Ultrasonics xxx (2015) xxx–xxx
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The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin Taksid Charasseangpaisarn a, Chairat Wiwatwarrapan a,b,⇑ a b
Department of Prosthodontics, Faculty of Dentistry, Chulalongkorn University, Bangkok 10330, Thailand Developing Research Unit in Dental Polymeric Materials in Prosthodontics, Faculty of Dentistry, Chulalongkorn University, Bangkok, Thailand
a r t i c l e
i n f o
Article history: Received 9 March 2015 Received in revised form 25 June 2015 Accepted 7 July 2015 Available online xxxx Keywords: Residual monomer Acrylic resin Frequency Ultrasonic High-performance liquid chromatography
a b s t r a c t Monomer remaining in denture base acrylic can be a major problem because it may cause adverse effects on oral tissue and on the properties of the material. The purpose of this study was to compare the effect of various ultrasonic cleaner frequencies on the amount of residual monomer in acrylic resin after curing. Forty-two specimens each of Meliodent heat-polymerized acrylic resin (M) and Unifast Trad Ivory auto-polymerized acrylic resin (U) were prepared according to their manufacturer’s instructions and randomly divided into seven groups: Negative control (NC); Positive control (PC); and five ultrasonic treatment groups: 28 kHz (F1), 40 kHz (F2), 60 kHz (F3) (M = 10 min, U = 5 min), and 28 kHz followed by 60 kHz (F4: M = 5 min per frequency, U = 2.5 min per frequency, and F5: M = 10 min followed by 5 min per frequency, U = 5 min followed by 2.5 min per frequency). Residual monomer was determined by HPLC following ISO 20795-1. The data were analyzed by One-way ANOVA and Tukey HSD. There was significantly less residual monomer in the auto-polymerized acrylic resin in all ultrasonic treatment groups and the PC group than that of the NC group (p < 0.05). However, the amount of residual monomer in group F3 was significantly higher than that of the F1, F4, and PC groups (p < 0.05). In contrast, ultrasonic treatment did not reduce the amount of residual monomer in heat-polymerized acrylic resin (p > 0.05). The amount of residual monomer in heat-polymerized acrylic resin was significantly lower than that of auto-polymerized acrylic resin. In conclusion, ultrasonic treatment at low frequencies is recommended to reduce the residual monomer in auto-polymerized acrylic resin and this method is more practical in a clinical situation than previously recommended methods because of reduced chairside time. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Acrylic resin is widely used in prosthodontics, as denture base material and for provisional crowns [1]. Denture base acrylic resin is used to support artificial teeth that replace missing teeth, and provisional crowns are used to provide immediate coverage of a prepared tooth to protect the pulp from thermal and chemical irritation, keep the tooth in position, maintain occlusal function, and provide esthetics before the definitive crown is delivered [2]. Denture base resin and provisional crowns are usually fabricated by the polymerization of pre-polymerized polymethyl methacrylate (PMMA) powder particles mixed with methyl methacrylate (MMA) monomer. When polymerization has occurred, the monomer remaining in the acrylic resin is known as residual monomer. ⇑ Corresponding author at: Department of Prosthodontics, Faculty of Dentistry, Chulalongkorn University, 34 Henri-Dunant Rd, Patumwan, Bangkok 10330, Thailand. Tel./fax: +66 2218 8532. E-mail address: [email protected] (C. Wiwatwarrapan).
The residual monomer content of denture acrylic is highest in the first 24 h after polymerization and decreases over time [3,4]. However, residual monomer can be detected in denture base polymer even after the denture has been worn for 5–20 years [5,6]. Many studies have reported that residual monomer acts as plasticizer, which affects the physical properties of acrylic resin (e.g., decreasing the impact strength and causing color changes) [7–9]. Moreover, residual monomer has been reported to be toxic and can irritate the oral mucosa and cause tissue sensitivity [10–13]. For these reasons, the residual monomer in acrylic resin should be minimized as much as possible. Many studies have demonstrated methods of reducing the residual monomer in acrylic resin that can be eluted into the environment using high watt microwave-polymerization, mechanical polishing, immersion in 55 °C water for 1 h or room temperature water for 24 h, coating the surface with resin, or curing under higher temperature and for a longer time [7,9,10,14–17]. However, the commonly used methods of immersion in room temperature water for heat-polymerized acrylic resin, performed during
http://dx.doi.org/10.1016/j.ultras.2015.07.005 0041-624X/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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T. Charasseangpaisarn, C. Wiwatwarrapan / Ultrasonics xxx (2015) xxx–xxx
