mechanism would be suppressed. 

The influence of the shell thickness has also been reported (Sue et al., 1996a). By varying the shell thickness from 15% to 35% of the total thickness, the dispersion of CSR particles changed from a random to a well-dispersed but locally clustered distribution. The latter produced a higher toughening effect, probably due to the promotion of a crack-deflection mechanism, in addition to the other toughening mechanisms. 

A crosslinked rubber may be synthesized at the surface of the glass beads to produce a core-shell structure (glassy core and rubbery shell). Thermosets modified with these particles showed a strong toughening effect for an optimum thickness of the rubbery shell (Amdouni et al., 1992). 

All else being equal, a toughening effect occurs if the crack tilts or twists away from a planar geometry because this reduces the net crack driving force. In homogeneous materials such as glass, cracks tend to propagate in... 

In general, the elastomer must be prereacted (adducted) with the epoxy for the toughening effect to take place. Adducts reduce the likelihood of early phase separation and maintain the solubility of the elastomer in the uncured resin system. For CTBN the reaction is carried out at high temperatures (150 to 160°C) and usually in the presence of a catalyst, such as tris-dimethylamino phenol or piperidine. The resulting epoxy-CTBN adducts are available from several suppliers, and they can be easily formulated into epoxy adhesives. 

In conclusion, the cohesive surface description presented in the foregoing sections appears suitable for capturing a ductile-to-brittle transition with increasing loading rate, and for predicting a toughening effect when the bulk is essentially elastic. These trends are reported experimentally and a calibration of the parameters used in the cohesive zone description is presented in [64],... 

It would appear that these rubbers have a similar toughening effect as the S-EB-S block copolymers. However, a synergistic effect is present when a mixture of both impact modifiers is employed [27]. 

---