Thermosetting resins fracture energies

Fig. 13.41 The temperature dependence of the critical fracture energy Gic in DGEBA epoxy-resin thermosets, modified either by rubber particles or by debonding glass spheres, either in tests of conventional extension rates or in Izod impact tests, compared with the generally flat behavior of unmodified epoxy resin (Kinloch (1985) courtesy of Springer).

The formation of such grafts must modify the interfacial tension in a similar way to block copolymers, as discussed in chapter 6. The mechanical effect of grafting has been studied by Norton et al. (1995), who looked at the effect on fracture energy of grafting polystyrene with a carboxy end-group at an interface between polystyrene and a thermosetting epoxy resin. The unmodified... 

However, this theory has several drawbacks. It can only explain the modest increase in toughness and cannot explain the dramatic increase in fracture energy reported in many rubber-toughened epoxy systems. There are many discrepancies between reported data for improvement in toughness for thermoset resins due to the sensitivity of the toughening effect to the curing condition and inherent matrix ductility [119, 120]. The effect of G on test temperature and strain rate cannot be explained using... 

Another method used to improve the fracture energy of BMI resins consists in mixing the thermosetting material with linear thermoplastic polymers. This can be illustrated by the behaviour of mixtures containing Compimide 796 and TM 123 BMI resins with GE Ultem 1000 , poly(ether-imide) 26 (Fig. 13) [70]. The critical stress intensity factor Kic of the linear polymer is six times higher than that of the BMI matrix and does follow the mixture law for aU BMI/Ultem combinations. The linear polyimide can also be added as 20- 0 [im spherical particles to the BMI resin before it is polymerised. In another example, particles of a soluble... 

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