Effects of three silane primers and five adhesive agents on the bond strength of composite material for a computer-aided design and manufacturing system

Abstract Objective The objective of this study was to evaluate the effects of combinations of silane primers and adhesive agents on the bond strength of a composite block for a computer-aided design and manufacturing system. Material and Methods Three silane primers [Clearfil Ceramic Primer (CP), Super-Bond PZ Primer (PZ), and GC Ceramic Primer II (GP)] were used in conjunction with five adhesive agents [G-Premio Bond (P-Bond), Repair Adhe Adhesive (R-Adhesive), Super-Bond D-Liner Dual (SB-Dual), Super-Bond C&B (SB-Self), and SB-Dual without tributylborane derivative (SB-Light)]. The surface of a composite block (Gradia Block) was ground with silicon carbide paper. After treatment with a silane primer, a adhesive agent was applied to each testing specimen. The specimens were then bonded with a light-curing resin composite. After 24 h, the shear bond strength values were determined and compared using a post hoc test (α=0.05, n=8/group). We also prepared control specimens without primer (No primer) and/or without adhesive agent (No adhesive). Results PZ/SB-Dual and GP/SB-Dual presented the highest bond strength, followed by GP/P-Bond, CP/SB-Dual, CP/R-Adhesive, No primer/SB-Dual, GP/R-Adhesive, CP/P-Bond, No primer/R-Adhesive, PZ/R-Adhesive, CP/SB-Self, PZ/P-Bond, PZ/SB-Self, and GP/SB-Self in descending order of bond strength. No primer/P-Bond, No primer/SB-Self, and all specimens in the SB-Light and No adhesive groups presented the lowest bond strengths. Conclusion A dual-curing adhesive agent (SB-Dual) containing a tributylborane derivative in combination with a silane primer (GP or PZ) presents a greater bond strength between the composite block and the repairing resin composite than the comparators used in the study.


Introduction
The advent of computer-aided design and computer-aided manufacturing (CAD/CAM) systems, the use of highly filled resin composite materials became increasingly common for crown restorations 16 .
Despite the improvement of the mechanical properties of composite blocks for CAD/CAM systems over time 8,13 , partial fractures still occur occasionally on restorations in the oral environment. Strong bonding between the machine-milled composite blocks and the resin composite veneering materials is required to repair defects in resin composite restorations or to modify the esthetics of the monochromatic composite blocks using the layering technique 15 .
Ceramic repair systems and pre-treatment agents have improved the bond strength of resin-based materials to composite blocks 4,6 . Certain studies recommend to treat the surface with silanes 1,5,19,20 , while others claim that silanization techniques do not increase bond strength 22, 23 . When a silane primer and an adhesive agent were applied to the surface of a composite block, the bond strength was greater than that obtained through the use of a lithium disilicate glass ceramic or a feldspar ceramic 3 . Studies show that the combined use of a silane primer and a lightcuring adhesive agent is effective to increase the bond strength between the layers of a resin composite 11,12 .
However, there is little information available on the effect of the type of polymerization initiator contained in the adhesive agent on the bond strength. The objective of this study was to evaluate the bond strength between a composite block and the light-curing resin composite comprising five different adhesive agents (three light-curing systems, one self-curing system, and one dual-curing system) in conjunction with three silane primers. The null hypothesis was that there is no significant difference among the bond strengths for the different combinations of the adhesive agents and silane primers.

Shear bond strength tests
The three silane primers (CP, PZ, and GP) and five adhesive agents (P-Bond, R-Adhesive, SB-Dual, SB-Self, and SB-Light) used are listed in Figure 1.
The testing specimens consisted of 192 rectangular specimens (8×10×3 mm) that were cut from a composite block (Gradia Block, A3, GC Corp., Tokyo, Japan) using a diamond saw (IsoMet Low Speed Saw, Buehler, Lake Bluff, IL, USA), they were divided into 24 groups (15 combinations of 3 primers and 5 adhesive agents and 9 controls) of 8 specimens each. All specimens were ground with 600-grit silicon-carbide abrasive paper (BuehlerMet2, Buehler, Lake Bluff, IL, USA), rinsed by spraying with water for 10 s, and air-dried. We attached a piece of masking tape with a circular hole of 2 mm in diameter to the surface of each specimen to delineate the bonding area ( Figure   2). One microliter of each of the silane primer and adhesive agent was then applied to the specimens with a micropipette (Eppendorf AG, Hamburg, Germany) and gently air-blown. Except for SB-Self, the adhesive agents used were light-cured for 10 s using a lightemitting diode handpiece (power density 1,000 mW/ cm 2 ; Pencure, J. Morita MFG. Corp., Tokyo, Japan). We also prepared control specimens without primer (No primer) and/or without adhesive agent (No adhesive).
We placed an acrylic ring (internal diameter: 4 mm, height: 2 mm) on the specimen and filled it with Corp., Tokyo, Japan) at a magnification of ×20 to assess bond failure. Failure modes were categorized as adhesive failure at the interface between the composite block and veneered resin composite (Ad) and a combination of Ad and crack propagation (Cr) inside the composite block (Ad/Cr).

Name (abbreviation)
The composite block used contained 73% inorganic filler and 3% organic-inorganic composite filler. Silane coupling agents react with inorganic components such as silica to form siloxane bonds; they also copolymerize with methacrylates 2,18 . However, in this study we did not obtain a high bond strength using a silane primer alone (CP/No adhesive, PZ/No adhesive, or GP/No adhesive). We hypothesize that the inorganic filler particles had already been treated with silane coupling agents during the manufacture process, therefore, the substrate surface contained few exposed inorganic components. This could be the reason for the limited effect of the silane primers on bond strength.
The mode of failure tended to shift from Ad to Ad/Cr as the bond strength increased (Table 2)

Conclusions
Within the limitations of this study, we conclude that the combination of a silane primer (GP or PZ) and a dual-curing adhesive agent (SB-Dual) improves the bond strength between a composite block and a lightcuring resin composite. Furthermore, the contribution of the coupling agent to the bond strength is greater than that of the silane primers.