Flow behaviour and foaming of recycled polyolefins and nanocomposites
thesisposted on 24.01.2014, 13:04 authored by Luis M. Conceicao
The flow behaviour of post-consumer recycled polyolefins, their blends and layered-silicate nanocomposites was studied by capillary rheometry and freesurface melt state elongational measurements to assess suitability for foaming applications. A novel, extrusion foaming technique and a flow simulation model were developed to attempt to correlate flow and foaming behaviour. The recycled polyolefins were high-density polyethylene (HOPE), low-density polyethylene (LOPE) and polypropylene (PP) of high-molecular weight, suitable for extrusion; that also contained also paper and inorganic fillers. LOPE-PP and HOPE-PP blends were prepared in a batch mixer with and without compatibilising agents, ethylene-propylene rubber (EPR) and ethylene-propylene (EP) copolymer. Several mixing conditions (temperature, time, rotor speed) were used to modify the morphology and the flow behaviour of the blend systems. In shear flow diverse pseudoplastic index, zero shear rate viscosity and the extensional viscosities, whilst, in uniaxial extensional flow the effects of process conditions on strain energy density, elongation at break and melt modulus were detected. The melt strength of uncompatibilised LOPE-rich systems is generally higher. The use of small quantities of EPR, 2.5 and 5.0% (w/w) can further improve the rheological properties in uniaxial extension, but shear flow behaviour is virtually unchanged by the compatibilisers. EP didn't improve the extensional flow behaviour of HOPE-PP blends. The influence of processing conditions and composition on the preparation of melt intercalated HOPE and HOPE-PP layered-silicate nanocomposites was conducted as a way of enhancing foaming behaviour; montmorillonite particles were dispersed with the aid of a compatibilising agent. Ideal mixing conditions (temperature, rotor speed and time) were determined for these nanocomposites based on rheological measurements. In HOPE nanocomposites, the melt elongation at break is reduced and the tensile modulus increased. The clay content has a significant influence on the melt tensile modulus. Wall slip behaviour appeared enhanced comparatively to unmodified HOPE. In HOPE-PP nanocomposite blends it is possible to modify the melt tensile modulus and elongation by changing blend composition and clay dispersion morphologies. A finite element model of capillary die flow was developed using commercial simulation software to assist the interpretation of an extrusion foaming technique. Experiments were carried out using azodicarbonamide as a blowing agent and a capillary rheometer and extruding the materials through a capillary die. Extruding at a shear rate of 300s-1 and at a temperature close to the melting point yielded the lowest foam densities; LOPE, the material with the highest melt strength and extensibility, achieved a density of 430 kg m-3. Blends of HOPE-PP, LOPE-PP and HOPE-PP nanocomposites didn't show an improved foaming behaviour with densities always above 600 kg m- 3 Oensities of 360kg m-3 and 500 kg m-3 were obtained with HOPE nanocomposites and unmodified HOPE, respectively.
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