Ultra-low Volumetric Shrinkage Dental Composites Based on in situ Nanocavitation

Polymerization shrinkage remains one of the major concerns for dental restorative composites because it induces stress and marginal gap at the composite-tooth interface. Internal and interfacial stress can cause composite failure, and the gap often results in undesirable discoloration and may lead to the formation of secondary caries. Objective: The objective of this study is to develop a strategy to reduce shrinkage, while not sacrificing other important properties of the composites significantly. Methods: A small amount of acetone dicarboxylic acid (ADCA) was added into the composite. (72% 400 nm silanized BaSiO3 glass, 25-27% ethoxylated bisphenol-A dimethacrylate (EBPADMA) with degree of ethoxylation ~ 30, 0.53-0.61% I819 initiator, 0.35-0.46% methacryloyloxyethyl succinate, and 0-1.98% ADCA by mass). Shrinkage was determined using dilatometry. Transmission electron microscopy and X-ray microcomputed tomography were used to visualize void formation. Flexural strength and modulus of composites were measured using an Instron via 3-point bending test. Results: A shrinkage reduction with increasing amount of ADCA is observed. Samples without ADCA exhibit shrinkage of 1.94%. With 0.28% ADCA, shrinkage is reduced to 0.65%. Shrinkage is further reduced to almost zero with the ADCA mass fraction ≥ 0.5% (Figure 1). This reduced shrinkage is due to the formation of CO2 upon in situ decomposition of ADCA by heat generated during polymerization. CO2 is trapped inside the composites as nanovoids, which resist the shrinkage. TEM results further confirm the formation of nanosized voids inside the resin film. The mechanical tests show that the mechanical properties of the composites are not deteriorated. Conclusions: A strategy to reduce shrinkage has been demonstrated. With small amounts of ADCA, shrinkage is reduced to almost zero, while mechanical properties are not sacrificed. Support: NIDCR/NIST Interagency Agreement Y1-DE-7005-01. Figure 1. The volume shrinkage of composites with different amounts of ADCA (inset: the decomposition reaction of ADCA).