Rubber-toughening of dimethacrylate dental composite resin

Abstract

Dimethacrylate dental composite resins exhibit inherently low toughness. Toughening of these materials may reduce the incidence of marginal and bulk fracture of composite restorations.

Objective: To determine if dimethacrylate dental restorative materials can be rubber-toughened, and if so, to identify a possible mechanism.

Methods: A filler composed of aggregates of polybutadiene/silica as well as irregularly-shaped silica slabs was produced by mixing silica with polybutadiene in dichloromethane. The dried filler was subsequently ground and sieved to < 25 microm. Polybutadiene/silica ratios were varied from 0:1 (control) to 0.5:1. EDAX analysis verified the composition of the complex filler. Filler was added to a bis-GMA/bis-EMA/TEGDMA resin system and fractured in three-point bend test mode at a crosshead speed of 1 mm/min. In addition, 1 bar was fractured at a crosshead speed of 0.001 mm/min to identify a possible mechanism for toughening.

Results: In specimens fractured at 1 mm/min, flexural modulus is increased or maintained and flexural strength and energy to break increase as the amount of polybutadiene in the aggregates increases. Cavitation of high-rubber-containing aggregates is demonstrated. In the one specimen fractured at 0.001 mm/min, a marked increase in size of high-rubber-containing aggregates along with severe shear damage in the surrounding matrix is shown, suggesting that cavitation with subsequent absorption of energy during shear yielding is the likely mechanism behind the increase in energy to break in bars fractured at 1 mm/min.

Significance: These results indicate that dimethacrylate dental composite materials can be rubber toughened, which may potentially reduce marginal and bulk fractures of composite restorations, and consequently extend their service lifetime.

Figure 1

EDAX results and SEM images of complex filler (0.25:1 polybutadiene/silica). The spherical particle, Fig. 1a, was composed of 83.2 atomic % carbon, with the remainder silica. The irregular slab, Fig. 1b, was composed of 11.1 atomic % carbon, with the remainder silica. Areas analyzed by EDAX are marked by (₊).

Figure 1

EDAX results and SEM images of complex filler (0.25:1 polybutadiene/silica). The spherical particle, Fig. 1a, was composed of 83.2 atomic % carbon, with the remainder silica. The irregular slab, Fig. 1b, was composed of 11.1 atomic % carbon, with the remainder silica. Areas analyzed by EDAX are marked by (₊).

Figure 2

Spherical polybutadiene/silica aggregates (0.25:1) of size a: 25μm and b: 10μm and several irregular silica slabs, components of the complex filler used in this work.

Figure 2

Spherical polybutadiene/silica aggregates (0.25:1) of size a: 25μm and b: 10μm and several irregular silica slabs, components of the complex filler used in this work.

Figure 3

Flexural modulus, flexural strength and energy to break in bis-GMA/bis-EMA/TEGDMA with 9 vol.% silica or aggregate. Arrows indicate groups with means significantly different from those of the controls (P<0.05). Crosshead speed – 1mm/min.

Figure 4

A specimen (0.5:1 polybutadiene/silica) fractured at 1mm/min. Note the large increase in size of many of the aggregates, possibly indicating cavitation of aggregates.

Figure 5

A specimen (0.00005:1 polybutadiene/silica) fractured at 1mm/min. Note the cracks originating at aggregate equators, indicating a brittle mode of deformation.

Figure 6

Cavitated aggregates (0.25:1 polybutadiene/silica) from a thin (600nm) section of a specimen fractured at 1mm/min. Silica plates are seen as blue stained areas. Toluidine blue stain.

Figure 7

Optical microscope images near (a,c) and far (b,d) from fracture surfaces. Note extensive shear damage and markedly increased size of aggregates, possibly due to cavitation, in (a). Much less shear damage is evident in (c). Some planes are out of focus. Examined under crossed polarizers. Crosshead speed – 0.001mm/min.

Figure 8

Optical micrograph using crossed polarizers showing shear interactions (red bands, shown in brackets) among particles. The interactions are possibly nano-sized shear bands.