Fillers toughening effect

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. 

With ultrafine S1O2 filler, there is no toughening effect, even at high filler content up to 5% vol. Both the critical energy Jo.2 2nd the crack propagation resistance dJ/dAa are reduced compared to unfilled PP, leading to a very stiff and brittle material. 

Fig. 2.9. Influence of temperature and filler content on flow curves (Ref. 14). (a) Temperature effect on 1-part hot cure toughened epoxy. (i>) Filler content effect on 3-part flexibilised epoxy polyamine.

These observations have been observed for composites of several other thermosets and fillers. Lange and Radford [41] studied an epoxy (ERL 2774 Union Carbide Co.) filled with aluminium hydroxide and, indeed, were the first to report the toughening effect of a rigid particulate filler in a polymer matrix. Brown [42] investigated an unsaturated polyester... 

The addition of grafted silica nanoparticles into PP can bring in both reinforcing and toughening effects at rather low filler contents. Such a simultaneous improvement in modulus, strength, and elongation to break is hard to observe in conventional micron-sized particulate composites. 

Nanoparticles are able to provide PP with stiffening, reinforcing and toughening effects at rather low filler concentration. The influence of processing conditions on the nanocomposite structure, i.e. intercalated or exfoliated, and on the enhancement of mechanical properties of PP nanocomposites was studied by different researchers." " Most polymer/clay nanocomposites studies report... 

The way in which a filler is incorporated has an effect on fracture resistance. Figure 8.33 shows a schematic representation of the microstructure of fillers. The rubber particles are generalized as rubber particles added in a toughening process (a), rubber or polymer coating in core-shell microstructure, bound polymer, or surface coating (b). 

The effects of ceramic particles and filler content on the thermal shock behavior of toughened epoxy resins have been studied. Resins filled with stiff and strong particles, such as silicon nitride and silicon carbide, show high thermal shock resistance, and the effect of filler content is remarkable. At higher volume fractions (Vf > 40%), the thermal shock resistance of these composites reaches 140 K, whereas that of neat resin is about 90 K. The highest thermal shock resistance is obtained with silicon nitride. The thermal shock resistance of silica-filled composites also increases with increasing filler content, but above 30% of volume fraction it comes close to a certain value. On the contrary, in alumina-filled resin, the thermal shock resistance shows a decrease with increasing filler content. 

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