Toughening of epoxy with WS2 nanoparticles
This Thesis deals with the toughening effect of tungsten disulfide (WS2) nanoparticles (NPs) on epoxy, in particular that of inorganic, fullerene-like WS2 (IF-WS2) NPs. IF-WS2 was treated with three different chemical modifiers, two of which functionalized the NPs successfully. When the NPs were dispersed within ethanol by sonication, the surface functionalization significantly deteriorated the dispersion quality. The final agglomerate size of 200nm is obtained after 10 min of sonication. The primary particle size is significantly smaller (approx. 100 nm), but aggregation seems to limit the dispersibility. Sonication is ineffective in dispersing the NPs within an epoxy resin, while three-roll milling gives good andwell reproducible dispersion quality. Flaky WS2 exhibits larger agglomerate sizes than IF-WS2, but improves the fracture toughness more effectively.
The elastic modulus distribution in epoxy can be measured with a novel atomic force microscopy (AFM) technology, provided that the investigated surface is smooth enough. Reported nodules are likely AFM artifacts caused by too rough surfaces. It is unlikely that epoxy has modulus inhomogeneities that differ by more than 150MPa from the bulk modulus and that are larger than 10nm in size. The epoxy’s modulus in the vicinity of IF-WS2 NPs does not seem to differ from its bulk modulus.
The addition of either kind of WS2 NPs considerably improves the fracture toughness of certain epoxy systems, but other epoxy systems are hardly affected by them or even embrittle. The NP surface functionalization does not seem to improve the dispersion quality nor the toughening effect. IF-WS2 leads to the formation of secondary cracks, which create additional fracture surface. This might be an important toughening mechanism. However, the fracture surface increases even if the toughening effect of the NPs is negative, so that more complex mechanisms are more likely.
The two major factors determining the toughening effect are the type of curing agent and its quantity. If only polyetheramine-cured epoxy systems are concerned, the toughening effect tends to be higher for sub-stoichiometric epoxy systems, but the same is not true for some other curing agent types. It is unlikely that this is due to the resulting higher fraction of molecular network defects. For a given epoxy system, IF-WS2 NPs do not seem to differ significantly fromother kinds of NPs with respect to their toughening effect.