Author's Accepted Manuscript
Resistance to impact of cross-linked denture base biopolymer materials: Effect of relining, glass flakes reinforcement and cyclic loading Luciano Elias da Cruz Perez, Ana Lucia Machado, Carlos Eduardo Vergani, Camila Andrade Zamperini, Ana Cláudia Pavarina, Sebastião Vicente Canevarolo Jr www.elsevier.com/locate/jmbbm
PII: DOI: Reference:
S1751-6161(14)00134-9 http://dx.doi.org/10.1016/j.jmbbm.2014.05.009 JMBBM1151
To appear in: Journal of the Mechanical Behavior of Biomedical Materials
Received date:24 April 2014 Accepted date: 3 May 2014 Cite this article as: Luciano Elias da Cruz Perez, Ana Lucia Machado, Carlos Eduardo Vergani, Camila Andrade Zamperini, Ana Cláudia Pavarina, Sebastião Vicente Canevarolo Jr, Resistance to impact of cross-linked denture base biopolymer materials: Effect of relining, glass flakes reinforcement and cyclic loading, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/j.jmbbm.2014.05.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Resistance to impact of cross-linked denture base biopolymer materials: effect of relining, glass flakes reinforcement and cyclic loading.
Luciano Elias da Cruz Perez*, Ana Lucia Machado**, Carlos Eduardo Vergani**, Camila Andrade Zamperini****, Ana Cláudia Pavarina**, Sebastião Vicente Canevarolo Jr*** a
Associate Professor, Department of Prosthodontics, UNIP Universidade Paulista, Goiânia, Goiás,
Brazil. bFull Professor, Department of Dental Materials and Prosthodontics, Univ Estadual Paulista UNESP, Araraquara, São Paulo, Brazil. *** Associate Professor, Materials Engineering Department, Universidade Federal de São Carlos UFSCar, São Carlos, São Paulo, Brazil. **** Postdoctoral Student, Department of Dental Materials and Prosthodontics, Univ Estadual Paulista - UNESP, Araraquara, São Paulo, Brazil. Corresponding author: Dr. Ana Lucia Machado Faculdade de Odontologia de Araraquara – UNESP Rua Humaitá, 1680 – Araraquara - São Paulo - Brazil – CEP - 14801-903 Phone: #55#016#33016410 - Fax: #55#016#33016406 e-mail:
[email protected] [email protected] Abstract Aims: The effect of reinforcement and cyclic loading on the resistance to impact (RI) of denture base biopolymer materials was evaluated using Charpy (C) and falling-weight (FW) impact tests. Methods: bar-shaped (60x6x2 mm) and denture-shaped specimens (2 mm) for the C and FD tests, respectively, were prepared with Lucitone 550 (L) and Vipi Wave (V) and relined (2 mm) using the same material or the autopolymerizing relining resins Tokuyama Rebase II (T) and Ufi Gel Hard
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(U). Bulk specimens (60x6x4 mm) of all materials (L, V, T and U) were also prepared and tested. To evaluate the effect of reinforcement, glass flakes were added to the powder of the relining resins T and U (5% by weight). Half of bar-shaped (n=320) and half of the denture-shaped specimens (n=480) were subjected to cyclic loading (10,000 cycles) before the impact tests. Results: data were analyzed by one-way ANOVAs (α = 0.05) and revealed that the RI of L and V were comparable and higher than those of U and T. Compared to L and V, the RI was increased by relining with T and decreased by relining with U. When relining was made using the same material (L and V) the RI was maintained. Flexural cyclic loading and the incorporation of glass flakes into the resins T and U did not cause any significant alteration in the RI. A high correlation between results from C and FW tests was observed (r=0.8854). Conclusion: Relining may exert effects on the RI of L and V denture base resins, which vary according to the relining material used. The high correlation between C and FW, suggests that the Charpy test, using bar-shaped specimens, can be a simple and reliable method for evaluating factors that may influence the RI of denture base polymers. Key words: acrylic resins; denture liners; dental prosthesis.
