Microcapsule synthesis for 3D printing of self-healing materials

Sanders, Polly

Abstract

3D printing allows for the manufacture of finished structures from starting components with essentially no specialist training required. The process is autonomous; the starting components, such as inks, are poured into the machine and the finished product is produced with no intervention from the user. Currently, most commercial printers only produce structures which have limited functionality and are often used to produce ‘prototypes’ rather than a finished product. There have been recent advancements in the utilisation of 3D printing to produce structures which have longer lifespans, wider material choice and improved functionality, in order to extend the utility of printed structures beyond prototyping and expand potential applications. Within the research reported in this thesis, the focus is primarily on the production of 3D printing resins with embedded capsules, which can be utilised in 3D printers for the production of microcapsule containing composites. Specifically, theses capsules contain solvents and polymers which can be used for self-healing with the intention of extending the lifetime and toughness of any structures built by 3D printers using these materials.

In this work, the 3D printing of self-healing materials is shown through the use of 3D printing resins modified with solvent and polymer containing urea-formaldehyde microcapsules. Urea- formaldehyde capsules are utilised widely in the literature for extrinsically self-healing microcapsule-based systems. The inclusion of the capsules showed no harmful effect on the quality of printing and mechanical testing showed the capability of fracture toughness recovery after healing via a classic solvent welding mechanism to be 48-59% depending on which solvent and concentration was used. The addition of poly(methyl methacrylate) chains into the core of the capsules and increasing the healing time to 72 hours enhanced the self-healing ability of the solvent anisole. Under these conditions, mechanical testing showed a capability for fracture toughness recovery of 87%.

A more novel production route for microcapsules was also explored. The ability to generate an optically transparent self-healing system would be beneficial for utilisation in applications which require optically transparent materials. Microfluidic devices were utilised to produce microcapsules with a transparent resin shell and liquid oligomer core.

Therefore, a key investigation for this work regarded the optimisation of capsule generation via droplet microfluidic devices. Droplet microfluidic devices generated the capsules in one at a time with sequential UV polymerisation in a continuous flow fashion, this proved an elegant solution to some of the problems seen when investigating production using a batch mixing process. Despite concerns with regards to high viscosities (230-11,000 mPa s) and the complex rheology associated with polymer blends, it was shown that droplet generation can be controlled via microfluidics. This fixed some issues seen when trying to optimise this process using batch synthesis routes, producing superior capsules than a due to the high degree of control afforded by microfluidic devices.

These capsules were successfully utilised in a 3D printer and the reduced light scattering from these microcapsules when compared to the urea-formaldehyde capsules was significant. Here, we show that this mechanism had the ability to recover fracture toughness of up to 83%.

Citation

Sanders, P. (2021). Microcapsule synthesis for 3D printing of self-healing materials. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4223143

• Thesis Type: Thesis
• Deposit Date: Apr 21, 2021
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4223143
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Apr 1, 2021