Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemistry

Major Professor

Norma Alcantar, Ph.D.

Co-Major Professor

Theresa Evans-Nguyen, Ph.D.

Committee Member

Jianfeng Cai, Ph.D.

Committee Member

Abdul Malik, Ph.D.

Committee Member

Sylvia Thomas, Ph.D.

Keywords

Additive Manufacturing, Projection Sintering, Laser Sintering, Three-dimensional Printing

Abstract

As the requirements posed to products have increased in recent years. The trend towards individualized serial products steps up the need for respective manufacturing techniques to be more and more flexible. Conventional techniques of serial production, such as injection molding, are unable to fully meet the requirements of this trend. Additive manufacturing techniques generate components directly from a CAD data set while requiring no specific mold, producing minimal waste, and reaching satisfactory geometric accuracy. This is how, as opposed to conventional techniques, they comply with these increased demands to processing technology. Over the last two decades, the research community has developed impressive AM products and processes and applied them in various industries such as aerospace, biomedical, automotive, military, and architectural industries. Global sales for additive manufacturing devices, materials and products have grown to 33% over the last three years. Additive manufacturing techniques can be broadly classified into three main categories based on raw materials used i.e., liquid-based, powder-based and solid-based additive manufacturing. The first part of this work introduced the commercially available AM processes including Laminated object manufacturing (LOM), Ink-Jet printing, Stereolithography (SLA), Multi-Jet molding (MJM), Selective laser sintering (SLS), Selective laser melting (SLM), Laser melt deposition (LMD) and the new concept of sintering i.e., Large Area Projection Sintering (LAPS) and how it can resolve the challenges regarding additive manufacturing today.

Selective Laser Sintering (SLS) is a powder-based additive manufacturing process in which parts are selectively sintered by focusing high intensity laser on the selected areas of powder layers on the sintering bed. Due to its processability of a wide-ranging material (polymers, ceramics and metals), SLS is one of the most speedily growing AM technology which has applications in the fields of simple prototyping to functional prototyping and even rapid manufacturing and rapid tooling processes. Commercial LS systems use a focused laser beam with small beam diameter to process the materials with high resolution. Additionally, a single point of the powder bed is exposed for micro to milliseconds. High optical intensities and brief exposure time generate high local temperatures during sintering which degrade the properties of the final parts. It is observed that SLS parts have extremely poor mechanical strength to satisfy the functional requirements of the industry. To improve the mechanical properties and reproducibility of printed parts relative to SLS, Large area projection sintering (LAPS), a novel powder bed-AM technology is developed in Mechanical engineering department of University of South Florida, which uses longer exposure time and sinters larger areas. However, further fundamental research in the field of LAPS is required, to completely understand the connection between LAPS process parameters such as sintering time, bed temperature, intensity of light etc. and the part properties. This will allow users of this technology to fabricate customized parts with predetermined properties by establishing trendlines to relate build settings to the part properties. The second chapter addresses this need by investigating the influence of LAPS sintering parameters on the crystal structure, chemical structure, thermal and mechanical properties. Tensile test specimens of Polyamide 12 (PA2202) were fabricated by LAPS at various sintering times and temperatures and were characterized using Fourier transformation infrared spectroscopy (FTIR), x-ray diffraction spectroscopy (PXRD), small angle x-ray spectroscopy (SAXS), differential scanning calorimetry (DSC), and micro-hardness testing. The observable differences in the chemical, mechanical, crystallinity and thermal behavior of the specimens with respect to sintering parameters are discussed.

Powder-based additive manufacturing systems reflect the current state of the art in additive manufacturing systems, whereas representative techniques like Selective Laser Sintering (SLS) are limited in their production capacity by the library of materials available. Polymers are of particular research interest to researchers for powder-based additive manufacturing techniques with regard to their low melt temperatures when compared to ceramics and metals. However, only around 30 polymer materials are known to meet the strict requirements of powder-based additive manufacturing techniques and 90% of these materials are of a single compound class, Polyamide. Although Polyamides meet the rudimentary physical requirements of laser sintering techniques such as, a wide sintering temperature window, low zero viscosity, and adequate optical properties, they lacks the UV resistance common to poly ethylene terephthalate (PET). Concentrated thermal energy imparted on small areas of powder in techniques like SLS leads to warping of final specimen materials made from polyamide and similar polymers due to non-uniform cooling and differential viscosity issues (possible due to polarizability). As a result, the handful of current polymeric materials available for laser sintering processes fail to meet the requirements of industry due to their unsatisfactory thermal, electrical, and mechanical properties. Furthermore, the development of novel polymeric materials is rather expensive in addition to energy intensive.

