Design, development and characterization of polypropylene clay nanocomposites
Creators: name: Pannirselvam, Muthu
Publisher: name: RMIT University
Year: 2024

description: Nanocomposites are attractive from an application point of view because they possess better tensile properties (higher stiffness, greater strength), better dimensional stability, improved barrier properties (lower liquid and gas permeability), higher heat distortion temperatures, and enhanced flame retardancy, with respect to the unfilled matrix material. Research on the preparation and characterization of polypropylene clay nanocomposites has increased continuously with the general aim of developing new polymeric materials with improved characteristics. Montmorillonite (MMT) clay is preferred over other fillers due to its large surface area to mass (780 m2/g) and high cation exchange capacity (~ 1 mol/kg monovalent cations). One gram of MMT clay provides nearly a square kilometre of surface. The special properties of MMT clay push the boundary of academic and industrial research in finding suitable material that will intercalate with clay layers and with non-polar polymers. Quaternary ammonium salts are a possible alternative for clay treatment; however amine treated clays break down through exhaustive methylation (Hoffmann elimination) at 200 °C. The processing temperature of PP is in the range of 200 °C, where the clay treated with amine leads to discoloration of the final composite and a decrease of clay layer spacing. The aim of this research is to select a suitable intercalant that will influence the interfacial adhesion between PP and unmodified clay. An objective is to choose an alternate intercalant with no amine group without sacrificing the hydrophobic (alkyl) molecule that is compatible with a non-polar polymer. There are many polar organic solvents and surfactants suitable for intercalation of clay layers. Initial experiments focused on the effect of clay treatment on intercalation with different polar organic solvents such as aldehyde, polyethers, alcohols, stearates, and oleates. Some of the intercalants were chosen for their potential to produce complex with sodium ions between the clay layers as well as their capacity to be adsorbed on the clay layers. It should be also noted that the neutral molecules with polar functional groups cannot react with PP. Some of the intercalants were deliberately selected because of their dual functionality to react with sodium ions of clay layers and to react with PP in presence of compatibilizer. XXI Characterization of treated clay d-spacing was studied by wide angle X-ray scattering (WAXS) technique, and mass loss was studied by thermogravimetry. Poly(ethylene glycol) monolaurate intercalated appreciably well satisfying initial objectives and requirements, compared with other intercalants. Poly(ethylene glycol) can form a complex with clay surfaces instead of ion-exchange of sodium ions. Poly(ethylene glycol) monolaurate (PEG-ML) has oxyethylene units that form complex with sodium ions of the clay gallery layer and has a long alkyl chain that is compatible with PP in presence of compatibilizer, maleic anhydride grafted Polypropylene (PP-g-MA). This prompted the selection of clay treated with poly(ethylene glycol) monolaurate for the preparation of nanocomposites. PEG –ML (Mn=400 and 600 g/mol) treated clay is denoted as PEGC and PEGM respectively. PEGC and PEGM were chosen as final candidates for the preparation of nanocomposites. PP-g-MA was added to a mixture of the treated clay and PP to facilitate dispersion and to act as a compatibilizer between the clay and the matrix hydrophobic PP. PEGC and PEGM (at 1, 2, and 5 parts per hundred (phr)) were used to prepare nanocomposites by solution blending. WAXS was used to elucidate the structural morphology of the nanocomposites. WAXS patterns indicated that PEGC and PEGM clay are well dispersed and preferentially embedded in the polymer matrix, at low clay ratios (1 and 2 phr). The exfoliation degree of the clay decreased with increasing organoclay content. Transmission electron microscopy (TEM) studies showed a better dispersion of clay in the PP matrix. The thermal stability enhancement of PP after adding treated clay was determined with thermogravimetry (TGA). The oxygen permeability values for all the hybrids prepared by solution blending were reduced by approximately 25 - 28 % of the corresponding values for pure PP. Dynamic mechanical analysis studies showed an increase in the storage modulus and a slight change in glass transition temperature for PP nanocomposite with respect to pure PP. Melt intercalation was employed to scale-up the process of preparing nanocomposites using clay treated with PEGM, as morphological, thermal, gas barrier, and dynamic mechanical properties was better than those for clay treated with PEGC. The treated clay was added to prepare nanocomposites by a melt intercalation technique using a twin-screw extruder (Brabender). In order to find the percolation threshold of PEGM in PP XXII nanocomposites, the clay loading was increased up to 7 phr in the melt intercalation process. The effect of treated clay on morphological, structural rheological, thermal, mechanical, and gas barrier properties of PP were analysed with transmission electron microscopy (TEM), wide angle X-ray scattering (Bruker AXS), and dynamic rheology (rheometer), mass loss and oxidation induction time (TGA), static mechanical analysis (tensile testing) and oxygen barrier property (Mocon) respectively. The exfoliation degree of the clay was observed as there was decrease in the intensity of the WAXS peak associated with interlayer spacing in all clay ratios (1, 2, 5 phr). Mixed intercalation, exfoliation and aggregation of clay layers were observed at 7 phr clay ratios. Furthermore, storage moduli (G'), and loss moduli (G') increased with polyether treated clay content. The presence of treated clay leads to pseudo-solid-like behaviour and slower relaxation behaviour of nanocomposites. The slope of G' and G' at 100 rad/s and 1 rad /s was calculated. The slope change indicated that the nanocomposites attained pseudo-solid like behaviour due to the nano-reinforcing effect of the intercalated / exfoliated clay. Temperature effects on the viscoelastic properties of PP-PEGM nanocomposites were studied at four melt temperatures ranging from 180 to 210 °C. Results indicated that the activation energy for PP-PEGM nanocomposites was higher than that of pure PP. The reason for increase in activation energy could be due to the higher concentration of polar molecules that increased the intermolecular forces leading to strong interactions between clay and PP. These strong interactions within the polymer matrix result in a higher resistance to flow and consequently higher activation energy. The mechanical properties of PP-PEGM nanocomposites showed 20 – 25 % increase in tensile modulus and tensile strength. The relationship between the gas barrier and thermal oxidation through the gas tortuous path theory are described. Thermal degradation of PP nanocomposites obtained by solution blending and thermal degradation, thermal oxidation and oxidation induction time of nanocomposites obtained by melt intercalation are higher compared with that of pure PP. Mass loss of nanocomposites prepared by solution blending and melt intercalation was compared. Temperature for 10 % and 50 % weight loss of nanocomposites was ~ 60 °C higher than that for pure PP.