Investigating the Process-Structure-Property Relationships in Vat Photopolymerization to Enable Fabrication of Performance Polymers
| dc.contributor.author | Meenakshisundaram, Viswanath | en |
| dc.contributor.committeechair | Williams, Christopher Bryant | en |
| dc.contributor.committeemember | Johnson, Blake | en |
| dc.contributor.committeemember | Zheng, Xiaoyu | en |
| dc.contributor.committeemember | Long, Timothy E. | en |
| dc.contributor.committeemember | Acar, Pinar | en |
| dc.contributor.department | Mechanical Engineering | en |
| dc.date.accessioned | 2022-07-02T06:00:12Z | en |
| dc.date.available | 2022-07-02T06:00:12Z | en |
| dc.date.issued | 2021-01-07 | en |
| dc.description.abstract | Vat photopolymerization's (VP) use in large-scale industrial manufacturing is limited due to poor scalability, and limited catalogue of engineering polymers. The challenges in scalability stem from an inherent process paradox: the feature resolution, part size, and manufacturing throughput cannot be maximized simultaneously in standard VP platforms. In addition, VP's inability to process viscous and high-molecular weight engineering polymers limits the VP materials catalogue. To address these limitations, the research presented in this work was conducted in two stages: (1) Development and modeling of new VP platforms to address the scalability and viscosity challenges, and (2) Investigating the influence of using the new processes on the cured polymer network structure and mechanical properties. First, a scanning mask projection vat photopolymerization (S-MPVP) system was developed to address the scalability limitations in VP systems. The process paradox was resolved by scanning the mask projection device across the resin surface while simultaneously projecting the layer as a movie. Using actual projected pixel irradiance distribution, a process model was developed to capture the interaction between projected pixels and the resin, and predict the resulting cure profile with an error of 2.9%. The S-MPVP model was then extended for processing heterogeneous UV scattering resins (i.e. UV curable polymer colloids). Using computer vision, the scattering of incident UV radiation on the resin surface was successfully captured and used to predict scattering-compensated printing parameters (bitmap pattern, exposure time , scanning speed). The developed reverse-curing model was used to successfully fabricate complex features using photocurable SBR latex with XY errors < 1.3%. To address the low manufacturing throughput of VP systems, a recoat-less, volumetric curing VP system that fabricates parts by continuously irradiating the resin surface with a movie composed of different gray-scaled bitmap images ( Free-surface movie mask projection (FreeMMaP)) was developed. The effect of cumulative exposure on the cure profile (X,Y,Z dimensions) was investigated and used to develop an iterative gray-scaling algorithm that generated a combination of gray-scaled bitmap images and exposure times that result in accurate volumetric curing (errors in XY plane and Z axis < 5% and 3% respectively). Results of this work demonstrate that the elimination of the recoating process increased manufacturing speed by 8.05 times and enabled high-resolution fabrication with highly viscous resins or soft gels. Then, highly viscous resins were made processible in VP systems by using elevated processing temperatures to lower resin viscosity. New characterization techniques were developed to determine the threshold printing temperature and time that prevented the onset of thermally-induced polymerization. The effect of printing temperature on curing, cured polymer structure, cured polymer mechanical properties, and printable aspect ratio was also investigated using diacrylate and dimethacrylate resins. Results of this investigation revealed increasing printing temperature resulted in improvements in crosslink density, tensile strength, and printability. However, presence of hydroxl groups on the resin backbone caused deterioration of crosslink density, mechanical properties, and curing properties at elevated printing temperatures. Finally, the lack of a systematic, constraint based approach to resin design was bridged by using the results of earlier process-structure-property explorations to create an intuitive framework for resin screening and design. Key screening parameters (such as UV absorptivity, plateau storage modulus) and design parameters (such as photoinitiator concentration, polymer concentration, UV blocker concentration) were identified and the methods to optimize them to meet the desired printability metrics were demonstrated using case studies. Most work in vat photopolymerization either deal with materials development or process development and modeling. This dissertation is placed at the intersection of process development and materials development, thus giving it an unique perspective for exploring the inter-dependency of machine and material. The process models, machines and techniques used in this work to make a material printable will serve as a guide for chemists and engineers working on the next generation of vat photopolymerization machines and materials. | en |
| dc.description.abstractgeneral | Vat Photopolymerization (VP) is a polymer-based additive manufacturing platform that uses UV light to cure a photo-sensitive polymer into the desired shape. While parts fabricated via VP exhibit excellent surface finish and high-feature resolution, their use for commercial manufacturing is limited because of its poor scalability for large-scale manufacturing and limited selection of engineering materials. This work focuses on the development of new VP platforms, process models and the investigation of the process-structure-property relationships to mitigate these limitations and enable fabrication of performance polymers. The first section of the dissertation presents the development of two new VP platforms to address the limitations in scalability. The Scanning Mask Projection Vat Photopolymerization (S-MPVP)) was developed to fabricate large area parts with high-resolution features and the Free-surface movie mask projection (FreeMMaP) VP platform was developed to enable high-speed, recoat-less, volumetric fabrication of 3D objects. Computer-vision based models were developed to investigate the influence of these new processes on the resultant cure shape and dimensional accuracy. Process models that can: (1) predict the cure profile for given input printing parameters (error < 3%), (2) predict the printing parameters (exposure time, bitmap gray-scaling) required for accurate part fabrication in homogeneous and UV scattering resins, and (3) generate gray-scaled bitmap images that can induce volumetric curing inside the resin (dimensional accuracy of 97% Z axis, 95% XY axis), were designed and demonstrated successfully. In the second portion of this work, the use of high-temperature VP to enable processing of high-viscosity resins and expansion of materials catalogue is presented. New methods to characterize the resin's thermal stability are developed. Techniques to determine the printing temperature and time that will prevent the occurrence of thermally-induced polymerization are demonstrated. Parts were fabricated at different printing temperatures and the influence of printing temperature on the resultant mechanical properties and polymer network structure was studied. Results of this work indicate that elevated printing temperature can be used to alter the final mechanical properties of the printed part and improve the printability of the high-resolution, slender features. Finally, the results of the process-structure-property investigations conducted in this work were used to guide the development of a resin design framework that highlights the parameters, metrics, and methods required to (1) identify printable resin formulations, and (2) tune printable formulations for optimal photocuring. Elements of this framework were then combined into an intuitive flowchart to serve as a design tool for chemists and engineers. | en |
| dc.description.degree | Doctor of Philosophy | en |
| dc.format.medium | ETD | en |
| dc.identifier.other | vt_gsexam:28907 | en |
| dc.identifier.uri | http://hdl.handle.net/10919/111091 | en |
| dc.publisher | Virginia Tech | en |
| dc.rights | In Copyright | en |
| dc.rights.uri | http://rightsstatements.org/vocab/InC/1.0/ | en |
| dc.subject | Additive Manufacturing | en |
| dc.subject | Vat Photopolymerization | en |
| dc.subject | Process Modeling | en |
| dc.subject | Stereolithography | en |
| dc.subject | Mask Projection | en |
| dc.title | Investigating the Process-Structure-Property Relationships in Vat Photopolymerization to Enable Fabrication of Performance Polymers | en |
| dc.type | Dissertation | en |
| thesis.degree.discipline | Mechanical Engineering | en |
| thesis.degree.grantor | Virginia Polytechnic Institute and State University | en |
| thesis.degree.level | doctoral | en |
| thesis.degree.name | Doctor of Philosophy | en |