1. Introduction Cross-linked biopolymer materials based on poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) are widely used, respectively, for denture base construction and direct relining of dental prostheses in the mouth (Reis et al., 2006; Seo et al., 2006; da Cruz Perez et al., 2010; Lombardo et al., 2012). These materials exhibit large variations in both chemical composition and properties (Reis et al., 2006; Seo et al., 2006; da Cruz Perez et al., 2010; Lombardo et al., 2012). In general, the denture base PMMA materials are heat-polymerized and present higher mechanical properties than the autopolymerizing PEMA reline resins (Reis et al., 2006; Seo et al., 2006; da Cruz Perez et al., 2010; Lombardo et al., 2012). However, even the heat-polymerized resins do not possess the ideal requirements. Impact failure due to accidental dropping of the
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prosthesis is still an unsolved problem (Choi et al., 2012; Yu et al., 2014) causing embarrassment and inconvenience to both the patients and their clinicians. One method of improving the mechanical properties of denture base PMMA may be the addition of glass flakes, which was found to significantly improve fracture toughness and hardness properties (Franklin et al., 2005; Nejatian et al., 2006). Also, unlike when using fibers as reinforcements (Vallittu et al., 1995; Chen et al., 2001; Jagger et al., 2001; Kim & Watts, 2004; Karacaer et al., 2003; Doğan et al., 2008; Köroğlu et al., 2009; Hari & Mohammed, 2011) the denture base reinforced with glass flakes can be processed in the usual way, requiring no additional technical time, accurate laboratory technique or new equipment outlay (Karacaer et al., 2003; Franklin et al., 2005; Nejatian et al., 2006; Choi et al., 2012; Alla et al., 2013; Yu et al., 2013). Thus, because of its simplicity, this method could be used to improve the impact strength, particularly of the PEMA hard direct reline resins. However, to date, there have been no reports on the effect of incorporation of glass flakes on the impact resistance of these materials. In a relined denture, the strength is dependent not only upon that of the individual parts (denture base and reline material) but also upon their adhesion at the interface (Reis et al., 2006; Seo et al., 2006; da Cruz Perez et al., 2010; Takahashi & Chai, 2001; Wady et al., 2011; Machado et al., 2012). Thus, besides evaluating and comparing the impact strength of the bulk denture base and reline materials, it is also important to analyze reline-base composite specimens (Reis et al., 2006; Seo et al., 2006; da Cruz Perez et al., 2010; Wady et al., 2011; Machado et al., 2012). Many of the impact studies have been undertaken using the Charpy test method with bar shaped specimens (Vallittu et al., 1995; Memon et al., 2001; Azarri et al., 2003; Karacaer et al., 2003; Faot et al., 2006; Ganzarolli et al., 2007; Köroğlu et al., 2009; da Cruz Perez et al., 2010; Wady et al., 2011; Choi et al., 2012; Machado et al., 2012). However, knowledge of the impact strength of specimens with the shape of removable dentures may also be relevant to predict clinical failure. In this aspect, the falling-weight impact test has been suggested as an acceptable alternative as it was designed to
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simulate impact conditions in service with dentures of conventional size (Kim & Watts, 2004; Hari & Mohammed, 2011; Abdulla, 2012). Another aspect that must be considered is that denture bases (intact or relined) are exposed to cyclic flexural stresses during mastication. Over time, these repetitive stresses can eventually lead to the formation of a crack, which propagates through the denture, resulting in fracture. It has been reported that the flexural properties of denture base and reline materials are detrimentally influenced by cyclic loading (Reis et al., 2006; Seo et al., 2006). However, the effect of mechanical stresses on the impact strength of these denture polymers has not been investigated. The aim of this study was to evaluate the impact strength of denture base acrylic resins and hard chairside reline resins, tested alone (bulk specimens) or in combination (relined specimens). The effect of reinforcement of the reline resins with glass flakes and cyclic loading on the impact strength was also studied for the first time. Impact strength was analyzed by Charpy test (barshaped specimens) or the falling-weight impact test (denture-shaped specimens). The correlation of results obtained with the two impact tests was also calculated.
