Ethyl-4-(Dimethylamino) benzoate initiation of the spontaneous polymerization of aqueous acidic adhesive formulations in the presence of a base

Tertiary Amine and Tooth Mineral Hydroxyapatite Facilely Trigger Self-cure of 10-MDP Based Adhesives

The present work aims to investigate if a novel self-cure system, mediated by tertiary amine ethyl 4-(dimethylamino)-benzoate (4E) and tooth mineral hydroxyapatite (HAp), would trigger polymerization of model adhesives based on a popular self-etch monomer, 10- methacryloyloxydecyl dihydrogen phosphate (10-MDP). The effect of 4E and HAp contents on degree of conversion (DC), polymerization rate (Rp), and induction period (IP) was investigated. The occurrence of such self-cure phenomenon in adhesives that underwent prior inadequate light cure was also evaluated. Model self-etch adhesives were prepared by using a monomer mixture of 10-MDP with 2-hydroxyethyl methacrylate at1:1 wt. ratio. 4E (0.3-1.3 wt%), and HAp (0.5-2 wt%) were added to the mixture. Benzoylperoxide and N,N-dihydroxyethyl-p-toluidine were used as conventional chemical-cure system, and trimethylbenzoyl-diphenylphosphine oxide as light-curing photoinitiator. The polymerization processes and mechanical properties of model adhesives were evaluated by real time ATR/FT-IR and nanoindentation, respectively. The 4E- HAp system successfully triggered self-cure of 10-MDP based model adhesives. DC, Rp and IP were apparently affected by both 4E and HAp contents. DC of self-cure of the model adhesives was much higher than that of the conventional chemical-cure system. 4E-HAp initiated self-cure further boosted DC to ~100% regardless of prior light exposure, and significantly improved elastic modulus and hardness, thus provided a novel polymerization method to effectively salvage curing of the adhesives after inadequate light-cure.

Light-Cured Self-Etch Adhesives Undergo Hydroxyapatite-Triggered Self-Cure

Light cure is a popular mode of curing for dental adhesives. However, it suffers from inadequate light delivery when the restoration site is less accessible, in which case a self-cure mechanism is desirable to salvage any compromised polymerization. We previously reported a novel self-cure system mediated by ethyl 4-(dimethylamino)-benzoate (4E) and hydroxyapatite (HAp). The present work aims to investigate if such self-cure phenomenon takes place in adhesives that underwent prior inadequate light cure and to elucidate if HAp released from the dental etching process is sufficient to trigger it. Model self-etch adhesives were formulated with various components, including bis[2-methacryloyloxy)ethyl]-phosphate (2MP) as acidic monomer and trimethylbenzoyl-diphenylphosphine oxide (TPO) as photoinitiator. In vitro evolution of degree of conversion (DC) of HAp-incorporated adhesives was monitored by infrared spectroscopy during light irradiation and dark storage. Selected adhesives were allowed to etch and extract HAp from enamel, light-cured in situ, and stored in the dark, after which Raman line mapping was used to obtain spatially resolved DC across the enamel-resin interface. Results showed that TPO+4E adhesives reached DC similar to TPO-only counterparts upon completion of light irradiation but underwent another round of initiation that boosted DC to ~100% regardless of HAp level or prior light exposure. When applied to enamel, TPO-only adhesives had ~80% DC in resin, which gradually descended to ~50% in enamel, whereas TPO+4E adhesives consistently scored ~80% DC across the enamel-resin interface. These observations suggest that polymerization of adhesives that underwent insufficient light cure is salvaged by the novel self-cure mechanism, and such salvaging effect can be triggered by HAp released from dental substrate during the etching process.