Additive manufacturing of high solid content lunar regolith simulant paste based on vat photopolymerization and the effect of water addition on paste retention properties

In recent years, the Moon, as the only satellite of the Earth, has once again become the primary destination for exploration by countries around the world. However, considering the limited carrying capacity and high cost of extraterrestrial space construction, additive manufacturing (AM) technology, such as the photopolymerization method, for in-situ resource utilization (ISRU) lunar regolith has attracted the favor of researchers for building lunar research bases. Still, the photopolymerization of lunar regolith simulant (LRS) systems has been greatly limited by the imbalance of high solid content and low viscosity of the prepared slurries. To address this challenge, previous studies employed a silane coupling agent (KH-570) or stearic acid (SA) to modify the surface of the powder, but the process is complicated and unsuitable for use in lunar microgravity environments. In this research, based on the principle of emulsification of two different solutions, by introducing a certain proportion of aqueous solution, under the action of disperbyk-111, the interface between water and resin forms a directional molecular layer to reduce surface tension. According to the Gibbs adsorption formula, the decrease in surface tension will increase the adsorption amount of the surface layer per unit area, which means that the closer the molecular arrangement is, the more stable the emulsion is. The macroscopic phenomenon reduces the viscosity while ensuring the retention properties of the material, which better solves the problem that high solid content LRS paste is difficult to achieve low viscosity. The comparative analysis of the viscosity shows that adding an aqueous solution has achieved the expected result. Herein, we developed LRS paste with 62 vol% (i.e., 80 wt%) solid content for the use of stereolithography (SL) vat photopolymerization (VP) and completed the printing successfully. The experimental results further suggested that the sample layer thickness of 50 µm and the placement angle of 0° were the most suitable for successful printing. The average flexural and compressive strength reached 132.21 MPa and 444.23 MPa, surpassing previous results. In addition, the phenomenon of interlayer bonding at different placement angles observed by scanning electron microscope (SEM) was explained by the secondary extrusion force per unit area. Finally, we also realized the printing of integral one-piece movable parts using LRS materials, achieving high precision and superior performance, which makes it possible to print high-strength structures on the Moon in the future.