Original article

Surface treatment with methyl formate–methyl acetate increased the shear bond strength between reline resins and denture base resin Rachanee Osathananda1 and Chairat Wiwatwarrapan1,2 1

Faculty of Dentistry, Department of Prosthodontics, Chulalongkorn University, Bangkok, Thailand; 2Developing Research Unit in Dental Polymeric Materials in Prosthodontics, Faculty of Dentistry, Chulalongkorn University, Bangkok, Thailand

doi: 10.1111/ger.12120 Surface treatment with methyl formate–methyl acetate increased the shear bond strength between reline resins and denture base resin Background: Chemical surface treatment increases the shear bond strength (SBS) between hard reline resins (HRRs) and denture base resin. Objective: To evaluate the effect of methyl formate–methyl acetate (MF-MA), when used as a surface treatment agent, on the SBS between denture base resin and different HRRs. Materials and Methods: One hundred and twenty specimens of heat-polymerised acrylic resin denture base (Meliodentâ) were divided into 12 groups. These groups comprised denture base relined with three self-polymerised HRRs [Unifast tradâ (UT), Tokuyamaâ RebaseII Fast (TR), Ufi gel hardâ(UG)], and treated with their respective Bonding Agent (BA) or by MF:MA solutions at ratios of 35:65, 25:75, and 15:85 for 15 s. The SBS was measured using a Universal Testing Machine. The data were analysed using two-way ANOVA and post hoc Tukey’s analysis at p < 0.05. Results: The highest SBS was in the UT treated with MF:MA at a ratio of 25:75 group, followed by UT treated with MF:MA at ratios of 15:85, 35:65, UT treated with BA, and all UG treated with MF:MA groups. The SBS of the UT treated with MF:MA at a ratio of 25:75 group was significantly higher than those of the groups treated with BA. The SBS of the UG treated with MF:MA groups was significantly higher than control. The TR groups treated with BA or MF:MA groups showed no significant difference in SBS. Conclusion: Surface treatment with MF-MA significantly enhanced the SBS of denture base resin and UT and UG compared to that of the groups treated with BA. Keywords: hard reline resin, denture base, shear bond strength, surface treatment. Accepted 14 January 2014

Introduction Relining a denture base is a common procedure to improve the fit of the denture. The direct relining of a denture base with a self-polymerised hard reline resin is convenient, easy, inexpensive, and can reproduce the morphological features of the oral soft tissue directly on the denture base without a significant change in its dimensions1,2. There are two types of reline materials. One type contains methyl methacrylate monomer in its liquid component (MMA-based). The other type contains various high molecular weight methacrylate monomers (Non-MMA based) that are less

irritating substances and generate less heat during polymerisation than methyl methacrylate monomer3–5. The success of a relined denture depends on the bond strength between the reline resin and the denture base resin. However, the high molecular weight of the non-MMA-based reline material results in less diffusion, and low penetration of the monomer from the reline resin to the denture base can cause weak bonding6,7. Weak bonding can result in staining or bacterial invasion, and ultimately delamination can occur between the reline resin and the denture base resin. Delamination results not only in the loss of tissue adaptation, but also decreases the mechani-

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Table 1 Trade name, manufacturer, and chemical composition of the tested materials. Composition Product name

Lot No. and Manufacturer

Powder

Heat polymerized denture base Meliodentâ 10FED017 Heraeus Kulzer, PMMA Sanden, Germany Self polymerized reline resins MMA based Unifast Tradâ (UT) MMA & EMA Powder: 0807151, Liquid: Copolymer 1003012 GC Dental Products Corp, Tokyo, Japan Non-MMA based Ufi Gel Hardâ (UG) 1023088 Voco, Cuxhaven, PEMA Germany Tokuyamaâ RebaseII 007E10 Tokuyama Dental PEMA Fast (TR) Corp, Tokyo, Japan Chemical solvents Methyl acetate A0259676 Acros Organics, Geel, Belgium Methyl formate 1239211 Fluka & Riedel-de Hean, Buchs, Switzerland

Liquid

Adhesive

MMA



MMA

MMA

1,6-HDMA

Acetone, 2-HEMA Ethyl acetate, Acetone

AAEMA 1,9-NDMA

PMMA, Poly(methyl methacrylate); MMA&EMA copolymer, Methyl methacrylate & Ethyl methacrylate copolymer; PEMA, Poly(ethyl methacrylate); 1,6-HDMA, 1,6-Hexanedial dimethacrylate; 2-HEMA, 2- Hydroxyethyl methacrylate; AAEMA, 2-(Acetoacetoxy) ethyl methacrylate; 1,9-NDMA, 1,9-Nonanediol dimethacrylate.