Polymer blending provides a practical and efficient strategy to fabricate materials with desirable, synergistic properties that could otherwise not be obtained by homo-polymers. Nevertheless, many binary pairs of polymers are thermodynamically immiscible due to their intrinsically low entropy of mixing. This inherent propensity toward immiscibility leads to morphology coarsening upon cooling of specimens and ultimately degrades their mechanical properties. This makes reflection upon the compatibility of polymer components a key issue in developing control over the micromorphology and physical properties of blends as the mechanism relies heavily on the interface between materials. Typically, immiscible materials exhibit a high interfacial tension between unlike molecules. A typical method to overcome this physical phenomenon is to strengthen the adhesive forces at the interface between molecules through the incorporation of a surface-active co-polymer referred to as a compatibilizer. Such copolymers under these conditions act as surfactants to reduce interfacial tensions as well as enhance interfacial adhesion, suppress particle coalescence, and achieve a uniform phase morphology between molecules. Prohibitive engineering costs challenge the merit of pursuing traditional compatibilizers to increase the inter-facial localization stability between polymers. Conversely, inorganic nanofillers are gaining attention in the literature as promising compounds capable of improving the interfacial miscibility of material blends. Given their high specific area and fine scale, inorganic nanofillers such as nano silica exhibit great potential to accommodate the polymer–polymer interfacial area with appropriate wetting parameters. Inherent rigidity of inorganic nanofillers provides an improved inclusion capacity in interfacial areas as well as improved interfacial stability to that of engineered copolymer species, even when subjected to intense shear fields. The thermodynamic compatibility provided by the nanoparticles not only improves interfacial adhesion between polymer molecules, but also works to enhance the interfacial stability between them, protecting against mechanical shear force.

The research work in chapter three demonstrates the polymer material library expansion required for LAPS powder-based additive manufacturing techniques and determination of chemical, thermal, microstructural, and mechanical properties of specimens. Physically mixed powderous thermoplastic polymer blends consisting of polymers with complementary properties could allow the successful fabrication of components with tailored and graded properties. In the third chapter, the powderous miscible blend system Poly (ethylene terephthalate) (PET), poly (butylene terephthalate) (PBT) and Polyamide-12 (PA-12) (PET/PBT and PET/PBT/ PA-12) compatibilized with SiO2 nanomaterials, was produced from mechanically grinded polymer pallets at various weight ratios. SiO2 nanoparticles have outstanding electrical, mechanical, and chemical properties. Large Area Projection sintering of polymer blends was used to fabricate light weight and robust components layer by layer. The well dispersed inorganic SiO2 nano compatibilizers in the polymer blends promoted uniform stress transfer from the material matrix to the nanomaterials, improving the mechanical performance of the polymers for engineering applications in which stiffness and rigidity are the most important parameters.

Poly (ethylene terephthalate) (PET) and poly (butylene terephthalate) (PBT) are important classes of commercial aliphatic-aromatic polyester resins. The cost-effective thermoplastic polymer PET is well-known for its excellent properties such as chemical resistance, high mechanical strength and electronic properties. Therefore, this polymer resin is an important precursor in the automotive and aerospace industry where greater rigidity, higher thermal resistance, and economy are required. However, the LAPS processing of PET is difficult because of its slow crystallization rate and poor moldability. PBT resins have a rapid crystallization rate and good melting strength. The PET/PBT/PA-12 blends exhibit many good chemicals, mechanical and thermal features. The incorporation of PBT and PA-12 to the PET matrix can increase the impact strength of the virgin matrix and tensile and flexural properties. Furthermore, inorganic nanoparticles are often organically modified to achieve homogeneous distribution. Notably, studies related to the LAPS of PET–PBT-PA-12-SiO2 blends have not yet been reported in the literature.

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