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Objectives.

To evaluate the effect of different silane coupling agents and air-drying temperatures on bond strength of translucent quartz fibre posts to composite resin.

Methods. The post surface was etched with 10 vol% hydrogen peroxide for 20 min. A two- liquid coupling agent containing 4-methacryolxyethyl trimellitate anhydride (4-META) and ‘Y-trimethoxysilyil propyl methacrylate (‘Y-MPTS) and two pre-hydrolyzed single component silanes containing 3-methacryloxypropyltrimethoxysilane (3-MPS) and glycid-oxi-propyl- trimetil-oxi-silane (GPS), respectively, were used for treating the fiber posts. Two different post-silanization drying temperatures were applied (21 and 38 ◦C). A dual-cure composite resin (Core Paste XP) was selected to build-up the core around posts, obtaining cylindrical specimens that were serially cut in beams and subsequently loaded in tension (µTBS) at a cross-head speed of 1 mm/min until failure. Bond strength data were statistically analyzed by two-way ANOVA and Student–Newman–Keuls tests (˛ = 0.05).

Results. Warm air-drying determined significantly higher bond strengths (p < 0.001) for glycid- oxi-propyl-trimetil-oxi-silane (11.6 MPa) and 4-methacryolxyethyl trimellitate anhydride/’Y- trimethoxysilyil propyl methacrylate silane (11.7 MPa). These two systems exhibited lower bond strengths (6.9 and 8.8 MPa, respectively) than 3-methacryloxypropyltrimethoxysilane (11.0 MPa) when dried at 21 ◦C. No statistical differences were recorded for 3- methacryloxypropyltrimethoxysilane when drying at 21 or 38 ◦C.

Significance.

The composition of the silane coupling agent in terms of acidic content, solvent rate or degree of hydrolysis may influence resin/post bond strength when dried at 21 ◦C. Drying at 38 ◦C most likely facilitates the evaporation of solvents present in the silane agent, resulting in increased bond strength of the composite resin to the fiber post.

1.  Introduction

Several studies suggested the use of silane coupling agents in coating applications to promote adhesion between inorganic surfaces and polymeric molecules [1,2].

Organosilanes have the formula R×-Si-(OR)₃ with an organic functional group (R×) and three alkoxy groups (R): the chemical reaction begins with the hydrolysis of the alkoxide groups (R) into silanols (SiOH) that may condense forming siloxane bonds [2,3].

Many factors (pH, presence of solvents, molecule size, etc.) may exert an influence in the way silane molecules can absorb, condense or interact with the substrate, influencing coupling effectiveness [4,5].

To accelerate the mechanism of chemical interaction between the silane and the inorganic surface, the reaction may be catalyzed by acid treatment or heating [6,7]. Heat treatment of silanated glass is routinely performed in the glass indus- try to maximize bond strength [8]. Silane has been proven   to increase ceramic-composite bond strength during luting procedures or when repairing chipped ceramic restorations [9–11]. Drying with hot air increases the effectiveness of some silane coupling agents when bonding ceramics to composite resins [7,12].

High temperature silane heat treatment (70–80 ◦C) is not feasible for chair-side procedures, but a stream of warm air (38 ◦C) may be used for this purpose [7].

Fibre posts are extensively used in combination with composite resins to directly restore endodontically treated teeth [13,14]. The efficacy of silane coupling agents increas- ing bond strength between fiber post and composite core restorations have been recently reported [15–18]. However, no information is available concerning the possible influence of different silane coupling agents’ composition or silanizing modalities on post/composite bond strength.  In  particu-  lar, the possible influence of heating on the condensation reaction of silane molecules on the post surface is still unknown. The aim of the study was to determine the effect of warm air drying and different silane coupling agents on the achieved bond strength between fiber posts and resin composite.

The null hypothesis is that silane composition and air- drying temperature do not influence the microtensile bond strength between fiber post and composite resin.

2.  Materials and methods

Thirty quartz fiber posts, with a maximum diameter of 1.80 mm in the cylindrical coronal portion and 1.0 mm at the radicular end (DT Light Post #2, batch no.100US0311A, RTD, St. Ege´ve, France) were used for this study. DT Light posts are made of unidirectional pre-tensed quartz fibers (60%) bound in an epoxy resin matrix (40%).

The posts were etched in 10 vol% hydrogen peroxide solu- tion (Panreac Qu´ımica, Barcelona, Spain) for 20 min at room temperature [19]. They were rinsed with tap water and ultra- sonically cleaned for 10 min in deionised water (P Selecta S.A. Abrera, Barcelona, Spain), subsequently immersed in 96% ethanol and dried with an air stream.

Six experimental groups (n = 5) were formed and three different silane coupling agents were used: a pre-hydrolyzed silane coupling agent containing 3-methacryloxypropyl- trimethoxysilane (3-MPS) (Monobond-S, batch no. E53184, Ivoclar-Vivadent, Schaan, Liechtenstein); a two-component silane coupling agent containing 4-methacryolxyethyl trimellitate anhydride (4-META) and trimethoxysilyil propyl methacrylate (‘Y-MPTS) (Porcelain Liner M, batch no. GF1, Sun Medical Co. Ltd., Japan); a pre-hydrolyzed silane coupling agent containing glycid-oxi-propyl-trimetil-oxi-silane (GPS) (Porcelain Silane, batch no. 4101PFS, BJM Lab, Or-Yenuda, Israel) at two different air-drying temperatures (21 and 38 ◦C). The tested materials were applied following manufacturers’ recommendations. The composition and application mode of the tested materials are described in Table 1.

pH measurements were performed for all tested silane cou- pling agents with a digital pH-meter and a glass electrode calibrated with standard buffer solutions (Micro pH 2000, Cri- son Instruments, Alella, Spain).