2. Experimental work The product names, codes, batch numbers, compositions, manufacturers, proportions and polymerization conditions of the materials used in this study are listed in Table 1. To better describe the various specimen groups, a designation system was created (Table 2).
Table 1 – Materials used in this study. Batch Product
Composition
Powder/Liq
Polymerization
uid ratio
conditions
Manufacturers
Code number
Polymer
Tokyuya
Monomer AAEM
Tokuyama
1,9-
Co.,
5.5 min at room ma
T
UF65886
PEMA
2.4 g/1 mL temperature
Rebase II
nonanediol Ltd.,Tokyo,
5
dimethacry Japan late Voco, Ufi Gel
1,6U
631742
PEMA
Hard
2.12 g/1.2
7 min at room
mL
temperature
2.1 g/1.0
90 min at 73 °C and
Cuxhaven, HDMA Germany Dentsply Co., MMA
Lucitone
Ltd.,
P- 379209 L
550
PMMA
EDGM Petrópolis, RJ, mL
L- 510862
30 min at 100 °C
A Brazil VIPI Ind Com Vipi
MMA
Exp Imp Prod
EDGM
Odontol Ltda.,
P- 75643 V
Wave
PMMA L- 70945 A
2.15 g/1.0
20 min at 180W and
mL
5 min at 540W
Pirassununga, SP, Brazil
PEMA, poly (ethyl methacrylate); PMMA, poly (methyl methacrylate); AAEM, 2-(acetoacetoxy) ethyl methacrylate; 1,6-HDMA, 1,6-hexanediol dimethacrylate; MMA, methyl methacrylate; EDGMA, ethylene glycol dimethacrylate.
Table 2 – Specimen group designation system Impact test
Experimental conditions
Code Bulk or intact
No reinforcement and no cyclic loading
Relined
L
L/L
V
V/V
T
L/T
U
L/U V/T V/U
No reinforcement but submitted to cyclic loading
LC
L/LC
VC
V/VC
Charpy (bar shaped-
TC
L/TC
specimens; n = 10)
UC
L/UC V/TC V/UC
With reinforcement but no cyclic loading
TR
L/TR
6 UR
L/UR V/TR V/UR
With reinforcement and submitted to cyclic loading
TRC
L/TRC
URC
L/URC V/TRC V/URC
No reinforcement and no cyclic loading
DSL
DSL/L
DSV
DSV/V DSL/T DSL/U DSV/T DSV/U
No reinforcement but submitted to cyclic loading
DSLC
DSL/LC
DSVC
DSV/VC DSL/TC DSL/UC
Falling-weight impact test (denture-shaped
DSV/TC
specimens; n = 20)
DSV/UC With reinforcement but no cyclic loading
Not Tested
DSL/TR DSL/UR DSV/TR DSV/UR
With reinforcement and submitted to cyclic loading
Not Tested
DSL/TRC DSL/URC DSV/TRC DSV/URC
L – denture base acrylic resin Lucitone 550; V – denture base acrylic resin Vipi Wave; T – reline resin Tokuyama Rebase II; U – reline resin Ufi Gel Hard. R – reinforcement; C – cyclic loading; DS – denture-shaped specimens.
