Defining Optimal Concentration of Photoinitiators in Dental Composites

Objectives: The aim of this study is to define the most appropriate concentration of the photoinitiators BAPO (bis(acyl)phosphine oxide) and CQ (camphorquinone) in experimental resin composites, regarding degree of conversion (DC) and mechanical properties.
Methods: Twenty experimental resin composites based on BISGMA / TEGDMA and 70wt% feldspar filler particles were prepared containing either CQ or BAPO at concentrations 0.25%, 0.5%, 1%, 1.5% and 2 %, combined or not with a tertiary amine (DMAEMA).Photoactivation was performed using a polywave LED ligh curing unit (1000mW/cm2) for 40 seconds. The degree of conversion (DC) was evaluated by Attenuated Total Reflection Fourier Transformed Infrared (ATR-FTIR) Spectroscopy. Knoop Hardness (KHN) was assessed using an indenter Shimadzu HMV with load of 50g for 15s. Elastic modulus was evaluated through a three point bending flexural test in a Universal testing machine. An indirect evaluation of crosslink density was performed by hardness measures before and after storage in absolute ethanol for 24h. Data were submitted to two-way ANOVA and Tukey's test (pre-set α=0.05).
Results: Unlike CQ, the BAPO photoinitiator suitably initiates polymerization in the absence of amine being more efficient than CQ. It allows proper polymerization even at low concentrations. When equal concentrations are considered, in general, BAPO resulted in a higher degree of conversion and mechanical properties similar or superior to CQ. Thus for QC concentration of 1% enabled satisfactory balance between mechanical properties and efficient polymerization, while for BAPO this balance was obtained with a concentration of 0.25% .

Conclusions: BAPO is an efficient photoinitiator, capable of initiating polymerization in the absence of amine, being a promising substitute for CQ. The most favorable concentration was 1% for the CQ/DMAEMA system and 0.25% for BAPO considering dental restoratice composites.