laboratory processing, and immersion in 55 °C water for auto-polymerized acrylic resin, done chairside, increase the amount of time before the prosthesis can be delivered to the patient. Ultrasonic waves have been applied in many industries. In dentistry, ultrasonic waves are usually used for scaling and for instrument cleaning. The generator of an ultrasonic cleaner sends high frequency sound waves through an ultrasonic cleaning solution, resulting in the formation of numerous gas bubbles. When these gas bubbles implode, resulting in cavitation, they release a large amount of impact energy that rapidly increases the local temperature and produces a high-energy liquid stream that collides with the surface of the object being cleaned [18]. The operating frequency of an ultrasonic transducer has an effect on the amount of bubbles and their implosion. Lower frequencies generate fewer bubbles that are larger and release more energy. In contrast, higher frequencies generate more bubbles that are smaller and less release energy. A higher frequency may have less cleaning ability but generate greater fluid movement. In industrial applications, a single-frequency ultrasonic cleaner usually uses a 40 kHz ultrasonic transducer. In many industries, ultrasonic waves are used to enhance the extraction rate of chemical substances from food and bacteria [19–21]. However, the effect of the frequencies used in dental ultrasonic cleaners to enhance the elution of residual monomer from acrylic resin has not been reported. The purpose of this study was to determine the effect of various ultrasonic frequencies on the amount of residual monomer eluted from heat-polymerized and auto-polymerized acrylic resin. 2. Materials and methods 2.1. Sample preparation Forty-two disc shaped specimens each of heat-polymerized acrylic resin (Meliodent, Heraeus Kulzer, Sandan, Germany) and auto-polymerized acrylic resin (Unifast Trad Ivory, GC Corp., Tokyo, Japan) were prepared by mixing the powder and liquid according to the manufacturer’s instructions (Meliodent, 2.2 g
powder (Lot No. 33May105) to 1 mL liquid (Lot No. 140411); Unifast, 2.0 g powder (Lot No. 1309122) to 1 mL liquid (Lot No. 1202011). At the dough stage, the resin was packed into circular stainless steel molds (50 mm diameter (3.0 ± 0.1) mm deep), and the molds were placed in dental stone in dental flasks (Internal diameter 10 ± 0.1 cm). The two parts of the flask were pressed in a hydraulic press at 300 kPa. Heat-polymerized acrylic resin was pressed for 1 h at 25 °C and 9 h at 73.9 °C; autopolymerized acrylic resin was pressed for 3 min at 25 °C. After processing, the specimens were kept in the dark for 24 ± 5 h. Both sides of the specimens were wet-ground to a thickness of 2.0 ± 0.1 mm with P500 metallographic grinding paper (TOA, Thailand), the edge was polished with P1200 paper until smooth, and stored at 28 °C until used. The specimens of each material were divided into seven groups (n = 6) as shown in Table 1. The specimens were stored in the dark for 24 ± 1 h prior to the monomer extraction procedure. 2.2. Residual monomer extraction procedure following ISO 20795-1 (2013) Each specimen disc was first broken into small pieces. A digital scale (Sartorius BP110s, Sartorius, Germany) was used to weigh approximately 650 mg of broken disc pieces to four decimal places that were added to a 10 mL volumetric flask (Duran, Germany) for each sample solution. The broken pieces from each specimen were distributed into three sample solutions for the pass/fail determination test for residual monomer following ISO 20795-1 (2013). Tetrahydrofuran diluting solution (Merck KGaA, Darmstadt., Germany) was added to a 10 mL final volume. Each flask was stirred using a clean 3-mm polytetrafluoroethylene-coated magnetic stirring bar (Cowie Technology, Middlesbrough, UK) on a magnetic stirrer (PMC 509C, Barnstead, USA) for 72 ± 2 h at room temperature. Two mL of the resultant slurry was transferred to another 10 mL volumetric flask with a micropipette. Methanol diluting solution (M, Bangkok, Thailand) was added to a 10 mL final volume and the solution shaken to precipitate the resin. Five mL of the solution from each flask was transferred to glass centrifugation tubes, centrifuged at 3000 rpm for 15 min at 25 °C (Avanti J-E,
Table 1 Groups of experiment and mean amount of residual monomer (%mg ± standard deviation). Groups*
Materials
Treatment
Residual Monomer Mean ± SD
Water Temperature (°C)
Ultrasonic frequency (kHz)
Time
– – 28 40 60 28 Followed 28 Followed – – 28 40 60 28 Followed 28 Followed
– 24 h 10 min 10 min 10 min 5 min 5 min 10 min 5 min – 1h 5 min 5 min 5 min 2.5 min 2.5 min 5 min 2.5 min
MNC MPC MF1 MF2 MF3 MF4
Meliodent Meliodent Meliodent Meliodent Meliodent Meliodent
– Room 50 50 50 50
MF5
Meliodent
50
UNC UPC UF1 UF2 UF3 UF4
Unifast Unifast Unifast Unifast Unifast Unifast
Trad Trad Trad Trad Trad Trad
– 50 50 50 50 50
UF5
Unifast Trad
50
by 60 by 60
by 60 by 60
1.20 ± 0.16 a 1.21 ± 0.08 a 1.16 ± 0.05 a 1.20 ± 0.10 a 1.25 ± 0.06 a 1.24 ± 0.12 a 1.23 ± 0.15 a 3.27 ± 0.09 C 2.03 ± 0.14 A 2.01 ± 0.08 A 2.11 ± 0.10 A,B 2.28 ± 0.08 B 2.07 ± 0.18 A 2.17 ± 0.08 A,B
The groups with identical letters were not significantly different (capital and small letters represent separate analyses). * MNC = Meliodent Negative Control; MPC = Meliodent Positive Control; MF1 = Meliodent, F1 ultrasonic treatments; MF2 = Meliodent, F2 ultrasonic treatments, MF3 = Meliodent, F3 ultrasonic treatments, MF4 = Meliodent, F4 ultrasonic treatments; MF5 = Meliodent, F5 ultrasonic treatments; UNC = Unifast Trad Negative Control; UPC = Unifast Trad Positive Control; UF1 = Unifast Trad, F1 ultrasonic treatments; UF2 = Unifast Trad, F2 ultrasonic treatments; UF3 = Unifast Trad, F3 ultrasonic treatments; UF4 = Unifast Trad, F4 ultrasonic treatments; UF5 = Unifast Trad, F5 ultrasonic treatments.