2.1. Impact Strength - Charpy Bar-shaped specimens (60×6×2 mm) were prepared as described elsewhere (da Cruz Perez et al., 2010). Briefly, L and V resins were proportioned, packed into stone molds in conventional or microwave dental flasks, respectively, and processed according to the manufacturer’s instructions (Table 1). The bars were stored in distilled water at 37 °C for 50 ± 2h (da Cruz Perez et al., 2010; Wady et al., 2011; Machado et al., 2012) and then the surfaces to be bonded with the reline materials were ground with 240-grit silicon carbide paper (da Cruz Perez et al., 2010), brushed with detergent for 20 s, washed in distilled water and blot dried. When the reline was made using the
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same material (L or V), the bars with 2 mm thickness were placed into preformed stone molds with 4 mm thickness. The remaining 2 mm was filled with L or V acrylic resins, and the relined specimens L/L and V/V (n=20) were stored in distilled water at 37 °C for 50 ± 2h (da Cruz Perez et al., 2010; Wady et al., 2011; Machado et al., 2012). For the hard reline resins, the bond surfaces of the L and V bars were treated with the bonding agents, which were supplied by the T and U manufacturers. Thereafter, bars were placed into a stainless steel mould (60×6×4 mm) on a glass plate. Materials T and U were inserted into the mould and covered with an acetate sheet and a glass plate until polymerization of the relined specimens L/T, L/U, V/T and V/U (n=20). For comparison purposes, bulk specimens of all materials (L, V, T and U) were prepared (4 mm) and tested (Table 2). To evaluate the effect of the reinforcement on the impact strength of the hard reline resins, additional specimens (n=20) of T and U were made (Table 2). In these specimens (60×6×4 mm), acryl silane micronized glass flakes (GF 003, Glassflake Ltd, Leeds, UK), with thickness of 1.9 – 2.5 μm, were added in amount of 5% by mass of powder with a constant amount of liquid. According to the manufacturer, the composition of the glass flakes is 64-70% SiO2, 8-13% Na2O, 37% CaO, 3-6% Al2O3, 2-5% B2O3, 1-1.5% ZnO, 1-4% MgO, 0-3% K2O and 0-1% TiO2. The flakes were first mixed with the powder, and then added to the liquid (Franklin et al., 2005). The reline resins were polymerized and the reinforced specimens TR and UR were obtained. The effect of reinforcement on the impact strength of relined specimens was also investigated. For this, specimens of each denture base/hard reline material combination were made (Table 2). The glass flakes were only added to the reline materials, producing the specimens L/TR, L/UR, V/TR and V/UR (n=20). To evaluate the effect of cyclic loading, half of the 320 specimens (bulk or relined, with or without reinforcement) was submitted to cyclic flexural tests (104 cycles at a frequency of 5 Hz) (Reis et al., 2006) in a water bath maintained at 37 ± 1 °C (Table 2). The applied load (N) during cycling was obtained at the proportional limit from flexural strength tests of additional specimens of
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each experimental condition (n=10) (Reis et al., 2006). These three point bend tests were conducted with a crosshead speed of 5 mm/min and distance between the specimen supports of 50 mm, in a servo-hydraulic universal testing machine (MTS 810, MTS Systems Corporation, Eden Prairie, MN, USA) (Reis et al., 2006). Before impact testing, V-notches were machined with a notch cutter (Notchvis; Ceast, Pianezza, Italy) into the reline materials (Figure 1), across the width of specimens with 0.8 mm depth leaving an effective depth under the 3.2 mm notch (da Cruz Perez et al., 2010). The impact strength was measured in a Charpy impact tester (RESIL 25R; Ceast), applying the force to the specimen from the unnotched side (da Cruz Perez et al., 2010; Wady et al., 2011) (Figure 2). The test was performed with 0.5 J pendulum and a 150 º lifting angle. The impact strength (IS), expressed in kJ/m2, was calculated as IS = EC/(hbA), where EC is the corrected absorbed energy, bA the remaining thickness at the notch tip, and h is the specimen width. Reline material Interface Denture base material
Figure 1 –Bar-shaped specimen with the V-notch.
Figure 2 – Charpy test showing the V-notched bar-shaped specimen placed on horizontal supports with the midpoint in the path of the pendulum.
