Eggshell-derived mineral and membrane fillers in a cold-cured PMMA matrix: phase transformation, local nanomechanics and multiscale mechanical response

Abstract Cold-cured poly(methyl methacrylate) is used for prosthetic, dental and repair components, where the stiffness of the resin is adequate while notch sensitivity and low energy absorption limit service. Hen eggshell is a waste generated in very large quantities and an attractive particulate filler for such systems. Native eggshell mineral (ES), the same mineral calcined at 900°C for 3 h (CES) and the separated membrane (ESM), all from one batch of shells, were cast into a cold-cured acrylic at 5, 10 and 15 wt% mineral and 1, 2.5 and 5 wt% membrane. Calcination converted calcite to CaO that partly rehydrated to portlandite and lowered the indentation modulus of the filler-associated regions from 33.0 to 8.3 GPa and raised the apparent porosity of the 15 wt% composite from 1.3 to 7.3%. Every filled formulation was stiffer in tension than the matrix, by 19–67%, and at 15 wt% the calcined composites were stiffer than the native ones despite their more compliant filler and higher porosity. Tensile strength stayed within 1.01–1.29 times the Nicolais–Narkis prediction for non-adhering inclusions. Flexural strength fell to 39–55% of the reference, while compressive yield stress was retained.