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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Beckman Coulter, USA). One lL of the supernatant of each sample was analyzed by a high performance liquid chromatography (HPLC) system (Shimadzu 20A Prominence HPLC, Shimadzu Corporation, Kyoto, Japan) using a reverse-phase LC-18 (5 lm particle diameter, 4.6 mm internal diameter 150 mm length) analytical column maintained at 40 °C with a 66% methanol and 34% water isocratic elution. The flow rate was 1.5 mL/min and the UV detection wavelength was 205 nm. 2.3. Residual monomer determination The amount of residual MMA was determined from a standard calibration curve (R2 > 0.99) that was prepared by plotting the peaks of known amounts of MMA (Approximately 6 mg, 60 mg, 150 mg, 300 mg, and 400 mg). The standard curve was used to determine the concentration in micrograms of MMA, CMMA, per milliliter of analyzed sample solution. The standard calibration curve was calculated from known concentrations of methyl methacrylate solution, which had the following equation (R2 > 0.995):
f ðxÞ ¼ ð1:33415 107 Þx þ 194085 where f(x) = absorbance area of MMA by UV detector and x = MMA concentration. The total quantity of MMA in the sample solution, mMMA, in micrograms, was calculated according to the following equation:
10 mMMA ¼ cMMA 10 2 where * was the tetrahydrofuran amount and ** was the methanol amount used for extraction. The residual monomer (%mg) was calculated using the following equation:
Residual monomerð%mgÞ ¼
Fig. 1. Representative MMA HPLC chromatogram: standard solutions (A), heatpolymerized acrylic resin (B) and auto-polymerized acrylic resin (C).
mMMA 100 cMMA
Each specimen that was divided into three solutions (nine solutions in total) was tested for pass/fail determination of residual monomer. The three solutions per sample were averaged to generate the representative value of each specimen. Therefore, six values were obtained for each experimental group. The data were analyzed using one-way analysis of variance (ANOVA) followed by the Tukey HSD test at the 95% confidence level. 3. Results Representative HPLC chromatograms of MMA standard and MMA in heat-polymerized acrylic resin, and auto-polymerized acrylic resin are given in Fig. 1. 3.1. Residual monomer in heat-polymerized acrylic resin The amount of residual monomer in the heat-polymerized acrylic resin samples ranged from 1.031–1.482 %mg. The mean amount of residual monomer (Table 1, Fig. 2) showed that there were no significant differences between ultrasonic treatment at different frequencies (MF1, MF2, MF3, MF4, or MF5), immersion in room temperature water (MPC), or the non-treatment group (MNC) (p > 0.05). 3.2. Residual monomer in auto-polymerized acrylic resin The amount of residual monomer in the auto-polymerized acrylic resin samples ranged from 1.835–3.400 %mg. The mean amount of residual monomer (Table 1, Fig. 2) in the ultrasonic
Fig. 2. The mean amount and standard deviation of residual monomer in heatpolymerized acrylic resin (A) and auto-polymerized acrylic resin (B).
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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treatment groups and the positive control group (UPC; immersion in 50 °C water for 1 h) was significantly less than that of the negative control group (p < 0.05). The amount of residual monomer in the UF1, UF2, UF4, and UF5 groups was not significantly different from the positive control (UPC) group (p > 0.05). In contrast, the amount of residual monomer in the UF3 group was significantly higher than that of the UPC, UF1, and UF4 groups (p < 0.05).
4. Discussion In the present study, the length of time of the ultrasonic treatment in the heat-polymerized acrylic resin (MF1–MF3) and auto-polymerized acrylic resin (UF1–UF3) groups were different because previous studies investigating the amount of residual monomer in acrylic resin after ultrasonic treatment used different treatment times for the different material types [22,23]. These studies recommended 10 min of ultrasonic treatment for heat-polymerized acrylic resin and 5 min of ultrasonic treatment for auto-polymerized acrylic resin. In our study, the effect of ultrasonic frequency was determined for two different ultrasonic treatment frequencies that were used for the same length of time within each material group (MF4 and UF4). Furthermore, the effect of low ultrasonic frequency (28 kHz) was observed by increasing the timing of low frequency ultrasonic treatment within each material group (MF5 and UF5).
4.1. Pass/fail determination of residual monomer Previous investigations into the amount of residual monomer focused on the elution of monomer from acrylic resin into the environment, i.e. saliva or water, and how much monomer remained in the acrylic resin [9,24–26]. However, in the present study, we analyzed the amount of remaining residual monomer in acrylic resin following the ISO method for the determination of residual monomer. The three specimens that were divided into nine solutions were tested for pass/fail determination of residual monomer, which is defined in ISO 20795-1. The maximum allowable residual monomer is 2.2 mg% and 4.5 mg% for heat-polymerized and auto-polymerized acrylic resin, respectively. Our results indicated that all groups passed this requirement [27].
4.2. Residual monomer in heat-polymerized acrylic resin Our results are in agreement with the study of Vallittu et al. [25] who showed that the amount of residual monomer in heatpolymerized acrylic resin after immersion in room temperature water for 24 h was not significantly different from the nonimmersion group, and there was no significant difference unless the immersion time was longer than 1 month. However, Vallittu et al. suggested immersing heat-polymerized acrylic resin in room temperature water for 24 h, because the amount of residual monomer that eluted from acrylic resin into the water was highest on the first day and decreased over time. Our finding that ultrasonic treatment did not affect the residual monomer remaining in heat-polymerized acrylic resin may be explained by the nonextractable monomer theory, as described by Smith and Bains, in which the residual monomer remains trapped in long polymer molecules and remains in acrylic resin after various treatments [28]. In addition, Grassie [29] found that small solvent molecules may be trapped in the interior of poly-methyl methacrylate chains and are released only under extreme conditions.