After etching and silanizing the post  surface,  compos-  ite build-up was performed following a technique previously described by Goracci et al. [15] and using a dual-curing resin composite (Core Paste XP, batch no. 030653101, Dent Mat, Santa Maria, CA, USA). Core Paste XP  is  a  low  viscosity  core material and contains glass fillers in a methacrylate matrix. Samples were stored 24 h at room temperature before testing.

Microtensile test specimens were prepared by sectioning each sample with a diamond saw under water cooling (Isomet 4000, Buehler, Lake Bluff, IL, U.S.A). A medium of 29 beams of 1-mm in thickness were tested for each group. For the microtensile bond strength test, each beam was glued with cianoacrylate (Zapit, Dental Ventures of America, Corona, USA) to the flat grip of a testing device (Bencor, Multi-T, Danville Engineering, San Ramon,  CA,  U.S.A.)  and  loaded  in tension at a cross-head speed of 1 mm/min until failure (Instron Model 4411, Instrom, Canton, MA, U.S.A.). The modes of failure were evaluated after testing under a stereomi- croscope (Olympus SZ-CTV, Olympus, Tokyo, Japan) at 40× magnification. Failure modes were classified as adhesive (at the post/core interface), cohesive (within the resin compos- ite) or mixed (combination of the two modes on the same surface).

Interfacial bond strength values were expressed in MPa using a mathematical formula previously described by Bouil- laguet et al. [20]. Data were analyzed by two-way ANOVA to evaluate the effect of the  factors  (silane  composition  and air-drying temperature) on the dependent variable (microtensile bond strength). Interactions were included in the analysis. Multiple comparisons were performed with Student–Newman–Keuls test. Statistical significance was set at ˛ = 0.05. The sample size was calculated to ensure a power of 0.8 in the statistical analysis.

3.  Results

Mean microtensile bond strength values are shown in Table 2. Bond strength was influenced by  the air-drying temperature   (p < 0.001), and by the application of different coupling agents (p < 0.001) on the bond strength. Interactions were also signif- icant (p < 0.001). 3-MPS performed similarly, regardless of air- drying temperature. If dried at 21 ◦C, GPS and 4-META/-y-MPTS silanes exhibited lower bond strengths than 3-MPS. Similar results were attained for all  coupling  agents  when  dried  at  38 ◦C. GPS and 3-MPS had a similar acidic pH (4.5 and 3.8, respectively),  while  4-META/-y-MPTS  silane  recorded  a  lower value (1.8).

The distribution and percentages of failures are described in Table 2. A percentage of adhesive failure between 90 and 100% was recorded in all tested groups.

4.  Discussion

The use of different air-drying temperatures on the silanated posts and the different composition of the silane coupling agents affected the bond strength of composite to fiber posts. Thus, the null hypothesis is rejected.

Silane enhances post-resin bond strength by promoting the wetting of the etched post surface and facilitating the diffusion of the fluid composite resin into the retentive spaces among the exposed fibers [19]. When silane is applied on the post surface and dried, two phases are created [21,22]: an outermost physisorbed layer with few siloxane bonds and a hydrolytically stable chemisorbed layer on the post surface [2,7].

Further reactions between silane molecules and the organic surface (fiber posts) have been proven to occur, enhancing condensation and providing a more tightly packed configuration of the coupler molecules on the post surface [2,23].

5. Discussion

The use of different air-drying temperatures on the silanated posts and the different composition of the silane coupling agents affected the bond strength of composite to fiber posts. Thus, the null hypothesis is rejected.

Silane enhances post-resin bond strength by promoting the wetting of the etched post surface and facilitating the diffusion of the fluid composite resin into the retentive spaces among the exposed fibers [19]. When silane is applied on the post surface and dried, two phases are created [21,22]: an outermost physisorbed layer with few siloxane bonds and a hydrolytically stable chemisorbed layer on the post surface [2,7].

Further reactions between silane molecules and the organic surface (fiber posts) have been proven to occur, enhancing condensation and providing a more tightly packed configuration of the coupler molecules on the post surface [2,23].mixing the silane coupler (-y-MPTS) with the acidic monomer (4-META) just before its application [24,25]. Pre-activated silane solutions are expected to exhibit a higher rate of hydrol- ysis compared to the two-component systems in which an incomplete reaction may eventually take place if the solvent is not completely evaporated, affecting bond strength [8,25].

3-MPS and GPS are considered neutral silanes with a similar degree of coupling power: under mildly acidic concentration,they condense rapidly to reach an equilibrium composition in a short time [7].

GPS silane is appropriate for epoxy resin coupling: however, its application on the post surface has been proven to decrease bond strength at least if dried at 21 ◦C [26].

Pre-activated silane primers have a limited shelf-life due to the rapid formation of oligomers [2,8] At high silane concen- tration (3% GPS in the tested silane) the potential of oligomer formation increases: moreover, the physiosorbed silane tends to form a weak boundary layer onto the quartz fibres that may act deficiently in the post/composite interface, causing a lubri- cation effect [2,5,27]. It explains the lower µTBS attained by the pre-hydrolyzed GPS-based silane when compared to 3- MPS silane when dried at 21 ◦C. The results from this study indicated that by using a GPS coupling on the post surface,  38 ◦C air-drying is advisable to facilitate solvent removal and to achieve chemical stability [28].

6.  Conclusions

Air-drying at 38 ◦C promotes the condensation process of the silane on the post surface and removes some of the loosely absorbed molecules probably by simple evaporation. Water- based silane coupling solutions were shown to be less influ- enced by air-temperature during drying.