Effects of unfolded and intercalated halloysites on mechanical properties of halloysite-epoxy nanocomposites

Phenylphosphonic acid (PPA) was used to unfold a unique tubular clay mineral, halloysite. By varying the durations of PPA treatment, halloysites with different levels of enfoldment and intercalation were obtained. Halloysites, particularly with the unfolded and intercalated halloysites, were effective additives in increasing the fracture toughness of the cured epoxies without sacrificing their basic properties. Fracture toughness of composites was increased with increasing intercalation levels of halloysite, with the improvement being 78.3% in KIC for the composite containing 10 wt.% of the fully PPA-intercalated and unfolded halloysite. There was a substantial increase in the contact area between halloysite and epoxy, and as a result the morphology changed from nanotubes to nano-platelets in the composites with the treated halloysites. The treated halloysites achieved a better dispersion in epoxy and promoted the formation of a large number of micro-cracks and plastic deformations in the interfaces, resulting in an improvement in fracture toughness.