Additive manufacturing (AM) is a form of production that directly processes raw materials into their final form by building the product in a layer-by-layer fashion. Numerous types of AM exist, including selective laser sintering (SLS) of polymeric powders, vat polymerization (VP) of low viscosity photocurable resins, and material extrusion (MatEx) of thermoplastic or high viscosity composite materials. Because of its ability to reduce material waste while printing complex geometries, AM has the potential to revolutionize the manufacturing industry for a diverse set of materials and products.
MatEx of thermoplastic feedstocks is most commonly performed using fused filament fabrication (FFF) – a form of melt extrusion. A solid filament is fed directly into a heated nozzle, where it melts onto a build bed before resolidifying in a matter of seconds. While this is the most common form of AM, especially among hobbyists, the material catalog is limited to thermoplastic polymers, and difficulties arise when fillers are introduced (e.g. reactions at elevated temperatures, clogging, disruption of polymer chain diffusion, and large increases in viscoelastic properties). To combat these challenges, direct ink write (DIW) AM extrudes highly viscous composites by applying pneumatic backpressure to a syringe, such that the material can be extruded in ambient conditions. This method enables processing of unreacted, thermosetting resins which have been filled with a large proportion of solid particulate fillers, called "highly filled" inks. The interparticle network formed from particle-particle interactions in the form of weak surface forces (e.g. Van der Waals forces) provides structural stability of the printed lines, such that they can sustain the weight of subsequent layers.
In the realm of DIW 3D printing material discovery and processing, there are currently three major challenges. First, the high shear region of the nozzle frequently disrupts the interparticle network through a de-agglomeration process, such that there is a finite timescale for the interparticle network to reestablish itself. During this timeframe, the deformation/reformation process causes printed lines to sag, which negatively impacts both print quality and mechanical properties. Second, printed parts require a post-processing step to develop adequate mechanical properties suitable for the final product. The kinetics of this cure process are extremely slow, often taking multiple days or weeks to reach completion. Third, high shear rheological characterization of highly filled inks is challenging because of the numerous artifacts of error associated with high shear testing environments (e.g. sample loss/edge fracture, slip, and large sample size requirements). A literature review in Chapter 2 outlines the most recent advances in highly filled polyurethane processing for DIW, with a particular focus on how interparticle network recovery – in the form of thixotropy – can be tailored using a variety of reactive inks.
The subsequent chapters of this dissertation address these challenges by systematically downselecting reactive inks appropriate for highly filled DIW extrusion while introducing numerous process relevant rheological protocols. An initial discussion in Chapter 3 covers the potential drawbacks of thermoplastic polyurethane (TPU) processing as it relates to industrial scale melt extrusion. Specifically, multiple side reactions and degradation processes are identified for a variety of TPU manufacturers. Such reactions elicit undesirable solid-like particulate buildup within the extrusion line, and the impacts/causes of these reactions are quantified using rheological criteria. These protocols offer evidence that differences in processability can arise not just between manufacturers, but also between lots of TPU from the same manufacturer.
To address these concerns, Chapter 4 offers an alternative form of polyurethane processing in the form of a thermosetting reaction between hydroxyl-terminated polybutadiene (HTPB) and isophorone diisocyanate (IPDI). When uncatalyzed at room temperature, full conversion takes place over the course of multiple weeks which necessitates an accelerated kinetic analysis. Hence, a combination of chemorheological and spectroscopic methods are used to rapidly probe for changes in isocyanate reactivity using limited sample quantities, which substantiate the advantages and disadvantages of chemorheology and spectroscopy in the context of curing studies.
While this synthetic pathway provides mechanical properties appropriate for the final printed product, a major concern is retention of green body strength post deposition. In order to maintain the shape of printed beads, ultraviolet (UV) light can be shined in-situ onto the nozzle of a DIW printhead, which actively cures the urethane acrylate ink through free radical polymerization. This technique, termed UV-assisted direct ink write (UV-DIW), assists recovery of the interparticle network. A novel rheological method proposed in Chapter 5, termed the "UV-assisted three interval thixotropy test" (UV-3ITT), quantifies the contribution of UV light towards structural stability and printability. This is accomplished by applying stepwise changes in strain on a torsional photorheometer, optionally applying UV light in the third interval, and then quantifying the contribution of UV light towards process-relevant recovery parameters. Resultingly, the threshold of solid particulate fillers required for UV light to improve print fidelity is determined.