4.3. Residual monomer in auto-polymerized acrylic resin The results of our study are in agreement with the recommendation of Tsuchiya [30], who suggested that immersing acrylic resin dentures in 50 °C water before insertion, especially for auto-polymerized acrylic resin, could significantly decrease the amount of residual monomer, which can minimize the risk of adverse reactions in patients. We found that higher ultrasonic frequency was associated with more residual monomer compared with lower frequency. This may be because low frequency causes high impact forces that extracted the residual monomer more than high frequency was able to. The study of Albu et al. [19] and Romdhanea et al. [31] showed there were no significant differences when using ultrasonic treatment at 20 and 40 kHz to assist extraction from food components, however, 20 kHz ultrasonic resulted in greater extraction compared with 40 kHz. González-Centenoa et al. [32] compared the effect of ultrasonic frequencies of 40, 80, and 120 kHz for extraction of phenols, flavonols, and antioxidants from grapes. They found that 40 kHz ultrasonic frequency was the best condition to obtain the highest rate of extraction. In our study, the UF4 group received the same total treatment time as that of the UF1 and UF3 groups to compare the effect of different frequencies; demonstrating that low frequency (28 kHz) had a greater effect in reducing the residual monomer than that of high frequency (60 kHz). The UF5 group was treated with the same frequencies as the UF4 group, however, the treatment time at low frequency (28 kHz) was increased to observe the effect of treatment time on the amount of residual monomer. Our results indicated that when treatment time was increased the amount of residual monomer also increased. We found that showed that 40 kHz ultrasonic frequency was the best condition to obtain the highest rate of extraction. Therefore, the immersion of auto-polymerized acrylic resin in 50 °C water using 28 kHz, 40 kHz for 5 min, 28 kHz for 2.5 min followed by 60 kHz for 2.5 min, or 28 kHz for 5 min followed by 60 kHz for 2.5 min are preferred because of reduced chairside time. Patil et al. [33] recommended using microwave treatment at 650 W for 5 min as a post-polymerization treatment for acrylic resin. This treatment uses the same chairside time as ultrasonic methods, but may affect the dimensional stability of the acrylic resin. Chia et al. [34] demonstrated that during polymerization using a microwave at 500 W, the temperature of the acrylic resin rose to approximately 100 °C. In addition, Ghassan [35] demonstrated a negative correlation between temperature and dimensional stability. Moreover, Wagner [36] found that post-polymerization treatment of acrylic resin with microwave curing at either 420 or 700 W for 3 min caused acrylic resin deformation. However, the dimensional stability of acrylic resin after ultrasonic treatment has not been investigated. Ultrasonic treatment may affect the amount of residual monomer in two ways. First, enhanced residual monomer reduction has been attributed to the propagation of ultrasound pressure and cavitation. The implosion of cavitation bubbles generates macro-turbulence, high-velocity inter-particle collisions, and perturbation in acrylic resin micro-pores, which accelerates the eddy diffusion and internal diffusion of monomer. In addition, the imploding bubbles release energy at the surface of the acrylic and may cause polymerization of the remaining monomer or de-polymerization of polymer chains. However, in the present study we did not determine how ultrasonic treatment affects the residual monomer. Future studies should focus on the elution of residual monomer into the environment coupled with the comparison of the degree of conversion of MMA before and after ultrasonic treatment. The surface of the acrylic resin should also be investigated, because the high energy of the implosive bubbles may damage the surface, increase its roughness, which is undesirable.
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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5. Conclusion In conclusion, the amount of residual MMA monomer in acrylic resin can be significantly minimized by the following treatments: 5.1. Heat-polymerized acrylic resin Immersion in 50 °C ultrasonicated water at either 28 kHz, 40 kHz or 60 kHz, or 28 kHz followed by 60 kHz for 5 min, or 28 kHz followed by 60 kHz for 7.5 min. 5.2. Auto-polymerized acrylic resin Immersion in 50 °C ultrasonicated water at either 28 kHz, 40 kHz, or 28 kHz followed by 60 kHz for 5 min, or 28 kHz followed by 60 kHz for 7.5 min. 5.3. Heat-polymerized versus auto-polymerized acrylic resin The amount of residual monomer in heat-polymerized acrylic resin was significantly lower than that of auto-polymerized acrylic resin (p < 0.05) in all experimental groups. Acknowledgements This study was supported by the Faculty of Dentistry, Chulalongkorn University Research Fund and Developing Research Unit in Dental Polymeric Materials in Prosthodontics. We also thank Professor Martin Tyas, Melbourne Dental School, University of Melbourne, and Dr. Kevin Tompkins, Faculty of Dentistry, Chulalongkorn University for their critical reviews of this manuscript. References [1] J.M. Powers, R.L. Skaguchi, Restorative Dental Materials, twelfth ed., CV Mosby, St.Louis, 2006. [2] H.T.J. Shillingburg, S. Hobo, L.D. Whitsett, R. Jacobi, S.E. Brackett, Fundamentals of Fixed Prosthodontics, third ed., Quintessence Publishing Co Inc, Illinois, 1997. [3] D.J. Lamb, B. Ellis, D. Priestley, Loss into water of residual monomer from autopolymerizing dental acrylic resin, Biomaterials 3 (1982) 155–159. [4] G.D. Stafford, S.C. Brooks, The loss of residual monomer from acrylic orthodontic resins, Dent. Mater. 1 (1985) 135–138. [5] W.H. Douglas, J.F. Bates, The determination of residual monomer in polymethylmethacryalte denture-base resins, J. Mater. Sci. 13 (1978) 2600– 2604. [6] S. Sadamori, H. Kotani, T. Hamada, The usage period of dentures and their residual monomer contents, J. Prosthet. Dent. 68 (1992) 374–376. [7] M.J. Azzarri, M.S. Cortizo, J.L. Alessandrini, Effect of the curing conditions on the properties of an acrylic denture base resin microwave-polymerised, J. Dent. 31 (2003) 463–468. [8] J. Arab, J.P. Newton, C.H. Lloyd, The effect of an elevated level of residual monomer on the whitening of a denture base and its physical properties, J. Dent. 17 (1989) 189–194. [9] S. Lee, Y. Lai, T. Hsu, Influence of polymerization conditions on monomer elution and microhardness of autopolymerized polymethyl methacrylate resin, Eur. J. Oral. Sci. 110 (2002) 179–183. [10] A.A. Fisher, Allergic sensitization of the skin and oral mucosa to acrylic resin denture materials, J. Prosthet. Dent. 6 (1956) 593–602. [11] A. Ali, J.F. Bates, A.J. Reynolds, D.M. Walker, The burning mouth sensation related to the wearing of acrylic dentures: an investigation, Br. Dent. J. 161 (1986) 444–447.
