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Bio-nanocomposites for packaging applications: physico-chemical properties of poly (lactic acid)/halloysite nanotube nanocomposite films
thesisposted on 22.02.2017, 00:56 by De Silva, Rangika Thilan
At present, petrochemical-based plastics are commonly used in many industries. Yet, environmental concerns over these materials have led to the search for biodegradable alternatives in several industries including packaging. In this regard, the renewable resource-based polymer, poly (lactic acid) (PLA), holds a greater demand due to its unique properties (such as biodegradability and biocompatibility). Despite the merits of PLA, limitations in mechanical, thermal and gas barrier properties have restricted the usage of PLA in a wider range of applications in the packaging industry. Therefore, it has become a necessity to reinforce PLA to overcome these drawbacks. The main objective of this research is to investigate the feasibility of halloysite nanotubes (HNTs) as a nanofiller to reinforce PLA. PLA/HNTs nanocomposite films were prepared using the solution casting method by varying the HNTs loading (from 2.5 – 10 (w/w %)). Evaluation of the physico-chemical properties of the films revealed that the mechanical and thermal properties of PLA films were enhanced with the addition of HNTs. The interfacial interaction was further investigated with Fourier transform infrared (FTIR) spectroscopy and the end-hydroxyl groups of PLA were found to chemically interact with outer surface siloxane groups of HNTs. Furthermore, a comparison study was conducted to investigate the influence of the nanocomposite processing methods, namely melt compounding and solution casting, on the thermo-mechanical properties of PLA/HNTs nanocomposite films. Moreover, the ductility of PLA/HNTs films was improved by incorporating a plasticiser, poly (ethylene) glycol (PEG), while the dispersion of HNTs in PLA at high filler loadings was further improved by modifying the outer surface of HNTs with a silane modifier, γ-aminopropyltriethoxysilane (APTES). In addition, after evaluating the effect of three different types of HNTs (which are structurally different) on the tensile properties, this study suggests the best type of HNTs that can be incorporated into the PLA matrix in order to obtain the optimum tensile properties. In this dissertation, an alternative finite element (FE) approach to accurately predict the elastic modulus of polymer/HNTs nanocomposites is proposed. A real-structure-based 3-D computational model with randomly oriented HNTs was developed and compared with the conventional, idealized modelling approach. The developed idealized model consists of nanotubes with fixed aspect ratio and the proposed alternative real-structure-based model takes the experimentally observed variations in HNTs sizes, impurities and aspect ratios into account. According to the parametric studies, a unit cell model with cylindrical reinforcements (representing HNTs) and at least 30 inclusions gave promising results, provided the model included actual information about HNT's size ranges and aspect ratios. Numerical studies were validated with experimental findings and the developed real-structure-based model gave more accurate results than idealized and analytical models. Furthermore, the reinforcing mechanism was carefully studied in terms of the stress distribution. As the final stage of this research, the PLA/HNTs nanocomposite films were further developed for better end user application by creating high performance, multifunctional, active packaging films. ZnO nanoparticles have been successfully investigated to remarkably enhance the antimicrobial properties of poly (lactic acid) (PLA) composite films for active packaging, where they can enhance the shelf life of goods, but the addition of ZnO into PLA decreases its thermo-mechanical properties. In this study, ZnO nanoparticles were deposited on the outer and inner surfaces of halloysite nanotubes (HNTs) using a novel solvothermal method and these ZnO deposited HNTs (ZnO-HNTs) were incorporated into the PLA matrix as a reinforcing filler. PLA nanocomposite films with ZnO had inferior mechanical properties compared to PLA films with ZnO-HNTs which showed significant improvements, with an increase in the tensile strength and modulus by 33% and 74%, respectively, with the addition of 5 (w/w %). Antimicrobial tests revealed that ZnO-HNTs can act as a promising antimicrobial agent against bacteria such as Escherichia coli and Staphylococcus aureus where the bacteria count reduced by more than 99%.