The purpose of this study was to assess the effects of an experimental primer containing acetone solution and a sulfur-containing functional monomer, 10-methacryloyloxydecyl-(2-thiohydantoin-4-yl)propionate (MDTHP), on the bonds between noble metals and acrylic resin.
The experimental primer used as the control for comparison consisted of 6-(4-vinylbenzyl- n -propyl)amino-1,3,5-triazine-2,4-dithione (VBATDT) in acetone. These primers were prepared as equimolar functional monomers (0.1 mol%). A self-polymerizing acrylic resin initiated with tri- n -butylborane (TBB) was used as the luting agent. Four elemental metal disks (silver, copper, palladium, and gold) were used as adherend specimens. All the disks were wet-ground with silicon carbide paper (#1500). Bonding reactions were performed on 12 combinations of the four metals, and the disks were either primed with MDTHP or VBATDT or were unprimed (control). Shear bond strengths were determined pre- and post-thermocycling (5–55 °C, dwell time 60 s, 20,000 cycles). The results were statistically analyzed via a non-parametric test ( α = 0.05).
The post-thermocycling shear bond strengths of the MDTHP primer were as follows (median, n = 11): 13.2 MPa on silver, 25.9 MPa on copper, 4.1 MPa on palladium, and 11.3 MPa on gold. The MDTHP primer showed higher post-thermocycling shear bond strength on all the four metals. Additionally, on silver and copper, the MDTHP bond strengths were higher than on the other metals.
Within the limitation of current of experimental setting, the MDTHP compound may be applicable as a functional monomer for bonding noble metal alloys.
Noble metal alloys are being extensively used in the fabrication of restorations and dental prostheses owing to their excellent mechanical and handling properties. However, it has been found that it is difficult for noble metals and their alloys to adhesively bond to tooth structures, ceramics, and other dental materials.
The use of several organic sulfur-containing compounds has been reported in order to facilitate the bonding of noble metals and of copper. Mori and Nakamura synthesized 6-(4-vinylbenzyl- n -propyl)amino-1,3,5-triazine-2,4-dithione (VBATDT) for use as a coating agent for copper plates [ ]. Kojima et al. reported on the bonding performance of 10-methacryloyloxydecyl dihydrogen thiophosphate (MDTP) when it is applied to metals and alloys [ ]; however, they did not publish details regarding the synthesis and purification procedures. Thereafter, Kojima et al. obtained a patent for several monomers, including 10-methacryloyloxydecyl-6,8-dithiooctanoate (10-MDDT) [ ]. The performances of primers containing VBATDT or 6-methacryloyloxyhexyl 2-thiouracil 5-carboxylate (MTU-6) have been investigated in relation to facilitating the bonding of elemental noble metals [ ] or of noble alloys [ ], the characteristics of their bonds to these metals have been analyzed [ ], and the clinical effectiveness of their use in facilitating bonds to dental noble metal alloys has been reported [ , ].
Synthesis of a sulfur-containing functional monomer, 10-methacryloyloxydecyl-(2-thiohydantoin-4-yl)propionate (MDTHP), was thereafter reported [ ]. Although MDTHP is expected to facilitate bonds between noble metal alloys and luting agents, limited information is available regarding the bonding properties of this compound.
The purpose of this study was to investigate the effects of an experimental primer containing MDTHP on the bonding between elemental noble metals and an acrylic resin.
Materials and methods
The materials are described in Table 1 . High-purity silver (Ag), copper (Cu), palladium (Pd) (The Nilaco, Tokyo, Japan), and gold (Au) (Kojundo Chemical Laboratory, Sakado, Japan) elemental metals were used as adherend materials. In evaluation of bonding performance of primers, two organic sulfur compounds were employed. MDTHP was synthesized according to the method described by Matsumura et al. [ ], whereas VBATDT was synthesized as a reference compound by the procedure reported by Mori and Nakamura [ ]. Both the MDTHP and VBATDT primers composed of 0.1 mol% MDTHP or VBATDT in acetone (Tokyo Chemical Industry, Tokyo, Japan). Fig. 1 depicts the structural formulae of the two monomers.
|Silver (Ag)||The Nilaco Corp., Tokyo, Japan||85,002,201||99.9 mass%|
|Copper (Cu)||The Nilaco Corp.||85,002,301||99.9 mass%|
|Palladium (Pd)||The Nilaco Corp.||44,151,201||99.9 mass%|
|Gold (Au)||Kojundo Chemical Laboratory Co. Ltd., Sakado, Japan||282,443||99.9 mass%|
|MDTHP primer||0.1 mol% MDTHP, acetone|
|VBATDT primer||0.1 mol% VBTDT, acetone|
|Super-Bond C&B Catalyst V||Sun Medical Co., Ltd.||RG23F||TBB, TBB-O, hydrocarbon|
|Super-Bond C&B Opaque Ivoly Powder||Sun Medical Co., Ltd.||MW1||PMMA, titanium oxide|
|Methyl methacrylate||Tokyo Chemical Industry Co., Ltd., Tokyo, Japan||YMVAA OL||MMA 99.8%|
A self-polymerizing resin consisting of partially oxidized tri- n -butylborane (TBB) initiator (Super-Bond Catalyst V; Sun Medical, Moriyama, Japan), methyl methacrylate (MMA) (Tokyo Chemical Industry), and poly(methyl methacrylate) (PMMA, Super-Bond Opaque Ivory powder; Sun Medical) was used as the luting agent that did not contain a functional monomer.
