Dissertation committee:
Ηλιάδης Γεώργιος, Καθηγητής, Τμήμα Οδοντιατρικής, Σχολή Επιστημών Υγείας, EKΠΑ
Ζηνέλης Σπυρίδων, Αν. Καθηγητής, Τμήμα Οδοντιατρικής, Σχολή Επιστημών Υγείας, EKΠΑ
Σιλίκας Νικόλαος, Καθηγητής, Οδοντιατρική Σχολή, University of Manchester
Ρούσσης Βασίλειος, Καθηγητής, Τμήμα Φαρμακευτικής, Σχολή Επιστημών Υγείας, ΕΚΠΑ
Πολυζώης Γρηγόριος, Καθηγητής, Τμήμα Οδοντιατρικής, Σχολή Επιστημών Υγείας, EKΠΑ
Ιωάννου Ευσταθία, Επίκουρη Καθηγήτρια, Τμήμα Φαρμακευτικής, Σχολή Επιστημών Υγείας, ΕΚΠΑ
Μασούρας Κωνσταντίνος, Επίκουρος Καθηγητής, Τμήμα Οδοντιατρικής, Σχολή Επιστημών Υγείας, EKΠΑ
Summary:
Reactivity and Stability of Silanes
Maria Dimitriadi CDT, MSc
A thesis submitted as partial fulfillment for the PhD Degree in Dental Biomaterials to the School of Dentistry of the National and Kapodistrian University of Athens, Greece
Aims:
1. To evaluate the stability and reactivity of γ- methacryloxypropyl trimethoxysilane (MPTMS) in commercially available silane primers and universal adhesives.
2. To assess the silane status in commercially available products and their bonding capacity with polished lithium disilicate glass-ceramic surfaces before and after hydrofluoric (HF) acid-etching.
3. To investigate the stability, reactivity and bond strength with the lithium disilicate glass-ceramic surfaces of a self-etch silane primer.
Materials and methods:
Three independent experiments were performed to address the aims of the study as follows:
1. Four representative primer formulations [prehydrolyzed silane: Calibra Silane Coupling Agent/Dentsply (CLB); silane-phosphate-thiophosphate primer: G-Multi Primer/GC (GMP); silane-resin monomer primer: Kerr Silane Primer/Kerr (KSP); silane-phosphate-disulfide primer Monobond Plus/Ivoclar Vivadent (MBP)], a silane containing universal adhesive: Scotchbond Universal/3M ESPE (SBU)], all containing MPTMS, and two controls (non-hydrolyzed MPTMS /Evonik Industries and 10-methacryloyloxydecyl dihydrogen phosphate 10-MDP/Ivoclar Vivadent) were analyzed spectroscopically. For the stability study, the silanol content was evaluated in bulk solutions as received (reference-RE) and after aging (AG, 48°C/1month) by 1H-, 13C-, 31P-NMR spectrometry and in fresh films applied on Ge windows by transmission FTIR spectrometry (T-FTIR). The reactivity, as expressed by the siloxane formation capacity of the RE products, was further evaluated by micro-multiple internal reflectance FTIR analysis (MIR-FTIR) of films applied on Ge crystals after drying and ethanol rinsing (t0) and following 1h (t1) and 24h (t24) storage (air/37oC).
2. The products tested were CLB, GMP, KSP, MBP and SBU. The silane status was studied by 13C nuclear magnetic resonance spectroscopy (13C-NMR). The roughness parameters of polished (group A) and 5% HF acid-etched (IPS Ceramic Etching Gel/Ivoclar Vivadent, group B) of lithium disilicate glass-ceramic surfaces (IPS e.max Press, Ivoclar Vivadent) were measured by optical profilometry (n = 5/group). The interaction of the products with group A and B ceramic surfaces was examined by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The shear strength (SBS) of a flowable composite (G-aenial Flo Universal/GC Int.) bonded to the ceramic surfaces (groups A, B) was assessed before (NS) and after silane treatment (n = 20/group, product).
3. The stability of a self-etching silane primer (Monobond Etch and Prime/MEP) was evaluated by 1H-, 31P-NMR spectroscopy (before/after aging), and the reactivity by micro MIR-FTIR spectroscopy on Ge crystals after drying and ethanol rinsing after 0, 1 and 24 h storage (air/37oC), using the prehydrolyzed silane primer CLB, as a control. The effect of MEP vs. 5% HF-etching on ceramic roughness was assessed by optical profilometry. The shear bond strength (SBS) of a resin composite bonded to polished lithium disilicate glass-surfaces treated with MEP, HF without silane (HF+NS), HF+CLB, and HF+MEP (n = 20/group) was evaluated after storage in water (A: 37oC/1 week, B: 5000×/5-55oC and C: 100 oC/24 h).
