Microstructural and interfacial factors leading to reduced mechanical properties in calcium orthophosphate-containing...

Resin-based restorative composites containing ion-releasing particles usually present inferior mechanical properties in relation to conventional composites. The objective of the present study is to estimate the contribution of dicalcium phosphate dihydrate (DCPD) particle characteristics (morphology and intrinsic mechanical behavior) and the lack of chemical bond to the polymer matrix on degree of conversion (DC), composite flexural strength (BFS) and modulus (FM), and Knoop microhardness (KHN). Three series of materials were tested (25 in total), consisting of decreasing silanized barium glass (Gf) fractions and increasing fractions of non-silanized glass (Gnf), non-functionalized DCPD (Dnf), or 10-MDP-functionalized DCPD (Df). Composite DC was determined using near-FTIR (n = 4). BFS (n = 10) and KHN (n = 5) tests were conducted after 24 h storage in water. Stress distribution in the flexural specimen during loading was simulated by finite element analysis (FEA). Experimental data were analyzed by Kruskal-Wallis (DC) and one-way ANOVA/Tukey test (BFS, FM and KHN, α ≤ 0.05). Particle type and functionalization did not affect DC. BFS showed a higher effect of functionalization than particle type, explained by the stress concentration in the polymer around non-functionalized particles. For FM, functionalization with 10-MDP compensated for the lower elastic modulus of the DCPD particle. For KHN, particle type showed a stronger effect for both functionalization conditions. In conclusion, the low mechanical properties of DCPD-containing composites can be ascribed both to the intrinsic characteristics of the DCPD particles and the absence of an effective bonding to the polymer matrix.