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2.2. Falling-weight impact test Denture-shaped specimens were prepared as previously described (Seo et al., 2006). A silicone mold (RTV 3120, Dow Corning, Midland, Michigan, USA) was made on a standard metal cast that simulates a maxillary edentulous arch with no undercuts. This mold was used for the preparation of the standard stone casts (Type IV, Polidental Ind Com Ltda, São Paulo, Brazil). To fabricate the intact denture bases, a denture base pattern was waxed on the standard metal cast with 4-mm-thick base plate wax (Wilson; Polidental Ind e Com Ltda, Sao Paulo, Brazil). Then, acrylic resin artificial denture teeth (VIPI Ind Com Exp e Imp de Prod Odontol Ltda) were set in the wax. A new silicone mold was obtained to ensure that specimens of standard size, shape and thickness could be made. The waxed denture replicas were obtained by first positioning a new set of the artificial teeth in the new silicone mold, pouring melted wax and fully seating a standard stone cast in the mold. Lateral holes in the mold provided vents for the excess melted wax. Following conventional laboratory procedures (Seo et al., 2006), the dentures were flasked and the resins L and V were processed (Table 1). For each PMMA resin, 40 intact denture-shaped specimens (DSL and DSV) were made in this manner, and stored in distilled water at 37 °C for 50 ± 2h (Seo et al., 2006). To fabricate the dentures intended to be relined, base plate wax (Wilson; Polidental Ind e Com Ltda) with a thickness of 2 mm was applied on the standard metal cast. The wax space provided a relief area to be filled with relining materials. The spaced metal cast was duplicated using RTV 1312 silicone and poured with type IV dental stone, giving a total of 400 spaced casts. To obtain the 2-mm dentures, a set of the artificial teeth was positioned in the silicone mold previously used to fabricate the 4-mm denture bases. Thereafter, melted wax was poured into the mold and the 2-mm spaced stone cast was positioned into the mold and the 2-mm-thick waxed base was reproduced. The dentures were then processed (Table 1) using the resins L (n=200) or V (n=200) and stored in distilled water at 37 °C for 50 ± 2h (Seo et al., 2006) Forty dentures of each resin L and V were relined using the same material (Table 2). The 2mm dentures were placed on the standard metal cast, sealed with wax and flasked in a conventional
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manner. The flask halves were separated, the wax removed, and the intaglio surface of the denture bases was cleaned using detergent (20 s), washed with distilled water and dried. L and V resins were applied to the cleaned surfaces and polymerized (Table 1) to fabricate the relined dentureshaped specimens DSL/L and DSV/V (n=40). When the reline was made using T and U reline materials, after cleaning and drying, the bonding agent of each respective reliner was applied with a brush to the intaglio surface of the denture base resin. Materials T and U were mixed and placed onto the intaglio surface of the 2-mm denture. The denture was then reinserted into its original position and kept in a hydraulic press (500 kgf) until polymerization of the relined denture-shaped specimens DSL/T, DSL/U, DSV/T and DSV/U (n=40). The effect of reinforcement on the impact strength of these specimens was also evaluated. For this, specimens of each denture base/hard reline material combination were made (Table 2). The glass flakes were only added to the reline materials, as previously described, producing the specimens DSL/TR, DSL/UR, DSV/TR and DSV/UR (n=40). Before the falling-weight impact tests, half of 480 the denture-shaped specimens (bulk or relined, with or without reinforcement) was submitted to 10,000 cycles flexural loading with a maximum force of 150 N and a minimum of 1 N in a universal testing machine at a frequency of 0.8 Hz (Seo et al., 2006). An area beneath the test specimens simulated the anatomical structure of a torus palatinus. Thus, no contact occurred between the specimens and the residual ridge of the standard brass model during flexural cyclic loading.2 The denture-shaped specimens (DSLC, DSVC, DSL/LC, DSV/VC, DSL/TC, DSL/UC, DSV/TC, DSV/UC, DSL/TRC, DSL/URC, DSV/TRC, DSV/URC; n = 20) were submitted to the cyclic flexural stresses in a water bath maintained at 37 ± 1 °C and the load force of 150 N was applied in the molar region, to simulate the occlusal force of patients wearing complete dentures (Seo et al., 2006). The dimensions of the specimens also closely approximated those of actual prostheses (Figure 3).
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Figure 3 –Denture base shaped specimen with the frenal notch. The falling-weight impact tests were performed using an impact testing apparatus (DT3P, DSM, Instrumentação Científica Ltda., São Paulo, SP, Brazil). In this test, an impactor was fastened to a load-carrying device to which weights could be attached (Kim & Watts, 2004; Hari & Mohammed, 2011; Abdulla, 2012). The denture-shaped specimens were placed on the table of the apparatus (Figure 4) and the impactor with an initial mass was released from a height of 66 cm to drop onto the denture.