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Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin Taksid Charasseangpaisarn a, Chairat Wiwatwarrapan a,b,⇑ a b
Department of Prosthodontics, Faculty of Dentistry, Chulalongkorn University, Bangkok 10330, Thailand Developing Research Unit in Dental Polymeric Materials in Prosthodontics, Faculty of Dentistry, Chulalongkorn University, Bangkok, Thailand
a r t i c l e
i n f o
Article history: Received 9 March 2015 Received in revised form 25 June 2015 Accepted 7 July 2015 Available online xxxx Keywords: Residual monomer Acrylic resin Frequency Ultrasonic High-performance liquid chromatography
a b s t r a c t Monomer remaining in denture base acrylic can be a major problem because it may cause adverse effects on oral tissue and on the properties of the material. The purpose of this study was to compare the effect of various ultrasonic cleaner frequencies on the amount of residual monomer in acrylic resin after curing. Forty-two specimens each of Meliodent heat-polymerized acrylic resin (M) and Unifast Trad Ivory auto-polymerized acrylic resin (U) were prepared according to their manufacturer’s instructions and randomly divided into seven groups: Negative control (NC); Positive control (PC); and five ultrasonic treatment groups: 28 kHz (F1), 40 kHz (F2), 60 kHz (F3) (M = 10 min, U = 5 min), and 28 kHz followed by 60 kHz (F4: M = 5 min per frequency, U = 2.5 min per frequency, and F5: M = 10 min followed by 5 min per frequency, U = 5 min followed by 2.5 min per frequency). Residual monomer was determined by HPLC following ISO 20795-1. The data were analyzed by One-way ANOVA and Tukey HSD. There was significantly less residual monomer in the auto-polymerized acrylic resin in all ultrasonic treatment groups and the PC group than that of the NC group (p < 0.05). However, the amount of residual monomer in group F3 was significantly higher than that of the F1, F4, and PC groups (p < 0.05). In contrast, ultrasonic treatment did not reduce the amount of residual monomer in heat-polymerized acrylic resin (p > 0.05). The amount of residual monomer in heat-polymerized acrylic resin was significantly lower than that of auto-polymerized acrylic resin. In conclusion, ultrasonic treatment at low frequencies is recommended to reduce the residual monomer in auto-polymerized acrylic resin and this method is more practical in a clinical situation than previously recommended methods because of reduced chairside time. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Acrylic resin is widely used in prosthodontics, as denture base material and for provisional crowns [1]. Denture base acrylic resin is used to support artificial teeth that replace missing teeth, and provisional crowns are used to provide immediate coverage of a prepared tooth to protect the pulp from thermal and chemical irritation, keep the tooth in position, maintain occlusal function, and provide esthetics before the definitive crown is delivered [2]. Denture base resin and provisional crowns are usually fabricated by the polymerization of pre-polymerized polymethyl methacrylate (PMMA) powder particles mixed with methyl methacrylate (MMA) monomer. When polymerization has occurred, the monomer remaining in the acrylic resin is known as residual monomer. ⇑ Corresponding author at: Department of Prosthodontics, Faculty of Dentistry, Chulalongkorn University, 34 Henri-Dunant Rd, Patumwan, Bangkok 10330, Thailand. Tel./fax: +66 2218 8532. E-mail address: [email protected] (C. Wiwatwarrapan).
The residual monomer content of denture acrylic is highest in the first 24 h after polymerization and decreases over time [3,4]. However, residual monomer can be detected in denture base polymer even after the denture has been worn for 5–20 years [5,6]. Many studies have reported that residual monomer acts as plasticizer, which affects the physical properties of acrylic resin (e.g., decreasing the impact strength and causing color changes) [7–9]. Moreover, residual monomer has been reported to be toxic and can irritate the oral mucosa and cause tissue sensitivity [10–13]. For these reasons, the residual monomer in acrylic resin should be minimized as much as possible. Many studies have demonstrated methods of reducing the residual monomer in acrylic resin that can be eluted into the environment using high watt microwave-polymerization, mechanical polishing, immersion in 55 °C water for 1 h or room temperature water for 24 h, coating the surface with resin, or curing under higher temperature and for a longer time [7,9,10,14–17]. However, the commonly used methods of immersion in room temperature water for heat-polymerized acrylic resin, performed during
http://dx.doi.org/10.1016/j.ultras.2015.07.005 0041-624X/Ó 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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laboratory processing, and immersion in 55 °C water for auto-polymerized acrylic resin, done chairside, increase the amount of time before the prosthesis can be delivered to the patient. Ultrasonic waves have been applied in many industries. In dentistry, ultrasonic waves are usually used for scaling and for instrument cleaning. The generator of an ultrasonic cleaner sends high frequency sound waves through an ultrasonic cleaning solution, resulting in the formation of numerous gas bubbles. When these gas bubbles implode, resulting in cavitation, they release a large amount of impact energy that rapidly increases the local temperature and produces a high-energy liquid stream that collides with the surface of the object being cleaned [18]. The operating frequency of an ultrasonic transducer has an effect on the amount of bubbles and their implosion. Lower frequencies generate fewer bubbles that are larger and release more energy. In contrast, higher frequencies generate more bubbles that are smaller and less release energy. A higher frequency may have less cleaning ability but generate greater fluid movement. In industrial applications, a single-frequency ultrasonic cleaner usually uses a 40 kHz ultrasonic transducer. In many industries, ultrasonic waves are used to enhance the extraction rate of chemical substances from food and bacteria [19–21]. However, the effect of the frequencies used in dental ultrasonic cleaners to enhance the elution of residual monomer from acrylic resin has not been reported. The purpose of this study was to determine the effect of various ultrasonic frequencies on the amount of residual monomer eluted from heat-polymerized and auto-polymerized acrylic resin. 2. Materials and methods 2.1. Sample preparation Forty-two disc shaped specimens each of heat-polymerized acrylic resin (Meliodent, Heraeus Kulzer, Sandan, Germany) and auto-polymerized acrylic resin (Unifast Trad Ivory, GC Corp., Tokyo, Japan) were prepared by mixing the powder and liquid according to the manufacturer’s instructions (Meliodent, 2.2 g
