Thermal and Nano-Additive Based Approaches to Modify Porosity, Crystallinity, and Orientation of 3D-Printed Polylactic Acid

Date of Award

2023

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Khalid Lafdi

Abstract

Additive manufacturing (AM) has evolved as a convenient technology for rapid fabrication of prototype tooling and complex geometry components. Among all AM techniques, FFF is the most widely used for making polymeric structures. However, the process consistency and control of properties in the manufactured articles remains a challenging issue. The current study aims to investigate physical changes in polylactic acid (PLA) during 3D printing. The correlations between porosity, crystallinity and mechanical properties of the printed parts were studied. Moreover, the effects of bed-plate temperature were investigated. Experimental results confirmed the anisotropy of printed objects due to the occurrence of orientation phenomena during filament deposition and the formation both of ordered and disordered crystalline structures (α and δ, respectively). A post-3D printing heat treatment cycle was demonstrated as an effective method to improve mechanical properties by optimizing the crystallinity (transforming the α form into the δ form) and overcoming the anisotropy of the 3D printed object. The second approach to enhance the physical and chemical properties of neat PLA is by using nano-additives such as carbon nanotubes (CNTs) and carbon black (CB). As the concentration of carbon nanotubes increased the mechanical and electrical properties were improved even with low volume ratio of CNTs. In molten polymer and under shear force CNTs tended to align parallel to the shear direction leading to significant increase in electrical properties in the direction of alignment. Also, a change in the enthalpy of cold crystallization was observed. The enthalpy of cold crystallization of PLA/CNT samples was lower than pure PLA because of a change in the type of crystallites formed during cold crystallization. The presence of carbon nanotubes reduced the crystallization domain leading to the formation of unstable crystalline phase δ, which was remarkably disordered compared to that of the regular α form. The increase in the rigidity of the material was attributed to the presence of CNTs restricting the mobility of the PLA chain mobility in the amorphous phase. A process model was developed to simulate the extrusion of the nanotube loaded polymer and the resulting electrical conductivity property; this brought new perspectives because it highlighted the role of shear force in the alignment of CNTs. The experimental and numerical simulation data were in excellent agreement. This study introduces, for the first time, the influence of semi-crystalline polymer crystals on the performance of products in 3D printing. It has inspired further research efforts in this field. The use of nano-carbon additives for modifying PLA to enhance its electrical and mechanical properties also alters its crystalline structure. Subsequent work mainly focuses on obtaining better thermodynamic properties, such as improved thermal conductivity and heat dissipation, as well as thermal shock resistance, while ensuring the mechanical and electrical performance of the material using additives.

Keywords

Materials Science, Fused filament fabrication, Polylactic acid (PLA), Carbon nanotubes, Porosity, Crystal structure

Rights Statement

Copyright © 2023, author

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