Figure 4 – Falling-weight impact test with the denture-shaped specimen placed with the teeth facing up. At least 20 specimens of each experimental condition were tested sequentially, whereby the dart mass used to a given specimen was dependent on the outcome (fail or not fail) of the previous data. The dart mass in each succeeding test was either increased or decreased by a given increment (15 g – ΔW) depending upon whether denture fracture was experienced or not. To calculate the mean-failure mass to fracture (W), the number of failures for each dart mass (ni) was recorded,
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considering the last 10 failures occurred during the test. The darts (i) were identified using a counting index (0, 1, 2…) where 0 (zero) represented the dart with the smallest mass at which an event (failure) occurred. The product of ni and i was calculated (ini) and then the sum of all ini was identified as A. Thus, the mean-failure mass was calculated from the test data obtained, as follow: W = W0 + [ΔW(A/N – ½)], where: W = mean-failure mass (g); ΔW = increment of dart mass (g); N = total number of failures; W0 = smallest mass at which an event occurred (g); and A = Σ ini. The failure energy in Joules was then calculated, as follow: E = Wgh, where: W = mean-failure mass (g); g = 9.81 m/s2; and h is the height from which the dart was dropped (66 cm).
2.3. Statistical Analysis Data from Charpy tests were analyzed by one-way analysis of variance and Dunnett C post hoc test. For the falling-weight impact tests, results were analyzed by one-way of variance and Tukey’s test. A significance level of p< 0.05 was used. In addition, to test for possible correlation between data from Charpy and falling-weight impact tests, a linear correlation ‘‘r’’ was calculated.
3. Results and discussion Figures 5 and 6 present the results from the Charpy and falling-weight impact tests, respectively. The heat-polymerized resin L and the microwave-polymerized resin V showed comparable Charpy impact resistance. Similar findings were obtained from the falling-weight impact test, with values of 738.82 J and 758.24 J for resins L and V, respectively.
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Figure 5 – Results (kJ/m2) from the Charpy impact test performed with the bar-shaped specimens. Note: for abbreviations, see Table 2.
Figure 6 – Results (J) from the falling-weigth impact test performed with the denture-shaped specimens. Note: for abbreviations, see Table 2.
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Studies that tested unotched specimens found significant differences in the Charpy impact strength between heat- and microwave-polymerized resins, according to the type of resin, power and period of microwave irradiation (Memon et al., 2001; Azarri et al., 2003; Faot et al., 2006; Ganzarolli et al., 2007). In this study, notched specimens were selected to simulate the effect of the frenal notches on the impact strength (Vallittu et al., 1995), concentrate the stress overcoming the effects of surface defects in the specimens (Jagger et al., 2001), and direct the crack in a specific direction. Therefore, due to the different experimental protocols it is difficult to compare the findings directly. With regard the falling weight impact test, the mean values obtained here are higher than those reported for high-impact denture base acrylic resins (with or without reinforcement), which ranged from 85.62 J to 277.9 J (Kim & Watts, 2004; Hari & Mohammed, 2011; Abdulla et al., 2012). This difference can be attributed to the fact that in these studies, the same specimen was loaded repeatedly until failure while in the present investigation the test was performed only once for each specimen and the event (failure or non-failure) was recorded. For both Charpy and falling-weight impact tests, the laboratory processed relined specimens (L/L, V/V, DSL/L and DSV/V) showed mean impact strength values equal to those of the bulk specimens (L, V, DSL and DSV). On the other hand, when reline was made with the autopolymerizing resin U, in general, the impact strength values were lower than that of the intact resins L and V. Relining with resin T resulted, generally, in increased impact strength compared to the intact denture base resins. Similar results were observed previously in Charpy impact tests of bar-shaped specimens made from resin L relined with different materials (da Cruz Perez et al., 2010; Wady et al., 2011; Machado et al., 2012). The findings were attributed to the strength of the bond between the two materials, which is based on dissolving and swelling the PMMA-denture base surface, diffusion and polymerization of monomers, and formation of interpenetrating polymer networks (Takahashi & Chai, 2001; Vallittu, 2009). L specimens relined using the same material