cal strength of the relined denture4,5,8–10. Chemical surface treatment of the denture base resin can enhance the diffusion and penetration of the monomer, and thus improve bonding6,7,11–30. Chloroform6,13,14 and methylene chloride6,12,14–19 have been suggested as chemical surface treatment agents, but there is evidence that they may be carcinogenic to humans31. Therefore, methyl methacrylate (MMA) 6,7,11,12,16,20–23, ethyl acetate17,21,24, acetone16,21,22,25–28, methyl formate29, methyl acetate29, and a mixture of methyl formate–methyl acetate (MF-MA)30 have been investigated as alternatives. The objective of this study was to evaluate the effect of MF:MA, at different ratios, as a surface treatment agent on the shear bond strength (SBS) of three self-polymerised hard reline resins bonded to denture base resin. The null hypothesis was that there would be no significant differences in SBS between the hard reline resins and denture base resins that received different surface treatments with MF-MA solutions or the bonding agents supplied by the manufacturer.

in Table 1. PVC tubes were used for the fabrication of 120 (22 9 20 mm) wax cylinders (Fig. 1a), which were invested in flasks. Heatpolymerised denture base (Meliodentâ; Heraeus Kulzer, Sanden, Germany) was mixed and packed according to the manufacturer’s directions. The specimens were polymerised in a water bath at 73.9°C for 9 h and then deflasked. Subsequently, all specimens were stored in distilled water at

(a)

(b)

(c)

(f)

(e)

(d)

Materials and methods The heat-polymerised denture base, three selfpolymerised reline resins, and their compositions, as well as the solutions used in this study are seen

Figure 1 Specimen preparation. (a) heat-cured denture base in PVC tube, (b) specimen with masking tape, (c) metal split mould, (d, e) specimen in mould, (f) prepared test specimen.

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Bond strength between reline and denture base

37  1°C for 50  2 h32. Before bonding, each specimen’s surface was polished with 600-grit silicon carbide paper to remove any irregularities. The denture base specimens were randomly divided into 12 groups (n = 10) by the type of reline material Unifast Tradâ (UT): GC Dental Products Corp, Tokyo, Japan, Ufi Gel Hardâ (UG): Voco, Cuxhaven, Germany, and Tokuyamaâ Rebase II Fast (TR): Tokuyama Dental Corp, Tokyo, Japan and the four types of surface treatment. The denture base surface was treated with either; (1) the UG or TR group’s respective BA supplied by the manufacturer and used following the manufacturer’s recommendation, or with MMA for 15 s for the UT group, or (2–4) MF:MA at ratios of 15:85, 25:75, and 35:65 for 15 s30. Masking tape with a 5.3 mm diameter hole was placed on the treated denture base surface (Fig. 1b), and a custom metal split mould (5.3 mm. diameter 9 5 mm. height) (Fig. 1c) was positioned over the hole (Fig. 1d, e). The reline resins were mixed per the manufacturer’s instructions, placed on the denture base surface, and polymerised in the metal mould. An acetate sheet was placed over the material and pressure was applied until polymerisation was complete, and the metal mould and the tape were removed (Fig. 1f). The relined specimens were stored in distilled water at 37  1°C for 50  2 h32. The shear bond test was carried out using a Universal testing machine (8827, INSTRON, High Wycombe, UK) at a 0.5 mm/min crosshead speed, with the contact point at the reline resin denture base interface. The SBS was calculated by dividing the failure force by the adhesion surface area. To identify any morphological changes of the denture base surface after surface treatment, specimens were sputter-coated with gold and observed under a scanning electron microscope (JEOL5410; JEOL Inc., Tokyo, Japan). Specimens with untreated surfaces served as control. The debonded surfaces were observed using an ML9300 stereo microscope (Meiji Techno, Saitama, Japan) at 79 magnification to identify the mode of failure. Failures that occurred at the reline denture base/ resin interface were recorded as adhesive failures. Failures that occurred within the reline material were recorded as cohesive failures. A combination of failure at the interface and cohesive fracture within the reline resin or the denture base material was recorded as a mixed failure. The data were statistically analysed using SPSS for Windows 17.0 (SPSS Inc., Chicago, IL, USA). The results were tested to determine the normality of distribution with the One-sample Kolmogorov–

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Smirnov test and the homogeneity of variance using the Levene’s test. The data were normally distributed (p > 0.05) and presented homogeneous variances (p > 0.05), which indicated that a parametric analysis should be performed. The means and standard deviations (SD) for the SBS were calculated and statistically analysed using one-way analysis of variance (ANOVA) and the post hoc Tukey’s test at a 95% confidence level.

