New Germanium-based Photoinitiator as an Alternative to Camphorquinone/Amine

Objectives: The initiator system Camphorquinone/Amine (CC) has been used in almost all composites worldwide for the past 30 years. This system has the disadvantage, of not being color-stable under artificial sunlight because of its amine content. The acyl-germanium compound Bis-(4-methoxybenzoyl)diethylgermane (BMBDG) absorbs in the visible wavelength region: l_max = 408 nm, e = 7240 dm²×mol^-1. In comparison for CC the values are: l_max = 467 nm, e = 380 dm²×mol^-1. An amine-type accelerator is not necessary for BMBDG to support the polymerization process. The purpose of this study was to investigate the influence of BMBDG concentration on the strength of a composite in comparison to CC.

Methods: A monomer mixture based on Bis-GMA, UDMA and TEGDMA was initiated separately with BMBDG (0.10 – 0.40%) and CC/Amine (0.33/0.60 %) respectively. Composites were made by mixing these monomers (30 w-%) with a 1.0 my Ba-Glassfiller (70 w-%). To test the strength of the composites, eight rectangular bars (2x2x25 mm) were cured from both sides for 20 seconds (Bluephase/IvoclarVivadent), and stored at 37 °C in water for 24 hours; Flexural strength and modulus were measured with an Instron Universal Testing machine with a crosshead speed of 0.8 mm/min.

Results: Flexural strength and modulus measurements showed the following results (different letters within a column indicating significant differences in post-hoc comparisons after a multivariate ANOVA):

 

Initiator in monomer	Flexural strength (MPa)	Flexural modulus (MPa)
0.10 % BMBDG	124±8      a	7200±300    a
0.15 % BMBDG	124±14    a	8500±200    b
0.20 % BMBDG	135±9      b	9400±600    c
0.40 % BMBDG	140±15    b	10300±300    d
0.33 % CC / 0.60 % Amine	138±10    b	9200±600    c

Conclusion: Composites initiated with the new germanium-based initiator BMBDG show strength values in part similar to those of the well known CC/Amine system.