Characterization and properties of rectorite and rectorite-based clay films and bionanocomposites

Abstract

This research aimed to characterise the physico-chemical and structural properties of rectorite from a local deposit, the Beatrix Gold Mine in South Africa and to investigate its unique material properties. The ultimate research target was to explore the capability of natural and ion-exchanged mixed-layer clay rectorite to form binderless, non-isotropic (i.e. two-dimensionally oriented), self-standing, and mechanically strong clay-only films as a precursor to bionanocomposites with very high clay loading. The primary goal was to develop a facile “green” method, i.e. an energy-efficient, environmentally-friendly process without organic modifications, binders, plasticisers, thermal pre-treatments or other property enhancers. At these conditions the extent of process and product limitations was evaluated via statistical analysis to estimate the limits for the ultimate material performance. Three rectorite-rich samples from the Beatrix Gold Mine, South Africa, were obtained, purified to > 90 % rectorite content and characterised. Structurally and chemically, the purified materials from the three samples were very similar and were categorised as rectorite, except where applicable specifics are highlighted. Based on chemical composition by X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) evidence, the rectorite from Beatrix Mine was identified as Na-Ca-rectorite, a dioctahedral regular interstratification of mica-smectite type. Scanning electron microscopy (SEM) revealed a layered morphology. High resolution transmission microscopy (TEM) showed well distinguished light and dark layers of about 2.20 nm consistent with the 1:1 interstratified mica-smectite nature. XRD measurements confirmed the basal spacing d001 of 2.20 nm, indicating a one-water-layer structure. Unit cell parameters, for a monoclinic unit cell with primitive lattice, refined to a = 5.177 Å; b = 8.980 Å; c = 22.489 Å and β = 97.335 °, with a mean crystallite size around 14 nm and a calculated cell volume of 1045 Å3. The Greene-Kelly test suggested that the expandable smectite layers have montmorillonite-beidellite composition. NMR indicated a high degree of Al substitution and the presence of different Al sites corresponding to six- and four-fold octahedral and tetrahedral aluminium respectively. The chemical composition and diffraction data suggest that the mica is Na-Ca-rich, i.e. of paragonite-margarite series. The fixed interlayer regions (mica interlayers) contain proportionally dominant Na+ and Ca2+ and minor amounts of K+. The exchangeable smectitic interlayers contain almost equal amounts of Na+ and Ca2+ ions. The distribution of the interlayer Na+ ions was quantified by 23Na solid-state NMR spectroscopy. It points to a three component mixedlayer structure with considerable variation in the composition of the mica layer of the different samples. Rectorite clay-only films (RecF) were prepared by the simple device of allowing slow evaporation of aqueous dispersions. In this process the exfoliated natural clay sheets self-assembled into cohesive films. The inorganic films from neat and ion-exchanged rectorite clay were translucent and flexible, with a layered microstructure and they featured excellent mechanical properties. The multi-layered films consist of continuous layers of well-aligned clay platelets parallel to the casting surface. Layers overlapped randomly in the lateral direction (plane) and joined vertically in an irregular manner by edge to face cross-links (bridging) forming coherent multi-layered sheets organised in platelet-void nanostructured films. The films with highest mechanical performance had thickness below 30 μm. Overall films from rectorite clay with monovalent interlayer content exhibited highest strength ranging from 10 - 44 MPa and Young’s Modulus 2 - 56 GPa. The corresponding experimental values for films with divalent interlayer cations were 8 - 23 MPa and modulus 11 - 25 GPa. The neat Na-Ca-rectorite films had strength 12 - 34 MPa and modulus 3 - 9 GPa. Of all cation-exchanged modifications, only NH4-Rec films showed increased tensile strength compared to the neat material used for the exchange. Ultimate strength and modulus values were estimated via extreme-value statistical analysis. Some of these values compare favourably with values featured by polymer films.