Results The mean SBS and standard deviation of each group, and the percentage of each failure type after shear testing, are presented in Table 2. The mean SBSs of the groups relined with UT (groups 1–4) were significantly higher than those of the groups relined with UG (groups 5–8) or TR (groups 9–12) (p < 0.05). The mean SBS of the group treated with MF:MA at a ratio of 25:75 and relined with UT (group 3) was significantly higher than that of the UT control group (p < 0.05). The mean SBSs of all groups treated with MF-MA and relined with UG (groups 6–8) were significantly higher than those of the UG control group and TR groups 9–12 (p < 0.05). There were no significant differences between the UG groups treated with MF-MA. The mean SBSs of all groups treated with MF-MA and relined with TR were not significantly different from the TR control group (p > 0.05). Analysing the mode of failure revealed that the UT groups did not demonstrate any adhesive failures, while the UG control group and the UG group treated with MF:MA at a ratio of 15:85 showed 100% adhesive failures. The UG groups treated with MF:MA at a ratio of 25:75 or 35:65 exhibited 70 and 60% adhesive failures, respectively. In the TR control group and the TR group treated with MF:MA at a ratio of 15:85, adhesive failures occurred at a rate of 90 and 60%, respectively, while the TR groups treated with MF:MA at a ratio of 25:75 or 35:65 resulted in 80% adhesive failures. Scanning electron microscopy images of the specimens are shown in Fig. 2. Compared with the untreated specimens (Fig. 2a), the specimens treated with MMA (Fig. 2b) did not show a clear difference. However, the specimens treated with UG bonding agent exhibited a smoother surface (Fig. 2c), while the surface of the specimens treated with TR bonding agent demonstrated a honeycomb appearance (Fig. 2d). Denture base treatment with MF:MA at all ratios used in our study produced a porous surface. The pores of the

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Table 2 Mean shear bond strength with standard deviation and type of failure (%) of the three reline resins and different surface treatments. Type of failure (%) Group

Reline resin

Surface treatment for 15 s

Mean SBS  SD (MPa)

Adhesive

Cohesive

Mixed

1 2 3 4 5 6 7 8 9 10 11 12

UT UT UT UT UG UG UG UG TR TR TR TR

MMA MF:MA MF:MA MF:MA BA MF:MA MF:MA MF:MA BA MF:MA MF:MA MF:MA

16.99  0.83b 18.65  0.60a,b 18.88  1.24a 18.35  1.47a,b 8.27  1.05d 10.87  1.42c 10.67  1.30c 11.73  1.56c 7.08  1.22d 7.69  0.78d 6.77  0.70d 7.33  1.57d

0 0 0 0 100 100 70 60 90 60 80 80

10 20 0 10 0 0 0 0 0 0 0 0

90 80 100 90 0 0 30 40 10 40 20 20

15:85 25:75 35:65 15:85 25:75 35:65 15:85 25:75 35:65

UT, Unifast Tradâ; UG, Ufi Gel Hardâ; TR, Tokuyamaâ RebaseII Fast; MF:MA, ratio of mixture of methyl formate–methyl acetate solution; BA, bonding agent supplied by the manufacturer and used following the manufacturer’s recommendation; SBS, shear bond strength; SD, standard deviation; MPa, MegaPascals. The same superscript letter indicates no significant difference (p > 0.05).

denture base surface treated with MF:MA at a ratio of 15:85 had pores of similar shape, which differed somewhat in size (Fig. 2e). Denture base treated with MF:MA at a ratio of 25:75 had the largest pore size, but the pores ranged in size and shape (Fig. 2f). We found that denture base treatment with MF:MA at a ratio of 35:65 produced the smallest pores (Fig. 2g) compared to those treated with lower MF:MA ratios.

