Enhancement of interfacial properties through surface modification in poly(lactic acid) wood-flour composites

In order to develop filled polymer composites and blends from sustainable resources, the primary objective that needs to be considered is the nature of the interface between the natural filler and the polymer matrix. A fundamental understanding of filler-filler interactions and the filler-matrix interface is critical to the design and manufacture of biopolymer composite materials and the interfacial properties of such materials. The improvements achieved can lead to increase market share of such materials in more demanding applications. The early part of this thesis focuses on the efficacy of lignocellulosic fillers like wood-flour as a low-cost filler in Poly(lactic) acid (PLA) wood-flour biocomposites. A primary emphasis is on evaluating the effect of wood-flour on the mechanical, impact and thermal performance of the resulting biocomposites. The early part of this thesis also investigates the role that surface modification can play in improving interfacial interactions between PLA and wood-flour. The hydrophilic nature of lignocellulosic fillers introduces challenges in developing biopolymer composites with superior mechanical and thermal performance. Hence, a critical part of materials design for developing biopolymer composites is to deploy surface modification strategies to overcome the hydrophilic nature of lignocellulosic fibres. The implementation of surface modification through a variety of wet and dry techniques and a combination of both, do not always lead to improvements in interfacial adhesion. This lack of effect is primarily attributed to the heterogeneous structure and variation in the chemical composition of lignocellulosic fibres, which can vary from species to species for different varieties of biomass sources. In order to address the technical challenges, this thesis focuses more closely on the role of surface modification. Initially, surface modification of the filler or polymer matrix is investigated and compared. In the middle chapters, the role of pre-treatment through chemical (alkaline) or physical processes (plasma) is conducted in conjunction with chemical treatment in order to evaluate the influence on the practical adhesion at a macro level. And finally, the influence of pre-treatment and chemical modification is assessed on a micro level for their effect on the PLA composites themselves. The interfacial properties of PLA/wood-flour composites were evaluated using two strategies: the first approach evaluated chemical modification of wood-flour using methylene diphenyl isocyanate (MDI), whilst the second strategy focused on the physical modification of PLA using polyethylene acrylic acid (PEAA) as an impact modifier. MDI was found to improve the interfacial adhesion between PLA and wood-flour and this was confirmed by Electron Probe Microanalysis (EPMA). The analysis indicated that MDI was present at the interface between PLA and wood-flour but also distributed to some degree within the PLA matrix. In comparison, PEAA was found to lower the tensile strength of the biocomposites but increased the impact performance. The enhancement in impact performance was due to the formation of a separate phase observed within the PLA matrix, which comprised spherical domains. The presence of these domains impeded crack propagation, resulting in higher impact strength. The heterogeneous nature of lignocellulosic materials and their varying chemical composition, contribute to their reduced surface functionality. In an effort to promote increased surface functionality, a pre-treatment process was used in conjunction with chemical modification. A macro-scale method was developed to evaluate the effectiveness of silane modification performed in conjunction with alkaline pre-treatment on the surface chemical and physical characteristics of a model wood substrate. Three silanes with different chemical end groups were used to modify the surface of wood. PLA film was bonded to the model wood surface using a melt press and the level of practical adhesion was assessed by peel adhesion testing. Surface characterisation studies proved that alkaline pre-treatment and subsequent chemical modification with an amino silane produced the highest peel fracture energy following peel adhesion. The macro scale, peel adhesion method for evaluating the effectiveness of different surface modification techniques was used to assess the suitability of plasma techniques for modifying the surfaces of a model wood substrate. Surface treatment using plasma based techniques has the potential to be a more benign approach to activate the surfaces of lignocellulosic substrates. Low-pressure plasma treatment was conducted on pinewood veneers using a custom built 13.5 MHz Inductively Coupled Plasma Chamber. A dual plasma pre-treatment process that involved a combination of continuous wave and pulse mode was found to be more conducive to optimising surface active groups. The results of XPS analyses showed that plasma treatment based on a dual process was more effect than a single process plasma treatment due to the higher levels of oxygen relative to carbon (O/C) and O functional groups. The main peaks that proved the benefits of the dual plasma process was the higher atomic ratio of oxygen, which was attributed to a higher incidence of surface hydroxyl groups. Plasma treatment alone of the wood veneer produced a more homogeneous surface morphology, compared to unmodified wood veneer. The appearance of nano-like nodules were more even in height and furthermore, evenly distributed across the wood veneer surface. AFM was successfully used to define the surface morphology that was created following plasma treatment and chemical grafting using an amino silane and two polyethyleneimines with different molecular weight. AFM also showed that grafting with the amino silane did not alter the surface morphology. AFM phase imaging showed variations in PEI grafting onto plasma modified wood veneer. Peel adhesion studies showed that plasma pre-treatment combined with chemical modification (ex-situ) lead to significant enhancements in the fracture energies of wood-PLA laminates that were superior to those following low-pressure plasma treatment alone. The findings of the macro scale studies were used to prepare PLA based composites and evaluate the effect on the mechanical and thermal performance. PLA biocomposites were produced from wood-flour subjected to pre-treatment using alkaline treatment or low-pressure plasma and then chemically modified (ex-situ) using an amino silane and two polyethyleneimines. A micro extruder was used to compound PLA wood-flour formulations and mini injection moulder was used to prepare sample test bars. The results of mechanical testing showed that pre-treatment using a low-pressure plasma dual process (CW+P) combined with chemical modification (ex-situ) produced composites with enhancements in the flexural and impact properties of the resulting composites. Low-pressure plasma treatment of wood-flour alone did not enhance the mechanical and impact performance, compared to untreated wood-flour and wood-flour subjected to alkaline treatment. DMA testing also showed the benefits of using low-pressure plasma treatment combined with chemical treatment, due to enhanced interfacial properties of the resulting biocomposites. Low-pressure plasma treatment of lignocellulosic fibre/fillers is a useful approach and an alternative to more conventional chemical pre-treatment processes including alkaline treatment.