While most discussions revolve around torsional rheology, this method has one major drawback: it cannot probe the high shear properties of high solids content materials due to sample loss/edge fracture during steady shear measurement. Capillary rheometers are able to probe the viscosity profiles of highly filled materials in high shear environments, but the cost of the device and the sample requirements are burdensome. To resolve this challenge, the "microcapillary rheometer" is developed in Chapter 6 using common laboratory equipment at a fraction of the cost of a full-scale capillary rheometer, which enables rapid characterization of high solids content materials at extrusion-relevant conditions while exploiting small sample quantities. This study illustrates the accuracy and precision of the microcapillary rheometer when comparing the high shear properties of several highly filled systems to the full-scale capillary rheometer. Results highlight that application of the Bagley and Weissenberg-Rabinowitsch corrections is possible using this novel device, which facilitates calculation of true shear viscosity of high solids content systems. The limited sample requirement facilitates characterization of novel or potentially hazardous materials in a much safer, efficient manner, which accelerates material discovery while improving safety standards. / Doctor of Philosophy / Subtractive manufacturing technologies, which reduce raw materials down from their bulk state into a final product, make up a significant portion of the manufacturing sector today due to the convenience and ease of material processing. Some of the most common forms of subtractive manufacturing include lathing, milling, cutting, drilling, and grinding; these methods are applicable for a diverse set of materials ranging from metals to plastics. By the nature of this process, subtractive manufacturing yields substantial material waste, while limiting the complexity of a final product's design. To combat these unintended consequences, a novel form of production termed additive manufacturing (AM) has grown dramatically in the past several decades. AM directly processes raw materials into their final form which reduces material waste while enabling complex geometries to be "printed." Although there are numerous types of additive manufacturing, the most common forms utilize material extrusion, whereby the raw material is deposited through a nozzle and stacked in a layer-by-layer fashion onto a build bed, thus constructing a final product.
For materials that melt and flow at elevated temperatures (i.e. thermoplastic materials), fused filament fabrication (FFF) is ideal since a solid filament can be fed into a heated nozzle, melted onto a build bed, and then quickly re-solidified. However, many polymers do not melt at elevated temperatures, and instead degrade; these materials are termed "thermosetting." To print these materials, unreacted thermosetting precursors, which are filled with a large proportion of solid fillers ("highly filled inks"), can be extruded by applying pneumatic back pressure to a syringe at ambient conditions. The process of extruding these materials layer-by-layer describes the direct ink write (DIW) technique.
The solid particulate fillers form structural "networks" due to weak electrostatic forces on the surface of the fillers. These forces provide structural stability and enable the printed lines to hold the weight of subsequent layers. Unfortunately, the high-pressure region of the nozzle disrupts this network, causing the printed lines to sag over time. This effect can be reduced by actively applying ultraviolet (UV) light onto the nozzle during extrusion, which helps to hold the particles in place by curing the resin, thus increasing the capacity for a line to sustain the weight of subsequent layers. This form of material extrusion is termed UV-assisted direct ink write (UV-DIW). Because UV light only partially cures the material during prints, a separate, slower thermosetting reaction can occur as the material rests in an oven or in ambient conditions, which completely cures the printed part and provides sufficient mechanical properties. The combination of UV-curable resins, thermosetting resins, and sufficiently large amounts of solid particulate fillers for material extrusion describes the dual-cure nature of this highly filled UV-DIW process.
To understand the curing patterns, flow behavior, and the amount of structural deformation that occurs within the nozzle, rheology becomes a powerful characterization tool. This branch of physics deals with the deformation and flow of matter ranging from simple fluids to complex polymer melts. As such, it is possible to probe reaction progress (chemorheology), structural deformation/reformation (thixotropy), and high-shear regimes representative of the DIW process.
The research contained within this dissertation provides a holistic understanding of the overlap between rheology and DIW material extrusion for dual-reactive materials. This process begins by evaluating challenges during melt extrusion of thermoplastic polyurethane while quantifying the rate of degradation side reactions. An alternative form of polyurethane synthesis in the form of a thermosetting reaction is then introduced, whereby the reaction progress is evaluated using both rheological and spectroscopic techniques. Next, a novel rheological protocol is introduced which can predict the structural deformation/reformation of an ink during UV-DIW. This research concludes by proposing a downscaled version of the high-shear capillary rheometer which requires only several grams of material in contrast to the dozens of grams required for full-scale capillary rheometry. In essence, the work presented here rapidly evaluates the complex flow behavior and cure progression of various materials relevant for extrusion processes by utilizing limited sample quantities, thus preserving valuable resources while improving the economics of material discovery.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/117233 |
| Date | 19 December 2023 |
| Creators | Reynolds, John Page |
| Contributors | Chemical Engineering, Bortner, Michael J., Johnson, Blake, Martin, Stephen Michael, Williams, Christopher Bryant |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
Page generated in 0.0163 seconds