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Composites of polymers with 2D materials : boron nitride nanosheets (bnns) and exfoliated graphite nanoplatelets (gnp) by melt-mixing
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Guerra, Valentina (2020) Composites of polymers with 2D materials : boron nitride nanosheets (bnns) and exfoliated graphite nanoplatelets (gnp) by melt-mixing. EngD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3520308~S15
Abstract
2D materials (nanomaterials) such as boron nitride nanosheets (BNNS) and exfoliated graphite/graphene nanoplatelets (GNP) are of increasing interest to both academia and industry due to their exceptional mechanical, thermal and electrical (GNP only) properties. Such properties could potentially be transferred to thermoplastic polymers, for the manufacture of functional composites for a variety of applications ranging from automotive to aerospace, electronics and energy. The morphology and geometry (i.e. thickness, platelet length, shape and, aspect ratio) of BNNS/GNP along with the surface chemistry, the state of filler dispersion/distribution in the polymer matrix, filler-filler/polymer-filler interfacial interactions and the processing methodology adopted to prepare the composites all affect the final properties. It is generally understood that high aspect ratio BNNS/GNP, strong polymer-filler interactions, and high levels of filler dispersion and distribution in the matrix are required to form a percolated 3D filler network in the matrix, essential to achieving enhanced mechanical, thermal and electrical (GNP) properties.
The inclusion of BNNS and GNP in commercial polyolefin such as polypropylene and high density polyethylene is a non-trivial task. BNNS/GNP tend to agglomerate upon mixing with the polymer matrix, due to the strong particle-particle interactions preferred over the particle-polymer interactions. In addition, the presence of functionalities and/or traces of additives derived from the manufacturing of the BNNS/GNP particles affect the state of distribution and dispersion thereof in the polymer matrix, thus the properties of the final composites (e.g. morphology, mechanical and rheological properties, thermal conductivity). Little information is available in the literature on the structure-properties correlation for composites of polymers, particularly polyolefin, with BNNS/GNP prepared by relevant industrial melt-mixing based processing techniques. This limits the application of such composites on industrial scale.
The present thesis focusses on the characterization of different grades of BNNS and GNP as provided by the industrial partner Thomas Swan & Co. Ltd (TS) and their application as functional additives in isotactic polypropylene (PP) and high density polyethylene (HDPE) prepared through extrusion and injection moulding. The complexity and challenges for the manufacturing of functional composites of PP and HDPE with TS BNNS/GNP lay on the morphology and surface chemistry of the filler particles, the mixing efficiency during extrusion, the melt-flow realised during injection moulding, the viscosity and crystallization mechanisms of the polymer matrices.
The chemical and physical characteristics of BNNS and GNP, prepared by high pressure homogenisation (HPH)- a process developed by TS- were determined and the morphology/crystalline structure (Scanning Electron Microscopy (SEM); X-Ray Diffraction (XRD); Raman spectroscopy), surface chemistry (X-Ray photoemission spectroscopy (XPS) and sessile tests) and thermal stability (Thermo-gravimetric analysis (TGA)) of both fillers reported. The pressure applied during HPH, the morphology of the bulk hexagonal boron nitride and graphite fed into the HPH, the solvent and the surfactant exploited for the exfoliation of the bulk materials all govern the final properties of the BNNS/GNP. The grades of fillers examined in this thesis showed a distribution of platelet sizes (from 100 nanometre (nm) up to tens of micrometre (µm)) layered as irregular flakes, with high crystalline structure and low defect concentration in the crystal lattice. The fillers contained functionalities and impurities on the surfaces (mostly derived from the surfactants used during HPH) which imparted hydrophilic characteristics to the particles. All the BNNS and GNP analysed were thermally stable under oxidative conditions up to 300-400°C.
Composites of PP and HDPE with both BNNS and GNP were prepared by 16 mm co-rotating twin-screws extruder followed by injection moulding, over a range of compositions with filler loadings up to 10wt%. The morphology, crystalline structure, rheological and mechanical properties of the as prepared composites were assessed by SEM, XRD, Differential Scanning Calorimetry (DSC), oscillatory rheology and tensile testing. The processing adopted to prepare the composites of PP/HDPE with the BNNS/GNP grades produced composites with different levels of filler particles dispersion and distribution.
The extent of dispersion and distribution of BNNS/GNP in PP appeared to be strongly affected by the mechanism of interactions with the surfactant present in trace amounts on the surfaces of the filler platelets, namely sodium cholate (SC), T and L. It was found that SC and L were more effective at distributing and dispersing BNNS in PP compared with T. On the other hand, SC was more effective at distributing GNP in PP than L. Consequently, an increase in the Young’s modulus of PP by up to 20% was obtained when the BNNS with SC and L and, GNP with SC were added to PP. In contrast, a decrease in the Young’s modulus of PP of ca 20% was observed when the BNNS prepared with T and GNP with L were included to PP.
The lateral dimension of the fillers play a key role in achieving rheological percolation. The BNNS particles with a lateral dimension up to 10 µm facilitated the formation of a 3D particle-particle network. However, no improvement in the thermal conductivity of the polymer was recorded due to the phonon-scattering at the polymer-filler interface, perhaps also caused by the presence of surfactant on the filler surface and/or by the PP chain entanglements near the BNNS particles.
The BNNS and GNP had a nucleating effect on the PP, manifested by an increase in the crystallization temperature by up to 14°C higher than the Tc of the neat PP. The different grades of BNNS and GNP examined also promoted the crystallization of the β-crystals (polymorph) of PP, particularly at low filler content (≤0.5wt%). The inclusion of the BNNS and GNP particles did not affect the morphology and crystalline structure of HDPE. A decrease in the Young’s modulus of the HDPE of ca 20% was observed in presence of the fillers, probably due to the agglomeration of the BNNS/GNP as the HDPE crystallizes rapidly during injection moulding excluding the nanoparticles from the crystalline phase.
In summary, this work confirms the importance of the dual non-trivial technical challenges of promoting polymer-filler interactions and achieving effective dispersion and distribution of the nano-filler throughout the polymer matrix when manufacturing composites by relevant industrial technologies i.e. extrusion and injection moulding. The outputs of the present research supported TS to understanding the properties of their BNNS/GNP and the requirements for the optimal inclusion thereof in polymers matrices.
Item Type: | Thesis (EngD) | ||||
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Subjects: | T Technology > TA Engineering (General). Civil engineering (General) | ||||
Library of Congress Subject Headings (LCSH): | Nanostructured materials -- Properties, Nanostructured materials -- Design and construction, Polymers, Thermoplastics, Boron nitride -- Mixing, Graphite -- Mixing | ||||
Official Date: | June 2020 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Warwick Manufacturing Group | ||||
Thesis Type: | EngD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | McNally, Tony (Nanotechnologist) | ||||
Sponsors: | University of Warwick ; Thomas Swan & Co Ltd | ||||
Format of File: | |||||
Extent: | ix, 164 leaves : illustrations (some colour) | ||||
Language: | eng |
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