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(indirect laboratory system) or the autopolymerizing resin U (direct relining) exhibited strong bond between the denture base and the reliner (Vergani et al., 2010). In such situation, when the impactor struck the specimens (Figs. 2 and 4), the crack started at the tip of the stress concentrator (V-notch and frenal notch - Figs. 1 and 3) may have propagated across the interface and through the resins L and V, leading to catastrophic failure without being arrested. Indeed, during the impact tests of L/L, V/V, DSL/L and DSV/V specimens and of the L and V specimens relined using material U, no delamination was observed. This further suggests that the interfacial bonding was strong enough so that the bilayered structure acted as a homogenous material and the crack initiated in the reline material crossed the interface. On the other hand, when the adhesion between the two materials is not so strong, the cracks are more likely to branch along the bimaterial interface, before propagating into the denture base resins. This was probably one factor influencing the results obtained here and in previous experiments (da Cruz Perez et al., 2010; Wady et al., 2011; Machado et al., 2012) for specimens relined with material T. It has been found that the bond strength of material T to denture base polymers was lower compared to other reline resins (Hayakawa et al., 2006; Ohkubo et al., 2009). In addition, as seen previously (da Cruz Perez et al., 2010; Wady et al., 2011; Machado et al., 2012), in the present investigation a large number of specimens relined with material T showed delamination, indicating that the bond between T and the resins L and V was not so strong. Delamination between the two resins may have consumed energy during the tests, leading to the increased impact strength of the bar- and denture-shaped relined specimens (Figs. 5 and 6). This increase was more pronounced when the denture base resin V was used with the reline resin T, with the exception of the conditions V/TR and L/TR, and DSL/T and DSV/T, which did not differ from each other. To the authors' knowledge, this is the first report on the effect of cyclic loading on the impact strength of denture base biopolymer materials. Flexural cyclic loading was chosen because it best simulates denture base exposure to flexural bending during occlusal function, and the tests were performed in water at 37 ± 1 °C in an attempt to mimic the clinical conditions (Johnston et al.,
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1981; Reis et al., 2006). The specimens were exposed to a finite number of load cycles (104) (Reis et al., 2006; Seo et al., 2006) to provide information on possible changes in the impact strength of the materials. For both Charpy and falling-weight impact tests, cyclic loading was applied so that the denture base material was submitted to tensile stresses and the reline material to compression forces. It has been reported that relatively small stresses over a period of time can lead to the development of microscopic cracks or microdamages of the polymer matrix. These cracks can grow and propagate through the denture, even when the loads are comparatively low, and they may fuse to an ever growing fissure that weakens the material (Narva et al., 2005; Doğan et al., 2008). Denture fracture results from a final loading cycle that exceeds the mechanical capacity of the remaining sound portion of the material (Jagger et al., 1999; Doğan et al., 2008). Cyclic loading had a weakening effect on denture base and reline acrylic resins, decreasing the flexural properties (Vallittu et al., 1994; Reis et al., 2006; Seo et al., 2006). However, in this study, for each experimental condition, comparison between the cycled and non-cycled specimens, did not demonstrate any significant differences in the impact strength values (Figs. 5 and 6). Charpy and falling-weight impact tests are high speed fracture tests which measure the energy to break the specimen when struck by a sudden blow from an impact instrument (Faot et al., 2006) with a weighted pendulum or dart, whereas in the flexural tests specimens are subjected to bending at constant speed until failure (Reis et al., 2006). Given such differences, it is conceivable that the results and trend obtained from flexural and impact tests may differ as well. Thus, the results obtained here suggest that two approaches (flexural and impact tests) may complement each other in determining the effect of cyclic loading on the mechanical performance of denture base polymers, not found by using only one approach. The incorporation of inorganic filler particles such as glass flakes to denture base resins has been proposed to enhance mechanical properties, with less expense and improved handling properties (Franklin et al., 2005; Nejatian et al., 2006). Hence, in this study, glass flakes were chosen as reinforcement for the autopolymerizing reline resins. The glass flakes used were treated
