Nanostructured polymers for additive manufacturing

Fused filament fabrication (FFF) is one of the most widely employed techniques of additive manufacturing, which produces three dimensional (3D) printed objects by the layering of melt-extruded thermoplastic-based filaments. Despite its ease of use and environmentally friendly nature, FFF has so far only provided a narrow range of potential applications due to the limited number of materials (mostly thermoplastic-based composites, with metal or ceramic fillers) compatible with this technique. Another obstacle for the wider application of 3D printed parts is their inferior mechanical performance compared to that of their conventionally-manufactured counterparts. A strategy for overcoming this deficiency is by merging polymer/clay nanocomposites with 3D printing. However, the incorporation of clays in the nanocomposite feedstock filament usually incurs several processing challenges, including clay agglomeration to the detriment of the formation of the printed part. This research aims to provide a systematic investigation and understanding of the influence of clay fillers and additives (coupling agents) on the mechanical properties and morphology of 3D printed nanocomposites. A series of polylactide (PLA)/clay nanostructured composite filaments were developed and successfully printed by an open-source 3D printer based on FFF. The effect of filament composition on the mechanical properties and morphology was investigated and correlated with the extent of intercalation of different clay types. The mechanical behaviour of the printed composite samples was influenced significantly by the clay type and content. For example, the samples containing organoclay with the same clay content exhibited a higher modulus of elasticity and strength than those with natural clay. In addition, the Halpin-Tsai model was found to be successful in predicting the moduli of the PLA/clay systems. Based on the experimental results, the mechanical properties of the PLA/clay composite systems were shown to be correlated to the extent of clay intercalation. An implication from the model is that clay intercalation was more effective as a reinforcement technique than raising the total clay content. Upon the introduction of Garamite clay in the polymer matrix, the flowability of the melt was improved followed by a decrease in the die swell ratio of the composite samples. As a consequence, the composite feedstock filaments provided an enhanced print resolution compared to neat PLA and resulted to a printed part with a more compact mesostructure. The research showed that the dispersibility of the nanophase was a general difficulty affecting nanocomposite performance. As a result, grafted PLA was added to act as a compatibiliser to the Garamite and Cloisite composite systems, in order to promote the dispersion of clays in the polymer matrix. It was found that the mechanism underlying the mechanical performance of the grafted PLA/PLA/clay composites was dependent on the clay morphology. Upon the addition of grafted PLA in the PLA/Cloisite composite, the mechanical properties were improved due to the increased interfacial interaction and wetting between PLA and Cloisite platelets. In the case of the PLA/Garamite system, however, the addition of various concentrations of grafted PLA did not substantially improve the mechanical properties. These findings could act as a guideline in the design and development of feedstock filaments for 3D printing.