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In this study, we performed alkaline degradation testing to investigate the microstructural properties of a new resin composite composed of a single paste containing no pigments that still has the same basic structure as conventional pigment-containing composites. Our aim was to identify factors that affect the matching of various shades of teeth using structural color technology. The effect of these structural properties on the surface properties of the resin composite was also investigated. We found that the structural properties of the new composite included a wide distribution of many spherical organic fillers of various sizes. In addition, spherical inorganic filler of 260 nm in diameter was uniformly distributed at almost the same density both within the organic filler and within the base resin surrounding it. The organic filler and base resin, which are regarded as difficult to couple, were strongly bound without any gaps forming even after alkaline degradation testing. Although the surface layer of the organic filler was vulnerable to alkaline degradation, the center part exhibited very low degradation similar to the matrix surrounding the organic filler. In the new resin composite, the bonding state between the base resin and the various fillers was significantly improved. Furthermore, the microstructural properties were inferred to be effective factors for producing structural color, including the shape, particle diameter, distribution mode and density of the fillers as well as the properties of the base resin. However, these structural properties were not found to affect surface properties such as the line roughness, surface roughness, gloss, discoloration, and wettability.

Resin composites employing structural coloration have recently been developed. These resins match to various tooth shades despite being a single paste. To accomplish this, the filler and base resin are tightly bonded, which is thought to provide excellent discoloration resistance. Here, we investigated the surface properties of one of these resins, including the discoloration of the repolished surface. We developed an innovative in vitro method to adjust the repolished surface, in which structural degradation is removed according to scanning electron microscopy (SEM) observation rather than by the naked eye. The resin samples (20 mm (length) × 10 mm (width) × 4 mm (depth)) were manufactured using this resin material. After accelerated aging of the resin by alkaline degradation, the resin was repolished and the discoloration (ΔE*ab), surface roughness (the arithmetic mean roughness (Ra)), and glossiness (the 60° specular) were measured. SEM observation showed that the appearance of the bond between the organic composite filler and base resin on the repolished surface was different from that on the mirror-polished surface. This revealed that according to our in vitro method it was difficult to make the repolished surface structurally identical to the mirror-polished surface. Among the properties of the repolished surface, the degree of discoloration did not change despite the rougher and less glossy surface. It can be concluded that the factors that induce discoloration in this resin composite are independent of the surface roughness and glossiness.

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