Effect of coinitiator and water on the photoreactivity and photopolymerization of HEMA/camphoquinone‐based reactant mixtures

Abstract

It was reported previously that, in the presence of water, a commercially available light-activated BisGMA/HEMA adhesive underwent physical separation into solid BisGMA-rich particles and a fluid-like HEMA-rich phase. The HEMA-rich phase exhibited limited monomer conversion suggesting that the photoinitiator is localized to the hydrophobic phase or that the photoinitiator is not compatible with the hydrophilic HEMA. The objective of the present study was to identify photoinitiators that are compatible with the hydrophilic HEMA-rich phase, when the mixtures are prepared without and with water added. The photoinitiator was camphoquinone (CQ, 0.5 mol %), and the coinitiators (0.5 mol %) were 2,2′-dihydroxyethyl-para-toluidine (DHEPT), dimethylaminoethyl methacrylate (DMAEMA), and N-phenylgly-cine (NPG), and (1 wt %) diphenyliodonium chloride (DPIC). Reactivities were evaluated using photodifferential scanning calorimetry, at 37°C, using visible light (>418 nm), with the parameters determined being enthalpy (ΔH), the induction time (herein defined as the time for 1% of the photopolymerization to be complete), and the time at which the maximum exotherm occurred. The degree of monomer conversion was measured using micro-Raman spectroscopy. It was shown that the reactivity ranking (based on time to exotherm peak maximum and total enthalpy) was HEMA/CQ/DHEPT < HEMA/ CQ/DMAEMA < HEMA/CQ/NPG. Reactivity was dramatically increased for CQ/DMAEMA and CQ/NPG in the presence of DPIC, but not for CQ/DHEPT. Water has a major effect on HEMA conversion. At 10% of water, the conversion level of HEMA formulated with CQ/DMEMA dropped from ~100% to 86%. In comparison, the conversion in 10% of water increased to nearly 96% when DPIC was used. The results suggest that DHEPT, which is commonly used in commercial adhesives, is not compatible with HEMA. Both NPG and DMAEMA appear compatible with the HEMA. The ionic hydrophilic iodonium salt, DPIC, enhances the polymerization of HEMA, even in the presence of water. Future studies on water-compatible photoinitiators should be performed to address the detrimental effects of water on dentin adhesive systems.

Effects of a solubility enhancer on penetration of hydrophobic component in model adhesives into wet demineralized dentin

“…This will affect the integrity of the adhesive and dentin interface. Meanwhile, water can inhibit polymerization of dentin adhesives [33], especially, the hydrophilic component of adhesives [4,34]. This is because the photoinitiators such as CQ and its co-initiators used in most dentin adhesives are relatively hydrophobic [34].…”

“…Meanwhile, water can inhibit polymerization of dentin adhesives [33], especially, the hydrophilic component of adhesives [4,34]. This is because the photoinitiators such as CQ and its co-initiators used in most dentin adhesives are relatively hydrophobic [34]. Thus, reducing phase separation of adhesives in the presence of water and keeping homogenous penetration of adhesives into wet demineralized dentin are very critical factors during the dentin bonding process.…”

“…On the other hand, due to the hydrophobic nature of the BisGMA-rich phase, this phase will not readily penetrate the wet demineralized dentin matrices. Since the CQ based photoinitiator system is hydrophobic, most of the photoinitiator will remain associated with the BisGMA-rich phase [4,34]. This will lead to poor polymerization of the HEMA which has penetrated the demineralized dentin zone.…”

Effects of water content and initiator composition on photopolymerization of a model BisGMA/HEMA resin

“…The conversion of the resin decreased from ~54% to ~25% when more than 20% water was added [17]. The extent of monomer conversion [19][20][21][22] and the presence of phase separations in the presence of water [6,[23][24][25][26] during polymerization are very influential on the structure and properties of the resulting polymer.…”

“…[20] The effectiveness of the systems depends on the H-atom donor ability of amines used as coinitiators and the compatibility of initiator components with monomers. CQ is relatively hydrophobic.…”