powder (Lot No. 33May105) to 1 mL liquid (Lot No. 140411); Unifast, 2.0 g powder (Lot No. 1309122) to 1 mL liquid (Lot No. 1202011). At the dough stage, the resin was packed into circular stainless steel molds (50 mm diameter (3.0 ± 0.1) mm deep), and the molds were placed in dental stone in dental flasks (Internal diameter 10 ± 0.1 cm). The two parts of the flask were pressed in a hydraulic press at 300 kPa. Heat-polymerized acrylic resin was pressed for 1 h at 25 °C and 9 h at 73.9 °C; autopolymerized acrylic resin was pressed for 3 min at 25 °C. After processing, the specimens were kept in the dark for 24 ± 5 h. Both sides of the specimens were wet-ground to a thickness of 2.0 ± 0.1 mm with P500 metallographic grinding paper (TOA, Thailand), the edge was polished with P1200 paper until smooth, and stored at 28 °C until used. The specimens of each material were divided into seven groups (n = 6) as shown in Table 1. The specimens were stored in the dark for 24 ± 1 h prior to the monomer extraction procedure. 2.2. Residual monomer extraction procedure following ISO 20795-1 (2013) Each specimen disc was first broken into small pieces. A digital scale (Sartorius BP110s, Sartorius, Germany) was used to weigh approximately 650 mg of broken disc pieces to four decimal places that were added to a 10 mL volumetric flask (Duran, Germany) for each sample solution. The broken pieces from each specimen were distributed into three sample solutions for the pass/fail determination test for residual monomer following ISO 20795-1 (2013). Tetrahydrofuran diluting solution (Merck KGaA, Darmstadt., Germany) was added to a 10 mL final volume. Each flask was stirred using a clean 3-mm polytetrafluoroethylene-coated magnetic stirring bar (Cowie Technology, Middlesbrough, UK) on a magnetic stirrer (PMC 509C, Barnstead, USA) for 72 ± 2 h at room temperature. Two mL of the resultant slurry was transferred to another 10 mL volumetric flask with a micropipette. Methanol diluting solution (M, Bangkok, Thailand) was added to a 10 mL final volume and the solution shaken to precipitate the resin. Five mL of the solution from each flask was transferred to glass centrifugation tubes, centrifuged at 3000 rpm for 15 min at 25 °C (Avanti J-E,
Table 1 Groups of experiment and mean amount of residual monomer (%mg ± standard deviation). Groups*
Materials
Treatment
Residual Monomer Mean ± SD
Water Temperature (°C)
Ultrasonic frequency (kHz)
Time
– – 28 40 60 28 Followed 28 Followed – – 28 40 60 28 Followed 28 Followed
– 24 h 10 min 10 min 10 min 5 min 5 min 10 min 5 min – 1h 5 min 5 min 5 min 2.5 min 2.5 min 5 min 2.5 min
MNC MPC MF1 MF2 MF3 MF4
Meliodent Meliodent Meliodent Meliodent Meliodent Meliodent
– Room 50 50 50 50
MF5
Meliodent
50
UNC UPC UF1 UF2 UF3 UF4
Unifast Unifast Unifast Unifast Unifast Unifast
Trad Trad Trad Trad Trad Trad
– 50 50 50 50 50
UF5
Unifast Trad
50
by 60 by 60
by 60 by 60
1.20 ± 0.16 a 1.21 ± 0.08 a 1.16 ± 0.05 a 1.20 ± 0.10 a 1.25 ± 0.06 a 1.24 ± 0.12 a 1.23 ± 0.15 a 3.27 ± 0.09 C 2.03 ± 0.14 A 2.01 ± 0.08 A 2.11 ± 0.10 A,B 2.28 ± 0.08 B 2.07 ± 0.18 A 2.17 ± 0.08 A,B
The groups with identical letters were not significantly different (capital and small letters represent separate analyses). * MNC = Meliodent Negative Control; MPC = Meliodent Positive Control; MF1 = Meliodent, F1 ultrasonic treatments; MF2 = Meliodent, F2 ultrasonic treatments, MF3 = Meliodent, F3 ultrasonic treatments, MF4 = Meliodent, F4 ultrasonic treatments; MF5 = Meliodent, F5 ultrasonic treatments; UNC = Unifast Trad Negative Control; UPC = Unifast Trad Positive Control; UF1 = Unifast Trad, F1 ultrasonic treatments; UF2 = Unifast Trad, F2 ultrasonic treatments; UF3 = Unifast Trad, F3 ultrasonic treatments; UF4 = Unifast Trad, F4 ultrasonic treatments; UF5 = Unifast Trad, F5 ultrasonic treatments.
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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Beckman Coulter, USA). One lL of the supernatant of each sample was analyzed by a high performance liquid chromatography (HPLC) system (Shimadzu 20A Prominence HPLC, Shimadzu Corporation, Kyoto, Japan) using a reverse-phase LC-18 (5 lm particle diameter, 4.6 mm internal diameter 150 mm length) analytical column maintained at 40 °C with a 66% methanol and 34% water isocratic elution. The flow rate was 1.5 mL/min and the UV detection wavelength was 205 nm. 2.3. Residual monomer determination The amount of residual MMA was determined from a standard calibration curve (R2 > 0.99) that was prepared by plotting the peaks of known amounts of MMA (Approximately 6 mg, 60 mg, 150 mg, 300 mg, and 400 mg). The standard curve was used to determine the concentration in micrograms of MMA, CMMA, per milliliter of analyzed sample solution. The standard calibration curve was calculated from known concentrations of methyl methacrylate solution, which had the following equation (R2 > 0.995):
f ðxÞ ¼ ð1:33415 107 Þx þ 194085 where f(x) = absorbance area of MMA by UV detector and x = MMA concentration. The total quantity of MMA in the sample solution, mMMA, in micrograms, was calculated according to the following equation:
10 mMMA ¼ cMMA 10 2 where * was the tetrahydrofuran amount and ** was the methanol amount used for extraction. The residual monomer (%mg) was calculated using the following equation:
Residual monomerð%mgÞ ¼
Fig. 1. Representative MMA HPLC chromatogram: standard solutions (A), heatpolymerized acrylic resin (B) and auto-polymerized acrylic resin (C).
