Degradation, fatigue, and failure of resin dental composite materials

Abstract

The intent of this article is to review the numerous factors that affect the mechanical properties of particle- or fiber-filler-containing indirect dental resin composite materials. The focus will be on the effects of degradation due to aging in different media, mainly water and water and ethanol, cyclic loading, and mixed-mode loading on flexure strength and fracture toughness. Several selected papers will be examined in detail with respect to mixed and cyclic loading, and 3D tomography with multi-axial compression specimens. The main cause of failure, for most dental resin composites, is the breakdown of the resin matrix and/or the interface between the filler and the resin matrix. In clinical studies, it appears that failure in the first 5 years is a restoration issue (technique or material selection); after that time period, failure most often results from secondary decay.

Figure 1

SEM of typical filer particles showing a colloidal filler, OX 50 and two micro fillers Z100 and a Sr-SiO₂ glass showing the differences in size and shape.

Figure 2

SEM of Renew, a micro hybrid by weight 28% resin and 72% glass filler particles with an average particle size distribution of 5% 0.004 mm, 62% 0.7 mm and 5% 3–7 mm particles The figures represent the fracture of specimens aged for 6 months in three media air, distilled water, and a 50/50 by volume mixture of ethanol and distilled water.

Figure 3

SEM of Filtek, a nano filler by weight 25–75 nm filler particles and 21.5% resin. The figures show how the nano particles are formed as 5 mm clusters and are pulled out of the resin matrix during fracture. The specimens aged in the 50/50 mixture demonstrate the degradation (weakening of mechanical properties) by a lack of sharpness in the fracture surface. The figures represent the fracture of specimens aged for 6 months in three media air, distilled water, and a 50/50 by volume mixture of ethanol and distilled water.

Figure 4

SEM of Restolux, a fiber filler by weight 85% filler and 15% resin with the filler composed of 3–4 mm particles (~ 27%) and 80–120 mm fibers (~52%). The SEMs indicate the relative large size of the fiber filler compared to the surrounding particle filler and the separation of the fiber filler from the resin matrix, c and f, compared to the other aging media. The separation is moist likely a combination of aging in the 50/50 mixture and polymerization shrinkage stress release. The figures represent the fracture of specimens aged for 6 months in three media air, distilled water, and a 50/50 by volume mixture of ethanol and distilled water.

Figure 5

SEM of fracture surfaces of specimens: (A) control A, (B) SiC whisker composite, and (C) Si₃N₄ whisker composite, all after 1-day immersion. The fracture surfaces of the controls were relatively flat. In contrast, the whisker composites had much rougher surfaces with fracture steps (large arrows) and whisker pullout (small arrows) from the paper Xu HHK (2003). Long-term water aging of whisker-reinforced polymer-matrix composites, J Dent Res, 82:48–52.

Figure 6

SEM of whisker pullout on fracture surfaces of Si₃N₄ composite: (A) 1 d, (B) 400 d, and (C) 730 d of water aging, with shorter whisker pullout at 400 d and 730 d. Polymer remnants were observed on the pulled-out whiskers (arrows), indicating good whisker-polymer matrix bonding even after 730 d of water aging from the paper Xu HHK (2003). Long-term water aging of whisker-reinforced polymer-matrix composites. J Dent Res, 82:48–52.

Figure 7

Fracture toughness versus number of cycles completed for Renew and Restolux using a Diametral (Brazilian) disc specimen of controls and aged specimens for 3 months. Cyclic loading had little effect on the fracture toughness for the control specimens, but in conjunction with aging had a major effect for the aged specimens. Aging in the 50/50 mixture of ethanol and distilled water caused the greatest decrease in the fracture toughness.

Figure 8

Reconstructions (a–d) of the images taken at the Advanced Photon Source of a Renew specimen subjected to multiaxial compression at a strain level of 12% demonstrating the crack pattern after loading. The 3D reconstruction of the same Renew specimen in (a–d) indicating the complexity and distribution of the cracking within the specimen with (e) looking down the axial axis and (f) off axis.