Discussion This study used the shear bond test to compare the bond strength between denture base resin and different reline materials after using MF:MA at ratios of 35:65, 25:75, and 15:85, or the reline resin surface treatment agents recommended by the manufacturers, as surface treatments of the denture base resin. When using the shear bond test, the force was applied to the interface of the two materials. This method applies force in a manner similar to the in vivo forces.19,33,34 In the Ufi Gel Hardâ groups, the SBS of groups 6–8 were significantly higher than that of the group 5. In the Unifast Tradâ groups, the SBS of group 3 was significantly higher than that of group 1. Based on these results, the null hypothesis was rejected. Generally, the solubility and swelling of polymers occurs when the polymer’s and solvent’s solubility parameters and polarities are close to each other29. The solubility parameter of PMMA is

18.3 MPa1/2, while those of MMA, MF, and MA are 18.0, 20.9, and 19.6, respectively35. In addition, MMA, MF, and MA have the same methyl ester group that would affect their ability to soften PMMA. MF and MA surface treatment increased the SBS between an acrylic rebasing material and PMMA29. Thus, we hypothesised that an MF-MA mixture would increase bond strength. Surface treatment of an acrylic denture base with MF-MA mixtures increased the transverse strength of heat-polymerised acrylic resin repaired with selfpolymerised acrylic resin. The MF:MA at ratio of 25:75 group had a significantly higher flexural strength than that of the 180 s MMA surface treatment group32. Studies have shown that chemical surface treatment improved the bond strength between a reline material and denture base resin4,8– 12,19,21,29,33,34 . It has been demonstrated that applying MMA as a surface treatment for 180 s resulted in the highest bond strength12,21,29,33. However, we applied MMA to UT for 15 s, because a 180-s treatment is not widely used in clinical practice. In our study, we used MF:MA at ratios of 15:85, 25:75, and 35:65, and these were applied for 15 s, based on the study of Thunyakitpisal et al.30. In this study, the 15 s duration of the surface treatment in the UT/MMA group was shorter than in other studies that used a 180 s treatment. Our previous study had shown that there was no significant difference in mean flexural strength of

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Bond strength between reline and denture base

Figure 2 SEM analysis of the surface characteristics of heat-cured acrylic resin denture base (Meliodentâ). (a) no treatment, (b) methyl methacrylate monomer, (c) Ufi Gel Hardâ bonding, (d) Tokuyamaâ Rebase II adhesive, (e–g) mixture of methyl formate and methyl acetate solution 15 s at ratios 15:85, 25:75, 35:65, respectively.

(a)

(b)

(c)

(d)

(e)

(f)

151

(g)

denture repair between the groups treated with MMA for 15 s (54.37  4.16 MPa) or 180 s (58.81  2.80 4.16 MPa)36. In that study, both groups demonstrated higher mean flexural strength than that of the untreated group (47.79  4.17 MPa)36. If a 180 s MMA treatment is used, the degree of swelling of the denture surface and the depth of the MMA penetrated-layer of the denture base might be increased. The effect of surface treatment time when using MMA or MF-MA solutions should be examined in future studies. The bonding mechanism between denture base and reline resin begins when the solvents in the surface treatment agent contact the denture base material, dissolving the denture base and causing swelling of the surface layers, followed by solvent

evaporation. Subsequently, the monomers of the reline material diffuse and penetrate into the pores of the denture base resin and form an interpenetrating polymer network. This mechanism is affected by time, temperature, type of solvent, polymer structure, and glass transitional temperature7. We found the SBSs of the UT groups were higher than those of the TR and UG groups, which is consistent with previous studies6,34. MMA, the monomer in UT, has a lower molecular weight (MMA = 100.12) than the monomers in TR (1,9-NDMA = 296.40, AAEMA = 214.21) or UG (1,6-HDMA = 254.32)37. Differential scanning calorimetry was used to measure the heat energy change during the polymerisation of these reline materials at 25°C38. In this study, the released energy of UT, TR, and UG was found to be 153.5,