mMMA 100 cMMA
Each specimen that was divided into three solutions (nine solutions in total) was tested for pass/fail determination of residual monomer. The three solutions per sample were averaged to generate the representative value of each specimen. Therefore, six values were obtained for each experimental group. The data were analyzed using one-way analysis of variance (ANOVA) followed by the Tukey HSD test at the 95% confidence level. 3. Results Representative HPLC chromatograms of MMA standard and MMA in heat-polymerized acrylic resin, and auto-polymerized acrylic resin are given in Fig. 1. 3.1. Residual monomer in heat-polymerized acrylic resin The amount of residual monomer in the heat-polymerized acrylic resin samples ranged from 1.031–1.482 %mg. The mean amount of residual monomer (Table 1, Fig. 2) showed that there were no significant differences between ultrasonic treatment at different frequencies (MF1, MF2, MF3, MF4, or MF5), immersion in room temperature water (MPC), or the non-treatment group (MNC) (p > 0.05). 3.2. Residual monomer in auto-polymerized acrylic resin The amount of residual monomer in the auto-polymerized acrylic resin samples ranged from 1.835–3.400 %mg. The mean amount of residual monomer (Table 1, Fig. 2) in the ultrasonic
Fig. 2. The mean amount and standard deviation of residual monomer in heatpolymerized acrylic resin (A) and auto-polymerized acrylic resin (B).
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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treatment groups and the positive control group (UPC; immersion in 50 °C water for 1 h) was significantly less than that of the negative control group (p < 0.05). The amount of residual monomer in the UF1, UF2, UF4, and UF5 groups was not significantly different from the positive control (UPC) group (p > 0.05). In contrast, the amount of residual monomer in the UF3 group was significantly higher than that of the UPC, UF1, and UF4 groups (p < 0.05).
4. Discussion In the present study, the length of time of the ultrasonic treatment in the heat-polymerized acrylic resin (MF1–MF3) and auto-polymerized acrylic resin (UF1–UF3) groups were different because previous studies investigating the amount of residual monomer in acrylic resin after ultrasonic treatment used different treatment times for the different material types [22,23]. These studies recommended 10 min of ultrasonic treatment for heat-polymerized acrylic resin and 5 min of ultrasonic treatment for auto-polymerized acrylic resin. In our study, the effect of ultrasonic frequency was determined for two different ultrasonic treatment frequencies that were used for the same length of time within each material group (MF4 and UF4). Furthermore, the effect of low ultrasonic frequency (28 kHz) was observed by increasing the timing of low frequency ultrasonic treatment within each material group (MF5 and UF5).
4.1. Pass/fail determination of residual monomer Previous investigations into the amount of residual monomer focused on the elution of monomer from acrylic resin into the environment, i.e. saliva or water, and how much monomer remained in the acrylic resin [9,24–26]. However, in the present study, we analyzed the amount of remaining residual monomer in acrylic resin following the ISO method for the determination of residual monomer. The three specimens that were divided into nine solutions were tested for pass/fail determination of residual monomer, which is defined in ISO 20795-1. The maximum allowable residual monomer is 2.2 mg% and 4.5 mg% for heat-polymerized and auto-polymerized acrylic resin, respectively. Our results indicated that all groups passed this requirement [27].
4.2. Residual monomer in heat-polymerized acrylic resin Our results are in agreement with the study of Vallittu et al. [25] who showed that the amount of residual monomer in heatpolymerized acrylic resin after immersion in room temperature water for 24 h was not significantly different from the nonimmersion group, and there was no significant difference unless the immersion time was longer than 1 month. However, Vallittu et al. suggested immersing heat-polymerized acrylic resin in room temperature water for 24 h, because the amount of residual monomer that eluted from acrylic resin into the water was highest on the first day and decreased over time. Our finding that ultrasonic treatment did not affect the residual monomer remaining in heat-polymerized acrylic resin may be explained by the nonextractable monomer theory, as described by Smith and Bains, in which the residual monomer remains trapped in long polymer molecules and remains in acrylic resin after various treatments [28]. In addition, Grassie [29] found that small solvent molecules may be trapped in the interior of poly-methyl methacrylate chains and are released only under extreme conditions.
