Sepiolite needle-like clay for PA6 nanocomposites: An alternative to layered silicates?

This paper presents a study of polyamide 6/sepiolite clay nanocomposite, obtained by melt compounding in a mini twin-screw extruder, and characterised in terms of morphology (SEM and TEM analysis) and mechanical properties of the composite, along with their crystallinity and crystal structure of the semi-crystalline matrix. The tensile test results are interpreted in terms of two micromechanical models: the Halpin-Tsai model for stiffness and the Pukanszky equation for yield stress, and compared with the results on polyamide 6/smectite montmorillonite (MMT) clay nanocomposites from selected publications. This comparison between sepiolite (fibre-like shape, 1D) and smectite (platelet-like, 2D) clays offers an interesting model-study on the effect of nanofiller shape and orientation on final mechanical properties. It shows that 1D fillers are expected to provide better reinforcement in the case of oriented systems, whereas platelets should be more efficient in isotropic composites. It was demonstrated that sepiolite has a higher reinforcing efficiency (higher effective aspect ratio) than MMT in PA6 and a good filler-matrix interface, due to strong hydrogen bonds between silanol groups on the sepiolite surface and amide groups of PA6.