Proposition of New Testing Procedure for the Mechanical Properties of Bulk-Fill Materials

Abstract

This study analysed flexural properties, microhardness, and the degree of conversion (DC) of five bulk-fill composites under clinically relevant conditions (4 mm thick specimens) in comparison to 2 mm specimens according to ISO 4049. Additionally, the effect of rapid polymerisation on 4 mm specimens was evaluated after accelerated aging. DC was measured using Fourier transform infrared spectrometry at 2 and 4 mm thick layers, while flexural properties and Vickers microhardness were tested using 16 × 2 × 2 mm or 16 × 2 × 4 mm specimens. Three polymerisation protocols were used: (I) “ISO”: 2 mm thickness, 1000 mW/cm2, double-sided; (II) “10 s”: 4 mm thickness, 1000 mW/cm2, one-sided; and (III) “3 s”: 4 mm thickness, 2600 mW/cm2, one-sided. Mechanical properties were tested after 1 day, after 10,000 thermocycles, and after 10,000 thermocycles followed by a 7-day immersion in absolute ethanol. The “ISO” protocol produced a higher DC and microhardness of all materials. Elastic modulus was significantly higher for the “ISO” protocol compared to the 4 mm specimens. The differences in flexural strength for all polymerisation protocols were equalised after thermocycling and immersion in absolute ethanol. All tested materials met the ISO 4049 flexural strength requirement (80 MPa) for all polymerisation methods and all aging conditions. Rapid polymerisation achieved nearly optimal properties (ISO), except for elastic modulus, which was significantly reduced in 4 mm samples.

One-Year Evaluation of High-Power Rapid Curing on Dentin Bond Strength

“…On the other hand, Filtek as the sculptable material with the highest filler volume in this study, has previously also shown the highest flexural strength, followed by PowerFill and SDR+ (which were statistically similar), and PowerFlow [43]. However, in the present study, Filtek did not exhibit the highest bond strength in any of the curing protocols.…”

“…However, in the present study, Filtek did not exhibit the highest bond strength in any of the curing protocols. This is probably due to its low DC value, as previously shown [42,43]. The distinctive feature of this material is a long, high-molecular-weight AUDMA molecule with only two methacrylate groups [44].…”

“…Their results showed higher solubility for all materials in the 3 s curing protocol compared to the 20 s curing protocol. Both SDR+ and PowerFlow reached complete water saturation after 7-14 days, but also had high solubility [43]. When constantly exposed to the water environment for 12 months, there is sufficient time for the permeation of the material with water and the subsequent hydrolytic degradation of the ester groups in the resin matrix.…”

“…The distinctive feature of this material is a long, high-molecular-weight AUDMA molecule with only two methacrylate groups [44]. Previous investigations have shown that the DC of this material significantly drops in 4 mm depth, especially if "3 s" curing is employed [43,44]. It appears that the AUDMA and high viscosity of this material slow down the polymerization kinetics, so that "3 s" curing is not efficient for Filtek [45], leaving unreacted methacrylate groups.…”