Results:
1. NMR and T-FTIR showed ∼60% MPTMS silanol groups in RE-CLB, with the remaining (∼40%) groups being methylated or ethoxylated. In AG-CLB, the silanol peaks further decreased, while ethoxylation and siloxane derivatization increased. In all other products and aging conditions no silanols were traced and formation of small- and large-size MPTMS derivatives was evident. Apart from the 10-MDP molecule, phosphorous impurities were identified in all RE specimens (2-5%), which after AG reached a maximum value of 15% (MBP). MIR-FTIR analysis showed siloxane formation in all products, regardless the presence of free silanols (t1), which further increased at t24 especially in CLB, GMP and MBP.
2. The NMR analysis showed the presence of silanol monomers only in CLB. Methoxylated-siloxane adducts were found in GMP, silanol-siloxane adducts in MBP, SBU, and siloxane polymers in KSP. Acid-etching greatly increased Sa, Sz, Sdr, Sc and Sv parameters (p<0.001) and ATR-FTIR analysis demonstrated evidence of bonding with the substrate in CS. Weibull analysis of SBS revealed the following rankings in characteristic life (p < 0.05): CLB>SBU,KSP,MBP>GMP > NS (group A) and CLB>GMP>SBU,KSP,MBP,NS (group B). The most reliable treatment in both groups was CLB. For the same silane treatment, the SBS of group B were significantly higher from group A. Failures were mainly of adhesive type, except of several partial resin cohesive failures found in group B.
3. Aging did not affect the silanol groups of MEP, but only the phosphate co-monomer. Silanols were reactive forming siloxanes, but exhibited lower consumption rate than CLB. HF-etching induced significantly higher values than MEP, in all the roughness parameters tested (Sa, Sz, Sdr, Sc, Sv), with the greatest differences found in Sdr and Sv. For SBS, MEP was inferior to all treatments/storage conditions, except of HF+NS in A, where the values were similar.
Conclusions:
1. The spectroscopic analyses confirmed that MPTMS silanols existed only in the reference state of the product (CLB) with prehydrolyzed silane, without evidence of condensation. In all other products, silanol polymerization and formation of low- and higher-order condensates were identified in the reference state. The thermal aging, although mild, led to pronounced hydrolysis and condensation in products with detectable silanols in the reference state. The condensates formed, retained residual activity for further siloxane polymerization, the extent of which varied among the products. These results demonstrate that silanols are more stable and reactive when used separately and not when combined with adhesive or conventional resin monomers. 2. The silane primers and adhesive tested contained various forms of MPTMS, including partially hydrolyzed silanol monomers, methoxy- and methoxysilanol-functionalized siloxane dimers and siloxane polymers. The primer with the silanol monomers (CLB) demonstrated chemical bonding via the Si–O− groups with the substrate, whereas carboxylate salts were formed between the adhesive (SBU) and the substrate. These reactions were more pronounced on polished surfaces. The primer with the silanol monomers (CLB) showed the highest bond strength to polished and to acid-etched glass ceramic surfaces, with the highest reliability. The number of products with statistically significant differences in the characteristic life on polished surfaces was reduced after acid-etching, because of the contribution of the micromechanical retention. This reinforcement was quite strong to balance the reduced chemical bonding capacity of several products with reduced or minimal silanol activity. 3. The self-etch silane primer MEP was stable, retaining the original silanol activity after aging. However, the stability of the phosphate co-monomer was affected. The silanol consumption rate of the functional (MPTMS) and crosslinking (BTSE) silanes in MEP was different from that of the same functional silane alone; prehydrolyzed MPTMS demonstrated increased in situ hydrolysis and faster silanol consumption rate. The MEP treatment resulted in significantly lower amplitude (Sa, Sz), hybrid (Sdr) and functional (Sc, Sv) roughness parameters in comparison with 5% HF-etching, when applied on the polished lithium disilicate ceramic surfaces. The greatest differences were found in Sdr (18×) and Sv (4×) in favor of the HF acid. By using the MEP on a HF-etched substrate, the values recorded for all the parameters increased at the levels provided by the HF acid. The SBS of the resin composite to the lithium disilicate as mediated by the MEP was lower from that of the functional silane (MPTMS) and of the same primer applied on HF-etched surfaces, under all storage conditions. Accelerated aging strongly affected the self-etch primer group, leading to values even lower than the negative control (HF-etched substrate without silane).
