Effects of storage temperature on the shelf life of one-step and two-step self-etch adhesives

Abstract

Recently, self-etch adhesive systems, the one-step (all-in-one) and two-step adhesive systems, have become widely utilized due to their simplified application procedures and low technique sensitivity. In the current study, in order to understand the effects on shelf life of the two types of self-etch adhesives, the effects from storage temperature and time period on the alteration stage of 10-methacryloxydecyl dihydrogen phosphate (MDP)-based, a one-step adhesive (all-in-one adhesive system) and an MDP-based self-etching primer (two-step adhesive system) were examined. Clearfil Tri-S Bond (TSB), an MDP-based one-step adhesive (all-in-one adhesive system), and Clearfil Mega Bond Primer (MBP), an MDP-based self-etching primer (two-step adhesive system), were used. Both TSB and MBP, received within two days after they were produced, were immediately stored at 8°C, 20°C or 40°C for 1, 3, 7 and 14 weeks, respectively. At the end of each storage period, 13C NMR observations were performed by an EX-270 spectrometer. In addition, NMR observations of TSB and MBP were immediately performed within two days, after both were produced as a control. The effects from the storage temperature and time period on the alteration rate and stage of TSB and MBP were examined by determining the amount of hydrolyzed 2-hydroxyethy1 methacrylate (HEMA) and MDP in both materials. Specimens bonded to dentin by using TSB or MBP in different alteration stages were prepared. The degradation stage effect of TSB or MBP on bond durability was examined by measuring the shear bond strength before and after thermocycling (20,000 times). With increases in storage temperature and time period, the relative intensities of the NMR peak "ε" assigned to both methylene carbons in the ethylene glycol (EG) and to the NMR peak 'ζ" assigned to the terminal methylene carbon bonded to the hydroxy group in the 10-hydroxydecyl dihydrogen phosphate (HDP) produced by the hydrolysis of the ester portion in HEMA or MDP, respectively, increased. The alteration stages of TSB and MBP were strongly dependent on storage temperature and time period. When TSB or MBP, stored at 40°C for 14 weeks, was applied to dentin, specific decreases in mean bond strength were observed in both adhesive systems. However, the application of thermocycling did not exhibit any specific decreases in the mean bond strength, even though the alteration stage of TSB and MBP progressed. From the results of the current study, storage temperature and time period significantly affect the alteration rate and stage of TSB and MBP. However, TSB and MBP exhibit expectant bond strength and bond durability when both are stored below 20°C.