[43] Smaller filler particles, such as microfills (0.01-0.1 ��m) and minifills (0.1-1.0 ��m), scatter more light than microhybrids, which contain a combination of different particle sizes.[43] As the light beam becomes scattered and reflected within the composite material, it loses intensity, with the consequent adverse effect on the degree of polymerization.[43] The nature of the resin compound library matrix has also been reported to affect the hardness and overall mechanical properties of composites. TEGDMA is known to create a much more dense network than Bis-GMA.[44] Newer formulations of composites incorporate an increased TEGDMA content, a more reactive diluent monomer (��-methylene-��-butyrolactone), carboxylic anhydrides, aldehydes, and diketones, all of which allow increased polymer matrix cross-linking, with the consequent improved mechanical and physical properties.

[43] However, manufacturers normally do not disclose proprietary information regarding the specific composition of their materials, and thus, correlations based merely on material composition cannot be established. The type and concentration of the photoinitiator is also known to influence the curing efficiency of the composite.[45] All composites evaluated in our study use CQ as their photoinitiator. Nevertheless, the concentration of CQ and the presence of any other unreported photoinitiators in the mixture are both unknown, and thus, no associations may be drawn with the observed results. A study demonstrated that two of the composites evaluated in our study, Tetric EvoCeram and Vit-l-escence, contain TPO in their composition.

[46] The manufacturers of these products, however, do not report this information in their product description. Photoinitiators such as TPO have a lower absorption peak of around 380 nm,[45] and hence, a less cross-linked polymer may be the result of polymerization with a narrow bandwidth LED unit. In our study, a poly-wave LED unit with a broadband spectrum between 380 and 515 nm was used. The wider emission spectrum, comparable to that of halogen lights, allows curing of composites containing all photoinitiator systems; thus, the differences in hardness values observed for Tetric EvoCeram polymerized with either halogen or LED cannot be attributed to these additional unreported co-initiators.

Effect of the storage time Hardness tests most commonly report results obtained immediately after polymerization, at 24 hours, and a few days following initial photoactivation. Typically, an increase in hardness values is observed in the first few hours/days following initial photoactivation, due to a continued polymerization reaction. Results from long-term storage, conversely, Brefeldin_A provide information regarding the effect of different aging conditions, such as water sorption, thermal variations, and wear, in the stability of the polymer network.