4.3. Residual monomer in auto-polymerized acrylic resin The results of our study are in agreement with the recommendation of Tsuchiya [30], who suggested that immersing acrylic resin dentures in 50 °C water before insertion, especially for auto-polymerized acrylic resin, could significantly decrease the amount of residual monomer, which can minimize the risk of adverse reactions in patients. We found that higher ultrasonic frequency was associated with more residual monomer compared with lower frequency. This may be because low frequency causes high impact forces that extracted the residual monomer more than high frequency was able to. The study of Albu et al. [19] and Romdhanea et al. [31] showed there were no significant differences when using ultrasonic treatment at 20 and 40 kHz to assist extraction from food components, however, 20 kHz ultrasonic resulted in greater extraction compared with 40 kHz. González-Centenoa et al. [32] compared the effect of ultrasonic frequencies of 40, 80, and 120 kHz for extraction of phenols, flavonols, and antioxidants from grapes. They found that 40 kHz ultrasonic frequency was the best condition to obtain the highest rate of extraction. In our study, the UF4 group received the same total treatment time as that of the UF1 and UF3 groups to compare the effect of different frequencies; demonstrating that low frequency (28 kHz) had a greater effect in reducing the residual monomer than that of high frequency (60 kHz). The UF5 group was treated with the same frequencies as the UF4 group, however, the treatment time at low frequency (28 kHz) was increased to observe the effect of treatment time on the amount of residual monomer. Our results indicated that when treatment time was increased the amount of residual monomer also increased. We found that showed that 40 kHz ultrasonic frequency was the best condition to obtain the highest rate of extraction. Therefore, the immersion of auto-polymerized acrylic resin in 50 °C water using 28 kHz, 40 kHz for 5 min, 28 kHz for 2.5 min followed by 60 kHz for 2.5 min, or 28 kHz for 5 min followed by 60 kHz for 2.5 min are preferred because of reduced chairside time. Patil et al. [33] recommended using microwave treatment at 650 W for 5 min as a post-polymerization treatment for acrylic resin. This treatment uses the same chairside time as ultrasonic methods, but may affect the dimensional stability of the acrylic resin. Chia et al. [34] demonstrated that during polymerization using a microwave at 500 W, the temperature of the acrylic resin rose to approximately 100 °C. In addition, Ghassan [35] demonstrated a negative correlation between temperature and dimensional stability. Moreover, Wagner [36] found that post-polymerization treatment of acrylic resin with microwave curing at either 420 or 700 W for 3 min caused acrylic resin deformation. However, the dimensional stability of acrylic resin after ultrasonic treatment has not been investigated. Ultrasonic treatment may affect the amount of residual monomer in two ways. First, enhanced residual monomer reduction has been attributed to the propagation of ultrasound pressure and cavitation. The implosion of cavitation bubbles generates macro-turbulence, high-velocity inter-particle collisions, and perturbation in acrylic resin micro-pores, which accelerates the eddy diffusion and internal diffusion of monomer. In addition, the imploding bubbles release energy at the surface of the acrylic and may cause polymerization of the remaining monomer or de-polymerization of polymer chains. However, in the present study we did not determine how ultrasonic treatment affects the residual monomer. Future studies should focus on the elution of residual monomer into the environment coupled with the comparison of the degree of conversion of MMA before and after ultrasonic treatment. The surface of the acrylic resin should also be investigated, because the high energy of the implosive bubbles may damage the surface, increase its roughness, which is undesirable.
Please cite this article in press as: T. Charasseangpaisarn, C. Wiwatwarrapan, The effect of various frequencies of ultrasonic cleaner in reducing residual monomer in acrylic resin, Ultrasonics (2015), http://dx.doi.org/10.1016/j.ultras.2015.07.005
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5. Conclusion In conclusion, the amount of residual MMA monomer in acrylic resin can be significantly minimized by the following treatments: 5.1. Heat-polymerized acrylic resin Immersion in 50 °C ultrasonicated water at either 28 kHz, 40 kHz or 60 kHz, or 28 kHz followed by 60 kHz for 5 min, or 28 kHz followed by 60 kHz for 7.5 min. 5.2. Auto-polymerized acrylic resin Immersion in 50 °C ultrasonicated water at either 28 kHz, 40 kHz, or 28 kHz followed by 60 kHz for 5 min, or 28 kHz followed by 60 kHz for 7.5 min. 5.3. Heat-polymerized versus auto-polymerized acrylic resin The amount of residual monomer in heat-polymerized acrylic resin was significantly lower than that of auto-polymerized acrylic resin (p < 0.05) in all experimental groups. Acknowledgements This study was supported by the Faculty of Dentistry, Chulalongkorn University Research Fund and Developing Research Unit in Dental Polymeric Materials in Prosthodontics. We also thank Professor Martin Tyas, Melbourne Dental School, University of Melbourne, and Dr. Kevin Tompkins, Faculty of Dentistry, Chulalongkorn University for their critical reviews of this manuscript. References [1] J.M. Powers, R.L. Skaguchi, Restorative Dental Materials, twelfth ed., CV Mosby, St.Louis, 2006. [2] H.T.J. Shillingburg, S. Hobo, L.D. Whitsett, R. Jacobi, S.E. Brackett, Fundamentals of Fixed Prosthodontics, third ed., Quintessence Publishing Co Inc, Illinois, 1997. [3] D.J. Lamb, B. Ellis, D. Priestley, Loss into water of residual monomer from autopolymerizing dental acrylic resin, Biomaterials 3 (1982) 155–159. [4] G.D. Stafford, S.C. Brooks, The loss of residual monomer from acrylic orthodontic resins, Dent. Mater. 1 (1985) 135–138. [5] W.H. Douglas, J.F. Bates, The determination of residual monomer in polymethylmethacryalte denture-base resins, J. Mater. Sci. 13 (1978) 2600– 2604. [6] S. Sadamori, H. Kotani, T. Hamada, The usage period of dentures and their residual monomer contents, J. Prosthet. Dent. 68 (1992) 374–376. [7] M.J. Azzarri, M.S. Cortizo, J.L. Alessandrini, Effect of the curing conditions on the properties of an acrylic denture base resin microwave-polymerised, J. Dent. 31 (2003) 463–468. [8] J. Arab, J.P. Newton, C.H. Lloyd, The effect of an elevated level of residual monomer on the whitening of a denture base and its physical properties, J. Dent. 17 (1989) 189–194. [9] S. Lee, Y. Lai, T. Hsu, Influence of polymerization conditions on monomer elution and microhardness of autopolymerized polymethyl methacrylate resin, Eur. J. Oral. Sci. 110 (2002) 179–183. [10] A.A. Fisher, Allergic sensitization of the skin and oral mucosa to acrylic resin denture materials, J. Prosthet. Dent. 6 (1956) 593–602. [11] A. Ali, J.F. Bates, A.J. Reynolds, D.M. Walker, The burning mouth sensation related to the wearing of acrylic dentures: an investigation, Br. Dent. J. 161 (1986) 444–447.
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