Performance and Microstructural Features of Sodium Silicate Shell Cured via Microfluidic Droplets of Citric Acid Solution

The large solid waste discharge of investment casting shells is closely related to its unstable performance of shells and uncontrolled curing reaction. The unique advantage of microfluidic technology is that it can effectively control the chemical reaction process. In the present work, an environmentally friendly citric acid widely used in food as hardening agent was employed to cure a sodium silicate shells. Droplets of citric acid solution with a concentration of 1.5 × 10⁻³ mol/L were generated by microfluidic technology and spread on the surfaces of shell specimens driven by air flow to induce the shell hardening. The green-, fired-, residual-strength, high temperature self-weight deformation, and gas to permeability of the shell were investigated. The results showed that the peak strength reached 30.38 MPa for green shell and 10.61MPa for the fired shell, about 26.4% and 17.4% higher than the immersion method, respectively. The fracture morphology of the shell observed by SEM (Scanning Electron Microscopy) confirmed that the more uniform, and far less cracks and micropores formed on the sodium silicate film of shells hardened by microfluidic droplets than the dipping method. The analysis of FTIR (Fourier Transform Infrared Spectroscopy) reveals that the final products of sodium silicate gel cured by microfluidic droplets achieved a high degree of polymerization and generated a relatively dense Si–O–Si cellular structure. The phase composition and thermal stability of the shell were analyzed by XRD (X-ray Diffraction) and TG-DSC (Thermogravimetric Differential Scanning Calorimetry). The results demonstrate that the improvement in hardening performance of shells is due to reaction microfluidic technology that can precisely control the volume and rate of hardening agent droplets, which can improve the repeatability and stability of the reaction in a short time, as well as reduce the damage of the gel film during the hardening process, and improve the quality of the gel film. The resulting shell can achieve higher strength and stability.