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78.08, and 40.39, J/g, respectively. Thus, UT generates more heat during polymerisation, allowing for better monomer diffusion and penetration into the denture base material. The solubility of specific surface treatment agents to heat-polymerised denture base can be estimated by the relative closeness of their respective solubility parameters and their respective polarities35. In addition, lower molecular weight materials have better solubility than those of higher molecular weight39. Methyl methacrylate is closer to the solubility and polarity of the denture base material than are methyl formate and methyl acetate29,37. However, MMA has a higher molecular weight than methyl formate (60.05) and methyl acetate (74.08), thus MMA has lower solubility to the denture base material. This may be why the SBSs of the groups treated with MF-MA were higher than that treated with MMA. In addition, methyl formate and methyl acetate contain methyl groups that could make them relatively closer in polarity to the denture base material than ethyl acetate, acetone, or 2-HEMA. This could contribute to the SBS values of the UG groups treated with MF-MA being higher than those treated with the control bonding agent, which is composed of acetone and 2-HEMA. Thunyakitpisal et al.30 found that MF-MA treatment resulted in higher bond strength than that of treatment with the TR adhesive, which consists of ethyl acetate and acetone29,30. However, in our study, we found no significant differences between the two solutions. This may be because our study mainly used reline materials with non-MMA based components. We found that the SEM appearance of the denture base treated with MMA was not markedly different from that of the untreated denture base. However, treating denture base resin with the UG bonding agent produced a smoother surface, suggesting there may be some chemical agent coating the surface. This might be because 2-HEMA, a component of the UG bonding agent, has a high boiling point (198°C). Thus, the 2-HEMA could not evaporate, and remained as a visible layer on the surface of the denture base. This might affect the bonding between the reline material and the denture, and further study is required to investigate this. Using SEM analysis, pores were seen on the surface of the denture base resin treated with the TR bonding agent or MF-MA. This indicates that the TR bonding agent and MF-MA first dissolved the surface of the PMMA denture base, which became soft and swelled, and finally evaporated. This left behind a porous surface allowing

penetration of the monomer of the reline material, leading to better mechanical interlocking than in the samples treated with the UT or UG adhesive agents. Considering the fracture patterns of the tested specimens, the UT specimens exhibited only mixed and cohesive fractures, while most of the UG and TR specimens exhibited adhesive fractures. These results showed that UT had a greater monomer-penetrating activity than that of UG and TR. It is noteworthy that the molecular weight of the MMA monomer of the UT resin was lower than those of the non-MMA-based monomers of UG and TR. Some correlations were found in UG and TR groups between the mode of failure and scanning electron microscopy images of the treated surfaces of the specimens. The groups treated with MF:MA solution likely had more mixed mode of failure than the groups treated with BA because the treatment with MF:MA at all ratios produced more porous surfaces (in both shape and size) than those treated with BA. This could result in better interlocking between the two materials. When used in denture reline as a surface treatment, a methyl formate and methyl acetate solution acts by swelling and dissolving the denture surface, and then evaporating. Thus, there is no residual solution on the denture surface before the reline material is applied. If there were to be residual solution at the denture surface, it would obstruct the interlocking of the reline resin polymer chains and the denture base, and the shear bond strength would be reduced. In addition, there are no carbon–carbon double bonds (C=C) in methyl formate or methyl acetate molecules to polymerise with the monomer in self-polymerised reline denture materials. According to the National Fire Protection Association (NFPA) 704 code: Standard system for the identification of the hazards of materials for emergency response40, the health hazard level of methyl methacrylate41 and methyl formate42 are the same (level 2), but greater than that of methyl acetate43 (level 1). Level 1 means ‘exposure would cause irritation with only minor residual injury’ and level 2 means ‘intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury’. Thus, the hazard of methyl formate and methyl acetate are equal to or less than that of methyl methacrylate (the liquid component of self-polymerised acrylic resin materials). In addition, the workplace airborne exposure limits of the American Conference of Governmental Industrial Hygienists

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for 8 h to MMA, MF, and MA are 50, 100, and 200 ppm, respectively44–46. This indicates that MF and MA are not only good solvents, but are also safer than MMA.

adhesive agents directions.

Conclusion

This study was supported by the Dental Research Fund 5302, Faculty of Dentistry, Chulalongkorn University and the Developing Research Unit in Dental Polymeric Materials in Prosthodontics. We also thank Dr. Kevin Tompkins, Faculty of Dentistry, Chulalongkorn University for his critical review of this manuscript.

Surface treatment with a MF:MA solution at a ratio of 25:75 significantly enhances the SBS between denture base resin and Unifast Tradâ, or Ufi gel hardâ compared with the SBS of the groups treated with the recommended

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Correspondence to: Chairat Wiwatwarrapan, Faculty of Dentistry, Department of Prosthodontic, Chulalongkorn, University, Henry Dunant Road, Pathumwan, Bangkok 10330, Thailand. Tel.: +66 81 5717031 Fax: +662 2188532 E-mail: [email protected]

© 2014 John Wiley & Sons A/S and The Gerodontology Association. Published by John Wiley & Sons Ltd, Gerodontology 2016; 33: 147–154

Surface treatment with methyl formate-methyl acetate increased the shear bond strength between reline resins and denture base resin.

Chemical surface treatment increases the shear bond strength (SBS) between hard reline resins (HRRs) and denture base resin...
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