Shear bond strength testing
Disk-shaped metals (10 mm in diameter and 3 mm in thickness) were cut to 66 specimens from each metal rods. All the disks were wet-ground with silicon carbide paper (Wet or Dry Tri-M-ite Sheet; 3M Corp., St. Paul, MN, USA) in the order of #800, #1000, and #1500. Subsequently, all the specimens were washed with acetone (Tokyo Chemical Industry) in an ultrasonic bath (SUC-110; Shofu, Kyoto, Japan).
MDTHP primer (MP) or VBATDT primer (VP) was applied to the adherend surfaces, and the specimens were air-dried. Unprimed specimens were prepared for use as the control sample. A piece of masking tape with a circular hole measuring 5 mm in diameter was placed on the disk surface in order to define the bonding area. A stainless-steel ring (inner diameter of 6.0 mm, 2.0 mm in height, and 1.0 mm in thickness) was placed around the circular hole with the diameter of 5 mm. The ring was filled with the luting agent using the brush-dip technique. After 30 min of allowing the bonding to occur, the specimens were immersed in distilled water at a temperature of 37 °C for 24 h. This state was considered to be the “0 thermocycles” state, and half of the specimens ( n = 11) were tested at this stage. The remaining half of the specimens ( n = 11) were subsequently thermocycled in water between temperatures of 5−55 °C for 20,000 cycles with a 60 s dwell time per bath (Thermal Shock Tester TTS-1 LM; Thomas Kagaku, Tokyo, Japan). The specimens were then set in a steel mold and were seated in a bond test jig. The shear bond strengths were determined using a mechanical testing device (Type 5567; Instron, Canton, MA, USA) at a crosshead speed of 0.5 mm/min.
Failure mode analysis
Following the shear bond testing procedure, the debonded surfaces of the specimens were observed under an optical microscope (Stemi DV4; Carl Zeiss, Jena, Germany) at ×32 magnification. The cohesive failure ratios were calculated using the following equation: Cohesive failure ratio (%) = Cohesive failure area (pixel) × 100/Bonded area (pixel).
Statistical analyses were performed using the application (Kyplot 5.0; KyensLab, Tokyo, Japan). For shear bond strength, the median values and interquartile range of the eleven specimens were calculated. All the groups were primarily analyzed by performing the Kolmogorov–Smirnov normal distribution test. When the results did not indicate a normal distribution in some groups, the statistical significance was assessed by performing a non-parametric test. The Kruskal–Wallis test and Steel–Dwass multiple comparisons were performed to evaluate the difference in the effects of the two primers among the metals. The Mann–Whitney U test was performed to evaluate the difference in primer effects between 0 and 20,000 thermocycles. The statistical significance level was set at α = 0.05.
X-ray photoelectron spectroscopic (XPS) analysis
The primed and unprimed surfaces of the Cu disks were analyzed using X-ray photoelectron spectroscopy (XPS, ESCA-3400; Shimadzu Co. Ltd., Kyoto, Japan). The Cu disks were wet-ground with silicon carbide paper (#800, #1000, #1500, and #2000) and polished using a felt pad (TexMet 1500; Buehler, Lake Bluff, IL, USA) and monocrystalline diamond suspension (3 μm and 1 μm, MetaDi; Buehler). They were then washed in the same manner as the shear bond strength testing specimens. Three polished disk surfaces each were primed with a single drop of MP or of VP or were unprimed, and they were rinsed three times with acetone.
The following XPS parameters were used: an Mg K α X-ray excitation source of 1,253.6 eV was used to produce photoelectrons and was operated at 10 kV and 20 mA. A wide spectrum scan (0–1,200 eV) and narrow scans (Cu 2 p , O 1 s , N 1 s , C 1 s , S 2 p ) were performed to examine the four specimens.
Shear bond strength testing
The results of the shear bond strength tests are summarized in Table 2 . This table lists median, interquartile range (IQR), and grouping results for all the metals acquired by Steel-Dwass multiple comparisons, the difference between the pre- and post-thermocycling shear bond strengths acquired by performing the Mann–Whitney U test, and the calculated post-/pre-thermocycling bond strength ratios (post-/pre-BS ratio %). MP was categorized as producing the highest post-thermocycling shear bond strength in the specimen groups of all the four metals. Additionally on Ag and Cu, the shear bond strength with MP was higher than on the other metals. The bond strength in all the pre-thermocycling groups was significantly higher than that in the post-thermocycling ones.