Hardness Mapping of Light-Cured Composites to Identify Inhomogeneous Curing

Objective:

Light-curing units for dental materials exhibit an inhomogeneous intensity distribution. This leads to locally different curing processes and mechanical properties. The aim of this study is to map the surface hardness of cured samples to identify inhomogeneous curing and to measure the temporal evolution of post-curing.

Methods:

The dental composite Arabesk Top OA2 (VOCO) was shaped into plates of diameter 8mm and thickness 1mm and cured with the Halogen polymerization unit (Polofil LUX, VOCO) for 40 seconds. Subsequently, top and bottom surfaces were scanned with a Vickers micro-hardness tester (MacroMet 5103, Buehler) with a force of 2N (200gf) according to the grid shown in Fig.1 (left). The hardness measurements were started after 10min.

Results:

Within two hours after light-curing the hardness of the composites increased by approximately 40%. This post-curing effect is superimposed by hardness differences due to inhomogeneous intensity distributions as the total hardness scan takes approximately two hours, Fig.1 (right). To extract intensity distribution effects all individual hardness measurements have to be referred to a certain time requiring a correcting post-curing function. The hardness at the bottom surface significantly exceeds the hardness of the top surface.

Conclusion:

The correcting post-curing function allows for the mapping of inhomogeneous curing states for certain times via the local hardness. This hardness pattern of the surface can be correlated to intensity distribution pattern of the light-curing unit. The slower cured bottom surface exhibits a higher hardness than the top surface. This allows for the assumption that the higher the concentration of activated initiator molecules is the more low-molecular components are captured in cured network.