Tensile and fracture behaviour of isotropic and die-drawn polypropylene-clay nanocomposites. Compounding, processing, characterization and mechanical properties of isotropic and die-drawn polypropylene/clay/polypropylene maleic anhydride composites
As a preliminary stage of the present study, the physical and mechanical properties of polypropylene nanocomposites (PPNCs) supplied by Queen’s University Belfast were evaluated. Subsequently, polymer/clay nanocomposite materials were produced at Bradford, where mixing and processing routes were explored and the mechanical properties of the compounded samples were examined. Particular attention was given to the clay intercalation structure to support the overarching objective of optimising the tensile and fracture behaviour of both isotropic and die‑drawn PPNCs. Solid‑state molecular orientation was introduced into the PPNCs using the die‑drawing process. Tensile stress–strain measurements with video extensometry and tensile fracture tests on double‑edge‑notched tensile specimens were used to determine the Young’s modulus at three strain rates and the total work of fracture toughness at three notch lengths. The polymer composites were characterised using differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy, wide‑angle X‑ray diffraction, and transmission electron microscopy. Nanocomposite systems containing 3% and 5% clay with various compatibilizer (PPMA) loadings were prepared using three different mixing routes for isotropic sheets produced by compression moulding and tensile bars produced by injection moulding. Die‑drawn oriented tensile bars were drawn to draw ratios of 2, 3, and 4. Results from the Queen’s University Belfast samples showed a reduction in tensile strength at yield, likely attributable to poor bonding associated with inadequate dispersion. The presence of voids, potentially supported by intercalated PP/clay phases, may explain the observed improvement in elongation at break. The use of PPMA combined with an intensive mixing regime employing a two‑step masterbatch process successfully addressed compatibility issues and produced increases in modulus of approximately 40% and 50% for the 3% and 5% clay systems, respectively. After drawing, these improvements were reduced to around 15% and 25% relative to drawn neat PP. The work of fracture increased either through the addition of nanoclay, through drawing to low draw ratios, or through a combination of both. At moderate and high draw ratios, however, PPNCs may experience an increase in microvoid size at low clay loadings or microvoid coalescence at high clay loadings, ultimately leading to earlier failure compared with neat PP. Appropriate PPMA loading, combined with an optimised mixing route and clay content, can achieve a balance between the stiffness contribution of PPMA and the degree of bonding between clay particles and the isotropic or oriented polymer matrix. Control of spherulite size, silicate layer d‑spacing, and nanoparticle distribution—particularly the presence of intercalated microtactoids with possible semi‑exfoliated structures—has been suggested as key to optimising the final properties of PPNCs.
Tensile modulus, Video-extensometer, Fracture toughness, Die-drawing, Oriented polymers, Polypropylene, Nanocomposites, Polypropylene maleic anhydride
Networked Digital Library of Theses and Dissertations