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with 3-methacryloxypropyltrimethoxysilane (γ-MPS), which contains tri-alkoxy groups and has been widely used in commercial silanes in dentistry (Elshereksi et al., 2009; Tham et al., 2010; Lung et al., 2012). Silane treated filler are able to disperse more favorably in the polymer matrix and to have better adhesion to the matrix (Tham et al., 2010; Lung et al., 2012). No information could be identified by the authors on the addition of glass flakes to PEMA-based denture reline polymers. Ayad et al. (2008) added zirconia (ZrO2) powder, treated with zirconate coupling agent in acetone, to PMMA. They found that the flexural strength increased with ZrO2 at concentrations of 5% and 15%. However, there was no improvement for hardness and the impact strength slightly decreased. In the case of BaTiO3 filler, the neat PMMA exhibited slightly higher fracture toughness properties than the PMMA composite (Elsesheksi et al., 2009). The incorporation of hydroxyapatite (HA) filler (untreated or treated with γ-MPS at concentrations of 2, 4, 6 and 8%) (Tham et al., 2010) and Al2O3/ZrO2 (Alhareb et al., 2011) to PMMA was also evaluated. Although the fracture toughness was increased, a reduction in flexural and tensile properties was observed. The addition of Al2O3 powder, as well as ZrO2 nanotubes or ZrO2 nanoparticles, was found to increase the strength of PMMA when tested in flexural tests (Ellakwa et al., 2008; Yu et al., 2014) in comparison to the unreinforced control, but the authors did not measure other mechanical properties. The glass flakes used in the present investigation has been found to improve the hardness (Nejatian et al., 2006) and the fracture toughness (Franklin et al., 2005; Nejatian et al., 2006) of denture base resins. Figures 5 and 6 show that, in general, when the glass flakes were added to the reline resins, the impact strength was maintained, and, in the case of bar-shaped specimens, the impact strength of L/TR was significantly higher than that of L/T. Several studies investigating the relationship between the strength of denture base biopolymer materials and different reinforcements were performed with bar-shaped specimens. However, an evaluation of the effect of reinforcements on the strength using denture base-shaped specimens may better simulate the clinical situation. In this aspect, the falling weight impact test method has been pointed as an acceptable alternative to measure the impact strength of specimens
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with the geometrical shape of complete dentures, making the results more relevant clinically. Thus, it is of special interest to note that, in this study, a high correlation between the results from the Charpy and the falling-weight impact tests was observed (r = 0.8854). These findings, together with the relative simplicity of the procedure, lower cost and time saving to prepare the specimens, suggest that the Charpy test using bar-shaped specimens can be used as an in vitro evaluation of a variety of factors that may influence the impact strength of denture base polymers.
4. Conclusions In this in vitro study, the denture base acrylic resins Lucitone 550 and Vipi Wave displayed significantly higher Charpy impact strength than the reline materials Ufi Gel Hard and Tokuyama Rebase II. For both Charpy and falling-weight impact tests, no significant differences were found between intact (bulk) and relined specimens, when the same denture base material (Lucitone 550 or Vipi Wave) was used to produce and reline the specimens. In general, relining with Ufi Gel Hard resulted in significantly lower impact strength compared with the intact denture base resins Lucitone 550 and Vipi Wave, in both impact tests used. With few exceptions, the specimens relined with the material Tokuyama Rebase II exhibited significantly higher Charpy and falling-weight impact strength than those of the intact denture base resins Lucitone 550 and Vipi Wave. Cyclic loading did not adversely affect the impact strength of the materials. After the incorporation of glass flakes to the reline resins Ufi Gel Hard and Tokuyama Rebase II, the impact strength was maintained. And, finally, a high positive linear correlation was observed between the two methods used to measure the impact strength (Charpy and falling-weight impact tests).
Acknowledgements This investigation was supported by Brazilian Council for Scientific and Technological Development (CNPq - Grant 301042/2004-7) and Sao Paulo State Research Foundation (FAPESP Grants 05/04101-0 and 06/00773-6).
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