Resin viscosity determines the condition for a valid exposure reciprocity law in dental composites

Abstract

Objective-To provide conditions for the validity of the exposure reciprocity law as it pertains to the photopolymerization of dimethacrylate-based dental composites Methods-Composites made from different mass ratios of resin blends (Bis-GMA/TEGDMA and UDMA/TEGDMA) and silanized micro-sized glass fillers were used. All the composites used camphorquinone and ethyl 4-dimethylaminobenzoate as the photo initiator system. A cantilever beam-based instrument (NIST SRI 6005) coupled with NIR spectroscopy and a microprobe thermocouple was used to simultaneously measure the degree of conversion (DC), the polymerization stress (PS) due to the shrinkage, and the temperature change (TC) in real time during the photocuring process. The instrument had an integrated LED light curing unit providing irradiances ranging from 0.01 W/cm 2 to 4 W/cm 2 at a peak wavelength of 460 nm (blue light). Vickers hardness of the composites was also measured.Results-For every dental composite there exists a minimum radiant exposure required for an adequate polymerization (i.e., insignificant increase in polymerization with further increase in the radiant exposure). This minimum predominantly depends on the resin viscosity of composite and can be predicted using an empirical equation established based on the test results. If the radiant exposure is above this minimum, the exposure reciprocity law is valid with respect to DC for highfill composites (filler contents > 50% by mass) while invalid for low-fill composites (that are clinically irrelevant).Significance-The study promotes better understanding on the applicability of the exposure reciprocity law for dental composites. It also provides a guidance for altering the radiant exposure, *

Conversion kinetics of rapid photo-polymerized resin composites

“…The high energy delivered in a short time via the wide-spectrum LCU was sufficient to produce an adequate DC, due to the materials' ability to absorb energy from both types of wavelengths (violet and blue spectra) and its high reactivity to the lower wavelength [13,27]. Having a type I photoinitiator in the evaluated materials, our findings were generally in line with the Bunsen-Roscoe exposure reciprocity law, which assume that the photoresponse depends on the radiant exposure rather than irradiance power and time [28][29][30].…”

“…Furthermore, high irradiance received by the resin composite materials can cause a slight delay in auto-acceleration to occur at higher DC leading to generally higher DCRPmax, as seen in our study [30,37]. Furthermore, a strong positive correlation (r=0.96) seen between higher RPmax and DCRPmax, indicating sufficient conversion even with high conversion rate.…”

Outcomes of ultra-fast (3 s) photo-cure in a RAFT-modified resin-composite

“…More recently, the Stansbury-Bowman group has established that, for mechanistic reasons, exposure reciprocity may be achievable using Norrish Type I photoinitiators [6]. Very recently, a NIST group has established empirically that, within appropriate ranges of irradiance and resin-composite viscosity, conditions may be met for valid exposure reciprocity [7]. Although this evidence is empirical it has been known for some time that system viscosity has a significant influence by reducing the rate of free-radical termination reactions.…”

Minimum Radiant Exposure and Irradiance for Triggering Adequate Polymerization of a Photo-Polymerized Resin Cement

“…In the present study, the specimens were inadequately polymerized when the irradiance was 50 mW/cm 2 , regardless of the irradiation time of 120 s. This apparent violation of the ERL is similar to what occurred in previous studies. Although the RE plays an important role, the irradiance and irradiation time independently affect the polymerization of resin [ 29 , 30 , 31 , 32 , 33 ]. Therefore, both time and irradiance should be considered critical in the polymerization process.…”